Table of Contents
Adsorption measurements on all the solids were carried out by the methods described above. Figure 2 shows a typical isotherm for the adsorption of polymer A on coal. The adsorption density rises monotonically until an apparent plateau is reached. For the case shown in Figure 2, the plateau occurs at an adsorption density of about 10 -4 mg/cm² which corresponds to a “parking area” of about 8×10 5 A² per molecule. If we use the radius of gyration calculated from the viscosity data to estimate the cross-sectional area of the molecule in solution, we obtaine a value, for this system, of about 1.25×10 7 A°². Thus, it appears that the adsorbed polymer molecule occupies considerably less area on the surface than in solution. Three possible explanations can be offered for this apparent discrepancy:
- Multiple adsorbed layers are formed. This seems unlikely for the case of soluble polymer flocculants.
- Significant changes in molecular configuration accompany the adsorption process.
- The use of Equation 1 to estimate the radius of gyration from viscosity data is inappropriate for these systems and leads to a gross over-estimation.
Realistically, we might expect some combination of (ii) and (iii) to apply to these systems. It is interesting to note that if we calculate the theoretical radius of gyration for such a molecule in the form of a true random coil, the calculated value is significantly less than that obtained from viscosity data and corresponds roughly to the observed “parking area”. This result is probably fortuituous, however, since it is unlikely that polyacrylamides form random coils in aqueous solution.
Effect of Surface Area
The total surface area effect was determined by carrying out adsorption measurements on coal suspensions of different pulp densities and of varying specific surface areas. The results show that the amount of polymer adsorbed is directly proportional to the pulp density and when the isotherms are plotted on a unit area basis (Figure 2) they all fall on one curve.
The specific surface area effect is shown in Figure 3 which exhibits a direct proportionality between the adsorption density and the specific surface area. These results imply that particle size and pulp density are important factors in polymer adsorption although they also show that the mechanism for polymer adsorption is independent (at least within the range studied) of these two variables.
Effect of Solid Adsorbate
The results of polymer adsorption measurements on coal, clay and quartz indicate that the adsorption capacities and affinities tor these solids are not identical. Figure 4 shows that polymer C has a stronger affinity for coal than for the ocher two minerals. It should be noted that, due to the use of different methods for determining the surface area of these solids direct comparison of the adsorption densities may be misleading. Nevertheless, it is clear that there are significant differences between the three systems. The differences are even larger at low concentrations. The concentration at which saturation is approached seems to be lower for clay and quartz than for coal. This may reflect differences in the adsorption mechanisms. Hydrophobic bonding may be playing a significant role in the adsorption on coal while it would be non-existent in the case of polar solids such as quartz and clay.
Effect of pH and Ionic Strength
The results of the polymer characterization studies indicate that the hydrodynamic volumes of polymer vary with pH and ionic strength and suggest that the polymer adsorption characteristics might undergo some kind of changes with these variables. Consequently, particular attention was paid to the effects of pH and ionic strength on the adsorption process. Some typical results for the adsorption of polymer A on coal and polymer B on quartz at different pH’s and ionic strengths are shown in Figures 5-7. The principal effect seems to be on the adsorption capacity of the solid surfaces. The response of coal and quartz suspensions to changes in pH appears to be very similar. At normal pH the maximum capacity of both minerals is smaller than at acidic pH. For example, when the pH is between 6.5 and 10.3 maximum adsorption of polymer A on coal is around 1×10 -4 mg/cm² while at pH 3.3 or in 1M NaCl, the maximum capacity is well over 4×10 -4 mg/cm². Similar results were obtained for the adsorption of polymer C on coal and quartz.
The observed pH and ionic strength effects can be attributed to changes in the polymer configuration in response to changes in the environment (see Figure 1). That this is the case can be easily observed when the maximum adsorption (extrapolated from the Langmuir model) is qualitatively correlated with the radius of gyration, Rg (calculated from Equation 1). When changes of Rg with pH are small (Figure 8) the maximum capacity undergoes very little variation, while large changes in the maximum adsorption capacity are observed for polymer A (Figure 9) the apparent radius of gyration of which is markedly influenced by pH and ionic strength. These correlations can be explained in terms of the configuration of the polymer molecules in solution. At low pH, where the polymer is essentially in the anionic form, the neutral molecules are tightly coiled. At high pH the dissociation of carboxylic groups tends to give the molecule a negative charge. This leads to uncoiling of the molecule due to electrostatic repulsion between the charged groups and brings about an increase in the molecular hydrodynamic volume. Naturally, the larger the hydrodynamic volume the more space each molecule will occupy on the solid surface. Hence, low adsorption capacities are observed at high pH’s and larger values at low pH’s.
The adsorption behavior of clay suspensions was also investigated with respect to the pH and Ionic strength and it was found chat their response to these variables is markedly different. In fact the general trends seen for coal and quartz appear to be reversed for this system. For example, the maximum adsorption of polymer A is four tines higher at pH 10.3 than at acidic pH. This effect cannot be attributed to changes in the configuration of the polymer molecules. The crystallographic structure of kaolinite provides a good explanation for the clay system behavior. In particular, the ability of the 2- layer aluminosilicate crystal to carry two opposite charges simultaneously can bring about mutual coagulation at certain pH’s. Thus at acidic pH, the edges and the faces of the clay crystals are oppositely charged resulting in mutual coagulation of the clay. This tends to decrease the apparent adsorption capacity since the effective surface area is reduced. Around neutral pH the negatively charged face and the slightly negative edge of the clay crystal stabilize the suspension somewhat thereby freeing more surface for adsorption. This trend will become more pronounced when better dispersion is present at higher pH. Since polymer adsorption is directly proportional to surface area, as was demonstrated previously, adsorption capacity must therefore increase with pH. Similar arguments can be used to explain the effect of ionic strength on adsorption.
Unlike the coal and quartz systems, no correlation could be found between the molecular dimensions of the polymer and the maximum adsorption on clay. In fact the results show (Figure 10) that the maximum adsorption rises with molecular dimensions. It is clear then that the polymer configuration is not the controlling factor in the adsorption characteristics. In support of this hypothesis, adsorption measurements were carried out in which the order of addition of the pH and ionic strength modifiers was changed. These measurements were made by making adjustment through the polymer solution rather man through the suspension. Figure 11 shows that when the pH modifier is added to the polymer solution the adsorption isotherm obtained is very close to that at neutral pH. The slight difference observed in the adsorption is probably due to improved dispersion of the clay particles as the pH is increased during the polymer addition.
Effect of Molecular Weight
The effect of the molecular weight on adsorption was investigated by adsorbing polymers A, B and C on coal and clay at normal pH. The results plotted on a molar basis in Figures 12 and 13 show that adsorption decreases with increasing molecular weight except for polymer A which is partially hydrolyzed as was discussed previously. Because of this chemical difference, it is questionable to try to evaluate the molecular weight effect using these three polymers. Nevertheless, it is clear that the adsorption on both clay and coal is greater for the lower molecular weight polymers. These results seem to indicate a definite molecular size effect: the larger the molecule, the more surface sites it can interact with. Hence, for a fixed number of sites, more molecules of polymer B can be accommodated on the surface than polymer C as shown in Figure 13.
