CALCITE GROWTH-INHIBITION BY IRON

This paper presents a study of how various solution conditions (i.e., iron concentration, calcite supersaturation, pH, alkalinity, seed surface area, and ionic strength) influence the effectiveness of both ferrous (Fe(II)) and ferric (Fe(III)) ions as inhibitors of the growth of calcite, a common scale-forming mineral. It shows that when Fe(II) concentrations greater than 8 x 10(-5) M or Fe(III) concentrations greater than 5 x 10(-6) M are used, calcite growth is completely inhibited. Lesser concentrations of iron cause the calcite growth process to slow, and also cause growth to almost stop at a calcium concentration which is still significantly higher than the calcium concentration which would be in equilibrium with calcite seeds in the absence of iron. The extent of growth inhibition depends on the iron to seed ratio; a larger ratio results in increased growth inhibition. Lower supersaturations require less iron for the same extent of growth inhibition. Decreasing the alkalinity greatly reduces the inhibiting effectiveness of Fe(III); the inhibiting effectiveness of Fe(II) is also reduced at lower alkalinity but to a lesser extent. Fe(III) inhibits calcite growth more effectively at pH 7 than at pH 8; for Fe(II), studies of the effect of pH were inconclusive because very fast oxidation occurs at pH values greater than 7. Changes in ionic strength have only negligible effects. At the experimental conditions studied, Fe(III) is a much better inhibitior than Fe(II). However, adding oxygen to a solution that contains Fe(II) results in a dramatic increase in growth inhibition, so much so that the inhibition is even better than that observed on adding the same amount of iron directly as Fe(III).