Alkalinity in tunnel construction wastewater as a promoter: boosting the Fe(II)/CaSO3 system for as(III) oxidation and immobilization

Alkaline As(III)-contaminated wastewater, which is prevalent at tunnel-construction sites, poses a serious risk to the environment and human health. However, the existing research on the treatment of this alkaline wastewater is scarce. Herein, we proposed a treatment method based on a Fe(II)/CaSO 3 system enhanced by highly alkaline As(III)-contaminated wastewater (pH: ~12) itself. Mechanistic investigations revealed that alkaline conditions enhanced the process in two ways: (i) by accelerating the Fe(II)/Fe(III) cycle to sustain · OH and SO 4 · − radical generation and (ii) by promoting the reaction between M(Ca/Fe)–OH and the oxidation products of AsO 4 3− species. These dual effects promoted the formation of Ca 5 (AsO 4 ) 3 OH and FeAsO 4 precipitates that immobilized As(V). Furthermore, the Fe(II)/CaSO 3 system exhibited high resistance against interference from suspended solids and other coexisting substances. The system achieved >98 % removal of 30 mg·L −1 As(III) within 10 min at pH ~12. Notably, the final effluent pH value could be reduced to an ecofriendly value of ~8, and the residual SO 4 2− concentration was lower (140 mg·L −1 ) than that observed in common sulfur-based systems (e.g., peroxomonosulfate, peroxydisulfate, and sodium sulfite). This study presents a perspective shift in treating alkaline wastewater streams by viewing a dominant waste characteristic as a resource rather than as a barrier. Moreover, it provides an effective method for As(III) decontamination of alkaline wastewater.