Adsorption plateau, calcium concentration

Adhesive force, non-Brownian particles, 549 Admicelle formation, 277 Adsorption flow rate, 514 mechanism, 646-647 on reservoir rocks, 224 patterns, on kaolinite, 231 process, kinetics, 487 reactions, nonporous surfaces, 646 surface area of sand, 251 surfactant on porous media, 510 Adsorption-desorption equilibria, dynamic, 279-239 Adsorption plateau, calcium concentration, 229... 

Typical adsorption isotherms are shown in Figs. 16 and 17. Despite the large experimental scatter, a steep increase in adsorption can be seen at low concentrations, followed by a plateau at concentrations exceeding the CMC. Similar behavior has been observed before with model surfactants [49-54] and has also been predicted by modem theories of adsorption [54]. According to Fig. 16, adsorption increases modestly with salinity provided that the calcium ion concentration remains low. The calcium influence, shown in Fig. 17, cannot be explained by ionic strength effects alone but may be due to calcium-kaolinite interactions. 

Calcium chloride, hydrochloric acid, or ethanol at low concentrations do not modify the adsorption plateau nevertheless, the saturation is attained at a different equilibrium concentration due to the changes in the solubility behavior of the surfactant in the presence of these additives. When the concentration of ethanol is 25% (6 molar), the adsorption maximum is reduced to 0.8 p moles m indicating that for this rather high concentration of ethanol, the surfactant does not completely dislocate the alcohol from the solid surface. 

Nevertheless, for waste paper recycling the plateau values of the isotherm are not decisive. With 0.2% dosage of the cationic polymer and 1 % stock consistency, the initial concentration is only 20 mg/1. Due to the steeper slope of the adsorption isotherm on the fillers, the polymer should preferentially be adsorbed on the fillers. Figure 5 shows the flotation recovery of all three solid materials — calcium carbonate, kaolin, and cellulose. These flotation tests were performed with pure systems. The solid content was the same as in waste paper recycling 1 and 0.2% for fibers and fillers, respectively. At a concentration of 20 mg/1, the recovery of kaolin is higher than 90%, while 70% of calcium carbonate are floated. However, with the same initial concentration the recovery of the cellulose fibers is below 20%. In comparison with the adsorption isotherms it can be concluded that the adsorbed amount of the cationic polymer is too low in order to hydrophobize the cellulose surface. Thus, the cationic polymer is a selective collector with respect to filler pigments. This fact has been confirmed by flotation tests with synthetic mixtures of cellulose and fillers [30]. 

In the presence of 286 ppm CaCOj the onset of the adsorption isotherm starts at much lower equilibrium concentrations, reaching a fivefold higher plateau value (Fig. 6). As a result, calcium ions improve the adsorption of anionic surfactant molecules on the kaolin surface. However, under these circumstances the precipitation of Ca-surfactant complexes is noticeable. The adsorption... 

The flotation behavior of kaolin in distilled water is poor. Even with high initial concentrations, only 30 to 40% of the kaolin are recovered with the foam (Fig. 8). On the other hand, in hard water the recovery increases more strongly even at low surfactant concentrations, reaching a recovery of 75% in the plateau region. The high recovery in hard water shows that the adsorption of the surfactant on the surface predominates the precipitation in solution. A comparison of the recovery of fillers with a technical collector in hard water (Fig. 8) leads to the conclusion that it is distinctly easier to flotate calcium... 

Graphon) [43]. Similar isotherms have been reported by Abe and Kuno [44,45] for the adsorption of polyoxyethylated nonylphenols onto carbon and calcium carbonate and by Elworthy and Guthrie [46] for the adsorption of a series of polyoxyethylene non-ionics onto the water-insoluble, antifungal antibiotic, griseofulvin. Other authors have examined the adsorption of polyoxyethylene ethers on paraffin wax [47] and silver iodide particles [48,49]. In general, the plateau region commences at a concentration at or near the CMC. 

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