Attenuation of heavy metals and sulfate by aluminium precipitates in acid mine drainage?

During the mixing of acid mine waters with nearly neutral tributaries, ofte n precipitates are formed which are high in iron or aluminium. These precip itates cover the river bed for many kilometres. Near the town of Lehesten ( Thuringian slate mining area), leachates of slate quarries and waste rock d umps contain high amounts of aluminium, sulfate, copper, nickel, zinc, mang anese, and H+ ions as a result of the oxidation of incorporated pyrite. The se leachates enter the brooks Loquitz, Kleine Sormitz, and Rehbach leading to the phenomenon named above. The contribution of the forming aluminium-rich precipitates on the attenuat ion of sulfate and heavy metals by sorption or coprecipitation was studied by analysing the composition of water and sediment samples as well as sampl es of suspended matter. Sulfate is often considered as conservative tracer in acid mine drainage. H owever, sulfate does not behave conservatively in this system what might be explained by the adsorption of sulfate to the aluminium precipitates. Inst ead, conservative behaviour was found for calcium, potassium, chloride, zin c, manganese, and nickel. A formation of jurbanite can be excluded because of the low sulfate content s. The sulfate content of the sediment depends on the pH. At low pH values (4.8) the S/Al ratio corresponds to the theoretical ratio in basaluminite a nd decreases with rising pH. Sulfate is weakly bound to the solid phase and can easily be replaced by OH- ions. A Formation of basaluminite is possibl e at low pH values with a fluent transition to aluminium hydroxide. Therefo re the precipitates are assumed to consist predominantly of aluminium hydro xide with sulfate being adsorbed to the surface. The concentration of zinc and nickel is almost not affected by the precipit ation of aluminium. Tn contrast copper is partially coprecipitated with alu minium and bound in a non-exchangeable form. But less than 40% of total cop per were bound at pH 6.4. Therefore, adsorption and coprecipitation play on ly a minor role in the attenuation of heavy metals for the studied system w ith the greater part (60% for copper and nearly 100% for nickel and zinc) b eing transported downstream in dilute form.