Mechanistic insights into sulfate and phosphate-mediated hexavalent chromium removal by tea polyphenols wrapped nano-zero-valent iron

Nano zero-valent iron via green synthesis (g-nZVI) has great potential in removing toxic hexavalent Cr(VI) from industrial wastewater. Sulfate and phosphate in wastewater can influence Cr(VI) removal by g-nZVI. In this study, the Cr(VI) removal kinetics by different g-nZVI materials were investigated with the existence of sulfate and/or phosphate, and the corresponding mechanisms were first revealed using multiple characterizations, including X-ray absorption near-edge spectra (XANES) and X-ray photoelectron spectroscopy (XPS). The results showed that Cr(OH)(3) was the dominant species initially formed on the surface of g-nZVI particles before transforming to Cr2O3 during the reaction of g-nZVI with Cr(VI). Sulfate in wastewater can promote the reduction from Cr(VI) to Cr(OH)(3) by g-nZVI, because sulfate triggers the release of Fe(II) and tea polyphenols (fromtea extracts) fromthe g-nZVI surface due to the corrosion of Fe-0 core, which is in line with an obvious increase in pseudo-second-order rate constant (k(2)) and subtle change in Cr (VI) removal capacity (q(e)). However, phosphate impedes the g-nZVI corrosion and inhibits qe because of the innersphere complexation of phosphate onto g-nZVI decreasing the released Fe(II) for Cr2O3 production. When sulfate and phosphate coexisted in contaminated water, the inhibition effect of phosphate in Cr(VI) removal by g-nZVI was stronger than the promotion of sulfate. Accordingly, qe value of g-nZVI declined from 93.4 mg g(-1) to 77.5 mg g(-1), while k(2) remained constant as the molar ratio of phosphate/sulfate increased from 0.1 to 10 in water. This study provides new insights into applying g-nZVI in efficient Cr(VI) removal from contaminated water with enrichment of sulphates and phosphates.

Automated summary

This study investigates the effects of sulfate and phosphate on the removal kinetics of hexavalent chromium (Cr(VI)) by different nano zero-valent iron materials. The results reveal that sulfate promotes the reaction while phosphate inhibits it. When both sulfate and phosphate are present, the inhibition effect of phosphate is stronger than the promotion effect of sulfate. This study provides new insights into the efficient removal of Cr(VI) from contaminated water using nano zero-valent iron.