Heterogeneous Production of Perchlorate and Chlorate By Ozone

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Heterogeneous Production of Perchlorate and Chlorate By Ozone Oxidation of Chloride: Implications on the Source of (Per)Chlorate in the Solar System. William Andrew Jackson, Sixuan Wang, Balaji Rao, Todd A. Anderson, and Nubia Luz Estrada ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00087 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Earth and Space Chemistry

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Heterogeneous Production of Perchlorate and Chlorate By Ozone Oxidation of

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Chloride: Implications on the Source of (Per)Chlorate in the Solar System.

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W. Andrew Jackson*,a, Sixuan Wanga, Balaji Raoa, Todd Andersonb, Nubia Luz Estradaa

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a

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University, Lubbock, Texas, 79409-1023 USA

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b

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Health (TIEHH), Lubbock, Texas 79409-1163, USA

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*Corresponding Author. E-mail: [email protected]. Phone: (806) 834-6575

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Key Words: Heterogeneous Oxidation, Oxychlorine, Mars, Perchlorate, Chlorate

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Department of Civil, Environmental, and Construction Engineering, Texas Tech

Department of Environmental Toxicology, The Institute of Environmental and Human

ABSTRACT The occurrence of chlorate (ClO3-) and perchlorate (ClO4-) in the terrestrial and

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extra-terrestrial environment has been partly attributed to ozone (O3)-mediated oxidation

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of chlorine bearing compounds. This is based on varying elevated ∆17O values in all

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measured terrestrial natural ClO4- as well as the nearly universal co-equal occurrence of

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ClO3- and ClO4-, which has only been reported to occur for dry oxidation of Cl-, a process

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for which little information is available. In this study we examine possible factors

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influencing ClO4- and ClO3- formation by O3 oxidation of sodium chloride (NaCl) salt

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and hydrochloric acid (HCl) gas in glass reactor vessels. We show that longer reaction

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times increase production of ClO4- and ClO3-, with ClO3- produced generally being lower

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than ClO4- by 1-2 orders of magnitude. For 1 day oxidation periods ClO4-/ClO3- ratios

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were relatively constant (~ 50) for low Cl- masses and decreased over 3 orders of

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magnitude for higher (~100X) Cl- masses. Perchlorate mass increased with increasing

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glass reactor surface areas but not salt surface area. Increasing the relative humidity

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(RH %) from 2 % to 67 % increased ClO3- production, but did not affect the amount of

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ClO4- produced, confirming previous reports that free water will promote additional ClO3-

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but not ClO4- production pathways. Additionally, oxidation of HCl (g) produced ClO4- at

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higher yields than oxidation of NaCl, but produced less ClO3-. Our findings suggest that

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sufficient O3 saturation and availability of active sites is essential for heterogeneous

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formation of ClO4- and ClO3-. While glass surfaces per se are not relevant to

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environmental production, catalytic surfaces (silicate or others) abound in terrestrial and

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extraterrestrial environments. The Cl- form oxidized and amount of water vapor present

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will also significantly impact the ClO4-/ClO3- ratio, which could be helpful in evaluating

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the sources of ClO4- and ClO3- in extra-terrestrial material with important implications on

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the availability of water during formation.

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1.

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INTRODUCTION Perchlorate (ClO4-) has gained increasing attention due to its widespread

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terrestrial and Martian occurrence. Terrestrially, ClO4- has been measured in groundwater,

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soils, precipitation, and evaporite deposits throughout the world (1-2). Terrestrial ClO4-

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occurrence is usually associated with nitrate (NO3-) occurrence, likely due to their

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common atmospheric origin as well as the similar processes that impact their fate,

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including microbial reduction. In hyper-arid areas such as the Atacama Desert, Antarctica

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Dry Valleys (ADV), and Turpan Hami, ClO4- is highly correlated with the concentration

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of unprocessed atmospheric NO3- and is enriched in comparison to less arid environments

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(1). On Mars, ClO4- has been measured by both the Phoenix Mars Lander and Mars

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Science Laboratory in the Gale Crater and in a Martian meteorite, suggesting that ClO4- is

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distributed across the Martian landscape (3-6). Chlorate was measured in the Martian


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meteorite but no instruments on Mars are able to differentiate between ClO4- and ClO3-

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and it is likely that some reported ClO4- on Mars is likely ClO3- (5). Combined ClO3- and

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ClO4-concentrations (ClOx-) measured by the Mars Science Laboratory are also correlated

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to NO3-, although the ratios of NO3-/(ClOx-) are much lower than any terrestrial locations

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(7). In contrast to other locations, the occurrence of ClOx- on Mars is unusual due to its

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prevalence and very high abundance compared to other anions. Perchlorate and ClO3- are

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also present in chondrite meteorites and lunar regolith (8).

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Terrestrial ClO4- has been attributed to atmospheric production and deposition

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based on its presence in precipitation and dry deposition as well as the isotopic

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composition of ClO4- measured in indigenous terrestrial ClO4- (2, 9-11). Terrestrial,

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indigenous ClO4- has positive but varying ∆17O values and an elevated 36Cl/Cl ratio,

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supporting stratospheric production, while ClO4- from the southwest U.S. is characterized

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by a lower ∆17O range but similarly elevated 36Cl/Cl ratios (2, 12). These isotopic

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characteristics have been attributed to combinations of 1) varying stratospheric

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production mechanisms including ozone (O3) and ultraviolet (UV) mediated oxidation

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mechanisms of stratospheric chlorine (Cl) species, 2) surface oxidation of Cl-, and 3)

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post-depositional alteration of stratospherically produced ClO4-. On Mars the source of

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ClO4- is purely speculative but both atmospheric and heterogeneous reactions have been

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discussed (13-16). No oxygen (O) isotopic data are available for Martian ClO4-, but SO4-2

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from Mars is attributed to photochemical oxidation reactions due to a positive ∆17O value

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in SO4-2 from Martian meteorites (17).

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Perchlorate production has been demonstrated in aqueous solutions by the

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irradiation (> 254 nm) of hypochlorous acid/hypochlorite (HOCl/OCl-), chlorite (ClO2-),



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and chlorine dioxide (ClO2) (18,19). Perchlorate production was not observed when Cl-

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or ClO3- were the starting reactants in the absence of catalyst but Cl- was oxidized to

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ClO4- in the presence of anatase or rutile and a broader spectrum UV photon source (20).

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No definitive pathway has been identified but previous work has implicated the

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production of intermediate compounds such as ClO2 and higher chlorine oxide (ClxOy)

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compounds. Chlorate and Cl- were the dominant reaction products. Perchlorate and ClO3-

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were also produced in a simulated Martian atmosphere by UV irradiation of Cl- (as NaCl)

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coated SiO2 (15). The role of aqueous phase production is unclear, given the aridity of

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many environments with ClOx- present (e.g. Lunar regolith, Meteorites, Mars, Atacama)

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but at least in some of these locations, thin water films are possible (21).

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Perchlorate production has also been demonstrated by O3 oxidation of aqueous

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solutions of Cl-, HOCl/OCl-, ClO2-, and ClO2 but was not produced by O3 oxidation of

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ClO3-(22). For all ClOx starting species, Cl- and ClO3- were the dominant reaction

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products, while ClO3- was the major product with Cl- as the starting species. Perchlorate

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and ClO3- can also be produced in non-aqueous systems by O3 oxidation of Cl- coated

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sand or glass (23). Rates of ClO4- production were small compared to oxidation of ClxOy

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in solution but the relative ratio of ClO4- to ClO3- is closer to 1:1, as observed in both

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terrestrial and extraterrestrial systems (8, 24).

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While previous research has shown the potential for ClO4- formation from

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heterogeneous systems, no information is available concerning the controlling factors of

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O3 oxidation of non-aqueous Cl- forms. The objectives of this study were to determine

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the ClO4- and ClO3- generation potential by O3 mediated oxidation of Cl- in dry systems

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including: the impact of reaction time, initial Cl- mass reacted, water vapor, surface area


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(reactor and salt), and Cl form (gas phase or solid phase). Results of this study will

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increase our understanding of the sources of ClOx- both terrestrially and extra-terrestrially

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and the environments that support their production.

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2.

MATERIALS AND METHODS

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2.1

Experiment Set-up

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Perchlorate and ClO3- production were investigated by evaluating O3 mediated oxidation of sodium chloride (NaCl- ACS reagent 99+%; Sigma Aldrich) solids and

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hydrochloric acid gas (HCl (g)) under conditions of constant O3 concentration (150 mg/L ̶

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180 mg/L) and continuous flow (5 mL/min). The impact of reaction time, initial Cl- mass,

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glass surface area, relative humidity, and Cl form were systematically evaluated. An

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overview of the experimental parameters investigated is presented in Table 1.

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Glass tubes (1 m in length and 1 cm in diameter) used as the reactors were pre-

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washed with distilled water followed by distilled de-ionized (DDI) water and dried using

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compressed oxygen (O2) gas. The reactor was connected to a corona discharge O3

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generator (Model EOZ-330Y; AC 120V, 50HZ) that received O2 (Ultra high purity, >

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99.99 %) from a mass flow controller (5 mL/min). Because of the potential for the

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production of reactive gas species (e.g. ClO2, Cl2O6, or Cl2O7), the gas exiting the reactor

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was passed through a trapping flask containing either 500 mL DDI water or 500 mL of

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sodium thiosulfate (Na2S2O3) solution. Sodium thiosulfate was initially selected to

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quench O3 and other oxidants and thus, prevent secondary production of ClO4- in the

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trapping flask by oxidation of lower ClxOy compounds (e.g. ClO2- and OCl-). This flask

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was serially connected to two final gas traps containing 500 mL and 250 mL of 0.1 M

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sodium hydroxide (NaOH). Pre-washed glass tubes were used for connections (Figure 1).



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Varying masses of NaCl salt were evenly distributed into the reactor. Pre-washed and

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dried glass wool was inserted into the opening of each end of the reactor to prevent the

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loss of NaCl crystals.

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The impact of initial Cl- mass and exposure time on ClO4- and ClO3- production

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were evaluated by systematically varying the exposure of NaCl to O3 (150 ̶ 180 mg/L)

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in the glass reactor (Table 1). The impact of surface area on ClO4- and ClO3- production

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was evaluated in two ways: 1) by varying the ratios and maintaining a constant ratio of

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glass surface area to salt mass but increasing total area, and 2) by increasing the salt

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surface area at a constant salt mass. Varying ratios of glass surface area to salt mass was

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achieved using 0.5 m and 1 m glass tubes connected in series each with equal salt mass

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(11.20 g Cl- as NaCl). Higher total glass surface area at a constant ratio of glass surface

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area to salt mass was achieved by connecting three 1 m tubes in series each with equal

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salt masses (1.12 g Cl- as NaCl). Finally, higher salt surface area was evaluated by

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crushing 1.12 g Cl- as NaCl into a fine powder and reacting in a 1 m glass tube. To

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investigate the impact of water availability, 1.12 g Cl- as NaCl was placed into the glass

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reactor and exposed to O2 gas that had been bubbled though DDI water. The NaCl was

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exposed to the humidified O2 gas for 2 hours, at which point the relative humidity (RH %)

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measured 67 %. The glass tube(s) was then exposed to O3 as described above.

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Heterogeneous O3 oxidation of dry HCl (g) was conducted using a modified

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experimental setup (Figure 2) (25). It should be noted that we did not measure the RH of

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the produced gas due to its corrosive nature. Dry HCl (g) was produced by dripping 37.3 %

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HCl solution into calcium chloride (CaCl2) to adsorb water. Helium (He) (Ultra high

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purity, > 99.99 %) was used to purge the anhydrous HCl (g) produced and was mixed


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with an O3 (g) line prior to entry into a 1 m glass reaction tube. The mixing of the He gas

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containing HCl had the effect of lowering the O3 concentration compared with previous

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experiments. The gas exiting the reactor was passed through a NaOH trap which was

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used to neutralize the HCl and allow quantification of total Cl-. Several control experiments were selected to check for system contamination.

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These experiments were conducted under the same experimental conditions as the other

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experiments, including the same O3 concentration (150 mg/L ̶ 180 mg/L), O3 flow rate (5

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mL/min), glass tubes (1 meter or 0.5 meter), and outlet traps without addition of NaCl or

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with NaCl but without power to the ozone generator. For all control experiments, ClO3-

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and ClO4- concentrations were below detection.

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At the end of each experiment, the glass tube reactors were washed into storage

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vials using ~ 30 mL of DDI water. This solution and aliquots of each trapping solution

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were stored at 4°C. All solutions were evaluated for concentrations of Cl-, ClO3-, and

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ClO4- as described below.

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2.2

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Analysis An iodometric method was used to measure the gaseous O3 concentration

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produced by the O3 generator. The O3 laden gas was bubbled into a phosphate buffered

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potassium iodide (KI) solution (2 wt/wt %) for 30 minutes. The exposed phosphate

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buffered KI solutions were titrated with 0.01 N Na2S2O3. The O3 concentration produced

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by the O3 generator was calculated based on flow rate (5 mL/min), exposure time, and

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amount of 0.01 N Na2S2O3 used for titration.

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Details of the ClO4- and ClO3- analysis have been previously published (18, 24)

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and are briefly summarized here. Perchlorate and ClO3- were quantified using a Dionex



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LC 20 ion chromatography system coupled with a triple quadrupole mass spectrometer

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equipped with a Turbo-IonSprayTM source (MDS SCIEX API 2000TM). The eluent was

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45 mM NaOH for ClO4- and a gradient for ClO3-, which was pumped at a rate of 0.3

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mL/min. For both ClO4- and ClO3- an AS19 analytical and guard column was used. A 90%

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acetonitrile solution (0.3 mL/min) was used as a post-column solvent. All samples were

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spiked with an oxygen-isotope (18O) labeled ClO4- or ClO3- internal standard. Chloride

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was analyzed following EPA method 300.0 (26) by using a Dionex LC 20 ion

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chromatography system, an IonPac AG14A guard column (4 X 250 mm) connected to an

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AS14A analytical column for species separation, a CD25A conductivity detector, an 8

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mM sodium carbonate/1 mM sodium bicarbonate (Na2CO3/NaHCO3) eluent with a flow

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rate of 1.0 mL/min, and an Anion Atlas Electrolytic Suppressor.

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3.

RESULTS

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3.1

Effect of Reaction Time on Perchlorate and Chlorate Production by Ozone

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Oxidation of Dry Chloride

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Two masses of NaCl (0.112 g and 1.12 g as Cl-) were exposed to a pure O2 gas

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stream (RH ~ 2 %) containing O3 for varying time periods (1 hr ̶ 20 d) to evaluate the

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impact of reaction time on ClO4- production (Table 2 and Figure 2). For both exposed Cl-

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masses, the ClO4- produced increased with exposure time and was similar for exposure

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times less than 3 days. For the longest exposure times (10 and 20 d), ClO4- production

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was ~10 X higher for the higher starting Cl- mass. Chlorate was produced in all

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experiments but at smaller masses (< 10 ̶ 100 X) than ClO4- and there was a much less

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consistent increase in production with time with the exception of the 10 and 20 day

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exposure time for 0.112 g initial Cl-, for which ClO3- production increased substantially


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but remained less than ClO4- by 2-3 fold. For these experiments, we did not detect

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measurable ClO4- (< 0.25 µg/L), ClO3- (< 0.5 µg/L), or Cl- in the Na2S2O3 trap (Table 2).

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A subset of experiments was conducted using DDI water as the trapping solution

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(detection limit = 0.025 µg and 0.05 µg for ClO4- and ClO3-, respectively). After the

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reaction period, Cl- was generally detected in the DDI water for experiments with higher

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initial Cl- masses and longer reaction times. For these experiments ClO4- and ClO3- in the

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trapping flasks generally increased with reaction time for all initial reacted Cl- masses.

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The inability to detect ClO4- and ClO3- in the Na2S2O3 solution even when their

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concentrations in the DDI trapping solution were greater than the Na2S2O3 solution

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detection limit, often by 1-2 orders of magnitude, strongly suggests that the ClO4- and

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ClO3- in the DDI water were produced in the trapping flask and not transported directly

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from the reactor vessel. This is consistent with the presence of Cl- in the flasks at the

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termination of the experiment and our previous work demonstrating ClO4- and ClO3-

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production from aqueous solutions of Cl-, HOCl/OCl-, ClO2-, and ClO2 but not ClO3-

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exposed to O3 (22). However, Cl may have been transported in various forms (ClXOY) as

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Cl- and ClO3- are the major products of ClOx ozone oxidation.

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3.2

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The Impact of Initial Chloride Mass on Perchlorate and Chlorate Production The effect of initial Cl- mass on the production of ClO4- and ClO3- from the O3

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oxidation of NaCl crystals was evaluated for a one day reaction period (Figure 3).

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Produced ClO4- mass did not consistently vary with an increase in Cl- mass (Table 3).

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Initial Cl- mass varied over almost 4 orders of magnitude (0.0112 g ̶ 86.78 g) while

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ClO4- mass varied by a factor of 5 (0.729 µg ̶ 3.40 µg) with maximum production rates

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observed for intermediate masses, possibly due to an inability to saturate active sites at



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high Cl- but low reactions times. Chlorate production was lower than ClO4- production

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for all initial Cl- masses except the highest (86.78 g) for which ClO3- production greatly

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exceeded ClO4- production. The ClO4-/ClO3- ratio is essentially constant (~ 50) for

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reacted Cl- masses of 0.0112 g to 1.12 g but then consistently decreased with further

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increases in exposed Cl- mass. One possible explanation is generation of ClO4- from

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ClO3- which at salt masses may favor incomplete or partial oxidation of intermediate

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products. Chlorate crystals doped with nitrate and irradiated (254nm) have been shown to

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produce O1(D) with subsequent production of ClO4- (27). As in previous experiments, no ClO4- or ClO3- was detected in the Na2S2O3

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trapping solutions but were present in all DDI trapping solutions at concentrations much

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greater than the detection limits of the Na2S2O3 trapping solutions, again indicating

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production in the trapping solution rather than transport of ClO3- or ClO4- from the

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reaction vessel. Chloride was detected (detection limit = 0.5 mg/L) in the DDI water

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traps at the highest reacted Cl- masses (Table 3) and was generally > 100 X the mass of

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ClO3- or ClO4-, suggesting carryover of other Cl species.

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3.3

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The Impact of Moisture on Perchlorate and Chlorate Production In a past study ClO3- and ClO4- yields were roughly similar for NaCl oxidation by

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O3 in the absence of free water (23). In experiments that evaluated the O3 mediated

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oxidation of Cl- in solution, the ClO3- yield was 1000 X the yield of ClO4-. We evaluated

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the impact of moisture on ClO4- and ClO3- formation by increasing the relative humidity

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(RH %) of the O2-O3 mixture.

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The RH was increased to approximately 67 % compared to the normal RH 2% used to conduct our previous experiments. Limited condensation (visual observation of


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small patches of fog on glass) was found to form on the tube inner surface during the

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elevated moisture experiment. Perchlorate production at elevated RH (6.10 ± 3.34 µg)

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was similar to the production for “dry” systems (3.40 ± 1.77 µg) under similar conditions

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(1 day oxidation of 1.12 g NaCl as Cl-). However, ClO3- production (3.65 ± 0.18 µg) in

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the reactor at elevated RH was approximately two orders of magnitude higher than in the

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dry system (0.069 ± 0.040 µg). Our data are therefore consistent with previous research

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(22,23) and suggests that even limited water adsorbed onto the salt surface supports ClO3-

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formation by a mechanism that does not contribute to ClO4- formation.

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3.4

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The Impact of Surface Area on Perchlorate and Chlorate Production One possible limitation to ClO4- production, unrelated to Cl- mass, is the potential

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for active sites on the salt crystal or glass reactor tube to limit production. Perchlorate

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and ClO3- production by O3 oxidation of 1.12 g crushed NaCl as Cl- (increased salt

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surface area) were similar to experiments with un-powdered NaCl of the same mass.

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Perchlorate production for each of the three reactors connected in series and containing

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1.12 g NaCl as Cl- was also similar (4.99 ± 1.16 µg, 5.14 ± 1.55 µg, and 3.26 ± 2.42 µg,

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respectively) (Table 4) and the mass generated for a given tube was also similar to the

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mass generated from a single reactor tube in previous experiments (3.40 ± 1.77 µg)

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(Table 3). However, the total mass generated by the three tubes combined (13.39 ± 3.10

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µg) was greater than the mass (2.17 µg ± 1.71 µg) generated for a single tube containing a

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similar mass of NaCl (5.58 g as Cl-). Chlorate production decreased in each sequential

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tube and, with the exception of the first tube in series, was similar to ClO3- production in

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a single tube (Table 4). In all cases the mass of ClO3- produced was less than the mass of

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ClO4-. Finally, ClO4- production in a 0.5 m tube was one order of magnitude lower than



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for the 1 m tube (0.22 ± 0.02 µg and 3.28 ± 0.82 µg, respectively), each packed with

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11.20 g Cl- as NaCl (Table 4). Overall, the lack of increase in ClO4- production due to increased NaCl surface

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area, the increase in produced ClO4- for more NaCl mass at constant salt to glass ratio

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(compared to only increasing salt mass), and the decrease in production at lower glass to

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salt surface area ratios, suggest that the glass surface area may play a critical role in the

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amount of ClO4- produced.

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3.5

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Acid Gas

Formation of Perchlorate and Chlorate by Ozone Oxidation of Hydrochloric

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Hydrochloric acid gas is a potential important atmospheric reactant and thus, was

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also evaluated as a reactant for ClO4- and ClO3- production in a glass reactor. Of the total

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Cl- that flowed through the reactor, only a small quantity remained in the glass reactor

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(169 ± 149 µg) with most being trapped in the NaOH trap (12,256 ± 12,115 mg). The

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mass of ClO4- recovered in the reactor (2.55 ± 3.47 µg) was approximately two orders of

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magnitude higher than the mass of recovered ClO3- (0.0275 ± 0.0247 µg). Perchlorate

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produced was comparable to the mass of ClO4- recovered from experiments conducted

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using solid NaCl at comparable total masses (5.58 ̶ 11.20 g as Cl-) for reaction times of

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1 day. Given the relatively small mass of Cl- that remained in the reactor, the lower

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concentration of O3, and the relatively short exposure time period, the gas phase and/or

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glass catalyzed oxidation of HCl appears to be much more efficient both per unit time

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and unit reactant at producing ClO4- than oxidation of dry NaCl by O3. ClO3- mass

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generated during the HCl (g) experiment is one order of magnitude lower than the mass

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formed by the O3 oxidation of solid NaCl (0.043 ± 0.038 µg to 0.58 ± 0.27 µg for


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0.0112 ̶ 11.20 g NaCl as Cl-, respectively), although ClO3- is unstable at low pH (< -1)

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and so it is unclear whether production was lower or produced ClO3- was decomposed.

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As expected given the very large Cl- mass recovered in the trapping flask, ClO3-

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concentrations in the trapping flask were exceedingly high (1,240 ± 622 µg) but ClO4-

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mass (1.14 ± 1.4 µg) was similar to that recovered in the reactor. This is consistent with

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our previous work evaluating O3 oxidation of Cl- in solution.

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4.

DISCUSSION Our experiments do not attempt to evaluate perchlorate production for relevant

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environmental conditions (O3 concentrations, temperature, Cl reactants, etc), whether

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atmospheric (Mars or Earth) or surface, but rather explored the impact of specific

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variables on the relative production of ClO3- and ClO4-. Our data suggest that ClO4-

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formation by O3 mediated oxidation of Cl- is likely catalyzed by glass surface sites

290

occupied by a volatile Cl species. This is supported by the increase in ClO4- production

291

with increasing ratio of glass surface area to salt mass and lack of increase due to

292

increased salt surface at constant mass. In addition, the presence of Cl- and production of

293

ClO4- and ClO3- in the DDI trapping flasks supports the volatile loss of Cl from the

294

reactor vessel. The reaction of a volatile species on the glass surface is also supported by

295

production of ClO4- by reactions of O3 with dry HCl (g) at comparable rates to solid

296

phase salt experiments, even with a much lower Cl- mass in the reactor. A loss of Cl

297

atoms in a heterogeneous system, due to pyrex glass wall reactions, was observed in one

298

study (28). While glass surfaces are not relevant to environmental production of ClOx-,

299

there are numerous potential reaction surfaces in the atmosphere (e.g. dust, ice crystals,



300

acid droplets) and the role of catalytic surfaces on ClOx- production has also been

301

explored for other oxidation mechanisms such as photolysis (15,20). Surface catalyzed

302

reactions are also very relevant to ClOx- measured in meteorites and lunar regolith as gas

303

phase reactions are unlikely to be relevant.

304

The increase of ClO4- with increasing exposure time and with salt mass but

305

decreasing production of ClO4- at high salt masses and low reaction times is unlikely to

306

be due to O3 limitations given the high concentrations (150 mg/L ̶ 180 mg/L) of O3

307

applied and reactor flushing rates (5 ml/min). The lower production at high salt masses

308

and relatively low reaction times could be due to adsorption of a reactive species on the

309

salt surface, reducing Cl on glass active sites. Chlorate production from dry oxidation

310

was almost always lower than ClO4- (except high Cl- mass, low reaction time) and less

311

dependent on reaction time. Increased ClO3- production, relative to ClO4- production,

312

occurred only for experiments with very high salt mass and low reaction time and at

313

elevated RH. Previous research (23) has examined the influence of humidity (RH
1) is of importance given the reported ratios of ClO4- and ClO3- in

347

natural systems (8,24). Terrestrially ClO3-/ ClO4- molar ratios in locations with

348

environmental conditions suitable for ClO3- preservation vary but generally range from

349

0.1 to 10 (10). Lunar regolith and both non-planetary and Martian meteorites have a

350

similar range of reported ClO3-/ ClO4- molar ratios although generally > 1. Given the

351

positive ∆17O values of ClO4- for some terrestrial ClO4- and the range of ClO3-/ ClO4-

352

molar ratios, it is likely that terrestrial ClO4- is produced from heterogeneous reactions of

353

a Cl precursor with O3 in the absence of bulk water but in the presence of adsorbed water.

354

Given the very low production rates of ClO4- formation from dry oxidation and elevated

355

36

356

major source of indigenous terrestrial ClO4-. The similarity between terrestrial samples

357

and a Martian meteorite might suggest similar production mechanisms and water

358

availability, but this is speculative given only the single sample, lack of isotopic

359

composition to eliminate non-O3 photochemical production, and lack of data on the

360

relative ratio of ClO3- to ClO4- on the Mars surface.

361

Acknowledgements

362

This work was supported by the U.S. Department of Defense Strategic Environmental

363

Research and Development Program (SERDP -1435).

Cl content of terrestrial ClO4-, surface oxidation of Cl bearing salts is unlikely to be a

364 365

6. REFERENCES

366 367 368 369 370 371 372

1. Jackson, W.A.; Böhlke, J.K.; Andraski, B.J.; Fahlquist, L.; Bexfield, L.; Eckardt, F.D.; Gates, J.B.; Davila, A.F.; McKay, C.P.; Rao, B.; Sevanthi, R.; Rajagopalan, S.; Estrada, N.; Sturchio, N.; Hatzinger, P.B.; Anderson, T.A.; Orris, G.; Bentancourt, J.; Stonestrom, D.; Latorre, C.; Li, Y.; Harvey, G.J. Global patterns and environmental controls of perchlorate and nitrate co-occurrence in arid and semi-arid environments. Geochim. Cosmochim. Acta. 2015a, 164, 502-522.


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2. Jackson, W.A.; Böhlke, J.K.; Gu, B.; Hatzinger, P.B; Sturchio, N.C. Isotopic composition and origin of indigenous natural perchlorate and co-occurring nitrate in the southwestern United States. Environ. Sci. Technol. 2010, 44(13), 4869-4876. 3. Hecht, M. H.; Kounaves, S. P.; Quinn, R. C.; West, S. J.; Young, S. M. M.; Ming, D. W.; Catling, D.C.; Clark, B.C.; Boynton, W.V.; Hoffman, J.; DeFlores, L.P.; Gospodinova, K.; Kapit, J.; Smith, P. H. Detection of perchlorate and the soluble chemistry of Martian soil at the Phoenix Lander Site. Science. 2009, 325(5936), 6467. 4. Kounaves, S.P.; Chaniotakis, N.A.; Chevrier, V.F.; Carrier, B.L.; Folds, K.E.; Hansen, V.M.; McElhoney, K.M.; O’Neil, G.D.; Weber, A.W. Identification of the perchlorate parent salts at the Phoenix Mars landing site and possible implications. Icarus. 2014a, 232, 226-231. 5. Kounaves, S.P.; Carrier, B.L.; O’Neil, G.; Stroble, S.T.; Claire, M.W. Evidence of Martian perchlorate, chlorate, and nitrate in Mars Meteorite EETA79001:Implications for oxidants and organics. Icarus. 2014, 229, 206-213. 6. Glavin, D.P.; Freissinet, C.; Miller, K.E.; Eigenbrode, J., Brunner, A.E.; Buch, A.,; Sutter, B.; Archer, P.D.; Atreya, S.K.; Brinckerhoff, W.B. Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater. J Geophys Res-Planet., 2013, 118(10),1955-1973. 7. Stern, J.C.; Sutter, B.; Jackson, W.A.; Navarro-González, R.; McKay, C.P.; Ming, D.W.; Archer, P.D.; Mahaffy, P.R. The nitrate/(per)chlorate relationship on Mars. Geophys. Res. Lett. 2017, 44(6), 2643–2651. 8. Jackson, W.A.; Davila, A.; Sears, D.; Coates, J.; McKay, C.; Brundrett, M.; Estrada, N.; Bohlke, J.K. Widespread occurence of (per)chlorate in the solar system. Earth Planet. Sci. Lett. 2015b, 430, 470476. 9. Rajagopalan, S.; Anderson, T.; Cox, S.; Harvey, G.; Cheng, Q.; Jackson, W. A. Perchlorate in wet deposition across North America. Environ. Sci. Technol. 2009, 43(3), 616-622. 10. Andraski, B. J.; Jackson, W. A.; Welborn, T. L.; Böhlke, J. K.; Sevanthi, R.; Stonestrom, D. A. Soil, plant, and terrain effects on natural perchlorate distribution in a desert landscape. J. Environ. Qual. 2014, 43(3), 980-994. 11. Jackson,W.A.; Davila, A.F.; Böhlke, J.K.; Sturchio, N.C.; Sevanthi, R.; Estrada, N.; Brundrett, M.; Lacelle, D.; McKay, C.P.; Phoghosyan, A.; Pollard, W.; Zacny, K. Deposition, accumulation, and alteration of Cl-, NO3-, ClO4- and ClO3- salts in a hyper-arid polar environment: mass balance and isotopic constraints. Geochim. Cosmochim. Acta. 2016, 182, 197-215.



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12. Sturchio N.C., Caffee M., Beloso A.D., Heraty L.J., Böhlke J.K., Hatzinger P.B., Jackson W.A., Gu B., Heikoop J.M., Dale M. Chlorine-36 as a tracer of perchlorate origin. 2009 Environ. Sci. Technol. 43, 6934–6938.

13. Catling, D.C.; Claire, M.W.; Zahnle, K.J.; Quinn, R.C.; Clark, B.C.; Hecht, M.H.; Kounaves, S. Atmospheric origins of perchlorate on Mars and in the Atacama. J. Geophys. Res- Planet. 2010, 115(E1). 14. Smith, M.; Claire, M.W.; Catling, D.C.; Zahnle, K.J. The formation of sulfate, nitrate and perchlorate salts in the Martian atmosphere. Icarus. 2014, 231, 51-64. 15. Carrier, B. L. and Kounaves, S. P. The origins of perchlorate in the Martian soil, Geophys. Res. Lett. 2015, 42(10), 3739–3745. 16. Wilson, E. H.; Atreya, S. K.; Kaiser R.I.; Mahaffy, P.R. Perchlorate formation on Mars through surface radiolysis-initiated atmospheric chemistry: A potential mechanism, J. Geophys. Res-Planet. 2016, 121(8), 1472–1487. 17. Farquhar, J. and Thiemens, M.H. Oxygen cycle of the Martian atmosphere-regolith system: ∆17O of secondary phases in Nakhla and Lafayette. J. Geophys. Res-Planet. 2000, 105(E5), 11991-11997. 18. Rao, B.; Estrada, N.; McGee, S.;Mangold, J.; Gu, B.; Jackson, W.A. Perchlorate production by photodecomposition of aqueous chlorine solutions. Environ. Sci. Technol. 2012, 46(21), 11635-11643. 19. Kang, N.; Anderson, T.A.; Rao, B.A.; Jackson, W.A. Characteristics of perchlorate formation via photodissociation of aqueous chlorite. Environ. Chem. 2009, 6(1), 53– 59. 20. Schuttlefield, J.D.; Sambur, J.B.; Gelwicks, M.; Eggleston, C.M.; Parkinson, B.A. Photooxidation of chloride by oxide minerals: Implications for perchlorate on Mars. J. Am. Chem. Soc. 2011, 133, 17521-17523

21. Boxe, C.S., Hand, K.P., Nealson, K.H., Yung, Y.L., Yen, A.S., Saiz-Lopez, A.. Adsorbed water and thin films on Mars. International Journal of Astrobiology. 2212, 11(3), 169-175. 22. Rao, B.A.; Anderson, T.A.; Redder, A.; Jackson, W.A. Perchlorate formation by ozone Oxidation of aqueous chlorine/oxy-chlorine species: role of ClxOy radicals. Environ. Sci. Technol. 2010, 44(8), 2961–2967.


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23. Kang, N.; Jackson, W. A.; Dasgupta, P. K.; Anderson, T. A. Perchlorate production by ozone oxidation of chloride in aqueous and dry systems. Sci. Total Environ. 2008, 405(1), 301-309. 24. Rao, B., Hatzinger, P., Bohlke, J.K., Sturchio, N., Andraski, B., Eckardt, F., Jackson, W.A. Natural chlorate in the environment: application of a new IC-ESI/MS/MS Method with a Cl18O- Internal Standard. Environ. Sci.Technol. 2010b, 44(22), 84298434. 25. Arnáiz, F.J. A convenient way to generate hydrogen chloride in the freshman lab. Journal of Chemical Education 1995 72 (12), 1139 26. Pfaff, J.D. Determination of inorganic anions by ion chromatography-method 300.0. 1993. DOI: 10.1016/b978-0-8155-1398-8.50022-7. 27. Anan’ev, V.; Miklin, M.; Kriger, L., Reactions of atmic oxygen with th echlorate ion an dthe perchlorate ion. Chem. Phsy. Lett., 2014, 607, 39-42 28. Martin, L.R.; Wren, A.G.; Wu, M. Chlorine atom and ClO wall reaction products. Int. J. Chem. Kinet. 1979, 11(5), 543-557. 29. Finlayson-Pitts, B.J. The tropospheric chemistry of sea salt: a molecular-level view of the chemistry of NaCl and NaBr. Chem. Rev. 2003, 103(12), 4801-4822. 30. Buxton, G. V. and Subhani, M. S. Radiation chemistry and photochemistry of oxychlorine ions. Part 2. - Photodecomposition of aqueous solutions of hypochlorite ions. J. Chem. Soc., Faraday Trans. 1. 1972, 68, 958-969. 31. Quiroga, S. L. and Perissinotti, L. J. Reduced mechanism for the 366nm chlorine dioxide photodecomposition in N2-saturated aqueous solutions. J. Photobio. AChem. 2005, 171(1), 59-67. 32. Wiberg, E. Inorganic Chemistry, 1st English ed./ed.; Academic Press: San Diego, 2001. 33. Hubler, D. K.; Baygents, J. C.; Chaplin, B. P.; Farrell, J. Understanding chlorite, chlorate and perchlorate formation when generating hypochlorite using boron doped diamond film electrodes. ECS Transactions. 2014, 58(35), 21-32. 34. Simonatis, C. and Heicklen, J. Perchloric acid: A possible sink for stratospheric chlorine. Planet. Space Sci. 1975, 23(11), 1567-1569.



509 510 511 512


Page 20 of 30

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

513 514


Table 1. Experimental matrix of system parameters investigated. Mass of Cl- (g)

NaCl

0.0112

1

1

1

Dry

NaCl

0.112

0.042

1

1

Dry

NaCl

0.112

0.125

1

1

Dry

NaCl

0.112

0.5

1

1

Dry

NaCl

0.112

1

1

1

Dry

NaCl

0.112

3

1

1

Dry

NaCl

0.112

10

1

1

Dry

NaCl

0.112

20

1

1

Dry

NaCl

1.12

0.042

1

1

Dry

NaCl

1.12

0.125

1

1

Dry

NaCl

1.12

0.5

1

1

Dry

NaCl

1.12

1

1

1

Dry

NaCl

1.12

3

1

1

Dry

NaCl

1.12

10

1

1

Dry

NaCl

1.12

20

1

1

Dry

NaCl

1

1

1

RH (67.7%)

1

1

2

Dry

1

1

3

Dry

NaCl

1.12 2.24 (1.12/tube) 3.36 (1.12/tube) 5.58

1

1

1

Dry

NaCl

11.2

1

1

1

Dry

NaCl

11.2

1

0.5

1

Dry

HCl (g)

12.26

1

1

1

Dry

NaCl

22.4

1

1

1

Dry

NaCl

86.78

1

1

1

Dry

NaCl NaCl

515

Reaction Tube Number Experiment Time Length of tubes System (day) (meter)

Cl Form

RH: Relative Humidity 21 ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

516 517

Page 22 of 30

Table 2. Average mass of ClO4- and ClO3- detected in the glass reactor, including the mass of ClO4-, ClO3- and Cl- in the trapping solutions after O3 oxidation of two NaCl masses (0.112 g and 1.12 g as Cl-). Reaction Time (day)

Reactor

Trapping Solutions

Mass Of Cl- (g)

Mean ClO4(µg)

ClO4SD

Mean ClO3(µg)

ClO3SD

1.12

0.128

0.030

0.0285

0.005

0.042 0.112

1.12

0.130

0.524

0.102

0.122

0.00765

0.033

1.12

0.342

1.93

0.093

1.16

0.0128

0.085

1.12

1.022

3.40

0.19

1.77

0.00815

0.069

2.39

1.37

0.0525

Cl(mg)

DDI

NA

NA

NA

Na2S2O3

< 0.25

< 0.5

NA

DDI

0.25

0.22

< 0.25

Na2S2O3

< 0.25

< 0.5

NA

DDI

0.16

0.62

< 0.25

Na2S2O3

< 0.25

< 0.5

NA

DDI

0.23

0.36

< 0.25

Na2S2O3

< 0.25

< 0.5

NA

DDI

NA

NA

NA

Na2S2O3

< 0.25

< 0.5

NA

DDI

0.25

1.7

0.32

Na2S2O3

< 0.25

< 0.5

NA

DDI

0.84

4.6

0.6

Na2S2O3

< 2.5

Heterogeneous Production of Perchlorate and Chlorate By Ozone

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