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

Growth behavior of heavy metal sulfide particles: A comparison between gas-liquid and liquid-liquid sulfidation

Sulfide precipitation is an effective method for treating acidic heavy metal wastewater. However, the process often generates tiny particles with poor settling performance. The factors and mechanisms influencing particle size and settling performance remain unclear. In this study, the growth behavior of CuS particles generated by two sulfide precipitation methods, gas-liquid and liquid-liquid sulfidation, was investigated. The effects of acidity, sulfur-to-copper molar ratio, and temperature on particle size were analyzed. The results showed that increasing the temperature had an adverse effect on CuS particle growth. Additionally, we found that acidity and sulfur-to-copper molar ratio had a more significant impact on particle growth in the liquid-liquid sulfidation system than in the gas-liquid sulfidation system. Based on supersaturation calculations and XPS analysis, it is found that particle growth in gas-liquid sulfidation systems is mainly influenced by supersaturation, while particle growth in liquid-liquid sulfidation systems is mainly affected by surface charge. This study provides valuable insights into the factors that influence particle growth in sulfide precipitation and can inform the development of strategies to improve the effective precipitation of sulfide nanoparticles in 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.

Sulfidation-reoxidation enhances heavy metal immobilization by vivianite

Iron minerals in nature are pivotal hosts for heavy metals, significantly influencing their geochemical cycling and eventual fate. It is generally accepted that, vivianite, a prevalent iron phosphate mineral in aquatic and terrestrial environments, exhibits a limited capacity for adsorbing cationic heavy metals. However, our study unveils a remarkable phenomenon that the synergistic interaction between sulfide (S2-) and vivianite triggers an unexpected sulfidation-reoxidation process, enhancing the immobilization of heavy metals such as cadmium (Cd), copper (Cu), and zinc (Zn). For instance, the combination of vivianite and S2- boosted the removal of Cd2+ from the aqueous phase under anaerobic conditions, and ensured the retention of Cd stabilized in the solid phase when shifted to aerobic conditions. It is intriguing to note that no discrete FeS formation was detected in the sulfidation phase, and the primary crystal structure of vivianite largely retained its integrity throughout the whole process. Detailed molecular-level investigations indicate that sulfidation predominantly targets the Fe(II) sites at the corners of the PO4 tetrahedron in vivianite. With the transition to aerobic conditions, the exothermic oxidation of CdS and the S sites in vivianite initiates, rendering it thermodynamically favorable for Cd to form multidentate coordination structures, predominantly through the Cd-O-P and Cd-O-Fe bonds. This mechanism elucidates how Cd is incorporated into the vivianite structure, highlighting a novel pathway for heavy metal immobilization via the sulfidation-reoxidation dynamics in iron phosphate minerals.

Application of dirty-acid wastewater treatment technology in non-ferrous metal smelting industry: Retrospect and prospect

Dirty-acid wastewater (DW) originating from the non-ferrous metal smelting industry is characterized by a high concentration of H2SO4 and As. During the chemical precipitation treatment, a significant volume of arsenic-containing slag is generated, leading to elevated treatment expenses. The imperative to address DW with methods that are cost-effective, highly efficient, and safe is underscored. This paper conducts a comprehensive analysis of three typical methods to DW treatment, encompassing technical principles, industrial application flow charts, research advancements, arsenic residual treatment, and economic considerations. Notably, the sulfide method emerges as a focal point due to its minimal production of arsenic residue and the associated lowest overall treatment costs. Moreover, in response to increasingly stringent environmental protection policies targeting new pollutants and carbon emissions reduction, the paper explores the evolving trends in DW treatment. These trends encompass rare metal and sulfuric acid recycling, cost-effective H2S production methods, and strategies for reducing, safely disposing of, and harnessing resources from arsenic residue.

Selective separation of metals from wastewater using sulfide precipitation: A critical review in agents, operational factors and particle aggregation

Extensive research has been conducted on the separation and recovery of heavy metals from wastewater through the targeted precipitation of metal sulfides. It is necessary to integrate various factors to establish the internal correlation between sulfide precipitation and selective separation. This study provides a comprehensive review of the selective precipitation of metal sulfides, considering sulfur source types, operating factors, and particle aggregation. The controllable release of H2S from insoluble metal sulfides has garnered research interest due to its potential for development. The pH value and sulfide ion supersaturation are identified as key operational factors influencing selectivity precipitation. Effective adjustment of sulfide concentration and feeding rate can reduce local supersaturation and improve separation accuracy. The particle surface potential and hydrophilic/hydrophobic properties are crucial factors affecting particle aggregation, and methods to enhance particle settling and filtration performance are summarized. The regulation of pH and sulfur ion saturation also controls the zeta potential and hydrophilic/hydrophobic properties on the particles surface, thereby affecting particle aggregation. Insoluble sulfides can decrease sulfur ion supersaturation and improve separation accuracy, but they can also promote particle nucleation and growth by acting as growth platforms and reducing energy barriers. The combined influence of sulfur source and regulation factors is vital for achieving precise separation of metal ions and particle aggregation. Finally, suggestions and prospects are proposed for the development of agents, kinetic optimization, and product utilization to promote the industrial application of selective precipitation of metal sulfides in a better, safer, and more efficient way.

Componential and molecular-weight-dependent effects of natural organic matter on the colloidal behavior, transformation, and toxicity of MoS2 nanoflakes

The potential widespread applications in water processing have rendered the necessity for investigations of the fate and hazard of molybdenum disulfide (MoS2) nanosheets. Herein, it was found that humic acid (HA) had better performances toward stabilizing pure 2H phase MoS2 and chemical-exfoliated MoS2 (ce-MoS2) in electrolyte solutions than fulvic acid (FA), and molecular weight (MW)-dependent manners were disclosed due to steric repulsions. Compared with darkness, the extent to which the aggregation and sedimentation of ce-MoS2 facilitated by visible light irradiation was greater in the presence of HA and FA fractions, likely due to the introduction of stronger plasmonic dipole-dipole interaction and Van der Waals attraction forces. HA-triggered structural disintegration of nanosheets was performed after irradiation and it was observed to be more significant with the increase in MWs, whereas the MW-dependent dissolution of MoS2 caused by FA was much quicker than that by HA owing to the higher generation of singlet oxygen. Moreover, FA lowered the bioavailability of MoS2 and relieved its toxicity to zebrafish more effectively than HA. Our findings boost the insights into the effects of organic molecules on the fates and hazards of MoS2, providing guidance for the MoS2-based nanotechnological development on environment.

Ozonation/UV irradiation of dispersed Ag/AgI nanoparticles in water resources: stability and aggregation

Proliferation of nanoparticles (NPs) as aqueous pollutants is a matter of growing concern today. The aggregation kinetics of colloidal bare silver (Ag, 20.5 nm) and silver iodide (AgI, 15.3 nm) NPs were investigated during ozone/ultraviolet (O₃/UV) oxidation. Dynamic light scattering was applied to monitor the aggregation of NPs, and the z-average of treated samples was considered aggregate diameter. The effect of temperature, pH, and initial concentration of NPs was investigated on the aggregation rate constant and stability ratio. At a short oxidation period of approximately 1 min, the lower stability ratio was achieved for Ag NPs (< 50) than AgI NPs (> 100). Under acidic conditions, the negative surface charge of both NPs was neutralized that resulted in faster aggregation. In contrast, the impact of temperature and initial concentration of NPs on the aggregation rate was different for both NPs, which was due to the type of O₃/UV interaction with the surface of NPs and the thickness of the electrical double layer surrounding the NPs. The aggregation behavior of Ag NPs obeyed diffusion-limited regime, while an intermediate regime between diffusion- and reaction-limited was observed for AgI NP aggregation. The resulting aggregate morphologies showed that the clusters were ramified for Ag and compressed for AgI NPs. Applying the O₃/UV oxidation process for water treatment purposes leads to a significant reduction in aggregation time for inherently unstable Ag and stable AgI toxic NPs from several hours or days to several minutes.

H2S release rate strongly affects particle size and settling performance of metal sulfides in acidic wastewater: The role of homogeneous and heterogeneous nucleation

Sulfide precipitation is an extensively used method to precipitate metal and arsenic from acidic wastewater, whereas the tiny and negatively-charged metal sulfides with poor settling performance are generated. The factors and mechanisms that influence particle size and settling performance remain unclear. Herein, the effects of sulfuration factors, e.g., reagent dosage, acidity and H₂S release rate on the particle size and settling performance of metal sulfides were investigated, and involved mechanisms were systematically revealed. The results showed that the reagent dosage and acidity had a limited effect on particle size and settling performance while the H₂S release rate played a critical role. Under homogeneous conditions, the decrease in H₂S release rate, which can reduce the initial supersaturation and supply the sustainable supersaturation, increased the particle size of metal sulfides generated using Na₂S solution. Under heterogeneous conditions, the decrease in H₂S release rate further increased the particle size of metal sulfides generated using low-solubility CaS/FeS and further improved settling performance, in which heterogeneous nucleation played a crucial role besides supersaturation. The developed dissolution-diffusion-growth model qualitatively explained the negative relationship between H₂S release rate and particle growth. This work provides implications for improving the settling performance of metal sulfides in acidic 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.

Synergistic Effect of Metal Cations and Visible Light on 2D MoS2 Nanosheet Aggregation

Aggregation significantly influences the transport, transformation, and bioavailability of engineered nanomaterials. Two-dimensional MoS₂ nanosheets are one of the most well-studied transition-metal dichalcogenide nanomaterials. Nonetheless, the aggregation behavior of this material under environmental conditions is not well understood. Here, we investigated the aggregation of single-layer MoS₂ (SL-MoS₂) nanosheets under a variety of conditions. Trends in the aggregation of SL-MoS₂ are consistent with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) colloidal theory, and the critical coagulation concentrations of cations follow the order of trivalent (Cr³⁺) < divalent (Ca²⁺, Mg²⁺, Cd²⁺) < monovalent cations (Na⁺, K⁺). Notably, Pb²⁺ and Ag⁺ destabilize MoS₂ nanosheet suspensions much more strongly than do their divalent and monovalent counterparts. This effect is attributable to Lewis soft acid-base interactions of cations with MoS₂. Visible light irradiation synergistically promotes the aggregation of SL-MoS₂ nanosheets in the presence of cations, which was evident even in the presence of natural organic matter. The light-accelerated aggregation was ascribed to dipole-dipole interactions due to transient surface plasmon oscillation of electrons in the metallic 1T phase, which decrease the aggregation energy barrier. These results reveal the phase-dependent aggregation behaviors of engineered MoS₂ nanosheets with important implications for environmental fate and risk.

Chemical solidification/stabilization of arsenic sulfide and oxide mixed wastes using elemental sulfur: Efficiencies, mechanisms and long-term stabilization enhancement by dicyclopentadiene

Large amounts of hazardous arsenic sulfide (As₂S₃) wastes are generated in many industries. These wastes, which are extremely unstable and can partially transform into highly soluble arsenic oxide (As₂O₃) and then transform into As₂S₃ and As₂O₃ mixed wastes (ASOW), are difficult to be solidified/stabilized using common binders. This study proposed a thermally initiated copolymerization method employing elemental sulfur (S₈) to chemically solidify/stabilize ASOW. Under thermal conditions (140-200 °C), the elemental sulfur rings break and polymerize into diradical polymeric sulfur chains (•S-(S)_m-S•). The ASOW is solidified/stabilized not only by transforming As₂S₃ into poly(As₂S₃-r-S) copolymers through copolymerization of •S-(S)_m-S• with As₂S₃ but also by transforming As₂O₃ into As₂S₃ in the presence of poly(As₂S₃-r-S) copolymers. However, the sulfur chain in poly(As₂S₃-r-S) copolymers gradually crystallizes into S₈ after long-term aging, resulting in the depolymerization of copolymers. Dicyclopentadiene (DCP) greatly improves the long-term stability of the solidified body through maintaining the sulfur chain form by forming highly stable poly(As₂S₃-r-S-r-DCP) copolymers. The solidified body showed high compressive strength (25.7 MPa) and low leaching concentration of arsenic (<1.2 mg L^-1) even after 732 days of aging. This study provides a theoretical foundation for the S₈-based chemical solidification/stabilization of ASOW as well as other sulfide-containing wastes.

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.

Recovery of EDTA from soil-washing wastewater with calcium-hydroxide-enhanced sulfide precipitation

It is cost effective and thermodynamically feasible to recover EDTA and remove potential toxic elements (PTEs) with sulfide precipitation from soil-washing wastewater produced from EDTA washing PTEs-contaminated soil. However, poor solid-liquid separation and EDTA recovery restrict its application due to a large number of fine particles formed during the precipitation process. This study investigated the effect of single factor on PTEs (Cu, Pb, Cd, and Zn) removal and solid-liquid separation from wastewater. The results showed that Zn was the most difficult to remove compared with Cu, Pb, and Cd; with the aid of Ca(OH)₂, Zn removal efficiency was improved from 22.16% to 92.45%, and over 70.98 min, its average rate was 4.2 times that obtained without Ca(OH)₂ dosage; undissolved Ca(OH)₂ adsorbed suspended particles, acted as condensation nucleus, and promoted similar flocculation effect (self-flocculation); dissolved Ca(OH)₂ modified the charge on the surface of suspended particles by changing the zeta potential from -36.77 ± 1.2 mV to -25.39 ± 3.06 mV and weakened the electrostatic repulsion between the suspended particles, and promoted their adsorption and flocculation precipitation, thereby improving the solid-liquid separation. The acid-recovered EDTA was analyzed in the protonated form (H₄EDTA) using Fourier transform infrared (FT-IR) spectroscopy, and it maintained the same ability to extract PTEs from the soil as that of fresh EDTA over several cycles. This indicates that Ca(OH)₂-enhanced sulfide precipitation can effectively treat soil-washing wastewater and recover EDTA and potentially reduce the cost of remediation techniques for PTEs-contaminated soil with EDTA-enhanced soil washing.

UV light irradiation improves the aggregation and settling performance of metal sulfide particles in strongly acidic wastewater

Tiny metal sulfide particles are usually generated in the metal sulfide precipitation process, and residual dissolved sulfide (H₂S, HS^- and S^2-) generally makes the aggregation and settling performance of these particles worse. In this work, under strongly acidic conditions with 0.102 M H₂SO₄, the characteristics of CuS, CdS and mixed metal sulfide particles (CuS and CdS) produced in strongly acidic wastewater by H₂S were initially studied. Then, UV light irradiation was employed to improve the aggregation and settling performance of the metal sulfide particles, and relevant mechanisms were investigated. The results showed that the residual H₂S in suspension can deteriorate the settling performance of metal sulfide particles. After H₂S was removed, CuS particles exhibited good settling performance, but the settling performance of CdS and mixed sulfide particles remained poor. Subsequent studies showed that UV light irradiation effectively improved the aggregation and settling performance of CdS and mixed metal sulfide particles in the presence of H₂S. After 30 min of UV irradiation, the average hydrodynamic diameter of CdS particles increased approximately 166-fold in the presence of 131.40 mg/L H₂S, and that of mixed sulfide particles increased approximately 105-fold with the existence of 129.30 mg/L H₂S. Further study revealed that H₂S in suspensions played an important role in the aggregation of metal sulfide particles under UV irradiation. HS• generated by the photolysis of H₂S oxidized a portion of S^2- on the surface of CdS particles and a portion of S^2- and S_n^2- on the surface of mixed sulfide particles to S₂^2- and S⁰, which resulted in a decrease of negative charge and an enhanced aggregation of these particles. Finally, this study showed that the settling performance of CdS and mixed sulfide particles was effectively improved at high initial H₂S concentrations under UV irradiation. This study provides a clean and effective method to improve the metal sulfide precipitation process for strongly acidic wastewater treatment.

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.