High Performance Capacitive Deionization Electrode Materials for the Selective Recovery of Hazardous Metal Ions from Wastewater

The current study focused on designing and preparing novel CDI electrode materials with high adsorption capacity, high selectivity and excellent cyclic performance for hazardous metal ions (HMI) recovery from wastewater. And the key process for practical application, enriching target ions from low concentration of wastewater to concentrated solution is elaborately designed and achieved. Mesoporous carbon electrode materials derived from pomelo peel were functionalized with pyrrolic-N (BNC-5 electrode) and pyridinic-N (BNC-6 electrode) for enhanced selective electroadsorption of Pb2+ and Cu2+. The preferential adsorption of Pb2+ from multi-ionic solution can be attributed to the interaction between soft acid ions (Pb2+) and soft base sites (pyrrolic-N), contributing to elevated electroadsorption capacity (2.0 mmol g-1). The effect of applied voltages on electrosorption capacities was studied to understand the adsorption behaviours of electrode materials, and adsorption–desorption hysteresis of BNC-5 confirmed the nature of chemisorption. BNC-5 electrode still maintained a capacity retention ≥ 80% after ~400 cycles, and such electrode could be almost fully restored (98.7%) by 0.1 M HNO3 washing to achieve much longer cyclic performance (over 1200 cycles).

An ethylenediamine triacetic acid (EDTA) and 2D MXene (EDTA-MXene) was utilized as CDI electrode to recover Cs+ from strongly acidic process water. EDTA enriched the adsorption sites on MXene to increase its recovery capacity, and prevented the oxidation of electrode to enhance its long-term cyclic performance. EDTA-MXene exhibited high Cs+ adsorption capacity of 2.07 mmol g-1 at 1.2 V and outstanding cyclic performance with a capacity retention > 80% after 320 cycles. Moreover, the electrode maintained satisfactory adsorption capacity (0.66 mmol g-1) in strongly acidic solution (2M HNO3), and presented great ability to enrich Cs+ from low concentration of process water into a highly concentrated solution for element recovery or safe disposal.

A sulfur and nitrogen co-doped MXene (S/N-MXene) electrode material was fabricated to recover palladium from wastewater. 2D MXene material was modified by abundant S and N-containing functional groups, which greatly increased the adsorption sites for Pd2+, and acted as pillars to expand the interlayer spacing of MXene for Pd2+ accommodation, thus contributing to high recovery capacity, high recovery efficiency and satisfactory long-term cycling performance. The low concentration of Pd2+ in industrial wastewater was enriched to highly concentrated solution (around 100 mmol L-1), and the selectivity was also evaluated in multi-ionic solution with almost 98% recovery efficiency for Pd2+. Furthermore, the adsorption mechanism was revealed using density functional theory (DFT) to show that the downward shift of the p-band centre of the MXene OH groups induced by pyrrolic-N and -C=S in S/N-MXene contributes to enhancement of the soft-soft interactions, which may enlighten the fabrications of doped MXene materials and their applications.

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