Influence of flow rate and organic carbon content on benzene transport in a sandy soil

The influence of organic carbon content at various flow rates on the transport of aqueous benzene in a sandy soil was investigated. Column experiments were conducted for a sandy soil with different carbon contents (0%, 0.5%, and 2% by weight) and flow rates (0-25, 0.625, and 1.25 ml min^-1) to observe breakthrough curves of KCl and benzene using a step application. Three types of convection-dispersion transport model (equilibrium, reversible two-site, and reversible-irreversible sorption) were used to determine the appropriate model that best described the observed benzene transport. The modelling results revealed that the reversible-irreversible transport model was suitable for describing the transport and sorption behaviour of benzene in the sandy soil studied. Retardation and irreversible sorption coefficients increased with increasing carbon content, since the increment of carbon content resulted in the enhancement of sorption capacity for benzene. With increasing flow rate, decreased retardation of aqueous benzene was observed due to less reaction time; however, the model parameter for irreversible sorption increased unrealistically. This was attributed to the interference of velocity change on the irreversible sorption coefficient during parameter estimation. In order to avoid this problem, we introduced a Damköhler number (Da₁), which took into account the velocity changes in estimating the irreversible sorption coefficient. A reasonable relationship was then found between Da₁ and flow rate, which showed a decrease of Da₁ with the increase in flow rate.