Insect-Derived Chitosan for Phosphate Recovery and Application as a Sustainable Fertilizer.

Abstract

It is necessary to remove excessive phosphate in treated wastewater before its discharge into inland waters, as phosphate is known to cause eutrophication. Phosphate, being an important nutrient in agriculture, the reuse of recovered phosphate as fertilizer is highly desirable. This article discusses the development of a phosphate-selective biodegradable adsorbent using black soldier fly (BSF) exuviae, which serve as a source of chitosan. The adsorbent, comprised of chitosan and carboxy methyl cellulose (CMC) dispersed with hydrated ferric oxide (HFO) nanoparticles, proved effective in phosphate adsorption through inner sphere complex formation between HFO surface and phosphate ions. The adsorbent showed a capacity of 2 mg /g, and followed Freundlich adsorption model and pseudo-second order kinetics. The adsorbent, named as FeCC, could absorb water about 35 times of its own dry weight. It got 85% biodegraded within 90 days when placed in soil. The near-complete release of adsorbed phosphate (>97%) was observed within 10 days in aqueous solutions having pH 5, 7, and 9. These findings indicated that the release of nutrients occurs gradually over a period of time. A plant growth study indicated that the phosphate-rich hydrogel adsorbent beads with phosphate content as small as 5% of the phosphorus (as P) contained in single super phosphate were sufficient to promote growth similar to the conventional fertilizer. The chitosan of the FeCC matrix also provided nitrogen, which is necessary for plant growth. Thus, gradual and simultaneous release of phosphate and nitrogen from FeCC provided essential nutrients to plants to grow more efficiently than those treated with conventional fertilizer. The study demonstrates the circular economy principle, in which waste materials were recycled to tackle a waste-related problem, with treatment residue reused in agricultural field to be assimilated back into the ecosystem, also providing a win-win solution for environmental issues, food security, and sustainability.