Separating the Kinetic and Sorption Parameters of Mixed Chlorinated Solvents in Contact with Granular Iron

Abstract

Chlorinated solvents and nitroaromatic solvents in drinking-water supplies are an important concern for public health. Granular iron, the most common medium in permeable reactive barriers (PRBs), is very effective at removing organic chemicals, such as chlorinated solvents and nitroaromatic compounds, from groundwater. In an effort to improve barrier designs, studies have been undertaken to examine the iron surface, as well as the reaction kinetics of granular iron. The development of the kinetic iron model (KIM) in 2009, which was derived specifically for PRB settings, made it possible for the first time to assess the simultaneous contributions of sorption and reaction to contaminant degradation rates in iron PRBs, providing a new tool to improve PRB design. This work extended the previous studies that used the KIM by applying the kinetic model to study the effects of iron aging on the reaction kinetics of chlorinated solvents and nitroaromatic solvents. It was found that over time and exposure to water and oxidizing organics, iron tended to lose sorption sites associated with the highest reactivities , but gained reactive sorption capacity to sites with lower reactivity. In the short term, the increasing sorption capacity led to overall faster reaction rates than were observed with new iron. The results also indicated that the KIM parameters were more than simple fitting parameters. As expected, the nitroaromatic compounds tested (4ClNB and 4AcNB) reacted faster than the chlorinated solvents tested (PCE and TCE). Analysis of the data with the KIM indicated the rate differences were due to the surface reaction rate constant, not sorption. This result matched expectations based on earlier studies of these classes of organic chemicals. To test the accuracy of the estimated kinetic and sorption parameters, determined with novel methods in this work, a one dimensional transport model with Langmuir sorption and KIM kinetics was developed to generate synthetic data sets. The model was prepared with the ability to assess intra- and interspecies competition between TCE and PCE in the column experiments. Synthetic data were analyzed with the methods used to interpret the laboratory data and accurate estimates of the input parameters were calculated, validating the methodology. Finally, the activation energy of the 4-chloronitrobenzene reacting with two types of granular iron, Connelly iron and QMP, in batch reactors was obtained to assess the role of mass transfer in controlling the kinetics. Previous work had indicated that mass transfer was not rate controlling with Connelly iron, but QMP was a texturally different form of granular iron that needed further testing. QMP exhibited slower reaction rates compare to Connelly iron. Based on the estimated activation energies (Ea) of the reduction reactions, the reaction mechanism(s) for 4ClNB transformation on Connelly iron and QMP iron were both electron transfer controlled, and the result also suggest that the different transformation rates were therefore related to phases on the solid surface.