Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration 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.
References (22)
- et al.
Characterization of synthetic membranes by Raman spectroscopy, electron spin resonance and atomic force microscopy: a review
Polymer
(2000) - et al.
Surface structure and phase separation mechanism of polysulfone membranes by atomic force microscopy
J. Membr. Sci.
(1999) - et al.
Biofouling potentials of microporous polysulfone membranes containing a sulfonated polyether-ethersulfone/polyethersulfone block copolymer: correlation of membrane surface properties with bacterial attachment
J. Membr. Sci.
(1999) - et al.
Correlations of chemical structure, atomic force microscopy (AFM) morphology, and reverse osmosis (RO) characteristics in aromatic polyester high-flux RO membranes
J. Membr. Sci.
(1997) - et al.
Use of atomic force microscopy and solid-state NMR spectroscopy to characterize structure-property-performance correlation in high-flux reverse osmosis (RO) membranes
J. Membr. Sci.
(1999) - et al.
Influence of membrane structure on fouling layer morphology during apple juice clarification
J. Membr. Sci.
(1998) - et al.
Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes
J. Membr. Sci.
(1997) - et al.
Effect of skin layer surface structures on the flux behaviour of RO membranes
J. Membr. Sci.
(1996) - et al.
Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes
J. Membr. Sci.
(1996) - et al.
The effect of CA membrane properties on adsorptive fouling by humic acid
J. Membr. Sci.
(1999)
XPS characterization of poly(methylhydrosiloxane)-modified cellulose diacetate membranes
J. Membr. Sci.
Cited by (1044)
Water purification advances with metal–organic framework-based materials for micro/nanoplastic removal
2024, Separation and Purification TechnologyTailoring the pore size distribution of nanofiltration membranes via surfactants with different alkyl chain lengths: Towards efficient molecular separation
2024, Separation and Purification TechnologyTailorable metal–organic framework based thin film nanocomposite membrane for lithium recovery from wasted batteries
2024, Separation and Purification TechnologyEnhancing the properties and performance of polysulfone ultrafiltration membranes using citric acid based deep eutectic solvent additives
2024, Journal of Environmental Chemical EngineeringEffect of temperature on organic fouling and cleaning efficiency of nanofiltration membranes for loch water treatment
2024, Separation and Purification Technology