Enhanced capacitive deionization of toxic metal ions using nanoporous walnut shell-derived carbon

Zheng Cao ^{a, 1}, Shen Hu ^{a, 1}, Jian Yu ^a, Luyao Wang ^a, Qi Yang ^a, Haiou Song ^{b, *}, Shupeng Zhang^{a, *}

Abstract

Capacitive deionization (CDI) technology differs from traditional demineralization technology. Its desalination effect is mainly dependent on the electrode material and the porous nanostructure which can enhance capacitive deionization performance. Here, a low-cost and eco-friendly CDI technology has been developed to remove divalent ions from water. Using different activators such as KOH, ZnCl₂ and K₂C₂O₄ to reconstruct the biomass carbon skeleton, porous carbon material derived from walnut shells was used as symmetric electrodes. The maximum removal capacities in the CDI electrosorption were 17.1 mg/g for Mg²⁺, 11.99 mg/g for Ca²⁺, 18.56 mg/g for Pb²⁺ and 15.52 mg/g for Cd²⁺. The best effect on formation of micropores and construction of adsorption channels was observed using KOH as activator. The KOH-C had a specific surface area of 1600.34 m²/g and a cumulative pore volume of 0.3561 cm³/g, which provides favorable conditions for ion storage. These results demonstrate that KOH-activated carbon biomass can improve electrosorption performance and has potential as an advanced, inexpensive electrode material for capacitive deionization.

In this work, we have used electrodes prepared from waste walnut shells activated by KOH, ZnCl₂ and K₂C₂O₄ for electrosorption of different metal ions. Images from SEM and TEM confirmed that the biomass had a fluffy porous network structure. From the Brunauer-Emmett-Teller (BET) test and calculation of the pore size distribution, KOH-C, ZnCl₂-C, and K₂C₂O₄-C all exhibited ideal mesoporous structures, while average adsorption pore sizes of KOH-C and ZnCl₂-C were smaller, indicating that these activators induced a larger amount of microporous structure. In terms of electrochemical performance, K₂C₂O₄-C had the highest specific capacitance due to the presence of more sites distributed on its surface. In the CDI experiments, porous carbon material was used as symmetric electrodes and achieved maximum capacities of 17.1 mg/g for Mg²⁺, 11.99 mg/g for Ca²⁺, 18.56 mg/g for Pb²⁺ and 15.52 mg/g for Cd²⁺ removal. The adsorption capacities and rates of all the materials for divalent ions were higher than those for monovalent sodium ions. The comprehensive adsorption capacity of KOH-C for different ions was highest and its regeneration performance was also relatively good, indicating its potential as a promising electrode material for future CDI applications and paving the way to eliminate multiple inorganic contaminants from waste water.

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