Preparation and characterization of biogenic CaCO₃-reinforced polyvinyl alcohol–alginate hydrogel as controlled-release urea formulation

A slow-release fertilizer formulation was prepared by incorporation of urea in polyvinyl alcohol–alginate hydrogel core followed by deposition of calcium carbonate coating using HCO₃⁻/CO₃²⁻-rich alkaline cell-free ureolytic culture broth. High urea encapsulation efficiencies were evident in both CaCO₃-reinforced (23.48% w/w) and regular (38.71% w/w) hydrogel matrices. Deposition of carbonate over the matrix surface in the reinforced hydrogel was evident from scanning electron microscopy and typical FTIR band near 1465 cm⁻¹ due to C–O stretch of carbonate ion. Aqueous phase release behaviour was not significantly different between the two hydrogel variants. In the soil column, two-stage urea release pattern (i.e. a slow followed by a fast) was eminent that can extend into hours. Therefore, the modified hydrogel for controlled release could be expected to have wide potential application as slow-release fertilizer in agriculture.