Properties and mechanism of Cd(II) adsorption from waste water by modified banana peel

Abstract: In order to explore the method of preparing green adsorption material from agricultural biomass to treat waste-water containing Cd2+, the banana peel was modified by NaOH to adsorb Cd2+. The condition of modification and adsorption process were discussed through static adsorption experiment combined with isotherm (involve Langmuir, Freundlich, Temkin and D-R) and kinetics models (involve First-order, Pseudo Second-order, Elovich and Intraparticle diffusion). The surface structure and properties were characterized by using Brunauer-Emmett-Teller (BET) surface area and pore size analysis, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), element analyzer (EA) and Fourier transform infrared spectrometer (FTIR), which were used to explore the mechanism of modification and adsorption. The results of optimized modification conditions showed that NaOH was 0.25 mol/L, modification time was 30 min. Static adsorption experiment showed that adsorption capacity of modified banana peel reached 87.15 mg/g and the Equilibrium time reached 45 min. The results showed that under the same conditions, the dosage of 4 g/L and the pH value of 6 were better than those of unmodified banana peel (adsorption capacity = 37.61 mg/g, equilibrium time = 60 min). BET specific surface area and pore size analysis confirmed that the specific surface area and total pore volume increased 108 times and 3.6 times respectively after modification. Scanning electron microscopy showed that the surface became uneven and wrinkled after modification. The changes mentioned above were conducive for Cd2+ to enter the interior of the material along through diffusion，thus contacting and reacting fully with adsorbent. According to the change of element composition, the H/C ratio of the modified banana peel decreased from 0.158 to 0.102, while the K content decreased by 4.38 percentage points, while the Na content increased by 1.55 percentage points after the modification, indicating the exchange reaction between K and Na. Furthermore, FTIR denoted active functional groups such as O-H (3 320 cm-1), C-H (2 910 cm-1), C=O (1 600 cm-1), C-C (1 390 cm-1), C-O (1 020 cm-1) were found on the surfaces of banana peel whether modified or not, indicated good adsorption potential for Cd2+. The results showed that the isothermal adsorption and adsorption kinetics of banana peel under different modification conditions were described by Langmuir isotherm model and pseudo-second order model, respectively. The results showed that Cd2+ ions were adsorbed by unmodified and modified banana peel through a single layer membrane, and the mechanism of Cd2+ was complex interaction of physical and chemical factors. Freundlich, D-R isotherm model and Elovich kinetic fitting results showed that banana peel had a strong affinity for Cd2+and the adsorption capacity was enhanced after modification. Combined with characterization and model analysis, it was concluded that the adsorptive mechanism of raw banana for Cd2+ was multiple, including ion exchange/ligand exchange, cation-π and electrostatic interaction, while the adsorptive mechanism of modified banana peel for Cd2+ was ion exchange. The results showed that NaOH modified banana peel had good adsorbability for Cd2+ in aqueous solution. This conclusion will provide an important theoretical basis for the preparation of environmental friendly adsorbents from lignocellulose biomass to remove heavy metals from polluted water.