Effect of Temperature on the Adsorption of Nitrogen, Phosphorus, and Potassium Onto Potassium Ammonium Phosphate-Modified Watermelon Rind

The utilization of agricultural wastes as sustainable soil amendments has gained significant attention due to their environmental and economic benefits. This study investigated the effect of temperature on the adsorption capacity of nitrogen (N), phosphorus (P), and potassium (K) during the fortification of watermelon rind (Citrullus lanatus) using potassium ammonium phosphate (KNH₄PO₄) for potential soil amendment applications. Watermelon rind was processed into a powdered adsorbent and subjected to batch adsorption experiments at three temperatures: 298 K (25°C), 303 K (30°C), and 308 K (35°C). The adsorption capacities of N, P, and K were determined using the Kjeldahl method, molybdenum blue spectrophotometric method, and flame photometry, respectively. Results showed that adsorption capacity increased with both contact time and temperature for all nutrients, reaching equilibrium at approximately 17–18 minutes. The highest adsorption capacities were recorded at 308 K, with values of 3.53 mg g⁻¹ for nitrogen, 3.67 mg g⁻¹ for phosphorus, and 3.08 mg g⁻¹ for potassium. Among the nutrients studied, phosphorus exhibited the highest adsorption affinity, followed by nitrogen and potassium. The observed increase in adsorption capacity with temperature indicates that the adsorption process was endothermic and favored by enhanced ion mobility and improved accessibility of adsorption sites. The findings demonstrate that watermelon rind is an effective, low-cost, and environmentally friendly biosorbent for nutrient enrichment. The optimum temperature for fortification was identified as 308 K (35°C), suggesting its suitability for producing nutrient-enriched biomass that can function as a slow-release soil amendment for sustainable agricultural practices.