On the chemistry of the Keggin Al13 polymer:: Kinetics of proton-promoted decomposition

The decomposition of the polynuclear Keggin Al-13 species (Al13O4(OH)(24)(H2O)(12)(7+) or Al-13) has been examined as a function of pH (2.06 less than or equal to pH less than or equal to 3.50), ionic strength (I = 0.1 and 1.0) and temperature (10 degrees C less than or equal to T less than or equal to 65 degrees C) using batch and flow-through reactors. The overall decomposition rates were found to be positively correlated with the activity of H+, ionic strength and temperature with half-lives ranging from 350 to 43,000 s. The decomposition rate was interpreted as a function of two parallel reactions, one first-order and one second-order with respect to [H+]: -d[Al-13]/dt = R-1 + R-2 (1) where R-1 = k(1) [H+] [Al-13] and R-2 = k(2) [H+](2) [Al-13]. For 25 degrees C and I = 0.1, the rate constants k(1) and k(2) were determined as 0.0333 +/- 0.0016 M-1 s(-1) and 2.59 +/- 0.62 M-2 s(-1), respectively. The activation energies and the Arrhenius factors for an ionic strength of 1.0 were found to be Ea(1) = 13.3 +/- 1.9 kJ mol(-1), Ea(2) = 44.9 +/- 4.9 kJ mol(-1), A(1(I=1.0)) = 25.2 +/- 19.2 M-1 s(-1) A(2(I=1.0)) = 1.62 10(9) +/- 3.12 10(9) M-2 s(-1). While Ea(1) and Ea(2) do not depend on the ionic strength, the Arrhenius factors for I = 0.1 were obtained as A(1(I=0.1)) = 7.23 +/- 0.19 M-1 s(-1) and A(2(I=0.1)) = 1.86 10(8) +/- 1.9 10(7) M-2 s(-1). The apparent charge of the Al-13 complex in the encounter reaction with a hydronium ion was calculated from the Arrhenius factors at various ionic strengths as 0.78. We postulate Al-13 with a protonated bridging OH group as the precursor for the decomposition reaction, first-order in [H+]. The low value of the activation energy for the one-proton pathway is explained by an exothermic formation of the precursor HAl138+. The larger value of Ea(2) indicates that the formation of a precursor with two adjacent protonated site involves a positive reaction enthalpy. The half-life of several hundred hours at pH 5 indicates that Al-13 may exist in natural waters, however, it may also be subject to continuous decomposition and reformation. (Copyright (C) 1999 Elsevier Science Ltd.