DYNAMIC INTERACTIONS BETWEEN SORPTION AND BIODEGRADATION OF HALOGENATED COMPOUNDS IN CONTAMINATED GROUNDWATER

Abstract

Groundwater is one of the most important freshwater sources on Earth. Toxic dissolved chemicals from anthropogenic activities have been seeping into the groundwater, posing a risk for drinking and irrigation water resources. The contaminants form a plume that travels underground spreading those chemicals to a wider area. Permeable Reactive Barriers (PRBs) can be implemented underground to intercept the plume via sorption and biodegrade the contaminants. Traditionally, Granular or Powdered Activated carbon (GAC or PAC) were used as a fill material. Within the past decade, Colloidal Activated Carbon (CAC) has been produced, reducing the cost of PRBs installations. Despite years of research on sorption and biodegradation, some aspects of the dynamic interaction between adsorption, desorption and biodegradation have not been fully understood. Therefore, in this study, the kinetics of biodegradation in presence and absence of sorptive material, the extent of biomediated desorption, and bioregeneration efficiency were examined. Extent of biomediated desorption reached 93.5 ±4.4% on CAC materials, showing high bioavailability of sorbed TCE. This means that initially sorbed contaminants are able to desorb and biodegrade. Longevity of PRBs can be extended with the carbon acting as a repository. Moreover, biofilm morphology in presence of this novel material was characterized. It was shown that small CAC particles are taken up and embedded in biofilm. Different CAC products were compared to a reference GAC to examine the effect of particle size on kinetics, biomediated desorption extent and bioregeneration efficiency. The results showed a slower biodegradation kinetics on GAC compared to CAC. All CAC samples had a significantly (p < 0.05) higher extent of biomediated desorption than GAC samples. Additionally, bioregeneration efficiency of GAC decreased significantly post biodegradation, with GAC showing 41.5 ±3.5%, while CCAC samples exhibited an 88.4 ±11.8% efficiency.It is commonly reported that biodegradation is the rate-limiting factor for the coupled sorption and biodegradation. Therefore, an equilibrium model was compared to a dynamic kinetics model, to evaluate this assertion. The results showed that biodegradation and sorption have an interplay, and biodegradation alone is not the rate-limiting factor for the overall process. This will allow practitioners and engineers to have a more realistic prediction of the PRBs remediation efficacy. Lastly, methanogenic activity was investigated to assess the Greenhouse Gas (GHG) emissions from bioremediation of chlorinated solvents. Methane is produced, with no difference between GAC and CAC. Future studies should assess these findings in a column or field applications.