Influence of NO2 and dissolved iron on the S(IV) oxidation in synthetic aqueous solution

The influence of dissolved NO₂ and iron on the oxidation rate of S(IV) species in the presence of dissolved oxygen is presented. To match the conditions in the real environment, the concentration of iron in the reaction solution and trace gases in the gas mixture was typical for a polluted atmosphere. The time dependence of HSO₃⁻, SO₄²⁻, NO₂⁻ and NO₃⁻ and the concentration ratio between Fe(II) and total dissolved iron were monitored. Sulphate formation was the most intensive in the presence of an SO₂/NO₂/air gas mixture and Fe(III) in solution. The highest contribution to the overall oxidation was from Fe-catalysed S(IV) autoxidation. The reaction rate in the presence of both components was equal to the sum of the reaction rates when NO₂ and Fe(III) were present separately, indicating that under selected experimental conditions there exist two systems: SO₂/NO₂/air and SO₂/NO₂/air/Fe(III), which are unlikely to interact with each other. The radical chain mechanism can be initiated via reactions Fe(III)–HSO₃⁻ and NO₂–SO₃²⁻/HSO₃⁻.     

Redox reactions in the Fe-As-O2 system

We have examined two redox reactions involving arsenic and iron at near-neutral pH: the reduction of As(V) by Fe(II) under anoxic conditions, and the co-oxidation of As(III) during Fe(II) oxygenation. We also considered the impact of goethite, pH buffers, and radical scavengers on these reactions. In a series of anoxic experiments, Fe(II) was found to reduce As(V) in the presence of goethite, but not in homogeneous solution. The reaction rate increased with increasing pH and Fe(II) concentration, but in all cases was relatively slow. In aerobic experiments, the kinetics of Fe(II) oxygenation at neutral pH, and the corresponding oxidation of As(III) were found to depend heavily on pH buffer type and concentration. The classic formulation of Fe(II) oxidation by oxygen, involving four single-electron transfers, was reviewed and found to be inadequate for explaining observed oxidation of Fe(II) and As(III). Widely cited rate constants for Fe(II) oxygenation originate from experiments conducted in carbonate buffer, and do not match observations made in phosphate, MES, or HEPES systems. In phosphate buffer, Fe(II) oxidation is rapid and dependent on phosphate concentration. In MES and HEPES buffers, Fe(II) oxidation is much slower due to the lack of labile ferrous iron species. Oxygenation of Fe(II) appears to proceed through different mechanisms in phosphate and MES or HEPES systems. In both cases, reactive intermediary species are produced which can oxidize As(III). These oxidants are not the hydroxyl radical, but may be Fe(IV) species.     

Arsenic(III) and iron(II) co-oxidation by oxygen and hydrogen peroxide: Divergent reactions in the presence of organic ligands

Iron-catalyzed oxidation of As(III) to As(V) can be highly effective for toxic arsenic removal via Fenton reaction and Fe(II) oxygenation. However, the contribution of ubiquitous organic ligands is poorly understood, despite its significant role in redox chemistry of arsenic in natural and engineered systems. In this work, selected naturally occurring organic ligands and synthetic ligands in co-oxidation of Fe(II) and As(III) were examined as a function of pH, Fe(II), H₂O₂, and radical scavengers (methanol and 2-propanol) concentration. As(III) was not measurably oxidised in the presence of excess ethylenediaminetetraacetic acid (EDTA) (i.e. Fe(II):EDTA < 1:1), contrasting with the rapid oxidation of Fe(II) by O₂ and H₂O₂ at neutral pH under the same conditions. However, partial oxidation of As(III) was observed at a 2:1 ratio of Fe(II):EDTA. Rapid Fe(II) oxidation in the presence of organic ligands did not necessarily result in the coupled As(III) oxidation. Organic ligands act as both iron speciation regulators and radicals scavengers. Further quenching experiments suggested both hydroxyl radicals and high-valent Fe species contributed to As(III) oxidation. The present findings are significant for the better understanding of aquatic redox chemistry of iron and arsenic in the environment and for optimization of iron-catalyzed arsenic remediation technology.     

Experimental study and steady-state simulation of biogeochemical processes in laboratory columns with aquifer material

Packed bed laboratory column experiments were performed to simulate the biogeochemical processes resulting from microbially catalyzed oxidation of organic matter. These included aerobic respiration, denitrification, and Mn(IV), Fe(III) and SO(4) reduction processes. The effects of these reactions on the aqueous- and solid-phase geochemistry of the aquifer material were closely examined. The data were used to model the development of alkalinity and pH along the column. To study the independent development of Fe(III)- and SO(4)-reducing environments, two columns were used. One of the columns (column 1) contained small enough concentrations of SO(4) in the influent to render the reduction of this species unimportant to the geochemical processes in the column.The rate of microbially catalyzed reduction of Mn(IV) changed with time as evidenced by the variations in the initial rate of Mn(II) production at the head of the column. The concentration of Mn in both columns was controlled by the solubility of rhodochrosite (MnCO(3(S))).In the column where significant SO(4) reduction took place (column 2), the concentration of dissolved Fe(II) was controlled by the solubility of FeS. In column 1, where SO(4) reduction was not important, maximum dissolved Fe(II) concentrations were controlled by the solubility of siderite (FeCO(3(S))). Comparison of solid-phase and aqueous-phase data suggests that nearly 20% of the produced Fe(II) precipitates as siderite in column 1. The solid-phase analysis also indicates that during the course of experiment, approximately 20% of the total Fe(III) hydroxides and more than 70% of the amorphous Fe(III) hydroxides were reduced by dissimilatory iron reduction.The most important sink for dissolved S(-II) produced by the enzymatic reduction of SO(4) was its direct reaction with solid-phase Fe(III) hydroxides leading initially to the formation of FeS. Compared to this pathway, precipitation as FeS did not constitute an important sink for S(-II) in column 2. In this column, the total reacted S(-II) estimated from the concentration of dissolved sulfur species was in good agreement with the produced Cr(II)-reducible sulfur in the solid phase. Solid-phase analysis of the sulfur species indicated that up to half of the originally produced FeS may have possibly transformed to FeS(2).     

Kinetics of oxidation of chlorobenzenes and phenyl-ureas by Fe(II)/H2O2 and Fe(III)/H2O2. Evidence of reduction and oxidation reactions of intermediates by Fe(II) or Fe(III)

The rates of degradation of 1,2,4-trichlorobenzene (TCB), 2,5-dichloronitrobenzene (DCNB), diuron and isoproturon by Fe(II)/H2O2 and Fe(III)/H2O2 have been investigated in dilute aqueous solution ([Organic compound]0 approximately 1 microM, at 25.0 +/- 0.2 degrees C and pH < or = 3). Using the relative rate method with atrazine as the reference compound, and the Fe(II)/H2O2 (with an excess of Fe(II)) and Fe(III)/H2O2 systems as sources of OH radicals, the rate constants for the reaction of OH* with TCB and DCNB were determined as (6.0 +/- 0.3)10(9) and (1.1 +/- 0.2)10(9) M(-1) s(-1). Relative rates of degradation of diuron and isoproturon by Fe(II)/H2O2 were about two times smaller in the absence of dissolved oxygen than in the presence of oxygen. These data indicate that radical intermediates are reduced back to the parent compound by Fe(II) in the absence of oxygen. Oxidation experiments with Fe(III)/H2O2 showed that the rate of decomposition of atrazine markedly increased in the presence of TCB and this increase has been attributed to a regeneration of Fe(II) by oxidation reactions of intermediates (radical species and dihydroxybenzenes) by Fe(III).     

Synthesis of iron composites on nano-pore substrates: Identification and its application to removal of cyanide

Two types of nano-pore substrates, waste-reclaimed (WR) and soil mineral (SM) with the relatively low density, were modified by the reaction with irons (i.e. Fe(II):Fe(III) = 1:2) and the applicability of the modified substrates (i.e. Fe-WR and Fe-SM) on cyanide removal was investigated. Modification (i.e. Fe immobilization on substrate) decreased the BET surface area and PZC of the original substrates while it increased the pore diameter and the cation exchange capacity (CEC) of them. XRD analysis identified that maghemite (γ-Fe₂O₃) and iron silicate composite ((Mg, Fe)SiO₃) existed on Fe-WR, while clinoferrosilite (FeSiO₃) was identified on Fe-SM. Cyanide adsorption showed that WR adsorbed cyanide more favorably than SM. The adsorption ability of both original substrates was enhanced by the modification, which increased the negative charges of the surfaces. Without the pH adjustment, cyanide was removed as much as 97% by the only application of Fe-WR, but the undesirable transfer to hydrogen cyanide was possible because the pH was dropped to around 7.5. With a constant pH of 12, only 54% of cyanide was adsorbed on Fe-WR. On the other hand, the pH was kept as 12 without adjustment in Fe-WR/H₂O₂ system and cyanide was effectively removed by not only adsorption but also the catalytic oxidation. The observed first-order rate constant (k_obs) for cyanide removal were 0.49 (±0.081) h⁻¹. Moreover, the more cyanate production with the modified substrates indicated the iron composites, especially maghemite, on substrates had the catalytic property to increase the reactivity of H₂O₂.     

Efficient abatement of an iodinated X-ray contrast media iohexol by Co(II) or Cu(II) activated sulfite autoxidation process

Efficient abatement of an iodinated X-ray contrast media iohexol by an emerging sulfite autoxidation advanced oxidation process is demonstrated, which is based on transition metal ion–catalyzed autoxidation of sulfite to form active oxidizing species. The efficacy of the combination of sulfite and transition metal ions (Ag(I), Mn(II), Co(II), Fe(II), Cu(II), Fe(III), or Ce(III)) was tested for iohexol abatement. Co(II) and Cu(II) are proven to show more pronounced catalytic activity than other metals at pH 8.0. According to the quenching studies, sulfate radical (SO₄^??) is identified to be the primary species for oxidation of iohexol. Increasing dosages of metal ion or sulfite and higher pH values are favorable for iohexol abatement. Inhibition of iohexol abatement is observed in the absence of dissolved oxygen, which is vital for the production of SO₅^?? and subsequent formation of SO₄^??. Overall, activation of sulfite to produce reactive radicals with extremely low Co(II) or Cu(II) concentrations (in the range of μg L^?1) in circumneutral conditions is confirmed, which offers a potential SO₄^??-based advanced oxidation process in treatment of aquatic organic contaminants.

     

The formation of dithionate during the iron(III)-catalysed autoxidation of sulfur(IV)-oxides

The iron(III)-catalyzed autoxidation of sulfur(IV)-oxides results in the formation of two different oxidation products of sulfur(IV): dithionate, S₂O₆²⁻, and sulfate, SO₄²⁻. The yield of these reaction products depends on the experimental conditions. Under the studied conditions ([Fe(III)] : [SIV)] = 1:10, pH = 2–4) dithionate is the minor reaction product. The formation of dithionate is influenced by the initial pH but not by the initial O₂ concentration. The presence of CO²⁺, Mn²⁺, and Ni²⁺ have no influence on the yield of dithionate, whereas in the presence of Cr³⁺ less and, in the presence of Cu²⁺, no dithionate is formed.     

Determination of sulfur oxides formed during the S(IV) oxidation in the presence of iron

The reaction products (i.e., sulfate (SO4(2-)) and dithionate (S2O6(2-))) of S(IV) oxidation in the presence of iron(III) under different experimental conditions were investigated. Ion-interaction chromatography was used for the separation of sulfate and dithionate using tetrabutylammonium hydroxide (TBAOH) as an ion-pair reagent. The chromatographic method was optimized by varying the composition of the mobile phase (i.e., concentration of TBAOH, acetonitrile and Na2CO3) and by varying the flow rate of the mobile phase. The method was successfully applied to the determination of dithionate formed during the S(IV) oxidation in the presence of Fe(III). In air-saturated solutions sulfate was observed as the only product, while in N2-saturated solutions dithionate was also determined, but it is the minor reaction product and represents about 4% of the total amount of oxidized HSO3- under the studied conditions.     

Simultaneous oxidation of EDTA and reduction of metal ions in mixed Cu(II)/Fe(III)-EDTA system by TiO2 photocatalysis

Both the photooxidation of EDTA and the photoreduction of metal ions in metal-EDTA systems were investigated. EDTA oxidation by TiO(2) photocatalysis occurred sequentially as Cu(II)-EDTA>Cu(II)/Fe(III)-EDTA>Fe(III)-EDTA. For Cu(II)-EDTA, EDTA was completely decomposed after only 60min of irradiation. The rate of EDTA decomposition was directly correlated with the initial Cu(II) concentration in the case of a mixed Cu(II)/Fe(III)-EDTA system. The metal ions in a single metal-EDTA complex were removed following significant decomposition of EDTA. For a mixed Cu(II)/Fe(III)-EDTA system, however, no copper was removed whereas almost all of the iron was removed. The iron and copper species deposited on the TiO(2) surface were identified via EPR and XPS as mixed FeO/Fe(3)O(4) and Cu(0)/Cu(2)O, respectively.     

Photocatalytic oxidation of xenobiotics in water with immobilized TIO2 on agitator

Abstract

A novel photocatalytic oxidation reactor, using Degussa P‐25 T_iO₂ as a stationary phase with a thickness of 1.5–2.0 um on the blades of agitator, was developed to study the photocatalytic oxidation of xenobiotics. Particularly in this device, separation of photocatalyst from the purified water after oxidation reaction was not necessary, and no other aeration equipment was required to supply oxygen. To examine the efficiency of this device, photocatalytic degradation of xenobiotic organics such as carbofuran was studied as an example. Results indicated that carbofuran could be degraded completely with mineralization efficiency of 20 % after 6 hours of oxidation under the imposed conditions. The mineralization rate of carbofuran was found to follow the pseudo‐first order reaction kinetics. Moreover, the rate constant of mineralization was found to be proportional to T_iO₂ film area and the square root of UV light intensity. These results implied the mineralization efficiency of carbofuran could be improved through increasing T_iO₂ film area and UV light intensity. Accordingly, this novel device showed potential application for degrading xenobiotics in water.     

New perspectives on the passive treatment of ferruginous circumneutral mine waters in the UK

This paper examines major physico-chemical processes during the passive treatment of ferruginous circumneutral drainage from abandoned coal mines in the UK. Data collected over several years of studies on mine water treatment systems shed new light on the relative importance of hydraulics, settling velocity, Fe(II) oxidation rates and cascade aeration, which, in turn, informs the design of future systems. This paper demonstrates that (1) the complex settling behaviour of Fe(III) precipitates may be described by a first-order volumetric process and that settling rate is different for different mine waters; (2) the hydraulic efficiency (ratio of time to peak tracer concentration to nominal residence time) of the settling ponds studied was widely variable at low flow rates in comparison to constructed wetlands; (3) aeration cascades contribute dissolved oxygen and lead to a rise in pH due to CO₂ degassing, which are very important in reducing the required time for iron oxidation and removal; (4) for at least 10 of the 30 sites examined, modelling of the rates of Fe(II) oxidation and particulate settling reveals that removal of iron is primarily dependent on settling rate; and (5) that substantial increases in pH can be brought about by forced aeration of mine water over several hours. Findings of this study apply to the majority of coal mine water treatment sites in the UK and may have broader application to other ferruginous waters with circumneutral pH or after treatment to increase pH.     

Fate of a stilbene-type fluorescent whitening agent (DSBP) in the presence of Fe(III) aquacomplexes: from the redox process to the photodegradation

The behaviour of 4,4′-bis(2-sulfostyryl)biphenyl (DSBP), a fluorescent whitening agent, was investigated in the presence of Fe(III) aquacomplexes at room temperature. In the dark, a two-step reaction was observed when adding Fe(III) to a solution of DSBP: an initial fast redox reaction between DSBP and the monomeric species Fe(OH)²⁺ and a slower reaction leading to the coagulation of oxidised DSBP and iron. This phenomenon is due to the formation of a complex or an ion-pair between Fe(II) and/or Fe(III) with oxidised DSBP and it probably occurs by charge neutralisation in our experimental conditions. The precipitation of DSBP depends on the initial concentration in Fe(OH)²⁺ and is achieved for a ratio [Fe(OH) ²⁺]/[DSBP] of 5 approximately. Under irradiation at 365 nm, a complicated behaviour was observed: a complexation of iron by oxidised DSBP favoured by irradiation and a degradation of DSBP induced by an intramolecular electron transfer in the complex or by a photoredox of Fe(OH)²⁺ species generating OH radicals in the supernatant. The complete degradation of DSBP is reached four times faster in the presence of Fe(III) with respect to the direct photolysis of DSBP alone. Moreover, the total mineralization of DSBP obtained in less than 120 h upon irradiation at 365 nm is only observed in the presence of the ferric ions, enlightening the efficiency of the method involving Fe(III) and UV irradiation.     

Phenol nitration upon oxidation of nitrite by Mn(III,IV) (hydr)oxides

An interesting aspect of the chemistry of nitrite is the possibility for this compound to interact with other environmental factors and many oxidising species, which results in the oxidation of nitrite to nitrogen dioxide. This is a potentially interesting process that can lead to the formation of nitroaromatic compounds in the environment. In previous papers we have shown that nitrite can interact with dissolved Fe(III) and nitrate under irradiation, Fenton and heterogeneous photo-Fenton reagents, and semiconductor oxides such as TiO2, alpha-Fe2O3, and beta-FeOOH under irradiation. This paper reports on the interaction between nitrite/nitrous acid and the Mn(III,IV) (hydr)oxides beta-MnO2 and gamma-MnOOH, both in neutral solution under irradiation and in acidic conditions in the dark. beta-MnO2 and gamma-MnOOH originate from the oxidation of Mn(II) and play a key role in the redox cycling of manganese in the environment. These Mn(III,IV) (hydr)oxides show some photocatalytic activity, and they can act as thermal oxidants at acidic pH. The photoinduced oxidation of nitrite and the thermal oxidation of nitrous acid by Mn(III,IV) (hydr)oxides yield nitrogen dioxide and lead to the formation of nitrophenols in the presence of phenol. These processes can take place at the water-sediment or water-colloid interface in natural waters and on the surface of atmospheric particulate. Furthermore, the phenol/gamma-MnOOH/HNO2 system in dark acidic solution is an interesting model due to the formation of phenoxyl radical upon phenol monoelectronic oxidation by gamma-MnOOH. The kinetics of nitrophenol generation under such conditions indicates that phenol nitration is unlikely to take place upon reaction between phenoxyl and *NO2 and suggests a solution to a literature debate on the subject.     

A novel application of H2O2-Fe(II) process for arsenate removal from synthetic acid mine drainage (AMD) water

Batch experiments were carried out to investigate the influences of H₂O₂/Fe(II) molar ratio, pH, sequence of pH adjustment, initial As(V) concentration, and interfering ions on As(V) removal in H₂O₂-Fe(II) process from synthetic acid mine drainage (AMD). The optimum H₂O₂/Fe(II) molar ratio was one for arsenate removal over the pH range of 4-7. Arsenate removal at pH 3 was poor even at high Fe(II) dosage due to the high solubility of Fe(III) formed in situ. With the increase of Fe(II) dosage, arsenate removal increased progressively before a plateau was reached at pH 5 as arsenate concentration varied from 0.05 to 2.0 mg L⁻¹. However, arsenate removal was negligible at Fe/As molar ratio <3 and then experienced a striking increase before a plateau was reached at pH 7 and arsenate concentration ≥1.0 mg L⁻¹. The co-occurring ions exerted no significant effect on arsenate removal at pH 5. The experimental results with synthetic AMD revealed that this method is highly selective for arsenate removal and the co-occurring ions either improved arsenate removal or slightly depressed arsenate removal at pH 5-7. The extended X-ray absorption fine structure (EXAFS) derived As-Fe length, 3.27-3.30 Å, indicated that arsenate was removed by forming bidentate-binuclear complexes with FeO(OH) octahydra. The economic analysis revealed that the cost of the H₂O₂-Fe(II) process was only 17-32% of that of conventional Fe(III) coagulation process to achieve arsenate concentration below 10 μg L⁻¹ in treated solution. The results suggested that the H₂O₂-Fe(II) process is an efficient, economical, selective and practical method for arsenate removal from AMD.     

Investigation of the removal mechanism of Cr(VI) in groundwater using activated carbon and cast iron combined system

Zero-valent iron (Fe⁰) has been widely used for Cr(VI) removal; however, the removal mechanisms of Cr(VI) from aqueous solution under complex hydrogeochemical conditions were poorly understood. In this research, the mixed materials containing cast iron and activated carbon were packed in columns for the treatment of aqueous Cr(VI)-Cr(III) in groundwater with high concentration of Ca²⁺, Mg²⁺, HCO₃ ⁻, NO₃ ⁻, and SO₄ ²⁻. We investigate the influences of those ions on Cr(VI) removal, especially emphasizing on the reaction mechanisms and associated precipitations which may lead to porosity loss by using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) techniques. The results show that the precipitations accumulated on the material surface were (Fe/Cr) (oxy)hydroxide, mixed Fe(III)-Cr(III) (oxy)hydroxides, Fe₂O₃, CaCO₃, and MgCO₃. During these reactions, the Cr(VI) was reduced to Cr(III) coupled with the oxidated Fe⁰ to Fe(II) through the galvanic corrosion formed by the Fe⁰-C and/or the direct electron transfer between Fe⁰ and Cr(VI). In addition, Cr(VI) could be reduced by aqueous Fe(II), which dominated the whole removal efficiency. The primary aqueous Cr(III) was completely removed together with Cr(III) reduced from Cr(VI) even when Cr(VI) was detected in the effluent, which meant that the aqueous Cr(III) could occupy the adsorption sites. In general, the combined system was useful for the Cr(VI)-Cr(III) treatment based on galvanic corrosion, and the hardness ions had a negative effect on Cr(VI) removal by forming the carbonates which might promote the passivation of materials and decrease the removal capacity of the system.

     

The influence of iron and sulfur mineral fractions on carbon tetrachloride transformation in model anaerobic soils and sediments

The objective of this research was to identify the dissolved species or solid phase mineral fraction(s) best correlated with rates of carbon tetrachloride (CT) reductive transformation in systems modeling sulfate-reducing and iron oxide-rich soils and sediments. We used sulfide (S(-II))-treated goethite as our model system, but also studied Fe(II) and S(-II)-treated goethite, Fe(II)-treated goethite, pure FeS, and Fe(II)-treated FeS in order to isolate and evaluate the influence of different mineral fractions on reaction rates. Initial rates of CT transformation were measured for different pH values and concentrations of added Fe(II), as well as different aging times and conditions. The following dissolved species and iron and sulfur mineral fractions were quantified and compared with CT transformation rates: aqueous Fe(2+) and S(-II), surface associated Fe(II) (including weakly and strongly bound Fe(II)), FeS(s), and Cr(II) reducible solid phase S. Over the pH range of 6-10, CT transformation rates were correlated with surface associated Fe(II), while at pH 8, rates were correlated with weakly bound Fe(II). Aging of S(-II)-treated goethite led to oxidation of surface sulfur and a change in the concentration of weakly bound Fe(II), but did not change the relationship between initial rates and weakly bound Fe(II). The results of this research suggest that surface associated Fe(II) and weakly bound Fe(II) could serve as indicators of the potential for abiotic CT dechlorination in natural soils under sulfate-reducing conditions.     

Degradation of Acid Orange 7 in aqueous solution by dioxygen activation in a pyrite/H2O/O2 system

Increasing attention has been paid to pyrite due to its ability to generate hydroxyl radicals in air-saturated solutions. In this study, the mineral pyrite was studied as a catalyst to activate molecular oxygen to degrade Acid Orange 7 (AO7) in aqueous solution. A complete set of control experiments were conducted to optimize the reaction conditions, including the dosage of pyrite, the AO7 concentration, as well as the initial pH value. The role of reactive oxygen species (ROS) generated by pyrite in the process was elucidated by free radical quenching reactions. Furthermore, the concentrations of Fe(II) and total Fe formed were also measured. The mechanism for the production of ROS in the pyrite/H₂O/O₂ system was that H₂O₂ was formed by hydrogen ion and superoxide anion (O₂ ^·?) which was produced by the reaction of pyrite activating O₂ and then reacted with Fe(II) dissolved from pyrite to produce ^·OH through Fenton reaction. The findings suggest that pyrite/H₂O/O₂ system is potentially practical in pollution treatment. Moreover, the results provide a new insight into the understanding of the mechanism for degradation of organic pollutants by pyrite.     

Spectroscopic investigation of magnetite surface for the reduction of hexavalent chromium

The reduction of Cr(VI) to Cr(III) by magnetite in the presence of added Fe(II) was characterized through batch kinetic experiments and the effect of Fe(II) addition and pH were investigated in this study. The addition of Fe(II) into magnetite suspension improved the reductive capacity of magnetite. Eighty percent of Cr(VI) was reduced by magnetite (6.5 g l(-1)) with Fe(II) (80 mg l(-1)) within 1 h, while 60% of Cr(VI) was removed by magnetite only. However, the extent of improved reductive capacity of magnetite with Fe(II) was less than that predicted by the summation of each reduction capacity of magnetite and Fe(II). The reduction of Cr(VI) in the magnetite suspension with Fe(II) increased with the increase of molar ratio of Fe(II) to Cr(VI) (0.6, 1, 1.5, 2.3) in the range of 0-2.3 and with the decrease of pH in the range of pH 8.0-5.5. The speciation of chromium, iron, and oxygen on the surface of magnetite was investigated by X-ray photoelectron spectroscopy. Cr 2p3/2, Fe 2p3/2, and O 1s peaks were mainly observed at 576.7 and 577.8 eV, at 711.2 eV, and at 530.2 and 531.4 eV, respectively. The results indicates that Cr(III) and Fe(III) were the dominant species on the surface of magnetite after reaction and that the dominant species covered the magnetite surface and formed metal (oxy)hydroxide.