Effect of fulvic acid on the crystallization of aluminum hydroxides

Abstract

Effects of addition of increasing amounts of fulvic acid (FA) on the crystallization of aluminum hydroxides from AlCl 3, solution, neutralized to different degrees, were investigated. All systems were allowed to age at 30°C for 70 days. In the absence of FA, gibbsite was formed at pH 6, a mixture of nordstrandite and bayerite at pH 8, and bayerite crystallized at pH 10. At pH 6 and 8, the addition of increasing amounts of FA (up to 10 mg/l or FA/Al molar ratio ⋍ 10 −2 ) first delayed and then inhibited the crystallization of these aluminum-hydroxide polymorphs but tended to favor the crystallization of pseudoboehmite, an aluminum oxyhydroxide. At pH 10, the addition of FA totally inhibited crystallization and precipitation. Our data suggest that FA so strongly complexes Al that it prevents its full hydroxylation to Al(OH) 3. Another explanation is that the action of FA on the crystallization of aluminum hydroxides resembles that of salts in that FA appears to favor the formation of Al-O-Al (oxo) over that of ▪ (ol) linkages. Either reaction mechanism throws new light on the genesis of some bauxites, which consist of submicroscopic boehmite-like particles which are very similar to pseudoboehmite.