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 Cd, Ni and NH by fresh and artificially ageing biochars produced from rice straw at 400 and 700 °C (RB400, RB700, HRB400 and HRB700) were investigated. Cd competed with Ni and NH for the overlapped adsorption sites on the biochars. For Cd and Ni adsorption, cation exchange (Q) and mineral co-precipitation (Q) were the primary mechanisms for the low-temperature and high-temperature biochars, respectively. However, the other potential mechanisms (Q) made the greatest contributions to NH adsorption (>60%). Cd and Ni competition increased the proportions of mineral co-precipitation (Q) and other potential mechanisms (Q) but decreased that of cation exchange (Q) mechanism. Biochar ageing increased the contribution of surface complexation (Q) 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.