Rapid reduction of Cr(VI) with plant leaves: Implications for ex-situ phytoremediation of chromium-polluted waters in cold region

Removal of toxic Cr(VI) with waste biomass via adsorption is considered an eco-friendly technology for remediation of chromium-polluted water. However, the roles of dissolved organic matter (DOM) released from waste biomass are often neglected in the overall Cr(VI) depletion. In this study, the utilization of plant leaves (PLs), eco-friendly waste biomass resources, is investigated for Cr(VI) reduction. The abiotic reduction rates of Cr(VI) by 10 types of PLs in aqueous (25 °C) and frozen (−20 °C) solutions are compared. The higher efficiencies (18.3%–54.0%) of Cr(VI) depletion are achieved with most PLs at −20 °C under acidic conditions. The freeze-enhanced Cr(VI) reduction with PLs is also demonstrated in practical Cr(VI)-polluted industrial wastewater. Interestingly, unlike previous chemically modified leaves, almost no Cr(VI)/Cr(III) species is adsorbed on these original PLs surface. It is found that Cr(VI) reduction rate is determined by both the concentration and component of DOM. D-(−)-quinic acid from Neolitsea sericea (Bl.) Koidz and lignin/carboxyl-rich acyclic molecules (CRAM) are identified as the main components responsible for Cr(VI) reduction. It may provide an eco-friendly and cost-effective technology for ex-situ phytoremediation of Cr(VI)-polluted waters, especially in cold regions/season.