University of Saskatchewan
Department of Soil Science
College of Agriculture and Bioresources

Environmental Soil Chemistry

Si and Al XANES Spectroscopic Studies of Clay Liners Subjected to Hyper-acidic Solutions


Natural and constructed clay liners are routinely used to contain waste and wastewater. Although widely investigated in relation to acid mine drainage systems at pH > 1.0, we know little about the impact of sulphuric acid (H2SO4) on the geochemistry and mineralogy of clays at pH < 1.0 (including negative pH values). This is a relevant research question for western Canada's Oil Sands production, as large blocks of zero-valent sulphur are commonly stored on clay liners and leachate can routinely be present at pH < 1.0.

Figure 1.Elemental sulfur block from Syncrude's Fort McMurray operation (taken from www.folc.ca).

To understand the mineral transformations that occur when clay liner materials are subjected to sulphuric acid solutions, aluminum (Al) and silicon (Si) K-edge total electron yield, fluorescence yield (FY), and L-edge FY X-ray absorption near edge spectroscopy (XANES) analyses were conducted. These experiments were performed at the VLS-PGM beamline 11ID-2 (Al and Si L-edge) and SGM (Al K-edge) beamline 11ID-1 at the Canadian Light Source as well as the SRC-ALADDIN's CSRF-DCM (Si K-edge) beamline. XANES analysis was performed on a variety of Si and Al mineral standards as well as three natural clays to investigate the impact of H2SO4 on the short-range coordination of Al and Si between pH 1.0 to -3.0 and 14 to 365 day reaction times.

Results:

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Acidic dissolution of clays results in the measurable alteration of the short-range Al and Si order from aluminosilicate coordination. The results confirm the dissolution of aluminosilicate Al-octahedral layers at pH < 1.0 and t > 14 d. Moreover, four-fold coordinated Al persists relative to six-fold coordinated Al under increasingly acidic conditions.

Figure 2. Al K-edge XANES data for Kc liner at pH -3 from 2 weeks to 1 year. Note the gradual shift from spectral features similar to montmorillonite to those of aluminum sulfate as contact time increases.

The Si XANES results (not shown) also demonstrated the dissolution of Si-tetrahedral layers and subsequent precipitation of a higher ordered tectosilicate Si coordination at pH < 0.0 and t > 90 d. The precipitation of a secondary Al-SO4 rich phase was observed in all three clays through a qualitative comparison of known standards with the spectra of the altered clays. An overall description of how mineral dissolution and reprecipitation occurs in clay liners subjected to hyperacidic conditions could then be developed:

Figure 3. Reaction scheme for clay-H2SO4 interactions consistent with results from Si and Al XANES studies.


Important Findings


  • Si dissolution resulted in formation of a new tectosilicate structure rather than a persistence of a phyllosilicate sheet
  • Increasingly larger amounts of tetrahedral Al are present in samples as they react, which seems to be caused by formation of new Aluminum Sulfate solid phases.


Publications:
Shaw, S., Peak, D., and Hendry, J. 2008."Aluminum and Silicon K- and L-edge X-ray absorption spectroscopy of clays exposed to sulfuric acid (pH 1.0 to -3.0). " Geochim. Cosmochim. Acta (in review).