Effects of molecular size fraction of DOM on photodegradation of aqueous methylmercury

•We examined the effect of DOM in humic substances on the photodegradation of MeHg.•DOM fraction of MW < 3.5 kDa showed the highest photodegradation rate of MeHg.•MeHg photodegradation increased at alkaline pH due to OH radical production.•Photo-degradation of MeHg increased with increasing DOM concentration.•Radicals produced by DOM under MW < 3.5 kDa increased photodegradation of MeHg.

This study investigated the photodegradation kinetics of MeHg in the presence of various size fractions of dissolved organic matter (DOM) with MW < 3.5 kDa, 3.5 < MW < 10 kDa, and MW > 10 kDa. The DOM fraction with MW < 3.5 kDa was most effective in MeHg photodegradation. Increasing UV intensity resulted in the increase of photodegradation rate of the MeHg in all size of DOM fractions. Higher rates of MeHg degradation was observed at higher pH. For the portion of MW < 3.5 kDa, the photodegradation rate of MeHg increased with increasing DOM concentration, indicating that radicals such as singlet oxygen (1O2) radicals can be effectively produced by DOM. At higher portion of MW > 3.5 kDa, the inhibition of MeHg degradation was observed due to the effect of DOM photo-attenuation. Our result indicates that radical mediated reaction is the main mechanism of photodegradation of MeHg especially in the presence of MW < 3.5 kDa. Our results imply that the smaller molecular weight fraction (MW < 3.5 kDa) of DOM mainly increased the photodegradation rate of MeHg.

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