Selenium removal from water using modified biochar: A critical review and insights to adsorption mechanisms through computational analyses

Selenium (Se) is an essential micronutrient for human and animal health and becomes toxic at elevated concentrations. Escalating Se concentration in water bodies has become a growing serious global issue. Biochar (BC) is recognized as a green adsorbent for the removal of potentially toxic metals including Se. However, the practical application of pristine BC is hindered by its limited efficacy for Se oxyanions. To improve the removal efficiency of BC for Se oxyanions various techniques have been employed to modify BC and enrich it with a range of physiochemical attributes. This systematic review aimed to critically analyze the efficacy of various modified biochar (MBC) for Se oxyanions removal from water, comprehend their adsorption mechanisms at the molecular level, and evaluate adsorption influencing parameters. The literature (2010–2024) showed that Se oxyanions removal using MBCs better fitted with Langmuir isotherm model and their removal kinetics followed the pseudo-second-order model. Moreover, the solution pH plays a critical role in the removal of Se oxyanions using MBCs; maximum removal was reported under acidic conditions. The potential removal mechanisms include surface complexation, reduction, and electrostatic interactions. Simulations (molecular dynamics and density functional theory) were conducted to elucidate the removal mechanism at the molecular scale and demonstrate alignment between experimental and computational findings. Moreover, the Bader charge analysis was employed, and its findings revealed the transfer of electrons from the surface of MBCs to Se oxyanions. This review delivers a simulation methodology for screening MBCs for removal of Se oxyanions from water prior to experimental efforts.