Photochemical coupling reactions between Fe(III)/Fe(II), Cr(VI)/Cr(III), and polycarboxylates: inhibitory effect of Cr species

The roles of chromium species on photochemical cycling of iron and mineralization of polycarboxylates are examined in the presence of Cr(VI) or Cr(III) at pH 2.2-4.0. Under UV irradiation, Cr(III) altered the redox equilibrium of iron species, leading to the shift of the photosteady state toward Fe(II). After a longer time of illumination, total organic carbon (TOC) approached a steady state in the presence of Cr(III) or Cr(VI), whereas oxalate was thoroughly mineralized in the absence of Cr species. The TOC of steady state was closely related to the kind of polycarboxylates, Cr species dosages, pH and O2 atmosphere, but hardly affected by more addition of Fe(III). ESI-MS data indicates that several Cr-oxalate complexes formed in the photochemical reactions, which are responsible for protecting oxalate against further oxidation. A mechanism is proposed for the inhibitory effect of Cr species on oxidation of oxalate and Fe(II). The present study may provide a new insight into the dual environmental effects induced by Cr contaminants especially at heavily chromium-contaminated and dissolved organic matter (DOM)-rich sites.     

Cr(VI)/Cr(III) and As(V)/As(III) ratio assessments in Jordanian spent oil shale produced by aerobic combustion and Anaerobic Pyrolysis

With the increase in the awareness of the public in the environmental impact of oil shale utilization, it is of interest to reveal the mobility of potentially toxic trace elements in spent oil shale. Therefore, the Cr and As oxidation state in a representative Jordanian oil shale sample from the El-Lajjoun area were investigated upon different lab-scale furnace treatments. The anaerobic pyrolysis was performed in a retort flushed by nitrogen gas at temperatures in between 600 and 800 °C (pyrolytic oil shale, POS). The aerobic combustion was simply performed in porcelain cups heated in a muffle furnace for 4 h at temperatures in between 700 and 1000 °C (burned oil shale, BOS). The high loss-on-ignition in the BOS samples of up to 370 g kg(-1) results from both calcium carbonate and organic carbon degradation. The LOI leads to enrichment in the Cr concentrations from 480 mg kg(-1) in the original oil shale up to 675 mg kg(-1) in the ≥ 850 °C BOS samples. Arsenic concentrations were not much elevated beyond that in the average shale standard (13 mg kg(-1)). Synchrotron-based X-ray absorption near-edge structure (XANES) analysis revealed that within the original oil shale the oxidation states of Cr and As were lower than after its aerobic combustion. Cr(VI) increased from 0% in the untreated or pyrolyzed oil shale up to 60% in the BOS ash combusted at 850 °C, while As(V) increased from 64% in the original oil shale up to 100% in the BOS ash at 700 °C. No Cr was released from original oil shale and POS products by the European compliance leaching test CEN/TC 292 EN 12457-1 (1:2 solid/water ratio, 24 h shaking), whereas leachates from BOS samples showed Cr release in the order of one mmol L(-1). The leachable Cr content is dominated by chromate as revealed by catalytic adsorptive stripping voltammetry (CAdSV) which could cause harmful contamination of surface and groundwater in the semiarid environment of Jordan.     

Bifunctionalized mesoporous silicas for Cr(VI) reduction and concomitant Cr(III) immobilization

Surfactant-templated thiol-functionalized mesoporous silica adsorbents have been prepared by cocondensation of mercaptopropyltrimethoxysilane and tetraethoxysilane in the presence of cetyltrimethylammonium bromide, which were then partially oxidized to get bifunctionalized materials containing both thiol and sulfonic acid moieties (MCM-41-SH/SO3H). The resulting organic-inorganic hybrid was applied to the uptake of chromium species according to a reduction-sorption mechanism involving reduction of Cr(VI) by thiol groups and immobilization of Cr(III) onto sulfonic acid moieties. These processes were strongly affected by pH, and the optimal conditions for effective chromium sequestration resulted from a compromise between pH values low enough to ensure quantitative reduction of Cr(VI) and not too low to enable Cr(III) binding to sulfonate groups, which was best achieved at pH 2-3. The effect of the solid-to-solution ratio and the relative amounts of -SH and -SO3H groups was also discussed. Even if Cr(VI) reduction by thiol groups resulted in the formation of sulfonic acid moieties, their contentwas not high enough to ensure quantitative Cr(III) immobilization, which was only attained with materials containing already some sulfonic acid groups prior to contacting Cr(VI) solutions. Redox speciation of sulfur and chromium species was analyzed by X-ray photoelectron spectroscopy (XPS) and used to support the proposed mechanism.     

Spectroscopic investigation of Cr(III)- and Cr(VI)-treated nanoscale zerovalent iron

The reaction of hexavalent chromium (Cr(VI)) with zerovalent iron (Fe0) during soil and groundwater remediation is an important environmental process. This study used several techniques including X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy to investigate nanometer scale Fe0 particles (nano Fe0) treated with Cr(III) and Cr(VI). X-ray diffraction and XPS analyses of oxidized nano Fe0 showed the crystalline Fe(III) phase is composed of lepidocrocite (gamma-FeOOH). Results of XPS Cr 2p data and Cr K-edge X-ray absorption near edge spectroscopy (XANES) provided evidence that Cr(VI) was entirely reduced to Cr(III) by nano Fe0 with no residual Cr(VI) after reaction. In addition, XPS and XANES results of Cr(III) precipitated as Cr(OH)3 in the presence of corroding nano Fe0 were nearly identical to the Cr(VI)-nano Fe0 reaction product. Detailed analysis of XPS O 1s line spectra revealed that both Cr(III)- and Cr(VI)-treated nano Fe0 yielded a predominantly hydroxylated Cr(OH)3 and/ or a mixed phase CrxFe(1 - x)(OH)3 product. The structure of the Cr(III)- and Cr(VI)-treated nano Fe0 determined using extended X-ray absorption fine structure spectroscopy (EXAFS) revealed octahedral Cr(III) with Cr-O interatomic distances between 1.97 and 1.98 A for both Cr(III) and Cr(VI) treatments and a pronounced Cr-Cr second interatomic shell at 3.01 A. Our results suggest that the reaction product of Cr(VI)-treated nano Fe0 is either a poorly ordered Cr(OH)3 precipitate or possibly a mixed phase CrxFe(1 - x)(OH)3 product, both of which are highly insoluble under environmental conditions.     

Role of dissolved organic matter composition on the photoreduction of Cr(VI) to Cr(III) in the presence of iron

The photochemical reduction of Cr(VI) by iron and aquatic dissolved organic matter (DOM) was investigated. DOM sampled from a number of surface waters (a eutrophic wetland, a blackwater stream, and river water from a mix-use watershed) was used in this study. Moreover, a fulvic acid from Lake Fryxell, Antarctica, was also used to represent a DOM derived from a strictly autochthonous source. Cr(VI) reduction to Cr(III) at pH 5.5 was observed for all target DOMs used in this study, but rates varied widely. In general, photoreduction rates increased with increasing iron concentrations, but the type of DOM appeared to influence the kinetics to a larger degree. The rate of reduction was significantly greater for DOM derived from terrestrial systems than from predominantly autochthonous materials even if additional iron was added to the later. A positive correlation was observed between rates of Cr(VI) photoreduction and properties of the isolated DOM samples whereby faster reduction was observed for larger more aromatic substrates. On the basis of the fast rates reported for the dark reduction of Cr(VI) to Cr(III) by Fe(II)-organic ligands, we hypothesize that the rate-limiting step in these reactions is the photoreduction of Fe(III) to Fe(II) by a ligand-to-metal charge-transfer pathway after absorption of light by Fe(III)-DOM complexes or by reduction of Fe(III) by superoxide or other intermediates formed after light absorption by DOM. Thus, the rate of Cr(VI) photoreduction to Cr(III) in natural sunlit waters is dependent upon both the amount of iron present and the nature of the dissolved organic matter substrate.     

Electroreduction of Cr(VI) to Cr(III) on reticulated vitreous carbon electrodes in a parallel-plate reactor with recirculation

The reduction of Cr(VI) to Cr(III) is achieved in a flow-by, parallel-plate reactor equipped with reticulated vitreous carbon (RVC) electrodes;this reduction can be accomplished by the application of relatively small potentials. Treatment of synthetic samples and field samples (from an electrodeposition plant) results in final Cr(VI) concentrations of 0.1 mg/L (i.e., the detection limit of the UV-vis characterization technique used here) in 25 and 43 min, respectively. Such concentrations comply with typical environmental legislation for wastewaters that regulate industrial effluents (at presenttime = 0.5 mg/L for discharges). The results show the influence of the applied potential, pH, electrode porosity, volumetric flow, and solution concentration on the Cr(VI) reduction percentage and on the required electrolysis time. Values for the mass transfer coefficient and current efficiencies are also obtained. Although current efficiencies are not high, the fast kinetics observed make this proposed treatment an appealing alternative. The lower current efficiency obtained in the case of a field sample is attributed to electrochemical activation of impurities. The required times for the reduction of Cr(VI) are significantly lower than those reported elsewhere.     

Visible light Cr(VI) reduction and organic chemical oxidation by TiO2 photocatalysis

Here we report the simultaneous Cr(VI) reduction and 4-chlorophenol (4-CP) oxidation in water under visible light (wavelength > 400 nm) using commercial Degussa P25 TiO2. This remarkable observation was attributed to a synergistic effect among TiO2, Cr(VI), and 4-CP. It is well known that TiO2 alone cannot remove either 4-CP or Cr(VI) efficiently under visible light. Moreover, the interaction between Cr(VI) and 4-CP is minimal if not negligible. However, we found that the combination of TiO2, Cr(VI), and 4-CP together can enable efficient Cr(VI) reduction and 4-CP oxidation under visible light. The specific roles of the three ingredients in the synergistic system were studied parametrically. It was found that optimal concentrations of Cr(VI) and TiO2 exist for the Cr(VI) reduction and 4-CP oxidation. Cr(VI) was compared experimentally with other metals such as Cu(ll), Fe(lll), Mn(IV), Ce(IV), and V(V). Among all these metal ions, only Cr(VI) promotes the photocatalytic oxidation of 4-CP. The amount of 4-CP removed was directly related to the initial concentration of Cr(VI). The system was also tested with four other chemicals (aniline, salicylic acid, formic acid, and diethyl phosphoramidate). We found that the same phenomenon occurred for organics containing acid and/or phenolic groups. Cr(VI) was reduced at the same time as the organic chemicals being oxidized during photoreaction under visible light. The synergistic effect was also found with pure anatase TiO2 and rutile TiO2. This study demonstrates a possible economical way for environmental cleanup under visible light.     

Arsenic(III) oxidation by iron(VI) (ferrate) and subsequent removal of arsenic(V) by iron(III) coagulation

We investigated the stoichiometry, kinetics, and mechanism of arsenite [As(III)] oxidation by ferrate [Fe(VI)] and performed arsenic removal tests using Fe(VI) as both an oxidant and a coagulant. As(III) was oxidized to As(V) (arsenate) by Fe(VI), with a stoichiometry of 3:2 [As(III):Fe(VI)]. Kinetic studies showed that the reaction of As(III) with Fe(VI) was first-order with respect to both reactants, and its observed second-order rate constant at 25 degrees C decreased nonlinearly from (3.54 +/- 0.24) x 10(5) to (1.23 +/- 0.01) x 10(3) M(-1) s(-1) with an increase of pH from 8.4 to 12.9. A reaction mechanism by oxygen transfer has been proposed for the oxidation of As(III) by Fe(VI). Arsenic removal tests with river water showed that, with minimum 2.0 mg L(-1) Fe(VI), the arsenic concentration can be lowered from an initial 517 to below 50 microg L(-1), which is the regulation level for As in Bangladesh. From this result, Fe(VI) was demonstrated to be very effective in the removal of arsenic species from water at a relatively low dose level (2.0 mg L(-1)). In addition, the combined use of a small amount of Fe(VI) (below 0.5 mg L(-1)) and Fe(III) as a major coagulant was found to be a practical and effective method for arsenic removal.     

Research Watch: Cr(VI) removal

Remediation.     

Effect of coupled dissolution and redox reactions on Cr(VI)aq attenuation during transport in the sediments under hyperalkaline conditions

Aluminum-rich, hyperalkaline (pH > 13.5) and saline high-level nuclear waste (HLW) fluids at elevated temperatures (>50 degrees C), that possibly contained as much as 0.41 mol L(-1) Cr(VI), accidentally leaked to the sediments at the Hanford Site, WA. These extreme conditions promote base-induced dissolution of soil minerals which may affect Cr(VI)aq mobility. Our objective was to investigate Cr(VI)aq transport in sediments leached with HLW simulants at 50 degrees C, under CO2 and O2 free conditions. Results demonstrated that Cr(VI)aq fate was closely related to dissolution, and Cr(VI)aq mass loss was negligible in the first pore volumes but increased significantly thereafter. Similar to dissolution, Cr(VI)aq attenuation increased with increasing fluid residence time and NaOH concentration but decreased with Al concentrations in the leaching solutions. Aqueous Cr(VI) removal rate half-lives varied from 1.2 to 230 h with the fastest at the highest base concentration, lowest Al concentration, greatest reaction time, and lowest Cr(VI) concentration in the leaching solution. The rate of Cr(VI) removal (normalized to 1 kg of solution) varied from 0.83 x 10(-9) (+/-0.44 x 10(-9)) to 9.16 x 10(-9) (+/-1.10 x 10(-9)) mol s(-1). The predominant mechanism responsible for removing Cr(VI) from the aqueous phase appears to be homogeneous Cr(VI) reduction to Cr(III) by Fe(II) released during mineral dissolution. Cr(VI)aq removal was time-limited probably because it was controlled by the rate of Fe(II) release into the soil solution upon mineral dissolution, which was also a time-limited process, and other processes that may act to lower Fe(II)aq activity.     

Reduction of Cr(VI) at a polyaniline film: influence of film thickness and oxidation state

Hexavalent chromium [Cr(VI)] is highly toxic, carcinogenic, and mutagenic to living organisms. In this paper, the reduction of Cr(VI) to the much less toxic trivalent state [Cr(III)] was studied at polyaniline films grown to different thickness. Much higher rates of Cr(VI) reduction were observed for the "thick" polyaniline films. This was explained in terms of the morphology of the polymer and the higher surface area of polymer in contact with the Cr(VI) solution. For "thin" polyaniline films, the Cr(VI) reduction reaction was found to obey pseudo-first-order kinetics for the duration of exposure. However, in the case of thick polyaniline layers, the Cr(VI) reduction reaction followed a two-stage process, with each stage obeying pseudo-first-order kinetics. This was explained in terms of oxidation of the polymer from the leucoemeraldine to the emeraldine state and then further oxidation of the polymer from the emeraldine to the pernigraniline state. Much higher rates of Cr(VI) reduction were observed on oxidation of the polymer from the leucoemeraldine to the emeraldine state.     

Nanocomposite pyrite-greigite reactivity toward Se(IV)/Se(VI)

A nanopyrite/greigite composite was synthesized by reacting FeCl(3) and NaHS in a ratio of 1:2 (Wei et al. 1996). Following this procedure, the obtained solid phases consisted of 30-50 nm sized particles containing 28% of greigite (Fe(2+)Fe(3+)(2)S(4)) and 72% pyrite (FeS(2)). Batch reactor experiments were performed with selenite or selenate by equilibrating suspensions containing the nanosized pyrite-greigite solid phase at different pH-values and with or without the addition of extra Fe(2+). XANES-EXAFS spectroscopic techniques revealed, for the first time, the formation of ferroselite (FeSe(2)) as the predominant reaction product, along with elemental Se. In the present experimental conditions, at pH 6 and in equilibrium with Se(0), the solution is oversaturated with respect to ferrosilite. Furthermore, thermodynamic computations show that reaction kinetics likely played a significant role in our experimental system.     

氧化淀粉对Cr(VI)的吸收研究

用玉米淀粉合成了一种带羧基的氧化淀粉,研究该合成氧化淀粉和玉米淀粉对Cr(VI)的吸收。玉米淀粉和氧化淀粉对Cr(VI)的吸收均受反应pH、温度、时间和吸收剂用量的影响。通过实验发现玉米淀粉对Cr(VI)的吸收各因素影响主次是pH>温度>吸收剂用量>时间;氧化淀粉对Cr(VI)的吸收各因素影响主次是pH>时间>温度>吸收剂。两者影响因素主次稍有不同,这可能是由羧基所造成的。实验结果表明氧化淀粉的吸收效果优于玉米淀粉。     

Photocatalytic reduction of Cr(VI) in the presence of NO3- and Cl- electrolytes as influenced by Fe(III)

Photoreduction of Cr(VI) involving Fe is strongly affected by the presence of organic or inorganic compounds in an acidic environment. In this study, we have found a new pathway of Cr(VI) photoreduction in the presence of Fe-(III) that is influenced by two inorganic electrolytes (i.e., NO3- and Cl-) and the pH. In NO3- and Cl- systems without Fe(III), Cr(VI) photoreduction could occur and was independent of the Cr(VI) concentration. The zero-order rate constant of the photoreduction reaction increased when the solution pH was decreased from 2 to 1; the reaction rate was higher in the NO3- system than in the Cl- system. The higher reaction rate in the NO3- system was attributed to the photolysis of NO3-, which resulted in the formation of NO2- for reduction of Cr(VI). Conversely, the effect of Fe-(III) addition on the increase in Cr(VI) photoreduction rate in the Cl- system was more significant than that in the NO3- system. The addition of Fe(III) to the Cl- system caused the formation of [Fe(OH2)5Cl]2+, the photolysis of which subsequently resulted in the formation of Fe(II) for reduction of the Cr(VI). This study suggests that the photolysis of NO3- and Fe-Cl complex may contribute significantly to Cr(VI) reduction in surface water that receives electroplating wastewater containing high levels of NO3-, Cl-, and Fe-(III). Therefore, under the acidic conditions that are favorable for Fe-Cl complex formation or in the presence of NO3-, the effects of inorganic components on Cr(VI) photoreduction cannot be ignored for the precise evaluation of the transformation of Cr in the environment.     

Ferrate(VI) oxidation of weak-acid dissociable cyanides

Cyanide is commonly found in electroplating, mining, coal gasification, and petroleum refining effluents, which require treatment before being discharged. Cyanide in effluents exists either as free cyanide or as a metal complex. The kinetics of the oxidation of weak-acid dissociable cyanides by an environmentally friendly oxidant, ferrate(VI) (Fe(VI)O4(2-), Fe(VI)), were studied as a function of pH (9.1-10.5) and temperature (15-45 degrees C) using a stopped-flow technique. The weak-acid dissociable cyanides were Cd(CN)4(2-) and Ni(CN)4(2-), and the rate-laws for the oxidation may be -d[Fe(VI)]/dt = k[Fe(VI)][M(CN)4(2-)]n where n = 0.5 and 1 for Cd(CN)4(2-) and Ni(CN)4(2-), respectively. The rates decreased with increasing pH and were mostly related to a decrease in concentration of the reactive protonated Fe(VI) species, HFeO4(-). The stoichiometries with Fe(VI) were determined to be: 4HFeO4(-) + M(CN)4(2-) + 6H2O --> 4Fe(OH)3 + M(2+) + 4NCO(-) + O2 + 4OH(-). Mechanisms are proposed that agree with the observed reaction rate-laws and stoichiometries of the oxidation of weak-acid dissociable cyanides by Fe(VI). Results indicate that Fe(VI) is effective in removing cyanide in coke oven plant effluent, where organics are also present.     

Reduction of uranium(VI) by mixed iron(II)/iron(III) hydroxide (green rust): formation of UO2 nanoparticles

Green rusts, which are mixed ferrous/ferric hydroxides, are found in many suboxic environments and are believed to play a central role in the biogeochemistry of Fe. Analysis by U LIII-edge X-ray absorption near edge spectroscopy of aqueous green rust suspensions spiked with uranyl (U(VI)) showed that U(VI) was readily reduced to U(IV) by green rust The extended X-ray absorption fine structure (EXAFS) date for uranium reduced by green rust indicate the formation of a UO2 phase. A theoretical model based on the crystal structure of UO2 was generated by using FEFF7 and fitted to the data for the UO2 standard and the uranium in the green rust samples. The model fits indicate that the number of nearest-neighbor uranium atoms decreases from 12 for the UO2 structure to 5.4 forthe uranium-green rust sample. With an assumed four near-neighbor uranium atoms per uranium atom on the surface of UO2, the best-fit value for the average number of uranium atoms indicates UO2 particles with an average diameter of 1.7 +/- 0.6 nm. The formation of nanometer-scale particles of UO2, suggested by the modeling of the EXAFS data, was confirmed by high-resolution transmission electron microscopy, which showed discrete particles (approximately 2-9 nm in diameter) of crystalline UO2. Our results clearly indicate that U(VI) (as soluble uranyl ion) is readily reduced by green rust to U(IV) in the form of relatively insoluble UO2 nanoparticles, suggesting that the presence of green rusts in the subsurface may have significant effects on the mobility of uranium, particularly under iron-reducing conditions.     

Iron(VI) and iron(V) oxidation of thiocyanate

Thiocyanate (SCN-) is used in many industrial processes and is commonly found in industrial and mining waste-waters. The removal of SCN- is required because of its toxic effects. The oxidation of thiocyanate (SCN-) by environmentally friendly oxidants, Fe(VI) and Fe(V), has been studied anaerobically using stopped-flow and premix pulse radiolysis techniques. The stoichiometry with Fe(VI) was determined to be 4HFeO(4-) + SCN(-) + 5H2O-->4Fe(OH)3 + SO4(2-) + CNO(-) + O2 + 2OH-. The rate law for the oxidation of SCN- by Fe(VI) was found to be -d[Fe(VI)]/dt = k11([H+]/([H+] + Ka,HFeO4)) [Fe(VI)][SCN-] where k11 = 2.04 +/- 0.04 x 10(3) M-1 s-1 and pKa,HFeO4 = 7.33. A mechanism is proposed that agrees with the observed reaction stoichiometry and rate law. The rate of oxidation of SCN- by Fe(V) was approximately 3 orders of magnitude faster than Fe(VI). The higher reactivity of Fe(V) with SCN- indicates that oxidations by Fe(VI) may be enhanced in the presence of appropriate one-electron-reducing agents. The results suggest that the effective removal of SCN- can be achieved by Fe(VI) and Fe(V).     

Uptake and reduction of Cr(VI) to Cr(III) by mesquite (Prosopis spp.): chromate-plant interaction in hydroponics and solid media studied using XAS

Chromium (Cr) is a well-established carcinogen that is a contaminant at half of the EPA Superfund sites in the United States. Two separate studies were performed to investigate the possibility that mesquite (Prosopis spp.), which is an indigenous desert plant species, can remove Cr from the environment via active transport systems to the aerial portions of the plant. The first study was performed by growing mesquite on solid media (agar) at Cr(VI) concentrations of 75 and 125 ppm. The accumulation found in the leaves under the present conditions indicated that mesquite could be classified as a hyperaccumulator of chromium. The second study was conducted to investigate the differences between the type of Cr ligand involved in Cr uptake with agar and hydroponic cultures. We used X-ray absorption spectroscopy (XAS) to determine the mechanisms involved in the uptake and binding of Cr(VI) in live mesquite tissue. The XAS results for this study showed that some of the supplied Cr(VI) was uptaken by the mesquite roots; however, the data analyses of the plant tissues demonstrated that it was fully reduced to Cr(III) in the leaf tissues. Experiments are currently being performed to evaluate the behavior of the Mesquite plant using lower Cr concentrations.