Abstract
Pyrolysis temperature-dependent changes in pH, ionic strength, and metal ion-coordinating dissolved organic carbon (DOC) control the sorption of CdII and other transition metals on biochar. Sorption of CdII 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” CdII species engaging in the surface interaction with biochar. Sorption of CdII 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 ([CdII]:[citrate] molar ratios of 4–24) dramatically enhanced CdII sorption on lower temperature (300–500 °C) biochars; higher citrate concentrations diminished CdII sorption. Citrate-enhanced CdII 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 fun...
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