Photodegradation is the major pathway of methyl mercury (MeHg) degradation in surface water. However, the mechanism underlying MeHg photodegradation is still not completely understood. Previous reports suggest four potential pathways are responsible for MeHg photodegradation, including direct photodegradation, hydroxyl radical dominant indirect photodegradation, singlet oxygen dominant indirect photodegradation, and comprehensive action of various free radicals/reactive oxygen species (ROS). Recently, we reported that dissolved organic matter (DOM)-MeHg complex play the dominant role in the photodegradation of MeHg in the Florida Everglades wetland water. Another very different pathway, involving direct photodegradation of MeHg-DOM complexes via intramolecular electron transfer, was proposed as the dominant mechanism for MeHg photodegradation in the Everglades water. Taking the differences in water chemistry in aquatic system into consideration, MeHg photodegradation in different waters should be investigated furtherly to inspect the dominant mechanism for MeHg photodegradation in normal aquatic system. In this study, based on our experimental results reported about the methyl mercury (MeHg) photodegradation in the Everglades wetland water, MeHg photodegradation in different other typical waters, including sea water, creek water and DOM contained solution were studied by the methods of N2/O2 purging and scavengers adding for further understanding of MeHg photodegradation mechanism. The influence of MeHg/DOM ratio and different MeHg ligands adding on MeHg photodegradation were also investigated. It was found that MeHg photodegradation rate constant in sea water ((17.95±0.70)×10-3 E-1 m2), creek water ((13.62±0.75)×10-3 E-1 m2) and DOM contained solution ((6.08±0.33)×10-3 E-1 m2) differed greatly. However, the influences of N2/O2 purging and scavengers adding on MeHg photodegradation in three waters were very similar to that in the Everglades water previously reported. Firstly, MeHg photodegradation was promoted under the condition of purging N2 in all the studied waters. Secondly, the addition of scavengers of hydroxyl radical and hydrated electron had little influence on MeHg photodegradation, while addition of sodium azide inhibited MeHg photodegradation. As the production of reactive oxygen species is expected to increase under oxic conditions and decrease under anoxic conditions, the significant enhancement of MeHg photodegradation under anoxic conditions indicates that reactive species, especially singlet oxygen, may not play an important role in MeHg photodegradation in the studied waters. This was confirmed furtherly by the MeHg photodegradation in presence of scavengers of hydroxyl radical and hydrated electron. The inhibition of azide on MeHg photodegradation in studied waters was because the competition of N3- with DOM for complexing MeHg rather than scavenging effect of N3- on singlet oxygen. This was confirmed by the MeHg photodegradation in presence of cysteine (CYS) and N3-, which showed that MeHg can be photo-degraded in presence of cysteine at a relatively high rate, while would be inhibited under dark or in presence of N3- as competing ligand. It was also found that MeHg photodegradation rate constant was greatly influenced by MeHg/DOM ratio. With DOM concentration fixed as 2 mg L-1, MeHg photodegradation rate constant remained almost unchanged with MeHg concentration increasing from 1 to 100 ng L-1, then increased quickly when MeHg concentration was between 100 and 1000 ng L-1, and then dropped sharped. This indicated that different of photodegradation pathways might play dominant role under the condition of different MeHg/DOM ratio. In the case of normal environ water with very low MeHg concentration and relatively high DOM concentration, MeHg mainly existed in the form of MeHg-DOM complex. Results above indicated that direct photodegradation of DOM-MeHg complexes via intra-molecular electron transfer was perhaps the dominant mechanism for MeHg photodegradation in normal environmental water.
