In water treatment and industrial resource recovery, conventional separation methods like reverse osmosis (RO) and electrodialysis (ED) encounter operational challenges including high energy consumption, membrane fouling, and low removal efficiencies for trace heavy metal species. Capacitive Deionization (CDI) has emerged as an electrochemical water purification technology. Offering low operational energy requirements, freedom from chemical regenerants, and resistance to particulate fouling, CDI represents an approach for brackish water desalination and industrial wastewater recycling. The operating principle of CDI relies on electrostatic ion sorption within the Electric Double Layer (EDL) formed at the interface of porous electrodes under an applied electric field. Reversing or removing the cell voltage causes the adsorbed ions to desorb into a concentrate stream, regenerating the electrode matrix.

The operational capacity of a CDI system is dictated by the electrochemical properties of its porous electrode materials. An ideal CDI electrode substrate must combine high electrical conductivity, an accessible specific surface area, surface wettability, and an optimized pore size distribution. Conventional activated carbons or carbon black powders often exhibit poor intra-particle conductivity and contain closed or tortuous micropores that restrict ion ingress, yielding experimental salt adsorption capacities far below theoretical predictions. Carbon Aerogels (CAs)—characterized by their continuous, three-dimensional carbon networks and tunable hierarchical porosity—represent a class of high-performance CDI electrode materials.

Pyrolyzed carbon aerogels possess specific surface areas ranging from 800 to 1,500 m²/g paired with low bulk electrical resistivities. Within their nanostructure, micropores (<2 nm) provide charge-storage sites for double-layer formation, while interconnected mesopores (2–50 nm) act as low-resistance channels for rapid ion transport. Under modest cell voltages (typically around 1.2 V), aqueous ions—such as $text{Na}^+$, $text{Cl}^-$, and heavy metal species—migrate into the internal porosity of the carbon aerogel electrode. This hierarchical network raises both the Salt Adsorption Capacity (SAC) and the Average Salt Adsorption Rate (ASAR).

In industrial wastewater treatment, carbon aerogels enable selective electrosorption of toxic heavy metal ions (e.g., $text{Pb}^{2+}$, $text{Cd}^{2+}$, $text{Cu}^{2+}$, $text{Cr}^{6+}$). By introducing heteroatom dopants (such as nitrogen, sulfur, or phosphorus) or grafting chelating moieties (such as amine, carboxyl, or thiol groups) onto the graphitic aerogel backbone, the electrode surface acquires targeted chemical selectivity. Under an applied potential, heavy metal ions experience both electrostatic attraction and coordination bonding with the surface functional sites. This dual mechanism enables targeted removal of trace heavy metals from complex industrial effluents containing high concentrations of competing background ions like $text{Na}^+$ and $text{Ca}^{2+}$. CDI technology powered by tailored carbon aerogel electrodes is establishing new parameters for electrochemical water purification and metal resource recovery.