| Abstract | Polycyclic aromatic hydrocarbon (PAH) contamination in soil continues to be one of the biggest environmental challenges because of the persistence and carcinogenicities of the contaminants. There is a need to seek updated environmental friendly ways to remediate these pollutants and vermiremediation provides a promising solution.
Earthworms, because of their burrowing activity result in the accumulation of several lipophilic organic pollutants from their surrounding environment via the absorption of the contaminant through their body wall and also through intestinal uptake when soil passes through the gut. This makes them suitable model organisms in the remediation of PAHs from contaminated land. Biosurfactant produced by bacteria is known to aid mobility and bioavailability of contaminants which enhances bioremediation of hydrocarbons. The toxicity of these hydrocarbon compounds are yet to be fully understood. This research investigates the effect of biosurfactant on vermiremediation using anecic earthworm species (Lumbricus terrestris) and epigeic species (Eisenia hortensis) and studies the joint impact of PAHs and biosurfactant on the biochemical processes in both species by examining the monooxygenase activities (EROD and MROD activity of cytochrome P450 [CYP1A1 and CYP1A2]) and the antioxidase activity (GST activity). A 28-day experiment was conducted by exposing the two species of earthworms in soil spiked with selected 3, 4 and 5 ringed hydrocarbons [phenanthrene (PH), fluoranthene (FL) and benzo(a)pyrene (BAP) at 180 mg Kg-1 combined (60 mg Kg-1 each) and BAP alone at 60 mg Kg-1 under the same conditions. A time response relationship was established between the concentration of the three PAHs and length of exposure. In the presence of biosurfactant (0.1 g L-1), E. hortensis removed on average (mean ± stdev) 91% ± 5.2% PH, 70% ± 8.4% FL and 27% ± 4% BAP after 7 days whereas L. terrestris removed approximately 89% ± 0.5%, 67% ± 6.4% FL and 30% ± 1.4% BAP. These values were significantly higher than the control without the addition of biosurfactant (p = <0.05), E. hortensis removed only 26% ± 1.7% PH, 17% ± 6.2% FL and 8.4% ± 0.7% and L. terrestris 25% ± 4.5% PH 24% ± 8.8% FL and 6.4% ± 3% BAP. A one-way ANOVA analysis showed that there was a significant difference in the degradation of FL and PH in control soils in the presence of biosurfactant compared to the control soils without biosurfactant. Also, a significant difference between degradation of PH, FL and BAP by E. hortensis in the presence of biosurfactant, compared to its control (absence of biosurfactant, degradation by L. terrestris with biosurfactant was observed compared to the control (without biosurfactant). In the presence of biosurfactant (0.1 g L-1), the degradation was similar in both types of hydrocarbons with R-sq values of 85.6%, 85.9% and 94.1% for removal of PH, FL and BAP. This shows that the integration of vermiremediation and biosurfactant hold a promising approach to optimized and accelerated biodegradation of heavy molecular weight (HMW) PAHs in soil.
Further investigation of enzymatic activities in a time response relationship over a 28-day period was conducted. Ultra-performance liquid chromatography (UPLC) analysis showed that both EROD and MROD activities were induced between days 0 and 2 of exposure. The level of increase in enzymatic activities were similar in both genera (between 2 and 2.5-fold higher in EROD and between 2 and 2.5-fold increase in MROD activity). Enzymatic activities decreased by day 7, with complete mortality of L. terrestris by day 28, indicating the presence of monooxygenases in earthworms further indicating the potential in degrading PAHs.
Results obtained indicate that an epigeic species when used with biosurfactant performed slightly (≤5%) better because of their tolerance limits than anecic species which are more sensitive to several parameters such as temperature, moisture content, contaminants. Vermiremediation (using either species) was enhanced with biosurfactant. It is a promising, sustainable and quick technique to remediate PAH contaminants when compared to some of the other conventional approaches such as land filling, soil vitrification or incineration that could be expensive or pose some negative effects in the environment over a period of time.
Overall the integrated approach of vermiremediation with biosurfactant completely removed 3 and 4 ringed hydrocarbons and 80% of 5-ringed hydrocarbons as stated earlier, where the application of rhamnolipid biosurfactant (0.1 mg Kg-1) posed no deleterious effect in either specie of earthworms, thus indicating that this could be a promising technique in remediating organic pollutants such as PAHs. |
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