Characterisation of the rhizoremediation of petroleum-contaminated soil: effect of different influencing factors

Pilot experiments were conducted to analyse the effect of different environmental factors on the rhizoremediation of petroleum-contaminated soil. Different plant species (cotton, ryegrass, tall fescue and alfalfa), the addition of fertilizer, different concentrations of total petroleum hydrocarbons (TPH) in the soil, bioaugmentation with effective microbial agents (EMA) and plant growth-promoting rhizobacteria (PGPR) and remediation time were tested as influencing factors during the bioremediation process of TPH. The results show that the remediation process can be enhanced by different plant species. The order of effectiveness of the plants was the following: tall fescue > ryegrass > alfalfa > cotton. The degradation rate of TPH increased with increased fertilizer addition, and a moderate urea level of 20 g N (Nitrogen)/m<sup>2</sup> was best for both plant growth and TPH remediation. A high TPH content is toxic to plant growth and inhibits the degradation of petroleum hydrocarbons. The results showed that a 5% TPH content gave the best degradation in soil planted with ryegrass. Bioaugmentation with different bacteria and PGPR yielded the following results for TPH degradation: cotton+EMA+PGPR > cotton+EMA > cotton+PGPR > cotton > control. Rapid degradation of TPH was found at the initial period of remediation caused by the activity of microorganisms. A continuous increase of degradation rate was found during the 30–90 days period followed by a slow increase during the 90–150 days period. These results suggest that rhizoremediation can be enhanced with the proper control of different influencing factors that affect both plant growth and microbial activity in the rhizosphere environment