Effects Of Interaction Of Al-F On The Adsorption Characteristics And Distribution Of Fluoride In Tea Garden Soil

Fluorine is a kind of important elements for human health, and tea plant is known to be a typical fluoride accumulator. 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 Al-F, phosphate and low-molecular-weight organic acids. Therefore, there will be of the theoretical and practical significance to study systematically the adsorption characteristics and form distribution of fluoride in tea garden soil under the interaction of Al-F and the existing of phosphate and the low-molecular-weight organic acids for assessing reasonably the activities and form distribution of fluoride in tea garden soil and adjusting and controlling the content of fluoride in tea leaves by changing the soil conditions. The adsorption characteristics and form distribution of fluoride in tea garden soils under the interactions of Al-F and the existing of phosphate and the low-molecular-weight organic acids, i.e. oxalic acid,citric acid, malic acid and succinic acid separately were studied in the paper by the simulate test and form analysis.There were not obvious change in the time of adsorbing equilibrium of fluoride and the kinetics equations of Elovich and double-constant could be used to describe the characteristics of kinetics of fluoride under the different beginning concentrations of fluoride and different ratios of water to soil. The adsorbing rate of fluoride was different for the different tea garden soils, but all was fast at the beginning and later on was slow, and the time of adsorption equilibrium were in the range of from 25min to 50min. The dynamics equations of Second grade, Parabola, Elovich and Double-constant could be used to describe the characteristics of dynamics of fluoride in the different tea garden soils.The adsorbing rate increased and the equilibrium time was shorten to 10min and the dynamics characteristics of fluoride could be described with the dynamics equations of First grade, Second grade, Parabola, Elovich and Double-constant under the existing of the low-molecular-weight organic acids, i.e. oxalic acid, citric acid, malic acid and succinic acid separately. The equilibrium time was shorten to 25min and 20min under the existing of lower concentration and higher concentration of phosphate respectively due to the less of fast adsorption time. The equilibrium time protracted to 50min under the different ratios of F/Al due to the increase of fast adsorption time. Nevertheless, the adsorption dynamics characteristics of fluoride could be described with the equations of Second grade, Parabola, Elovich and Double-constant.The thermodynamics equations of Langmuir, Freundlich and Temkin all could be used to describe the characteristics of thermodynamics of fluoride. The maximum amounts of adsorption for fluoride in the tea garden soils were in the range of 263～769mg/kg obtained from the value of M of Langmuir equation. There were the positive correlations between content of OM, aluminum and manganese and the adsorption amount of fluoride and the negative correlation between the soil pH and the adsorption amount of fluoride. The adsorption amount of fluoride increased with the beginning concentration of fluoride under the existing of the low molecular weight organic acids. This proved the promoting role of low molecular weight organic acids for the adsorption of fluoride in tea garden soil. The adsorption amount of fluoride decreased under the existing of phosphate or aluminum comparing to control. The effect was more significant for the existing of phosphate or aluminum at the high concentration than that at the low concentration.There was little affected for the lower concentration of organic acids and obviously increase for the higher concentration of organic acids under the coexisting of low molecular weight organic acids and aluminum. The equations of Freundlich and Temkin could be used to describe the thermodynamics characteristics of fluoride in the tea garden soil under the existing of the low molecular weight organic acids or phosphate or aluminum and the coexisting of low molecular weight organic acids and aluminum. Besides the equations of Freundlich and Temkin, Langmuir equation could also be used to describe the thermodynamics characteristics of fluoride in the tea garden soil under the coexisting of phosphate and aluminum. The percentage of residual fluoride decreased and that of water soluble fluoride increased with the increase of adsorption amount of fluoride for the control. The transformation rate of residual fluoride decreased and that of water soluble fluoride increased under the phosphate comparing to the control.The transformation rate of residual fluoride increased and that of the rest forms decreased with increasing adsorption amount of fluoride under the low molecular weight organic acids.The transformation rate of residual fluoride decreased and that of water soluble increased when the increase of the adsorption amount of fluoride under the different ratios of F/Al and the decreasing ratios of F/Al when the same beginning concentration of fluoride. The same tendency for the residual and water soluble fluoride was obtained with the decrease of ratios of F/Al and the increase of phosphate under the coexisting of phosphate and aluminum. The same situation was also found with the decrease of ratios of F/Al under the coexisting of these organic acids and aluminum respectively. The transformation rate of residual fluoride were higher and that of water soluble fluoride were lower at the high concentrations than that at the low concentrations for citric acid, oxalic acid and succinic acid, however, the adverse trend was obtained for malic acid. Under the different test conditions, the contents of various forms of fluoride increased with the adsorption amount of fluoride and there were different transformation rate of forms and different distribution of fluoride, but the residual and water soluble fluoride were nearly the main part of adsorbing fluoride in soil in general.