Effect of precursor type on the reduction of concentrated nitrate using zero-valent copper and sodium borohydride

In this study, we demonstrated that the choice of precursor has a strong effect on the reduction of nitrate (NO₃^-) using zero-valent copper (Cu⁰) synthesized by sodium borohydride (NaBH₄). Different precursors: CuSO₄, CuO, Cu₂O, Cu powder, and Cu mesh were used to reduce NO₃^- at 677 mg-N/L under the reducing conditions of NaBH₄. Compared with the prehydrolyzed samples, those prepared without prehydrolysis exhibited lower reduction rates, longer times and higher concentrations of nitrite (NO₂^-) intermediate. It was found that one-time addition of NaBH₄ resulted in higher reduction rate and less NO₂^- intermediate than two-step addition. Results showed that Cu⁰ from CuSO₄ possessed the smallest particle size (890.9 nm), highest surface area (26.0 m²/g), and highest reaction rate (0.166 min^-1). Values of pseudo-first-order constant (k_obs) were in the order: CuSO₄ > CuO > Cu₂O > Cu powder >Cu mesh. However, values of surface area-normalized reaction rate (k_SA) were approximately equal. It was proposed that NO₃^- was reduced to NO₂^- on Cu⁰, and then converted to NH₄⁺ and N₂, respectively; H₂ generated from both NaBH₄ hydration and Cu (II) reduction contributed to NO₃^- reduction as well.