Removal and environmentally safe disposal of As(III) and As(V)-loaded ferrihydrite/biosilica composites

Pure ferrihydrite and ferrihydrite-biosilica composite were synthesized and studied for the removal of As(III) and As(V). The synthesized materials have an adsorption capacity higher than some reported materials in the literature - 140 and 90 mg g^-1 for As(III) and As(V), respectively. The pH of the solution was shown to impact greatly on As(V) adsorption, but not on As (III), which is stable as a protonated, uncharged oxyanion, at pH < 9.2. The adsorption products were subjected to thermal treatment (500 °C for 2 h), promoting ferric arsenate formation. The adsorbed As on ferrihydrite (Fh) was shown to inhibit the phase transformation of Fh to hematite. More so, thermal treatment was shown to oxidize As(III) to As (V). The changes in the adsorption residues after thermal treatment also had an impact on As mobility. The As (III) associated with the Fh phase increased from 42 to 95%, according to a sequential extraction protocol. Therefore, this work presents a process for As removal, followed by thermal treatment of arsenic-loaded ferrihydrites which enables environmentally safe disposal of As residues.

Partitioning and transformation behavior of arsenic during Fe(III)-As(III)-As(V)-SO42- coprecipitation and subsequent aging process in acidic solutions: Implication for arsenic mobility and fixation

The coprecipitation and subsequent aging of Fe(III)-As(III)-As(V)-SO₄^2- play an important role in controlling As behavior in acidic systems, such as acid mine drainage and hydrometallurgical acid waste. In this study, we investigated the redistribution and transformation of As in the Fe(III)-As(III)-As(V)-SO₄^2- system (As(III)/As(V) ≈ 1) at different Fe/As molar ratios (i.e., 0.25, 0.5, and 1) and pH (1.2 and 1.8) at 60 °C. The results showed that As(III) and SO₄^2- can be incorporated into the amorphous ferric arsenate and scorodite host phases by forming a Fe(AsO₄)_x(AsO₃)_y(SO₄)_z solid solution. As(III) contents in the freshly coprecipitated solids increased with pH and initial As(III) concentrations. During aging process, As(III) contents in the solid products with Fe/As molar ratios of 0.5 and 1 increased with aging time at pH 1.8. In contrast, As(III) was gradually expelled from aging products with aging time at pH 1.2, regardless of Fe/As molar ratio. X-ray diffraction (XRD), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy characterization results showed that an As(III)-SO₄^2--doped scorodite was formed at Fe/As molar ratio ≤0.5 during the aging process. It was also found that As(III) had an inhibitory effect on the transformation of poorly crystalline ferric arsenate to scorodite. The present study may have important implications for understanding the geochemical cycle of As, Fe, and SO₄^2- in acidic solutions and give further understanding on the mechanisms involved in As removal and fixation in hydrometallurgical unit operations.

Arsenic removal procedure for the electrolyte from a hydro-pyrometallurgical complex

Commercial copper (Cu) is obtained by a hydro-pyrometallurgical process, where the Cu anodes obtained in the furnaces (Cu > 99.5%) are enriched up to 99.99% in "cathodes" by electrorefining at an electrolysis plant. During this process, some impurities accumulate in the electrolyte, mainly arsenic (As), which decrease the quality of the Cu cathode. For this reason, the electrolyte is sent to an electrolyte cleaning plant (ECP) for its purification. Electrolyte sludge (ES) is produced in the last stage of purification and is recirculated back to the furnace due to the high Cu content. This recirculation involves a severe problem of As accumulation in the industrial process. The objective of this work was to develop a procedure to fully dissolve the ES, removing the As and recovering its Cu content. The ES dissolution process was optimised (dissolution efficiency > 99%) in H₂SO₄ (1.4 M)/HNO₃ (1.8 M) medium using a 1:20 g mL^-1 solid-to-liquid ratio. As was removed from the ES solution by its precipitation as iron (III) arsenate, with high efficiency (more than 70%). After As removal, the Cu can be precipitated as copper sulphate, which is used in several applications.

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.

Minimization and stabilization of smelting arsenic-containing hazardous wastewater and solid waste using strategy for stepwise phase-controlled and thermal-doped copper slags

Minimization and stabilization of arsenic-containing smelting wastewater and residue is of crucial issue to resolve the arsenic contamination. Calcium arsenate is a typical precipitate produced from disposal of smelting acid wastewater. However, it suffers from poor stability and large quantity in the aqueous environment. Copper slags, as for rich-iron species materials, are disposed of in landfills or open-air tailing ponds, which are another waste material that have not been effectively utilized for reuse application. In this study, strategy for sequence of phase-controlled and thermal-doped copper slag technique was used as the efficient means of minimization and stabilization of arsenic-bearing resides. Detailed results were showed that stepwise phase precipitation significantly reduced the formation of hazardous solid waste; the total solid waste was reduced 47.0 wt% because the gypsum was separated from arsenic calcium residues through two-step methods. Subsequently, solid waste stabilization was achieved by using thermal-doped slag, and the high yield of magnetite (75.6 wt%) and fayalite (22.7 wt%) was produced from copper slags. It was proved that these iron-rich species displayed the remarkable performance to stabilize arsenic due to the formation of Fe-As-Ca-O complex; compared with the raw solid waste, the arsenic leachability was decreased from 280.75 to 1.05 mg/L via copper slag stabilization process. The immobilized arsenic content was 25.0 wt%. Overall, the proposed strategy for stepwise phase-controlled and thermal-doped copper slags was a potentially effective strategy for reducing emissions and pollution of arsenic-containing wastewater and residue.

Rapid oxidation and immobilization of arsenic by contact glow discharge plasma in acidic solution

Arsenic is a priority pollutant in aquatic ecosystem and therefore the remediation of arsenic-bearing wastewater is an important environmental issue. This study unprecedentedly reported simultaneous oxidation of As(III) and immobilization of arsenic can be achieved using contact glow discharge process (CGDP). CGDP with thinner anodic wire and higher energy input were beneficial for higher As(V) production efficiency. Adding Fe(II) in CGDP system significantly enhanced the oxidation rate of As(III) due to the generations of additional OH and Fe(IV) species, accompanied with which arsenic can be simultaneously immobilized in one process. Arsenic immobilization can be favorably obtained at solution pH in the range of 4.0-6.0 and Fe(II) concentration from 250 to 1000 μM. The presence of organics (i.e., oxalic acid, ethanol and phenol) retarded the arsenic immobilization by scavenging OH or complexing Fe(III) in aqueous solution. On the basis of these results, a mechanism was proposed that the formed ionic As(V) rapidly coprecipitated with Fe(III) ions or was adsorbed on the ferric oxyhydroxides with the formation of amorphous ferric arsenate-bearing ferric oxyhydroxides. This CGDP-Fenton system was of great interest for engineered systems concerned with the remediation of arsenic containing wastewater.