Fifty milligram of raw biochars or de-ashed biochars were added into 23 mL of deionized (DI) water in 50 mL vials. These vials were shaken at 200 rpm at 25 C for 3 day equilibrium. Then the pHs of the mixtures in these vials were adjusted to 2–10. Certain amount of Zn(NO3)2 stock solution was dripped into each vial to make the initial concentration of Zn2+ to be 150 mg L−1 for raw biochar and 50 mg L−1 for de-ashed biochar. DI water was then added into the vial to make the solution volume up to 25 mL. The following procedure was the same as that of isotherms experiments and the sorption capacity of biochar was obtained by Supplementary Equation S6.