Microwave-Assisted Synthesis of a Semi-interpenetrating Polymer Network Slow-Release Nitrogen Fertilizer with Water Absorbency from Cotton Stalks

A semi-interpenetrating polymer network (semi-IPN) slow-release nitrogen fertilizer with water absorbency was synthesized via microwave irradiation. Urea was incorporated as the nitrogen source into a cotton stalk-g-poly(acrylic acid)/bentonite (CS-g-PAA/bent) network and linear polyvinylpyrrolidone (PVP). A reaction mechanism for synthesis of CS-g-PAA/bent/PVP/urea was proposed. Variables influencing water absorbency, such as microwave power, irradiation time, and contents of CS, PVP, and bent were evaluated and optimized. The sample prepared under optimized conditions had a water absorbency of 1018.4 g/g in distilled water and 71.3 g/g in 0.9 wt % NaCl solution. The structure and properties of samples were characterized through various analysis technologies. Swelling measurements and water-retention study indicated that the sample possessed excellent water absorbency in different environments and enhanced the water-retention capacity of soil by introduction of bent and formation of a semi-IPN structure. Investigation of leaching behavior indicated that the sample has the potential to carry the necessary N. The results indicated that about 60% of nutrients were released after 30 days based on anomalous transport, and the sample improved the seed germination rate and promoted cotton growth. Thus, microwave irradiation is a possible method to produce slow-release fertilizers that could be widely applicable in agriculture and horticulture.