The Regulatory Effects And Mechanisms Of Phytoremediation On Migration And Transformation Of Heavy Metals In Manganese Residue

Heavy metals in manganese residue will migrate to soil,surface water and groundwater around the disposal sites under the action of rainwater,which will have serious impacts on the ecological environment of the region.Plants can absorb heavy metals,or influence their geochemical behaviors through root exudates such as organic acids and amino acids.Therefore,studying the effects and mechanisms of phytoremediation on the migration and transformation of heavy metals in manganese residue is important for controlling their pollution in manganese residue disposal sites.In this study,the composition of naturally settled plants and the accumulation characteristics of dominant plants to heavy metals in manganese mine residue disposal sites and electrolytic manganese residue disposal sites were analyzed through field investigation.Then the plant rhizosphere effects on heavy metal forms,organisms and minerals composition of manganese residue were investigated through inductively coupled plasma mass spectrometry（ICP-MS）,high-performance liquid chromatography（HPLC）,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,and fourier transform infrared spectroscopy（FTIR）.On this basis,six organic compounds（citric acid,lactic acid,isoleucine,glycine,threonine,and humic acid）and four minerals（kaolinite,dolomite,albite,and chlorite）with significant differences in manganese residue between plant rhizosphere and non rhizosphere were selected to conduct incubation experiments and simulated leaching experiments to analyze their effects on the dissolution behaviors and mechanisms of heavy metals in manganese residue.At the same time,the impact of plant growth on the diversity of bacterial communities in manganese residue was studied by using high throughput sequencing technology,and its correlation with migration and transformation of heavy metals were discussed.Finally,a pot experiment was conducted to study the regulatory effects and mechanisms of plant growth on heavy metals migration in the manganese residue-water system.The main results of this study are as follows:（1）The dominant plants in the manganese mine residue disposal site and electrolytic manganese residue disposal site were Erigeron annuns,Miscanthus floridulu and Buddleja lindleyana Fortune during natural vegetation restoration.The Mn content in the three dominant plants were 32.68～723.36 mg/kg while the enrichment coefficient were 0.001～0.022.The content of Mn in the roots and leaves of M.floridulus and B.lindleyana grown in manganese mine residue disposal site,and in the roots of B.lindleyana grown in the electrolytic manganese disposal site exceeded the normal upper limit of the content level of ordinary plants.The content order of other heavy metals in plants was Zn>Cu>Cr>Pb>Cd,and the enrichment coefficient of Cd in B.lindleyana stems of the two manganese residue disposal sites were larger than 1.According to the distribution of the six heavy metal elements measured in the plant body,the root of E.annuus and the aboveground（stem,branch,leaf,flower）of B.lindleyana were the main accumulation parts for this two kinds of plants,while the heavy metals content in organs of M.floridulus not showed significant accumulation.（2）The main speciation of Mn in manganese mine residue was the reducible（accounting for 66.01%～74.43%of the total amount）while it was the residual（accounting for 59.73%～72.33%of the total amount）in electrolytic manganese residue.The rhizosphere effects of dominant plants increased the proportion of oxidizable Mn in the two types of manganese residue while the exchangeable fractions decreased.The influence on Mn speciation was in order M.floridulus>E.annuus>B.lindleyana.In addition,the main speciation of Cu,Zn,Pb,Cd,and Cr in the two types of manganese residue was the residual.Compared with non-rhizosphere regions,the proportion of the exchangeable and residual Cu,Zn,and Cd in plant rhizosphere manganese residue decreased,while the proportion of oxidizable Cu,Pb,and oxidizable or reducible Zn,Cd increased.The specific influencing extent related to element types,plant types,and manganese residue types.（3）The determination results of organic components in manganese residue showed that the humic acid content of manganese residue in the rhizosphere region was higher than that in the non-rhizosphere region,especially for manganese mine residue.The content of citric acid,isoleucine and threonine in plant rhizosphere of manganese mine residue increased,while the content of lactic acid and glycine in plant rhizosphere of electrolytic manganese residue increased.The SEM results revealed that the surface of manganese residue in the rhizosphere was more rough and porous.The XRD results indicated that the content of dolomite and albite in rhizosphere manganese residue were lower than that in non-rhizosphere manganese residue,but the content of secondary minerals（kaolinite and chlorite）were significantly higher.The FTIR results showed that the peak strengths of dominant plants rhizosphere manganese residues around 1033 cm^-1and 473 cm^-1were greater than those of non-rhizosphere manganese residues,indicating that the number of functional groups such as-COO-,-OH,-CHO,and-C=O were more abundant.（4）The culture experiment and simulated leaching experiment results showed that humic acid could inhibit the release of heavy metals in manganese residue and reduce the elements concentration in the solution by 22.63%～38.68%compared to the control.The order of influence degree was Cu>Pb>Cr>Zn>Mn>Cd.Low-molecular-weight organic acids and amino acids could promote the release of Mn,Cu,Zn,Cd in the two types of manganese residue,especially for the dissolution of heavy metals from manganese mine residue.There were significant differences in the ability of different types of low-molecular-weight organic acids and amino acids to dissolve heavy metals in manganese residue.Specifically as follows:citric acid>lactic acid,glycine>threonine and isoleucine.The overall impact of changes in organic components caused by plant rhizosphere effects on the leaching and migration of heavy metals was inhibitory.The four minerals all had inhibitory effects on the leaching and migration of heavy metals in manganese residue,which were shown in the following order:chlorite>kaolinite>dolomite>albite.The rhizosphere effects of plants promoted the formation of chlorite and kaolinite,so the adsorption of heavy metals by minerals in manganese residue was strengthened.It was beneficial to reduce the leaching and migration of heavy metals in manganese residue.（5）The diversity of the microbial community in the manganese mine residue increased significantly due to plant growth,with the Shannon index and ACE index of bacteria significantly increased,while the Simpson index significantly decreased.In addition,the relative abundance of the Firmicutes phylum was significantly higher in the plant rhizosphere manganese mine residue than in the control and that of the Thiobacillus genus was lower.The changes in these microbial populations affected the formation and transformation of manganese oxide minerals,and had an important impact on the proportion of heavy metals forms in manganese residue.（6）The concentrations of Mn,Cu,Zn,Pb,Cd,Cr in the permeated water of the two types of manganese residues decreased by 8.62%～79.34%,31.17%～98.33%,15.90%～83.67%,39.23%～93.30%,5.13%～84.13%and 23.31%～88.21 respectively compared to the control when planting E.annuus,M.floridulus,and B.lindleyana in the pot experiment.The concentrations of Cu,Zn,Pb,and Cr in the permeated water of all treatment groups were lower than the Class III-V water quality limit specified in the Environmental Quality Standard for Surface Water（GB3838-2002）.Plant growth had reduced the maximum concentration of Cd in manganese residue permeated water（10.32～11.57μg/L）to below the Class V water limit in the above standard,but the minimum concentration of Mn was also 3.28 mg/L,which still carried a high pollution risk.The growth of dominant plants could reduce the leaching rate of heavy metals from manganese residue to permeated water,with the overall largest decrease was M.floridulus.In summary,the naturally growing dominant plants on the manganese residue disposal sites had increased the content of humic acid and secondary minerals through rhizosphere effects.They had increased the fixation of heavy metal elements through pathways such as chelation,competitive adsorption,ion exchange,and oxidation-reduction.In addition,the participation of microorganisms and the absorption of some heavy metals activated by low-molecular-weight organic acids and amino acids by plants under rhizosphere effects had significantly reduced migration of elements such as Mn,Cd from the manganese residue into water.The application prospects of M.floridulus in the treatment of heavy metals pollution in manganese residue disposal sites were the most noteworthy due to factors such as biomass,prevention to heavy metal migration,environmental adaptability,and difficulty in planting management.