Investigating polysaccharides role in superabsorbent hydrogel composites with rice husk ash for Pb(II) and Cu(II) adsorption: Experimental and theoretical insights.

Polysaccharide-based hydrogels and hydrogel composites are effective materials for removing metals from contaminated water and wastewater. However, comprehensive comparisons of their composition and performance are limited. To address this, superabsorbent hydrogel composites based on starch (ST) and chitosan (CTS), grafted with poly(acrylic acid) (PAAc) and reinforced with rice husk ash (RHA) (5 % w/w), were developed and tested for Pb(II) and Cu(II) adsorption from aqueous solutions. The influence of the polysaccharide matrix and RHA incorporation on physicochemical properties, adsorption behavior, and reusability was systematically evaluated. ST-based hydrogels and composites showed superior swelling capacity and metal uptake compared to CTS-based ones, attributed to higher grafting efficiency and greater carboxyl group density. RHA addition improved structural stability, increased active adsorption sites, and enhanced overall performance. Adsorption kinetics followed the pseudo-second-order model, and isotherms fit the Langmuir type I model, indicating monolayer adsorption. Thermodynamic analysis confirmed the process was spontaneous and exothermic, while competitive adsorption tests revealed preferential uptake of Pb(II) over Cu(II). Reusability tests showed that the hydrogels retained over 85 % of their initial adsorption capacity after 6 cycles, with minimal loss of performance. Spectroscopic analysis and density functional theory (DFT) calculations indicated metal ion adsorption occurred mainly via coordination with carboxylate groups, with hydroxyl and amine groups playing a notable role in CTS-based composites. These findings demonstrate the potential of these polysaccharide-based hydrogel composites as sustainable and efficient materials for heavy metal removal and provide guidance for optimizing polysaccharide matrix selection.