Humic acid promotes pathway of chloronitrobenzene cathodic reduction shunting from atomic H* to direct electron...

Progress mechanism of humic acid (HA) interacting with chloronitrobenzenes (ClNBs) and affecting their reduction and degradation was investigated. A two-stage chamber with graphite felt as cathode and anode was constructed, and 2,4-dichloronitrobenzene (2,4-DCNB) was target pollutant. Result showed that HA increased 36.15 % of 2,4-DCNB removal efficiency at pH 5.0 after 4-h cathodic reduction, while at pH 7.0 and 10.0, it showed much less effect. Meanwhile, HA reduced cathodic H₂ and atomic H* production via competing electron with H⁺. Combined with result of HA alleviating inhibition of tert‑butyl alcohol on 2,4-DCNB removal, it was supposed that HA shunted dominant pathway of cathodic reduction of 2,4-DCNB from atomic H* to direct electron transfer (DET). This was proved by HA increasing electron transfer efficiency (k) of 2,4-DCNB reduction from 0.50±0.01 to 0.63±0.01, which indicated rate-limiting step of 2,4-DCNB reduction changed from electron transfer (ET) to bond breaking kinetics. HA-mediated DET pathway initially reduced nitro group, followed by dechlorination, while atomic H* pathway was randomly dechlorinated and nitro reduced. The pH significantly affected agglomeration of HA and 2,4-DCNB. Molecular dynamics simulation showed that hydrogen bond and Van der Waals force dominated agglomeration of HA and 2,4-DCNB in acidic condition, while electrostatic force was main driving force in alkaline condition. Less effect of HA on 2,4-DCNB removal efficiency at high pH could be related to its reduced conductivity and weaker molecular interactions with 2,4-DCNB. This study provides comprehensive insight into role and impact of HA in remediation of ClNBs-contaminated sediment and water by (bio)electrochemical technology.