United States Environmental Protection Agency Municipal Environmental Research v Laboratory Cincinnati OH 45268 Research and Development EPA-600/S2-83-047 Aug. 1983 vvEPA Project Summary Predicting Preferential Adsorption of Organics by Activated Carbon Georges Belfort, Gordon L. Altshuler, Kusuma K. Thallam, Charles P. Ferrick, Jr., and Karen L Woodfjeld Preferential adsorption of organic compounds from dilute aqueous solu- tions onto activated carbon (AC) was studied to develop a comprehensive theoretical basis for predicting adsorp- tion of multicomponent solutes. The research program investigates why some solutes are strong adsorbers, and others weak, and why some solutes displace others during aqueous phase adsorption. The overall objectives were to develop, test, and simplify the theo- retical basis for prediction. The fundamental, multidimensional approach of the solvophobic thermo- dynamics theory was used to correlate the extent of adsorption for the com- prehensive theory with the overall stan- dard free energy change for the associa- tion adsorption reaction in solution, and for the simplified theory with the cavity surface area of the solute. Experimental adsorption isotherms of two homologous series (alkyl phenols and alkyl alcohols) were measured and used to test the theory. Differences resulting from simplestructural modifi- cations of solutes were predicted theo- retically and confirmed experimentally. Several experimental innovations for equilibrium adsorption studies have been introduced to reduce solute loss by extraneous adsorption and vaporiza- tion. Small negative activation energies for intraparticle pore and surface diffu- sion of alkyl phenols were also calcu- lated from a temperature study. This Project Summary was developed by EPA's Municipal Environmental Re- search Laboratory, Cincinnati, OH, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Background The ability to predict the effects of even simple structural modifications on the ad- sorption of organic molecules from dilute aqueous solutions onto AC (or other ad- sorbents) could be of great value in the design and operation of large-scale com- mercial water and wastewater treatment plants. Structural modifications such as those found between isomers of the same homologous series are weU known to make the difference between benign and toxic compounds. At present, most theo- retical approaches either rely on single- solute isotherms originally derived from gas and vapor phase systems to predict mixed-solute isotherms, or they rely on solubility theory. Although Traube and others more than 90 years ago recognized the need to include the solvent interactions, only recently have attempts to quantitize these effects been made. Recent attempts to include the solvent effect in aqueous-phase adsorption include a semi-empirical approach based on partial solubility parameters and some arbitrarily chosen parameters called the net adsorp- tion energy approach. Another approach called the "thick compressed film theory" (or "Polanyi adsorption potential theory," as it is often called) has been used to describe adsorption isotherm behavior. The problem with this three-dimensional adsorbed film model is the difficulty in defining the properties of the film and the need to use "scaling factors" for construct- ing the so-called characteristic equation. Objectives The objectives of this study were: 1. Develop a comprehensive formalism of dilute aqueous-phase adsorption ------- of organics, including fundamental formulations of all dominant inter- actions between solute, solvent, and sorbent; 2. Test the use of this formalism to predict a ranking order of adsorption capacity of members of two homo- logous series; 3. Measure experimentally the equilibri- um adsorption isotherms at constant pH and temperature for a statistically significant number of members of two homologous series; 4. Propose and apply certain simplifying assumptions to the comprehensive model that would result, for special cases, in a simplified analytical ex- pression; 5. Compare correlations of adsorption capacity from the comprehensive and simplified theories with those ob- tained with other independent vari- ables such as the molecular weight, density, index of refraction, molar volume, molar refraction, octanol- water partition coefficient, parachor, and polanzability; and 6. Examine the possibilities of using the derived theory for predicting multicomponent adsorption. Objective 1 is fundamental to the other objectives. The theoretical formalisms developed during this study were tested both with experimental data obtained from the literature and that suggested m Objec- tive 3. Objective 4 is crucial to the practicability and usefulness of the sug- gested approach. Objectives 5 and 6 were included because the usefulness of the theory will eventually depend on its ability to rank-order the adsorption of solutes with betterpredictability(i.e., higherlinear coefficients of correlation) than other in- dependent variables, and from multicom- ponent systems, respectively. Materials and Methods Crushed granular activated carbon (PAC) (U.S. Sieve Series No. 200 to No. 400 range and 1031 m2/g) was the major adsorbent used in this study. Comparison of adsorption isotherms of five alkyl phenols on granular activated carbon and on graph- itized carbon (89 m2/g) was undertaken. Cleaning procedures of the PAC were an important aspect in obtaining reproduc- ible isotherms. The adsorbates used included two homo- logous series of 19 alkyl phenols and 1 2 aliphatic alcohols all of the highest grade available (>99% purity). Stock solutions of the individual compounds were made up with 0.01m phosphate buffer and stored in covered bottles at pH = 7.0. From a practical viewpoint, both homologous series are commonly found in surface wastewaters, i.e., alkyl phenols are widely found in coal-conversion process waste- waters and alcohols are found in industrial effluents because of their wide use as solvents and reactants. To minimize ex- traneous solute-loss and maximize solute/ sorbent contact, several innovations have been introduced into the adsorption iso- therm procedures in an attempt to im- prove reproducibility and accuracy and to reduce solute losses. Thus, completely filled and capped stain- less steel tubes containing the adsorbate/ adsorbent mixture were rotated 360° end- over-end at 2 rpm for 24 hours at 20 ± 0.5°C. The tubes were then ultracentri- fuged at 20,000 rpm for 20 to 30 mm at 20°C, thereby spinning down the carbon. The tubes were then opened, and the supernatant was analyzed directly. Theoretical General Solvophobic Approach The solvophobic (c$) theory describes the tendency of a surrounding solvent medium to influence aggregation or dis- sociation of those molecules with consider- able microsurface areas exposed to the solvent medium. In the solvophobic treatment, adsorp- tion is considered as a reversible reaction between the adsorbate molecules, S,, and the activated carbon, C, to form the ad- sorbed complex, S,C, at the surface of the carbon, S, + C ^ S,C. The effect of the solvent on this reaction is obtained by subtracting the standard free energy change for the reaction in the gas phase from that in the presence of the solvent (taking as standard states X°k = 1, p°k = 1 atmosphere ideal gas). This process results in a net free energy change; AG [Solvent effect) .ex- pressing the effect of the solvent on the association adsorption reaction. Conceptually, Sinanoglu proposed a two-step dissolution process. 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. Quan- titatively this process is expressed as follows: or AG AG net (solvent effect) Ar assoc Arassoc Alj (solvent) ~AU (gas) RT net _ '(solvent effect) ~~ . ,, net . ~net . -net AGJ,S,C-AGJ,S, - AGJ,C (2) where kk = pk/Xk is the Henry's constant for the kth species, and j represents each type of interaction. After specifying each interaction such as the cavity, van der Waal's, and electrostatic terms plus two correction terms for polymer mixing and reduced electrostatic effects because of the presence of the solvent, the following expression is obtained from Eq. (1) and (2) for the overall standard free energy change, viz. A f^ AG assoc (solvent) - [AGcav + AGvdw assoc (gas) net , AGredl S.C-S,-C - RT In (RT/P0V) (3) (1] where the last term is called the cratic term and results from an entropy or free volume reduction. AG floiventi is related to the experimental equilibrium constant, Ksolvent i - XS,C/Xs,X0 which itself wil1 be related to the experimental adsorption capacity'p, for solute S, later in this analysis. Each term in the square bracket in Eq. (3) can be calculated explicitly from known physiochemical parameters obtained from the literature. Explicit formulae do this and a discussion on the relevance of each term to the adsorption association reaction are presented in Appendix A m the compre- hensive report. For the comprehensive c<£-model, each of the terms in Eq. (3) is calculated explicitly; for the simplified model, the "thermo- dynamic microsurface area change of the reaction," AA, in the cavity term is as- sumed to be proportional to the cavity surface area, TSA, of the specific sorbate and homologous series, AA = g TSA. Thus in this study. In Q°b is correlated with TSA, where Q°b is the initial slope at low solute concentration for the Langmuirian adsorption isotherm. In addition, competi- tive adsorption as related to surface tension of a multicomponent solution, diffusional kinetics, and a comparison of various isotherm models for use in the Ideal Ad- sorbed Solution (IAS) theory are all included in the comprehensive report. Experimental Results In the comprehensive report, the experi- mental batch adsorption isotherm results are presented and discussed. After dis- ------- cussing adsorption sensitivity and the range of solution concentration used, ad- sorption onto powdered activated carbon (PAC) is compared with adsorption onto graphitized carbon (GC). Kinetic studies are then discussed ipso facto and in con- nection with the time to reach pseudo- equilibrium and the effects of temperature. Thereafter, both single and multicom- ponent adsorption results are presented and analyzed with respect to the solvo- phobic thermodynamic (c0) approach. Some of these results are presented below. Kinetic Studies The time to reach a plateau or pseudo- equilibrium adsorption capacity for phenol, 2-cresol, 2-ethyl phenol, 4n-propyl phenol, 2-butyl phenol, 2 pentyl phenol, and 2 hexyl phenol is 1, 4, 8, 9, 11, 12, and 14 hours, respectively. Plots, showing the experimental data points superimposed onto the theoretical curves based on the Freundlich model obtained from Suzuki and Kawazoe, are used to obtain the surface and pore diffu- sion models. The results obtained from this procedure are summarized in Table 1; the surface and pore diffusion coefficients for 4n- propyl phenol are unexplainably too low. The rest of the results follow the expected trend of decreased diffusion rate with increased solute cavity surface area (TSA). Since De ^ D^ the De values are within an acceptable range of values, and the Ds values are comparable to previously re- ported values in the literature. For example, for phenol. Van Vliet et al., using a single parameter approach, report Ds = 1.24 x 10'9 cm2 sec'1 in Filtrasorb400, * whereas Peel et al., using their branched-pore kinetic model, report Ds % 7.75 to 9.01 x 10'8 cm2 sec"1 for rapid diffusion in the macro- pores of Filtrasorb 400. 2-Butyl phenol was sorbed onto PAC at four different temperatures, 20 °C, 30 °C, 40 °C, and 50 °C, to determine the effect of temperature on the adsorption dynamics. Both the single parameter surface and pore diffu- sion models to fit the experimental data, Ds(t) and De(t) were obtained and plotted in an Arrhenius plot to obtain the activation energy, Ea. From the plots, Ea values equal to-2.79 cal gmole"1 and 3.20 cal gmole'1 were obtained for surface and pore diffu- sion, respectively. Two competing phe- nomena can be hypothesized; internal diffusion is expected to increase with temperature, while for the exothermic re- actions such as adsorption of organics 'Mention of trade names or commercial products does not constitute endorsement or recommendation for use Table 1. Surface and Pore Diffusion Coefficients Obtained from Adsorption Kinetics Diffusion coefficients, x 1O6 Solute 1. phenol 2. 2-cresol 3. 4-n-propyl phenol 4. 4-ethyl phenol 5. 2-butyl phenol 6. 2-pentyl phenol 7. 2-hexyl phenol Molecular weight 94.1 108.2 122.3 136.4 150.2 164.3 178.4 Bulk Db cm2 sec'1 9.127 8.122 7.366 6.773 6.292 5.893 5.520 Surface Ds cm2 sec'1 5.26x1 Or4 4.62x1 0-4 1.28x1Cr4 4.36x1 0-4 3.59x1 0-4 3.21x1 Or4 2.695x1 Q-4 Port?' De cm2 sec'1 8.88 8.15 2.16 8.09 6.97 6.62 5.56 onto PAC the reverse is true. The linear plots and the small negative activation energies that result highlight both the competition and the relative importance of the exothermic adsorption reaction. Single Solute Adsorption The major experimental effort of this study involved the careful measurement and accumulation of statistically relevant data of single solute aqueous phase ad- sorption isotherms for two homologous series. The main purpose for doing this is to establish a reliable data base for evalua- ting the efficacy of the solvophobic thermo- dynamic treatment in ranking the adsorp- tion intensity of the different members of different homologous series. Homologous series of 19 alkyl phenols and 12 aliphatic alcohols were chosen to represent aromatic and aliphatic organic groups in water, respectively. The adsorption results in terms of In Q°b versus molecular weight and TSA are summarized in Figs 1 and 2. One of the major findings shown in Fig 1 is that branched compounds have lower adsorbability than linear or normal com- pounds. For the phenols, however (Fig 2), fragmented compounds (alkyl group dis- tributed around the ring) exhibit higher adsorbability. Also above a molecular weight of about 1 50 D, adsorbability is independent of molecular weight. Comparison between the goodness-of- fit for correlating adsorbabilities with MW and TSA (i.e., checking the simplified c0- model) shows that for 11 alkyl phenols, rmw = 0.93(7) is different from rTSA = 0.97(5) with a confidence of 70%. For all 12 alcohols, rmw = 0.72 is different from rTSA = 0.93 with a confidence of 95%. In summary, the adsorption capacity decreases (slope increases) for each isomer of a homologous series with in- creased branching or decreasing cavity surface area. By measuring the gas-phase adsorption of the same homologous series of com- pounds as that measured from the liquid- phase, AJ^o'vent effect) could be checked. With the study of additional homologous series and the knowledge of the molecular struc- ture and volume of each member, the res- triction of adsorption because of steric hinderance, as recently suggested by Benedek (slow adsorption), could be de- termined. Further work is also necessary to couple the theory to Myers' new charac- teristic dimensionless adsorption para- meter. With respect to equilibrium adsorption isotherm measurements, the limited data base should be extended to include a comparison of different activated carbons with different pore sizes and surface activities. Adsorption of ionizable homo- logous groups should also be measured. Competitive adsorption effects should be extended to include additional homologous series. Finally, we recommend that the theory be evaluated for competitive ad- sorption of some industrial effluent streams, such as coal-based effluents containing many isomers of phenols. The full report was submitted in fulfill- ment of Cooperative Agreement No. CR-80664801 by Rensselaer Polytechnic Institute under sponsorship of the U.S. Environmental Protection Agency. ------- I 3 Alcohols slope ~ 0.057 int ~ -2.842 r = 0.72(0) 2-M-1-B • 1-B 1-Hx 4-E-1-Pe 2-E-1-B 2-M-3-Pe 1-Hp 2,4-diM-3-Pe 3-E-3-Pe 70 80 1-Pe 3,3-diM-1-B 2,3-diM-2-B = n Alcohols = Branched 90 100 110 Molecular Weight 120 5.0 4.0 3.0 2.0 1.0 0.0 1-Hp Alkyl Alcohols 1-Hx 4-M-1Pe 2-E-1-B • • •• 2,4-diM-3-Pe 2-M-3-Pe 3-E-3-Pe 2-M-1-B • 1-Pe 3,3-diM-1-B 2.3-diM-2-B r = 0.5264 SL = 0.0335 INT = -8.561 #PTS. = 12 ST. DEV. = 0.3974 i 250 260 270 280 290 300 310 320 330 340 350 TSA (fc) Figure 1. A dsorption (1nQ°b) versus molecular weight (MW) and total cavity surface area (TSA) for linear and nonlinear alkyl alcohols. ------- 7.5 7.0 6.5 6.0 I 5.5 5.0 4.5 4.0 4-pentyl • Alky I Phenols 2-butyl • 4-butyl • 2,3,6-trimethyl A 2-pentyl 2-hexyl • 4-tert pentyl m 4-tert butyl 2,6-dimethyl A $^2.3,S-trimethyl 2,3-dwethyl£34al£ , 4-propyl 3,5-dimethyl 2,5-dimethyl* • 4-ethyl • 4-isopropyl 2-methyl • phenol • linear A fragmented m branched r = .87(9) H9 compounds) r = .93(7) (12 compounds) 90 100 110 120 130 MW 140 150 160 170 ? c 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 Alkyl Phenols 4-pentyl 2-butyl 2-pentyl 2-hexyl '4-butyl • 4-tert-pentyl 2,3,6-trimethyl A -/" • 4-tert butyl 2,6-dimethyl A A/ 2,3,5-trimethyl 2.3-dimethyl 4.isopropy, 4-ethyl • 2-methyl • linear A fragmented • branched phenol r = .88(1) 119 compounds) r = .97(5) (12 compounds) 220 240 260 280 300 320 340 360 380 400 420 440 TSA (k) Figure 2. Adsorption (1nQ°b) versus molecular weight (MW) and total cavity surface area (TSA) for linear, nonlinear, and fragmented alkyl phenols. ------- Georges Be/fort, Gordon L Altshuler, Kusuma K. Thallam, Charles P. Ferrick, Jr., and Karen L. Woodfield are with Rensselaer Polytechnic, Troy, NY 12181. Richard A. Dobbs is the EPA Project Officer (see below). The complete report, entitled "Predicting Preferential Adsorption of Organ ics by A ctivated Carbon." (Order No. PB 83-222 778; Cost: $ 13.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Municipal Environmental Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 -&U. S. 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