Ice-water distribution of antibiotics and antiviral drugs during the freezing–thawing process: influencing factors and related mechanisms

Seasonal freezing and thawing of surface waters are common phenomena in mid- and high-latitude regions. During freezing and thawing processes, the pollutants in surface waters migrate between ice and water. Antibiotics and antiviral drugs (ATVs) have been widely detected in surface waters and even in river ice. To reveal their ice-water distribution and migration patterns, tetracycline (TC) and ribavirin (RBV) were chosen as model compounds and simulated freezing-thawing experiments were conducted. The results showed that the influencing factors regulated their ice-water distribution by influencing the solution freezing rate and solute diffusion rate. The ice-water distribution coefficient (<i>K</i>_IW) was enhanced with the increasing freezing temperature (<i>T</i>) and salinity (<i>S</i>), while the increasing freezing time (<i>t</i>) and initial concentration (<i>C</i>₀) resulted in a decrease in <i>K</i>_IW. Moreover, it was found that RBV had stronger migration capability than TC. During the thawing process, their release from ice declined with melting time, and the release amount of RBV declined more significantly in comparison with that of TC. The related mechanisms involve freeze concentration, solute diffusion, competition with salt in the freezing process, and the release of solutes through channels in the thawing process. These findings contribute to better understanding the migration and distribution of pharmaceuticals in frozen water bodies in cold regions.