Generation of Gaseous ClO2 from Thin Films of Solid NaClO2 by

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Generation of Gaseous ClO from Thin Films of Solid NaClO by Sequential Exposure to Ultraviolet Light and Moisture

Rishabh Jain, Reza Abbasi, Kevin Nelson, David Busche, David M. Lynn, and Nicholas L. Abbott ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16570 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 16, 2017

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ACS Applied Materials & Interfaces

Generation of Gaseous ClO2 from Thin Films of Solid NaClO2 by Sequential Exposure to Ultraviolet Light and Moisture

Rishabh Jain

†‡§

(shared first author), Reza Abbasi

‡

†

†§

David Busche , David M. Lynn , Nicholas L. Abbott

‡

(shared first author), Kevin Nelson , †*

† Department of Chemical and Biological Engineering, University of Wisconsin–Madison, 1415 Engineering Drive, Madison, WI 53706, USA ‡ Bemis Company, Inc., 2301 Industrial Drive, Neenah, WI 54956, USA § These authors contributed equally * Corresponding Author; E-mail: [email protected] KEYWORDS: Chlorine dioxide, triggered release, sodium chlorite, ultraviolet (UV) light, disinfectant gas, reactivity

Abstract We report that thin films of solid sodium chlorite (NaClO2) can be photochemically activated by irradiation with ultraviolet (UV) light to generate gaseous chlorine dioxide (ClO2) upon subsequent exposure to moisture. The limiting role of water in the reaction is evidenced by an increase in yield of ClO2 with relative humidity of the gas stream passed over the UVactivated salt. The UV-activated state of the NaClO2 was found to possess a half-life of 48 1 ACS Paragon Plus Environment


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hours, revealing the presence of long-lived UV activated species that subsequently react with water to produce gaseous ClO2. The yield of ClO2 was determined to be proportional to the surface area of NaClO2 particles projected to the incident illumination, consistent with activation of a ~10nm-thick layer of NaClO2 at the surface of the micrometer-sized salt crystals (for an activation wavelength of 254 nm). We also found that the quantity of ClO2 released can be tuned ~10 fold by varying wavelength of UV-irradiation and relative humidity of the gas stream passed over the UV-activated NaClO2. The UV-activated species were not detectable by electron paramagnetic resonance spectroscopy, indicating that the activated intermediate is not an excited triplet state of ClO2-. Additionally, neither X-ray photoelectron spectroscopy, Raman spectroscopy nor attenuated total reflection infrared spectroscopy revealed the identity of the activated intermediate species. The ability to pre-activate solid phase chlorite salt for subsequent generation of ClO2 upon exposure to moisture suggests the basis of new materials and methods that permit triggered release of ClO2 in contexts that use its disinfectant properties.

Introduction Chlorine dioxide (ClO2) is a powerful oxidizing agent and is widely used in bleaching processes in the paper pulp industry and as a disinfectant for water treatment 1. ClO2 has also been shown to be potentially useful as a biocide for food processing

2-5

, fungus and mold

fumigation 6-8, biofilm treatment 9-11 , the killing of bedbugs 12 and disinfection of anthrax spores 13

. For applications in this latter context, however, ClO2 cannot be stored or transported safely in

concentrated form14 and, instead, must be produced in situ before use 14-15. Production of gaseous ClO2 needs to be carefully controlled since ClO2 is toxic (it has a threshold limit value (TLV) classification by the Occupational Safety and Health Administration of 0.1 ppm), flammable and

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decomposes explosively at concentrations >10% (v/v) in air

15

. This study reports the new

finding that ClO2 can be evolved from solid NaClO2 in a two-step process in which it is first ‘activated’ by irradiation with UV light (200-400 nm) under dry conditions, and subsequently exposed to moisture to produce ClO2. We note that a variety of light activated materials (inorganic and organic) that show antimicrobial activity have been recently report16-21. These materials undergo physical or chemical changes upon activation with light, which subsequently results in release of antimicrobial agents (e.g., singlet oxygen (1O2) 19-21, reactive oxygen species (ROS)21, nitric oxide (NO)16-17) or photothermal heat18. Our discovery potentially adds new functionality to light-activated materials and provides new ways to produce ClO2 (that from materials incorporate NaClO2) in a manner that is convenient and safe, thus enabling the use of ClO2 as a broad spectrum gaseous biocide

22-24

. We comment also that the life time of singlet

oxygen and ROS is typically short in the presence of organics (the life time can range from a few microseconds to seconds).25. Although the lifetime of ClO2 will also vary depending on conditions (e.g., what organics are present),26.the lifetime of ClO2 is likely to be generally longer than that of singlet oxygen and ROS thus facilitating its transport from the point of generation.26.Past studies have reported that UV light irradiation can generate ClO2 from aqueous solutions of NaClO2 according to the general reaction scheme 27-28 + + ℎ → , , , , 1 A brief summary of other methods that have been used to generate ClO2 is included in Supporting Information.29-34 However, in all these studies, ClO2 generation ended upon cessation of UV-irradiation

27-29, 35

. Results reported in this paper, in contrast, reveal that UV-

activated samples of solid NaClO2 can retain their activity after UV-irradiation and that ClO2 can be produced upon subsequent exposure of the activated samples to moisture. We additionally 3 ACS Paragon Plus Environment


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document experimental conditions that provide insight into the physical and chemical processes that permit ClO2 to be produced subsequent to UV-irradiation upon reaction with water. We characterize the lifetime of the activated samples and reveal that the half-life is approximately 48 hours under the conditions used in our experiment. Additional experiments explore the yield of ClO2 and provide mechanistic insights based on varying the wavelength of UV irradiation, the projected area of the irradiated salt crystals, and the use of characterization methods such as electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and attenuated total reflection infrared (ATR-IR) spectroscopy. Overall, we conclude that ClO2 gas is evolved from a nanoscopically thin layer at the surface of the NaClO2 crystals that is directly illuminated with UV light. We anticipate that the principles and approaches developed in this study to achieve triggered release of ClO2 from UV-activated NaClO2 crystals may be useful in a range of applications that involve biocidal thin films and interfaces.

Materials and Methods Materials NaClO2 (80 wt.% pure technical grade) was obtained from Sigma-Aldrich (Saint Louis, Missouri). According to the manufacturer, the salt impurity is predominantly sodium chloride (NaCl; (NaClO2 wt.% ≈ 80, NaCl wt.% ≈ 18 and Na2CO3 wt.% ≤ 2)). We placed 100 mg of NaClO2 powder inside a small vial and then crushed and ground the powder thoroughly with a spatula (see Table S1 in Supporting Information for size distribution of NaClO2 crystals). Unless noted otherwise, this finely ground NaClO2 powder (chlorite salt with NaClO2 mass fraction of 80%) was used for all experiments. In addition, we performed a few experiments 4 ACS Paragon Plus Environment

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using high purity NaClO2 (purity ≥ 95 wt.%) obtained from Chemours company (Wilmington, Delaware; Chemours company provides sodium chlorite solution under the trade name Headline™ 3875). According to the manufacturer, the high purity NaClO2 is free of measurable Na2CO3 (NaClO2 wt.% ≥ 95 and NaCl wt.% ≈ 4). We dried aqueous solutions of the high purity NaClO2 to obtain the salt crystals. We placed 100 mg of NaClO2 salt crystals inside a small vial and then crushed and ground the powder thoroughly with a spatula (see Table S1 in Supporting Information for characterization of the size distribution of the NaClO2 crystals). In this paper, we adopted a simplified notation to denote the size of the NaClO2 crystals. Specifically, samples of NaClO2 crystals with sizes between 105 µm and 150 µm are denoted by the mean particle size D = 128 µm. Similarly, 1) samples containing NaClO2 crystals with sizes less than 105 µm are denoted as D < 105 µm, 2) samples of NaClO2 crystals with sizes 150 µm to 250 µm or 250 µm to 850 µm are denoted as D = 200 µm and D = 550 µm, respectively, and 4) sample of NaClO2 crystals with sizes greater than 850 µm are denoted as D > 850µm.

UV-Activation of NaClO2 and Subsequent Exposure to Humid Vapor A gas flow cell (internal volume: 4 cm × 4 cm × 1 cm = 16 cm3) was fabricated from stainless steel with 3 ports (radius = 2 mm) and two openings for windows (5 cm × 5 cm = 25 cm2, see Figure S1 in Supporting Information). Two of the ports were fitted with stainless steel tubing along with valves (Swagelok) to serve as gas inlets and outlets. The third port was fitted with a humidity sensor (Honeywell Analytics, #HIH-4602-C; Poole, United Kingdom). The two window openings were fitted with UV-transparent windows made of fused silica (Edmund Optics; Barrington, New Jersey). Most experiments were performed with a stream of N2 gas (Industrial grade, AirGas, Inc. ; Radnor Township, Pennsylvania) flowing through the gas flow cell at a flow rate of 1000 5 ACS Paragon Plus Environment


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cm3/min. The gas inlet was either connected (i) directly to a N2 gas cylinder (dry gas flow condition) or (ii) to a dew point generator (LI-610, LI-COR Inc.; Lincoln, Nebraska) that was connected to a N2 gas cylinder (moist gas flow condition). The moisture content of the N2 gas flowing into the cell was adjusted to 15, 30, 45, 60 or 75% relative humidity (RH) using the dew point generator (temperature = 25 °C). The gas outlet from the flow cell was connected to a ClO2 detector (GasAlert Extreme, Honeywell Analytics; Poole, United Kingdom) via a fixture provided with the detector. For each experiment, fresh samples of powdered NaClO2 (100 mg) were spread in a thin layer (radius ~1 cm) on a small plastic boat placed on the bottom window of the gas flow cell (see Figure S1 in Supporting Information). Samples inside the gas flow cell were ‘activated’ by exposure to ultraviolet (UV) light introduced through the UV-transparent window on top of the flow cell. UV light of three different wavelengths was used to illuminate the sample: (i) 254 nm (short-wave, UV-C), (ii) 312 nm (medium-wave, UV-B) and (iii) 365 nm (long-wave, UV-A). UV light was generated using UV lamps (Spectroline, #EN-280L (UV - 365 nm), #EBF-280C (UV - 312/254 nm); Westbury, New York) which were placed at a distance of 6 cm from the sample (irradiance at sample 1-1.5 mW/cm2, as discussed below). Moist N2 gas was passed over activated/non-activated (control) NaClO2 samples, and ClO2 in the gaseous stream exiting the flow cell was quantified by the ClO2 detector, which logged one data point every five seconds. The data were transferred to a computer for further analysis. The total mass of ClO2 produced in any experiment was calculated by integration of the release profile. All experiments reported in this paper were repeated at least three times. The UV intensity at a distance of 6 cm from each lamp was measured using a radiometer (UV Power Puck® II, EIT Inc.; Leesburg, Virginia, (N=3)) and it was found to be 1.15 mW/cm2 (UV – 254 nm), 1.43 mW/cm2 (UV – 312 nm) and 1.01 mW/cm2 (UV – 365 nm), respectively.


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We also note that the UV lamps used to illuminate NaClO2 samples are not monochromatic and have a spectral spread. Information provided by the manufacturer of the lamps indicated that the limits of the spectral spread for UV-254 nm, UV-312 nm and UV-365 nm lamps were 240-280 nm, 280-320 nm and 320-400 nm respectively. In the remainder of this paper, we refer to these lamps by the wavelength of the peak of the emission spectrum of each lamp (namely 254 nm, 312 nm, and 365 nm).

Determining the Initial Moisture Content of NaClO2 Samples (80 wt.% pure NaClO2) Using Karl-Fisher Titration and Thermogravimetric Analysis (TGA) We determined the initial moisture content of NaClO2 samples (80 wt.% pure NaClO2; prior to exposure to flowing N2 gas) using Karl-Fisher titration and thermogravimetric analysis (TGA) to be 0.13±0.01% and 0.16±0.01%, respectively (see Figure S2 for the TGA profile). NaClO2 can exist in an anhydrous and a hydrated form (NaClO2·3H2O), the latter of which dehydrates at 37.4 0C (the expected weight percent of water in NaClO2·3H2O is 37%) 36-37. Our measurements of water content indicate that the NaClO2 samples used in this study are in anhydrous state. In addition, the TGA profile indicates that desorption of water from our NaClO2 samples occurs at two distinct desorption temperatures (one peak around 80 0C and one around 120 0C; Figure S2). Both temperatures are higher than the dehydration temperature of NaClO2·3H2O, which provides further evidence that NaClO2 samples used in this study are in anhydrous state.We also note that the 80 wt.% pure NaClO2 samples contain less than 2% Na2CO3 impurity. Na2CO3 is known to form several hydrates. The dehydration temperature of the known Na2CO3 hydrates are 33.5 °C (heptahydrate;

Na2CO3.

7H2O),

34 °C

(monohydrate; Na2CO3. H2O) respectively

(decahydrate; 38-39

Na2CO3.

10H2O),

and

100 °C

. None of these temperatures correspond to the



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TGA peaks observed at 80 0C and 120 0C. Therefore, we believe that the two observed TGA peaks correspond to physically adsorbed water in chlorite samples.

Additional Characterization As detailed in the Supporting Information, we used (i) Karl-Fisher titration to determine the moisture content of NaClO2 samples (80 wt.% pure NaClO2) that were in contact with N2 having different levels of relative humidity (RH=0, 15, 30, 45, 60, or 75%, flow rate= 250 cm3/min, duration of exposure=one hour) , (ii) thermogravimetric analysis (TGA) to independently verify the moisture content of powdered NaClO2 samples (80 wt.% pure NaClO2) that were in contact with N2 having different levels of relative humidity (RH=0, 15, 30, 45, 60, or 75%, flow rate= 250 cm3/min, duration of exposure=one hour) and to investigate the bound state of water (loosely versus strongly bound water), and (iii) EPR spectroscopy, XPS spectroscopy, confocal Raman microscopy, and IR spectroscopy to analyze NaClO2 samples before and after exposure to UV light.

Results and Discussion Evolution of Gaseous ClO2 from UV-Activated NaClO2 Finely ground NaClO2 powder (100 mg) was spread in a thin layer (approximately circular area, with radius of 1 cm) on a small plastic boat placed on the bottom window of the gas flow cell (see Materials and Methods for details). After closing the flow cell, dry N2 gas was passed through the gas flow cell for 15 min. Subsequently, the samples were exposed to UV light for 15 min. After UV exposure, unless otherwise indicated, the samples were kept in darkness for 15 min under dry N2. A flow of moist N2 was then initiated, and the concentration of ClO2 in the


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gas exiting the flow cell was measured using an electrochemical detector (see Table S2 in Supporting Information for experimental protocol). Figure 1A shows the ClO2 concentrations measured in the gas exiting the flow cell when using samples activated with UV light (λ = 254 nm) and exposed subsequently to varying levels of relative humidity (RH). Inspection of Figure 1A reveals that no ClO2 was detected when samples were exposed only to moist N2 (without pre-exposure to UV light) or only to UV light (without post-exposure to moisture). However, samples that were exposed sequentially to both UV light and moist N2 gas generated concentrations of ClO2 that increased with the relative humidity of the flowing N2 (15-60% RH). We note that RH levels greater than 75% generated a burst release of ClO2 from the samples that resulted in saturation of the electrochemical detector response and thus prevented quantification of the amount of ClO2 released (data not shown). Overall, Figure 1A reveals that NaClO2 samples can be photochemically activated to produce ClO2 upon subsequent exposure to moist N2 gas. As noted in the Introduction, past studies have reported that ClO2 can be generated from aqueous solutions of NaClO2 by UV irradiation

29-32

. In contrast to our observations using

NaClO2 salt particles, the production of ClO2 in those past studies ceased as soon as UVirradiation was turned off.

The results in Figure 1A, however, suggest that our experimental

approach generates an activated intermediate species that subsequently leads to the production of ClO2 upon exposure to water. We also note that the total mass of ClO2 potentially available for release in our experimental system is ~60 mg (assuming that one mole of NaClO2 produces one mole of ClO2), which is > 1000 times higher than the highest total mass of ClO2 (~12 µg) that we measured over 15 min. This observation implies that our experiments activated only a small portion of the NaClO2 powder, a point that we discuss in detail below. 9 ACS Paragon Plus Environment


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Past studies have demonstrated that gaseous ClO2 is effective as an antimicrobial gas at concentrations between 50 ppm and 300 ppm 40-43. In our experiments, we calculate the mass of ClO2 (~12 µg) produced during exposure to air at RH=60% for 15 mins establishes concentrations of 50 ppm and 300 ppm in volumes of 90 cm3 and 15 cm3, respectively (temperature = 25 °C and pressure = 1 atm). We conclude, therefore, that the above method of production of ClO2 gas can potentially be used in applications requiring antimicrobial activity. We note also that, as will be shown below, it is possible to increase the yield of ClO2 yield not only by increasing the RH of the moist N2 flow but also by increasing the NaClO2 dispersion.

Lifetime of UV-Activated NaClO2 We quantified the lifetime of the UV-activated NaClO2 samples described above by storing them in a closed chamber containing dry N2 gas for periods of time ranging from one hour to 48 hours prior to exposure to N2 gas containing 60% RH for 15 minutes (duration of illumination = 15 min, λ = 254 nm; see Table S2 in Supporting Information for details). Figure 1B shows the total mass of ClO2 released from these samples and leads to two key observations. First, storage of the activated NaClO2 samples for as long as 15 hours did not diminish the yield of ClO2 generated from the samples upon subsequent exposure to moist N2 gas. Second, the mass of ClO2 generated after storage for 48 hours decreased to about one-half of the maximum amount of ClO2 released following activation. We also performed a series of experiments to investigate whether the generation of ClO2 from UV-activated NaClO2 was a consequence of NaClO2 impurity (in these experiments, we used high purity (purity > 95%) NaClO2 obtained from Chemours company). Figure 1B shows results obtained using the high purity NaClO2 salt. The purity of the NaClO2 did not substantially impact the lifetime of the activated state of the salt in our experiments.

Overall, these results support the hypothesis that a long-lived activated 10 ACS Paragon Plus Environment

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intermediate (half-life of ~48 hours) forms as a result of UV-irradiation, and that this reactive intermediate subsequently reacts with water upon exposure to moist N2 flow to produce ClO2.

Effect of Dose of Radiation on Yield of ClO2 As noted above, based on the mass of NaClO2 salt used in our experiments, we calculated the overall yield of ClO2 following exposure of the UV-activated NaClO2 samples to water to be small. The highest mass of ClO2 released in 15 min was ~12 µg, which corresponds to a $%&'( )&* ,-%./0'.

+ fractional yield of only 0.02% ( ! " = $%&'( 12)&*

+ 2324&25&'

× 100 )). To determine

whether the dose of radiation limited the yield of ClO2, we changed the duration of UV exposure (see Table S2 in Supporting Information; The RH of the N2 gas stream used to generate ClO2 was kept constant at 60% in these experiments). Figure 2 shows the total mass of ClO2 produced from NaClO2 samples that were activated with UV light (λ = 254 nm) for different lengths of time prior to exposure to the humid N2 for 15 min. Inspection of Figure 2 reveals that the mass of ClO2 generated from the activated NaClO2 increased almost linearly (from ~0.9 µg to ~12 µg) with increase in the UV exposure time from 1 min to 10 min. For longer exposure times, however, an increase in the dose of radiation had little effect on the mass of generated ClO2. Thus, we conclude that, for a UV exposure time greater than 10 min, the dose of radiation is not limiting the ClO2 yield. As detailed below, our measurements suggest that the upper bound on the yield of ClO2 evidenced in Figure 2 arises because only a thin layer of NaClO2 on the surface of the salt crystals is activated by the UV-illumination. We hypothesize that it is this thin layer of salt that subsequently reacts with water to yield ClO2.



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Evidence that a Thin Layer of NaClO2 on the Crystals is Activated by UV light As noted above, we determined that the maximum amount of ClO2 generated in our experiments is ~12 µg and that the yield (i.e., 0.02%) is not limited by UV dose for exposure times longer than 10 min (see Figure 2). Below we report the results of several experiments performed to test the hypothesis that only a thin layer of NaClO2 on each salt grain that is directly exposed to UV-radiation is activated and subsequently reacts with water to produce ClO2. First, we performed a series of experiments in which we repeatedly cycled NaClO2 samples between activation with UV light (λ = 254 nm) and exposure to moist N2 gas. As shown in Figure 3A, after four cycles of photoactivation and exposure to moisture, no additional ClO2 was produced. However, by reorganizing and reorienting the grains of salt in the flow cell using a spatula, we found that it was possible to produce additional ClO2 upon subsequent exposure to UV light and moist N2 gas. The amount of ClO2 produced from the reorganized salt grains was comparable to that generated from freshly prepared NaClO2 samples. These experiments suggested that ClO2 was evolved from only the upper layer of the NaClO2 sample that was exposed to UV light and that after four activation cycles a passivating thin film forms on the +

-

-

surface of the NaClO2 crystals. We speculate that this film consists of Na , Cl , ClO and ClO3

-

ions. We further tested the hypothesis that it is the surface regions of the salt grains onto which UV light is incident that generate ClO2 by varying the size of the NaClO2 particles presented to the incident UV light while keeping the projected area of the NaClO2 sample constant. If the above mentioned proposal is true, the amount of ClO2 produced should be independent of particle size when the projected area is held constant. To perform these experiments, we 12 ACS Paragon Plus Environment

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distributed multilayers of NaClO2 particles (total mass = 100 mg) with different mean sizes over the same circular area (radius ~1 cm) and then we measured the amount of ClO2 released. We used samples with particle sizes of 1) D < 105 µm 2) D = 128 µm and 3) D = 200 µm (see Materials and Methods for definition of D, and Table S2 in Supporting Information). The results of these experiments, which are summarized in Figure 3B, demonstrate that the amount of ClO2 produced was independent of the mean size of the NaClO2 crystals. This observation is consistent with the hypothesis that the amount of ClO2 released is proportional to the projected area of the multilayer of NaClO2 particles, independent of the mean size of the NaClO2 particles. Next, we performed experiments using either 100 mg, 80 mg, 60 mg, 40 mg, 20 mg, 10 mg or 5 mg of NaClO2 particles (D = 128 µm) to generate monolayers of particles with distinct projected areas. In these experiments, the projected area scales with the mass of NaClO2 (see Table S2 in Supporting Information). The results, which are summarized in Figure 4A, reveal that the amount of ClO2 produced varies linearly with the mass of NaClO2 and that the yield of ClO2 is approximately independent of the mass of NaClO2. This observation also supports our hypothesis that only regions of the surface of the NaClO2 particles that receive UV light are activated. Finally, we performed a series of experiments in which we distributed NaClO2 particles (80 mg) as monolayers (i.e., the layers were one particle thick) and then measured the ClO2 produced from the monolayers (see Table S2 in Supporting Information). Three different particle sizes were used, namely 1) D = 128 µm, 2) D = 200 µm and 3) D = 550 µm (the surface area projected by these monolayers are different). According to our hypothesis, the ClO2 released from each monolayer should be directly proportional to the number of particles (Np) and the projected area of each particle (AProjected), with Np and AProjected calculated as 13 ACS Paragon Plus Environment


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Initial Mass of NaClO VW N9 = A9MSTQPNQU = 2 ρGHIJK+ V9HMNOPJQ 4 where V9HMNOPJQ =

Z[ \ ]

is the volume of each NaClO2 particle and ρGHIJK+ is the mass density of

the NaClO2 particles. Because the number of particles is inversely proportional to the volume of _

each particle N9 ∝ [\ and because the projected area of each particle is directly proportional to diameter squared (A9MSTQPNQU ∝ W ), the amount of ClO2 is predicted to vary as (

_ [

). Inspection

of Figure 4B reveals that the amount of ClO2 produced from the monolayers does vary in an _

approximately linear manner with ([). This third set of measurements are, therefore, in close agreement with the above prediction and provide support for our hypothesis. Overall, the above results are consistent with UV-activation of only a thin outer layer of the NaClO2 particles that is directly exposed to UV-irradiation. By assuming that the salt crystals are spherical particles and that one mole of activated NaClO2 produces one mole of ClO2, the mass of ClO2 produced can be used to calculate the thickness of the activated layer. We estimate the thickness of this layer to be around 10 nm (see Supporting Information and Table S3 for more details). We note that the molar extinction coefficient of NaClO2 dissolved in water is 154 M-1cm-1 at 260 nm (the UV-Visible spectra of aqueous solution of NaClO2 shows a maximum at 260 nm) 44. Using the density (2.5 g/cm3)

44

and molecular weight of NaClO2 (90.44 g/mole)

44

and

assuming that the absorption of ClO2- in both the solid and solution are the same, a 4.8 µm thick layer of NaClO2 crystal would transmit 1% of light at 260 nm (neglecting scattering). Since the NaClO2 crystals used in our experiments are much larger than 4.8 µm, this implies that a top layer of NaClO2 crystals could shield a lower layer from activation. We note also that our 14 ACS Paragon Plus Environment

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estimate of the thickness of the activated layer of NaClO2 is much less than 4.8 µm, suggesting that the depth of penetration of light into the crystals does not limit the reaction leading to ClO2 gas. We finish this section by commenting on the relative importance of the rate of mass transport of water to the surface of NaClO2 crystals compared to the rate of solid phase reaction which generates ClO2. Our calculations show that the rate of mass transport of water to the surface of NaClO2 crystals is much faster than the rate of the reaction in the solid phase that generates ClO2 and therefore the reaction rate in the solid phase is the limiting factor in ClO2 generation (see Supporting Information for more details).

Effect of the Initial Moisture Content of NaClO2 on ClO2 Generation We reported above that increasing the RH of the nitrogen gas following UV activation increased the yield of ClO2 (for RH values between 15% and 60%). This observation indicates that water plays a limiting role in ClO2 generation at low humidity. Here we describe a series of experiments that provides additional insight into the role of water in determining the release of ClO2 from UV activated NaClO2 samples. Specifically, we investigated the role that the initial moisture content of the NaClO2 samples plays in the generation of ClO2. To determine if the initial moisture content of the salt impacts the amount of ClO2 generated, we contacted NaClO2 samples with moist N2 gas with a range of RH values (0, 15, 30, 45, 60, 75 %) for one hour (flow rate = 250 cm3/min) and then subsequently activated the NaClO2 samples with UV light (λ = 254 nm). Measurements of the release of ClO2 in these experiments were performed using dry N2 (the head space moisture was eliminated before activation by flowing dry N2 (flow rate = 1000 cm3/min) for 20 seconds, see Table S2 in Supporting Information for the experimental scheme). The results in Figure 5A show that the 15 ACS Paragon Plus Environment


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amount of ClO2 released from the activated NaClO2 samples increased with the RH of the nitrogen gas used to pre-treat the samples. We used Karl-Fischer titration and thermogravimetric analysis (TGA) to determine the moisture content of the NaClO2 samples pre-equilibrated with moist N2 gas (as described above) and to determine the state of the bound water (loosely bound water versus strongly bound water). The moisture content of the NaClO2 samples is summarized in Table 1 and the TGA profiles are included in the Supporting Information. Inspection of Table 1 reveals that the moisture content of the NaClO2 samples generally increased with increasing the RH of the nitrogen gas to which the samples were exposed (this trend is more obvious in the TGA results). This observation, when combined with the results discussed in the previous section, leads us to conclude that the amount of ClO2 produced increases with the initial moisture content of NaClO2 samples (see Figure 5B). We note, however, that the amount of ClO2 produced is a strongly non-linear function of the moisture content of the NaClO2 samples. These results hint that the reaction mechanism giving rise to ClO2 generation may be complex, with water involved in several different steps similar to the photochemical reaction mechanisms suggested for ClO2 generation from aqueous NaClO2 solutions (see Supporting Information for the photochemical reaction mechanisms suggested for ClO2 generation from aqueous NaClO2 solutions at different wavelengths)27-28. In addition, inspection of the TGA profiles in Figure S2 reveals two peaks in the weight derivative profile for NaClO2 samples contacted with moist N2 gas with RH values of either 0, 15, 30, 45, or 60% (contact time = one hour; flow rate = 250 cm3/min). One peak is observed around 80 °C (weakly bound water) and the other peak is observed around 120 °C (strongly bound water) (the water weight percentage related to each of these peaks is summarized in Table


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2). Close inspection of Table 2 demonstrates that most of the moisture content of the NaClO2 samples (~80%) is in the form of weakly adsorbed water. For the case of NaClO2 samples contacted with moist N2 gas with RH = 75% (contact time = one hour; flow rate = 250 cm3/min), only one peak is observed in the TGA profile around 65 °C and therefore for these samples all of the moisture content is in the form of weakly adsorbed water (see Figure S3). We calculated the amount of ClO2 that would be generated if the water within the NaClO2 samples reacted to form ClO2 (in the absence of the detailed mechanism of the reaction, we assumed that one mole of water reacts with one mole of NaClO2 to produce one mole of ClO2; our conclusions are not strongly dependent on this assumption). The results in Table S4 reveal that the amounts of ClO2 produced during the above-described experiments are much lower (by a factor of 360) than the amounts of the ClO2 that would be produced if all the water in the NaClO2 sample reacted to form ClO2. This result is consistent with our earlier results and supports the proposal that only a thin layer of NaClO2 on the surface of the grains is activated by UV-irradiation and that only water within and/or near this thin layer participates in ClO2 production.

Effect of Wavelength of UV Irradiation on Yield of ClO2 from Activated NaClO2 Samples Past studies of the generation of ClO2 from aqueous solutions have demonstrated that the ClO2 yield (defined as moles of ClO2 produced per moles of C1O2- consumed during the reaction) is strongly dependent on the wavelength of the UV irradiation

27-28

. For example, it

was shown that light with wavelengths of 253.7 nm or 313 nm are much more efficient in producing ClO2 from aqueous solutions of NaClO2 than light with a wavelength of 365 nm (i.e., the ClO2 yield is ~0.48, ~0.48 and ~0.19 for λ = 253.7, 313, and 365 nm respectively) 17 ACS Paragon Plus Environment

27-28

.


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Although the mechanism leading to ClO2 production in aqueous solution has not yet been fully elucidated, it has been proposed that illumination of aqueous solutions of NaClO2 at λ = 365 nm results in the formation of oxygen radicals in ground state, while illumination of aqueous solutions of NaClO2 at λ = 253.7 or 313 nm generates oxygen radicals in excited state. Furthermore, it was suggested that the different excited states of oxygen radicals may result in different reaction pathways for photodecomposition of C1O2-, which in turn result in different ClO2 yields (see Supporting Information)

27-28

. We emphasize, however, that no experimental

evidence exists to directly confirm the existence of oxygen radicals (either in the ground or excited states) 27-28. Motivated by the above-reported effects of UV wavelengths on ClO2 generation from aqueous NaClO2 solutions, we quantified the effect of wavelength of UV irradiation on ClO2 release from activated grains of NaClO2. The experimental protocol used in these experiments was identical to that described above in the context of Figure 1A using λ = 254 nm except that the NaClO2 samples were activated at longer UV wavelengths (λ = 312 nm, and λ = 365 nm; see Table S2 in Supporting Information). Figure 6 reveals that the amount of ClO2 produced decreases with increasing UV wavelength.

Furthermore, (i) the minimum RH at which

detectable levels of ClO2 (> 0.01 ppm) were produced was found to be dependent upon the wavelength of UV activation (30% for 254 nm, 45% for 312 nm and 45% for 365 nm), and (ii) the total ClO2 produced decreased from ~2.5-12 µg (30-60% RH) for λ = 254 nm, to ~1.7-6.6 µg (45-60% RH) for λ = 312 nm and to ~0.9-1.5 µg (45-60% RH) for λ = 365 nm. These results clearly demonstrate that shorter UV-wavelengths are more efficient at producing ClO2 from activated NaClO2 powders. Although we do not yet fully understand the origin of the difference, we note that our observation that λ = 254 nm is more efficient than λ = 313 nm at producing


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ClO2 from activated NaClO2 powders differs from the results reported previously with aqueous NaClO2 solutions for which these two wavelengths yielded similar amounts of ClO2 27-28. We comment that λ = 254 nm is higher in energy in comparison with λ = 313 nm and λ = 365 nm and therefore one might hypothesize that the higher energy of short wavelength light is responsible for the difference in ClO2 yield. We tried, however, to activate NaClO2 samples with X-rays (λ = 0.83435 nm) but were not able to detect ClO2 from the X-ray activated samples. In these experiments, we activated a monolayer of NaClO2 particles (mass NaClO2 = 50 mg; D = 128 µm) by scanning an X-ray beam over the monolayer (X-ray intensity = 1.5×107 mW/cm2 = 6.296×1019 photons/(s.cm2); beam spot size = 400 µm × 600 µm) for different exposure times (namely one, five or ten seconds) and then exposed the samples to moist N2 flow (RH = 75%, flow rate = 1000 cm3/min and delay time between X-ray activation and exposure to moist N2 flow ~ 5 min). We note that the intensity of UV-irradiation used in the UV-activation experiments were 1.15 mW/cm2 = 1.469×1015 photons/(s.cm2) (UV – 254 nm), 1.43 mW/cm2 = 2.244×1015 photons/(s.cm2) (UV – 312 nm) and 1.01 mW/cm2 = 1.855×1015 photons/(s.cm2) (UV – 365 nm). Therefore, the X-ray radiation dose (photons/cm2) was ~30 (for exposure time of 1 s), ~150 (for exposure time of 5 s) and ~310 (for exposure time of 10 s) times higher than the UV-irradiation dose used in the UV-activation experiments. This observation hints that the energy level of the radiation does not directly determine the ClO2 yield.

Physicochemical Characterization of Activated NaClO2 Samples A key finding reported in this paper is that a long-lived activated species is formed as a result of the UV activation of solid NaClO2, and that the activated species can react with water to produce ClO2. Buxton and Subhani reported a semi-quantitative reaction mechanism to predict the product distribution of photo-decomposition of NaClO2 in aqueous solutions upon UV 19 ACS Paragon Plus Environment


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irradiation at different wavelengths 27-28. In their proposed reaction mechanism, they included an elementary reaction step in which a reactive intermediate facilitates transfer of an electron from one chlorite ion (ClO2-) to another resulting in the formation of ClO2. Although they did not present experimental evidence to support the existence of this reactive intermediate, they speculated that it might be an excited triplet state of the chlorite ion (ClO2-) (see Supporting Information) 27-28. Motivated by the above mentioned studies, we checked for the possible presence of an excited triplet state of ClO2- using EPR spectroscopy. In brief, EPR is a spectroscopic technique that is widely used to identify the presence of species that contain unpaired electrons (e.g. triplet state molecules, free radicals, transition metal ions and complexes, and odd-electron compounds) 45

. The EPR first derivative spectra of non-activated NaClO2 samples and samples that were

activated at three wavelengths of UV light (namely 254 nm, 312 nm, and 365 nm) are compared in Figure 7A. Although close inspection of Figure 7A reveals absorption in the non-activated NaClO2 samples (we note that Na+ and ClO2- ions do not have unpaired electrons, and are therefore not EPR active), EPR first derivative spectra of NaClO2 samples before and after activation at wavelengths of 254 nm, 312 nm, and 365 nm are almost identical. We conclude that we are not able to detect the presence of any activated triplet state associated with UVirradiation. The EPR set up used in this study can detect EPR active species with concentrations as low as 10 ppm. We expect the concentration of the activated intermediate in our study to be higher than 200 ppm (which corresponds to a yield of 0.02 %). Overall, we interpret the absence of a measurable difference between the EPR spectra of activated and non-activated NaClO2 samples to indicate that the activated intermediate cannot be an excited triplet state of ClO2-.


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We also used XPS to analyze activated and non-activated NaClO2 samples. We found that XPS is destructive toward NaClO2 by comparing the XPS spectra recorded at different levels of X-ray exposure (one scan versus twenty scans; see Supporting Information for more details). Accordingly, we only present XPS spectra recorded at minimal X-ray exposure (i.e., one scan, see Figure 7B). As is apparent from Figure 7B, we are not able to detect any signature of a new species in UV-activated NaClO2 samples (see Table 3 for peak assignments; the chloride peak assignment was performed based on a past study 46). Although we have not been able to find past XPS studies of chlorite- or hypochlorite-containing compounds, we speculate that the strongest two peaks observed in the spectra recorded with one scan (binding energies of 203.5 and 205.1 eV) correspond to the chlorite ion and the two peaks that are only observable when the spectra are recorded with twenty scans (binding energies of 206.5 and 208.1 eV) correspond to the hypochlorite ion (see Supporting Information for more details).Finally, we performed attenuated total reflection (ATR) spectroscopy and Raman spectroscopy to characterize the activated NaClO2 samples. In both cases, we were not able to find any spectroscopic signature of a new species after UV activation (see Supporting Information). In these cases, the lack of measurable signal of the activated intermediate may be due to limits in the instrument sensitivities.

Conclusions The key discovery reported in this paper is that ClO2 can be generated from solid NaClO2 by first photoactivating it with UV light (200-400 nm) and then exposing it to moisture. Importantly, the activated NaClO2 samples exhibited a half-life of 48 hours. Our results are consistent with a mechanism in which a thin layer of the NaClO2 that is directly exposed to UV



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irradiation is activated and subsequently reacts with water to produce ClO2 (based on this mechanism, we expect the yield of ClO2 to increase by increasing the dispersion of NaClO2 crystals). Whereas our results document the effects of wavelength of UV irradiation, relative humidity, and dose of UV irradiation, additional studies are needed to elucidate the photochemical pathways responsible for the observations reported in this paper. Future experiments that activate and characterize highly dispersed NaClO2 particles (particle sizes of the order 10 nm or less) may provide additional insight into the reaction mechanisms. Our results demonstrate that ClO2 can be generated in a facile and controlled manner from NaClO2 using UV-irradiation and subsequent exposure to moisture. Several past studies have reported the use of gaseous ClO2 to sterilize or disinfect fresh produce (fruits, vegetables and poultry products)

2, 4, 41-42

. Implementation of the use of ClO2 in these contexts, however, has

been limited by the challenge of generating ClO2

33-34

. The methods reported in this paper may

provide the basis of an easily implemented method for generation of ClO2. For example, sodium chlorite could be integrated into polymeric films used in packaging, and activated prior to introducing a product into the package. (because of the widely adjustable physical and chemical attributes of polymers, antimicrobial polymers hold promise for controlling release rate of antimicrobial agents and mitigating the potential toxicity of residual agents

16, 47-50

). Subsequent

exposure of the photoactivated package to moisture would lead to release of ClO2 and sterilization/disinfection 2, 4, 8, 22, 24, 41-42.

Supporting Information. Detailed experimental procedures and additional results discussed in the text are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. 22 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Financial support was provided, in part, by Bemis Company, Inc., and the Wisconsin MRSEC (DMR-1121288). Facilities used in the research were also supported by the Wisconsin MRSEC. R. Abbasi acknowledges Bird-Stewart-Lightfoot (BSL) fellowship from the University of Wisconsin-Madison.

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Figure 1. (A) Concentration of gaseous ClO2 released from powdered NaClO2 (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01) that was activated with UV light (λ = 254 nm) and subsequently exposed to moist N2 gas (the gas flow rate was 1000 cm3/min) (N=3). (B) Total mass of gaseous ClO2 released over 15 minutes from UV-activated (λ = 254 nm) NaClO2 samples (the gas flow rate was 1000 cm3/min) that were stored for different times after UV-activation and before exposure to moist N2 gas (60% RH) (N=3).



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Figure 2. Total ClO2 released in 15 minutes from NaClO2 samples (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) at 60% RH that were pre-activated with UV light (λ = 254 nm) for different times (0-30 minutes) (N=3). The smooth curve is drawn to guide the eye.


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Figure 3. (A) Concentration of ClO2 released from NaClO2 samples (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) in experiments where the NaClO2 samples were repeatedly cycled between activation with UV light (λ = 254 nm) and exposure to moist N2 gas (the gas flow rate was 1000 cm3/min) (N=3) (B) Total mass of ClO2 released from multilayers of NaClO2 particles (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) with different mean sizes in 15 minutes (λ = 254 nm, and RH = 60%; mean diameter is indicated as D) (N=3).



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Figure 4. (A) Total mass of ClO2 released from a monolayer of activated NaClO2 particles (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) in 15 minutes as a function of mass of NaClO2 (average NaClO2 particle size =128 µm, λ=254 nm, and RH=60%). The yield of ClO2 was calculated from the mass of NaClO2 and the amount of ClO2 produced from each sample (N=3). The line is drawn to guide the eye. (B) Total mass of ClO2 released from a monolayer of NaClO2 particles (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) in 15 minutes as a function of mean diameters of the NaClO2 particles (mass NaClO2=80 mg, D (mean diameter) =128 µm, 200 µm, or 550 µm, λ=254 nm, and RH=60%) (N=3). The line is drawn to guide the eye.


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Figure 5. (A) Total mass of ClO2 released in 15 minutes from NaClO2 samples (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) that were contacted with a stream of N2 for one hour (RH=0, 15, 30, 45, 60 and 75 %) and activated subsequently with UV light (λ=254 nm) for 15 minutes. The ClO2 was generated in dry N2 (headspace moisture was eliminated by flowing dry N2 (1000 cm3/min) for 20 seconds before UV activation) (N=3). (B) Total mass of ClO2 released as a function of the initial moisture content of NaClO2 sample (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) (the moisture content was determined by TGA) (N=3). The curve is drawn to guide the eye.



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Figure 6. Mass of ClO2 released from NaClO2 samples (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) following activation with 3 different wavelengths of light (λ=254 nm, 312nm, and 365 nm) and subsequent exposure to moist N2 with different RH values (N=3).


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Figure 7. (A) EPR first derivative spectra of NaClO2 samples (80 wt.% pure NaClO2; initial weight

percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) before and after activation at λ=254 nm, 312 nm, or 365 nm (N=3) (B) XPS spectra of NaClO2 samples (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) =

0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) before and after activation at λ=254 nm, 312 nm, or 365 nm (number of scans=1 scan) (N=3).



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Tables Table 1. Quantification of water (Karl-Fischer and TGA) (wt.%) in NaClO2 samples (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) that were contacted with N2 having different levels of relative humidity (RH=0, 15, 30, 45, 60, or 75%, flow rate= 250 cm3/min, duration of exposure=one hour). Sample specification

Weight percent of water (Karl Fischer)

Weight percent of water (TGA)

RH 0%

0.130±0.010

0.160±0.002

RH 15%

0.135±0.005

0.166±0.004

RH 30%

0.159±0.013

0.183±0.001

RH 45%

0.169±0.018

0.190±0.002

RH 60%

0.227±0.012

0.211±0.007

RH 75%

12.759±1.948

14.620±0.520


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Table 2. Thermogravimetric analysis of water in NaClO2 samples (80 wt.% pure NaClO2; initial weight percent of water (Karl-Fischer) = 0.13±0.01% and initial weight percent of water (TGA) = 0.16±0.01%) that were contacted with moist N2 with different levels of relative humidity (RH=0, 15, 30, 45, or 60%, flow rate= 250 cm3/min, duration of exposure=one hour) (the number inside the parenthesis=water content of sample related to a specific desorption peak/total water content of sample×100). Sample specification

Weight percent of water (TGA) related to the peak at 80 0C

Weight percent of water (TGA) related to the peak at 120 0C

RH 0%

~0.126 (~79%)

~0.034 (~21%)

RH 15%

~0.140 (~85%)

~0.025 (~15%)

RH 30%

~0.151 (~83%)

~0.032 (~17%)

RH 45%

~0.157 (~83%)

~0.033 (~17%)

RH 60%

~0.157 (~86%)

~0.029 (~14%)

Table 3. Expected binding energies of chlorine based ions. Chemical state

Binding energy Cl2p3/2 / eV

Binding energy Cl2p1/2 / eV

Metal chloride (Cl-) or Organic Cl (-C-Cl)

~198.5-201

~200.1-202.6

Chlorite (ClO2-) (the peak positions are inferred from the XPS spectra recorded with one scan)

~203.5

~205.1

Hypochlorite (ClO-) (the peak positions are inferred from the XPS spectra recorded with twenty scans)

~206.5

~208.1

Cl2p spin-orbit splitting (∆=1.6 eV).



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Table of Contents Graphic


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Generation of Gaseous ClO2 from Thin Films of Solid NaClO2 by

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