Investigation of Biofoulant-Membrane Surface Interactions Through Advanced Surface-Sensitive Techniques

Introduction

Membrane water treatment processes suffer severely from (bio)fouling phenomena, defined as an undesired deposition and build-up of adsorbed materials, which alters the membrane surface and causes the clogging of membrane pores and hence a reduction of the membrane flux. Elucidating the phenomena that take place on the membrane surface during (bio)fouling requires the use of adequate characterization techniques. Membrane autopsy is the most common technique used to identify and determine the causes of fouling events in membrane systems making use of fouled membranes that already exhibit a poor performance, and certainly provide some hints on how and when to apply corrective measures. However, the control of membrane (bio)fouling is directly related to first the (bio)foulant agent – membrane surface interactions arising at a much earlier stage during the process, and neither the early-stage biofoulant-membrane surface interactions nor the conditions leading to their occurrence are completely understood, yet. Therefore, there is a strong need for real time and on line monitoring of these phenomena providing a fundamental understanding and an appropriately fast response for operators to act and adjust their operational conditions in order to minimize the effect of (bio)fouling on the membrane process. This study presents surface plasmon resonance (SPR), quartz crystal microbalance with dissipation monitoring (QCM-D) and ellipsometry as advanced surface-sensitive techniques for the real time and non-invasive characterization of (bio)foulant-membrane surface interactions. These techniques will also be shown to prove powerful for characterizing and monitoring antifouling measures, such as responsive membrane surface layers.

 

Methods

QCM-D, SPR and ellipsometry techniques were used for the on-line characterization of the biofouled polymer membrane surface. QCM-D is a mass sensitive technique that simultaneously monitors the resonant frequency (Df) and the dissipation factor (D) of the quartz crystal sensor. The novelty of using QCM-D as a non-invasive technique for characterizing (bio)fouling of polymers is that in addition to quantitative information on the adsorbed mass on the polymer surface, it provides information on the viscoeleasticity of the adsorbed layer which can be correlated to the compactness of the fouling layer. On the other hand, it also provides important information on the efficiency and kinetics of cleaning procedures. SPR and ellipsometry are complementary optical methods which, based on changes in refractive index, allow determining the amount of adsorbed material per unit membrane area. In this preliminary work, the interaction of two types of biofoulant agents (bovine serum albumin as protein and alginic acid as polysaccharide) with the membrane polymer surface will be presented in terms of kinetics, adsorbed mass and thickness of the fouling layer. Furthermore, poly(4-vinylpyridine)  (P4VP) was used as a responsive polymer and its pH responsiveness was evaluated using the above mentioned techniques.

 

Results

The adsorption of bovine serum albumin (BSA) and alginic acid on P4VP surface was evaluated by QCM-D and SPR. The combination of these two techniques allowed determining the “dry” and “wet” masses of both BSA and alginic acid on the P4VP surface, yielding in this way information on the “softness” of the layers adsorbed.  The adsorbed “dry” mass of BSA on P4VP was measured using SPR while the “wet” mass during the adsorption of BSA was detected by QCM-D. SPR and QCM-D determined adsorbed “dry” and “wet” masses of BSA on P4VP to be 12ng/cm² and 188ng/cm², respectively, indicating that the adsorbed BSA layer contained more than 90% of water.  In contrast, alginic acid was found to adsorb as a “rigid” layer with the adsorbed masses/membrane area being very similar between SPR and QCM-D. With regard to the responsiveness of P4VP to changes in pH, SPR, QCM-D and ellipsometry proved to provide quantitative and reproductive data on the reversible swelling of the polymer.

 

It could be shown that SPR, QCM-D and ellipsometry are powerful and complementary techniques for elucidating early stage adsorption phenomena leading to membrane fouling. These techniques can also serve for determining the reversible responsiveness of polymers to external stimuli such as changes in pH.