*Article* **Stimulating Nitrate Removal with Significant Conversion to Nitrogen Gas Using Biochar-Based Nanoscale Zerovalent Iron Composites**

**Siyuan Liu , Xiao Han, Shaopeng Li, Wendi Xuan and Anlei Wei \***

**\*** Correspondence: alwei@nwu.edu.cn

**Abstract:** For efficient and environmentally friendly removal of nitrate from groundwater, biocharbased nanoscale zerovalent iron composites were prepared, where biochar was derived from pine sawdust at 4 different pyrolysis temperatures. The results show that biochar with different pyrolysis temperatures played a great role in both nitrate removal efficiency and nitrate conversion rate to nitrogen gas for the prepared composites. Specifically, the composite with biochar pyrolyzed at 500 ◦C, ZB12-500, showed the best performance in both nitrate removal and conversion to nitrogen gas. With an initial solution pH from 5 to 10, ZB12-500 maintained high removal efficiencies varying from 97.29% to 89.04%. Moreover, the conversion of nitrate to nitrogen gas increased with the initial nitrate concentration, and it reached 31.66% with an initial nitrate concentration of 100 mg/L. Kinetics analysis showed that the nitrate removal process fit well with a two-compartment firstorder kinetic model. Meanwhile, the test of nitrate removal by ZB12-500 in synthetic groundwater showed that HCO<sup>3</sup> − and SO<sup>4</sup> <sup>2</sup><sup>−</sup> limited nitrate removal but improved nitrate conversion to nitrogen gas. Furthermore, the nitrate removal mechanism suggested that biochar could facilitate electron transfer from zero valent iron to nitrate, which led to high nitrate removal efficiency. In addition, the interaction of ferrous ions and the quinone group of biochar could increase the nitrate conversion to nitrogen gas. Therefore, this study suggests that ZB12-500 is a promising alternative for the remediation of nitrate-contaminated groundwater.

**Keywords:** nano zero-valent iron; biochar; nitrate removal; nitrogen gas
