Artificial weathering of some soil clays

Abstract

An artificial weathering experiment was set up to test for reversibility within the system. 2:1 – 2:2 Al-intergrade <->vermiculture <-> montmorillonite. Strongly chelating (0.20M and 0.02M citrate) and weakly chelating (0.02M acetate) solutions buffered at pH 3.5, 4.5 and 5.5, were leached for varying periods of time up to 6 monhts, through B2 horizon. Craigieburn silt loam. X-Ray diffraction of the leached soil clay fraction showed that the period of leaching was too short to cause a clay transformation from the initial 2:1 – 2:2 Al-intergrade into a 2:1 layer silicate. However, X-Ray and electron microscopy studies showed removal of some surface coatings and interlayer hydroxy Al. Analyses leachates showed that the citrate (chelating) buffers were more effective at removing Al and Fe from the solid phases than acetate (non-chelating) buffers. Quantities removed and rates of solution of Al and Fe increase with decreasing pH. Rapid initial loss of Al and Fe probably involved disordered gels and surface coatings. Increasing the citrate concentration above a certain limit did not increase the ability of the solution to remove Al and Fe. Leaching with 1.5M AlCl3 solution at pH’s 3.5, 4.5 and 5.5 caused rapid fixation of Al in the A2 horizon of the Katrine silt loam. A vermiculite-montmorillonite intergrade was rapidly transformed into a 2:1 – 2:2 Al-intergrade, suggesting that an increase in pH leading to build up in Al species in the soil would result in montmorillonite being transformed into 2:1 – 2:2 Al-intergrade. It was concluded that leaching for longer periods, using a lower concentration of chelating solution, that would more closely simulate the natural system should be attempted. Developments of the method are suggested.