Removal of hexavalent chromium using one-step synthesized metal-polyphenol composites epigallocatechin gallate/titanium: Performance, interference, and mechanisms

Chromium-containing wastewater intensifies the shortage of water resources, and how to solve the problem has become a research hotspot. In this study, epigallocatechin gallate and titanium salt were combined to prepare epigallocatechin gallate/titanium composites by one-step synthesis, and the effectiveness and mechanisms of removing hexavalent chromium from wastewater were studied. Based on the characterization results, the epigallocatechin gallate/titanium nanometer-scale materials were irregularly spherical, and their surface was rich in phenolic hydroxyl groups. The average particle size, average pore size, and BET surface area were 448 nm, 6.23 nm, and 15.40 m2/g, respectively. The Freundlich isothermal model for adsorption and pseudo-second-order adsorption kinetics were both applicable to the hexavalent chromium adsorption. The maximum adsorption capacity of hexavalent chromium was 105.8 mg/g, and the adsorption process was more inclined to endothermic reaction. The salinity resistance of epigallocatechin gallate/titanium materials to different salts was sodium chloride > magnesium sulfate heptahydrate. In the organic acid-coexisting system, the removal efficiency of epigallocatechin gallate/titanium materials for hexavalent chromium with 20 mg/L fulvic acid and citric acid was 82.7% and 81.5%, respectively. The epigallocatechin gallate/titanium materials also showed excellent removal ability for hexavalent chromium, lead(II), copper(II), and nickel(II), with removal efficiencies all above 95%. Hexavalent chromium was first adsorbed and then reduced to trivalent chromium with the help of phenol hydroxyl groups on the materials. The trivalent chromium was further immobilized through electrostatic attraction and complexation. This work aimed to provide a new idea for handling hexavalent chromium-contaminated wastewater by using green and versatile polyphenol nanomaterials.