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

With the growth of batteries, electroplating, and mining industries, heavy metal ions such as cadmium (Cd 2+ ) are being discharged on a massive scale, thus posing a severe threat to the environment. Conventional techniques for removing Cd 2+ 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 m 2 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 Cd 2+ 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 Cd 2+ is captured by both electrical-double-layer and pseudocapacitive mechanisms. Additionally, CdCO 3 precipitate is also responsible for Cd 2+ removal, which might be ascribed to the reaction of dissolved CO 2 in aqueous media with Cd 2+ 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.