Investigations Of Separation Performances, Electrokinetical Properties And Transport Coefficients In Charged Porous Membranes

Charged porous membranes are widely used in water treatment, which are very important to the more and more intense water resources problems. Studies on the transport phenomena of charged porous membranes can provide guidance in optimizing the existing membrane processes or developing and designing a novel membrane processes with high performance. These studies can be divided into three aspects, transport coefficients, separation performances and electrokinetical properties. However, there are some problems to be solved in each aspect, such as the estimation of transport coefficients for highvalent single salts solutions, the evaluation of separation performances for mixed salts solutions, and the estimation of electrokinetical properties for highvalent single salts solutions. In this paper, these problems were solved by model building, numerical simulation and experimental measurements.Firstly, the calculating methods of the space charged (SC) model and the fixed charged (TMS) model were studied and developed, which enabled the two models to describe the transport coefficients (the reflection coefficient, the solute permeability coefficient, the electrical conductance and the electrical transport number) for multivalent salt solutions and not only for 1-1 salt solutions.Moreover, the irreversible thermodynanics model and the TMS model were extended to mixed salts solutions, and the expression of reverse osmosis (σ) for total mixed salts in mixed salts solutions was obtained. Through simplified treatment, the approximate expression ofσfor single salt andσfor total mixed salts was achieved. Based on the approximate expression, a novel model for the observed transmissions of total mixed salts and ions was proposed. Compared with the experiment, the model could exactly predict the separation performances for bi-component salts solutions and complex mixed salts solutions with 6 ions.With the established calculating methods in this paper, the SC model and the TMS model could predict the electrokinetical properties (membrane potential and streaming potential) for multivalent salt solutions and not only for 1-1 salt solutions.As for the membrane potential, the TMS model was used to fit the experimental data of five commercial nanofiltration membranes (NTR 7450, ESNA 1, ESNA 1-LF, LES 90 and UTC 60). The simulation were calculated and compared under two kinds of initial conditions: a uniform and an exponential variation of fixed charge density (X_m) with the bulk salt concentration. Based on the TMS model, the membrane potential and the induced X_m could be fitted better under the condition of the exponential variation of X_m with the bulk salt concentration.Concerning the streaming potential, the analysis method to separate streaming potential from the measured transmembrane potential was established. According to the TMS model, a newly modified Helmholtz-Smoluchowski (HS) equation was proposed, which could be used to characterize the charged properties of membranes based on the separated streaming potential. The modified HS equation was employed to fit the streaming potential of three commercial nanofiltration membranes (NTR 7250, ESNA 1-K and LES 90), and the fitted X_m was agreed well with that obtained by membrane potential methods.