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Title: Influence of pH, ionic strength, and multidentate ligand on the interaction of CdII with biochars

item Uchimiya, Sophie

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2014
Publication Date: 7/7/2014
Citation: Uchimiya, M. 2014. Influence of pH, ionic strength, and multidentate ligand on the interaction of CdII with biochars. ACS Sustainable Chemistry & Engineering. 2(8):2019-2027.

Interpretive Summary: Heavy metal contaminants like cadmium, nickel, and lead react with water to form different “species” in both solution and solid phases. This study investigated how different cadmium species react with biochar (charcoal made from agricultural wastes) prepared at low (300 °C) and high (700 °C) temperatures. The 700 °C biochar most effectively immobilized cadmium under alkaline conditions. For biochars prepared at lower temperatures (300-500 °C), dissolved organic carbon (DOC) played an important role. The DOC can originate from biochar or environment (e.g., soil, river water). Depending on the conditions, DOC can immobilize or mobilize cadmium in the presence of biochar. These and additional influence of DOC must be taken into account whenever biochar is used as a soil amendment.

Technical Abstract: Cadmium (II) is intrinsically more soluble and less sensitive to stabilization by biochar, compared to Pb(II) and Cu(II). Carboxyl ligands are expected to form unusually strong hydrogen bond with the surface functional group of biochar having a similar pKa. In this study, we employed citrate as a model dissolved organic carbon (DOC) to examine its interaction with low pyrolysis temperature biochars enriched with carboxyl functionalities in the presence of CdII. Sorption of CdII on 700 °C (lowest O/C) biochar progressively increased from pH 3 to 7, but was not affected by the citrate concentration. In contrast, low concentration of citrate ([Cd]/[citrate] molar ratios of 4-24) dramatically enhanced CdII sorption on 300-500 °C biochars, while higher citrate concentrations diminished CdII sorption. Enhanced Cd sorption was accompanied by a several-fold increase in DOC as well as pH buffering (near 7) at equilibrium. We hypothesize that citrate sorption displaced biochar’s polyaromatic, weakly associated DOC. Sorbed multidentate ligand citrate served as a new sorption site for Cd. Equilibrium calculations suggested that 1:1 complexes with hydroxide and phosphate were the reactive Cd species engaging in surface interaction with biochars.