The release of aluminum from aluminosilicate minerals. II. Acid-base potentiometric titrations

Acidified suspensions of Al-saturated kaolinite, montmorillonite, mica, illite, and biotite in 10 -a M NaNO3 were potentiometrically titrated with 0.1 N NaOH and 0.1 N HNO3 in succession in a COz-free nitrogen atmosphere. The resulting curves were compared with those for AI(NO3)3 solutions of similar AI concentration in the supernatant solution and corrected for Al in the entrained solution in the clay. Base titrations of AI ions adsorbed on all the minerals, except montmorillonite, showed two pH inflec- tions separated by a buffering range. With montmorillonite, there were three pH inflections similar to those for AI in solution. The first inflections in the titration of suspensions occurred at lower pHs and were less pronounced than for AI in solution. These represent the titration of HaO § sorbed during the pretreatment. The buffering by adsorbed AI ions is also less than that by A1 in solution. The OH- used up by adsorbed Al ions between the first and last inflections was equal to, or slightly greater than, the CEC of the minerals, except for mica where it was more than twice the CEC, because new interlayer surfaces were formed during the acid pretreatment. Acid titration curves of Al ions in the adsorbed and solution states showed hysteresis when related to the base titration curves. The use of two titration speeds (3 and 0.3 pH units/hr) only slightly affected the titration curves of the minerals suggesting that the observed effects were not caused by lack of equilibrium with added base or acid.

F. Cabrera thanks the Ramsay Memorial Fellow- ships Trust and the Consejo Superior de Investiga- ciones Cientificas (C.S.I.C.) for financial support.

Peer Reviewed