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46099-G2
Are Natural Chars Important for the Sorption of PAHs? A Field Study With Lake Sediments and Soils in Oriole Lake

Rainer Lohmann, University of Rhode Island

Progress to date

We successfully completed field sampling of sediment cores from Oriole Lake (CA) in July 2007. A total of three surface (biological, ca 1 m) and three deep (Livingstone-style, ca 4 m cores) cores were taken from the Lake and shipped back to URI. Initially, we worked on the surface cores first, by sectioning every one cm interval. Subsamples have been sent out for ²¹⁰Pb determination of sediment ages (the results of which are still pending). Subsamples were analyzed for % moisture, loss on ignition, organic (OC) and soot black carbon (BC) content and polycyclic aromatic hydrocarbons (PAHs). BC was isolated via a thermal oxidation method at 375 °C.

We routinely included NIST SRM 1649a (marine sediment) for BC analysis. Our results (average 0.62% ± 0.06%) are not significantly different from the average published by the international BC ring trial (0.51% ± 0.14%). Our analysis of the top 50 cm of the cores showed minor fluctuations in BC content (around 0.5 %), and a very high OC content of ca. 20%. We therefore decided to bulk the top 15 cm, and use the homogenized sediment samples to derive porewater concentrations and sorption equilibrium of the PAHs involved. Sediment samples were incubated with different masses of polyethylene (PE) samplers to derive

In a modification of our proposal, I decided to use the null hypothesis (H₀) that:

Black carbon and organic carbon are sufficient to explain the sorption of PAHs to Oriole Lake sediments.

Alternatively, if the sorption of PAHs to sediments is significantly underpredicted in Lake Oriole, then we will quantify chars as a potential 3^rd sedimentary phase.

According to H₀, the overall partitioning of PAHs can be attributed to absorption into the OC fraction (f_oc) and to adsorption onto the BC fraction (f_BC) via:      

K_d = f_ocK_oc + f_BCK_BCC_w^n-1                                                                         (1)

with     K_oc the HOC's OC-water partitioning constant,

K_\BC the HOC's soot BC-water adsorption constant,

n the Freundlich coefficient,

and C_w the dissolved concentration in µg/L.

Values of the Freundlich coefficient n, K_ocs and K_BCs were taken from the literature. C_w was calculated after equilibrating the sediments with the PE samplers.

In more detail, K_oc was calculated based on a linear free-energy relationship

Log K_oc = 0.989*log K_ow - 0.346                                                          (2).

K_\BC was derived based on a linear free-energy relationship between the activity coefficients of the PAHs in water (g_w) and average K_\BCs reported by Lohmann et al (2005) for harbor sediments in Boston and New York:

Log K_BC = 0.59*log g_w + 2.33                                                  (3).

Activity coefficients were estimated based on a linear free-energy relationship between log K_ow and g_w (Schwarzenbach et al., 2005):

g_w = (log K_ow + 0.13)/0.75)                                                       (4).

As expected, explaining the observed sorption in Oriole Lake sediments (K_d = f_ocK_oc ) was insufficient to account for the observed partitioning. In contrast, the combined OC-absorption and BC-adsorption model using equation (1) gave a satisfactory result for the sediments – 8 out of 12 PAHs were within a factor of 3 of the predicted results. For most PAHs, sorption was overpredicted using equation (1), implying that the charred sediments exerted no additional sorption.

Our next step is to verify these results with sediments from deeper layer that represents previous fire-events, to see if sorption of the PAHs changes over time.

Training and education

The project provided training and education to two undergraduate students at URI-GSO: Julia Sullivan and Kevyn Bollinger (URI-GSO). Kevyn was involved in the field sampling, while Julia has been in charge of the OC, BC and PAH analysis, and the sediment equilibrations. Both undergraduate students excelled in the project, and rose to the challenge of being responsible for their own part of the research to be carried out.

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