电场对导电陶瓷微滤膜离子截留的作用及机理研究

It was found that the ion rejection rate of conductive ceramic microfiltration (MF) membranes could be increased from less than 20% to more than 90% with external electric field in our previous experiments. In this research, the interactions between the ions and conductive ceramic MF membranes and their influences on the ion distribution and transportation and rejection will be investigated in order to analyze the mechanisms of ion rejection with the electric field. For this purpose, the influences of electric field on the electrokinetic and ion rejection rate of conductive ceramic MF membranes will be explored systematically under different parameters, such as membrane structure (including pore size, porosity, tortuosity and thickness), permeation rate, types of ion and ion concentration, pH value of the solutions. The focus will be put on the clarification of the specific roles of external electric field and the corresponding key parameters. The interrelation between the separation performance and electrokinetic and membrane structure parameters would be study to establish the mathematical model to describe the transport phenomena (ion rejection) of conductive ceramic MF membranes based on the Space Charge model. The established model will consist of three equations to describe the potential and ion concentration distributions, ion transportation and permeate flux, respectively. The boundary conditions and the corresponding numerical analytic methods will also be established. The researches of this project are expected to overcome the technological bottlenecks which limited the applications of MF membranes in water desalination, and could provide scientific theory basis for the applications of MF membranes in desalination.

针对前期实验研究中首次发现的导电陶瓷微滤膜在外电场作用下可明显提高膜的离子截留率的现象（从无电场时的≤20%提高至90%以上）。本项目拟研究导电陶瓷微滤膜在电场作用下膜与离子间的相互作用关系及其对离子分布、迁移及截留的作用，并揭示其离子截留作用机理。详细研究不同的膜结构参数（包括膜孔径、孔隙率、孔曲折度和膜厚度）、渗透液流速、溶液中离子种类和浓度及pH值等条件下，电场对导电陶瓷微滤膜的动电性能和离子截留率的影响规律，分析外电场的有效作用及范围。研究膜的分离性能与其动电性能和结构参数之间的相关性，借助空间电荷模型推导出适合于描述电场作用下导电陶瓷微滤膜孔内电位分布和离子浓度分布、离子传递及渗透通量等的相关基本方程，建立微滤膜的传递现象模型及其边界条件和解析方法，突破陶瓷微滤膜不能在水脱盐处理领域应用的科学技术瓶颈，为指导微滤膜在脱盐领域中应用提供科学理论依据。

在前期实验的基础上，采用La0.7Ca0.3CrO3 /Al2O3、La1-xCaxCrO3（x=0~3）和氧化石墨烯改性Al2O3等不同材料制备了导电陶瓷微滤膜。研究了制备工艺条件对膜支撑体和分离膜层的物相组成、结构和性能的影响，在优化的工艺条件下制备出不同结构参数和电导率的导电陶瓷微滤膜。详细研究了外电场作用下，外电场强度和方向、操作压力、盐溶液pH值、盐离子种类和浓度、膜孔径、膜电导率、过滤方式等参数条件对导电陶瓷微滤膜的离子截留率和动电性能的影响过程与规律，分析了外电场的有效作用及范围。结果表明，外电场作用下，膜在酸性条件下(pH≤4)的Zeta电位和膜面电荷密度明显提高，且随着电场强度提高，膜的离子截留率表现出先增大后减小的变化趋势，其中在外电场为9V( pH=3.5)时，92nm的片状La0.85Ca0.15CrO3陶瓷膜对1mmol/L 的CaCl2溶液的离子截留率最高为82.6%，而80nm的片状氧化石墨烯改性Al2O3膜对1mmol/L MgCl2盐溶液的离子截留率最高达到87.3%，进一步提高外电场强度后，离子截留率明显降低。为有效提高膜的离子截留率，微滤膜的电导率和膜孔径应分别控制在0.1~0.6S/m和50~100nm。外电场作用下，导电陶瓷微滤膜离子截留率显著提高的机理与膜的动电性能变化密切有关。外电场作用显著提高了膜面电荷密度，膜/溶液界面双电层排斥作用力大大增强，从而在道南效应的协同作用下使膜的离子截留率显著提高。膜电荷密度提高引起的双电层德拜长度增大和膜有效孔径减小也是膜的离子截留率提高的重要原因。借助经典的理论模型，初步建立了可描述电场作用下导电陶瓷微滤膜离子分离过程的模型方程。项目成果突破了陶瓷微滤膜不能应用于溶液中离子分离的科学技术瓶颈，为微滤膜在金属离子(特别是重金属)污染废水处理中的应用提供了实验和理论依据。