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

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Abstract
Montmorillonite from Upton, Wyoming (< 2 μn) was aged 30 days in the presence of Al-citrate solutions both in the absence of NaOH and at an initial NaOH/Al molar ratio of 2.5. The effect of citric acid on the formation of hydroxy-Al-montmorillonite complexes depended greatly on the initial citrate/Al molar ratio. A low citrate/Al ratio of 0.1 did not significantly affect the CEC and X-ray diffraction patterns of hydroxy-Al-montmorillonite complexes. An intermediate citrate/Al ratio of 0.5 reduced the amount of Al depleted from solution and increased the proportion of 1 M KCl-exchangeable Al and 0.2 M HCl-extractable Al and the CEC of the samples; the structural perturbation in the interlamellar hydroxide sheet as revealed by XRD is attributed to the fixation of hydroxy-Al-citrate in the interlamellar spaces. At a high citrate/Al ratio of 1.0, most of the Al remained in solution and was prevented from being adsorbed by montmorillonite. The complexation between Al and citrate is a competitive reaction for the fixation of hydroxy-Al by clay. The close association between hydroxy-Al and citrate in the system is demonstrated by the existence of organic carbon associated with the 1 M KCl-exchangeable Al and 0.2 M HCl-extractable Al. Key words: Exchangeable-Al-associated organic carbon, hydroxy-Al interlayers, structural perturbation, organic acid, smectite