Article

Direct Electrochemical Pathways for Selenium Reduction in Aqueous Solutions

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Abstract

Direct electrochemical reduction provides a novel strategy for selenium removal from complex wastewaters. While electrochemical Se(IV) reduction is thermodynamically favorable, anion structure reorganization hinders process kinetics and the phase of reduced Se(0) determines process performance. This study evaluates the thermodynamic and kinetic performance of Se(IV) removal via direct electrochemical reduction (SeDER) and proposes moderate heating to promote efficient and continuous process operation. We find that SeDER is a robust process that can handle 0.001–10 mM Se(IV) in a weakly acidic solution (pH 4–7). Se(IV) can be electrochemically removed from the aqueous phase through either a four- or six-electron pathway, with the former generating Se(0) directly attached to the electrode surface and the latter producing Se(-II) that is subsequently converted to Se(0). The four-electron pathway is a surface-limited process below 70 °C and terminates when the cathode is fully covered with the insulative amorphous Se(0). We demonstrate that raising the solution temperature to 80 °C deposits Se(0) in a conductive crystalline form and enables continuous reduction on the electrode surface. In a simple batch process design, we observe Se(IV) removal rates of up to 89 mg h–1 m–2 of electrode surface area, up to 10% Faradaic efficiency, and up to 95% removal, although we observe moderate trade-offs between these metrics depending on the electron pathway and the initial concentration of Se(IV). Our results suggest value in future work to enhance Faradaic efficiency via better reactor and electrode design, investigate parasitic reactions among competing ions, and select cost-effective electrodes for an economically competitive SeDER process.

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... To understand factors affecting DER performance for Se , 2021a)). recovery, a three-electrode electrochemical system with gold as a working electrode was employed to evaluate Se reduction's thermodynamic and kinetic performance (Zou and Mauter, 2021a). This work found that Se reduction via DER is a robust process that can deal with weakly acidic solutions (pH 4 − 7) containing 0.001 − 10 mM Se(IV). ...
... Reducing SeO 4 2to SeO 3 2is a critical challenge in applying DER in wastewater or natural water treatment due to the necessity of anion structure change and the high activation energy required to break the Se--O double bond. Other oxyanions, e.g., SO 4 , and metal oxyanions that complex water matrices can trigger cathodic parasitic reactions to compete with Se removal via DER or lead to codeposition with Se (Zou and Mauter, 2021a). However, DER approaches offer several advantages over indirect electrochemical Se removal, including selective Se removal when the cathode potential is precisely controlled, less solids generation, and direct Se recovery on the cathode. ...
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... The developed analytical protocol was applied to 60 mL of synthetic FGD wastewater, in which different concentrations of Se species were spiked. The Se concentrations in the synthetic wastewater were set at 5.0 and 10 µmol L -1 (0.39 and 0.78 mg L -1 ) because Se concentrations in contaminated FGD wastewater fall within 0.10-10 mg L -1 (Zou and Mauter, 2021). The spiked Se IV was 100-fold concentrated through the adsorption-desorption procedure, which enabled the detection and quantification of Se via LEP-OES. ...
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Speciation of selenium (Se) is typically carried out using a sophisticated technique such as ICP-MS after preconcentration using an adsorbent; however, the separation and preconcentration of inorganic Se has not been realized in the solutions containing high concentrations of SO42-. A dithiocarbamate-modified cellulose (DMC) was used in this study for the selective extraction and preconcentration of inorganic Se in wastewater, with a portable liquid electrode plasma-optical emission spectrometry (LEP-OES) being employed for quantification. DMC was found to selectively and quantitatively adsorb selenite (SeIV) over a wide range of pH (1.0-8.0); however, less than 3.0% of selenate (SeVI) was adsorbed in a pH range of 3.0-11. Quantitative extraction of SeIV was achieved even in the presence of 3.5 mol L-1 SO42-. The maximum sample volume from which 10 mg of DMC could quantitatively extract SeIV was found to be 500 mL. KOH (0.60 mL, 1.5 mol L-1) was found to quantitatively desorb SeIV retained on the adsorbent and yielded an enrichment factor of 833. The recovery of Se species from synthetic flue-gas desulfurization wastewater containing SeIV and SeVI at concentrations of 5.0 µmol L-1 was 96.2 ± 1.8% and 105.8 ± 1.8%, respectively.
... We have recently reported an alternative strategy for Se direct electrochemical reduction (SeDER) that requires no chemical additives and significantly reduces solids generation. 14 SeDER is thermodynamically favored due to a very positive standard reduction potential for both Se(VI) and Se(IV) oxyanions, but Se(VI) reduction is significantly hindered by molecular-level structure reorganization and high activation energy. On the other hand, Se(IV) is readily electrochemically reduced and separated from the aqueous solution through a four-or six-electron pathway, with the former plating Se(0) directly onto the electrode surface and the latter producing Se(-II) that is chemically converted to Se(0). ...
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Laboratory algal cultures exposed to selenate were shown to produce and release selenomethionine, selenomethionine oxide, and several other organic selenium metabolites. Released discrete organic selenium species accounted for 1.6 - 13.1 % of the selenium remaining in the media after culture death, with 1.3 - 6.1 % of the added selenate recovered as organic metabolites. Analysis of water from an industrially-impacted river collected immediately after the death of massive annual algal blooms showed that no selenomethionine or selenomethionine oxide was present. However, other discrete organic selenium species, including a cyclic oxidation product of selenomethionine, were observed, indicating the previous presence of selenomethionine. Industrial biological treatment systems designed for remediation of selenium-contaminated waters were shown to increase both the concentration of organic selenium species in the effluent, relative to influent water, and the fraction of organic selenium to up to 8.7 % of the total selenium in the effluent, from less than 1.1 % in the influent. Production and emission of selenomethionine, selenomethionine oxide, and other discrete organic selenium species were observed. These findings are discussed in the context of potentially increased selenium bioavailability caused by microbial activity in aquatic environments and biological treatment systems, despite overall reductions in total selenium concentration.
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Selenium is an essential trace element for many organisms, including humans, but it is bioaccumulative and toxic at higher than homeostatic levels. Both selenium deficiency and toxicity are problems around the world. Mines, coal-fired power plants, oil refineries and agriculture are important examples of anthropogenic sources, generating contaminated waters and wastewaters. For reasons of human health and ecotoxicity, selenium concentration has to be controlled in drinking-water and in wastewater, as it is a potential pollutant of water bodies. This review article provides firstly a general overview about selenium distribution, sources, chemistry, toxicity and environmental impact. Analytical techniques used for Se determination and speciation and water and wastewater treatment options are reviewed. In particular, published works on adsorption as a treatment method for Se removal from aqueous solutions are critically analyzed. Recent published literature has given particular attention to the development and search for effective adsorbents, including low-cost alternative materials. Published works mostly consist in exploratory findings and laboratory-scale experiments. Binary metal oxides and LDHs (layered double hydroxides) have presented excellent adsorption capacities for selenium species. Unconventional sorbents (algae, agricultural wastes and other biomaterials), in raw or modified forms, have also led to very interesting results with the advantage of their availability and low-cost. Some directions to be considered in future works are also suggested. Copyright © 2015 Elsevier B.V. All rights reserved.
Article
Increasing evidences suggest that nanoscale zero-valent iron (nZVI) is an effective agent for treatment and removal of selenium from water. For example, 1.3 mM selenite was quickly removed from water within 3 min with 5 g/L nZVI. In this work, reaction mechanisms of selenite [Se(IV)] in a single core–shell structured nanoscale zero-valent iron (nZVI) particle were studied with the method of spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) integrated with X-ray energy dispersive spectroscopy (XEDS). This method was utilized to visualize solid phase translocation and transformation of Se(IV) such as diffusion, reduction, deposition and the effect of surface defects in a single nanoparticle. Se(IV) was reduced to Se(-II) and Se(0), which then formed a 0.5 nm layer of selenium at the iron oxide-Fe(0) interface at a depth of 6 nm from the surface. The results provided near atomic-resolution proof on the intraparticle diffusion-reduction of Se(IV) induced by nZVI. The STEM mapping also discovered that defects on the surface layer accelerate the diffusion of selenium and increase the capacity of nZVI for selenium sequestration.
Article
Selenium (Se) is one of contaminants required to be regulated during drinking water treatment, however, little information has been collected to date regarding Se removal by coagulation. In this study, the performance of Se removal by coagulation has been evaluated with respect to the dependence on Se species, coagulant type, water pH and interfering ions. The results showed that a Fe-based coagulant was much more efficient than Al-based coagulants in Se removal. The removal of selenite (Se(IV)) by coagulation was much more pronounced than that of selenate (Se(VI)). With an FC dosage of more than 0.4 mM Fe/L, Se(IV) removal efficiency of more than 98% could be achieved when the initial Se(IV) concentration was 250 μg/L. For Al-based coagulants (AlCl3 (AC) and polyaluminum chloride (PACl)) Se removal efficiency was positively correlated with the content of Al13 species during the coagulation process. Adsorption onto hydroxide flocs was the most active coagulation mechanism for Se removal and precipitation also played specific roles at low dosage, especially for Se(IV) removal and with Fe coagulant. High coagulant dosage and weakly acidic pH could enhance the formation of hydroxide flocs having more active adsorption sites and high zeta potential, and thus favored Se removal. These findings are important to understand the efficiency and mechanisms of Se removal by coagulation.
Article
The electrical conductivity of liquid selenium and selenium‐tellurium mixtures was studied in detail. Measurements in the temperature range 230° to 500°C from dc to 1 mc/sec indicated the conductivities to be frequency‐independent at low field strength (<5 v/cm). For larger fields (5 to 250 v/cm) the conductivity increases and higher dc than ac values result. Adding increasing amounts of tellurium to selenium produced a continuous rise in conductivity from 1.26×10−4 ohm−1 cm−1 (pure Se) to 2.08×10−1 ohm−1 cm−1 (50.3 percent Se, 49.7 percent Te) at 480°C and a decrease in activation energy from 1.14 ev to 0.84 ev. Comparatively good reproducibility was obtained in the measurements. An attempt to study ionic migration in liquid selenium by tracer methods has thus far not given satisfactory results. The conductivity values obtained for pure selenium are in good agreement with those presented by Henkels but disagree with those of Borelius. The electrical characteristics of liquid selenium and selenium‐tellurium mixtures indicate that hole conduction as pictured for selenium in the solid state cannot be considered as an adequate explanation of the conduction process in these liquids. A complex mixture of conduction by electrons, holes, and ions is believed to exist.
Article
The effects of various anions on the oxidation of glucose on single crystal gold electrodes were studied in HClO4, CF3SO3H, HNO3, H2SO4, H3PO4 and HCl solutions. Contrary to common belief, glucose can be oxidized on gold in acid solutions, but only in the absence of strongly adsorbed anions such as chlorides, sulfates and phosphates. The reaction is partly or completely inhibited in solutions containing specifically adsorbed anions. The data show some indication of an (OH)ads layer on gold at potential cathodic to +1.2 V vs rhe in HClO4 and CF3SO3H solutions. Based on the rates of oxidation of glucose, the following sequence has been found for the inhibition of the glucose oxidation by adsorbed anions: ClO−4 ≈ CF3SO−3 ⪡ NO−3 ⪡ HSO−4 (SO2−4) < H2PO−4 (HPO2−4 < Cl−.
Article
The electroplating of metallic selenium from acid baths has been achieved and reduced to practice. The paper discusses in succession the polymorphism of selenium; selenium ions in selenious acid solution; cathodic deposition as the amorphous and metallic phase; the nucleation characteristic and crystal growth of the metallic modification; the properties of various types of plating baths; a selenium-carbon anode for the stabilization of the bath composition; and finally a number of influences affecting the crystallization habit of the deposit. The current density of 200 amp./ft.2 attainable in a well-balanced plating bath compares favorably with standard baths for true metals.
Article
The electrochemical oxidation of glucose has been studied in phosphate buffer (pH = 7.4) on single crystal and polycrystalline gold electrodes using electrochemical techniques, ex situ NMR, in situ FTIR spectroscopy, and isotope labeling. Under these conditions, the results indicate that the rate determining step for the electro-oxidation of glucose is bond breaking between the hydrogen atom and the C 1 carbon atom. Gluconolactone appears to be an intermediate and sodium gluconate is the reaction product. First order kinetics with respect to glucose were also found. Gold would be a useful electrode for sensor applications, if the inhibition of the glucose oxidation by the adsorption of Cl - could be avoided
Article
Removal of the toxic selenium compounds selenite and selenate from waste water before discharge is becoming increasingly imperative in industrialized countries. Bacteria can reduce selenate to selenite, but also further to elemental selenium, selenide or organic selenium. In this paper, we aim to exclusively bio-reduce selenate to selenite in an open high-rate bioreactor. This conversion could be part of a two-stage process in which the selenite is subsequently reduced by chemical means under optimal conditions to produce a biomass-free selenium product. In the process, yield and reduction rate of biological selenate to selenite should be maximized, while formation of elemental selenium, selenide and organic selenium compounds should be avoided. Fed-batch experiments with a liquid volume of 0.25-0.75 L at different temperatures 20-30-40-50 °C, pH settings 6-7-8-9, initial biomass concentration of 1 or 5 g wet weight granular Eerbeek sludge and various lactic acid concentrations were performed to determine their effect on the biological conversion of selenate to selenite. Furthermore, the effect on selenite losses by further biological reduction or, if present, chemical reduction was investigated as well. Optimal selenate reduction to selenite was found at 30 °C and pH 6 or 7 or 8 with 25 mM selenate and 13.75 mM lactic acid in the influent, with a selenite yield of 79-95%. In all the other conditions, less selenate was reduced to selenite. Also a 5 times higher electron donor concentration resulted in less selenite production, with only 22% of the selenate converted to selenite. The high yield and the high biological reduction rate of at least 741 mg Se/g initial VSS/day detected in the 1 g initial biomass experiment implicate that biological selenate conversion to selenite is a feasible process.
Article
Selenium (Se) is an important element belonging to the chalcogen family. Electrochemistry is of significant importance in various applications of Se and its compounds, ranging from non-vacuum processed solar cells, direct methanol/polymer electrolyte membrane fuel cells, electrocatalysts, batteries, and metallic alloys. However, rich information on Se electrochemistry remains unexplored in the last few decades. Here we made an attempt to assess the reported studies on electrochemical behaviours of Se. Fundamental concepts of Se electrochemistry in aqueous media and ionic liquids at various electrodes, polarographic and cyclic voltammetric analysis, cathodic and anodic stripping analysis, electrodeposition mechanism and electrocatalysis are discussed. Electrochemical atomic layer epitaxy (ECALE) in engineering Se atomic layers and fabrication of Se nanostructures by electrochemical methods are highlighted. Se electrochemistries in application areas are reviewed. Voltammetric studies of Se electrodeposition on molybdenum (Mo) electrode are presented with a view of its applicability in electrodeposited CuInSe2/CuIn(Ga)Se2 (CIS/CIGS) solar cells.
Article
Removal of selenate from solution is investigated in batch electrochemical systems using reactive iron anodes and copper plate cathode in a bicarbonate medium. Iron anodes produce ferrous hydroxide, which is a major factor in the removal of selenate from solution. Iron anodes also generate a significant decrease in the oxidation-reduction potential (ORP) of the solution because it prevents generation of oxygen gas at the anode by electrolysis. The removal rates varied from 45.1 to 97.4%, depending on current density and selenate concentration. The transformation of selenate by the process is modeled based on a heterogeneous reaction coupled with electrochemical generation of ferrous and hydroxide. The rates are optimized at lower initial concentrations, higher electrical currents, and the presence of anions. Presence of dissolved oxygen does not cause any significant effects the removal of selenate.
Article
Studies of the electrodeposition of Se atomic layers on Au(111) and Au(110) are presented. Three electrochemical methods of forming Se atomic layers were investigated:  reductive deposition, oxidative stripping of bulk Se, and reductive stripping of bulk Se. The resulting Se atomic layers were studied using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). LEED indicated the formation of Au(111)(√3×√3)R30°-Se and Au(110)(2×3)-Se structures. STM analysis confirmed the presence of those structures along with several others. At low Se coverages on Au(111), a mosaic structure was formed, composed of a large number of small domains of a (√3×√3)R30°-Se structure, separated by areas void of Se. At higher coverages, near 1/3, the (√3×√3)R30° structure covered most of the surface, except for a number of linear phase boundaries. Commensurate with completion of the (√3×√3)R30° structure, some domains of square Se8 rings were usually evident, as well. At still higher coverages, a heterogeneous surface was formed, composed of a complex network of rings, chains, clusters, and pits. This heterogeneity appears to result from slow deposition kinetics, probably the result of both a low exchange current and low Se surface mobility. Some of the kinetic sluggishness may have resulted from the need to translate whole domains of Se atoms from one site to another, in order to remove phase boundaries. STM studies of the Au(110) surface indicated that only the (2×3) structure was formed at coverages much below 1 monolayer and that it was formed homogeneously. At monolayer coverages and above, a honeycomb structure composed of chains of Se atoms was observed, which filled in at still higher coverages to complete a second Se layer.
Article
The electrodeposition of Se(IV) at a Au electrode in 0.1 M HClO4 is concluded to produce Se in three distinct states of activity, and three anodic stripping peaks are observed for large quantities of deposited Se. Approximately a mono-layer is initially deposited which is apparently stabilized by one-dimensional interaction with the Au electrode surface. The formation of a bulk deposit of Se produces a large activity gradient which is the driving force for irreversible diffusional transport of Se into the Au electrode forming a AuSe alloy of unknown stoichiometry. Application of stripping voltammetry for determination of trace Se(IV) in 0.1 M HClO4 is possible if the total deposit does not exceed the equivalent of one monolayer. The detection limit of the technique is approximately 0.04 ppb Se(IV) in 0.1 M HClO4.
Article
The review deals with the principal advances in the electrochemistry of selenium and tellurium. Problems concerning the kinetics and mechanisms of the cathodic reduction of selenium and tellurium to different valence states and their anodic oxidation in aqueous and non-aqueous solutions are discussed. The practical applications of electrochemistry in the isolation, refining, and determination of small amounts of the two elements are described. The bibliography includes 178 references.
Article
Selenium is a natural trace element that is of fundamental importance to human health. The extreme geographical variation in selenium concentrations in soils and food crops has resulted in significant health problems related to deficient or excess levels of selenium in the environment. To deal with these kinds of problems in the future it is essential to get a better understanding of the processes that control the global distribution of selenium. The recent development of analytical techniques and methods enables accurate selenium measurements of environmental concentrations, which will lead to a better understanding of biogeochemical processes. This improved understanding may enable us to predict the distribution of selenium in areas where this is currently unknown. These predictions are essential to prevent future Se health hazards in a world that is increasingly affected by human activities.
Article
This work presents a simple, reproducible and low cost method, employing differential pulse anodic stripping voltammetry, for determination of selenium(IV). A gold electrode obtained from recordable compact disks (CD-R) was used to evaluate the voltammetric behavior of the metallic ion in 0.1molL(-1) HClO(4). To evaluate the voltammetric behavior of Se(IV), parameters such as deposition potential and deposition time were optimized. A wide linear response range, from 0.5 to 291ngmL(-1), was obtained using a 5.0mm diameter gold electrode. Recovery tests for Se(IV) utilizing standard reference solutions provided values between 94 and 96%.
Article
The essential trace mineral, selenium, is of fundamental importance to human health. As a constituent of selenoproteins, selenium has structural and enzymic roles, in the latter context being best-known as an antioxidant and catalyst for the production of active thyroid hormone. Selenium is needed for the proper functioning of the immune system, and appears to be a key nutrient in counteracting the development of virulence and inhibiting HIV progression to AIDS. It is required for sperm motility and may reduce the risk of miscarriage. Deficiency has been linked to adverse mood states. Findings have been equivocal in linking selenium to cardiovascular disease risk although other conditions involving oxidative stress and inflammation have shown benefits of a higher selenium status. An elevated selenium intake may be associated with reduced cancer risk. Large clinical trials are now planned to confirm or refute this hypothesis. In the context of these health effects, low or diminishing selenium status in some parts of the world, notably in some European countries, is giving cause for concern.
Article
In many environmental contaminant situations selenium has become the primary element of concern because of its bioaccumulative nature in food webs. Initial concerns about selenium were related to fish kills at Belews Lake, NC, Martin Lake, TX, and Kesterson Reservoir, CA, and to bird deformities at Kesterson Reservoir. Additional concerns were identified under the National Irrigation Water Quality Program at Salton Sea, CA, Kendrick, WY, Stewart Lake, UT, and Grand Valley and Uncompahgre Valley, CO. Recent studies have raised concerns about selenium impacts on aquatic resources in Southeastern Idaho and British Columbia. The growing discomfort among the scientific community with a waterborne criterion has lead the US Environment Protection Agency to consider a tissue-based criterion for selenium. Some aquatic ecosystems have been slow to recover from selenium contamination episodes. In recent years, non-governmental researchers have been proposing relatively high selenium thresholds in diet and tissue relative to those proposed by governmental researchers. This difference in opinions is due in part to the selection of datasets and caveats in selecting scientific literature. In spite of the growing selenium literature, there are needs for additional research on neglected organisms. This review also discusses the interaction of selenium with other elements, inconsistent effects of selenium on survival and growth of fish, and differences in depuration rates and sensitivity among species.
Article
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.
Article
Enterobacter cloacae SLD1a-1 is capable of reductive detoxification of selenate to elemental selenium under aerobic growth conditions. The initial reductive step is the two-electron reduction of selenate to selenite and is catalyzed by a molybdenum-dependent enzyme demonstrated previously to be located in the cytoplasmic membrane, with its active site facing the periplasmic compartment (C. A. Watts, H. Ridley, K. L. Condie, J. T. Leaver, D. J. Richardson, and C. S. Butler, FEMS Microbiol. Lett. 228:273-279, 2003). This study describes the purification of two distinct membrane-bound enzymes that reduce either nitrate or selenate oxyanions. The nitrate reductase is typical of the NAR-type family, with alpha and beta subunits of 140 kDa and 58 kDa, respectively. It is expressed predominantly under anaerobic conditions in the presence of nitrate, and while it readily reduces chlorate, it displays no selenate reductase activity in vitro. The selenate reductase is expressed under aerobic conditions and expressed poorly during anaerobic growth on nitrate. The enzyme is a heterotrimeric (alphabetagamma) complex with an apparent molecular mass of approximately 600 kDa. The individual subunit sizes are approximately 100 kDa (alpha), approximately 55 kDa (beta), and approximately 36 kDa (gamma), with a predicted overall subunit composition of alpha3beta3gamma3. The selenate reductase contains molybdenum, heme, and nonheme iron as prosthetic constituents. Electronic absorption spectroscopy reveals the presence of a b-type cytochrome in the active complex. The apparent Km for selenate was determined to be approximately 2 mM, with an observed Vmax of 500 nmol SeO4(2-) min(-1) mg(-1) (kcat, approximately 5.0 s(-1)). The enzyme also displays activity towards chlorate and bromate but has no nitrate reductase activity. These studies report the first purification and characterization of a membrane-bound selenate reductase.
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