Interactions of Real Urine with Modified Palygorskite and Zeolite Focusing on Adsorption Mechanisms, Nutrient Bioavailability and Soil Conditioner Upgrade

Nutrient recovery from urine can contribute to sustainable crop production. However, the efficient reuse of adsorbed nutrients depends on their bioavailability, a characteristic that is typically disregarded. In this study, Ca(OH)2-treated zeolite (CaT-Zeol) and palygorskite (CaT-Pal) were compared as one-time adsorbents of PO4-P and NH4-N in aqueous solutions and real human urine. Desorption tests were conducted to determine the real solid concentration of nutrients after adsorption in fresh (FU) and hydrolyzed urine (HU). They revealed that (i) CaT-Zeol immobilized more P (6.27–6.79 mg/g) than CaT-Pal (5.35–5.68 mg/g), and (ii) drying the loaded adsorbents at 35 or 105 °C slightly affected the total P desorption. Both materials adsorbed less P from the alkaline HU (1.92 mg/g CaT-Zeol and 0.50 mg/g CaT-Pal) than from the slightly acidic FU. The calcium bound P predominated on the FU-loaded adsorbents, followed by the plant-available NaHCO3-P. Compared to the CaT-Pal (0.09–0.61 mg N/g), the desorption of plantavailable NH4 N from the FU- and HU-loaded CaT-Zeol (1.72–8.07 mg N/g) was significantly higher. SEM–EDS analyses confirmed the formation of calcium phosphate phases on the FU-loaded adsorbents. The IR-ATR spectra indicated the NH4- N presence on the FU- and HU loaded CaT-Zeol as well as the phosphate adsorption via ligand exchange on both geosorbents. In conclusion, both materials are better suited for P adsorption in FU than in HU, with CaT-Zeol being more effective than CaT-Pal as a dual adsorbent in FU and HU. The overall findings demonstrate the upgrade of both soil-friendly materials after adsorbing plant-available nutrients.