Solubility characteristics of proto-imogolite sols: how silicic acid can de-toxify aluminium solutions

Proto-imogolite sols can be considered as highly dispersed forms of proto-imogolite allophane, the most widespread type of allophane in volcanic and non-volcanic soils world-wide. The solubility characteristics of such sols define the conditions of precipitation of allophanes in soils, and the maximum concentrations of aluminium released during acidic episodes from soils, such as podzols, that contain allophane. Direct measurement of Al, Si and pH values in equilibrium with proto-imogolite sols, approached from higher and lower pH, indicated a solubility equation: log{Al 3+ } + 0.5 log{H 4 SiO 4 } + 3pH = log * K so where log *K so lay in the range 7.14 to 7.23 after equilibration for 4-24 weeks at 22 ± 2°C in 17 of the 20 systems studied. The mean value of log *K so at 298 K was calculated as 7.02. This value indicates that proto-imogolite will be more stable than amorphous aluminium hydroxides at H 4 SiO 4 concentrations above 5 x 10 -6 M, but less stable than bayerite below 10 -3 M H 4 SiO 4 , and than gibbsite below 10 -2 M. Proto-imogolite is more stable than micro-crystalline gibbsite in 10 -4 M H 4 SiO 4 , a typical minimum concentration in soil solutions and streams in landscapes where podzols are present. The rapid formation of proto-imogolite effectively prevents the formation of gibbsite seeds in soil, except in highly leached and warm environments, i.e. in older landscapes in the tropics. Although the presence of 10 -4 M silicic acid has been found to eliminate the acute toxicity to fish exhibited by solutions containing 6-7 μM Al at pH 4.96, little or no proto-imogolite would form under these conditions. Silicic acid would, however, prevent the precipitation of aluminium hydroxides, and could inhibit the formation of the Al 13 polycation. These polymeric species are a likely cause of the increased toxicity exhibited by partially neutralized aluminium solutions.