A laboratory experiment was conducted to investigate and evaluate the sorption-adsorption of heavy metals (cadmium and cobalt) on sandy soil treated with biochar at rates of 0, 1, 2.5, and 5% (w/w). The sorption-adsorption data were fitted on some sorption-adsorption isotherm models using the software Iso-Fit and Excel softwires. The isotherm models were Linear, Freundlich, Langmuir and proposed new models. Measured and simulated data were compared statistically to evaluate model reliability. It noticed that the sorption-adsorption of heavy metals was more pronounced in sandy soil treated with biochar especially at 5%. The sorption-adsorption percentage was decreased by increasing the initial concentration of heavy metals and also increased with increasing the biochar rates.
The results showed that the average sorption-adsorption percentage were 89.03% for (Cd) and 72.66% for (Co) at 0% biochar. The corresponding values for 5% biochar were 92.15% for (Cd) and 91.12% for (Co).
Also, the experimental data was analyzed to examine the adsorption isotherm models. The distribution coefficients (Kd) obtained from the linear sorption-adsorption isotherm model were 172.77 and 66.56 L/kg for Cd and Co respectively at 0% biochar suggesting a relatively weaker interaction with heavy metals in comparison to sandy soil treated with 5% biochar in which the Kd account as 292.11 and 260.56 L/kg for Cd and Co respectively. The soil treated with biochar demonstrates a greater affinity for heavy metals. Metals can be arranged according to their Kd values, i.e., their affinities for the soil, in the following relative sequence: Cd >Co.