One hundred milligram of raw biochars or de-ashed biochars were added into 25 mL of Zn(NO3)2 solutions in 50 mL vials. These vials were shaken at 200 rpm at 25 °C. For raw biochars, the Zn2+ concentration in Zn(NO3)2 solution was 150 mg L−1 and the pH of the solution was not adjusted. For the sorption experiments of de-ashed biochars, the Zn2+ concentration was 50 mg L−1 and the pH of the solution was adjusted to 7 buffered by 5 mM MOPs. The samples were withdrawn at appropriate time intervals, then the mixture was filtered with a 0.45 μm membrane, and the Zn2+ concentrations of the filtrates were determined. The sorption capacity of biochar in each time interval was obtained by Supplementary Equation S1. The concentrations of CO32−, Si species, PO43−, Ca2+, and Mg2+ were also determined in the samples withdrawn at 120 h, then concentrations of these five species in the filtrates of blank samples (0.1 g of biochar + 25 mL of water) at 120 h were determined for comparison. The concentrations of Zn2+, Ca2+ and Mg2+ were determined using Atomic Absorption Spectrometer (AAS, Hitachi-Z2000, High-Technologies Corporation, Japan); the concentrations of CO32− were determined using titration method44; the concentrations of Si species were determined using Inductively Coupled Plasma − Atomic Emission Spectrometer (ICP-AES, OPTIMA 8000, PerkinElmer, USA); and the concentrations of PO43− were determined using molybdate-ascorbic acid method. Two parallel samples were applied in sorption samples. Sorption isotherms were performed the same way as sorption kinetics except for the initial concentrations of Zn2+ applied (see Supplementary S6).