Hydrotalcite (HT) is a unique Mg-Al hydroxide mineral having excess surface positive charge. Because of its high selectivity for carbonate ion, the positive surface charge is mostly neutralized by adsorbing carbonate ion under CO2containing atmosphere. Since the adsorbed carbonate is essentially exchangeable, anion exchange reactions with other anions, e.g. nitrate, Y2CO3+2NO3-= 2YNO3+ CO3 2(1) take place depending on the concentrations of nitrate and carbonate in the milieu. In Eq. 1, Y is a fraction of HT carrying a unit positive charge. If the coexisting cation is calcium the reaction in Eq. 1 will advance to a larger extent because of the elimination of released carbonate as calcium carbonate and, as a result, the salt concentration in a bathing solution will decrease.
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