Chitosan-based hydrogels: bentonite supported architectures explored for removal of cadmium(ii) from aqueous solution

Abstract

To maintain ecosystem integrity, the use of hydrogels as adsorbents is a straightforward and cost-effective approach towards heavy metal removal from drinking water and wastewater. Heavy metal ions have lethal effects on all life forms by entering the food chain through the waste disposal in water channels. Among various metal ions, cadmium ion Cd(II) is at the top of the toxicity list. Herin, we report the synthesis and Cd(II) removal efficiency of chitosan-based hybrid hydrogel CPAB {chitosan (C)/poly(vinyl alcohol) (P)/3-aminopropyltriethoxysilane (A)/bentonite (B)}. Bentonite clay was used as a filler in four different amounts in CPAB-15, CPAB-30, CPAB-45, and CPAB-60 compositions. FTIR, SEM and XRD, and TGA characterization indicated structural, surface, and morphological modifications and significant enhancement in the thermal stability. Swelling analysis revealed that, in water, CPAB swells rapidly in the presence of bentonite. A gradual increase was observed as a function of bentonite concentration. In addition, these hydrogels showed better swelling in acidic pH. The prepared hydrogels were subjected to batch tests and experimental evidence revealed that CPAB-15 is the best composition for Cd(II) removal from water. The maximum percentage removal and equilibrium adsorption efficiency (q_e) results obtained from batch experiments are in-agreement and assured that 90 mg adsorbent dose adsorbed 25 mg of Cd(II) in 120 min at pH 6. Theoretical modelling indicated that the experimental data for Cd(II) removal was well fitted to Langmuir isotherm and pseudo-second order kinetic models. The adsorption kinetics revealed chemisorption to be the most probable adsorption process, while the equilibrium adsorption capacity (q_max) of 354.61 mg g⁻¹ evaluated from the Langmuir model was found to be much greater than most of the reported chitosan-based hydrogels and endorsed the appropriateness of the CPAB-15 hydrogel to be used as an efficient adsorbent system for Cd(II) removal from water.