The hydrolysis of Al(III) in NaCl solutions--a model for Fe(III)

There is a lot of interest in the behavior of Fe(III) in natural waters because of its importance in the productivity of phytoplankton. The hydrolysis of Fe(III) in natural waters limits the solubility of Fe(III) in aqueous solutions. At the present time, reliable hydrolysis constants for Fe(III) are limited to low temperatures (5-50 degrees C) in NaCl to 5 m. The hydrolysis constants of Fe(III) and Al(III) are linearly related over a wide range of temperatures (0-50 degrees C) and ionic strengths (0-5 m NaCl). The near linear correlations allow one to make reasonable estimates for the values of Fe(III) from 0 to 300 degrees C in dilute solutions and from 0 to 100 degrees C to 5 m in NaCl solutions. In this paper, the stoichiometric (beta(i)) and thermodynamic (K(i)) hydrolysis constants for Al(III) in NaCl have been fit to equations of the form log beta(i) - log K(i) = a0I(0.5) + a1I(0.5)/T + a3I + a4I/T + a5I2. This equation has been used to estimate the hydrolysis constant for Fe(III) in dilute solutions to 300 degreesC and in NaCl solutions to 5 m and 100 degrees C. The cause of the correlation in water is due to the differences in the free energies of deltaG0(Al3+) - deltaG0 (Al(OH)j(3-j)) being almost equal to the values of deltaG0(Fe3+) - deltaG0(Fe(OH)(3-j)). An examination of the activity coefficients of Fe3+ and Al3 and the complexes show that the correlation at higher ionic strengths is due to the fact that the activity coefficients ratios are similar gamma(Fe3+)/gamma(Fe(OH)j(3-j))approximately equal to gamma(Al3+)/gamma(Al(OH)j(3-j) in NaCI solutions. These linear correlations appear to also hold for other trivalent metals (Cr3, As3+). The results of this study should be useful in examining the speciation of Fe(III) in hydrothermal brines.