Combined Organic Fouling and Inorganic Scaling in Reverse Osmosis

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Environmental Processes

Combined Organic Fouling and Inorganic Scaling in Reverse Osmosis: Role of Protein-Silica Interactions Amanda Quay, Tiezheng Tong, Sara M. Hashmi, Yu Zhou, Song Zhao, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02194 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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Environmental Science & Technology

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Combined Organic Fouling and Inorganic Scaling in Reverse Osmosis: Role of Protein-Silica Interactions

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†

14

†‡

Amanda N. Quay ∥, Tiezheng Tong

∥ *,

†

Sara M. Hashmi , Yu Zhou§, Song Zhao⊥, †#

and Menachem Elimelech

15

*

†

16 17

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut, 06520-8286 ‡

18 19 20 21 22 23 24 25 26 27 28 29 30 31

Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, Colorado, 80523 § Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut, 06511 ⊥ School of Chemical Engineering and Technology, Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin, P. R. China, 300072 # Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, 06520-8286 ∥

These authors contribute equally.

* Corresponding authors: email: [email protected]; Tel. +1 (203) 432-2789 email: [email protected]; Tel. +1 (970) 491-1913

1 ACS Paragon Plus Environment


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ABSTRACT

33

We investigated the relationship between silica scaling and protein fouling in reverse osmosis

34

(RO). Flux decline caused by combined scaling and fouling was compared with those by

35

individual scaling or fouling. Bovine serum albumin (BSA) and lysozyme (LYZ), two proteins

36

with opposite charges at typical feedwater pH, were used as model protein foulants. Our results

37

demonstrate that water flux decline was synergistically enhanced when silica and protein were

38

both present in the feedwater. For example, flux decline after 500 minutes was far greater in

39

combined silica scaling and BSA fouling experiments (55 ± 6% decline) than those caused by

40

silica (11 ± 2% decline) or BSA (9 ± 1% decline) alone. Similar behavior was observed with

41

silica and LYZ, suggesting that this synergistic effect was independent of protein charge.

42

Membrane characterization by scanning electron microscopy (SEM) and Fourier transform

43

infrared spectroscopy (FTIR) revealed distinct foulant layers formed by BSA and LYZ in the

44

presence of silica. A combination of dynamic light scattering (DLS), transmission electron

45

microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX) analyses further suggested

46

that BSA and LYZ facilitated the formation of aggregates with varied chemical compositions. As

47

a result, BSA and LYZ were likely to play different roles in enhancing flux decline in combined

48

scaling and fouling. Our study suggests that the coexistence of organic foulants, such as proteins,

49

largely alters scaling behavior of silica, and that accurate prediction of RO performance requires

50

careful consideration of foulant-scalant interactions.

51

TOC Arts

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INTRODUCTION

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Organic fouling and inorganic scaling are both primary barriers that significantly constrain the

55

performance of membrane desalination

56

membrane surface causes severe flux decline and an increase in transmembrane pressure,

57

compromising the cost and energy efficiency of membrane desalination. As a result, numerous

58

studies have been performed to understand the behaviors and mechanisms of membrane fouling

59

and scaling in desalination processes 1, 2, 4, 5. However, those studies primarily test feed solutions

60

containing individual foulants or scalants, and only a limited number of studies have investigated

61

combined fouling and scaling 6-10.

1-3

. The accumulation of foulants and scalants on the

62

Organic foulants and inorganic scalants commonly coexist in feedwaters of membrane

63

desalination. For example, in the Groundwater Replenishment System (GWRS) treatment facility

64

(Orange County, CA), the secondary wastewater effluent used as the feedwater of reverse

65

osmosis (RO) contains sufficient organic matter to sustain biofilm growth as well as inorganic

66

components with significant scaling potential

67

reported in the wastewater treated by high efficiency reverse osmosis (HERO) for a coal-to-

68

chemical facility (Inner Mongolia, China)

69

organic fouling and inorganic scaling results in distinct membrane performance as compared to

70

individual fouling or scaling 6, 7, 9. A recent study reported a remarkable increase in RO filtration

71

resistance only when silica and organic matter coexisted in the effluent of a membrane bioreactor

72

6

73

imposed a mitigating effect on silica scaling of RO membranes 7. Additionally, Liu and Mi

74

observed that the presence of alginate facilitated gypsum scaling in forward osmosis 9. Despite

75

the importance of combined organic fouling and inorganic scaling in membrane desalination

76

operation, we still lack adequate knowledge about foulant-scalant interactions and the influence

77

of these interactions on the overall membrane performance.

78

11

12

. A high fouling and scaling tendency was also

. A few studies have suggested that combined

. In another study, it was shown that alginate, a model organic foulant of polysaccharide,

Proteins represent a major group of organic foulants in membrane desalination, particularly 13-16

79

for wastewater reclamation

80

mechanism of protein fouling, as indicated by the most severe flux decline observed at pHs near

81

their isoelectric points (IEPs)

82

scalants. Silica scale is difficult to remove from the membrane surface 17, and most commercially

. Interactions between protein macromolecules are the primary

13-16

. Meanwhile, silica is one of the most common inorganic



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available anti-scalants, which disrupt the crystallization process of scale formation 1, are

84

ineffective in preventing amorphous silica scale

85

problem facing membrane desalination, and is widely considered the ‘Gordian knot’ of water

86

treatment processes

87

involving polymerization (or condensation) of monomeric silicic acids 17, 19, 20. Silicic acids have

88

been found to interact with proteins, peptides, and amino acids, influencing the kinetics and

89

structure of silica particle formation

90

feedwater is likely to pose a marked influence on individual silica scaling and protein fouling.

91

Several studies have investigated combined organic fouling and colloidal silica fouling

92

However, due to the dramatic difference between silica scaling (caused by polymerization of

93

silicic acids) and colloidal silica fouling (caused by deposition of silica particles), these studies

94

shed no light on the impacts of proteins on silica scale formation. To date, combined protein

95

fouling and silica scaling in membrane desalination have not been explored in the literature.

18

18

. Hence, silica scaling remains a challenging

. The chemistry associated with silica scale formation is complex,

21-24

. Therefore, the coexistence of silica and proteins in 25-29

.

96

In this work, we investigated the relationship between silica scaling and protein fouling in

97

RO, and protein-silica interactions were probed to reveal the underlying mechanisms. Two

98

proteins with opposite charges at typical feed pH, bovine serum albumin (negatively charged)

99

and lysozyme (positively charged), were used as model protein foulants. Our study represents

100

one of the first efforts to delineate combined effects of protein fouling and silica scaling on RO

101

performance. The resulting findings demonstrate a synergistic effect between protein fouling and

102

silica scaling, thereby highlighting the importance of considering scalant-foulant interactions

103

when predicting membrane performance in RO.

104

MATERIALS AND METHODS

105

Materials and Chemicals. Sodium metasilicate pentahydrate (Na2SiO3·5H2O, >95.0%),

106

bovine serum albumin (BSA, ≥ 98%), lysozyme (LYZ, from chicken egg white), and oxalic acid

107

dihydrate (HO2CCO2H·2H2O, ≥ 99%) were purchased from Sigma-Aldrich. Sodium chloride

108

(NaCl), hydrochloric acid (HCl, 36.5-38.0%), and magnesium chloride hexahydrate

109

(MgCl2·6H2O) were purchased from J.T. Baker. Calcium chloride dihydrate (CaCl2·2H2O) was

110

purchased from Alfa Aesar. Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) were

111

purchased from EMD Chemicals. Commercial thin-film composite (TFC) RO membranes


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(SW30 XLE) were provided by Dow Chemical. Deionized (DI) water was produced from a

113

Milli-Q ultrapure water purification system (Millipore).

114

RO Membrane Scaling and Fouling Tests. All the feed solutions were composed of

115

7.0 mM CaCl2, 3.5 mM MgCl2, and 35 mM NaCl as background electrolytes. The feed solution

116

used in silica scaling tests also contained 2.8 mM Na2SiO3·5H2O, resulting in a saturation index

117

(defined as the ratio of ion activity product to solubility product) of 1.5 for amorphous silica as

118

used in our previous study 30. BSA and LYZ were selected as representative proteins due to their

119

opposite surface charge at near neutral pH (BSA has an IEP of 4.7-4.9, while the IEP of LYZ is

120

~10.4

121

molecular weight of 14.3 kDa 13, 16). For protein fouling experiments 35 mg/L BSA or LYZ was

122

added into the feed solutions. The feed solutions used in combined scaling and fouling tests

123

contained both 2.8 mM sodium silicate and 35 mg/L protein. The solution pH was adjusted to

124

6.50 ± 0.05 for all the experiments.

13, 31

) as well different sizes (BSA has a molecular weight of 66 kDa, while LYZ has a

125

The RO membrane scaling and/or fouling tests were performed with a bench-scale crossflow

126

RO system, which has been described in our previous publications 30, 32. Membrane coupons with

127

an effective area of 20.02 cm2 were compacted for ≥12 hours using DI water under a pressure of

128

31.7 bar (460 psi), after which pure water flux was recorded at 27.6 bar (400 psi) with a constant

129

crossflow velocity of 8.5 cm/s. DI water was then replaced by the prepared feed solution,

130

initiating the scaling and/or fouling tests. The hydraulic pressure was adjusted to 30.3 ± 1.3 bar

131

(440 ± 20 psi) to create an initial water flux of 56±2 L·m-1·h-1. The water flux was continuously

132

monitored for 1400 minutes with a constant crossflow velocity at 4.25 cm/s and temperature at

133

22 ± 1 °C. A relatively low crossflow velocity was chosen to facilitate silica scaling and protein

134

fouling, so that observable flux decline could be achieved within a short time (< 1 day). A

135

recycling mode was applied to all the tests, in which the RO permeate was recycled back to the

136

feed solution reservoir.

137

After 1400-minute scaling and/or fouling tests, physical membrane cleaning was conducted

138

immediately with a high crossflow velocity (21.3 cm/s) for 30 minutes. The pure water flux was

139

then re-measured at 27.6 bar (400 psi) at a crossflow velocity of 8.5 cm/s, in order to calculate

140

the flux recovery ratio.



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The foulant/scalant layers present on the membrane surface after physical cleaning were

142

characterized by scanning electron microscopy (SEM, Hitachi SU-70) and attenuated total

143

reflectance-Fourier transform infrared spectroscopy (ATR-FTIR, Thermo Nicolet 6700).

144

Membrane samples were air dried and sputter-coated with a thin layer of iridium (Denton Desk

145

IV) prior to SEM imaging. Before acquiring ATR-FTIR spectra, the membrane samples were

146

also air dried, and 32 scans were completed for each sample.

147

Dynamic Light Scattering Analysis. Dynamic light scattering (DLS) was employed to

148

analyze potential formation of particle aggregates in the RO feed solutions. DLS was performed

149

with a fixed So-SIPD optical detector on CGS-5000F goniometer setup (ALV GmBH) and a

150

Verdi V2 continuous wave DPSS laser (COHERENT) operating at 532 nm

151

scattered light (I) was collected at 150° to minimize the effect of stray large particles, and

152

normalized by the incident intensity I0. The resulting I/I0 ratio was used as a proxy to indicate the

153

extent of aggregate formation. The DLS data were collected for 30 seconds, and 10 independent

154

concurrent runs were employed for each sample.

33

. The intensity of

155

Two sets of solutions were analyzed by DLS in our study. The first set of solutions

156

contained identical composition to the feed solution used in the scaling/fouling RO tests (denoted

157

as bulk solutions). However, the concentration of each chemical component near the membrane

158

surface was elevated compared to that in the bulk solution due to concentration polarization 34.

159

Thus, another set of solutions was prepared to take the effects of concentration polarization into

160

consideration (denoted as CP solutions). The detailed procedure of calculating the concentration

161

polarization modulus for each chemical component is described in the Supporting Information,

162

and the chemical compositions of bulk and CP solutions are presented in Table S1. The

163

concentration polarization modulus values for BSA and LYZ were as large as ~4600 and ~610,

164

respectively, which were ~1200 and ~160 times greater than those for sodium silicate. The

165

resulting high protein concentrations on the membrane surface (Cm) led to significant

166

aggregation of proteins and intrinsically high DLS signal, shielding the effects of protein-silica

167

interactions on aggregate formation. Therefore, lower concentrations of BSA (10-2 and 10-3 of Cm)

168

and LYZ (10-1 and 10-2 of Cm) were applied to capture the potential change of DLS signal

169

intensities due to silica-protein interactions. If large aggregates were observed in DLS

170

measurements, they were collected by centrifugation (8200 g or 10000 rpm for 10 min) and

171

washed thoroughly with DI water. Transmission electron microscopy (TEM) analysis was 6 ACS Paragon Plus Environment

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performed with a JEOL JEM-2100F TEM operating at 200 kV, and energy dispersive X-ray

173

spectroscopy (EDX) was conducted to generate elemental maps that indicated the chemical

174

composition of the collected aggregates.

175

Quantification of Molybdate-reactive Silica in Solution. The silicomolybdate test 18, 35

176

was used to measure soluble silicic acids with a low level of polymerization

177

only monomeric, dimeric, and possibly trimeric forms of silicic acids are able to react with

178

ammonium molybdate to produce colorimetrically detectable products, whereas highly

179

polymerized silica species cannot be measured

180

reactive silica is inversely proportional to the extent of silicic acid polymerization.

35, 36

. In this test,

. Therefore, the concentration of molybdate-

181

The quantification of molybdate-reactive silica was performed following the method

182

reported by Preari et al 36. In brief, 0.4 mL of ammonium molybdate solution (100 g/L, pH 7.5)

183

and 0.2 mL of HCl (~18.5%) were added to 10 mL of sample solution. The solution was mixed

184

thoroughly and left undisturbed for 10 min. Then, 0.4 mL of oxalic acid solution (87.5 g/L) was

185

added. After 2 min, the sample absorbance at 420 nm was measured by ultraviolet-visible (UV-

186

Vis) spectroscopy (Varian Cary 50 Bio); the absorbance was linearly proportional to the

187

concentration of molybdate-reactive silica (expressed as mg/L SiO2) in solution. In our study, the

188

quantitative detection range of this protocol was 6-75 mg/L SiO2. Similar to DLS analysis, both

189

bulk solution and CP solution were analyzed. Because the high Cm of proteins (i.e., after

190

considering concentration polarization) created precipitates in the presence of the molybdate

191

reagents, we reduced the protein concentrations until negligible precipitation was observed (i.e.,

192

the absorbance of protein solution after adding the molybdate reagents was indistinguishable

193

with that of DI water). Accordingly, BSA and LYZ at 10-3 and 10-2 of Cm were used to

194

understand the effect of proteins on molybdate-reactive silica in the CP solutions.

195

RESULTS AND DISCUSSION

196

Combined Protein Fouling and Silica Scaling are Synergistic. Dynamic protein

197

fouling and silica scaling tests were carried out in a bench-scale crossflow RO system, and the

198

corresponding water fluxes are shown in Figure 1. When silica was present alone in the feed

199

solution (with a saturation index of 1.5), the water flux decreased gradually with a total decline

200

of ~30% after 1400 min. Consistent with previous findings

201

after physical membrane cleaning (Figure 2), indicating that silica scaling was irreversible in RO.

30, 32


, no flux recovery was achieved


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In fouling tests with protein as the sole foulant, the presence of BSA and LYZ resulted in a total

203

flux decline of ~35% and ~15% after 1400 min, respectively (Figure 1A and B). For both BSA

204

and LYZ fouling, physical membrane cleaning partially restored water fluxes (Figure 2). A

205

smaller extent of membrane fouling by LYZ compared to BSA was also observed in the

206

literature 13.

207

FIGURE 1

208

FIGURE 2

209

When BSA and silica coexisted in the feed solution, a synergistic effect was observed for

210

protein fouling and silica scaling. The water flux experienced a severe flux decline of ~75% after

211

1400 min (Figure 1A). This flux decline was greater than the additive flux decline of BSA

212

fouling and silica scaling, and was particularly noticeable in the initial stage of the test. For

213

example, the flux decline in combined BSA fouling and silica scaling was 55 ± 6% after 500 min,

214

significantly higher than those caused by silica scaling (11 ± 2%) or BSA fouling (9 ± 1%)

215

individually (Figure 2A). Also, the flux decline due to combined BSA fouling and silica scaling

216

was irreversible after physical membrane cleaning (Figure 2A). Synergistic and irreversible flux

217

decline was also observed for combined LYZ fouling and silica scaling, albeit to a lesser extent

218

(Figures 1B and 2B). Due to their different IEPs (pH of 4.7-4.9 for BSA 13, 31 and pH of 10.4 for

219

LYZ

220

results demonstrate that proteins facilitated silica scaling in RO regardless of their surface charge.

221

BSA and LYZ Form Different Foulant Layers in the Presence of Silica. ATR-

222

FTIR analysis was performed to characterize the foulant/scalant layers on the membrane surface

223

after RO fouling and scaling tests followed by physical membrane cleaning (Figure 3). The

224

ATR-FTIR spectrum of the pristine RO membrane represented spectra from both the polysulfone

225

support layer and polyamide active layer. The well-defined peaks observed at 1600-1700 cm-1

226

and 1540 cm-1 corresponded to the typical amide I and amide II bands of polyamide, respectively

227

37

228

enhanced after 1400-min BSA fouling, indicating that a BSA layer was firmly attached to the

229

membrane even after physical membrane cleaning. However, these two peaks remained nearly

230

unchanged after LYZ fouling, suggesting a low amount of LYZ on the membrane surface. After

231

silica scaling, the ATR-FTIR signal was largely enhanced in the region between 1050 and 1100

232

cm-1, which was attributed to the Si-O-Si bonds

13

), BSA and LYZ carried opposite charges at the experimental pH of 6.5. Therefore, our

. These amide peaks, which are also characteristic features of proteins

39, 40

38

, were significantly

, but the peaks for amide bands were not


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visible. This provided evidence of a silica layer thicker than the penetration depth of infrared

234

radiation. With the coexistence of BSA and silica in the feed solution, the resultant membrane

235

exhibited characteristic peaks of both protein and silica, indicating a foulant layer that consisted

236

of both BSA and silica on the membrane surface. After combined LYZ fouling and silica scaling,

237

however, only the peak associated with silica was clearly amplified in the ATR-FTIR spectrum.

238

The features for the amide bands were much weaker for combined LYZ fouling and silica

239

scaling than those after combined BSA fouling and silica scaling.

240

FIGURE 3

241

SEM analysis was used to observe the surface morphologies of membranes after fouling and

242

scaling tests. After silica scaling, the membrane surface was fully covered with a layer of silica

243

particles, with the typical ridge-and-valley surface structure of TFC membranes (Figure 4A) no

244

longer visible (Figures 4B). The chemical composition of the observed silica layer has been

245

confirmed by ATR-FTIR analysis (Figure 3) and energy-dispersive X-ray spectroscopy (EDX) in

246

our previous study

247

membrane surface morphologies. The membrane fouled by BSA demonstrated a dense layer of

248

proteins (Figure 4C and S1A), whereas such a foulant layer was not found on the LYZ-fouled

249

membrane surface (Figure 4E and S1C). Furthermore, BSA and LYZ also led to different

250

membrane surface morphologies after combined protein fouling and silica scaling. For combined

251

BSA fouling and silica scaling, a dense BSA layer and silica particles were both observed on the

252

membrane surface (Figures 4D and S1B). In contrast, only silica particles were found on the

253

membranes subjected to combined LYZ fouling and silica scaling (Figures 4F and S1D). These

254

observations were also consistent with the ATR-FTIR result that only combined BSA fouling

255

and silica scaling resulted in an increase of FTIR signal associated with both protein and silica

256

(Figure 3). Therefore, both SEM and ATR-FTIR analyses indicated that BSA and LYZ formed

257

different foulant layers, and thus they were likely to facilitate silica scaling via different

258

mechanisms.

30

. However, protein fouling by BSA and LYZ resulted in contrasting

259

FIGURE 4

260

Protein-Silica Interactions Differ for Bovine Serum Albumin and Lysozyme.

261

Dynamic light scattering (DLS) was employed to investigate the influence of protein-silica

262

coexistence on the formation of aggregates in the feed solutions, and the effect of concentration 9 ACS Paragon Plus Environment


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polarization was considered (Table S1, Supporting Information). In the absence of silica, the

264

DLS signal intensities remained at low levels for all the tested solutions (Figures 5A and 5B),

265

suggesting that negligible aggregates were formed in both BSA and LYZ fouling tests. However,

266

the addition of silica to the protein solutions induced aggregate formation, as evidenced by the

267

enhancement of DLS signal intensities (Figures 5C and D). For combined BSA fouling and silica

268

scaling (Figure 5C), the presence of silica increased the DLS signal moderately in the bulk feed

269

solution. However, a rapid and significant increase of DLS signal was observed when

270

concentration polarization was considered, accompanied by the formation of white precipitates in

271

the corresponding solutions (Figure S2A). The co-presence of LYZ and silica also promoted

272

aggregate formation (Figure 5D and Figure S2B), albeit to a much lesser extent as compared to

273

that of BSA and silica. This result was in accordance with the less extreme synergistic effect

274

observed in the combined LYZ fouling and silica scaling test.

275

FIGURE 5

276

The white precipitates formed by protein-silica interactions were collected and characterized

277

by TEM coupled with EDX (Figures 6, 7, S3-S5, and Table S2). TEM images showed that the

278

precipitates formed by LYZ-silica interaction exhibited a well-defined particle morphology, with

279

particle sizes of ~100 nm (Figure 6B and D). In contrast, the precipitates formed from BSA-

280

silica interaction were mostly amorphous (Figure 6A and C). Elemental mapping demonstrated

281

that both precipitates were enriched with C, N, Si, and O (Figures 7 and S3), indicating that they

282

were composed of both silica and proteins. However, further analysis of elemental percentage

283

(Figure S4 and Table S2) revealed that the silica contents in the LYZ-silica precipitates (35.5%-

284

42.4% of Si) were much higher than those in the BSA-silica precipitates (2.3%-19.1% of Si).

285

FIGURE 6

286

FIGURE 7

287

We used the C/Cu EDX signal ratio of the protein-silica precipitates as a proxy of protein

288

content (Table S2). The C signal was derived from both proteins and a thin carbon film of the

289

TEM copper grid, whereas the Cu signal was exclusively from the copper grid. Thus, a higher

290

C/Cu signal ratio indicates of a higher content of proteins. The C/Cu signal ratio of the LYZ-

291

silica precipitates (0.6-1.2) was smaller than that of the BSA-silica precipitates (1.7-2.3) and

292

close to that of the TEM copper grid (0.7), suggesting a lower protein content in the LYZ-silica 10 ACS Paragon Plus Environment

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precipitates. We also calculated the C/Si signal ratio for each sample (Table S2). Similar to the

294

C/Cu ratio, the BSA-silica precipitates exhibited a much higher C/Si ratio, confirming that the

295

BSA-silica precipitates were enriched with protein whereas LYZ-silica precipitates were mainly

296

composed of silica.

297

The TEM-EDX results demonstrated that BSA and LYZ resulted in aggregates with

298

different structure and chemical compositions when interacting with silica near the membrane

299

surface. BSA formed amorphous co-precipitates with silica, while LYZ was likely to facilitate

300

silica particle precipitation.

301

Since silicic acid polymerization plays an important role in silica scaling on TFC 30, 32, 41

302

membranes

, the effect of protein-silica coexistence on silicic acid polymerization was

303

examined by measuring the concentrations of molybdate-reactive silica in the feed solutions.

304

Results show that the presence of BSA did not affect silicic acid polymerization, even after

305

considering concentration polarization (Figure S6 and S7). Therefore, the facilitated aggregate

306

formation due to BSA-silica interaction (Figure 5C) did not involve additional generation of

307

silica polymers or particles. LYZ also posed a negligible influence on silicic acid polymerization

308

in the bulk feed solution (Figure S6), but it slightly increased the concentration of molybdate-

309

reactive silica after considering concentration polarization (Figure S7). This phenomenon was

310

consistent with the findings of Coradin et al. 21, 42, who also observed an increase of molybdate-

311

reactive silica when LYZ induced silica precipitation at pH of 7.4. The authors suggested that

312

large oligomers of silicic acids, which carried more negative charges than monomeric silicic

313

acids but could not be detected by the silicomolybdate test, interacted with positively charged

314

LYZ and preferentially participate in silica polymerization and precipitation. The resulting

315

decrease of free silica species in solution induced de-polymerization of silica, producing

316

monomeric and dimeric forms of silicic acids that led to the increase of molybdate-reactive silica

317

concentration 42.

318

Proposed Mechanisms of Combined Protein Fouling and Silica Scaling. We

319

have demonstrated a synergistic effect for protein fouling and silica scaling in RO. The

320

coexistence of protein and silica, regardless of the protein surface charge, resulted in an

321

enhanced and irreversible water flux decline compared to individual protein fouling or silica



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scaling. However, the underlying mechanisms associated with different proteins (i.e., BSA and

323

LYZ) appear to differ.

324

The BSA-silica interaction generated a large amount of protein-rich aggregates near the

325

membrane surface, and their attachment to the membrane surface created a compact and dense

326

silica-protein layer. This foulant layer not only enhanced hydraulic resistance, but also caused an

327

elevated osmotic pressure due to the hindrance of salt back diffusion, a phenomenon referred to

328

as cake-enhanced osmotic pressure 43. Hence, rapid and significant flux decline was observed in

329

combined BSA fouling and silica scaling. Coradin et al. 21 reported that co-presence of silica and

330

BSA did not induce precipitation at near neutral pH in the absence of divalent cations. In our

331

study, divalent cations did not facilitate aggregation unless silica was present (Figure 5). Thus,

332

the formation of BSA-silica precipitates must be due to interactions among BSA, divalent ions,

333

and silica. Silicic acid polymerization produced silica oligomers and polymers with a high

334

density of ionized silanol groups

335

repulsion between BSA macromolecules as well as between BSA and these negatively charged

336

silica species. Enhanced interactions between silica and BSA, such as via hydrogen bonding 23 or

337

binding of silica with the positively charged BSA residues (positively charged residues are

338

present in BSA despite the overall negative charge of the macromolecule

339

and salt out BSA from the feed solution.

44

. The presence of Ca2+ and Mg2+ reduced electrostatic

45

), could destabilize

340

LYZ is less prone to aggregation than BSA, with its comparatively smaller molecular weight,

341

lower concentration polarization modulus, and higher stability 46. LYZ was positively charged at

342

the feed solution pH due to its high IEP (pH ~10.4 13). The electrostatic attraction between LYZ

343

and negatively charged silica species has been shown to promote silica precipitation at neutral

344

pH 21, consistent with the expedited formation of silica-rich precipitates in our study (Figure 5D).

345

The facilitating effect of positively charged molecules on silicic acid polymerization has been

346

also observed in the field of biomineralization

347

groups in peptides and polyamines were associated with biogenic formation of silica in diatoms

348

22, 47

22, 47-50

. For example, positively charged amino

, probably via bringing ionized silica species close enough for condensation occurrence 22.

349

The dramatic flux decline in combined BSA fouling and silica scaling was probably

350

attributed to the enhanced protein aggregation in the presence of silica, whereas the synergistic

351

effect of combined LYZ fouling and silica scaling was mainly due to expedited formation of 12 ACS Paragon Plus Environment

Page 13 of 24


352

silica particles. This difference was consistent with the lower silica content but higher protein

353

content of BSA-silica precipitates compared to LYZ-silica precipitates (Figure S4 and Table S2),

354

and was also supported by SEM and ATR-FTIR results (Figure 3 and 4). In addition, our

355

previous study has shown that positively charged membrane surfaces accelerated silica scaling in

356

RO 30. The deposition of LYZ may impart the RO membrane surface with more positive charges,

357

thereby further enhancing the extent of silica scaling.

358

Environmental Implications. Our study indicates the importance of scalant-foulant

359

interactions in determining RO performance. The presence of organic foulants, such as proteins,

360

significantly expedites silica scaling in RO. A similar synergistic effect between silica scaling

361

and organic fouling was also reported by Kimura et al., who applied RO to the treatment of

362

effluent generated from a membrane bioreactor 6. They found that the coexistence of silica and

363

organic matter caused severe flux decline and a marked increase of filtration resistance, whereas

364

minimal fouling was observed when silica or organic matter was present alone in the feedwater.

365

Thermodynamic calculation solely based on the scalant solubility is likely insufficient to

366

evaluate the flux decline potential of RO feedwaters, so it is imperative to consider scalant-

367

foulant interactions when predicting the efficacy of RO. Interactions between other foulants (e.g.,

368

humic acid) and scalants (e.g., gypsum and calcite) and their impacts on RO performance remain

369

to be understood. This knowledge gap requires more studies focusing on combined fouling and

370

scaling in membrane desalination. Further, although effective antifouling membranes have been

371

successfully developed using organic foulant-only feedwaters, it is still unknown whether those

372

membranes are able to maintain their high performance when treating feedwaters with both high

373

scaling and fouling potential. Additional research is needed to challenge antifouling membranes

374

in combined fouling and scaling tests, thereby identifying favorable membrane properties in this

375

complex but realistic scenario.

376

SUPPORTING INFORMATION

377

Details on the calculation of salt concentration at the membrane surface; concentrations of

378

background electrolytes, silica, and proteins with and without considering concentration

379

polarization (Table S1); SEM micrographs of membranes after 1400-min protein fouling and

380

combined protein fouling and silica scaling (Figure S1); the formation of white precipitates when

381

proteins coexisted with silica (Figure S2); elemental maps of the precipitates produced by 13 ACS Paragon Plus Environment


382

protein-silica interactions (Figure S3); the areas within TEM images of the precipitates produced

383

from silica-protein interactions, from which the elemental percentages were calculated (Figure

384

S4); elemental percentages of the white precipitates produced by proteins and silica (Table S2);

385

EDX spectra of protein-silica precipitates (Figure S5); concentrations of molybdate-active silica

386

as a function of time when silica coexists with proteins in the bulk solution (Figure S6);

387

concentrations of molybdate-active silica as a function of time when silica coexists with proteins

388

after considering concentration polarization (Figure S7).

389

ACKNOWLEGMENT

390

We acknowledge the support received from the National Science Foundation Nanosystems

391

Engineering Research Center for Nanotechnology-Enabled Water Treatment (EEC-1449500).

392

Parts of the experiments were performed with the support received from the start-up funds of T.T.

393

at the Department of Civil and Environmental Engineering, Colorado State University. We also

394

thank Dr. Michael Rooks and Dr. Roy Geiss for their technical assistance on SEM (supported by

395

the Yale Institute for Nanoscience and Quantum Engineering) and TEM (supported by the

396

Central Instrumental Facility at Colorado State University) analyses, respectively. Zeta potential

397

measurements and ATR-FTIR analysis were performed in the Facility for Light Scattering (FLS)

398

and Chemical and Biophysical Instrumentation Center at Yale University, respectively.

399


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17. Milne, N. A.; O'Reilly, T.; Sanciolo, P.; Ostarcevic, E.; Beighton, M.; Taylor, K.; Mullett, M.; Tarquin, A. J.; Gray, S. R., Chemistry of silica scale mitigation for RO desalination with particular reference to remote operations. Water Res 2014, 65, 107-133. 18. Neofotistou, E.; Demadis, K. D., Use of antiscalants for mitigation of silica (SiO2) fouling and deposition: fundamentals and applications in desalination systems. Desalination 2004, 167, 257-272. 19. Makrides, A. C.; Turner, M.; Slaughter, J., Condensation of silica from supersaturated silicic-acid solutions. J Colloid Interf Sci 1980, 73, (2), 345-367. 20. Wallace, A. F.; DeYoreo, J. J.; Dove, P. M., Kinetics of silica nucleation on carboxyland amine-terminated surfaces: insights for biomineralization. J Am Chem Soc 2009, 131, (14), 5244-5250. 21. Coradin, T.; Coupé, A.; Livage, J., Interactions of bovine serum albumin and lysozyme with sodium silicate solutions. Colloids and Surfaces B: Biointerfaces 2003, 29, (2-3), 189-196. 22. Coradin, T.; Durupthy, O.; Livage, J., Interactions of amino-containing peptides with sodium silicate and colloidal silica: A biomimetic approach of silicification. Langmuir 2002, 18, (6), 2331-2336. 23. Coradin, T.; Livage, J., Effect of some amino acids and peptides on silicic acid polymerization. Colloid Surface B 2001, 21, (4), 329-336. 24. Kroger, N.; Deutzmann, R.; Sumper, M., Polycationic peptides from diatom biosilica that direct silica nanosphere formation. Science 1999, 286, (5442), 1129-1132. 25. Arkhangelsky, E.; Wicaksana, F.; Tang, C. Y.; Al-Rabiah, A. A.; Al-Zahrani, S. M.; Wang, R., Combined organic-inorganic fouling of forward osmosis hollow fiber membranes. Water Res 2012, 46, (19), 6329-6338. 26. Contreras, A. E.; Kim, A.; Li, Q. L., Combined fouling of nanofiltration membranes: Mechanisms and effect of organic matter. Journal of Membrane Science 2009, 327, (1-2), 87-95. 27. Kim, Y.; Elimelech, M.; Shon, H. K.; Hong, S., Combined organic and colloidal fouling in forward osmosis: Fouling reversibility and the role of applied pressure. Journal of Membrane Science 2014, 460, 206-212. 28. Qin, W. L.; Zhang, J. H.; Xie, Z. L.; Ng, D.; Ye, Y.; Gray, S. R.; Xie, M., Synergistic effect of combined colloidal and organic fouling in membrane distillation: Measurements and mechanisms. Environ Sci-Wat Res 2017, 3, (1), 119-127. 29. Xie, M.; Luo, W. H.; Gray, S. R., Synchrotron Fourier transform infrared mapping: A novel approach for membrane fouling characterization. Water Res 2017, 111, 375-381. 30. Tong, T.; Zhao, S.; Boo, C.; Hashmi, S. M.; Elimelech, M., Relating silica scaling in reverse osmosis to membrane surface properties. Environ Sci Technol 2017, 51, (8), 4396-4406. 31. Nakamura, K.; Matsumoto, K., Properties of protein adsorption onto pore surface during microfiltration: Effects of solution environment and membrane hydrophobicity. Journal of Membrane Science 2006, 280, (1-2), 363-374. 32. Mi, B.; Elimelech, M., Silica scaling and scaling reversibility in forward osmosis. Desalination 2013, 312, 75-81. 33. Xie, M.; Bar-Zeev, E.; Hashmi, S. M.; Nghiem, L. D.; Elimelech, M., Role of reverse divalent cation diffusion in forward osmosis biofouling. Environ Sci Technol 2015, 49, (22), 13222-9. 34. Bhattacharjee, S.; Kim, A. S.; Elimelech, M., Concentration polarization of interacting solute particles in cross-flow membrane filtration. J Colloid Interf Sci 1999, 212, (1), 81-99.


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35. Icopini, G. A.; Brantley, S. L.; Heaney, P. J., Kinetics of silica oligomerization and nanocolloid formation as a function of pH and ionic strength at 25°C. Geochimica et Cosmochimica Acta 2005, 69, (2), 293-303. 36. Preari, M.; Spinde, K.; Lazic, J.; Brunner, E.; Demadis, K. D., Bioinspired insights into silicic acid stabilization mechanisms: The dominant role of polyethylene glycol-induced hydrogen bonding. J Am Chem Soc 2014, 136, (11), 4236-44. 37. McClellan, S. J.; Franses, E. I., Adsorption of bovine serum albumin at solid/aqueous interfaces. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2005, 260, (1-3), 265-275. 38. McClellan, S. J.; Franses, E. I., Adsorption of bovine serum albumin at solid/aqueous interfaces. Colloid Surface A 2005, 260, (1-3), 265-275. 39. Christl, I.; Brechbuhl, Y.; Graf, M.; Kretzschmar, R., Polymerization of silicate on hematite surfaces and its influence on arsenic sorption. Environ Sci Technol 2012, 46, (24), 13235-43. 40. Tripp, C. P. H., M.L., Reaction of chloromethylsilanes with silica - A low-frequency infrared study. Langmuir 1991, 7, (5), 923-927. 41. Xie, M.; Gray, S. R., Silica scaling in forward osmosis: From solution to membrane interface. Water Res 2017, 108, 232-239. 42. Coradin, T.; Eglin, D.; Livage, J., The silicomolybdic acid spectrophotometric method and its application to silicate/biopolymer interaction studies. Spectrosc-Int J 2004, 18, (4), 567576. 43. Hoek, E. M. V.; Elimelech, M., Cake-enhanced concentration polarization: A new fouling mechanism for salt-rejecting membranes. Environmental Science & Technology 2003, 37, (24), 5581-5588. 44. Belton, D. J.; Deschaume, O.; Perry, C. C., An overview of the fundamentals of the chemistry of silica with relevance to biosilicification and technological advances. FEBS J 2012, 279, (10), 1710-1720. 45. Baler, K.; Martin, O. A.; Carignano, M. A.; Ameer, G. A.; Vila, J. A.; Szleifer, I., Electrostatic unfolding and interactions of albumin driven by pH changes: A molecular dynamics study. J Phys Chem B 2014, 118, (4), 921-930. 46. Lepoitevin, M.; Jaber, M.; Guegan, R.; Janot, J. M.; Dejardin, P.; Henn, F.; Balme, S., BSA and lysozyme adsorption on homoionic montmorillonite: Influence of the interlayer cation. Appl Clay Sci 2014, 95, 396-402. 47. Menzel, H.; Horstmann, S.; Behrens, P.; Barnreuther, B.; Krueger, I.; Jahns, M., Chemical properties of polyamines with relevance to the biomineralization of silica. Chem Commun 2003, (24), 2994-2995. 48. Belton, D. J.; Patwardhan, S. V.; Annenkov, V. V.; Danilovtseva, E. N.; Perry, C. C., From biosilicification to tailored materials: Optimizing hydrophobic domains and resistance to protonation of polyamines. P Natl Acad Sci USA 2008, 105, (16), 5963-5968. 49. Brunner, E.; Lutz, K.; Sumper, M., Biomimetic synthesis of silica nanospheres depends on the aggregation and phase separation of polyamines in aqueous solution. Phys Chem Chem Phys 2004, 6, (4), 854-857. 50. Knecht, M. R.; Wright, D. W., Amine-terminated dendrimers as biomimetic templates for silica nanosphere formation. Langmuir 2004, 20, (11), 4728-4732.



1.0

0.8 0.6 0.4 Silica only BSA only Silica + BSA

0.2 0.0

0

A

Normalized Water Flux


1.0

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0.8 0.6 0.4

0.0

200 400 600 800 1000 1200 1400

Silica only LYZ only Silica + LYZ

0.2

0

B

200 400 600 800 1000 1200 1400

Time (min)

Time (min)

Figure 1. Representative water flux decline curves for silica scaling, protein fouling, and combined scaling and fouling. (A) Bovine serum albumin (BSA) and (B) lysozyme (LYZ) were used as model protein foulants with different surface charge at the feed pH (6.50 ± 0.05). The flux decline tests were conducted with a cross-flow velocity of 4.25 cm/s and an initial water flux of 56 ± 2 L·min-1·h-1 for 1400 minutes at a constant temperature of 22 ± 1 °C. All feed solutions were composed of 7.0 mM CaCl2, 3.5 mM MgCl2, and 35 mM NaCl as background electrolytes. Feed solutions related to silica scaling included 2.8 mM Na2SiO3·5H2O, while those related to protein fouling included 35 mg/L protein.




A

500-min Flux Decline 1400-min Flux Decline After Physical Cleaning

1.0 0.8 0.6 0.4 0.2 0.0

Silica Only

B Normalized Water Flux

Page 19 of 24

1.0 0.8 0.6 0.4 0.2 0.0

BSA Only Silica + BSA

Silica Only

LYZ Only

Silica + LYZ

Figure 2. Normalized water flux after fouling and/or scaling and after physical cleaning, for model protein foulants: (A) BSA and (B) LYZ. The data after 500 min- and 1400-min fouling and/or scaling were chosen to represent the effects of fouling/scaling at the initial and final stages of the experiments. Membrane cleaning involved DI water rinsing at a crossflow velocity of 21.3 cm/s for 30 minutes. After cleaning, pure water flux of the tested membrane under 27.6 bar (400 psi) was measured to calculate the flux recovery ratio. The error bars represent standard deviation from triplicate independent experiments. The compositions of the feed solutions were consistent with those described in Figure 1.



Figure 3. FTIR spectra of RO membranes after 1400 minutes of scaling/fouling experiments followed by physical membrane cleaning. Absorbance peaks at 1050 to 1100 cm-1 are associated with Si-O-Si bonds and serve as a proxy for the presence of silica. Absorbance peaks at 16001700 cm-1 and 1540 cm-1 are associated with N-C=O and C-N-H vibrations in amide groups and serve as a proxy for the presence of protein. The details of scaling/fouling experiments can be found in the main text and Figure 1. The “background electrolytes” represent RO tests using feed solutions containing background electrolytes (CaCl2, MgCl2, and NaCl), but without either silica or protein.


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A

B

1 µm

1 µm

D

C

1 µm

1 µm

E

F

1 µm

1 µm

Figure 4. SEM micrographs of (A) pristine RO membrane, and membranes after 1400 min (B) silica scaling, (C) BSA fouling, (D) combined BSA fouling and silica scaling, (E) LYZ fouling, and (F) combined LYZ fouling and silica scaling, followed by physical membrane cleaning.



Figure 5. Normalized relative scattering intensity, I/I0, of dynamic light scattering (DLS) to analyze the formation of aggregates when proteins coexist with silica. The bulk solution composition was consistent with the concentration of each component used in the RO feedwater (Figure 1). Those solutions considering concentration polarization (CP) were composed of silica and background electrolytes at the calculated concentration at the membrane surface (Cm, Table S1), except that the concentrations of protein were 10-3 to 10-1 of Cm. The choice of those solutions has been explained in the main text, and the calculation of Cm is detailed in the Supporting Information. Low intensity of DLS signal from both proteins was observed in the absence of silica (A and B). However, the coexistence of silica and BSA induced rapid and significant aggregation (C). Although DLS signal was enhanced when both LYZ and silica were present (D), the extent of aggregate formation was less than that for BSA.


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A

B

C

D

Figure 6. Bright-field TEM images of the white precipitates formed by silica coexisted with (A and C) BSA and (B and D) LYZ. The solutions used to form those precipitates were composed of 27.2 mM CaCl2, 13.5 mM MgCl2, and 117.2 mM NaCl as background electrolytes, 10.4 mM Na2SiO3·5H2O, and 1.62 g/L BSA (10-2 of Cm) or 2.14 g/L LYZ (10-1 of Cm). Higher magnification TEM images of C and D show the areas indicated by the red dashed squares in A and B, respectively. Note that nano-scale particles were observed clearly in the silica-LYZ precipitates, while silica and BSA resulted in amorphous precipitates with no defined particle morphology.



A

Page 24 of 24

C

N

Si

O

C

N

Si

O

1 µm

B

2.5 µm

Figure 7. Elemental maps obtained by energy-dispersive X-ray spectroscopy indicating the location of C and N (the proxy for proteins) and Si and O (the proxy for silica) for the white precipitates produced by (A) silica and BSA and (B) silica and LYZ. The scale bars in A and B represent 1 µm and 2.5 µm, respectively. The solutions used to form those precipitates are detailed in Figure 6.


Combined Organic Fouling and Inorganic Scaling in Reverse Osmosis

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