Competitive adsorption behaviour and mechanisms of cadmium, nickel and ammonium from aqueous solution by fresh and ageing rice straw biochars.

In this study, competitive adsorption behaviour and mechanisms of Cd2+, Ni2+ and NH4+ by fresh and artificially ageing biochars produced from rice straw at 400 and 700 °C (RB400, RB700, HRB400 and HRB700) were investigated. Cd2+ competed with Ni2+ and NH4+ for the overlapped adsorption sites on the biochars. For Cd2+ and Ni2+ adsorption, cation exchange (Qci) and mineral co-precipitation (Qcp) were the primary mechanisms for the low-temperature and high-temperature biochars, respectively. However, the other potential mechanisms (Qco) made the greatest contributions to NH4+ adsorption (>60%). Cd2+ and Ni2+ competition increased the proportions of mineral co-precipitation (Qcp) and other potential mechanisms (Qco) but decreased that of cation exchange (Qci) mechanism. Biochar ageing increased the contribution of surface complexation (Qcf) mechanism, especially for the low-temperature biochars. This study indicated that biochar aging and types and states of adsorbates should be considered when biochars were applied to remove contaminants.