Chapter I Introduction

1. INTRODUCTION

I.1- Water pollution

Water is a vital and essential finite resource worldwide. Hence, the
availability of safe drinking water should be a crucial priority for sustaining
quality life. In this regard, water pollution and its purification are currently a
major concern for governments, research scientists, and industries. The
critical challenges are to develop an effective method for the removal of
pollutants to ultra-low level sand to recycle wastewater through purification [1]
[2] [3]
.
I.1.1- Heavy metals pollution
Due to the industrial waste of this modern society from mining, metallurgy,
foundries, etc. as well as the increased use of toxic substances such as
pesticides, heavy-metal intoxication has become an usual problem for
population[4]. Also, the increasing development of catalytic processes and
batteries has forced the arising of new factory protocols on waste materials
and fumes. Moreover, exploitation of aquifer resources in underdeveloped
areas, as for example in Asia and South America, deals with waters not
suitable for consumption in their natural state [5].As already well known, heavy
metal absorption by the body can cause serious damages in vital organs that
even could provoke their failure. Still nowadays, several studies have
demonstrated the presence of these ions in consumable waters, in higher
concentrations than the ones allowed by the organism [6] [7]
. Research for
ensuring a more sustainable management of the environment and its resources
is catalogued as one of the main technologies and services to be funded and
studied, but until that happens, new technologies should be emerging for
treating the consequences. There are several patents and previous studies on
purification treatments for waste and consumption waters describing ion
immobilizers using new technologies as zeolites or phosphoric acid [8]. Also
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 Chapter I Introduction

macroporous systems have been proposed as heavy metal water purifying

[9]
materials including hydroxyapatite (HA) systems, being efficient capturers
of Cadmium (Cd) and Lead (Pb) among others [10]. In some of these systems,
where the apatite is applied in the form of dissolutions/foams/powders/rocks,
capturing results showed that immobilization takes place at different pHs, and
their efficiency depends on the ion concentration, ion nature and temperature
in soils and waters, for example[11] [12]. It has also been reported that the use of
HA/polyurethane composite as efficient lead removers [13]. The aim of all these
phosphate systems is immobilizing metals through the formation of metal
phosphates with reduced solubility in a wide range of environmental
conditions. Here we present novel 3D-macroporous biopolymer-coated HA
foams as a potential alternative for the treatment of heavy-metal polluted
waters. The proposed materials have recently proved viability and success to
be used as a fast intoxication treatment after heavy metal ingestion, with the
principal quality of maintaining their morphology after the digestion process
leading to an easy expulsion by the body. For the first time, these foams have
been studied in terms of isotherm models and sorption kinetics analysis at
neutral pH in order to consider their application as heavy metal removers
from water. These foams could provide a new treatment with high
effectiveness without high financial costs or the need of water [14].

I.1.2- Classification of Heavy Metals

The definition of heavy metals is an ongoing discussion and may be quite
ambiguous, yet an element can be classified as a heavy metal if its density is
very much higher than that of water and it is of natural origin and primarily
occurs in the earth's crust (Nickel (Ni) and water). The main heavy metal
pollutants present in surface waters are Lead (Pb2+), Zinc (Zn2+), Cadmium
(Cd2+), Copper (Cu2+), Nickel (Ni2+), Arsenic (As3+), Cobalt (Co2+), Iron
(Fe3+), Manganese (Mn2+), Mercury (Hg2+), Chromium (Cr(VI)), Silver

2
 Chapter I Introduction

(Ag+), Gold (Au3+), Palladium (Pd2+), Platinum (Pt3+), Uranium (U(VI)),

Cesium (Cs+) and Rubidium (Rb+). Among them, Cu2+, Cr (VI), Mn2+,
Ni2+, Fe3+, Cd2+, Pb2+ and Zn2+ ions are highly toxic in nature especially
in wastewater[15].
Although some heavy metals such as zinc, barium, lead, silver are important
to human health when they are under the regulatory limit (Zn< 5 ppm; Ba<2
ppm; Pb<0.015 ppm; Ag<0.001 ppm) as recommended by the environmental
protection agency (EPA); most of these heavy metals are considered to be
harmful at different levels and excessive exposure to them can be fatal [16].
Heavy metal exposure may occur either through air, surface water, or food.
The presence of heavy metals in surface water sources in many areas is often
caused by human operations, mainly by mining, automobile emissions, and
the employment of heavy metals containing compounds utilized in
manufacturing. The wastewater created by industries is unleashed into the
environment, mainly in the watercourses. In water of industrial operations,
there are few origins of anthropogenic heavy metals including electroplating,
[17]
metal smelting, chemical and manufacturing industries . Homemade,
commercial, and agricultural wastewater contains high metal concentrations
that are frequently disposed off in many developed countries into the
atmosphere [15]. Zheng discovered that metals like mercury and lead are stored
in the soil around the reservoir after they are unleashed into the air by heavy
air emissions from automobiles and factories. Weathering and volcanic
eruptions are other natural phenomena that lead to heavy metal emissions in
certain parts of the world. Therefore, water contamination by heavy metals
often relies on the location's geographical, biological, and industrial activity.
The risks of the entry of these heavy metals into the food chain are well
documented. Thus, preparing and extracting as many as possible from
wastewater before letting them into the atmosphere is highly necessary.
Owing to the introduction of more strict laws and regulations on wastewater

3
 Chapter I Introduction

handling and discharge over the past decades, heavy metals have become key
pollutants for the environment, and treatment methods to extract heavy metals
from wastewater are of great concern and significance [18] .

I.1.3- Heavy metals removal techniques

Different methods have been used to remove heavy metals from wastewater
including physical, chemical and biological techniques. These also include
ultra-filtration, precipitation, oxidation and solvent extraction with electrolytic
methods, membrane filtration, ion exchange, biological systems and
adsorption methods. Traditional adsorbents, including alumina, zeolite, AC,
silica gel and bauxite, have been used on a small scale. In non-conventional
adsorbents, the most frequently used is agricultural and industrial waste,
natural materials and biomass for removal of heavy metals. Large quantities
of metal ion polluted wastewater are discharged into the environment from
various industries. Heavy metal ions such as Cd2+, Cr(VI), Cu2+, Ni+, As3+,
Pb2+, and Zn2+ are highly toxic in nature, which are released from chemical
industries. They have shown better solubility in water and also intake by
living organisms. The heavy metal ions tend to accumulate in the human
[19] [20]
body. They are the cause of serious human health problems . Hence the
need for removal and treatment of heavy metals from wastewater prior to
discharge in the environment is therefore a matter to consider. The adsorption
process has not effectively removed heavy metal ions from inorganic
effluents. Inorganic effluents containing heavy metal ions are eliminated by
using various methods such as electrochemical, chemical precipitation and
ion exchange. These methods presented some disadvantages such as
incomplete removal, huge amount of energy needed and large quantity of
toxic sludge produced[21] . Nowadays, the adsorption method has improved the
quality treatment of effluent waste and wastewater treatment due to low–cost
adsorbents having good binding abilities with heavy metals [22]. A great

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 Chapter I Introduction

number of adsorbents have been utilized for the removal of heavy metals.
However, the key drawbacks of these adsorbents are their poor capacity to
adsorb, weak connections with metallic ions, and their complexities in
removing and regenerating some of them from water. Ion-exchange resins can
significantly extract metal ions; however, they have low selectivity and
display a significant level of swelling associated with poor mechanical
stability[23]. Hence, attention in the past has been on carbohydrate
biopolymers. Heavy metals and common techniques used for their removal
Heavy metals are basically described as elements that have atomic weights
[24]
between 63.5 and 200.6 and a density greater than 5 g per cubic meter .
Some heavy metals such as copper (Cu), zinc (Zn), manganese (Mn), iron
(Fe), and cobalt (Co) play important roles in biochemical processes in the
human body. However, excessive exposure to these metal ions can result in
hazardous impacts. Other heavy metals such as arsenic (As), cadmium (Cd),
lead (Pb), mercury (Hg), and chromium (Cr) are toxic, even at trace levels
(parts per billion, ppb) because they are non-degradable and can bio-
accumulate in the main systems of human body [25]. Under normal
circumstances, human body can tolerate trace amounts of metals without
experiencing severe health problems. However, long-term exposure to heavy
metals can cause high levels of toxin accumulation in the body, which leads
to the failure of body systems and eventually fatality [26]. Therefore, the
maximum contamination levels (MCLs) have been set by the World Health
Organization (WHO) to ensure that zero or only the threshold limit of heavy
metals is allowed in a water source. In water sources, the MCL for Pb, Cd,
As, and Cr is set at 10 ppb, 3 ppb, 10 ppb, and 100 ppb, respectively [27] [28]
.
Any excessive release of heavy metals beyond the MCL limit is, therefore,
not safe for human use or consumption. To date, the common techniques used
to sequester heavy metals from water are precipitation, coagulation–
flocculation, ionic exchange, adsorption, and membrane separation. The

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 Chapter I Introduction

[29]
descriptions of each method are given in Table 1. Barakat , different
[30]
treatment methods, focusing on the adsorption process. Ali et al. ,
meanwhile, reviewed the removal of metallic ions from polluted soil, water,
[31]
and sediments using a phytoremediation process. Recently, Burakov et al.
provided a comprehensive review on the adsorptive removal of heavy metals
using conventional and nanostructured materials. From these reviews, it can
be concluded that each technique has advantages as well as limitations such
as infeasibility, high operational cost, slow operation duration, secondary
pollutant generation, non-reusabil- ity, and incompatibility for large-scale
application[32].

Due to the discharge of large amounts of metal-contaminated wastewater,

industries bearing heavy metals, such as Cd, Cr, Cu, Ni, As, Pb, and Zn, are
the most hazardous among the chemical-intensive industries. Because of their
high solubility in the aquatic environments, heavy metals can be absorbed by
living organisms. Once they enter the food chain, large concentrations of
heavy metals may accumulate in the human body. If the metals are ingested
beyond the permitted concentration,they can cause serious health disorders [33].
Therefore, it is necessary to treat metalcontaminatedwastewater prior to its
discharge to the environment.
Heavy metal removal from inorganic effluent can beachieved by conventional
treatment processes such as chemicalprecipitation, ion exchange, and
electrochemical removal.These processes have significant disadvantages,
which are,for instance, incomplete removal, high-energy requirements and
production of toxic sludge[21].Recently, numerous approaches have been
studied for thedevelopment of cheaper and more effective technologies,
bothto decrease the amount of wastewater produced and to improve the
quality of the treated effluent. Adsorption has become one of the alternative
treatments, in recent years; the search for low-cost adsorbents that have metal-

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 Chapter I Introduction

binding capacities has intensified[22]. The adsorbents may beof mineral,

organic or biological origin, zeolites, industrial by products agricultural
wastes,biomass,and polymeric materials[34] . Membrane separation has been
increasingly used recently for the treatment of inorganic effluent due to its
convenient operation. There are different types of membrane filtration such as
ultra filtration (UF), nano filtration(NF) and reverse osmosis (RO) Kurniawan
et al., 2006. Electro treatments such as electrodialysis (Pedersen, 2003) has
also contributed to environmental protection. Photocatalytic processes an
innovative and promising technique for efficient destruction of pollutants in
water.Although many techniques can be employed for the treatment of
inorganic effluent, the ideal treatment should be not onlysuitable,appropriate
and applicable to the local conditions[35].

I.2- Carbohydrate Biopolymers

Polymers also called macromolecules are known as large molecular structures
that contain one or more units, which are repeated within the molecule. The
term ‘polymer’ has its origin in Greek and is derived from the term ‘poly’ that
means many and ‘mer’ mean ingunit. Polymers are usually prepared by a
reaction between identical or similar molecules in order to produceatheo-
retically infinite chain. In reality, reactions usually
resultinarangeoflongchainsastheyarehinderedbymolecularmotionsandsidereact
ions.Biopolymersst and among the kinds of polymers, which are generated
through them eans of living organisms and can be acknowledged as polymeric
biomolecules. The term “Biopolymer”stands for a biodegradable polymer,
which is recognized as a biodegradable chemical compound and considered
them as organic composite throughout the ecosphere. All of the existing
biopolymers(DNA,proteins,carbohydrates,lipids,polysaccharides,polyphenols,
andnucleicacids)holdagreatportionofthehumanbodyandecosphere.Sincethewh
olestructureofbodyandgeneticbehaviorsthatistransportedfromparentstochildren

7
 Chapter I Introduction

isbaseduponDNAbiopolymer,theyarequitesignificantformankind.Representing
themostfrequentlyutilizedbiopolymer[36] [37].

Carbohydrate biopolymers are generated from renewable resources of plants

and chemicals synthesized from starch and sugar [38] . Due to their unique
structure, physico-chemical properties, chemical stability, great sensitivity
and high selectivity arising from the inclusion of chemical reactive groups
such as hydroxyl, acetamide or amino functions in polymer chains, these
biopolymers provide a relevant and desirable route to sorbents. In addition,
these materials are plentiful, reusable, biodegradable and capable of binding a
large range of molecules physically and chemically [39] [40]
. A diversity of
carbohydrate biopolymers has been used extensively as adsorbents for the
removal of heavy metals in wastewater. Among them, Chitosan, Lignin,
Carbox- ymethyl cellulose and Alginate based adsorbents have been
extensively used due to their low cost, efficiency, biocompatibility and
biodegradability. They contain different functional groups such as amine,
hydroxide, phenolic hydroxyl, methoxyl carboxyl acid and also good
hydrophilicity. These functional groups of biopolymer based adsorbents
enhance the removal efficiency of heavy metals from water samples due to
chelating metal ions producing the complex.

I.2.1- Chitosane
Chitosan is naturally a carbohydrate biopolymer with hetero- polymers of D-
glucosamine and N-acetyl-D-glucosamine moiety. It is prepared from the
deacetylation of chitin. Chitosan was applied in various applications such as
biomedical and industrial due to non-toxicity, biocompatibility,
biodegradability, high mechanical strength and physical-chemical properties
[41]
.

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 Chapter I Introduction

Chitosan is a major building group of amines and binds with heavy metal ions
from wastewater by using adsorption methods. Chitosan is generally used as
an adsorbent for the elimination of heavy metal ions from water samples and
it has amine and hydroxyl groups used to absorb the heavy metal ions from
wastewater. It was applied in the removal of heavy metal ions from aqueous
solution with pH 6–6.5 due to the cationic nature of Chitosan and cationic
nature of heavy metal ions [42].

Chitosane

I.2.2- Chitosane nanocomposites

Furthermore, among the bio-based natural polymer nanocomposites, previous
studies have demonstrated that chitosan-based nanocomposites are promising
biosorbents for water purification due to their high affinity for different water
pollutants [43] [44] . The main advantage is the existence of hydroxyl and amino
groups in the chitosan chemical structure, which favors the functionalization
of chitosan and their further modifications with nanomaterials. These
modifications of chitosan with nanomaterials such as carbon nanotubes and
inorganic nanoparticles such as ZnO, Fe3O4, and TiO2 are suitable to
[45]
increase the chitosan’s mechanical properties useful for water treatment .
Besides, these functional chitosan’s modified with nanomaterials to introduce

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 Chapter I Introduction

new functional groups are needed to increase the chelating properties of

chitosan, to change the pH range for the adsorption of pollutants, and to
improve the adsorption sites density as well as to enhance the adsorption
selectivity for the different water contaminants [46] [47]
. This is because
nanomaterials such as carbon nanostructured and inorganic nanoparticles have
the large surface area and many other excellent multifunctional properties
vital to achieving a high adsorption capacity resulting from an increase in
interactions between the adsorbate (pollutants) and active sites of the
chitosan-based nanocomposites. Currently, research studies on the use of
functional chitosan modified with nanomaterials such as carbon nanotubes,
graphene oxides, Ag, ZnO, Fe3O4, and TiO2, to eliminate the three classes of
water pollutants (organic, inorganic, and pathogens) by fixed-bed continuous
flow column adsorption technique are limited. A literature search conducted
from google scholar shows that most of the research works published in the
last ten years on functional chitosan-based nanomaterials focus on their
antimicrobial activity and application in drug delivery, biomedical, and water
treatment by batch adsorption studies. Hence, this article's focus is to review
the most recent development on the synthesis of chitosan-based
nanocomposites (i.e., functional chitosan modified with both carbon
nanostructured and inorganic nanoparticles) and their application in fixed-bed
continuous flow column adsorption for water purification. The different
approaches to functionalize chitosan materials and their further modifications
with nanomaterials to produce chitosan-based nanocomposites used for water
treatment are discussed. Critical attention has also been pointed to the use of
nonlinear mathematical expressions of conventional models and new artificial
intelligence models to assess the breakthrough adsorption curve. Moreover,
the factors affecting the quality of water used for purification by column
adsorption are evaluated. The possible adsorption or disinfection mechanism

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 Chapter I Introduction

of water pollutants by these chitosan-based nanocomposites are also

highlighted[48] [49].

1.2.3- Chitosan and physicochemical properties

Chitosan is one of the polymers that are abundant in nature and can be
considered as an eco friendly polymers due to its low toxicity. One of the
advantages of CH is that its surface can be modified chemically by many
functional groups. Also, it has bacterial inhibiting properties, biodegradable,
high permeable to water, good gel-forming, and adhesive characteristics due
to the high hydrophilicity. These characteristics were reflected in many
applications, especially in the fields of medicine and engineering, where it
was indicated that CH is one of the most promising materials in the field of
biological sensing after modifying a surface with BREs to sense some
chemical and biological molecules. Therefore, CH based biometric sensors
have gained tremendous success as an effective analytical device consisting
of BREs (e.g., DNA, proteins, enzymes, antibodies,etc.) connected to
transducer surface to translate the signal to a detector as an electrical signal,
and this signal can be quantitatively measured. The type of BRE immobilized
on CH-based nanocomposites determines the signal types such as intensity,
color, etc. and also played a key role in the fabrication of biosensors
(conductimetric, amperometric, and potentiometric sensors). Given the
reported characteristics of CH, CH was considered one of the most important
and most popular polymers, the materials used in biological sensors, as it was
successfully used in much detection, as will come later. One of the
advantages of CH, being an environmentally friendly polymer, is that it is
suitable for modifying the surface of the transducer through its ability to
immobilized biological recognition elements[50] [51] [52] [53] [54].

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 Chapter I Introduction

1.3- A short note on sodium alginate

Sodium alginate is a natural polysaccharide which can be extracted from
brown algae. Sodium alginate has some outstanding features as high bio-
compatibility, bio-degradable, and renewable. Moreover, the plentiful
hydroxyl and carboxyl groups have high adsorption affinity for heavy metal
ions. However, the mechanical strength, stability, heat resistance of sodium
[55] [56] [57]
alginate is relatively low . Therefore, physical or chemical
modification is usually applied to enhance its applicability in heavy metal
adsorption. Conventionally, the modification methods involved in sodium
alginate based adsorbents consists of surface grafting, cross-linking, and
composite with other materials[58] [59]
. Surface grafting mainly enhances the
selectivity towards targeted metal ions and increases the metal uptake
capacities while cross-linking can alter the chemical resistance and
[60]
mechanical strength . Compositing sodium alginate with other materials,
further, can increase the adsorption capacities as well as the physical
properties of the composite materials [61] [62].

The

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 Chapter I Introduction

adsorption mechanism of sodium alginate on heavy metal ions has been

[15, 63]
thoroughly studied by many researchers . The adsorption behavior was
generally determined by the physical and chemical properties of the adsorbent
and the influences of the external environment, such as pH value and co-
existing pollutants. Currently, some characterization methods including X-ray
photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy
(FT-IR), and scanning electron microscopy (SEM) have been applied to
analyze the adsorbent before and after heavy metal adsorption to elucidate the
adsorption mechanism. Based on the interactions between functional groups
and heavy metal ions, the adsorption mechanism can be divided into physical
adsorption and chemical adsorption. In addition, changes in pH values and/or
solution components can significantly alter the adsorption mechanism. Many
researchers also suggested that sodium alginate based adsorbent can reduce
metal ions to low states or metallic forms [64].
However, the redox mechanism is controversial and lack of direct evidence.
Moreover, considerable attention has been paid to theoretical investigations
on the molecular structure as well as the interactions between the adsorbent
and adsorbate in past few years with the development of density functional
theory (DFT) calculations DFT can be used to evaluate the binding energies
of heavy metal ions between the exposed surface functional groups,
representing the affinity and the selectivity order of adsorbent towards heavy
metal ions[65].
The chemical and physical properties of sodium alginate Sodium alginate can
be extracted by crushing of seaweed followed by alkaline extraction,
precipitation with calcium chloride, and reaction with sodium carbonate
solution. Sodium alginate is a linear anion copolymer with homopolymeric
blocks of (1–4)-linked β-Dmannuronate (M) and C-5 epimer α-L-guluronate
(G) residues, respectively,covalently linked together in different sequences or

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 Chapter I Introduction

blocks. The composition and structure difference of M and G blocks would

significantly change the gelation properties. The polymer with moreMblocks
would result in good elasticity while more G blocks lead to high hardness
and brittle gels. It is reported that the carboxyl groups and the pyranose
oxygen atoms of sodiumalginate can form stable five membered chelates with
metal ions, therefore providing binding sites for adsorption process [41].
.
1.4- Hydrogels of biopolymers
Hydrogels are three-dimensional polymeric networks that swell in contact
with water but maintaining their structural integrity [66]. This growing interest
in hydrogels is due to their excellent biocompatibility, easy preparation, and
versatile applications. In particular, they have shown widespread applications
in nutrient/drug delivery, tissue engineering, bioadsorbent, and separation
systems[67]. Recently, research on hydrogels made from various biomaterials
has been on the rise. Natural biopolymers, in particular polysaccharides such
as chitosan, pectin, sodium alginate and starch, are well-developed as the
matrix to fabricate hydrogels to meet different demands. Chitosan, obtained
by the deacetylation of chitin, is a popular polysaccharide biomaterial
composed of β-(1–4)-2-acetamido-2- deoxy-β-D-glucopyranose and 2-amino-
2-deoxy-β-D-glucopyranose with pKa value varying from6.3 to 6.5 [68].
Chitosan has received considerable attention due to its biocompatibility,
degradability, nonmammalian origin, and antibacterial properties [69] . The
antimicrobial property of chitosan was ascribed to its positive surface charge
from the amino groups of the glucosamine monomer at pH b 6.3, which
allows interactions with negatively charged microbial cell membranes leading
to the leakage of intracellular constituents [70]. As chitosan has limited chain
flexibility and poor mechanical strength, various methods such as
crosslinking, grafting and blending have been developed to broaden its
applications. In recent years, chitosan and its derivatives have been widely

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 Chapter I Introduction

used in food science[71] [72]

. One attractive application of chitosan is as a food
grade excipient in encapsulation technology. Chitosan matrices are used to
protect the encapsulated materials, i.e., bioactive compounds, additives, or
flavors, control their release to the outside, reduce their toxicity, and promote
targeted delivery after absorption from the gastrointestinal (GI) tract. This is
in part due to its much adhesive ability, permeation enhancing effect across
the biological surfaces, ease of chemical modification and biocompatibility [73].
Moreover, the cationic characteristic of chitosan in acidic condition may also
simplify the preparation of capsules for nutrients encapsulation since
electrostatic interactions between chitosan and anionic molecules are the main
driving forces in chitosan-based encapsulation systems [71] . Applications of
chitosan-based hydrogel beads in agriculture have attracted increasing
attention in recent years due to its abundance and low price, as well as rich
amino and hydroxyl groups. Promising results have already been achieved in
delivery of pesticides, micronutrients and fertilizers as well as its application
as adsorbent in wastewater treatment[74] [75].
Raw chitosan, however, is only soluble in a dilute acid solution, which limits
its wide applications. In addition, several drawbacks, such as low mechanical
strength, low thermal resistance and low adsorption selectivity, also hinder its
practical use [76]. Raw chitosan could be physically modified via conversion to
different forms, such as powder, flakes, and hydrogel (beads, membranes,
film, etc.), based on the consideration of ideal structure for special
utilization .However, employing chitosan in the form of flake or powder as
adsorbents has drawbacks, such as low adsorption capacity mainly because of
its crystallized structure. As a result, adsorption merely occurs on the
amorphous region of the crystals which limits its adsorption capacity [77].

This problem may be solved by making chitosan into hydrogel beads. In

addition to the simplicity in preparation and higher loading capacity, chitosan-

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 Chapter I Introduction

based hydrogel beads have increased porosity, expanded polymer chains,

increased surface area, decreased crystallinity and improved access to internal
sorption sites, which are desirable features for delivery and adsorption
applications. Considering the rapid advancement in research, it is important to
provide an updated review on the most recent progress in chitosan based
hydrogel beads for food and agriculture researches. This review specifically
focuses on the development in recent five years (2015–2019) and presents
three aspects of chitosan-based hydrogel beads: (a) preparation
andmodification of desirable chitosan-based hydrogel beads via physical or
chemical methods, and recent and new applications of chitosan-based
hydrogel beads in (b) food and (c) agriculture sectors.
2. Preparation andmodification of chitosan-based hydrogel beads There are
various structures of chitosan beads and each one can be synthesized by its
distinct technique, such as emulsion crosslinking, template-sacrifice, ion-
imprinting, extrusion etc. The schematic illustrations for five different forms
of chitosan-based hydrogel beads and their preparation methods are
represented Generally, the selections of different structures of chitosan beads
and preparation methods are dependent upon several requirements such as the
dimension, shape, thermal stability, release time of the active ingredients, and
residual toxicity of the final product[75] .
2.1. Physical modifications
2.1.1. Blending with polymers
Polymer blending has become a promising method for providing chitosan
with desirable properties in practical applications. Incorporating two or more
polymers into chitosan hydrogel matrix could combine properties of those
different materials with unique characteristics of chitosan. It has been
reported that chitosan blended with synthetic polymers (such as polyvinyl
alcohol (PVA) and polyvinyl chloride (PVC)) has shown good mechanical
and chemical properties. Physically crosslinked hydrogels are preferred to

16
 Chapter I Introduction

those prepared chemically due to easiness in formation of physical polymeric

network[78]. Among synthetic polymers, the main advantage of PVA is its
ability to form intra and intermolecular hydrogen bonds through freezing-
thawing cycles. The crystallite, formed via hydrogen bonding, acts as physical
crosslinking points among PVA chains leading to the formation of PVA
hydrogels[79]. It was reported that chitosan-PVA beads showed no cytotoxic
effect on human cells, and no hemolyzing and erythrocyte aggregating effect
in vitro[80].
Polysaccharide-based hydrogels composed of chitosan with other natural
biopolymers, such as sodium alginate and cellulose, have received increasing
attention because they are biodegradable, biocompatible, bioadhesive and
biofunctional[74]. For example, alginate beads can be easily prepared using
polyvalent cations as crosslinking agents, leading to the development of
hydrogelswith an egg-box structure. Alginate and chitosan can form
polyelectrolyte complex (PEC) together by ionic interaction via the carboxyl
residues of alginate and amino groups of chitosan. Schematic representation
of alginate and chitosan interactions is shown in Fig. 1A. Typically, the
chitosan-alginate hydrogel beads are core-shell structured and prepared by
adding alginate solutions dropwise into chitosan solution containing metal
cations, e.g., Ca2+, Zn2+, Cu2+, etc., under slowmagnetic stirring.
Comparedwith hydrogel beads prepared with chitosan alone, the PEC beads
possess improved physicochemical properties, such as enhanced stability in
swelling media of different pH, improved structural strength and mechanical
stability.
[81]
.

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 Chapter I Introduction

[1] A. L. Taka, K. Pillay and X. Y. Mbianda, Carbohydrate

polymers 2017, 159, 94-107.
[2] S. D. Mhlanga, B. B. Mamba, R. W. Krause and T. J. Malefetse,
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382-388.
[3] K. Salipira, R. Krause, B. Mamba, T. Malefetse, L. Cele and S.
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[4] K. Krishnamurthi, S. S. Devi, J. Hengstler, M. Hermes, K.
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 Chapter I Introduction

[7] M. Vallet-Regi and J. M. González-Calbet, Progress in solid

state chemistry 2004, 32, 1-31.
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