Temperature effects on bioremediation of PAHs and PCP contaminated south Louisiana soils: A laboratory mesocosm study

Abstract

Goal, Scope and Background

Temperature and soil moisture content are important environmental variables in bioremediation technologies. Optimizing these variables in-situ would enhance and maintain remediation of hazardous wastes during cold winter seasons or in cold regions and may lead to reduced maintenance and/or cost. The effect of elevated temperature and soil moisture on bioremediation efficiency was investigated using a laboratory mesocosm approach. Selected polycyclic aromatic hydrocarbons (PAHs) and phenols degradation in contaminated flooded soils, commonly found in Superfund sites situated in coastal plains sediments/soils, were evaluated in the mesocosms.

Material and Methods

Four laboratory mesocosm treatments in triplicate simulating in-situ bioremediation of contaminated site soils using an immobilized microbe bioreactor system, i.e., bioplug, were established to evaluate temperature effects. Elevated temperature treatments of site soils with and without contaminant-specific microorganisms were established at a temperature of 42±2°C. Similarly, treatment of site soils with and without contaminant-specific microorganisms were established at an ambient temperature of 21±1°C. Composite samples were analyzed for selected PAHs and chlorinated phenols to determine rates of mineralization and overall remediation efficiency for different temperature regimes.

Results

Mesocosm studies indicated that the high temperature inoculated treatment demonstrated a significant reduction in mean total PAHs and total phenols with a kinetic rate (KR) of 76±13 ng g⁻¹ d⁻¹ in 49 days (approximately 84% reduction; p<0.01) The KR for low temperature inoculated treatment was 54±1 ng g⁻¹ d⁻¹ in 49 days (approximately 66% reduction; p<0.01). High temperature non-inoculated mesocosms exhibited significant mineralization of all constituents with KR of 15±6 ng g⁻¹ d⁻¹ (approximately 65% reduction; p<0.01) in 49d compared to 54% reduction for low temperature non-inoculated treatment with KR of 12±3 ng g⁻¹ d⁻¹ (p=0.1794). Phenol compounds in inoculated treatments were also significantly reduced (65%, p<0.01) at elevated temperatures compared to ambient (52%, p<0.01).

Discussion

Increased bioavailability and desorption were noted for elevated temperature and moisture in the soil laboratory mesocosms simulating a field in situ remediation protocol. This protocol employing the application of immobilized microflora indicated that in situ systems provide an economical advantage if optimal elevated temperature and moisture are controlled properly. Results also suggested that temperature and moisture optimization needs to be combined with efficient nutrients delivery systems for impacted soils/sediments.

Conclusions

The study demonstrated that temperature and soil moisture contents are important factors in the success of in-situ bioremediation techniques at hazardous waste sites situated in a coastal zone. Kinetic rates were significantly enhanced to remediate known recalcitrant compounds (PAHs and phenols) in aged soil.

Recommendations and Perspectives

The placement of a preferred microbial consortia such as an immobilized microbial population in an entrained bioreactor, i.e., bioplug, can significantly reduce constituents of concern in a timely manner for contaminated soils/sediments. However, frequent monitoring of the soil temperature, moisture content, nutrient level, and dissolved oxygen is necessary to achieve predictable kinetic rates of mineralization.