Optimization of facile synthesis of xanthan gum xanthate based hydrogel for the capturing of heavy metal ions from aqueous solutions

Abstract

This study investigated the elimination of Co²⁺, Ni²⁺ and Cu²⁺ ions from water using a mesoporous, cost-effective, reusable, biodegradable and efficient xanthan gum xanthate-based hydrogel (XGmXHs hydrogel) as an adsorbent. The XGmXHs hydrogel was prepared via free radical polymerization process with varying the ratios of 2-hydroxyethyl methacrylate (HEMA) and acrylic acid (AA) as monomers. These ratios were optimized based on grafting efficiency, swelling capacity and point of zero charge (ΔpH_PZC) analysis. The results demonstrate that the XGmXHs hydrogel containing copolymer of HEMA: AA in a 1:3 ratio is optimal for grafting on xanthan gum (XGm) during polymerization process. The 1:3 ratio of monomer grafted on xanthan gum xanthate (XGmX) is referred to as XGmXHs-3 hydrogel. The XGmXHs-3 hydrogel showed a consistently negative surface charge across a diverse pH range, resulting the AA content was enhanced. Consequently, the swelling and adsorption capacity of the XGmXHs-3 hydrogel is significantly high. The optimum swelling capacity of the XGmXHs-3 hydrogel was found 40,128 % in water at pH 11. The maximum removal efficiency was 92.21 % for Co²⁺, 95.45 % for Ni²⁺ and 98.30 % for Cu²⁺ ions at pH 7. According to isothermal studies, the adsorption data aligns most closely with Langmuir isotherm model, resulting the adsorption capacities of 358.42, 469.48 and 555.55 mg/g Co²⁺, Ni²⁺ and Cu²⁺ ions, respectively. The adsorption kinetics were consistent with a pseudo-first-order kinetic model, showing rate constant of −2.6 × 10⁻², −4.0 × 10⁻² and − 6.3 × 10⁻² min⁻¹ for Co²⁺, Ni²⁺ and Cu²⁺ ions, respectively. The desorption efficiencies were 64.76 % for Co²⁺, 68.83 % for Ni²⁺ and 72.11 % for Cu²⁺ ions to the XGmXHs-3 hydrogel after being regenerated for the fifth cycle.