A sustainable approach to fertilizer management: Microwave-assisted synthesis of slow-release ammonium-natural clinoptilolite

Abstract

Chemical fertilizers are crucial for sustainable agriculture, but they present challenges such as low nutrient efficiency, groundwater contamination, and health risks. This study proposes a greenness method for synthesizing slow-release fertilizers using natural zeolite (NH₄-clinoptilolite). This study compared microwave-assisted synthesis of NH₄-zeolite A (hydrated ammonium aluminosilicate) with conventional methods to assess the impact of synthesis parameters like temperature, NH₄Cl content, and reaction time on material properties. Temperature variations between 140 and 160 °C showed no significant effect on NH₄⁺ (9.3–9.67 %) and Na (7.0–7.7 %) contents. However, NH₄Cl content and hydrothermal treating duration significantly influenced outcomes. The microwave method proved faster and more efficient, particularly at 150 °C for 7.5 min using 4.5 g of NH₄Cl. Both synthesized materials were characterized using XRD, SEM, and FTIR to analyze ammonium release kinetics affected by ionic strength and particle size. Microwave-assisted ion exchange was more effective for loading ${NH}_4^{+}$ than conventional methods requiring 2 h. The resulting zeolites A and clinoptilolite contained 10.86 % and 6.04 % ${NH}_4^{+}$, respectively, with release kinetics evaluated across varying ionic strengths. Four mathematical models were tested, with the Elovich equation best describing the ${NH}_4^{+}$ release process; microwave-treated zeolites exhibited slower, more controlled release patterns than those treated with conventional methods. This enhanced retention positions them as promising slow-release fertilizers. Through innovative microwave-assisted functionalization of zeolites as a controlled-release nitrogen source, they could improve the efficiency of this and other nutrients while minimizing leaching and environmental pollution risks.