Hexavalent chromium removal by green synthesized nano-size iron particles combined with iron sulfides: Effects of dissolved oxygen and phosphate

The removal of hexavalent chromium (Cr(VI)) from wastewater using the green synthesized nano-zero-valent-iron (GnZVI) has attracted great attention. However, the Cr(VI) removal capacity of GnZVI remains limited by dissolved oxygen (DO) or phosphate in wastewater. Therefore, Cr(VI) removal by GnZVI combined with iron sulfides (i.e., FeS or FeS 2 ) impacting by DO and phosphate was investigated at pH 2.0 and 6.0. Experimental results showed that DO inhibited the Cr(VI) removal by GnZVI with FeS at pH 2.0, because high DO saturation was beneficial to Fe(II) oxidization into Fe(III). Nevertheless, DO promoted the corrosion and dissolution of FeS 2 but GnZVI for Cr(VI) reduction at pH 2.0. Especially, phosphate inhibited the Cr(VI) reduction by the combination of GnZVI and iron sulfides at pH 2.0, likely because phosphate inhibited Fe(II) release from GnZVI hybrid materials during the reduction process from Cr(VI) to Cr (III). Differently, at pH 6.0, phosphate mainly promoted Cr(VI) removal by FeS compared with GnZVI via the formation of phosphate complexation, while it inhibited Cr(VI) removal using GnZVI combined with FeS 2 through the competitive adsorption. This study reveals the potential inherent mechanisms and emphasizes a sustainable strategy for the effective removal of Cr(VI) using a hybrid technique that combines GnZVI and iron sulfides under complex wastewater conditions.