Influence of pH, Ionic Strength, and Multidentate Ligand on the Interaction of CdIIwith Biochars

Pyrolysis temperature-dependent changes in pH, ionic strength, and metal ion-coordinating dissolved organic carbon (DOC) control the sorption of Cd II and other transition metals on biochar. Sorption of Cd II on 300–700 °C pecan shell biochars was strongly pH- and ionic strength-dependent. Equilibrium speciation calculation suggested that CdOH + and other hydrolysis products were the “reactive” Cd II species engaging in the surface interaction with biochar. Sorption of Cd II on 700 °C (lowest O:C) biochar progressively increased from pH 3 to 7 and was not affected by the citrate concentration. In contrast, low concentration of citrate ([Cd II ]:[citrate] molar ratios of 4–24) dramatically enhanced Cd II sorption on lower temperature (300–500 °C) biochars; higher citrate concentrations diminished Cd II sorption. Citrate-enhanced Cd II sorption was accompanied by (i) a several-fold increase in DOC and (ii) pH buffering near 7 at equilibrium. Carboxyl ligands form an unusually strong hydrogen bond with the surface functional group of biochar having a similar p K a . Sorbed ligand (citrate) could (i) provide new Cd II sorption sites on biochar, (ii) buffer pH near 7, and (iii) facilitate the release of higher molecular weight DOC from biochar.