Kinetic and isothermal studies of naproxen adsorption from aqueous solutions using walnut shell biochar

In this study, biochar was obtained from walnut shells (WS) using one- and two-stage pyrolysis processes at different temperatures (500, 600, 700, 700 + 700 °C). The biochar obtained was used as an adsorbent for the removal of naproxen, an organic pollutant, from aquatic systems. The highest removal efficiency was obtained with 700 + 700-WSB obtained by the two-stage pyrolysis method. The physicochemical properties of walnut shell biochar (WSB) were investigated using SEM–EDX, FTIR, XRD, BET, elemental analysis, and TGA. 700 + 700-WSB had a high surface area of 649 m²/g and a pronounced porous structure, according to the BET and SEM analysis results. Various experimental parameters (pH, contact time, amount of adsorbent, temperature, initial concentration, and pH_pzc) were investigated to study the adsorption of naproxen by 700 + 700-WSB in an aqueous solution. The point of zero charge of the 700 + 700-WSB adsorbent was calculated as 7.15. The 700 + 700-WSB obtained at optimum parameter levels, such as 25 °C and 4 h time has an adsorption capacity of 58.8 mg/g. To analyze the equilibrium results, the Langmuir, Sips, Freundlich, and Temkin adsorption isotherm models were utilized. The equilibrium data fit the Langmuir (R² = 0.984) and Sips (R² = 0.979) models among the isotherm models. Investigating the adsorption characteristics of naproxen on WSB involved the study of the kinetic models and thermodynamic components of the adsorption process. The psuedo-second order kinetic model (PSO) was shown to be consistent with the experimental data through kinetic investigations, and the thermodynamic results (ΔG° =  − 21.87 kJ/mol, ΔH° =  − 29.52 kJ/mol and ΔSº =  − 25.73 J/mol K) indicated that the adsorption process was exothermic and spontaneous.