At low equilibrium pH values (1.0 and 2.0e2.1), the biocharsexhibited very low immobilized Pb amount (1.25e1.91 and4.12e4.30 mg/g). This was due to the electrostatic repulsion as thebiochars' surfaces were intensively surrounded by protons undersuch low pH conditions, which repelled he positively charged Pbcations (Gao et al., 2013). At initial solution pH 6, although theequilibrium solution pH values were not significantly different fromthose at 5, the immobilized Pb amount was significantly enhanced.This was due to the surface precipitation of Pb on the biochars.Although the equilibrium solution pH values were at 5.6e5.8 whichmay not aid the precipitation of Pb, there were alkaline minerals onthe biochars’ surfaces forming local hot spots with high alkalinity.Therefore, the Pb in the solution can precipitate on these hot spots.This phenomenon was not observed for initial solution pH 4 or 5,because more alkaline minerals were consumed under these lowerpH conditions to buffer the solution to the equilibrium pH within5e6. In general, the lower Pb removal for the biochars under lowersolution pH values suggests the limitations of them in treatingacidic wastewater (e.g., acid mine drainage) containing Pb.