The kinetics of mercury vaporization in soil during low-temperature thermal treatment

Equilibrium vapor-phase Hg concentrations and vaporization kinetics at temperatures from 100 to 400 °C were evaluated in field soil naturally contaminated with Hg0, as well as sand artificially contaminated with HgCl2, HgO, and HgS. The calculated change in the standard enthalpy of Hg vaporization (ΔH0) was 4.73 (±1.52), 2.11 (±0.17), 5.59 (±0.25), and 4.87 (±0.46) kcal mol−1 for naturally Hg0-contaminated field soil and sand artificially contaminated with HgCl2, HgO, and HgS, respectively. The measured ΔH0 was 30% of the theoretical ΔH0, which suggests that higher temperatures are required to remove Hg from contaminated soil when compared to pure chemical states. Thermal vaporization and desorption kinetics tended to increase upon increasing the temperature; however, the rates at 300 and 400 °C were similar to each other due to kinetic limitations. Our theoretical calculations showed that 90% Hg removal from field contaminated soil at 100, 300, and 400 °C would require 204 days, 3.5 h, and 2.7 h, respectively. At low temperature (i.e., approximately 100 °C), an unrealistically long time was required for Hg removal from soil; however, increasing the temperature up to 400 °C did not necessarily decrease the remediation time. Thus, optimal remediation temperature needs to be evaluated based on Hg thermal desorption and volatilization kinetics.