Geochemical Characteristics of Selenium
Selenium is a non-metallic element with an average content of approximately 0.05 mg/kg in the Earth's crust, classifying it as a rare element. It mainly exists as isomorphous forms in sulfide minerals (such as pyrite and galena), and can also be found in shale, coal-bearing strata, and volcanic rocks. Selenium exists in various valence states in nature (-2, +4, +6), and its bioavailability is closely related to its form: organic selenium (such as selenomethionine and selenocysteine) is more easily absorbed and utilized by organisms than inorganic selenium (selenite and selenate), and is also less toxic.
Conditions for the Formation of Selenium-Enriched Mineral Water
The formation of natural selenium-enriched mineral water requires three core conditions:
Selenium-rich parent rock or soil: The regional geological background must contain rocks or sediments with high selenium content. For example, the black shale (carbonaceous siliceous shale), coal-bearing strata, and volcanic clastic rocks widely distributed in southern China can have selenium contents ranging from hundreds to thousands of micrograms per gram.
Suitable hydrogeochemical environment: Groundwater requires a weakly reducing or neutral pH level during its flow to facilitate the migration of selenium from the solid to the liquid phase. If the water body is strongly oxidizing or acidic, selenium tends to remain as insoluble precipitates.
Long-term stable groundwater circulation system: Deep confined water or karst fissure water flows slowly in a closed environment for decades or even centuries, fully exchanging ions with the surrounding rocks, thus enriching various beneficial minerals, including selenium.
Typical selenium-rich geological units in China:
The northern margin of the Yangtze Block black shale belt: Covering southern Shaanxi, western Hubei, eastern Chongqing, and western Hunan, this is China's largest and most typical selenium-rich area. The selenium content in the Cambrian-Silurian black shale in this region is generally >3 mg/kg, and locally exceeds 10 mg/kg.
South China red soil selenium-rich area: Deep red soil has developed from the weathered crust of granite in Jiangxi, Guangdong, and Fujian provinces. In some areas, the soil selenium content is >0.4 mg/kg, and groundwater can carry selenium into springs through leaching.
Selenium-rich basins in arid Northwest China: Such as the Ili River Valley in Xinjiang and the Tongren Basin in Qinghai, have relatively high selenium concentrations in groundwater due to evaporation and concentration effects.
Selenium-rich volcanic rock areas: Such as parts of Zhejiang and Fujian provinces, where volcanic ejecta are rich in trace elements, forming selenium-rich water bodies after weathering.
