Study On Geochemical Characteristics And Purification Mechanism Of Fe And Mn In AMD-wetland

In China,1.2-1.5 billion tons of AMD are discharged from coal mines every year,accounting for about 10%of industrial wastewater.AMD pollution is a global problem and how to deal with AMD has been widely concerned by experts and scholars all over the world.Conventional treatment methods(such as adding lime,limestone,sodium hydroxide or other alkaline substances)are applied to increase p H by chemical reaction and then reduce the concentration of heavy metals.Consturcted wetland is a new technology instead of traditional method by physical and chemical adsorption,chemical reduction and microbial reduction of Fe,Mn in AMD-wetland,where Fe and Mn or other heavy metals are precipitated into the sediments or transformed into organism components.In AMD-wetlands,more studies have focused on substrate improvement and screening hyper-accumulated plants,ignoring that wetlands are an comprehensive treatment system.At present,some achievements have been made in substrate improvement and plant selection.But under the coupling effect of substrace and plant,few studies have been done on the migration and transformation of heavy metals,which limits the application of wetland treatment system in heavy metal wastewater,especially in AMD treatment field.In addition,a large number of litters,including roots,stems and leaves of wetland plants,provide abundant labile organic matter for wetlands,which have an important impact on the heavy metals entering the sediment environment.Litter decomposition also have an significant impact on the activity of heavy metals stored in sediment and especially the pollution caused by the secondary release of heavy metals from sediments has become an environmental issue concerned by the academia and engineering fileds.In this thesis,AMD-wetlands were taken as the research object to explore the spatial-temporal variability of Fe,Mn as well as other main related elements in the overlying water,sediments,pore water and plant in a natural and a constructed wetland.Through pot experiments,the characteristics of Fe enrichment in plants in different conditions(Fe²⁺,p H and Mn)were studied.Batch experiments were conducted to investigate the adsorption characteristics of Fe,Mn on the main components of sediments(Fe oxides,Mn oxides and organic matter),as well as the sediment with different types of organic matter(no,insoluble and dissolved organic matter).In addition,effects of living plant,litter decomposition and reducing bacteria on the distribution of Fe,Mn in the AMD-wetland were systematically explored by constructing different wetlands.The main conclusions of study were as follows:(1)The purification effect of AMD-wetland system on Fe,Mn and other heavy metals was greatly affected by different seasons.In the dry season,the concentrations of Fe,Cu,Zn and Al at the outlet of wetland were 6.7,1.2,9.6 and 31 times higher than those in the control group,respectively.In the constructed wetland,Fe and Mn of AMD in the normal water period were higher than those in dry and wet water periods.In dry season,Cu,Zn,Al and SO₄^2-were 1.67-1.83,9.25-13.82,16.7-38.2 and9.07-12.67 times higher than those in the control group,respectively.At the outlet of the wetland,Fe did not exceed environmental quality standard for surface water(GB3838-2002)in the wet period but 1.3 times above the standard in the dry periods and 14.23 times in the wet period.Mn exceeded 4 times in the normal water period.Pb?Cd?Cr and As exceeded the standard seriously and those exceeded 7.58,19.4,1.47 and 24.2 times,respectively in the dry period.Cu and Zn did not exceed the standard at all the periods.(2)Heavy metal contents varied greatly both in sediment and in pore water of AMD-wetland.In the natural wetland,heavy metal contents in the surface sediment was Fe>Al>Mn>Zn>Cr>Pb>As>Cu>Cd>Ni.The average contents of Fe,Mn,Cu,Zn,Pb,Cd,Cr,Al,As in the sediments of the natural wetland exceeded the geochemical background values of surface sediment in Guizhou Province,exceeding the multiples of 9,13.9,2.6,3.9,3.4,1.6,1.5,4.8,5.6 times respectively and 5.6,8.8,2.4,2.7,3,10.6,1.5,3.7 and 3.7 times in the contructed wetland.The distribution characteristics of heavy metals in the pore water was Fe>Mn>Al>As>Pb>Zn>Cu>Cd>Ni>Cr,in which Fe and Mn content in rhizosphere pore water was obviously less than that in non-rhizosphere pore water.(3)Wetland plant had a strong ability to accumulate heavy metals in AMD,and were significantly affected by the characteristic pollutants in AMD.Wetland plant had a strong ability to enrich Fe,Mn,Cu,Zn and other heavy metals.Many elements were enriched in the root surface of plants,and Mn had a high concentration in the root surface,root and aboveground parts of the plant.(4)The enrichment and transport characteristics of Fe in plants was controlled by Fe²⁺,p H and Mn.Results showed that the amount of plant enrichment increased as the Fe²⁺concentration when the Fe concentration was 0-150 mg/L and the amount of Fe enrichment on plant decreased significantly when the Fe concentration was 200 mg/L.Fe concent on root surface and in the root and aboveground parts reached the maximum when p H=6.The low concentration of Mn(0-5mg/L)promoted the increase of Fe content in the root surface,and facilitated the transport of Fe from the root surface to the root and aboveground parts.When the concentration of Mn was 20 mg/L,the high concentration of Mn inhibited the enrichment of Fe in the plant.(5)The adsorption of Fe and Mn by various mineral components in sediment and organic matter produced by wetland plants had obvious regularity.Both Langmuir and Freundlich isotherms could be applied to describe the Fe and Mn adsorption process on major mineralized components.The Fe adsorption process on major mineralized components could be better denoted by the first-order,Elovich and double constant,whice first-order and Elovich could be applied to describe Mn adsorption process.In the surface sediments,Mn oxides and organic matter were the main adsorption sites for Fe,whice Fe oxides and organic matter played an important role in Mn adsorption.Freundlich and Langmuir equations could be better described Fe with different types of organic matter,which Langmuir could be applied to denote Mn.Both first-order kinetics and Elovich can well characterize the adsorption kinetics of Fe and Mn in various sediments.In the surface sediments,Insoluble organic matter played an important role in the Fe,Mn adsorption.The dissolved organic matter significantly reduced the adsorption effect of sediments on Fe and Mn.(6)The wetland plant had a significant effect on the distribution of Fe and Mn in surface sediments.Plants had obvious rhizosphere accumulation effect on Fe in surface sediments,while Mn content increased in surface sediments,but on obvious rhizosphere effect.The main reason for the increase of Fe on the sediments was the increase of iron-manganese oxides-bound Fe while the main reason for the increase of Mn was the increase of the exchangble Mn.Plants had a significant effect on Fe in pore water,but have no significant effect on Mn in pore water.In the experimental process,Fe content on root surface and in the root and aboveground parts increased gradually and Mn was mainly concentrated on the root surface and aboveground parts(7)The plant litter had an obvious effect on the migration of Fe and Mn from AMD to sediment.Litter decomposition inhibited Fe and Mn removal from AMD to sediments.OC,DOC in sediments increased significantly or highly significantly by adding plant litter.Litter decomposition not only promoted the dissolution and release of Fe and Mn from sediments,but also promoted the transformation of iron-manganese oxides-bound Fe and Mn to exchangble and orgnic-bound Fe and Mn.Litter had a certain effect on Fe and Mn contents in roots and above ground parts,but the effect was not obvious.(8)SRB inoculation had obvious influence on the chemical speciation of heavy metals in the sediments.SRB inoculation decreased the concentration of Fe,Mn and Cu in the overlying water,but had no significant effect on TOC in the overlying water.SRB inoculation could decrease the contents of iron-manganese oxides-bound and exchangble Fe,Mn and Cu and increase the contents of organic-bound Fe,Mn and Cu in the sediments.Exchangble Zn in sediments first increased and then decreased in the SRB inoculated group.SRB inoculation promoted the transformation between exchangble Zn and iron-manganese oxides-bound Zn.(9)The SRB inoculation in the sediments could significantly or extremely affect physical and chemical characters in AMD and pore water,speciation of heavy metals in the sediments,but had no significant effect on the absorption,enrichment and transport of Fe and Mn in wetland plants.SRB inoculation in the sediment could significantly reduce Fe²⁺and total Fe,Mn,SO₄^2-and TOC in AMD,and finally the pollutants could be transferred into sediments.At the same time,the contents of OC and DOC in the SRB inoculation group were significantly or significantly reduced,and Fe,Mn contents significantly increased.Organic-bound Fe,Mn increased,and the conversion of Fe,Mn forms was realized from unstable state to the stable state in the sediment.