The electrochemical regeneration of granular activated carbons in situ of permeable reactive barriers

Abstract

Permeable reactive barriers have proven to be an effective and cost efficient remediation technique for the clean-up of petroleum hydrocarbon contaminated sites in extreme regions such as the Antarctic. The materials within these barriers, namely granular activated carbon, decontaminate migrating groundwaters via adsorption processes and prevent further spread of pollutants into the environment. However, with long operational periods, the activated carbon becomes saturated and is no longer effective at capturing contaminants. In an effort to prevent this, this thesis investigated the possibility of using in situ electrochemical treatments as a means of regenerating the activated carbon in these barriers such that the continuous replacement of saturated material is not necessary.

Aqueous phase studies were first conducted to assess which electrochemical reactions aid in the degradation of solubilized petroleum hydrocarbons. Due to the natural presence of chloride and iron at the contaminated sites in the Antarctic and sub-Antarctic, the active chlorine and electro-Fenton pathways were chosen. Similarly, naphthalene, a high priority pollutant for removal in these regions, was chosen as a model compound to investigate the efficacy of the selected reactions. Upon application of an electric current in a near-saturated naphthalene solution, both reaction pathways achieved full contaminant removal within 3 hours of treatment. Further analysis showed that the naphthalene was electrochemically transformed into species of lesser toxicity with minimal energy usage that is appropriate for use in remote regions. Varying operational conditions were assessed to determine the underlying mechanism for which naphthalene was removed, and a dynamic kinetic model was developed for each reaction that could accurately predict treatment outcomes over a range of reagent concentrations, treatment timeframes, and applied electric currents.

Due to the success for which the active chlorine and electro-Fenton pathways degraded naphthalene in the aqueous phase, the reactions were applied to naphthalene loaded granular activated carbon to determine the extent of regeneration that could be achieved. Regardless of the reaction applied, only 30 % regeneration could be achieved under any of the regenerative trials conducted, indicating that only the exterior surface of the porous granular activated carbon was likely being regenerated. As the micropores within the activated carbon were essentially unaffected by electrochemical treatments, macroporous or non-porous materials may be better suited for achieving high regeneration efficiencies.

Although complete regeneration of the activated carbon was not reached, the developed technology can still prolong the longevity for which granular activated carbon can perform within permeable reactive barriers; over four cycles of treatment, the exterior surface was continually restored and freed up adsorptive sites for further adsorption processes. Thus, an ideal method for applying electrochemical treatments in situ of existing permeable reactive barriers is recommended.