摘要/Abstract

首先制备了α-MnO₂纳米花簇、β-MnO₂纳米针和δ-MnO₂微米颗粒三种不同晶型的MnO₂粉末材料，对其结构、形貌及吸附除铵能力进行了表征和测试.结果表明，层间距（7.2Å）大于NH₄⁺直径（2.96Å）和水合NH₄⁺直径（6.62Å）的δ-MnO₂相比其他两种晶型的MnO₂有更高的NH₄⁺吸附量；接着研究采用KMnO₄原位氧化还原法在石墨毡（GF）上直接生长超薄δ-MnO₂纳米片（MnO₂NPs）阵列构筑了石墨毡载纳米MnO₂（MnO₂NPs/GF）多级结构材料，制备简单，无须成型造粒就可直接用作除铵净水材料，研究结果表明，MnO₂NPs/GF不仅具有较高的吸附量（15 mg·g^－1）与良好的选择性，同时还展现了优异的快速吸附和稳定的循环使用性能.MnO₂NPs/GF对水中NH₄⁺的吸附符合准二级动力学模型，其吸附等温线符合Langmuir吸附等温式，是吸附-离子交换法除铵的理想材料.
关键词: 纳米δ-MnO2, 除铵净水, 吸附材料, 吸附动力学
We synthesized three kinds of MnO₂ powder with different crystalline phases including α-MnO₂ nanoflowers, β-MnO₂ nanorods and δ-MnO₂ micro-particles. The structure and morphology of prepared MnO₂ were studied by XRD (X-ray diffraction), SEM (Scanning Electron Microscope), TEM (Transmission Electron Microscope) and XPS (X-ray photoelectron spectroscopy), systematically. Adsorption process was conducted in NH₄Cl solution (40 mg·L^-1 NH₃-N) and actual water samples containing NH₄⁺, Ca²⁺, Mg²⁺, K⁺ and Na⁺, respectively. The results demonstrate that δ-MnO₂ with 7.2 Å interlayer spacing which is a little larger than the diameter of hydrated ammonium (6.62 Å) has high adsorption capacity; α-MnO₂ with[2×2] tunnel of 4.6 Å has less adsorption capacity than that of δ-MnO₂, and β-MnO₂ whose[1×1] tunnel is just 1.89 Å, barely has adsorption capacity. Then MnO₂NPs/GF (MnO₂ nanoplates decorated graphite felt) was prepared via a facile in-situ redox process. Graphite felt (GF) was immersed in KMnO₄ solution (4 g·L^-1, pH=2) at 65℃ for 5 h to get MnO₂NPs/GF. GF not only reacted as the reductant of KMnO₄, but also acted as 3D framework to support the in-situ deposited MnO₂NPs. MnO₂NPs/GF shows high adsorption capacity (15 mg·g^-1) and good selectivity (86.7%). In repetitive adsorption-desorption experiments, MnO₂NPs/GF not only exhibits good stability after 20 cycles, but also decreases the concentration of NH₃-N to as low as 1 mg·L^-1. The thermodynamics experiment demonstrates that the adsorption isotherm fit well with Langmuir isotherm, and the adsorption process corresponds to the pseudo-second-order model. The excellent performance of MnO₂NPs/GF is attributed to the following three aspects. Firstly, the 7.2 Å interlayer spacing of δ-MnO₂ is suitable for the exchange-adsorption of NH₄⁺. Secondly, the ultra-thin MnO₂ nanoplate arrays, which vertically grow on the graphite felt substrate, provide fast path and convenient interface for ion exchange. Finally, the interlaced nanoplates with self-supported structure ensure its high stability. In a conclusion, MnO₂NPs/GF has a bright future in the field of ammonium removal.
Key words: δ-manganese dioxide nanoplates, removal of ammonium, adsorption materials, adsorption kinetics


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