论文部分内容阅读
Heavy metal pollution of soil is increasingly becoming a global problem with impetus onindustrialization recognized as a global policy for rapid economic growth.Heavy metals，after being processed by mall in industry,are transported into the soil via polluted air，liquid and solid waste and some agricultural chemicals.In recent years，widespreadconcem has arisen over the implications to human health of increasing amounts of heavymetals in the environment.In a soil matrix，heavy metals are often strongly retained，withtheir adverse impact on the environment and human health persisting for substantialperiods.Dabao mountain districts in Wengyuan County，Shaoguan，Guangdong province，China,have severely contaminated about 1.7 million ha of farmland，which accounted for7.3％of the total cultivated land in this area,are contaminated by heavy metals releasedfrom mining activities and irrigation of mine wastewater.The residence time of metals insoils is thousands ofyears，SO they create a permanent dsk for humans and environmentalhealth.The risks of its potential entry in the food chain need to remediate thesecontaminated areas.
Physical and chemieal remediation techniques，destroy the soilsbiological and structural properties and not suitable for vast，diffusely polluted areas.Phytoremediation is an emerging technology that should be considered forremediation of contaminated sites because of its cost effectiveness，aesthetic advantages，and long-term applicability.One important approach of phytormediation is to use ofhyperaccumulators which can be defined as a plant capable of accumulating traceelements at tissue concentration approximately 10 to 100 times higher than that of thenormal plants from contaminated soils.Distribution and accumulation of heavy metalsamong different parts of plant and the mechanisms still remain unknown.In this study,anewly found Zn／Cd hyperaccumulating plant species Potentilla grifithii Hook Was usedto investigate the Cd and Zn interaction，accumulation and subcellular distribution in1eaves.
(1)In hydroponic experiments，P griffithii plants grow healthy and without showing any toxic symptoms during the experiment period in all treatments except 0.4mM Cd and 2.5mM Zn with 0.4raM Cd.The stimulator),effect on plant biomass WaS noticed with moderate addition of Cd and Zn in nutrient solution.These results suggested that certain heavy metals，whether they arc beneficial or toxic，can be stimulatory to some extent,for the growth of several hyperaccumulators but inhibitory at higher concentrations.
(2)The accumulation results shows that P griffithii is able to accumulate Cd up to 654 mg k91 in leaves，2645 mg kg-1 in petiole and Zn up to 9496 mg kg-1 in leaves and 1 57 1 6 mg kg-1 in petioles without showing any toxic symptoms. Further，accumulation of Zn in shoots was more than that of the roots；indicate that P.griffithii has a Zn hyperaccumulator.
(3)Zn addition significantly decreased the Cd accumulation in roots and alleviates the Cd phytoxicity.This phenomenon could have been interpreted as simple competition between Zn and Cd ions at the entry point for metals into the plant.In shoots Zn addition increased the Cd translocation from root to shoot.This stimulation of shoot Cd accumulation due to the Zn addition suggests that both Cd and Zn transport via similar transporters and efficiently detoxified intemally.
(4)In subcellular distribution，the results showed the 63％of the total Cd and 94％of the total Zn in the leaves was present in the protoplasts.In this.94％of total Cd and all most 95％of the total Zn in the protoplasts was present in the vacuoles.These results suggest that vacuolar compartmentation is an important mechanism for Zn and Cd detoxification and hyperaccumulation of leaves of P griffithii. Further,75 to 86％Cd and 71 to 85％ofZn in the leaves ofP griffthii Was present in the soluble form.The high proportion of soluble Zn and Cd in the shoots ofP.griffithii rules out apoplast precipitation as an important detoxification mechanism，but suggests that the excess Zn and Cd are complexed with soluble organic compounds and stored in the vacuoles.
Phytoextraction is applicable only to sites that contain low to moderate levels ofmetal pollution，because elevated concentration of metals in the top soil layer Can bestrongly phytoxie and increase adverse environmental effects such as ground waterpollution due to leaching.Further，hyperaccumulators are small biomass and are slowgrowth prevents the optimization of the phytoextraetion technique and its furthercommercial application.Under such conditions，a solution to the problem Can be the useof metal tolerant plant ecotypes，in combination with an enhanced immobilization of themetals on the soil complex，through the additions of metal—immobilizing soil additives.The fundamental premise of the immobilization technique is to reduce the‘bioavailability’of metals in the soil，bioavailability being defined as that fraction of thetotal metal content that Can interact with the biological target.Subsequently，food andfeed can be grown safer,and on more heavily contaminated sites，a vegetation cover Candevelop to stabilize the soil.A combination of contaminant immobilization andrevegetation thus may limit the environmental impact of contaminated areas.
Chemical amendments such as limestone，dolomite，fly ash and compost were appliedto fix heavy metals in soil and raise the plantation in the agro-discard soil polluted byacidic mine water irrigation.The effect of these amendments on metal immobilization，migration，transfer,distribution and accumulation of heavy metals in the soil—rice systemwere assessed in field conditions.Addition of amendments had no significant changes intotal metal concentrations but it reduced the bioavailability to rice and decreased themetal accumulation in grains.The health risk caused by heavy metals in diets thatresidents in the polluted area was also reduced.This study provides significant andpractical solutions to remediate the large areas of metal contaminated soils and reducingthe health risk of animals and human beings along the downstream of mining areas.Main findings ofthis study，
(1)A field experiment WaS carried out to evaluate the effectiveness of limestone，dolomite，fly ash and compost on the bioavailability of heavy metals in acidicmine water contaminated soils and to study the growth，yield and metalaccumulation by rice(Oryza sativa L).Treatment of heavy metal contaminatedsoil with limestone，dolomite，fly ash or compost to acidic mine contaminated soil，significantly increased the soil pH and made this soil to suitable for plant growth.Moreover,amendments improved the soil physical and chemical properties to releasing essential elements(N，P，K，Ca,Mg，etc.)and helped to immobilize the Cd，Pb and Zn in soil.Decreased migration and bioavailability of heavy metals to rice plants，contribute the reduction of heavy metal concentrations in rice straw and grains.
(2)Toxicity and bioavialbility of Cd，Cu，Pb and Zn,in rum，depend in part on reactivity and solubility，speciation，solubility and specific characteristics of soil and amendments.In this study，Fly ash Was significantly increased the concentration of Cd，Pb and Zn in the Fe／Mn fraction.Limestone，dolomite and compost were decreased the Cd，Pb and Zn exchangeable fraction and increased Fe／Mn and organic matter fractions in soil.All amendments transferred the copper from exchangeable to organic and Fe／Mn oxides fraction.It Was found that many of these amendments contain significant amounts of oxide，organic matter,carbonates and phosphates that reduce the bioavailable fractions.
(3)Among the amendments，compost with limestone showed best in increasing the rice straw and grain yield followed by fly ash，whereas dolomite and fly ash treatments appeared to be more efficient in reducing Cd，Pb and Zn in rice grains.The concentrations of Cd，Cu，Pb and Zn in different parts of rice followed the order：root>straw>grain.Rice grew in contaminated soil，metals were absorbed by roots and most of the metals were retained in roots，and only very little was transferred to the above ground parts of rice such as grain.In conclusion all amendments could decrease the migration and bioavailability of Cd，Pb and Zn in rice significantly but no significant changes noted in Cu availability and accumulation.
(4)All amendments influenced positively the dry matter yield of fice and effectively fixed the bioavailable metal concentrations in acidic soil.Applied amendments could effectively help to reduce the health risk of metal toxicity to the food chain. Further long-term monitoring was needed to test the sustainability of the amendments in metal fixation.