Comparative Hg(II) Adsorption on Coffee Husk and Sugarcane Bagasse Biochars: Structure–Function Relationships

Mercury contamination in water poses a significant environmental and public health concern because it is toxic, persistent, mobile, and bioaccumulative. This study investigated biochars derived from the perisperm (silver skin) fraction of coffee husk (CH) and sugarcane bagasse (SB) as sustainable adsorbents for Hg(II) removal from water. The biochars were produced at 800 °C, characterized by N2 adsorption–desorption measurements at approximately −198 °C, SEM–EDS, XRD, XRF, FTIR, and XPS, and evaluated through kinetic, equilibrium, and thermodynamic studies of Hg(II) adsorption. SB exhibited a higher BET surface area (599 vs. 525 m2 g−1), larger pore volume (0.298 vs. 0.216 cm3 g−1), and lower micropore fraction (63.4 vs. 86.3%) than CH. These textural differences were consistent with the faster Hg(II) adsorption observed for SB. The Avrami and Elovich models best described the adsorption kinetics (R2 = 0.980–0.996). At the same time, Freundlich and Langmuir adequately described the equilibrium data; their relative performance depended on biochar type and temperature. The Langmuir-estimated maximum adsorption capacity (qm) increased with temperature, reaching 45.05 mg g−1 for SB and 45.25 mg g−1 for CH at 318.15 K. The thermodynamic analysis yielded negative enthalpy and Gibbs free-energy changes for both biochars, with ΔH° values of −71.4 and −52.8 kJ mol−1 and ΔG°(298) values of −39.3 and −37.6 kJ mol−1 for SB and CH, respectively. FTIR and XPS supported the involvement of oxygen-containing surface environments in Hg(II) retention.