Behavior of Tio2 and Ceo2 Nanoparticles and Polystyrene Nanoplastics in Bottled Mineral, Drinking and Lake Geneva Waters. Impact

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Article Behavior of TiO2 and CeO2 Nanoparticles and Polystyrene Nanoplastics in Bottled Mineral, Drinking and Lake Geneva Waters. Impact of Water Hardness and Natural Organic Matter on Nanoparticle Surface Properties and Aggregation

Lina Ramirez 1, Stephan Ramseier Gentile 2, Stéphane Zimmermann 2 and Serge Stoll 1,*

1 Université de Genève, Institute F.A. Forel, Physico-chimie de l’environnement, Uni Carl Vogt, 66 Bd Carl-Vogt, 1211 Genève 4, Switzerland; [email protected] 2 SIG, Industrial Boards of Geneva 2 Ch. du Château-Bloch, Le Lignon, 1211 Genève-2, Switzerland; [email protected] (S.R.G.); [email protected] (S.Z.) * Correspondence: [email protected]; Tel.: +41223790333

Received: 28 February 2019; Accepted: 2 April 2019; Published: 6 April 2019

Abstract: Intensive use of engineered nanoparticles (NPs) in daily products ineluctably results in their release into aquatic systems and consequently into drinking water resources. Therefore, understanding NPs behavior in various waters from naturel to mineral waters is crucial for risk assessment evaluation and the efficient removal of NPs during the drinking water treatment process. In this study, the impact of relevant physicochemical parameters, such as pH, water hardness, and presence of natural organic matter (NOM) on the surface charge properties and aggregation abilities of both NPs and nanoplastic particles is investigated. TiO2, CeO2, and Polystyrene (PS) nanoplastics are selected, owing to their large number applications and contrasting characteristics at environmental pH. Experiments are performed in different water samples, including, ultrapure water, three bottled mineral waters, Lake Geneva, and drinking water produced from Lake Geneva. Our findings demonstrate that both water hardness and negatively charged natural organic matter concentrations, which were measured via dissolved organic carbon determination, are playing important roles. At environmental pH, when negatively charged nanoparticles are considered, specific cation adsorption is promoting aggregation so long as NOM concentration is limited. On the other hand, NOM adsorption is expected to be a key process in NPs destabilization when positively charged PS nanoplastics are considered.

Keywords: TiO2 nanoparticles; CeO2 nanoparticles; polystyrene nanoplastics; water hardness; NOM concentration; nanoparticle stability and aggregation

1. Introduction Manufactured nanoparticles (NPs) are widely incorporated and used into a large number of consumer products and industrial applications due to their specific physicochemical properties [1,2]. Examples include the use of TiO2 NPs in food, cosmetics [3], and photovoltaics [4], and the use of CeO2 NPs as a fuel additives [5], in electronic devices, and cosmetics [6]. With increasing production and applications in our daily life [7], NPs are expected to diffuse in aquatic systems, through industrial discharges, wastewater treatment effluents, or surface runoff from soils [8]. Studies have shown that NPs can pose an ecotoxicological risk in aquatic environments, and possible risks to humans via, for example, their presence in water compartments, which are used for drinking water production [9,10].

Water 2019, 11, 721; doi:10.3390/w11040721 www.mdpi.com/journal/water Water 2019, 11, 721 2 of 14

Once released in the environnement, the fate, mobility, and persistence of NPs in aquatic systems largely depend on their stability and possibility to form aggregates [2]. Aggregation is an important process, since aggregates containing NPs will sediment or otherwise be immobilized (via filtration process), while dispersed NPs will be able to diffuse, be more mobile, more bioavailable, and potentially more toxic. NPs stability in aquatic systems is governed by their physicochemical properties (i.e., size, coating surface charge, acid-base, and redox behavior), as well as aquatic chemical conditions, such as pH, ionic composition, and presence of natural organic matter (NOM) [11,12]. NOM is present in the majority of natural aquatic ecosystems at concentrations typically ranging from 0.1 to 10 mg/L, and is generally expected to enhance NPs stability [13–15]. Adsorbed NOM imparts a negative charge and it modifies the surface properties of NPs by raising their absolute surface potential [16,17]. Keller et al. [18] studied the behavior of three different metal oxide nanoparticles (TiO2, ZnO, and CeO2) in eight different aqueous media. They showed that the adsorption of NOM onto the NPs surface significantly reduced their aggregation, stabilizing them under many environmental conditions. The presence of NOM can also induce NPs destabilization, depending on the physicochemical conditions prevailing in the medium and NOM concentration [19]. According to Wu and Cheng [15], adsorption of NOM on TiO2 NPs is pH dependent. At low pH (< 5), NOM strongly adsorbs on positively charged TiO2 NPs and promotes aggregation. On the other hand, high NOM concentrations can reverse the NP surface charges and promote NPs stabilization. In such conditions, divalent cations, in particular, Ca2+ and Mg2+ ions, can neutralize NP electrostatic stabilization that is enhanced by NOM and induce aggregation through cation bridging and charge neutralization mechanisms [13,17]. Studies are mainly focusing attention on the behavior and fate of metal oxide NPs in aquatic systems. However, nanoplastic particles can also represent an important source of pollution by themselves, but also via the transport, adsorption, and release of contaminants, such as heavy metals and persistent organic pollutants [20,21]. It should be noted that the environmental impact of nanoplastics differ from microplastics, because of their important surface area ratio and small size that can penetrate tissues and accumulate in organs [20,22]. Researches concerning nanoplastic particles [23] are surprisingly limited and are mainly focused on marine systems, even though fluvial systems are the main source of plastic contaminants to marine waters [24,25]. In the present work, we are studying the effect of pH, presence of NOM, and water hardness on two widespread metal oxide manufactured nanoparticles, TiO2 and CeO2. Positively charged amidine Polystyrene latex particles in the nano range are also considered as nanoplastics to better understand their surface charge transformations and behavior in aquatic systems with regards to their aggregation and water chemistry. Metal oxide NPs and nanoplastic particles that were used in this study exhibit different characteristics at the pH of natural waters and consequently different chemical scenario. Indeed, at environmental pH, the surface charge of CeO2 NPs is close to zero, whereas TiO2 NPs exhibit negative surface charges. On the other hand, nanoplastic particles that were used in this work are positively charged in a wide range of pH and, as a result, are expected to better interact with the negatively charged NOM. The use of positively charged nanoparticles is important, since several studies have demonstrated that nanoplastic particles positively charged are more toxic to living organisms when compared to negatively charged particles [26]. First, TiO2, CeO2 NPs, and nanoplastic particles are characterized in ultrapure water in a large pH domain to gain insight into their pH-responsive and aggregation behavior. Subsequently, NPs behavior in mineral, Lake Geneva, and drinking waters is evaluated by focusing on aggregation, owing to the importance of such a process in the NPs risk assessment evaluation. Water 2019, 11, 721 3 of 14

2. Materials and Methods

2.1. Methods

2.1.1. Zeta Potential and Size Distribution Measurements Determination of zeta (ζ) potential and z-average hydrodynamic diameter values of NPs in mineral, drinking, and Lake Geneva waters was achieved by laser doppler velocimetry and dynamic light scattering (DLS) methods, respectively [27]. The ζ potential was measured with a Zetasizer Nano ZS (Malvern Instruments Ltd: Malvern, UK) using the Smoluchowski approximation model and from the electrophoretic mobility determination [28]. The z-average hydrodynamic diameter was calculated from Brownian motion measurement via the Stokes-Einstein equation while using the same instrument. For each sample, ﬁve measurements of ten runs, with a delay of 5 s between them to stabilize the system, were performed to determine the ζ potential and z-average hydrodynamic diameter.

2.1.2. Scanning Electron Microscopy Imagining (SEM) A JEOL JSM-7001FA scanning electron microscope (JEOL: Tokyo, Japan) was used to obtain images of NPs in ultrapure and Lake Geneva waters. The SEM samples were prepared by dropping 10 µL of suspension on one aluminum stub that was covered with 5 5 mm silica wafer Agar Scientiﬁc × (G3390) and wrapped with platinum coating.

2.2. Materials

2.2.1. Nanoparticles

Two manufactured nanoparticles TiO2 anatase, CeO2 NPs, and one amidine polystyrene latex nanoplastic were used in this study. TiO2 NPs, with a determined manufacturer x-ray diffraction diameter of 15 nm and a specific surface area of 40 60 m2/g, were purchased as a powder from − Nanostructured & Amorphous Material Inc-5430MR (Los Alamos, NM, USA). The CeO2 NPs were also purchased as a powder from Sigma-Aldrich-MKBN9764V (Buchs, Switzerland). BET determined the CeO2 primary particle diameter < 50 nm [29], according to the manufacturer with a specific surface area of 30 m2/g. Surfactant-free polystyrene latex nanospheres with amidine functional groups on the surface were obtained from Thermo Fisher Scientific-1862725 (Rheinach, Switzerland). The aqueous dispersion contains 40 g/L of positively charged particles with a diameter that was equal to 0.09 µm 2 (TEM) and a specific surface area of 63 m /g. 1 g/L stock suspensions of TiO2 and CeO2 NPs was prepared by diluting the previously weighted powder with ultrapure water (Milli Q water, Millipore, Switzerland, with R > 18 MΩ.cm, TOC < 2 ppb), previously adjusted to pH 3.0 0.2 to obtain a ± stable stock suspension. Stock suspension of TiO2 and CeO2 NPs were sonicated during 15 and 5 min, respectively, with an ultrasonic probe (Sonic Vibra cell, probe model CV18, Blanc Labo S.A., Switzerland) before use. Polystyrene latex nanoplastics stock suspension of 1 g/L was prepared by diluting the original suspension with ultrapure water at pH 3.0 0.2. The stock suspension of PS ± nanoplastics was sonicated during 15 min with a sonication bath (Bransonic ultra cleaner, Branson 5510 model, Switzerland) before use. A sonication bath was used in that case, because it was found that the ultrasonic probe promoted the aggregation of the nanoplastic particles rather than dispersion. All of the stock solutions were stored in a dark place at constant temperature of 4 ◦C and were used to prepare diluted suspensions for further experiments.

2.2.2. Water Samples Five different water samples having different chemical identities, including three types of commercial bottled mineral waters (MW1, MW2, MW3), Lake Geneva, and drinking waters were selected for NPs behavior studies. The physicochemical properties of all water samples, including pH, conductivity, water hardness, and NOM concentrations as Dissolved Organic Carbon (DOC) Water 2019, 11, 721 4 of 14 are given in Table1. The choice of the three mineral waters was based on their di fferences in ionic composition and, in particular, water hardness. The total hardness calculation was based on Ca2+ and 2+ Mg concentrations and expressed in mg CaCO3/L. NOM was measured by considering the DOC concentrations while using a Shimadzu TOC-L instrument (Shimadzu Scientific Instruments: Kyoto, Japan). pH and conductivity were measured using a Hach Lange HQ40d portable meter (Hach Lange, Switzerland). All of the water samples were stored in a dark place and constant temperature of 4 ◦C.

Table 1. Bottled mineral, drinking, and Lake Geneva water properties.

Conductivity Ionic Strength Water Hardness Samples pH DOC 0.05 mg C/L (20 ◦C) 5 µS/cm meq/L 2 mg CaCO3/L ± ± ± Ultrapure water adjusted 2 0 0 0.004 Mineral water 1 7.0 0.1 201 2.66 63 0.28 ± Mineral water 2 7.2 0.1 557 9.70 307 0.03 ± Mineral water 3 7.4 0.2 2229 56.3 1474 0.25 ± Lake Geneva Water 8.1 0.1 298 4.97 138 1.12 ± Drinking water 8.2 0.1 310 5.04 138 0.40 ±

2.2.3. Experimental Procedures 10 mg/L NP and nanoplastic suspensions were considered in the different water samples and they were prepared from the stock suspensions. Such a NP concentration was used in all of the experiments so as to stay in the measurement detection limit of the Zetasizer Nano. pH-titration in ultrapure water was performed for pH values in a range of 3 to 11 in order to characterize NPs and nanoplastic particles surface charge and aggregation behavior in changing pH conditions. Suspensions of NPs at pH 3.0 0.1 were prepared by first adding the appropriate amount of NPs from the stock suspension ± to obtain a final concentration of 10 mg/L and then the pH was increased. To adjust the suspension pH, small amounts of diluted hydrochloride acid and sodium hydroxide (HCl and NaOH, Titrisol® 113, Merck, Switzerland) were used. Measurements of ζ potential and z-average hydrodynamic diameters were performed every 10 min after pH modification and stabilization. For comparison of the behavior of NPs and nanoplastics in mineral, Lake Geneva, and drinking waters with a reference system, experiences in ultrapure water were first conducted by adjusting the pH to 7.0 0.1 and 8.0 0.1. ± ± The aliquots of the NP stock suspension were added to the solution to obtain a final concentration of 10 mg/L. Next, experiences in mineral, Lake Geneva, and drinking waters were performed at their natural pHs. Water from Lake Geneva was filtered while using a membrane filter with pore size that was equal to 0.2 µm (Merck Millipore Ltd, Schaffhausen, Switzerland) before each experiment. The measurements of ζ potential and z-average hydrodynamic diameters were performed as a function of time during 135 min. For suspension homogenization, gentle agitation was applied during all of the experiments with a magnetic Stirrer (Lab-Mix 15, Fisher Scientific, Reinach, Switzerland) and a rotational speed equal to 100 rpm.

3. Results

3.1. Characterization of NPs and Nanoplastic Particles in Ultrapure Water In order to evaluate the surface charge pH dependence and stability of the NPs and nanoplastic particles, the pH-titration curves in ultrapure water are determined by measuring the ζ potential and z-average hydrodynamic diameter variations as a function of pH. Figure1a presents the results for TiO NPs. The surface of TiO NPs is found to be strongly positively charged at pH 3.0 (+31 0.5 mV). 2 2 ± By further increasing the pH, the ζ potential decreases until the point of zero charge (PZC) at pHpzc = 5.8 0.1. This value is close to the experimental value of pHpzc = 6.2 that was obtained by ± Loosli et al. [30]. When the pH is higher than pHpzc, the surface charge becomes negative and stable at 40 2 mV at pH 10 0.1. The z-average hydrodynamic diameter of TiO NPs is found stable − ± ± 2 below pH 5.0 with values that are equal to 430 10 nm. Subsequently, fast aggregation is observed ± Water 2019, 11, x FOR PEER REVIEW 5 of 14

Water 2019, 11, x FOR PEER REVIEW 5 of 14 5.0 with values that are equal to 430 ± 10 nm. Subsequently, fast aggregation is observed with a maximumWater 2019, 11 value, 721 of 887 ± 40 nm in the PZC region. A further pH increase leads to the decrease of5 ofthe 14 5.0 with values that are equal to 430 ± 10 nm. Subsequently, fast aggregation is observed with a TiO2 hydrodynamic diameter, indicating reversible aggregation. When pH ≥ 8.0, TiO2 NPs exhibit a maximum value of 887 ± 40 nm in the PZC region. A further pH increase leads to the decrease of the stable domain with values of z-average hydrodynamic diameter equal to 405 ± 10 nm in good TiOwith2 hydrodynamic a maximum value diameter, of 887 indicating40 nm in reversible the PZC region.aggregation. A further When pH pH increase ≥ 8.0, leadsTiO2 NPs to the exhibit decrease a agreement with the SEM image± of TiO2, as presented in Figure1b. TiO2 NPs destabilization is stableof the domain TiO2 hydrodynamic with values of diameter, z-average indicating hydrodynamic reversible diameter aggregation. equal to When 405 pH± 10 nm8.0, TiOin good2 NPs observed in a ζ potential range between +20 and −25 mV, as indicated in the gray area≥ (Figure 1a). agreementexhibit a stablewith the domain SEM with image values of TiO of z-average2, as presented hydrodynamic in Figure1b. diameter TiO2 NPs equal destabilization to 405 10 nm is in ± observedgood agreement in a ζ potential with the range SEM between image of +20 TiO and2, as −25 presented mV, as indicated in Figure in1b. the TiO gray2 NPs area destabilization (Figure 1a). is observed in a ζ potential range between +20 and 25 mV, as indicated in the gray area (Figure1a). 40 − 1000

30 40 zeta potential 1000900 z-average diameter (b) 20 30 zeta potential 900800 (b) 10 z-average diameter 20 (a) 800 0 700 10 (a) -10 0 700600

-20 potential (mV) -10 600500 ζ− -30 -20 potential (mV) 500400 ζ− -40 -30 z-averagehydrodynamic (nm)diameter -50 400300 -40 234567891011 z-averagehydrodynamic (nm)diameter -50 pH 300 234567891011 pH Figure 1. (a) Zeta (ζ) potential and z-average hydrodynamic diameter variation of TiO2 nanoparticles ζ (NPs)Figure (10 1. mg/L)(a) Zeta as ( a) function potential of and pH z-average in ultrapure hydrodynamic water. The diameterpoint of zero variation charge of TiO(PZC)2 nanoparticles is found at Figure 1. (a) Zeta (ζ) potential and z-average hydrodynamic diameter variation of TiO2 nanoparticles pH(NPs)pzc = (105.8 mg± 0.1./L) Aggregation as a function is of observed pH in ultrapure in the PZC water. region The (gray point domain) of zero charge(b). SEM (PZC) image is foundof TiO at2 (NPs)pH (10= mg/L)5.8 0.1. as a Aggregation function of pH is observed in ultrapure in the water. PZC regionThe point (gray of domain)zero charge (b). (PZC) SEM image is found of TiO at NPspzc in ultrapure water at pH > pHpzc. Aggregates diameters are found consistent with dynamic light2 pHpzc = 5.8 ± 0.1.± Aggregation is observed in the PZC region (gray domain) (b). SEM image of TiO2 scatteringNPs in ultrapure (DLS) analysis. water at Lines pH > connectpHpzc. Aggregatesing data are diametersgiven to guide are found the eye. consistent with dynamic light NPsscattering in ultrapure (DLS) water analysis. at pH Lines > pH connectingpzc. Aggregates data diameters are given toare guide found the consistent eye. with dynamic light scattering (DLS) analysis. Lines connecting data are given to guide the eye. As shown in Figure 2a, CeO2 NPs exhibit a stable and positive charge (+21 ± 1 mV) from pH 3.0 As shown in Figure2a, CeO 2 NPs exhibit a stable and positive charge (+21 1 mV) from pH 3.0 to to 5.0. Subsequently, ζ potential rapidly decreases to reach the PZC at pHpzc = 6.9± ± 0.1, which is similar 5.0.As Subsequently, shown in Figureζ potential 2a, CeO rapidly2 NPs decreasesexhibit a stable to reach and the positive PZC at charge pHpzc =(+216.9 ± 10.1, mV) which fromis pH similar 3.0 to the previously reported studies [31]. An increase of pH leads to charge reversal± with ζ potential toto 5.0. the Subsequently, previously reported ζ potential studies rapidly [31 decreases]. An increase to reach of pH the leadsPZC at to pH chargepzc = 6.9 reversal ± 0.1, which with ζispotential similar values equals to -32 ± 2 mV at pH 10 ± 0.1. As illustrated in Figure 2a, the z-average diameter of CeO2 tovalues the previously equals to reported32 2 mV studies at pH [31]. 10 An0.1. increase As illustrated of pH in leads Figure to2 chargea, the z-average reversal with diameter ζ potential of CeO NPs is stable below− pH± 6.0 with values± that are equal to 350 ± 50 nm. Near the PZC region z-average2 valuesNPs isequals stable to below -32 ± pH2 mV 6.0 at with pH 10 values ± 0.1. that As areillustrated equal to in 350 Figure50 2a, nm. the Near z-average the PZC diameter region z-averageof CeO2 diameter increases until values that are greater than 1.5µm,± a further pH increase leads to charge NPsdiameter is stable increases below pH until 6.0 valueswith values that arethat greater are equal than to 1.5350µ ±m, 50 a nm. further Near pH the increase PZC region leads z-average to charge reversal. However, CeO2 aggregates are found to be stable and are not affected by pH increase, as diameterreversal. increases However, until CeO values2 aggregates that are are greater found than to be 1.5µm, stable a and further are not pH a ffincreaseected by leads pH increase,to charge as observed for TiO2, since no significant decrease of the z-average diameter is observed after the PZC, reversal.observed However, for TiO ,CeO since2 aggregates no significant are decreasefound to ofbe the stable z-average and are diameter not affected is observed by pH afterincrease, the PZC, as which is also confirmed2 by SEM (Figure 2b). observedwhich is for also TiO confirmed2, since no by significant SEM (Figure decrease2b). of the z-average diameter is observed after the PZC, which is also confirmed by SEM (Figure 2b). 40 zeta potential 2500 z-average diameter 4030 (a) (b) zeta potential 20 2500 30 z-average diameter (a) 2000 (b)

2010 2000 1500 10 0 1500 -100 1000 potential (mV)

ζ− -20 -10 1000

potential (mV) 500

ζ− -20-30

500 (nm) diameter hydrodynamic z-average -30-40 0 234567891011 z-average hydrodynamic diameter (nm) diameter hydrodynamic z-average -40 pH 0 234567891011 Figure 2. (a) ζ potential andpH z-average hydrodynamic diameter variation of CeO2 NPs (10 mg/L) as a Figure 2. (a) ζ potential and z-average hydrodynamic diameter variation of CeO2 NPs (10 mg/L) as a function of pH in ultrapure water. The point of zero charge (PZC) is found equal to pHpzc = 6.9 0.1. function of pH in ultrapure water. The point of zero charge (PZC) is found equal to pHpzc = 6.9 ±± 0.1. FigureStrong 2. aggregation(a) ζ potential is observedand z-average around hydrodynamic the PZC region diameter (gray domain) variation in of agreement CeO2 NPs with (10 mg/L) SEMimage as a

function(b) SEM of image pH in of ultrapure CeO2 NPs water. aggregates The poin int ultrapure of zero charge water (PZC) at pH >is pHfoundpzc. equal to pHpzc = 6.9 ± 0.1.

Water 2019, 11, x FOR PEER REVIEW 6 of 14

Strong aggregation is observed around the PZC region (gray domain) in agreement with SEM image

Water 2019(b), SEM11, 721 image of CeO2 NPs aggregates in ultrapure water at pH > pHpzc. 6 of 14

The surface charge and stability domain of polystyrene latex nanoplastics versus pH are illustratedThe surface in Figure3a. charge and PS stabilitynanoplastics domain are offound polystyrene to be strongly latex nanoplastics positively versuscharged pH and are stable illustrated at pH in< Figure7.0 with3a. values PS nanoplastics that are equal are found to +50 to be± 1 strongly mV. By positively increasing charged pH, surface and stable charge at pHof nanoplastics< 7.0 with valuesdecrease that to are the equal PZC toat +pH50pzc =1 9.9 mV. ± By0.1 increasingas (Figure 3a), pH, which surface is chargeclose to of the nanoplastics value that was decrease obtained to the by ± PZCCross at pHet al.pzc [32]= 9.9 with0.1 pH aspzc (Figure = 9.5. 3Subsequently,a), which is close charge to the reversal value that occurs was by obtained further by increasing Cross et al.the [ 32pH.] ± withNo pHaggregationpzc = 9.5. is Subsequently, observed below charge pH reversal9.0 and occursnanopl byastics further remain increasing stable with the pH.a z-average No aggregation diameter isthat observed is equal below to 98 pH ± 2 9.0 nm. and Such nanoplastics a pH-surface remain charge stable behavior with a z-average is related diameterto the high that pKa is equal value to of 98amidine2 nm .functional Such a pH-surface group, which charge is behaviorapproximately is related equal to theto 12 high [33]. pKa Aggregation value of amidine is observed functional in the ± group,PZC region which with is approximately a maximum equalz-average to 12 diameter [33]. Aggregation value of 1260 is observed ± 190 nm. in theSEM PZC picture region (Figure3b) with a maximumindicates that z-average the PS diameter nanoplastics value are of spherical 1260 190 and nm. highly SEM monodispersed. picture (Figure3 b) indicates that the PS ± nanoplastics are spherical and highly monodispersed.

60 1600

50 (a) 1400 (b) 40 1200 zeta potential 30 z-average diameter 1000 20

10 800

0 600 -10 potential (mV) 400 ζ− -20 200 -30

-40 z-average hydrodynamic diameter(nm)

23456789101112 pH

ζ FigureFigure 3. 3.(a (a)) ζpotential potential and and z-average z-average hydrodynamic hydrodynamic diameter diameter variation variation of of amidine amidine PS PS nanoplastic nanoplastic particles (10 mg/L) as a function of pH in ultrapure water. The point of zero charge (PZC) is found at particles (10 mg/L) as a function of pH in ultrapure water. The point of zero charge (PZC) is found at pH = 9.9 0.1. Aggregation is observed around the PZC region (gray domain), and (b) SEM image of pH = 9.9± ± 0.1. Aggregation is observed around the PZC region (gray domain), and (b) SEM image of PS nanoplastic aggregates in ultrapure water at pH > pHpzc. SEM analysis indicates spherical and PS nanoplastic aggregates in ultrapure water at pH > pHpzc. SEM analysis indicates spherical and monodispersed nanoplastics. monodispersed nanoplastics. 3.2. Behavior of NPs and Nanoplastic Particles in Mineral, Lake Geneva and Drinking Waters 3.2. Behavior of NPs and Nanoplastic Particles in Mineral, Lake Geneva and Drinking Waters To understand the impact of the physicochemical properties of mineral, drinking, and Lake GenevaTo waters, understand such the as waterimpact hardness of the physicochemica and dissolvedl organic properties matter of mineral, concentration drinking, on NPsand andLake nanoplasticsGeneva waters, stability, such time-resolvedas water hardness measurements and dissolved were madeorganic to matter provide concentration information on particleNPs and surfacenanoplastics charge stability, and z-average time-resol diameterved measurements variations. Experiments were made with to provide 10 mg/ Linformation NPs and nanoplastic on particle suspensionssurface charge in five and di z-averagefferent water diameter samples variations. (Table1) wereExperiments conducted with to 10 evaluate mg/L NPs their and stability nanoplastic with time.suspensions Ultrapure in water five different was used water as a reference samples system(Table 1) to were better conducted explain the to changes evaluate with their time, stability effects with of watertime. hardness, Ultrapure organic water was matter, used and as ionica reference strength system in the to di betterfferent explain waters. the changes with time, effects of water hardness, organic matter, and ionic strength in the different waters.

3.2.1. TiO2 Nanoparticles 3.2.1. TiO2 Nanoparticles The stability of TiO2 NPs in bottled mineral waters as a function of time is ﬁrst considered. The ζ 2 potentialThe and stability z-average of TiO hydrodynamic NPs in bottled diameter mineral of waters TiO2 NPs as a in func MW1,tion MW2, of time and is MW3first considered. are presented The forζ comparisonpotential and in Figurez-average4a.In hydr ultrapureodynamic water diameter that was of adjusted TiO2 NPs at the in pHMW1, of the MW2, mineral and waters MW3 i.e., are pHpresented 7.0 0.1, forζ potential comparison and in z-average Figure 4a. hydrodynamic In ultrapure diameterswater that arewas found adjusted to be at stable the pH with of time.the mineral TiO ± 2 NPswaters are negativelyi.e. pH 7.0 charged ± 0.1, ζ withpotential a ζ potential and z-average value equal hydrodynamic to 28 3 mV diameters and z-average are found hydrodynamic to be stable − ± diameterwith time. in TiO the2 range NPs are of 466negatively4 nm. charged The reason with ofa ζ such potential stability value is dueequal to to the -28 presence ± 3 mV and of repulsive z-average hydrodynamic diameter in the± range of 466 ± 4 nm. The reason of such stability is due to the presence forces between negatively charged TiO2 particles. of repulsive forces between negatively charged TiO2 particles.

Water 2019, 11, 721 7 of 14 Water 2019, 11, x FOR PEER REVIEW 7 of 14

7000 Ultrapure water 20 Ultrapure water MW1 [TiO2] = 10mg/L MW1 [TiO2] = 10mg/L (b) 6000 MW2 (a) MW2 MW3 MW3 10 5000

0 4000

-10 3000 potential (mV) potential 2000 ζ− -20

1000 -30 z-average hydrodynamic diameter (nm) diameter hydrodynamic z-average 0 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Time (min) Time (min)

Figure 4. (a) ζ potential and (b) z-average hydrodynamic diameter variation of TiO2 as a function of Figure 4. (a) ζ potential and (b) z-average hydrodynamic diameter variation of TiO2 as a function of time in ultrapure water at pH = 7.0 0.1 and mineral waters at their natural pHs. TiO2 surface charge time in ultrapure water at pH = 7.0± ± 0.1 and mineral waters at their natural pHs. TiO2 surface charge is found to be reduced and neutralized in mineral waters hence resulting in TiO2 aggregation. is found to be reduced and neutralized in mineral waters hence resulting in TiO2 aggregation. By considering MWs, a systematic decrease in ζ potential, i.e., less negative values, is observed, whichBy is foundconsidering to be proportionalMWs, a systematic to the mineral decrease water in ζ hardness.potential, i.e The less presence negative of divalentvalues, is ions, observed, such which2+ is found2 to+ be proportional to the mineral water hardness. The presence of divalent ions, such as Ca and Mg , is found here to strongly modify the stability of negatively charged TiO2 NPs via as Ca2+ and Mg2+, is found here to strongly modify the stability of negatively charged TiO2 NPs via specific adsorption. As shown in Figure4a, at a low concentration of divalent ions (MW1), TiO 2 NPs arespecific found adsorption. to be less stable As shown than in in ultrapure Figure4a, water, at a whichlow concentration is in good agreement of divalent with ions their (MW1), surface TiO charge2 NPs ( are16 found1 mV). to Asbe aless result, stable a continuous than in ultrapure increase wate of aggregater, which sizeis in is good observed agreement within with the experimental their surface − ± timecharge (135 (− min),16 ± with1 mV). a maximum As a result, value a continuous equal to 1200 increase26 nm. of Aggregationaggregate size is more is observed pronounced within when the experimental time (135 min), with a maximum va±lue equal to 1200 ± 26 nm. Aggregation is more TiO2 NPs are added into MW3. As the water hardness increases (from MW1 to MW3), surface charge ispronounced more effectively when neutralized TiO2 NPs are due added to both into the MW3. specific As adsorption the water and hardness surface increases charge screening (from MW1 effect, to MW3), surface charge is more effectively neutralized due to both the specific adsorption and surface resulting in the formation of larger TiO2 aggregates with z-average hydrodynamic diameters of up to 2.5chargeµm. screening effect, resulting in the formation of larger TiO2 aggregates with z-average hydrodynamic diameters of up to 2.5 µm. Regarding the stability of TiO2 NPs in Lake Geneva and drinking water (Figure5a), a significant decrease in ζ potential is also observed with values equal to 13 1 mV and 8 1 mV, respectively. 20 − ± − ± 5000 These values are close to the zeta potential measurements Ultrapure that water were made by Loosli et al. [30] and Ultrapure water (b) [TiO ] = 10mg/L Lake Geneva [TiO2] = 10mg/L Graham10 et Lake al. [Geneva34] for TiO2 NPs2 and natural(a) colloids fromDrinking thewater Geneva Lake. Lake Geneva water Drinking water 4000 contains multivalent cations (Ca2+ and Mg2+), but also natural organic matter. As natural organic 0 matter is negatively charged, the Ca2+ ions are expected to promote the adsorption of NOM and act as 3000 bridges-10 between the NOM and the TiO2 NPs surface [12,13,35]. As shown in Figure5b, TiO 2 NPs are stable with time in ultrapure water with z-average diameters that are equal to 460 12 nm. NPs are ± 2000 found-20 to be unstable in surface waters, especially in drinking waters where aggregates are rapidly potential (mV) formedζ− -30 with sizes that are greater than 2000 nm after 60 min. On the other hand, aggregation is less 1000 important for NPs in Lake Geneva water with z-average diameters equal to 1550 23 nm after 135 min. -40 ±

This can be mainly explained by the higher DOC concentration in Lake Geneva water when compared (nm) diameter hydrodynamic z-average 0 to drinking0 water, 20 as 40 indicated 60 in 80 Table 1001. In 120 such 140 conditions,0 20406080100120140 NOM is expected to reduce the impact of 2+ 2+ Time (min) Ca and Mg ions onTime the (min) ﬁnal TiO2 surface charge, as shown by Loosli et al. [30]

Figure 5. (a) ζ potential and (b) z-average hydrodynamic diameter variation of TiO2 as a function of time in ultrapure water at pH = 8.0 ± 0.1, drinking, and Lake Geneva waters at their natural pHs. In comparison to ultrapure water, ζ potential decreases towards less negative values when TiO2 NPs are introduced in drinking and Lake Geneva water, hence promoting aggregation.

Regarding the stability of TiO2 NPs in Lake Geneva and drinking water (Figure 5a), a significant decrease in ζ potential is also observed with values equal to -13 ± 1mV and -8 ± 1mV, respectively. These values are close to the zeta potential measurements that were made by Loosli et al. [30] and Graham et al. [34] for TiO2 NPs and natural colloids from the Geneva Lake. Lake Geneva water contains multivalent cations (Ca2+ and Mg2+), but also natural organic matter. As natural organic matter is negatively charged, the Ca2+ ions are expected to promote the adsorption of NOM and act

Water 2019, 11, x FOR PEER REVIEW 7 of 14

7000 Ultrapure water 20 Ultrapure water MW1 [TiO2] = 10mg/L MW1 [TiO2] = 10mg/L (b) 6000 MW2 (a) MW2 MW3 MW3 10 5000

0 4000

-10 3000 potential (mV) potential 2000 ζ− -20

1000 -30 z-average hydrodynamic diameter (nm) diameter hydrodynamic z-average 0 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Time (min) Time (min)

Figure 4. (a) ζ potential and (b) z-average hydrodynamic diameter variation of TiO2 as a function of time in ultrapure water at pH = 7.0 ± 0.1 and mineral waters at their natural pHs. TiO2 surface charge

is found to be reduced and neutralized in mineral waters hence resulting in TiO2 aggregation.

By considering MWs, a systematic decrease in ζ potential, i.e less negative values, is observed, which is found to be proportional to the mineral water hardness. The presence of divalent ions, such as Ca2+ and Mg2+, is found here to strongly modify the stability of negatively charged TiO2 NPs via specific adsorption. As shown in Figure4a, at a low concentration of divalent ions (MW1), TiO2 NPs are found to be less stable than in ultrapure water, which is in good agreement with their surface charge (−16 ± 1 mV). As a result, a continuous increase of aggregate size is observed within the experimental time (135 min), with a maximum value equal to 1200 ± 26 nm. Aggregation is more pronounced when TiO2 NPs are added into MW3. As the water hardness increases (from MW1 to MW3), surface charge is more effectively neutralized due to both the specific adsorption and surface charge screening effect, resulting in the formation of larger TiO2 aggregates with z-average Water 2019, 11, 721 8 of 14 hydrodynamic diameters of up to 2.5 µm.

20 5000 Ultrapure water Ultrapure water (b) [TiO ] = 10mg/L Lake Geneva [TiO2] = 10mg/L 10 Lake Geneva 2 (a) Drinking water Drinking water 4000

3000 -10

Water-20 2019, 11, x FOR PEER REVIEW 20008 of 14 potential (mV)

ζ− -30 as bridges between the NOM and the TiO2 NPs surface [12,13,35]. As shown in Figure5b, TiO10002 NPs are -40stable with time in ultrapure water with z-average diameters that are equal to 460 ± 12 nm. NPs z-average hydrodynamic diameter (nm) diameter hydrodynamic z-average are found to be unstable in surface waters, especially in drinking waters where aggregates are rapidly0 0 20 40 60 80 100 120 140 0 20406080100120140 formed with sizes that are greater than 2000 nm after 60 minutes. On the other hand, aggregation is Time (min) Time (min) less important for NPs in Lake Geneva water with z-average diameters equal to 1550 ± 23 nm after ζ 135 Figureminutes.Figure 5. 5.( aThis )(a) potentialζ can potential be mainly and and (b )(b) z-averageexplained z-average hydrodynamic byhydrodynamic the higher diameter DOCdiameter concentration variation variation of of TiO TiOin2 asLake2 as a functiona Genevafunction of waterof time in ultrapure water at pH = 8.0 0.1, drinking, and Lake Geneva waters at their natural pHs. In when timecompared in ultrapure to drinking water at water, pH = 8.0 as± ± indicated 0.1, drinking in ,Table and Lake 1. In Geneva such conditions, waters at their NOM natural is expected pHs. In to comparison to ultrapure water, ζ potential decreases towards less negative values when TiO NPs are reducecomparison the impact to of ultrapure Ca2+ and water, Mg2+ ζ ions potential on the decreases final TiO towards2 surface less charge, negative as values shown when by LoosliTiO2 2 NPs et areal. [30] introducedintroduced in in drinking drinking and and Lake Lake Geneva Geneva water, water, hence hence promoting promoting aggregation. aggregation.

3.2.2. CeO2 nanoparticles 3.2.2. CeO2 Nanoparticles Regarding the stability of TiO2 NPs in Lake Geneva and drinking water (Figure 5a), a significant decreaseInIn Figure Figure in ζ6 a,potential6a, are are presented presented is also theobserved theζ ζpotential potential with values and and z-average z-average equal to hydrodynamic -13hydrodynamic ± 1mV and diameter -8diameter ± 1mV, variation variationrespectively.of of CeOCeOThese22NPs NPs values inin mineralmineral are close waterswaters to the as as a zetaa function function potential of of time. time. measurem TheThe surfacesurfaceents that charge charge were of of CeOmade CeO22 NPs byNPs Loosli inin ultrapureultrapure et al. [30] water water and isisGraham negative negative et with with al. a[34] aζ ζpotential potentialfor TiO2 value valueNPs thatand that isnatural is equal equal tocolloids to –77 ± 1 1from mV.mV. As theAs expected, expected,Geneva Lake. a a decrease decrease Lake in Genevainζ ζpotential potential water − ± valuevaluecontains is is observed observed multivalent in in MWs, MWs, cations which which (Ca can can2+ and also also Mg be be related 2+rela), butted to alsoto water water natural hardness. hardness. organic Indeed, Indeed, matter. when when As NPs naturalNPs are are added organicadded totomatter MW2 MW2 andis and negatively MW3, MW3, charge charge charged, inversion inversion the Ca is achieved2+is ions achieved are and expected and surface surface to charge promote charge is found theis found adsorption to be to slightly be slightly of positive NOM positive and with act ζwithpotential ζ potential values values equalto equal+2.5 to 1+2.5 mV ± and 1mV+5 and1 mV,+5 ± respectively.1mV, respectively. Such a Such behavior a behavior indicates indicates strong ± ± astrongﬃnity betweenaffinity between the divalent the cationdivalent and cation CeO 2andnegative CeO2 negative surface. Aggregationsurface. Aggregation is observed is forobserved all water for samplesall water with samples z-average with z-average hydrodynamic hydrodynamic diameters diameters greater thangreater 1 µ thanm within 1 µm within the experimental the experimental time (Figuretime (Figure6b). The 6b). highly The highly variable variable CeO 2 CeOdiameters2 diameters in ultrapure in ultrapure water water range range from from 500 nm 500 to nm 3000 to nm,3000 whichnm, which is the is result the result of a ζ ofpotential a ζ potential close toclose zero to( zero8 mV), (−8 mV), and is and thus is situatedthus situated in the in PZC the region.PZC region. −

6000 Ultrapure water Ultrapure water 20 MW1 [CeO ] = 10 mg/L MW1 [CeO2] = 10 mg/L 2 (a) (b) MW2 MW2 5000 MW3 MW3

10 4000

3000 0

potential (mV) 2000 ζ− -10 1000 z-average hydrodynamic diameter (nm) (nm) diameter hydrodynamic z-average -20 0 0 20406080100120140 0 20406080100120140 Time (min) Time (min)

Figure 6. (a) ζ potential and (b) z-average hydrodynamic diameter variation of CeO as a function of Figure 6. (a) ζ potential and (b) z-average hydrodynamic diameter variation of CeO2 2 as a function of time in ultrapure water at pH = 7.0 0.1 and mineral waters at their natural pHs. CeO NPs surface time in ultrapure water at pH = 7.0± ± 0.1 and mineral waters at their natural pHs. CeO2 2 NPs surface charge is found to be reduced in all mineral waters, hence promoting CeO aggregation. charge is found to be reduced in all mineral waters, hence promoting CeO2 2 aggregation.

The results for CeO2 NPs stability in Lake Geneva and drinking waters are presented in Figure7. The results for CeO2 NPs stability in Lake Geneva and drinking waters are presented in Figure 7. A decrease in ζ potential is observed in both waters with values that are equal to 11 1 mV and A decrease in ζ potential is observed in both waters with values that are equal to –11− ± ±1mV and – 6 6 1 mV, respectively. CeO2 NPs are aggregated in all water samples, as shown in Figure7b. −± 1± mV, respectively. CeO2 NPs are aggregated in all water samples, as shown in Figure 7b. As discussed for TiO NPs, the adsorption of divalent cations are also expected here to interact 2 with CeO2 NPs and NOM, forming bridges between them, resulting in NPs destabilization. As

Water 2019, 11, x FOR PEER REVIEW 9 of 14

Water202019, 11, 721 50009 of 14 Ultrapure water Ultrapure water [CeO ] = 10 mg/L Lake Geneva [CeO2] = 10 mg/L (a) Lake Geneva 2 (b) Drinking water Drinking water 10 4000 shown for drinking water, 0.4 mg/L DOC lead to CeO2 NPs aggregation with z-average diameters that are greater than 2 µm. However, limited aggregation is observed for NPs in Lake Geneva water 0 3000 with z-average diameters of less than 1 µm due to the presence of DOC at signiﬁcant concentrations (1.12 mg/L), enhancing NPs stability. Our results are in agreement with those that were obtained by -10 2000

Oriekhovapotential (mV) and Stoll [31]. The authors found CeO2 NPs aggregated in Lake Geneva water with time ζ− andWater negative 2019, 11, x surface FOR PEER charge REVIEW with a zeta potential equal to 13.3 0.6 mV. 9 of 14 -20 − ± 1000

20 5000 z-average hydrodynamic diameter (nm) Ultrapure water 0 -30 Ultrapure water CeO = 10 mg/L 0Lake Geneva 20 40[CeO 602] = 10 80mg/L 100 120 140 0Lake 20406080100120140Geneva [ 2] (a) (b) Drinking water Drinking water Time (min) 10 Time (min) 4000

0Figure 7. (a) ζ potential and (b) z-average hydrodynamic diameter variation of CeO2 as a function3000 of time in ultrapure water at pH = 8.0 ± 0.1, drinking and Lake Geneva waters at their natural pHs.

-10Limited aggregation is observed due to the presence of DOC at a significant concentration (1.12 mg/L)2000 potential (mV) when CeO2 NPs are introduced in Lake Geneva water. ζ−

-20 1000 As discussed for TiO2 NPs, the adsorption of divalent cations are also expected here to interact z-average hydrodynamic diameter (nm) with CeO2 NPs and NOM, forming bridges between them, resulting in NPs destabilization.0 As -30 0 20 40 60 80 100 120 140 shown for0 drinking 20406080100120140 water, 0.4 mg/L DOC lead to CeO2 NPs aggregation with z-average diameters Time (min) that are greater than 2 µm.Time However,(min) limited aggregation is observed for NPs in Lake Geneva water withFigure z-average 7. (a) diametersζ potential of and less (b) than z-average 1 µm hydrodynamicdue to the presence diameter of variationDOC at significant of CeO2 as aconcentrations function Figure 7. (a) ζ potential and (b) z-average hydrodynamic diameter variation of CeO2 as a function of (1.12of mg/L), time in enhancing ultrapure water NPs atstabilit pH =y.8.0 Our0.1, results drinking are andin agreement Lake Geneva with waters those at that their were natural obtained pHs. by time in ultrapure water at pH = 8.0 ±± 0.1, drinking and Lake Geneva waters at their natural pHs. OriekhovaLimited and aggregation Stoll [31]. is observed The authors due to found the presence CeO2 ofNPs DOC aggregated at a signiﬁcant in Lake concentration Geneva water (1.12 mg with/L) time Limited aggregation is observed due to the presence of DOC at a significant concentration (1.12 mg/L) andwhen negative CeO 2surfaceNPs are charge introduced with in a Lakezeta Genevapotential water. equal to −13.3 ± 0.6 mV. when CeO2 NPs are introduced in Lake Geneva water. 3.2.3. Polystyrene Nanoplastic Particles 3.2.3. Polystyrene nanoplastic particles As discussed for TiO2 NPs, the adsorption of divalent cations are also expected here to interact Nanoplastics behavior in mineral waters is presented in Figure8 and a comparison is made with with NanoplasticsCeO2 NPs and behavior NOM, in forming mineral watersbridges is between presented them, in Figure8 resulting and ina comparison NPs destabilization. is made with As ultrapure water. In ultrapure water, the surface charge is found to be stable and PS nanoplastics are ultrapureshown for water. drinking In ultrapure water, 0.4 water, mg/L theDOC surface lead chargeto CeO 2is NPs found aggregation to be stable with and z-average PS nanoplastics diameters are strongly positively charged (+41 3 mV), with z-average hydrodynamic diameter values that are equal stronglythat are greater positively than charged 2 µm. However, (+ ±41 ± 3 limitedmV), with aggregation z-average is hydrodynamic observed for NPs diameter in Lake values Geneva that water are to 98 3 nm. When nanoplastics are added in mineral waters, a decrease of ζ potential is observed equalwith± z-average to 98 ± 3 diametersnm. When of nanoplastics less than 1 µm are due added to the in presence mineral ofwaters, DOC ata decreasesignificant of concentrations ζ potential is because of the presence of small amounts of negatively charged DOC (Table1). Despite the decrease observed(1.12 mg/L), because enhancing of the NPspresence stabilit ofy. small Our amountsresults are of innegatively agreement charged with those DOC that (Table were 1.) obtainedDespite the by of ζ potential, nanoplastics are stable in almost all cases and no aggregation is observed due to the decreaseOriekhova of andζ potential, Stoll [31]. nanoplastics The authors are found stable CeO in almost2 NPs aggregatedall cases and in no Lake aggregation Geneva wateris observed with time due positive amidine functional groups, making nanoplastics resistant in MW1 and MW2 to aggregation toand the negative positive surface amidine charge functional with a zeta groups, potential maki equalng nanoplastics to −13.3 ± 0.6 resistant mV. in MW1 and MW2 to eﬀects of divalent ions and the presence of NOM. aggregation effects of divalent ions and the presence of NOM.

3.2.3.60 Polystyrene nanoplastic particles 6000 Ultrapure water 50 (a) MW1 (b) Nanoplastics behavior in mineral waters is presented in Figure8 [PS and] = 10mg/L a comparison is made with MW2 5000 ultrapure40 water. In ultrapure water, the surface charge MW3is found to be stable and PS nanoplastics are

strongly30 positively charged (+ 41 ± 3 mV), with z-average hydrodynamic diameter values that4000 are

equal20 to 98 ± 3 nm. When nanoplastics are added in mineral waters, a decrease of ζ potential is observed because of the presence of small amounts of negatively charged DOC (Table 1.) Despite3000 the 10 decrease of ζ potential, nanoplastics are stable in almost all cases and no aggregation is observed due 2000 0 potential (mV)

toζ− the positive amidine functional groups, making nanoplastics resistant in MW1 and MW2 to -10 Ultrapure water aggregation MW1 effects of divalent ions and the presence of NOM. 1000 MW2 [PS] = 10mg/L -20 60 MW3 6000 z-average hydrodynamic diameter (nm) diameter hydrodynamic z-average Ultrapure water 0 -30 (a) 50 0 MW1 20 40 60 80 100 120(b) 140 0 20 40 60 80 100 120 140 [PS] = 10mg/L MW2 5000 Time (min) 40 Time (min) MW3

30 4000 FigureFigure 8. 8.(a )(a)ζ potential ζ potential and (band) z-average (b) z-average hydrodynamic hydrodynamic diameter diameter variation ofvariation amidine of PS nanoplasticamidine PS 20particles as a function of time in ultrapure at pH = 7.0 0.1 and mineral waters at their natural pHs. A nanoplastic particles as a function of time in ultrapure± at pH = 7.0 ± 0.1 and mineral waters at their3000 10decrease of ζ potential is observed for all mineral waters. However, nanoplastics are stable and no

2000 aggregation0 is observed. Limited aggregation is only observed for MW3. potential (mV) ζ− -10 Ultrapure water MW1 1000 MW2 [PS] = 10mg/L -20 MW3 z-average hydrodynamic diameter (nm) diameter hydrodynamic z-average 0 -30 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Time (min) Time (min)

Figure 8. (a) ζ potential and (b) z-average hydrodynamic diameter variation of amidine PS nanoplastic particles as a function of time in ultrapure at pH = 7.0 ± 0.1 and mineral waters at their

Water 2019, 11, x FOR PEER REVIEW 10 of 14

natural pHs. A decrease of ζ potential is observed for all mineral waters. However, nanoplastics are stable and no aggregation is observed. Limited aggregation is only observed for MW3.

As shown in Figure 8b, limited aggregation is only observed for MW3, which is in agreement with the corresponding ζ potential values, which are found less than + 20 mV. On the other hand, when nanoplastic particles are added in Lake Geneva and drinking water, a significant decrease of ζ Water 2019, 11, x FOR PEER REVIEW 10 of 14 Waterpotential2019, 11 ,is 721 observed. As can be seen in Figure 9a, the surface charge of nanoplastics in drinking10 ofwater 14 significantly decreases but remains positive at about +10 mV, resulting in aggregation, as indicated in Figurenatural 9b. pHs. A decrease of ζ potential is observed for all mineral waters. However, nanoplastics are stable and no aggregation is observed. Limited aggregation is only observed for MW3. AsIn shown Lake Geneva in Figure water,8b, limited high concentrations aggregation is of only NOM observed lead to for charge MW3, reversal which is(–13 in agreement± 1 mV), and withnanoplastics the corresponding remain relativelyζ potential stable values, with whicha z-average are found hydrodynamic less than +diameter20 mV. Onof 450 the ±other 100 nm. hand, This As shown in Figure 8b, limited aggregation is only observed for MW3, which is in agreement whenindicates nanoplastic that NOM particles coating are added in Lake in LakeGeneva Geneva is reducing and drinking nanoplastic water, aaggregation signiﬁcant decrease due to ofstericζ with the corresponding ζ potential values, which are found less than + 20 mV. On the other hand, potentialstabilization is observed. effects. AsNOM can coating be seen is in illustrated Figure9a, thefrom surface a morphological charge of nanoplastics point of view in in drinking the SEM water image when nanoplastic particles are added in Lake Geneva and drinking water, a significant decrease of ζ signiﬁcantlythat is presented decreases in Figure but remains 10. PS positive nanoplastic at about shapes+10 are mV, no resulting more spherical, in aggregation, butslightly as indicated ellipsoidal in potential is observed. As can be seen in Figure 9a, the surface charge of nanoplastics in drinking water Figureand fuzzy9b. membranes surrounding the particles are producing less contrasted images. significantly decreases but remains positive at about +10 mV, resulting in aggregation, as indicated in Figure60 9b. 5000 Ultrapure water Ultrapure water 50In Lake Geneva water, high concentrations of NOM Drinking lead water to charge reversal (–13 ± 1 mV), and Drinking water [PS] = 10mg/L [PS] = 10mg/L Lake Geneva (a) Lake Geneva (b) 4000 nanoplastics40 remain relatively stable with a z-average hydrodynamic diameter of 450 ± 100 nm. This indicates that NOM coating in Lake Geneva is reducing nanoplastic aggregation due to steric 30 stabilization effects. NOM coating is illustrated from a morphological point of view in the SEM 3000image 20 that is presented in Figure 10. PS nanoplastic shapes are no more spherical, butslightly ellipsoidal 10 and fuzzy membranes surrounding the particles are producing less contrasted images. 2000 0 potential (mV) potential

ζ− 60 5000 -10 Ultrapure water Ultrapure water 1000 50 Drinking water Drinking water [PS] = 10mg/L [PS] = 10mg/L -20 Lake Geneva (a) Lake Geneva (b) 4000

40 (nm) diameter hydrodynamic z-average -30 0 0 20 40 60 80 100 120 140 30 0 20 40 60 80 100 120 140 Time (min) Time (min) 3000 20 Figure 9. ζ(a) ζ potential and (b) z-average hydrodynamic diameter variation of amidine PS 10 Figure 9. (a) potential and (b) z-average hydrodynamic diameter variation of amidine PS nanoplastic nanoplastic particles as a function of time in ultrapure at pH = 8.0 ± 0.1, drinking and Lake Geneva2000 particles as a function of time in ultrapure at pH = 8.0 0.1, drinking and Lake Geneva waters at 0 potential (mV) potential waters at their natural pHs. Charge reversal is observed± when PS nanoplastic particles are introduced

ζ− their natural pHs. Charge reversal is observed when PS nanoplastic particles are introduced in Lake -10 in Lake Geneva water. Natural organic matter (NOM) coating is expected and results in limited1000 Geneva water. Natural organic matter (NOM) coating is expected and results in limited aggregation -20 ofaggregation nanoplastics. of nanoplastics. z-average hydrodynamic diameter (nm) (nm) diameter hydrodynamic z-average -30 0 In0 Lake 20 Geneva 40 water, 60 80 high 100 concentrations 120 140 of NOM0 20 lead 40 to charge 60 80 reversal 100 ( 12013 1401 mV), and Time (min) Time (min) − ± nanoplastics remain relatively stable with a z-average hydrodynamic diameter of 450 100 nm. Figure 9. (a) ζ potential and (b) z-average hydrodynamic diameter variation of amidine± PS This indicates that NOM coating in Lake Geneva is reducing nanoplastic aggregation due to steric nanoplastic particles as a function of time in ultrapure at pH = 8.0 ± 0.1, drinking and Lake Geneva stabilization eﬀects. NOM coating is illustrated from a morphological point of view in the SEM image waters at their natural pHs. Charge reversal is observed when PS nanoplastic particles are introduced that is presented in Figure 10. PS nanoplastic shapes are no more spherical, butslightly ellipsoidal and in Lake Geneva water. Natural organic matter (NOM) coating is expected and results in limited fuzzy membranes surrounding the particles are producing less contrasted images. aggregation of nanoplastics.

Figure 10. Scanning Electron Microscopy (SEM) image of amidine PS nanoplastic particles (10 mg/L)

in filtered (0.2 µm) Lake Geneva water at pH 8.1 ± 0.1 < pHpzc. When compared to ultrapure water

(Figure 3b) where, pH > pHpzc. PS nanoplastic aggregates are found coated with a complex organic matter matrix resulting in a fuzzy particle contrast.

. Figure 10. Scanning Electron Microscopy (SEM) image of amidine PS nanoplastic particles (10 mg/L) inFigure ﬁltered 10. (0.2Scanningµm) Lake Electron Geneva Microscopy water at pH(SEM) 8.1 image0.1 < ofpH amidinepzc. When PS nanoplastic compared toparticles ultrapure (10 watermg/L) ± (Figurein filtered3b) where,(0.2 µm) pH Lake> pH Genevapzc. PS water nanoplastic at pH 8.1 aggregates ± 0.1 < pH arepzc found. When coated compared with ato complex ultrapure organic water matter(Figure matrix 3b) where, resulting pH in> pH a fuzzypzc. PS particle nanoplastic contrast. aggregates are found coated with a complex organic matter matrix resulting in a fuzzy particle contrast.

Water 2019, 11, 721 11 of 14 Water 2019, 11, x FOR PEER REVIEW 11 of 14

3.2.4.3.2.4. ImpactImpact of Water Water Ha Hardnessrdness and and Organic Organic mMatter Matter

ToTo better better highlight highlight the the influence influence of of both both water water hardness hardness and and NOM NOM on on the the aggregation aggregation behavior behavior of NPsof NPs and nanoplastics,and nanoplastics, stability stability diagrams diagrams were calculated were calculated from the from z-average the z-average hydrodynamic hydrodynamic diameter valuesdiameter that values were obtainedthat were inobtained the diff erentin the conditions, different conditions, and they areand presented they are presented in Figure 11in Figurefor TiO 112 andfor TiO CeO2 and2 NPs, CeO and2 NPs, nanoplastics. and nanoplastics. All of All the of results the results dealing dealing with with aggregate aggregate formation formation obtained obtained in MWs,in MWs, drinking drinking and and Lake Lake Geneva Geneva waters waters have have merged merged together. together. Red Red color color indicates indicates that that NPs NPsare are dispersed,dispersed, whereas whereas blue blue color color indicates indicates strong strong aggregation aggregation i.e., i.e. thethe formationformation of of aggregatesaggregates with with sizes sizes thatthat are are greater greater than than 2000 2000 nm. nm. For For TiO TiO22NPs, NPs, aggregationaggregation isis foundfound toto bebe promotedpromotedby by increasingincreasing water water hardnesshardness so so long long as as the the DOC DOC concentration concentration is less is less than than 0.6 mg0.6/ L.mg/L. This This indicates indicates that divalentthat divalent ions, suchions, 2+ 2+ assuch Ca as andCa2+ Mgand Mg, are2+, eareffective effective in neutralizing in neutralizing TiO TiO2 NPs2 NPs negative negative surface surface charge. charge. As As TiO TiO2 NPs2 NPs are are 2+ 2+ stronglystrongly negatively negatively charged charged at at environmental environmental pH, pH, Ca Ca2+and and Mg Mg2+ areare stronglystronglyadsorbed adsorbed onto onto TiO TiO2 2due due toto the the attractive attractive electrostatic electrostatic forces forces between between NPs NPs and and divalent divalent cations, cations, resulting resulting in in TiO TiO2 2NPs NPs charge charge neutralization.neutralization. IncreasingIncreasing DOCDOC concentration concentration is is found found to to reduce reduce the the aggregation aggregation of of TiO TiO2 2NPs, NPs, which which isis probably probablydue due toto thethe formationformation ofof Ca22++ andand Mg Mg2+2 complexes+ complexes reducing reducing the the impact impact of ofthese these ions ions on onthe theTiO TiO2 surface2 surface charge. charge. On Onthe theother other hand, hand, water water hardness hardness is found is found to have to have a less a less significant significant role role on onCeO CeO2 NPs2 NPs aggregation. aggregation. At At environmental environmental pH, pH, the the surface surface charge charge of of CeO 2 NPsNPs is is close toto thethe PZCPZC (pH(pHpzcpzc == 6.9 ± 0.1)0.1) and, and, consequently, consequently, in most conditions, NPsNPs areare alreadyalreadyaggregated. aggregated.The Thestability stability of of ± CeOCeO22NPs NPs withwith regardsregards toto waterwater hardnesshardness isis causedcaused byby thethe repulsiverepulsive forcesforces betweenbetween NPs NPs and and divalent divalent ionsions (Ca (Ca22++ andand Mg Mg2+2).+ ).DOC DOC increase increase (above (above 0.8 0.8 mg/L) mg/ isL) also is also found found to reduce to reduce aggregation. aggregation. On the On other the otherhand, hand, polystyrene polystyrene nanoplastics, nanoplastics, which which are are strongly strongly positively positively charged charged at at environmental environmental pH,pH, areare foundfound to to be be more more stable stable with with regards regards to to water water hardness hardness and and natural natural organic organic matter. matter. PS PS nanoplastics nanoplastics areare stabilized stabilized against against water water hardness hardness due due to to electrostatic electrostatic repulsive repulsive interaction interaction between between the the amidine amidine functionalfunctional groupgroup andand ions ions Ca Ca2+2+ andandMg Mg2+2+.. Negatively charged NOMNOM cancan adsorbadsorb ontoonto positivelypositively chargedcharged NPs NPs promoting promoting aggregation aggregation by by charge charge neutralization neutralization and and via via bridging bridging mechanisms mechanisms that that are are inducedinduced by by the the presence presence of of divalent divalent cations cations [ 33[33].]. However, However, PS PS nanoplastics nanoplastics aggregation aggregation is is observed observed inin a a limited limited domain domain with with specific specific water water hardness hardness and and DOC.DOC. ThisThis is is an an important important issue, issue, indicating indicating that,that, in in general, general, the the polystyrene polystyrene nanoplastics nanoplastics stability stabilityand and dispersion dispersionstate state are are more more pronounced pronounced in in aquaticaquatic systems. systems.

TiO2 CeO2 Polystyrene Latex 100 500 1.0 1.0 1.0 1000 1500 2000 2500 0.8 0.8 0.8 3000 > d.nm

0.6 0.6 0.6

0.4 0.4 0.4 Total organic carbon (mg/L) carbon organic Total

Dissolved organic carbon (mg/L) carbon Dissolved organic 0.2 0.2 0.2

0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 Water hardness (mg CaCO /L) Water hardness (mg CaCO3/L) 3 Water hardness (mg CaCO3/L)

FigureFigure 11. 11.Aggregation Aggregation intensity intensity diagrams diagrams of of TiO TiO2,2, CeO CeO2,2, and and amidineamidine PSPS nanoplasticnanoplastic particlesparticles asas a a functionfunction of of water water hardness hardness and and Dissolved Dissolved Organic Organic Carbon Carbon (DOC) (DOC) in in MW MW and and SW SW at at a a 10 10 mg mg/L/L TiO TiO22,, CeO and PS nanoplastic concentration. The color and respective values in the legend represent the CeO22 and PS nanoplastic concentration. The color and respective values in the legend represent the z- z-averageaverage diameter diameter of of NPs NPs and and PS PS nanoplastic particles in nanometersnanometers thatthat werewere obtained obtained after after 1h. 1h. Blue Blue colorcolor indicates indicates signiﬁcant significant aggregation, aggregation, whereas whereas red red color color indicates indicates limited limited aggregation. aggregation. 4. Conclusions 4. Conclusions In this study, the importance of several relevant environmental parameters on the stability of NPs In this study, the importance of several relevant environmental parameters on the stability of and PS nanoplastics in various waters was evaluated. In ultrapure water, TiO2 and PS nanoplastic NPs and PS nanoplastics in various waters was evaluated. In ultrapure water, TiO2 and PS were found stable, while CeO2 NPs was found to aggregate with time. In bottled mineral waters, TiO2 nanoplastic were found stable, while CeO2 NPs was found to aggregate with time. In bottled mineral waters, TiO2 and CeO2 NPs were found to be aggregated. The presence of divalent ions, such as Ca2+

Water 2019, 11, 721 12 of 14

2+ 2+ and CeO2 NPs were found to be aggregated. The presence of divalent ions, such as Ca and Mg , induced the aggregation of negatively charged NPs by adsorption and charge neutralization, hence reducing electrostatic forces. In contrast, positively charged PS nanoplastics, were found to remain stable against aggregation. In surface waters, such as drinking and Lake Geneva waters, NPs, as well as nanoplastic particles, were systematically destabilized, especially in drinking water with low NOM concentration when compared with Lake Geneva water. This indicates that high NOM concentrations, such as in Lake Geneva, is limiting NPs and PS nanoplastic aggregation due to steric stabilization effects. Although nanoplastic particles were destabilized in specific conditions, such positive particles have longer residence times and form small aggregates that will be difficult to eliminate via filtration processes. Our research indicates that fate and transport of nanoparticles, including nanoplastics, in aquatic systems are strongly governed by their surface properties and physicochemical conditions of the medium (water hardness, pH, NOM concentration). Future research will focus on coagulation processes that provide an effective way for NPs removal during water treatment.

Author Contributions: Methodology, L.R.; Software, L.R.; Validation, S.S., S.R.G. and S.Z.; Writing—original draft preparation, L.R.; Writing—review and editing, S.S., S.R.G. and S.Z.; Supervision, S.S. Funding: This research was funded by FOWA from Société Suisse de l’Industrie du Gaz et des Eaux SSIGE/SVGW. Acknowledgments: The authors acknowledge support received from FOWA fund from Société Suisse de l’Industrie du Gaz et des Eaux SSIGE/SVGW, Service Industriel de Genève (SIG) and University of Geneva. We are also grateful to Agathe Martignier for her support during the SEM measurements. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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