Therefore, the following five scenarios may explain
the mechanism of Cd adsorption: (I) an increase in surface area
upon calcination; (II) ion-exchange with Ca2þ or Kþ; (III) surface
precipitation; (IV) bulk precipitation at pH > 9; (VI) incorporation
of Cd into Ettringite crystal (Ca6Al2 (SO4)3(OH)12$26H2O).