Tannin-derived vacancy-deficient nitrogen-doped porous carbon for highly selective adsorption of Cu2+ in capacitive deionization

Abstract

The application of capacitive deionization (CDI) for the selective removal of Cu²⁺ offers a promising avenue for the efficacious treatment of copper-laden wastewater, thereby mitigating significant environmental pollution concerns. Using tannin as a base material and Cu²⁺ as the target ion, we constructed a two-dimensional planar material with selective Cu²⁺ ion imprinting sites. Through a simple acid treatment, we successfully prepared vacancy-deficient nitrogen-doped porous carbon (TDNC-VD). Additionally, the biocarbon materials exhibited a high adsorption capacity with discernible selectivity towards Cu²⁺ in both single-component and mixed-solution systems. Notably, TDNC-VD has high selectivity coefficients (S_c) of 6.32, 11.38, and 18.20 compared to Zn²⁺, Mn²⁺, and Co²⁺ in mixed solutions. Finally, employing density functional theory (DFT) calculations, the TDNC-VD exhibited pronounced selectivity towards Cu, as evidenced by an adsorption energy of −9.25 eV for Cu, surpassing that of Zn (−8.70 eV), Mn (−8.43 eV), and Co (−8.31 eV). The selective adsorption of Cu by TDNC-VD primarily stemmed from the dimensions of the two-dimensional cavity structure established within the CuN coordination environment. The vacancy defects within the material exhibited greater compatibility with the structural dimensions of copper ions, facilitating the formation of stable bonds with the cavities. Conversely, the selective adsorption of copper ions onto the material was attributed to the incapacity of other ions to firmly bind to the adsorption sites. This study is anticipated to offer an environmentally friendly solution for the treatment of copper-containing wastewater.