Characterisation of arsenic levels in acid-treated arsenic-containing sludge after steel slag-fly ash gel curing.

Arsenic-containing sludge (ABG) is a common hazardous waste in the metallurgical industry and poses a serious threat to environmental safety. However, its instability and mobility have a significant impact on the environment. Traditional curing methods are time-consuming and costly, often resulting in incomplete curing. In this study, we introduce a curing/stabilisation method with a steel slag-fly ash gel material after ABG acid treatment. The toxic leaching of arsenic from ABG was reduced to 220 mg/kg by treating the sludge with acids (H2 SO4 -H3 PO4 ) at different solid-to-liquid ratios. Afterward, H2 O2 was added to oxidise As(III) to As(V). The ABG was cured/stabilised using an alkali-activated steel slag-fly ash gel material. The cured product exhibited optimal arsenic fixation under an ABG/steel slag/fly ash mass ratio of 1:4:2, a curing temperature of 60°C, a curing time of 20 h, and an ambient pH of 12.5. Under these conditions, steel slag-fly ash facilitated Ca-As precipitation, resulting in a hydration reaction that produced C-S-H gel. Additionally, the reaction generated calcium hydroxide, calcium and iron pyroxene, silica, and iron ferrite, which adsorbed part of the free arsenic, completing the curing of the acid-treated ABG and stabilising arsenic leaching toxicity. The leaching of arsenic in the ABG was much lower than the Chinese 'Hazardous Wastes Leaching Toxicity Identification Standard' (GB5085.3-2007) (5 mg/L), with an arsenic curing rate exceeding 99%. The mechanism of arsenic solidification involves the combined effects of chemical precipitation, physical encapsulation, and adsorption. Collectively, our findings demonstrated that the use of steel slag-fly ash gel as a functional material for ABG curing holds considerable environmental and economic benefits. Therefore, this study provides theoretical guidance and provides insights into the experimental feasibility of ABG treatment.