INTERACTIONS BETWEEN CITRATE AND MONTMORILLONITE AL HYDROXIDE POLYMERSYSTEMS

Citrate forms strong complexes with Al ions and may thus influence the stability and formation of Clay-Al hydroxide polymer systems (CAlHO). We studied the adsorption of citrate to CAlHO and the influence of ci trate on the stability and formation of CAlHO at different Al:clay and Al:citrate ratios and pH values and at a total salt concentration of 0.01 M monovalent anions. The amount of citrate sorbed to the aged CAl HO was independent of the Al fixed to the clay as Al hydroxide polymer s (AlHO) at 5 < pH < 6.6. The added citrate seemed not to sorb to the AlHO but only to the edges of the clay. As the citrate:AI ratio increa sed from 1:5-1:1 at pH 6, more of the AlHO of the aged CAlHO systems d issolved. The change in the CEC of the clay indicated that the nature of the remaining AlHO is independent of the amount of Al dissolved. Ci trate influenced the formation of CAlHO systems as measured at pH 6.6, to an extent which depended on the citrate:Al ratio. At a small ratio (1:5), AlHO formed and all citrate was incorporated in the AlHO, prob ably leading to a coprecipitate. The amount of citrate incorporated de pended linearly on the amount of AlHO present. Go-precipitation of AlH O and citrate probably led to the formation of a separate phase, which was only weakly bound to the clay particles. At a large citrate:Al ra tio (1:I) soluble Al-citrate complexes became dominant, and only a sma ll part of the added Al was present as AlHO.