The Migration And Transformation Rules Of Fluoride In Water-Soil System In The Typical Area Of North China Plain

Fluoride is a chemical element which is essential for human growth. It is one of the construction elements of teeth and skeleton, the right amount of fluoride can prevent dental caries. However, fluorine is a pro-bone element, excessive intake can cause endemic fluorosis, such as dental fluorosis, fluorosis of bone. Endemic fluorosis is widely distributed in the world, and North China belong to high fluoride water area, its absolute quantity and relative proportions of drinking fluoride water population are among the first in our country. The excess fluorine content in the soil and groundwater has caused great concerns of the government and scientists.Currently, the research about the migration and transformation rules of fluoride in water-soil system at high F groundwater irrigated area is still rare. This study selected Langfang, Cangzhou, Hengshui and Yucheng four typical areas in the North China Plain. To grow wheat-com farmland soils as the research object, through the field sample collection and laboratory experiments, study the distribution characteristics and migration rules of fluorine in groundwater-soil System in the typical area of north China plain. This study can provide a scientific evidence for soil fluorine pollution, prevention and a theoretical basis for soil fluorine mobility and its influence on ecology and environment.Through field investigation and laboratory experiments, This study analyzed fluorine speciation and its distribution characteristics, adsorption-desorption characters of fluoride, fluoride distribution in the groundwater and its effect on the soil fluoride release in the typical area of the north china plain. The main conclusions are as follow:1. It was found thatdeep groundwater and shallow groundwater fluorine content vary accordinglyin different typical areas. The deep groundwater fluorine content was quite different in different typical areas, the fluorine of deep groundwater content in Cangzhou is obviously higher than in Langfang. Groundwater chemical composition is complex in the study area, mainly containing Na+, Mg2+,HCO3-,SO42-and so on, which are in favor of the fluorine migration and enrichment. The relationship beteween fluoride and other chemical composition in groundwater had shown that the fluorine content of groundwater was significant positive with the pH. For shallow groundwater, the correlation beteween Ca2+and F was strongest, meaning the Ca2+influences greatly fluorine content of the shallow groundwater. For deep groundwater, the main effects on the fluoride content were the cation Na+and the anion HCO3-.2. The soil total fluorine (T-F) content in the cultivated soils of wheat-corn fields at the typical areas was ranged from338.31mg·kg-1to781.67mg·kg-1, with a mean of430.46mg·kg-1. The highest contents of the soil fluorine speciation was Residual-Fluorine (Res-F), with a mean of402.73mg-kg-1.The average content of Water soluble Fluorine (Ws-F) was14.39mg·kg-1. It indicates that the cultivated soil in the study area was at a higher fluoride pollution level, which may be harmful to human health and ecological environment. Then it followed by Organic Fluorine (Or-F) and Fe/Mn Oxide-Fluorine (Fe/Mn-F), with a mean of8.90mg·kg-1and4.10mg·kg-1, respectively. The exchangeable fluorine (Ex-F) only had a very small amount of0.33mg·kg-1.3. According to analyzing correlation between contents of fluorine speciation and some soil properties, The results showed that Soil Ws-F was positively correlated with soil pH and CEC, while it was negatively correlated with percentage of soil clay. The content of soil Fe/Mn-F was positively correlated with soil pH, CEC and the sand grain content percentage, while it was negatively correlated with the clay grain content percentage. The soil pH value has the most significant influence on the water soluble fluorine (Ws-F) and Fe/Mn Oxide-Fluorine (Fe/Mn-F). On the other hand, the soil CEC has the most significant influence on the soil total fluorine (T-F) and residual-Fluorine (Res-F) by the stepwise regression analysis.4.1n the soil profiles, the T-F content appeared as peaks and valleys represented the changes of the soil lithology in the vadose zone. The Ws-F in the soil profiles mainly changed in the depth of0～100cm near the surface soil and was little influenced by the soil lithology. But it was strongly influenced by the soil pH, and was positively correlated with soil pH. While the soil lithologys were different, the fluorine content in soils were also significantly different. The T-F contents were decreased in the order:silt loam>loam>sandy loam>loamy sand, the Ws-F contents were decreased in the order:loam> silt loam>sandy loam>loamy sand.5. When adding a lower F initial concentration solution to the soil, the fluorine in the soil present negative adsorption phenomenon that F released from the soil to the solution. For the different types of soils, the fluorine adsorption phenomenon was not immediately started at the beginning of the experiment, but after a certain time, the fluorine in the soil present adsorption phenomenon. The F adsorption amount demonstrated an increasing trend with the time prolonged, the higher the initial concentration was, the faster the adsorption capacity increased. With F initial concentration5mg·L-1and100mg·L-1, the preudo-second order equation was the best dynamic modelfordescribe the soil S14(loam) dynamic process; With F initial concentration50mg·L-1, the Elovich model was best for soil S14;With F initial concentration50mg·L-1and100mg·L-1, the preudo-second order equation was also fitting to describe the dynamic processes of adsorption of F for soil S13in the study area.6.When the initial concentration range of7～100mg·L-1, The soils studied showed different abilities in fluorine adsorption. The order is silt loam>loam>sandy loam. F adsorption isotherms of the soils in the study area all can be described by Linear Adsorption Isotherm Equations, Freundlich Equation and Dubinin-Radushkevich Equation. Over all Freundlich Equation fits best, Dubinin-Radushkevich Equation fits worst, and Langmuir Equation could not fit to describe the fluoride adsorption isotherm by the soils in the study area. The migration of fluoride in water-soil system was affected greatly by the initial solution pH, the decrease in initial solution pH was conducive to alkaline soil adsorption of fluoride ions.7. The fluoride release from the cultivated soils affected by the deep and shallow groundwater at different volum ration was analyzed, The result showed that with the increasing ration of the deep groundwater, the increase rate of fluorine concentration in the leaching solution was faster in the soils of Langfang typical area (S01-S03);the soil in the Hengshuitypical area showed opposite law (S11); changes of the fluorine concentration in the leaching solution was little in the soils of Cangzhou typical area (S04-S07). The initial fluorine concentration of the mixing groundwater could be affect the release amount of soil fluoride, the release amount of fluorine from the soils increased with the F initial concentration in the mixing groundwater in Langfang typical area. The pH, EC,anionic and cationic of the leaching solution all affectthe fluorine concentration, that fluorine concentration was positively correlated with pH in the leaching solution; that fluorine concentration was negatively correlated with EC after leaching experiment in Langfang and Cangzhou typical areas, Hengshuitypical area showed opposite law. In different typical areas, the relationship between fluorine concentration and anionic and cationic were different in the leaching solution.The innovation are:by combining field investigation, laboratory experiments, test analysis and mathematical statistics mothods, the speciationand distribution of fluoride in water-soil system in the typical area of North China Plain wereanalysed. Theadsorption-desorptionstudieson typical areas soils, the adsorption-desorption law, adsorption kineticsand mechanism were discussed.