Efficient remediation of PAH-metal co-contaminated soil using microbial-plant combination: A greenhouse study

A 2-year greenhouse study was conducted to remediate an actual wastewater-irrigated soil co-contaminated with polycyclic aromatic hydrocarbons (PAHs) and heavy metals (Cd and Zn). The remediation methods included microbial remediation, phytoremediation, and microbe-assisted phytoremediation. The maximum PAH removal (96.4%), PAH mineralization, and metal phytoextraction (36.1% Cd and 12.7% Zn) were obtained by interplanting ryegrass with Seduce alfredii with regular re-inoculation with Microbacterium sp. KL5 and Candida tropicalis C10 in the co-contaminated soil. The plants shoots were harvested at a 4-month interval. After 2 years, the concentrations of 16 individual PAHs were reduced to below the limit of Chinese soil quality standard for agricultural use (grade II, pH 6.5-7.5), and the metal concentrations in ryegrass shoots were below the Chinese national limit for animal feeds (GB13078-2001). The exogenous microbes gradually disappeared with time, and thus a 2-month re-inoculation interval was applied for a purpose to maintain high cell density and activity of the inoculants. KL5 introduction increased soil enzyme activity, plant growth, PAH removal and metal phytoextraction, while C10 promoted soil enzyme activity and removal of high-molecular-weight PAHs. Interplanting with S. alfredii reduced metal concentrations in ryegrass tissues. Ryegrass showed stronger rhizosphere effects than S. alfredii did.