A substantial cost of granular iron permeable reactive barriers is that of the granular iron itself. Cutting the iron with sand can reduce costs, but several performance issues arise. In particular, reaction rates are expected to decline as the percentage of iron in the blend is diminished. This might occur simply as a function of iron content, or mass transfer effects may play a role in a much less predictable fashion. Column experiments were conducted to investigate the performance consequences of mixing Connelly granular iron with sand using the reduction kinetics of trichloroethylene (TCE) to quantify the changes. Five mixing ratios (i.e., 100%, 85%, 75%, 50%, and 25% of iron by weight) were studied. The experimental data showed that there is a noticeable decrease in the reaction rate when the content of sand is 25% by weight (iron mass to pore volume ratio, Fe/Vp = 3548 g/L) or greater. An analysis of the reaction kinetics, using the Langmuir‐Hinshelwood rate equation, indicated that mass transfer became an apparent cause of rate loss when the iron content fell below 50% by weight (Fe/Vp = 2223 g/L). Paradoxically, there were tentative indications that TCE removal rates were higher in a 15% sand + 85% iron mixture (Fe/Vp = 4416 g/L) than they were in 100% iron (Fe/Vp = 4577 g/L). This subtle improvement in performance might be due to an increase of iron surface available for contact with TCE, due to grain packing in the sand‐iron mixture.

In this study, the sorption behavior of a wide variety of N-, S-, and O-heterocyclic compounds (NSOs) to reference soils (Eurosoils 1-5) was characterized by a soil column chromatography (SCC) approach. The major goal was to identify the compound specific and environmental factors influencing sorption processes. The sorption of S- and O-heterocyclic compounds (thiophene, benzothiophene, 5-methylbenzo[b]thiophene, benzofuran, 2-methylbenzofuran, and 2,3-dimethylbenzofuran) was generally controlled by nonspecific interactions with soil organic carbon (OC). With regard to non-ionizable N-heterocyclic compounds, pyrrole, 1-methylpyrrole, and pyrimidine were hardly retarded in any soil. The sorption of indole, 2-hydroxyquinoline, and benzotriazole was dominated by specific interaction (e.g., complexation of surface-bound cations) rather than partition to soil OC. The sorption of ionizable N-heterocyclic compounds (quinoline, isoquinoline, quinaldine, 2-methylpyridine, and pyridine) can be described by a conceptual model including partitioning to soil OC, cation exchange, and an additional sorption process (probably surface complexation of the neutral species). Cation exchange was usually the dominant mechanism in the sorption of ionizable compounds if the protonated fraction of the compound exceeded 5%. Otherwise, surface complexation became dominant. Soil pH was the most important factor influencing the sorption of ionizable NSOs. Our study suggests that a fairly precise assessment of sorption in most soils can be expected for N-, S-, and O-heterocyclic compounds if the three sorption mechanisms are taken into accountwhere appropriate. Deviations from this behavior indicated special cases where additional soil specific properties (e.g., accessible surface, CEC, charge density) need to be considered such as for 2-methylpyridine and pyridine sorption to Eurosoil 1.

Heterocyclic organic compounds containing nitrogen, sulfur, or oxygen (NSOs) are an important class of groundwater contaminants related to the production and use of manufactured gas, heavy oils, and coal tar. Surprisingly little is known about the processes that control sorption and transport of NSOs in the subsurface. In this study, the effects of various environmental factors including temperature, ionic strength, and dissolved/sorbed ion composition on the sorption of NSOs have been investigated by means of a soil column chromatography approach. For the investigated compounds, increased temperature normally decreases their sorption to soil. The enthalpy change of the sorption process corroborates earlier findings that van der Waals forces dominate the sorption of S- and O-heterocyclic compounds such as thiophene, benzothiophene, benzofuran, and 2-methylbenzofuran. Ionic strength and ion composition (Ca2+ vs K+ at given ionic strength) of the aqueous phase show no significant effects on the sorption of these compounds. Previous studies demonstrated that for N-heterocyclic compounds, cation exchange and surface complex formation rather than partitioning into soil organic matter control their overall sorption. In contrast to S- and O-heterocyclic compounds, increasing ionic strength reduced the sorption of ionizable N-heterocyclic compounds (pyridine, 2-methylpyridine, quinoline, 2-methylquinoline, and isoquinoline), due to increased electrostatic competition by cations. At given ionic strength, an increase of the K+/Ca2+ ratio in the mobile phase enhanced the sorption of N-heterocyclic compounds, consistent with cation exchange of the protonated organic species as the dominating sorption process. Among the investigated N-heterocyclic compounds sorption of benzotriazole showed a peculiar feature in that ternary surface complexation with Ca2+ appears to be the dominant sorption mechanism.

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