Efficient groundwater defluorination over a wide concentration gradient through capacitive deionization with a...

Area of Science:

• Environmental Science & Engineering
• Materials Science
• Electrochemistry

Background:

• Groundwater fluoride pollution is a significant environmental concern.
• Capacitive deionization (CDI) is a promising technology for defluorination, but faces challenges with electrode performance at varying fluoride concentrations.
• Existing electrodes exhibit poor salt adsorption capacity (SAC) due to electrical double-layer (EDL) overlap at low concentrations and co-ion effects at high concentrations.

Purpose of the Study:

• To develop a novel three-layer structured electrode for enhanced capacitive deionization (CDI) performance in groundwater defluorination.
• To overcome the limitations of existing electrodes, such as EDL overlap and co-ion effects, across a wide range of fluoride concentrations.
• To improve the salt adsorption capacity (SAC) and stability of CDI electrodes for efficient fluoride removal.

Main Methods:

• Fabrication of a three-layer electrode: membrane/carbon nanotube (CNT)/activated carbon (AC), termed CNT-MCE, via electrospinning CNT onto AC followed by polymer membrane coating.
• Evaluation of electrode performance using capacitive deionization (CDI) experiments with varying sodium fluoride (NaF) concentrations.
• Characterization of electrode structure and analysis of EDL overlap and co-ion effects.
• Testing stability and performance in simulated groundwater defluorination.

Main Results:

• The CNT-MCE electrode demonstrated a significantly higher SAC compared to AC and membrane-coated electrodes, achieving 40.8 mg g^-1 at 100 mg L^-1 NaF.
• At high NaF concentration (1500 mg L^-1), CNT-MCE showed an improved SAC of 58.8 mg g^-1 by effectively inhibiting co-ion effects.
• The electrode maintained excellent stability and high SACs (200-800 mg L^-1 NaF) over a wide concentration gradient, resisting oxidation.
• Successfully reduced fluoride concentration in simulated groundwater from 3.4 to 1.1 mg L^-1.

Conclusions:

• The developed membrane/carbon nanotube/activated carbon (CNT-MCE) electrode offers an efficient strategy for broadening the applicability of capacitive deionization (CDI) for groundwater defluorination.
• The mesopore-dominated structure and unique design of CNT-MCE effectively mitigate EDL overlap and co-ion effects, leading to superior SAC.
• CNT-MCE exhibits excellent stability and performance across a wide range of fluoride concentrations, making it a viable solution for environmental remediation.