Mechanistic Insights into Sulfate and Phosphate Mediated the Dissolution of Nano-Zero-Valent Iron Wrapped by Tea Polyphenols for Hexavalent Chromium Removal

Water chromium (Cr) contamination is prevalent due to the rapid development of industry, thus making it urgent in toxic hexavalent Cr(VI) removal from wastewater with efficient methods. Green tea polyphenols (TPs) wrapped nano-zero-valent iron (GT-nZVI) has great potential in the removal of most Cr(VI). Here, the ubiquitous sulfate and phosphate in water were proved to play important roles in controlling the Cr (VI) removal efficiency. In specific, sulfate in wastewater could promote Cr (VI) removal by GT-nZVI as sulfate favors Fe(II) release and TPs dissolution from GT-nZVI surface thereby facilitating the reduction of Cr(VI) to Cr(III). When phosphate coexisted in the water, Cr (VI) removal by GT-nZVI declined from 88.9 to 72.4 mg g −1 coupled with the gradually enlarged sizes of GT-nZVI particles, as the molar ratio of phosphate/sulfate in water increased from 0.1 to 10. This change was associated with the depressed pseudo-second-order rate constant for Cr (VI) removal of 0.00083−0.00081 g mg −1 min −1 . Mechanically speaking, the predominant inhibition of phosphate on GT-nZVI dissolution was stronger than of sulfate promotion due to the inner-sphere adsorption complexation of phosphate on GT-nZVI. X-ray absorption near-edge spectra (XANES) and X-ray photoelectron spectroscopy (XPS) confirmed that the dominant Cr(OH) 3 initially generated before the Cr 2 O 3 formation on GT-nZVI after reacting with Cr(VI). In particular, sulfate favors the conversion of Cr(VI) to Cr(OH) 3 by GT-nZVI, while phosphate significantly inhibited Cr 2 O 3 production, therefore decreasing Cr(VI) reduction. This study furnishes new insights into applying GT-nZVI in efficient Cr(VI) removal via mediating the sulfate and phosphate conditions in contaminated water