Reclaiming Phosphate from Waste Solutions with Fe(III)–Polysaccharide Hydrogel Beads for Photo-Controlled-Release Fertilizer

Photoresponsive hydrogels from polysaccharides and Fe(III) were used as a new system to capture and release PO 4 3– from waste solutions. Uptake of 0.6–1.5 mg of phosphate per gram of hydrogels was determined from 800 ppm phosphate solutions (pH 4.8−9.0). These beads also captured 1.2 mg g –1 of phosphate from animal waste (raw manure, 727 ppm phosphate, pH 7.6), which accounted for above 80% phosphate uptake. Irradiation of phosphate-loaded hydrogels degraded the gels due to the photochemistry of the Fe(III)–carboxylates, giving controlled phosphate release (∼81% after 7 days). No release (<2% after 7 days) was seen in the dark. Kale plant trials showed complete degradation of the hydrogels in ∼2 weeks under greenhouse conditions. Biomass analysis of kale treated with phosphate-loaded beads compared to controls indicated no signs of toxicity. These results show that Fe(III)–polysaccharide hydrogels were able to reclaim phosphates from waste solutions and can be used as a controlled-release fertilizer.