Chemicals: Reagent grade H2SO4 and KOH were used for pH adjustment. Aluminum sulfate was used for coagulation and zeta potential studies. Percol 368 a solid grade highly cation flocculant by Allied Colloids Inc. was used for flocculation and zeta potential measurements.
Methods: Zeta potential measurements of the slurry as received and/or after treatment with different chemicals, were conducted using Laser Zee meter by PenKem model 500. The turbidity of the slurry or the supernatant from treated samples was measured by measuring light transmitted.
After conditioning (by agitation for 10 minutes) of the received slurry in presence of desired reagent, the slurry was left standing still for another 10 minutes or as indicated. The supernatant was used for turbidity measurements and the residue was used for zeta potential determinations.
The zeta potential of the suspension as a function of pH is given in Figure 1. Values obtained in presence of Aluminum Sulfate are also shown in the same figure. Turbidity values are reported in the top portion of Figure 1. The data suggest that the solids are negatively charged all over the pH range with a decrease in the negative charge in the neutral pH range. The corresponding turbidity values indicate lower turbidity in the neutral solutions probably as a result of attractive Van der Vaal forces dominating in this region in absence of strong electrostatic repulsive forces. The dispersion of the slurry (high turbidity) in the high and low pH ranges, however, can be attributed to the strong electrostatic repulsion between highly charged particles in these pH ranges. Addition of Aluminum Sulfate reduces both the charge and the pH as shown in the same figure.
The zeta potential values can be explained if the solid material (clay) particle is visualized as shown in Figure 2. In neutral solutions the net charge could be neutral as a result of the negatively charged surface is balanced by the values of the positive charges on the edges. In alkaline solutions the OH- groups are adsorbing on the edges leaving the particle with a net negative charge. In Acidic solutions (H2SO4) both hydrogen cations and the sulfate anions adsorb on the face and the edge respectively. However, due to the double negative charge on the sulfate anions the net charge on the particle is negative as shown in Figure 1. Addition of aluminum sulfate reduces the charge due to adsorption of sulfate species on the edges and hydroxy aluminum species (AlOH++) of high positive charge and high surface activity leading to the observed decrease in the value of the zeta potential.
The above changes in the zeta potential values in presence of this flocculant are reflected in the turbidity of the suspension as a function of Percol 368 dosage as given in Figure 5. The data indicate higher turbidity both at lower and higher dosages of the cationic flocculant. This could be explained on the basis of high negative charge in absence of the flocculant and the decrease in the charge in presence of low dosage of the reagent. However, at higher dosage the charge in the particles are highly positive leading to electrostatic repulsion and high stability (turbidity) of the suspension. It should be noted that the values reported in Figure 5 are obtained after only one minute of settling time which indicates that the formed floccs are large and subsequently settle fast. Recording turbidity as a function of time in presence of 3.0 ppm of the cation polymer has shown that 1.0 minute is a good settling time that may be used for the field applications. These values are plotted in Figure 6. These data about the settling time and the polymer dosages are utilized to design the field application as shown below.
