2.3 Electrospinning

The previous prepared solution was injected in a 5 mL syringe with a 20-gauge needle for electrospinning. The syringe was set to the syringe pump and voltage was applied between the needle and the collector. The collector was a non-stick cookie sheet giving good electrospinning, reusability and easy recovery of the nanofibres. The electrospinning parameters were based on results obtained previously [26]. The conditions were a flow rate of 0.1 mL/h with an applied DC voltage of 15 kV. The collector was placed 20 cm away from the tip of the needle. The temperature was kept at 22°C and relative humidity maintained between 10 and 40%. A razor blade was used to recover the nanofibrous mat from the collector. All experiments were conducted in a customized electrospinning box. The electrospun nanofibre mat was then kept overnight in a laboratory oven at 80°C for drying and stabilization purposes.

#### 2.4 Nanofibre stabilization

Due to electrospun nanofibres' high solubility in water, AL:PVA nanofibres mats were stabilized using two consecutive techniques. Both techniques are based on previous works [26]. The first method used the glass transition temperature of polymers to raise their crystallinity and hence their water resistance. Therefore, nanofibres were heated in a laboratory oven at 160°C for 3 h. Next, the membranes were immersed in a 0.5 M sodium citrate buffer pH 4.5 for a period of 3 h. This process aims to protonate AL's phenol groups which were previously deprotonated during the preparation of the electrospinning solution in a method similar to the extraction methods of black liquor [15]. During this step, the morphology of the membrane changes drastically due to the dissolution of a part of the PVA. The dissolution causes a rise in the concentration of AL (the membranes become browner) and cross-linking of the nanofibres. After exposure to the buffer, the membranes were washed several times with purified water, stretched and dried on a metallic surface. Finally, the nanofibrous mats were recovered using a razor blade.

#### 2.5 Adsorption tests

In this section, three types of tests were performed: adsorption of a single contaminant on AL:PVA membranes as well as commercial adsorbents, and adsorption of multiple contaminants on AL:PVA membranes. All adsorption tests were conducted in batches by adding a defined amount of adsorbent to a stirred solution containing a specific concentration of contaminants. All tested solutions were composed of purified water with 5% of methanol and contaminants adjusted at the targeted concentration. The organic solvent's purpose was to ensure that pharmaceuticals were solubilized in water. Separate 2500 ppm standard solutions of FLX, VEN, CAR and IBU were prepared by dissolving the corresponding stock solutions in methanol. Those solutions were then diluted for adsorption tests. Before, during and after the adsorption test, aliquots of 500 μL of the contaminated water were sampled, diluted with 500 μL of mobile phase, vortexed and injected in HPLC-DAD to determine the concentration of contaminants in solution. For tests using one contaminant on AL:PVA membranes, 50 ppm FLX solution was used as a model contaminated water. For tests with commercial adsorbents, adjustments were made to compensate for the size difference between adsorbents. Therefore, solutions of 250 ppm FLX in 10 mL were prepared to keep the same contaminant to Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical… DOI: http://dx.doi.org/10.5772/intechopen.88621

adsorbent mass ratio. For tests with multiple contaminants, 12.5 ppm of FLX, IBU, CAR and VEN were added to water to simulate contaminated water.

The adsorption tests were initialized by the addition of 25 mg of adsorbent (nanofibres or commercial adsorbent) to the solution. The tests were conducted over a period of 150 min to ensure that equilibrium was reached. Using a calibration curve and the area under the peaks on the chromatograms, the remaining concentration of the solution was calculated. From this value, the adsorption capacity at time t (Qt) was calculated using the following equation:

$$Q\_t = \frac{(C\_0 - C\_t)}{m} \times V \tag{1}$$

where Ct is the concentration of the contaminant (ppm) at time t (min), C<sup>0</sup> is the initial concentration of contaminants (ppm), V is the volume of the solution (L), and m is the mass of adsorbents (g).

For samples containing one contaminant, samples injected in HPLC-DAD were eluted using a mobile phase composed of acetonitrile and a 0.1% solution of phosphoric acid (60:40% v/v ratio). The flow rate was adjusted at 0.45 mL/min for 3.75 min with a detection at 230 nm. For samples containing more than one contaminant, the mobile phase was composed of acetonitrile and 0.1% phosphoric acid with a ratio of 40:60% v/v. The flow rate was adjusted to 0.5 mL/min for a period of 20 min with a detection still at 230 nm. In all cases, 10 μL of the samples were injected using an autosampler. For all contaminants, a 10-point (0.5–100 ppm) calibration curve was established to determine the concentration. All tests were performed in triplicate.

#### 2.6 Kinetic studies

1 month. Before use, the refrigerated solution was brought to room temperature in a

The previous prepared solution was injected in a 5 mL syringe with a 20-gauge needle for electrospinning. The syringe was set to the syringe pump and voltage was applied between the needle and the collector. The collector was a non-stick cookie sheet giving good electrospinning, reusability and easy recovery of the nanofibres. The electrospinning parameters were based on results obtained previously [26]. The conditions were a flow rate of 0.1 mL/h with an applied DC voltage of 15 kV. The collector was placed 20 cm away from the tip of the needle. The temperature was kept at 22°C and relative humidity maintained between 10 and 40%. A razor blade was used to recover the nanofibrous mat from the collector. All experiments were conducted in a customized electrospinning box. The electrospun nanofibre mat was then kept overnight in a laboratory oven at 80°C for drying and stabilization purposes.

Due to electrospun nanofibres' high solubility in water, AL:PVA nanofibres mats were stabilized using two consecutive techniques. Both techniques are based on previous works [26]. The first method used the glass transition temperature of polymers to raise their crystallinity and hence their water resistance. Therefore, nanofibres were heated in a laboratory oven at 160°C for 3 h. Next, the membranes were immersed in a 0.5 M sodium citrate buffer pH 4.5 for a period of 3 h. This process aims to protonate AL's phenol groups which were previously deprotonated during the preparation of the electrospinning solution in a method similar to the extraction methods of black liquor [15]. During this step, the morphology of the membrane changes drastically due to the dissolution of a part of the PVA. The dissolution causes a rise in the concentration of AL (the membranes become browner) and cross-linking of the nanofibres. After exposure to the buffer, the membranes were washed several times with purified water, stretched and dried on a metallic surface. Finally, the nanofibrous mats were recovered using a razor blade.

In this section, three types of tests were performed: adsorption of a single contaminant on AL:PVA membranes as well as commercial adsorbents, and adsorption of multiple contaminants on AL:PVA membranes. All adsorption tests were conducted in batches by adding a defined amount of adsorbent to a stirred solution containing a specific concentration of contaminants. All tested solutions were composed of purified water with 5% of methanol and contaminants adjusted at the targeted concentration. The organic solvent's purpose was to ensure that pharmaceuticals were solubilized in water. Separate 2500 ppm standard solutions of FLX, VEN, CAR and IBU were prepared by dissolving the corresponding stock solutions in methanol. Those solutions were then diluted for adsorption tests. Before, during and after the adsorption test, aliquots of 500 μL of the contaminated water were sampled, diluted with 500 μL of mobile phase, vortexed and injected in HPLC-DAD to determine the concentration of contaminants in solution. For tests using one contaminant on AL:PVA membranes, 50 ppm FLX solution was used as a model contaminated water. For tests with commercial adsorbents, adjustments were made to compensate for the size difference between adsorbents. Therefore, solutions of 250 ppm FLX in 10 mL were prepared to keep the same contaminant to

hot water bath for an hour.

2.4 Nanofibre stabilization

2.5 Adsorption tests

28

2.3 Electrospinning

Sorption in 2020s

Kinetic curves were obtained by sampling at intervals during the adsorption process. Samples were collected at 0, 5, 10, 15, 20, 30, 40, 50, 60, 75, 90, 120 and 150 min after the addition of the adsorbent. The equilibrium time of 150 min was determined by an initial kinetic test. The same sampling and injection processes as traditional adsorption tests were conducted for kinetic studies. By calculating the adsorption capacity through time, it is possible to obtain a kinetic curve which can be compared to adsorption kinetic models. Adsorption kinetic models give crucial information on the adsorption parameters and the limiting processes occurring during the adsorption. Typically, three steps occur during the adsorption: transfer of the adsorbate to the external surface of the adsorbent, internal diffusion of the adsorbate to active sites and sorption reaction with the adsorbent [27, 28]. In this study, three models were compared: pseudo-first order, pseudo-second order and Elovich. In all cases, the kinetic constants were calculated using Matlab's curve fitting app. To determine the best fitting model, determination coefficients and root of mean square errors (RMSE) were compared. The pseudo-first order model is represented by Eq. (2):

$$Q\_t = Q\_\varepsilon \left(1 - e^{-k\_1 t}\right) \tag{2}$$

In Eq. (2), Q <sup>e</sup> corresponds to the adsorption capacity at equilibrium (mg/g), Qt the adsorption capacity (mg/g) at time t (min) and k<sup>1</sup> the pseudo-first order kinetic adsorption constant (min�<sup>1</sup> ) [10, 29]. The pseudo-second order model is represented by Eq. (3):

$$Q\_t = \frac{k\_2 Q\_\epsilon \,^2 t}{1 + k\_2 Q\_\epsilon t} \tag{3}$$

where k<sup>2</sup> is associated to the pseudo-second order kinetic adsorption constant (g mg�<sup>1</sup> min�<sup>1</sup> ) [10, 29]. The Elovich model is represented by Eq. (4):

$$Q\_t = \frac{\ln\left(a\beta\right) + \ln t}{\beta} \tag{4}$$

where ΔS° is the standard entropy (J mol�<sup>1</sup> K�<sup>1</sup>

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

salts, and then dried in a vacuum desiccator.

3.1 Electrospinning and nanofibre stabilization

3. Results and discussion

31

where ΔG° is the standard Gibbs's free energy (J mol�<sup>1</sup>

give information on the amount of thermal energy produced, the energy of the bonds, the spontaneity of the reaction and the favourability of an adsorption

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

The capacity of an adsorbent to be desorbed is also an important characteristic

Mixed solutions of AL and PVA were prepared for the production of membranes for adsorption tests. Using the specified electrospinning parameters, it was possible to

obtain steady nanofibre formation for periods of a few hours to produce thin nanofibrous mats. Those were thermally stabilized giving the nanofibres a brownish colour and more rigidity. Their immersion in a sodium citrate buffer finalized the stabilization process to provide fibres stability at various pHs enabling their use for adsorption. As shown in Figure 2, the stabilization process had a slight impact on the visual aspects of the membranes. However, the impact is more obvious when seen by scanning electron microscopy. Figure 2b shows that nanofibres of 183 � 5 nm in diameter were obtained by the electrospinning with a low number of beads or defects. This size does not technically correspond to nanofibres (0–100 nm), but the adsorption properties shall be akin to real nanofibres considering the small difference. Figure 2d shows the nanofibres after a thermal process. This image shows a similar nanofibrous aspect with small variations of the nanofibre diameter (156 � 5 nm). However, nanofibres seem to be closer to each other with slight cross-linking giving it

value since it can have a significant contribution on economics and life cycle assessment of the process. Therefore, the reusability of the AL:PVA membranes was evaluated by desorption. For this purpose, multiple conditions were tested to recover the contaminant safely. Hence, the nanofibres were exposed to solutions of methanol (to create an environment in which the contaminant is highly soluble), purified water (to verify the risk of desorption due to equilibrium), heated solutions (might be able to revert the sorption reaction), salts (ion exchange and/or competition) and combined techniques (except heated methanol solutions). In all cases, the membranes were immersed in 50 mL of solution for 4 h. For temperature effect, solutions were heated to 60°C to verify desorption. The salt used for desorption was sodium chloride since it is a simple and non-toxic substance, largely found in typical wastewater. Concentrations of sodium chloride of 1, 2 and 3 M were tested. Initial and final samples were injected in HPLC-DAD to determine the concentration of FLX recovered. The desorption solution showing the best desorption efficiency was used for repeated adsorption/desorption cycles to evaluate the reusability. Between each test, the membranes were recovered and dried in a vacuum desiccator to prevent humidity from interfering with the measured masses. For membranes exposed to salts, these were washed several times with purified water to dissolve

enthalpy (J mol�<sup>1</sup>

2.9 Desorption tests

reaction.

) and ΔH° is the standard

) [31]. These values will

) [31]. The second thermodynamic law equation corresponds to:

ΔG° ¼ ΔH° � TΔS° (6)

where α is the initial adsorption rate constant (mg g�<sup>1</sup> min�<sup>1</sup> ) and β is the initial desorption rate constant (g mg�<sup>1</sup> ) [10, 29].

#### 2.7 Isotherms

The adsorption isotherms were performed for AL:PVA membranes to obtain information on the adsorption sites and the type of reaction occurring. For these tests, 50 ppm solutions of FLX were prepared as typical adsorption tests. Samples were collected at 0 and 180 min (equilibrium). From those samples, the concentration (Ce) and adsorption capacity (Q <sup>e</sup>) at equilibrium were calculated. Isotherms are obtained by varying the mass of adsorbents (resulting in a varying adsorption capacity and concentration at equilibrium) at fixed temperatures. For our tests, adsorbent masses of 5, 10, 15, 20, 25, 30 and 35 mg were tested and temperatures of 25, 40 and 60°C were compared. The curves obtained by plotting the Ce versus Q <sup>e</sup> are then compared to isotherm models (Freundlich, Langmuir, Sips, Redlich-Peterson) to gain important information. Table 1 shows the different equations for the models studied.

Here, Q <sup>e</sup> is the adsorption capacity at equilibrium (mg/g), Ce is the concentration in solution at equilibrium (ppm), kF is the Freundlich isotherm constant (mg/g [L/mg]1/n), n is the heterogeneity factor (dimensionless), Q max is the maximum adsorption capacity (mg/g), kL is the Langmuir isotherm constant (L/mg), kS is the Sips isotherm constant ([L/mg]1/n), kR is the Redlich-Peterson isotherm constant (L/g), aR being the Redlich-Peterson isotherm constant ([L/g]bR) and bR is the Redlich-Peterson model exponent (dimensionless).

#### 2.8 Thermodynamic study

The thermodynamic parameters (enthalpy, entropy and Gibbs's free energy) are calculated through a thermodynamic study. These parameters are obtained by using the Ce and Qe recovered from isotherms adsorption tests and the Van't Hoff and 2nd thermodynamic law equations. The Van't Hoff equation corresponds to:

$$\ln\frac{Q\_{\epsilon}}{C\_{\epsilon}} \ast 1000 \text{ g} \frac{\text{g}}{L} = \frac{\Delta S^{\circ}}{R} - \frac{\Delta H^{\circ}}{RT} \tag{5}$$


Table 1.

Isotherm models non-linear equations [10, 30].

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

where ΔS° is the standard entropy (J mol�<sup>1</sup> K�<sup>1</sup> ) and ΔH° is the standard enthalpy (J mol�<sup>1</sup> ) [31]. The second thermodynamic law equation corresponds to:

$$
\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ \tag{6}
$$

where ΔG° is the standard Gibbs's free energy (J mol�<sup>1</sup> ) [31]. These values will give information on the amount of thermal energy produced, the energy of the bonds, the spontaneity of the reaction and the favourability of an adsorption reaction.

## 2.9 Desorption tests

where k<sup>2</sup> is associated to the pseudo-second order kinetic adsorption constant

Qt <sup>¼</sup> ln ð Þþ αβ ln <sup>t</sup>

The adsorption isotherms were performed for AL:PVA membranes to obtain information on the adsorption sites and the type of reaction occurring. For these tests, 50 ppm solutions of FLX were prepared as typical adsorption tests. Samples were collected at 0 and 180 min (equilibrium). From those samples, the concentration (Ce) and adsorption capacity (Q <sup>e</sup>) at equilibrium were calculated. Isotherms are obtained by varying the mass of adsorbents (resulting in a varying adsorption capacity and concentration at equilibrium) at fixed temperatures. For our tests, adsorbent masses of 5, 10, 15, 20, 25, 30 and 35 mg were tested and temperatures of 25, 40 and 60°C were compared. The curves obtained by plotting the Ce versus Q <sup>e</sup> are then compared to isotherm models (Freundlich, Langmuir, Sips, Redlich-Peterson) to gain important information. Table 1 shows the different equations for

Here, Q <sup>e</sup> is the adsorption capacity at equilibrium (mg/g), Ce is the concentration in solution at equilibrium (ppm), kF is the Freundlich isotherm constant (mg/g [L/mg]1/n), n is the heterogeneity factor (dimensionless), Q max is the maximum adsorption capacity (mg/g), kL is the Langmuir isotherm constant (L/mg), kS is the Sips isotherm constant ([L/mg]1/n), kR is the Redlich-Peterson isotherm constant (L/g), aR being the Redlich-Peterson isotherm constant ([L/g]bR) and bR is the

The thermodynamic parameters (enthalpy, entropy and Gibbs's free energy) are calculated through a thermodynamic study. These parameters are obtained by using the Ce and Qe recovered from isotherms adsorption tests and the Van't Hoff and 2nd

<sup>L</sup> <sup>¼</sup> <sup>Δ</sup>S°

1þkLCe

1þaRCe bR

1=n 1þkSCe 1=n

Models Non-linear equation Equation

<sup>R</sup> � <sup>Δ</sup>H°

RT (5)

(6)

(7)

(8)

<sup>1</sup>=<sup>n</sup> (5)

thermodynamic law equations. The Van't Hoff equation corresponds to:

<sup>∗</sup> <sup>1000</sup> <sup>g</sup>

ln Qe Ce

Freundlich Qe <sup>¼</sup> kFCe

Langmuir Qe <sup>¼</sup> Qmax kLCe

Sips Qe <sup>¼</sup> Qmax kSCe

Redlich-Peterson Qe <sup>¼</sup> kRCe

Isotherm models non-linear equations [10, 30].

where α is the initial adsorption rate constant (mg g�<sup>1</sup> min�<sup>1</sup>

) [10, 29].

<sup>β</sup> (4)

) and β is the initial

) [10, 29]. The Elovich model is represented by Eq. (4):

(g mg�<sup>1</sup> min�<sup>1</sup>

Sorption in 2020s

2.7 Isotherms

the models studied.

2.8 Thermodynamic study

Table 1.

30

Redlich-Peterson model exponent (dimensionless).

desorption rate constant (g mg�<sup>1</sup>

The capacity of an adsorbent to be desorbed is also an important characteristic value since it can have a significant contribution on economics and life cycle assessment of the process. Therefore, the reusability of the AL:PVA membranes was evaluated by desorption. For this purpose, multiple conditions were tested to recover the contaminant safely. Hence, the nanofibres were exposed to solutions of methanol (to create an environment in which the contaminant is highly soluble), purified water (to verify the risk of desorption due to equilibrium), heated solutions (might be able to revert the sorption reaction), salts (ion exchange and/or competition) and combined techniques (except heated methanol solutions). In all cases, the membranes were immersed in 50 mL of solution for 4 h. For temperature effect, solutions were heated to 60°C to verify desorption. The salt used for desorption was sodium chloride since it is a simple and non-toxic substance, largely found in typical wastewater. Concentrations of sodium chloride of 1, 2 and 3 M were tested. Initial and final samples were injected in HPLC-DAD to determine the concentration of FLX recovered. The desorption solution showing the best desorption efficiency was used for repeated adsorption/desorption cycles to evaluate the reusability. Between each test, the membranes were recovered and dried in a vacuum desiccator to prevent humidity from interfering with the measured masses. For membranes exposed to salts, these were washed several times with purified water to dissolve salts, and then dried in a vacuum desiccator.
