What are the characteristics of the hazards of heavy metals
The heavy metals mentioned in terms of environmental pollution actually mainly refer to metals or metalloids such as mercury, cadmium, lead, chromium, and arsenic, as well as general heavy metals with certain toxicity, such as copper, zinc, nickel, cobalt, and tin. We briefly discuss the hazards of heavy metals from the aspects of naturalness, toxicity, activity and persistence, biodegradability, bioaccumulation, and additive effects on organisms. (1) Naturalness: Human beings who have lived in the natural environment for a long time have a strong ability to adapt to natural substances. Someone analyzed the distribution law of more than 60 common elements in the human body, and found that the percentage content of most of the elements in human blood is very similar to their percentage content in the earth's crust. However, humans have much less tolerance to synthetic chemicals. Therefore, distinguishing the natural or artificial attributes of pollutants is helpful to estimate their harmfulness to human beings. Heavy metals such as lead, cadmium, mercury, and arsenic are enriched in the surrounding environment of humans due to the development of industrial activities, enter the human body through the atmosphere, water, food, etc., accumulate in certain organs of the human body, cause chronic poisoning, and harm human health. (2) Toxicity: The main factors that determine the toxicity of pollutants are their material properties, content and existing forms. For example, chromium has three forms of divalent, trivalent and hexavalent, among which hexavalent chromium is very toxic, and trivalent chromium is one of the important elements of human metabolism. In natural water bodies, the toxic range of general heavy metals is about 1-10 mg/L, while the range of mercury, cadmium, etc. is 0.01-0.001 mg/L. (3) Spatiotemporal distribution: After pollutants enter the environment, they are diluted and diffused with the flow of water and air, which may cause a wider range of pollution from point sources to surface sources. and intensity distributions vary with time. (4) Activity and persistence: Activity and persistence indicate the stability of pollutants in the environment. Pollutants with high activity are prone to chemical reactions in the environment or during the treatment process, and their toxicity is reduced, but they may also generate more toxic pollutants than the original ones, which constitute secondary pollution. For example, mercury can be converted into methylmercury, which is highly toxic. Contrary to activity, persistence means that some pollutants can maintain their harmfulness for a long time, such as heavy metals such as lead and cadmium, which are toxic and difficult to degrade in nature, and can produce bioaccumulation, long-term threats to human health and survival. (5) Biodegradability: Some pollutants can be absorbed, utilized and decomposed by living organisms, finally producing harmless and stable substances. Most organic substances have the possibility of being biodegraded, but most heavy metals are not easily biodegradable, so once heavy metal pollution occurs, it will be more difficult to control and more harmful. (6) Bioaccumulation: Bioaccumulation includes two aspects: one is that pollutants accumulate in the environment through food chains and chemical and physical actions. The second is the accumulation of pollutants in certain organs and tissues of the human body due to long-term intake. For example, cadmium can accumulate in the liver, kidney and other organs and tissues of the human body, causing damage to various organs and tissues. Another example is the Minamata disease incident that occurred in Japan from 1953 to 1961. Inorganic mercury was transformed into methylmercury in seawater, which was ingested and accumulated by fish and shellfish. After biomagnification in the food chain, local residents were poisoned after eating it. (7) Additivity of action on organisms: Multiple pollutants exist at the same time and interact with organisms. There are two types of additive effects of pollutants on organisms: one is synergistic effect, where mixed pollutants make the harm to the environment more serious than the simple addition of pollutants; the other is antagonistic effect, where pollutants coexist Sometimes the hazards weaken each other.