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Distributions and bioavailabilities of iron and other trace metals associated with environmental colloids

thesis
posted on 2014-04-16, 00:00 authored by Keshia Kuhn

Fe associated with colloids such as natural organic matter (NOM; e.g., humic and fulvic acids), clay minerals, and Fe(oxy)(hydr)oxides, may represent potentially bioavailable Fe pools. These colloids, including nanoparticle (NP) forms, are ubiquitous and their high surface areas make them potentially important sorbents of contaminants and nutrients. My dissertation research applied state-of-the-art methods and approaches to characterize metal associations with colloids, including environmental NPs, and explored interactions of aerobic bacteria with colloid-associated metals in abiotic and biotic experiments.

Asymmetrical flow field-flow fractionation (AsFlFFF) was used to characterize the distributions of metals and rare earth elements (REEs) associated with aquatic NOM from the 'type locality' Suwannee River (GA, USA) and several other NOM-rich surface waters (Chapter 2). NOM samples with naturally associated metals were reacted with the commercially available siderophore desferrioxamine B (DFOB) to quantify metal removal by this high-metal-affinity microbial ligand (Chapter 3). This work showed that Fe could be easily stripped from NOM by a microbial siderophore. The NOM samples were also used in Fe-limited growth experiments with the siderophore-producing aerobic soil bacterium Pseudomonas mendocina ymp and a genetically engineered siderophore(-) mutant (Chapter 4). These biotic experiments confirmed the bioavailability of NOM-bound Fe and further demonstrated that while siderophores can be useful for Fe acquisition from NOM, they are not required, most likely because P. mendocina has a cell-associated reductant with the potential to reduce NOM-bound Fe. Overall, this work suggested that Fe and other metals bound to NOM may be far more bioavailable to aerobic microorganisms than previously thought.

Bioavailability of Fe associated with mineral colloids, including the clay mineral montmorillonite (MMT; Chapter 5) and two phases of the nanomineral ferrihydrite (Fh; Chapter 6) was also investigated in abiotic dissolution and biotic growth/Fe acquisition experiments. In biotic experiments using P. mendocina, siderophores were able to acquire Fe associated with MMT for use by cells. In the absence of siderophores, P. mendocina was able to acquire Fe via cell-associated, enzymatic reduction of clay-associated Fe(III), a process requiring direct cell-clay mineral contact. This contact, and hence Fe acquisition, was increased by the presence of biofilms. Fe acquisition from the two Fh phases (a smaller, more poorly ordered '2-line' form and a larger, more well ordered '6-line' form) was aided by siderophores, but cell-associated metalloreductases were also capable of supplying Fe to cells. The copious addition of a biofilm polysaccharide had no effect on Fe acquisition from the 2-line Fh phase and a slight enhancing effect on the 6-line Fh phase.

Siderophores can bind not just Fe(III) but a host of other metals such as Al, Cd and Pb, strongly. The effects of siderophores on Cd sorption to NPs of the Fe oxide mineral hematite were also investigated in batch reaction vessels (Chapter 7). In the absence of siderophores, Cd sorption was largely controlled by nanoparticle size (i.e., 8 vs. 40 nm) and inherent size-dependent reactivity rather than simply the total reactive surface area. Siderophores could either inhibit or enhance Cd sorption to nanohematite, depending on the solution pH. Results suggest the importance of additional research on the mechanism (adsorption vs. precipitation) of Cd sorption using molecular-based synchrotron techniques.

History

Date Modified

2017-06-05

Defense Date

2014-04-07

Research Director(s)

Patricia Maurice

Committee Members

Bruce Bunker Jeremy Fein Joshua Shrout

Degree

  • Doctor of Philosophy

Degree Level

  • Doctoral Dissertation

Language

  • English

Alternate Identifier

etd-04162014-121752

Publisher

University of Notre Dame

Program Name

  • Civil and Environmental Engineering and Earth Sciences

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