Diffusive transport of 2,4,6-trinitrotoluene (TNT) from contaminated soil to overlying water

Abstract

Understanding the transport mechanism of 2,4,6-trinitrotoluene (TNT) and other compounds is necessary in order to implement an effective phyto- or bioremediation scheme for explosives-contaminated soil. The transport of TNT from two contaminated soils into overlying water was investigated using a laboratory sheet-flow leaching bed reactor (SLBR). Soil I had a low contamination of TNT (11±1 mg kg −1), whereas Soil II had very high contamination of TNT (22 874±518 mg kg −1). The results showed a decrease in aqueous effluent TNT concentration and flux with time in both cases indicating the diffusive nature of the process ( t −1/2 dependance). The flux from the sediment to the water column was used to obtain the effective diffusivity of TNT by fitting the data on Soil I to a mathematical model. The average effective diffusivity value obtained was 1.18×10 −6±8.32×10 −7 cm 2 s −1. This suggests slow diffusive transport of TNT through the soil. The initial flux from Soil II was approximately 50 times higher than from Soil I. The final steady state flux was larger than that from Soil I since the initial porewater TNT concentration was much higher for Soil II. From our experiments, we calculated that the characteristic time for transport is estimated to be ca. 7 years whereas the first order degradation rate by plant enzymes is ca. 70 min. The slow transport of TNT from the soil bed will be a limiting factor in the phyto- or bioremediation of explosives-contaminated soils.