The pH of aqueous bentonite suspensions

Abstract

The pH of aqueous bentonite suspensions is known to be influenced by carbonates present even in minor amounts. On the other hand, at high solid:liquid ratios (at standard pH measurement conditions: 2% w/w suspension), the type of exchangeable cation in the smectite is also known to determine pH (particularly Na⁺ or Ca²⁺). By cation-exchange tests we proved that exchanging the Ca²⁺ for Na⁺ results in an increase in the pH. However, this increase in pH was only found if excess salts were removed from the system (by washing or dialysis, respectively). The effect of the type of exchangeable cation can, at least partially, be explained by hydrolysis of Ca²⁺. On the other hand, a pronounced alkalinity of Na bentonites is observed which can, at least partially, be attributed to the hydrolysis of montmorillonite (Na⁺ is exchanged for H⁺ of water). The increase in the volume of the Stern layer, caused by increasing the degree of delamination, is also suggested to play a role. H⁺ and Na⁺ are concentrated in the Stern layer. Hence, increasing the Stern layer volume decreases the amount of H⁺ and Na⁺ in solution and thus increases pH. Unfortunately, both processes, montmorillonite hydrolysis and delamination, depend on the ionic strength. Distinguishing the processes quantitatively, therefore, is an analytical challenge, and impossible based on the data presented here.

To model the pore-water chemistry of clays and clay stones, all of the above-mentioned processes have to be considered. It is possible that other reactions, not identified in the present work, contribute toward the pH values of aqueous bentonite suspensions.