Changes in photo-reactivity and optical properties of dissolved organic matter during the freezing process of water

The freezing of water in cold areas is a universal phenomenon. Dissolved organic matter (DOM) is ubiquitous in environmental ices. DOM in ice can produce reactive species (RS) including singlet oxygen (O₂1), triplet excited-state DOM (DOM^*3), and hydroxyl radical (•OH) upon light absorption, and induce indirect photolysis of contaminants in ice. In order to investigate the changes in photo-reactivity and optical properties of DOM during the freezing process of water, the freezing experiments were conducted on two natural water samples (2 L) at -10 and -20 °C with the freezing time of 60 h and 120 h in a freezer, respectively. The results showed that DOM was rejected from the ice phase and enriched in the unfrozen liquid phase during the freezing process of water. The freezing process caused changes in optical properties of DOM. The steady-state concentrations of RS ([RS]_ss) in the original samples were lower than those in all unfrozen liquid samples, but higher than those in all melted ice samples, which were collected from the freezing experiments [RS]_ss in liquid phase increased with increasing freezing time during freezing, which were mainly attributed to the enhanced light absorption of DOM in liquid phase.