| In recent years,the problem of soil pollution caused by heavy metal because of the wide-spread using fertilizer,pesticide and sewage irrigation has gradually given rise to one of the problems which environmental scientists focused on and studied.Elusive,posteriority,cumulative and coupled with the complexity of the governance process are the characters of heavy metal contamination in soil,so the harm caused by heavy metal to soil is more serious than atmosphere and water pollution.Soil is the main place where people performed agriculture production,and soil contamination is directly concerning the safety of food chain and human health.Undoubtedly,research and treatment of heavy metal contamination of soil have great theoretical and practical significance to us.Compared with physical,chemical and biological treatment techniques,people are more interested in cheap and safe bioremediation technology.Phytoremediation is a kind of green remediation technology in removing environment pollutants and has developed quickly in recent years.And it has been studied by many environmental researchers for its safe and cheap characters.Phytoremediation has a broad prospect of new technology and it is particularly suitable for use in developing countries.In view of this, based on the research at home and abroad,the author selected 22 species of plants which the predecessors have not studied,performed model pot-culture under various density of cadmium. Determining biomass,the Cd content of plant and calculating the BCF,TF and the quality of different parts of the plants,it analyses the ability of absorption,accumulation and transference of cadmium from soil to the body of plants.Through analyzing the correlation between the density of cadmium in soil and BCF,TF of the plants studied the influence degree on the ability of accumulate and transfer Cd from soil to plants with change of the density of Cd.The main results are the following:(1)The effect on the plants caused by the density of soil is different for the kind of plants.The tested 22 kinds of plants present three rulers.Firstly,biomass of plant increases while the density of Cd in the soil under low level(Cd＜50 mg·kg-1),such as Daucus carota,Humulus scandens,Abutilon theophrasti,Orychophragmus violaceus,Brassica campestris,yellow heart cabbage;Secondly, biomass decreases with the rising of density of Cd in soil in high level(Cd＞75 mg·kg-1)such as the above-ground tissue biomass of Acalypha lanceolata and the down-ground tissue of Sonchus oleraceus;Thirdly,the biomass of Datura stramonium and Xanthium Sibiricum still increase even in high density of Cd in soil(Cd＞75 mg·kg-1).(2)The critical concentration of toxicity of Cd to plants is different.For example the critical concentration of Ipomiea aquatica Forsk is 25 mg·kg-1,but the critical concentration of Phaseolus vulgeris and Sorghum bicolor is 50 mg·kg-1.(3)Characters of absorption of the tested plants are the following:①No or little accumulation. Only Oxalis rubra presents this character in the test.When the density of Cd in soil is 125 mg·kg-1 the Cd contents of the above-ground tissue and the down-ground tissue of Oxalis rubra reach up to maximum amount which are 20.13 mg·kg-1 and 9.64 mg·kg-1.Oxalis rubra does not show any poisoning symptoms under any density of Cd in soil,so that it has strong resistant ability to Cd.②A lot of Cd is accumulated in down-ground tissue but the Cd content of above-ground tissue is low.For instance,when the density of Cd in soil is 125 mg·kg-1,the Cd content of above-ground tissue of Sesamum indicum reaches up to maximum amount which is 13.77 mg·kg-1,BCF is 0.28,and the Cd content of down-ground tissue Sesamum indicum also reaches up to maximum amount which is 231.32 mg·kg-1,BCF is 1.85,and TF is 0.06.There are some other plants which have the same characteristics as Sesamum indicum,such as Sorghum bicolor,Alopecurus aequalis,Humulus scandens,butilon theophrasti,Setaria viridis,Oxalis rubra and Phaseolus vulgeris.③The Cd contents of above-ground tissue and down-ground tissue are both high,and the Cd content of the plant is rising with the rising Cd density in soil.The plants grow normally and don't show any toxic symptoms.Just like the Cd content of above-ground tissue of yellow heart cabbage is lower than 100 mg·kg-1 when the density of Cd in soil is lower than 50 mg·kg-1,but its BCF is more than 1;while the density of Cd in soil is 75 mg·kg-1,100 mg·kg-1,125 mg·kg-1 the Cd content of above-ground tissue of yellow heart cabbage is 106.44 mg·kg-1,165.79 mg·kg-1 and 144.72 mg·kg-1 respectively,all of them surpass the standard of hyperaccumulator(the Cd content of above-ground tissue is 100 mg·kg-1),meanwhile within these densities of Cd the BCF of the plant are 1.42,1.66 and 1.16 accordingly,they are all more than 1.And TF are 0.89,0.94,0.60,they are all lower than 1.Based on these,conclusion is that yellow heart cabbage has good ability of Cd accumulation and enrichment but poor ability of transference.(4)Biomass of plant affects the absolute amont of Cd which absorbed by plant to a certain degree.By calculating the absolute amount of Cd it is found that the biomass of Chlorophytum comosum is the biggest one in the tested 22 kinds of plants and the absolute number of Cd is the highest.The quality of Cd of Chlorophytum comosum is 0.170～0.325mg under the tested density of Cd while the Cd content of above-ground tissue is 45.18～65.38 mg·kg-1 which not reaches the standard of Cd of hyperaccumulator.But the Cd content of above-ground tissue of the selected plants in the test reach up to 165.79 mg·kg-1 and 115.02 mg·kg-1,but the quality of Cd in above-ground tissue are 0.029mg and 0.077 mg,they are far lower than Chlorophyum comosum,the quality of Cd of above-ground tissue of all other plants are also lower than Chlorophytum cornosurn.All these show that biomass of plant has great influence on the absolute number of Cd that plant absorbed.(5)There is a density of Cd in soil under which the Cd content of above-ground tissue is higher than others.When the density of Cd reached this value the plant capable of Cd,enrichment is strongest and repairing ability is best.Yellow heart cabbage and Datura stramonium are two kinds of plants which are selected from the test have strong capacity of accumulating Cd.The Cd content of above-ground tissue of yellow heart cabbage and Datura stramonium both reach up to high When the density of Cd in soil is 100 mg·kg-1,these show that the ability of a plant absorb Cd would be best under some certain density,if the density is lower or higher than the amount,the restoration effect would decline.(6)Analyzing the influence factors it is found that the kind of plant is the key factor for plant absorbing Cd capability.The effect ion of plant absorbing Cd ability caused by the density of Cd is different because the kinds of plants are different.Soil,climate and environment indirectly affect the capacity of plant by influencing the growth condition of plants.(7)Analyzed by synthesis all the factors including the Cd content of above-ground tissue,BCF and TF of the 22 tested plants,and taken them as the indexes to measure the plant capable of cadmium enrichment,from these conclusions is that the ability of yellow heart cabbage and Datura stramonium absorbing Cd are the strongest.Secondly,Solanum melongena,Sonchus oleraceus, Brassica campestris,Bidens parviflora,Daucus carota,Ipomiea aquatica Forsk and Acalypha lanceolata are stronger.Thirdly,Coriandrum sativum,Sorghum bicolor,Abutilon theophrasti, Chlorophytum comosum,Phaseolus vulgaris,Xanthium Sibiricum and Orychophragmus violaceus are normal.Lastly,the plant capable of cadmium enrichment of Sesamum indicum,Humulus scandens,Alopecurus aequalis,Oxalis rubra,Phaseolus vulgeris,Setaria viridis are the worst. |
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