On the nonlinear relationship between k(obs) and reductant mass loading in iron batch systems

A first-order relationship between reaction rate and reductant loading is commonly invoked (but rarely verified) for granular iron batch systems. In this study, a linear relationship between the pseudo-first-order rate constant (k(obs)) for polyhalogenated alkane reduction and iron mass loading (rhom) was only obtained when reduction appeared to be mass-transport-limited. For most alkyl polyhalides, a nonlinear relationship was observed under various experimental conditions, causing surface-area-normalized rate constants (k(SA)) to decrease by as much as an order of magnitude with increasing rhom. While a detailed explanation for this nonlinearity remains unclear, the reaction order in rhom (from plots of log(k(obs)) versus log(rhom)) approached unity as the system pH decreased, suggesting that the behavior may be linked to changes in the thickness of a passive oxide overlayer. We obtained strong linear correlations between k(obs) values and the concentration of aqueous iron(ll) generated over a 24 h period in batch systems with that same rhom. Equally strong correlations were obtained when k(obs) was related to the summed concentration of protons and water reduced over an equivalent time interval at that same rhom. Such correlations suggest a possible link between those sites responsible for proton/water reduction and the surface species participating in alkyl polyhalide reduction by granular iron.