Experimental study and kinetic analysis of oxidant-free thermal-assisted UV digestion utilizing supported nano-TiO2 photocatalyst for detection of total phosphorous☆

Tian Dong *,Jianhua Tong ,Chao Bian ,Jizhou Sun ,Shanhong Xia

1 State Key Laboratory of Transducer Technology,Institute of Electronics,Chinese Academy of Sciences,Beijing 100190,China

2 University of Chinese Academy of Sciences,Beijing 100190,China

Keywords:Thermal-assisted Ultra-violet digestion TiO2 Oxidant-free Kinetic analysis

ABSTRACT A novel thermal-assisted ultra-violet(UV)photocatalysis digestion method for the determination of total phosphorus(TP)in water samples was introduced in this work.The photocatalytic experiments for TP digestion were conducted using a 365 nm wavelength UV light and TiO2 particles as the photocatalyst.Sodium tripolyphosphate and sodium glycerophosphate were used as the typical components of TP and the digested samples were then determined by spectrophotometry after phosphomolybdenum blue reaction.The effects of operational parameters such as reaction time and temperature were studied for the digestion of TP and the kinetic analysis of two typical components was performed in this paper.The pseudo- first-order rate constants k of two phosphorus compounds at different temperatures were obtained and the Arrhenius equation was employed to explain the effect of temperature on rate constant k.Compared with the conventional thermal digestion method for TP detection,it was found that the temperature was decreased from 120 °C to 60 °C with same conversion rate and time in this thermal-assisted UV digestion method,which enabled the digestion process work at normal pressure.Compared with the individual ultra-violet(UV)photocatalysis process,the digestion time was also decreased from several hours to half an hour using the thermal-assisted UV digestion method.This method will not lead to secondary pollution since no oxidant was needed in the thermal-assisted UV photocatalysis digestion process,which made it more compatible with electrochemical detection of TP.

1.Introduction

Phosphorus is an essential and limiting nutrient element for the growth of all life forms[1].In recent years elevated level of phosphorus has been recognized to be a major contributing factor to algal blooms and eutrophication[2].This has caused increasing concerns on the discharge of large quantities of phosphorous into the aquatic environment in the form of industrial and domestic effluent[3].Phosphorus determination is of great importance from both the environmental and nutritional point of view.Total phosphorus(TP)is defined as the sum of orthophosphates,condensed phosphates and organically bound phosphorus[4],and it is one of the key indicators of eutrophication in water environment.The analysis ofTP in water samples(natural,waste,etc.)is especially complex owing to the fact that the phosphorus exists in both inorganic and organic forms.Thus,before the detection of TP,a digestion pretreatment procedure of the water sample must be proceeded to convert all types of phosphorus to orthophosphate,the readily analyzable form.Subsequently,either electrochemical or optical inspection method is employed to detect the concentration of orthophosphate and complete the detection of TP.

The common used procedures for water treatment[5]mainly rely on an elevated temperature digestion with mineral acids and/or oxidizing agents such as perchloric acid[6],nitric-sulphuric acid[7]and peroxydisulfate[8].Among them,the autoclave persulfate oxidation technique is the most common used method for the digestion of TP.This technique was recommended as one of the standard methods for the examination of water and wastewater by the Water Environment Federation(Method4500-P)[9]and the Environmental Protection Agency(Method365)[10].This traditional autoclave persulfate oxidation method is a high temperature oxidation(HTO)process.Another common used digestion method is microwave digestion[7,11],which is also a HTO process.Although microwave digestion method has many advantages such as high speed and high efficiency,strong oxidant agents such as potassium peroxodisulfate,nitric acid and perchloric acid are still needed.Huang and Zhang proposed a neutral persulfate digestion atsub-boiling temperature for totaldissolved phosphorus determination[12].This method enabled the digestion process work at the sub-boiling temperature,which minimized the sample loss due to boiling.However,a relatively long time(16 h)was required in this process.Benson et al.proposed an on-line UV/thermal induced digestion method and flow-injection system for the detection of total phosphorus[13].The developed method used a two-stage photooxidation/thermaldigestion procedure together with oxidant and hydrolyzing reagents.

In the pastdecades,electrochemical sensors have been studied extensively,and some of them have been applied in water quality monitoring[14,15].The properties such as low limit of detection,wide linear range and high selectivity are drawing attentions.However,in the digestion processes described above,oxidants such as potassium peroxodisulfate are often used.The peroxodisulfate could not decompose completely at the condition lower than 100°C and the residual peroxodisulfate will have a significant influence on the electrochemical detection of orthophosphate.The existing methods are not suitable for the electrochemical method for orthophosphate detection.

Photocatalysis has recently appeared as a new emerging technique with the advantages of high efficiency and environmental friendly[16-23],and it has the capability to degrade samples without oxidants.Among many kinds of photo-catalyst,titanium dioxide(TiO2)has often been proposed for the elimination of organic pollutants due to the merits of relatively inexpensive,safe,chemically stable and high photocatalytic activity[22-27].The photocatalytic process is initiated by the photogeneration of hole/electron pairs(e?/h+)by absorption of UV light with energy equal to or higher than the corresponding band gap[27],as shown in Fig.1.An electron will be promoted from the valence band to the conduction band when TiO2absorbs a photon containing the energy equal to or larger than its band gap(energy of the band gap,Eg=3.2 eV).The activation of electrons(e?)results in the generation of“holes”(h+,electron vacancy)in the valence band.In this reaction,h+and e?are the powerful oxidizing and reducing agents,respectively.The strong oxidation power of h+enables it to react with H2O or OH?in water to generate the highly active hydroxylradical(OH·)which is also a powerful oxidant.Most phosphorous compound can be oxidized completely by either the hydroxyl radicals or the holes themselves to innocuous final products(e.g.PO43?,CO2and H2O).

Fig.1.Principle scheme of TiO2-based photocatalysis.

In many photo-decomposition reaction systems,TiO2powders are often used as a photo-catalyst[26,28,29].However,powder catalysts have several problems,such as difficulties in the separation of the catalyst from suspension after the reaction and the prevention of aggregation in high concentration suspensions.To avoid the agglomeration,suspension must be diluted and then the overall reaction rate tends to be slow.These problems can be solved by the immobility of catalyst.TiO2thin films prepared by Sol-Gel method[30],sputter method[31]and chemical vapor deposition(CVD)method[32].But the TiO2film made by these methods has a bad-controlled composition and relatively low catalytic activity.To avoid these problems,the commercial TiO2powders with high catalyze activity are immobilized by epoxide-resin glue on silicon wafer in this work.

This paper presented an oxidant-free thermal-assisted ultra-violet(UV)photocatalysis oxidation method for the total phosphorus digestion.A 365 nm wavelength UV light was used as the UV light source,and the TiO2particles,with high catalyze activity and immobilized on silicon wafer,were used as the catalyst.The digested samples were then determined by spectrophotometry after phosphomolybdenum blue reaction.Sodium tripolyphosphate and sodium glycerophosphate were used as the typical TP components to conduct the digestion experiments.Compared with the conventional thermal digestion method for TP detection,this thermal-assisted UV digestion method decreases the temperature from 120 °C to 60 °C with same conversion rate and time,which enable the digestion process work at normal pressure.Compared with the individual ultra-violet(UV)photo catalysis process[17,26],the thermal-assisted UV digestion method decreases the digestion time from several hours to half an hour.

2.Experimental Section

2.1.Materials and equipments

Analyticalgrade of sodium glycerophosphate(C3H7Na2O6P),sodium tripolyphosphate(Na5P3O10),ascorbic acid(C6H8O6),ammonium molybdate[(NH4)6Mo7O24·4H2O],potassium antimony tartrate(KSbC4H4O7·1/2H2O)and H2SO4was purchased from Beijing Chemical Reagent Company(China).Ascorbic acid,ammonium molybdate,potassium antimony tartrate and H2SO4were used to prepare the colour reagent for the digested water samples.Sodium glycerophosphate and sodium tripolyphosphate were used to prepare the water samples and also to construct the criterion curves for the determination of orthophosphate concentration.Nano titanium dioxide powders were purchased from Yunnan University(China).

All commercial products were used as received without any further purification.The aqueous solution was prepared by dissolving specific amounts of sodium glycerophosphate,sodium tripolyphosphate,ascorbic acid ammonium molybdate and potassium antimony tartrate in ultra-pure water(18 MΩ).Mother solutions containing either Na5P3O10or C3H7Na2O6P were prepared at 100.0 mg·L?1(by mass of P)concentration and then diluted to several concentrations[0.5,1.0,2.0,3.0 and 4.0 mg·L?1(by mass of P)].Na5P3O10is one of the condensed phosphorus compounds and C3H7Na2O6P is one of the organic phosphorus compounds.All the aqueous solution was prepared weekly and stored at 4°C.The solutions were all used without pH adjustment.The initial pH values for Na5P3O10and C3H7Na2O6P with the concentration of 4 mg·L?1(by mass of P)were 7.9 and 8.2,respectively.

The UV light was supplied by a middle-pressure lamp(SB-100P/F/J from Brightstars Corporation,China).A hot plate(KW-4AH from Chemat Technology Corporation,China)was used as the heating equipment.Ultraviolet spectrophotometer(Biospec-nano from Shimadiu Corporation,Japan)was employed to measure the concentration of orthophosphate.

2.2.Catalysts preparation

TiO2film was prepared by spinning coating on silicon wafer.The silicon wafer from the forty-sixth research institute of Electronic Equipment has a diameter of 50 mm.A 300 nm thick silicon dioxide layer was made by thermal oxidation,and then a 300 nm thick silicon nitride layer was made by low pressure vapor deposition(LPCVD).Firstly,epoxide-resin glue was spun on the surface of the wafer with the spin rate of 2000 r·min?1for 1 min.Then 0.05 g TiO2nano-particles were dispersed on the surface of the glue.The TiO2/silicon was then dried at 100°C for 2 h.

The function of the epoxide-resin glue was only to fix the TiO2particles on surface of the silicon wafer.The epoxide-resin glue was inert,resistant to acid and alkali.The characteristics of the epoxide-resin glue were not changed at the temperature less than 150°C.Basically,epoxide-resin glue had no influence on the photo-catalyst property.

2.3.Photo-reactor and irradiation procedures

The photo-catalytic activity of TiO2film was investigated with respect to the photocatalysis digestion of Na5P3O10and C3H7Na2O6P.The reactor was a cylinder made of quartz with the height of15 mm and inner diameter of 55 mm.The solution contained in the reactor for digestion was 9 ml in volume,about5 mm thick.The digestion process of water sample was started by immersing the TiO2nanoparticles-attached silicon chip into the cylinder reactor,and then turned on the UV light and heater.The radiated power of UV light on the surface of the chip was around 7000 μW·cm?2.The temperature of the reaction was controlled by the heater.The digestion temperature was preset by the heater.The temperature of the solutions elevated with the increase of the time after the heater turned on.At the beginning,the temperature elevated quickly as the increase of the time.The acceleration of the temperature became slower and slower as the increase of the time till the preset temperature achieved.4 min,6 min and 8 min were needed for the solutions to achieve 40 °C,50 °C and 60 °C,respectively.

Before the colour reaction,4.5 ml digested water samples were transferred from reactor to centrifuge tube.The TiO2film was used repetitively and showed long-term stability.The silicon wafers with TiO2film was swashed with ultra-pure water for 2 min after each digestion process.After the digestion process,the solutions were cooled down for 15 min,and then 1 ml ascorbic acid solution and 1 ml ammonium molybdate solution were added into the digested solution in order.This mixed solution was kept stable for 10 min after mixed thoroughly.The orthophosphate concentration(by mass of P)of the digested solution was measured by the ultraviolet spectrophotometer at the wavelength of 700 nm against a reagent blank.

The digestion method proposed in this work does not need oxidant agent.Since the rudimental oxidant in the water sample has a remarkable interference for the electrochemical detection of TP,this method proposed is favored for the electrochemical detection method.

2.4.X-ray diffusion(XRD)and scanning electron microscopy(SEM)study

X-ray diffusion(XRD)was performed on a Dmax 12 kW powder diffraction instrument operated at 40 kV and 8°(2θ)scanning rate to study the composition of the TiO2powder.The surface morphology analysis was carried out on S-4800 field emission scanning electron microscope(SEM)produced by Hitachi(Japan).

3.Results and Discussion

3.1.XRD and SEM examination of TiO2 film on silicon wafer

XRD analysis was performed to identify the phase structure and composition of nanotitanium dioxide powders purchased from Yunnan University,China,as shown in Fig.2.The TiO2used in this work is a mixture of anatase and rutile[anatase:84%;rutile:16%(by mass)].It has been reported that the mixture of anatase and rutile with a certain proportion has the best catalyze activity[33].The reason could be that the mixture of two different crystal structures has a larger disfigurement density in the crystal lattices,and the disfigurements form the carrier traps,in which the electron orhole could be trapped.In this way the lifetimes of both hole and electron were extended.Fig.3 shows the SEM image of nano-TiO2on the surface of silicon wafer and the corresponding particle size distribution histogram.And the average size of the TiO2particle is about 50 nm.

Fig.2.XRD pattern of TiO2 nano-particles.

3.2.The photocatalysis digestion process

In order to examine the effect of photocatalysis oxidation and individual UV irradiation,experiments were conducted under room temperature using initial concentration 4 mg·L?1(by mass of P)Na5P3O10and C3H7Na2O6P,respectively.When the UV irradiation(365 nm)was applied alone without TiO2catalyst,the experimental results showed that the concentrations of the two phosphorous compounds were not decreased in the solutions,which indicated that the individual UV light could not oxidize the phosphorus compounds.However,a significant increase of the orthophosphate concentration,which indicated a significant reduction of the concentration of Na5P3O10and C3H7Na2O6P,was observed after the treatment of UV irradiation and TiO2catalysis,implying that the phosphorus compounds were oxidized.The oxidation of the phosphorus compounds could be attributed to the combination of the UV irradiation and TiO2through which hydroxyl radical(OH·)was added to the solutions.The generation of ·OH depends on the absorption of UV irradiation on TiO2.Fig.4 shows the conversion rates(the ratio of the concentration of P converted into orthophosphate to the initial concentration of P in phosphorus compound)of the two typical phosphorus substances for different irradiation times at room temperature(25°C).The conversion rates elevate with an increase of the irradiation time for both substances,but not in a linear relationship.It is observed that the conversion rates increase rapidly during the first 30 min and then with increasing irradiation time the acceleration of the conversion rate becomes slower and slower.After 120 min,the conversion rates of Na5P3O10and C3H7Na2O6P reach 78.8%and 92.6%,respectively.

3.3.Thermal-assisted photocatalysis digestion process

To reduce the digestion time and improve the efficiency,the thermal assisted UV photocatalysis digestion method was proposed in this work.The experiments were conducted at reaction temperature of 40°C,50 °C and 60 °C,respectively,and the effect of temperature on the digestion process was studied.Conversion rates of the two typical phosphorus substances at different irradiation times and different temperatures are shown in Fig.5.It can be seen that the trends of change on the conversion rate versus irradiation time at these reaction temperatures(40 °C,50 °C and 60°C)are similar to those at the room temperature,and the conversion rates are larger at higher reaction temperatures than those at room temperature,which means the oxidation speed of phosphorus compounds to orthophosphate is much faster at a higher reaction temperature.It can be concluded that heating has an enhanced effect on the photocatalysis oxidation digestion process.At the condition of 30 min and 60°C,the conversion rates of Na5P3O10and C3H7Na2O6P are 95.9%and 96.1%,respectively.

Fig.3.SEM image of TiO2 nano-particles on surface of silicon wafer(a)and the corresponding particle size distribution histogram(b).

Fig.4.Conversion rates of two typical phosphorus substances vs.irradiation time at room temperature(25°C).

When the two kinds of compounds were kept at 40 °C,50 °C and 60°C for30 min,almostno orthophosphate was detected in the samples,which indicated that the substances chosen in the work were relatively stable during the temperature of 40 °C to 60 °C.

3.4.Reaction kinetics of TP digestion process

3.4.1.Pseudo- first order equation

Several experimental results indicated that the oxidation rates of photocatalytic oxidation of various compounds in the heterogeneous system over illuminated TiO2were in accord with the Langmuir-Hinshelwood(L-H)kinetics model[34-37]:

where r is the oxidation rate of the reactant(mg·L?1·min?1),c is the concentration of the reactant(mg·L?1),k1is the adsorption constant of the reactant(L·mg?1),the absorption constants for the compound modules adsorbed on the TiO2surface,and k2is the reaction rate constant(mg·L?1·min?1).

When the initial concentration of the compound to be oxidized is small enough(e.g.mg·L?1),the term k2c could be ignored.Eq.(1)could be simplified to a pseudo- first order rate equation format:

where k represents the product of k1and k2,and k could be defined as a pseudo- first-order rate constant.A common form of the pseudo- first order rate kinetic equation can be obtained by integrating the equation above[34,36]:

Fig.5.Conversion rates of Na5P3O10(a)and C3H7Na2O6P(b)vs.irradiation time at different reaction temperature.

where k is pseudo- first-order rate constant,t is the reaction time,and ctand c0are the compound concentrations at the irradiation time t and initial concentration,respectively.The slope of the plot of ln(ct/c0)versus time gives the value of rate constant k(min?1).

The plot of ln(ct/c0)versus t for the photocatalysis digestion process for the phosphorus compounds is shown in Fig.6,and the rate constants are 0.0136 min?1and 0.0208 min?1for Na5P3O10and C3H7Na2O6P,respectively.

Fig.6.Kinetic study of photocatalysis digestion of Na5P3O10 and C3H7Na2O6P.

Fig.7 shows the relationship between ln(ct/c0)and time under the thermal-assisted photocatalysis digestion process for Na5P3O10and C3H7Na2O6P solutions.The linearity of the plots indicates that the thermal-assisted photocatalysis digestion approximately follows the pseudo- first order rate kinetics model.Table 1 shows the pseudo- first order rate constants of two phosphorus compounds at different temperatures.

3.4.2.Effect of temperature on pseudo- first order rate constant k

The plots of k versus temperature for two typical TP components during the thermal-assisted photocatalysis digestion process are shown in Fig.8.With an increase of the reaction temperature,the values of kincrease but not linear.The increase of the rate constant becomes sharper at higher temperatures.It can be concluded that temperature has a significant enhanced effect on the reaction rate of the photocatalysis digestion.The relationship between the rate constant k and the temperature is determined quantitatively by the Arrhenius equation which is used to describe the nonlinear relationship between k and temperature[36,38].

Table 1 Pseudo- fist order rate constants of different phosphorus compounds at different temperatures

The Arrhenius equation is based on the collision theory and it is an experiential equation proposed according to the experiential results[39].Arrhenius argued that when reactants transform into products,they must first acquire a minimum amount of energy to overcome the repulsive force between electron clouds due to the in finitely approach of molecules.This minimum energy is called the activation energy Ea.At an absolute temperature T,the fraction of molecules that have a kinetic energy greater than Eacould be calculated from statistical mechanics,the Maxwell-Boltzmann distribution.The Arrhenius equation is a simple,but remarkably accurate formula for the temperature dependence of the reaction rate constant,and it is available in the elementary reaction,non-elementary reaction and heterogeneous reaction[36].

The logarithmic formula is written as follows:

where k is the rate constant,T is the absolute temperature in degrees Kelvin,Eais the activation energy,A is the pre-exponential factor and R is the gas constant(8.314 J·mol?1·K?1).The unit of the preexponential factor A is identical to those of the rate constant.Given the small temperature range that kinetic studies occur in,it is reasonable to approximate the activation energy and pre-exponential factor as being independent of the temperature.Thus,the plot of ln k versus 1/T produces a straight line according to Eq.(4).

Arrhenius plot is obtained by plotting ln k versus 1/T for the two compounds as shown in Fig.9.

Fig.7.Kinetic study of thermal-assisted photocatalysis digestion:Na5P3O10(a)and C3H7Na2O6P(b).

Fig.8.Effect of temperature on rate constant k.

Fig.9.Arrhenius plot of thermal-assisted UV digestion process for Na5P3O10 and C3H7Na2O6P.

3.5.Analytical performance in real water samples

The proposed thermal-assisted UV digestion method was also used to digest real water samples collected from lakes and rivers around Beijing.The experimental results were compared with those using standard autoclave persulfate oxidation methods.The digestion condition of thermal-assisted digestion process was 60°C and 30 min.The measurement results are shown in Table 2.It is found that the conversion rate using thermal-assisted UV digestion method is consistent with that using standard autoclave persulfate oxidation method.

4.Conclusions

In this work,an oxidant-free thermal-assisted ultra-violet(UV)photocatalysis oxidation method for total phosphorus(TP)detection is proposed.Experimental and kinetic analyses of this digestion process are presented in this paper.A 365 nm UV light is used as the light sourceand TiO2film modified on the surface of silicon wafer is used as the photo-catalyst.The experimental results show that the conversion rates of sodium tripolyphosphate and sodium glycerophosphate could reach above 90%at the condition of 30 min and 333 K(60°C)using thermal-assisted ultra-violet(UV)photocatalysis oxidation method.

Table 2 Total phosphorus(TP)analysis with thermal-assisted UV digestion process in real water samples

The thermal-assisted photocatalysis digestion approximately follows the pseudo- first order rate kinetics model,and the value of rate constant k increases with the increase of reaction temperature.The Arrhenius equation is employed to describe the nonlinear relationship between k and temperature T.The absence of strong oxidant can avoid the influences of rudimental persulfate on the detection of phosphate by oxidation-reduction reactions,which makes it more applicable to the electrochemical detection of TP.

Nomenclature

A pre-exponential factor,min?1

c the concentration of the reactant,mg·L?1

c0initial concentration of compound,mg·L?1

ctcompound concentrations at the irradiation time t,mg·L?1

Eaactivation energy,kJ·mol?1

k pseudo- first-order rate constant,min?1

k1the adsorption constant of the reactant,L·mg?1

k2the reaction rate constant,mg·L?1?min?1

R the gas constant,8.314 J·mol?1·K?1

r the oxidation rate of the reactant,mg·L?1?min?1

T absolute temperature,K

t the reaction time,min