Exploiting the ion-exchange ability of titanate nanotubes in a model water softening process

Titanate nanotubes were utilized in Ca2+ and Mg2+ removal in a continuous ion-exchange unit. Three consecutive water softening–regeneration cycles were performed. The highest measured value of the total ion-exchange capacity was 1.2 mmol g−1 which decreased to 0.66 mmol g−1 in the third cycle. The capacity loss was due to the irreversible binding of Ca2+ ions to very strong adsorption sites, while the Mg2+/Na+ exchange was reversible. A relatively fast initial adsorption step and a subsequent slower concentration decrease were the two processes that governed the kinetics of the ion exchange reaction.