Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes

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Abstract

Recent studies have shown that membrane surface morphology and structure influence permeability, rejection, and colloidal fouling behavior of reverse osmosis (RO) and nanofiltration (NF) membranes. This investigation attempts to identify the most influential membrane properties governing colloidal fouling rate of RO/NF membranes. Four aromatic polyamide thin-film composite membranes were characterized for physical surface morphology, surface chemical properties, surface zeta potential, and specific surface chemical structure. Membrane fouling data obtained in a laboratory-scale crossflow filtration unit were correlated to the measured membrane surface properties. Results show that colloidal fouling of RO and NF membranes is nearly perfectly correlated with membrane surface roughness, regardless of physical and chemical operating conditions. It is further demonstrated that atomic force microscope (AFM) images of fouled membranes yield valuable insights into the mechanisms governing colloidal fouling. At the initial stages of fouling, AFM images clearly show that more particles are deposited on rough membranes than on smooth membranes. Particles preferentially accumulate in the “valleys” of rough membranes, resulting in “valley clogging” which causes more severe flux decline than in smooth membranes.

Introduction

Successful utilization of membrane technology has been greatly limited by membrane fouling. Fouling increases operation and maintenance costs by deteriorating membrane performance and ultimately shortening membrane life. Numerous studies in recent years have investigated the causes and control of membrane fouling, and substantial progress has been made. However, in many applications colloidal fouling of membranes continues to be a serious problem, thus pointing out to the paramount importance of understanding the fundamental physical and chemical mechanisms that govern colloidal fouling of membranes.

Recent studies have shown that membrane surface morphology and structure influence performance characteristics of membranes [1], [2], [3], [4], [5], [6], [7], [8], [9]. Hirose et al. [9] suggested an approximately linear relationship between membrane surface roughness and permeate flux for crosslinked aromatic polyamide reverse osmosis (RO) membranes, where permeability increased with increasing surface roughness. The linear relationship was attributed to surface unevenness of the RO membrane skin layer, which resulted in enlargement of the effective membrane area. Kwak et al. [4] showed that substitution of bisphenol biphenyl rings with either methyl or halogen strongly influenced rejection and permeability of aromatic polyester RO membranes. Higher flux and lower rejection were associated with the smoother membrane surfaces obtained from methyl substitution, while lower flux and higher rejection were associated with the rougher membrane surfaces resulting from halogen substitution. Additional work by Kwak and Ihm [5] coupling nuclear magnetic resonance (NMR) spectroscopy and atomic force microscopy (AFM) has shown an important relationship between proton spin–lattice relaxation times and RO permeability, regardless of surface morphological features. The latter two studies suggest that membrane performance (flux and rejection) is strongly influenced by the structure of the polymer network which constitutes the thin-film active layer.

Several fundamental investigations of membrane fouling have explored the effects of membrane surface properties such as pore size and pore size distribution, surface roughness and structure, electrokinetic (zeta potential) characteristics, chemical properties (hydrophobic/hydrophilic), and specific chemical structure [1], [3], [6], [7], [8]. Various analytical techniques have been employed for elucidating specific physical and chemical surface properties of membranes, including Raman spectroscopy (structure) [1], electron spin resonance (solute mobility in membrane polymer matrix and pores) [1], AFM (surface morphology, structure, and pore size) [1], [2], [3], [4], [5], [6], [7], [8], [9], streaming potential (membrane surface zeta potential) [10], [11], NMR spectroscopy (permeability) [5], contact angle [12], and X-ray photoelectron spectroscopy (XPS) for surface chemical functional groups [13], [14]. Despite these efforts, however, the role of membrane surface properties in colloidal fouling of RO/NF membranes is still not well understood.

This investigation relates several key membrane surface properties of four commercial RO/NF membranes to their initial colloidal fouling behavior during crossflow membrane filtration. The aromatic polyamide thin-film composite membrane surfaces were characterized for morphological properties (AFM), contact angle, zeta potential, and specific chemical structure (XPS). Membrane fouling, determined by percent of flux decline for a specific volume of permeate filtered, was correlated to the measured membrane surface properties. It was demonstrated that colloidal fouling of the RO and NF membranes was strongly correlated only with membrane surface roughness. A novel mechanistic explanation for the striking effect of membrane surface roughness on colloidal fouling behavior is proposed.

Section snippets

Membranes

Four commercial RO/NF thin-film composite membranes were used in this study. The RO membranes were Hydranautics LFC-1 (Oceanside, CA) and Trisep X-20 (Goleta, CA). The NF membranes were Dow-FilmTec NF-70 (Minneapolis, MN) and Osmonics HL (Minnetonka, MN). All membranes were stored in deionized (DI) water at 5°C with water replaced regularly. The membranes were characterized for intrinsic physical and chemical properties such as zeta potential, roughness (AFM), contact angle, chemical

Membrane fouling experiments

Several fouling experiments were performed to provide a basic understanding of the influence of physical and chemical operating (feed) conditions on colloidal fouling behavior of the test membranes. The effects of varying initial flux, ionic strength, and crossflow velocity (wall shear rate) were systematically investigated. The expected behavior for each condition was observed, whereby decreasing the initial flux or ionic strength or increasing the crossflow velocity (shear rate) significantly

Conclusion

Laboratory-scale experiments were conducted to investigate the role of membrane surface properties on the initial rate of RO/NF membrane colloidal fouling. Membranes were characterized for key physical and chemical surface properties, and those properties were correlated with fouling data. In all cases, regardless of physical and chemical operating conditions, the rate and extent of colloidal fouling was most significantly influenced by the physical roughness of membrane surfaces. It was

Acknowledgements

The authors thank the American Water Works Association Research Foundation for supporting this project through Project No. 2514. The authors would also like to thank Dr. Amy Childress of the University of Nevada, Reno for performing the contact angle measurements; Hydranautics, Dow-FilmTec, Trisep, and Osmonics for supplying membranes; and Nissan Chemical America Corp. for providing the colloidal silica particles.

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