Buffering of Acidic Mine Lakes: The Relevance of Surface Exchange and Solid- Bound Sulphate

Abstract

Buffering mechanisms in an acidic mine lake in Lusatia, Germany were investigated. The titration curve has four sections with different buffering mechanisms: (1) buffering by free hydrogen ions and hydrogen sulphate (pH = 2.55-2.9), (2) buffering by Fe with bound SO4 (pH = 2.9-4.3), (3) buffering by Al with bound SO4 (pH = 4.3-5.5), and (4) buffering by surface exchange of SO4 and basic cations (pH > 5.5). Three different phase models were applied to simulate the titration curve: (1) an iron and aluminium hydroxide model; (2) an iron and aluminium hydroxysulphate model; and (3) an iron hydroxide model with surface exchange for SO4, Ca, and Mg, coupled with an aluminium hydroxysulphate model. The uncertainty of model input parameters was accounted for in a sensitivity analysis. Only the third model, which considers surface exchange, was able to adequately reproduce the measured titration curve.