Energy from Garbage: Recycling Heavy Metal-Containing Wastewater Adsorbents for Energy Storage

Abstract

The global challenge of heavy metal contamination in wastewater necessitates efficient adsorbents, which, while they can effectively remove contaminants, ultimately become toxic waste themselves. Recycling wastewater adsorbents loaded with heavy metals is an alternative to their disposal as toxic garbage. This study presents a genuine recycling strategy for heavy metal-loaded adsorbents, repurposing them as electrode materials for energy storage applications, that is, electrochemical capacitors, and thus synergistically tackles water purification and waste valorization. Graphene oxide was prepared via the improved Hummers’ method, and subsequently, a reduced graphene oxide (rGO) foam was hydrothermally synthesized. This rGO-based foam adsorbed >95% of the Hg²⁺ ions in a 100 ppm solution, using a dose = 500 mg L⁻¹, and up to 240 mg of Hg²⁺ ions/g on average at 25 °C, which are among the highest values reported so far. Electrochemically, the Hg-loaded rGO (rGO/Hg_ads) exhibits mercury redox activity and a 15% increase in capacity as compared to pristine rGO in an aqueous cell. Overall, this demonstrates the potential of reprocessed wastewater adsorbents as efficient and sustainable electrodes for high-power energy storage (time constant τ = 11 s), offering a compelling solution to add more value and extend the life cycle of waste materials.