Removal and reuse of phosphorus from plant nursery irrigation return water with reclaimed iron oxides

Abstract Development of low-cost strategies enabling phosphorus (P) use, recovery, and reuse across multiple economic sectors is needed to ensure its long-term availability for those industries that require P to enhance plant productivity. Irrigation return water from conventional agriculture and specialty crop production remains a source of excess P loading to aquatic ecosystems. Low-cost and technology treatment options are critical for the long-term economic and environmental sustainability of agricultural producers. To address P removal with a sustainable lens, we identified iron oxides (FeOx, a waste product from treating coal mine drainage) as a potential substrate for P sorption, and further evaluated the potential for reuse of P-saturated FeOx as a fertilizer source. The overall objectives of this study were to quantify P sorption, desorption, and plant availability of P adsorbed to FeOx. Batch sorption experiments were conducted to measure the effect of mixing FeOx with silica sand (1:1; FeOx:sand, by mass) on P sorption capacity. The sorption capacity of the FeOx:sand mixture was 21.5 mg P/g FeOx using deionized water. Less than 1% of initially adsorbed P desorbed over 96 h in treatments using irrigation reservoir water adjusted to either pH 6.0 or pH 10.0, demonstrating the stability of the sorption complexes against pH changes expected in irrigation return water systems. Plant available P, which we define as P that desorbed after contact with a CaCl2 solution, ranged from 0.10–0.12 mg/L P; this range is within nutrient recommendations for most plants. Phosphorus-saturated FeOx may be suitable as a fertilizer amendment, as well as a sustainable option for P removal from water. This research provides the foundation for a circular P economy, with viable P treatment and reuse paired with managing a waste product from another widespread industry.