Thermodynamics and kinetics of hydrophobic organic compound sorption in natural sorbents and quantification of black carbon by electron microscopy

Abstract

The sorption behaviors of hydrophobic organic compounds (HOCs) in sediments were investigated using pyrene. Native pyrene desorbed slowly, taking from weeks to months to equilibrate. The end-point data suggested that, at nanogram-pyrene-per-liter porewater levels, sorption was much stronger than conventionally expected. The non-linearity of the isotherm may indicate physical occlusion of native sorbate and/or sorption onto micropore surfaces of char/charcoal. Between 30-70% of the native pyrene may be occluded. Conceptual pictures for both hypotheses were presented with supportive evidence from experiments and literature. Analysis of experimental and literature data suggested logKoc (organic-carbon normalized partition coefficient) and logKBC (black-carbon normalized partition coefficient) values were fairly constant across different geosorbents (around 4.5-5.7 and 5.6-6.3, respectively), while the non-linearity exponent varied substantially. This may explain the orders of magnitude scatter in logKoc's and logKBC's reported in recent reviews. An a priori non-linear numerical model based on Intra-particle Porewater Diffusion (IPD) was constructed and successfully predicted the desorption kinetics of native pyrene. Fitted kinetic parameters correlated with system and sorbate/sorbent properties. This suggested the empirical approach can be replaced by the a priori model and the diverse HOC desorption rates in the literature can be reconciled if relevant physicochemical properties are known. The regional fate of pyrene in Boston Harbor was evaluated with a box model using derived kinetic and equilibrium properties. Realistic predictions can be obtained when assuming pseudo steady state conditions, but not equilibrium partitioning, for the bed sediment and the water column. Furthermore, model results and literature evidence suggested that sediment resuspension may be a significant mobilization mechanism for sedimentary HOCs in estuaries and harbors. A new BC quantification method based on energy dispersive X-ray spectroscopy (EDX) was developed. The method identified/quantified Organic Carbon (OC) or Black Carbon (BC) by analyzing the elemental ratios of C, N, and 0 of the sample. Agreeable OC/BC estimates on a variety of carbonaceous materials were obtained using the method. The good analytical potential of the method warranted further exploration and methodological refinement. This study has great implications for the sequestration and bioavailability of HOCs in the environment.