Investigation on the performance and mechanism of Cr(VI) immobilization with improved amorphous iron polysulfide: Batch experiments and numerical simulation

Abstract

Amorphous iron sulfides (FeS_x), which are more powerful than crystalline FeS_x in reduction, are commonly found on the surfaces of many sulfidated nanoscale zero-valent iron (nZVI). However, due to the interference of nZVI, the reduction performance of FeS_x is still unknown. In this paper, we synthesized efficient amorphous iron polysulfide (AIPS) with FeSO₄ and CaS_x and checked its reduction ability using Cr(VI) as a model contaminant, and found that the reactivity and amorphous structure of AIPS were significantly affected by the titration rate of CaS_x during synthesis. The removal of Cr(VI) by AIPS synthesized at a titration time of 120 min was 81.9 mg/g, which was 10.6 times higher than that of AIPS synthesized within 6 min (7.7 mg/g). The improved AIPS can be applicable for Cr(VI) removal via simultaneous adsorption, reduction and precipitation at wide pH range and in the coexistence of ionic and natural organic matter (NOM). Using a reactive transport model, it can be found that FeS and FeS₂ in AIPS contribute 85.01 % and 14.99 % to the Cr(VI) reduction, respectively. FeS_x (x > 2), also as a reaction by-product of the Cr(VI) reduction by FeS and FeS₂, hinders the reduction of Cr(VI).