EFFECTS OF GRAIN-SCALE MASS-TRANSFER ON THE TRANSPORT OF VOLATILE ORGANICS THROUGH SEDIMENTS .1. MODEL DEVELOPMENT

In the first paper of this two-paper series, we present a new model th at attributes nonequilibrium sorption of moderately hydrophobic, volat ile organic compounds to intragranular diffusion. The model differs fr om those of previous researchers in that for the first time, it combin es the following elements: (1) We account for two distinct intragranul ar rate-limiting diffusion processes, occurring in series and at widel y different timescales; (2) we describe the slower of the two processe s with a gamma distribution of diffusion rates; and (3) we use the dis parity of timescales of the two processes to approximate a boundary co ndition for the distributed diffusion equation, allowing it to be solv ed analytically. The slower diffusion process is attributed to activat ed diffusion through very small pores, called micropores. In paper 2 [ Werth et al., this issue] we evaluate the capabilities of the model an d use it to interpret experimental results.