Destruction of methyl bromide sorbed to activated carbon by thiosulfate and electrolysis

Authors: YANG, YU - Stanford University; YUANQING, LI - Stanford University; Walse, Spencer; MITCH, WILLIAM - Stanford University

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2015
Publication Date: 3/19/2015
Citation: Yang, Y., Yuanqing, L., Walse, S.S., Mitch, W.A. 2015. Destruction of methyl bromide sorbed to activated carbon by thiosulfate and electrolysis. Environmental Science and Technology. 49:4515-4521. doi: 10.1021/es505709c.

Interpretive Summary: Methyl bromide (MB) is a postharvest fumigant that is highly effective against insect and microorganism pests. MB is also an atmospheric source of reactive bromine gases, which deplete stratospheric ozone. Anthropogenic utilization of MB is regulated by international agreement under the Montreal Protocol. In instances where postharvest chamber fumigations are permitted, contribution(s) to ozone depletion can be minimized, or eliminated, by removing MB from the ventilation effluent via activated carbon sorbent. As part of a larger research project to optimize activated carbons for this use, we conducted experiments to determine how sorbed methyl bromide was destroyed by aqueous solutions of thiosulfate as well as by electrolysis. Aqueous solutions of thiosulfate effectively destroyed methyl bromide, yet a relatively costly amount of thiosulfate was required and disposal of methyl thiosulfate generated as a reaction byproduct would be required. Electrolysis, a potentially cheaper approach, resulted in >80% debromination of methyl bromide in ~30 h. Finding cost-effective techniques for eliminating methyl bromide emissions into the atmosphere may help ensure that the continued use of MB has minimal environmental impact.

Technical Abstract: Methyl bromide is widely used as a fumigant for post-harvest and quarantine uses at port facilities due to the low treatment times required, but it is vented to the atmosphere after its use. Due to the potential contributions of methyl bromide to stratospheric ozone depletion, technologies for the capture and degradation of the methyl bromide are needed to enable its continued use in these applications. While activated carbon has been used for methyl bromide capture, techniques for destruction of the methyl bromide while sorbed to the carbon have not been explored. Submerging the activated carbon in an aqueous thiosulfate solution achieved reductive debromination of sorbed methyl bromide with half-lives ~2 h, but required molar concentrations of thiosulfate due to the high methyl bromide loading. Cycling experiments demonstrated that the sorptive capacity of the activated carbon was rapidly depleted. Submergence of the activated carbon in water, and use of the activated carbon as the cathode of an electrolysis unit also destroyed sorbed methyl bromide with bromide as the product. Destruction rates increased with decreasing applied voltage down to ~ -1.2 V. The Coulombic efficiency dropped with decreasing voltage, particularly below -1.2 V, likely due to competition for electrons by proton reduction to generate hydrogen. Cycling experiments conducted at -1.0 V indicated that debromination of methyl bromide was achieved over ~6 h half-lives with ~40% Coulombic efficiency, while maintaining sorptive capacity and degradation efficiency over at least 3 cycles.