Surface Modification of Water Purification Membranes

Abstract

Polymeric membranes are an energy-efficient means of purifying water, but they suffer from fouling during filtration. Modification of the membrane surface is one route to mitigating membrane fouling, as it helps to maintain high levels of water productivity. Here, a series of common techniques for modification of the membrane surface are reviewed, including surface coating, grafting, and various treatment techniques such as chemical treatment, UV irradiation, and plasma treatment. Historical background on membrane development and surface modification is also provided. Finally, polydopamine, an emerging material that can be easily deposited onto a wide variety of substrates, is discussed within the context of membrane modification. A brief summary of the chemistry of polydopamine, particularly as it may pertain to membrane development, is also described.

Membranes with Surface-Enhanced Antifouling Properties for Water Purification

“…As an energy-efficient and low-cost technology, polymeric membranes permeate pure water and reject contaminants ranging from bacteria in microns to ions in angstroms [1,2,3,4,5]. For example, microfiltration (MF) membranes with pore sizes of 1–100 μm can remove microbes, cells and bacteria [1,3]; ultrafiltration (UF) membranes with pore sizes of 1–100 nm can remove small contaminants, such as proteins and viruses [3,6]; nanofiltration (NF) membranes having pore sizes of a few angstroms can remove divalent ions (e.g., Ca 2+ , Mg 2+ , Fe 2+ ) and small molecules with a molecular weight of 200–1000 Da [3]; and reverse osmosis (RO) membranes with a dense selective layer that can desalinate brackish water and seawater [4,5,7]. The core of membrane technology is high performance membranes with high water permeance and high selectivity in a practical environment [1,2,3,4,5].…”

“…While the skin layer has been designed to be as thin as possible to increase water permeance, contaminants in the feed water may deposit and accumulate on the membrane surface (i.e., external fouling, as shown in Figure 1a), which would dramatically decrease water flux (as shown in Figure 1b) [1,3]. For MF and UF membranes, the contaminants may even block the internal pores (i.e., internal fouling, as shown in Figure 1a).…”

“…Two general approaches have been widely explored: coating a thin film on the membrane surface (as shown in Figure 2a) and grafting of polymer chains on the surface (as shown in Figure 2b) [1,3,10]. The materials for coating or grafting do not have affinity towards the foulants (e.g., proteins, emulsion and organic compounds), thus avoiding any favorable interactions between the foulants and membranes.…”

“…For example, membranes with a rough surface are more susceptible to fouling because foulants can deposit in valleys on the membrane surface, preventing it from being removed by hydrodynamic force [1,11,12], though membranes with a nanopatterned surface have also been shown to improve antifouling properties [13,14]. Surface charge can promote fouling from foulants with counter-charges due to favorable electrostatic interactions.…”

Fit-for-purpose block polymer membranes molecularly engineered for water treatment

“…This development alone, however, will likely not be sufficient to meet the world's thirst for water in a sustainable manner due to the high energy use associated with desalination processes. 98 This need for less energy-intensive methods of producing usable water has led to interest in potable and non-potable water reuse as well as resource recovery processes. In these cases, the indiscriminate removal of all solutes is often unnecessary or undesired.…”