Adsorption of Citric Acid by Synthetic Pseudoboehmite

The adsorption of citrate, at 10 4_10 -3 M initial concentration, by pseudoboehmite suspended in 0.02 M NaC104 was investigated at varying pH. Citrate shows a strong affinity for the pseudoboehmite surface, as seen in the adsorption isotherm at pH 6. Adsorption envelopes of adsorbed citrate vs. pH for a given initial citrate concentration are characteristic for the adsorption ofa polyprotic acid by a variablecharge mineral. The envelope data were fit well by the Constant Capacitance Model assuming a ligandexchange adsorption mechanism, three monodentate surface species of citrate, and a reactive surface OH density of 0.4 mol kg-L Aqueous speciation calculations suggest that solubility equilibrium with pseudoboehmite was attained at pH > 9 and that particulate or polymeric Al may have existed at 6 < pH < 9. Dissolved A1 appeared to reduce the adsorption of citrate at pH < 5.5 via solution eomplexation reactions. Key Words--Citrate, Pseudoboehmite, Adsorption reactions. I N T R O D U C T I O N The adsorpt ion of inorganic and organic anions by metal oxides has been investigated intensively because of applications in agronomic, environmental , and industrial sciences (Sposito, 1984; Stumm, 1987). The results of the many experimental equil ibrium studies that have been published are characterized reasonably well by recent parametric models (Sposito, 1985; Schindler and Stumm, 1987; Barrow, 1987; Hayes et al., 1988). Usually, these models are based on postulated chemical reactions controlling the equilibria between the adsorpt ive anion and an average hydroxyl group on the surface of a crystalline oxide, such as goethite, gibbsite or a-alumina. Because of its wide distr ibution in earth surficial materials, gibbsite [7-Al(OH)3] has been the adsorbent of choice in studies of hydrous a luminum oxide reactions in soils (Hsu, 1989). However, evidence is accumulating that poorly-crystall ine boehmite (7-A1OOH), or pseudoboehrnite, may be an impor tant adsorbent as well (Huang, 1988). Pseudoboehmite differs from boehmite because of smaller particle size, greater hydration, and lack of three-dimensional, longrange structural order (Hsu, 1989). Gastuche and Herbil lon (1962) have shown that neutralization of aluminum chloride by sodium hydroxide and aging of the resulting solution favor the formation of a poorly-ordered boehmite, as compared to washing or dialyzing the precipitate, which favor gibbsite formation. High concentrations o f chloride during the aging of AI-OH solutions (Hsu, 1967), and the presence of many other inorganic and organic ligands, including sulfate and Copyright 9 1991, The Clay Minerals Society citrate, are known to stabilize pseudoboehmite (Huang and Violante, 1986; Huang, 1988; Hsu, 1989). The present paper is intended to provide the first experimental data and chemical modeling of the adsorption of citrate by pseudoboehmite. Citrate is a highly reactive organic ligand that is common in soils (Huang and Violante, 1986), and that has been studied often in conneCtion with mineral weathering, anion adsorpt ion competi t ion, and A1 toxicity effects (Earl et al., 1979; Bruckert, 1970; Violante and Huang, 1985; Huang and Violante, 1986; Traina et al., 1987). Pseudoboehmite can persist in weathering environments i f soluble anion concentrations and/or temperatures are high, and i f alternate wetting and drying periods occur (Huang, 1988; Hsu, 1989). Under these conditions, pseudoboehmite could present an impor tant reactive hydroxylated surface for citrate adsorption by the ligand exchange mechanism (Sposito, 1984). MATERIALS A N D METHODS