Abstract: The sorption of Co and Zn by layer silicates was studied in dilute mineral suspensions containing, in most cases, about 10−6 M Co or Zn. Electrostatic adsorption was eliminated by the presence of 0.1 N CaCl2. Co and Zn were determined by a radioisotopie technique.
Detailed studies on montmorillonite and to a lesser extent vermiculite, muscovite and biotite revealed the presence of at least two forms of specifically sorbed Co, one of which was exchangeable by certain ions such as Cu, Ni, Zn, Fe, Mn or more Co and the other, occurring in much smaller amounts, was not exchangeable by these cations. The latter form is considered to result from lattice penetration; the former is associated with surface groups. These forms may be separated quantitatively by several desorption procedures including successive extractions with dilute acetic acid.
The total amount and relative proportion of these forms of sorbed Co and Zn depend on the pH of the system, time of reaction, mineral species used, and amount of Co or Zn added. Equilibrium is not readily attained but tends to approach a slow steady state after several days. Isotherms indicate a variation in bonding energy with surface coverage. The sorption of Co and Zn from dilute solutions by any mineral is related to its stability.
These studies, considered in conjunction with published data, suggest that a common mechanism may be involved in the specific sorption of many heavy metal cations by many minerals.