Continuous electrochemical deionization by utilizing the catalytic redox effect of environmentally friendly riboflavin-5'-phosphate sodium

https://doi.org/10.1016/j.mtcomm.2020.100921Get rights and content

Highlights

  • The environmentally friendly and non-toxic FMN-Na was used as redox catalyst.

  • The redox-catalysis deionization can continuously work.

  • The 100 ppm water product can be obtained from brackish water with 98.1 % removal efficiency.

  • The active electrode regeneration is not required.

Abstract

In electrochemical desalination, an efficient ions transfer between positive and negative electrodes is necessary and highly desirable. Herein, a redox media, edible riboflavin-5′-phosphate sodium salt (FMN-Na), is successfully applied to carry out continuous desalination as flow electrolyte in a desalinated device. The desalination process is achieved by trapping Na+/Cl at positive/negative steams respectively with help of cation/anion exchange membranes. The removal efficiency can be up to 98.1 %. The salt removability and consumption rate are investigated under different current densities and salt concentrations. The flow rate of electrolyte and cyclic-ability are also examined. This green and pollution-free electro-catalytic technology will offer great potential for the safe and effective seawater desalination.

Introduction

With global warming and the climate change, the evolution of land desertification and freshwater shortage has become severe. Growing of population and the development of industrialization undoubtedly speed up the freshwater storage. Nearly 97.5 % of the world's total water resource is ocean water, and more than 70 % of the world's population lives within 70 km along the coastline. Since 1950s, desalination has been considered as the most practical method of sustainably providing fresh water sources and to remediate the enlarging in demands of fresh water. The desalination technology currently used in large-scale industrial applications includes multi-stage flashing, reverse osmosis, capacitive deionization and electrodialysis deionization [1]. Multi-stage flash is thermal treatment process with the advantages of high water quality, safe and reliable operation. However, acid and a scale inhibitor are required to do the regeneration. The thermal power consumption is large, and corrosion occurs in the system in previous developed methods [2]. RO is widely used in industry all over the world, however the energy consumption is still considered high [3]. Recently, CDI has been paid much attention as an emerging desalination technology based on the electrical double layer absorption from high-surface-area electrode materials such as carbon [4]. Conventional CDI [5,6], membrane capacitance deionization (MCDI) [7,8], hybrid capacitive deionization (HCDI) [9,10] and flow-electrode capacitive deionization (FCDI) [[11], [12], [13]] are widely investigated. When electrically applied, cation/anion can be electrostatically absorbed by the negative/positive electrodes respectively, resulting in deionization process [14]. Lots of research works were focused on electrode materials such as improvement on the surface area and conductivity in the last decade. The desalination technology with commercial potential possesses the embodiment of high salt removal capacity, economic energy consumption, and low maintenance cost. However, the desalination ability of CDI devices is not particularly desirable because of the limited capacitance of these carbon materials. In addition, another restriction of the conventional CDI or Faradic CDI is it can only work in intermittent desalination/salination exchanged mode [15]. In the practical application, the continuous process is highly demanded. Therefore, in our latest research work, an electrocatalytic redox desalination technique using TEMPO/TEMPO+ was proposed, which enables continuous desalination by the circulation of redox species between the electrode reservoirs [16]. However, these chemicals are potentially toxic in some extent (Oral-category 4, skin irritation-category 2, eye irritation-category 2A, specific target organ toxicity-category 3). Therefore, utilizing cheap and nontoxic candidate is necessary to develop desalination technology. FMN-Na has been proposed as a versatile electroactive molecule that catalyze diverse redox reactions in various biological organizations [17,18], and redox flow battery [17,[19], [20], [21]]. It serves as a cofactor in many enzymes in tissue and cell. In this current work, as a great importance to be environmentally friendly in desalination process, FMN-Na is utilized as a nontoxic redox mediator to achieve the continuous electrochemical deionization by circulating the electrode material between positive and negative electrodes. The salt removal efficiency up to 98.1 % can be attained by applying FMN-Na as flow electrolyte. By controlling the current density and salt feed concentrations, the desalination performance is examined. The cyclability and flow rate are also performed. In addition, energy consumption and salt feed concentration and other important electrochemical tests are also performed. This study suggests that pollution-free electro-catalytic technology will be huge significant for the future low cost and safe desalination.

Section snippets

Material and experimental method

The detailed preparation, device structure and desalination setup, electrochemical tests are provided in the supplementary information

Results and discussion

Continuous desalination of ED was shown in Fig. 1a. FMN-Na at the positive stream gains electron while the generation of FMN-Na at negative chamber. The sodium ions in stream A transport to positive chamber through CEM while the chloride ions move to stream B, occurring a salt removal in stream A. With the acceptance of sodium ions from negative chamber through CEM, the salt content in stream B becomes concentrated. Overall, the salt in stream A is removed to stream B, and the constituents of

Conclusion

The edible FMN-Na is utilized firstly for continuous desalination process in the ED cell that consists of the flow FMN-Na between positive and negative electrodes, two salt feeds, separated by two CEMs and one AEM. The electrochemical catalytic function of FMN-Na achieves an uninterrupted desalination effect, reducing the salt feed from 5200 ppm to 100 ppm drinking water level. A series of experiments were designed to explore the influence of applied current density, feed concentration, and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This project was supported by Outstanding Young Scholar Project (8S0256), and the Scientific and Technological Plan of Guangdong Province (2018A050506078), and the Project of Blue Fire Plan (CXZJHZ201709).

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