Abstract:
The solubility of gibbsite was measured at 35 and 80°C, and the solubility of portlandite was measured at 25, 35, and 80°C, each as a function of pH and malonate concentration. The gibbsite solubility data can be satisfactorily modelled using only one aqueous Al-malonate species. The data rule out all possible Alp(OH)q(Mal)3p-q-2r)r and Alp(OH)q(HMal)3p-q-r)r stoichiometries except Al(Mal)-2 and Al(OH)2(HMal)-2, each of which provides an excellent fit to all of the experimental solubility data. If Al(Mal)-2 is the dominant species, the data constrain the value for the log of the dissociation constant of Al(Mal)-2 to be -11.3 +/- 1.0 and -14.5 +/- 0.5 at 35 and 80°C, respectively. If Al(OH)2(HMal)-2 is the dominant species, the values of its dissociation constant at 35°C and at 80°C are -27.3 +/- 1.0 and -27.6 +/- 0.5, respectively. The portlandite solubilities indicate that Ca(Mal)0 is the dominant Ca-malonate complex, with values for the log of the dissociation constant of Ca(Mal)0 of -2.4 +/- 0.3, -2.4 +/- 0.3, and -2.9 +/- 0.3, at 25, 35, and 80°C, respectively. Thermodynamic modelling, using these results, indicates that Al-malonate complexation can potentially enhance the solubility of Al-bearing minerals by up to five orders of magnitude.