Nitrate Removal by Expanded Graphite Loaded with Zero-valent Iron : Performance and Mechanism

Zero-valent iron (ZVI) loaded onto expanded graphite (EG) as a composite material (EG-ZVI) was prepared through chemical deposition method to remove nitrate. The EG/ZVI was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It is observed that the EG was successfully loaded with sub-micron scale zero-valent iron. Batch experiments with variable theoretical ZVI mass loading, pH values and temperature were conducted to evaluate the removal performance of EG-ZVI. The results showed that the removal rate of nitrate was 2.3 times higher than that for EG. Furthermore, the removal of nitrate by EG-ZVI shows little dependence on solution pH. Even at pH = 9, the nitrate removal efficiency could attain 65% after reaction for 90 min. Galvanic cell effect and high specific surface area are the main reasons for the enhanced removal of nitrate by EG-ZVI with chemical reduction and physical adsorption. And third-order kinetic model is well fitted. XPS were used to analyze the mechanisms of nitrate removal, which indicated that ZVI loaded on the surface was oxidized, and the removed nitrate was reduced to NH4+-N and NO2--N. In addition, EG-ZVI could overcome the shortcomings of ZVI which formed undesirable precipitation of ferrous hydroxide on the surface during reaction frequently. High removal capability, excellent stability and broad applicability make EG-ZVI ideal candidates for nitrate removal in practical application