Catalytic removal of organic colorants from water using some transition metal oxide nanoparticles synthesized under sunlight

Article abstract of DOI:10.1039/c6ra17555d

Transition metal oxides (TMO) constitute a most amazing class of materials with a wide range of properties and applications; therefore, their synthesis using a green approach is a necessity. As such, sunlight irradiation was employed to synthesize various TMO nanostructures (ZnO, CuO, Co₃O₄, NiO and Cr₂O₃) using water as a solvent. Nanoparticles obtained with distinct morphologies, such as nanotubes (ZnO; <35 nm), nanorods (CuO; 7-50 nm), nanorods, triangles and hexagons (Co₃O₄; 45-90 nm), needle-shaped (NiO; 2-25 nm) and nanobeads (Cr₂O₃; ～17 nm) were confirmed by TEM analysis. The significance of the synthesis is in its quick approach with no thermal heat involvement, reusable catalyst, cost effectiveness and ability to fabricate almost uniformly distributed nanoparticles with small sizes. The potential of the synthesized nanoparticles was examined in the treatment of simulated water containing hazardous dyes: Alizarin Red S (ARS) + Methylene Blue (MB). Interestingly, in a short period of 180 min, 88.24% of the dye mixture was, for the most part, completely degraded using Cr₂O₃ nano-needles, followed by 87.96% (ZnO) > 86.86% (CuO) > 85.89% (NiO) > 80.35% (Co₃O₄), depending on the sizes of the respective TMO nanoparticles. This is also supported by the finding of small and non-toxic by-products such as but-2-enal, sulfur trioxide and benzoquinone. With high potential observed in the removal of dyes, TMO nanoparticles have a bright future with respect to their use as important adsorbents in waste water treatment. The advantage of the present work lies in the green synthesis of nanoparticles and their application in helping to make our environment green.

Full text of DOI:10.1039/c6ra17555d

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Catalytic removal of organic colorants from water using some transition
metal oxide nanoparticles synthesized under sunlight
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Uma Shanker,* VidhishaJassal and Manviri Rani
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*Department of Chemistry
Dr B R Ambedkar National Institute of Technology
Jalandhar, Punjab, India-144011
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* Corresponding Author
Dr Uma Shanker
(Assistant Professor)
Office Number-CE-306
Department of Chemistry
Dr B R Ambedkar National Institute of Technology Jalandhar,
Jalandhar, Punjab, India-144011
Email: shankeru@nitj.ac.in, umaorganic29@gmail.com
Contact number: +91- 7837-588-168 (Mobile)
+91-0181-269-301-2258 (Office)
Fax: +91-0181-269-0932
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Abstract
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Transition metal oxides (TMO) constitute most amazing class of materials with wide range of
properties and applications. Therefore, their synthesis usinggreen approach is the need of present
time.
Hence, based on it, sunlight irradiation was employed to synthesize
variousTMOnanostructures (ZnO, CuO, Co3O4,NiO and Cr2O3) using water as a
solvent.Nanoparticles obtained with distinct morphologiessuch asnanotubes (ZnO; <35 nm),
nanorods (CuO; 7ꢀ50 nm), nanorods, triangles and hexagons (Co3O4; 45ꢀ90 nm), needleꢀshaped
(NiO; 2ꢀ25 nm) and nanobeads (Cr2O3;~17 nm) were confirmed by TEM analysis.The
significance of synthesis is in its quick approach with no thermal heat involvement, reusable
catalyst, cost effectiveness and ability to fabricate almost uniformly distributed nanoparticles
with small sizes.The potential of nanoparticles synthesized was examined in treatment of
simulated water containing hazardous dyes: Alizarin Red S (ARS) + Methylene Blue (MB).
Interestingly, in a short period of 180 min 88.24% of the dyes mixture was more or less
completelydegraded using Cr2O3nanoꢀneedles followed by 87.96% (ZnO) > 86.86% (CuO) >
85.89% (NiO) > 80.35% (Co3O4), depending on the sizes of respective TMO nanoparticles. This
is also supported by finding of small and nonꢀtoxic byꢀproductssuch as butꢀ2ꢀenal, sulfur trioxide
and benzoquinone. With high potential observedin removal of dyes, TMO nanoparticles can be
used as important adsorbents in waste water treatment with a bright future. The advantage of
present work lies in its green synthesis of nanoparticles and their application in making our
environment green.
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Keywords: green synthesis, sunlight, TMO nanoparticles, morphology, dyes removal.
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1. Introduction
Transition metal oxides (TMOs) are an important class of materials due to their several attractive
properties such as magnetic, electronic and optical.1,2 This makes them effective to be used in the
variety of applications such as catalysis,3 sensors,4 lithium ion batteries4,5 and many more.6,7
Concerning to these applications, TMOs are advantageous as compared to the other
conventionally used materials. Therefore, their synthesis using advanced methods is the key
interest of researchers around the globe.
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In general, various methods based on the dry and wet procedures have been developed for the
synthesis of TMO nanoparticles.8,9 However, such methods have disadvantages in terms of
temperature and requirement of toxic chemical reagents that may harm the environment. To
overcome the negative effects of these problems, the best possible way is to move towards green
chemistry, i.e. avoiding the use of hazardous reagents and replacing them with the natural ones in
all the technological processes.10 Along with the reduction in environmental pollution,11 green
routes yield contaminants free nanoparticles with small size and distinct morphology.12,13
Following the green chemistry principles, authors decided to employ a sunlight assisted approach
for the synthesis of various TMO nanoparticles, namely, zinc oxide (ZnO), chromium oxide
(Cr2O3), cobalt oxide (Co3O4), copper oxide (CuO) and nickel oxide (NiO) nanoparticles.
Recently, the applications of TMO nanoparticles in the treatment of wastewater containing
organic colorants have gained immense interest owing to their high surface area and
semiconducting properties.14 Irregular and unsystematic use of dyestuffs is a glaring evidence of
environmental pollution. Textile and paper industries discharge the enormous volume of
pollutants, nonꢀdegradable and carcinogenic natured colored dye effluents into the water bodies
without any proper treatment, thus leading to the contamination of water resources.15 Therefore,
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it is a matter of utmost importance to eliminate the dyestuffs from water bodies. Recently,
authors reported the degradation of malachite green and eriochrome black T using potassium
zinc hexacyanoferrate nanoparticles.16 This motivated us to explore the effective use of other
nanoparticles such as TMO in the degradation/removal of hazardous organic dyes.
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The dye Alizarin red S (ARS) has been considerably used in the textile industry since ancient
times.17ARS (1,2ꢀdihydroxyꢀ9, 10ꢀanthraꢀquinonesulfonic acid sodium salt) is a waterꢀ soluble,
widely used anthraquinone dye with carcinogenic effects and its release in environment can
cause serious problems to human beings and animals. ARS is resistant to degradation because of
its complex aromatic structure, high thermal and optical stability. Therefore, an effective catalyst
is required for the degradation of ARS.18 Methylene blue (MB) is widely used in coating for
paper stocks, coloring paper, dyeing cottons, wools, etc. Acute exposure of MB will result in
increased heart rate, vomiting, shock, Heinz body formation, cyanosis, jaundice and quadriplegia
and tissue necrosis in humans.19,20 Due to their wide use, toxicity, high persistence and
bioaccumulation, authors selected these dyes for observing the potential of synthesized TMO
nanoparticles in removal from simulated water.
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In the light of above facts, it was planned to carry out green synthesis of various TMO
nanoparticles using sunlight irradiation technique. In this method reaction was carried out by
exposing the metal salt solution in sun light for one hour followed by addition of aqueous NaOH
solution. Immediate precipitation of metal oxide nanoparticles occurred. Further characterization
reveals the wellꢀdefined morphologies and uniform distribution of nanoparticles. The green
synthesized TMO nanoparticles were employed for the treatment of simulated water containing
ARS+MB dye mixture, which constitutes the hazardous discharge from industries into the water
bodies. Various process parameters such as concentration of mixed dye solution, dose amount of
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catalysts used and pH of mixed dye solution were optimized. To the best of our knowledge, so
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far no such method has been reported for the sunlight assisted green synthesis of TMO
nanoparticles and their use in removal of these dyes from simulated water.
2. Experimental
Zinc nitrate (Zn(NO3)2), copper nitrate (Cu(NO3)2), cobalt nitrate (Co(NO3)2), nickel nitrate
(Ni(NO3)2), chromium nitrate (Cr(NO3)3) and sodium hydroxide (NaOH) used in this study were
of analytical grade and procured from Merck. All solutions were prepared using double distilled
water.
2.1 Design of Experiment
2.1.1 Green synthesis of TMO nanoparticles
The aqueous solutions (0.05M, 100 ml) of different metal salts were prepared using double
distilled water. The solution as such was kept in sunlight for 1 hour followed by addition of
aqueous NaOH solution. Immediate precipitation of metal oxide nanoparticles occurred, which
were then filtered, washed and dried in oven for further characterization.
Reactions were investigated under both with and without photoꢀillumination. No precipitation
was observed without photoꢀillumination, whereas immediate precipitation of the TMO
nanoparticles was observed within 1 hour under photoꢀillumination. The reason might be the
presence of greater number of photons of certain wavelength in direct sunlight which initiate the
reaction, thus promoting the formation of TMO nanoparticles.21ꢀ23
Initially the metal nitrate reacts with the sodium hydroxide to form [M(OH)x]+, as shown below:
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When the aqueous mixture was kept under sunlight irradiation, following process occurs:
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The solvated electrons and free radicals are highly active species which helps in generating TMO
nanoparticles.
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2.1.2 Photocatalyticdegradation studies in simulated water
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The mixture of two dyes, namely, ARS (λmax = 510 nm) and MB (λmax = 665 nm),were used as a
model system for photocatalytic dye removal experiments. Practicality of this particular
application lies in the fact that industrial effluents contain a mixture of colored dyes which are
being discharged untreated into the water reservoirs. A 50ꢀml of dye solution at various
concentrations (10ꢀ50 mg/L) was prepared and metal oxide (5ꢀ25 mg) nanoparticles were added
to it. The solutions were kept under bright sunlight. At different time intervals (20 minutes), 2ꢀml
of the aliquots was taken and centrifuged at 10,000 rpm for 10 min. The absorbance spectrum of
the supernatant was subsequently measured and continued upto 180 minutes.
Different parameters (Figure 5) such as catalyst amount, initial concentration and initial pH of
mixed dye solution were optimized in order to get maximum dye degradation (%) by synthesized
TMO nanoparticles. These collected samples were also analyzed for the identification of possible
degradation products.
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2.2 Instrumentation
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Powder Xꢀray diffraction (PXRD) was recorded on a PAN analytical XꢀPRT PRO instrument
using CuKα (λ=1.5406 Ǻ) radiation. FieldꢀEmission Scanning Electron Microscopy (FEꢀSEM)
was performed in order to find out the surface morphology and average particle size of the
synthesized TMO nanoparticles (Quanta 200 FEG). Exact particle size and shape of the
nanoparticles was studied using Transmission Electron Microscopy (TEM) conducted with
Hitachi (Hꢀ7500) instrument operating at 120kV. Malvern Zetasizer (Nano ZS) instrument was
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used to measure the zeta potential of nanoparticles.
A spectrophotometer (Agilent Pro) was used for quantitative measurement of dyes. A gas
chromatograph GC 1300 coupled with mass spectrometer TSQ8000 was used for identification
of oxidative degradation products. Column used was TG 5MS having dimensions 30m × 0.2mm
× 0 × 2mm. The conditions for GC were: injector temperature, 280 ºC and transfer line
temperature, 250 ºC. The capillary column temperature was programmed 80 ºC for 2 min; from
80 to 260 ºC at 10 ºC min−1 and holding at 250 ºC for 15 min. Helium was used as a carrier gas
with a flow rate of 2 ml min−1. The mass spectrometer conditions were ion source at 250 ºC and
ionization energy 40 eV.
3. Results and Discussion
Employing a green pathway based on sunlight and water, various TMO nanoparticles were
synthesized successfully. Their detailed structural morphology and potential for degradation of
hazardous dyes mixture are discussed. The PXRD, FEꢀSEM/EDS and TEM images are shown by
Figures 1, 2 and 3, respectively.
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3.1 Powder X-Ray Diffraction
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PXRD data was found to be concordant with the JCPDS data (ZnO Card No.79ꢀ2205, CuO Card
No. 80ꢀ1917, Co3O4 Card No. 76ꢀ1802, NiO Card No. 22ꢀ1189, Cr2O3 Card No.ꢀ76ꢀ0147). The
peaks of the XRD spectra confirm the formation of respective products (Figure 1). The
symmetry of the as synthesized TMO nanoparticles obtained was: Hexagonal (ZnO), Monoclinic
(CuO), Cubic (Co3O4), Rhomohedral (NiO) and Rhombohedral (Cr2O3). Absence of secondary
peaks ensures high purity level of the samples. In case of ZnO, CuO, Co3O4, NiO and Cr2O3 the
maximum relative intensity (%) was observed at 17.47º, 25.26º, 25.04º, 25.02º and 33.67º at 2θ
scale, respectively (Table 1ꢀS). A comparison was also carried out with traditionally used
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methods to confirm the effectiveness of the current approach. Nugroho and Kim 2014
synthesized Co3O4 nanoparticles with high crystalline size and low crystallanity at low
concentrations of KOH using supercritical water.24 A thermal treatment method was also
employed to synthesize crystalline Co3O4 nanoparticles with no peaks of impurities.25 Various
other methods are also reported for the synthesis of cobalt oxide nanoparticles such as
mechanochemical, microꢀemulsion, etc.26, 27 Employing a solꢀgel technique NiO nanoparticles
having average size of 5 nm and rockꢀsalt structurewere synthesized.28 Heatꢀtreated and cubic
nickel oxide nanoparticles with high level of purity have also been synthesized.29, 30 Gibot and
Vidal synthesized wellꢀcrystalline Cr2O3 nanoparticles (with no impurities of silica)using Cr (III)
nitrate and nanometric silica spheres as precursor and template agent, respectively.31 Magnetic
chromium oxide nanoparticles were also synthesized via simple approach.32 Through
electrochemical method ZnO nanoparticles with high crystallanity were synthesized.33 Various
other techniques like microꢀemulsion, thermal treatment, etc also resulted in monodisperse ZnO
nanoparticles.34ꢀ36 Good crystallanity of CuO nanoparticles was obtained using Cu(NO3)2 as
precursor as compared to CuCl2 precursor, with increase in crystallanity after annealing.37 A
thermal decomposition route was employed for synthesizing copper oxide nanoparticles with
high purity level.38 Using carbon nanotubes as template, Wu et al. (2002) synthesized CuO
nanoparticles with average size ~ 37.2 nm, as confirmed by XRD analysis.39 Though our results
are similar to these conventional methods such as electrochemical or solꢀgel providing the
nanosize crystaline TMOs but green approach is best among because of easy to perform and a
good alternative to the earlier time consuming and costly methods.
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3.2 FE-SEM
It is interesting to note here that even though all the TMOs were synthesized via same method
but they exhibited different morphologies and were distributed almost uniformly throughout the
network(Figure 2). Long uniform channels of ZnO nanoparticles(<50 nm) were formedin shape
of tubes. Long rods nanoparticles of CuOhaving average size less than 100 nm were observed.
Almost hexagonal shaped Co3O4 nanoparticles (50ꢀ100 nm) were formed. The average particle
size in case of NiO and Cr2O3 nanoparticles was extremely small (<10 nm).
The EDS (Energy Dispersive XꢀRay Spectroscopy) analysis of all the samples indicated the
presence of oxygen along with concerned metal ion (Figure 2). In ZnO, Zn and O showed 80.07
and 19.93 weight (%) and 49.58 and 50.42 atomic (%), respectively. In case of CuO, weight (%)
of Cu and O were 72.99 and 27.01, respectively, whereas atomic (%) were found to be 40.49 and
59.51, respectively. In Co3O4, Co and O exhibited 69.57 and 30.43 weight (%), on the other hand
atomic (%) was found to be 38.29 and 61.71, respectively. Also, in case of NiO, weight (%) of
Ni and O correspond to 66.52 and 33.48 and atomic (%) was 35.13 and 64.87, respectively. In
Cr2O3, Cr and O exhibited 49.95 and 50.05 weight (%) and 23.49 and 76.51 atomic (%),
respectively.
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3.3 TEM
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Small nanotubes of ZnO having size less than 35 nm were formed. Attractive morphology of
CuO i.e. long nanorods (diameter7ꢀ50 nm) was observed. It is important to note that in case of
Co3O4, three types of nanostructures namely, nanorods, triangular and hexagonal shapedwere
observed. The length of the nanorods varied between 50ꢀ100 nm whereas diameter was less than
50 nm. The particle size of triangular and hexagonal shaped Co3O4 nanoparticles were also less
than 50 nm. Needleꢀshaped NiO nanoparticles having extremely small size (length and width less
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than 25 and 2 nm, respectively) were formed. Moreover, nanobeads of Cr2O3were formed with
size ~17 nm. The most interesting feature of this study is the formation of various TMO
nanoparticles with unique morphologies and sizes less than 100 nm (Figure 3).
3.4Application of TMO nanostructures in removal of hazardous dyes
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The photocatalytic activity of the above synthesized TMO nanoparticles was evaluated for the
removal of mixture of two dyes, namely, ARS and MB. Concentration was found to decrease
continuously with increase in time interval. Under optimized conditions, the photocatalytic
removal capacity of various nanostructures was observed to be: Cr2O3 (88.24%) >ZnO (87.96%)
>CuO (86.86%) >NiO (85.89%) > Co3O4 (80.35%), depending on the size of nanoparticles
formed.
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Previous studies reported by researchers shows that maximum dye (Congo Red) degradation of
81.33% was achieved using ZnO nanoparticles synthesized via solꢀgel method.40 Also,
degradation efficiency of ~72% was achieved using ZnO as a photocatalyst.41 87% of methyl
violet was degraded using ZnO nanoparticles using precipitation technique.42 Degradation of
ARS under UV light irradiation was carried out using ZnO nanoparticles (~77% degradation in
90 minutes).43 Moreover, CuO nanostructures synthesized via hydrothermal route were able to
degrade methylene bue only in the presence of H2O2 as an oxidizing agent.44 This proves the
effectiveness of the above synthesized nanoparticles which can degrade dyes in the absence of
any external oxidizing agent. Vaseem et al. (2008) utilized CuO nanoparticles for degrading MB
dye. Results revealed the lower catalytic efficiency of the catalyst as negligible amount of dye
got degraded.45 Thus, it is clear from the above discussion that TMO nanoparticles synthesized
via sunlight mediated green route have better catalytic efficiency for the removal of organic dyes
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than the conventional TMO nanoparticles. Also, these nanomaterials are costꢀeffective, hence,
can be employed on a larger scale for degrading hazardous organic dyes
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3.4.1 Effect of dye concentration
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To estimate the initial concentration of dyes which can be effectively degraded using synthesized
nanoparticles, experiments were carried out for various concentrations of the dye solution (10ꢀ50
mg/L; catalyst loadingꢀ15mg/10 ml; pH=7.0). Maximum removal of dyewas obtained at 10 mg/L
(Figure 5a), which is probably due to the fact that at low dye concentration more number of
active sites are available for adsorption by catalyst. As the concentration increases, the surface
area of the catalyst becomes limited, i.e. catalyst:reactant ratio decreases. Hence, responsible for
the decreased rate of dye removal.46
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3.4.2 Effect of pH
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pH of solution is the most important parameter for measuring the photocatalytic efficiency of
nanoparticles. It was observed that the dye removal increased with the raising pH upto 7.0, after
that it got decreased (dye concentrationꢀ10mg/L; catalyst loadingꢀ15mg/10 ml) (Figure 5b). This
may be due to the ion screening effects of H+ ions under acidic conditions and OHꢀ ions in
alkaline medium which mayhave the adverse impact on the adsorption and hence less dye
decoloration was observed.47 In acidic medium H+ ions may get attracted towards the dye
molecules and in alkaline medium OHꢀ ions may get adsorbed on the surface of photocatalyst.
Thus, decreasing the rate of photodegradation. Therefore, at neutral pH interference from other
ions is not observed and best results were obtained. Moreover, according to the point of zero
charge (Pzc), the surface charge properties of TMOs changes with change in pH of the solution.
The Pzc for TMO is around 6.5ꢀ8.0, thus, the surface is presumably positively charged in acidic
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medium (pH<Pzc) and negatively charged in basic medium (pH>Pzc).48, 49 MB is cationic dye
whereas ARS is anionic, thus mixing the two dyes of equal concentrations is believed to give a
neutral solution. For better understanding of effect of pH, Zeta potential, of the TMOs at neutral
pH was recorded and as per established fact that surface charge can greatly influence the stability
of nanoparticles through the electrostatic repulsion between the particles. Greater the zeta
potential, more is the probability of particles to be stable because the charged particles repel one
another and hence overcome the usual tendency to aggregate.50 The negative zeta potential
values of ZnO, CuO, Co3O4, NiO and Cr2O3, i.e. ꢀ16.5, ꢀ11.5, 3.07, ꢀ11.6 and ꢀ25.2 mV,
respectively were observed at neutral pH (Figure 4). The high absolute zeta potential value
specifies a strong repellent force among the nanoparticles which ultimately prevents
aggregation .51, 52 Among the nanoparticles used, Cr2O3 is having the highest absolute value of
zeta potential, thus, justifying its greater surface activity.
3.4.3 Effect of catalyst loading
The rate of removal of hazardous dyes as a function of initial amount of photocatalyst (5mg/10
ml to 25 mg/10 ml) has been examined. The results(Figure 5c) show that removal of dye
continuously increased with increasing dosage of the catalysts, reached the higher value (at
15mg/10 ml) and then decreased. The probable reason could be that as the dose increases, the
number of active sites on catalystpenetratedbyphotons gets blocked, thereby, decrease in the rate
of removal of dyes were observed.53
3.5 Reaction mechanism
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TMOs (ZnO, CuO, Co3O4, NiO and Cr2O3) being semiconducting in nature are able to generate
electronꢀhole pair (in conduction and valence band, respectively) upon photoillumination. The
interaction of these electronꢀhole pairs with water produces active species OH• which breaks the
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large harmful organic dyes (Figure 6). First, adsorption of the dye takes place on the surface of
adsorbent (TMO nanoparticles), followed by dye degradation process.54 Overall, the generation
of electronꢀhole pairs cum OH• is responsible for the photodegradation of different organic dyes
as well as other organic pollutants. Electrons in the conduction band on the surface of catalyst
can reduce to molecular oxygen into superoxide ion, which ultimately forms hydrogen peroxide.
The holes generated in situ are responsible for the generation of hydroxyl radicals which are the
primary agents responsible for the cleavage of dye minerlization.47
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3.5.1 Adsorption isotherms
Adsorption isotherms (Figure 7a) of ARS+MB clearly show that adsorption is fast in all the
cases and the isotherms are regular, positive and concave to the concentration axis. Adsorption
data can be represented through Langmuir adsorption isotherms which assume the formation of a
monolayer of solute molecules on the surface of the adsorbent.55 A typical graph of Ce/Xe v/s Ce
of all the dyesis a straight line (Figure 7b). The adsorption data was fitted in Langmuir
adsorption equation:
ꢀꢁ
ꢂꢁ
1
ꢀꢁ
=
+
ꢃꢄꢂꢅ ꢂꢅ
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or
1
1
1
1
=
ꢆ
ꢇ +
ꢂꢁ ꢀꢁ ꢃꢄꢂꢅ
ꢂꢅ
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Where, Ce is the equilibrium concentration of the dye solution; Xe the amount of dye adsorbed
per gram weight of adsorbent; Xm the amount of dye adsorbed at saturation; kL the Langmuir
adsorption constant. Xm, kL and R2 values were determined to be: ZnO (18.587; 4.744; 0.985),
CuO (19.696; 4.896; 0.933), Cr2O3 (18.663; 4.969; 0.988), Co3O4 (22.183; 13.254; 0.777) and
NiO (22.306; 7.686; 0.905).
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3.5.2Photocatalytic degradation products of ARS and MB using TMO Nanoparticles
It has been found that under neutral conditions (pH~7.0) solidꢀsupport oxidative process resulted
in the formation of colorless products which got deposited over the surface of TMO
nanoparticles. Possible degradation pathway of oxidative degradation of ARS and MB and their
representative mass spectrum are shown in Figures 8 and 9, respectively.
3.5.3 Identification of products by GC-MS analysis:
An attempt has been made to propose the degradation pathway of the major products formed via
hydrolysis or oxidation of the dyes mixture. The identification of the products has been done
using earlier reported literature as well as NIST library inbuilt with GCꢀMS system. The major
oxidative agent is OH• which is highly active species and is known to degrade the complex dyes
into smaller byꢀproducts.47 Complete mechanism leading to the generation of OH• is represented
in Figure 10. ARS (mol.wt. = 342) present in the dye mixture undergoes attack by O2 radical at
Cꢀ12 position under visible irradiation (1; sodiumꢀ3,4ꢀdihydroxyꢀ9,10ꢀdioxoꢀ9,10ꢀ
dihydroanthraceneꢀ2ꢀsulfonate). This causes the dye to cleave into two parts: one part gets
oxidized to phthalic acid (1a; molecular ion peak at m/z=166) and the other part is oxidized to
2ꢀ
hydroxyl intermediates (1b) which finally gets mineralized into CO2 and SO4 ions.56 After the
hydrolysis of MB (mol.wt.=320), (2; 3,7ꢀbis(dimethylamino)phenothiazinꢀ5ꢀium) is generated
having CꢀS+=C bond functional group. Attack of OH• on this functional group leads to the
cleavage of bonds and (2a; 3ꢀ(3ꢀ(dimethylamino)cyclohexaꢀ2,4ꢀdienꢀ1ꢀyl)sulfinylꢀN,Nꢀ
dimethylbenzeneꢀ1,4diamine) is generated having m/z=303. The oxidation state of S now
changes from ꢀ2 to 0 by the attack of OH•. This conversion to CꢀS(=O)ꢀC ultimately leads to the
opening of the central aromatic ring. Subsequent attack of OH• finally leads to the formation of
+
an intermediate (2b; phenol) with the release of NH4 and SO42ꢀ ions. The intermediate (2b) then
gets converted into benzoquinone (2c). This is further supposed to get mineralized into stable
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CO2.57 Degradation of MB also results in the formation of 2Hꢀ1,4ꢀthiazene (3) having m/z=98,
ꢀ
+
2ꢀ
which ultimately gets mineralized into CO2, NO3 , NH4 and SO4 ions. Attack of active OH•
species can also take place at Cꢀ5 position, leading to the formation of intermediate (4; 3,4,5ꢀ
trihydroxyꢀ9,10ꢀdioxoꢀdihydroanthraceneꢀ2ꢀsulfonic acid), which gets converted into 6ꢀformylꢀ
3,4ꢀdihydroxyꢀ5,8ꢀdioxoꢀ5,8ꢀdihydrophthaleneꢀ2ꢀsulphonic acid (4a; m/z=295). This ultimately
gets cleaved into butꢀ2ꢀenal (4b; m/z=71). In another proposed pathway for the degradation of
MB, an intermediate (5; 3,7ꢀbis(dimethylamino)ꢀ9ꢀformylꢀ4aHꢀphenothiazineꢀ1ꢀcarboxylic acid)
having m/z=355 is generated, which after series of oxidative and bond cleavages results in the
formation of (5a; (Z)ꢀ 2ꢀaminoethenethiol; m/z=73). This species is also supposed to get
ꢀ
+
2ꢀ
mineralized into NO3 , NH4 and SO4 ions. In addition to this, various other byꢀproducts are
also formed such as sulfur trioxide (6; m/z=83), 2,3ꢀdimethyl cyclohexaꢀ2,5ꢀdieneꢀ1,4ꢀdione (7;
ꢀ
+
2ꢀ
m/z=140) and benzoquinone (2c; m/z=108). Formation of CO2, NO3 , NH4 and SO4 ions
suggest complete mineralization of the dyes mixture.
345
4. Reusability of catalyst
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To test the reusability of the catalystin simulated water treatment containing ARS and MB, the
decolorization reaction was run with a 180 min cycle. At the end of each cycle, TMO
nanoparticles were separated, heated to 60 ºC for 30 min, followed by washing with acetone as
well as distilled water, and further used for next cycle. The results showed that TMO
nanoparticles could be reused at least ten times (Figure 1S) without a significant loss in its
adsorbing properties.
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Industries such as textiles, cosmetics, paper, printing, etc., involve the use of dyes in form of
their aqueous solution and discharge them untreated into the water reservoirs. The amount of
dyes discarded each year is constantly increasing which is the major reason for environmental
pollution. Moreover, use of reactive dyes worldwide has increased from 60,000 tonnes in 1988 to
178,000 tonnes in 2004.58 Clinical and immunological investigation revealed that about 15% of
400 workers exposed to reactive dyes experienced nasal and respiratory problems.59 Exposure to
such dyestuffs causes serious health hazards since these have a property of binding to the body
proteins.60 In early 1981 some sulphonyl ethyl sulphatederivates were observed to be
carcinogenic60 and various textile dyes and their raw materials including benzidine and oꢀ
toluidine based are included in the international register of cancerꢀcausing chemicals.61
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In order to regulate this negative impact of dyes on living species, several methods such as
physical separation, chemical processes and biological degradation are being carried out. Due to
the growing awareness worldwide researchers are showing keen interest in the development of
effective removal techniques. Some important techniques are widely used including adsorption,
oxidation, coagulation–flocculation, biological treatment, electrochemical treatment and
membrane filtration. This needs further advancement so that it can be employed on a large scale.
6. Conclusions
A quick green approach using sunlight and water for synthesis ofZnO, CuO, Co3O4, NiO and
Cr2O3 nanostructures was successfully established. Almost uniformly distributed TMO
nanoparticles were obtained with distinct morphologies such as nanotubes (ZnO), nanorods
(CuO), nanorods, triangles and hexagons (Co3O4), needleꢀshaped (NiO) and nanobeads (Cr2O3).
These TMOs were found to be potential catalystin treatment of simulated water containing
hazardous dyes: ARS + MB. Among all, the degradation efficiency of Cr2O3 was highest
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(10mg/L of dye; 15 mg catalyst loading and neutral pH) followed by ZnO > CuO > NiO > Co3O4,
depending on the size of respective TMO. The finding of small and nonꢀtoxic byꢀproducts like
butꢀ2ꢀenal, sulfur trioxide and benzoquinone indicates the positive aspects of using synthesized
nanoparticles in removal of toxic dyes. Some oxidativebyꢀproducts such as 2Hꢀ1,4ꢀthiazeneꢀ
2,3,6ꢀtriol, 2Hꢀ1,4ꢀthiazene, 2,3ꢀdimethylcyclohexaꢀ2,5ꢀdieneꢀ1,4ꢀdione, maleic acid and
malealdehyde were identified. Further spread of TMO nanoparticles utilization in waste water
treatment for the removal of several other harmful pollutants such as polycyclic aromatic
hydrocarbons (PAHs), aromatic amines, various types of phenols and other carcinogenic
materialsneeds to be explored more effectively. In addition to this, the reusability of the catalyst
can decrease the cost of process.
Acknowledgements
Authors are thankful to DSTꢀFIST New Delhi for providing the equipment (UVꢀVIS
spectrohpotometer) used in characterization of the samples. Authors are also thankful to NIPER
Mohali for Zeta potential analysis and SAIF Panjab University, Chandigarh for TEM analysis.
One of the authors Ms. Vidhisha Jassal is thankful to Ministry of Human Resource Development
(MHRD), New Delhi for providing fellowship.
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Abstract Graphic
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Synopsis: Novelty of work lies in utilizing sunlight irradiated green synthesis of TMO
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List of Figures
Figure 1. PXRD pattern of (a) ZnO (b) CuO (c) Co3O4 (d) NiO (e) Cr2O3 nanoparticles

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Figure 2. FE-SEM and EDS analysis of (a) ZnO (b) CuO (c) Co3O4 (d) NiO (e) Cr2O3 nanoparticles

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Figure 3. TEM images of (a) ZnO (b) CuO (c) Co3O4 (d) NiO (e) Cr2O3 nanoparticles

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Figure 4. Zeta potential measurement of (a) ZnO (b) CuO (c) Co3O4 (d) NiO (e) Cr2O3 nanoparticles

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(a)
(b)
(c)
Figure 5. Dye degradation (%) at different (a) dye concentrations (b) pH (c) catalyst dose
Figure 6. Schematic diagram of photocatalytic dye degradation

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Figure 7. (a) Adsorption isotherm; (b) Langmuir isotherms for adsorption of Alizarin Red S (ARS) +Methylene Blue (MB) on different
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Figure 8. Proposed Degradation pathway of ARS and MB using TMO nanostructures