**3. Configuration of CA membranes**

### **3.1 Flat sheet**

Flat sheet membranes in general take the shape of flat sheets. The membrane's top surface is an active layer for fluids or particulate/fluid separation. The membrane surface is the top upper layer that performs the function of separation. However, flat membranes are often prepared by pouring the polymeric solution onto a substrate. The most important characteristics of this type of membrane are high specific flow rates, high production rates, and easy placement of the membrane in the module. There are several approaches for preparing flat sheet membranes, but generally phase inversion or thin film composite and interfacial polymerization are the essential methods used to manufacture these membranes.

### *3.1.1 Phase inversion*

Generally, asymmetric porous polymer membranes are developed by the phase inversion process. This occurs when an initially homogeneous polymer solution changes its conditions thermodynamically.

During phase inversion, a single-phase casting solution causes the formation of two different phases: a polymer-rich phase or a solid membrane matrix and a poor phase in terms of polymer or liquid polymer [45]. This technique enables the change from a liquid to a solid state. The transition from one liquid to two liquids starts the solidification process. Slowly, the rich polymer phase forms the solid matrix during this liquid-liquid transition. The poor phase is then removed by successive washing steps and leaves room for the pores of the membrane. The morphology of a membrane has a significant impact on its characteristics and performance.

Therefore, membrane synthesis's major objective by phase inversion process is to obtain membranes with specific properties such as microfiltration, nanofiltration, or ultrafiltration that ensure good thermal, chemical, and mechanical resistance properties and to develop adequate membranes properties with a surface layer for reverse osmosis and gas separation [46].

The CA membranes prepared by using this technique have been used as microfiltration [46], ultrafiltration [47], nanofiltration [42], forward osmosis [48], reverse osmosis [49, 50], and gas separation membranes [51].

In order to create an asymmetric microfiltration membrane structure, a selective layer and a prefilter layer were combined in one membrane to examine the effect of the CA-based solvent system on membrane morphology during membrane

#### *Cellulose Acetate Membrane Preparation for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.109416*

manufacturing. A pseudo-ternary mixture of CA, methyl formate (MF), 2-propanol, and water was used to create asymmetric microfiltration of CA membranes by using a single-layer dry casting process. The created membranes were composed of uniform pore size isotropic regions that are covered by protective skin.

The selective layer of these newly created CA microfiltration membranes fronted the solid-liquid interface in contrast to other asymmetric membranes, or the upper area facing the liquid-gas interface showing larger pore sizes. The denser bottom layer acted as a selective layer while the upper layer functioned as a prefilter. The membranes' structural analyses were carried out using scanning electron microscopy. A theory regarding the actions occurring in the casting solution during membrane development is addressed in depth along with the membrane structures that have been observed [52].

Using the phase inversion method, Ounifi et al. [53] successfully synthesized ultrafiltration membranes from a casting solution containing CA at concentrations ranging from 15 to 21 wt.%. With the rise in CA, the membrane's performance, such as permeability, contact angle, and water content, decreased, but the PEG rejection rose at higher polymer concentrations during membrane formation (**Figure 3**).

CA-NF membranes [54] were prepared via the NIPS inversion process. The asymmetric structure of CA-NF membranes, which are composed of a dense top layer supported by a porous sublayer, was clearly elucidated by the SEM images (**Figure 4**). By increasing the CA concentration in the polymeric doped solution used for membrane synthesis, membrane characterization demonstrated the membrane's hydrophilic nature and decreased porosity. The membrane's permeability to pass water through it was decreased due to reduced porosity, while the salt retention was increased. The size exclusion mechanism dominates the transport mechanism through the membrane.

#### **Figure 3.**

*Scanning electron microscopy (SEM) image of ultrafiltration membranes with different polymer concentrations [53].*

**Figure 4.** *SEM image of nanofiltration membranes with different polymer concentrations [54].*

The order of rejection is determined by the hydration energy and hydrated radii of the ions and is as follows: R(Na2SO4) > R(CaCl2) > R(NaCl).

In numerous studies, the phase inversion method used in the CA membranes preparation is carried out in conjunction with the inclusion of certain additives such as composites, nano-materials, surfactants, and polymer additives to change the prepared membranes with this approach.

TiO2 nanoparticles were dispersed in the CA casting solutions to prepare hybridCA/TiO2 membranes by phase inversion. Investigations were done into how TiO2 affected the morphology and thermal stability of CA/TiO2 membranes. According to SEM findings, the addition of TiO2 nanoparticles makes the hybrid membrane more porous due to increase in the average pore size. The TGA results show that the interaction between TiO2 nanoparticles and CA exists. Thus, the hybrid membrane's thermal stability was enhanced by the addition of TiO2 nanoparticles. In addition, PWF membranes show that the addition of TiO2 nanoparticles leads to an increase in water permeation [49].

CA-silica (SiO2) hybrid membranes (CA-SiO2) were synthesized by promoting in situ condensations between silanols of the SiO2 precursor and COH or acetate groups of the CA polymer. The results showed molecular water strongly linked to hydrogen bonds with SiOH groups, which leads to a drastic drop in the membrane hydraulic permeability, from 57 to 10 kg h−1 m−2 bar−1. In comparison with the CA membrane, the inclusion of 5 and 10 mol% silica enhanced the hydraulic permeability from 32 to 82 kg h−1 m−2 bar−1 [55].

In order to remove Cd2+, Ounifi et al. developed fouling-resistant UF membranes. Phase inversion was used to prepare the UF membranes by combining PAA and CA. The strong hydrogen interactions between the two polymers were observed with FT-IR. The membranes showed highly porous and asymmetric structures with

#### *Cellulose Acetate Membrane Preparation for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.109416*

affected morphologies as a result of PAA concentration. In fact, by increasing the PAA concentration from 0 wt.% to 15 wt.%, the porosity increased from 44.5% to 75.6%. Contact angle and pure water flow measurements showed that adding PAA to the CA structure improved the membrane hydrophilicity. The result ultimately demonstrated that the prepared membrane with a higher PAA content (M15) showed superior permeability and HA rejection characteristics [56].

#### *3.1.2 Interfacial polymerization*

Interfacial polymerization (IP) is a very practical and useful simple technique to create a thin layer for nanofiltration and reverse osmosis membranes. Interfacial polymerization always has been of a particular interest in industrial manufacturing.

The thin layer on the membrane surface is formed by the reaction and co-polymerization of two reactive monomers. In recent years, the development of thin-film composite by interfacial polymerization has received attention due to significantly improved membrane properties such as selectivity and fouling resistance. This method involves several steps, and it is easy to apply. To create the hydrophilic NF membranes, the substrate, typically used in MF or UF (cellulose acetate, polysulfone, etc.), must first be dissolved in a solution containing a highly reactive, water-soluble, and bifunctional or trifunctional monomer. In the second step, the membrane is submerged in a solution comprising an additional mono- or polyfunctional monomer that is soluble in an organic water-miscible solvent. The two monomers thus combine to form a dense polymer layer at the water/organic solvent interface. To finish the polymerization reaction and crosslink the resulting structure, this layer is then frequently heated. The layer that forms on the surface of the membrane limits the progression of the reagents, resulting in a dense thin layer of around 50 nm. This is an advantage of interfacial polymerization.

The porous layer is usually made from PSF polymer. However, interfacial polymerization can also be performed on CA-based support, or CA can be used as a selective layer on a support.

According to Ounifi et al., the interfacial polymerization of M-phenylenediamine (MPD) and trimesoyl chloride (TMC) on CA-UF successfully produced innovative flat-sheet thin film composite membranes for nanofiltration (TFC-NF) (**Figure 5**). The thin film composite TFC had a smooth surface, as evidenced by the SEM and atomic force microscopy (AFM) pictures. The polyamide thin top layer preparation conditions, such as reaction time and TMC concentration, have a significant impact on the membrane performance, specifically the water permeability and salt rejection of the TFC-NF. When TMC concentration and IP reaction time were increased, the water permeability fell from 23.35 to 10.44 L m−2 h−1 bar−1. Additionally, the salt rejections of these TFC membranes were adjusted from 69.23% to 92.43% and from 19% to 55.54% by using Na2SO4 and NaCl, respectively. A cadmium solution retention test has been done. The test result showed that this type of membrane could be used for the removal of heavy metals. The rejection of cadmium depends on the nature of the associated anion: (Cd(NO3)2) < R(CdCl2) < R(CdSO4), and it can reach a maximum of 97.76% for CdSO4 [57].

According to Ounifiet al., the interfacial polymerization technique was used to successfully prepare a number of polyamide TFC membranes. The CA-UF was used as a substrate. The PA layer over the CA-UF substrate was created by using the monomers of 1,3- cyclohexanebis (methylamine) (CHMA) and TMC in the aqueous phase and organic phase, respectively (**Figure 6**). Many concentrations of CHMA, namely 0, 0.2, 0.5, 1, and 2 wt.%, were used on membranes preparation. (The CHMA concentration

**Figure 5.** *Mechanism of polymerization interfacial and AFM image of membrane CA and TFC.*

**Figure 6.**

*Mechanism of polymerization interfacial and SEM image of membrane CA and TFC.*

effect on the membranes' physicochemical characteristics was investigated by using SEM, FT-IR, water absorption, water permeability, porosity, contact angle, specific salt rejection rate, and dye molecules.) SEM analyses showed that increase of the CHMA concentration increased the membrane's rugosity and thickness. The TFC-2 membrane exhibited the typical polyamide membrane morphology identified by the "ridge and valley" structure. The FT-IR findings revealed novel absorption bands specific to polyamide, which ensure the successful use of interfacial polymerization. The hydrophilicity of the membrane was enhanced by the increase in CHMA concentration, but the porosity and permeability of the membrane decreased. For example, the membrane water permeability varied from 36.02 for the CA-UF membrane to 17.09 L h−1 m−2 bar−1

#### *Cellulose Acetate Membrane Preparation for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.109416*

for TFC-2. All TFC membranes had a higher rate of salt and dye rejection than the CA-UF membrane. The TFC-2 membrane demonstrated a desalination rate of up to 89%, whereas the malachite green and Congo red were rejected at rates of 89 and 85%, respectively. The concentration of the molecule and the feed solution's pH affect the separation performance. Overall, the CHMA could be a potential candidate to develop TFC membranes for desalination and water treatment [58].

Ounifi et al. studied the influence of support pores size and monomer type on the thin-film composite membranes performance. By using CA membrane as support and the cyclohexane-1,3,5tricarbonylchloride (HTC) monomer as an organic phase, a series of polyamide-cellulose acetate thin-film composite nanofiltration (TFC-NF) membranes were created for this purpose by the interfacial polymerization technique. The effect of the cellulose acetate concentration of 15, 16.5, 18, 19.5, and 21.5% on the pore size of the membrane support was studied. As monomers, m-phenylenediamine (MPD), piperazine (PIP), and 1,3-cyclohexanebis (methylamine) (CHMA) were investigated. In comparison with other produced membranes, the results showed that the PIP/HTC membrane is more hydrophilic and has a more intense granular and convex structure with a rougher surface. The concentration of cellulose acetate affected both the porosity and the water flow. The reduction in porosity was consistent with the results of the SEM analysis (**Figure 7**), contact angle, and permeability. The retention of NaCl, CaCl2, and Na2SO4 was evaluated for the membrane performances. Comparing the Na2SO4 rejection with CaCl2 and NaCl, the former is higher than the latter [55].

#### **3.2 Hollow fiber**

Another membrane configuration, in which CA can be used as the main polymer, is a hollow fiber membrane. The hollow fiber membrane (HF) has three main advantages, which have led to the hollow fiber design becoming a popular choice:


The two main approaches for preparing HF are thermally induced phase separation (TIPS) and non-solvent-induced phase separation (NIPS). For NIPS, the solvent is extracted from the solution using a non-solvent, which is a substance that is miscible with the solvent but cannot solubilize the polymer. When the solvent is removed from the solution, the polymer concentration rises to its solubility limit, causing precipitation. The membrane structure can be tuned by adjusting preparation parameters such as polymer concentration, non-solvent system strength, and the use of additives [60].

For TIPS, phase separation is achieved by lowering the temperature of the dope solution. To obtain a homogeneous solution, the polymer is dissolved in one or more solvents, commonly referred to as diluents, at a high temperature. The dope solution is then extruded at high temperature through a spinneret and cooled in a low-temperature-quenching bath. At low temperatures, the diluents are unable to solubilize the polymer, and the solution undergoes phase separation [61]. After the hollow fiber membrane is formed, the remaining solvent is removed with cleaning baths [62].

**Figure 7.**

*SEM image of TFC nanofiltration membranes with different monomers [55].*

The diluent properties and solution viscosity both play a significant role in determining the final membrane morphology.

A homogeneous braid-reinforced CA hollow fiber membrane was formed through a non-solvent-induced phase separation process, and the membrane was prepared using an HMR method, in which CA solution was coated on the outer surface of the CA braid [63].

CA hollow fiber membranes were fabricated via various spinning processes such as dry-jet wet spinning [64], well-known dry-jet wet spinning process, dry-wet phase inversion method [65], coating a separation layer on the high-strength hollow tubular braid, melt spinning, and stretching process [66].

Most CA hollow fiber membranes were prepared using the dry-wet phase inversion method (**Figure 8**), but their poor mechanical properties limited their use in engineering practice. As a result, a hollow fiber membrane with high mechanical properties was required. Many studies have been conducted in order to improve the mechanical properties of hollow fiber membranes. For example, coating a separation layer on the high-strength hollow tubular braid was a relatively simple and effective method. There were two interfaces between the braid and the separation layer: the homogeneous-reinforced (HMR) interface and the heterogeneous-reinforced (HTR) interface. According to Fan et al. [63], the homogeneous-reinforced (HMR) method and the heterogeneous-reinforced (HTR) technique were combined to reduce the weaknesses of each and achieve the best results. Fan et al. produced a BR CA hollow fiber membrane by coating the "hybrid" braid with CA solution. The two-dimensional braided technique was used to prepare the "hybrid: braid, which consisted of CA filaments as the homogeneous fiber and PAN filaments as the heterogeneous fiber as shown in **Figure 9** [63]. SEM image results illustrated the cross section of BR CA membranes made by CA filaments and PAN filaments process in various braid compositions as shown in **Figure 10**.

*Cellulose Acetate Membrane Preparation for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.109416*

**Figure 9.**

*BR CA membrane process with CA filaments and PAN filaments.*

**Figure 10.** *SEM images of cross section of BR CA membranes with various braid compositions.*

The mechanical properties and permeation performance of BR CA hollow fiber membranes with different fiber ratios in the braid and different CA concentrations in the coating solution were investigated, giving a new approach to obtaining BR CA membranes with desirable performance.

The potential and application of commercial hollow fiber membrane products are majorly affected by three factors:


CA-based NF hollow fiber membranes have been studied and explored for FO applications. Heat treatment at 60°C for 60 min slightly affects the membrane pore radius and the thickness of the outer skin layer. Further, heat treatment at 95°C for 20 min results in the significant shrinkage of pores on the membrane surface with a denser outer skin layer. The resultant fiber has a high rejection to NaCl and MgCl2 and low pure water permeability in the NF tests. The CA-NF hollow fiber heat-treated at 95°C shows promising FO performance. However, this type of CA-NF hollow fiber membrane shows a decrease in performance ratio with increasing saline concentration in the feed due to the more severe internal concentrative concentration polarization. These preliminary results manifest the potential of the CA NF hollow fiber membranes for concentrating various feed solutions in the FO process. Future works should aim at the reduction of the dense layer thickness to enhance the flux while maintaining a low salt leakage rate [67].

#### **3.3 Electrospinning**

Electrospinning has drawn much attention and has already been as one of the most important techniques for manufacturing nonwoven polymer nanofiber membranes (synthetic or bio-based) with diameters ranging from nanometers to microns. Many geometries of the membrane are available for a diverse range of applications, along with agri-food, biomedical, cosmetics energy, and even textiles. For producing fibers from a polymer solution and synthesizing sophisticated three-dimensional structures, the electrospinning process employs thousands to tens of thousands of volts of high-voltage electrostatic repulsion [68]. A large number of synthetic polymers, including polycaprolactone (PCL), poly-lactic acid, and poly(lactic-co-glycolic acid), as well as more than 200 different types of natural polymers and composites, such as gelatin, silk fibroin, chitosan, and collagen, have been used in electrospinning technology systems. With this technology, the synthesized nanofibers have a high surface area-to-volume ratio, excellent plasticity, and flexibility, which can improve cell adhesion, proliferation, and differentiation activities. Lyu and his coworkers [69] studied the electrospun cellulose acetate (CA) membranes with optimized porous structures. They demonstrated that the synergy between pore size and porosity is extremely crucial for moisture-induced electricity generation from porous AC membranes. They will also prove that the pore size and porosity of CA membranes can be easily adjusted using a compression process. Several investigations are made to apply the membranes manufactured by electrospinning. For example, Cheng et al. [70] developed a new type of deacetylated cellulose acetate (DA)-polydopamine (PDA) composite nanofiber membrane by electrospinning. The membrane was employed as a highly efficient adsorbent for removing methylene blue (MB) from an aqueous solution. The results demonstrated the successful development of a uniform PDA coating layer on the surface of DA nanofibers. After 30 h of adsorption, the

*Cellulose Acetate Membrane Preparation for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.109416*

DA-PDA nanofiber membrane had an adsorption capacity of 88.2 mg/g at a temperature of 25°C and a pH of 6.5, which is approximately 8.6 times greater than that of DA nanofibers. The experimental results revealed that the DA-PDA composite nanofiber's adsorption behavior followed Weber's intraparticle diffusion model, the pseudo-second-order model, and the Langmuir isothermal model. According to a thermodynamic analysis, endothermic, spontaneous, and physisorption processes were observed. Furthermore, Acid black 172 was removed from simulated wastewater using electrospun CA/P(DMDAAC-AM) composite nanofibrous membranes [71, 72]. The equilibrium adsorption capacities were 116, 159, and 192 mg/g, respectively, when the proportion of P(DMDAAC-AM) to CA was 20, 30, and 40wt.%. When the proportion of P(DMDAAC-AM) was 40%, the average fiber diameter was 185 nm, and the maximum adsorption capacity could reach 231 mg/g. CA/P(DMDAAC-AM) nanofibrous membrane adsorption capacities had a well-linear relationship with P(DMDAAC-AM) content and fiber diameter.

## **4. Additives**

CA has been widely used in the manufacture of membranes due to its hydrophilicity, adsorption properties for dyes, heavy metals, and low cost. But in order to improve permeability, separation efficiency, and antifouling properties, in many studies, the CA membranes were prepared by adding certain additives such as composites, nanomaterials, surfactants, and additive polymers to modify the membranes [73, 74].

Ounifi et al. [56] developed new UF membranes incorporating polyacrylic acid (PAA) as an additive. The strong hydrogen interactions between the two polymers observed with FT-IR and the presence of a single peak in the DSC results confirmed the homogeneity of the mixture of CA and PAA in the prepared membranes. The membranes presented asymmetric and highly porous structures with morphologies affected by the PAA concentration. Indeed, the porosity and permeability increase proportionally with the concentration of PAA and inversely proportionally with the contact angle. The benefits of blending CA with PAA have also been observed in terms of antifouling properties, allowing efficient flux recovery from the membrane fouled by the treatment.

CA/TiO2 hybrid membranes were prepared by phase inversion method, by dispersing titanium dioxide (TiO2) nanoparticles in the CA casting solutions [49]. The influence of TiO2 on the morphology and thermal stability of CA/TiO2 membranes has been studied. The SEM results show that the addition of TiO2 nanoparticles causes the hybrid membrane to become more porous due to the increase in average pore size. The thermogravimetric analysis results show that the interaction exists between TiO2 nanoparticles and AC. Moreover, the thermal stability of the hybrid membrane was improved by the addition of TiO2 nanoparticles. PWF membranes show that the addition of TiO2 nanoparticles leads to an increase in water permeation.

Ultrafiltration membranes based on CA modified with zinc oxide (ZnO) were prepared [75]. Membrane performance was improved. For example, mechanical stability, morphology, contact angle, and porosity were evaluated on modified CA and ZnO samples. The increase in ZnO concentration in the membrane leads to a decrease in permeability and an increase in hydrophobicity compared with the pure CA membrane.

To improve antifouling properties and shedding, PVP has been added to CA to produce a membrane [76]. PVP is a hydrophilic additive polymer and blowing agent. From the results, it was significant that the membrane mixed with the hydrophilic additive PVP was improved and showed better properties in terms of surface roughness, hydrophilicity, and thermal and mechanical stability.

CA membranes modified with various surfactant additives, including Span-20 (sorbitan monolaurate), Tween-40 (polyoxyethylene sorbitan monopalmitate), and Tween-60 (polyoxyethylene sorbitan monostearate), were prepared [77]. The effect of these additives has been studied. The membranes' behavior depends on different concentrations of surfactant. It was found that even adding small amounts of the studied surfactant additives could affect the separation behavior and cause the formation of macro voids and finger-like structures. In addition, the effect of surfactant hydrophobicity on the contact angle and water permeation flux of the prepared


**Table 1.** *Some of the additives used with CA in membrane preparation.* membranes was investigated. It was found that the membrane derived from tween-60 has the lowest flux and the highest contact angle (77.2°) (**Table 1**).
