Zirconium Oxide Based Fibers for Effective Antimonate and Pertechnetate Removal

Department of Chemistry – Radiochemistry University of Helsinki Finland

ZIRCONIUM OXIDE BASED FIBERS FOR EFFECTIVE ANTIMONATE AND PERTECHNETATE REMOVAL

Satu Lönnrot

DOCTORAL DISSERTATION

To be presented for public discussion with the permission of the Faculty of Science of the University of Helsinki, in Auditorium D101 at Physicum, on the 20th of November, 2020 at 12 o’clock.

Helsinki 2020

Supervisor

University Researcher, Risto Koivula Ion Exchange for Nuclear Waste Treatment and for Recycling Department of Chemistry – Radiochemistry Faculty of Science University of Helsinki, Finland

Pre-examiners

Associate Professor, Eveliina Repo Hydrometallurgy for Urban Mining Department of Separation and Purification Technology LUT University, Finland

Chair Professor, Jurate Kumpiene Waste Science and Technology Geosciences and Environmental Engineering Department of Civil, Environmental and Natural Resources Engineering Luleå University of Technology (LTU), Sweden

Dissertation Opponent

Professor, Ulla Lassi Applied Chemistry Sustainable Chemistry Faculty of Technology University of Oulu, Finland

ISBN 978-951-51-6740-8 (paperback) ISBN 978-951-51-6741-5 (PDF) http://ethesis.helsinki.fi

Unigrafia Helsinki 2020

ABSTRACT

During recent years the emerging threat caused by climate change has directed energy production towards carbon-free technologies including nuclear power with a stable energy production. However, the nuclear energy production generates radioactive fission and activation products that are posing a threat to human health and environment without proper handling of nuclear waste streams. Among the radionuclides, activated antimony isotopes: 122Sb, 124Sb and 125Sb, and a high fission yield 99Tc are of special interest due to their high activities, anionic species and redox chemistry.

Zirconium dioxide (ZrO2) is a ceramic material with a high chemical and physical resistance. Therefore, ZrO2 is highly suitable for a treatment of radioactive waste solutions in which it might experience elevated temperatures, high radiation, presence of corrosive chemicals and fast changes in the conditions. Due to the different crystal structures and morphologies of ZrO2 and its suitability for doping, the material can be applied to separation of different metal ions. The materials selectivity and other separation properties can be tuned by modifying the structure and chemical composition of ZrO2. In this thesis, the ZrO2 is used in different forms to remediate Sb(V) and Tc(VII). The main purpose of this thesis was to develop electroblown ZrO2 submicron fibers for the separation. At first, the effect of morphology on the Sb(V) uptake was studied by comparing fibrous and microparticulate zirconia with each other and special focus was targeted to adsorption kinetics and pressure development during the column operation. Secondly, the influence of ZrO2 crystal structure (amorphous, tetragonal, monoclinic) on the Sb(V) adsorption was investigated. Finally, the tetragonal ZrO2 fibers were functionalized with Sb doping to efficiently and selectively remove Tc(VII) from aqueous solutions. The electroblowing and calcination synthesis produced separate, cylindrical and long fibers with a diameter of 300 – 800 nm. In the Sb(V) adsorption experiments, the ZrO2 showed good adsorption performance by retaining Sb(V) on a broad pH range and having fast adsorption kinetics. Compared to microsized zirconia granules, the fibers maintained lower pressures during column operation indicating improved utility of nano- and microsized materials that would allow fast feeding rates of purifiable solution through the column. In addition, the tetragonal ZrO2 crystal structure was found to be beneficial for Sb(V) adsorption as the tetragonal fibers had higher adsorption capacity than the amorphous or monoclinic ZrO2 fibers. The superiority of the tetragonal ZrO2 fibers was expected to originate from the largest specific surface area generating higher number of adsorption sites, and from a higher adsorption energy inducing more stable surface complexes. According to conducted experiments, an inner-sphere complexation was assumed to be the dominating separation mechanism but Sb(V) was also partially retained by an outer-sphere complexation or ligand exchange.

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– Since the undoped ZrO2 fibers showed only poor TcO4 separation, the material was – functionalized with the Sb doping. The Sb doping improved the TcO4 separation as several orders of magnitude higher Kd values were obtained with the Sb-doped ZrO2 fibers than with the undoped ZrO2 fibers. The superior separation probably originated from the reduction of Tc(VII) to Tc(IV) by the Sb(III) dopant, and then Tc(IV) was retained on the ZrO2 – surface. The material showed higher selectivity towards TcO4 than many other published – materials as it was not interfered with even by stereostructurally similar ClO4 . In addition, – the Sb-doped ZrO2 fibers showed only limited ReO4 uptake as these anions were not reduced. Instead, they were assumed to be bound on the zirconia surface with the – electrostatic interaction mechanism that was also the probable mechanism for TcO4 adsorption on the undoped ZrO2 fibers.

Overall, the electroblown ZrO2 fibers demonstrated good radionuclide separation performance that can be addressed to separation of Sb(V) and Tc(VII) and potentially some other elements. Furthermore, the simple electroblowing synthesis can be applied to the synthesis of other inorganic fibers extending the separation potential of the fibers to an even higher number of target compounds.

ACKNOWLEDGEMENTS

The research presented in this thesis was conducted in the ion exchange group, Department of Chemistry – Radiochemistry at University of Helsinki in 2016 – 2020. The research was a part of the NanoFib project that was funded by Fortum Power and Heat Oy. The thesis writing was supported with a grant provided by the Doctoral School in Natural Sciences. Additionally, I would acknowledge the travel grant provided by the doctoral program in chemistry and molecular sciences (CHEMS). First of all, I would like to thank my supervisor Risto Koivula who offered me the possibility to work in this research project and start my doctoral studies. I appreciate your guidance and support throughout these years. I thank Leena Malinen, Pasi Kelokaski and Jussi-Matti Mäki from Fortum for guiding the project and giving valuable feedback. I’m grateful to Johanna Paajanen, Valtteri Suorsa, Mikko Ritala and Risto Koivula, who were actively involved in the NanoFib -project. I am thankful for your support, advices, and assistance with the publications. I also thank current and former members of the ion exchange group and all the members of Radiochemistry group for nice discussions and assistance with the research. I thank my former officemates Elmo Wiikinkoski and Ilkka Välimaa for helping me with various matters and enjoyable cola breaks. I also appreciate the support of my former officemate Wenzhong Zhang who offered his advice whenever I was asking it and I thank him for daily conversations that enlightened my days. I thank professor Gareth Law and the radiochemistry seniors for advices, support and teaching throughout my master’s and doctoral studies. I am especially grateful to the late professor Risto Harjula whose outstanding lecturing skills got me interested in radiochemistry. Ilman vanhempieni tukea ja kannustusta en olisi päässyt opinnoissani näin pitkälle. Vaikka ala-asteella aapiset ja ehkä muutkin kirjat lensivät kaaressa seinään, te kärsivällisesti jaksoitte tukea minua kotitehtävissä. Sen jälkeen opiskelusta on tullut paljon helpompaa. Kiitos kaikesta tuesta! I appreciate the patience of my sister Kati who listened my complaints about unsuccessful experiments and shared her thoughts on them. Coffee breaks with you absolutely relieved my stress and kept me continuing with my experiments in a better mood. I also appreciate your help in many matters including research. I am also grateful to my husband Peter for his support during my studies. You offered your shoulder when I needed it, listened my worries and encouraged me to continue. Thank you!

“When the snows fall and the white winds blow, the lone wolf dies but the pack survives.” George R.R. Martin

Satu Lönnrot Helsinki 2020

TABLE OF CONTENTS

ABSTRACT ...... iii ACKNOWLEDGEMENTS ...... v TABLE OF CONTENTS ...... vi LIST OF ORIGINAL PUBLICATIONS ...... viii ABBREVIATIONS ...... ix 1 INTRODUCTION ...... 1 1.1 Antimony and Technetium ...... 1 1.1.1 Antimony ...... 1 1.1.2 Technetium ...... 4 1.2 Separation techniques for antimony and technetium ...... 7 1.2.1 Extraction ...... 7 1.2.2 Electrochemical methods ...... 8 1.2.3 Coagulation and flocculation ...... 8 1.2.4 Membrane separation ...... 9 1.2.5 Adsorption and ion exchange ...... 9

1.3 Zirconium dioxide (ZrO2) materials ...... 11 1.3.1 Structure and morphology ...... 11 1.3.2 Properties ...... 12 1.4 Nano- and submicron fibers ...... 14 1.4.1 Properties and uses ...... 14 1.4.2 Synthesis by spinning techniques ...... 14 1.4.3 Synthesis of inorganic fibers by spinning techniques ...... 19 2 AIM OF THE STUDY ...... 20 3 EXPERIMENTAL ...... 21 3.1 Synthesis ...... 21 3.2 Characterization ...... 22 3.3 Uptake experiments ...... 24 3.3.1 Batch experiments ...... 24 3.3.2 Column experiments ...... 25 3.3.3 Results calculation ...... 26

4 RESULTS AND DISCUSSION ...... 27 4.1 Materials characterization ...... 27

4.1.1 Characterization of ZrO2 and Sb-doped ZrO2 submicron fibers ...... 27 4.1.2 Characterization of precipitated Zr-oxyhydroxide particles ...... 33

4.2 Effect of ZrO2 morphology on Sb(V) separation performance ...... 35

4.3 Effect of the ZrO2 fibers crystal structure on Sb(V) adsorption ...... 40 – 4.4 Functionalization of the ZrO2 fibers by Sb doping: Uptake of TcO4 ...... 46 5 CONCLUSIONS ...... 53 REFERENCES ...... 55

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LIST OF ORIGINAL PUBLICATIONS

This thesis is based on the following publications: I. Lönnrot, S., Suorsa, V., Paajanen, J., Hatanpää, T., Ritala, M. and Koivula, R., Submicron fibers as a morphological improvement of amorphous zirconium oxide particles and their utilization in antimonate (Sb(v)) removal, RSC Advances, 2019, 9, 22355-22365 II. Paajanen, J.,# Lönnrot, S.,# Heikkilä, M., Meinander, K., Kemell, M., Hatanpää, T., Ainassaari, K., Ritala, M. and Koivula R., Novel electroblowing synthesis of submicron zirconium dioxide fibers: effect of fiber structure on antimony(v) adsorption, Nanoscale Advances, 2019, 11, 4373-4383 III. Lönnrot, S., Paajanen, J., Suorsa, V., Zhang, W., Ritala, M. and Koivula, R., Sb-doped - zirconium dioxide submicron fibers for separation of TcO4 , Separation Science and Technology, 2020, https://doi.org/10.1080/01496395.2020.1826967 # Equal contribution The publications are referred to in the text by their roman numerals.

Contribution on these articles:

I. S.L. conducted synthesis of the amorphous ZrO2, most of the characterization and all the adsorption experiments. S.L. wrote the manuscript. V.S. synthesized the fibers based on recipe designed by J.P. and V.S. performed the SEM imaging. T.H. analyzed the TGA-MS. The BET was measured as a service. V.S., J.P., M.R. and R.K. commented the manuscript. II. S.L. conducted the Sb(V) adsorption experiments and the zeta potential analysis and wrote the parts of the manuscript covering those topics. J.P. synthesized characterized the fibers with SEM, TEM and XRD and wrote these sections. Abstract, introduction and conclusions sections S.L. and J.P. contributed equally. M.H. conducted the Rietveld refinement of XRD data, K.M. conducted the XPS analysis, M.K. assisted with the TEM analysis and T.H. conducted the TGA and K.A. the BET measurement. M.R. and R.K. commented the manuscript. III. S.L. with assistance of J.P. synthesized the fibers and S.L. wrote the manuscript. S.L. conducted most of the experiments and characterizations except that J.P. conducted the SEM imaging, W.Z. and V.S. measured the XAS of the samples provided by S.L. and the BET was measured as a service. J.P., W.Z., V.S., M.R. and R.K. commented the manuscript.

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ABBREVIATIONS

BET Brunauer-Emmett-Teller (specific surface area calculation method) BF Batch factor BJH Barrett-Joyner-Halenda (porosity calculation method) BT Breakthrough CPM Counts per minute DFT Density functional theory DL Detection limit et al. et alia (and others) FE-SEM Field emission – scanning electron microscopy

FTIR-ATR Fourier transform infra-red-attenuated total reflectance

FZR Fibrous ZrO2

GZR Granular ZrO2 ICP-MS Inductively coupled plasma – mass spectrometry PVA Poly(vinyl alcohol) PVP Poly(vinyl pyrrolidone) PZC Point of zero charge TEM Transmission electron microscopy TGA Thermogravimetric analysis XANES X-ray absorption near edge structure XRD X-ray diffraction XRF X-ray fluorescence YSZ Yttria-stabilized zirconia ZVI Zero valent iron

1 INTRODUCTION

1.1 Antimony and Technetium

Radioactive antimony and technetium are generated in nuclear reactors during energy production. The main production routes of these elements are different, fission for technetium and neutron activation for antimony, but they both have oxy-anionic species that predominate in oxidizing conditions and thereby dictates their chemistry in such environment. Antimony has both stable and radioactive isotopes whereas technetium has only radioactive isotopes with a varying half-life. As these two elements have different half- lifes and types of ionizing radiation, they are creating different problems as well. The major hazards created by 122Sb, 124Sb and 125Sb isotopes are their high gamma energies and intensities that are elevating radiation doses of a nuclear power plant personnel but these isotopes are rather short lived and not causing long-term problems. Instead, a long-lived 99Tc can cause health risks by β– emission when in a direct contact with living organisms and due to its long half-life requires safe final disposal.

1.1.1 Antimony

Antimony (Sb) is a metalloid with an elemental number of 51 that belongs to a group 15 called pnictogens together with arsenic and phosphorus that have a reminding chemistry. The origin of the name antimony is uncertain but it is suggested to originate from the Greek words ‘anti’ and ‘monos’ meaning ‘not alone’, as Sb usually occurs with sulfur and heavier metals, or from the French word ‘antimoine’ meaning ‘monk-killer’ as alchemists, who were mainly monks, discovered toxicity of Sb.1 The element symbol Sb originates from the Latin name of antimony, stibium. In ancient times, antimony (Sb2S3) was used as a make-up, for example, to contour eyes but nowadays Sb has found applications from many fields such as metallurgy, electronics, flame-proof retardants and as a drug for treatment of leishmaniasis caused by a leishmania parasite.2-4 Due to an extensive use of Sb, it has been estimated that 160,000 tons of Sb was produced in 2019 of which over 60% was extracted by China.5 Sb has several oxidation states in the solids but only +III and +V are found from aqueous solutions in mildly reducing or oxidizing conditions.4,6,7 Antimonite (Sb(III)) prevails in the reducing conditions whereas antimonate (Sb(V)) occurs in the oxidized waters. However, Sb(III) can occur in the oxidized waters and Sb(V) in the reducing systems for some time before they undergo oxidation or reduction as their redox kinetics in natural conditions is 4 2+ + – rather slow. Sb(III) has four hydroxyl species Sb(OH) , Sb(OH)2 , Sb(OH)3 and Sb(OH)4 whose occurrence follows an increasing pH value. In addition, free Sb3+ ions can appear in acidic solutions. Correspondingly, Sb(V) has two hydroxyl species in the normal pH range, – – neutral Sb(OH)5 and negatively charged Sb(OH)6 (or SbO3 ) with the pKa of 2.7. The + positively charged Sb(OH)4 is present only in highly acidic solutions. The speciation of

Sb(III) and Sb(V) is demonstrated in Fig. 1. Because Sb(III) and Sb(V) have multiple species that occurrence change along the pH, Sb remediation is influenced by the pH value and the redox conditions of the target solution. Typically, the Sb concentration in the natural waters is less than 1 μg/L but human activities such as a mining and smelting industry can raise the concentration over 100 times.4 As Sb is a toxic element, the pollution has raised some concerns of its effects on the human health and the environment.8 For that reason, several authorities have set the maximum Sb concentration limits for drinking water.9,10 For example, the EU drinking water directive 98/83/EC about quality of water intended for human consumption regulated the maximum Sb concentration limit to 5 μg/L.9

3+ 2+ + - - Sb Sb(OH) Sb(OH)2 Sb(OH)3 Sb(OH)4 Sb(OH)5 Sb(OH)6 100

40 Species (%)

0 0123456789101112

pH Fig. 1. Speciation of 10-8 M Sb(III) and Sb(V) as function of pH. The speciation was calculated using the PhreeqC program and its ANDRA Thermochimie -database.11

Antimony has two stable isotopes: 121Sb (57.21%) and 123Sb (42.79%). In addition, it has 122 124 125 several radioactive isotopes of which Sb (t½ 2.72 days), Sb (t½ 60.2 days) and Sb (t½ 2.76 years) are considered the most important ones due to their reasonably long half-lifes and relatively high generation in the nuclear reactors.12,13 Since these three isotopes emit high-energy gamma radiation (Table 1), they are causing a significant radiation dose to the nuclear power plant personnel together with other gamma-emitting nuclides 60Co, 58Co, 110mAg, 59Fe and 54Mn particularly during reactor shutdowns when maintenance is conducted in a close proximity of activated components.14 From these nuclides, 124Sb and 110mAg are evaluated to be of special concern during the reactor shut-down operations and requiring additional dosimetry especially in the case of contamination due their high concentration, gamma energies and intensities, and mobilization during a reactor oxygenation.13

Table 1. The main activation products in the primary cooling system with the activation reaction, half-life, the most intensive gamma energies and the major sources.12,13

Radio- Activation Half-life Gamma energies Sources nuclide reaction keV, (%) 51Cr 50Cr(n,γ)51Cr 27.70 d 320.0 (9.91) Stainless steel and nickel based alloy 54Mn 54Fe(n,p)54Mn 312.2 d 834.8 (99.98) Stainless steel and nickel based alloy 55Fe 54Fe(n,γ)55Fe 2.744 y 126.0 (1.28·10-7) Stainless steel and nickel based alloy 58Co 58Ni(n,γ)58Co 70.86 d 810.8 (99.45), 864.0 (0.69), 1675 Nickel alloys (0.52) 59Fe 58Fe(n,γ)59Fe 44.49 d 192.3 (3.08), 1099 (56.8), 1292 Stainless steel and nickel based alloy (43.2) 59Ni 58Ni(n,γ)59Ni 7.6·104 y – Stainless steel and nickel based alloy 60Co 59Co(n,γ)60Co 5.275 y 1173 (99.85), 1332 (99.98) StelliteTM and Co-bearing components 65Zn 64Zn(n,γ)65Zn 243.9 d 1116 (50.0) Natural zinc injection 95Nb 95Zr decay 34.99 d 765.8 (99.8) Fuel cladding (Zircaloy, ZirloTM etc.) 95Zr 94Zr(n,γ)95Zr 64.03 d 724.2 (44.3), 756.7 (54.4) Fuel cladding (Zircaloy, ZirloTM etc.) 110mAg 109Ag(n,γ)110mAg 249.8 d 657.8 (95.6), 937.5 (35.0), 1384 Ag-In-Cd control rod wear, (25.1) HelicoflexTM seals 122Sb 121Sb(n,γ)122Sb 2.724 d 564.0 (70.7), 692.7 (3.85) Sb-Be source, RCP bearings, impurities 124Sb 123Sb(n,γ)124Sb 60.20 d 602.7 (97.8), 722.8 (10.8), 1691 Sb-Be source, RCP bearings, (47.6) impurities 125Sb 124Sb(n,γ)125Sb 2.759 y 427.9 (29.6), 600.6 (17.7), 636.0 Fuel cladding impurities and neutron 125Sn decay (11.2) capture by 124Sb 181Hf 180Hf(n,γ)181Hf 42.39 d 133.0 (43.3), 345.9 (15.1), 482.2 Fuel cladding impurities (80.5)

Regardless of the isotope, the chemistry of antimony is the same as long as Sb occurs as a dissolved species, not as a particulate matter. During the energy production, the coolant water of the nuclear reactor is maintained close to the neutral pH and slightly reductive to minimize the corrosion of metal components. Despite the efforts, still some corrosion occurs and dissolved antimony among other metals mobilizes and flows through the reactor core. In the reactor, the dissolved antimony can come into contact with a zircaloy cladding that tends to oxidize to zirconium dioxide (ZrO2) during the operation. Since ZrO2 shows affinity to the Sb sequestration, Sb accumulates as Sb(III) on the zirconia cladding leading to the activation of Sb.14-16 When the chemical environment changes – during the reactor shutdown, Sb remobilizes as Sb(V)O3 because of H2O2 induced oxidation that elevates the radiation dose rates outside the reactor core and thereby creating a radiation hazard to the personnel.14 As mentioned, antimony belongs to the activated corrosion products, Table 1, that form as a result of dissolution or corrosion of reactor materials that get activated in the reactor core.13 In recent decades, a major source of antimony in the coolant water has been Sb-impregnated graphite bearings and seals of primary coolant pumps or smaller pumps.13 Even though the problem has been realized, the replacement of these parts has been rather slow and expensive process, and therefore these parts are still used on some nuclear power plants. Another source of antimony is an external Sb-Be neutron source which purpose is an initiation of nuclear fission in the nuclear reactors by the neutron

activation of 123Sb(n,γ)124Sb that causes a photo induced neutron emission from 9Be(γ,n)8Be.13,17 Unlike is for 124Sb, 125Sb commonly originates from the activation reaction of tin (Sn) that is present as a minor constituent in a fuel cladding (Zircaloy and ZIRLOTM). The activation can be described with the reaction equation 1

124Sn(n,ɣ)125Sn → 125Sb. (1) Additionally, 125Sb is generated when 124Sb absorbs a neutron, but it is not as significant formation mechanism as the activation of tin. The neutron activation reactions for 122Sb and 124Sb are given below:13 121Sb(n,ɣ)122Sb (2) 123Sb(n,ɣ)124Sb. (3) Furthermore, the three isotopes 122Sb, 124Sb and 125Sb are formed through a fission of 235U but their fission yields are low (4.8·10-9 – 2.7·10-7) making the fission reaction a less important formation mechanism.12 Moreover, the fission products inside the nuclear fuel rods are less likely to dissolve in the primary coolant water and mobilize.

1.1.2 Technetium

Technetium (Tc), with the elemental number of 43, is the lightest element without stable isotopes.12 Tc is a transition metal and its chemistry resembles those of manganese and rhenium that belong to the same 7th group of periodic table. Dmitri Mendeleev, who named the element as ekamanganese (Em), predicted existence and properties of Tc for the first time in 1871.18 Not until decades later, Tc was artificially produced. Therefore, its name originating from the Greek language means synthetic or artificial. As even the 98 longest-lived Tc isotope ( Tc with t½ 4.2 million years) have a short half-life compared to the Earth’s age, all Tc that was present when the Earth was formed has already decayed. Thus, before anthropogenic pollution, only negligible amount of Tc occurred on the planet because of spontaneous fission of 238U. Therefore, almost all Tc present today originates from anthropogenic sources. Tc has only radioactive isotopes but three of them are very long-lived when compared 97 6 98 6 to human age. Therefore, these three isotopes Tc (t½ 2.6·10 y), Tc (t½ 4.2·10 y) and 99 5 Tc (t½ 2.11·10 y) are causing radiation hazard for a long time. However, as they are not emitting significant gammas, the radiation hazard is caused by their beta emission. Because the beta emission has only a short range, Tc needs to be almost in a direct contact with the skin or to be ingested to cause significant damage. Therefore, the contamination of the environment should be avoided as much as possible. The major source of Tc is the nuclear energy production. During the reactor operation, Tc isotopes are generated by 235U fission and they accumulate inside fuel rods. The isotope of the highest concern is 99Tc due to its high cumulative yield of 6.13%, Fig. 2. In addition, 99Tc is formed through the neutron

activation of 98Mo, which is impurity in the reactor materials such as stainless steel. However, the quantity of 99Tc generated by the neutron activation is negligible compared to fission yield but the activated corrosion product is more abundant in the primary coolant water if no fuel leakages occur. Despite, the challenges that Tc is causing in the nuclear 99m industry, metastable Tc (t½ 6.0 h) has found practical use in medicine where it is utilized for imaging of different organs and tumors.

-2 2.0∙10

-2 -2 3.6∙10 10 -3 -3 4.4∙10 10 -4 -4 4.3∙10 10

-5 10 Fission yield -7 -6 2.7∙10 10

-7 -7 10 1.0∙10 -8 -9 10 4.8∙10 -9 -9 105 10 1.5∙10 100 95 -10 90 10 Others 85 -11 25 80 Y-99 10 Zr-99 30 75 70 Nb-99 35 Z (Atomic40 number) 65 60 Mo-99 45 55 Tc-99 50 N (Neutron number) 50 Sb-122 55 45 60 Sb-124 40 Sb-125 65

Fig. 2. Independent fission yields of 235U. 99Tc and its mother nuclides are highlighted as well as 122Sb, 124Sb and 125Sb. The whole decay chain from 99Kr to 99Tc and finally to 99Ru is presented below the 3D image with half-lifes and independent fission yields.12,19

The most stable oxidation state of Tc in oxidizing conditions is Tc(VII) mainly occurring – 20 – as pertechnetate anion (TcO4 ), Fig. 3. As an anionic species, TcO4 interacts only little with soils and sediments and is therefore highly mobile in groundwater and other water environments. In reducing environment, the prevalent oxidation state is Tc(IV) with its four 2+ + – 21 species: TcO , TcO(OH) , TcO(OH)2 and TcO(OH)3 . As the neutral TcO(OH)2 species prevails in the near neutral pH range, the environmental chemistry of Tc(IV) is mostly dictated by the properties of TcO(OH)2. In presence of other ions of low solubility, Tc(IV)

can co-precipitate and be immobilized until the conditions change again to more oxidizing.22-24 In specific conditions, also other oxidation states can be achieved but Tc(VI) and Tc(V) tend to disproportionate back to Tc(VII) and Tc(IV), and oxidation states III and lower are highly prone to oxidation.20

2+ + - - TcO TcO(OH) TcO(OH)2 TcO(OH)3 HTcO4 TcO4 100

40 Species (%)

0 0123456789101112 pH

Fig. 3. Speciation of 10-6 M Tc(IV) and Tc(VII) as function of pH. The speciation of Tc(IV) was calculated using the PhreeqC program and its ANDRA Thermochimie-database,11 and speciation of

Tc(VII) was calculated by using the pKa value of 0.032.

During the nuclear power production, 99Tc is accumulating inside the fuel rods.25 Hence, Tc leaches into the primary coolant water only in the case of fuel rod failure. Instead, activation of corroded 98Mo generates 99mTc and 99Tc into the primary coolant water even though the fuel rods would remain intact.13 Hence, it has been estimated that 4.4 – 178·1012 Bq/year of 99Tc was released from the Sellafield nuclear fuel reprocessing plant in 1978 – 198325 whereas Holm26 calculated that a cumulative release from nuclear power plants was only 4 GBq by 1990. The Chernobyl accident created approximately 0.75 TBq 99Tc fallout ad the fallout from nuclear weapons testing was 180 TBq of which 40 TBq was deposited on close surroundings and the rest spread through the stratosphere.26 Thus, the dissolution and reprocessing of nuclear fuel has caused significantly larger 99Tc discharges than the effluents from the nuclear power plants or releases from weapons or accidents. As a consequence of decades lasting releases into the natural waters, nuclear weapons testing and nuclear accidents, Tc is now widely spread in the environment. For example, the long-term discharge of 99Tc from the Sellafield reprocessing plant has been detected from subtidal sediments of the Irish Sea in 2005 – 200627 and even from surface seawater of the Greenland in 2000 – 200128 although 99Tc discharges have substantially declined. In addition, a leakage of nuclear waste from geological containments and tanks, for example, in Hanford in the United States has led to a local contamination of surroundings.29

1.2 Separation techniques for antimony and technetium

Several separation methods for Sb and Tc remediation has been developed since the elements were discovered. However, only some of them have been used in real water purification applications. The most suitable method for the separation depends on many factors including the concentration and speciation of the target element, and a solution matrix. The separation methods can be divided into five main categories: extraction, electrochemical methods, coagulation and flocculation (or precipitation), membrane separation, and adsorption and ion exchange methods. The separation procedure can also combine some of these methods to achieve more efficient purification.

1.2.1 Extraction

Liquid extraction is based on the transfer of solute from solution to another. Commonly, the solvents are immiscible with each other such as aqueous and nonpolar organic solvent, which can be mixed with a suitable ligand. The solute distributes between the solvents with a certain distribution coefficient and by repeating the extraction cycle several times, a product with high purity can be obtained. For example, Sarkar et al.30 separated Sb(III) from bismuth (Bi(III)) with acids and bis(2,4,4-trimethylphenyl) monothiophosphinic acid in toluene with a good selectivity. In their study, Bi(III) was stripped from the organic phase with 2 M HNO3 and Sb(III) with 8.5 M H2SO4 yielding over 98% respective recoveries. Even though several ligand-solvent combinations for antimony separation have been published such as ammonium pyrrolidine dithiocarbamate in methanol or xylene,31,32 N-benzoyl-N-phenylhydroxylamine (BPHA) - chloroform33,34 and 1- butanol – n-heptane – water ternary system,35 the large scale extraction is usually done by 36 37 NaOH or Na2S leaching. Gu et al. extracted As and Sb from arsenic smelter ash by alkaline pressure oxidative leaching and further Na2S leaching separated Sb from As by precipitating Na3SbS4 salt with over 97% efficiency. The liquid extraction is used for the nuclear fuel reprocessing to separate uranium and plutonium from other actinides, lanthanides and fission products including 99Tc. In the plutonium uranium extraction process (PUREX), the nuclear fuel is dissolved into nitric acid and uranium and plutonium are extracted from other nuclides with tributyl phosphate in kerosene.38,39 The remaining acid solution can be treated by using the diamide extraction – selective actinide extraction (DIAMEX-SANEX) process, among other methods, utilizing dimethyl-dioctyl-hexylethoxy malonamide to separate Am and Cm from the fission products.38-40 Instead, in a technetium oxidation state analysis, tetraphenyl arsoniumchloride is used to extract Tc(VII) from an acid solution to an organic solvent such as chloroform while Tc(IV) remains in the aqueous phase.41,42 The major challenge of liquid extraction in the separation of antimony and technetium on a larger scale is the high volumes of extractant needed for the separation, and additionally numerous cycles might be needed to gain satisfactory separation. As a result,

large volumes of potentially toxic and contaminated organic waste is generated upon separation needing further treatment.

1.2.2 Electrochemical methods

Electrochemical methods are based on redox reactions and coagulation or deposition of the target element. For some reason, this removal technique is still scarcely studied for Sb and Tc. However, Zhu et al.43 used electrolytic coagulation-reduction method to remove Sb from flotation-water of Sb mine with aluminum electrodes. They studied the effect of current density, time, and pH on the removal and they succeeded to reach the emission standards of the State Administration of China with below 1 mg/L concentration. In addition, Koparal et al.44 electrodeposited Sb from copper electrorefining and spent battery solutions effectively with nearly 100% removal by using electrochemical cell. The electrochemical separation method has also been applied for Tc separation on a small scale. For example, electrodeposition of Tc on a cathode electrode has been demonstrated in the Tc separation from a nitric acid solution.45 A main disadvantage of the electrochemical methods is their high operation cost as the electrolytic separation is an energy intensive method but as a positive side, the method produces only a small volume of waste compared to some other methods such as coagulation.

1.2.3 Coagulation and flocculation

In coagulation process, a coagulant is added to water to agglomerate existing colloids and to adsorb and precipitate chemical compounds. The coagulation is followed by flocculation that is a gentle stirring process. The flocculation enhances the production of larger agglomerates called flocs which are more easily sedimented or filtrated from the solution. The coagulation and flocculation process is the main method for purification of drinking water from Sb. Typically, the method uses aluminum and ferric salts as coagulants, 46-49 but the ferric compounds such as FeCl3 are more effective and more commonly used. The separation by coagulation is generally more efficient for Sb(III) than for Sb(V) but good separation levels can be reached with a proper coagulant dosing.47 However, other compounds including sulfate and phosphate anions and humic acid interferes the Sb(V) coagulation.48 The benefit of ferric coagulation is the low price of the coagulation chemicals and an operation simplicity but during the process, large volumes of Sb containing iron precipitate is generated and needing to be disposed.50 The coagulation of Tc commonly utilizes sulfide chemicals such as Na2S and H2S, which can co-precipitate – 51-53 TcO4 with other metals. The main precipitation product is a reduced tetravalent TcS2 but also heptavalent Tc2S7 can be achieved. However, possible remobilization and large volumes of radioactive sludge are considered as disadvantages of the method.

1.2.4 Membrane separation

Membrane separation can be divided into filtration (nano-, ultra- and microfiltration) and reverse osmosis. These methods are only scarcely studied for the antimony and technetium removal but some publications have reported effective separation. Kang et al.54 reported more effective separation of Sb(V) compared to Sb(III) by reverse osmosis apart from pH conditions. Moreover, chelating and polyol-ligand containing hollow-fiber membranes showed respective efficient Sb(III) decontamination.55,56 In addition, Ma et al.57 used Fe flocs combined with the ultrafiltration and achieved good Sb(V) separation but the membrane was prone to fouling. Also, Du et al.58 used combined coagulation- flocculation with the ultrafiltration to remove Sb(III) effectively.

– Some membrane separation materials have also been used for TcO4 separation such as a graphene oxide membrane,59 a tetraphenylarsonium perchlorate loaded polypropylene based supported liquid membrane60 and a tri-n-octylamine-xylene based 61 – supported liquid membrane that have shown high selectivity for TcO4 . Instead, Hodgson et al.62 reported a low rejection of Tc by the reverse osmosis and nanofiltration. Generally, amount of publications about Tc membrane separation is low. The benefits of membrane separation are the simplicity of operation, and low waste volumes. However, a high- pressure separation process can be expensive to operate and install. In addition, the membranes can be prone to fouling and surface damage.

1.2.5 Adsorption and ion exchange

Adsorption and ion exchange are appealing methods for purification of water due to the simplicity of operation, low-cost, minimal waste production and possible regeneration of the material.7,63 The ion exchangers are commonly organic resins but also inorganic materials have been developed. Adsorbents, instead, can be divided in different groups: activated carbons and chars, natural and synthetic minerals, waste materials and biomaterials, and hybrid materials that contain more than one of these materials.7 Activated carbons and chars are used as general adsorbents to remove multiple contaminants from waste solutions and therefore they are not showing high selectivity towards certain ion or compound.7,64 This also concerns the utilization of bio- and waste materials if they are not modified before use. In addition, the organic resins and other biobased materials can suffer from radiation damage and they are not ideal for direct disposal as a nuclear waste. The benefit of natural minerals as adsorbents is their low price when compared to synthetic materials but their adsorption performance is often weak. For example, the Sb(V) adsorption capacity of bentonite was only 0.50 mg/g probably due – 65 to its low point of zero charge of 2.6 repelling Sb(OH)6 anion above that pH. As the adsorption properties of synthetic minerals and products can be controlled, they are now vastly studied for the Sb and Tc separation.

Both Fe- and Zr-based materials have drawn special attention in the Sb and Tc removal from aqueous solutions. For example, ferric hydroxide66, goethite (FeO(OH)),67-69 hematite 70 66 71,72 (Fe2O3) coated magnetic nanoparticles, Fe-Zr binary oxide, Fe-Mn binary oxide and Fe(III)-loaded saponified orange waste73 have been used in the Sb sequestration with success. Respectively, iron based materials such as zero valent iron (ZVI, Fe(0)),41,74,75 41 41 – Fe2O3 and Fe3O4 have been utilized in the TcO4 separation. From these iron materials, ZVI has shown the highest separation with a reductive uptake mechanism. Despite the good separation properties of the iron based materials, the materials are prone to surface passivation and leaching, which facilitates remobilization of adsorbate. Additionally, ZVI usually has a low surface area limiting the uptake capacity.76,77 For that reason, chemically and physically more resistant Zr-based materials have been developed for the Sb and Tc removal. Zirconium oxide has been effectively used for the Sb separation in different 66,I.,II. 66 78 forms. Moreover, Fe-Zr binary oxides , ZrO2 decorated carbon nanofibers and Zr- based metal-organic frameworks79 have been studied for the Sb removal from water solutions. Similarly, Zr-based materials such as ZrO2 nanoparticles anchored onto reduced graphene oxides,80 Zr metal-organic framework,81 zirconium sulphide carbide embedded 82 III within carbon framework and the Sb-doped ZrO2 fibers have been developed for the – – separation of TcO4 or its common research surrogate ReO4 anion.

1.3 Zirconium dioxide (ZrO2) materials

1.3.1 Structure and morphology

Zirconium dioxide is a versatile ceramic material used in many applications. For example, cubic single crystal ZrO2 (zircon) is used in jewelry as synthetic gemstone being more affordable option for a diamond.83 Instead, the white, hard and tough stabilized tetragonal zirconia polycrystalline has been utilized in dental prostheses with success.84 85 Beside these applications, ZrO2 in different forms has been utilized in coatings, fuel cells,86,87 optical materials,88,89 and sensors90,91.

ZrO2 has five polymorphs: cubic, tetragonal, monoclinic and two orthogonal 92-95 structures. From these polymorphs, only cubic (c-ZrO2), tetragonal (t-ZrO2) and monoclinic (m-ZrO2) zirconia crystal lattices (Fig. 4.) can be formed in ambient pressures whereas the orthogonal phases require high pressures to be formed.96 The stability of zirconia polymorphs depends on the temperature. The bulk m-ZrO2 is stable from a room temperature to 1170°C, t-ZrO2 at 1170–2370°C, and c-ZrO2 from 2370°C to the melting point of 2690°C.97 Due to the low temperature of upper earth’s crust, the baddeleyite mineral occurring in the nature is in the form of m-ZrO2. However, the tetragonal and cubic ZrO2 structures can be obyained in a metastable state at lower temperatures than a phase diagram of bulk zirconia indicates by limiting the crystallite size or by using dopant ions. 98 Garvie studied metastable t-ZrO2 at the room temperature and observed that when the crystallite size was below 30 nm, t-ZrO2 remained in the metastable form because of the surface energy stabilization. When the crystallite was larger than 30 nm, the metastable t- ZrO2 transformed to m-ZrO2. After this publication, also other researchers have investigated the formation of metastable t-ZrO2 and the possible influence of crystallite size on its stabilization.99-101

A great number of studies have been targeted to doping of ZrO2 and its structure stabilizing influence on c- and t-ZrO2. Most of the studies have used lower than IV valency metal ions but also metals with the same or even higher valency have been 96,102-104 3+ investigated. Probably the most studied ZrO2 dopant is yttrium (Y ) that is used to form yttria-stabilized zirconia (YSZ). This material is very hard and used for example in the aforementioned dental prosthesis, and in ceramic knifes.

I-III 105 106 ZrO2 can be produced in different morphologies such as fibers, films, spheres, nanocrystals107 and large single crystals83. The morphology of zirconia affects significantly on its properties such as conductivity, hardness, and optical properties that determine the 108 most suitable form of ZrO2 for the specific application. For example, ZrO2 thin films have been applied to optical filters and memory devices due to an appropriate band gap energy 109 whereas, high surface area ZrO2 nanocrystals have been tested as a catalyst to form dimethyl carbonate from CO2 and methanol.

96 Fig. 4. Cubic, tetragonal and monoclinic ZrO2 crystal structures. Dashed lines and arrows in the lower panel represent deformation of the cubic lattice to tetragonal and monoclinic structures. Large red spheres present oxygen and small grey spheres zirconium atoms. Reproduced with permission from the Royal Society of Chemistry, Copyright (2011).

1.3.2 Properties

ZrO2 is chemically and physically resistant material as it tolerates rather strong acids and bases, heat, and radiation. In addition, the material is non-toxic enabling its use in multiple applications. ZrO2 is practically insoluble to water but it can be dissolved in 110,111 concentrated sulfuric acid (H2SO4) and hydrofluoric acid (HF). Zirconia can also withstand high temperatures but it can change the crystal structure upon heating over the transformation temperature and it finally melts at 2670°C. The temperature tolerance coupled with the low thermal conductivity of zirconia facilitates its utilization as a ceramic insulating material in high temperature processes, for example, in gas turbines.85 In elevated temperatures, ZrO2 is used as an oxygen sensor because of its good oxygen conductivity that can be further enhanced with dopant ions such as Ce(IV) and Y(III).102,103 Furthermore, a high refractive index of the cubic ZrO2 crystals (zircon gemstone) makes the material suitable for jewelry because of its resemblance with the diamond.83

The crystal structure and the morphology have a great impact on properties of ZrO2. Tetragonal polycrystalline ZrO2 has a higher fracture toughness than single crystals that 112 can be even improved by using dopant ions such as Ce(IV) or Y(III). In contrast, m-ZrO2 is has commonly fewer application in ceramics as it is prone to crumbling and 96 microcracking during great temperature changes. In addition, c-ZrO2 and t-ZrO2 have a

higher surface reactivity than m-ZrO2 and for that reason, it has been used as a catalyst, and adsorbent for organic compounds and metal ions. For example, a cubic CeO2-ZrO2 based material has been used as a soot oxidation catalyst.113

In metal ion removal applications, many authors have observed that t-ZrO2 is a more 78 active adsorbent than m-ZrO2. DFT calculation by Luo et al. demonstrated higher energy for Sb(III) and Sb(V) adsorption on the (111) surface of t-ZrO2 than on the (111) surface of m-ZrO2. Accordingly, the adsorption experiments with the ZrO2 submicron fibers showed II that t-ZrO2 separated Sb(V) more effectively than m-ZrO2. The difference between uptake properties of t-ZrO2 and m-ZrO2 have been also observed for other anions. For example, 80 – the DFT calculation conducted by Gao et al. showed that the adsorption energy of ReO4 was higher for t-ZrO2 (111) plane than for m-ZrO2 (111) plane.

Besides the crystal structure, the morphology has a high influence on the adsorption properties of zirconia. By producing zirconia with a small crystallite size, a large specific surface area can be created which is beneficial for the adsorption. For that reason, ZrO2 nanocrystals or aggregated ZrO2 precipitates have drawn special attention in the metal ion 114,115 remediation. However, the down side of the small ZrO2 particles is their poor performance in separation columns as they have a high flow resistance increasing the pressures inside the separation columns possibly leading to system failures.I Despite the challenges, mesoporous amorphous ZrO2 nanoparticles with the high surface area have shown efficient arsenic adsorption from aqueous solutions by achieving capacity over 34 mg/g for As(V) and 83 mg/g for As(III), respectively.114 The authors proposed that As(III) and As(V) in the solution was bound by the surface hydroxyl groups of zirconia that have been generally supposed to be responsible for the adsorption capability of zirconia.116 As ZrO2 is an amphoteric material, meaning that it can act as an acid or base, it can adsorb both cations and anions depending on the solution pH. The protonation and deprotonation reactions can be presented in the following forms:

≡ZrO– + H+ ↔ ≡ZrOH (1)

+ + ≡ZrOH + H ↔ ≡ZOH2 (2)

As demonstrated by the above reactions, the ZrO2 surface is positively charged in acidic conditions and negatively charged in alkaline solutions. At certain pH value, which slightly varies between different ZrO2 materials, the net surface charge is zero. This point is called point of zero charge (PZC) which can also be marked as pHPZC. Below the pHPZC, the adsorbent material mainly adsorbs anionic compounds whereas above pHPZC, the material is an effective adsorbent for cationic compounds due to electrostatic attraction between the oppositely charged adsorbent surface and the adsorbate ion.116,117 When the hydrated adsorbate is retained by the surface only with the physical electrostatic attraction, the mechanism is classified as an outer-sphere complexation, but when chemical bond is formed between the adsorbent and adsorbate, the adsorption mechanism is called an inner-sphere complexation.

1.4 Nano- and submicron fibers

A nanofiber is according to CEN ISO/TS 27687 standard118 an object with two similar external dimensions in nanoscale and one much larger dimension and therefore it is commonly considered as a 1D material. Usually, the fiber with a diameter between 1 and 100 nm are classified as a nanofiber whereas the fibers having diameter between 100 and 1000 nm are categorized as submicron fibers and the fibers with the diameter of 1 – 10 μm as microfibers familiar, for example, from cleaning cloths.

1.4.1 Properties and uses

Nanofibers are finding uses in many applications especially on medical design. For example, a potential of hydroxyapatite-polymer composite nano- and submicron fibers as a bone scaffolds has been extensively studied during recent years.119-122 Moreover, other biomimetic fiber materials have been developed for cartilage, ligament, skin and vessel tissue engineering.123 Another potential application of organic fibers is their use as drug carriers due to a large surface area of the nanofibers that can bind large quantities of drug molecules on its surface and release it slowly when the carrier polymer decomposes.123 Outside the medical field, nano and submicron fibers have found uses as air filters124, optical fibers125 and lithium-sulfur batteries126 among other applications. Some studies have been conducted using different nano- and submicron fibers as adsorbents. For example, ZrO2 decorated carbon nanofibers have been tested for antimony and arsenic remediation.78,127 Accordingly, electrospun chitosan nanofibers showed efficient Cu(II) and Pb(II) adsorption capability.128 The authors stated that the nanofibers had ~6 and ~11 times higher capacities for Cu(II) than the best chitosan microspheres and plain chitosan, respectively. Because of the high specific surface area of nano- and submicron fibers, they are highly suitable for the adsorptive separation technologies. A self-supporting structure of the fibers further facilitates their use in separation columns and filters as their porous network structure allows a flow of solution through the material with lower resistance than the nanoparticles that are packing tightly in the column without additional support.129

1.4.2 Synthesis by spinning techniques

The first electrospinning patent was filed by John Francis Cooley in 1899.130 Only decades later, Anton Formhals attempted nanofiber production in practice and patented the electrospinning process and apparatus in 1934 for the production of artificial filaments and threads.131 However, the interaction between a solution and charged objects was noticed already centuries before patenting.

The electrospinning process is probably the most common method for the nanofiber production. The apparatus used for the electrospinning can be very simple. All that one needs for the fiber production is a polymer solution confinement such as a syringe, metallic needle, high voltage supply, and collector that can be, for example, a metal plate (Fig. 5).132 The high voltage is applied between the spinneret needle and the grounded collector to emit a polymer jet that is deposited as dry fibers on the collector.132 But commonly, the electrospinning apparatus is also equipped with a pump that feeds the solution to the needle, and often a rotating cylinder collector is used instead of the plate-like collector since the cylinder is capable of gathering larger fiber sheets than the plate.133 Additionally, another high voltage supply can be used to apply an opposite charge on the collector and thereby increasing a potential difference between the solution and the collector. In the electrospinning process, the high voltage source is connected to the metallic needle that conducts charge to the polymer solution. As the polymer solution gets charged, the electrostatic repulsion begins to stretch solution droplet from the needle tip (Fig. 5). The Taylor cone forms when a polymer droplet starts to stretch in the electric field and finally overcomes the critical surface tension of the solution erupting a solution jet.132 The emitted jet can be divided into two parts. The first part emitted from the Taylor cone is called as a straight or stable part as the jet is straight and no stretching or thinning occurs.134 On the second part, the solution jet starts to whip due to electrostatic repulsion between small bends of the jet that causes the thinning of the liquid jet and formation of uniform nano- or submicron fibers.132,134 This part of the jet is called instable region or whipping region that continues until the fiber reaches the collector.134 During the travel from the spinneret needle to collector, solvents of the polymer solution evaporates that results in solid fibers at latest on the collector.132 However, drying during the flight is desired as this produces more uniform and cylindrical fibers. The morphology of the fibers also depends on other solution parameters as well as apparatus parameters and ambient conditions.132

Fig. 5. The electrospinning scheme in nanofiber production and the Taylor cone.132 Reproduced with permission from Wiley-VCH Copyright (2004).

A blow spinning process reminds in many ways the electrospinning synthesis but the polymer solution is ejected from the needle because of gas flow instead of the electric potential. Usually, the blow spinning apparatus composes of a syringe and needle, nozzle that feeds pressurized air and a collector, Fig. 6a. In this setup, the needle is pushed through the nozzle and the gas flow stretches the polymer drop in the needle tip to the shape of cone that emits the polymer solution jet towards the collector. This process is based on the Bernoulli’s effect in which the changing pressure converts to kinetic energy. Thus, when the pressurized gas (P1) exits the outer nozzle, the pressure drops rapidly to the ambient pressure (Patm) that induces acceleration of the gas, Fig. 6b. This acceleration decreases the pressure (P2) in front of the inner nozzle (or needle) which induces a driving force. The accelerated gas also creates a shear force on the gas/solution boundary deforming the solution into a conical shape. When the driving force and the shear force together exceed the force of the surface tension, the polymer solution erupts. The solvents of the polymer solution evaporate during the flight and the dry fibers are deposited on the collector. The fibers produced with the blow spinning method tend to form fiber bundles instead of single fibers as the polymer jets do not experience the electrostatic repulsion with each other, unlike it is with the electrospinning technique, Fig. 8. However, the production rate of the fibers is commonly faster than in the electrospinning process. Also, the working distance in the blow spinning is longer than in the electrospinning due to the high flow rate of the gas.135,136

Fig. 6. The blow spinning apparatus. a) Presents common a blow spinning setup and b) demonstrates a functioning of the nozzle.135 Reproduced with permission from Wiley Periodicals, Inc, Copyright (2009).

An electroblowing technique (also gas-assisted electrospinning) is a fiber production technique that combines the favorable properties of the electrospinning and the blow spinning methods, Fig. 7. The electroblowing synthesis utilizes the electric potential between the polymer solution and the grounded collector to form non-woven webs of thin and uniform fibers and the additional gas flow is used to enhance the production rate of the fibers. The gas flow is assisting the electric field by pulling the fiber jet from the spinneret with the reduced pressure in front of the nozzle and at the same time it speeds up the evaporation of solvents.137 The gas jet can also be used to direct the solution jet towards the collector. Similar to the blow spinning process, the collector distance is greater than in the electrospinning process as gas flow shortens the flight time of the solution. The increased flight time facilitates a full evaporation of the solvents preventing the flattening and fusion of the fibers on the collector and enables the production of thin fibers.

Fig. 7. Eletroblowing synthesis unit scheme. In the synthesis unit a high voltage is applied between the spinneret needle and the collector and additional air flow is used to enhance the production rate of the fiber.

As the polymer solution is charged by external electric field, the bending solution jet starts whipping as a consequence of electrostatic repulsion between the bends. This allows the formation of singe uniform fibers like in the electrospinning process, which is more difficult with the blow spinning technique often producing fiber bundles, Fig. 8. However, the fiber production rate by the electroblowing is in similar order of magnitude with the blow spinning process and approximately ten times the production rate of electrospinning synthesis. For example, Liu et al.138 enhanced the production rate of poly(vinyl alcohol) fibers 11 times by using an assisting gas jet in the spinning process. The obtained fibers can be thinner than those of fabricated with the electrospinning technique but the diameter of the fibers also depends on the other process parameters such as the collector distance and the applied voltage.139

Fig. 8. Poly(caprolactone) fibers produced with A) blow spinning technique and B) electrospinning technique. Reprinted with permission from (ACS Appl. Mater. Interfaces 2016, 8, 51, 34951-34963) 136. Copyright (2016) American Chemical Society.

The nano- and submicron fiber production by spinning techniques is affected by many factors that also influence the fiber morphology. The process parameters can be divided into three categories: solution parameters, process parameters and ambient parameters. For example, a viscosity is an essential solution parameter. When viscosity is too low, fiber formation fails because of the spray formation whereas too high viscosity inhibits the jet formation as the solution is too viscose to be emitted. Only at certain range of viscosity, the uniform fibers can be achieved. An example of process parameters is the voltage. Too low voltage is not inducing the Taylor cone whereas the high voltage shortens the flight time and path that can lead to production of thickened fibers or in some cases thinner fibers. Also, the ambient conditions affect the spinning process. Possibly, the most obvious effect originates from gas humidity when too high can prevent the solvent evaporation and fuse the fibers on the collector whereas a too low humidity can dry out the solution on the tip of the spinneret. Other parameters are listed in Table 2.135,140

Table 2. Solution, process and ambient parameters affecting the fiber spinning process. The table is compiled from the book of Ramakrishna 140 and the article of Medeiros et al.135

Solution parameters Process parameters Ambient parameters

Molar mass of polymer (viscosity) Spinneret size and design Humidity Concentration of polymer (viscosity) Voltage (electric field) Atmosphere Dielectric constant Solution feed rate Pressure Conductivity Collector distance Temperature Surface tension Collector design Vapor pressure of the solvent Gas flow rate and nozzle design (Inorganic precursors) (Temperature)

1.4.3 Synthesis of inorganic fibers by spinning techniques

The spinning techniques have been initially developed for the production of organic polymer fibers. However, during recent years the utilization of nanomaterials has increased on many fields and for that reason, research efforts has been targeted to the production of inorganic nanofibers. As a consequence, also the different spinning techniques have been exploited and further developed for the synthesis of inorganic nanofibers. The majority of published articles about the production of inorganic nano- or submicron fibers are utilizing electrospinning process. The inorganic precursor is added to the spinning solution and mixed with the polymer to form viscous solution that produces uniform solution jets and even fibers. The polymer used in the synthesis solution has to be soluble into the same solvents as the inorganic precursors. Therefore, water and ethanol soluble poly(vinyl pyrrolidone) (PVP) and poly(vinyl alcohol) (PVA) are the most frequently used polymers.141,142 The production of purely inorganic fibers requires a removal of the polymer after the spinning. The removal commonly happens upon the calcination. In the beginning of the calcination process, the remaining solvents evaporate which is followed by burning of the polymer that is observed as a shrinkage of the fiber, Fig. 9.143,II. When the polymer carrier burns, the ceramic material starts to form as the precursor salts come into contact. This sintering process is highly critical when considering the crystal structure and the morphology of the fiber. The sintering increases the crystallite size and potentially transforms the crystal structure. Zhao et al.144 observed that the crystal structure of electrospun PVP-ZrO2 nanofibers transformed from amorphous to tetragonal and finally monoclinic along the increasing calcination temperature. Too high calcination temperature increases the crystallite size and at the same time, decreases the specific surface area that is usually the pursued property of the nanofibers.141,II. However, too low calcination temperature is unable to burn out the polymer leaving organic residues, which possibly decrease the surface activity of the material.II.

Fig. 9. Calcination process of polymer-inorganic fibers. Yellow color presents the polymer carrier and red inorganic precursor or crystallites

2 AIM OF THE STUDY

Inorganic materials have been used for a metal ion removal for years. A significant challenge in the use of inorganic materials as ion exchangers and adsorbents has been confronted in their uptake kinetics and use of the materials in the packed bed columns. When the adsorbent material is in the form of large particles, they create only slight flow resistance in the separation column but the adsorption kinetics is slow as only small fraction of the material is exposed to solution (small specific surface area) and the diffusion inside small pores and into the crystal lattice is a slow process. On the other hand, small nano- and microparticles have large surface areas exposed to the solution but the tightly packed particles are generating higher flow resistance in the column use that can escalate as a system failure. Therefore, the aim of this study was to develop nano- or microsized fibers with fast adsorption kinetics and low flow resistance that is enabled by the loose web-like structure. For this research, ZrO2 was chosen to fiber production as it has good chemical and physical properties and previously shown anion uptake capability. Additionally, inorganic ZrO2 withstands ionizing radiation and can be rather easily disposed as a radioactive waste. The aim of the study can be divided into four sub-subjects which are presented below:

1) Develop nano- or submicron fibers by electroblowing technique for metal oxy-anion separation. (Articles I-III)

2) Determine the differences between the zirconia fibers and the microparticles in the antimonate separation process. (Article I)

3) Determine the effect of calcination process on the ZrO2 fiber crystallinity and the antimonate removal capability. (Article II)

4) Functionalize the ZrO2 submicron fibers to effectively separate pertechnetate from aqueous solutions. (Article III)

3 EXPERIMENTAL

3.1 Synthesis

The ZrO2 and Sb-doped ZrO2 submicron fibers were synthesized with the electroblowing method. The production was started by preparing a precursor solution. The Mw 1 300 000 g/mol PVP-polymer was dissolved in absolute ethanol under magnetic stirring. After the homogenous polymer solution was achieved, ZrOCl2 was dissolved in water and added to the polymer solution. The stirring was continued until the solution became homogenous. For the Sb-doped ZrO2 fibers, SbCl3 was dissolved in ethanol in fixed amount to achieve targeted doping levels. This solution was added to PVP-ZrOCl2 solution and stirred to homogeneity. More accurate recipes are provided in the articles I – III. The precursor solution was placed into a 10-ml syringe and a blunt spinneret needle (0.21 mm ø) was attached to the syringe tip. The syringe was inserted to a syringe pump feeding solution at a rate of 15 mL/h and the needle was pushed through a nozzle that fed pressurized dry air at a rate of 30 – 40 NL/min. A fiber collector that composed of a cylindrical side collector and a planar back collector was placed in front of the needle at 10 cm distance and the collector was grounded. A grounded high voltage supply was connected to the nozzle-needle system to apply 15 kV potential between the solution and the collector. The collector was confined in a polycarbonate box to enable humidity control during the spinning process. The relative humidity was kept approximately at 15% to speed up the solvent evaporation and the room temperate was 22 °C. The synthesis apparatus which is similar to one designed by Holopainen et al.145 is presented in Fig. 10.

Fig. 10. The electroblowing synthesis unit used for the ZrO2 fibers production. On the left side is the syringe pump and a corner of the high voltage supply and on the right side the fiber collector composing of the cylindrical side collector and the planar back collector that are sealed to the polycarbonate box. The orange tubes bring pressurized air to the nozzle attached to the box, and to the sides of the box for the humidity control.

The fibers deposited on the collector were gathered, and folded loosely on a shallow evaporating dish or a silicon wafer and calcined in a furnace in ambient air. The heating rate (1–15°C/min) and the calcination temperature (300–800°C) was controlled to obtain fibers with the wanted crystallinity and to burn out the PVP support. The fibers were calcined for 6 hours, and the furnace cooled back to the room temperature approximately in 12 hours. Granular zirconium oxyhydroxide (GZR) used as a reference material was produced by a precipitation method. In this method, ZrCl4 precursor was dissolved in 3 M HCl solution in a plastic beaker under constant stirring. This highly acidic solution was then slowly neutralized with concentrated ammonia that was added into solution until pH 7.8 was achieved. The neutralized solution was let to settle for 30 min followed by decantation of the clear supernatant solution. An equal amount of deionized water was added, solution stirred for 5 min and precipitate was let to settle for 1 h. This washing cycle was repeated until the solution remained turbid even after 1 h settling time. Then the turbid supernatant was discarded and precipitate transferred to an evaporation dish. The product was dried in a convection oven at 70°C for 4 days yielding a hard cake. This cake was ground in an agate mortar and sieved to under 200-mesh (75 μm) particles.

3.2 Characterization

The produced ZrO2 granules and submicron fibers for the articles I-III were comprehensively characterized to discover the influence of morphology, crystal structure and chemical composition of the materials on the antimonate and pertechnetate separation efficiency.

The morphology of GZR and all the ZrO2 based fibers were examined with a field emission - scanning electron microscopy (FE-SEM) Hitachi S-4800. Before the imaging, the samples were secured to a carbon tape and coated with 4 nm of Au-Pd alloy using a sputtering technique to increase conductivity and the image quality of the samples. In addition, the ZrO2 submicron fibers of different crystallinity (article II) were imaged with a transmission electron microscopy (TEM) to investigate the density differences in the fibers. Furthermore, an elemental mapping was conducted with an Oxford Inca 350 energy dispersive X-ray spectroscopy (EDX) system to examine the distribution of antimony after adsorption on the ZrO2 fibers. The crystal structure of the synthesized products was determined with a powder X-ray diffraction (XRD). The materials were ground before the analysis to prevent alignment of the fibers. The analysis was carried out either with a PANalytical X'Pert3 Powder or a PANalytical X'Pert PRO MPD with a Cu Kα1 radiation. More detailed description is provided in the articles I-III. The specific surface areas and porosities of the synthesized materials were characterized with a N2 adsorption/desorption method as the specific surface area and the

porosity are key factors in the adsorption process. The samples were degassed in vacuum at elevated temperature prior to analysis at 77 K with a Micromeritics ASAP 2020 gas analyzer. The specific surface area was calculated with the N2-BET method and the porosity was estimated with the BJH method from the gas adsorption branch. The thermogravimetric analysis (TGA) was conducted to determine a water content of GZR and to examine the calcination process of the ZrO2 and Sb-doped ZrO2 submicron fibers by analyzing the calcined and as-spun fibers. The analysis was conducted either with a Mettler Toledo TA8000 or a NETZSCH STA 449 F3 Jupiter®. The former was combined with a simultaneous differential thermal analysis (STDA) and the later with a differential scanning calorimetry (DSC) and an Agilent 7890B/5977A gas chromatograph-mass selective detector. More comprehensive descriptions of the analyses are given in the articles I-III.

Infra-red spectroscopy was used to examine if the calcined Sb-doped ZrO2 fibers still contained organic impurities from PVP. The powdered fiber samples were measurement with a Fourier transform infra-red – attenuated total reflectance (FTIR-ATR) Bruker Alpha Platinum ATR from 400 to 4000 cm-1 with a 2 cm-1 resolution using 50 scans average. For comparison, also the as-spun fibers and the PVP source chemical were analyzed. X-ray fluorescence (XRF) was used to analyze the Sb content and the amount of other elemental components in the Sb-doped ZrO2 fibers. The powdered fibers were analyzed with a wavelength-dispersive XRF spectrometer PANalytical Axios mAX. X-ray absorption near edge structure (XANES) was used to determine the oxidation state of antimony both adsorbed on the surface of ZrO2 particles and fibers, and used as a dopant in the Sb-doped ZrO2 fibers. The former was measured in a PETRA III synchrotron facility in Hamburg, Germany, and the later with a self-assembled laboratory XAS. At PETRA III, the measurement was conducted on the K-edge of the antimony in a cryostat whereas with the self-assembled XAS, the analysis was done on the Sb LI-edge at room temperature. More detailed description is offered in articles I and III. The zeta potentials of the materials were determined with an electrophoresis analysis. For the analysis, the powdered materials were weighed into vials and a solution was added. The solution was either 0.01 M NaCl or NaNO3 electrolyte, which were used to compensate salt concentration difference caused by the pH adjustment, or 0.01 M NaNO3 + Sb(V) to investigate the effect of Sb(V) on the zeta potential and hence to determine the Sb(V) complexation mechanism. The pH of the samples was adjusted with the small volume of either NaOH or HCl and the samples were equilibrated for 24 hours before the analysis. The equilibrium pHs were measured, and the zeta potentials determined with a Malvern Zetasizer Nano ZC after few minutes settling time.

3.3 Uptake experiments

The performance of the ZrO2 granules and fibers in antimonate separation was tested by batch experiments, and a pressure development during the column operation was monitored. Also, the performance of the ZrO2 and Sb-doped ZrO2 submicron fibers in pertechnetate separation was investigated by the batch and the column studies.

3.3.1 Batch experiments

The batch experiment can be classified as a stationary experiment since the adsorbent and the adsorbate remain in the closed system, Fig. 11. In common batch experiment, a certain amount of solid adsorbent is mixed with the constant volume of adsorbate solution. This ratio is also called a batch factor (BF). After a preset time, which may be equilibrium or not, the solution and the solid is separated by centrifugation and filtration. The remaining adsorbate concentration in the solution is determined to calculate the amount of adsorbate bound by the adsorbent. The batch experiments of this thesis can be divided to different categories: pH effect, isotherm, kinetics, competition and reusability.

Fig. 11. Schematic of the batch experiment at beginning, during the experiment and at the equilibrium.

99 – 188 – The effect of pH on the uptake of Sb(V) and TcO4 (and ReO4 ) was studied by preparing a set of samples with the constant BF and anion concentration. The pHs of the solutions were set into a range of 1 – 10 using a small volume of either NaOH or HCl. The samples were equilibrated for 24 hours in a constant rotary mixer before the phase separation by centrifugation (2 100 g) and a 0.2-μm syringe filtration. The remaining antimonate in the filtrate was measured with an inductively coupled plasma – mass spectrometry (ICP-MS, Agilent 7800) after acid dilution. Respectively, the remaining 99Tc activity was determined with a liquid scintillation counting by using a Perkin Elmer Tri-Carb 2910T. Before the measurement, the filtrate was mixed with an Optiphase HiSafe 3 scintillation cocktail. The activity of 188Re was in turn analyzed with a PerkinElmer Wallac Wizard 3’’ 1480 automatic gamma counter.

The isotherm (capacity) experiments were conducted in a similar manner as the pH effect experiments but the pH of the solution was kept constant whereas the adsorbate concentration in the solution was varied. This experiment was done only for Sb(V) as the determination of 99Tc uptake capacity would have required too high activities. The obtained results were used for an adsorption capacity calculation and fitted with the Langmuir and Freundlich models provided in the articles I and II. The effect of competing ions and compounds on the uptake of Sb(V) and Tc(VII) was also studied by the batch method. In this experiment the solution pH and the adsorbate concentration were kept constant but the concentration of competing species was increased within the sample series. After the equilibration, only the target adsorbate concentration was determined. The kinetic experiment was conducted for Sb(V) and Tc(VII) in a beaker under magnetic stirring. The adsorbent was weighed and the solution poured into the beaker that started the time taking. The sample aliquots were withdrawn and filtered at the preset time points and analyzed for the result calculation. The beaker was covered with a parafilm to avoid solution evaporation during the experiment.

Reusability of the ZrO2 fibers in the Sb(V) separation was investigated via a cyclic use. The material was first loaded with Sb(V) by equilibrating samples for 18 hours in the Sb(V) solution. The equilibrium concentration was analyzed and remaining solution discarded. Then, the material was rinsed with water and 1 M NaOH added in the samples that were equilibrated for 6 hours to regenerate the fibers. This solution was discarded and the fibers rinsed with deionized water until solution became pH neutral. The cycle was repeated four more times.

3.3.2 Column experiments

The pressure build-up in the separation columns packed with the granular and fibrous ZrO2 was investigated in the laboratory scale column system. In this experiment, 0.25 g of each material was packed into a 0.7 cm ø column. A peristaltic pump was used to pump solution through the column at different flow rates. The pressure build-up was monitored with a pressure sensor, connected to a 3-way valve above the column, for 30 minutes after starting the peristaltic pump. After 30 minutes, the pump was stopped and the pressure released before monitoring the pressure development at another solution feeding rate.

– The TcO4 separation performance of the Sb-doped ZrO2 submicron fibers in a dynamic use was investigated by the column experiments. In this experiment, 0.25 g of the scissor cut fibers were packed into the 1 cm ø low pressure Bio-Rad Econo column. A solution 99 – 188 – containing TcO4 or ReO4 in 0.01 M NaCl at pH 4.0 was fed through the column with the peristaltic pump at a constant rate of 33 mL/h. The solution that passed the column was collected with a fraction collector and the activity of technetium was determined with the liquid scintillation counting and the activity of rhenium with the gamma counting.

3.3.3 Results calculation

The metal uptake in the equilibrium was calculated with the following equation:

∙ = (3) where qeq(mg/g or mol/g) is the amount of adsorbed metal, C0 and Ceq are the initial and the equilibrium concentrations (mg/L or mol/L), respectively and V(L) is the volume of the solution and m(g) is the mass of the dry adsorbent.

The distribution coefficient (Kd) determines the distribution of the adsorbate between the solid and the solution phases that can be presented as follows

∙⁄ ∙ = = = (4) ∙ The parameters are same as above and the units are otherwise the same but the unit of volume in this thesis is mL. However, also net activity (Bq/L or CPM) can be used in the calculation as the units are cancelled in the equation.

The breakthrough (BT) of the column was presented as a percentage (Ceq/C0·100%) remaining in the feed solution. Some other results were presented as a sorption percentage ((1 – Ceq/C0)*100%).

The uncertainty calculation of the uptake results was executed by using the general error propagation law, Equation 5. However, in some cases when Ceq was extremely close to background giving over 100% uncertainties for Kd, the uncertainty was estimated to be 90% of the value for the figures.

= + ⋯ + (5) Here x, … and z are the independent parameters of q, and , … and are the uncertainties of these parameters. The uncertainty of the zeta potential, instead, was given as standard deviation of three parallel measurements.

4 RESULTS AND DISCUSSION 4.1 Materials characterization

The comprehensive characterization is necessary to evaluate the influence of the chemical and structural features of the adsorbents on the antimonate and pertechnetate separation. As reported by others, the morphology, crystal structure, and the specific surface area have a great impact on the metal ion adsorption. Therefore, the SEM, XRD and BET results among other observations are presented in this chapter and their influence on the Sb(V) and Tc(VII) uptake discussed later in this thesis.

4.1.1 Characterization of ZrO2 and Sb-doped ZrO2 submicron fibers

The synthesized ZrO2 fibers formed even non-woven fiber mats that were easy to gather from the collector and fold on the silicon wafer for the calcination. During the calcination the mass of the fibers decreased significantly and the size of the fiber mats shrunk as seen in Fig. 12. The shrinkage of the fiber mats originated from the solvent evaporation and burning of the PVP support that also decreased the diameter of the fibers. The color of the calcined ZrO2 fibers was white when PVP had completely burned during the calcination, otherwise a hint of brown color could be observed.

Fig. 12. Images of the as-spun electroblown PVP-ZrO2 fibers on the left side and the ZrO2 fibers after the calcination at 700°C for 6h on the right side.

The SEM images of the electroblown ZrO2 fibers showed that almost all the synthesized fibers (Fig. 13 a-o) had smooth surfaces. The exception was the ZrO2 fiber calcined at 800°C (also the fibers calcined at 600 and 700°C that are not presented here) whose surface seemed to be composed of agglomerated particles (Fig. 13i) with visibly porous structure. However, the ZrO2 fibers with the highest magnification (Fig 13b) showed surface roughness possibly originating from the crystallites that were not observed in the lower magnification images. Any kind of surface roughness is known to increase the specific surface area that offers more surface sites for the adsorption and thereby increases the adsorption capacity.

Fig. 13. The a) low magnification and b) high magnification SEM images of the ZrO2 calcined at

350°C with the 5°C/min heating rate (article I). The low magnification images of ZrO2 fibers calcined at c) 400°C, d) 500°C, and e) 800°C with the heating rate of 1°C/min and the high magnification images of f) the as-spun fibers, and the ZrO2 fibers calcined at g) 400°C, h) 500°C, and i) 800°C

(article II). The images of j) the as-spun 10%Sb:ZrO2 fibers and k) the calcined 10%Sb:ZrO2 fibers with the low magnification, and the high magnification images of l) ZrO2, m) 5%Sb:ZrO2, n)

10%Sb:ZrO2 and o) 15%Sb:ZrO2 fibers calcined at 350°C with the heating rate of 15°C/min (article III).

The electroblowing synthesis produced single fibers that were not fused together nor joint to bundles, and only few beads were observed. The fibers had a high length-to- diameter ratio which indicates that the synthesis was successful. The fibers had a rather high tensile strength as they tolerated a robust handling when the fibers were taped on an Al-disk for the SEM imaging. This is an extremely important feature as breakage of the fibers could negatively affect their column separation performance. The shape of the fibers was mostly cylindrical. Only with the Sb-doped fibers some flattened fibers were observed probably due to slow evaporation of solvents during the synthesis but still the majority of them were cylindrical. The diameter of the calcined fibers varied between 300 and 800 nm and were classified as submicron fibers. Before the calcination, the diameter of the ZrO2 fiber was approximately 1 260 nm which decreased along the increasing calcination temperature as the polymer was more efficiently burned. The average diameter of the ZrO2 fibers calcined at 300 and 400°C was 720 nm and for fibers calcined at 500–800°C the diameter was 570 nm when the slow 1°C/min heating rate was used. Instead, the Sb content did not seem to influence the diameter of the Sb-doped fibers. Moreover, the TEM analysis showed that the fiber calcined at 400, 500 and 800°C were solid. The TGA measurement was conducted to investigate the calcination process of the PVP supported ZrO2 and Sb-doped ZrO2 fibers even though the analysis of few milligram sample differs significantly from the calcination of the large fiber mats. The obtained results (Fig. 14) showed that the mass of the fibers decreased in four steps. The first step at 25–200°C was caused by evaporation of solvents, water and ethanol, that is also observed as an endothermic peak in the STDA, Fig 14 b. The next two steps at approximately 200–380°C and 380–550°C are related to the combustion of pyrrolidone side groups that is also seen as exothermic peaks in the STDA.146 The last step probably originated from the combustion 146 of the alkane chain of PVP. The mass loss steps of the undoped ZrO2 (Fig. 14 a,b) and the Sb-doped ZrO2 fibers (Fig. 14 b) are highly similar despite the differences in the analysis gas (air or air+N2) and presence of Sb. However, the mass stabilization temperature seems to follow Sb content since the mass of the 15%Sb:ZrO2 fibers stabilizes first and the ZrO2 fibers last.

Fig. 14. TGA results of a) the ZrO2 fibers analyzed in the mixture of air (80%) and N2 (20%) and b) the TGA-STDA analysis of the Sb-doped ZrO2 fibers in air.

The actual calcination process of the fibers is more efficient than the TGA results indicate as the exothermic combustion of PVP drastically elevates the temperature inside the fiber mats. In addition, the heating rate affects the PVP removal efficiency due to a limited heat transfer. With the low heating rate, the combustion of PVP releases heat slowly so that the thermal conduction and convection are maintaining the temperature inside the fiber mats close to the set temperature ramp during heating. Therefore, the temperature might exceed the set end temperature only slightly. Instead, the fast heating burns PVP rapidly leading to significantly higher temperatures inside the material than the preset temperature suggests since the heat production is much faster process than the heat transfer out of the material. However, the temperature drops relatively fast to the set value when all PVP has burned.

The undoped ZrO2 fibers were calcined using the slow heating rate of 1°C/min to control the temperature inside the fibers mats. By using the low heating rate, the effect of calcination temperature on the fiber crystallinity is reliably observed as the temperature should remain close to the preset value. From the fibers calcined at 300–800°C, the ZrO2 fibers calcined at 300 and 400°C are showing amorphous structure in the XRD (Fig. 15a) as no clear diffractions were seen. Instead, the fibers calcined at 500°C mostly compose of small tetragonal crystallites. Above 500°C, the portion of monoclinic crystallites increases along temperature, and the fibers calcined at 800°C are already fully monoclinic. The sintering effect also grows the crystallite size along elevating temperature that was noticed as a narrowing XRD diffractions. The ratios of tetragonal and monoclinic crystallites, and the crystallite sizes determined by the Rietveld Refinement are given in Table 3.

Fig. 15. The XRD diffractograms of a) the ZrO2 fibers calcined at 300–800°C with the heating rate of

1°C/min and b) the 0-15 metal% Sb-doped ZrO2 fibers calcined at 350°C using the heating rate of

15°C/min and c) the ZrO2 submicron fibers calcined at 350°C using the heating rate of 5°C/min.

Table 3. The crystallite ratios and the crystallite sizes of the ZrO2 submicron fibers calcined at 500, 600, 700 and 800°C with the heating rate of 1°C/min. T = tetragonal and M = monoclinic.

Calcination T T/M ratio (wt%) Approximate crystallite size T/M (nm) 500°C 90/10 9/13 600°C 60/40 26/26 700°C 15/85 29/40 800°C 0 /100 - /63

When the diffractogram of the ZrO2 fibers calcined at 350°C with the heating rate of 5°C/min (Fig 15c) are compared with the slowly heated fibers (Fig. 15a), a significant difference can be observed. With the slow heating rate, the fibers would be fully amorphous at 350°C but by using the faster heating rate, the PVP support burns faster and raises the temperature momentarily. This excess heat is high enough to form crystals into the fibers but because the temperature drops rather rapidly back to the set value, the crystallites do not grow that much in size, which is observed as a broader XRD diffractions. The size of the tetragonal crystallites was estimated with the Scherrer’s equation to be 7 nm which is smaller than the crystallite sizes of the slowly heated fibers. The broadest diffractions and so the smallest crystallites were observed for the Sb-doped ZrO2 fibers with the highest 15°C/min heating rate, Fig. 15b. As the diffractions were very broad, the identification of the monoclinic and the tetragonal crystallites is hampered. However, it seems that the heating rates of 5 and 15°C/min produced fibers that were mostly tetragonal but also small portion of monoclinic crystallites was present. Instead, the Sb content in the doped ZrO2 fibers did not seem to influence the crystallinity of ZrO2 nor form own crystallites that could be detected with the XRD. The specific surface areas and the porosities of the selected fibers were analyzed with the N2 gas adsorption/desorption method combined with the BET and the BJH calculation, Table 4. The fibers calcined with the slowest heating rate of 1°C/min had the lowest specific surface areas and the porosities. Among these fibers, the tetragonal fibers calcined at 500°C had the largest specific surface area that could originate from the smallest crystallites observed in the XRD, and meso- and microporosity which could not be seen in the SEM images. Because of the large crystallite size of the monoclinic fibers calcined at 800°C, their specific surface area was low despite the large pores that were seen in the SEM images. Probably, the sintering process was destroying the surface roughness, and micro- and mesopores of the fibers by generating large, smooth and solid crystallites. The specific surface area of the amorphous fibers (400°C) was the lowest that could be due to the polymer residues covering the fiber surface and its pores.

The ZrO2 fiber calcined at 350°C with the heating rate of 5°C/min had the largest specific surface area and porosity of the all fibers that could originate from the small crystallite size and a great number of micro- and mesopores. However, the fibers calcined at 350°C with even higher heating rate of 15°C/min had even smaller crystallite size but yet, the specific

surface area was somewhat smaller. Within the fastest heated fibers, there are no significant differences in the specific surface areas and the pore volumes as they go through the same calcination treatment. The large surface area should facilitate efficient adsorption by offering more surface sites for adsorption if the adsorbate has affinity to the surface.

Table 4. The N2-BET specific surface areas and pore volumes of the ZrO2 and the Sb-doped ZrO2 submicron fibers.

Fiber BET Surface area BJH Pore volume 2 3 ZrO2, 350°C, 5°C/min 72 m /g 0.06 cm /g 2 3 ZrO2, 400°C, 1°C/min 0.93 m /g 0.004 cm /g 2 3 ZrO2, 500°C, 1°C/min 14 m /g 0.02 cm /g 2 3 ZrO2, 800°C, 1°C/min 1.7 m /g 0.02 cm /g 2 3 ZrO2, 350°C, 15°C/min 49 m /g 0.04 cm /g 2 3 5%Sb:ZrO2, 350°C, 15°C/min 59 m /g 0.03 cm /g 2 3 10%Sb:ZrO2, 350°C, 15°C/min 43 m /g 0.02 cm /g 2 3 15%Sb:ZrO2, 350°C, 15°C/min 54 m /g 0.05 cm /g

The XRF analysis was conducted to estimate the Sb doping ratios in the Sb-doped fibers (supplementary material of the article III) as losses during the synthesis could hamper the – materials TcO4 uptake properties. The potential loses and errors could originate from the evaporation and segregation of antimony, hygroscopic nature of SbCl3 and other loses during the synthesis.147,148 The semiquantitative XRF analysis showed the Sb doping rates of 3.7, 6.0 and 11.5 metal% (Zr+Sb) when 5, 10 and 15 metal% were the respective target values. Hence, the analyzed values are rather close to those targeted when taking into account the semiquantitative nature of the analysis. However, some Sb loss had probably occurred but the amount of antimony should be sufficient for the separation of trace – amounts of TcO4 from the solution. The XRF analysis showed also presence of Hf and Cl in the fibers that originate from the synthesis chemicals. Nevertheless, they should not – hamper the TcO4 separation properties as Hf is chemically similar to Zr and Cl does not – compete with the TcO4 uptake. Additionally, the fibers contained only minute amount of carbon demonstrating efficient calcination process. The burning of PVP was also investigated with the FTIR-ATR analysis before and after the calcination of the fibers (supplementary material of the article III). Before the calcination, distinctive IR-peaks of PVP were observed but after the calcination at 350°C these peaks disappear. The peaks of -1 calcined Sb-doped ZrO2 fibers at 3300, 1575 and 1405 cm are related to bending and stretching vibrations of readsorbed and coordinated water, and 750 cm–1 to Zr-O vibration.

The XANES analysis on the Sb-LI edge was conducted to determine the oxidation state of Sb in the Sb-doped ZrO2 fibers after the synthesis, Fig. 16. Antimony was added to the synthesis solution as Sb(III) but Fig. 16 shows that a partial oxidation of Sb(III) occurred,

most probably during the calcination process, as the edges of the fiber samples do not remind the edge of Sb2O3 reference sample. The oxidation seemed to be stronger for the 5%Sb:ZrO2 and 15%Sb:ZrO2 fibers than for the 10%Sb:ZrO2 fibers but the amount of Sb(III) – in all fibers should be high enough to separate trace amounts of TcO4 from the solution. Unfortunately, a rather limited data quality of the laboratory X-ray source-based setup originating from low signal statistics and thermal broadening disables the calculation of more accurate oxidation state ratios. After the 10%Sb:ZrO2 fibers were loaded to – exhaustion with TcO4 in the column, the oxidation state of Sb shifted from mostly Sb(III) – to mainly Sb(V) as a consequence of Sb(III) oxidation by TcO4 and a dissolved O2 in the solution.

Fig. 16. The XANES Sb-LI edge analysis of the Sb-doped ZrO2 fibers, and Sb2O3 and KSb(OH)6 Sb- – oxidation state standards. The dashed line presents the 10%Sb:ZrO2 fiber after TcO4 loading in the column.

4.1.2 Characterization of precipitated Zr-oxyhydroxide particles

The precipitation synthesis of GZR yielded small separate zirconium hydroxide particles that aggregated during the drying stage of the synthesis as the free and structural water evaporated. The resulted Zr-oxyhydroxide GZR product was a dense hard cake that was ground and sieved with 200 mesh (75 μm) sieve which limited at least the smallest dimensions below this size. However, the particles can be much smaller as a lower limit was not controlled. In the SEM image (Fig. 17 a) some larger and smaller particles can be seen with large variation in the size and shape. The sharp edges of the particles seen in the images were formed during grinding and the surfaces of the particles are coarse because of aggregated structure that can be observed even better from the higher magnification SEM image (Fig. 17 b). This kind of surface roughness and aggregated structure increases the specific surface area significantly which is highly beneficial for the antimonate separation.

Fig. 17. The SEM image of Zr-oxyhydroxide granules a) with the low magnification and b) with the higher magnification.

The XRD diffractogram of the precipitated GZR particles shows amorphous or nanocrystalline structure, Fig. 18. The precipitate went through only a relatively low temperature drying (70°C) so that the small seed crystals were unable to grow in size and therefore, the material is lacking the typical ZrO2 diffractions. If the material would have been heated to higher temperatures, it would have first transformed to t-ZrO2 and later to m-ZrO2 as the crystal size would have grown. However, excess heating would have decreased the specific surface area and the adsorption capability of the material. Despite the heating at 70°C, the material contained still a lot of adsorbed water which was seen from the steady mass loss in the TGA-MS at 25–200°C provided in the supplementary information of the article I. Therefore, the material is considered rather as Zr-oxyhydroxide or hydrous oxide than ZrO2. Based on the mass loss of 25.9%, the loss is closer to the value 149 that Huang et al. observed for ZrO2·nH2O (21.5%) than the mass loss of Zr(OH)4·nH2O (32.2%). Thus, it is deduced that OH groups of Zr(OH)4 precipitate have partially condensed during the drying process and the product is probably in the form of ZrOx(OH)y·nH2O.

Fig. 18. The powder XRD of GZR after drying at 70°C for four days.

Because of the agglomerated structure of GZR, the specific surface area of 248 m2/g 3 and the pore volume of 0.1 cm /g are high and significantly higher than those of the ZrO2 fibers. The difference is expected to originate from the synthesis and calcination process. The fibers are denser as they experience higher temperature than GZR, which increases the crystallite size of the fibers. In addition, the spinning process produces originally dense fibers. Instead, GZR is precipitated as small Zr(OH)4 particles that polymerize and condense to porous oxide structure upon drying.

4.2 Effect of ZrO2 morphology on Sb(V) separation performance

The morphology of the adsorbent affects greatly on its adsorption properties. The nano- and microparticles have large specific surface areas that can result in high adsorption capacities but their utilization in the separation columns is challenging because of high flow resistance caused by the tightly packed particles. Instead, large particles have a lower flow resistance in the packed columns but their adsorption kinetics is slower as a significant portion of the reactive sites are inside the material and thereby kinetics is limited by the intraparticle diffusion. For that reason, the performance of the self-supported ZrO2 submicron fibers calcined at 350°C with the heating rate of 5°C/min (FZR) were compared with the granular Zr-oxyhydroxide (GZR) to point out their differences (article I). The adsorption of Sb(V) on zirconia surface is highly influenced by the solution pH since – it affects both the speciation of Sb(V) (Sb(OH)5 and Sb(OH)6 ) and the surface charge of the zirconia. For that reason, the zeta potentials of GZR and FZR were determined to clarify the influence of their surface charges on the Sb(V) adsorption, Fig. 19a,b. The pHPZC of the both materials were close to each other as the values of 7.4 and 7.2 were determined for FZR and GZR, respectively. At this pH, the net surface charges of the materials are zero and – they neither electrostatically repel nor attract the anionic Sb(OH)6 . Below pHPZC, the – – materials should electrostatically attract Sb(OH)6 and above the value, repel the Sb(OH)6 anion. This sort of adsorption trend was actually noticed in the pH effect experiment, Fig 19c. Below the pHPZC values of the FZR and GZR, the highest Kd values (over 600 000 mL/g) – were obtained as the surface charges of the materials and Sb(OH)6 were opposite. However, below pH 2, the Kd values decreased as the prevailing electrically neutral Sb(OH)5 species was not electrostatically attracted by the surface. Above pH 6 and 7 for FZR and GZR, respectively, the Kd values decreased towards the more alkaline conditions as the electrostatic repulsion increased. The fact that the Kd value of FZR decreased already below – its pHPZC is probably due to the limited amount of surface sites. When Sb(OH)6 is adsorbed, the pHPZC declines hampering the adsorption and additionally, the adsorbed Sb(V) probably – repels the solute Sb(OH)6 retarding the further adsorption. Therefore, GZR with the higher amount of hydroxyl groups (article I) and the specific surface area is less influenced by the – already adsorbed Sb(OH)6 .

Fig. 19. The zeta potential of a) GZR and b) FZR and c) the distribution coefficients of Sb(V) for GZR and FZR as a function of pH (10 mg/L Sb(V) in 0.01 M NaNO3, BF 500 mL/g) and the ratio of Sb(V) species calculated with the PhreeqC-program.

The adsorption capacity is an important factor when adsorbents are designed and produced as the material with the high capacity facilitates long operation time which reduces the operation costs. Thus, the Sb(V) adsorption capacities of GZR and FZR were determined at three temperatures: 13, 22 and 33 °C, Fig. 20. The adsorption capacity of FZR was practically unaffected by temperature since all adsorption points were the same within the uncertainties. Instead, the elevating temperature increased the adsorption capacity of GZR significantly meaning that the adsorption reaction was endothermic. This means that the GZR would perform better in elevated temperatures which can be beneficial in the nuclear separation application where the nuclear reaction and radiation is heating the surrounding solution. In turn, in cold solutions, the performance of GZR would be hampered. Then again, the increase in the capacity along the elevating temperature can also be attributed to the reaction kinetics. The increasing Brownian motion can potentially expedite the Sb(V) diffusion into the small pores of the material and speed up the adsorption kinetics resulting in a higher adsorption capacity because of limited equilibration time of 24 h. The Sb(V) adsorption capacity of GZR was higher than the adsorption capacity of FZR. At 22 °C, the end point value for the adsorbed Sb(V) were 69 ± 2 mg/g for FZR and 102 ± 5 mg/g for GZR. The higher adsorption capacity of GZR is probably due to its higher specific

surface area and amount of hydroxyl groups than those of FZR. The difference in these properties originates from the synthesis. The FZR experienced higher temperature treatment causing the condensation of structure whereas the precipitation and the low temperature drying of GZR maintained the high specific surface area and most of the hydroxyl groups of zirconia.

Fig. 20. The isotherms of a) GZR and b) FZR at 13, 22 and 33 °C (1-30 mg/L Sb(V) in 0.01 M NaNO3 at pH 4.0, BF 4000 mL/g. The solid lines describe Langmuir and the dashed lines Freundlich fittings.

The calculated Langmuir single layer adsorption capacities were 58 and 113 mg/g for FZR and GZR, respectively. At least, the calculated value for FZR is too low as the correlation with the Langmuir model is low (R2 0.887) and the end point of result is higher than the fitted value. The Freundlich model (R2 are 0.959 and 0.971 for FZR and GZR, respectively) seems to be more suitable for describing the adsorption of Sb(V) on these materials than the Langmuir model. This could indicate that the adsorption process is heterogenous and adsorption occurs on various sites or with different mechanisms, such as outer- and inner- sphere complexation.150 However, the model cannot be used as a direct prove for a certain mechanism as the model is only empirical. Nevertheless, according to other observations, it is possible that the adsorption is facilitated by the inner-sphere and outer-sphere complexation, and ligand exchange. The zeta potential decrease that was observed after – adsorption of Sb(OH)6 (Fig. 19a,b) has been reported to be due to the inner-sphere complexation66,151,152 whereas the noticed slight increase in the pH (0.1 units for FZR and 0.3 for GZR) along the Sb(V) concentration in the isotherm experiment could indicate – – 153,I Sb(OH)6 ion exchange with the OH ions. The XANES analysis was conducted to detected possible oxidation state chances of Sb(V) before and after adsorption on FZR and GZR since the oxidation reactions cannot be fully excluded, Fig 21a. For example, ZVI has been observed to reduce Sb(V) to Sb(III) during the adsorption process.154,155 However, Sb adsorbed on the surfaces of GZR and FZR seemed to remain as Sb(V) since the edge energies of the samples are the same as the

energy of the Sb2O5 reference sample and is not to be deformed by Sb(III) edge presented by Sb2O3. On this basis, the involvement of redox reactions in the Sb adsorption process can be excluded. The competing ions and compounds could significantly hamper the adsorption of – – 2- – Sb(OH)6 on ZrO2 and therefore, the effect of NO3 , H2CO3, SO4 and H2PO4 on the Sb(V) adsorption on GZR and FZR was examined, Fig. 21b,c. Despite the multiple times higher – concentration of NO3 and H2CO3, they did not hamper the Sb(V) adsorption markedly. Neither did the Cl– anion originating from the HCl that was used for the pH adjustment of – – the samples. The singly charged NO3 and Cl ions generally have a low retarding influence on Sb(V) adsorption as they are mainly retained by the electrostatic attraction whereas – Sb(OH)6 is proposed to bind on ZrO2 surface with the stronger inner-sphere complexation 66,73,78 2– mechanism. The SO4 anion weakened the Sb(V) adsorption capability of the both materials only slightly but the influence was higher for FZR. The Sb(V) adsorption percentage of FZR decreased from 100% to 99.5% with the highest 10 mM concentration sample and the drop was even smaller for GZR. The most significant decrease in Sb(V) – adsorption was caused by H2PO4 for both materials even though the effect was higher for – – FZR. The adsorption competition between the chemically reminding H2PO4 and Sb(OH)6 probably originates from their affinity to the same adsorption sites. Su et al.156 discovered that OH groups had a significant influence on phosphate removal and that phosphate formed inner-sphere complexes with Ce0.2Zr0.8O2. For that reason, the phosphate anions could be bound to the OH groups of GZR and FZR with the same inner-sphere complexation mechanism as Sb(V) and occupy the adsorption sites, which retards the Sb(V) adsorption. Due to the higher surface area and amount of OH groups on the GZR surface, its Sb(V) – adsorption is less influenced by H2PO4 than the adsorption on FZR.

Fig. 21. a) The XANES Sb-K edges of Sb(V) adsorbed on GZR and FZR, and the Sb2O3 and Sb2O5 – 2- – reference samples. The influence of NO3 , H2CO3, SO4 and H2PO4 (prevalent species at pH 4) on the Sb(V) adsorption on b) GZR and c) FZR. 10 mg/L Sb(V) at pH 4.0, BF 500 mL/g

The main purpose of this study was to develop ZrO2 fibers with improved properties in kinetics and pressure development that could be proven only by comparing the adsorption kinetics, and the pressure build-up in the separation columns between the zirconia microgranules and the submicron fibers, Fig. 22.

Fig. 22. a) The Sb(V) adsorption kinetics of FZR and GZR (10 mg/L Sb(V) in 0.01 M NaNO3 at pH 4.0, BF 500 mL/g). b) The overpressure developed in the columns packed with 0.25 g of FZR or GZR with different flow rates during the 30-minute period.

The fast adsorption kinetics is essential when large volume of contaminated solution is to be purified. Therefore, the adsorption kinetics of Sb(V) on the GZR and FZR were investigated. The Sb(V) adsorption kinetics of both materials was fast since at the first time points, the Kd values were 28 700 mL/g (98.3%) for FZR and 4 600 mL/g (90.2%) for GZR. In addition, FZR reached the 99% adsorption in 2 minutes and GZR in 11 minutes, later being several times longer. Hence, the adsorption kinetics of FZR seems to be faster at the beginning of the reaction. However, the GZR reached the detection limit of ICP-MS analysis first as the slope of the GZR curve is steeper at the end part of the reaction, Fig. 22a. This could originate from the higher adsorption capacity of GZR. However, as the reaction time in the column operation is usually less than an hour, the reaction rate of FZR shows better potential for the column separation regardless of its lower Sb(V) adsorption capacity. The pressure build-up during the column operation was measured to determine the potential of GZR and FZR in the column separation, Fig. 22b. The observed difference between GZR and FZR pressure build-ups was considerable. The FZR maintained low pressures even with the highest flow rates but instead, the pressure in the GZR column increased steadily when the flow rate was raised. With the lowest flow rate of 5 mL/h, which can be considered as a normal speed for this size column, the difference was still rather small since the overpressures were 1 mbar for FZR and 51 mbar for GZR. When the flow rate through the columns was increased, the differences spiked up. The overpressure in the FZR column elevated slowly to 17, 48 and 71 mbar in the last time points of series with the respective 25, 50 and 75 mL/h flow rates. Instead, the pressure in the GZR column

built up rapidly yielding 266, 611 and 926 mbar overpressure with the same flow rates. The difference in the pressure build-up originates from the morphologies of the materials. The fibrous FZR composes of the self-supported loose 3D network that allows solution flow through the structure with low resistance. On the contrary, the small < 75 μm particles are packing tightly inside the column as the tiniest particles fill the gaps of larger ones leaving only narrow and fewer pathways for the solution to flow through. As a result, the dense packing greats high flow resistance and elevates the pressures inside the column. Based on the superior performance of FZR in the column, it is seen highly promising material for solving the problems of the nano- and microsized materials in the packed bed column use.

4.3 Effect of the ZrO2 fibers crystal structure on Sb(V) adsorption

ZrO2 has three crystal structures that can be produced in the ambient pressures: cubic, tetragonal and monoclinic. As discussed in the chapter 1.3.2, the crystal structure greatly affects the properties and the reactivity of ZrO2. However, experimental studies published about the influence of ZrO2 of polymorphs on the Sb(V) adsorption are scarce. Therefore, the influence of amorphous, tetragonal and monoclinic structures of the ZrO2 submicron fibers on Sb(V) adsorption was carefully studied to discover the most suitable structure for the separation and further material development.

The zeta potentials of the ZrO2 fibers were determined as a function of pH to examine its influence on the Sb(V) adsorption, Fig. 23a. The obtained results show that the zeta potentials of the ZrO2 fibers calcined at 300°C were the lowest throughout the studied pH range. Similarly, the zeta potential of the amorphous fibers calcined at 400°C was lower above pH 5 than the potentials of the fibers calcined at 500–800°C. The lower zeta potential of the amorphous fibers (300 and 400°C) could be due to the organic residues from the unburned PVP as the calcination was not as effective as with the higher temperatures. Another possibility is that the lower zeta potential is caused by the amorphous structure (Fig. 15a) but this is not seen as probable explanation. The zeta potentials of the other four fibers seems to follow the same trends with each other by having the highest zeta potential at pH 3 that decreases towards more acidic and alkaline conditions. The estimated pHPZC values of the fibers calcined at 300–800°C were 5.3, 6.4, 6.8, 6.8, 6.8 and 6.9 in order of increasing calcination temperature that demonstrates the lower values of the amorphous fibers. The highest pHPZC values are also quite close to those of FZR (7.4) and GZR (7.2).

The Sb(V) adsorption trends of all the synthesized ZrO2 fibers were similar even though the separation efficiencies of the fibers varied, Fig. 23b. The optimal pH for the Sb(V) separation was between pH 2 and 3 because on that range the oppositely charged fiber – surface and Sb(OH)6 are electrostatically attracting each other with the highest potential difference. Outside that pH range, the adsorption of Sb(V) decreases towards the higher

and the lower pH values as the attraction decreases due to the lowering zeta potential and the changing Sb(V) speciation. Below pH 2, the electrically neutral Sb(OH)5 becomes increasingly prevalent. At the same, the zeta potential decreases which reduces the interaction between the fibers and Sb(V), and lowers the Sb(V) adsorption significantly. Above pH 3, the zeta potential gradually declines resulting in lower interaction with – Sb(OH)6 which is seen as a lower uptake. Hence, the Sb(V) uptake decreases already well before the pHPZC of the fibers as the already adsorbed Sb(V) lowers the zeta potential and thereby, interferes further Sb(V) uptake. Instead, the highest uptake of t-ZrO2 fibers (500°C) is expected to derive from the largest specific surface area, detected with BET, that generates the highest number of adsorption sites.

Fig. 23. a) The zeta potentials of the ZrO2 fibers in 0.01 M NaNO3 and after Sb(V) adsorption for the

ZrO2 fiber calcined at 500°C (BF 2000 mL/g). b) The amount of adsorbed Sb(V) as a function of pH

(10 mg/L Sb(V) in 0.01 M NaNO3, BF 2000 mL/g).

After Sb(V) was adsorbed onto t-ZrO2 fibers (500°C), the zeta potential plummeted and consequently, the pHPZC decreased from 6.8 to approximately 2.0, Fig. 23a upward triangles. This kind of drop in the zeta potential have been seen as an indication of inner- 66,73,78 sphere complexation in which the antimonate forms a chemical bond with the ZrO2 surface, generally thought as a Zr-O-Sb single bond or as a bidentate complex. However, as the Sb(V) adsorption results seemed to follow the trends of electrostatic attraction and repulsion, the outer-sphere complexation could not be excluded. These inner-sphere (6) and outer-sphere (7) complexation reactions can be expressed with the following reactions 157 that are obtained from the complexation reactions of Sb(V) on the Al(OH)3 surface:

− − ≡ZrOH + Sb(OH)6 ≡ZrOSb(OH)5 + H2O (6)

+ − + − ≡ZrOH + H + Sb(OH)6 ≡ZrOH2 ··· Sb(OH)6 (7)

The Sb(V) uptake efficiency of the ZrO2 fibers calcined at 300–800°C varied significantly. The lowest uptake values were obtained with the amorphous fibers (Fig. 15a) calcined at 300 and 400°C and the highest uptake values were achieved with the most tetragonal ZrO2 fiber calcined at 500°C. The adsorption performance declined from 500°C upwards along the increasing portion of monoclinic crystallites in the fiber structure. As a consequence, the Sb(V) uptake by the monoclinic ZrO2 fibers calcined at 800°C was rather close to those of the poorly performing amorphous fibers. Thus, it seems that the tetragonal crystal structure is highly beneficial for the Sb(V) adsorption as the fibers calcined at 500°C had the highest portion of tetragonal crystallites and the amount of adsorbed Sb(V) decreased along the declining portion of tetragonal phase. Accordingly, the Sb(V) uptakes at pH 2, were 1.8, 1.7, 9.5, 5.7, 3.4 and 2.4 mg/g for the fibers in the temperature order (300–800°C) 78 pointing the superiority of t-ZrO2. Also, Luo et al. have observed that ZrO2 with a high tetragonal-to-monoclinic ratio had a higher Sb(V) adsorption capacity than ZrO2 with lower tetragonal-to-monoclinic ratios. According to their DFT calculation, the Sb(V) adsorption on the (111) plane of t-ZrO2 had a higher adsorption energy than on the (111) plane of m- ZrO2 meaning more stable complex with t-ZrO2. The poor adsorption properties of the amorphous fibers (300 and 400°C) is probably caused by the organic residues, which were blocking the ZrO2 surface from Sb(V), and to their low specific surface area. The amorphous or nanocrystalline structure itself should not prevent the adsorption as seen in the case of GZR in the chapter 4.2. When large volumes of water are to be purified, the fast adsorption kinetics is necessity. For that reason, the Sb(V) adsorption kinetics of the ZrO2 submicron fibers calcined at 300–800°C were studied, Fig. 24. The results showed that the kinetics of the fibers were rather fast even though the uptake increased during the whole experiment. For example, the ZrO2 fibers calcined at 500°C reached 30% of the 7 days’ uptake in a minute. For other fibers, this percentage varied between 10 and 40%. However, already at the beginning of the reaction, the higher absolute uptake of the t-ZrO2 fibers (500°C) makes them superior compared to the other fibers as it could remove trace concentrations of

Sb(V) more rapidly because of a faster mass transfer. Also here, the results seem to follow the portion of tetragonal crystallites. The reason for the continuous increase in the adsorption of Sb(V) throughout the experiment could derive from several reasons. The first of them is the changing pH during the reaction. For all the fibers, the pH of the solution begins to drop from 7.0 at the moment the solution comes into contact with the fibers. The decrease is the highest for the amorphous fibers calcined at 300°C as the pH has dropped to 4.4 after 7 days. For the fibers calcined at 400 and 500°C, the pH dropped to 6.0 and for the fibers calcined at 600– 800°C, the end pH was 6.3 being the highest. As seen from the pH results (Fig. 23), the lower pH results in higher uptake than the initial pH 7.0. When the pH changes, the additional time is needed to reach a new equilibrium state that slows down the adsorption reaction. The second reason is the repulsion between the adsorbed and solute Sb(V) that retards the adsorption reaction and decreases the zeta potential. Despite that the adsorption reaction did not reach the equilibrium with the high initial concentration, the fibers are seen suitable for the removal of trace amounts of Sb(V). For example, in the nuclear solutions, the Sb(V) concentration is very low and therefore, the mass transfer is sufficient to those conditions. In addition, the material withstands radiation originating from the activated isotopes enabling its use in highly active solutions.

Fig. 24. The adsorption of Sb(V) on the ZrO2 fibers calcined at 300–800°C as a function of time. (10 mg/L Sb(V) in 0.01 M NaNO3, pHi 7.0, BF 2000 mL/g)

The isotherm study was conducted to determine the Sb(V) adsorption capacities of the ZrO2 fibers calcined at 300–800°C, Fig. 25. The experiment was conducted at pH 6 as it is seen relevant for many applications even though this is outside of the optimal range for the Sb adsorption (Fig. 23b). The Sb(V) adsorption capacities based on the highest equilibrium concentration points were 3.2, 2.9, 8.6, 5.4, 3.4 and 2.5 mg/g in order of

increasing calcination temperature from 300 to 800°C. However, none of the fibers reached the full plateau in this experiment indicating that the maximum capacity is slightly higher and cannot be described with the Langmuir monolayer adsorption model. Instead, the correlation with Freundlich model was higher suggesting heterogenous adsorption. The uptake trends were almost the same as observed in the pH experiment meaning that the tetragonal fibers performed well whereas the amorphous and monoclinic fibers worked less efficiently. However, some of the capacity values were lower than in the pH experiment as the experimental pH was only slightly below pHPZC of the fibers and further – Sb(V) adsorption decreased the pHPZC causing stronger repulsion between Sb(OH)6 and the fiber surface. Probably, lower pH and a more concentrated solution would have offered higher adsorption capacity. According to EDX mapping images, Sb(V) was distributed evenly on the ZrO2 fiber surface revealing involvement of the whole surface to the adsorption (supplementary material of article II). This observation supports the importance of the surface area to the adsorption. A large specific surface area generates more adsorption sites and causes less repulsion between the adsorbed and solute ions.

The respective Sb(V) adsorption capacity of FZR (350°C, 5°C/min) was 69 mg/g that is significantly higher than any of the slowly heated ZrO2 fibers presented here. The difference can be partially explained with the lower pH 4.0 in the isotherm experiment of FZR. However, the seemingly high tetragonal-to-monoclinic ratio, probably higher amount of crystal defects, and the significantly higher specific surface area (72 vs 14 m2/g) are seen as main reasons for the higher capacity. This observation indicates that the heating rate should not be ignored in the design of calcination process.

Fig. 25. The Sb(V) adsorption isotherms of the ZrO2 fibers calcined at 300–800°C. (1-25 mg/L Sb(V) in 0.01 M NaNO3 at pH 6.0, BF 2000 mL/g)

The reusability of two of the best performed ZrO2 fibers calcined at 500 and 600°C, were examined for five adsorption/desorption cycles using 1 M NaOH as a regeneration agent due to the good regeneration performance of NaOH in the literature (Fig. 26).71,153,158 During the first adsorption cycle, the Sb(V) uptakes were 6.2 and 3.9 mg/g for the fibers calcined at 500 and 600°C, respectively. The capacity dropped most dramatically after the first desorption cycle as the amount of adsorbed Sb(V) decreased to respective 54 and 52% of the uptake of the first cycle. After this the amount of adsorbed Sb(V) declined more slowly and consequently, at the last adsorption cycle, the Sb(V) uptake values of both fibers were 40% of the initial uptake. According to these results, the fibers can be reused with some compromise in the uptake capability. This is definitely a benefit in the decontamination of inactive Sb(V) solutions as it lowers the operation costs but in the nuclear industry, the regeneration capability is not required as the inorganic fibers are disposable after confinement as a radioactive waste. Therefore, the lowering capacity does not hinder the usability of the fibers. The fact that the fibers are only partially regenerable could be attributed to the different adsorption sites of ZrO2 such as the strong inner-sphere complexation sites and the weak outer-sphere complexation sites. Probably, Sb(V) adsorbed with the outer- sphere complexation presents the elutable fraction of Sb(V). This fraction can be desorbed with the OH– ions as the interaction is only electrostatic and can be reversed by changing the charge of the ZrO2 surface. Instead, the increasing fraction that cannot be eluted with OH– could be due to the strong chemical binding to the zirconia sites (inner-sphere complexation) which is much more difficult to reverse.

Fig. 26. The Sb(V) adsorption on the ZrO2 fibers calcined at 500 and 600°C during the five adsorption/desorption cycles. The Sb(V) solution was equilibrated for 18 hours and the desorption cycle with 1 M NaOH eluent lasted 6 hours. (Adsorption: 10 mg/L Sb(V) in 0.01 M NaNO3 at pH 6.0, BF 2000 mL/g)

– 4.4 Functionalization of the ZrO2 fibers by Sb doping: Uptake of TcO4

The ZrO2 submicron fibers showed good performance in the Sb(V) separation but the – initial experiments for the TcO4 removal from aqueous solutions revealed that the ZrO2 – fibers cannot be directly used in TcO4 removal due to their limited uptake performance. For that reason, the ZrO2 fibers were functionalized with Sb(III) doping since in literature, – 159 Sb(III)-doped SnO2 has shown promising TcO4 uptake performance. Thus, in this section – the influence of Sb doping and the doping level (5, 10 and 15 metal%) on the TcO4 removal was investigated and compared with the results of the undoped ZrO2 fibers. Also, the – – removal of ReO4 , which is often used as an inactive surrogate for TcO4 , was examined – and compared with the TcO4 separation capabilities to point out their differences.

The zeta potential of the undoped and the three Sb-doped ZrO2 fibers were analyzed to determine the influence of the pH on the surface charge of the fibers and to observe the differences between the doping rates, Fig. 27. The highest zeta potentials were achieved at pH 3 and the potentials declined towards more acidic and alkaline pH values. The estimated pHPZC values in the 0.01 M NaCl solution were 6.3, 6.1, 5.9 and 5.8 for the fibers in the order of increasing Sb doping level. Thus, it seems that the increasing Sb content decreases the pHPZC value of the fibers. A similar trend was observed in the – absolute zeta potential values below pHPZC. Therefore, if the TcO4 uptake would be only – due to electrostatic attraction between the TcO4 anion and the positively charged fiber surface, the removal efficiency should be the highest with the undoped ZrO2 fibers that is – not the case, Fig. 28. However, the zeta potential trend seems to correlate with the TcO4 – and ReO4 uptakes indicating electrostatic repulsion above pHPZC and electrostatic – attraction below pHPZC. Hence, the electrostatic interaction between the TcO4 anion and – the charged surface could partially explain the TcO4 uptake behavior but it cannot be the reason for a great uptake difference between the undoped and the Sb-doped ZrO2 fibers. – The effect of pH on the TcO4 adsorption on the Sb-doped ZrO2 fibers was studied as – the solution pH affects both the surface charge of the fibers and the speciation of TcO4 . – The TcO4 anion is very stable but with the pKa of 0.032, the increasing portion of Tc(VII) 160 occurs as the neutral HTcO4 when the pH decreases. At pH 1.0, already 9.7% of Tc(VII) occurs as HTcO4 that can have a retarding influence on the Tc(VII) uptake.

– The electrostatic interaction between TcO4 and the fiber surface most probably affects – – the TcO4 separation as the TcO4 uptake is the highest at the pH range of 2 – 6 where, the charges of the anion and the surface are opposite, Fig. 28. The uptake decreases below pH 2 as the fraction of electrically neutral HTcO4 becomes more significant and the zeta potential decreases. Above pH 6, the uptake decreases as the zeta potential becomes – negative creating repulsion between the fiber surface and the TcO4 anion. Therefore, it – can be concluded that TcO4 is partially bound on the fibers surface with the electrostatic attraction mechanism which is demonstrated below:

+ – + – ≡SOH2 + TcO4 ≡SOH2 ···TcO4 (8)

Fig. 27. The zeta potentials of the undoped and the Sb-doped ZrO2 fibers as a function of pHeq in 0.01 M NaCl, BF 1000 mL/g.

99 – 188 – Fig. 28. Kd of TcO4 and ReO4 as a function of pHeq for the ZrO2 and Sb-doped ZrO2 fibers. DL for 99 –7 99 – -14 188 – Tc is 0.04 Bq/mL. (2.22·10 M [20 kBq/L] TcO4 or 2.62·10 M [250 kBq/L] ReO4 in 0.01 M NaCl, BF 1000 mL/g)

– The electrostatic attraction could fully explain the TcO4 adsorption on the undoped ZrO2 fibers, which was very weak, but it cannot be the reason for the high uptake difference between the undoped and the Sb-doped ZrO2 fibers as the zeta potentials were close to each other and even higher for the undoped fibers. Therefore, Sb(III) doping has to be responsible for the great uptake difference between the materials. However, the Kd values of all the Sb-doped ZrO2 fibers were almost the same as even a low quantity of Sb(III) in – the fibers is sufficient for the reaction with the trace TcO4 present in the solution. Most

probably, the Sb(III) dopant is reducing Tc(VII) to Tc(IV) that is then bound on the fiber surface probably as TcO2/TcO(OH)2. The assumption about Tc(VII) reduction is supported by the XAS results (Fig. 16) in which the oxidation of Sb(III) to Sb(V) was observed after Tc loading even though the dissolved oxygen was probably partially responsible for the Sb(III) oxidation. In addition, the redox potential of Sb(III) is sufficient to reduce Tc(VII) in the solution to Tc(IV). The general redox half reactions for Sb and Tc, and their potentials are given below:110,161

– – + – 0 Sb(OH)4 + H2O ↔ Sb(OH)6 + 2 H + 2 e (E = -0.363 V) (9)

+ – 0 Sb2O3 + 2 H2O ↔ Sb2O5 + 4 H + 4 e (E = -0.649 V) (10)

– + – 0 TcO4 + 4 H + 3 e ↔ TcO2 + 2 H2O (E = 0.738 V) (11)

– When the redox half reaction of Sb2O3 (10) and TcO4 (11) are combined, the resulting full redox reaction (12) is slightly positive meaning spontaneous redox reaction between the redox couple.

– + 0 3 Sb2O3 + 4 TcO4 + 4 H ↔ 3 Sb2O5 + 4 TcO2 + 2 H2O (E = 0.089 V) (12)

However, as the Sb(III) was not presents as Sb2O3 in the fiber but as a dopant ion, the redox potential can be significantly higher and even higher than for the dissolved Sb(III). Some publications have concluded that the surface bound Fe(II) is more efficient reductant than dissolved Fe2+ or Fe(II) oxides.74,162,163 This could also be possible for Sb(III) that is mixed in the ZrO2 lattice and therefore located both inside and on the surface of the ZrO2 fiber structure allowing the formation of surface bound species. According to the Nernst equation, the redox potential of the reaction decreases when pH increases that can explain the declining Kd value along the pH together with the electrostatic repulsion between the – negatively charged fibers and TcO4 . However, if the Tc(VII) is first reduced to Tc(IV), the prevalent TcO2/TcO(OH)2 is not repelled by the negatively charged Sb-doped ZrO2 fibers and it could precipitate to the surface or form inner-sphere complexes.

– – The ReO4 and TcO4 uptakes were compared with each other by using the 10%Sb:ZrO2 fibers that had the highest capacity in the column experiment presented later. As seen – – from Fig. 28, the uptake of ReO4 was very low compared to the TcO4 uptake despite the – significantly lower concentration of Re in the solution. Instead, the ReO4 uptake results – reminded the TcO4 uptake performance of the undoped ZrO2 fibers. Therefore, it can be – deduced that ReO4 is bound on the fiber surface only with the electrostatic adsorption – mechanism. Probably, the redox potential of Sb(III) is insufficient to reduce ReO4 which redox half reaction is given below:

– + – 0 ReO4 + 4 H + 3 e ↔ ReO2 + 2 H2O (E = 0.510 V) (13)

– – Therefore, ReO4 should not be used as a TcO4 surrogate in adsorbent experiments before proper testing particularly when the redox reactions are considered as potential mechanisms for the separation.

The uptake properties of the fibers were compared with the commercial ZrO2, Sb2O5 – and Sb2O3 materials to clarify the influence of zirconia, Sb(III) and Sb(V) on the TcO4 – uptake, Table 5. The results showed that the commercial materials separated TcO4 very poorly. Among these materials, the highest uptake was obtained with Sb2O5 with a clear – difference but still the uptake by Sb2O5 is too weak to be used in the TcO4 separation. Thus, it seems that the materials containing only Zr or Sb are performing poorly including – the undoped ZrO2 fibers whereas the high TcO4 uptakes were obtained with the Sb-doped

ZrO2 fibers. Therefore, it is concluded that both Sb(III) and ZrO2 are needed for the efficient – TcO4 removal. The Sb(III) dopant is assumed to reduce Tc(VII) to Tc(IV) that is then bound on the zirconia surface.

– Table 5. The TcO4 uptake by the commercial ZrO2, Sb2O5 and Sb2O3 materials and the synthesized fibers in 0.01 M NaCl at pH 4.0.

–1 Chemical Uptake (%) Kd (mL g )

m-ZrO2 (Alfa Aesar, 99.978%) 0.2 ± 1.1 2 ± 15

Sb2O5 (Aldrich, 99.995%) 15.1 ± 0.9 176 ± 18

Sb2O3 (Sigma-Aldrich, 99.99%) 0.7 ± 1.1 7 ± 15

15%Sb:ZrO2 fiber 99.98 ± 0.09 6 500 000 ± 37 600 000

10%Sb:ZrO2 fiber 99.98 ± 0.09 6 600 000 ± 38 000 000

5%Sb:ZrO2 fiber 99.98 ± 0.09 6 700 000 ± 38 400 000

ZrO2 fiber 5.8 ± 0.7 61 ± 13

– – – 2– – – The effect of Cl , NO3 , ClO4 , H3BO3, SO4 and H2PO4 on the TcO4 uptake by the

10%Sb:ZrO2 fibers was examined to detect competition between the compounds, Fig. 29. – – – – The Cl , NO3 and ClO4 anions did not seem to decrease the TcO4 uptake noticeably on the studied concentration range. Also, in the case of FZR, the Sb(V) adsorption was – I – uninfluenced by the increasing NO3 concentration. In the same manner, ClO4 did not – 164 interfere the TcO4 uptake unlike was observed for activated carbon by Galambos et al. – – They suggested that the stereo chemically similar ClO4 was competing with TcO4 from the same adsorption sites and thereby decreasing the adsorption efficiency. The boric acid, – H3BO3, decreased the Kd values of TcO4 only slightly even with the highest concentrations. – The low competition between TcO4 and H3BO3 is an important property since in the nuclear reactors H3BO3 is used in large quantities as a neutron absorber. As a result, the solutions to be decontaminated can contain rather high concentrations of H3BO3 and yet, – 2– not cause too high disturbance in the TcO4 removal by the Sb-doped ZrO2 fibers. The SO4 – – – and H2PO4 anions disturbed the TcO4 uptake the most. The influence of H2PO4 was even

2– higher than the effect of SO4 anion which was seen also in the case of FZR and GZR in 2– – Sb(V) removal (Fig. 21b). The binding of SO4 and H2PO4 lowered the zeta potential of the – fibers (article III, supplementary material) that repulses negative TcO4 and prevents the – – uptake on the surface. From the examined ions and compound, only NO3 and ClO4 would have an adequate redox potential to oxidize Sb(III) to Sb(V) and as a result hamper the – – – TcO4 adsorption. However, due to slow redox reaction kinetics of NO3 and ClO4 , their – 165 influence on TcO4 can be regarded as insignificant.

– – – 2– – – Fig. 29. The effect of Cl , NO3 , ClO4 , H3BO3, SO4 and H2PO4 on TcO4 uptake of the 10%Sb:ZrO2 –7 99 – fibers as a function of compound concentration. (2.22·10 M [20 kBq/L] TcO4 in 0.001 – 0.1 M solution at pH 4.0, BF 1000 mL/g) DL is 0.04 Bq/mL.

– The TcO4 uptake kinetics of the Sb-doped ZrO2 fibers was investigated to estimate their suitability for the purifications of large solution volumes, Fig. 30. The kinetics of the all fibers was fast as they exceeded Kd 1000 mL/g before one minute had passed. At the beginning of the reaction, the uptake kinetics of the 15%Sb:ZrO2 fibers was fastest and the

5%Sb:ZrO2 fibers the slowest since the uptake percentage were 67, 80, and 86% for the fibers in the increasing Sb content order. However, after few minutes, the order of Kd values turned to opposite since the 5%Sb:ZrO2 fibers reached the detection limit first and the 15%Sb:ZrO2 fibers last. The initial uptake order of the fibers could be due to larger Sb content in the fiber that enables faster reduction of Tc(VII) to Tc(IV) than the fibers with the lower content. However, Sb(III) can have an inhibitory effect on the Tc(IV)/Tc(VII) uptake as well because Sb2O5 and Sb2O3 reference samples showed only weak Tc uptake.

Hence, the 5%Sb:ZrO2 fibers with the lowest Sb content could have the highest concentration of active Zr surface sites that facilitates the fastest uptake at the end part of the reaction.

– –7 99 – Fig. 30. The TcO4 uptake kinetics of the Sb-doped ZrO2 fibers (2.22·10 M [20 kBq/L] TcO4 in 0.01 M NaCl at pH 4, BF 2000 mL/g). DL is 0.02 Bq/mL.

The separation columns are one of the simplest ways to purify trace amounts of radionuclides from the large solution volumes. Therefore, such a column separation – experiment was conducted for TcO4 with the Sb-doped ZrO2 fibers in the laboratory scale, Fig. 31. The results showed that the performance of the fibers before the breakthrough 99 was outstanding as the Tc activity remained below the detection limit. The 5%Sb:ZrO2 exhausted first after passing through approximately 2 L of the simulant. Surprisingly, the next product to break through was 15%Sb:ZrO2, and 10%Sb:ZrO2 was the last. This was also – the order of the TcO4 uptake capacities: 2.1±0.1, 3.6±0.1 and 3.5±0.1 μmol/g. One could assume that the fibers with the highest initial Sb content would offer the best separation performance but this was not the case. Instead, the performance was attributed to Sb(III) content in the fibers. According to XANES analysis (Fig. 16), the 10%Sb:ZrO2 fibers seemed to have the highest Sb(III)/Sb(V) ratio which probably enabled the most effective separation. The portion of Sb(III) in the 10%Sb:ZrO2 fibers decreased after the column was – exhausted indicating that the TcO4 uptake capacity run out when the accessible Sb(III) was – consumed by TcO4 and the dissolved O2 in the solution. However, as the XANES measurement was conducted with the laboratory XAS setup with a limited statistics, the – Sb(III)/Sb(V) ratios could not be accurately calculated and their influence on the TcO4 uptake capacity quantitatively evaluated.

In addition to the total uptake capacity of the separation material, also the degree of – utilization (qBT/qTOT) affects its usability. The steepness of the TcO4 breakthrough slopes were in order 5%Sb:ZrO2, 10%Sb:ZrO2 and 15%Sb:ZrO2 which was also the order in which the fibers reached the detection limit in the kinetic experiment. Because the column setup was similar for all the materials, the reaction kinetics and the uptake capacity dictated the shape of the breakthrough slope. The material with the fastest kinetics is the closest to the full exhaustion before the breakthrough starts as it naturally has the shortest reaction

zone. When the reaction zone reaches the bottom of the column, the remaining capacity is filled rapidly resulting in the steep rise of the breakthrough. Therefore, the 5%Sb:ZrO2 fibers with fastest adsorption kinetics had the steepest slope. Despite the fastest kinetics of the 5%Sb:ZrO2 fibers, the 10%Sb:ZrO2 fibers are seen the most prominent for the separation due to their highest uptake capacity and the high degree of utilization. The situation could change if all Sb in the fibers would occur as Sb(III). This would probably enhance the uptake performance of all the Sb-doped fibers and especially that of the 15%Sb:ZrO2 fibers with the seeming lowest Sb(III)/Sb(V) ratio. Furthermore, the column – operation in anaerobic conditions would probably improve the TcO4 uptake performance of all the Sb-doped ZrO2 fibers as the dissolved O2 is assumed to participate in the Sb(III) oxidation despite its slow redox reaction kinetics.166

– Fig. 31. a) The TcO4 breakthrough after the columns packed with 0.25 g of the Sb-doped ZrO2 fibers –7 99 – – – (1.11·10 M TcO4 in 0.01 M NaCl at pH 4.0, 33 mL/h) b) the breakthrough of TcO4 and ReO4 of -15 188 – the column packed with 10%Sb:ZrO2 fibers (5.51·10 M ReO4 ).

– – The separation of TcO4 and ReO4 with the 10%Sb:ZrO2 fibers was examined to determine their uptake differences in the column use, Fig. 31b. The observed difference in – – – the breakthrough of TcO4 and ReO4 was enormous. The ReO4 was breaking through already in the very first fraction (0.015 L) whereas the same 10% breakthrough occurred – after 5.0 L for TcO4 meaning over 300-fold volume difference. The difference in the – -15 capacity is even higher as the concentration of ReO4 (5.51·10 M) in the test solution was – –7 – several orders of magnitude lower than the concentration of TcO4 (1.11·10 M). The TcO4 selectivity is a significant benefit that is in high importance in specific solutions that contain – – both TcO4 and ReO4 . The obtained result also confirms the observation from the pH – – – experiment that the ReO4 cannot be used as the TcO4 surrogate when studying the TcO4 uptake performance of the reductive separation materials.

5 CONCLUSIONS

Inorganic ion exchangers and adsorbents have shown good metal ion uptake performance in many applications, for example, in a purification of contaminated waters at the Fukushima nuclear power plant. However, a major challenge has been confronted in their column use. Large granules allow fast flow rates through the column but their uptake kinetics is limited due to slow adsorbate diffusion into the structure. Instead, nano- and microsized particles have good adsorption capacities and kinetics but they create high flow resistance in the column that can lead to system failures. Therefore, in this thesis, the submicron fibers have been developed and tested for Sb(V) and Tc(VII) remediation to facilitate fast uptake kinetics and still enable high flow rates through the column.

The ZrO2 and Sb-doped ZrO2 fibers were synthesized with the electroblowing method which enabled a faster production rate compared to the more common electrospinning method. The spinning process was simple to operate and it resulted in separate and long fibers with a cylindrical shape. After the spinning, the fibers were calcined to remove PVP support and to obtain fully inorganic fibers. This calcination process is seen as a critical step in the synthesis since it affects the specific surface area and porosity, crystallinity, oxidation state of elements, and the amount of polymer residues.

The t-ZrO2 fibers were tested for Sb(V) separation and compared with the microsized zirconia particles. Due to the larger surface area and porosity of the precipitated granular zirconia, it had higher Sb(V) adsorption capacity than the fibers. This also enabled a wider pH range for operation. However, the ZrO2 fibers showed faster adsorption kinetics at the beginning of the reaction and the fibers maintained low pressures during the column operation that was facilitated by the loose 3D packing of the fibers. Therefore, the submicron fibers are seen to be more prominent for the column separation of Sb(V) than the nano- or microparticles.

Six different ZrO2 fibers were developed for Sb(V) removal from aqueous solutions. The fibers were synthesized by electroblowing and calcination at 300–800°C that produced amorphous, tetragonal and monoclinic fibers. The t-ZrO2 fibers calcined at 500°C performed well because of the highest specific surface area among these fibers and more stable inner-sphere complexes between Sb(V) and the t-ZrO2 surface. Instead, the amorphous ZrO2 fibers (300 and 400°C) had weak adsorption capabilities most probably due to PVP residues on the ZrO2 surface. Similarly, when the portion of monoclinic crystallites increased (600–800°C), the Sb(V) adsorption properties weakened. The lowered adsorption capability of the fibers calcined at 600–800°C is expected to originate from weaker adsorption complexes between Sb(V) and m-ZrO2 and from condensation of the fiber structure and OH groups that are essential for the Sb(V) remediation. Hence, the calcination process is seen to be highly critical for obtaining efficient adsorbent. Too low a fiber calcination temperature is unable to combust PVP support efficiently whereas too high a temperature leads to unnecessary condensation of structure and formation of undesirable m-ZrO2 structure.

– The ZrO2 fibers were functionalized with Sb doping to enhance TcO4 separation as the undoped ZrO2 fibers showed only modest uptake capability. By doping the fibers with – – Sb(III), high Kd values for TcO4 were achieved in a wide pH range. The separation of TcO4 was most probably based on the reduction of Tc(VII) to Tc(IV) by Sb(III), and then Tc(IV) present as TcO2/TcO(OH)2 was retained by the ZrO2 surface. The fibers also showed good column performance since the 99Tc activity remained below the detection limit until the – column was approaching the full exhaustion. On contrary, the ReO4 column experiment – showed only negligible uptake by Sb-doped ZrO2 fibers even though ReO4 is used in – – research as a TcO4 surrogate. In addition, the Kd values for TcO4 were many orders of – magnitude higher than those of ReO4 . The dissimilarity is expected to arise from their difference in the redox potentials. The Sb(III) dopant is sufficient to reduce Tc(VII) to Tc(IV) – but unable to reduce Re(VII) to Re(IV) and therefore, ReO4 is bound on ZrO2 only with the – – weak electrostatic interaction mechanism. For that reason, comparison of TcO4 and ReO4 – uptakes is advisable before using ReO4 as a surrogate particularly if the adsorbent has – – reducing properties. The Sb-doped fibers also showed high selectivity over NO3 , Cl , stereo – structurally similar ClO4 , and boric acid, which is an important chemical in the operation of a nuclear reactor.

Based on the results presented here, the ZrO2-based fibers are seen to be prominent – materials for decontamination of Sb(V) and TcO4 bearing waters in specific conditions and they could facilitate effective radioactive wastewater treatment. However, synthesis and separation upscale studies would be required before commercialization. The simple electroblowing synthesis was rather easily applied to the production of the ZrO2 and the Sb-doped ZrO2 fibers and it can be extended to the production of other materials. There is actually still ongoing development of other metal oxide fibers for decontamination of wastewaters from other metal ions. In future, the metal uptake abilities could be further improved by surface functionalization after the calcination process for example, by surface etching or – hydrothermal treatment. In addition, the TcO4 uptake mechanism on the Sb-doped ZrO2 surface could be further examined with synchrotron quality XAS measurements on the Sb, Zr- and Tc-edges, which would reveal the oxidation state changes and the coordination environment of these elements. The deeper understanding of the uptake mechanism would allow further development of the fibers potentially leading to improved uptake performance.

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