Immobilizing arsenic-enriched wastewater from utilization of crude antimony oxides as scorodite using a novel...

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 FeSO₄·7H₂O 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_nAsO₄^(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 △rG_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.