Hydrothermal synthesis of hierarchically structured birnessite-type MnO₂/biochar composites for the adsorptive removal of Cu(II) from aqueous media

In this study, hierarchical birnessite-type MnO₂/biochar composites (δ-MnO₂/BCs) were synthesized by a hydrothermal technique, and their Cu(II) removal performance was examined in aqueous solution. Morphological characterization confirmed that a three-dimensional flower-like structure of δ-MnO₂ was formed, which results in effective adsorption affinity towards Cu(II). The effects of solution pH, adsorbent dosage, and ionic strength on the adsorption behavior of the prepared materials were systemically investigated. The adsorption kinetics indicated that Cu(II) adsorption onto δ-MnO₂/BCs follows a pseudo-second-order model. Analysis of possible adsorption/diffusion mechanisms suggested that the adsorption process is controlled by both film and pore diffusion. The adsorption isotherms fit closely to the Sips isotherm model, and the theoretical maximum adsorption capacities of Cu(II) on the synthesized δ-MnO₂/BCs are approximately 124, 154, 199, and 230 mg/g at 15, 25, 35, and 45 °C, respectively. Adsorption-desorption studies demonstrated the recyclability of the δ-MnO₂/BCs for the removal of Cu(II) from aqueous solutions.