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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 less than can be accounted for by simple competitive light absorbance and suggests that the DOM may be sensitizing the photolysis of the PAH.  In this grant, we proposed extending our analytical methodologies, developed for pyrene and appropriate for undergraduate research, to 5 other common PAHs in natural water systems (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, students set up a second HPLC system for this project, GC/MS/SPME and fluorescence binding analytical protocols for the sorption studies were developed, photolysis experiments as a function of salt and humic acid concentration were initiated for phenanthrene and anthracene and initial binding experiments carried out. In year two, we did not reach our research goals. This was because most of our research is conducted over the summer with undergraduate students and in Summer 2010, which would have been our primary research time for year 2, we had to move our research labs due to 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. We report here results from year three of the grant when we completed planned work from year two. We have received a no-cost extension through August 2012 to complete this project.

Four undergraduate junior chemistry majors worked on this project in 10/11: two worked over the summer. Three had been involved in this project the year before.

Results from Fall 2010/Spring 2011

Photolysis experiments as a function of solution medium were completed for phenanthrene and anthracene. We evolved our research methodology to incorporate three dimensional excitation-emission matrix fluorescence to study photolysis products. An undergraduate student co-authored paper was written and submitted on the phenanthrene results: Warren De Bruyn, Catherine D. Clark, Kattie Otelle, Paige Aiona. Photochemical degradation of phenanthrene as a function of natural water variables modeling fresh to marine environments. Marine Pollution Bulletin, 2012. in press.

Phenanthrene photolysis followed first order kinetics, with an estimated photodegradation half-life in sunlight in pure water of 10.3 ± 0.7 hours, in the mid-range of previously published results. Photolysis rate constants decreased by a factor of 5 in solutions with humic acid concentrations from 0 to 10 mg C L^-1. This decrease could be modeled entirely based on competitive light absorption effects due to the added humics. No significant ionic strength or oxygen effects were observed, consistent with a direct photolysis mechanism. In the absence of significant solution medium effects, the photodegradation lifetime of phenanthrene will depend only on solar fluxes (i.e. temporal and seasonal changes in sunlight) and not vary with a freshwater to marine environment.

Like phenathrene, anthracene photolysis followed first order kinetics, however the photolysis rate was significantly faster than phenanthrene.  The anthracene half-life was estimated to be 1.5 minutes.  Once again, the lifetime of anthracene decreased with humic concentration at a rate consistent with competitive light absorption.  No significant ionic strength or oxygen effects were observed, consistent with a direct photolysis mechanism.  As with phenathrene, the photodegradation lifetime of anthracene  depends only on the variability in solar flux. Another student co-authored paper on anthracene is in preparation for submission in Spring 2012.

A full suite of photolysis experiments as a function of solution medium is now underway for fluoranthene, chrysene, benzo(a)pyrene and benzo(a) anthracene

Solid phase micro-extraction GC/MS and fluorescence quenching approaches to measuring DOM-PAH binding coefficients were investigated.  The fluorescence quenching approach was found to be more sensitive and suitable for undergraduate student capabilities.  Binding coefficients of 0.0352 for chrysene and 0.056 L/mg humic acid for benzo(a) anthracene were measured in pure water. 

Future work:

Over the next year (one-year no-cost extension), we plan to:

1.    Complete photolysis experiments on the remaining PAHs and prepare a manuscript.

2.    Binding coefficients will be determined for the remaining three PAHs and a manuscript on DOM – PAH binding in air-equilibrated solutions will be prepared.

3.    Extend the binding studies to explore ionic strength of the binding coefficient.