Molecular Mechanisms of Ultrafiltration Membrane Fouling in Polymer

Article pubs.acs.org/est

Molecular Mechanisms of Ultrafiltration Membrane Fouling in Polymer-Flooding Wastewater Treatment: Role of Ions in Polymeric Fouling Guicai Liu,† Shuili Yu,*,† Haijun Yang,‡ Jun Hu,*,‡ Yi Zhang,‡ Bo He,§ Lei Li,† and Zhiyuan Liu† †

School of Environmental Science and Engineering; State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai, China, 200092 ‡ Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, PO Box 800-204, Shanghai 201800, P.R. China § Shandong Academy of Environmental Science; Jinan, China, 250013 S Supporting Information *

ABSTRACT: Polymer (i.e., anionic polyacrylamide (APAM)) fouling of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes and its relationships to intermolecular interactions were investigated using atomic force microscopy (AFM). Distinct relations were obtained between the AFM force spectroscopy measurements and calculated fouling resistance over the concentration polarization layer (CPL) and gel layer (GL). The measured maximum adhesion forces (Fad,max) were closely correlated with the CPL resistance (Rp), and the proposed molecular packing property (largely based on the shape of AFM force spectroscopy curve) of the APAM chains was related to the GL resistance (Rg). Calcium ions (Ca2+) and sodium ions (Na+) caused more severe fouling. In the presence of Ca2+, the large Rp corresponded to high foulant−foulant Fad,max, resulting in high flux loss. In addition, the Rg with Ca2+ was minor, but the flux recovery rate after chemical cleaning was the lowest, indicating that Ca2+ created more challenges in GL cleaning. With Na+, the fouling behavior was complicated and concentration-dependent. The GL structures with Na+, which might correspond to the proposed molecular packing states among APAM chains, played essential roles in membrane fouling and GL cleaning.

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and HA deposited on the membrane surface. Their finding first demonstrated that foulant−foulant adhesion plays a key role in determining the rate and extent of HA fouling of NF membranes. Subsequently, the same research group obtained a striking correlation between the maximum foulant−foulant adhesion forces and the flux decline rate of organic (i.e., alginate and natural organic materials) fouling of RO membranes,12 which suggested that intermolecular adhesion forces can serve as an indicator of the organic fouling potential. Similar results were reported for FO membranes using a carboxylate-modified AFM colloid probe as surrogates of alginate of bovine serum albumin (BSA), Aldrich humic acid (AHA) and EPS.13,14 Alternatively, membrane−foulant adhesion corresponds to the antifouling property of membranes. Low adhesion forces between the organic foulant (i.e., BSA, alginate and NOM) and membrane surface were observed for superhydrophilic FO membranes that exhibited high antifouling ability.2 Similarly, large membrane−-

INTRODUCTION Organic polymers, as major foulants of ultrafiltration (UF) membranes, are widespread in various waters, such as source water containing algal organic matter (AOM),1,2 municipal and industrial wastewater containing extracellular/intracellular polymer substances (EPSs),3,4 laundry wastewater containing cationic polymers,5 coagulation effluent containing synthetic polymeric coagulants,6 and polymer-flooding wastewater containing anionic polyacrylamide (APAM).7−9 In particular, APAM, as an important polymer additive for enhanced oil recovery from existing brown fields, is abundant in polymerflooding wastewater (e.g., at a volume of 3 000 000 m3/d in the Daqing oilfields in China), with typical APAM concentrations of 320−600 mg/L.7,10 Therefore, interpreting the fouling behavior of organic polymers (e.g., APAM) is of paramount practical significance for the sustainable application of UF membrane technology. Membrane−foulant and foulant−foulant intermolecular forces have been highlighted in investigations of organic fouling mechanisms. Elimelech et al.11 utilized atomic force microscopy (AFM) in conjunction with a carboxylate-modified colloid probe to quantify the adhesion forces between bulk humic acid (HA) © XXXX American Chemical Society

Received: August 24, 2015 Revised: December 20, 2015 Accepted: January 5, 2016

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DOI: 10.1021/acs.est.5b04098 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Scheme 1. Schematic Illustration of Preparations for APAM-tip

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foulant adhesion forces led to more severe flux declines at the initial stage of UF in organic foulant treatment.15 Force curve shapes can provide information on the single molecular configuration and intermolecular packing properties,16−26 which can be applied to explore the fouling layer structure. For this purpose, foulant molecules should be covalently bound to the tip and substrate surface using two types of surface chemistries based on either the strong chemisorption of thiols on gold surfaces or the covalent attachment of silanes or alcohols on silicon oxide surfaces.27 Both the curve shape and force value in AFM force spectroscopy are possibly related to membrane fouling by the gel layer (GL) and concentration polarization layer (CPL), which are two essential factors in the resistance-in-series model.28 However, the distinct relations between the intermolecular interactions and fouling resistance over the CPL and GL have been seldom explored. In particular, GL resistance is closely related to the flux loss during filtration, and the GL stability corresponds to the chemical cleaning complexity; thus, both properties are typically utilized to evaluate irreversible fouling. Nevertheless, there are limited reports of the distinction between intermolecular interaction and irreversible fouling over GL resistance and GL stability. In this study, probe tips and silicon substrates were aminofunctionalized through the covalent attachment of silanes on the silicon oxide surfaces27,29−34 and then covalently bound with APAM carboxyl functionalities, which were activated with 1ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) and Nhydroxysuccinimide (NHS) in aqueous solution.27,35−38 AFM force spectroscopy of APAM−APAM and APAM−membrane was used to predict the polymer fouling behavior in the UF process for water treatment. The objectives of this study were as follows: (i) to confirm the major foulants (i.e., APAM) in a plant−scale UF facility during polymer-flooding wastewater treatment and investigate the role of ions in APAM fouling of polyvinylidene fluoride (PVDF) UF membranes (using simulated solutions); (ii) to discuss foulant−foulant/membrane intermolecular force spectroscopy in terms of force value, curve shape and interaction distance; (iii) to distinguish the relationships between the membrane fouling and foulant−foulant intermolecular interaction over the GL and CPL; and (iv) to investigate relations between the GL stability and foulant− foulant intermolecular interaction through cleaning experiments.

MATERIALS AND METHODS Materials and Chemicals. N-Ethyl-N′-(3-(dimethylamino)-propyl)carbodiimide hydrochloride (EDC) (98%), Nhydroxysuccinimide (NHS) (98%), 3-aminopropyltriethoxysilane (APTES) and 4-morpholine ethanesulfonic acid hydrate (MES) were obtained from Aladdin. Toluene (99.99%, HPLC grade) was purchased from Sigma (U.S.). Other reagents used in all experiments were of analytical grade. Milli-Q purified water (18.2 MΩ·cm) was used in all experiments. Anionic polyacrylamide (APAM) was of analytic grade, and its general structure, molecular weight and degree of hydrolysis can be found in the Supporting Information (SI), Figure S1. Flat sheet polyvinylidene fluoride (PVDF) UF membranes with a mean pore size of 50 nm were fabricated at the Shanghai Institute of Applied Physics (SINAP, China), and their surfaces were characterized by X-ray photoelectron spectroscopy (XPS) (see Figure S2). XPS was performed with a RBD upgraded PHI5000C ESCA system (PerkinElmer) with Mg Kα radiation (hν = 1253.6 eV) or Al Kα radiation (hν = 1486.6 eV). Covalent Modification of Probes and Substrates. According to previously reported procedures,31,32,36−38 the covalent modification of the probe tips and substrates is presented in Scheme 1. This novel method included the activation of APAM and the silanization of wafers (single-crystal p-type silicon) and tips (spring constant of 0.08 N/m, CSG11, NT-M DT, Russia). The silanized wafers and probes were immersed in the APAM (activated by EDC/NHS) solution (20 μm/mL in MES buffer at pH 8.5) and incubated at room temperature for 1 h to facilitate amidation. The details of the covalent modification of the probes and substrates are described in the SI when presenting Figures S3 and S4. AFM Force Spectroscopy Measurements. A MultiMode 8 atomic force microscope equipped with a NanoScope V controller (Bruker, Germany) was employed to perform the intermolecular force measurements in a liquid cell filled with freshly prepared inorganic salt solution consisting of pure water; 10, 20, 40, and 80 mM NaCl; and 1.4, 7.0, and 14 mM CaCl2. Force measurements were obtained between the APAMfunctionalized tips and APAM-modified wafers and between the APAM-modified tips and PVDF UF membranes. Foulant Analysis. Element contents of the fouling layer and polymer-flooding wastewater were obtained on an S-2360N B


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Environmental Science & Technology Table 1. Element Content and Foulant Content of the Actual Feeding Water and Membrane Foulants element content (atom percent (at%))

foulant content ((weight percent (wt %))

sample

C

N

O

Na

Ca

APAM

Oil

Na

Ca

actual feeding water membrane foulants

29.95 48.73

19.29 17.23

30.39 19.59

15.15 6.91

0.13 1.81

29.42 66.34

2.56 1.62

63.96 13.58

3.58 15.92

Figure 1. Membrane flux decline curves: (a) comparison of Na and Ca with identical ionic strength, (b) role of the Ca concentration, (c) role of the Na concentration, and (d) the comparison between the CPL resistance (Rp) and GL resistance (Rg). For (a), Without: APAM solution without addition of salts, Na (i.e., 40Na): APAM solution with addition of 40 mM NaCl, Ca (i.e., 14Ca): APAM solution with addition of 14 mM CaCl2; for (b), Without: APAM solution without addition of salts, 14Ca, 7Ca and 1.4Ca: APAM solution with addition of 14 mM CaCl2, 7 mM CaCl2+20 mM NaCl and 1.4 mM CaCl2+37.0 mM NaCl; for (c), Without: APAM solution without addition of salts, 10Na, 20Na, 40Na, 80Na: APAM solution with addition of 10 mM, 20 mM, 40 mM, and 80 mM Na; for (d), Without: APAM solution without addition of salts, 10Na, 20Na, 40Na, 80Na, and Ca: APAM solution with addition of 10 mM Na, 20 mM Na, 40 mM Na, 80 mM Na, and 14 mM Ca. J0 is the permeation flux of the virgin membrane with pure water.

CPL was further rinsed with 200 mL of DI water at a stirring speed of 300 rpm for 15 min. Subsequently, this membrane was placed in a beaker containing 300 mL of a sodium hypochlorite solution (at a mass percent of 0.2% and a pH of 12, adjusted with sodium hydroxide), where it remained for 1 h at ambient temperature (25−30 °C) to clean the GL and then rinsed with adequate DI water to remove the residual chemical solution. The permeation fluxes of this membrane with DI water were recorded (under 0.05 MPa) after the physical rinsing of the CPL and the chemical cleaning of the GL, respectively, which were applied to estimate the flux recovery rate and calculate the resistance of the fouling layer. At least three replicates of the fouling and cleaning experiments were performed for each electrolyte. Fouling Resistance. The resistance-in-series model, which is based on Darcy’s law, was applied to evaluate the characteristics of membrane fouling.28 For constant pressure filtration, the resistance-in-series model is expressed as40

system (HITACHI). Correspondingly, the concentrations of calcium and sodium were determined on an inductively coupled plasma emission spectrometer (ICP-Agilent 720E), and the concentrations of APAM and oil were determined based on previous reports.39 The fouling layer was obtained from a plant− scale UF facility during polymer-flooding wastewater (see SI Table S1) treatment in Daqing oilfield in China. Fouling and Cleaning Experiments. The UF fouling experiments were conducted in a dead-end filtration system.39 First, the feeding solutions were obtained by adding 400 mg/L APAM to the various electrolytes, which were identical to those used in the AFM force measurements. Then, the feed solution was filtered through a PVDF membrane under a constant transmembrane pressure (TMP) of 0.05 MPa for 200 min. At the end of filtration, the fouled membrane was detached and rinsed with deionized (DI) water to clean the CPL; this membrane was then installed in the filtration system, and the C


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Environmental Science & Technology

Figure 2. Impacts of Na+ and Ca2+ on the APAM−APAM intermolecular interactions: (a) and (b): comparison of the influence of Na+ and Ca2+ (with identical ionic strength) on approaching force curves and rupture force curves, respectively; (c) and (d): impacts of the concentration of Ca2+ on approaching force curves and rupture force curves, respectively; (e) and (f): impacts of the concentration of Na+ on approaching force curves and rupture force curves, respectively. The force curves in (e) and (f) were measured in pure water, 10 mM, 20 mM, 40 mM, and 80 mM Na, sequentially, using the same tip and substrate both of which were covalently grafted by the APAM molecule. Herein, the tested solutions were the inorganic salt solutions (without any addition of APAM), which had the same concentrations as the feeding solution used in Figure 1.

J=

ΔP μ(R m + R f )

R f = R rev + R irev (1)

(2)

Here, the CPL resistance (Rp) and the GL resistance (Rp) are denoted by Rrev and Rirrev, respectively. The permeation flux of the membrane with pure water as the virgin membrane, the fouled membranes without any cleaning, and the fouled membranes after physical rinsing are denoted by J0, Jf, and Jg, respectively, and expressed as in eqs 3−5:

where J is the permeation flux, P is the trans-membrane pressure, μ is the permeate viscosity, Rm is the inherent membrane resistance, and Rf is the total fouling resistance. Rf is the sum of hydraulically reversible (Rrev) and irreversible (Rirrev) fouling resistances: D


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Environmental Science & Technology J0 =

ΔP μR m

(3)

Jf =

ΔP μ(R m + R p + R g)

(4)

Jg =

ΔP μ(R m + R g)

(5)

the corresponding intermolecular force curves and surface morphology. There were relatively slight changes in the flux loss for Ca2+ with different concentrations, as shown in Figure 1(b); however, the flux loss varied with different concentrations of Na+ (see Figure 1(c)), which can be better expressed by a relative flux (flux with pure APAM solution acted as the denominator). At the end of filtration, the relative flux was 1.00 in pure APAM water, and was increased to 1.13, then was restored to 0.92, finally was decreased further and reached a minimum of 0.68, followed by an increase to 0.91, respectively, by the addition of 10 mM, 20 mM, 40 mM, and 80 mM NaCl. The Rp and Rg of all feeding solutions used in the fouling experiments are presented in Figure 1(d). Rp was larger than Rg in all cases, indicating that the concentration polarization had a greater impact on the flux loss than the GL, which is the reason for the sharp flux decline at the initial filtration stage. Rp was enhanced and Rg was decreased by adding Ca2+ compared to pure water; conversely, Rp and Rg exhibited different tendencies in the presence of Na+. Comparing Rp and Rg with the flux loss indicates that the aggravation of flux loss in the presence of Ca2+ and Na+ was attributed to the increase in Rp and Rp, respectively, based on Equation 4. Accordingly, a distinction was made between intermolecular interaction and membrane fouling over Rp and Rg to understand the molecular mechanisms of the polymer fouling of UF membranes, as described later in this paper. Intermolecular Force Curves and Molecular Packing Property. Corresponding to the membrane flux loss and fouling resistance, the foulant−foulant and foulant−membrane intermolecular force curves and the molecular packing property of the foulant are presented below. (i). Roles of Na+ and Ca2+ in Foulant−Foulant Intermolecular Interactions. As shown in Figure 2(a), the APAM− APAM approaching intermolecular forces were repulsive (+2.57 nN at 5.4 nm), slightly repulsive (+1.49 nN at 5.4 nm), and significantly attractive (−4.11 nN at 4.9 nm) for pure water (i.e., Without), Na, and Ca, respectively, and the interaction distances were 147.9, 19.9, and 27.8 nm, respectively, as the tip approached the surface of the membrane and substrate. Moreover, the attractive force curve had a typical single sawtooth pattern in the presence of Ca2+. Considering the interaction distance and curve shape of these aforementioned approaching forces in pure water, the major repulsive force was the electrostatic repulsion17,18,24 between the carboxylates of the APAM molecules covalently grafted on the tip and those on the substrate. By adding 40 mM NaCl (i.e., Na), sodium ions (Na+) were bound as counterions around these negatively charged carboxyl groups, and the Debye screening length of this electrolyte was reduced from 960 nm for pure water to 1.52 nm (see SI Table S2); thus, both the value and interaction distance of the electrostatic repulsion were reduced. In the presence of 14 mM CaCl2 (i.e., Ca), divalent calcium ions (Ca2+) acted as a bridge to coordinate the carboxylates on the tip with those on the substrate, and the overall intermolecular Ca2+ binding force became much larger than the electrostatic repulsions,11,12 resulting in the net attractive approaching force. Upon retracting the tip from the sample, the curve often displays a hysteresis referred to as the rupture force (i.e., adhesion force). As shown in Figure 2(b), the maximum adhesion forces (Fad,max) for APAM−APAM were −13.23 nN for pure water (i.e., Without), −9.10 nN for Na, and −26.68 nN for Ca. In the presence of Ca2+, Fad,max was largest due to the strong coordination bonds11,12,45 of the Ca2+-bridging carboxylates

After measuring J0, Jf, and Jg under a TMP of 0.05 MPa, Rm, Rg, and Rp are given by eqs 6−8: Rm =

K ΔP = J0 μJ0

(6)

Rg =

K (J0 − Jg ) ΔP ΔP − = J0 Jg μJg μJ0

(7)

K (Jg − Jf ) ΔP ΔP − = μ Jf μJg Jg Jf

(8)

Rp =

where K is equal to ΔP/μ. In the calculation of the fouling resistance, μ is the permeate viscosity of pure water, and ΔP is the TMP, which is constant; thus, the parameter K is a constant. In this paper, Rg/K and Rp/K are applied to represent Rg and Rp, respectively.

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RESULTS AND DISCUSSION Major Foulants and Polymeric Fouling. The element contents and foulant contents of the feeding water (i.e., the actual wastewater) and of the fouling layer for the PVDF UF membrane are presented in Table 1. The element contents (atom percent (at%)) of C and Ca increased from 29.95 and 0.13 in the feeding water to 48.73 and 1.81 in the fouling layer, respectively, during UF in the polymer-flooding wastewater treatment. Correspondingly, the contents (weight percent (wt %)) of APAM and Ca in the eluate from the fouled membranes increased from 29.42 and 3.58 to 66.34 and 15.92, respectively, whereas that of oil and Na decreased from 2.56 and 63.96 to 1.62 and 13.58, respectively. Accordingly, APAM7,41,42 and the coexisting divalent metal ions had dominant roles in membrane fouling. The role of Na+ in APAM fouling cannot be ignored because it can bind around the interface of the carboxylates of APAM in the fouling layer,43,44 in addition the high concentration of Na+ (at a wt % of 63.96) in the feeding water. Herein, the impacts of metal ions (e.g., Ca and Na) on APAM fouling of UF membranes (with simulated feeding solutions) are presented in Figure 1, including membrane flux loss and fouling resistance. The comparison of flux loss with and without Na and Ca is presented in Figure 1(a). The total ionic strength of inorganic salt added in the APAM feeding solution was 40 mM for NaCl (i.e., Na) and CaCl2 (i.e., Ca), and no metal ions were added in the “Without” experiment as a control. Similar to previous studies,1,11 Ca2+ enhanced the flux loss by forming complexes with APAM and neutralizing the negative charges of APAM molecules more effectively than monovalent cations. Unexpectedly, Na+, as a monovalent cation, resulted in nearly the same flux loss as with Ca2+, even though the total ionic strength of Na was identical to that of Ca. This serious flux loss with Na is attributed to the fouling layer structure of the interpenetrating polymer (i.e., APAM) network, which is discussed in detail when presenting E


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Figure 3. Schematic representation of the roles of Na+ concentration in the APAM−APAM intermolecular interactions for (a) Without, (b) 10 Na, (c) 20 Na, (d) 40 Na, and (e) 80 Na. Upper part: the schema of the foulant−foulant interaction at the moment of the maximum adhesion force acting upon the tip; middle part: the schema of the foulant−foulant attraction; lower part: the schema of the foulant−foulant repulsion. Red, blue, gray and white spheres are oxygen, nitrogen, carbon and hydrogen atoms, respectively. Additionally, (a): IHBs I: −N−H···−OOC−, IHBs II: −C−H···−OOC−, and (d): Coil-up I: cohesion entanglement, Coil-up II: topological entanglement. The number of “+” represents the strength of the electrostatic repulsion among the APAM carboxyl groups.

mentioned above. Compared to pure water, the presence of Na+ in water weakened the adhesion and repulsion forces between APAM molecules, instead of strengthening the adhesion force as expected, indicating the destruction of certain bonds that can be generated in pure water. Hydrophobic attraction is typically the favorable interaction among the nonpolar hydrocarbon segments of the APAM chains.2 Alternatively, the hydrogen bonds of −N− H···OCO−−43,44 and −C−H···OCO−−46 and the specific ionic hydrogen bonds (IHBs)47 of −N−H···−OOC−, H−O− H···−OOC−, and −C−H···−OOC− typically contribute to the APAM−APAM rupture force. The Fad,max and force curve shape suggests that the hydrophobic attraction plus hydrogen bonding was stronger than the electrostatic repulsion, rendering a net attractive rupture force in pure water. However, the formation of the hydrogen bonds was more likely impeded by the addition of sodium ions,48 thus resulting in the decrease of Fad,max with Na compared to pure water. There are three major sequential plateaus in the APAM−Na−APAM rupture force curve: Plateau 1, Plateau 2, and Plateau 3 with average values of −8.4 nN, −4.4 nN, and −1.9 nN, respectively, as shown in Figure 2(b). The plateau-like pattern indicates that the APAM−APAM interactions were under a nonequilibrium condition,18,20 which should represent the overlap among the tip-APAM and substrate-APAM chains, and the number of plateaus approximately corresponded to the number of APAM chains that were entangled together.17,20 For the same reason, the interaction distance of APAM−Na−APAM was 820 nm longer than that of APAM−water−APAM or APAM−Ca−APAM.23 This wide interaction distance, in conjunction with the multiple plateaus, likely indicates that more than three APAM chains significantly coiled up intermolecularly with the addition of Na.17−21,23−26,49 In

contrast, the typical single sawtooth pattern for Ca and pure water indicates that APAM−APAM interactions were under equilibrium conditions, which may correspond to less intermolecular overlap.17−20 Na+ might have played an important role in the molecular configuration and the molecular packing property of APAM chains, which was elaborately discussed for Figure 3. The impacts of the concentration of Ca2+ on APAM−APAM interactions are presented in Figure 2(c) and (d). The maximum force value, interaction distance and curve shape were nearly the same for all tested concentrations of Ca2+ for the rupture and approaching force curves, indicating that the molecular packing property and intermolecular adhesion were independent of the Ca2+ concentration. This is consistent with the similar flux loss measured for a series of Ca2+ concentrations (see Figure 1(b)). However, the APAM−APAM intermolecular force curves varied with the concentrations of Na+ (see Figure 2(e) and (f)), which is consistent with the changes in flux loss (see Figure 1(c)). As presented in Figure 2(f), the Fad,max for pure water (i.e., Without) was larger than that for all of the tested NaCl electrolytes likely due to Na+ impeding the formation of the intermolecular hydrogen bonds, as mentioned above. For the same reason, the destruction of the IHBs led to more free − OOC− groups on the APAM molecules, and the 10 mM and 20 mM Na+ provided slight double layer compression on these free APAM carboxylates,12 enhancing the approaching repulsion compared to that with pure water (see Figure 2(e)). However, as the concentration of NaCl increased to 40 mM or 80 mM, the double layer compression became prominent,11,12 thereby decreasing the APAM−APAM approaching repulsion even though there were more −OOC− groups than in pure water. Moreover, the significant reduction of electrostatic repulsions for F


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Figure 4. Relationships between membrane fouling and intermolecular interaction. (a) correlation between Rp and the maximum APAM−solution−APAM adhesion forces, (b) relation between Rg and the proposed molecular packing property. For (b), “R & P” represented “regular and porous”, and the number of “+” and “−” represented the increase and decrease of regularity and porosity, respectively; D-rods: dispersed rods, A-rods: agminated rods, E-coils: expanded random-coils, S-coils: sphere-shaped coils.

40 Na or 80 Na led to the larger Fad,max compared to 10 Na or 20 Na. In contrast to the insignificant changes in the repulsion values between 10 Na and 20 Na and between 40 Na and 80 Na, there were prominent changes in Fad,max attributed to the difference in molecular packing states, rather than electrostatic repulsions.11 For the same reason, the rupture force curves experienced significant changes in curve shape with all of the tested NaCl solutions in contrast to the approaching force curves (see Figure 2(e) and (f)). Based on the relations between the shapes and interaction distances of the rupture force curves discussed above, the intermolecular overlap was more extensive with 20 Na than with 10 Na, rendering a larger Fad,max for the higher sodium concentration. Unexpectedly, the overlap with 80 Na was abated, but Fad,max was enhanced compared to that with 40 Na, which had the most extensive overlap among APAM chains. This is likely due to the change in molecular configuration of single APAM chains (see Figure 3). (ii). Roles of Na+ and Ca2+ in Foulant−Membrane Intermolecular Interactions. The APAM−membrane intermolecular interactions in all of the tested electrolytes are presented in SI Figure S5. The influencing mechanism of Na+ and Ca2+ on the APAM−membrane interaction was identical to that for APAM−APAM, on account of the similar carboxylate groups on the membrane surface (see SI Figure S2) as APAM. Nevertheless, there were differences between the APAM−APAM and APAM−membrane interactions under different experimental conditions. In pure water, the repulsive rupture force for APAM−membrane, which differed from the attractive rupture force for APAM−APAM, indicates that the intermolecular hydrogen bonds plus hydrophobic attractions were not sufficient to neutralize the electrostatic repulsion between APAM and the membrane. There were almost no changes in the interaction distance, curve shape, and Fad,max of the APAM−membrane rupture force curve for all concentrations of Na+, which indicates that the molecular packing state of polymeric organics (e.g., APAM) on the surface differed from that among the polymer chains. (iii). Roles of Na+ and Ca2+ in the Molecular Packing Properties. The molecular packing properties of APAM in all tested electrolytes are proposed in this section based on all of the above-mentioned discussion of the AFM force curves and on the

scanned AFM imagines of the fouled membrane surfaces (see SI Figure S6). In the presence of Ca2+, the APAM molecular packing state can be explained by the “egg-box” model50 in which calcium ions preferentially bind to the oxygen atoms of carboxylates in a highly organized manner and form bridges between adjacent APAM molecules, leading to the egg-boxshaped gel network.1 The schematic illustration of the proposed molecular packing property of APAM in the presence of Na+ is presented in Figure 3. In Figure 3, the polymerization25 of hydrogen-bonds (between − CO/−OOC− and −N−H/−C−H) in pure water (i.e., Without) led to a macromolecular structure (i.e., hydrogen bonded network46). The addition of Na+ impeded the formation of intermolecular hydrogen bonds, and the APAM molecular packing state then depended mainly on electrostatic repulsion11,20 and the salting-out effect of Na+ and Cl−.25,51−53 In the presence of 10 mM Na+, the prominent electrostatic repulsion caused the single chains to expand11,53 and simultaneously led to the enhancement of molecular stiffness, as well as the reduction of the intermolecular overlap in the GL; thus, the APAM molecules could be viewed as dispersed “rigid rods”. The addition of 20 mM Na+ enhanced the molecular flexibility of the APAM “rigid rods” due to an enhanced salting-out effect,25,52 which facilitated the intermolecular overlap, although the detected electrostatic repulsion was still notable; thus, the APAM molecular chains in the GL could be viewed as agminated “rigid rods”. When the concentration of Na+ was increased to 40 mM, the electrostatic repulsion was significantly abated; thus, the molecular flexibility was further enhanced, and the “rigid rods” were spontaneously converted into “expanded randomcoils”22,25 according to the principle of increasing entropy. Simultaneously, the random APAM coils entangled together intermolecularly to create the interpenetrating polymer network. Finally, in the 80 mM NaCl solution, the expanded random coil collapsed into a sphere-shaped structure likely due to an enhanced attraction among the Gaussian segment24 of an APAM chain, around which the amount of free solvent (i.e., water) molecules was reduced because more water molecules were orientationally bound to Na+ and Cl− to form hydrated ions (i.e., salting-out effect).25,51,52 In summary, APAM−APAM adhesion was a net force of electrostatic repulsions among carboxylate groups, hydrogen G


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Figure 5. The flux recovery rate for the GL was significantly higher than that for the CPL with all tested feeding solutions,

bonds and hydrophobic attractions in pure water (i.e., Without). By adding NaCl, the IHBs were destroyed, and the electrostatic repulsions were reduced by double layer compression. Moreover, the Fad,max (between APAM molecules), molecular packing property, and flux loss were dependent on the concentrations of Na+. The strong bonds formed by Ca2+ coordinating the carboxylate groups were crucial to the flux loss in the presence of Ca2+. The relations between intermolecular interactions and fouling need further investigation. Relationship between Fouling and Foulant−Foulant Intermolecular Interactions. The foulant−foulant, rather than foulant−membrane, intermolecular interactions were closely related to membrane fouling. Rp was strongly correlated with the foulant−foulant Fad,max (see Figure 4(a)), and Rg was closely related to the GL structure that corresponded to the molecular packing property of the foulant (see Figure 4(b) and SI Figure S6). There was a striking correlation between the foulant−foulant Fad,max and Rp, as displayed in Figure 4(a). The enhancement of the APAM−APAM Fad,max could have rendered the increase in Rp, resulting in more severe flux loss by the CPL. The Ca2+ bridging force, electrostatic double layer compression effect, and salting-out effect were responsible for the changes in Rp. In fact, Rp is of the nature of osmotic pressure caused by the concentration gradient between the CPL (at a high concentration probably exceeding the dilute/semidilute threshold C*54) and the permeation (at a low concentration), which was determined by the diffusion coefficient (D) of foulant under constant extrinsic conditions. The large foulant−foulant adhesion force may have corresponded to a small D and thereby caused the high concentration gradients between the CPL and bulk solution and between the CPL and permeation (according to Fick’s first law), as well as the high Rp (based on the Van’t Hoff equation). The proposed molecular packing properties, in conjunction with the measured AFM images of the fouling layers (see SI Figure S6), were related to the GL resistance, as shown in Figure 4(b). Rg increased according in the following order: Ca < Without < 80 Na < 10 Na < 40 Na < 20 Na. The packing layer of agminated rods that formed with 20 Na provided the highest resistance (Rg/K) of 1.40205. The egg-box-shaped gel network of Ca exhibited the lowest resistance of 0.50303, although the total flux loss for Ca was higher. The regular and well-organized pore structure, such as the egg-box-shaped gel network formed by Ca2+ bridging carboxylates in a highly organized manner,50 and the macromolecular structure created by hydrogen bond polymerization in pure water46 could render a low Rg. In contrast, the slightly porous structure that was formed by intermolecular overlap in a random manner, such as the interpenetrating polymer network of 40Na and the agminatedrod packing layer of 20 Na, corresponded to a high Rg. Overall, the strong correlation between Fad,max and Rp indicates that Fad,max can serve as an indicator of CPL fouling potential. Alternatively, the molecular packing property was responsible for the GL structure, which can be used to predict Rg. The sum of Rp and Rg determines the membrane flux based on eq 4. An overview of the molecular mechanisms of APAM fouling for the PVDF UF membranes is presented in SI Figure S7. Flux Recovery Rate and Chemical Stability. The flux recovery rate after cleaning was investigated to further understand the specific roles of sodium and calcium ions in APAM fouling. The flux recovery rates after physical flushing of the CPL and after chemical cleaning of the GL are presented in

Figure 5. Flux recovery rate of APAM-fouled membranes for Without, 10 Na, 20 Na, 40 Na, 80 Na, and Ca, after physically rinsing with water and chemical cleaning with agents (same cleaning method used in all cases). Herein, “Baseline” is the normalized flux at the end of filtration.

although Rg > Rp. The GL (stably adsorbed onto the membrane surface) was crucial to membrane flux recovery according to Darcy’s law. As shown in Figure 5, the flux recovery rates after chemical cleaning in the same method increased as Ca < 40 Na < 10 Na ≈ 20 Na < Without ≈ 80 Na, which was different from the order of Fad,max for APAM−APAM or interactions. With Ca, although Rg was minor, the flux recovery rate was the lowest in all of the tested electrolytes due to the chemically stable coordination bonds that corresponded to the largest APAM−APAM Fad,max. This indicates that nature of the intermolecular force played an essential role in GL stability that had no direct relation to Rg. In contrast, although the Fad,max of 40Na was not the largest in all of the tested NaCl solutions, the flux recovery rate was the lowest likely due to the stable structure of the interpenetrating polymer network that was created through the expanded random-coils coiling up. Moreover, in spite of the higher Fad,max with 80 Na, the flux recovery rate was the highest, possibly due to the sphereshaped structure of GL. The GL structure could have also played an important role in the GL stability, under the Fad,max of identical nature (e.g., hydrogen bonds, hydrophobic attractions, and coordination bonds). In this study, a distinction between intermolecular interactions and fouling behavior over the CPL and GL was made to understand the molecular mechanisms occurring in the fouling of UF membranes by APAM in polymer-flooding wastewater. Because Fad,max was strongly correlated to Rp, APAM−APAM adhesion must be taken into account in controlling concentration polarization. Alternatively, the molecular packing property of APAM, which was responsible for Rg, could be used to predict the irreversible fouling potential. In addition, the GL stability depended on the nature of the intermolecular forces, which can provide a fundamental basis for the choice of reagent for cleaning APAM GL. These results are promising for the H


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prediction and control of membrane fouling by organic polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b04098.

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Additional information as noted in the text, Figures S1− S7, Table S1−S2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(S.Y.) Phone: 02165982708; e-mail: [email protected]. *(J.H.) Phone: 02139196392; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (No. 51578390), Shanghai Pujiang Program (No. 14PJ1432400), and the National Water Pollution Control and Treatment Key Technologies R&D Program (No. 2012ZX07403-001).

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Molecular Mechanisms of Ultrafiltration Membrane Fouling in Polymer

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