Adsorption of silver nanoparticles by activated carbon from Eragrostis plana Nees: kinetics, equilibrium, and catalytic application in the degradation of 4-nitrophenol

Eragrostis plana Nees is an invasive species in Brazilian territory, known for its high levels of lignin, cellulose, and hemicellulose, making it a valuable raw material for activated carbon (AC) production. In this study, AC derived from Eragrostis plana Nees leaves was investigated as an adsorbent for silver nanoparticles (AgNPs). Kinetics, equilibrium, and thermodynamic assays were conducted to assess AgNPs adsorption onto AC. The AC exhibited a substantial surface area of 1030 m² g⁻¹ and demonstrated significant adsorption capacity for AgNPs. Both the pseudo-second-order and Langmuir models were found to best describe the kinetics and equilibrium of adsorption, with the highest adsorption capacity observed at 55 °C, reaching 140.19 mg g⁻¹ according to the Langmuir model. Thermodynamic analysis revealed an enthalpy change (∆H°) of 60.75 kJ mol⁻¹ and an entropy change (∆S°) of 0.2711 kJ mol⁻¹ K⁻¹, indicating that the adsorption process is spontaneous and endothermic. Additionally, the AgNPs/AC composite exhibited excellent catalytic activity in the 4-nitrophenol reduction, achieving a conversion rate of 97% within 10 min.