Influence Mechanism Of Nanobubbles On The Remediation Of Cadmium-contaminated Water By Alternaria Chinensis

In recent years,the problem of cadmium（Cd）pollution has become increasingly serious due to the discharge of industrial waste and the abuse of chemical fertilizers.Cd is highly mobile and easily absorbed by plants,and it will pose a serious threat to biological organisms after entering the food chain.Due to the limitations of traditional physicochemical methods for remediating Cd pollution,green and low-cost phytoremediation technology has received more and more attention,but its development is limited by the shortcomings of low efficiency and long time.Therefore,it needs to be combined with other methods to enhance phytoremediation.Nanobubbles（NBs）are fine bubbles with diameters less than or equal to submicron,which can promote plant growth.In this thesis,the effects of NBs on the absorption of Cd by Alternanthera philoxeroides（A.philoxeroides）under hydroponic conditions were studied,the physical and chemical characteristics of NBs were analyzed,and the growth,photosynthetic performance,and different plant parts of A.philoxeroides under different NBs content and Cd concentration combinations were compared and investigated.Changes in Cd enrichment ability and antioxidant system,using excitation-emission matrix spectroscopy（EEM）and Fourier transform infrared spectroscopy（FTIR）techniques to explore the rhizosphere mechanism of NBs to enhance A.philoxeroides Cd tolerance.The main results are as follows:1.The main physical and chemical characteristics of NBs with different contents are different.The dilution method was used to generate NBs with different concentrations,where the DO levels were between 10 mg·L^-1 and 11 mg·L^-1.The mean particle sizes of NBs-25%,NBs-50%,NBs-75%and NBs-100%are 215.8,201.4,184.4 and 176 nm,respectively,and the Zeta potentials are-17.9,-18.1,-19.3 and-23.4 mv respectively.The concentration of·OH produced by NBs at each concentration was 3.11,2.51,11.13 and16.9 times that of the initial time,respectively.The number of nanoparticles in NBs-100%is 2.8×10~8 particles·m L^-1.2.NBs-25%can relieve the growth inhibition of Cd on A.philoxeroides and improve the Cd accumulation ability of each tissue.Compared with Cd alone,the fresh weight of roots,stems and leaves increased by 23.31%,21.33%and 32.25%,respectively.NBs-25%alleviated the reduction of photosynthetic parameters of A.philoxeroides leaves caused by Cd pollution,and further improved the photosynthetic activity.Moreover,the average soluble protein content in the roots,stems and leaves of the NBs-25%group was the highest,which increased by 24.64%,31.20%and 9.62%compared with the DI water group,respectively.However,high concentrations of NBs inhibited the growth of A.philoxeroides.Therefore,NBs25%(7×10~7particles·m L^-1)is the optimum concentration for strengthening phytoremediation of heavy metal Cd-contaminated water:the maximum enrichment in roots,stems and leaves in the NBs-25%group was 280.80mg·kg^-1,204.76 mg·kg^-1 and 18.98 mg·kg^-1,which increased by 0.6%,38.2%and 53.4%,respectively,compared with the DI water group.3.NBs-25%can strengthen phytoremediation by increasing the antioxidant enzyme activity and the content of antioxidant non-enzymatic substances in A.philoxeroides.NBs-25%treatment can effectively reduce the Cd toxicity and lead to lipid peroxidation and accumulation of reactive oxygen species.Among them,under Cd poisoning,NBs-25%significantly decreased the MDA and O₂^·-contents in roots,stems and leaves compared with Cd alone treatment,and decreased by 30.6%,60.8%,22.7%,26.5%,41.0%and 12.4%compared with DI water group,respectively.In addition,NBs-25%maintains the homeostasis of oxygen reduction by increasing the activities of antioxidant enzymes（SOD,POD,CAT）and increasing the content of antioxidants（GSH,As A）,and reduces the accumulation of active oxygen in A.philoxeroides.Reduce the oxidative toxicity of Cd poisoned to plants,so that the cell structure in plants is protected from damage.However,high concentrations of NBs will adversely affect the antioxidant system;therefore,NBs-25%is an appropriate concentration for strengthening phytoremediation of Cd-contaminated water.4.The fluorescence regional integration method（FRI）and the parallel factor analysis method（PARAFAC）were used to analyze the fluorescence spectral characteristics and fluorescence components of the rhizosphere organic matter（r DOM）.The distribution areas are tyrosine proteins,microbial metabolic proteins and humic acids.The average fluorescence volume of the Q_IV and Q_V regions,which accounted for the highest proportion in NBs-25%,was 1.54 and 1.60 times that of the DI water group,respectively.Four kinds of fluorescent components were obtained by PARAFAC analysis:components C1 and C4 were protein-like components,and C2 and C3 were humic acid-like components.The Fmax of each fluorescent component in NBs-25%increased by35.92%,0.56%,12.55%and 11.53%,respectively,compared with the DI water group.5.FTIR technology was used to analyze the change of wavelength of the root components of A.philoxeroides,and to further clarify the effect of different parts of A.philoxeroides on NBs in improving the cadmium tolerance of A.philoxeroides.The FTIR results verified that adding NBs may increase the number of carboxyl and amino groups to bind Cd by promoting the secretion of carbohydrates,proteins,amino acids,etc.,so as to reduce the toxic effect of cadmium on A.philoxeroides.In conclusion,the appropriate concentration of NBs can alleviate the photosynthesis and growth inhibition caused by cadmium poisoning to a certain extent.It mainly alleviates cadmium toxicity by regulating the antioxidant non-enzymatic system and enzymatic system and rhizosphere dissolved organic matter of A.philoxeroides.The results provide a theoretical basis for the application of NBs in enhancing phytoremediation of Cd-contaminated water.