石墨毡载纳米δ-MnO_2高性能除铵净水材料研究

Research on High Performance Ammonium Removal Materials Based on δ-MnO_2 Nanoplate Arrays Decorated Graphite Felt

首先制备了α-MnO_2纳米花簇、β-MnO_2纳米针和δ-MnO_2微米颗粒三种不同晶型的MnO_2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(7.2?)大于NH_4^+直径(2.96?)和水合NH_4^+直径(6.62?)的δ-MnO_2相比其他两种晶型的MnO_2有更高的4NH+吸附量;接着研究采用KMnO_4原位氧化还原法在石墨毡(GF)上直接生长超薄δ-MnO_2纳米片(MnO_2NPs)阵列构筑了石墨毡载纳米MnO_2(MnO_2NPs/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,MnO_2NPs/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.MnO_2NPs/GF对水中NH_4^+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.

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 ？ interlayer spacing which is a little larger than the diameter of hydrated ammonium（6.62 ？） has high adsorption capacity; α-MnO2 with [2×2] tunnel of 4.6 ？ has less adsorption capacity than that of δ-MnO2, and β-MnO2 whose [1×1] tunnel is just 1.89 ？, barely has adsorption capacity. Then MnO2 NPs/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 MnO2 NPs/GF. GF not only reacted as the reductant of KMnO4, but also acted as 3D framework to support the in-situ deposited MnO2 NPs. MnO2 NPs/GF shows high adsorption capacity（15 mg·g-1） and good selectivity（86.7%）. In repetitive adsorption-desorption experiments, MnO2 NPs/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 MnO2 NPs/GF is attributed to the following three aspects. Firstly, the 7.2 ？ 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, MnO2 NPs/GF has a bright future in the field of ammonium removal.