香蕉皮改性材料对废水中二价Cd离子的吸附特性与机理

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

为探究农业生物质制备绿色吸附材料处理含Cd^(2+)废水方法,以香蕉皮为原料,制备改性吸附材料。采用单因素试验,优化了改性工艺条件。通过静态吸附试验,结合等温模型和吸附动力学模型探讨了其对Cd^(2+)吸附过程。利用比表面及孔径分析(brunner-emmet-teller)、扫描电镜(scanning electron microscope)、能谱仪(energy disperse spectroscopy)、元素分析仪、傅里叶变换红外光谱(Fourier transform infrared spectroscopy)等手段对改性前后材料的表面形态和结构进行表征,并分析了改性和吸附过程的机理。结果表明:改性较佳工艺条件为Na OH浓度为0.25 mol/L,改性时间为30 min。在此条件下香蕉皮改性后,对水中Cd^(2+)的理论饱和吸附量由37.61 mg/g提高到87.15 mg/g,平衡时间由60 min缩短到45 min。吸附符合Langmuir等温模型(R^2=0.998)和准二级动力学模型(R2=0.999)。改性后的香蕉皮对水中Cd^(2+)的吸附以离子交换吸附为主。研究结果可为木质纤维素生物质改性制备绿色吸附材料处理含重金属废水提供理论依据。

In order to explore the method of preparing green adsorption material from agricultural biomass to treat waste-water containing Cd^2+, the banana peel was modified by NaOH to adsorb Cd^2+. 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 Na OH 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 Cd^2+ 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 Cd^2+. 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 Cd^2+ ions were adsorbed by unmodified and modified banana peel through a single layer membrane, and the mechanism of Cd^2+ 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 Cd^2+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 Cd^2+ was multiple, including ion exchange/ligand exchange, cation-π and electrostatic interaction, while the adsorptive mechanism of modified banana peel for Cd^2+ was ion exchange. The results showed that NaOH modified banana peel had good adsorbability for Cd^2+ 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.