Photostationary State in Photoelectrochemical Generation of Perchlorate

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Photostationary State in Photoelectrochemical Generation of Perchlorate: Relevance to Mars Alero Gure, Thomas Sorenson, Janet Dewey, Theodore J. Kraus, Carrick M. Eggleston, and Bruce A Parkinson ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00106 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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ACS Earth and Space Chemistry

Photostationary State in Photoelectrochemical Generation of Perchlorate: Relevance to Mars Alero J. Gure*†, Thomas Sorenson†‡, Janet C. Dewey†, Theodore Kraus‡, Carrick M. Eggleston†§, Bruce A. Parkinson‡§ †Department

of Geology and Geophysics, ‡Department of Chemistry and §School of Energy

Resources, University of Wyoming, Laramie, Wyoming 82071, United States

ABSTRACT The discovery of abundant perchlorate (ClO4-) on Mars has prompted renewed interest in the production, accumulation and transport of ClO4- and other oxychlorine species in natural systems. Here we focus on the role of semiconducting minerals in the photochemical generation and destruction of ClO4- and chlorate (ClO3-). Illumination of single crystal, or nanocrystalline films, of titanium dioxide polymorphs - rutile and anatase - in chloride (Cl-) solutions can both generate and destroy ClO4- depending on starting ClO4- and Cl- concentrations. For single crystal anatase, we observe an apparent photostationary state in which ClO4- production and destruction reach a near-steady state. We observe more ClO3- production and less ClO4- at higher Clconcentrations. An inventory of measured dissolved chlorine (Cl) species indicates that some Cl was lost to a volatile form or to a dissolved form not measured. Our experiments were performed in an aqueous medium under Earth atmosphere and temperature conditions; further experiments under Mars-like conditions are in progress. Photochemical processes as described here, in which activation energy is provided by photons, are particularly important under cold conditions with limited thermal activation. KEYWORDS: perchlorate, Mars, photochemistry, titanium dioxide, chlorine chemistry

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INTRODUCTION The origin of ClO4- on Mars has been under investigation since the Phoenix Mars Lander’s Wet Chemistry Laboratory provided evidence of ClO4- in Mars regolith in an amount equivalent to 0.6 weight percent. The ClO4-/Cl- ratio measured at this lander’s site was 5:1 or greater (there was some Cl- contamination from a terrestrial reagent.1). In comparison, the ClO4-/Cl- ratio in the Antarctic Dry Valleys – a terrestrial environment used as a Mars analog – is 0.00067:1.2 The Curiosity Rover’s Sample Analysis at Mars (SAM) instrument detected up to 0.9 weight percent ClO4- or other oxychlorine species (possibly ClO3- and dichlorine heptoxide, Cl2O7) in an evolved gas analysis of Mars regolith and drilled core samples.3 Cl-O redox chemistry is complex,4-6 and the SAM results underscore the importance of reaction intermediates and “dead ends” on pathways to ClO4- production. Several hypotheses have been proposed to explain the presence of ClO4- on Mars, including (1) Gas phase reactions between chlorine species and photochemically generated oxidants like hydrogen peroxide and ozone;1,7 or (2) oxidation of chloride-bearing solids by ozone produced by electrostatic discharge or atmospheric oxygen photochemistry.8,9 Hypothesis (1) has been shown to be inadequate to explain the high concentrations of ClO4- in Mars regolith because of the roughly 3000-fold lower ozone concentration on Mars than on Earth, and the lack of Cl sources to Mars’ current atmosphere.10,11 Schuttlefield et al. (2011)12 experimentally tested an alternative to the atmospheric hypothesis. They illuminated natural titanium dioxide (TiO2) in aqueous NaCl solutions with light from a Xe arc lamp to generate ClO4-. Titanium dioxide and titanium-bearing minerals like ilmenite are present on Mars in amounts sufficient to contribute significantly to the photochemical reactions proposed. Mariner 9 data indicated anatase as a likely major component of Martian dust.13 Titanium oxides constitute up to 1 weight percent of Mars surface material as quantified by Page | 2 ACS Paragon Plus Environment

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various spectrometers on the Viking, Pathfinder and Curiosity missions.14-16 Anatase has also been found in Mars SNC meteorites.17 Light-driven production of oxidizing valence band holes (hvb+) and reducing conduction band electrons (ecb-) in some oxide and sulfide minerals is wellestablished,18-21 and we propose that valence band holes provide the potential for Cl- oxidation, whether to ClO4- or ClO3-. We emphasize here that photon-driven production of hvb+is not a thermally activated process. Any Cl-, even if frozen in solution, can be oxidized by hvb+; water molecules, even if frozen, can also be oxidized by hvb+, potentially providing reactants and species that can be mobile and therefore reactive in thin aqueous films on surfaces.22 Thus, while our experiments were not done under Mars conditions, the fundamental processes under study are not ruled out by lack of liquid water or brines on Mars – they could even take place in atmospheric dusts with adsorbed water and solutes. Under Mars-like conditions23, ClO4- formation by illumination of dry NaCl increased in the presence of successively added SiO2, Fe2O3, Al2O3 and TiO2 solids. ClO3- production23 was greater than that of ClO4-. A study under simulated Mars atmosphere but not temperature24 shows UV-driven production of ClO4- from Cl- in the presence of silica beads, possibly by generation of oxidant radicals at mineral surfaces; in some cases, ClO4- loss occurred via unknown mechanisms. A study done using a CO2 and HCl atmosphere at 77 K with an Hg lamp in the presence of anatase, rutile, montmorillonite and the Nakhla meteorite revealed both ClO4and ClO3- after 3500 hours, along with chloromethane species (CO2 acting as electron acceptor)25. These were not time-series experiments, so compositional evolution over time cannot be discerned. In Ref. 12, ClO4- production was initially fast, but slowed after a few hours, raising questions about sustained ClO4- production over time. One way to explain this is through



a photostationary state in which the rate of photochemical ClO4- production equals the rate of photochemical ClO4- degradation. There is a complex array of redox reactions and intermediates for oxidation of Cl- to ClO4-. From known pathways6 in ozone oxidation of Cl- to ClO4-, chlorine dioxide (ClO2) and other ClxOy species are important intermediates. ClO3- and ClO4- are products of two separate oxidation pathways, and interconversion between ClO3- and ClO4- is slow.26 A key question, then, is what selects between the ClO3- and ClO4- pathways? The purpose of this paper is to establish whether the concept of photostationary state applies to photochemical ClO4- production by illumination of TiO2 in aqueous Cl- solutions. A description of experimental methods is presented in the Supporting Information (SI). Briefly, experiments used three different light-absorbing materials: single crystal anatase, single crystal rutile, and a powder mixture of anatase and rutile (Degussa P25). Starting [Cl-], [ClO4-], [ClO3-] and pH (Table S-1) were varied. Experiments were conducted in solutions of pH ranging from 9.18 – 9.96 for comparability to Ref. 12. We measured ClO4-, Cl-, ClO2- and ClO3- at low ppb levels. The experiments reported here were done in a room temperature air-saturated aqueous medium and do not represent Mars conditions. They explore processes that might also occur on Mars, where photon-activated processes may be particularly important relative to thermally activated processes under cold conditions. Further study of ClO4- formation, degradation and photostationary state under more Mars-like conditions is underway. RESULTS AND DISCUSSION The results show that: (a) ClO4- can be produced from zero starting concentration, but also degraded from higher starting concentration, achieving a near-photostationary state; (b) ClO3- is Page | 4 ACS Paragon Plus Environment

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also produced under some circumstances, and sometimes produced in higher quantities than ClO4-; (c) in accounting for total Cl in some of the experiments, some Cl is lost – possibly to a volatile form. Results are presented in Figure 1 (a-e) where similar symbols (shape and color) represent data from the same experiment. Analytical error is smaller than symbol size. Figure. 1a includes data from Ref. 12 for comparison (see caption). For single crystal anatase and rutile, [ClO4-] increases until an apparent photostationary state is reached, with similar kinetics. Figure 1a includes the results of (a) an experiment under conditions similar to those of Ref. 12 using a P25 substrate in 0.5M NaCl, and (b) an experiment with P25 in much lower (2.8 mM) [Cl-]. Despite the substantial difference in surface areas, similar initial kinetics was observed for all three substrates in 0.5 M Cl- solution within the first 50 hours. Starting from lower initial [Cl-] with P25, steady state was not attained before the end of the experiment and ClO4- was detected in relatively small amounts. For illuminated single crystal rutile, Figure 1a shows ClO4degradation to a near-steady state in a solution of relatively low [Cl-] (4.8 mM Cl- and 0.1 mM initial ClO4). ClO4- can be both produced and destroyed in the presence of illuminated rutile.



Figure 1: (a), (b) ClO4- production and loss using illuminated P25, single crystal anatase and rutile in solutions of different initial [Cl-], [ClO3-] and [ClO4-]; open black symbols in panel (a) are data from Ref. 12. (c) ClO3- production and loss using illuminated P25, single crystal anatase and rutile in solutions of different initial [Cl-], [ClO3-] and [ClO4-]. (d) Average and range of [Cl-] over the course of each experiment. (e) Cl deficit calculated for experiments with no Cl- leaks.

With illuminated single-crystal anatase (Fig. 1b), ClO4- is generated from a solution with zero starting [ClO4-], but degraded from a higher starting [ClO4-], all with a similar [ClO4-] after


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350-400 hours (and comparable to data from Ref. 12 in Figure 1a). This is consistent with achievement of a near-photostationary state. No ClO2- was detected in any of our experiments (similar to the results of Ref. 24). ClO3- was produced (Figure 1c) in several experiments involving all substrates. For example, while ClO4was produced in relatively small quantities with the P25 powder in 2.8 mM Cl- (Fig. 1a), ClO3was produced in greater quantities (Figure 1c). ClO3- was not measured in Ref. 12, so we cannot make comparisons, but in this study illuminated single-crystal anatase in 7.2 mM Cl- produced ClO3- as the dominant reaction product (Figure 1c) with very small amounts of ClO4- (Figure 1b, inset), and in 0.45 M Cl- large amounts of ClO3- were detected but no ClO4- (Figure. 1c). ClO3was found with illuminated single-crystal rutile in 4.8 mM Cl- and 0.1 mM ClO4- (Figure 1c). The [Cl-] may play a role in selecting the pathway6 toward either ClO3- or ClO4-. In Figure 1d we show average [Cl-] pertinent to each experiment along with the range of [Cl-] measured over the course of each experiment. In some (but not all) experiments, quantification of photochemically driven [Cl-] changes was impossible because of Cl- contamination within the system (see SI). However, [Cl-] > 3 mM tended to result in less ClO4- and more ClO3- production for all substrates. For example, at lower [Cl-] with single-crystal anatase (Figure 1b and 1d - blue, red and green squares), ClO4- was the only oxychlorine species detected. At higher [Cl-], ClO3became the more prevalent product. For example, note the data represented by green and black squares (Runs 4 and 9 respectively; Figs. 1b, 1c and 1d). These experiments used single crystal anatase under similar starting light intensity, pH and initial [Cl-] (2.8 mM). However, [Cl-] decreased in Run 4 and increased in Run 9. No ClO3- was found in Run 4, but [ClO3-] increased and reached a near-steady state in Run 9 (Figure 1c). The fact that higher Cl- favors the ClO3pathway has been noted by others.27 An experiment with illuminated single crystal anatase in a Page | 7 ACS Paragon Plus Environment


solution of high initial [ClO3-] (Figs. 1b-d) showed a small amount of ClO3- degradation but almost no corresponding ClO4- production (Fig. 1b inset); ClO3- apparently does not convert directly to ClO4-. Some studies6 suggest that ClO2-, ClO2, and chloroxy radicals (e.g., Cl2O4, Cl2O6) are precursors in the formation of ClO3- and ClO4-. Results from previous studies suggest possible loss of volatile Cl from the aqueous phase during redox processes involving Cl- and Cl-oxyanions.6,9,28-30 We attempted to measure volatiles using a residual gas analyzer (RGA), but were unable to detect volatile Cl species in RGA runs using gas sampled from the experimental chamber headspace. We detected no masses above 44 (CO2); no chloromethanes were therefore found. We calculated a Cl deficit (the loss of Cl to forms for which we did not analyze or to volatilization) based on a careful Cl inventory. Deficits for all experiments with single crystal anatase and with similar [Cl-] were similar, but the deficit for single crystal anatase in higher [Cl-] (Fig. 1b-e) was higher than for lower initial [Cl-]. We excluded other experiments from this analysis because they were affected by Cl- contamination into the system. Photochemical production of ClO4- appears to reach a photostationary state as a balance with ClO4- degradation. ClO3- is an important reaction product, and can be the dominant product at higher [Cl-], consistent with Ref. 25. ClO2- oxidation to ClO2 is the likely reaction pathway6 that can branch toward either ClO3- or ClO4- products. Because the near photostationary state occurs at a ClO4-/Cl- ratio much less than 1, the 5:1 ratio found by the Phoenix lander is difficult to explain unless, once ClO4- is photochemically produced, freezing point depression (as low as 206 K) in the presence of sufficient adsorbed water (even if initially frozen)22, at the eutectic of e.g. Mg perchlorate brines, could result in slow transport of high ClO4-/Cl- ratio brines to the subsurface on a centimeter scale. Although the experiments were performed under Earth Page | 8 ACS Paragon Plus Environment

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temperature, pressure and air composition conditions, the results suggest a role for semiconducting minerals in both light-driven production and degradation of ClO4- and other oxychlorine species. ASSOCIATED CONTENT Supporting Information Available Experimental method and matrix (Table S-1), Analytical methods AUTHOR INFORMATION Corresponding Author Telephone: 307 223 2312

Email: [email protected]

Present Address: Department of Geology and Geophysics, University of Wyoming, Laramie 82071, United States. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by NASA Grant NNX16AG11G to C.M. Eggleston and B.A. Parkinson. We thank the Department of Geology and Geophysics and the School of Energy Resources at the University of Wyoming for the use of the instruments in the Geochemistry Analytical Laboratory. REFERENCES 1. 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 Regolith at the Phoenix Lander Site. Science. 2009, 325, 64–67. 2. Kounaves, S. P., Stroble, S. T., Anderson, R. M., Moore, Q., Catling, D. C., Douglas, S., McKay, C. P., Ming, D. W., Smith, P. H., Tamppari, L. K., Zent, A. P. Discovery of Natural Perchlorate in the Antarctic Dry Valleys and its Global Implications. Environ. Sci. Technol. 2010, 44, 2360–2364. 3. Clark, B.C.; Kounaves, S.P. Evidence for the Distribution of Perchlorates on Mars. Int. J. Astrobiol. 2016, 15(4) 311-318. Page | 9 ACS Paragon Plus Environment


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