Carbon isotope fractionation of benzene and toluene by progressive evaporation

Evaporation is one of the key attenuation processes for near-surface volatile organic compounds (VOCs) in the upper soil zone. Evaporation experiments were performed to investigate the carbon isotope fractionation of benzene and toluene during progressive and non-equilibrium evaporation at room temperature. Considerable carbon isotope fractionation occurred during evaporative enrichment of benzene and toluene. The carbon isotope compositions of residual compounds increased exponentially with increasing evaporation. Thus, the remaining liquids become isotopically heavier, and the process follows a Rayleigh trend. This result is compatible with the direction of isotopic changes associated with both microbial degradation and volatilization of hydrocarbons previously observed in soil columns, but shows exactly the opposite behavior to previous equilibrium volatilization findings.