Treatment of dairy wastewater by electrocoagulation process: Advantages of combined iron/aluminum electrodes

ABSTRACT

The objective was to assess the efficiency of electrode material in an electrocoagulation (EC) process for wastewater treatment by comparing the efficiency of aluminum (Al–Al), iron (Fe–Fe) and combined Fe–Al electrodes. The treatment of synthetic dairy wastewater, characterized by high levels of 5-day biological oxygen demand (BOD₅) and chemical oxygen demand (COD), was used to compare electrode materials. Experimental results showed that all electrodes materials achieved the same final removal yield in the range of current studied (55% COD, 60% total organic carbon, 90% total nitrogen, and nearly 100% turbidity) when equilibrium was achieved. But at fixed current density and initial concentration of dairy waste, the Al–Al assembly exhibited the fastest elimination, whereas the slowest removal rate was observed with the Fe–Fe electrodes, even though adsorption was always the main removal mechanism. Finally, an Fe–Al system using an Fe anode with an Al cathode emerged as a techno-economic trade-off because of the low price of iron: both metals contributed to the removal of dairy waste, and the treatment time to achieve equilibrium values was closer to the Al–Al assembly at fixed current density. Moreover, experimental results proved the additivity of the mechanisms reported for Al–Al and Fe–Fe systems with Fe–Al.     

Treatment of paint manufacturing wastewater by electrocoagulation

Treatability of paint manufacturing wastewater (PMW) by electrocoagulation (EC) process was investigated. Effects of operating parameters for the EC process such as electrode type (Al or Fe), initial pH (2–10), current density (5–80 A/m²) and operating time (0–50 min) were evaluated for optimum operating conditions. The highest removal efficiencies for COD and TOC in PMW were obtained with 93% and 88% for Fe and 94% and 89% for Al electrodes at the optimum conditions (35 A/m², 15 min and pH 6.95). Operating costs for removal of PMW at the optimum conditions were calculated for Fe and Al electrodes as 0.187 €/m³ and 0.129 €/m³. Toxicity test was carried out to obtain information about toxic effect of the raw and treated wastewaters at optimum operating conditions. The samples measured by respirometric method contained hardly toxicities. Performance of Al electrode was better than that of Fe electrode in terms of removal efficiency and operating cost.     

Characterization of the removal of Chromium(VI) from groundwater by electrocoagulation

A batch electrocoagulation system has been evaluated for the removal of Cr(VI) from brackish groundwater under different operating conditions. The influence of electrode type, applied current density, initial pH, initial chromium concentration, conductivity and temperature were evaluated. The experimental results indicated that chromium removal increased with increasing the applied current density and conductivity. The efficiency of different electrode arrangements (iron, aluminum) was also assessed, and indicated that Fe–Fe electrode pair was the most efficient arrangement and was able to achieve 100% Cr removal at an electrocoagulation time of 5 min, a current density of 7.94 mA/cm², and pH of 8 at room temperature 25 °C. The generated sludge for the iron electrodes was characterized using EDS, X-ray fluorescence (XRF) and FE-SEM. The analysis confirmed the formation and precipitation of Fe(OH)₃ and Cr(OH)₃ as solids. Overall, the study affirmed that electrocoagulation is a reliable technique for the purification of groundwater with an estimated energy consumption of 0.6 kWh/m³.     

Removal of arsenic from drinking water by the electrocoagulation using Fe and Al electrodes

A novel technique of electrocoagulation (EC) was attempted in the present investigation to remove arsenic from drinking waters. Experiments were carried out in a batch electrochemical reactor using Al and Fe electrodes with monopolar parallel electrode connection mode to assess their efficiency. The effects of several operating parameters on arsenic removal such as pH (4–9), current density (2.5–7.5 A m⁻²), initial concentration (75–500 μg L⁻¹) and operating time (0–15 min) were examined. Optimum operating conditions were determined as an operating time of 12.5 min and pH 6.5 for Fe electrode (93.5%) and 15 min and pH 7 for Al electrode (95.7%) at 2.5 A m⁻², respectively. Arsenic removal obtained was highest with Al electrodes. Operating costs at the optimum conditions were calculated as 0.020 € m⁻³ for Fe and 0.017 € m⁻³ for Al electrodes. EC was able to bring down aqueous phase arsenic concentration to less than 10 μg L⁻¹ with Fe and Al electrodes. The adsorption of arsenic over electrochemically produced hydroxides and metal oxide complexes was found to follow pseudo second-order adsorption model. Scanning electron microscopy was also used to analyze surface topography of the solid particles at Fe/Al electrodes during the EC process.     

Comparison of Electrocoagulation and Chemical Coagulation for Heavy Metal Removal

Copper (Cu), chromium (Cr), and nickel (Ni) removal from metal plating wastewater by electrocoagulation and chemical coagulation was investigated. Chemical coagulation was performed using either aluminum sulfate or ferric chloride, whereas electrocoagulation was done in an electrolytic cell using aluminum or iron electrodes. By chemical coagulation, Cu‐, Cr‐, and Ni‐removal of 99.9 % was achieved with aluminum sulfate and ferric chloride dosages of 500, 1000, and 2000 mg L^–1, respectively. Removal of metals by electrocoagulation was affected by the electrode material, wastewater pH, current density, number of electrodes, and electrocoagulation time. Electrocoagulation with iron electrodes at a current density of 10 mA cm^–2, electrocoagulation time of 20 min, and pH 3.0 resulted in 99.9 % Cu‐, 99.9 % Cr‐, and 98 % Ni‐removal.     

Removal of Cr(VI) from Aqueous Solution: Electrocoagulation vs Chemical Coagulation

Abstract

Hydrolyzed products of Al(III) have affinity below pH_zpc for oppositely charged mono and bi‐nuclear species of hexavalent chromium. This study investigates the comparative performance of electrocoagulation (EC) and chemical coagulation (CC) for the removal of Cr(VI) from aqueous solution. The highest removal of Cr(VI) achieved with EC was about 42% with 4.36 mA/cm² current density. Cathodic adsorption of chromium boosted up Cr(VI) removal during EC. Simultaneous electro‐ and chemical‐dissolution lead to high current efficiency of about 178%. Both the pH and the coagulant dosage have a significant impact on Cr(VI) removal in the pH ranges from 4.9 to 7.0. CC with alum and aluminum sulfate (AS) removed about 11% and 12% of Cr(VI). Co‐adsorption of divalent SO₄ ^2? ions with Cr(VI) is responsible for the lower removal observed with chemical coagulants. About 0.061 and 0.099 mole of SO₄ ^2? was adsorbed per mole Al in the precipitate in the pH range 4.9 to 7.0 with AS and alum. A higher coagulant dosage increases the removal of Cr(VI) but adversely affects the removal efficiency (Cr(VI) removed per unit of Al dosing). Cell current density (CD) has shown little effect on Cr(VI) removal and the pH elevation at the same charge density. Higher initial Cr(VI) concentration improves the removal efficiency as the species of Cr(VI) is acidic in solution and decreases the pH elevation rate.     

Trivalent chromium removal by electrocoagulation and characterization of the process sludge

Removal of chromium compounds from wastewater is a known pollution control challenge for environmental engineers. In this present work Cr³⁺ was removed from wastewater by electrocoagulation (EC) using Al electrodes in a batch cell. Results indicate that EC with an Al electrode can reduce Cr³⁺ concentration below 2.0 mg L^?1, its discharge limit. At higher stirrer speed, Cr³⁺ removal increases owing to enhanced contact with the Al³⁺ species. Cell current density controls the rate of Al dissolution and solution pH that affects the Cr³⁺ removal significantly. pH elevation during EC is due to accumulation of OH^? ions forming a supersaturated solution of Al³⁺ species. Supersaturation with respect to Al(OH)₃(s) is attributed to incomplete precipitation of aluminum hydroxide in the dynamic (transient) state and subsequent precipitation when cell current is stopped. Sludge produced in the process can be classified as non‐hazardous according to the European Waste Catalogue. Disposal cost of this dried sludge is estimated to be $ 0.144 (INR 7.20) per m³ of tannery effluent treating an initial solution of 1000 mg L^?1 Cr³⁺ to about 2.0 mg L^?1 Cr³⁺. Batch gravity settling characteristics of the electrocoagulated metal hydroxide sludge (EMHS) at different initial sludge loadings (as generated at different current density) is also investigated. Batch sedimentation flux is reported from experimental settling velocity and concentration of sludge. Copyright © 2007 Society of Chemical Industry     

Chemical and electrochemical considerations on the removal process of hexavalent chromium from aqueous media

Two methods were used to remove Cr(VI) from industrial wastewater. Although both are based in the same general reaction: 3Fe(II)_(aq) + Cr(VI)_(aq) ; 3Fe(III)_(aq) + Cr(III)_(aq) the way in which the required amount of Fe(II) is added to the wastewater is different for each method. In the chemical method, Fe(II)_(aq) is supplied by dissolving FeSO₄ · 7(H₂O)_(s) into the wastewater, while in the electrochemical process Fe(II)_(aq) ions are formed directly in solution by anodic dissolution of an steel electrode. After this reduction process, the resulting Cr(III)_(aq) and Fe(III)_(aq) ions are precipitated as insoluble hydroxide species, in both cases, changing the pH (i.e., adding Ca(OH)_2(s)). Based on the chemical and thermodynamic characteristics of the systems Cr(VI)–Cr(III)–H₂O–e^– and Fe(III)–Fe(II)–H₂O–e^– both processes were optimized. However we show that the electrochemical option, apart from providing a better form of control, generates significantly less sludge as compared with the chemical process. Furthermore, it is also shown that sludge ageing promotes the formation of soluble polynuclear species of Cr(III). Therefore, it is recommended to separate the chromium and iron-bearing phases once they are formed. We propose the optimum hydraulic conditions for the continuous reduction of Cr(VI) present in the aqueous media treated in a plug-flow reactor.     

Nickel-dimethylglyoxime complex modified graphite and carbon paste electrodes: preparation and catalytic activity towards methanol/ethanol oxidation

Nickel-dimethylglyoxime complex (abbreviated as Ni(II)(DMG)₂) modified carbon paste and graphite electrodes were prepared by mixing Ni(II)(DMG)₂ with graphite paste, and coating Ni(II)(DMG)₂ to the graphite surface. It is necessary to cycle the electrode potential to a high value (e.g. 0.8 V versus SCE) for the preparation of the modified electrodes. The electrochemical reaction was originally assumed to be a one-electron process converting Ni(II)(DMG)₂ to [(DMG)₂(H₂O)Ni(III)ONi(III)(OH)(DMG)₂]⁻. [(DMG)₂(H₂O)Ni(III)ONi(III)(OH)(DMG)₂]⁻ showed a strong catalytic activity toward electro-oxidation of methanol and ethanol. The electrocatalytic oxidation currents consistently increase with the increase in Ni(II)(DMG)₂ loading, OH⁻, and alcohol concentrations. Rotating disk electrode results obtained with a Ni(II)(DMG)₂ coated graphite disk electrode showed that the electrocatalytic oxidation of alcohol is a 4-electron process producing formate anion (methanol oxidation) or acetate anion (ethanol oxidation). A mechanism for the electrocatalytic oxidation of methanol/ethanol was proposed, and a rate-determining step was also discussed.     

Sorption of Cr(III)‐containing cations on strongly basic anion exchangers

It is shown that strongly basic anion exchangers AV‐17 and Varion‐AD in definite conditions are able to retain Cr(III)‐containing ions from Cr(III) sulfate solution. It is found that the sorption of Cr(III)‐containing ions on the polymers is essentially dependent on the pH, temperature, and Cr(III) sulfate concentration. The maximum temperature dependence of sorption was found to be about 60°C. The sorption isotherms are well described by Langmuir's equations. The sorption kinetics is determined by the diffusion of Cr(III)‐containing ions into polymer's phase. It is assumed that the Cr(III)‐containing ions are retained through formation, in polymer's phase, of the jarosite‐type mineral compounds: R₄N[Cr₃(OH)₆(SO₄)₂], H₃O[Cr₃(OH)₆(SO₄)₂], and K[Cr₃(OH)₆(SO₄)₂]. For comparison of sorptional capacities, the sorption of Cr(III)‐containing ions was determined on different cation and anion exchangers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3978–3985, 2006     

Modulated Cr(III) oxidation in KOH solutions at a gold electrode: Competition between disproportionation and stepwise electron transfer

The electrochemical oxidation of aqueous Cr(III) was examined using cyclic voltammetry with a polycrystalline Au electrode in KOH solutions of varying pH and Cr(III) concentration. The mechanism and kinetics for the oxidation of Cr(III) is a quasi-reversible diffusion-controlled reaction and is largely dependent on the solution pH. The reaction mechanism is initiated by an irreversible electrochemical electron transfer to form Cr(IV) which is the rate-determining step (RDS). Following the RDS, subsequent oxidation of Cr to its hexavalent state occurs by the disproportionation of Cr(IV) at low KOH concentrations and electron transfer at high KOH concentrations due to the involvement of OH⁻ in the disproportionation reaction. As the solution pH increases, the Cr(III) oxidation peak potential shifts negatively owing to the involvement of OH⁻ in the RDS. The competitive adsorption of OH⁻ and CrO₂⁻ on the electrode surface also plays an important role in the oxidation behavior.     

Synthesis and characterization of novel twin‐tailed hydrophobically associated copolymers and their applications to Cr(III) removal from aqueous solutions

Water pollution is one of the most serious environmental problems facing society. The objective of this study was to provide an efficient and inexpensive way of removing Cr(III) from polluted water. Novel twin‐tailed hydrophobically associating copolymers (PAAD) were prepared by micellar copolymerization of acrylamide (AM) and sodium 2‐acrylamido‐2‐methylpropane sulfonate with N,N‐dibutylmethacrylamide in an aqueous solution. The PAAD series, comprising copolymers of different molecular weights (ranged from 1.42 to 1.78 × 10⁶ g/mol) and compositions was characterized by gel permeation chromatography–multiangle laser light scattering, ¹H NMR and FTIR spectroscopy, and elemental analysis. The surface activity, morphology, and aggregation properties of PAAD samples were measured by surface tension and dynamic light scattering. A schematic representation of the formation of Cr(OH)₃ and its precipitation using PAAD samples as coagulant aid is discussed in the article. In addition, the Cr(OH)₃–copolymer interaction, as investigated by precipitation experiments, demonstrated that these copolymers have good Cr(OH)₃ binding capacity and potential as coagulant aid for the removal of Cr(III) from polluted water. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41028.     

Facile synthesis of MTaO4 (M = Al,Cr and Fe) metal oxides and their application as anodes for lithium-ion batteries

Ternary metal oxides have attracted much attention in energy storage fields. Herein, tantalum-based oxides MTaO₄ (M = Al, Cr and Fe) are synthesized by a facile co-precipitation method, and their performance as lithium-ion battery anodes are evaluated. Among them, the FeTaO₄ electrode presents superior electrochemical performance compared to the MTaO₄ (M = Al and Cr) and a reversible capacity of more than 200?mAh?g^?1 can be maintained after 100 cycles, while a capacity of 53.6 and 128.9?mAh?g^?1 can be obtained at the same condition for AlTaO₄ and CrTaO₄, respectively. The explanation that FeTaO₄ exhibits the excellent electrochemical performance in MTaO₄ (M = Al, Cr and Fe) are further discussed.     

Electrotreatment of industrial copper plating rinse effluent using mild steel and aluminum electrodes

BACKGROUND: Heavy metals from aqueous streams can be removed effectively using electrochemical techniques. Although both mild steel (MS) and aluminum (Al) electrodes have long been considered for the treatment of different waste‐waters, unfortunately the reported optimum treatment time, current density, pH and background electrolyte concentration vary greatly. In this work, an electrochemical technique was used for the removal of Cu from electroplating rinse water collected from a local plating industry using MS and Al as electrode materials. RESULTS: Removal of Cu during electrotreatment is due to the combined effect of hydroxide precipitation and electrodeposition. The discharge limit of Cu (3 mg L^?1) was achieved with both the electrodes and electrode arrangements. Both MS and Al electrodes showed similar performance for removing copper, however, a lower increase in pH was observed with the MS electrode under identical experimental conditions. NaCl added to increase the solution conductivity decreases Cu removal and inhibits oxide layer formation during electrotreatment. The effluent contains about 6.8 mg L^?1 Zn and its discharge limit (4 mg L^?1) was achieved within the first 10 min of treatment. CONCLUSIONS: Results obtained from experiments demonstrate that the stipulated limit of both Cu and Zn can be achieved during treatment of industrial plating effluent using MS and Al as electrode materials, depending on current density and treatment time. Copyright © 2009 Society of Chemical Industry     

The oxidation of trivalent chromium at the reconstructed Au(100) electrode

The electro-oxidation of Cr(III) to Cr(VI) species at the reconstructed Au(100) electrode has been investigated in a highly alkaline solution. The subsequent comparison of the results obtained with those at Au(111) electrode points to the structural sensitivity of this oxidation process related to different adsorbability of OH⁻ anions on both electrodes. Finally, it was found that the addition of Cr(III) ions does not shift the reconstruction peak to less positive values of potential indicating that for Au(100) electrode the oxidation process under consideration has no impact on the lifting of the surface reconstruction.     

Removal of Cu(II) from Aqueous Solution by Oil-Water Interfacial Emulsion Technique with Adsorbing Colloids

Abstract

Experimental investigations on the removal of Cu(II) from an aqueous solution were carried out by an interfacial emulsion technique with an adsorbing colloid (Al(OH)₃, FE(OH)₃), Cu(II) from the aqueous solution was segregated into a compact emulsion between water and a water-immiscible oil phase by an interfacial emulsion technique that uses the adsorptive power of the oil-water interface. Trimethylamine was effective as a surfactant for the removal of Cu(II), and the optimum pH for the removal of Cu(II) was found at 9.0 when using Fe(OH)₃ and at 10.0 when using Al(OH)₃ as an adsorbing colloid, respectively. The effects of pH, mixing time, initial surfactant concentration, initial Fe(III) concentration, and foreign ions (Na⁺, Ca²⁺, CI^?, NO₃ ^?, HPO₄ ^2?) on the removal efficiency were investigated. The adsorption and separation mechanisms for the removal of Cu(II) by the interfacial emulsion technique of adsorbing colloids were observed.     

Electrochemical oxidation of aliphatic hydrocarbons promoted by inorganic radicals. I. OH radicals

The OH initiated oxidation of aliphatic hydrocarbons by the simultaneous electrochemical reduction of O₂ and of Fe(III) at controlled potential was investigated in the liquid phase over a Fe(III) concentration range 0.5–5 mM. OH radicals were generated by the reaction: Fe(II)+H₂O₂Fe(III)+^.OH+OH^– The compounds studied were the linear alkane hydrocarbons from C₅ to C₁₀ and 3-methyl pentane. The results showed that the ketones are the only reaction products and that the yields decrease with increasing number of carbonium atoms of the hydrocarbon. Decreasing yields were also observed with increasing Fe(III) concentration.     

Container washing wastewater treatment by combined electrocoagulation–electrooxidation

Treatment of container washing wastewater (CWW) by using combined electrocoagulation (EC)–electrooxidation (EO) process was studied. CWW contains many organic compounds such as surfactants used in cleaning agents. Wastewater was first treated by EC with iron (Fe) and aluminum (Al) electrodes. The process performance was measured according to the removal efficiencies of soluble chemical oxygen demand (sCOD) and color. Maximum sCOD removal efficiency was found 82% and color removal efficiencies were 95%, 95% and 98% at 436, 525 and 620 nm, respectively, with Fe electrodes under 25 mA/cm² current density, initial pH of 5 and 120-min operation time. Because of the low sCOD removal efficiency, EO was used as post-treatment process by using boron doped diamond electrode (BDD). sCOD removal efficiency was increased to 89% and color removal efficiencies decreased to 72%, 64%, 71% at 436, 525 and 620 nm, respectively, under 25 mA/cm² current density, initial pH of 3 and 300-min operation time. This study showed that electrochemical processes caused new complex molecules formation in the CWW, which caused deterioration of water color and limited the process efficiency.     

In Situ Soft X‐ray Microscopy Study of Fe Interconnect Corrosion in Ionic Liquid‐Based Nano‐PEMFC Half‐Cells

This in situ soft X‐ray scanning microscopy electrochemical study of model proton exchange cathodic and anodic nano‐fuel cells is exploring the evolving structure and chemical composition of key cell components represented by Au and Fe electrodes in contact with Nafion‐ionic liquid composite electrolyte containing Pt black catalyst particles. Morphological and chemical changes of the electrodes as well as the chemical state and fate of the Fe species released into the electrolyte are monitored in short circuit and with applied cathodic or anodic polarization. The in situ X‐ray absorption images of the cathodic cell fed with 2.5 × 10^–5 mbar O₂ have revealed corrosion‐induced morphology changes in the Fe electrode, being more pronounced in the vicinity of Pt‐black particles, and deposition of the Fe species released into the electrolyte, onto the intact Au counter electrode upon cathodic polarization. The Fe electrodes of the anodic cell containing NaBH₄ in the electrolyte appear relatively more corrosion resistant. The Fe L₃ absorption spectra taken in different locations within the Fe electrode have shown lateral variations in the relative ratio between Fe²⁺ and Fe^3&4+ oxidation states, whereas the Fe species released into the RTIL electrolyte are only in the high Fe^3&4+ oxidation states.     

A comparative investigation of chromium deposition at air electrodes of solid oxide fuel cells

Deposition processes of chromium (Cr) species were investigated for the O₂ reduction on (La,Sr)MnO₃ (LSM), Pt and (La,Sr)(Co,Fe)O₃ (LSCF) electrodes in the presence of chromia-forming alloy metallic interconnect at 900^oC under air flow. For the reaction on LSM electrodes, deposition of Cr species preferentially occurred on the zirconia electrolyte surface, forming a distinct deposit ring at the edge of the LSM electrode while at LSCF electrodes, Cr species deposited on the electrode and electrolyte surface, forming isolated Cr particles. In contrast, there was no detectable deposition of Cr species either on the electrode or electrolyte surface for the O₂ reduction reaction on Pt electrodes. The results clearly demonstrated that deposition of Cr species in solid oxide fuel cells is not an electrochemical reduction of high valent Cr vapor species to Cr₂O₃ in competition with O₂ reduction. Cr deposition at SOFC cathodes is basically a chemical dissociation reaction and is controlled by the nucleation reaction between the nucleation agent and the gaseous Cr species. The nature of the nucleation agent strongly depends on the electrode material and impurities which may be introduced during electrode and electrolyte fabrication processes.