**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 N<sup>2</sup> 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 HCO<sup>3</sup> <sup>−</sup> or SO<sup>4</sup> <sup>2</sup><sup>−</sup> in the solution can reduce the removal efficiency of NO<sup>3</sup> −-N to 57.00% or 80.20% and increase the conversion ratio of N<sup>2</sup> to 37.01% or 29.49%, respectively. The removal of nitrate by ZB12 was in 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, thus affecting the removal of nitrate by the ZB12 composite and the formation of products. Therefore, the results of this study provide further references for the eco-friendly removal of nitrate from groundwater.

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

**Funding:** This research work was funded by the National Natural Science Foundation of China (NSFC) (Grant No. 51208424).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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