Interactions and Reductive Reactivity in Ternary Mixtures of Fe(II), Goethite, and Phthalic Acid Based on a Combined Experimental and Modeling Approach

Abstract

The interactions between organic ligands, Fe­(II), and iron oxides are important in biogeochemical redox processes. The effect of phthalic acid (PHA) on the reductive reactivity of Fe­(II) associated with goethite was examined using batch adsorption and kinetic studies, attenuated total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR), and surface complexation modeling (SCM). PHA significantly inhibited the reductive reactivity of Fe­(II)/goethite, as quantified by the pseudo-first-order reduction rate constants (k) of p-cyanonitrobenzene. The k value decreased from 1.68 ± 0.03 to 0.338 ± 0.14 h–1 at pH 6.0 as the PHA concentration increased from 0 to 1000 μM. The effects of the co-adsorption of Fe­(II) and PHA onto goethite were then investigated to study the inhibition mechanism. The adsorption experiments showed that Fe­(II) slightly enhanced PHA adsorption, whereas PHA did not affect Fe­(II) adsorption, suggesting that the inhibition was not due to different amounts of Fe­(II) adsorbed. The ATR–FTIR spectra of the adsorbed PHA in the ternary mixtures demonstrated that the major surface species was outer-sphere species, with minor inner-sphere complexes formed. SCM results showed that the presence of PHA (L) led to the formation of a type A ternary species ((FeOFe+)2···L2–) on the goethite surface, decreasing the abundance of the reactive species (FeOFeOH). Moreover, the adsorption of PHA on the surface of goethite might block the reactive sites and inhibit the electron transfer between Fe­(II) and goethite, thus decreasing the reactivity. Overall, these findings provided new insights into the reaction mechanisms of surface-adsorbed Fe­(II), which will facilitate the development of new technologies for site remediation and more accurate risk assessment.

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

“…Nitrobenzene reduction was coupled to aniline production, and a stoichiometric amount of aniline was produced when the reaction was provided sufficient time to proceed to completion (Figure S3). In control experiments lacking goethite, no nitrobenzene disappearance or aniline production occurred over a 24 h period regardless of the carbonate concentration (Figure S4), confirming that goethite-bound Fe 2+ was the responsible species for nitrobenzene reduction rather than dissolved Fe 2+ species. , …”

“…In control experiments lacking goethite, no nitrobenzene disappearance or aniline production occurred over a 24 h period regardless of the carbonate concentration (Figure S4), confirming that goethitebound Fe 2+ was the responsible species for nitrobenzene reduction rather than dissolved Fe 2+ species. 30,64 We compared measured k obs values with reported values from the literature. In the absence of carbonate, the k obs value (25.8 ± 0.8 h −1 ), collected with 0.2 mM total Fe 2+ and 0.5 g/L goethite, agreed well with our previously reported value (32.3 ± 0.7 h −1 ) collected under similar experimental conditions (0.5 mM total Fe 2+ and 1 g/L goethite).…”

“…Ferrous iron (Fe 2+ ) can reduce and simultaneously alter the toxicities and/or solubilities of several classes of environmental pollutants found in groundwater. − Consequently, Fe 2+ is a critical reductant to account for in naturally attenuated and engineered remediation systems. − Prior work has established that aqueous Fe 2+ in the presence of an iron (oxyhydr)­oxide (i.e., an “iron oxide”) reduces pollutants far more quickly than aqueous Fe 2+ alone, ,,,− with reduction rates depending on the redox properties of the iron oxide present. ,− ,,,,− This effect is a result of the iron oxide influencing what Fe 3+ oxidation product forms. ,,, When aqueous Fe 2+ is oxidized in the absence of an iron oxide, it tends to form an aqueous Fe 3+ complex or ferrihydrite . When Fe 2+ is oxidized in the presence of a crystalline iron oxide, however, it tends to form a more thermodynamically stable iron oxide phase, typically via homoepitaxial growth. ,, Thus, the presence of an iron oxide alters the reduction potential ( E H ) value of the Fe 3+ /Fe 2+ redox couple by changing the Fe 3+ speciation (i.e., the iron oxide that forms).…”

Fe(II) Redox Chemistry in the Environment

“…In order to explore Fe­(II) speciation on the surfaces of metal oxides and their reductive reaction mechanisms, surface complexation modeling (SCM) is commonly applied, where the amounts of Fe­(II) sorbed at different pH values were measured in order to study speciation at the mineral surfaces. ,,,, As mentioned earlier, based on SCM, two stable surface sorbed Fe­(II) species ((Fe III OFe II ) + and Fe III OFe II OH) are formed on iron oxide surfaces, , with Fe III OFe II OH being the dominant reactive species in the reduction of 4-chloronitrobenzene and uranium­(VI) . However, the relevance of SCM when the underlying mineral is an iron oxide is unresolved because the complexity of interfacial electron exchange dominates the reactive behavior in these systems.…”

Effects of Phosphate Competition on Arsenate Binding to Aluminum Hydroxide Surfaces

“…While the effect of phosphate competition on arsenate adsorption can be assessed by the differences in the curvature and plateau of their adsorption isotherm data, such empirical descriptions lack the ability to extrapolate to water compositions outside the experimental conditions as the parameters of simple isotherm models (e.g., Langmuir) are not intrinsic. To address this ambiguity, surface complexation models (SCMs) can provide a unified framework with a set of mole balance and mass action equations that can reproduce the data of both arsenate and phosphate adsorption, individually or coexisting. ,− In addition, SCM can account for the surface electrostatic effects through which the surface complexation of oxyanions could introduce negative surface charge rendering further oxyanion binding less favorable. − Karamalidis and Dzombak compiled the double-layer SCM parameters for gibbsite, which include phosphate and arsenate. However, these thermodynamic equilibrium constants in the model were calculated based on the experimental data of single ion adsorption.…”