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Insights on carbonaceous materials tailoring for effective removal of the anticancer drug 5-Fluorouracil from contaminated waters Eduardo Macedo, Mónica S. F. Santos, Francisco J. MaldonadoHodar, Arminda Alves, and Luis Miguel Miguel Madeira Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05145 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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Industrial & Engineering Chemistry Research

Insights on carbonaceous materials tailoring for effective removal of the anticancer drug 5-Fluorouracil from contaminated waters

Eduardo Macedoa, Mónica S. F. Santosa *, F.J. Maldonado-Hódarb, Arminda Alvesa and Luis M. Madeiraa * a

LEPABE – Laboratory for Process, Environmental, Biotechnology and Energy Engineering, Faculty of Engineering, University of Porto, R. Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal b

Department of Inorganic Chemistry, Faculty of Sciences – University of Granada, 18071, Granada, Spain

ABSTRACT This work presents for the first time a comprehensive analysis of the influence of the textural and elemental properties of different carbonaceous materials on the 5Fluorouracil (5-FU) removal efficacy from water by adsorption. A detailed textural and elemental characterization of the adsorbent materials was performed, based on which their kinetics and equilibrium adsorption performances for 5-FU uptake were compared. The equilibrium data revealed that the adsorption capacities are generally in line with the surface area of the carbonaceous materials; the carbon blacks, BP2000 and Vulcan, exhibit the highest (112 mg/g) and lowest (19 mg/g) adsorption capacities, respectively. The linear driving force model was successfully used to describe the rate of 5-FU adsorption on the carbonaceous materials and the effect of some parameters (agitation speed, adsorbent particle size and initial 5-FU concentration) on the sorption kinetics was evaluated. It was found that the oxygen content and the mesopore volume are the most relevant properties of the adsorbent affecting the kinetics.

Keywords: 5-Fluorouracil, activated carbon, carbon black, adsorption, isotherms, kinetics

*

Corresponding author Mónica S. F. Santos; Tel.: +351-225084848; E-mail address: [email protected]

*

Corresponding author Luis M. Madeira; Tel.: +351-225081519; E-mail address: [email protected] 1

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INTRODUCTION Cancer is currently the leading cause of death in the world, after heart and infectious diseases 1. Cancer incidence in population is gradually increasing, being expected to rise by about 70% over the next two decades 1. Consequently, the demand and consumption of anticancer drugs, which are used in chemotherapy, have proportionally increased.

Anticancer drugs and most of pharmaceuticals are not

completely metabolized by the human body, whereby they are excreted unchanged or only slightly transformed into the sewage system 2. Since relatively low degradation rates have been reported for these compounds in conventional wastewater treatment plants (WWTPs), they are reaching water courses 3-5. As a result, cytostatics are being accumulated in water bodies, potentiating their spread into other environmental compartments: soil, sediments and air 4. Their pharmacological potencies, along with their fetotoxic, genotoxic (mutagenic) and teratogenic properties, potentially make cytostatic drugs the most dangerous contaminants of our water system 3. Predicted environmental concentrations, based on the total national consumption of cytostatics, have been calculated in France 6, UK 3, 4, 7, Portugal 8, Spain 9-11 and Poland 12

. From these studies, priority cytostatics have been identified, whose concentrations

in river waters were recognized to induce chronic effects to aquatic organisms. The present work is focused in the cytostatic 5-fluorouracil (5-FU). Three aspects were in the basis of such selection: (1) according to the prescription data provided by Autoridade Nacional do Medicamento e Produtos de Saúde, I.P. (INFARMED), 5-FU is one of the most used cytostatics in Portugal 8; (2) 5-FU and its pro-drug capecitabine are among the highest consumed cytostatics in Europe 11, 13 and (3) despite of its large prescription and consumption rates, few studies related to 5-FU degradation/removal from water were found in the literature. These are restricted to the use of the Fenton process 14 and photo-assisted oxidation under different conditions 14-17; only one study was found regarding the removal of 5-FU by adsorption on carbonaceous materials 18. Kovalova and co-workers found that activated carbons exhibited lower adsorption capacity for polar 5-FU than for less polar endocrine disrupting micropollutants (17-αethinylestradiol, bisphenol A, cytarabine), even though attractive Coulombic interactions between the positively charged powdered activated carbon surface and anionic 5-FU may have contributed to 5-FU adsorption 18. Therefore, it is important to 2


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screen the performance of different carbonaceous materials and, through a comprehensive textural and elemental characterization of these materials, to understand which properties might influence on the 5-FU removal efficiency, envisaging the subsequent adsorbent tailoring and process optimization. This is the main objective of this work, aiming the efficient removal of 5-FU from wastewaters.

MATERIALS AND METHODS Reagents and materials 5-FU with a purity of 97.7% (w/w) was purchased from Sigma-Aldrich (St. Louis, MO, USA). The distilled water used to prepare 5-FU solutions was filtered through 0.45 μm nylon membrane filters obtained from Supelco, Sigma-Aldrich (Pennsylvanis, USA). For the adsorption assays, four commercial activated carbons commercialized in the form of grains (Merck and Wittco) or pellets (Norit RX3 Extra and Ceca AC40) and two carbon blacks as fine powders (samples BP2000 and Vulcan XC72, from CABOT) were used.

Characterization of the carbonaceous materials The chemical characterization of the carbonaceous materials was carried out by elemental analysis; their pHpzc (pH point of zero charge ) values were determined according to the method proposed by Leon y Leon et al. 1992

19

. The nature of the

oxygenated surface groups was inspected by Thermal Programmed Desorption (TPD) in helium until 1000 °C. The ash content was determined by Thermogravimetric Analysis (TGA) in a Mettler-Toledo thermobalance, burning the organic fraction in flowing air until constant weight. Moreover, the nature of the inorganic content was analysed by X-ray diffraction (DRX) using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. On the other hand, textural characterization was carried out by N2 and CO2 adsorption at −196 °C and 0 °C, respectively, using a Quantachrome Autosorb-1 equipment. The Brunauer-Emmett-Teller (BET) and Dubinin–Radushkevich (DR) equations were applied to determine the apparent surface area (SBET) and the micropore volume (W0), the mean micropore width (L0) and the microporous surface (Smic), respectively

20-23

total pore volume was considered as the volume of N2 adsorbed at P/P0 = 0.95

. The 24

.A 3



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broad pore size distribution was also obtained analysing the N2 adsorption isotherms in different adsorption ranges: 0.0 < P/P0 < 0.1 which corresponds to the adsorption into primary micropores (smaller than 0.8 nm), 0.1 < P/P0 < 0.4 into secondary micropores (0.8 – 2 nm), and 0.4 < P/P0 < 0.95 corresponds to the adsorption in mesopores 25. The Barrett-Joyner-Halenda (BJH) method was applied to the desorption branch of the N2-isotherms to determine the mesopore size distribution of the samples. More details regarding the methods used here can be found in the Supporting Information section (part 2).

Standards and adsorbents preparation A 5-FU stock solution of 250 mg/L was prepared by dissolving the appropriate amount of powder in filtered distilled water. The stock solution was used to prepare working solutions with different 5-FU concentrations (1.0, 2.5, 5.0, 7.5 and 10 mg/L), which were selected taking into account the adsorption capacities of the different carbon materials and the analytical methodology used for the quantification of 5-FU in water. All solutions were preserved by refrigeration until their use. The six carbonaceous materials were dried, grinded and sieved, in order to obtain particle sizes in the range of 38–63 μm and 212-600 μm.

Adsorption kinetics runs Preliminary tests showed that adsorption capacities and kinetics were different among the several carbonaceous materials, whereby different adsorbent concentrations were used for a given initial 5-FU concentration. Therefore, the appropriate amount of each adsorbent (40 mg/L for Merck; 60 mg/L for BP2000 and Norit; 90 mg/L for Ceca, Vulcan and Wittco) and 250 mL of 5-FU solutions of different concentrations (1 mg/L and 10 mg/L) were mixed in 500 mL erlenmeyers. The erlenmeyers were placed in an OLS200 orbital/linear

shaking

bath

purchased

from

Grant

Instruments

(Shepreth,

Cambridgeshire), which allows the control of agitation (88 rpm, unless otherwise specified) and temperature (30 °C in this work). Samples of 1 mL were withdrawn during the 24 h experiments, filtered and collected in vials. Syringe filters with 0.2 μm of polytetrafluoroethylene (PTFE) membrane were purchased from VWR (Wester

4


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Chester, USA) to filtrate the samples. The vials with the solutions were preserved in a freezer for further analysis by molecular absorption spectrophotometry.

Adsorption equilibrium runs The equilibrium runs were carried out in a similar way to the kinetic experiments. A given amount of adsorbent (40 mg/L for Merck; 60 mg/L for BP2000 and Norit; 90 mg/L for Ceca, Vulcan and Wittco) was put in contact with solutions of different 5-FU concentrations and samples were collected at the beginning (t = 0) and after 24 h (equilibrium conditions). Then, the samples were centrifuged in an Eppendorf® Minispin® at 3000 rpm (604 g-Force) during 5 minutes. The resulted supernatant was kept in vials and preserved in a freezer for further analysis.

Spectrophotometric analysis of 5-FU The 5-FU remaining in the liquid phase was followed by UV-Vis spectrophotometry. Absorbance was measured at 266 nm (which is the characteristic wavelength for 5-FU molecule) in a Helios gamma UV-Vis spectrophotometer from Thermo Electron Corporation. Calibration curve was constructed with 5-FU standards in water in the range 0.15 -12.5 mg/L. Details of the analytical method validation are present in the Supporting Information section (part 1). A high precision cell made of Quartz Suprasil® with 10 mm by Hellma Analytics was used in the analysis.

Storage, fate and treatment of wastes Liquid and solid wastes containing 5-FU were generated during the experimental activity. These wastes were collected appropriately to be further treated by a specific system through different specialized entities – solid wastes by Ambimed and liquid wastes by Sistema de Gestão Ambiental da FEUP (EcoFEUP).

RESULTS AND DISCUSSION Materials characterization Activated carbons used here (Merck, Norit, Ceca and Wittco) are no crystalline materials and contain heteroatoms, mainly O, but also N, S, halogens, etc., as well as inorganic impurities (mineral matter or ash). On the other hand, carbon blacks (in this 5



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series, samples BP2000 and Vulcan) are pure materials, composed by C, H and small amounts of O atoms. More information can be found in the Supporting Information (section 2). The elemental analyses of the carbonaceous materials are displayed in Table 1. Oxygenated surface groups can contribute simultaneously for the acidic (carboxylic, lactones, anhydrides) or basic (carbonyl, semiquinones) character of the material surface, influencing in such a manner the pHpzc. In general, acid groups evolve during TPD experiments as CO2, being the carboxylic acid groups the less thermally stable, while basic surface groups evolve as CO and typically at higher temperatures 24, 26. The TPD-profiles obtained for Norit are shown in Figure S2 of the supporting information as an example. All activated carbons, except the Norit sample, present neutral pHpzc values or slightly acid, while carbon blacks (BP2000 and Vulcan) are clearly basic materials. Characteristics of carbon materials can range from typically basic (electron donor) and hydrophobic materials with low oxygen content (after thermal treatments in inert or reducing atmosphere), to acid and hydrophilic materials after oxidation processes 24. In this case, the highest pHpzc values are generally associated to carbon blacks (BP2000 and Vulcan), especially to BP2000, because these samples are clearly free of oxygen surface groups and ash content (cf. Table 1). The nature of the inorganic fraction was tentatively analysed by XRD (Figure S3b of the supporting information). In all cases the widest diffraction peaks at around 25 and 42°, which correspond to the 002 and 101 diffraction peaks of graphite, respectively, confirms that activated carbons are mainly amorphous materials. Only the XRD patterns of Norit and Merck samples showed low intensity diffraction peaks corresponding to the inorganic phase. This fraction should be amorphous in the rest of activated carbons. The peaks at 31.4, 45.1 and 55.9° in the Norit sample denotes the presence of carbonates (dolomite, PDF 11-78), while the small peaks at 20.8 and 26.6° in the XRD pattern of the Merck sample were ascribed to quartz impurities (PDF 14260). The presence of dolomite in the Norit sample can contribute to the higher pHpzc of this sample compared to other activated carbons. The textural characteristics of the carbonaceous materials are summarized in Table 2. It is well known that N2 and CO2 adsorption provides complementary information about microporosity. In the absence of diffusion restrictions the total microporosity is 6


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obtained from N2 isotherms. The volume of N2 adsorbed close to saturation (P/P0 = 0.95; V0.95) was considered as the total pore volume of the samples while the CO2 adsorption is usually performed to characterize the micropores with diameter below 0.7 nm 27. BP2000 and Vulcan present the highest and the lowest S BET of the samples series, respectively, on contrary to expectations, since they are both carbon blacks. When analysing the microporosity, it is noteworthy that the micropore volume determined with CO2 (W0 (CO2)) is quite similar between all the samples except Vulcan. These narrow micropores strongly contribute to the high surface area of the samples. Thus, with only 0.05 cm3 of narrow micropores per gram of material, Vulcan exhibits the lowest surface area. It is also noteworthy that W0(N2) > W0(CO2) in all cases. Therefore, microporosity is large enough to avoid diffusion restrictions to the interior of the narrowest micropores, and W0(N2) > W0(CO2) due to the adsorption of N2 on large micropores and mesopores. This is confirmed by the large values for the mean micropore size (L0), which in some cases are very close to the mesopore size limit (2 nm) - Table 2. A clear difference in the porosity of the samples is observed at a glance taking into account the shape of the N2-adsorption isotherms and the total volume adsorbed. All N2 adsorption isotherms obtained for activated carbons (Merck, Wittco, Norit and Ceca) correspond mainly to the type I (Figure S4 of the supporting information), while in the case of carbon blacks (BP2000 and Vulcan) the isotherms are a mixture of the type I and type II isotherms. Although these results point out the microporous character of activated carbons, the micropore distribution is quite different. Bellow P/P0 0.8, indicating the presence of large mesopores that can be associated also to the interparticle voids (Figure S4 of the supporting information)

28

. The

contribution of these large pores strongly enhances the total pore volume of the 7



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sample (Table 2) but evidently does not contribute in the same proportion to the surface area values. In both carbon blacks, the application of the BJH method to the desorption isotherms confirms that the main porosity is located in large mesopores and macropores, corresponding in general to pores of diameter larger than 10 nm (Figure S5 of the supporting information), which probably includes, as commented, the interparticle voids. Activated carbons, on the contrary, showed their pore size distribution mainly located in the micropore range, although some mesopore volume is detected (Table 2 and Figure S5).

Adsorption isotherms Preliminary tests, not presented here for brevity reasons, showed that adsorption capacities and kinetics were different among the several carbonaceous materials and also that 24 h of contact is enough to reach the equilibrium conditions. The adsorption isotherms of 5-FU on carbonaceous materials were obtained by plotting the amount of 5-FU adsorbed in the solid phase against the 5-FU concentration in the liquid phase, both under equilibrium conditions (i.e., after 24 h of contact) – Figure 1. Langmuir, Freundlich, and Temkin isotherm models (Eqs. (1) to (3), respectively) were considered to describe the experimental results:

=

. . 1 +

(1)

In Eq. (1), q e (mg/g) and C e (mg/L) are the equilibrium concentration of adsorbate on the adsorbent and in the solution, respectively, q L (mg/g) is the maximum monolayer capacity, and K L (L/mg) is the Langmuir adsorption equilibrium constant 29.

=

(2)

Here, K F (mg/g/(mg/L)1/n) stands for the adsorption equilibrium constant, which is a measure of the adsorption capacity, and nF is the Freundlich exponent, indicative of the affinity of the adsorbate to the adsorbent 30.

8


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=

ln ( )

(3)

In Eq. (3), K T is the equilibrium binding constant (L/mg), b is related to the heat of adsorption (J g/(mg mol)), R is the ideal gas constant (8.314 J/K mol) and T is the absolute temperature (K) 31. The parameters of the isotherms were determined by fitting the models to the experimental data through non-linear regression. The results obtained are compiled in Table 3. The best fits were identified applying the mathematical/model selection criterion (MSC) – Eq. (4)

32, 33

. Besides the correlation between the experimental data

and the theoretical results, MSC also takes into account the number of experimental points and the number of parameters of the fitted model. A higher number for MSC is synonym of a better fit.

∑ ! )" 2) #$( − = ' − ∑#$( − %&% )" *

(4)

In this equation, %&% is the calculated adsorbate concentration in the solid phase by

any of the previous models and is the measured adsorbate concentration in the solid phase; ! is the mean of the measured adsorbate concentrations in the solid

phase, * is the number of experimental points and p is the number of fitting parameters.

The results compiled in Table 3 show that Temkin is the worst model to describe the equilibrium data from the adsorption of 5-FU on carbonaceous materials. On the other hand, while the adsorption of 5-FU on Merck, BP2000, Norit, Vulcan and Witco seems to be better described by Freundlich model, the adsorption on Ceca seems to be slightly better fitted by Langmuir equation. To decide for the best model, the statistic

F -test was applied, determining the ratios between the variances of the two models and comparing them with the tabulated F -values for 95% confidence level. As shown in Table 3, the calculated F -values are all below the tabulated F -value ( Fα =0.05 ),

9



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meaning that both models (Langmuir and Freundlich) can be used to describe the experimental data because they are not statistically different. The q L parameter of the Langmuir model describes quantitatively the formation of a monolayer of adsorbate on the surface of the adsorbent 34. Observing Figure 2a, which relates BET surface area with q L for every carbonaceous material, it is verified a tendency: an increase of surface area allows a higher uptake of 5-FU. The deviations from this close to linear relationship may result from surface chemistry influence. BP2000 and Vulcan correspond to the materials with highest and lowest surface area and adsorption capacity, respectively (Table 2 and Table 3). Since they have very identical compositions (Table 1 and Table 2), surface area is the predominant factor influencing adsorption among these two carbon blacks. Regarding the other carbonaceous materials, a graph comparing oxygen content and q L was plotted (Figure 2b), in order to better understand the influence of superficial chemistry. As observed, q L value increases almost linearly with the oxygen content. The presence of oxygen groups, even in small quantities, may affect the surface properties such as surface acidity, polarity or hydrophobicity, and surface charge. Certain oxygencontaining surface functionalities such as chromene, ketone, and pyrone can contribute to the carbon basicity, while carboxylic acid or carboxylic anhydride, lactone, and phenolic hydroxyl, have been postulated as sources of surface acidity 35. The several types of oxygen groups to be found on adsorbent surfaces, because of the electronegativity of the oxygen atoms, possess dipole moments and their presence has a marked effect on the shapes of adsorption isotherms of polar adsorbates 36. Concerning the other parameter of the Langmuir equation, K L , the highest and lowest adsorption affinities were noticed in the systems 5-FU/Norit and 5-FU/Ceca, respectively (Table 3). Curiously, these observations can be correlated with the pHpzc of the materials: the highest affinity was attained for the material with the highest pHpzc (Norit) and vice versa (Table 1 and Table 3). The pHpzc defines the surface charge of an adsorbent and, depending on its value and the amount of adsorbent used in the process, the pH of the solution could change, along with the ionization state of the adsorbate. The surface of Norit is positively charged in water solutions of pH99.0 100

S 3.0 0.4 0.9 0.0

O 4.2 19.6 9.1 10.0

Ash 0.3 8.3 5.2 4.8

Insights on Carbonaceous Materials Tailoring for Effective Removal of

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