||The purpose of this study was to treat soils contaminated by pyrene through phytoremediation. The plant species selected were Phragmites communis Trin., Typha orientalis Presl, Vetiveria zizanioides, Rohdea japonica (Thunb.) Roth et Kunth, Cyperus malaccensis. Lam. subsp. monophyllus (Vahl) T. Koyama, Bolboschoenus planiculmis (F. Schmidt) T. Koyama and Bidens pilosa respectively. The degradation efficiencies of pyrene in soils and concentration of pyrene in the plant tissues were evaluated in this study. In addition, the change of microbial biota in soils was investigated in the tests of this study.|
The experimental results indicated that after twenty-two weeks, soils planted with V. zizanioides, R. japonica and T. orientalis have better pyrene degradation efficiencies. Especially, after fourteen weeks the pyrene degradation efficiencies were 86%, 84% and 77% respectively, which showed that the efficiencies 10% to 20% higher than those unplanted control experiments, which was 66%. In addition, the pyrene degradation efficiencies in summer were found to be higher than those in winter. The degradation efficiencies of pyrene in sterilized soil with and without T. orientalis were found equal to 59% and 55%, respectively. These values were found lower than those in the experiment without sterilization, in which the pyrene degradation efficiencies with and without T. orientalis were 77% and 66%, respectively, after the fourteen weeks experiment. Hence the rhizospheric microorganisms had a significant effect on the degradation of pyrene in soils.
The pyrene degradation efficiencies were improved with application of fertilizer (HYPONeX No.2, HYPONeX Co., USA). After fourteen weeks, it was found that the experiment with fertilizer and with or without T. orientalis planted were 7% higher, which were 84% and 73% respectively, compared to 77% and 66% with no application of fertilizer.
Proper surfactants have positive effect on phytoremediation. In this study, we found that addition of the surfactant Triton X-100 or combined surfactants (Triton X-100, Tween 20 and sodium dodecylbenzene sulfonate) both presented better pyrene degradation efficiencies than the system without adding surfactant. After ten weeks, soils planted with V. zizanioides and added with surfactants showed the pyrene degradation efficiencies equal to 85% and 87% (combined) respectively, which showed that 4% and 6% higher rate than the system without adding surfactants (81% ). After twenty two weeks, soils planted with V. zizanioides and added with surfactants showed that the pyrene degradation efficiencies were 96% and 96% (combined) respectively. They were all higher than the system without adding surfactants (94%). Soils planted with R. japonica also showed the same results.
In this study, it was also found that the degradation efficiencies were higher at the surface layer of the soil than subsurface layer due to better oxygen content there. Hence the activities of microorganisms in the surface layer were higher than those in the subsurface layer of soils. After twenty two weeks, soils planted with V. zizanioides showed the residual concentration of pyrene were 5.7mg/Kg (surface layer) and 10.8 mg/Kg (subsurface layer). The difference between them was about 50%. Soils planted with R. japonica, T. orientalis or unvegetation also showed the same results.
The pyrene concentrations of the roots, stems and leaves were analyzed and the results showed that pyrene did not exist in the plant stems or its leaves. Since pyrene could not be absorbed into plant’s tissues by plants, the phytoextraction and phytovolatilization did not occur in this study. It was concluded that the degradation of pyrene in soils was mainly in rhizoremediation.
The soils planted with V. zizanioides showed that the inhibition of Lactobacillus sp, while the soils planted with R. japonica and T. orientalis showed unfavorable conditions to Rhibopus sp.. Four weeks after this experiment, both Lactobacillus sp. and Rhibopus sp. were not existent. The soils planted with V. zizanioides, R. japonica and T. orientalis showed an increase of the number of bacteria (CFU), and thus the pyrene degradation efficiency was increased.