Urea solution absorbency and release properties of phytic acid-modified corn stalk composite superabsorbents

Abstract: The growth of plants and vegetables is mainly a function of the quantity of fertilizer and water. It is very important to improve the utilization of water resources and fertilizer nutrients. However, about 40-70% of the nitrogen of the applied normal fertilizers is lost to the environment, and cannot be absorbed by plants and vegetables, causing not only large economic and resource losses, but also very serious environmental pollution. It has been reported that about 1/3 of nitrogen fertilizers enter into the atmosphere, where N2O destroys the ozone layer, and about 1/3 of nitrogen fertilizers leach into the water, leading to eutrophication of water resources. As a by-product of corn crops, corn stalk is an abundant biodegradable resource whose main components are cellulose, hemicellulose, and lignin. Up to now, most of the crop residues have been focused on burying them back to improve the fertility of the soil, using them as the materials for the paper industry and producing protein feed, alcohol, and methane by microbial fermentation. Corn stalk can be used as a bioabsorbent, due to its large surface area and a number of hydrophilic hydroxyl groups on the macromolecular chains. However, its fertilizer adsorption capacity is still very low. Therefore, modification of corn stalk is needed to enhance its fertilizer adsorption capacity. Up to now, straw was either gelatinized, or modified by acid or/and base treatment and then grafted with some monomers to prepare straw composite superabsorbents. But there are no reports on the phytic acid -modified corn stalk composite superabsorbents. The present work aims to investigate the urea absorbency, urea retention, and urea release properties of phytic acid-modified corn stalk composite superabsorbents (PCS-SA) prepared by graft copolymerization of acrylamide (AM), vinyl pyrrolidone (VP), and itaconic acid (IA) with phytic acid-modified corn stalk (PCS) in an aqueous solution, using ammonium persulfate and sodium bisulfite as a redox initiator. The effect of the mass ratio of IA to VP on the urea absorbency, urea retention, and urea release rate of PCS-SA was investigated. The structure and morphology of PCS-SA were characterized by FTIR and SEM, respectively. The results showed that with an increasing IA and NVP mass ratio, the urea solution absorption rate, urea solution retention, and urea release rate of PCS-SA are improved. However, when the IA and NVP mass ratio was greater than 70:30, the urea solution absorption rate, urea solution retention, and urea release rate of PCS-SA decreased. PCS-SA, synthesized with an IA and VP mass ratio of 70:30, had a urea absorption capacity of 3.81g/g, urea absorbency of 360g/g within 40min, urea release ratio of 66.14% in distilled water after 3d, and urea release ratio of 47.6% in soil after 5 d, resulting in a significantly decreased urea release rate and an increasing urea retention of soil. FTIR spectrum of PCS-SA appeared as characteristics of absorption peaks which might be ascribed to a PCS structural unit, VP unit, IA unit and AM unit, respectively, as indicative of the target structure for the prepared PCS-SA. SEM showed that PCS-SA had an irregular and rugged surface, accompanied by many small and large pores, which can facilitate the permeation of urea solution into the polymeric network.