Shao Linlin,  Yang shengke,  Wang wenke

Environmental science and engineering College, Chang^’an University, xi^’an 710054,China)

Abstract:The effect conditions of the photocatalytic degradation of caffeine has been studied by using UV lamp as light-source and nano-TiO₂ in solution as catalyst. The results indicated that the optimum physicochemical conditions necessary for the degradation of caffeine were 10mg/L of caffeine,1g/L of  nano-TiO₂,10.89 of pH and 1g/L of H₂O₂.Moreover,photodegradation kinetics of caffeine has been explained in terms of Langmuir-hinshelwood kinetics model. The degradation rate constant and adsorption equilibrium constant of caffeine were calculated as k 0.1003mg/(L·min) and K 0.0274 L/min.

Keywords: nano- TiO₂,caffeine,photocatalytic，degradation

1 Introduction

In recent years, applications of semiconductor photocatalysts for the total destruction of organic compounds in polluted air and wastewaters have been one of most active areas in heterogeneous photocatalysis[1]. The use of TiO₂  photocatalyst for environmental clean-up has been of great interest due to its non-toxic nature, photochemical stability and low lost when sunlight is used as the source of irradiation. When TiO₂ is exposed to light of energy equal to or greater than the band gap energy (3.2ev for anatase TiO₂),an electron is excited from the TiO₂ valance band into the TiO₂ surface where they initiate the redox reactions with organic species adsorbed on the surface. Many organic contaminants can be oxidized into CO₂ and H₂O and so on by UV/ TiO₂[2-7].The aim of this study is to evaluate the effect conditions of photodecomposition caffeine in aqueous suspension of illuminated TiO₂.Photodegradation kinetics was also studied and discussed in terms of Langmuir-hinshelwood model.

2 Material and Methods

2.1Material

UV-Visspectrophotometer(UV-2450,ShimadzuCorporation),quartzUVlamp(ZSZ-20,Beijing),magneticstirrer(QZD-1,chongqing),pHmeter(LP115,Mettler-Tolendo ),centrifuge(TGL-16C,shanghai).TiO₂ used asanatase (particle size<30nm,jiangsu).Caffeine was purchased from the second reagent factory of shanghai.All chemicals used in the work were reagent grade. Deionized and doubly distilled water was used for the preparation of all solution.

2.2Methods

In each experiments, the glass trough (245×195×148mm)as reactor filled with 1L caffeine solution and some dosage of nano-TiO₂.A magnetic stirrer was located at the reactor’s base such that a heterogeneous TiO₂ suspension could be maintained during the operation. The distance between the surface of the UV lamp and the solution was 10.0cm.The samples were cengtrifuged at 12000rpm for 5 min. The clarity solution of caffeine was used to obtain the UV spectrum, the maximum absorption wavelength(λ_max) selected to detected caffeine was 206 nm ,calculating the concentration of caffeine according as Lambert-Beer's law.

3 Result and Discussion

3.1The UV-VIS spectra of caffeine

The observed of UV spectra of caffeine under different concentrations of H₂O₂ is shown in Fig.1.The spectra of solution which only has caffeine shows two maximum absorption peak at206 and 207nm.As the solution is irradiated by UV lamp, the concentration of caffeine become diminution for nano- TiO₂ photodecomposition. After the H₂O₂ is injected into the solution, the absorption spectra of caffeine has been varied, one is that absorption peak of 273nm disappeared while the concentration of H₂O₂ is raised; two is the 206 nm absorption peak gradual red-shift, the biggest red-shift reaches to 7 nm; three is that the adsorption spectral intensity of caffeine increases significantly when the H₂O₂ is joined into the solution , the spectral intensity become a direct proportion to the joining quantity of H₂O₂.Because when the H₂O₂ is joined into the solution, it bores abundantly hydroxyl radical(OH•) to impact the structure of caffeine, which lead to the absorption peak variation. Comparing curve 2 and curve 6 to find, two absorption peaks at206 and 207nm disappearance in the photodecomposition caffeine solution when the H₂O₂ is joined into the system, indicating that the decomposition of H₂O₂ and irradiation generating hydroxyl radical(OH•) is the main factor to cause the caffeine degradation .

1—0g/LH₂O₂；2—0g/LH₂O₂ irradiation 6h；

3—1g/LH₂O₂；4—2g/LH₂O₂；

5—5g/LH₂O₂；6—1g/LH₂O₂ irradiation 6h；

Fig1.Absorption spectra of caffeine

3.2Effect conditions of the degradation of caffeine

3.2.1Effect of irradiation time and the initial concentration

Fig.2 shows the times courses of the photodecomposition of caffeine for different initial

concentrations with normalized concentration C/Co. Under certain conditions along the time variation, the caffeine is decomposed gradually ,the highest degradation rate is in 0-3h scope; the degradation rate is relative slowly exceed 3h;the lowest degradation rate is after 5h,so the irradiation time is selected 6h in the experiments. The degradation ratio of different initial concentration of caffeine indicated that the degradation rate of caffeine is relative highly as the initial concentration of caffeine ≤10mg/L,but as the initial concentration of caffeine≥10mg/L,the degradation ratio of caffeine is marked droply. Because the absorption catalyst is saturated in the high concentration of caffeine, the degradation ratio of caffeine isn’t controlled by the concentration of caffeine but to determine by the quantity of electron-hole pairs on the surface of TiO₂.Electron-hole pairs generation is determined by the dosage of TiO₂. Therefore when the dosage of TiO₂ was definition ,the total photodecomposition of caffeine is relative stably, but the degradation ratio of caffeine is descended.

Fig2.Effect of irradiation time and the initial concentration

3.2.2Effect of dose of nano- TiO₂

Figure 3 illustrates photodecomposition of 10 mg/L caffeine with the illumination of a UV lamp in the presence of different dosage of TiO₂. The photodecomposition rates of caffeine was increased as the loading of TiO₂ increasing from 0 to 1g/L.Because the quantity of hydroxyl radical (OH•) and the absorption of TiO₂ are increased when the dosage of TiO₂ is increased. So the photodecomposition rates of caffeine are influenced by the active site and the

photo-absorption of the catalyst. Adequate loading of TiO₂ increases the generation rate of electron-hole pairs for promoting the degradation of caffeine and the absorption of TiO₂ is also increased [8-9]. However, when the dose of TiO₂ exceedes 1g/L, although the absorption of TiO₂ is increased, the UV light penetrates lowly and scatters which lead to the generation rate of electron-hole pairs slowly, so the degradation ratio of caffeine is fell down.

Fig,3 Effect of the dosage of nano-TiO₂

3.2.3Effect of pH value

Figure 4 illustrates the degradation of caffeine in TiO₂/UV system under various pH values. The degradation rate of caffeine is the highest as 10.89 of pH. Because the pH of the aqueous solution influences the physicochemical properties of TiO₂, including the charge on the particle, the aggregation number of particles, and the position of the conduction and valence band. The pH has a great effect on the photodecomposition efficiency of caffeine in presence of TiO₂.TiO₂ surface carries a net positive charge (pH_zpc=6.25) at low pH value, while the caffeine and intermediates are primarily negatively and neutrally charged. Therefore, low pH values can facilitate the adsorption of the organic molecule and promote better photocatalytic degradation. But the degradation rate of caffeine is decreased when the pH is so low that the electron-hole pairs recombine easily. However better removal efficiency of caffeine in TiO₂/UV system under alkali condition is found in our experiments. One possible explanation is that the photocatalytic transformation of caffeine does not involve hydroxyl radical (OH•) exclusively. Direct electron transfer and surface sorption reactions also contribute significantly to the disappearance of caffeine in TiO₂ suspensions. Although there is no semiconductor exited in the solution of direct photolysis system, hydroxyl radical (OH•) and hydrated electrons can be formed when water is irradiation with high energy UV light[10-11]. Therefore, higher pH value can provide higher concentration of hydroxyl ions to react with holes to form hydroxyl radicals, subsequently enhancing the photodecomposition rate of caffeine. But the degradation ratio of caffeine is inhibited when the pH value is so high that hydroxyl ions compete with caffeine ions on the surface absorption of TiO₂.

Fig4 Effect of pH value

3.2.4Effect of H₂O₂ concentration

H₂O₂ is added to enhance production of hydroxyl radical (OH•) through a series of redox reaction[1].Figure 5 shows the degradation ratio of caffeine is increased when the concentration of H₂O₂ increases from 0 to 1g/L. The degradation rate of caffeine is the highest as 1g/Lof H₂O₂. The electron-hole pairs recombining rate is reduced by H₂O₂. So H₂O₂ can enhance the removal of caffeine.

H₂O₂+hv→2·OH

H₂O₂+e →·OH+OH^-

However H₂O₂ is a OH^- scavenger, so the excess H₂O₂ , OH^- produced was quenched by the H₂O₂ rather

reacting with the caffeine[].Moreover, as the rate of OH^- generation increased, the reaction between OH^- became faster; even faster than the rate of oxidation of the organic pollutants by OH^-.As a result, the OH^- were quenched by themselves and not available to oxidized the organize pollutants[12-13].

H₂O₂+·OH→H₂O+·HO₂

·HO₂+·OH →H₂O +O₂

Fig.5 Effect of H₂O₂ concentration

3.3Kinetics of photocatalytic degradation of caffeine

The kinetics of caffeine degradation under the above conditions was stated by measuring the quantity of caffeine remaining in the solution as a function of UV irradiation time. The decrease in the concentration of caffeine could be accounted for by a Langmuir-Hinshelwood kinetic model[14-16]:

r=kθ=        (1)

where r is the oxidation rate of the reactant,θthe surface coverge, C the concentration of the reactant, k the rate constant and K the equilibrium constant for the adsorption of the reactant. When the surface adsorption of catalyst is saturated in the high concentration of reactant, the rate of reactant is controlled by the quantity of electron-hole pairs, but the initial concentration has no effect on it. So the degradation react follows the zero-order decay kinetics. When the surface adsorption of isn’t saturated in the low concentration of reactant (KC«1), the rate of reactant is mainly controlled by the initial concentration of reagent, and the quantity of electron-hole pairs has some effect on it. So the degradation rate follows the first-order decay kinetics. Fig 6a shows the photodecomposition of caffeine for different initial concentrations ranging from 1to 30mg/L with the initial degradation rates r_o(r= ). Inversion of this rate expression provides a linear plot:      (2)

In fig 6b,the plot of reciprocal initial rates versus reciprocal initial caffeine concentrations is presented , with which are obtained the slope (kK)^-1 and the intercept k^-1.So the values of the rate constant k 0.1003mg/(L•min) and equilibrium absorption constant K 0.0274 L•mg^-1 could be estimated, respectively, with correlation coefficient 0.9925.

The basic assumption of the L-H kinetic model are that only one substrate may bind at each surface site and there is no interaction between adjacent adsorbed molecules[15].While in the photocatalytic reaction, three possible situations exist[16]:(ⅰ)adsorbed photoactive oxygen interacts with the reactant adsorbed or in solution; (ⅱ) hydroxyl groups and water molecules cover a TiO₂ solid surface, they compete for the active sites of the catalyst; (ⅲ) substrate photodecomposes giving rise to intermediates competitively on the surface of catalyst. All these reflect in not ideal adsorption isotherms and mass transfer problems, so these reactions will require further study in detail. However ,the above mentioned behavior is mainly to recombination reactions of active species but not adsorption properties of the substrate, the use of Lanfmuir-Hinshelwood equation could provide reasonable simulations to the observed degradation kinetics since the behavior of the reaction rate versus reactant concentration could be adjusted to a mathematical expression with it.

                             Fig.6 a The relation between r_o and c_o             Fig.6b Linear transform of reciprocal initial 

                                                                                                 rates  vs reciprocal initial concentrations

4 conclusion

(1) The optimum physicochemical conditions necessary for the degradation of caffeine were 10mg/L of caffeine,1g/L of nano-TiO₂, 10.89 of pH and 1g/L of H₂O₂.

(2)Photocatalytic degradation of caffeine follow first-order decay kinetics, calculating the degradation rate constant k 0.1003mg/(L•min) and absorption equilibrium constant K 0.0274 L•mg^-1.

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