Aluminum release rates from allophanic and nonallophanic Andosols

Abstract

Aluminum release rates were studied for subsoil horizons of allophanic and nonallophanic Andosols and for imogolite gel films. Release rates were determined using a stirred, flow-through reaction vessel with a 10⁻³ M acetate buffer adjusted to pH 3.3, 3.8, or 4.2. To determine the sources of dissolved AI, soils were pretreated with Kel, Na-pyrophosphate, citrate-bicarbonate-dithionite, and acid oxalate to remove various solid-phase pools of active AI. Rates of Al release for imogolite gel films during the first minute of reaction ranged from 74.8 nmol/gls at pH 4.2 to 113.3 nmol/g/s at pH 3.3. Rates of AI release for nonallophanic Andosols were greater than for allophanic Andosols due to the rapid release of exchangeable Al from nonallophanic Andosols. Aluminum release rates showed fractional order H⁺ -dependence ranging from 0.23 to 0.54. The primary source of dissolved Al in allophanic Andosols was from AI-humus complexes and allophane/imogolite. In nonallophanic Andosols, exchangeable Al and AI-humus complexes were the primary sources of dissolved AI. Treatment with KCl increased Al release rates for allophanic Andosols, possibly due to precipitation of polymeric-AI on soil surfaces. Iron oxyhydroxide coatings substantially attenuated Al release rates from allophane and imogolite in allophanic Andosols.