*2.4. Experiments for Chemical Reduction of Nitrate by ZB12*

Initial concentrations of nitrate were investigated in the range of 30–100 NO<sup>3</sup> −-N mg/L, which was set according to a survey of nitrate-contaminated groundwater in China [42]. In addition, nitrate removal experiments were conducted in 40 mL nitrate solution (30 NO<sup>3</sup> −-N mg/L) with 0.2 g of the prepared samples (nZVI or ZB12 samples). Moreover, the role of dosage in a 40 mL solution (30 NO<sup>3</sup> −-N mg/L) was investigated with 0.08, 0.12, 0.16, 0.2, 0.24, and 0.28 g of ZB12 samples. The impact of the solution pH was studied between

5 and 10, which was adjusted with HCl or NaOH solutions. On the other hand, the effects of co-existing ions were also investigated according to Table 1, which was obtained from a survey of groundwater samples in Weinan City, Shaanxi Province, China. In the above experiments, all the reactions happened in anaerobic bottles, which were sealed with butyl rubber stopper.


**Table 1.** Water quality data of groundwater and experimental configuration water.

Note: K<sup>+</sup> , Na<sup>+</sup> , Mg2+, Ca2+, Cl−, SO<sup>4</sup> <sup>2</sup>−, HCO<sup>3</sup> <sup>−</sup>, NO<sup>3</sup> − are in mg/L.

Kinetic and mechanistic experiments were carried out as follows: 0.2 g of ZB12 samples was reacted with nitrate with an initial concentration of 30 NO<sup>3</sup> −-N mg/L in 40 mL solution in anaerobic bottles. Samples in each bottle were measured in turn at presetting time points. The measurement included pH, DO, oxidation-reduction potential (ORP), and the concentrations of nitrogen species (NO<sup>3</sup> −-N, NO<sup>2</sup> −-N, NH<sup>4</sup> + -N and total nitrogen (TN)). The values of DO, pH, and ORP were monitored by a portable DO meter and a pH meter, respectively. The concentrations of NO<sup>3</sup> <sup>−</sup>-N, NO<sup>2</sup> <sup>−</sup>-N, NH<sup>4</sup> + -N and TN were determined by UV-spectrophotometric method, spectrophotometric method, Nessler's reagent spectrophotometry, and alkaline potassium persulfate digestion spectrophotometry, and the instrument for determination was spectrophotometer. To explore the selectivity of nitrate reduction, gas samples were gathered using a microsyringe. N<sup>2</sup> and N2O in the gas samples were analyzed by GC-TCD with a Molecular Sieve 5A column and a Porapak Q column, respectively [43]. NO<sup>x</sup> (NO<sup>2</sup> and NO) were analyzed using the chemiluminescence detection method [44]. The models of all the instruments used in the experiment are shown in Table 2. All the experiments were conducted at room temperature (25 ± 2 ◦C) with 120 rpm shaking.

**Table 2.** Models of experimental instruments.

