**2. Methodology**

**1. Introduction**

256 Desalination and Water Treatment

membrane surface.

microorganisms.

**Figure 1.** Scheme of composite membrane.

metals, and reduce the salt content of feed water [4].

The capability of several human activities, like industry and mining, has expanded throughout the last decades [1], which has led to a similar pace of improvements in membrane technology for separation. Drinking water treatment processes have developed to the point of requiring more advanced pretreatment processes [2]. Nanofiltration (NF) might function as a pretreatment for reverse osmosis desalination [3], as it can remove hardness, specific heavy

In addition, one of the main problems faced by desalination processes is the inevitable appearance of biofouling on its membranes [5, 6]. This causes problems such as decreases in permeate flux and salt rejection, as well as an increase in transmembrane pressure [7]. In an effort to reduce the most damaging effects of biofouling, there have been many studies for testing numerous anti-fouling agents and solutions added during membrane fabrication. However, not all these attempts include organic-based preparation, suggesting that there may be possible hazards for human health, after consumption [8, 9]. Pepper extract, the source of the capsaicin molecule, has proven to limit bacterial growth [10]; therefore, its addition during the membrane fabrication process may help control biofilm formation on the

In this study, nanofiltration membranes were prepared (**Figure 1**) for brackish water treatment. It is desirable for the desalination membranes to have hydrophilic properties, as this usually correlates with the tendency of a membrane for allowing water to permeate instead of rejecting it (hydrophobic) [11]. Contact angle measurements are usually carried out to determine the degree of hydrophilicity or hydrophobicity of a surface [12]. This study includes an analysis of the fabrication, as well as the characterization of four nanofiltration membranes, before and after their performance testing on a cross-flow module, operated with an aqueous salt solution to determine salt rejection. Membrane characterization is also performed through atomic force microscopy (AFM) and attenuated total reflectance infrared spectroscopy (ATR-IR) [13]. Membranes prepared with capsaicin will later be assessed for their anti-biofouling properties, to evaluate whether they are resistant to biofouling by seawater

#### **2.1. Polymers and monomers**

Polysulfone (PS purity > 99%), polyether sulfonyl (PES; purity > 99%), piperazine (PP; purity > 99%), and 1,3,5-Benzenetricarbonyl chloride (TMC; purity > 98%) were obtained from Sigma-Aldrich. Chiltepin pepper extract [9] (capsaicin extract) was obtained in the laboratory as a source of the capsaicin molecule (**Figure 2**). This extract was used to prepare nanofiltration membranes.

To corroborate the extraction of capsaicin, infrared spectroscopy was performed, showing the following signals:


**Figure 2.** Chemical structure of the monomers for preparation nanofiltration composite membranes: piperazine, capsaicin and 1,3,5-benzenetricarbonyl chloride (TMC).

#### **2.2. Preparation of the microporous polysulfone membrane**

Sheets of microporous polysulfone membrane supports were prepared, following the methodology by Perez-Sicairos et al. [14] and Lin et al. [15]. Four sheets were prepared through a phase inversion process; a polymer solution including 18% w/w of dried polysulfone (12 g) was dissolved in a mixed solvent of n-methyl-pyrrolidone (NMP, 82 g) and an independent addition of sulfonated polysulfone (6 g), inside a clear jar and sealed with a Teflon cap. This glass bottle was subjected to a rotator device, spinning at 52 rpm under a heating light, to achieve a homogeneous polymer solution. A 71 × 28 cm Holytek paper piece (0.12 mm thick) was cut and held onto the membrane casting machine, where the polymeric solution was spread through the casting blade, allowing a solution spreading to a depth of 0.004 mm. Submersion speed was set at full capacity (11 m/min). The tank was filled with distilled water, providing a bath to induce phase inversion and a resting pace for 6 min. Finally, the membranes proceeded to a rinsing stage with distilled water for 2 min, in order to eliminate the excess of floating polymer.

Once the porous support for the membranes was prepared, two of the pieces were designated as composite membranes. These membranes received the concentrations shown in **Table 2**.

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Atomic force microscopy (AFM) was performed with a PS 50–50-15 (50 μm) scanning probe microscope instrument (AFM workshop) to determine the surface morphology of the nanofiltration membrane. The software Gwyddion 2.4 was used to obtain 3D images and roughness measurements (root mean square, RMS). A sample size of 0.5 × 0.5 cm was used to analyze the

Attenuated total reflectance infrared (ATR-IR) characterization of the nanofiltration membrane surface was carried out with a Nicolet iS5 Fourier transform infrared spectrometer and the accessory iD3 ATR (thermo Fisher scientific). For ATR-IR analysis of the nanofiltration membranes samples, a germanium crystal was employed. A sample size of 0.5 × 0.5 cm was

Contact angle measurements were performed with a dataphysics contact angle system (OCA15SEC), camera iDs and injection system ES. A sample size of 3 × 2 cm was used to analyze the nanofiltration membranes. The water used was previously distilled and filtered (0.2 μm). The water drop was placed on different locations on the membrane surface, at a

The operation of the cross-flow cell unit (**Figure 3**) was as follows. Starting from a 20 L feed

SO4

**Capsaicin (g) Trimesoyl** 

**chloride (mL)**

**Polysulfone support Polyamide layer**

A 12.0 6.0 NA 100 0.25 NA B 12.0 6.0 NA 100 0.25 1 C 11.5 5.5 1 100 0.25 NA D 11.5 5.5 1 100 0.25 1

**polysulfone (g)**

at the different concentrations) is conducted

**Piperazine (mL)**

**Capsaicin (g)**

**2.4. Characterization of nanofiltration membranes**

*2.4.2. Attenuated total reflectance infrared spectroscopy*

used to analyze the nanofiltration membranes.

tank, the feed solution (NaCl, MgSO4, and Na2

**Membrane Polysulfone (g) Sulfonated** 

**Table 2.** Polysulfone and polyamide composition.

*2.4.1. Atomic force microscopy*

nanofiltration membranes.

*2.4.3. Contact angle*

temperature of 22°C.

*2.4.4. Cross-flow equipment*

#### **2.3. Preparation of nanofiltration membranes with capsaicin**

Nanofiltration membrane production started with the preparation of porous polysulfone membrane via the phase inversion method, as reported by [14, 15]. The NF membrane was prepared by interfacial polymerization, over the porous polysulfone membrane. Two chemical solutions were prepared, an aqueous solution containing piperazine (solution A) and an organic solution containing 1, 3, 5 benzene-tricarbonyl trichloride (TMC). The composition of each of these solutions was as follows. The composition of the aqueous solution (A) was piperazine (0.25% w/w), polyvinyl alcohol (0.25% w/w), and sodium hydroxide (0.5% w/w). This solution was prepared as follows: for each 250 g of solution A, 1.25 g of sodium hydroxide were weighed and placed inside a 250 mL flask, with 100 mL of distilled water. The preparation was stirred. Moreover, 0.63 g of polyvinyl alcohol was added until full dilution. Then, 0.63 g of piperazine was added to the flask and filled to 250 mL. The composition of the organic solution (solution B) was TMC (1.0% w/w) and hexane. To prepare it, the procedure was as follows: for each 250 mL of solution B, 1 g of capsaicin extract and 1.5 g of TMC were weighed and the solution was filled to 250 mL with hexane. The first of the membranes, labeled control, was prepared from solution A and solution B.

In order to prepare the experimental membranes with distinctive characteristics, capsaicin extract was added during the preparation process, using four different concentrations, as shown in **Table 1**.


**Table 1.** Composition for nanofiltration membrane preparation.

Once the porous support for the membranes was prepared, two of the pieces were designated as composite membranes. These membranes received the concentrations shown in **Table 2**.

#### **2.4. Characterization of nanofiltration membranes**

#### *2.4.1. Atomic force microscopy*

**2.2. Preparation of the microporous polysulfone membrane**

258 Desalination and Water Treatment

**2.3. Preparation of nanofiltration membranes with capsaicin**

labeled control, was prepared from solution A and solution B.

**Table 1.** Composition for nanofiltration membrane preparation.

shown in **Table 1**.

Sheets of microporous polysulfone membrane supports were prepared, following the methodology by Perez-Sicairos et al. [14] and Lin et al. [15]. Four sheets were prepared through a phase inversion process; a polymer solution including 18% w/w of dried polysulfone (12 g) was dissolved in a mixed solvent of n-methyl-pyrrolidone (NMP, 82 g) and an independent addition of sulfonated polysulfone (6 g), inside a clear jar and sealed with a Teflon cap. This glass bottle was subjected to a rotator device, spinning at 52 rpm under a heating light, to achieve a homogeneous polymer solution. A 71 × 28 cm Holytek paper piece (0.12 mm thick) was cut and held onto the membrane casting machine, where the polymeric solution was spread through the casting blade, allowing a solution spreading to a depth of 0.004 mm. Submersion speed was set at full capacity (11 m/min). The tank was filled with distilled water, providing a bath to induce phase inversion and a resting pace for 6 min. Finally, the membranes proceeded to a rinsing stage with distilled water for 2 min, in order to eliminate the excess of floating polymer.

Nanofiltration membrane production started with the preparation of porous polysulfone membrane via the phase inversion method, as reported by [14, 15]. The NF membrane was prepared by interfacial polymerization, over the porous polysulfone membrane. Two chemical solutions were prepared, an aqueous solution containing piperazine (solution A) and an organic solution containing 1, 3, 5 benzene-tricarbonyl trichloride (TMC). The composition of each of these solutions was as follows. The composition of the aqueous solution (A) was piperazine (0.25% w/w), polyvinyl alcohol (0.25% w/w), and sodium hydroxide (0.5% w/w). This solution was prepared as follows: for each 250 g of solution A, 1.25 g of sodium hydroxide were weighed and placed inside a 250 mL flask, with 100 mL of distilled water. The preparation was stirred. Moreover, 0.63 g of polyvinyl alcohol was added until full dilution. Then, 0.63 g of piperazine was added to the flask and filled to 250 mL. The composition of the organic solution (solution B) was TMC (1.0% w/w) and hexane. To prepare it, the procedure was as follows: for each 250 mL of solution B, 1 g of capsaicin extract and 1.5 g of TMC were weighed and the solution was filled to 250 mL with hexane. The first of the membranes,

In order to prepare the experimental membranes with distinctive characteristics, capsaicin extract was added during the preparation process, using four different concentrations, as

**Membrane Support layer Composite layer Capsaicin (g)**

A Polysulfone Polyamide 0 B Polysulfone Polyamide + capsaicin 1 C Polysulfone + capsaicin Polyamide 1 D Polysulfone + capsaicin Polyamide + capsaicin 2 Atomic force microscopy (AFM) was performed with a PS 50–50-15 (50 μm) scanning probe microscope instrument (AFM workshop) to determine the surface morphology of the nanofiltration membrane. The software Gwyddion 2.4 was used to obtain 3D images and roughness measurements (root mean square, RMS). A sample size of 0.5 × 0.5 cm was used to analyze the nanofiltration membranes.

#### *2.4.2. Attenuated total reflectance infrared spectroscopy*

Attenuated total reflectance infrared (ATR-IR) characterization of the nanofiltration membrane surface was carried out with a Nicolet iS5 Fourier transform infrared spectrometer and the accessory iD3 ATR (thermo Fisher scientific). For ATR-IR analysis of the nanofiltration membranes samples, a germanium crystal was employed. A sample size of 0.5 × 0.5 cm was used to analyze the nanofiltration membranes.

#### *2.4.3. Contact angle*

Contact angle measurements were performed with a dataphysics contact angle system (OCA15SEC), camera iDs and injection system ES. A sample size of 3 × 2 cm was used to analyze the nanofiltration membranes. The water used was previously distilled and filtered (0.2 μm). The water drop was placed on different locations on the membrane surface, at a temperature of 22°C.

#### *2.4.4. Cross-flow equipment*

The operation of the cross-flow cell unit (**Figure 3**) was as follows. Starting from a 20 L feed tank, the feed solution (NaCl, MgSO4, and Na2 SO4 at the different concentrations) is conducted


**Table 2.** Polysulfone and polyamide composition.

**3. Results and discussion**

**3.1. Test on cross-flow equipment**

properties on the membrane surface.

**Figure 4.** Permeate flux results for the membranes, with Na<sup>2</sup>

5000 ppm respectively.

permeate flux.

of membrane B.

This can be appreciated for the three salts tested (Na<sup>2</sup>

The data in **Figure 4** point to a decrease in permeate flux as the salt concentration increases.

pressure (80 psi). Comparing the control membrane A with the rest of the membranes that include capsaicin in their formulation, it is observed that capsaicin addition tends to increase

For the NaCl solution at 1000 ppm, membrane B presented a permeate flux increase of 9.1% compared to membrane A, whereas at 5000 ppm, the increase was 14.81%. Membrane B presented the highest permeate flux increment, with membranes C and D also having permeate flux increases compared to membrane A but not as high as those

Due to the hydrophilic properties of capsaicin, the flux increments observed can be attributed to the capsaicin present in the membrane elaboration process for membranes B, C, and D. The capsaicin molecule has two main functional groups: hydroxyl, a polar group which presents affinity to water, and amide, which has nitrogen and a couple of free electrons generating polarity, also resulting in affinity to water. Hence, both characteristics result in hydrophilic

SO4, MgSO<sup>4</sup>

SO4

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, MgSO4, and NaCl) under constant

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and NaCl solutions at (a) 1000, (b) 3000 and (c)

**Figure 3.** Cross-flow equipment for membrane testing.

to the cross-flow cell, with an applied and stable pressure (80 psi). A 3.5 × 2 membrane piece is placed in the interior of the cell. Once feed water reaches the membrane cell, two outcomes occur: some water permeates through the membrane, leaving impurities behind and producing purified water, and the rest of the water that does not cross the membrane flows to a retentate tank. This latter effluent will have a distinctive characteristic of higher salinity than the feed flow.

The thickness of the membranes was measured using a micrometer (Holytex paper 0.12 mm). This information can be related to the uniformity of the membrane surface. Finally, the salt rejection by the membrane is calculated considering the conductivity in the feed flow and in the permeate flow (after 60 min). For concentrations in the range of different salts used, the conductivity is proportional to solution concentration, so salt rejection can be determined using the following equation:

$$\% \text{Conductivity rejection} = \frac{\Omega\_\circ - \Omega\_\circ}{\Omega\_\circ} \times 100\% \tag{1}$$

where Ω0 represents the conductivity of the feed and Ω60 represents conductivity of the permeate after 60 min of permeation. In terms of concentration, the rejection percentage is given by:

$$\%R = \frac{C\_{\gamma} - C\_{r}}{C\_{\gamma}} \times 100\% \tag{2}$$

where %*R* is salt rejection, *Cf* is the concentration of salt in the feed, and *Cp* is the concentration of salt in the permeate.
