5. Characterization of pH-responsive membranes

The current chapter deals with the adsorption/membrane integrated systems. As could be seen in Figure 2, some promising advantages of adsorption/membrane integrated systems could be

4. Straightforwardness of handling and fast control compared to conventional treatments

9. Low-energy feed requirements versus adsorption columns, NF and RO systems

Intended for the pre-synthesis of pH-responsive polyacrylamide zirconium titanosilicate (PAm-ZTS) membranes, liquid titanium(IV)chloride (98%), TiCl4, 189.68 [g/mol], 1.728 g/cm3 (20C), and zirconium(IV)oxychloride octahydrate powder (>99.5%), ZrOCl2.8H2O, 321.26752 [g/mol], 1.91 g/cm3 (20C), pH value ~1 (50 g/l, H2O, 20C), were picked up from Merck Chemicals,

obtained. They include:

4 Wastewater and Water Quality

1. Expanding separation efficiency

3. Diminished membrane fouling in some cases

Figure 2. Membrane/adsorption hybrid process with adsorption pretreatment.

6. Potential request of beneficial biosorbents

7. Reusability of both membranes and adsorbents

10. Low-pressure drop against adsorption columns

4. Fabrication of pH-responsive membranes

2. Diminishing process cost

5. Lower volume of discharge

8. Firm removal kinetics

RO polymerized membranes are different in a couple of characteristics such as material, morphology, transport/separation mechanism, and applications [21–24]. Therefore, a large number of methodologies are required for their characterizations. They can be generally divided into three major tests, that is, methods used for chemical analysis, methods used for physical analysis, and filtration process for assessing membrane separation performance. Depending on the applicable utilization of RO membranes, their stability assessments against chlorination, organic solvent, thermal, and fouling can also be performed to examine their sustainability under specific environments.

Table 1 describes some instrumental methods used in depicting RO membranes with respect to their chemical and physical characteristics, as well as their separation performances and stability. In a wide range, before conducting RO experiments, various techniques can be employed for their characterization in order to obtain a good knowledge of their parameters that are prominent for manufacturing a membrane with the right integration of water flux and solute rejection. For reverse osmosis pH-responsive membranes, zeta potential is well-thought-out as one of the significant parameters to determine the routes and mechanisms that the membranes behave according to its chemical properties.


Table 1. Assessments on membrane properties and performances based on different analytical instruments/methods.

#### 5.1. Zeta potential

Zeta potential is a surface charge property for RO membranes at different pH environments. The analysis is particularly significant to help recognize the acid–base features of RO membranes and to predict their separation productivity, as well as to consider the fouling propensity of RO at different water pHs [25–27]. Based on the Helmholtz-Smoluchowski equation with the Fairbrother and Mastin approach, zeta potential can be persistent from the measurement of the streaming potential using Eq. (1):

$$
\zeta = \frac{\Delta E}{\Delta P} \,\frac{\mu k}{\varepsilon \varepsilon \circ \bullet} \tag{1}
$$

conduct this investigation using two identical freshly prepared ROMs, that is, one for acid titration (pH 6 down to pH 2) followed by another identical membrane for alkali titration

Figure 3. Surface zeta potential as a function of pH for pH-responsive membranes made of ZTS, PAm-ZTS, and Pam,

Recent Drifts in pH-Sensitive Reverse Osmosis http://dx.doi.org/10.5772/intechopen.75897 7

PAm-ZTS RO membranes tended to have more positive charge owing to the protonation of the amine functional groups. In contrast, the negative charge of RO membranes at higher pHs can be attributed to the loss of functional groups [28–30]. Deprotonation of amine functional groups coupled with either dissociation of the carboxylic acid group or sulfonic acid group on the membrane surface may occur. In brief, in the membranes with organic origin, PAm is more negatively charged than that of that made up of ZTS and PAm-ZTS till pH 7 [25, 31, 32]. Besides showing the positive and negative charge values of a membrane, zeta potential profile can also reveal the isoelectric point (IEP) of the RO membrane at which the membrane surface

Depending on the functional groups of RO surface, a highly positively charged RO membrane could also be prepared, in which this membrane displays a positive zeta potential over a wide range of pH values (pH 2–11). The phenomenon is mainly due to the presence of pendant tertiary amine groups in some polymers used to fabricate the membranes. It was also reported to cause the membrane to be positively charged for pH ranging from 3 to 9 [33, 34]. A summary of the surface zeta potential of some RO membranes made of different monomers

(pH 6 up to pH 12).

measured at 25C.

carries no net electrical charge (i.e., neutral).

at two different pH environments is presented in Table 2 [25–27].

where ΔE is the streaming potential, ΔP is the applied pressure, μ is the solution viscosity, κ is the solution conductivity, and ε and ε ∘ are the permittivity of the test solution and free space, respectively. Several assumptions are inherent in this equation. They are (1) flow is laminar, (2) surface conductivity has no effect and has homogeneous properties, (3) width of the flow channel is much larger than the thickness of the electric double layer, and (4) no axial concentration gradient occurs in the flow channel.

The surface zeta potential of ZTS, PAm, and PAm-ZTS, as pH-Responsive membranes, measured at 25� C, are shown in Figure 3. Taking a horizontal section at the zero point of charge shows that the isoelectric point of ZTS, PAm, and PAm-ZTS was about 4.01, 5.7, and 7.6, respectively; throughout membrane testing, an electric potential is induced when cations and anions enclosed by the electrical double layer are forced to migrate along with the flow tangential to the ROM surface; in consequence, a potential difference could be initiated. Mostly, the streaming potential of the membrane surface is being measured. The typical pH range applied for determining surface zeta potential of a ROF membrane used to fall within pH 2–12, more preferably, between pH 3 and 9. The pH of the background electrolyte (5 mM KCl, 25�C) can be adjusted by the addition of either an acid, 0.1 M HCl (or HNO3), or a suitable base electrolyte, 0.1 M NaOH (or KOH) solution. Owing to the workable irreversible change of membrane surface characteristics, it is highly urged to

5.1. Zeta potential

measured at 25�

ment of the streaming potential using Eq. (1):

Chemical properties ATR-FTIR spectroscopy

6 Wastewater and Water Quality

XPS

Zeta potential analysis

X-ray diffractometry (XRD) Nuclear magnetic resonance (NMR) spectroscopy

tration gradient occurs in the flow channel.

Zeta potential is a surface charge property for RO membranes at different pH environments. The analysis is particularly significant to help recognize the acid–base features of RO membranes and to predict their separation productivity, as well as to consider the fouling propensity of RO at different water pHs [25–27]. Based on the Helmholtz-Smoluchowski equation with the Fairbrother and Mastin approach, zeta potential can be persistent from the measure-

Table 1. Assessments on membrane properties and performances based on different analytical instruments/methods.

Property assessment Instrument/method Property assessment Instrument/method

Separation performance Permeability selectivity Stability test Chlorination

<sup>ζ</sup> <sup>¼</sup> <sup>Δ</sup><sup>E</sup> ΔP

where ΔE is the streaming potential, ΔP is the applied pressure, μ is the solution viscosity, κ is the solution conductivity, and ε and ε ∘ are the permittivity of the test solution and free space, respectively. Several assumptions are inherent in this equation. They are (1) flow is laminar, (2) surface conductivity has no effect and has homogeneous properties, (3) width of the flow channel is much larger than the thickness of the electric double layer, and (4) no axial concen-

The surface zeta potential of ZTS, PAm, and PAm-ZTS, as pH-Responsive membranes,

charge shows that the isoelectric point of ZTS, PAm, and PAm-ZTS was about 4.01, 5.7, and 7.6, respectively; throughout membrane testing, an electric potential is induced when cations and anions enclosed by the electrical double layer are forced to migrate along with the flow tangential to the ROM surface; in consequence, a potential difference could be initiated. Mostly, the streaming potential of the membrane surface is being measured. The typical pH range applied for determining surface zeta potential of a ROF membrane used to fall within pH 2–12, more preferably, between pH 3 and 9. The pH of the background electrolyte (5 mM KCl, 25�C) can be adjusted by the addition of either an acid, 0.1 M HCl (or HNO3), or a suitable base electrolyte, 0.1 M NaOH (or KOH) solution. Owing to the workable irreversible change of membrane surface characteristics, it is highly urged to

C, are shown in Figure 3. Taking a horizontal section at the zero point of

μk

εε <sup>∘</sup> (1)

Physical properties SEM/FESEM TEM

PAS

Solvent Thermal Filtration

Atomic force microscopy (AFM) Contact angle analysis

Figure 3. Surface zeta potential as a function of pH for pH-responsive membranes made of ZTS, PAm-ZTS, and Pam, measured at 25C.

conduct this investigation using two identical freshly prepared ROMs, that is, one for acid titration (pH 6 down to pH 2) followed by another identical membrane for alkali titration (pH 6 up to pH 12).

PAm-ZTS RO membranes tended to have more positive charge owing to the protonation of the amine functional groups. In contrast, the negative charge of RO membranes at higher pHs can be attributed to the loss of functional groups [28–30]. Deprotonation of amine functional groups coupled with either dissociation of the carboxylic acid group or sulfonic acid group on the membrane surface may occur. In brief, in the membranes with organic origin, PAm is more negatively charged than that of that made up of ZTS and PAm-ZTS till pH 7 [25, 31, 32]. Besides showing the positive and negative charge values of a membrane, zeta potential profile can also reveal the isoelectric point (IEP) of the RO membrane at which the membrane surface carries no net electrical charge (i.e., neutral).

Depending on the functional groups of RO surface, a highly positively charged RO membrane could also be prepared, in which this membrane displays a positive zeta potential over a wide range of pH values (pH 2–11). The phenomenon is mainly due to the presence of pendant tertiary amine groups in some polymers used to fabricate the membranes. It was also reported to cause the membrane to be positively charged for pH ranging from 3 to 9 [33, 34]. A summary of the surface zeta potential of some RO membranes made of different monomers at two different pH environments is presented in Table 2 [25–27].


aAEPPS—N-aminoethyl piperazine propane sulfonate, MPD—m-phenylenediamine, mm-BTEC—3, 3<sup>0</sup> , 5, 5<sup>0</sup> -biphenyl tetraacyl chloride, MWCNT—multi-walled carbon nanotube, GO—graphene oxide, PES–TA—poly (arylene ether sulfone) with pendant tertiary groups, PIP—piperazine, PVAm—polyvinylamine, and TMC—trimesoyl chloride. bThese NF membranes are positively charged over a wide pH range.

Table 2. Summary of the surface zeta potential of some NF membranes at different pH environments.

It should be noted here that besides surface zeta potential measurement, the conventional titration method can also be employed to evaluate the ion exchange capacity of the RO membrane. Any change in the membrane ion exchange capacity can be related to the amount of charged groups that exist on a membrane.

#### 5.2. Surface topography of PAm-ZTS pH-responsive membranes

In white-LED illumination focused by AFM, as shown in Figures 4a and 5a, the surface topography of the prepared PAm-ZTS was different as the pH of the treatment was switched from three to eight. Figures 4b and 5b explain the three-dimensional image of the pHresponsive membranes. The surface roughness was depicted by the histograms in Figures 4c and 5c, with a broad distribution from less than 50 nm to more than 290 nm, and has a median value of roughly 130 nm in the case of PAm-ZTS treated at pH = 3, while PAm-ZTS treated at pH = 8 has a spread-out distribution between about 20 and 300 nm with an average value of circa 115 nm.

The dissection of Figures 4a–c and 5a–c illuminates the photomicrograph of the cross section in the compact layer morphology of dry/wet phase inversion shear to cast PAm-ZTS asymmetric membrane, in a strained convection dwelling time for 15 s, at pHs 3 and 8, separately. This microstructure had the relatively fit dense skin layer with inconspicuous flaws backed on a highly open nanoporous sublayer containing not only nanovoids but also micro-voids. These

Figure 4. Surface topography of PAm-ZTS as pH-responsive membrane, measured at 25C after treatment at pH 3.

Recent Drifts in pH-Sensitive Reverse Osmosis http://dx.doi.org/10.5772/intechopen.75897 9

It should be noted here that besides surface zeta potential measurement, the conventional titration method can also be employed to evaluate the ion exchange capacity of the RO membrane. Any change in the membrane ion exchange capacity can be related to the amount

tetraacyl chloride, MWCNT—multi-walled carbon nanotube, GO—graphene oxide, PES–TA—poly (arylene ether sulfone)

, 5, 5<sup>0</sup>


aAEPPS—N-aminoethyl piperazine propane sulfonate, MPD—m-phenylenediamine, mm-BTEC—3, 3<sup>0</sup>

with pendant tertiary groups, PIP—piperazine, PVAm—polyvinylamine, and TMC—trimesoyl chloride.

Table 2. Summary of the surface zeta potential of some NF membranes at different pH environments.

Type of NF Membranea IEP (pH) ζ (mV) at pH 3 ζ (mV) at pH 9

MPF-34 (Koch Membrane Systems) 4.5 ~13 ~ –34 Desal-5DK (GE Osmonic) 3.9 ~18 ~ –50 NF 270 (DOW FILMTEC) 3.2 ~5 ~ –75 BW30 (DOW FILMTEC) 3.6 ~2 ~ –10 NF90 (DOW FILMTEC) 4.2 ~14 ~ –24 PIP–TMC–MWCNT NF membrane 2.6 ~ –1.2 ~ –7 MPD–TMC NF membrane 6.0 ~28 ~ –11 PIP–TMC–GO NF membrane 5.4 ~25 ~ –32 PVAm–TMC NF membrane 6.5 ~19 ~ –12 AEPPS–PIP–TMC NF membrane 4.1 ~1.3 ~5.6 PES–TA NF membrane<sup>b</sup> 10.7 ~32 ~6 PIP–mm-BTEC NF membrane<sup>b</sup> – ~28 ~4 PEG600–NH2–TMC NF membrane<sup>b</sup> ~8.9 ~19 0

In white-LED illumination focused by AFM, as shown in Figures 4a and 5a, the surface topography of the prepared PAm-ZTS was different as the pH of the treatment was switched from three to eight. Figures 4b and 5b explain the three-dimensional image of the pHresponsive membranes. The surface roughness was depicted by the histograms in Figures 4c and 5c, with a broad distribution from less than 50 nm to more than 290 nm, and has a median value of roughly 130 nm in the case of PAm-ZTS treated at pH = 3, while PAm-ZTS treated at pH = 8 has a spread-out distribution between about 20 and 300 nm with an average value of

The dissection of Figures 4a–c and 5a–c illuminates the photomicrograph of the cross section in the compact layer morphology of dry/wet phase inversion shear to cast PAm-ZTS asymmetric membrane, in a strained convection dwelling time for 15 s, at pHs 3 and 8, separately. This microstructure had the relatively fit dense skin layer with inconspicuous flaws backed on a highly open nanoporous sublayer containing not only nanovoids but also micro-voids. These

of charged groups that exist on a membrane.

bThese NF membranes are positively charged over a wide pH range.

circa 115 nm.

8 Wastewater and Water Quality

5.2. Surface topography of PAm-ZTS pH-responsive membranes

Figure 4. Surface topography of PAm-ZTS as pH-responsive membrane, measured at 25C after treatment at pH 3.

were truly similar to those found in the aqueous quenched asymmetric ROMs [35–37]. The nanovoids did not span the width of the ROM evoking that these nanovoids are provoked by disparate mechanisms. In this case, the creation nanopores were formulated by intrusion of non-solvent through defects in the surface layer during wet phase separation, in a step for membrane reinforcement. Additionally, no surface pores could be observed on the outer surface of RO membrane, even at 5000X magnifications 5000X magnifications. This indicated that the diameters of any surface pores were at least less than 20 A, which would be helpful to

Recent Drifts in pH-Sensitive Reverse Osmosis http://dx.doi.org/10.5772/intechopen.75897 11

The natural water resources contain solids in two forms, suspended and dissolved [16, 38, 39]. Suspended solid-state matters exist in insoluble particulates, debris, seawater microorganisms, silt, or colloids. Dissolved matters are present as ions, preferably as chloride, sodium, calcium, or magnesium. Principally, all desalination plants incorporate two-key treatment steps, sequentially

The first step of pretreatment removes the suspended solids from water resources or the naturally occurring soluble solids that may turn into a solid form and precipitates on the ROMs during separation processes. The second step of the RO system separates the dissolved solids from the pretreated saline source water, thereby producing fresh low-salinity water convenient for human utilization agricultural purposes and industrial implementations.

Subsequent pretreatment is designed for the left solids in the source stream; it includes the dissolved minerals. As long as the desalination system is operated in a manner that prevents these minerals from precipitating on the membrane surface, the ROMs could operate and produce freshwater of persistent nature at a high rate deprived of the need to clean these

Notwithstanding pretreatment systems remove most but not all the insoluble solids contained in the saline source water and may not always effectively protect some of the soluble solids from precipitating on the membrane surface, the suspended solids, silt, and natural organic matter (NOM) that remained which may accumulate on ROM surface causing the loss of membrane productivity. In inclusion, saline water contains microorganisms as well as dissolved organics that could serve as food for these microorganisms. Consequently, a biofilm could form and grow on the ROM surface, causing loss of mem-

The protocol of reduction/loss of productivity of ROMs due to agglomeration of suspended solids and NOM, precipitation of dissolved solids, and/or establishment of biofilm on the ROMs surface is known as membrane fouling (MF). Excessive MF is undesirable since it has a negative impact on ROM productivity; it could also result in an increased consumption of

energy for salt separation and in deterioration of product water quality.

be applied for reverse osmosis separation rather than ultrafiltration or nanofiltration.

6. Modeling of pressure-driven membranes

ROMs for long periods.

brane productivity as well.

designed to remove suspended and dissolved matters from their sources.

Figure 5. Surface topography of PAm-ZTS as pH-responsive membrane, measured at 25C after treatment at pH 8.

were truly similar to those found in the aqueous quenched asymmetric ROMs [35–37]. The nanovoids did not span the width of the ROM evoking that these nanovoids are provoked by disparate mechanisms. In this case, the creation nanopores were formulated by intrusion of non-solvent through defects in the surface layer during wet phase separation, in a step for membrane reinforcement. Additionally, no surface pores could be observed on the outer surface of RO membrane, even at 5000X magnifications 5000X magnifications. This indicated that the diameters of any surface pores were at least less than 20 A, which would be helpful to be applied for reverse osmosis separation rather than ultrafiltration or nanofiltration.
