Investigating desorption of native pyrene from sediment on minute- to month-timescales by time-gated fluorescence spectroscopy

We investigated desorption of native pyrene from field-aged sediments using time-gated, laser-induced fluorescence (LIF) spectroscopy. LIF is superior to conventional analytical methods for the measurement of quickly changing dissolved pyrene because it allows observations at minute-scale resolution, has a low detection limit (∼1 ng/L), and minimizes sample loss and disturbance since it requires no system subsampling and chemical analysis. The efficacy of LIF was demonstrated in studies of pyrene desorption from Boston Harbor sediment segregated into different size-fractions (38-75, 75-106, and 180-250 μm diameter) and used in varying solid-to-water ratios (20, 70, and 280 mg _solids/L). The effects of particle size and solid loading on desorption were consistent with diffusion physics. For suspension conditions between 20 and 280 mg_solids/L, we observed desorption continuing toward an apparent plateau level over the course of weeks to months. This implies that the characteristic desorption time of pyrene from fine sediments and, by inference, other sediment-bound hydrophobic organic compounds (HOCs) of similar hydrophobicity, exceeds the typical characteristic times for pore waterflushing and resuspension events. Consequently, the assumption of local sorption equilibrium in modeling efforts would be inappropriate.