Nutrient Capture from Aqueous Waste and Photocontrolled Fertilizer Delivery to Tomato Plants Using Fe(III)-Polysaccharide Hydrogels.

Inexpensive and sustainable methods are needed to reclaim nutrients from agricultural waste solutions for use as a fertilizer while decreasing nutrient runoff. Fe(III)-polysaccharide hydrogels are able to flocculate solids and absorb nutrients in liquid animal waste from Confined Animal Feeding Operations (CAFOs). Fe(III)-alginate beads absorbed 0.05 mg g-1 NH4 + and NO3 - from 100 ppm solutions at pH = 7, with > 80% phosphate uptake and ∼30% uptake of ammonium and nitrate. Ammonium uptake from a raw manure solution (1420 ppm NH4 +) showed a significant 0.7 mg g-1 uptake. Tomato plant trials carried out with Fe(III)-alginate hydrogel beads in greenhouse conditions showed controlled nutrient delivery for the plants compared to fertilizer solution with the same nutrient content. Plants showed an uptake of Fe from the gel beads, and Fe(III)-alginate hydrogel beads promoted root growth of the plants. The plants treated with nutrient-loaded Fe(III)-alginate hydrogels yielded comparable tomato harvest to plants treated with the conventional fertilizer solution.

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 PO43- 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.