The formation of iron(II) sulfides in aqueous solutions

The kinetics of the spontaneous formation of iron sulfides were studied in low ionic strength deaerated aqueous solutions at 25°C at both nearly neutral and moderately acidic pH. The constant supersaturation approach was adopted for this investigation. In nearly neutral pH (7.1–7.4) mackinawite precipitated directly, while at pH 5.4–5.6 amorphous FeS formed, showing marked sensitivity to oxidation by atmospheric oxygen. The rates of precipitation were found to be proportional to the relative solution supersaturation and the value m = 2 was found for the apparent rate order of precipitation. The investigation was extended at 80°C at comparable supersaturations with the 25°C experiments using the free drift method. At pH > 7, only iron oxides were produced while at pH 5.5 amorphous FeS precipitated. The rates of precipitation measured, showed a first order dependence on the relative solution supersaturation. The apparent orders of 2 and 1 for the precipitation of amorphous iron sulfide at 25°C and 80°C, respectively, in combination with an estimated activation energy of 40 kJ mol −1 and with the insensitivity of the kinetics measured on the fluid dynamics suggested a surface controlled process.