Surfactant-Enhanced Guava Seed Biosorbent for Lead and Cadmium Removal: Kinetics, Thermodynamics, and Reusability Insights

Heavy metal pollution in water resources represents a severe environmental and public health challenge, with cadmium (Cd) and lead (Pb) being some of the most toxic contaminants. Finding new sustainable, efficient, and cost-effective methods for water treatment is essential to address such problems. Therefore, this study investigates the biosorption capabilities of natural (SN) and surfactant-modified (SM) guava seed biosorbents to remove Cd and Pb from aqueous solutions. To enhance their adsorption efficiency, Guava seeds, an agricultural waste material, were treated with hexadecyltrimethylammonium bromide (HDTMA-Br). The biosorbents were characterized by FTIR, SEM-EDS, and zeta potential analysis to understand the surface modifications and their influence on the adsorption mechanisms. Batch experiments were performed to evaluate the effects of pH, contact time, temperature, biosorbent dosage, and concentration on Cd and Pb removal efficiencies. Adsorption isotherm and kinetic data were analyzed using mathematical models to obtain the basic parameters of the systems under study. The results showed that SM exhibited superior adsorption capacities of 328 mg/g for Cd and 594 mg/g for Pb at 25 °C, significantly outperforming SN. The study also analyzed the thermodynamic parameters of adsorption systems, revealing endothermic and exothermic properties for SN and SM. Functional groups, such as hydroxyl and carbonyl, were crucial in metal ion binding. HDTMA-Br introduced additional active sites and enhanced surface charge interactions. Regeneration tests showed the reusability of both biosorbents, maintaining an efficiency of over 85% after four cycles. Guava seeds, after surfactant modification, could be cost-effective and sustainable biosorbents for heavy metal removal in water treatment.