Surface Charge and Electrophoretic Properties of Colloids Obtained From Homoionic and Natural Bentonite
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functional groups (SOH) at the solid surface according to the following equilibria: SOH+H
K.---,-•
SOH
SO- + H÷
K
SOH 2÷
E.A E.2
The main contribution to the clay mineral charge is permanent (intrinsic) and it is represented by an excess of negative charge due to isomorphic substitutions within their sheet structure. This negative charge is usually compensated by the adsorption of cations in the layer surface that, in presence of water, can be easily exchanged with other cations present in solution. The total amount of these cations is called the cation exchange capacity (CEC) of the material and represents the degree of charge substitution and therefore the magnitude of the intrinsic charge. [2] However, at the edge of the particles surface functional groups similar to those present on the surface of an alumina/silica particle exist. Due to these surface functional groups, an additional electrical double layer can be formed at these edges by the adsorption/desorption of protons. The properties of these surface sites have been found to be similar to those of oxi-hydroxides surfaces [3] [4], therefore they are supposed to follow the equilibria expressed by equations E. l and E.2. The aim of this work is to study the surface chemical properties of colloids obtained from purified and homoionised FEBEX[5] bentonite, to determine the pKs of the E.1 and E.2 255 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
reactions, and to calculate the surface potential at different salt concentration, considering the contributions due to both edge and structural charge. This parameter is of importance in colloidal stability predictions and in surface complexation models. The pH - dependent surface charge, both in oxy - hydroxides [6] and in clay [7] [8] is usually determined by potentiometric titrations and different methods can be used, trying to optimise the technique depending on the material to be studied [9]. The proton adsorption vs. pH behaviour was therefore studied using different acid/base titration techniques, namely 'fast" and "batchback" titrations in order to analyse the difference that can arise from the two different experimental approaches. The effects of the salt impurities were studied using as - received bentonite. MATERIALS AND METHODS FEBEX bentonite is a clay with a smectite content greater than 90% (93±2%), with quartz (2±1%), plagioclase (3±1%), cristobalite (2±1%), potassic feldspar, calcite and trydimite as accessory minerals. More details about this clay can be found elsewhere.[ 10]
Bentonite colloids from the as-received material were prepared by putting in contact, for a week, the clay sieved at a fraction < 64 gtm and water in an appropriate solid to liquid ratio. For electrokinetics measurements a suspension with a solid to liquid ratio 1:20 was used. The suspension was then centrifuged for 20 min at 7000 rpm and the supernatant, which contained the colloidal fraction, finally collected. A solid to liquid ratio of 1:1000 was used in titration experiments and the colloid
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