Nanoporous pvdf based membranes for lonic dye removal from aqueous solutions and filtration machanisms therein

Owing to the additive effects of GO and PVP, the PPG membrane achieved outstanding high permeance 953.0 – 1353.0 L m-2 h-1 MPa-1 and excellent dye removal efficiencies 83.2 – 91.4%, whic

長庚大學化工與材料工程學系

博士論文 Department of Chemical and Materials Engineering

Chang Gung University Doctoral Dissertation

具奈米微孔的聚偏二氟乙烯及其複合薄膜用於離子性染料的

去除及其分離機制的探討

Nanoporous PVDF-based Membranes for Ionic Dye Removal

from Aqueous Solutions and Filtration Mechanisms Therein

指導教授：呂幸江 博士 Advisor: Shing-jiang Jessie Lue, Ph.D

研究生：鄭氏德芳 Graduate Student: Tran Thi Tuong Van

中華民國 108 年 12 月

December 2019

Special thanks go to Professors of Chemical and Materials Engineering Department who provided me plenty of precious knowledge and warm encouragement I am very appreciative to nice and helpful officers at every section in Chang Gung University

Sincere thanks go to willing generations of labmates in Microcontamination & Membrane Separation Laboratory who enthusiastically helped me in lab works and many other things In particular, deep thanks come to Doctor S Rajesh Kumar who supplied me invaluable advices during my writing process I am so thankful to my Taiwanese and international friends for sharing with and supporting me during classes, experiments and life Thank you all for turning

my time in CGU into unforgettable memories ever

Sincere gratitude goes to my managers and colleagues at Institute of Environmental Science, Engineering and Management, Industrial University of Ho Chi Minh City in Vietnam who facilitated my Ph.D study and sent me many kind encouragements

Last but not least, I would like to wholeheartedly thank my dear family and parents for endless love and support This thesis is dedicated to them

摘要

本文探討了各種有機染料在聚偏氟乙烯（PVDF）超濾膜上的過濾行為，發現 Donnan 排斥機制在染料分離中為主要分離機制。PVDF 膜對兩性離子若丹明 B（RhB，1.1%）的截留率極低，而對陽離子亞甲基藍（MB，45.6%）的截留 率中等；但對陰離子染料，鉻黑 T（EBT，83.5%）和萘酚藍的截留率高。黑 色（NBB，89.1%）。兩性離子或相同電荷的混合染料溶液（RhB-MB，RhB- NBB 和 EBT-NBB）顯示出的染料去除率與其成分的單一染料溶液相似。相 反，由於分子間的相互作用，帶相反電荷的染料（MB-NBB）的混合溶液形 成微米級的聚集體，導致染料幾乎完全被去除、並提高了滲透率。在多次循 環過濾過程中證明了膜過濾性能的穩定性。接下來，本文研究了氧化石墨烯

（GO）和聚乙烯基吡咯烷酮（PVP）添加劑在 PVDF 混成膜的個別和加成作 用。結果顯示 PVP 的主要作用是增強膜的孔隙率，而 GO 的顯著貢獻是降低 了膜表面電荷。含有 GO 和 PVP 的 PVDF 膜在膜孔隙率，水接觸角和 Zeta 電 位特性方面表現出累加效應，並使 PVDF 的結晶型態由轉變至較親水的 型。在 pH 值為 4-10 的純水和染料溶液（NBB 和 MB-NBB）中，濾液的滲透 率從純 PVDF（P），GO 混合 PVDF（PG），PVP 混合 PVDF（PP）和 GO- PVP 混合 PVDF（PPG）膜逐漸增加。其中，GO 和 PVP 的引入分別使滲透率 增加了幾倍和幾十倍。由於 Donnan 排斥機理，所有膜均表現出高於 80%的染 料排斥率，與微弱的染料吸附能力。由於 GO 和 PVP 的累加作用，PPG 膜具 有出色的高滲透性（953.0–1353.0 L m-2 h-1 MPa-1）和傑出的脫色率（83.2– 91.4%），本研究證實這種複合 PVDF 膜在實際染料廢水處理應用中開了一條 可行的路。

關鍵字：離子染料；聚偏二氟乙烯；氧化石墨烯；聚乙烯吡咯烷酮；Donnan 排除；分子間相互作用；加乘作用；膜通透性

Abstract

In this thesis, filtration behaviors of various organic dyes across a poly(vinylidene fluoride) (PVDF) ultrafiltration membrane were explored The Donnan exclusion mechanism played a major role in dye separation The PVDF membrane had extremely low rejection for zwitterionic rhodamine B (RhB, 1.1%) and medium rejection for cationic methylene blue (MB, 45.6%), but high rejections for anionic dyes, eriochrome black T (EBT, 83.5%) and naphthol blue black (NBB, 89.1%) The mixed zwitterionic or same-charged dye solutions (RhB-MB, RhB-NBB and EBT-NBB) exhibited dye removal rates similar to their constituents’ single dye solutions Conversely, the mixed solution of opposite-charged dyes (MB-NBB) formed micron-sized aggregates due to intermolecular interaction, leading to almost complete dye removal and enhanced permeance value The stable membrane filtration performance was demonstrated during multi-cycle filtration Next, individual and simultaneous effects of graphene oxide (GO) and poly(vinyl pyrrolidone) (PVP) additives on PVDF based membranes were investigated It was demonstrated that the principal role of PVP was to enhance membrane porosity and -to- phase transformation The noticeable contribution of GO was to lower membrane surface charge PVDF membrane containing GO and PVP showed additive effects in membrane porosity, polar crystallization, water contact angle and zeta potential properties In both pure water and dye solutions (NBB and MB-NBB) at pHs 4-10, the filtrate permeance increased from the pure PVDF (P) to GO blended PVDF (PG), PVP blended PVDF (PP) and GO-PVP blended PVDF (PPG) membranes Therein, GO and PVP introduction provided permeance increases of several times and several tens of times, respectively All membranes exhibited high dye rejections of  80.0% due to the Donnan exclusion mechanism The membranes showed minimal dye adsorption capacities Owing to the additive effects of GO and PVP, the PPG membrane achieved outstanding high permeance (953.0 – 1353.0 L m-2 h-1 MPa-1) and excellent dye removal efficiencies (83.2 – 91.4%), which opened an avenue for application of this composite PVDF membrane to actual dye wastewater treatment

Keywords: Ionic dyes; Poly(vinylidene fluoride); Graphene oxide; Poly(vinyl pyrrolidone);

Donnan exclusion; Intermolecular interaction; Additive effect; Membrane permeance

Table of Contents

Recommendation Letter from the Thesis Advisor

Dissertation Oral Defense Committee Certification

Acknowledgements iii

摘要 iv

Abstract v

Table of Contents vi

List of Figures ix

List of Tables xiii

Chapter 1 Literature Review 1

1.1 Dye removal from wastewater 1

1.1.1 Dye structure and classification 1

1.1.2 Wastewater containing dyes 2

1.1.3 Treatment of dye wastewater 4

1.2 Membrane processes 11

1.2.1 Introduction to membrane processes 11

1.2.2 Membrane modifications 13

1.2.3 Pressure-driven membrane processes for dye removal 16

1.2.4 PVDF membranes for dye removal and their modifications 18

1.3 Rationale of this research 20

1.4 Objectives and scopes of this research 22

Chapter 2 Materials and Methods 25

2.1 Materials 25

2.1.1 Commercial materials 25

2.1.2 GO preparation 28

2.2 Membrane preparation 28

2.3 Characterization 29

2.3.1 GO and PVP characterization 29

2.3.2 Membrane characterization 30

2.3.3 Dye characterization and dye concentration evaluation 31

2.4 Membrane performance experiment 32

2.4.1 Membrane filtration system 32

2.4.2 Filtration performance evaluation 34

2.4.3 Static adsorption 34

2.4.4 Multi-cycle filtration 35

Chapter 3 Filtration Mechanisms of Binary Dye Mixtures Using a PVDF Membrane 36

3.1 Membrane characterization 36

3.2 Membrane filtration performance for single dyes 39

3.3 Membrane filtration performance for binary dye mixtures 43

3.3.1 RhB-MB mixture 43

3.3.2 EBT-NBB mixture 46

3.3.3 RhB-NBB mixture 46

3.3.4 MB-NBB mixture 47

3.4 Membrane adsorption performance 50

3.5 Stability of membrane performance 50

Chapter 4 GO and/or PVP Blended PVDF Membranes for Enhanced Dye Filtration 55

4.1 GO and PVP characteristics 55

4.2 Membrane morphology and pore size distribution 58

4.3 Membrane chemical composition 65

4.4 Membrane hydrophilicity and surface charge 72

4.5 Membrane filtration performance 73

4.5.1 Pure water 73

4.5.2 NBB dye 76

4.5.3 MB-NBB dye mixture 83

4.6 Membrane adsorption performance 84

Chapter 5 Conclusions and Future Recommendations 89

5.1 Conclusions 89

5.2 Future recommendations 90

References 92

Appendix A Dye concentration evaluation 112

Appendix B Curriculum Vitae 116

List of Figures

Fig 1-1 Schematic representation of (a) a membrane process and (b) a two-phase system

separated by a membrane [59] 11

Fig 1-2 Fabrication of composite membranes through phase inversion process and main effects

of nanomaterials on final products [62] 16

Fig 1-3 Pressure-driven membrane processes classified by pore size, target species and

operating pressure [61] 17

Fig 1- 4 Investigated membrane and dye subjects in this thesis 24 Fig 2-1 (a) Photograph and (b) schematic diagram of the membrane filtration unit 33 Fig 3-1 FESEM micrographs of the PVDF membrane showing (a, c) differently magnified top

surfaces, (b, d, e) overall, enlarged upper and lower cross-sections 37

Fig 3-2 (a) Pore size distribution, (b) FTIR spectrum, (c) surface zeta potential over a pH range

and (d) water contact angles at different drop ages of the PVDF membrane 39

Fig 3-3 (a) Dye rejection, TOC rejection and (b) permeance performance of the PVDF

membrane with single dyes 39

Fig 3-4 Dye rejection, TOC rejection and permeance performance of the PVDF membrane

with binary dye mixtures of (a) RhB-MB, (b) EBT-NBB, (c) RhB-NBB and (d) MB-NBB 44

Fig 3-5 Absorbance spectra of feed and permeate after 120 min of filtration with binary dye

mixtures of (a) RhB-MB, (b) EBT-NBB, (c) RhB-NBB and (d) MB-NBB 45

Fig 3-6 Proposed separation mechanism for MB-NBB mixture on the PVDF membrane 49 Fig 3-7 Photographs of the pristine and adsorbed PVDF membranes with dyes 51 Fig 3-8 (a) Dye rejection and (b) permeance performance in multi-cycle filtration process for

single dyes on the PVDF membrane 52

Fig 3-9 Dye rejection and permeance performance in multi-cycle filtration process for binary

dye mixtures on the PVDF membrane: (a) RhB-MB, (b) EBT-NBB, (c) RhB-NBB and (d) NBB 52

MB-Fig 3-10 FESEM micrographs of the (a) used and (b) cleaned PVDF membranes after

multi-cycle filtration with RhB-MB mixture showing (i) top surfaces and (ii) cross-sections 54

Fig 4-1 (a) FESEM and (b) TEM images, (c) Raman and (d) XRD spectra of GO 56 Fig 4-2 (a) Full XPS spectra of GO and PVP, (b-c) C1s XPS spectra of GO and PVP, (d) FTIR

spectra of GO and PVP, and (e) zeta potential versus pH profiles of GO and PVP 20 mg L-1

solutions (in DI water) 58

Fig 4-3 (a) Top surface photographs of the prepared membranes Top and cross-section FESEM

images of the (b, c) P, (d, e) PG, (f, g) PP and (h, i) PPG membranes 61

Fig 4-4 Pore size distribution histograms from top surface FESEM image analysis using

ImageJ software of the (a) P, (b) PG, (c) PP and (d) PPG membranes 62

Fig 4-5 Pore size distribution histograms from CFP analysis of the (a) P, (b) PG, (c) PP and (d)

PPG membranes 64

Fig 4-6 (a) Full XPS spectra, (b) chemical composition and (c, d, e, f) deconvoluted C1s XPS

spectra of the prepared membranes (P membrane(c), PG(d), PP(e) and PPG(f)) 67

Fig 4-7 (a) FTIR transmittance spectra of PVDF-based films and (b-d) ATR-FTIR spectra of

composite membranes on nonwoven support of the prepared membranes 68

Fig 4-8 FESEM-EDX elemental mapping in (a) top surface and (b) cross-section of the

Fig 4-12 Zeta potential versus pH profile of NBB 20 mg L-1 solution 78

Fig 4-13 Robeson plot showing NBB filtration performance of the prepared membranes in

comparison with other membranes reported in literature [95-97] 79

Fig 4-14 Filtrate permeance and dye rejection performance of the prepared membranes with

MB-NBB mixture 84

Fig 4-15 Photographs of the adsorbed membranes with NBB 88 Fig 4-16 FTIR-ATR spectra of the membranes (a) before and (b) after NBB adsorption 88 Fig A-1 Absorbance spectra of 10 mg L-1 single dye solutions and corresponding λmax of (a) RhB, (b) MB, (c) EBT and (d) NBB 112

Fig A-2 Calibration curves of (a) RhB at λmax = 553 nm, (b) MB at λmax = 654 nm, (c) EBT

at λmax = 534 nm and (d) NBB at λmax = 618 nm 113

Fig A-3 (a) Absorbance spectra and (b) 1st derivative spectra of the RhB solution (5 mg L-1),

MB solution (5 mg L-1) and RhB-MB mixture (total concentration of 10 mg L-1) 113

Fig A-4 (a) Absorbance spectra and (b) 1st derivative spectra of the EBT solution (5 mg L-1), NBB solution (5 mg L-1) and EBT-NBB mixture (total concentration of 10 mg L-1) 113

Fig A-5 (a) Absorbance spectra and (b) 1st derivative spectra of the RhB solution (5 mg L-1), NBB solution (5 mg L-1) and RhB-NBB mixture (total concentration of 10 mg L-1) 114

Fig A-6 (a) Absorbance spectra and (b) 1st derivative spectra of the MB solution (5 mg L-1), NBB solution (5 mg L-1) and MB-NBB mixture (total concentration of 10 mg L-1) 114

Fig A-7 Calibration curves of (a) RhB (based on 1st derivative values at wavelength 505 nm)

and (b) MB (based on 1st derivative values at wavelength 682 nm) in its binary mixture 114

Fig A-8 Calibration curves of (a) EBT (based on 1st derivative values at wavelength 474 nm)

and (b) NBB (based on 1st derivative values at wavelength 537 nm) in its binary mixture 115

Fig A-9 Calibration curves of (a) RhB (based on 1st derivative values at wavelength 474 nm)

and (b) NBB (based on 1st derivative values at wavelength 645 nm) in its binary mixture 115

Fig A-10 Calibration curves of (a) MB (based on absorbance values at wavelength 695 nm)

and (b) NBB (based on absorbance values at wavelength 419 nm) in its binary mixture 115

List of Tables

Table 1-1 Characteristics of typical untreated textile wastewater [14] 3

Table 1-2 Dye removal performance using different treatment processes 7

Table 2-1 Characteristics of experimental polymers and solvent 26

Table 2-2 Characteristics of experimental dyes 27

Table 2-3 Casting solution components of the prepared membranes, g (mass ratio in wt.%) 29 Table 3-1 pH and zeta potential of feeds with a total dye concentration of 20 mg L-1, and PVDF membrane surface zeta potential working with the corresponding feeds 41

Table 3-2 Dye adsorption capacity of the PVDF membrane (after 24 h) 51

Table 4-1 Structural parameters of the prepared membranes 63

Table 4-2 Elemental composition based on EDX analysis of the prepared membranes 69

Table 4-3 Water permeance values of the prepared membranes obtained from physical equations and experimental data, L m-2 h-1 MPa-1 75

Table 4-4 Comparison on membrane characteristics and NBB filtration performance between the prepared membranes and other membranes reported in literature 80

Table 4-5 Dye concentration (mg L-1) remaining in the NBB solution (1) over adsorption time 85

Table 4-6 Comparison on dye adsorption capacity between the prepared membranes and other membranes reported in literature 86

Chapter 1 Literature Review

1.1 Dye removal from wastewater

1.1.1 Dye structure and classification

Dyes, also known as colorants, are highly colored substances and can be used to impart color to an infinite variety of materials described technically as substrates [1] Preparation and usage of dyestuff are one of the oldest forms of human activities The usage of natural dyestuff dates back to the bronze age The dyes were obtained from natural sources such as animal, vegetable or mineral origin with no or little processing The first synthetic organic dye (mauveine, in purplish lilac color) was discovered by William Henry Perkin in 1856 [2] In the early 20th century, synthetic dyes had almost completely replaced natural dyes [1] Color provides a vital enhancement to the world in which we live, with approximately 20% of all known chemical compounds being colored, amounting to over 1 million colorants [3] Synthetic colors are used in everyday materials, including cloths, paints, plastics, photographs, cosmetics, pharmaceutical, food and ceramics, on television, film and in a broad range of multicolored printed materials such as posters, magazines and newspapers [1] The routine use of colors in day-to-day life is increasing due to rapid industrialization, most widely in textile, rubber, enamel, plastic, cosmetic, pharmaceutical, home and personal care and many other industries [1, 3] Dyes contain chromophores (delocalized electron systems with conjugated double bonds) and auxochromes (electron-withdrawing or electron-donating substituents) [1, 4] The chromophore, usually C=C, C=N, C=O, N=N, NO2 and quinoid rings, imparts color to the dye molecule by absorbing radiation in the visible part of the electromagnetic spectrum [1, 4] The auxochrome, such as NH3 (NH2, NHR, NHR2), COOH, SO3H, SR and OH (OR), intensifies the chromophore color by altering overall energy of electron system [1] A dye may have multiple chemical groups Chemical structures determine colors, properties and uses of dyes, and provide the rational basis of a classification of these compounds [4]

Dyes are usually classified in two separate ways, either according to dye structure or according to how they are used in the dyeing process Hence, a dye belongs to more than one chemical class In the classification according to chemical structure, a number of chemical classes of dyes are reported in Color Index, developed by the Society of Dyers and Colorists [1, 4] Dyes are also classified according to how they are used in the dyeing process in textile industries, including acid dyes, basic dyes, direct dyes, disperse dyes, vat dyes, reactive dyes, mordant dyes, azoic dyeing and sulfur dyes [1]

1.1.2 Wastewater containing dyes

Currently, there are more than 100,000 commercial synthetic dyes [5] Over 7×105 tons of dyestuffs are manufactured and consumed annually in the world [6] Approximately 10-20% of the dye usage is released into water sources as untreated dye effluents [7, 8] The presence of colorant substances in the water bodies impedes light transmission, thus adversely influencing the photosynthesis process of aquatic flora [9] There is a fact that many dyes are visible in water

at even very low (1 mg L-1) concentrations [1] Moreover, the majority of dye compounds are toxic and nonbiodegradable due to stable aromatic structures, accounting for their toxic effects

on the food chain [10-12] It has been reported that some dyes induce allergy, cancer, and damage of the kidney, liver, digestive, reproductive and central nervous systems in human beings [7, 9, 10] Hence, dye removal from wastewater is required prior to discharging into receiving waters in developed and developing countries

Dye production and application creates large quantities of dye wastewater The effluents from dyeing industries are difficult to treat due to their high biological oxygen demand (BOD), chemical oxygen demand (COD), color, turbidity, pH and presence of metals, salts and residual chemicals [1, 12, 13] Table 1-1 shows typical characteristics of the untreated textile effluents

Table 1-1 Characteristics of typical untreated textile wastewater [14]

Color (Pt-Co) 50-2,500 Dyes (mg L-1) 10-250 BOD (mg L-1) 80-6,000 COD (mg L-1) 150-12,000 Total Kjeldahl nitrogen (mg L-1) 70-80 Oil & grease (mg L-1) 10-30 Silica (mg L-1) < 15 Total suspended solids (mg L-1) 15-8,000 Total dissolved solids (mg L-1) 2,900-3,100

1.1.3 Treatment of dye wastewater

Several primary, secondary and tertiary treatment processes with various physical, chemical and biological techniques have been used to treat the dye-containing wastewaters [14-16] Primary treatment aims at the removal of insoluble organic and inorganic solids by screening, sedimentation, flotation, coagulation and flocculation Secondary treatment involves degradation and removal of biodegradable, dissolved and colloidal organic matter using biological oxidation (e.g aerobic and anaerobic approaches) and/or chemical oxidation (e.g chemical oxidizing agents, advanced oxidation processes) There are several technologies used

in tertiary treatment including photocatalytic degradation, adsorption, ion exchange and membrane processes, etc

The first step in dye wastewater treatment is the removal of coarse suspended solids, gritty materials, oils and greases using bar and fine screens [17] Screening is followed by gravity sedimentation to settle the suspended particles such as clay or silts [14] The floating particles are removed using mechanical scraping systems [14] Coagulation process uses some quickly dispersed chemicals to change the characteristics of the colloidal and suspended particles in dye wastewater so that they can coalesce to form flocs and then sink down [16] The chemicals to promote flocculation and rapid settling include aluminum sulfate, aluminum sulfate/sulfuric acid, ferric chloride/hydrochloric acid, ferrous sulfate/sulfuric acid and lime, etc [14, 16] By applying the coagulation-flocculation technique, COD and color removal of different dyes lies

in the range of 19.0-95.4% and 22.6-98.6%, respectively [18]

Biological oxidation processes are carried out with the help of microorganisms under aerobic or anaerobic conditions Aerobic bacteria use organic matters as a source of energy and nutrients They oxidize dissolved organic matters into CO2 and water and degrade nitrogenous organic matters into ammonia Anaerobic bacteria break down high organic loads into biomass,

CH4 and CO2 in the absence of air Many bacteria are employed in dye decolorization, such as

Brevibacillus sp., Klebsiella pneumoniae, Acetobacter liquefaciens and Pseudomonas

desmolyticum [16] Microorganisms can interact with dyes in many ways, including dye binding

to the cell surface by ionic, hydrophobic and covalent interactions, dye transport into the cell, interactions with enzymes that change the redox characteristics of dyes, interactions with enzymes that decompose dyes and transport of altered dyes (redox-altered or decomposition products) out of cells [16] Aerated lagoons, trickling filters and activated sludge systems are aerobic systems commonly used in secondary treatment BOD removal and phosphorus removal efficiencies for aerated lagoons are up to 99% and 15-25%, respectively [14] Aerobic activated sludge process can obtain a BOD removal efficiency of 90-95%, however, this process is time-consuming [19] Anaerobic systems usually are up-flow anaerobic filters, up-flow anaerobic sludge blankets and anaerobic fluidized/fixed-bed reactors [20] Dye decolorization under anaerobic conditions ranges broadly from 0% to 99% depending on dye structure and operating conditions [20]

Advanced oxidation processes have a powerful ability to treat almost all solid components

in textile effluents The basic mechanism is the production of OH radicals capable of destroying hard oxidized components [14] OH radicals are considered as reactive electrophiles due to their preference to electrons and hence they react quickly towards electron-rich organic compounds [21] Generation of OH radicals are generally accelerated by the combinations of

H2O2, UV, O3, TiO2, Fe2+, electron beam irradiation and ultrasound [22] In an H2O2/UV system [23], total organic carbon and decolorization efficiencies after 5 h can achieve 100% and 85%, respectively With sufficient O3 dosage, COD and color removal for various dyes vary in the range of 34.1-100% and 11.5-100%, respectively [18]

Photocatalytic degradation method can decolorize and mineralize a wide range of dyes depending on their molecular structures Some common photocatalysts are TiO2, ZnO and their composites [14, 24, 25] The first step of photocatalytic degradation of the aromatic compounds results in the formation of hydroxyl derivatives, which in turn modify the product distribution

in the solution [14] Photocatalytic degradation is carried out by two types of oxidizing species:

the hydroxyl radicals and the positive holes [14] The dye decolorization can achieve > 99% after 180-min UV exposure in the photocatalytic process [24]

Adsorption can change or destroy the dye chromophore to remove color effectively [26] Some common absorbents are clay minerals (kaolinite, montmorillonite) and activated carbon (in a powdered, granular or extruded form) [16] Adsorption efficiency depends on the type of adsorbents and dyes For instance, activated carbon provides excellent color removal rates (> 90%) for cationic, mordant and acid dyes; meanwhile, moderate efficiency (> 40%) for direct, sulfur, dispersed and reactive dyes, and low removal ability (< 20%) for vat dyes [18]

Ion exchange method involves the passage of effluents through the beds of ion exchange resins which are either cationic or anionic charged [27] The effluent passing through a cationic resin has its cations removed by the resin and substituted with hydrogen ions making it acidic [27] When the acid solution is passed through anion resin, the anions would be replaced with hydroxyl ions [27]

However, these methods suffer from certain serious disadvantages which need future improvements Physical and chemical treatment processes, such as coagulation, adsorption, chemical transformation, lead to the formation of toxic sludge and even more toxic metabolites [1, 12] Advanced oxidation processes, e.g ozonation and chlorination, are not economically attractive due to oxidant demand [12] Physicochemical treatment methods also require large space [14] Limitations of biological treatment processes are microorganism growth adversely affected by the presence of toxic constituents in the effluent, non-biodegradable nature of most

of the dyes used and long treating time required [12, 14]

Membrane processes occupy the center stage in tertiary treatment [15] and will be introduced in detail in the next section The dye removal performance using different treatment processes from the literature are presented in Table 1-2

Table 1-2 Dye removal performance using different treatment processes

(in mg L -1 )

Dye removal performance

Aerated stirred tank with activated

sludge containing Bacillus cereus M1

16, HRT = 72 h

Anaerobic methane-based hollow fiber

membrane bioreactor, HRT = 0.5-2 day

Methyl orange (400-800) 88-100 % [37]

Process and material Dye

(in mg L -1 )

Dye removal performance

[46]

Process and material Dye

(in mg L -1 )

Dye removal performance

TDPA-KCl (80 mg / 50 mL) Reactive blue 49 119.10 mg g−1 [48]

Co3O4 nanoparticle (0.5 g / 50 mL) Methyl orange (50) 46.08 mg g−1 [49]

Acid black dye 194 (300) 147.27 mg g-1 [53]

P81 and ICE 450 supported cation

exchange membranes

Methyl violet 2B (15-30) 84-100% [54]

Process and material Dye

(in mg L -1 )

Dye removal performance

PACl  Polyaluminum chloride

HRT  Hydraulic retention time

WWTP  Wastewater treatment plant

k – Process constant

TDPA-KCl  Thermal decomposition product of alunite-potassium chloride mixture

PES – Polyethersulfone

Ag NPs/ZnO NRs/3DG  Ag nanoparticles/ZnO nanorods/three-dimensional graphene

rGO  Reduced graphene oxide

C-g-PSS  Poly(sodium pstyrenesulfonate) modified ion exchange cellulose monoliths

C-g-APTAC  Poly[(3- acryloylaminopropyl)trime thylammonium chloride] modified ion exchange cellulose monoliths

AM-CPGMA  Amino-functionalized cellulose-poly(glycidyl methacrylate) graft copolymer

MCNC  Modified cellulose nanocrystals

MDA  Melamine-based dendrimer amine

CNF  Carbon nanofibers

1.2 Membrane processes

1.2.1 Introduction to membrane processes

Membrane processes are operations where a membrane interface is employed as selective layer and feed stream is divided into two streams, i.e a retentate or concentrate stream and a permeate stream (Fig 1-1a) [59] If the goal is concentration, the retentate often is the product stream For the aim of purification, both the retentate and the permeate can yield the desired product depending on which contaminants have to be eliminated As the heart of every membrane process, the membrane can be defined as a permselective barrier or interphase between the two phases, feed and permeate (Fig 1-1b) [59] The performance of a given membrane is characterized by two parameters; its selectivity and flow through the membrane The former is generally expressed in the retention or separation factor The latter, usually denoted as the flux or permeation rate, is determined as the volume flowing through the membrane per unit area and time