Correlation between adsorption behavior

Malcolm et al. (14) and Thurman et al. (15) noticed that the adsorption of solutes onto XAD-8 macroreticular resin could be predicted by means of a linear correlation between the logarithm of the capacity factor and the inverse of the logarithm of the water solubility of each compound. Their investigation, however, was limited to approximately 20 selected organic compounds in individual aqueous solutions. By comparing the results shown in Table II and the water solubility properties of each model compound used in this study (see Table I), it appears that the predictive model could serve for a first estimate of the recovery of multisolute solutions at trace levels. However, low recoveries and the erratic behavior of several compounds included in this study suggest that additional factors need to be considered. 

For the accurate characterization of the adsorption phenomena, it is necessary to obtain accurate information on pore stmctures. However, most of ordinary microporous carbons and mesoporous carbons are obtained with amorphous stmctures that are characterized by irregular arrangements of non-uniform pores. X-ray (or electron) diffraction (XRD) techniques are not useful for such carbons because there are no well-defined stmctural factors to correlate with the adsorption behavior. Moreover, porous carbons exhibit wide varieties of the surface functional groups and the thickness of the pore walls, depending on the details of the synthesis conditions. The lack of distinct XRD lines makes it difficult to distinguish stmctural differences between samples which causes many works to depend empirically on specific samples. 

Finally, in the group of acids whose 0 < pKai < 4, sigmoidal uptake curves and uptake curves with a maximum are reported in different sources, and there is no apparent correlation between the type of uptake curve, and the nature of the adsorbent (actually most available data on anion adsorption are for aluminum and iron III oxides and hydroxides as adsorbents) or the experimental conditions (e.g. the initial concentration of the adsorbate). Arsenate V, chromate VI, phosphate, and molybdate are typical examples of such behavior. For three former anions the number of publications reporting sigmoidal uptake curves on the one hand and uptake curves with a maximum on the other are approximately equal, but for molybdate the sigmoidal curves are more abundant. Comparison of molybdate with other anions in terms of pKa, is difficult in view of tendency to form polyacids (condensation). 

The following correlation log K (surf)= log K (soln) +1.5 was obtained for adsorption of five anions on goethite [615], but the K(surf) calculated from this equation for H2PO4 is lower than the observed one by two orders of magnitude. Another limited correlation between the stability of surface and solution complexes of anions has been reported by the same authors in Ref, [672]. It should be emphasized, that the literature data on stability constants of aqueous complexes involving A1 and Fe and specifically adsorbing anions is scarce, and for a few anions whose sorption behavior has been extensively studied, the stability constants of aqueous complexes with Fe and A1 are not known. 

In this paper we examine the role of mixed surfactants in the demulsification of water-in-Leduc oil emulsion by application of the spreading rate method which is then correlated with the electroacoustic results and centrifugation. Microelectrophoresis using the reverse emulsion was also used to investigate the adsorption process. The results show both a very good correspondence between the various techniques and provide insight on the synergistic adsorption behavior of the hydrophobic and hydrophilic surfactants. 

The removal of heavy metal ions from both natural water supplies and industrial wastewater streams is becoming increasingly important as awareness of the environmental impact of such pollutants is fiilly realized. In particular, the likelihood of such metal ions precipitating out of solution and/or coating other materials can have a profound effect on both aqueous and nonaqueous environments. There is considerable evidence in the literature that the primary mechanism for transportation of metal contaminants in aquatic systems is the movement of suspended particulate material containing the adsorbed pollutant metals [1,2]. It is also known that a strong correlation exists between the concentration of trace metals in the (aquatic) environment and the extent to which those metal ions adsorb onto colloidal substrates present in the environment [2,3], A similar correlation between the concentration of trace metals in the (aquatic) environment and their precipitation behavior is not so clear. There is, then, a well-founded need to study adsorption-related phenomena in order to understand and predict the behavior of toxic metals in the environment. 

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