Adsorption of Cr(Ⅵ) and dyes by plant leaves: Effect of extraction by ethanol, relationship with element contents and adsorption mechanism

Plant leaves are eco-friendly, low-cost and renewable biosorbents. This study utilized the recyclable ethanol solvent to pre-treat plant leaves against coloring the treated effluent and explore its adsorption performance for Cr(Ⅵ) and dyes. Six types of plant leaves and six corresponding extracted leaves were characterized by elemental analyzer, FT-IR and BET and leaf powders adsorbed Cr(Ⅵ) were characterized by SEM, FTIR and XPS. After the extraction treatment, the adsorption amounts of Cr(Ⅵ) declined, conversely, those of Congo red and Rhodamine B onto most leaves increased. For leaf powders, the adsorption of the three pollutants was mainly related with hydroxyl, nitrogen contents and the O/C value. Therefore, it could be concluded that the adsorption performance of plant leaves could be predicted through the element contents, in favor of the wide application of leaves in wastewater treatment. In addition, adsorption behaviors of leaf powders were obtained via kinetic, isotherm and thermodynamic studies. It was found that the difference in the adsorption rate of leaf powders on Cr(VI), Congo red and Rhodamine B was mainly related to whether the adsorption process needed the protonation of leaf powders. The maximum adsorption capacities of Cr(Ⅵ), Congo red and Rhodamine B on the E-FS-LP were 80.00, 232.56 and 74.63 mg/g (308 K). The possible adsorption mechanism of the three pollutants onto leaf powders was proposed.