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 m(2) g(-1) and magnetization from 30.5 to 23.1 emu g(-1) for ZrO2@SiO2@Fe3O4 compared to Zr02@Fe304. 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 SO42-)to phosphate molar ratio of 100:1. Cyclic experiments suggest that Zr02@Fe304 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 Zr02@Fe304 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. (C) 2017 Elsevier B.V. All rights reserved.