Influence of pH on the aluminum speciation in freeze-dried poly(hydroxo aluminum) intercalated bentonites

Two freeze-dried poly(hydroxo aluminum) intercalated Wyoming bentonites were prepared starting from two different aluminum pillaring solutions. Their charge versus pH properties were studied by zeta potential measurements, potentiometric titrations and calcium sorption measurements in order to determine the speciation of the intercalated Al-species as a function of pH. The structural changes of the intercalated bentonite induced by the pH increase were studied by XRD analysis and N₂ adsorption/desorption measurements. At pH 5 the permanent charge (0.75 meq/g) of the original Wyoming bentonite is completely neutralized by positively charged Al-hydroxo species. With increasing pH the Al-species deprotonate and aggregate until their complete neutralization at pH 9.4. Above this pH the Al-polymers start to dissolve from the clay as Al(OH)₄⁻.