Influence of pH, Concentration, and Chelating Power of Organic Anions on the Synthesis of Aluminum Hydroxides and Oxyhydroxides

Abstract

Abstract--Chelating organic acids hampered the hydrolytic reactions of A1 and affected the nature of the crystalline aluminum hydroxides. Chemical composition, structure, size, nature of functional groups, and concentration of each organic anion, as well as the pH of the system, controlled the rate of AI(OH)3 crystallization. The order of effectiveness of the various acids was: glutaric < succinic = phthalic < glycine < malonic < glutamic < aspartic < oxalic < salicylic = malic < citric < tartaric. An increase in the stability of complexes formed between the organic ligands and AI decreased the rate of crystallization and changed the final aluminous products from bayerite to nordstrandite and/or gibbsite and then to pseudoboehmite and/or amorphous material. In the presence of anions with a great affinity for AI, particularly at pH equal to or less than 9.0, the reaction products were commonly poorly crystalline or structurally distorted. In the range of pH 8.0 to 10.0 moderately or strongly chelating anions acted to retard or prevent olation and facilitated the formation of stable pseudoboehmite or X-ray-amorphous products. The stronger the chelating power or the higher the concentration of organic anions, the easier was the formation of pseudoboehmite or amorphous material.

Morphology of Nordstrandite

“…The particular examples quoted are those which yielded a high percentage (>90%) of nordstrandite and only a small percentage of gibbsite and/or bayerite. Citric acid more effectively inhibits the crystallization of AI(OH)3 than malic acid because of its slightly higher chelating power for A1 ions (Violante and Violante, 1980). However, under the conditions employed here, the resulting morphology was independent of whether malic or citric acid was used.…”

“…TEM micrographs of Pt/C-shadowed replicas (Figure lb) and SEM micrographs (Figure lc) of the same sample show that the crystals have a tabular morphology. The crystals formed at pH 11.0 were usually larger (as large as 5/~m) than those obtained at lower pH because the inhibiting effect on crystallization of the chelating carboxylic acid anion decreased as the pH increased (Violante and Violante, 1980).…”

“…Nordstrandite has since been synthesized in a pure state by reacting slightly amalgamated Al foil or noncrystalline, precipitated aluminum hydroxide gel in an ethylenediamine solution (Hauschild, 1963), or by ageing hydrolytic precipitation products of aluminum with aqueous ethylene glycol (Aldcroft and Bye, 1967). Recently, organic chelating anions and clay mineral surfaces, particularly at pH >8.0, have been shown to play important roles in favoring the crystallization of pure nordstrandite Jackson, 1979, 1981;Violante and Violante, 1980).…”

“…Lippens (1961), Hauschild (1963), and Schoen and Roberson (1970) claimed that nordstrandite occurs as rectangular plates or in the form of elongate, platy parallelograms. However, Jackson (1979, 1981) and Violante and Violante (1980) found that the synthesis of nordstrandite in the presence of chelating carboxylic acid anions produced Copyright 9 1982, The Clay Minerals Society roughly rectangular crystals and that, with the addition of clay to the system, either ovoidal particles or clusters of acicular crystals were obtained, depending on pH. The aim of the present work was to determine by electron microscopy the crystal morphology of nordstrandite synthesized under different conditions, so that the mechanisms of formation of this mineral in soils and sediments may be better understood.…”

FORMATION OF HYDROXY-Al-MONTMORILLONITE COMPLEXES AS INFLUENCED BY CITRIC ACID

“…The ubiquitous nature of the non-humic, low molecular weight organic acids as products ofdecomposition of plant materials and metabolites of organisms (Stevenson 1967(Stevenson , 1982 (Kwong and Huang,l9'75,197'7,l979a,b,c). This perturbation leading to structural distortion is so strong that it occurs even in the alkaline range of pH (Violante and Violante 1980 Alexiades and Jackson (1965 …”