In Situ Bioremediation of Toluene‐Polluted Vadose Zone: Integrated Column and Wetland Study

Abstract

A vertical soil column setup integrated with wetlands is developed to study the biodegradation and transport of toluene, a light non‐aqueous phase liquid (LNAPL), in the subsurface environment. LNAPL‐contaminated water is applied to infiltrate from the top of the soil column. The observed and simulated breakthrough curves show high equilibrium concentration at top ports rather than at lower ports, indicating effective toluene biodegradation with soil depth. The observed equilibrium concentration of toluene is higher in the case of unplanted wetland, asserting an accelerated biodegradation rate in the planted case. A difference in the relative concentration of toluene between input and output fluxes at 100 h is found as 13.34% and 30.86% for planted and unplanted wetland setups, respectively. Estimated biodegradation rates show that toluene degradation is 2.5 times faster in the planted wetland setup. In addition, the difference in the observed bacterial count and dissolved oxygen prove that toluene degraded aerobically at a faster rate in the planted setup. Simulations show that as time reached 80–100 h, there is no significant change in concentration profile, thereby confirming the equilibrium condition. The results of this study will be useful to frame plant‐assisted bioremediation techniques for LNAPL‐contaminated soil–water resources in the field.

Bioremediation of Organic Pollutants in Soil–Water System: A Review

“…Plant-assisted biostimulation and bioaugmentation performed in a controlled environment at room temperature (21 °C sustaining the initial substrate concentration as 8 mg/L), exhibited the maximum rate of degradation with accrual of 97.93% juxtaposition to natural biodegradation separately. Locally available wetland plants of Canna generalis were utilised to increase the removal efficiency of the LNAPL, as the root exudates play a dominant role in the biostimulation of the rhizosphere, providing a benign carbon source for toluene degraders [ 4 , 35 ]. The batch experiment showed the decay of toluene in every phase, whereas sterile batches demonstrated comparatively small losses of the LNAPL compared to biodegradation assays over the whole experimental period.…”

“…Percolation of BTEX compounds into the groundwater table is a perilous concern due to their lethal toxicity, so it is indispensable to seek and examine better remediation options that are environmentally nonfatal; among all other methods of remediation, such as physical containment, booming and skimming, soil–water extraction and a host of others, this paper [ 4 ] examines the removal efficacy of plant-assisted engineered bioremediation techniques using pot-scale wetlands to quantify toluene absorption by root and shoot biomass. Dzantor [ 38 ] observed that plants may create a suitable environment for the metabolism of microorganisms, in turn intensifying the rate of biodegradation in the contaminated root zone, as the phytoremediation of organic contaminants transpires sharply via root uptake and eventual translocation to shoot biomass, phytodegradation and rhizodegradation.…”

“…Considering their viscosity, NAPLs have been classified as light non-aqueous phase liquids (LNAPLs) and dense non-aqueous phase liquids (DNAPLs) [ 1 , 2 , 3 ]. As, NAPLs are immiscible with water, hence even if small quantity of NAPL got introduced into the soil, it can create a plume in the subsurface system through vertically downward flow, which is enough to degrade the soil’s quality ( Figure 1 ) [ 2 , 4 ]. Later, it also reaches the groundwater table where it contaminates the groundwater, which is used for different purposes such as irrigation and drinking water supply, on a large scale.…”

“…Hence, understanding the transient flow of NAPLs in the subsurface heterogeneities is the precursor to approaching remedial actions. Therefore, this paper presents an in-depth study on field-scale behaviour, laboratory studies and analytical and numerical modelling in multiphase flow to understand immiscible fluid behaviour and characterize remedial actions [ 1 , 2 , 3 , 4 ].…”

Impact of Nonaqueous Phase Liquid on Cr(VI) Removal by Nano Zerovalent Iron Particles: Effects of Contact Time, Pollution Load, and pH

“…The presence of toluene in soil and water resources causes a long-term threat to the receiving environment. When released at the surface, toluene moves downward through the soil and generally retains on top of the groundwater table (Yadav and Hassanizadeh 2011;Basu et al 2020). Though toluene is immiscible with water, its solubility (535 mg/L) is several orders of magnitude higher than the permissible level (0.7 mg/L) in drinking water (World Health Organization 2004.…”