THE FORMATION OF IRON(II) SULFIDES IN AQUEOUS-SOLUTIONS

The kinetics of the spontaneous formation of iron sulfides were studie d in low ionic strength deaerated aqueous solutions at 25 degrees C at both nearly neutral and moderately acidic pH. The constant supersatur ation approach was adopted for this investigation. In nearly neutral p H (7.1-7.4) mackinawite precipitated directly, while at pH 5.4-5.6 amo rphous FeS formed, showing marked sensitivity to oxidation by atmosphe ric oxygen. The rates of precipitation were found to be proportional t o the relative solution supersaturation and the value m = 2 was found for the apparent rate order of precipitation. The investigation was ex tended at 80 degrees C at comparable supersaturations with the 25 degr ees C experiments using the free drift method. Ar pH>7, only iron oxid es were produced while at pH 5.5 amorphous FeS precipitated. The rates of precipitation measured, showed a first order dependence on the rel ative solution supersaturation. The apparent orders of 2 and 1 for the precipitation of amorphous iron sulfide at 25 degrees C and 80 degree s C, respectively, in combination with an estimated activation energy of 40 kJ mol(-1) and with the insensitivity of the kinetics measured o n the fluid dynamics suggested a surface controlled process.