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

•The adsorption sites for Cd2+, Ni2+ and NH4+ on the biochars were competitive.•Cation exchange and mineral co-precipitation dominated Cd2+ and Ni2+ adsorption.•The other potential mechanisms dominated NH4+ adsorption.•Competition between Cd2+ and Ni2+ decreased cation exchange mechanism.•Biochar aging increased the contributions of surface complexation mechanism.