Mechanisms And Enhancement Of Phytoremediation Process In Soil Contaminated With Polycyclic Aromatic Hydrocarbons

Over the past centuries, rapid growth of population, mining, industrialization, etc., have significantly contributed to extensive soil contamination. Polycyclic aromatic hydrocarbons (PAHs), a class of persistent organic pollutants (POPs), are widely distributed in the soil, water and air environment. Soil contamination with PAHs pose great threat worldwide to the agricultural food quality and human health and calls for an immediate action to remediate the contaminated sites. The prospect of phytoremediation for soil organic contaminants is an attractive cost effective alternative to traditional engineering approaches. Many studies have demonstrated that plants have potential to enhance the dissipation of PAHs when compared to unplanted controls. However, the basic mechanisms involved are not well elucidated. Although using plants for remediation of persistent contaminants have advantages over other methods, efficiency of phytoremediation of PAH contaminated soil is always limited by the poor water solubility and strong adsorption of PAHs to soil particles, resulting in lower biodegradation rates and longer time to achieve the standards. The use of surfactants to enhance the apparent aqueous solubility, desorption of organic compounds from solids and microbial bioavailability of PAHs in soil solution has been well documented. However, there is a lack of experimental data to elucidate the effect of surfactants on the phytoremediation efficiency of PAHs.In this dissertation, we examined the PAH removal in the presence of plants to contribute to the technology for field phytoremediation in practice. The PAH dissipation mechanisms were investigated to determine if phytodegradation, especially rhizodegradation plays a role in PAH removal. In addition, the effect of surfactants on the phytoremediation efficiency was also addressed. The present study can be summarized as follows.The capability of four plant species (Festuca arundinacea, Lolium perenne, Medicago sativa and Brassica napus) was investigated for phytoremediation of PAH contaminated soil. The plants were grown alone and in combination to see the effect of combined plantation on phytoremediation process. After 65 day of plant growth, plants were harvested and plant biomass, dehydrogenase activity, water-soluble phenolic (WSP) compounds and residual concentrations of phenanthrene and pyrene were determined. The results showed that PAHs had an inhibitive effect on the biomass yield of all plants. The greatest reduction in biomass was observed in Medicago sativa, which produced approximately 35% of the biomass of control. Brassica napus was the most resistant to the presence of PAHs. Significant higher WSP compounds were observed in planted soils than unplanted controls in PAHs polluted soils. WSP compounds of the PAH polluted soils were 3.71-5.63 ug vanillic acid g^-1 of soil, which were 1.05-1.68 times higher than in uncontaminated soil (i.e. 3.06-4.03 ug vanillic acid g^-1 soil). Similarly, higher dehydrogenase activities were observed in the planted soil compared to unplanted soil. The results also indicated higher rates of PAHs loss in planted soils (i.e., with a rhizosphere) than in unplanted control soils. This effect was especially marked with the combined plant cultivation. Rape seed, tall fescue and rye grass were statistically at par in degradation of PAHs; however, the degradation rates were different. Rape seed displayed the highest PAHs degradation rate (98% of phenanthrene and 86% of pyrene) followed by rye grass and tall fescue whereas alfalfa showed the lowest degradation rate, i.e. 97 % of phenanthrene and 79.8% of pyrene. Among combined plant cultivation, the combination of tall fescue and rape seed had the highest removal rate of PAHs, i.e. 99.1% for phenanthrene and 95.7% for pyrene. The correlation analysis indicated a significant inverse relation (P < 0.01) between biological activities (dehydrogenase activity and water-soluble phenolic compounds) and residual concentrations of phenanthrene and pyrene in soils planted with different plant combinations. Phytoremediation with tall fescue, alfalfa, and rape seed could be a feasible choice for PAHs contaminated soils. Moreover, the combined plant cultivation has potential to enhance the process.Tall fescue (Festuca arundinacea) was grown in soil artificially contaminated with varying concentrations of phenanthrene (11-344 mg kg^-1) and pyrene (15-335 mg kg^-1) to evaluate its phytoremediation potential for PAHs contaminated soil. After 65 day of tall fescue growth, plant biomass, microbial viable counts, dehydrogenase activity, water soluble phenolic compounds, bacterial diversity by using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and PAHs residual concentrations were determined. The results showed that target PAHs (phenanthrene and pyrene) did not affect plant biomass at lower concentrations but a reduced biomass (only 53.46% of shoot and 29.74 % of root compared to control) was observed at higher concentrations. Higher biological activities (microbial viable counts, water soluble phenolic compounds, dehydrogenase activity) and PAHs degradation rates were detected in planted soils than those of unplanted controls. Shannon diversity index for planted soils were 3.65-3.79 which were significantly higher compared to unplanted soils i.e. 3.51-3.68 suggesting that presence of plants increased the bacterial diversity. It is also evident from the results that increasing levels of PAHs reduced the bacterial diversity. After harvest, 91.70-97.78% of phenanthrene and 70.80-89.97% of pyrene degraded in planted soils which were 1.88-3.19% and 8.85-20.69% larger than those in corresponding unplanted soils. This enhanced dissipation of target PAHs in planted soils might be derived from increased biological activities in the rhizosphere.Uptake, accumulation and translocation of phenanthrene and pyrene were comparatively investigated to assess the uptake and accumulation contribution in plant promoted degradation of PAHs. Root concentration factors (RCFs) of phenanthrene and pyrene for plants grown in contaminated soils with initial phenanthrene of 199.97 mg kg^-1 and pyrene of 199.34 mg kg^-1 were 0.33-0.46 and 0.56-1.21, whereas the shoot concentration factors (SCFs) of these compounds were 0.15-0.22 and 0.017-0.0837, respectively. Alfalfa exhibited the highest root concentrations of phenanthrene (8.17 mg kg^-1) and pyrene (105.8 mg kg^-1), followed by rape seed which accumulated 5.33mg kg^-1 of phenanthrene and 70.48 mg ^-1 of pyrene, while tall fescue contained the lowest portion of these compounds (phenanthrene of 1.6 mg kg^-1 and pyrene of 25.62 mg kg^-1). The contributions of plant off-take of these chemicals to the plant promoted dissipation were only 0.10-1.42% for phenanthrene and 0.18-2.04% for pyrene. Plants promoted the biodegradation of PAHs mainly were the predominant contribution to the remediation enhancement for soil phenanthrene and pyrene in the presence of vegetation.In order to enhance the phytoremediation process, a study was conducted to evaluate the effects of two non ionic surfactants (Tween 80 and triton x-100) a biosurfactant (Soya Lecithin) and randomly methylated-β-cyclodextrins (RAMEB) on the removal of pyrene from soil cultivated with tall fescue (Festuca arundinacea). Soils with pyrene concentration of about 243 mg kg^-1 grown with the tall fescue and were individually amended with 0, 200, 600, 1000 and 1500 mg kg^-1 of tween 80, triton x-100, biosurfactant, and RAMEB. Unplanted (with and without surfactants) and sterile microcosms (covered and uncovered) were prepared as the controls. Plant biomass dehydrogenase activity and pyrene concentrations were quantified after 60 d of plant growth. The results indicated that all surfactants have significant greater yields of plant biomass compared to unamended soil. Results of dehydrogenase activities showed that with the application of all surfactants, significant higher microbial activities were recorded; however, some surfactants (triton x 100 and RAMEB) inhibited the microbial activities at higher concentration levels. Only 3.9% and 3.2% of pyrene was disappeared in the uncovered and covered abiotic sterile controls, indicating that the removal mechanism of PAHs in soil was mainly due to microbial degradation of the PAHs in soil. In the planted treatment receiving no surfactant amendment, the removal rate of pyrene was 45% which is significantly higher than that of corresponding unplanted control soil, suggesting that the cultivation of tall fescue alone could significantly improve the overall removal of pyrene in soil. All surfactants had significantly higher rates of PAH degradation compared to the unamended planted soil. Overall RAMEB displayed the highest degradation rates (1.4-1.8 times higher than unamended planted control) followed by the triton X-100 (1.34-1.66 times higher than unamended planted control). The positive effect of surfactants on pyrene removal could probably due to their capacities to enhance bioavailability in soil. This study suggests that the addition of surfactants could facilitate phytoremediation of PAH contaminated soil.