Reactions between Fe salts and CaCO 3 are important in influencing the nutrient availability of Fe in calcareous soils and are also important in soil pedogenic processes. The objective of this study was to investigate the reaction between Fe 2+ or Fe 3+ and CaCO 3 , especially with regard to influence of CaCO 3 surface area and P CO2 on rate of reaction and Fe oxide product. Ferrous or ferric perchlorate, 0.01 M , was reacted with calcite in stirred aqueous suspensions while bubbling vigorously with an oxidizing purge gas. Redox potential and pH were monitored continuously during the reactions. Upon addition of Fe(ClO 4 ) 2 to an aqueous calcite suspension, pH was initially controlled by the solution phase Fe at approximately 5.0 to 6.0; however, as the Fe salt was completely precipitated there was a rapid rise in pH to the equilibrium pH of calcite. In all cases, pH and redox endpoints corresponded closely. Rate of reaction was strongly dependent on CaCO 3 surface area. With the Fe(ClO 4 ) 2 system and with air as the purge gas, rate of reaction was linearly related to calcium carbonate surface area. Increased P CO2 resulted in reduced rates of reaction and deviation from the linear relationship between surface area and rate of reaction. With the Fe(ClO 4 ) 2 system, mineralogy of the Fe oxide product as determined by x‐ray diffraction was influenced by P CO2 . Lepidocrocite and goethite were the dominant products formed in the presence of air and 20% CO 2 (balance air), respectively. With the Fe(ClO 4 ) 3 system, poorly crystalline ferrihydrite was the only product obtained.
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Loeppert et al. (1984) studied this question.