Effects Of Interaction Of Ca-F On Adsorption Capability And Form Distribution Of Fluoride In Tea Garden Soil

Fluorine is the most active chemical characteristics of non-metal elements. Fluoride in soil comes mainly from the decency of rock, the subsidence of substance containing fluoride in atmosphere and the activities of industrial manufacture. There will be the significant impact of fluoride on the growth and development of plants and the human health. Tea plant is known to be a typical fluoride accumulator, and consequently it will be an important source of assimilating fluoride for human body through drinking tea. 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, which may be affected by the interaction of among elements in soils. There will be the interaction not only between Al and F but also between Ca and F when the content of Ca in soil increases according to the characteristics of element circulation and environmental chemistry of elements in the system of tea plant/soil. It can provide an important theoretical foundation to study systematically the adsorption characteristics and distribution of fluoride in tea garden soil under the interaction of Ca-F for understanding comprehensively the migration and transformation of fluoride in the system of plant/soil as well as the effects of environmental chemistry factors on the uptake of fluoride from soil into tea plant, and adjusting and controlling the content of fluoride in tea leaves by changing the soil conditions. Furthermore, there will be of the theoretical and practical significance for improving the people's living quality and protecting human health.The adsorption characteristics and the form distribution of fluoride in tea garden soils were studied in the paper under the interactions of Ca-F, as well as different level of pH and organic matter by the simulate test and form analysis.The adsorbing rate of fluoride in tea garden soils were all fast at the beginning and slow at the last, and reached the equilibrium within 30-50min. However, the rate of fluoride adsorption increased and the equilibrium time shorten under the interaction of Ca-F as well as under the different pH except the different contents of organic matter, which were not distinct change in the adsorption equilibrium time. The adsorption dynamics characteristics of fluoride in tea garden soils could be described with the equations of Double-constant, Elovich , Parabola and First grade.The adsorption amount of fluoride increased significantly with the beginning concentration of fluoride. There was the difference in the adsorption amount of fluoride under the different contents of calcium. The adsorption amount did not have obvious difference at the low beginning concentration of fluoride under different level of pH. However, there were distinct differences in the adsorption amounts of fluoride at the high beginning concentration of fluoride under different pH, the order was pH=5>pH=4>pH=3. The desorption percentage of fluoride increased with increasing the amount of calcium added in the soil and decreased significantly under the different pH especially there was no calcium added. Although there were different effects of different pH under the existing of calcium on the adsorption amount, the desorption percentage raised obviously.The adsorption amount of fluoride in tea garden soil increased with the beginning concentration of fluoride increased under the different content of organic matter added in the tea garden soil, but the desorption percentage decreased evidently with increasing the content of organic matter. The average adsorption amount of fluoride for the different treatments of adding calcium increased with increasing organic matter under the co-existing of organic matter and calcium. Accoeding to the average desorption percentage,with the increasing of the amount of calium,the desorption percentage has been a downward trend at the same level of exogenous organic matter.The addition of calcium increased the desorption percentage of fluoride especially under the lower content of calcium comparing to non calcium. The thermodynamics equations of Freundlich and Temkin all could be used to describe the thermodynamics characteristics of the adsorption and desorption of fluoride in tea garden soil.The contents of F forms increased distinctly with the increase of F adsorption amount after F added into the soil. The distribution of F adsorbed in soil was related to the adsorption amount of F, and the main forms of F for all treatments were the water soluble and residual. The transform percentage increased for the water soluble and decreased for the residual compared with natural soil. The percentage increased for the water soluble F and decreased for other forms, i.e. Fe-Mn oxide, exchangeable, organic and residual with the increasing of adsorption amount of F under the different amount of calcium added in the soil. The percentage decreased at first and increased at last and for the water soluble F and increased at first and decreased at last for the residual F with the increasing of calcium amount added in soil under the low and high beginning concentration of F.The percentage increased both for the residual F with raising pH under the same F beginning concentration and for the water with increasing the amount of calcium under the different pH. The percentage of the residual F decreased with increasing calcium when the value of pH was 3. The percentage of the residual F increased for pH 4 and increased at first and decreased at last for pH 5 with increasing calcium under the low beginning concentrations of F. However, the percentage of the residual F decreased with the increase of calcium amount under the value of pH 4 and 5. The percentage decreased for the water soluble F and increased for the residual F with increasing the content of organic matter under the same beginning F concentration. The percentage increased for the water soluble F and decreased at first and increased at last for the residual F with adsorption amount of F under the 5% organic matter and different calcium amount (except the 100 mg/kg calcium).When the beginning concentration of F was lower, the percentage of the water soluble decreased at first and increased at last under 10% organic matter treatment and decreased under 15% organic matter treatment with increased calcium amount, however, there was a reverse change for the percentage of the residual F under 10% and 15% organic matter. When the beginning concentration of F was higher, the percentage of the water soluble F increased at first and decreased at last under 10% organic matter and increased under 15% organic matter with increased calcium amount, the percentage of the residual F increased for treatment of 10% organic matter and decreased at first and increased at last for the treatment of 15% organic matter with increasing calcium amount. In brief, the contents of various forms F increased obviously with the increasing of adsorption amount of F under different affecting factors when fluoride entered soil. No matter there was a distinct difference in the percentage of various forms of F and distribution of F, the main forms of F in the soil was the residual and water soluble in the all treatments of tea garden soil.