3.3 Kinetic studies

Kinetic studies give interesting data about the reaction order, type and time necessary to reach equilibrium. For this purpose, kinetic curves were plotted by observing the adsorption capacity at multiple times for FLX alone and for the simultaneous adsorption of contaminants. First, adsorption capacity for fluoxetine was measured at 0, 5, 10, 15, 20, 30, 40, 50, 60, 75, 90, 120 and 150 min to obtain a kinetic curve. Using Matlab®, the kinetic models' parameters were calculated using non-linear regression analysis. These results are shown in Table 4. From Figure 3, it is possible to observe the different kinetic curves corresponding to pseudo-first and pseudo-second order and Elovich kinetic models as well as the experimental values. From Figure 3 and Table 4, it is clear that the pseudo-first order best fitted the experimental data. The pseudo-first order indicates that the adsorption occurs in one step. Good correlation with this model also shows that the reaction is regulated by the time necessary for the reaction and not by the diffusion in the nanofibrous

material. However, the literature shows that no assumptions can be made from the kinetic model to determine the adsorption mechanism (physisorption or chemisorption for instance) [28]. Still, considering the chemical structure of lignin, physisorption is more logical. For physisorption, the main possible interaction forces are van der Waals, π-stacking, hydrogen bonds, hydrophobicity, steric and

Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical…

More information can also be obtained from Figure 3. For instance, following the observation of the graph, it is possible to conclude that equilibrium is obtained within 1 h and that most of the adsorption occurs in the first 20 min. Such a fast adsorption could allow multiple applications to the adsorbent in addition to the retention of contaminants in wastewater. Also, due to the sampling, the total amount of contaminant available is lower which causes a lower adsorption capacity during kinetics. During the test, it was also possible to follow the adsorption process by monitoring of the pH becoming more acid as the alkaline FLX was removed from the solution.

A kinetic experiment was also conducted with simultaneous contaminants. The kinetic curve for each contaminant is presented in Figure 4. For FLX, the curve was almost identical to the individual curve which suggests that no significant competition occurred for FLX. For VEN, the adsorption was significantly longer with equilibrium at 90 min. Its final adsorption capacity, however, remained similar by roughly adsorbing half of the initial concentration. For IBU and CAR, the adsorption was fast (equilibrium within 5 min) and their adsorption capacity was low. Their low adsorption capacities show that AL:PVA membranes might be ineffective

for such contaminants except for really low quantities. Coupling AL:PVA

nanofibres with other adsorbents could be an alternative to adsorb a wider pharmaceutical contaminants spectrum. For instance, recent studies on chitosan and poly (ethylene oxide) showed good adsorption capacities for IBU in water [32].

The goal of the experiment is to understand the behaviour of adsorption sites while specific changes are made. By monitoring the variation of concentration and

polarity interactions [10].

Kinetic curve for the adsorption of FLX on AL:PVA nanofibres.

DOI: http://dx.doi.org/10.5772/intechopen.88621

Figure 3.

3.4 Isotherm studies

35


Table 4.

Kinetic parameters for pseudo-first order, pseudo-second order and Elovich models.

Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical… DOI: http://dx.doi.org/10.5772/intechopen.88621

Figure 3. Kinetic curve for the adsorption of FLX on AL:PVA nanofibres.

material. However, the literature shows that no assumptions can be made from the kinetic model to determine the adsorption mechanism (physisorption or chemisorption for instance) [28]. Still, considering the chemical structure of lignin, physisorption is more logical. For physisorption, the main possible interaction forces are van der Waals, π-stacking, hydrogen bonds, hydrophobicity, steric and polarity interactions [10].

More information can also be obtained from Figure 3. For instance, following the observation of the graph, it is possible to conclude that equilibrium is obtained within 1 h and that most of the adsorption occurs in the first 20 min. Such a fast adsorption could allow multiple applications to the adsorbent in addition to the retention of contaminants in wastewater. Also, due to the sampling, the total amount of contaminant available is lower which causes a lower adsorption capacity during kinetics. During the test, it was also possible to follow the adsorption process by monitoring of the pH becoming more acid as the alkaline FLX was removed from the solution.

A kinetic experiment was also conducted with simultaneous contaminants. The kinetic curve for each contaminant is presented in Figure 4. For FLX, the curve was almost identical to the individual curve which suggests that no significant competition occurred for FLX. For VEN, the adsorption was significantly longer with equilibrium at 90 min. Its final adsorption capacity, however, remained similar by roughly adsorbing half of the initial concentration. For IBU and CAR, the adsorption was fast (equilibrium within 5 min) and their adsorption capacity was low. Their low adsorption capacities show that AL:PVA membranes might be ineffective for such contaminants except for really low quantities. Coupling AL:PVA nanofibres with other adsorbents could be an alternative to adsorb a wider pharmaceutical contaminants spectrum. For instance, recent studies on chitosan and poly (ethylene oxide) showed good adsorption capacities for IBU in water [32].

#### 3.4 Isotherm studies

The goal of the experiment is to understand the behaviour of adsorption sites while specific changes are made. By monitoring the variation of concentration and

completely prevent any ionic bonding between lignin's phenols and cationic groups from pharmaceuticals. AL is also a weak acid which would hardly make any ionic

(mg/g)

Fluoxetine (FLX) 22.85 0.28 78.24 1.35 Venlafaxine (VEN) 11.05 1.02 49.76 2.80 Carbamazepine (CAR) 1.02 0.02 8.04 0.01 Ibuprofen (IBU) 0.62 0.39 5.00 0.46

Affinity comparison of the AL:PVA nanofibres for various pharmaceutical contaminants.

Individual adsorption capacity\*\* (mg/g)

Kinetic studies give interesting data about the reaction order, type and time necessary to reach equilibrium. For this purpose, kinetic curves were plotted by observing the adsorption capacity at multiple times for FLX alone and for the simultaneous adsorption of contaminants. First, adsorption capacity for fluoxetine was measured at 0, 5, 10, 15, 20, 30, 40, 50, 60, 75, 90, 120 and 150 min to obtain a kinetic curve. Using Matlab®, the kinetic models' parameters were calculated using non-linear regression analysis. These results are shown in Table 4. From Figure 3, it is possible to observe the different kinetic curves corresponding to pseudo-first and pseudo-second order and Elovich kinetic models as well as the experimental values. From Figure 3 and Table 4, it is clear that the pseudo-first order best fitted the experimental data. The pseudo-first order indicates that the adsorption occurs in one step. Good correlation with this model also shows that the reaction is regulated by the time necessary for the reaction and not by the diffusion in the nanofibrous

Kinetic model Parameter Value Pseudo-first order R<sup>2</sup> 0.9989

Pseudo-second order R<sup>2</sup> 0.9790

Elovich R<sup>2</sup> 0.9229

Kinetic parameters for pseudo-first order, pseudo-second order and Elovich models.

K<sup>1</sup> (min<sup>1</sup>

RMSE 0.7117

Qe (mg/g) 63.98

RMSE 3.046 K<sup>2</sup> (g/mg/min) 0.0017 Qe (mg/g) 71.14

RMSE 5.832 α (mg/g min) 33.37 β (g/mg) 0.08453

) 0.086

bonds with an acidic compound such as IBU.

Contaminant Simultaneous adsorption capacity\*

3.3 Kinetic studies

Initial concentration of 12.5 ppm. \*\*Initial concentration of 50 ppm.

Sorption in 2020s

\*

Table 3.

Table 4.

34

Figure 4. Kinetic curve for simultaneous adsorption of FLX, VEN, CAR and IBU.

adsorption capacity at equilibrium while varying the mass of adsorbents, it is possible to obtain an isotherm curve which can be compared to isotherm models. The Freundlich, Langmuir, Sips (or Langmuir-Freundlich) and Redlich-Peterson models were compared to the data. The isotherm constants and statistical analysis of the fitting at various temperatures are presented in Table 5.

> range normally associated with hydrogen bonds (4–50 kJ/mol) and π-stacking (8–12 kJ/mol) [34, 35]. For entropy, a value of 42.01 J/mol. k was obtained. A positive value indicates that there is a gain in entropy and that the reaction is favourable. For ΔG° (at 25°C), a value of 20.51 kJ/mol was calculated which shows that the reaction is spontaneous. Moreover, values close to 20 kJ indicate that

Isotherm parameters for various isotherm models at 25, 40 and 60°C.

Isotherm Parameter 25°C 40°C 60°C Freundlich R<sup>2</sup> 0.9655 0.9510 0.8486

Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical…

DOI: http://dx.doi.org/10.5772/intechopen.88621

Langmuir R<sup>2</sup> 0.9783 0.9374 0.8242

Sips R<sup>2</sup> 0.9899 0.9510 0.8877

Redlich-Peterson R<sup>2</sup> 0.9855 0.9510 0.8669

RMSE 4.716 6.200 6.113 kF 11.22 2.298 8.093 N 0.6441 1.131 0.6326

RMSE 3.746 7.006 5.893 Q max 249.2 3.242e+4 175.3 kL 0.02292 1.083e4 0.02223

RMSE 2.859 6.933 6.079 Q max 140.3 9.285e+4 82.32 kS 0.05902 8.513e5 0.06166 N 1.83 1.132 3.277

RMSE 3.423 6.933 6.620 kR 4.499 117.5 2.983 aR 3.122e4 50.23 1.523e4 bR 2.021 0.135 2.183

Hence, the adsorption seems to be an appropriate method for water remediation against pharmaceutical contaminants since it consumes low to no energy. Its exothermic nature is also advantageous in cold climate countries like Canada since heating costs would be higher. In addition, it is possible to use the information obtained through thermodynamic to develop a desorption method for nanofibres. For this reason, the use

of heated solutions for desorption will be investigated in the next section.

One of the benefits of sorption is the possibility of desorption. In this way, multiple adsorption and desorption cycles are possible, and the material is reusable. For these reasons, various desorption solutions were tested on AL:PVA nanofibres for the recovery of FLX. This step's purpose also was obtaining a simpler matrix in which the contaminants can easily be recovered (dried form) or disposed safely. The solutions used had to be either non-toxic or easily evaporated and reused. The effect of temperature was also investigated for a simple desorption method. Results obtained for each tested desorption method are presented in Table 6. As shown in this table, the use of an organic solvent such as methanol has the disadvantage of causing

physisorption is prevalent [31].

Table 5.

37

3.6 Desorption and reusability study

Results show that, for all temperatures, the Sips model best fitted the experimental data with the highest R2 and RMSE coefficients. This model indicates that a contaminant will link to multiple adsorption sites simultaneously. Also, this model can also be reduced to both Freundlich and Langmuir isotherms depending on the concentration of contaminants (low concentration and high concentration respectively) [10, 27, 30]. Considering that in real remediation conditions the concentrations are lower, it would be appropriate to predict the adsorption to be closer to a Freundlich isotherm. In the Freundlich model, the adsorption occurs in multi-layers with heterogenous sites adsorbing a single molecule [10]. In the Langmuir model, however, the adsorption is in monolayers on homogenous sites [10]. In these types of models, the pH, the temperature and the concentration remain the dominating factors affecting adsorption. This is further observed when the temperature increased and the adsorption capacity accordingly got lower. However, the isotherm models had much lower correlation as the temperature went up.

#### 3.5 Thermodynamic studies

Thermodynamic parameters give interesting information on energy transfers during adsorption. Using the experimental data obtained from fluoxetine's isotherms and Eqs. (9) and (10), standard enthalpy, standard entropy, standard Gibbs's free energy were calculated. For enthalpy, a value of 7987 J/mol or 7.99 kJ/mol was obtained which indicates that the adsorption reaction is exothermic. This means that no heat is necessary for efficient adsorption and that supplying heat would be unfavourable for adsorption in this case. Also, this value falls into the energy


Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical… DOI: http://dx.doi.org/10.5772/intechopen.88621

#### Table 5.

adsorption capacity at equilibrium while varying the mass of adsorbents, it is possible to obtain an isotherm curve which can be compared to isotherm models. The Freundlich, Langmuir, Sips (or Langmuir-Freundlich) and Redlich-Peterson models were compared to the data. The isotherm constants and statistical analysis of the

Results show that, for all temperatures, the Sips model best fitted the experimental data with the highest R2 and RMSE coefficients. This model indicates that a contaminant will link to multiple adsorption sites simultaneously. Also, this model can also be reduced to both Freundlich and Langmuir isotherms depending on the concentration of contaminants (low concentration and high concentration respectively) [10, 27, 30]. Considering that in real remediation conditions the concentrations are lower, it would be appropriate to predict the adsorption to be closer to a Freundlich isotherm. In the Freundlich model, the adsorption occurs in multi-layers with heterogenous sites adsorbing a single molecule [10]. In the Langmuir model, however, the adsorption is in monolayers on homogenous sites [10]. In these types of models, the pH, the temperature and the concentration remain the dominating factors affecting adsorption. This is further observed when the temperature increased and the adsorption capacity accordingly got lower. However, the iso-

therm models had much lower correlation as the temperature went up.

Thermodynamic parameters give interesting information on energy transfers during adsorption. Using the experimental data obtained from fluoxetine's isotherms and Eqs. (9) and (10), standard enthalpy, standard entropy, standard Gibbs's free energy were calculated. For enthalpy, a value of 7987 J/mol or 7.99 kJ/mol was obtained which indicates that the adsorption reaction is exothermic. This means that no heat is necessary for efficient adsorption and that supplying heat would be unfavourable for adsorption in this case. Also, this value falls into the energy

3.5 Thermodynamic studies

36

Figure 4.

Sorption in 2020s

fitting at various temperatures are presented in Table 5.

Kinetic curve for simultaneous adsorption of FLX, VEN, CAR and IBU.

Isotherm parameters for various isotherm models at 25, 40 and 60°C.

range normally associated with hydrogen bonds (4–50 kJ/mol) and π-stacking (8–12 kJ/mol) [34, 35]. For entropy, a value of 42.01 J/mol. k was obtained. A positive value indicates that there is a gain in entropy and that the reaction is favourable. For ΔG° (at 25°C), a value of 20.51 kJ/mol was calculated which shows that the reaction is spontaneous. Moreover, values close to 20 kJ indicate that physisorption is prevalent [31].

Hence, the adsorption seems to be an appropriate method for water remediation against pharmaceutical contaminants since it consumes low to no energy. Its exothermic nature is also advantageous in cold climate countries like Canada since heating costs would be higher. In addition, it is possible to use the information obtained through thermodynamic to develop a desorption method for nanofibres. For this reason, the use of heated solutions for desorption will be investigated in the next section.

#### 3.6 Desorption and reusability study

One of the benefits of sorption is the possibility of desorption. In this way, multiple adsorption and desorption cycles are possible, and the material is reusable. For these reasons, various desorption solutions were tested on AL:PVA nanofibres for the recovery of FLX. This step's purpose also was obtaining a simpler matrix in which the contaminants can easily be recovered (dried form) or disposed safely. The solutions used had to be either non-toxic or easily evaporated and reused. The effect of temperature was also investigated for a simple desorption method. Results obtained for each tested desorption method are presented in Table 6. As shown in this table, the use of an organic solvent such as methanol has the disadvantage of causing


Table 6.

Impact of various desorption solutions on desorption of fluoxetine and AL:PVA nanofibres.

degradation of the membranes in addition to the desorption. Even, it is possible that the desorption detected is due to the degradation of nanofibres. Therefore, pure methanol and 50% methanol solution were discarded. Using pure water, almost no desorption occurred which means that the strength of the bond is sufficiently strong to prevent a new equilibrium. When pure water is heated, however, FLX can be desorbed to some extent (30%) which follows the assumptions made in thermodynamic study. Then, sodium chloride was tested for the desorption of FLX. First, 1 M NaCl solution was tested at room temperature which indicated that salts were effective to recover the pharmaceutical contaminant. Afterwards, the same desorption was tested at 60°C. With this method, more than 90% of the fluoxetine could be recovered without affecting the membrane's integrity. The concentration of salt was also varied to verify if a higher concentration would give a better desorption. Instead, higher salt concentrations reduced desorption capacity. Considering the difference in size of FLX compared to NaCl, this might be due to the saturation of the solution in which the fluoxetine cannot be dissolved.

membranes produced could be incorporated in dynamic systems in wastewater treatment plants or even at the source in hospitals or medical centre effluents. In this way, most of the potentially harmful pharmaceutical residues would be

Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical…

Meanwhile, our research is also investigating the use of nanofibres for the collection and analysis of illicit drugs such as cocaine or methamphetamine. Results obtained from this study will be the object of a future study. However, encouraging results with more than 90% of retention was obtained for both drugs (unpublished results). Moreover, desorption is also efficient. In this way, illicit drugs in complex matrices could be transferred to a simpler matrix for direct analysis in liquid chro-

As of now, the efficiency of the nanofibres was proven for alkaline pharmaceutical contaminants. However, its efficiency is rather poor for contaminants that are neutral or acidic such as CAR or IBU. Therefore, an interesting avenue would be the coupling or sequential use of AL:PVA nanofibres and another biosorbent such as chitosan. In fact, works from our research group showed that chitosan nanofibres are efficient for adsorption of IBU in aqueous medium [32]. Therefore, it would be interesting to test a nanofibrous structure composed of AL and chitosan or a sandwich-like structure made of both types of fibres on mixtures of contaminants. In addition, surface chemical modifications are considered in the near future.

Novel alkali lignin and poly (vinyl alcohol) (AL:PVA) nanofibrous membranes were tested for adsorption of pharmaceutical contaminants. Its efficiency to adsorb was first studied on a model contaminant, fluoxetine. An adsorption capacity of 78 mg/g was obtained which corresponds to the adsorption of 78% of fluoxetine present in the water. With further adsorption cycles, the membranes can adsorb up to 90% of contaminants. Compared to commercially available adsorbents (ionexchange resins, zeolites and silica), the results are similar to costly ion-exchange resins (75–80 mg/g). Using kinetic and isotherm models, it is possible to conclude that nanofibres follow a pseudo-first order kinetic model and Sips' isotherm model

removed and would not enter aquatic ecosystems.

Adsorption/desorption cycles for FLX using 60°C 1 M NaCl solution.

DOI: http://dx.doi.org/10.5772/intechopen.88621

matography during forensic investigation.

4. Conclusions

39

Figure 5.

To attest the reusability of the membranes, the best desorption method (1M NaCl solution heated to 60°C) was tested for three adsorption/desorption cycles (see Figure 5). In this process, the membranes were dried and weighed before each adsorption or desorption tests to observe possible mass loss.

To simplify comparison, Figure 5 shows the mass of FLX instead of the adsorption capacity. First, it is interesting to observe that the amount desorbed is increasing with desorption cycles. This can be due to a higher number of FLX molecules on the membranes on the second and third cycle (the amount not desorbed on the previous cycles plus the amount adsorbed on the current cycle). Moreover, the adsorption capacity of the membranes was not affected by the desorption as shown by the small rise in mass adsorbed on the third cycle. The small weight gain could have been caused by a slight rise in porosity of the material due to stretching during adsorption and desorption tests. During this test, no significant mass losses were measured through the 3 cycles. Since the membranes are not degrading and do not lose adsorption capacity after the third cycle, it would be logical to assume that the synthetized membranes could still be used for even more cycles.

#### 3.7 Applications and perspectives

The potential of AL:PVA nanofibres was clearly demonstrated through our study. This promising new technology can be exploited in many fields that require adsorption. For instance, the main application dedicated in this study is the adsorption of pharmaceutical contaminants in wastewater. In such application, the

Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical… DOI: http://dx.doi.org/10.5772/intechopen.88621

#### Figure 5.

degradation of the membranes in addition to the desorption. Even, it is possible that the desorption detected is due to the degradation of nanofibres. Therefore, pure methanol and 50% methanol solution were discarded. Using pure water, almost no desorption occurred which means that the strength of the bond is sufficiently strong to prevent a new equilibrium. When pure water is heated, however, FLX can be desorbed to some extent (30%) which follows the assumptions made in thermodynamic study. Then, sodium chloride was tested for the desorption of FLX. First, 1 M NaCl solution was tested at room temperature which indicated that salts were effective to recover the pharmaceutical contaminant. Afterwards, the same desorption was tested at 60°C. With this method, more than 90% of the fluoxetine could be recovered without affecting the membrane's integrity. The concentration of salt was also varied to verify if a higher concentration would give a better desorption. Instead, higher salt concentrations reduced desorption capacity. Considering the difference in size of FLX compared to NaCl, this might be due to the saturation of the solution in

Impact of various desorption solutions on desorption of fluoxetine and AL:PVA nanofibres.

Desorption solution Fluoxetine recovered (%) Qualitative result 100% methanol 89 High mass loss 50% methanol 36 Slight mass loss 100% water 1 None 100% water 60°C 30 None 1M NaCl 25°C 52 None 1M NaCl 60°C 92 None 2M NaCl 60°C 76 None 3M NaCl 60°C 19 None

To attest the reusability of the membranes, the best desorption method (1M NaCl solution heated to 60°C) was tested for three adsorption/desorption cycles (see Figure 5). In this process, the membranes were dried and weighed before each

To simplify comparison, Figure 5 shows the mass of FLX instead of the adsorption capacity. First, it is interesting to observe that the amount desorbed is increasing with desorption cycles. This can be due to a higher number of FLX molecules on the membranes on the second and third cycle (the amount not desorbed on the previous cycles plus the amount adsorbed on the current cycle). Moreover, the adsorption capacity of the membranes was not affected by the desorption as shown by the small rise in mass adsorbed on the third cycle. The small weight gain could have been caused by a slight rise in porosity of the material due to stretching during adsorption and desorption tests. During this test, no significant mass losses were measured through the 3 cycles. Since the membranes are not degrading and do not lose adsorption capacity after the third cycle, it would be logical to assume that the

which the fluoxetine cannot be dissolved.

Table 6.

Sorption in 2020s

3.7 Applications and perspectives

38

adsorption or desorption tests to observe possible mass loss.

synthetized membranes could still be used for even more cycles.

The potential of AL:PVA nanofibres was clearly demonstrated through our study. This promising new technology can be exploited in many fields that require adsorption. For instance, the main application dedicated in this study is the adsorption of pharmaceutical contaminants in wastewater. In such application, the

Adsorption/desorption cycles for FLX using 60°C 1 M NaCl solution.

membranes produced could be incorporated in dynamic systems in wastewater treatment plants or even at the source in hospitals or medical centre effluents. In this way, most of the potentially harmful pharmaceutical residues would be removed and would not enter aquatic ecosystems.

Meanwhile, our research is also investigating the use of nanofibres for the collection and analysis of illicit drugs such as cocaine or methamphetamine. Results obtained from this study will be the object of a future study. However, encouraging results with more than 90% of retention was obtained for both drugs (unpublished results). Moreover, desorption is also efficient. In this way, illicit drugs in complex matrices could be transferred to a simpler matrix for direct analysis in liquid chromatography during forensic investigation.

As of now, the efficiency of the nanofibres was proven for alkaline pharmaceutical contaminants. However, its efficiency is rather poor for contaminants that are neutral or acidic such as CAR or IBU. Therefore, an interesting avenue would be the coupling or sequential use of AL:PVA nanofibres and another biosorbent such as chitosan. In fact, works from our research group showed that chitosan nanofibres are efficient for adsorption of IBU in aqueous medium [32]. Therefore, it would be interesting to test a nanofibrous structure composed of AL and chitosan or a sandwich-like structure made of both types of fibres on mixtures of contaminants. In addition, surface chemical modifications are considered in the near future.
