Efficient removal of Cr(VI) by mechanochemically modified pyrites: Self-regulated neutralization behaviors

The pH of the aqueous environment commonly occurs during the application of natural/modified pyrite in water/soil remediation. In this study, mechanochemical modification of natural pyrite (M−pyrite) was employed to remove Cr(VI) over a wide pH range. Self-regulated neutralization was observed during Cr(VI) removal by M−pyrite in the initial pH range of 2.0–12.3. The reductive immobilization of total Cr occurred effectively when the solution self-regulated to a circumneutral pH (pH 5.0–9.0), but only efficient Cr(VI) reduction occurred under strongly acidic or alkaline conditions. Three pathways, i.e., Fe(II) removal, surface sulfide reduction and both combinations, could be revealed for Cr(VI) removal by M−pyrite at circumneutral pH and under strongly alkaline and acidic conditions. As Cr(VI) is reduced by ferrous and sulfide species, the protons generated from pyrite oxidation by O2 are consumed, causing neutral restoration to resist the initial sharp decrease in pH. Effective immobilization of the reduced Cr(VI) was confirmed at an initial pH of 5.0–9.0, but this immobilization did not occur at more acidic and alkaline pH values due to strong Fe(II) dissolution or surface precipitation. The M−pyrite was successfully applied in treating four types of saline industrial Cr(VI) wastewaters, demonstrating the advantage of self-regulated neutralization resistance in complex wastewater matrices.