Biomass-derived activated carbon from carica papaya and vernonia amygdalina with enhanced adsorptive and surface properties for sustainable water treatment

Water pollution and the increasing cost of commercial adsorbents have intensified the search for sustainable biomass-derived materials for water treatment applications. This study compares the production of activated carbon (AC) from underutilized biomass feedstocks, Carica papaya (pawpaw leaf) and Vernonia amygdalina (bitter leaf), as sustainable alternatives to fossil-based adsorbents. Emphasizing resource efficiency, low-cost processing, and agro-waste valorization, both biomasses were carbonized under controlled pyrolysis conditions at 400 °C for papaya leaves and 600 °C for Vernonia leaves, and chemically activated with 0.3 M HCl to enhance porosity and remove mineral residues. Structural and surface properties were critically analyzed using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). Quantitative results show that PP-AC achieved a yield of 28.4 ± 1.2%, a BET surface area of 892 ± 15 m² g⁻¹, and dominant microporosity (average pore diameter 1.9 nm). VA-AC yielded 32.7 ± 1.1%, with a BET surface area of 946 ± 12 m² g⁻¹, a mesoporous structure (3.8 nm), and minor crystalline inclusions of quartz and calcite. Pb²⁺ adsorption data fitted well to Langmuir and Freundlich models (R² = 0.983 and 0.971, respectively), with maximum capacities of 95.2 mg g⁻¹ (PP-AC) and 90.8 mg g⁻¹ (VA-AC). The adsorption behavior further suggested that oxygen-containing surface functional groups and chemisorption mechanisms significantly contributed to Pb²⁺ uptake efficiency. These result shows that precursor-specific composition strongly determines porosity and functionality, with PP-AC suited for trace-organic removal and VA-AC for industrial effluents. Overall, the findings demonstrate the feasibility of converting underutilized agro-wastes into efficient activated carbon materials suitable for decentralized and sustainable water treatment systems.