Date of Award

1998

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

S.F. Korom

Abstract

Nitrate (N03) contamination due to anthropogenic sources is a serious and widespread health problem. In many groundwater systems, N03-can be reduced to a harmless gas, N2, through the bacterially mediated process of denitrification. During this process, bacteria gain energy by transferring electrons from an "electron donor" to nitrate, the "electron acceptor". It is typically believed that organic carbon is the most important electron donor in the denitrification process; however, increasing amounts of research suggest the importance of inorganic electron donors. The most extensive studies involving the reduction ofN03- by inorganic species have focused on pyrite (FeS2) and Fe(II). Few studies have considered the possibility of denitrification by recently formed sulfide species, such as hydrogen sulfide (H2S), iron monosulfide (FeS), mackinawite (FeS), and greigite (Fe3S4). The objective of this research was to design and begin implementing a methodology for investigating the effects of recently formed sulfides on denitrification.

In order to begin investigating the denitrification potential of recently formed sulfides, a 4-foot high, 6-inch diameter Plexiglas column was constructed, with tubing ports on the top and bottom. The column was filled with sediment and groundwater collected from a field site in Fertile, MN. Groundwater from the field site was spiked with organic carbon (glucose) and sulfate. The spiked water was injected into the column, until two pore-volumes of the origina1-column water were flushed out. The spiked water was left in the column for several months, during which the concentrations of several groundwater species were monitored, including: total organic carbon (TOC), total inorganic carbon (TIC), dissolved oxygen (DO), reduced iron (Fe2+), reduced manganese (Mn2l, nitrate (N03-), sulfate (So/-), and pH.

The results of the laboratory study revealed that water in the column was reducing according to the thermodynamic model of reduction sequences. The dissolved oxygen concentrations decreased from 8.4 mg/L to 0.8 mg/L. Once concentrations of oxygen became limiting, dissolved manganese and then dissolved iron concentrations increased as the manganese and iron minerals in the sediment were reduced. Dissolved manganese concentrations rose from 2.07 mg/L to 4.40 mg/L, and dissolved iron concentrations increased from< 0.03 mg/L to 2.56 mg/L. Concentrations of total inorganic carbon increased concurrently with the oxidation of organic carbon and the reduction of oxygen, manganese, and iron. The next step in the sequence, sulfate reduction, was not observed during the course of this study. Once enough sulfate is reduced to form sufficient reserves of sulfides, the next step of the methodology is to add nitrate to the column. The expectation is that the sulfides will reduce the nitrate and be oxidized to sulfate.

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