# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 2 A Brief Review of Electrochemical Methods  
Electrocoagulation (EC) is currently an efficient technology in wastewater treatment [13]. EC is a process involving the in situ generation of coagulants such as aluminum/iron ions and corresponding hydroxides from the electrical dissolution of \"sacrificed anodes\" under direct electrical currents [14]. Generated iron or aluminum flocs serve as coagulants and subsequently precipitate HMs, and an aluminum plate, steel/iron plate/mesh, and Ti plate/mesh are applied as anode and cathode materials to remove Cr(VI), Cu(II), Ni(II), Cd(II), Pb(II), Zn(II), Ba(II), and As(III) [15; 16; 17; 18; 19; 20; 21; 22; 23]. The EC process can dispose of multiple contaminants in one process; has low sludge production and maintenance costs; can easily handle water quality variations; and can achieve over 95% removal efficiency for HMs. [15; 18; 22]. The disadvantages of EC are as follows: (1) The dissolution of \"sacrificial electrodes\" into wastewater occurs due to oxidation, which requires frequent disposal. (2) The passivation of electrodes over time has limited its implementation. (3) The use of electricity may be expensive in many places. (4) High conductivity of the wastewater suspension is required [24; 25; 26]. Figure 1: Electrochemical methods for heavy metal removal. Electrofolotation (EF) is suitable for separating complex heavy metal wastewater with heterotic contaminants such as sludge. In this process, water is split into oxygen and hydrogen electrodes. Gas bubbles are generated, and the suspended contaminants are raised to the water surface. In this complicated process, bubble size and gas generation rate are two crucial factors in metal efficiency, deeply dependent on the current density used [27]. EF is gaining interest because its system produces a fine gas bubble without turbulence, the bubbles exhibit good coverage of the whole area, and the system is suitable for full automatization. The EF process has been applied in the food industry; oily wastewater; chemical wastewaters; effluents from the sugar and leather industries; and farms as well as in the dairy industry, and thus has been successfully commercialized [28]. Gas bubbles are the workhorse of EF, and bubble size is the key parameter for determining efficiency. Recently, a coupled method combining EF and EC to remove and separate heavy ions has been scaled up in industry [29]. In such a process, metal ions are coagulated and floated over the water surface [30]. The drawbacks of EF are the insufficiently high productivity of its units; the release of H\({}_{2}\) bubbles; the cost of electrodes and maintenance; and the formation of sludge by volume [31]. The EF system relies on hydrogen and oxygen/chloride produced at the cathode and anode, respectively, which are referred to as the hydrogen evolution reaction and oxygen/chloride evolution reaction, respectively. These processes are particularly energy-consuming due to the water-splitting process, requiring a potential over 1.5 V even when using the best-developed catalysts. Presently, the most widely applied anode materials in EF systems are stainless steel, nickel plates, titanium, and carbon cloths due to their cheap price and relatively good HER ability [32; 33]. Anode materials should be chemically stable since the electrodes are working at high potentials (1-3 V). Iron, aluminum, and stainless steel are cheap but suffer from corrosion [34]. Graphite and other carbon-based materials are relatively stable but exhibit high overpotential for OER [35]. Dimensionally stable anodes (DSA) are the most attractive anodes for EF [36; 37]. The electrodes are developed using conductive precious RuO\({}_{2}\), IrO\({}_{2}\), and their composite metal oxides as the electrocatalysts, normally using TiO\({}_{2}\), Nb\({}_{2}\)O\({}_{5}\), Ta\({}_{2}\)O\({}_{5}\), and ZrO\({}_{2}\) as the stabilizing agents supported on a Ti-, Ta-, W-, etc. -based metal substrate [37; 38].