**1. Introduction**

Groundwater plays an important role in human freshwater resources. About 20% of the world's fresh water supply is provided by groundwater. Therefore, ensuring the safety of groundwater is of great importance to human beings. However, as a result of rapid expansion in modern agriculture and industry, inorganic nitrogen pollution has become a major problem in the world. The main types of nitrogen present in water are nitrate, nitrite, and ammonia, but the most common pollutant is nitrate, and nitrate nitrogen pollution has been reported as a worldwide pollution problem [1–3]. Nitrate nitrogen entering water may lead to eutrophication or other negative effects on water quality [4–7]. On the other hand, nitrate can also end up in the human body through groundwater and cause methemoglobinosis, also known as "blue baby syndrome," and can be converted into carcinogenic nitrite amine preforms [8]. The WHO limited the concentration of NO<sup>3</sup> −-N to 10 mg/L [9]. Therefore, advanced treatment technologies are needed to remove nitrate from groundwater in an economical and environmentally friendly manner.

At present, mainstream nitrate removal technologies in groundwater include chemical catalysis of nitrate reduction, anion exchange, low-pressure reverse osmosis, and microbial methods. Among them, the anion-exchange method and reverse osmosis method both have the disadvantage of requiring frequent regeneration medium and producing secondary pollutants [10,11], while biological denitrification requires a long repair time and generates

**Citation:** Liu, S.; Han, X.; Li, S.; Xuan, W.; Wei, A. Stimulating Nitrate Removal with Significant Conversion to Nitrogen Gas Using Biochar-Based Nanoscale Zerovalent Iron Composites. *Water* **2022**, *14*, 2877. https://doi.org/10.3390/w14182877

Academic Editor: Jianhua Wu

Received: 24 August 2022 Accepted: 11 September 2022 Published: 15 September 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

College of Urban and Environmental Sciences, Northwest University, Xi'an 710127, China

sludge, which requires a large amount of maintenance cost [12]. Compared with other methods, the chemical reduction method provides the benefits of quick effect, lower cost, and no secondary pollution, and is more suitable for nitrate remediation in groundwater [13].

Nanoscale zero-valent iron (nZVI) has been reported to effectively remedy groundwater contamination in recent years because of its finer particle size, higher reactivity, and larger specific surface area to remove pollutants. Although nZVI can effectively reduce nitrate, it also has some disadvantages, such as easy agglomeration, easy corrosion, poor electronic selectivity, low reactivity, and poor product selectivity [14]. For these reasons, a variety of modification methods for nZVI have been studied. The bimetallic method is to dope some high-potential metals into the nZVI so that nZVI and the doped metals can make up micro-macroscopic coupled electrode systems to get rid of more nitrate and product selectivity of the material. Sparis et al. used ZVI-5%Cu particles doped with Cu ions into ZVI to reduce more than 80% of nitrate in 20 min and completely remove it in 1 h, whereas ZVI alone removed only 74.5% of nitrate in 1 h [15]. Lubphoo et al. found that a trimetal (Pd-Cu)-ZVI material using Cu and Pd as catalysts was more efficient in reducing nitrate than pure nZVI, as well as an increased ratio of N<sup>2</sup> production [16]. Zhang et al. found that the addition of palladium increased the gas production of nitrate, nitrite, and ammonia recovered from aqueous and solid-phase supports [17]. Although the dopped metal materials could be beneficial for nitrate removal and its selective conversion to nitrogen gas, they also mean increased material cost and environmental risk. Pre-magnetization could also boost nZVI's reactivity due to its superiority in magnetic memory. The nitrate removal rate of the pre-magnetized Fe<sup>0</sup> system was 1.99 times better than that of the non-pre-magnetized one [18]. During nitrate reduction, the magnetic field gradient force drove nitrate gathering at the surface of the pre-magnetized Fe<sup>0</sup> system, and meanwhile led to more nitrate conversion to nitrogen gas [19]. However, it is difficult to apply the pre-magnetization method on a large scale in the field of groundwater treatment engineering. In fact, nZVI prepared by the loading method can prevent aggregation of ZVI in the reaction, provide more active sites, and have a wider range of pH application conditions [20]. The application scenario of the material is more suitable for practical groundwater remediation.

The carrier materials commonly used in the loading method include organic materials such as alginate matrix [21], polymeric styrene anion exchanger [22], and inorganic materials such as activated carbon [23], biochar [24], zeolite [20], etc. Biochar often presents with better porosity and specific surface area, which makes it a promising carrier for nanoscale materials [25]. Namasivayam et al. used a waste coconut shell to prepare ZnCl<sup>2</sup> activated carbon to recover nitrate from water. The results showed that pH had a great impact on the recovery of nitrate, and the desorption rate could reach 58% and 92% at pH 2 and 11, while the desorption rate was negligible at pH 3–10 [26]. Kamyar et al. prepared TBC by impregnating magnetic nanoparticles on tea biochar to remove heavy metals and nutrients in water, up to 147.84 mg/g of Ni2+, 160.00 mg/g of Co2+, 49.43 mg/g of NH<sup>4</sup> + and 112.61 mg/g of PO<sup>4</sup> <sup>3</sup><sup>−</sup> could be adsorbed onto tested biochar [27]. For nZVI, nitrate was mainly conversed to NH<sup>4</sup> + (93.5%) instead of N<sup>2</sup> (5.7%), while the N<sup>2</sup> conversion ratio of ZVI/BC composite can reach 60.1% [28]. Oh et al. used straw as raw material to prepare biochar-loaded nZVI material at 900 ◦C and reaction results showed that NO<sup>3</sup> −-N was almost completely removed and the selectivity of the N<sup>2</sup> product was also very high [29]. Gao et al. produced biochar-loaded ZVI at 400 ◦C to remove Cr6+ from aqueous solutions and reached a maximum removal capacity of 126 mg/g at pH 2.5, whereas ZVI was highly agglomerated at the same pH [30]. Wei et al. prepared BC/nZVI composites with different mass ratios from straw to remove nitrate nitrogen from water. The prepared composite presented superiority to nZVI, and its removal capacity was 229 mg NO<sup>3</sup> −-N/g [28]. Some studies have concluded that the process of nitrate removal by ZVI composite materials can be explained by Equations (1)–(10) [31–34]. Obviously, biochar-supported nZVI composites have great potential for remediation of nitrate contamination. As a low-cost and environmentally friendly natural material, biochar could also mediate environmentally

related abiotic redox processes [35,36], so it is desirable to use composite materials for nitrate treatment in groundwater. When biochar is used as the carrier of composite materials, pyrolysis temperature affects the properties of composite materials by changing the physicochemical properties of biochar (including specific surface area, functional group, hydrophobicity, and graphitization) [37–39]. However, little is known about the effect of the pyrolysis temperature of biochar support on the product selectivity for nitrate removal by nZVI/BC composites.

$$\text{Fe}^{0} + 2\text{H}^{+} \rightarrow \text{Fe}^{2+} + \text{H}\_{2}\uparrow \tag{1}$$

$$2\text{Fe}^{0} + \text{O}\_{2} + 2\text{H}\_{2}\text{O} \rightarrow 2\text{Fe}^{2+} + 4\text{OH}^{-} \tag{2}$$

$$\text{Fe}^{0} + \text{NO}\_{3}^{-} + 2\text{H}^{+} \rightarrow \text{Fe}^{2+} + \text{NO}\_{2}^{-} + \text{H}\_{2}\text{O} \tag{3}$$

$$4\text{Fe}^{0} + \text{NO}\_{3}^{-} + 10\text{H}^{+} \rightarrow 4\text{Fe}^{2+} + \text{NH}\_{4}^{+} + 3\text{H}\_{2}\text{O} \tag{4}$$

$$\text{3Fe}^{0} + \text{NO}\_{2}^{-} + \text{8H}^{+} \rightarrow \text{3Fe}^{2+} + \text{NH}\_{4}^{+} + \text{3H}\_{2}\text{O} \tag{5}$$

$$\rm{5Fe^{0}} + 2\rm{NO\_{3}^{-}} + 12\rm{H^{+}} \rightarrow \rm{5Fe^{2+}} + \rm{N\_{2}(g)} + 6\rm{H\_{2}O} \tag{6}$$

$$\mathrm{^08Fe}^0 + \mathrm{3NO\_3^-} + \mathrm{9H\_2O} \xrightarrow{\mathrm{BC}} \mathrm{4Fe\_2O\_3} + \mathrm{3NH\_4^+} + \mathrm{6OH^-} \tag{7}$$

$$3\text{ }10\text{Fe}^{0} + 6\text{NO}\_{3}^{-} + 3\text{H}\_{2}\text{O} \xrightarrow{\text{BC}} 5\text{Fe}\_{2}\text{O}\_{3} + 3\text{N}\_{2}(\text{g}) + 6\text{OH}^{-} \tag{8}$$

$$\text{3Fe}^{0} + \text{NO}\_{3}^{-} + \text{3H}\_{2}\text{O} \overset{\text{BC}}{\rightarrow} \text{Fe}\_{3}\text{O}\_{4} + \text{NH}\_{4}^{+} + \text{2OH}^{-} \tag{9}$$

$$18\text{Fe}^{0} + 3\text{Fe}^{2+} + 10\text{NO}\_{3}^{-} + 2\text{H}\_{2}\text{O} \overset{\text{BC}}{\rightarrow} 7\text{Fe}\_{3}\text{O}\_{4} + 5\text{N}\_{2}(\text{g}) + 4\text{OH}^{-} \tag{10}$$

Our previous study have proved the feasibility of the nZVI/BC for nitrate removal from groundwater [28]. In order to explore the effect of biochar prepared at different pyrolysis temperatures on the removal of nitrate from groundwater by nano zero-valent iron/biochar composite here, nZVI/BC composites with a mass ratio of 1:2 (ZB12) were prepared by using biochar at different pyrolysis temperatures. The prepared composites were characterized using surface analysis techniques, including a scanning electron microscope, X-ray diffraction pattern, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller (BET) specific surface area measurement. Moreover, nitrate removal efficiencies and their product selectivity were evaluated under different conditions of groundwater, including dosage, initial pH, initial nitrate concentration, and co-existing ions. In addition, the nitrate removal kinetics were investigated, and a mechanism of nitrate removal was proposed.
