Experimental determination of the porewater compositions in compacted bentonite

Abstract

  We have been developping techniques for measuring the chemical compositions of porewaters in compacted bentonite as a function of time. The present emphasis of this development effort is on pH measurements, which were carried out in the present study using low-decolorant pH test papers and high-absorbancy pads embedded in compacted bentonite (Kunigel-V1®=100%) having dry densitie of 1.6g/cm³. The experiments were conducted in a controlled-atmosphere glove box (O₂ (g) ≦1 ppm) using distilled water, synthesized seawater and a low-alkaline cement porewater (i.e., HFSC: High Flyash contained Silica fume Cement) as initial solutions. The pH of the distilled water and synthesized seawater was adjusted to pH 9 by adding NaOH solution. Distilled water was reacted with crushed HFSC to produce a solution representing low-alkaline cement porewater (pH≒11).
  The measured pH profiles based on this technique vary temporally and spatially. The variation of pH profiles depends on bentonite density and initial solution chemistry. In compacted bentonite contacted with distilled water, most of the profile did not vary significantly (pH=8.0 to 9.0) during the experiment, but the pH of porewaters near the infiltration interface slightly decreased (pH=7.5～8.5 → pH=7.0～8.0). In the experiment using synthesized seawater, the pH values were roughly constant at the interface between 6.5 and 7.0, but the pH of porewaters further away from the interface slightly decreased with time (pH=6.5～7.0 → pH=6.0～6.5). Like the experiment with involving distilled water, the pH of porewaters contacted with HFSC solution further away from the interface were roughly constant between 8.5 and 10.0, but the pH of porewaters near the infiltration surface significantly decreased with time from pH=9.5 to pH 7.5～8.5.
  The pH values measured are reasonably consistent with values measured by an alternative technique using pH-indicator with spectrophotometer, and are also consistent with the results of mineral-water equilibrium modeling. This favorable comparison suggests that this technique can be used with confidence to obtain accurate measurements of the pH of bentonite porewaters.
  Preliminary thermodynamic calculation using geochemical code PHREEQC suggests that the pH decrease near the infiltration interface observed in experiments using distilled water can be due to partial oxidation of trace amounts of pyrite in Kunigel-V1®.