# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 1 Introduction  
Industrialization brings about the rapid transformation of manufacturing and leads to higher productivity, raising living standards. However, this process also brings threats to human beings such as the pollution of air, water, and soil, which can result in significant deterioration of quality of life and life expectancy [1]. Heavy metals (HMs) are regarded as one of the most important categories of pollutants and have caused wide concern due to their high toxicity and bioaccumulative properties. The bioaccumulative process leads to the pollution of air, water, and soil and eventually leads to adverse effects on humans. HMs have no rigorous definition on a scientific and chemical basis [2]. Generally, HMs have a density of over 5 g cm\({}^{-3}\) (5 times \(>\) H\({}_{2}\)O), meaning a metal with a density below this lacks heavy metal chemistry. Thus, HM properties are partially related to density as well as their chemical properties; they contain transition and post-transition elements along with metalloids, namely arsenic and selenium. Moreover, HMs are regarded as \"toxic elements\" priority contaminants listed by the United States Environmental Protection Agency [3]. A large number of HMs, with the exceptions of Cd, Hg, and Pb, are also essential micronutrients for living organisms; they only become toxic when the concentration is over the limit [4; 5]. The dosage that determines either HM toxicity or micronutrient properties is the critical criterion with which to identify the toxicity of HMs [6].  
Sources of HMs can be natural such as landslides, weathering, and volcanic eruptions, but they mainly originate from anthropogenic processes such as mining, agriculture, electroplating fossil fuel combustion, and waste disposal. For instance, agricultural use of HMs in the form of pesticides and fertilizer as well as corrosion and industrialization releases HMs into the environment [7; 8]. Untreated wastewater containing heavy metals discharged into the natural environment can pollute water bodies and soil, endangering food safety and human health through the food chain [8]. An increase in concern for public health has led to the enaction of varied legislation to control the emission of HMs [9]. This increase in public health concern has led to the enaction of varied HM emissions controllegislation. This leads to an eruption of interest in developing new technologies to control emissions. Widely applied technologies for HM removal are based on solid-phase extraction using precipitation, coagulation, and flocculation [10]. However, a major concern with this technology is its output of heavy metal solids, which become solid waste. Conversely, liquid-phase extraction based on new technologies such as adsorption, ion exchange, and membrane separation has gained much interest because these processes concentrate metal ions directly devoid of a huge amount of solid heavy metal-containing waste. However, new advanced liquid phase extraction methods are highly dependent on emerging new materials such as adsorbents and ion exchangers, membranes, and electrochemical adsorption materials. [11; 12]. In the past several decades, great efforts have been made toward developing new materials for heavy metal removal. With the exception of adsorption and membrane filtration processes, electrochemical treatments for HM removal are gaining increasing interest. These methods use electrostatic fields to dry metal ions and enhance precipitation, adsorption, and separation. And this paper reviews the electrochemical treatment technology of heavy metal wastewater, which can provide a reference for the development of efficient and energy-saving heavy metal wastewater treatment technology. Figure 1 summarizes electrochemical methods for HM removal. These electrochemical methods can be classified into electrocoagulation, electroflotation, electrodialysis, and electrodeposition depending on their mechanism.