Study On Translocation And Geochemistry Characteristics Of Fluoride In The Tea Garden Soil/Tea Leaves System

Fluoride is an important element that can impact human health which can cause dental caries and fluorosis when the unbalance of fluoride in the human body. Tea plant (Camellia sinensis L.), one of the most popular beverages in the world, has been found to contain higher concentrations of fluoride than many other plants. Therefore, tea would be a potentially important source of dietary fluoride. The content of fluoride in tea leaves is related closely with both the adsorption characteristics and the form distribution of fluoride in tea garden soils. There are many kinds of tea garden soils in our country and the soil properties are different, and hence there is significantly different behavior of geo-environmental chemistry for fluoride in tea garden soils. The fluoride in soil is the main the source of fluoride accumulation in tea leaves and there is distinctly difference for the content of fluoride in tea leaves derived from different tea gardens. Consequently, it will be of theoretical and practical significance to study the fluoride contents in tea plants and factors influencing the uptake fluoride from soils into tea plants for adjusting the condition of soil in tea garden and decreasing contents of fluoride in tea plants and furthermore ensuring that the human body is healthy. The adsorption characteristics and the distribution of fluoride in tea garden soils and the relationships between fluoride in tea leaves and tea garden soils were studied in the paper by sampling soils (0–20cm, 20–40cm and 40–60cm) and tea leaves with different age (old leaf and young leaf) with sampling method of"Z-shaped"from 12 tea gardens located main tea production areas in Central and Southwest China, determining the samples and simulated test.The characteristics of adsorption and desorption of fluoride in the different tea garden soils were studied by the simulated test in laboratory. The adsorption of fluoride in the different tea garden soils could reach the equilibrium at about 20min. The amounts of adsorption for fluoride in the tea garden soils increased with the increasing of the initial concentration of fluoride, but the adsorption amounts were different for different tea garden soils. Moreover, the adsorption amounts increased slowly and tended to the adsorption equilibrium for the tea garden soils from Hunan and Hubei and reached a top and later on decreased obviously for that from Sichuan, Chongqing and Yunnan with the initial fluoride concentrations. The characteristics of dynamics of fluoride in the tea garden soils could be described with the dynamics equations of Second grade, Parabola, Elovich and Double-constant, and the characteristics of adsorption thermodynamics of fluoride in the tea garden soils could be described with the equations of Freundlich and Temkin, furthermore the Freundlich equation was more appropriated. The biggest adsorption amounts and average desorption percentages of fluoride for the several tea garden soils were in the range of 1160～2700mg/kg and 7.6%～30.8%, respectively. The characteristics of desorption thermodynamics of fluoride in the tea garden soils could be described with the equations of Freundlich. The correlation analysis showed that the sorption capacity of the tea garden for fluoride increased with the cation exchangeable capacity increasing and desorption capacity, however, decreased with the raising of pH and the cation exchangeable capacity.There were not the unified changing tendency for each form of fluoride in different soil layer, but the forms of fluoride existing for the water soluble, exchangeable and Fe-Mn oxide decreased with deepening soil layer and the organic fluoride was higher in the layers of 0-20 cm and 20-40 cm than that in the 40-60cm layer by comparing the average contents. The concentrations of different F forms for each soil layer were all the order of residual>>Fe-Mn oxide>organic>water>exchangeable. The residual was the primary form of fluoride in the garden soils. The water soluble fluoride decreased significantly with the raising of soil pH and the cation exchangeable capacity and Fe-Mn oxide fluoride increased with the raising of soil pH, organic matter content and the cation exchangeable capacity. The organic fluoride increased obviously with increasing the organic matter and the cation exchangeable capacity. Nevertheless, the relationship between the exchangeable fluoride and soil pH, organic matter and the cation exchangeable capacity were not significant.The contents of fluoride in tea plants ranged from 221 mg/kg to 1504 mg/kg for old leaves and 49.0 mg/kg to 602 mg/kg for young leaves, respectively, the fluoride contents in old leaves were higher 2.5～8.1 times than that in young leaves. The contents of fluoride in tea leaves increased significantly with increasing the leaf age. The concentrations of fluoride in the tea leaves increased significantly with the water soluble fluoride, but other forms of fluoride were not of the same correlations with the contents of fluoride in tea leaves.