Reduction of TCE and Chromate by Granular Iron in the Presence of Dissolved CaCO3

I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. Abstract Iron permeable reactive barriers (PRBs) have been installed at sites contaminated with various reducible organic and inorganic chemicals, particularly chlorinated solvents, worldwide. Many geochemical factors can affect the performance of iron PRBs. Chemicals such as nitrate and Cr(VI) may act as competing oxidants when co-exist with chlorinated solvents. Previous studies observed declines in the rate of TCE degradation by granular iron in the presence of nitrate. Passive oxide formation on the iron surface and an increase in corrosion potential of the iron were determined to be the mechanisms for the decline. Cr(VI), being a stronger oxidant than nitrate, may have a similar but greater effect on the iron reactivity. In addition, chromium oxide as well as dissolved CaCO 3 , a common groundwater constituent, form secondary precipitates and are likely to further affect the iron reactivity. The primary objective of this study was to determine the effects of Cr(VI) and dissolved CaCO 3 on the iron reactivity towards TCE and Cr(VI) reduction and to provide mechanistic explanations for the observation. In addition, the applicability of a modified reactive transport model (Jeen, 2005) to the system in which chromate and CaCO 3 co-exist with TCE was evaluated. Column experiments, including measurements of corrosion potential and surface film composition using Raman spectroscopy were conducted. Five column tests were carried out with input solutions consisting of different combinations of TCE (5 mg/L), Cr(VI) (10 mg/L) and dissolved CaCO 3 (300 mg/L) for eight months. The results from the column receiving only Cr(VI) showed that Cr(VI) was reduced rapidly by the granular iron and was not detected beyond 10 cm from the influent end of the column by the end of the experiment. However, Cr(VI) profiles migrated from the influent end further into the column overtime, suggesting progressive passivation of the iron near the influent end of the column. The gradual increase in corrosion potential (up to 180 mV positive shift) at the port 3 cm from the inlet with the migration of Cr(VI) profiles suggests the formation and accumulation of higher valent iron oxides, such as hematite and goethite, together with Cr(III) products on the iron surface, passivating iron …