A Proposed Improvement for Measuring Hydroxylamine in Seawater

Abstract

Hydroxylamine is a chemical intermediate in the nitrogen cycle that can quickly react via biotic and abiotic processes to yield nitrous oxide, a potent greenhouse gas. Because of its high reactivity, hydroxylamine tends to be present at low concentrations in aquatic ecosystems. High reactivity and low concentrations also make hydroxylamine difficult to measure. Our goal was to improve the method for measuring environmental concentrations of hydroxylamine. The current method involves converting hydroxylamine to nitrous oxide with ferric ammonium sulfate at low pH and analyzing the nitrous oxide produced by gas chromatography. This method requires a recovery curve because the conversion of hydroxylamine to nitrous oxide does not always go to completion. Here, we propose a new method using a manganese oxide mineral pyrolusite, which rapidly oxidizes hydroxylamine and completely converts it to nitrous oxide, thus eliminating the need for a recovery curve and sample acidification. The method involves (1) crushing and sieving commercial pyrolusite to increase reactive surface area; (2) adding crushed pyrolusite to airtight bottles containing hydroxylamine in artificial seawater at neutral pH; (3) incubating for two hours; and (4) analyzing of the oxidized product, nitrous oxide, by gas chromatography. Hydroxylamine concentrations are calculated from the concentration of nitrous oxide in headspace before and after pyrolusite addition. Addition of crushed pyrolusite resulted in complete conversion of hydroxylamine to nitrous oxide within two hours, whereas minimal conversion occurred without pyrolusite. This method has a shorter reaction time and goes to completion, allowing for more rapid and accurate measurements of hydroxylamine in aquatic environments.