Development of laboratory-scale sequential electrokinetic and biological treatment of chronically hydrocarbon-impacted soils

•Bioremediation failed to decontaminate a chronically oil-polluted soil.•Electrokinetic remediation (EKR) was combined with bioremediation.•Bioremediation was applied downstream of the EKR.•The sequential treatment exhibited high degradation performance.•The EKR induced a partial, albeit reversible, inhibition of the microbiota. This study focused on the remediation of a chronically diesel-polluted soil by combining an electrokinetic treatment with a variety of bioremediation approaches. Priority within the sequential treatment was given to electrokinetic remediation (EKR) since the application of natural attenuation (NA), biostimulation and site-specific bio-augmentation resulted in very low degradation performance for total petroleum hydrocarbons (TPH) and polycyclic hydrocarbons (PAH). The application of 20-day EKR (1.0 V cm−1 with polarity reversal) led to 47.2 % and 46.2 % removal of TPH and PAH, respectively, and exerted a negative impact on bacterial abundance, as determined indirectly by quantitative PCR of 16S rRNA genes and community function, as investigated by community-level physiological profiling. These adverse effects were transient and, after a 50-day NA treatment applied downstream from EKR, bacterial abundance was an order of magnitude higher than that found in the initial soil and TPH and PAH removals were significantly higher than those attained by EKR (64.1 % and 56.3 %, respectively). The combination of EKR with site-specific bioaugmentation led to the greatest TPH and PAH degradation (76.0 % and 78.6 %, respectively). The results indicate that bioremediation can be successfully applied downstream from EKR and that the adverse effects exerted by this physico-chemical approach on soil microbiota are reversible.