Photocatalytic degradation of dyes by CdS microspheres under near UV and blue LED radiation

Separation and Purification Technology 120 (2013) 206–214

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Photocatalytic degradation of dyes by CdS microspheres under near UV and blue LED radiation Eveliina Repo a,⇑, Selvaraj Rengaraj b,⇑, Susanna Pulkka a, Emmanuelle Castangnoli a, Sami Suihkonen c, Markku Sopanen c, Mika Sillanpää a a b c

Laboratory of Green Chemistry, LUT Savo Sustainable Technologies, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland Department of Chemistry, College of Science, Sultan Qaboos University, Muscat, Oman Department of Micro and Nano Sciences, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 11 December 2012 Received in revised form 3 October 2013 Accepted 4 October 2013 Available online 15 October 2013

Nanostructured CdS microspheres were prepared by hydrothermal synthesis and used as photocatalyst in the degradation of dyes under near UV and blue LED radiation. Nearly complete degradation of methylene blue, phenol red and methyl red was achieved in 3 h. Catalytic performance of the microspheres remained unchanged during five recycling steps. Different modification methods were tested to prevent photocorrosion of the CdS particles. Using glucose as protecting agent gave the most promising results. Furthermore, photocatalysis under LED radiation tested in this study showed quite good energy efficiency, which makes this method economically appealing. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Cadmium sulfide LED Photocatalysis Dye Water treatment

1. Introduction Wastewaters generated from textile industries contain considerable amounts of dyes that can cause harm to the ecosystem when released into the environment [1,2]. Especially, some of the dyes have very stable molecular structures and therefore are not easily removed from the wastewaters by conventional purification methods [3]. Heterogeneous photocatalysis is one of the most promising techniques in degradation of these organic pollutants from wastewaters [2,4,5]. UV/TiO2-based photocatalysis is commonly used in different water treatment applications [1,4,5]. However, especially the utilization of UV-lamps, which are typically based on continuous mercury vapor, has shown to be disadvantageous. They are instable due to overheating, have low mechanical stability, low photonic efficiency and relatively short life-time, need high voltages to operate, and contain toxic mercury [6]. Therefore, alternative lightsources should be considered and in recent years, especially, visible light sources have gained a lot of attention. Even if the solar radiation is the most attractive in visible light photocatalysis, in many practical applications it cannot be directly used to illuminate the solution to be treated. Due to the increasing ⇑ Corresponding authors. Tel.: +358 40 355 3707 (E. Repo). E-mail addresses: (S. Rengaraj).

Eveliina.Repo@lut.fi

(E.

Repo),

[email protected]

1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.10.008

demands to reduce energy consumption, LED-lights (light emitting diodes) could provide a viable solution. These semiconductor p–n junction devices are more and more used in different lightning applications due their long-term stability and high energy efficiency. More recently, LED-technology has also increased its popularity as a potential light source in water treatment applications [6–15]. Another disadvantage involved in conventional UV/TiO2-based photocatalytic water treatment is that TiO2 catalyst is inefficient under visible light irradiation. This has promoted the researchers to either modify TiO2 to reduce its band gap or use other semiconductor based catalysts with lower band gap energy [16]. One of the most potential semiconductor visible light active catalysts is CdS. Different kinds of CdS particles have been synthesized and tested for photocatalytic degradation of organic pollutants [17–21] or on the other hand for the evolution of hydrogen gas by water splitting [22]. A considerable drawback of CdS based catalysts is, however, a photocorrosion induced by the radiation causing leaching of Cd into the solution. Therefore, many studies have also focused on the prevention of photocorrosion of CdS materials [23–27]. In recent study, we demonstrated the synthesis and physicochemical properties of the cauliflower-like CdS microspheres composed of nanocrystals [28]. In this study we investigate the photocatalytic properties of these novel materials in degradation of dyes such as methylene blue, phenol red, and methyl red under near UV and blue LED radiation. Visible LED/CdS photocatalytic

E. Repo et al. / Separation and Purification Technology 120 (2013) 206–214

system has not been studied previously. The effects of wavelength of LED radiation and the synthetic conditions of CdS microspheres are investigated. The inhibition of the photocorrosion is tested using different modification techniques. On the whole, the combination of visible light active catalyst and LED-technology is hypothesized to decrease the operational costs and increase the energy efficiency of the water treatment processes. 2. Materials and methods 2.1. Synthesis of CdS microspheres CdS microspheres were synthesized according to our previous paper [28] by hydrothermal treatment of the mixture of 2 g of cadmium nitrate (Cd(NO3)24H2O, Sigma–Aldrich), 0.8 g of thioacetamide (Sigma–Aldrich), and 1.2 g of poly(ethylene glycol) (PEG-1000, -2000, and -4000, Merck, number (1000, 2000, 4000) refers the average molecular weight of PEG, higher number means longer chain length). All the reagents were separately dissolved in ultra pure water and added sequentially in the reaction mixture resulting the final total volume of 100 mL. Synthetic products were washed with water and ethanol and dried at 60 °C for at least 12 h. The crystalline properties of the CdS microspheres were studied by X-ray powder diffraction using a Bruker (D5005) X-ray diffractometer. Raman spectroscopy measurements (Bruker SENTERA 200 LX model) were performed to confirm the structure of the CdS microspheres. The morphology of the microspheres was characterized by SEM Hitachi S-4800 high-resolution field emission scanning electron microscope (HR-SEM) equipped with an energy-dispersive X-ray spectrometer (EDX). The morphology of the microspheres was also studied with a transmission electron microscope (JEOL JEM-3010). The chemical states and the relative composition of the samples were studied by X-ray photoelectron spectroscopy (XPS). The absorption spectra of the samples in diffuse reflectance spectrum (DRS) mode were recorded in the wavelength range of 2001000 nm using a spectrophotometer (Jasco V670) with BaSO4 as a reference. The band gap values were calculated by extrapolation from the absorption edge. 2.2. Degradation studies Photocatalytic degradation of three different dyes: methylene blue (MB), phenol red (PR), and methyl red (MR) (Fig. 1), was conducted using two different LEDs with wavelengths of 405 and

207

450 nm. The power consumptions of the lamps were 24 W and 17.5 W, respectively. Radiation efficiencies applied upon catalyst/ dye water mixture were 1.2 W and 3.2 W evaluated from the densities of the radiation efficiencies, which were 39 and 73 mW/cm2 for the distance of 5 cm between the lamp and the solution. The full width at half maximum of the LED spectrum was 34 nm for both LEDs. Experimental set-up is presented in Fig. 2. 200– 600 mg of CdS-catalyst and 200 mL of dye solution (3–10 mg/L) resulting doses of 1, 2, and 3 g/L of catalyst were placed in a borosilicate glass beaker and mixed with a magnetic stirrer for 30 min. After taking a sample, LED-light was switched on and stirring was continued. Samples were taken from the solution at first every 15 min and then after the first hour between every 30 min. To test the recycling of the catalysts, used catalysts were collected after experiment, washed with ultra pure water, and dried at 60 °C.

2.3. Chemical analysis Concentrations of the dyes were measured by UV–vis spectrophotometer. The amount of leached Cd(II) into solution after experiments was measured with an inductively coupled plasma optical atomic emission spectrometry (ICP-OES) model iCAP 6300 (Thermo Electron Corporation, USA). The total organic carbon (TOC) was measured by a TOC analyzer (Shimadzu 5000A) equipped with an autosampler (ASI 5000). Degradation products of MB were analyzed by a gas chromatography–mass spectrometry (GC–MS, Agilent 6890N, 5975 inert Mass Selective Detector) using DB-5 capillary column (5% diphenyl/95% dimethyl-siloxane), 60 m, 0.25 mm i.d. and 1.0 lm thick film. Splitless injector was used with 1 lL injection volume. Injector temperature was 280 °C, split flow 10 mL/min, and the helium carrier gas flow 1 mL/min. Oven temperature program was 4 min at 40 °C, 4 °C/min to 80 °C (2 min), and 8 °C/min to 280 °C (9 min). Electron impact (EI) mass spectra were monitored from 10 to 300 m/z. The ion source and inlet line temperatures were 220 and 280 °C, respectively. Solid-phase extraction (SPE) was used to preconcentrate samples prior to the analysis. SPE columns had a volume of 3 mL and amount of C18 phase 500 mg (Chromabaond C18, octadecyl-modified silica). Column was conditioned by 5 mL of methanol and 5 mL of de-ionized water. Then 200 mL of the sample was loaded in the column at a flow rate of around 10 mL/min. 8 mL of methanol was used in elution and gentle nitrogen stream to evaporate the sample to final volume of 1 mL.

Fig. 1. Structures of the studied dyes.

208

E. Repo et al. / Separation and Purification Technology 120 (2013) 206–214

composed of the agglomeration of nanocrystals. Size of the microparticles varied from 3 to 10 lm and size of the nanocrystals from 24 to 40 nm. 3.2. Adsorption of dyes by CdS microspheres Fig. 3 shows the adsorption behavior of different dyes on the surface of CdS microspheres. Adsorption of MB on the CdS surface was more efficient than adsorption of MR and PR. This can be attributed to the cationic nature of MB and negatively charged CdS surface at the pH of the experiments (around 6.3). The point of zero charge of CdS particles is around 3.5 [30] above which the surface charge is negative and therefore attractive for cationic species. The PEG chain length did not affect significantly on the adsorption properties of CdS microspheres. This can be attributed to the similar characteristics of the catalysts despite the type of PEG used in the synthesis (Section 3.1). 3.3. Photocatalytic degradation of dyes by CdS microspheres

Fig. 2. Experimental set-up.

Extraction was conducted under vacuum in a chamber obtained from J.T. Baker (BAKER spe-12G). 2.4. Modification of CdS microspheres Several modifications of CdS microspheres were tested in this study. (1) 1 g of CdS2000 microspheres were ultrasonically mixed in 10 mL of 0.2 wt% aqueous solution of poly(diallyl dimethylammonium) chloride (PDDA) and keeping at ambient temperature for 2 h and washing with water [23]. After filtration the sample was dried at 60 °C. (2) 1 g of CdS2000 microspheres were mixed with polyaniline PANI (tetrahydrofurane, THF) solution (10 mL, 0.45 g/L) in ultrasonic bath for 30 min following stirring for 24 h. The mixture was filtrated and washed with deionized water, and dried at 60 °C for at least 12 h [24]. (3) 150 mg of CdS2000 in 100 mL of 0.05 M glucose solution was refluxed for 5 h and 20 h [29]. Products were washed with water and dried at 60 °C for at least 12 h. (4) 0.9 g of glucose was directly added with PEG-2000 (0.4 g) in the reaction mixture of 1 g of CdNO3 and 0.4 g of thioacetamide prior to hydrothermal treatment as in the case of preparation of unmodified CdS particles (combination from Refs. [28,29]).

The effect of LED radiation on the degradation of MB by CdS microspheres is shown in Fig. 4a. It is seen that MB degradation was almost 100% after 3 h. Moreover, there were not significant differences between the results obtained with 405 and 450 nm LEDs. This indicates that the energy of the both lamps was sufficient to excite the electrons of the catalyst. Fig. 4a also shows the degradation efficiency obtained using CdS1000 calcinated at 200 °C for 2 h. The calcinated CdS did not adsorb MB molecules, but after LED radiation performed as efficient MB degradation as non-calcinated samples. Overall, the calcination did not improve the catalytic performance of the microspheres and therefore noncalcinated samples were used in all further experiments. Fig. 4b shows the effect of PEG chain length on the degradation of MB. The band gaps of CdS microspheres analyzed previously [28] did not show significant dependency on the PEG chain length and therefore the photocatalytic activities of these microspheres were also rather similar. The photocatalytic degradation of different dyes is compared in Fig. 4c and Table 1. Complete decolorization was obtained for MB, which can be attributed to its efficient adsorption on the catalyst’s surface. Fig. 4c also shows that LED radiation without the catalyst was not able to degrade the target compounds. Thus, it can be concluded that both catalyst and LED radiation were necessary in order to degrade the target compounds effectively. Increasing intensity seen for MR (Fig. 4c) may be due to the presence of some undissolved MR, which was released in the solution phase during mixing (solubility of MR was not as good as for MB and PR).

1.0 0.8

3.1. Characterization of CdS microspheres Throughout characterization of the CdS microspheres is presented in our earlier work [28]. Shortly, crystal structure of CdS microspheres was identified mainly as cubic and hexagonal phases independently on the PEG chain length with crystalline size ranging from 35 to 26.9 nm. EDX-analysis revealed the ratio of Cd:S to be 1:1 indicating stoichiometric composition for studied materials. High purity of CdS microspheres was also confirmed by XPS spectra. The band gap values determined from UV–vis DRS spectra showed a slight increase from 1.99 to 2.06 eV with increasing PEG chain length. SEM and TEM images showed a cauliflower-like structure of microspheres

C / Ci

3. Results and discussion

0.6 0.4

CdS1000 MB CdS2000 MB CdS4000 MB

0.2

CdS2000 MR CdS2000 PR

0.0

0

40

80

120

160

200

240

time (min) Fig. 3. Adsorption of different dyes on the CdS microspheres. Dye concentration: 3 mg/L, dose of the catalyst: 1 g/L.

209

E. Repo et al. / Separation and Purification Technology 120 (2013) 206–214

(a)

1.2 1.0

1.0

C / Ci

0.6 0.4

0.8

C / Ci

CdS1000 405 nm CdS1000 450 nm CdS1000 calcinated 450 nm CdS1000 450 nm TOC

0.8

0.4 0.2

0.0

0.0 0

40

80

120

160

200

240

0

time (min)

1.0

(b)

0.0

0.0 120

160

200

240

time (min)

200

240

Dose: 1 g/L Dose: 2 g/L Dose: 3 g/L

(d)

0.4 0.2

80

160

0.6

0.2

40

120

time (min)

1.0

C / Ci

0.4

0

80

0.8

0.6

lamp on

40

lamp on

CdS1000 405 nm CdS2000 405 nm CdS4000 405 nm

0.8

MB MR PR MB without catalyst MR without catalyst PR without catalyst

0.6

0.2

lamp on

C / Ci

(c)

1.2

0

40

80

lamp on

120

160

200

240

time (min)

Fig. 4. (a) Effect of LED wavelength on the degradation of MB by CdS1000 microspheres. (b) Effect of PEG chain length on the degradation of MB under LED radiation. (c) Degradation of different dyes by CdS2000 microspheres under 450 nm LED radiation. Dye concentration: 3 mg/L (a–c) and 5 mg/L (d), dose of the catalyst: 1 g/L.

Table 1 also shows that degradation of MB was clearly suppressed when the concentration of MB was higher (5–10 mg/L). Most likely, due to the intensive color of MB, LED radiation did not effectively reached the catalyst’s surface. However, increasing the dose of the CdS2000 in 5 mg/L MB solution resulted better degradation efficiency (Fig. 4d) presumably due to the higher amount of adsorption sites as well as better contact with LED radiation under intensive mixing. 3.4. Modeling of degradation kinetics Photocatalytic degradation of dyes by CdS microspheres was modeled using the first (Eq. (1)) and second order (Eq. (2)) kinetic models:

C ¼ C i ek1 t

ð1Þ

where C (mg/L) is the concentration of dye at time t, Ci (mg/L) is the initial concentration of dye, and k1 (1/min) the first-order rate constant.

1 1 ¼  k2 t C Ci

ð2Þ

where k2 (L/mg min) the second-order rate constant. Non-linear regression was applied using Excel Solver option on the experimental points from the lamp ignition (after adsorption) to 90 min reaction time. These data points were selected to exclude the effect of

pure adsorption and because in part of the experiments reaction was terminated after 90 min. Results are shown in Fig. 5 and Table 1. In the case of degradation of MB the second-order model was better fitted than the first-order model and for the other dyes the first-order model fitted generally better. The applicability of the second-order model can be attributed to the dimerization of MB [31]. Furthermore, in most of the experiments kinetics of the degradation of dyes was faster with 450 nm lamp indicating a good catalytic performance of CdS microspheres at the visible light region. Comparison of the rate constants for different MB concentrations and different doses of the catalyst are also shown in Table 1. Higher concentration of MB resulted slower degradation rate due to the inefficient contact between the LED radiation and catalyst’s surface. Therefore a formation of hydroxyl radicals was also inhibited. Increasing the dose of the catalyst, however, did not affect significantly the degradation rates. Fig. 4d shows that adsorption efficiency was also rather similar for different amounts of catalyst, which could resulted similar degradation behavior within the first 90 min.

3.5. Reuse of spent catalysts One of the key factors in selecting a proper photocatalyst is its recyclability. Therefore, in this study, the spent catalysts were

210

E. Repo et al. / Separation and Purification Technology 120 (2013) 206–214

Table 1 Degradation efficiency of different dyes by CdS microspheres and kinetic parameters obtained by non-linear regression. Catalyst

Dye

Light source (nm)

Dye concentration 3 mg/L, dose of the catalyst: 1 g/L CdS1000 MB None 405 450 CdS2000 None 405 450 CdS4000 None 405 450 CdS1000

Degradation efficiency within 3 h (%)

k1 (1/min)

R

k2 (L/mg min)

R

49.5 99.4 100 49.2 100 100 55.0 100 100

– 0.0287 0.0372 – 0.0314 0.0212 – 0.0319 0.0134

– 0.9608 0.9804 – 0.9756 0.9514 – 0.9773 0.9829

– 0.0354 0.0407 – 0.0274 0.0150 – 0.0418 0.0155

– 0.9944 0.9996 – 0.9990 0.9882 – 0.9989 0.9914

MR

405 450 None 405 405 450

92.0 91.7 23.4 88.8 90.3 89.8

0.0221 0.0250 – 0.0177 0.0188 0.0294

0.9559 0.9982 – 0.9992 0.9952 0.9978

0.0112 0.0149 – 0.0107 0.0108 0.0198

0.9832 0.9878 – 0.9884 0.9911 0.9767

PR

405 450 None 405 450 405 450

89.9 89.5 28.2 85.1 98.0 90.0 93.8

0.0177 0.0321 – 0.0169 0.0371 0.0174 0.0221

0.9985 0.9968 – 0.9975 0.9972 0.9908 0.9996

0.0086 0.0565 – 0.0075 0.0260 0.0071 0.0116

0.9934 0.9893 – 0.9904 0.9717 0.9950 0.9875

Dye concentration 5 mg/L, dose of the catalyst: 1 g/L CdS2000 MB 450

84.8

0.0195

0.8886

0.0070

0.9542

Dye concentration 10 mg/L, dose of the catalyst: 1 g/L CdS2000 MB 450

69.7

0.0058

0.9938

0.0009

0.9904

Dye concentration 5 mg/L, dose of the catalyst: 2 g/L CdS2000 MB 450

90.5

0.0172

0.9112

0.0070

0.9583

Dye concentration 5 mg/L, dose of the catalyst: 3 g/L CdS2000 MB 450

99.8

0.0179

0.9857

0.0084

0.9986

CdS2000 CdS4000 CdS1000 CdS2000

CdS4000

3.6. Photocorrosion of CdS microspheres

3.0

MB experiment MR experiment First-order model Second-order model

2.5

C / Ci

2.0 1.5 1.0 0.5 0.0 0 lamp on

20

40

60

80

100

time (min)

Fig. 5. The first and second-order kinetic model fittings on the experimental data.

collected and their catalytic activities tested again in similar experimental conditions. It was noticed that the catalyst did not lose its efficiency during five first cycles (Fig. 6a). Furthermore, the adsorption properties of microspheres remained similar during the recycling (Fig. 6a). SEM-images presented in Fig. 6b and c show that sequential use of the same catalyst did not effect on the shape of the microspheres even though the surface nanostructure showed some decrease of crystalline size (Fig. 6d and e). This is probably due to photocorrosion, which is discussed in the next section.

Even though the results above seem promising, notable leaching of Cd was observed during the experiments (Table 2). This is due to the photocorrosion of CdS, which is the main problem of CdS-based photocatalysis. To prevent photocorrosion different modifications of CdS-microspheres were tested. Table 2 shows that either PDDA or PANI treatment did not protect the CdS microspheres from photocorrosion even though the previous studies showed that both of these treatments resulted a formation of protective layer around the catalytic particles and leaching of Cd was prevented [23,24]. In the earlier study (PDDA case), CdS nanoparticles were pre-incorporated in hexagonal mesoporous silica sphere and PDDA formed a layer around this sphere [23] and most likely interacted with silica and not with CdS. This can explain the unsuccessful modification in the present study. PANI was earlier used for the coating of CdS nanoparticles and it was reported that a monolayer of PANI was formed through chemical bonds around the particles [24]. However, when coating of microparticles was tested in the present study, it was possible that polyaniline aggregated rather than reacted with the surface. Presence of free nanoparticles has been reported to prevent aggregation [32], but in the present study nanostructures were on the surfaces of microspheres. Moreover, treatment with PANI-solution did not improve the photocatalytic activity of CdS microspheres as observed earlier [24], which indicates that PANI did not form uniform layers on the CdS microspheres. As seen from Table 2, glucose seems to be the most promising protecting agent for the CdS microspheres. However, coating of prepared CdS particles with glucose (CdS–Glu1 and Glu2) did not

211

E. Repo et al. / Separation and Purification Technology 120 (2013) 206–214

1. use 450 nm (a) 2.use 450 nm 3. use 450 nm 4. use 450 nm 5. use 450 nm 3. use without lamp

1.0

C / Ci

0.8 0.6 0.4 0.2 0.0

0 lamp on

40

80

120

160

200

240

time (min)

(b)

(c)

(d)

(e)

Fig. 6. (a) Recycling of CdS4000 catalyst. Dye concentration: 3 mg/L, dose of the catalyst: 1 g/L. SEM images of (b and d) unused CdS2000 sample and (c and e) CdS2000 after the fourth recycling step.

Table 2 Effect of modification of CdS microspheres on their photocorrosion and photocatalytic performance.

a

Catalyst

Modification

k2 (L/mg min)

Degradation within 3 h (%)

Cd released within 3 h (%)

CdS2000 CdS1000 calcinated CdS–PDDA CdS–PANI CdS–Glu1 CdS–Glu2 CdS–Glu3 CdS–Glu3 CdS–Glu3

– – CdS2000 mixed with PDDA-solution CdS2000 mixed with PANI-solution CdS2000 mixed with 0.05 M glucose solution for 5 h CdS2000 mixed with 0.05 M glucose solution for 20 h 0.9 g of glucose added with PEG before hydrothermal treatment Once regenerated CdS–Glu3 Twice regenerated CdS–Glu3

0.0150 0.0265 0.0047 0.0320 – – 0.0772 0.106 0.059

100 100 99.8 87.7 – – 100 100 100

15.8 14.3 12.9 10.4 0.8a 0.9a 0.5 4.1 5.1

Cd leaching after 90 min.

give as good results as when glucose was added with PEG in the reaction mixture before the hydrothermal synthesis of CdS catalysts (CdS–Glu3). Table 2 also indicates that degradation of MB enhanced when glucose was used in the synthesis. This is probably related to the better adsorption efficiency of CdS–Glu3 (see

Fig. 7) compared to original CdS microspheres enhancing the degradation of organic pollutants on the catalyst’s surface. For further comparison, CdS–Glu3 was also tested using different doses and MB concentrations. It is clear that higher MB concentrations resulted slower degradation (Fig. 7a and Table 3), which can

212

E. Repo et al. / Separation and Purification Technology 120 (2013) 206–214

3 mg/L (a) 5 mg/L 10 mg/L 3 mg/L 2. use 3 mg/L 3. use 5 mg/L without lamp

1.0

C / Ci

0.8 0.6 0.4 0.2 0.0 0 lamp on

40

80

120

200

Dose: 1 g/L Dose: 2 g/L Dose: 3 g/L

1.0 0.8

C / Ci

160

240

time (min)

(b)

0.6 0.4 0.2 0.0 0 lamp on

40

80

120

160

200

240

Fig. 8. SEM-images of (a) unused CdS–Glu3 and (b) CdS–Glu3 after second recycling step.

time (min)

Fig. 7. Degradation of MB by CdS–Glu3. Dose of the catalyst: 1 g/L.

be attributed to the darker solution inhibiting LED radiation to reach the catalyst’s surface. However, degradation results were better than those obtained for CdS2000 (Table 1) at higher concentrations, which is most likely due to the better adsorption properties of CdS– Glu3. Higher catalyst’s dose further enhanced MB removal by adsorption (Fig. 7b) increasing also the degradation rates (Table 3). Reusing of CdS–Glu3 was also tested in order to study the effect of photocorrosion. Clearly, the amount of released Cd increased due to the recycling (Table 2), but still the catalytic activity of the material remained unchanged (Fig. 7a). Photocorrosion also decreased the size of the nanocrystals on the CdS–Glu3 surface (Fig. 8) as in the case of unprotected CdS microspheres (Fig. 6d and e). However, photocorrosion was still lower for recycled CdS–Glu3 than for unused CdS2000.

be observed that TOC increased during the degradation. This means that more organic species appeared in the solution after experiments. This could be explained by the release of PEG due to the photocorrosion or from the PEG as impurity remained in the material after synthesis. Increase of TOC was lower for the CdS–Glu3 comparable to the lower release of Cd. GC–MS results after MB degradation are shown in Fig. 9. Peaks related to the main degradation products (phenols, aromatic amines, and sulfates) observed earlier [33] could not be detected. It is possible, that also intermediate products were adsorbed on the catalyst’s surface. Presence of organic compounds observed in TOC measurements could be related to the degradation products of PEG, which were not extracted by used SPE method. On the whole, formation of toxic degradation products was not observed as illustrated in Fig. 9.

3.7. TOC results and intermediates formation

4. Cost evaluation and comparison of the studied method with literature

Table 4 and Fig. 4a show the TOC results before and after degradation of MB by both CdS2000 and CdS–Glu3. Generally, it can

Cost evaluation (electrical costs) was conducted according to the method presented by Mahamuni and Adewuyi [34]. First-order

Table 3 Degradation efficiency of MB by CdS–Glu3 and kinetic parameters obtained by non-linear regression. Light source: 450 nm. Dose of CdS–Glu3 (g/L)

Concentration of MB (mg/L)

Degradation efficiency within 3 h (%)

k1 (1/min)

R

k2 (L/mg min)

R

1 1 1 2 3

3 5 10 5 5

100 96.9 94.2 99.8 99.9

0.0415 0.0216 0.0153 0.0476 0.0441

0.9954 0.9993 0.9999 0.9953 0.9942

0.0772 0.0105 0.0032 0.0546 0.0984

0.9815 0.9807 0.9891 0.9706 0.9662

213

E. Repo et al. / Separation and Purification Technology 120 (2013) 206–214 Table 4 Change in TOC during degradation of MB by CdS–Glu3 and CdS4000. Light source: 450 nm. Catalyst

Dose of catalyst (g/L)

Concentration of MB (mg/L)

TOC initial (mg/L)

TOC final (mg/L)

CdS2000

1 1 1 2 3

3 5 10 5 5

1.34 2.12 4.01 2.12 2.12

3.05 3.93 5.16 3.56 3.19

CdS–Glu3

1 1 1 2 3

3 5 10 5 5

1.34 2.12 4.01 2.12 2.12

2.75 2.90 3.65 4.22 3.73

Once regenerated CdS–Glu3

1

3

1.34

2.64

Twice regenerated CdS–Glu3

1

3

1.34

3.05

14000

Abundance

12000 10000 8000 6000 4000 4

8 12 16 20 24 28 32 36 40 44 48 52 56

time Fig. 9. GC–MS chromatogram obtained for a SPE extract of MB solution after 180 min of degradation.

rate-constants were taken or evaluated from the literature and used to calculate the time required for the 90% degradation of MB. In the case of first-order model time (min) required can be calculated by dividing ln(10) by k1 (see Eq. (1)). Then following equation was used to estimate the electric energy per order EE/O, which is electric energy in kWh required to degrade MB by one order of magnitude in a unit volume (1000 L).

EE=O ¼

Pel  t  1000 V  60  logðC MBi =C MB Þ

ð3Þ

In this case: log (CMBi/CMB) = log 10 = 1 for the 90% degradation and Eq. (3) turns into:

EE=O ¼

Pel  t  1000 V  60

ð4Þ

where Pel is the input power (kW) to the system and V is the volume (L) of the MB solution to be treated. Table 5 summarizes some estimated electrical costs of photocatalytic degradation of MB based on the literature and this study. Based on the estimations in Table 5 it seems that electrical costs of the method presented in this study are quite low. The concentration of MB used in this study was also lower compared to the other studies, which makes comparison rather difficult. It should also be noted that other costs such as investments, maintenance, and operating costs were not included in this simple cost estimation. However, even though LEDs are relatively expensive at the moment their stability and long lifetime are definitely advantageous. Therefore, their applicability in the water treatment solutions should be further studied. Besides the cost, removal efficiencies for dyes obtained in this study are rather good compared to the literature (Table 5). However, comparison is not straightforward because different experimental conditions have been used by different authors. For example, high intensity Xe lamp has been applied for around 40 times higher MB concentration compared to the present study (Table 5). Clear advantage of use of LEDs over Xe lamps is energy efficiency, which can be further improved due to the constantly developing LED technology. Visible LED photocatalysis can also be compared to the other oxidation processes, membrane technology, and commonly used activated carbon adsorption. Ozonation is often used for degradation of dyes, but a short half-life of ozone decreases its applicability. Addition of chemicals in chemical oxidation is obvious disadvantage and some of the dyes are too stable for microbial oxidation. The problems in membrane technology arise from its high cost and unsuitability for treating of large volumes of wastewater. Adsorption technology simply transfers the dyes from one phase to another resulting a problem with the used adsorbent. Furthermore, in the case of activated carbon, its fabrication as well as regenera-

Table 5 Evaluation of the electrical costs of the MB degradation.

a

Catalyst

Lightsource

Catalyst dose (g/L)

k1 (1/ min)

Degussa TiO2 CdS/ZnS nanoparticles mixture CdS nanospheres CdS PANI CdS2000

UV Visible halogen Visible Xe Visible Xe 450 nm LED

0.5 0.5

0.053 0.125 0.0033a 0.5

1 0.5 1

CdS-Glu3

450 nm LED 1

Evaluated from the given data.

Pel (kW)

t V (min) (L)

C0 (mg/ C (mg/ Energy density L) L) (W/mL)

EE/O (kW h/ 1000 L)

Electrical cost ($/ 1000 L)

Ref.

43 300

0.75 27 0.6 10

2.7 1

0.17 0.83

121 4167

10 333

[1] [35]

0.033a 0.008 0.0212

0.3 70 0.5 288 0.0175 109

0.05 44 0.2 20 0.2 3

4.4 2 0.3

6.00 2.50 0.09

6978 11,993 158

558 959 13

0.0415

0.0175 426

0.2

0.3

0.09

622

[21] [24] This study This study

3

6.6

214

E. Repo et al. / Separation and Purification Technology 120 (2013) 206–214

tion are highly energy consuming [35–37]. Observable disadvantages of the present method are photocorrosion and suitability mainly for rather low dye concentrations. However, these obstacles could be overcome by increasing the amount of the catalyst and further optimizing the synthesis of glucose protected CdS microspheres. 5. Conclusions Nanostructured CdS microspheres were successfully applied in the photocatalytic degradation of dyes under near UV and blue LED radiation. MB, PR, and MR were nearly completely decolorized after 3 h. Furthermore, the studied catalysts were recyclable at least five times and observed photocorrosion of CdS microspheres was successfully diminished by using glucose as protecting agent. Finally, reasonably low electrical costs were evaluated for the studied method. Acknowledgment The authors are grateful to the Finnish Funding Agency for Technology and Innovation (TEKES) for financial support. References [1] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UVirradiated titania, Appl. Catal. B – Environ. 39 (2002) 75–90. [2] S.K. Kansal, M. Singh, D. Sud, Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts, J. Hazard. Mater. 141 (2007) 581–590. [3] F.M. Huang, L. Chen, H.L. Wang, Z.C. Yan, Analysis of the degradation mechanism of methylene blue by atmospheric pressure dielectric barrier discharge plasma, Chem. Eng. J. 16 (2010) 250–256. [4] G. Sivalingam, K. Nagaveni, M.S. Hegde, G. Madras, Photocatalytic degradation of various dyes by combustion synthesized nano-anatase TiO2, Appl. Catal. B – Environ. 45 (2003) 23–38. [5] R.J. Tayade, P.K. Surolia, R.G. Kulkarni, R.V. Jasra, Photocatalytic degradation of dyes and organic contaminants in water using nanocrystalline anatase and rutile TiO2, Sci. Technol. Adv. Mater. 8 (2007) 455–462. [6] R.J. Tayade, T.S. Natarajan, H.C. Bajaj, Photocatalytic degradation of methylene blue dye using ultraviolet light emitting diodes, Ind. Eng. Chem. Res. 48 (2009) 10262–10267. [7] S. Vilhunen, M. Sillanpää, Ultraviolet light-emitting diodes and hydrogen peroxide in the photodegradation of aqueous phenol, J. Hazard. Mater. 161 (2009) 1530–1534. [8] S. Vilhunen, H. Särkkä, M. Sillanpää, Ultraviolet light emitting diodes in water disinfection, Environ. Sci. Pollut. Res. 16 (2009) 439–442. [9] W. Wang, Y. Ku, Photocatalytic degradation of reactive red 22 in aqueous solution by UV-LED radiation, Water Res. 40 (2006) 2249–2258. [10] J.P. Ghosh, G. Achari, C.H. Langford, Characterization of an LED based photoreactor to degrade 4-chlorophenol in an aqueous media by using coumarin (C-343) sensitized TiO2, J. Phys. Chem. A 112 (41) (2008) 10310– 10314. [11] J.P. Ghosh, R. Sui, C.H. Langford, G. Achari, C.P. Berlinguette, A comparison of several nanoscale photocatalysts in the degradation of a common pollutants using LEDs and conventional UV light, Water Res. 43 (2009) 4499–4506. [12] J.-L. Shie, C.-H. Lee, C.-S. Chiou, C.-T. Chang, C.-C. Chang, C.-Y. Chang, Photodegradation kinetics of formaldehyde using light sources of UVA, UVC and UVLED in the presence of composed silver titanium oxide photocatalyst, J. Hazard. Mater. 155 (2008) 164–172. [13] T.S. Natarajan, K. Natarajan, H.C. Bajaj, R.J. Tayade, Energy efficient UV-LED source and TiO2 nanotube array-based reactor for photocatalytic application, Ind. Eng. Chem. Res. 50 (2011) 7753–7762.

[14] S. Vilhunen, M. Sillanpää, Recent developments in photochemical and chemical AOPs in water treatment: a mini review, Rev. Environ. Sci. Biotechnol. 9 (2010) 323–330. [15] A.-C. Chevremont, A.-M. Farnet, B. Coulomb, J.-L. Boudenne, Effect of coupled UV-A and UV-C LEDs on both microbiological and chemical pollution of urban wastewaters, Sci. Total Environ. 426 (2012) 304–310. [16] D. Chatterjee, S. Dasgupta, Visible light induced photocatalytic degradation of organic pollutants, J. Photochem. Photobiol. C: Photochem. Rev. 6 (2005) 186– 205. [17] S. Rengaraj, S.H. Jee, S. Venkataraj, Y. Kim, S. Vijayalakshmi, E. Repo, A. Koistinen, M. Sillanpää, CdS microspheres composed of nanocrystals and their photocatalytic activity, J. Nanosci. Nanotechnol. 11 (2011) 2090–2099. [18] Y. Yu, Y. Ding, S. Zuo, J. Liu, Photocatalytic activity of nanosized cadmium sulfides synthesized by complex compound thermolysis, Int. J. Photoenergy (2011). http://dx.doi.org/10.1155/2011/762929. [19] X. Di, S.K. Kansal, W. Deng, Preparation, characterization and photocatalytic activity of flowerlike cadmium sulfide nanostructure, Sep. Purif. Technol. 68 (2009) 61–64. [20] V. Taghvaei, A. Habibi-Yangjeh, M. Behboudnia, Simple and low temperature preparation and characterization of CdS nanoparticles as a highly efficient photocatalyst in presence of a low-cost ionic liquid, J. Iran. Chem. Soc. 7 (2010) S175–S186. [21] G.F. Lin, J.W. Zheng, R. Xu, Template-free synthesis of uniform CdS hollow nanospheres and their photocatalytic activities, J. Phys. Chem. C 112 (2008) 7363–7370. [22] M. Sathish, B. Viswanathan, R.P. Viwanath, Alternate synthetic strategy for the preparation of CdS nanoparticles and its exploitation for water splitting, Int. J. Hydrogen Energy 31 (2006) 891–898. [23] Y. Yang, N. Ren, Y. Zhang, Y. Tang, Nanosized cadmium sulfide in polyelectrolyte protected mesoporous sphere: a stable and regeneratable photocatalyst for visible-light-induced removal of organic pollutants, J. Photochem. Photobiol. A: Chem. 201 (2009) 111–120. [24] H. Zhang, Y.F. Zhu, Significant visible photoactivity and antiphotocorrosion performance of CdS photocatalysts after monolayer polyaniline hybridization, J. Phys. Chem. C 114 (2010) 5822–5826. [25] P.K. Khanna, R.R. Gokhale, V.V.V.S. Subbarao, N. Singh, K.-W. Jun, B.K. Das, Synthesis and optical properties of CdS/PVA nanocomposites, Mater. Chem. Phys. 94 (2005) 454–459. [26] J. Jang, S.H. Kim, K.J. Lee, Fabrication of CdS/PMMA core/shell nanoparticles by dispersion mediated interfacial polymerization, Chem. Commun. (2007) 2689–2691. [27] D. Jing, L. Guo, Novel Method for the preparation of a highly stable and active CdS Photocatalyst with a special surface nanostructure, J. Phys. Chem. B 110 (2006) 11139–11145. [28] S. Rengaraj, S. Venkataraj, S.H. Jee, Y. Kim, C.W. Tai, E. Repo, A. Ferancova, M. Sillanpää, Cauliflower-like CdS microspheres composed of nanocrystals and their physicochemical properties, Langmuir 27 (2011) 352–358. [29] M.J. Pawar, S.S. Chaure, Synthesis of CdS nanoparticles using glucose as capping agent, Chalcogenide Lett. 6 (2009) 689–693. [30] E. Borgarello, K. Kalyanasundaram, M. Gratzel, Visible light induced generation of hydrogen from H2S in CdS-dispersions, hole transfer catalysis by RuO2, Helv. Chim. Acta 65 (1982) 243–248. [31] M.A. Rauf, M.A. Meetani, A. Khaleel, A. Ahmed, Photocatalytic degradation of Methylene Blue using a mixed catalyst and product analysis by LC/MS, Chem. Eng. J. 157 (2010) 373–378. [32] X. Li, G. Wang, X. Li, Surface modification of nano-SiO2 particles using polyaniline, Surf. Coat. Technol. 197 (2005) 56–60. [33] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, J.-M. Herrmann, Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B: Environ. 31 (2001) 145–157. [34] N.N. Mahamuni, Y.G. Adewuyi, Advanced oxidation processes (AOPs) involving ultrasound for waste water treatment: a review with emphasis on cost estimation, Ultrason. Sonochem. 17 (2010) 990–1003. [35] E. Forgacs, T. Cserhati, G. Oros, Removal of synthetic dyes from wastewaters: a review, Environ. Int. 30 (2004) 953–971. [36] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [37] G. Crini, Non-conventional low-cost adsorbents for dye removal: a review, Bioresour. Technol. 97 (2006) 1061–1085.