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Effect of humic acid on pharmaceuticals adsorption using sulfonic acid grafted chitosan
Journal of Molecular Liquids 230 (2017) 1–5
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Short Communication
Effect of humic acid on pharmaceuticals adsorption using sulfonic acid grafted chitosan George Z. Kyzas a, Dimitrios N. Bikiaris b, Dimitra A. Lambropoulou c,⁎ a b c
Hephaestus Advanced Laboratory, Eastern Macedonia and Thrace Institute of Technology, Kavala GR-65404, Greece Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-65404, Greece Laboratory of Environmental Pollution Control, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki GR-65404, Greece
a r t i c l e
i n f o
Article history: Received 21 December 2016 Received in revised form 4 January 2017 Accepted 5 January 2017 Available online 7 January 2017 Keywords: Humic acid Chitosan derivatives Pharmaceuticals Grafting Isotherms
a b s t r a c t A modified polymer (chitosan grafted with sulfonic acid and cross-linked with glutaraldehyde) was synthesized in order to examine the effect of humic acid (HA) on the adsorption equilibrium of a model pharmaceutical compound (pramipexole). The presence of humic acid in water intended for potable or industrial use can have a significant impact on the treatability of that water and the success of chemical disinfection processes. Therefore, industrial effluents are even mis-treatable. The results showed that increasing the concentration of HA (0, 2.5, 5.0, and 20 mg/L), the maximum adsorption capacity decreases. However, in both materials, it seems that there is a crucial concentration of humic acid (5.0 mg/L), which results the higher reduction of adsorption capacity (Qm. found after fitting to Langmuir and Langmuir-Freundlich models). The presence of 5 mg/L of HA in solution causes slightly different Qm than that of 20 mg/L. Also, techniques as BET analysis, SEM images and swelling experiments were applied for the characterization of the material. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The increasing occurrence of pharmaceuticals in the environment has raised a number of critical concerns, including their removal, fate and elimination in wastewater treatment plants (WWTPs). Hence, several advanced technologies have been evaluated as options to treat these contaminants in wastewaters, including adsorption, advanced oxidation, biological treatment and so on. Among these methods, adsorption offers a number of advantages such as low cost, easy operation and no sludge formation, and therefore is widely applied in not only labscale fundamental studies but also large scale industrial applications [1–25]. Commonly used adsorbents which have been successfully applied for removal of different organic pollutants are activated carbon and synthetic polymer resins etc. Despite these, alternative biopolymer based adsorbents (i.e. chitosan and cellulose) have been received recently considerable attention. The adsorption of pharmaceuticals by biopolymers is dependent on their surface morphology, physical and chemical properties of pharmaceuticals, and environmental conditions. Although adsorption assays targeting pharmaceuticals are steadily increased in recent years, not much data is currently available on the adsorption interferences from environmental conditions such as the effect and mechanisms of natural organic matter (NOM) on the removal of ⁎ Corresponding author. E-mail address: [email protected] (D.A. Lambropoulou).
http://dx.doi.org/10.1016/j.molliq.2017.01.015 0167-7322/© 2017 Elsevier B.V. All rights reserved.
pharmaceuticals from waters and wastewaters. NOM is ubiquitous in aquatic environment and plays a predominant role in the sorption of pharmaceuticals onto sorbent particles. Humic acid (HA) is a common model compound of NOM and widely exist in the aquatic environment. It is a principal component of humic substances, which are the major organic constituents of soil (humus), peat, coal, many upland streams, dystrophic lakes, and ocean water. It is produced by biodegradation of dead organic matter. It is not a single acid; rather, it is a complex mixture of many different acids containing carboxyl and phenolic groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. The ubiquitous occurrence of HA in waters and wastewaters greatly affects the pharmaceutical adsorption process and thus have a significant impact on the success of both, treatment and chemical disinfection processes. In this regard, a better understanding of the interactions of pharmaceuticals with HA is of great importance and may provide guidance for a more distinctive selection of polymer materials to be applied in water/wastewater treatment systems. In the present study, chitosan derivative (poly-β-(1 → 4)-2-amino2-deoxy-D-glucose) grafted with sulfonic groups was synthesized and tested as adsorbent for the removal of a particular pharmaceutical compound. Chitosan is a widely used polymer which has repeatedly applied from our team in past as super-adsorbent for dyes, heavy metals, ions, pharmaceuticals, giving impressively high adsorption capacities for the removal of various pollutants from aqueous media [5,6,26–33]. As model pharmaceutical pollutant, pramipexole dihydrochloride ((6S)-
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N6-propyl-4,5,6,7-tetrahydro-1,3-benzothiazole-2,6-diamine, denoted as PRM) was used. It is widely all over the world for its unique pharmaceutical activity and on the basis of recent drug usage trends [34]; therefore, treatment of wastewater by high polluted levels of PRM is required and urgent needed. The novelty of this study is based on the co-existence of humic acids on the adsorbate (PRM) in various concentrations and the first approach in that type of studies. Which is the effect of humic acids on the adsorbent use of chitosan derivatives? How did humic acids influence the crucial parameters of adsorption (isotherm etc.)? The latter are some of crucial questions replied with this study. 2. Materials and methods 2.1. Materials Pramipexole dihydrochloride (C10H21Cl2N3OS, MW = 302.26 g/mol) was purchased from Amino Chemicals Ltd. (Malta) (assay 99.2%). Stock solutions of PRM (1000 mg/L) were prepared by weighing and dissolving the suitable amount of the corresponding substance in water; stock solutions were stored at −20 °C and were stable for at least 1 week, as assessed by spectrophotometric assays. Working solutions of PRM were prepared daily by diluting the corresponding stock solutions in water. High molecular weight chitosan was purchased by Sigma-Aldrich (purification in Soxhlet apparatus by extraction overnight with acetone). Then, the drying of chitosan particles was carried out under vacuum at 20 °C. Its average molecular weight was estimated at 3.55 × 105 g/mol and the degree of deacetylation was 85 wt%, according to the FTIR method described in literature [35,36]. Glutaraldehyde (GLA, 50 wt% in water), formamide (≥99.5%), sodium carbonate (≥99%), and chlorosulfonic acid (≥ 99%) were purchased from Sigma-Aldrich. Dichloroacetic acid (≥98.5%) was obtained from Fluka. 2.2. Methods For the synthesis of sulfonic acid-grafted chitosan adsorbent (CsSLA) [37,38], a mixture of dichloroacetic acid (5 mL) and formamide (50 mL) was added into chitosan (4.0 g) and stirred to be an homogenized solution. This was then mixed with a complex of chlorosulfonic aciddimethylformamide and stirred for 1 h in a water bath at 50 °C. The reaction mixture was then diluted by a small quantity of deionized water, filtered, and precipitated by pouring into ethanol 95% (400 mL). The precipitate was dissolved in deionized water, neutralized by a saturated Na2CO3 solution, and dialyzed against deionized water. After dialysis, the product was dried and stored in a desiccator. Then, a cross-linking procedure was realized with GLA as reagent (0.5 wt%) at 60 °C for 1.5 h and the final product is obtained (Fig. 1). The final grafting degree (GD) was determined on the basis of the percentage weight increase of the final product relative to the initial weight of chitosan GD = (W2 − W1) / W1 (where W1 and W2 denote the weight of chitosan before and after grafting reaction, respectively). So, the grafting degree was found 2.2. The water retention into materials caused their swelling. To calculate it, experiments were carried out using phosphate buffer pH 7.4 as immersion medium (n = 3). Firstly, each material was carefully weighed (Wsw,1) and then immersed in phosphate buffer pH 7.4. The sponge remnants were wiped off excess surface water using filter paper and weighed (Wsw,2) at different time intervals. Swelling percentage (SP) was calculated using Eq. (1): SP ð%Þ ¼
Wsw;1 −Wsw;2 100% Wsw;2
ð1Þ
Adsorption/desorption experiments were conducted in 20-mL amber vials using a batch approach. All experiments were run in duplicate. The residual concentration of PRM was measured
Fig. 1. Chemical structure of CsSLA.
spectrophotometrically by monitoring its UV absorbance at 263 nm (model U-2000, Hitachi). A detailed description of experimental procedure is given below, where C0 (mg/L) is the initial PRM concentration, pH is the pH of the aqueous solutions (fixed with micro-additions of HCl or NaOH), T (°C) is the temperature, m (g) is the mass of the adsorbent used, V (mL) is the volume of adsorbate, N (revolutions or full rotations per minute, abbreviated as rpm) is the agitation rate of the shaking machine and t (h) is the contact time. For all experiments three different values of HA concentrations (2.5, 5.0, 20 mg/L) were selected to investigate their influence to adsorption of PRM onto CsSLA. The selection of the lower tested HA concentrations (0.0, 2.5, and 5.0 mg/L) was due to the fact that HA exists in waters in relatively low concentration (~5 mg/L). Having checked the aforementioned low HA concentration (0.0, 2.5, and 5.0 mg/L), another value should be selected (by far higher than the others) in order to check the adsorption behavior of CsSLA under those HA conditions. So, the extreme value of 20 mg/L HA was then selected. The adsorption experiments had the following conditions: C0,PRM = 0–500 mg/L; CHA = 0.0, 2.5, 5.0, 20.0 mg/L; pH = 10; m = 0.02 g; V = 20 mL; T = 25 °C; N = 160 rpm; t = 24 h. The equilibrium data were fitted to the Langmuir [39] (Eq. (2)) and Langmuir-Freundlich (L-F) (Eq. (3)) isotherm model [40]:
Qe ¼
Qe ¼
Q m KL Ce 1 þ KL Ce Q m KLF Ce 1=n 1 þ KLF Ce 1=n
ð2Þ
ð3Þ
where Qe (adsorbed PRM weight/adsorbent weight) is the equilibrium concentration in the solid phase; Qm is the maximum amount of adsorption (adsorbed PRM weight/adsorbent weight); KL (L/mg) is the Langmuir adsorption equilibrium constant; n (dimensionless) is the constant depicting the adsorption intensity; KLF (L/mg)1/n is the Langmuir-Freundlich constant; n (dimensionless) is the LangmuirFreundlich heterogeneity constant. The adsorption capacity in equilibrium (Qe) was calculated using the mass balance equation:
Qe ¼
ðC0 −Ce ÞV m
ð4Þ
G.Z. Kyzas et al. / Journal of Molecular Liquids 230 (2017) 1–5
3
Fig. 2. SEM images of (a) Cs and (b) CsSLA.
where C0 and Ce (PRM weight/liquid volume) are the initial and equilibrium PRM concentrations in the liquid phase, respectively.
3. Results and discussion 3.1. Characterization The effect of grafting reactions to the physical structure and the appearance of chitosan can be observed from SEM images (Fig. 2). All derivatives of chitosan (Cs, CsSLA) had an irregular shape owing to the grinding. The nearly total smooth surface of Cs was changed to CsSLA. It is readily observed that the drying method caused a change to the porous microstructure of the particles. This is possibly due to hydrophilic interactions between the water molecules and the sulfonic, hydroxyl and amino groups on the macromolecular chains of the prepared materials. According to BET analysis, the surface area of CsSLA was 2.9 ± 0.3 m2/g, while the non-grafted derivative (Cs) had only 0.9 ± 0.2 m 2 /g. The above values belong to the typical ones of non-porous materials, as bibliographically chitosan is characterized [41]. Regarding swelling, CsSLA showed swelling ~300% at pH = 10 (the above percentage for Cs was found even higher ~650% [42]). The above percentages are commonly observed in chitosan adsorbents, given the powdered-nature of the adsorbents and the single cross-linking method with GLA followed in the current study (and not dual cross-linking with GLA and some ionic reagent as sodium tripolyphosphate).
3.2. Adsorption experiments Based on previous study, the optimum pH for the system Cs (or CsSLA) – PRM was alkaline (10) [28]. Therefore, all isotherm tests were carried out at pH = 10. The most crucial parameter of each adsorbent-adsorbate system is the adsorption isotherms. The maximum theoretical adsorption capacity shows how suitable is a material for the pollutant studied. Fig. 3 shows the effect of the presence of humic acid on the adsorption isotherm of Cs (error-bars included). The experimental data were fitted to Langmuir and L-F model; the best correlation was found for L-F model (0.932 b R2L b 0.962; 0.994 b R2LF b 0.999). So, based on L-F theoretical calculation, Qm was 181 mg/g without existence of HA in the adsorbate solution (CHA = 0 mg/L). However, increasing the HA concentration to 2.5 mg/L, a decrease was observed for the maximum adsorption capacity of Cs (Qm = 151 mg/g), which corresponds to 17% reduction. Increasing the HA concentration to 5.0 and 20.0 mg/L, a sharp decrease was observed for the maximum adsorption capacity of Cs (CHA = 5.0 mg/L: Qm = 74 mg/g, ΔQm = 51%; CHA = 20.0 mg/L: Qm = 50 mg/g, ΔQm = 33%). As it was shown, after a gradual decrease in the case of 2.5 mg/L HA, the next reduction was very intense, implying complex antagonistic interactions between HA and PRM for the adsorption onto Cs. Similar observations were taken for the case of CsSLA. (CHA = 0.0 mg/L: Qm = 339 mg/g; CHA = 2.5 mg/L: Qm = 252 mg/g, ΔQm = 25%; CHA = 5.0 mg/L: Qm = 146 mg/g, ΔQm = 43%; CHA = 20.0 mg/L: Qm = 134 mg/g, ΔQm = 8%). All above are listed in Table 1. Similar results (decrease of pollutants removal with other materials) were also presented in literature [43–46].
Fig. 3. Isotherms of (a) Cs (a) and (b) CsSLA under various concentrations of HA.
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Table 1 Theoretical adsorption capacity calculated after L-F fitting for the adsorption of PRM onto Cs and CsSLA. Adsorbent
CHA (mg/L)
Qm (mg/g)
Cs
0.0 2.5 5.0 20.0 0.0 2.5 5.0 20.0
181 151 74 50 339 252 146 134
CsSLA
ΔQm (%) 17 51 33 25 43 8
The above findings may be related to a potential adsorption mechanism. The interaction between PRM and CsSLA is mainly between the + sulfonic groups (SO− 3 ) of CsSLA and the amino groups (NH3 ) of PRM. But, when humic acid exists into the aqueous solution, the presence of their carboxylic groups (COO−) immediately influence (weaken) the aforementioned interaction between amino and sulfonic groups. As a result, increasing the concentration of HA, the concentration of carboxylic groups is up and then the weakening of the fully electrostatic bond of amino-sulfonic groups reduces more. This explanation seems to be normal. However, in both materials, it seems that there is a crucial CHA which can cause the higher Qm reduction. The presence of 5 mg/L of HA in solution causes slightly different Qm than that of 20 mg/L. So, it is clear that the “border” of this crucial CHA is approximately close to 5 mg/L. A special comment should be added about the selection of grafting negatively charged groups (sulfonic) and not alkaline (i.e. extra amino groups). So, the idea to use acid-modified chitosan was based on the structure of the model pharmaceutical compound use. The molecule of pramipexole has clear excess of positive charge due to the existence of amino groups, so the optimum was to graft negatively charged functional groups in chitosan in order to improve the electrostatic forces as clearly indicated from Fig. 4. In general, chitosan was also studied in past for pharmaceuticals removal by Kyzas' group. Cross-linked with glutaraldehyde and grafted with sulfonic or N-(2-carboxybenzyl) groups chitosan were synthesized and investigated as efficient and environmental-friendly adsorbents for removing pramipexole from polluted water matrices. That was the only until now work regarding pramipexole removal from any adsorbent. The adsorption capacities resulted (L-F fitting) were 337, 307, and 181 mg/g at 25 °C for sulfonic-, carboxybenzyl- and neat chitosan, respectively [28]. In another study, a graphite oxide/poly(acrylic acid) grafted chitosan nanocomposite was prepared and used as biosorbent
Fig. 4. Antagonistic adsorption of PRM versus HA for adsorption onto CsSLA.
for the removal of dorzolamide from biomedical synthetic wastewaters [26]. The nanocomposite presents higher adsorption capacity (334 mg/g) than that of initial adsorbents (graphite oxide 175 mg/g and poly(acrylic acid)-grafted chitosan 229 mg/g). 4. Conclusions Chitosan grafted with sulfonic groups and cross-linked with glutaraldehyde was synthesized in order to examine the effect of HA on the adsorption equilibrium of PRM. Our findings demonstrate that when even low HA concentrations are present in water, they could reduce the adsorption capacity (Qm) of organic pollutants like pharmaceuticals. In particular, for the tested biopolymer materials, the higher reduction was for CsSLA: (CHA = 2.5 mg/L: ΔQm = 25%; CHA = 5.0 mg/L: ΔQm = 43%; CHA = 20.0 mg/L: ΔQm = 8%). Also, the presence of 5 mg/L of HA in solution causes slightly different Qm than that of 20 mg/L. References [1] I. Anastopoulos, G.Z. Kyzas, Agricultural peels for dye adsorption: a review of recent literature, J. Mol. 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Short Communication
Effect of humic acid on pharmaceuticals adsorption using sulfonic acid grafted chitosan George Z. Kyzas a, Dimitrios N. Bikiaris b, Dimitra A. Lambropoulou c,⁎ a b c
Hephaestus Advanced Laboratory, Eastern Macedonia and Thrace Institute of Technology, Kavala GR-65404, Greece Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-65404, Greece Laboratory of Environmental Pollution Control, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki GR-65404, Greece
a r t i c l e
i n f o
Article history: Received 21 December 2016 Received in revised form 4 January 2017 Accepted 5 January 2017 Available online 7 January 2017 Keywords: Humic acid Chitosan derivatives Pharmaceuticals Grafting Isotherms
a b s t r a c t A modified polymer (chitosan grafted with sulfonic acid and cross-linked with glutaraldehyde) was synthesized in order to examine the effect of humic acid (HA) on the adsorption equilibrium of a model pharmaceutical compound (pramipexole). The presence of humic acid in water intended for potable or industrial use can have a significant impact on the treatability of that water and the success of chemical disinfection processes. Therefore, industrial effluents are even mis-treatable. The results showed that increasing the concentration of HA (0, 2.5, 5.0, and 20 mg/L), the maximum adsorption capacity decreases. However, in both materials, it seems that there is a crucial concentration of humic acid (5.0 mg/L), which results the higher reduction of adsorption capacity (Qm. found after fitting to Langmuir and Langmuir-Freundlich models). The presence of 5 mg/L of HA in solution causes slightly different Qm than that of 20 mg/L. Also, techniques as BET analysis, SEM images and swelling experiments were applied for the characterization of the material. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The increasing occurrence of pharmaceuticals in the environment has raised a number of critical concerns, including their removal, fate and elimination in wastewater treatment plants (WWTPs). Hence, several advanced technologies have been evaluated as options to treat these contaminants in wastewaters, including adsorption, advanced oxidation, biological treatment and so on. Among these methods, adsorption offers a number of advantages such as low cost, easy operation and no sludge formation, and therefore is widely applied in not only labscale fundamental studies but also large scale industrial applications [1–25]. Commonly used adsorbents which have been successfully applied for removal of different organic pollutants are activated carbon and synthetic polymer resins etc. Despite these, alternative biopolymer based adsorbents (i.e. chitosan and cellulose) have been received recently considerable attention. The adsorption of pharmaceuticals by biopolymers is dependent on their surface morphology, physical and chemical properties of pharmaceuticals, and environmental conditions. Although adsorption assays targeting pharmaceuticals are steadily increased in recent years, not much data is currently available on the adsorption interferences from environmental conditions such as the effect and mechanisms of natural organic matter (NOM) on the removal of ⁎ Corresponding author. E-mail address: [email protected] (D.A. Lambropoulou).
http://dx.doi.org/10.1016/j.molliq.2017.01.015 0167-7322/© 2017 Elsevier B.V. All rights reserved.
pharmaceuticals from waters and wastewaters. NOM is ubiquitous in aquatic environment and plays a predominant role in the sorption of pharmaceuticals onto sorbent particles. Humic acid (HA) is a common model compound of NOM and widely exist in the aquatic environment. It is a principal component of humic substances, which are the major organic constituents of soil (humus), peat, coal, many upland streams, dystrophic lakes, and ocean water. It is produced by biodegradation of dead organic matter. It is not a single acid; rather, it is a complex mixture of many different acids containing carboxyl and phenolic groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. The ubiquitous occurrence of HA in waters and wastewaters greatly affects the pharmaceutical adsorption process and thus have a significant impact on the success of both, treatment and chemical disinfection processes. In this regard, a better understanding of the interactions of pharmaceuticals with HA is of great importance and may provide guidance for a more distinctive selection of polymer materials to be applied in water/wastewater treatment systems. In the present study, chitosan derivative (poly-β-(1 → 4)-2-amino2-deoxy-D-glucose) grafted with sulfonic groups was synthesized and tested as adsorbent for the removal of a particular pharmaceutical compound. Chitosan is a widely used polymer which has repeatedly applied from our team in past as super-adsorbent for dyes, heavy metals, ions, pharmaceuticals, giving impressively high adsorption capacities for the removal of various pollutants from aqueous media [5,6,26–33]. As model pharmaceutical pollutant, pramipexole dihydrochloride ((6S)-
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N6-propyl-4,5,6,7-tetrahydro-1,3-benzothiazole-2,6-diamine, denoted as PRM) was used. It is widely all over the world for its unique pharmaceutical activity and on the basis of recent drug usage trends [34]; therefore, treatment of wastewater by high polluted levels of PRM is required and urgent needed. The novelty of this study is based on the co-existence of humic acids on the adsorbate (PRM) in various concentrations and the first approach in that type of studies. Which is the effect of humic acids on the adsorbent use of chitosan derivatives? How did humic acids influence the crucial parameters of adsorption (isotherm etc.)? The latter are some of crucial questions replied with this study. 2. Materials and methods 2.1. Materials Pramipexole dihydrochloride (C10H21Cl2N3OS, MW = 302.26 g/mol) was purchased from Amino Chemicals Ltd. (Malta) (assay 99.2%). Stock solutions of PRM (1000 mg/L) were prepared by weighing and dissolving the suitable amount of the corresponding substance in water; stock solutions were stored at −20 °C and were stable for at least 1 week, as assessed by spectrophotometric assays. Working solutions of PRM were prepared daily by diluting the corresponding stock solutions in water. High molecular weight chitosan was purchased by Sigma-Aldrich (purification in Soxhlet apparatus by extraction overnight with acetone). Then, the drying of chitosan particles was carried out under vacuum at 20 °C. Its average molecular weight was estimated at 3.55 × 105 g/mol and the degree of deacetylation was 85 wt%, according to the FTIR method described in literature [35,36]. Glutaraldehyde (GLA, 50 wt% in water), formamide (≥99.5%), sodium carbonate (≥99%), and chlorosulfonic acid (≥ 99%) were purchased from Sigma-Aldrich. Dichloroacetic acid (≥98.5%) was obtained from Fluka. 2.2. Methods For the synthesis of sulfonic acid-grafted chitosan adsorbent (CsSLA) [37,38], a mixture of dichloroacetic acid (5 mL) and formamide (50 mL) was added into chitosan (4.0 g) and stirred to be an homogenized solution. This was then mixed with a complex of chlorosulfonic aciddimethylformamide and stirred for 1 h in a water bath at 50 °C. The reaction mixture was then diluted by a small quantity of deionized water, filtered, and precipitated by pouring into ethanol 95% (400 mL). The precipitate was dissolved in deionized water, neutralized by a saturated Na2CO3 solution, and dialyzed against deionized water. After dialysis, the product was dried and stored in a desiccator. Then, a cross-linking procedure was realized with GLA as reagent (0.5 wt%) at 60 °C for 1.5 h and the final product is obtained (Fig. 1). The final grafting degree (GD) was determined on the basis of the percentage weight increase of the final product relative to the initial weight of chitosan GD = (W2 − W1) / W1 (where W1 and W2 denote the weight of chitosan before and after grafting reaction, respectively). So, the grafting degree was found 2.2. The water retention into materials caused their swelling. To calculate it, experiments were carried out using phosphate buffer pH 7.4 as immersion medium (n = 3). Firstly, each material was carefully weighed (Wsw,1) and then immersed in phosphate buffer pH 7.4. The sponge remnants were wiped off excess surface water using filter paper and weighed (Wsw,2) at different time intervals. Swelling percentage (SP) was calculated using Eq. (1): SP ð%Þ ¼
Wsw;1 −Wsw;2 100% Wsw;2
ð1Þ
Adsorption/desorption experiments were conducted in 20-mL amber vials using a batch approach. All experiments were run in duplicate. The residual concentration of PRM was measured
Fig. 1. Chemical structure of CsSLA.
spectrophotometrically by monitoring its UV absorbance at 263 nm (model U-2000, Hitachi). A detailed description of experimental procedure is given below, where C0 (mg/L) is the initial PRM concentration, pH is the pH of the aqueous solutions (fixed with micro-additions of HCl or NaOH), T (°C) is the temperature, m (g) is the mass of the adsorbent used, V (mL) is the volume of adsorbate, N (revolutions or full rotations per minute, abbreviated as rpm) is the agitation rate of the shaking machine and t (h) is the contact time. For all experiments three different values of HA concentrations (2.5, 5.0, 20 mg/L) were selected to investigate their influence to adsorption of PRM onto CsSLA. The selection of the lower tested HA concentrations (0.0, 2.5, and 5.0 mg/L) was due to the fact that HA exists in waters in relatively low concentration (~5 mg/L). Having checked the aforementioned low HA concentration (0.0, 2.5, and 5.0 mg/L), another value should be selected (by far higher than the others) in order to check the adsorption behavior of CsSLA under those HA conditions. So, the extreme value of 20 mg/L HA was then selected. The adsorption experiments had the following conditions: C0,PRM = 0–500 mg/L; CHA = 0.0, 2.5, 5.0, 20.0 mg/L; pH = 10; m = 0.02 g; V = 20 mL; T = 25 °C; N = 160 rpm; t = 24 h. The equilibrium data were fitted to the Langmuir [39] (Eq. (2)) and Langmuir-Freundlich (L-F) (Eq. (3)) isotherm model [40]:
Qe ¼
Qe ¼
Q m KL Ce 1 þ KL Ce Q m KLF Ce 1=n 1 þ KLF Ce 1=n
ð2Þ
ð3Þ
where Qe (adsorbed PRM weight/adsorbent weight) is the equilibrium concentration in the solid phase; Qm is the maximum amount of adsorption (adsorbed PRM weight/adsorbent weight); KL (L/mg) is the Langmuir adsorption equilibrium constant; n (dimensionless) is the constant depicting the adsorption intensity; KLF (L/mg)1/n is the Langmuir-Freundlich constant; n (dimensionless) is the LangmuirFreundlich heterogeneity constant. The adsorption capacity in equilibrium (Qe) was calculated using the mass balance equation:
Qe ¼
ðC0 −Ce ÞV m
ð4Þ
G.Z. Kyzas et al. / Journal of Molecular Liquids 230 (2017) 1–5
3
Fig. 2. SEM images of (a) Cs and (b) CsSLA.
where C0 and Ce (PRM weight/liquid volume) are the initial and equilibrium PRM concentrations in the liquid phase, respectively.
3. Results and discussion 3.1. Characterization The effect of grafting reactions to the physical structure and the appearance of chitosan can be observed from SEM images (Fig. 2). All derivatives of chitosan (Cs, CsSLA) had an irregular shape owing to the grinding. The nearly total smooth surface of Cs was changed to CsSLA. It is readily observed that the drying method caused a change to the porous microstructure of the particles. This is possibly due to hydrophilic interactions between the water molecules and the sulfonic, hydroxyl and amino groups on the macromolecular chains of the prepared materials. According to BET analysis, the surface area of CsSLA was 2.9 ± 0.3 m2/g, while the non-grafted derivative (Cs) had only 0.9 ± 0.2 m 2 /g. The above values belong to the typical ones of non-porous materials, as bibliographically chitosan is characterized [41]. Regarding swelling, CsSLA showed swelling ~300% at pH = 10 (the above percentage for Cs was found even higher ~650% [42]). The above percentages are commonly observed in chitosan adsorbents, given the powdered-nature of the adsorbents and the single cross-linking method with GLA followed in the current study (and not dual cross-linking with GLA and some ionic reagent as sodium tripolyphosphate).
3.2. Adsorption experiments Based on previous study, the optimum pH for the system Cs (or CsSLA) – PRM was alkaline (10) [28]. Therefore, all isotherm tests were carried out at pH = 10. The most crucial parameter of each adsorbent-adsorbate system is the adsorption isotherms. The maximum theoretical adsorption capacity shows how suitable is a material for the pollutant studied. Fig. 3 shows the effect of the presence of humic acid on the adsorption isotherm of Cs (error-bars included). The experimental data were fitted to Langmuir and L-F model; the best correlation was found for L-F model (0.932 b R2L b 0.962; 0.994 b R2LF b 0.999). So, based on L-F theoretical calculation, Qm was 181 mg/g without existence of HA in the adsorbate solution (CHA = 0 mg/L). However, increasing the HA concentration to 2.5 mg/L, a decrease was observed for the maximum adsorption capacity of Cs (Qm = 151 mg/g), which corresponds to 17% reduction. Increasing the HA concentration to 5.0 and 20.0 mg/L, a sharp decrease was observed for the maximum adsorption capacity of Cs (CHA = 5.0 mg/L: Qm = 74 mg/g, ΔQm = 51%; CHA = 20.0 mg/L: Qm = 50 mg/g, ΔQm = 33%). As it was shown, after a gradual decrease in the case of 2.5 mg/L HA, the next reduction was very intense, implying complex antagonistic interactions between HA and PRM for the adsorption onto Cs. Similar observations were taken for the case of CsSLA. (CHA = 0.0 mg/L: Qm = 339 mg/g; CHA = 2.5 mg/L: Qm = 252 mg/g, ΔQm = 25%; CHA = 5.0 mg/L: Qm = 146 mg/g, ΔQm = 43%; CHA = 20.0 mg/L: Qm = 134 mg/g, ΔQm = 8%). All above are listed in Table 1. Similar results (decrease of pollutants removal with other materials) were also presented in literature [43–46].
Fig. 3. Isotherms of (a) Cs (a) and (b) CsSLA under various concentrations of HA.
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Table 1 Theoretical adsorption capacity calculated after L-F fitting for the adsorption of PRM onto Cs and CsSLA. Adsorbent
CHA (mg/L)
Qm (mg/g)
Cs
0.0 2.5 5.0 20.0 0.0 2.5 5.0 20.0
181 151 74 50 339 252 146 134
CsSLA
ΔQm (%) 17 51 33 25 43 8
The above findings may be related to a potential adsorption mechanism. The interaction between PRM and CsSLA is mainly between the + sulfonic groups (SO− 3 ) of CsSLA and the amino groups (NH3 ) of PRM. But, when humic acid exists into the aqueous solution, the presence of their carboxylic groups (COO−) immediately influence (weaken) the aforementioned interaction between amino and sulfonic groups. As a result, increasing the concentration of HA, the concentration of carboxylic groups is up and then the weakening of the fully electrostatic bond of amino-sulfonic groups reduces more. This explanation seems to be normal. However, in both materials, it seems that there is a crucial CHA which can cause the higher Qm reduction. The presence of 5 mg/L of HA in solution causes slightly different Qm than that of 20 mg/L. So, it is clear that the “border” of this crucial CHA is approximately close to 5 mg/L. A special comment should be added about the selection of grafting negatively charged groups (sulfonic) and not alkaline (i.e. extra amino groups). So, the idea to use acid-modified chitosan was based on the structure of the model pharmaceutical compound use. The molecule of pramipexole has clear excess of positive charge due to the existence of amino groups, so the optimum was to graft negatively charged functional groups in chitosan in order to improve the electrostatic forces as clearly indicated from Fig. 4. In general, chitosan was also studied in past for pharmaceuticals removal by Kyzas' group. Cross-linked with glutaraldehyde and grafted with sulfonic or N-(2-carboxybenzyl) groups chitosan were synthesized and investigated as efficient and environmental-friendly adsorbents for removing pramipexole from polluted water matrices. That was the only until now work regarding pramipexole removal from any adsorbent. The adsorption capacities resulted (L-F fitting) were 337, 307, and 181 mg/g at 25 °C for sulfonic-, carboxybenzyl- and neat chitosan, respectively [28]. In another study, a graphite oxide/poly(acrylic acid) grafted chitosan nanocomposite was prepared and used as biosorbent
Fig. 4. Antagonistic adsorption of PRM versus HA for adsorption onto CsSLA.
for the removal of dorzolamide from biomedical synthetic wastewaters [26]. The nanocomposite presents higher adsorption capacity (334 mg/g) than that of initial adsorbents (graphite oxide 175 mg/g and poly(acrylic acid)-grafted chitosan 229 mg/g). 4. Conclusions Chitosan grafted with sulfonic groups and cross-linked with glutaraldehyde was synthesized in order to examine the effect of HA on the adsorption equilibrium of PRM. Our findings demonstrate that when even low HA concentrations are present in water, they could reduce the adsorption capacity (Qm) of organic pollutants like pharmaceuticals. In particular, for the tested biopolymer materials, the higher reduction was for CsSLA: (CHA = 2.5 mg/L: ΔQm = 25%; CHA = 5.0 mg/L: ΔQm = 43%; CHA = 20.0 mg/L: ΔQm = 8%). Also, the presence of 5 mg/L of HA in solution causes slightly different Qm than that of 20 mg/L. References [1] I. Anastopoulos, G.Z. Kyzas, Agricultural peels for dye adsorption: a review of recent literature, J. Mol. 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