Sequential biostimulation and bioaugmentation treatments of a diesel-contaminated soil: effect on hydrocarbon degradation and soil bacterial communities

Abstract

Bioremediation is considered a safe, economical and environmentally friendly approach for the treatment of contaminated soils. In this study, two aerobic biostimulation processes, landfarming (LF) and bioventing (BV), associated with nutrient addition (N) and followed by bioaugmentation (BA), were compared to assess the remediation of a contaminated soil. The experimental study was conducted over a 180-day period, with 120 days of biostimulation followed by 60 days of bioaugmentation, with a selected consortium of hydrocarbon (HC) degrading Actinobacteria, for 60 days. Microbiological analyses were carried out to characterize the diversity and composition of the microbial communities by cultivation on HC, and by 16S rDNA Illumina-MiSeq sequencing. Total petroleum HC (TPH), measured by Gas-Chromatography FID, was progressively reduced up to 40.8% in the LFNBA microcosm, after 180 days of landfarming and nutrient biostimulation followed by bioaugmentation. The quality of the treated soil was assessed by a phytotoxicity test that confirmed a progressive reduction of phytotoxicity. The contaminated soil was dominated by Acidobacteria, Actinobacteria, and Alphaproteobacteria. HC degrading bacteria were isolated and identified by 16S rDNA sequencing. After 180 days of treatment, an increase of Actinobacteria, Alphaproteobacteria and Bacilli in BV microcosms was observed, while TM7-3 and Gammaproteobacteria phyla increased in LF treatment. More than 40% of the bacteria detected in LF and BV microcosms were affiliated to HC degrading genera. Molecular investigations confirmed the presence of the alkane monooxygenase encoding gene, alkB for alkane biodegradation. The achieved results showed the feasibility of biostimulation coupled with bioaugmentation for the removal of hydrocarbons in contaminated soils.