Adsorption of Cd²⁺ and Pb²⁺ onto coconut shell biochar and biochar-mixed soil

Permeable reactive barrier (PRB) systems containing effective and low-cost adsorbents for heavy metals are expected to function as in situ treatment methods for leachate from waste landfills and contaminated groundwater surrounding the landfills in developing countries. This study was conducted to characterize the adsorption of Cd²⁺ and Pb²⁺ onto coconut shell biochar (fine granules), a local soil from Sri Lanka (Entisol), and a biochar-mixed soil (1:1 mixture of biochar and soil) as potential adsorbents for PRB systems. Batch experiments were carried out to investigate the effects of solution pH, contact time, initial ion concentration, and competitive ions on the Cd²⁺ and Pb²⁺ adsorption. Results showed that the adsorption kinetics of Cd²⁺ and Pb²⁺ onto all adsorbents were well described by the pseudo second order kinetics model and that adsorption isotherms followed the Langmuir model. In the range of pH ≥3, the initial solution pH had a minor effect on efficiency of metal removal and the removal of metals mostly exceeded 80 % for all adsorbents. Measured maximum adsorptions onto soil and biochar-mixed soil were 30.1 mmol/g for Cd²⁺ and 44.8–46.7 mmol/g for Pb²⁺. These adsorption capacities are similar to or higher than the values of biosorbents tested for wastewater treatment in previous studies, suggesting our tested materials would be useful as adsorbents of Cd²⁺ and Pb²⁺ in PRB systems. Additional analysis by scanning electron microscopy linked with energy dispersive X-ray revealed that both Cd²⁺ and Pb²⁺ exhibited high adsorption affinity towards soil particles while adsorbing randomly to biochar granules.