Interaction of clays with dilute fluoride solutions

Abstract

Interaction between dilute (mg L⁻¹) NaF solutions and clay suspensions (0.08 % w/v) has been examined as a function of pH (range 3 to 8), clay type (Na⁺- or Ca²⁺-kaolinite, illite, montmorillonite) and NaF concentration. No F loss from solution was detected at pH > 6.5, while enhanced uptake was found on decreasing the pH, especially in the 4 to 3 region. Removal of F from 1 to 6 × 10⁻⁴ M NaF was only slightly dependent on weight of solid, but did increase with [F⁻]. It is proposed that F losses are due to the formation of sparingly soluble Al species (e.g. cryolite, Na fluoro silicate), occasionally augmented with CaF₂ formation (Ca²⁺-clays). The Al is released by proton attack on the lattice, following conversion of the suspended solids into the unstable H⁺-form, either through acid addition (pH < 5) or through hydrolysis of the Na⁺-form material. The latter process was most pronounced with the illite and montmorillonite samples. The amount of F fixed by montmorillonite was roughly double that held by the other two clays, and had a maximum value (pH 3) of ∼ 4 mg g⁻¹, using 11 mg L⁻¹ NaF solutions. Soluble fluoro-complexes, similar in quantity to the retained F, were detected, in many of the studies. It was concluded that contact of the clay components of soils or sediments with mg L⁻¹ levels of F in adjacent aqueous phases would result in only a minor proportion being retained.