Stability of Ca2+, Cd2+, and Cu2+-illite complexes

In this study, using ion selective electrode techniques, we investigated the influence of pH on metal-ion, Ca2+, Cd2+, and Cu2+, adsorption by Fithian illite. The results showed that Fithian illite exhibited at least two types of metal-ion adsorption sites, high and low strength with the strength of metal-ion-illite surface complexes following the order of Cu2+ > Cd2+ > Ca2+ (strongest to weakest acids) at any of the pH values tested. The ability of metal-ions to form complexes with illite surfaces was affected by type of metal-ions and pH, especially for low metal-ion affinity sites. These sites formed stronger metal-ion complexes at high pH than at low pH, which implicated clay edge sites and indicated that H+ competed with metal-ions for available complexation sites. The data also showed that illite functional groups forming the strongest metal-ion complex did not appear to be pH-sensitive, which implicated wedge siloxane cavities or extremely low pKa clay-edge OH functional groups, but the total number of such sites were very small. The magnitude of the metal-ion-illite stability constants, as metal-ion solution concentration approached zero, on a log-scale, varied from 3.52 to 4.21 for Ca2+, 4.38 to 5.18 for Cd2+, and from 5.23 to 5.83 for Cu2+. These constants were approximately an order of magnitude smaller than those representing illite with sorbed humic fractions. The above results along with the results from our previous studies imply that metal-ion mobility and bioavailability would be affected by soil mineral surface properties, which would be significantly influenced by sorption of humic substances.