EFFECT OF IRON ON THE NATURE OF PRECIPITATION PRODUCTS OF ALUMINUM

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Abstract
Solutions Of AlCl3 (5 × 10−3 mol L−1) and FeCl2 were mixed and aged for 3 yr under ambient conditions at the initial pH of 7.0 and Fe:Al molar ratios of 0, 0.1, 0.3, 0.5 and ∞ (Fe at 5 × 10−3 mol L−1). Fe2+ was readily oxidized in this experiment so that only the mutual interaction between Fe3+ and Al3+ can be reported. The initial Fe:Al molar ratio was an important parameter in determining the rate and amount of hydrolysis and precipitation of Fe and Al. At Fe:Al molar ratios ≤ 0.3, hydrolysis was delayed when compared to that in pure Al suspensions. At higher ratios more acidity was produced by hydrolytic reactions. The acidity was contributed by both Al and Fe. The fractions of Fe and Al precipitated were greater at lower Fe:Al molar ratios. Aluminum and iron were mutually interfering cations during crystallization. In mixed systems, the fractions of crystalline materials were greatly reduced. Between the crystalline compounds, gibbsite was more abundant than bayerite and this evolution was governed by pH and the interference of Fe on the subsequent rate of hydrolysis and crystallization. Iron substitution was not evident in the crystalline Al(OH)3 polymorphs. Crystalline Fe oxides could not be detected even at the Fe: Al molar ratio of 0.1. Two discrete oxide phases were formed in the mixed Fe-Al systems consisting of crystalline Al(OH)3 and a mixed Fe-Al oxide of indeterminate composition. In the pure ferric suspension, maghemite was formed. The results presented are used to explain the distribution of aluminum hydroxide polymorphs in soil and the genesis of gibbsite and iron oxide containing horizons of Oxisols. Key words: Iron, aluminum, gibbsite, bayerite, maghemite, Fe-Al surface coatings
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