A novel glutaraldehyde cross-linked chitosan@acid-activated bentonite composite for effective Pb (II) and Cr (VI) adsorption: Experimental and theoretical studies

In this study, a novel glutaraldehyde cross-linked chitosan@acid-activated bentonite composite (CsG@AAB) was synthesized from crab waste, glutaraldehyde, and acid-activated bentonite (AAB). The as-obtained composite was used as an adsorbent for the efficient removal of Pb(II) and Cr(VI) from the aqueous solutions. The adsorbent was characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Braeuer–Emmett–Teller (BET) analyses. The effects of factors such as initial metal ion concentration, contact time, temperature, adsorbent dosage, and pH on Pb(II) and Cr(VI) adsorption were investigated, and related adsorption parameters were determined. Characterization results indicate that chitosan entered into the bentonite interlayer. BET surface area and point of zero charge (pH_PZC) of the biocomposite were 85.2 m²/g and 6.3, respectively. Both metal ions uptake was rapid while adsorption was highly dependent on solution pH. The monolayer adsorption capacity was 452.78 mg g⁻¹ for Pb(II) and 284.14 mg g⁻¹ for Cr(VI), respectively at 25 °C. The adsorption kinetics were explained using the PSO model. The adsorption thermodynamic process was endothermic and spontaneous. The adsorption mechanism was explained by theoretical calculations and was in good agreement with the experimental results. Physisorption (van der Waals force, pore-filling), chemisorption (both inner and outer-sphere complexes), ion exchange, electrostatic interaction, and surface precipitation were the main mechanisms of metal ions adsorption. The results of this study suggest that the CsG@ AAB composite is promising for Pb(II) and Cr(VI) uptake and can be potential adsorption for decontamination of other pollutants from real wastewater streams.