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Scorodite is an important mineral not only for arsenic (As) removal from industrial wastewaters but also in the mobility and final fate of As in waste rocks, contaminated soils and sediments, and mine tailings. Because of the mineral's high As-loading capacity and stability, numerous studies have been done to understand its formation. Unfortunately, most of these studies were limited to elevated temperatures (>70 °C), so the processes involved in scorodite formation under ambient conditions remain unclear. This study provides evidence of the catalytic effects of hematite on the formation of scorodite at 25 °C in a pyrite-rich natural geologic material. Scorodite peaks were detected in the XRD patterns of the leaching residues with and without hematite, but those in the former were stronger and more pronounced than the latter. These results suggest that the formation of scorodite was catalysed by hematite, a generalisation that is further supported by strong characteristic IR absorption bands of scorodite at 819 cm–1 (As–O bending vibration), 785 and 725 cm–1 (As–O stretching vibrations), and 2990 cm−1 (OH-vibration) as well as the distinct XPS binding energies of Fe(III)–As (709.7 eV), As(V)–O (44.8, 44.31 and 43.7 eV), O2− (530.5 eV) and coordinated water (531.3 eV) in scorodite. This phenomenon could be attributed to three possible mechanisms: (1) more rapid precipitation promoted by the "seeding" effect of hematite particles, (2) additional supply of Fe3+ from hematite dissolution under acidic conditions, and (3) enhanced oxidations of Fe2+ to Fe3+ and As(III) to As(V) on the surface of hematite. Image 1 • Scorodite formation was promoted in the presence of hematite. • Hematite particles acted as "seeds" that enhanced scorodite precipitation. • Partial hematite dissolution supplied Fe3+ that enhanced scorodite formation. • Hematite catalysed Fe2+ and As(III) oxidations that promoted scorodite formation. • Hematite could be used to catalyse scorodite formation at room temperature. [ABSTRACT FROM AUTHOR]

Arsenic–enriched wastewater (A–EW) is a hypertoxic sewage from the utilization of crude antimony oxides in lead anode slime metallurgy. In traditional methods, the H+ accumulation inhibits the arsenic immobilization during scorodite synthesis. In this study, a novel multivalent iron source comprised of Fe(OH) 3 and FeSO 4 ·7H 2 O was proposed to resolve the adverse effects of pH fluctuation during immobilizing A–EW as scorodite. Various approaches, such as scanning electron microscopy and X–ray photoelectron spectroscopy, were applied to characterize the synthesized scorodite. This work was divided into two parts. In thermodynamics, H n AsO 4 (3−n)- (n = 1, 2, 3) and Fe(OH) n (3−n)+ (n = 0, 1, 2, 3) can feasibly coprecipitate as scorodite according to their △ r G m , T θ ranged from −111.10 kJ mol−1 to −33.53 kJ mol−1. In experimental research, A–EW was immobilized as scorodite by optimizing conditions as initial pH = 2.0, molar ratio of Fe to As = 1.2, molar ratio of Fe(II) to Fe(III) = 4:6, arsenic concentration = 40 g/L, and temperature = 95 °C. The arsenic precipitation ratio is 99.60%, and the micromorphology of synthesized scorodite presents a regular octahedron having size of 5–10 μm. The low leachability of As (0.41 mg/L) in toxicity characteristic leaching procedure (TCLP) confirmed that the prepared scorodite is nonhazardous. The solution pH is stable at 2.0 as the H+ depletion (0.5660 mol) by Fe(OH) 3 dissolution and Fe2+ oxidization balanced with that (0.5657 mol) generated from As(V)–Fe(III) coprecipitation. In general, the A–EW was effectively immobilized by proposed multivalent iron source, and can be potentially applied to safely dispose other industrial effluents, such as high arsenic leachates and arsenic–bearing waste acid from nonferrous metallurgy. Scorodite formation mechanism and protons balance in weak acidic solution using a multivalent iron source. [Display omitted] • H n AsO 4 (3−n)- and Fe(OH) n (3−n)+ can coprecipitate as scorodite from △ r G m , T calculations. • 99.60% of A–EW was immobilized as 5–10 μm octahedral scorodite at a stable pH. • Fe(OH) 3 neutralizes extra H+ from oxidative coprecipitation of Fe(II) and As(V). • Sodium salts were recovered from solution by an evaporative crystallization. [ABSTRACT FROM AUTHOR]

Reductive dissolution of scorodite results in the release and migration of arsenic (As) in groundwater. The purpose of this study was to explore the possible abiotic and biotic reduction of scorodite in groundwater environment and the effect of microbial-mediated sulfur cycling on the bio-reduction of scorodite. Microcosm experiments consisting of scorodite with bacterium Citrobacter sp. JH012-1 or free sulfide were carried out to determine the effects of thiosulfate on the mobilization of As/Fe. The results show arsenic release is positively correlated with iron reduction. The arsenate [As(V)] released can agglomerate with Fe(II) on the surface of scorodite to form crystalline parasymplesite, while no parasymplesite was detected in the abiotic reduction of scorodite by sulfide. The reduction of scorodite and As(V) was affected by thiosulfate. When 0.5 mM thiosulfate was added, the Fe(III) reduction rate increased from 32% to 82%, and the As(V) reduction rate rose from 54% to 64%. When the addition of thiosulfate was increased from 0.5 mM to 2 mM and 5 mM, Fe(III) reduction rate added 4% and 8%, and As(V) reduction rate increased 11% and 16%, respectively. In addition, the presence of thiosulfate promoted the scorodite almost completely converting to parasymplesite. Therefore, the effect of microbial-mediated sulfur cycling should be considered in arsenic migration and reduction from scorodite. [Display omitted] • The release of arsenic was positively correlated with the reduction of scorodite. • Parasymplesite was the main product of scorodite bio-reduction. • Thiosulfate promoted the bio-reduction of scorodite. • The formation of parasymplesite was affected by thiosulfate. [ABSTRACT FROM AUTHOR]

High-arsenic wastewater has long been considered a major threat to ecological balance and human health because of its strong toxicity and high mobility. Herein, an environmentally friendly process was proposed for As removal and fixation in the form of As-stabilized mineral, using Lead–Zinc smelting (LZS) slag as the in situ Fe donor, neutralizer, and crystal seed. The slag was dissolved in the wastewater and released Fe and Ca ions, while simultaneously increasing the pH value of the solution to help scorodite synthesis. The dissolved Ca2+ ion preferentially reacted with SO 4 2− ion in the form of CaSO 4 ·2H 2 O precipitate as in situ "seeds" for As precipitation. The dissolved Fe(II) and As(III) ions were oxidized to Fe(III) and As(V) ions by H 2 O 2 , and later reacted with each other to generated amorphous ferric arsenate on the surface of CaSO 4 ·2H 2 O, and then evolved into scorodite crystals with high stability. With a Fe/As molar ratio of 2, a reaction temperature of 90 °C, and a reaction time of 12 h, 98.42% of As was effectively precipitated from the wastewater with an initial As concentration of 7530.00 mg/L. Moreover, the leached As concentration of the As-bearing precipitate in the TCLP test was 3.46 mg/L. The concentration of the residual As and heavy metals ions in the final filtrate was lower than local wastewater discharge standards, successfully realizing the treatment of smelting wastewater. In summary, a prospective process successfully shows a great potential for co-treatment of LZS wastewater and slag, which could advance the large-scale disposal of LZS plants. [Display omitted] • LZS slag as an in situ Fe donor and neutralizer in the form of scorodite for arsenic removal. • The newly generated CaSO 4 ·2H 2 O precipitate provided abundant nuclear sites for scorodite synthesis. • Arsenic was almost completely removed from LZS wastewaters after deep purification. • The co-treatment of LZS wastewater and slag can mitigate arsenic contamination risk. [ABSTRACT FROM AUTHOR]

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 H 2 SO 4 (1.4 M)/HNO 3 (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. [Display omitted] • New procedure for arsenic removal in the electrolyte wastewater from cathode copper. • Fully dissolve electrolyte sludge was achieved. • High arsenic removal efficiency was obtained. [ABSTRACT FROM AUTHOR]