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Heavy metals widely exist in wastewater, which is a serious threat to human health or water environment. Highly efficient removal of heavy metal ions from wastewater is a major challenge to wastewater treatment. In this work, capacitive removal of heavy metal ions from wastewater via an electro-adsorption and electro-reaction coupling process was originally demonstrated. The removal efficiency of heavy metal ions in the binary-component solutions containing metal nitrate (10 mg/L) and NaCl (100 mg/L) can reach 99%. Even the removal efficiency of heavy metal ions can be close to 99% in the multi-component solution containing all the seven metal nitrates (10 mg/L for each) and 100 mg/L NaCl. Meanwhile, the electro-adsorption and electro-reaction coupling process maintained excellent regeneration ability even after 20 cycles. Furthermore, the heavy metal ions removal mechanism was proven to be the pseudocapacitive intercalation of heavy metal ions into the layered structure of the employed W18O49/graphene in the electro-adsorption and electro-reaction coupling process. This work demonstrates great potential for general applicability to wastewater treatment.

Capacitive deionization (CDI) has received wide attention as an emerging water treatment technology because of its low energy consumption, low cost, and high efficiency. However, the conventional carbon electrode materials for CDI have low densities, which occupy large volumes and are disadvantageous for use in limited space (e.g., in household or on offshore platforms). In order to miniaturize the CDI device, it is quite urgent to develop high volumetric adsorption capacity (VAC) electrode materials. To overcome this issue, we rationally designed and originally developed high VAC MoS2–graphene hybrid electrodes for CDI. It is interesting that MoS2–graphene hybrid electrode has a much higher NaCl VAC of 14.3 mg/cm3 with a gravimetric adsorption capacity of 19.4 mg/g. It has been demonstrated that the adsorption capacity is significantly enhanced because of the rapid ion transport of MoS2 and high electrical conductivity of graphene. In situ Raman spectra and high-angle annular dark-field scanning transmission electron microscopy tests demonstrated a favorable Faradaic reaction, which was crucial to enhancing the NaCl VAC of the MoS2–graphene hybrid electrode. This work opens a new avenue for miniaturizing future CDI devices.

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