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Formation of Ferric Oxyhydroxides from Ferrous and Ferric Perchlorate in Stirred Calcareous Systems¹

ABSTRACT

Reactions between Fe salts and CaCO₃ 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 Fe²⁺ or Fe³⁺ and CaCO₃, especially with regard to influence of CaCO₃ surface area and P_CO2 on rate of reaction and Fe oxide product. Ferrous or ferric perchlorate, 0.01M, 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(ClO₄)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 CaCO₃ surface area. With the Fe(ClO₄)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(ClO₄)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% CO₂ (balance air), respectively. With the Fe(ClO₄)₃ system, poorly crystalline ferrihydrite was the only product obtained.

¹ Contribution from the Texas Agric. Exp. Stn. as Journal Series no. 18188. This research was partially supported by a grant from the Center for Energy and Mineral Resources.

² Assistant Professor, Professor, and Research Associate, Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843.

Received for publication October 17, 1983. Accepted for publication December 21, 1983.