Pyrite oxidation in a sediment sample of an open-cut brown coal mine: mineral formation, buffering of acidity and modeling of cations and sulfate

Secondary reactions occurring in pyrite-containing sediments under aerobic conditions are complex and are not fully understood. Objectives were to (i) study the formation of secondary minerals using x-ray diffraction (XRD) an d ion activity product (IAP) calculations; (ii) to obtain a budget of acidi ty producing and consuming processes; and (iii) to study the performance of a chemical equilibrium model (including kinetic reactions) using sequentia l batch experiments with varying input solutions on samples of different py rite oxidation states. A sediment sample from the open pit coal mine Garzwe iler, Germany, was oxidised in the laboratory to obtain four different pyri te oxidation states. Sequential batch experiments were carried out using H2 O, 100 mM CaCl2 and 10 mM NaOH as input solutions. A coupled equilibria mod el was used to describe the experiments. The model (PHREEQC) included inorg anic complexation, redox reactions, precipitation/dissolution of sparingly soluble salts, multiple cation exchange and pyrite oxidation using a simple input, function. IAP calculations and XRD showed the formation of large amounts of gypsum wi th increasing pyrite oxidation and for the highly oxidised sample also the formation of hydroniumjarosite. The budget of acidity consuming processes f ollowed the order (i) release of Fe(III) into the solution phase (51% of pr oduced acidity); (ii) Al release into solution and exchangeable phases (pro bably mainly due to silicate weathering, 22% of produced acidity); and (iii ) CEC reducing processes (11% of produced acidity). Modeling of the sequential equilibration experiments with water and CaCl2 g ave satisfactory agreements between modeled and measured pH and sorption va lues, indicating that the main processes governing pH and ion sorption were quite well understood. However, model results of the alkaline additions at larger pyrite oxidation states differed considerably from the experimental results.