Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Diane M. McKnight

Second Advisor

Fernando Rosario-Ortiz

Third Advisor

Mark Williams

Fourth Advisor

Robert Runkel

Fifth Advisor

Joseph Ryan

Abstract

Abstract

Duren, Sabre Miel (Ph.D., Civil, Environmental, Architectural Engineering Department)

Wetland Photochemistry as a Major Control on the Cycling of Dissolved Organic Matter, Rare Earth Metals and Trace Metals in an Acid Mine Drainage Impacted Watershed

2013

Thesis directed by Associate Professor Diane M. McKnight

Throughout Colorado, acid mine drainage (AMD) is a pervasive problem affecting the integrity of surface waters. Several studies have investigated iron photochemistry in acid mine drainage streams and lakes (e.g. McKnight et al., 1988; Hrncir and McKnight, 1998; McKnight and Duren, 2004; Gammons et al., 2005; Wood et al., 2006; Parker et al., 2008). Wetlands are potential "hot spots" for dissolved organic matter (DOM) photochemistry because the shallow waters are influenced by high light intensity and high DOM concentrations with long residence times for reactions to take place.

DOM is key in understanding dominant diel processes on H2O2, iron speciation, trace metals, and rare earth elements. H2O2 can be produced through photolysis of DOM (Voelker and Sulzberger, 1996). While Fe2+ is produced by photoreduction, it is also consumed in the photo-Fenton reaction (Voelker and Sulzberger, 1996).

A diel study was performed on October 14, 2011 in a wetland system located downstream of Pennsylvania Mine in Summit County, Colorado to quantify the concentrations and reaction rates of DOC, H2O2, Fe2+/Fe3+, and other metals of interest. The pH throughout the experiment ranged from 3.41 - 3.97. The results confirmed photochemistry is a major control on the oxidation and reduction of iron in the AMD-impacted wetlands. At midday the H2O2 concentrations reached a maximum and then decreased in the afternoon. The dissolved Fe2+ concentrations were a mirror image of the H2O2 concentrations due to consumption in the photo-Fenton reaction.

The diel fluctuations of dissolved iron concentrations driven by changing light intensity were associated with nearly identical trends in the concentrations of several metals, all of which increased as Fe2+ decreased. In addition to metals commonly found in AMD streams (Cu, Cd, and Pb), these metals included a number of rare earth elements (Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sc, Sm, Tb, Tm, Y, Yb) some of which occurred in concentrations exceeding 200 ug/L. The data collected during the experiment confirmed the role of photochemistry in controlling the oxidation and reduction of iron, and the effect iron speciation has on other metal concentrations in the wetland.

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