Methyl halide hydrolysis rates in natural waters

Abstract

International regulations are under consideration for methyl bromide because of its high time dependent ozone depletion potential. Geocycling of the species is not well understood, and removal may occur in several types of natural water incuding the oceanic and those in soils. The hydrolysis reaction is a dominant loss pathway in environmental aqueous systems, but rate constants have generally been reported only in distilled water and at greater than room temperature. Here we present measurements in sodium chloride solutions and in seawater in addition to pure water, and at temperatures across the oceanographic range. The reaction could be followed even in solutions near the freezing point because product methanol was monitored in the method of initial rates. Time constants for methyl bromide hydrolysis fall between 10 and 1000 days over the temperatures of the sea, and are always within an order of magnitude of the fastest abiotic destruction mode, chlorination. Activation energies for the two processes are similar so that the ratio of their time scales does not vary with oceanic location. Hydrolysis rate constants are also listed for the closely related compounds methyls iodide and chloride. Solvolysis of the methyl halides in natural waters acts as a source of methanol to the ocean and atmosphere.