MARCASITE DISSOLUTION KINETICS IN ACIDIC CONDITIONS

Marcasite (FeS₂) is a pyrite's polymorphous whose dissolution can produce large amounts of acid rock drainage (ARD) and acid mine drainage (AMD). In this communication, the effects of pH (1≤pH≤3), dissolved oxygen (0.2, 2 and 8.7 mg L^-1), and temperature (25, 50 and 70 ºC) on marcasite dissolution are studied. Stirred and non-stirred flow-through experiments containing marcasite powder (10<size fraction<100µm) were used to obtain steady-state dissolution rates.

Under these experimental conditions, marcasite dissolution was characterized by a systematic deficit in aqueous sulfur compared with the bulk mineral composition, yielding a Fe/S ratio in solution of 0.7-1.7. Consequently, steady-state dissolution rates were only based on the iron released to the solution. Within this pH range, marcasite dissolution rate is only slightly pH dependent, decreasing with decreasing pH. On the contrary, dissolution rate increases noticeably with increasing dissolved oxygen and temperature.

Under these conditions it is possible to propose an empirical rate law accounting for the effects of pH, dissolved oxygen and temperature on marcasite dissolution:

Rate = k₊[O₂(aq)]^maⁿ_H+exp^Eapp/RT

where Rate is marcasite dissolution rate in (mol g s^-1), k+ is the dissolution rate constant (10^-7.17±0.11), [O₂(aq)] and (a_H+) are the activities of dissolved oxygen and hydrogen ions, respectively, m and n are empirical factors (0.48 ± 0.02 and -0.12 ± 0.03, respectively) and Eapp is the apparent activation energy of the overall reaction (20 ± 2 kJ mol^-1). According to these results, marcasite dissolution rate at very acidic conditions is similar to published values for pyrite (Williamson and Rimstidt, 1994).

References

Williamson M. A. and Rimstidt J. D. (1994) The Kinetics and Electrochemical Rate-Determining Step of Aqueous Pyrite Oxidation. Geochimica et Cosmochimica Acta 58(24), 5443-5454.