Bi2S3 nanocomposites on the methylene blue photodecolorization process

Colloids and Surfaces A: Physicochem. Eng. Aspects 328 (2008) 107–113

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Adsorption and catalytic properties of SiO2 /Bi2 S3 nanocomposites on the methylene blue photodecolorization process Rita Albuquerque a , Márcia C. Neves b , Maria H. Mendonc¸a a , Tito Trindade b , Olinda C. Monteiro a,∗ a b

CCMM and Department of Chemistry and Biochemistry, Faculty of Sciences, University of Lisbon, Campo Grande, 1749-016 Lisboa, Portugal CICECO and Department of Chemistry, University of Aveiro, 3810 - 193 Aveiro, Portugal

a r t i c l e

i n f o

Article history: Received 19 February 2008 Received in revised form 16 June 2008 Accepted 17 June 2008 Available online 26 June 2008 Keywords: Methylene blue Adsorption Photodecolorization SiO2 /Bi2 S3 nanocomposites

a b s t r a c t The decolorization of aqueous solutions methylene blue (C.I. Basic Blue 9), due to the presence of nanocrystalline Bi2 S3 , supported on SiO2 submicron particles, was investigated here. For this decolorization process, two distinct characteristics, though related, associated to the role of SiO2 /Bi2 S3 were identified: (i) high methylene blue adsorption capability and (ii) photocatalytic activity to methylene blue photodecolorization. Effects of experimental parameters on the decolorization process, such as methylene blue and nanocomposite concentrations, pH and Bi2 S3 particle size were investigated. The maximum adsorption ability of the SiO2 /Bi2 S3 was approximately 15.6 mg methylene blue per gram. The complete decolorization of a 16 ppm organic dye solution can be achieved, by an adsorption process, in an extremely short time (less than 5 min), using 1.6 g/L of SiO2 /Bi2 S3 nanocomposite. The study of the decolorization of the dye by an adsorption–photoassisted decolorization process was carried out by irradiation of a suspension prepared with 100 mL of methylene blue solution (8 ppm) and 50 mg of SiO2 /Bi2 S3 . In these conditions the complete decolorization of the dye, adsorbed and in the solution, was achieved in 40 min. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In the past decades we have witnessed a growing interest in photoassisted catalysis over semiconductors, as an advanced oxidation technique, for the elimination of many organic pollutants in wastewater systems [1–5]. The light irradiation of nanocrystalline semiconductors systems has been successful for the photodegradation of a wide variety of organic compounds [6–12]. When a semiconductor is illuminated with light of an appropriate wavelength it generates highly active oxidizing sites, which can potentially oxidize a large number of organic wastes such as dyes, pesticides and herbicides. Even if heterogeneous photocatalytic processes have shown to be very effective, there are many real situations that they cannot cope with. The pollutants are frequently present in concentrations that are too low to allow their efficient removal. In addition, natural organic matter, present in water systems, can occupy the catalytically active surface sites causing final low degradation efficiencies [13,14]. In order to improve the photodegradation processes performance, heterogeneous photocatalysis has been combined with other technologies [15–17]. These combinations can increase the

∗ Corresponding author. Tel.: +351 217500000; fax: +351 217500088. E-mail address: [email protected] (O.C. Monteiro). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.06.036

efficiency of the heterogeneous photocatalysis, by decreasing the reaction time or the costs. If the heterogeneous photocatalysis is associated, for instance with ultrasonic irradiation [18], photoFenton reaction [19], ozonation [15] or electrochemical treatment [20], the photocatalytic mechanisms will be directly affected. When the combination is made with a biological treatment [21], physical adsorption [22–24], or membrane reactor [25] only the global degradation process efficiency will be improved but not the photocatalytic mechanism. In the combination adsorption–photocatalysis, a suitable adsorbent, such as silica, alumina, zeolites, clays or activated carbon, selectively adsorbs the toxic pollutants present in the water. Once the adsorbent looses its ability to remove the organic contaminants, it will be separated from the clean/treated water. Afterwards it can be added to a photocatalyst, TiO2 powder, etc., in aqueous medium to produce a composite suspension. Irradiating this suspension, the desorbing component is photocatalytically oxidized and the adsorbent can be regenerated and reused in further adsorption cycles. This stepwise process is a good strategy to achieve the remediation goal, the purification of the water systems, without irradiation as a prerequisite. Other alternative of photocatalysis–adsorption association, with many advantages, consists in fixing the catalyst on the adsorbent surface, thereby producing a catalyst–adsorbent system. In this case, the adsorption and oxidation of organic compounds occur continuously, avoiding the need of two independent processes.

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The use of sunlight should be the final objective for some of these applications. However to achieve this goal, a systematic investigation on bench scale, using artificial light, is needed in order to have a better knowledge of the influence of physicochemical and operational parameters on the photo-reactivity of the systems [8,10]. In this work, with the objective of exploring new materials for future applications in coupling pollutant adsorption– photodecolorization processes, we studied adsorption and photocatalytic properties of SiO2 /Bi2 S3 . The preparation of morphological well-defined metal chalcogenide nanostructures, including nanocrystals supported on silica micro particles, was carried out by the metal dithiocarbamato thermal decomposition [26,27]. SiO2 /Bi2 S3 nanocomposites, prepared by this chemical method, were used to decolorize aqueous solutions of methylene blue (MB), by an adsorption–photodecolorization process. 2. Materials and methods All the reagents were of analytical grade (Aldrich or Fluka) and were used as received without further purification. The solutions were prepared using Millipore Milli-Q ultra pure water. 2.1. Materials synthesis 2.1.1. SiO2 sub-microparticles The SiO2 was prepared using the Stöber method reported in the literature [28]: 0.73 g of tetraethoxysilane (TEOS) was added to 5 mL of absolute ethanol containing 0.06 g of distilled water, and the mixture was allowed to stand for 30 min. Subsequently, 2 mL of NH4 OH solution (25%) was added, and the solution was left to stand for 30 min. The SiO2 colloid formed was filtered and washed thoroughly with water and ethanol. The SiO2 particles were used after thermal treatment at 700 ◦ C for 4 h. This thermal treatment increased the degree of hydroxylation of the silica surface. 2.1.2. Bi2 S3 precursor The Bi2 S3 precursor, bismuth(III) tris-diethyldithiocarbamato, {Bi[S2 CN(C2 H5 )2 ]3 }, was prepared as previously reported [29]: 40 mmol of the diethylamine and 50 mmol of carbon disulphide were added to a suspension containing 6 mmol of bismuth(III) oxide in methanol (20 mL). The mixture was stirred over 24 h and a yellow solid was obtained. The solid was crystallized in hot chloroform/methanol (3:1) and the Bi(III) complex was identified by IR spectroscopy. 2.1.3. SiO2 /Bi2 S3 nanocomposite The nanocomposite particles of SiO2 /Bi2 S3 were prepared [27], by adding drop-wise ethylenediamine (2.5 mL) to an acetone solution (25 mL) containing 0.125 mmol {Bi[S2 CN(C2 H5 )2 ]3 }and 0.250 g of SiO2 particles. The dark grey solid obtained was collected by centrifugation, washed with acetone and dried at room temperature in a desiccator over silica gel. 2.1.4. Bi2 S3 nanocrystals The nanocrystalline particles of Bi2 S3 were prepared by the procedure described above (Section 2.1.3) without the presence of the SiO2 particles. 2.1.5. Bi2 S3 bulk material Macrocrystalline particles of Bi2 S3 were prepared by the thermal decomposition of bismuth(III) tris-diethyldithiocarbamato, {Bi[S2 CN(C2 H5 )2 ]3 }, in a furnace at 320 ◦ C for 3 h. The dark grey solid was used after XRD characterization.

Fig. 1. Experimental photochemical setup.

2.2. Adsorption studies Before the photocatalytic experiments adsorption studies have been carried out, using 25 mL of a MB-nanocomposite aqueous suspension, under stirring for 1 h in dark conditions. The sampling has been made periodically and after centrifugation, the MB concentration was estimated by measuring the absorbance at 665 nm, the MB chromophoric peak. 2.3. Photodecolorization experiments The photodecolorization experiments have been conducted using an Ace Glass refrigerated photoreactor (Fig. 1). The reaction vessel (250 mL) was made of borosilicate glass and suitable to accommodate an immersion well. The quartz immersion well, was a double-walled, with inlet and outlet tubes for cooling. The inlet tube extends down the annular space and ensures the upward flow of coolant from the bottom of the well upward to the outlet. The reactor had one angled joint for the sparger tube, one vertical joint for the condenser and one Ace-Thread side arm for the thermometer. The reactor bottom is flat to allow the use of a magnetic stirrer. The radiation source was a 450-Watt medium-pressure mercury-vapour lamp (from Hanovia). The radiated watt density was 0.37 W/cm2 . MB-nanocomposite aqueous suspensions were prepared by adding SiO2 /Bi2 S3 powders into a 100 mL of 8 ppm (0.025 mM) MB aqueous solution, at natural pH. Prior to irradiation, the suspensions were sonicated and magnetically stirred in dark conditions for 15 min (graphically represented as time −15 min). During irradiation, the suspensions were sampled at regular intervals and centrifuged. The MB concentrations were estimated by measuring their absorbance at the maximum absorption peak (665 nm). 2.4. Instrumentation X-ray powder diffraction was performed using a Philips X-ray diffractometer (PW 1730) with automatic data acquisition (APD Philips v3.6B) using Cu K␣ radiation ( = 0.15406 nm) and working at 30 kV/40 mA. The diffraction patterns were collected in the range 2 = 20–60◦ with a 0.01◦ step size and an acquisition time of 2.5 s/step. The UV–vis spectrophotometer, a Jasco V560, was used for monitoring the absorption of the MB solutions and the powders diffuse reflectance spectra, in the range 300–900 nm at a scanning speed of 400 nm/min. Scanning electron microscopy

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(SEM) images and energy-dispersive X-ray spectroscopy (EDX) were performed using a FEG-SEM JEOL 7001F microscope. Transmission electron microscopy (TEM) was carried out on a JEOL 200CX microscope operating at 300 kV. The samples were prepared as follows: an aliquot containing the nanocomposite dispersed in acetone was placed on a copper grid coated with an amorphous carbon film and then the solvent was evaporated. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were performed on a Nicolet 6700 with a 4 cm−1 resolution in the 400–4000 cm−1 range. The XPS spectra were taken in CAE mode (30 eV), using an Al (non-monochromate) anode. The accelerating voltage was 15 kV. The specific surface areas were obtained from the analysis of the adsorption isotherms by the Brunauer–Emmett–Teller (BET). Nitrogen adsorption–desorption isotherms were measured at 77 K, using an automatic ASAP 2000 adsorption apparatus. The BET specific surface area was calculated on the basis of nitrogen adsorption data in the relative pressure range from 0.05 to 0.30. The samples Zeta potentials were measured with a Zetasizer instrument. The suspensions were prepared using ultra pure water and the pH value was adjusted by adding 0.01 M NaOH or HCl into the solution. The suspension pH was taken as the isoelectric point (IEP) at which the zeta potential was zero. 3. Results and discussion 3.1. Materials characterization The reaction of metal dithiocarbamato complexes with ethylenediamine in the presence of amorphous silica particles originates metal sulphide/silica nanocomposites [27]. In order to establish a correlation between the Bi2 S3 phases and their adsorption and photocatalytic properties, the preparation of bulk and nanocrystalline Bi2 S3 samples were also performed, as described in Section 2.1. The identification of the samples was made by XRD and EDX (Fig. 2), and also by XPS. The Bi2 S3 loading on SiO2 was estimated by XPS, the value found was 19% (w/w). The morphology of the Bi2 S3 structures was analysed by SEM or TEM. Fig. 3a (SEM) and Fig. 3b (TEM) show typical images for the SiO2 /Bi2 S3 nanocomposites. These images show a fibre-like network of Bi2 S3 nanocrystals grown at the SiO2 surface. The Bi2 S3 nanopowder morphology presents also a fiber-like network although in this case the formation of spherical agglomerates is clearly seen (Fig. 3c). The anisotropic growth, typical of this semiconductor, is also visible in the bulk material morphology (Fig. 3d).

Fig. 3. SiO2 /Bi2 S3 SEM (a) and TEM (b) images. Bi2 S3 SEM images nanoparticles (c) and Bi2 S3 macrocrystalline powder (d).

Fig. 2. EDX spectra and XRD pattern (inset) of a SiO2 /Bi2 S3 nanocomposite sample.

The band gap energy of the SiO2 /Bi2 S3 sample was calculated from the Kubelka–Munk (KM) spectrum shown in Fig. 4 (after conversion of diffuse-reflectance to KM). R is related to the Kubelka–Munk function FKM by the following relationship: FKM (R) = (1 − R)2 /2R [30]. By plotting the function fKM = (FKM h)2

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Fig. 5. Kinetics of MB adsorption onto SiO2 /Bi2 S3 (1 g/L) for distinct MB initial concentrations. Fig. 4. Electronic spectrum of the SiO2 /Bi2 S3 nanocomposite. Inset: Plot of the (FKM h)2 versus h.

3.2.1.1. Adsorption isotherms. The typical kinetics of MB adsorption, onto SiO2 /Bi2 S3 surface (Fig. 5) can be considered as Langmuir type, usually associated with a monolayer adsorption process. The Langmuir isotherms show that, as long as adsorption sites are available, adsorption will increase for increasing concentrations of adsorbate. Once all the sites are occupied a further increase on the adsorbate concentration will not increase the adsorbed amount, i.e. the saturation point is achieved. The Langmuir equation can be used to estimate the specific surface area of the nanocomposite [35]. The general form of the Langmuir equation is:

versus h, the linear part of the curve was extrapolated to fKM = 0 to get the direct band gap energy (Eg ) of the sample (Fig. 4, inset). The estimated Eg value was 1.78 eV. This value is blue shifted from the Eg of the bulk Bi2 S3 (Eg = 1.3 eV). This result is in agreement with other published work, reporting that as the particles become small to nanosize, their optical absorption band shows a blue shift compared to that of the bulk semiconductor [31,32]. Since the photo-oxidation takes place at the catalyst surface, the piezoelectric point and the surface area are very important parameters concerning the photocatalysts efficiency; these properties were also determined for the SiO2 /Bi2 S3 nanocomposite. The SiO2 /Bi2 S3 determined zero charge potential (PZC), pH 6.75 ± 0.25, shows that the nanocomposite surface is positively charged in acidic media and negatively charged in alkaline media. The BET specific total surface areas were found to be 16.59 m2 /g for the SiO2 /Bi2 S3 , 19.34 m2 /g for the nanocrystalline Bi2 S3 and 8.38 m2 /g for the bulk material.

where Y is the fraction of the nanocomposite surface covered by adsorbed MB molecules, K is a constant and Ce is the MB equilibrium concentration. On the other hand, Y = Xe /Xm , where Xe represents the mg of MB adsorbed per gram of SiO2 /Bi2 S3 at equilibrium concentration, Ce , and Xm is the mass of MB (mg) per gram of SiO2 /Bi2 S3 needed to form a monolayer. After rearranging Eq. (1), we obtain:

3.2. Adsorption process

Ce 1 Ce = + Xm KXm Xe

The ability of the MB to be adsorbed on the nanocomposite surface was tested using suspensions of MB and SiO2 /Bi2 S3 in dark conditions. The dye was adsorbed onto the SiO2 /Bi2 S3 particles surface and after less than 10 min a clear decrease on the MB solution concentration was visualized. The dark grey nanocomposite color did not allow the visualization of any change on its color related with the MB adsorption. After this period the adsorption–desorption equilibrium was achieved and no more decrease on the MB concentration occurred.

Y=

KCe 1 + KCe

(2)

For the MB adsorption isotherm, the plot Ce /Xe versus Ce originates a straight line (R2 = 0.9999) with a slope equal to 1/Xm (0.0685) and the interception with the x-axis equal to 1/KXm (0.0025). These results confirm that the Langmuir isotherm can be used to describe the adsorption of MB onto SiO2 /Bi2 S3 nanocomposites and it shows that the required amount of MB (mg per gram of SiO2 /Bi2 S3 ) to form a monolayer (Xm ) is 14.6 and the constant K is equal to 27.4. The specific surface area was estimated by the following equation [36,37]: SMB =

3.2.1. Influence of the catalyst amount The effect of the catalyst amount on the MB adsorption was investigated. As expected, increasing the nanocomposite quantity an increase on the adsorbed MB was observed. The use of very high amounts of SiO2 /Bi2 S3 led to the complete MB adsorption and no dynamic equilibrium has been achieved. The complete decolorization of a 16 ppm (50 mM) MB solution, corresponding to a 100% of MB adsorbed onto the nanocomposite surface, was achieved in less than 5 min, for a solid content of 1.6 g/L. When compared with other methods described in the literature [33,34], this decolorization method involves a shorter contact time and a much lower quantity of adsorbent to decolorize identical aqueous MB solutions.

(1)

Xm × aMB × N M

(3)

where SMB is the specific surface area in m2 /g; Xm is the amount of MB molecules adsorbed at the monolayer (mg of MB per gram of SiO2 /Bi2 S3 ); aMB is the occupied surface area of one molecule of MB (0.55E−18 m2 ) [37], N is the Avogadro constant (6.02E+23 mol−1 ) and M the molecular weight of MB (g/mol). The estimated value for the specific surface area, using this method, was 12.93 m2 /g. This value is in agreement with the one obtained by the BET method (16.59 m2 /g). 3.2.2. Influence of the adsorbent The ability of distinct Bi2 S3 materials to adsorb MB was also tested using identical amounts of SiO2 , Bi2 S3 nanocrystals and

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Fig. 6. Amount of the adsorbed MB (%) over time, for different materials: a) Bi2 S3 macrocrystalline; b) SiO2 ; c) SiO2 /Bi2 S3 nanocomposite and d) Bi2 S3 nanocrystalline powder. Experimental: 25 mg of material, 25 mL of 16 ppm MB aqueous solution.

macrocrystalline semiconductor (Fig. 6). In the presence of macrocrystalline Bi2 S3 , as adsorbent, only slight changes color intensity of the MB solution were observed (Fig. 6, curve a). On the other hand the SiO2 adsorption capacity was close to 20% (Fig. 6, curve b). The use of SiO2 /Bi2 S3 increases this value to nearly 60% (Fig. 6, curve c). However, as Fig. 6 (curve d) shows identical quantity of Bi2 S3 nanocrystallites conducts to a 100% MB adsorption occurrence. In face of these results it can be considered that the nanocrystalline Bi2 S3 is the best adsorbent material to use for the MB adsorption. However the difference on the surface areas of the supported and non-supported nanocrystalline Bi2 S3 must also be considered (identical weight for all the samples was used). A more detailed examination shows that the SiO2 /Bi2 S3 has a surface area that is practically 86% of the Bi2 S3 nanopowder surface area, 16.59 m2 /g and 19.34 m2 /g respectively, possessing although only 19% (w/w) of the nanocrystalline Bi2 S3 material. Besides this fact, the use of the SiO2 /Bi2 S3 as a replacement for the Bi2 S3 nanocrystalline powder on the MB decolorization process presents also other advantages: the SiO2 /Bi2 S3 preparation process is easier and faster, considering the same material quantity; the recovery of the solid, usually by centrifugation, and by dimensions reasons, is also easier for the nanocomposite system. It seems that these are good reasons for the use of SiO2 /Bi2 S3 instead of Bi2 S3 nanocrystallites on the MB adsorption process.

3.2.3. Influence of the pH In aqueous solution, the SiO2 /Bi2 S3 surface is positively charged in acidic media and negatively charged in alkaline media, with a PZC of about pH 6.5–7 (determined in this work). Thus, adsorbates would prefer to adsorb on the SiO2 /Bi2 S3 surface by a negatively charged or electron-abundant group in acidic solutions and by a positively charged group in alkaline solutions due to the electrostatic interaction. According to this, the efficiency of the MB decolorization, in the presence of SiO2 /Bi2 S3 , is expected to increases with pH owing to the electrostatic interactions between the negative nanocomposite surface and the MB cations [38]. Fig. 7 shows the MB adsorption kinetics for a 16 ppm MB aqueous solution with different initial pH. For pH 3 and natural pH (near 7) MB adsorption, onto the SiO2 /Bi2 S3 surface, close to 80% was visualized. For pH 10 a 100% dye adsorption was achieved. The strong dye adsorption onto the catalyst surface, at pH 10, should be correlated with the cationic configuration of MB and the anionic behaviour of the nanocomposite surface.

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Fig. 7. MB adsorption kinetics for 16 ppm MB aqueous solution (25 mL) using 25 mg of SiO2 /Bi2 S3 and different pH values.

3.3. Photodecolorization Several studies have shown that MB can be decomposed by light irradiation in the presence of appropriated catalysts [39–41]. Taking this into account, we have investigated the MB photodecolorization process after being submitted to adsorption onto SiO2 /Bi2 S3 . The MB decolorization in the absence of catalyst (photodecolorization) and in the presence of SiO2 has also been evaluated. The photocatalytic efficiency of 50 mg of SiO2 /Bi2 S3 material on the decolorization of an 8 ppm MB solution, 0.05% (w/w), was tested during 180 min (Fig. 8). These experimental conditions were chosen to analyse the possibility of the simultaneous MB photodegradation when adsorbed onto the SiO2 /Bi2 S3 surface and in solution. After 135 min of photodecolorization, the blue color of the MB solution disappeared. In the presence of the SiO2 , 180 min were not enough for the complete MB decolorization, and after this experiment a blue coloration remained in the solid. The minimum time needed for the complete MB decolorization, 40 min (nearly 95 min faster than the photodecolorization process), was achieved using the SiO2 /Bi2 S3 nanocomposite material. The degradation of methylene blue in the presence of SiO2 /Bi2 S3 , could be described by first-order kinetic Langmuir–Hinshelwood mechanism (Eq. (4)): − ln

C  t

C0

= kap t

(4)

where kap is the apparent reaction rate constant, and C0 and Ct are respectively the methylene blue initial and at time t. The apparent first-order reaction rate constant value, calculated from the slop of the curve, was 0.0905 min−1 . In the presence of SiO2 /Bi2 S3 , the pathway of the MB photoassisted decolorization, as the inset in Fig. 8 suggests, is slightly different from the photodecolorization pathway. On the other hand, the initial photodecolorization rate is lower when SiO2 /Bi2 S3 is present. These two facts suggest that photodecolorization, using SiO2 /Bi2 S3 , was not the unique process occurring in the reactor during irradiation and some photocatalytic activity of the solid must also be considered. The occurrence of a MB desorption process during irradiation is not out of question, however no experimental evidence has been observed. The dark grey color of the solid SiO2 /Bi2 S3 , did not allow the visualization of potential alterations associated with the MB presence on the nanocomposite surface. The hypothesis of MB being adsorbed during and after irradiation was checked by DRIFTS (Fig. 9). Before irradiation, and after MB adsorption on

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Fig. 8. Amount of MB (%) during photo-irradiation: () direct photolysis, () in the presence of SiO2 and () SiO2 /Bi2 S3 assisted photodecolorization (50 mg of solid/100 mL of 8 ppm MB solution). Inset: normalization of the MB direct photolysis () and SiO2 /Bi2 S3 assisted photodecolorization ().

the SiO2 /Bi2 S3 surface, several DRIFTS measurements were carried out in order to detect the presence of the dye. Identical measurements were made after irradiation. Comparing the DRIFTS spectra of SiO2 /Bi2 S3 , before and after adsorption, it is possible to correlate some peaks, especially from 1400 to 1700 cm−1 , with the MB presence. After being submitted to irradiation, the nanocomposite DRIFTS spectrum was identical to the SiO2 /Bi2 S3 one without any peaks suggesting the MB existence. In face of these results it can be concluded that the complete decolorization of MB aqueous solutions and the simultaneous SiO2 /Bi2 S3 recovery were reached. In face of these results, the adsorption of MB onto the SiO2 /Bi2 S3 nanocomposite surface has been proved to be a fast and efficient method for the MB solutions decolorization, without any photoequipment requirements. Moreover, it was shown that the subsequent irradiation of the suspension allows the complete photocatalytic decolorization of the total MB, adsorbed onto SiO2 /Bi2 S3 and in solution. Simultaneously the recovery of the nanocomposite could be achieved.

One of the great advantages of using the present material over others (in particular TiO2 ) is the possibility to perform the MB decolorization without the unquestionable need of a light source. After achieving the maximum adsorption capacity of the SiO2 /Bi2 S3 , this material can be recovered by using a photoassisted dye degradation process. Experimental work using the nanocomposite SiO2 /Bi2 S3 and aqueous dye solutions simulating real dye wastes (contain mixtures of dyes, electrolytes, auxiliaries buffers, etc.) is now in progress in our laboratory. 4. Conclusions In conclusion, the combination of heterogeneous photocatalysis–physical adsorption appears to be a promising tool for developing real applications for decontamination processes. This study indicates that the use of a stepwise process, involving the SiO2 /Bi2 S3 as MB adsorbent, followed by a photoirradiation step, is a fast, useful and low cost approach to obtain water decolorization systems. The strong affinity of the MB to adsorb onto Bi2 S3 supported nanomaterials is useful to decolorize aqueous solutions without the need of any specific requirements, especially light sources. Acknowledgments Rita Albuquerque thanks Fundac¸ão para a Ciência e Tecnologia (FCT) for a grant. O.C. Monteiro thanks Fundac¸ão para a Ciência e Tecnologia (FCT) for the grant SFRH/BPD/14554/2003. Márcia C. Neves thanks the University of Aveiro for a PhD grant. This work was supported by FCT, POCI and co-financed by FEDER (POCI/QUI/59615/2004). References

Fig. 9. DRIFTS spectra of MB, SiO2 /Bi2 S3 and SiO2 /Bi2 S3 -MB before and after photoirradiation.

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