Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX
http://pubs.acs.org/journal/aesccq
The Role of Titanium Dioxide (TiO2) in the Production of Perchlorate (ClO4−) from Chlorite (ClO2−) and Chlorate (ClO3−) on Earth and Mars Dongyu Liu and Samuel P. Kounaves* Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States
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S Supporting Information *
ABSTRACT: The widespread presence of perchlorate (ClO4−) on Mars has significant implications for the alteration or destruction of indigenous organic compounds that may have been or still be present on Mars. The intermediary products of the UV-driven production of ClO4− include oxychlorines (ClOx) such as chlorite (ClO2−), chlorate (ClO3−), and chlorine-dioxide (ClO2) gas. The objective of this study was to start with ClO2− or ClO3− under Mars ambient and vary temperature, humidity, and UV wavelengths in order to isolate the reaction pathways leading to ClO4−. We also investigated the role of titanium dioxide (TiO2) as a catalyst for these reactions. We show here that the production of ClO4− from ClO2− and ClO3− proceeds through different pathways. The ClO2− is rapidly converted to stable levels of ClO3− and Cl−, suggesting that the amount present on Mars will likely be very low compared to other ClOx. We also observed that temperature does not affect ClO4− production when starting with NaClO2 but causes a decrease in ClO4− production when starting with NaClO3, and production of ClO4− using UV > 300 nm with O2 present does not involve an ozone (O3) pathway. We have also shown that adding TiO2 to the SiO2/ClOx mixture has a catalytic effect in the production of ClO4− under terrestrial conditions but shows primarily a shielding effect under Mars ambient; ClO3− is stable under Mars ambient even in the presence of TiO2 and is not affected by temperature or humidity. Finally, it was shown that water is necessary for generation of ClO2(g) during perchlorate production from either NaClO2 or NaClO3. This suggests that production of ClO2 might be occurring on Mars in areas where ice can provide increased humidity levels. KEYWORDS: Mars, UV, perchlorate, chlorate, chlorite, titanium dioxide Tindouf Basin (Morocco),17 and many others.18 It has also been shown to be present on the Moon, the Murchison and Fayetteville chondrite meteorites, and likely across the entire solar system.19 Isotopic analysis has shown that terrestrial perchlorate is produced globally via atmospheric ozone (O3) oxidation of chlorine,13,20,21 and thus it was initially assumed that the same process was likely responsible for the perchlorate found on Mars.13 However, the high levels of perchlorate in the soil and the lack of chlorine in the Martian atmosphere have made it clear that the same process cannot be solely responsible for the perchlorate on Mars.22 Laboratory experiments have demonstrated a number of potential ClO4− production mechanisms, including O3 and Cl− reactions in aqueous systems,12,23 by electrolysis,24 lightning discharges,20 UV irradiation of Cl− solutions and oxide minerals,25 and photodecomposition of aqueous chlorine solutions.12,26 Chlorate and chlorite (ClO2−) have also been produced under irradiation by 5 keV electrons.27
1. INTRODUCTION Perchlorate (ClO4−) was first measured in three Martian soil samples by the Wet Chemistry Laboratory (WCL) on board the Phoenix Mars lander in the summer of 2008 at concentrations of ∼0.6 wt %1−3 and subsequently at similar levels by the Sample Analysis at Mars (SAM) instrument suite on the Curiosity rover.4 Perchlorate and chlorate (ClO3−) have also been found in the Mars meteorites Tissint and EETA79001.5,6 The presence of perchlorate salts on Mars is of significance for several reasons: (1) their ability to depress the freezing point of water to −70 °C and allow for the possible presence of liquid brines over a broad area of the Martian surface;7−9 (2) their potential to interfere with analyses requiring high temperatures; and (3) their ability under UV and/or galactic cosmic radiation to alter or destroy organic compounds by generating reactive intermediary oxychlorine (ClOx−) species such as hypochlorite (ClO−) and chlorine dioxide (ClO2) gas, and radicals such as •OCl, •Cl, and •OH.10,11 On Earth natural perchlorate (often accompanied by chlorate) is ubiquitously formed in the atmosphere and widely distributed by both wet and dry deposition.12,13 It is typically found at higher concentration levels in arid locations across the globe such as the Atacama Desert (Chile),14 the McMurdo Dry Valleys (Antarctica),15 the Mojave Desert (U.S.A.),16 the © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
May 13, 2019 June 25, 2019 July 12, 2019 July 12, 2019 DOI: 10.1021/acsearthspacechem.9b00134 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry
MSC consisted of a Mars simulation gas (Airgas) composed of 95.3% CO2, 2.8% N2, 1.8% Ar, and 0.10% O2, less than 0.1% H2O, delivered at a constant flow rate of 120 cm3/min and maintained at pressure of 10 Torr. Before turning on the Mars gas flow, the MSC was pumped down to 0.08 Torr to remove laboratory ambient gases and water vapor. A mass spectrometer (SRS QMS-200) was connected to monitor the gas composition inside the MSC. The UV irradiation was provided by a 450 W xenon lamp (Osram XBO-450W) through a 22 cm fused silica port. The UV radiation (∼200− 400 nm) was monitored using a UV spectrophotometer probe (StellarNet) every 1 min. A long-pass filter with a cutoff at ∼300 nm (Spectroline SB-100P UV) was inserted in the UV output beam to study perchlorate production dependence on wavelength. Both UV spectra are shown in Figure S2 and Table S5. Production of ClO2 was detected using a BW Technologies GAXT-V-DL GasAlert Extreme Chlorine Dioxide (ClO2) Single Gas Detector. 2.2.1. Control of Temperature and Humidity. The humidity under Mars conditions was roughly controlled by placing or removing a 9 cm diameter Petri dish filled with 50 mL of water ice on the cold plate ∼ 30 cm from the sample, creating water vapor with ∼1 Torr partial pressure at −15 °C. The presence or absence of the ice is referred to as humidMars or dry-Mars conditions, respectively. Terrestrial conditions were produced by opening the MSC to the laboratory ambient atmosphere with a temperature of ∼ 25 °C and RH of ∼ 25%. In both cases, all other conditions were the same as described above. 2.3. Sample Preparation and Irradiation Procedures. The samples irradiated in the MSC consisted of mixtures of (1) sodium chlorite (NaClO2, Sigma-Aldrich technical grade, containing 80 wt % NaClO2, ∼18 wt % NaCl, and ∼2 wt % Na2CO3); (2) sodium chlorate (NaClO3, ACS reagent, ≥99.0%); (3) silica sand (SiO2, 50−70 mesh); (4) titanium dioxide (TiO2, anatase form, powder, 99.8%, Sigma-Aldrich #232033, average particle size 325.); (5) chlorite (ClO2−) 1000 ppm IC standard; or (6) chlorate (ClO3−) 1000 ppm IC standard. Prior to use, the SiO2 sand was rinsed five times with Nanopure 18.2 MΩ-cm deionized (DI) water and then dried overnight at ∼60 °C. Each compound was weighed out to the nearest 0.1 mg and homogenized using mortar and pestle. The mixture was spread into a 9 cm diameter Pyrex dish. The salts were dissolved in DI water and added to the sample, dried overnight at ∼60 °C, homogenized, and stored in a 20 mL plastic vial. The salts used in the experiment were in excess, and it was confirmed using a chlorite IC standard (>99.9%) that the NaClO3 in technical grade NaClO2 did not affect the amount of perchlorate formed in a 48 h experiment most likely due to the low concentration and slow reaction rate. A series tests were conducted with NaCl as starting material under the same conditions with no chlorite, chlorate, or perchlorate detected after 7 days, thus the effect of NaCl in 48 h experiments should be negligible. Four custom-made ∼4 mL vials were each filled with ∼1 g of the appropriate mixture stock and placed symmetrically under the UV light spot. Vial configuration under the UV beam can be found in Supporting Information Figure S1. Another vial filled with 1 g of SiO2 sand was placed next to the samples but out of the UV light spot as reference. The reference vial was used to monitor if any gaseous intermediary products were formed during the reaction. A dark control vial was made at the
However, none of them are likely to be occurring under the severely arid Mars ambient conditions (i.e., −40 to 5 °C, 96% CO2 and 0.1% oxygen, ∼10 Torr). Recently, it was shown that under the present Mars environment and UV irradiation, ClO4− and ClO3− can be produced from either powdered sodium chloride (NaCl) or at even higher levels with mixtures of NaCl, silica (SiO2), iron oxide (Fe2O3), aluminum oxide (Al2O3), and titanium dioxide (TiO2).10 Although the effects of the metal oxides were not fully investigated, it was effectively shown that the SiO2 by itself was sufficient to promote the reaction. One other important observation was that this photochemical mechanism did not appear to require the presence of O3. In order to understand the production of perchlorate under Mars ambient conditions, it is important to understand not only what other oxychlorines are produced and their reaction mechanisms and pathways leading to the fully oxidized ClO4− oxychlorine species but also any potential catalysts that may be involved in its production. In this study we have specifically focused on the role and effects of titanium dioxide (TiO2) as a semiconductor photocatalyst, not only because it is known to be present on Mars (probably as the minerals anatase and/or rutile) at ∼1−2 wt %28−32 but more importantly because it has the potential to catalytically promote the photochemical oxidization of chloride. Previous results under Martian ambient conditions showed a substantial increase in the production of ClO4− and ClO3− (×20 and ×6, respectively) when SiO2 was added to the NaCl substrate. It was suggested that this was most likely due to the SiO2 acting as a semiconducting photocatalyst, generating free O2− radicals from O2 which then reacted with the Cl− in the sample.10,33 Zhao et al. showed that under 254 nm UV irradiation with chloride bearing evaporative brines that a larger silica bead surface area led to higher ClO4− and ClO3− production.34 Because TiO2 (anatase) has a smaller band gap (3.2 eV, ∼390 nm) than SiO2 (8.5 eV, ∼145 nm), its ability to oxidize Cl− should be even greater than that of SiO2 and it should still function as a photocatalyst in UVB (280−315 nm) and UVA (315−400 nm) regions where SiO2 cannot.35,36 Because the environments on Mars and Earth are very different, it is possible that TiO2 could also behave very differently in each case. In this study, we determined the effects of water on perchlorate production under both dry and wet martian environments. The amount of water available in perchlorate formation could change the results significantly. It has been shown that under UV irradiation a “dried” yet still hydrated chloride/bromide brine produces orders of magnitude more ClOx or BrO3− than a liquid brine.34 For TiO2, water is generally considered to be the electron donor for photocatalysis, and its requirement for effective TiO2 photocatalysis has been shown in many systems including wastewater treatment plants.37 However, it has also been shown that increasing the humidity above the optimized level can hinder its catalytic ability,38 most likely due to the competition between water and reactants for available sites on the TiO2 surface.39
2. MATERIALS AND METHODS 2.1. The Mars Simulation Chamber. The Mars simulation chamber (MSC) consisted of a stainless steel cylindrical chamber (∼5 × 104 cm3) with a 25 cm diameter cold plate at the bottom which was maintained at −15 °C while gases above were at ∼0 °C. The atmosphere inside the B
DOI: 10.1021/acsearthspacechem.9b00134 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry Table 1. Production of Perchlorate under Various Conditions and Mixturesa Mars ambient (dry-Mars) sample compositionand wavelength used
ClO4 in sample (mol)
#1 NaClO2+ SiO2 @ λ = 200−400 nm #2 NaClO2+ SiO2+TiO2 @ λ = 200−400 nm
5 × 10−7 (±1 × 10−7) 9 × 10−9 (±2 × 10−9)
#3 NaClO2+ SiO2 @ λ = 300−400 nm #4 NaClO2+ SiO2+TiO2 @ λ = 300−400 nm #5 NaClO3+ SiO2 @ λ = 200−400 nm #6 NaClO3+ SiO2+TiO2 @ λ = 200−400 nm
6 × 10−8 (±1 × 10−8) >LOD but < LOQ 2.7 × 10−7 (±1 × 10−9) 9 × 10−9 (±1 × 10−9)
