Thiol functionalized kaolin pellets: Development and optimization for mercury ion removal from aqueous solutions

Mercury contamination in aqueous media poses a severe environmental and health risk due to its high toxicity and bioaccumulation potential. In this study, a novel thiol-grafted kaolin pellet adsorbent was developed for efficient Hg²⁺ remediation. The pellet production involved a combination of acid-base treatment, 3-mercaptopropyltrimethoxysilane grafting, and extrusion with polyvinyl alcohol serving as a binder. Additionally, a novel approach was developed to assess biofilm formation on the produced pellets. Characterization results confirmed the successful grafting of thiol groups, providing high-affinity binding sites for Hg²⁺ ions. Optimal operating conditions were identified using 2-mm pellets, 1.5 wt% PVA, 5 g/L dosage, and an initial solution pH of 5, achieving 74.2 % removal and adsorption capacities of up to 12.5 mg/g at 313 K. Kinetic studies revealed that Hg²⁺ uptake followed a pseudo-second-order model (R² = 0.986), while isotherm studies indicated the Freundlich model (R² = 0.987, 0.993, and 0.997 for 293, 303, and 313 K, respectively) as the best model. Antibacterial assays demonstrated that raw and acid-base-treated kaolin pellets inhibited biofilm formation for E. coli (BL21), P. aeruginosa, S. epidermidis, and S. aureus, whereas thiol-grafted pellets promoted biofilm development, highlighting the need to balance adsorptive enhancement with potential microbial colonization. These findings confirm the potential of kaolin-based pellet adsorbents for Hg²⁺ remediation, offering a viable framework for scaling up into industrial applications.