Evaluation of a pH- and time-dependent model for the sorption of heavy metal cations by poultry litter-derived biochar
Biochars with high phosphate (P) contents are promising amendments to remediate metal-contaminated soils due to their ability to form stable metal-P precipitates. However, their performance is usually assessed at a single pH. This study investigated the sorption of Pb, Cu, Zn, Cd, and Ni by poultry litter (PL) biochar across a pH range using sorption edge, isotherm, and kinetics experiments. Metal sorption was strongly pH-dependent with increased sorption at higher pH. The affinity of the PL biochar for the metals decreased in the order of Pb>>Cu>Zn>Cd>Ni. In all cases, ≤21% of the sorbed metals were exchangeable, indicating that stable metal-biochar associations were formed. Sorption kinetics experiments demonstrated that reaction rates were slower at pH 4.5 than 6.5 for Pb, Cu, and Cd whereas those for Zn and Ni were unaffected by pH. The results suggested that metal-P precipitation was favored for Cu, Cd, and Zn at pH≥5.5, or Pb at any pH. This indicates that PL-derived biochars can be effective amendments for contaminated soils given that the soils are not too acidic. Experimental data were described using a pH-dependent Freundlich-type isotherm and its kinetic analog. Sorption edges and isotherms were reasonably described for Pb, Zn, Cd, and Ni (r2≥0.83). Kinetics data were best described using model parameters obtained from sorption edge experiments due to similarities between the input metal concentrations. This modeling approach has superior descriptive capabilities than traditional empirical approaches while maintaining relative simplicity. Moreover, pH-dependent equilibrium and kinetic sorption can be described using a single set of parameters.