Synthesis of urea-containing sodium alginate-g-poly(acrylic acid-co-acrylamide) superabsorbent-fertilizer hydrogel reinforced with carboxylated cellulose nanocrystals for efficient water and nitrogen utilization

A novel slow-release nitrogen fertilizer hydrogel nanocomposite was fabricated by in-situ free radical copolymerization between sodium alginate (SA), acrylic acid (AA), and acrylamide (AM) in an aqueous media using N, N’methylene bis-acrylamide (MBA) as a crosslinker, citric acid-functionalized cellulose nanocrystals (C-CNC) as a nanofiller and urea as a source of nitrogen. Several variables influencing the water absorbency of the fabricated hydrogels were studied and optimized. The optimized Hyd/C-CNC and Hyd/C-CNC/urea were fully characterized to determine their structure, thermal stability, and morphology. Swelling behavior in saline solutions such as (NaCl, CaCl2, and FeCl3) and at various pHs (2−12) as well as the swelling kinetics were investigated. The incorporation of functionalized cellulose nanocrystals into the hydrogel was very beneficial since it boosts the water absorbency from 260 ± 8 g/g for hydrogel without C-CNC to 316 ± 10 g/g for the hydrogel with 1.5 wt% of C-CNC. The soil experiment showed an enhanced soil water-retention capacity after the addition of 1 wt% of the Hyd/C-CNC polymer. The addition of urea confers to the Hyd/C-CNC material an enhanced water retention property and a slow-release nitrogen property. Under optimized conditions, the Hyd/C-CNC/urea had a total nitrogen amount of 13.66%, a high swelling degree (412 ± 4 g/g), and prolonged nitrogen release duration (20 days for complete N-release). Thus, the good water adsorption capacity, as well as the good slow-release fertilizer property, make this material a suitable water-saving material for agriculture applications.