A novel approach to sustain Fe 0 -electrocoagulation for Cr(VI) removal by optimizing chloride ions

Hexavalent Chromium Removal from Water and Wastewaters by Electrochemical Processes: Review

Hexavalent chromium (Cr(VI)) is a toxic, mutagenic, teratogenic, and carcinogenic species. Its origin is in industrial activities. Therefore, its effective control is realized on a source basis. Although chemical methods proved effective in removing Cr(VI) from wastewaters, more economic solutions with a minimum sludge production have been sought. Among them, the use of electrochemical processes has emerged as a viable solution to the problem. Much research was conducted in this area. The aim of this review paper is to make a critical evaluation of the literature on Cr(VI) removal by electrochemical methods, particularly electrocoagulation with sacrificial electrodes, and to assess the present data as well as to point out the areas that need further elaboration. Following the review of the theoretical concepts of electrochemical processes, the literature on the electrochemical removal of Cr(VI) was evaluated on the basis of important elements of the system. Among them are initial pH, initial Cr(VI) concentration, current density, type and concentration of supporting electrolyte, and the material of electrodes and their operating characteristics and process kinetics. Dimensionally stable electrodes that realize the reduction process without producing any sludge were evaluated separately. Applications of electrochemical methods to a wide spectrum of industrial effluents were also assessed.

The role of the current waveform in mitigating passivation and enhancing electrocoagulation performance: A critical review

Electrocoagulation (EC) can be an efficient alternative to existing water and wastewater treatment methods due to its eco-friendly nature, low footprint, and facile operation. However, the electrodes applied in the EC process suffer from passivation or fouling, an issue resulting from the buildup of poorly conducting materials on the electrode surface. Indeed, such passivation gives rise to various operational problems and restricts the practical implementation of EC on a large scale. Therefore, it has been suggested that using pulsed direct current (PDC), alternating pulse current (APC), and sinusoidal alternating current (AC) waveforms in EC as alternatives to conventional direct current (DC) can help mitigate passivation and alleviate its associated detrimental effects. This paper presents a critical review of the impact of the current waveform on the EC process towards the capabilities of the PDC, APC, and AC waveforms in de-passivation and performance enhancement while comparing them to the conventional DC. Additionally, current waveform parameters influencing the surface passivation of electrodes and process efficiency are elaborately discussed. Meanwhile, the performance of the EC process is evaluated under different current waveforms based on pollutant removal efficiency, energy consumption, electrode usage, sludge production, and operating cost. The proper current waveforms for treating various water and wastewater matrices are also explained. Finally, concluding remarks and outlooks for future research are provided.

A novel electrocoagulation process with centrifugal electrodes for wastewater treatment: Electrochemical behavior of anode and kinetics of heavy metal removal

Anodic passivation is a key problem to impair the efficiency of in the electrocoagulation (EC) process. Process intensification of EC has attracted increasingly greater attention. In this work, a novel centrifugal electrode reactor was designed and applied in EC process to enhance the treatment of simulated heavy metal wastewater using aluminum anode. Results showed that the removal efficiency of heavy metals was significantly improved by the centrifugal electrodes, compared with the stationary electrodes. Electrochemical behavior of centrifugal electrodes was analyzed by an improved rotating disk electrode system. Anodic polarization behavior of aluminium showed a typical characteristic of dissolution in centrifugal electrodes, rather than passivation in static condition. Anode dissolution was controlled by the diffusion of Cl^- ion that was enhanced by centrifugal electrodes. Thus, anode passivation was reduced. In addition, the kinetics analysis indicated that the removal of heavy metals in EC by centrifugal electrodes conformed to Variable-Order-Kinetic (VOK) model based on the Langmuir adsorption.

Anode passivation mitigation by homogenizing current density distribution in electrocoagulation

Electrode passivation is the most challenging technical problem in electrocoagulation (EC) water treatment process, but research on understanding and mitigating passivation evolution are still lacking. Herein, homogenization of current density (CD) distribution was found to be a critical factor in alleviating the anode passivation during EC process. Decreasing electrode area decelerated the growth of passivation layer on anode through homogenizing CD distribution, which was quantified by the ratios of CD distributed at the electrode edges and centers. When aluminum anode area decreased from 8 cm² to 2 cm² with a constant CD, the homogenization degree increased by 24.0%, and passivation was reduced by 24.3%. The depth profiles of passivated anodes confirmed the inhomogeneity of the anode passivation. Thicker passivation layers were observed at edges due to high CD distributions, which originated from the "edge effect" of electric field distribution between parallel plate electrodes. A facile strategy to homogenize CD distribution by splitting electrodes into smaller electrodes is then proposed for passivation mitigation, which can save energy consumption by 21.8% with unchanged removal efficiency. This study provides a unique insight into anode passivation mitigation and a feasible electrode design in EC.

Efficient adsorption of strontium by in-situ electrochemical synthesis of monohydric phosphate intercalated layered double hydroxides

In this study, in-situ electrochemical synthesis of monohydric phosphate intercalated layered double hydroxide was used to treat the simulated strontium-containing low-level waste liquids. The removal rate of Sr2+ was 99.24%...

How does periodic polarity reversal affect the faradaic efficiency and electrode fouling during iron electrocoagulation?

Electrocoagulation (EC) is a promising electrochemical water treatment technology. However, a major challenge to sustaining effective long-term EC operation is controlling the precipitation of materials on the electrodes, commonly referred to as fouling. Periodically reversing electrode polarity has been suggested as an in-situ fouling mitigation strategy and is often implemented in EC field applications. However, the utility of this approach has not been investigated in detail. In this study, the effect of polarity reversal (PR) on the performance of EC using iron electrodes was examined under different water chemistry conditions and at a range of reversal frequencies. It was observed that the faradaic efficiency in PR-EC was always lower than that in the EC systems operated with a direct current (i.e., DC-EC). It was also observed that the faradaic efficiency progressively decreased as the current reversal frequency increased, with the faradaic efficiency dropping as low as 10% when the PR interval was 0.5 min. Results from fouling layer, chronopotentiometric, and cyclic voltammetric investigations indicated that the decrease in the faradaic efficiency was caused by (i) increased electrode fouling by iron precipitates and (ii) electrochemical side reactions at the electrode-electrolyte interface. The extent of these effects was dependent on the solution chemistry; oxyanions and sulfide were found to be particularly detrimental to the performance of PR-EC, causing severe electrode fouling while decreasing the faradaic efficiency. Fouling could be mitigated by increasing the solution convection rate, resulting in a shear on the electrode surface that removed iron and other electrochemically reactive species from the electrodes.

Mitigating Electrode Fouling in Electrocoagulation by Means of Polarity Reversal: The Effects of Electrode Type, Current Density, and Polarity Reversal Frequency

One of the biggest issues in electrocoagulation (EC) water treatment processes is electrode fouling, which can cause decreased coagulant production, increased ohmic resistance and energy consumption, and reduced contaminant removal efficiency, among other operational problems. While it has been suggested that switching the current direction intermittently (i.e., polarity reversal, PR) can help mitigate electrode fouling, conflicting results about the utility of this approach have been reported in the literature. The objective of this study was to systematically investigate the effects of PR frequency and current density on the performance of Fe-EC and Al-EC. It was found that operating Fe-EC under the PR mode reduced neither electrode fouling nor energy consumption. Notably, the Faradaic efficiency (ϕ) in Fe-EC decreased with increasing PR frequency; ϕ was as low as 10% when a PR frequency of 0.5 minutes was employed. Unlike Fe-EC, operating Al-EC under the PR mode resulted in high coagulant production efficiencies, reduced energy consumption, and diminished electrode fouling. In addition to comparing PR-EC and DC-EC, a novel strategy to minimize electrode fouling was investigated. This strategy involved operating Fe DC-EC and Al DC-EC with a Ti-IrO₂ cathode, whose fouling by Ca- and Mg-containing minerals could be readily avoided by periodically switching the current direction.

Effect of polarity shift on the performance of electrocoagulation process for the treatment of produced water

A novel electrocoagulation (EC) system for the treatment of produced water (PW), with a provision to shift polarity is designed with the aim of reducing the problems resulting from electrode passivation. The enhancement of performance in terms of the rate and completion of the pollutant removal, energy and electrode consumption, sludge formation and ultimately the operating cost has been investigated by performing statistically designed experiments using RSM. The switching mode operation of EC had brought about significant improvement in the oil removal than the conventional mode. The polarity changeover in short span of batch recirculation time (BRT) was found to be more effective. Greater current density (CD) and supporting electrolyte concentration (SC) were found to enable a still lower switchover time (SOT) so that changeover frequency can further be increased. It was possible to remove a maximum of 99% COD and 98% Oil & Grease from a PW sample having initial concentration of 360 mgL^-1of oil (1280 mgL^{- 1}COD) at BRT of 15 min, CD of 1.6 Adm^-2, SC of 3 gL^-1 and SOT of 1 min. The combination of operating variables (BRT- 3 min, CD- 2.14 Adm^-2SC- 3 gL^-1and SOT- 1.9 min) giving minimum operating cost (0.65 US$/kg COD removed) achieving oil removal (88%) meeting discharge standards was found out by optimizing the RSM models for cost and oil removal. The method can be considered as an effective alternative for treating PW especially in offshore oil basins where time and space are the major constraints.

Removal of strontium by in situ electrochemical synthesis of Zn–Al LDHs using a bidirectional pulse method

The Sr2+ removal rate is 99.26%, and the energy consumption of the reaction process is 5.01 kW h m−3.

Electrode passivation, faradaic efficiency, and performance enhancement strategies in electrocoagulation-a review

Treating water and wastewater is energy-intensive, and traditional methods that require large amounts of chemicals are often still used. Electrocoagulation (EC), an electrochemical treatment technology, has been proposed as a more economically and environmentally sustainable alternative. In EC, sacrificial metal electrodes are used to produce coagulant in-situ, which offers many benefits over conventional chemical coagulation. However, material precipitation on the electrodes during long term operation induces a passivating effect that decreases treatment performance and increases power requirements. Overcoming this problem is considered to be the greatest challenge facing the development of EC. In this critical review, the studies that have examined the nature of electrode passivation, and its effect on treatment performance are considered. A fundamental approach is used to examine the association between passivation and faradaic efficiency, a surrogate for EC performance. In addition, the strategies that have been proposed to remove or avoid passivation are reviewed, including aggressive ion addition, AC current operation, polarity reversal, ultrasonication, and mechanical cleaning of the electrodes. It is concluded that the success of implementing each method is dependent on critical operating parameters, and careful consideration should be taken when designing an EC system based on the phenomena discussed in this article. In conclusion, this review provides insight into passivation mechanisms, delivers guidelines for sustaining high treatment performance, and offers an outlook for the future development of EC.

Comparative performance of green rusts generated in Fe0-electrocoagulation for Cd2+ removal from high salinity wastewater: Mechanisms and optimization

The treatment of wastewater containing high concentration of inorganic salts has always been one of the focuses of environmental researchers. In this work, the effect of Cl^- and SO₄^2- on the removal of Cd²⁺ from wastewater using Fe⁰-electrocoagulation (Fe⁰-EC) were investigated by evaluating the transformation of Fe mineral. The experimental results indicated that the removal of Cd²⁺ from wastewater was depended on the property of Fe minerals. The generation of sulfate green rust (GRSO₄) produced in the presence of SO₄^2- showed stronger adsorption than the chloride green rust (GRCl) for Cd²⁺, and GRSO₄ was obtained even in the mixture Cl^- and SO₄^2- solutions, because Fe(II)-Fe(III) GRs (layered double hydroxides, LDHs) showed stronger affinity for divalent SO₄^2- than monovalent Cl^-. High concentration of inorganic anions in wastewater resulted in the negative charged Fe flocs. High concentration of Cl^- promoted the oxidation of Fe(II) to Fe(III) by chlorine-containing oxidants, and increased the proportion of Fe(III)/Fe(II) in Fe flocs, secondary Fe mineral magnetite (Fe₃O₄) was formed because of the increase of pH. Therefore, the presence of GRSO₄ intermediate increased the Cd²⁺ removal by adsorption (coagulation and coprecipitation), and then the generated GRSO₄ were gradually transformed into lepidocrocite (γ-FeOOH) by oxygen from air. Finally, the parameter optimization were conducted by adjusting the ratio of Cl^- and SO₄^2- (R_C:S), current density (j), initial pH (pH_i), initial Cd²⁺ concentration (C₀), and temperature (T₀). The removal efficiency of Cd²⁺ reached 99.5% after 10 min Fe⁰-EC under the optimal parameters: R_C:S = 25:50 mmoL/mmol, j = 6 mA/cm², pH_i = 7-9, and T₀ = 40 °C.

Reusable and removable PmPD/PVA membrane for effective Cr(vi) adsorption and reduction

The PmPD/PVA membranes with facile reusability for and highly effective removal of Cr(vi).

Simultaneous removal of cadmium, zinc and manganese using electrocoagulation: Influence of operating parameters and electrolyte nature

In the present study, the influence of operating parameters and electrolyte nature on the simultaneous removal of toxic metals (cadmium, zinc and manganese) from synthetic smelting wastewater by batch electrocoagulation was investigated. This wastewater contained high concentrations of anion-cation electrolytes. Results indicated that the efficiency of heavy metals removal can be enhanced by increasing the solution pH and current density. The Fe-Fe electrode combination is more effective than the other combinations (Al-Al, Al-Fe and Fe-Al). The interaction of heavy metal ions showed that the increase of initial Zn²⁺ concentration adversely affects on Cd²⁺ removal. In addition, the single chloride system exhibits the optimum removal efficiency on Mn²⁺. Single sulfate and binary anion systems exert a more positive effect on Cd²⁺ and Zn²⁺ removal because of the stronger charge neutralization and destabilization of iron hydroxide flocs. Increases of Ca²⁺ and Mg²⁺ ions exert a significant negative effect on metal removal. However, the addition of a small amount of sodium chloride into a high sulfate and hardness solution can accelerate the removal of heavy metals. Finally, the sludge samples generated from electrocoagulation were characterized by XRD and SEM-EDS analyses.

Formation of macroscopic surface layers on Fe(0) electrocoagulation electrodes during an extended field trial of arsenic treatment

Extended field trials to remove arsenic (As) via Fe(0) electrocoagulation (EC) have demonstrated consistent As removal from groundwater to concentrations below 10 μg L(-1). However, the coulombic performance of long-term EC field operation is lower than that of laboratory-based systems. Although EC electrodes used over prolonged periods show distinct passivation layers, which have been linked to decreased treatment efficiency, the spatial distribution and mineralogy of such surface layers have not been investigated. In this work, we combine wet chemical measurements with sub-micron-scale chemical maps and selected area electron diffraction (SAED) to determine the chemical composition and mineral phase of surface layers formed during long-term Fe(0) EC treatment. We analyzed Fe(0) EC electrodes used for 3.5 months of daily treatment of As-contaminated groundwater in rural West Bengal, India. We found that the several mm thick layer that formed on cathodes and anodes consisted of primarily magnetite, with minor fractions of goethite. Spatially-resolved SAED patterns also revealed small quantities of CaCO3, Mn oxides, and SiO2, the source of which was the groundwater electrolyte. We propose that the formation of the surface layer contributes to decreased treatment performance by preventing the migration of EC-generated Fe(II) to the bulk electrolyte, where As removal occurs. The trapped Fe(II) subsequently increases the surface layer size at the expense of treatment efficiency. Based on these findings, we discuss several simple and affordable methods to prevent the efficiency loss due to the surface layer, including alternating polarity cycles and cleaning the Fe(0) surface mechanically or via electrolyte scouring.