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

Efficient reductive recovery of arsenic from acidic wastewater by a UV/dithionite process

The removal of arsenic (As(III)) from acidic wastewater using neutralization or sulfide precipitation generates substantial arsenic-containing hazardous solid waste, posing significant environmental challenges. This study proposed an advanced ultraviolet (UV)/dithionite reduction method to recover As(III) in the form of valuable elemental arsenic (As(0)) from acidic wastewater, thereby avoiding hazardous waste production. The results showed that more than 99.9 % of As(III) was reduced to As(0) with the residual concentration of arsenic below 25.0 μg L-1 within several minutes when the dithionite/As(III) molar ratio exceeded 1.5:1 and the pH was below 4.0. The content of As(0) in precipitate reached 99.70 wt%, achieving the purity requirements for commercial As(0) products. Mechanistic investigations revealed that SO2·‒ and H· radicals generated by dithionite photolysis under UV irradiation are responsible for reducing As(III) to As(0). Dissolved O2, Fe(III), Fe(II), Mn(II), dissolved organic matter (DOM), and turbidity slightly inhibited As(III) reduction via free radicals scavenging or light blocking effect, whereas other coexisting ions, such as Mg(II), Zn(II), Cd(II), Ni(II), F(-I), and Cl(-I), had limited influence on As(III) reduction. Moreover, the cost of treating real arsenic-containing (250.3 mg L-1) acidic wastewater was estimated to be as low as $0.668 m-3, demonstrating the practical applicability of this method. This work provides a novel method for the reductive recovery of As(III) from acidic wastewater.

Ice-templated synthesis of tungsten oxide nanosheets and their application in arsenite oxidation

Tungsten oxide (WO₃) nanosheets were prepared as catalysts to activate hydrogen peroxide (H₂O₂) in arsenite (As(III)) oxidation. Ice particles were employed as templates to synthesize the WO₃ nanosheets, enabling easy template removal via melting. Transmission electron microscopy and atomic force microscopy revealed that the obtained WO₃ nanosheets were plate-like, with lateral sizes ranging from dozens of nanometers to hundreds of nanometers and thicknesses of <10 nm. Compared to that of the WO₃ nanoparticle/H₂O₂ system, a higher efficiency of As(III) oxidation was observed in the WO₃ nanosheet/H₂O₂ system. Electron spin resonance spectroscopy, radical quenching studies, and As(III) oxidation experiments under anoxic conditions suggested that the hydroperoxyl radical (HO₂^●) acted as the primary oxidant. The WO₃ nanosheets possessed numerous surface hydroxyl groups and electrophilic metal centers, enhancing the production of HO₂^● via H₂O₂ activation. Various anions commonly present in As(III)-contaminated water exhibited little effect on As(III) oxidation in the WO₃ nanosheet/H₂O₂ system. The high oxidation efficiency was maintained by adding H₂O₂ when it was depleted, suggesting that the catalytic activity of the WO₃ nanosheets did not deteriorate after multiple catalytic cycles.

The arsenic species in the sulfidic environments: Determination, transformation, and geochemical implications

The species and fate of arsenic (As) are closely related to sulfide (S^-II) in the anaerobic and sulfidic environment. In this work, the mechanisms and kinetics of arsenate (As^V) reduction by S^-II at different pHs, S^-II/As^V molar ratios, and initial As^V concentrations in the absence (or presence) of Al-hydroxide were studied, where the concentrations of various kinds of As species, namely As^V, arsenite (As^III), and thioarsenics (ThioAs) were qualitatively and quantitatively determined by liquid chromatography with atomic fluorescence spectrophotometry. The results showed that under acidic or neutral conditions, ThioAs may act as intermediate(s), where amorphous As₂S₃ precipitate was observed at pH 5 in high S^-II condition. By comparison, at pH 9, As^V was probably directly reduced to As^III with polysulfide as the byproduct. The reaction rate was faster at mildly acidic pH than that of neutral or alkaline pH, as well as in the presence of Al-hydroxide. The findings may give further insights about the role of ThioAs in the biogeochemical cycle of As.

Clean and effective removal of Cl(-I) from strongly acidic wastewater by PbO2

Recycling strongly acidic wastewater as diluted H₂SO₄ after contaminants contained being removed was previously proposed, however, Cl(-I), a kind of contaminant contained in strongly acidic wastewater, is difficult to remove, which severely degrades the quality of recycled H₂SO₄. In this study, the removal of Cl(-I) using PbO₂ was investigated and the involved mechanisms were explored. The removal efficiency of Cl(-I) reached 93.38% at 50℃ when PbO₂/Cl(-I) mole ratio reached 2:1. The identification of reaction products shows that Cl(-I) was oxidized to Cl₂, and PbO₂ was reduced to PbSO₄. Cl₂ was absorbed by NaOH to form NaClO, which was used for the regeneration of PbO₂ from the generated PbSO₄. Cl(-I) was removed through two pathways, i.e., surface oxidation and •OH radical oxidation. •OH generated by the reaction of PbO₂ and OH^- plays an important role in Cl(-I) removal. The regenerated PbO₂ had excellent performance to remove Cl(-I) after six-time regeneration. This study provided an in-depth understanding on the effective removal of Cl(-I) by the oxidation method.

Multi-roles of SO2 to enhance the removal of arsenic from wastewater in sulfidation processes

Sulfidation has been an efficient method for arsenic (As) removal from acid wastewater, yet it is inefficient under neutral and weak acid conditions. The higher pH values resulted in the formation of the unstable As-S precipitates, especially employing Na₂S as the vulcanizing agent as it can increase the pH value dramatically. Here, we found that SO₂ exhibited excellent multi-roles in As removal when applying H₂S-sulfidation method. The acidification effect of SO₂ lead to the decreasing of pH values, guaranteed the stable As-S precipitates formation. Through the SO₂ pre-treatment method, the results indicated that the pH values decreased from 7 to 2.8, with the increased H₂S utilization efficiencies for As(III) removal from 20.9% to 92.0%. Moreover, SO₂ post-treatment not only increased the As(III) removal efficiency, but also eliminated the excessive sulfides in solution. The reaction mechanism analysis indicated that the liquid comproportionation reaction between SO₂ and excessive sulfides plays a vital role. The generated nascent sulfur (N-S⁰) can adsorb arsenic species and promote the agglomeration of As(III)-S precipitates. Furthermore, the SO₂ and H₂S co-treatment exhibited excellent As(V) removal performance. This study provides a new alternative method to improve the H₂S-sulfidation process with SO₂ for As removal from wastewater.

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.

Adsorption and photocatalysis removal of arsenite, arsenate, and hexavalent chromium in water by the carbonized composite of manganese-crosslinked sodium alginate

The preparation of easily synthesized and cheap composite materials for the efficient removal of toxic oxoanions still remains challenging in sewage treatment. Herein, a new carbonized manganese-crosslinked sodium alginate (Mn/SA-C) was fabricated for the removal of arsenite (As(III)), arsenate (As(V)) and hexavalent chromium (Cr(VI)) in water. The results indicated that the Mn/SA-C pretreated with MnSO₄ solution (Mn/SA-C-S) exhibited a rapid adsorption toward As(III) and As(V) with the removal efficiency of >98% within 10 min, and had a high adsorption capacity toward As(III), As(V), and Cr(VI) with the maximum value of 189.29, 193.29, and 104.50 mg/g based on the Langmuir model, respectively. The removal efficiency of As(III), As(V), and Cr(VI) could be further significantly enhanced by coupling a photocatalytic process. For example, the time in which >98% of Cr(VI) (10 mg/L) was removed dramatically shortened from 360 min (adsorption) to 45 min (adsorption-photocatalysis), and the removal efficiency of As(III) increased by ∼10% within initial 5 min. This was primarily attributed to the Mn-catalyzed production of the photocatalytic excitons for Cr(VI) reduction, and the superoxide (•O₂^-) and hydroxyl (•OH) radicals for As(III) oxidation. The adsorption removal of arsenic (As) was primarily ascribed to surface complexation with MnO and precipitation by MnS₂, and oxidative adsorption because of Mn valence cycle. The removal mechanisms of Cr(VI) mainly contained reduction by MnO and MnS₂, complexation with MnO and carboxyl/hydroxyl groups as well as Cr(OH)₃ precipitation. Our research provides a promising Mn/SA-C-S material for rapid and efficient removal of As(III), As(V), and Cr(VI) in contaminated water through an adsorption-photocatalysis synergistic strategy.

Removal of Arsenate From Groundwater by Cathode of Bioelectrochemical System Through Microbial Electrosorption, Reduction, and Sulfuration

Arsenate [As(V)] is a toxic metalloid and has been observed at high concentrations in groundwater globally. In this study, a bioelectrochemical system (BES) was used to efficiently remove As(V) from groundwater, and the mechanisms involved were systematically investigated. Our results showed that As(V) can be efficiently removed in the BES cathode chamber. When a constant cell current of 30 mA (I_cell, volume current density = 66.7 A/m³) was applied, 90 ± 3% of total As was removed at neutral pH (7.20–7.50). However, when I_cell was absent, the total As in the effluent, mainly As(V), had increased approximately 2–3 times of the As(V) in influent. In the abiotic control reactor, under the same condition, no significant total As or As(V) removal was observed. These results suggest that As(V) removal was mainly ascribed to microbial electrosorption of As(V) in sludge. Moreover, part of As(V) was bioelectrochemically reduced to As(III), and sulfate was also reduced to sulfides [S(–II)] in sludge. The XANES results revealed that the produced As(III) reacted with S(–II) to form As₂S₃, and the residual As(III) was microbially electroadsorbed in sludge. This BES-based technology requires no organic or chemical additive and has a high As(V) removal efficiency, making it an environment-friendly technique for the remediation of As-contaminated groundwater.

Recovery of Re(VII) from strongly acidic wastewater using sulphide: Acceleration by UV irradiation and the underlying mechanism

Strongly acidic wastewater generated from the molybdenum and copper smelting process is of great value for recycling sulfuric acid and valuable metals, such as rhenium (Re). Herein, a high Re(VII) (HReO₄) recovery efficiency of 99% within 35 min from strongly acidic wastewater was successfully achieved by using sulphide coupled with ultraviolet (UV) light, and soluble Re(VII) precipitated as Re₂S₇ in this process. Mechanistic experiments showed that the intermediate Re-S species (i.e., HReO₃S) was the dominant limitation responsible for Re(VII) precipitation in the dark, and UV irradiation dramatically accelerated the generation and conversion of HReO₃S by inducing the formation of HS• and H•. The H• produced from the photodissociation of H₂S promoted HReO₄ transformation to H₂ReO₄•, which rapidly reacted with HS• to produce HReO₃S, accelerating the conversion of HReO₄. The radical-induced acceleration can also take place during the HReO₃S conversion by slowly introducing H₂S into the strongly acidic wastewater to continuously produce H• and HS•. This work offers an insight into the improvement of Re(VII) recovery by UV light, which can be potentially applied into resource recovery from strongly acidic wastewater.

Leaching characteristics and stability assessment of sequestered arsenic in flue dust based glass

Arsenic (As), a toxicant, present in flue dust, tailings, and mine drainages generated from mineral processing and smelting processes represents high environmental risk due to its high mobility. Around 42-50% As is found in flue dust in the form of As₂O₃. The vitrification of As results in the formation of stable inert glass material and supposed to reduce the risk of As release to the environment. In this study, a glass material produced by vitrification of As bearing flue dust via DST GlassLock™ Process was received from Dundee Sustainable Technologies, Canada and was subjected for As stability assessment using United States Environmental Protection Agency (EPA) leaching methods-1311,1312,1313,1314,1315 and 1316. The released arsenic concentration was found to be less than the recommended TCLP hazardous waste limit for arsenic i.e., 5 mg/L in most of the test conditions. The experimental data were analyzed using LeachXS Lite™, a data management software that showed the goodness of the DST GlassLock™ Process for As stabilization and safe landfill deposition of the resulting product.

Removal of Ni(II) from strongly acidic wastewater by chelating precipitation and recovery of NiO from the precipitates

Strongly acidic wastewater produced in nonferrous metal smelting industries often contains high concentrations of Ni(II), which is a valuable metal. In this study, the precipitation of Ni(II) from strongly acidic wastewater using sodium dimethyldithiocarbamate (DDTC) as the precipitant was evaluated. The effects of various factors on precipitation were investigated, and the precipitation mechanism was also identified. Finally, the nickel in the precipitates was recovered following a pyrometallurgical method. The results show that, under optimised conditions (DDTC:Ni(II) molar ratio = 4:1; temperature = 25 °C), the Ni(II) removal efficiency reached 99.3% after 10 min. In strongly acidic wastewater, the dithiocarbamate group of DDTC can react with Ni(II) to form DDTCNi precipitates. Further recovery experiments revealed that high-purity NiO can be obtained by the calcination of DDTCNi precipitates, with the nickel recovery efficiency reaching 98.2%. The gas released during the calcination process was composed of NO₂, CS₂, H₂O, CO₂, and SO₂. These results provide a basis for an effective Ni(II) recovery method from strongly acidic wastewater.

Production of H2S with a Novel Short-Process for the Removal of Heavy Metals in Acidic Effluents from Smelting Flue-Gas Scrubbing Systems

Direct sulfidation using a high concentration of H₂S (HC-H₂S) has shown potential for heavy metals removal in various acidic effluents. However, the lack of a smooth method for producing HC-H₂S is a critical challenge. Herein, a novel short-process hydrolysis method was developed for the on-site production of HC-H₂S. Near-perfect 100% efficiency and selectivity were obtained via CS₂ hydrolysis over the ZrO₂-based catalyst. Meanwhile, no apparent residual sulfur/sulfate poisoning was detected, which guaranteed long-term operation. The coexistence of CO₂ in the products had a negligible effect on the complete hydrolysis of CS₂. H₂S production followed a sequential hydrolysis pathway, with the reactions for CS₂ adsorption and dissociation being the rate-determining steps. The energy balance indicated that HC-H₂S production was a mildly exothermic reaction, and the heat energy could be maintained at self-balance with approximately 80% heat recovery. The batch sulfidation efficiencies for As(III), Hg(II), Pb(II), and Cd(II) removal were over 99.9%, following the solubilities (K_sp) of the corresponding metal sulfides. CO₂ in the mixed gas produced by CS₂ hydrolysis did not affect heavy metals sulfidation due to the presence of abundant H⁺. Finally, a pilot-scale experiment successfully demonstrated the practical effects. Therefore, this novel on-site HC-H₂S production method adequately achieved heavy metals removal requirements in acidic effluents.

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.

Removal of Cl(-I) from strongly acidic wastewater containing Cu(II) by complexation-precipitation using thiourea: Efficiency enhancement by ascorbic acid

Strongly acidic wastewater produced by copper smelting industries contains high concentrations of Cl(-I), Cu(II) and H₂SO₄. The common method for the treatment of this type of wastewater is neutralization, which produces large amounts of solid waste. To avoid the production of solid waste, it was proposed to selectively remove contaminants and then recycle the wastewater as diluted sulfuric acid. This study proposed a new complexation-precipitation method to effectively remove Cl(-I) using thiourea (TU) under the promotion of ascorbic acid (AC). The Cl(-I) removal efficiency was optimized, important effecting factors were investigated and the mechanisms of the AC-improved removal of Cl(-I) were studied. The results showed that, Cl(-I) removal efficiency reached 87.4 % under a TU/AC/Cl(-I) mole ratio of 1:3:1 and the residual Cl(-I) concentration was lowered from 1000 mg/L to 126.4 mg/L. The mechanism investigation showed that, AC first reduces Cu(II) to Cu(I), then, the produced Cu(I) is quickly complexed by TU to form the [Cu(I)_x(TU)_y]^x+; finally, [Cu(I)_x(TU)_y]^x+ precipitates with Cl(-I) in the form of [Cu(I)_x(TU)_y]Cl_x. This study provided a theoretical foundation of complexation-precipitation of Cl(-I) under strongly conditions and developed an effective method for removal of Cl(-I) from strongly acid waster.

Specific H2S Release from Thiosulfate Promoted by UV Irradiation for Removal of Arsenic and Heavy Metals from Strongly Acidic Wastewater

The removal of arsenic and heavy metals (HMs) from strongly acidic wastewater by hydrogen sulfide (H₂S) is an efficient method. However, traditional sulfuration reagents (Na₂S, FeS, CaS, etc.) rapidly release H₂S under acidic conditions via spontaneous hydrolysis, leading to serious H₂S pollution. Herein, a H₂S release process employing thiosulfate as a sulfuration reagent was proposed to eliminate H₂S pollution. We found that thiosulfate can release H₂S with specificity both in the dark and under ultraviolet (UV) irradiation under acidic conditions. In the absence of arsenic/HMs, H₂S is not released because the formed H₂S is consumed by a thiosulfate decomposition product, sulfite, or by its photolysis. In the presence of arsenic/HMs, H₂S is released because the formed H₂S immediately reacts with arsenic/HMs to generate sulfide precipitates rather than being consumed. The efficiency of transforming thiosulfate to H₂S under UV irradiation is 2.5-fold the efficiency in the dark, because UV irradiation promotes the transformation of "effective sulfur" in thiosulfate molecules to form H₂S through the transformation of HS· and S₂O₃^• - radicals. Moreover, more than 99.9% of arsenic/HMs were removed from strongly acidic wastewater without producing H₂S pollution under UV irradiation. This thiosulfate-based H₂S-specific release process solves the problem of H₂S pollution under acidic conditions.

Design and analysis of continuous-flow reactors for copper sulfide precipitation process by a computational method

Sulfide precipitation method has been widely applied in heavy metal-polluted wastewater treatment, due to the low solubility of most metal sulfide precipitates. Nevertheless, the relevant hydrodynamics studies on the metal sulfide precipitation process are rarely found in the literature. In this study, three continuous-flow sulfide precipitation reactors (CFSPRs) were designed and evaluated by a computational method. To characterize the process efficiency of copper sulfide precipitation in different reactors, fluid velocity field, species concentration distribution, and reaction rate distribution maps were acquired as simulation results. A two-factor designed set of boundary conditions was used to determine their effects on processing efficiency. The model results indicate that the inflow rate and reactor layout have significant effects on the copper sulfide precipitation process. The layout of reactor no. 3 and the inflow rate of 0.75 m/s prove to have higher treatment efficiencies than those at other conditions. Possible explanations for the simulation results were proposed. The model data of effluent concentration were compared and statistically analyzed with the measured concentrations of copper ion and sulfur ion in the outlet stream, and the results demonstrate a strong correlation between them, which suggests the model is reasonably accurate.

Release Behaviors of Arsenic and Heavy Metals from Arsenic Sulfide Sludge during Simulated Storage

Non-ferrous metal smelting enterprises produce hundreds of thousands of tons of arsenic sulfide sludge (ASS) each year in China. Most of the ASS are stored at the companies without enough preventive measures. During the storage and natural drying process, arsenic sulfide is easily oxidized, thereby causing secondary pollution and increasing environmental risks. In this paper, experiments of simulated storage were used to study the release characteristics of heavy metals. During the simulated storage, the release concentrations of As, Pb, and Cd increased rapidly at first and then slowly. Although the total amount of arsenic released was the largest, the release ratio was in the order of Cd > Pb > As. The effects of different atmospheres and conditions on the release of arsenic and heavy metals were explored. The more the H2SO4 in the sludge, the higher the release concentration, and the addition of an appropriate amount of Ca(OH)2 is beneficial for reducing the release of heavy metals. Finally, SEM, XRD and TG-DTG techniques were carried out to confirm that the release of heavy metals was caused by the oxidation process resulting from the residual H2SO4 in the ASS and the air.

Removal of Chloride Ions from Strongly Acidic Wastewater Using Cu(0)/Cu(II): Efficiency Enhancement by UV Irradiation and the Mechanism for Chloride Ions Removal

Strongly acidic wastewater, which is usually generated from nonferrous metal smelting industries, has the ability to be recycled as sulfuric acid. Before this wastewater is recycled, the removal of chloride ions is necessary to improve the quality of the recycled sulfuric acid. At present, the widely used method to remove chloride ions from acidic wastewater in the form of CuCl precipitate has several disadvantages, including low removal efficiency, high temperature, long treatment time, and high dosage of Cu(II). This study proposed an improved new method of removing Cl(-I) using Cu(0)/Cu(II) under UV irradiation, and the mechanism was investigated. The Cl(-I) concentration was lowered to below 50 mg/L at a Cu(II) dosage of 1200 mg/L. Under UV irradiation, ligand-to-metal charge transfer takes place, thereby resulting in the formation of Cl^•. Next, CuCl precipitates form through the reaction between Cu(0) and Cl^• and produce h⁺/^•OH under UV irradiation, which can oxidize Cl(-I) to Cl^•. Simultaneously, Cl₂ gas also forms directly from Cl^•. This study offered a theoretical foundation for the application of UV irradiation for the enhanced removal of chloride ions from strongly acidic wastewater.

UV-Light-Induced Aggregation of Arsenic and Metal Sulfide Particles in Acidic Wastewater: The Role of Free Radicals

The removal of arsenic and metals by sulfide (S(-II)) from acidic wastewater is an efficient method. However, the small sulfide particles formed in such a process make solid-liquid separation difficult, which greatly hinders its application. This study investigated the aggregation behavior of different sulfide particles (As₂S₃, CuS and CdS) under ultraviolet (UV) irradiation. In the dark, the aggregation rate of the arsenic sulfide (As₂S₃) particles was extremely slow. However, under UV irradiation, the growth of the As₂S₃ particles was significantly enhanced. A possible mechanism of UV-light-induced aggregation of As₂S₃ particles was proposed. The HS· and ·OH radicals formed by a series of photochemical reactions can efficiently attack the S(-II) in the As₂S₃ particle, leading to the formation of an intermediate species, [As₂S₂-S·]⁺. Then, two [As₂S₂-S·]⁺ species combine to form [As₂S₂-S-S-S₂As₂]²⁺. The formation of [As₂S₂-S-S-S₂As₂]²⁺ results in the attenuation of the electronegativity and the rapid aggregation of the sulfide particles. In addition, the small S⁰ particles generated in irradiated As₂S₃ system can efficiently coalesce into As₂S₃ particles. The CuS and CdS particles should have similar aggregation mechanisms. This study proposed a potential method for sulfide particle aggregation and provided a theoretical foundation for the development and application of UV-light-induced sulfide particle aggregation technology.

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.

Monothioarsenate Transformation Kinetics Determining Arsenic Sequestration by Sulfhydryl Groups of Peat

In peatlands, arsenite was reported to be effectively sequestered by sulfhydryl groups of natural organic matter. To which extent porewater arsenite can react with reduced sulfur to form thioarsenates and how this affects arsenic sequestration in peatlands is unknown. Here, we show that, in the naturally arsenic-enriched peatland Gola di Lago, Switzerland, up to 93% of all arsenic species in surface and porewaters were thioarsenates. The dominant species, monothioarsenate, likely formed from arsenite and zerovalent sulfur-containing species. Laboratory incubations with sulfide-reacted, purified model peat showed increasing total arsenic sorption with decreasing pH from 8.5 to 4.5 for both, monothioarsenate and arsenite. However, X-ray absorption spectroscopy revealed no binding of monothioarsenate via sulfhydryl groups. The sorption observed at pH 4.5 was acid-catalyzed dissociation of monothioarsenate, forming arsenite. The lower the pH and the more sulfhydryl sites, the more arsenite sorbed which in turn shifted equilibrium toward further dissociation of monothioarsenate. At pH 8.5, monothioarsenate was stable over 41 days. In conclusion, arsenic can be effectively sequestered by sulfhydryl groups in anoxic, slightly acidic environments where arsenite is the only arsenic species. At neutral to slightly alkaline pH, monothioarsenate can form and its slow transformation into arsenite and low affinity to sulfhydryl groups suggest that this species is mobile in such environments.

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.