Analytical determination by standardized method

Nitrate and nitrite were analyzed using ion chromatography. Water samples of effluent were filtered through a 0.2 μm Minisart® Plus syringe membrane filter (NY, USA). The available analytical standards for sodium nitrite, sodium nitrate, and sodium carbonate provided by Panreac (Barcelona, Spain). Distilled, de-ionized water (DDW, 18 Ω cm<sup>−</sup>1) was used in all sample preparation procedures.

The samples were injected into the IC system consisting of a Metrohm pump (Herisau, Switzerland) coupled by a transfer line to a Metrohm conductimetric detector (Herisau, Switzerland). The column used was Metrosep A Supp 7, Metrohm (Herisau, Switzerland). The injection volume was 20 μL and the mobile phase flow (*Na*2*CO*3, 3.6 mM) was 0.7 mL.min<sup>−</sup>1. The anions were identified and quantified according to their retention times (13.90 min ± 0.4 min and 21.19 min ± 0.5 min for nitrite and nitrate, respectively).
