Fast adsorption of methylene blue and crystal violet from aqueous solution by sustainable biosorbent (pine needle of Pinus nigra Arn.): Kinetics, equilibrium, and thermodynamics study

Methylene blue (MB) and crystal violet (CV) dyes, which are toxic to the ecosystem, were removed by Pinus nigra Arn. tree (Pn A.) waste needle powders (Ptwnd), which is a natural, easily available, and cheap adsorbent. The physicochemical composition of Ptwnd was carried out by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and energy dispersive X-ray (EDX), thermogravimetry–differential thermal analysis (TGA-DTA), UV–vis spectroscopy, X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, and point of zero charge (pH_pzc). In adsorption studies, the effects of pH, adsorbent amount, time, initial dye concentration, and temperature were determined. The results were tested by kinetics (pseudo first order [PFO], pseudo second order [PSO], Elovich, and intra-particle diffusion [I-PD]) and isotherm (Freundlich, Langmuir, Temkin, Dubinin–Radushkevich [D-R]) models and tested with 5 different error functions. Accordingly, the average pore diameter and pH_pzc value were measured as 68.87 Å and 6.13, respectively. Also, the mass loss of 4.6%–28.7% and 24.3% at three temperatures was 121.2–533.5°C and 766.2°C, respectively. The adsorption mechanism was endothermic, and the removal efficiencies exceeded 99% in the first 10 min. Also, the most suitable models were determined to be Langmuir and PSO for both dyes, respectively. Maximum adsorption capacity (q_max) calculated as 95.767 (for MB) and 151.657 (for CV) mg/g respectively. In this study, very promising results were achieved in the removal of two different dyes from water with the biosorbent obtained from pine needles, which we think will contribute to the sustainability of the forest ecosystem.