Major anion effects on the kinetics and reactivity of granular iron in glass-encased magnet batch reactor experiments

The relative effects of sulfate (SO4(2-)), chloride (Cl-), nitrate (NO3-), and bicarbonate (HCO3-) (8 mM ionic strength solutions, adjusted to pH 10) on the reactivity of Master Builders iron was investigated using a low-abrasion batch reactor with a glass-encased magnet (GEM). Reactivity of the granular iron surface was assessed by measuring the reduction rate of 4-chloronitrobenzene (4ClNB) as a function of initial 4CINB concentration and anion type. Relative to a similarly prepared perchlorate (ClO4-) solution, in which perchlorate was assumed not to interact with the iron surface, nitrate and bicarbonate inhibited the reduction of the probe compound (4ClNB). Chloride and sulfate enhanced reactivity. Thus, the anions were ranked SO4(2-) > Cl- > or = ClO4- > NO3- > HCO3 (from most enhanced to most inhibited) in their influence on granular iron reactivity toward 4ClNB. Kinetic studies of 4CINB were conducted under conditions that caused the iron surface to saturate with the reacting compound (saturation kinetic studies). These experiments, conducted in the various anion solutions indicated above, showed that the gains in reactivity that occurred in the presence of Cl- and SO4(2-) were due to either increased surface reactivity or sorption capacity. The losses in reactivity that occurred in the presence of NO3- were due to decreases in one or both of these same two factors. However, reactivity declines in the presence of CO3(2-) appear to have been due, in large part, to a reduced affinity of 4ClNB for the iron surface.