Can epicatechin gallate increase Cr(VI) adsorption and reduction on ZIF-8?

The most important thing to do with hexavalent chromium (Cr(VI)) removal in wastewater through adsorption-reduction method is to find an adsorbent that has both strong adsorbability and reducibility. Herein, an eco-friendly material ZIF-8-EGCG, which was a combination of epicatechin gallate (EGCG) and ZIF-8, was synthesized to treat Cr(VI)-containing wastewater. At room temperature, the maximum adsorption amount of Cr(VI) reached 136.96 mg·g−1, and the Cr(VI) reduction rate (96%) was much better than that of the original ZIF-8 (37%). Such outcome was among the best in the existing studies using MOFs to treat Cr(VI). In addition, the characterization results showed that the surface of ZIF-8-EGCG was electronegative. The material was irregular sphere with pore size of 7.12 nm and BET surface area of 113.02 m2·g−1. The well-fitted Langmuir adsorption isotherm and pseudo first/second-order kinetics model suggested that the adsorption of Cr(VI) on ZIF-8-EGCG was governed by both physical and chemical processes. Stability results revealed that the overall stability of ZIF-8-EGCG due to the etched structure was slightly inferior to ZIF-8, especially at higher temperatures (>35 °C). Simultaneously, the reaction between ZIF-8-EGCG and Cr(VI) was a spontaneous, exothermic, entropic decrease process.