Calcium sulfide-organosilicon complex for sustained release of H2S in strongly acidic wastewater: Synthesis, mechanism and efficiency

Comparative study of organosilicon and inorganic silicon in reducing cadmium accumulation in wheat: Insights into rhizosphere microbial communities and molecular regulation mechanisms

Silicon is widely used as a "quality element" and "stress resistance element" in crop production and the remediation of heavy metal-contamination soils. Compared to inorganic silicon, organosilicon has unique properties such as amphiphilicity, low surface energy and high biocompatibility. Our previous research has confirmed the effectiveness of organosilicon-modified fertilizers in inhibiting Cadmium (Cd) absorption in wheat. Therefore, it is of great importance to further explore the potential mechanisms and comprehensive benefits of organosilicon. In this study, the microbiological and molecular mechanisms by which organosilicon reduces Cd concentration in wheat compared to inorganic silicon were investigated in depth. The findings indicated that, in comparison with inorganic silicon, organosilicon exhibited a more remarkable efficacy. Specifically, it was more effective in reducing the Cd concentration in wheat grains, achieving a reduction range of 35-39 % as opposed to the 23-28 % reduction achieved by inorganic silicon. Moreover, it manifested a greater ability to mitigate health risks, with a reduction range of 33-42 % compared to the 25-30 % reduction of inorganic silicon. Furthermore, organosilicon contributed to a significant increase in wheat yield, with a growth range of 11-14 % in contrast to the 8-11 % increase from inorganic silicon. Additionally, it enhanced the quality of the grains, substantially improving the protein content and amino acid content. The comparative advantages of organosilicon over inorganic silicon would be firstly due to the reduction of the bioavailability of soil Cd by increasing the available silicon content in the soil and improving the soil microbial ecology (increasing the abundance of Bacillus, Pseudomonas, Massilia and Talaromyces and reducing the enrichment of Fusarium). Secondly, organosilicon achieved vacuolar compartmentalization of Cd by upregulating the expression of the ABC transporter gene (TaABCB7), thereby alleviating Cd toxicity and restricting Cd transport from leaves to grains. Meanwhile, organosilicon increased the wheat yield by optimizing the availability of soil nutrients and enhancing photosynthesis. These results demonstrate the immense potential of organosilicon in mitigating heavy metal contamination in crops.

Ascorbic acid-induced digenite (Cu9S5) formation: A strategy to enhance sulfidation efficiency for copper recovery from acidic wastewater

Sulfide precipitation is an effective method for copper recovery from acidic wastewater. However, excessive use of sulfide reagents leads to secondary pollution, which poses a significant challenge. This study demonstrates that leveraging the reducing properties of ascorbic acid (AA) and adding Na2S to acidic copper-containing solutions (pH 2-4) at a Cu:S molar ratio of 1.6:0.8 enhances Cu(II) removal efficiency from 50 % to 100 %, doubling sulfide reagent utilization. Solution chemistry, electron microscopy, and X-ray spectroscopy analyses indicate that AA primarily promotes copper sulfide precipitation with higher copper content. Addition of AA at pH 2 at a Cu:S:AA molar ratio of 1.6:0.8:1 lowers the oxidation-reduction potential (ORP) from 0.5 V to 0.1 V, forming digenite (Cu9S5). Compared to covellite (CuS) formed without AA, digenite exhibits a higher Cu oxidation state and a lower S oxidation state. Furthermore, coagulation kinetics studies show that solution pH, Na2SO4, and FeCl2 concentrations influence copper sulfide aggregation. At pH 2, Cu9S5 coagulates at least 4.2 times faster than CuS within 20 min. Cost analysis shows that the cost per ton of copper recovered from wastewater using this method is about one-third of the cost of conventional methods. More importantly, this study minimizes residual sulfide, offering a novel strategy for dose control in copper sulfide recovery.

Mechanism study of the effect of copper ions on the stability of As(III) sulfuration precipitation in acidic copper smelting wastewater

Sulfide precipitation is considered as a very efficient method for removing arsenic from actual copper smelting acidic wastewater. However, the arsenic removal process can be affected by copper ions. This study focuses on the mechanism of copper ions' influence on the stability of As(III) sulfuration precipitation. Sulfuration reaction experiments are carried out using Na2S in three different simulated highly acidic wastewaters with initial As(III) concentrations of 2000 mg/L (LAs), 5000 mg/L (MAs), and 10000 mg/L (HAs), and the implications of processes variables of S/As ratio and copper concentration on the stability of As(III) sulfuration precipitation are discussed. The results show that the As(III) sulfuration precipitation is significantly affected by copper ions in the LAs reaction systems, whereas in the MAs and HAs reaction systems, which can be noticeably affected by copper ions only when the S/As is not greater than 1.5 (≤ 1.5), i.e., when the amount of Na2S is insufficient. Person correlation analysis also demonstrates a remarkable negative correlation (correlation coefficient is around - 0.96) between the S/As ratio and copper ion concentration on As(III) removal efficiency in the LAs reaction systems. The effect of copper ions on As2S3 is further investigated, and it is detected that copper ions cause approximately 3.3% of the precipitated As2S3 to be re-dissolved. This study proves that copper ions not only compete with As(III) for S2-, but also cause the precipitated As2S3 to re-dissolve. Therefore, in the actual manufacturing process, it is essential to control not only the sulfiding dose, but also the copper ions. This study provides a specific reference for actual enterprises to sulfurize As(III) from highly acidic wastewater and is of great significance for controlling actual industrial processes.

Controllable H2S supply via membrane contactors for safe and efficient arsenic precipitation from acidic wastewater

Sulfide precipitation is a popular technique for the treatment and resource recovery of arsenic-containing dirty acid wastewater. However, this approach frequently suffers from toxic hydrogen sulfide (H2S) escape and agent wastage issues. Herein, a safe and efficient technique is proposed based on a polytetrafluoroethylene membrane contactor wherein the mass transfer resistance of the gas, membrane, and liquid phases can be easily and independently regulated to control the H2S supply. Additionally, H2S is transferred in bubble-free form, fundamentally avoiding the issue of gas escape, and uniform membrane distribution contributes to evenly distributed H2S delivery. The results demonstrated that 99.3 % arsenic precipitation could be achieved and the H2S emission ratio was limited to < 0.12 %, which was significantly better than traditional methods. Thus, highly efficient separation and resource recovery of heavy metals in wastewater can be achieved using this unique and controllable low-dose H2S supply method. The separation factor between copper and arsenic was as high as 7490.9. This study provides a safe, reliable, and promising new approach for the treatment of arsenic-containing acidic wastewater.

Artificial intelligence optimization and controllable slow-release iron sulfide realizes efficient separation of copper and arsenic in strongly acidic wastewater

Iron sulfide (FeS) is a promising material for separating copper and arsenic from strongly acidic wastewater due to its S2- slow-release effect. However, uncertainties arise because of the constant changes in wastewater composition, affecting the selection of operating parameters and FeS types. In this study, the aging method was first used to prepare various controllable FeS nanoparticles to weaken the arsenic removal ability without affecting the copper removal. Orthogonal experiments were conducted, and the results identified the Cu/As ratio, H2SO4 concentration, and FeS dosage as the three main factors influencing the separation efficiency. The backpropagation artificial neural network (BP-ANN) model was established to determine the relationship between the influencing factors and the separation efficiency. The correlation coefficient (R) of overall model was 0.9923 after optimizing using genetic algorithm (GA). The BP-GA model was also solved using GA under specific constraints, predicting the best solution for the separation process in real-time. The predicted results show that the high temperature and long aging time of FeS were necessary to gain high separation efficiency, and the maximum separation factor can reached 1,400. This study provides a suitable sulfurizing material and a set of methods and models with robust flexibility that can successfully predict the separation efficiency of copper and arsenic from highly acidic environments.

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.

Removal of hydrogen sulfide in the gas phase by carbide slag modified bentonite

Bentonite-based adsorbents for the removal of hydrogen sulfide (H₂S) were prepared by a wet-mixing method using carbide slag as the active component. The effects of carbide slag content, calcination temperature, calcination time, and reaction temperature on the H₂S adsorption capacity were investigated. The results showed that compared with the blank bentonite adsorbent, the carbide slag-modified bentonite-based adsorbent enhanced the chemisorption of H₂S. The adsorption capacity of the carbide slag modified bentonite adsorbent (2.50 mg g^-1) was more than 40 times higher than that of the blank bentonite-based adsorbent (0.06 mg g^-1) under optimal conditions. The optimal conditions for H₂S removal were 3 : 5 ratio of carbide slag-to-bentonite, calcination temperature of 450 °C for 2 h, and reaction temperature of 95 °C. H₂S was mainly removed in the mesopores and macropores of the adsorbent and was finally transformed to CaS and sulfate on the adsorbent surface. The adsorption process of H₂S followed the Freundlich adsorption isotherm equation and Bangham adsorption kinetic model.

Recovery of zinc and extraction of calcium and sulfur from zinc-rich gypsum residue by selective reduction roasting combined with hydrolysis

A novel process that includes selective reduction roasting followed by hydrolysis was proposed in this work to recover zinc, and efficiently extract calcium and sulfur from hazardous zinc-rich gypsum residue (ZGR) waste for high-purity of CaCO₃ and sulfur production. The selective reduction behaviors of ZGR during the reduction roasting were investigated in detail based on thermodynamic analysis and roasting experiments. The effect of roasting temperature, carbon dosage and time on the selective reduction of ZGR was comprehensively investigated, and the results indicated that ZnO and CaSO₄ in the ZGR can be selectively reduced to Zn(g) and CaS, respectively. The volatile Zn(g) was oxidized to ZnO and enriched in the dust, which can be used as a secondary zinc resource. Moreover, the hydrolysis behaviors and leaching kinetic of CaS during hydrolysis were studied intensively. Results depicted that in the H₂S-H₂O system, the CaS in the roasted product can be selectively and efficiently dissolved into the leachate. Furthermore, the kinetic analysis revealed that the hydrolysis of CaS conformed to the internal diffusion reaction control model in the shrinking core model and the apparent activation energy Ea = -12.02 kJ/mol. The obtained hydrolysate with low impurities could be used to capture CO₂ for the production of high-purity sulfur and CaCO₃. Iron and other impurities in the roasted product were concentrated into the leaching slag in the form of metallic iron and akermanite. The whole process realized the recovery of zinc, and the selective and effective extraction of calcium and sulfur, which could provide an alternative process for the large-scale treatment of these hazardous wastes.

Metal Sulfide Precipitation: Recent Breakthroughs and Future Outlooks

The interest in metal sulfide precipitation has recently increased given its capacity to efficiently recover several metals and metalloids from different aqueous sources, including wastewaters and hydrometallurgical solutions. This article reviews recent studies about metal sulfide precipitation, considering that the most relevant review article on the topic was published in 2010. Thus, our review emphasizes and focuses on the overall process and its main unit operations. This study follows the flow diagram definition, discussing the recent progress in the application of this process on different aqueous matrices to recover/remove diverse metals/metalloids from them, in addition to kinetic reaction and reactor types, different sulfide sources, precipitate behavior, improvements in solid–liquid separation, and future perspectives. The features included in this review are: operational conditions in terms of pH and Eh to perform a selective recovery of different metals contained in an aqueous source, the aggregation/colloidal behavior of precipitates, new materials for controlling sulfide release, and novel solid–liquid separation processes based on membrane filtration. It is therefore relevant that the direct production of nanoparticles (Nps) from this method could potentially become a future research approach with important implications on unit operations, which could possibly expand to several applications.