Bentonite surface charge

Changing of the surface charge on bentonite due to changing groundwater composition thus effecting its adsorption and agglomeration properties. 

The parameters obtained by others for SWy-2, BSAB, and MX-80 cannot be compared to the previously discussed data because the silanol and aluminol sites as well as the deprotonation processes (Equations 2.4 and 2.5) were treated together. Calcium bentonite (Istenmezeje) shows similar intrinsic stability constant for SWy-1 bentonite, but the number of edge sites is different. Note, however, that the specific external surface areas are also very different 21.4 m2/g for SWy-1, and 93.5 m2/g for Istenmezeje montmorillonite (Table 2.1). The ratio of the specific surface area (Istenmezeje/SWy-1) is 4.4, and the ratio of the total number of edge sites (silanol + aluminol) is 5.3, which are in fairly good agreement if the surface charge density is the same. 

Reference [766] reports selected parameters of a model of surface charging of MX-80 bentonite. 

The application of ion exchangers to dextrose process liquors involved considerable experimental work because of a number of factors which do not enter into their application to water purification. The accumulation of fats and proteins on the resin surfaces must be guarded against by proper clarification of the liquors to be treated. Such accumulation may result from precipitation as the neutralization progresses, and may soon destroy the effective acid-removing capacity of the anion exchange resin. This difficulty can effectively be eliminated by prior precipitation of thfe refinery residue from the acid liquor by bentonite, a colloidal clay of opposite electrical charge to the colloids,21 followed by filtration. 

The sorption of the anionic Tc (as TcOi " in an aerated system) and I (as I") is low, both on granite and bentonite, which is expected since the active surfaces and sites of the sorbents would be negatively charged. 

Similarly, the specific surface area and CEC of B-II.b. lower sample (Table 3.2) are below that expected from the montmorillonite content even if a greater uncertainty in montmorillonite content determination ( 20%) is taken into account. Similarly, low specific surface areas are observed for some other bentonite rocks (e.g., SWy-1, SWy-2, and MX-80 Table 3.2). In the case of SWy-2, the CEC is low as well. This bentonite consists of large particles with small layer charge. 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

The second mechanism, first proposed by van Olphen (8,11-14), assumed structure formation in bentonite gel to be due to edge-to-flat surface asssociation of the plate-like particles as a result of electrostatic attraction between the oppositely charged double layers at the surface. This so-called "house of cards structure" is likely to occur provided the pH of the suspension is below the isoelectric point of the edges, which are then positive and become attracted to the negatively charged faces. 

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