Formation of tooeleite and the role of direct removal of As(III) from high-arsenic acid wastewater

Novel insights into the kinetics and mechanism of arsenopyrite bio-dissolution enhanced by pyrite

Arsenopyrite and pyrite often coexist in metal deposits and tailings, thus simultaneous bioleaching of both sulfides has economic (as well as environmental) significance. Important targets in bio-oxidation operations are high solubilization rates and minimized accumulation of Fe(III)/As-bearing secondary products. This study investigated the role of pyrite bioleaching in the enhancement of arsenopyrite dissolution. At a pyrite to arsenopyrite mass ratio of 1:1, 93.6% of As and 93.0% of Fe were solubilized. The results show that pyrite bio-oxidation can promote arsenopyrite dissolution, enhance S0 bio-oxidation, and inhibit the formation of jarosites, tooeleite, and amorphous ferric arsenate. The dry weight of the pyrite & arsenopyrite residue was reduced by 95.1% after bioleaching, compared to the initial load, while only 5% weight loss was observed when pyrite was absent. A biofilm was formed on the arsenopyrite surface in the presence of pyrite, while a dense passivation layer was observed in the absence of pyrite. As(III) (as As2O3) was a dominant As species in the pyrite & arsenopyrite residue. Novel and detailed findings are presented on arsenopyrite bio-dissolution in the presence of pyrite, and the presented approach could contribute to the development of novel cost-effective extractive bioprocesses. ENVIRONMENTAL IMPLICATION: The oxidation of arsenopyrite presents significant environmental hazards, as it can contribute to acid mine drainage generation and arsenic mobilization from sulfidic mine wastes. Bioleaching is a proven cost-effective and environmentally friendly extractive technology, which has been applied for decades in metal recovery from minerals or tailings. In this work, efficient extraction of arsenic from arsenopyrite bioleaching was presented through coupling the process with bio-oxidation of pyrite, resulting in lowered accumulation of hazardous and metastable Fe(III)/As-bearing secondary phases. The results could help improve current biomining operations and/or contribute to the development of novel cost-effective bioprocesses for metal extraction.

Curious Behavior of Fe3+-As3+ Chemical Interactions and Nucleation of Clusters in Aqueous Medium

The simultaneous presence of Fe³⁺ and As³⁺ ions in groundwater (higher ppb or lower ppm level concentrations at circumneutral pH) as well as in acid mine drainages (AMDs)/industrial wastewater (up to few thousand ppm concentration at strongly acidic pH) are quite common. Therefore, understanding the chemical interactions prevalent between Fe³⁺ and As³⁺ ions in aqueous medium leading to nucleation of ionic clusters/solids, followed by aggregation and growth, is of great environmental significance. In the present work, we attempt to probe the nucleation process of Fe³⁺-As³⁺ clusters in solutions of various concentrations and pHs (from AMD to groundwater-like) using a combination of experimental and theoretical techniques. Interestingly, our study reveals nucleation of primary FeAs clusters in nearly all of them independent of concentration or pH. Theoretical studies employed density functional theory (DFT) to predict the primary clusters as stable Fe₄As₄ units. The surprising resemblance of these clusters with known Fe³⁺-As³⁺ minerals at the local level was observed experimentally, which provides an important clue about solid-phase growth from a range of Fe³⁺-As³⁺ solutions. Our experimental findings are further supported by a stepwise reaction mechanism established from detailed DFT studies.

Enhancement of 15% calcium oxide doped nano zero-valent iron on arsenic removal from high-arsenic acid wastewater

Nano zero-valent iron (nZVI) has a great potential for arsenic removal, but it would form aggregates easily and consume largely by H⁺ in the strongly acidic solution. In this work, 15%CaO doped with nZVI (15%CaO-nZVI) was successfully synthesized from a simplified ball milling mixture combined with a hydrogen reduction method, which had a high adsorption capacity for As(V) removal from high-arsenic acid wastewater. More than 97% As(V) was removed by 15%CaO-nZVI under the optimum reaction conditions of pH 1.34, initial As(V) concentration 16.21 g/L, and molar ratio of Fe/As (n_Fe/n_As) 2.5:1. The effluent pH solution was weakly acidic 6.72, and the secondary arsenic removal treatment reduced the solid waste and improved arsenic grade in slag from the mass fraction of 20.02% to 29.07%. Multiple mechanisms including Ca²⁺ enhanced effect, adsorption, reduction, and co-precipitation coexisted for As(V) removal from high-arsenic acid wastewater. Doping of CaO might lead to improving cracking channels which was benefit for electronic transmission and the confusion of atomic distribution. The in situ weak alkaline environment generated on the surface of 15%CaO-nZVI would increase the content of γ-Fe₂O₃/Fe₃O₄, which was in favor for As(V) adsorption. In addition, H⁺ in the strongly acidic solution could accelerate corrosion of 15%CaO-nZVI and abundant fresh and reactive iron oxides continuously generated, which would provide plenty specific reactive site and fast charge transfer and ionic mobility for arsenic removal.

FeSx@MOF-808 composite for efficient As(III) removal from wastewater: behavior and mechanism

Arsenic is extremely toxic to humans with water as its carrier. One challenge for arsenic control is the complete elimination of As(III) due to its high toxicity, mobility, and solubility. Herein, an active FeS_x@MOF-808 composite was fabricated to enhance the As(III) removal for wastewater remediation. The FeS_x@MOF-808 showed better As(III) adsorptive performance (Q_e = 73.60 mg/g) compared with Fe₂S₃ (Q_e=12.38 mg/g), MOF-808 (Q_e = 27.85 mg/g), and Fe@MOF-808 (Q_e=34.26 mg/g). This can be attributed to an improved porous structure provided by MOF-808 and abundant reactive sites provided by FeS_x. Calculated by the Langmuir model (R² =0.9965), the maximum adsorption capacity (Q_max) of FeS_x@MOF-808 for As(III) removal at 298 K and pH = 7 was 203.28 ± 6.43 mg/g, which is beyond most of the traditional materials and MOFs. Additionally, FeS_x@MOF-808 exhibited good stability in a wide pH range (1-13). Results also showed that the different Fe/S ratios (1:0-1:8) and FeS_x loading amount (0.00625-0.25 mmol) have effects on the FeS_x@MOF-808 performance. By kinetics studies, XPS, and DFT calculation, the mechanisms for arsenic by FeS_x@MOF-808 were proposed. Multiple reaction mechanisms combine the adsorption by the MOF-808 support, the co-precipitation of iron oxides via hydroxyl (Fe-OH) groups, and most importantly, the precipitation through the break of Fe-S and the bond of As-S.

Enhanced removal of high-As(III) from Cl(-I)-diluted SO4(-II)-rich wastewater at pH 2.3 via mixed tooeleite and (Cl(-I)-free) ferric arsenite hydroxychloride formation

An advanced cost-saving method of removal of high-As(III) from SO₄(-II)-rich metallurgical wastewater has been developed by diluting the SO₄(-II) content with As(III)-Cl(-I)-rich metallurgical wastewater and then by the direct precipitation of As(III) with Fe(III) at pH 2.3. As(III) removal at various SO₄(-II)/Cl(-I) molar ratios and temperatures was investigated. The results showed that 65.2‒98.2% of As(III) immobilization into solids occurred at the SO₄(-II)/Cl(-I) molar ratios of 1:1‒32 and 15‒60 °C in 3 days, which were far higher than those in aqueous sole SO₄(-II) or Cl(-I) media at the equimolar SO₄(-II) or Cl(-I) and the same temperature. SO₄(-II)/Cl(-I) molar ratio of 1:4 and 25 °C were optimal conditions to reach the As removal maximum. Mixed aqueous SO₄(-II) and Cl(-I) played a synergetic role in the main tooeleite formation together with (Cl(-I)-free) ferric arsenite hydroxychloride (FAHC) involving the substitution of AsO₃^3- for Cl(-I) for enhanced As fixation. The competitive complexation among FeH₂AsO₃²⁺, FeSO₄⁺ and FeCl²⁺ complexes was the main mechanism for the maximum As(III) precipitation at the SO₄(-II)/Cl(-I) molar ratio of 1:4. Low As(III) immobilization at high temperature with increased Fe(III) hydrolysis was due to the formation of As(III)-bearing ferrihydrite with the relatively high Fe/As molar ratio at acidic pH.

Effects of nitrate and Fe/As molar ratio on direct iron(III)-arsenite precipitation in high-sulfate-chloride wastewaters

The addition of an arsenite-chloride solution into an arsenite-sulfate solution is extremely beneficial for the removal of As(III) via Fe(III) salt precipitation at pH 2.3. However, the applicability of this method to complicated high-As(III) metallurgical wastewaters still requires further verification. This work investigated the effects of nitrate and Fe/As molar ratio on As(III) immobilization using Fe(III) in three acid radical media including sulfate, chloride, and nitrate at pH 2.3. Our results indicated that 72.1‒93.5% of As(III) was precipitated, which was 5‒10% less than those obtained in the nitrate-free systems. The Fe/As molar ratio of 4 was the optimal condition with an average of 93% As(III) removal based on a broad sulfate/chloride molar ratio range (1:1‒16). However, a maximum of 96% As(III) removal was observed under the Fe/As molar ratio of 1.5 and the sulfate/chloride condition of 1:16. The negative correlation between complexation and precipitation was attributed to the enhanced initial complexation by the synergistic effect of the mononitratoiron complex and FeH₂AsO₃²⁺. The variation of Fe/As molar ratios resulted in the diverse solid species, thus further affecting the As(III) removal efficiency. Despite producing tooeleite as a major As(III) host phase, ferrihydrite and poorly crystalline ferric arsenite hydroxysulfate formed simultaneously at the Fe/As molar ratio of 4 participated in As(III) immobilization compared with the solid products at Fe/As molar ratios ≤ 2.

Release and fate of As mobilized via bio-oxidation of arsenopyrite in acid mine drainage: Importance of As/Fe/S speciation and As(III) immobilization

Mining activities expose sulfidic minerals including arsenopyrite (FeAsS) to acid mine drainage (AMD). The subsequent release of toxic arsenic (As) can have great negative implications for the environment and human health. This study investigated the evolution of secondary products and As speciation transformations during arsenopyrite bio-oxidation in AMD collected from a polymetallic mine. Immobilization of the As solubilized via arsenopyrite bio-oxidation using red mud (RM) was also studied. The results show that the high ionic strength (concentrations of dissolved Fe³⁺, SO₄^2-, and Ca²⁺ reached values up to 0.75, 3.38, and 0.35 g/L, respectively) and redox potential (up to +621 mV) of AMD (caused primarily by Fe³⁺) enhanced the dissolution of arsenopyrite. A high [Fe]_aq/[As]_aq ratio in the AMD favored the precipitation of tooeleite during arsenopyrite bio-oxidation, and the formation of other poorly crystalline products such as schwertmannite and amorphous ferric arsenate also contributed to As immobilization. Bacterial cells served as important nucleation sites for the precipitation of mineral phases. Arsenopyrite completely dissolved after 12 days of bio-oxidation in AMD and the [As]_aq (mainly present as As(III)) reached 1.92 g/L, while a greater [As]_aq was observed in a basal salts medium (BSM) assay (reaching 3.02 g/L). An RM addition significantly promoted As(III) immobilization, with final [As(III)]_aq decreasing to 0.16 and 1.43 g/L in AMD and BSM assays respectively. No oxidation of As(III) was detected during the immobilization process. These findings can help predict As release from arsenopyrite on contact with AMD and, on a broader scale, assist in designing remediation and treatment strategies to mitigate As contamination in mining.

Tooeleite Transformation and Coupled As(III) Mobilization Are Induced by Fe(II) under Anoxic, Circumneutral Conditions

Tooeleite [Fe^III₆(As^IIIO₃)₄SO₄(OH)₄.4H₂O] is an important As(III) host phase in diverse mining-impacted environments. Tooeleite has also received attention as a target phase for immobilizing As(III) in environmental and engineered settings. However, little is known regarding tooeleite's environmental stability, with no previous research examining the possible role of Fe(II) in inducing tooeleite transformation (as occurs for Fe(III) oxide minerals). We investigated shifts in solid-phase Fe and As speciation and associated As mobilization into the aqueous phase during exposure of tooeleite to aqueous Fe(II) under anoxic conditions at pH 4 to 8. Our results demonstrate that environmentally relevant concentrations of aqueous Fe(II) (i.e., 1 to 10 mM) induce significant mobilization of As(III) from tooeleite under near-neutral pH conditions, with greater As(III) mobilization occurring at higher pH. Extended X-ray absorption fine structure spectroscopy at both the As and Fe K-edge reveals that the observed As(III) mobilization was coupled with partial Fe(II)-induced transformation of tooeleite to As(III)-bearing ferrihydrite at pH 6 to 8. These results provide new insights into the environmental stability of tooeleite and demonstrate a novel pathway for As(III) mobilization in tooeleite-bearing systems.

Improving removal rate and efficiency of As(V) by sulfide from strongly acidic wastewater in a modified photochemical reactor

Employing ultraviolet light to enhance the removal of As(V) by sulfide (S(-II)) from strongly acidic wastewater is a potential method. However, we found the arsenic trisulfide (As₂S₃) and elemental sulfur (S₈) particles formed in this method not only vastly hinder light transmission in the wastewater but also undergo light-induced redissolution, leading to a decrease in removal rate and efficiency of As(V). Herein, As(V) removal by sulfide from strongly acidic wastewater was performed in a modified photochemical reactor to weaken the effect of the formed particles on As(V) removal. It was found that in this study, the formed particles could be efficiently removed from the photoreactor by three operations, i.e. circulation-filtration, septum setting, and lamp sleeve cleaning. The removal of As(V) was approximately 11-fold faster than that without three operations, saving 90.9% of the reaction time and 89.4% of energy consumption. The removal efficiency of As(V) also increased through weakening the light-induced redissolution of the formed particles. This study facilitates the practical application of the UV light promoted As(V) removal technology and also provides a new method to lessen the light-blocking effect in the particle-forming photochemical reaction systems.

Effects of Fe(II)-induced transformation of scorodite on arsenic solubility

Scorodite (FeAsO₄·2H₂O) is a pivotal secondary ferric arsenate that immobilizes most of arsenic (As) in acidic As-contaminated environments, but secondary As pollution may occur during dissolution of scorodite in environments involving redox changes. Reductive dissolution of scorodite by coexisting dissolved Fe²⁺ (Fe(II)_aq) under anaerobic conditions and its effects on the behavior of As have yet to be examined. Here, this study monitored the changes in mineralogy, solubility and speciation of As during scorodite transformation induced by Fe(II) under anaerobic conditions at pH 7.0 and discussed the underlying mechanisms. Mössbauer and X-ray diffraction (XRD) analysis showed the formation of parasymplesite and ferrihydrite-like species during scorodite transformation, which was highly controlled by Fe(II)_aq concentrations. 1 mM Fe(II)_aq enhanced As mobilization into the solution, whereas As was repartitioned to the PO₄^3--extractable and HCl-extractable phases with 5 and 10 mM Fe(II). The neo-formed parasymplesite and ferrihydrite-like species immobilized dissolved As(V) through adsorption and incorporation. Additionally, As(V) reduction occurred during Fe(II)-induced scorodite transformation. Our results provide new insights into the stability and risk of scorodite in anaerobic environments as well as the geochemical behavior of As in response to Fe cycling.

Yeast Extract Affecting the Transformation of Biogenic Tooeleite and Its Stability

Highly toxic As(III) is the main form of As in wastewater. The retention of As by tooeleite has gradually attracted attention in recent years due to its great potential for the direct removal of As(III). The existence of natural As-bearing minerals is closely related to microorganisms and organic matters. In this study, yeast extract was found to enhance the stability of biogenic tooeleite by Acidithiobacillus ferrooxidans (A. ferrooxidans). The effects of pH, Fe/As and yeast extract concentration were systematically studied, and the toxicity characteristic leaching procedure (TCLP) was conducted to evaluate the short-term stability of tooeleite. The mineral synthesized in the presence of yeast extract showed that the As leaching concentration decreased from 13.78 mg/L to 7.23 mg/L and the stability increased by more than 40%. In addition, various characteristics confirmed that the precursor was changed from amorphous schwertmannite to basic ferric sulfate in the presence of yeast extract, and then transformed to relatively purer tooeleite with less hollow structure and excellent dispersion, which is favorable for the stability of tooeleite. This result indicated that yeast extract resulted in the formation of different precursors and thus affected the transformation and stability of tooeleite.

Contrasting effects of dry-wet and freeze-thaw aging on the immobilization of As in As-contaminated soils amended by zero-valent iron-embedded biochar

Zero-valent iron-embedded biochar (ZVI/BC) is considered as an effective material for arsenic (As) immobilization in soil, but the stability of As after remediation against aging remains unknown. Herein, the effects of dry-wet and freeze-thaw aging on the immobilization of As in two As-contaminated soils amended by ZVI/BC were evaluated. ZVI/BC showed high immobilization capacity for As-contaminated soils with an over 82% decrease of bioavailable As, mainly due to the As-Fe co-precipitation accompanied with ZVI oxidation. The aging of dry-wet and freeze-thaw had an opposite effect on the bioavailability of As. After 35 rounds of dry-wet aging, bioavailable As concentration increased from 1.25-9.50 to 1.83-21.75 mg/kg, because of the oxidation dissolution of ZVI and the formation of mobile reduced As(III). By contrast, the crystallization of amorphous iron with the structural incorporation of sorbed As and the oxidation of As(III) into stable As(V) occurred during the 35 rounds of freeze-thaw aging, leading to the decrease of bioavailable As concentration from 9.50-1.25 to 5.42-0.45 mg/kg. Our results revealed that the stability of soil As after remediation by ZVI/BC varied with the different aging process, which needs more consideration for the long-term soil As immobilization in the different whether areas.

Partitioning and transformation behavior of arsenic during Fe(III)-As(III)-As(V)-SO42- coprecipitation and subsequent aging process in acidic solutions: Implication for arsenic mobility and fixation

The coprecipitation and subsequent aging of Fe(III)-As(III)-As(V)-SO₄^2- play an important role in controlling As behavior in acidic systems, such as acid mine drainage and hydrometallurgical acid waste. In this study, we investigated the redistribution and transformation of As in the Fe(III)-As(III)-As(V)-SO₄^2- system (As(III)/As(V) ≈ 1) at different Fe/As molar ratios (i.e., 0.25, 0.5, and 1) and pH (1.2 and 1.8) at 60 °C. The results showed that As(III) and SO₄^2- can be incorporated into the amorphous ferric arsenate and scorodite host phases by forming a Fe(AsO₄)_x(AsO₃)_y(SO₄)_z solid solution. As(III) contents in the freshly coprecipitated solids increased with pH and initial As(III) concentrations. During aging process, As(III) contents in the solid products with Fe/As molar ratios of 0.5 and 1 increased with aging time at pH 1.8. In contrast, As(III) was gradually expelled from aging products with aging time at pH 1.2, regardless of Fe/As molar ratio. X-ray diffraction (XRD), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy characterization results showed that an As(III)-SO₄^2--doped scorodite was formed at Fe/As molar ratio ≤0.5 during the aging process. It was also found that As(III) had an inhibitory effect on the transformation of poorly crystalline ferric arsenate to scorodite. The present study may have important implications for understanding the geochemical cycle of As, Fe, and SO₄^2- in acidic solutions and give further understanding on the mechanisms involved in As removal and fixation in hydrometallurgical unit operations.

Hydrothermal treatment of arsenic sulfide slag to immobilize arsenic into scorodite and recycle sulfur

Arsenic sulfide slag (ASS) is typically by-produced from arsenic-containing wastewater treatment. In this work, a novel hydrothermal treatment method with the assistance of Fe(NO₃)₃ (HT-Fe(NO₃)₃) was developed to detoxify ASS by transforming arsenic into scorodite and extracting sulfur in one step. After hydrothermal treatment, As(III) in ASS was oxidized and immobilized into the stable scorodite with a high As immobilization efficiency (~99%), and the toxicity leachability of arsenic-containing solid waste significantly reduced from 634.2 to 2.5 mg/L, well below the discharge standard of solid waste. Further study reveals that the nucleation and growth process was fit well by Avrami-Erofeev model and followed Ostwald step rule, which involved the As₂S₃ dissolution, formation of amorphous ferric arsenate and then crystallization within the amorphous precursor. In this process, sulfur originated from As₂S₃ played an important role by serving as the heterogeneous nuclei to decrease the barrier for the formation of amorphous ferric arsenate, and facilitated the transformation of as-formed scorodite from nano-sheet aggregates to the bulk and dense spherical polymorph, which further increased the stability of the arsenic contained solid product. This study will shed light on the development of new technologies for treatment of industrial solid waste and recycle of useful resources.

Modeling of microbial kinetics and mass transfer in bioreactors simulating the natural attenuation of arsenic and iron in acid mine drainage

Natural attenuation in acid mine drainage (AMD) due to biological iron and arsenic oxidation offers a promising strategy to treat As-rich AMD in passive bioreactors. A reactive transport model is developed in order to identify the main controlling factors. It simulates batch and flow-through experiments that reproduce natural attenuation in a high-As AMD. The 2-D model couples second-order microbial kinetics (Fe- and As- oxidation) and geochemical reactions to hydrodynamic transport. Oxidation only occurrs in the biofilm with an oxygen transfer from the air through the water column. The model correctly simulates the Fe(II)-Fe(III) and As(III)-As(V) concentrations in the outlet waters and the precipitates, over hydraulic retention times from 30 min to 800 min. It confirms that the natural attenuation at 20 °C is driven by the fast Fe(II) oxidation and slow As(III) oxidation that favors arsenite trapping by schwertmannite over amorphous ferric arsenate (AFA) formation. The localization of iron oxidation in the biofilm limits the attenuation of arsenic and iron as the water column height increases. The change in the composition of the bacterial iron-oxidizer community of the biofilm at the lowest pH boundary seems to control the Fe(II) oxidation kinetic rate besides the bacterial concentration.

Dual-functional Sites for Selective Adsorption of Mercury and Arsenic ions in [SnS4]4-/MgFe-LDH from Wastewater

Heavy metals existed as multiple types in wastewater, enhanced the difficulty for disposal, and aroused huge environmental issues. High selective adsorption of the most hazardous heavy metals is one important method for water purification and resource utilization. In this study, we assembled the [SnS₄]^4- clusters and MgFe-based layered double hydroxide (LDH) to synthesize the [SnS₄]^4-/LDH composites, to capture mercury and arsenic ions simultaneously. The results indicated that such composite exhibited excellent mercury and arsenic removal performance with higher than 99% removal efficiency at a wide pH range. The uptake of mercury was ascribed to the [SnS₄]^4- clusters sites while the arsenic removal was mainly due to the existence of Fe site in LDH composite. The inserted [SnS₄]^4- clusters can enlarge the surface areas and create a hierarchical pore channel due to the increased interlayer spacing of LDH, which can enhance the adsorption capacity. The different adsorption mechanisms were also indicated by dynamic analysis. Pseudo-second-order kinetic model was more suitable for both Hg(II) and As(III) adsorption in the dual-heavy metal solution, and neither Langmuir isotherm model nor Freundlich isotherm model fitted the Hg(II) and As(III) adsorption in the mixed solution. The adsorption progress was influenced due to the coexistence of another heavy metal. Besides, mercury can be collected from the spent materials using a thermal-heating method. Such composite exhibits promising potential for mercury recycling.

As(III) removal from wastewater and direct stabilization by in-situ formation of Zn-Fe layered double hydroxides

In order to remove and stabilize As(III) simultaneously from wastewater, a novel and effective method based on the in-situ formation of As(III)-containing Zn-Fe layered double hydroxides (ZnFe-As-LDHs) was developed. The influence of pH, Zn/Fe, Fe/As and adding rate on the formation of ZnFe-As-LDHs were investigated. Under the optimal conditions, the concentration of As(III) decreased from 100 to 0.13 mg/L and As leaching concentration of the ultimate sludge was 1.87 mg/L, which could meet the arsenic leaching criteria (5 mg/L) regulated by US EPA. Compared with the "ex-situ" sludge obtained by As(III) adsorbed on the pre-formed ZnFe-LDHs, the As(III) removal efficiency increased by 21.6 % and the stability of the sludge increased by 94.2 % on the in-situ formation of LDHs, which mainly attributed to 55.06 % oxidation of As(III) and co-precipitation of As with Zn and Fe. Additionally, a possible in-situ formation pathway for ZnFe-As-LDHs was illustrated. At the beginning of the process, non-crystalline ferric arsenate formed and then transformed to amorphous ferrihydrite as precursors, followed by the formation of LDHs. This work demonstrated that co-precipitating As with Zn and Fe in the wastewater to in-situ form LDHs exhibited excellent potential for removal and direct stabilization of As(III).

A combined abiotic oxidation-precipitation process for rapid As removal from high-As(III)-Mn(II) acid mine drainage and low As-leaching solid products

A combination process of Fenton-like and catalytic Mn(II) oxidation via molecular oxygen-induced abio-oxidation of As(III)-Mn(II)-rich acid mine drainage (AMD) is developed to rapidly and efficiently remove As and obtain low As-leaching solids in this study. The effect of pH, temperature, oxygen flow rate and neutralization reagent on As removal was investigated. The results showed that pH was important to As removal efficiency, which achieved maximum in 0.25-2 h, but decreased from ∼100 % to ∼92.6 % with the increase of pH 5-9. pH, temperature and oxygen flow rate played key roles in As(III) oxidation. The increase of As(III) oxidized from 16.8 to 67.1% to 98.6-99.0 % occurred as increasing the pH 5-9, 25-95 °C and oxygen flow rate of 0-2.4 L min^-1. NaOH or Ca(OH)₂ as base was less important to As removal. The mechanism involved Fenton-like reaction between Fe(II) and O₂ for produced Fe(III) (oxy)hydroxide association with As(III + V) and Mn(II), catalytic Mn(II) oxidation for the formation of Mn(III, IV) oxides, and further As(III) oxidation by Mn(III, IV) oxides. As-bearing six-line ferrihydrite was the main solid product for low As-leaching fixation. pH 8, 95 °C and oxygen flow rate of 1.6 L min^-1 were optimal for As removal.

Dissolution, Stability and Solubility of Tooeleite [Fe6(AsO3)4(SO4)(OH)4·4H2O] at 25–45 °C and pH 2–12

Tooeleite [Fe6(AsO3)4(SO4)(OH)4·4H2O] was synthesized and characterized to investigate its possible immobilization for arsenic in acidic and alkali environments by a long-term dissolution of 330 d. The synthetic tooeleite was platy crystallites of ~1μm across, giving the lattice parameters of a = 6.4758 Å, b = 19.3737 Å and c = 8.9170 Å. For the tooeleite dissolution, the dissolved arsenic concentration showed the lowest value of 427.3~435.8 mg/L As at initial pH 12 (final pH 5.54). The constituents were dissolved preferentially in the sequence of SO42− > AsO33− > Fe3+ in the aqueous medium at initial pH 2–12. The dissolved iron, arsenite and sulfate existed mainly as FeSO4+/Fe3+, H3AsO30 and SO42− at initial pH 2, and in the form of Fe(OH)30/Fe(OH)2+, H3AsO30 and SO42− at initial pH 12, respectively. The tooeleite dissolution was characterized by the preferential releases of SO42− anions from solid surface into aqueous medium, which was fundamentally controlled by the Fe-O/OH bond breakages and the outer OH− group layers. From the data of the dissolution at 25 °C and initial pH 2 for 270–330 d, the ion-activity product [logˍIAP], which equaled the solubility product [Ksp] at the dissolution equilibrium, and the Gibbs free energy of formation [ΔGfo] were estimated as −200.28 ± 0.01 and −5180.54 ± 0.07 kJ/mol for the synthetic tooeleite, respectively.

Photoredox catalysis of As(III) by constructed CSnS bonds: Using biomass as templates leads to bio‑carbon/SnS2 nanosheets capable of the efficient photocatalytic conversion of As(III) and calcium arsenate capture

In this paper, a new interface design strategy for bio‑carbon/SnS₂ nanosheets equipped with CSnS bonds was proposed by using biomass as a template for the efficient photocatalytic conversion of As(III). The characterization results illustrated that the CSnS bonds could effectively prevent the agglomeration of SnS₂, expand the photoresponse range and improve the hydrophilicity of the bio‑carbon/SnS₂ composites while reducing their transfer resistance. Therefore, the construction of CSnS bonds could more efficiently promote the photoredox catalysis of As(III) to As(V) compared with pure SnS₂, attributing to the polarization and conjugation effects of the CSn bonds. Meanwhile, CaSO₄·nH₂O (n = 0, 0.5, 2) could rapidly convert AsO₄^3- into Ca₃(AsO₄)₂ precipitates to eliminate arsenic from the aqueous solution in one step. In particular, 7500 μg/L As(III) could not only be photocatalyzed into As(V) but also be converted to Ca₃(AsO₄)₂ to achieve the removal of arsenic within only 55 min in the coexistence of CaSO₄. In addition, the electron transfer path in the photocatalytic oxidation system on arsenite was proposed according to the Mott-Schottky (MS) plots of SnS₂ and graphitic carbon. The electron paramagnetic resonance (EPR) results implied that O₂^- and h⁺ were the main active substances in the photooxidation arsenic system and the effect of OH could be negligible. Thus, the possible mechanism of the photocatalytic conversion of As(III) was discussed.

Arsenic(III) biotransformation to tooeleite associated with the oxidation of Fe(II) via Acidithiobacillus ferrooxidans

Tooeleite (Fe₆(AsO₃)₄(SO₄)(OH)₄·4H₂O), the only known ferric arsenite sulfate bearing mineral, has great potential for arsenic remediation due to its structure favoring incorporation of As(III). Based on the natural attenuation of removing As(III) directly by the formation of tooeleite via microorganisms, an iron-oxidizing bacterial strain Acidithiobacillus ferrooxidans ATCC 23270 (At.ferrooxidans) was selected to facilitate the formation of tooeleite. The optimized condition for the biogenic tooeleite was obtained at pH of 2.0, 30 °C and an initial arsenic of 500 mg/L. The process of biological mineralization is accompanied by the removal of 95.4% arsenic. What's more, biosynthetic tooeleite crystallization via a three-stage process was revealed using a combination of liquid and solid analyses (ICP-OES, XRD, XPS, FT-IR, SEM, STEM, particle distribution). The three stages included Fe²⁺ oxidation by At.ferrooxidans, Fe³⁺ hydrolysis and an initial Fe-As amorphous precursors formation, and finally transforming to tooeleite crystal. Moreover, RT-qPCR was used to reveal the relationship between functional gene expression of At.ferrooxidans and the mineral formation. The results showed the biogenic tooeleite exerts significant control on the geochemistry of arsenic contaminated systems.

Leaching of Oxide Copper Ores by Addition of Weak Acid from Copper Smelters

In this study, weak acid in the curing and leaching stages of copper ore was incorporated, and we analyzed its effect on the dissolution of copper and final impurities. The weak acid corresponds to a wastewater effluent from sulfuric acid plants produced in the gas treatment of copper smelting processes. This effluent is basically water with high acidity (pH-value low at 1), which contains several toxic elements and some valuable metals. The results indicated that there is no positive or negative effect on the incorporation of the weak acid in the curing stage, while the case of the leaching stage is favored. Toxicity characteristic leaching procedure (TCLP) and synthetic precipitation leaching procedure (SPLP) toxicity tests were performed on the solid leaching residues, determining that they accomplish the stability ranges of the impurities (Pb, Cd, Hg, Cr, Ba, Se, As, and Ag).

Formation and stability of biogenic tooeleite during Fe(II) oxidation by Acidithiobacillus ferrooxidans

Tooeleite is the only known ferric arsenite sulfate mineral and has environmental significance for arsenic remediation. This study investigated the formation and stability of biogenic tooeleite in Fe(II)-As(III)-SO₄^2- environment using Acidithiobacillus ferrooxidans under the ambient conditions. The results show that bacteria facilitated the formation and crystallization of tooeleite owing to the microbial oxidation of Fe(II) to Fe(III). Due to the better growth of bacteria, the higher removal of As(III) by tooeleite formation was achieved under 8.978-10.806 g/L initial Fe(II) concentration and 2.00-3.00 initial pH, and the highest efficiency was ~95%. Fe(III) and As(III) precipitated simultaneously into two types of tooeleite. The relatively stable tooeleite is featured by the developed (020) crystal face and the bulk-like structure with thick flakes. This study yields a better understanding of biogenic tooeleite, and the importance of tooeleite formation in As(III)-rich environment for arsenic remediation.

Characterization of Fe5(AsO3)3Cl2(OH)4·5H2O, a new ferric arsenite hydroxychloride precipitated from FeCl3-As2O3-HCl solutions relevant to arsenic immobilization

Tooeleite (Fe₆(AsO₃)₄SO₄(OH)₄·4H₂O) is widely precipitated for direct As(III) removal from sulfate-rich industrial effluents. However, whether or not Fe(III)-As(III)-Cl(-I) precipitate is produced in chloridizing leaching media for As immobilization is almost unknown. This work founded the existence of ferric arsenite (hydroxy)chloride as a new mineral for As(III) removal. Its chemical composition and solid characterization were subsequently studied by using scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), infrared (FT-IR), Raman spectroscopy and thermogravimetric (TG) curve. The results showed the formation of a yellow precipitate after 3-days reaction of Fe(III)/As(III) with molar ratio ≈ 1.7 in chloride solution at pH 2.3 neutralized with NaOH. Compared with tooeleite, chemical analysis and solid characterization indicated that Cl(-I) replaces SO₄(-II) producing ferric arsenite hydroxychloride with formula Fe₅(AsO₃)₃Cl₂(OH)₄·5H₂O. This new plate shaped solid showed better crytallinity than tooeleite, although it has similar morphology and characteristic bands to tooeleite. The FT-IR bands at 628, 964 cm^-1 and the Raman bands at 448, 610, 961 cm^-1 were assigned to Fe-O or As(III)-O-Fe or As(III)-O bending/stretching vibration, indicating that both arsenite and chloride substituted for the position of sulfate for ferric arsenite hydroxychloride produced due to the lack of the SO₄^2- vibrations. Cl-(I) also contributed to increase As removal efficiency in aqueous sulfate media under acidic pH conditions via the probable formation of sulfate-chloride ferric arsenite.

Arsenic removal from highly-acidic wastewater with high arsenic content by copper-chloride synergistic reduction

A synergistic combination of chloride and copper powder was proposed as a new method to reductively remove arsenic from highly-acidic wastewater with high arsenic content (HAWA). As(III) was reduced to As(0) by copper powder in the presence of chloride and were effectively removed from HAWA. The procedure to remove arsenic was optimized as follows: initial H⁺ concentration of 5 mol L^-1, Cu-to-As molar ratio of 8, Cl-to-As molar ratio of 10, a reaction temperature of 60 °C, copper powder particle size of 68-24 μm, and a stirring speed of 300 r min^-1. Under these optimal conditions, the removal rate of arsenic was close to 100%. Kinetics results suggested that the arsenic removal process was controlled by both diffusion and chemical reactions with an apparent activation energy of 29.78 kJ mol^-1. The XRD results showed that the removed arsenic in the residue existed primarily in the form of AsCu₃ alloy.

Disposal of high-arsenic waste acid by the stepwise formation of gypsum and scorodite

The typical disposal of high-arsenic waste acid is at the expense of discharging a large quantity of hazardous solid waste, resulting in secondary pollution of arsenic. We propose a modified lime/ferric salt method for high-arsenic waste acid disposal by the stepwise formation of gypsum and scorodite at atmospheric pressure. The sulfuric acid in the high-arsenic waste acid is first removed by calcium carbonate generating gypsum, and then the arsenic in the solution is precipitated in form of scorodite. Gypsum with an arsenic leaching concentration below 5 mg L⁻¹ is obtained at a final pH of 0.5 in the calcium carbonate neutralization stage. In the second stage, the optimal conditions including a starting pH of 2.0, an Fe/As ratio of 1.5, a reaction temperature in the range of 80–90 °C and a reaction time equal to or longer than 8 hours provide an arsenic removal efficiency of 95.34% by the formation of well-crystallized and environmentally stable scorodite with grain sizes in a range of 1–5 μm. The proposed process offers a promising and facile solution for the low-cost disposal of high-arsenic waste acid in the nonferrous metallurgical industry, which enables an efficient arsenic removal with the good accessibility of chemical reagents and facilities.

Arsenic is removed from high arsenic waste acid via the stepwise formation of gypsum and scorodite.

Effects of acid and phosphate on arsenic solidification in a phosphogypsum-based cement backfill process

Phosphogypsum (PG) produced during phosphoric acid production contains significant amounts of arsenic and can potentially cause adverse environmental and health effects. Cement backfill technology is an effective management technique that is used to store PG to prevent such problems. The goal of this paper is to study the influencing factors and mechanism of arsenic stabilization in a PG-based cement backfill process. First, a leaching toxicity test was conducted, which showed that the arsenic concentration in PG batches ranged from 129.1 μg L⁻¹ to 407.1 μg L⁻¹, which were all far above the standard limit (10 μg L⁻¹) set by GB/T 14848-93. In addition, the arsenic content was higher in samples with larger PG particles. Secondly, hydrogen and phosphate ions were added to the backfill to investigate how they influenced arsenic solidification, and the results indicated that phosphate ions, rather than hydrogen ions, delayed the arsenic solidification process. This suggests that controlling the soluble phosphate in PG will help reduce arsenic pollution during backfilling. A toxicity leaching test was carried out after backfill samples were cured for 28 d. All arsenic concentrations were below the standard limit, indicating that the cement backfill technology ensured the long-term solidification and stabilization of arsenic.

Phosphogypsum (PG) produced during phosphoric acid production contains significant amounts of arsenic and can potentially cause adverse environmental and health effects.

Formation of high crystalline LDH sludge for removing Cu and Zn from wastewater by controlled double-jet precipitation

In order to improve the heavy metal wastewater treatment by avoiding formation of amorphous sludge phase, we develop a faster formation of high crystalline layered double hydroxide (LDH) sludge to remove Cu and Zn from wastewater by controlled double-jet precipitation (CDJP) without hydrothermal or heat aging post-treatment. A series of experimental procedures are conducted to determine the optimal parameters. Results show that the optimal adding rate, pH value, and stirring rate is 0.5 mL min^-1, 9.0, and 500 rpm, respectively. The CuZnAl-LDH phase sludge is formed in a well-crystallized hexagonal platelet, which assembled into a flower-like architecture. Comparative studies show that the formation of amorphous LDH sludge in conventional precipitation (CP) could be divided roughly into two stages-from the mixed copper hydroxide, zinc hydroxide, and scarbroite to the mixed low crystallinity CuAl-LDH and ZnAl-LDH. However, in CDJP method, the high crystalline LDH sludge evolved from a new four-step evolution process that is the formation of an amorphous (quasi-)multinary metastable ternary CuZnAl-LDH phase, followed by the indiffusion of cations and substitution of anions to fabricate crystalline LDH, the integrated LDH hexagonal platelets assembled into a flower-like architecture by the screw dislocation growth mechanism, the coarsening growth of each ternary LDH platelet, respectively. Thereinto, the formation of (quasi-)multinary metastable LDH phase instead of metal hydroxide in initial stage would be an obvious advantage of the CDJP method compared to CP method due to the former skipping the sequential precipitation.

Simultaneous adsorption of As(III), Cd(II) and Pb(II) by hybrid bio-nanocomposites of nano hydroxy ferric phosphate and hydroxy ferric sulfate particles coating on Aspergillus niger

To develop an efficient, convenient and cost-effective method to simultaneously remove pollution of As(III), Cd(II) and Pb(II) in wastewater, a strategy to fabricate hybrid bio-nanocomposites ((n-HFP + n-HFS)@An) of nano hydroxy ferric phosphate (n-HFP) and hydroxy ferric sulfate (n-HFS) particles coating on Aspergillus niger was applied. The scanning electron microscope and energy dispersive spectrum analyses showed that (n-HFP + n-HFS)@An composites had been successfully developed which well solved the self-agglomeration problem of the nano particles. Comparing to the bulk nanoparticles, the adsorption rates of the (n-HFP + n-HFS)@An composites for the three metals were promoted 145.34, 28.98 and 25.18% and reached 76.84, 73.62 and 94.31%, respectively. Similarly, the adsorption capacities for As(III), Cd(II), and Pb(II) were 162.00, 205.83 and 730.79 mg/g, respectively. Moreover, the pseudo-second-order kinetic model was more relevant to the adsorption on the three metals by (n-HFP + n-HFS)@An, and adsorbing As(III) was fitted to the Freundlich isotherm model, while the adsorption on Cd(II) or Pb(II) was related to the Langmuir isotherm model. In addition, the adsorption of Cd(II) and Pb(II) was associated with transformation of hydroxyl groups and precipitation with phosphate. As(III) was adsorbed through exchange between AsO₂^- and SO₄^2- in the (n-HFP + n-HFS)@An composites.

Structural substitution for SO4 group in tooeleite crystal by As(V) and As(III) oxoanions and the environmental implications

Two different SO₄-free tooeleite were prepared for the first time through structural substitution for SO₄ group by As(V) and As(III). As(III)-tooeleite and As(V)-tooeleite have similar crystalline structure to SO₄-tooeleite but incorporate different anions in the interlayer space. The removal of As can reach 94% by forming SO₄-free tooeleite crystals, and As leaching in TCLP tests can be much lower than that of SO₄-tooeleite. Therefore, SO₄-free tooeleite crystals are of great potential in As removal and immobilization. Moreover, our study indicates the different affinities of Fe(III) towards As(III), As(V) and SO₄, which can explain that a) the coordination structure of As(III)-tooeleite is much closer to the ideal crystal structure but easily affected by As(V) and SO₄ group; b) tooeleite mineral found in natural environments is commonly a SO₄-containing mineral and associated with scorodite due to the abundance of As(V) and SO₄ group.

Improved waste-sourced biocomposite for simultaneous removal of crude oil and heavy metals from synthetic and real oilfield-produced water

Oil- and gas-produced water (PW) which contains various pollutants is an enormous threat to the environment. In this study, a novel low-cost bio-adsorbent was prepared from shrimp shell and acid-activated montmorillonite. The results of FT-IR spectroscopy, energy dispersive X-ray (EDX) analysis, and SEM-EDX technique indicated that the chitosan-activated montmorillonite (CTS-A-MMT) was prepared successfully. The synthesized CTS-A-MMT was applied to remove simultaneously five cationic and anionic metal species and crude oil from synthetic and real oilfield PW. The adsorption data indicated that crude oil and all studied metals (except As) were adsorbed to CTS-A-MMT in a monolayer model (best fitted by Langmuir model), while As adsorption fits well with Freundlich model. Kinetic models' evaluation demonstrated that the adsorption kinetics of metals on CTS-A-MMT are initially controlled by the chemical reaction (film diffusion) followed by intra-particle diffusion. Application of the prepared CTS-A-MMT in real oilfield PW indicated removal efficiency of 65 to 93% for metals and 87% for crude oil in simultaneous removal experiments. Presence of additional ions in PW decreased the removal of studied metals and crude oil considerably; however, the concentration of the investigated pollutants in treated PW is less than the ocean discharge criteria. It is concluded that the prepared CTS-A-MMT composite is a low-cost and effective adsorbent for treating wastewater contaminated with crude oil and heavy metals (i.e., PW).

Hydrothermal Treatment of Arsenic Sulfide Residues from Arsenic-Bearing Acid Wastewater

Arsenic sulfide residue (ASR), a by-product from the treatment of arsenic-bearing acidic wastewater, is abundantly generated but not properly disposed of in China. The utilization of such high-content arsenic waste residue is limited by the market. The traditional methods of stabilization/solidification (S/S) by lime cement or iron salt have a large mass/volume addition, high dumping cost and secondary pollution risk. In this paper, hydrothermal technology was used to treat three kinds of ASRs obtained from different smelters to minimize waste. The leaching toxicity and chemical speciation of the generated products was also evaluated by TCLP and BCR analyses. It was found that the hydrothermal treatment could greatly reduce the volume and moisture content of the ASRs. TCLP tests showed that the leachability of arsenic and heavy metals significantly decreased after the treatment. According to the BCR analysis, most of the unstable As, Cd and Cr transformed into a residual fraction. Finally, XRD, SEM, Raman and XPS techniques were carried out to reveal the mechanism. As a result, hydrothermal treatment can efficiently achieve the dehydration, volume reduction and stabilization/solidification of ASRs.

Solid-liquid separation: an emerging issue in heavy metal wastewater treatment

Solid-liquid separation (SLS) plays a dominant role in various chemical industries. Nowadays, low efficiency of SLS also become a significant problem in heavy metal (HM) wastewater treatment, affecting the effluent quality (HM concentration and turbidity) and overall process economy. In this context, we summarize here the occurrence of solids in HM wastewater, as well as typical SLS operations used in HM wastewater treatment, including sedimentation, flotation, and centrifugation. More important, this article reviews the improvement of the SLS operations by some technologies, including coagulation, flocculation, ballasted method, seeding method, granular sludge strategy, and external field enhancement. It is noted that abiological granular sludge strategy and magnetic field enhancement often possess higher SLS efficiency (faster settling velocity or shorter separation time) than other methods. Hence, the two strategies stand out as promising tools for improving SLS in HM wastewater treatment, but further research is required regarding scalability, economy, and reliability.

Removal of Arsenic from Strongly Acidic Wastewater Using Phosphorus Pentasulfide As Precipitant: UV-Light Promoted Sulfuration Reaction and Particle Aggregation

Strongly acidic wastewater (H₂SO₄) with a high arsenic concentration is produced by many industries. The removal of arsenic by traditional sulfide (e.g., Na₂S, FeS) from strongly acidic wastewater introduces cations (Na⁺ and Fe²⁺) to the solution, which may prevent the recycle of acid. In this study, a new sulfuration agent, phosphorus pentasulfide (P₂S₅) was employed, and its feasibility in arsenic removal from strongly acidic wastewater was investigated. In the dark, As(III) was efficiently removed, but the removal rate of As(V) was rather slow, which was the crucial defect for this method. We found that this defect can be efficiently overcome by UV irradiation through accelerating the formation and transformation of an intermediate species, monothioarsenate (H₃AsO₃S) in the As(V) removal process. In addition, the hydrolysis of P₂S₅ was enhanced under UV irradiation, which resulted in the increase of the arsenic removal efficiencies. Besides, the aggregation of the formed particles was also promoted. Different from FeS and Na₂S, P₂S₅ introduces H₃PO₄ instead of cations to the solution, which can facilitate the recycle and reuse of arsenic and acid in strongly acidic wastewater.

Co-treatment of flotation waste, neutralization sludge, and arsenic-containing gypsum sludge from copper smelting: solidification/stabilization of arsenic and heavy metals with minimal cement clinker

Flotation waste of copper slag (FWCS), neutralization sludge (NS), and arsenic-containing gypsum sludge (GS), both of which are difficult to dispose of, are major solid wastes produced by the copper smelting. This study focused on the co-treatment of FWCS, NS, and GS for solidification/stabilization of arsenic and heavy metals with minimal cement clinker. Firstly, the preparation parameters of binder composed of FWCS, NS, and cement clinker were optimized to be FWCS dosage of 40%, NS dosage of 10%, cement clinker dosage of 50%, mill time of 1.5 h, and water-to-binder ratio of 0.25. On these conditions, the unconfined compressive strength (UCS) of the binder reached 43.24 MPa after hydration of 28 days. Then, the binder was used to solidify/stabilize the As-containing GS. When the mass ratio of binder-to-GS was 5:5, the UCS of matrix can reach 11.06 MPa after hydration of 28 days, meeting the required UCS level of MU10 brick in China. Moreover, arsenic and other heavy metals in FWCS, NS, and GS were effectively solidified or stabilized. The heavy metal concentrations in leachate were much lower than those in the limits of China standard leaching test (CSLT). Therefore, the matrices were potential to be used as bricks in some constructions. XRD analysis shows that the main hydration products of the matrix were portlandite and calcium silicate hydrate. These hydration products may play a significant role in the stabilization/solidification of arsenic and heavy metals.

Mechanisms of UV-Light Promoted Removal of As(V) by Sulfide from Strongly Acidic Wastewater

Strongly acidic wastewater with a high arsenic concentration is produced by a number of industries. The removal of As(V) (H₃AsO₄) by sulfide from strongly acidic wastewater remains a difficult issue. This study proposed a UV-assisted method to efficiently remove As(V) by sulfide, and the involved mechanisms were systematically investigated. In the dark, the low removal efficiency of As(V) by sulfide was attributed to the slow formation and transformation of an intermediate species, i.e., monothioarsenate (H₃AsO₃S), in the As(V) sulfuration reaction, which were the rate-controlling steps in this process. However, UV irradiation significantly promoted the removal efficiency of As(V) not only by promoting the formation of H₃AsO₃S through light-induced HS^• and •H radicals but also by enhancing the transformation of H₃AsO₃S through a charge-transfer process between S(-II) and As(V) in the H₃AsO₃S complex, leading to the reduction of As(V) to As(III) and the oxidation of S(-II) to S(0). The formed As(III) species immediately precipitated as As₂S₃ under excess S(-II). Kinetic modeling offered a quantitative explanation of the results and verified the proposed mechanisms. This study provides a theoretical foundation for the application of light-promoted As(V) sulfuration removal, which may facilitate the recycling and reuse of arsenic and acid in strongly acidic wastewater.