Citation for published version (APA): Fischer, V. M. (2001). In situ electrochemical regeneration of activated carbon [Groningen]: s.n.

Transcription

1 University of Groningen In situ electrochemical regeneration of activated carbon Fischer, Vincent Marco IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 21 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Fischer, V. M. (21). In situ electrochemical regeneration of activated carbon [Groningen]: s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 1 maximum. Download date:

2 C HAPTER 3 ELECTROSORPTION ISOTHERM DATA 3.1. The potential dependent isotherm Langmuir-like isotherms The isotherm gives the relation between bulk and surface concentration. This relation is usually based on model assumptions. In the simplest model every adsorption site is equivalent, only monolayer adsorption occurs and no interactions exist between molecules at adjacent sites. If the equilibrium in Eq with ν = 1 is considered, the rate of adsorption of component B is found to be proportional to the fraction of the surface not occupied by it (1-θ) and to the bulk fraction x B. The rate of desorption is proportional to θ. At equilibrium both rates are equal: ads ( θ) x = k θ k B des Replacing the kinetic constants k ads and k des with the potential dependent equilibrium constant (Eq. 2.22) gives: θ f ( θ) = = Kx B = K( φ) x B θ 39

3 The result is the Langmuir isotherm, often only useful as a first approximation or for simple systems. Parsons (1963), Delahay (1965, chapter 5), Gileadi (1967, chapter 1) and Damaskin et al. (1971, chapter 3) gave isotherms for more complex systems. A number of them are listed in Table 3.1. They can all be derived from the basic Langmuir isotherm by changing one or more assumptions. Name isotherm Equation f(θ) Henry Langmuir Langmuir (ν > 1) = θ θ = 1 θ θ = (1 θ) ν [ θ + ν(1 θ) ] ν ν ν 1 Volmer = 1 θ θ exp 1 θ θ Helfand-Frisch- exp 2 θ Lebowitz 1 θ ( 1 θ) θ = 2 θ 1 θ Frumkin = exp( 2aθ) Virial = θexp( 2a θ) with a < Temkin = exp( 2aθ) Table 3.1: Langmuir like isotherm equations. The constant a is the Frumkin interaction parameter that can be positive (attraction) or negative (repulsion). For low surface coverage the Langmuir isotherm is identical to the linear Henry isotherm (1 θ 1). If the adsorbed molecule B occupies more then one site (ν >1), the Langmuir isotherm becomes more complex (Gileadi, 1967 chapter 1). The Temkin isotherm takes into account interactions between adsorbed molecules. Each additional molecule will adsorb with less ease as the heat of adsorption does not remain constant but changes (linearly) with coverage. The Volmer and the HFL isotherms consider the adsorbed phase as a two 4

4 dimensional fluid of rigid particles. The Frumkin and Virial isotherms take into account long-range interactions between adsorbed molecules that can be attractive or repulsive. Both the logarithmic Temkin and the Langmuir isotherm can be considered special cases of the Frumkin isotherm. Typical plots of these isotherms are given in Figure 3.1 for phenol on activated carbon. Excluding the Henry isotherm, the Langmuir isotherm gives the highest surface coverage for a certain bulk concentration. 1 Henry Langmuir Frumkin θ [-].5 Volmer Virial HFL c [g/m ] 15 Figure 3.1: Various isotherms for phenol on activated carbon. Parameters used: K =.439 and a = The Freundlich isotherm Generally, the empirical Freundlich isotherm describes adsorption of organic compounds on activated carbon better than the Langmuir-like isotherms. A number of attempts were made to provide the Freundlich isotherm with a more theoretical background. Halsey (1952) and Rudnitski and Alexeyev (1975) could obtain it from the Langmuir isotherm using the following assumptions: 1) The adsorbent surface is heterogeneous. 2) The site energies are distributed exponentially. 3) For all sites with the same energy a Langmuir isotherm is applicable. Summation of all these Langmuir isotherms yields the Freundlich isotherm: 41

5 n q = K F F x B 3.3 with K F and n F semi-empirical constants and generally <n F <1. The Freundlich isotherm does not contain the equilibrium constant and hence is no function of the electrical potential. In order to describe electrosorption the isotherm must be modified. Because of the empirical nature of the isotherm it is simply assumed that K F depends on the potential similarly to the equilibrium constant: K F ( φ) K( φ) and n F is assumed to be independent of the potential. These assumptions are supported by the observation of McGuire et al. (1985) that K F was much more sensitive to φ than n F. In order to improve the fit of Langmuir type isotherms it is possible to use x n instead of x. This results in Langmuir-Freundlich, Volmer-Freundlich etc. isotherms that tend to fit data much better, however at the cost of introducing an additional (empirical) fit parameter Fitting literature data Loading versus potential curves The electrosorption model (Eq. 2.36) predicts bell shaped curves when K(φ) and q(φ) are plotted versus φ. The position of the maximum depends on the potential difference at open circuit conditions caused by adsorbed solute dipoles. For a neutral molecule, the open circuit potential will be close to zero. In Figure 3.2, the theoretical equilibrium loading of phenol on activated carbon as a function of φ is plotted for both the Langmuir and the Freundlich isotherm. The steepness of the loading curve depends not only on molecular properties and characteristics of the double layer, but also on the type of isotherm used. Increasing the φ N value (in Eq. 2.36) will cause both a shifting and an increase of the maximum surface loading of the carbon. If the concentration B is high, the Langmuir isotherm becomes a horizontal line, the monolayer coverage q max. In our electrosorption model the monolayer coverage is no function of the applied potential. This means that switching to a higher potential no longer causes desorption of B. However the adsorption of 42

6 organic compounds on activated carbon usually shows a Freundlich type of behaviour. The Freundlich and to a lesser extend the Langmuir-Freundlich do not have the problem of a constant, potential independent, monolayer coverage at higher concentrations. Electrosorption enhances desorption but Figure 3.2 suggests it is possible to enhance adsorption as well, be it under certain conditions only. If the solute has a permanent dipole and adsorbs with a specific orientation (not completely random) this generates a potential difference. Applying an equal and opposite external potential will nullify this and adsorption of the solute will be enhanced. If the solute bears a charge, the open circuit potential will be displaced more significantly. Negatively charged particles will be attracted to positive surfaces due to coulombic interactions. The (contact) adsorption of ions is treated in appendix A. 8 6 q [mg/g] φ [V] Figure 3.2: The theoretical change in surface coverage as function of the potential for two isotherms. Black lines, Freundlich (KF, = 41.72, nf =.377), grey lines, Langmuir (K =.439, qmax = 71.2). Concentration is 5 kg/m 3, φn = and 1 mv for solid and dotted lines. The collection of experimental electrosorption data available from literature is very small. Most of the data is presented in the form of isotherms at various applied potentials. These isotherms can be converted to q(φ) curves by keeping the bulk concentration constant and taking the corresponding surface loads for 43

7 all applied potentials. The bulk concentrations used were between ½ and ¾ of the maximum concentration reported θ [-] φ [V] θ [-] φ [V] Figure 3.3a) and b): Overview of available electrosorption data in literature. Series are scaled, for zero potential the loading is set to unity. In a): squares, phenol, McGuire et al. (1985); rectangles, naphtalenesulfonic acid anion; circles, benzylalcohol; plusses, methylquinolinium, Bán et al. (1998); pentagrams, o- nitrophenol, Chue et al. (1992); black diamonds β-naphtol, Alkire and Eisinger (1983b); white diamonds, 1,8-dichloro- 9,1-anthraquinone; triangles, phenanthrene quinone, Strohl and Dunlap (1972). In b): diamonds, 9-1-Anthraquinone-1- sulfonic acid; squares, 1.2-naphtaquinone-4-sulfonic acid, Strohl and Dunlap (1972); triangles, EDA, Eisinger and Keller (199). 44

8 The results are plotted in Figure 3.3a) and b). For better comparison, all series are scaled, by setting the surface loading to unity at zero applied potential. In Figure 3.3b) three data series are plotted that have a more strongly displaced maximum of adsorption. From these, 9-1-Anthraquinone-1-sulfonic acid and 1,2-naphtaquinone-4-sulfonic acid have a negatively charged group and their adsorption is strongly enhanced if the carbon is positively charged. The third series (EDA) does not show the (expected) bell shape. EDA adsorption is low for small potentials and increases for larger potentials. An explanation for the anomalous behaviour of EDA could be the occurrence of Faradaic reduction/oxidation already at low potentials. Eisinger and Keller (199) did report that potentials of only +15 mv were sufficient to cause oxidation of EDA. An unaccounted for reaction interferes with determining adsorption, as it is calculated from the difference in bulk liquid concentrations before and after the experiment. Any reactions will contribute to the apparent adsorption. The data series in Figure 3.3a) vaguely show a bell shape. Adsorption is highest around zero potential and decreases with increasing potential. However, most series contain only a few data points or cover only one side of the bell. This strongly reduces the reliability of the data. Despite the experimental uncertainty, an attempt was made to fit the data with the electrosorption model (Eq. 2.36). In Figure 3.4 the results are presented for a selection of the data. Marks represent measurements of various authors, lines are calculated using the Freundlich isotherm with a potential dependent K F constant unless stated otherwise. The required K F, and n F, are taken from the reported open circuit isotherms and are summarised in Table 3.2. The displacement of the maximum φ N is kept (close to) zero for neutral molecules. For charged molecules large values were needed to obtain a reasonable fit. Values for α, µ, ρ and M needed for calculating the polarisability P and the dielectric constant are reported in Table 2.1 for water, phenol and benzyl alcohol. For the other components the first two properties had to be estimated. The error made by this estimation is relatively small as the electrosorption Gibbs free energy only weakly depends on α and µ if the molecule is relatively large. 45

9 All Kirkwood constants are set to a value of 1.5 as was explained in chapter 2. The values for r, c and z regarding the supporting electrolyte are obtained from the various articles. Molecular dimensions are needed to estimate the thickness of the first layer in the Helmholtz capacitor (Eq. 2.2) and to convert differential capacities into integral capacities. The following relations are used: S mol r = N A The molecular radius r depends on the molar surface area S mol, which in turn is calculated from: S 2 3 M 1 3 = 3.5 ρ mol N A the molar mass M and the density ρ. The molar surface area depends on the orientation of the adsorbed molecule. Mattson et al. (1969) report a value of.4 nm 2 for planar adsorbed phenol, yielding an area of about m 2 /mole. Eq. 3.5 predicts a value of m 2 /mole, which is in the same order of magnitude. While fitting the experimental data it was found that the measured electrosorption effects were always smaller then the theoretical effects. In order to account for this, two efficiency factors are introduced. The first is the bed efficiency: ( 1 η) K + ηk( φ) K exp( φ) = 3.6 If the bed efficiency η is smaller than one, part of the packed bed electrode area is electrically inaccessible because the pore size is too small and there is not enough room for an electrical double layer. The adsorption equilibrium constant in these parts of the bed (no changes in potential) remains K. For graphite and glassy carbons η is probably higher than for activated carbon because of the more extensive micropore structure of the latter. 46

10 Another source of inefficiency is the loss of part of the applied potential difference due to ohmic resistances in the system. In order to compensate for these, higher potentials have to be applied..4 naphtalenesulfonic acid q [g/g].2 N+ CH 3 methyl quinolinium SO 3 - benzyl alcohol -1 1 φ [V] CH OH 2 q [mg/g] OH phenol phenolate O φ [V] β-naphtol 6 K [1 ].5 OH φ [V] 2 47

11 1 phenanthrenequinone capacity of bed [ml] 5 O O Langmuir- Freundlich Freundlich -1 1 φ [V] 2 Figure 3.4a) to d) experimental data and model fits: a) three compounds on activated carbon (Bán et al. 1998). b) phenol (and phenolate?) on activated carbon (McGuire et al., 1985). c) β- naphtol on glassy carbon (K values) (Alkire and Eisinger, 1983b). d) Breakthrough curves of phenanthrenequinone (Strohl and Dunlap, 1972). In Figure 3.4a) three data series are plotted: The change in carbon loading as function of potential for benzyl alcohol (neutral), methyl quinolinium (positively charged) and naphtalenesulfonic acid anion (negatively charged). The displacement of the maximum is perfectly illustrated. An extensive set of variables is needed to calculate the model lines: α, µ, ρ, M, k KW, r ion, c ion, z, K F,, n F,, φ N and η. Parameters not found in literature (Weast and Astle, 1979) were estimated. In Table 3.2 an overview is given of some of the values used to obtain a reasonable fit between electrosorption model and measured data. An attempt was made to keep the two efficiency parameters as constant as possible. Figure 3.4b) shows electrosorption data for phenol. These data points cannot be fitted adequately with our model. This could be the result of a Faradaic stimulated transition from phenol to the phenolic anion due to an increasingly positive charged carbon. To illustrate this, two model lines are drawn in the figure: One represents phenol, the other the phenolic anion. The only difference between the curves is a different φ N value used. 48

12 Name component q max [g/kg] K [varies] n F, [-] η φ/φ appl φ N benzyl alcohol methylquinolinium naphtalene sulfonic acid phenol (1) (2) β-naphtol E Phenanthrenequinone Table 3.2: Adsorption constants (Langmuir-Freundlich or Freundlich) and efficiencies used for fitting the experimental data in Figure 3.4. In Figure 3.4d) experimental results from Strohl and Dunlap (1972) are plotted. The markers do not represent isotherm data, but column breakthrough curves. For small potentials, the column performance is much better than for higher potentials, also the big plateau is remarkable. The Freundlich isotherm is unable to describe such a plateau but the Langmuir-Freundlich isotherm is. A cautious conclusion from the fitting of these experimental results is that they do not give a cause to doubt the validity of the electrosorption model. A more distinct conclusion is not justified at this point considering the poor quality of the available data. It was found that the bed efficiency strongly influences the shape of the loading curve. At higher potentials K will become constant instead of decreasing towards to zero. This feature can be used to estimate the bed efficiency from experimental results obtained at higher potentials. Generally the changes in loading predicted by the model are too large. The experimental changes are a factor of two smaller. To explain this it is assumed that half of the applied potential is lost due to ohmic resistances. A potential that is twice as high is needed to obtain the wanted result. Furthermore a substantial part of the bed is considered electrochemically inactive. It can be 49

13 concluded that the experiments give about 4% of the effects predicted by our theory Physical feasibility of electrosorption It is important to establish the theoretical maximal effects that can be induced on the adsorption equilibrium with an applied potential. The maximal potential that can be used is the potential where electrochemical conversion of water occurs. At standard conditions the thermodynamic potential difference to convert water into oxygen and hydrogen gas is V. The actual potential difference is higher due to over-potentials and ohmic losses (Prentice, 1991). Also the reaction can be kinetically hampered, proceeding at a very slow rate. To remain on the safe side however, applied potentials must not be too high to prevent electrolysis. In order to benchmark the changes in adsorption equilibrium due to electrosorption, they are compared to experimental chemical regeneration data published by Suzuki (199). Suzuki reports isotherm data for phenol on activated carbon in presence of methanol 1. The methanol causes a decrease in phenol adsorption as can be seen from Figure 3.5. The experimental data is fitted with the potential dependent Freundlich isotherm. The potentials needed to produce the same decrease of the isotherm are calculated and shown in the graph. The bulk concentration is assumed to remain at the same value. A single constant n F value was used and K F = K F (φ). Furthermore: φ N is zero, η is set to 1% and.5 N KCl is the fictive electrolyte. The Freundlich isotherm with one potential dependent parameter fits the experiments very well. The deviation at the % methanol series is due to the use of an average n F value instead of the best value. The theoretical potentials for this idealised benchmark (1% bed efficiency) are, with exception of the 1% MeOH line, below the maximum allowable potential (see also section 1 In a second text o-chlorobenzoic acid is mentioned instead of phenol. It is not clear which of the two texts is in error so it is assumed that the experimental data represents phenol. Calculated model data for o-chlorobenzoic acid is in the same order of magnitude as for phenol but are far more uncertain because electrical properties are unknown and have to be estimated 5

14 4.1). Reducing the bed efficiency will of course increase the required potentials or the installed surface area MeOH % 2% φ V.43V q [mg/g] % 6% 8%.67V.93V 1.15V 2 4 1% V 8 1 c B [g/m ] Figure 3.5: Adsorption isotherms for phenol on activated carbon in the presence of methanol. Dots are data reported by Suzuki (199, chapter 9). The applied potentials give a similar decrease of the isotherm due to the electrosorption phenomenon. Comparison of methanol induced desorption and electro-desorption indicates that, at least in theory, the latter can be as effective a tool for regeneration as the chemical method. Additionally, our model makes it possible to predict the isotherm for various potentials, using only the open circuit isotherm constants and the molecular properties of the solute and the solvent Economical feasibility Electrical energy requirements Electrosorption has to be competitive with alternative regeneration techniques. This means that not only technical but also economic considerations determine the success of the method. The energy requirements for electrosorption, apart from those needed for pumping and other secondary process operations can be divided into two parts. Electrical energy is needed to: Polarise the surface of the packed bed electrode. Replace the adsorbed component (B) by water (A). 51

15 A schematic view of the regeneration process is given in Figure 3.6. In situation Q, the activated carbon is saturated with organic material B. Part of the surface remains covered with A, depending on the bulk liquid concentration and the adsorption equilibrium. If the packed bed is polarised situation R is obtained. The polarising energy is given by: 2 charge = 2 1 C tot φ U 3.7 After the desorbing potential has been established throughout the bed, water molecules will replace part of the adsorbed organic molecules (situation S). 1 organic compound Charging 2 3 Desorption Carbon surface water Saturated bed Formation of double layer Exchange of dielectric Figure 3.6: Schematic picture of the carbon surface. Part of it is covered with water and part with an organic compound. Two energy-consuming steps occur: formation of the double layer and replacing the organic by water. Not all B will desorb, some molecules will remain on the surface. Replacing B by A alters the overall dielectric constant and hence the system capacity. The change in capacity requires an additional amount of energy equal to: ( C C ) 2 U 3.8 des = 2 1 tot, A tot, B φ Desorbing one mole of phenol The amount of electrical energy needed to desorb one mole of phenol can be calculated using Eq. 3.7 and Eq The values of three variables as function of concentration and applied potential are examined: U charge, U des and m the 52

16 amount of carbon in the bed. This is the bed size from which exactly one mole of B will desorb. It is not equal to the molar surface S mol,b because the surface coverage is lower than unity and desorption of B is never 1%. Lower surface coverage and poor desorption characteristics increase the required bed size. In order to describe a surface covered by a binary mixture the capacity model has to be refined. The surface is represented by two parallel capacitors. One for the surface area covered with A, the second for the area covered with B. See also the left circuit in Figure 3.7. The relative size of both capacitors depends on the surface coverage of B. C H,B C GC C tot,b θ B B A A A C tot,a (1- θ) A A A C H,A C GC Figure 3.7: A polarised carbon electrode in contact with a water/phenol mixture is described by 6 capacitors. The top branch represents the surface area with adsorbed B, the lower branch represents the surface area with adsorbed A. In chapter 2 it was found that in order to describe the double layer more accurately, three capacitors in series are needed. Two capacitors for the compact or Helmholtz double layer and one for the diffuse or GC double layer. This means that a total of six capacitors is required (the right circuit in Figure 3.7). Only one of these six capacitors contains the dielectric B, the others are filled with water. The overall capacity can be calculated from: C tot S = q BET M q 1 B 1 C H, B θb, 1 + C GC + 1 θb, C C H, A GC

17 where q is the surface load at open circuit conditions and q 1 is the surface load if a potential is applied. Also: q Smol, B θb, = 3.1 S M BET B The surface coverage must be determined experimentally before Eq. 3.9 can be used to calculate energy requirements. Suzuki (199, chapter 9) provides this data. See section The first term in Eq. 3.9 ensures that exactly one mole is desorbing. S BET is the BET surface area of 1 kg activated carbon (assumed to be m 2 /kg). The C H and C GC represent the modified Helmholtz and the Gouy-Chapman capacities. They are calculated using Eq and Eq Results The energy needed for charging the bed depends on the surface coverage and the applied potential, whereas the energy needed to desorb B is a function of potential only. The surface coverage in turn depends on the isotherm used and the bulk liquid concentration. In Figure 3.8 the energy requirements versus potential and concentration are given. A higher concentration means a higher surface coverage and therefore less waste of charging energy. charge [kj/mol] U c is varied [g/m 3] : B U des φ is varied [ V]: φ [V] c B [g/m 3 ] Figure 3.8: The change of electrical energy with c and φ. Dotted line denotes the desorption energy, other lines the charging energy. If the energy requirements are plotted against the potential, a minimum can be observed around.65 V if c B = 25 g/m 3. At higher concentrations no 54

18 minimum is found. If the applied potentials are small, a larger bed needs to be installed, increasing charging costs. This effect becomes even more pronounced for lower bulk concentrations. From the graphs it can be seen that energy costs go down with decreasing potential. This is because the stored electrical energy depends quadratically on the potential (Eq. 3.7 and 3.8). A numerical overview of the required surface area, charging and desorption energies is given in Table 3.3. c B [g/m 3 ] φ [V] Property U charge [kj/mol] U desorb [kj/mol] m [kg] U charge [kj/mol] U desorb [kj/mol] m [kg] U charge [kj/mol] U desorb [kj/mol] m [kg] U charge [kj/mol] U desorb [kj/mol] m [kg] U charge [kj/mol] U desorb [kj/mol] m [kg] Table 3.3: Energy costs and installed amounts of carbon for electro-desorption of one mole phenol at various bulk concentrations and applied potentials. The required charging energy needed to desorb phenol lies between 4.3 kj/mol and 3.7 kj/mol. The desorption energy lies between 2.6 kj/mol and 15 kj/mol, about one third of the total energy demand of 7 to 45 kj/mol phenol desorbed. 55

19 The actual energy needed for charging the bed is higher than this theoretical amount because in real systems electrical losses will occur due to electrical resistances in the solid and the liquid phase. Higher potentials must be applied to account for this. These do not change the amount of stored electrical energy, as the potential drop over the solid liquid interface remains unchanged. Furthermore the counter electrode must be charged. The total potential drop will be distributed over the WE and the CE. If the electrodes are equal, the same goes for the potential drop. System configurations The energy use of the system depends on configuration aspects as well. If the adsorption constant depends symmetrically on the applied potential and the maximum lies close to zero, it is better to use two packed beds instead of one. The first packed bed is our normal working electrode and the second bed is the counter electrode. For a symmetrical dependence on the potential a positively and a negatively charged bed both reduce the adsorption constant to the same extent. This means no energy is lost on faraday reactions at an inert counter electrode. If two electrosorption units are used in series, part of the electrical energy stored inside the double layer can be re-used. This stored energy of the first electrosorption unit can be used to polarise the electrodes of the second unit (at most) halfway. Conclusions and discussion Eisinger and Keller (199) reported energy requirements of kwh per kg EDA depending on process configuration for the recovery of one mole of EDA from brine solution. This translates to kj/mol EDA. These values are 15-3 times higher then found for the electrosorption of phenol in this work. It must be stated that EDA loading on activated carbon was a factor of 1 lower then that of phenol at higher bulk concentrations. Also it is uncertain if evaporation costs for the concentrated EDA effluent were included in their calculation. None the less, their energy costs seem to be rather high. For comparison, heating an amount of water containing 1% phenol from 298K to 373 K, requires about 3 kj/mol phenol. 56

20 3.3. New electrosorption data Experimental details: Materials used The carbons used in this work are ROW.8 SUPRA from Norit N.V. (The Netherlands) and Ambersorb Carbonaceous Adsorbent type 572 from Rohm and Haas co. (USA). The ROW.8 SUPRA is an extruded type of activated carbon with a diameter of.7 to.8 mm and lengths between 3 and 1 mm. The Ambersorb 572 is a granular synthetic carbon made from a highly sulfonated styrene-divinylbenzene precursor. The beads are between mesh sizes 2/5 and possess an excellent mechanical strength. No attrition could be observed for the Ambersorb 572 where the ROW.8 SUPRA did produce some fine carbon dust easily. Additional precautions had to be made in order to prevent the carbon powder to interfere with the analytical UV method. Surface areas of the carbons were determined using adsorption of nitrogen. The BET areas found were 78 m 2 /g for ROW and 115 m 2 /g for Ambersorb close to the reported values of the manufacturer. For Ambersorb a particle density of.49 g/ml and a micro/meso/macro pore porosity (in ml/g) of.41,.19 and.24 was reported by the manufacturer. Although the Ambersorb is rather hydrophobic compared to activated carbons from a natural precursor, spontaneous wetting of the pores with water did occur. All chemicals used were obtained from Merck Company. The phenol and the benzyl alcohol were both P/A grade (>99%). The KCl was extra pure grade (99%). The water used to make the solutions is reverse osmosis water that has been de-ionised prior to use Experimental details: The set-up Open circuit isotherms were measured using a batch method. In a thermostatic bath, held at 298 K, a number of sample vials were rotated slowly head over tail, see also Figure 3.9. The slow speed ensured that the carbon particles moved from top to bottom twice during each rotation. The movement of the carbon enhances the contact with the liquid phase, greatly reducing the time to reach equilibrium, to about half an hour. The residence time in the bath was 57

21 however at least 12 hours. All vials were filled with the same liquid containing small amounts of benzyl alcohol or phenol. The amount of carbon added differed per vial. After equilibrium was reached, the remaining organic molecules in the solution are determined by measuring the UV adsorption at 254 nm using an UV photospectrometer from Pharmacia Biotech, type UV-1. thermostated bath Figure 3.9: Side view of the batch set-up for determining open circuit isotherms. Each vial contains the same solution and a different amount of carbon. The movement of the carbon particles during rotation ensures good mixing. In order to measure the influence of the electrical potential on the adsorption equilibrium a different set-up is required. The cell must allow for good electrical contact between carbon particles and external circuit. A number of cells were designed and tested before the final set-up evolved. A schematic overview of the set-up used is given in Figure 3.1. The tank is filled with.5 N KCl electrolyte and can be stirred. Nitrogen gas is added to the liquid to remove dissolved oxygen. A peristaltic pump pumps the brine through the cell and then back to the tank. The cell itself contains two packed beds: the working (WE) and counter electrode (CE). Both are connected to an Amel general-purpose potentiostat, type 251, that is used to set a certain potential difference between the WE and the reference electrode (RE). If no reference electrode is present, the RE outlet is connected directly to the CE outlet. 58

22 Otherwise the RE is an Ag/AgCl electrode with an external 3 N KCl solution reservoir, supplied by Metrohm. The amount of current that flows through the apparatus is monitored by the potentiostat or by an additional ammeter (Keithley multimeter or picoammeter) that can be included in the circuit. PC addition of pollutant N gas 2 potentiostat data V CE UV - spectrometer RE Cell WE A Ammeter Mixing tank peristaltic pump Figure 3.1: The experimental set-up used for determining electrosorption isotherms. A small amount of solvent is added to the mixing tank. After equilibrium is obtained, more solvent is added. This is repeated 5-1 times to construct an isotherm. In the small cell, shown in Figure 3.11, both beds are separated by a glass frit but are in contact with the same liquid. The bed diameter is 52 mm and the bed thickness is about 5 mm. For the large cell, shown in Figure 3.12, a NAFION 45 membrane separates the working and the counter electrode. The counter electrode is connected to a separate mixing tank and separate fluid circuit containing no organic compound. The bed diameter for the large cell is 9 mm and the thickness between 5 and 1 mm. 59

23 liquid out glass wool Pt-wire WE glass frit CE carbon bed liquid in Figure 3.11: Close up side view of the small electrochemical cell for determining electrosorption isotherms. Glass wool is used to reduce the bed porosity. Platinum wire is used as current collector. liquid outlets membrane holder RE connector filter graphite plate RE connector cross section WE CE CE lead connector liquid inlets membrane bolts Carbon bed Figure 3.12: Close up view of the large cell. A Nafion 45 membrane separates both beds. This way any asymmetry of the loading curve can be investigated. The beds can be mechanically pressed. Current collectors are graphite disks on the outside of the beds. 6

24 A pulse of benzyl alcohol or concentrated phenol solution can be added to the mixing tank. At the same moment the measurements start and the decrease in concentration is monitored online with the UV photospectrometer. To this end a second, peristaltic pump is used to continuously pump a small flow from tank to UV, and back again. The measured currents and absorption signals are converted to voltages, collected and stored by a data acquisition unit connected to a personal computer Experimental details: Procedures For each new measurement fresh carbon is used. The compartments in the small cell can hold g of carbon. For the large cell this is 2-25 g. In combination with the small cell 4 litres of electrolyte are used, for the large cell this is 2 L. After filling the electrodes, the potentiostat is set to a specific value up to 1.5 V. Next the apparatus and carbon are rinsed by 4 or 2 L of.5 KCl for at least 24 hours. This is done to remove impurities and carbon dust from the bed, to allow time for the double layer to form and charging currents to die out and to make sure that the carbon beds are free of entrapped air. To obtain the first point of the isotherm, the washing liquid is replaced by fresh electrolyte solution. A certain amount of benzyl alcohol or phenol is then added to the mixing tank. Part of the added impurity will adsorb on the activated carbon and (pseudo) equilibrium is reached within a few hours. The recorded c B (t) and i(t) data are stored in the computer. The final concentration is noted and a small liquid sample is retrieved for later (batch) analysis. Then a new pulse of solute is added in order to determine a second point of the isotherm. This procedure is repeated 5 to 9 times in order to get the isotherm over a broad concentration range. After the isotherm at the set potential has been derived, the whole measurement is repeated a number of times for different potentials, two different solutes (phenol and benzyl alcohol) and two different carbons Results Batch experiments With the batch set-up, up to ten isotherm data points can be produced at the same time, therefore the measurement is completed within 24 hours. The data 61

25 obtained with the batch set-up was found to be very sensitive to experimental inaccuracies. Furthermore it can only be used for determining open-circuit isotherms. Relative errors are in the order of 5% and a significant percentage of the data yielded nonsense. Attempts to improve the reliability of the method did not succeed completely. Purified results are shown in Figure q [mg/g] 1 phenol on Norit ROW.8 SUPRA phenol on Ambersorb 572 benzyl alcohol on Ambersorb 572 phenol (Suzuki, 199) phenol (Tien, 1994) c [g/m ] Figure 3.13: Results obtained with the batch method. Points are measurements from this work, lines are reported isotherms from literature. Although a log scale is used, the scatter in the data is still significant. For comparison two isotherms equations reported in literature (Suzuki, 1994; Tien, 199) are plotted in the same graph. The data seems to be in the same order of magnitude. Continuous experiments From the packed bed configuration (Figure 3.1) much better data is obtained. The disadvantage of this method is that it also takes far more time to determine the complete isotherm. All data points must be measured one after the other, determining an isotherm takes one to two weeks. An example of the output obtained with this set-up is shown in Figure

26 UV signal [mv] t [ks] (a).4 (b) current [ma] Figure 3.14: Typical result obtained using a flow through cell: (a) current, (b) concentration. Both the current and the concentration as function of time are recorded for approximately 1 hours. The concentration curve (b) clearly shows the initial addition of benzyl alcohol. After 8 hours about 95% of the adsorption equilibrium is reached and the liquid concentration becomes constant. The charging curve is not a horizontal line but shows a peak at the beginning of the measurement, indicating a change in the electrical properties of the system. This change seems to be related to the changes in bulk concentration as both have the same time dependency. If absolute changes in concentration are large, the current flowing through the system is also high. In chapter 2 it was shown that the system capacity changes when component B is adsorbing. In chapter 4 it will be shown that this results in a charging current. The occurrence of mass and charge transfer at the same time will be examined in chapter 7. The charging current does not decrease to zero but to a constant value of.4 ma. The reason for this is a phenomenon called streaming-current. The physical background of this phenomenon and experimental data are presented in chapter 7. Benzyl alcohol on Ambersorb 572 The solid phase concentration q was calculated from the difference in initial and final bulk concentration. To get the isotherm the solid concentration was 63

27 plotted versus the bulk concentration. Isotherms where measured for a number of different potential drops. The results for benzyl alcohol on Ambersorb 572 can be seen in Figure The following conclusions were drawn: Changing the potential changes the position of the isotherm. If the applied potential becomes 5 mv or lower, the isotherm starts to decrease after an initial increase. This decrease is more pronounced if the potential is lower. For a simple one component system, a decreasing isotherm is in variance with thermodynamics. It means that if more benzyl alcohol is added to the bulk phase, more adsorbed benzyl alcohol will desorb. This result is illogical and some error must have been made and overlooked. q [g/kg] mv -3 mv -5 mv -6 mv -8 mv c B [g/m ] Figure 3.15: Adsorption isotherms for benzyl alcohol on Ambersorb 572. Decreasing isotherms were found. GC-FID analysis of the samples revealed traces of benzaldehyde in the mixture. It seems that some of the benzyl alcohol is oxidised to form benzaldehyde. The overall reaction is: 2 CH 2 OH + O 2 (aq) 2 CH O + 2H 2 O A small amount of oxygen is dissolved in the electrolyte. This oxygen allows the conversion of the alcohol- to the aldehyde group by taking the remaining protons. The concentrations of benzaldehyde found in the samples were 64

28 relatively low, only a few percent of the benzyl alcohol was converted. This could not explain the large deviations of the isotherms. Analysis of the UV spectrum of benzaldehyde revealed that at 254 nm the absorption was 88 times higher than that of benzyl alcohol. As the UV method cannot discriminate between the various molecules absorbing, all contributions are added. The resulting too high absorption leads to too high apparent liquid concentrations. Because the solid concentration is calculated from the changes in liquid concentration it can explain the decreasing isotherms. The increased production of benzaldehyde at higher potentials suggests an electrochemical contribution to this process although the potentials applied are relatively low for Faraday reactions to occur q [g/kg] mv -3 mv -5 mv -6 mv -8 mv c B [g/m ] Figure 3.16: Adsorption isotherms for benzyl alcohol on Ambersorb 572 for oxygen free conditions. All isotherm measurements for benzyl alcohol on Ambersorb 572 were repeated while applying a nitrogen atmosphere to the system. Bubbling nitrogen gas through the solution was found to remove almost all dissolved oxygen from the system and hence the oxidation of benzyl alcohol is prevented. The obtained results are shown in Figure

29 q [g/kg] mv -3 mv -5 mv -6 mv -8 mv c B [g/m ] Figure 3.17: As Figure 3.16 but on a semi-log scale. Removing the oxygen prevents the reaction to benzaldehyde. The decreasing isotherms have disappeared but unfortunately with it most of the potential dependent effects. A bell shaped decrease of the surface loading, as function of the applied potential was not found and no other trend could be abstracted from these data. The large effects of the potential on the adsorptive behaviour of benzyl alcohol found by Bán et al. (1998) could not be reproduced. q [g/kg] mv 5 mv open circ mv -5 mv -1 mv -15 mv c B [g/m ] 1 Figure 3.18: Electrosorption isotherms for phenol on Ambersorb 66

30 Phenol on Ambersorb 572 The experiments described above were repeated with phenol instead of benzyl alcohol and applied potentials were taken between 1.5 V and +1. V. The resulting isotherms are plotted in Figure A graph almost identical to Figure 3.16 was found. Obtained carbon loads were slightly lower than for the benzyl alcohol. If the loads at a surface concentration of 5 g/m 3 are put in decreasing order it is found that: +5 mv > +1 mv > -5 mv > -1 mv > V and open circuit > +1 mv. The large effects reported by McGuire et al. (1985) could not be reproduced q [g/kg] c B [g/m ] open circ +1 mv Figure 3.19: Phenol adsorption on Norit ROW.8 SUPRA. Phenol on Norit ROW.8 SUPRA The last system examined was phenol adsorbing on Norit ROW.8 SUPRA. Only two lines were measured due to experimental difficulties. The carbon load for +1 mv was found to be somewhat lower than the loading at the open circuit potential. Desorption experiments A number of desorption experiments were done to check the validity of the isotherms measured. The bed was saturated with component B (phenol) prior to the measurement. After equilibrium was reached, the applied potential was suddenly changed and the effects on the bulk concentration were monitored. 67

31 2.5 2 UV signal [ABS] t [s] Figure 3.2: As a result of suddenly switching off the applied potential of 15 mv, the bulk concentration decreases 23%. The double layer no longer hampers adsorption of phenol. It was found that some of these experiments produced hardly any change in the bulk concentration, while others gave larger effects than expected from the corresponding isotherm measurements. The maximal effects found are in the order of 2-25%. An example is the plot in Figure 3.2 where the bulk concentration decreases 23% due to the enhanced adsorption of B q [mg/g] φ [mv] Figure 3.21: The loading of benzyl alcohol and phenol onto Ambersorb 572 as a function of the applied potential. No significant effects were found. 68

32 Discussion If the various results for q, obtained at identical bulk concentrations are plotted as function of the applied potential, the electrosorption model predicts a bell shaped function. Comparing Figure 3.2 to Figure 3.21 reveals that these theoretical effects could not be determined experimentally. Only a slight decrease of loading with increasing absolute potential is visible Looking back Theoretically the Langmuir type isotherms are the ideal choice for describing electrosorption data as these isotherms contain the equilibrium constant. The biggest disadvantage of these isotherms, their poor fit with experimental data, can be solved by raising the bulk fraction x to the power of n, where 1 > n >. n However has only an empirical meaning. The electrosorption model predicts a bell shaped dependence of the surface loading with applied potential. The position of the maximum is not fixed at zero applied potential but it can shift to more positive or negative potentials. An additional potential drop created by a specifically adsorbing component B causes this shift. If the component B has a free charge, the shift of the maximum will be more pronounced. Plotting all available literature data in the same graph reveals a number of things. The data is generally of poor quality and only one potential branch is usually examined experimentally. A bell shaped dependence is only visible if one wishes hard enough to see it. Charged molecules indeed shift their maximum loading according to the theory. Fitting these experimental data with the model gives reasonable results if the effectiveness of the phenomenon is decreased. This is done by introducing a bed efficiency (the fraction of the electrochemically accessible area and the total area) and assuming that part of the applied potential is lost due to ohmic resistances in the bed. 69

33 In order to benchmark the effects of the potential on the loading, they are compared with the influence of added methanol to the bulk phase. It was concluded that the required potentials to achieve similar decreases of the isotherm are feasible. The theoretical energy requirements needed for desorbing one mole of phenol range between 4.3 and 3.7 kj/mol. It was attempted extensively to reproduce the electrosorption experiments conducted in literature but with little success. Early erroneous effects suggested a large influence of the applied potential on the adsorption of benzyl alcohol on Ambersorb 572. It was however discovered that the higher applied potentials caused oxidation of the benzyl alcohol to benzaldehyde. These type of errors can occur for adsorption experiments in a solid-liquid system as the solid concentration cannot be measured directly (easily). Instead solid concentrations are usually derived from changes in the bulk concentration so that unaccounted for mechanisms, such as evaporation or Faraday reactions tend to increase the apparent total adsorption. After special precautions where taken to prevent this, the influence of the potential on the adsorptive behaviour diminished as well. Differences between isotherms obtained for various potentials are in the order of 1% at best, literature results could not be reproduced. Two interesting features were encountered though. Desorption experiments suggest a potential effect that is (somewhat) larger then the measured effect from the isotherms experiments. Furthermore, analysis of the current going through the system suggests that the model for describing the electrosorption phenomena is correct, as there seems to be a strong relation between the amounts of charge and mass transferred. 7

34