Modelling plant-aided in-situ bioremediation of soils contaminated with a volatile organic compound (VOC)

Abstract

In this study, bioremediation of volatile organic compounds (VOCs) in soil, associated with plant root systems, was simulated by developing a mathematical model based on the assumption of homogeneous, isothermal and isotropic soil conditions, with mobile water but without density-driven gas flow. The proposed model includes root interactions with soil and water and VOCs in time and space, as well as advective and dispersive transport in unsaturated soil. The developed model considers liquid-gas mass exchanges and permits root growth in the soil. For the simulation of temporal and spatial changes of the root behaviour in soil and water and with VOCs, the time-specific distribution of root quantity through the soil was incorporated into the simulation model. VOC absorption and subsequent uptake into the roots with water were simulated using empirical equations. In addition, microbial activity in the rhizosphere, a zone of unique interactions between the roots and the soil microorganisms, was included in the model. Cotton was seeded on a hypothetical site contaminated with trichloroenthylene (TCE), one of the ubiquitous volatile contaminants at many hazardous waste sites. The soil was assumed to be a sandy loam. Model parameters were independently estimated from the simulation of real data and a range of literature values to obtain a realistic simulation. The simulation results showed a greater degradation of TCE in the presence of cotton compared to the degradation in the absence of cotton. Significant sensitivity of the model to the diffusion in the gas phase shows that the degree of change of volatile organic distribution in the soil may be influenced by the transport of the gas phase as well as the plant-enhanced biodegradation. This mathematical model for understanding and predicting the fate and the transport of volatile compounds in plant-aided remediation will assist the effective application of plant-aided remediation to field contamination.