Study On Phytoremediation Of Heavy Metal Pollution In Soil

It was reported that the soil around some factories are contaminated by heavy metals Cd and As in 2004,which is paid great attention by environment bureau and government of this province.Cd and As is a poisonous heavy metal element,which poses long-term latency,strong mobility,lower concentration and extensive hazardous areas.Therefore,it must be prior to be removed from the environment.It's very essential to find it out in early days and repair it in time.Nowadays,the technique of Phytoremediation,because of its safe,low-priced characters,is becoming a hot subject, and is relatively suitable for adopting in developing countries,so it's a very new promising technology.On the basis of the research about this subject both at home and abroad,we gave priority to experiment of field,and make potted experiment in laboratory as a supplement.We chose field far way from the factory 700m under the dominant wind direction,which was found that the content of heavy metal exceed the standard in soil environment.And the field is surrounded by walls,which make it facilitate managed. We selected local crops of maize,soybeans,sunflowers,which are facilitate to collect and cultivate,and Brassica juncea and Thlaspi caerulescens L,which has strong capabilities of tolerance and concentration of cadmium,to phytoremediation.The selected field was partitioned into different experiment areas to conduct experiment with different plant and corresponding potted cultivation experiment(experiment for improving soil conditions);the absorption of heavy metals at different growth stage in different plant parts was found out,and the relationship between absorption and external conditions were studied to look for the best soil conditions for plant absorbing heavy metals in the effect of promotion or inhibition,to determine the best soil rehabilitation program of heavy-metal contamination,and to predict the phytoremediation period.Studies have shown that the content of As and Cd in the whole maize and soybeans showed a progressive trend,when the EDTA was watered into the soil.The content of As in maize roots in blank experiment is most,and the maize seed absorb a maximum of As with No.4 concentration of EDTA.The maize stalk absorbed the largest amount of Cd,and the content of Cd in maize seed is 0 in the blank experiment.But the content of Cd in maize seed absorb the largest amount of Cd,far more than it of maize stalk with No.3 concentration of EDTA.Compared with the blank experiment,each part of soybean absorb much As with the No.4 concentration of EDTA,and the soybean leaf absorbed the most.Each part of soybean absorbed much Cd at No.3 concentration of EDTA,and and its absorption order:soybean seed＞soybean stalk＞soybean pod＞soybean root＞soybean leaf.The reasons can be analyzed as the mode of occurrence of heavy metal elements was changed by adding EDTA into the soil,which affect the plants' capacity of absorption.The main absorption site and the possible mechanism as follows:①EDTA because of the addition of EDTA,plants can efficiently absorb the heavy metals,which became metal complexes.Then they can pass through cell wall,were absorbed by the plants,and were free to migrate in the plants.It made the EDTA and metal complexes be partly absorbed by the plant root,and partly migrate to other parts of the plants.②It can be seen that content of As and Cd in the plants showed an increasing trend,but with a small increase,from the content of heavy metal of the whole plant changes corresponding with the content of EDTA changes.The main absorption site and distribution in the plants has weak regularity.The results show that,EDTA and metal complexes were easily absorbed by the seeds,and we should pay special attention to the changes of heavy metal content in the seeds to prevent harm to humans and animals.The content of As and Cd in the maize and soybeans showed increase trend with fulvic acids adding to the soil.The order of amount of As absorbed in the different parts of maize in the blank experiments is:maize root＞maize stalk＞maize seed＞leaf of maize＞maize＞maize leaf.With the No.4 concentration of fulvic acids,the order changed as:maize seed＞maize root＞maize stalk＞maize＞leaf of maize＞maize leaf.And the amount of As in the maize seed is biggest.Compared with the blank experiment,the content of Cd in maize increased,and it in maize seed significantly increased with No.4 concentration of fulvic acids.The soybeans absorbed much As with No.4 concentration of fulvic acids,and each part absorbed many As except soybean root. The amount of Cd absorbed by soybean increased obviously with adding No.4 concentration of fulvic acids into the soil.It also happened in the each part of soybean except soybean root,and the soybean leaf absorbed most of Cd.The main absorption site and the possible mechanism listed as below:①decrease of heavy metals in the roots,which may be due to the addition of fulvic acid can be efficiently combined with the heavy metals,which made the complex pass across the cell wall,absorbing by the plants.②As the fulvic acid concentration increased,content of As and Cd in the plants showed an upward trend.And the content of Cd in the plants increased obviously,when the concentration of fulvic acids increased to a certain amount.And As and Cd was mainly enriched at the ground.Therefore,we should pay special attention to the changes of content of heavy metal in the seeds to prevent harm to humans and animals,when we used fulvic acids to carry out phytoremediation of heavy metal pollution.The soybean absorb the most of As and Cd with adding EDTA to the soil in this experiment.Because of only Cd exceeding the standards,the amount of Cd the plants absorbed in exceeded that in the dust landing on the same place where EDTA or fulvic acid is added into,if the plants were all carried away to be disposed of together or moved away,without participating in the heavy metal circle next year.The characteristics of this thesis is that,although the local heavy-metal contamination exist for a long time,there is no treatment carried out,this paper is the first soil phytoremediation experiment carried out to the local heavy metal contaminated soil.We use local experimental and potted experiment with the polluted soil as compared experiments,using local crop,such as maize,soybean,brassica juncea and Thlaspi caerulescens L,instead of hyperaccumulation,to facilitate local people in phytoremediation.