A Dual-Function Hydrogel Platform for Cu(II)/Fe(III) Remediation and Controlled Nutrient Release

The development of multifunctional materials that address multiple environmental challenges simultaneously represents a paradigm shift in sustainable water and soil management. This study presents a dual functional hydrogel consisting of urea-biochar encapsulated crosslinked hydrogel (U-BECA) designed for Cu(II)/Fe(III) removal and controlled nutrient release. The hydrogel was synthesized through calcium ion crosslinking of sodium alginate incorporating biochar and urea, characterized using FTIR, SEM, EDS, DMA, XRD and evaluated under single and binary metal ion systems. Results demonstrated effective adsorption with selectivity order Fe³⁺ > Cu²⁺. The urea-loading strategy also exhibited sustained urea release over extended periods while maintaining metal ion adsorption capacity, offering integrated pollution remediation and nutrient delivery. The Fe³⁺ and Cu²⁺ adsorption respectively followed Langmuir and Sips model with theoretical adsorption capacities of 131.38 mg/g for Fe³⁺ and 95.63 mg/g for Cu²⁺. Dynamic mechanical analysis (DMA) confirmed the hydrogel's structural integrity, exhibiting a storage modulus of 319 kPa and no gel-to-sol transition, indicative of a mechanically stable and highly elastic network. SEM-EDS analysis revealed a dense, Ca²⁺-crosslinked microstructure that became rougher and enriched with particulate deposits following Cu²⁺ and Fe³⁺ adsorption. The persistence of characteristic metal peaks in the EDS spectra further confirmed successful metal uptake and retention within the hydrogel matrix. The effect of pH on urea release was evaluated, revealing pH-responsive release behavior. The Korsmeyer-Peppas model accurately described the release kinetics at pH 4 (n = 0.347), indicating a Fickian diffusion-controlled mechanism. The hydrogel also retained substantial adsorption capacity for both Cu²⁺ and Fe³⁺ over three consecutive adsorption-desorption cycles. These findings highlight hydrogel as a promising multifunctional material for integrated metal ion remediation and controlled nutrient delivery.