Manganese-Oxide-Modified Porous Carbon from Ulva prolifera for Efficient Cd(II) Removal: Adsorption Behavior and Mechanisms

Biochar materials exhibit promising prospects in adsorption–separation owing to their large specific surface area and well-developed pore structures. Herein, a series of manganese-oxide-modified porous carbon adsorbents (PEC-MnO_x) were fabricated via the pyrolysis method, using Ulva prolifera as the carbon source and KMnO₄ as the activator. XRD, SEM, TEM, N₂ adsorption–desorption, FT-IR and XPS were employed to characterize the morphology, crystal phase, pore structure and surface chemical properties of the as-prepared materials. The results demonstrated that KMnO₄ activation effectively optimized pore architecture, increased specific surface area, and generated abundant unsaturated -Mn-OH sites and oxygen vacancies. Batch adsorption experiments were carried out to evaluate Cd(II) removal performance. PEC-MnO_x-6 displayed the optimal adsorption capability with an equilibrium adsorption uptake of 152.4 mg/g at 298.15 K. Kinetic fitting suggested that the pseudo-first-order model better described the adsorption behavior. Intraparticle diffusion analysis confirmed the multi-step mass-transfer nature, and liquid-film diffusion acted as the primary rate-limiting step. Both Langmuir and Freundlich isotherm equations obtained high correlation coefficients, indicating the coexistence of homogeneous active sites and heterogeneous pores/defects on the carbon matrix. Combined with characterization results, Cd(II) sequestration was realized through synergistic effects mainly including surface complexation and ion exchange, with the assistance of electrostatic interaction. This work provides a reference for high-value utilization of Ulva prolifera biomass and remediation of Cd(II)-contaminated wastewater.