Capacitive removal of Pb ions via electrosorption on novel willow biochar-manganese dioxide composites

Biochar derived from lignocellulosic biomass has been used as a low-cost adsorbent in wastewater treatment applications. Due to its rich porous structure and good electrical conductivity, biochar can be used as a cost-effective electrode material for capacitive deionization of water. In this work, willow biochar was prepared through carbonization of shrub willow chips, activated with potassium hydroxide, and loaded with manganese dioxide (WBC-K-MnO₂ nanocomposite). The prepared materials were used to electrochemically adsorb Pb²⁺ from aqueous solutions. Under the applied potential of 1.0 V, the WBC-K-MnO₂ electrode exhibited a high Pb²⁺ specific electrosorption capacity (23.3 mg/g) as compared to raw willow biochar (4.0 mg/g) and activated willow biochar (9.2 mg/g). KOH activation followed by MnO₂ loading on the surface of raw biochar enhanced its BET surface area (178.7 m²/g) and mesoporous volume ratio (42.1%). Moreover, the WBC-K-MnO₂ nanocomposite exhibited the highest specific capacitance value of 234.3 F/g at a scan rate of 5 mV/s. The electrosorption isotherms and kinetic data were well explained by the Freundlich and pseudo-second order models, respectively. The WBC-K-MnO₂ electrode demonstrated excellent reusability with a Pb²⁺ electrosorption efficiency of 76.3% after 15 cycles. Thus, the WBC-K-MnO₂ nanocomposite can serve as a promising candidate for capacitive deionization of heavy metal contaminated water.