The Sorption of Toxic and Radiotoxic Heavy Metals by Inorganic Colloids in Mine Waters

Inorganic colloids are ubiquitous in mine waters. They are primarily 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+. Colloidal particles are able to adsorb heavy metals and to influence trace heavy metal transport as for instance the transport of uranium. However, colloid formation and heavy metal adsorption are strongly dependent on pH. From a colloid-chemical point of view it can be differentiated between two principal types of mine waters:

(i) Type "acidic pore water" [1]
These waters can be separated from the rock by centrifugation or can be collected in the mines as acid rock drainage (ARD) samples. The waters possess high salt concentrations and are very acidic (pH 3 to 1). Ultrafine particles of <5 nm in size are the main colloidal component (based on photon correlation spectroscopy and ultrafiltration). The colloid concentration reaches the g/L range. The particles consist mainly of hydronium jarosite and schwertmannite (based on EXAFS). As and Pb show a high tendency to adsorb onto these particles.

(ii) Type "bulk water" [2]
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 (based on photon correlation spectroscopy and scanning electron microscopy). The particles consist of Fe(III) and Al oxyhydroxides (ferrihydrite, alumogel). Their electrostatic stabilization is weak (zeta potential only about -10 mV), i. e. they show a larger tendency to coagulate and aggregate than the colloids found in the type (i) waters. Contaminants such as As, Pb, Cu, Th, U(IV), Po, and in the absence of carbonate complexation also U(VI), are strongly bound onto these particles. Uranium(VI) adsorption is often suppressed due to the formation of dissolved uranyl carbonato complexes in mine waters [2].

Important colloid-chemical processes are observed during the transition of type (i) waters to type (ii) waters which occurs in the course of the flooding of abandoned ore mines (dilution of the type (i) waters). Huge amounts of iron(III) particles with a size of 100 to 300 nm are formed under such conditions due to O2 ingress and pH increase. The adsorption rises drastically when the pH reaches the near-neutral region (cf. [3]). The sorption behavior of U(VI) in flooding waters differs significantly from that of U(VI) in both typical "acidic pore waters" and typical steady-state "bulk waters". Typical pH values of the flooding waters are in the range between 4 and 6 after some flooding time. Uranyl adsorption to the iron(III) particles is neither suppressed by high acidity nor by uranyl carbonate complexation in this pH region and the assumption of an unretarded migration of uranyl is "over-conservative". Scavenging by Fe(III) particles and colloid coagulation plus sedimentation can significantly immobilize the U(VI) in waters of the transition type. This scavenging mechanism refers also to trace elements such as Cu, As, Mo, Sb, Y, and Ce. Also the radiotoxic heavy metals Po, Pb, Th and Ac are colloid-borne above pH 4. Ra is usually truly dissolved.

[1] Zänker, H.; Moll, H.; Richter, W.; Brendler, V.; Hennig, C.; Reich, T.; Kluge, A.; Hüttig, G. (2001)
The colloid chemistry of acid rock drainage solution from an abandoned Zn-Pb-Ag mine. Appl. Geochem. (accepted).

[2] Zänker, H.; Richter, W.; Brendler, V.; Nitsche, H. (2000)
Colloid-borne uranium and other heavy metals in the water of a mine drainage gallery. Radiochim. Acta 88, 619-624.

[3] Hsi, C.-K. D.; Langmuir, D. (1985)
Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model. Geochim. Cosmochim. Acta 49, 1931-1941.