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3 Selective Adsorption of Organic Homologues onto Activated Carbon from Dilute Aqueous Solutions

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: March 15, 1983 | doi: 10.1021/ba-1983-0202.ch003

Solvophobic Interaction Approach— Branching and Predictions G O R D O N A L T S H U L E R and G E O R G E S B E L F O R T Rensselaer Polytechnic Institute, Department of Chemical and Environmental Engineering, Troy, NY 12181

The testing of the solvophobic (cø) theory was extended to include adsorption of branched alkyl alcohols and slightly ionized linear carboxylic acids. Correlations with adsorption capacity for both the comprehensive and the simplified c ø theories were compared with similar correlations for other independent variables including molecular weight, density, index of refraction, molar volume, and group contribution parameters such as molar refraction, octanol-water partition coefficient, and parachor. The best correlative parameters were the octanol-water partition coefficient, three solvophobic terms, and the total cavity surface area. A simplified expression requiring only the total cavity surface area was used to predict the unknown adsorbability of two branched alcohols.

T

HE ABILITYTOPREDICT THE EFFECTS of even simple structural modifica-

tions on the adsorption of organic molecules from dilute aqueous solutions onto activated carbon (or other adsorbents) could be valuable in the design and operation of large-scale commercial water and wastewater treatment plants. This capability would assist in gaining a better understanding of competitive adsorption between and chromatographic elution by different organic solutes during adsorption. This is Part III in a series.

0065-2393/83/0202-0029$09.25/0 © 1983 American Chemical Society

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

30

TREATMENT OF WATER BY GRANULAR ACTIVATED CARBON

To accomplish this goal, a general comprehensive solution interaction approach, originally developed by Sinanoglu (1), was recently adapted to the adsorption of organic homologues from dilute aqueous solutions by Belfort (2-4). By including solvent effects in the equilibrium adsorption (association) process, this solvophobic theory (c0) differs from most other equilibrium theories in that the latter were originally derived from gas and vapor phase adsorption and thereby a priori ignored the presence of the solvent during adsorption (5). Previously (3) the solvophobic theory was adapted to adsorption and the free energies of solvent-mediated adsorption for a series of linear aliphatic alcohols, ketones, aldehydes, and acids. Both the comprehensive (AG /RT) and simplified (total surface area, TSA, A ) theories were tested with adsorption of linear aliphatic and aromatic alkyl compounds for single and solute solutions. In this chapter, the testing of the solvophobic theory is extended to include the adsorption of branched aliphatic alcohols and slightly ionized linear carboxylic acids. Correlations from the comprehensive and simplified c 0 theories are compared with correlations of other independent variables such as the molecular weight, density, index of refraction, molar volume, molar refraction, octanol-water partition coefficient, parachor, and polarizability. Total solute surface cavity areas (TSA) are calculated with a newly developed molecular-build program coupled to Hermann s method of intersecting atomic spheres with appropriate crowding factors (6). The theory also can be used to predict the adsorption capacity of organic compounds whose experimental adsorption capacity has not been measured but whose homologues have. With respect to solid-liquid adsorption, a major limitation of the various equilibrium theories of adsorption (4,5) is that they were originally derived from gas and vapor phase adsorption, and thereby a priori ignored the presence of the solvent during solute adsorption. Also, single solute adsorption data are needed to predict multisolute competitive adsorption, while other empirical mathematical equations represent the data without attempting to establish a physical model (5). Freundlich, in his classic monograph (7), describes the first attempts 90 years ago by Traub and others to include the solvent-solute interaction effect through interfacial tension at the solid-liquid interface. Although Defay et al. (8) updated this approach with a major emphasis on the simplified Gibbs adsorption model especially under nonequilibrium conditions, a comprehensive interaction theory between solute, solvent, and sorbent was still lacking. Recent attempts to include the solvent effect in aqueous phase adsorption include a semi-empirical quasi-theoretical approach based on partial solubility parameters called the net adsorption energy approach (9-11). In spite of the limitations of this approach (see


net

2



3.

ALTSHULER AND BELFORT

Adsorption of Organic Homologues

31

Reference 4 for a detailed discussion) it does provide a useful semiquantitative screening method for estimating the relative adsorption capacity (rank order) of different organics. Although these approaches provide useful insight with respect to solute-solvent adsorption onto solids, a comprehensive formalism of aqueous phase adsorption including fundamental formulations of all known interactions among solute, solvent, and sorbent is needed. By invoking and adapting the c0 theory of Sinanoglu (I), Belfort (2-5) and more recently Melander and Horvath (12) attempted to provide such a formalism for describing the solvent effect on the solute-solid adsorption association reaction. The results presented in this chapter represent a continuing effort to use, test, and adapt this formalism to predict a priori a ranking order of adsorption capacity especially for homologues without previously measured single solute adsorption data Here, specifically the testing of the c 0 theory is extended to branched and slightly ionized organics in dilute aqueous solution.

Theory General Solvophobic Approach. The solvophobic (c) theory describes the tendency of a surrounding solvent medium to influence aggregation or dissociation of molecules with considerable microsurface areas exposed to the solvent medium. Examples of these molecules include amino acids, nucleotide bases in biopolymers, various drug molecules, antigens and substrates, and relatively low molecular weight organic homologues and an adsorption surface. The c 0 theory also has introduced a new measurable quantity called "the thermodynamic microsurface area change of a reaction" (13) which is now finding applications in protein structure (14), high-performance liquid chromatography (HPLC) (15), protein salting-in and salting-out (16), the conformational structure of organic compounds (17), isomerization reactions (18), stacking of bases and solvent denaturation of DNA (19), and association adsorption reactions (3). The approach used here follows closely that used by Belfort (3) in which he adapted the c0 theory, originally presented by Sinanoglu (1) and applied to H P L C by Horvath et al. (15), to the association adsorption reaction. For a comprehensive review of the adaptation of this approach to the adsorption of organic substances from dilute aqueous solutions by nonpolar adsorbents, the reader is referred to the literature (3, 4, 12). Since this is an equilibrium theory, kinetic effects such as convective mass transfer and solute diffusion processes are not considered. In the solvophobic treatment, the effect of the solvent on the reversible association reaction of the adsorbate molecules S , with t



Figure 1. A pictorial representation of the association adsorption reaction in the gas and, liquid phase.

SOLVENT PHASE

GAS PHASE


3.



33

activated carbon adsorbent, C, at the surface to produce adsorbed complex S C is obtained by subtracting the standard free energy change for the reaction in the gas phase from that in the solution phase under unitary standard state (x°k= 1, P°k = 1 tm ideal gas). This process is diagrammed in Figure 1 and gives rise to a net free energy, A G " expressing the effect of the solvent in the association adsorption process. Conceptually, Sinanoglu proposed a two-step dissolution process (i). First, a hole or cavity needs to be prepared in the solvent to accommodate the solute, carbon, or adsorbed complex "molecule"; second, after the "molecule" is placed into the cavity, it interacts with the solvent. Quantitatively this process is expressed as follows: t

a


t t s o lvent

^^Mvent effect)

=

AG£X t) ~~ ^Qgas) en

=

RT In [kg-c/k".^^]

effect)
i

- K' AA 2

(25)

where K" = K[ + 0, K' = Ny, as before, and AG^j is given by Equation 5. The subscript S is replaced by i. 2

t

Adsorption Data Adsorption data of typical aliphatic organics on activated carbon were obtained from two sources (32, 38). Single-solute equilibrium adsorption data (32) were obtained by contacting 100-mL aliquots of 1000 mg/L stock solution with 5 g/L of pulverized Westvaco Nuchar GAC, Grade WV-G, for 2 h. The molar amount of aliphatic solute adsorbed during this period is designated Q' . The single solute Freundlich (Kf,n) and Langmuir (Q°,b) parameters for the isothermal adsorption of aliphatic organics onto pulverized Filtrasorb 400 (Calgon Corp., Pittsburgh, PA) were obtained by contacting 200 mL of a 500 mg/L phosphate-buffered solute in distilled water for 2 h m


3.



43

with 0.05-16.0 g carbon (38). Solute concentrations were measured for total organic carbon calibrated with standard solute solutions.

Solvent-Solute Data Various parameters that characterize the solvent (water), solute, and adsorbent (activated carbon) are needed to calculate the free energy terms in Equation 15. In addition, a series of solute and solute-solvent parameters, such as molecular weight (MW), density (p), index of refraction (n ) molar volume (V), molar refraction (MR), octanol-water partition coefficient log P, parachor [P], and polarizability (a), are correlated with adsorption capacity (In p ). A summary of the values used for the parameters of each solute is presented in Table III. The major reference sources are included in the table, while the definitions of several parameters are given in the footnotes. Croup Contribution Parameters. Group contribution methods have been successfully used to predict among other properties, the critical temperature (47), critical pressure (47, 48), critical volume (48), and normal boiling point (47). They have also been used to predict heat capacity (50), enthalpy of formation (51), heat of vaporization (52), and activity coefficients (53). Another application of group contribution methods is quantitative structure and activity relationship (QSAR) where structure of a solute is correlated with its biological or pharmacological effect The appearance of MR, log P, [P], and a in some or many of these group contribution methods suggests that these parameters might be considered with respect to the adsorption of homologous series. The Clausius-Mosotti equation for atomic and electronic polarization (P and P ) may be written as follows:


D

f

a

e

e - 1 MW 4 P = P + P = - — — = -7rIVa e+ 2 p 3 fl

g

/

x

(26)

By using Maxwell's equation which equates the dielectric constant, e, with the square of the index of refraction, n , for nonpolar molecules without a permanent dipole, Equation 26 is transformed into the Lorenz-Lorentz equation to give molar refraction: 2

D

n% ~ 1 MR = - f n + 2 2

D

MW 4 =-nNa p 3

, (27)

where P is total polarization, p is density, N is Avogadro's number, a is polarizability, and M W is molecular weight MR has been widely used in QSAR (54, 55).


x

44


Table III. Solute Density

Ionization Potential I

Weight, MW

(39,60) eV

Refract. Index, n (40,61)

32.0 46.1 60.1 60.1

10.97 10.65 10.49 10.52

74.1 74.1

Molecular

11

P (40,61) g/cm

Volume

1.32652 1.35941 1.3752 1.3837

0.7866 0.7851 0.7813 0.7998

67.55 97.51 127.74

10.5 10.35

1.3851 1.3950

0.7812 0.8026

157.51 153.31

74.1 74.1

10.47 10.44

1.3939 1.3973

0.7978 0.8060

154.24

9. 2-Methyl-2hutanol 10. 1-Pentanol 11. 1-Hexanol 12. 3-Methyl-l-

88.2 88.2 102.2

10.16

1.4024

0.8044

10.42

—

1.4079 1.4161

0.8112 0.8159

181.97 180.33 208.00

pentanol

102.2

—

1.4175

0.8200

206.96

46.0 60.1 74.1 88.1 102.1

11.05 10.35 10.24 10.16 10.12

1.3714 1.3716 1.3869 1.3980 1.4085

1.220 1.0492 0.9930 0.9577 0.9391

62.61 95.12 123.92 152.76 180.54

18.0

15.59

1.33250

0.9971

29.99

No. Compounds

Molecular

D

V A

3

3

Aliphatic Alcohols 1. 2. 3. 4.

Methanol Ethanol 2-Propanol 1-Propanol


5. 2-Methyl-2propanol 6. 2-Butanol 7. 2-Methyl-lpropanol 8. 1-Butanol

124.79

152.67

Carboxylic Acids/ 13. Formic acid 14. Acetic acid 15. Propanoic acid 16. Butanoic acid 17. Pentanoic acid Solvent 18. Water

« v = ( M W / N p ) 1 0 in A . ''For nonpolar hydrocarbon of same geometry. 7 M R r (MW/PH"?)- !)/("&+ 2) = (4/3)7rAte. [P] — 7 MW/(p/ — p ), where y = surface tension, pj = liquid density, and p = vapor density. For p, + p , [?] « y ' V and for y = 1, [P] = V. log P= log (concentration of solute in octanol phase/concentration of solute in water phase), i.e., P is octanol-water distribution coefficient. /The acid dissociation constants 10 K a(H. 9 O) and 25°C for Compounds 13-17 are 17.12, 1.76, 1.34, 1.50, and 1.38, respectively. " " ^Estimated. 24

1

3

v

v

v

1

4

e

5

H

The partition coefficient, log F, of a solute distributed between a nonpolar phase such as octanol and a polar phase such as water is an obvious choice for correlating the adsorption of organics onto the nonpolar activated-carbon sorbent from the polar aqueous solution. The parachor [F] was developed in the 1920's as a means for calculating molecular volumes (56). It is not dimensionally correct to consider [F] as a volume. More accurately, the ratio of the parachors of


3.



45

and Solvent Properties Accentric^ Factor, w (39)


0.098

Dipole Moment, p. (40) Dehye

Molar

0

Refraction, MR cm /mol 3

Parachor^

Partition^

[p] calc (42)

Coeff. fogP (43,44)

Polarizatic

1.78 1.68 1.72 1.63

3.26

12.9402 17.6182 17.5579

85.3 125.3 161.6 165.3

-0.75

0.152 0.176 0.193

-0.21 0.11 0.33

5.13 6.98 6.96

0.197 0.227

1.67 1.66

22.2339 22.1341

197.9 201.6

0.43 0.65

8.81 8.77

0.227 0.251

1.67^ 1.60

22.2123 22.1543

201.6 205.3

0.65 0.87

8.81 8.78

0.231 0.296

1.59^ 1.59 1.55

26.7219 26.8171 31.4288

237.9 245.3 285.3

0.97

10.59 10.63

31.3755

281.6

1.82

12.44

8.5572 13.0064

94.0 131.2 169.0 208.6 247.0

-1.09 -0.20 0.34 0.88 1.42

3.39 5.16 6.96 8.80 10.64

— —

—

0.152 0.176 0.227 0.279 0.330

1.46 1.05 0.89 0.94 0.948

0.023

1.84

8.2194

17.5642 22.2023 26.8502

1.41 1.95

12.46

two compounds is equal to the ratio of the molecular volumes under conditions such that the surface tension is equal or unity. Hence, the parachor may be thought of as a molecular volume at a corresponding state with respect to surface tension, rather than temperature or pressure (44). It should also be noted that the MR, a, and molecular volume, v, parameters are included in the comprehensive c0 theory. (See Equations 4 and 15). Molecular Cavity Surface Area. As described during the development of the c theory, the thermodynamic microsurface area change, AA, resulting from the association of solute and sorbent is an important newly introduced measurable quantity. This contact area between solute and sorbent is not directly known. It depends not only on the interactive forces between solute and sorbent but also between solute and solvent It also depends on the position of a polar moiety on the solute, on the solute's


46


morphological characteristics (flexibility and length to diameter ratio), and the sorbents surface morphology and cavity volume. Thus, the system will try to maximize both the contact area and the solvent interaction (12). To calculate AG™[ for species i an estimate of AA is necessary. See, for example, the fourth term on the right-hand side of Equation 15 (NyAA). Horvath et al. (15) plotted log p (for HPLC) versus the hydrocarbonaceous surface area (HSA) of the solutes and obtained linear parallel plots for closely related aromatic solutes, carboxylic acids, amino acids, and amines. The identity of the slopes for constant solvent surface tension y(= 32.6 dynes/cm) indicated that A A was proportional to HSA [i.e., AA = g(HSA)] for homologous series. By using the same approach, very low g values are obtained for adsorption of alkyl aliphatic compounds. This is probably due to two factors: the unlikely assumption that the g is constant, and the fact that the curvature correction to the surface tension falls away. Using a phenomenological approach, Altshuler and Belfort (24) derived an equation of the form ti


{

logp.^mA. +

fe'

(28)

where the slope is given by: m = -^[(l-

Selective Adsorption of Organic Homologues onto ... - ACS Publications

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