From effluent to fertilizer: Phosphorus recovery and reuse using granulated iron-oxyhydroxide-montmorillonite composites

Recovering phosphorus (P) from wastewater offers a sustainable solution to managing the anthropogenic P cycle by reducing water pollution and enhancing its use efficiency in agriculture. This study developed a P recovery platform using montmorillonite‑iron-oxyhydroxide composites (FeOx-MMT) granulated with a polycation. The adsorption properties of the granules were compared to a commercially available granulated ferric hydroxide (GFH) and the powdered FeOx-MMT. Then, the spent material (P-loaded FeOx-MMT granules) was tested as a P-fertilizer in comparison to triple-super-phosphate, a commercial fertilizer. Batch adsorption tests showed that GFH had a higher maximum adsorption capacity than FeOx-MMT granules (10 mg∙P∙g^-1 vs 3.6 mg∙P∙g^-1), yet this trend is reversed when normalized to iron content (32.4 mg∙P∙g-Fe^-1 vs 61 mg∙P∙g-Fe^-1). In addition, the adsorption of P to FeOx-MMT granules was significantly faster than GFH. In terms of desorption, the FeOx-MMT granules showed higher ability compared to GFH. Consequently, in column experiments with real secondary effluent these trends translated to higher overall removal efficiencies by the FeOx-MMT granules: FeOx-MMT specifically adsorbed phosphate ions while leaving other anions like sulfate, nitrate, and chloride in solution. Positively charged ions such as Ca²⁺, Mg²⁺ and NH₄⁺, were also removed by the granulated FeOx-MMT. The result was high volumes of effluent with P concentrations below 0.1 ppm (regulation limit), and granulated composites loaded with high concentrations of P and essential cations. The bioavailability of P from the spent granules in soil was then tested in funnel and tomato-pot experiments. The results suggest that the adsorbed P is released in a manner that can support growth of agricultural crops. This approach supports sustainable phosphorus management, preserving resources, reducing pollution, and promoting food security while enhancing ecological and economic health.