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Membrane fouling: microscopic insights into the effects of surface chemistry and roughness

Fouling is a major obstacle and challenge in membrane-based separation processes. Caused by the sophisticated interactions between foulant and membrane surface, fouling strongly depends on membrane surface chemistry and morphology. Current studies in the field have been largely focused on polymer membranes. Herein, we report a molecular simulation study for fouling on alumina and graphene membrane surfaces during water treatment. For two foulants (sucralose and bisphenol A), the fouling on alumina surfaces is reduced with increasing surface roughness; however, the fouling on graphene surfaces is enhanced by roughness. It is unravelled that the foulant-surface interaction becomes weaker in the ridge region of a rough alumina surface, thus allowing foulant to leave the surface and reducing fouling. Such behavior is not observed on a rough graphene surface because of the strong foulant-graphene interaction. Moreover, with increasing roughness, the hydrogen bonds formed between water and alumina surfaces are found to increase in number as well as stability. By scaling the atomic charges of alumina, fouling behavior on alumina surfaces is shifted to the one on graphene surfaces. This simulation study reveals that surface chemistry and roughness play a crucial role in membrane fouling, and the microscopic insights are useful for the design of new membranes towards high-performance water treatment.


Introduction
In recent years, water scarcity has become a major concern worldwide due to ever-growing population, climate change and water pollution [1,2]. Clean water is indispensable for global health and economic development. It has become a grand challenge to provide clean water in a reliable and affordable manner. As an energy efficient and cost effective technology, membrane-based water treatment has a wide variety of applications in seawater or brackish water desalination, municipal or industrial wastewater purification. However, one of the key issues in membrane-based water treatment is membrane fouling. It adversely affects membrane performance by reducing water permeability, deteriorating water quality, lowering membrane lifetime, and increasing energy consumption [3,4].
Microscopically, fouling is caused by sophisticated interactions between membrane surface and foulant, which strongly depends on the chemistry and morphology of membrane surface.
In this respect, surface modification is widely adopted to improve membrane anti-fouling capacity [5][6][7]. At present, most studies in this field have primarily investigated the effect of surface chemistry on fouling [8]. Nevertheless, surface morphology or roughness also has a significant impact. A number of studies have shown that surface roughness can enhance fouling.
For example, Elimelech et al. found that colloidal particles were preferentially accumulated in the valleys of rough aromatic polyamide thin-film composite membranes, which resulted in "valley clogging" and severely reduced water flux [9,10]. Based on a mathematical model, they explained that the valleys created by surface roughness produced a lower interaction well in which colloidal particles might preferentially deposit [11]. Using both experimental and simulation techniques, Jun et al. investigated the fouling of poly(methyl methacrylate) (PMMA) colloidal particles on polyvinylidene fluoride (PVDF) membrane surface with periodically spaced prism shape. A stagnant flow zone was found to form in the valley of patterned surface with more particles deposited [12]. On three commercial polyamide thin-film composite reverse osmosis (RO) membranes with different roughness and hydrophilicity, Yin et al. observed that the fouling increased with an increase in surface roughness [13].
Interestingly, the opposite effect of surface roughness on fouling (i.e., reduce fouling) was also reported. Gohari et al. fabricated flat sheet poly(ether sulfone) (PES) membranes with aligned patterns by incorporating hydrophilic nanoparticles for the ultrafiltration of bovine serum albumin (BSA). When feed solution flowing perpendicular to the patterns, they observed that the fouling could be mitigated; however, the fouled surface was washed more effectively for parallel feed solution [14]. Using a newly developed layered interfacial polymerization, Choi et al. produced a sharklet-patterned membrane, which exhibited remarkably lower biofouling compared to conventional membranes with irregular roughness and simple patterns [15]. The sharklet patterned membrane was revealed to possess optimal anti-biofouling effect if the unit and pattern spacings were both 2 μm, as attributed to a balance between intrinsic biofouling propensity and surface flow characteristics such as vortex and primary/secondary flows [16]. Through the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and density functional theory (DFT) calculations, Li et al. examined the effect of surface roughness on membrane fouling caused by alginate adhesion. A rough membrane was found to be less alginate adhesion and adhesive fouling than a smooth counterpart [17].
As discussed above, fundamental understanding in the effect of surface roughness on membrane fouling is elusive and most of the current studies are focused on polymer membranes.
In the past decade, ceramic membranes have been increasingly applied for industrial/municipal wastewater and drinking water treatment [18]. Compared to polymer membranes, ceramic membranes particularly alumina-based exhibit stronger mechanical strength, higher water permeability, better chemical resistance, as well as longer lifetime. Recently, Lyu et al.
developed a surface-patterned alumina ceramic membrane with gradient porous structure by using a 3D-printing technology, and the line-patterned membrane showed a notable increase in water flux and a significantly enhanced antifouling ability [19]. On the other hand, carbon membranes like graphene have also received attention due to their unique advantages such as low toxicity, flexibility in storage and handling, easy processing and modification [20].
Practically, the surface roughness of alumina and graphene membranes has been revealed to significantly affect water transport and separation performance [21][22][23][24]; however, the impact on fouling is less clear. Herein, a molecular simulation study is reported to investigate the effects of both surface chemistry and roughness on membrane fouling during water treatment. As illustrated in Fig.   1a and 1b, we consider two types of membranes (hydrophilic alumina and hydrophobic graphene), thus representing different surface chemistry. For each type, the surface roughness is tuned by varying amplitude A (A = 0, 3 and 5 Å, respectively). Two organic molecules, sucralose (SUC) and bisphenol A (BPA), as illustrated in Fig. 1c and 1d are used as foulants.
SUC is a chlorinated carbohydrate, while BPA is a xenoestrogen [25]. Both foulants are commonly present in wastewater and have a similar size of about 1.2 nm. Following this introduction, the simulation models and methods are briefly described in Section 2. In Section 3, foulant mobility is first examined on the two types of membrane surfaces; then, the effects of surface chemistry and roughness on fouling are discussed in detail. Finally, the concluding remarks are summarized in Section 4. and y-axes. The alumina surface was considered to be fully hydroxylated with 15 hydroxyl group per nm 2 [26]. Along the z-axis, the alumina surface consisted of two layers of aluminum (Al) and oxygen (O) atoms underneath hydrogen (H) atoms [23,24].

Models and Methods
For rough alumina and graphene surfaces, the roughness was introduced onto a flat surface by using a sinusoidal function [24] = A sin (   2   40   ) (1) were A is the wave amplitude, is the atomic position along the x-axis. For each type of surface, three different amplitudes (A = 0, 3 and 5 Å represented by A0, A3 and A5, respectively) were examined. The atoms in alumina and graphene were described by the nonbonded Lennard-Jones (LJ) and electrostatic potentials: where and are the potential well and collision diameter, is the distance between atom i and j, is the charge of atom i, and 0 is the permittivity of vacuum. The LJ parameters and atomic charges of alumina were adopted from the CLAYFF force field [27], which is widely used for hydroxylated oxide materials such as alumina, aluminosilicates and silica. The LJ parameters of graphene were taken from the OPLS-AA force field [28]. Water was mimicked by the SPC/E model [29], because of its simplicity and capability to reproduce water diffusivity [30]. The LJ and bonded parameters of SUC and BPA were based on the AMBER force field [31], while their atomic charges were estimated from density functional theory calculations using GAUSSIAN 09 package [32].
Each system was first equilibrated by molecular dynamics (MD) simulation for 100 ps, with a pressure of 1 bar applied on the plate above the water compartment. Then, the MD simulation continued for 10 ns with the plate fixed. The simulations were performed at 300 K using velocity-rescaling thermostat and the time step was 2 fs. The electrostatic interactions were calculated by the Particle-Mesh Ewald method, while the LJ interactions were estimated using a cutoff of 1.2 nm. During the 10 ns MD simulation, the foulant molecule was free to move and its mean-squared displacement (MSD) was averaged from the second 5 ns trajectory: where t is time and () r t  is the displacement of the foulant molecule. For each system, six independent runs were performed with different random seeds to generate initial velocities. In addition, the potentials of mean force (PMFs) for foulant on flat alumina graphene surfaces were calculated separately. More specifically, a force was applied to steer foulant molecule moving towards the flat surface. Umbrella sampling [33] was used involving 21 simulations with 0.05 nm interval along the z-axis. In each simulation, the position of foulant was restrained in the z-axis by a harmonic potential with a force constant of 8000 kJ mol −1 nm −2 . The PMF was then generated from 3 ns simulation using the weighted histogram analysis method [34].
All the simulations in this study were conducted using GROMACS 5.1.4 [35].

Foulant mobility
The MSD of foulant can be used to assess membrane fouling. When a foulant molecule is less mobile, it tends to deposit on membrane surface and leads to fouling [36,37]. fouling, which will be discussed in detail below.

Effect of surface chemistry
To quantitatively examine the effect of surface chemistry on fouling, we focus on the flat alumina and graphene surfaces.    Fig. 3 and Fig. 4, apparently, the PMFs of both foulants on the graphene surface are much deeper than on the alumina surface. The reason is that the hydrophobic graphene surface interacts more strongly with the two organic foulants compared to the hydrophilic alumina counterpart. Interestingly, as shown in Fig. 2, the MSDs of the two foulants appear to be larger on the graphene surface than on the alumina surface. With a closer look at the respective MSDs in the z and xy directions, as shown in Fig. S1, we can find that the foulant has a negligible MSDz but a significant MSDxy. This is attributed to the atomic smooth surface of graphene, thus the foulant can move quite freely parallel to the surface. However, the movement perpendicular to the surface is largely prohibited due to strong interaction. Such a phenomenon was also observed in a simulation study for polymer chains adsorbed on the graphene surface [38].

Effect of surface roughness
We use a harmonic potential to steer a foulant molecule moving along different surfaces by varying roughness (see Fig. 5a). In this way, the foulant is used as a probe to quantify the foulant-surface interaction in the x-axis. Fig. 5b and Fig. S2b show the interaction energies of Thus, the surface roughness has a significant impact on foulant-surface interaction. The different interactions in the valley and ridge regions are closely related to the variation in foulant-surface contact areas. Fig. 5c and Fig. S2c show the contact areas between the two foulants and graphene surfaces. On the flat surface, the contact area is nearly a constant, as also seen in the interaction. On the rough surface, the contract area increases in the valley and accordingly, the interaction becomes stronger. This leads to "valley clogging" effect as experimentally observed [10]. By contrast, the contact area in the ridge decreases, which causes weaker foulant-surface interaction. In principle, the ridge is the place where foulant is most likely to depart from the surface. Nevertheless, the foulant interacts strongly with graphene surface; the interaction energies are about -110 kJ mol -1 for SUC and -75 kJ mol -1 for BPA even in the ridge of graphene surface A5, thus preventing foulant from leaving the surface. For alumina surfaces, the interaction energies of BPA and SUC are plotted in Fig. 6b and On the graphene surface A5, there is always strong interaction between foulant and surface in the ridge (see Fig. 5b and Fig. S2b) and the foulant is always adsorbed on the surface. On the alumina surface A5, however, the foulant may leave the surface in the ridge region as illustrated in Fig. S5, and hence the interaction may decrease to 0 (see Fig. 6b and Fig. S4b). Fig. S6 shows the simulation snapshots of BPA on the alumina surface A5 without imposing a harmonic potential. The foulant is initially adsorbed on the surface at 1.4 ns, a bit away from the surface at 2.6 ns, and eventually leave the surface at 8.9 ns. Fig. 6c shows the contact areas between BPA and alumina surfaces. As also observed on graphene surfaces, the contact area increases in the valley and decreases in the ridge. On alumina surface A5, the contact area may become 0, which implies the foulant leaves the surface and the foulant-surface interaction decreases to 0 accordingly. From these results, apparently, fouling on alumina surfaces is reduced upon increasing roughness. This prediction is accord with a recent experimental study, in which a 3D-printed line-patterned alumina membrane was observed to be less fouling [19]. To further quantify the effect of roughness on hydrophilic alumina surfaces, we analyse the hydrogen bonds between water and alumina surfaces as illustrated in Fig. 7a. Fig. 7b shows the numbers of hydrogen bonds on different alumina surfaces. With increasing roughness and hence increasing surface area, more hydrogen bonds are formed between water and the surface.
The dynamics of hydrogen bonds can be evaluated by autocorrelation function:  It is instructive to explore how the fouling behavior on hydrophilic alumina surfaces changes to that on hydrophobic graphene counterparts. To achieve this, we manipulate the hydrophilicity of alumina surfaces by scaling the atomic charges of alumina. For example, if a scaling factor is 0.5, the atomic charges are only 0.5 times of the actual alumina. Such a simple scaling method was used in the literature to change the hydrophilicity of MgO surface [39].

Fig. S7
shows the interaction energies between water and alumina surfaces with different scaling factors. By decreasing the scaling factor, the water-surface interaction becomes weaker and apparently the surface is less hydrophilic. Fig. 8 shows the MSDs of BPA on alumina surface with different scaling factors. On the actual alumina surfaces (i.e., scaling factor = 1), as shown in Fig. 2a, the MSD rises upon increasing roughness and hence fouling is reduced.
When the scaling factor = 0.7, the MSD is nearly equal on A5 and A0 surfaces. When the scaling factor = 0.1, at which the surface is highly hydrophobic, the trend of MSD resembles that on graphene surfaces in Fig. 2b; i.e., the MSD drops on a rougher surface and fouling is enhanced. Clearly, these results reveal that the fouling behavior can be tailored by changing the hydrophilicity or hydrophobicity of surfaces.

Conclusions
We have conducted molecular simulations to investigate the effects of surface chemistry and roughness on membrane fouling. For two foulants (sucralose and bisphenol A), the mobility on alumina surfaces is found to rise with increasing roughness and hence fouling is reduced by roughness. The opposite trend is observed on graphene surfaces, i.e., the mobility of foulant drops and fouling is enhanced with increasing roughness. These interesting effects of roughness on fouling are attributed to the different foulant-surface interactions and contact areas between alumina and graphene surfaces. It is also found that upon increasing roughness, more hydrogen bonds with longer lifetime are formed between water and alumina surface.

Notes
The authors declare no competing financial interest.