Photocatalytic mineralization and degradation kinetics of ampicillin and oxytetracycline antibiotics using graphene sand composite and chitosan supported BiOCl

Article abstract of DOI:10.1016/j.molcata.2016.07.043

In precedent work, nano-sized BiOCl was immobilized onto graphene sand composite (GSC) and chitosan (CT) to report efficient photocatalytic system for wastewater treatment. GSC was prepared by graphitization of sugar over river sand. The supported BiOCl was prepared by modified hydrolysis method to report BiOCl/GSC and BiOCl/CT photocatalysts. The citric acid directed nucleation and growth process resulted in well dispersed BiOCl nanoplates over GSC and CT. The supported catalysts were characterized by FESEM, TEM, HRTEM, FTIR, XRD, EDX, BET, Raman, photoluminescence and UV–vis diffuse reflectance spectral analysis. The optical band gap of BiOCl/GSC and BiOCl/CT was given by 3.31 and 3.33?eV, respectively. The size of BiOCl/GSC and BiOCl/CT was found to be 50 and 70?nm respectively. The catalytic efficiency of BiOCl/GSC and BiOCl/CT was tested for ampicillin (AMP) and oxytetracycline (OTC) removal. The adsorption of AMP and OTC followed pseudo second order kinetics. Both BiOCl/GSC and BiOCl/CT exhibited significant photocatalytic activity for the mineralization of ampicillin (AMP) and oxytetracycline (OTC) antibiotics under solar light. Simultaneous adsorption and degradation process (A+P) process showed higher antibiotic degradation rate. The applicability of power law model showed the complex nature of mineralization process. During A+P process, both antibiotics were mineralized to CO₂, H₂O and NO₃^? ions. BiOCl/GSC and BiOCl/CT exhibited significant recycle efficiency for 10 catalytic cycles in comparison to native BiOCl.

Full text of DOI:10.1016/j.molcata.2016.07.043

Accepted Manuscript
Title: Photocatalytic mineralization and degradation kinetics
of ampicillin and oxytetracycline antibiotics using graphene
sand composite and chitosan supported BiOCl
Author: Bhanu Priya Pooja Shandilya Pankaj Raizada Pankaj
Thakur Nahar Singh Pardeep Singh
PII:
S1381-1169(16)30293-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.07.043
MOLCAA 9977
To appear in:
Journal of Molecular Catalysis A: Chemical
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Accepted date:
6-5-2016
20-7-2016
24-7-2016
Please cite this article as: Bhanu Priya, Pooja Shandilya, Pankaj Raizada, Pankaj
Thakur, Nahar Singh, Pardeep Singh, Photocatalytic mineralization and degradation
kinetics of ampicillin and oxytetracycline antibiotics using graphene sand composite
and chitosan supported BiOCl, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.07.043
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Photocatalytic mineralization and degradation kinetics of ampicillin and oxytetracycline
antibiotics using graphene sand composite and chitosan supported BiOCl
Bhanu Priyaa, Pooja Shandilyaa, Pankaj Raizadaa, Pankaj Thakura, , Nahar Singhb, Pardeep
Singha*
aSchool of Chemistry, Faculty of Basic Sciences, Shoolini University, Solan (Himachal
Pradesh)-173212, India
bMetrology in Chemistry (ALSIM); CSIR-National Physical Laboratory, Dr K.S. Krishnan Marg,
New Delhi-110012
*Corresponding Author
Tel: +91-1792-308000,Fax: +91-1792-30800, Email: pardeepchem@gmail.com (Pardeep Singh
)
1

Highlights:

BiOCl nanoplates were supported over graphene sand composite(GSC) and chitosan(CT).
Photocatalytic activity of BiOCl/GSC and BiOCl/CT was tested for antibiotic removal.
Simultaneous adsorption and photocatalysis had synergistic effect on process.
Complete photo-mineralization of antibiotics was attainted under solar light.
Power law model validated the complex kinetics of long term degradation process.
.




.
Abstract
In precedent work, nano-sized BiOCl was immobilized onto graphene sand composite (GSC)
and chitosan (CT) to report efficient photocatalytic system for wastewater treatment. GSC was
prepared by graphitization of sugar over river sand. The supported BiOCl was prepared by
modified hydrolysis method to report BiOCl/GSC and BiOCl/CT photocatalysts. The citric acid
directed nucleation and growth process resulted in well dispersed BiOCl nanoplates over GSC
and CT. The supported catalysts were characterized by FESEM, TEM, HRTEM, FTIR, XRD,
2

EDX, BET, Raman, photoluminescence and UV-visible diffuse reflectance spectral analysis.
The optical band gap of BiOCl/GSC and BiOCl/CT was given by 3.31 and 3.33 eV, respectively.
The size of BiOCl/GSC and BiOCl/CT was found to be 50 and 70 nm respectively. The catalytic
efficiency of BiOCl/GSC and BiOCl/CT was tested for ampicillin (AMP) and oxytetracycline
(OTC) removal. The adsorption of AMP and OTC followed pseudo second order kinetics. Both
BiOCl/GSC and BiOCl/CT exhibited significant photocatalytic activity for the mineralization of
ampicillin (AMP) and oxytetracycline (OTC) antibiotics under solar light. Simultaneous
adsorption and degradation process (A+P) process showed higher antibiotic degradation rate.
The applicability of power law model showed the complex nature of mineralization process.
-
During A+P process, both antibiotics were mineralized to CO2, H2O and NO3 ions. BiOCl/GSC
and BiOCl/CT exhibited significant recycle efficiency for 10 catalytic cycles in comparison to
native BiOCl.
Keywords: Supported BiOCl; Antibiotics; Adsorptional-photocatalysis; Mineralization kinetics;
Recycle efficiency.
1. Introduction
The aquatic contamination from the release of antibiotics in water bodies is a major
challenge for the environmental engineering around the globe. Ampicillin (AMP) is a semi-
synthetic penicillin widely used in human and veterinary medicine. It possesses antimicrobial
properties due to presence of β-lactam ring in its structure. Oxytetracycline is one of the detected
tetracyclines in water sources and sediments. These antibiotics enter in aqueous system usually
3

from discharge of hospital waste, pharmaceutical manufacture, livestock and agriculture. Their
presence exerts unfavorable effects on aquatic systems, not only resulting in the development of
a variety of resistant bacteria but also disturbing the aquatic animal life [1, 2]. Nowadays, various
physico-chemical methods such as membrane separation, chemical oxidation, adsorption,
coagulation, flocculation, ozone treatment, reverse osmosis and filtration have been used to
eliminate antibiotics form aquatic environment. However, these methods either fail to attain
complete breakdown of antibiotic or produce large volume of toxic sludge which results in
disposal problem [3]. Therefore, more competent treatment techniques are needed for complete
elimination of antibiotics form wastewater.
From the aforesaid perspective, solar light assisted photocatalysis is emerging as a green
technology for waste water treatment [3]. Since the exploration of photocatalytic activity of
TiO2, it is being used in laboratories and industries for significant photocatalytic activity [4].
However its wide band gap and rapid recombination of photo-generated electron holes-pair
limits its applications under solar light which contains only 5% of UV light [5]. In order to
exploit 52% visible light of solar spectrum, visible light active photocatalyst is highly needed for
photocatalysis properties [6]. Although variety of narrow-band gap semiconductor photocatalyst
such as Ag3PO4, CdS, Cu2O, ZnS and PbS possess high visible light activity. However,
instability in aqueous medium and toxicity put thrust on the development of more robust
photocatalytic system for water purification [7].
During last decade, BiOCl has gained more and more attention as a promising
photocatalyst for photocatalytic energy conversion and environmental remediation [8]. The
layered structure of BiOCl endows it with fascinating physicochemical properties, suitable band
gap, non-toxicity, high chemical and corrosion resistance [9]. The positive conduction band and
4

layered structure (Cl-Bi-O-Bi-Cl) along „c‟ axis enable easier separation of photo generated
electron and hole pairs. In last decades several nanostructures like nanowires, nanoplates,
nanofibers, nanobelts, nanoflakes, nanospheres, 2D, 3D hierarchical structures have been
synthesized by hydrothermal, solvothermal, ionothermal routes [10]. However sheet like
structure has been obtained mainly by hydrolysis method using Bi3+-Cl- aqueous system[11]. The
use of capping agent has proven an effective mean to control the activity of cations and retarding
the grain growth of inorganic semiconductors [12]. Citric acid is a non-toxic and stable organic
compound that has been proven as a good capping agent for the complexation with Bi3+, Al3+,
Ca2+, Fe3+, Zn2+and Mn2+ ions [13]. Xiong et al. prepared mannitol assisted well crystalline
square like plate structure for effective photo degradation of rhodamine B dye [14]. Song el al.
synthesized hierarchical BiOCl crystals via pyridine mediated solvothermal routes[15].
From application prospective, the main drawbacks of nano-sized inorganic nanoparticles
are complex filtration processes and highly turbid solution that decrease the photoactive surface
area during photodegradation process [16]. Secondly, preliminary adsorption of aqueous phase
pollutants is required for efficient surface photodegradation reactions [17]. These concerns have
motivated researchers to develop supported photocatalytic systems in which nano-sized BiOCl
nanoparticles can be immobilized on different supports. The support should have long term
stability, capability to delocalize electrons and endure the oxidative radical attack during
exposure to light [18]. As the most recently discovered carbonaceous material, graphene and its
derivatives have attracted remarkable attention due to high surface area and electron mobility.
Graphene is a single layer sp2 hybridized carbon organized in honeycomb structure [19, 20]. Fu
et al. synthesized magnetically separable ZnFe2O4/graphene nanocomposite which exhibited dual
functions as a photo electrochemical degrader and an OH· generator via photo electrochemical
5

decomposition of H2O2 [21]. Hu et al. prepared graphene supported ZnS nanocomposite using
microwave radiation assisted method [22]. Liu et al. synthesized BiOI/GR composite through
hydrothermal method and claimed their improved photocatalytic activity and stability for
methylene green dye degradation [23]. In recent time, chitosan, a non-toxic, biocompatible and
abundant biopolymer has emerged as an absorbent for heavy metals, organic and inorganic
pollutants [24]. Chitosan possesses amine(-NH2) and hydroxyl (-OH) groups in water that can
serve as reaction sites for immobilization of inorganic nanoparticles and adsorption of aqueous
phase contaminant [25]. Farzana et al. synthesized zinc impregnated chitosan beads for the
removal of methylene blue and rhodamine B dye [25].
This research was focused on the preparation of GSC and CT supported BiOCl nano
composites using simple hydrolysis method. GSC was prepared form highly carbonaceous
material sucrose which is non-toxic to human and environment [26]. Both BiOCl/GSC and
BiOCl/CT were characterized using FESEM TEM, HRTEM, BET, XRD, EDX, Raman, photo
luminescence and UV-visible analysis. The photocatalytic performance of BiOCl/GSC and
BiOCl/CT was tested for the complete mineralization of AMP and OTC antibiotics. The effect of
adsorption on photocatalytic antibiotic degradation was also investigated. More specifically, the
co-operative effect between adsorption and photocatalysis was explored to optimize the
degradation rate. The power law model was applied to understand the intricacies of long term
mineralization process. The recycle efficiency of BiOCl/GSC and BiOCl was also evaluated for
ten cycles to ensure the applicability of BiOCl/GSC and BiOCl/CT as effective photocatalysts.
2. Experimental
2.1. Preparation of graphene sand composite (GSC)
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In this work, GSC was prepared with modification in earlier reported method [27]. The
common sugar cubes was used as the carbon source for the preparation of GSC. 1.0 g of sugar
cubes were dissolved in water to obtain a homogenous solution. To this solution, 0.5 g of sand
was added and mixture was stirred for 7 h to get slurry. The obtained mixture was dried at 90 ˚C
to obtain sugar-coated sand. The sugar-coated sand was heated at 750 ˚C for 3 h to ensure the
graphitization of sugar molecules. The obtained material was allowed to cool at room
temperature and the resultant black sample was leveled as GSC.
2.2. Synthesis of BiOCl/GSC and BiOCl/CT
BiOCl/GSC was prepared using modified hydrolysis method [13]. Typically, 1g of BiCl3
was dissolved in 80 mL of distilled water. To this solution 20 mL of concentrated hydrochloric
acid was added and mixture was stirred to obtain a transparent BiCl3-HCl aqueous solution.
Subsequently, 20 mL of citric acid (1.0 × 10-3 mol/L) was added to reaction mixture. To this
solution 0.2 g of GSC was added. The reaction mixture was heated at 60 C with continuous
stirring for 3 h to obtain precipitates of BiOCl/GSC. The precipitates were washed with distilled
water and then dried at 90 ˚C. The BiOCl/GSC was preserved for further use. BiOCl/CT was
prepared using same methodology with addition of CT in place of GSC. . BiOCl/CT precipitates
were dried at 50 ˚C to prevent the decomposition of chitosan. During synthesis of BiOCl, neither
GSC nor CT was added to reaction mixture.
Field emission scanning electron analysis was carried using model Nava Nano SEM-
45(USA). Sample surfaces were observed at different magnification and images were recorded.
HRTEM and Energy dispersive X-ray analysis was performed at randomLy selected areas under
vacuum conditions using model FP/5022 – Tecnai G2 20 S-TWIN (USA). Fourier- transform
7

infrared (FTIR) analysis was performed using Perkin – Elmer Spectrometer (Spectrum RX-I).
Panalytical‟s X‟Pert Pro-diffractrometer having Cu K-α-1 radiation source along with beta filter
made of nickel metal was used for XRD pattern. The optical absorption performance of catalysts
was estimated by using a diffuse reflectance spectrophotometer (UV 3600, Shimadzu) using
BaSO4 as reference. The specific surface area of samples was estimated by nitrogen adsorption–
desorption isotherms according to Brunauer–Emmett–Teller analysis (Autosorb I; Quatachrome
Corp.). The photoluminescence emission analysis was performed using inductively coupled
plasma-optical emission spectrometer (ICP-OES), Thermo-iCAP 6000 Series model to find the
very low concentration of elements.
2.3. Photocatalytic and adsorption experiments
The adsorption and photocatalytic activity of BiOCl/GSC, BiOCl/CT and pure BiOCl was
evaluated using double walled pyrex cylinder surrounded by thermostatic water circulation
arrangement (30±5 °C) (Supplementary Fig. S1). The suspension containing antibiotics and
photocatalyst was kept under solar light with continuous stirring. The aliquot (3 mL) was
withdrawn and centrifuged to separate catalyst from reaction solution. The absorbance of
ampicillin (AMP) and oxytetracycline (OTC) in supernatant liquid was measured at 260 and 350
nm, respectively [1, 2]. The solar light intensity was measured by digital lux-meter (35×103±
1000lx). All the degradation and adsorption experiments were undertaken in triplicate with errors
below 5 %. The chemical oxygen demand (COD) was estimated by potassium dichromate reflux
method where unreacted oxidant was determined by titrating the sample with ferrous ammonium
sulfate using ferroin indicator [28]. The CO2 was determined using NaOH as titrant in the
presence of phenolphthalein indicator [29]. The inorganic ions were detected using total dissolved
solid (TDS) benchtop ion analyzer. After the photodegradation experiment, the photocatalyst was
8

separated form reaction solution. The samples were preserved in dark bottles for COD, CO2 and
inorganic ion estimation. The removal efficiency was calculated using Eq. 1:
Where, C0 is the initial concentration/COD and Ct is instant concentration/COD of sample.
2.4. Determination of pH of zero point charge (pHpzc)
The pH of zero point charge was determined by previously reported method [30, 31].
During zero point charge determination, 50 mL of 0.01 M NaCl was adjusted to successive
initial pH value between 2 to 12 at 25 ˚C by titrating it with HCl (0.1 M) and NaOH (0.1 M). 0.1
g of adsorbent was added to each solution. After 48 h, final pH of each solution was measured
and plotted against initial pH. The pH at which curve crossed the line of equality, was taken as
pHzpc of samples.
3. Results and discussion
3.1. Morphological and structural characterization of BiOCl/GSC and BiOCl/CT
FESEM imaging was used to investigate the surface morphology of BiOCl/GSC and BiOCl/CT.
Fig. 1 (a) & (b) depicts representative FESEM images of GSC and CT respectively. Both GSC
and CT exhibited quite rough, uneven and irregular surface having different sized
pores[32].These morphological features provided good leeway for the fixation of BiOCl over its
surface and also enhanced the adsorption of antibiotics onto catalysts surface.Fig.1 (c) &(d)
displayed FESEM images of BiOCl/GSC and BiOCl/CT respectively. The dispersion of BiOCl
over GSC and CT could be seen clearly. The dissimilar sized BiOCl nanoplates were observed
on the surface of GSC and CT. The magnified images clearly indicated that BiOCl nanoplates
were developed over the surface of GSC and CT, respectively (Fig. 1 e & f).
9

The crystalline structure of nanocatalysts was characterized by powder X-ray diffraction
analysis. XRD pattern of BiOCl/GSC, BiOCl/CT, GSC and CT is represented in Fig. 1(g) & (h).
The diffraction peak at 26.63 was attributed to plane (002) of graphene like structure [33]. XRD
pattern of CT as given in Fig. 1h (3) exhibited characteristic broad peak between 10-20˚
indicating presences of chitosan molecule [34]. In both BiOCl/GSC and BiOCl/CT, the major
characteristic diffraction peaks of BiOCl at12.02˚, 26.00˚, 33.33˚, 34.00, 41.00˚ and
49.85˚correspondto (001), (011), (110), (012), (003) and (020) planes Fig.1g(3) &1h(3).These
peaks are identical to those of tetragonal phase BiOCl (space group:P4/nmm) [35]. The major
peak of GSC overlapped with BiOCl peak at 26.00˚. While no peak of other impurities like
monoclinic Bi3O4Cl, orthorhombic Bi12O15Cl6 and narrow band-gap Bi2O3 was observed; the
intensity of major peaks of BiOCl (JCPDS-06-0249) decreased in both BiOCl/GSC and
BiOCl/CT. The decrease in reflection peaks was attributed to interaction between BiOCl and
GSC or CT. From Debye Scherer equation, the respective size of BiOCl/GSC and BiOCl/CT was
found to be 50 and 70 nm.
The TEM images of BiOCl/GSC and BiOCl are shown in Fig. 2 (a)-(d). The morphology
of both BiOCl/GSC and BiOCl/CT was in agreement with FESEM analysis. Fig. 2 (c) & (d)
depicts magnified TEM images of BiOCl/GSC and BiOCl/CT. The irregular tiny BiOCl
nanoplates with explicit edges were clearly seen over GSC and CT as given in Fig. 2 (c) & (d).
HRTEM investigation provided additional information concerning the growth of BiOCl
nanoplates over GSC and CT. HRTEM images of BiOCl/GSC and BiOCl/CT are shown in Fig.
2 (e) & (f), respectively. The lattice fringes with inter planar spacing of 0.037 nm corresponded
to (110) plane were observed for BiOCl in prepared nanocomposite [36]. The presence of GSC
in BiOCl was confirmed by the presence of (002) plane having interplanar spacing of 0.02 nm
10

[27]. The presence of (110) plane was in accordance with XRD data [37]. A corresponding
selected area electron diffraction (SAED) pattern of BiOCl/GSC and BiOCl/CT was also
examined. Fig. 2(g) & (h) depicts SAED pattern of BiOCl/GSC and BiOCl/CT composites.
SAED pattern indicated semi crystalline nature of prepared nanocomposites. The acquired
diffraction pattern of spot could be indexed to (110) and (002) reflection planes [37, 38]. The
indexed (110) and (200) planes indicated tetragonal phase of BiOCl in prepared nanocomposite.
Moreover, SAED patterns were in good agreement with X- ray diffraction and HRTEM studies.
EDX analysis of GSC, CT, BiOCl/GSC and BiOCl/CT was recorded for elemental
analysis. In EDX of GSC as given in Fig.3 (a), presence of Si, P, C, Cl and O indicated the
stacking of graphene with river sand. The presence of these species are mainly due to existence
4-
of SiO2, PO3 and Cl- ions in river sand.EDX of BiOCl/GSC as given in Fig. 3 (a) & (b) clear
displays the presence of Si, Bi, C, Cl and O in test material. Chitosan was mainly composed of
carbon and oxygen as depicted in Fig. 3 (c). The presence of Bi, C, Cl and O as major elements
in BiOCl/CT confirmed the formation of BiOCl/CT as shown in Fig. 3(d).
FTIR analysis was used to find main functional groups in BiOCl/GSC and BiOCl/CT.
FTIR spectra of GSC, CT, BiOCl/GSC and BiOCl/CT are shown in Fig. 3 (e). In GSC spectrum,
characteristics peaks of carbon-oxygen functionalities at 1941 and 1632 cm−1 were due to C=O
stretching of COOH groups and C–O–C stretching vibrations, respectively. The peak at 1080
cm−1 was attributed to the stretching mode of C=C bond [39-41]. The band at 1632 cm-1 was
ascribed to skeletal vibration of unoxidized graphitic molecules [37]. The peak at 3410 cm-1 was
originated due to O–H stretching vibration or adsorption of water molecules onto GSC [32]. The
peak at 777 cm-1 was assigned to Si-O stretching mode [42]. In case of BiOCl/GSC, the peak at
535 cm-1 was due to Bi-O stretching vibrations present in BiOCl/GSC [43]. The shift in
11

characteristics peaks of GSC towards lower wave number (3410 to 3398, 1941 to 1920, 1632 to
1615, 1080 to 1077 and 778-765 cm-1) indicated the formation of BiOCl/GSC.
In FTIR spectrum of CT, absorption band at 3429 cm-1 was attributed to its -NH2
functional group [44]. The stretching modes of amine I (C-O stretching along N-H deformation)
and amine II were assigned at 1648 cm-1 and 1597 cm-1, respectively. The band at1421 cm-1
indicated C-N axial deformation of amine group. The peak at 1082 cm-1 corresponded to C-O-C
stretching vibration in glucose [45]. The respective shift in peaks from 3429 to 3398, 1648 to
1618, 1421 to 1415, 1087 to 1076 and 620 to 537 cm-1 confirmed the interaction between CT and
BiOCl. The appearance of bands at 530 cm−1 was attributed to Bi-O vibration in BiOCl/CT [35].
In present study, pHzpc of BiOCl/GSC and BiOCl/CT was found to be 7.8 and 6 .8, respectively as
shown in (Supplementary Fig S2 (a) & (b). UV-visible diffuse reflectance analysis and band gap
analysis as given in (Supplementary Fig S2 (c) & (d). BiOCl, BiOCl/GSC and BiOCl/CT
exhibited absorption at 321 nm. The band gap was calculated using Tauc relation [46] (Eq. 2):
where α = absorption coefficient = 2.303 A/l, Eg= optical band gap, β= band tailing parameter, hν
is the photon energy and n = ½ for direct band gap. The optical band gap is estimated by
extrapolating the straight portion of curve between (αhν)2 and hν when α= 0. The respective band
gap of BiOCl/GSC and BiOCl/CT was found to be 3.31 eV. Both BiOCl/GSC and BiOCl/CT
had narrower band gap than theoretical value of BiOCl (3.46 eV) [47]. According to Unlu
theory, the decrease in band gap was due to phonon confinement effect. When the size and
dimensionality of BiOCl/GSC and BiOCl/CT were reduced, the phonon confinement would
transpire [48].
12

The porous nature of BiOCl/GSC and BiOCl/CT further examined by Isotherm
adsorption–desorption (BET) measurements. Fig. 4(a) & (b) depicts N2 adsorption/ desorption
isotherm curve and pore volume distribution curve of BiOCl/GSC and BiOCl/CT. The isotherm
exhibited type IV isotherm which is typical characteristics of mesoporous materials. The BET
surface area of BiOCl/GSC and BiOCl/CT is 14.5 and 13.5 cm2, respectively. The pore size of
BiOCl/GSC and BiOCl/CT was less than 20 nm.
Raman spectroscopy is a powerful technique for probing structural properties of
carbonaceous materials. Fig. 4(c) & (d) displays Raman spectra of BiOCl/GSC, BiOCl/CT, GSC
and CT. In spectrum of GSC, two typical bands were observed at 1347 cm-1 (D) and 1597 cm-1
(G). The G band was attributed to E2g phonon of sp2 bonded carbon of graphene [49]. The D
band is due to local defects and disorders found on graphene sheet. Whereas in chitosan Fig. 4(d)
main peaks found at 471 cm-1 (C-C-O bending vibration) and 893 cm-1, 1149 cm-1 (C-C-O and
C-N stretching vibration) [50]. A special peak at 2837 cm-1 was due to C-H stretching vibration
[51]. The bands at 1373, 1456 and 1565 cm-1 were corresponded to C-H stretching, (-NH2) band
and CH3 bonding n chitosan [52]. Usually, symmetric vibrations give rise to more intense bands
than asymmetric vibrations. In both BiOCl/GSC and BiOCl/CT, strong band at 144 cm-1 was
assigned to A1g internal Bi-Cl stretching mode while band at 202 cm-1 was masked to Eg external
mode of Bi-Cl indicated the presence of BiOCl in BiOCl/GSC and BiOCl/CT. The
characteristics peaks of GSC were observed at 1598 and 1347 cm-1 in BiOCl/GSC and for CT
were at 1370, 1454 and 1563 cm-1 in BiOCl/CT.
Photoluminescence is an important technique to check the separation of photo induced
electron-hole pair. The BiOCl exhibited the emission peaks around [54] (Supplementary Fig S3).
Though the shape and peak position of emission peak of BiOCl/GSC are similar to that of pure
13

BiOCl, the emission intensity [55] BiOCl/GSC decreases. The indicated the GSC had suppressed
the recombination rate of photo induced electron-hole pair by accepting the conduction band
electrons [56]. However, in case of BiOCl/CT, both shape and peak position of emission peaks
are same as that of pure BiOCl.
On dissolving BiOCl3 in hydrochloric acid, colorless and transparent solution containing
Bi3+ ions was formed. Citric acid was hydrolyzed to [C6O7H5]3- and H+ ions. It was proposed that
[C6O7H5]3- may chelate with free Bi3+ to produce [C6O7H5]3−Bi3+ in reaction medium [57].
During chelation, hydrogen bonds were formed between carboxylic acid of citric acid and
hydroxyl ions in [C6O7H5]3−Bi3+. Free Bi3+ions combined with citric anions via chelating effect
to form [C6O7H5]3- Bi3+. In next step, [C6O7H5]3- Bi3+ was hydrolyzed slowly to form BiOCl
nanoplates due to intrinsic nucleation and anisotropic growth of BiOCl [58, 59].
3.3. Removal of AMP and OTC under different catalytic systems
The catalytic efficacy of BiOCl/GSC and BiOCl/CT was tested for the removal of AMP and
OTC antibiotics present in aqueous phase. The experiment were conducted to optimize reaction
conditions [Catalyst dose = 50 mg/50 mL, OTC/AMP concentration = 1×10-4 mol dm-3, solar
light intensity= 35×103± 100 lx, pH=6.0 and reaction time=60 min (AMP) and 120 min (OTC)]
and experiments were performed under these reaction conditions. Fig 5 illustrates the results of
AMP and OTC removal under different catalytic systems. Direct photolysis in the absence of
catalyst resulted in negligible removal of AMP and OTC from aqueous phase. However, the
decrease in OTC and AMP concentration was noteworthy in the presence of BiOCl/GSC and
BiOCl/CT under solar light. Fig.5 (a) depicts photocatalytic investigations for AMP removal in
the presence of solar light. During 60 min exposure to solar light, 70 %, 63%,38 %, 26% and
20% of removal efficiency was achieved utilizing BiOCl/GSC, BiOCl/CT, BiOCl, GSC and CT,
14

respectively. However, in the absence of solar light, BiOCl/GSC, BiOCl/CT, GSC, CT and
BiOCl displayed 39%, 29%, 26%, 20%, and 11 % of AMP of removed from reaction solution.
85% and 70% of OTC was removed under solar light in 120 min using BiOCl/GSC and
BiOCl/CT, respectively. While in dark, 39 % and 28 % of removal efficiency was attained for
BiOCl/GSC and BiOCl/CT, respectively. Efficiency trend followed by adsorbents was:
BiOCl/GSC > BiOCl/CT > GSC > CT > BiOCl. These investigations indicated that OTC and
AMP removal mainly occurred via BiOCl/GSC and BiOCl/CT assisted photocatalytic process.
Even so the photocatalytic evaluation of both BiOCl/GSC and BiOCl/CT was quite different
from the conventional binary or ternary inorganic metal oxide photocatalyst. This was due to
substantial adsorption of antibiotics onto BiOCl/GSC and BiOCl/CT.The adsorption capacity of
adsorbent can play a crucial role during the photodegradation reaction occurring on the catalyst
surface. Further studies were undertaken to understand the real behavior of BiOCl/GSC and
BiOCl/CT for antibiotic removal.
3.4. Adsorption behavior of BiOCl/GSC and BiOCl/CT.
The amount of antibiotics adsorbed onto BiOCl/GSC and BiOCl/CT at time t (min) was
calculated using following equation [60, 61]:
V
qe


Co  Ce

(3)
M
where qe (mg g-1) is the amount of adsorbed AMP/OTC per gram of adsorbent at time t (min), C0
is the initial concentration of AMP/OTC ( mol/L), Ct is the concentration of AMP/OTC (mol/L)
at time t(min), V volume of the solution (50 mL) and M is the mass of the adsorbent (g). The
pseudo first order rate expression is given as [60, 61]:
15

Where qe and qt are the amount of antibiotic adsorbed on sorbent at equilibrium and time t(mg/g).
k1is first order rate constant (min-1). A plot of ln (qe-qt) versus t gave a linear relationship from
which the value of k1 and qe can be estimated from the slope and intercept. The linearized form
of pseudo second order rate expression can be described using Eq. 5 and 6 [60, 61]:
(6)
where qe is the amount of adsorbate adsorbed per unit mass of sorbent at equilibrium (mg/g), qt is
the amount of antibiotic adsorbed at contact time t(mg/g) and k2 pseudo second order constant
(g/mg min). A plot between t/qt versus t gave a linear relationship from which qe and k2 can be
determined. The kinetic data for OTC and AMP adsorption is represented in Table 1. Both
BiOCl/GSC and BiOCl/CT had quite higher adsorption capacity as compared BiOCl. The
obtained data indicated the value of correlation coefficient for second order was found to be
greater than for first order reaction (Table 1). The adsorption of antibiotics onto BiOCl/GSC,
BiOCl/CT and BiOCl followed pseudo second order kinetics. The respective adsorption capacity
of BiOCl/GSC, BiOCl/CT and BiOCl was obtained as 62.31, 60.22 and 15.67 mg/g for AMP
adsorption. In case of OTC adsorption, the adsorption capacity (qe) of BiOCl/GSC, BiOCl/CT
and BiOCl was found to be 64.35, 61.39 and 14.00 mg/g, respectively. These results clearly
indicated that incorporation of BiOCl with GSC/chitosan resulted in augmented adsorption
capacity. The pseudo second order kinetics for both AMP and OTC adsorption indicated
complex nature of adoption process.
The initial pH of reaction solution is an important variable which influences the
adsorption of liquid phase pollutants onto the surface of adsorbent. The association /dissociation
of surface functional group can be explained by surface charge for electrostatic interaction and
16

reactive sites for catalyst and antibiotic [30, 60]. The pH of zero point charge (pHZPC) of
BiOCl/GSC, BiOCl/CT and BiOCl was found to be 6.8, 7.0 and 6.4 respectively. Above pHzpc,
surface of absorbents was negatively charged, while below pHzpc, the positively charged species
dominated the catalyst surface [30].
At the same time, the ionic species of both AMP and OTC completely changed from
positive charge to negative charge with change in pH form acidic to basic [62]. At lower pH
both, absorbent and antibiotics were positively charged and adsorption of AMP and OTC was
retarded [63, 62]. At pH 6.0, OTC existed as H2OTC and HOTC- . The ionic species AMP was
mainly found as HAMP- at near neutral and slightly basic pH. However, the surface of
BiOCl/GSC and BiOCl/CT was positively charged below neutral pH. The negatively charged
species of OTC and AMP were strongly attracted towards positively charged absorbent
molecules [62, 63]. This resulted in higher adsorption of OTC and AMP onto BiOCl/GSC and
BiOCl/CT at pH 6 [Table 2]. The retardation in adsorption at basic pH was due to repulsion
between negatively charged adsorption and antibiotic molecules [62, 63]. From above
discussion, we claimed that pH 6 was the optimal pH for the AMP and OTC adsorption onto
BiOCl/GSC and BiOCl/CT.
The real antibiotic photo degradation process involves two catalytic reactions: (1) a
pseudo-first order reaction taking place on the surface of catalyst particles and (2) a very slow
reaction occurring in catalyst free solution [64]. According to Langmuir-Hinshelwood model, the
photodegradation of organic compounds is a surface phenomenon [64]. Therefore, the decrease
in concentration of adsorbed antibiotics must be considered during real photodegradation
process. In our case, the antibiotic degradation was negligible in the absence of catalysts.
Therefore, BiOCl/GSC and BiOCl/CT free photocatalysis did not have any significant effect on
17

degradation process. Further investigations were carried out to explore the role adsorption on
photodegradation of AMP and OTC.
3.5. Role of adsorption in solar light assisted photodegradation of AMP and OTC
The removal of AMP and OTC using BiOCl/GSC, BiOCl/CT and BiOCl was
investigated under three reaction conditions i.e. adsorption in dark (DA), adsorption followed by
photocayalysis (A-P) and simultaneous adsorption and photodegradation process (A+P). Fig. 6
(a) depicts the removal efficiency of BiOCl/GSC, BiOCl/CT and BiOCl as a function of time.
The First portion of graph displays adsorption of AMP and OTC using BiOCl/GSC, BiOCl/CT
and BiOCl.40%, 30% and 10 % of AMP removal was observed for BiOCl/GSC, BiOCl/CT and
BiOCl, respectively in 120 min. While during OTC removal, 38%, 27% and 8 % of removal
efficiency was attainted for BiOCl/GSC, BiOCl/CT and BiOCl, respectively.
During adsorption followed by catalysis (A-P), 70%, 55% and 14 % of AMP removal
was attained using BiOCl/GSC, BiOCl/CT and BiOCl as photocatalyst in 240 min. For OTC
removal, efficiency of BiOCl/GSC, BiOCl/CT and BiOCl was found tobe 60%, 47% and 10 %,
respectively. During A-P process, the saturated adsorption of antibiotics onto catalysts caused the
retardation in antibiotic removal process. The increased antibiotic concentration onto catalyst
surface acted as a filter for incident solar light. It resulted in decreased effective photoactive
volume leading to lower production of OH· radicals.
In case of simultaneous adsorption and photodegradation (A+P), 90%, 80%, and 40 % of
AMP removal efficiency was obtained using BiOCl/GSC, BiOCl/CT and BiOCl as photocatalyst
in 120 min, respectively. For OTC removal, 84%, 69% and 30 % of removal efficiency was
recorded for BiOCl/GSC, BiOCl/CT and BiOCl photocatalyst, respectively. On comparing
different removal processes, A+P process was most efficient process for the degradation of both
18

AMP and OTC. During A+P process, the antibiotics adsorbed onto catalyst surface were
simultaneously degraded by photocatalytic process. So, the photoactive volume did not decrease
during A+P process and solar light assisted BiOCl/GSC/A+P and BiOCl/CT/A+P catalytic
process was the most effective for the removal of the OTC and AMP degradation from aqueous
phase.
To study the kinetics OTC and AMP degradation using solar light assisted
BiOCl/GSC/A+P and BiOCl/CT/A+P processes. Langmuir-Hinshelwood model was used to
elucidate the kinetics of antibiotic degradation (Eq. 7)[65, 66]:
Where C is the concentration of antibiotics at time tand k is rate constant or reaction. K is
adsorption coefficient of reaction. However, at milli molar concentration C<<1, the equation can
be simplified by integrating the above between limits (0, C0) and (t, C), we get (Eq. 8).
The graph between –ln(C/C0) and time were plotted for AMP and OTC degradation. The
linearity of plots suggests that photocatalytic degradation approximately followed pseudo first
order kinetics. The rate constant for AMP was found to be 2.7 × 10-2 min-1 and 2.0 ×10-2 min-1
using BiOCl/GSC/A+P and BiOCl/CT/A+P, respectively in 60 min. For OTC degradation, the
respective rate constants for BiOCl/GSC/A+P and BiOCl/CT/A+P catalytic system were found
to 1.0×10-2 min-1 and .9×10-3 min-1 under solar light for 120 min. The kinetics studies indicated
that degradation process followed pseudo first order kinetics during initial hours of degradation
process.
19

3.6. AMP and OTC mineralization and long term degradation kinetics using BiOCl/GSC/A+P
and BiOCl BiOCl/CT/A+P process
From above discussion, it can be inferred that overall OTC and AMP photo degradation
using BiOCl/GSC/A+P and BiOCl/CT/A+P catalytic systems is the combined effect of
photocatalytic and adsorption process. Consequently, the degradation of both OTC and AMP
was subjected to photo catalysis (A+P) for 10 hours in the presence of solar light to obtain
complete mineralization of antibiotics. The COD and CO2 estimation as depicted in Fig. 7 (a) &
(b) were performed to investigate the extent of OTC and AMP mineralization during long term
photocatalytic process. During 10 hours of exposure, 99% and 60% of COD removal was
attained for AMP degradation using BiOCl/GSC/A+P and BiOCl/CT/A+P process, respectively.
For OTC mineralization, 70% and 60 % of COD was removed utilizing BiOCl/GSC/A+P and
BiOCl BiOCl/CT/A+P process, respectively. The evolution of CO2 during mineralization
-
2-
process was an indicative of degradation process. Furthermore, the formation of NO3 and SO4
ions during degradation of antibiotics confirmed the mineralization of AMP and OTC.
During long term photocatalytic process, kinetics is an important parameter to understand
the overall antibiotic degradation process. The real degradation process can be represented as the
combination of two parallel branches namely, photo catalysis and photolysis (Eq. 9) [66]:
where net photo degradation (R), photocatalysis (R1) and photolysis rate (R2), respectively. In
precedent study, the alone photolysis has no significant effect on the antibiotics degradation.
Therefore, only photo catalysis was considered as dominant process during long term
photocatalysis for ten hours. In this work, power law model was applied to explore the kinetics
20

of OTC and AMP degradation during mineralization process using BiOCl/GSC/A+P and BiOCl
BiOCl/CT/A+P catalytic system (Eq. 10):
Where k1 is rate constant for photocatalysis and n1denote the order of the reaction for photo
catalysis and to find above parameters, the differential method of data analysis was applied to
Eq. 10. The obtained results are presented in Fig. 7 (c) to (f). The obtained plots presented the
goodness of fit of rate equation 12.The obtained experimental results are presented in Table 3.
For AMP degradation, order of reaction (n1) was found to be -1.48 and-1.69 for
BiOCl/GSC/A+P and BiOCl/CT/A+P catalytic system, respectively. BiOCl/GSC/A+P and
BiOCl/CT/A+P catalytic systems had respective rate constants of 1.03 × 10-7 (mol dm-3)-0.48 h-1
and 6.08 × 10-8 (mol dm-3)-0.69 h-1 (Table 3). In case of OTC removal, the order of reaction was
given by -1.54 and -1.78 for BiOCl/GSC/A+P and BiOCl/CT/A+P catalytic processes,
respectively. The rate constant for BiOCl/GSC/A+P and BiOCl/CT/A+P catalytic process was
given by 8.05 ×10-8 (mol dm-3)-0.54 h-1 and 2.0×10-8 (mol dm-3)-0.78 h-1. The negative value of
reaction order signifies the slow and complex nature of mineralization process. The overall
degradation rate R (mol dm-3 h-1) for OTC and AMP degradation are presented in Table 3.These
results clearly indicated that long term degradation did not follow simple pseudo first order
kinetics. The degradation process tends to slower with increased degradation time and overall
process of mineralization is complicated in nature. Solar light assisted BiOCl/GSC/A+P process
displayed higher efficiency in comparison to BiOCl/CT/A+P process.
When aqueous suspension of BiOCl/GSC or BiOCl/CT was exposed to solar light,
charge pair separation caused the formation of electron-hole pair [67, 65]. The conduction band
electron (e-CB) and valance band hole subsequently reacted with H2O and dissolved oxygen to
21

form different oxidizing species. It is well established that during heterogeneous
photodegradation, hydroxyl radical with oxidizing potential of 2.8 eV are the main the oxidizing
agent [67]. To probe the role of hydroxyl radicals during photo degradation process, degradation
experiments were conducted with hydroxyl radical scavenger, isopropanol. The second order rate
9
constant (6 ×10 M-1 S-1) of isopropanol with hydroxyl radical was reported by Buxton and co-
workers [68]. AMP removal efficiency was reduced to 10% and 8% for BiOCl/GSC/A+P and
BiOCl/CT/A+P process in the presence of isopropanol as given in Fig. 8 (a) & (b). While during
OTC photodegradation, BiOCl/GSC/A+P and BiOCl/CT/A+P process had respective efficiency
-4
of 8%and 7 % with isopropanol (2.0 × 10 mol dm-3) as presented in Fig. 8 (c) & (d). The
degradation experiments were performed in the presence of hole scavengers, NaCl [69, 65]. The
reduction in removal efficiency was very minor in presence of NaCl. Moreover, the oxygen
scavenger, tertiary butyl alcohol had no significant effect on the degradation of AMP and OTC
[62]. These results indicated AMP and OTC degradation occurred mainly via hydroxyl radical
assisted process.
On the basis of A-P experiments and characterization, a mechanistic view of antibiotic
degradation is anticipated and described in Fig. 9. In BiOCl/GSC and BiOCl/CT, both GSC and
chitosan provided surface functionalities for the adsorption of OTC and AMP. In case of GSC,
electrostatic interaction between electron donor groups (C=O, SiO2, O-CH3) and amphoteric
+
OTC and AMP resulted in effective adsorption of antibiotics [21]. In case of chitosan, NH2 and
OH-functionalities are responsible for adsorption of OTC and AMP [36, 70]. The higher
adsorption in case of GSC was due to the presence of graphene sheet present in BiOCl/GSC [23].
In the meantime, the adsorbed OTC or AMP was degraded rapidly during A+P process. Upon
irradiation of BiOCl/GSC and BiOCl/CT with solar light, the electron-hole pair formed [70, 71,
22

36, 66, 67]. The conduction band electrons reacted with O2 to form OH·. The valance band
electrons combined with H+/H2O to form hydroxyl radicals. The hydroxyl radicals subsequently
mineralized AMP and OTC in CO2 and respective ions. In GSC, graphene sheet acted as an
electron sink for condition band electron and minimized the electron-hole pair recombination
[23, 54, 55]. Under precedent experimental conditions, alone photolysis had no effect on AMP
and OTC degradation.
The reusability of photo catalyst is an important factor for real time applicability of the
process. The recycle efficiency of both BiOCl/GSC and BiOCl/CT was investigated for AMP
and OTC removal. The separation of both BiOCl/GSC and BiOCl/CT from reaction solution was
quick and easy due to low agglomeration. Both BiOCl/GSC and BiOCl/CT were separated from
reaction solution through simple sedimentation process and catalyst loss was 5 % after ten
catalytic cycles. However, the separation of native BiOCl was difficult and resulted in 40 % loss
of catalyst with during 10 cycles. For AMP removal, the removal efficiency of BiOCl/GSC was
reduced to 85% from 92%as shown in Fig. 10 (a), whereas removal efficiency of BiOCl/CT was
decreased to 67% from 75 %as presented in same figure. During OTC removal for ten successive
catalytic cycles, the removal efficiency of BiOCl/GSC and BiOCl/GSC was found to be 77% and
62 %, respectively as depicted in Fig.10 (b). The removal efficiency of native BiOCl was
reduced to 4 %.However no significant changes in FTIR and XRD spectra of BiOCl/GSC and
BiOCl/CT were observed after 10 catalytic cycles (Fig. 10 c and d).
Conclusion
Supported photocatalysis is emerging as an important technique for wastewater treatment. In
this work, GSC and CT supported BiOCl nanoplates (BiOCl/GSC and BiOCl/CT) were
successfully prepared by modified hydrolysis method. The characterization results indicated the
23

formation of BiOCl nanoplates over GSC and CT. The size of BiOCl/GSC and BiOCl/CT was
found to be 50 and 70 nm, respectively. The interplanar fringes for {110} plane of tetragonal
BiOCl were clearly observed in HRTEM images in BiOCl/GSC and BiOCl/CT. The supported
photocatalyst had substantial adsorption capacity for AMP and OTC removal. The adsorption
had significant effect photo degradation of AMP and OTC. The simultaneous adsorption and
photocatalysis (A+P) process had synergistic effect of photo degradation of OTC and AMP. The
complete mineralization of AMP was obtained in 10 h using BiOCl/GSC/A+P catalytic system.
-
While 70% of OTC was mineralized to CO2, H2O and NO3 ions in 10 hours. Both BiOCl/GSC
and BiOCl/CT exhibited reasonable recycle efficiency for 10 catalytic processes. According to
precedent work, graphene sand composite or chitosan supported BiOCl can be used as both
adsorbent and photocatalyst for treatment of wastewater that contains. The main advantage of
supported BiOCl, in comparison to BiOCl is easier separation from reaction solution without
applying external magnetic field and centrifugation process.
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Fugure capations:
Fig. 1. FESEM images and XRD analysis of BiOCl/GSC and BiOCl/CT. (a) FESEM image of
native GSC, (b) FESEM image of native Chitosan, (c) FESEM image of BiOCl/GSC, (d)
FESEM images of BiOCl/CT, (e) magnified FESEM image of Fig.1 (c), (f) magnified FESEM
image of Fig.1 (d), (g) XRD spectra of (1) BiOCl, (2) BiOCl/GSC, (3) native GSC and (h) XRD
spectra of (1) BiOCl, (2) BiOCl/CT (3) native CT.
Fig.2.TEM, HRTEM and SAED pattern of BiOCl/GSC and BiOCl/CT. (a) TEM image of
BiOCl/GSC, (b) TEM image of BiOCl/CT, (c) magnified TEM image of Fig. 2 (a) , (d)
magnified TEM image of Fig. 2(b) , (e) HRTEM images of BiOCl/GSC, (f) HRTEM image of
BiOCl/CT, (g) SAED pattern of BiOCl/GSC and (h) SAED pattern of BiOCl/CT.
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