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Evaluation of a pH- and time-dependent model for the sorption of heavy metal cations by poultry litter-derived biochar
Citation:
Padilla, J., D. Watts, A. Szogi, AND M. Johnson. Evaluation of a pH- and time-dependent model for the sorption of heavy metal cations by poultry litter-derived biochar. CHEMOSPHERE. Elsevier Science Ltd, New York, NY, 347:140688, (2024). https://doi.org/10.1016/j.chemosphere.2023.140688
Impact/Purpose:
Manure-based 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 generally assessed at a single pH which limits the interpretation of their performance across a range of soil pH conditions found at metal-contaminated sites. This study investigated the sorption of Pb, Cu, Zn, Cd, and Ni by poultry litter (PL) biochar across a pH range using advanced methods. We found that metal sorption was strongly pH-dependent with increased sorption at higher pHs. The affinity of PL biochar for these metals was greatest for Pb and decreased in the following order of Pb>>Cu>Zn>Cd>Ni. pH also affects the kinetics (rate) of metal sorption with sorption slower at low pHs and much faster at higher pHs. Precipitation of metal phosphates was favored for Cu, Cd and Zn at pHs ≥ 5.5, but for Pb at any pH. These results suggest that PL-derived biochars are effective amendments for metal-contaminated soils given that the soils are not too acidic. Therefore, adjusting metal-contaminated soils or residuals of mining to pHs above 5.5 with lime and then amending with PL biochar is a tool that can be used at metal-contaminated site to reduce metal bioaccessibility.
Description:
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.
URLs/Downloads:
DOI: Evaluation of a pH- and time-dependent model for the sorption of heavy metal cations by poultry litter-derived biochar