1. Adsorption of several emerging contaminants in fixed bed columns by clay and activated carbon

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1 Transworld Research Network 37/661 (2), Fort P.O. Trivandrum Kerala, India Review Article Recent Res. Devel. Chemical Engg., 7(214): 1-17 ISBN: Adsorption of several emerging contaminants in fixed bed columns by clay and activated carbon José Luis Sotelo, Gabriel Ovejero, Araceli Rodríguez Silvia Álvarez and Juan García Grupo de Catálisis y Procesos de Separación (CyPS), Departamento de Ingeniería Química, Facultad C.C. Químicas, Avda Complutense s/n, Universidad Complutense, 284 Madrid, Spain Abstract. In this work the availability of a granular activated carbon and a clay material, sepiolite, on the removal of several emerging contaminants, such as caffeine, diclofenac and flumequine in water has been studied. Batch adsorption experiments by activated carbon and sepiolite was developed. Equilibrium time and adsorption isotherms (q e versus C e ) were obtained for the three compounds in aqueous solution. Experimental data from isotherms were fitted to several mathematical models, such as Langmuir, Freundlich and Dubinin- Radsuhkevich equations. The adsorption kinetic data of caffeine removal onto sepiolite were fitted better by the pseudo-second order equation. Therefore, intra-particle diffusion mechanism, Boyd and McKay models were applied in order to describe the adsorption mechanism, indicating that both film and intra-particle diffusion control the adsorption rate. Correspondence/Reprint request: Dr. Juan Garcia, Department of Chemical Engineering, Faculty of Chemical Sciences, Complutense University, Avda. Complutense s/n, 284 Madrid, Spain juangcia@quim.ucm.es

2 2 José Luis Sotelo et al. To our knowledge, this is one of the first studies on the elimination of these compounds by activated carbon and sepiolite fixed-bed columns. In the present study, the effect of several operation conditions as flow rate, bed height and inlet adsorbate concentration on the adsorption process was studied. There is evidence that flumequine removal by activated carbon is ph dependent, founding the maximum adsorption capacity and percentage saturation values at ph 6.3. The regeneration by ethanol-water (5:5, v/v) provided lower elution efficiency values, since the elution time was not enough to remove totally the active sites in the column. Therefore, a maximum adsorption concentration factor of 2 was obtained. 1. Introduction The so-called emerging contaminants present in waters are a unique challenge for researchers and regulatory agencies in regard to most of them are not yet regulated and their impacts on human life are not quite known. The number of emerging compounds detected is very high and continuously increasing. Studies about these compounds have documented a large class of chemical contaminants including endocrine disrupting compounds (EDCs), pharmaceutical active compounds (PhACs), personal care products (PPCPs), pesticides, surfactants, flame-retardants, fuel additives, plasticizers and numerous other industrial pollutants. Among the pharmaceutical products, anti-inflammatory drugs (diclofenac, ibuprofen, indometacine, naproxen and phenazone), lipid regulators (bezafibrate, gemfibrozil, clofibric acid, fenofibric acid), ß-blockers (metoprolol, propranolol, atenolol), antiepileptic (carbamazepine), antimicrobial, cytostatic agents and 17ß-estradiol, have been found to be ubiquitously present in streams and source waters [1]. Most of these micropollutants, though are detected in the environment in a concentration range from ng.l -1 up to μg.l -1, tend to bioaccumulate and can suppose a potential risk in both human health and aquatic life [2, 3]. Some recent researches indicated that the low concentrations of pharmaceutical compounds and other contaminants present in drinking water are not harmful to humans from a toxicological point of view, but their presence is still not desirable as a precautionary principle. Therefore, recent surveys indicated that EDCs are suspected to produce reproductive, hormonal and neurobehavioral disorders on human health, even at low concentrations [4-6]. Micropollutants are usually released into the environment via wastewater discharges or leakage from conventional methods of wastewater treatment and they can contaminate the water sources that are used as drinking water resources. Up to date, due to the inadequate analytical methods and the lack of accurate information about the effects of these

3 Adsorption of several emerging contaminants 3 compounds and their metabolites in the aquatic environment, there has been a relevant lack of studies on this field [7, 8]. Therefore, in recent years a rising interest on this subject is developing and the research on the occurrence of emerging micropollutants in the environment is being particularly intensive in Europe and USA. In this sense, it has been estimated that hundreds of tons of pharmaceutical compounds are produced and consumed in developed countries each year, and they can end up in water sources usually from human excretion, where the administered pharmaceuticals are passed through the human body unaltered [9-11]. Due to this review work, in the European Union, more than 1 registered substances it have been quantified, of which 3-7 are in commonly used, daily use, and are related in the European Inventory of Existing Chemical Substances (EINECS). The European Water Framework Directive 28/15/EC identify 33 priority hazardous substances, including benzenic compounds, polycyclic aromatic hydrocarbons, pesticides, such as isoproturon, and solvents. Some of these 33 compounds are recognized as potential EDCs. Therefore, a proposed revision of the list includes several PhACs as diclofenac [12, 13]. Due to this subject, it is necessary the implementation of more advanced treatment technologies in order to reduce the concentration levels of these contaminants in drinking water. Among these techniques, advanced oxidation by ozone, photo-catalysis, electro and photoelectron-fenton processes, membrane technologies or activated carbon adsorption can be used. Activated carbon adsorption is a well-known process for removing organic contaminants. It is most commonly used as a powdered carbon or in granular form (GAC) in fixed bed columns. The late is widely used in many WWTPs of United States and Canada, as a tertiary treatment. The efficiency of granular activated carbon for the removal of trace organic micropollutants from water has been demonstrated by several researchers, providing in general high adsorption capacities [14, 15]. However, the major drawback of this material comes from economical consideration; the commercial activated carbons are expensive, leading to its infeasibility for a full-scale operation. Therefore, alternative adsorbents which come from nature, such as some agricultural wastes, such as wool, rice straw, coconut husk, saw dust and peat moss, are of interest. However, the adsorption capacities of the above-mentioned adsorbents are not very high. Sepiolite is a natural hydrated magnesium silicate clay mineral which provides great retention capacities of micropollutants, such as heavy metal cations, dyes and others [15, 16].

4 4 José Luis Sotelo et al. The present work is a review of several studies focused in the removal of emerging contaminants from aqueous solutions by granular activated carbon adsorption in fixed-bed column operation. For this purpose, 3 pharmaceuticals were studied: caffeine, diclofenac and flumequine. Caffeine is an alkaloid of the methylxanthine family, which is commonly used as a stimulant. It is considered a chemical marker for surface water pollution due to its regular and highly consumed, 7 mg per person, over the world. Caffeine has several physiological effects such as stimulation of the central nervous, respiratory and cardiovascular system. It is considered to be a risk factor for cardiovascular diseases and may cause depression and hyperactivity [17, 18]. Diclofenac sodium belongs to the pharmaceutical substances that are commonly found in aquatic environments. It is a non-steroidal antiinflammatory drug (NSAID) that is widely used to reduce inflammation as well as analgesic. The removal percentage of diclofenac by WWTPs ranges from 21% to 4%, which explains their usual detection in surface water, groundwater or even in drinking water [19]. Therefore, it has been demonstrated the synergistic toxicity of diclofenac, ibuprofen, naproxen and acetylsalicylic acid [2]. Flumequine is an antibiotic which belongs to fluoroquinolone antibacterial agents, a broad spectrum class of synthetic antibiotics that are widely used for human and veterinary purposes. Flumequine has been detected in rivers at concentrations up to 5 ng.l -1 and it can be very persistent in the sediments [21]. 2. Adsorption equilibrium and kinetic studies First For the successful further design of full-scale adsorption processes, it is necessary as a starting point to quantify equilibrium time and adsorption capacity of adsorbate-adsorbent, which is identified by the adsorption isotherm. Several modeling equations can be used for fitting the experimental data, being Freundlich, Langmuir or Dubinin-Radushkevich models the most frequently used. Therefore, for caffeine removal by activated carbon and sepiolite adsorption, equilibrium time and adsorption isotherm were obtained for both systems. The results are showed in Fig. 1a-b. All the batch experiments were developed in ultrapure water, at 25 ºC. For activated carbon, equilibrium was reached after 2 days of operation, since no significant changes in the adsorption capacity values were observed for further adsorption times. It can be observed that the equilibrium time for

5 Adsorption of several emerging contaminants 5 sepiolite is quite higher, 1 days, than in the case of activated carbon. This result is not in concordance with the theoretical proposal that the microporous materials as activated carbon, present slower kinetics, represented by higher equilibrium time values. So, these results can be attributed to the significance difference between the initial caffeine concentration values used with each adsorbent. Therefore, the adsorption capacity value for caffeine-sepiolite system, 21. mg.g -1, is quite lower than those obtained in activated carbon adsorption, 28. mg.g -1, since sepiolite is mainly a mesoporous material, which provides lower adsorption capacity values [22]. In the case of activated carbon, adsorption appears to proceed in two stages (Fig. 1b). First, caffeine is quickly adsorbed in the macro and mesopores; afterwards, slower migration of the adsorbed caffeine molecules to less accessible sites on the activated carbon takes place [22]. For sepiolite adsorption isotherm, caffeine adsorption capacity rises favorably with the equilibrium concentration, but there is not a saturation limit corresponding to the total filling of the pores within the adsorbent, that is characteristic of isotherms of microporous materials. In this case, there is no formation of a monolayer as in the case of activated carbon. Diclofenac by activated carbon adsorption was defined by equilibrium time and adsorption isotherm of the system (Fig. 2 a-b). In Fig. 2a the equilibrium time of diclofenac can be compared to that obtained for caffeineactivated carbon. As it can be seen in Fig. 2a, for caffeine, the equilibrium concentration quickly approaches a plateau, while for diclofenac the plateau is reached more gradually and in a longer time (since caffeine concentration is higher Figure 1. (a) Equilibrium time and (b) Adsorption isotherms for caffeine activated carbon and sepiolite adsorption.

6 q eq / mg.g -1 6 José Luis Sotelo et al. a) b) q e / mg.g Caffeine Diclofenac 1 5 Diclofenac Time / min 1, 1,2 1,4 1,6 1,8 2, C e / mg.l -1 Figure 2. (a) Equilibrium time for caffeine and diclofenac activated carbon adsorption and (b) Adsorption isotherm for diclofenac activated carbon adsorption. than diclofenac concentration). As in the case of caffeine, adsorption isotherm of diclofenac is L1-type according to Giles classification, indicating a high affinity between adsorbate and adsorbent. The aromatic rings of the diclofenac molecules are being adsorbed in parallel to the carbon surface and, due to the hydrophobic nature of the molecules, there is no major competition between diclofenac and water molecules for the active sites within activated carbon [22]. Finally, for flumequine adsorption by activated carbon, equilibrium time and adsorption isotherm have been obtained. These results are compared to those obtained for diclofenac in Fig. 3a-b. Both adsorption isotherms are L1-type (Giles classification), showing a good affinity adsorbates-activated carbon. Since the concentration range is similar in both cases, it can be seen from adsorption isotherm that flumequine adsorption capacity, compared to diclofenac saturation capacity, increased from 33 mg.g -1 to 54 mg.g -1. Finally, it can be suggested that adsorption efficiencies in microporous materials are highly conditioned by the log D (distribution coefficient) values, which are related to log K ow values and the pk a values of the solute. In general, authors suggest that log K ow values serve as an indicator of molecule removal by adsorption. In this case, between of the 3 compounds, flumequine presents the highest log K ow value (log K ow = 1.31), leading to a highest adsorption efficiency, which can be quantified by the adsorption capacity provided by the isotherm. Therefore, in regard to the kinetic of the process, in activated carbon the adsorption process is proportional to the attraction forces between adsorbate and adsorbent, which mainly depend on the molecular size of the compound. So, in microporous adsorbents the

7 C e / mg.l -1 q e / mg.g -1 Adsorption of several emerging contaminants 7 a) b) Diclofenac Flumequine 1 Diclofenac Flumequine C e / mg.l -1 Figure 3. (a) Equilibrium time and (b) Adsorption isotherm for diclofenac and flumequine activated carbon adsorption. adsorption kinetic will be highly conditioned by the internal transport, intraparticle diffusion, within the smallest carbon micropores, due to steric hindrance [13, 22]. Regarding to this concept, adsorption kinetic studies of caffeine removal onto sepiolite were developed. Authors suggest that the adsorption rate controls the migration of the molecules and the residence time of the adsorbate at the adsorbent-solution interaction [16]. In order to investigate the adsorption kinetic processes of caffeine on sepiolite, three kinetic models, including pseudo-first order model, Elovich model and pseudosecond order model, were used. There was a very accuracy between experimental data and those predicted by pseudo-second order model, showing correlation coefficients in the range , since pseudo-second order fitted more properly to the experimental data. Pseudo-second order model fitting confirmed the faster adsorption kinetic in mesopores, which can be considered as transport pores, favoring the diffusion into the smaller pores [13]. The adsorption mechanism of an organic/inorganic compound by a porous adsorbent can be assumed following three mainly consecutive steps: (i) transport of the compound from the bulk to the solid interface (film diffusion), (ii) transport of the adsorbate within the active sites of the adsorbent (intra-particle diffusion) and (iii) adsorption of the compound to solid phase adsorbent, by physical or chemical bonding. It is known that this latter step is very rapid and in any case does not represent the rate-limiting step in the removal by adsorption. So, in order to distinguish between the film diffusion and particle diffusion controlled

8 8 José Luis Sotelo et al. adsorption rate, several models, including Weber intra-particle diffusion model, film diffusion model of Boyd and McKay plot, have been applied. Weber suggested that if diffusion in the pores is the control step of the process, adsorption capacity will vary with the square root of time. In Weber and Morris plot can be distinguished two linear sections with different slopes, k i,1 and k i,2. (Fig. 4a). This multilinearity indicates that there are two distinguished steps in the adsorption process, phase I and phase II. Phase I is the bulk diffusion, this is the transport of the solute from aqueous phase through the boundary layer of the adsorbent; and then, intra-particle diffusion begins, where caffeine penetrates through the pores, macro and mesopores and finally, into the less accessible pores, micropores, increasing the diffusion resistance. So, phase II corresponds to the internal diffusion through the adsorbent micropores. Otherwise, the Boyd plot showed in Fig. 4b illustrates this idea, through the non-linearity of the plot of B t versus t in the period studied. The plots are linear in the initial period of adsorption and then, they are curved, which supports the assumption that the rate of removal of caffeine onto sepiolite is governed by external transport at initial times of the process, since further operation times the intraparticle diffusion mechanism is of greater importance. To further verify the above considerations, McKay plots were drawn at different concentrations (Fig. 4c), finding the same above assumptions. Similar results were reported by other authors [23-25]. Therefore, the slopes found from piecewise linear regressions, B, of Boyd plot were used to obtain the effective diffusion coefficient values for different caffeine concentrations, D i (cm 2.h -1 ), by this equation, which relates the two slopes of the Boyd plot and the radius of the adsorbent particle, r (cm). 2. Di B (1) 2 r The values of intra-particle rate parameters (k i,1 and k i,2 ) and diffusion coefficients (D 1 for film diffusion and D 2 for pore diffusion) for different concentrations are shown in Table 1. Both film diffusion and pore diffusion coefficients (D 1 and D 2, cm 2 h -1 ) presented large negative exponential values, being higher intra-particle diffusion coefficients. Since both mechanisms control the adsorption process, pore diffusion appears to control the adsorption rate, especially at lower caffeine concentrations [25].

9 ln (1-F) Bt Adsorption of several emerging contaminants 9 q / mg.g -1 a) b) =.84 mg.l -1 =1.77 mg.l -1 =2.9 mg.l -1 phase I t.5 / h.5 c) -2 phase II =.84 mg.l -1 film diffusion =.84 mg.l -1 intra-particle diffusion =1.77 mg.l -1 film diffusion =1.77 mg.l -1 intra-particle diffusion =2.9 mg.l -1 film diffusion =2.9 mg.l -1 intra-particle diffusion =.84 mg.l -1 =1.77 mg.l -1 =2.9 mg.l -1 t / h t / h Figure 4. (a) Intra-particle diffusion plot (b) Boyd model plot and (c) McKay plot for different inlet concentrations in the caffeine removal by sepiolite. Table 1. Intra-particle diffusion model rate parameters and diffusion coefficients at different concentrations for adsorption of caffeine into sepiolite. / mg.l -1 k i,1 / mg g -1 h -.5 k i,2 / mg g -1 h -.5 D 1 x 1-7 / cm 2 h D 2 x 1-6 / cm 2 h Adsorption column studies. Breakthrough curves Breakthrough curves for caffeine, diclofenac and flumequine adsorption present an S-shaped profile, since when both internal and external resistances

10 1 José Luis Sotelo et al. are significant the breakthrough curve is S-shaped. So, this concept is related to the length of the mass transfer zone, which depends on the adsorption isotherm type, nonlinear and favorable or unfavorable isotherm. In the case of a favorable isotherm, the most common case, a constant profile of the mass transfer zone can be established while adsorption proceeds through the column. In most cases, there is a combination of a driving force to narrow the adsorption front (mass transfer zone) and the spreading of the breakthrough curves by mass transfer resistance and dispersion effects. Therefore, the mass transfer zone proceeds in a constant S-shape after these two counteracting effects are balanced [26, 27]. The effect of the operation conditions, such as inlet adsorbate concentration, volumetric flow rate and mass of adsorbent on the adsorption parameters was studied. The breakthrough curves obtained for caffeine removal by granular activated carbon and sepiolite are shown in Fig. 5 a-c and Fig. 6 a-c, C/ a) b) c) g AC.6 g AC 1. g AC Q = 3 ml.min -1 = 15 mg.l -1 C/ mg.l mg.l -1 Q = 2. ml.min -1 m =.8 g AC C/ ml.min ml.min -1 = 15 mg.l -1 m =.8 g AC 1 mg.l mg.l -1 Q = 2. ml.min -1 m =.8 g AC Time / h c) ml.min ml.min -1 = 15 mg.l -1 m =.8 g AC Time / h C/ Time / h Time / h Figure 5. Breakthrough curves of caffeine removal by granular activated carbon fixed-bed columns of: (a) different mass of adsorbent (b) different inlet caffeine concentration and (c) different volumetric flow rate.

11 Adsorption of several emerging contaminants 11 a) b) c) 1, W =.8 g AC W = 1.2 g AC W = 1.6 g AC 1, =.84 mg.l -1 =1.77 mg.l -1 =2.9 mg.l -1 1, Q= Q=1 C/ C/ C/ b) c),,8 =.84 mg.l -1 =1.77 mg.l -1 =2.9 mg.l -1 1, Q=.6 ml.min -1 Q=1. ml.min ,6,4 C/,2, Figure 6. Breakthrough curves of caffeine removal by sepiolite fixed-bed columns of: (a) different mass of adsorbent (b) different inlet caffeine concentration and (c) different volumetric flow. respectively. Those obtained for diclofenac and flumequine adsorption by activated carbon are depicted in Fig. 7 a-b and Fig. 8 a-c, respectively. For the three studied compounds, the increase in the inlet concentration led to lower breakthrough times. This is due to a faster saturation of the available adsorption sites. Higher initial concentrations led to decrease the mass transfer resistance in the adsorption columns, so the adsorbent reached saturation more quickly, which resulted in a decrease of the exhaustion time. These results demonstrate that a change in the gradient concentration affects the diffusion process in the adsorbent, since at higher inlet concentrations the driving force for mass transfer increases, which results in a decrease in the mass transfer zone [28]. Both the breakthrough and exhaustion times increased when mass of adsorbent increased, as more binding sites are available for adsorption.

12 12 José Luis Sotelo et al. a) b) C/.4 C/ g AC.4 g AC Time / h.2 2. ml.min ml.min Time / h Figure 7. Breakthrough curves of diclofenac removal by granular activated carbon fixed-bed columns of: (a) different mass of adsorbent and (b) different volumetric flow rate. = 1.2 mg.l -1 = 2.3 mg.l -1 = 2.7 mg.l -1 C/ a) b) c),7,5,3,1 m =.4 g AC m =.6 g AC m =.8 g AC c) 1, Q = 1. ml.min -1 Q = 3. ml.min -1 Q = 2. ml.min -1 C/,9,7,5,3,1 = 1.2 mg.l -1 = 2.3 mg.l -1 = 2.7 mg.l C/ 1, Q = 1. Q = 3. Q = 2. 2 C/ Figure 8. Breakthrough curves of flumequine removal by granular activated carbon fixed-bed columns of: (a) different mass of adsorbent (b) different inlet flumequine concentration and (c) different volumetric flow rate.

13 Adsorption of several emerging contaminants 13 Therefore, with increasing bed weight, especially at lower inlet concentrations, the steepness of the breakthrough curve decreased, probably due to the influence of the diffusion process. Otherwise, this trend is due that the bed lengths used are not high enough to ensure a fully developed profile, reaching a constant pattern behavior where there is no changes on the shape of the breakthrough curves [22, 26]. In general, as volumetric flow rate increases, the breakthrough and exhaustion times occurred significantly faster. When at higher flow rate, the external film mass transfer resistance at the surface of the adsorbent tended to decrease and the residence time decreased, since the breakthrough curves became steeper. This tendency was consistent to other researches [28-3]. 4. ph influence on adsorption and desorption studies The adsorption of organic molecules from aqueous solutions is a complex mechanism in which electrostatic and non-electrostatic interactions are involved, since both forces depend on the properties of the adsorbent and adsorbate, as well as the chemical properties of the solution. The effect of ph of the solution above the adsorption of flumequine from aqueous solutions by activated carbon was studied at ph 3. and 8., comparing the results to those obtained at natural ph, 6.3. The adsorption isotherms and breakthrough curves are depicted in Fig. 9 a-b, respectively. In the Fig. 9c is shown the influence of the tested ph values on the adsorption capacities, from adsorption isotherms, and the saturation levels obtained in breakthrough curves. From the Figure 9, it can be concluded that the adsorption capacity of flumequine onto activated carbon is highest at around ph 6 (ph>pk a, anionic form) and much lower with either increase or decrease in ph values, showing a dramatically decreasing in the adsorption capacity at ph 8. The same trend was observed in other research [31]. Finally, in order to investigate the re-use of the activated carbon, the regeneration of activated carbon columns saturated of flumequine was developed, using a mixture ethanol-water (5:5, v/v). The elution curves and the breakthrough curves for the second cycle adsorption are shown in Fig. 1 a-b. Characteristic parameters of the desorption in saturated columns, such as elution efficiency, E, and overall adsorption process concentration factor, CF, were estimated and are shown in Table 2. Elution efficiencies achieved are not the expected values, since elution percentages near to 1% can be found in the literature [32, 33]. This behavior

14 14 C / mg.l -1 José Luis Sotelo et al. a) b) c) 6 5 1, ph = 6.3 ph = 3. ph = ph = 6.3 ph = 3. ph = 8. q e / mg.g ph = 6.3 ph = 3. ph = C e / mg. L -1 c) 55 5 C/ , q e / mg.g q e / mg.g ph C/ Figure 9. Effects of ph on the adsorption of flumequine by activated carbon: (a) Adsorption isotherms (b) Breakthrough curves (c) Adsorption capacity and saturation level versus ph. a) b) 2 m =.4 g AC m =.6 g AC,7.4 g AC/ 1 st Cycle.4 g AC/ 2 nd Cycle 15,5.6 g AC/ 1 st Cycle.6 g AC/ 2 nd Cycle 1 5 C/, , Figure 1. (a) Elution concentration profile for flumequine-saturated column desorption by EtOH-water (5:5, v/v) (b) Breakthrough curves for flumequine adsorption onto activated carbon. Cycles #1 and #2. Conditions: volumetric flow rate = 2. ml.min -1, inlet flumequine conc. = 1. mg.l -1.

15 Adsorption of several emerging contaminants 15 Table 2. Desorption parameters for cycle #1 of adsorption-desorption in flumequine removal onto activated carbon. mass of adsorben / g Q / cml.min -1 Elution E /% t p /h C p /mg.l -1 CF can be explained by the fact that the elution time was not enough to remove all flumequine molecules from active sites of activated carbon. So, in Fig. 1b can be observed that, in the second cycle adsorption, the breakthrough times was much lower, leading to much lower adsorption capacities and bed utilization values, since there are binding sites previously occupied in the first cycle which have not properly desorbed. Better results are reached about the overall adsorption concentration factor, since this value is maximum for the second column tested (mass of adsorbent =.6 g). In order to desorb the maximum mass of flumequine in a shorter time, it is desirable that this parameter is as high as possible [34]. 5. Conclusion Adsorption removal of the studied emerging contaminants onto granular activated carbon and sepiolite was investigated in batch and fixed-bed column mode. It was found that: (i) adsorption efficiency and adsorption rate are dependent on the properties of the adsorbate (hydrophobicity, size, water solubility, charge, polarizability, presence of functional groups, etc.) and on the porous structure of the adsorbent. Since, the mesopore surface area of sepiolite plays a main role on the adsorption capacity values obtained, 21. mg.g -1, compared to the microporous material values, which is featured by higher specific surface area, higher micropore volume and larger adsorption capacity (28 mg.g -1 ) and a lower adsorption rate; (ii) in the literature, it can be confirmed that log K ow values of the adsorbate strongly influence the adsorption removal. Flumequine presented a value of log K ow = 1.31, leading to a highest adsorption efficiency, which can be quantified by the adsorption capacity provided by the isotherm. (iii) pseudo-second order kinetic model reproduced experimental kinetic studies, being connected this fact to the presence of mesopores in the structure of the adsorbent, since sepiolite is a mainly mesoporous material. The existence of mesopores favors the diffusion of relatively large dimensions molecules, such as caffeine; (iv) the Weber and Morris

16 16 José Luis Sotelo et al. equation, Boyd and McKay models confirmed that both film diffusion and intra-particle diffusion are rate-controlling steps involved in the caffeine adsorption removal by sepiolite (v) adsorption capacities and saturation levels of flumequine onto activated carbon are highly dependent on ph value; (vii) in the regeneration studies with ethanol-water (5:5, v/v), since the elution time was not enough to produce higher elution efficiencies, a maximum overall adsorption concentration factor value was obtained. Acknowledgements The authors gratefully acknowledge the financial support from Ministerio de Economía y Competitividad CTQ , and Comunidad de Madrid through REMTAVARES Network S29/AMB Also, the authors would like to thank TOLSA, S.A. for providing the sepiolite. References 1. Bendz, D., Paxéus, N. A., Ginn, T. R., Loge, F. J. 25,. J. Hazard Mater., 122, Bolong, N., Ismail, A. F., Salim, M. R., Matsuura, T. 29, Desalination, 239, Ashton, D., Hilton, M., Thomas, K. V. 24, Sci. Total Environ., 333, Valladares-Linares, R., Yangali-Quintanilla, V., Li, Z., Amy, G. 211, Water Res., 45, Murray, A. and Örmeci, B. 212, Environ. Sci. Pollut. Res., 19, Stackelberg, P. E., Furlong, E. T., Meyer, M. T., Zaugg, S. D., Henderson, A. K., Reissman, D. B. 24, Sci. Total Environ., 329, Carballa, M., Omil, F., Lema, J. M. 28, Chemosphere, 72, Lishman, L., Smyth, S. A., Sarafin, K., Kleywegt, S., Toito, J., Peart, T., Lee, B., Servos, M., Beland, M., Seto, P. 26, Sci. Total Environ., 367, Kümmerer, K. 29, J. Environ. Manag., 9, Enick, O. V. and Moore, M. M. 27, Environ. Impact Asses., 27, Ternes, T. A. 1998, Water Res., 32, Loos, R., Gawlik, B. M., Locoro, G., Rimaviciute, E., Contini, S., Bidoglio, G. 29, Environ. Pollut., 157, Delgado, L. F., Charles, P., Glucina, K., Morlay, C. 212, Science Sci. Total Environ., , Snyder, S. A., Adham, S., Redding, A. M., Cannon, F. S., DeCarolis, J., Oppenheimer, J., Wert, E. C., Yoon, Y. 27, Desalination, 22, Putra, E. K., Pranowo, R., Sunarso, J., Indraswati, N., Ismadji, S. 29, Water Res., 43, Dogan, M., Özdemir, Y., Alkan, M. 27, Dyes and Pigments, 75, Buerge, I. J., Poiger, T., Müller, M. D., Buser, H-R. 23, Environ. Sci. Technol., 37, 691.

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