Relating Empirically Derived and Mechanistic Kinetic Models for Solute Reduction and Formation of Precipitates in Reactive Barriers of Iron Metal

To further our understanding of contaminant degradation rates and particle surface aging in iron metal permeable reactive barriers, we have developed an expanded reaction scheme and a set of accompanying rate laws. In this analysis, the particle surface is parceled into surface sites where reactions occur via sorption of an aqueous species, transformation of the sorbed species (and possibly the reactive site), and desorption of products. The resulting rate law is made tractable by grouping surface sites into classes based on the similarity of rate constants and/or physico-chemical structures associated with each site. Evolution of the surface due to redox reactions, corrosion, precipitation, and interaction with non-reactive species is modeled by defining rules, both arbitrarily and by derivation from film growth theory, for transformation of a surface site from one class to another. The variety of possible surface site groupings and transformation rules gives rise to a number of alternative expanded rate laws, the relative merits of which will be discussed. We show how the expanded rate law, in its various forms, relates to previous work in this area and how it may be simplified to yield the heuristic rate laws commonly encountered in the analysis of experimental data and in the engineering design of granular iron treatment schemes. Elements of the mechanistic model are carried through the simplification procedure thereby providing a framework from which structural effects on, and temporal variations in, heuristic model rate constants may be examined. The implications of this work are discussed, both to the design and interpretation of experiments and to the need for further theoretical development.