# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Separation-Based Technologies
#### 4.1.5 Capacitive Deionization  
Capacitive deionization (CDI) is another emerging technology for the removal of ionic pollutants from waterbodies [82; 83]. The basis of the CDI process is the removal of dissolved ions by the application of a constant potential between two porous electrodes immersed in water. By polarizing the electrodes, one positively and the other negatively, the ions move from the solution to the electrode surface: the cations migrate to the cathode and the anions to the anode [84]. The salt ions are electrosorbed by pore filling the surface of the electrodes and afterwards released into a concentrated brine solution by shorting the system or switching the voltage. Therefore, it can be stated that by means of CDI, deionization is carried out and simultaneously, energy storage is produced. This fact is one of the key aspects of capacitive deionization since it allows recovering part of the stored energy as a supercapacitor by means of a discharge stage, also called regeneration [85]. The name regeneration refers to the fact that the electrodes, by means of discharge, desorb the ions, leaving them in a state equivalent to the initial state, without ions electrochemically adsorbed on their surface. In CDI, low voltage (<1.5 V) must be applied to avoid water hydrolysis and Faradaic reactions, causing a decrease in energy efficiency, desalination yield, and electrode lifespan [83]. In terms of energy requirement, CDI may compete cost-effectively with membrane-based technologies when operating with water streams of medium or low concentrations. [84]. The nitrate ion is characterised by its planar configuration with a tendency to solvate with H\({}_{2}\)O molecules [86], which facilitates its inclusion in the slit-shaped micropores [87; 88] present in carbonaceous electrodes used in CDI. Thus, CDI eliminates more than 90% of the nitrate ions from brackish groundwaters in only 15 min. [89]. To enhance the processing efficiency, the electrodes used in CDI can be activated by using phosphoric acid solutions that increase the number of mesopores and hence their specific surface area [90]. To improve their selectivity for nitrate ions, the electrodes can also be treated with surfactants such as cetyl-trimethyl-ammonium bromide, or anion exchange resins [20; 91]. Conventional CDI with carbonaceous electrodes tends to have a significantly lower energy efficiency than expected due to co-ion expulsion [92]. This phenomenon occurs when ions of the same polarity as that of the electrode interfere by being adsorbed and desorbed from the pores continuously, which thereby reduces the number of counterions that may be eliminated from the water stream [82]. To ameliorate these issues, membrane capacitive deionization (MCDI), as a derivation of the original CDI, has been receiving increasing attention in recent years. In MCDI, a cation exchange membrane is arranged over the cathode, and an anion exchange membrane over the anode, which serves to inhibit the respective transport of anions and cations. This is to reduce the cited co-ions effects and enhance the salt removal rate, as well as to provide a protection barrier to prevent adverse redox reactions on the electrode surface [93]. The MCDI treatment of contaminated groundwater was able to reduce nitrate levels from 233 mg/L to 4.7 mg/L with a low potential (0.86 V) and reduced energy demand (0.4 kWh/m\({}^{3}\)) [94]. A novel approach referred to as catalytic capacitive deionization (CCDI), which combines targeted heterogeneous catalysis with CDI to substantially minimize residual waste products, has recently been announced [95]. In CCDI, porous electrodes containing catalytic In and Pd nanoparticles are used to achieve electrostatic adsorption of nitrate anions and transform them into innocuous N\({}_{2}\). This CCDI equipment, working at a potential of 1.5 V, eliminated 92% of the nitrate present in the feed water, with a successful selectivity of 91% to N\({}_{2}\) of the adsorbed nitrate. [95].