PHYSICO-CHEMICAL STUDIES ON THE INTERACTION OF PROTEINS AND DYES WITH CLAY MINERALS

Abstract

The process of soil formation is primarily a process of disintegration or •weathering*. In the course of geolo gical ages nearly 4$ of the igneous rocks of the eaths crust have been weathered to clays,shales and surface soUs. All these substaaces are made up of finely divided components exhbitlng properties of association and dlspersion,floceu- 1ation and peptlsatlon or of undergoing surface reactionsproperties which rightly categorise them under the well known form of matter named colloid. The four components of soil occurlng in the state of fine sub-division are minerals, organic matter,water and air. The last thirty years or so have witnessed great advancement in the field of soil science. Scientists from different disciplines have contributed to the overall development which has been jossible with the help of new research tools like X-ray,differential thermal analysis, electron microscopy,infra red spectroscopy etc. The growing economic importance of clays has also been responsible for the rapid progress in this direction. Mineral arecies. According to Fauling*,who was the first to eluci date the structure of clay minerals,each plate like clay particle consists of a stack of parallel unit layers. The principal building elements of the clay minerals nre twoO OXYCENS O HYDROXYLS £ ALUMINIUM OO SILICON FIG. I. DIAGRAMMATIC SKETCH OF THE STRUCTURE OF KAOLINITE. 0oxTs:?0roxYL#ALuM,N,uM''RoN^-- •SILICON .OCCASIONALLY ALUMINIUM FIG. II. DIAGRAMMATIC SKETCH OF THE STRUCTURE OF MONTMORILLONITE. 2 dimensional array of silicon-oxygen tetrahedra and two-dimensional arrays of aluminium or magnesiumoxygen- hydroxyl octahedra.In the sheet,three oxygen atoms are shared by the neighbouring tetrahedra and the fourth oxygen atom protruding from the tetrahedra! sheet is shared by octahedral sheet. In the octahedral sh^et Al or Mg atoms are coordinated with six oxygen atoms or oH groups which are located around the Al or Mg atom with their centres on the six corners of a regular octahedron. The symmetry and almost identical dimensions in the tetrahedral and octahedral sheets allow the sharing of oxygen atoms between these sheets. This sharing may occur between one silica and one alumina sheet,as in a two layer mineral like kaollnite or between alumina and two silica sheets giving rise to a three layer mineral like montmorillomite,illite etc. The combination of one octahedral and one or two tetra hedral sheets is called a unit layer. Within each unit layer a certain unit of structure repeats Itself in a lateral direction and is known as unit cell. Such unit layers stacked parallel to each other constitute various clay minerals, Montaorillonlte clays. (Fig.3 )These are three layer minerals wherein a tetravalent silica is replaced by trivalent Al or Al 3 in the octahedral sheet is partly replaced by divalent Mg without the third vacant position being filled.Such substitutions by elements of lower valence results in an excess of negative charge on the lattice. The adsorption of catlons,both on the interior and the exterior surfaces of the stack takes place to compensate the net negative charge on the lattice. These compensating cations may be exchanged for other cations and are known as exchange able cations of the clay. The most typical property of montmorillonites is the phenomenon of interlayer swelling with water. Water penetrates between the unit layers and pushes the* apart a distance equivalent to 1-4 monomolecular layers of water,which increases the c spacing from 10 A9 to 12.6-20 A*. AttH* These have the same basic structure as montmori llonites. The total amount of lattice substitution is largor than that for montmorillonites and is predominan tly that of Si by Al in the tetrahedral sheet. The striking feature of these clays is that the compensating cations are mainly potassium ions. These minerals do not show interlayer swelling which Is attributed to the strong electrostatic attraction between the potassium ions and the two charged unit layers on each side. In the absence of interlayer swelling these cations are not available for exchange. It is a two layer mineral consisting of sheet units ( of the type already described) continuous in the a and b directions and stacked one above the other in the c direction. The exchangeable ions are quite low and are situated on the broken edges of the kaolinite plates where they would compensate charge deficiencies owing to broken bonds. The charge defi ciency owing to isomorphous substitution is absent in this case. Phenomenon of ion-exchange. The clay minerals have the property of sorbing certain cations and anions and retaining them in an exchangeable position. These ions are held around the outside of the silica-alumina clay-mineral structural unit,and the exchange reaction generally does not affect the structure of the silica-alumina packet.The exchange reaction is stoichiometric and the exchange capacity is measured in terms of miliiequivaients per 100 gm. of clay. This property is of fundamental importance in the study of clay minerals. In agricultural solis,plant foods are frequently 5 held in the soils as exchangeable lons,and consequent ly their persistence in the soil and their availability for plant growth depends on exchange reactions.The r tention and availability of potash added In fertili sers depends on cation exchange between the potassium salt and the clay mineral of the soil. The replacement of the :*a* by another ion,usualiy Ca+*will make the soil more suitable for agriculture. Similarly in the field of geology,during weath ering process the liberation of alkalies may or may not be retained in the secondary material depending on exchange reactions. In ceramics the plastic properties of clay can be adjusted according to needs,by changing the exchange able cations. There are a number of factors on which the exchange adsorption normally dependsi (i) valency of the ion,(11) hydration of the ion,(iii) ionic radii and (iv) structural configuration of the clay micelle. The valence and hydration of ions are the most important factors in determining the energy of adsorption and release. Ion exchange will be difficult when the adsorbed ions have got a higher valency or are weakly hydrated. It Is so,because ions of higher valency 6 are adsorbed more strongly than those of the lower valency and weakly hydrated ions are more tightly bound up than those containing a large water hull, feigner and Jenny8•• and Alten and Kurmies4 have overemphasised the influence of hydration. Hendricks® and his colleag ues,after careful dehydration studies have shown that Na*fH* and S!4" are not hydrated when adsorbed by clay minerals while Ca++,Mg"M' and LI"*" undergo hydration. Another factor,which controls exchange phenomenon, is the size of the exchanging ion. It is normally observed,that ions of smaller ionic radii are easily replaced by ions of larger Ionic radii and vice versa. Apart from this the nature of the clay surface, according to Gleseking and Jenny9,Jerusov* etc.,guides the energy with which a given ion is held. Basically there are three causes of the cation exchange phenomenons 1. Substitution within the lattice structure of silicon and aluminium by ions of lover valence results In unbalanced charges in the structural units of some clay minerals. 2. Broken bonds around the edges of the silica-alumina units would give rise to unsatisfied charges,which would be balanced by adsorbed cations. 3. The hydrogen of the exposed hydroxyls may be replaced 7 by a cation which would be exchangeable. Some oH groups would be exposed around the broken edges of all the clay minerals,and cation exchange due to these would be through the replacement of hydrogen.