A Suppression Method for Elution of F − , [ B ( OH ) 4 ] − , AsO 43 − , and CrO 42 − from Industrial Wastes Using Some Inhibitors and Crushed Stone Powder

The disposal and the reuse of industrial wastes have become increasingly difficult due to the elution of hazardous anions, such as F−, [B(OH)4], AsO4, and CrO4. Effective methods for removing hazardous ions and reusing solid wastes are urgently required. In this study, Ca(OH)2, MgCl2, and BaCl2 were added to reduce the elution concentrations of F, B, As, and Cr by coprecipitating insoluble inorganic salts. After this, ordinary Portland cement (OPC) was added to the ion exchange and solidified with these hazardous ion-containing substances. The addition of crushed stone powder (CSP), which was a by-product of the process of crushing aggregates or sawing stone, inhibited the elution of hazardous ions and improved the inhibition effect of OPC. The elution concentrations of F, B, As, and Cr were successfully reduced from their maximum elution concentration of 10 mg/L to below the environmental standards values of Japan. A simultaneous inhibition method for the elution of F, B, As, and Cr from industrial wastes has been developed successfully and would be able to promote the reuse and recycling of CSP and other industrial wastes.


Introduction
Fluoride (F, F − ), boron (B, [B(OH) 4 ] − ), arsenate (As(V), AsO 4 3− ), and chromium (Cr(VI), CrO 4 2− ), which are eluted from industrial wastes, such as slag, incineration ash, and so on, are usually present as anions and oxyanions.The environmental standards of the Soil Contamination Countermeasures Law of Japan [1] stipulate that the maximum concentration values of F, B, As(V), and Cr(VI) are 0.8, 1.0, 0.01, and 0.05 mg/L, respectively.It is difficult to remove these harmful ions due to their high solubility.
The disposal and the reuse of hazardous ion-containing industrial wastes have become increasingly important.In Japan, intermediate waste treatment, such as burning, crushing, and so on, has been carried out to reduce the quantity of the industrial wastes.However, there are still about 17.2 million cubic meters of wastes in 2014, which are left in landfills due to the presence of hazardous materials [2].A stabilization/solidification (S/S) process using cement and other cementitious materials has become increasingly popular in the immobilization of hazardous ions and oxyanions [3][4][5][6][7][8].The immobilization of wastes or contaminates has been reported that may involve the three following mechanisms: (a) chemical fixation of the contaminate, which occurs by chemical reaction between hydration products of the cement and the contaminate; (b) physical adsorption of the contaminate on the surface of the hydration products of the cements; and (c) encapsulation of the contaminate in the pores of the cement [8].In addition, ettringite [Ca 6 Al 2 (SO 4 ) 3 (OH) 12 •26H 2 O], which is formed during the hydration of ordinary Portland cement (OPC), showed a high removal preference for F [9][10][11], B [11][12][13][14][15], As [16,17], and Cr [12,[18][19][20][21] due to its ion exchange capacity by replacing SO 4 2− of ettringite.In our precious study, F concentration eluted from CaF 2 pure reagent could be reduced from 288.5 mg/L to 0.47 mg/L in alkaline regions with the addition of Ca(OH) 2 , MgCl 2 , and OPC [22].The coprecipitation effect of Ca 2+ , ion exchange effect of ettringite, and solidification effect of Ca-bearing hydrates contribute to the decrease in the elution concentration of F. This has been successfully applied for the inhibition of F eluted from paper sludge and coal ash.This method allows treated fluorine-containing wastes to be applied as earth cover or road bed materials.We also studied the solidification/stabilization of arsenic in red mud and gypsum [23,24].The As concentrations eluted from these wastes were successfully reduced to below the required environmental standards (As ≤ 0.01 mg/L) using Fe(III) and/or Fe(II).The elution of As was reduced by forming insoluble Fe-As compounds, such as FeAsO 4 , using Fe(III), and/or Fe(II).Gypsum waste was successfully reused as a soil conditioner without arsenic contamination.However, this method was only carried out under weak acidic conditions and Fe-As compounds subsequently have a strong dependence on pH [25][26][27][28].Moreover, there are usually many hazardous ions, such as F − , [B(OH) 4 ] − , AsO 4 3− , and CrO 4 2− , that coexist in these industrial wastes.
These hazardous ions usually need to be treated separately under certain conditions with different reagents, which further complicates the disposal and the reuse of industrial wastes.An effective method for the simultaneous suppression of these hazardous ions and the reuse of industrial wastes is urgently required.Crushed stone powder (CSP) occurs as a waste by-product from the process of crushing aggregates or sawing stone.The generation rate of CSP during manufacturing processes was 1-25% [29][30][31][32][33][34].This suggests that a considerable quantity of CSP would be generated in countries that are rich in the rock deposits, such as Portugal, Spain, and so on.It is difficult to utilize or dispose of this CSP because of its small grain size, the mixing of metals, and so on [31].Therefore, it is generally disposed of in landfills.However, because of the continuous depletion of natural resources, the limited landfill land available, and increased transportation costs, there is a demand for the effective utilization of CSP.In recent years, there have been an increasing number of studies that focus on the recycling of CSP in concrete instead of in silica powder or sand [35][36][37][38][39].However, further utilization of CSP in inhibiting the elution of hazardous ions is rarely found.
The aim of this study is to develop an effective method to suppress the elution of F − , [B(OH) 4 ] − , AsO 4 3− , and CrO 4 2− from industrial wastes and to improve the reuse and recycling of CSP and other industrial wastes.

Methods
For the hazardous ions, a solution containing 200 mg/L of F, B, As, and Cr was made from 1000 mg/L of the abovementioned standard solutions of hazardous ions.A total of 5 mL of hazardous ions solution was mixed with 7 g of CSP, 3.0 g of OPC, 0.5 g of Ca(OH) 2 , 0.4 g of MgCl 2 , and 0.00, 0.10, 0.20, 0.30, and 0.40 g of BaCl 2 .The mixed samples were stirred for 30 min with a stirring glass rod in a polypropylene bottle before being dried at room temperature for 48 h in a phenol culture dish (Φ = 85 mm).The dried samples were crushed into a powder and subsequently placed in polypropylene bottles with water so that the water-to-sample ratio was 10:1.After this, they were shaken with a laboratory shaker (SA300, Yamato, Japan) at 200 rpm for 6 h before being filtered through 5C filter paper after centrifuging at 3000 rpm for 20 min with a low-speed centrifuge (LC-120, TOMY, Tokyo, Japan).The elution examination of dried samples was conducted according to the testing methods for industrial wastewater of Japan (JIS K 0102).
The pH measurements of the filtrates were obtained with a pH Meter (F-22, Horiba, Japan).The elution concentration of F was measured using IC.B and Cr were measured using ICP-MS.As was measured using AAS with a hydride generator.In addition, the crystal structure and phase composition of dried samples were characterized by XRD.The microstructures of the dried mixtures were examined using SEM.

Results and Discussion
The solutions of hazardous ions (200 mg/L) were added and mixed with CSP and OPC so that the maximum elution concentrations of these hazardous ions during elution examination were 10 mg/L in the case where all hazardous ions had been eluted.
A decrease in the elution concentrations of F, B, As, and Cr was observed with an addition of CSP (Table 1).The elution concentrations of these hazardous ions were 7.15, 8.60, 2.75, and 4.46 mg/L (pH = 8.6), respectively, after the elution experiment.CSP is mainly comprised of components of quartz (SiO 2 , File No. 01-046-1045) and albite (NaAlSi 3 O 8 , File No. 01-001-0739) (Figure 1).Furthermore, it has a high content of SiO 2 (52.9%) and Al 2 O 3 (14.4%)(Table 2).In recent years, geopolymer as a type of alkali-activated aluminosilicates, such as calcium aluminosilicate hydrate and alkali aluminosilicate, was reported to immobilize hazardous ions [5,8,42,43].Albite belongs to a type of sodium aluminosilicate.Geopolymer created from albite would form during the mixing of CSP and water.The forming of the geopolymer would greatly contribute to the immobilization of hazardous ions.The pH values of the filtrates of all the samples were above 12.2 with an addition of OPC.The inhibitory effects of OPC and CSP on the elution of F, B, As, and Cr are illustrated in Table 1.•Quartz ◇Albite

Suppression of F Elution
A significant decrease in the elution concentration of F was observed when only OPC was added.The elution concentration of F was reduced to 0.45 mg/L with an addition of 2.0 g of OPC, which was a value below its environmental standards (F ≤ 0.8 mg/L).Portlandite (Ca(OH) 2 ), calcium silicate hydrates (Ca 3 SiO 5 and Ca 2 SiO 4 ), and ettringite (Ca 6 Al 2 (SO 4 ) 3 OH 12 •26H 2 O) are formed during the hydration of OPC.F could be reduced by coprecipitating CaF 2 [9] due to the high content of Ca 2+ , which originates from cement hydrates.Our previous studies have shown that the solubility of CaF 2 increases with an increase in pH in the alkaline range due to the competition between F − and OH − [22].However, ettringite with a high ion exchange capacity was formed during the hydration of OPC, and therefore, the elution of F was inhibited due to the replacement of the exchangeable anion, such as SO  2b).The generation of calcium aluminosilicate could not be confirmed by XRD, which is possibly due to the reflections assigned to calcium aluminosilicate being too weak to be seen.Ettringite with a high ion exchange capacity is a rod-like particle with 1-2 µm length.These particles were confirmed in the SEM images of the mixtures of OPC and the hazardous ions with/without an addition of CSP (Figure 3).Consequently, ettringite would form in the mixtures of OPC and the hazardous ions with/without an addition of CSP.Hence, the coprecipitation effect of Ca 2+ , ion exchange effect of ettringite, and solidification effect of Ca-bearing hydrates would contribute to the decrease in elution concentrations of these hazardous ions.•Quartz ◇Albite

Suppression of B Elution
B was reported to be precipitated with Ca(OH) 2 and OPC as calcium borate (CaO•B 2 O 3 •6H 2 O) [44-46].The solution would become alkaline with an addition of OPC.B would be reduced by the formation of calcium borate.In this study, the elution concentration of B was reduced to 0.01 mg/L, which is far less than its environmental standards (B ≤ 1.0 mg/L), with an addition of 2.0 g of OPC.The coprecipitation effect of Ca 2+ , ion exchange capacity of ettringite, and solidification effect of Ca-bearing hydrates contribute to the inhibition of the elution of B. However, the coexistence of F and B possibly leads to the generation of BF 4 − , which further complicates the treatment of F and B [47].In this study, the elution concentration of BF 4 − was not detected by IC as the generation of BF 4 − requires certain conditions.The elution concentrations of F and B were all successfully reduced with an addition of OPC.

Suppression of As Elution
As could be reduced by forming insoluble Ca-As precipitates with Ca 2+ [48][49][50][51], such as Ca This elution concentration meets the environmental standards of As (As ≤ 0.01 mg/L).Due to the high content of Ca 2+ originating from OPC, the elution of As was inhibited by the coprecipitation effect of Ca 2+ , ion exchange capacity of ettringite, and solidification effect of Ca-bearing hydrates.Additionally, the presence of carbonate (≥0.3 mol/L) would capture Ca 2+ to generate CaCO 3 , suppressing the generation of Ca-As compounds and resulting in the release of As into the aqueous solution [51].Therefore, the prevention of the carbonation of Ca 2+ could be a way of promoting the inhibitory effect on As.

Suppression of As Elution
As could be reduced by forming insoluble Ca-As precipitates with Ca 2+ [48][49][50][51], such as Ca3(AsO4)2•xH2O, Ca5(AsO4)3OH, and Ca4(OH)2(AsO4)2•4H2O.In alkaline regions, the dominant species of As(V) in solution are HAsO4 2− and AsO4 3− at pH > 7 (Figure 4).Zhu et al. studied the solubility and stability of calcium arsenates under different pH values [48].The results showed that Ca3(AsO4)2•xH2O, Ca5(AsO4)3OH, and Ca4(OH)2(AsO4)2•4H2O were identified in the experiment over a wide range of pH (3.0 < pH < 13.4) and for Ca/As molar ratios between 1.0 and 4.0.In this study, the elution concentration of As was reduced to 0.01 mg/L with an addition of 2.0 g of OPC.This elution concentration meets the environmental standards of As (As ≤ 0.01 mg/L).Due to the high content of Ca 2+ originating from OPC, the elution of As was inhibited by the coprecipitation effect of Ca 2+ , ion exchange capacity of ettringite, and solidification effect of Ca-bearing hydrates.Additionally, the presence of carbonate (≥0.3 mol/L) would capture Ca 2+ to generate CaCO3, suppressing the generation of Ca-As compounds and resulting in the release of As into the aqueous solution [51].Therefore, the prevention of the carbonation of Ca 2+ could be a way of promoting the inhibitory effect on As.

Suppression of Cr Elution
The immobilization of Cr(VI) by OPC was achieved due to the formation of the low solubility complex compounds, such as CaCrO4 [53,54] and CrO4-ettringite.CaCrO4 was formed by the reaction between Cr and Ca 2+ at a high pH value.CrO4-ettringite was formed by the ion exchange between CrO4 2− and SO4 2− in ettringite.Some researchers studied the inhibitory effect of calcium silicate hydrate on Cr [55][56][57].In the study of Zhang et al., in terms of calcium silicate hydrate (C-S-H), one of the most important hydration products of OPC, it seemed that the chemical incorporation degree of Cr(VI) was relatively low and just a sorption mechanism was more possible [21].In this study, although the elution concentration of Cr decreased with an addition of OPC, it was still higher than its environmental standards (Cr ≤ 0.05 mg/L).
Ca(OH)2, MgCl2, and BaCl2 were added to decrease the quantity of eluted Cr by forming insoluble salts.The addition of Ca(OH)2 results in a reduction in the elution of F, B, As, and Cr.MgCl2 was able to prevent the carbonation of Ca 2+ [58,59] and coprecipitate with F and B to create MgF2 and Mg[B(OH)4]2.BaCl2 could suppress the elution of Cr by coprecipitating into the form of BaCrO4 [60].Additionally, As could also react with Ba 2+ to form the insoluble salt Ba3(AsO4)2 (Ksp = 2.59 × 10 −9 , 20 °C), which is stable within the pH range of 12-14 [61].FeSO4 was added to compare the inhibitory effect on Cr with BaCl2.
The pH values of the filtrates of all the samples were above 12.1 with the additions of CSP, OPC, Ca(OH)2, MgCl2, and BaCl2.The elution concentration of Cr decreased significantly with these

Suppression of Cr Elution
The immobilization of Cr(VI) by OPC was achieved due to the formation of the low solubility complex compounds, such as CaCrO 4 [53,54] and CrO 4 -ettringite.CaCrO 4 was formed by the reaction between Cr and Ca 2+ at a high pH value.CrO 4 -ettringite was formed by the ion exchange between CrO 4 2− and SO 4 2− in ettringite.Some researchers studied the inhibitory effect of calcium silicate hydrate on Cr [55][56][57].In the study of Zhang et al., in terms of calcium silicate hydrate (C-S-H), one of the most important hydration products of OPC, it seemed that the chemical incorporation degree of Cr(VI) was relatively low and just a sorption mechanism was more possible [21].In this study, although the elution concentration of Cr decreased with an addition of OPC, it was still higher than its environmental standards (Cr ≤ 0.05 mg/L).Ca(OH) 2 , MgCl 2 , and BaCl 2 were added to decrease the quantity of eluted Cr by forming insoluble salts.The addition of Ca(OH) 2 results in a reduction in the elution of F, B, As, and Cr.MgCl 2 was able to prevent the carbonation of Ca 2+ [58,59] and coprecipitate with F and B to create MgF 2 and Mg[B(OH) 4 ] 2 .BaCl 2 could suppress the elution of Cr by coprecipitating into the form of BaCrO 4 [60].
The pH values of the filtrates of all the samples were above 12.1 with the additions of CSP, OPC, Ca(OH) 2 , MgCl 2 , and BaCl 2 .The elution concentration of Cr decreased significantly with these additives and further decreased with an increase in the added amount of BaCl 2 (Figure 5).When 0.2 g of BaCl 2 was added, the Cr elution concentration was reduced to 0.01 mg/L, which is below its environmental standard value.In contrast, the lowest elution concentration of total Cr, which was in the forms of Cr(VI) and Cr(III), was 0.14 mg/L with an addition of FeSO 4 .BaCl 2 is obviously superior to FeSO 4 in reducing the elution of Cr in this study.On the one hand, FeSO 4 inhibited Cr(VI) by reducing it to Cr(III).This process was generally carried out under acidic conditions.Otherwise, Fe(II) would react with OH − to form Fe(OH) 2 before oxidizing in the air to form Fe(OH) 3 and weakening its reduction ability.Moreover, Cr(VI) could be separated from Fe(II) by the generation of floccules, which are namely Fe(OH) 2 and/or Fe(OH) 3 , resulting in a decrease in the inhibiting effect on Cr [62].On the other hand, additional operations are required to suppress Cr(III) after reduction reaction.Therefore, the treatment of Cr by BaCl 2 was simpler and more effective than FeSO 4 .In addition, it was easy to react BaCl 2 with SO 4 2− to generate the insoluble salt BaSO 4 (K sp , BaSO 4 = 2.45 × 10 −4 , 20 • C) due to its low solubility.This would lead to a reduction in the amount of SO 4 2− of gypsum, which plays an important role in the composition of ettringite, resulting in the suppression of the hydration of OPC [63,64].Therefore, the added amount of BaCl 2 should be controlled.In this study, the elution concentrations of F, B, and As were 0.20, 0.18, and 0.00 mg/L, respectively, with additions of CSP, OPC, Ca(OH) 2 , MgCl 2 , and 0.2 g of BaCl 2 , while there were no obvious changes with an increase in the added amount of BaCl 2 .The elution concentrations of F, B, As, and Cr were successfully inhibited to be lower than the environmental standards values with additions of CSP, OPC, Ca(OH) 2 , MgCl 2 , and 0.2 g of BaCl 2 .
Technologies 2018, 6, x FOR PEER REVIEW 7 of 10 additives and further decreased with an increase in the added amount of BaCl2 (Figure 5).When 0.2 g of BaCl2 was added, the Cr elution concentration was reduced to 0.01 mg/L, which is below its environmental standard value.In contrast, the lowest elution concentration of total Cr, which was in the forms of Cr(VI) and Cr(III), was 0.14 mg/L with an addition of FeSO4.BaCl2 is obviously superior to FeSO4 in reducing the elution of Cr in this study.On the one hand, FeSO4 inhibited Cr(VI) by reducing it to Cr(III).This process was generally carried out under acidic conditions.Otherwise, Fe(II) would react with OH − to form Fe(OH)2 before oxidizing in the air to form Fe(OH)3 and weakening its reduction ability.Moreover, Cr(VI) could be separated from Fe(II) by the generation of floccules, which are namely Fe(OH)2 and/or Fe(OH)3, resulting in a decrease in the inhibiting effect on Cr [62].
On the other hand, additional operations are required to suppress Cr(III) after reduction reaction.Therefore, the treatment of Cr by BaCl2 was simpler and more effective than FeSO4.In addition, it was easy to react BaCl2 with SO4 2− to generate the insoluble salt BaSO4 (Ksp, BaSO4 = 2.45 × 10 −4 , 20 °C) due to its low solubility.This would lead to a reduction in the amount of SO4 2− of gypsum, which plays an important role in the composition of ettringite, resulting in the suppression of the hydration of OPC [63,64].Therefore, the added amount of BaCl2 should be controlled.In this study, the elution concentrations of F, B, and As were 0.20, 0.18, and 0.00 mg/L, respectively, with additions of CSP, OPC, Ca(OH)2, MgCl2, and 0.2 g of BaCl2, while there were no obvious changes with an increase in the added amount of BaCl2.The elution concentrations of F, B, As, and Cr were successfully inhibited to be lower than the environmental standards values with additions of CSP, OPC, Ca(OH)2, MgCl2, and 0.2 g of BaCl2.

Conclusions
The elution concentrations of F, B, As, and Cr were successfully reduced from their maximum elution concentrations of 10 mg/L to below the environmental standards values with the additions of Ca(OH)2, MgCl2, BaCl2, OPC, and CSP.Metal salts reduced the elution concentrations of F, B, As, and Cr by coprecipitation into insoluble inorganic salts.After this, OPC was added to the ion exchange and solidified with these hazardous ion-containing substances.The addition of CSP inhibited the elution of hazardous ions and improved the inhibition effect of OPC.A simultaneous inhibition method for the elution of F, B, As, and Cr from industrial wastes has been developed successfully and would be able to promote the reuse and recycling of industrial wastes.

Conclusions
The elution concentrations of F, B, As, and Cr were successfully reduced from their maximum elution concentrations of 10 mg/L to below the environmental standards values with the additions of Ca(OH) 2 , MgCl 2 , BaCl 2 , OPC, and CSP.Metal salts reduced the elution concentrations of F, B, As, and Cr by coprecipitation into insoluble inorganic salts.After this, OPC was added to the ion exchange and solidified with these hazardous ion-containing substances.The addition of CSP inhibited the elution of hazardous ions and improved the inhibition effect of OPC.A simultaneous inhibition method for the elution of F, B, As, and Cr from industrial wastes has been developed successfully and would be able to promote the reuse and recycling of industrial wastes.
4 2− in ettringite by F. The coprecipitation effect of Ca 2+ , ion exchange effect of ettringite, and solidification effect of Ca-bearing hydrates contribute to the inhibition of the elution of F. Additionally, under the conditions of the same added amount of OPC, the elution concentrations of F were lower than those without additions of CSP.SiO 2 and Al 2 O 3 in CSP could improve the solidification effect of Ca-bearing hydrates by becoming involved in the pozzolanic reaction.Furthermore, it is possible for CSP to react with OPC to generate calcium aluminosilicate hydrate and alkali aluminosilicate to immobilize F and other hazardous ions.Moreover, due to its small particle size, CSP could also fill in the gaps of cement paste, enhancing the density of the cement paste and improving the inhibitory effects of OPC.The reflections assigned to quartz, albite, tricalcium silicate (Ca 3 SiO 5 , ICSD file No. 00-055-0738), dicalcium silicate (Ca 2 SiO 4 , ICSD file No. 00-039-0298), calcite (CaCO 3 , ICSD file No. 01-072-1937), and portlandite (Ca(OH) 2 , ICSD file No. 01-076-0571) were observed in the mixture with OPC and CSP OPC (Figure

Figure 2 .
Figure 2. XRD patterns of hazardous ions mixed with 8.0 g of OPC (a) with CSP and (b) without CSP.

Figure 3 .
Figure 3. SEM images of (a) CSP and (b) OPC before reaction; and hazardous ions mixed with (c) 8.0 g of OPC and (d) 3.43 g of OPC and 8.0 g of CSP after reaction.

Figure 4 .
Figure 4. Eh-pH diagram for aqueous As species in the system As-O2-H2O at 25 °C and 1 bar total pressure.Referred from [52].

Figure 4 .
Figure 4. Eh-pH diagram for aqueous As species in the system As-O 2 -H 2 O at 25 • C and 1 bar total pressure.Referred from [52].

Figure 5 .
Figure 5.The elution concentrations of Cr with additions of 7.0 g of CSP, 3.0 g of OPC, 0.5 g of Ca(OH)2, 0.4 g of MgCl2, and a certain amount of BaCl2 and FeSO4.

AuthorFigure 5 .
Figure 5.The elution concentrations of Cr with additions of 7.0 g of CSP, 3.0 g of OPC, 0.5 g of Ca(OH) 2 , 0.4 g of MgCl 2 , and a certain amount of BaCl 2 and FeSO 4 .

Table 1 .
Experimental conditions and the elution concentrations of F, B, As, and Cr with additions of crushed stone powder (CSP) and/or ordinary Portland cement (OPC).

Table 2 .
Chemical compositions of CSP and elution concentrations of the hazardous ions by the testing methods for industrial wastewater of Japan (JIS K 0102).