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.     

A new electrode reactor with in-built recirculation mode for the enhancement of methylene blue dye removal from the aqueous solution: Comparison of adsorption,electrolysis and combined effect

The removal of basic dye such as methylene blue (MB) dye from the synthetic wastewater was experimentally investigated using an electrolytic cell (EC), adsorption and the combined effect of EC and adsorption technology called a three-phase three-dimensional electrode reactor (TPTDER). The performance of the each technology was checked on the basis of the efficiency of the systems. The experimental results are expressed in terms of the removal efficiency of the dye molecules. The results show that the TPTDER could efficiently remove the dye molecules from the aqueous solutions when compared with the EC and adsorption process. The removal efficiency reached as high as about 99% for an initial MB dye concentration in the range of 100–1,000 mg/L by TPTDER for 10 min at 12 V cell voltage and at specific airflow conditions. It was also observed that the removal of dye molecules depends upon the initial solution pH, applied cell voltage, contact time, and initial dye concentration. The recyclability of the particle electrodes in the TPTDER process was also checked. These findings suggest that TPTDER is a promising technology for the removal of dyes from the aqueous solution, and can be applied to the removal of dyes from the industrial effluents.     

Decolorisation of aqueous reactive dye Remazol Red 3B by electrocoagulation

The present study demonstrated the applicability of the electrocoagulation method for the removal of reactive dye, Remazol Red 3B, in a batch study. Iron electrode material was used as a sacrificial electrode in monopolar parallel mode in this study. The effects of the initial pH, current density, conductivity, initial concentration of dye and electrolysis time on the removal of Remazol Red 3B were investigated to determine optimum operating conditions. High decolorisation efficiency (>99%) for Remazol Red 3B dye solution was obtained with optimal value of process parameters, such as 15 mA cm^?2 of current density, 10 min of electrolysis time, pH 6 and 500 mg l^?1 dye concentration. The energy consumption, electrode consumption and operating costs under optimum operating conditions were calculated as 3.3 kW h kg dye^?1, 1.2 kg Fe kg dye^?1 and 0.6 € m^?3, respectively.     

Arsenic removal from drinking water by electrocoagulation using iron electrodes

Arsenic removal from drinking water was investigated using electrocoagulation (EC) followed by filtration. A sand filter was used to remove flocs generated in the EC process. Experiments were performed in a batch electrochemical reactor using iron electrodes with monopolar parallel electrode connection mode to assess their efficiency. The effects of several operating parameters on arsenic removal such as current density (1.5–9.0 mA cm^?2), initial arsenic concentration (50–500 μg L^?1), operating time (0–15 min), electrode surface area (266–665 cm²), and sodium chloride concentrations (0.01 and 0.02M) were examined. The EC process was able to decrease the residual arsenic concentration to less than 10 μg L^?1. Optimum operating conditions were determined as an operating time of 5 min and current density of 4.5 mA cm^?2 at pH of 7. The optimum electrode surface area for arsenic removal was found to be 266 cm² taking into consideration cost effectiveness. The residual iron concentration increased with increasing residence time, and maximum residual iron value was measured as 287 μg L^?1 for electrode surface area of 266 cm². The addition of sodium chloride had no significant effect on residual arsenic concentration, but an increase in current density was observed.     

Evaluation of electro-coagulation method for decolorization and degradation of organic dyes in aqueous solutions

This research has two parts: at first part electro coagulation (EC) method was used to remove the dye Direct Black 22 (DB22 (in aqueous media. All experiments were done in a 2 liter reactor with two electrodes made of steel (SS-304) as cathodes and one aluminum electrode as anode. Parameters affecting the process, such as anode material, electrolyte concentration, current density, initial pH of solution and the initial dye concentration, were investigated. Total amount of consumed energy was used for determination of optimal conditions. According to the results obtained for DB22 at optimized conditions, color and COD removal percentage were 92% and 85%, respectively. In addition, the current efficiency for aluminum anode in removal of DB22 was 90%. At the second part of the research work, color removal by EC for six different dyes was evaluated. The results showed that dye structure and anode type are very important factors on performance of the process.     

Simultaneous recovery of copper and surfactant by an electrolytic process from synthetic solution prepared to simulate a concentrate waste stream of a micellar-enhanced ultrafiltration process

Electroplating method was employed to recover copper and surfactant simultaneously from synthetic solutions prepared to simulate concentrated waste stream generated from micellar-enhanced ultrafiltration. Effects of surfactant and copper (II) concentrations, surfactant to copper (II) molar ratio (S/M), electroplating voltage and time, solution pH, and ionic strength on metal recovery and electrical current efficiency were investigated. Results show that at a fixed S/M ratio of 5, the first-order kinetic constant for Cu (II) removal by electroplating increases with decreasing SDS concentration. At fixed SDS concentration of 8.5 mM, increasing initial Cu (II) concentration increases both Cu (II) recovery and current efficiency. Electrolyte pH has profound effects on the metal recovery and current efficiency due to the difference in solution conductivity after pH adjustment and the extent of Cu (II) adsorption onto SDS at different pHs.     

Comparative study of electrocoagulation and electrochemical Fenton treatment of aqueous solution of benzoic acid (BA): Optimization of process and sludge analysis

Benzoic acid containing synthetic solution was pretreated by acid precipitation at various pH (1-3) and temperature (15-60 °C). Pre-treated solution was further treated by electrocoagulation (EC) and electrochemical Fenton (EF) processes using iron anode and graphite cathode. Optimization of independent operating parameters, namely, initial pH: (3-11), current density (A/m²): (15.24-76.21), electrolyte concentration (mol/L): (0.03-0.07) and electrolysis time (min): (15-95) for EC process and pH: (1-5), current density (A/m²): (15.24-76.21), H₂O₂ concentration (mg/L): (100-500) and electrolysis time (min): (15-95) for EF process, was performed using central composite design (CCD) in response surface methodology (RSM). Maximum removal efficiencies of BA- 76.83%, 88.50%; chemical oxygen demand (COD) - 69.23%, 82.21% and energy consumption (kWh/kg COD removed) - 30.86, 21.15 were achieved by EC and EF processes, respectively, at optimum operating conditions. It was found that EF process is more efficient than EC process based on removal of BA and COD with lower energy consumption. The sludge obtained after EC and EF treatments was analyzed by XRD, FTIR, DTA/TGA and SEM/EDX techniques.     

Comparative Study of Electrocoagulation and Electrooxidation Processes for the Degradation of Ellagic Acid From Aqueous Solution

A comparative study of electrocoagulation and electrooxidation processes for the degradation of ellagic acid from aqueous solution was carried out. For the electrocoagulation process, metallic iron was used as electrodes whereas graphite and RuO₂/IrO₂/TaO₂ coated titanium electrodes were used for the electrooxidation processes. The effect of the process variables such as initial pH, concentration of the supporting electrolyte, applied current density, electrolysis time, and anode materials on COD removal were systematically examined and discussed. Maximum COD removal of 93% was obtained at optimum conditions by electrocoagultion using an iron electrode. The ellagic acid was degraded completely by electrooxidation using graphite electrodes under the optimum conditions. During electrooxidation, the chloride ion concentration was estimated and the effect of the Cl^? ion was discussed. The finding of this study shows that an increase in the applied current density, NaCl concentration, and electrolysis time enhanced the COD removal efficiency. The UV–Vis spectra analysis confirms the degradation of ellagic acid from aqueous solution.     

Treatment of Endocrine Disrupting Chemical From Aqueous Solution by Electrocoagulation

The removal of endocrine disrupting chemical (BPA; Bisphenol–A) from aqueous solution was experimentally investigated by electrocoagulation process. The effects of different combinations of aluminum (Al) and iron (Fe) electrode pair, supporting electrolyte type, supporting electrolyte concentration, initial pH and applied current density and initial BPA concentration on the Chemical Oxygen Demand (COD), and energy consumption performances were critically evaluated. The experiment results indicate that Al–Al electrode pair is the most efficient choice of the four electrode pairs. The COD removal efficiency was increased when NaCl was used as the supporting electrolyte instead of Na₂SO₄ and NaNO_3. The optimum supporting electrolyte type and its concentration, initial pH, applied current density and treatment time were found to be NaCl, 0.05 M, pH 7.0, 12 mA cm^?2 and 40 min, respectively. Energy consumption was found to decrease with increase of NaCl concentration while it increases with increasing applied current density. The initial and treated sample was characterized by UV–vis spectroscopy to confirm the treatment efficiency. The sludge formed during electrocoagulation was characterized by XRD and SEM/EDAX analysis.     

Multi-Objective Optimization of Indigo Carmine Removal by an Electrocoagulation/GAC Coupling Process in a Batch Reactor

A coupling process between Electrocoagulation (EC) and GAC was employed to separate dyes from aqueous solutions. The removal of an indigoid dye, namely C.I. Acid Blue 74, was tested. A novel approach for optimizing EC-based techniques is presented. In addition to maximizing removal efficiency, minimizing consumptions of energy and electrode materials were also targeted by means of multi-objective optimization in order to reduce the specific costs. A very good cost-efficiency feature of EC/GAC coupling process operated under optimal conditions to treat wastewater from dyestuff has been revealed. The independent variables considered were the current density, influent pH, contact time, granular activated carbon dose, and initial dye concentration. Simple maximization of color removal efficiency and multi-objective optimization were compared. Two different constraints were considered for each type of optimization. The determined costs outline the cheapness feature of the EC/GAC system as a potential dye wastewater treatment technology.     

Investigation of shipyard wastewater treatment using electrocoagulation process with Al electrodes

In this study, shipyard oily wastewater treatment was investigated by electrocoagulation (EC) using aluminum electrodes in a batch reactor by evaluating different operation conditions. The maximum chemical oxygen demand (COD) removal efficiency of 88.83% was obtained at current density of 3 mA/cm². The removal efficiency was gradually improved with increasing current density and decreased with increasing COD concentration. However, initial pH value was not determinant factor for this process. Total energy and electrode cost were calculated as $0.88 per m³ treated wastewater. The result of this research shows that EC process seems to be an efficient method for the oily wastewater treatment.     

Removal of Cyanide from Liquid Waste by Electrochemical Oxidation in a New Cell Design Employing a Graphite Anode

Removal of CN^? from liquid solutions was studied experimentally by an anodic oxidation technique under various operating conditions. A new cell configuration was used, the cell consists of four vertical graphite rod anodes distributed at 90° on the circular circumference at a distance of 3 cm from the wall and surrounded by two cylindrical stainless steel screen cathodes from the inner and the outer side. Parameters investigated are the initial concentration of cyanide ions, applied current density, initial pH of the electrolyte, and electrolyte conductivity. All parameters were investigated at an operating time of 3 h. The % removal increased upon increasing the applied current density, increasing the electrolyte conductivity, and decreasing the initial concentration of CN^?. Increasing initial pH from 4 to 8 increased the % removal by a factor ranging from 55 to 73%, further increase of pH up to 12 slightly increased the % removal (3–8%).

Application of the present electrochemical reactor in treating cyanide containing effluents from different industries was highlighted.     

Electrocoagulation of Soluble Oil Wastewater: Parametric and Kinetic Study

Soluble oils (cutting oils) in aqueous solution are used extensively in the engineering and metal working industries, as coolants, metal forming fluids, and/or lubricants. Hysol-X is a general-purpose cutting oil used in 1–10% concentration in aqueous solution. This paper investigates demulsification of soluble-oil-water emulsion using electrocoagulation with aluminum electrodes. It examines the current efficiency and the pH variation of the system due to electrocoagulation and the effect of pH of the oil-water emulsion, current density, and temperature on the demulsification process. A simple first order kinetics fits the experimental data at a constant value of current density. The optimum pH and current density were found to be 6.50 and 138.8 A/m², respectively. Apparent electrode current efficiency was found to be 115.5%. Temperature has significant effect on emulsion stability and solution pH. At a lower temperature of 20°C or a higher temperature of 60°C, the turbidity (or oil) removal is found to be very high. More than 99% oil separation is achieved in 3 h at 20°C. The residual aluminum concentration in the treated water at an initial pH 6.5 is found to be 0.001 mg/l, well below the statutory discharge standard value.     

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.     

Electrocoagulation studies on removal of cadmium using magnesium electrode

The removal of cadmium from aqueous solution was carried out by electrocoagulation using magnesium as anode and stainless steel as cathode. Various operating parameters on the removal efficiency of cadmium were investigated, such as initial cadmium ion concentration, initial pH, current density and temperature. The optimum removal efficiency of 98.6% was achieved at a current density of 0.2 A dm⁻² at a pH of 7.0. The experimental data were tested against different adsorption isotherm models for describing the electrocoagulation process. The adsorption of cadmium preferably fitting the Langmuir adsorption isotherm suggests monolayer coverage of adsorbed molecules. First and second-order rate equations were applied to study adsorption kinetics. The adsorption process follows second order kinetics model with good correlation. Temperature studies showed that adsorption was endothermic and spontaneous in nature.     

Numerical simulation of the electrodeionization (EDI) process accounting for water dissociation

The electrodeionization process (EDI) is usually operated at overlimiting current density, and is thus characterized by water dissociation and concentration polarization. We attempt to study the useful and harmful effects of water dissociation on the EDI process. A numerical steady state model was established to simulate the process of EDI, accounting for the effects of water dissociation. The differences in concentration polarization of membranes were investigated to study the effects of water dissociation on cation and anion membranes. Protons produced by water dissociation caused the resin to transform into the H-form. The H-form resin, which has high conductivity and high transport number, depletes protons in the interstitial solution. This explains the experimentally detected phenomenon that at high current densities, the pH value of the effluency of the dilute compartment (DC) stops decreasing when current increases. We suggest that the useful role of water dissociation in EDI is due to the H-form resin bringing more salt cations of the interstitial solution into the resin phase, thus producing a high conductivity channel for the electro-migration of the salt cations. This mechanism avoids the decrease in salt ion conductivity brought about by concentration polarization. The disadvantageous effect of concentration polarization on the transportation of salt ions in interstitial solution is thus lessened. An intermediate point between the useful and harmful effects of water dissociation was determined by the dependence of current efficiency and removal rate for both cations and anions as a function of current density.     

Using electrocoagulation-electroflotation technology to treat synthetic solution and textile wastewater, two case studies

The purpose of this study was to investigate the effects of the operating parameters, such as pH, initial concentration (C_i), duration of treatment (t), current density (j), interelectrode distance (d) and conductivity (κ) on the treatment of a synthetic wastewater in the batch electrocoagulation (EC)-electroflotation (EF) process. The optimal operating conditions were determined and applied to a textile wastewater. Initially a batch-type EC-EF reactor was operated at various current densities ranging from 11.55 to 91.5 mA/cm² and various electrode gaps (1, 2 and 3 cm). For solutions with 300 mg/L of silica gel, good turbidity removal (89.6%) was obtained without any coagulant when the current density was 11.55 mA/cm², and with initial pH at 7.6, conductivity at 2.1 mS/cm: the treatment time was hold for 10 min and the electrode gap was 1 cm. Application of the optimal operating parameters on a textile wastewater showed a high removal efficiency for the following variables: suspended solid (SS) 85.5%, turbidity 76.2%, biological oxygen demand (BOD₅) 88.9%, chemical oxygen demand (COD) 79.7%, and color over 93%.     

Kinetics and equilibrium studies of removal of an azo dye from aqueous solution by adsorption onto scallop

In the present work removal of an azo dye (Reactive Black 5) was investigated from aqueous solution by adsorption onto scallop as a low-cost and widely available adsorbent. The effect of various operational parameters, such as contact time, pH, initial dye concentration and adsorbent dosage on the removal efficiency of dye was studied. Removal efficiency declined with the increase in solution pH and initial dye concentration but with the decrease in adsorbent dosage. Experimental equilibrium and kinetics data were fitted by Langmuir and Freundlich isotherms and pseudo-first-order and pseudo-second-order kinetic models, respectively.     

DSA电极电催化氧化降解四环素废水工艺优化

以盐酸四环素(TC·HCl)为研究对象,钛基钌铱涂层(IrO2-RuO2/Ti)为阳极,钛板为阴极,电催化氧化降解TC·HCl模拟废水,探讨了初始质量浓度、电流密度、pH、电解质硫酸钠浓度对电催化降解TC·HCl效率的影响。结果表明,电催化氧化可有效降解水中的TC·HCl。提高电流密度,降低TC·HCl初始质量浓度、电解质硫酸钠浓度,可增大TC·HCl去除率。反应的前90 min,pH对去除率无影响,反应90 min后降解效果出现明显差别,pH为3、7和12时,反应300 min去除率分别为92%、100%和72%。降解过程遵循一级反应动力学模型。通过综合各工艺参数下的去除率、能量消耗和电流效率,得出最佳工艺参数为:TC·HCl初始质量浓度300 mg/L、电流密度10 mA/cm2、硫酸钠浓度0.05 mol/L、pH=7,在该条件下反应180 min后TC·HCl去除率达94%。该实验结果为电化学处理制药工业废水提供了基础数据和科学参考。     

悬浮态TiO_2光催化降解腐殖酸的影响因素研究

在悬浮态TiO2光催化体系中,探讨了光催化氧化降解腐殖酸的规律,考察了催化剂投加量、腐殖酸溶液初始浓度、初始pH值、光强、反应时间等因素对腐殖酸去除效果的影响。结果表明,光催化氧化法降解腐殖酸的效率比直接光解法有显著提高,腐殖酸的去除率从34.73%增加到65.61%(反应时间为3 h);TOC的去除率也大幅度提高,直接光解反应对TOC几乎无去除作用,而光催化氧化对TOC的去除率达37.5%;腐殖酸的初始浓度增加,初始pH值降低,光强增大,光照时间延长均能提高腐殖酸的降解率,其中pH值和光强的影响较为显著。另外,催化剂的投加量存在一个最佳值。