Restoration of manufactured gas plant site soil through combined ultrasound-assisted soil washing and bioaugmentation

• EDDS + MCD could efficiently remove PAHs/heavy metals simultaneously.
• Inoculation enhanced enzyme activity and PAH removal.
• Ultrasonication promoted pollutants desorption.
• Biodegradation activity was inhibited by the presence of toxic metals.

An effective ex situ soil remediation technology was developed in this study to remove polycyclic aromatic hydrocarbons (PAHs) and heavy metals in a mixed contaminated site. Ultrasonication (20 kHz, 45 min) combined with methyl-β-cyclodextrin (75 g/L) and S,S-ethylenediaminedisuccinic acid (25 g/L) were efficient in extracting mixed pollutants from the soil. After two successive washing cycles, the removal efficiency of PAHs and heavy metals were approximately 84.5% and 81.3%, respectively. The high removal of metals remarkably reduced soil microtoxicity and thus activated biodegradation activity towards PAHs. Inoculation of PAHs-degrading bacterial strains with nutrients addition further removed 86.8% of residual PAHs in 16 weeks. These results were indicated by the significant increase in the number of PAH degraders and soil enzyme activity. After treatment, the residual levels of individual PAHs and heavy metals could meet Chinese soil quality standard for residential use. The proposed combined cleanup strategy proved to be effective and environmentally friendly for remediation of mixed-contaminated site.