Intermittently mixed batch reactor systems for treatment of contaminated soils and sediments: Enhancement of mass transfer and bioavailability of polycyclic aromatic hydrocarbons.

Abstract

A unique intermittently mixed batch reactor (IMBR) system was developed to address mass transfer and biodegradation of polycyclic aromatic hydrocarbons (PAHs) sorbed to soil organic matter (SOM) or associated with non-aqueous phase liquids in soils and sediments. Mixing of dense geosorbent slurries (57--67% of solids content, w/w) that exhibited a non-Newtonian fluid behavior using a helical screw impeller (auger) was quantified via dimensional analyses with respect to power requirement, indicating that a small increase in the degree of mixing is subject to a significant increase in torque and power demands. The intermittent auger mixing at relatively low degrees effectively enhanced the phenanthrene desorption from the labile and rapid desorption fraction SOM and biodegradation of sorbed phenanthrene under either aerobic or anaerobic conditions. The phenanthrene desorption from the resistant and slow desorption fraction SOM was not noticeably influenced by the auger mixing that controls reactor-scale processes. The similarity between enhancement pattern for mass transfer and that for biodegradation of sorbed PAHs suggests that the overall PAH biodegradation is limited by its mass transfer from the solid phase to the aqueous phase. The effects of surfactant introduction on the bioavailability of PAHs in soil-water systems were evaluated using biodegradable nonionic ethoxylated sorbitan fatty ester surfactants. High dosages of surfactants (>20 g/L) enhanced the mobilization of PAHs from a creosote-contaminated field-site soil via micellar solubilization, but the micellar solubilized PAHs in the dispersed phase were not directly bioavailable to PAH-degrading microorganisms. Microbial cultures selectively and preferentially utilized the surfactants as a carbon source, resulting in release of PAHs from the micellar-solubilized pseudophase to the aqueous phase. The PAHs so released were readily re-crystallized, associated with sorbed phase surfactants, or re-adsorbed to SOM too rapidly to be accessed by the PAH-degrading microorganisms. Preliminary cost analyses for the<italic> in-situ</italic> IMBR system yielded estimates ranging from $153 to $223 per cubic yard of soil, which are competitive with other current technologies. The IMBR technology developed in this work appears to be both performance-efficient and cost-effective for the treatment of PAH-contaminated soils and sediments.