Experimental and modelling studies on the pH buffering of MX-80 bentonite porewater

Almost invariably the compositions of porewaters given for highly compacted bentonite are calculated values because reliable water samples from compacted material are virtually impossible to obtain, even by squeezing under very high pressures. Assumptions and simplifications are made in the geochemical models used to perform such calculations and the predictions are seldom if ever tested. One of the main hypotheses in a recently proposed model for calculating the porewater in compacted bentonite was that the initial pH is determined by the state of the amphoteric surface hydroxyl groups, triple bond; length of mdashSOH type sites, on the montmorillonite component. The calculations indicated that the initial pH of the porewater is a value close to 8. The aims of the work reported here are to check the predictions of the model in terms of the pH, which is one of the most important parameters for any porewater, and the strong buffering effect of the triple bond; length of mdashSOH type sites. The concept behind the tests was to take a powdered bentonite and a background electrolyte (0.2 M CaCl2) containing known quantities of acid or base (pH values between 1.5 and 12), mix them together at a high solid to liquid ratio (312 g L−1) and then allow them to react in the absence of air in a closed system. If the model concepts and parameter values are valid, then the model should be able to predict the pH measured in such experiments. The model calculations agreed well with the measured pH values and confirmed that it is highly likely that the triple bond; length of mdashSOH type sites determine the initial pH of the porewater in compacted bentonite systems. Further, these amphoteric surface hydroxyl groups at the edges of montmorillonite provide an extremely high buffer capacity in the compacted system for maintaining the bentonite porewater pH at a value of not, vert, similar8.