Photochemically induced formation of the "Al-13" tridecameric polycation in the presence of Fe(III) and organic acids

We found that light-induced Fe(III) reduction associated with the oxidation of a simple hydroxy-carboxylic acid (lactate) caused the formation of the AlO4Al12(OH)(24)(H2O)(12) polycation ("Al-13"). Initial conditions were a l actate:Al:Fe ratio of 1:0.76:0.11 in a partially neutralized solution. Base was added rapidly and no Al-13 was detected in samples kept in the dark. W ith exposure to light, Fe(III) reduction was rapid and Fe(II) reached a max imum within 1 day. After the maximum, steady-state Fe(II) declined from 54% to 43% over eight days. During this same time period, the lactate concentr ation fell to 2% of the original, pH rose from 4.05 to 4.46, and the Al-13 detectable by Al-27 NMR increased to 2.3 mmol 1(-1) (51% of the total solut ion Al). The formation of Al-13 is attributed to the pH rise resulting from the removal of the organic acid buffer. Similar photo-induced chemical cha nges occur in natural waters and may promote the formation of Al-13, condit ions permitting. (C) 2001 Elsevier Science Ltd. All rights reserved.