Groundwater nitrate contamination is challenging and requires efficient solutions for nitrate removal. This study aims to investigate nitrate removal using a novel adsorbent, biochar-supported aluminum-substituted goethite (BAG). The results showed that an increase in the initial Al/(Al + Fe) atomic ratio for BAGs from 0 to 20% decreased the specific surface area from 115.2 to 75.7 m²/g, but enhanced the surface charge density from 0.0180 to 0.0843 C/m². By comparison, 10% of Al/(Al + Fe) led to the optimal adsorbent for nitrate removal. The adsorbent’s adsorption capacity was effective with a wide pH range (4–8), and decreased with increasing ionic strength. The descending order of nitrate adsorption inhibition by co-existing anions was SO₄²⁻, HCO₃⁻, PO₄³⁻, and Cl⁻. The adsorption kinetics and isotherms agreed well with the pseudo-first-order equation and Langmuir model, respectively. The theoretical maximum adsorption capacity was 96.1469 mg/g. Thermodynamic analysis showed that the nitrate adsorption was spontaneous and endothermic. After 10-cycle regeneration, the BAG still kept 92.6% of its original adsorption capacity for synthetic nitrate-contaminated groundwater. Moreover, the main adsorption mechanism was attributed to electrostatic attraction due to the enhancement of surface charge density by Al substitution. Accordingly, the BAG adsorbent is a potential solution to remove nitrate from groundwater.

For efficient and environmentally friendly removal of nitrate from groundwater, biochar-based 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 first-order kinetic model. Meanwhile, the test of nitrate removal by ZB12-500 in synthetic groundwater showed that HCO₃⁻ and SO₄²⁻ 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.