ON THE INFLUENCE OF CARBONATE IN MINERAL DISSOLUTION .1. THE THERMODYNAMICS AND KINETICS OF HEMATITE DISSOLUTION IN BICARBONATE SOLUTIONS AT T = 25-DEGREES-C

We have studied the thermodynamics and kinetics of hematite dissolution in bicarbonate solutions under constant pCO2. The solubility of hematite is increased in the presence of bicarbonate. We have established that the complexes responsible for this increase are FeOHCO3 (aq) and Fe(CO3)2-. The stability constants of these complexes at the infinite dilution standard state are log beta-11 = -3.83 +/- 0.21 and log beta-2 = 7.40 +/- 0.11, respectively (all errors are given at 2-sigma confidence level through this work). The rate of dissolution of hematite is enhanced in bicarbonate solutions. This rate of dissolution can be expressed as R(diss) = k1[HCO3-]0.23 (mol m-2 h-1), with k1 = 1.42 10(-7) h-1. The combination of the study of the surface complexation and kinetics of dissolution of hematite in bicarbonate solutions indicate that the dissolution of hematite is surface controlled and bicarbonate promoted. The rate of dissolution follows the expression R(diss) = kHCO3-{= FeOH - HCO3-}, where kHCO3- = 1.1 10(-3) h-1. The implications of these findings in the oxic cycle of iron in natural waters are discussed, most importantly in order to explain the high-Fe(III) concentrations measured in groundwaters from the Pocos de Caldas complex in Brazil.