Title: The role of foulant?foulant electrostatic interaction on limiting flux for RO and NF membranes during humic acid fouling?Theoretical basis, experimental evidence, and AFM interaction force measurement
Journal: Journal of Membrane Science
Authors: Chuyang Y. Tanga,b ,Young-Nam Kwona,c, James O. Leckiea
Corresponding author: Chuyang Y. Tang
Institute:
a Environmental Engineering and Science, Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305-4020, USA
b School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore
c Ulsan National Institute of Science and Technology, Ulsan 689-805, South Korea
The original and creativity of paper: The paper aims to establish a theoretical and quantitative model for the limiting flux. They limit flux measurements for both RO and NF membranes to verify the model. In this study, purified Aldrich humic acid (PAHA) was used as a model macromolecular foulant. After operated membrane processes, interaction force between surrogate foulant and a membrane surface was also measured by an atomic force microscope (AFM). The results show that AFM interaction force correlated well with the limiting flux values.
Summary:
A limiting flux model was developed for predicting the fouling behavior of reverse osmosis and nanofiltration membranes by organic macromolecules. They found that the limiting flux did not depend on the properties of membranes as well as the limiting flux had strong dependence on the feed water composition, such as pH, ionic strength, and divalent ion concentration. This paper considers the dependence of limiting flux on intermolecular interaction between foulant molecules. They found that the limiting flux was directly proportional to the intermolecular electrostatic repulsive force that enhances foulant-deposited-foulant repulsion promoted greater limiting flux value. The summary of research is follow:
Effect of solution composition on limiting flux
1. The limiting flux showed independence on membrane initial flux. Moreover, the limiting flux not likely to be affected by membrane properties.
2. At lower pHs, the limiting flux decreased radically due to carboxylic and phenolic functional groups of humic acid appear as a significant portion, causing a reduced charge density of the humic acid molecules resulted to a significant reduction in foulant?foulant electrostatic interaction.
3. The greater divalent calcium ions concentration affected limiting flux reduction because this condition may introduce partial charge neutralization, effectively reducing the foulant?foulant electrostatic repulsion.
4. Humic acid concentration had no effect on the limiting flux. It affects only the frequency of foulant molecules approaching a membrane surface and also the rate of foulant deposition.
AFM interaction force measurement
1. Using AFM, they found that at higher pHs the interaction force became more repulsive due to increased charge density resulted to electrostatic interaction.
2. Higher concentration of calcium creased more attractive force due to the formation of carboxylate?Ca2+ complex and partial charge neutralization.
3. AFM interaction force showed good correlation with limiting flux measurement. Therefore, AFM interaction force can be used to forecast organic fouling. However, large uncertainties of AFM measurement, due to the non-homogeneity of membrane and foulant layers at submicron scales, should be considered in addition to provide a solution.
Application & further study: For the better understanding of fouling development, interaction force between foulant ?foulant and foulant-deposited foulant is meaningful to investigate.
By Monruedee Moonkhum
Email: moon@gist.ac.kr
