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Article Removal from Groundwater and Wastewater Using -Supported Nanoscale Zero-Valent Iron

Borys Kornilovych 1,2,*, Iryna Kovalchuk 2 , Viktoriia Tobilko 1 and Stefano Ubaldini 3

1 National Technical University of “Igor Sikorsky Kyiv Polytechnic Institute”, 37 Peremogy Av., 03056 Kyiv, Ukraine; [email protected] 2 Institute for Sorption and Problems of Endoecology, National Academy of Science of Ukraine, 13 General Naumov Str., 03164 Kyiv, Ukraine; [email protected] 3 Istituto di Geologia Ambientale e Geoingegneria, CNR, Area della Ricerca di Roma RM 1—Montelibretti-Via Salaria Km 29,300—Monterotondo Stazione, 00015 Roma, Italy; [email protected] * Correspondence: [email protected]; Tel.: +38-050-8407970

 Received: 17 September 2020; Accepted: 23 October 2020; Published: 26 October 2020 

Abstract: The peculiarities of sorption removal of uranium (VI) compounds from the surface and mineralized groundwater using clay-supported nanoscale zero-valent iron (nZVI) composite materials are studied. Representatives of the main structural types of clay are taken as clays: kaolinite (Kt), (MMT) and (Pg). It was found that the obtained samples of composite sorbents have much better sorption properties for the removal of uranium from surface and mineralized waters compared to natural clays and nZVI.It is shown that in mineralized waters uranium (VI) is mainly in anionic form, namely in the form of carbonate complexes, which are practically not extracted by pure clays. According to the efficiency of removal of uranium compounds from surface and mineralized waters, composite sorbents form a sequence: montmorillonite-nZVI > palygorskite-nZVI > kaolinite-nZVI, which corresponds to a decrease in the specific surface area of the pristine clay minerals.

Keywords: uranium; clays; nanoscale zero-valent iron; groundwater; wastewater

1. Introduction Pollution of the water basin in sites where uranium ore is mined and processed is one of the most important environmental problems, requiring an effective and rational solution [1]. The sources of surface and groundwater pollution are, first of all, the areas of development of uranium deposits, storages of wastes from hydrometallurgical processing of uranium ores, sites where uranium where enriched [2–4]. Water pollution with uranium compounds is often accompanied by pollution of other toxic elements, for example, arsenic [5]. Contaminated groundwater in places of extraction and processing of uranium ores, in addition to the high content of compounds, is characterized by high mineralization. The latter is a few grams per litre, mainly due to sulfates of Ca and Mg, which are formed due to the use of sulfuric acid in the technological processes of leaching [6]. In such waters, it is mainly in the composition of negatively charged sulfate or carbonate complexes, which significantly complicates the processes of water purification [7]. Sorption methods using exchange and synthetic inorganic sorbents are most widely used for deep purification of sewage and surface waters from uranium complexes [8–10]. At the same time, if it is necessary to treat large amounts of treated water, the economic factor becomes principal. Some of the new effective environmental protection technologies that havebeen developed and widely tested in recent years are technologies based on the use of highly reactive nanoscale zero-valent

Metals 2020, 10, 1421; doi:10.3390/met10111421 www.mdpi.com/journal/metals Metals 2020, 10, 1421 2 of 12 iron. The use of coarsely dispersed zero-valent iron is quite common in environmental practice in the construction of permeable reaction barriers that are installed in the path of contaminated groundwater and serve as a collector of various toxicants [11,12]. The advantage of using nZVI is the possibility of its use for groundwater purification without the application of permeable reaction barriers, the construction of which requires significant capital costs. Suspensions based on nZVI can be pumped into the ground along the path of groundwater directly in front of contaminated areas. Further, they are carried with underground streams, promoting the decomposition of organic toxicants or sorption of inorganic pollutants on themselves. The first successful results on the use of nZVI were obtained during the decomposition of organic toxicants (chlorinated solvents, pesticides, dyes) [13,14]. Subsequently, the effectiveness of this technology was shown for a variety of inorganic pollutants, including heavy metals [15] and natural radionuclides [16,17]. However, the potential risks for natural ecosystems when using nZVI is insufficiently studied so far [18]. The problem with the use of nZVI for in situ and ex situ technologies is the insufficient colloidal stability of its suspensions, easy aggregation, and difficulty in separating nZVI from the treated solution. To solve it, various polymers and surfactants are commonly used [19,20]. Another approach to increase the stability of suspensions is their anchoring onto a , which allows not only to increase their stability and stability to oxidation, but also to expand the scope of their application in traditional sorption technologies [21]. Active carbons [22], amorphous silica [23,24], layered double [25], carbonized fungi [26], graphene and composites based on it [27,28] and others were successfully investigated. Some of the most widely used matrices as the supports of nZVIare clay minerals, which have a significant specific surface area and rather high sorption properties in relation to radionuclides and heavy metal [29,30]. Iron-containing composite sorbents based on clays have enhanced sorption characteristics in relation to heavy metals and radionuclides compared to the initial minerals [31]. Representatives of almost all major classes of clay minerals were studied as matrixes: kaolinite [32], montmorillonite [33,34], [35], palygorskite [36]. While considerable experimental material accumulated on the purification of uranium-containing water using nZVI, there was no comparative study of the sorption properties of iron-containing composites of different composition based on clay minerals. Taking into account the importance of environmental studies based on natural minerals, the removal of uranium by nZVI supported on kaolinite, montmorillonite and palygorskite was investigated, including the removal efficiency of uranium from contaminated groundwater with low and high mineralization.

2. Materials and Methods

The object of the study was a layer kaolinite (Al4Si4O10(OH)8) with a structure of 1:1 type. This clay was taken from the Glukhovets deposit (Ukraine), and has the most perfect among the kaolinites from other numerous kaolin deposits of Ukraine. Among the representatives of layered with a 2:1 structure (smectites), montmorillonite from the Cherkassk deposit (Ukraine) with a structural formula (Ca Na K ) (Al Mg Fe ) (Si Al ) O (OH) nH O 0.12 0.03 0.03 0.18 1.39 0.13 0.44 1.96 3.88 0.12 4.0 10 2 × 2 was chosen. Fibrous clay palygorskite (also known as attapulgite) with a structural formula Mg Si O (OH) 4H O was also taken from the Cherkassk deposit (Ukraine). 5 8 20 2 × 2 The purification of natural minerals from impurities of quartz, , carbonates, and iron oxides was carried out.X-ray powder diffraction (XRD) patterns were recorded on X-ray diffractometer DRON-4-07 (Russia) in the range of 2–40◦ (2θ) using CuKα irradiation. The parameters of the porous structure: specific surface area (SSA), total pore volume (V), average pore radius (r) of the natural and composite sorbents were determined by the BET method from nitrogen adsorption isotherms obtained on a Nova 2200e gas analyzer (USA), pore distribution(Rmax)—by BJH and DFT methods [37]. Metals 2020, 10, x FOR PEER REVIEW 3 of 13

The parameters of the porous structure: specific surface area (SSA), total pore volume (V), average pore radius (r) of the natural and composite sorbents were determined by the BET method from nitrogen adsorption isotherms obtained on a Nova 2200e gas analyzer (USA), pore distribution(Rmax)—by BJH and DFT methods [37].

MetalsTo2020 obtain, 10, 1421 composite sorbents “/nZVI” with a mass ratio nZVI: clay mineral~0.2:3 of 121 modified method was used [38,39]. To do this, in a 0.2M solution of Fe(NO3)3. 9H2O, weighed clay was introduced.The resulting suspension (pH2) was quantitatively transferred to a three-necked 3+ flask Toand obtain the ion composite Fe reduction sorbents process “clay mineral was performed/nZVI” with with a a mass solution ratio of nZVI: sodium clay borohydride mineral~0.2: ( 1 modified method was used [38,39]. To do this, in a 0.2M solution of Fe(NO ) . 9H O, weighed clay NaBH ) in a nitrogen atmosphere. The resulting composite sorbent was then3 3 separated2 from the was introduced.The4 resulting suspension (pH2) was quantitatively transferred to a three-necked flask liquid phase by3+ centrifugation and washed three times with alcohol. The resulting precipitate was and the ion Fe reduction process was performed with a solution of sodium borohydride (NaBH4) in drieda nitrogen under atmosphere. vacuum at a The resulting of composite 60 °C and sorbent crushed was to obtain then separated a fraction from ≤0.1 themm liquid phase by centrifugationWater purification and washed from threeuranium times ions with was alcohol. performed The resulting using precipitatenatural clay was minerals dried under and compositevacuum at sorbents. a temperature Solutions of 60 wereC and prepared crushed with to obtain distilled a fraction water and0.1 mm.uranyl trihydrosulfate salt ( ⋅ ◦ ≤ UO24Water SO purification 3H 2 O ). 1M from NaCl uranium solution ions to was create performed ionic strength using natural (~0.01) clay was minerals used. The and pH composite of the solutionssorbents. Solutionswas adjusted were with prepared 0.1M withsolutions distilled NaOH water and and uranylHCl . trihydrosulfate salt (UO SO 3H O). 2 4· 2 1M NaClMineralizedsolution waters to create were ionic used strength solutions, (~0.01) whose was used. composition The pH ofcorresponded the solutions to was the adjusted composition with of0.1M underground solutions NaOH mineralized and HCl. water near the liquid waste storage facility for hydrometallurgical processingMineralized uranium waters ores were at the used Centre solutions, of Ukrainian whose composition Uranium Industry corresponded (Zhovti to theVody) composition by anions: of underground− mineralized− water2 near− the liquid waste− storage facility for hydrometallurgical2+ processing2+ HCO3 —450; Cl —180; SO4 —2830; NO3 —130 mg/l and by cations: Ca —576; Mg uranium ores++ at the Centre of Ukrainian+ Uranium Industry (Zhovti Vody) by anions: HCO —450; + 2+ 2+ 2+ 3− —209; (Na2 K ) —391; NH4 —0.92; Ni —<0.05; Cu2+—<0.03; Co2+ —<0.06; +Mnsum+ —0.10; Cl−—180; SO4−—2830; NO3−—130 mg/l and by cations: Ca —576; Mg —209; (Na + K )—391; Zn 2++ 2Pb+ 2+ 2Cd+ 2+ 2+Fe 2+ 2+ NH4 —0.92;—<0.01;Ni —<—<0.19;0.05; Cu —<—<0.01;0.03; Co —sum<0.06;—0.05mg/LMnsum—0.10; [40]. TheZn output—<0.01; solutionsPb —

3. Results Results X-ray analysis of samples shows the presence of only small impurities in pristine minerals (Figure1 1).).

Pg-nZVI

0.446 1.054 0.425 0.319 0.254 Pg 0.629 0.541

MMT-nZVI 1.532 0.424 0.333 0.446 0.245 0.197 MMT Intensity, % 0.358 0.715 0.252 0.202 Kt-nZVI

0.334 0.426 0.238

Kt

10 20 30 40 50 θ 2 , deg. Figure 1. X-rayX-ray powder powder diffraction diffraction (XRD) (XRD) patterns patterns of of pristine pristine and nanoscale zero-valent iron (nZVI)—modified(nZVI)—modified clay minerals.

On diffractograms (XRD) of all modified minerals, there are weak reflections at 0.252 and 0.202 nm corresponding to crystalline phases of zero-valent iron (α-Fe), (FeO), and also, in time, smaller quantities of goethite (FeOOH). Clays kept their structures in mixtures, since no structural changes between neat and supported clays were found.The amount of iron applied to the surface Metals 2020, 10, x FOR PEER REVIEW 4 of 13

On diffractograms (XRD) of all modified minerals, there are weak reflections at 0.252 and 0.202 Metalsnm corresponding2020, 10, 1421 to crystalline phases of zero-valent iron (α-Fe), iron oxide (FeO), and also, in 4time, of 12 smaller quantities of goethite (FeOOH). Clays kept their structures in mixtures, since no structural changes between neat and supported clays were found.The amount of iron applied to the surface is, is,according according to tothe the chemical chemical analysis analysis of of the the solution solution after after treatment treatment of of the the modified modified samples with ,acid, 0.170.17 mgmg/g/g forfor allall minerals.minerals. By , the nitrogennitrogen sorptionsorption isotherm on thethe pristinepristine kaolinite (Figure2 2),), accordingaccording toto thethe modifiedmodified dede Boer Boer classification, classification, belongs belongs to typeto ty IIpe (b) II isotherms (b) isotherms and are and typical are fortypical nonporous for nonporous sorbents withsorbents a small with macroporous a small macroporous component component [41]. [41].

250 350

300 200 MMT-nZVI Pg 250 (STP) (STP) -1 150 MMT -1 g

g 200 3 3

150 100 Kt-nZV I 100 Volume, cm Volume, cm 50 Kt 50 Pg-nZVI

0 0 0 0,2 0,4 0,6 0,8 1 0 0,2 0,4 0,6 0,8 1 p/p 0 p/p0

(a) (b)

Figure 2.2. NitrogenNitrogen adsorption–desorptionadsorption–desorption isotherms of pristine and nZVI—modifiednZVI—modified clay minerals: ((aa)—kaolinite)—kaolinite andand montmorillonite,(montmorillonite,(bb)—palygorskite.)—palygorskite.

The narrow hysteresis hysteresis loop loop of of type type H3 H3 on on isotherm isothermss is isthe the result result of capillary of capillary condensation condensation in the in thestructural structural aggregates aggregates of kaolinite of kaolinite between between weakly weakly interconnected interconnected flat flatelementary elementary packages packages of the of themineral. mineral. The Thenature nature of ofthe the nitrogen nitrogen adsorption adsorption curves curves on on the the samples samples of of montmorillonite andand paligorskitepaligorskite are similar to kaolinite and, thus, also belong to type II (b)(b) isothermsisotherms with aa hysteresishysteresis looploop typetype H3.H3. TheThe calculatedcalculated characteristicscharacteristics of of the the porous porous structure structure of of the the samples samples are are given given in in Table Table1. As1. As can can be be seen seen from from the the nitrogen nitrogen isotherms, isotherms, the specificthe specific surface surface area area of samples of samples sharply sharply decreases decreases after theafter surface the surface modification. modification. Such reductionSuch reduction is stipulated is stipulated by aggregation by aggregation processes processes of small clayof small particles clay byparticles nZVI andby nZVI practically and practically complete closingcomplete of microporesclosing of micropores with nZVI filmswith nZVI and the films resultant and the blocking resultant of theblocking access of of the nitrogen access moleculesof nitrogen to these pores. to these pores.

TableTable 1. 1.CharacteristicsCharacteristics of of the the porous structure structure of of the the samples. samples.

DistributionDistribution ofof PorePore Size,Size, nm nm 2 3 3 SampleSample S, m /gS, m2V/gP , cmV∑,/ gcm3/gV µ,V cmμ, cm/g 3/g VµV%μ,%%, % BJHBJH dV dV (r) DFT DFT dV dV (r) (r) r1 r2 r1 r2 r1 r2 r1 r2 Kaolinite 8.98 0.124 0.003 2.1 - - 2.36 4.91–8.58 Kaolinite 8.98 0.124 0.003 2.1 - - 2.36 4.91–8.58 Kaolinite-nZVI 11.72 0.093 0.003 3.1 - - 2.51 2.63–3.52 Kaolinite-nZVIMontmorillonite 11.7289.11 0.0930.081 0.0030.016 3.119.8 1.41 - - -1.91 2.51 2.63–3.522.82 MontmorilloniteMontmorillonite-nZVI 89.11 24.74 0.081 0.384 0.016 0.008 19.8 2.1 1.412.11 - -2.84 1.91 8.96 2.82 Montmorillonite-nZVIPalygorskite 24.74213.15 0.3840.512 0.0080.084 2.116.6 2.111.90 6.26 - - 2.84 8.96 - PalygorskitePalygorskite-nZVI 213.1571.68 0.5120.274 0.0840.002 16.60.8 1.908.89 6.26- 6.73 - 8.64–9.95 - Palygorskite-nZVI 71.68 0.274 0.002 0.8 8.89 - 6.73 8.64–9.95 Sorption (q, µmol/g) of uranium ions on pristine and modified samples are shown in Figure 3. On the kaolinite surface, sorption can occur on active centres of two types: ditrigonal siloxane wells on theSorption basal surfaces (q, µmol /ofg) kaolinite of uranium particles ions on and pristine hydroxyl and modifiedgroups on samples broken are bonds shown in intetrahedral Figure3. On(Si-O-Si) the kaolinite and octahedral surface, sorption(Al-O-Al) can sheets occur on on edge active surfaces centres of of the two particles types: ditrigonal [42]. siloxane wells on the basal surfaces of kaolinite particles and hydroxyl groups on broken bonds in tetrahedral (Si-O-Si) and octahedral (Al-O-Al) sheets on edge surfaces of the particles [42].

Metals 2020, 10, 1421 5 of 12 Metals 2020, 10, x FOR PEER REVIEW 5 of 13 Metals 2020, 10, x FOR PEER REVIEW 5 of 13

150 150 Kt-nZVI Kt-nZVI 100 100 nZVI

mol/g nZVI μ mol/g μ q, 50

q, 50 Kt Kt 0 0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Ceq, μmol/l μ Ceq, mol/l

Figure 3. Sorption isotherms of U(VI) ions on pristine kaolinite andand kaolinite-nZVI.kaolinite-nZVI. Figure 3. Sorption isotherms of U(VI) ions on pristine kaolinite and kaolinite-nZVI. However, forfor thethe pristinepristine GlukhovetsGlukhovets kaolinite, due to itsits perfectperfect structurestructure andand thethe absenceabsence ofof However, for the pristine Glukhovets kaolinite, due to its perfect structure and the absence of heterovalent isomorphicisomorphic substitutionssubstitutions in in the the mineral mineral structure, structure, the the charge charge of of the the structural structural packages packages is heterovalent isomorphic substitutions in the mineral structure, the charge of the structural packages closeis close to zeroto zero and, and, therefore, therefore, sorption sorption at low at concentrationslow concentrations of uranium of uranium ions in ions solution in solution occurs mainlyoccurs is close to zero and, therefore, sorption at low concentrations of uranium ions in solution occurs onmainly hydroxyl on hydroxyl groups locatedgroups onlocated edge on surfaces edge surfaces of the particles. of the particles. mainly on hydroxyl groups located on edge surfaces of the particles. Such sorption centres on thethe edgeedge surfaces,surfaces, depending on the pH of thethe medium,medium, cancan havehave thethe compositionSuch sorption>Si-OH centres and >Si-O on theon edge the brokensurfaces, tetrahedral depending sheet on ofthe the pH mineral of the andmedium,>Al-OH can +have; Al-OH the composition >Si-OH and >Si-O−− on the broken tetrahedral sheet of the mineral and >Al-OH22+; Al-OH composition >Si-OH and >Si-O− on the broken tetrahedral sheet of the mineral and >Al-OH2+; Al-OH and >>Al-OHAl-OH−− on aa brokenbroken octahedral octahedral sheet sheet of theof the mineral mine andral sorptionand sorption of uranium of uranium ions on ions the surfaceon the and >Al-OH− on a broken octahedral sheet of the mineral and sorption of uranium ions on the occurssurface with occurs the with formation the formation of strong of inner-sphere strong inner-sphere surface complexes.surface complexes. surface occurs with the formation of strong inner-sphere surface complexes. The resultsresults obtainedobtained forfor the the modified modified samples samples indicate indicate that that the the sorption sorption values values of of uranium uranium ions ions at The results obtained for the modified samples indicate that the sorption values of uranium ions pHat pH 6 are 6 are several several times times higher higher than than those those for for the the pristine pristine minerals minerals (Figure (Figure3 ).3). As As it it was was established, established, at pH 6 are several times higher than those for the pristine minerals (Figure 3). As it was established, synthesizedsynthesized nZVInZVI hashas aa core–shellcore–shell structure structure and and the the thin thin film film on on the the surface surface of of the the particles particles consists consists of ironsynthesized oxides such nZVI as has FeO, a core–shell Fe O and structure Fe O and and its hydroxidesthe thin film [14 on,17 the,43 surface,44]. Fe(0) of the is in particles the centre consists of the of iron oxides such as FeO,2 Fe32O3 and3 Fe43O4 and its hydroxides [14,17,43,44]. Fe(0) is in the centre of of iron oxides such as FeO, Fe2O3 and Fe3O4 and its hydroxides [14,17,43,44]. Fe(0) is in the centre of particles.the particles. As aAs result a result of such of such composite composite structure, structure, the removal the removal of uranium of uranium (VI) from (VI) waterfrom water by nZVI by the particles. As a result of such composite structure, the removal of uranium (VI) from water by isnZVI also is may also be may affected be affected by the redoxby the transformationredox transformation of a highly of a solublehighly soluble form of form U(VI) of to U(VI) less soluble to less nZVI is also may be affected by the redox transformation of a highly soluble form of U(VI) to less U(IV)withsoluble U(IV)with direct electron direct electron transfer transfer at the Fe(0) at the surface Fe(0) [surface16,17,45 [16,17,45]:]: soluble U(IV)with direct electron transfer at the Fe(0) surface [16,17,45]: 0 0 2+ 2+ ++ 33++ 4+4+ Fe +Fe1.5UO+1.5UO2 2++6H 6H = = FeFe ++ 1.51.5 U U + +3H3H2O2 O(1) (1) Fe0+1.5UO22++ 6H+ = Fe3+ + 1.5 U4++ 3H2O (1) In order to elucidate the synergetic effect for the Kt-nZVI composite sorbents, the removal of InIn orderorder toto elucidate elucidate the the synergetic synergetic eff ecteffect for fo ther the Kt-nZVI Kt-nZVI composite composite sorbents, sorbents, the removal the removal of U(VI) of U(VI) by nZVI alone was also investigated. As shown in Figure 3, the removal efficiencies of U(VI) byU(VI) nZVI by alone nZVI wasalone also was investigated. also investigated. As shown As inshow Figuren in3 ,Figure the removal 3, the eremovalfficiencies efficiencies of U(VI) byof U(VI) nZVI by nZVI alone wassignificantly lower than that of Kt-nZVI. aloneby nZVI wassignificantly alone wassignificantly lower than lower that than of Kt-nZVI. that of Kt-nZVI. The dependence of the sorption values of uranium ions on the surface of kaolinite particles The dependencedependence ofof the the sorption sorption values values of of uranium uranium ions ions on theon surfacethe surface of kaolinite of kaolinite particles particles from from pHis presented in Figure 4. The corresponding curves have a characteristic form with a marked pHisfrom pHis presented presented in Figure in Figure4. The 4. The corresponding corresponding curves curves have have a characteristic a characteristic form form with with a a marked marked minimum in the acidic and alkaline pH areas. minimum inin thethe acidicacidic andand alkalinealkaline pHpH areas.areas.

100 100 80 80 Kt-nZVI Kt-nZVI 60 60 mol/g μ mol/g 40 μ q, 40

q, nZVI 20 nZVI 20 Kt 0 Kt 0 13579 13579 pH pH

Figure 4. Sorption of U(VI) ions on pristine kaolinite and kaolinite-nZVI as a function of pH. Figure 4. Sorption of U(VI) ions on pristine kaolinitekaolinite and kaolinite-nZVI as a function ofof pH.pH.

Metals 2020, 10,, 1421x FOR PEER REVIEW 6 of 1213 Metals 2020, 10, x FOR PEER REVIEW 6 of 13 The last is due to the peculiarities of the chemistry of the clay surface and the complex The last is due to the peculiarities of the chemistry of the clay surface and the complex chemistryThe last of is aqueous due to the solutions peculiarities of ofuranium the chemistry salts [46]. of the In clay the surface acidic and pH the area, complex the chemistrysorption chemistry of aqueous solutions of uranium salts [46]. In the acidic pH area, the sorption ofcharacteristics aqueous solutions of clays of in uranium relation salts to uranyl [46]. In cations the acidic UO pH22+are area, determined the sorption by characteristicsthe neutral and of positive clays in characteristics of clays in relation2+ to uranyl cations UO22+are determined by the neutral and positive relationsurface charge to uranyl resulting cations from UO2 protonationare determined of surface by the groups neutral and and the positive formation surface of >> chargeSi-OH, resulting >Al-OH surface charge resulting from protonation of surface groups and the formation of >>Si-OH,+ >Al-OH fromand >Al-OH protonation2+ groups. of surface In the groupsalkaline and area, the dissociated formation >Si-O of >>−Si-OH, and >Al-O>Al-OH−groups and predominate>Al-OH2 groups. on the and >Al-OH2+ groups. In the alkaline area, dissociated >Si-O− and >Al-O−groups predominate on the Inclay the surface, alkaline and area, uranium dissociated in solution>Si-O exists− and mainly>Al-O− ingroups the form predominate of neutral onUO the2(OH) clay2, surface,UO2CO3 and clay surface, and uranium in solution exists mainly in the form of neutral UO2(OH)2, UO2CO3 and uraniumnegatively in charged solution species exists mainly (UO2) in2CO the3(OH) form3− of[7,47]. neutral This UO causes2(OH) insignificant2, UO2CO3 and values negatively of sorption charged of negatively charged species (UO2)2CO3(OH)3− [7,47]. This causes insignificant values of sorption of speciesuranium (UO compounds2)2CO3(OH) in3− [these7,47]. ThispH causesareas,because insignificant positively values ofcharged sorption uranium of uranium species compounds which uranium compounds in these pH areas,because positively charged uranium species which inpredominated these pH areas, in the because acidic positively pH areas chargedare not adsorbed uranium speciesonto the which positively predominated charged clay in the surface, acidic and pH predominated in the acidic pH areas are not adsorbed onto the positively charged clay surface, and areasnegatively are not charged adsorbed uranium onto thespecies positively in the alkaline charged pH clay areas surface, are andnot adsorbed negatively onto charged the negatively uranium negatively charged uranium species in the alkaline pH areas are not adsorbed onto the negatively speciescharged in clay the surface. alkaline In pH the areas neutral are notpH adsorbedarea, the charge onto the of negativelythe kaolinite charged surface clay and surface. the charge In the of charged clay surface. In the neutral pH area, the charge of the kaolinite surface and the charge of neutraluranium pH ions area, have the opposite charge of signs, the kaolinite which surfaceinfluenc andes the the proceedings charge of uranium of sorption ions have processes opposite and signs, the uranium ions have opposite signs, which influences the proceedings of sorption processes and the whichappearance influences of maximum the proceedings on the curves of sorption of the processes dependence and theof sorption appearance values of maximumfrom pH. on the curves appearance of maximum on the curves of the dependence of sorption values from pH. of theMontmorillonite dependence of sorptionhas a labile values structure from pH. with the possibility of significant expansion of the Montmorillonite has a labile structure with the possibility of significant expansion of the inter-packageMontmorillonite distance has during a labile the structurepenetration with of thepolar possibility water molecules of significant between expansion flat structural of the inter-package distance during the penetration of polar water molecules between flat structural inter-packagepackages. This distancedetermines during the availability the penetration for ion of ex polarchange water processes molecules not only between of the flatouter structural but also packages. This determines the availability for ion exchange processes not only of the outer but also packages.of the inner This surface determines of the particles the availability of this clay for mineral, ion exchange which processes determines not a onlysignificant of the increase outer but in also the of the inner surface of the particles of this clay mineral, which determines a significant increase in the ofsorption the inner of uranium surface of ions the compared particles of to this kaolinite clay mineral, (see Figure which 5). determines The nature a significantof the dependence increase inof sorption of uranium ions compared to kaolinite (see Figure 5). The nature of the dependence of thesorption sorption values of uraniumfrom pH ions for comparedmontmorillonite to kaolinite corresp (seeonds Figure to that5). Thefor naturekaolinite, of thehowever, dependence is less sorption values from pH for montmorillonite corresponds to that for kaolinite, however, is less ofpronounced sorption values (see Figure from 6). pH This for is montmorillonite due to the smaller corresponds relative contribution to that for of kaolinite, the edge however, surfaces of is lessthis pronounced (see Figure 6). This is due to the smaller relative contribution of the edge surfaces of this pronouncedmineral to its (see total Figure specific6). This surface is due area to the and smaller the increased relative contribution role of sorption of the centres edge surfaces on the of basal this mineral to its total specific surface area and the increased role of sorption centres on the basal mineralsurfaces toof itsparticles total specific (ditrigonal surface wells) area whose and the behavior increased is roleindependent of sorption of centrespH [42]. on the basal surfaces surfacesof particles of particles (ditrigonal (ditrigonal wells) whose wells) behavior whose behavior is independent is independent of pH [42 of]. pH [42].

350 350 300 300 MMT-nZVI 250 MMT-nZVI 250 200

mol/g 200

μ 150 mol/g μ q, 150

q, 100 100 MMT 50 MMT 50 0 0 0 50 100 150 200 0 50 100 150 200 Ceq, μmol/l C , μmol/l eq

Figure 5. Sorption isotherms of U(VI) ions on pristi pristinene montmorillonite and montmorillonite-nZVI. Figure 5. Sorption isotherms of U(VI) ions on pristine montmorillonite and montmorillonite-nZVI.

200 M 200 M 150 M MMT-nZVI 150 M MMT-nZVI

mol/g 100 μ

mol/g 100 μ q,

q, 50 50 MMT MMT 0 0 16 16 pH pH

Figure 6. Sorption of U(VI) ions on pristine montmor montmorilloniteillonite and montmorillonite-nZVI as a function Figure 6. Sorption of U(VI) ions on pristine montmorillonite and montmorillonite-nZVI as a function of pH. of pH.

Metals 2020,, 10,, 1421x FOR PEER REVIEW 7 of 1213 Metals 2020, 10, x FOR PEER REVIEW 7 of 13 Particles of chain silicate paligorskite have an elongated rod-like shape, the volume of which is Particles of chain silicate paligorskite have an elongated rod-like shape, the volume of which is piercedParticles by -like of chain channels silicate paligorskite [48]. Sorption have of anions elongated occurs on rod-like the outer shape, surface the of volume the mineral, of which the pierced by zeolite-like channels [48]. Sorption of ions occurs on the outer surface of the mineral, the isvalue pierced of which by zeolite-like is greater channels than that [48 of]. kaolinite Sorption ofand ions montmorillonite occurs on the outerestablished surface by of N the2 sorption. mineral, value of which is greater than that of kaolinite and montmorillonite established by N2 sorption. theHowever,montmorillonite value of which is greater particles than that show of kaolinite an outstanding and montmorillonite property to delaminate established into by N individual2 sorption. However,montmorillonite particles show an outstanding property to delaminate into individual However,silicate layers montmorillonite or thin packets particles of them show in solution an outstandings. So, the property values of to the delaminate specific intosurface individual area of silicate layers or thin packets of them in solutions. So, the values of the specific surface area of silicatedispersions layers of ormontmorillonite thin packets of samples them in in solutions. solutions So,are the much values higher of the than specific of air-dried surface samples area of dispersions of montmorillonite samples in solutions are much higher than of air-dried samples dispersions(maximum crystallographic of montmorillonite value samples of SSA in are solutions ~750–780 are m much2/g [41]). higher Therefore, than of air-driedthe values samples of the (maximum crystallographic value of SSA are ~750–7802 m2/g [41]). Therefore, the values of the (maximumsorption of crystallographicuranium on both value the of pristine SSA are and ~750–780 modified m / gminerals [41]). Therefore, are the theaverage values values of the between sorption sorption of uranium on both the pristine and modified minerals are the average values between ofthose uranium for kaolinite on both and the montmorillonite pristine and modified (see Figures minerals 7 and are 8). the average values between those for thosekaolinite for kaolinite and montmorillonite and montmorillonite (see Figures (see7 andFigures8). 7 and 8).

200 200 Pg-nZVI Pg-nZVI 150 150

mol/g 100 μ

mol/g 100 μ q,

q, 50 50 Pg Pg 0 0 0 50 100 150 200 0 50 100 150 200 С , μmol/l eq μ Сeq, mol/l

Figure 7. Sorption isotherms of U(VI) ions on pristine paligorskite and paligorskite-nZVI. FigureFigure 7. 7.SorptionSorption isotherms isotherms of ofU(VI) U(VI) ions ions on on pristine pristine paligorskite paligorskite and and paligorskite-nZVI. paligorskite-nZVI.

140 140 120 120 100 100 80 Pg-nZVI mol/g 80

μ Pg-nZVI mol/g 60 μ

q, 60

q, 40 40 Pg 20 Pg 20 0 0 246810 246810 рН рН

FigureFigure 8. 8. SorptionSorption of of U(VI) U(VI) ions ions on on pristine pristine paligorskite paligorskite andand paligorskite-nZVIpaligorskite-nZVI as as a a function function of of pH. pH. Figure 8. Sorption of U(VI) ions on pristine paligorskite and paligorskite-nZVI as a function of pH. When considering the the sorption sorption processes processes in in mineralized mineralized waters, waters, it is it important is important to analyze to analyze the When considering the sorption processes in mineralized waters, it is important to analyze the theforms forms in which in which uranium uranium exists exists under under these theseconditions. conditions. MedusaMedusa software,software, which is which widely is used widely in forms in which uranium exists under these conditions. Medusa software, which is widely used in usedanalytical in analytical practice, practice,was used was for usedcalculations for calculations [49]. The [ 49main]. The solid main phases solid of phases uranium of uraniumin aqueous in analytical practice, was used for calculations [49]. The main solid phases of uranium in aqueous aqueous systems are insoluble hydrates ⋅UO3 H2O or UO2(OH)2 (lgKsp= 20.34 – 23.5) [47,50] and systems are insoluble hydrates UO32⋅ H O· or UO22 (OH) (lgKsp= −−20.34 – −−23.5) [47,50]and systemscarbonate areUO insoluble2CO3 (lgK sphydrates= 13.21 UO – 14.26)32 H O [51 ].Ator UO the22 same (OH) time, (lg sulfate,Ksp= −20.34 carbonate, – −23.5) phosphate [47,50]and and UO CO sp− − nitratecarbonate complexes 23 of uranium(lgK = −13.21 may be– − the14.26) main [51].At species the insame water.The time, sulfate, composition carbonate, of main phosphate anions carbonate UO23 CO (lgKsp= −13.21 – −14.26) [51].At the same time, sulfate, carbonate, phosphate ofand underground nitrate complexes mineralized of uranium water may near be the the main liquid species waste in storage water.The facility composition for hydrometallurgical of main anions and nitrate complexes of uranium may be the main species in water.The composition of main anions of underground mineralized water near the liquid waste storage facility for hydrometallurgical processing uranium ores at the Centre of Ukrainian Uranium Industry (Zhovti Vody)are: HCO3−—450; of underground2 mineralized water near the liquid waste storage facility for hydrometallurgical− Clprocessing−—180; SO uranium−—2830; oresNO at− the—130 Centre mg/ L.of TheUkrainian high a ffiUraniumnity of uranylIndustry ions (Zhovti to the Vody)are: nitrate is known,HCO− processing uranium4 ores at 3the Centre of Ukrainian Uranium Industry (Zhovti Vody)are: HCO 3 but the content− of nitrate-ions2− in Zhovti− Vody mineralized water is much smaller thanthe content of3 —450; Cl− —180; SO24− —2830; NO−3 —130 mg/L. The high affinity of uranyl ions to the nitrate is —450;carbonate-ions Cl —180; and SO sulfate-ions.4 —2830; Therefore,NO3 —130 we mg/L. did not The include high nitrate-ionsaffinity of uranyl complexes ions into thethe speciationnitrate is known, but the content of nitrate-ions in Zhovti Vody mineralized water is much smaller thanthe known,calculations. but the Zhovti content Vody-mineralized of nitrate-ions in waters, Zhovti evenVody with mineralized a sufficiently water highis much content smaller of uranium thanthe content of carbonate-ions and sulfate-ions. Therefore, we did not include nitrate-ions complexes in contentin them, of are carbonate-ions characterized and by sulf almostate-ions. complete Therefore, binding we ofdid uranium not include to sulfate nitrate-ions complexes complexesUO2SO in4 the speciation calculations. Zhovti Vody-mineralized waters, even with a sufficiently high content of the speciation calculations. Zhovti Vody-mineralized waters, even with a sufficiently high content of

Metals 2020, 10, x FOR PEER REVIEW 8 of 13 Metals 2020, 10, x FOR PEER REVIEW 8 of 13 Metalsuranium2020 ,in10 ,them, 1421 are characterized by almost complete binding of uranium to sulfate complexes8 of 12 uranium in them, are characterized2− by almost complete binding of uranium to sulfate complexes UO24 SO and UO242 (SO )2− in the acidic area, as well as in carbonate complexes UO23 CO , UO24 SO and2− 2 UO242 (SO ) in4− the acidic area, as well as in carbonate complexes UO223 CO , and UO (SO ) − in the acidic area, as well as in carbonate complexes UO CO , UO (CO ) − and UO232 (CO2 )24− 2and UO233 (CO )4− in neutral and alkaline areas (see Figure 29). 3 2 3 2 UO (CO4 ) and UO (CO ) in neutral and alkaline areas (see Figure 9). UO2232(CO3)3− in neutral and233 alkaline areas (see Figure9).

Figure 9. U(VI) speciation in mineralized groundwater. Figure 9. U(VI) speciation in mineralized groundwater. Sorption processes in mineralized waters were studied using montmorillonite and paligorskite. Sorption processes in mineralized waters were studied using montmorillonite and paligorskite. The pHSorption dependence processes curves in mineralized of uranium waters sorption were values studied have using a characteristic montmorillonite form andfor both paligorskite. types of The pH dependence curves of uranium sorption values have a characteristic form for both types of Theclays pH with dependence a maximum curves in the of pH uranium area of sorption5–7 (Figure values 10). have a characteristic form for both types of clays with a maximum inin thethe pHpH areaarea ofof 5–75–7 (Figure(Figure 1010).).

MMT-nZVI 60 MMT-nZVI 60 50 Pg-nZVI 50 Pg-nZVI 40 40

mol/g 30 μ

mol/g 30 MMT q, μ 20 MMT q, 20 10 Pg 10 0 Pg 0 3456789 3456789pH pH

Figure 10. Sorption of U(VI) ions on pristine and nZVI-modified montmorillonite and palygorskite Figure 10. Sorption of U(VI) ions on pristine and nZVI-modified montmorillonite and palygorskite from mineralized waters as a function of pH. fromFigure mineralized 10. Sorption waters of U(VI) as a ionsfunction on pristine of pH. and nZVI-modified montmorillonite and palygorskite from mineralized waters as a function of pH. Sorption of uranium complexes occurs primarily due to the exchange of hydroxyl ions of the hydroxideSorption film of on uranium the surface complexes of nanoscale occurs iron primarily particles: due to the exchange of hydroxyl ions of the hydroxideSorption film of on uranium the surface complexes of nanoscale occurs iron primarily particles: due to the exchange of hydroxyl ions of the film on the surface of nanoscale iron2 particles: M-(OH)n + UO (SO ) −2− M-(OH)n UO (SO ) + 2OH− − (2) M-(OH)n + UO2 2(SO4 24)2 →→ M-(OH)n-2-2UO2(SO44)2 2+ 2OH (2) M-(OH)n + UO2(SO4)22−→ M-(OH)n-2UO2(SO4)2 + 2OH− (2) 2 2− − M-(OH)M-(OH)n +n UO+ UO2(CO2(CO3)23)2− → M-(OH)M-(OH)nn-2-2UO2(CO(CO33))2 2++ 2OH2OH − (3)(3) M-(OH)n + UO2(CO3)22−→→ M-(OH)n-2UO2(CO3)2 + 2OH− (3) When considering the mechanism of sorption of uranium compounds on the surface of clays, When considering the mechanism of sorption of uranium compounds on the surface of clays, especially ofof rather rather high high concentration concentration of uranium,of uraniu them, possibilitythe possibility of precipitation of precipitation of sparingly of sparingly soluble especially of rather high concentration of uranium, the possibility of precipitation of sparingly hydroxidessoluble hydroxides and others(hexavalent and others(hexavalent uranium uranium compounds compounds schoepite, schoepite, carnotite, carnotite, tyuyamunite, tyuyamunite, etc. [52]) soluble hydroxides and others(hexavalent uranium compounds schoepite, carnotite, tyuyamunite, cannotetc. [52]) be cannot ruled out.be ruled In mineralized out. In mineralized waters, in waters, contrast in to contrast diluted to waters, diluted the waters, solubility the solubility of insoluble of etc. [52]) cannot be ruled out. In mineralized waters, in contrast to diluted waters, the solubility of saltsinsoluble of uranium salts isof significantly uranium is increased. significantly Therefore, increased. at concentrations Therefore, of at salts concentrations corresponding of to thosesalts insoluble salts of uranium is significantly increased. Therefore, at concentrations of salts incorresponding mineralized watersto those from in mineralized 3–5 to 10–20 waters g/L[52 ],fr theom precipitation3–5 to 10–20 ofg/L uranium [52], the solid precipitation phases is not of corresponding to those in mineralized waters from 3–5 to 10–20 g/L [52], the precipitation of expected and the most probable mechanism of uranium binding is only the sorption mechanism.

Metals 2020, 10, x FOR PEER REVIEW 9 of 13

Metals 2020, 10, 1421 9 of 12 uranium solid phases is not expected and the most probable mechanism of uranium binding is only the sorption mechanism. SorptionSorption isothermsisotherms of of uranium uranium from from mineralized mineralize watersd waters were obtained were obtained at pH 7.2, at which pH corresponds7.2, which correspondsto the pH value to the of realpH groundwatervalue of real (Figuregroundwater 11). (Figure 11).

140 MMT-nZVI 120 100 80 Pg-nZVI 60 40 20 mol/g

μ 1.5 MMT q, 1.0 Pg 0.5 0.0 020406080100 C , mol/l eq μ FigureFigure 11. SorptionSorption isotherms isotherms of of U(VI) U(VI) ions ions on on pristine pristine and and nZVI-modified nZVI-modified montmorillonite and and palygorskitepalygorskite from from mineralized mineralized waters.

SorptionSorption isotherms isotherms were were analyzed analyzed using using Langmuir Langmuir and and Freundlich Freundlich equations. The The results results of of the the calculationscalculations of of the the corresponding corresponding co coeefficientsfficients are shown in Table2 2..

Table 2. Langmuir and Freundlich parameters for the adsorption of uranyl ions onto pristine and Table 2. Langmuir and Freundlich parameters for the adsorption of uranyl ions onto pristine and nZVI-modified minerals. nZVI-modified minerals. Sample MMT MMT-nZVI Pg Pg-nZVI Sample MMT MMT-nZVI Pg Pg-nZVI LangmuirLangmuir 1 a∞a, μ, molµmol g g−1− 125125 452 452 59 59 325325 ∞ 1 K ,L µmol−1 0.0084 0.1093 0.0096 0.1221 KL, LL μmol − 0.0084 0.1093 0.0096 0.1221 R2 0.956 0.964 0.975 0.938 R2 0.956 0.964 0.975 0.938 FreundlichFreundlich KKF F 5.35.3 92.0 92.0 3.23.2 116.5116.5 1/n1/ n 1.971.97 3.28 3.28 2.132.13 3.353.35 R2 0.887 0.825 0.850 0.892 R2 0.887 0.825 0.850 0.892

TheThe obtainedobtained isothermsisotherms are are well well described described by thebyequation the equation of monomolecular of monomolecular Langmuir Langmuir sorption 2 sorption(correlation (correlation coefficient coefficient R = 0.968–0.996) R2 = 0.968 which – 0.996) assumes which the assumes energy homogeneity the energy homogeneity of the active centresof the activeaccordingly. centres For accordingly. the empirical For Freundlich the empirical equation, Freundlich which is suitableequation, mainly which for is describing suitable mainly the starting for 2 describingareas of isotherms, the starting the areas correlation of isotherms, coefficient the iscorrelation lower (R coefficient= 0.934–0.991). is lower (R2 = 0.934 – 0.991).

4.4. Conclusions Conclusions Thus,Thus, different different in in structure structure clay clayss are are effective effective cheap matrices for for obtaining efficient efficient sorption sorption materialsmaterials basedbased on on zero zero valence valence iron foriron purification for purification of uranium-contaminated of uranium-contaminated surface and surface mineralized and mineralizedgroundwater, groundwater, which is typical which for is areas typical of uraniumfor areas of ore uranium mining andore mining processing.In and processing.In terms of sorption terms ofcapacity, sorption the compositecapacity, the samples composite form asample sequence:s form montmorillonite-nZVI a sequence: montmorillonite-nZVI> paligorskite-nZVI >> paligorskite-nZVIkaolinite-nZVI, which > kaolinite-nZVI, corresponds to which a decrease corresponds in the specific to a decrease surface area in the of the specific pristine surface clay minerals. area of the pristineRemoval clay of minerals. U(VI) from mineralized waters occurs primarily due to the exchange of hydroxyl ionsRemoval of the hydroxide of U(VI) from film mineralized on the surface waters of nanoscaleoccurs primarily iron particles due to the by exchange uranium of sulfate hydroxyl and ionscarbonate of the complexes. hydroxide film on the surface of nanoscale iron particles by uranium sulfate and carbonateAnother complexes. possible mechanism of immobilization of uranium compounds is the reduction of uranium (VI) to uranium (IV) by electron transfer from the volume of nZVI particles through a

Metals 2020, 10, 1421 10 of 12 hydroxide film to their surface with the formation and deposition of much less soluble compounds of the latter.However, it is necessary to specify the main reactions in the removal of uranium by immobilized ZVI for its further practical applications.

Author Contributions: Conceptualization, B.K.; methodology, I.K. and V.T.; validation, B.K. and I.K.; formal analysis, I.K.; investigation, I.K., V.T.; writing-original draft preparation, B.K., S.U.; writing—review & editing, B.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: Author is grateful to the staff of Institute for Sorption and Problems of Endoecology and Igor Sikorsky Kyiv Polytechnic Institute for the help and support. Conflicts of Interest: The authors declare no conflict of interest.

References

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