Kinetics of Cd adsorption by biochar, activated carbon, and zeolite in some calcareous soils

Different methods have developed to reduce the risks of potentially toxic elements in contaminated soils. Among them, adsorption is one of the most important and effective strategies. In this research, kinetics of Cd adsorption by biochar, activated carbon, and zeolite in some calcareous soils were investigated. A factorial experiment was conducted based on a completely randomized design with three replications. The factors included three types of adsorbents (sunflower's biochar, activated carbon, and zeolite), four levels of Cd (0, 20, 50, and 100 mg/L as Cd (NO₃)₂), and three soil samples, differing in their cation exchange capacity, soil organic matter, and calcium carbonate equivalent. Batch experiments were carried out to evaluate Cd adsorption isotherms and kinetic models. Furthermore, the effect of adsorbent dosage, contact time, and initial pH on the adsorption efficiency was examined. Optimizing studies revealed that the best pH for Cd adsorption was 5, while the optimal equilibrium time was achieved at 24 h. The results showed that the Freundlich model fitted to the experimental data slightly better than the Langmuir model. Moreover, the pseudo-second-order kinetic model better described the kinetic behavior of Cd adsorption for the investigated adsorbents. The maximum Cd removal efficiency (99%) belonged to soil No. 2 with biochar. Lastly, it was concluded that sunflower biochar, a cheap and cost-effective adsorbent, had high efficiency in Cd adsorption in calcareous soils.