Sustainable Biomass\nActs as an Electron Donor for\nCr(VI) Reduction during the Subcritical Hydrothermal Process: Molecular\nInsights into the Role of Hydrochar and Liquid Compounds

Heavy metal pollution is a critical environmental issue\nthat has\ngarnered significant attention from the international community. Subcritical\nhydrothermal liquefaction (HTL) as an emerging green technology has\ndemonstrated remarkable promise in environmental remediation. However,\nthere is limited research on the remediation of highly toxic Cr(VI)\nusing HTL. This study reveals that the HTL reaction of biomass enables\nthe simultaneous reduction and precipitation of Cr(VI). At 280 °C,\nthe reduction of Cr(VI) was nearly complete, with a high reduction\nrate of 98.9%. The reduced Cr as Cr(OH)<sub>3</sub> and Cr<sub>2</sub>O<sub>3</sub> was primarily enriched in hydrochar, accounting for\nover 99.9% of the total amount. This effective enrichment resulted\nin the removal of Cr(VI) from the aqueous phase while simultaneously\nyielding clean liquid compounds like organic acids and furfural. Furthermore,\nthe elevated temperature facilitated the formation of Cr(III) and\nenhanced its accumulation within hydrochar. Notably, the resulting\nhydrochar and small oxygenated compounds, especially aldehyde, served\nas electron donors for Cr(VI) reduction. Additionally, the dissolved\nCr facilitated the depolymerization and deoxygenation processes of\nmacromolecular compounds with lignin-like structures, leading to more\nsmall oxygenated compounds and subsequently influencing Cr(VI) reduction.\nThese findings have substantial implications for green and sustainable\ndevelopment.