The Effect of Various Hydroxide and Salt Additives on the Reduction of Fluoride Ion Mobility in Industrial Waste

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Article The Effect of Various Hydroxide and Salt Additives on the Reduction of Fluoride Ion Mobility in Industrial Waste

Tadas Dambrauskas 1,* , Kestutis Baltakys 1, Agne Grineviciene 1 and Valdas Rudelis 2

1 Department of Silicate Technology, Kaunas University of Technology, LT-50270 Kaunas, Lithuania; [email protected] (K.B.); [email protected] (A.G.) 2 JSC “Lifosa”, LT-57502 Kedainiai, Lithuania; [email protected] * Correspondence: [email protected]

Abstract: In this work, the inﬂuence of various hydroxide and salt additives on the removal of F−

ions from silica gel waste, which is obtained during the production of AlF3, was examined. The leaching of the mentioned ions from silica gel waste to the liquid medium was achieved by the application of different techniques: (1) leaching under static conditions; (2) leaching under dynamic conditions by the use of continuous liquid medium flow; and (3) leaching in cycles under dynamic conditions. It was determined that the efficiency of the fluoride removal from this waste depends on the w/s ratio, the leaching conditions, and the additives used. It was proven that it is possible to reduce the concentration of fluorine ions from 10% to <5% by changing the treatment conditions and by adding alkaline compounds. The silica gel obtained after the leaching is a promising silicon dioxide source.

Keywords: ﬂuorine ions; silica gel waste; leaching; hydroxide additives

Citation: Dambrauskas, T.; Baltakys, K.; Grineviciene, A.; Rudelis, V. The 1. Introduction Effect of Various Hydroxide and Salt Waste management and the reduction of pollution are the priority areas of environ- Additives on the Reduction of mental protection in the World [1–4]. According to the data of Eurostat [5], the European Fluoride Ion Mobility in Industrial Union generates approximately 180 million tons of manufacturing waste every year. From Waste. Sustainability 2021, 13, 1554. an economic point of view, it is preferable to store waste in landﬁll sites, but they occupy https://doi.org/10.3390/su13031554 large areas, and are concentrated sources of air, groundwater and soil pollution [6–10].

Academic Editor: Dimitrios Komilis Therefore, the search for attractive industrial waste recycling treatments, which will allow Received: 15 January 2021 us to develop products of high value, has become very important. Accepted: 29 January 2021 The global production of aluminum ﬂuoride (AlF3) reaches 1 million tons per year, Published: 2 February 2021 while the amount of generated waste is more than 3 million tons [11–14]. It is worth noting that more than 60% of this waste is contaminated with ﬂuorine ions, the concentrations of

Publisher’s Note: MDPI stays neutral which can reach up to 10%. According to the literature, more than 20% of the aluminum with regard to jurisdictional claims in fluoride in the world is produced by neutralizing hexafluorosilicic acid with aluminum published maps and institutional affil- hydroxide [11,13,15]: iations. H2SiF6 + 2Al(OH)3 → 2AlF3 + SiO2·nH2O + H2O + Q (1)

However, during the acid neutralization, about 0.67 tons of silica gel contaminated with fluoride ions are generated and stored in the landfill sites [11,13,15–17]. Furthermore, Copyright: © 2021 by the authors. many manufacturers use an amount of hexafluorosilicic acid (H2SiF6) in excess of the Licensee MDPI, Basel, Switzerland. amount that the added aluminum hydroxide would fully react, and the yield of aluminum This article is an open access article fluoride would increase [11–18]. However, very low pH values of the obtained system distributed under the terms and are produced, leading to the strong chemisorption properties of silica gel for fluoride conditions of the Creative Commons Attribution (CC BY) license (https:// ions [17,19–21]. For this reason, the purification of this material has become a great creativecommons.org/licenses/by/ challenge [19]. Few attempts have been made to reduce the amount of fluoride ions in 4.0/). silica gel waste by leaching [22,23]. None of the experiments were successful enough.

Sustainability 2021, 13, 1554. https://doi.org/10.3390/su13031554 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 1554 2 of 17

For this reason, the scientific literature is mostly focused on the search for alternative routes, using silica gel waste as a raw material for the synthesis of commercial value products, or as an additive in binding materials [17,20–22,24,25]. Vaiciukyniene et al. [17,26] investigated the possibility of the utilization of silica gel waste in cement stone. The authors found that the compressive strength of the standard concrete samples decreased by increasing the amount of silica gel waste (up to 15%). Furthermore, these authors determined that the addition of 10% silica gel waste calcined at 800 ◦C (1 h) had a positive effect on the compressive strength values of the samples, which increased by ~5–7 MPa. However, these studies did not provide any data on the stability of the fluoride ions in cement stone or during calcination. In other research, silica gel waste was used for the production of alkali-activated blends [27]. It was found that the compressive strength values of hardened samples of silica gel and clay, which were additionally treated with sodium hydroxide and calcined at 600 ◦C (3 h), were equal to 6–17.50 MPa. Krivenko et al. [28] investigated the influence of sodium silicate (Na2SiO4) activated silica gel waste on the hydration characteristics of cement samples. It was found that the additive changes the mechanism of early hydration: (1) increase the amount of heat released during the main hydration reaction; (2) prolong the duration of the induction period; and (3) promote the crystallization of both crystalline and amorphous calcium silicates hydrates and calcium aluminate hydrates. The discussed studies [17,26–28] did not determine the durability of the samples or the stability of the fluorine ions. R. Kaminskas et al. [21,25,29,30] found that, after the calcination of a mixture containing 20% clay and 80% silica gel waste at 600 ◦C, its pozzolanic activity increased two times, equal to 230–260 mg CaO/g. Further studies have shown that, in order to bind fluorine ions to the stable compounds, 10–20% limestone must be used in the production of the pozzolanic additive. It was also found that by replacing a part of the cement with a synthetic additive (5–15 wt%) in the concrete products, the compressive strength values of the samples increased to 5%. Another potential application of silica gel waste is the synthesis of calcium silicates hydrates [21,31]. However, the obtained products contain fluoride ions which limit their application. Furthermore, silica gel waste can be used for the synthesis of NaA zeolite [20]. The authors found that this zeolite can be synthesized by the sonication of suspensions of silica gel, aluminum hydroxide, sodium hydroxide, and water. Finally, Sarkar et al. [32] found that the use of silica gel with impurities in the production of white porcelain can reduce the burning temperature of products without compromising their performance. However, the applicability of the mentioned products is limited due to the presence of fluorine ions in the structure of the final product. Therefore, another method could be to leach it from silica gel waste, which later on could be used as an adsorbent or silica source. In the previous work [19], it was determined that it is possible to reduce the quantity of F− ions by more than two times by applying different leaching techniques. However, due to the strong adsorption properties of silica gel waste, the amount of water required for the leaching process is very high (w/s = 100). It is known that, by increasing the pH values of system up to >4, the framework of silica gel is destroyed and it loses its adsorption properties. In order to increase the mentioned value, silica gel waste should be treated using alkaline (various synthetic or natural) additives, which could improve the physicochemical properties of amorphous silicon dioxide (SiO2·nH2O). On the other hand, by using additives at the same time, the formation of other by-products could be avoided. For this reason, the aim of this work is to determine the influence of various hydroxide and salt additives on the removal of F− ions from the silica gel waste obtained from the production of AlF3. Sustainability 2021, 13, x FOR PEER REVIEW 3 of 16

2. Materials and Methods Sustainability 2021, 13, 1554 2.1. Materials 3 of 17

Silica gel waste (SGW)—i.e., a waste product of the production of AlF3 in the chemical plant of JSC ‘Lifosa’ (Kėdainiai, Lithuania)—was dried for 48 hours at 50 °C. The XRF 2. Materials and Methods analysis2.1. Materials data showed that SGW consists of 36.2% silica (which is equivalent to 78.9% SiO2), 3+ 2.5%Silica Al gel ions waste, (SGW)—i.e.,and traces a waste of other product elements. of the production Meanwhile, of AlF3 in the the chemical chemical analysis data showedplant of JSC that ‘Lifosa’ SGW (Kedainiai,˙ contains Lithuania)—was 10.0% F− ions. dried These for 48 h results at 50 ◦C. Thewere XRF in analysis good agreement with the data showed that SGW consists of 36.2% silica (which is equivalent to 78.9% SiO2), 2.5% data of the XRD analysis: aluminum fluoride trihydrate (AlF3·3H2O) and amorphous sili- Al3+ ions, and traces of other elements. Meanwhile, the chemical analysis data showed that conSGW dioxide, contains 10.0% which F− ions.corresponds These results to were a broad in good basal agreement reflection with the in data the of 18 the–37º diffraction angle range,XRD analysis: were aluminumobserved ﬂuoride in the trihydrate XRD pattern (AlF3·3H of2O) SGW and amorphous (Figure 1). silicon The dioxide, results of the particle size distributionwhich corresponds analysis to a broad showed basal reﬂection that the in the diameter 18–37º diffraction of the angleSGW range, particles were varied within the observed in the XRD pattern of SGW (Figure1). The results of the particle size distribution 0.analysis03–170 showed μm range, that the while diameter particles of the SGW with particles a size varied of within34–72 the μm 0.03–170 wereµ dominant.m It was also determinedrange, while particles that withthe asurface size of 34–72 areaµm and were the dominant. density It was of also the determined silica gel that waste were equal to 2 281.01the surface m2/kg area and and the2141 density kg/m of3 the, respectively. silica gel waste The were detailed equal to 281.01description m /kg and of the SGW’s chemical 2141 kg/m3, respectively. The detailed description of the SGW’s chemical and physical andproperties physical are available properties in reference are available [19]. in reference [19].

40 A 35 A 30 25 A A 20 A 15 A AA A A A Intensity, cps. Intensity, 10 A 5 0 3 13 23 33 43 53 2q, deg. Figure 1. XRD patterns of the SGW. Indexes: A—AlF ·3H O. Figure 1. XRD patterns of the SGW. Indexes:3 2A—AlF3∙3H2O; g—Al(OH)3.

Chemicals of the following characteristics were used: aluminum hydroxide (Al(OH)3, ‘Honeywell’,Chemicals Düsseldorf, of the Germany) following purity characteristics > 99%; calcium hydroxide were used: (Ca(OH) aluminum2, ‘Stanchem hydroxide (Al(OH)3, Sp. J.’, Niemce, Poland), purity > 97%; potassium hydroxide (KOH, ‘Reachem’, Petrzalka, ‘Honeywell’, Germany) purity > 99%; calcium hydroxide (Ca(OH)2, ‘Stanchem Sp. J.’, Po- Slovakia), purity > 99%; sodium hydroxide (NaOH, ‘Reachem’, Petrzalka, Slovakia), pu- land)rity > 99%;, purity potassium >97 % chloride; potassium (KCl, ‘Honeywell’, hydroxide Düsseldorf, (KOH, Germany),‘Reachem purity’, Slovakia) > 99%; , purity >99%; so- diumammonium hydroxide hydroxide (NaOH, (NH4OH, ‘Lachema’,‘Reachem Reckovice,’, Slovakia) Czech, Republic,purity and >99% JSC; ‘Lifosa’).potassium chloride (KCl,

‘2.2.Honeywell Methodology’, Germany), purity > 99%; ammonium hydroxide (NH4OH, ‘Lachema’, Czech RepublicIn this, work,and theJSC following ‘Lifosa leaching’). experiments were performed: (1) The leaching of F− ions under static conditions: silica gel waste was mixed with 2.2.the requiredMethodology amount of additive and liquid medium, and kept for 24 h at 25 ◦C. After the leaching, the suspensions were filtered off, and the products were dried at 50 ± 5 ◦C for 24 h. In this work, the following leaching experiments were performed: (2)1) TheThe leaching leaching of F of− ions F− ions by the under use of continuousstatic conditions liquid medium: silica (by gel applying waste was mixed with the vacuum 0.6–0.7 bar) flow, which was applied to silica gel waste. After the process, the requiredproducts were amount dried at of 50 ±additive5 ◦C for 24 and h. liquid medium, and kept for 24 h at 25 °C. After the leaching,(3) The the leaching suspensions of F− ions inwere cycles filtered by the use off, of and continuous the products liquid medium were flow.dried at 50 ± 5 °C for 24 h.In total, 10 g SGW was treated with 50 mL liquid medium in each step until the w/s 2) The leaching of F− ions by the use of continuous liquid medium (by applying vacuum 0.6–0.7 bar) flow, which was applied to silica gel waste. After the process, the products were dried at 50 ± 5 °C for 24 h. 3) The leaching of F− ions in cycles by the use of continuous liquid medium flow. In total, 10 g SGW was treated with 50 ml liquid medium in each step until the w/s ratio reached 10–100. After each cycle, the obtained products were filtered off and dried (50 ± 5 °C; 24 h) at the end of the process. The X-ray diffraction analysis (XRD) was performed on a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) operating at U = 40 kV and I = 40 mA. The X-ray beam Sustainability 2021, 13, 1554 4 of 17

ratio reached 10–100. After each cycle, the obtained products were filtered off and dried (50 ± 5 ◦C; 24 h) at the end of the process. The X-ray diffraction analysis (XRD) was performed on a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) operating at U = 40 kV and I = 40 mA. The X-ray beam was filtered with Ni 0.02 mm filter in order to select the CuKα wavelength. The diffraction patterns were recorded in a Bragg–Brentano geometry by the use of a fast counting detector Bruker LynxEye. The samples were scanned over the range 2θ = 3–70◦ at a scanning speed of 6◦ min−1. The quantitative chemical composition analysis of the samples was performed using an X-ray fluorescence spectroscopy (XRF) on a Bruker X-ray S8 Tiger WD spectrometer equipped with an Rh tube with an energy of up to 60 keV. The powder samples were measured in the Helium atmosphere, and the data were analyzed with SPECTRAPlus QUANT EXPRESS standardless software. For the determination of the fluoride in the solid, a 1 g sample was placed on a platinum plate and mixed with 10 g NaOH and KOH mixture (5:7). After that, the platinum plate was placed on the sand bath and heated until the mixture was melted. During the heating, the mixture was vigorously stirred with a platinum spatula before being left to cool down. Afterward, 150 cm3 distilled water was poured into the platinum plate and heated on the sand bath until the formed salts were melted. Then, in order to bind the fluoride ions, 15 g chemically pure (NH4)2CO3 powder was added into the solution and evaporated until a dry salt was formed. After that, the residue was mixed with 150 cm3 distilled water and heated. A hot solution with precipitates was poured into a 250 cm3 flask and cooled down. Then, it was diluted with water to the indicated level and filtered off. Before the measurements were taken, TISAB II was added to each sample (volume ratio 1:1). The concentrations of the fluoride ions in the solution were measured by the use of a Metler Toledo T70 potentiometer. The error of the selective electrode for F− ions is <1 ppm (0.0001%). The concentration of the F− ions was calculated as the arithmetic mean of the 3 individual results. The measurements’ deviations were below 3%.

3. Results and Discussion 3.1. The Effect of Aluminium Hydroxide on the Mobility of F− Ions in the Silica Gel Waste Samples It is known that aluminum hydroxide has amphoteric properties, i.e., it is able to donate or accept protons and react with both acids and bases. Furthermore, Al(OH)3 dissolves in water, and—under certain conditions—produces H+ or OH− ions:

+ 3− 3+ − 3H + AlO3 ↔ Al + 3OH (2)

As the pH of the silica gel waste is strongly acidic, different amounts of aluminum hydroxide, which were equivalent to 2.5, 5, 7.5 and 10% Al3+ ions from the mass of the dry materials, were added to the SGW samples and mixed in a homogenizer (Turbula type T2F) for 1 h at 30 rpm. Later, the samples were treated with water under static and dynamic conditions. It was determined that, after 24 h of the treatment of the SGW with Al(OH)3 additive (2.5–10%) under static conditions, the intensity of the diffraction peaks characteristic to AlF3·3H2O only slightly decreased, when the water-to-solid ratio was equal to 6 (Figure2). Meanwhile, the intensity of the main diffraction maximum of Al(OH) 3 (d-spacing: 0.547 nm) increased from 0 (without additive) to 185 cps (when 10% additive was used). Meanwhile, the increment of the water-to-solid ratio to 100 had a positive 3+ effect on the decomposition of the AlF3·3H2O in the mixtures with 2.5% Al ions, because the intensities of the diffraction maximums typical to the latter compound decreased more than ﬁve times (Figure2). It should be noted that, when the Al(OH) 3 additive is not used and the SGW is treated with water (w/s = 100, 24 h), the intensity of the diffraction peaks characteristic of AlF3·3H2O decreased only three times [19]. Sustainability 2021, 13, x FOR PEER REVIEW 5 of 16 Sustainability 2021, 13, 1554 5 of 17

w/s = 100 g g Al3+ = 2.5% A g g g g w/s = 6 g A g A A g g g gg ggg A g g g w/s = 6 Intenisty, cps. Intenisty, Al3+ = 2.5% A A A A g g A A A

3 13 23 33 43 53 2θ, deg. FigureFigure 2.2.XRD XRD patterns patterns of theof the products products obtained obtained after the after treatment the treatment of SGW withof SGW Al3+ withions underAl3+ ions under staticstatic conditions.conditions. Indexes: Indexes:A—AlF A—3AlF·3H32∙3HO; g2—Al(OH)O; g—Al(OH)3. 3.

Moreover, it was found that the pH values of the liquid medium separated from the SGWMoreover, samples after it thewas treatment found that with the water pH (w/s values = 6) of depended the liquid on medium the amount separated of Al3+ from the 3+ SGWions used samples (Table after1). When the treatment 2.5% of the with mentioned water ions (w/s were = 6) added depend to theed system,on the amount the of Al ionspH of used the liquid (Table medium 1). When reached 2.5% 2.54; of meanwhile,the mentioned at higher ions concentrations were added ofto Althe3+ system,ions the pH of(≥ 5%),the liquid this parameter medium was reached increased 2.54 to 4.41–4.73; meanwhile, (Table 1at). Ithigher should concentrations be noted that, in theof Al3+ ions (≥ same treatment conditions, the pH value of the liquid medium was ~2.4 times higher in 5%), this parameter was increased to 4.41–4.73 (Table 1). It should be noted− that, in the samecomparison treatment with theconditions samples, without the pH aluminum value of ionsthe [liquid19]. However, medium the was amount ~2.4 of times F higher in ions released to the liquid medium was insigniﬁcant (Table1). comparison with the samples without aluminum ions [19]. However, the amount of F− Table 1. The amount of F−ionsions andreleased the pH to values the ofliquid the liquid mediu mediumm was after insignificant the treatment of (Table the SGW 1). with Al3+ ions under static conditions. Table 1. The amount of F− ions and the pH values of the liquid medium after the treatment of the The Amount of Al3+ The Concentration of F− The Amount of The pH Value of the w/sSGW Ratio with Al3+ ions under static conditions. Additive, % Ions in the Sample, % Released F− Ions, % Liquid Medium 2.5The 6 Amount of -The Concentration - The Amount 2.54of The pH Value of 5 63+ - - 4.41 7.5Al 6 Additive, w/s -ratio of F− Ions in the - Released F− Ions 4.60, the Liquid Me- 10 6% 9.51Sample, % 5.18 % 4.73 dium 2.5 1002.5 4.476 - 55.43- 3.06 2.54 5 6 - - 4.41 3+ Furthermore,7.5 when the6 w/s ratio was increased- to 100 and 2.5%- Al ions were used,4.60 the pH value of the liquid medium reached 3.06 (Table1). In this case, more than 55% of the initial10 amount of F− ions6 were released9.51 from the structure of the5.18 sample into the liquid4.73 medium2.5 (Table 1). It should100 be noted that this4.47 is ~6% more than in55.43 the samples without an3.06 additive [19]. Furthermore, when the w/s ratio was increased to 100 and 2.5% Al3+ ions were used, Since the amount of Al(OH)3 additive does not affect the stability of AlF3·3H2O, the theleaching pH value in cycles of the under liquid dynamic medium conditions reached (10 g3.06 SGW (Table was treated 1). In withthis case, 50 mL more liquid than 55% of themedium) initial was amount achieved of F− by ions the usewere of released Al(OH)3, from the quantity the structure of which of corresponded the sample into to the liquid 3+ medium2.5% Al ions.(Table It was1). It determined should be that noted the intensities that this of is the ~6% diffraction more than peaks in characteristic the samples without · − anto AlFadditive3 3H2O [19]. only slightly decreased, and the concentration of F ions decreased only to ~9% when the w/s ratio was equal to 20 (Figure3, Table2). By increasing the w/s ratio Since the amount of Al(OH)3 additive does not affect the stability of AlF3∙3H2O, the to 100, it was observed that a small amount of AlF3·3H2O was still present in the sample, leaching in cycles under dynamic conditions (10 g SGW was treated with 50 ml liquid medium) was achieved by the use of Al(OH)3, the quantity of which corresponded to 2.5 % Al3+ ions. It was determined that the intensities of the diffraction peaks characteristic to AlF3∙3H2O only slightly decreased, and the concentration of F− ions decreased only to ~9% when the w/s ratio was equal to 20 (Figure 3, Table 2). By increasing the w/s ratio to 100, it was observed that a small amount of AlF3∙3H2O was still present in the sample, because Sustainability 2021, 13, x FOR PEER REVIEW 6 of 16

SustainabilitySustainability 20212021, 13, 13, x, 1554FOR PEER REVIEW 6 of 17 6 of 16 only low intensity diffraction peaks were identified in the XRD patterns (Figure 3). More- over, the quantity of the fluorine ions released into the liquid medium was estimated to be ~2.5% higher (Table 2) than in the samples treated under static conditions (Table 1). onlybecause low only intensity low intensity diffraction diffraction peaks peaks were were identified identiﬁed in inthe the XRD XRD patterns patterns (Figure (Figure3 3).). More- Moreover, the quantity of the ﬂuorine ions released into the liquid medium was estimated over, the quantityg of the fluorine ions released into the liquid medium was estimated to to bew/s ~2.5% = 100 higher (Table2) than in the samples treated under static conditions (Table1). be ~2.5% higher (Tableg 2) than in the samples treated under static conditions (Table 1).

. A Ag w/s = 100 A A g cps g A

, , A Ag . A w/s = 20 A g A A g

cps A A , ,

Intensity A g A w/s = 20 A A g A Ag A A g A A

Intensity A A A A A g g A 3 13 23 A33 43A A 53 2θ, deg.

Figure3 3. XRD13 patterns23 of the products33 obtained43 after53 the treatment of SGW with Al3+ ions in cycles under dynamic conditions.2θ, deg. Indexes: A—AlF3∙3H2O; g— Al(OH)3. Figure 3. XRD patterns of the products obtained after the treatment of SGW with Al3+ ions in cycles FigureTable 2.3. TheXRD concentration patterns of the of products F− ions in obtained the SGW after with the 2.5 treatment % Al3+ ions of afterSGW treatment with Al3+ ionsin cycles in cy- under dynamic conditions. Indexes: A—AlF3·3H2O; g—Al(OH)3. clesunder un derdynamic dynamic conditions conditions.. Indexes: A—AlF3∙3H2O; g—Al(OH)3. Table 2. The concentration ofNumber F− ions in of the Cy- SGW with 2.5% Al3+ ionsThe after Concentration treatment in cycles of underF− dynamicThe Amount conditions. of Released F− Table 2. The concentrationw/s ratio of F− ions in the SGW with 2.5 % Al3+ ions after treatment in cycles Number of Cycles w/s RatiounderThe dynamiccles Concentration conditions of. F − Ions in theIons Sample, in the % Sample The Amount, % of Released FIons− Ions,, % % 4 204 20 9.06 9.06 9.67 9.67 Number of Cy- The Concentration of F− The Amount of Released F− 20 10020 w/s100 ratio 4.22 4.22 57.93 57.93 cles Ions in the Sample, % Ions, % The4 results of the20 liquid medium analysis9.06 showed that the duration9.67 of one cycle (filtration/interaction)The results of the was liquid equal medium to ~40 analysisseconds showed; thus, the that total the interaction duration of time one cycle was 13 min, (ﬁltration/interaction)20 100 was equal to ~40 s; thus,4.22 the total interaction time was57.93 13 min, which is significantly shorter in comparison to the static conditions (24 h). Moreover, the which is signiﬁcantly shorter in comparison to the static conditions (24 h). Moreover, the The results of the liquid medium analysis showed that the duration of one cycle (fil- pH valuevalue of of the the liquid liquid medium medium reached reached ~3 when ~3 when the w/s the ratiow/s wasratio equal was toequal 10 (Figure to 10 4(Figure). 4). tration/interaction)TheThe furtherfurther increment increment was in in the equal the w/s w/s to ratio ~40 ratio (30–100) seconds (30–100) resulted; thus resulted, inthe a total pH in valuea interactionpH ofval 4.2.ue of time 4.2. was 13 min, which is significantly shorter in comparison to the static conditions (24 h). Moreover, the pH value5 of the liquid medium reached ~3 when the w/s ratio was equal to 10 (Figure 4). The further increment in the w/s ratio (30–100) resulted in a pH value of 4.2. 4.5 54 4.53.5 43

3.52.5 Value of pH pH of Value 32 2.51.5 Value of pH pH of Value 1 5 9 13 17 21 2 Number of cycles . 1.5 Figure 4.4.The The1 change change5 in in the the pH 9pH values values of13 the of liquidthe liquid17 medium medium21 during during the treatment the treatment in cycles. in cycles. Number of cycles . Figure 4. The change in the pH values of the liquid medium during the treatment in cycles. Sustainability 2021, 13, x FOR PEER REVIEW 7 of 16

In a previous work, it was determined that, by the use of treatment with water at 25 °C , the F− ion concentration in silica gel waste can be reduced by up to 49% [19]. Mean- Sustainability 2021, 13, 1554 while, by adding 2.5% Al3+ ions to SGW, the F− ion concentration can be7 of reduced 17 by up to 58% (Table 1 and Table 2). Thus, it can be stated that the Al(OH)3 additive positively affected the removal of F− ions from the structure of the silica gel in the liquid medium. ◦ InIn a previousthe next work, part it of was this determined work, it that, was by decided the use of treatmentto investigate with water the at influence 25 C, of another the F− ion concentration in silica gel waste can be reduced by up to 49% [19]. Meanwhile, hydroxideby adding 2.5% (calcium Al3+ ions hydroxide) to SGW, the on F− ionthe concentrationmentioned canprocess. be reduced by up to 58% (Tables1 and2). Thus, it can be stated that the Al(OH) 3 additive positively affected the 3.2.removal The ofEffect F− ions of C fromalcium the structureHydroxide of theon silicathe M gelobility in the of liquid F− ions medium. in Silica Gel Waste Samples In the next part of this work, it was decided to investigate the inﬂuence of another According to the literature [33,34], the application of Ca(OH)2 can not only induce hydroxide (calcium hydroxide) on the mentioned process. the release of F− ions, but can also bind these ions into a stable calcium fluoride compound − (3.2.CaF The2). Effect of Calcium Hydroxide on the Mobility of F Ions in Silica Gel Waste Samples

AccordingThe treatment to the literatureof contaminated [33,34], the silica application gel with of Ca(OH) a Ca(OH)2 can2 notadditive only induce was first performed − underthe release static of Fconditions.ions, but can In alsoorder bind to thesedecrease ions intothe aamount stable calcium of F− ions, fluoride 6.5 com- and 20% Ca(OH)2 poundwere added (CaF2). to the SGW sample, along with a w/s ratio equal to 10 and a treatment tem- The treatment of contaminated silica gel with a Ca(OH)2 additive was first performed perature of 25 °C. It is worth noting that the maximum− amount of the additive (20%) was under static conditions. In order to decrease the amount of F ions, 6.5 and 20% Ca(OH)2 weresufficient added enough to the SGW to combine sample, along all of with the a fluorine w/s ratio ions equal into to 10 CaF and2. aIt treatment was determined that, ◦ aftertemperature the treatment of 25 C. It of is the worth SGW noting samples that the with maximum 6.5% amount Ca(OH) of the2, the additive AlF3∙3H (20%)2O was fully de- was sufficient enough to combine all of the fluorine ions into CaF . It was determined composed, and the F− ions were bound into CaF2 (Figure2 5). Furthermore, aluminum flu- that, after the treatment of the SGW samples with 6.5% Ca(OH)2, the AlF3·3H2O was fully oride hydroxide hydrate− (AlF1.5(OH)1.5∙0.375H2O; d-spacing: 0.568, 0.296, 0.284 nm), a decomposed, and the F ions were bound into CaF2 (Figure5). Furthermore, aluminum 3 2 productfluoride hydroxide of AlF ∙3H hydrateO decomposition, (AlF1.5(OH)1.5·0.375H was2O; identifiedd-spacing: in 0.568, the 0.296, XRD 0.284 patterns. nm), a Moreover, the productincrement of AlF in3 ·the3H2 quantityO decomposition, of the additive was identified to 20% in the did XRD not patterns. affect the Moreover, mineral the composition of theincrement formed in the compounds quantity of the(Figure additive 5). to The 20% results did not affectof chemical the mineral analysis composition showed of that, in both casesthe formed, F− ions compounds were not (Figure released5). The resultsinto the of chemicalliquid medium analysis showed. Furthermore, that, in both it was found that cases, F− ions were not released into the liquid medium. Furthermore, it was found that the amountamount of of the the additive additive determined determin the pHed ofthe the pH liquid of the medium, liquid the medium, values of whichthe values of which

were equal equal to to 7.1 7.1 (by (by theuse the of use 6.5% of Ca(OH) 6.5% Ca(OH)2) and 11.72) (byand the 11.7 use (by of 20% the Ca(OH) use of2 ).20% Ca(OH)2).

. ity,cps s x n k Inten n

3 13 23 33 43 53 2θ, deg.

Figure 5. 5.XRD XRD patterns patterns of the of productsthe products obtained obtained after the after treatment the treatment of SGW with of 6.5%SGW Ca(OH) with 26.5% Ca(OH)2 under static static conditions conditions (24 h, (24 w/s h, = w/s 10). = Indexes: 10). Indexes:n—CaF2 ;nx——AlFCaF1.52;(OH) x—AlF1.5·0.375H1.5(OH)2O;1.5k∙0.375H—CaCO32.O; k—CaCO3.

It can be stated that the Ca(OH)2 additive has a positive effect on AlF3·3H2O decom- It can be stated that− the Ca(OH)2 additive has a positive effect on AlF3∙3H2O decomposition; however, the F ions were bound into CaF2, or were absorbed into the silica position;gel sample however, and remained the inF− the ions solid were material. bound Similar into resultsCaF2, or were were obtained absorbed by Iljina into the silica gel sampleet al. [22]. and These remained authors showed in the that, solid by treatingmaterial. SGW S underimilar static results conditions were atobtained different by Iljina et al. − [22].temperatures These authors and w/s showed ratios, the that F ion, by concentration treating SGW can under be reduced static to conditions only 6–7 wt%. at different tem- For this reason, in the next stage of this work, the silica gel samples were treated with a peratures and w/s ratios, the F− ion concentration can be reduced to only 6–7 wt%. For saturated Ca(OH)2 solution under dynamic conditions. this reason, in the next stage of this work, the silica gel samples were treated with a saturated Ca(OH)2 solution under dynamic conditions. It was determined that the treatment conditions affect the stability of AlF3∙3H2O and the removal of F− ions. After the leaching of the F− ions by the use of continuous liquid medium flow (w/s = 25, 50, 100), low diffraction peaks characteristic of AlF3∙3H2O and its Sustainability 2021, 13, 1554 8 of 17 Sustainability 2021, 13, x FOR PEER REVIEW 8 of 16

It was determined that the treatment conditions affect the stability of AlF3·3H2O − − decompositionand the removal ofproduct, F ions. AlF After1.5(OH) the leaching1.5∙0.375H of2 theO, Fwereions still by theobserved use of continuous in the XRD patterns liquid medium ﬂow (w/s = 25, 50, 100), low diffraction peaks characteristic of AlF ·3H O (Figure 6, curves 1–4). As expected, together with the mentioned compounds,3 2 CaF2 and and its decomposition product, AlF1.5(OH)1.5·0.375H2O, were still observed in the XRD CaCO3 (formed due to the interaction between Ca(OH)2 and atmospheric CO2) were de- patterns (Figure6, curves 1–4). As expected, together with the mentioned compounds, tected. Meanwhile, when the silica gel waste was treated in cycles (w/s = 100), AlF3∙3H2O CaF2 and CaCO3 (formed due to the interaction between Ca(OH)2 and atmospheric CO2) waswere completely detected. Meanwhile, decomposed. when However, the silica gel alongside waste was CaF treated2 and in CaCO cycles3 (w/s, two =diffraction 100), peaks (AlFd-spacing3·3H2O:was 0.762, completely 0.385 nm) decomposed.—which did However, not correspond alongside to CaF any2 and compound CaCO3, twoindexed in the PDdiffractionF−4 database peaks (d—-spacing:were also 0.762, noted 0.385 nm)—which(Figure 6). didCalcium not correspond aluminates to any with compound unknown struc- − tureindexeds and in composition the PDF 4 database—weres were probably also formed noted (Figure during6). the Calcium leaching. aluminates with unknown structures and compositions were probably formed during the leaching.

b k b b 248 b k . b b b

5 1

n n 627 . b b 0 n k k

w/s = 100; 20 cycles k 385

4 .

762 0 . n 0 n n n nn

w/s = 100; 1 cycle k

ity, spc. ity, 3 A A n

x n n n nn Intens w/s = 50; 1 cycle 2 k A A n x n n nn w/s = 25; 1 cycle 1 k A n x n

3 13 23 33 43 53 2q, deg.

FigureFigure 6.6.XRD XRD patterns patterns of theof productsthe products obtained obtained after the after treatment the treatment of SGW with of saturatedSGW with Ca(OH) saturated2 Ca(OH)solution2 (curves solution 1–4) (curves under dynamic 1–4) under conditions, dynamic and conditions the precipitates, and formed the precipitates in the liquid formed medium in the liquid medium(curve 5). Indexes:(curve 5).A—AlF Indexes:3·3H2 AO;—n—CaFAlF3∙3H2; x2—AlFO; n—1.5CaF(OH)21.5; x·—0.375HAlF1.52O;(OH)b—Ca1.5∙0.375H3Al2.85O2.552O;(OH) b—9.45; Cak—CaCO3Al2.85O3. 2.55(OH)9.45; k—CaCO3.

Regardless of the chosen treatment method (in cycles or not), fine solid particles wereRegardless formed after of the the filtration chosen in treatment the liquid medium method (Figure (in cycles7). For or this not), reason, fine solid in order particles were formedto induce after the sedimentation the filtration and in chemicalthe liquid reaction, medium the liquid(Figure medium 7). For was this additionally reason, in order to inducemaintained the for sedimentation 24 h under static and conditions. chemical Later reaction, on,the the obtained liquid precipitates medium was were additionally maintainedfiltered and driedfor 24 at h 50 under± 5 ◦C static for 24 conditions. h. The XRD Later analysis on, of the the obtained precipitates precipitates showed were fil- teredthat, during and dried the leaching at 50 experiments,± 5 °C for 24 Ca(OH) h. The2 reacted XRD analysis with both of fluoride the precipitates and aluminum showed that, ions (formed by the decomposition of AlF3·3H2O) ions, which lead to the formation of during the leaching experiments, Ca(OH)2 reacted with both fluoride and aluminum ions CaF2 and katoite (Ca3Al2.85O2.55(OH)9.45, d-spacing: 0.509, 0.333, 0.279 nm) in the products (formed by the decomposition of AlF3∙3H2O) ions, which lead to the formation of CaF2 and katoite (Ca3Al2.85O2.55(OH)9.45, d-spacing: 0.509, 0.333, 0.279 nm) in the products (Figure 6, curve 5). It is worth mentioning that CaCO3 and two diffraction maximums (d-spacing: 1.248, 0.627 nm), which did not correspond to any compound indexed in the PDF−4 database, were observed in the XRD patterns of the obtained precipitates as well (Figure 6, curve 5). Based on the obtained results, it can be stated that the rate of CaF2 crystallization was great; therefore, this compound was detected in both the solid material and the precipitates. The obtained results were in good agreement with the literature data [23,33,34]. Sustainability 2021, 13, 1554 9 of 17

(Figure6, curve 5). It is worth mentioning that CaCO 3 and two diffraction maximums (d-spacing: 1.248, 0.627 nm), which did not correspond to any compound indexed in the PDF−4 database, were observed in the XRD patterns of the obtained precipitates as well (Figure6, curve 5). Based on the obtained results, it can be stated that the rate of CaF 2 crystallization was great; therefore, this compound was detected in both the solid material Sustainability 2021, 13, x FOR PEER REVIEW 9 of 16 and the precipitates. The obtained results were in good agreement with the literature data [23,33,34].

(a) (b)

Figure Figure7. The 7.opticalThe optical images images of the of theliquid liquid medium medium obtained obtained afterafter the the treatment treatment of SGWof SGW with with saturated saturated Ca(OH)2 solution under dynamic conditions: (a) leaching by the use of continuous liquid Ca(OH)2 solution under dynamic conditions: (a) leaching by the use of continuous liquid medium mediumﬂow flow (w/s (w/s = 100); = 100); (b) leaching(b) leaching in cycles in cycles (w/s =(w/s 100). = 100).

It wasIt determined was determined that, that, after after the the treatment treatment of of the the SGW SGW with thethe saturated saturated Ca(OH) Ca(OH)2 2 solutionsolution under under dynamic dynamic conditions, conditions, the the pH pH values values of of the the liquid liquid medium medium werewere greater greater than than 8, i.e., 8,the i.e., liquid the liquid medium medium had had alkaline alkaline properties properties (Table (Table 33).). Meanwhile,Meanwhile, in in a a case case of of the the samples with Al(OH)3, the pH of the liquid medium was acidic (Table1). It was measured samples with Al(OH)3, the pH of the liquid medium was acidic (Table 1). It was measured that the duration of the interaction (ﬁltration) depends on the amount of water, and it that theincreased duration from of 55 the s (w/s interaction = 25) to 283(filtration) s (w/s = 100)depends (Table on3). Thethe chemicalamount analysisof water data, and of it increasedthe treatedfrom 55 SGW s (w/s (w/s = 25) = 100) to 283 showed s (w/s that = the100) amount (Table of3). F −Theions chemical released analysis into the liquid data of the treatedmedium SGW was (w/s equal = to100) 35.19% showed (Table that3). Itthe is worthamount mentioning of F− ions that released the mentioned into the value liquid mediumwas was two equal times to higher 35.19% in comparison(Table 3). It to is the worth value mentioning obtained after that the the treatment mentioned of SGW value was twowithout times the higher additives in comparison (16.9%) [19]. to the value obtained after the treatment of SGW without the additives (16.9%) [19]. Table 3. The amount of F− ions and the pH values of the liquid medium in the SGW samples after the treatment with a saturated Ca(OH) solution under dynamic conditions. 2 Table 3. The amount of F− ions and the pH values of the liquid medium in the SGW samples after No. of w/s Thethe Concentrationtreatment with of a F saturated− The Ca(OH) Amount2 solution of underThe dynamic pH Value conditions of the . The Duration of Cycles Ratio Ions in the Sample, % Released F− Ions, % Liquid Medium Interaction, s The Concentration The Amount of The pH Value 25 1No. of - - 8.61The Duration 55 of w/s ratio of F− Ions in the Released F− of the Liquid 50 1Cycles - - 11.67Interaction 107 , s 100 1 6.50Sample, 35.19 % Ions, % 12.03Medium 283 100 2025 6.021 - 39.98- -8.61 - 55 50 1 - - 11.67 107 100 It was1 determined6.50 that, after the treatment35.19 of the samples12.03 in cycles—already283 at the beginning of the process (w/s = 10)—the pH of the liquid medium was alkaline, i.e., pH > 7 100 20 6.02 39.98 - - (Figure8, curve 2). Meanwhile, after 4 cycles (w/s = 20), the values of the pH of the liquid It was determined that, after the treatment of the samples in cycles—already at the beginning of the process (w/s = 10)—the pH of the liquid medium was alkaline, i.e. pH > 7 (Figure 8, curve 2). Meanwhile, after 4 cycles (w/s = 20), the values of the pH of the liquid medium became equal to the ones of the initial solution, and the further increment in the w/s ratio had no effect on the latter values. It was observed that, under these experimental conditions, the duration of the interaction (filtration) depended on the number of cycles, i.e., the duration increased with each cycle, and was more than five times longer at the end of the process (112 s) than at the beginning (21 s) (Figure 8, curve 1). Presumably, this was caused by the crystallization of CaF2 and/or Ca3Al2.85O2.55(OH)9.45 in the filter pores. Although the duration of the interaction (filtration) was longer than it was in the samples with Al(OH)3 (40 s), the amount of F− ions released into the liquid medium (39.98%) during the treatment in cycles was ~18% lower than the amount formed by the use of the Al(OH)3 additive (Table 2 and Table 3). The greater amount of residual F− ions can be Sustainability 2021, 13, 1554 10 of 17

medium became equal to the ones of the initial solution, and the further increment in the w/s ratio had no effect on the latter values. It was observed that, under these experimental conditions, the duration of the interaction (filtration) depended on the number of cycles, i.e., the duration increased with each cycle, and was more than five times longer at the Sustainability 2021, 13, x FOR PEER REVIEW 10 of 16 end of the process (112 s) than at the beginning (21 s) (Figure8, curve 1). Presumably, this was caused by the crystallization of CaF2 and/or Ca3Al2.85O2.55(OH)9.45 in the filter pores. Although the duration of the interaction (filtration) was longer than it was in the samples − with Al(OH)3 (40 s), the amount of F ions released into the liquid medium (39.98%) during explainedthe treatment inby cycles the wasfact ~18% that lower CaF than2 remains the amount in formedthe sample by the use (Figure of the Al(OH) 6). The3 chemical analysis additive (Tables2 and3). The greater amount of residual F − ions can be explained by the data of the liquid medium showed that all of the F− ions were combined to CaF2, because fact that CaF2 remains in the sample (Figure6). The chemical analysis data of the liquid − themedium mentioned showed that ions all of’ concentration the F ions were combineddid not toexceed CaF2, because 0.001%. the Thus, mentioned the calcium ions influ- encedions’ concentration both the didcomposition not exceed 0.001%. (mineral Thus, and the calciumchemical) ions influencedand the bothproperties the of the silica gel wastecomposition and (mineralthe liquid and medium. chemical) and the properties of the silica gel waste and the liquid medium.

120 14 2 100 12 10 80

8 H 60 p 6 40 1

4 Value Value of

20 2 Duration Duration filtration, of s 0 0 1 6 11 16 21 Number of cycles Figure 8. 8.The The change change in the in duration the duration of the filtration of the (curvefiltration 1) and (curve the pH 1 values) and ofthe the pH liquid values of the liquid me- medium (curve 2) during the treatment of the SGW with a saturated Ca(OH) solution in cycles. dium (curve 2) during the treatment of the SGW with a saturated2 Ca(OH)2 solution in cycles. As can be seen from the results obtained in parts 3.1 and 3.2, the hydroxide additives not onlyAs promote can be the seen release from of fluorine the results ions into obtained the liquid mediumin parts but 3.1 also and alter 3.2, the min-the hydroxide additives noteral compositiononly promote of the silicathe gelrelease waste. of Therefore, fluorine it was ions decided into to the treat liquid the investigated medium but also alter the samples with soluble alkaline solutions. mineral composition of the silica gel waste. Therefore, it was decided to treat the investi- gated3.3. The samples Effect of Alkaline with Solutions soluble on alkaline the Mobility solutions. of F− Ions in Silica Gel Waste Samples In this part of work, the NaOH and NH4OH solutions were used for the leaching of the F− ions in the experiments. At the beginning, the treatment of the SGW with 0.01% 3.3.and 0.05%The E NaOHffect of solutions Alkaline under Solutions dynamic conditionson the Mobility was performed. of F− ions It was in determinedSilica Gel Waste Samples that theIn NaOH this part solutions of work, (w/s = 20)the do NaOH not have and a significant NH4OH influence solutions on the were stability used of for the leaching of · AlFthe3 F−3H 2ionsO, because in the the experiments. intensities of the diffractionAt the beginning, maximums typical the treatment to the latter onlyof the SGW with 0.01% slightly decreased (Figure9). Furthermore, low intensity diffraction peaks characteristic of AlFand1.5 0.05(OH)%1.5· 0.375HNaOH2O solutions were detected. under dynamic conditions was performed. It was determined that the NaOH solutions (w/s = 20) do not have a significant influence on the stability of AlF3∙3H2O, because the intensities of the diffraction maximums typical to the latter only slightly decreased (Figure 9). Furthermore, low intensity diffraction peaks characteristic of AlF1.5(OH)1.5·0.375H2O were detected.

w/s = 20 c = 0.05 % A A A A A x A A A AA w/s = 20

ity, cps. ity, c = 0.01 % A A

Intens A A A A A A A A

3 13 23 33 43 53 2q, deg. Figure 9. XRD patterns of the products obtained after the treatment of SGW with an NaOH solution in cycles. Indexes: A—AlF3∙3H2O; v—Al(OH,F)3; x—AlF1.5(OH)1.5·0.375H2O. Sustainability 2021, 13, x FOR PEER REVIEW 10 of 16

explained by the fact that CaF2 remains in the sample (Figure 6). The chemical analysis data of the liquid medium showed that all of the F− ions were combined to CaF2, because the mentioned ions’ concentration did not exceed 0.001%. Thus, the calcium ions influ- enced both the composition (mineral and chemical) and the properties of the silica gel waste and the liquid medium.

120 14 2 100 12 10 80

8 H 60 p 6 40 1

4 Value Value of

20 2 Duration Duration filtration, of s 0 0 1 6 11 16 21 Number of cycles Figure 8. The change in the duration of the filtration (curve 1) and the pH values of the liquid medium (curve 2) during the treatment of the SGW with a saturated Ca(OH)2 solution in cycles.

As can be seen from the results obtained in parts 3.1 and 3.2, the hydroxide additives not only promote the release of fluorine ions into the liquid medium but also alter the mineral composition of the silica gel waste. Therefore, it was decided to treat the investigated samples with soluble alkaline solutions.

3.3. The Effect of Alkaline Solutions on the Mobility of F− ions in Silica Gel Waste Samples

In this part of work, the NaOH and NH4OH solutions were used for the leaching of the F− ions in the experiments. At the beginning, the treatment of the SGW with 0.01% and 0.05% NaOH solutions under dynamic conditions was performed. It was determined that the NaOH solutions (w/s = 20) do not have a significant influence on the stability of AlF3∙3H2O, because the intensities of the diffraction maximums typical to the latter only Sustainability 2021, 13, 1554 slightly decreased (Figure 9). Furthermore, low intensity diffraction peaks11 of 17 characteristic of AlF1.5(OH)1.5·0.375H2O were detected.

w/s = 20 c = 0.05 % A A A A A x A A A AA w/s = 20

ity, cps. ity, c = 0.01 % A A

Intens A A A A A A A A

3 13 23 33 43 53 2q, deg. FigureFigure 9. 9.XRD XRD patterns patterns of the of products the products obtained obtained after the treatmentafter the of treatment SGW with anof NaOHSGW solutionwith an NaOH solu- · · tionin cycles. in cycles. Indexes: Indexes:A—AlF3 A3H—2AlFO; x—AlF3∙3H21.5O;(OH) v—1.5Al(OH,F)0.375H2O.3; x—AlF1.5(OH)1.5·0.375H2O. Moreover, it was found that the duration of the interaction (Table4) was very close to the one obtained by the treatment of the SGW sample with distillated water (20–30 s). Meanwhile, the pH value of the liquid medium had already reached more than 3 at the beginning of the process (w/s = 10). It should be indicated that the pH values of the initial NaOH solutions were equal to 11.17 (0.01%) and 11.73 (0.05%).

Table 4. The change in the pH values of the liquid medium and the duration of the interaction during the treatment with an NaOH solution in cycles.

The Concentration of NaOH Solution, % 0.01 0.05 No. of Cycles The pH Values of the The Duration of The pH Values of the The Duration of Liquid Medium Interaction, s Liquid Medium Interaction, s 1 2.08 29 2.13 24 2 3.03 28 3.96 22 3 4.15 27 6.92 34 4 6.38 30 7.06 30

The results of the chemical analysis showed that, after treatment with a 0.05% NaOH solution (w/s = 20), the concentration of fluorine ions in the SGW sample decreased by ~1.6 times, i.e., to 6.23%. Although the diffraction peaks characteristic of AlF3·3H2O remained intensive, the intense release of fluorine ions into the liquid medium proceeded. Due to the intense release of fluorine ions into the liquid medium, during the leaching experiments with the NaOH solution, it was decided to use 0.01% and 0.5% ammonia water solutions—which are currently applied in JSC ‘Lifosa’—for further research. The treatment of the SGW samples was performed in cycles when the maximum w/s ratio 20 was used (four cycles in total). It was determined that a slight decrease in the intensities of the diffraction peaks typical to AlF3·3H2O was observed in the XRD patterns when the sample was treated with a 0.01% ammonia water solution (w/s = 20) (Figure 10). Meanwhile, after the treatment with a more highly-concentrated solution (0.5%, w/s = 10), it was noted that AlF3·3H2O fully recrystallized to Al(OH,F)3 (d-spacing: 0.570, 0.289, 0.284, 0.189 nm). Sustainability 2021, 13, x FOR PEER REVIEW 11 of 16

Moreover, it was found that the duration of the interaction (Table 4) was very close to the one obtained by the treatment of the SGW sample with distillated water (20–30 s). Meanwhile, the pH value of the liquid medium had already reached more than 3 at the beginning of the process (w/s = 10). It should be indicated that the pH values of the initial NaOH solutions were equal to 11.17 (0.01%) and 11.73 (0.05%).

Table 4. The change in the pH values of the liquid medium and the duration of the interaction during the treatment with an NaOH solution in cycles.

The Concentration of NaOH Solution, % 0.01 0.05 No. of The pH Values of Cycles The Duration of The pH Values of the The Duration of the Liquid Me- Interaction, s Liquid Medium Interaction, s dium 1 2.08 29 2.13 24 2 3.03 28 3.96 22 3 4.15 27 6.92 34 4 6.38 30 7.06 30 The results of the chemical analysis showed that, after treatment with a 0.05% NaOH solution (w/s = 20), the concentration of fluorine ions in the SGW sample decreased by ~1.6 times, i.e., to 6.23%. Although the diffraction peaks characteristic of AlF3∙3H2O remained intensive, the intense release of fluorine ions into the liquid medium proceeded. Due to the intense release of fluorine ions into the liquid medium, during the leaching experiments with the NaOH solution, it was decided to use 0.01 % and 0.5 % ammonia water solutions—which are currently applied in JSC ‘Lifosa’—for further research. The treatment of the SGW samples was performed in cycles when the maximum w/s ratio 20 was used (four cycles in total). It was determined that a slight decrease in the intensities of the diffraction peaks typical to AlF3∙3H2O was observed in the XRD patterns when the sample was treated with a 0.01% ammonia water solution (w/s = 20) (Figure 10). Meanwhile, after the treatment with a more highly-concentrated solution (0.5 %, w/s = 10), Sustainability 2021, 13, 1554 it was noted that AlF3∙3H2O fully recrystallized to Al(OH,F)3 (d-spacing12: of0.570, 17 0.289, 0.284, 0.189 nm).

w/s = 10 v c = 0.5 % v v

v v v v ity, cps. ity,

w/s = 20 A

Intens A c = 0.01 % A A A A A A A A

3 13 23 33 43 53 2q, deg.

FigureFigure 10. 10.XRD XRD patterns patterns of products of products obtained obtained after the after treatment the oftreatment SGW with of NH SGW4OH solutionwith NH in4OH solution in cycles. Indexes: A—AlF ·3H O; v—Al(OH,F) . cycles. Indexes: A—AlF3 32∙3H2O; v—Al(OH,F)3 3. It was measured that, when the silica gel waste was treated with 0.01% ammonia water,It the was filtration measured time and that, the pHwhen values the increased silica gel with waste the increasing was treated w/s ratio, with and at0.01% ammonia water,the end the of the filtration experiment time (w/s and = 20)the were pH equalvalues to 55increased and 6.54 s,with respectively the increasing (Table5). w/s ratio, and Meanwhile, after the treatment with a higher concentration of the solution, the structure of the silica gel was destroyed, and fine particles clogged the pores of the filter, because even by applying vacuum 0.6–0.7 bar, ~900 s (cycle 1) and ~6000 s (cycle 2) filtration durations were reached (Table5). Thus, the w/s ratio was not increased to 20. It is worth noting that the pH values obtained in these conditions were >8.

Table 5. The change in the pH values of the liquid medium and the duration of the interaction during the treatment with

NH4OH solution in cycles.

The Concentration of NH4OH Solution, % 0.01 0.5 No. of Cycles The pH Values of the The Duration of The pH Values of the The Duration of Liquid Medium Interaction, s Liquid Medium Interaction, s 1 2.02 26 8.51 900 2 3.36 34 9.59 6000 3 6.30 40 - - 4 6.54 55 - -

It was determined that the amount of F− ions released from the structure of the SGW sample into the liquid medium depends on the concentration of the NH4OH solution (Table6). It was found that, after treatment with 0.01% ammonia water (w/s = 20), the amount of F− ions released was almost three times lower that than in the sample which was treated with 0.5% ammonia water (w/s = 10) (55.04%). It should be noted that a similar amount of ﬂuorine ions were released into the liquid medium (>50%) obtained by treating the SGW only with a high volume of water (w/s = 100) (Tables1 and2). Thus, it can be stated that the structure of the SGW decomposes in NH4OH solution, and that the SGW lost its adsorption properties. Sustainability 2021, 13, 1554 13 of 17

− Table 6. The amount of F ions in contaminated silica gel samples after treatment with NH4OH solution under dynamic conditions.

The Concentration of The Concentration of F− The Amount of Released No. of Cycles w/s Ratio − NH4OH Solution, % Ions in the Sample, % F Ions, % 4 20 0.01 8.02 20.04 2 10 0.5 4.51 55.04

3.4. The Effect of a Combined Treatment on the Mobility of F− Ions in Silica Gel Waste Samples As can be seen from the results discussed above that the pH value of the liquid medium obtained after the treatment increases signiﬁcantly with the addition of different hydroxide additives (NH4OH, Ca(OH)2, NaOH) (Tables3–6 and Figure8). This is likely due to the peculiarities of the AlF3 production process; a signiﬁcant amount of one of the starting materials, H2SiF6 acid, remained in the investigated sample, and it can theoretically be converted into soluble compounds only by the use of KCl and NaOH solutions:

2KCl + H2SiF6 → K2SiF6 + 2HCl (3)

4NaOH + K2SiF6 → 2KF + 4NaF + SiO2 + 2H2O (4) Sustainability 2021, 13, x FOR PEER REVIEW 13 of 16 Therefore, in the last stage of this research, the SGW was treated with 5% KCl and 5% NaOH solutions: the KCl solution (w/s = 0.5) was poured onto the sample, stirred vigorously for 5 min (60 rpm), and filtered off; later, the same procedure was repeated with anan NaOH solution (w/s = = 0.5), 0.5), and, and, in in the the final final step, step, the the sample sample was was treated treated with water (w/s(w/s = = 0.5). 0.5). TheThe results results of of the XRD analysis show that a slight decrease in the intensintensitiesities of AlFAlF33∙3H·3H2O2O diffraction diffraction peaks peaks was was observed, observed, when when the theSGW SGW sample sample was wastreated treated with withKCl solutionKCl solution (Figure (Figure 11). 11 Meanwhile,). Meanwhile, AlF AlF3∙3H3·23HO 2 recrystallizedO recrystallized to to a a related related compound, AlAl22(OH)(OH)2.762.76F3.24F3.24(H(H2O),2O), after after treatment treatment with with an NaOH an NaOH solution solution (Figure (Figure 11). It11 was). It found was found that, duethat, to due the to presence the presence of Al of3+ ions Al3+ inions the in silica the silicagel samples, gel samples, the F− the ions F− wereions were bound bound to low to solubilitylow solubility compounds compounds—K—K2NaAlF2NaAlF6 (d-spacing6 (d-spacing:: 0.287, 0.234, 0.287, 0.203 0.234, nm) 0.203 and nm) Na3 andAlF6 Na (d-3spac-AlF6 ing(d-spacing:: 0.194, 0.275, 0.194, 0.157 0.275, nm) 0.157—therefore, nm)—therefore, they were they identified were identified even aft evener the after treatment the treatment with waterwith water (Figure (Figure 11). 11).

SGW+KCl+NaOH+H2O x x ec e e c x SGW+KCl+NaOH

x ity, cps. ity, x e c ec e c x Intens SGW+KCl A A AA A A A A A A A

3 13 23 33 43 53 2q, deg. FigureFigure 11. 11. XRDXRD patterns patterns of of the the products products obtained obtained after after the the combined combined treatment treatment of of SGW SGW with with KCl KCl 3 2 2 2.76 3.24 2 2 6 3 6. andand NaOH. NaOH. Indexes: Indexes: AA——AlFAlF 3∙3H·3HO;2O; xx——AlAl (OH)2(OH)2.76F F3.24(H(HO);2 O);e—Ke—KNaAlF2NaAlF; c—6;Nac—NaAlF3AlF 6.

As JSC ‘Lifosa’ preferentially uses NH4OH in the production process, the NaOH solution was replaced with 5% ammonia water. It was found that the AlF3∙3H2O remained stable, as a slight decrease in the intensities of the diffraction peaks typical to the mentioned compound was observed in the XRD patterns (Figure 12). Besides this, K2NaAlF6 or Na3AlF6 did not form under these treatment conditions.

A A SGW+KCl+ NH OH+H O A A 4 2 A A A A A A A ity, ity, cps. A

Intens A SGW+KCl+NH4OH AA A A A A A A A

3 13 23 33 43 53 2q, deg.

Sustainability 2021, 13, x FOR PEER REVIEW 13 of 16

an NaOH solution (w/s = 0.5), and, in the final step, the sample was treated with water (w/s = 0.5). The results of the XRD analysis show that a slight decrease in the intensities of AlF3∙3H2O diffraction peaks was observed, when the SGW sample was treated with KCl solution (Figure 11). Meanwhile, AlF3∙3H2O recrystallized to a related compound, Al2(OH)2.76F3.24(H2O), after treatment with an NaOH solution (Figure 11). It was found that, due to the presence of Al3+ ions in the silica gel samples, the F− ions were bound to low solubility compounds—K2NaAlF6 (d-spacing: 0.287, 0.234, 0.203 nm) and Na3AlF6 (d-spacing: 0.194, 0.275, 0.157 nm)—therefore, they were identified even after the treatment with water (Figure 11).

SGW+KCl+NaOH+H2O x x ec e e c x SGW+KCl+NaOH

x ity, cps. ity, x e c ec e c x Intens SGW+KCl A A AA A A A A A A A

3 13 23 33 43 53 2q, deg. Figure 11. XRD patterns of the products obtained after the combined treatment of SGW with KCl Sustainability 2021, 13, 1554 and NaOH. Indexes: A—AlF3∙3H2O; x—Al2(OH)2.76F3.24(H2O); e—K2NaAlF6; c—14Na of 173AlF6.

As JSC ‘Lifosa’ preferentially uses NH4OH in the production process, the NaOH so-

lutionAs was JSC ‘Lifosa’replaced preferentially with 5% ammonia uses NH4OH water. in the It production was found process, that the the AlF NaOH3∙3H2O remained stable,solution as was a replacedslight decrease with 5% ammonia in the water.intensities It was foundof the that diffraction the AlF3·3H peaks2O remained typical to the men- stable, as a slight decrease in the intensities of the diffraction peaks typical to the mentioned tioned compound was observed in the XRD patterns (Figure 12). Besides this, K2NaAlF6 compound was observed in the XRD patterns (Figure 12). Besides this, K2NaAlF6 or or Na3AlF6 did not form under these treatment conditions. Na3AlF6 did not form under these treatment conditions.

A A SGW+KCl+ NH OH+H O A A 4 2 A A A A A A A ity, ity, cps. A

Intens A SGW+KCl+NH4OH AA A A A A A A A

3 13 23 33 43 53 2q, deg.

Figure 12. XRD patterns of the products obtained after the combined treatment of SGW with KCl

and NH4OH. Indexes: A—AlF3·3H2O.

It was also found that the SGW sample was neutralized when KCl and NaOH solutions were used together, as the pH value of the liquid medium was equal to 6.69 after the treatment with water (Table7). Meanwhile, when the NaOH solution was replaced with 5% ammonia, the pH values were equal to 2.36 and 3.01. It should be noted that, after the leaching of the silica gel samples with ammonia water (without KCl), the pH value of the liquid medium had already reached >6 at the beginning of the process (Table5).

Table 7. The amount of F− ions and the pH values of the liquid medium in the SGW samples after the treatment with

saturated Ca(OH)2 solution under dynamic conditions.

The Treatment The Concentration of F− The Amount of Released F− The pH Value of the Liquid Conditions Ions in the Sample, % Ions, % Medium SGW + KCl - - 1.96 SGW + KCl + NaOH - - 8.13 SGW + KCl + NaOH + H2O 8.44 15.86 6.69 SGW + KCl + NH4OH - - 2.36 SGW + KCl + NH4OH + H2O 8.73 12.97 3.01

Thus, a combined treatment method was found to have no significant effect on the fluorine ion content in the investigated sample, as the concentration of these ions in the sample was reduced only by 13–16% (Table7), which was four times lower than that in the sample which was treated with NH4OH (Table6). To summarise the results, it can be stated that, by the use of an Al(OH)3 additive, it is possible to reduce the concentration of F− ions in the sample from 10.0% to ~4.5%; however, a large amount of water is required for this process (w/s > 100) (Table8). The amount of liquid medium required can be significantly reduced by the use of an NH4OH solution (Table8). To our knowledge, the more successful results were obtained only by Sustainability 2021, 13, 1554 15 of 17

Krysztafkiewicz et al. [23]; the F¯ ions’ concentration was reduced by eight times; however, the initial concentration of the mentioned ions was equal only to 1.6%, and there is no data about the water-to-solid ratio.

Table 8. The treatment conditions under which the concentration of F− ions in the sample was reduced twice.

Treatment The Duration of The pH Value of the The Concentration of F− Additive w/s Ratio Conditions Interaction Liquid Medium Ions in the Sample, % - - - - - 10.0 Al(OH)3 100 Static 24 h 3.06 4.47 Al(OH)3 100 Dynamic 13 min 4.2 4.22 NH4OH solution 10 Dynamic 2 h >7 4.51

In the future research, the treated silica gel waste will be used as an SiO2 source for the production of environmentally-friendly cementitious materials.

4. Conclusions (1) The treatment method and additives were found to have a significant effect on the mineralogical composition of the silica gel waste samples. It was determined that the amount of the Al(OH)3 additive did not affect the stability of the AlF3·3H2O; however, by increasing w/s ratio from 20 to 100, only a small amount of AlF3·3H2O was present in the sample. Meanwhile, when a Ca(OH)2 additive or a saturated Ca(OH)2 solution were used, the formation of CaF2 in the silica gel waste was observed. (2) It was found that, in the case of Al(OH)3 or Ca(OH)2 additives, a significant reduction (~2 times) of the fluorine ions in the silica gel waste can be achieved only by applying the treatment with a high quantity of liquid medium (w/s = 100). Meanwhile, by the use of NaOH or NH4OH solutions (w/s = 20), the concentration of fluorine ions in the silica gel waste can be reduced by 1.6 and 2.2 times, respectively. (3) It was determined that the compounds containing aluminum and fluorine ions— such as AlF3, CaF2 and/or Ca3Al2.85O2.55(OH)9.45—were crystallized in the liquid medium, which was separated from the silica gel by the application of leaching with alkaline solutions. (4) The fluoride ions were found to be most efficiently removed from the silica gel waste when treatment with an NH4OH solution under dynamic conditions (w/s = 10) was applied, as the amount of fluorine ions released into the liquid medium was >50%. The resulting reaction medium, after the precipitation of aluminum fluoride trihydrate, can be reused for the treatment of contaminated silica gel samples.

Author Contributions: This paper was written using contributions from all of the authors. Concep- tualization, T.D. and K.B.; methodology, T.D.; formal analysis, V.R., A.G. and T.D.; investigation, V.R.; data curation, K.B.; writing—original draft preparation, V.R. and T.D.; writing—review and editing, T.D. and A.G.; visualization, A.G. and V.R.; supervision, K.B.; funding acquisition, T.D. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Research and Innovation Fund of Kaunas University of Technology (No. PP59/2002) and by JSC ‘Lifosa’. The APC was funded by Faculty of chemical technology, Kaunas university of technology. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author, T.D., upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest. Sustainability 2021, 13, 1554 16 of 17

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