Photocatalytic Conversion of Gaseous Nitrogen Trichloride into

Article pubs.acs.org/est

Photocatalytic Conversion of Gaseous Nitrogen Trichloride into Available ChlorineExperimental and Modeling Study F. Gérardin,*,† A. Cloteaux,†,‡ M. Guillemot,† M. Faure,†,‡ and J. C. André‡ †

Institut National de Recherche et de Sécurité, Rue du Morvan, CS60027, 54519 Vandœuvre Cedex, France Laboratoire Réactions et Génie des Procédés, UPR 3349 CNRS, 1 rue Grandville BP20451, 54001 Nancy Cedex, France

‡

S Supporting Information *

ABSTRACT: In water, chlorine reacts with nitrogen-containing compounds to produce disinfection byproducts such as nitrogen trichloride which induces ocular and respiratory irritations in swimming pool workers. A technical solution has been used to reduce NCl3 exposure to acceptable levels, by adding a stripping step to the water recycling loop. The pollutants extracted are currently rejected into the atmosphere without treatment. However, the physical properties of NCl3 could be harnessed to induce its controlled degradation by direct or indirect light. This paper describes the way to transform NCl3 into oxidizing chlorine by photocatalysis under laboratory conditions. Photocatalytic oxidation efficiently degrades gaseous nitrogen trichloride, producing compounds such as HClO. About 60% of NCl3 decomposed was converted into HClO which could be used as a disinfection compound. A kinetic model is proposed for the photocatalytic process based on a convection/diffusion model. The Langmuir−Hinshelwood model was applied to the chemical part of the mechanism. The apparent quantum yield was also estimated to assess the optimal irradiance for NCl3 transformation. The results show that photocatalysis performs much better than photolysis alone for NCl3 removal, i.e. at least 25 times more efficient.

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very volatile.3 Nitrogen trichloride induces ocular and respiratory irritations in lifeguards and other swimming pool workers.4−6

INTRODUCTION Nitrogen-containing compounds (urea, ammonia, amino acids, etc.) are provided in pools by swimmers and react with chlorine which is widely used in water as a disinfection compound.1 The nitrogen-containing molecules are progressively degraded by chlorine giving rise to components such as haloforms, aldehydes, and chloramines,2 of which the most halogenated form, nitrogen trichloride (trichloramine, NCl3), is © 2013 American Chemical Society

Received: Revised: Accepted: Published: 4628

February 5, 2013 April 3, 2013 April 6, 2013 April 8, 2013 dx.doi.org/10.1021/es400588m | Environ. Sci. Technol. 2013, 47, 4628−4635

Environmental Science & Technology

Article

Figure 1. Experimental setup.

on a convection/diffusion model. The monomolecular/ monolayer Langmuir−Hinshelwood (LH) model was tested for the chemical part14 and the global quantum yield was also estimated.

To reduce NCl3 exposure to acceptable levels a stripping step was added to the recycling loop of water treatment.7,8 This process provides an interesting response in terms of both occupational and public health, but in environmental terms it is not entirely satisfactory. The nitrogen trichloride extracted by this process should be treated before rejecting it into the atmosphere, this would make the technique acceptable to employees, facility owners, and citizens alike. Alternatives to adsorption processes are possible thanks to the physical properties of NCl3 which should make it possible to induce controlled degradation of this compound by irradiation with direct or indirect light. Photocatalysis is a recently developed, effective, relatively cheap technique which is becoming more commonly used to treat a large range of pollutants that can be decomposed into mineral components.9 Photocatalysis could provide an original means to break down gaseous NCl3 into nitrogen and chlorine, thus contributing to the formation of available chlorine, i.e., gaseous chlorine, or hypochlorous acid, with its well-known bactericidal properties. Various authors have studied gaseous nitrogen trichloride photodissociation.10,11 Their results showed that NCl3 could be broken down using short-wave UV light irradiation. Indeed, the absorption spectrum for this compound revealed an absorption maximum at 220 nm. The free radical based photolytic mechanism for NCl3 breakdown suggested by Gilbert et al.10 at 249 or 308 nm leads to the formation of Cl2, which itself absorbs UV light irradiation to varying extents between 250 and 350 nm, producing free Cl● atoms. These radicals also can enhance NCl3 decomposition, leading to a quantum yield for NCl3 degradation greater than one.12 The main drawback of this process is that NCl3 only weakly absorbs the wavelengths emitted by the most common medium- or low-pressure mercury vapor lamps (Supporting Information Figure S1).13 The combination of the low energy yield of these lamps with the low absorption by NCl3 at the wavelengths emitted makes this process not entirely economically viable for most swimming pools. This paper describes the way to decompose NCl3 by heterogeneous photocatalysis in an annular reactor under laboratory conditions. This process allows very efficient conversion of gaseous nitrogen trichloride into recoverable compounds such as gaseous chlorine and hypochlorous acid. A kinetic model is proposed for the photocatalytic process based

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EXPERIMENTAL SECTION Generation of Gaseous Nitrogen Trichloride. Nitrogen trichloride is formed in aqueous solution by the action of chlorine, in a +1 or 0 oxidation state, on nitrogen-containing materials. The kinetics and reaction mechanism are known for the reaction of sodium hypochlorite with ammonia or ammonium ion.15,16 For this study, nitrogen trichloride was synthesized in an open, continuous stirred-tank reactor (CSTR) (Figure 1). A hypochlorite solution and a solution of ammonium sulfate ((NH4)2SO4) were supplied continuously to the reactor using a peristaltic pump.17 The hypochlorite solution was adjusted to 12 mol m−3 from a stock solution of bleach (NaClO, Merck, 6−14% active chlorine). The ammonium sulfate solution (4 × 10−1 mol m−3), was prepared from ammonium sulfate crystals (Merck, > 99.5%, A.C.S. reagent). The pH of these two solutions was adjusted to 4 using phosphoric acid (Fluka, 85%). This stabilized the NCl3 produced, and enhanced the yield of this compound compared to the other chloramine forms.15 The chosen reagent concentrations provide a Cl/N ratio of 15. The outlet flow from the CSTR was carried toward a stripping column fed with dry air. The Henry’s constant for NCl3 is 435 at 20 °C.3 About 95% of the nitrogen trichloride was transferred from solution to the air. Residual species such as monochloramine, dichloramine, or hypochlorous acid present in the stripping flow were trapped in a solution of sulfamic acid.5 The outlet air flow from this step, loaded with nitrogen trichloride, was then mixed with dry air which had been humidified in variable proportions to adjust the relative humidity (RH) of the system to a target value. RH ranging from 1 to 85% can be obtained. Photoreactor. The experiments described here were performed using a test bench equipped with an adaptable photoreactor to study both the photolytic and photocatalytic processes (Figure 1). The annular plug-flow reactor consists of two concentric borosilicate glass cylinders (Øinternal = 9.05 × 10−2 m and Øexternal = 0.1 m, L = 1.2 m, and V = 1.71 × 10−3 m3). The hydraulic diameter dh is 9.5 × 10−3 m. The gaseous effluent flows through the interannular space and the length of the reactive area is Lr = 1.0 m. Axial dispersion of NCl3 in the reactor was verified by 4629

dx.doi.org/10.1021/es400588m | Environ. Sci. Technol. 2013, 47, 4628−4635


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calculating the specific Peclet number for each flow value. The relation used to determine the Peclet number is defined for a tubular reactor as described in 18.

P=

UmLr D*

Reactor Configuration for Photocatalysis. The 250-μm thick photocatalytic support used in this study consisted of cellulose fibers coated with 18 g m−2 TiO2 (Ahlstrom 1048 PC 500 Millennium, 350 m2 g−1 specific surface, anatase form; 20 g m−2 SiO2, 2 g m−2 zeoliths). The physical adsorption of gas molecules on the catalyst surface is characterized by the determination of Brunauer−Emmett−Teller (BET) surface area. For TiO2, It was measured by N2 adsorption and was found to be 317 m2 g−1; the median diameter of deposited pellets on the medium is 1.4 μm.20 The outer side of the inner tube was covered with the photocatalytic medium to produce a reactive zone of length 1.0 m, with a catalytic surface of 0.286 m2 and a total of 5.1 g of TiO2. Photocatalysis of nitrogen trichloride was performed in the same conditions as the photolysis study, but three NCl3 concentrations were tested. These concentrations at the reactor inlet were C0 ≈ 4.5 × 10−5/10−4/2 × 10−4 mol m−3. Sampling Methods and Analyses. Samples of nitrogen trichloride and chlorinated compounds were taken at the inlet and outlet of the reactor, and at each sampling point along the reactor’s length. Sampling and analyses were performed by the specific method described by Héry et al.6 The sampling device was composed of two successive parts, i.e. a tube containing silica gel coated with sulfamic acid and a cassette containing quartz filter soaked with a solution of sodium carbonate and diarsenic trioxide. This allows separate assessment of nitrogen trichloride and all oxidizing chlorine species without distinction, including hypochlorous acid, dichlorine, monochloramine, and dichloramine. The sampling time was 15 min. Sampling was started 90 min after the experiment was initiated on the test bench. Byproducts of degradation were identified using a range of analytical techniques. The first of these relies on samples taken using multibed supports (Carbotrap 300). These sampling supports contain several successive adsorbent layers. Sampling was performed over approximately 120 min at 100 mL min−1. Compounds trapped on these tubes were analyzed by thermal desorption (Perkin-Elmer ATD 400) coupled to gas phase chromatography (Perkin-Elmer Clarus 500) and mass spectrometry (Perkin-Elmer Turbo mass Gold). This technique allows identification fo hydrochloric acid and many other compounds. The analytical conditions were

(1)

where D* = Dm +

Um 2dh 2 192Dm

(2)

Um and Dm respectively represent the mean flow velocity in the reactor (m s−1) and the molecular diffusion coefficient of NCl3 (Dm = 9.6 × 10−6 m2 s−119). D* is the axial dispersion coefficient (m2 s−1) calculated from the Taylor equation (eq 2). It takes into account the dispersion of the compound under the simultaneous action of molecular diffusion and variation of the velocity of the gaseous effluent. Axial dispersion is considered negligible for annular photoreactor with P > 100.18 Illumination was provided by a UV-A lamp (Sylvania Blacklight F40W/350 BL) placed at the center of the inner cylinder. UV-A irradiance was measured with a light detector (Gigahertz Optik radiometer model X11-XD-9511, range 315/ 400 nm). The detector head is fitted with the cosine diffuser to receive the incoming light signal. The detector allows polychromatic measurements with uniform-wavelength-dependence in its response to incident radiation. The maximum wavelength for the lamp used was 350 nm, and 99% of the power emitted ranged from 315 to 388 nm (Figure S1). The UV-A unit photon flow of the lamp is F0 = 3.7 × 10−5 E s−1 (or mol of photons s−1), e.g. 12.8 W. The conversions between E s−1 and W were made by integrating the spectrum of the lamp. The irradiance measured at the outer surface of the inner tube was I0 = 34 W m−2. The reactor was fitted with ten sampling openings to monitor the concentrations of the different species over the entire reactive area. The reactor temperature was monitored and it remained constant during the experiment. Some authors have shown that nitrogen trichloride breaks down after UV light absorption.10−12 As photocatalysis with TiO2 is also due to UV-A light irradiation with wavelength λ < 388 nm,9 the photolytic and photocatalytic processes of NCl3 decomposition were studied separately. The first part of this work focuses on the photolytic effect, while the second part describes the heterogeneous photocatalysis of nitrogen trichloride. All the experiments were carried out in laboratory conditions, i.e., with synthetic NCl3. Reactor Configuration for Photolysis. Photolysis of nitrogen trichloride was studied in the basic reactor configuration, i.e., in homogeneous phase, with the following operating conditions: • Inlet flow rate: Q0 = 1.35 × 10−4 m3 s−1 • Mean velocity: Um = 0.095 m s−1, D* = 4.33 × 10−4 m2 s−1, and specific Peclet number P = 219 • Residence time: τ = 10.5 s • Concentration at the reactor inlet: C0 = 2 × 10−4 mol m−3 • Temperature in the reactor: T = 27 °C • Total pressure: 105 Pa • Relative humidity: RH = 80% • Irradiance: 9.2, 22.8, and 34 W m−2. The nitrogen trichloride concentration and relative humidity used in these experiments were based on the average values measured at the outlets of existing stripping devices.7,8

• Thermal desorption: Tube heated at 320 °C during 20 min with 50 mL min−1 of helium. • Gas chromatography: · Column: GS-Gaspro plot, 60 m diameters 0.32 mm/0.53 mm) · Helium flow rate =1.4 mL min−1 · Oven: 15 min at 30 °C, ramp at 10 °C min−1, 5 min at 220 °C. • Mass spectrometry: · Scan mod with 20 < m/z < 100 · Source temperature: 200 °C. Monochloramine and dichloramine compounds were trapped by passing the outflow gas through a bubbler containing 40 mL of carbon tetrachloride (CCl4) with a flow rate of 2 L min−1 for 60 min. The contents were then analyzed by UV spectrometry (Perkin-Elmer Lambda 950) according to the protocol proposed by Czech et al.21 Hypochlorous acid and/or dichlorine contained in the reactor exhaust were identified by absorption in two bubblers in series, each containing 60 mL of ultrapure water. Sampling was performed over 45 min with a flow rate of 300 mL min−1. The solution was then analyzed by N,N-diethyl-paraphenylenediamine 4630



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(DPD) colorimetry.22 As this technique cannot distinguish between hypochlorous acid and Cl2, the results obtained represent the total concentration for both compounds. Assays for chloride ions that may be present at the surface of the photocatalytic support were performed after photocatalytic degradation of NCl3. Ions were desorbed from samples of the photocatalytic medium by washing with 20 mL of ultrapure water. Chloride ions were identified in an aliquot of wash solution using a colorimetric method.23

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C in − Cout × 100 C in

(3)

NCl 2• + NCl3 → N2 + 2Cl 2 + Cl•

(6)

Cl• + NCl3 → NCl 2• + Cl 2

(10)

(11)

⎛ ∂ 2C ∂C 1 ∂C ⎞ = −Dm⎜ 2 + ⎟ ∂z r ∂r ⎠ ⎝ ∂r

(12)

where r is radial coordinate (m), z is axial coordinate, u is flow velocity (m s−1), and C is the concentration of NCl3 (mol m−3). The model takes the velocity profile developed in the annular space into account, as expressed by the following relationship:25 u(r ) =

⎡ ⎛ ⎞2 ⎛ ⎞2 ⎤ ⎢1 − ⎜ r ⎟ + β ln⎜ r ⎟ ⎥ 1 + α 2 − β ⎢⎣ ⎝ re ⎠ ⎝ re ⎠ ⎥⎦ 2Um

(13)

with

Thermodynamically, if the first step corresponds to the creation of an NCl2● radical and an atom of chlorine (eq 4), it is possible that the latter can react with NCl3 to generate a second NCl2● radical (eq 5) according to the mechanism suggested by Briggs and Norrish:12 (5)

(9)

u(r )

(4)

Cl + NCl3 → NCl 2 + Cl 2

2NCl 2• → N2 + Cl 2 + 2Cl•

These reactive species then react with NCl3 according to reactions 5 and 7 to increase NCl3 decomposition through the mechanism proposed by Briggs and Norrish.12 With regard to byproducts, the different analytical techniques presented above did not reveal the presence of other chlorine species. Dichlorine is trapped, in part, in bubblers containing ultrapure water as hypochlorous acid. Photocatalytic Degradation. The annular tubular photocatalytic reactor can be modeled based on three main assumptions. The first is to consider the reactor as a steadystate reactor, i.e. inlet and outlet parameters are not timedependent. The second is that the radial velocity profile in the annular space is fully developed in laminar conditions. All experiments were performed at Reynolds number Re = 60 with dh as characteristic length. The third assumption is that the internal mass transfer into the catalyst is negligible.20 From these assumptions, a two-dimensional (2D) model is proposed. This model is based on a convection/radial diffusion principle in the annular space of the reactor. The mass balance in the gas phase can be written as eq 12:

A nonlinear relationship is observed between the NCl3 removal rate and the irradiance, I. Some authors have suggested reaction mechanisms that may explain these experimental observations.10−12 The decomposition reaction may involve a “step by step” mechanism, or even a more complex process involving branching chain decomposition.24 Additional studies will be necessary to determine the precise nature of the reaction mechanism observed. However, based on our experimental results, it is possible to suggest the following reaction mechanism. The absorption of a photon by NCl3 leads to the formation of NCl2● and Cl● radicals

•

(8)

Cl 2 + hν → 2Cl•

Figure 2. Photolytic removal rate at 1 m vs irradiance.

•

2NCl4• → N2 + 4Cl 2

It must be remembered that once the reaction has proceeded for some removal rate, the Cl2 concentration is no longer negligible in the reactor. This Cl2 can then play a part in the process. The absorption of photons by gaseous chlorine leads to the formation of free Cl● atoms.

Cin and Cout are, respectively, the inlet and outlet NCl3 concentration (mol m−3). Irradiances measured at the outer side of the inner tube in the direction of the lamp of 9.2, 22.8, and 34 W m−2 were used. The results of these tests are shown in Figure 2, illustrating how nitrogen trichloride breaks down after absorbing UV-A irradiation.

NCl3 + hν → NCl 2• + Cl•

(7)

X and X′ are unidentified products. The photon can thus be absorbed by a single NCl3 molecule but will ultimately lead to degradation of six NCl3 molecules. It is, however, possible to suggest a branching chain decomposition, complementary to steps 5, 6, 7, and 8:24

RESULTS AND DISCUSSION Photolytic Degradation. Tests were performed to assess the removal rate (X) for NCl3 at different irradiances. X=

Cl• + NCl3 + X → NCl4• + X′

α=

β=

ri re

(14)

1 − α2

( α1 )

2ln

(15)

where ri is the internal radius (m) and re is the external radius of reactor (m). 4631



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Figure 3. Evolution of NCl3 concentration in the annular space of the reactor (a) and the corresponding removal rate (b).

monomolecular/monolayer LH model describing the kinetics of NCl3 decomposition is

Boundary conditions are as follows: • at the inner surface of the external cylinder, r = re: ⎛ ∂C ⎞ ⎜D ⎟ = 0 ⎝ m ∂r ⎠r

rp = I nk p (16)

e

i

1 + K pC

(18)

where I is the irradiance (W m−2), n is the order with respect to I, and kp and Kp are the LH rate constant (mol s−1 W1−) and the adsorption constant (m3 mol−1), respectively. Figure 3 shows how NCl3 degradation evolves in the reactor with an irradiance of 9.2 W m−2 and initial NCl3 concentrations of 2 × 10−4 (case 1), 1.1 × 10−4 (case 2), and 4.5 × 10−5 mol·m−3 (case 3). The compound almost completely disappears with a reactor length of around 0.6 m or longer. Photocatalysis is very effective compared to photolysis (Figure 2). It must be remembered that, with an irradiance of 9.2 W m−2, the contribution of photolysis to the degradation process is negligible in the cases presented here. In addition, the results indicate that the convection/diffusion/reaction model presented is quite well adapted to the process for all the concentrations tested. The removal rate (Figure 3b) seems not really dependent on the initial concentration of NCl3. Tests were performed to assess how irradiation of the catalyst affects the NCl3 removal rate (Figure 4a and b). The NCl3 concentration used for these tests was 2 × 10−4 mol m−3, while the irradiance varied between 0.09 and 9.2 W m−2. The kinetics of nitrogen trichloride decomposition depends on the irradiance emitted, and how many photons are absorbed by the catalyst. Figure 4c represents the experimental and modeled rates at 0.25 m for different irradiances and shows that two kinetic phenomena are apparent. Both of these are well predicted

• at the catalyst surface, r = ri, ⎛ ∂C ⎞ rp = ⎜ −Dm ⎟ ⎝ ∂r ⎠r

K pC

(17)

where rp is the photocatalytic rate (mol m−2 s−1). The kinetics of NCl3 degradation at the surface of the catalyst relies on a monomolecular and monolayer Langmuir−Hinshelwood model.14 Initially, the internal material transport of NCl3 into the catalyst is considered to be nonlimiting with respect to the reaction process. Some authors have shown that the internal material transport into the pores of each TiO2 grain in the medium used in tests can be neglected.20 The irradiance is taken into account in the kinetic law expression (eq 18) and varies between 0.09 and 9.2 W m−2 to limit NCl3 photolysis. Indeed, with an irradiance of 9.2 W m−2, the rate of photolytic NCl3 decomposition between the inlet and the outlet of the reactor is less than 2% for the three concentrations tested. NCl3 photolysis in the photocatalytic medium was therefore neglected. With relatively constant oxygen concentration and relative humidity, considering that the products formed are only partially and noncompetitively absorbed by the catalyst, the 4632



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Figure 4. Influence of irradiance on NCl3 decomposition case 1: I = 9.2 W m−2; case 2: I = 0.09 W m−2.

by the model. The results indicate that for an irradiance of greater than about 0.02I0, e.g. 0.6 W m−2, the kinetics is only affected by radial diffusion. The flux of photons absorbed by the catalyst does not limit NCl3 photocatalysis. For this range of light intensities, the process is considered to be of 0 order relative to I. However, chemical reactivity is apparent from an irradiance below 0.2 W·m−2. The constants of the LH model described above were determined from these experiments by the parametric optimization based on the least-squares method. Thus, n = 1, kp = 2.6 × 10−6 mol s−1 W1−, and Kp = 3 × 103 m3 mol−1. The LH model constants determined here only apply to the Ahlstrom 1048 photocatalytic medium. These results show that NCl3 photocatalysis is very effective relative to photolysis. For example, for an irradiance of 0.6 W m−2, the degradation yield is over 95% in the presence of the photocatalytic medium with a reactor length of 0.5 m. In these conditions, photolysis does not function at all. The effect of humidity on the photocatalytic degradation of NCl3 was studied with the same flow-rate as in the studies above, with an irradiance, I, of 9.2 W m−2. The NCl3 concentration at the reactor inlet was approximately 2 × 10−4 mol m−3. A relative humidity of 24% was used in this experiment. In this case, degradation was significantly less efficient than with a RH = 78% (Figure 5). Indeed, the relative humidity is a key parameter in the photocatalytic process as it is the main source of OH• radicals.9 Although an excess of humidity can, in some cases, inhibit the reaction process by competing for adsorption with the pollutant to be degraded, it contributes in the tests presented here to increasing the reaction speed. Thus, the OH• concentration, which is considered constant, was integrated into the kinetic constant kp of the LH model presented above. The overall apparent quantum yield for photocatalysis is defined by

Φ=

Figure 5. Influence of relative humidity on NCl3 decomposition.

where Φ is the overall quantum yield for photocatalysis, rp̅ is the average photocatalytic rate (mol m−2 s−1), and Fa (E m−2 s−1) is the light flux absorbed by the catalyst, with 388

Fa =

Faλdλ

(20) −2 −1

where Faλ is the light flux absorbed by the catalyst (E m s nm−1) for a given wavelength, λ. I Faλ = aλ NAhc (21) Here, Iaλ is the absorbed flux (W m−2) for a given wavelength, λ, NA is Avogadro’s number (mol−1), h is Planck’s constant (j s−1), and c is the speed of light (m s−1). Iaλ was calculated from light irradiation and optical properties of materials.26,27 About 10% of the light emitted was absorbed by the Pyrex tube; the TiO2 on the medium absorbed 62% of photons emitted.26,27 These values were integrated for 300 nm < λ < 388 nm.

rp Fa

∫300

(19) 4633



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Given the thickness of the photocatalytic medium, we considered photons to be homogeneously absorbed in the volume of medium. The quantum yield for the photocatalytic reaction indicates that a chain mechanism is not involved. However, the maximum quantum yield for the process as a whole can be determined. The maximum quantum yield can be determined based on the minimum number of photons that must be absorbed by the photocatalytic support to cause the desired rate of removal. Table 1 shows the relationship between the overall quantum yield of NCl3 degradation and the light flux absorbed by the catalyst.

The analytical techniques described in Section 2.5 indicate that the main products of the reaction are HClO (around 60%) and/or Cl2. HClO can be partially broken down by photolysis, even in the near UV range, according to the mechanism suggested by Vogt and Schindler:28 HClO + hν → OH• + Cl• ●

Cl and OH radicals can then react with NCl3 as part of reactions 10 and 29, or they can form Cl2 and increase the rate of NCl3 elimination. The presence of gaseous HCl is also confirmed by mass spectrometry; analysis of the contents of the CCl4-containing bubbler indicates that monochloramine represents about 10% (molar). Chloride was detected on the filters, representing around 20% (molar) of the initial nitrogen trichloride concentration. With regard to nitrogen-containing byproducts, the theoretical mechanism indicates the presence of N2. It is, however, likely that nitrogen oxides (NO and NO2) are formed by the reaction of HClO and OH ● with NCl2 ●. Given the wavelengths emitted by the light sources, it is also possible that traces of ozone are formed from nitrogen dioxide. The results presented in this study confirm that nitrogen trichloride can be readily degraded by a photocatalytic mechanism. Although the precise reaction mechanism has not been fully elucidated, the photocatalytic process, by improving photon capture, significantly increases NCl3 decomposition compared to photolysis alone. The convection/diffusion/reaction model proposed in this study predicts the evolution of nitrogen trichloride concentrations in an annular photocatalytic reactor working under laboratory conditions. This model could be easily adapted to design a full scale reactor. The constants of the Langmuir− Hinshelwood kinetic model were also determined for the photocatalytic support used in the tests described here. Beyond the efficacy of the process, this work demonstrates that nitrogen trichloride can largely be transformed into oxidizing chlorine, mainly in the form of gaseous chlorine and hypochlorous acid. Nitrogen-containing byproducts were not extensively characterized, and compounds such as ozone were not sought in this study. Thus, taken together, the results presented in this study show that it is possible to design a device to treat the gaseous effluent from the buffer tank of swimming pools. This type of procedure would make it possible to recycle the treated effluent after the stripping step, by reintroducing the chlorinated byproducts with known bactericidal properties into the water treatment circuit. However, complementary studies must be performed to assess the incidence of potential copollutants such as chloroform. Further study is also required to characterize the impact of chloride deposits on catalyst activity.

Table 1. Overall Apparent Quantum Yield at 0.25 m for Each Irradiance Tested absorbed light flux Fa (E m−2 s−1)

irradiance I (W m−2) 0 0.1 0.7 1.9 6.3 9.2

1.8 1.3 3.4 1.1 1.6

rp̅ (mol m−2 s−1) (× 107)

overall apparent quantum yield Ø

0 1.7 3.0 3.9 3.3 3.1

0.9 0.2 0.1 0.03 0.02

0 × 10−7 × 10−6 × 10−6 × 10−5 × 10−5

It appears, in this case, that the maximal overall quantum yield of the process is close to 0.9. This value was obtained in conditions where the chemical processes produced in each nanoparticle of TiO2, linked to multiphotonic effects, are negligible. The results presented in Table 1 suggest that the kinetics of photocatalysis can be maintained even with a significantly reduced irradiation of the light source fitted to the reactor, close to 0.6 W m−2. However, this observation is applicable only for the photocatalytic medium tested (Ahlstrom 1048). Based on the results obtained in these studies, a reaction mechanism for NCl3 decomposition can be suggested. The mechanism of catalyst activation and OH● radical production described in the literature, involving a single photon of adequate energy, is as follows:9 TiO2 + hν → TiO2 (e− + h+)

(22)

H 2O → OH− + H+

(23)

OH− + h+ → OH•

(24)

−

O2 + e → O2 O2

•−

•−

+

(25) •

+ H → HOO

(26)

•

2HOO → O2 + H 2O2 H 2O2 → 2OH

•

(27)

■

(28)

•

NCl3 + OH → NCl 2 + HClO •

2NCl 2 → N2 + 2Cl 2

S

Figure S1. Extinction coefficient of gaseous NCl3 and emission spectrum of the experimental lamp between 230 and 400 nm. This material is available free of charge via the Internet at http://pubs.acs.org.

(29)

■

(30)

According to this mechanism, the main product is chlorine gas, although this can be partially transformed in a wet medium into HClO and HCl, in line with eq 31. The stoichiometry obtained is three moles of Cl2 produced (or equivalent) for every two moles of NCl3 degraded. Cl 2 + H 2O → HClO + HCl

ASSOCIATED CONTENT

* Supporting Information

For the nitrogen trichloride degradation mechanism, we suggest the following steps: •

(32)

●

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +33 383 509 820; fax: +33 383 502 184. Notes

The authors declare no competing financial interest.

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Photocatalytic Conversion of Gaseous Nitrogen Trichloride into

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