Using cation exchange resin for ammonium removal as part of sequential process for nitrate reduction by nanoiron

Ammonium was reported as a major undesired product in the nitrate reduction by nanoscale zero-valent iron (nZVI). Hence, the focus of this study was aimed to remove ammonium by the cation exchange resin (Purolite C160H). The preliminary study involved optimizing pH condition (4–10) for nitrate (100 mg NO3− L−1) removal by nZVI (0.25 g L−1). The highest nitrate removal of 77% was obtained at pH 4, which was followed by 70% at pH 6, 69% at pH 8, and 64% at pH 10. In addition, the maximum NH4+–N formation of 60% (based on the initial NO3−–N) occurred at pH 4. As for the nitrate reduction, it appears that the pseudo-second order reaction has been followed for both cases of various nitrate concentrations (50–250 mg L−1) and initial pHs (4–10). The rate constants for the former fall within the range of (0.058–2.1) × 10−3 M−1 min−1, and the range of (0.24–0.56) × 10−3 M−1 min−1 for the latter. In view of formation of ammonium in the nZVI preliminary study, two consecutive experiments were performed to determine the effects of competitive ions (Fe2+/Fe3+, NO2−/NO3−) and initial pH's on its removal. As a result, the equilibrium time for the removal of ammonium was 10 min at initial pH 4, 7, and 9. Fe2+ and Fe3+ significantly reduced the ammonium removal efficiency by 40%, while the NO3− and NO2− residues showed insignificant effect on its removal. In addition, there is no significant effect on the removal of ammonium when various initial pHs were applied.