| The rapid development of agriculture and industries has led to the constant release of phenol in wastewater into the aquatic environment. Our urban environment faces increasing threat. Phenol has a characteristic pungent smell and taste in water, and easy to react with the protein in the cell protoplasmic. It usually causes the degeneration and coagulation of cell, and brings teratogenic, carcinogenic, even deadly harm to fish and humans. Nowadays, treatment of phenol-contained wastewater has been a global hot issue. Many traditional physical, physico-chemical, chemical, and biological methods have been used to remove phenol from wastewater, but these methods have some limitations, including high costs, high energy consumption, complex, secondary pollution. Phytoremediation as an efficient technology for phenol removal has recently been receiving increasing attention. However, the new method is being concentrated on microbial field. Many efficient higher plants have not been exploited, but it could not meet the comprehensive treatment of modern water environment. In this study, we selected Polygonum orientale, Canna indica and Phragmites australis as the material for phenol removal, we investigated the tolerant concentrations, physiological responses, removal efficiency and processes, groped the optimum application conditions and combinations of the three species of plants. The results are as follows.1. According to the concentration range (0-80mgL-1) of phenol near the Fenhe River, we screened nine kinds of common plants for phenol removal. As a result, Polygonum orientale, Canna indica and Phragmites australis plants were selected and determined which have strong tolerance of phenol stress and good landscape effect. Although Typha orientalis, Hemerocallis fulva ’Golden Doll’ and Apocymum venetum plants have some degree of effect for removing phenol, the toxicity symptoms were emerged. Humulus scandens plants have high tolerant ability for phenol, but the removal rate was poor. Chenopodium glaucum and Kochia scoparia plants could not live even in a low concentration of phenol.2. Accroding to the results of growth and physiological responses, we determined the concentrations of phenol,≤80mgL-1 of Polygonum orientale, ≤20mgL-1 of Canna indica and≤5 mgL-1 of Phragmites australis plants respectively. The three species of plants could keep or recover a normal growth and physical activity under these concentrations of phenol stresses. Although growth indicators had a slight decrease, the morphology characters were satisfied for ornamental and landscape purposes.3. The harms for plants in this study were concentrated on the follows: (1) promoting the excessive generation of free radicals, aggravating the cell membrane lipid peroxidation, generating electrolyte exosmosis of cell; (2) The antioxidant enzyme activities were inhibited; (3) The composition of osmotic regulators were hindered and protein disintegrated; (4) The photosynthetic level declined and the chlorophyll content decreased. In order to resist the damage of phenol stress, plants could use some mechanisms to avoided injury. Within the maximum tolerated concentrations of the three species of plants respectively, they could enhance the activities of antioxidant enzyme to clear free radicals. They changed the contents of osmotic regulators to reduce the osmotic potential. The stomatal openness was extended and the content of carotenoid was increased. The combined action of different protective mechanisms made the plants resist the harms brought by phenol.4. Within the maximum tolerated concentrations respectively, phenol could be removed completely by the three species of plants, and the effective concentration ranges were consistent with the tolerated concentration ranges. Polygonum orientale plants could remove≤80mgL-1 of phenol in 12 days. Canna indica plants could remove≤20 mgL-1 of phenol in 30 days, while Phragmites australis plants need 16 days to remove≤5 mgL-1 of phenol. However, the efficience of phenol removal decreased significantly when the concentrations of phenol exceeded the tolerance degree of plants.5. The contact time and initial phenol concentrations significantly influenced the adsorption efficiency of phenol. The adsorption process data were analyzed using adsorption isotherm and kinetics models. The adsorption processes of Polygonum orientale and Phragmites australis plants were fitted Langmuir isotherm model and Lagergren’s pseudo-first-order kinetic model. The adsorption processes of these two species of plants belonged to monomolecular physical absorption. However, the data of Canna indica plants were supported by Freundlich isotherm model and Ho’s preudo-second-order kinetic model. The adsorption process was belonged to a physico-chemical course. According to the comparison of the removal pathways, both adsorption and metabolism played roles in removing phenol from water. Adsorption appeared to play a key role in phenol removal, especially during the first few days, when the plants needed some time to adapted phenol stress within the tolerant concentrations. However, the metabolism efficiency could be the main phenol removal process later, with a crucial role in the final removal of phenol. In order to guarantee the good results, we suggested the plants should be used as materials of phenol phytoremediation within the maximum tolerated concentrations.6. We investigated the optimum application conditions of the three species of plants by orthogonal test, respectively. For phenol removal, the optimum conditions of Polygonum orientale were to have a 5 mgL-1 initial phenol concentration,100% daylight, and a 12-day treatment time. The optimum conditions for Canna indica to remove phenol were to have a 2.5 mgL-1 initial phenol concentration,100% daylight, and a 30-day treatment time. And 1 mgL-1 initial phenol concentration,100% daylight, and a 16-day treatment time were the optimum conditions of Phragmites australis for phenol removal. Phenol concentration, light condition, and treatment time as three factors had different impacts on the removal effect of phenol. The affecting sequences for Polygonum orientale were phenol concentration> light condition> treatment time; for Canna indica were treatment time> light condition> phenol concentration; for Phragmites australis were light condition> phenol concentration> treatment time. As a result, we suggested that the concentration of phenol should be reduced to the tolerant range of each specie of plant by some physical or chemical method first, avoiding shade form buildings and high trees. In addition, the phenol treatment time should be considered if necessary to achieve efficient phenol removal.7. With the advantages of phytoremediation and ornamental value, we carried out the floating bed experiment for phenol removal. The results confirmed that, the optimum plant configuration was two Polygonum orientale plants, one Canna indica plant and one Phragmites australis plant on 0.5m2 floating bed. Considering the interaction effect, mixed planting form of the three species of plants and the single form of Polygonum orientale plants significantly affected the phenol removal effect. We suggested mixed plant form for phenol-remove floating bed was priority selected for phytoremediation, and secondary was the single form of Polygonum orientale plants and embellished with Canna indica and Phragmites australis plants.In conclusion, we found three species of plants with good effect for phenol removal. Within the respective tolerant concentrations of phenol, they resisted the damage of phenol stress, maintaining normal growth by the antioxidant enzyme system, osmotic and pigment adjustment. The two main pathways contributing to the total phenol removal rate of the plants were adsorption and metabolism. The optimum condition for each species of plant was determined to remove phenol. Also, we found the optimum plant configuration for the arrangement of phenol-removal floating bed. Above all, this study made up the insufficient of plant materials for phenol removal and provided the references and examples for the phytoremediation of phenol. We believed that these three species of plants could play an important role in the treatment of wastewater in Fenhe basin. |
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