Analysis of adsorption equilibrium for aqueous solution of weak organic electrolyte-activated carbon system

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1 112 Int. J. Environment and Pollution, Vol. 23, No. 1, 2005 Analysis of adsorption equilibrium for aqueous solution of weak organic electrolyte-activated carbon system Joshua Ifeanyichukwu Ume* Department of Pure and Industrial Chemistry, University of Nigeria, Nsukka, Nigeria *Corresponding author Godian O. Mbah Enugu State University of Science and Technology, Enugu, Nigeria Abstract: Several models for the adsorption of weak organic electrolytes on activated carbons from dilute aqueous solutions have been reported recently. It is apparent, however, that the electrokinetics of carbon surface has not been sufficiently addressed by these studies. The present treatment therefore employed the fundamental concepts provided by these studies, in conjunction with electrokinetic measurements and mass titration, to predict experimentally observed adsorption data. Important and convenient parameters for characterisation of activated carbon surfaces were thus evaluated. The interplay between reduced potential, ph and adsorption capacity were examined for adsorption of weak acidic electrolytes on untreated, oxidised and nitrided activated carbons. The best-fit parameters for aqueous adsorption on the carbon samples were also acquired. Keywords: electrokinetics; electrophoretic mobility; surface and reduced potentials. Reference to this paper should be made as follows: Ume, J.I. and Mbah, G.O. (2005) Analysis of adsorption equilibrium for aqueous solution of weak organic electrolyte-activated carbon system, Int. J. Environment and Pollution, Vol. 23, No. 1, pp Biographical notes: Joshua Ume received his BS and MSc degrees in Chemical Engineering from the University of Lagos in 1978 and 1982, respectively. He subsequently received his PhD in Fuel Science and Engineering from the Pennsylvania State University, University Park in He is presently a Senior Lecturer at the University of Nigeria, an Associate Lecturer at ESUT, Enugu, and a consultant chemical engineer. He is presently the Zonal Chairman of the Nigerian Society of Chemical Engineers at Enugu, Nigeria. Godian Mba obtained his BSc and MSc degrees in Chemical Engineering from the Aristotle University of Thesolonica, Greece and has taught at the Enugu state University of Science and technology for over 20 years now. He is presently the Head of Department of Chemical Engineering. His research interests are in kinetics, adsorption and chemical rection engineering. Copyright 2005 Inderscience Enterprises Ltd.

2 Analysis of adsorption equilibrium for aqueous solution Introduction Adsorption on activated carbons is proven technology for the effective removal of organic pollutants from municipal and industrial wastewaters (Weber 1985, 1972). Excellent literature reviews of adsorption of organics on carbon from aqueous and non-aqueous systems have been provided by Derylo-Marczewska and Janroneic (1987) and Bansal et al. (1988). It is interesting to note that there is a virtual absence of adsorption studies at ph values above 10, despite the observation by several authors that ph has a dramatic effect on adsorption of weak electrolytes (Bansal et al., 1988; Derylo-Marczewska, 1993; Muller et al., 1980, 1985). This may have been due to the fact that there had been no practical need for such an investigation. Several workers (Derylo-Marczewska, 1993; Muller et al., 1980, 1985) had noted: decrease in adsorption capacity of carbons for weak electrolytes with increase in solution ph an increase in adsorption with decrease in ph a maximum adsorption capacity at some intermediate ph. Weber and coworkers (1969; 1997) found a strong dependence of adsorption of phenols and sodium benzenesulphonate on ph and also observed that this effect was reduced by the addition of salts. There is a general consensus that increase in surface acidity of carbons suppresses the adsorption of phenols (Mahajan et al., 1908; Puri, 1980; Coughlin et al., 1968; Coughlin and Ezra, 1969; Oda et al., 1981; Urano et al., 1981; Hemly et al., 1987; Giusti, 1974). Getzen and Ward (1969) used a binary Langmuir model to describe the adsorption of both the molecular ionic forms of weak electrolytes in aqueous solution. These authors reported synergistic adsorption effects at low concentrations in the ph range where ion and bulk concentrations were comparable. Rosene and Manes (1977) applied the Polanyi potential theory to account for the ph effects on the hydrolytic adsorption from aqueous mixtures of organic acids and their salts. Using solubility data and the single component isotherms of the acids and their salts, they calculated adsorption isotherms for binary systems. Wang et al. (1975) reported that the ph effect on the effectiveness of carbon adsorption mainly depends on the nature of the adsorbate. The implication would be that the adsorption of ionic organic solutes depends mostly on the effect of ph on the degree of ion isation of the solute. In all these studies, adequate consideration was not given to the effect of ph on adsorbent surface chemistry. It is also apparent that the electrokinetics of carbon surface has not been sufficiently addressed by these studies. The objective of the present treatment was therefore to employ the fundamental concepts provided by these studies, in conjunction with electrokinetic measurements and mass titration, to predict experimentally observed adsorption data. 2 Theoretical considerations 2.1 Adsorption on a homogeneous surface The dissociation of weak organic acids and bases can be represented by the following equilibria:

3 114 J.I. Ume and G.O. Mbah M ph ( ) M 10 + M M + H K = (acid) [ M] ph [ M] ( 10 ) + + M M + H K M = (base) + M where, K M is the dissociation constant, and M, M +, M represent the concentrations of molecular, anionic and cationic species, respectively. The Langmuir model for competitive adsorption gives the total surface coverage (F) as: ± M [ M] + Kq C1 Φ=Φ M +Φ ± = = M ± M C1 + K K+ [ M ] + K q where, F M and F M are surface coverages for molecular and ionic species, and K q and K are the equilibrium constants for adsorption of ionic and molecular species, and C 1 is a measure of the total adsorbed species at equilibrium, respectively. The overall equilibrium constant for the adsorption process is (Muller et al., 1980): 0 K = KKq = Exp( G + Z Fψ M ± 0)/RT = [A exp( U / RT)][exp(z F ψ )/ RT] 0 M ± 0 where, G 0 is the standard Gibbs free energy of adsorption (independent of charge of species), z is solute charge, F is Faraday s constant, ψ 0 is the surface charge of the adsorbent, T is the absolute temperature and U the adsorption potential for the molecular species (due to dispersion forces). The pre-exponential factor A 0 is assumed to be the same for both molecular and ionic species. It is seen from the above equation that the ionic solute will adsorb (i.e., K > 1) only when the dispersion attractive term exceeds the repulsive electrostatic term. 2.2 Adsorption on heterogeneous surface The Freundlich model has been applied successfully to describe experimental data for heterogeneous surfaces (Derylo-Marczewska and Jaronec, 1987; Urano et al., 1981). A modified Freundlich isotherm, which accounts for the adsorption of both molecular and ionic adsorbates has been derived by Muller et al. (1980): θ n C C α 1 = C 1 C1 + K 0 with C α [M] + α[m ± ]/K q and K 0 A 0 exp( U 0 /RT). Here, α represents the fraction of the surface available for the adsorption of the ionic solute. When the surface and the ionic solute have opposite charge, α = 1. When they have the same charge, α is unity as long as the electrostatic potential is lower than the dispersion potential. As the repulsive electrostatic potential increases, e.g., as ph varies, α decreases. The heterogeneity of the

4 Analysis of adsorption equilibrium for aqueous solution 115 adsorbent surface is characterised by the exponent n. The dispersion equilibrium constant K 0 therefore contains the adsorption potential, U 0 for a heterogeneous surface. The modified Freundlich isotherm reduces to the form C1 θ = C + K 1 0 for a heterogeneous surface (n = 1) that is entirely available for the adsorption of ionic solutes (α = 1). 3 Experimetal 3.1 Materials The adsorbent employed in the study was Calgon BPL (brand name is Calgon Corporation, product briefs, Pittsburgh, PA (1991) activated carbon. It is a bituminous coal-based carbon with ash content of 8 wt%, total pore volume of 0.7 cc/g and mesh size of (US standard sieve). The adsorbate was phenol at initial concentrations ranging from to M. 3.2 Modification of carbon adsorbents The procedure for reaction of BPL carbon with ammonia consisted of purging the sample in ultra high purity (UHP) nitrogen for ca. 30 minutes in a tubular furnace and then heating to 600 o C (also in nitrogen flux). Ammonia flow (~150 ml/min) was then maintained for ~3 hours after which the sample was cooled to 200 o C in ammonia. Nitrogen flow was subsequently restored to cool the sample to ambient temperature. Nitric acid oxidation of carbon samples consisted of a sequence of steps designed to maintain the consistency and integrity of product samples and has been described in details elsewhere (Ume, 1994). 3.3 Electrophoretic mobility measurements The ionic strength of each slurry was controlled with 10 3 M KNO 3 solution in distilled-deionised water. Samples were equilibrated on Burnell wrist-action shaker for 24 hours, prior to taking electrophoretic mobility measurements on a Zeta-metre System 3.0 +, using the automatic sample transfer mode. Details of sample preparation, instrument set-up and initial adjustments, limitations imposed by the problems of thermal overturn and particle velocity as well as the automatic sample transfer mode are described elsewhere (Ume, 1994). ph adjustments were accomplished by the addition of HCl and NH 4 OH drop-wise. All measurements were conducted at 25 o C. 3.4 Physical properties of adsorbents The physical properties of the carbon adsorbents were acquired on a Quantachrome Corporation Autosorb 1 system, while elemental analysis was determined using a Leco CHN-600 instrument. Details of the experimental approach and data reduction have been described elsewhere (Ume, 1994).

5 116 J.I. Ume and G.O. Mbah 3.5 Adsorption studies Equilibrium adsorption measurements consisted of mixing various amounts of adsorbents (0 5 g) with a fixed mass of test liquid, in stoppered flasks and shaking the contents for at least 12 hours. (Contact time experiments had established the equilibrium time for each system.) Preliminary purging was necessary to eliminate the effects of oxidative coupling of adsorbates. The adsorbent was subsequently separated by filtration and the filtrate analysed by UV spectroscopy for residual adsorbate concentration. 4 Results and discussion Figure 1 shows the electrophoretic mobility-ph isotherms for untreated, oxidised and nitrided BPL carbons. It is observed that there is a general decline in electrophoretic mobility with ph. It is also evident that, in agreement with previous reports, it is indicated that aqueous phase oxidation lowered the electrophoretic mobility of carbons (Leon et al., 1990; Solar et al., 1990; Lau et al., 1986), while reaction with ammonia had the opposite effect (Lau et al., 1986). These results are consistent with the facts that oxidation incorporates acidic functional groups on carbons and hence renders the carbon surface more acidic (hydrophilic). Reaction with ammonia incorporates basic groups onto the carbon surface and at the same time results in partial removal of acidic functional groups. Electrophoretic mobility data were converted to zeta potential values based on the Smoluchowsk s equation: µ = Dξ 4πη where, D is the dielectric constant, ξ is the zeta potential, µ is the electrophoretic mobility, and η the viscosity. For water at 25 o C, the equation reduces to (Ume, 1994): ξ(mv) = 12.8µ (µm/sec per volt/cm) Figure 1 Electrophoretic mobilities for as-received, oxidised and nitrided BPL activated carbons (particle size = 60 mesh, sample temperature = 25 o C, electrolyte = 10 3 M KNO 3 solution in distilled-deionised water) The pk a of the surface functional group is acquired from the method of Jacobash (1989) and Lopez-Ramon et al. (1993) as follows:

6 Analysis of adsorption equilibrium for aqueous solution 117 Fξplateau pk a = (ph) ζ = ζ at plateau/ RT Mass titration results are shown in Figure 2. The point of zero charge (PZC) of each adsorbent is taken as the ph value at the plateau of each plot. The point of zero charge provides a convenient index of the propensity of the total surface of a particulate system to become either positively or negatively charged as a function of ph. The ph at zero electrophoretic mobility is referred to as the isoelectric point (IEP). The electrophoretic mobility function was shifted by the difference between the PZC and IEP to obtain the Zeta potential function. The reduced Stern and surface potentials were calculated for the carbon adsorbents from the relationship (Yopps and Fuerstenau, 1964; Hunter, 1981): Fψ Reduced Potential = RT where ψ is the surface potential (zeta potential function at PZC), F is the Faraday constant ( C mol 1 ), R is the gas constant ( Jmol 1 K 1 ), and T the temperature (K). Figure 3 shows the variation of the reduced Stern and surface potentials with ph for untreated carbon activated carbon. Figure 4 depicts adsorption isotherms for phenol as a function of ph. It is indicated that the adsorption isotherms and reduced potential functions follow similar patterns. This is anticipated since reduced potential is a measure of adsorption capacity of an adsorbate; which has been shown to decrease with increase in ph of solution. The apparent differences may be attributed to the experimental zeta potential measurements, which are averages of multiple tests. Several important parameters for the characterisation of activated carbon surface have therefore been established and are summarised in Table 1. Direct relationships were found to exist between the percentage carbon content on one hand, and point of zero charge, isoelectric point and pk a of activated carbon samples on the other. This trend is to be expected. An increase in carbon content of a sample implies decreasing oxygen content, synonymous with lowering acidity. Low acidity again is indicative of high pk a. Figure 2 Mass titration results for as-received, oxidised and nitrided BPL activated carbons (no background electrolyte)

7 118 J.I. Ume and G.O. Mbah Figure 3 Effect of ph on reduced stern and surface potentials for untreated BPL activated carbon Figure 4 Phenol adsorption on as-received BPL activated carbon as a function of ph and equilibrium concentration Table 1 Important parameters for characterisation of activated carbon surface Parameter Nitrided BPL carbon Untreated BPL carbon Oxidised BPL carbon % Carbon % Hydrogen % Nitrogen % Oxygen H/C N/C O/C BET surface area, m 2 /g Micropore volume, cc/g Micropore area, m 2 /g Average pore width, nm PZC IEP PK a

8 Analysis of adsorption equilibrium for aqueous solution 119 pk a values provide information on surface acidity of a carbon sample. Information on surface acidity is necessary for proper understanding of the nature of adsorption interactions between ionic adsorbates and carbons. It is remarkable that pk a values of carbon surfaces acquired using the method of Jacobash are in good agreement with the literature values, as outlined in Table 2 for carboxyl and pyrylium groups: oxidised activated carbon, a highly acidic surface, has a pk a value of 2.6 while carboxyl groups were shown to possess values ranging from 3 to 6. Table 2 Brönsted acidity of some functional groups and BPL activated carbons Activated carbon sample/functional group pk a at ambient temperature Carboxyl a 3 6 Pyrylium a 3 6 Nitrided activated carbon 5.9 Untreated activated carbon 5.3 Oxidised activated carbon 2.6 a Adapted from (Ume, 1994; Solar, 1990). A non-linear least squares regression method (Marquardt, 1963) was employed to fit data on adsorption of predominantly molecular species to the modified Freundlich isotherm. This treatment yields K 0 and n. U 0 was assumed to be zero for all calculations since temperature dependence of adsorption was not of interest in our study. Table 3 provides the Freundlich best-fit parameters for phenol adsorption on a homogeneous surface (n = 1) at 25 o C. The maximum uptakes were computed from values of total surface areas and assuming that phenol occupies a molecular area of 52.2 Å 2. The physical significance of the dispersion equilibrium constant, K 0 is, however, unclear. Figure 5 describes the effect of electrostatic potential on the fraction of surface sites available to ionic sites for untreated, oxidised and nitride BPL activated carbons. Enhanced reduced potential, as is expected, lowered the fraction of surface available for adsorption. Reduced potential functions, therefore, provide a measure of the propensity of an adsorbent surface to function effectively, and were found to exhibit functional relationships analogous to conventional adsorption isotherms. Table 3 Adsorption isotherm parameters for phenol adsorption on BPL carbons at 25 o C Activated carbon type a n max (mmol/g) K o (mmol/l) n Untreated BPL Oxidised BPL Nitrided BPL a Maximum uptake of adsorbate.

9 120 J.I. Ume and G.O. Mbah Figure 5 Effect of electrostatic potential on fraction of sites available to untreated and modified BPL activated carbons 5 Conclusions Several important and convenient parameters for the characterisation of activated carbon surfaces have been investigated for as-received and modified activated carbon samples. Direct relationships were observed between the percentage carbon content on one hand, and point of zero charge, isoelectric point and pk a of activated carbon samples on the other. pk a values for BPL activated carbons investigated are in good agreement with literature values of Brönsted acidity for functional groups usually present on activated carbon surface. Reduced potential functions, an indication of the propensity of a carbon surface to function effectively, were found to exhibit functional relationships analogous to conventional adsorption isotherms. References Bansal, R.C., Donnet, J.B. and Stoeckli, F. (1988) Active Carbon, Marcel Dekker, New York. Coughlin, R.W. and Ezra, F.S. (1969) Environ. Sci. Tchnol., Vol. 2, p.291. Coughlin, R.W., Ezra, F.S. and Tan, R.N. (1968) J. Colloid Interf. Sci., Vol. 28, p.386. Derylo-Marczewska, A. (1993) Langmuir, Vol. 9, p Derylo-Marczewska, A. and Jaronec, M. (1987) Surf. Colloid Sci., in Matijevic, E. (Ed.) Plenum Press, New York, Vol. 14, p.301. Getzen, F.W. and Ward, T.M. (1969) J. Colloid Interf. Sci., Vol. 31, p.441. Giusti, D.M., Conway, R.A. and Lawson, C.T. (1974) J. Water Pollut. Cont. Fed., Vol. 46, p.947. Hemly, A.K., Ferreiro, E.A. and de Bussetti, S.G. (1987) Ads. Sci. Technol., Vol. 4, p.211. Hunter, R.J. (1981) Zeta Potential in Colloid Science-Principles and Applications, Academic Press, New York. Jacobash, H. (1989) Prog. Org. Coatings, Vol. 17, p.115. Jawaid, M.N.A. and Weber, T.W. (1997) Carbon, Vol. 17, pp Lau, A.C. et al. (1986) Colloids Surf., Vol. 18, p.93.

10 Analysis of adsorption equilibrium for aqueous solution 121 Leon, Y. and Leon, C.A. et al. (1990) Carbon, Vol. 28, p.369. Lopez-Ramon, M.V., Moreno-Castilla, C., Rivera-Utrilla, J. and Hidalgo-Alvarez, J. (1993) Carbon, Vol. 31, p.815. Mahajan, O.P., Moreno-Castilla, C. and Walker Jr., P.L. (1980) Separation Science and Technology, Vol. 15, pp Marquardt, D.W. (1963) J. Ind. Appl. Math., Vol. 11, p.431. Muller, G., Radke, C.J. and Prausnitz, J.M. (1980) J. Phys. Chem., Vol. 84, p.369. Muller, G., Radke, C.J. and Prausnitz, J.M. (1985) J. Colloid Interf. Sci., Vol. 103, p.466, 484. Oda, H., Kishida, M. and Yokokawa, C. (1981) Carbon, Vol. 19, p.243. Puri, B.R. (1980) Activated carbon adsorption from the aqueous phase, in Suffet, I.H. and McGuire, M.J. (Eds.): Ann Arbor Science, Michigan, Vol. 1, p.353. Rosene, M.R. and Manes, M. (1977) J. Phys. Chem., Vol. 81, p Snoeyink, V.L., Weber Jr., W.J. and Mark, H.B. (1969) Environ. Sci. Technlo., Vol. 3, p.918. Solar, J.M. et al. (1990) Carbon, Vol. 28, p.369. Ume, J.I. (1994) PhD Thesis, The Pennsylvania State University. Urano, K., Koichi, Y. and Nakazawa, Y. (1981) J. Colloid Interf. Sci., Vol. 81, p.477. Wang, L.K., Leonard, R.P., Wang, M.H. and Groupli, D.W. (1975) J. Appl. Chem. Biotechnol., Vol. 25, p.49. Weber Jr., W.J. (1972) Physicochemical Processes for Water Quality Control, Wiley-Interscience, New York. Weber Jr., W.J. (1985) Adsorption Technology: A Step-by-step Approach to Process Evaluation and Applications, in Slejko, F.L. (Ed.) Marcel Dekker, New York, Vol. 19, p.1. Yopps, J.A. and Fuerstenau, D.W. (1964) J. Colloid Sci., Vol. 19, p.61.

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