Heavy metals in plant Wars: cadmium rice crisis leads to soil pollution

The increasing crisis of cadmium rice has led to the fact that the land is polluted by heavy metals According to the research on the current situation and development strategy of rice quality and safety in China issued by China Rice Research Institute and the rice and product quality supervision, inspection and testing center of the Ministry of agriculture in 2010, 1 / 5 of China's cultivated land is polluted by heavy metals, among which the cultivated land polluted by cadmium involves 11 provinces and 25 regions Heavy metal pollution of soil seeps into our life like an invisible ghost These ghosts are hidden in the soil They not only reduce the yield of crops, but also climb along the roots of plants, invade into rice, vegetables and other common crops, and finally reach the human body, and accumulate enough doses to cause disease, causing serious harm to human health They are scattered, wide in scope and strong in concealment Eliminating them is a time-consuming and costly project After more than half a century of exploration, scientists are now forming a plant corps to resist the attack of heavy metal pollution They are more than 400 kinds of super enriched plants, good at absorbing various heavy metals from the soil, and have a long time to deal with different types of heavy metal pollution Equipped with chemical or microbial enhanced weapons, their combat effectiveness is more superior However, if the pollution sources such as mine waste water and industrial waste water are not contained, the ghost of heavy metal pollution will not retreat from the earth This will be a never-ending war, and the human and plant side will never win Using plant to eat heavy metals phytoremediation technology is to reduce the content and effective state of heavy metals in the soil by using the absorption, decomposition, volatilization or fixation of heavy metals in the soil, so as to reduce the harm of heavy metals Chen Tongbin, a researcher at the Institute of geography of the Chinese Academy of Sciences, once said in an interview with the media that the soil in the suburbs of most cities in China has been polluted to varying degrees, and the contents of cadmium, chromium, arsenic, lead and other heavy metals in grains, vegetables, fruits and other foods in many areas exceed or approach the critical value Heavy metals in various chemical states or chemical forms will remain, accumulate and migrate after entering the environment or ecosystem, causing harm But it is more hidden in the soil, the time lag of pollution exposure is longer, it is easy to be ignored, and it is more dangerous than water pollution and air pollution Dangzhi, vice president of South China University of technology and director of the Key Laboratory of pollution control and ecological restoration of the Ministry of education in the industrial cluster, introduced that there are many ways to restore the land polluted by heavy metals, mainly physical, chemical and biological methods However, it has been proved that the method of physics and chemistry is not practical The physical method is to take away the land of a certain place and replace it with clean soil when it is polluted This method has a large amount of work and high cost It is only effective for the local enterprise land pollution, and can not solve the problem of large area of cultivated land pollution The chemical method is to change the physical and chemical properties of soil, so that heavy metals can not be absorbed by plants "Only the ionic and complex heavy metals that can dissolve in water will be absorbed by plants We can change the pH value of soil by adding some chemicals such as lime to make it alkaline Heavy metals and hydroxyl ions can form precipitates, become insoluble in water, and plants won't absorb them " Party Zhi said However, the roots of plants will continuously secrete some substances, most of which are acidic After a long time, the alkali added into the soil will be neutralized and will not work Moreover, it is not always possible to add chemicals to the soil, which will change the nature of the soil and make it difficult for crops to grow "The biggest problem with chemical methods is that no matter what is added, heavy metals still exist in the soil, only limiting its activity and not removing it." Party will say Therefore, phytoremediation technology stands out among many technologies for remediation of heavy metal contaminated land, among which plant extraction is the most studied and most promising method Plant extraction is to extract heavy metals from the soil by using the root system of hyperaccumulator, transfer and store them to the aboveground part of the plant, then harvest the aboveground part, and continuously plant hyperaccumulator to reduce the heavy metals in the soil to an acceptable level The core of plant extraction is the super enrichment plant with super combat power Hyperconcentrated plants are plants that can absorb heavy metals and transport them to leaves, fruits and other parts of the ground Generally speaking, their heavy metal content in the aboveground part can reach more than 100 times that of common crops under the same conditions As early as 1848, scientists found that the content of nickel in the leaves of a plant named "betotine Mustard" reached 7900mg / kg However, it was not until 1977 that biologist Robert Richard Brooks first proposed the concept of "super enriched plants" Since the 1970s, especially in the 1980s, in the United States, the super fund has started to repair the soil polluted by mines, heavy metals and organic substances through this biological method, Party Zhi said At present, many people are engaged in the research of soil pollution remediation by using phytoremediation technology, looking for more economical and more suitable super enriched plants, and exploring the joint phytochemical and phytomicrobial remediation technology With the deepening of research, it is found that after heavy metals are absorbed by plants, most of them gather in the roots of plants After the above ground parts are harvested, heavy metals still return to the soil with the root decay Therefore, the researchers proposed that the super enrichment plants need to meet three conditions: the concentration of heavy metals in the above ground part of the plant reaches a certain amount; the content of heavy metals in the above ground part of the plant should be higher than the root; the growth of the plant is not significantly inhibited under the high content of heavy metals In 1989, biologist Baker et al Formulated the "critical content" for screening super enriched plants, in which zinc and manganese were 10000mg / kg, cadmium 100mg / kg, gold 1mg / kg, copper, lead, nickel and cobalt 1000mg / kg At present, there are more than 400 species in 45 families, of which 73% are nickel accumulating plants Cauliflower of Cruciferae is a super accumulating plant of zinc and cadmium It is a "master of war" in the super accumulating plant army According to the researcher's experiment, the content of zinc and cadmium in the aboveground part of cauliflower can reach 36000 mg / kg and 1140 mg / kg respectively, and when the content of zinc in the aboveground part is as high as 26000 mg / kg, the plant does not show toxic symptoms The cadmium in industrial polluted soil with 19 mg / kg cadmium concentration could be reduced to 3 mg / kg by harvesting natural cauliflower 6 times However, it is not good at absorbing heavy metals that can be used in the remediation of heavy metal soil What kind of plants can be recognized to join the "super enrichment plant corps"? Scientists have a strict set of screening criteria The hyperaccumulator that can be used for phytoremediation of polluted soil should have the following characteristics: high accumulation efficiency even when the concentration of pollutants is low; high concentration of pollutants can be accumulated in the body; a variety of heavy metals can be accumulated at the same time; fast growth, large biomass; strong resistance to insects and diseases, etc At present, many people are engaged in the research of soil pollution remediation by using plant remediation technology, looking for more economical and more suitable super enriched plants, and exploring the joint phyto chemical and plant microbial remediation technology, dangzhi said According to dangzhi, only the heavy metals in the ionic and complexed states are absorbed by the plants after the plants are planted, but the other states are not absorbed by the plants, and the detection concentration is still very high The researchers have adopted the plant chemical strengthening joint repair technology, and by adding some chemicals, the heavy metals in other states in the soil are transformed into active states Some microorganisms secrete some biosurfactants during their growth, which can activate heavy metals By screening out such microorganisms, or extracting the active agents from the microorganisms and putting them into the soil, it is the technology of plant microbial joint remediation In 2011, a project named "Research on environmental chemical behavior and remediation mechanism of pollutants in soil" presided over by dangzhi won the first prize of natural science category of Guangdong science and technology award, one of which is to restore cadmium polluted soil by planting corn and chemical strengthening "Western countries have fewer people and more land, so they mainly use super accumulated herbs to control the polluted land, but this road is not feasible in China." The party records that there are more people and less land in our country We hope that we can not only control pollution, but also bring benefits to farmers Therefore, they turn their attention to corn, sunflower, tobacco and other economic crops After more than two years of repeated experiments, the research team selected "super sweet 38" from more than 20 corn varieties, which can absorb heavy metals in the soil into the stems and leaves At that time, dangzhi said, they considered several plants, one of which is sunflower, which has a strong ability to absorb heavy metals Another is corn "We found dozens of kinds of corn to do experiments, and finally determined a kind of corn, which has a strong absorption capacity for heavy metals In addition to the chemical strengthening mechanism, we did experiments in the vegetable base of Baiyun District, and the repair effect is good." Encounter war or protracted war? Most of the task of soil remediation is to do the remediation of the contaminated soil, but in fact, if the pollution is not cut off at the source, it is only done at the end This work has no great significance Although plant remediation technology is an economic and effective way to control heavy metal pollution, it still faces many problems to be solved For example, a plant can only tolerate or absorb one or two heavy metals, while other heavy metals with high concentrations in the soil show some toxic symptoms, which limits the application of phytoremediation technology in the treatment of heavy metal contaminated soil Moreover, plants are living organisms and need suitable growth conditions, which have certain requirements for natural conditions such as soil fertility, climate, water, salinity, drainage and irrigation system When plants are attacked by diseases and insect pests, their restoration ability will be affected In addition, the root system of plants is generally shallow, which is the most effective for the remediation of shallow soil pollution, but poor for the remediation of deep soil pollution There is also a problem that puzzles researchers like dangzhi, "what about so many plants after soil restoration?" Dangzhi said the heavy metal plants must be disposed of The current treatment method is only burning, and the ash will be used as hazardous waste to landfill But this is clearly not a satisfactory approach According to the party's annals, fighting against heavy metal contaminated soil is not only a technical problem, but also a social management problem to a greater extent "There is a difference between soil remediation and air, sewage and other remediation For example, the factory has the obligation to treat the river polluted by the factory sewage, and the air pollutant power plant can also handle it by itself But the soil is different because it is polluted