Influence of solution pH, ionic strength, and humic acid on cadmium adsorption onto activated biochar: Experiment and modeling

Biochar was prepared by pyrolysis of torrefied loblolly pine and used as a sorbent for the removal of Cd(II) ions from aqueous solutions. The effects of pH, ionic strength, and humic acid (HA) on Cd(II) adsorption on biochar were investigated in comparison with other sorbents, commercially available powdered activated carbon (PAC) and multi-walled carbon nanotubes (MWNT). The order of overall adsorption for Cd(II) was biochar > PAC > MWNT for the adsorbents. In addition, the results indicated that the sorption behaviors of Cd(II) were strongly dependent on pH and ionic strength. The maximum adsorption capacity of Cd(II) on biochar at pH 7.5 ± 0.1 and T = 25 °C was calculated to be 167.3 mg/g, much higher than other reports. The presence of HA modifies the surface properties of biochar and enhances complexation of Cd(II) with adsorbed HA. Surface complexation modeling gave an excellent fit with the predominant mononuclear monodentate >SOCd²⁺ in the absence of HA and the ternary surface >SOL₁Cd species in the presence of HA as well as binuclear bidentate >(SO)₂Cd species at higher pH. The results of this study highlight that biochar is a suitable material for the removal of environmental cadmium pollution.