*4.7. Nitrate Reduction Mechanism*

During the nitrate removal process, ZVI nanoparticles in ZB12 also corroded. In the XRD pattern, the disappearance of Fe<sup>0</sup> peak and the appearance of Fe3O<sup>4</sup> and Fe2O<sup>3</sup> peaks in the composite also indicate the corrosion of nZVI and the appearance of iron oxides. The infrared spectrum shows that the peak strength and peak width of the –OH functional group increased significantly, indicating that –OH was involved in the reaction between ZB12 and NO<sup>3</sup> <sup>−</sup>-N. XPS survey spectra shows that Fe2+ was consumed and generated during the reaction.

In order to deeply explore the reaction process between ZB12 and NO<sup>3</sup> −-N, this section analyzes the nitrogen species (NO<sup>3</sup> <sup>−</sup>-N, NO<sup>2</sup> <sup>−</sup>-N, NH<sup>4</sup> + -N and TN), pH, ORP, and DO during the reaction process. The experimental results are shown in Figures 11 and 12. According to the change in the concentration of each substance in the reaction process, the reaction can be basically divided into three stages.

proceeded at a very low rate.

**Figure 11.** Evolution of the concentration of nitrogen species during the reaction (initial NO3−-N concentration: 30 mg/L, pH 6, Dosage: 5 g/L, ZB12 sample: ZB12-500). **Figure 11.** Evolution of the concentration of nitrogen species during the reaction (initial NO<sup>3</sup> −-N concentration: 30 mg/L, pH 6, Dosage: 5 g/L, ZB12 sample: ZB12-500). *Water* **2022**, *14*, 2877 18 of 23

NH4+-N and TN at the time of 0.5 h were 0.5 mg/L, 10.18 mg/L and 19.53 mg/L, respectively, indicating that nitrite and ammonia were generated during this stage (Equations (3) and (4)). The decrease in TN concentration indicated that N2 was generated during the reaction (Equation (6)). Additionally, the pH increased from 7.5 to 9.67, indicating that H+ in the system participated in the reaction (Equation (1)); DO decreased from 7 mg/L to 0.9 mg/L, indicating that dissolved oxygen in the system competed with NO3−-N and participated in the reaction (Equation (2)); and ORP decreased, indicating that the system was

The second stage occurred at 0.5–8 h. Here, the concentration of NO3−-N decreased from 8.85 mg/L to 1.65 mg/L, which slowed the reaction speed. The concentration of NO2−- N decreased from 0.50 mg/L to 0.05 mg/L, indicating that nitrite was an intermediate product of the reaction (Equation (5)). The concentrations of NH4+-N increased from 10.18 mg/L to 18.88 mg/L, becoming the main component of TN. The pH dropped from 9.67 to 8.9, probably buffered by oxygen-containing functional groups in the biochar. The rise in ORP indicated the consumption of Fe2+ in the system (Equation (10)). The rise in DO was due to the release of adsorbed oxygen or the buffering effect of functional groups from the biochar. Combined with the characterization analysis, the slow reaction rate was affected by the formation of iron oxide in the reaction process. In addition, functional groups in

In the last stage (8–24 h). The concentration of NO3−-N, TN and NO2−-N were still decreasing, and the concentration of NH4+-N was slightly increasing, indicating that the reduction reactions still occur. pH, ORP, and DO leveled off, indicating that the reaction

transformed into a reducing environment and Fe2+ was generated.

biochar were also involved in the reaction (Equations (7)–(9)).

**Figure 12.** Evolution of DO, pH, and ORP during reaction (initial NO3−-N concentration: 30 mg/L, pH 6, Dosage: 5 g/L, ZB12 sample: ZB12-500). **Figure 12.** Evolution of DO, pH, and ORP during reaction (initial NO<sup>3</sup> −-N concentration: 30 mg/L, pH 6, Dosage: 5 g/L, ZB12 sample: ZB12-500).

The reaction mechanism of the ZB12 composite with NO3−-N is shown in Figure 13. The nZVI particles loaded on the surface and pores of biochar can react with NO3−-N or DO in solution. During this period, electrons escape from the core and shell of nZVI to form Fe2+ and generate NO2−, NH4+ and N2, with NH4+ being the main product and NO2<sup>−</sup> being partially converted to NH4+ as an intermediate. nZVI is converted into iron oxide to cover the surface of biochar or exist in solution, and the Fe2+ generated by the reaction contributes to the reduction of nitrate [79]. Carboxyl groups on the surface of biochar will exist in the form of esters [80], while ester groups mainly undergo hydrolysis reaction, so ester groups in biochar will react with H2O, nZVI, and Fe2+ to generate quinone groups (Equation (10)), which is consistent with the results of the FTIR spectrum. The generated quinone group can also provide H+ for the reaction, which converts NO3−-N to N2 and The first stage occurred in 0–0.5 h. Here, the concentration of NO<sup>3</sup> −-N decreased to 8.85 mg/L in a straight line because the active sites of ZVI on ZB12 were abundant at this time, which could fully react with nitrate in the solution. The concentrations of NO<sup>2</sup> −-N, NH<sup>4</sup> + -N and TN at the time of 0.5 h were 0.5 mg/L, 10.18 mg/L and 19.53 mg/L, respectively, indicating that nitrite and ammonia were generated during this stage (Equations (3) and (4)). The decrease in TN concentration indicated that N<sup>2</sup> was generated during the reaction (Equation (6)). Additionally, the pH increased from 7.5 to 9.67, indicating that H<sup>+</sup> in the system participated in the reaction (Equation (1)); DO decreased from 7 mg/L to 0.9 mg/L, indicating that dissolved oxygen in the system competed with NO<sup>3</sup> −-N and participated in the reaction (Equation (2)); and ORP decreased, indicating that the system was transformed into a reducing environment and Fe2+ was generated.

itself into an ester group. In addition, ester groups on the surface of biochar can dissociate in a wide pH range, thus buffering the pH of the reaction and slowing down the reaction inhibition at a higher pH. In addition to the role of biochar in regulating the pH of the system and increasing the reactivity, the conductivity of the carbon matrix and the electron-mediated ability of The second stage occurred at 0.5–8 h. Here, the concentration of NO<sup>3</sup> −-N decreased from 8.85 mg/L to 1.65 mg/L, which slowed the reaction speed. The concentration of NO<sup>2</sup> −-N decreased from 0.50 mg/L to 0.05 mg/L, indicating that nitrite was an intermediate product of the reaction (Equation (5)). The concentrations of NH<sup>4</sup> + -N increased from 10.18 mg/L to 18.88 mg/L, becoming the main component of TN. The pH dropped from

functional groups may also contribute to the reduction of nitrate nitrogen. The electrical conductivity of biochar contributes to the transfer of electrons from nZVI to nitrate, and

the carbon matrix [81]. The electron-mediating capacity of functional groups can be divided into electron donation capacity and electron acceptor capacity, among which the electron donation capacity is attributed to phenolic functional groups, and the electron acceptor capacity is attributed to quinone groups and concentrated aromatic hydrocarbons [82]. These REDOX functional groups can mediate the electron transfer process

In modified ZVI materials, carbon with high potential is used as the cathode and iron as the anode. When the two contact, many microscopic galvanic cells will be generated. The potential difference causes electrons to transfer from the ZVI core shell to the carbon. Some studies have shown that nitrate nitrogen reduction occurs primarily on the carbon surface with a higher electric potential, which reduces the hindering effect of iron oxides on nitrate reduction [15,83]. Therefore, for the ZB12 composite material in this paper, a huge number of miniature galvanic couples can be formed between biochar and nZVI. nZVI acts as the anode, losing electrons and being oxidized to ferrous ions, and biochar

among nZVI, biochar, and nitrate.

acts as the cathode for nitrate reduction.

9.67 to 8.9, probably buffered by oxygen-containing functional groups in the biochar. The rise in ORP indicated the consumption of Fe2+ in the system (Equation (10)). The rise in DO was due to the release of adsorbed oxygen or the buffering effect of functional groups from the biochar. Combined with the characterization analysis, the slow reaction rate was affected by the formation of iron oxide in the reaction process. In addition, functional groups in biochar were also involved in the reaction (Equations (7)–(9)).

In the last stage (8–24 h). The concentration of NO<sup>3</sup> <sup>−</sup>-N, TN and NO<sup>2</sup> −-N were still decreasing, and the concentration of NH<sup>4</sup> + -N was slightly increasing, indicating that the reduction reactions still occur. pH, ORP, and DO leveled off, indicating that the reaction proceeded at a very low rate.

The reaction mechanism of the ZB12 composite with NO<sup>3</sup> −-N is shown in Figure 13. The nZVI particles loaded on the surface and pores of biochar can react with NO<sup>3</sup> −-N or DO in solution. During this period, electrons escape from the core and shell of nZVI to form Fe2+ and generate NO<sup>2</sup> <sup>−</sup>, NH<sup>4</sup> <sup>+</sup> and N2, with NH<sup>4</sup> <sup>+</sup> being the main product and NO<sup>2</sup> − being partially converted to NH<sup>4</sup> <sup>+</sup> as an intermediate. nZVI is converted into iron oxide to cover the surface of biochar or exist in solution, and the Fe2+ generated by the reaction contributes to the reduction of nitrate [79]. Carboxyl groups on the surface of biochar will exist in the form of esters [80], while ester groups mainly undergo hydrolysis reaction, so ester groups in biochar will react with H2O, nZVI, and Fe2+ to generate quinone groups (Equation (10)), which is consistent with the results of the FTIR spectrum. The generated quinone group can also provide H<sup>+</sup> for the reaction, which converts NO<sup>3</sup> <sup>−</sup>-N to N<sup>2</sup> and itself into an ester group. In addition, ester groups on the surface of biochar can dissociate in a wide pH range, thus buffering the pH of the reaction and slowing down the reaction inhibition at a higher pH. *Water* **2022**, *14*, 2877 19 of 23

**Figure 13.** Reaction mechanism of ZB12 composite with NO3−-N. **Figure 13.** Reaction mechanism of ZB12 composite with NO<sup>3</sup> −-N.

**5. Conclusions** On the basis of the 1:2 mass ratio of nZVI to biochar studied in our previous work [28], ZB12 was successfully prepared. The effects of different biochar pyrolysis temperatures on nitrate removal by the ZB12 composite were explored. It was found that the best biochar pyrolysis temperature was 500 °C. In addition, ZB12 has a higher N2 conversion ratio (21.9~27.13%) in a wide pH range (5–10) under the premise of high nitrate removal efficiency (89.04–97.59%), which is more environmentally friendly in practical application. Increasing the initial concentration of nitrate would lead to a decrease in the removal efficiency, but a higher density of N-species on the surface of the composite might lead to an increase in the conversion ratio of N2. The co-existence of HCO3<sup>−</sup> or SO42− in the solution can reduce the removal efficiency of NO3−-N to 57.00% or 80.20% and increase the conversion ratio of N2 to 37.01% or 29.49%, respectively. The removal of nitrate by ZB12 was in In addition to the role of biochar in regulating the pH of the system and increasing the reactivity, the conductivity of the carbon matrix and the electron-mediated ability of functional groups may also contribute to the reduction of nitrate nitrogen. The electrical conductivity of biochar contributes to the transfer of electrons from nZVI to nitrate, and the higher the pyrolysis temperature of biochar, the stronger the electrical conductivity of the carbon matrix [81]. The electron-mediating capacity of functional groups can be divided into electron donation capacity and electron acceptor capacity, among which the electron donation capacity is attributed to phenolic functional groups, and the electron acceptor capacity is attributed to quinone groups and concentrated aromatic hydrocarbons [82]. These REDOX functional groups can mediate the electron transfer process among nZVI, biochar, and nitrate.

accordance with the two-compartment first-order kinetics. Biochar plays a mediating role in the reduction of nitrate by nZVI. The pyrolysis temperature of biochar will affect its electrical conductivity and the electron-mediated ability of its surface functional groups,

Therefore, the results of this study provide further references for the eco-friendly removal

**Author Contributions:** Conceptualization, A.W.; methodology, S.L. (Siyuan Liu) and A.W.; software, S.L. (Siyuan Liu); validation, S.L. (Shaopeng Li) and W.X.; formal analysis, S.L. (Siyuan Liu); investigation, W.X.; resources, X.H. and S.L. (Shaopeng Li); data curation, S.L. (Siyuan Liu); writing—original draft preparation, S.L. (Siyuan Liu) and X.H.; writing—review and editing, A.W.; visualization, X.H.; supervision, A.W. and X.H.; project administration, A.W.; funding acquisition,

**Funding:** This research work was funded by the National Natural Science Foundation of China

**Conflicts of Interest:** All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or

A.W. All authors have read and agreed to the published version of the manuscript.

materials discussed in this manuscript. The authors declare no conflict of interest.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable.

of nitrate from groundwater.

(NSFC) (Grant No. 51208424).

In modified ZVI materials, carbon with high potential is used as the cathode and iron as the anode. When the two contact, many microscopic galvanic cells will be generated. The potential difference causes electrons to transfer from the ZVI core shell to the carbon. Some studies have shown that nitrate nitrogen reduction occurs primarily on the carbon surface with a higher electric potential, which reduces the hindering effect of iron oxides on nitrate reduction [15,83]. Therefore, for the ZB12 composite material in this paper, a huge number of miniature galvanic couples can be formed between biochar and nZVI. nZVI acts as the anode, losing electrons and being oxidized to ferrous ions, and biochar acts as the cathode for nitrate reduction.
