Arsenic-bearing Wastes Research Articles

Kinetic features and mechanism of scorodite formation via multivalent iron source from arsenic-bearing solution

Arsenic-bearing wastes from non-ferrous metallurgy exhibit poor stability during long-term storage, and are transferred as pollutants to the atmosphere and groundwater, thereby, threatening human health and ecological security. Immobilising arsenic as scorodite crystals via a multivalent iron source (Fe(OH)3-FeSO4·7 H2O) is one effective approach for controlling arsenic pollution. However, the kinetic features and mechanisms of this method have not yet been revealed, thereby, necessitating further research. Herein, the kinetics of scorodite generation influenced by the initial pH, Fe/As ratio, Fe(III)/Fe(II) ratio, and reaction temperature were investigated. Prominent features included variations in the minimum pH, oxidation–reduction potential (ORP) platform, Fe/As ratio (1.0), and the delay or advance of scorodite generation. The complicated mechanism of scorodite formation can be divided into coordinated polymerisation, acidic Fe(OH)3 dissolution, polymer oxidation, Fe(II) oxidation, and scorodite crystallisation. The fast coordinated polymerisation of precursors ([Fe(H2O)4(H2AsO4)]nn+) in stage I resulted in a decrease in pH; Fe(OH)3@scorodite and Fe(H2O)2AsO4 basic units were formed via the oxidation of ionic Fe(III) in stage II. Furthermore, regular octahedral scorodite was generated from two sources: the crystallisation of Fe(H2O)2AsO4 basic units and Ostwald ripening of flaky scorodite from Fe(III)–As(V) precipitates at stage III. The Fe(II) in the polymers was mainly oxidised by ferric ions generated from the oxidation of ferrous ions via dissolved O2 from the oxygen gas. Fe(II) ions as carriers for transferring electrons from O2 to polymers. Scorodite formation was controlled by the oxidation of Fe(II) by oxygen gas. Overall, this study provides guidance for the effective immobilisation of arsenic wastes into scorodite via multivalent iron sources to eliminate arsenic pollution.

Mechanism and thermodynamics of scorodite formation by oxidative precipitation from arsenic-bearing solution

Arsenic pollution is a challenging environmental issue caused by arsenic-bearing wastes from nonferrous metallurgy. Oxidative precipitation via introducing O2 into an ionic Fe(II)–As(V) solution is an advanced method for arsenic immobilization. However, the underlying mechanism is still not well understood. This study proposed a mechanism for scorodite formation by oxidative precipitation, and its thermodynamics were calculated using Gaussian software. Scorodite formation was divided into three stages: precursor formation (3–90 min), oxidative conversion (90–270 min) and crystallization (270–720 min) from the variation in precipitates and solution characterization and parameters such as initial pH, arsenic concentration, and ferrous dosage. In the scorodite formation mechanism, the precursors originate from the coordination polymerization of aqueous Fe(H2O)62+ and H2AsO4−, which contributes to the oxidative conversion of coordinated polymers ([Fe(H2O)4(H2O)]nn+) to basic Fe(H2O)2AsO4 until regular octahedral crystals are formed via nucleation and growth during crystallization. The ΔrGmθ for polymerization varied from −491.96 kJ mol−1 to −33.30 kJ mol−1, and the ΔrGmθ of oxidative conversion changed from −982.16 kJ mol−1 to −224.82 kJ mol−1, demonstrating the feasibility in scorodite formation. This research is significant for understanding scorodite formation in As(V) solutions. It can provide schemes for controlling and modifying the conditions of arsenic-bearing waste immobilization in the laboratories and industries.

A novel process of immobilizing sodium arsenate crystals as scorodite using Fe(OH)3 as an iron source

Sodium arsenate crystals are a hazardous waste produced from the utilization of arsenic–alkali residue in antimony metallurgy that pose a serious threat to the environment. In this article, to eradicate the damage caused by sodium arsenate and enhance the sustainable development of nonferrous metallurgy, a novel process was proposed to immobilize arsenic as flaky scorodite using Fe(OH)3 as an iron source. Various methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and inductively coupled plasma atomic emission spectrometry (ICPAES), were applied to characterize these synthesized products. The whole process is divided into three parts. In the thermodynamic analysis, the stable area of scorodite in the Pourbaix diagram significantly expands at high ionic activity and temperature, which favors the decrease in △rG for scorodite synthesis, and Fe–As coprecipitation is feasible in solution at 0.18 ≤pH≤ 5.30. In the scorodite synthesis, 99.88% of the arsenic was converted to a flaky scorodite with a size of 2–5 µm by optimizing the parameters. In the removal of arsenic, residual arsenic and iron were completely coprecipitated into an iron salt by dropping NaOH solution to a final pH of 5.0. The final products include scorodite and sodium sulfate in this process. Overall, this new approach successfully eliminates the potential arsenic pollution from the utilization of arsenic–alkali residue in antimony metallurgy and can potentially be applied to dispose of other high arsenic-bearing wastes.

Comparative Study for Flue Dust Stabilization in Cement and Glass Materials: A Stability Assessment of Arsenic

Arsenic is a poisonous element and its super mobility can pose a major threat to the environment and human beings. Disposed arsenic-bearing waste or minerals over time may release arsenic into the groundwater, soil and then the food chain. Consequently, safe landfill deposition should be carried out to minimize arsenic bleeding. Cement-based stabilization/solidification and glass vitrification are two important methods for arsenic immobilization. This work compares the stability and intrinsic leaching properties of sequestered arsenic by cement encapsulation and glass vitrification of smelter high-arsenic flue dust (60% As2O3) and confirms if they meet or exceed the requirement of landfill disposition over a range of environmentally relevant conditions. The toxicity characterization leaching procedure (TCLP, 1311), synthetic precipitation leaching procedure (SPLP, 1312) and Australian standard (Aus. 4439.3) in short-term (18 h) and mass transfer from monolithic material using a semi-dynamic leaching tank (1315) in longer-term (165 days) were employed to assess arsenic immobility characteristic in three arsenic-cement (2%, 8.4% and 14.4%) and arsenic-glass (11.7%) samples. Moreover, calcium release from different matrices has been taken into consideration as a contributor to arsenic bleeding. Based on the USEPA guidelines, samples can be acceptable for landfilling only if As release is <5 mg/L. Results obtained from short-term leaching were almost similar for both cement and glass materials. However, high calcium release was observed from the cement-encapsulated materials. The pH of leachates after the test was highly alkaline for encapsulated materials; however, in glass material it was near neutral or slightly acidic. Method 1315 tests made a huge difference between the two materials and confirmed that cement encapsulation is not the best method for landfilling arsenic waste due to the high arsenic and calcium release over time with alkaline pH. However, glass material has shown promising results, i.e., the insignificant release of arsenic over time with an acceptable change in pH value. Overall, arsenic sequestration in glass is a better option compared with the cement-based solidification process.

A shortcut approach for cooperative disposal of flue dust and waste acid from copper smelting: Decontamination of arsenic-bearing waste and recovery of metals

Recovering harmful elements (As, Pb) and metals (Cu, Bi, Zn) from copper smelting flue dust (CSFD) is a critical subject and task for arsenic contamination control and resource sustainability. In this work, a two-step pyrometallurgical process was developed to preferentially separate arsenic and recover metals from CSFD. During the low-temperature roasting, arsenic-bearing waste acid (AWA) from copper industry was used as an additive and effective removal of arsenic (97.8 %) was obtained at 350 °C, which follows the idea of “treating waste with waste”. Subsequently, the recovery and separation of metals were well-achieved based on the affinity between metals and sulfur in the second stage of roasting, by which 91.28 % of Pb and 95.65 % of Bi were recovered as an alloy (Pb 86.48 %, Bi 13.21 %), while 82.62 % of Cu was enriched in the matte. The migration rules of metal elements and phase transformation in the whole process were studied in-depth from theory and experiments. This process can realize the efficient removal of arsenic as well as effective recovery of metals via cooperative disposal of CSFD and AWA, and minimize the environmental impacts.

A novel method for dearsenization from arsenic-bearing waste slag by selective chlorination and low-temperature volatilization.

Because of the widespread presence of arsenic in various smelting waste slags, it not only hinders the recycling and utilization of waste slag, but also causes serious pollution to the ecological environment. In this study, As2O3, the main form of arsenic in non-ferrous metal smelting slag, was used as the research object, and FeCl3 was used as the chlorination agent. As2O3 was selectively chlorinated to low-boiling-point AsCl3 gas which was easy to be volatilized and removed by chlorination roasting. According to the thermodynamic calculation results, the feasibility of FeCl3 as the chlorination agent for selective chlorination and low-temperature volatilization of dearsenization was analyzed. The TG-DTA diagram was analyzed by thermogravimetric experiment, and the mass change, endothermic and exothermic behaviors of the As2O3-FeCl3 system during the linear heating process were studied. The effects of roasting temperature, roasting time, and molar ratio of reactants on the chlorination-volatilization of the As2O3-FeCl3 system were investigated. The optimal chlorination roasting conditions were determined with a roasting temperature of 290-300 ℃, a roasting time of 50min, and a reactant FeCl3/As2O3 molar ratio of 4:1. The results of this study provided a novel idea for the removal of arsenic from smelting slag and dust, but the mechanism and process conditions of chlorination still need to be further studied and optimized.

Effect of Rhamnolipids and Lipopolysaccharides on the Bioleaching of Arsenic-Bearing Waste

The adsorption of biosurfactants and polysaccharides changes the surface properties of solid particles, which is important for controlling the release of arsenic compounds from the solid phase and preventing undesirable bioleaching. Microbial leaching and scorodite adhesion experiments, including pure and modified mineral material, were conducted in a glass column with a mineral bed (0.8–1.2 mm particle size) to test how rhamnolipids (Rh) and lipopolysaccharides (LPS) affect surface properties of mineral waste from Złoty Stok (Poland) and secondary bio-extraction products (scorodite). Adsorption tests were conducted for both solid materials. The adsorption of Rh and LPS on the solids was shown to modify its surface charge, affecting bioleaching. The highest bio-extraction efficiency was achieved for arsenic waste with adsorbed rhamnolipids, while the lowest, for the LPS-modified mineral. Under acidic circumstances (pH~2.5), the strongly negative zeta potential of arsenic-bearing waste in the presence of Rh creates conditions for bacteria adhesion, leading to the intensification of metal extraction. The presence of a biopolymer on the As waste surface decreases leaching efficiency and favours the scorodite’s adhesion.

An environmentally-friendly process for recovering, solidifying As and preparing bronze material from tin-bearing middling

Tin-bearing middling, a hazardous solid waste produced during beneficiation of tin deposits and tin-bearing tailings, presents a serious environmental pollution risk due to high levels of As. This paper aims to explore an environmentally friendly technology to reduce toxic substances and prepare a high value-added bronze material from tin-bearing middling. The co-process for tin-bearing middling, anthracite, and copper is first proposed at 1073–1123 K. Variables like temperature, amount of added anthracite, and holding time were optimized. Those results showed that 95.56% Sn and 92.79% As were recovered; levels of Fe in the residue reached 73.52 wt%. During roasting, SnO2 in the tin-bearing middling was initially transformed into SnO(g), then into an Fe–Sn spinel (Fe3-xSnxO4) and Ca2SnO4, and finally a Cu–Sn alloy. Generation of the Fe–Sn spinel came primarily from substitution of Fe2+ by Sn2+, but small amounts of Sn4+ replaced Fe3+ in Fe3O4. Meanwhile, FeAsS and FeAsO4 were initially transformed into As4(g) and then solidified as a Cu–As alloy. Interestingly, the residual As existed as Fe–As alloy, but As leaching levels did not exceed 0.02 mg/L. After initial treatment, the Cu alloy can be prepared for bronze. Meanwhile, subsequent volatilization and solidification of As resulted in a significant reduction of arsenic-bearing waste that minimizes environmental risk.

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.

Safe and ecological performance of mining and processing industry

Mineral mining and processing activities result in highly toxic arsenic-bearing waste water discharged in natural water bodies. The promising methods of arsenic removal from waste water use adsorption at natural sorbents and oxidizers. The tests of arsenic removal by adsorption at magnesium- and manganese-bearing minerals show that the use of these natural raw materials can greatly enhance efficiency of the environmental activities in the mining and processing industry.

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.

Removal and fixation of arsenic by forming a complex precipitate containing scorodite and ferrihydrite

For the metallurgical industries, the safe disposal of arsenic-bearing waste is animportant issue. In this study, a two-stage arsenic precipitation process was proposed, which consists of formation of scorodite at lower pH followed by precipitation of arsenical ferrihydrite at higher pH. The experimental results show that arsenic can be removed effectively from a solution with arsenic concentration of 5 g/L as precipitate of crystalline scorodite through oxidizing ferrous ions with air stream at pH of 2at first. Then, the pH value was raised to 3 and 4, and almost all the residual arsenic was removed from the solution due to adsorption onto amorphous ferrihydrite at the molar ratio of Fe/As between 2 and 4. The United States Environmental Protection Agency's Toxicity Characteristic Leaching Procedure tests demonstrate that the complex precipitates generated under these conditions (Fe/As ratio of 4, final pH between 3 and 4) were very stable and no leached arsenic can be detected using acetic acid buffer solution (pH 4.93 ± 0.05) in tests of 72 h. Experimental results suggest that a high Fe/As ratio was necessary, so that the excess iron promotes the formation a product of good stability, arsenical ferrihydrite (AsFH: AsO43−·Fe(OH)(H2O)1+x) under a relatively high pH in acidic solution.

Arsenic(V) adsorption on ferric oxyhydroxide gel at high alkalinity for securely recycling of arsenic-bearing copper slag

Arsenic contamination has raised great concern around the world as the cause of many severe health issues, including internal and external cancers. Discharging arsenic waste into the environment before proper handling is strictly forbidden. In order to treat the arsenic-bearing wastes in a safe way, we used the oxidation alkali leaching to dissolve arsenic in the low-risk state arsenate (AsO43−) firstly, followed by recycling of the alkali solution after arsenate removal. In this paper, we synthesized high-concentrated ferric oxyhydroxide gels (HFGs) with different supersaturation in a hydrothermal system to adsorb arsenate anions at high alkali without consuming the hydroxyls. The arsenate adsorption onto HFGs followed the pseudo second-order and intra-particle diffusion kinetics and fitted Langmuir isotherms model well. Through characterization of the synthesized HFGs particles by XRD, Raman spectra, SEM and TGA, it was confirmed that the surface hydroxyl sites on the ferric oxyhydroxide reacted with arsenates to form the bidentate surface complexation. In addition, the synthesized HFG at lower supersaturation had lower crystallinity and more surface hydroxyl sites, resulting in higher adsorption capacity for arsenate. Using HFG as the adsorbent, the arsenic (V) concentration in the copper slag alkali leaching solution can be reduced from 2084 mg/L to 71.8 mg/L, which can realize stabilization of the high-concentrated arsenate at high alkali and alkali solution recycling.

Anaerobic Disposal of Arsenic-Bearing Wastes Results in Low Microbially Mediated Arsenic Volatilization.

The removal of arsenic from drinking water sources produces arsenic-bearing wastes, which are disposed of in a variety of ways. Several disposal options involve anaerobic environments, including mixing arsenic waste with cow dung, landfills, anaerobic digesters, and pond sediments. Though poorly understood, the production of gaseous arsenic species in these environments can be a primary goal (cow dung mixing) or an unintended consequence (anaerobic digesters). Once formed, these gaseous arsenic species are readily diluted in the atmosphere. Arsenic volatilization can be mediated by the enzyme arsenite S-adenosylmethionine methyltransferase (ArsM) or through the enzymes involved in methanogenesis. In this study, methanogenic mesocosms with arsenic-bearing ferric iron waste from an electrocoagulation drinking water treatment system were used to evaluate the role of methanogenesis in arsenic volatilization using methanogen inhibitors. Arsenic volatilization was highest in methanogenic mesocosms, but represented <0.02% of the total arsenic added. 16S rRNA cDNA sequencing, qPCR of mcrA transcripts, and functional gene array-based analysis of arsM expression, revealed that arsenic volatilization correlated with methanogenic activity. Aqueous arsenic concentrations increased in all mesocosms, indicating that unintended contamination may result from disposal in anaerobic environments. This highlights that more research is needed before recommending anaerobic disposal intended to promote arsenic volatilization.

Leaching characteristics of encapsulated controlled low-strength materials containing arsenic-bearing waste precipitates from refractory gold bioleaching

We report on the leaching of heavy elements from cemented waste flowable fill, known as controlled low-strength materials (CLSM), for potential mine backfill application. Semi-dynamic tank leaching tests were carried out on laboratory-scale monoliths cured for 28 days and tested over 64 days of leaching with pure de-ionised water as leachant. Mineral processing waste include flotation tailings from a Spanish nickel-copper sulphide concentrate, and two bioleach neutralisation precipitates (from processing at 35 °C and 70 °C) from a South African arsenopyrite concentrate. Encapsulated CLSM formulations were evaluated to assess the reduction in leaching by encapsulating a ‘hazardous’ CLSM core within a layer of relatively ‘inert’ CLSM. The effect of each bioleach waste in CLSM core and tailings in CLSM encapsulating medium, are assessed in combination and in addition to CLSM with ordinary silica sand. Results show that replacing silica sand with tailings, both as core and encapsulating matrix, significantly reduced leachability of heavy elements, particularly As (from 0.008–0.190 mg/l to 0.008–0.060 mg/l), Ba (from 0.435–1.540 mg/l to 0.050–0.565 mg/l), and Cr (from 0.006–0.458 mg/l to 0.004–0.229 mg/l), to below the ‘Dutch List’ of groundwater contamination intervention values. Arsenic leaching was inherently high from both bioleach precipitates but was significantly reduced to below guideline values with encapsulation and replacing silica sand with tailings. Tailings proved to be a valuable encapsulating matrix largely owing to small particle size and lower hydraulic conductivity reducing diffusion transport of heavy elements. Field-scale trials would be necessary to prove this concept of encapsulation in terms of scale and construction practicalities, and further geochemical investigation to optimise leaching performance. Nevertheless, this work substantiates the need for alternative backfill techniques for sustainable management of hazardous finely-sized bulk mineral residues.

Evaluating the cement stabilization of arsenic-bearing iron wastes from drinking water treatment

Cement stabilization of arsenic-bearing wastes is recommended to limit arsenic release from wastes following disposal. Such stabilization has been demonstrated to reduce the arsenic concentration in the Toxicity Characteristic Leaching Procedure (TCLP), which regulates landfill disposal of arsenic waste. However, few studies have evaluated leaching from actual wastes under conditions similar to ultimate disposal environments. In this study, land disposal in areas where flooding is likely was simulated to test arsenic release from cement stabilized arsenic-bearing iron oxide wastes. After 406 days submersed in chemically simulated rainwater, <0.4% of total arsenic was leached, which was comparable to the amount leached during the TCLP (<0.3%). Short-term (18h) modified TCLP tests (pH 3–12) found that cement stabilization lowered arsenic leaching at high pH, but increased leaching at pH<4.2 compared to non-stabilized wastes. Presenting the first characterization of cement stabilized waste using μXRF, these results revealed the majority of arsenic in cement stabilized waste remained associated with iron. This distribution of arsenic differed from previous observations of calcium–arsenic solid phases when arsenic salts were stabilized with cement, illustrating that the initial waste form influences the stabilized form. Overall, cement stabilization is effective for arsenic-bearing wastes when acidic conditions can be avoided.

Arsenic Waste Management: A Critical Review of Testing and Disposal of Arsenic-Bearing Solid Wastes Generated during Arsenic Removal from Drinking Water

Water treatment technologies for arsenic removal from groundwater have been extensively studied due to widespread arsenic contamination of drinking water sources. Central to the successful application of arsenic water treatment systems is the consideration of appropriate disposal methods for arsenic-bearing wastes generated during treatment. However, specific recommendations for arsenic waste disposal are often lacking or mentioned as an area for future research and the proper disposal and stabilization of arsenic-bearing waste remains a barrier to the successful implementation of arsenic removal technologies. This review summarizes current disposal options for arsenic-bearing wastes, including landfilling, stabilization, cow dung mixing, passive aeration, pond disposal, and soil disposal. The findings from studies that simulate these disposal conditions are included and compared to results from shorter, regulatory tests. In many instances, short-term leaching tests do not adequately address the range of conditions encountered in disposal environments. Future research directions are highlighted and include establishing regulatory test conditions that align with actual disposal conditions and evaluating nonlandfill disposal options for developing countries.

Integrated Multimedia Decision-Making for Human and Ecologic Risk Assessment: A National-Scale Study of Land Application of Arsenic-Bearing Wastes

Integrated Multimedia Decision-Making for Human and Ecologic Risk Assessment: A National-Scale Study of Land Application of Arsenic-Bearing Wastes

Reactions between cement and As(III) oxide: The system CaOSiO 2As 2O 3H 2O at 25°C

Arsenic-bearing wastes are common by-products of mineral processing and smelting. As arsenic is well known to be highly toxic, it is important that these wastes should be treated in an ecologically acceptable manner. As(III) is the species of principal concern. One treatment system uses Portland cement as a containment and chemical immobilization matrix. A review of the literature revealed the solid phases of relevance, and solubility data were critically evaluated. Based upon these data, computer-based modelling was used to identify stable phase assemblages in the CaOSiO 2As 2O 3H 2O system at 25°C and calculate equilibrium solubilities. An experimental program was undertaken to verify the modelling predictions within the CaOAs 2O 3H 2O sub-system, and experimentally-obtained solubility data were, in turn, used by the modelling program. Solubility data thus derived were critically compared with those of the literature and although there are some differences between the data sets recast in the form of phase diagrams, the two are numerically in good agreement. The solubility of As is limited by reaction with calcium hydroxide and CSH gel to ∼0.0007 mmol kg −1, or approx. 0.075 mg l −1 as arsenite (AsO 2 −), by formation of CaAsO 2OH.

Removal of Arsenic from Wastewaters and Stabilization of Arsenic Bearing Waste Solids: Summary of Experimental Studies

One of the major research efforts at Montana College of Mineral Science and Technology has been the investigation of techniques for the treatment of arsenic bearing solutions and metallurgical solid waste materials. Experimental results from several Master of Science thesis studies and other research studies are summarized in this presentation. Studies that are discussed include those that relate to: arsenic removal from solution by precipitation techniques; stabilization of arsenic bearing waste materials; and recovery of arsenic from metallurgical waste and by-products.