TiO2 nanoparticles aggregation and

3 downloads 0 Views 3MB Size Report
Title: TiO2 nanoparticles aggregation and disaggregation in presence of alginates and humic acids. pH and concentration effects on nanoparticle stability.
Elsevier Editorial System(tm) for Water Research Manuscript Draft Manuscript Number: Title: TiO2 nanoparticles aggregation and disaggregation in presence of alginates and humic acids. pH and concentration effects on nanoparticle stability. Article Type: Research Paper Keywords: Titanium dioxide nanoparticles, Alginate, Suwannee river humic acid, Nanoparticle stability, Aggregate disaggregation, Aquatic systems Corresponding Author: Dr Serge Stoll, Corresponding Author's Institution: University of Geneva First Author: Frederic Loosli Order of Authors: Frederic Loosli; Philippe Lecoustumer, Dr; Serge Stoll Abstract: The behavior of manufactured TiO2 nanoparticles is studied here in a systematic way as a function of pH and in the presence of Suwannee river humic acids and alginate, at variable concentrations, which represent two major components found in aquatic systems. TiO2 nanoparticles aggregation, disaggregation and stabilization are investigated using dynamic light scattering and electrophoretic experiments allowing the measurement and evolution determination of z-average hydrodynamic diameters and zeta potential values. Stability of the TiO2 nanoparticles is carried out by considering three pH-dependent electrostatic scenarios (below the point of zero charge of the nanoparticles, at the point of zero charge and above it). In the first scenario, when pH is below the point of zero charge of the TiO2 nanoparticles, nanoparticles exhibit a positively charged surface whereas alginate and Suwannee river humic acids are negatively charged. Fast adsorption at the TiO2 nanoparticles occurs, promotes surface charge neutralization and aggregation and, by increasing further alginate and Suwannee river humic acids, results in charge inversion and thus stabilization of TiO2 nanoparticles. In the second electrostatic scenario, at the pH of the TiO2 surface charge neutralization, TiO2 nanoparticles are rapidly forming aggregates and adsorption of alginate and Suwannee river humic acids on aggregates surface leads to the partial disaggregation of aggregates. In the third electrostatic scenario, when nanoparticles, alginates and Suwannee river humic acids are negatively charged a small amount of Suwannee river humic acids is adsorbed via hydrophobic interactions. It is found that the fate and behavior of individual and aggregated TiO2 nanoparticles in presence of environmental compounds are strongly dependent in a non linear on the electrostatic, concentration ratio, and to a less extend to steric interactions and amphiphilic compound character way and that environmental aquatic concentration ranges of humic acids and biopolymers are expected to largely modify the stability of aggregated or individual TiO2 nanoparticles. Suggested Reviewers: Mohammed Baalousha Dr. School of Geography, Earth and Environmental Science, University of Birmingham, UK [email protected] Expert in the field of nanoparticles behavior , such as aggregation and disaggregation, and transport in water Gregory V Lowry Prof.

Center for the Environmental Implications of Nano Technology, Carnegie Mellon University, US [email protected] Expert in the interaction processes between manufactured nanoparticles and natural organic matter Xiaojing Leng Prof. M-Director, China agricultural university, China [email protected] Expert in the field of nanoparticle chracterization using electrophoretic measurements Martin Hassellhov Prof Environmental Nanochemistry Dep, University of Gothenburg [email protected] Expert studying nanoscale processes in the environment, and how the reactivity of nanoscale materials is impacting the environment.

Cover Letter, For Editor only

Dr. Serge STOLL (Corresponding Author) University of Geneva, F.-A. Forel Institute Group of Environmental Physical Chemistry 10 Route de Suisse, 1290 Versoix, Switzerland tel ++ 41 22 379 0333; Email : [email protected] Editor Water Research Prof. Mark van Loosdrecht Kluyver Inst. For Biotechnology Technische Universiteit Delft Julianalaan 67 2628 BC Delft, Netherlands

February 06/02/2013

Dear Professor van Loosdrecht,

We would like to publish in Water Research, as a regular article, a manuscript entitled: “TiO2 nanoparticles aggregation and disaggregation in presence of alginates and humic acids. pH and concentration effects on nanoparticle stability.” by Fredéric Loosli, Philippe Lecoustumer and Serge Stoll. We believe that understanding the role of humic substances and polysaccharides in the evolution, tranformation and stability of manufactured nanoparticles is an essential aspect in aquatic systems. In this manuscript, the adsorption of humic acids and alginates at the surface of titanium dioxide nanoparticles is investigated to achieve a systematic, quantitative and significant description of the physico chemical behavior of these nanoparticles. Surface charge variations and electrostatic transformation (charge neutralisation, overcharging), kinetics of disaggregation are investigated as a function of the solution pH, humic acids, alginates and nanoparticle concentration ratio. Stability diagrams are also given and clearly indicates the rapid stabilisation and dispersion of nanoparticle aggregates and isolated monomers. The manuscript has not been previously published, in whole or in part, and is not under consideration by any other journal. All authors are aware of, and accept responsibility for, the manuscript.

We are looking forward to hearing from you soon.

Yours Sincerely,

Serge Stoll (on behalf of the authors)

*Highlights (for review)

Highlights 

TiO2 nanoparticles (NPs) stability is found pH dependent



Alginate and humic acids (NOM) adsorption modify the NPs TiO2 surface charge



When aggregates are initially present NOM promotes disaggregation



Electrostatic, steric and hydrophobic forces play key roles in TiO2 stability



Typical environmental NOM concentrations disperse TiO2 NPs in aquatic systems

Graphical Abstract (for review)

Graphical abstract

*Manuscript Click here to download Manuscript: Manuscript-Loosli-Lecoustumer-Stoll.docx

Click here to view linked References

TiO2 nanoparticles aggregation and disaggregation in presence of alginates and humic acids. pH and concentration effects on nanoparticle stability.

1 2 3 4 5 6 7

a

8

b,c

Frédéric Loosli , Philippe Le Coustumer , Serge Stoll

a,*

9 10 11 12 13 14 15 16 17

a

University of Geneva, F.-A. Forel Institute, Group of Environmental Physical Chemistry, 10 route de Suisse, 1290 Versoix, Switzerland b

Université Bordeaux 3, EA 4592 Géoressources & Environnement, ENSEGID, 1 allée F. Daguin, 33607 Pessac, France c

Université Bordeaux 1, UFR STM, B.18 Av. Des facultés 33405 Talence, France

18 19 20 21 22

Frédéric Loosli:

23

Tel: + 41 22 379 0332 / email:[email protected]

24

Philippe Le Coustumer:

25

Tel: +33 5 40 00 87 98 / email:[email protected]

26

Serge Stoll *:

27

Tel: + 41 22 379 0333 / email:[email protected]

28

Fax: + 41 22 379 0302 1

29

Abstract

30

The behavior of manufactured TiO2 nanoparticles is studied here in a systematic way as a

31

function of pH and in the presence of Suwannee river humic acids and alginate, at variable

32

concentrations, which represent two major components found in aquatic systems. TiO2

33

nanoparticles aggregation, disaggregation and stabilization are investigated using dynamic

34

light scattering and electrophoretic experiments allowing the measurement and evolution

35

determination of z-average hydrodynamic diameters and zeta potential values. Stability of the

36

TiO2 nanoparticles is carried out by considering three pH-dependent electrostatic scenarios

37

(below the point of zero charge of the nanoparticles, at the point of zero charge and above it).

38

In the first scenario, when pH is below the point of zero charge of the TiO2 nanoparticles,

39

nanoparticles exhibit a positively charged surface whereas alginate and Suwannee river humic

40

acids are negatively charged. Fast adsorption at the TiO2 nanoparticles occurs, promotes

41

surface charge neutralization and aggregation and, by increasing further alginate and

42

Suwannee river humic acids, results in charge inversion and thus stabilization of TiO2

43

nanoparticles. In the second electrostatic scenario, at the pH of the TiO2 surface charge

44

neutralization, TiO2 nanoparticles are rapidly forming aggregates and adsorption of alginate

45

and Suwannee river humic acids on aggregates surface leads to the partial disaggregation of

46

aggregates. In the third electrostatic scenario, when nanoparticles, alginates and Suwannee

47

river humic acids are negatively charged a small amount of Suwannee river humic acids is

48

adsorbed via hydrophobic interactions. It is found that the fate and behavior of individual and

49

aggregated TiO2 nanoparticles in presence of environmental compounds are strongly

50

dependent in a non linear on the electrostatic, concentration ratio, and to a less extend to steric

51

interactions and amphiphilic compound character way and that environmental aquatic

52

concentration ranges of humic acids and biopolymers are expected to largely modify the

53

stability of aggregated or individual TiO2 nanoparticles. 2

54

Keywords: Titanium dioxide nanoparticles, Alginate, Suwannee river humic acid,

55

Nanoparticle stability, Aggregate disaggregation, Aquatic systems

56

1.Introduction

57

Manufactured nanoparticles (NPs) represent today an important class of potential emerging

58

pollutants due to their release already occurring into the environment. Contrary to natural

59

aquatic colloids manufactured NPs are produced to have surface properties that have never

60

been encountered by man and other organisms in nature. Depending on physical and chemical

61

conditions prevailing in aquatic systems, and possible interaction processes with natural

62

aquatic colloids, manufactured NPs will occur either as aggregated or dispersed materials.

63

Aggregates containing NPs will sediment and become immobilized, while dispersed NPs will

64

be able to significantly diffuse in aqueous environments, and will be more mobile,

65

bioavailable and toxic. Interactions between manufactured NPs with, in particular, natural

66

organic matter such as biopolymers and humic substances, leading to surface coating, surface

67

charge modifications, aggregation, flocculation, or stabilization and dispersion, will then

68

strongly alter their dynamic properties (immobilization versus diffusion).

69

Nanoparticles are commonly defined as particles with a characteristic dimension in the 1-100

70

nm range. This class of colloidal particles includes a large number of different type of

71

compounds such as metal NPs (Au, Ag), metal oxide NPs (TiO2, ZnO, Fe2O3) and carbon

72

based NPs (nanotubes, fullerenes). They exhibit various chemical surface properties and

73

reactivities depending on their sizes, shapes and chemical compositions (Ju-Nam and Lead

74

2008). Their particular properties concern redox and catalytic properties to high performance

75

UV filtering capacity. Among oxide based NPs, titanium dioxide is one of the most produced

76

and common nanomaterial. TiO2 is used in photovoltaic, photocatalytic sensor and energy

77

storage domains, plastic, fibers, foods, pharmaceuticals, cosmetics, antimicrobial applications, 3

78

and painting industry where TiO2 is primarily used as pigment and UV protective agent (Chen

79

and Mao 2007). The global production of TiO2 is of the order of thousands of tons per year

80

(Weir et al. 2012). As a result many sources of anthropogenic TiO2 are expected to enter into

81

aquatic compartments such as surface waters via sewage systems. Different studies on the

82

behavior of TiO2 NPs and the role of pH, ionic strength, salt valency, and surface charge on

83

their aggregation and dispersion have been reported (French et al. 2009). Studies indicate that

84

TiO2 NPs aggregation occurs at pH near the point of zero charge and that they are stable at

85

other pH. Moreover high ionic strength and presence of divalent salt is shown to facilitate

86

aggregation process (French et al. 2009, Guzman et al. 2006).

87

Impact, fate and reactivity of NPs and their possible interaction with natural organic matter in

88

aquatic system is thus a topic of high interest (Nowack and Bucheli 2007) for environmental

89

risk evaluation (Auffan et al. 2009, Colvin 2003, Wiesner et al. 2006). Natural organic matter

90

(NOM), composed of humic substances (HSs) and non humic substances such as

91

polysaccharides, is ubiquitous in the environment. HSs are mainly present as fulvic acids

92

(FAs) and humic acids (HAs). HAs are heterogeneous macromolecules from a conformational

93

and chemical point of view with mainly carboxylic acid and phenolic functional groups. HAs

94

are principally composed of fibrils generated by the interconnection of basic molecules of 20

95

nm diameter with some isolated larger (50-300 nm) globular macromolecules (Baalousha et

96

al. 2005). Alginate is a polysaccharide linear block copolymer extracted from the cell walls of

97

brown seaweed that comprises 1,4-linked β-D-mannuronic acid and α-L-guluronic acid

98

residues. Alginate is widely used in the pharmaceutical and food industry. In food industry

99

alginate is mainly employed as thickening agent and stabilizer whereas as efficient drug

100

carriers in biomedecine (Liu et al. 2008). Alginate was also utilized in the recovery of heavy

101

metal ions and Ngomsik showed the efficiency of Ni2+ removal from wastewater with alginate

102

microcapsules containing extractant and magnetic NPs (Ngomsik et al. 2006). 4

103

A substantial amount of work has been reported to have a better understanding of the effect of

104

NOM on NPs stability and bioavailability. Aggregation processes of metal, metal oxide and

105

carbon based NPs in presence of FAs, HAs and alginate clearly indicate the importance of

106

both electrostatic and steric stabilization when NOM are adsorbed on their surfaces (Chen and

107

Elimelech 2008, Domingos et al. 2009, Keller et al. 2010, Ottofuelling et al. 2011). Moreover,

108

effect of salt valence and presence of divalent salt facilitate bridging processes of NPs

109

(fullerenes, hematite and TiO2) even once coated with NOM by formation of NOM-divalent

110

cation complexes (Chen and Elimelech 2007, Chen et al. 2006, Domingos et al. 2010).

111

Another important role of NOM adsorption on NPs is the enhancement of their photocatalytic

112

properties and capacity to adsorb heavy metal and hydrophobic organic compounds (Chae et

113

al. 2012, Chen et al. 2012, Papageorgiou et al. 2012, Yang and Xing 2009). Adsorption of

114

NOM on mineral surfaces takes mainly place by ligand exchange between the carboxylic acid

115

and phenolic groups of NOM even if other mechanisms such as hydrophobic interaction,

116

hydrogen bonding, anion exchange or cation bridging are playing a role (Pallem et al. 2009,

117

Yang et al. 2009).

118

The objective of this study consists to investigate the stability of TiO2 NPs by considering two

119

important but different organic compounds found in aquatic systems, namely HAs and

120

alginates. Conformational structures of alginate and HAs are different. Alginate is a semi-

121

flexible linear polyelectrolyte with an almost constant charge density along its chain whereas

122

HAs are heterogeneous and semi-rigid macromolecules. In this study, the influence of pH

123

solution, which will play a key role on the electrostatic, NOM/NPs concentration ratio and

124

NOM structure, is investigated to have a better overview of the possible behavior of TiO2 NPs

125

when released in complex aquatic environments with regards to aggregation and dispersion.

126

The influence of these two organic compounds on TiO2 NPs stability is systematically studied

127

by adjusting the pH and NOM concentration and by the measurements of z-average 5

128

hydrodynamic diameters and electrophoretic mobilities at a constant electrolyte background.

129

Focus is also made on the disaggregation processes of TiO2 NP aggregates, which is a central

130

aspect in the NPs dispersion in aquatic systems, and relation between the specific structural

131

and chemical properties of NOM and the stabilization of TiO2 NPs. Effect of NOM

132

concentration and structure on the kinetic of aggregate disaggregation and variation of TiO2

133

NPs stability is investigated at the point of zero charge of TiO2 NPs and recording the

134

aggregate fragment sizes as a function of NOM concentration and time.

135

2.Materials and methods

136

2.1.Materials

137

TiO2 anatase NPs (Nanostructured & Amorphous Material Inc) with a nominal particle

138

diameter of 15 nm were purchased as a 170 g/L TiO2 suspension in water with a specific

139

surface area of 240 m2/g. Suwannee river humic acid (SRHA) (standard II, International

140

Humic Substances Society) 500 mg/L pH 9.8, and low viscosity alginate (A2158, Sigma

141

Aldrich) 1 g/L pH 6.2 stock solutions were stirred overnight and filtrated through a 0.45 μm

142

cellulose acetate filter (VWR). Hydrochloric acid (1 M HCl, Titrisol®, Merck) and sodium

143

hydroxide (1 M NaOH, Titrisol®, Merck) were used to adjust the solutions pH. Sodium

144

chloride (NaCl, 99.5%, Acros Organics) was employed to adjust the final ionic strength to

145

0.001 M. All the solutions were prepared to the target experimental concentrations with

146

deionized Milli Q water (R > 18 MΩ cm). Experiments were performed in 25 mL

147

polypropylene (PP) tubes 25 × 90 mm (Millan) with a crosshead single 8 × 10 mm magnetic

148

stirrer (VWR).

149

2.2. Zeta potential and size distribution measurements

150

A Zetasizer Nano ZS (Malvern Instrument) was used to determine the zeta (ζ) potential values

151

as well as size distributions of the TiO2 NPs, alginate, SRHA solutions and their 6

152

corresponding mixtures. For each situation triplicate measurements were performed and each

153

sample was measured 3 times to determine the zeta potential values and the size distributions.

154

For the zeta potential determination 15 sub-runs, with a delay of 5 s between them, to relax

155

and stabilize the system, whereas 12 sub-runs of 10 s with a 5 s delay, for the size distribution

156

measurements, were made. For the ζ potential calculations the Smoluchowski approximation

157

model, i.e. when particle are large as compared to the double layer thickness, was applied. 50

158

mg/L solutions for the TiO2 NPs and mixtures and a 100 mg/L solution for the NOM (alginate

159

and SRHA) were used. Solutions were vigorously stirred before analysis.

160

2.3. Aggregate size evolution determination

161

The TiO2 aggregate z-average hydrodynamic diameter determination in presence of alginate

162

and SRHA was performed in 50 mL PP tubes 29 × 115 mm (VWR) under agitation with a

163

crosshead single 8 × 10 mm magnetic stirrer (VWR). Equilibrium time before measurement

164

was set to 45 min for alginate and 24 hours for SRHA after each successive addition of NOM.

165

Variation of aggregate z-average hydrodynamic diameters and zeta potential values as a

166

function of alginate and SRHA mass concentrations were recorded with a Zetasizer Nano ZS

167

(Malvern Instrument) for an initial TiO2 mass concentration of 50 mg/L and a background

168

NaCl salt concentration equal to 0.001 M. The pH of all solutions was, before starting the

169

experiments, adjusted to the pH of interest.

170

2.4. TEM image analysis

171

Image analysis was also realized with a 120 kV Hitachi A7650 Transmission Electron

172

Microscope (TEM). TEM samples were prepared by dropping 20 μL of solutions containing a

173

TiO2 mass concentration equal to 100 mg/L on a 200 square mesh copper grid coated with a

174

thin film (30-50 nm) of pure carbon (Electron Microscopy Sciences, CF200-Cu). The

175

different drops were left to dry for a minimum of one hour before TEM analysis. For mixtures

7

176

analysis, i.e. TiO2 in presence of alginate and SRHA, one minute treatment with osmium

177

tetroxide as staining agent was realized to enhance the contrast of NOM. The image

178

resolution was set to 3284 × 2600 pixels with an acquisition time equal to 5 sec.

179

3. Results and discussion

180

3.1. Material characterization

181

TiO2 NPs. The TiO2 NPs surface charge pH dependence and the resulting stability of the NPs

182

in solution were determined through pH titration curves for pH values in the range 2-11 and a

183

background salt concentration of 0.001 M (adjusted with NaCl). As shown in Fig. 1a, TiO2

184

NPs exhibit a stable and positive zeta potential (+40.0 ± 3.1 mV) from pH 2 to 5. Then the ζ

185

potential value decreases to the Point of Zero charge (PZC) at pH 6.2 ± 0.1 in good agreement

186

with the literature (Parks 1965, Preocanin and Kallay 2006). By increasing further the pH, the

187

zeta potential becomes negative and is found to stabilize at -44.2 ± 1.2 mV at pH 9.4. The z-

188

average diameter evolution with pH is also determined and presented in Fig. 1b. The NPs size

189

is found to increase from 52 ± 9 nm in the pH 2-5 domain to reach a maximum value at the

190

PZC with values greater than 10 µm indicating strong aggregation when the NPs surface

191

charge is neutralized. TEM images of TiO2 NPs stabilized at pH < pHPZC,TiO2, as well as TiO2

192

NPs aggregates at the PZC are presented in Fig. 2a and 2b respectively. Fig. 2a indicates the

193

presence of small aggregates composed of few particles in good agreement with DLS

194

measurements. By further increasing the pH, the z-average diameter is found to rapidly

195

decrease to reach stable values again at 57 ± 7 nm. As shown by the gray domains in Fig. 1

196

TiO2 NPs destabilization is found to occur in a zeta potential domain comprised between +30

197

and -30 mV.

198

Alginate. Titration curves of a 100 mg/L alginate solution with a background ionic strength

199

of 0.001 M (NaCl) were performed from pH 11 to 3 with HCl at variable concentrations. 8

200

Alginate exhibits negative zeta potential values and constant z-average values on the whole

201

domain of pH. As shown in Fig. 3a, a ζ potential plateau of about -30.3 ± 1.5 mV is found

202

between pH 11 and 5.75. Then, at lower pH values, the zeta potential increases due to the

203

continuous protonation of the carboxylic moieties of the α-L-guluronate and β-D-

204

mannuraonate alginate monomers to reach a value of -13.0 ± 1.1 mV at pH 3. No PZC is thus

205

observed here in good agreement with the α-L-guluronate and β-D-mannuraonate pKa values

206

of respectively 3.6 and 3.2 (Stefansson, 1999). Owing to the low pKa value of carboxylic

207

groups (pKa < 4) alginate is hence stabilized against aggregation upon pH variations at low

208

monovalent electrolyte concentration. The z-average diameter of alginate was found constant

209

from pH 3 to 11 and equal to 178 ± 21 nm (Fig. 3b).

210

Suwannee river humic acids. To get an insight into the zeta potential variation of the SRHA

211

macromolecules in solution as a function of pH, the corresponding titration curves were

212

determined with the measurement of the electrophoretic mobilities. 100 mg/L SRHA

213

solutions with a background NaCl salt concentration (10-3 M) were used and the pH was

214

adjusted from pH 11 to pH 3 with HCl at variable concentrations. As shown in Fig. 4a, SRHA

215

exhibits a strong negative structural charge from -69.0 ± 2.4 mV at pH 11 to -30.2 ± 0.8 mV

216

at pH 3 mainly due to the presence of carboxylic acid and phenolic functional groups. The

217

zeta potential decreases continuously owing to the heterogeneity of the functional groups. As

218

shown in Fig. 4b, SRHA z-average diameter is found constant on the whole pH domain with a

219

z-average diameter value of 379 ± 19 nm. Such a value, in comparison with few nanometers

220

range values obtained by transmission electron microscopy (Baalousha et al. 2005), is related

221

to the aggregation of the heterogeneous single nanometric entities that gives large-scale

222

supramolecular structures in solution.

9

223

3.2. TiO2 NPs stability in presence of alginate and SRHAs

224

As previously shown in Fig. 1 stability of TiO2 NPs is strongly pH dependant whereas

225

alginate and SRHA exhibit a negative surface charge in a large pH domain (Fig. 3 and Fig. 4).

226

According to this observation three main different electrostatic scenarios can be defined. In

227

the first one, when pH < pHPZC,TiO2, TiO2 NPs are positively charged with zeta potential

228

values greater than +30 mV. As a result they are stabilized against aggregation owing to

229

electrostatic repulsions between the NPs. On the other hand alginate and SRHA are negatively

230

charged and thus strong electrostatic adsorption of NOM is expected at the oppositely charged

231

TiO2 NPs surface hence modifying the surface charge and resulting stability of the TiO2 NPs.

232

In the second electrostatic scenario, when pH = pHPZC,TiO2, TiO2 NPs are neutral and strong

233

aggregation is observed; attractive Van-der-Waals interactions will be the predominant

234

interaction phenomena, and the adsorption of negatively charged alginate and SRHA is

235

expected to restabilize the TiO2 NPs by enhancement of the electrostatic repulsions and steric

236

effects. Finally, in the third scenario, when pH > pHPZC,TiO2, TiO2 NPs are negatively charged

237

as well as alginate and SRHA, and NOM adsorption is expected to be limited due to

238

electrostatic repulsions. Thus no significant changes in TiO2 stability should be observed.

239

Positively charged TiO2 NPs in presence of negatively charged alginate and

240

SRHA (pH < pHPZC,TiO2)

241

Alginate-TiO2 mixtures. Experiments at pH = 4.5 were conducted to get an insight on NOM

242

capacities to modify the surface charge of the TiO2 NPs. As shown in Fig. 5, in the case of

243

zeta potential variation of TiO2 NPs as a function of alginate mass concentration, before

244

addition of alginate, TiO2 NPs exhibit a zeta potential value of +34.1 ± 1.4 mV with a z-

245

average diameter of 48 ± 1 nm. During alginate addition, three stability domains are observed.

246

For alginate mass concentration below 1.1 mg/L, the TiO2 NPs are still strongly positively

10

247

charged with a zeta potential greater than +28 mV and z-average value smaller than 200 nm.

248

Even if TiO2 NPs are stabilized against strong aggregation due to the electrostatic repulsions,

249

the z-average diameter increases in this domain, as a consequence of alginate adsorption.

250

Then, in the second domain (grey area in Fig. 5), alginate mass concentration in the range 1.1-

251

2.4 mg/L strongly influences the surface charge of the TiO2 NPs and destabilizes the TiO2

252

NPs. Full charge neutralization and IsoElectic Point (IEP) of TiO2 NPs is then observed for an

253

alginate mass concentration of 1.68 ± 0.01 mg/L with a corresponding charge ratio of alginate

254

over TiO2 found here equal to 1.25 ± 0.21. At charge neutralization, the z-average diameter is

255

found to correspond to the maximum of the size distribution curve with a value of 1082 ± 90

256

nm (Fig. 5b). After the IEP, charge inversion is observed and both zeta potential and z-

257

average diameter values rapidly decrease until an alginate mass concentration equal to 2.4

258

mg/L, which corresponds to a zeta potential value of about -28 mV. Finally, for alginate mass

259

concentration greater than 2.4 mg/L, the system is stabilized and full charge inversion is

260

observed at 5 mg/L with a zeta potential value of -34.0 ± 0.7 mV corresponding to a z-average

261

diameter of 105 ± 1 nm.

262

SRHA-TiO2 mixtures. Three important domains in Fig. 6 are also observed here. For SRHA

263

mass concentration below 1.5 mg/L, the NOM is continuously adsorbed at the TiO2 NPs

264

surface, the z-average diameter slightly increases to 247 ± 19 nm at 1.5 mg/L, whereas the

265

zeta potential value at this concentration is found to be +19.5 ± 2.6 mV. Then, significant

266

destabilization occured for SRHA concentration comprised between 1.5 mg/L and 4 mg/L as

267

shown in Fig. 6 (grey area). This second domain corresponds to zeta potential values ranging

268

from +20 mV to -10 mV. IEP is obtained at 2.80 ± 0.25 mg/L corresponding to a charge ratio

269

of SRHA over TiO2 equal to 1.40 ± 0.23 and a maximum z-average diameter value of 2196 ±

270

254 nm. Finally, for SRHA concentrations above 4 mg/L the TiO2 NPs are found to be

11

271

stabilized and complete charge inversion is achieved above 8 mg/L leading to a z-average

272

diameter plateau value at 95 ± 2 nm.

273

When comparing the effects of the SRHA and alginate on TiO2 NPs stability some significant

274

differences between alginate and SRHA are notifiable. On the one hand, the aggregation

275

domain (destabilization) of TiO2 NPs is larger in the presence of alginate than with SRHA if

276

ones consider the zeta potential values. Such observation can be related to the architecture

277

difference between alginate and SRHA. Indeed the alginate is a linear semi-flexible

278

polyelectrolyte whereas SRHA is a more globular heterogeneous and semi-rigid

279

macromolecule which can exhibit important steric hindrance in comparison to alginate. On

280

the other hand, the alginate is more efficient than SRHA for charge neutralization when

281

considering the charge ratio to reach the IEP. It is again the consequence of the alginate

282

flexibility and charge distribution which is expected to facilitate its adsorption at the TiO2

283

surface.

284

TiO2 NPs aggregates in presence of negatively charged alginate and SRHA (pH =

285

pHPZC,TiO2)

286

In this second electrostatic scenario the pH is adjusted to pHPZC,TiO2 = 6.2. In such conditions,

287

TiO2 NPs are uncharged and are forming large aggregates due to predominance of attractive

288

Van-der-Waals forces. On the other hand NOM are negatively charged and their adsorption at

289

the TiO2 aggregates should result in a negatively charged surface for the TiO2-NOM system

290

and thus partial restabilization of TiO2 NPs is expected.

291

Alginate-TiO2 mixtures. At a pH value of 6.2, alginate has a zeta potential value of -28.9 ±

292

3.5 mV and a z-average diameter of 154 ± 34 nm (Fig. 3). As shown in Fig. 7, for successive

293

alginate additions and after an equilibrium time of 45 min between each measurements, a

294

significant negative surface charge acquisition and decrease of the zeta potential are observed. 12

295

For an alginate mass concentration of 3 mg/L or greater a plateau at around -30 mV is

296

reached. Partial but significant disaggregation of the TiO2 NP aggregates is also observed to

297

reach a z-average diameter plateau at around 1 μm for alginate concentrations ≥ 3 mg/L.

298

SRHA-TiO2 mixtures. SRHA exhibits a zeta potential value of -50.8 ± 5.6 mV and a z-

299

average diameter of 378 ± 14 nm (Fig. 4). In presence of SRHA, after an equilibrium time of

300

one day between successive SRHA additions, the zeta potential value decreases as the

301

concentration of SRHA is increasing to finally stabilize to -40 mV for a SRHA concentration

302

of 10 mg/L (Fig. 8a). The size of TiO2 aggregates is also decreasing via disaggregation to

303

reach a plateau value at 300 nm for [SRHA] ≥ 5 mg/L as shown in Fig. 8b.

304

TiO2 NPs disaggregation kinetics

305

The kinetics of disaggregation are investigated here by analyzing the evolution of zeta

306

potential and z-average diameter values as a function of time after addition of given amounts

307

of NOM at t = 0.

308

Alginate-TiO2 mixtures. As shown in Fig. 9, owing to the stabilization of the zeta potential

309

and z-average diameter to constant values, the time to reach equilibrium is equal to 45 min

310

after addition of alginate. Two different regimes of aggregate fragmentation are observed. The

311

first regime corresponds to a rapid decrease of the z-average diameter whereas the second

312

regime to its stabilization. Significant variation of zeta potential values in the first regime of

313

TiO2 NPs aggregate disaggregation (t < 45 min) is only observed at high alginate

314

concentration (≥ 3 mg/L). This suggests a continuous alginate adsorption at the TiO2

315

aggregates during aggregate fragmentation. Alginate concentration clearly influences the

316

disaggregation process and maximum disaggregation is obtained when [alginate] ≥ 3 mg/L

317

with final z-average diameters at 500 nm. It should be noted that only partial disaggregation is

13

318

observed here with the formation of fragments having z-average diameters larger than the

319

initial TiO2 NP sizes (50 nm).

320

SRHA-TiO2 mixtures. The time requested to equilibrate the system after adding a given

321

amount of SRHA, at t = 0, is one day as shown in Fig. 10. The maximum of fragmentation is

322

found for [SRHA] ≥ 5 mg/L. In this condition, the SRHA disaggregates the TiO2 NP

323

aggregates and a z-average diameter of about 250 nm is obtained. Continuous adsorption of

324

SRHA is also observed for high concentrations owing to a significant decrease of the zeta

325

potential values in the first regime of disaggregation (t < 24 h).

326

From the comparison of Fig. 9 and Fig. 10 three main results are emerging. i) At low

327

concentrations (≤ 4 mg/L) alginate is more effective in the disaggregation of the TiO2

328

aggregates than SRHA. Indeed for a concentration of 3 mg/L the z-average is found equal to

329

500 nm after alginate addition and 5.5 μm for the same SRHA concentration. Moreover, the

330

kinetics of disaggregation are greater in the case of alginate with an equilibrium time of

331

approximately 45 min whereas the addition of SRHA required one day before reaching size

332

equilibrium. The semi-flexible and linear character of alginate and thus its ability to adsorb at

333

the TiO2 surface and interpenetrate the aggregate structures is then expected to play an

334

important role in the disaggregation process. Alginate through conformational changes can

335

easily reach the inner structure of the TiO2 aggregates, whereas SRHA diffusion within

336

aggregate is probably more difficult owing to important steric hindrances. ii) When working

337

at higher concentrations (≥ 5 mg/L), SRHA is able to disaggregate the TiO2 NPs more

338

efficiently than alginate with regards to the z-average diameter values after system

339

equilibrium. For a 5 mg/L mass concentration, the z-average diameter is equal to 250 nm after

340

addition of SRHA and 500 nm in the case of alginate. Such a difference can be attributed to

341

the SRHA charge, which is higher than alginate resulting in higher electrostatic and steric

342

repulsions (Baalousha 2009, Christian et al. 2008, Pefferkorn 1995). Disaggregation kinetics 14

343

are also expected to be controlled by the compactness of the aggregates and corresponding

344

fractal dimensions. From a mechanistic point of view low fractal dimension aggregates are

345

supposed to disaggregate by splitting into fragments of similar sizes, whereas high fractal

346

dimension and thus more compact aggregates to undergo attrition, i.e. aggregates erosion.

347

Baalousha (2009) described the disaggregation kinetics of iron oxide in presence of SRHA by

348

two regimes; first disaggregation of aggregates resulting in fractions of similar size with a

349

relative high kinetic rate followed by a second regime of attrition. Pefferkorn when studying

350

disaggregation processes of latex colloids in presence of polyvinylpyridine suggested that

351

after a consequent lag time, a first regime of attrition followed by a second regime where

352

attrition as well as internal rupture of aggregates was taking place. According to our results,

353

aggregates disaggregation is also expected to result from random aggregates breakup leading

354

to a rapid decrease of the z-average diameter. Aggregates attrition resulting in the release of

355

individual NPs is however not excluded. iii) For high NOM concentrations continuous

356

adsorption occurs owing to the decrease of zeta potential values in the first regime of

357

disaggregation. Aggregate disaggregation, in particular aggregate internal break-up, is thus

358

expected to release some free surface for further NOM adsorption.

359

Negatively charged TiO2 NPs in presence of negatively charged alginate and

360

SRHA (pH > pHPZC,TiO2)

361

In this third electrostatic scenario, negatively charged TiO2 NPs and negatively charged NOM

362

are considered at pH 9.8. Electrostatic repulsions are expected to prevent and/or limit the

363

adsorption of NOM at the TiO2 NPs surface.

364

Alginate-TiO2 mixtures. In such condition alginate has a zeta potential value equal to -32.1

365

± 3.4 mV and 150 ± 15 nm as z-average diameter (Fig. 3). As shown in Fig. 11, TiO2 NPs

366

zeta potential values and z-average diameter are constant (-45.5 ± 1.4 mV and 55 ± 1 nm

15

367

respectively) as function of alginate concentration. No significant interactions are thus

368

obtained between TiO2 NPs and alginate.

369

SRHA-TiO2 mixtures. Z-average diameter and zeta potential value of SRHA are equal to

370

393 ± 19 nm and -63.0 ± 4.1 mV respectively (Fig. 4). Contrary to the previous situation, Fig.

371

12 indicates a significant, however limited, decrease of the zeta potential values as the SRHA

372

concentration increases whereas the z-average diameter is found to slightly increase. Such a

373

behavior denotes limited SRHA adsorption at the TiO2 NPs and importance of the

374

amphiphilic character of SRHA as shown by Ait Akbour (Ait Akbour and Jada 2013).

375

Stability diagrams

376

To give an overview of such a complex TiO2 NPs behavior as a function of alginate and

377

SRHA concentrations, three relevant pH values (pH < pHPZC,TiO2, pH = pHPZC,TiO2 and pH >

378

pHPZC,TiO2) were considered and schematic stability diagrams reported in Fig. 13 and Fig. 14

379

respectively. They summarize the aggregate formation and disaggregation domains resulting

380

from the surface charge variation of TiO2 NPs in presence of NOM (§3.2). The domains of

381

destabilization (aggregation) are represented by cross area and the changes in TiO2 NPs

382

stability (aggregation versus disaggregation) by arrows. At pH 9.8 (blue area), TiO2 are stable

383

from an electrostatic point of view and further addition of alginate has no effect on TiO2 NPs

384

stability. On the other hand, in presence of SRHA, limited adsorption of SRHA at the TiO2

385

NPs occurs owing to hydrophobic interactions. Such an interaction is nonetheless promoting

386

the NPs stability and overcharging. At pH 6.2 (yellow area), TiO2 NPs form aggregates and

387

subsequent addition of NOM results in the partial disaggregation of TiO2 aggregates as

388

confirmed by TEM analysis in Fig. 15. At pH 4.5 (red area), the presence of NOM gradually

389

leads to charge neutralization of the TiO2 NPs and thus rapid destabilization of the system is

390

observed (i.e. aggregate formation). The destabilization range is comprised between 1.1 and

16

391

2.4 mg/L and 1.5 to 4 mg/L for alginate and SRHA respectively. By increasing further NOM

392

concentration, electrostatic repulsions due to charge inversion of NPs become predominant

393

and stabilization of the NPs takes place again. Stability diagrams clearly indicate here that

394

three important mechanisms are controlling the NPs stability; NOM adsorption and resulting

395

surface charge, aggregation and disaggregation mainly controlled by the electrostatic and to a

396

lesser extent by steric and hydrophobic interactions.

397

3. Conclusions

398

Zeta potential and z-average hydrodynamic diameter measurements were used to investigate

399

in a systematic way the interaction processes and stability of solutions containing TiO2 NPs in

400

presence of two important environmental compound models, i.e. alginate and Suwannee river

401

humic acid. To the best of our knowledge, it is the first time that such a systematic

402

comparison has been done on the impact of two different components on both individual NPs

403

and aggregates. Our results clearly indicate that aggregation, disaggregation and stabilization

404

of TiO2 NPs (isolated or aggregated) are depending on the solution pH, concentration ratio of

405

environmental compounds over TiO2 NPs and equilibrium time. Our study also suggest that

406

typical environmental concentrations of humic acids (> 4 mg/L) and alginate (> 2.4 mg/L) are

407

sufficient to stabilize TiO2 nanoparticles even if present at high concentration (50 mg/L) and

408

that they can play important roles in the disaggregation and dispersion of already formed

409

nanoparticle aggregates. In all cases the negatively charged SRHA and alginate adsorb on

410

positive and neutral NPs and charge inversion is observed. This indicates that the driving

411

force for adsorption and NPs behavior is a complex combination of electrostatic attractive and

412

repulsive interactions. Hydrophobic interactions and steric effects are also playing key roles

413

in the NPs stability in particular when amphiphilic compounds such as SRHA are considered.

414

Our results also indicates that times required to partially disaggregate NPs aggregates are

17

415

relatively short and involves different mechanisms when comparison is made between linear

416

and globular polyelectrolytes. The conclusions reported here should, however, considered as a

417

first step for several reasons since aquatic systems consist in heterogeneous systems and the

418

concomitant presence of humic compounds, biopolymers as well as monovalent and

419

multivalent salts and natural inorganic colloidal particles, may also significantly modify the

420

final stability, transport and transformation processes of manufactured nanoparticles.

421 422

Acknowledgments

423

The authors are grateful to Sabrina Lacomme for technical assistance and the contribution of

424

BioImaging Center at the University of Bordeaux 2. We also acknowledge the financial

425

support received from the Swiss National Foundation (project 200021_135240 and

426

206021_13377).

18

References Ait Akbour, R. and Jada, A. (2013) Effects of Solution Chemistry on the Fluorescence and Electrophoretic Behaviours of Humic Acid. Journal of Colloid Science and Biotechnology 2, 11-18. Auffan, M., Rose, J., Bottero, J.-Y., Lowry, G.V., Jolivet, J.-P. and Wiesner, M.R. (2009) Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective. Nature Nanotechnology 4(10), 634-641. Baalousha, M. (2009) Aggregation and disaggregation of iron oxide nanoparticles: Influence of particle concentration, pH and natural organic matter. Science of the Total Environment 407(6), 2093-2101. Baalousha, M., Motelica-Heino, M., Galaup, S. and Le Coustumer, P. (2005) Supramolecular structure of humic acids by TEM with improved sample preparation and staining. Microscopy Research and Technique 66(6), 299-306. Chae, S.-R., Xiao, Y., Lin, S., Noeiaghaei, T., Kim, J.-O. and Wiesner, M.R. (2012) Effects of humic acid and electrolytes on photocatalytic reactivity and transport of carbon nanoparticle aggregates in water. Water Research 46(13), 4053-4062. Chen, K.L. and Elimelech, M. (2007) Influence of humic acid on the aggregation kinetics of fullerene (C-60) nanoparticles in monovalent and divalent electrolyte solutions. Journal of Colloid and Interface Science 309(1), 126-134. Chen, K.L. and Elimelech, M. (2008) Interaction of Fullerene (C-60) Nanoparticles with Humic Acid and Alginate Coated Silica Surfaces: Measurements, Mechanisms, and Environmental Implications. Environmental Science & Technology 42(20), 7607-7614. Chen, K.L., Mylon, S.E. and Elimelech, M. (2006) Aggregation kinetics of alginate-coated hematite nanoparticles in monovalent and divalent electrolytes. Environmental Science & Technology 40(5), 1516-1523. Chen, Q., Yin, D., Zhu, S. and Hu, X. (2012) Adsorption of cadmium(II) on humic acid coated titanium dioxide. Journal of Colloid and Interface Science 367, 241-248. Chen, X. and Mao, S.S. (2007) Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chemical Reviews 107(7), 2891-2959. Christian, P., Von der Kammer, F., Baalousha, M. and Hofmann, T. (2008) Nanoparticles: structure, properties, preparation and behaviour in environmental media. Ecotoxicology 17(5), 326-343. Colvin, V.L. (2003) The potential environmental impact of engineered nanomaterials. Nature Biotechnology 21(10), 1166-1170. Domingos, R.F., Peyrot, C. and Wilkinson, K.J. (2010) Aggregation of titanium dioxide nanoparticles: role of calcium and phosphate. Environmental Chemistry 7(1), 61-66. 19

Domingos, R.F., Tufenkji, N. and Wilkinson, K.J. (2009) Aggregation of Titanium Dioxide Nanoparticles: Role of a Fulvic Acid. Environmental Science & Technology 43(5), 12821286. French, R.A., Jacobson, A.R., Kim, B., Isley, S.L., Penn, R.L. and Baveye, P.C. (2009) Influence of Ionic Strength, pH, and Cation Valence on Aggregation Kinetics of Titanium Dioxide Nanoparticles. Environmental Science & Technology 43(5), 1354-1359. Guzman, K.A.D., Finnegan, M.P. and Banfield, J.F. (2006) Influence of surface potential on aggregation and transport of titania nanoparticles. Environmental Science & Technology 40(24), 7688-7693. Ju-Nam, Y. and Lead, J.R. (2008) Manufactured nanoparticles: An overview of their chemistry, interactions and potential environmental implications. Science of the Total Environment 400(1-3), 396-414. Keller, A.A., Wang, H., Zhou, D., Lenihan, H.S., Cherr, G., Cardinale, B.J., Miller, R. and Ji, Z. (2010) Stability and Aggregation of Metal Oxide Nanoparticles in Natural Aqueous Matrices. Environmental Science & Technology 44(6), 1962-1967. Liu, Z., Jiao, Y., Wang, Y., Zhou, C. and Zhang, Z. (2008) Polysaccharides-based nanoparticles as drug delivery systems. Advanced Drug Delivery Reviews 60(15), 1650-1662. Ngomsik, A.F., Bee, A., Siaugue, J.M., Cabuil, V. and Cote, G. (2006) Nickel adsorption by magnetic alginate microcapsules containing an extractant. Water Research 40(9), 1848-1856. Nowack, B. and Bucheli, T.D. (2007) Occurrence, behavior and effects of nanoparticles in the environment. Environmental Pollution 150(1), 5-22. Ottofuelling, S., Von der Kammer, F. and Hofmann, T. (2011) Commercial Titanium Dioxide Nanoparticles in Both Natural and Synthetic Water: Comprehensive Multidimensional Testing and Prediction of Aggregation Behavior. Environmental Science & Technology 45(23), 10045-10052. Pallem, V.L., Stretz, H.A. and Wells, M.J.M. (2009) Evaluating Aggregation of Gold Nanoparticles and Humic Substances Using Fluorescence Spectroscopy. Environmental Science & Technology 43(19), 7531-7535. Papageorgiou, S.K., Katsaros, F.K., Favvas, E.P., Romanos, G.E., Athanasekou, C.P., Beltsios, K.G., Tzialla, O.I. and Falaras, P. (2012) Alginate fibers as photocatalyst immobilizing agents applied in hybrid photocatalytic/ultrafiltration water treatment processes. Water Research 46(6), 1858-1872. Parks, G.A. (1965) Isolelectric points of solid oxides, solid hydroxydes and aqueous hydroxo complex systems. Chemical Reviews 65(2), 177-198. Pefferkorn, E. (1995) The role of polyelectrolytes in the stabilization and destabilization of colloids. Advances in Colloid and Interface Science 56, 33-104.

20

Preocanin, T. and Kallay, N. (2006) Point of zero charge and surface charge density of TiO2 in aqueous electrolyte solution as obtained by potentiometric mass titration. Croatica Chemica Acta 79(1), 95-106. Weir, A., Westerhoff, P., Fabricius, L., Hristovski, K. and von Goetz, N. (2012) Titanium Dioxide Nanoparticles in Food and Personal Care Products. Environmental Science & Technology 46(4), 2242-2250. Wiesner, M.R., Lowry, G.V., Alvarez, P., Dionysiou, D. and Biswas, P. (2006) Assessing the risks of manufactured nanomaterials. Environmental Science & Technology 40(14), 43364345. Yang, K., Lin, D. and Xing, B. (2009) Interactions of Humic Acid with Nanosized Inorganic Oxides. Langmuir 25(6), 3571-3576. Yang, K. and Xing, B. (2009) Sorption of Phenanthrene by Humic Acid-Coated Nanosized TiO2 and ZnO. Environmental Science & Technology 43(6), 1845-1851.

21

Figure

Fig. 1 - a) Zeta potential variation of TiO2 NPs as a function of pH. The PZC is found here to be equal to pH = 6.2 ± 0.1. b) TiO2 particles z-average diameter variation as a function of pH. TiO2 NPs aggregation domain is found important and comprised between -30 mV and +30 mV (gray domain); [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 2 - a) Transmission Electron Microscopy image of dispersed TiO 2 NPs at pH 4.5 (positively charged NPs) with a 500000× magnification. Isolated and small aggregates are observed. b) TEM image of TiO2 NPs aggregate at pH = 6.2 (point of zero charge) with a 60000× magnification. Large aggregates are observed.

Figure

Fig. 3 - a) Zeta potential variation of alginate as a function of pH. No PZC is found here. b) Alginate z-average diameter variation as a function of pH. The z-average diameter is found constant with a value of 178 ± 21 nm. [Alginate] = 100 mg/L; I = 0.001 M.

Figure

Fig. 4 - a) Zeta potential variation of SRHA as a function of pH. No PZC is found here. b) SRHA z-average diameter variation as a function of pH. The z-average diameter is found constant with a value of 379 ± 19 nm. [HA] = 100 mg/L; I = 0.001 M.

Figure

Fig. 5 - a) Zeta potential variation of TiO2 NPs as a function of alginate concentration at pH 4.5. Charge neutralization (IEP) is found here for [alginate] = 1.68 ± 0.01 mg/L and a charge ratio of alginate over TiO2 equal to 1.25 ± 0.21. b) TiO2 z-average diameter variation as a function of alginate concentration at pH 4.5. Aggregation domain is found important and comprised between +28 mV and -28 mV (gray domain) and maximum aggregation is observed at the IEP. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 6 - a) Zeta potential variation of TiO2 NPs as a function of SRHA concentration at pH 4.5. Charge neutralization (IEP) is found here for [SRHA] = 2.80 ± 0.25 mg/L and a charge ratio of SRHA over TiO2 equal to 1.40 ± 0.23 . b) TiO2 z-average diameter variation as a function of SRHA concentration at pH 4.5. Aggregation domain is found important and comprised between +20 mV and -10 mV (gray domain) and maximum aggregation is observed at the IEP. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 7 - a) Zeta potential variation of TiO2 NPs as a function of alginate concentration at pH 6.2 (PZC) for successive alginate addition. b) TiO2 z-average diameter variation as a function of alginate concentration at pH 6.2 (PZC). Maximum of disaggregation is found for [alginate] ≥ 3 mg/L. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 8 - a) Zeta potential variation of TiO2 NPs as a function of SRHA concentration at pH 6.2 (PZC) for successive SRHA addition. b) TiO2 z-average diameter variation as a function of SRHA concentration at pH 6.2 (PZC). Maximum of disaggregation is found here for [SRHA] ≥ 5 mg/L. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 9 - a) TiO2 aggregate z-average diameter variation as a function of time at pH 6.2 (PZC) for alginate mass concentration of: ▲ 0.5 mg/L, ▼ 1 mg/L, ◄ 2 mg/L, ► 3 mg/L and ● 5 mg/L. b) Zeta potential variation of TiO2 aggregates in presence of alginate as a function of time at pH 6.2 (PZC). Equilibrium time for disaggregation is found to be equal to 45 min. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 10 - a) TiO2 aggregate z-average diameter variation as a function of time at pH 6.2 (PZC) for a SRHA mass concentration of: ▲ 3 mg/L, ▼ 4 mg/L, ◄ 5 mg/L, ► 7 mg/L and ● 10mg/L. b) Zeta potential variation of TiO2 aggregates in presence of SRHA as a function of time at pH 6.2 (PZC). Equilibrium time for disaggregation is found to be equal to 24 h. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 11 - a) Zeta potential variation of TiO2 NPs as a function of alginate concentration at pH 9.8 for successive alginate addition. b) TiO2 z-average diameter variation as a function of alginate concentration at pH 9.8. No specific interaction between alginate and TiO2 NPs are observed owing to the electrostatic repulsions. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 12 - a) Zeta potential variation of TiO2 NPs as a function of SRHA concentration at pH 9.8 for successive SRHA addition. b) TiO2 z-average diameter variation as a function of SRHA concentration at pH 9.8. Limited adsorption of SRHA at TiO2 surface is observed due to the amphiphilic character of SRHA. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 13 - Stability diagram for TiO2 NPs in presence of alginate at pH = 4.5 < pHPZC,TiO2, pH = 6.2 = pHPZC,TiO2 and pH = 9.8 > pHPZC,TiO2. The two only destabilization domains are represented by shaded areas. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 14 - Diagram of stability for TiO2 NPs in presence of HA for pH = 4.5 < pHPZC,TiO2, pH = 6.2 = pHPZC,TiO2 and pH = 9.8 > pHPZC,TiO2. The two only destabilization domains are represented by shaded regions. [TiO2] = 50 mg/L; I = 0.001 M.

Figure

Fig. 15 - a) TEM image of TiO2 NPs aggregate fragments at pH 6.2 in presence of alginate with a 120000× magnification. b) TEM image of TiO2 NPs aggregate fragments at pH = 6.2 in presence of SRHA with a 100000× magnification. Presence of NOM is found to result in aggregate fragmentation.