Potential of zero charge-mediated electrochemical capture of cadmium ions from wastewater by lotus leaf-derived porous carbons

Abstract

With the growth of batteries, electroplating, and mining industries, heavy metal ions such as cadmium (Cd²⁺) are being discharged on a massive scale, thus posing a severe threat to the environment. Conventional techniques for removing Cd²⁺ from wastewater with low concentrations still suffer from slow kinetics and secondary pollution. A carbon-based capacitive deionization (CDI) system is highly desired but encounters a severe co-ion expulsion effect. Herein, we developed CDI systems based on surface charge-modulated porous carbon and an asymmetric configuration. This was achieved by first preparing porous carbons through facile microwave pyrolysis of lotus leaf followed by KOH activation. The morphology, pore structure, heteroatom content, surface charge, and electrochemical behavior of porous carbons were investigated by adjusting the mass ratio of KOH to carbon. The lotus leaf-derived carbons show a morphology of nanosheet-like thin carbon (NSTC), with their specific surface areas increasing with the amount of KOH used for activation. In contrast, the heteroatom (i.e., nitrogen and oxygen) contents decrease with the increase in the mass ratio of KOH to carbon, resulting in a more positive surface charge. Notably, the NSTC with a mass ratio of 3 for KOH/carbon (NSTC-3) displays an ultrahigh specific surface area of 3705.0 m² g⁻¹, and a specific capacitance of 92.5 F g⁻¹ at a current density of 0.5 A g⁻¹ when coupled with a commercial activated carbon in an asymmetric YP-50F//NSTC-3 supercapacitor. Consequently, the CDI cell equipped with a YP-50F as the anode and a NSTC-3 as the cathode exhibits a high specific adsorption capacity of 88.6 mg_Cd·g_cathode⁻¹ at 1.2 V in a 100 mg L⁻¹ Cd²⁺ solution, which is about 36.3 % higher than that of the symmetrical configuration NSTC-3//NSTC-3. Furthermore, 71 % of the initial removal capacity of the YP-50F//NSTC-3 system is retained after 7 cycles of charging and discharging. Characterizations of the cathode after the adsorption process indicate that the Cd²⁺ is captured by both electrical-double-layer and pseudocapacitive mechanisms. Additionally, CdCO₃ precipitate is also responsible for Cd²⁺ removal, which might be ascribed to the reaction of dissolved CO₂ in aqueous media with Cd²⁺ under the electrified action. The high removal performance and excellent cycling stability are attributed to the tunability of the surface charge properties and the asymmetric configuration, which minimizes the co-ion expulsion and modulates potential distribution. This study provides a novel avenue to design biochar-based configurations for electrified water treatment.