Sustainable Hybrid Alginate Hydrogels with Mineral and Lignocellulosic Fillers: Structure–Property Relationships and Rhodamine B Adsorption

Hybrid alginate hydrogels offer a versatile platform for integrating the structural properties of biopolymeric networks with the functionality of inorganic and biomass-derived fillers. In this study, calcium-alginate hydrogels incorporating thermally treated bentonite, olive pomace, or kraft lignin were prepared by ionotropic gelation to investigate how filler identity influences hydrogel properties and Rhodamine B (RhB) adsorption. The hybrid materials were characterized by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, and determination of the point of zero charge (pHpzc). Their adsorption behavior was evaluated through kinetic, equilibrium, pH, ionic-strength, and regeneration studies. Despite the very low N2-accessible surface areas measured in the dry state, the hydrogels exhibited markedly different adsorption performances, indicating that dry-state BET surface area alone does not account for RhB uptake and highlighting the importance of filler chemistry, surface-charge characteristics, and the accessibility of adsorption domains within the hydrated network. Adsorption kinetics were better described by the pseudo-second-order model, while intraparticle diffusion analysis indicated a multistep mass-transfer process. Equilibrium data were empirically well described by the Langmuir model, with fitted maximum adsorption capacities ranging from 12.4 to 212.7 mg·g−1. Bentonite-containing hydrogels exhibited the highest capacities, following the order AB110 > AB200 > AB400. Increasing the bentonite thermal-treatment temperature decreased the fitted maximum adsorption capacity while increasing the Langmuir affinity parameter, revealing a capacity–affinity trade-off. The lignocellulosic fillers produced lower-capacity adsorption systems with distinct apparent affinities and responses to solution conditions. After five adsorption–desorption cycles, the bentonite- and lignin-containing hydrogels retained more than 80% of their initial adsorption performance, whereas the olive-pomace-containing formulations retained approximately 70%. Overall, the results demonstrate that incorporating chemically distinct mineral and lignocellulosic fillers into a common calcium-alginate network provides an effective strategy for tuning the structure–property–adsorption relationships and reusability of hybrid hydrogels, supporting their further development as sustainable adsorbent materials for water-remediation applications.