Effect of solution chemistry on the removal of Co(II) on rice straw-derived biochar

Herein, the sorption of Co(II) onto rice straw-derived biochar as a function of various environmental conditions was investigated in batch experiments. The removal efficiency was strongly affected by pH but not by ionic strength over the whole pH range. At low pH values, inner-sphere surface complexation dominated the sorption. While at high pH values, both precipitation and inner-sphere surface complexation controlled sorption process. The kinetics was simulated well by the pseudo- second-order model and was affected by both the film diffusion and the intraparticle diffusion model. The initial Co(II) concentration significantly enhanced the sorption amount of Co(II) on biochar, but weakly affected the sorption rate. Any of the Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models could simulate sorption isotherms well, while the Langmuir model fitted it the best. The thermodynamic data implied the Co(II) sorption was enhanced at high temperature and was a spontaneous and endothermic process. In addition, the biochar can be employed repeatedly in Co(II) sorption process. These results demonstrate that rice straw-derived biochar can be a suitable adsorbent for Co(II) removal from aqueous solutions.