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Speciation and availability of selenium in soil
Huang Huaizeng Liu Zhang Li Qi

(Research Center of Bioenvironmental Geochemistry, Chinese Academy of Geological Sciences, Beijing 100037)

Wu Shaoxing

(Nanjing Institute of Soil Research, Chinese Academy of Sciences, Nanjing 2 10008)

From arid soil in northwest China, humus soil and eluviation soil in semi-arid and semi-humid areas in central China, to iron-rich soil and iron-aluminum soil in humid areas in southeast China, the general trend of total selenium content in topsoil reflects the characteristics that selenium is easy to disperse in oxidizing environment and easy to enrich in reducing state. The contents of water-soluble selenium and exchangeable selenium decreased almost in turn (arid soil > eluviated soil > homogeneous soil > iron-rich soil > iron-alumina soil), and the contents were very low, and their proportions in total selenium changed from middle to high and then to low in the corresponding range. The contents of organic selenium and acid-soluble selenium increased obviously, and the proportion of organic selenium and acid-soluble selenium in total selenium increased from northwest to southeast, while water-soluble selenium and exchangeable selenium were opposite. Residual selenium and its proportion in total selenium change from northwest to southeast. No matter what soil, residual selenium and its proportion are in the first place.

What is meaningful to plants is selenium that can be directly absorbed by them, that is, selenium can be used. The available selenium released by soil is controlled by many factors. The redox potential and the composition of soil solution are the most important parameters to determine the solubility and chemical form of selenium, while the physical and chemical properties of soil are secondary, which means that the connotation of available selenium is different in different environmental backgrounds. Microbial action mainly produces polymer organic slow-acting selenium, which is an important supply source of water-soluble and quick-acting selenium. Because of the constant change of hydrothermal state in natural environment, the difference of biological action and the interference of human activities, the soil solution in dynamic chemical equilibrium is the driving force of selenium speciation transformation and selenium migration. Speciation analysis and practical investigation of selenium show that the total amount of inorganic selenium in rocks and soil is not a low selenium area from the northeast Sanjiang (Heilongjiang-Songhua River-Wusuli River) to the southwest Sanjiang (Nujiang-Lancang River-Jinsha River). Non-encapsulated selenium is composed of water-soluble selenium, exchangeable selenium and organic selenium, which is a low selenium area. Although the humid area in the southeast of China is in an acidic reducing atmosphere, the biological action is strong and the content of organic selenium is quite high. The oxidation environment in the arid area of northwest China is conducive to the oxidation of selenium into high-priced water-soluble inorganic selenium that can be directly absorbed by plants; In the semi-humid and semi-arid area between these two areas, the water-soluble selenium is lower than that in the northwest area and the organic selenium is lower than that in the southeast area, which leads to the unclosed low-selenium area in central China and gives the low-selenium area a new meaning.

Soil; Speciation of selenium

In the past, most of the research on selenium focused on the total content and inorganic content. Through the total analysis of selenium in rocks, soil, crops and human body, it is concluded that there is a low selenium area from the northeast Sanjiang (Heilongjiang, Songhua River and Wusuli River) to the southwest Sanjiang (Nujiang, Lancang River and Jinsha River), and the southeast and northwest areas located on the east and west sides are high selenium areas. In the low selenium area, Keshan disease is caused by selenium deficiency in various media and has always been an important pillar of academic circles. The internal relationship between Keshan disease and selenium revealed by modern molecular biology research also supports this argument.

In recent years, in cooperation with Britain, we have carried out selenium research in Zhangjiakou, Hebei, Handan and Enshi, Hubei, and conducted an epidemiological investigation in Zhangjiakou where Keshan disease has not disappeared. It is found that the total selenium content in rocks and soil does not completely correspond to the selenium content in crops and human body. Some areas with high soil total selenium content still have Keshan disease patients, while some areas with low soil total selenium content may not have Keshan disease patients. The investigation also found that Keshan disease is related to selenium. The forms of selenium in soil include water-soluble, exchangeable, organic, acid-soluble and residual. Se6+ is the main water-soluble selenium, with a small amount of Se4+ and low molecular organic selenium. The exchangeable selenium is mainly Se4+, followed by Se6+. There are two kinds of organic selenium: high molecular and low molecular. The former is slow-acting selenium, which exists in acid-insoluble protein and high molecular amino acids, but can be degraded into low molecular selenium amino acids and inorganic selenium with valence of +4 and +6 in alkaline solution. The latter is the available selenium amino acid, that is, the organic water-soluble part. Acid-soluble selenium refers to selenium combined with iron, manganese and aluminum oxides. Residual selenium, selenide, selenium hidden in sulfide in isomorphic form and selenium-containing substances wrapped in silicate minerals. Obviously, it is not only a challenge but also an urgent need for further research to re-understand the relationship between selenium and Keshan disease in geographical distribution and explain various contradictions reasonably.

Speciation analysis of selenium in 1 soil

The occurrence state of selenium in soil includes valence state and bound state. Selenium can constitute inorganic and organic compounds. We chose a typical soil for analysis, and the results are as follows.

Water soluble selenium: extracted with water. Selenite is the main species, followed by selenite. In fact, the meaning of water-soluble selenium is different in different environments. In an oxidizing environment, water-soluble selenium is alkali-hydrolyzed selenite and selenite. In strong reducing environment, it includes trace acid soluble, bound selenium, secondary selenide and some volatile selenides dissolved in mixed solution of water and acid. In addition, it also contains water-soluble part of organic selenium and interconversion part with exchange state. It can be seen that the water-soluble selenium with the lowest content in the same form changes complicated under different conditions.

Exchangeable selenium: selenite and selenite ions adsorbed in oxides, clay minerals and organic matter. Extracting with potassium dihydrogen phosphate. In practical analysis, it is found that selenite ions are hardly adsorbed and become wandering ion groups in the solution, and selenite is the main adsorption object. Although the adsorption capacity of selenite ion is enhanced with the decrease of pH value to 7 ~ 4.7, it is still dominated by selenite ion and maintains a stable adsorption capacity.

Organic selenium: extracted with sodium pyrophosphate. According to molecular weight, there are macromolecular selenium and micromolecule selenium. The former is slow-acting selenium, which exists in acid-insoluble protein and macromolecular amino acids, but can be degraded into low-molecular selenium amino acids and inorganic selenium with valence of +4 and +6 in alkaline solution. The latter is the available selenium amino acid, which is the organic water-soluble part mentioned above.

Acid-soluble selenium refers to selenium combined with iron, manganese and aluminum oxides. Adding hydrochloric acid destroys the structure of oxides and carbonates, so that selenite is first decomposed from the bound state, and then selenium dioxide and elemental selenium are generated, and hydrogen selenide can also be generated under strong reduction conditions.

Residual selenium: Selenium contained in selenides, sulfides with isomorphism and selenium-containing substances wrapped in silicate minerals are digested and dissolved.

From the table 1, it can be seen that the total selenium content in the soil in the arid areas of northwest China, from humus soil and leaching soil in the semi-arid and semi-humid areas in the middle to iron-rich soil and iron-aluminum soil in the humid areas in the southeast, is in a saddle-shaped sequence. However, the statistical analysis of the measured data of China National Environmental Monitoring Center shows (Table 3) that the total selenium content in the topsoil still increases step by step from northwest to southeast, reflecting that selenium is easy to disperse in the oxidized environment. The contents of water-soluble selenium and exchangeable selenium decreased almost in turn (arid soil > eluvial soil > homogeneous soil > iron-rich soil > iron-alumina soil), and both of them were low, not exceeding 65,438+05% and 65,438+00% of the total selenium, and their proportions in the total selenium changed from middle to high and then to low in the corresponding range. The contents of organic selenium and acid-soluble selenium increased obviously, and the proportion of organic selenium and acid-soluble selenium in total selenium increased from northwest to southeast, while water-soluble selenium and exchangeable selenium were opposite. Residual selenium and its proportion in total selenium are different from northwest to southeast. No matter what kind of soil, residual selenium and its proportion are in the first place. Obviously, it plays a leading role in determining the distribution of total selenium. These residual inorganic selenium mainly comes from parent materials, which shows how important selenium in primary bedrock is to the distribution of selenium in soil.

Table 1 Speciation Analysis of Selenium in Typical Soils (Layer A) in China (ng/g)

① Non-closed storage selenium includes water-soluble selenium, exchangeable selenium and organic selenium.

2 Relationship between soluble selenium in soil and pH value

Fig. 1 relationship between dissolved state of selenium and Eh and pH value

From the chemical equilibrium experiment (figure 1), we can know that the negative logarithm (pe) of the electronic activity of the solution is a function of pH: when pH+PE > 14.5, it is a strong oxidation environment, which is obviously dominant in the solution; In the medium redox environment with pH+pe of 7.5 ~ 14.5, alkaline solution is dominant, but acidic solution is dominant. In the low redox environment with pH+PE < 7.5, HSe- is the main form in the solution. It is known that the solubility in soil aqueous solution is quite high, but the solubility of HSe- is very small, and HSe- is easy to escape in gaseous form, so the effective selenium concentration in soil increases with the increase of pH+pe value. No matter the typical soil in Table 1 or the comprehensive statistical data in Table 3, the pH value of soil solution changes from alkaline to weakly alkaline-weakly acidic from northwest to southeast, and the change of water-soluble selenium content (Table 1) with pH (Table 2) is consistent with the experimental results in Figure 1. The effective rate of soil selenium (soluble selenium/total selenium) was positively correlated with pH, and the correlation coefficient was 0.89 (P < 0.05).

Exchange state and adsorption of selenium in soil

The adsorption of exchangeable selenite and selenite by soil mainly depends on pH value, properties of active iron and clay minerals. At present, the comprehensive analysis results of five typical soils and China soil categories described in Table 1 are taken as examples, and their physical and chemical properties and main element oxide contents are recorded in Table 2 and Table 3 respectively.

Table 2 Physical and chemical properties of soil surface layer

Table 3 Physical and chemical properties and selenium content of main soil types in China

Note: The total selenium in topsoil is the data of environmental monitoring station (1990).

There are two kinds of adsorption, one is polar adsorption, or electrostatic adsorption, such as adsorption of selenium anion group by cationic group of active iron gel; The other is ordinary adsorption, that is, the adsorption of free field on the surface of solid matter. The adsorption of selenium ion groups by clay minerals and organic matter generally belongs to this type. From its acting force, the electrostatic adsorption is greater than the free field, which is also the reason why the adsorption capacity of activated iron gel is stronger than that of clay minerals, which is also confirmed by the experimental results (Figure 2). From northwest to southeast, with the change of climatic conditions, the activity and organic matter content of Fe2O3 and Al2O3 in different weathering stages and regions increase correspondingly. The adsorption capacity of clay minerals from northwest to southeast is hydromica and chlorite in arid area, hydromica and montmorillonite in semi-arid area, vermiculite and hydromica in semi-humid area, and kaolinite in humid area. In addition to mineral composition and content, clay particle size is another important parameter to measure adsorption coefficient. The finer the particles, the larger the surface area of the material, and the more obvious the adsorption effect. From northwest to southeast of China, with the strengthening of chemical weathering and biological weathering, the clay content in soil gradually increased, and the analysis results in Table 2 and Table 3 reflect this change. For these reasons, the soil adsorption capacity should increase from northwest to southeast, but the exchange status of different soil types in table 1 can not fully reflect this change. This is because selenium has its unique chemical properties. Southeast coastal areas have hot climate, abundant rainfall and lush plants. The production and decomposition of organic matter make the water rich in O2 and CO2. However, the molar ratio of CO2 to ions dissolved in water helps to reduce the OH- concentration, resulting in a low pH background, and selenium is easily reduced to non-exchangeable elements selenium and selenide. In the arid area of northwest China, although it is in a good alkaline environment, it is mainly weakly alkaline, with a pH value of 8 < 10, and the soil is still dominated by selenite ions, which becomes the material basis of exchange state. Therefore, the exchange adsorption capacity is the product of many comprehensive factors, and pH value plays a more important role. Fig. 2 The change of adsorption capacity of four substances at different pH values also confirms this point.

Effects of pH value and time on the amount of Se4+ adsorbed by clay minerals and Fe2O3.

4 Relationship between soil organic selenium and availability

Primary organic selenium is residual selenium in plant residues; The newly-born organic selenium is directly converted from inorganic selenium to organic selenium under the action of microorganisms, which also contains volatile selenium compounds. Both forms are products of biological metabolism. Generally speaking, the abundance of organic selenium increases with the increase of organic matter content, but the soil in Sanjiang area in Northeast China is fertile, with organic matter content as high as 4% ~ 10%, and the absolute content of organic selenium is low, such as the residual soil in Yichun, Heilongjiang Province is only 20ng/g, even lower than some dry soils. Compared with the total amount of selenium, it is about 22.5%, and its relative content is not low, sometimes it can be higher than that of arid soil 10%, which is obviously caused by the low total amount of selenium in primary parent material. The average annual temperature and precipitation in the arid northwest are low, and the vegetation can not flourish, resulting in low organic matter content, while the southeast is the opposite; On the other hand, microorganisms play a huge and irreplaceable role in promoting the natural circulation of biomass. Because of the activity of fungi, inorganic selenium is transformed into organic selenium, and the reproduction of fungi is far better than oxidation. It is these reasons that lead to the high background of organic selenium in the southeast coast of China. However, the real water-soluble effective organic selenium content is very small, and most of it is contained in humic acid and fulvic acid. Only a small amount of organic selenium combined with fulvic acid and inorganic selenium with valence of +6 and +4 contained in it can be utilized by plants, and a large amount of organic selenium is only the source of available selenium supply.

5 discussion

What is meaningful to plants is selenium that can be directly absorbed by them, that is, selenium can be used. However, no matter the theoretical analysis or the actual analysis results, the content of water-soluble selenium in soil decreases step by step from northwest to southeast, which is not consistent with the low selenium area of crops in Central China. In the early 1990s, 25 provinces, municipalities and autonomous regions in China carried out dietary sampling surveys. The results show that at present, the food circulation in China is very high except in cities, and the local plant food is the main food in rural areas. The element intake level of residents is basically controlled by material sources and economic development level. At the same time, it is confirmed that there is indeed a low selenium belt in Central China, but its scope is wider than that described by Qin Jian 'an and others.

How to explain this contradiction? We noticed that the above typical soil samples were collected in the late 1980s and early 1990s. After standing for a long time, the analysis done in the past one or two years can be regarded as a static analysis result. What is the difference between these results and the dynamic soil in nature today? In order to solve this problem, different methods were used to treat samples at room temperature. All kinds of soil samples in table 1 are boiled in water bath 1 hour, and the test results are quite different (table 4), that is, the water-soluble selenium content is inclined from northwest to southeast, that is, Fe-Al soil > Fe-rich soil > eluvial soil > homogeneous soil > arid soil. The reason is that under the action of heat, the chemical activity suddenly becomes active, which not only desorbs some adsorbed substances, but also promotes the decomposition of organic matter and the increase of pH value, thus leading to the sharp rise of water-soluble selenium in iron-alumina soil and iron-rich soil. The dry soil is different, and the water-soluble selenium treated with boiling water is much lower than that treated at room temperature. Table 2 lists the oxide contents of iron, aluminum, calcium, magnesium and phosphorus in various soils. From northwest to southeast, Fe and Al increase, while Ca and Mg decrease, which accords with the distribution characteristics of elements in different zones and weathering stages. The contents of Ca and Mg in arid soil are about 15% higher than those in Fe-Al soil and Fe-rich soil, and the pH value is 8.50. Obviously, carbonate and sulfate are important components. This soil is usually rich in available copper, molybdenum, calcium and magnesium, and often forms sulfate and carbonate compounds with lead and nickel. Under the action of boiling water heat, calcium bicarbonate and magnesium bicarbonate are transformed into calcium carbonate and magnesium carbonate, CO2 is reduced, and the concentrations of metal ions such as Cu, Pb and Ni are increased, which combine with selenite and selenite to form white chalcopyrite [[PbSeO3]], blue selenocite [[[cuseo 3·2H2O]]] and nickel selenite [(Ni Co) Se03 2h2o]. Mineralogy also points out that these minerals are generated in a weak oxidation state. Because of the low abundance and complex composition of selenium in soil, it is difficult to confirm this analysis by the relationship and mechanism between the form and solubility of selenium.

People try to understand the change of selenium availability through experiments, such as adding phosphate, gypsum and lime to alkaline soil. Ravikovitch and Margolin think it can reduce the utilization rate of selenium, while Brown and Carter think it can improve the utilization rate of selenium. Although there are completely different views, it does not deny the important role played by the change of soil solution composition, and it still needs great efforts to correctly understand the chemical balance in soil solution.

Table 4 Physical and chemical properties and available selenium content of main soils in China.

As mentioned above, different forms of selenium are always dissolved and leached, adsorbed and desorbed, oxidized and reduced, hydrolyzed and hydrated with the change of external conditions, and are in a constant dynamic balance. In cultivated soil, not only the concentration of water-soluble selenium changes with the change of hydrothermal state all year round, but also its internal meaning is different. Moreover, different forms can be transformed into each other in different environments, especially the transformation of water-soluble selenium, exchangeable selenium and organic selenium in non-closed selenium storage is more frequent. Human factors have aggravated this change. For example, the application of potassium, nitrogen and phosphate fertilizer will not only change the redox degree of soil, but also have other effects. Potassium ion has a large radius and strong polarization, which plays a desorption role, so potassium dihydrogen phosphate is used as the extraction solution of adsorption state; After applying nitrogen fertilizer, it becomes NH4OH. This alkaline solution may promote the instantaneous hydrolysis of organic selenium at the contact point, and Kari and Allawi extract organic selenium with ammonia water accordingly. According to the 75Se tracer experiment reported by Li Shuding and Yang, the organic selenium compounds and the extraction rate (extraction amount/total amount) in black soil, meadow brown soil and dark brown soil are 3.80% ~ 6. 1% and 6.23% ~ 8.33% respectively. The strong binding energy and affinity with soil solid phase also affect the availability of selenium in soil. In particular, it is pointed out that methyl selenium and dimethyl selenium volatiles, which are transformed from inorganic selenium to organic selenium under biological action, can be re-oxidized to +4 and +6 valence inorganic selenium under the catalysis of OH- and biology. Although people have noticed their migration and transformation of selenium, which plays an important role in plant absorption and soil adsorption, it is a pity that it is still impossible to quantitatively describe its true appearance. In addition to the change of available selenium content, it also involves the conversion rate. It has been pointed out that a form that never adapts to a certain condition has changed into a form that adapts to a new environment. For example, under alkaline conditions, selenite is oxidized to selenite; In the acidic state, selenite is reduced to elemental selenium, which starts quite quickly and then enters a slow transformation period, which is why the analysis results of static samples for a long time are different from those of dynamic samples. This inference is confirmed by another analysis result. At room temperature, the contents of water-soluble selenium, exchangeable selenium, organic selenium and total selenium in iron-rich soil in Yangchun, Guangdong Province and selenium-enriched leaching soil in Enshi, Hubei Province were 9.8ng/g, 65,438+06 ng/g, 44.6ng/g and 78ng/g, 65,438+029.8 ng/g and 450ng/g, respectively. It tells us that apart from natural conditions and parent material factors, different analysis results will be obtained due to different sampling time and sample treatment methods.

It is in this changing environment that effective selenium can exist in the humid areas of southeast China. Sufficient reserve sources-a large amount of organic selenium and a large amount of chemical fertilizers, especially nitrogen fertilizers, coupled with the strong action of microorganisms and the change of hydrothermal state, may be the comprehensive effect of intermittently increasing the effective amount of selenium and raising the selenium level of crops in this area. In the arid area of northwest China, because the content of +6 valence selenium is high and relatively stable, the selenium absorbed by crops may be high. The +6 valence selenium in the semi-arid and semi-humid areas in central China is lower than that in the northwest, and the organic selenium is much lower than that in the southeast, so the soluble selenium can not be supplemented immediately, so the absorption of crops is relatively low, so that the selenium in people's diet is also low.

To sum up, the available selenium released by soil is controlled by many factors. Redox potential and soil solution composition are the most important parameters to determine the solubility and chemical form of selenium, while physical and chemical properties are secondary. Microorganisms may play an important role in some forms transformation of selenium, producing slow-acting organic selenium, which is an important supply source of effective selenium. Due to the constant change of hydrothermal state in natural environment, the difference of biological action and the interference of human activities, the soil solution in dynamic chemical equilibrium is the direct factor affecting selenium speciation transformation and selenium migration. When the environment is suitable, high molecular organic selenium will be instantly transformed into low molecular organic selenium that can be absorbed by plants, and inorganic selenium that can be used will be released, thus improving the effective utilization rate of selenium by plants. The available selenium in the arid area of northwest China is mainly inorganic selenium that can be directly absorbed by plants, thus forming a distribution pattern that the selenium content in crops and meals in China is low in the middle and high in the southeast and northwest, which is different from the trend that the total selenium content decreases step by step from southeast to northwest.

The speciation analysis of regional selenium is far from enough, especially the solubility of different selenium-containing substances in soil liquid phase and the transformation between different chemical forms are not perfect, so it is difficult for us to quantitatively consider the physical and chemical reactions in the process of dissolution and precipitation, adsorption and desorption, and transformation of inorganic and organic selenium in soil solution.

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