Efficient removal of Cr(VI) by spent coffee grounds: Molecular adsorption and reduction mechanism

Spent coffee ground (SCG), a byproduct from the soluble coffee industry, is usually discarded as waste. The reutilization of SCG for the removal of toxic heavy metal ions is a novel research direction. Until recently, the molecular adsorption and reduction mechanism of Cr(VI) on SCG was barely investigated. In this study, SCG was used for the efficient removal of Cr(VI) at a concentration range of 2`-100 mg/L, with a maximum Cr(VI) uptake up to 36.2 mg/ g. Structural characterization and ATR-FTIR analysis indicated that SCG possessed abundant surface O and N- containing functional groups. The corresponding adsorption and reduction effects on the Cr(VI) removal were investigated by the carboxyl and hydroxyl groups elimination experiments and ATR-FTIR characterization, respectively. The results revealed that HCrO4 ions were preliminarily adsorbed on SCG surfaced-COOH/-OH/-NH by the formation of hydrogen bond (SCG surfaced-COOH/-OH/-NH-HCrO44), and quickly reduced to Cr(III) by the electron denoted by phenolic compounds, and then in-situ immobilized on the surface of SCG. The effect of Cr(VI) concentration, coexisting ions, and humic acid was systematically studied to optimize the removal of Cr(VI) wastewater. Column experiments provided a new substitution to restore the Cr(VI)-containing groundwater for the permeable reactive barrier application. Thus, the proposed study uncovered the intrinsic Cr(VI) removal mechanism at the molecular level and explored the application of SCG for the efficient removal of Cr(VI).