Starch-g-poly(acrylic acid-co-acrylamide) composites reinforced with natural char nanoparticles toward environmentally benign slow-release urea fertilizers

Loss control of nutrients and water has been considered as a global challenge of agriculture because it faced with concurrent management of these two necessities of crop production. Herein, a bio-based slow-release fertilizer (SRF) has been developed using reinforcement of starch-g-poly(acrylic acid-co-acrylamide) superabsorbent polymer with natural char nanoparticles (NCNPs). Incorporation of urea in the matrix of nano-biocomposite resulted in the efficient SRF formulation with remarkable water absorbency (215.1 g/g). Variables including starch, monomers, cross-linker, and nano-filler dosages were optimized using screening the urea release behavior of samples in various pHs (3-10) and salt solutions (NaCl, CaCl2, and FeCl3). NCNPs acted as physical cross-linker as well as nano-filler and formed H-bondings between the oxygenated groups of the polymer and nanoparticles. Urea release measurements in water indicated a well slow-release property of SRF formulations, 70 % of nitrogen released during 21 days. The release time was prolonged with increasing the amount of NCNPs because the favorable interfacial polymer-filler interactions resulted in slower nitrogen diffusion and consequently, slower release rate at neutral and basic pHs. The water-retention of soils containing two different SRFs (with and without nano-filler) showed that NCNPs could double the water-retention compared with the neat polymer. While nitrate leaching rate in the soil for pure urea was measured to be 591.8 mg/L, the starch-g-poly (acrylic acid-co-acrylamide)/NCNPs/Urea decreased the leaching loss of N up to 49.5 mg/L. The soil burial degradation test indicated that the presence of NCNPs in the network of polymer could facilitate the degradation process of the SRF samples.