Rapid and long-effective removal of phosphate from water by zero-valent iron in combination with hypochlorite (ZVI/NaClO)

Abstract

Rapid and long-effective removal of phosphate (P) from water was achieved by simply using zero-valent iron coupled with hypochlorite (ZVI/NaClO). Two irons, steel iron (ZVI-S) and pig iron (ZVI-P) were tested. The removal rate of P by ZVI-P alone was observed to 94.2 (94.2), 96.5 (88.3) and 76.2 (33.8) %, respectively, at 0.5, 1.0 and 5.0 mg/L of initial P concentrations under oxic (anoxic) conditions. The combination of NaClO with ZVI-P remarkably enhanced the percentages of P removal to 94.8–98.2%, regardless of initial-P concentrations and aeration conditions. At 5.0 mg/L of initial P, the coupling of ZVI-P and NaClO also achieved a rapid and long-effective removal of P, resulting in a 96.4–97.8% of P removal during 12 batch recycles, with residual concentration of P averaged at 0.04 mg/L during the long-term of column running with only 12 min of empty-bed contact time (EBCT). The oxidant, NaClO, drove a rapid increase in iron corrosion, which was probably favored by microscopic galvanic effect between elemental carbon and iron, and less degree of iron crystallinity with more crystal defects. However, a limited enhancement of P removal by ZVI-S was observed in presence of oxygen and even spiked with NaClO. The formation of a stable passivation layer composted of ferric oxides and FePO4 on ZVI-S surface was strongly indicated, which might block the oxidative attack from dissolved O2 and even NaClO.