Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe²⁺

Abstract

The reduction of environmental pollutants by Fe 2+ bound to iron oxides is an important process that determines pollutant toxicities and mobilities. Recently, we showed that pollutant reduction rates depend on the thermodynamic driving force of the reaction in a linear free energy relationship that was a function of the solution pH value and the reduction potential, E H , of the interfacial Fe 3+ /Fe 2+ redox couple. In this work, we studied how carbonate affected the free energy relationship by examining the effect that carbonate has on nitrobenzene reduction rates by Fe 2+ bound to goethite (-FeOOH). Carbonate slowed nitrobenzene reduction rates by inducing goethite particle aggregation, as evidenced by surface charge and particle size measurements. We observed no evidence for carbonate affecting Fe 3+ /Fe 2+ reduction potentials or the mechanism of nitrobenzene reduction. The linear free energy relationship accurately described the data collected in the presence of carbonate when we accounted for the effect it had on the reactive surface area of goethite. The findings from this work provide a framework for determining why common groundwater constituents affect the E H -dependence of reaction rates involving oxide-bound Fe 2+ as a reductant.

Reduction of 3-Nitro-1,2,4-Triazol-5-One (NTO) by the Hematite–Aqueous Fe(II) Redox Couple

“…A recent study found that carbonate ion lowered the k Obs for NB reduction by the goethite–Fe 2+ (aq) redox couple due to carbonate-induced goethite particle aggregation . This would decrease the surface area and reactive sites available and result in a downward shift of the LFER (i.e., a lower intercept), similarly to the effects observed in our study, i.e., complexing Fe 2+ (shift from green to pink data in Figure ) and changing oxide surface charge (shift from blue to green data in Figure ).…”

“…Zeta potentials of hematite particles in type I (both with and without added sulfite) and type II reactors were measured using a Zetasizer Nano ZS (Malvern Panalytical, Westborough, MA). Aliquots of these suspensions were taken after 24 h incubation and transferred to disposable folded capillary Zeta cells for charge measurement …”

“…Aliquots of these suspensions were taken after 24 h incubation and transferred to disposable folded capillary Zeta cells for charge measurement. 42…”

Disentangling the size-dependent redox reactivity of iron oxides using thermodynamic relationships

“…As expected, E H values varied as a function of pH values and activities for all the hematite samples. At a single pH value, plotting the measured E H values vs. the logarithm of the activities yielded linear relationships for all the hematite batches, consistent with the system reaching equilibrium according to the following half-reaction ( 33 , 34 ): …”

“…Only recently have researchers developed free-energy relationships capable of explaining these trends among minerals. Specifically, advancements in the use of electrochemical techniques have made it possible to measure and calculate reduction potential ( E H ) values for mineral-bound Fe 2+ , and those values have been found to relate to environmental contaminant reduction rates in relationships consistent with transition state theory ( 29 – 35 ).…”