Adsorption mechanism of Cu(II) and Pb(II) by starch biochar co-modified with plant extract and chitosan

To investigate the adsorption mechanisms of heavy metal ions by starch (S)-based materials, S biochar (SB) was prepared by anaerobic calcination of S at 400 °C for 2 h. Plant extract (E) and chitosan (C) were subsequently used for single and combined modifications of SB, yielding E-modified SB (E-SB) and E/C co-modified SB (E/C-SB), respectively. The morphological features of E-SB and E/C-SB were characterized using multiple analytical techniques. Thermodynamic and kinetic behaviors of Cu(II) and Pb(II) adsorption were evaluated through batch experiments, examining the effects of temperature, pH, and ionic strength. E and C modification altered the physicochemical properties and introduced functional groups onto the SB. After modification, the void structures of E-SB and E/C-SB became more porous, forming a mesh-like structure with uneven pore distribution. The thermoplastic properties of modified SB was enhanced compared to unmodified SB. Adsorption isotherms were better described by the Langmuir model than by the Freundlich model. The maximum adsorption capacities for Cu(II) and Pb(II) ranged from 696.29 to 1290.98 mmol kg⁻¹ and 238.16–643.60 mmol kg⁻¹, respectively, following the order: E/C-SB > E-SB > SB. Thermodynamic and kinetic analyses indicated that Cu(II) and Pb(II) adsorption was endothermic, spontaneous, and entropy-driven, primarily governed by transfer resistance. Higher temperature and pH, and lower ionic strength, promoted adsorption. After three regeneration cycles, E/C-SB retained 74.9 % and 67.0 % of its original adsorption capacity for Cu(II) and Pb(II), respectively.