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

Degradation of fluoride in groundwater by electrochemical fixed bed system with bauxite: performance and synergistic catalytic mechanism

Fluoride pollution in water has garnered significant attention worldwide. The issue of fluoride removal remains challenging in areas not covered by municipal water systems. The industrial aluminum electrode and natural bauxite coordinated defluorination system (IE-BA) have been employed for fluoride removal. The experiment investigated the effects of pH, current density, and inter-electrode mineral layer thickness on the defluorination process of IE-BA. Additionally, the study examined the treatment efficiency of IE-BA for simulated water with varying F- concentrations and assessed its long-term performance. The results demonstrate that the defluorination efficiency can reach 98.4% after optimization. Moreover, irrespective of different fluoride concentrations, the defluorination rate exceeds 95.2%. After 72 hours of continuous operation, the defluorination rate reached 91.9%. The effluent exhibited weak alkalinity with a pH of around 8.0, and the voltage increased by 2.0 V compared to the initial moment. By analyzing the characterization properties of minerals and flocs, this study puts forward the possible defluorination mechanism of the IE-BA system. The efficacy of the IE-BA system in fluoride removal from water was ultimately confirmed, demonstrating its advantages in terms of defluorination ability under different initial conditions and resistance to complex interference. This study demonstrates that the IE-BA technology is a promising approach for defluorination.

Mo(VI) removal from water by aluminum electrocoagulation: Cost-effectiveness analysis, main influencing factors, and proposed mechanisms

Mo(VI) (MoO42-) removal by aluminum electrocoagulation (Al EC) with Al as anodes and cathodes was studied for the first time. At the initial Mo concentrations of 0.3 - 150 mg/L, kinetic analysis and effects of main factors (electrode connection modes, current density (CD), initial pH, and electrolytes) were examined, and potential mechanism of Mo(VI) removal were elucidated. Results showed that CD had significant impacts on anode weight loss, cathode weight loss, and total electrode weight loss (p value < 0.05). Cathode weight loss was higher than anode weight loss. XRD analysis results showed lower crystallinity of scums than that of precipitates. Boehmite was the most prevalent oxide in scums. An appropriate amount of NaCl was beneficial for enhancing the Mo(VI) removal efficiency and reducing the energy consumption of the Al EC process. Electrostatic attraction, surface complexation, hydroxyl exchange, flocculation, and coprecipitation were the main mechanisms involved in the Mo(VI) removal process by Al EC. Al EC outperformed conventional chemical coagulation in terms of Mo(VI) removal at the same dosage of Al. The Mo(VI) removal efficiencies in two real water samples (lake water and river water) reached up to 89.2% and 71.2%, respectively. This study provides novel insights into the strategies for the removal of oxoanionic metal pollutants and reduction of operating cost by Al EC technology.

Oily wastewater treatment by a continuous flow electrocoagulation reactor with polarity switch: Assessment of the relation between process variables and the aluminum released to the environment

Electrocoagulation with electrical polarity inversion was used to treat oil in water emulsions (145 ± 5 mg dm-3) using a cylindrical 4.8 dm3 reactor in continuous mode. The effects of spatial time and time between polarity inversion were explored using a three-level full factorial design (32), followed by Spearman correlation (ps), which has shown that the aluminum concentration in the treated effluent is not directly dependent on the mass of aluminum released by the electrodes. Nonetheless, the loss of mass of the electrodes is correlated (ps = 0.6970) to oil removal and to less electric power consumption (ps = -0.6909). Surface response analysis revealed that increasing the number of inversion cycles reduces electrode degradation. The treatment reduced the effluent's chemical oxygen demand by over 92.8%. Regarding environmental impact, there is an inverse statistical correlation between aluminum in the treated effluent and oil removal (ps = -0.7426), indicating that removing more oil with less environmental impact is possible. The better condition, considering oil removal and lower electrode consumption, was obtained with a spatial time of 36 min and a polarity inversion time of 10 s; for this condition, oil removal reached 87.0% with an energy expenditure of about 7.21 kW h.m-3.

Efficient removal of norfloxacin from water using batch airlift-electrocoagulation reactor: optimization and mechanisms analysis

In this study, we developed an airlift-electrocoagulation (AL-EC) reactor to remove norfloxacin (NOR) from water. Six parameters influencing NOR removal were investigated, and the possible removal mechanism was proposed based on flocs characterization and intermediates analysis. The performances for treating different antibiotics and removing NOR from 3 types of water were also evaluated. The best NOR removal efficiency was obtained with the iron anode and aluminum cathode combination, a current density of 2 mA cm-2, an initial pH of 7, a treatment time of 32 minutes and an air flow rate of 200 mL min-1, the supporting electrolyte type was NaCl, and the initial NOR concentration was 10 mg L-1. Flocs adsorption and electrochemical oxidation were the main ways to remove NOR from water. The average removal efficiency of the AL-EC reactor exceeded 60% of the different antibiotic concentrations in artificial and real water. The highest NOR removal rate reached 93.48% with an operating cost of 0.153 USD m-3. The present work offers a strategy for NOR removal from water with high efficiency and low cost, showing a huge potential for the application of the AL-EC in antibiotic contaminated water treatment.

Enhanced H2O2 formation and norfloxacin removal by electro-Fenton process using a surface-reconstructed graphite felt cathode: New insight into synergistic mechanism of defective active sites

The efficient catalytic activity and strong durability possibility of carbon-based three-dimensional fiber materials remains an important challenge in Electro-Fenton advanced oxidation technology. Graphite felt (GF) is a promising electrode material for 2-electron oxygen reduction reaction but with higher catalytic inertia. Anodizing modification of GF has been proved to enhance it electro-catalytic property, but the disadvantages of excessive or insufficient oxidation of GF need further improved. Herein, the surface reconstituted graphite felt by anodizing and HNO₃ ultrasonic integrated treatment was used as cathode to degrade norfloxacin (NOR) and the substantial role of different modification processes was essentially investigated. Compared with the single modification process, the synergistic interaction between these two methods can generate more defective active sites (DASs) on GF surface and greatly improved 2-electron ORR activity. The H₂O₂ can be further co-activated by Fe²⁺ and DASs into •OH_{(ads and free)} and •O₂^- to efficiently degrade NOR. The treated GF with 20 min anodizing and 1 h HNO₃ ultrasound had the highest electrocatalytic activity in a wide electric potential (-0.4 V to -0.8 V) and pH range (3-9) in system and the efficient removal rate of NOR was basically maintained after 5 cycles. Under optimal reaction conditions, 50 mg L^-1 NOR achieved 93% degradation and almost 63% of NOR was completely mineralized within 120 min. The possible NOR degradation pathways and ecotoxicity of intermediates were analyzed by LC-MS and T.E.S.T. theoretical calculation. This paper provided the underlying insights into designing a high-efficiency carbon-based cathode materials for commercial antibiotic wastewater treatment.

Challenges and Future Roadmaps in Heterogeneous Electro-Fenton Process for Wastewater Treatment

The efficiency of heterogeneous electro-Fenton technology on the degradation of recalcitrant organic pollutants in wastewater is glaringly obvious. This green technology can be effectively harnessed for addressing ever-increasing water-related challenges. Due to its outstanding performance, eco-friendliness, easy automation, and operability over a wide range of pH, it has garnered significant attention from different wastewater treatment research communities. This review paper briefly discusses the principal mechanism of the electro-Fenton process, the crucial properties of a highly efficient heterogeneous catalyst, the heterogeneous electro-Fenton system enabled with Fe-functionalized cathodic materials, and its essential operating parameters. Moreover, the authors comprehensively explored the major challenges that prevent the commercialization of the electro-Fenton process and propose future research pathways to countervail those disconcerting challenges. Synthesizing heterogeneous catalysts by application of advanced materials for maximizing their reusability and stability, the full realization of H2O2 activation mechanism, conduction of life-cycle assessment to explore environmental footprints and potential adverse effects of side-products, scale-up from lab-scale to industrial scale, and better reactor design, fabrication of electrodes with state-of-the-art technologies, using the electro-Fenton process for treatment of biological contaminants, application of different effective cells in the electro-Fenton process, hybridization of the electro-Fenton with other wastewater treatments technologies and full-scale analysis of economic costs are key recommendations which deserve considerable scholarly attention. Finally, it concludes that by implementing all the abovementioned gaps, the commercialization of electro-Fenton technology would be a realistic goal.

Graphical Abstract

Application of sono-electrocoagulation in arsenic removal from aqueous solutions and the related human health risk assessment

Among the contaminants found in groundwater, arsenic poses a great threat to human health and the ecosystem. Therefore, it is vital to eliminate arsenic from water sources. This study utilizes one of the most efficient and emerging decontamination techniques known as the sono-electrocoagulation method. In recent years, sono-electrocoagulation has attracted many scientists due to its unique features, such as being cost-effective, rapid process, and high efficiency. The required groundwater samples were artificially synthesized in the laboratory, where the anode and cathode were determined to be Fe, Ti/PbO2, and Al, respectively. During the experiment, the impact of pH (5,6,7,8), various initial concentrations (100, 200, 300,400, 500, 600 μg/l), exposure times of 5,10,15,20,25 min, electrode distances of 1.5,2,2.5,3,3.5 cm and different current intensities of 5,10,15,20,25 mA/cm2 were examined. The ambient temperature of the laboratory was kept at 30 and 40 °C. Furthermore, this study showed that the system containing Ti/PbO2 as the anode and Al as the cathode electrodes removed arsenic contamination more effectively in the base environment. The performance of arsenic removal was directly related to current intensity, pH, and time. Nevertheless, time elapse played a negative factor due to the corrosion of the electrodes' surface and the dissolution of floating materials in the solution. With the surge of arsenic concentration from 100 to 300 mg/L, the arsenic removal efficiency increased from 61.9 to 98.5 percent, where the maximum removal efficiency due to the rise of the current intensity was 84.16 percent. The sono-electrocoagulation method reduced the risk of carcinogenic and non-carcinogenicity from 5.15E-03 to 7.73E-05 and 26.71 to 0.40. Accordingly, it was found that a combination of ultrasonic and electrocoagulation processes is a promising approach for arsenic removal.

Synergistically Improved Catalytic Ozonation Process Using Iron-Loaded Activated Carbons for the Removal of Arsenic in Drinking Water

This research attempts to find a new approach for the removal of arsenic (As) from drinking water by developing a novel solution. To the author’s knowledge, iron-loaded activated carbons (Fe-AC) have not been previously applied for the removal of As in a synergistic process using ozonation and catalytic ozonation processes. The As was investigated using drinking water samples in different areas of Lahore, Pakistan, and the As removal was compared with and without using catalysts. The results also suggested that the catalytic ozonation process significantly removes As as compared with single ozonation and adsorption processes. Moreover, a feed ozone of 1.0 mg/min and catalyst dose of 10 g was found to maintain a maximum removal efficiency of 98.6% within 30 min. The results of the catalyst dose–effect suggested that the removal of As tends to increase with the increase in catalysts amount. Hence, it is concluded that the Fe-AC/O3 process efficiently removes As in water. Moreover, it was established that the Fe-AC/O3 process might be regarded as an effective method for removing As from drinking water compared to the single ozonation and adsorption processes.

Effect of additional Fe2+ salt on electrocoagulation process for the degradation of methyl orange dye: An optimization and kinetic study

The wastewater generated from textile industries is highly colored and contains dyes including azo dyes, which are toxic to human and water-living organisms. The treatment of these azo dyes using conventional treatment techniques is challenging due to their recalcitrant properties. In the current study, the effect of additional Fe²⁺ on electrocoagulation (EC) using Fe electrodes has been studied for the removal of methyl orange (MO) azo dye. pH between 4-5 was found to be optimum for EC and treatment efficiency decreased with increasing dye concentrations. With the addition of Fe²⁺ salt, dye removal for a certain concentration was increased with the increase of current density and Fe²⁺ up to a certain limit and after that, the removal efficiency decreased. The COD, color and dye removals were 88.5%, 93.1% and 100%, respectively, for EC of 200 mg.L⁻¹ dye solution using only 0.20 mmol.L⁻¹ Fe²⁺ for 0.40 mA cm⁻² current density, whereas for EC, the respective removal efficiencies were 76.7%, 63.4% and 82.4% for 32 min. The respective operating cost for EC was $768 kg⁻¹ removed dye ($0.342 m⁻³), whereas, for EC with additional Fe²⁺ salt, it was $350 kg⁻¹ removed dye ($0.189 m⁻³). The kinetic results revealed that the first-order kinetic model was fitted best for EC, whereas the second-order kinetic model was best fitted for Fe²⁺ added EC. For real textile wastewater, 57.6% COD removal was obtained for 0.15 mmol.L⁻¹ Fe²⁺ added EC compared to 27.8% COD removal for EC for 32 min. Based on the study we can conclude that Fe²⁺ assisted EC can be used for effective treatment of textile wastewater containing toxic compounds like azo dyes.

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EC represents limiting treatment performance for higher contaminant concentrations.

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0.20 mmol.L⁻¹ Fe²⁺ salt enhances the EC treatment performance of MO dye to 100%.

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EC followed first-order kinetic model, whereas Fe²⁺ added EC followed second-order kinetic model.

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Operating cost was reduced to $0.327 m⁻³ from $0.598 m⁻³ for EC with additional Fe²⁺.

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58% COD was removed for 0.15 mmol.L⁻¹ Fe²⁺ added EC for real textile wastewater.

Advances on removal of organophosphorus pesticides with electrochemical technology

Widespread use of organophosphorus pesticides (OPs), especially superfluous and unreasonable use, had brought huge harm to the environment and food chain. It is because only a small part of the pesticides sprayed reached the target, and the rest slid across the soil, causing pollution of groundwater and surface water resources. These pesticides accumulate in the environment, causing environmental pollution. Therefore, in recent years, the control and degradation of OPs have become a public spotlight and research hotspot. Due to its unique advantages such as versatility, environmental compatibility, controllability, and cost-effectiveness compatibility, electrochemical technology has become one of the most promising methods for degradation of OPs. The fundamental knowledge about electrochemical degradation on OPs was introduced in this review. Then, a comprehensive overview of four main types of practical electrochemical technologies to degrade pesticides were presented and evaluated. The knowledge contained herein should conduce to better understand the degradation of pesticides by electrochemical technology, and better exploit the degradation of pesticides in the environment and food. Overall, the objective of this review is to provide comprehensive guidance for rational design and application of electrochemical technology in the degradation of OPs for the safety of the environment and food chain in the future.

Removal of As(V) from wastewaters using magnetic iron oxides formed by zero-valent iron electrocoagulation

Electrocoagulation of zero-valent iron has been widely applied to the removal of dissolved arsenic, but the solid-liquid separation of arsenic-containing precipitates remains technically challenging. In this work, zero-valent iron was electrochemically oxidized to magnetic iron oxides for the removal of As(Ⅴ) from simulated and actual mining wastewaters. The results indicated that lepidocrocite was formed when zero-valent iron was oxidized by dissolved oxygen, but ferrihydrite and green rust were first formed and then transformed to magnetic iron oxides (mainly magnetite and maghemite) in the electrochemical oxidation from 0 to 0.9 V (vs. SCE), which facilitates the adsorption of As(V) and subsequent solid-liquid separation under a magnetic field. In simulated As(V)-containing solution with initial pH 7.0, zero-valent iron was electrochemically oxidized to magnetite and maghemite at 0.6 V (vs. SCE) for 2 h. The As(V) concentration first decreased from 5127.5 to 26.8 μg L^-1 with a removal ratio of 99.5%. In actual mining wastewaters, zero-valent iron was electrochemically oxidized to maghemite at 0.6 V (vs. SCE) for 24 h, and the As(V) concentration decreased from 5486.4 to 3.6 μg L^-1 with a removal ratio of 99.9%. The removal ratio of As(V) increased slightly with increasing potential, and increased first and then decreased with increasing initial pH. Compared with that of SO₄^2- and NO₃^-, the presence of Cl^- significantly enhanced the removal of As(V). This work provides a highly efficient, facile and low-cost technique for the treatment of arsenic-containing wastewaters.

How does arsenic speciation (arsenite and arsenate) in groundwater affect the performance of an aerated electrocoagulation reactor and human health risk?

Arsenic (As) occurrence in water resources has become one of the most critical environmental problems worldwide. The detrimental health impacts on humans have been reported due to the consumption of As-contaminated groundwater resources. Consumption of As-containing water over the long term can cause arsenicosis and chronic effects on human health due to its toxicity. Several treatment processes are available for As removals such as coagulation, ion exchange, adsorption, and membrane technologies but they have various major drawbacks. In the present work, therefore, an aerated electrocoagulation (EC) system with aluminum anodes was operated for simultaneous arsenate (As(V)) and arsenite (As(III)) removal to overcome the disadvantages of other processes such as, sludge formation, difficulties in operation, high operating costs, high energy consumption, and the requirement of pre-treatment process and to enhance the conventional EC process. The combined effects of the applied current (0.075-0.3 A), aeration rate (0-6 L/min), pH (6.5-8.5), and As speciation (As(V)-As(III)) were studied on As removal efficiency. The findings revealed that As removal mostly depended on the airflow rate and the applied current in the EC system. The highest As removal efficiency (99.1%) was obtained at an airflow rate of 6 L/min, a pH of 6.5, an initial As (V) concentration of 200 μg/L, and a current of 0.3 A, with an energy consumption of 2.85 kWh/m³ and an operating cost of 0.66 $/m³. The human health risk assessment of treated water was also examined to understand the performance of the EC system. At most of the experimental runs, the chronic toxic risk (CTR) and carcinogenic risk (CR) of As were within the permissible limits except for an airflow rate of 0-2 L/min, an initial pH of 8.5, and a current of 0.075-0.15 A for high initial As (III) concentrations. Overall, the As removal performance and groundwater risk assessment show that the EC process is a promising option for industrial applications.

Effects and behaviors of Microcystis aeruginosa in defluorination by two Al-based coagulants, AlCl3 and Al13

The complexity of natural water made it difficult to remove fluoride. Based on the environmental problems found in the investigation, the fluoride removal research in the water containing algal cells was carried out. In this study, AlCl₃ and [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃) were used to remove fluoride. Additionally, the role of aluminum speciation in fluoride removal and the effect of Microcystis aeruginosa on the fluoride removal by different aluminum species coagulants were elucidated. The results showed that AlCl₃ mainly removed fluoride by physical interactions, surface adsorption and enmeshment. When algal cells were added to the system, the fluoride removal rate increased from 22.75 % to 72.99 % at a dosage of 40.0 mg/L. This was because algal cells greatly increased the distribution of Al(OH)₃ in the flocs. In particular, the specific surface area of the flocs containing algal cells reached 160.77 m²/g, which allowed more fluoride to be adsorbed. However, excessive Al³⁺ led to serious damage to algal cells and release of intracellular organic matter (IOM), worsening the effect of defluoridation. F^- and Al³⁺ formed AlF²⁺ and AlF₂⁺ via complexation in water. These compounds were not conducive to defluoridation. Al₁₃ removed fluorine mainly through ion exchange, substitution and hydrogen bonding. Algal cells had an inhibitory effect on defluorination, which was observed in the process of coagulation by different Al dosages. Al₁₃ achieved agglomeration of algal cells and generated small and dense flocs through charge neutralization and electrostatic patch mechanism. Once Al₁₃ combined with algal cells and algae organic matter (AOM), the reaction between Al₁₃ and fluoride would be weakened. Al₁₃ not only maintained the defluoridation performance, but also did not damage the integrity of algal cells, even at high dosages.

Optimization of hybrid treatment of olive mill wastewaters through impregnation onto raw cypress sawdust and electrocoagulation

This research investigation proposes a new method for sustainable olive mill wastewater (OMW) treatment and handling. It is based on the combination of its impregnation onto raw cypress sawdust (RCS) followed by electrocoagulation. The retention of OMW compounds onto various RCS doses show an important decrease of its chemical oxygen demand (COD) and its main cation and anion content. The maximum retention efficiencies of COD, Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, [Formula: see text], and [Formula: see text] were about 51.0%, 75.3%, 28.7%, 77.9%, 84.7%, 41.1%, 98.3%, and 90.9%, respectively, for the highest RCS dose (200 g L^-1). This organic matter- and nutrient-loaded biomass could be thermochemically converted through pyrolysis into biofuel and biochar for energetic and agronomic purposes, respectively. The treatment by electrocoagulation of the pre-treated OMW using mild steel electrodes could be considered an attractive treatment method since 75.6% of COD removal efficiency was achieved. Besides, this approach permits a significant energy consumption reduction by 46% as compared with the electrocoagulation process alone. It allows also a significant improvement of the treated effluent quality in terms of both organic and mineral contents that could be reused for the irrigation of olive trees in the context of circular economy.

Arsenic and fluoride removal by electrocoagulation process: A general review

The environmental sector has expressed a growing interest in using electrocoagulation (EC) to treat groundwater/wastewater for drinking/recycling purposes. In the EC process, the electro-dissolution of sacrificial metallic anodes through direct application of current/cell potential dissolves the metals, which precipitate as oxides and hydroxides depending on the electrolyte pH. These particles have large surface areas and can remove pollutants by coagulation. The EC process has been considered an alternative technology due to its versatility, efficiency, low cost, and environmental compatibility. Unfortunately, the lack of knowledge about scaling-up this process has limited its implementation at the industrial scale. The aim of this study is to provide a review of the EC process used for removing arsenic and fluoride from groundwater and wastewater. Approximately 80 published studies were reviewed for this paper. The fundamentals of the EC process and importance of its operating conditions, i.e., electrode material, current density, supporting electrolyte, and pH, are reported in this paper. Additionally, overview of floc characterization and energy consumption are also presented. Finally, this paper also discusses the future perspectives.

Arsenic removal from groundwater using an aerated electrocoagulation reactor with 3D Al electrodes in the presence of anions

Co-occurrence of arsenic and anions in groundwater causes a severe health problems and combine effects of these pollutants significantly affect performance of treatment process. Thus, this study has been conducted to examine the combine effects of anions on arsenic removal using aerated electrocoagulation (EC) reactor with 3D Al electrodes in groundwater. A 3-level, six factors Box-Behnken experimental design (BBD) was applied to investigate the individual and combine effect of anions and operating time: phosphate (x₁: 1-10 mg L^-1), silica (x₂: 20-80 mg L^-1), bicarbonate (x₃: 130-670 mg L^-1), fluoride (x₄: 2-10 mg L^-1), boron (x₅: 5-10 mg L^-1), and operating time (x₆: 8-22 min) on desired responses. The specified responses were effluent arsenic concentration (C_f,As), removal efficiency of arsenic (R_e), consumptions of energy and electrode (ENC and ELC), operational cost (OC), and adsorption capacity (q_e). The optimum operating parameters predicted using BBD were found to be x₁: 1.0 mg L^-1, x₂: 26.0 mg L^-1, x₃: 651.5 mg L^-1, x₄: 2.0 mg L^-1, x₅: 9.9 mg L^-1, and x₆: 10.5 min considering highest removal efficiency of arsenic and lowest operational cost. Under these operating conditions, the experimental values of C_f,As, R_e, ENC, ELC, OC, and q_e were found to be 2.82 μg L^-1, 98.6%, 0.411 kWh m^-3, 0.0124 kg m^-3, 0.098 $ m^-3, and 17.65 μg As (mg Al)^-1, respectively. Furthermore, mathematical modelling was conducted using quadratic regression model and response surface analysis was performed to understand the relationship between independent parameters and responses.

Electrocoagulation for Arsenic Removal: Field Trials in Rural West Bengal

Arsenic contamination in drinking water is a great concern in different regions of the world as well as in India. Several technologies have been investigated to remove arsenic from water, such as coagulation and co-precipitation, ion exchange, adsorption, and reverse osmosis. In the present research, electrocoagulation with iron electrodes has been assessed as a treatment technology for arsenic removal from groundwater to reach concentrations below 0.01 mg/L (WHO limit) and which is technically effective, affordable for the local area, and easy to operate and maintain. Electrochemically generated iron is converted to hydrated ferric oxide within the contaminated water, which takes up the arsenic from water. A downstream filtration unit (sand or activated alumina) is applied to remove ferric hydroxide flocs produced during the process. The laboratory experiments were conducted in a batch reactor using iron plates as electrodes with monopolar configuration to study the effects of initial pH and electro-charge loading (ECL) on arsenic removal. The optimum operating condition was observed for an electro-charge loading of 25-30 Coulombs/L at pH 7.0 and an initial arsenic concentration of 0.2 mg/L. Two field trials were implemented in West Bengal after suitably designing the electrocoagulation system. Arsenic removal was significant (75-80%) delivering safe water with arsenic below 0.01 mg/L (acceptable limit). Passivation of the electrodes occurred during the operation and calcium-based (including iron) deposition was observed on the cathodes. Passivation is avoidable after running regular polarity reversal of the electrodes.

Arsenic Removal by Advanced Electrocoagulation Processes: The Role of Oxidants Generated and Kinetic Modeling

Arsenic (As) is a naturally occurring element in the environment that poses significant risks to human health. Several treatment technologies have been successfully used in the treatment of As-contaminated waters. However, limited literature has explored advanced electrocoagulation (EC) processes for As removal. The present study evaluates the As removal performance of electrocoagulation, electrochemical peroxidation (ECP), and photo-assisted electrochemical peroxidation (PECP) technologies at circumneutral pH using electroactive iron electrodes. The influence of As speciation and the role of oxidants in As removal were investigated. We have identified the ECP process to be a promising alternative for the conventional EC with around 4-fold increase in arsenic removal capacity at a competitive cost of 0.0060 $/m3. Results also indicated that the rate of As(III) oxidation at the outset of electrochemical treatment dictates the extent of As removal. Both ECP and PECP processes reached greater than 96% As(III) conversion at 1 C/L and achieved 86% and 96% As removal at 5 C/L, respectively. Finally, the mechanism of As(III) oxidation was evaluated, and results showed that Fe(IV) is the intermediate oxidant generated in advanced EC processes, and the contribution of •OH brought by UV irradiation is insignificant.

Nanofiltration for Arsenic Removal: Challenges, Recent Developments, and Perspectives

Arsenic (As) removal is of major significance because inorganic arsenic is highly toxic to all life forms, is a confirmed carcinogen, and is of significant environmental concern. As contamination in drinking water alone threatens more than 150 million people all over the world. Therefore, several conventional methods such as oxidation, coagulation, adsorption, etc., have been implemented for As removal, but due to their cost-maintenance limitations; there is a drive for advanced, low cost nanofiltration membrane-based technology. Thus, in order to address the increasing demand of fresh and drinking water, this review focuses on advanced nanofiltration (NF) strategy for As removal to safeguard water security. The review concentrates on different types of NF membranes, membrane fabrication processes, and their mechanism and efficiency of performance for removing As from contaminated water. The article provides an overview of the current status of polymer-, polymer composite-, and polymer nanocomposite-based NF membranes, to assess the status of nanomaterial-facilitated NF membranes and to incite progress in this area. Finally, future perspectives and future trends are highlighted.

Comparison between sinusoidal AC coagulation and conventional DC coagulation in removing Cu2+ from printed circuit board wastewater

A new Electrocoagulation (EC) technique, sinusoidal AC coagulation (SACC), is creatively put forward for Cu²⁺ removal in the wastewater from the printed circuit board (PCB) production in this paper. The removal efficiency of Cu²⁺ from PCB wastewater and energy consumption are compared by SACC and conventional direct current coagulation (DCC). The optimal process parameters were established through analysis of response surface methodology (RSM). The coagulations containing Cu²⁺ was characterized by SEM, EDS, TEM，BET, XRD and FTIR. The nano-ferrum collosol, mainly composed of goethite (α-FeOOH) and magnetite (γ-Fe₂O₃), absorbs the Cu²⁺ and coagulates to remove Cu²⁺. The results show that the removal rates of Cu²⁺ by SACC and DCC are 99.86％ and 98.21％, respectively, and the energy consumption is 2.76 × 10^-2 kWh⋅m^-3 for SACC and 4.42 × 10^-2 kWh⋅m^-3 for DCC under the optimal process conditions of c₀ (Cu²⁺) = 41.99 mg⋅dm^-3, pH = 7.14, j = 0.293 A⋅m^-2, t = 16.7 min. The pilot tests indicate that the SACC technique is feasible in industrial application. Cu²⁺ removal were completed through electrodeposition of Cu²⁺ on iron electrode, the deposition of Cu(OH)₂ and the adsorption of Cu²⁺ by ferrum collosol. The adsorption follows the pseudo-second order kinetics model well. The maximum saturated adsorption capacity (q_max) of Cu²⁺ on ferrum collosol produced by SACC is larger than that by DCC. The adsorption of Cu²⁺ on the ferrum collosol prepared by SACC and DCC are in accordance with Langmuir's adsorption isotherms. The novel SACC technique is a promising technique for the highly-efficient treatment of Cu²⁺ from PCB wastewater.

Arsenite removal from groundwater by aerated electrocoagulation reactor with Al ball electrodes: Human health risk assessment

The application of conventional electrocoagulation (EC) process for removal of As(III) from groundwater suffers from the need of external oxidation agent for oxidation of As(III) to As(V). To tackle this limitation, an aerated EC reactor for the removal of As(III) from groundwater was evaluated in this study. The effect of initial pH_i, air flow rate, applied current, and electrode height in the EC reactor was examined. The experimental results showed that removal of arsenic mostly dependent on the applied current, electrode height in EC reactor, and air flow rate. The As(III) removal efficiency (99.2%) was maximum at pH_i of 7.5, air flow rate of 6 L min^-1, applied current of 0.30 A, and electrode height in EC reactor of 5 cm, with an total operating cost of 0.583 $ m^-3. Furthermore, the carcinogenic risk (CR) and non-carcinogenic risk of arsenic (As) was in the range of tolerable limits at all operating conditions except applied current of 0.075 A at the end of the aerated EC process to remove As from groundwater. The present EC reactor process is able to remove As(III) from groundwater to below 10 μg L^-1, which is maximum contaminant level of arsenic in drinking water according to the World Health Organization (WHO).

Arsenic (III) Removal from a High-Concentration Arsenic (III) Solution by Forming Ferric Arsenite on Red Mud Surface

Arsenic (As) is considered one of the most serious inorganic pollutants, and the wastewater produced in some smelters contains a high concentration of arsenic. In this paper, we purified the high-concentration arsenic solution with red mud and Fe3+ synergistically. In this system, arsenite anions reacted with Fe(III) ions to form ferric arsenite, which attached on the surface of red mud particles. The generated red mud/Fe1−x(As)x(OH)3 showed a better sedimentation performance than the pure ferric arsenite, which is beneficial to the separation of arsenic from the solution. The red mud not only served as the carrier, but also as the alkaline agent and adsorbent for arsenic treatment. The effects of red mud dosage, dosing order, pH, and molar ratio of Fe/As on arsenic removal were investigated. The efficiency of arsenic removal increased from a pH of 2 to 6 and reached equilibrium at a pH of 7. At the Fe/As molar ratio of 3, the removal efficiency of arsenic ions with an initial concentration of 500 mg/L reached 98%. In addition, the crystal structure, chemical composition, and morphological properties of red mud and arsenic removal residues (red mud/Fe1−x(As)x(OH)3) were characterized by XRD, XPS, X-ray fluorescence (XRF), SEM-EDS, and Raman spectroscopy to study the mechanism of arsenic removal. The results indicated that most of the arsenic was removed from the solution by forming Fe1−x(As)x(OH)3 precipitates on the red mud surface, while the remaining arsenic was adsorbed by the red mud and ferric hydroxide.

An Electrochemical Process Comparison of As(III) in Simulated Groundwater at Low Voltage in Mixed and Divided Electrolytic Cells

A relatively low voltage can be favor of e- transfer and peroxide generation from dominant 2e--reduction of O2 on carbon materials as cathode, with low energy loss. In this study the conversion of As(III) in simulated high arsenic groundwater at low voltage was compared in a mixed and a anode–cathode separated electrolytic system. With applied voltages (the potential difference between cathode and anode) from 0.1 V to 0.8 V, As(III) was found to be efficiently converted to As(V) in the mixed electrolytic cells and in separated anodic cells. The complete oxidation of As(III) to As(V) at 0.1–0.8 V was also achieved on graphite in divided cathodic cells which could be long-running. The As(III) conversion process in mixed electrolytic cells, anodic cells and cathodic cells all conformed to the pseudo first-order kinetics equation. The energy consumed by As(III) conversion was decreased as the applied voltage declined. Low voltage electrolysis is of great significance for saving energy consumption and improving the current efficiency and can be applied to in-situ electrochemical pre-oxidation for As(III) in high arsenic groundwater.

Environmental exposure of arsenic and fluoride and their combined toxicity: A recent update

Environmental exposure to arsenic (As) and fluoride (F) in the recent year has been increased because of excessive use of naturally contaminated ground water. Surface water is also regularly contaminated with these elements in various industrial areas. Arsenicosis and fluorosis upon individual exposure of As and F are reported in many studies. A syndrome of endemic As poisoning and fluorosis occurs during concurrent exposure of As and F. Previous reports showed synergistic, antagonistic and independent effects of these two compounds, although few recent reports also revealed antagonistic effects after co-exposure. Interaction during intestinal absorption and influence of F on As metabolism might be the cause of antagonism. The synergism/antagonism is thought to depend on the dose and duration of the co-exposure. However, the detailed mechanism is still not fully understood and needs further studies. Removal technologies of As and F from contaminated water is available but removal of such contaminants from food is yet to be developed. Antioxidants are useful to mitigate the toxic effects of As and F. This review focused on the effect of co-exposure, amelioration as well as removal techniques of As and F.

Treatment of re-refining effluent from lubricating oils by combining electrocoagulation and coagulation-flocculation processes

A combination of electrocoagulation and coagulation-flocculation processes was used for re-refining effluent from lubricating oils. The efficiency of the process was evaluated based on the chemical oxygen demand (COD), color, and turbidity of the refined effluent. Electrocoagulation (EC) and coagulation-flocculation parameters, such as the initial pH (3.00, 4.41, and 9.00), and current density (4, 9, and 16 A/m²), and the use of aluminum polychloride coagulant and superfloc A300 flocculant were studied. EC performed at pH 9, with a current density of 16 A/m² and 7 V, resulted in removal efficiencies of 85.14%, 99.81%, and 99.85%, for COD, color, and turbidity, respectively. The removal efficiencies increased to 96%, 99.87%, and 99.94% for COD, color, and turbidity, respectively, by the further coagulation-flocculation treatment in the presence of 13.8 mg/L aluminum polychloride coagulant and 80 mg/L Superfloc A300 flocculant.

Emerging investigator series: interdependency of green rust transformation and the partitioning and binding mode of arsenic

We investigated the impact of aging-induced structural modifications of carbonate green rust (GR), a mixed valent Fe(ii,iii) (hydr)oxide with a high oxyanion sorption affinity, on the partitioning and binding mode of arsenic (As). Suspensions of carbonate GR were produced in the presence of As(v) or As(iii) (i.e. co-precipitated with As(iii) or As(v)) and aged in anoxic and oxic conditions for up to a year. We tracked aqueous As over time and characterized the solid phase by X-ray absorption spectroscopy (XAS). In experiments with initial As(v) (4500 μg L-1, As/Fe = 2 mol%), the fresh GR suspension sorbed >99% of the initial As, resulting in approximately 14 ± 8 μg L-1 residual dissolved As. Anoxic aging of the As(v)-laden GR for a month increased aqueous As to >60 μg L-1, which was coupled to an increase in GR structural order revealed by Fe K-edge XAS. Further anoxic aging up to a year transformed As(v)-laden GR into magnetite and decreased significantly the aqueous As to <2 μg L-1. The As binding mode was also modified during GR transformation to magnetite from sorption to GR particle edges to As substitution for tetrahedral Fe in the magnetite structure. These GR structural modifications altered the ratio of As partitioning to the solid (μg As/mg Fe) and liquid (μg As per L) phase from 2.0 to 0.4 to 14 L mg-1 for the fresh, month, and year aged suspensions, respectively. Similar trends in GR transformation and As partitioning during anoxic aging were observed for As(iii)-laden suspensions, but occurred on more rapid timescales: As(iii)-laden GR transformed to magnetite after a day of anoxic aging. In oxic aging experiments, rapid GR oxidation by dissolved oxygen to Fe(iii) precipitates required only an hour for both As(v) and As(iii) experiments, with lepidocrocite favored in As(v) experiments and hydrous ferric oxide favored in As(iii) experiments. Aqueous As during GR oxidation decreased to <10 μg L-1 for both As(v) and As(iii) series. Knowledge of this interdependence between GR aging products and oxyanion fate improves biogeochemical models of contaminant and nutrient dynamics during Fe cycling and can be used to design more effective arsenic remediation strategies that rely on arsenic sorption to GR.

Fluoride removal from natural volcanic underground water by an electrocoagulation process: Parametric and cost evaluations

Excessive fluoride content in groundwater can cause serious risks to human health, and sources of groundwater intended for human consumption should be treated to reduce fluoride concentrations down to acceptable levels. In the particular case of the island of Tenerife (Canary Islands, Spain), the water supply comes mainly from aquifers of volcanic origin with a high content of fluorides that make them unacceptable for human consumption without prior conditioning treatment. The treatments that generate a high rejection of water are not acceptable because water is a scarce natural resource of high value. An electrocoagulation process was investigated as a method to treat natural groundwater from volcanic soils containing a hazardously high fluoride content. The operating parameters of an electrocoagulation reactor model with parallel plate aluminum electrodes were optimized for batch and continuous flow operations. In the case of the batch operation, acidification of the water improved the removal efficiency of fluoride, which was the highest at pH 3. However, operation at the natural pH of the water achieved elimination efficiencies between 82 and 92%, depending on the applied current density. An optimum current density of 5 mA/cm² was found in terms of maximum removal efficiency, and the kinetics of fluoride removal conformed to pseudo-second-order kinetics. In the continuous-flow operation, with the optimal residence time of 10 min and a separation of 0.5 cm between the electrodes, it was observed that the current density that would be applied would depend on the initial concentration of fluoride in the raw water. Thus, an initial fluoride concentration of 6.02 mg/L required a current density >7.5 mA/cm² to comply with the legal guidelines in the product water, while for an initial concentration of 8.98 mg/L, the optimal current density was 10 mA/cm². Under these operating conditions, the electrocoagulation process was able to reduce the fluoride concentration of natural groundwater to below 1.5 mg/L according to WHO guidelines with an operating cost between 0.20 and 0.26 €/m³ of treated water.

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.

Efficiency analysis of the electrocoagulation and electroflotation treatment of poultry slaughterhouse wastewater using aluminum and graphite anodes

The application of electrocoagulation (EC) and electroflotation (EF) was investigated for the treatment of poultry slaughterhouse wastewater in a bench scale unit cell electrolyzer with different EC-to-EF ratios at current densities of 3, 9, and 15 mA cm^-2. The EC-to-EF ratio was controlled by current reversal using aluminum and graphite electrodes. The electrochemical treatment showed satisfactory removal efficiencies for Al coagulant loads greater than 51.8 mg L^-1. The 4/5 EC to EF ratio (69.1 mg L^-1 Al and 32.2 NmL L^-1 additional EF gas) and 3/5 (51.8 mg L^-1 Al/64 NmL L^-1 additional EF gas) presented the best results for the removal of COD (76-85%), color (93-99%), and turbidity (95-99%), with the additional benefit of reducing the electrode consumption and sludge disposal costs proportionally to the EC-to-EF ratio. The effects of the EC-to-EF ratio and the current density on efficiency of the electrochemical treatment for the removal of COD, apparent color, turbidity, TSS, TSD, and NH₃-N were discussed in the light of the physicochemical and electrochemical processes underlying the removal mechanism for each parameter. In particular, the blow-off mechanism seems to play an important role in the NH₃-N removal, whereas indirect electrooxidation mechanism accounts for a fraction of the soluble COD removal for the electrodes configuration used in the treatment.

House hold unit for the treatment of fluoride, iron, arsenic and microorganism contaminated drinking water

A first of its kind hybrid electrocoagulation-filtration prototype unit was fabricated for the removal of fluoride, iron, arsenic and microorganisms contaminated drinking water. The unit comprised of 3 chambers, chamber A consisting of an inlet for the water to be treated and an outlet for the treated water along with one block of aluminum electrodes. Chamber B consisted of ceramic membrane filtration assembly at the bottom over a metallic support which filters the flocs so produced in chamber A and chamber C consisting of space to collect the treated water. Operating parameters were maintained as current density of 625 A m^-2 and an electrode distance of 0.005 m. Contaminated drinking water containing mixture of fluoride (10 mg L^-1), iron (25 mg L^-1), arsenic (200 μg L^-1) and microorganisms (35 CFU ml^-1) was used for the experiment. A removal of 98.74%, 95.65%, 93.2% and 100% were obtained for iron, arsenic, fluoride and microorganisms, respectively. The apparatus and method made it possible to efficiently treat contaminated drinking water to produce drinkable water as per WHO specification. By-products obtained from the electrocoagulation bath were analyzed using SEM, EDX and XRD and explained.