Photocatalytic Degradation of Rhodamine-B Under UV-Visible … · 2016-07-07 · Photocatalytic Degradation of Rhodamine-B Under UV-Visible Light Irradiation Using Different Nanostructured

____________________________________________________________________________________________

*Corresponding author: Email: [email protected];

American Chemical Science Journal3(3): 178-202, 2013

SCIENCEDOMAIN internationalwww.sciencedomain.org

Photocatalytic Degradation of Rhodamine-BUnder UV-Visible Light Irradiation Using

Different Nanostructured Catalysts

María del C. Cotto-Maldonado1*, Teresa Campo2, Eduardo Elizalde2,Arancha Gómez-Martínez2, Carmen Morant2 and Francisco Márquez1

1School of Science and Technology, Universidad del Turabo, 00778-PR, USA.2Departamento de Física Aplicada C-XII, Universidad Autónoma de Madrid, Cantoblanco,

28049 Madrid, Spain.

Authors’ contributions

This work was carried out in collaboration between all authors. Author MdCCM wrote thedraft of the manuscript and manage the analysis of the experimental data of the research.

Authors TC, EE, AGM and CM contributed to the chemical characterization of the catalysts,author FM supervised the synthesis, characterization, and analysis of catalytic results. All

authors read and approved the final manuscript.

Received 4th December 2012Accepted 30th March 2013

Published 3rd May 2013

ABSTRACT

Aims: The goal of this research is to determine the efficiency of different catalysts for thedegradation of organic compounds as possible alternative for wastewater treatments. Toreach this goal, many objectives should be previously satisfied including the synthesis ofdifferent catalysts and the catalytic tests for the different processes.Study Design: A multifactorial design was used for the experimental study.Place and Duration of Study: The present study was conducted between January 2011to December 2011 at the School of Science and Technology, Universidad del Turabo andthe Department of Applied Physics at the Universidad Autónoma de Madrid, Spain.Methodology: Different catalysts were synthesized and the photocatalytic activity wasmeasured. Catalysts were characterized by XRD, FE-SEM, SBET and TGA. For thephotocatalytic activity a cylindrical reactor with continuous stirring was used. The dye (10-5

M) was previously dissolved in water and 0.6g L-1 of the corresponding catalyst was added

Research Article

American Chemical Science Journal, 3(3): 178-202, 2013

179

to the reaction mixture. An irradiation of 60 watts was applied. An aliquot of 10 mL wastaken every 10 min during a period of an hour from the solution and diluted forcharacterizing by UV-vis and fluorescence spectroscopies and TOC.Results: The catalytic tests indicate that TiO2NWs is the most efficient catalyst andeventually could be used for alternative wastewater treatments.Conclusion: Synthesized (TiO2NWs, TiO2@MWCNTs and ZnO) and commercial catalystswere fully characterized by FE-SEM, TGA, specific surface area (BET) and XRD. The mostefficient catalyst was TiO2NWs (with approximately 96.44% of degradation). All catalystsused were able to degrade the Rhodamine-B and could eventually be used to removalpollutants from water.

Keywords: Photocatalysis; Rhodamine-B; TiO2; nanowires; advanced oxidation process;pollutants.

1. INTRODUCTION

Water is an important resource in our society. Less than a 0.7% of the total of water in thePlanet is fresh water and only 0.01% is accessible to be used [1]. Today, some of the mostdiscussed issues around the world are the sanitation, soil and water chemical pollution, airpollution, the degradation of water sources and natural resources because organic,inorganic, bionutrients and microorganisms are some of the most common contaminants inwater [1,2].

The production and use of synthetic chemical products have experienced an importantincrease during the last century. These products imply a challenge to the environment [2],due to the fact that the environment does not have the ability to degrade these pollutants.New water treatment technologies are necessary to remove or degrade hazardouscontaminants present in effluents, making the water resources both safe and potable tohuman consumption. For example, to maintain the aesthetic and diminish the environmentalimpact of industrial effluents is necessary the discoloration of the wastewaters [3] and one ofthe most relevant pollutants are dyes. The dyes have different applications in paperindustries, leather, cosmetics, drugs, electronics, plastics and printing and approximately80% of the synthetic dyes are consumed by the textile industry [4]. Some researchersdetermined that the annual discharge of waters containing dyes ranges from 30,000-150,000tons [5] and also other chemicals used during the processes [4]. According to somestatistical results approximately 12% of the synthetic textile dyes used during a year are“lost” during the manufacturing and operational procedures and from that 12%, the 20% willbe finally released to the ecosystem through the industrial water discharges [6]. In the textileindustry, more than 10,000 different dyes and pigments are available in the market and 20-30% of them are reactive dyes [7,8].

Dyes are non biodegradable compounds [9] and industrial wastewaters that containbiorefractory compounds are normally limited to the use of chemical treatments because thechemicals are toxic to the microorganisms used in the conventional biological treatments[10]. Potential human exposure to wastewater which contains dyes is a concern because arecarcinogenic compounds, showing high resistance against biological, physical and chemicalreactions [5]. The common techniques used to remove the dyes include chemical, physicaland biological processes [8] but are inefficient because these compounds have highmolecular weight and biochemical stability (aromatic rings) [11,12]. This characteristicnecessarily implies the use of treatments by unconventional methods [6]. The adsorption


180

processes using activated carbon to eliminate the contaminants have the advantage that arevery easy to use but they are expensive [13] and produce additional problems during thedisposition of the contaminated materials. Other methods include the adsorption of the dyesby polymers and other materials [8,9]. The conventional treatments do not reduce the toxicityof the dyes [10]. Some of the principal disadvantages of the physical methods ascoagulation, precipitation and adsorption are the sludge formation, possible toxic by-products and the fact that the chemical processes involved are expensive [9,11,12,14].

The Advance Oxidation Processes (AOPs) use chemical procedures based on the use ofcatalysts or photochemical compounds which generate highly reactive transient species asthe hydroxyl radical which possesses high affectivity for the oxidation of organic compounds[15]. Processes as ozonation, Fenton, photochemistry, hydrogen peroxide oxidation, wet-airoxidation, radiolysis or even sonolysis generate highly reactive hydroxyl radicals forbleaching, finally arising to the mineralization of recalcitrant compounds [1,13,16]. In theboom of the eco-conservation and the eco-friendly techniques to degrade the pollutants inwater and wastewater, the AOPs are seen as alternative techniques [13] to the traditionalprocesses.

AOPs have many advantages as: the complete mineralization of the pollutants, are non-selective processes, can be used in low concentration of contaminants and can be combinedwith other methods [1]. The use and development of photocatalytic processes for theremoval of harmful contaminants, as a treatment for wastewater and air pollutants isbecoming increasingly popular [17]. Heterogeneous photocatalysis is one of the AOPs and isbased on the direct or indirect absorption of photons from ultraviolet (UV) or visible light by asemiconductor that possesses the appropriate energy gap. The semiconductor photocatalystshould be chemical and biological inert, stable, inexpensive, of easy synthesis andproduction, and without human and environmental risks [1]. When a dye is used, themechanism of photodegradation involves the excitation of the dye and the transference ofthe electrons to the conduction band of the photocatalyst (i.e. TiO2) to generate the dyeradicals. These radicals react with the oxygen on the surface of the catalyst generatingoxygen radical species as O2

•-, H2O2 and O2• remaining the valence band unaffected [17].

The photochemical process generated by using these photocatalysts transforms thepollutants in CO2, H2O and inorganic acids without generation of secondary compounds thatcould be toxic [18].

In the last decades, new applications for the use of nanoparticles in homogeneous andheterogeneous catalytic reactions were developed because these materials show a highefficiency and a high surface-to-volume ratio along with high surface energy [19] and are apart of the so-called new green chemistry technologies [20].

A model organic dye has been selected for this research due to their structures (functionalgroups) and their presence in the environment. The degradation reaction of a dye by ahydroxyl radical generated by UV-visible light or ultrasound irradiation using titanium oxideas catalyst is as follow [4]:

TiO2(OH·)ads – Dads + TiO2 – Dads(or D) → intermediates (P) → CO2 + H2O

Where D is the model organic dye and P is an intermediated product of the reaction.

Rhodamine-B (RhB) is a dye that belongs to a class of compounds called xanthenes, with aMol. Wt. 479.02 gmol-1 and a formula C28H31N20O3Cl. It is extensively used as model


181

compound because it shows a strong absorption band in the visible region of theelectromagnetic spectrum (555 nm) and this dye is characterized by having a high stability atdifferent pH values. This dye is currently used as dye laser material [21] and is part of thetriphenylmetane family of dyes that contain four N-ethyl groups at both sides of the xanthenerings [22]. It is stable in aqueous solution, used as a dye in textiles, food, cosmetics and asanalytical reagent during the determination of metals in solution, especially alkali andalkaline earth metals but can cause aesthetic pollution in the aquatic environments showinghigh resistance to biological and chemical degradation [22,23]. Currently this dye has beenprohibited for the use as food color because as many basic dyes it is suspected that RhBcould be a carcinogenic substance [13,24], inducing mutagenesis and teratogenesis in rats[25]. Their stability, resistance to biological and chemical degradation, presence in textileindustry effluents, and the suspect that this dye could be a possible carcinogenic substance,implies that RhB is an excellent model compound for this study.

2. MATERIALS AND METHODS

2.1 Materials

All chemicals used during this research are of analytical grade and were used as receivedwithout any further purification, unless otherwise described. HCl, RhB and NaOH (97+%ACS Reagent) were provided by Fisher Scientific (New Jersey, USA). TiCl4 andZn(CH3COO)2 (98+% purity ACS Reagent) were provided by Aldrich Chemical. Multi-WalledCarbon Nanotubes or MWCNTs were provided by CheapTubes Inc. (Brattleboro, USA).Ethyl alcohol (95% purity) was provided by Acros Organic. Milli-Q water (>18.2 MΩ.cmresistivity at 25oC) was used for all experiments.

2.2 Catalysts Synthesis

2.2.1 Synthesis of titanium oxide nanowires

TiO2 nanowires (TiO2NWs) have been synthesized by a catalyst-free hydrothermalprocedure. For a typical synthesis, 75 mL of concentrated hydrochloric acid and 75 mL ofMilli-Q water were mixed. Because the reaction of the hydrochloric acid and water isexothermic, it is necessary be careful during the preparation of the solution. After thesolution was cooled down to room temperature, 5 mL of the titanium precursor (titaniumtetrachloride, Aldrich Chemical) was added by dripping under agitation at room temperature.The mixture was magnetically stirred until all solid particles were dissolved and the solutionacquired a uniform color. The solution was placed in 30 ml Teflon-lined stainless steelautoclaves. Autoclaves were maintained at 150oC by 4 hours. After that, the autoclaves wereleft to cool down to room temperature. The synthesized TiO2NWs were washed at least 5times with Milli-Q water and dried overnight at 60oC.

2.2.2 Synthesis of zinc oxide nanoparticles

In a typical synthesis, 0.2 mol of Zn(CH3COO)2 (Aldrich, 98+% ACS Reagent) and 0.2 mol ofNaOH (Fisher Scientific, 97+% ACS Reagent) were previously dissolved in a few milliliters ofMilli-Q water and subsequently added to a 200 mL Erlenmeyer flask. After that, 100 mL ofethanol (Acros Organic, 95%) were added to the mixture. The solution was magneticallystirred at room temperature for approximately 2 h. The synthesized ZnO nanoparticles wereseparated from the solution by centrifugation (7,000 rpm) for 10 min and washed five times


182

with Milli-Q water. Next, the powder was dried overnight at 60oC and maintained in sealedcontainers.

2.2.3 Synthesis of titanium oxide@multiwalled carbon nanotubes

The synthesis of multiwalled carbon nanotubes (MWCNTs) coated with titanium oxideparticles in rutile phase consists principally of two steps. The first one is the modification ofthe carbon nanotubes to produce actives sites (OH- groups) on the surface. The second oneis the synthesis of the titanium oxide and the incorporation of this material into the activessites previously generated on the surface of the carbon nanotubes.

2.2.3.1 Carbon nanotubes modification

In a typical synthesis, 5 g of MWCNTs were refluxed in concentrated nitric acid at 100oC for24 hrs. After that, the nanotubes were separated by centrifugation (6,000 rpm, 10 min) andwashed repeatedly with Milli-Q water until pH neutral. The modified nanotubes were dried at60oC and maintained in sealed containers.

2.2.3.2 Synthesis and incorporation of titanium oxide on the MWNT surface

In a typical synthesis, 75 mL of concentrated hydrochloric acid (Fisher Scientific, 35%) and75 mL of Milli-Q water were mixed and magnetically stirred in an Erlenmeyer flask. When thereaction mixture cooled down to room temperature, 5 mL of the titanium oxide precursor(titanium tetrachloride, Aldrich Chemical) was carefully added dropwise over 10 min. Thereaction mixture was kept stirring until any solid particle was observed (approximately 30min). To synthesize TiO2 nanoparticles on the MWCNTs surface, 0.5 g of the chemicallymodified MWCNTs were added to this reaction mixture and the solution was kept stirring for30 min. Next, this solution was transferred to 30 ml Teflon-lined autoclaves. The autoclaveswere introduced in an oven for 4 hours at 150oC. After cooling down to room temperature,the synthesized material, namely TiO2@MWCNTs, was washed with Milli-Q water at least 5times and finally washed with ethanol. The product was dried overnight at 60oC andmaintained in sealed containers.

2.3 Characterization Techniques

X-ray powder diffraction patterns (XRD) were collected using an X´Pert PRO X-raydiffractometer (PANalytical, The Netherlands) in Bragg-Brentano goniometer configuration.The thermogravimetric analyses were done using a TGA Q-500 instrument (TA Instruments)under an inert atmosphere of nitrogen. The specific surface areas of the catalysts used inthe present research were determined by the BET method using a Micromeritics ASAP2020. Field emission scanning electron microscopy (FE-SEM) images were obtained using aJEOL JM-6400 microscope. The instruments used to determine the TOC concentration werea Tekmar Dohomann, Phoenix 8000 UV-Persulfate TOC Analyzer and a Leco CHNS-932.To characterize the absorption properties of the samples and to study the catalyticdegradation of the organic compounds a UV-Vis CARY 3 Varian spectrophotometer wasused. The fluorescence spectroscopy was studied by using a Varian Cary Eclipsespectrophotometer.


183

3. RESULTS AND DISCUSSION

3.1 Characterization of the Photocatalysts

3.1.1 Titanium oxide (TiO2, rutile phase)

The titanium oxide (rutile phase) is a commercial catalyst (Alfa Aesar, 97%) that has beenused for comparative purposes. The specific surface area, as determined by the BETmethod, was 41 m2g-1 (Table 1). Fig. 1 shows the FE-SEM micrographs corresponding tothis catalyst. As can be seen there, the rutile nanoparticles show diameters ranging from ca.200 to 500 nm. Thermogravimetric studies (Table 1) revealed that only 11% of weight lostwas observed under thermal treatment from RT to 450oC. Fig. 2 shows the XRD pattern oftitanium oxide indicating that this catalyst has rutile phase.

Fig. 1. FE-SEM image of titanium oxide particles (rutile phase)

Table 1. Surface area (BET) and thermogravimetric analysis of synthesized andcommercial catalysts

Sample SBET (m2g-1) TGA (%)TiO2 (Rutile Phase)* 41 11

TiO2 (Anatase Phase)* 48 27.8TiO2NWs 480 5.65TiO2@MWCNTs 620 25.27ZnO 68 25.27

* Commercial catalysts.


184

Fig. 2. XRD diffraction pattern corresponding to TiO2 (rutile phase)

3.1.2 Titanium oxide (TiO2, anatase phase)

The titanium oxide (in anatase phase) is a commercial catalyst used for comparisonpurposes. The specific surface area (SBET), as determined using the BET method, was 48m2g-1 (Table 1). The FE-SEM images of the anatase catalyst (Fig. 3) shows particles withdiameters at ca. 100 nm and the presence of small aggregates. According to the TGanalysis, the aggregates of the anatase particles could loss approximately the 27.8% of theirweight under thermal treatment from RT to approx. 500oC (Table 1). Fig. 4 shows thediffraction pattern of the commercial anatase catalyst, indicating that the anatase structure isthe predominant phase in this catalyst.

Fig. 3. FE-SEM image of titanium oxide particles (anatase phase)

(110)

(101)

(111)

(211)(002)


185

Fig. 4. XRD diffraction pattern of commercial TiO2 (in anatase phase)

3.1.3 Titanium oxide nanowires (TiO2NWs)

Titanium oxide nanowires (TiO2NWs) were synthesized according to the proceduredescribed previously. Fig. 5 shows different images obtained by FE-SEM of this catalyst.The wires are composed by smaller wires of nanometric dimensions (Fig. 5). The specificsurface area (SBET), as determined by the BET method, was 480 m2g-1 (Table 1). This valuewas unexpectedly high and could have relevant effects on the catalytic properties of thismaterial. According to the TG analysis of the as-synthesized TiO2NWs, only a weight loss of5.65% (Table 1) was observed during thermal treatment (from RT to approx. 400°C),indicating the compact and non-porous structure of this material. The XRD pattern (Fig. 6)unambiguously reveals that this material is exclusively composed by TiO2 in rutile phase.

0 10 20 30 40 50 60 70 80 90

Inte

nsity

(cps

)

2 Theta (Degree)

(004)

(101)

(200)(105)

(211)

(116)


186

Fig. 5. FE-SEM images of the as-synthesized TiO2NWs at different magnifications:5,000X (a), 10,000X (b), 25,000X (c) and 150,000X (d)

Fig. 6. XRD pattern of the as-synthesized TiO2NWs

3.1.4 Titanium oxide@multiwalled carbon nanotubes

Multiwalled carbon nanotubes were coated with particles of titanium oxide in rutile phase(TiO2@MWCNTs) (Fig. 7). The synthesis of this material has been carried out according to

a) b)

c) d)

20 30 40 50 60 70

Inte

nsity

(cps

)

2 Theta (Degree)

(310)

(002)(220)

(211)

(210)

(111)

(101)

(110)


187

the experimental procedure described previously. TiO2@MWCNTs were characterized byFE-SEM (Fig. 7). As can be seen there, the arrows indicate the presence of aggregates andclusters of carbon nanotubes coated by TiO2. The specific surface area of this hybridmaterial (SBET), as determined by the BET method, was 620 m2g-1 (Table 1). TG analysisperformed on this material from RT to approx. 500oC shows that approximately the 25.27%of weight is lost during the thermal process (Table 1). The XRD pattern (Fig. 8) indicates thatthe TiO2 coating as aggregates and clusters over the carbon nanotubes has rutile phase.

Fig. 7. FE-SEM image of the as-synthesized TiO2@MWCNTsArrows indicate the presence of small clusters of TiO2

Fig. 8. XRD pattern of the as-synthesized TiO2@MWCNTs

0 10 20 30 40 50 60 70 80 90

(002)

(220)(211)

(210)

(101)

Inte

nsity

(cps

)

2 Theta (Degree)


188

3.1.5 Zinc oxide nanoparticles

The synthesis of ZnO nanoparticles has been carried out according to the experimentalprocedure described previously. ZnO nanoparticles were characterized by FE-SEM (Fig. 9).As can be seen there, ZnO nanoparticles are characterized by having irregular forms anddimensions ranging from several hundred nanometers to no more than one-micrometerlength. According to the TG analysis, the weight-loss was approximately 25.27% (Table 1).The weight loss observed from RT to approx. 350oC can be possibly justified as due to theloss of water and the removal of surplus reagents of the sample. The specific surface area(SBET), as determined by the BET method, was 68 m2g-1 (Table 1). The most characteristiccrystallographic lattice planes are shown in the diffractogram of Fig. 10 [26,27] andcorrespond to reflections of the hexagonal phase [26].

Fig. 9. FE-SEM images of the as-synthesized ZnO particles at different magnification:25,000X (a), 50,000X (b)

Fig. 10. XRD diffraction pattern of the as-synthesized ZnO particles

0 10 20 30 40 50 60 70 80 90

Inte

nsity

(cps

)

2 Theta (Degree)

(100)

(002)

(101)(103)(110)


189

3.2 Catalytic Tests

The experimental setup used for the photocatalytic reactions was adapted from the literature[14,28]. Rhodamine B (RhB) is used in this research as model pollutant to evaluate thephoto degradation behavior of each catalyst (synthesized and commercial ones) under solarlight irradiation in the presence of H2O2. A cylindrical reactor (semi-batch type) withcontinuous stirring was placed in the center of a solar simulator, as the irradiation source.The solar simulator was composed by two annular white bulb lights with a total irradiationpower of 60 watts. A vessel of 1 L was used during the irradiation of the sample. The samplewas mechanically stirred with a paddler to maintain a homogeneous mixture during theirradiation of the sample. During the photocatalytic process, the sample was at roomtemperature and ambient pressure. Hydrogen peroxide was added to the sample batch toincrease the oxygen source during the reaction avoiding the decrease of the catalytic activityduring the reaction due the lack of oxygen [28]. The volume of hydrogen peroxide (at 50%v/v) added to the reaction mixture, the concentration of the dye and the catalystconcentration was 0.1 mL, 10-5 M and 0.6 gL-1 respectively [18,26,28]. Before irradiation, thecatalyst was dispersed in the solution and kept in the dark under stirring for 10 min to reachthe adsorption-desorption equilibrium [27,29,31]. The irradiation started after the adsorption–desorption equilibrium was reached. The irradiation time ranged from 0 to 60 min.

During the photocatalytic process, the system was covered to avoid any other irradiation onthe sample. Only the light from the solar simulator could reach the sample. Every 10 minutesan aliquot of 10 mL was taken out from the solution. The sample was filtered at vacuum toremove the particles of catalyst, and after filtering, dilution (1:5) was prepared to obtain theUV and fluorescence spectra and to determine the TOC concentration.

Fluorescence, UV-visible absorption and TOC were determined for each sample collectedevery 10 minutes during a period of one hour. A decrease in the intensity of the absorptionand fluorescence spectra was observed along the degradation process (Fig. 11). The resultsclearly show how the area of the curves decreases along the reaction time. Additionally, asmooth displacement of the maximum absorption peak could be observed. This behaviorhas been justified as due to the formation of intermediates during the degradation process[20]. Different intermediates have absorption peaks at different wavelengths than the originalorganic compounds and for this reason additional absorption peaks are observed during thedegradation process.

As shown in Fig. 11, the relevance of the hydrogen peroxide and the photocatalyst for thedegradation of the RhB was determined at the beginning of the research. To study theseparameters the photocatalytic degradation process of RhB was done in the absence of thecatalyst and in the absence of the hydrogen peroxide (Fig. 11c). In both cases, anysignificant reduction in the pollutant concentration was observed during the irradiationprocess even after 60 min of irradiation. Otherwise, when the photocatalyst and the H2O2were added to the reaction mixture during the photocatalytic process a significantdegradation was observed, indicating the relevance of the photocatalyst and the H2O2 for theefficiency of the process. As shown in the literature, H2O2 is used as a source of OH·produced during the catalytic breakdown. The hydrogen peroxide can accept thephotogenerated electrons from the conduction band allowing the formation of the OH·radicals that prevent the recombination of the charges and increasing the photocatalyticprocess [32].


190

Hydrogen peroxide increases the degradation process of the pollutants allowing theformation of complexes on the surface of the TiO2. These complexes shift the spectrumtowards the visible region, being this behavior relevant because the TiO2 and the H2O2 donot absorb in these portions of the visible light [32]. The decomposition of the H2O2 for theformation of radicals during the degradation process would be [32]:

H2O2 + e -→OH-+ OH∙

H2O2 + O2∙- →OH-+ OH∙+ O2

Fig. 11. Decrease of the UV-visible absorption (a), fluorescence (b) and TOC (c) duringthe photocatalytic degradation process of RhB using TiO2NWs as catalyst

Many degradation reactions occur simultaneously in the same reaction mixture due todifferent pathways and to the presence of new reaction intermediates that are generatedduring the degradation process. For this reason to define a reaction rate for all the differentprocesses is extremely difficult. Therefore, the degradation process is normally defined as apseudo-kinetic reaction [33]. For the photocatalytic process studied in this research, the besttype of kinetic reaction adapted is a pseudo-first order reaction (Fig. 12). Because the initialconcentration of the pollutant is very small (millimolar), the pseudo-first-order kinetic reactionfollows a Langmuir-Hinshelwood mechanism for degradation [32,34], according to the nextequation:

r = - dC/dt = kKC /(1+KC) Equation 1


190


H2O2 + e -→OH-+ OH∙

H2O2 + O2∙- →OH-+ OH∙+ O2





190


H2O2 + e -→OH-+ OH∙

H2O2 + O2∙- →OH-+ OH∙+ O2





191

where r is defined as the reaction rate, k the reaction rate constant, K is the absorptioncoefficient and C is the concentration of the reactant. If C is small we can say that ln C0/C =kKt = kt.

The pseudo-first order reaction model is defined as [18, 35]:

ln(C0/C) = kt Equation 2

where C0 and C are the initial concentration and the concentration at any time, respectively(Fig. 12). The semilogarithmic plots of the concentrations vs time give straight lines, wherethe slope represents the value of k (reaction rate) (Fig. 12a-e). The values of k and R2 areshown in the graphics. For all catalyst, k values are in the range of 10-2 min - 1.

Anatase

y = -0.0488xR² = 0.9589

a)

0 10 20 30 40 50 60-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Ln (C

/C0)

Irradiation Time (min)0 10 20 30 40 50 60

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Ln (C

/C0)

Irradiation Time (min)

Rutile

y = -0.0547xR² = 0.9429

b)

0 10 20 30 40 50 60

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Ln (C

/C0)


TiO2@MWCNts

y = -0.0276xR² = 0.7102

c)

0 10 20 30 40 50 60

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Ln (C

/C0)


TiO2NWs

y = -0.0578xR² = 0.9939

d)


192

Fig. 12. Graphical determination of the pseudo-first order reaction rate andregression line of RhB in presence of anatase (a), rutile (b), TiO2@MWCNTs (c),

TiO2NWs (d) and ZnO (e)

Synthesized and commercial catalysts used in this investigation could be compared withother systems. In this research, the synthesized catalysts were compared with thecommercial catalysts as the TiO2 in two phases namely rutile and anatase. All catalysts weresubjected to the same photocatalytic process under identical experimental conditions. TheTiO2NWs showed the highest degradation percent (Table 2). The order of efficiency of thephotocatalysts was TiO2NWs>TiO2 (rutile)> TiO2 (anatase)> ZnO > TiO2@MWCNTs (Table2).

According to the data of Table 2, the TiO2NWs has the highest degradation rate, reaching avalue of 96.44%. Investigations demonstrated that pure TiO2NWs reflect near the 95% of thevisible light irradiated to the catalyst; most of the light absorbed is UV-light [37]. The use oftitanium oxide as photocatalyst is a very interesting option because it is nontoxic,photostable and has a high oxidation potential, but its activity is limited to the UV region ofthe spectrum [22,37].

The two more effective catalysts studied in the present research (TiO2NWs and TiO2-rutile)are in the same crystalline phase. A relevant difference between both catalysts is the particlesize; one of them has nanosized dimensions (TiO2NWs), and TiO2-rutile is composed byparticles with micrometric sizes. The nanowires have a surface area greater than anataseand rutile, increasing the contact area between the catalyst and the pollutant and facilitatingthe photocatalytic process [36]. Studies with other catalysts such as In2O3-TiO2 nanofibers,demonstrated the importance of the surface area on the catalytic process [36].

The use of composite materials is a technique used to reduce the hole-electron pairrecombination induced by the photocatalytic processes. Materials such as metals, metaloxides and organic molecules can be used for these purposes [36]. According to theliterature, is expected that the use of carbon nanotubes (i.e. TiO2@MWCNTs) shows asynergistic effect between the materials, allowing the free movement of the electrons fromthe valence band across the surface of the nanotubes decreasing the recombination of thehole-electron pair and increasing the degradation of the compounds [34].

0 10 20 30 40 50 60

-2.0

-1.5

-1.0

-0.5

0.0

Ln (C

/C0)


ZnO

y = -0.0333xR² = 0.9633

e)


193

Table 2. Degradation percent of dye solutions during the Photocatalytic Process

Organic contaminant Anatase Rutile TiO2@MWCNT TiO2NWs ZnORhB 92.72% 94.37% 74.54% 96.44% 84.45%

On the surface of the catalyst the semiconductor is excited by a photon of light and anelectron-hole pair is generated.

The valence band hole has a high oxidative potential producing the oxidation of the dye andgenerating the hydroxyl radical from the water molecule. Consecutive reactions allow theoxidation of the dye and the complete photodegradation:

Oxidation of the dye

OHOHh .VB

dyeOH photodegradation of the dye

The conduction band electron ejected from the surface produces radicals of the oxygenmolecule in the solution . The oxygen radicals react with the hydrogenperoxide producing hydroxyl radical and ions. At the same time regenerate the O2 tocontinue with the reaction [18,38].

Another possible explication to justify the high activity of this catalyst is the presence ofanchoring groups on the surface of the catalyst that facilitate the anchorage of groupsavailable in the dye, increasing the degradation processes [18].

According with some investigations one possible mechanism for the degradation of RhB iscomposed by many deethylations and carboxylations of the dye and the intermediates (Fig13) [30] until the compound is mineralized to CO2 and H2O. Additional studies are required todetermine the degradation mechanism involved with the different catalysts.

In a typical heterogeneous catalytic reaction, at the beginning of the process the decayobserved in the concentration of RhB was part of the adsorption-desorption process prior toreach the equilibrium. Studies with other catalysts as NaBiO3 demonstrated the importanceof the adsorption-desorption processes, in a heterogenous reaction, to finally produce thedegradation of RhB [22]. A decrease in the degradation rate was observed between 30 and60 min (Fig. 14). According with some investigations, this decrease in the degradation ratecould be part of i) the dissociative molecules that are present in the solution limiting the lightabsorbed by the catalyst; ii) the presence of intermediate that compete with the actives sitesof the catalyst [4,39], or iii) possibly as part of the desorption process of the dye from thesurface of the catalyst.

VBCB22 heTiOhνTiO

dyedyehVB

OHHOHh 2VB

22CB OOe

2222 OOHOHOHO


194

Fig. 13. Possible degradation intermediates of RhB during the photocatalyticprocess [28,30]

O N+ CH3

CH3

NCH3

CH3

COOH

O N+ CH3

CH3

NCH3

H

COOH

O N+ CH3

CH3

N

CH3

HOOC

O N+COOH

NHOOC

COOH

H H

O N+

CH3

NHOOC

H

H

.

O N+COOH

NHOOC

COOH

H H

.OH

CarboxylationDeethylation

Carboxylation

DeethylationCarboxylation

Deethylation

Hydroxylation

CO2, H2O, Low Molecular Weight Byproducts

.OH


195

Fig. 14. Decrease of the UV-vis absorption during the degradation of RhB usingTiO2@MWCNTs

The ZnO catalyst has a lower degradation rate when this catalyst is compared with TiO2catalysts (rutile, anatase and TiO2NWs). These results were similar to the results obtainedby other researchers [40]. The lower efficiency of this catalyst (84.45%) could be due to thepossible photodecomposition of the catalyst during the photoreaction. Although ZnO is oneof the most common semiconductors used for the degradation of contaminants, this catalysthas poor stability [41-42]. Some investigations determine a difference between the activationenergy of TiO2 (21+1 kJmol-1) and ZnO (24+1 kJmol-1). These results could be related to thedegradation rate of the dye [37]. In the case of the ZnO, the poor efficiency of thephotocatalytic process of this material is given by the poor quantum yield which allows arapid recombination of the electron-hole pair generated in the same material [34].

The lowest degradation rate was observed using the TiO2@MWCNTs (74.54%) (Fig. 15).Carbon nanotubes were used to reduce the band gap, the electron-hole recombination andto shift the optical response of the photocatalyst [23]. For decreasing the possibility ofrecombination of the electron-hole it is necessary to eliminate the agglomeration of TiO2 overthe carbon nanotubes during the synthesis process [23]. A low degradation rate wasobserved when TiO2@MWCNTs was used in solution. According to the literature, thecomposite materials present some difficulties such as thermal instability, the possibility ofphotocorrosion, distortion of the crystal lattice framework and the possibility of an increase inthe probability of recombination, decreasing the catalytic efficiency [34]. TiO2@MWCNTsshows a reduced efficiency with respect to the other catalysts due possibly to the instabilityof this material during the catalytic reaction.

0 10 20 30 40 50 600.3

0.4

0.5

0.6

0.7

Abso

rban

ce

Time (min)


196

Fig. 15. Graphical representation of the photodegradation of RhB by the differentphotocatalysts studied in this research

Table 3 shows the data of the kinetic reaction rates for RhB and the catalysts used duringthe degradation processes. The mean velocity for the reaction is approximately 10-2 min.The values of R2 ramped from 0.99 to 0.71. The difference among the R2 values can bejustified as part of the adsorption-desorption process of the dye by the catalyst (Fig. 14).Adsorption of the dye by the catalyst can occur during the first minutes of contact, until anadsorption–desorption equilibrium is reached.

Table 3. Kinetic reaction rates and R2 values for the degradation reaction of RhBduring the photocatalytic process

Organic contaminantAnatase Rutile TiO2@MWNT TiO2NWs ZnO

RhB 4.88 x 10-2

R2= 0.95895.47 x 10-2

R2= 0.94292.76 x 10-2

R2= 0.71025.78 x 10-2

R2= 0.99393.33 x 10-2

R2= 0.9633

The differences between the reaction rates could be based on the adsorption capacity of thedifferent catalysts [14], differences in the specific surface area and stability of the crystallinephase during the process.

Some investigations demonstrated the effectiveness of the adsorption as a treatment for theremoval of dyes from wastewaters [24]. Investigations using bioadsorbents as Azadirachtaindica leaf (neem) demonstrated the possibility to used biological waste for the adsorption ofthe contaminants in wastewater [24]. Meanwhile, adsorption studies for decontaminationprocesses using bottom ash and de-oiled soya has the disadvantages of more time foradsorption (than using photocatalytic processes), high operational cost and the regenerationof the column [43-46]. The photocatalytic processes present some advantages over theadsorption processes. Some of the advantages of the photocatalytic processes include thecomplete mineralization of the contaminant, the low energy consumption and no formation ofsludge [47].

Studies to compare photocatalytic processes versus adsorption processes using activatedcarbon and rice husks demonstrated that both processes were efficient but according with

A natase R utile T iO 2@ M W C N T s T iO 2N W s ZnO

74 .54%

84.45%

92.72%

94.37%

96.44%

Degr

adat

ion (%

)

C a ta lys ts


197

the COD results, the decrease in concentration of the contaminant (RhB) is more effectiveduring the photocatalytic process (1608 to 232mgL-1). Meanwhile, the COD values foractivated carbon and rice husks were 267 mgL-1 and 276mgL-1 [48].

Different studies have been published on photocatalytic processes using RhB as pollutantmodel (Table 4). One example of these catalysts is BiVO4. Studies demonstrated that BiVO4reached the 55% of degradation of the contaminant after 80 min of reaction [49]. Bi2O3showed a 80% degradation of the pollutant after irradiation during 80 min [49].

Table 4. Comparison of degradation of RhB between different catalysts

Compound Degradation time Degradation (%) ReferencesBiVO4 80 min 55 [49]Bi2O3 80 min 80 [49]Au-ZnO (nanopyramids) 5 min - [51]ZCT (Cd/Zn=3:1) 120 min 96 [52]CCA/Pd-TiO2 180 min 100 [53]In2O3/TiO2 240 min 86-90 [36]

A similar system based on nanostructures of TiO2 with flower shape [32] was compared withthe TiO2NWs system. Both systems use H2O2 during the process. In this regard, after 80 minof exposure of the sample to the simulated solar irradiation, RhB was totally degraded[32]. Both photocatalysts are in the same crystalline phase (rutile), even though they showrelevant differences in the surface area (ABET). The surface area of the nanostructured TiO2flower shape was 60.2 m2g-1 [32], while the TiO2NWs have a surface area of 480 m2g-1,which is approximately 8 times the surface area of the other catalyst. This demonstrates theimportance of the surface area in the photocatalytic process.

Doped nanostructured materials have also been used by other researchers [50]. An exampleis Au-ZnO, characterized by having nanopyramids shape [51]. In this case, 5 min is enoughtime for the degradation of the RhB [51], but the study do not specified how much is thepercentage of degradation obtained with this nanomaterial. InO/TiO2 is another example ofdoped catalyst and in this case the catalytic reaction reached approximately 86 to 90%degradation after 240 min of irradiation [36].

Composites are other alternative under study for different photocatalytic processes includingthe shifting of water for hydrogen production. A catalyst ZnxCd1-xS/TiO2 (ZCT), with a Cd/Zncomposite ratio 3:1, showed 96% of degradation of RhB after 120min of irradiation [52].

Other interesting materials are those formed by composites on oxide substrates. An exampleis carbon covered alumina supported Pd dopped TiO2 (CCA/Pd-TiO2) with a ratio 1:1 [53]. A100% of degradation could be observed in presence of this catalyst, after 180 min ofirradiation. When these photocatalysts are compared with the catalysts studied in thepresent research (Table 2) it is possible to observe that all photocatalysts (commercial orsynthesized) showed more efficiency during the degradation of RhB. In all cases, thedegradation processes required lower irradiation times with respect to the investigationsfrom the literature. The catalysts studied in the present research showed high degradation ofRhB, ranging from 74.54% to 96.44% after 60 min of irradiation.

Further studies using different organic pollutants and photocatalysts are currently in process.


198

4. CONCLUSION

During the present research, different catalysts (TiO2 nanowires, TiO2@MWCNTs, ZnOnanoparticles) were synthesized and fully characterized by different techniques as FE-SEM,TGA, specific surface area (BET) and XRD. Commercial (rutile and anatase) andsynthesized catalysts were used with the aim to study the photocatalytic removal of RhB inwater.

In all cases, the catalysts used in the present research were able to degrade the pollutant,even though not all demonstrated the same efficiency during the photocatalytic process.Under similar reaction conditions, the most effective catalyst was TiO2NWs (approximately96.44% of degradation) and the less effective was TiO2@MWCNTs (with approximately74.54% of degradation) according to the TOC analysis. Hence, it is deduced that thecatalytic reactions studied in this research can be efficiently used for the degradation anddecolorization of organic pollutants. The catalytic processes can be suitably and costeffectively employed (compared with the adsorption processes) for the removal of pollutantsfrom wastewaters in a short period of time. Degradation processes using other catalystsfrom literature were compared with the catalysts used in the present study. According to ourresults, the TiO2NWS shows the highest efficiency. The degradation result could be relatedto the high surface area of the nanostructured catalyst. When all systems (adsorption andsome common photocatalysts presented in literature as BiVO4, Bi2O3, Au-ZnO(nanopyramids), ZCT (Cd/Zn=3:1), CCA/Pd-TiO2, In2O3/TiO2) were compared with thephotocatalytic systems studied in this research, the photocatalytic process using TiO2NWSwere more effective. In processes based on adsorption mechanisms, the pollutant is onlyadsorbed by the material and this process normally has some disadvantages as the highadsorption time that is required, high operational cost due principally to the regeneration ofthe adsorbent materials, and the appropriated disposition of these materials. Even thoughthere are many publications concerning different photocatalytic systems none of themcompare in efficiency with the TiO2NWS because for RhB a 96.44% of degradation isachieved in a short period of time (60 min), it has a high recyclability and is non-toxic,facilitating the disposition of the material. According with the results presented in thisresearch we can predict that, with high probability, these catalytic processes could beimplemented as appropriate chemical procedures for pollutant removal from water.

CONSENT

Not applicable.

ETHICAL APPROVAL

Not applicable.

ACKNOWLEDGEMENTS

The authors gratefully recognize the financial support provided by MEC through the grantMAT2010-19804. Financial support from US Department of Energy through the MasseyChair project at University of Turabo and from the National Science Foundation under theproject CHE-0959334 is also acknowledged. The “Servicio Interdepartamental deInvestigación (SIdI)” from Universidad Autónoma de Madrid and “Centro de Microscopia Luis


199

Bru” from Universidad Complutense de Madrid are acknowledged for the use of the FESEMfacilities.

COMPETING INTERESTS

Authors declare that no competing interests exist.

REFERENCES

1. Garriga I Cabo, C. Estrategias de optimización de procesos de descontaminación deefluentes acuosos y gaseosos mediante fotocatalisis heterogénea, Dissertation,Universidad de Gran Canarias; 2007. Spanish.

2. UNEP, UNICEF and WHO: United Nations Environment Programme, United NationsChildrens Fund and World Health Organization (2002a). Children in the NewMillenium: Environmental Threats to Children [Internet]. Accessed April 2012.Available: http://www.unep.org/ceh/main01.html.

3. Zein, SHS, Boccaccini AR. Synthesis and Characterization of TiO2 Coated MultiwalledCabon Nanotubes Using a Sol Gel Method. Material and Interfaces. Ind Eng ChemRes. 2008;47(17):6598-6606.

4. Vinu R, Madras G. Kinetic of Sonophotocatalytic Degradation of Anionic Dyes withNano-TiO2. Environ Sci Technol. 2009;43(2):473-479.

5. Vanhulle S, Trosvaslet M, Enaud E, Lucas M, Taghavi S, van der Lelie D, van Aken B,Foret M, Onderwater RCA, Wesenberg D, Agathos SN, Schneider YJ, Corbisier AM.Decolorization, Cytotoxicity, and Genotoxicity Reduction During a CombinedOzonation/Fungal Treatment of Dye-Contaminated Wastewater. Environ Sci Technol.2008;42(2):584-589.

6. Torres Martinez LM, Juarez Ramirez I, Garcia Montelongo XL, Cruz Lopez A.Desarrollo de semiconductores con estructuras tipo perovskitas para purificar el aguamediante oxidaciones avanzadas. [Internet] Ciencia UANL. 2010;23(4):376-388.Available : http://redalyc.uaemex.mx/redalyc/pdf/402/40215505008.pdf.

7. Karadag D, Tok S, Akgul E, Ulucan K, Evden H, Kaya MA. Combining Adsorption andCoagulation for the Treatment of Azo and Anthraquinone Dyes from AqueousSolution. Ind Eng Chem Res. 2006;45(11):3969-3973.

8. Dafnopatidou, EK, Gallios GP, Tsatsaroni, EG, Lazaridos NK. Reactive DyestuffsRemoval from Aqueous Solution by Flotation, Possibility of Water Reuses, andDyestuff Degradation. Ind Eng Chem Res. 2007;46(7):2125-2132.

9. Mahanta D, Madras G, Radhakrishnan S, Patil S. Adsorption of Sulfonated Dyes byPolyaniline Emeraldine Salt and Its Kinetic. J Phys Chem B. 2008;112(33):10153-10157.

10. Barrera-Díaz C, Linares-Hernández I, Roa-Morales G, Bilyeu B, Balderas-HernándezP. Removal of Biorefractory Compounds in Industrial Wastewater by Chemical andElectrochemical Pretreatments. Ind Eng Chem Res. 2009;48(3):1253-1258.

11. Panizza M, Barbucci A, Ricotti R, Cerisola G. Electrochemical degradation ofmethylene blue. Separation and Purification Technology. 2006;54(3):382–387.

12. Ma H, Zhuo Q, Wang B. Characteristic of CuO-MoO3-P2O5 Catalyst and Its CatalyticWet Oxidation (CWO) of Dye Wastewater under Extremely Mild Conditions. EnvironSci Technol. 2007;41(21):7491-7496.

13. Gupta VK, Suhas Ali I, Saini VK. Removal of Rhodamine B, Fast Green, andMethylene Blue from Wastewater Using Red Mud, an Aluminum Industry Waste. IndEng Chem Res. 2004;43(7):1740-1747.


200

14. Hernández Enrique JM, Garcia Serrano LA, Zeifert Soares BH. Síntesis yCaracterización de Nanopartículas de N-TiO2 – Anatasa. Superficies y Vacío.2008;21(4):1-5. Spanish.

15. Marín JM, Montoya J, Monsalve E, Granda CF, Ríos LA, Restrepo G. Degradación denaranja de metilo de un nuevo fotorreactor solar de placa plana con superficiecorrugada. Scientia Et Technica. 2007;13(34):435-440. Spanish.

16. Ozen AS, Aviyente V, Tezcanli-Guyer G, Ince NH. Experimental and ModelingApproch to Decolorization of Azo Dyes by Ultrasound: Degradation of the HydrazoneTautomer. J Phys Chem A. 2005;109(15):3506-3516.

17. Yin M, Li Z, Kou J, Zou Z. Mechanism Investigation of Visible Light-InducedDegradation in a Heterogeneous TiO2/Eosin Y/Rhodamine B System. Environ SciTechnol. 2009;43(21):8361-8366.

18. Asiri AM, Al-Amoudi MS, Al-Talhi TA, Al-Talhi AD. Photodegradation of Rhodamine6G and phenol red by nanosized TiO2 under solar irradiation. J Saudi Chem Soc.2011;15:121-128.

19. Pattapu S, Saha S, Jana S, Pal A. Novel Resin Bound MnO2 Nanocomposite for theDegradation of Crystal Violet Dye in Aqueous Medium. World J. Eng. InternationalConference on Composite/Nano Engineering (ICCE-16). Kumming (CHN):Sun LightPublishing. Accessed April 2012; 2008.Available: http://wjoe.hebeu.edu.cn/mulu.sup.2.2010.html.

20. Cao SW, Zhu YJ, Cheng GF, Huang YH. Preparation and photocatalytic property of α-Fe2O3 Hollow core/shell hierarchical nanostructures. J Phys Chem Solids.2010;71:1680-1683.

21. Aarthi T, Madras G. Photocatalytic Degradation of Rhodamine Dyes with Nano-TiO2.Ind Eng Chem Res. 2007;46(1):7-14.

22. Yu K, Yang S, He H, Sun C, Gu C, Ju Y. Visible Light-Driven PhotocatalyticDegradation of Rhodamine B over NaBiO3: Pathways and Mechanisms. J Phys ChemA. 2009;113(37):10024-10032.

23. Mahlambi MM, Mishra AK, Mishra SB, Krause RI, Mamba BB, Raichur AM. The effectof metal-ions (Ag, Co, Ni and Pd) on the visible light degradation of Rhodamine B bycarboncovered alumina supported TiO2 in aqueous solutions. Ind Eng Chem Res.DOI: 10.1021/ie3025505 • Publication Date (Web): 09 Jan 2013.

24. Sarma J, Sarma A, Bhattacharyya, KG. Biosorption of Commercial Dyes onAzadirachta indica Leaf Powder: A Case Study with a Basic Dye Rhodamine B. IndEng Chem Res. 2008;47:5433–5440.

25. Lu Q, Gao W, Du J, Zhou Li, Lian Y. Discovery of Environmental Rhodamine BContamination in Paprika during the Vegetation Process. J Agric Food Chem,2012;60:4773−4778.

26. Velegraki T, Mantzavinos D. Conversion of benzoic acid during TiO2-mediatedphotocatalytic degradation in water. Chem Eng J. 2008;140:15-21.

27. Hong RY, Li JH, Chen LL, Liu DQ, Li HZ, Zheng Y, Ding J. Synthesis, surfacemodification and photocatalytic property of ZnO nanoparticles. Powder Technol.2009;189:426-432.

28. Cotto-Maldonado MC. Heterogeneous Catalysis Applied To Advanced OxidationProcesses (AOPs) For Degradation of Organic Pollutants, Dissertation, Universidaddel Turabo; 2012.

29. Zhou J, Zhang Y, Zhao XS, Ray AK. Photodegradation of Benzoic Acid over Metal-Doped TiO2. Ind Eng Chem Res. 2006;45(10):3503-3511.

30. Sun M, Li D, Chen Y, Chen W, Li W, He Y, Fu X. Synthesis and Photocatalytic Activityof Calcium Antimony Oxide Hydroxide for the Degradation of Dyes in Water. J PhysChem C. 2009;113(31):13825-13831.


201

31. Xiao FX. Construction of Highly Orderes ZnO-TiO2 Nanotubes Arrays (ZnO/TNTs)Herterostructure for Photocatalytic Application. ACS Appl Mat Interfaces. 2012;4:7055-7063.

32. Ge M, Li JW, Liu L, Zhou Z. Template-Free Synthesis and Photocatalytic Applicationof Rutile TiO2 Hierarchical Nanostructures. Ind Eng Chem Res. 2011;50:6681-6687.

33. Pey Clemente J. Aplicaciones de Procesos de Oxidación Avanzada (FotocatalisisSolar) para Tratamiento y Reutilizacion de Efluentes Textiles, Dissertation,Universidad Politecnica de Valencia; 2008. Spanish.

34. Mu J, Shao C, Guo Z, Zhang Z, Zhang M, Zhang P, Chen B, Liu Y. High photocatalyticActivity of ZnO-Carbon Nanofiber Heteroarchitectures. ACS Appl Mat Interfaces.2011;3:590-596.

35. Wang XH, Li JG, Kamiyama H, Moriyoshi Y, Ishigaki T. Wavelength-SensitivePhotocatalytic Degradation of Methyl Orange in Aqueous Suspension over Iron (III)-doped TiO2 Nanopowders under UV and Visible Light Irradiation. J Phys Chem B,2006;110(13):6804-6809.

36. Mu J, Chen, Zhang M, Guo Z, Zhang P, Zhang Z, Sun Y, Shao C, Liu Y.Enhancemente of the Visible-Light Photocatalytic Activity of In2O3-TiO2 NanofiberHeteroarchitectures. ACS Appl Mat Interfaces, 2012;4:424-430.

37. Wang C, Yin L, Zhang L, Liu N, Lun N, Qi Y. Platinum-Nanoparticle-Modified TiO2Nanowires with Enhanced Photocatalytic Property. Appl Mat Inter, 2010;2(11):3373-3377.

38. SrideviD, Rajendran KV. Synthesis and optical characteristics of ZnO nanocrystals.Bull Mater Sci, 2009;32(2):165-168.

39. Yin M, Li Z, Kou J, Zou Z. Mechanism Investigation of Visible Light-InducedDegradation in a Heterogeneous TiO2/EosinY/Rhodamine B System Environ SciTechnol. 2009;43:8361–8366.

40. Attia, AJ, Kadhim SH, Hussein FH. Photocatalytic Degradation of Textile DyeingWastewater Using Titanium Dioxide and Zinc Oxide. E-Journal of Chemistry,2008;5(2):219-223.

41. Hernández-Alonso MD, Fresno F, Suarez S, Coronado JM. Development ofalternative photocatalysts to TiO2: Challenges and opportunities. Energy Environ Sci.2009;2:1231-1257.

42. Lo SC, Lin CF, Wu CH, Hsieh PH. Capability of coupled CdSe/TiO2 for photocatalyticdegradation of 4-chlorophenol. J Haz Mat. 2004;B114:183-190.

43. Mittal A, Mittal J, Malviya A, Gupta VK. Removal and recovery of Chrysoidine Y fromaqueous solutions by waste materials. J Colloid and Interface Sci. 2010;344:497–507.

44. Mittal A, Mittal J, Kurup L, Singh AK. Process development for the removal andrecovery of hazardous dye erythrosine from wastewater by waste materials—BottomAsh and De-Oiled Soya as adsorbents. J Hazard Mater B. 2006;138:95–105.

45. Mittal A, Kaur D, Mittal J. Batch and bulk removal of a triarylmethane dye, Fast GreenFCF,from wastewater by adsorption over waste materials. J Hazard Mater.2009;163:568–577.

46. Mittal A, Gajbe V, Mittal J. Removal and recovery of hazardous triphenylmethane dye,Methyl Violet through adsorption over granulated waste materials. J Hazard Mater.2008;150:364–375.

47. Gupta VK, Jain R, Mittal A, Saleh TA, Nayak A, Agarwal S, Sikarwar S. Photo-catalyticdegradation of toxic dye amaranth on TiO2/UV in aqueous suspensions. Mater SciEng C. 2012;32:12–17.

48. Jain R, Mathur M, Sikarwar S, Mittal A. Removal of the hazardous dye rhodamine Bthrough photocatalytic and adsorption treatments. J Environ Manag, 2007;85:956–964.


202

49. Saison T, CheminN, Chanéac C, Durupthy O, Ruaux V, Mariey L, Maugé F, BeaunierP, Jolivet JP. Bi2O3, BiVO4, and Bi2WO6: Impact of Surface Properties onPhotocatalytic Activity under Visible Light. J Phys Chem C. 2011;115:5657-5666.

50. Udawatte N, Lee M, Kim J, Lee D. Well-Defined Au/ZnO Nanoparticles CompositeExhibiting Enhanced Photocatalytic Activities. ACS Appl Mater Interfaces,2011;3:4531-4538.

51. Li P, Wei Z, Wu T, Peng Q, Li Y. Au-ZnO Hybrid Nanoparticles and TheirPhotocatalytic Properties. J Am Chem Soc, 2011;133:5660-5663.

52. Li W, Li D, Meng S, Chen W, Fu X, Shao Y. Novel Approach To EnhancePhotosensitized Degradation of Rhodamine B under Visible Irradiation by the ZnxCd1-xS/TiO2 Nanocomposite.Environ SciTechnol. 2011;45:2987-2993.

53. Mahlambi MM, Mishra AK, Mishra SB, Krause RW, Mamba BB, Raichur AM. Effect ofMetal Ions (Ag, Co, Ni, and Pd) on the Visible Light Degradation of Rhodamine B byCarbon-Covered Alumina-Supported TiO2 in Aqueous Solution. Ind eng Chem Res.2013;52:1783-1794.

_________________________________________________________________________© 2013 Cotto-Maldonado et al.; This is an Open Access article distributed under the terms of the CreativeCommons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited.

Peer-review history:The peer review history for this paper can be accessed here:

http://www.sciencedomain.org/review-history.php?iid=230&id=16&aid=1329

Documents

Photocatalytic Degradation of Rhodamine-B Under UV-Visible … · 2016-07-07 · Photocatalytic Degradation of Rhodamine-B Under UV-Visible Light Irradiation Using Different Nanostructured