Surface charge modulation in metal-crosslinked sodium alginate-chitosan composite adsorbent: Optimized anionic-to-cationic polysaccharide component ratio for Cd and Cr removal from water

Carbohydrate-based polymers such as sodium alginate (SA) and chitosan (CS) are emerging as eco-friendly sorbents for heavy metal removal from water; however, their limited water stability often compromises regenerability. Moreover, optimizing the cationic-to-anionic polysaccharide ratio is critical for enhancing the surface negative charge and boosting metal ion affinity. In this study, we prepared a robust Zr/Fe-crosslinked SA-CS composite by carefully tuning the SA-to-CS ratio to 2:1, which exhibits an anionic surface charge (pH_ZPC = 4.3 vs 6.7 for a 1:1 ratio), making it effective for cation adsorption. The Zr/Fe-SA/CS beads were characterized by FTIR, XRD, SEM-EDX, BET, and TGA to confirm that the polysaccharides were well incorporated and functionalized within the adsorbent. The composite beads were employed in batch mode for adsorptive removal of Cd²⁺ and Cr³⁺, offering the maximum uptakes of 77.88 mgCd²⁺/g and 66.36 mgCr³⁺/g. The adsorption process was predominantly led by the Langmuir isotherm (R² = 0.99) and the pseudo-second-order kinetic (R² = 0.99) models for both Cd²⁺ and Cr³⁺ adsorption, implying chemical adsorption on a homogeneous monolayer surface. The thermodynamic parameters revealed a negative value of ΔG° and positive values of ΔH° and ΔS°, indicating that the Cd²⁺ and Cr³⁺ adsorption on the adsorbent is spontaneous, endothermic, and characterized by increased randomness. The molecular dynamics simulation study revealed an adsorption energy of 140.7 kcal/mol for Cd²⁺ and 21.9 kcal/mol for Cr³⁺, substantiating the feasibility of adsorption. The work introduces a novel approach to tuning the polysaccharide-based composite adsorbent’s surface towards negative charge, leading to rapid and enhanced adsorption of Cd²⁺ and Cr³⁺ from water. The developed robust composite provides an eco-friendly solution for the efficient removal of heavy metals from water.