STUDIES ON THE INTERACTION OF SOLOCHROME MORDANT DYES WITH CLAY MINERALS

Abstract

Soil mineralogy began with the study of sands separated from whole soils by sieving or sedimentation in water. As is clear from the fascinating historical account given by Steinriede(l) much of the early work sought to explain ecological and fertility differences through variations in the proportions of minerals found in the sands. Long before the nature of the soil clays was established, Steinriede had a clear concept of the joint importance of specific surface and mineralogical constitution as they enter into soil formation processes and the delivery of nutrients to plants. Surprisingly enough in this appraisal he was greatly in advance of his time. Over forty years had to pass before his broad objectives came within sight of experimental realization. The last thirty years or so have witnessed great advancement in the field of soil science, particularly because of the better understanding of the structure of clay minerals by modern physical methods such as X-ray differaction, thermal analysis, electron microscopy etc. Although the organic constituents of soils remain less characterised even now, it is hoped that the tempo of research in this direction would result in its resolution soon. The crystal structure and charge distribution of minerals; Clay minerals are composed of small plate-like particles ranging in diameter from a few hundredths of a micron to several microns. Making up each such platelet are one or more unit layers, stacked like a deck of cards; the structure of unit layers consists of sheets of tetrahedrally coordinated silica in combination with sheets of octahedral alumina or magnesia. Three layer clays such as montmorillonite and illite have a unit layer consisting of a sheet of octahedral alumina or magnesia sandwiched between two sheets of tetrahedral silica(2> 3). Aschematic representation of the structure of the prototype silica-alumina mineral, pyrophyllite, is shown in Fig. 1, an example of a clay mineral with similar structure is montmorillonite. This structure is termed dioctahedral, as only two out of every three octahedral sites are occupied. In contrast, illites and the montmorillonoids, vermiculite and hectorite are trioctahedral; the central sheet of the unit layer is predominantly magnesia, and ideally all octahedral sites are occupied. The prototype trioctahedral mineral is mica. The exposed basal faces of these threelayer clays consist of siloxane, that is, Si-O-Si. In a two layer clay, e.g., kaolin, the unit layer is composed of one sheet of silica and a sheet of alumina, the basal faces of these clays are thus half uiloxane and half hydroxylated alumina. Electrokinetic studies reveal that the clay particles carry a net negative charge which is compensated by the presence of positive counterions. The origin of this charge on the clay lattice is believed to be isomorphic substitution in the lattice, broken bonds at the edges of the particles and exposed structural hydroxyls. The main source of the observed negative charge on the clay ra u (2a) particle is isomorphic substitution according to van -Iphen which implies areplacement of metal ions of the lattice by cations of lower valence. In a clay with high cation exchange capacity e.g. montmorillonite, the chemical analysis reveals a substitution of the o D A _ Si _ 0 _ Oh - At Crystal structure of prototype of three -layer stlicaxtlumwa clay (schematic) . FIG.1 3 order of 30 - 100 me,/l00g of clay and this is in the central alumina sheet(2b>. fa aclay with low cation exchange capacity e.g., kaolinite, the isomorphic substitution does occur to asmaU extent and is difficult to be measured accurately. ft has been suggested that the broken bonds at the edges of the particles may be in part responsible for the negative charge on the lattice. However, there is astrong evidence to indicate that the edges of clay ^ particles are positively charged in the neutral and acid pH ranges 2c' . These positive charges are believed to arise from reactions of the type. , H,0, H1" + -Al-O-Al »(-Al-0-+ - Al+) -^ »2-Al-OH2 The negative charge on the clay particles is compensated by the adsorption of cations, fa days which sweU in the presence of water the counterions are held on the external surfaces of the aggregates and between the unit layers, whereas in nonswelling clays,the counterions are sorbed on to the external surfaces only. In aqueous suspension, some of these cations may remain in aclosely held Stem layer; others diffuse away from the surface and form adiffuse double layer. If these cations are not fixed up irreversibly by certain strong and specific bonding with clay, they can undergo ion-exchange with other cations present in the system. The magnitude of the cation exchange capacity of clay depends largely on the type of clay and to alesser extent on the source of particular sample. Tne exchange capacity of various montmormonites is o, the order of 85 - 160 mc/iOOg clay while that of kaolinite is 3to 10 meq/lOOg clay. Aithough the layer surface is the largest part of the particle surface, the relatively small surface of the e<iges of the clay plates 4 should also be considered. As emphasised earlier, at the edges the siUca and alumina sheets are broken and the situation is analogous to that at the surface of silica and alumina sol particles and apositive double layer with anions as counterions are created. This also accounts for the anion exchange capacity<M> of each clay mineral althougn it is much smaller in comparison to cation exchange capacity. The electrical double layer: The application of the diffuse double layer theory to describe the ionic distribution in the vicinity of clay particle has met with mixed success. Eriksson'7' calculated the distributions of sodium and calcium counterions competing at the clay-water interface and concluded that diffuse double layer theory can satisfactorily account for the physical situation. However, Bolt<8> and Ottewill<9> tound it necessary to assume the adsorption of counterions in the Stern layer in order to interpret their results. This was also supported by Weiss et. al.(10) who noted that the extent of the diffuse double layer at the clay-solution interface^^ depended upon the affinity of the cation for the clay. Geuze and Retail ' showed that the double layer theory qualitatively explains edge-to-face flocculation, although the agreement with the experiments may be somewhat fortuitous considering the relative simplicity of their model. Recently the coagulation and peptization of montmorUlonite-type clays has been treated in terms of DLVO theory by using the platelet model and assuming an asymmetrical charge d,.ist,ribvut.i1on(12,13) interaction between inorganic cations and anions withcUyJurface: ine factors affect the interactions between inorganic The following cations and clay surface: i) The unhydrated radius ii) The charge iii) The hydration energy iv) The specific interactions e.g., H*" and NH4 . It has been stated that larger are the first two parameters and the lower is the third, the stronger are the interactions. Also the exchange ability increases with decreasing hydrated radius, increasing polarizability and increasing counterion charge. In agreement with the above observa tions, the order of increasing preference of alkali ions for ion exchange on montmorillonite*14' l5), vermiculite*^ and kaolinite^) , is reported to be Li+< Na+< K+< Rb+< Cs+. The exchange of ammonium ion is complicated by the physical adsorption of ammonia and the fixation of ammonium ion(I9) (fs„p~e^c4i«firc. iinntteerraaccttiioonnss);.. Another observed »i*> Na+/ NH+/ K+ (20), although there are cases in order is Li < Na <^ inw4 <^ *v j + which NH4+ seems to be more strongly held than K. The exchange of H+ is difficult to measure, because it attacks the clay lattice, freeing aluminium and magnesium ions which may be taken up by the exchange sites and consequently some of the exchange attributed to H+ could be caused by metal ions dissolved from the clay<21)- W The order of exchange of bivalent ions on clays is m2+< Mn2V Ca2V Sr2+< Ba2+. The reverse order is N (23) sometimes found in the case of vermiculite Several investigations deal with the exchangeability of divalent transition metal cations on clays, for which orders .+ 6 Mn2+^ Ni2+< Co2+< Zn2+ <Cu2+ Mand Z„2+< Mn2^ Ni2V Co2+ / Cu2+ (25) have been reported. Studies comparing the exchange of mono, di and trivalent cations on clay have shown in principle, a preference for cations of higher charge. However, this trend is not always followed, especially when a strongly held monovalent ion is compared with arelatively weakly held divalent ion. The surface charge properties of clays play a special role in their interactions with anions. The strong negative charge on the faces repels them, whereas the weak positive charge on edges attracts anions. In addition, specific bonding with cations of the clay surface may also occur. The nature of charge distribution on clay explains why, under various conditions, both negative and positive adsorption of anions has been reported. Bolt and Warkentin(26) measured the negative adsorption of chloride ions by montmorillonite, but the amount found was lower than predicted by the double layer theory. This was explained by the neglect of making corrections for activities and for positive adsorption on edges. Positive adsorption of chloride on clay was found to take place at Phosphate ions have aparticular strong effect on the properties of clay dispersions. This involves electrokinetic, stability and rheological phenomena. Lyons(28) reported aLangmuir-type isotherm on the adsorption of triphosphate on kaolinite. Adirect correlation between I the point of maximum uptake of the anion and the amount of triphosphate required for full deflocculation of the sol by neutralization of edge charge was observed. (29) Studies of anion exchange cited by Wayman are in good agreement with the work of Bingham et. al. ' .He reports the preference for anion exchange on clays as CNS < I < NOg <Br"^ CHgCOO" < Cl" < H2P04"< OH" < F . Deflocculation of clays can be accomplished by a variety of anionic species when these adsorb in sufficient amounts to neutralize the edge charge e.g., hexadecyl sulphate at pH-3 renders kaolinite particles more negative(31) and causes deflocculation. Interaction with organic molecules: Association of an organic molecule with a clay particle may take place in several ways. The molecule may be adsorbed on the clay lattice by ion-dipole forces, by vander Waal's forces or by hydrogen bonding. It may also complex with a counterion of the clay or if it is ionized, it may undergo cation or anion exchange with the original counterions. Dowdy and Mortland(32) studied the adsorption of ethanol and ethylene glycol on montmorillonite and suggested the conditions under which H-bond is formed. Brindley and Ray1 studied the increase in the basal spacings of calcium montmorillonite in the presence of straight chain alcohols with 2to 18 Catoms. They reported that mono- and . bilayers are formed between the unit layers of the clay with the chain of the alcohol parallel to the surface of the clay.