Naphthalene and phenanthrene sorption to very low organic content diatomaceous earth: modeling implications for microbial bioavailability.

Naphthalene and phenanthrene sorption was investigated on microporous/high surface area and low-microporous/low surface area particles with very low organic (f(oc)) content. Partitioning coefficients (K(p)) for naphthalene were similar to those predicted from the Karickhoff equation in both competitive and non-competitive sorption isotherms, even given the very low f(oc). In contrast, phenanthrene K(p) values in competitive isotherms were 10-fold higher than predicted by Karickhoff, suggesting phenanthrene out-competes naphthalene for sorption sites. Naphthalene exhibited greater non-competitive K(p) at higher concentrations on the microporous particles, as evidenced by a Freundlich n=0.74. Both compounds had 100-fold lower adsorption and desorption mass flux on the microporous particles. Adsorption followed first order kinetics, with phenanthrene adsorbing at 1.5 and 3 times the rate of naphthalene on the low surface area and high surface area particles, respectively. Naphthalene and phenanthrene desorption kinetics were well-described by a Fickian diffusion model with observed diffusivities (D(obs)) of 1.7-1.9 x 10(-8) and 0.93-1.9 x 10(-8) cm(2) s(-1) for naphthalene and phenanthrene, respectively. Phenanthrene D(obs) were 3-5 orders of magnitude faster than those reported in organic-rich sediments. Naphthalene D(obs) were 100-fold lower than fast-domain diffusivities, indicating access to micropores. Naphthalene sorption non-linearity was investigated via simulations with two coupled desorption-biodegradation models. Results indicate that non-linearity would not significantly affect bioavailability in low f(oc) geosorbents. In contrast, sorption non-linearity would result in greatly decreased bioavailability in organic-rich geosorbents, indicating that desorption non-linearity should be considered for surface soils and sediments but may not be critical for low f(oc) aquifer material.