Experimental study of zero-valent iron induced nitrobenzene reduction in groundwater: The effects of pH, iron dosage, oxygen and common dissolved anions

Nitrobenzene (NB) in groundwater has posed significant health risks because of its persistence and high toxicity. In this study, batch experiments were performed to evaluate the effects of initial pH, iron dosage, oxygen and common dissolved anions including Cl−, SO42−, NO3−, HCO3−, PO43− and humic acid on the NB reduction by zero-valent iron (ZVI). The NB removal increased with the decrease of initial solution pH and a removal efficiency of 88.7% was obtained at an initial pH value of 7.0. The reduction process followed the pseudo-zero-order kinetic model when the iron dosage was 0.5 g L−1 and the corresponding rate constant k0 was 1.76 mg L−1 h−1. But it showed a better fit with the pseudo first order kinetics when iron dosage ranged from 1.0 g L−1 to 5.0 g L−1 with the corresponding rate constant k1 between 0.16 h−1 and 0.76 h−1. Oxygen inhibited NB reduction due to its competing for electrons from ZVI and the generation of iron oxides. In comparison to the anaerobic system, a 17.1% decrease of NB removal was observed in the air-water system. Improvement of NB removal was observed in the present of Cl−, SO42− and NO3−, while bicarbonate would be an inhibitor if its concentration exceeds 50 mg L−1. The NB reduction was significantly inhibited by PO43− and humic acid since they could form inner-sphere complexes on iron surface. The reaction even ceased as their concentrations reached 8 and 18 mg L−1, respectively.