Electrokinetic Transport of CrVI and Integration with Zero-Valent Iron Nanoparticle and Microbial Fuel Cell Technologies for Aquifer Remediation

Hexavalent chromium (CrVI) is a carcinogenic heavy metal that is a product of many industrial processes. The study presented here investigates alternative remediation options for CrVI through the integration of effective contaminant transport and novel treatment technologies. It was determined that contaminant transport by electrokinetic technology can be used in conjunction with zero-valent iron nanoparticles (nZVI) for Cr VI reduction, and also integrated with microbial reduction in a microbial fuel cell (MFC). NZVI have been proposed as an economic and effective in situ remediation technology, capable of rapidly reducing Cr VI species to insoluble and non-toxic CrIII species. MFC technology is capable of reducing CrVI through microbiological mechanisms, and is viewed as a potential renewable energy resource. Both of these technologies are environmentally conscious alternatives to many conventional remediation approaches.;Electrokinetic contaminant transport has been proposed to be effective for contaminated groundwater in soils of low hydraulic conductivity such as clay. Thus, initial electrokinetic investigations were performed using two different clay soils, EPK kaolin and kaolinite. Bench-scale isotherm and rate studies were performed to characterize the interaction between the two clay soils and CrVI. It was found that EPK kaolin had an extremely high capacity for reducing CrVI through chemical reduction and physical adsorption, resulting from natural organic matter (NOM) bound to the EPK kaolin particle surface. Comparisons between the soils indicated the importance of NOM on the fate and transport of CrVI in aquifers, and also highlighted the significance of surface bound NOM relative to dissolved NOM in the water matrix. Surface bound NOM was shown to rapidly interact with CrVI driving its reduction to CrIII species, while dissolved NOM and CrVI formed complexes over relatively long time periods. Electrokinetic column studies with the two clay soils reflected these initial findings, and also highlighted transport inhibition due to pH gradient formation, as described in literature.;To circumvent inherent problems associated with electrokinetic transport through clay soils, a sandy soil was used for the continuous-flow column studies. This work focused on the integration of electrokinetic transport with nZVI addition for CrVI remediation, and evaluated the effects of NOM. Since transport of nZVI through subsurface formations has been shown to be difficult, it was decided that using electrokinetic transport to move contaminants to the point of nZVI injection could be effective. A series of adsorption/reduction studies and rate experiments determined the reduction capacity of nZVI to be nearly two times greater under anoxic conditions in comparison to oxygenated water containing naturally occurring groundwater constituents such as humic acid and calcium ion, and indicated rapid reaction kinetics. Electrokinetic transport experiments with nZVI injection showed that transporting CrVI to the site of nZVI injection was beneficial in terms of reducing the amount of nZVI required for treatment, and overall treatment time. Additionally, the continuous-flow system eliminated pH gradient formation, allowing for continual CrVI transport. The presence of humic acid in these studies did not effect electrokinetic transport of CrVI, however did lower its removal by nZVI. In a pilot- or full-scale nZVI treatment system, it seems appropriate to place a number of nZVI injection wells at strategic locations in the aquifer to optimize treatment, however for the sake of simplicity and ease of evaluation only one injection location was used in the experimental studies. (Abstract shortened by UMI.).