Adsorption and degradation of Dyes by Novel Ti-peroxy gel (TPG) 1. Introduction Textile industries are among the most polluting industries in terms of the volume and the complexity of treatment of its effluents discharge [37]. Dyes are widely used in dye stuffs and textile industries and because of their toxicity, mutagenisity and nonbiodegradability has danger to the environment. Grzechulska and Morawski were considered that the removal of color from wastewater is often more important than the removal of other organic colorless chemicals [45]. The removal of dyestuff from textile wastewaters is a serious environmental problem which is difficult by conventional methods [46]. The decolourization of dye effluents has received increasing attention, thus various chemical, physical and biological treatment methods have been developed for the removal of dyes from aqueous solutions, including precipitation, coagulation-flocculation, reverse osmosis, oxidation with ozone, chlorine or hydrogen peroxide, use of anion exchange membranes and bacterial cells [38-40]. Adsorption has proven to be a reliable treatment methodology due to its low capital investment cost, simplicity of design, ease of operation and insensitivity to toxic substances, but its application is limited by the high price of some adsorbents and the large amounts of wastewater normally involved [37]. It is therefore highly desired to develop an efficient and cost effective process for the treatment of dye industries effluent. The ineffectiveness of the conventional methods to destroy pollutants such as dyes has led to the development of other efficient wastewater treatment processes such as advanced oxidation processes (AOPs) [41,42]. TiO2 is the most commonly used effective photocatalyst for such environmental applications [42] and its adsorption capacity is a very important factor in heterogeneous photocatalysis because it determines the degradation rates [37]. In most cases, the enhanced photoactivity of semiconductors has attributed to their superior adsorption capacity [43]. Photocatalytic oxidation technology based on TiO2 has been proved effectively for the degradation and mineralization almost all of the organic compounds, and is regarded as one of the most prospectively environmental clearing technologies [1]. However, the conventional TiO2 power materials have the disadvantage of requiring stirring during the reaction process and are difficult to separate out after the reaction [35].Recently; TiO2 films have been gaining much attention as useful photocatalysts

to offer practical benefits. But these approaches are complicated, high-cost. Besides, coatings produced by these ways are weakly adhered to substrate and not uniform [35]. Although, a lot of studies are revealed that TiO2 can not make use of the visible and solar light efficiently, to the best of our knowledge we could successfully find efficient way for dye removal comparing to other methods, also it has higher efficiency among photocatalytic degradation methods, and suggest a simple and efficient solution to solve environmental and energy problems specially in the field of dyes effluent treatment. For fabrication of titania photocatalysts, the sol-gel method has been used almost exclusively; however, it requires a subsequent thermal treatment to induce crystallization of the formerly amorphous titania gel, which results in particle agglomeration and grain growth and hence reduction in specific surface area [2,3]. The effect of adsorption of organics substrate on the surface of TiO2 has become a focus of attention since the early study of the photocatalytic oxidation. Both the reaction mechanisms, under either UV or visible irradiation, suggest that preliminary adsorption of organic substrate on the TiO2 surface exhibits an advantage for high efficient oxidation [1]. The adsorption modes of the organics substrate on the surface of TiO2 have been found playing an important role in the photodegradation pathways of organics [4, 5]. Electron holes, OH, and O2_ are extremely reactive upon contact with organic mpounds. Complete oxidation of organic compounds to carbon dioxide could be achieved [6, 7]. In this study, Titania peroxy gels (TPG) were prepared, using hydrogen peroxide solutions and used without any further thermal treatment and calcinations to remove the dyes from aqueous solution. Here this catalyst activity examined with 11 dyes (Methylene Blue (MB), Methyl orange (MO), Methyl Red (MR), Methyl Violet (MV), Malachate Greeen (MG), Bromophenol Blue (BB), Dimethyl yellow (DMY), Ink, rhodamine B (RB), a xanthene dye, Basic magenta (BM) and Para base (Pb) solution in water) and actual effluent of industry. The effects of the zeta potential and viscosity of the gel and abilities to adsorb and photodegrade dyes solution in water under sunlight and room condition are both studied in detail. The adsorption of all mentioned dyes, Considerable degradation (100%) of all dyes over TPG under both light sources is achieved, this novel gel has amazing activity

under both condition and the degradation of dyes is very fast in the sunlight. It is necessary to clarify the mechanism of TPG and the different photocatalytic degradation mechanism in room condition and sunlight for different dyes. To the best of our knowledge, there have not been reports on the study on degradation of organic pollutants using TPG. Song et al [23] studied the photodegradation of aqueous RB (100ml, 2.5*10-6M= 1.2mg/l) by low-temperature deposited anatase thin films, while in this study we used initial concentration of 47.9mg/l, almost 50 times of their concentration. Li et al [24] studied degradation of RB in a solution containing 5mg/l by a PhotoElectrochemical Process under visible light and achieved 87 % removal RB (5mg/l) after 120min. 2. Experimental 2.1. Materials Titanium titanium butoxide (Aldrich make) and 30 % aqueous hydrogen peroxide (Qualigen make) were of analytical reagent grade quality. The suggested composition of the chelate is [Ti (IV)O22- -Ti(IV)O2- -Ti(IV)(OH)4]aq [8,9,10]. The dye Rhodamine B (RB), Basic Magenta (basic violet 14) (BM) and Para Base (Vinyl Sulphone) (PB) were bought from Deccand dye industry and dyes solutions 10 4

molar were made by this powders. The structure of the RB, BM and PB molecules

are illustrated below. The RB, xanthene dye, has become a common organic pollutant [26]. It is a very stable non-volatile dye usually used in factories, has a comparatively high resistance to photo and oxidation degradation [27, 28]. Basic magenta also called Fuchsine, is brown red crystal and aqueous red, water, ethanol and acid-soluble. For cotton and man-made fibers, paper, printing and dyeing leather, but also for the painting, such as ink. Magenta can combine with sulfur dioxide into instability colorless material, a longer period of time or when the heat decomposition, and red. Magenta can also serve as a distinction and Ketones a reagent. Aniline, o-toluidine, a pair of nitrobenzene, aniline iron and zinc chloride in the presence of heat may produce.

Molecular Structure of Rhodamine B

Molecular Structure of Basic magentaO H 2N S O O O S O OH

Molecular structure of Para Base

No. Name 1 Rodamine RB B

Formula C28H31ClN2O3

Structure

Formula Type max weight 479 Basic, 551nm Xanthene dyes 337.85 Basic 211nm

2

Basic magenta Para base

BM C20H20ClN3O H 2N S O O O S O OH

3

PB

C8H11NO6S2

281.31

Basic

538

No . 1

Name Methylene blue (MB)

Formula C16H18ClN3S

Structure

Formula weight 319.85

Type Basic

max

2 Methyl orange(MO) 3 Methyl red(MR) 4 Bromophen ol blue(BB) C19H10Br4O5S C15H15N3O2 C14H14N3NaO3S

327.32

Azo dye Acidi c, Azo dye Acidi c dye

654 ,61 2 464 .5

269.3

669.96

592 ,43 5

5 Dimethyl yellow(DM Y) 6 Basic magenta(B M) 7 Para base(PB) 8 Rodamine B(RB) C28H31ClN2O3 C8H11NO6S2 C20H20ClN3O H 2N S O O O S O OH

225.29 C14H15N3 337.85

Azo dye Basic

515

211

281.31

Basic

538

479

Basic, Xanth ene dyes

552

9 Methyl violet(MV) 10 Malachite green(MG) C23H25ClN2 C24H28N3Cl

394

Basic

581

365

Basic (Trip henyl metha ne dye)

619

11 Ink

590 , 302

2.2. Synthesis of photocatalyst (Titanium peroxy gel) by sol-gel technique

TPG was prepared by sol-gel technique using titanium peroxide. 10.4 g titanium butoxide, Ti[O(CH2)3CH3]4 (Aldrich) was hydrolyzed with 50ml distilled water. 100 ml 30 % aqueous hydrogen peroxide (RanChem) was added to get a, transparent orange sol of titanium peroxide, 500ml distilled water also added during reaction of H2O2 with titanium hydroxide to maintain the temperature and dilution of the gel as well. A transparent yellow gel was formed and this only used as catalyst i.e. drying, deposition and calcinations steps are deleted for the preparation of catalyst and the gel was directly used without any further thermal treatment. 2.3. Photoreaction procedures and analytical methods Aqueous solution of the dye (50 ml, 104 M) was placed in the presence of photocatalyst (10gr) in a 250ml Pyrex vessel. Prior to irradiation, the dispersion was magnetically stirred in the dark for about 5 min to establish the adsorption/desorption equilibrium between the dye, photocatalyst and oxygen and also to measure the capacity of catalyst to adsorb the dye. Irradiation of the dispersions was carried out at two conditions, room light and under the sunlight. All the photocatalytic experiments were carried out at initial pH without any pH adjustment and the pH of catalyst itself was measured about 3. The -potential of photocatalysts was measured during formation of the gel with a Zeta Puls Analyzer (.). Photocatalysts were placed in a cell to determining of zeta potential in different interval time. All water used for the -potential measurement was deionized water. The dye concentration in solution can be determined by measuring the absorbance using a UVVis spectrophotometer. Calibration curves were obtained by using the standard dye solutions with known concentrations. The temporal absorption of the filtered reaction solution was analyzed by UV-VIS spectroscopy using a UV-VIS spectrophotometer (UV-2101). The dyes and intermediates generated in the photooxidative process were analyzed in IR and GC. Mineralization rate of dyes was determined by COD analysis.

2.4. Dark adsorption studies Sorption experiments

All sorption experiments were performed on shaker with 10g of the TPG in a 250mL beaker containing 50mL of dye solutions. Adsorption experiments were conducted at lab temperature to examine the sorption kinetics and equilibrium. In the sorption kinetic, the concentration of the three dyes was 104 mol L1. The concentration of the dyes after adsorption was determined by measuring their characteristic absorbance using UVvis spectrometer, and the characteristic absorbance of RB, BM and PB are 551, 211 and 538 m, respectively. All equilibrium sorption experiments were conducted at room condition by contacting 50 ml of dye solution with 10gr of adsorbent (TPG). The solutions were stirred until equilibrium is reached. Results of adsorption kinetic experiments indicated that almost 99% of dye sorption was achieved within 5min stirring. After equilibrium, the samples were filtered and analyzed by UVvisible spectroscopy to determine the residual equilibrium liquid-phase dye concentration. The equilibrium adsorption capacity, qe (mg/g), at different dye concentrations was determined by a mass balance on the dye: qe = V(C0- Ce)/m Where C0 (mg/L) is the initial concentration, Ce (mg/L) is the equilibrium concentration in the liquid phase, V is the volume of liquid phase (L), and m is the mass of the adsorbent (g). 2.5. Sunlight experiment Photocatalytic degradation experiments The photocatalytic activity of the catalyst was evaluated by measuring the degradation ratio of dyes. The light sources used were sunlight and room light. The catalyst TPG (10 g) was added to 50 mL of dye (10-4 mol/L) solution in beaker and then stirred with a magnetic stirrer prior to irradiation in the dark for adsorption/desorption equilibrium. The degradation ratio was determined using UV-vis spectrophotometer and the absorbance at different interval time was recorded.

2.6. Photocatalytic degradation of RB Rhodamine B (C28H31N2O3.Cl) with molecular weight of 479.02 is an amphoteric dye, although usually listed as basic as it has an overall positive charge. Figure 1 is a typical photocatalytic degradation process of RB. The concentration changes of the RB (initial concentration: 10-4 mol/L, 50 mL) taking place in the presence of TPG (10g). Before the irradiation, the maximum absorption of RB was 552 nm. The absorption diminished very fast and disappeared after first 5min stirring. Examination of the spectral variations of RB suggest that RB is firstly adsorbed to the catalyst and then decomposed in a short period of time. Figure 4 shows the degradation ratio of RB under sunlight. The degradation velocity under sunlight irradiation is much faster than room condition. The absorbance of the adsorbed Rhodamine B can be used to calculate the surface concentration of the dye and to determine the quantum efficiency of electron transfer [11]. 2.7. Treating dyeing industry wastewater 10g of the Ti-peroxy gel was added into 50mL dye wastewater obtained from Deccan dyeing industry in Pune, India. After adsorption for different interval time, the colored gel was separated, and the concentration of the dyes in the filtrate was analyzed by UVvis spectrophotometer.

3. Result and discussion 3.1. Characterization of photocatalysts and Photooxidation pathway of RhB on the surface of TPG The results illustrate the presence of three different coordination of the Ti-superoxide radical in aqueous Ti-peroxy systems. In the TPG the radical is observed both at low and at high pH [13]. An aqueous titanium gel is formed when excess peroxides have been consumed [14-16]. Since one important component of this gel may be the titanium-peroxy-radical, we call the gel a Ti-peroxy gel [17].

The composition of the gel is according to Tengwall et al [16] suggested to be:Ti(IV)O22-(OH-)x - Ti(IV)O2- (OH-)y - Ti(IV)(OH-)4 - Ti(IV)O2 . n. H2O

The gel thus contains a superoxide or perhydroxyl radical, which may be formed in the gel and is stable for a long period of time[3,4].The gel has oxidizing properties with a redox potential larger than that of the Fe(II)/Fe(III)-couple (0.77 V)[17]. The pH value strongly influences the surface charge properties of metal-oxide particles in the aqueous dispersion. Normally, surface of the net TiO2 is positively charged in acid media and negatively charged in alkaline one with an isoelectric point of about pH 6 -7 [12]. Thus, adsorbates in the aqueous solution tend to adsorb on the surface of TiO2 by the negatively charged or electron abundant group in acidic solution because of the electrostatic interaction. In this study the pH of the catalyst in the form of TPG was about 3 and it was negatively charged in this acidic condition versus to the net TiO2 powder which mentioned in other study [1]. The isoelectric point of TiO2 is relative acidic (only about pH 3-4) comparing with that of TiO2 powder.Therefore, under the experimental condition (pH 3), the surface of TPG is negatively charged. However, the net TiO2 is positively charged. Accordingly, due to the electrostatic interaction, adsorption of RB molecules on the surface of TiO2 proxy gel is through positively charged diethylamino group, but in the case of net TiO2, RB molecules tend to adsorb via the negatively charged carboxyl group. The speculation model of simple adsorption for the RB molecules on the surface of TPG in the experimental condition is illustrated in figure 1.

TPG

(-)

Figure 1: Adsroption pathway of RB on TPG surface (RB+ & TPG- )

3.1.1. -potential of Titanium peroxy gel

Figure 2: Mobility of a TPG with 40 zeta potential Gels are dispersion of macrmolecules in liquid and a colloidal system. Zeta Potential is a very good index of magnitude of the interaction between colloidal particles and Zeta Potential measurement are used to assess the stability of colloidal systems. Strong negative or positive surface charges stabilize a suspension and avoid particle aggregation [44]. It says that the stability of a colloidal system is determined by sum of the electrical double layer repulsive and van de Waals attractive forces which the particle experience as they approach one another. The -potential of TPG, photocatalysts, was measured during formation of the gel in different interval time. Figure 2 shows the mobility of TPG with high zeta potential and consequently high performance. Different batches of gel prepared by different portion of water, H2O2 and Titanium Butoxide, to study the variation of Zeta Potential with different concentration and to determine the capacity of gel to hold the water molecules. The zeta potential highly related to the viscosity of the gel, and as the viscosity of gel increases, zeta also increases. Apart from this, the zeta is highly depending on the method of preparing the gel and controlling of the heat during preparation. For the comparison of the ability of gels with different potential, same dye solution with the same volume at the same time were added to the catalyst and it was absorbed that TPG with higher zeta potential adsorb the dye faster and also ability and capacity of the gel to adsorb, degraded and regeneration is faster. Therefore, as the zeta potential increase the capacity of catalyst to adsorb the dyes

also increase. Another advantage of this gel is that it has acidic pH around 3 and it is the point which shows higher zeta potential.

3.1.2. UVVisible analysis The UV of prepared TPG was recorded before and after colour removal. 3.1.3. SEM analysis From the result of SEM it absorbed that the dyes adsorbed to the surface of catalyst only. 3.1.4. Process Features and mechanistic aspects Mechanism of Titanium peroxy gel The reaction product of H2O2 and metallic titanium, said reaction product being a titanium peroxy radical and titanium peroxide, together with titanium hydroxide as important in its structure [18]. The gel like product offers optimal properties, thanks to its double-oxidizing effect, i.e. by means of both radical and H2O2. The gel proposed according to invention decomposes in two stages as follows: Ti(IV)O2- + e- Ti(IV)O22Ti(IV)O22- + 2H+ Ti(IV)(OH)x- +H2O2 The Ti-peroxy radical (Ti (IV)O2- ) has approximately the same redox-potential as H2O2 [18]. Metallic titanium is suitably immersed in H2O2 solution and, when the solution has ceased to give off oxygen, the gel formed can be used for the colour removal. The gel formation time is depend on the ratio of Titanium butoxide, hydrogen peroxide and water and most important heat control of solution during reaction of Titanium hydroxide with hydrogen peroxide. As the time of gel formation increase quality of formed gel also increases. The reaction occurs within a pH interval of 1-4, preferably value 3.5, and the gel is formed without intermediary steps. A catalytic decomposition of peroxide, i.e. hydrogen peroxide and titanium peroxide occurs at the surface. When the peroxide has been decomposed the solution forms a

gel at pH value 3 or above [18].The transparent yellowish gel thus obtained is free from salts and vital elements of polymeric structure. The gel thus produced has been found to be free other complexing ions such as sulphate ions, chloride ions, etc. the gel is decomposed through chemical reduction to hydrogen peroxide and titanium hydroxide. In principal the gel thus acts as slowrelease hydrogen peroxide reservoir [18]. The gel has also ability to oxidized thiol groups [18]. This gel is soft, hydrophilic, and insoluble in water. In the first step the dyes molecules adhere to the surface of TPG, during biding processes. Its encapsulation in the gel also ensure that the titanium body is hydrophilic, free from carbon impurities and has a surface that is saturated with respect to hydrated titanium dioxide. This means that in a pure water solution, the titanium will not leach out more titanium ions before those already present are transported away [18]. Thanks to the oxidizing agent action of the titanium peroxy radicals in this gel, as well as the decomposition product-hydrogen peroxide- in corporation with the peroxadasehalogen system. 3.2. Adsorption kinetics The mechanism for the sorption of the three dyes on the biomass may be assumed to involve the following sequential steps [29, 30]: 1. Migration of dyes from bulk of the solution to the boundary layer of the adsorbent. 2. Diffusion of dyes through the boundary layer to the surface of the sorbent. 3. Intraparticle diffusion of dyes into the interior pores of the sorbent particle. 4. Sorption of dyes at an active site on the exterior or interior surface of biosorbent [31]. Fig. 6 illustrates the sorption kinetics curves of the three dyes. The removal rates were all very rapid during the initial stages of the sorption process. After a very rapid sorption, uptake rates slowly declined with lapse of time and reached equilibrium values at about 5 min for all three dyes. The three phases of the dyes sorption could be attributed to boundary layer diffusion sorption, intraparticle diffusion sorption and sorption equilibrium, respectively. The high initial uptake rate and the short adsorption equilibrium time demonstrated that the surfaces of gel have a high density

of active sites for dyes adsorption. Hence, a practical advantage of using the Tiperoxy gel as an adsorbent would be in its ability to remove more dyes in a much shorter adsorption time. On the other hand, it could be seen that the adsorption rate followed this order: PB > BM> RB. Higher molecular weight, larger ionic size and the presence of carboxylic group were the probable reasons for the lower sorption of RB [31]. The adsorption occurred mainly on the gel surface, it could be observed by SEM (Scanning Electron Microscopy) and also by simply breaking down the pieces of gel we could see that the dyes adsorbed on the surface of the gel and therefore stirring of dye solution make the gel into small particle and consequently increase the surface area and the speed and rate of dyes removal, although this happen without stirring but in a longer time. Adsorption occurred mainly through bonding between the TPG and positive groups in the dyes (eg. Diethylamino group in RB), and some electrostatic interactions between the negatively charged groups of gel surface and the cationic dye molecules. 3.3. Photocatalytic activity study Here we can assume 2 step pseudo-first-orders for adsorption of dye to catalyst and in the second part first order for degradation of dye. The photocatalytic degradation reaction of RB in water can be assumed to follow a pseudo-first-order expression [34], LnC0/C= kt Where c/c0 is the normalized RB concentration, t is the reaction time, and k is the apparent reaction rate in term of min-1. Therefore, the pseudo-first-order reaction rate constants can be calculated using the corresponding photodegradation data. Figure 4 illustrates the pseudo-first-order reaction rate constants of the photodegradation reaction in the presence of the TPG. (Note : here because of sharp adsorption of dyes into TPG finding the order is difficult but if we use less gel with the same volume of dys solution we can find the order.) Figure 3 and 4, show the absorption spectra of standards dyes solutions and relation between dye concentration- absorption respectively. The concentration residual of dye solution after reaction therefore can be calculated by this equation: Y= 14.012947X-0.78576144 (y= 14x-0.8) This equation is found by plotting regression line of standard solutions concentration.

As we can see from table 3 , after only 5minthere is a sharp reduction of RB and we achieved 99.3% reduction of RB in the solution and after 2hr almost 100% removal.

No. Time 1 2 3 4 5 6 (min) 0 5 15 30 60 120

Conc. (mg/l) 46.1 3.64 2.7 2.08 1.16 0.124

Conc. (molar) 10-4

Absorption 3.35 0.317 0.250 0.206 0.140 0.066

Reduction (%) 0 99.3 99.5 99.6 99.7 99.9

Table 3: Residual of RB after adsorption experiment by TPG

3.5 3.0 2.5

RB, 10(-4)M RB, 0.5*10(-4)M RB, 0.2*10(-4)M RB,10(-5)M RB, 10(-6)M RB, 10(-7)M discolored outlet of TiO2 gel column

Absorption

2.0 1.5 1.0 0.5 0.0 -0.5 200 300 400 500 600 700 800

Wave length(nm)

Figure 3: Standards Curve of Rhodamine B solution

S = 1.56479606 r = 0.996786465 2. 4 3. 69 91 13 35 56 8 0

Absorption

3 5. 2 6. 1 7.

8 .7 0 .0

0 .0

0 .6

1 .2

1 .8

2 .5

3 .1

3 .7

C o n c e n tr a t i o n ( m g /l )Figure 4: relation between concentration and absorption for Rodamine B

3.5 3.0 2.5

RB10-4M RB5min RB15min RB30min RB1hr RB2hr RB24hr

Absorption

2.0 1.5 1.0 0.5 0.0 -0.5 200 300 400 500 600 700 800

Wavelenght(nm)

Figure 5: Absorption spectrum of RB solution (10-4mol/L, 50mL) before and after reaction with TPG at room

0.40 0.35 0.30 0.25

RB5min RB15min RB30min RB1hr RB2hr

Absorption

0.20 0.15 0.10 0.05 0.00 200 300 400 500 600 700 800

Wavelenght(nm)

figure 6: Absorption spectrum of RB solution (10-4mol/L, 50mL) before and after reactionwith TPG at room

3.3.1. Effect of the photocatalyst amount With increasing the amount of photocatalyst the adsorption of dyes and regeneration of the photocatalyst is faster. 3.3.2. Effect of irradiation time With the increase of irradiation time the rate of dye removal increase

3.3.3. Effect of temperature on the adsorption The equilibrium uptakes as a function of temperature are given in Table 3 for the studied dyes. In this experiment, the concentration of the three dyes was 104 mol L1, and the solution pH was not controlled. For BM and PB the equilibrium uptakes increases with increasing temperature up to 35 C. Temperature could affect desorption step and consequently the reversibility of the adsorption equilibrium. In general, an increase in temperature is followed by an increase in the diffusivity of the ion, and consequently an increase in the adsorption rate if diffusion is the rate

controlling step [31, 33]. So, BM, PB and RB sorption may be controlled diffusion, the adsorption capacity increased with the increasing temperature.

0.20

BM5min BM15min BM30min BM1hr BM2hr

0.15

Absorption

0.10

0.05

0.00 200 300 400 500 600 700 800

Wavelenght (nm)

Figure 7: Absorption spectra for BM at different interval time of stirring at 25C

0.35 0.30 0.25

BM, 5min stirring BM,15min stirring BM,30min stirring BM, 1hr stirring BM,2hr stirrring

Absorption

0.20 0.15 0.10 0.05 0.00 200 300 400 500 600 700 800

Wavelenght (nm)

Figure 8: Absorption spectra for BM at different interval time of stirring at 35C

99.70

reduction % at 25C Reduction % at 35C

99.65

Removal of BM (%)

99.60

99.55

99.50

99.45

99.40 0 20 40 60 80 100 120

Time interval (min)

Figure 9: Removal percentage of Basic Magenta at 2 different Temperature

3.4. Treating actual wastewater The adsorption capacity of the Ti-peroxy gel in treating dye wastewater was studied. The UVvis spectrum of the diluted wastewater, whose concentration is one-tenth of that of the original dye wastewater, is shown in Fig. 10. After dealing with the Tiperoxy gel, the wastewater became colorless, and the peaks of the dyes disappeared, indicating the efficiency of the gel in treating dye wastewater. These results indicate that the Ti-peroxy gel would have great potential in practical applications for the removal of dyes from wastewater. 3.5. Photodegradation mechanism The adsorption of organic molecules on the surface of photocatalysts had been proved to influence greatly the photocatalytic degradation of the organics [19-22]. TPG was investigated in this work to understand the influence of adsorption mode and intensity in the degradation of different dyes. The degradation of dyes in the presence of TPG under the sunlight irradiation and in room condition was depicted. Whats more, in the presence of sunlight, degradation of dyes was greatly accelerated.

The ratio among the UV-vis absorption disappearance rate of RB, BM and PB was . at the beginning of the adsorption (in the first 5 min). One of the main reasons may be due to the much stronger adsorption of dye on the surface of TPG, more than 99% of the dyes adsorbed on the surface of TPG the first 5 min stirring. Moreover, the reacted solution of RB exhibited a much more evident hypsochromic shift of absorption bands (Figure 2b) than that in the PB and BM. While the RB solution left the % of the original absorbence, there were 55nm hypsochromic shift for the RB and nm hypsochromic shift for the PB and BM. IR spectrum was then employed to further study the difference in treated dyes of the photooxidation intermediates. Figure shows the IR spectral patterns for treated RB and the degradation products in the presence of TPG. There was no extra peak related to the intermediate and it was blow detetion limit of IR By identification of the intermediates can be confirmed that the deethylation process was proceeded: RB molecule lost the ethyl groups step by step to transform to products and the final mineralization. The adsorption modes of RB on the surface of TPG greatly influence the photocatalytic degradation mechanism of the RB. RB molecules adsorb on the surface of TPG through diethylamino group, while they tend to adsorb via carboxyl group in the case of RhB-TiO2 powder [1]. Therefore, active oxygen species predominantly attack the chromophore ring structure and induce the cleavage of the ring structure of RB molecule in the RB-TiO2 case [1]. And those in the RB-TPG case mainly attack the auxochromic groups and induce the deethylation of the alkylamine group as shown in Scheme below.

COO-

N(C 2H5 )2

+N(C2H5 )2

----------------------------------TPG(negatively Charged) The same phenomena were also found in the RB degradation under sunlight. In comparison with that under the room condition, the degradation rates of dyes are higher under the sunlight. These differences could be due to the difference of the active oxygen species generation mechanism between these two cases. Thus, in this

case, the adsorption of RB is very important since it influences the electron injecting process from the excited RB to TiO2. Under sunlight irradiation, TPG itself absorbs the photons and then directly react with H2O, O2 and OH group to produce active oxygen species. As shown in Figure , TPG under sunlight exhibited a higher activity than TiO2 under room light. It is well-known that the RB degradation occurs via two competitive processes: one is N-demethylation, and the other is the destruction of the conjugated structure [25]. 3.6. Mineralization studies of dyes As the reduction of COD reflects the extent of degradation or mineralization of an organic species, the percentage change in COD was studied for dyes samples before and after reaction [36]. The results presented in Figure show that the photocatalytic process leads, apart from decolorization, to a substantial decrease in the COD of the solution. The COD reduction is less than percentage decolorization which may be due to the formation of smaller uncoloured products. Therefore, it seems that to achieve complete mineralization of dyes, a longer irradiation time is required. 3.7. Reuse of photocatalyst Reuse of TPG was separately studied, after regeneration under the sunlight or whitener. During this study, after adsorption and degradation of dye, when the Colored TPG also becomes colorless, under the sunlight irradiation, photoreaction solution was filtered. Filtrate was used for analysis COD determination or UV spectrophotometery and TPG residue was used for dye removal for more cycles. In the case of whitener after the color was remove by TPG, the colored TPG was washed firstly with diluted whitener and then after decolorization of TPG, it was washed several times with distilled water to remove the interference of whitener. Recovered TPG was then reused for new photodegradation batch, without any further treatment. Adsorption and photo catalytic degradation experiments are carried out in duplicate, under sunlight. Activity of recycled TPG was found to retain almost same after 3rd regeneration. Note (I ll give data here) 4. Conclusions

TPG (Ti-peroxy gel) was prepared through a simple method. Kinetics experiment illustrated that the adsorption process could be completed in 5 min. The obtained gel has great potential in practical use. Removal efficiency of TPG was found to be greatly affected by zeta potential and the acidic pH of the gel also helps to stabilize the zeta potential along time. Easy preparation and regeneration of TPG without any thermal treatment and calcinations were favorable for catalytic degradation. Dyes solutions of 10-4M concentration were completely mineralized by TPG. It can be regenerated and reused for more times. 7. References [1] F. Chen, J. Zhao, H. Hidaka1. International Journalof Photoenergy(5)2003 [2] Lee, S. H.; Kang, M.; Cho, S. M.; Han, G. Y.; Kim, B. W.; Yoon, K. J.; Chung, C. H. J. Photochem. Photobio. A: Chem. 2001, 146, 121-28. [3] Kang, M. Appl. Catal. B: Environ. 2002, 37, 187-96. [4] G. Liu, X. Li, J. Zhao, H. Hidaka, and N. Serpone, Environ. Sci. Technol. 34 (2000), 3982. [5] P. Qu, J. Zhao, T. Shen, H. Hidaka, and N. Serpone,J. Mol. Catal. A 129 (1998), 257. [6] Jacoby, W. A., P. C. Maness, E. J. Wolfrum, D. M. Blake, and J. A. Fennel. 1998. Environ. Sci. Technol. 32:26502653 [7] Legrini, O., E. Oliveros, and A. M. Braun. 1993. Chem. Rev. 93:671698. [8] Tengvall, P., Lundstr6m, I., Sj6kvist, L., Elwing, H., and Bjursten, L. M., iomaterials 10, 166 (1989). [9] Naccache, C., Meriadeau, P., Che, M., and Tench, A. J., J. Trans. Faraday Soc. 67, 506 (1971). [10] Shiotani, M., Moro, G., and Freed, J. H., J. Chem. Phys. 74, 2616 ( 1981 ). [11] M. Spitler and M. Calvin, The Journal of Chemical Physics, Vol. 67, No.11, 1977. [12] J. Zhao, H. Hidaka, A. Takamura, E. Pelizzetti, and N. Serpone, Langmuir 9 (1993), 1646. [13] Letter to the Editor, Journal of Colloid and Interface Scienced, Vol. 143, No. 2, 1991

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fig: standards graph of Basic Magenta fig: standards graph of Para Base

5

4

bm bms1 bms2 bms3 bms4 bms5 bms6

Y Axis Title

3

2

1

0 200 300 400 500 600 700 800

X axis title

0.40 0.35 0.30 0.25

bms1 bms2 bms3 bms4 bms5 bms6

Y Axis Title

0.20 0.15 0.10 0.05 0.00 -0.05 200 300 400 500 600 700 800

X axis title

5.0 4.5 4.0 3.5 3.0

Para base 10-4M PB5min PB15min PB30min PB1hr PB2hr PB24hr

Absorption

2.5 2.0 1.5 1.0 0.5 0.0 200 300 400 500 600 700 800

Wavelength(nm)

fig: Absorption of Para Base after stirring in different interval time