Microwave-Enhanced Tea Polyphenol Reduction of Natural Iron Minerals and Effective Immobilization of Hexavalent Chromium in Soil

Abstract

Remediation of hexavalent chromium (Cr(VI)) contaminated soil has become a global challenge, not only because of the carcinogenicity and high toxicity of Cr(VI), but also because the remediation of chromium contaminated soils is often affected by the complexity of different external factors. Tea polyphenols (TP), as an eco-friendly material, can not only reduce hexavalent chromium in the soil, but also efficiently reduce Fe(III) in the soil, and further promote the reduction and immobilization of soil Cr(VI). In this study, a new cost-effective, sustainable and efficient soil remediation strategy was proposed to enhance the detoxification efficiency and long-term stability of Cr(VI) by utilizing TP to complex and reduce iron minerals in the soil with the aid of microwave (MW). Microwave-assisted TP (0.5 wt% TP+MW (700 W, 10 min)) enhanced the removal of Cr(VI) from iron-enriched soil from 70.49% to 99.94% as compared to TP (0.5 wt%) reduction alone. SEM-EDS, XRD, FTIR and XPS studies indicated that the increase in Cr(VI) removal efficiency by tea polyphenols was attributed to the accelerated reduction and fixation of Cr(VI) by tea polyphenols promoting the reduction of soil iron minerals and the conversion of Cr stabilized on the mineral surface to Cr2O3 and FeCr2O4 under MW irradiation. TP can directly complex and reduce solid-phase Fe minerals leading to a significant increase in the Fe(II) content, which is further enhanced by MW irradiation by the formation of a stable chromite. In addition, the toxic leaching concentration of Cr(VI) in soil remained below 0.05 mg·L-1 after 360 days of immobilization of microwave-assisted tea polyphenol-treated soils placed in the atmosphere. The technology offers an effective alternative for remediation Thus, the technology provides an effective alternative for The technology offers an effective alternative for remediation of soil Cr(VI).

Keywords: Heavy metal, Tea polyphenols, Soil remediation, Microwave-facilitated reaction, Metal contamination