Investigation of pyrite-weathering processes in the vadose zone using optical oxygen sensors.

Pyrite weathering in the unsaturated zone of overburden material was investigated in column experiments. Optodes were used to monitor changes in oxygen concentrations. Oxygen recharge in the vadose zone and effects of secondary reactions were investigated. Oxygen transport limits thickness and the rate of the depyritisation. The overburden material (thickness 0.6 m, initial pyrite content 1.2 wt%) was depyritised within 200 days. The position of the pyrite decomposition front, the momentary release of reaction products and the remaining acidification potential can be determined. Buffer capacity, speciation and seepage water velocity are crucial for the distribution of the reaction products and the resulting acidification (minimum pH 1.6). A total of 75% of the protons were consumed by buffering reactions and 15% were fixed to complexes. Only 10% of the protons are represented by the measured pH value. In oxygen-consuming environments optical oxygen measurements can be used to identify and quantify structures and processes. [ABSTRACT FROM AUTHOR]