Photochemical generation of methyl chloride from humic aicd: Impacts of precursor concentration, solution ph, solution salinity and ferric ion

Abstract

Methyl chloride (CH3 Cl) is presently understood to arise from biotic and abiotic processes in marine systems. However, the production of CH3 Cl via photochemical processes has not been well studied. Here, we reported the production of CH3 Cl from humic acid (HA) in sunlit saline water and the effects of the concentration of HA, chloride ions, ferric ions and pH were investigated. HA in aqueous chloride solutions or natural seawater were irradiated under an artificial light, and the amounts of CH3 Cl were determined using a purge-and-trap and gas chromatography-mass spectrometry. CH3 Cl was generated upon irradiation and its amount increased with increasing irradiation time and the light intensity. The formation of CH3 Cl increased with an increase of HA concentration ranging from 2 mg L−1 to 20 mg L−1 and chloride ion concentration ranging from 0.02 mol L−1 to 0.5 mol L−1. The photochemical production of CH3 Cl was pH-dependent, with the highest amount of CH3 Cl generating near neutral conditions. Additionally, the generation of CH3 Cl was inhibited by ferric ions. Finally, natural coastal seawater was irradiated under artificial light and the concentration of CH3 Cl rose significantly. Our results suggest that the photochemical process of HA may be a source of CH3 Cl in the marine environment.