Valorization of waste pine needle biomass into biosorbents for the removal of methylene blue dye from water: Kinetics, equilibrium and thermodynamics study

This work demonstrates sustainable valorization of pine needle biomass as a biosorbent for dye removal. Pine needle biomass was prepared by pyrolysis at 550 °C and functionalized by treating with a mixture of strong acids (sulphuric acid and nitric acid) and a mixture of weak acids (acetic acid and phosphoric acid) to obtain activated biochar. The influence of acid activation on biochar was studied by ultimate and proximate analyses, surface morphology, and textural properties. The adsorption potential of native and acid-activated biochar was studied for the removal of cationic dye methylene blue from an aqueous solution. Results indicate that acid activation increased the carbonyl functional groups on the biochar surface. Treatment with strong acids led to an increased surface area (217 m2g−1) in comparison to weak acid (173 m2g−1) and native biochar (164 m2g−1). The adsorption isotherm was best fitted with the Langmuir model (R2 > 99) and the adsorption kinetics followed the pseudo-second-order model. Thermodynamic parameters revealed that the process was endothermic and spontaneous. The maximum adsorption capacity was observed to be 106.38, 153.84, and 113.63 mg g−1 for native, strong, and weak acid-treated biochar respectively. Strong acid-treated biochar retained more than 60% dye removal rate after five cycles. The adsorption mechanism appeared to be governed by the co-existing phenomenon of hydrogen bonding (between nitrogen on dye and hydroxyl group on biochar surface), electrostatic interaction (between charged moieties of dye and reactive oxygen on biochar surface), and aromatic interactions between the benzene rings of dye and biochar surface.