Effect of the Alcohol Cosolvent in the Removal of Caffeine by Activated

Article pubs.acs.org/IECR

Effect of the Alcohol Cosolvent in the Removal of Caffeine by Activated Carbons Ana S. Mestre, Susana C. R. Marques, and Ana P. Carvalho* Departamento de Química e Bioquímica and Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal S Supporting Information *

ABSTRACT: The effect of the addition of methanol and 2-propanol as cosolvents on the caffeine adsorption onto activated carbons was evaluated by both kinetic and equilibrium studies. The kinetic results presented a good adjustment to the pseudosecond-order kinetic model and reveal that the alcohol cosolvent has a double negative effect on caffeine adsorption: it enhances caffeine’s solubility lowering its affinity toward the adsorbent and it seems to compete with caffeine for the adsorption active sites. The change in the isotherm shape from L to F type, when the mixture 2-propanol/water was used, was justified in terms of the higher interaction between the caffeine and 2-propanol molecules. This solvent presents the most similar Hansen threedimensional solubility parameters when compared to the ones of caffeine. The overall adsorption results point out that when the sample with acidic surface chemistry is used, the water adsorption is favored and consequently the effect of the cosolvent is less marked and is mainly explained by the solubility. For the carbons with basic surfaces the addition of the alcohol cosolvent reflects in the adsorption mechanism due to the competitive adsorption of the alcohol, especially when the cosolvent is 2-propanol.

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INTRODUCTION Caffeine is an alkaloid present in numerous beverages and in many food products (chocolate, pastries, dairy desserts). Besides being a cardiac, cerebral, and respiratory stimulant and also a diuretic,1 caffeine enhances the effect of certain analgesics in cough, cold, and headache medicines.2 Due to its high consumption and low degradation, caffeine is one of the most frequently detected compounds in influents and effluents of sewage treatment plants (STPs),3−5 and numerous studies report its occurrence in surface water.3,5−7 Moreover, caffeine has a clear anthropogenic origin, so several authors have considered it as a possible chemical marker for surface water pollution by domestic wastewater.8,9 Adsorption on activated carbons is a well-established and effective methodology for the removal of micropollutants and is widely used in wastewater treatment.10 The extensive use of these solid materials is linked with their large accessible surface area and pore volume as well as the possibility of regeneration.11,12 In recent years, our group has developed several studies to evaluate the potentialities of commercial and laboratory-made activated carbons for the adsorption of pharmaceutical and personal care products. The activated carbons obtained present high adsorption capacities for ibuprofen13,14 and acetaminophen,14−16 both analgesic compounds, and also for clofibric acid,17 a metabolite of blood lipid regulators. In line with these works, the present study deals with the use of activated carbons for the removal of caffeine. To attain this goal, two commercial carbons were used and one of them was oxidized to evaluate the effect of surface chemistry on the caffeine adsorption process. In the literature there are some studies where the effect of the solvent (water or organic solvents) in the adsorption of organic compounds onto activated carbons is evaluated.18−20 However, © 2012 American Chemical Society

to the best of our knowledge, the effect of a cosolvent in the adsorption of organic solutes onto activated carbons was never explored. Even considering other solid adsorbents, only in the work developed by Wei et al.21 the effect of cosolvents in the adsorption of nitrobenzene in a hydroxyapatite was studied. The competitive adsorption of organic compounds and water onto carbon adsorbents is a complex phenomenon since the results of multicomponent adsorption depend on the textural characteristics of the adsorbents, content and type of the surface functionalities, concentration and chemical structure of the organic adsorbates, and temperature.22 While the process of water molecule adsorption onto carbon adsorbents has been well studied and documented,23−28 and in some works the adsorption of light aliphatic alcohols (namely methanol) has been considered,29,30 there are no studies regarding the competitive adsorption of liquid organic/water mixtures on these solid materials. Nevertheless, this research topic allows a closer approach to the complexity of the real aqueous matrixes, namely those of the wastewater treatment plants. Therefore, the objective of this study was to evaluate the effect of two organic cosolvents, methanol and 2-propanol, in the caffeine adsorption process onto activated carbons with distinct textural and surface properties. The adsorption assays were performed with caffeine solutions in water and in water solutions with 10% (v/v) of each of the two organic solvents.

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EXPERIMENTAL SECTION Chemicals. Caffeine (1,3,7-trimethylpurine-2,6-dione) was provided by Normapur (batch 08J300011). The caffeine

Received: Revised: Accepted: Published: 9850

March 15, 2012 May 10, 2012 June 20, 2012 June 20, 2012 dx.doi.org/10.1021/ie300695a | Ind. Eng. Chem. Res. 2012, 51, 9850−9857

Industrial & Engineering Chemistry Research

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Table 1. Apparent Surface Area (ABET) and Total (Vtotal), Mesoporous (Vmeso) and Microporousa Volumes, and pH at the Point of Zero Charge (pHPZC) of all the Carbons αs method sample

ABET (m2 g−1)

Vtotal (cm3 g−1)

Vmeso (cm3 g−1)

Vα total (cm3 g−1)

Vα ultra (cm3 g−1)

Vα super (cm3 g−1)

VDR N2 (cm3 g−1)

VDR CO2 (cm3 g−1)

pHPZC

GAC NS NSox

917 1065 856

0.52 0.70 0.52

0.16 0.31 0.22

0.36 0.40 0.36

0.04 0.00 0.00

0.32 0.40 0.36

0.36 0.39 0.30

0.26 0.27 0.13

8.9 8.4 3.7

b

c

a Microporous volumes evaluated by the DR equation and αs method. bEvaluated at relative pressure = 0.95 in the N2 adsorption isotherm at −196 °C. cCalculated by the difference between Vtotal and VDR N2.

the point of zero charge (pHPZC), using the mass titration procedure.13,34 Caffeine Adsorption: Kinetic and Equilibrium Assays. To study the adsorption kinetics, 10 cm3 of caffeine solution (initial concentration 180 mg dm−3) was mixed with ca. 6.7 mg of activated carbon in glass vials. A magnetic stir bar was introduced, the vials were placed in a water bath at 30 °C (Grant GD100 controller), and the caffeine solution was added. At that moment, the stirring (using a multipoint agitation plate (Variomag Poly)) at 700 rpm and the time recording were started and several samples were collected between 1 min and 24 h. After filtration, the amount of caffeine in solution was determined. The initial and residual concentrations of the solutions were determined by UV−vis spectrophotometry (JascoV560) at the wavelength corresponding to the maximum absorbance, 273 nm, considering calibration plots previously made for each of the three solvents used. Caffeine uptake was calculated according to eq 1:

solutions were prepared in three different solvents: water, water with 10% (v/v) methanol (Panreac for HPLC, 99.9%), and water with 10% (v/v) 2-propanol (Panreac pro analysis, 99.8%), that in the following will be briefly named “W”, “M/ W”, and “P/W”, respectively. The caffeine solutions were prepared with ultrapure water obtained from Milli-Q water purification systems. The solutions used in kinetics and equilibrium assays were prepared without pH adjustment, presenting values around 5 units when water was used as solvent and around 5.5 units when mixtures M/W and P/W were used. Activated Carbons. Two commercial activated carbons, GAC (granular, fraction between 0.297 and 0.420 mm) and NS (powder), were selected for the adsorption of caffeine from aqueous phase. The activated carbon NS was oxidized with HNO3 (Sigma-Aldrich, 65%). For the oxidation, 1 g of NS was mixed with 10 cm3 of HNO3; the mixture was boiled to dryness and then washed with boiling distilled water up to neutral pH. The sample obtained will be designated “NSox”. Nanotextural and Chemical Characterization of Activated Carbons. The nanotextural properties of the carbon materials were characterized by N2 and CO2 adsorption at −196 and 0 °C, respectively. The CO2 isotherms were carried out in a conventional volumetric apparatus equipped with a MKS Baratron (310BHS-1000) pressure transducer (0− 133 kPa), while the N2 isotherms were made in an automatic volumetric apparatus (ASAP 2020 from Micromeritics). Before the isotherm acquisitions, the samples (∼50 mg) were outgassed overnight at 120 °C, under vacuum better than 10−2 Pa. The isotherms were used to calculate the specific surface areas and pore volumes. The apparent specific surface area, ABET, was assessed applying the BET equation (in the range 0.05 < p/p0 < 0.15)31 and the microporous volume, Vα total, was assessed using the αs method, taking as reference the isotherm reported elsewhere.32 With this method the volumes of ultramicropores (width less than 0.7 nm), Vα ultra, and supermicropores (width between 0.7 and 2 nm), Vα super, were also obtained.31 The microporous volume was also assessed applying the Dubinin−Radushkevich (DR) equation31 to N2 and CO2 data, using β values of 0.33 and 0.36, respectively. The micropore volumes obtained by N2 and CO2 data will be designated VDR N2 and VDR CO2, respectively. The micropore size distributions were assessed from CO 2 adsorption isotherms at 0 °C with the Dubinin−Radushkevich−Stoeckli (DRS) formula.33 The ash content of the activated carbons was estimated by the mass residue left after the combustion of the samples in air, according to the procedure described elsewhere.13 The samples were further characterized by the determination of the pH at

C0 − Ct V (1) W where qt is the amount (mg g−1) of caffeine adsorbed at time t, C0 is the caffeine initial concentration (mg dm−3), Ct is the caffeine concentration at time t (mg dm−3), V is the volume (dm3) of the adsorbate solution, and W is the weight (g) of dried carbon. Equilibrium adsorption studies were carried out at 30 °C varying the adsorbent doses (3.3−6.7 mg), the solution volumes (10−30 cm3), and the caffeine concentrations (20− 180 mg dm−3). After stirring for 6 h (GAC) or 4 h (NS and NSox), the concentration of caffeine remaining in solution at equilibrium (Ce) was determined and the uptake (qe) was calculated using eq 1. The stirring times for the equilibrium assays were selected according with the kinetic results. All the adsorption assays were made in triplicate. Blank experiments were also carried out in the three solvents assayed to verify that losses by volatilization or adsorption on the flask walls and magnetic stirrer do not occur. The solubility of caffeine in the different solutions assayed was experimentally determined. For this, a known amount of caffeine was added to 20 cm3 of each of the solvents in glass vials with a magnetic stir bar. The vials were sealed and placed in a water bath at 30 °C (Grant GD100 controller) and stirred overnight. This procedure was repeated until solution saturation. Then the liquid phase was filtered and aliquots were collected to determine the concentration of caffeine by UV−vis spectrophotometry. qt =

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RESULTS AND DISCUSSION Characterization of the Activated Carbons. The N2 adsorption isotherms at −196 °C of the carbon samples studied 9851

dx.doi.org/10.1021/ie300695a | Ind. Eng. Chem. Res. 2012, 51, 9850−9857


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organic solvent−organic solvent, and water−organic solvent), solvent−adsorbate, solvent−adsorbent, and adsorbent−adsorbate must be taken into account.37 Kinetic Studies. The rate of caffeine adsorption was studied in three solvents: water (W), water with 10% methanol (M/W), and water with 10% 2-propanol (P/W). The results of C/C0 vs t are displayed in Figure 2, and the curves qt vs t are presented in the Supporting Information (Figure 1S). In all cases, the experimental data were fitted by pseudosecond-order kinetics,38 with determination coefficients higher than 0.99 (Table 2). The experimental data were also fitted to the pseudo-first-order kinetic model, but the correlations obtained (not shown) were very unfavorable.

(not shown) belong to type I isotherm combined with type IV isotherm, according to the BDDT classification,35 indicating that all the solids are microporous carbons presenting also a mesoporous structure and/or external surface area. All the N2 adsorption isotherms have a hysteresis loop of type H4, characteristic of the presence of slit-shaped pores. The textural parameters presented in Table 1 confirm the well-developed mesoporous structure of the carbons since the Vmeso of sample GAC corresponds to 30% of the total pore volume, and for samples NS and NSox it attains 42% of the total pore volume. The αs method results reveal that the micropore volumes of samples NS and NSox correspond totally to supermicropores. Carbon GAC has a different microporous structure since besides supermicropores it also has ultramicropores (Vα ultra). The micropore size distribution, obtained from the DRS equation applied to the CO2 adsorption isotherms at 0 °C (Figure 1) also show that this sample has

Figure 1. Micropore size distribution obtained from DRS equation applied to the CO2 adsorption isotherms at 0 °C.33

the maximum at smaller size width. The oxidation procedure made over sample NS led to an activated carbon with lower apparent surface area and pore volumes than the original material, which is in line with results reported by other authors.36 As pointed out by Ania et al.,36 the decrease of the pore volume upon oxidation results from the destruction of some thin pore walls and/or the blocking of the pore entrances by oxygen functional groups. From the micropore size distributions presented in Figure 1, it is possible to see that the oxidation process (sample NS versus sample NSox) led to the decrease of the micropore volume, with both distributions centered at the same pore width. These findings suggest that, in this particular case, the smaller textural parameters of the oxidized sample are most likely due to the blocking of some pore entrances since no shift in the micropore size distribution is observed. Regarding the surface chemistry, the pHPZC values revealed that the as-received carbons GAC and NS have basic nature. The oxidation process led to a sample with an acidic surface since the pH at the point of zero charge decreased more than 4 pH units. Caffeine Adsorption from Solution. The adsorption of pollutants from wastewater is an extremely complex system because of the following: (i) water is itself a reactive solvent where different species are presentH2O, H+, and OH− whose concentration depends on the pH; (ii) besides water, wastewater also contain varying amounts of organic solvents (these solvents may change the pollutant solubility and can also compete with the solute to the carbon adsorption sites); (iii) the interactions between solvent−solvent (water−water,

Figure 2. Kinetic results of caffeine adsorption at 30 °C. Symbols correspond to the experimental data, whereas lines represent the fitting to the pseudo-second-order kinetic equation. Error bars are included. 9852



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Table 2. Pseudo-Second-Order Caffeine Adsorption Parameters at 30 °C: qe,calc and Ce,calc, Respectively, Caffeine Uptake, and That Remaining in Solution at Equilibrium, Both Calculated by the Pseudo-Second-Order Kinetic Model and Removal Efficiency (re) GAC k2 (g mg−1 h−1) h (mg g−1 h−1) t1/2 (h) qe,calc (mg g−1) Ce,calc (mg dm−3) rea (%) R2 a

NS

NSox

W

M/W

P/W

W

M/W

P/W

W

M/W

P/W

0.103 6250 0.039 246.1 16.1 91.0 0.999

0.011 691 0.356 245.9 16.2 91.0 0.999

0.020 342 0.382 130.5 93.1 48.3 0.997

0.385 23810 0.010 248.7 14.4 92.0 0.999

0.239 13889 0.017 241.8 19.4 89.2 0.999

0.538 10989 0.013 143.0 84.8 52.9 0.999

0.364 7407 0.019 142.7 84.9 52.8 0.999

0.439 7042 0.018 126.7 95.6 46.9 0.999

0.552 5650 0.018 101.2 112.6 37.4 0.999

re = ((C0 − Ce)/C0)·100.

suggesting that the effect of surface chemistry prevails over the negative effect of the cosolvent. In fact, the oxidized carbon is the only acidic sample, so it has a more hydrophilic surface than the as-received samples, favoring the adsorption of water molecules which can lead to pore blocking. The competitive adsorption of water molecules must play an important role in the overall process of caffeine removal by the NSox sample, which can explain the lower removal efficiencies obtained. When a cosolvent is added, the complexity of the system considerably increases. Having in mind that solutions with 180 mg dm−3 caffeine were prepared, and that the 10% of the alcohol corresponds to around 79 000 mg dm−3, the competition of the cosolvent for the adsorption onto the active sites cannot be disregarded. While the water adsorption is favored in the acidic carbon due to the higher amount of surface polar oxygenated groups, the nonpolar chain (hydrocarbon moiety) of organic solvents has more affinity for the adsorption onto the graphene layers of the activated carbons porous network (in both basic and acidic carbons).40 Therefore, in the case of sample NSox the competitive adsorption of caffeine and the solvent molecules occurs in all the solvents assayed, whereas for carbons GAC and NS, where the water molecule adsorption is not so favored, it seems to become significant only when the alcohol cosolvent is added. Carbon NS presents the higher initial adsorption rate regardless of the solvent used, which is in accordance with its textural properties, since this sample has the more developed mesoporous and supermicroporous structures. As previously mentioned, the surface modification (carbon NSox) had a strong negative effect both on the removal efficiencies, especially when W and M/W were used as solvents, and on the initial adsorption rate. Nevertheless, NSox carbon has higher initial adsorption rates than sample GAC, with this difference well pronounced when the mixtures of water/alcohol were used as solvents, highlighting the influence of the particle size. In fact, it must be remembered that GAC carbon is a granular sample while besides sample NSox being a powder has a high Vmeso value which also favors a better diffusion of caffeine molecule through the porous network. Summarizing, the addition of the two light aliphatic alcohol cosolvents has two negative effects on the adsorption of caffeine onto activated carbons: first, it enhances the solubility of caffeine, lowering its adsorption affinity toward the solid material; second, the cosolvents compete with caffeine for the adsorption active sites. This last effect is more pronounced for the carbons with a basic surface chemistry, since in this case water adsorption is less favored than for acidic carbons, highlighting the cosolvent competition.

The kinetic results reveal that the presence of the cosolvent has great influence in the adsorption process of caffeine. When M/W and P/W mixtures are used the amount of caffeine removed by the carbons with basic surfaces (GAC and NS) decreases from ≈90 to ≈50%. This result can be linked both to the distinct solubility of caffeine in the various solvents (see Table 3) and to the competition of the cosolvent for the active Table 3. Solubility of Caffeine in the Mentioned Solvents at 30 °C solvent

solubility of caffeine (g dm−3)

W M/W P/W

30 32 42

adsorption sites. In fact, the study developed by Salame and Bandosz39 reveals that methanol adsorbs onto activated carbons by both dispersive interactions and hydrogen bonding, due to the hydrocarbon moiety and OH group, respectively. Since 2propanol also has the OH group, but the hydrocarbon moiety is longer than that of methanol, it would be expected that the dispersive interactions of 2-propanol with the carbon surface would be greater. Although the distinct solubility of caffeine in the solvents studied might, at least partially, justify the pattern observed for the removal efficiencies on carbons GAC and NS, the adsorption rates cannot be justified by this parameter. In fact, there is practically no change in the solubility when 10% methanol is added, but when the same amount of 2-propanol is used, the solubility of caffeine increases around 30%. However, the initial adsorption rate, h, presents the more important change when water is replaced by the mixture M/W. This is especially noted in the case of sample GAC. A less abrupt decrease of h values was observed for carbon NS when the alcohol cosolvent was introduced. These results seem to point out that the slower adsorption rates can be linked to the competitive adsorption between all the components (water− alcohol−caffeine) of the system for the active adsorption sites of the carbons. The slower initial adsorption rates obtained for sample GAC are the consequence of its granular morphology that hinders the diffusion pathway of the caffeine molecules toward the active sites. In carbon NS the diffusional constraints are less effective as this is a powder. Concerning the oxidized carbon (NSox), the introduction of any of the alcohol cosolvent assayed had no significant effect on the kinetic adsorption of caffeine. On the other hand, this is also the solid that presents the lower removal efficiency, 9853



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Equilibrium Adsorption Isotherms. Figure 3 displays the caffeine adsorption isotherms for each solvent on the studied

developed supermicropore structure. Sample NSox has a slight lower volume of larger micropores than the pristine material, but it presents the lowest adsorption capacities due to the inhibitory effect of acidic surface chemistry, as discussed above when the removal efficiencies were considered. Regardless of the activated carbon, the isotherms obtained using W or the mixture M/W as solvent are L type, while the curves obtained with the mixture P/W are of the F type.11,41 These results clearly show that when the P/W mixture is used as solvent a different adsorption mechanism of caffeine occurs. Besides the analysis of the isotherm shape, it is crucial to apply also theoretical models to enhance the analysis of the experimental results. In this sense the equilibrium adsorption data were fitted to the most commonly used models: the theoretical Langmuir equation42 and the empirical Freundlich equation.43 The Langmuir and Freundlich parameters are quoted in Table 4, along with the coefficients of determination (R2) of the linear plots and the results of the nonlinear chisquare test analysis, χ2.44 If the values calculated from the model are similar to experimental data, χ2 should be a small number and vice versa. When W or the mixture M/W are used as solvent, caffeine adsorption isotherms show a rather good agreement with the Langmuir model, while the isotherms obtained with the mixture P/W present a better fitting to the Freundlich equation, which is in accordance with the former analysis based to the isotherm configuration. This change in the adsorption mechanism when P/W mixture is used can be explained in terms of the different assumptions of the two models. The Langmuir equation is based on the assumption that the surface of the adsorbent is energetically homogeneous and that a monolayer is formed, with no interactions between the molecules adsorbed.42 Therefore, this equation gives good fittings for the experimental isotherms with a concave curvature at low equilibrium concentrations followed by a plateau or saturation limit. Freundlich theory takes into account the surface heterogeneity and considers that intermolecular interactions between the adsorbate molecules may exist.43 The Freundlich model fits to isotherms with a less concave curvature, where the amount adsorbed steadily increases, and the saturation limit at low adsorbent doses is not attained.43 The equilibrium results (see Figure 3 and Table 4) show that when W or M/W mixture is used as solvent, caffeine has a high affinity towards the activated carbons (Langmuir equation). On the other hand, when P/W mixture is used the better fitting to the Freundlich equation indicates that caffeine has a lower affinity to the carbons, and that in the complex system of three components (water, 2propanol, and caffeine) under study intermolecular interactions may occur. Although the curves obtained with the P/W mixture fit better to the Freundlich model, for comparison purposes we will consider the results obtained by the Langmuir equation. Obviously a possible misinterpretation can be made, but it must be noted that even in the less favorable case the R2 value of the fitting to the Langmuir equation is 0.909. Figure 2S in the Supporting Information displays the fitting of the experimental data obtained in the solvent P/W to the Langmuir and Freundlich equations for the three activated carbon samples under study, showing that the fitting to both models is very close. However, when Langmuir equation presents the best adjustment, the fitting to Freundlich model is clearly

Figure 3. Caffeine adsorption isotherms on the studied carbons at 30 °C. Symbols correspond to the experimental data, whereas lines represent the fitting to the most adequate theoretical equation (broken lines, fitting to the Langmuir equation; solid lines, fitting to the Freundlich equation). Error bars are included.

carbons. The stirring times used were 6 h for GAC and 4 h for NS and NSox, since according to the results of the kinetic assays the caffeine uptake remained unchanged between 4 and 24 h (for carbon GAC) and 2−24 h (for carbons NS and NSox). From the analysis of the curves it is clear that, for all the solvents assayed, the adsorption capacities of caffeine on carbon NS are the highest, which is in agreement with its more 9854



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Table 4. Langmuir and Freundlich Isotherm Parameters for the Adsorption of Caffeine in the Various Solvents onto the Activated Carbons, Linear Regression Coefficient of Determination (R2), and Nonlinear Chi-Square Test Analysis (χ2) GAC Langmuir equation qm (mg g−1) b (dm3 mg−1) R2 χ2 a Freundlich equation 1/n KF (mg1−1/n (dm3)1/n g−1) R2 χ2 a

NS

NSox

W

M/W

P/W

W

M/W

P/W

W

M/W

P/W

271.1 0.639 0.996 13.85

238.3 0.478 0.998 12.10

205.5 0.028 0.934 14.10

296.3 0.389 0.999 2.03

267.6 0.499 0.998 10.16

232.8 0.025 0.967 4.88

156.0 0.251 0.993 2.07

132.5 0.272 0.991 2.79

177.2 0.017 0.909 9.67

0.261 91.8 0.759 55.77

0.223 91.3 0.930 46.00

0.496 15.5 0.996 0.89

0.215 115.9 0.844 28.45

0.081 149.1 0.316 58.03

0.561 13.0 0.998 0.68

0.128 81.1 0.879 3.56

0.240 44.1 0.959 7.68

0.538 8.98 0.992 2.30

χ = ∑[(qe − qe,m)2/qe,m], where qe is the experimental equilibrium uptake and qe,m is the equilibrium uptake calculated from the model.44

a 2

δt 2 = δd 2 + δp2 + δ hb 2

unfavorable (see the example displayed in Figure 3S in the Supporting Information). The Langmuir constant, b, is a direct measure of the adsorption affinity; thus it is possible to conclude that the values obtained corroborate the different affinity of caffeine toward the carbons when different solvents are used. The b values calculated for the results obtained with W and M/W mixture are reasonably similar and 1 order of magnitude higher than those assessed for data obtained using the P/W mixture. As already mentioned, the presence of alcohol has a significant effect on the kinetic results, leading to an abrupt change of h values due to the introduction of methanol or 2propanol as cosolvent. However, the equilibrium data reveal that the modification of the adsorption mechanism is only evident when the mixture P/W is used as solvent. To understand the several factors that lead to a considerable increase of caffeine solubility in the mixture P/W comparatively with the mixture M/W, and most of all to the change of the adsorption mechanism, it is necessary to take into account some physicochemical properties of all the species involved in this process: caffeine, water, and the alcohols, methanol and 2propanol. All the molecules are polar species, with caffeine being the one presenting the highest dipole moment (≈3.6−3.7 D45). However, caffeine is a large molecule and the partial charges are apart from each other. Thus, the molecule has hydrophilic polar zones (nitrogen and oxygen) that allow the establishment of hydrogen bonds and also interactions by electrostatic forces. Caffeine also has nonpolar zones that are hydrophobic and interact by dispersion forces. The alcohols have similar dipole moments (1.70 and 1.58 D for methanol and 2-propanol, respectively46) that are slightly lower than that of water (1.85 D46). Hence, the marked change in the caffeine adsorption mechanism observed when the P/W mixture was used cannot be justified by the differences in the dipole moments. To account for the different polarities of caffeine and solvent molecules, Hansen’s three-dimensional solubility parameters (HSPs) can be a very useful tool.47 Hansen assumed that the cohesive energy arises from the contributions of nonpolar interactions, polar interactions, and hydrogen bonding interactions. In this approach the total solubility (δt) is subdivided into components which express the contributions from the different types of interatomic/intermolecular forces, namely dispersion (nonpolar) forces (δd), polar interactions (δp), and hydrogen bonds (δhb), according to eq 2:

(2)

The Hansen solubility parameters (HSPs) allow a more detailed characterization of the system under study, and they can be used to assess the relative affinity of a molecule to a solvent.48,49 The solubility parameters have found their greatest use in, for example, the coatings industry to aid in the selection of solvents.46 Therefore, this theory can be also helpful for a better understanding of caffeine dispersion in the solvents and also caffeine and solvent interaction with the activated carbon surface. The HSPs parameters quoted in Table 5 show that 2propanol is the solvent for which the values of the different Table 5. Hansen Solubility Parameters (HSPs) of Water, Methanol, 2-Propanol, and Caffeine47 HSPa

water

methanol

2-propanol

caffeine

δd δp δhb δt

15.5 16.0 42.3 47.8

15.1 12.3 22.3 29.6

15.8 6.1 16.4 23.6

19.5 10.1 13.0 25.5

δd, dispersion; δp, polarity; δhb, hydrogen bond; δt, total. δt2 = δd2 + δp2 + δhb2.

a

interaction components (dispersion, polarity, and hydrogen bonding) are more similar to the ones presented by caffeine, maximizing the extension of their interaction and consequently their affinity (higher solubility). The activated carbon surface is constituted by graphitic layers that are highly hydrophobic; however, the edges of the basal planes generally contain higher amounts of heteroatoms that favor electrostatic interactions. Thus, water molecules are preferentially adsorbed at the entrance of the pores, not competing with caffeine for the adsorption active sites. However, as already mentioned, the water adsorption can lead to pore blocking due to the formation of water clusters that limit the adsorption of the caffeine. The introduction of the alcohol cosolvent, even at 10% (v/v), greatly increases the system complexity due to the large number of new possible interactions. The equilibrium isotherm reveals that when methanol is added the adsorption capacity of the carbons slightly decreases, which is likely due to the slight increase of solubility. When 2propanol is added, the increase of solubility also explains the lower adsorption capacity for caffeine observed for all the 9855


Industrial & Engineering Chemistry Research carbons, but as mentioned, the major effect is the change of the caffeine adsorption mechanism. This behavior is probably due to the fact that 2-propanol competes with caffeine for the adsorption active sites due to its slightly higher dispersion component value (δd) that allows the interaction with the nonpolar surface of the activated carbons by van der Waals forces. The distinct mechanism of caffeine adsorption when the alcohol cosolvents were used can be related to the fact that, although the dispersion parameter value (δd) of 2-propanol is not significantly higher than that of methanol, the polar (δp) and the hydrogen bond (δhb) component values are much lower, allowing a better interaction of this solvent with the adsorption active sites. In line with the conclusions from the kinetic results, the equilibrium data also reveal that the addition of 2-propanol as cosolvent has a double negative effect in caffeine adsorption: first, it enhances caffeine solubility in the solvent mixture and, second, it competes with caffeine for the adsorption active sites.

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REFERENCES

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CONCLUSIONS The different textural and surface properties of the activated carbons used allowed the evaluation of the effect of the alcohol cosolvent (methanol and 2-propanol) on the adsorption of caffeine. Both kinetic and equilibrium results revealed that the cosolvent has a double negative effect on caffeine adsorption onto the solids, with this effect being more pronounced when 2-propanol was used. In fact, the solubility of caffeine in the mixture 2-propanol/water is 30% higher than that in water or in the mixture methanol/water, which lowers the caffeine affinity towards the activated carbon. The introduction of 2-propanol as cosolvent results in the change of the isotherm from L to F type, which is indicative of a change in the adsorption mechanism. The Hansen solubility parameters allowed a deeper understanding of the interactions in the complex systems studied. These parameters justified the higher solubility of caffeine in the mixture 2-propanol/water and a high affinity of 2-propanol for the carbon surface that will result in a competitive adsorption between this alcohol and the caffeine molecules and consequently in the change of the adsorption mechanism. When acid carbons are considered, the effect of the water adsorption prevails over the effect of the cosolvent. The results obtained in this study point out that the presence of an alcohol cosolvent, even at the low concentration assayed, has a great influence in the adsorption process of this organic molecule: it decreases the adsorption rate, lowers the removal efficiency, and changes the adsorption mechanism. ASSOCIATED CONTENT

S Supporting Information *

Kinetic curves qt versus time (Figure 1S), and Freundlich and Langmuir adjustments to the isotherm experimental data (Figures 2S and 3S). This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work was supported by the FCT (Portugal) pluriannual program of CQB through strategic project PEst-OE/QUI/ UI0612/2011. A.S.M. thanks AdI for a postdoctoral grant (QREN- 5523-WaterCork). The authors thank Salmon & Cia for the supply of carbon samples.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +351 217500897. Fax: +351 217500088. E-mail: ana. [email protected]. Notes

The authors declare no competing financial interest. 9856



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Effect of the Alcohol Cosolvent in the Removal of Caffeine by Activated

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