Cadmium(II) and Ni(II) are intrinsically more soluble and less sensitive to surface complexation/precipitation than Pb(II) and Cu(II) in both biochar and soil matrices.  Depending on the solution chemistry, biochar can release dissolved organic carbon (DOC) of diverse molecular weight, structure, and concentration.  In particular, carboxyl-enriched DOC of 350-500 �C biochars can have diverse environmental consequences, including the mobilization of heavy metals.  Both biochar-borne DOC and naturally-occurring ligands, e.g., citrate, will control the Cd(II) speciation and reactivity towards surface sites.  In this study, batch sorption experiments were combined with equilibrium speciation calculations to quantitatively predict the Cd(II) speciation-sorption relationships on pecan shell biochars pyrolyzed at 300-700 �C.  Experiments were designed to systematically investigate the influence of Cd(II) speciation and biochar property.  The Cd(II) speciation was studied as a function of pH, ligand structure and concentration, and ionic strength.  The biochar property was studied as a function of pyrolysis temperature (and corresponding changes in DOC, ash, pH, and O/C), and by monitoring DOC and dissolved P concentrations at equilibrium.  The Cd sorption on 500 and 700 �C biochars progressively increased as a function of pH, and was strongly ionic strength dependent.  At low [citrate]/[Cd] (=1/4 molar) ratio, citrate enhanced Cd sorption on 300-500 �C biochars, but not higher temperature biochars.  In addition, trace ligands dramatically increased the concentration of polyaromatic DOC similar to SOC at advanced stage of humification.  Multidentate ligands likely competed with biochar's DOC for available surface sites, and sorbed ligand served as the new reactive sites for Cd.  The new surface sites buffered equilibrium pH to a constant value, regardless of biochar type or initial Cd/ligand concentrations.

Multidentate ligand-assisted sorption of Cd(II) on pyrogenic carbonaceous materials

248th National Meeting (2014)

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