Heterogeneous Production of Perchlorate and Chlorate by Ozone Oxidation of Chloride: Implications on the Source of (Per)Chlorate in the Solar System

W. Andrew Jackson, Sixuan Wang, Balaji Rao, Todd Anderson, Nubia Luz Estrada

Research output: Contribution to journalArticlepeer-review

9 Scopus citations

Abstract

The occurrence of chlorate (ClO3 -) and perchlorate (ClO4 -) in the terrestrial and extraterrestrial environments has been partly attributed to ozone (O3)-mediated oxidation of chlorine-bearing compounds. This is based on varying elevated Δ17O values in all measured terrestrial natural ClO4 - as well as the nearly universal co-equal occurrence of ClO3 - and ClO4 -, which has only been reported to occur for dry oxidation of Cl-, a process for which little information is available. In this study, we examine possible factors influencing ClO4 - and ClO3 - formation by O3 oxidation of sodium chloride (NaCl) salt and hydrochloric acid (HCl) gas in glass reactor vessels. We show that longer reaction times increase production of ClO4 - and ClO3 -, with ClO3 - production generally being lower than ClO4 - by 1-2 orders of magnitude. For 1 day oxidation periods, ClO4 -/ClO3 - ratios were relatively constant (∼50) for low Cl- masses and decreased over 3 orders of magnitude for higher (∼100×) Cl- masses. Perchlorate mass increased with increasing glass reactor surface areas but not the salt surface area. Increasing the relative humidity (RH) from 2 to 67% increased ClO3 - production but did not affect the amount of ClO4 - produced, confirming previous reports that free water will promote additional ClO3 - but not ClO4 - production pathways. Additionally, oxidation of HCl (g) produced ClO4 - at higher yields than oxidation of NaCl but produced less ClO3 -. Our findings suggest that sufficient O3 saturation and availability of active sites are essential for heterogeneous formation of ClO4 - and ClO3 -. While glass surfaces per se are not relevant to environmental production, catalytic surfaces (silicate or others) abound in terrestrial and extraterrestrial environments. The Cl- form oxidized and amount of water vapor present will also significantly impact the ClO4 -/ClO3 - ratio, which could be helpful in evaluating the sources of ClO4 - and ClO3 - in extraterrestrial material, with important implications on the availability of water during formation.

Original languageEnglish
Pages (from-to)87-94
Number of pages8
JournalACS Earth and Space Chemistry
Volume2
Issue number2
DOIs
StatePublished - Feb 15 2018

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