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 degrees 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 degrees C at comparable supersaturations with the 25 degrees C experiments using the free drift method. Ar 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 degrees C and 80 degrees 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.