Permeable Reactive Barriers For Treatment Research Articles

Fifteen-year assessment of a permeable reactive barrier for treatment of chromate and trichloroethylene in groundwater

The fifteen-year performance of a granular iron, permeable reactive barrier (PRB; Elizabeth City, North Carolina) is reviewed with respect to contaminant treatment (hexavalent chromium and trichloroethylene) and hydraulic performance. Due to in-situ treatment of the chromium source zone, reactive and hydraulic longevity of the PRB has outlived the mobile chromate plume. Chromium concentrations exceeding 3μg/L have not been detected in regions located hydraulically down-gradient of the PRB. Trichloroethylene treatment has also been effective, although non-constant influent concentrations of trichloroethylene have at times resulted in incomplete dechlorination. Daughter products: cis-1,2-dichloroethylene, vinyl chloride, ethene, and ethane have been observed within and down-gradient of the PRB at levels <10% of the influent trichloroethylene. Analysis of potentiometric surfaces up-gradient and across the PRB suggests that the PRB may currently represent a zone of reduced hydraulic conductivity; however, measurements of the in-situ hydraulic conductivity provide values in excess of 200m/d in some intervals and indicate no discernible loss of bulk hydraulic conductivity within the PRB. The results presented here are particularly significant because they provide the longest available record of performance of a PRB. The longevity of the Elizabeth City PRB is principally the result of favorable groundwater geochemistry and hydrologic properties of the site.

The effect of silica on the degradation of organohalides in granular iron columns

Dissolved silica species are naturally occurring, ubiquitous groundwater constituents with corrosion-inhibiting properties. Their influence on the performance and longevity of iron-based permeable reactive barriers for treatment of organohalides was investigated through long-term column studies using Connelly iron as the reactive medium. Addition of dissolved silica (0.5 mM) to the column feed solution led to a reduction in iron reactivity of 65% for trichloroethylene (TCE), 74% for 1,1,2-trichloroethane (1,1,2-TCA), and 93% for 1,1,1-trichloroethane (1,1,1-TCA), compared to columns operated under silica-free conditions. Even though silica adsorption was a gradual process, the inhibitory effect was evident within the first week, with subsequent decreases in reactivity over 288 days being relatively minor. Lower concentrations of dissolved silica species (0.2 mM) led to a lesser decrease (70%) in iron reactivity toward 1,1,1-TCA. The presence of dissolved silica species produced a shift in TCE product distribution toward the more highly chlorinated product cis-dichloroethylene ( cis-DCE), although it did not appear to alter products originating from the trichloroethanes. The major corrosion products identified were magnetite (Fe 3O 4) or maghemite (γ-Fe 2O 3) and carbonate green rust ([Fe 4 2+Fe 2 3+(OH) 12][CO 3·2H 2O]). Iron carbonate hydroxide (Fe(II) 1.8Fe(III) 0.2(OH) 2.2CO 3) was only found in the silica-free column, indicating that silica may hinder its formation. A comparison with columns operated under the same conditions, but using Master Builder iron as the reactive matrix, showed that Connelly iron is initially less reactive, but performs better than Master Builder iron over 288 days.

Modeling the Influence of Decomposing Organic Solids on Sulfate Reduction Rates for Iron Precipitation

The influence of decomposing organic solids on sulfate (S04(2-)) reduction rates for metals precipitation in sulfate-reducing systems, such as in bioreactors and permeable reactive barriers for treatment of acid mine drainage, is modeled. The results are evaluated by comparing the model simulations with published experimental data for two single-substrate and two multiple-substrate batch equilibrium experiments. The comparisons are based on the temporal trends in SO4(2-), ferrous iron (Fe2+), and hydrogen sulfide (H2S) concentrations, as well as on rates of sulfate reduction. The temporal behaviors of organic solid materials, dissolved organic substrates, and different bacterial populations also are simulated. The simulated results using Contois kinetics for polysaccharide decomposition, Monod kinetics for lactate-based sulfate reduction, instantaneous or kinetically controlled precipitation of ferrous iron mono-sulfide (FeS), and partial volatilization of H2S to the gas phase compare favorably with the experimental data. When Contois kinetics of polysaccharide decomposition is replaced by first-order kinetics to simulate one of the single-substrate batch experiments, a comparatively poorer approximation of the rates of sulfate reduction is obtained. The effect of sewage sludge in boosting the short-term rate of sulfate reduction in one of the multiple-substrate experiments also is approximated reasonably well. The results illustrate the importance of the type of kinetics used to describe the decomposition of organic solids on metals precipitation in sulfate-reducing systems as well as the potential application of the model as a predictive tool for assisting in the design of similar biochemical systems.

A Permeable Reactive Barrier for Treatment of Heavy Metals

Historical storage of ore concentrate containing sulfide minerals at an industrial site in British Columbia, Canada, has resulted in widespread contamination of the underlying soil and ground water. The oxidation of sulfide minerals has released significant quantities of heavy metals, including Cu, Cd, Co, Ni, and Zn, into the ground water. A pilot-scale, compost-based, sulfate-reducing permeable reactive barrier was installed in the path of the dissolved heavy-metal plume. The permeable reactive barrier uses sulfate-reducing bacteria to promote precipitation of heavy metals as insoluble metal sulfides. Monitoring over a 21-month period indicated significant removal of heavy metals within the barrier. Copper concentrations declined from a mean concentration of 3,630 pg/L in the influent to a mean concentration within the barrier of 10.5 microg/L, Cd from 15.3 microg/L to 0.2 microg/L, Co from 5.3 microg/L to 1.1 microg/L, Ni from 131 pg/L to 33.0 microg/L, and Zn from 2,410 microg/L to 136 pg/L. Within the lower half of the barrier where tidal influences were more limited and sulfate-reducing conditions were better maintained, mean treatment levels of 2.9 microg/L (Cu), 0.1 microg/L (Cd), 0.4 microg/L (Co), 2.7 microg/L (Ni), and 6.3 microg/L (Zn) were observed.

817. Evaluation of medical plastics: Mühlemann, H. (1965). Eignung und Prüfung von Kunststoffen in der Pharmazie. Pharm. Acta Helv.40, 129

Dissolved silica species are naturally occurring, ubiquitous groundwater constituents with corrosion-inhibiting properties. Their influence on the performance and longevity of iron-based permeable reactive barriers for treatment of organohalides was investigated through long-term column studies using Connelly iron as the reactive medium. Addition of dissolved silica (0.5 mM) to the column feed solution led to a reduction in iron reactivity of 65% for trichloroethylene (TCE), 74% for 1,1,2-trichloroethane (1,1,2-TCA), and 93% for 1,1,1-trichloroethane (1,1,1-TCA), compared to columns operated under silica-free conditions. Even though silica adsorption was a gradual process, the inhibitory effect was evident within the first week, with subsequent decreases in reactivity over 288 days being relatively minor. Lower concentrations of dissolved silica species (0.2 mM) led to a lesser decrease (70%) in iron reactivity toward 1,1,1-TCA. The presence of dissolved silica species produced a shift in TCE product distribution toward the more highly chlorinated product cis-dichloroethylene (cis-DCE), although it did not appear to alter products originating from the trichloroethanes. The major corrosion products identified were magnetite (Fe3O4) or maghemite (γ-Fe2O3) and carbonate green rust ([Fe42+Fe23+(OH)12][CO3·2H2O]). Iron carbonate hydroxide (Fe(II)1.8Fe(III)0.2(OH)2.2CO3) was only found in the silica-free column, indicating that silica may hinder its formation. A comparison with columns operated under the same conditions, but using Master Builder iron as the reactive matrix, showed that Connelly iron is initially less reactive, but performs better than Master Builder iron over 288 days.