Development of Eco-Friendly Hydrogels Loaded with Stoichiometric and Calcium-Deficient Hydroxyapatites for...

This study presents the development and characterization of eco-friendly hydrogel composites based on sodium alginate (SA) and two types of hydroxyapatite─stoichiometric (HA) and calcium-deficient (CDHA)─for potential agricultural applications, including soil conditioning and cadmium ion immobilization. The hydrogels were synthesized with varying concentrations of calcium chloride (0.25-0.5%) as a cross-linking agent and different HA/CDHA loadings. Comprehensive physicochemical analyses (X-ray diffractometer, FTIR, scanning electron microscopy/TEM, N₂ adsorption/desorption) confirmed the successful incorporation of mineral fillers into the alginate matrix and revealed structural differences between HA- and CDHA-filled composites. Swelling studies demonstrated that both filler type and cross-linking density strongly affected water uptake and diffusion mechanisms. HA-filled hydrogels exhibited polymer relaxation-dominated swelling at low cross-linker concentrations, while CDHA-filled systems were governed primarily by diffusion. For Alg/HA gels with a filler concentration of 20%, 33.3%, and 50%, the equilibrium swelling degrees (Q_∞) were 83.02 g/g, 59.36 g/g, and 31.60 g/g, respectively, while the corresponding values for CDHA-filled gels were 80.22 g/g, 81.62 g/g, and 46.48 g/g. When the cross-linker concentration was increased to 0.5 wt %, a substantial reduction in Q_∞ was observed across all composite samples. Sorption experiments showed that the composites effectively sorbed cadmium ions (Cd²⁺) with maximum capacities of 157.7 mg/g (HA-filled) and 190.6 mg/g (CDHA-filled). Experimental data were better described by the Langmuir model. The specific surface area of CDHA was significantly higher than that of stoichiometric HA and equaled 123.6 m²/g, whereas the total pore volume of both hydroxyapatites remained similar. The mesopore surface area of CDHA was 102.8 m²/g, indicating better sorption properties than in the case of HA, for which this parameter was 57.7 m²/g. Biosafety tests using Pisum sativum and Lepidium sativum seeds confirmed the nontoxic nature of the materials. The composite hydrogels stimulated root elongation, and its high swelling degree supported water retention in soil. Owing to the ability to sorb Cd²⁺ ions, they promoted immobilization of heavy metals, limiting transfer of toxic species to crops. The applied hydroxyapatite improved soil fertility through the supply of mineral components. All these effects indicated that alginate-hydroxyapatite hydrogels can serve as multifunctional materials supporting more sustainable soil management in agriculture.