Evaluation of a pH- and time-dependent model for the sorption of heavy metal cations by poultry litter-derived biochar

Authors: Padilla, Joshua - Josh; Watts, Donald - Don; Szogi, Ariel; JOHNSON, MARK - Us Environmental Protection Agency (EPA)

Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2023
Publication Date: 11/15/2023
Citation: Padilla, J.T., Watts, D.W., Szogi, A.A., Johnson, M.G. 2023. Evaluation of a pH- and time-dependent model for the sorption of heavy metal cations by poultry litter-derived biochar. Chemosphere. 347.Article 140688. https://doi.org/10.1016/j.chemosphere.2023.140688.
DOI: https://doi.org/10.1016/j.chemosphere.2023.140688

Interpretive Summary: Biochar (charcoal-like material made from biomass) can bind heavy metals in contaminated soils or water, preventing their movement into environmentally sensitive ecosystems. Biochar made from poultry litter (PL) is high in phosphate (P) and is an especially promising material because P forms solid compounds with various heavy metals. Solution pH and reaction time can significantly affect the ability of biochar to bind heavy metals, and therefore, the ability of biochar to remove heavy metals should be evaluated across a wide range of chemical conditions. In this study, we investigated the ability of PL-derived biochar to bind cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) across a wide range of pH and reaction times. We found that metal binding by biochar was more significant at high pH (alkaline, higher than pH 7.0) than at low pH (acidic, lower than pH 7.0). We also found that reactions were faster at high pH for Cd, Cu, and Pb. Furthermore, we demonstrated that a mathematical model was able to predict the metal binding ability of PL-derived biochar across the tested pH and reaction times. We then used the model to predict chemical conditions required to meet water quality standards using PL-derived biochar.

Technical Abstract: Common isotherm and kinetic models cannot describe the pH-dependent sorption of heavy metal cations by biochar. In this paper, we evaluated a pH-dependent, equilibrium/kinetic model for describing the sorption of cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) by poultry litter-derived biochar (PLB). We performed sorption experiments across a range of solution pH, initial metal concentration, and reaction time. The sorption of all five metals increased with increasing pH. For Cd, Cu, and Pb, kinetics experiments demonstrated that sorption rates were greater at pH 6.5 than at pH 4.5. For each metal, all sorption data were described using single set of four adjustable parameters. Sorption edge and isotherm data were well described with R2>0.93 in all cases. Time-dependent sorption was well described (R2=0.90) for all metals except Pb (R2=0.77). We then used the best-fit model parameters to calculate linear distribution coefficients (KD) and equilibration times as a function of pH and initial solution concentration. These calculations provide a more robust way of characterizing biochar affinity for metal cations than Freundlich distribution coefficients or Langmuir sorption capacity. Because this model can characterize metal cation sorption by biochar across a wider range of reaction conditions than traditional isotherm or kinetic models, it is better suited for estimating metal cation/biochar interactions in engineered or natural systems.