The ability of chitin fermentation products to promote tetrachloroethene (PCE) reduction was evaluated in a continuous-flow column system to identify how different electron donors affect reductive dechlorination. Natural chitin fermentation products were initially used to support PCE reduction. Acetate (3.5mM) was the dominant fermentation product, followed by propionate (0.1mM), butyrate (0.1mM), and hydrogen (100nM). After chlorinated ethene concentration profiles reached pseudo steady state, the ability of individual fermentation products (acetate, acetate+propionate, propionate, or formate) to support PCE reduction was evaluated. None of the fermentation products tested stimulated dechlorination as well as the suite generated from chitin (kPCE=6.9day−1); however, acetate-stimulated PCE dechlorination the best (kPCE=5.3day−1), followed by formate (kPCE=2.4day−1), acetate+propionate (kPCE=1.8day−1), and propionate (kPCE=1.2day−1). Similar trends were observed for the PCE daughter products trichloroethene and dichloroethene. Free energies of individual fatty acid reactions were calculated and shown to be useful predictors of dechlorination performance, except for the case of acetate+propionate. Hence, acetate is the dominant fatty acid controlling dechlorination in the chitin-enhanced system, propionate appears to have an inhibitory effect when present with acetate alone, and other unidentified nutrients produced during chitin fermentation likely contribute to dechlorination activity as well.
Transverse dispersion across adjacent streamlines can control the amount of mixing and reaction between one or more contaminants and a limiting substrate along the fringes of groundwater plumes. Streamlines in groundwater converge and diverge in heterogeneous porous media, depending on the permeability distribution. When flow is focused in a high‐permeability zone, the distance required for a solute to cross a given number of streamlines decreases, and the time allowed for mixing and reaction is reduced. Because the first effect outweighs the latter, the overall result is an enhancement of transverse mixing and reaction. Here we develop a conceptual model of heterogeneous two‐dimensional structures facilitating flow focusing. We use the conceptual model to develop simple analytical expressions quantifying the extent to which mixing and reaction are enhanced when flow focusing occurs and compare these to results of numerical simulations. Significant enhancement of transverse mixing and reaction by flow focusing is observed; for the cases considered, flow focusing enhances the amount of reaction by a factor ranging from 1.8 to 11.9. The relatively simple analytical expressions demonstrate that the fraction of the domain height made up by high‐permeability inclusions, the fraction of flow that passes through the inclusions, and the fringe bypassing of inclusions determine the amount of mixing and reaction enhancement for the permeability distributions considered. These results partially explain why field‐scale dispersivities are larger than laboratory derived dispersivities, where homogeneous and isotropic sediments are typically used. Further work is needed to verify the theoretical results presented here with laboratory and field experiments and to expand the relatively simple analytical expressions to consider more heterogeneous three‐dimensional permeability fields.
Chitin, corncobs, and a mixture of chitin and corncobs were tested as potential electron donor sources for stimulating the reductive dechlorination of tetrachloroethene (PCE). Semi-batch, sand-packed columns were used to evaluate the donors with aerobic and anaerobic groundwaters containing varying degrees of alkalinity. In all experiments, acetate and butyrate were the dominant fatty acids produced, although propionate, valerate, formate, and succinate were also detected. From a multivariable regression analysis on the data, the presence of chitin, limestone, and dechlorinating culture inoculum were determined to be the most positive predictors of dechlorination activity. Chitin fermentation products supported the degradation of PCE to trichloroethene (TCE), cis-1,2-dichloroethene (DCE), and vinyl chloride (VC), even in columns containing PCE DNAPL, whereas dechlorination activity was not observed in any of the columns containing corncobs alone. The longevity and efficiency of chitin as an electron donor source demonstrates its potential usefulness for passive, in situ field applications.
Catalytic nitrate reduction was evaluated for the purpose of drinking water treatment. Common anions present in natural waters and humic acid were evaluated for their effects on NO3- hydrogenation over a bimetallic supported catalyst (Pd−Cu/γ-Al2O3). Groundwater samples, with and without powder activated carbon (PAC) pretreatment, were also evaluated. In the absence of inhibitors the NO3- reduction rate was 2.4 × 10-01 L/min g cat. However, the addition of constituents (SO42-, SO32-, HS-, Cl-, HCO3-, OH-, and humic acid) on the order of representative concentrations for drinking water decreased the NO3- reduction rate. Sulfite, sulfide, and elevated chloride decreased the NO3- reduction rate by over 2 orders of magnitude. Preferential adsorption of Cl- inhibited NO3- reduction to a greater extent than NO2- reduction. Partial regeneration of catalysts exposed to SO32- was achieved by using a dilute hypochlorite solution, however Cu dissolution occurred. Dissolved constituents in the groundwater sample decreased the NO3- reduction rate to 3.7 × 10-03 L/min g cat and increased ammonia production. Removal of dissolved organic matter from the groundwater using PAC increased the NO3- reduction rate to 5.06 × 10-02 L/min g cat and decreased ammonia production. Elemental analyses of catalysts exposed to the natural groundwater suggest that mineral precipitation may also contribute to catalyst fouling.