The Performance and Characterization of Polymeric Inclusion Membranes (PIMs) Containing 2-Nitro Phenyl Octyl Ether as Plasticizer on Phosphate Transport

Polymer Inclusion Membranes (PIMs) have been fabricated for diffusive passive sample layers. A study of various concentrations of plasticizers and characterization of PIM performance on phosphate transport has been carried out. The composition of PIM consisted of cellulose triacetate (CTA) as the base polymer, Aliquot 336-Cl as a carrier


INTRODUCTION
Phosphate is one of the essential nutrients that are useful for plants (Jiao et al., 2012). Phosphate has two forms in water, namely 90% in organic phosphate and 10% in inorganic phosphate (Bennett & Schipanski, 2013). One form of dissolved inorganic phosphate is orthophosphate with the species of PO 4 3-, HPO 4 2-, and H 2 PO 4 -, which depend on the pH of the water. Orthophosphate is one of the nutrients that cause algae blooms or eutrophication (Feng et al., 2018). It is indicated by high phosphate concentration in aquatic. Eutrophication is when a water body undergoes an excess of nutrients, causing plants to bloom (Tan et al., 2020). One of the negative impacts of eutrophication is algae blooms causing hypoxia in aquatic organisms (Godoy et al., 2019). Therefore, a device is needed for monitoring phosphate concentration in the water systems as one of the eutrophication prevention periodically.
Phosphate measurement can be carried out by spot sampling. The measurement of this method can only determine the phosphate concentration at that time. Therefore, spot sampling provides less representative investigation due to the fluctuation of phosphate concentration during measurement (Wilson et al., 2022). On the other hand, using the passive sampling method Time-Weighted Average Concentration (CTWA) of phosphate can be measured. This method can be carried out by deploying passive samplers in the water system for several hours (Cristóvão et al., 2021). During deployment, phosphate will be transferred from the bulk to the internal phase inside a passive sampler (Górecki & Namienik, 2002). Passive samplers based on membranes have been developed for analytes measurements, such as semipermeable membrane device (SPMD) (Lima et al., 2019), polar organic chemical integrative sampler (POCIS) (Jeong, 2020), Chemcatcher (Garnier et al., 2020), polyethylene (PE) (McKay et al., 2020), diffusion gradient in thin (DGT) (Kreuzeder et al., 2015) and Polymeric Inclusion Membrane (PIM) (Maiphetlho et al., 2021). Polymeric inclusion membranes (PIM)s have been used for a diffusive layer of passive samplers to measure metal ions (Ni, Co, Zn) (Keskin et al., 2021), ammonia (Almeida et al., 2016), pesticides (Mwakalesi & Potter, 2021). The advantages of PIMs are simple synthesis, efficient carrier immobilization, adaptability, and high mechanical characteristics (Rodríguez de San Miguel, 2022).
PIM is the development of SLM (supported liquid membranes). Unlike SLM, where weak capillary forces hold the liquid phase within the micro pores of the hydrophobic microporous membrane, PIM integrates the membrane liquid phase into the entangled chains of the base polymer to the adjacent aqueous phase. Therefore, PIM is much more stable and robust than SLM (Almeida et al., 2017). PIM consists of the base polymer, plasticizer, and carrier. Base polymerics frequently used are polyvinyl chloride (PVC) (Keskin et al., 2021) and cellulose triacetate (CTA) (Ait Khaldoun et al., 2018). Base polymer holds the carrier as a liquid phase and gives strength to the membrane (Nitti, Cendana et al., 2021). 2-nitro phenyl octyl ether (2-NPOE) is a plasticizer commonly used with CTA base polymer instead of tris (2-ethylhexyl) phosphate (TEHP) and dibutyl phthalate (DBP) (Wang et al., 2016). The suitable plasticizer has to present a high dielectric constant because the dielectric constant will affect the ion transport through the PIM-containing amines group in the carrier (Monroy- Barreto et al., 2021). Both TEHP and DBP show lower dielectric constant than 2-NPOE. PIM also contains a carrier that facilitates molecule or ion transport through PIM. Keskin et al. (2021) reported that aliquot 336-Cl as the carrier could be used to monitor metal ions. PIM formed will be thin, flexible, and transparent. Some research presented PIM can be used to determine ammonia (Almeida et al., 2016), metal ions (Keskin et al., 2021), and sulfamethoxazole (Garcia-Rodríguez et al., 2016). Phosphate transport has been studied using PIMs consisting of PVC/Aliquat 336-Cl/1-decanol (Rumhayati et al., 2021), where the sampling rate of this PIM containing 55:15:30 (% w/w) was 8 x 10-5 L/minutes. However, PIM that is consisted of CTA/Aliquat 336-Cl/2-NPO has not been investigated for phosphate measurement. Therefore, in this study, we purposed to determine the performance and characteristics of PIMs containing CTA/Aliquat 336-Cl/2-NPOE for phosphate transport.

MATERIALS AND METHODS Materials
Chemicals needed to fabricate PIM were cellulose triacetate (CTA) (Merck), 2nitro phenyl octyl ether

Instrumentation
VTSYIQI Ultrasonic Thickness Gauge Tester was used to measure the thickness of the membrane. Genesys 10 Spectrophotometer UV-Vis was used to detect phosphate concentration. Scanning electron microscopy (SEM, FEI Inspect-S50) was used to observe the surface section and cross-section of PIMs. The contact angle was measured by the instrument of OCA 15EC with a video system. The mechanical measurement of PIMs was carried out by digital force gauge push-pull gause with RS232 output (BAOSHISHAN ZP-50N).

Procedure PIM Preparation and Characterization
This study produced PIMs with two different compositions, as shown in Table 1, based on the variation of plasticizers. The total weight of the composition was 300 mg (Rumhayati et al., 2021). PIMs solution was prepared by diluting some CTA in 10 mL chloroform. Afterwards, Aliquot 336-Cl and 2-NPOE were added and stirred thoroughly for 2-3 hours to dilute the compounds well. The membrane solution resulted was 15.2 g. The solution was then put into a 6 cm diameter molded glass. The molded glass was covered with filter paper and watch glass to evaporate the chloroform gradually at room temperature. After 24 hours, the membrane could be peeled out of the molded glass. The thickness and weight of membranes were then measured using VTSYIQI Ultrasonic Thickness Gauge Tester, and the elasticity of PIMs was determined based on the tensile strength value. The characteristic of hydrophobicity-hydrophilicity was tested based on the contact angle measurement. The surface morphology and cross-section were determined using SEM. All the produce membranes were kept in sealed plastic before being used as a diffusive layer of passive samplers.

PIM Passive Sampler Assemble
PIM passive sampler devices used in this research have been shown in Figure 1. The device consists of a Teflon ring, rubber ring, cover in the top part, cover with the middle hole in the bottom part, and passive sampler bottle with a volume total of 15 ml. The membrane was cut with a diameter of 2.2 cm and placed between a rubber ring and a Teflon ring (Rumhayati et al., 2021).
The assembled sandwich was then positioned at the bottom opening of the passive sampler device and closed tightly. Then, the bottle of the passive sampler was filled with 15 mL of NaCl solution 0.1 M as an internal phase before it was closed tightly. The possibility of leakage should be checked and avoided before using the device for phosphate measurement.

The Effect of Plasticizer Concentration on the Phosphate Sampling Rate
The sampling rate (Rs) determines the equivalent phosphate volume transported (Veq) through PIM during experiment time (t). It becomes a specific characteristic of a PIM as a passive sampler device's diffusive layer. For the experiment in this section, a tank of water was filled with 1 L of phosphate solution of 0.6 mg/L. The solution was bubbled using an aquarium bubble pump overnight to ensure homogeneity. PIM at specific composition was positioned inside passive samplers. The devices filled with 15 mL of NaCl 0.1 M were placed floatingly with the PIM part faced inside the bulk solution. 5 mL of bulk solution and 5 mL of internal phase were pipetted out at 0 -48 hours from each device. Then, the concentrations of phosphate in the bulk phase and internal phase were detected. All experiments were conducted three times for replication. The sampling rate (Rs) was then obtained from the slope value of the straightline equation between equivalent volume (Veq) and time (t), as shown in Eq. 1 below V eq = R s . t …………………………………. (1)

Performance of PIM Passive Sampler for Phosphate Measurement
In this research, the phosphate measurement was conducted using a passive sampling method in the laboratory by deploying some PIM passive sampler devices. A water tank was filled with 1 L of phosphate solution 0.6 mg/L. An aquarium pump was installed inside the water tank to imitate the water flow in the entire aquatic system. Bubbling was conducted overnight to homogeneous the system. Two to three passive sampler devices were then deployed facing down and floating in the water tank for 48 hours. In the last experiment, the bulk of phosphate in the water tank and the internal solution inside passive sampler devices were pipetted as much as 5 mL. Phosphate Concentrations were detected using a spectrophotometer UV-Vis at a wavelength of 691 nm for the bulk phase and 710 nm for the internal phase. In this study, phosphate was detected using the SnCl 2 .  (2) Cr is the concentration of phosphate in the internal phase (mg/L), Vr is the volume of the internal phase (mL), Rs is the sampling rate (mL/hour), and t is the deployment time. CTWA of phosphate was then compared to the phosphate concentration in the bulk solution (C s ) to provide an accurate measurement. All the experiments were conducted in triplicate.

RESULTS AND DISCUSSION The Characterization of PIM Morphology of PIMs
PIM was made using thermoplastic base polymer, for instance, CTA. CTA has a group of hydroxyl and acetyl and causes the polymer becomes highly both polar (Sanders et al., 2013) and crystalline polymer able to give tremendous mechanical strength . On the other hand, Figure 2 shows that the part of crystalline is due to the acetylation process of the cellulose hydroxyl group, and the CTA polymorph's crystalline consists of parallel chains (Sikorski et al., 2004).  Figure  3, had thicknesses of 174 and 159.67 µm, respectively. The weight of PIMs was relatively the same, i.e., 0.440 ± 0.009 grams. Their homogenous, transparent elasticity and hydrophilicityhydrophobicity specified the PIMs Govindappa et al., 2022)). Visually, both membranes did not have a significant difference in either the surface or transparency. The homogenous PIM depends on the compatibility of plasticizer binding between the base polymer and carrier used (Bonggotgetsakul et al., 2015). 2-NPOE can penetrate the polymer strands and neutralize the polar groups of CTA  by greasing the section motions of the polymer, which will give the flexibility of membrane (Casadellà et al., 2016). SEM can study the surface and cross-section morphology. SEM pictures of the surface and cross-sections were taken to examine the effect of the membrane composition on the morphology (Sellami et al., 2019). Figure 4 depicts the morphology of PIM by comparing the surface section and the cross-section. PIM A and PIM B were no pores, similar to those studied in another research (Ait Khaldoun et al., 2018). It can be seen that PIMs A has a break section compared to PIMs B. In addition, PIMs B was more homogeneous than PIMs A. Those showed that the more increase of 2-NPOE as a plasticizer, the more decrease in the roughness of the membrane (Wang et al., 2016). However, the membrane can be rigid and brittle with low plasticizer concentration. Comparing the plasticizer and base polymer determines the minimum plasticizer concentration, and a plasticizer variated predisposes the distinguish in membrane pores (Kiswandono et al., 2013). Furthermore, PIMs based on CTA were compact and homogenous (Xu et al., 2022).

Hydrophilicity-Hydrophobicity
The hydrophilic or hydrophobic properties of the membrane can be assigned from the contact angle. The higher contact angle of the membrane (> 90 o ) shows the hydrophobic membrane, whereas the lower contact angle of the membrane (< 90 o ) presents the hydrophilic (Soo et al., 2021). The contact angles of PIM A and PIM B were observed, and those membranes were hydrophilic ( Table  2). There was an increase in contact angle, but it was still classified as hydrophilic membranes. In addition, Aliquot 336-Cl as a carrier determines the hydrophilicity characteristic of membranes. Aliquot 336-Cl has aliphatic chains (with eight carbons) oriented upward (in the air) and is deposited on the surface of the membrane. The aliphatic chains will prevent the membrane from interacting with water (Benosmane et al., 2022). However, the presence of the charged quaternary ammonium groups in Aliquot 336-Cl induces hydrophilicity in the membrane (Sellami et al., 2019). The membrane with high hydrophilicity will have higher wettability and higher tendencies of water uptake (Soo et al., 2021). On the other hand, adding plasticizer also increases surface hydrophobicity because of the aromatic ring and alkyl groups (Sedkaoui et al., 2021). Hence, PIM B is better than PIM A because PIM B is more hydrophobic than PIM A, with 10% NPOE as a plasticizer.

Tensile Strength
PIM's mechanical properties like tensile strength, elongation, modulus young are essential to determine (Kotsilkova et al., 2018). Tensile strength is used to assess membrane mechanical properties by measuring deformation resistance, where mechanical properties depend on the base polymer (Upitis et al., 2009). The tensile mechanical strength of the tested membrane is highly dependent on its quality and structural homogeneity (Kotsilkova et al., 2018). Furthermore, this tensile strength test is also used to identify the elasticity of the membrane measured by modulus young. The tensile strength can be seen from the compositions of the membrane, for instance, the plasticizer (Upitis et al., 2009). Figure 5 shows that the maximum force of PIM A and PIM B was 25.24 N and 24.79 N. The modulus of young of PIM A and PIM B were 2.90 MPa, and 1.86 MPa were obtained from the division between stress and strain. The tensile strength showed that PIM B was higher than PIM A because the addition of plasticizer will increase the membrane's tensile strength and physical strength (Kiswandono et al., 2013). The modulus young is used to indicate the elasticity of the membrane, where the higher the modulus young, the higher film's stiffness (Hazrati et al., 2021).  The modulus young of PIM A is more significant than PIM B, resulting in PIM A being more rigid than PIM B. If the resulting membrane is stiffer, it will be more easily broken if it is subjected to continuous pressure (Souisa, 2011). The addition of plasticizer causes strong hydrogen bonds produced by intermolecular activity, so the PIM B with plasticizer is more elastic than PIM A without plasticizer, indicating the comparison of modulus young. Furthermore, the tensile strength also depends on the membrane's quality and structural homogeneity (Sanyang et al., 2015). It can be concluded that PIM B was better than PIM A for elasticity, so PIM B Extension Time (0.5 sec/cm) F max (N) showed that the membrane applied would not be easily ruptured.

The Effect of PIM Plasticizer on Phosphate Sampling Rate.
The intramolecular forces included in PIM are mostly Van der Waals forces (Rumhayati et al., 2021). Polymers composed of cellulose units, such as CTA, form crystalline films where polymer chains do not follow a systematic order, increasing the distance between polymer chains. Therefore, water molecules in the CTA film accumulate in the amorphous regions (larger spaces), leading to hydrogen bonding between CTA and water so that voids in the structure are reduced (Casadellà et al., 2016).
Accordingly, the plasticizer is added because plasticizer can reduce van der Waals and hydrogen bonds between polymer and water molecules. Polar groups in plasticizers are essential for good compatibility. When plasticizer molecules are presented into the polymer mass, the polymer chains are separated by small liquid molecules that can align dipoles with polymer dipoles. Dipoles aligned in this way are a compelling force of attraction. Liquid molecules can have polarizable ring structures that form points of mutual attraction. Polymer chains that are separated in this way are more mobile relative to each other than polymer chains that are tightly bound (Marcilla & Beltrán, 2012).
The plasticizer is one of the membrane components that also influences membrane stability and plays a role in condensing or hardening the membrane. PIM contained by plasticizers is more flexible, and there is an increase in substance transport (Keskin et al., 2021).
The effect of the base polymer and plasticizer on the phosphate transport rate is shown in Table 4. It can be seen that phosphate has good transport through PIM B with a sampling rate was 0.0003 ± 0.0001 L/hour. PIM-containing plasticizers will increase the ability of ion transport because PIMs are not rigid and easily damaged (Gibbons & Kusy, 1998).
That is concluded that the plasticizer influenced ion transport (Djamila et al., 2011). Nevertheless, the excessive plasticizer will interrupt ion transport (Kiswandono et al., 2013) and become a barrier because the membrane forms a film on the surface of the membrane (Kiswandono et al., 2013). The base polymer in PIM A became dominant because no addition of the plasticizer increased the membrane's ability to transport ions. The effect of the base polymer and plasticizer on the phosphate transport rate is shown in Table 4. It can be seen that phosphate has good transport through PIM B with a sampling rate was 0.0003 ± 0.0001 L/hour. PIM-containing plasticizers will increase the ability of ion transport because PIMs are not rigid and easily damaged (Gibbons & Kusy, 1998).
That is concluded that the plasticizer influenced ion transport (Djamila et al., 2011). Nevertheless, the excessive plasticizer will interrupt ion transport (Kiswandono et al., 2013) and become a barrier because the membrane forms a film on the surface of the membrane (Kiswandono et al., 2013). The base polymer in PIM A became dominant because no addition of the plasticizer increased the membrane's ability to transport ions.

Performance of PIM Passive Sampler for Phosphate Measurement
Transport of phosphate through PIM assisted by Aliquot 336-Cl (R 4 N + Cl -) as the carrier that exchanged ions and formed an ion pair. Clion in the internal phase will assist the ion-exchanged so phosphate can be measured as below: The carrier in PIM binds the target analyte forming either an anion or cation. Figure 6 shows that the bulk phase released H 2 PO 4 while Clwas released in the internal phase. The bubbling process during the deployment of the passive sampler will reduce the second diffusion layer in front of the PIMs so phosphate transport will not be influenced. The concentration of the internal phase has to be higher than the bulk phase because the difference in the driving force of ion transport from the bulk phase to the internal phase was also against the existing concentration gradient (Almeida et al., 2017). Transport of phosphate through PIMs B is more accurate than through PIMs A ( Table 5). Accuracy was obtained by dividing the CTWA and phosphate concentration in the bulk phase. However, both PIMs showed low accuracy, where the accuracy of PIM A was 28.38 % (v/v), and PIM B was 52.15% (v/v). That is because CTA is a polar polymer (Maiphetlho et al., 2021) than PVC, where the carboxyl group of CTA binds with the ammonium group of aliquot 336-Cl strongly (Sellami et al., 2019). On the other hand, the research conducted by Benavente et al. (2018) showed that membranes containing Aliquot-Cl/CTA have a low amount of Cl ion, and phosphate ion exchange with Cl-is minimal, resulting in low phosphate transport accuracy.

CONCLUSIONS
The transport of phosphate into a passive sampler containing PIMs as the diffusive layer is affected by the performance and characteristics of PIMs themselves. PIMs containing plasticizer (10% w/w 2-NPOE) showed good agreement for a diffusive layer. The PIMs have no pores, no break section, and homogeneous membranes that can transport phosphate. PIMs B was more elastic and hydrophobic than PIMs A. Phosphate was more easily transported through PIMs B, indicated by the high sampling rate of 0.0003±0.0001 L/hour. Even if the transport is low, the PIM passive sampler could be used to measure phosphate with an accuracy of 52.15%. This low accuracy was due to membranes containing Aliquot-Cl/CTA having a low amount of Clion; this causes low phosphate ion exchange with Clion.

ACKNOWLEDGMENTS
The