Adsorption Equilibria in Multisolute Mixtures of Known and Unknown

11 Adsorption Equilibria in Multisolute Mixtures of Known and Unknown Composition Downloaded by FUDAN UNIV on January 11, 2017 | http://pubs.acs.org Publication Date: March 15, 1983 | doi: 10.1021/ba-1983-0202.ch011

B E R N D R. FRICK and HEINRICH SONTHEIMER Bereich Wasserchemie am Engler-Bunte-Institut der Universität Karlsruhe, Richard-Willstätter-Allee 5, D-7500 Karlsruhe 1, Federal Republic of Germany

The influence of the composition of multisolute mixtures on the adsorptive uptake of individual compounds and on the adsorption of total organics is demonstrated. Based on these dependencies, adsorption isotherms can be interpreted relative to mixture composition. A method is proposed that characterizes multisolute mixtures as a solution of three compounds with defined adsorption properties, and computation results based on this procedure are presented. The prediction of adsorption equilibria can be improved greatly as can the prediction of granular activated-carbonfilterperformance when multisolute mixtures are adsorbed. Experimental data for humic acid solutions and groundwater are used to confirm the applicability of the proposed method.

K

NOWLEDGE ON THE PROCESS of activated-carbon adsorption from the

aqueous phase has been enlarged during the past 15 years. The major results of intensive research are, from our point of view, several mathematical models that describe adsorption equilibria and adsorption kinetics for single solute systems as well as for mixtures containing two or three organic solutes (1-5). These models provide a basis for the design of granular activated-carbon (GAC) treatment plants and cost estimates, and they can be used to evaluate pilot plant studies. As most theories and models need single solute adsorption data for compounds present in water, the most important provision in applying such models is to know the components of the mixture and their concentrations. However, these requirements restrict, or even prevent, the application of this design tool when the prediction of GAC performance in water treatment is desired. The reason for this is the variety of 0065-2393/83/0202-0247$0.700/0 © 1983 American Chemical Society

McGuire and Suffet; Treatment of Water by Granular Activated Carbon Advances in Chemistry; American Chemical Society: Washington, DC, 1983.

Downloaded by FUDAN UNIV on January 11, 2017 | http://pubs.acs.org Publication Date: March 15, 1983 | doi: 10.1021/ba-1983-0202.ch011

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T R E A T M E N T O F W A T E R BY G R A N U L A R A C T I V A T E D C A R B O N

compounds in the water and the lack of analytical methods to detect them. To overcome this difficulty, the total of the organics present frequently was treated mathematically as a single compound by using equilibria and kinetic data obtained from experiments where the organics concentration was measured as a sum by determining dissolved organic carbon (DOC) [total organic carbon (TOC)] by UV absorbance or fluorescence. With this method, the interactions between the different compounds are neglected, and the prediction of chromatographic effects or of the decreased adsorption capacity of the carbon, which influences GAC performance drastically, is impossible (3,6). It was the aim of this study to find a method by which the models developed for predicting adsorption behavior of defined multisolute mixtures with a limited number of components could be applied to the calculation of adsorption equilibria in multisolute mixtures of unknown composition. To overcome the drawback of missing information on the composition and adsorption properties of the mixture constituents, a procedure grouping compounds with similar adsorption behavior, i.e., reducing the number of components, seemed to be a way to consider the heterogeneity of those mixtures. By means of this hypothetical mixture and the models referred to previously, the actual adsorption behavior can be simulated as a response of competitive adsorption. Before one can define such a reduced mixture, the number of Active components, and their adsorption properties, it is necessary to know more about the influence of competitive adsorption on the uptake of individual compounds and the adsorption behavior of total organics. This knowledge is essential as the selection of such a hypothetical mixture should be based on experimental equilibrium data determined with the unknown mixture.

Adsorption Equilibria of Individual Solutes in Mixtures Data on adsorption equilibria in mixtures containing two solutes have been reported primarily as adsorption isotherms for the individual species (7). To illustrate the influence of the other solute present, the equilibrium concentration or the initial concentration of this compound is used as a parameter according to the general isotherm equations: q = f(c {

h

Cj)Cj = constant or q = f(c {

i9

C )C 0 J

0 j

= constant

(1)

where the subscript i denotes the component for the adsorption isotherm of interest and subscript j denotes the other solute present.



11.

FRICK AND SONTHEIMER Adsorption Equilibria in Multisolute Mixtures

In general, the batch determination of the isotherms for mixture adsorption is carried out by varying the composition of the initial solution and by holding constant the ratio of the solution volume, L, to the amount of carbon, m. The experimental procedure to examine adsorption equilibria for mixtures of unknown composition is to add different amounts of carbon to constant volumes of the solution which is identical in initial composition for all points of the resulting isotherm. To interpret results of batch equilibrium experiments with complex mixtures of undefined composition, adsorption isotherms with defined mixtures containing two or three organic solutes were determined using the method of variable liquid volume—carbon amount ratio (L/ra) and constant initial solution composition The isotherms so obtained show a different shape from those presented in the literature. An explanation of this effect is given in Reference 7. Based on the characteristic shape of the isotherms, a qualitative interpretation can be made, which evaluates the adsorbability of the other mixture components relative to that of the individual species. An example of adsorption equilibria of individual compounds in a ternary mixture is shown in Figure 1. Six solutions, each containing different initial concentrations of p-nitrophenol, p-hydroxybenzoic acid, and 4-phenolsulfonic acid were agitated with four different amounts of powdered activated carbon (Norit ROW 0.8 S, Table I). The liquid phase concentrations of the three individual compounds at equilibrium were determined by UV photometric analysis. Figure 1 illustrates the individual isotherms of these compounds when adsorbed simultaneously. The numbers attached to the isotherms refer to the six different initial solutions. The adsorbability of the compounds as a single solute decreases from p-nitrophenol to sulfonic acid as shown in Table II where the Freundlich isotherm parameters are listed. In a log-log scale, all six individual isotherms for p-nitrophenol are straight lines, while the isotherms for the more weakly adsorbed hydroxybenzoic acid and phenolsulfonic acid are curved. The curved isotherms of individual species adsorbed from a mixture are typical if one or more of the other mixture constituents have a better adsorbability as a single solute than that of the species of concern. The most strongly adsorbed component in a mixture always shows a straight line in a log-log isotherm plot, i.e., the Freundlich isotherm equation can be applied. However, the Freundlich constant K decreases as the adsorption capacity is decreased by the other solutes and the isotherm exponent n increases in comparison to that of the single solute adsorption isotherm.


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T R E A T M E N T O F W A T E R BY G R A N U L A R A C T I V A T E D C A R B O N


250

Figure 1. Adsorption isotherms of the individual components when adsorbed simultaneously from a ternary mixture; initial composition of the mixture is identical for the isotherms with the same numbers attached.


11. FRICK AND SONTHEIMER Adsorption Equilibria in Multisolute Mixtures Table I. Activated-Carbon Properties ROW 0.8 S , Cylindrical Shape 8

Property Particle density Apparent density Pore volume Mean diameter (based on a sphere of identical volume)

0.64 0.40 1.03

1.24

g/cm g/cm cm /g 3

3

3

F30&, Irregular Shape 0.85 0.54 0.66

1.40

mm

g/cm g/cm cm /g 3

3

3

mm


"Norit, The Netherlands. "Chemviron, Belgium

These dependencies were confirmed by equilibrium data in mixtures of up to six compounds (8). An extreme example of the influence of competitive adsorption on isotherms is given in Figure 2. The two solutes, p-nitrophenol and tetrachloroethane, have very different single solute adsorbabilities as indicated by the two single solute adsorption isotherms. If adsorbed simultaneously, the weakly adsorbed tetrachloroethane is strongly influenced by p-nitrophenol, resulting in an extremely curved isotherm. These results indicate that it is possible to get an estimate of the mixture composition in view of the adsorbability of the components, if one can determine the adsorption isotherm of an individual and known compound already present in the mixture or specially added for this purpose. Table II. Freundlich Isotherm Parameters and Properties of the Single Solutes (Norit R O W 0.8 S) K (g/kg)

n

mw

pK

60.8

0.12

174.2

Adsorption Equilibria in Multisolute Mixtures of Known and Unknown

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