Surface Catalytic Oxidation Research Articles

Mineral surface-catalyzed oxidation of Mn(II) by bromate: Implications for the occurrence of Mn oxides on Mars

The occurrence of manganese (Mn) oxides on Mars is believed to be an indicator of an O2-rich paleoenvironment of Mars because Mn oxides often form through the oxidation of Mn(II) by O2 on the surface of Earth. An alternative formation pathway was recently proposed, in which Mn(II) is oxidized by bromate (BrO3−), a common oxidant in contemporary Martian regolith. However, the oxidation of Mn(II) by bromate in solution is kinetically controlled and slow unless using very high concentrations (100 mM) of reactants that may be irrelevant to the conditions of Mars. We conducted laboratory simulations to determine whether iron (Fe) oxides (hematite and goethite) and a phyllosilicate (montmorillonite), abundant minerals on the surface of Mars, could catalyze the oxidation of Mn(II) by bromate. Hematite and goethite, but not montmorillonite, dramatically accelerated the oxidation with a low concentration (1 mM) of Mn(II) and bromate under various solution conditions. The reaction system was autocatalytic with Fe oxides initiating the oxidation of Mn(II) at the early stage and the subsequent catalysis mainly provided by the Mn oxide products. In contrast to producing Mn(IV)O2 only during the homogeneous oxidation of Mn(II) by bromate in solutions, the heterogeneous mineral-surface catalyzed oxidation resulted in a mixture of Mn(III)OOH and Mn(IV)O2 phases. Mn(III)OOH was an intermediate product and can be further oxidized by bromate to Mn(IV)O2. The occurrence and accumulation of the intermediate product MnOOH can be attributed to its rapid formation due to surface-enhanced nucleation and growth on Fe oxide surfaces and to its higher resistance to oxidation by bromate than Mn(III) ions or clusters. Overall, mineral-surface catalyzed oxidation of Mn(II) by bromate is favorable from both thermodynamic and kinetic perspectives, and can be a major pathway for the occurrence of Mn oxides on Mars where microorganisms are lacking to catalyze the reaction. Our study further improves our understanding of the thermodynamic and kinetic controls on Mn(II) oxidation.

Dual-functional effect encompassing adsorption and catalysis by Mn-modified iron-based sorbents for arsenic removal: Experimental and DFT study

Arsenic oxidation plays a crucial role in its removal, which has been identified in numerous studies. However, the mechanisms, especially reaction pathways of arsenic oxidation on sorbent surfaces remain inadequately explored. In this work, the effects of Mn doping on arsenic adsorption and oxidation were first verified by adsorption experiments. Subsequently, DFT calculations were carried out to identify alterations in the adsorption energies, active sites, and oxidation pathways. By integrating the experimental and simulation results, a dual-functional framework encompassing adsorption and catalysis of Mn-modified Fe-based material was distinctly established. For adsorption, the introduction of manganese into iron-based sorbent considerably enhanced As2O3 adsorption owing to the increased active sites available for As2O3 chemisorption and the promotion of surface nucleophilicity. Concerning oxidative catalysis, the incorporation of MnO2 augmented surface catalytic oxidation and provided a substantial amount of active Oload. Consequently, the arsenic oxidation occurring on the Mn-modified sorbent surfaces possessed a lower oxidation RDS energy barrier and a shorter oxidation pathway than those on the bare sorbent surfaces. These experimental and simulation results provide a theoretical basis for the design and application of efficient gaseous arsenic adsorbents.

Mechanistic insights into surface catalytic oxidation of fluoroquinolone antibiotics on sediment mackinawite

Fluoroquinolone antibiotics (FQs) have been widely detected in the sediments due to vast production and consumption. In this study, the transformation of FQs was investigated in the presence of sediment mackinawite (FeS) under ambient conditions. Moreover, the role of dissolved oxygen was evaluated for the enhanced degradation of FQs induced by FeS. Our results demonstrated that typical FQs (i.e., flumequine, enrofloxacin and ciprofloxacin) could be efficiently adsorbed and degraded by FeS under neutral pH conditions. As indicated by the results of electron paramagnetic resonance analysis (EPR) and free radicals quenching experiments, hydroxyl radical and superoxide radical anions were identified as the dominant reactive species responsible for FQs degradation. Based on the results of product analysis and theoretical calculation, the degradation of FQs mainly occurred at the piperazine ring and quinolone structure. Our results show that FQs could be efficiently removed by FeS, which benefits understanding the transformation of antibiotics in the sediments, and even sheds light on the remediation of organic pollutants contaminated soils.

Alteration of birnessite reactivity in dynamic anoxic/oxic environments

Although the oxidative capacity of manganese oxides has been widely investigated, potential changes of the surface reactivity in dynamic anoxic/oxic environments have been often overlooked. In this study, we showed that the reactivity of layer structured manganese oxide (birnessite) was highly sensitive to variable redox conditions within environmentally relevant ranges of pH (4.0 – 8.0), ionic strength (0–100 mM NaCl) and Mn(II)/MnO2 molar ratio (0–0.58) using ofloxacine (OFL), a typical antibiotic, as a target contaminant. In oxic conditions, OFL removal was enhanced relative to anoxic environments under alkaline conditions. Surface-catalyzed oxidation of Mn(II) enabled the formation of more reactive Mn(III) sites for OFL oxidation. However, an increase in Mn(II)/MnO2 molar ratio suppressed MnO2 reactivity, probably because of competitive binding between Mn(II) and OFL and/or modification in MnO2 surface charge. Monovalent cations (e.g., Na+) may compensate the charge deficiency caused by the presence of Mn(III), and affect the aggregation of MnO2 particles, particularly under oxic conditions. An enhancement in the removal efficiency of OFL was then confirmed in the dynamic two-step anoxic/oxic process, which emulates oscillating redox conditions in environmental settings. These findings call for a thorough examination of the reactivity changes at environmental mineral surfaces (e.g., MnO2) in natural systems that may be subjected to alternation between anaerobic and oxygenated conditions.

Equilibrium and Kinetic Simulation of Groundwater Deironing and Demanganation

Chemical equilibria of iron and manganese aqueous species were analyzed at various Eh–pH and aqueous CO2 concentration. Thermodynamic and equilibrium-kinetic simulations were conducted for the oxidations of iron and manganese aqueous species with reference to groundwater demanganation and deironing, using the most probable rate constants of congruent dissolution reactions of iron and manganese carbonates, rate constants of homogeneous iron oxidation, and surface catalytic oxidation of manganese on suspension of iron hydroxide in aqueous solution. A kinetic–thermodynamic model is suggested for iron and manganese oxidation in groundwater under the effect of dissolved oxygen. The numerical simulations show that deironing is relatively effective, whereas the dissolved manganese(II) concentration increases because of slower manganese oxidation and the dissolution of manganese-bearing siderite. Hence, effective groundwater demanganation is possible if the host rocks contain no manganese-bearing siderite.

Kinetic Study on Clogging of a Geothermal Pumping Well Triggered by Mixing-Induced Biogeochemical Reactions.

The sustainability of ground-source geothermal systems can be severely impacted by microbially mediated clogging processes. Biofouling of water wells by hydrous ferric oxide is a widespread problem. Although the mechanisms and critical environmental factors associated with clogging development are widely recognized, effects of mixing processes within the wells and time scales for clogging processes are not well characterized. Here we report insights from a joint hydrological, geochemical, and metagenomics characterization of a geothermal doublet in which hydrous ferric oxide and hydrous manganese oxide deposits had formed as a consequence of mixing shallow groundwater containing dissolved oxygen and nitrate with deeper, anoxic groundwater containing dissolved iron (FeII) and manganese (MnII). Metagenomics identify distinct bacteria consortia in the pumping well oxic and anoxic zones, including autotrophic iron-oxidizing bacteria. Batch mixing experiments and geochemical kinetics modeling of the associated reactions indicate that FeII and MnII oxidation are slow compared to the residence time of water in the pumping well; however, adsorption of FeII and MnII by accumulated hydrous ferric oxide and hydrous manganese oxide in the well bore and pump riser provides "infinite" time for surface-catalyzed oxidation and a convenient source of energy for iron-oxidizing bacteria, which colonize the surfaces and also catalyze oxidation. Thus, rapid clogging is caused by mixing-induced redox reactions and is exacerbated by microbial activity on accumulated hydrous oxide surfaces.

Effect of oxygen pretreatment on the surface catalytic oxidation of HCHO on Ag/MCM-41 catalysts

The effect of oxygen pretreatment on HCHO catalytic oxidation of Ag/MCM-41 catalysts has been investigated from 500°C to 800°C in this paper. The highest HCHO surface reaction activity is achieved on the silver sample pretreated at 700°C. The state and the dispersion of silver species on the support are strongly related to the pretreatment temperature. The increase of the O2 pretreatment temperature for Ag/MCM-41 catalysts results in the diffusion of Ag2O clusters from outside to inside of the channels. And the amount of Ag2O clusters inside the channels becomes maximum at 600°C. Interestingly, it is found that the smaller metallic Ag nanoparticles coexist with large particles when the catalyst is pretreated at 700°C. It is suggested that Ag2O clusters inside of MCM-41 pores improves the HCHO adsorption capacity and accelerates the formation of HCHO adsorption intermediates, particularly dioxymethylene(DOM) ad-species. And the formed meallic silver particles with smaller size at 700°C plays an essential effect on the formation of formate intermediate and the enhancement of surface reaction activity. It is also concluded that CO2 formation rate from formate is higher than that from DOM species in HCHO oxidation. The decrease of HCHO surface reaction activity after oxygen pretreatment at 800°C may be due to the decrease of the Ag2O clusters inside MCM-41 pores and the aggregation of metallic Ag nanoparticles.

Catalytic Reactions on the Surface of Ag Nanoparticles: A Photochemical Effect and/or Molecule Property?

Using time-dependent surface-enhanced Raman scattering (TDSERS), we demonstrate the surface-catalyzed oxidation of 3-hydroxy anthranilic acid (HAA) to its azo derivatives. No external source either in the form of laser excitation or heat was required for the surface-catalyzed reaction, which clearly established that thermal energy alone was sufficient. But the possibility of enhanced reaction rate due to the surface plasmon resonance could not be ignored. To mimic in vivo conditions that prevail in healthy and unhealthy cells, SERS measurements were recorded under different environmental conditions. It is shown that the surface-catalyzed azo formation and its isomerization were strongly dependent on the external conditions, namely, temperature, pH, and environment. A plausible mechanism based on electron transfer to the adsorbed O2 is proposed for the trans–cis isomerization. This study can lead toward a new strategy of surface-catalyzed reactions not only for the synthesis of azo dyes but also to disting...

Improved arsenic(III) adsorption by Al2O3 nanoparticles and H2O2: Evidence of oxidation to arsenic(V) from X-ray absorption spectroscopy

We have investigated the oxidation of inorganic As(III) with H2O2 catalysed by Al2O3, using X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy. The effects of different reaction conditions (pH, time and initial H2O2 concentration) were also studied as were the kinetics of the oxidation reaction. We demonstrated that As(III) was oxidized to As(V) in the presence of H2O2 and Al2O3. Furthermore, all arsenic species found on the Al2O3 surface were in the As(V) state. The presence of both Al2O3 and H2O2 was necessary for oxidation of As(III) to take place within the period of time studied. The oxidation kinetics indicate a mechanism where reversible As(III) binding to the alumina surface is followed by irreversible oxidation by H2O2 leading to strongly bound As(V). Results from this study indicate that there is a surface-catalysed oxidation of As(III) on Al2O3 by H2O2, a reaction that can take place in nature and can be of help in the development of novel treatment systems for As(III) removal.

Sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions

The sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions were investigated as a function of pH, Se(IV) concentration, and Fe(II) concentration under an anoxic condition. The sorption of Se(IV) onto chlorite surfaces followed the Langmuir isotherm regardless of the presence of Fe(II) ions in the solution. The Se(IV) sorption was observed to be very low at all pH values when the solution was Fe(II)-free or the concentration of Fe(II) ions was as low as 0.5 mg/L. However, the Se(IV) sorption was enhanced at a pH > 6.5 when the Fe(II) concentration was higher than 5 mg/L because of the increased sorption of Fe(II) onto the chlorite surfaces. XANES (X-ray absorption near edge structure) spectra of the Se K-edge showed that most of the sorbed Se(IV) was reduced to Se(0) by Fe(II) sorbed onto the chlorite surfaces, especially at pH > 9. The combined results of field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) also showed that elemental selenium and goethite were formed and precipitated on the chlorite surfaces during the sorption of selenite. Consequently it can be concluded that Se(IV) can be reduced to Se(0) in the presence of Fe(II) ions by the surface catalytic oxidation of Fe(II) into Fe(III) and the formation of goethite at neutral and particularly alkaline conditions. Thus the mobility of selenite in groundwater is expected to be reduced by the presence of a relatively higher concentration of Fe(II) in subsurface environments.

Preparation of Si–Al/α-FeOOH catalyst from an iron-containing waste and surface–catalytic oxidation of methylene blue at neutral pH value in the presence of H2O2

The tested Si–Al/α-FeOOH composite was prepared by dissolution, precipitation and aging of a high iron-containing grinding wheel ash (GWA). The obtained Si–Al/α-FeOOH composite was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscope/energy dispersive X-ray spectrometer (SEM/EDS) analyses. The characterized composite was then evaluated for the decolorization of methylene blue (MB) in the presence of H2O2 at various pH conditions. The extent of MB decolorization was characterized using UV–Vis spectra, total organic carbon (TOC) and Fourier transform infrared spectroscopy (FTIR). The surface–catalytic oxidation mechanism was discussed and the roles of silica and alumina were also evaluated through preparation of four kinds of catalysts from analytical reagents (ARs). The results showed that α-FeOOH (75.40wt% Fe) was the predominate content in Si–Al/α-FeOOH composite with 2.26wt% Si and 1.02wt% Al. The Fenton-like process (Si–Al/α-FeOOH composite combined with H2O2) can effectively oxidize MB at pH 6.46% and 72% of TOC removal can be achieved after 60min catalytic oxidation. The surface–catalytic oxidation by Si–Al/α-FeOOH/H2O2 may proceed mainly through a non-radical mechanism. The introduction of silica played an important role in the enhancement of the α-FeOOH adsorptive capacity by the inhibition of the formation of crystalline structure of FeOOH, and the introduction of alumina might enhance the α-FeOOH catalytic activity and also prevent the iron from leaching from Si–Al/α-FeOOH catalyst.

Application of XPS and Solution Chemistry Analyses to Investigate Soluble Manganese Removal by MnOx(s)-Coated Media

X-ray photoelectron spectroscopy (XPS) was applied to investigate Mn(II) removal by MnO(x)(s)-coated media under experimental conditions similar to the engineered environment of drinking water treatment plants in the absence and presence of chlorine. Macroscopic and spectroscopic results suggest that Mn(II) removal at pH 6.3 and pH 7.2 in the absence of chlorine was mainly due to adsorption onto the MnO(x)(s) surface coating, while removal in the presence of chlorine was due to a combination of initial surface adsorption followed by subsequent surface-catalyzed oxidation. However, Mn(III) was identified by XPS analyses of the Mn 3p photoline for experiments performed in the absence of chlorine at pH 6.3 and pH 7.2, suggesting that surface-catalyzed Mn oxidation also occurred at these conditions. Results obtained at pH 8.2 at 8 and 0.5 mg·L(-1) dissolved oxygen in the absence of chlorine suggest that Mn(II) removal was mainly due to initial adsorption followed by surface-catalyzed oxidation. XPS analyses suggest that Mn(IV) was the predominant species in experiments operated in the presence of chlorine. This study confirms that the use of chlorine combined with the catalytic action of MnO(x)(s) oxides is effective for Mn(II) removal from drinking water filtration systems.

Reductive Transformation of Birnessite by Aqueous Mn(II)

Reaction of aqueous Mn(II) with hexagonal birnessite at pH 7.5 causes reductive transformation of birnessite into feitknechtite (β-Mn(III)OOH) and manganite (γ-Mn(III)OOH) through interfacial electron transfer from adsorbed Mn(II) to structural Mn(IV) atoms and arrangement of product Mn(III) into MnOOH, summarized by Mn(II) + Mn(IV)O(2) + 2 H(2)O → 2 Mn(III)OOH + 2 H(+). Feitknechtite is the initial transformation product, and subsequently converted into the more stable manganite polymorph during ongoing reaction with Mn(II). Feitknechtite production is observed at Mn(II) concentrations 2 orders of magnitude below thermodynamic thresholds, reflecting uncertainty in thermodynamic data of Mn-oxide minerals and/or specific interactions between Mn(II) and birnessite surface sites facilitating electron exchange. Under oxic conditions, feitknechtite formation through surface-catalyzed oxidation of Mn(II) by O(2) leads to additional Mn(II) removal from solution relative to anoxic systems. These results indicate that Mn(II) may be an important moderator of the reductive arm of Mn-oxide redox cycling, and suggest a controlling role of Mn(II) in regulating the solubility and speciation of phyllomanganate-reactive metal pollutants including Co, Ni, As, and Cr in geochemical environments.

Novel passive treatment for mine water discharges

This paper describes the research and development of a novel passive treatment system to remove iron and acidity from water discharging from coal mines. The system aims to remove Fe(aq) at the point of discharge, rather than allowing the dissolved iron load to impact the ecology of receiving streams. Field-test results show that the surface-catalysed oxidation of iron occurs and is best suited to alkaline mine water discharges with the influent pH, alkalinity and the Fe(II)aq concentration having a strong influence on the performance of this technology. The results also demonstrate the suitability of a model developed to assist in understanding the processes that have occurred in the reactor. The data presented show that by using this reactor system, some dissolved iron may also be removed at acidic discharges and demonstrate the importance of the microbially mediated oxidation of Fe(II)aq as expected at low pH.

English

Settling lagoons are a common feature of passive mine water treatment schemes but tend to require large surface areas, this makes them unfeasible in the numerous locations where space is limited. A novel pilot-scale vertical flow reactor has been built at the Taff Merthyr former colliery site. The vertical flow system is designed to replace conventional settling lagoons and decrease the surface-area requirements. To achieve this, the system is designed to take advantage of the surface-catalysed oxidation of Fe(II) on ochre surfaces and the (self)filtration of an ochre bed. This paper presents results from the first four months of operation and details iron and manganese removal. The system receives net-alkaline water from the abandoned colliery, the water quality is typically pH 6.9, with Fe TOT of 9 mg/l. The results show that the vertical flow reactor achieves higher iron removal rates (typically between 10 and 20 g/m 2 /d) compared to the settling lagoon (typically < 5 g/m 2 /d). The vertical flow reactor achieves these removal rates with a smaller residence time, often half that of the settling lagoon. Manganese is present in the influent in low concentrations (~0.8 mg/l). Operating data show that about 50 % of Mn can be removed in the vertical flow reactor in 24 hrs compared to just 5 % in 24 hours in the settling lagoon. Longevity of the reactor will depend on maintaining an acceptable permeability in the bed. Although the permeability fell rapidly at first, the indications are that it is stabilising near to 10 -4 m/s. Additional

Characterisation of the surface chemistry of magnesium exposed to the ambient atmosphere

The changes in the surface chemistry of the oxidised surface of evaporated magnesium metal stored in the ambient atmosphere are studied with water contact angle (WCA) measurement, polarisation-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and X-ray photoelectron spectroscopy (XPS). Upon exposure to the ambient atmosphere, the surface picks up volatile organic compounds (VOC), which cause a significant increase in the WCA values. The PM-IRRAS and XPS analyses indicate that the adsorbates contain hydrocarbons, carboxylates and carbonate functionalities. After long ambient storage times, the composition of the carbon-containing functionalities on the surface changes significantly. This change could be caused by the build-up and/or surface-catalysed oxidation of adsorbed organic species. Thickening of the air-formed oxide/hydroxide layer was also noted, ascribed to the reaction of adsorbed atmospheric moisture with the magnesium surface. Copyright © 2006 John Wiley & Sons, Ltd.

Investigations of chemisorption and reaction at non‐aqueous electrochemical interfaces by in situ surface‐enhanced Raman spectroscopy

AbstractSurface‐enhanced Raman scattering (SERS) spectroscopy has been successfully extended to transition metal/non‐aqueous electrochemical interfaces recently in our laboratory. Some interesting and new findings are presented with the objective of illustrating the virtues of SERS as a versatile technique to examine not only aqueous but also non‐aqueous interfaces. Emphasis is placed on highly catalytic roughened platinum metal surfaces. Static interfacial structure information of Pt (Ag, Au)/acetonitrile was extracted from the relevant SERS spectral features of the surface species, including interfacial trace water, the production of dissociation reactions of acetonitrile (CN−), and some effects on these spectra exerted by interfacial inorganic ions. The simultaneous competitive adsorption and reorganization processes between these interfacial species and some intentionally added small organic molecules were also systematically investigated. The important aspect of the corrosion inhibition mechanism relevant to non‐aqueous media was then probed based on the success of observing SERS from non‐aqueous interfaces. These included mainly widely used N‐ and S‐containing inhibitors for iron, nickel, silver and gold. Finally, the archetypal surface catalytic oxidation mechanism of C1 molecules (formic acid and methanol) on Pt group metals was investigated by using SERS combined with traditional electrochemical techniques. The non‐linear vibrational Stark tuning effect of the poisoning intermediate CO and its oxidation processes are also described in comparison with experimental observations from aqueous solution and other metal surfaces. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Stochastic resonance in surface catalytic oxidation of carbon monoxide induced by colored noise

The dynamical behavior of surface catalytic oxidation reaction of Pt(110)/CO+O2 modulated by colored noise, under the condition of specific temperature, has been investigated when the partial pressure of CO gas is near the supercritical Hopf bifurcation point. By computer simulation the oscillation and stochastic resonance induced by colored noise are observed. The influences of the intensity and correlation time of colored noise on stochastic resonance are discussed. The range of sensitivity of the system to the environmental fluctuation is analyzed.

A geochemical model for removal of iron(II)(aq) from mine water discharges

A steady state geochemical model has been developed to assist in understanding surface-catalysed oxidation of aqueous Fe(II) by O 2(aq), which occurs rapidly at circumneutral pH. The model has been applied to assess the possible abiotic removal of Fe(II)(aq) from alkaline ferruginous mine water discharges using engineered reactors with high specific-surface area filter media. The model includes solution and surface speciation equilibrium, oxidation kinetics of dissolved and adsorbed Fe(II) species and mass transfer of O 2(g). Limited field data for such treatment of a mine water discharge were available for model development and assessment of possible parameter values. Model results indicate that an adsorption capacity between 10 −6 and 10 −5 mol l −1 is sufficient for complete removal, by oxidation, of the Fe(II)(aq) load at the discharge. This capacity corresponds approximately to that afforded by surface precipitation of Fe(III) oxide onto plastic trickling filter media typically used for biological treatment of wastewater. Extrapolated literature values for microbial oxidation of Fe(II)(aq) by neutrophilic microbial populations to the simulated reactor conditions suggested that the microbially-mediated rate may be several orders-of-magnitude slower than the surface-catalysed oxidation. Application of the model across a range of mine water discharge qualities shows that high Fe(II)(aq) loadings can be removed if the discharge is sufficiently alkaline. Additional reactor simulations indicate that reactor efficiency decreases dramatically with pH in the near acid region, coinciding with the adsorption edge for Fe 2+ on Fe oxyhydroxide. Alkaline discharges thus buffer pH within the range where Fe(II)(aq) adsorbs onto the accreting Fe hydroxide mineral surface, and undergoes rapid catalytic oxidation. The results suggest that the proposed treatment technology may be appropriate for highly ferruginous alkaline discharges, typically associated with abandoned deep coal mines.

Iron oxide surface-catalyzed oxidation of ferrous iron by monochloramine: implications of oxide type and carbonate on reactivity.

The maintenance of monochloramine residuals in drinking water distribution systems is one technique often used to minimize microbial outbreaks and thereby maintain the safety of the water. Reactions between oxidizable species and monochloramine can however lead to undesirable losses in the disinfectant residual. Previous work has illustrated that the Fe(II) present within distribution systems is one type of oxidizable species that can exert a monochloramine demand. This paper extends this prior work by examining the kinetics of the reactions between Fe(II) and monochloramine in the presence of a variety of iron oxide surfaces. The identity of the iron oxide plays a significant role in the rate of these reactions. Surface area-normalized initial rate coefficients (k(init)) obtained in the presence of each oxide at pH approximately 6.9 exhibit the following trend in catalytic activity: magnetite > goethite > hematite approximately = lepidocrocite > ferrihydrite. The differences in the activity of these oxides are hypothesized to result from variations in the amount of Fe(II) sorbed to each of the oxides and to dissimilarities in the surface site densities of the oxides. The implications of carbonate on Fe(II) sorption to iron oxides are also examined. Comparing Fe(II) sorption isotherms for goethite obtained under differential carbonate concentrations, it is apparent that as the carbonate concentration (C(T,CO3)) increased from 0 to 11.7 mM that the Fe(II) sorption edge (50% sorption) shifts from a pH of approximately 5.8 to a pH of 7.8. This shift is hypothesized to be the result of the formation of aqueous and surface carbonate-Fe(II) complexes and to competition between carbonate and Fe(II) for surface sites. The implications of these changes are then discussed in light of the variable oxide studies.