Removal of trichloroethene by iron-based biochar from anaerobic water: Key roles of Fe/C ratio and iron carbides

Abstract Nanoscale zero-valent iron (nZVI) is widely studied for environmental remediation in recent years. However, its easy agglomeration, vulnerable oxidation, and runoff limit its practical application. Herein, iron-based lignin-derived biochar composites (Fe-LBC) with different Fe/C ratios were synthesized using a facile carbothermal reduction method and applied for trichloroethene (TCE) removal from anaerobic water. The synthesized Fe-LBC was equipped with higher specific surface area, more uniform nanoparticles distribution, and higher iron carbide content with the decrease of Fe/C ratio from about 1:5 (Fe-LBC5) to 1:30 (Fe-LBC30). TCE removal by Fe-LBC firstly increased as the Fe/C ratio increased from 1:106 to 1:103, and then decreased with the Fe/C ratio further increased to 1:5. The removal efficiency remained largely stable (97.0-98.5%) within the Fe/C ratio range of 1:104-1:30. Fe-LBC30 (1.0 g L-1) could remove 97.4% of TCE (10.4 mg L-1), and owned the highest dechlorination efficiency (30.0%). Iron carbides but not Fe0 could mainly contribute to the dechlorination. The quenching experiment reveals atomic hydrogen (H*) formed under the catalysis of iron carbides, and 1,2-dichloroethene was identified as the main degradation product derived from hydrogenolysis; electrochemical analyses suggest iron carbides could also serve as electron donors to TCE. Moreover, Fe-LBC30 maintained excellent TCE removal efficiency in various water matrixes and pH range of 3-9, revealing its high application potential in practical groundwater remediation. This work firstly demonstrated the reductive dechlorination action of iron carbides rather than nZVI in iron-biochar hybrid materials, boosting our knowledge and potential application of iron-based materials.