Impregnation of Phosphorus- and Nitrate-Selective Anion Exchangers with Zirconium Oxide Nanoparticles for Nutrient Recovery from Wastewater

Nitrogen (N) and phosphorus (P) influx into receiving water bodies leads to cultural eutrophication and, therefore, poses a major environmental challenge. Consequently, increasingly stringent discharge limits have been established, necessitating advanced wastewater treatment methods. In addition, phosphorus is a finite resource, with current reserves projected to last less than a century. The principles of the circular economy therefore emphasize not only the removal of N and P from wastewater, but also their recovery and reuse as fertilizers. This article presents two strong-base anion-exchangers impregnated with zirconium oxide (ZrO2) nanoparticles to create the following: (1) a phosphorus-selective resin (Hybrid Anion Exchanger with ZrO2 (HAIX-Zr)), with a Zr loading of 2.5–3% (m/m) and an ion-exchange capacity (IEC) of ≈35 mg PO43− − P/g resin, and (2) a resin selective for both phosphorus and nitrate (Nitrate-Selective Resin with ZrO2 (NSR-Zr)), with a Zr loading of 1.5–2% (m/m), a simultaneous phosphate IEC of ≈41.3 mg PO43− − P/g resin, and a nitrate IEC of ≈41.3 mg NO3− − N/g resin. When loaded in a fixed-bed column, HAIX-Zr can treat > 300 Bed Volumes (BV) of wastewater to below the phosphate detection limit of 0.02 mg/L when the initial phosphate–phosphorus concentration is ≈11.0 mg/L and the two most common competing anions, Cl− and SO42−, are present at ≈225 mg/L and ≈160 mg/L, respectively. Regeneration of the exhausted HAIX-Zr column with a 2% NaCl + 2% NaOH solution resulted in >95% P recovery, which can be processed to generate MgNH4PO4 (struvite), a slow-release fertilizer. A fixed-bed NSR-Zr column can treat ≈150 BV of wastewater to below the phosphate detection limit of 0.02 mg/L and nitrate detection limit of 0.05 mg/L when the initial phosphate–phosphorus concentration is 31 mg/L, the nitrate–nitrogen concentration is 12 mg/L, and the competing anions Cl− and SO42− are present at ≈175 mg/L and ≈155 mg/L, respectively. Regeneration of the exhausted NSR-Zr column with 2% KOH solution resulted in >92% recovery of nitrogen and phosphorus in a solution rich in nitrogen, phosphorus, and potassium, the essential ingredients of a fertilizer.