Factors affecting the movement and distribution of fluoride in aquifers

Abstract

This dissertation presents the results of laboratory experiments in which essential aspects of the movement of F- in saturated media have been addressed. The interactions between F-solutions and quartz, vermiculite, and kaolinite were studied through batch and column experiments. Quartz was found to react slightly with F⁻ , giving data described by a quasi-linear isotherm. Vermiculite adsorbed only minute amounts of F. A large uptake of F⁻ by kaolinite was measured. Ion exchange F⁻ by 0H⁻. may not have been the exclusive mechanism operating under the experimental conditions. The kinetics of the dissolution of fluorite (CaF₂) were investigated by means of batch and column tests. There appears to be a relationship between the dissolution rate and the mean flow velocity. Solution pHs greater than 5-6 may accelerate the dissolution process. For temperatures between 15 and 30°C, the dissolution is characterized by an activation energy of about 7 Kcal/mole, which would indicate that both surface reaction and transport are the rate-limiting step. The percolation of columns containing quartz, vermiculite, and kaolinite with multi-component solutions (including F⁻) resulted in a late breakthrough of F⁻ when compared with that of the other species. Circulation of distilled water led to an almost complete recovery of the F⁻ injected, which exited the column with relative concentrations greater than 1. When the packing included fluorite and distilled water was flushed through the column for 2 days, a concentration of F⁻ of about 1.3 pprn was rapidly reached and remained constant throughout the run. Multivariate analysis techniques (factor and correspondence analyses) were applied to data from two aquifers known to carry high F-waters. Although helpful in discriminating major and minor associations of species, none of those techniques could help unravel the behavior of F⁻ in the study aquifers. It is suggested that laboratory and field studies be continued and that, for the successful modeling of the movement and distribution of F⁻ in aquifers, non-linear source/sink terms should be included in the pertinent differential equation governing the transport of solutes.