Purification of heavy metal chromium in saturated sand by artificial freezing: Mechanism and method optimization

Heavy metal pollution of soil has become one of the most common hazards in human development. The artificial freezing method, especially the progressive freezing method, can reduce heavy metal pollutants in the soil and promises to be an effective in-situ treatment of contaminated sites. This study analyzes the freezing purification mechanism of heavy metal contaminants in saturated sand and identifies three main factors that impact the effects of purification: freezing rate, initial concentration, and diffusion coefficient. Moreover, one-dimensional freezing tests are carried out by different freezing modes. The experimental results show that the heavy metal chromium could only be removed effectively with a slow freezing rate. By optimizing the freezing mode and freezing rate, a long section of soil was frozen and purified, with the maximum purification rate reaching 65.8%. This study shows that it is feasible to treat contaminated saturated sand by a gradual-cooling freezing method.