The effect of initial pH on sorption capacities of Zn2+ on biochars were shown in Supplementary Fig. S4. As can be seen from Fig. S4, the sorption capacities of PN550, WS350, and WS550 decreased gradually with the decrease of initial pH. Unlike the sorption behaviors of raw biochars, sharp decreases occurred on the sorption capacities of de-ashed biochars, when the initial pH decreased from 9 to 7. This may be caused by the absence of buffer effect of ash. The low sorption capacity of Zn2+ on PN350 under the whole pH range could be due to the low ash content.