Optimization of phosphorus-loaded Ni–ZnO crosslinked carboxy methyl cellulose-based biodegradable nanocomposite hydrogel beads for the slow release of P, Ni and Zn: a kinetic approach

Abstract

Nutrient (macro and micro) deficiency retards and restricts plant growth whereas over use of certain fertilizers is not only detrimental to growing plants but also to the environment. To address the two associated problems, the present study decerns a novel synthetic protocol for the preparation of nickel–zinc oxide (plant micronutrients) crosslinked carboxy methyl cellulose (CMC) nanocomposite hydrogel beads loaded with phosphorus (P) as a slow release phosphatic fertilizer. A 1 : 0.15 ratio of 4% CMC solution and zinc chloride (ZnCl₂) was found to be optimum for the formation of stable spherical hydrogel beads when added dropwise in nickel chloride (NiCl₂) solution (3 g in 10 mL). For the phosphate loading and release study, CMC (2 g) was introduced into disodium hydrogen phosphate solution (4%) during the synthesis process keeping the rest of the procedure the same. The prepared composite beads having P, Ni and Zn were characterized using Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and SEM-energy dispersive X-Ray (SEM-EDX) mapping analysis. Rheological parameters such as the storage modulus (G′) and loss modulus (G′′) value indicate that the visco-elastic properties of the hydrogel were closely associated with the CMC concentration. The swelling kinetics of the hydrogels followed Schotts’ second order kinetic model with a high connection co-efficient (R² = 0.99). Under the optimized conditions, the hydrogel beads showed the maximum swelling ratio of 185%, 3770% and 5552% in distilled water, pH 7 buffer and pH 9.2 buffer, respectively. The prepared composite beads were found to be a potential slow-release phosphatic fertilizer for agricultural delivery within a time span of 21 days. The degradability of the composite hydrogel beads at low indicative soil pH of 4.5 was also evaluated. After degradation, the hydrogel beads potentially release the cross-linker Zn and Ni in the form of plant available micronutrients. The findings coincide with other slow-release studies for sustainable nutrient release for agricultural applications.