Aluminum Precipitation and Dissolution Rates in Spodosol Bs Horizons in the Northeastern USA

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Abstract
The kinetics of Al precipitation/dissolution reactions in Spodosol Bs horizons from sites at Hubbard Brook, NH and Bear Brook, ME were examined. A mechanical vacuum extractor was employed to draw solutions through soil columns at solution/soil residence times between 0.3 and 100 h. Equilibrium was approached from conditions of both undersaturation and oversaturation to determine if a particular soil mineral controlled Al3+ activity. Column leachates were analyzed for major solutes followed by chemical speciation and calculation of mineral saturation indices. Our results showed that apparent equilibrium with respect to Al(OH)3 solubility was readily obtained from both conditions of undersaturation and oversaturation within 0.3 h. Computed saturation indices, for an Al(OH)3 mineral with log K25C = 8.1, were identical at all examined residence times. Aquo Al (Al3+) within soil solutions may be regulated by the hydroxy‐Al interlayer of expansible 2:1 layer silicates. Soil solutions from Bear Brook also reached apparent equilibrium with imogolite at residence times in excess of 10 h. The laboratory results compared favorably with saturation indices calculated from stream and soil solutions at these sites.