Synthetic wastewater was made up using deionized water with analytical grade KNO3 and KH2PO4. In order to simulate ionic strength of real wastewater the concentration of KNO3 was 0.01 mol/L [12]. 2 g pyrrhotite particles with the size of 0.3–0.6 mm and 50 ml wastewater were added in plastic tubes which were then flushed with N2 for 1 min before sealed. The tubes were placed on a rotator with a rotation speed of 20 rpm. This speed was selected, as it was fast enough to ensure complete mixing but not too fast so as to avoid centrifugation of particles.