壳聚糖改性4A沸石分子筛颗粒去除低温水中硝酸盐的机理研究

为解决我国北方严寒地区常规工艺难以有效去除低温地下水中硝酸盐污染的问题，以4A沸石分子筛为载体，通过壳聚糖改性制备出一种具有低温去除水中硝酸盐功能的吸附颗粒，有望作为吸附剂应用于常规水处理工艺的提标改造.通过小试试验，研究壳聚糖改性4A沸石分子筛颗粒去除低温水中硝酸盐的反应机理，分析其去除硝酸盐过程的吸附动力学、吸附等温线、吸附热力学，并通过pH_PZC（零电点）测定、FT-IR（傅里叶红外光谱）和XPS（X射线光电子能谱）进一步分析其反应前、后表面物化特性.结果表明:①在温度为8~10℃下，壳聚糖改性4A沸石分子筛颗粒对硝酸盐的吸附平衡时间为8 h，平衡吸附量为1.88 mg/g.②吸附过程符合准二级动力学模型，吸附符合Langmuir和Freundlich吸附等温模型，吸附热力学参数△G0 < 0、△H0>0、△S0>0，表明吸附为熵增加的吸热过程.③壳聚糖改性4A沸石分子筛颗粒的pH_PZC为10.0，FT-IR结果表明，-NH₂和-OH官能团吸附了水中的硝酸根离子，XPS结果表明，N（1s）和O（1s）元素在吸附过程中参与反应.研究显示，壳聚糖改性4A沸石分子筛颗粒分子结构中的-NH₂和-OH基团通过静电结合和氢键作用与硝酸根离子进行吸附反应. 

Mechanism of Nitrate Removal in Drinking Water by 4A Zeolite Molecular Sieve Modified with Chitosan at Low Temperature

A new zeolite molecular sieve modified with chitosan was prepared and investigated for the adsorption of nitrate from underground water at low temperature. The adsorption kinetics, isotherms, and thermodynamics of this adsorbent were studied through batch adsorption experiments of nitrate. The physicochemical characteristics before and after the adsorption reactions were demonstrated by point of zero charge (pH_PZC), Fourier infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) methods. The experimental results showed that:(1) At the water temperature of 8-10℃, the adsorption equilibrium time and adsorption capacity were 8 h and 1.88 mg/g, respectively. (2) The kinetic study indicated that the pseudo second-order dynamic model could fit the experimental data well and it was also observed that the adsorption isotherms could be well described by both Langmuir and Freundlich isotherm models. The thermodynamic results of △G0 < 0, △H0>0 and △S0>0 indicated that the adsorption was an endothermic process with increased entropy. (3) The pH_PZC of the particles was 10.0. And functional groups of -NH₂ and -OH were found to adsorb the nitrate ions based on the FT-IR results. N(1s) and O(1s) elements participated in the reactions during adsorption process according to the XPS results. Studies have shown that the adsorbed nitrate ions were associated with the -NH₂ and -OH groups in the molecular structure of the 4A zeolite molecular sieve modified with chitosan by electrostatic binding and hydrogen bonding. The adsorbents can be applied to the upgrading of the conventional water treatment processes to improve the removal of nitrate in low-temperature groundwater in the cold regions of northern China.