Dear Mr. Jamshidi,

the core of your question is rather obscure and needs some clarification. You deal with two clay mineral groups, the kaolinite-group minerals which are very different as to the Al/Si disorder and rather low in their cation absorption capacity (CEC). The other clay minerals of the smectite-group are very sensitive to redox changes as far as the Fe-bearing varieties are concerned which may accomodate trivalent Fe such as nontronite or bivalent iron plus Mg ,e,g., saponite This is the part where bivalent Ni comes in. Some members such as ferrosaponite, hectorite, sauconite, spadaite, stevensite, swinefordit and zinksilite may show the complexity of this group. In other words it is not only the considerable difference in the CEC between kaolinite- and smectite-group minerals which is very high and are controlled by the pH but also the Eh, which has, however, only implications on the composition of smectite. Concerning the heavy metals in these clay minerals only Ni, Zn, Cu and Cr play a part, substituting for either bivalent Mg and / or Fe or trivalent Fe, resepctively. Pb has no say in the built-up of these clay minerals. You will have to conduct a precise clay mineralogical investigation so as to do not run into trouble or get your soil system to become a matter of conjecture.

With kind regards

H.G.Dill

Well,

Low cationic exchange in Kaolinite than Smectite or/ and bentonite.

Regards

Dear Mr. Jamshidi,

Crystalline structure of the smectite (i.e. montmorillonite) (Na, Ca)0.33(Al, Mg)2(Si4O10)(OH)2·nH2O is relatively weak and contains isomorphic substitutions of Mg, & Fe for some Al in the octahedral sheet of smectites and are more common than in the crystalline structure of a 1:1 type of mineral such as kaolinite. Smectites are a 2:1 type of phyllosilicate mineral, which means that there is 1 Al-octahedral sheet sandwiched between 2 Si-tetrahedral sheets. The layers in smectite crystals are loosely held together by cation-to-oxygen bonds. Hence, water molecules and various ions are attracted between layers (into the interlayer space) causing expansion of the crystal. The effective surface of montmorillonite includes both internal and external surface area. Kaolinite with a chemical composition of Al2Si2O5(OH)4, is a 1:1 phyllosilicate mineral where in a layer of a kaolinite crystal there is 1 Al-octahedral sheet bonded to 1 Si-tetrahedral sheet by shared (apical) oxygen atoms of the tetrahedral sheet. Two adjacent layers are held together by hydrogen bonding. Consequently, the structure is fixed, and no expansion occurs between layers when the clay is wetted. The effective surface of kaolinite is restricted to its external surface area. Because of the strong binding forces between their structural layers, crystalline structure of kaolinite is quite stable and as such there are limited numbers of isomorphic substitutions in the crystalline structure of kaolinite. As a result, this leads to a low CEC for kaolinite as compared to smectites.

When you add contaminates such as lead, zinc and copper metals to kaolinite and smectite the Al and Si sites act as lewis bases (i.e. (OH)- sites) and facilitate sorption of the minerals and because both kaolinite and smectites are abundant in Al and Si there is no decrease in soils adsorption with increasing concentration of these metals. However, when you add nickle as the contaminant nickle exhibits a higher affinity to univalent competitive cations (K & Na) more commonly found in the smectite inter layers. As a result, there would be an increase in the amount of soil adsorption with increasing concentrations of these metals by smectite as compared to kaolinite.

Read;

Hizal and Apak, 2006, Modeling of Cu(II) and Pb(II) adsorption on kaolinite-based clay minerals individually and in the presence of humic acid, J. Colloid Inter Sci, 295, pp 1-13.

Rosskopfovas et al., 2015, Adsorption of Ni on illite/smectite Bolna Ves, J. Radioanal Nuel Chem, 307, pp 179-185

Regards,

Michael E. Bishop

Hello Thanks for the help of all my friends.