High-efficiency As(III) oxidation and electrocoagulation removal using hematite with a charge-discharge technique

¹Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River), Ministry of Agriculture and Rural Affairs, Hubei Key Laboratory of Soil Environment and Pollution Remediation, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China.

|November 28, 2019

Summary

This study explored a new method for removing arsenite (As(III)) from water using hematite electrodes in a charge-discharge system. The researchers found that periodic redox reactions in hematite may prevent electrode passivation and enhance As(III) oxidation. Chloride intermediates and the counter electrode played a key role in converting As(III) to As(V), which was then adsorbed onto ferrihydrite to form precipitates. The method achieved high removal efficiency, especially at pH 7.0 and with 15 mg of hematite. The system may also function as a supercapacitor, offering dual benefits of water treatment and energy storage. This approach may provide a more efficient and sustainable solution for arsenic removal compared to traditional electrocoagulation methods.

Area of Science:

Environmental chemistry and water treatment
Electrochemical remediation techniques
Heavy metal removal in aquatic systems

Background:

Arsenite removal typically requires oxidation to arsenate followed by adsorption or precipitation. Electrocoagulation is a promising method for arsenic removal but faces challenges like electrode passivation and high energy use. Prior research has shown that adsorption and coprecipitation are common approaches, but they rely on prior oxidation. No prior work had resolved the issue of electrode passivation during electrochemical arsenic removal. This gap motivated the exploration of alternative electrochemical strategies. The need for a sustainable and efficient method remains unmet. The role of chloride intermediates in oxidation is not well established. This study addresses these limitations. The potential of hematite in charge-discharge systems had not been fully explored.

Purpose Of The Study:

The aim of this study was to evaluate a charge-discharge technique using hematite for As(III) oxidation and removal. The researchers propose that this method could improve efficiency and reduce electrode passivation. The specific problem is the inefficiency of traditional electrocoagulation for arsenic removal. The motivation is to develop a sustainable and cost-effective solution. The study focuses on hematite's role in periodic redox reactions. The researchers suggest that this could avoid electrode degradation. The method also aims to utilize the system as a supercapacitor. This approach may offer a dual benefit of water treatment and energy storage.

Main Methods:

The study used a microwave-assisted hydrothermal reaction to prepare hematite electrodes. A multi-cycle galvanostatic charge-discharge technique was applied in As(III) solutions with NaCl as the electrolyte. The potential window was set between -0.8 and 0 V relative to the standard calomel electrode. The role of ClO⁻ intermediates and the counter electrode was analyzed. The pH of the solution was varied to assess its impact on removal efficiency. The hematite mass was adjusted to 4, 10, and 15 mg for testing. The number of charge-discharge cycles was set at 600. The formation of FeAsO₄ precipitate was monitored to evaluate As(V) adsorption.

Main Results:

The highest As(T) removal ratio of 98.6% was achieved at 15 mg hematite after 600 cycles. At pH 7.0, the removal efficiency was higher than at pH 5.0 and 9.0. The charge-discharge process promoted periodic redox reactions in hematite. ClO⁻ intermediates and the counter electrode at high potential contributed to As(III) oxidation. FeAsO₄ precipitate formed from As(V) adsorption on ferrihydrite. The system functioned as a supercapacitor during power output. Electrode passivation was effectively avoided in this setup. The results suggest that this method may outperform traditional electrocoagulation.

Conclusions:

The authors propose that the charge-discharge technique using hematite may improve As(III) removal efficiency. The periodic redox reactions may prevent electrode passivation. The system's dual role as a supercapacitor may enhance its practicality. The highest removal ratio was observed at 15 mg hematite and pH 7.0. The role of ClO⁻ intermediates and the counter electrode was significant. The method may offer a novel strategy for arsenic immobilization. The findings suggest that this approach may be more effective than conventional electrocoagulation. The study highlights the potential of hematite in electrochemical water treatment.

Keywords:

Arsenic Electrochemical removal Electrocoagulation FeAsO(4)Hematite Redox arsenic removal methods electrochemical water treatment hematite electrode applications charge-discharge water purification

Frequently Asked Questions

The authors propose that As(III) is oxidized to As(V) via ClO⁻ intermediates and high-potential counter electrode reactions. As(V) is then adsorbed onto ferrihydrite, forming FeAsO₄ precipitate.

Hematite undergoes periodic redox reactions during each charge-discharge cycle, which may prevent electrode passivation and enhance As(III) oxidation.

The researchers suggest that NaCl facilitated the formation of ClO⁻ intermediates, which contributed to As(III) oxidation in the charge-discharge process.

The electrochemical system may store and release energy during the charge-discharge cycles, potentially allowing it to function as a supercapacitor for power output.

The highest removal ratio was observed at pH 7.0, suggesting that solution pH significantly influences As(III) oxidation and As(V) adsorption.

The authors propose that this method may offer a novel and efficient strategy for As(III) immobilization and removal from aqueous solutions.

Related Concept Videos