**3. Cyclodextrins in membrane technology**

Membrane technology is one of the methods that have been used in water treatment for decades. Its merits have made its use continuous throughout the decades; however, membranes have several demerits, which cannot be ignored. Membranes suffer from fouling, selectivity and low flux among other problems. This results in poor performance, which implies high operational cost for poor water output. Modifying membranes with materials such as nanomaterials and polymers via surface functionalization greatly improves the overall performance of membranes. Currently, polymeric membranes are the most used in water treatment because of their advantages, which include higher flexibility, easy pore formation mechanism, smaller footprint for installation and low cost compared to other membrane types [81–83]. When it comes to energy efficiency and cost-effectiveness, it is required that membranes should have high permeability, high rejection and good fouling properties [81].

The use of CDs and their derivatives has been found to enhance membrane performance in terms of improved porosity, flux, rejection and efficiency, which makes the use of CDs considerably effective. Porous CD-based membranes have interconnected pores with high permeability and find application in various filtration processes [84, 85]. Even though the use of CDs in membrane technology is not as advanced as the use of other polymers, several researchers have dedicated their time into studying and exploring new properties and advantages brought by CDs in various membrane types. The use of CDs in water treatment has been motivated by the ability of CDs to allow water to pass through their cavities and their surface functionalities, which greatly improve the hydrophilicity and permeability of membranes [86]. Mixed matrix membranes (MMMs) and thin film composite (TFC) membranes are two types of membranes where CDs and their derivatives have been used as modifying agents to improve their total performance.

#### **3.1. Mixed matrix membranes**

**3. Cyclodextrins in membrane technology**

prepared without βCD. Reproduced with permission from [77].

148 Cyclodextrin - A Versatile Ingredient

Membrane technology is one of the methods that have been used in water treatment for decades. Its merits have made its use continuous throughout the decades; however, membranes have several demerits, which cannot be ignored. Membranes suffer from fouling, selectivity and low flux among other problems. This results in poor performance, which implies high operational cost for poor water output. Modifying membranes with materials such as nanomaterials and polymers via surface functionalization greatly improves the overall performance of membranes. Currently, polymeric membranes are the most used in water treatment because of their advantages, which include higher flexibility, easy pore formation mechanism, smaller footprint for installation and low cost compared to other membrane types [81–83]. When it comes to energy efficiency and cost-effectiveness, it is required that membranes should have high permeability, high rejection and good fouling properties [81]. The use of CDs and their derivatives has been found to enhance membrane performance in terms of improved porosity, flux, rejection and efficiency, which makes the use of CDs considerably effective. Porous CD-based membranes have interconnected pores with high permeability and find application in various filtration processes [84, 85]. Even though the use of CDs in membrane technology is not as advanced as the use of other polymers, several researchers have dedicated their time into studying and exploring new properties and advantages brought by CDs in various membrane types. The use of CDs in water treatment has been motivated by the ability of CDs to allow water to pass through their cavities and their surface

**Figure 13.** Micrographs showing multipetals of (a–c) ZnO nanomaterials prepared with βCD and (d) ZnO nanofibers

Mixed matrix membranes (MMMs) are known for their high flux and low pressure drops. MMM high flux capacity and selectivity are often a result of functionalized modifying agents [87]. MMMs are used mostly for the removal of heavy metals, natural organic matter (NOM), EMPs and disinfection by products such as trihalomethanes, haloacetic acids, trihaloacetaldehydes, haloacetones and trihalonitromethanes in water [88]. Adams et al. prepared MMMs using polysulfone/βCD-polyurethane (PSf/βCD/PU) for the selective removal of Cd2+ ions and improved structural properties of PSf MMMs. Upon studying their characteristics, it was found that βCD-polyurethane enhanced the water sorption and hydrophilicity and achieved 70% removal of Cd2+ ions [18]. Adams et al. used the same material (PSf/βCD/PU) in 2014 to study the effect of βCD/PU on the rejection of NOM and fouling resistance of PSf MMMs. It was concluded that βCD/PU improved the effective pore sizes and molecular-weight cut-off of PSf membranes due to their conical structure and larger pore sizes, which allows water molecules to pass easily [89]. Other workers used ceramic membranes modified with cross-linked silylated dendritic polymers and CDs for the removal of organic pollutants in water. The modified membranes removed pollutants such as monocyclic aromatic hydrocarbons (93%), pesticide (43%), polycyclic aromatic hydrocarbons (99%) and trihalogen methanes (81%). The high removal percentage was attributed to the dendritic polymers and CDs [90]. **Figure 14** shows

**Figure 14.** SEM images comparing morphologies for (a) PSf and (b) PSf/βCD outside surface and cross-section. Reproduced with permission from [91].

comparison between PSf and PSf/βCD membranes and it is shown that βCDs improved the surface and cross-sectional morphology in terms of pore size dimensions and size distribution.

#### **3.2. Thin film composite membranes**

Nanofiltration (NF) membranes are pressure-driven membranes and are mostly thin film composite (TFC) membranes [86]. TFC membranes include reverse osmosis (RO) and ultrafiltration (UF) membranes prepared by interfacial polymerization. The unique structure of TFC membranes that consists of a UF support, a nonwoven support and a polymer membrane brings advantages such as low operation pressure, high retention of multivalent ions or salts, low maintenance cost and high permeation flux [86, 92, 93]. This type of membranes is mostly used in water treatment for the production of drinking water from wastewater, seawater and brackish water [94]. Other areas of application include organic solvent nanofiltration and pharmaceuticals and biochemical industries [86, 93]. The layers of TFC membranes can be modified independently for maximum preferred properties such as water uptake, fouling resistance, chemical resistance, thermal stability, hydrophilicity and mechanical strength [92]. The excellent properties of TFC membranes are due to modifying agents such as CDs and their derivatives.

Wu et al. prepared NF TFC membranes using polyester/βCD as a polymer material [81]. The addition of βCDs was found to improve the membrane performance as shown by double flux and high rejection of Na<sup>2</sup> SO<sup>4</sup> compared to bare membranes. When sulfated-βCDs were used, the membrane had improved negative charge density and salt rejection. Both membranes were reported to have enhanced antifouling properties [86]. On the subject of TFC membranes, Mbuli et al. used amino-CDs and diethylamino-CDs to modify polyamide TFC membranes. The addition of modified CDs enhanced the membrane's permeability because of improved hydrophilicity and additional water channels. In a separate study, modified TFC membranes containing CDs also demonstrated high flux and good NaCl rejection [94]. Mao et al. prepared CD-modified PEI membranes for organic solvent nanofiltration. In the study, they

> prepared a membrane with dual pathway nanostructures from CDs (hydrophobic pathway) and the fractional free volume of PEI (hydrophilic pathway). Toluene permeation was improved from 0.13 to 2.25 L/mhbar when CD loadings were increased [93]. **Figure 15** shows m-phenylenediamine (a) and amine (b) f-CD-modified PES membranes. The presence of uniform pores is observed on membranes (b) due to the addition of amine f-CDs, while the addition of m-phenylenediamine produced membranes with layered structures on top (a). The incorporation of amine f-CDs improved the general performance of the membrane in terms of hydrophilicity, flux and salt rejection [95]. In **Table 3**, we show recent works on the use of CDs and their derivatives in the production of MMMs and TFC

**Material Type Method Application Ref.** PSf-βCD-polyurethane MMM Phase inversion Nanofiltration for removal of Cd2+ ions [18] βCD-polyurethane MMM Phase inversion Rejection of NOM (humic acid) [89]

PSf-βCD MMM Phase inversion Removal of endocrine disruptive chemicals [91] CD-polymers — — Adsorption and separation of pesticides [97]

Detection of chloroform,

Rejection of NaCl and Na<sup>2</sup>

SO<sup>4</sup>

Rejection of Na<sup>2</sup>

Rejection of MgSO<sup>4</sup>

properties

studies

and dichloromethane

Cyclodextrin-Based Nanofibers and Membranes: Fabrication, Properties and Applications

1,3-dichloropropane, 1,2-dichloroethane, 1,2-dichloropropane, 1,1,2-trichloroethane

Degradation of trichloroethylene [96]

http://dx.doi.org/10.5772/intechopen.74737

and antifouling

Organic solvent nanofiltration [93]

Rejection of NaCl [94]

and fouling-resistant

[88]

151

[86]

[92]

SO<sup>4</sup> [95]

MMM Polymerization and phase inversion

MMM Polymerization and precipitation

polymerization

polymerization

polymerization

polymerization

polymerization

TFC Interfacial

TFC Interfacial

TFC Interfacial

**Table 3.** CDs and CD derivatives used as additives in traditional membranes.

The ability of CDs to form inclusion complexes with other materials and alter their properties has enabled electrospun CD-based materials and membranes to be used in many applications

membranes.

Azo dye-modified βCD-epichlorohydrin

Fe-Ni/f-CNT/ βCD-polyurethane

and PES/mphenyldiamine/ amine-f-CDs

PA/amino-βCDs PA/diethylamino-βCDs

PA/amino-α and βCDs PA/diethylamino-α and

βCDs

PES/m-phenyldiamine

PE/βCDs TFC Interfacial

PEI/α, β and γCDs TFC Interfacial

**4. Other applications of CD-based materials**

**Figure 15.** SEM images of (a) unmodified and (b) amine f-CD m-phenyldiamine TFC membranes. Reproduced with permission from [95].


**Table 3.** CDs and CD derivatives used as additives in traditional membranes.

comparison between PSf and PSf/βCD membranes and it is shown that βCDs improved the surface and cross-sectional morphology in terms of pore size dimensions and size distribution.

Nanofiltration (NF) membranes are pressure-driven membranes and are mostly thin film composite (TFC) membranes [86]. TFC membranes include reverse osmosis (RO) and ultrafiltration (UF) membranes prepared by interfacial polymerization. The unique structure of TFC membranes that consists of a UF support, a nonwoven support and a polymer membrane brings advantages such as low operation pressure, high retention of multivalent ions or salts, low maintenance cost and high permeation flux [86, 92, 93]. This type of membranes is mostly used in water treatment for the production of drinking water from wastewater, seawater and brackish water [94]. Other areas of application include organic solvent nanofiltration and pharmaceuticals and biochemical industries [86, 93]. The layers of TFC membranes can be modified independently for maximum preferred properties such as water uptake, fouling resistance, chemical resistance, thermal stability, hydrophilicity and mechanical strength [92]. The excellent properties of TFC membranes are due to modifying agents such as CDs and their

Wu et al. prepared NF TFC membranes using polyester/βCD as a polymer material [81]. The addition of βCDs was found to improve the membrane performance as shown by double flux

the membrane had improved negative charge density and salt rejection. Both membranes were reported to have enhanced antifouling properties [86]. On the subject of TFC membranes, Mbuli et al. used amino-CDs and diethylamino-CDs to modify polyamide TFC membranes. The addition of modified CDs enhanced the membrane's permeability because of improved hydrophilicity and additional water channels. In a separate study, modified TFC membranes containing CDs also demonstrated high flux and good NaCl rejection [94]. Mao et al. prepared CD-modified PEI membranes for organic solvent nanofiltration. In the study, they

**Figure 15.** SEM images of (a) unmodified and (b) amine f-CD m-phenyldiamine TFC membranes. Reproduced with

compared to bare membranes. When sulfated-βCDs were used,

**3.2. Thin film composite membranes**

150 Cyclodextrin - A Versatile Ingredient

derivatives.

and high rejection of Na<sup>2</sup>

permission from [95].

SO<sup>4</sup>

prepared a membrane with dual pathway nanostructures from CDs (hydrophobic pathway) and the fractional free volume of PEI (hydrophilic pathway). Toluene permeation was improved from 0.13 to 2.25 L/mhbar when CD loadings were increased [93]. **Figure 15** shows m-phenylenediamine (a) and amine (b) f-CD-modified PES membranes. The presence of uniform pores is observed on membranes (b) due to the addition of amine f-CDs, while the addition of m-phenylenediamine produced membranes with layered structures on top (a). The incorporation of amine f-CDs improved the general performance of the membrane in terms of hydrophilicity, flux and salt rejection [95]. In **Table 3**, we show recent works on the use of CDs and their derivatives in the production of MMMs and TFC membranes.
