Structural dependent Cr(VI) adsorption and reduction of biochar: hydrochar versus pyrochar

Hydrochar and pyrochar are two typical biochars, and possess different intrinsic structures and chemical properties as well as pollutant removal abilities. However, their structural dependent pollutant removal performances and the related mechanisms are far less studied. In this study, we systematically compared the Cr(VI) removal processes of hydrochar and pyrochar in dark and under simulated sunlight at pH 5.7 ± 0.1, aiming to clarify the structural dependent Cr(VI) removal of biochar. In dark, hydrochar could remove 19.0% of Cr(VI) only via adsorption within 8 h, less than that (23.5%) of pyrochar via both adsorption and indirect solution •O₂^- reduction pathway. Although simulated sunlight irradiation could significantly promote the Cr(VI) reduction performances of both hydrochar and pyrochar, the Cr(VI) reduction percentage (88.1%) of hydrochar via both direct surface electron reduction and indirect solution •O₂^- reduction pathways, was much higher than that (30.2%) of pyrochar only via indirect solution •O₂^- reduction pathway. This different Cr(VI) reduction pathway of hydrochar and pyrochar was arisen from their structural dependent Cr(VI) adsorption models, as revealed by ATR-FTIR characterization and DFT calculation. More phenolic -OH group on hydrochar surface provided abundant sites for Cr(VI) chemical adsorption to form a strong inner-sphere complex, favoring the interfacial electron transfer for the direct surface Cr(VI) reduction. In contrast, more micropores in pyrochar were responsible for the Cr(VI) physical adsorption via intra-particle and boundary layer diffusion, which hampered the surface Cr(VI) direct reduction because of the weak interfacial interaction between Cr(VI) and pyrochar. This study clarifies the influence of surface structure on the Cr(VI) adsorption and reduction pathways of biochar, and also provides an efficient Cr(VI) removal strategy with sunlight and hydrochar.