Elimination of cadmium rice requires both internal and external treatment

With the launch of the "Collaborative Innovation Action for Comprehensive Prevention and Control of heavy Metal pollution of Rice in South China", the issue of cadmium rice has once again become a hot spot. Public awareness of cadmium rice is mostly focused on the harm of cadmium to the human body, while "how does cadmium in rice come into being, and is there any way to eliminate it?" There are some deviations in our understanding. Some people simply think that cadmium rice is because the rice fields are irrigated with industrial wastewater containing cadmium, and some even think that the rice fields that produce cadmium rice are replaced by the method of "new soil for old soil". The author tries to give some objective and comprehensive answers to the problems related to cadmium rice through this article.

1 cadmium is the first heavy metal pollution in farmland.

In geochemistry, cadmium is a dispersed element, which is difficult to form ore independently. At the same time, the content of cadmium in ore is very low, so it is not easy to recover. Therefore, in the process of mining, it is easy to discharge into the environment with tailings, especially in the presence of acidic mineral water, cadmium is easy to enter rivers and farmland with acidic mineral water. The melting point of cadmium is 321 ℃ and the boiling point is 767 ℃. Therefore, in the absence of effective cadmium removal equipment, non-ferrous metal smelting, iron and steel plants, municipal solid waste incineration plants and thermal power plants are easy to release cadmium into the atmosphere, resulting in cadmium pollution in a wide area with the plant as the center and the most serious downwind pollution.

Soil pH (pH) has a very important effect on the adsorption capacity of heavy metals such as cadmium in soil organic matter, iron and manganese oxides and clay minerals. When the soil is acidified, the ability of these substances to adsorb cadmium is very weak. Compared with other heavy metals such as zinc, copper, lead and mercury, cadmium is naturally higher in the parts that can be absorbed by plants (water-soluble and exchangeable) than other heavy metals, so cadmium is easier to migrate in the soil-plant system. Although the background value of cadmium in Chinese soil is only 0.097mg/kg, and the exotic cadmium soil is not easy to be fixed by soil colloid, especially under the condition of soil acidity, so the total cadmium content in many cultivated soils is not high, but its effectiveness to plants is very high.

2 why it is easy to produce cadmium rice

Many people think that cadmium rice is produced because rice is the strongest crop that absorbs cadmium. In fact, some scientists classify the tolerance of plants to cadmium into three categories, and the tolerance of rice is only moderate. The growth environment and the chemical change process of cadmium in soil play a key role in the easy absorption of cadmium by rice.

The cadmium in the soil is mainly adsorbed on the clay part. In the process of rice cultivation, the clay part is easy to precipitate on the soil surface, resulting in a high cadmium content of 1 cm to 3 cm on the surface. In the polluted area, there is atmospheric deposition of cadmium, at the same time, irrigation will also bring cadmium, cadmium will also be deposited on the surface of paddy soil. Rice is a bearded root crop, and most of the roots are within 5 cm of the surface layer. The overlapping distribution of cadmium content and root system creates conditions for rice to absorb more cadmium. Although under flooded conditions, an iron film will be formed on the surface of rice roots, which will help to prevent the absorption of cadmium by rice roots. However, rice is an ammonium-loving crop, and the absorption of ammonium nitrogen by roots will acidify the root surface, which is conducive to the absorption of cadmium.

Using isotope markers, some scientists found that about 70% of the cadmium in rice comes from the absorption and entry of rice roots during the reproductive period, and the rest comes from the accumulation of cadmium in stems and leaves. This is because under flooded conditions, cadmium and negative bivalent sulfide ions formed under reduction conditions form cadmium sulfide, which is very insoluble and difficult to be absorbed by roots. However, in the later stage of rice harvest, in order to facilitate the harvest, farmers deliberately drain the water from the field, and cadmium sulfide is oxidized to cadmium ion and sulfate ion by the oxygen infiltrated into the surface of the soil, and cadmium is easily absorbed by the root system. this chemical process can usually be completed within four days of water drainage. Therefore, the distribution of cadmium in the root system and the surface layer, coupled with the lack of water, caused a large amount of cadmium uptake by rice.

Temperature and variety have certain effects on cadmium uptake by rice. The results of studies in Japan and Taiwan showed that indica rice had stronger cadmium enrichment ability than japonica rice on the whole. The mainland has also done a lot of analysis on different varieties, probably because there are too many varieties or hybrid rice has not been well traced and distinguished, different types of rice cadmium absorption capacity is contradictory, can not make a clear distinction. Usually, when the temperature is high, the evaporation of plant leaves is large, and the cadmium ion in the soil is easy to enter the root system with water. However, there are many early rice Rain Water in southern China, and there is no shortage of water in the field. Despite the high temperature, cadmium in early rice is generally lower than that in late rice.

(3) there is no close relationship between rice cadmium and soil cadmium.

The main sources of cadmium in farmland are industrial wastewater, atmospheric deposition, livestock and poultry manure, chemical fertilizer, irrigation water and so on. However, people usually think that the reason why the cadmium in rice exceeds the standard is that the soil is polluted by cadmium. The more cadmium in the soil, the more cadmium in rice. This result can be obtained under various conditions under pot control. Scientists try to find this relationship in order to infer the content of heavy metals in rice from the content of heavy metals in the soil.

However, under the condition of field production, there was no such correlation between rice cadmium content and soil cadmium content. The cadmium content of rice plants with water head and tail in a paddy field may be different. On the one hand, it is difficult not to have any heavy metals in irrigation water, even if the concentration is as low as more than 0.0%, the incoming heavy metals are also easy to be absorbed by the soil near the inlet, making their heavy metal content slightly high. On the other hand, in order to facilitate the harvest, irrigation is stopped in the field before harvest, the soil at the inlet is relatively dry, and the soil at the outlet is relatively wet. These two factors as a whole make the rice at the inlet contain a little more heavy metals. If the concentration of cadmium in irrigation is higher under the condition of pollution, the difference is even more obvious.

Under the condition of field production, investigations in Japan, Thailand, South Korea, Chinese mainland and Taiwan all found that there was no close relationship between rice cadmium and soil cadmium, so the cadmium standard of farmland in Japan was replaced by the content of cadmium in brown rice.

4 how to "eliminate" cadmium rice

The various characteristics of cadmium make the soil vulnerable to widespread pollution, and once contaminated, cadmium is easy to enter rice. Therefore, it is necessary to strengthen source control, including the control of air pollution and industrial wastewater discharge, the use of phosphate fertilizer with low cadmium content, rational and appropriate use of livestock and poultry manure, etc., so as to reduce or even eliminate the pollution of exogenous cadmium to paddy soil. This is the key to eliminate cadmium rice caused by exogenous pollution.

Soil acidification is one of the major reasons for excessive cadmium in rice. China has a large population and little land, high multiple cropping index and large amount of fertilizer application, resulting in soil acidification, coupled with acid rain from the atmosphere and acid mineral water from sulfur-bearing mining areas, resulting in serious soil acidity, especially in acid rain areas and around sulfur-bearing mining areas. Therefore, the improvement of soil by alkaline substances such as lime is an important measure to eliminate cadmium rice. Of course, the requirement of lime needs to be calculated scientifically, and the regulation of soil pH should reach more than 6.5.

Water management in paddy field is another important measure to control cadmium in rice. The ideal condition is flooding throughout the process, which helps to minimize cadmium in rice. However, the whole process of flooding is not realistic in field production. First, it consumes a lot of water, and the usual rice growth management needs medium-dry drainage to control rice tillering and change the soil environment, and release nutrients through the middle-dry drainage process. In Japan, the soil polluted by medium and low cadmium can greatly reduce the cadmium content of rice by keeping the 2cm water layer of the field surface three weeks before and after heading to prevent the surface layer of soil from being oxidized. For paddy fields with soft soil, this kind of water management will bring difficulties in mechanical tillage, and it can also achieve a better effect on reducing cadmium in rice keeping a water layer of 2 cm to 3 cm 10 days after heading.