Biochars and modified-biochars for toxic-metal/metalloid ions sorption in various mixed solution systems: A review on kinetic and isotherm models

Both biochars (BCs) and modified-biochars (MBCs) for toxic-metal/metalloid ions (TMIs) removal from water have caught much attention owing to their exceptional properties such as eco-friendly, low-carbon emission and cost-effective. However, reviews on the exploration of theoretical models of BCs and MBCs in various systems are unobtainable. To give a summary of theoretical models on BCs and MBCs for TMIs removal in various solution systems, this review will center on some typical kinetic and isotherm equations of BCs and MBCs for TMIs sorption in a single, mixed, mixed metal ions/organic substance solutions. Typically, two-parameter and three- parameter kinetic and isotherm equations of BCs and MBCs for TMIs sorption were stressed. The comparisons of these models in various solution systems were drawn. It was found that P-2nd-O kinetic model, Langmuir and Freundlich isotherm models are the dominating adsorption manners for TMIs in various MSSs. Especially, only the kinetic models of Cu(II), Pb(II), Cr(VI) and Cd(II) sorption, as well as the isotherms of Cu(II) and Cd(II) sorption on BCs and MBCs were reported in these MSSs. We hope such a summary will be favorable for the accurately selecting of optimal models to explain the sorption processes in different metal-solution systems.