Pollutants removal from wastewater using nanocellulosic materials

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uef.fi

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THE UNIVERSITY OF EASTERN FINLAND Dissertations in Forestry and Natural Sciences

Dissertations in Forestry and Natural Sciences

DISSERTATIONS | EMMANUEL KAFUI ABU-DANSO | POLLUTANTS REMOVAL FROM WASTEWATER... | No 356

EMMANUEL KAFUI ABU-DANSO

POLLUTANTS REMOVAL FROM WASTEWATER USING NANOCELLULOSIC MATERIALS

PUBLICATIONS OF

THE UNIVERSITY OF EASTERN FINLAND

The presence of traditional and emerging pollutants in water bodies has increased in the recent past. Various research efforts are aimed towards finding an efficient treatment

method to ameliorate this phenomenon.

This thesis presents the results of studies on the use of nanocelluloses extracted from absorbent cotton, and discarded cigarette butts

as adsorbent materials for the removal of selected aquatic pollutants (toxic heavy metal ions, and pharmaceutically active compound).

The synthesized adsorbents were found to be promising and their attributes can be further explored at the pilot-scale to make the studied

materials an integrated part of adsorption- based wastewater treatment technology.

EMMANUEL KAFUI ABU-DANSO

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POLLUTANTS REMOVAL FROM WASTEWATER USING

NANOCELLULOSIC

MATERIALS

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Emmanuel Kafui Abu-Danso

POLLUTANTS REMOVAL FROM WASTEWATER USING

NANOCELLULOSIC MATERIALS

Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences

356

University of Eastern Finland Kuopio

2019

Academic dissertation

To be presented by permission of the Faculty of Science and Forestry for public examination in the Auditorium Sn201 in the Snellmania build- ing of the University of Eastern Finland, Kuopio, on December 10, 2019,

at 12 o’clock

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Grano Oy Jyväskylä, 2017

Editors: Pertti Pasanen, Raine Kortet, Jukka Tuomela, Matti Tedre, Nina Hakulinen

Distribution: University of Eastern Finland / Sales of publications www.uef.fi/kirjasto

ISBN: 978-952-61-3244-0 ISBN: 978-952-61-3245-7 (PDF)

ISSNL: 1798-5668 ISSN: 1798-5668 ISSN: 1798-5676 (PDF)

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Author’s address: Emmanuel Kafui Abu-Danso University of Eastern Finland

Depart. of Environmental and Biological Sciences P.O. Box 1627

FI-70211 KUOPIO, FINLAND email: emmanuel.abu-danso@uef.fi

Supervisors: Professor Amit Bhatnagar, Ph.D.

University of Eastern Finland

FI-70211 KUOPIO, FINLAND email: amit.bhatnagar@uef.fi

Professor Jorma Jokiniemi, Ph.D.

University of Eastern Finland

FI-70211 KUOPIO, FINLAND email: jorma.jokiniemi@uef.fi

Reviewers: Professor Ulla Lassi, Ph.D.

University of Oulu

Research Unit of Sustainable Chemistry P.O. Box 4300

FI-90014 OULU, FINLAND email: ulla.lassi@oulu.fi

Professor Angeles Blanco Suarez, Ph.D.

University Complutentse of Madrid Depart. of Chemical Engineering 280040 MADRID, SPAIN

email: ablanco@ucm.es

Opponent: Professor Paula Marques, Ph.D.

University of Aveiro

Depart. of Mechanical Engineering 3810-193 AVEIRO, PORTUGAL email: paulam@ua.pt

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Abu-Danso, Emmanuel Kafui

Pollutants removal from wastewater using nanocellulosic materials Kuopio: University of Eastern Finland, 2019

Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences 2019; 356 ISBN: 978-952-61-3244-0 (print)

ISSNL: 1798-5668 ISSN: 1798-5668

ISBN: 978-952-61-3245-7 (PDF) ISSN: 1798-5676 (PDF)

ABSTRACT

The presence of traditional and emerging pollutants in water bodies has increased in the recent past due to increases in activities that lead to their release. Detrimental health effects due to the exposure of living organisms to these substances have been well documented. Lead and cadmium have been identified as cancer-causing agents, while environmental health issues due to the presence of diclofenac have also been reported. Various research efforts are aimed towards reducing the presence of these pollutants in the environment. Among these efforts, is the search for an efficient treatment method to replace conventional treatments that have been overwhelmed by the increased levels of both traditional and emerging pollutants. Use of nanotechnology in adsorption-based treatment solutions has been studied and gained wide attention in the past decade, with demonstrated potential as an efficient treatment option.

This thesis presents the results of studies on syntheses, modifications, characterization and use of cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) extracted from absorbent cotton, and discarded cigarette butts as adsorbent materials for the removal of selected aquatic pollutants (toxic heavy metal ions, and pharmaceutically active compound). Chemical treatments viz SC(NH²)² tethering, H³PO⁴ modification, and clay fabrication to obtain clay-cellulose biocomposites were applied to the extracted CNFs. The pollutant removal processes were investigated using laboratory-scale batch, and fixed-bed column adsorption procedures by studying the effects of parameters such as solution pH, adsorbent dose, temperature, ionic strength, flow rate, adsorption kinetics, and isotherms. The evaluation of the stability of the synthesized adsorbents was conducted through regeneration experiments using adsorption-desorption procedures. The adsorbents were also analyzed for their surface morphology, functional groups, crystalline phase, size dimension, surface charge, surface area, pore volume, pore diameter, chemical state, and elemental composition before and after adsorption of the pollutants.

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The syntheses approach to extraction of celluloses from their precursors resulted in negatively charged CNCs and CNFs surfaces, and the desired porous CNFs.

Subsequently, the tethering of the CNFs with sulfur, functioning as an anionic ligand, revealed applicability of CNFs as an efficient adsorbent material for the removal of Pb(II) and Cd(II) from simulated wastewater, as well as simultaneous removal of varied heavy metals from industrial wastewater. Phosphate functionalized CNFs (HPO-CNFs) from discarded cigarette butts using weak phosphoric acid as the phosphate source exhibited potential as an adsorbent for the removal of diclofenac from an aqueous medium. The CNFs fabrication with halloysite clay to synthesize clay-cellulose biocomposite (CCB) exhibited applicability of the adsorbent for the removal of Pb(II) and Cd(II) from an aqueous medium. The adsorption kinetics for all the synthesized adsorbents during the removal of the pollutants were found to be fast and ranged between 0 to ca. 60 min. Different theoretical models used to assess the adsorption kinetics and isotherms gave further insight on the adsorption mechanisms and the adsorbent characteristics.

The maximum adsorption capacity of S-tethered CNFs for Cd(II) and Pb(II) was found to be 92.32 and 239.64 mg g^-1 respectively, while the removal efficiency of the metal ions from industrial wastewater ranged from 90-98%. The maximum adsorption capacity of CCB for Cd(II) and Pb(II) was 115.96 and 389.78 mg g^-1, while the maximum capacity of HPO-CNFs to remove diclofenac was found to be 107.90 mg g⁻¹.

The synthesized nanocellulose-based materials in this thesis study were found to be promising adsorbents due to their large surface area, chelating ability, strong electrical charges, and high adsorption capacity.These attributes can be further explored at the pilot-scale to make the studied materials an integrated part of adsorption-based treatment technology, rendering them a cheap and cost-effective technological alternative for the removal of the designated pollutants from wastewater. The adsorbents can further be modified with other nano sized materials or chemical agents as modified cellulose or cellulose nanocomposite adsorbents to increase their adsorption capacity for aquatic pollutants.

Universal Decimal Classification: 544.723, 547.458.81, 621.039.324, 628.316, 628.349, 661.728

CAB Thesaurus: wastewater; wastewater treatment; polluted water; water pollution;

pollutants; metal ions; heavy metals; lead; cadmium; diclofenac; removal; adsorption;

adsorbents; cellulose; nanomaterials; clay; synthesis; kinetics; sorption isotherms

Yleinen suomalainen ontologia: jätevesi; jäteveden käsittely; saasteet; raskasmetallit; lyijy;

kadmium; lääkkeet; poistaminen; puhdistus; adsorptio; nanoselluloosa; komposiitit; savi

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ACKNOWLEDGEMENTS

I would like to express my special gratitude to Professor Amit Bhatnagar, main supervisor, for his hard work in guiding me throughout my doctoral work. I would like to thank Professor Jorma Jokiniemi, my co-supervisor, for taking time to supervise and evaluating this thesis for its suitability as a doctoral thesis. I would like to acknowledge the financial support from Nesslingin Säätiö, (201600405, 201700276, 201800058), Anja ja Aimo Eerolan säätiö and the University of Eastern Finland (226/2019). I would like to express my profound gratitude to Sirpa Peräniemi for her help throughout the work. I thank Jari Leskinen for the training in characterization techniques. I would like to thank all my coauthors who contributed to the high quality publications in this thesis. I am grateful to Associate Professor James Blande for the critical proof reading of this thesis. Special thanks goes to all my colleagues who in diverse ways helped for me to get to this point. To friends who are still around.

To my son, my friend, my dad, my super boy; Noah Nana Olavi Abu-Danso; your presence in this world has had an effect I could not have imagine. To you Oona, thank you for the love and care. I would like to thank Sewaa, for being there for me, to Aku, Eyram, for the support, to my mother Vivian Obuobi for the life. To you, the late Major (Rtd) Emmanuel Abu-Danso (GH 1459), I wish I could undersatand life better.

Dedicated to super boy: Noah Nana Olavi Abu-Danso.

To the Almighty.

E.K. Abu-Danso Kuopio, 2019.

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LIST OF ABBREVIATIONS

AGU Anhydro glucopyranose unit

Al²Si²O⁵(OH)⁴•nH²O Halloysite clay

BET Brunauer–Emmett–Teller

BJH Barrett-Joyner-Halenda

C Carbon

CCB Clay-cellulose biocomposite

Cd(II) Cadmium ion

CNCs Cellulose nanocrystals

CNFs Cellulose nanofibers

CNFs-D Dewaxed-Cellulose nanofibers

CO(NH2)2 Urea

CTAB Cetyltrimethylammonium bromide

DP Degree of polymerization

EDS Energy dispersive x-ray spectroscopy

EHT Electron high tension

FO Forward osmosis

FTIR Fourier transform infrared spectroscopy

H³PO⁴ Phosphoric acid

H-bond Hydrogen bond

HPO-CNFs Phosphate functionalized CNFs

-O- Glycosidic link

OH Hydroxy group

PACs Pharmaceutically active compounds

Pb(II) Lead ion

PEG Polyethylene glycol

RO Reverse osmosis

SC(NH²)² Thiourea

SEM Scanning electron microscope

STPs Sewage treatment plants

TEM Transmission electron microscopy

WHO World Health Organization

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

ZPC point of zero charge

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LIST OF ORIGINAL PUBLICATIONS

This thesis is based on data presented in the following articles, referrred to by the Roman Numerals I-IV.

I. Abu-Danso, E., Srivastava, V., Sillanpää, M., & Bhatnagar, A. (2017).

Pretreatment assisted synthesis and characterization of cellulose nanocrystals and cellulose nanofibers from absorbent cotton. International Journal of Biological Macromolecules, 102: 248-257.

II. Abu-Danso, E., Peräniemi, S., Leiviskä, T., & Bhatnagar, A. (2018). Synthesis of S-ligand tethered cellulose nanofibers for efficient removal of Pb (II) and Cd (II) ions from synthetic and industrial wastewater. Environmental Pollution, 242: 1988-1997.

III. Abu-Danso, E., Bagheri, A., & Bhatnagar, A. (2019). Facile functionalization of cellulose from discarded cigarette butts for the removal of diclofenac from water. Carbohydrate Polymers; 219, 46-55.

IV. Abu-Danso, E., Peräniemi, S., Leiviskä, T., Kim, T., Tripathi, K. M., &

Bhatnagar, A. (2020). Synthesis of clay-cellulose biocomposite for the removal of toxic metal ions from aqueous medium. Journal of Hazardous Materials, 381; 120871.

The above publications have been included in this thesis with their copyright holders’ permission.

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AUTHOR’S CONTRIBUTION

I) E A-D planned and conducted all experiments and was responsible for the partial characterization of the materials and analyzed all data. VS also conducted partial characterization of the materials. E A-D wrote the manuscript with input from MS and VS. AB edited the manuscript and supervised all the experiments.

II) E A-D was responsible for planning and conducting all the experiments with the advice from AB. E A-D and TL conducted the characterization of adsorbents and E A-D analyzed the data. SP provided guidance on AAS analysis.

E A-D wrote the manuscript with input from TL and SP. AB edited the manuscript with coauthors and supervised all the experiments.

III) E A-D and BA were responsible for planning and conducting all experiments. BA conducted all diclofenac spectrophotometric analysis. E A-D conducted the characterization of adsorbents and analyzed all the data, E A-D wrote the manuscript with input from BA. AB edited the manuscript and supervised all the experiments.

IV) E A-D designed and conducted all the experiments. E A-D, KMT, and TL conducted the characterization of the adsorbents. E A-D analyzed the data with input from TL and KMT. E A-D wrote the manuscript with input from TL, SP, KMT and TK. AB was responsible for editing the manuscript and supervised all the experiments.

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1 INTRODUCTION

1.1 BACKGROUND

Water plays a crucial role in life sustainability, hence there is a need for an adequate and safe supply of water to all living organisms (WHO, 2011). According to the WHO (2006), infants, young children and the elderly are the group of humans most at risk of unsafe water. To eradicate the dangers of unsafe water to these vulnerable groups, and to preserve natural water bodies, management frameworks including stringent guidelines for treatment of drinking water and wastewater before discharge, must be set. However, finding the most suitable wastewater treatment option to meet regulatory requirements remains a challenge. This challenge is due to the overwhelming rate of pollution arising from population growth and rapid industrialization, which has resulted in the elevation of both traditional and emerging pollutants in the environment. Rapid industrialization has resulted in increased production of toxic heavy metals (WHO, 2006; WHO, 2011).

Population growth has increased the demand for improved living standards, which has also led to an increase in the production, usage and irresponsible disposal of numerous personal health care products which contain pharmaceutically active compounds (PACs). PACs have been detected in wastewaters globally (Heberer, 2002). The level of production of these emerging pollutants has put a strain on wastewater treatment infrastructures, thereby compromising the treatment efficiency (Qu et al., 2013). Pollutants, such as toxic heavy metals and pharmaceuticals, have all been designated as recalcitrant in the environment (WHO, 2011; Heberer, 2002). One such recalcitrant pharmaceutical pollutant is diclofenac. It is estimated that 75 tons of diclofenac is produced in Germany annually (Ternes, 2001), and it has been identified as one of the most prevalent pharmaceutical pollutants in German waters, with a low removal rate by sewage treatment plants (STPs) (Heberer et al., 2002a; Buser et al., 1998).

Pharmaceutical products enter the environment via municipal sewage after it has been disposed of in domestic and public sewage systems, such as in hospital effluents (Kümmerer, 2001). Their detection in both raw water and treated drinking water (Putschew et al., 2001) suggest that these contaminants are persistent as conventional treatment infrastructures are not designed to remove these emerging pollutants (Qu et al., 2013). Similarly, exposure to heavy metals is largely through contaminated water (WHO, 2011), although atmospheric exposures persist in some areas of the world as a result of activities that release heavy metal-laden particles into the air (Barton, 1981). A known detrimental effect of exposure to heavy metals is metal poisoning, which can lead to cancer, while exposure to pharmaceutical contamination can result in drug resistance among other things. Currently, the world’s population is facing a water crisis due to increasingly stressed and scarce

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freshwater resources owing to the prevalence of these pollutants, and other water security concerns (Oki and Kanae, 2006; Vörösmarty et al., 2010). This phenomenon is expected to worsen as demand for modernization from both developing and developed worlds persists (WHO, 2011). Therefore, there is a need for effective management schemes to sustain natural water resources. One critical management option is efficient wastewater treatment before discharge.

However, the current overwhelming rate of aquatic pollution and the urgent need to address the treatment challenges associated with emerging contaminants has made the selection of efficient treatment options a challenging task. Therefore, a state-of the art treatment method that combines energy efficiency, economic feasibility, flexible operation, and highly efficient treatment performance with less or no residual contamination is highly desired. One such potential treatment option is the use of nanocomposites as an integral part of an adsorption-based treatment process.

1.2 TECHNOLOGIES IN WASTEWATER TREATMENT

Wastewater treatment methods vary in efficiency and operation (Deegan et al., 2011).

Many treatment options have been used over the years with varying degrees of efficiency and success. Conventional treatment methods, such as the activated sludge process, have been used in wastewater treatment (Abu-Danso, 2015). However, high production of sludge and high energy consumption as well as bulking and the inability to function during high organic loads, have been reported as drawbacks (LaPara et al., 2001; Oz et al., 2004; Deegan et al., 2011). Membrane processes have been reported to function less effectively without pressure and fail to effectively treat organic contaminants (Deegan et al., 2011). Other treatment options such as reverse osmosis (RO) and forward osmosis (FO) have been reported as being complex in operation and not feasible at high total dissolved solids loads (Gregory et al., 2011;

Estrada and Bhamidimarri, 2016).

Adsorption, however, has become a preferred wastewater treatment option for removal of both organic (Sotelo et al., 2014) and heavy metal (Qin et al., 2016) contaminants due to attributes that include, simple operation, reliability, cost effectiveness, and reusability. The material utilised in the adsorption process is also crucial for effective removal. Recent findings have shown that nanosized materials can be effective in the adsorption process (Hu et al., 2005).

Nano technology is gaining acceptability as a next-generation wastewater treatment technology that could replace conventional wastewater treatment technologies (Tratnyek and Johnson, 2006; Mauter and Elimelech, 2008). Treatments with nanosized materials have evolved from the pre “trial and error” stage in water treatment to an operational stage. A rational design and synthesis of nanoscale materials for wastewater treatment requires the combined knowledge of chemistry and functional capabilities of nanoscale materials. This combination can lead to the

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syntheses of surface modified functional nano materials, and incorporation of other materials to form nanocomposites based on material composition and their manipulation (Mauter and Elimelech, 2008). One such material identified as a potential candidate in the application of environmentally benign nanotechnology to water/wastewater treatment is cellulose (Kalia et al., 2014; Wan et al., 2018).

Cellulose is a ubiquitous organic raw material that is almost inexhaustible in nature.

Under controlled conditions, cellulose can be extracted from different sources in the nano dimension as nanocelluloses. The fundamental composition of cellulose is a chain of repeated units of glucopyranose inter-connected covalently by β-1,4 glycosidic bonds with numerous active hydroxy (OH) groups attached to the glucopyranose units (Klemm et al., 1998; Klemm et al., 2005; Eichhorn et al., 2010).

These OH groups impart a high surface reactivity property to the cellulose, which can be exploited for chemical modification and fabrication with compactible materials as nanocomposites to realise the intended application (Klemm et al., 2005;

Lin et al., 2012). Applying nanotechnology in the form of modified nanocelluloses and cellulose nanocomposites in the adsorption process has enormous potential as an effective environmentally sustainable wastewater treatment solution.

1.3 AIM OF THE THESIS

The overall aim of the thesis was to provide knowledge on the extraction of celluloses, syntheses of modified celluloses and cellulose nanocomposites, and characterization and application of the synthesised materials for the removal of different pollutants from aqueous media using laboratory scale batch and column adsorption experiments. The work in this thesis has relevance to advancing the use of facile procedures to chemically modify the surface of cellulose nanofibers and to fabricate cellulose nanocomposites. This was done to impart new functional groups that will give the modified cellulose unique properties including stable surface electrostatic charges, chelation capability, and porosity. These properties were expected to enhance the sorption capacity of the synthesized materials for metal ions, and diclofenac. The synthesized materials can then be assessed for suitability in real wastewater treatment.

1.4 OBJECTIVES AND OVERVIEW OF THE THESIS

The overall objective of this thesis was to synthesize different nanocelluloses from different sources using eco-friendly approaches (Figure 1). The synthesized cellulose nanofibers (CNFs) were chemically modified with sulphur from thiourea (SC(NH₂)₂) as the source of anionic ligands, phosphates from weak phosphoric acid (H3PO4) to obtain variably charged cellulose nanofibers (HPO-CNFs), and cellulose biofabricated with halloysite clay (Al²Si²O⁵(OH)⁴•nH²O) to achieve negatively charged cellulose biocomposite (CCB). The specific objectives were;

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• to extract different nanocelluloses from absorbent cotton by studying the effect of pretreatment and different experimental conditions: (1) Pretreatment assisted synthesis and characterization of cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) (Paper I); to synthesize sulphur ligand modified CNFs as a negatively charged adsorbent for the removal of toxic cationic heavy metal ions from wastewater; (2) Synthesis of S-ligand tethered cellulose for efficient removal of Pb (II) and Cd (II) ions from synthetic and industrial wastewater (Paper II); to modify CNFs extracted from discarded cigarette butts with phosphates from weak phosphoric acid to obtain a variably charged adsorbent for diclofenac removal; (3) Facile functionalization of cellulose for the removal of diclofenac from an aqueous medium (Paper III) to biofabricate CNFs with halloysite clay for removal of heavy metal ions; (4) Synthesis of clay-cellulose biocomposite for the removal of toxic metal ions from aqueous medium (Paper IV).

• to characterize the materials to gain insight into the changes from the precursor to the synthesized nanomaterials. In this regard, FTIR, XPS, EDS, and Raman spectroscopy were used to analyze the changes in the functional groups, chemical state, atomic composition and crystal structures of the materials before, and after adsorption of the pollutants. The crystalline phase analysis was conducted using XRD. Zeta potential and pHzpc techniques were used to assess the surface charge of synthesized materials at different pH levels. Pore size, pore volume, and surface area analyses of the materials were conducted using N² adsorption-desorption from BET measurements.

Surface morphology analysis was conducted with SEM and TEM techniques.

• to study the effect of solution pH, adsorbent dosage, ionic strength, and competing cations and anions on the adsorption capacity of the synthesized adsorbents. The adsorbent-adsorbate contact time, and the adsorption isotherms were also studied. The reusability of the adsorbents was also studied, and the plausible adsorption mechanisms governing the adsorption processes of the different adsorbents were explained.

• to interpret the type of adsorption by applying different kinetic and isotherm models to the adsorption experiment data.

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Figure 1. Schematic overview of the thesis.

O P OH

OH OH

O OH OH O OH

OH

O O

OH

O O

O

Cl

N H Na O

Pb Cd Co

cellulose

S HN²

Extraction

H

Discarded cigarette butts Cotton

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2 LITERATURE REVIEW

2.1 WASTEWATER POLLUTANTS

Wastewater can be broadly defined as water containing substances that which are classified as not suitable for consumption by living organisms. Wastewater is generated from activities in homes, public institutions, and industries. Varied forms of wastewater from these sources are carried to sewage systems for treatment before discharge into natural waterbodies as regulated by law. Some industries and public outfits, such as hospitals, may also have their own treatment plants to treat their specific wastewater. Natural phenomena including runoff from rainstorms may also carry pollutants, which may have to be collected as wastewater for treatment plants.

Wastewater from different sources varies in composition depending on the activities at the source (Riffat, 2012). Exposure to a certain amount of some of these components may be harmful to life and aquatic bodies, if they are released in quantities beyond the regulatory limit. As a result of their detrimental effect, these components have been classified as pollutants. Accordingly, the WHO (2006) defines pollutants as components of wastewater/water whose concentration levels can be detrimental to living organisms and cause environmental health problems.

Pollutants can be classified as organic, such as excess nutrients (Peavy and Tchobanoglous, 1985), or inorganic, such as heavy metals including lead (Pb) and cadmium (Cd) (Huang et al., 2012). Organic pollutants can biodegrade, while inorganic pollutants are non-biodegradable.

According to Järup (2003), an inorganic compound with specific density over 5 g cm^-

3 is classified as a heavy metal. In the periodic table, heavy metals are spread across the transition elements to the metalloids. Clarkson et al., (2003) and the WHO (2011) have reported that heavy metals exist as inorganic compounds, including Pb and Cd, or organic compounds with varied toxicity. Some heavy metals exist naturally as ores in the earth crust. They are used in a wide range of processes including production of alloys, material stabilizers, batteries, and in the petrochemical industry. From these processes, they are introduced into the environment via different routes. The exposure to heavy metals such as Pb and Cd continues to increase, therefore, policies for environmental protection against the toxic effects of the metals are frequently reviewed. The pathways of heavy metal exposure have been reported (Järup, 2003;

WHO, 2011) as occurring through different processes, including the transport of heavy metal-laden particles through air, water, and food. However, some routes are specific, for example, high consumption of shark, tuna, and swordfish can result in higher exposure to methyl mercury compared to other heavy metals (Järup, 2003).

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2.1.1 Lead (Pb)

Lead has been identified as one of the heavy metals that poses a threat to humans and other organisms due to its cumulative poisoning properties (WHO, 2011). Goyer (1996) has reported that the toxicity of Pb has made it one of the most studied pollutants in environmental remediation. Lead belongs to group 14 in the periodic table with an atomic weight and number of 207.19 g mol^-1 and 82, respectively. The exposure routes of Pb include air, water, and food. Exposure to Pb via air occurs as a result of emission of Pb particles from mines that are inhaled into the lungs and diffuse into the blood stream. Places where Pb-laden fuel are still used in automobiles are exposed to Pb particles from the fumes emitted by the automobiles. These airborne Pb particles are also a source of aquatic Pb in addition to discharged Pb contaminated effluents (Citak and Tuzen, 2010). Other living organisms are exposed to Pb through the food chain when Pb accumulated in plants and aquatic organisms are consumed by other organisms. Apart from these exposure routes, cooking and storage pots have been reported as sources of Pb exposure (Järup, 2003).

2.1.2 Forms and reactions of Pb

It is imperative to study Pb hydrolysis and speciation in aqueous media since its complexation with a material’s surface in an adsorption system depends on the charge characteristics and precipitation phases. Brown and Ekberg (2016) reported that Pb may exist as Pb(IV) and Pb(II) cations with their oxide forms in aqueous solution, however, in strong alkaline solutions Pb(IV) reacts with OH to form negative ions (Pb(OH))_X²⁻. This suggests that the adsorption capacity for Pb by negatively charged adsorbents will be reduced at high solution pH. Different adsorption studies have confirmed this phenomenon. Abu-Danso et al. (2018; 2020) found a reduction in adsorption capacity for Pb by negatively charged adsorbents in solutions beyond pH 8 and reasoned that this was due to a charge repulsion between the speciated negative Pb ions and the negatively charged adsorbents. Stability constants for Pb interaction with other ions at zero ionic strength shows a non-linear relationship with the inverse of absolute temperature (Ziemniak et al., 2005). Further hydrolysis of Pb(II) in aqueous solution forms other monomeric Pb species including Pb²⁺, which can further hydrolyze to form four positive polymeric species Pb₃(OH)₄²⁺, Pb₃(OH)₅⁺, Pb4(OH)44+, and Pb6(OH)84+ (Brown and Ekberg, 2016). This makes Pb(II) a suitable form of Pb to study in water remediation due to its capacity to speciate and produce varied Pb species with varied oxidation states.

The International Agency for Research on Cancer (IARC) classifies Pb as a possible and probable human carcinogenic agent. Exposure to Pb causes lead poisoning.

According to the WHO (1995), the half-life of Pb in the human blood and skeleton range from a month to beyond twenty years. Children are most at risk from lead- poisoning and have been reported to suffer reduced IQs as blood level Pb increases (WHO, 1995). Other effects in children include, general behavioral abnormalities

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from a malfunctioning nervous system (Memon et al., 2005). Lidsky and Schneider (2003) and Steenland and Boffetta (2000) have reported that prolonged exposure to Pb causes kidney damage, effects on neural development and memory deterioration and cancers of the lungs, stomach, skin and glial cells (Lidsky and Schneider, 2003;

Steenland and Boffetta, 2000). As a result of these health effects, the set permissible level of Pb in drinking water is 0.01 mg L^-1.

2.1.3 Cadmium (Cd)

The use of cadmium (Cd) dates back to the 1800s. In recent times, it has been used in many applications including re-chargeable batteries, material stabilizers, as metal alloys, and as an anti-corrosion agent (Directive, 1991). Cadmium belongs to group 12 in the periodic table with an atomic weight and number of 112.4 g mol^-1 and 48, respectively. Cadmium occurs naturally in ores and is also mined. Exposure routes of Cd are similar to those of Pb, however, (Järup et al., 1998) cigarette smoking is recognized as a source of blood and atmospheric Cd (Järup et al., 1998). Additionally, anthropogenic activities are a major source of Cd that is adsorbed by plants and fed on by other organisms. Dumping of Cd-laden products in the environment is also a source of Cd exposure to organisms.

2.1.4 Forms and reactions of Cd

The high toxicity of Cd in the environment has necessitated the need for its removal from the environment. In this regard, knowledge of its complexation within different media is required prior to choosing the most appropriate removal option and strategy. The behavior of Cd in different media is predominately influenced by pH of the media. In a neutral to weak alkaline (pH < 8.6) aqueous medium, Cd speciation is dominated by Cd²⁺, however, the speciation of Cd from CdCO³ in the same environment is dominated by chloride-complexes (Powell et al., 2011). These chloride-complexes, when formed in that medium, tend to inhibit the production of Cd²⁺due to the instability of CdCO³. It has been reported that Cd(II) in solution has a high affinity for sulphur ligands compared to oxygen-donors (Powell et al., 2011).

This phenomenon suggests that complexation of Cd(II) with a sulphur ligand on a material will be more favorable compared to the complexation with an oxygen-based entity (Powell et al., 2011; Brown and Ekberg, 2016). It has also been reported that in a weak acid aqueous medium, Cd(II) speciation is dominated by Cd²⁺, except for wheninterference from organic ligands of humic nature occurs (Powell et al., 2011).

Exposure to Cd has been reported to cause kidney damage (Järup, 2003). Skeletal damage (Osteoporosis) due to interference in the normal bone mineral formation has also been reported to occur as result of comparatively low Cd exposure (Staessen et al., 1999). Although there are conflicting studies on the implication of Cd as an agent for prostate and kidney cancers, IARC classifies Cd as a group 1 human carcinogenic

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agent. According to the WHO (1992), 200 mg Cd/kg kidney cortex is defined as a critical limit which require immediate remedial measures.

2.1.5 Pharmaceuticals

Pharmaceutical compounds have a variety of active components, therefore their presence in the environment is likely to cause different detrimental effects on the consumers. The exposure routes of pharmaceutically active compounds (PACs) include unsafe disposal of unused drugs to sewage, excretion through feces and urine of consumers, and various treatments of livestock (Halling-Sørensen et al., 1998). PACs from these sources end up in water bodies as leachates through aquifers, and wastewater treatment plants. The presence of pharmaceutical compounds including personal care products in aquatic habitats is a relatively new phenomenon and a growing concern for water treatment (Heberer, 2002). The threat posed by pharmaceutical compounds to life, and their persistence and estimated duration in the environment, has been reported (Boxall et al., 2012). The occurrence of pharmaceutical compounds and other drug metabolites in aquatic environments has been monitored in different countries and continents (Heberer et al., 2002) and concentrations of diclofenac, ciproflaxin, and ibuprofen have been found to be high in both influent and effluent samples from treatment plants (Heberer et al., 2002).

The numerous environments in which diclofenac has been detected suggests that the compound could be difficult to remove or treat in wastewater treatment plants (Heberer, 2002; Sim et al., 2011).

2.1.6 Diclofenac

Diclofenac, (2-[(2,6-dichlorophenyl) amino]), considered to be an acetic acid, is part of the family of drugs commonly referred to as pain relievers (Tuncay et al., 2000).

Diclofenac has no steroidal effect and is one of the most commonly used drugs (Schmidt et al., 2017). As a pollutant, diclofenac has been frequently detected and is mainly unchanged in the aquatic environment, which suggests that it can be a recalcitrant (Schmidt et al., 2017). Further research on this property of diclofenac revealed that an average of 2.51 µg L^-1 was retained in the effluent of sewage treatment plants (Heberer, 2002), thus establishing the recalcitrant nature of diclofenac. Removal efficiency of diclofenac by STPs is very poor (17%), providing more evidence of the difficulty in removing diclofenac from water treatment systems (Buser et al., 1998). The adverse effects of diclofenac in the environment has led to its inclusion on the list of EU Water Framework watch (Ribeiro et al., 2015). Low concentrations (ng - µg L^-1 levels) have been shown to have negative effects on aquatic organisms (Triebskorn et al., 2007). Consequently, further studies on the behavior of a group of pharmaceutical compounds in the environment, including diclofenac, are required (Zhang et al., 2008; Vieno and Sillanpää, 2014).

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2.1.7 Forms and reactions of diclofenac

A critical challenge in removing pharmaceutically active compounds from water is overcoming their high solubility, which can reduce the removal efficiency of treatment facilities. One critical behavior of ionizable pharmaceutically active compounds, however, is their dependency on pH (Verlicchi et al., 2012). Diclofenac has an H-bond acceptor count of 3 (PubChem), which suggests that it can be easily ionized to assume a negative, positive, or neutral charge. These changes can affect its removal efficiency. Verlicchi et al. (2012) assigned this behavior to the speciation ability of the compound (diclofenac). A strong acidic medium, however, can also be detrimental to the compound (Abu-Danso et al., 2019). The efficient sorption of these ionizable compounds is usually in weak acidic conditions compared to alkaline or strong acidic media (Kümmerer, 2009).

The health effects of diclofenac exposure depend on the species exposed, the endpoint usage, and the duration of the exposure (Haap et al., 2008). Studies on the interactions of diclofenac with DNAs have shown that at a 1:1 molecular ratio, diclofenac has a controlling inference on DNA (Wei et al., 2012). Potential detrimental effects from environmentally relevant doses of diclofenac include estrogenic disorders, genotoxicity, and cellular toxicity (Nassef et al., 2010). In the aquatic habitat, a 1 µg L^-1 dose of diclofenac has been implicated in kidney and liver damage of fishes (Triebskorn et al., 1997). The presence of 12 ng L^-1 diclofenac in fish eggs has been reported to reduce the egg survival rate by 67 % (Nassef et al., 2010), while a toxic effect of diclofenac on aquatic invertebrates has also been reported (Baccar et al., 2012). Different toxicological effects of diclofenac on aquatic organisms are summarized by Vieno and Sillanpää (2014).

2.2 ADSORPTION PROCESS IN THE REMOVAL OF POLLUTANTS FROM WASTEWATER

Wastewater management and regulation trends are becoming stricter in response to increasing environmental challenges. The conventional dumping of wastewater in treatment facilities for treatment processes and subsequent discharge into natural water bodies is gradually shifting to treatment and reuse, and/or treatment and injection into deep wells beyond aquifer interactions (Lutz et al., 2013). There are different options available for removing pollutants from an aqueous medium.

Various methods, such as membrane filtration processes, biological treatments, advanced oxidation processes, coagulation-flocculation, activated sludge processes etc (Stephenson and Lester, 1987; Boyd et al., 2003; Klavarioti et al., 2009; Abu-Danso, 2015; Xiong et al., 2016), have been used to remove nutrients, pharmaceutical waste and toxic heavy metals. Different drawbacks and bottlenecks have been reported during the operation of these processes. To overcome most of these drawbacks, an

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efficient alternative method is required. Adsorption process is one of the alternatives which can effectively remove pollutants from water.

Adsorption is an established technology and one of the alternatives for the treatment of both pharmaceutical and heavy metal contaminated wastewater. The adsorption process is broadly defined as the accumulation of dissolved substance at the interface of a solid and a liquid driven by surface forces (Dubinin, 1977). However, it can also occur at the interface of gas and solid. The solid material in the adsorption process is the adsorbent and it is the adsorbent surface on which the liquid molecules or the dissolved substance (adsorbate) accumulate.

A feasible adsorption process is fundamentally controlled by the surface energy of the adsorbent, although there are parameters that can affect the overall performance of the adsorbent (Dubinin, 1977). Adsorption is classified as chemical or physical.

Chemical adsorption occurs when the accumulation of the adsorbate on the surface of the adsorbent is controlled by a reaction that involves the exchange of electrons between the surface of the adsorbent and the adsorbate (Roque-Malherbe, 2018).

However, when the adsorbate accumulates on the adsorbent surface without the exchange of electrons, but is instead driven by forces such as van der Waals forces, the adsorption process is a physical one (Roque-Malherbe, 2018). The deposition of molecules in either form of adsorption is influenced by the operational parameters within the reaction system on one hand, and the adsorbent and adsorbate characteristics on the other hand. The adsorbate removal efficiency is influenced by the presence of competing ions (adsorbates), the adsorbate polarity, and the concentration (Tuutijärvi, 2013). Other determinant factors of adsorption, and by extension adsorption capacity, include the adsorbent type, temperature, pH, solubility of the adsorbate in an appropriate solvent, and concentration (Rouquerol, 1999).

2.2.1 Modes of adsorption

Adsorption can be operated in different systems and modes depending on the goal of the adsorption process. A batch mode system is a closed adsorption system in which adsorbent and adsorbate are kept in an enclosed chamber until the determined duration of the process. An open adsorption system is a flow through dynamic fixed- bed reactor with an outlet from which the effluent can be collected and analyzed if required. In a dynamic adsorption system, pressure is constant while pressure must be determined in a batch mode system. To operate an adsorption process, a reaction system designed to allow dynamic mass transfer of particles and subsequent analyses with mathematical calculations is required.

To understand and interpret adsorption dynamics, the interrelation of mass transfer, coefficients of diffusion, and adsorption equilibrium are crucial (Myers, 1989). These interrelations are based on different mathematical theories applied to the processes.

The nature of adsorbent can also influence the adsorption process. One of the most

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critical attributes of an adsorbent in an adsorption process is its porosity. The pore structure and distribution can be rate controlling depending on the scale upon which it can resist mass transfer of particles (Ruthven, 1989). This implies that microporous (≤ 2 nm), mesoporous (2 nm and 50 nm), and microporous (> 0.05 µm) adsorbents will influence the overall adsorption capacity due to different specific surface areas, and pore volume dynamics (Ruthven, 1989). The adsorption capacity (q^e) of a studied adsorbent is determined by the equation, (Eq. 1):

𝑞𝑒=^(𝐶^𝑖^−𝐶^𝑒^)•𝑉

𝑚 (1) where qe is the adsorption capacity (mg g^-1), Ce is the concentration after equilibration time, Cⁱ is initial concentration (mg L^-1), V represents the volume of adsorbate (L) and m is the amount of adsorbent (g). The transfer rate, however, is always defined by a model notwithstanding the actual transfer mechanism (Roque-Malherbe, 2018).

2.2.2 Adsorption kinetics

Studies on adsorption kinetics can be conducted in both batch mode and fixed-bed adsorption systems. However, Myers (1989) has reported that the batch mode system allows a thorough mixing that ensures adequate contact between adsorbate and adsorbent. To study and predict the type of adsorption kinetics, modelling equations can be used to distinguish the rate determining step. Over the years, different kinetic models have been used to describe the order of reaction and hence the adsorption mechanisms of an adsorption process (O'Shannessy and Winzor, 1996). The pseudo- first-order adsorption kinetic model proposed by Lagergren (1898) is determined from Eq. 2:

𝑞𝑡= 𝑞𝑒 (1 − 𝑒^−𝑘¹) (2)

where q^e is the equilibrium concentration (mg g^-1) at time (t) and k¹is the pseudo-first order rate constant.

The pseudo-second-order kinetic model by Ho and McKay (1999) can be written as Eq. 3:

𝑞_𝑡= ^𝑘²^𝑞^𝑒²^𝑡

1+ 𝑘₂𝑞_𝑒 𝑡 (3)

where k2 is the pseudo-second-order rate constant (g mg min^-1).

These models are based on the plot of experimental adsorption capacity (q^{e exp}) against time to determine the calculated equilibrium capacity (q^{e cal}) as well as the regression values of k1 and k2. The common interpretation of the best fitted model is based on the values obtained for the correlation coefficient (R²) and the level of

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disparity between qe cal and qe exp which is expected to be minimal to be accepted as the best fitting kinetic model. The Avrami model (Avrami, 1939), is based on experimentally supported assumptions of kinetics of particles based on temperature- time and transformation-time curves. Lopes et al. (2003) have, however, suggested the use of the Avrami model, where the adsorption mechanism is more than one and the adsorption rate is slow, although other validations were also recommended. The Avrami model can be written as Eq 4:

𝑞_𝑡= 𝑞_𝑒 (1 − 𝑒^{( − (𝐾}^𝑎𝑣^𝑡)^𝑛⁾) (4)

where K^av (min ^-1) is the Avrami constant.

Intraparticle diffusion influences the rate of adsorption due to adsorbent pore movement and therefore, it can be a controlling step in the investigation of an adsorption process. The assumptions of the effect of adsorbent porosity (the internal void fraction) is critical in the determination of the rate limiting effect of pores (Weber and Chakravorti, 1974). The resistance to mass transfer during adsorbate diffusion can be ignored under high agitation of a nonflow adsorption system (Edeskuty and Amundson, 1952). The intra-particle diffusion model is determined from Eq. 5 (Weber and Morris, 1963):

𝑞𝑡= 𝑘𝑃𝑡¹²+ 𝐶 (5) 2.2.3 Adsorption isotherms

Adsorption isotherms of a system reveal information on the quantitative adsorption equilibrium and the adsorbent involved in the adsorption process. Adsorption isotherms can be broadly defined as the empirical data of equilibrium adsorption of an amount of adsorbent as a function of different adsorbate concentrations under constant experimental condition(s). It is essential to note that adsorption in liquid always reaches saturation due to the interference of solvents (Ruthven, 1989), therefore it is important to investigate the isotherm equilibrium of an adsorbent in an aqueous medium to determine the maximum removal capacity of a potential adsorbent. From this information, practical application of the adsorbent can be studied in an adsorption system with an operational design to result insuccessful application of the adsorbent. The adsorption capacity is calculated as stated in Eq 1.

Adsorption isotherms can be described using several empirical adsorption isotherm models in both single and multicomponent environments (Keller and Staudt, 2005).

Widely used empirical adsorption isotherm models have been based on three basic approaches (Malek and Farooq, 1996); a state of dynamic equilibrium controlled by particle kinetics (Langmuir, 1916), the interplay of temperature and kinetic energy (thermodynamics) on the adsorption equilibrium (Boer, 1968), and the theory of

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potential adsorption in which adsorption occurs on a surface continuously and rises as a growth curve (Dubinin, 1960). These fundamental principles have been combined into a single derivation to interpret adsorption isotherms based on different influences in an adsorption system (Ruthven, 1984). Some of the widely used isotherm models include the Langmuir, Sips, Freundlich, and Redlich-Peterson models, among others (Oscik, 1982; Malek and Farooq, 1996). Monolayer adsorption is one in which the adsorbates are adsorbed at fixed sites of constant/similar surface activation energies (homogenous surface) with no interaction between the adjacent sites (Langmuir, 1916). The Langmuir model equation (non-linear) can be written as Eq 6:

𝑞_𝑒=^𝑞^𝑚^𝐾^𝐿^𝐶^𝑒

1+ 𝐾_𝐿 𝐶_𝑒 (6)

where K^L is the Langmuir constant (L mg^-1), q^e represents the experimental equilibrium capacity in mg g^-1 or mmol g^-1, C^eis the adsorbate concentration at equilibrium (mg L^-1) and qm is the calculated maximum adsorption capacity (mg g^-1).

The Redlich-Peterson model (Redlich and Peterson, 1959) can be used in both homogenous and heterogenous systems. The model has been designated as a hybrid three parameter isotherm model involving features of Freundlich and Langmuir isotherms merged into a single empirical equation, which can cover a wide range of adsorbate concentrations (Redlich and Peterson, 1959; Ng et al., 2002; Rudzinski and Everett, 2012). The Redlich-Peterson isotherm model can be written as Eq 7:

𝑞_𝑒= ^𝐾^𝑅𝑃^𝐶^𝑒

1+ 𝑎_𝑅𝑃𝐶_𝑒^𝛽 (7)

where KRP (L g^-1) and aRP (L mg^-1) are the Redlich-Peterson constants. The other parameters are the same as defined previously.

The Sips adsorption isotherm model (Sips, 1948) is used for describing an adsorption process in a heterogenous systems. The Sips model uses three-parameters including qm, Ce, and Ks (Roque-Malherbe, 2018). The model combines Freundlich and Langmuir models to predict adsorption processes for both low and high adsorbate concentrations (Hokkanen, 2014). The Sips model equation is written as Eq 8:

𝑞_𝑒= ^𝑞^𝑚^(𝐾^𝑆^𝐶^𝑒⁾^𝑚𝑠

1 + (𝐾_𝑆𝐶_𝑒)^𝑚𝑠 (8)

where K^Sis the Sips affinity constant and all other parameters remain as defined previously.

The Freundlich isotherm model is a two-parameter model that describes a non-ideal multilayer adsorption on a heterogenous surface. The equation for the Freundlich model can be written as Eq 9:

𝑞_𝑒= 𝐾𝐹𝐶_𝑒^1/𝑛 (9)

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where KF is the Freundlich constant.

2.3 NANO-SCALE BIOMATERIALS

To advance wastewater treatment methods, there is a need to shift from bulk conveyance – which relies on extensive distribution and discharge systems – to a treatment system that has multifunctional, efficient treatment performance at an affordable price. In this regard, more research into next generation water remediation options is required. Nanotechnologically-designed wastewater remediation holds enormous potential for novel treatment capabilities, while taken into account economic feasibility (Qu et al., 2013; Wan et al., 2018) and environmental impact. This potential has led to a steady growth of interest in nanotechnology for water treatment as a distinct field of research (Tesh and Scott, 2014).

A typical material in water treatment nanotechnology has one dimension less than 100 nm. This size dimension is one of the main advantages of using nanotechnology in water treatment, since the nano dimension provides crucial size-based attributes over bulk material (Qu et al., 2013). From the nanoscale, an upscaling of materials to achieve desired attributes, such as pore size, high specific area, high reactivity, and sorption capacity, can be easily explored.

Two or more nanosized materials can also be combined as nanocomposites to exploit the vast advantages they provide. A nanocomposite is defined as a combination of heterogeneous materials fabricated through varied processing routes in which one or more of the combined materials is/are in the nanophase with specific properties that are different from bulk materials (Klimov, 2003; Grossman and Nwabunma, 2013). Nanostructured materials cover both naturally occurring materials and synthetically produced materials. Nanocomposites fabricated with a naturally occurring component are a biocomposite, in which the bio component is a natural polymer matrix e.g. cellulose (Grossman and Nwabunma, 2013).

Different nanocomposites that have been developed to conform to a specific form and function rely on interrelated attributes. Firstly, nano-sized materials possess various intrinsic attributes that make them “malleable” to achieving up-scaled materials targeted at a specific goal (Nanotubes, 2001). Secondly, biodegradable nano-sized materials can be synthesized within a sustainable framework to eliminate their potential eco-toxicological risk and the formation of secondary pollutants in the environment during their application (Lin et al., 2012). The former is achievable if the nanomaterials and the synthesis procedures are eco-friendly. Nanocomposite fabrication allows an easy understanding of the composition-function relationship based on knowledge of the active functional group of the materials involved.

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2.3.1 Cellulose

Of the different naturally occurring nanoscale materials capable of forming biocompactible products, cellulose is known to be the most abundant (Klemm et al., 2005; Abitbol et al., 2016). The compositions and hierarchical structure of cellulose makes it a versatile material, which can be manipulated at the nanoscale. Hence, cellulose has attracted a lot of attention over the years (Moon et al., 2011; Grossman and Nwabunma, 2013; Hoeng et al., 2016).

2.3.2 Sources of cellulose

Cellulose is classified as part of the polysaccharide group and in pristine form it is composed of fibers with amorphous and semi-crystalline regions (Lin et al., 2012). In addition, it has other components, such as lignin. Isolating nanocelluloses from their various sources has opened the flood gates for new materials and their applications in nanotechnology (Klemm et al., 1998; Henriksson and Berglund, 2007). Cellulose has been identified as the most abundant renewable natural biopolymer present in plant cell walls, as well as in some bacteria, algae, and animals (Tunicates) (Klemm et al., 1998; Henriksson and Berglund, 2007). The amount of cellulose present in different sources and the degree of polymerization is presented in Table 1.

Table 1. Percentage composition of cellulose from different sources and their degree of polymerization.

Source

% cellulose composition

Degree of

polymerization Reference

Cotton ca. 85-90 3000-15000 Willfor et al., 2011

Wood pulp ca. 20.50 600-1500 Van Dam and Gorshkova, 2003; Wang, 2008

Coir ca. 36 -43 - Mohanty et al., 2000

Hemp ca. 70-74 6500-8000 Willfor et al., 2011; Mohanty et al., 2000

Kenaf ca. 31-39 7.1 Mohanty et al., 2000

Jute ca. 61-71 6500-8000 Van Dam and Gorshkova, 2003 Agro residue ca. 35-45 - Willfor et al., 2011

Different synthesis procedures can be used to obtain different forms of cellulose.

However, a successful synthesis occurs under controlled conditions and the cellulosic product, thereafter, requires the appropriate terminology depending on the type of cellulose obtained. According to Lin et al. (2012), Kalia et al. (2014), and Jonoobi et al. (2015), cellulosic product obtained by removing the amorphous region from the cellulose structure is a crystalline nanocellulose (CNC), whereas cellulose obtained by maintaining both amorphous and crystalline regions is cellulose nanofibers (CNFs) (Lin et al., 2012; Kalia et al., 2014; Jonoobi et al., 2015).

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2.3.3 Structural hierarchy

Cellulose (C⁶H¹⁰O⁵)ⁿ) (where n ranges between 10 000 to 15 000) (Grossman and Nwabunma, 2013) is a chain of repeated units (Figure 2) of D-anhydro glucopyranose units (AGU) connected to each other by β (l, 4) glycosidic bonds of a covalent nature (Klemm et al., 1998). The extent of polymerization depends on the source of the cellulose and the extraction procedure used to obtain the cellulose. However, the glucopyranose chain can either be hydrolyzed to break up the polymerization or aggregated to form hierarchical structures using the extensive hydrogen bond network of the primary hydroxy functional group (Klemm et al., 2005).

Figure 2. Structural representation of cellulose (Source: Klemm et al., 2005).

2.3.4 Chemical interactions of cellulose

Solution, gel and solid-state structures of cellulose are of interest in nanotechnology applications. This is due to the complications of regenerating cellulose from the solution or gel state to the solid state (Klemm et al., 2005). Cellulose is insoluble in aqueous medium and organic solvents, but is easily hydrolyzed. The reactions occur with the OH groups of the glucopyranose chain at C2, C3 and C6 carbon positions with a reaction scheme similar to the reactions of primary and secondary alcohols, while the glycosidic links (-O-) are involved in degradation reactions (Klemm et al., 1998). Hence, most of the interactions of cellulose with other entities is controlled by the OH groups (Wang, 2008). It has been reported that urea prevents reaggregation in a study on the effects of urea on cellulose gel formation and aggregation (Wang, 2008). Urea hydrates donate and/or accept H-bonds across the solvent molecule, thus keeping the molecules in the solution apart (Wang, 2008).

2.3.5 Cellulose functionalization

To successfully bind other entities or modify the surface composition of cellulose, there is a need for substantial swelling or complete dissolution of the molecule.

According to Klemm et al. (1998), changing the state of cellulose before functionalization affects the intermolecular forces, which in turn eliminates the

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supramolecular structure of cellulose for easy access to the hydroxy groups and consequently the fibrils. The complete dissolution process is composed of two steps;

the first step is the activation state where the structure of the cellulose is disengaged, the second step proceeds with treatment of the cellulose with the appropriate chemical agent that can break up the H-bonds in the cellulose molecule (Wang, 2008).

As a result of the broken H-bonds, the intended modification can then proceed while the cellulose is in solution or gel state.

2.3.6 Cellulose solvents

One of the main hindrances to complete dissolution of cellulose is the large number of hydrogen bonds (Medronho and Lindman, 2014; Wan et al., 2018), hence the choice of dissolution agent must be based on the ability of the agent to interfere in the network of inter and intra molecular H-bond networks. Different swelling and dissolution agents with different effects on the cellulose molecules have been studied. Sulphuric acid (H²SO⁴) can be used to hydrolyse the cellulose molecule and introduces new functional groups to the cellulose surface (de Souza et al., 2002), which can interfere in the modification or functionalization process. In addition to H²SO⁴, solvents such as N-methylmorpholine-N-oxide, lithium chloride, and dimethyl sulfoxide have been investigated for the dissolution of cellulose. However, research suggests that formation of residual derivatives does not occur with other solvents (Wang, 2008). Various percentage compositions of NaOH and Urea (CO(NH²)₂) combinations have been used to dissolve cellulose (Cai and Zhang, 2005).

The results showed that 6 to 7% NaOH and 4 to 12% urea CO(NH²)₂ combinations did not yield any derivatives of the dissolution agents. Nevertheless, CO(NH2)₂ is a known chelator of positive chemical species; therefore, it is worth investigating the effect of the presence of the derivatives of CO(NH²)₂ on the cellulose surface modification and/or the material functionalization on intended applications, such as removal of metal ions.

2.3.7 Cellulose surface modification

The interfacial properties of cellulose can be altered using different modification strategies to develop new nanocelluloses with peculiar properties, such as adaptability, compatibility etc. One focus of this thesis is on the investigation of the effects of sodium hydroxide (NaOH) on the dissolution and regeneration of celluloses. The choice of NaOH as a component of the dissolution agents is due to the low toxicity of NaOH compared to other agents. Studies on NaOH cellulose dissolution by Wang (2008) showed that NaOH was able to transform cotton linter from the solid phase to a clear gel phase at specific temperatures, although the differences between mercerization action and cellulose fiber interaction with NaOH was not clearly defined. Nevertheless, the complexation of NaOH-cellulose has been reported to be a temporary reaction step (Kamida et al., 1984). The NaOH-cellulose

Pollutants removal from wastewater using nanocellulosic materials

uef.fi

Dissertations in Forestry and Natural Sciences

EMMANUEL KAFUI ABU-DANSO

POLLUTANTS REMOVAL FROM WASTEWATER USING NANOCELLULOSIC MATERIALS

EMMANUEL KAFUI ABU-DANSO

POLLUTANTS REMOVAL FROM WASTEWATER USING

NANOCELLULOSIC

MATERIALS

Emmanuel Kafui Abu-Danso

POLLUTANTS REMOVAL FROM WASTEWATER USING

NANOCELLULOSIC MATERIALS

ABSTRACT

ACKNOWLEDGEMENTS

LIST OF ABBREVIATIONS

LIST OF ORIGINAL PUBLICATIONS

AUTHOR’S CONTRIBUTION

CONTENTS

1 INTRODUCTION

1.1 BACKGROUND

1.2 TECHNOLOGIES IN WASTEWATER TREATMENT

1.3 AIM OF THE THESIS

1.4 OBJECTIVES AND OVERVIEW OF THE THESIS

2 LITERATURE REVIEW

2.1 WASTEWATER POLLUTANTS

2.2 ADSORPTION PROCESS IN THE REMOVAL OF POLLUTANTS FROM WASTEWATER

2.3 NANO-SCALE BIOMATERIALS