Facile synthesis of polyphenol-based spherical adsorbent for simultaneous adsorption and reduction of hexavalent chromium in an aqueous environment

Polyphenol-based materials have been regarded as a promising type of adsorbent due to their potential to simultaneously adsorb and reduce Cr(VI) in water. Herein, we report a new tea polyphenol-gallic acid-based adsorbent (TPGA), which can be facilely synthesized via polymerization using tea polyphenols and gallic acid as comonomers, and histidine and formaldehyde as crosslinking agents. Under the optimum conditions of an adsorbent dosage of 0.25 g L^-1, an initial solution pH of 2.0, and a contact time of 600 min, TPGA exhibited a maximum adsorption capacity of 408 mg g⁻¹ for Cr(VI) at 298 K. A removal rate of over 89 % for Cr(VI) at a concentration of 100 mg L^-1 was achieved. The adsorption kinetics of Cr(VI) conformed to the pseudo-second-order model and the adsorption isotherm data was in line with the Langmuir model. The adsorption process was spontaneous, endothermic, and accompanied by an increase in entropy. Multiple analysis including Zeta potential, FTIR, and XPS demonstrated that the rich phenolic hydroxyl groups on the surface of TPGA served as electron donors to reduce 63.92 % of Cr(VI) to Cr(III), and the excess hydroxyl groups could coordinate with the generated Cr(III), achieving concurrent adsorption and reduction of chromium pollutants in water.