Effects of pH and catalyst concentration on photocatalytic oxidation of aqueous ammonia and nitrite in titanium dioxide suspensions

Batch experiments were conducted to study the effects of titanium dioxide (TiO2) concentration and pH on the initial rates of photocatalytic oxidation of aqueous ammonium/ ammonia (NH4+/NH3) and nitrite (NO2-) in UV-illuminated TiO2 suspensions. While no simple kinetic model could fit the data at lower TiO2 concentrations, at TiO2 concentrations > or = 1 g/L, the experimental data were consistent with a model assuming consecutive first-order transformation of NH4+/NH3 to NO2- and NO2- to nitrate (NO3-). For TiO2 concentrations > or = 1 g/L, the rate constants for NO2 photocatalytic oxidation to NO3 were far more dependent on TiO2 concentration than were those for NH4+/NH3 oxidation to NO2-, suggesting that, without sufficient TiO2, complete oxidation of NH4+/NH3 to NO3- will not occur. Initial NH4+/NH3 photocatalytic oxidation rates were proportional to the initial concentrations of neutral NH3 and not total NH3(i.e., [NH4+] + [NH3]). Thus, the pH-dependent equilibrium between NH4+ and NH3, and not the pH-dependent electrostatic attraction between NH4+ and the TiO2 surface, is responsible for the increase in rates of NH4+/NH3 photocatalytic oxidation with increasing pH. Electrostatic adsorption, however, can partly explain the pH dependence of the initial rates of NO2- photocatalytic oxidation. Initial rates of NO2- photocatalytic oxidation were 1 order of magnitude higher for NO2- versus NH4+/NH3, indicating thatthe rate of NH4+/NH3 photocatalytic oxidation to NO3- was limited by NH4+/NH3 oxidation to NO2- under our experimental conditions.     

Competitive photocatalytic oxidation of Cu(II)--EDTA and Cd(II)--EDTA with illuminated TiO2

Competitive photocatalytic oxidation (PCO) of mixtures of Cu(II)-EDTA and Cd(II)-EDTA was studied with variation of molar ratio of these two complexes (1 x 10(-4):0, 8 x 10(-5): 2 x 10(-5), 5 x 10-5:5 x 10(-5), 2 x 10-5:8 x 10(-5), 0:1 x 10(-4) M) and in the pH range of 4-8. PCO rates for each compound can be described using a combined aqueous + adsorbed pathway: -dC/dt = k1Caq(1+ k2Caq)+ kadsCads. This expression is valid under both noncompetitive and competitive conditions. Differences in rates under competition result from differences in the partitioning of the two species between the TiO2 surface and the aqueous phase. Total initial complex degradation rates (rTT), obtained by summation of the total destruction rates for Cu(II)-EDTA and Cd(II)-EDTA, were relatively constant at pH 4 and 5 for all ratios. At these pH values, contribution of adsorbed pathways to rTT was important, and rates were similar to those of the aqueous phase pathways. From pH 6 to 8, the degree of adsorption, and thus the adsorbed pathway rate, diminished. Through the adsorbed pathway, no difference in rate constants was found between Cu(II)-EDTA and Cd(II)-EDTA; Cd(II)-EDTA is somewhat more reactive through the aqueous phase pathway.     

Design of a photocatalyst for bromate decomposition: surface modification of TiO2 by pseudo-boehmite

The rate of BrO3- reduction by a commercial TiO2 photocatalyst under UV illumination in an aqueous solution was increased by lowering the pH from 7 to 5. The effect is attributable to an enhancement of the electrical interaction between BrO3- and the positively charged surfaces of the TiO2 photocatalyst. The surface charge can be controlled by a surface modification of the TiO2 photocatalyst without controlling the pH of the water. In fact, the isoelectric point of surface-modified TiO2 was higher than that of the unloaded TiO2 photocatalyst, resulting in an increase in the rate of the photocatalytic reduction of BrO3- at a neutral pH. This increase is explained by an increase in the amount of adsorbed BrO3- on the photocatalyst surface.     

Adsorption mechanism of arsenic on nanocrystalline titanium dioxide

Arsenate [As(V)] and arsenite [As(III)] interactions at the solid-water interface of nanocrystalline TiO2 were investigated using electrophoretic mobility (EM) measurements, Fourier transform infrared (FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and surface complexation modeling. The adsorption of As(V) and As(III) decreased the point of zero charge of TiO2 from 5.8 to 5.2, suggesting the formation of negatively charged inner-sphere surface complexes for both arsenic species. The EXAFS analyses indicate that both As(V) and As(III) form bidentate binuclear surface complexes as evidenced by an average Ti-As(V) bond distance of 3.30 A and Ti-As(III) bond distance of 3.35 A. The FTIR bands caused by vibrations of the adsorbed arsenic species remained at the same energy levels at different pH values. Consequently, the surface complexes on TiO2 maintained the same nonprotonated speciation at pH values from 5 to 10, and the dominant surface species were (TiO)2AsO2- and (TiO)2AsO- for As(V) and As(III), respectively. The surface configurations constrained with the spectroscopic results were formulated in the diffuse layer model to describe the adsorption behavior of As in the pH range between 4 and 12. The study suggests that TiO2 is an effective adsorbent for As removal due to its high surface area and the presence of high affinity surface hydroxyl groups.     

Kinetics of Cr(VI) reduction by carbonate green rust

The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were studied for a range of reactant concentrations and pH values. Carbonate green rust, [FeII4FeIII2(OH)12][4H2O x CO3], was synthesized by induced hydrolysis (i.e., coprecipitation) of an Fe(ll)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 +/- 7 m2 g(-1) was measured for five separate batches of freeze-dried green rust precipitate. Heterogeneous reduction by Fe(II) associated with the carbonate green rust appears to be the dominant pathway controlling Cr(VI) loss from solution. The apparent stoichiometry of the reaction between ferrous iron associated with green rust ([Fe(II)GR]) and Cr(VI) was slightly higherthan the expected 3:1 ratio, possibly due to the presence of other oxidants, such as oxygen, protons, or interlayer carbonate ions. The rate of Cr(VI) reduction was proportional to the green rust surface area concentration, and psuedo-first-order rate coefficients (kobs) ranging from 1.2 x 10(-3) to 11.2 x 10(-3) s(-1) were determined. The effect of pH was small with a 5-fold decrease in rate with increasing pH (from 5.0 to 9.0). At low Cr(VI) concentrations (<200 microM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appearto deviate from first-order kinetics and approach a constant value. Estimated amounts of surface Fe(II) and total Fe(II) suggest that the deviation from first-order kinetics observed at higher Cr(VI) concentrations and the 50-fold decrease in rate observed upon three sequential exposures to Cr(VI) is due to exhaustion of available Fe(II).     

Spectroscopic evidence for Fe(II)-Fe(III) electron transfer at the iron oxide-water interface

Using the isotope specificity of 57Fe M?ssbauer spectroscopy, we report spectroscopic observations of Fe(II) reacted with oxide surfaces under conditions typical of natural environments (i.e., wet, anoxic, circumneutral pH, and about 1% Fe(II)). M?ssbauer spectra of Fe(II) adsorbed to rutile (TiO2) and aluminum oxide (Al2O3) show only Fe(II) species, whereas spectra of Fe(II) reacted with goethite (alpha-FeOOH), hematite (alpha-Fe2O3), and ferrihydrite (Fe5HO8) demonstrate electron transfer between the adsorbed Fe(II) and the underlying iron(III) oxide. Electron-transfer induces growth of an Fe(III) layer on the oxide surface that is similar to the bulk oxide. The resulting oxide is capable of reducing nitrobenzene (as expected based on previous studies), but interestingly, the oxide is only reactive when aqueous Fe(II) is present. This finding suggests a novel pathway for the biogeochemical cycling of Fe and also raises important questions regarding the mechanism of contaminant reduction by Fe(II) in the presence of oxide surfaces.     

Efficient removal of heavy metal ions with biopolymer template synthesized mesoporous titania beads of hundreds of micrometers size

We demonstrated that mesoporous titania beads of uniform size (about 450 μm) and high surface area could be synthesized via an alginate biopolymer template method. These mesoporous titania beads could efficiently remove Cr(VI), Cd(II), Cr(III), Cu(II), and Co(II) ions from simulated wastewater with a facile subsequent solid-liquid separation because of their large sizes. We chose Cr(VI) removal as the case study and found that each gram of these titania beads could remove 6.7 mg of Cr(VI) from simulated wastewater containing 8.0 mg·L(-1) of Cr(VI) at pH = 2.0. The Cr(VI) removal process was found to obey the Langmuir adsorption model and its kinetics followed pseudo-second-order rate equation. The Cr(VI) removal mechanism of titania beads might be attributed to the electrostatic adsorption of Cr(VI) ions in the form of negatively charged HCrO(4)(-) by positively charged TiO(2) beads, accompanying partial reduction of Cr(VI) to Cr(III) by the reductive surface hydroxyl groups on the titania beads. The used titania beads could be recovered with 0.1 mol·L(-1) of NaOH solution. This study provides a promising micro/nanostructured adsorbent with easy solid-liquid separation property for heavy metal ions removal.     

Siderophore-manganese(lll) Interactions. II. Manganite dissolution promoted by desferrioxamine B

Recent laboratory and field studies suggest that Mn(lll) forms persistent aqueous complexes with high-affinity ligands. Aqueous Mn(lll) species thus may play a significant but largely unexplored role in biogeochemical processes. One formation mechanism for these species is the dissolution of Mn(lll)-bearing minerals. To investigate this mechanism, we measured the steady-state dissolution rates of manganite (gamma-MnOOH) in the presence of desferrioxamine B (DFOB), a common trihydroxamate siderophore. We find that DFOB dissolves manganite by both reductive and nonreductive reaction pathways. For pH > 6.5, a nonreductive ligand-promoted reaction is the dominant dissolution pathway, with a steady-state dissolution rate proportional to the surface concentration of DFOB. In the absence of reductants, the aqueous Mn(lIl)HDFOB+ complex resulting from dissolution is stable for at least several weeks at circumneutral to alkaline pH and at 25 degrees C. For pH < 6.5, Mn2+ is the dominant aqueous species resulting from manganite dissolution, implicating a reductive dissolution pathway. These results have important implications for the biogeochemical cycling of both manganese and siderophores--as well as Fe(lll)--in natural waters and soils.     

Development of an E-H2O2/TiO2 photoelectrocatalytic oxidation system for water and wastewater treatment

In this study, an innovative E-H2O2/TiO2 (E-H2O2 = electrogenerated hydrogen peroxide) photoelectrocatalytic (PEC) oxidation system was successfully developed for water and wastewater treatment. A TiO2/Ti mesh electrode was applied in this photoreactor as the anode to conduct PEC oxidation, and a reticulated vitreous carbon (RVC) electrode was used as the cathode to electrogenerate hydrogen peroxide simultaneously. The TiO2/Ti mesh electrode was prepared with a modified anodic oxidation process in a quadrielectrolyte (H2SO4-H3PO4-H2O2-HF) solution. The crystal structure, surface morphology, and film thickness of the TiO2/Ti mesh electrode were characterized by X-ray diffraction and scanning electron microscopy. The analytical results showed that a honeycomb-type anatase film with a thickness of 5 microm was formed. Photocatalytic oxidation (PC) and PEC oxidation of 2,4,6-trichlorophenol (TCP) in an aqueous solution were performed under various experimental conditions. Experimental results showed that the TiO2/Ti electrode, anodized in the H2SO4-H3PO4-H2O2-HF solution, had higher photocatalytic activity than the TiO2/Ti electrode anodized in the H2SO4 solution. It was found that the maximum applied potential would be around 2.5 V, corresponding to an optimum applied current density of 50 microA cm(-2) under UV-A illumination. The experiments confirmed that the E-H2O2 on the RVC electrode can significantly enhance the PEC oxidation of TCP in aqueous solution. The rate of TCP degradation in such an E-H2O2-assisted TiO2 PEC reaction was 5.0 times that of the TiO2 PC reaction and 2.3 times that of the TiO2 PEC reaction. The variation of pH during the E-H2O2-assisted TiO2 PEC reaction, affected by individual reactions, was also investigated. It was found that pH was well maintained during the TCP degradation in such an E-H2O2/TiO2 reaction system. This is beneficial to TCP degradation in an aqueous solution.     

Oxidation mechanism of As(III) in the UV/TiO2 system: evidence for a direct hole oxidation mechanism

Although it is well-known that As(III) is oxidized to As(V) in the UV/TiO2 system, the main oxidant for that reaction is still not clear. Accordingly, the present study aims at reinvestigating the TiO2-photocatalyzed oxidation mechanism of As(III). We performed a series of As(II) oxidation experiments by using UV-C/H2O2 and UV-A/TiO2, focusing on the effects of competing compounds. The experiment with UV-C/H2O2 indicated that HO2*/O2-* is not an effective oxidant of As(III) in the homogeneous phase. The effects of oxalate, formate, and Cu(II) on the photocatalytic oxidation of As(III) contradicted the controversial hypothesis that HO2*/ O2-* is the main oxidant of As(III) in the UV/TiO2 system. The effect of As(III) on the TiO2-photocatalyzed oxidations of benzoate, terephthalate, and formate was also incompatible with the superoxide-based As(II) oxidation mechanism. Instead, the experimental observations implied that OH* and/or the positive hole are largely responsible forthe oxidation of As(III) in the UV/TiO2 system. To determine which species plays a more significant role, the effects of methanol and iodide were tested. Since excess methanol did not retard the oxidation rate of As(III), OH* seems not to be the main oxidant. Therefore, the best rationale regarding the oxidation mechanism of As(III) in the UV/TiO2 system seems to be the direct electron transfer between As(III) and positive holes. Only with this mechanism, it was possible to explain the data of this study. Besides the mechanistic aspect, an application method for this technology was sought. The usage of UV/TiO2 for oxidizing As(II) requires a posttreatment in which both As(V) and TiO2 should be removed from water. For this objective, we applied FeCl3 and AIK(SO4)2 as coagulants, and the result implied that the combined usage of TiO2 and coagulation might be a feasible solution to treat arsenic contamination around the world.     

Visible light-induced degradation of carbon tetrachloride on dye-sensitized TiO2

This study investigated an application of TiO2 photocatalyst sensitized with tris(4,4'-dicarboxy-2,2'-bipyridyl)ruthenium-(II) complex to CCl4 degradation under visible light irradiation. By injecting electrons from the photoexcited sensitizer to the conduction band, the sensitized TiO2 degraded CCl4 under the irradiation of lambda > 420 nm. The quantum yield of CCl4 dechlorination was about 10(-3). The dechlorination rate of CCl4 was reduced in the presence of dissolved O2 due to its competition for conduction band electrons. The photolysis rate was dependent on pH due to the strong pH dependence of the sensitizer adsorption on TiO2 surface with a maximum degradation rate achieved at pH approximately 3. A two-site Langmurian model successfully described the adsorption of the sensitizer on TiO2 particles. The monolayer coverage was achieved at the added sensitizer concentration of 10 microM at [TiO2] = 0.5 g/L. However, the photolysis rate of CCl4 showed a maximum at a sensitizer surface coverage of 0.3 monolayer. Since the photoinduced electron injection gradually depleted active sensitizer molecules on TiO2, sacrificial electron donors to regenerate the sensitizer were sought. 2-Propanol as an electron donor was efficient in the present RuIIL3/TiO2/CCl4 system, which showed no sign of deceleration in the dechlorination rate up to 6 h of irradiation.     

Oxidation of pentachlorophenol in manganese oxide suspensions under controlled Eh and pH environments

Abiotic oxidation of pentachlorophenol (PCP) by manganese(IV) oxide (MnO2) was examined in orderto understand the physiochemical environment(s) where PCP oxidation occurs. An Eh-pH potentiostat was used to simulate natural groundwater environments where MnO2 (0.025 g L(-1)) and PCP (0.020 g L(-1)) suspensions were incubated from Eh -300 to 300 mV and pH 4.5 to 7.0. The pH-Eh region where maximum PCP sorption occurred corresponded to the same region where the greatest concentrations of soluble Mn(II) where measured (Eh > -100 mV and pH <5.0). Reduced Mn species [Mn(II,III)] released by reductive dissolution were readsorbed and restricted further abiotic oxidation of PCP by the MnO2 surface. A greater transformation of PCP to primarily tetrachloro-1,4-benzoquinone (p-chloranil) and smaller amounts of lesser chlorinated phenols occurred under increasing pH and Eh conditions.     

Coupled Fe(II)-Fe(III) electron and atom exchange as a mechanism for Fe isotope fractionation during dissimilatory iron oxide reduction

Microbial dissimilatory iron reduction (DIR) is an important pathway for carbon oxidation in anoxic sediments, and iron isotopes may distinguish between iron produced by DIR and other sources of aqueous Fe(II). Previous studies have shown that aqueous Fe(II) produced during the earliest stages of DIR has delta56Fe values that are 0.5-2.0%o lowerthan the initial Fe(III) substrate. The new experiments reported here suggest that this fractionation is controlled by coupled electron and Fe atom exchange between Fe(II) and Fe(III) at iron oxide surfaces. In hematite and goethite reduction experiments with Geobacter sulfurreducens, the 56Fe/54Fe isotopic fractionation between aqueous Fe(II) and the outermost layers of Fe(III) on the oxide surface is approximately -3%o and can be explained by equilibrium Fe isotope partitioning between reactive Fe(II) and Fe(III) pools that coexist during DIR. The results indicate that sorption of Fe(II) to Fe(III) substrates cannot account for production of low-delta56Fe values for aqueous Fe(II) during DIR.     

Iron-mediated oxidation of antimony(III) by oxygen and hydrogen peroxide compared to arsenic(III) oxidation

Antimony is used in large quantities in a variety of products, though it has been declared as a pollutant of priority interest by the Environmental Protection Agency of the United States (USEPA). Oxidation processes critically affect the mobility of antimony in the environment since Sb(V) has a greater solubility than Sb(lll). In this study, the cooxidation reactions of Sb(lIl) with Fe(ll) and both O2 and H2O2 were investigated and compared to those of As(III). With increasing pH, the oxidation rate coefficients of Sb(lll) in the presence of Fe(ll) and O2 increased and followed a similar pH trend as the Fe(ll) oxidation by O2. Half-lives of Sb(lll) were 35 and 1.4 h at pH 5.0 and pH 6.2, respectively. The co-oxidation with Fe(ll) and H2O2 is about 7000 and 20 times faster than with Fe(ll) and O2 at pH 3 and pH 7, respectively. For both systems, *OH radicals appear to be the predominant oxidant below approximately pH 4, while at more neutral pH values, other unknown intermediates become important. The oxidation of As(lll) follows a similar pH trend as the Sb(lll) oxidation; however, As(lll) oxidation was roughly 10 times slower and only partly oxidized in most of the experiments. This study shows that the Fe(ll)-mediated oxidation of Sb(Ill) can be an important oxidation pathway at neutral pH values.     

Microbial reduction of Fe(III) and sorption/precipitation of Fe(II) on Shewanella putrefaciens strain CN32

The influence of Fe(II) on the dissimilatory bacterial reduction of an Fe(III) aqueous complex (Fe(III)-citrate(aq)) was investigated using Shewanella putrefaciens strain CN32. The sorption of Fe(II) on CN32 followed a Langmuir isotherm. Least-squares fitting gave a maximum sorption capacity of Qmax = 4.19 x 10(-3) mol/10(12) cells (1.19 mmol/m2 of cell surface area) and an affinity coefficient of log K = 3.29. The growth yield of CN32 with respect to Fe(III)aq reduction showed a linear trend with an average value of 5.24 (+/-0.12) x 10(9) cells/mmol of Fe(III). The reduction of Fe(III)aq by CN32 was described by Monod kinetics with respect to the electron acceptor concentration, Fe(III)aq, with a half-saturation constant (Ks) of 29 (+/-3) mM and maximum growth rate (micromax) of 0.32 (+/-0.02) h(-1). However, the pretreatment of CN32 with Fe(II)aq significantly inhibited the reduction of Fe(III)aq, resulting in a lag phase of about 3-30 h depending on initial cell concentrations. Lower initial cell concentration led to longer lag phase duration, and higher cell concentration led to a shorter one. Transmission electron microscopy and energy dispersive spectroscopy revealed that many cells carried surface precipitates of Fe mineral phases (valence unspecified) during the lag phase. These precipitates disappeared after the cells recovered from the lag phase. The cell inhibition and recovery mechanisms from Fe(II)-induced mineral precipitation were not identified by this study, but several alternatives were discussed. A modified Monod model incorporating a lag phase, Fe(II) adsorption, and aqueous complexation reactions was able to describe the experimental results of microbial Fe(III)aq reduction and cell growth when cells were pretreated with Fe(II)aq.     

Electrochemical study of arsenate and water reduction on iron media used for arsenic removal from potable water

Zerovalent iron filings have been proposed as a filter medium for removing As(III) and As(V) compounds from potable water. The removal mechanism involves complex formation of arsenite and arsenate with the iron surface and with iron oxides produced from iron corrosion. There is conflicting evidence in the literature on whether As(V) can be reduced to As(III) by iron filter media. This research uses electrochemical methods to investigate the redox reactions that occur on the surface of zerovalent iron in arsenic solutions. The effect of arsenic on the corrosion rate of zerovalent iron was investigated by analysis of Tafel diagrams for iron wire electrodes in anaerobic solutions with As(V) concentrations between 100 and 20,000 microg/L. As(V) reduction in the absence of surface oxides was investigated by analysis of chronoamperometry profiles for iron wire electrodes in solutions with As(V) concentrations ranging from 10000 to 106 microg/L. The effect of pH on As(V) reduction was investigated by analyses of chronopotentiometry profiles for iron wire electrodes at pH values of 2, 6.5, and 11. For freely corroding iron, the presence of As(III) and As(V) decreased the iron corrosion rate by a factor of 5 as compared to that in a 3 mM CaSO4 blank electrolyte solution. The decrease in corrosion rate was independent of the arsenic concentration and was due to the blocking of cathodic sites for water reduction by arsenic compounds chemisorbed to the iron surface. The chronoamperometry and chronopotentiometry experiments showed that elevated pH and increased As(III) to As(V) ratios near the iron surface decreased the thermodynamic favorability for As(V) reduction. Therefore, reduction of As(V) occurred only at potentials that were significantly below the apparent equilibrium potentials based on bulk solution pH values and As(III) to As(V) ratios. The potentials required to reduce more than 1% of the As(V) to As(III) were below those that are obtainable in freely corroding iron media. This indicates that there will be minimal or no reduction of As(V) in iron media filters under conditions relevant to potable water treatment.     

Arsenic removal and recovery from copper smelting wastewater using TiO2

Removal and recovery of high levels of arsenic (As) in copper smelting wastewater present a great environmental challenge. A novel approach was investigated for the first time using TiO(2) for As adsorptive removal from wastewater and subsequent spent adsorbent regeneration and As recovery using NaOH. EXAFS results demonstrate that As(III), as the only As species present in the raw water, does not form an aqueous complex with other metal ions. An average of 3890 ± 142 mg/L As(III) at pH 1.4 in the wastewater was reduced to 59 ± 79 μg/L in the effluent with final pH at 7 in the 21 successive treatment cycles using regenerated TiO(2). Coexisting heavy metals including Cd, Cu, and Pb with concentrations at 369 mg/L, 24 mg/L, and 5 mg/L, respectively, were reduced to less than 0.02 mg/L. As(III) adsorption followed pseudosecond-order rate kinetics, and the adsorption behavior was described with the charge distribution multisite surface complexation model. Approximately 60% As(III) in the waste solution after the TiO(2) regeneration process was recovered by thermo vaporization and subsequent precipitation of sodium arsenite, as suggested by the EDX and XPS analysis. This "zero" sludge process sheds new light on successful As remediation technology for acidic metallurgical industry wastewater.     

Reactivity of Fe(II)-bearing minerals toward reductive transformation of organic contaminants

Fe(II) present at surfaces of iron-containing minerals can play a significant role in the overall attenuation of reducible contaminants in the subsurface. As the chemical environment, i.e., the type and arrangement of ligands, strongly affects the redox potential of Fe(II), the presence of various mineral sorbents is expected to modulate the reactivity of surficial Fe(II)-species in aqueous systems. In a comparative study we evaluated the reactivity of ferrous iron in aqueous suspensions of siderite (FeCO3), nontronite (ferruginous smectite SWa-1), hematite (alpha-Fe2O3), lepidocrocite (gamma-FeOOH), goethite (alpha-FeOOH), magnetite (Fe3O4), sulfate green rust (Fe(II)4Fe(III)2(OH)12SO4 x 4H2O), pyrite (FeS2), and mackinawite (FeS) under similar conditions (pH 7.2, 25 m2 mineral/L, 1 mM Fe(II)aq, O2 (aq) < 0.1 g/L). Surface-area-normalized pseudo first-order rate constants are reported for the reduction of hexachloroethane and 4-chloronitrobenzene representing two classes of environmentally relevant transformation reactions of pollutants, i.e., dehalogenation and nitroaryl reduction. The reactivities of the different Fe(II) mineral systems varied greatly and systematically both within and between the two data sets obtained with the two probe compounds. As a general trend, surface-area-normalized reaction rates increased in the order Fe(II) + siderite < Fe(II) + iron oxides < Fe(II) + iron sulfides. 4-Chloronitrobenzene was transformed by mineral-bound Fe(II) much more rapidly than hexachloroethane, except for suspensions of hematite, pyrite, and nontronite. The results demonstrate that abiotic reactions with surface-bound Fe(II) may affect or even dominate the long-term behavior of reducible pollutants in the subsurface, particularly in the presence of Fe(III) bearing minerals. As such reactions can be dominated by specific interactions of the oxidant with the surface, care must be taken in extrapolating reactivity data of surface-bound Fe(II) between different compound classes.     

Role of Fe(III), phosphate, dissolved organic matter, and nitrate during the photodegradation of domoic acid in the marine environment

The photodegradation of domoic acid in model seawater containing varying amounts of total Fe(lll) (expressed as Fe(lll)), NO(3-), total phosphate (expressed as P04(3-)), and dissolved organic matter (DOM) is reported. A multivariate, microscale, high-throughput experimental approach is described for evaluating how these components interact to control the removal of domoic acid from natural waters. Under the nominal conditions of the study ([Fe(lll)]o 0-4 microM; [N03-]o 0-35 microM; [P04(3-)]o 0-4 microM; [DOM]o 0-10 mg/ L), it is apparent that Fe(lll) and DOM are significant promoters of domoic acid photooxidation. In contrast, P04(3-) interacts with Fe(lll) to inhibit the photooxidation of domoic acid, but P04(3-) alone does not act to slow or accelerate domoic acid photodegradation. No other variables (singly or interactively) have a statistically significant impact. At an incident light intensity of 765 W/m2 and initial domoic acid concentration of 0.96 microM, domoic acid half-lives range over 12-36 h, with half-life a function of [Fe(III)], [PO4(3-)], and dissolved organic matter loadings. An NMR based technique for measuring domoic acid-Fe(lll) binding (1.72 x 10(11)) is reported.