Hematite Thin Film Preparation for X-ray Spectroscopic Studies of the Reactivity and Distribution of Pb(II) and As(V) at Natural Organic Matter/Iron Oxide Interfaces
EMSL Project ID
2594
Abstract
We are conducting research on the effect(s) of organic coatings on the reactivity of mineral surfaces using synchrotron-based X-ray standing wave (XSW), soft x-ray spectroscopy and microscopy, and hard x-ray XAFS spectroscopy methods, as well as FTIR methods. The main hypothesis we are testing is that heavy metals such as Pb and metalloids such as As should react with sites on the mineral surface rather than with functional groups in the organic coatings at environmentally realistic concentrations. Proof of this hypothesis over a range of pH?s and Pb concentrations will result in revisions of current thinking regarding the effect of organic coatings on mineral surface reactivity and will add constraints to current reactive transport models. A key requirement in our studies is the availability of suitable single-crystal substrates in different orientations on which to deposit organic coatings. Because of the importance of hematite as a sorbent in natural systems, we request the growth of (0001) and (1-102) single-crystal a-Fe2O3 epitaxial thin-films (5 in each orientation @ 500 ? thick) by OPA-molecular beam epitaxy in S. Chambers? lab at EMSL. We have collaborated with Dr. Chambers over the past 6 years on projects that have utilized such samples in a variety of applications, including photoemission and soft XAS studies of the interaction of water (1,2), aqueous Cr(VI) (3-5), and aqueous Pb(II) (6) with thin-film hematite and magnetite. Biofilm-coated a-Fe2O3 (0001) thin films were also used in an XSW study of the distribution of Pb(II) at these interfaces (7), which is relevant to the current proposal, as described below. Davis (8) generalized the impact of natural organic matter (NOM) on metal ion uptake on mineral surfaces using two extreme cases. In surface waters with higher NOM/mineral ratio, mineral surfaces may be completely coated by adsorbed NOM. Thus, metal uptake in such systems may be dominated by adsorbed organic films. The opposite extreme might be found in a groundwater aquifer with low organic content, where the mineral surface site concentration is high and complexation between metal ion and mineral surface is dominant. However, contrary to Davis? generalizations, recent XSW studies by our group (7) have shown that Pb(II) adsorbs on mineral surface sites at a biofilm-metal oxide interfaces when Pb is at ?molar concentrations. This result indicates that (1) Pb(II) can penetrate the biofilm and associated exopolyssacharide; (2) reactive metal oxide surface sites are not ?blocked? by the biofilm coating; and (3) the intrinsic reactivities of metal oxide surfaces are not affected by biofilm coatings in the systems studied. Although these findings appear to refute Davis? hypotheses, it is dangerous to generalize based on a few experiments. Typical NOM layers may have different permeabilities and macromolecular structures than the B. cepacia biofilm used in our XSW study (7). This may be due to the fact that NOM is mainly composed of relatively small molecular units that display some hydrophobic character. Thus, there is a possibility that NOM may form dense, impermeable organic coatings on metal oxide surfaces that could prohibit penetration of ionic molecules to the metal oxide surfaces, as suggested by Davis (8). A copy of a recent highly rated SSRL beam time proposal submitted by our group is attached and provides additional details of the proposed experiments with the hematite thin films designed to test this hypothesis.
Project Details
Project type
Exploratory Research
Start Date
2002-08-01
End Date
2005-07-11
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Renslow RS, PD Majors, JS McLean, JK Fredrickson, B Ahmed, and H Beyenal. 2010. "In Situ Effective Diffusion Coefficient Profiles in Live Biofilms Using Pulsed-Field Gradient Nuclear Magnetic Resonance." Biotechnology and Bioengineering 106(6):928-937.