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Introduction

In a previous study funded by an ACS PRF Type G starter grant we measured the photo-oxidation of pyrene in aqueous solution as a function of environmentally relevant solution media, specifically ionic strength and humic acid concentration (Clark et al., 2007).  Key findings from this work were that photodegradation was inhibited by a commercial humic acid and rate constants varied by nearly an order of magnitude over the range of ionic strengths and humic acid concentrations found in natural waters.  The observed inhibition was much larger than can be accounted for by simple competitive light absorbance and could possibly result from a reduction in the lifetime of the PAH excited state due to binding to DOM.  A clear understanding of binding mechanisms is needed to assess the impact of DOM on PAH photolysis. Differences observed in DOM sorption properties have been attributed to differences in aromaticity, polarity, and molecular weight (Kopinke et al., 2001; references therein). Previous studies on the sorption of PAHs to DOM have given conflicting results, suggesting both a simple partitioning model (Kopinke et al., 2001) and site-specific interactions (Laor and Rebhun, 2002).

In this grant, we proposed extending our analytical methodologies, developed for pyrene and appropriate for undergraduate research, to 5 other common PAHs in natural waters (phenanthrene, anthracene, fluoranthene, chrysene, benzo(a)pyrene). Since DOM conformation (and hence possibly PAH binding) changes with ionic strength, photolysis rates, mechanisms and sorption processes/binding constants would be examined as a function of ionic strength and humic acid concentrations using a commercial humic acid as a DOM proxy.

In year one, the following was accomplished:

1) Students set up a second HPLC system for this project

2) GC/MS/SPME and fluorescence binding analytical protocols for the sorption

studies were developed

3) Photolysis experiments as a function of salt concentration and Suwannee  River humic acid concentration were completed for phenanthrene and anthracene. The SUMR scholar conducted anthracene photolysis experiments

4) Initial binding experiments were carried out for pyrene, phenanthrene, anthracene, benzo anthracene, fluoranthene, chrysene, and benzo(a)pyrene for both Swanee River Humic acid and Swanee River fulvic acid

We report here results from year 2 of the grant. We did not reach many of our research goals in year 2 and plan to apply for a no-cost extension at the end of year 3. Most of our research is conducted over the summer as undergraduate students (and their faculty mentors) have limited time during the academic semesters. Unfortunately, in Summer 2010, which would have been our primary research time, we had to move our research lab from two separate labs to one larger combined lab on the other side of the science building. These moves were necessitated by the hiring of new faculty and a major reorganization to carve out research space for them. Our labs and equipment were out of commission for 8 weeks due to these moves.

Three undergraduate chemistry majors worked on this project in 09/10: none worked over the summer

• two sophomores (now juniors) in Fall 2009 and Spring 2010 using course research credits  
• a freshman SUMR scholar in summer 2009 completed some research in the Fall semester 2009 as a sophomore.

Results from Fall2009/Spring 2010

1) Photolysis experiments as a function of pH were completed for phenanthrene and anthracene – no significant pH effect was seen

2) Initial photolysis experiments were conducted with the other proposed PAHs – aqueous solubility limits and instrument detection limits coupled with rapid decays only allowed for a limited range of measurements

3) A new student took over the binding project as the initial student graduated – she has developed new methodology based on titrating a single solution of the PAH with varying amounts of humic.  This approach is the more common approach in the literature.

Future work:

Over the next two years (year 3 + a one-year no-cost extension), we plan to:

1. Perform the same experiments on an extended range of other PAHs of environmental importance in aquatic systems
2. Complete binding studies for all 5 proposed PAHs in unpurged air-equilabrated solutions as previously done in the literature to compare results
3. Develop a dynamic flow-through system and then repeat the binding experiments in purged solutions using a flow through cell to remove potential oxygen artifacts that have not been accounted for in previous studies.