Phytoremediation Effect Of A Plant Community Model On A Manganese Contaminated Site

This study was aimed to explore the theory and engineering technology applied in ecological restoration of heavy metal contaminated soil.An ore tailing wasteland in Xiangtan manganese mine area was selected as the experimental site.Field investigation and soil sample analysis were carried out to determine both the physico-chemical properties and the degree of heavy metal contamination of the experimental site.Leaf,stem and root samples of native plant species distributed in the tailing site and surrounding areas were taken for comparison of their heavy metal tolerance and accumulation characteristics.Sixteen native plant species with ecological and economical values were selected as phytoremediation species for the target area.Pot experiments using tailing wastes as substrate were conducted to study the mechanisms of tolerance of the selected tree species under multiple metal contamination conditions.As an important focus,pot experiments were also designed to examine the amending effect of a specifically prepared organic manure containing a tolerant bacterium strain isolated from the mining wastes.Following the pot experiments,a set of on-site simulation devices using tailing wastes as substrate was constructed.The remediation effects of single and multiple tree species models were compared at pilot-scale test levels using the established simulation devices.Based on the results obtained from the above studies,a four-hectare ecological restoration plot using wood species P.fortunei and K.bipinnataas dominant phytoremediation plants was successfully established in the selected Xiangtan manganese-ore tailing site.The metal contaminated site was not sealed with normal soil.Before transplanting the seedlings of the wood species,a fixed amount of the organic manurespecifically prepared for the target site was applied to the rhizosphere.Both soil and plant samples were collected from the ecological restoration plotin the following five year period after its establishment.The growth and heavy metal accumulation of P.fortunei and K.bipinnata were compared based on sample analyses.The effect of seasonal climate changes on the structure and total metal uptake of the gradually formed plant community was also monitored and analyzed.By combining the basic theories of physical chemistry with those of ecology,an ecosystem thermodynamic model was established for describing the energy state of the recovered ecosystems at the metal contaminated site.The Gibbs free energy of the plant community and the chemical potential of individual plant species were calculated and compared using the established equations.The present study provided a new theoretical model with useful parameters for both implementation and assessment of ecological restoration engineering projects on heavy metal contaminated sites.The main results were summarized as follows:(1)The substrate soil samples collected from the Xiangtan manganese tailing site were poor in the water retention capacity and low in total nitrogen and phosphorus contents,which were unfavorable to plant growth.The highest total content of Mn found at the tailing site amounted to 24727 mg/kg while the total average contents of Pb,Zn,Cu and Cd of the collected samples were also much higher than the background values of Hunan province and China.The estimated Pollution index,Geo-accumulation index and potential ecological risk index indicated that the degree of site contamination fell into thesevere grade of multiple heavy metal pollutionwith potentially high ecological risk.Totally,63 native plant species belonging to 29 families were found in the tailing site and nearby surrounding areas,of which most were herbaceous plants with relatively low heavy metal uptake capacity.In pot cultivation experiments designed for screening of eco-economical plant species,the survival ratesof 7 species,Paulownia fortunei,Sapium sebiferum,Ricinus communis,Koelreuteria bipinnata,Ligustrum quihoui,Hibiscus cannabinus and Nerium indicum reached 100%.Among them,wood species P.fortunei and K.bipinnata possessed the highest bio-growth rates while P.fortunei,K.bipinnata,H.cannabinus and N.indicum possessed high Mn and Pb uptake potentials.Application of organic manure significantly increased both the biomass growth and manganese uptake for all selected plant species.Substrate containing 20%of organic manure was found to be the best cost-effective treatment.(2)In the pilot scale on-site simulation test,the total dry biomass,manganese uptake and water holding capacity of the multiple plant species system were,respectively,1.6,7.5 and 1.5 times those of the single plant species system.Addition of organic manure enhanced plant heavy metal uptake,improved substrate water retention capacity and reduced heavy metal diffusion rates.In simulated rainfall and leaching tests in the pilot scale on-site system,application of organic manure caused a slight increase in water soluble manganese content but a significant decrease in chelating-exchangeable manganese content in the substrate.In general,the quantity of heavy metal losses increased with increase in simulated rainfall intensity.For control of pollution dispersal,it is thus necessary to develop interception technology to prevent water loss and soil erosion from the metal contaminated sites.The multiple plant species-organic manure amending treatment was found to be the most effective ecological restoration model accounted for by its highest plant coverage degree,greatest biomass growth rate and largest heavy metal uptake quantity.Its gradually formed tree-shrub-grass space structure could be characterized by enriched biodiversity,improved water retention capacity and reduced heavy metal diffusion.(3)In the ecological restoration engineering site,the survival rates of both P.fortunei and K.bipinnata were both higher than 83%after transplanting.Accounted for by their fast-growing nature,P.fortunei and K.bipinnata were shown to be adequate wood species for rapid recover of the vegetation and landscape of the metal contaminated site.In the observed 5 year period,the average height,diameter,biomass and heavy metal uptake of P.fortunei were found to be significantly higher than those of K.bipinnata.The accumulated quantity of all measured metal elements in both wood species increased apparently with tree age.The distribution of the metal quantity in both species was found to follow the order:stem>roots>leaf.The change in contents of Mn and Pb in both species with age was insignificant.The distribution pattern of the metal content in both species followed the order:leaf>roots>stem.There were significant seasonal changes in density,coverage percentage,species richness and diversity index of the multiple plant species community,showing a clear descending trend from summer to fall,spring and winter.Due to defoliation of tree species and withering of herbs,there were also significant differences between seasons in quantities of biomass and heavy metal accumulation of the above-ground plants,of which the observed order was fall>summer>winter?spring.The water soluble manganese in the rhizosphere of the multiple plant community plot was found to be higher in summer,fall and winter than spring.The same dynamic seasonal changes were also found for the number of microbial species and their biomass quantities in soil samples collected from the root areas.The biomass growth and manganese uptake of the recovered plant community were positively correlated with the bioavailability of heavy metals and diversity of the plant community.(4)Among all monitored phytoremediation models,the multiple plant community-organic manure amending model had the highest values of coverage percentage,species richness,biodiversity index and evenness index.The order of annual increment in biomass growth among different models was:multiple plant species community-organic manure amending>grass plot-soil sealing>single K.bipinnata forest-soil sealing>control.The order of manganese uptake was:multiple plant species community-organic manure amending>single K.bipinnata forest-soil sealing>control>grass plot-soil sealing.In general,the multiple plant species community-organic manure amending model possessed the highest landscape value accounted for by its richness in layer space structure and beauty of scenery.(5)The quantity transfer factor TQ and bioaccumulation quantity factor BQF defined in this study were found to be adequate indexes for quantitative description of the plant ability to transport and remove heavy metals.The established ecosystem Gibbs free energy equations were shown to be useful models for both determination of the energy state of the recovered plant community on the ecological engineering site and assessment of the phytoremediation effect.The obtained results showed that the trends regarding the phytoremediation effect described by the changes in the Gibbs free energies and chemical potentials were basically in agreement with those by relevant ecological indexes.The highest values of Gibbs free energy and chemical potential with respect to manganese accumulation quantity and uptake potential,respectively,were obtained from the multiple plant species community-organic manure amending model.The Gibbs free energy(G)of an ecosystem contains integrated information concerning bio-productivity and diversity of the ecosystem.The chemical potential(?)is a thermodynamic parameter related not only to growth and element uptake potentials but also to relevant ecological characteristics of the plant species under study.In comparison with those traditionally applied ecological indices,the G and ? values can be used for analysis of the relationship between ecosystems and their surrounding site conditions.Therefore,the established ecosystem Gibbs free energy equations are of high values in guidance of the ecological engineering practice and assessment of the ecological remediation effect.The main progresses made in this study are(1)Development of a new plant community model using wood species as dominant tree species for ecological restoration of heavy metal contaminated sites.(2)Application of an organic manure containing a metal-tolerant strain isolated from the mining wastes with functions of metal toxicity reduction and nutrient supply.(3)Establishment of a thermodynamic equation for determination of theGibbs free energy and chemical potential of the restored plant community on the ecological engineering site.