Abstract: Understanding the impacts of freeze-thaw processes on the long-term stabilization of heavy metals in polluted soil in cold regions of northern China is an important basis for screening remediation techniques. In this study, we examined the effects of freeze-thaw frequency and water content on the synchronous stabilization of cadmium and arsenic in soil by ferromagnetic biochar (F-MBC) in the freeze-thaw process, taking the soil contaminated by in situ heavy metals for many years as the object. The results showed that as the frequency of freeze-thaw increased, soil pH and electrical conductivity showed a trend of first increasing and then decreasing, but both were higher than that before freeze-thaw. Under the same freeze-thaw frequency, the increase in initial water content did not have a significant impact on soil pH under F-MBC treatment, but soil electrical conductivity showed a downward trend. Compared with those before freeze-thaw, the contents of available Cd and As in soil experienced a pattern of first increasing and then decreasing during the freeze-thaw process, but both were significantly higher than those before freeze-thaw in the soil. With the increases of initial water content, soil available Cd content continued to decrease and was significantly lower than that in the control (non-freeze-thaw). Available As content first increased and then decreased, and was higher than that in the control (non-freeze-thaw). Our results demonstrate that F-MBC can still achieve synchronous and stable remediation of Cd and As contaminated soil after the freeze-thaw process, but the risk of As pollution would increase. These results provide theoretical support for optimizing the stabilization and remediation technology of cadmium and arsenic composite contaminated soil in cold regions.

Key words: freezing-thawing frequency, water content, ferromagnetic biochar, Cd, As, availability