基于道南渗析膜过程的饮用水除砷动力学及其强化研究

Limited by the actual situations of infrastructure and economics, conventional arsenic removal technologies always fails to work properly in rural China. Donnan dialysis is an ion-exchange membrane process driven by an electrochemical potential. Electronic fields or high pressures are not required, and the membrane is not susceptible to fouling; thus, Donnan dialysis is straightforward to operate, easy to maintain, and affordable to implement. These properties make Donnan dialysis an excellent candidate for developing novel point of use arsenic removal technology exclusively for rural and poor settings. However， the arsenic removal is slow in this process, and the arsenite is not able to be removed effectively. The present study focuses on the promotion of the dynamics of the Donnan membrane process for the arsenic removal. An arsenic mass transfer kinetics model is developed to explore the rate-limiting factors of the dialytic process and to reveal the specific intermembrane diffusion behavior of different arsenic ion species. Enhancement strategies are proposed based the conclusions of the kinetics study. A manganese dioxide loaded ion exchange membrane is fabricated and used to enhance the arsenite removal in the Donnan dialysis system. In addition, a Donnan dialysis-zerovalent iron coupled system is developed to improve the mass transfer and arsenic removal in the long-term run. Effects of ionic strength on the arsenic removal using zerovalent iron are thoroughly discussed to interpret the feature of the in situ regeneration of the arsenic-accumulating stripping solution, which is a high ionic strength solution. Results and conclusions on the kinetics and enhancement of the arsenic removal by the Donnan membrane process would provide a solution to the arsenic problem in rural areas and promote the progress of the Rural Drinking Water Safety Project.

受限于基础设施、经济技术水平等因素，传统除砷技术在应用于我国农村地区时多未能取得良好效果。道南渗析是以电化学势差为推动力的离子交换膜过程。该过程无外加电场、无膜污染、能耗低、操作简便，在农村终端净水除砷方面具有良好应用前景，但存在除砷速率较慢和无法有效去除三价砷的问题。本项目从提高道南渗析除砷传质速率角度入手，通过对该过程动力学的系统研究，解析除砷控速因素及不同形态砷离子的膜相传质行为，指导除砷过程强化策略。在动力学研究基础上，通过制备并使用负载二氧化锰功能化改性离子交换膜强化道南渗析去除三价砷效果；通过构建"道南渗析-零价铁"耦合系统实现富砷解吸液原位再生，强化道南渗析长期运行过程中传质和除砷效果，阐明盐含量对零价铁除砷行为的影响机制。项目研究所形成的基于道南渗析膜过程的饮用水除砷技术方案将有助于推动我国农村饮水安全问题的解决。

道南渗析（Donnan dialysis）是道南膜原理的典型应用，是以电化学势梯度为推动力的选择性膜过程，可以实现目标物质逆浓度梯度的迁移。尽管道南渗析家庭净水除砷装置具有结构简单、操作简便、易于维护、运行成本低廉等优点，但较低的五价砷去除效率以及无法去除三价砷限制了该项技术的应用。针对上述问题，本项目研究了道南渗析除砷动力学、制备了具有三价砷氧化能力的功能膜并构建适用于长期运行的“道南渗析-零价铁”耦合系统。主要完成内容如下：. 以能斯特–普朗克方程（Nernst–Planck equation）为基础，构建了描述稳定态下As(V)膜相扩散的简单模型，并结合实验数据计算出As(V)膜相自扩散系数。该系数在10-8 m2 h-1数量级，大小与系统pH和膜性能密切相关。As(V)膜相自扩散系数、As(V)在溶液和膜间的分配系数、膜厚度和膜有效面积等模型参数会在不同程度上影响道南渗析除砷速率或平衡时料液中残留的砷浓度。该模型可对不同实验条件下道南渗析除砷过程作出准确预测。. 通过将阴离子交换膜浸泡于KMnO4溶液或交替浸泡于KMnO4和MnCl2溶液的方式在膜表面负载MnO2薄层得到功能膜，并以扫描电子显微镜、X射线衍射和X射线光电子能谱等方式对该层MnO2形态进行表征。改性后的膜具有氧化三价砷（As(III)）为As(V)的能力。因此，道南渗析系统对含As(III)溶液的处理效果明显改善，24 h去除效率达到80%以上。As(III)的氧化为该除砷过程的速率控制步骤。转移至解吸液的砷中，大部分为以电化学势差推动传质的As(V)，少部分为以浓差扩散推动传质的As(III)。重复使用不会降低改性膜的运行效果。. 由于零价铁（Fe0）腐蚀产物具有吸附砷的作用，在解吸液中加入Fe0后，可以显著降低其中游离态砷浓度，从而在处理过程中实现解吸液的同步再生，改善道南渗析除砷系统长期运行效果。道南渗析–Fe0耦合工艺连续运行20批次后，解吸液砷浓度仅为出水的20倍，推动As(V)跨膜传质的电化学势差因此得到较好维持。