Monitoring TPH biodegradation in soil around Ray oil refinery by natural attenuation, biostimulation and bioaugmentation treatments

The removal mechanism of total petroleum hydrocarbons (TPHs) from contaminated soil around the Rey (South of Tehran, Iran) refinery complex was investigated. In this study, natural attenuation, biostimulation and bioaugmentation (separately and in combination) methods were  evaluated in the form of ۱۳ different treatments. The experiment was conducted on a laboratory scale using oil contaminated soils (۳۸% of the soil dry weight) by evaluation of TPHs and soil microbial respiration in ۲۱۰ days. The modified methods were applied through ۱۳ different treatments, including improving the environmental conditions for native bacteria (natural attenuation for treatments ۱-۸), adding non-native bacterial complex (bioaugmentation for treatment ۹) and intensifying and stimulating growth while adding non-native bacterial complex (biostimulation-bioaugmentation for treatments ۱۰-۱۳). Although, in all treatments, a significant decreasing trend was observed in reducing the TPHs over time, biostimulation-bioaugmentation treatments had the highest amount of TPH decomposition, the highest rate of bio-respiration, the lowest half-life times (t۱/۲), and the highest remediate efficiency and biodegradation rate constants. Among natural attenuation treatments, modifiers with manure and sawdust had the greatest effect on reducing TPHs and the highest rate of bio-respiration. The first-order kinetic model was fitted to the data related to biodegradation in a satisfactory manner. The results showed that there was a strong and positive linear correlation between TPH decrease and microbial respiration in all modifiers. Although in bacterial treatments in the early stages of inoculation, the rate of total respiration was low, over time and the adaptation of effective inoculated bacteria to contaminated soil, gradually increased it. The bioremediation technique proposed here for crude oil contaminated soils can be suitable on the field, due to its low cost and low environmental risk.