Chemosphere 111 (2014) 568–574

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Thermodynamic studies for adsorption of ionizable pharmaceuticals onto soil Joanna Maszkowska ⇑, Marta Wagil, Katarzyna Mioduszewska, Jolanta Kumirska, Piotr Stepnowski, Anna Białk-Bielin´ska ´ sk, ul. Wita Stwosza 63, 80-308 Department of Environmental Analysis, Institute for Environmental and Human Health Protection, Faculty of Chemistry, University of Gdan ´ sk, Poland Gdan

h i g h l i g h t s  Adsorption thermodynamics of three ionizable, polar compounds were analyzed.  Clear differences were observed in sorption thermodynamics depending on ionic form.  Sorption mechanisms of PRO, SGD and SSX were demonstrated.

a r t i c l e

i n f o

Article history: Received 26 February 2014 Received in revised form 30 April 2014 Accepted 2 May 2014

Handling Editor: I. Cousins Keywords: Beta-blockers Ionizable compounds Sorption Sulfonamides Thermodynamic

a b s t r a c t Although pharmaceutical compounds (PCs) are being used more and more widely, and studies have been carried out to assess their presence in the environment, knowledge of their fate and behavior, especially under different environmental conditions, is still limited. The principle objective of the present work, therefore, is to evaluate the adsorption behavior of three ionizable, polar compounds occurring in different forms: cationic (propranolol – PRO), anionic (sulfisoxazole – SSX) and neutral (sulfaguanidine – SGD) onto soil under various temperature conditions. The adsorption thermodynamics of these researched compounds were extensively investigated using parameters such as enthalpy change (DH°), Gibbs free energy change (DG°) as well as entropy change (DS°). These calculations reveal that sorption of PRO is exothermic, spontaneous and enthalpy driven, sorption of SGD is endothermic, spontaneous and entropy driven whereas sorption of SSX is endothermic, spontaneous only above the temperature of 303.15 K and entropy driven. Furthermore, we submit that the calculated values yield valuable information regarding the sorption mechanism of PRO, SGD and SSX onto soils. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction A diverse array of synthetic organic compounds are used in vast quantities by society for a range of purposes, including in the production and preservation of food, in industrial manufacturing processes, as well as in human and animal healthcare. More than 100,000 chemicals were introduced in the 20th century, with little realization of what their affects on the environment would be, and what the consequences either directly or indirectly would be for human health (Primel et al., 2012). Reliable data is required in order to be able to successfully evaluate the impact of these compounds on the environment. The predominant fate processes for pharmaceuticals in the different environmental compartments are sorption (e.g. tetracyclines and quinolones) and (bio)degradation. Photodeg⇑ Corresponding author. Tel.: +48 58 5235207. E-mail address: [email protected] (J. Maszkowska). http://dx.doi.org/10.1016/j.chemosphere.2014.05.005 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

radation and hydrolysis can also be significant (Kümmerer, 2009). However, making an assessment of the distribution of a chemical between its soil and aqueous phases is not a straightforward process. It depends on: (i) the basic chemistry of the compounds (ii) the amount of the substances involved; (iii) climatic factors such as intensity of ‘‘rainfall’’ events and temperature; and (iiii) soil type (e.g. pH, organic matter (OM) content, clay fraction content) (OECD, 2000). These features greatly affect the transport, reactivity, and bioavailability of organic compounds in the environment. The temperature is a useful tool from which valuable information about sorption mechanisms can be ascertained (DiVincenzo and Sparks, 2001; Kah and Brown, 2006). However up to date studies on adsorption thermodynamic among organic polar compounds have been mainly carried out for reactive dyes (Allen et al., 2004; Önal et al., 2007; Al-Degs et al., 2008; Zawani et al., 2009; He et al., 2010; Pansuk, 2011; Bajpai and Jain, 2012). Very limited data is available for pharmaceuticals, especially sulfonamides (Guo et al.,

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2013) and beta-blockers representing different ionic forms in this study. The objective of this research therefore was to evaluate the adsorption thermodynamics of three pharmaceuticals representing different ionic species in variable environmental conditions represented by temperature fluctuations.

from the region of Pomerania in northern Poland, the soil was airdried, ground in a mortar and passed through a 2 mm sieve, then re-ground in a mortar with a small rubber pestle. Soil pH was determined with a glass electrode in a 1:2.5 soil/water suspension using 1 M aq. KCl (exchangeable acidity) (6.22). Soil organic carbon (OC) was determined by loss-on-ignition (14.9%), and the cation exchange capacity (CEC) was determined using the BaCl2 Compulsive Exchange Method (53.1 cmol(+) kg1). The clay fraction of the soil was 7.2%.

2. Materials and methods 2.1. Chemicals Standards of propranolol hydrochloride (PRO), sulfisoxazole (SSX) and sulfaguanidine (SGD) (Table 1) as well as trifluoroacetic acid 99% (TFA) were purchased from Sigma–Aldrich (Steinheim, Germany). Deionized water was produced by HYDROLAB System (Gdan´sk, Poland). Acetonitrile (ACN), methanol (MeOH), hydrochloric acid (HCl), potassium chloride (KCl), calcium chloride (CaCl2) and potassium hydroxide (KOH) were purchased from POCH (Gliwice, Poland). Standard stock solutions of two of the researched compounds (PRO and SGD) at concentrations of 800 mg L1 were prepared by dissolving the pure compound in 0.01 M CaCl2. The spike solutions (8 points) were prepared from stock solutions in accordance with the serial dilution method in a proper CaCl2 solution, so that the other concentrations of PRO and SGD ranged between 6.25 mg L1 and 400 mg L1. The solutions were sonicated for 15 min for complete dissolution. Due to lower water solubility of SSX, the standard stock solution was prepared at a concentration of 80 mg L1. For this reason experiments involving this compound were carried out without previous equilibration. The other spiked solutions in the range from 0.625 to 40 mg L1 were prepared from a stock solution in accordance with the serial dilution method using aq. CaCl2.

2.3. Description of the thermodynamic adsorption test The sorption studies were carried out according to OECD Technical Guideline 106 (batch sorption experiments) (OECD, 2000). All of the samples were prepared in triplicate. Tests were carried out using a shaking water bath (Julabo SW22, Rose Scientific Ltd., USA), ensuring adjustment of temperature (293.15, 303.15 and 313.15 K) and constant contact with the soil sample solution containing analytes. To avoid photodegradation of the compounds being researched, the experiments took place in the dark. In our experiments we assumed constant structure/composition of the soil at the various temperatures. The selection of the optimum soil/solution ratios was based on the calculated percentage of chemical adsorbed to soil, which should be >20%, and preferably >50%. Selected ratios were 1:2, 1:5 and 1:50 for SSX, SGD and PRO respectively. Equilibrium time was achieved for all the pharmaceuticals in all three media after 24 h. The sorption experiment included the following steps: (1) 1 g (for SGD) or 0.5 g (for PRO) of air-dried soil samples were equilibrated by shaking with an appropriate volume (4.5 or 22.5 mL for SGD and PRO respectively) of CaCl2 at a concentration of 0.01 M overnight (12 h) before the day of the experiment; (2) a certain volume (0.5 or 2.5 mL to the soils derived from sorbent/solution ratio) of the spike solutions of the test substance was added to adjust the final volume and achieve a 10-fold dilution. In this step, soil samples without any previous equilibration were spiked using 2 mL of solutions containing SSX at a concentration in a

2.2. Soils The experiments were conducted using natural peat soil possessing different physicochemical properties (see below). Sampled Table 1 Structures and selected properties of the investigated pharmaceuticals. Substance abbreviation [CAS]

Chemical structure

Selected physico-chemical properties M = 295.8 g mol1

CH3

Propranolol hydrochloride PRO [318-98-9]

O OH

Sulfaguanidine SGD [57-67-0]

H2N

N H

O N HN H 2N

log P = 2.65 water solubility (25 °C) = 50 g L1

HCl

O S NH O

Sulfisoxazole SSX [127-69-5]

pKa = 9.53

CH3

S O O

C

NH

M = 214.2 g mol1

NH2

pKa3 = 12.0

pKa2 = 2.8 log P = 1.22 water solubility (25 °C) = 1 g L1 M = 267.3 g mol1 pKa2 = 2.15 pKa3 = 5.00 log P = 1.01 water solubility (25 °C) = 0.13 g L1

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range from 0.625 to 80 mg L1. Eight concentration levels for determining sorption isotherms were anticipated (0.625, 1.25, 2.5, 5, 10, 20, 40, 80 mg L1); (3) the mixture was shaken for 24 h until adsorption equilibrium was reached; (4) subsequently, samples were centrifuged at 4000 rpm for 10 min (MPW-250 Centrifuge, Warsaw, Poland), filtrated with 0.45 lm syringe filters (ChromafilÒ PET 15/25, Macherey–Nagel, Düren, Germany) and placed in HPLC vials. (4) The concentration of test substance in the supernatant was analyzed by reverse-phase HPLC with UV detection. 2.4. Instrumental analysis The filtrate samples of SSX and SGD were analyzed by isocratic reversed phase HPLC using a Phenomenex Gemini C18-110A column, 150 mm  4.6 mm i.d., 5 lm (Torrance, USA). The Perkin Elmer Series 200 analytical system consisted of a chromatographic interface (Link 600), a binary pump, a UV/VIS detector, a vacuum degasser and a Rheodyne injection valve. SSX and SGD were detected at a wavelength of 270 nm. The mobile phase for the determination of SGD was ACN:H2O (6:94, v:v) at a 0.5 mL min1 flow rate and ACN:H2O (with 0.0025% of TFA) (45:55, v:v) at a 0.7 mL min1 flow rate for SSX. The injection volume was 30 lL for SGD and 50 lL for SSX. The filtrate samples of PRO were analyzed by isocratic reversed phase HPLC using a Phenomenex Gemini C6-phenyl 110A column (150 mm  4.6 mm, 5 lm) (room temperature, wavelength 220 nm, injection volume 50 lL, flow rate 0.7 mL min1). Mobile phase A was 16% acetonitrile, and mobile phase B was buffered at pH 3, 56 containing H2O, 1 mM CH3COONH4 and CH3COOH. All chromatographic analyses were carried out on two replicates.

sorbent depends on temperature (Wang et al., 2010; Zhang et al., 2012). In this study, the sorption isotherms for PRO, SSX and SGD revealed that sorption dependency on temperature differs for various ionic forms of chemical compounds (Fig. 1, Table 2). Since PRO is considered to be a weak base (pKa = 9.42), this compound mainly exists as an organic cation. For this pharmaceutical, sorption distribution coefficients obtained for temperatures within a range from 293.15 K to 313.15 K, decrease with increasing temperatures (Table 2). Therefore sorption seems to be enhanced at lower temperatures. An inverse trend was observed for SSX. The values of Kd increased in the order 313.15 > 303.15 > 293.15 K (Table 2). This suggests that the adsorption process of SSX onto soil is endothermic. Although no study has been conducted previously for this compound, the yielded results are in agreement with studies reported for other sulfonamide – sulfamethazine (Guo et al., 2013) as well as for other organic anions, mainly reactive dyes (Önal et al., 2007; Al-Degs et al., 2008; Qu et al., 2008; Zawani et al., 2009; Cea et al., 2010; Pansuk, 2011; Salman and Al-Saad, 2012). Guo et al., 2013 – who reported endothermic sorption for sulfamethazine – also carried out thermodynamic studies for a

y = 2013.2x -2.6168 R² = 0.9986

PRO 4.3 4.2 4.1

ln Kd

570

4 3.9 3.8 3.7 0.0031

3. Results and discussion

0.0032

0.0033

0.0034

0.0035

-1

1/T [K ] 3.1. Sorption distribution coefficient and sorption isotherm at different temperatures

cs ¼ K d  cw

ð1Þ

where Kd – equilibrium distribution coefficient (L kg1), cs – content of test substance adsorbed on the soil at adsorption equilibrium (mg kg1), cw – mass concentration of test substance in the aqueous phase at adsorption equilibrium (mg L1). A plot of the cs values versus cw were obtained for different temperatures with the slope giving the values of Kd. The values of distribution coefficients which were obtained exhibited a general trend, indicating that sorption capacity of

0.9 0.85

ln Kd

0.8 0.75 0.7 0.65 0.6 0.0031

0.0032

0.0033

0.0034

0.0035

-1

1/T [K ] y = -2670x + 8.6113 R² = 0.9833

SSX 0.15 0

ln Kd

The measurement of isotherms at several temperatures is one of the techniques employed to determine thermodynamic parameters for sorption processes. The most popular methods used to calculate sorption thermodynamic parameters from isotherms are the partitioning model, the equilibrium constant model, the Langmuir sorption model or the Freundlich sorption equation (Salvestrini et al., 2014). Since the data obtained in our investigation were well fitted to linear isotherms (in most cases values of the correlation coefficient R2 > 0.99) the thermodynamic parameters were calculated on the basis of equilibrium distribution coefficient Kd. For this purpose, the amount of PRO, SSX and SGD remaining in the solution after the sorption experiments was determined. The concentrations of sorbed pharmaceuticals were determined by calculating the differences between the control and final drug concentrations. All the results were then applied to the linear sorption isotherm equation:

y = -748.06x + 3.2644 R² = 0.9613

SGD

-0.15 -0.3 -0.45 -0.6 0.0031

0.0032

0.0033

0.0034

0.0035

-1

1/T [K ] Fig. 1. Van’t Hoff plots of PRO, SGD and SSX adsorption onto peat soil.

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J. Maszkowska et al. / Chemosphere 111 (2014) 568–574 Table 2 Distribution coefficients as well as thermodynamic parameters of PRO, SGD and SSX sorption onto peat soil. T (K)

PRO SGD SSX

Kd (L kg1)

DG (kJ mol1)

293.13

303.13

313.13

293.13

303.13

313.13

70.49 ± 3.49 2.02 ± 0.21 0.62 ± 0.10

55.41 ± 6.62 2.26 ± 0.17 0.79 ± 0.75

45.47 ± 1.56 2.37 ± 0.10 1.11 ± 0.09

10.37 1.71 1.16

10.12 2.06 0.60

9.94 2.25 0.28

macrolide antibiotic – tylosin. Macrolide antibiotics contain a basic dimethylamine [–N(CH3)2] group, which is able to gain a proton (Babic´ et al., 2007). Thus, according to the chemical structure of these compounds, tylosin occurs as an organic cation. Although the experiment performed by Guo et al. (2013) embraced lower temperatures (288.15–308.15 K) than our investigation, the same trend was observed: that the sorption decreased with increasing temperatures. An explicit difference between the sorption thermodynamics of two compounds carrying an opposite charge was observed. In addition, the results reported for PRO tally with data published in publications for compounds which may exist as cation e.g. trimethoprim (Al-Bayati and Ahmed, 2011) or norfloxacin (Zhang et al., 2012). Sulfaguanidine is characterized by a relatively high value of pKa corresponding to the guanidine group. Therefore, it is able to lose a proton in extremely basic conditions (Babic´ et al., 2007; Białk-Bielin´ska et al., 2012b). Since the experiment was performed in calcium chloride, and the pHKCl of the soil used in the analysis was 6.22 SGD, molecules occurred in neutral form. For this compound, no apparent trend with increasing temperatures was observed. The partition coefficient increased only very slightly with increasing temperatures (Table 2). Studies by DiVincenzo and Sparks (2001) showed a clear decrease in Kd with increasing temperatures for the ionized form. Protonated species (neutral form) did not demonstrate an explicit trend, which tallies with results obtained for SGD. It is also in agreement with analysis by other researchers which has already been reviewed by Kah and Brown (2006). Although Thirunarayanan et al. (1985) reported an increase in Kd values for chlorsulfuron with a decrease in temperature from 303.15 K to 281.15 K, he also observed that the smallest effect temperature had on the amount adsorbed was with the lowest pH, where the neutral form dominate. In addition, for other pesticides, which exist mainly as neutral molecules, temperature seems to have a negligible effect on sorption (Fruhstorfer et al., 1993; Eberbach, 1998). However taking into consideration slight increases in Kd with increasing temperatures, this trend complies with the adsorption of the neutral pesticide, carbaryl, at different temperatures (Singh et al., 2011). Based on partition coefficients it can be concluded that the ionic form seems to have a major influence on the thermodynamic effects of sorption. Nevertheless, it is worth underlining that some authors have observed the opposite behavior for anionic compounds to that noted for negatively charged SSX in this study (DiVincenzo and Sparks, 2001; Vinod and Anirudhan, 2002; Antunes et al., 2012). However He et al. (2010) reported the endothermic nature of sorption for basic dye. Therefore, despite the thermodynamics of adsorption being clearly dependent on the charge carried by molecules, it cannot be considered to be the only factor. 3.2. Thermodynamic modeling Thermodynamic parameters such as changes in Gibbs free energy (as an indication of spontaneity of a chemical reaction and therefore important criterion for spontaneity of the process):

DH (kJ mol1)

DS (kJ mol1 K1)

16.74 6.22 22.20

0.022 0.027 0.072

DG° (kJ mol1), as well as enthalpy: DH° (kJ mol1) and entropy DS° (J mol1 K1) were determined by using the following equations. The free energy of an adsorption process is related to the linear sorption distribution coefficient (Kd) which is expressed in the Van’t Hoff equation (Eq. (2)) (Saha and Chowdhury, 2011; Salvestrini et al., 2014). 

DG ¼ RT ln K d 



DG ¼ DH  T DS 

ln K d ¼

ð2Þ 

DS DH  R RT

ð3Þ



ð4Þ

R is gas constant (0.008314 kJ mol1 K1) and T is temperature (K). The values of DH° and DS° were calculated in accordance to Eq. (4) from the slope and intercept respectively, by plotting the logarithmic value of the distribution coefficient against 1/T. In order to help clarify sorption mechanisms, the aforementioned thermodynamic parameters were calculated and they are presented in Table 2. The value of DH° obtained for PRO was negative, thus showing exothermic heat of adsorption. This indicates that the total energy absorbed in bond breaking is less than the total energy released in bond making between the adsorbate and the adsorbent, resulting in the release of additional energy in the form of heat (Saha and Chowdhury, 2011). Based on the magnitude of DH°, conclusions regarding the type of sorption process can be reached. For PRO it is 16.74 kJ mol1. The absolute value of DH° for PRO is less than 40 kJ mol1 which is consistent with an assumption of physisorption (Önal et al., 2007; Zulfikar and Materials, 2013). Moreover, this value falls into a range corresponding to heat of condensation (2.1–20.9 kJ mol1), which confirms that sorption of PRO onto peat soil may be considered to be physical in nature (Saha and Chowdhury, 2011; Zhang et al., 2012). In addition taking the magnitude of the enthalpy into account more thoroughly, hydrogen bonding can be regarded as an important interaction during the sorption process of PRO onto the analyzed soil (DiVincenzo and Sparks, 2001; Cea et al., 2010). Generally the value of Gibbs free energy change (DG°) is an indication of the spontaneity of a chemical reaction and therefore, based on this knowledge, it is possible to evaluate whether sorption is related to spontaneous interaction or not (Saha and Chowdhury, 2011). Negative values of this quantity would indicate the feasibility and spontaneous nature of the adsorption process under the conditions and the temperatures range (293.15–313.15 K) covered by the research. Although this is the first investigation of adsorption thermodynamics for PRO, the DG° value obtained in the research is in agreement with research for sorption of other pharmaceuticals possessing a positive charge (He et al., 2010; Al-Bayati and Ahmed, 2011; Cheng et al., 2013; Guo et al., 2013). The increase in DG° with increasing temperatures complies the assumptions of an exothermic adsorption process (Fig. 2). This indicates that sorption of PRO onto a soil surface is more favorable at lower temperatures, which also tallies with previously published studies for tylosin (Guo et al., 2013). The negative value of DS° (Table 2) suggests that adsorption of PRO is enthalpy driven.

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PRO

ΔG [kJ mol -1 ]

-9.9 -10 -10.1 -10.2 -10.3 -10.4 288.15

293.15

298.15

303.15

308.15

313.15

T [K]

SGD

ΔG [kJ mol -1 ]

-1.6 -1.8 -2 -2.2 -2.4 288.15

293.15

298.15

303.15

308.15

313.15

T [K]

SSX

ΔG [kJ mol -1 ]

1.2 0.8 0.4 0 -0.4 288.15

293.15

298.15

303.15

308.15

313.15

T [K] Fig. 2. Plots of Gibbs free energy change (DG) versus temperature for PRO, SGD and SSX adsorption onto peat soil.

It indicates that randomness at the soil/liquid interface during this process is lower. This may occur due to the formation of more than one layer of adsorption (Antunes et al., 2012). A decrease in adsorption coefficients with an increase of the temperature was also observed by Mrozik et al. (2008) for cationic ionic liquids onto clay mineral. The results obtained for cationic tylosin (Guo et al., 2013) and trimethoprim (Al-Bayati and Ahmed, 2011) are in agreement with this and therefore a hypothesis about the importance of ionic species to adsorption thermodynamics is justified. Although only a minor increase of sorption with increasing temperatures was observed for SGD, thermodynamic quantities were determined in order to try to better evaluate the sorption mechanism of SGD onto soil. Previously published data demonstrates the low sorption potential of SGD onto a soil surface (Białk-Bielin´ska et al., 2012a; Maszkowska et al., 2013). Białk-Bielin´ska et al. (2012a) indicates that weak bonding forces e.g. hydrogen bonds are possible in the sorption process of SGD molecules. The value of enthalpy DH° is less than 8 kJ mol1 which implies a physical nature of SGD adsorption as well as the contribution of London van der Waals interactions during this process. Nevertheless, results may include some errors due to the fact that they are derived from calculations based on obtained isotherms and not on direct calorimetric measurements, therefore meaning that hydrogen bonding cannot be excluded unambiguously. Although general principles of the adsorption process assume a negative value of enthalpy change, a positive value of DH° may

result from the necessity of replacing more than one water molecule from the adsorbent for adsorbate sorption to occur (Saha and Chowdhury, 2011). The negative value of DG° and its increased (Fig. 2) negativity tally with studies of carbaryl and neutral species of pentachlorophenol and is related to spontaneous process and a high affinity of SGD towards soil at higher temperatures (DiVincenzo and Sparks, 2001; Singh et al., 2011). Similar results were also obtained by Ghiaci et al. (2007) who observed endothermic nature of sorption of non-ionic surfactants. The positive value of entropy obtained for SGD reflects the affinity of the tested soil towards the SGD molecules. It also suggests increases in randomness in the investigated soil/solution system with some structural changes occurring in both the adsorbate and the adsorbent. It may be due to differences in translational entropy gained by the solvent (more) and lost by SGD (less) resulting in a slight increase in the disorder of the system (Saha and Chowdhury, 2011). The positive entropy may also come from the loss of the structured water surrounding the compound (DiVincenzo and Sparks, 2001), which is enhanced at higher temperatures. This is supported by Gibbs free energy and the endothermic nature of the process. Also Łuczak et al. (2009, 2011) studying the influence of temperature on critical micelle concentration of imidazolium ionic liquids in aqueous solution, highlighted that increased temperature affects the degree of hydratation around hydrophilic domain. The higher temperature, the lower degree of hydratation of polar head domain is observed, which leads to increasing hydrophobicity of the solute. This favors the aggregation process and possibly can also be referred to enhanced sorption, with increasing temperature, observed in our study. On the other hand the authors pointed out that with an increasing temperature the breaking down of the structured water surrounding the hydrophobic domain of ionic liquid can occur. This phenomenon is unfavorable for aggregation due to the fact that the low entropy of the structured water is principle driving force towards micellization. Although presented phenomena cannot be directly applied to the pharmaceuticals, due to different structures, influence of water from hydration layer around polar moieties on sorption might be taken into consideration. The thermodynamics of adsorption for SSX during this study exhibit an opposite trend to the positively charged PRO. The value of enthalpy change is positive which is consistent with an endothermic process. The magnitude of less than 40 kJ mol1 corresponds to physisorption (Önal et al., 2007; Zulfikar and Materials, 2013). The DH° value we obtained equals 22.20 kJ mol1 which clearly tallies with results for sulfamethazine (Guo et al., 2013). According to the calculations, the overall process for the studied soil seems to be endothermic and physical in nature. Nevertheless if the physisorption was the only adsorption process, the enthalpy of the system would be exothermic (Ajmal et al., 1998). The enthalpy change value is also in the range of values which imply the occurrence of hydrogen bonding. Since SSX occurs as negatively charged molecules, possible mechanisms of sorption are charge-transfer complexation or cation bridging (Schnitzer and Khan, 1978; MacKay and Vasudevan, 2012). In addition, for typical ion-exchange reactions DH° values are usually smaller than 8.4 kJ mol1 (Guo et al., 2013). Therefore, this indicates that sorption of SSX onto soil is a rather complex process. This further supports the above mechanism. The values of DG° display a decreasing trend with increasing temperatures (Table 2, Fig. 2). However, for temperatures of 293.15 and 303.15 K, positive values were obtained. Due to the energy barrier, this implies a non-spontaneous adsorption system at these range of temperatures. Such phenomena may result from repulsion between the soil surface and SSX – both of which are negatively charged (Antunes et al., 2012). The feasibility and spontaneity of the process was observed above 313.13 K, indicating a decrease of the energy barrier with

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increasing temperatures. The entropy obtained for SSX displays the same trend as for SGD (Table 2) indicating an entropy-driven adsorption of the anionic compound. This tallies with results for sulfamethazine (Guo et al., 2013) and other anionic compounds (Önal et al., 2007; Qu et al., 2008; Cea et al., 2010; Pansuk, 2011). Taking into consideration the same temperature effect and thereby comparing entropies for SGD and SSX, entropy is evident along with hydrophobicity of the compound. This has been previously reported for sorption of anionic pesticides onto soils at different temperatures (Cea et al., 2010). 4. Conclusions Thermodynamic calculations for PRO, SGD and SSX performed for the first time, suggest that there are differences in the partitioning mechanisms of chemicals and that they clearly depend on their ionic form. PRO sorption onto a soil surface is a spontaneous process and would be unfavorable at high temperatures. Negative entropy suggests a more specific surface reaction due to the presence of positively charged functional groups. However, neutral SGD and anionic SSX were much less sorbed and displayed endothermic sorption onto the tested soil. In the case of all the pharmaceuticals under study, a physical nature of sorption was observed which indicates a process which is fast and reversible due to the small energy requirements. Although PRO possesses a positive charge – which may lead to a much stronger attraction by the negatively-charged soil surface – the relatively low absolute value of the enthalpy change implies that other forces (weaker than ionexchange) may affect the sorption process to a great extent. Steric hindrance may be the reason for a decrease in the ion-exchange attraction, so far considered to be a major mechanism of betablockers sorption onto soils. Since opposing trends in adsorption thermodynamics have been observed for cationic and anionic compounds, this may aid in the development of more efficient methods of removal of ionizable compounds in different applications e.g. water treatment systems or remediation strategies. This should help in assessing the environmental risks of ionizable polar pharmaceuticals. In addition, due to the fact that some pharmaceuticals are fairly water-soluble, polar compounds, however quite persistent – resistant to biodegradation and hydrolysis like e.g. SAs (Białk-Bielin´ska et al., 2014) the thermodynamic data that has been attained may support predictions about their environmental fate. Acknowledgments Financial support was provided by the Polish National Science Centre under Grant DEC-2011/03/B/NZ8/03010. The authors would like to thank Marta Abramowicz for her lab work. References Ajmal, M., Hussain Khan, A., Ahmad, S., Ahmad, A., 1998. Role of sawdust in the removal of copper (II) from industrial wastes. Water Res. 32, 3085–3091. Al-Bayati, R.A., Ahmed, A.S., 2011. Adsorption – desorption of trimethoprim antibiotic drug from aqueous solution by two different natural occurring adsorbents. Int. J. Chem. 3, 21–30. Al-Degs, Y., El-Barghouthi, M., El-Sheikh, A., Walker, G., 2008. Effect of solution pH, ionic strength, and temperature on adsorption behavior of reactive dyes on activated carbon. Dye. Pigment. 77, 16–23. Allen, S.J., McKay, G., Porter, J.F., 2004. Adsorption isotherm models for basic dye adsorption by peat in single and binary component systems. J. Colloid Interface Sci. 280, 322–333. Antunes, M., Esteves, V.I., Guégan, R., Crespo, J.S., Fernandes, A.N., Giovanela, M., 2012. Removal of diclofenac sodium from aqueous solution by Isabel grape bagasse. Chem. Eng. J. 192, 114–121. Babic´, S., Horvat, A.J.M., Mutavdzˇic´ Pavlovic´, D., Kaštelan-Macan, M., 2007. Determination of pKa values of active pharmaceutical ingredients. Trends Anal. Chem. 26, 1043–1061.

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Thermodynamic studies for adsorption of ionizable pharmaceuticals onto soil.

Although pharmaceutical compounds (PCs) are being used more and more widely, and studies have been carried out to assess their presence in the environ...
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