Removal of heavy metals from water by macro-mesoporous calcium alginate–exfoliated clay composite sponges

The contamination of drinking water and freshwater by heavy metals has become a growing concern for human health and environment that calls for immediate remedial action. Adsorption process has been proved to be a promising technology for producing treated water that complies with drinking water standards. In this work, we describe a facile and low-cost approach to fabricate porous calcium alginate composite sponges containing exfoliated bentonite clay (denoted as CRAB) by saponin-assisted foaming and external gelation techniques for efficient removal of Cu²⁺ and Cr⁶⁺ ions from aqueous solution and real world water samples. The formation of exfoliated organoclay within the composite matrix was achieved by intercalation of saponin into the interlayer spacing of bentonite and subsequent penetration of alginate chains. Morphological and textural characterizations revealed that the resultant CRAB sponges mainly exhibited meso- and macropores in their interior. The formation of calcium alginate-organoclay exfoliated composites was also verified by the XRD results. The performance of CRAB sponges in removing Cu²⁺ and Cr⁶⁺ ions from aqueous solution was evaluated under different environmental conditions including temperature, pH, ionic strength, and background electrolyte. The results demonstrated that the uptake capacity of Cu(II) and Cr(VI) increased with increasing temperature, suggesting the endothermic nature of the adsorption process. The Freundlich isotherm model was found to better interpret the equilibrium adsorption behaviors of Cu²⁺ and Cr⁶⁺ ions onto the investigated sorbents than the Langmuir model. The sorption kinetics for these metal ions followed a pseudo-first-order model. Reusability tests for CRAB sponge demonstrated that the sorption capacity of the sorbent is well maintained over three successive adsorption–desorption cycles. Together, these results indicate the great potential of low-cost and sustainable CRAB sponges for remediation of water polluted with copper and hexavalent chromium ions.