Reclamation of a waste arsenic-bearing gypsum as a soil conditioner via acid treatment and subsequent Fe(II)[sbnd]As stabilization

Abstract

As a common hazardous waste in metallurgical industry, arsenic-bearing gypsum (ABG) is a great threat to ecological safety. However, the recycle of ABG has rarely been conducted due to its high arsenic mobility and worthless constituents. In this study, an ABG was successfully reclaimed as a soil conditioner via acid treatment and Fe(II)As stabilization. Firstly, the arsenic content of ABG was decreased below Japan standard (150 mg/kg) regarding safe materials by acid treatment (H2SO4H3PO4). Then upon heating oxidation, Fe2+ was used to stabilize the arsenic remaining in gypsum sludge at weakly acidic condition (pH 4) via forming insoluble FeAs compounds. In toxicity characteristic leaching procedure (TCLP), the arsenic leaching (0.125 mg/L) from reclaimed gypsum was much lower than TCLP limitation (5 mg/L). And in Japan standard leaching test (JSLT), it was suppressed below the Japan environmental criteria (<10 μg/L), remaining stable for three months upon air exposure. Furthermore, the possible influence factors regarding Fe-ABG stabilization including the destabilizing effects of acid treatment were examined. It showed that the oxidation was necessary for Fe(II)As immobilization. While keeping the weakly acidic condition was crucial important for that of Fe(III). Additionally, acid treatment would make troubles for subsequent FeAs fixation (mainly for Fe 3+). Especially, the introduction of free PO43− or Cl− ions would weaken the stability of insoluble FeAs compounds via forming iron phosphates or iron chloride hydrates and lead to the formation of soluble lollingite (FeAs2), causing arsenic leaching and compromising FeAs stabilization.