Comparison of two excilamps and two reactor configurations in the UV-H2O2 removal process of amaranth

Nowadays dyes are used in many industrial activities and their presence in wastewater is quite common and involves different environmental and health problems. In addition, dyes are usually recalcitrant compounds and conventional treatment are not appropriate for their removal. So there is great interest in finding alternative removal processes, as the one based on excimer lamp technology for the removal of amaranth dye described in the present work. Two excilamps and two reactor configurations have been tested: a batch reactor with KrCl or XeBr excilamp and a KrCl flow-through photoreactor. After comparing the results for both lamps in the batch reactor, the KrCl excilamp has proven to be more efficient both in terms of conversions achieved and energy consumption and, by this, it has been selected to be used in the flow-through photoreactor. For both reactor configurations, several experimental series were done to analyze the influence of the different operational variables on the photoprocess and the optimal mass ratio between hydrogen peroxide and dye has been established. Also, it has been proven that the use of Fenton reagent improves the process efficiency, particularly in the case of the XeBr lamp. In addition, a kinetic model, based on a previous one developed by the authors for a flow-through photoreactor and slightly modified to can also apply it to batch reactors, has been applied. The model has been validated with an excellent fitting of the experimental data for all series and both reactor configurations.

section_content: Nowadays dyes are used in many industrial activities and their presence in wastewater is quite common and involves different environmental and health problems. In addition, dyes are usually recalcitrant compounds and conventional treatment are not appropriate for their removal. So there is great interest in finding alternative removal processes, as the one based on excimer lamp technology for the removal of amaranth dye described in the present work. Two excilamps and two reactor configurations have been tested: a batch reactor with KrCl or XeBr excilamp and a KrCl flow-through photoreactor. After comparing the results for both lamps in the batch reactor, the KrCl excilamp has proven to be more efficient both in terms of conversions achieved and energy consumption and, by this, it has been selected to be used in the flow-through photoreactor. For both reactor configurations, several experimental series were done to analyze the influence of the different operational variables on the photoprocess and the optimal mass ratio between hydrogen peroxide and dye has been established. Also, it has been proven that the use of Fenton reagent improves the process efficiency, particularly in the case of the XeBr lamp. 

In addition, a kinetic model, based on a previous one developed by the authors for a flow-through photoreactor and slightly modified to can also apply it to batch reactors, has been applied. The model has been validated with an excellent fitting of the experimental data for all series and both reactor configurations.
section_name: Abstract

section_content: With the aim to increase sales, companies tend to offer more attractive products, something that can be achieved by means of color, since it allows us to differentiate the products, makes them more striking or customize them. Industries such as cosmetics, food or textile are just some examples of this tendency. Dyes can also be used for more technical purposes, such as those found in pharmacy or laboratories. 

The main problem derived from the multiple applications of dyes is the presence of colored compounds in wastewater that will end up being discharged into the natural environment. Despite the apparently innocuous aesthetic effects on the receptor medium, the transmission of solar energy decreases by the presence of dyes and this affects the photosynthesis and reduces the concentration of oxygen in the ecosystem. In addition, some of these compounds may be toxic to organisms of the aquatic medium [1, 2], affecting their development or behaviour [3, 4]. Another important aspect of dyes is that, in some cases, the products formed during their decomposition can become more toxic than the dyes themselves. 

Azo dyes are the most common and numerous dyes and among them amaranth has been the one selected for this research. It is used in the coloration of textile fibers, leather, paper, thermoplastics, wood, cosmetics, inks and in the food industry with the number E123 of the International Numbering System (INS) [5]. Amaranth also has applications in biological research, such as detecting changes in membranes potential [5]. According to some recent research studies, as one with in vitro human lymphocytes [6], it seems that amaranth can be toxic and, by this, it is not authorized anymore in EEUU as additive in the food, pharmaceutical and cosmetic industry. As a consequence, amaranth can be considered as a pollutant which must be removed from wastewaters. 

Different physical, chemical and biological methods have been used for the removal of amaranth from wastewaters. Adsorption treatments have been tested with good results with adsorbent materials such as zeolites or activated carbon [7]. 

However this technique involves the pollutant change of phase and by this, although almost total removal from wastewater can be achieved, the pollutant remains in the adsorbent, which must be treated further. Membrane processes have been applied as well among the physical treatments, using nanofiltration and reverse osmosis with polysulfone and polyethylamine membranes [8, 9]. In this case the main disadvantage is the high energy consumption and also the concentrate obtained that must be treated as well. Biological treatments have been used with bacterial culture such as Pseudomonas [10], fungi [11] or peroxidase enzymes extracted from Arundo donax, Typha angustifolia or Phragmites australis [12], however, due to the toxicity of pollutants, some of these treatments can be applied only for low dye concentrations. 

Chemical methods like coagulation-flocculation, ionic exchange or electro-coagulation have proven to be effective for the removal of azo dyes like amaranth [13, 14, 15]. Among the chemical methods, oxidation processes offer the important advantage that total mineralization of the pollutant can be achieved in most cases. Subcritical and supercritical water oxidation have been used attaining removal efficiencies of azo dyes between 80-90% [16, 17], although extreme temperature and pressure conditions are normally required for these treatments. 

Advanced oxidation processes (AOPs) offer an interesting alternative that has been attracting increasing interest over the last years. AOPs are based on the generation of strong oxidizing agents, like HO•, allowing oxidation and total mineralization of water pollutants even at very low concentrations due to their high reactivity and low selectivity [18]. Both photochemical and non-photochemical AOPs such as ozonation [19], Fenton treatment [20] or UV-Fenton [21] have been successfully used for amaranth removal from wastewater. 

Among the photochemical AOPs, excimer technology has been selected for the present work because in the literature there are no references about the use of this technology in the removal of this dye. Excimer lamps or excilamps are new UV sources based on transitions of exciplex (rare gas halides) or excimer molecules (rare gas or halogen dimmers) formed as a result of an electric discharge. They are attractive alternatives to commonly used mercury lamps because they have some important advantages including the absence of elemental mercury, long lifetime (from 1000 to 10000 h), geometric freedom and high photon flux. Their main advantage is the emission in a narrow-band UV radiation, nearly monochromatic and matching the dissociation energies of bonds of the main organic compounds, especially if barrier discharge lamps are used since they provide a narrower emission spectrum. From an environmental point of view, the absence of toxic mercury is a remarkable characteristic [22] [23] [24] [25]. Over the last years excimer technology has been applied to the removal of different organic pollutants that can be found in industrial wastewaters, using different lamp configurations [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]. 

In addition, different kinetic studies on oxidation treatments of amaranth and other dyes can be found in the literature [38] [39] [40] [41]. In most processes the progress curves can be successfully fitted to a pseudo first order kinetic model and, commonly, the first order kinetic parameter shows a dependence on the operational variables, mainly the concentration of pollutant, which is not the expected behaviour for a true first order kinetics. In a previous work [42] the authors studied the photodegradation of the methylene blue dye using an exciplex KrCl flow-through photoreactor, and a kinetic model, that explained the dependence of the first order kinetic parameter on the operational variables, was developed and successfully validated. 

In the present paper, a comparative study is carried out, firstly in a batch reactor with two different excilamps of KrCl and XeBr, to test their efficiency in the removal of amaranth. Once the best lamp is selected, the second part of the study is focused on the photodegradation of amaranth by using an exciplex KrCl flowthrough photoreactor. This is the reactor configuration that allows us to work, in the future, not only in discontinuous but also in semicontinuous or continuous mode, as a previous step for a further scale-up of the process. In addition, the previously developed kinetic model, with some small modifications to be suitable for batch reactors, is checked and validated with the data obtained from both reaction systems.
section_name: INTRODUCTION

section_name: MATERIALS AND METHODS

section_content: Amaranth (85-95% w/w) was purchased from Sigma-Aldrich Fine Chemicals, hydrogen peroxide (33% w/v) was purchased from Panreac, ferrous sulphate was purchased from Probus, catalase solution (1340 U/mg solid) and aluminium potassium sulphate (≥ 98,0%) were purchased from Sigma-Aldrich Fine Chemicals. Other chemicals were of analytical grade and were used without further purification.
section_name: Reagents

section_content: The equipment used for the experiments consist in two barrier discharge excilamps (purchased from the Institute of High Current Electronics of the Siberian Branch, Russian Academy of Sciences). The XeBr and KrCl excilamps emitting maximum UV radiation at 283 and 222 nm, respectively, were of cylindrical geometry covered by a metal case having an UV exit window with an area of 75 cm 2. The exit window was oriented at a fixed distance of 3 cm over a vessel of 100 mL of capacity and 4. 5 cm external diameter. The average radiation intensity delivered to the solution was 17. 12 and 2. 47 mWcm -2 for XeBr and KrCl excimer lamps, respectively. Also a flow-through KrCl photoreactor with an irradiation zone length of 30 cm and an internal diameter of 2 cm, which provides an irradiation area of 188. 5 cm 2 and with an average radiation intensity of 2. 38 mWcm -2 was used in the experiments. Data of radiation intensity, both for the two excilamps and for the flowthrough photoreactor, were provided by the manufacturer. 

For the analytical determination of amaranth concentration, an ultraviolet/visible Thermospectronic Helios Alpha spectrophotometer was used. COD was also measured with some specific equipment consisting of reactor and photometer, model HI 938800 and model 83099, respectively, from HANNA Instruments. An eppendorf centrifuge 5424 was used in Fenton's series.
section_name: Materials

section_content: Photodegradation assays were done in two configurations of reactors: a batch reactor with KrCl and XeBr excilamp and a KrCl flow-through photoreactor. 

In the first case a glass beaker is used as a batch reactor, excilamp is over the reactor and radiation directly falls on the treated volume. A system scheme is presented in Figure 1A. Amaranth, hydrogen peroxide and Fe 2+, at the required concentrations, were dissolved in distilled water and placed into the glass beaker and irradiated at room temperature (23-25 ºC), under static conditions and for exposure times of 120 min. For the different experiments, fluence was also determined by multiplying the radiation intensity and the exposure time of reaction volume. 

In the second case, the solution containing the amaranth and the rest of the reagents (hydrogen peroxide and Fe 2+ ), at the required initial concentration, was pumped from a stirred tank to the photoreactor and the effluent was continuously recycled to the tank, so that the system acts as a pseudo-batch reactor (flowthrough reactor). All the experiments were done at room temperature (23-25 ºC), with an operational time of 120 min. Experimental equipment, including the main components, is depicted in Figure 1B. 

In both cases, samples were taken at different reaction times (0, 2. 1 and 2.
section_name: Operational procedure

section_content: Samples were spectrophotometrically analyzed at the amaranth maximum absorption wavelength in the visible region (522 nm). Absorbance values were transformed to concentrations by using the amaranth calibration curve:

[Amaranth] (mg/L) = Absorbance 522 / 0. 0399 (r = 0. 9998).
section_name: Analytical determinations

section_content: The kinetic model applied in the present work is an adaptation of another one previously developed [42], with small modifications to be suitable for batch reactors. The model is based on the presence of two regions with different dye concentration in the liquid phase: a thin layer in the region where the radiation is delivered to the solution, where the photoprocess takes place, with a small residual concentration of non degraded dye, and the bulk solution, with a uniform concentration of dye, which decreases with time and is the measured concentration. The model takes into account the dependence of the first order kinetic constant on different experimental variables, a behaviour that does not correspond to a true first order kinetic, but to a pseudo-first order one. The model has been applied to the data from both the KrCl photoreactor and the batch reactor with KrCl and XeBr excilamps, being the equations valid for both systems. In the following, the model hypotheses and equations are presented.
section_name: KINETIC MODEL

section_content: I. Both the stirring in the glass beaker and the flow rate in the photoreactor are high enough and, as a consequence, the solution is well agitated and the amaranth concentration in the bulk solution, [A], only varies with time.
section_name: Model hypotheses

section_content: Different consecutive steps take place along the photodegradation process of amaranth, from the initial dye to intermediate compounds until the final products. The global reaction can be expressed as:
section_name: II.

section_content: 0 and [A] are the initial and current mass concentrations of the dye, respectively, and [Prod] i is the mass concentration of each one of the products formed, according to the mass conservation law it must be verified:

IV. The radiation intensity delivered to the sample from the KrCl photoreactor or from the KrCl and XeBr excilamps is constant at a fixed distance from the UV source and uniformly distributed. So a density of radiation intensity per mass unit can be expressed as the quotient I/[A] 0, where I is the average radiation intensity emitted.
section_name: III. At any time, if [A]

section_content: The radiation is quickly absorbed by the substrates and photoproducts and consumed during the photoprocess near the region where it is delivered to the solution. So it can be assumed that the photodegradation takes place in a thin
section_name: V.

section_content: In a similar way as explained in the above hypothesis for amaranth, there is a

However this mass transfer is faster than the one of amaranth and not limiting due to the much lower molecular weight of hydrogen peroxide.
section_name: VI.

section_content: The photodegradation of amaranth takes place both by direct photolysis and by reaction with hydrogen peroxide.
section_name: VII.

section_content: As previously indicated, in the assays with KrCl and XeBr excilamps a glass beaker was used as a batch reactor, and in the experiments carried out with the photoreactor, total recirculation was used, so the system acts as a batch reactor as well. As a result, for both reactor configurations, if r A is the amaranth photodegradation rate, V the total sample volume and V R the volume of the reaction zone, the mass balance can be expressed as follows:

According to hypothesis V, and being k L a the mass transfer volumetric coefficient, the mass transfer rate, r dif, can be expressed as:

Taking into account hypothesis IV, the energy received in the time unit by the mass of amaranth must be proportional to the mass transfer rate and to the density of radiation per mass unit, being k E the proportionality constant:

And taking into account the quantum yield of the process, the photodegradation rate can be defined as follows:

According to hypothesis VII, and if k c1 is the kinetic constant of direct photolysis and k c2 is the kinetic constant of the photodegradation with hydrogen peroxide, the following equation applies:

From these last two equations (3. 6) and (3. 7) the value of [A] lim can be obtained:

And by replacing this expression in equation (3. 7) a new equation for r A is obtained:

By substituting equation (3. 9) in equation (3. 3) of mass balance, the variation of [A] with time can be obtained as follows:

From Eq. (3. 10), a pseudo-first order kinetic constant, k r, can be defined as:

And since [ The definition of k r allows us to simplify the mass balance equation to the following one, matching the batch reactor equation for a pseudo-first order kinetic:

Additionally, from the definition of conversion:

[] 0 ( 3. 14)

Equation (3. 13) is modified to:

With the initial condition:

Finally, from the integration of equation (3. 15), it is obtained:

For the fitting of the experimental data to equation (3. 17), the software Curve Expert 1. 4 has been used.
section_name: Model equations

section_content: As indicated at the end of section 2. 3, four experimental series were carried out, in order to study the influence of the different operational variables on the process. In Figure 1 the experimental reaction system is shown, and the experimental conditions for the four series are shown on work and some others found in similar studies, has been made.
section_name: RESULTS AND DISCUSSION

section_content: In the first experimental series the influence of the variation of H 2 O 2 :amaranth mass ratio on the process has been analyzed. The results obtained are shown in Figures 2A and 2B for the KrCl and XeBr excilamp, respectively. As it can be observed, for the KrCl lamp the optimum mass ratio seems to be 5:1, and there is no significant improvement with the results corresponding to the 6:1 ratio. In a similar way, for the XeBr lamp the optimal ratio seems to be 6:1. 

Once the optimum mass ratio has been selected for both lamps the obtained results were compared, not only considering the photodegradation rate but also the energy consumption expressed as fluence. This can be seen in Figures 2C and 2D, proving that the KrCl excilamp is clearly more effective, both in terms of degradation rate and energy consumption. This result can be explained taking into account the wavelength of maximum UV emission of both excilamps, which is 222 nm for the KrCl excilamp, very close to the maximum absorption wavelength of amaranth in the UV region (225 nm), while this maximum is of 283 nm for the XeBr excilamp, far from amaranth absorption. 

It is important to notice that, although the average radiation intensity of the XeBr lamp is around 5 times higher than the one of the KrCl excilamp, the most determining factor to achieve an optimum photodegradation process is the matching between the maximum emission of the lamp and the maximum absorption of the target compound. This aspect is particularly relevant when monochromatic sources, such as excimer lamps, are used. 

In Figure 3A, which corresponds with the second series and where the variation of the initial dye concentration was studied, it can be seen that, for both excilamps, conversion decreases when increasing amaranth initial concentration, as expected, since the radiation intensity per mass unit is lower. As in the previous series, the best results are obtained with the KrCl lamp, also considering the energy consumption, as it can be seen in Figure 3B. 

The third experimental series, in which reaction volume was varied, is depicted in Figures 4A and 4B, showing the variation of conversion versus time and fluence, respectively. From these results it seems that this variable has no significant influence, probably due to the narrow range of volume variation used in this series. 

Once again, the KrCl has proven to be the best UV source, achieving high values of amaranth conversion with low energy requirements. 

Finally, for the four series, the results corresponding to the Photo-Fenton process are shown in Figures 5A to 5D. It is interesting to observe how the use of Fenton improves the performance of the XeBr excilamp. For the optimum Fe 2+ concentration selected, 5 mg/L, the results of the mentioned lamp are very close to the ones obtained with the KrCl excilamp, which is still a better option, especially in terms of fluence.
section_name: Experimental results in batch reactor with KrCl and XeBr excilamps

section_content: In Figures 2 to 5, the dots are the experimental values of conversion and the continuous lines correspond with the data calculated by using equation (3. 17). Additionally, in And by putting all constants together, the following dependence with [H 2 O 2 ] 0 is obtained:

Figure 6A shows the fitting to this previous equation using the different values of 3, where the high correlation coefficients obtained are also shown. In Figure 6A it is also important to point out the much higher values of k r obtained for the KrCl excilamp, in good agreement with the better performance of this UV lamp mentioned in section 4. 1. 

For the fitting to the model of experimental series 2, where the variation of initial dye concentration was studied, from equation (3. 12) the following new expression can be obtained:

Which can be expressed as follows:

The good fitting to this last equation is shown in Figure 6B. And once again, the values of k r are higher for the assays with the KrCl excimer lamp, as expected from the experimental results. 

In the third series, equation (3. 12) has been used again for the model fitting. A linear variation of k r with the inverse of the volume is expected initially according to the following expression:

However, as it can be seen in Figure 6C, there is not a significant variation of k r with the inverse of V, as predicted in equation (4. 5). This is probably due to a double and opposite effect. On one hand, as the volume is in the denominator of equation (4. 5), a higher volume has a negative effect on the kinetic constant. On the other hand, taking into account the experimental system used, a higher volume involves less distance between the free surface of reaction volume and the radiation source and, as a result, the sample receives higher radiation intensity and the value of k r increases. As a consequence of this double effect and considering, also, the narrow range obtained for the variation of k r, there is no clear tendency for this series. 

Finally, for the Photo-Fenton experiments, although the use of Fenton reagent is not considered in the model equations, the same kinetic model has been applied with good results, and a clear linear dependence between the obtained values of k r and the variation of the initial Fe 2+ concentration is presented in Figure 6D, being again the KrCl excilamp more efficient.
section_name: Model fitting of the batch reactor results with KrCl and XeBr excilamps

section_content: Results from this experimental reactor configuration are shown in Figure 7. Figure 7A depicts the results of variation in the H 2 O 2 :amaranth mass ratio. As it can be observed, the optimum mass ratio seems to be 2. 5:1. 0, because for a higher mass ratio (3. 0:1. 0) no significant improvements are obtained. After selecting the optimum mass ratio, the influences of variation of the initial dye concentration and the reaction volume were studied. Results are depicted in Figures 7B and 7C, respectively,

showing that conversion decreases when there is an increase in amaranth initial concentration and/or in the reaction volume, in good agreement with the behaviour predicted in equations (4. 4) and (4. 5). This is the expected result, as the radiation intensity is the same but lower per mass unit. 

Finally, Figure 7D shows the results corresponding to the Photo-Fenton process, where it can be noticed that, even with the lowest Fe 2+ concentration assayed, the reaction is very fast and conversion values close to 100% are reached from the first reaction minutes.
section_name: Experimental results in the KrCl flow-through photoreactor

section_content: As indicated in previous paragraphs, in Figure 7 solid lines correspond with the fitting to the model of the experimental data, for each experimental series. Also, the values of k r and correlation coefficients can be found in Table 2. Figures 7A to 7C show an excellent fitting of the experimental data to the model. 

Figure 8A shows the fitting to equation (4. 2) for the different values of k r obtained in the series of variation of H 2 O 2 :amaranth mass ratio. Table 4 shows the values of the different parameters of equation (4. 2), as well as the high correlation coefficient obtained. In the same way, Figure 8B shows the fitting of k r to equation (4. 4) for the experimental data corresponding to the variation of the initial dye concentration. A good fitting is observed and, also, a decreasing in k r values for increasing values of initial dye concentration, as expected. 

For the third series, and taken into account that in this reactor configuration there is no variation in the intensity of radiation, since the distance from the radiation source and the liquid surface is constant, according to equation (4. 5) a linear variation of k r with the inverse of the volume is expected. Figure 8C shows the obtained fitting. 

Finally, for the Photo-Fenton experiments, as it has been previously commented, the use of Fenton reagent is not considered in the model equations. Besides, the reaction occurs very fast and the progress curve is practically a constant line, as shown in Figure 7D.
section_name: Model fitting of the KrCl flow-through photoreactor results

section_content: Regarding the amaranth degradation rate, it must be taken into account that it depends on the different operational conditions: The dye concentration, the type of UV lamp used, the use of an oxidizing agent and the possible addition of a catalyst. As a consequence it is not easy to make a comparison between the results obtained in this work and the ones from other published manuscripts on similar studies. 

Nevertheless, a comparison has been made using the results of the present study and the ones obtained by Gomathi et al., [21], and Wu et al., [40]. These works have been selected because their results have been fitted to a pseudo-first order kinetic model as well, and the values of the pseudo-first order kinetic constant can be found, which facilitates the comparison. 

In this way, from equation (3. 17) of the manuscript, where the relationship between the conversion, X A, the reaction time, t, and the kinetic constant, k r, was established, the half-life time, t 1/2, can be calculated as follows:

This time corresponds with a conversion of 0. 5, and it is the time needed to degrade half of the initial dye concentration. So, the lower the half-life time is the higher is the efficiency of the lamp. Also, if the intensity of radiation is known, the fluence corresponding to the half-life time can be also calculated, and the lower the fluence, the lower the energy consumption. 

From these considerations, in Table 5, a comparison between the results obtained for the half-life time and fluence in this work and the ones calculated from the data shown in references [21] and [40] has been made. As it can be seen in Table 5, in terms of halflife time and fluence the higher efficiency corresponds with the KrCl excilamp, being the efficiency of the XeBr excilamp higher or lower than the one of reference [21],

depending on the initial dye concentration, and lower than the one of reference [40]. 

So, it can be affirmed that good removal performance of amaranth can be obtained with excilamps, mainly with the KrCl one. Additionally, as the use of mercury is avoided in these lamps, they are also a technology cleanest and more environmentally friendly than traditional UV sources.
section_name: Comparison with other similar studies.

section_content: Excimer technology has proven to be an excellent alternative to conventional treatments for the removal of complex organic compounds such as dyes, achieving a high elimination level under optimal reaction conditions. In this work, two reactor configurations have been used for the photodegradation of amaranth: a batch reactor with KrCl or XeBr excilamp and a KrCl flow-through photoreactor. 

Firstly, a detailed study comparing the results attained with the KrCl and XeBr excilamps in the discontinuous system has been carried out. The KrCl lamp has led to the best results, both in terms of removal efficiency, amount of oxidant needed (optimum mass ratio hydrogen peroxide: dye is 5:1 versus 6:1 for the XeBr) and energy requirements, although with the addition of Fenton reagent as oxidant there is a significant improvement in the results obtained with the XeBr excilamp, being very close to those achieved with the KrCl lamp. 

Once the KrCl excilamp has been selected, the flow-through photoreactor has been tested for the same experimental series. For this reactor configuration, the optimal mass ratio hydrogen peroxide:amaranth has been 2. 5:1 and, as previously, the photo-Fenton process significantly improves the removal efficiencies. 

In addition, a kinetic model, previously developed by the authors and slightly modified to be suitable for batch reactors, has been tested and validated with an excellent fitting of all experimental data for both reactor configurations. This model agrees with the observed dependence of the pseudo-first order kinetic constant on different experimental variables. 

Future research must be focused on the process scale-up and, for that, semicontinuous or continuous configurations of the flow-through photoreactor must be tested and some small aspects of the kinetic model must be improved, such as the inclusion of the Fenton reagent in the model equations or a deeper study of the influence of the reaction volume distance from the radiation source, allowing an even better process simulation and understanding.
section_name: CONCLUSIONS

section_content: Quantum yield defined in Eq. 3. 6, (mg L -1 W -1 )
section_name: ACKNOWLEDGEMENTS

section_content: Kinetic constant of direct photolysis in the film, (min -1 )

Kinetic constant with hydrogen peroxide in the film, (mg -1 L -1 min -1 )

Proportionallity constant defined in Eq.
section_name: Intensity of radiation, (W)