Fabrication of silica-free superparamagnetic ZrO₂@Fe₃O₄ with enhanced phosphate recovery from sewage: Performance and adsorption mechanism

To retrieve non-renewable phosphate from treated sewage effluent and prevent the occurrence of eutrophication, two core/shell zirconium oxide-based superparamagnetic adsorbents (i.e. ZrO₂@SiO₂@Fe₃O₄ and ZrO₂@Fe₃O₄) were developed and compared for phosphate recovery. Our synthesizing method results in a uniform ZrO₂-coating on SiO₂@Fe₃O₄ or Fe₃O₄, while the SiO₂ pre-coating leads to a notable reducing in surface area from 135.8 to 17.1 m² g⁻¹ and magnetization from 30.5 to 23.1 emu g⁻¹ for ZrO₂@SiO₂@Fe₃O₄ compared to ZrO₂@Fe₃O₄. The adsorption rate constant of 1.75 g mg⁻¹ min⁻¹ and adsorption capacity of 15.98 mg P g⁻¹ by ZrO₂@Fe₃O₄ are notably higher than those by ZrO₂@SiO₂@Fe₃O₄, with values of 0.24 g mg⁻¹ min⁻¹ and 6.33 mg P g⁻¹ respectively. Both adsorbents exhibit a high selectivity towards phosphate even at a competitive anion (i.e. Cl⁻, NO₃ ⁻ and SO₄ ²⁻) to phosphate molar ratio of 100:1. Cyclic experiments suggest that ZrO₂@Fe₃O₄ can consistently recover over 82.8% of phosphate from treated sewage effluent (1.9 mg P L⁻¹ as initial concentration), with high magnetic and chemical stability. Our results show the advantage of ZrO₂@Fe₃O₄ over ZrO₂@SiO₂@Fe₃O₄ for recovering phosphate. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) study indicates the formation of Zr-O-P inner-sphere surface complexation for phosphate adsorption. Linear combination fitting (LCF) quantitatively demonstrates that the phosphate adsorption on ZrO₂ accounts for about 91.1% of overall phosphate adsorption.