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Transformation Kinetics of Trace-Level Halogenated Organic Contaminants in a Biologically Active Zone (BAZ) Induced by Nitrate Injection

Laboratory experiments and numerical modeling were conducted to evaluate the secondary utilization of eight trace-concentration halogenated solvents in a denitrifying biologically active zone (BAZ) induced by nitrate injection into an acetate-fed porous-medium column. Results of column experiments indicated that carbon tetrachloride was removed most completely by the denitrifying BAZ, while bromoform, dibromoethane, tetrachloroethene, trichloroethene, and 1,2- and 1,3-dichlorobenzenes were removed, but to lesser degrees. 1,1,1-trichloroethane removal was slight. Compounds were removed to higher degrees when the BAZ contact time was increased. The steady-state, one-dimensional solute-transport equation was solved using an iterative finite-difference scheme and by employing a quasilinearization technique for the biofilm-reaction term. The model solved directly for the steady-state profiles of secondary substrates. One set of experimental results was used to obtain best-fit values of kinetic parameters, which were then used to predict the removal at different liquid flow velocities. The model predictions correctly described all experimental trends: removal of the halogenated compounds only in the BAZ, greater removal with increased BAZ contact time, and reduced specific removal rates caused by diffusion limitation in the biofilm.