The long-term stability of calcium arsenates: Implications for phase transformation and arsenic mobilization

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.

Effect of electrokinetic process on in situ stabilization and detoxification of arsenic-contaminated soil with high content of calcium

Owing to the high risk of human exposure to arsenic-contaminated soil, reducing its toxicity is essential. This study used the electrokinetic (EK) process with iron-rich electrodes to synchronously achieve the accumulate, stabilize and detoxify soil arsenic. Changes in arsenic valence, leaching toxicity, and microbial communities related to toxicity were comprehensively considered. The results demonstrated that arsenic was mainly transported toward the anode and accumulated by electromigration owing to the negatively charged arsenic anions under EK conditions. The cathode approaching effectively promote arsenic movement to the anode; the largest As(T) transportation rate of 30.61% was achieved near the cathode (S4). The transportation ratio of As(III) was 1.84 times more than that of As(V). The As(III) content and leaching toxicity of soil arsenic in all treatments decreased after applying the EK process. In particular, the anode approaching effectively elevated the average ratios of soil As(III) oxidation and stabilization to 37.88% and 61.73%, respectively. Correspondingly, the total phospholipid fatty acid content increased substantially after EK treatment and showed a pollution stress elimination effect. The electrokinetic effect can essentially cause highly active and easily migrated arsenic to accumulate near the anode and middle sections. The electric field mediated iron mineralization and stabilized arsenic by oxidizing As(III) and reacting with newly formed iron-rich phases (S). Meanwhile, the electric field regulated the form of soil calcium from CaCO3 to CaSO4 and caused calcium-bound arsenic to change to a more stable form. According to these results, in situ stabilization and detoxification of arsenic-contaminated soil can be realized by the EK process, avoiding stabilizer addition and excavation.

Application of a novel Ca-Fe-Si-S composite for the synchronous stabilization of As, Zn, Cu, and Cd in acidic arsenic slag

The control of multiple heavy metals (HMs) pollution in solid wastes, especially the co-contamination of As and other heavy metal cations, is of great importance to ecological and environmental health. To address this problem, the preparation and application of multifunctional materials have attracted wide attention. In this work, a novel Ca-Fe-Si-S composite (CFSS) was applied to stabilize As, Zn, Cu, and Cd in acid arsenic slag (ASS). The CFSS exhibited synchronous stabilization ability for As, Zn, Cu, Cd and owned strong acid neutralization capacity. Under simulated field conditions, the acid rain extracted HMs in ASS successfully decreased below the emission standard (GB 3838-2002-IV category in China) after incubated by 5% CFSS for 90 days. Meanwhile, the application of CFSS promoted the transformation of leachable HMs into less accessible forms, which was conductive to the long-term stabilization for HMs. There was competitive relation among the three heavy metal cations, following the stabilization sequence of Cu > Zn > Cd during incubation. And the stabilization mechanisms of HMs by CFSS were proposed as chemical precipitation, surface complexation, and ion/anion exchange. The research will be greatly conducive to the remediation and governance of field multiple HMs contaminated sites.

The role of doped-Mn on enhancing arsenic removal by MgAl-LDHs

To meet the challenges posed by global arsenic water contamination, the MgAlMn-LDHs with extraordinary efficiency of arsenate removal was developed. In order to clarify the enhancement effect of the doped-Mn on the arsenate removal performance of the LDHs, the cluster models of the MgAlMn-LDHs and MgAl-LDHs were established and calculated by using density functional theory (DFT). The results shown that the doped-Mn can significantly change the electronic structure of the LDHs and improve its chemical activity. Compared with the MgAl-LDHs that without the doped-Mn, the HOMO-LUMO gap was smaller after doping. In addition, the -OH and Al on the laminates were also activated to improve the adsorption property of the LDHs. Besides, the doped-Mn existed as a novel active site. On the other hand, the MgAlMn-LDHs with the doped-Mn, the increased of the binding energy, as well as the decreased of the ion exchange energy of interlayer Cl^-, making the ability to arsenate removal had been considerably elevated than the MgAl-LDHs. Furthermore, there is an obvious coordination covalent bond between arsenate and the laminates of the MgAlMn-LDHs that with the doped-Mn.

Long-Term Stabilization/Solidification of Arsenic-Contaminated Sludge by a Blast Furnace Slag-Based Cementitious Material: Functions of CaO and NaCl

Arsenic is a kind of element widely distributed in the environment that may pose a threat to the ecological environment and human health, while effective remediation and sustainable utilization of arsenic-containing sludge is a challenge. Based on stabilization/solidification blast furnace slag-based cementitious materials (BCMs), this study innovatively proposes to improve the arsenic (As) solidification efficiency and long-term stability by using the activation mode of CaO and NaCl. The effects of different factors on the properties of the BCM were measured by unconfined compressive strength (UCS) tests, X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The long-term stability and safety of the BCM were verified by leaching toxicity and improved three stage continuous extraction method (BCR) tests. Experimental results show that the addition of CaO provides conditions for the formation of ettringite (AFt), thus promoting the crystal growth of AFt. The addition of NaCl can promote the formation of Cl-AFt and play a good long-term stabilizing role. When the content of the alkali activator is 10% and the modulus is 1.0, the contents of CaO and NaCl are 10 and 1%, respectively. The BCM has the best efficiency in terms of UCS and As solidification. The UCS at 28 days was 5.4 MPa, and the leaching concentration of As was 0.309 mg/L, and the As solidification efficiency was up to 99.9%. In the improved BCR test, the proportions of residual and oxidizable states of arsenic increased by 19.6 and 13.5%, respectively, and the stability of heavy metals improved. These findings show that the BCM has good long-term stability and safety. Overall, this study shows that CaO and NaCl significantly increase the output of AFt and achieve the purpose of efficient and stable solidification of As by the BCM.

Natural field freeze-thaw process leads to different performances of soil amendments towards Cd immobilization and enrichment

Cadmium (Cd) soil pollution is a global issue affecting crop production and food safety. Remediation methods involving in-situ Cd immobilization have been developed, but their effectiveness can diminish under seasonal freeze-thaw aging processes. In this study, we assessed the field performance of four soil treatments at a seasonally frozen rice paddy. Amendments were applied at 2 wt%, including: (i) sepiolite (a 2:1 clay mineral), (ii) superphosphate, (iii) biochar (produced by rice husk at 500 °C for 2 h), and (iv) joint application of biochar & superphosphate (1:1 mixture by weight). Immobilization performance was determined as DTPA extractable Cd and plant uptake in various organs. Overall, the four treatments significantly reduced Cd bioavailability during the plant growth period, with average DTPA-extractable concentrations decreasing by 43%, 34%, 39% and 45% for the four treatments, respectively, relative to untreated soil (control). Rice grain yields from the superphosphate and the joint application treatments increased by 8.0% and 11.8%, respectively, and Cd accumulation within those grains reduced by 14.3% and 48.9%, respectively. During the winter non-growth period, freeze-thaw aging facilitated Cd mobilization, with DTPA-extractable Cd increasing by 16.9% in the control soil, relative to the initial period. However, this reduced to 10.9%, 14.4%, 7.6% and 5.0%, for the sepiolite, superphosphate, biochar and joint application treatments, respectively. Overall, the joint application of biochar and superphosphate provided the best performance in terms of both long-term Cd immobilization and rice production enhancement, offering a green remediation option for risk management at Cd contaminated rice paddies in seasonally frozen regions.

A novel method for in situ stabilization of calcium arsenic residues via yukonite formation

Stabilizing the hazardous calcium arsenic residues (CAR) and monitoring the subsequent fate of arsenic (As) are critical to reduce its risk to the environment. In this work, a novel in situ method has been proposed to stabilize CAR by adding Fe^III solution and subsequent formation of the secondary mineral (yukonite). The experiments were conducted at pH 6-9 with different Fe/As molar ratios (0.28-0.66) and the solid phases were characterized by using X-ray diffraction and scanning/transmission electron microscopy. Results showed that the stability of the CAR was significantly increased after the addition of Fe^III solution, indicating good fixation effectiveness. The dissolved As concentration in the treated CAR samples continuously decreased to <5 mg/L after 490 days of treatment at Fe/As molar ratio ≥ 0.54 and pH ≥ 8, with the leached As concentration lower than 5 mg/L (US EPA standard) for most of the treated CAR in the TCLP and HVM tests. The formation of yukonite under different experimental conditions is closely related to the enhanced stability of the treated CAR. This work provides a novel in situ method to treat CAR which might have potential for future industrial applications.

Insights into deep decline of As(III) leachability induced by As(III) partial oxidation during lime stabilization of As-Ca sludge

The enhancing effect of As(III) oxidation on As stabilization by lime is routinely attributed to the lower solubility of Ca arsenates than Ca arsenites. However, this routine explanation is insufficient for the scenario of As(III) partial oxidation, in which Ca arsenites still predominate As leachability due to the relatively high solubility. In this study, an As-Ca sludge with a high As(III) content (96 g/kg, 55% of the As(tot)) was treated by oxidant-lime to clarify the positive effect of As(III) partial oxidation. Lime alone only reduced As(tot) leaching concentrations from 541 to 4.9 mg/L (4.3 mg/L of As(III) and 0.6 mg/L of As(V)), failing to meet the regulatory limit (2.5 mg/L). After partial oxidation of As(III), lime treatment could further reduce As(III) leaching concentrations from 4.3 to below 1.9 mg/L, whereas As(V) remained stable at about 0.6 mg/L. Qualitative and quantitative analyses based on XRD, SEM-EDS, TG, and thermodynamic modeling suggested that the solubility of newly-formed amorphous Ca arsenites (CaHAs^IIIO₃•xH₂O) after lime treatment determined the final As(III) leachability. The CaHAs^IIIO₃•xH₂O formed at lower As(III) contents due to As(III) partial oxidation had lower solubility products and possibly higher crystallinity, resulting in the lower As(III) leachability. This study provides new insights into the role of As(III) partial oxidation in deep decline of As(III) leachability during lime stabilization, guiding the treatment of As-Ca sludge as well as other As(III)-bearing solid wastes.

Long-term stability of arsenic calcium residue (ACR) treated with FeSO4 and H2SO4: Function of H+ and Fe(Ⅱ)

Arsenic calcium residue (ACR) generated from the As-bearing wastewater treatment is highly hazardous due to high content of available As, which was seeking a suitable method for safe disposal such as stabilization treatment. In this study, the stabilization of available As in ACR was performed by combined treatment with FeSO₄ and H₂SO₄. After stabilization treatment, the As leaching concentrations extracted by China Standard Leaching Test (CSLT, HJ/T299-2007) decreased significantly from 162 mg/L to less than the Chinese regulation limit of 1.2 mg/L. And FeSO₄-H₂SO₄ treated ACR could maintain good long-term stability even after cured for 365 days. The stabilization mechanism for available As in ACR using leaching tests, sequential extraction analysis, XPS, XRD, and SEM-EDS was investigated. H⁺ from H₂SO₄ and Fe(Ⅱ) hydrolysis was committed to the full release of available As. Reactive oxygen species (ROSs) produced from Fe(Ⅱ) oxygenation drove the oxidation of As(Ⅲ) to As(Ⅴ). The release As was stabilized by forming stable Fe-O-As complexes (FeAsO₄·xFe(OH)₃). Moreover, this study also presented an effective and feasible method for ACR disposal.

Highly effective remediation of high-arsenic wastewater using red mud through formation of AlAsO4@silicate precipitate

High-arsenic wastewater derived from the metallurgical industry of nonferrous minerals is one of the most dangerous arsenic (As) sources that usually follow the emission of massive hazardous arsenic-bearing wastes. Considering the properties of red mud (RM), we propose an alternative and environmentally friendly method for the efficient remediation of high-arsenic wastewater using RM through formation of AlAsO₄@silicate precipitate, aiming at ''zero-emission of hazardous solid waste''. The results show nearly 100% of arsenic could be stepwisely removed from high-arsenic wastewater and reduce the arsenic concentration from 6100 mg/L to 40 μg/L using RM at room temperature. The highest arsenic removal capacity of RM reaches 101.5 mg/g at a RM-to-wastewater ratio of 40 g/L due to the superior arsenic adsorption and the co-precipitation of arsenate and Al³⁺ to form insoluble aluminum arsenate. The silicate shell of arsenic-loaded RM created at an alkaline condition acts as an arsenic stabilizer, resulting in a leached arsenic concentration of 1.2 mg/L in TCLP tests. RM acts as a highly effective arsenic remover and stabilizer for the disposal of high-arsenic wastewater. It shows great potential for the remediation of wastewater containing heavy metals with varying concentrations to produce clean water available for industrial purpose.

A novel arsenic immobilization strategy via a two-step process: Arsenic concentration from dilute solution using schwertmannite and immobilization in Ca-Fe-AsO4 compounds

Acid mine drainage (AMD) with toxic arsenic (As) is commonly generated from the tailings storage facilities (TSFs) of sulfide mines due to the presence of As-bearing sulfide minerals (e.g., arsenopyrite, realgar, orpiment, etc.). To suppress As contamination to the nearby environments, As immobilization by Ca-Fe-AsO₄ compounds is considered one of the most promising techniques; however, this technique is only applicable when As concentration is high enough (>1 g/L). To immobilize As from wastewater with low As concentration (~10 mg/L), this study investigated a two-step process consisting of concentration of dilute As solution by sorption/desorption using schwertmannite (Fe₈O₈(OH)_8-2x(SO₄)_x; where (1 ≤ x ≤ 1.75)) and formation of Ca-Fe-AsO₄ compounds. Arsenic sorption tests indicated that As(V) was well adsorbed onto schwertmannite at pH 3 (Q_max = 116.3 mg/g), but its sorption was limited at pH 13 (Q_max = 16.1 mg/g). A dilute As solution (~11.2 mg/L As) could be concentrated by sorption with large volume of dilute As solution at pH 3 followed by desorption with small volume of eluent of which pH was 13. The formation of Ca-Fe-AsO₄ compounds from As concentrate solution (2 g/L As(V)) was strongly affected by temperature and pH. At low temperature (25-50 °C), amorphous ferric arsenate was formed, while at high temperature (95 °C), yukonite (Ca₂Fe_3-5(AsO₄)₃(OH)_4-10·xH₂O; where x = 2-11) and johnbaumite (Ca₅(AsO₄)₃OH) were formed at pH 8 and 12, respectively. Among the synthesized products, johnbaumite showed strongest As retention ability even under acidic (pH < 2) and alkaline (pH > 9) conditions.

Fast and effective arsenic removal from aqueous solutions by a novel low-cost eggshell byproduct

Developing alternative green solutions for local and correct recycling of eggshells waste (ES) are needed by the egg-processing industries. In this study, we proposed transforming ES into a novel low-cost chemical compound named hydroxyl-eggshell (ES-OH) and investigated its capacity for arsenic (As) removal from aqueous solutions. Laboratory experiments were conducted to investigate the effects of ES-OH doses, pH, kinetics, and isotherms on As removal efficiency. The kinetics study showed that ES-OH removed nearly all As from solution in less than 15 min. The pseudo-second-order model described the process, and the maximum As removal capacity predicted by the Langmuir isotherm model was 529 mg g^-1. Using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy dispersive X-ray detector (SEM-EDS), and X-ray diffraction (XRD), we found that the As removal mechanism by ES-OH was due to vladimirite precipitation, followed by weak electrostatic interactions between the precipitate and arsenate ions. Finally, after an economic analysis, we conclude that besides being a novel and economical income source, egg-producing companies might implement the ES-OH production process as a local environmentally-friendly way of recycling eggshells and reducing water As contamination.

The adsorption of As(V) on poorly crystalline Fe oxyhydroxides, revisited: Effect of the reaction media and the drying treatment

Arsenic (As) adsorbed on Fe oxyhydroxides (adsorbent) is widely occurring in many environmental settings such as in acid mine drainage systems or in the hydrometallurgical operations to form Fe-As coprecipitates. However, the influence of the reaction media and the drying treatment on the microstructure of the directly formed adsorbents at various pHs was still not fully understood. In this work, As adsorption behaviors on various forms of Fe oxyhydroxides were systematically investigated by using XRD, FTIR, Raman, XANES, and HRTEM. The results revealed that at weak acidic pH, more As could adsorbed on the suspension adsorbent formed in sulfate and chloride media than that in nitrate media, possibly due to the microstructure alteration of the adsorbent in the presence of sulfate and chloride. Besides, the increasing crystallinity of the Fe oxyhydroxides and the aggregation effect after drying were the major reasons why less As could be hold by the dried adsorbents than that of the corresponding suspension adsorbents. These findings could shed more light on the nature of the Fe oxyhydroxides which may be helpful for more precisely predicting the fate of some toxic metal(loid)s in the environment.

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.

Immobilization and migration of arsenic during the conversion of microbially induced calcium carbonate to hydroxylapatite

Coprecipitation with calcium carbonate (CaCO₃) could decrease the bioavailability of arsenic (As). However, in a phosphate-rich environment, some CaCO₃ will be converted to hydroxylapatite (HAP). Currently, the behavior of carbonate-bound As during conversion is unclear. Therefore, we prepared bio-induced CaCO₃ in an As solution and converted it to HAP. The results showed that a high concentration of arsenate promoted vaterite precipitation and the conversion of CaCO₃ to HAP. The dissolution data verified the low solubility of As in HAP, though its As-bearing CaCO₃ precursor released up to 88.19% As during the conversion. Furthermore, HPLC-ICP-MS data showed partial oxidation of arsenite to arsenate, suggesting that CaCO₃ and HAP's structure favored the incorporation of arsenate. Our results demonstrated that the stability of heavy metal-bearing CaCO₃ should be considered, and the role of HAP in the immobilization of heavy metals such as As should not be overestimated.

Alternative Method for the Treatment of Hydrometallurgical Arsenic-Calcium Residues: The Immobilization of Arsenic as Scorodite

Arsenic-calcium residue (ACR) is one of the major hazardous solid wastes produced by the metallurgical industry that poses a serious threat to the environment. However, a suitable method for the effective treatment of ACR is still lacking. In this study, an alternative treatment method for ACRs via the immobilization of As as scorodite was proposed with the use of two types of ACRs (ACR_real directly collected from a Pb refinery and ACR_lab precipitated from waste sulfuric acid in the lab). The treatment of ACR included preparing the As-enriched solution via H₂SO₄ dissolution-neutralization of ACR at pH < 2, As(III) was oxidized by H₂O₂, and As(V) was immobilized as scorodite. The results showed that gypsum produced from ACR_lab in the dissolution-neutralization process contained 68 mg/kg of As, far below the Chinese national standard for hazardous solid wastes (<0.1 wt %, GB5085.62007). The gypsum produced from ACR_real contained 5400 mg/kg of As due to the presence of original high-As gypsum (1.6 wt %) in ACR_real. These results showed that the preliminary removal of SO₄ ^2- from waste sulfuric acid by lime neutralization-precipitation at pH ∼ 2 could produce pure-phase gypsum by avoiding the HAsO₄ ^2- isomorphic substitution for SO₄ ^2-. The scorodite produced from both ACRs displayed good As stability at pH 4.95 (0.9 and 0.5 mg/L) via the toxicity characteristic leaching procedure (TCLP) method and at pH 3-7 (0.4-3.0 mg/L) via a 15 day short-term stability test.

Long-term stability of the Fe(III)-As(V) coprecipitates: Effects of neutralization mode and the addition of Fe(II) on arsenic retention

The coprecipitation of arsenic with Fe(III) by lime neutralization is widely used in industrial practices to treat arsenic-containing waste waters generated from mineral processing operations. In this work, coprecipitation was conducted directly at pH 8 to simulate the operations in hydrometallurgical practices, which differed from the conventional laboratory operations. Moreover, although ferric is the major species of iron in arsenic-containing waste waters, the coexistence of ferrous ions cannot be ignored. Therefore, the effect of different neutralization modes, as well as the effect of ferrous ions on the removal of arsenic and the stability of the generated arsenic-bearing wastes, was systematically investigated. The result showed that arsenic was still released back into the liquid phase under alkaline conditions even for the samples formed directly at alkaline pH. It was found that the extra addition of Fe(II) may exert negative effect on the stability of the as-formed Fe(II)-Fe(III)-As(V) coprecipitates at pH 7 - 10. The concentration of ferrous ions in the liquid/solid phase decreased with increasing pH for each sample formed at different Fe(II)/Fe(tot). The results indicated that complete oxidation of the ferrous ions before coprecipitation with arsenic should be conducted to achieve optimal stability of the arsenic-bearing wastes for hydrometallurgical practice and waste disposal.