Adsorption and Photocatalytic Degradation of Reactive Brilliant Red K

Ind. Eng. Chem. Res. 2010, 49, 11321–11330

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Adsorption and Photocatalytic Degradation of Reactive Brilliant Red K-2BP by TiO2/AC in Bubbling Fluidized Bed Photocatalytic Reactor Qijin Geng* and Wenwen Cui Department of Chemistry and Chemical Engineering, Weifang UniVersity, Shandong ProVince, 261061, People’s Republic of China

An activated carbon-supported titanium dioxide photocatalyst (TiO2/AC) was prepared by a spinning coating method and applied in a designed bubbling fluidized bed photocatalytic reactor (BFBPR). Adsorption and photocatalytic degradation of reactive brilliant red K-2BP in BFBPR were investigated considering the pH value, Na2SO4 added, and initial dye concentration. The experimental results indicated that the adsorption and photocatalytic degradation efficiencies of K-2BP were influenced by the pH value, Na2SO4 added, and initial dye concentration. The adsorption and photocatalytic degradation of K-2BP was approximated to the maximum value at pH value 5.7. The complex influence of Na2SO4 added on photocatalytic degradation of K-2BP at alkaline suspension was observed and explained according to the adsorption models proposed and degradation mechanism of a new free radical (SO4•-) produced. In addition, the presence of Na2SO4 plays dual functions, i.e., salt bridge-role in adsorption for Na+ and competition adsorption between anion dye molecules and SO42-, conformed by adsorption models proposed and FT-IR spectra for dye adsorption on TiO2/AC. The mass-transfer limited and screening effect that resulted from variation of the initial dye concentration may be approximated to the minimum effect at concentration of 3.75 mg L-1, with the maximum degradation efficiency above 80%. The Langmuir-Hinshelwood kinetic model was applied to explore the adsorption and degradation. Finally, the special reaction paths were inferred with variation of experimental environments. 1. Introduction Textile and leather industries are large consumers of dyes, using them in conjunction with a wide range of auxiliary chemicals for various dyeing and finishing processes. If these dyes are discharged into the environment without any treatment, there are highly harmful to the local environment and people. Recently, new and tighter regulations coupled with increased enforcement concerning wastewater discharges have been established in many countries, especially in China. However, it is difficult to find a unique treatment that insures complete elimination of dye wastewater, including biological processes, physical methods of discoloration, and adsorption by some natural inexpensive materials.1-6 The main drawback of these physical methods is only to operate by transfer or concentration of these pollutants to another phase rather than to decompose them. The present research is focused on the reactive dyes because they represent an increasing market share, and a large fraction of the applied reactive dye is wasted due to dye hydrolysis in alkaline solution. Recent developments in advanced oxidation processes (AOPs) via heterogeneous photocatalysis in the presence of TiO2 have shown a promising prospective to completely mineralize such contaminates. Several studies have demonstrated complete destruction of the dye with reasonably high efficiencies.7-10 The initial dye concentration, light intensity, mass transportation rates/limitations, initial pH, catalyst size/loading, oxygen saturation, and temperature influencing photocatalysis must be considered. Wang7 et al. investigated the photocatalytic degradation of four dyes with positively charged nitrogen-alkyl groups using F-TiO2 and pure TiO2, and the relationship between surface fluorination and the degradation rate/pathway of dyes under visible irradiation was explored based on the effect of * Corresponding author. Phone: +86-532-84022757. Fax: +86-53284022757. E-mail: [email protected].

fluorination on the surface adsorption of dyes and on the energy band structure of TiO2. Aarthi and Madras8 used Rhodamine B, 6G, Blue, and 6G perchlorate as substrates and investigated the effect of organic solvents (ethanol and acetonitrile) and metal ions (Cu2+, Fe3+, Zn2+, and Al3+) on the photodegradation under UV irradiation. These results indicated that the presence of solvents and metal ions significantly reduced the degradation rate. Zhuang9 et al. investigated the photodegradation of RhB over the prepared TiO2 films and proposed that the defect sites at the surface or the interface of TiO2 films promote the separation of photogenerated electron-holes, leading to a higher photocatalytic activity of defective TiO2 films. Smith10 et al. studied the physicochemical parameters influencing photocatalytic degradation of methyl orange (MO) over TiO2 nanotubes. The overall initial dye degradation rate demonstrates three types of dependence on dye concentration over a range from 2.5 to 100 µM. In conclusion, the photocatalysis was obviously influenced by the catalyst characteristics, dye attributes, and additives. The suspended TiO2 powder enjoys free contact with pollutant molecules in a photocatalytic reactor; it can generally achieve better efficiency than the immobilized TiO2 catalysts. However, the separation and reuse of this catalyst powder from treated water often limit its application. Slurry reactors were the most used in photocatalysis research since these reactors were characterized by good contact between the reactant and catalyst, illustrated by the high illuminated surface area per unit of reaction gas or liquid volume inside the reactor.11 However, a separation step of the catalyst from the reaction products encountered major technical and economical problems.12 In addition, it was difficult to uniformly irradiate suspended particles, although some configurations might (partly) overcome this disadvantage.13,14 Some researchers, focusing on scaling-up of photocatalysis, advocated the use of immobilized catalysts.15,16 For example,

10.1021/ie101533x  2010 American Chemical Society Published on Web 10/12/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010

TiO2 immobilized on activated carbon was prepared using a modified sol-gel method over activated carbon15 or using a dip-coating method at low temperature to prepare TiO2 thin films deposited on granular activated carbon.16 Recent findings12 indicated that composites TiO2/AC resulted in more than a mere contact between both solid phases and that even in composites made by mixing TiO2 and activated carbon, changes in the physicochemical features of TiO2 were observed. Hence, the design of new photocatalysts based on activated carbon required a better understanding of the adsorption process in composites because various parameters were involved (such as catalyst dispersion, TiO2 particle size versus inert support, support surface structure, etc.). However, little in the literature was published openly to investigate the adsorption and photocatalytic degradation of reactive anion dye K-2BP in BFBPR using TiO2/ AC, so far considering the relationship between adsorption and degradation with variation of pH value, salt added, and initial dye concentration. In this paper, we fabricated the TiO2/AC about 200 mesh of diameter and little apparent density (about 0.5 g cm-3), which can suspend and fluidize in BFBPR very freely. Our challenges are to achieve effective degradation of commercial dye K-2BP wastewater, so the degradation and adsorption experiments were conducted considering the influence of initial dye concentration, pH value, and ions/anions added. Further to explore the relationship between adsorption and degradation, the corresponding kinetics was investigated based on the L-H model. 2. Experimental Section 2.1. Preparation of Catalyst. The granular superfine active carbon powder (granularity, ∼400 mesh; benzene adsorption value, 400 mg g-1; obtained from Nantong Tongsen Active Carbon Co., Ltd., China), used as the support material in the present experiment, has some excellent properties, such as suspended and adsorptive property. It was washed by distilled water three times to remove impurities and dried in the vacuum oven at 60 °C for 24 h. Titanium dioxide powder (Degussa P25, Shanghai, China) was supported on the exterior surface of the activated carbon by the spin coating method described everywhere in detail.17 In the present experiment, the calculated value of wt % TiO2/AC was 6.44%. 2.2. Design of BFBPR. The designed BFBPR is an annular coaxial double-cylinder-type reactor and consisted of an ultraviolet (UV) lamp (25 W, with the emission wavelength ranging from 228 to 400 nm and the maximum emission intensity at 253.7 nm, Shanghai Yaming Lighting Co., Ltd.), a reaction region (1.1 L, the maximal working volume × 500 mm height × 10 mm distance between the inner wall of the synthetic glass tube and the outer wall of the quartz glass tube), a tube-type gas distributor, a Cu sifter (200 mesh), a gas pump, a recycling pool (working volume including tube volume, 5 L), and a circulation pump as shown in Figure 1. The distance between the lamp surface and outer wall of the quartz glass tube is 10 mm, and the quartz glass tube (100 mm o.d. × 600 mm high × 1.8 mm wall thickness) located at the center of the synthetic glass tube (140 mm i.d. × 600 mm high × 4.5 mm wall thickness) and the UV-light lamp (18.5 mm o.d. × 640 mm length) coaxially. The minimum bubbling velocity (Umb) of TiO2/AC photocatalyst was controlled to make the TiO2/AC particles fluidized, suspend, and disperse uniformly in solution, determined as 0.4 L min-1. The Cu sifter was chosen and considered to have a threefold-role: (i) as a gas distributor, the Cu sifter (below) can avoid producing big air bubbles at aerating; (ii) Cu sifter (below) can support catalyst particles and efficiently

Figure 1. Schematic diagram of annular BFBPR: (1) UV-light; (2) Pyrex glass tube; (3) Synthetic glass tube; (4) Reaction pool; (5) Cu sifter; (6) Pipe-type gas distributor; (7) gas pump; (8) electrical source; (9) sample reservoir; (10) solution circulation pump; (11) sampling pore and pH detector; (12) sample solution inlet; (13) sample solution outlet; (14) catalyst particles.)

filter them out from suspension with the minimum energy of the overall process in a recycling application; (iii) Cu sifter (above) was used to prevent photocatalyst particles from drifting out of reaction region. The reactants and TiO2/AC particles in the reaction region can be mixed completely by aeration from the bottom to the top of the reaction pool and solution circulation from the inlet at the top of the reaction pool and flow out from the outlet at the bottom of the reaction pool. The gas velocity was regulated by a flowmeter (Matheson 603, 602). Dye solution is circulated at the volume flux of 0.2 L min-1 by a solution circulation pump (YZA5A5X, Baoding Prosys Precision Pump Co. Ltd. Hebei, China). The pH value of the circulation suspension was measured by a pH detector (pHS-3B, Shanghai Tianpu Analysis Equipment Co., Ltd., China). The major advantages of this annular BFBPR over a conventional batch reactor are as follows: (I) can circulate to handle large quantities of wastewater; (II) effective utilization of the catalyst and light; (III) higher gas-liquid and solid-liquid mass transfer; (IV) high surface area even at very low catalyst loading; and (V) catalyst particles can be filtered with the help of a filter and regenerated efficiently. Thereafter, this annular BFBPR was selected for the photocatalytic degradation of dye K-2BP in the present investigation. 2.3. Isotherms Absorption. Isotherms adsorption of reactive dye K-2BP onto TiO2/AC particles were carried out in the BFBPR under the dark and bubbling conditions at ambient. For these cases, the reaction region of BFBPR was fed at the fixed amount of 20.0 g of TiO2/AC, and 3 L of the dye solution (C.I. Reactive Red 24 (18208), K-2BP, without further purification, Jiangsu Shenxin Dyestuffs & Chemicals Co., Ltd.) was circulated from sample reservoir to reaction pool, which gave a bed height of 350 mm. At the adsorption equilibrium state after circulating 60 min, which was sufficient for achieving steadystate conditions, a 10 mL sample of K-2BP solution was taken to measure the absorbance by an ultraviolet-visible spectrophotometer (TU-1901, Beijing Purkinje General Instrument Co., Ltd., China) at the maximum absorption wavelength 522 nm. Calibration plots based on Beer-Lambert’s law can be established, relatign the absorbance to the concentration of dye. Thereafter, the adsorption efficiency of the dye was calculated by eq 1.


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Figure 2. Special molecular structure of K-2BP. (The maximum adsorption wavelength λmax, 522 nm; molecular weight, 808; iso-electric point (pI), 5.2.).

The solutions were prepared by dissolving a defined quantity of this commercial dye in distilled water. A pH value detector (pHS-3B, Shanghai Tianpu Analysis Equipment Co., Ltd., China) was used to measure the pH value of solution, adjusted by H2SO4 or NaOH solution. Attached, the special molecular structure of K-2BP was shown in Figure 2. 2.4. Photocatalysis of Dye in BFBPR. Aqueous dye dispersions (3 L) were circulated in BFBPR at ambient from the sample reservoir to the reaction region, with variation of the dye concentration ranging from 0.5 to 8 mg L-1 and the fixed amount of TiO2/AC as 20 g. Prior to irradiation, the suspensions were bubbling and circulating in the dark for 60 min to establish an adsorption-desorption equilibrium. After the lamp was switched on to initiate the photocatalytic reaction, at regular time intervals of 30 min, 10 mL of dye samples were collected to measure as mentioned before. The decomposition efficiency of the dye was calculated by eq 2. adsorption efficiency, ηA(%) )

C0 - Ce × 100 ) C0 A 0 - Ae × 100 A0

decomposition efficiency, ηD(%) )

Ce - Ct × 100 ) Ce A e - At × 100 Ae

(1)

(2)

where Ct, C0, and Ce are the t minute, original, and equilibrium concentrations of the dye, respectively; A0, Ae, and At were the corresponding absorbance of dye at the original, equilibrium, and t minute, respectively. 2.5. FT-IR Spectroscopy. FT-IR spectra were obtained by the KBr tablet compressing method employing a FT-IR spectrometer (FT-IR 380, Thermo Nicolet Corp.) equipped with a DTGS detector. The TiO2 sample adsorbed K-2BP was obtained from the suspension of the dye solution of K-2BP, then filtered, and dried in air at 70 °C. The spectrum was obtained with a 32-scan data acquisition at a resolution of 4 cm-1. It was worthwhile to mention that pure titanium dioxide powder was used as a background reference. All spectra were obtained at room temperature, with the powders being exposed to the atmosphere. 3. Results and Discussion 3.1. Adsorption of Dye on TiO2/AC in BFBPR. 3.1.1. Effect of pH Value. The effect of the initial pH value of the dye solution was studied in that the pH value could be considered as one of the most important parameters influencing the photo-oxidation process. Adsorption with variation of the

Figure 3. Influence of pH value and dye concentration on adsorption of K-2BP.

pH value ranging from 2.5 to 10.5, evaluated by the adsorption efficiency of K-2BP, was shown in Figure 3. It was observed that the solution pH has a significant effect on the adsorption of K-2BP on the TiO2/AC photocatalyst surface. The maximum adsorption efficiency of K-2BP occurred at a pH value of 5.7, which is intervenient between the iso-electric point (pI ) 5.2) of K-2BP and that of TiO2 (pI ) 6) used. A similar result18 was obtained in the case of 1,3-dinitrobenzene (m-DNB), where with initial pH values of 3, 7, and 11, as was observed that the solution pH had a significant effect on the adsorption of m-DNB and the maximum adsorption occurred at neutral pH, where the pI for the used m-DNB and TiO2 are 7 and 6, respectively. Theoretically, as a naked TiO2 particle is exposed to water, the surface is hydrated to satisfy the coordination of surface TiVI ions. Dissociation of the chemisorbed molecular water then gives rise to surface OH- groups (denoted here as tTisOH), consequently, the surface of TiO2 becomes positively charged in an acidic medium (eq 3). The surface specification modeling shows that tTisOH2+ species accounts for 89% at pH ) 3.19 Thus, a high adsorption efficiency of anionic dye, K-2BP, on TiO2 was observed at a pH ranging from 2.5 to 6.5. The similar phenomenon was observed in the case of adsorption of X3B anion dye on TiO2.20 It can be explained by the modification of the electrical double layer of the solid-electrolyte interface, which affects the adsorption-desorption processes.21 On the basis of the fact that TiO2 showed an amphoteric character, expressed as the following eqs 3 and 4, either a positive or a negative charge could be developed on its surface. tTisOH2+ T TisOH + H+ (pKa1 ) 3.9)

(3)

tTisOH T tTisO- + H+ (pKa2 ) 8.7)

(4)

Therefore, adsorption of dye onto the TiO2 surface was favored in the pH ranging from 5 to 6. However, the adsorption of dye on the photocatalyst surface may be involved to the additive of the solution, dye concentration, physical and chemical properties of the dye (i.e., polarizability, dipole moment, electron donation, acid-base interaction), which dominated the affinity and the adsorption/desorption of dye for the catalyst surface.22 Further to explore the mechanism of adsorption, the corresponding adsorption models should be developed. According to Figure 4a, the H+ linked catalyst adsorption site and the dye functional group act just as a “bridge”, and we can infer that the sulfonic group, as an active functional group of K-2BP, should mainly contribute to the

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Figure 4. Proposed adsorption models of K-2BP in TiO2/AC solution: (a) at acidic conditions (pH < pI); (b) at alkaline conditions (pH > pI); and (c) Na2SO4 added at alkaline conditions; (d) proposed in ref 7; and (e) proposed in ref 28.

affinity of it for the photocatalyst surface adsorption site. Solution pH had a significant effect on the uptake of dyes, since it determines the surface charge of the adsorbent, the degree of ionization, and speciation of the adsorbate. The hydrogen ion and hydroxyl ion were adsorbed strongly, and subsequently the adsorptions of other ions were affected. For example, in acidic suspension with respect to pH below pI, the sulfonate groups of reactive dyes can be almost protonated. All this maybe led to these dye molecules being naturally or positively charged and subsequently weakened the attraction between dye molecules and TiO2. In alkaline conditions, the lower adsorption efficiency was attributed to the obvious fact that the OH- added competed with anion dye molecules to adsorb onto the catalyst. The similar result was also investigated previously for hydroquinone adsorbed onto TiO2/AC photocatalyst at alkaline suspensions.17 3.1.2. Effect of Initial Dye Concentration. With the concentration of K-2BP in the bulk solution monitored after the adsorption/desorption equilibrium is reached with various concentrations, the isotherms adsorption of K-2BP in aqueous TiO2/AC suspensions were examined, shown in Figure 3. As was observed from Figure 3, the adsorption efficiency of K-2BP decreased with increasing the initial dye concentration ranging from 0.5 to 10 mg L-1. This result can be explained that, for the fixed amount of catalyst, the ratio of adsorption sites to the number of dye molecule decreased with the increase of K-2BP concentration. Further to explore the influences of pH value and initial concentration of dye on adsorption, the saturation amount of adsorption and the value of adsorption constant were obtained based on the Langmuir adsorption model, and the resultant

Figure 5. Regression curves and the corresponding results for adsorption of K-2BP.

parameters were presented in Figure 5. Generally, the Langmuir equation is expressed as [θ] )

NA KaCe ) NT 1 + KaCe

(5)

where θ is coverage, NA is the adsorption amount (millimoles), NT is the saturation amount of adsorption (millimoles), Ka represents the equilibrium constant (millimoles-1) for the adsorption process, and Ce is the equilibrium concentration of K-2BP in the bulk solution. The saturation amount of adsorption was estimated from the slope of the linear plot of NA-1 vs Ce-1,


Figure 6. Effect of Na2SO4 on K-2BP adsorbed onto TiO2/AC.

and the equilibrium constant was determined from the intercept of the fitting line. As was observed in Figure 5, at acidic conditions (pH < 6), the saturation amount of adsorption increased with increasing pH value, while the saturation adsorption amount decreased sharply after a pH value above 6. The variation of the saturation amount of adsorption was a reflection of the adsorption mechanism, i.e., at alkaline suspensions, the reduction of the saturation amount of adsorption was attributed to OH- adsorption on catalyst active sites to inhibit adsorption of the dye; at acidic conditions, the protonated sulfonate groups of reactive dyes and the catalyst surface maybe lead to weakening the attraction between the reactive dye and TiO2, as a result, the saturation amount of adsorption decreased with decreasing pH value. 3.1.3. Effect of Salt Added. Adsorption of dye molecules is influenced not only by the pH value but is also related to the presence of anions and ions. Lv and Xu20 observed obvious influences of polyoxometate and fluoride on the adsorption of organic dye X3B on TiO2. In the present research, the effect of salt on adsorption was explored by addition of Na2SO4. As was observed from Figure 6, the Na2SO4 added was an obvious influence on dye adsorption. The adsorption efficiency decreased with the amount of Na2SO4 added at acidic conditions (pH ) 4.5). This result was similar to the other reports published.23-27 It may be related to the competition adsorption between anions added and anionic dye molecules used. However, there is an interesting finding that an inflection point with respect to a pH value of 7.5 and Na2SO4 addition amount at 1.5 g L-1 was observed over the maximum adsorption efficiencies of 19.2%. It can be explained according to the proposed model in Figure 4b,c. For the given amount of Na2SO4 at basic suspension, Na+ maybe played the bridge-role as H+ in acidic conditions to enhance adsorption of the dye, where the adsorption occurred by the hinge of the ion (Na+) added to the sulfuric group anchored in the dye molecule and led to enhancement of the adsorption. The investigation of the adsorption processes of organic pollutants on the photocatalyst surface and its dependence on operational parameters was of great importance in elucidating the mechanism of photocatalytic reactions and in formulating appropriate kinetic expressions. Results can also improve the understanding of the interfacial phenomena which take place in photocatalytic reactions. Photocatalytic degradation rates and reaction pathways are expected to be strongly dependent on the specific molecular structure and the nature of the chemical bond between the adsorbent surface and the adsorbate. However, only

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a few investigations have been focused on the effect of the adsorption to the overall photocatalytic performance. Wang7 reported that RhB adsorbed on the F-TiO2 surface through the diethylamine functional group but hinged to the surface of pure TiO2 via the carboxylic groups (Figure 4d). In fact, the surface hydroxyl on the TiVI site is indispensable for the formation of an ester-like bond between the carboxylic group and surface Ti site of TiO2. Bourikas28 introduced the charge distributionmultisite complexation (CD-MUSIC) model and the corresponding adsorption state, which emphasizes the importance of the structure of both the photocatalyst surface and the adsorbed species. Surface complexes are not treated as point charges but considered to have a spatial distribution of charge in the interfacial region. In some cases, its predictions are consistent with physically realistic surface complexes found by spectroscopy. A schematic representation of this complex is given in Figure 4e. Further to validate the adsorption mechanism, the corresponding FT-IR spectra of the catalyst before and after adsorption of the dye were offered. Generally, the spectra show the Ti-O-Ti stretching vibration band at 400-600 cm-1;29 and sulfated metal oxide shows a broad band at 1200-900 cm-1, being the characteristic frequency of SO42-. As was observed from Figure 7, the broad peak at 1097.52 cm-1 should be the characteristics of inorganic chelating dentate sulfates, which are assigned to asymmetric and symmetric stretching vibration peaks of SdO.30 The peaks at 1620-1628 and 3372-3393 cm-1 are attributed to bending and stretching vibration frequencies of water molecules occluded in the sample.31 The Ti-O-Ti stretching vibration band is presented at 400-500 cm-1 in the FT-IR spectra. A series of peaks ranging from 1750 to 1450 cm-1 may be the reflection of other functional groups anchored in the K-2BP molecule. Briefly, the band at about 1500 cm-1 is attributed to the azo-bond of the dye, and the bands from 1650 to 1450 cm-1 are characteristic of phenyl ring vibrations. According to the performance of FT-IR spectra, K-2BP molecule adsorption onto the active sites of catalyst took place by sulfuric groups anchored in the dye molecule. There was good agreement with the adsorption models proposed in Figure 4. The influence of salt added on adsorption was further studied with variation of the pH value and dye concentrations simultaneously and presented in Figure 8. From Figure 8, the adsorption efficiency of K-2BP was larger than that of Na2SO4 added at pH ) 4.5, while the adsorption efficiency of K-2BP was less than Na2SO4 added at pH ) 7.5. It was reliable to conclude that there was a complex influence of Na2SO4 accompanied with the pH value and dye concentration on adsorption. According to the Langmuir adsorption equation, the saturation amount of adsorption was obtained and the fitting results are presented in Figure 8. As was observed in Figure 8, at acidic conditions (pH ) 4.5), the saturation amount of adsorption increased by addition of Na2SO4, while the adsorption equilibrium constant decreased obviously. In contrast, the saturation adsorption amount decreased by addition of Na2SO4, and the adsorption equilibrium constant increased obviously at basic conditions (pH ) 7.5). The decrease of saturation adsorption amount may be attributed to the competition adsorption of anions added (SO42-) and anion dye molecules onto the surface of TiO2/AC at alkaline conditions. At lower amounts of Na2SO4 added, the competition of anions (SO42-) and dye molecules can be neglected, while the ions (Na+) added may be played as a bridge-role to enhance the contact between dye molecules and TiO2/AC. At higher amounts of Na2SO4 added, the competition adsorption of anions and dye molecules onto the surface of TiO2/AC must be considered. In summary, the

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Figure 7. FT-IR spectra of TiO2 before (below) and after (above) adsorption of K-2BP.

Figure 8. Regression curves for the adsorption of dye molecules onto TiO2/ AC in BFBPR with various initial concentration of K-2BP, salt, and pH value.

variation of adsorption curves with pH value and Na2SO4 added was confirmed with the adsorption model proposed and the saturation adsorption amount and adsorption equilibrium constant obtained according to the Langmuir adsorption equation. In addition, addition of Na2SO4 played a complex role in adsorption of the dye K-2BP, which was related to adsorption environments, such as amount of additive, pH value, and dye concentrations. 3.2. Photocatalytic Decomposition of Dye in BFBPR. Elimination of organic pollutants using a TiO2 photocatalyt has been widely studied.7-10 Once the active species are generated at the catalyst surface, the subsequent reaction sequence, which leads to the final degradation of the pollutant, may be envisaged to occur through several steps as follows.32 If the reaction site is at the photocatalyst surface, the following processes can occur: (i) diffusion of the reactants from the bulk solution or from an “inert” surface to the photocatalyst surface, (ii) diffusion of the

reactants from surface or pore sites to the active centers at the photocatalyst surface, (iii) reaction at the catalytic centers, and (iv) diffusion of the reaction products from the surface to the bulk solution. If the reaction occurs exclusively (or also) in the bulk solution, the steps are (v) diffusion of photogenerated reactive species from the surface to the bulk solution and reactions in solution, (vi) if the compound is adsorbed on an “inert” surface, the reaction of the photogenerated reactive species originated from the catalyst at the surface of the “inert” support must migrate to the inert surface and react with the compound adsorbed. The reaction paths and steps involved were described in Figure 9 based on the reaction sequence. At all events, an increase in the surface concentration of appropriate scavenger or organic substrate adsorbed would favor the interfacial charge transfer and depress the charge recombination, consequently enhancing the desirable degradation of organic pollutants. 3.2.1. Effect of Initial Dye Concentration. The effect of initial concentration of K-2BP on the photocatalytic degradation was studied with variation of dye concentration from 0.5 to 8 mg L-1. The resultant degradation efficiency of K-2BP at the given conditions was shown in Figure 10, where the degradation efficiency reduced obviously with increasing dye concentration ranging from 4 to 8 mg L-1 after continuous illumination. Similar observations were reported with TiO2 nanoparticulate films deposited on different substrates.33,34 At a fixed UV-light intensity, the decrease of degradation efficiency with increasing dye concentration may be attributed to the competition among dye molecules for degradation, inhibition of intermediates produced in degradation processes, and/or the reduction in the light intensity that reaches the TiO2 surface by the screening effect. In particular, at higher dye concentrations above 6 mg L-1, more light was screened by the dye solution and fewer photons are able to reach the TiO2 surface. Thus, the generation of electron-hole pairs was greatly reduced, and in turn, the dye degradation was reduced due to absence of the oxidizing species. A similar finding was observed for the photocatalytic


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Figure 9. Photocatalytic reaction mechanism, step, and path based on ref 32.

Figure 10. Effect of initial concentration on photocatalytic degradation of K-2BP.

degradation of Remazol Black 5 and Procion Red MX-5B in the presence of commercial Degussa P25 TiO2.35 It was worthwhile to mention that the degraded products and intermediates, adsorbed on the surface of catalyst as scavengers of holes generated in photocatalytic process, were detrimental to the photocatalytic action. As Wang et al. reported that the deactivation of catalyst was observed since the adsorption of reaction intermediates on the TiO2 surface.36 In contrast, at lower concentration (below 4 mg L-1), the lower degradation efficiency may be attributed to the mass transfer resistance notwithstanding the mass-transfer intensified by bubbling and circulation of dye solution. In summary, on the basis of the experimental results, we can infer that the masstransfer limited and screening effect resulting from variation of the dye concentration may be approximated to the minimum effect at a concentration of 4 mg L-1, with the maximum degradation efficiency above 80%. Heterogeneous photocatalytic degradation of organic dyes at low concentrations often follows L-H kinetics with assumption of a first-order reaction, described by a linear plot of eq 6. ln C0/C ) kappt

(6)

This modified L-H equation was applied here, without consideration of a specific mechanism. All the plots of ln(C0)/ (C) versus t were satisfactorily linear (correlation coefficient above 0.996) presented in Figure 11 with the resultant apparent reaction coefficient (Kapp) as a function of dye concentration in Figure 11. It indicated that the apparent reaction rate coefficient varied with dye concentration and was approximated to the maximum value at a concentration of 3.75 mg L-1. Therefore, it is reliable to conclude that the photocatalytic degradation of K-2BP was a concentration-dependent reaction, and the masstransfer resistance occurred in lower dye concentrations, which may be the controlled step in photocatalysis under continuous illumination. Multifactors may be involved in the photocatalysis

Figure 11. Regression curves and apparent reaction coefficient of photocatalytic degradation with variation of the dye concentration.

Figure 12. Influence of pH value on photocatalytic degradation of K-2BP.

with respect to higher concentrations, such as inhabitation of intermediates produced in degradation processes and the screening effect. As a result, the possible reaction path and step at pI can be determined as path (I) described in Figure 9. 3.2.2. Effect of pH Value. Since photocatalysis is a surface phenomenon, the photocatalyst performance can be influenced by the pH of the stream. In acidic or alkaline conditions, the surface of TiO2 can, respectively, become positively or negatively charged based on the modification of the electrical double layer of the solid-electrolyte interface mentioned before. In the present experiments, the photocatalytic degradation of K-2BP was performed with variation of the pH value ranging from 2 to 11. It was observed from Figure 12 that the degradation efficiency increased under the continuous illumination from 0 to 3.5 h at acidic, neutral, and alkaline conditions, respectively, with the maximum degradation efficiency at a pH value of 5.7. Several studies have examined the similar effects of the pH value on photodegradation.36-40 On the basis of these studies, an accordant conclusion has been obtained that TiO2

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Figure 13. Regression curves and apparent reaction coefficients of photocatalytic degradation with variation of pH value.

photocatalyst demonstrated higher photocatalytic degradation efficiency of organic pollutants in acidic media than that in alkaline media. This finding was similar to the variation of adsorption efficiency with pH value and indicated that the adsorption enhanced the photocatalytic degradation of K-2BP. For K-2BP, as the literature39 mentioned methyl orange (MO) as an anionic molecule, an increase in degradation efficiency at pH < pI, was ascribed to a positively charged photocatalyst surface being more available for the absorption of the dye molecules. The dye structure of MO was prone to oxidation over the azo structure due to the sulfonic groups (-SO3-) aiding in capturing hydrogen protons and further enhancing the hydrophobicity of the TiO2 surface. Therefore, we can infer that the photocatalytic degradation reaction at acidic conditions may occur as described in the reaction step and path (III) in Figure 9. The dye K-2BP molecule, adsorbed on the AC surface by sulfonic groups (-SO3-) aiding in capturing hydrogen protons at acidic suspensions, may react with the special radicals, which originated from the catalyst surface and migrated quickly to the inert surface, over the azo functional group anchored in the dye structure. All the plots of ln(C0/C) versus t with variation of the pH value were satisfactorily linear (correlation coefficient above 0.997) presented in Figure 13, with the resultant parameters of apparent reaction coefficient (Kapp) as a function of pH value. It indicated that the apparent reaction rate coefficient varied with pH value and approximated to the maximum value at a pH value of 5.7. Therefore, it is reliable to conclude that the photocatalytic degradation of K-2BP was a pH-dependent reaction in BFBPR with the optimal pH value at 5.7. 3.2.3. Effect of Na2SO4 Added. As was observed from Figure 14 that, in the case of Na2SO4 added, the photocatalytic degradation efficiency decreased obviously with an increasing amount of Na2SO4 ranging from 0.3 to 2.7 g L-1 at acidic suspensions or the amount of Na2SO4 added from 0.3 to 1.75 g L-1 at alkaline suspensions. Addition of salt to the TiO2 dispersions can lead to a significant inhibiting effect on the photocatalysis of absorptive substrates in water under UV irradiation.26 However, the degradation efficiency increased gently after the amount of Na2SO4 was above 1.75 g L-1 at alkaline suspensions, which is in disagreement with the adsorption curves at the alkaline suspension presented in Figure 6. The adsorption of K-2BP decreased, but the observed photodecomposition reactivity tended to increase with the amount of Na2SO4 added above 1.75 g L-1 at alkaline suspensions. Therefore, it is reliable to conclude that the different degradation mechanisms occur at acidic and alkaline conditions. This is

Figure 14. Effect of Na2SO4 added on degradation efficiency of K-2BP.

partly because the photodecomposition of K-2BP does not follow a simple reaction between the mentioned active species and dye molecules. The decrease of degradation efficiency with SO42- anions introduced into TiO2/AC acidic suspension may be attributed to the competition adsorption between SO42- anions and anionic dye molecules onto the surface of catalyst, deactivating a portion of the catalyst.41 However, the observed degradation efficiency increased with the increasing amount of Na2SO4 above 1.75 g L-1 at alkaline suspensions, which can be explained by the formation of new reactive species SO4•- at high concentrations of SO42- according to the following reaction.42-44 hvb + SO42- f SO4•-

(7)

In the presence of Na2SO4 at the given amount of 1.75 g L-1, the competition adsorption of some anions, such as SO42-, OH-, with dye molecules would not be favorable to the dye oxidation by subsurface hvb+ and surface bound •OH radicals. However, SO42- adsorbed the photocatalyst surface, as a hole scavenger, and increases the generation of free radicals SO4•(eq 7), which could initiate the dye reactions either in solution or on the catalyst surface. As a result, the observed degradation efficiency of K-2BP increases gently with Na2SO4 added after the amount above 1.75 g L-1 (Figure 14). For the given amount of Na2SO4 of 1.75 g L-1, the photocatalysis mechanism was enhanced by SO4•- and may be confirmed by the theory as Minero32 put forward. For the strongly adsorbed species in the photocatalytic degradation process, their degradation rates were not strongly affected by their actual location, whether in solution, on the photocatalyst, or on another “inert” support since the exchange of these compounds between the two supports was very quick to contact the reactive sites on the catalyst surface, leading to a facile degradation. In the present investigation, the supported catalyst, TiO2/AC was composed of reactive sites TiO2 and inert support AC, the exchange of SO4•- may occur from AC to dye molecules in suspension or on the TiO2/AC surface quickly to initiate the radical reaction. Therefore, we can infer that the reaction path may be in agreement with path (II) mentioned in Figure 9. Conclusions The adsorption and photocatalytic degradation of reactive brilliant dye K-2BP using TiO2/AC photocatalyst have been investigated in BFBPR, and the following conclusions can be derived from the present study. (1) A high adsorption efficiency of reactive brilliant dye K-2BP onto the TiO2/AC photocatalyst


was achieved in BFBPR. The adsorption mechanism between dye molecules and photocatalysts was explored using the Langmuir adsorption law and proved by FT-IR spectra and adsorption models proposed. (2) Adsorption of reactive brilliant dye K-2BP onto the TiO2/AC photocatalyst was influenced by dye concentration, pH value, and Na2SO4 added. The maximum adsorption efficiency of K-2BP occurred at pH ) 5.7, and the adsorption efficiency decreased with increasing the initial dye concentration ranging from 0.5 to 10 mg L-1. In particular, an inflection point with respect to the pH value of 7.5 and Na2SO4 addition amount at 1.5 g L-1 was observed over the maximum adsorption efficiencies of 19.2%. (3) A high photocatalytic degradation efficiency of K-2BP using a TiO2/AC photocatalyst in BFBPR was achieved under UV light illumination. The apparent reaction rate coefficient varied with dye concentration and was approximated to the maximum value at a concentration of 3.75 mg L-1. The degradation efficiency was approximated to the maximum value at a pH value of 5.7. (4) The observed degradation efficiency of K-2BP increases gently with Na2SO4 added after the amount above 1.75 g L-1 at alkaline suspension. For the added amount of Na2SO4 of 1.75 g L-1, the photocatalysis mechanism was introduced by SO4•- initiation of the dye reactions either in solution or on the catalyst surface. Acknowledgment This investigation was supported by the Doctorate Programs Foundation of Ministry of Education of China (Contract No. 200804260002) and the Construction Project of Taishan Scholar of Shandong Province (Grant JS200510036). The authors also wish to acknowledge the financial support from Open Foundation of Chemical Engineering Subject, Qingdao University of Science & Technology, China. Literature Cited (1) Martin, R. W., Jr.; Baillod, C. R.; Mihelcic, J. R. Low-temperature inhibition of the activated sludge process by an industrial discharge containing the azo dye acid black 1. Water Res. 2005, 39, 17–28. (2) Alinsafi, A.; Motta, M.; Bonte´, S. L.; Pons, M. N.; Benhammou, A. Effect of variability on the treatment of textile dyeing wastewater by activated sludge. Dyes Pigments 2006, 69, 31–39. (3) Fritjers, C. T. M. J.; Vos, R. H.; Scheffer, G.; Mulder, R. Decolorizing and detoxifying textile wastewater, containing both soluble and insoluble dyes, in a full scale combined anaerobic/aerobic system. Wat. Res. 2006, 40, 1249–1257. (4) Joo, D. J.; Shin, W. S.; Choi, J. H.; Choi, S. J. K.; Han, M. H.; Ha, T. W.; Kim, Y. Decolorization of reactive dyes using inorganic coagulants and synthetic polymers. Dyes Pigments 2007, 73, 59–64. (5) Mo, J.; Hwang, J. E.; Jegal, J.; Kim, J. Pretreatment of a dyeing wastewater using chemical coagulants. Dyes Pigments 2007, 72, 240–245. (6) Ramakrishna, K. R.; Viraraghavan, T. Dye removal using low cost adsorbents. Wat. Sci. Technol. 1997, 36, 189–196. (7) Wang, Q.; Chen, C.; Zhao, D.; Ma, W.; Zhao, J. Change of Adsorption Modes of Dyes on Fluorinated TiO2 and Its Effect on Photocatalytic Degradation of Dyes under Visible Irradiation. Langmuir 2008, 24, 7338–7345. (8) Aarthi, T.; Madras, G. Photocatalytic Degradation of Rhodamine Dyes with Nano-TiO2. Ind. Eng. Chem. Res. 2007, 46, 7–14. (9) Zhuang, J.; Dai, W.; Tian, Q.; Li, Z.; Xie, L.; Wang, L.; Liu, P. Photocatalytic Degradation of RhB over TiO2 Bilayer Films: Effect of Defects and Their Location. Langmuir 2010, 26, 9686–9694. (10) Smith, Y. R.; Kar, A.; Subramanian, V. R. Investigation of Physicochemical Parameters That Influence Photocatalytic Degradation of Methyl Orange over TiO2 Nanotubes. Ind. Eng. Chem. Res. 2009, 48, 10268–10276. (11) Ray, A. K.; Beenackers, A. A. C. M. Development of a new photocatalytic reactor for water purification. Catal. Today. 1998, 40, 73. (12) Gerven, T. V.; Mulc, G.; Moulijn, J.; Stankiewicz, A. A review of intensification of photocatalytic processes. Chem. Eng. Process. 2007, 46, 781–789.

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ReceiVed for reView July 18, 2010 ReVised manuscript receiVed September 12, 2010 Accepted September 14, 2010 IE101533X

Adsorption and Photocatalytic Degradation of Reactive Brilliant Red K

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