Experimental Study On Migration Of High Concentration Fluoride In Unsaturated Red Clay

Yunnan is located in the carbonate karst area.A large number of karst depressions are distributed around most tailings reservoirs,causing a significant risk of contamination of downstream groundwater once leakage occurs in the tailings pond.However,the karst areas where these tailings ponds are located usually have red clay overburden with uneven thickness,and the groundwater level is deep,so there are often unsaturated zones with different thickness between the groundwater diving surface and the upper tailings ponds.Therefore,the study on the transport of high concentrations of fluoride in unsaturated red clay has important guiding significance for the assessment of groundwater pollution risk in karst areas of China.In this paper,the research focused on red clay with a wide distribution in southwest china karst region.It researches the factors affecting the adsorption of high concentration fluoride in a batch and column study,and the adsorption-migration law of high concentration fluoride in unsaturated red clay.The following main conclusions were obtained as follow :(1)Static adsorption with initial step concentration method showed that red clay had the best adsorption effect on 1000mg/L fluoride in the p H range of 8.0-8.7(2)The determination data of fluorine content in solution and red clay during static adsorption test is analyzed.The results show that the average removal rate of fluorine in solution is over 95% in the optimal p H.When the p H value was less than the optimal p H value,the removal rate showed an increasing trend.When the p H value is greater than the optimal p H value,it shows a decreasing trend.(3)The quasi-first-order kinetics and quasi-second-order kinetic equation models were used to fit the adsorption processes at 500 mg/L,1000 mg/L,and 2000 mg/L,respectively,the results show that the adsorption of high-concentration fluoride by red clay is more consistent with the pseudo-second-order kinetic equation model,and its R2 is greater than 0.98.(4)The concentration of fluorine in the red clay samples collected during the soil column test showed that the adsorption of fluorine by red clay was divided into three stages.There were a lot of strong adsorption sites in the surface layer in the early stage,and the adsorption capacity was strong.After 40 hours,the adsorption mode was converted from site adsorption to electrostatic adsorption,and the adsorption strength was decreased.With the increase of desorption strength in the later stage,the adsorption rate gradually reaches the dynamic equilibrium stage.(5)During the vertical migration of high-concentration fluoride solution in the unsaturated red clay soil column,the migration rate and depth show a certain regularity with the equation y=7.0338ln(x)-17.483.In the early stage of migration,gravity is greater than the upward capillary force between particles,and the migration rate is faster.Later,with the increase of capillary force,the migration rate is gradually smaller.But the migration is still going downward,because gravity is always greater than capillary water.(6)Under the combined action of adsorption,ion exchange and complexation,the fluoride ion content of the solution in unsaturated red clay increases regularly with time: The penetration time increases with the increase of depth,which is mainly reflected in that the time of four holes with different depths in the test is 120 h,124h,128 h and 136 h successively.The deeper the detection hole contacts the solution at the early stage,the longer the lower the concentration.