Waste-to-Wealth Strategy: Ti₃AlC₂-Max-Supported WS₂/Halloysite Nanocomposite for the Removal of Nickel Metal Ions from Wastewater with Machine Learning Simulation and Subsequent Application in Supercapacitors

We proposed an innovative approach employing the "waste-to-wealth" concept, transforming expended adsorbent into electrode materials for energy storage with a focus on sustainability. Ti3AlC2 MAX phase-decorated WS2 and halloysite (Mx-WS2-Hal NCs) were synthesized hydrothermally for the efficient removal of nickel metal ions from wastewater. Subsequently, the spent adsorbent, enriched with Ni2+ (Mx-WS2-Hal@Ni-adsorbed), was repurposed for supercapacitor applications, thus minimizing the environmental impact. The batch adsorption results revealed that WS2-Hal and 1, 3, and 5% Mx functionalized WS2-Hal NCs show removal efficiencies of 79.82, 86.146, 95.59, and 93.66%, respectively, for Ni2+. Furthermore, machine learning models were employed to predict the removal efficiency of Ni2+ onto the Mx-WS2-Hal NCs. The column research showed that the removal efficiency of Ni2+ was higher at low inlet solution concentration, low flow rate, or high column depth. The column experimental data presented the better fitting of Thomas and Yoon-Nelson model for the Ni2+ adsorption onto the Mx-WS2-Hal NCs. After adsorption, the expended Ni adsorbed Mx-WS2-Hal adsorbent was employed for energy storage to minimize the risk of causing additional pollution. The fabricated Mx-WS2-Hal@Ni-adsorbed NCs based symmetric supercapacitor device displayed 59.47 W h/kg of energy density at 0.583 kW/kg of power density. Moreover, it maintained a capacitance retention of 90% even after undergoing 10,000 cycles, highlighting the long-term sustainability and reliability of the proposed approach.