Conclusions

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Reductive dechlorination of chlorinated solvents in the ZVI system is a surface-mediated process. Adsorption of the chlorinated compounds takes place prior to the reduction, but the overall rate of reduction is limited by the electron transfer from the surface to the chlorinated compounds. The adsorption can occur on either reactive or nonreactive sites, while the reduction rate is directly proportional to the amount adsorbed onto the reactive sites. The proportion adsorbed onto reactive sites to the nonreactive sites is related to the nature of chlorinated compounds. Higher chlorinated ethylenes such as PCE and TCE are likely to have a larger portion going to the nonreactive sites compared to less chlorinated ethylenes like vinyl chloride. A two-site model incorporating the known observations related to the ZVI system has been developed and such a model can be applied to explain the adsorption and reduction of chlorinated solvents in the presence of competing coadsorbates.

ACKNOWLEDGEMENTS

We gratefully acknowledge two reviewers for their constructive comments. This work was supported in part by Waste Management-Education and Research Consortium (WERC) and by New Mexico Institute of Mining and Technology.

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APPENDIX A

A "model" file used in Scientist1® software package to obtain the simulation of the single site model in the present of a coadsorbate (Figure 2b).

// MicroMath Scientist Model File

// TCE degradation on metallic iron: single site model with competitive ads├│rbate // Based on adsorption equilibrium and assume that the reaction rate is directly

// proportional to the amount of TCE sorbed.

// S=free site concentration; TS=total site conc.; SX=TCE adsorbed; Sl= inhibitor // adsorbed; XT = total TCE conc.; X=TCE conc in aqueous phase; P=product conc.; // IT=total inhibitor conc.; I = aqueous inhibitor conc.; Sl= inhibitor adsorbed. k1, k2, // k3 - rate constants IndVars: t

DepVars: S, X, SX, I, SI, P Params: k1, k2, k3 KX=2.0*10A4 KI=2.0*10A4

// Initial conditions t=0.0

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