Converting Okara to Superabsorbent Hydrogels as Soil Supplements for Enhancing the Growth of Choy Sum (Brassica sp.) under Water-Limited Conditions

Okara is a food waste produced from the soybean milk and soybean curd industries and has been identified as a suitable candidate for development of bio-based functional materials. Here, we report the design, synthesis, and characterization of superabsorbent hydrogels through graft copolymerization of okara with acrylic acid (AA) and acrylamide (AAm). The suspension of okara fine particles was activated with ammonium persulfate, which then initiated the free radical polymerization to randomly graft poly­(acrylic acid) (PAA) and polyacrylamide (PAAm) onto okara particles in the presence of a small amount of N,N′-methylenebisacrylamide (MBA) as a cross-linker, producing okara-based superabsorbent hydrogels. The properties of the hydrogels, including their water absorbency and water holding and retention capacities in potting mix, were studied for application as soil supplements. Five superabsorbent hydrogels, with the okara content fixed at 33%, were synthesized with different ratios of monomers and cross-linker. Among the five hydrogels synthesized, three hydrogels were selected and evaluated for plant growth. Using choy sum (Brassica rapa subsp. chinensis var. parachinensis), a common green Asian leafy vegetable, as the test model, we demonstrated that the crop grown in potting mix supplemented with one of the three hydrogels, which was synthesized with the feed ratio AA/AAm/MBA at 7:3:0.05, showed more advanced growth stage and significantly higher shoot fresh weight and larger leaf areas. Further, seedlings grown in potting mix containing the hydrogel showed better survival than those without the hydrogel under extreme water-stressed conditions. More than 80% growth enhancement was observed in crop under water-limited conditions when 2% (w/w) hydrogel was added to the potting mix. Therefore, the okara-based superabsorbent hydrogels hold promise for potential agricultural and farming applications.