| dc.description.abstract | BTEX (benzene, toluene, ethylbenzene, and xylene isomers) are monoaromatic, volatile organic compounds (VOCs) with single benzene ring and various degree of alkylation. US EPA classifies benzene and TEX (toluene, ethylbenzene, and xylene) as Class A (Carcinogenic) and Class D pollutants, respectively. A major source of BTEX contamination in groundwater is leakage from storage tanks in gasoline filling stations and oil-refineries. Perusal of the groundwater quality data in problem areas of India shows availability of data for inorganic contaminants, microbial contaminants and pesticides but do not contain data on BTEX levels in groundwater.
Results of the study indicate that BTEX contamination of Bengaluru aquifers from leaky UST’s in petrol filling stations is not extensive as only 6 % of groundwater samples showed benzene presence in excess of the permissible limit, while TEX compounds were below permissible limit in all the 124 groundwater samples. BTEX levels in contaminated groundwater were sensitive to seasonal variations as higher BTEX concentrations were generally observed during post-monsoon than pre-monsoon season. Lower BTEX concentration during pre-monsoon is attributed to reduced leaching of organic compounds from the overlying soil overburden to aquifers.
A contaminant transport model using MODFLOW is created to obtain insight into the role of hydrodynamic dispersion and diffusion in the spread of BTEX plume in Bengaluru aquifer. Variations in Ss(specific storage) or Sy(specific yield) at given kx(aquifer permeability) did not affect the predicted BTEX concentrations in the observation well at any migration period. However, reduction in kx affected the BTEX concentration in the observation well. BTEX
migration modeled using the low kx (5E-7 m/s) generally agree well with the measured benzene, toluene, and total xylenes concentration values in the observation well.
Among various methods to treat BTEX contaminated water, adsorption has attracted considerable attention of researchers. Recycled materials such as tyre crumb rubber (TCR) have an advantage over other adsorbents, in terms of low cost and large availability. Permeable reactive barriers (PRB) are used for in-situ treatment of contaminated groundwater. The use of TCR particles as reactive media in PRB’s to treat BTEX contaminated groundwater is an attractive option, that has not been previously explored.
Results of the study showed that TCR particles are mainly composed by SBP (52.7%) and CB (24.5%) constituents. Despite a lesser presence (24.5%), the CB component of TCR accounts for bulk (83%) of toluene adsorption by the TCR-sand mix. Analysis of adsorption data using distribution coefficients (Kc), Scatchard plot and Freundlich isotherm implied that toluene molecules are adsorbed at energetically heterogeneous sites of the TCR particles. Adsorption in pores of different sizes was responsible for the heterogeneity of adsorption sites. Interactions between toluene molecules and the adsorption sites of TCR particles involve weak van der Waals attraction forces. Despite the weak bonding, bulk (91%) of the adsorbed toluene molecules is not released upon water leaching as the spatial arrangement of toluene molecules in the medium and fine pores of TCR particles hinders desorption. Adsorption isotherms identified 2.5% TCR + 97.5% sand mix as the optimum mix to treat toluene contaminated water. Batch experiment results predict that unit PRB column constructed with 2.5% TCR-97.5% sand mix can remediate 11013 KL of toluene contaminated (0.15 mg/L) groundwater near instantaneously.
Besides adsorption, biodegradation is extensively used to treat BTEX contaminated groundwater, wastewater, and subsurface soils. Microbial consortia isolated from activated sludge, gasoline contaminated soils, sea-tidal flat were successful in biodegradation of BTEX compounds. The feasibility of exploring bacterial cultures cultivated from gasoline contaminated soils and groundwater in Bengaluru city is an interesting bioremediation option.
Bioremediation is accomplished by enrichment of bacterial culture in minimal media with toluene as carbon source. Results of the study showed that Pseudomonas sp isolated from petroleum contaminated soil and Bacillus haynesii isolated from BTEX contaminated groundwater are capable of toluene biodegradation. At toluene concentrations of 5-300 mg/L, Pseudomonas could degrade 40 to 100% of the hydrocarbon concentration. Comparatively, Bacillus haynesii could degrade 22 to 66 % of available toluene concentration. Toluene degradation by the microbes followed first order kinetics with first order rate constant of 0.127 h-1. The lower toluene degradation ability of Bacillus haynesii is attributed to greater substrate inhibitory effect. Review of microbial facilitated toluene degradation studies indicate that Pseudomonas species have higher degradation efficiency than other bacteria strains. | en_US |
