INORGANIC COLLOIDS IN MINE WATERS

Inorganic colloids in mine waters are mainly produced by the formation of oxyhydroxides and oxyhydroxy sulfates of iron(III) and aluminum due to the oxidation of Fe²⁺ and the hydrolysis of Fe³⁺ and Al³⁺. They are of relevance as they are able to influence heavy metal transport (emphasis is laid on the behavior of uranium here). Two extremely different types of mine waters can be observed:

(i) Type "acidic pore water". These waters possess high salt concentrations and are very acidic (pH 1 to 3). Ultrafine particles of <5 nm are the main colloidal component. The colloid concentration reaches the range of >1 g/L. The particles consist mainly of H-jarosite and schwertmannite. As and Pb show a high tendency to adsorb onto these particles.

(ii) Type "bulk water". This type of waters mainly refers to the flowing mine waters like adit waters which possess a near-neutral pH. Typical colloid contents of such waters are about 1 mg/L, typical particle sizes are 100 to 300 nm. The particles consist of Fe(III) and Al oxyhydroxides. Their electrostatic stabilization is weak, i. e. they show a larger tendency to coagulate. Contaminants such as As, Pb, Cu, Th, U(IV) and Po are bound onto these particles. Uranium(VI) adsorption is often suppressed due to the formation of uranyl carbonato complexes in mine waters.

An interesting colloid chemistry is observed during the transition of type (i) waters to type (ii) waters (flooding of abandoned ore mines). Huge amounts of iron(III) particles with a size of 100 to 300 nm are formed under such conditions. The adsorption rises drastically when the pH reaches the near-neutral region. Uranyl adsorption to the iron(III) particles is neither suppressed by high acidity nor by uranyl carbonate complexation in the pH region 4 to 6. Therefore, most of the uranium(VI) is colloid-borne in waters of the transition type.