In Situ Evaluation of Benzene and Toluene Biodegradation in a Large Block of Gneissic Residual Soil

Abstract

Numerous socio-economic benefits are associated with the oil industry. However, problems cannot be neglected, resulting in many accidents that can occur at any production stage of this industry. A serious problem created by this industry is environmental contamination by hydrocarbons. Gasoline in contact with infiltration water, the aromatic constituents, in particular the BTEX group, partially dissolve being the first contaminants to reach the groundwater. Studies published in the literature indicate that biological degradation is the main mechanism responsible for reducing the concentration of these toxic pollutants. Laboratory studies of this kind have been conducted in small scale columns, where the biological field conditions are not well reproduced. However, studies conducted in a larger scale with structured soil, remaining closer to field conditions, evidenced biodegradation processes of BTEX. This paper presents the in situ construction, instrumentation and monitoring of a large rectangular block of undisturbed residual gneissic soil, subjected to a horizontal flow of a contaminant solution composed of water, benzene and toluene to evaluate the biodegradation of these compounds in different points of the block over time. During the period of 85 days, samples of the percolating solution were periodically collected from pre-established monitoring points and the concentrations of benzene and toluene were determined using a coupled technique of gas chromatography and mass spectrometry (GC-MS). Based on the monitoring results, decay in the concentrations of the compounds over time and along the length of the block was observed. These results indicated, consistently with the biological characterization of the soil that showed the existence of microbial degrading activity, the occurrence of biodegradation of benzene and toluene, and, by an extraction process, the retention of these compounds in soil was also verified.

Sensitivity of Benzene Biodegradation to Variations in Kinetic Parameters in a Brazilian Residual Soil

“…However, the presence of oxygen would have been more readily noticeable, which would have inhibited the anaerobic degradation process along the entire block until there was a significant concentration of aerobic bacteria. Consequently, until a balance was reached between aerobic and anaerobic degradation, there might have been a delay in benzene degradation in the other sampling points, more consistent with the observations made by Martins et al (2012).…”

“…The block of residual soil used in this study (Martins et al, 2012) was set up in the Geotechnical Experimental Field of the Federal University of Viçosa (UFV), Brazil. The block was 1.5 m long, 0.6 m high and 1.0 m wide and it was carved in an exposure of residual soil from gneissic origin (Martins, 2010, Martins et al 2012. The stages performed by Martins (2010) during the extraction, preparation and contamination tests of the block (Figure 2) were:…”

“…We considered a constant input of 305 mg/L of benzene and 5 mg/L of oxygen. The concentration of oxygen was determined according to the value of oxygen dissolved in the water used to prepare the solution of benzene and in the block saturation (Martins et al 2012). Therefore, the initial concentration of oxygen used in this study was also 5 mg/L, concentration found in the groundwater.…”

“…In addition to sampling point T2, three other points were included in the model: T5, T8, and T11, located at 0.60 m, 0.9 m and 1.2 m from the source, respectively, shown in (Figures 2a and 2c). The hydraulic parameters used in the analysis were determined based on the characteristics observed in the experimental study, with values of 0.327 m/d for the saturated hydraulic conductivity, 0.3 m/m for the hydraulic gradient, 0.098 m/d for the water velocity, 0.54 for the total porosity, and 1.63 kg/L for the soil's specific gravity (Martins et al 2012). The values of longitudinal (αL) and transversal (αT) dispersivity were estimated to be around 0.1 m and 0.04 m, respectively (Romero Velásquez 2015).…”

“…Substrate degradation was conservatively assumed to occur only in the liquid phase, ignoring the potential decay of the absorbed contaminants (Gomez et al 2008). All the biomass was considered to be stationary and adhered to the porous medium, according to the results found by Martins et al (2012), which indicated that in the block of soil, the biodegradation occurred mainly for the contaminants adsorbed on the soil. The selection of the parameters was determined by the site-specific aspect of bioremediation and, consequently, by the wide variety of parameters found in the literature (Table 2).…”