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Chapter 7
Photocatalytic Degradation
of Pharmaceuticals Using Graphene
Based Materials
William W. Anku, Ephraim M. Kiarii, Rama Sharma,
Girish M. Joshi, Sudheesh K. Shukla and Penny P. Govender
Abstract Pharmaceutical products are produced purposely for the treatment of
diseases with the aim of improving human health. Despite their usefulness to
human and animal health, pharmaceuticals are now being regarded as emerging
environmental pollutants. This is due to their increased use and the fact that they are
indiscriminately discharged into the aquatic environment from hospitals, house-
holds, industries, pharmacies, as well as leakages and leachates from municipal
wastewater treatment plants and landfill sites. Moreover, the conventional methods
of wastewater treatment were not designed with these emerging pollutants in mind
resulting in the discharge of untreated or incomplete treated wastewater into water
bodies. Pharmaceuticals in water are believed to exert deleterious effects on humans
W. W. Anku E. M. Kiarii S. K. Shukla (&)P. P. Govender
Department of Applied Chemistry, University of Johannesburg,
Doornfontein Campus, P. O. Box 17011, Johannesburg 2028, South Africa
e-mail: sudheeshkshukla010@gmail.com
W. W. Anku
e-mail: williamanku85@gmail.com
E. M. Kiarii
e-mail: kiariiem@gmail.com
P. P. Govender
e-mail: pennyg@uj.ac.za
R. Sharma
Department Agriculture Science, AKS University, Satna 485001, Madhya Pradesh, India
e-mail: rama81093@gmail.com
G. M. Joshi
Polymer Nanocomposite Laboratory, Center for Crystal Growth, VIT University, Vellore
632014, Tamilnadu, India
e-mail: varadgm@gmail.com
S. K. Shukla
Department of Biomedical Engineering, Ben-Gurion University of the Negev, Beer Sheva,
Israel
©Springer International Publishing AG, part of Springer Nature 2019
M. Naushad (ed.), A New Generation Material Graphene: Applications in Water
Technology, https://doi.org/10.1007/978-3-319-75484-0_7
187
and aquatic organisms. The concern to remove these pharmaceutical wastes and
their metabolites from wastewater before their final discharge into water bodies has
culminated in the development of a wide variety of other treatment technologies
such as adsorption, chemical oxidation, liquid extraction, biodegradation, and so
on. However, because these pharmaceuticals are mostly water soluble and
non-biodegradable, most of the treatment techniques are inappropriate for their
effective removal. The deployment of an appropriate technique for effective
degradation of pharmaceutical wastes in water has therefore become a necessary
requirement. This chapter therefore provides a detailed discussion on pharmaceu-
ticals in general, their occurrence in water and their health consequences. It also
delved into the photocatalytic degradation of these chemicals in water with
emphasis on the use of graphene based materials.
Keywords Pharmaceuticals Photocatalytic degradation Emerging environ-
mental pollutants Water treatment methods Graphene pollutants
1 Introduction
The world is faced with a number of challenges. Among these changes is the
difficulty in guaranteeing access to potable water. This problem is worsened by the
increasing world population, and proliferation of many emerging pollutants which
are difficult to remove (Qu et al. 2012). It is estimated that about 884 million people
across the world require access to potable water; and about 1.8 million children die
annually from water-related illnesses (Qu et al. 2012). These figures are likely to
double in the not so distant future due to increase in the quantity and diversity of
contaminants being discharged into the natural water bodies driven by a quest for
better human life. This scenario presents a big challenge for the current and future
generations, posing a significant threat to the well-being of mankind (Dosch 2009).
Environmental and water pollution advocates have raised a big concern over the
high rate of water pollution, particularly with pharmaceutical wastes.
Pharmaceutical products, despite their usefulness to human and animal health, are
now being regarded as emerging environmental pollutants as many of these
chemicals are now being detected in water bodies. The main sources of these
pollutants into the environment, particularly water bodies, include their indis-
criminately discharged through production processes or industrial activities, leak-
ages and leachates from municipal wastewater treatment plants and landfill sites
(White and de Groot 2006). In addition, only a few of the drugs consumed get
wholly metabolized. The remainders are excreted from the body though the skin as
sweat, as urine and feces, and finally find their way into the wastewater (White and
de Groot 2006). Some of the drugs identified in water included antidepressants,
antibiotics, heart medications such as ACE inhibitors, digoxin, calcium-channel
blockers. Others are blood thinners, hormones, painkillers, caffeine, carbamazepine,
188 W. W. Anku et al.
antiseizures and some fragrance chemicals (tonalide and galaxolide) (Mompelat
et al. 2009; Wang et al. 1993).
Even though the health effects of consuming pharmaceutical waste contaminated
water is not well documented, toxicity emanating from the prolonged exposure to
lower concentrations of mixtures of these pollutants can be devastating
(Rodriguez-Mozaz and Weinberg 2010). In addition, many of these pollutants
interact with other components of the water and transform into different interme-
diate products which may exert toxic ecological effects (Tijani et al. 2013). Fishes
and some other aquatic organisms have been identified to exhibit both male and
female sex features in heavily polluted waters owing to the increase in disposal of
estrogen and chemicals that affect the male or female fish species (Woodling et al.
2006). Traces of antibiotics in the water can induce drug resistance tendencies of
microorganism resulting in more rampant hospitalization (Guardabassi et al. 2002).
Thus, the quality of water for domestic, industrial, and agricultural use is of
paramount concern. The deployment of an appropriate technique for effective
degradation of pharmaceutical wastes in water has therefore become a necessary
requirement.
Because these pharmaceuticals are mostly water soluble and non-biodegradable,
most of the conventional treatment techniques are inappropriate for their effective
removal. The conventional methods of wastewater treatment were not designed
with these emerging pollutants in mind resulting in the discharge of untreated or
partially treated wastewater into water bodies (Tambosi et al. 2010). The concern to
effectively remove these pharmaceutical wastes and their metabolites from
wastewater before their final discharge into water bodies has culminated in the
development of a wide variety of other treatment technologies such as membrane
separation, chemical oxidation/precipitation, electrochemical separation, liquid
coagulation/extraction, biodegradation, and photocatalysis which is the main focus
of this chapter.
Photocatalysis has become a powerful way to mitigate the effects of environ-
mental pollution owing to its simple and comprehensive pollutants decomposition
tendency (Chen and Ye 2008; Liu et al. 2013; Xu et al. 2011). Metal oxides
photocatalysts have attracted interest in research due to their tailor-made bandgap
and high photodegradation efficiencies for many organic and inorganic water pol-
lutants (Liu et al. 2014). Combination of graphene materials with metal oxide
photocatalysts are known to improve the performance of the catalysts by helping to
overcome their inherent problems of ultraviolet light activity, and rapid
electron-hole recombination rates (Khan et al. 2017). Numerous favourable prop-
erties of graphene makes it a potential promising material for applications in
composites for water treatment. It’s large surface area, chemical purity, and the
possibility for its easy functionalization which makes it link easily with other
inorganic and organic materials (Yao et al. 2014,2012) allow graphene to provide
opportunities for water treatment.
Several graphene material based nanocomposites have been synthesized and
used efficiently in the photocatalytic degradation of pharmaceuticals in water. The
aim of this chapter is to provide a general overview on the use of graphene material
7 Photocatalytic Degradation of Pharmaceuticals …189
based composite for the degradation of pharmaceuticals in water. The chapter also
takes into consideration the sources of occurrence of these pharmaceuticals in
water, their potential health effects and the unique properties of graphene materials
that make them appropriate components of such composites.
2 Pharmaceuticals
Pharmaceuticals are a specific class of synthetic and natural chemicals with
bioactive characters that are designed and manufactured purposely to exert thera-
peutic effects against diseases, or to prevent the occurrence and spreading of disease
in humans and animals (Tijani et al. 2013). They may be available as
over-the-counter, prescription or veterinary drugs. Increasing number of these
chemicals are distributed or dispensed and consumed each year throughout the
world due to the emergence of new diseases and discovery of new drugs to cure
these emerging diseases. Other factors that cause increasing distribution of phar-
maceutical products include increasing population, accessibility/availability of
inexpensive drugs as more patents reach their expirations (Daughton 2003). Most of
these chemicals are now being regarded as water pollutants since their presence is
increasingly being determined in water bodies due to industrial, agricultural,
domestic, and veterinary practices; and their likelihood of producing toxic effects
on humans, non-target organisms, and microorganisms in the water bodies (Segura
et al. 2009).
2.1 Occurrence of Pharmaceuticals in Water
The presence of pharmaceutical products in the aquatic environment has been
mainly attributed to anthropogenic activities as research indicates that all pharma-
ceutical products detected in water bodies, apart from caffeine, are synthetic
(Marques et al. 2013; Seiler et al. 1999). Though pharmaceutical products have
played significant roles in the prevention, and the treatment of diseases in humans,
as well as animals, their presence in the aquatic environment has been a great
concert to various organization across the world; including environmental and
health organizations (Benotti et al. 2008; Xagoraraki and Kuo 2008). This is
because since these chemicals are design specifically to be toxic to disease-causing
microorganisms, and interact effectively with receptors in both humans and ani-
mals, they have the potential to cause harm to microorganisms and other organisms
in the water bodies (Jelićet al. 2012). It is therefore a necessity to understand the
modes/routes of entry of pharmaceutical products into water bodies so as to be able
to devise appropriate means of avoiding their likely adverse effects on humans,
animals, and aquatic lives. Among these routes of entry of pharmaceuticals into
190 W. W. Anku et al.
water bodies are industrial activities, municipal waste or sewage treatment outlets,
landfill leachates and wastewater from hospitals and pharmacies.
2.1.1 Industrial Activities
Industrial activities is a major route of entry of pharmaceutical products into water
bodies. Studies conducted in recent times have recognized direct discharges or
emissions from manufacturing industries as a major source of pharmaceutical
wastes into the environment. Though pollution through industrial activities is not
widespread, discharges with the potential to enhance drug-resistant tendencies of
microorganisms can create global problems (Larsson 2014). Wastewater emanating
from pharmaceutical industries contain a number of these chemicals and their
mixtures. These pollutants, either non-degraded or partially degraded (because the
treatment plants are ineffective against these micro-pollutants) is finally released
into the receiving water bodies. Possible leakages from the sewer system from the
industry to the treatment site may also contaminate the land. Runoffs during rain
then wash these pollutants into the water bodies. Gaseous or volatile pharmaceu-
ticals may also escape into the atmosphere and bound to air borne particles. They
can then directly contaminate surface water bodies through aerial deposition during
rain, or settle on land and washed by flood into the water bodies (Burkhardt-Holm
2011). Research conducted in Patancheru, an important area for drugs manufac-
turing in India, confirmed the accession that drugs production activities contribute
largely to the presence of pharmaceutical wastes in the environment. The con-
centration of pharmaceuticals, particularly ciprofloxacin, in the effluent of the plant
treating wastewater from these industries was identified to be much higher than
those present in patients’blood (Fick et al. 2009; Larsson et al. 2007).
2.1.2 Municipal Waste/Sewage Treatment Plant Outlets
Municipal waste treatment plant effluent is documented as one of the main sources
of entry of many pharmaceuticals, and their intermediate products of metabolic
reaction into the aquatic environment (Kümmerer 2009). The conventional
municipal waste treatment plants were not designed with pharmaceutical wastes in
mind, as these immerging pollutants were not an issue of environmental concern at
the time these plants were being designed. Some of these pollutants are therefore,
either not affected or degraded by the treatment processes. Others may also be
converted into other intermediate products which may even be more harmful than
the parent chemical (Ellis 2006; Ternes et al. 2004). They then become part of the
effluent, which finally is released into the receiving water bodies. According to Ellis
(2006), Sewer leakages from drainage systems resulted in the contamination of
water bodies with some pharmaceuticals such as diclofenac, clofibric acid,
ibuprofen, and iopamidol. One of the ways to handle the sludge generated through
the sewage treatment is its application as manure in agricultural fields.
7 Photocatalytic Degradation of Pharmaceuticals …191
This approach, apart from managing the generated sludge, reduces the cost and the
quantity of synthetic fertilizers used on agricultural fields. However, the pharma-
ceutical waste components of this sludge are washed by agricultural runoff into the
nearby water bodies with their subsequent pollution with these chemicals (Passuello
et al. 2010). In their study to assess the possible occurrence of pharmaceuticals in
wastewater treatment plant streams in Dublin, Lacey et al. (2012) identified pro-
pranolol, mefenamic acid, carbamazepine, nimesulide, diclofenac, furosemide,
gemfibrozil, clotrimazole and metoprolol as pharmaceuticals present in the three
streams assessed.
2.1.3 Landfill Sites/Leachates
In situations where expired or unused pharmaceuticals or medication are not flushed
down the toilet, they normally get thrown into the waste bin or household waste and
disposed of to landfill sites. At the landfill site, these chemicals are likely to undergo
degradation and/or reaction with other reactive pollutants resulting in the generation
of other intermediate compounds, which may equally be injurious to human health
and the environment. These chemicals then form part of the landfill leachate and
enter the ground or nearby surface water (Boxall et al. 2004; Walker et al. 2012).
2.1.4 Wastewater from Hospitals and Pharmacies
Hospitals and pharmacies are major sources of occurrence of pharmaceutical in the
aquatic environment. In most cases, drugs do not completely metabolized in the
human body. The remaining portions of these drugs and their metabolites are then
excreted through feaces and urine into the sewer system (Aksu 2005; Rivera-Utrilla
et al. 2013). In addition, some expired and unused/left-over drugs are discarded into
the water closet. All these pharmaceutical products then get flushed down through
the sewer system into water treatment plants and finally into the surface and
groundwater bodies. They can also be directly discharged into the nearby water
bodies by floodwater when the unused or expired drugs and urines are indiscrim-
inately discharged into drains. A study conducted in Taiwan by Lin and Tsai (2009)
confirmed the occurrence of pharmaceuticals in receiving rivers and wastewaters
from hospitals and pharmacies. The study, which was conducted in three rivers and
waste streams of six hospitals identified acetaminophen, erythromycin-H
2
O, sul-
famethoxazole and gemfibrozil as contaminants of the streams and water bodies,
with acetaminophen being the most ubiquitous with higher concentrations.
192 W. W. Anku et al.
2.2 Health Effects of Pharmaceutical Drugs in Water
Pharmaceutical products, upon entry into the environment, can undergo degradation
through biotic and/or abiotic means and adsorb onto suspended particles and sed-
iment. Some can also accumulate into aquatic organism (Ramirez et al. 2009) and
become part of the food chain. As far as humans are concerned, drinking of con-
taminated water is acknowledged as the main route of exposure to pharmaceutical
products (Daughton 2001).
There is currently no documented evidence of human health effect in relation to
the exposure to pharmaceuticals in water (Jones et al. 2005). However, the likeli-
hood of a potential health effect upon continuous, and low dose chronic exposure to
these pollutants is not ruled out (Schwab et al. 2005). This is because individual
pharmaceutical products are intended to exert specific effects on targeted parts of
the body over a specific period. Therefore, continuous exposure to mixtures of these
chemicals over a long period can gradually induce effects that could be noticeable
with time. In addition, some of these drugs are gender or age specific. Therefore, the
situation where the exposure cuts across age and gender is suspected to create
health problem with prolonged exposure. Sensitive groups such as children, the
elderly, and pregnant women are at high risks (Kolpin et al. 2002).
The possible effects of these chemicals on aquatic lives is also not clearly
understood. The probable effects of these chemicals on the organisms in field
situations are often based on laboratory toxicity data. These data may, however, not
be the exact representation of what happens in the field (Versteeg et al. 2005).
These experiments normally consider the effects of a single pharmaceutical product
on one organism. Meanwhile, real-life situations involved simultaneous exposure to
mixtures of the pollutants across generations (Trudeau et al. 2005). Unlike humans
who experience limited exposure to these compounds usually through drinking of
contaminated water, aquatic organisms experience life-time exposure and are
therefore more at risk compared to humans. Laboratory studies have indicated that
these pollutants have the potential to inflict adverse health effects on aquatic
organisms (Brooks et al. 2003). Research has shown that effluents emanating from
pharmaceuticals wastewater treatment plants with traces of birth control drugs
induced reproduction problems in fish upon exposure. Effluents containing endo-
crine disrupting drugs, particularly hormones and synthetic steroids, have also been
identified as having the tendency of altering sex ratios in aquatic organism,
including fish (Schultz et al. 2003). They can also cause male fish to exhibit
feminine features (feminization) and other changes that affect the overall health of
the aquatic organisms (Schultz et al. 2003; Zillioux et al. 2001). Another concern is
the possibility that pathogens may develop resistance to antibiotics making these
drugs ineffective in curing several diseases.
7 Photocatalytic Degradation of Pharmaceuticals …193
3 Graphene and Graphene Materials
Graphene is a modern from of carbon nanomaterials derived from graphite. Its two
dimensional structure coupled with its auspicious structural, electrical, optical, and
mechanical characteristics make it favorable for use in many fields. It is used as a
nano-electric constituent, a component of nano-electrochemical products, energy
storage devices, sensors among others (Dosch 2009).
Graphene can be described as the backbone of carbon nanomaterials (Castro
Neto et al. 2009). Its structure is described as a hexagonal lattice and is viewed as
two interleaving lattices as illustrated in Fig. 1.
Graphene is a very stable carbon material due to the closely packed atoms of
carbon and their SP
2
hybridized orbitals in addition to the S, P
X
, and P
Y
orbitals
constituting the a-bond (Cooper et al. 2012). The pbond is made up of the final P
Z
electrons. This pbond forms the pband and p* bands that are accountable for
graphene’s distinguished electronic properties as there is free movement of elec-
trons from the half-filled band (Cooper et al. 2012).
Graphene oxide (GO) (Fig. 2a), is the functionalization product of graphene
sheet. Presently, Hummers’method (KMnO
4
, NaNO
3
,H
2
SO
4
) is the technique
mostly used for the preparation of graphene oxide (Marcano et al. 2010). (Xiang
et al. 2010). Graphene oxide is an electrical insulator; it has the sp
2
bonding of
graphene disrupted. To improve the electrical conductivity and retain the honey-
comb hexagonal lattice, reduction of the graphene oxide is done to obtain reduced
graphene oxide (rGO) (Fig. 2b). The reduced graphene oxide obtained is indis-
pensable. It restores the sp
2
bonding present in pure graphene and produces two
dimensional carbon material that allows free movement of charges across con-
Fig. 1 Lattice structure of
graphene
194 W. W. Anku et al.
ductive nanomaterials (Lightcap et al. 2010). Graphene oxide has been used in
water purification, optoelectronics, coating, and battery electrode and so on. The
main advantages of graphene oxide over graphene is its ease of dispersion in
organic solvents, water and other inorganic solvents because of the oxygen func-
tionalities (Xiang et al. 2010).
Graphene further yields nanostructures which can be grouped into; quantum dots
(GQDs) and nanoribbons (GNR). Graphene nanoribbons can be cut into different
widths. This is achieved through electron-beam lithography and an etching mask
(Chen et al. 2007; Han et al. 2007). GQDs can be described as small fragments of
graphene with all the properties of graphene. However, GQDs normally have
dimensions 20 nm in diameter and below. The theoretical study of graphene
quantum dots has followed the work on graphene very closely shedding light of the
experimentally obtained results. Quantum dots can be made through cutting gra-
phene sheets using a top-down method (Simpson et al. 2002; Wu et al. 2004).
GQDs have been used in energy conversion (Zhang et al. 2012), sensors production
(Dong et al. 2010) and bioanalysis (Dong et al. 2010).
O O
HO
O O
OH
OH
OH
HO
HO
O OHO
OO
(b)
O
O
O
O
O
O
O
OO
OO
(a)
OH
OH
OH
O
O
OH
OO
OOH
OH
OH
(c)
OH
O
-
O
-
OH
OH
OH
O
-
O
-
OH
O
-
OH
OH
OH
OH
O
-
OH
OH
(d)
Fig. 2 Structural models of GO proposed by aHofmann, bScholz–Boehm, cRuess and
dNakajima-Matsuo (Redrawn from Szabóet al. 2006)
7 Photocatalytic Degradation of Pharmaceuticals …195
3.1 Properties of Graphene Materials for Use
in Environmental Applications
3.1.1 Optical Properties
Graphene absorbs maximally in the ultraviolet (UV) wavelength (Fig. 3). However,
there is a significant absorbance in the visible region of the solar spectrum as shown
in the wavelength between 320 and 800 nm. This shows graphene’s ability to make
maximum utilization of the solar energy which span a whole range of spectrum
(Falkovsky 2008). Thus, the tendency of graphene to absorb light across a wide
range of the electromagnetic spectrum enables it to enhance the degradation
properties of photocatalyst when combined with them.
3.1.2 Electrical Properties
Perfect graphene has a zero bandgap in band structure (Fig. 4). The zero band gap
of pristine graphene makes it a perfect conductor with high conductivity. Electron
arrangement in the graphene lattice is dominated by the S and P orbitals. Much
research has been done on how to tune the band gap in graphene-based semicon-
ductor materials (Gupta et al. 2013) in order to convert graphene from a perfect
conductor into a semiconductor. The band gap tuning can be achieved in pristine
graphene via nanopatterning (the fabrication of a nanoscale pattern, especially as
part of an electronic component) (Giovannetti et al. 2007; Han et al. 2007),
Fig. 3 Graphene optical properties
196 W. W. Anku et al.
application of gate voltage (Avetisyan et al. 2009; Dragoman et al. 2010; Model
2009; Molitor et al. 2008), or chemical functionalization (Elias et al. 2009; Jung
et al. 2008; Wu et al. 2008).
The electronic properties of GO depend on its chemical structure (He et al. 1998;
Kumar et al. 2014; Mermoux et al. 1991). The epoxide functional groups on GO
significantly induce the local distortion of graphene with a new bond formed by
graphene and oxygen atoms. This affects the bonding characteristics of carbon,
changing from planar sp
2
to partial sp
3
hybridization. Considering an arrangement
of epoxy functional groups in fully oxidized graphene sheet and the effect of epoxy
arrangement on electronic properties, there was significant induction of bandgap of
0.529 eV (Lin-Hui et al. 2001). Hybridization of graphene materials with photo-
catalysts will therefore result in high photodegradation process as the graphene
materials exhibit efficient electron conductivity coupled with the ability to minimize
charge carriers recombination rate through acceptance of photogenerated electrons.
3.1.3 Mechanical Properties
Graphene is the thinnest and lightest compound known to man (one atom thin) and
thus grows into paper-like sheets. It forms the strongest compound discovered to
date (graphene’s ultimate tensile strength is 130 billion Pascal compared to 400
million for A36 steel or 375.7 million for Aramid (kelwar)) (Dhiman and Ashutosh
Fig. 4 Graphene zero bandgap
7 Photocatalytic Degradation of Pharmaceuticals …197
2014). Graphene is the lightest at 0.77 mg per square sheet (Rodriguez-Mozaz and
Weinberg 2010). This can be qualified into three key features which determines the
general mechanical properties of graphene and its nanocomposites: (1) interlayer
crosslinks involving mostly the functionalised graphene where the side atoms are
involved in covalent bonding mostly with other side atoms in the sheet, or some
weak Van Der Waals forces between the carbon-carbon graphene in two separate
sheets forming layers, and (2) intralayer mechanical properties, defined by sp
2
carbon-carbon covalent bonds and crosslinks at graphene edges (Liu et al. 2012).
Graphene materials are therefore regarded as appropriate materials for catalyst
support as a result of their outstanding mechanical properties and high specific
surface area which promotes their propensity to adsorb and disperse pollutants for
resounding degradation process.
4 Photocatalysis
Photocatalysis refers to the occurrence of a chemical reaction through the use of
light as the source of energy, and metal oxide catalysts (photocatalysts) as absor-
bents of photons of the light to initiate and speed up the reaction (Divya et al. 2014).
Photocatalysis is known to have been deployed in solving many environmental
problems including the degradation of organic and inorganic pollutant in water as
well as disinfection of water (Saggioro et al. 2011). This technique is hailed as the
appropriate method for the degradation of many water pollutants because of its
ability to completely degrade the pollutant to less harmful products without the
generation of any secondary pollutants such as sludge. Other advantages of pho-
tocatalysis include its environmental friendliness, cost effectiveness and the fact that
it can be operated in areas without electricity (Yu et al. 2011).
Metal oxide photocatalysts normally have filled valence band and empty con-
duction bands. When these photocatalysts are irradiated with photons of appropriate
energy, electrons on the valence band acquire enough energy and become exited
and promoted to the conduction band. This occurrence leads to the creation of
positive charges (holes) on the valence band and the accumulation of electrons on
the conductions band (Fig. 5) (Tryk et al. 2000). It is the creation of the
electron-hole pairs that initiates the photocatalytic degradation process. The excited
electrons on the conduction band have the tendency to fall back to the valence band
and recombine with the holes causing the reaction to cease. Otherwise, they par-
ticipate in reactions with oxygen molecules (O
2
) and reduce them to superoxides
(
O
2
−
).
O2þe!O
2
On the other hand, the holes on the valence band also attacks water molecules
and oxidize them to hydroxyl radicals (
OH).
198 W. W. Anku et al.
H2Oþhþ!HOþHþ
The resultant
O
2
−
and
OH (excellent oxidizing agents) then oxidize the pol-
lutants or degrade them into various products. The end products of the pho-
todegradation process is usually carbon dioxide and water provided the reaction is
allowed to occur over a long enough period of time (Dung et al. 2005).
4.1 Photocatalytic Degradation of Pharmaceutical Products
As long as various domestic activities, recreation, food supply, and so on continue
to rely on the use of surface and ground water bodies, the accumulation of phar-
maceuticals in these water bodies will remain a matter of high public health con-
cern. Conventional methods of wastewater treatment, most of which use activated
sludge procedure, have been identified to exhibit limited efficiency in the removal
of pharmaceutical wastes from polluted wastewater (Li and Randak 2009). Thus,
the development of innovative wastewater treatment techniques that are environ-
mentally benign and capable of complete degradation of these pollutants has
become a paramount issue in the scientific domain. Heterogeneous semiconductor
photocatalysis has been aggressively tested in recent years for its ability to com-
pletely irradiate these pollutants from wastewater dues to its numerous advantages
as discussed in the previous section.
VB
CB O
2
O
2-
H
2
O
OH
-
pollutant
Photocatalyst
Recombination
Excitation
Visible light
e
-
h
+
h
+
h
+
e
-
e
-
CO
2
+ H
2
O
Fig. 5 Mechanism of photocatalytic degradation of pollutants in water
7 Photocatalytic Degradation of Pharmaceuticals …199
4.1.1 Graphene Based Materials for Pharmaceutical Products
Photocatalytic Degradation
Despite the numerous advantages of photocatalytic degradation of pharmaceuticals
in water, the effectiveness of the process in hindered by certain factors. Among
these factors are the fast rate at which the photogenerated electrons and holes
recombine. Another problem is the fact that most of the photocatalysts are only
active under ultraviolet (UV) light rather than visible light (which is more abun-
dant). Thus, in order to attain an efficient photocatalytic degradation process, the
photocatalyst must be active under visible light with reduced electron-hole
recombination rate. It should be non-toxic with excellent pollutants adsorption
property (Doll and Frimmel 2005). It has been identified that one of the best ways to
produce an effective photocatalyst is to modify the traditional photocatalyst with
graphene materials. Graphene has excellent pollutants adsorption property due its
large specific surface area coupled with delocalized pi-electron system that
enhances its interaction with the pollutants (Wang et al. 2013). Graphene materials
based photocatalysts have been identified to exhibit higher photocatalytic degra-
dation efficiencies compared to the non-graphene based ones due to fact that the
later possess large specific surface area with profound pollutants adsorption prop-
erty, ability to minimize the rate of recombination of the photogenerated
electron-hole pairs, and outstanding visible light activity (Oppong et al. 2016).
Different types of graphene based materials modified metal oxide photocatalysts
have been employed in the degradation of pharmaceuticals in water. The subse-
quent sections give detail discussions on the use of graphene based TiO
2
photo-
catalysts, coupled graphene based metal oxide photocatalysts, and other graphene
materials based photocatalysts in photocatalytic degradation of pharmaceutical in
wastewater.
Use of Graphene Based TiO
2
Photocatalysts
Titanium dioxide (TiO
2
) is undoubtedly the most widely used photocatalyst for the
degradation of many pollutants in water as a result of fact that this catalyst has high
photocatalytic activity, is less toxic and relatively cheap. Modification of TiO
2
with
graphene materials is known to improve its visible light activity and reduce the
photogenerated electron-hole recombination rate thereby improving its photocat-
alytic activity. The use of graphene based titanium dioxide photocatalyst in the
degradation of pharmaceuticals in water is regarded as one of the appropriate
approaches for the effective irradiation of these pollutants. Lin et al. (2017) con-
ducted a study by comparing the effectiveness of TiO
2
-rGO and TiO
2
-Fe in the
degradation of ibuprofen, carbamazepine and sulfamethoxazole in aqueous solution
under visible light. TiO
2
-rGO was noted to demonstrate higher photodegradation
activity against all the pharmaceutical products. The comparatively higher perfor-
mance of the TiO
2
-rGO was attributed to the presence of rGO in the composite. The
rGO accepted the photogenerated electrons and thus reduced the recombination rate
200 W. W. Anku et al.
of the electron-hole pairs. Photocatalytic degradation of 10 ppm carbamazepine
aqueous solution with rGO-TiO
2
was also observed to achieve 99% efficiency
within 90 min. This value was twice that achieved by the bare TiO
2
due to the
availability of abundant surface area for adsorption of the pollutants and effective
charge carriers separation and transportation (Nawaz et al. 2017). A graphene oxide
modified TiO
2
composite (GO-TiO
2
) was synthesis through liquid phase deposition
and employed to photocatalytically degrade diphenhydramine under ultraviolet and
visible light irradiation (Pastrana-Martínez et al. 2012). The GO content was noted
to be responsible for the composite’s large surface area, and its porous nature.
The GO was also noted to improve the photocatalytic activity of the GO-TiO
2
compare to that of the bare P25. This was due to the ability of GO to effectively
capture the photogenerated electrons from the TiO
2
and prevent them from
recombining with the holes, enhancing the adsorption of the pollutants due to its
large surface area, and by extending the visible light absorption of the composite to
longer wavelengths. Similar research work involving the use of GO-TiO
2
in the
degradation of diphenhydramine has also been conducted by Morales-Torres et al.
(2013) with the GO-TiO
2
demonstrating higher photocatalytic activity against the
pollutant.
Use of Coupled Graphene Based Metal Oxide Photocatalysts
Coupled metal oxide photocatalysts immobilized on graphene materials have also
been effectively used for the photocatalytic degradation of pharmaceuticals in
water. An example is the ZnFe
2
O
4
coupled Ag nanocomposites synthesized by
Khadgi et al. (2016). The coupled nanocomposites were immobilized on reduced
graphene oxide (rGO) to form ZnFe
2
O
4
-Ag/rGO nanocomposite, which was
effectively deployed in the photocatalytic degradation of 17a-ethinylestradiol, an
endocrine disrupting compound. The ZnFe
2
O
4
-Ag/rGO was observed to be pho-
tocatalytically effective than the ZnFe
2
O
4
alone and the ZnFe
2
O
4
-Ag. They
attributed the effectiveness of the ZnFe
2
O
4
-Ag/rGO nanocomposite to the presence
of the rGO, which minimized agglomeration of the nanocomposites. The rGO also
enhanced the composite’s visible light utilization, its effective charge carriers
generation and transfer/separation, its specific surface area with the accompanying
higher pollutant adsorption capacity. Comparatively, the ZnFe
2
O
4
-Ag/rGO
demonstrated about 5.6 time higher photocatalytic activity than the ZnFe
2
O
4
-Ag.
Similarly, oxytetracycline and ampicillin have been successfully degraded in
aqueous solution through visible light photocatalysis using a nanocomposite con-
sisting of Bi
2
WO
6
/Fe
3
O
4
immobilized on graphene and sand (Bi
2
WO
6
/Fe
3
O
4
/GSC)
(Raizada et al. 2017). The Bi
2
WO
6
/Fe
3
O
4
/GSC was noted to degrade 95 and 94%
of ampicillin and oxytetracycline respectively. On the other hand, the nanocom-
posite without graphene (Bi
2
WO
6
/Fe
3
O
4
) degraded 74 and 71% respectively, of
ampicillin and oxytetracycline, all within 60 min. The results pointed out the sig-
nificant role played by graphene in enhancing the pollutant adsorption capacity of
the composite. The effectiveness of coupled graphene based photocatalysts in the
7 Photocatalytic Degradation of Pharmaceuticals …201
photocatalytic degradation of pharmaceuticals in water has also been demonstrated
by the use of a silica coated Fe
3
O
4
-TiO
2
for efficient degradation of caffeine and
carbamazepine in aqueous solution. The composite exhibited higher photocatalytic
activity compared to commercial P25 with additional advantages of high recover-
ability and reusability (Linley et al. 2014). Fe
3
O
4
/Mn
3
O
4
-rGO nanocomposite was
also used for a comprehensive photocatalytic degradation of aqueous sulfamet-
hazine solution. The results revealed 99% sulfamethazine degradation efficiency at
optimum conditions of 0.07 mm/L sulfamethazine concentration, 0.5 g/L of Fe
3
O
4
/
Mn
3
O
4
-rGO nanocomposites, 35 °C, pH3 and hydrogen peroxide concentration of
6 mM (Wan and Wang 2017). Other studies carried out includes tracycline pho-
todegradation with rGO-CdS/ZnS (Tang et al. 2015) and quinolone photodegra-
dation with Fe
3
O
4
@Bi
2
O
3
-rGO (Zhu et al. 2017).
Use of Other Graphene Materials Based Photocatalysts
A nanocomposite consisting of graphene oxide decorated cerium molybdate
nanocubes (Ce (MoO
4
)
2
/GO)) was synthesized and utilized in the photocatalytic
degradation of chloramphenicol, an antibiotic, under visible light irradiation
(Karthik et al. 2017). The Ce(MoO
4
)
2
/GO composite displayed excellent pho-
todegradation potential against the drug and showed higher degradation efficiency
(99% within 50 min) than the pure Ce(MoO
4
)
2
nanocubes. The impressive per-
formance of the composites was assigned to the excellent separation of the pho-
togenerated electrons and holes. With the same intension of photocatalytic
degradation of pharmaceuticals in mind, (Anirudhan et al. 2017) fabricated a gra-
phene oxide modified nanohydroxyapatite composite through facile chemical
modification of graphene oxide by triethyltetramine and applied it in the removal of
aureomycine hydrochloride through adsorption and subsequent photocatalytic
degradation. The band gap of GO was observed to improve to 2.4 eV after its
modification with the triethyltetramine and enhanced the composites interaction
with the pollutant. A sample of the real aureomycine hydrochloride polluted
wastewater from the nearby poultry and animal farms was used as the source of
aureomycine hydrochloride. The photodegradation experiment was performed
under visible light irradiation with successful results. The aureomycine
hydrochloride photodegradation was noted to have followed the pseudo first-order
kinetics.
Similarly, Anirudhan et al. (2017) degraded ciprofloxacin hydrochloride aque-
ous solution through visible light photocatalysis using a nano zinc oxide incorpo-
rated graphene oxide/nanocellulose composites (ZnO-GO/NC). They observed that
the band gap of the GO was tunable to 2.4 eV as a result of incorporation of the
ZnO into the GO and further increased to 2.8 eV upon the introduction of the
nanocellulose. Their analysis revealed that the composite displayed a maximum
photocatalytic degradation efficiency of 98% at optimum pH of 6. The composite
exhibited the tendency of being recycled and reused five consecutive times without
significantly losing its efficacy. The improved photocatalytic efficiency of the
202 W. W. Anku et al.
ZnO-GO/NC was attributed to the improvement in its excited electrons generation
and conduction resulting in minimized charge carriers recombination rate due to the
presence of GO. Tang et al. (2017) examined the effectiveness of BiVO
4
sensitized
graphene quantum dots (GQD/BiVO
4
) in the photocatalytic degradation of carba-
mazepine while Aalicanoglu and Sponza (2015) used graphene oxide magnetite
(GO-Fe
3
O
4
) in the photocatalytic degradation of ciprofloxacin. In both cases, the
GQD/BiVO
4
and GO-Fe
3
O
4
showed improved optical properties and were iden-
tified to be more effective than the bare BiVO
4
and Fe
3
O
4
due to the presence of the
graphene materials. Inorganic salts assisted hydrothermal method was also
deployed in the synthesis of Bi
3.84
W
0.16
O
6.24
-graphene oxide (BWO-GO). The
composite showed successful results when used for the degradation of tetracycline
(Song et al. 2016).
5 Conclusion
The need for the development of more effective methods for eradication of phar-
maceuticals from water has become imperative as these pollutants possess the
tendency to exert devastating health effects on humans as well as aquatic organ-
isms; and due to the inefficiency of the traditional water treatment techniques to
effectively remove these pollutants from polluted water. The routes of entry of
pharmaceuticals in water, and their health effects on both human and wildlife have
been discussed in this chapter. Detailed discussion on graphene and graphene
materials, and the importance of using graphene materials based photocatalysts
have also been discussed. Our analysis revealed that application of graphene
materials based photocatalyst for the degradation of pharmaceuticals in water is an
effective approach as most of the available data indicate outstanding results.
However, we observed that limited data is available in this regard. We therefore
recommend the synthesis and application of other graphene materials based metal
oxide photocatalyst, apart from TiO
2
, in the photocatalytic degradation of phar-
maceuticals in water.
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