<p>Carboxymethyl cellulose stabilized and sulfidated nanoscale zero-valent iron (CMC-S-nZVI) has demonstrated considerable promise for the in-situ dechlorination of trichloroethylene (TCE) in groundwater. However, the effects of natural groundwater components, notably humic acid (HA), on the reactivity and electron efficiency (ε_e) of CMC-S-nZVI remains unclear. This study investigated the degradation of TCE by both sulfidated and unsulfidated CMC-nZVI particles in the presence of HA. Compared to CMC-nZVI, the dechlorination rate constant of TCE by CMC-S-nZVI increased by 40 times (from 0.01&#xa0;h⁻¹ to 0.47&#xa0;h⁻¹), enabling the complete conversion of TCE to ethylene within 10&#xa0;h. The dechlorination rate of CMC-S-nZVI decreased with the addition of HA, while the electron efficiency increased from 44 to 75%. This phenomenon is attributed to HA decreasing the reduction of water by CMC-S-nZVI, thereby optimizing the process and enhancing its efficiency for TCE degradation. Furthermore, HA inhibited the hydrogenation of acetylene to ethylene by CMC-S-nZVI during TCE dechlorination, as it suppressed H₂ generation by CMC-S-nZVI. The effect mechanism of HA on the dechlorination of CMC-S-nZVI provides a scientific foundation for optimizing the application in actual polluted groundwater remediation.</p> Graphical Abstract <p></p>