OTHER NPS

In this section, some NPs, other than those mentioned in previous sections, that are used as additives for membrane modifications are presented. These essentially include WS2 (tungsten disulfide) and PANI (polyaniline)

Preparation of Polyaniline (PANI)

Polyaniline (PANI) is one of the most important conductive polymers and it is prepared by various methods, namely, chemical synthesis (oxidative polymerization) and electrochemical synthesis (Stejskal et al., 1999).

Chemical Synthesis (Oxidative Polymerization)

Synthesis of PANI by chemical oxidation way involves the use of acid in the presence of ammonium persulfate as the oxidizing agent in the aqueous medium Chemical synthesis requires three reactants: aniline, an acidic medium (aqueous or organic) and an oxidant. The more common acids are essentially hydrochloric acid (HC1) and sulfuric acid (H2S04). Ammonium persulfate ((NH4)2S208), potassium dichromate (K2Cr207), cerium sulfate (Ce(S04)2), sodium vanadate (NaV03), potassium ferricyanide (K3(Fe(CN)6)), potassium iodate (КЮ3), hydrogen peroxide (H202) are recommended as oxidants (Malinauskas, 2001)

Electrochemical Synthesis

The electrochemical synthesis of conducting polymers is similar to the electrodeposition of metals from an electrolyte bath; the polymer is deposited on the electrode surface and also in the in situ doped form. Three electrochemical methods can be used to PANI synthesis Nalwa (1997)

  • • Galvanostatic method when applied a constant current,
  • • Potentiostatic method with a constant potential, and
  • • Potentiodynamic method where current and potential varies with time.

Effect on Morphology

Lin et al. (2013), showed that the EDX spectrum indicates that oxygen element existing in the nanoparticles samples, accounted for 5.53±0.98%., the sulfur 18.2111. 66%, less than the theoretical weight (25.8%) in pure nanoWS2,

TABLE 2.7

Effect of nanocellulose additives on membrane characteristics and performance

olname="col47>

Application base material morphology Mechanical performance References flux LMH %R/Qe

cellulose nanocrystals (CNCs)

MF/ads *CS **Cs/GA

#thickness increase 250-270 pm # Average pore size increase I3-I7nm, # surface area increase 2.9-3. n2/g #tcnsilc stress increase .98-1.1 Mpa #Strain slightly decrease from 0.28 to 0.23 %

#Modulus elasticity increase from 128 to 318 MPa 64 Lm-2h-l

98%, 84%, 70%, for positively dye Karim ct al., 2014 RO

PES/PA

#Thickness decrease from 360 to 311 nm, # roughness increase 180 to 158 nm (-)

# LMH increase from 9.4 to 11.4 Lm-2 h-1

# % R no change 98.8 for NaCl Smith ct al., 2019

MF

PVDF

#surface area increase from 4.6 to 5.2 m2/g #contact angle decrease 42-126

  • (-)
  • (-)

# increase adsorption capacity of dye (Gopakumar et al., 2017)

Cellulose Nanowhisker

MF

PAN

nanohbrous scaffold/poly(ethylenc terephthalate) (PET)

#maximum pore size decrease from 0.7 to 0.4 pm #mean pore size decrease from 0.38 to 0.22 pm # C decrease from 50.6° to 16.9°

#tensile stress increase from 8.5 to 1.3 MPa #Young’s Modulus increase from 226 to 375 MPa ffLMII decrease from 83 to 59 Lm-2 h-l kPa-1

# increase adsorption capacity for dye,

(Mact al., 2012)

cellulose Nanocrystals (CNCs)/ l'OCNs RO

PES/PA because of the presence of WOj. In general, the membrane morphology changes as the nanofiller content increased.

Fan et al. (2008), found that the peaks of PANI aqueous dispersion were centered on 340. 440, and 800 nm. The spectra showed that PANI nanofibers were in the emeraldine oxidation state. The emeraldine oxidation state of PANI contains half imine and half amine nitrogen and could be represented by the formula [-N C6H4 N-C6H4-N H-C6H4-N H-C6H4 ]n. The author used SEM images of PS membrane surface, PANI/PS Nanocomposite membrane surface and cross section. It can be seen that PANI nanofibers are assembled evenly on the PS membrane surface and porous structure is formed. The nanofiber layer cracks might result from the distortion during the preparation of SEM samples (Figure 2.9).

Kajekar et al. (2015), found that PANI-nanofibers have high surface energy and hydrophilicity, thus when the hollow fiber membrane was immersed in a water bath, the PANI-nanofibers may migrate from the polymer matrix towards the water bath to reduce the interfacial energy between the two phases. The migration would leave cavities in the polymer matrix, which in turn would increase the membrane porosity and have interconnection between the finger like pores and macrovoids.

Fan et al. (2008), depicts SEM images of the membranes

FIGURE 2.9 Fan et al. (2008), depicts SEM images of the membranes: (a) PS membrane surface; (b) PANI/PS nanocomposite membrane surface; and (c) PANI/PS nanocomposite membrane cross-section.

Characteristics and Performance

Lin et al. (2013), reported that at nanofiller contents above 0.1% the permeation of membrane decreased although the hydrophilicity still increased. The increase of membrane permeability by 36% at a lower amount of WS2 was attributed to the improvement of the membrane hydrophilicity and change in membrane structure.

Fan et al. (2008), found that the reduction of contact angle to 37°, was mainly caused by the hydrophilicity of PAN1 nanofibers and the surface roughness of the nanofiber layer. According to the surface AFM image of nanocomposite membrane, it was estimated that about 12.5° decrease of the contact angle was attributed to the surface roughness difference between the PAN1/PS composite membrane and the PS membrane. Thus, about 24.5° decrease of the contact angle resulted from the hydrophilicity of PAN I nanofibers.

Kajekar et al. (2015), found that the contact angle reduces with increasing concentration of PANI-nanofiber indicating an increase in the hydrophilicity of the of PSF/PVP/PANl-nanofiber hollow fiber membrane cross-sections. This may be caused by PAN I nanofibers, which migrate towards the polymer- water interface and aggregate at the membrane surface. The PAN 1-nanofibers being highly hydrophilic in nature may be responsible for reducing the water contact angle on the membrane surface.

Table 2.8 summarizes some of the reported data concerning the effects of WS2 and nano- PANI additive on membranes characteristics and performance.

TABLE 2.8

Effect of WS2 and nano - PANI additives on membranes characteristics and performance

Application

Polymer

Additive

Morphology

Performance

Reference

UF

PES/

NMP

WS2 (0.025- 0.25%) WS/ PES

# Nanoparticle size is around

80-140 nm,

# CA: increase 36-54 WS/PES ratio%: 0-0.25 #Porosity max at 0.1% of nanofillcrs

U Permeability increase by 36% at lower WS2

Lin ct al., 2013

UF

PS

PANI

# The average diameter and length of the fibers were 43 and 259 nm

# contact angle decrease to 37°

Fan ct al., 2008

NF

PS/

NMP

PANI (0-1%)/ PVP(2%)

# asymmetric pores and nonuniform distribution of pores

# CA decrease from 93 to 88

Kajckar et al. (2015)

ACKNOWLEDGEMENT

This work was undertaken by the National Research Centre, Dokki, Giza, Egypt through a project entitled “Development of a solar powered, zero liquid discharge Integrated desalination membrane system to address the needs for water of the Mediterranean region” within the scope of ERANETMED, an EU FP7 initiative dealing with the Mediterranean region. The work is funded by the local agency Science and Technology development Fund, STDF, Ministry of Scientific Research. Egypt; Project No. 30280.

 
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