The difference on the concentrations of PO43− between B and B + Zn implied the formation of Zn3(PO4)2 on biochars, however the concentrations of PO43− in B were no more than 0.06 mmol L−1, thus the precipitation role of PO43− in the studied biochars can be ignored. The concentrations of Ca2+ and Mg2+ in B + Zn were much higher than those of Ca2+ in B; expect for the condition of PN350, the concentrations of Mg2+ in B + Zn were also higher than those of Mg2+ in B. This may be caused by the competition of Zn2+ for released OH−, CO32−, and some sorption sites. When raw biochar was added into water, plenty of ions such as Ca2+, Mg2+, OH−, and CO32− were released from biochar to bulk solution. With the gradual increase in concentrations of Ca2+, Mg2+, and anions such as OH−, CO32− in the solution close to biochar surface, some of these ions would be over-saturated, Ca, Mg-precipitates would form and deposit on the surface of biochar. In addition to forming precipitates, a portion of Ca2+ and Mg2+ could also be captured by oxygen-containing functional groups of biochar. Thus, in raw biochar-water systems, the newly released Ca2+ and Mg2+ can be immobilized rapidly. When Zn2+ was present in the solution, Zn2+ would diffuse to the area close to biochar surface, they could also react with the newly released anions (i.e. OH− and CO32−) and occupy the sorption sites (i.e. oxygen-containing functional groups of biochar). This process would decrease the concentrations of OH−, CO32−, and PO43− and sorption sites of Ca2+ and Mg2+, which further resulted in the increased amounts of released Ca2+ and Mg2+ in raw biochar-Zn solution systems (B + Zn). The low sorption capacity of Zn2+ on PN350 and inhomogeneous distribution of Mg on biochar may lead to the lower concentration of Mg2+ in B(PN350) + Zn, when compared with those of Mg2+ in B(PN350) + Zn.