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• Simultaneous oxidation and removal of As(III) to form scorodite. • Arsenic removal efficiency reach 97% for high As(III) concentration of 15 g/L. • A critical role of HO•, Fe(III) and Fe(IV) in the oxidation of As(III). • A synthetic transformation mechanism of scorodite via a four-stage process. Due to the lack of affordable and green disposal technologies, high arsenic-containing waste acid from the heavy nonferrous metallurgical smelter causes serious environmental pollution and threatens human health. In this study, a cost-effective and environmentally-friendly solution for simultaneous oxidation and precipitation of arsenic as well as formation of scorodite by adding ferrous sulfate as an iron source is proposed under elevated temperature (170 °C) and pressurized oxygen (1 MPa) conditions. And that the total reaction can be interpreted as 4Fe2+ + 4HAsO 2 + 3O 2 + 10H 2 O → 4FeAsO 4 ·2H 2 O + 8H+. Consequently, the final oxidation and removal efficiency of arsenic reached 98.95% and 97.42%, respectively, resulting in scorodite with an arsenic leaching concentration of 1.4 mg/L in the TCLP leaching test. The results prove that the HO•, Fe(III) and Fe(IV) produced by the oxidation of Fe(II) is of capable oxidizing As(III) to As(V). Most importantly, the synthetic transformation mechanism of scorodite via a four-stage process is revealed, including the oxidation of As(III), formation of a mixture of amorphous hydronium jarosite and iron arsenate as precursors, partial dissolution of the precursor as well as nucleation and growth of scorodite particles. [ABSTRACT FROM AUTHOR]

[Display omitted] • An economical and efficient method for the treatment of As in WSA is proposed. • The removal efficiency of As from WSA can reach up to 99.99%. • The leaching concentration of As of products is as low as 0.13 mg/L. • Different iron arsenate phases formed during the reaction. • Both ferric arsenate and adsorbed As contribute to the high As removal efficiency. High concentration of arsenic (As) in waste sulfuric acid (WSA) generated from hydrometallurgical mineral processing must be removed and immobilized for protecting local environments. However, the economical and efficient treatment method for WSA is still lacking. We proposed the precipitation of As in WSA as scorodite by using economical siderite (FeCO 3) as an Fe source through a three-step process. Firstly, the WSA is neutralized to pH ~ 1 with Ca(OH) 2 via precipitation of gypsum with low As content (~ 7.1 × 10-2 < 1 g/kg As). Secondly, As(III) in the WSA is oxidized to As(V) with H 2 O 2. Thirdly, the produced As(V) is immobilized as scorodite through a mutual cycle composed of siderite dissolution-oxidation by O 2 (g) and Fe(III)-As(V) precipitation-crystallization. Slow dissolution-oxidation of siderite produces a low supersaturation of Fe(III), ensuring the continuous Fe(III)-As(V) precipitation as well as scorodite crystallization. Under the optimal condition (pH initial = 1.1, Fe/As = 2, Temperature = 95 °C, and Time = 10 h), the removal efficiency of As in WSA reached up to 99.99%. The produced solid residue displayed a good stability with leaching As concentration as low as 0.13 mg/L via the TCLP methods due to the formation of different ferric arsenate minerals and the adsorption of As on undissolved siderite and FeOOH minerals. Our findings provide an economical method for safe treatment of As-bearing hydrometallurgical WSA and shed new light on the understanding of As-Fe precipitation in acidic solution. [ABSTRACT FROM AUTHOR]

[Display omitted] • Arsenic conversion efficiency about 99% yielded scorodite as the main solid phase. • The high stability of the synthesized biogenic reflected the low arsenic leachability during the extended leaching test. • Acidianus spp. were the dominant species in the continuous reactor system. Herein we report the findings of an integrated green process for the continuous oxidation and biomineralization of the most toxic As(III) from acidic streams using a laboratory-scale airlift bioreactor operated at thermoacidophilic conditions and fed with Fe(II) as electron donor. As(III) oxidation catalyzed by granular activated carbon (GAC), biological Fe(II) oxidation and scorodite crystallization took place simultaneously in the reactor, allowing the treatment of influent solutions containing 0.65 g·L−1 As(III). At a hydraulic retention time of 2.2 days, a stable arsenite oxidation efficiency of 99% was achieved, while the removal of total arsenic was 93%. Scorodite was yielded as the main solid product whose physical characteristics such as average size (250 µm) and the developed crystalline structure allowed the easy harvesting from the reactor and reflected the high stability by the low arsenic release of 0.4 mg.L−1 after 60 days of leaching. The analysis of the microbial composition in the reactor suspension and the precipitates indicated the dominance of thermoacidophilic archaeon of the genus Acidianus. Similarly, the attachment of microorganisms to the precipitates observed by scanning electron microscopy, suggested that the precipitation in our system was biologically mediated. The simultaneous arsenic oxidation and removal through catalyzed oxidation and biological processes provide the basis for a new and cost effective green methodology for arsenic fixation from acid As(III)-containing wastewaters. [ABSTRACT FROM AUTHOR]

• Pyrite as an in situ Fe(II) and S(-II) donator in the form of scorodite and As 2 S 3 precipitate for arsenic removal. • Almost 100% of As was removed from high arsenic-bearing wastewaters after deep purification. • A process for the effective disposal of nonferrous smelting wastewaters was proposed. Nonferrous smelting wastewaters containing high levels of arsenic and sulfuric acid threaten human health and ecological environments due to insufficient disposal technologies and/or possible leakages during transportation to long-term storage facilities. Chemical precipitation is one important route for arsenic removal and immobilization via the formation of arsenic-stabilized minerals. In this study, we propose a new strategy for the treatment of wastewaters containing high levels of arsenic using pyrite as an in situ S and Fe source to prepare environmentally friendly scorodite and As 2 S 3 precipitates. Dissolved pyrite release (S-II) and Fe (II) ions in highly acidic wastewater, both of which drive arsenic precipitation. Sulfide ions readily react with As(III) to generate the stable As 2 S 3 while Fe(II)/As(III) were oxidized to Fe(III)/As(V) by H 2 O 2. The reaction between Fe(III) and As(V) took place on the pyrite surface as a nucleation site to form amorphous ferric arsenate that ultimately transformed into crystalline scorodite. Sulfide ions were oxidized to H 2 SO 4 that further promoted pyrite dissolution. Up to 99.4% of arsenic was successfully removed from copper smelting wastewaters with an initial arsenic concentration of 11200 mg/L. The concentration of leached As from the As-bearing precipitate was 1.78 mg/L under optimized conditions, which were: a 1.4 FeS 2 /As molar ratio, an initial solution pH of 2, an H 2 O 2 /As molar ratio of 1%, a reaction temperature of 90 °C, and a reaction time of 12 h. In summary, a prospective process successfully shows arsenic removal and immobilization from smelting wastewater, and further demonstrates the potential for the efficient and affordable disposal of wastewater. [ABSTRACT FROM AUTHOR]