Optimizing contaminant desorption and bioavailability in dense slurry systems. 1. Rheology, mechanical mixing, and PAH desorption.

Intermittently mixed batch reactor (IMBR) systems were employed to evaluate the effects of mechanical mixing and corresponding power consumption on rates of phenanthrene desorption from natural and synthetic model sorbent phases to the aqueous phase in dense slurry reactors. Sorbent slurries comprising 57-67% (w/w) solids exhibited non-Newtonian (pseudoplastic) fluid behaviors, with apparent viscosities varying with shear rate. Dimensionless power numbers varied inversely with the Reynolds number under laminar flow conditions, indicating that small increases in mixing revolution number and auger size effect significant increases in power and torque requirements for the mechanical mixing of dense slurries. Rates of release of phenanthrene associated with rapidly desorbing or labile fractions of sorbent organic matter (SOM) to the aqueous phase were markedly enhanced by relatively low-level auger mixing, but significantly less further enhancement was observed as higher levels of mixing were applied. Conversely, desorption of phenanthrene associated with slowly desorbing or resistant fractions of SOM was relatively unaffected by auger mixing, being limited as it is by slow intraparticle-scale diffusion processes that are not enhanced by reactor-scale mixing. The experimental results lead to and support a conclusion that auger mixing at relatively low intensity is an attractive strategy for optimizing dense slurry reactor systems for remediation of hydrophobic organic contaminants associated with labile (rapidly desorbing) fractions of SOM with respect to performance efficiency and cost-effectiveness.