*2.2. Synthesis of ZB12*

Biochar was prepared by pyrolysis. Briefly, the crucible was filled with pre-treated pine sawdust, which was then put into a muffle furnace for carbonization. The carbonization was under a nitrogen atmosphere with a nitrogen flow rate of 80 mL/min and a heating rate of 10 ◦C/min. The carbonization process lasted 4 h at target temperatures of 350, 500, 650 and 800 ◦C, respectively. Then the heating stopped, and the muffle furnace was cooled

down to room temperature. Next, the biochar was removed and pulverized through a 150-mesh sieve for later use. cooled down to room temperature. Next, the biochar was removed and pulverized through a 150-mesh sieve for later use. The synthesis of ZB12 was conducted based on a sodium borohydride reduction

Biochar was prepared by pyrolysis. Briefly, the crucible was filled with pre-treated pine sawdust, which was then put into a muffle furnace for carbonization. The carbonization was under a nitrogen atmosphere with a nitrogen flow rate of 80 mL/min and a heating rate of 10 °C/min. The carbonization process lasted 4 h at target temperatures of 350, 500, 650 and 800 °C, respectively. Then the heating stopped, and the muffle furnace was

Ferrous sulfate heptahydrate (FeSO4·7 H2O, 99.0%) was purchased from Sheng Ao Chemical Reagent (Tianjin, China). The raw pine sawdust was collected from Weinan, Shaanxi Province, China. The sawdust was rinsed three times with deionized (DI) water and then dried overnight in an oven at 80 °C. nZVI used for comparison were purchased from

*Water* **2022**, *14*, 2877 4 of 23

Xiangtian Nanomaterials Co., LTD., Shanghai, China.

*2.2. Synthesis of ZB12*

The synthesis of ZB12 was conducted based on a sodium borohydride reduction method, as in Equation (11) [40]. According to our previous study [28], 1:2 was the optimal mass ratio of iron content to biochar, so this ratio was used in the sample preparation in this study. Briefly, 1.39 g FeSO4·7H2O was dissolved in 50 mL deionized water, then 0.56 g of the prepared biochar sample was added, and next, the mixture was placed in an anaerobic flask for 1 h under ultrasound to make full contact with the iron solution. Then, nitrogen was purged for 30 min to create an anoxic environment. According to the reaction formula, NaBH<sup>4</sup> with a slightly excess amount was dissolved in deionized water of 20 mL, and then added dropwise. The Fe2+ was completely reduced to Fe<sup>0</sup> after reaction at 120 rpm in a shaker for half an hour. The prepared composite was rinsed using deionized water and then anhydrous ethanol, and such rinsing was repeated three times and then dried in a constant temperature water bath under nitrogen protection. The composites prepared from biochar pyrolysis at 350, 500, 650 and 800 ◦C were labeled ZB12-350, ZB12-500, ZB12-650, and ZB12-800, respectively. The preparation process for ZB12 is shown in Figure 1. method, as in Equation (11) [40]. According to our previous study [28], 1:2 was the optimal mass ratio of iron content to biochar, so this ratio was used in the sample preparation in this study. Briefly, 1.39 g FeSO4·7H2O was dissolved in 50 mL deionized water, then 0.56 g of the prepared biochar sample was added, and next, the mixture was placed in an anaerobic flask for 1 h under ultrasound to make full contact with the iron solution. Then, nitrogen was purged for 30 min to create an anoxic environment. According to the reaction formula, NaBH4 with a slightly excess amount was dissolved in deionized water of 20 mL, and then added dropwise. The Fe2+ was completely reduced to Fe0 after reaction at 120 rpm in a shaker for half an hour. The prepared composite was rinsed using deionized water and then anhydrous ethanol, and such rinsing was repeated three times and then dried in a constant temperature water bath under nitrogen protection. The composites prepared from biochar pyrolysis at 350, 500, 650 and 800 °C were labeled ZB12-350, ZB12- 500, ZB12-650, and ZB12-800, respectively. The preparation process for ZB12 is shown in Figure 1.

$$\text{Fe}^{2+} + 2\text{BH}\_4^- + 6\text{H}\_2\text{O} \rightarrow \text{Fe}^0 + 2\text{B(OH)}\_3 + 7\text{H}\_2\uparrow\tag{11}$$

**Figure 1.** Preparation process of ZB12. **Figure 1.** Preparation process of ZB12.

### *2.3. Characterization 2.3. Characterization*

According to previous studies, the nitrate removal reaction can reach about half of the reaction process after 1 h, and the reaction is almost complete after 24 h [41]. Therefore, the sample that reacted for 1 h is selected as the sample during the reaction, and the sample that is reacted for 24 h is the sample after the reaction. According to previous studies, the nitrate removal reaction can reach about half of the reaction process after 1 h, and the reaction is almost complete after 24 h [41]. Therefore, the sample that reacted for 1 h is selected as the sample during the reaction, and the sample that is reacted for 24 h is the sample after the reaction.

The morphology of the samples before and after the reaction was investigated using a scanning electron microscope (SEM). The specific surface area (SSA) was measured using a surface area BET analyzer. X-ray diffraction (XRD) patterns were determined using an X-ray diffractometer. The chemical composition of the samples before, during, and after the reaction was analyzed according to the diffraction peaks. The scanning range 2θ was 10◦~70◦ . In addition, the surface function groups were analyzed using Fourier transform infrared spectroscopy (FTIR). Meanwhile, the surface composition of the samples was investigated using X-ray photoelectron spectroscopy (XPS).
