| When treating river and lake with large volumes,low pollution concentrations,and large fluctuations in water quality,constructed wetland(CW)technology not only has good Cadmium(Cd)purification capabilities,but also can repair river ecological environment,improves species diversity,and perfectly integrates pollution control and ecological restoration.It is a truly green and ecological water treatment technology.However,there are still some deficiencies in the current CW technology,such as the lack of Cd hyperaccumulation aquatic plants,and the lack of research on the removal effect of Cd in CW.In this study,the mechanism of Cd uptake,transport and tolerance of the potential Cd hyperaccumulator,Myriophyllum aquaticum,was studied by hydroponic experiments to verify its Cd accumulation ability.Secondly,five strains of plant growth-promoting rhizobacteria(PGPR)were selected according to the wetland environment,and their growth promoting ability and Cd tolerance were tested for application in the CW.Finally,the surface flow constructed wetland(SFCW)was constructed to study the purification ability of the constructed wetland under the different Cd concentrations,the distribution of Cd in CW,and the effects of PGPR on the purification ability of Cd,plants,substrates and microorganisms.The enhanced effect of the CW on the purification efficiency of Cd was explored,and the effects of the CW and PGPR on Cd pollution were verified the application potential of water remediation.The main conclusions are as follows:(1)The results showed that Myriophyllum aquaticum has strong tolerance and enrichment ability to Cd.under the treatment of 50 mg/L Cd,the root length,plant height and biomass were inhibited.The highest Cd content in roots,stems and leaves of Myriophyllum aquaticum was 642.10 mg/kg,719.83 mg/kg,and 774.33 mg/kg,respectively;the highest Cd accumulation amount was 974.17 μg/plant.Most of the Cd in the roots,stems and leaves of Myriophyllum aquaticum was stored in the soluble fractions and cell wall fractions(85.61%-91.97%).In addition,the main chemical forms of Cd in the organs of Myriophyllum aquaticum were in the less toxic Na Cl extractable Cd,HAC extractable Cd and HCl extractable Cd(83.22%-90.76%).The forms of Cd with high toxicity and high migration only accounted for8.08%-14.95%.It was suggested that the cell wall and vacuole are the important places for the distribution and detoxification of Cd in the cells of Myriophyllum aquaticum,and the transformation of Cd into a less toxic form is the main detoxification way.(2)Five strains of PGPR were screened in the wetland root soil,namely Aeromonas media,Raoultella ornithinolytica,Aeromonas hydrophila,Bacillus megaterium and Kurthia gibsonii.Among them,Aeromonas media,Raoultella ornithinolytica,Aeromonas hydrophila,Bacillus megaterium have the ability to produce IAA,solubilize phosphorus,produce ACC deaminase,and nitrogen fixation;The Kurthia gibsonii has the ability to produce IAA,ACC deaminase,and nitrogen fixation.Five rhizosphere growth-promoting bacteria have good Cd tolerance and can grow at a concentration of 100 mg/L Cd.(3)Myriophyllum aquaticum CW has good Cd removal capacity.The average removal rate under different Cd concentrations was 87.32%-93.21%.The colonization of PGPR increased the Cd removal rate of the first-level wetland by12.51%-14.10%,and the Cd removal rate of the second-level wetland by10.79%-13.52%.17.39% of the Cd entering the CW system was enriched by Myriophyllum aquaticum,and the PGPR increased it to 22.53%.It may be because PGPR significantly improved the growth status and Cd accumulation capacity of Myriophyllum aquaticum.PGPR increased plant height by 4.46%-15.05%,root length increased by 1.44%-26.30%,biomass increased by 11.32%-23.20%.PGPR increased the Cd content of Myriophyllum aquaticum by 4.42%-18.61%,and increased the Cd accumulation by 16.01%-37.24%.(4)PGPR affected the physiological response of Myriophyllum aquaticum to Cd stress,and also increased the content of available Cd in the substrate.Under Cd stress,the photosynthetic system,antioxidant system and osmotic system of Myriophyllum aquaticum were significantly affected,and the impact range was related to the Cd concentration and plant Cd accumulation.PGPR enhanced the resistance of photosynthetic system,antioxidant system and osmotic system of plants.PGPR increased the available Cd content in the first-class wetland by 31.12%-57.43%,and increased the second-class wetland by 10.19%-51.59%.Improving the resistance of plants under Cd stress and increasing the available Cd content of the substrate may be one of the reasons why PGPR promote plant growth and enhance the ability of Cd accumulation.(5)PGPR affect the microbial community structure in the process of wetland operation.Cd decreased the abundance and diversity of microorganisms,while PGPR increased it.The microbial community structure of different wetlands in different operation periods is also different.Gamma Proteobacteria,alpha Proteobacteria and actinobacteria are the dominant species in wetlands.Among the five strains,Aeromonas media and Aeromonas hydrophila had the best colonization effect and were suitable for living in this environment.However,the colonization effect of Raoultella ornithinolytica and Bacillus megaterium was poor,and the colonization effect of Kurthia gibsonii was the worst,with the abundance below 0.5%. |
PDF Full Text Request