Behavior of TiO 2 and CeO 2 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

: 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 e ﬃ cient 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. TiO 2 , CeO 2 , and Polystyrene (PS) nanoplastics are selected, owing to their large number applications and contrasting characteristics at environmental pH. Experiments are performed in di ﬀ erent 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.


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 TiO 2 NPs in food, cosmetics [3], and photovoltaics [4], and the use of CeO 2 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].

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.

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.

Nanoparticles
Two manufactured nanoparticles TiO 2 anatase, CeO 2 NPs, and one amidine polystyrene latex nanoplastic were used in this study. TiO 2 NPs, with a determined manufacturer x-ray diffraction diameter of 15 nm and a specific surface area of 40−60 m 2 /g, were purchased as a powder from Nanostructured & Amorphous Material Inc-5430MR (Los Alamos, NM, USA). The CeO 2 NPs were also purchased as a powder from Sigma-Aldrich-MKBN9764V (Buchs, Switzerland). BET determined the CeO 2 primary particle diameter < 50 nm [29], according to the manufacturer with a specific surface area of 30 m 2 /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 (TEM) and a specific surface area of 63 m 2 /g. 1 g/L stock suspensions of TiO 2 and CeO 2 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 TiO 2 and CeO 2 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.

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)  Table 1. The choice of the three mineral waters was based on their differences in ionic composition and, in particular, water hardness. The total hardness calculation was based on Ca 2+ and Mg 2+ concentrations and expressed in mg CaCO 3 /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. 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.

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. Figure 1a presents the results for TiO 2 NPs. The surface of TiO 2 NPs is found to be strongly positively charged at pH 3.0 (+31 ± 0.5 mV).
By further increasing the pH, the ζ potential decreases until the point of zero charge (PZC) at pH pzc = 5.8 ± 0.1. This value is close to the experimental value of pH pzc = 6.2 that was obtained by Loosli et al. [30]. When the pH is higher than pH pzc , the surface charge becomes negative and stable at −40 ± 2 mV at pH 10 ± 0.1. The z-average hydrodynamic diameter of TiO 2 NPs is found stable below pH 5.0 with values that are equal to 430 ± 10 nm. Subsequently, fast aggregation is observed with a maximum value of 887 ± 40 nm in the PZC region. A further pH increase leads to the decrease of the TiO 2 hydrodynamic diameter, indicating reversible aggregation. When pH ≥ 8.0, TiO 2 NPs exhibit a stable domain with values of z-average hydrodynamic diameter equal to 405 ± 10 nm in good agreement with the SEM image of TiO 2 , as presented in Figure 1b. TiO 2 NPs destabilization is observed in a ζ potential range between +20 and −25 mV, as indicated in the gray area (Figure 1a). TiO2 NPs destabilization is observed in a ζ potential range between +20 and −25 mV, as indicated in the gray area ( Figure 1a). As shown in Figure 2a, CeO2 NPs exhibit a stable and positive charge (+21 ± 1 mV) from pH 3.0 to 5.0. Subsequently, ζ potential rapidly decreases to reach the PZC at pHpzc = 6.9 ± 0.1, which is similar to the previously reported studies [31]. An increase of pH leads to charge reversal with ζ potential values equals to -32 ± 2 mV at pH 10 ± 0.1. As illustrated in Figure 2a, the z-average diameter of CeO2 NPs is stable below pH 6.0 with values that are equal to 350 ± 50 nm. Near the PZC region z-average diameter increases until values that are greater than 1.5µm, a further pH increase leads to charge reversal. However, CeO2 aggregates are found to be stable and are not affected by pH increase, as observed for TiO2, since no significant decrease of the z-average diameter is observed after the PZC, which is also confirmed by SEM (Figure 2b).  As shown in Figure 2a, CeO 2 NPs exhibit a stable and positive charge (+21 ± 1 mV) from pH 3.0 to 5.0. Subsequently, ζ potential rapidly decreases to reach the PZC at pH pzc = 6.9 ± 0.1, which is similar to the previously reported studies [31]. An increase of pH leads to charge reversal with ζ potential values equals to −32 ± 2 mV at pH 10 ± 0.1. As illustrated in Figure 2a, the z-average diameter of CeO 2 NPs is stable below pH 6.0 with values that are equal to 350 ± 50 nm. Near the PZC region z-average diameter increases until values that are greater than 1.5 µm, a further pH increase leads to charge reversal. However, CeO 2 aggregates are found to be stable and are not affected by pH increase, as observed for TiO 2 , since no significant decrease of the z-average diameter is observed after the PZC, which is also confirmed by SEM (Figure 2b).
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 maximum value of 887 ± 40 nm in the PZC region. A further pH increase leads to the decrease of the TiO2 hydrodynamic diameter, indicating reversible aggregation. When pH ≥ 8.0, TiO2 NPs exhibit a stable domain with values of z-average hydrodynamic diameter equal to 405 ± 10 nm in good agreement with the SEM image of TiO2, as presented in Figure1b. TiO2 NPs destabilization is observed in a ζ potential range between +20 and −25 mV, as indicated in the gray area ( Figure 1a). As shown in Figure 2a, CeO2 NPs exhibit a stable and positive charge (+21 ± 1 mV) from pH 3.0 to 5.0. Subsequently, ζ potential rapidly decreases to reach the PZC at pHpzc = 6.9 ± 0.1, which is similar to the previously reported studies [31]. An increase of pH leads to charge reversal with ζ potential values equals to -32 ± 2 mV at pH 10 ± 0.1. As illustrated in Figure 2a, the z-average diameter of CeO2 NPs is stable below pH 6.0 with values that are equal to 350 ± 50 nm. Near the PZC region z-average diameter increases until values that are greater than 1.5µm, a further pH increase leads to charge reversal. However, CeO2 aggregates are found to be stable and are not affected by pH increase, as observed for TiO2, since no significant decrease of the z-average diameter is observed after the PZC, which is also confirmed by SEM ( Figure 2b).  The surface charge and stability domain of polystyrene latex nanoplastics versus pH are illustrated in Figure 3a. PS nanoplastics are found to be strongly positively charged and stable at pH < 7.0 with values that are equal to +50 ± 1 mV. By increasing pH, surface charge of nanoplastics decrease to the PZC at pH pzc = 9.9 ± 0.1 as (Figure 3a), which is close to the value that was obtained by Cross et al. [32] with pH pzc = 9.5. Subsequently, charge reversal occurs by further increasing the pH. No aggregation is observed below pH 9.0 and nanoplastics remain stable with a z-average diameter that is equal to 98 ± 2 nm. Such a pH-surface charge behavior is related to the high pKa value of amidine functional group, which is approximately equal to 12 [33]. Aggregation is observed in the PZC region with a maximum z-average diameter value of 1260 ± 190 nm. SEM picture (Figure 3b) indicates that the PS nanoplastics are spherical and highly monodispersed. Strong aggregation is observed around the PZC region (gray domain) in agreement with SEM image (b) SEM image of CeO2 NPs aggregates in ultrapure water at pH > pHpzc.
The surface charge and stability domain of polystyrene latex nanoplastics versus pH are illustrated in Figure3a. PS nanoplastics are found to be strongly positively charged and stable at pH < 7.0 with values that are equal to +50 ± 1 mV. By increasing pH, surface charge of nanoplastics decrease to the PZC at pHpzc = 9.9 ± 0.1 as (Figure 3a), which is close to the value that was obtained by Cross et al. [32] with pHpzc = 9.5. Subsequently, charge reversal occurs by further increasing the pH. No aggregation is observed below pH 9.0 and nanoplastics remain stable with a z-average diameter that is equal to 98 ± 2 nm. Such a pH-surface charge behavior is related to the high pKa value of amidine functional group, which is approximately equal to 12 [33]. Aggregation is observed in the PZC region with a maximum z-average diameter value of 1260 ± 190 nm. SEM picture (Figure3b) indicates that the PS nanoplastics are spherical and highly monodispersed.

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 Geneva waters, such as water hardness and dissolved organic matter concentration on NPs and nanoplastics stability, time-resolved measurements were made to provide information on particle surface charge and z-average diameter variations. Experiments with 10 mg/L NPs and nanoplastic suspensions in five different water samples (Table 1) were conducted to evaluate their stability with time. Ultrapure water was used as a reference system to better explain the changes with time, effects of water hardness, organic matter, and ionic strength in the different waters.

TiO2 Nanoparticles
The stability of TiO2 NPs in bottled mineral waters as a function of time is first considered. The ζ potential and z-average hydrodynamic diameter of TiO2 NPs in MW1, MW2, and MW3 are presented for comparison in Figure 4a. In ultrapure water that was adjusted at the pH of the mineral waters i.e. pH 7.0 ± 0.1, ζ potential and z-average hydrodynamic diameters are found to be stable with time. TiO2 NPs are negatively charged with a ζ potential value equal to -28 ± 3 mV and z-average hydrodynamic diameter in the range of 466 ± 4 nm. The reason of such stability is due to the presence of repulsive forces between negatively charged TiO2 particles.

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 Geneva waters, such as water hardness and dissolved organic matter concentration on NPs and nanoplastics stability, time-resolved measurements were made to provide information on particle surface charge and z-average diameter variations. Experiments with 10 mg/L NPs and nanoplastic suspensions in five different water samples (Table 1) were conducted to evaluate their stability with time. Ultrapure water was used as a reference system to better explain the changes with time, effects of water hardness, organic matter, and ionic strength in the different waters.

TiO 2 Nanoparticles
The stability of TiO 2 NPs in bottled mineral waters as a function of time is first considered. The ζ potential and z-average hydrodynamic diameter of TiO 2 NPs in MW1, MW2, and MW3 are presented for comparison in Figure 4a. In ultrapure water that was adjusted at the pH of the mineral waters i.e., pH 7.0 ± 0.1, ζ potential and z-average hydrodynamic diameters are found to be stable with time. TiO 2 NPs are negatively charged with a ζ potential value equal to −28 ± 3 mV and z-average hydrodynamic diameter in the range of 466 ± 4 nm. The reason of such stability is due to the presence of repulsive forces between negatively charged TiO 2 particles.  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 Ca 2+ and Mg 2+ , 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 hydrodynamic diameters of up to 2.5 µm. 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 (Ca 2+ and Mg 2+ ), but also natural organic matter. As natural organic matter is negatively charged, the Ca 2+ ions are expected to promote the adsorption of NOM and act  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 Ca 2+ and Mg 2+ , is found here to strongly modify the stability of negatively charged TiO 2 NPs via specific adsorption. As shown in Figure 4a, at a low concentration of divalent ions (MW1), TiO 2 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 TiO 2 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 TiO 2 aggregates with z-average hydrodynamic diameters of up to 2.5 µm.
Regarding the stability of TiO 2 NPs in Lake Geneva and drinking water (Figure 5a), a significant decrease in ζ potential is also observed with values equal to −13 ± 1 mV and −8 ± 1 mV, respectively. These values are close to the zeta potential measurements that were made by Loosli et al. [30] and Graham et al. [34] for TiO 2 NPs and natural colloids from the Geneva Lake. Lake Geneva water contains multivalent cations (Ca 2+ and Mg 2+ ), but also natural organic matter. As natural organic matter is negatively charged, the Ca 2+ ions are expected to promote the adsorption of NOM and act as bridges between the NOM and the TiO 2 NPs surface [12,13,35]. As shown in Figure 5b, TiO 2 NPs are stable with time in ultrapure water with z-average diameters that are equal to 460 ± 12 nm. NPs are found to be unstable in surface waters, especially in drinking waters where aggregates are rapidly formed with sizes that are greater than 2000 nm after 60 min. On the other hand, aggregation is less important for NPs in Lake Geneva water with z-average diameters equal to 1550 ± 23 nm after 135 min. This can be mainly explained by the higher DOC concentration in Lake Geneva water when compared to drinking water, as indicated in Table 1. In such conditions, NOM is expected to reduce the impact of Ca 2+ and Mg 2+ ions on the final TiO 2 surface charge, as shown by Loosli et al. [30] 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 hydrodynamic diameters of up to 2.5 µm. 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 (Ca 2+ and Mg 2+ ), but also natural organic matter. As natural organic matter is negatively charged, the Ca 2+ ions are expected to promote the adsorption of NOM and act

CeO 2 Nanoparticles
In Figure 6a, are presented the ζ potential and z-average hydrodynamic diameter variation of CeO 2 NPs in mineral waters as a function of time. The surface charge of CeO 2 NPs in ultrapure water is negative with a ζ potential value that is equal to −7 ± 1 mV. As expected, a decrease in ζ potential value is observed in MWs, which can also be related to water hardness. Indeed, when NPs are added to MW2 and MW3, charge inversion is achieved and surface charge is found to be slightly positive with ζ potential values equal to +2.5 ± 1 mV and +5 ± 1 mV, respectively. Such a behavior indicates strong affinity between the divalent cation and CeO 2 negative surface. Aggregation is observed for all water samples with z-average hydrodynamic diameters greater than 1 µm within the experimental time ( Figure 6b). The highly variable CeO 2 diameters in ultrapure water range from 500 nm to 3000 nm, which is the result of a ζ potential close to zero (−8 mV), and is thus situated in the PZC region. as bridges between the NOM and the TiO2 NPs surface [12,13,35]. As shown in Figure5b, TiO2 NPs are stable with time in ultrapure water with z-average diameters that are equal to 460 ± 12 nm. NPs are found to be unstable in surface waters, especially in drinking waters where aggregates are rapidly formed with sizes that are greater than 2000 nm after 60 minutes. On the other hand, aggregation is less important for NPs in Lake Geneva water with z-average diameters equal to 1550 ± 23 nm after 135 minutes. This can be mainly explained by the higher DOC concentration in Lake Geneva water when compared to drinking water, as indicated in

CeO2 nanoparticles
In Figure 6a, are presented the ζ potential and z-average hydrodynamic diameter variation of CeO2 NPs in mineral waters as a function of time. The surface charge of CeO2 NPs in ultrapure water is negative with a ζ potential value that is equal to -7 ± 1 mV. As expected, a decrease in ζ potential value is observed in MWs, which can also be related to water hardness. Indeed, when NPs are added to MW2 and MW3, charge inversion is achieved and surface charge is found to be slightly positive with ζ potential values equal to +2.5 ± 1mV and +5 ± 1mV, respectively. Such a behavior indicates strong affinity between the divalent cation and CeO2 negative surface. Aggregation is observed for all water samples with z-average hydrodynamic diameters greater than 1 µm within the experimental time (Figure 6b). The highly variable CeO2 diameters in ultrapure water range from 500 nm to 3000 nm, which is the result of a ζ potential close to zero (−8 mV), and is thus situated in the PZC region. 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 ± 1mV and -6 ± 1 mV, respectively. CeO2 NPs are aggregated in all water samples, as shown in Figure 7b. The results for CeO 2 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 −6 ± 1 mV, respectively. CeO 2 NPs are aggregated in all water samples, as shown in Figure 7b.
As discussed for TiO 2 NPs, the adsorption of divalent cations are also expected here to interact with CeO 2 NPs and NOM, forming bridges between them, resulting in NPs destabilization. As shown for drinking water, 0.4 mg/L DOC lead to CeO 2 NPs aggregation with z-average diameters that are greater than 2 µm. However, limited aggregation is observed for NPs in Lake Geneva water 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 Oriekhova and Stoll [31]. The authors found CeO 2 NPs aggregated in Lake Geneva water with time and negative surface charge with a zeta potential equal to −13.3 ± 0.6 mV. As discussed for TiO2 NPs, the adsorption of divalent cations are also expected here to interact with CeO2 NPs and NOM, forming bridges between them, resulting in NPs destabilization. As 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 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 Oriekhova and Stoll [31]. The authors found CeO2 NPs aggregated in Lake Geneva water with time and negative surface charge with a zeta potential equal to −13.3 ± 0.6 mV.

Polystyrene nanoplastic particles
Nanoplastics behavior in mineral waters is presented in Figure8 and a comparison is made with ultrapure water. In ultrapure water, the surface charge is found to be stable and PS nanoplastics are strongly positively charged (+ 41 ± 3 mV), with z-average hydrodynamic diameter values that are equal 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.) Despite the decrease of ζ potential, nanoplastics are stable in almost all cases and no aggregation is observed due to the positive amidine functional groups, making nanoplastics resistant in MW1 and MW2 to aggregation effects of divalent ions and the presence of NOM.

Polystyrene Nanoplastic Particles
Nanoplastics behavior in mineral waters is presented in Figure 8 and a comparison is made with ultrapure water. In ultrapure water, the surface charge is found to be stable and PS nanoplastics are strongly positively charged (+41 ± 3 mV), with z-average hydrodynamic diameter values that are equal 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). Despite the decrease of ζ potential, nanoplastics are stable in almost all cases and no aggregation is observed due to the positive amidine functional groups, making nanoplastics resistant in MW1 and MW2 to aggregation effects of divalent ions and the presence of NOM. As discussed for TiO2 NPs, the adsorption of divalent cations are also expected here to interact with CeO2 NPs and NOM, forming bridges between them, resulting in NPs destabilization. As 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 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 Oriekhova and Stoll [31]. The authors found CeO2 NPs aggregated in Lake Geneva water with time and negative surface charge with a zeta potential equal to −13.3 ± 0.6 mV.

Polystyrene nanoplastic particles
Nanoplastics behavior in mineral waters is presented in Figure8 and a comparison is made with ultrapure water. In ultrapure water, the surface charge is found to be stable and PS nanoplastics are strongly positively charged (+ 41 ± 3 mV), with z-average hydrodynamic diameter values that are equal 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.) Despite the decrease of ζ potential, nanoplastics are stable in almost all cases and no aggregation is observed due to the positive amidine functional groups, making nanoplastics resistant in MW1 and MW2 to aggregation effects of divalent ions and the presence of NOM. 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 potential is observed. As can be seen in Figure 9a, the surface charge of nanoplastics in drinking water significantly decreases but remains positive at about +10 mV, resulting in aggregation, as indicated in Figure 9b.
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 potential is observed. As can be seen in Figure 9a, the surface charge of nanoplastics in drinking water significantly decreases but remains positive at about +10 mV, resulting in aggregation, as indicated in Figure 9b.
In Lake Geneva water, high concentrations of NOM lead to charge reversal (-13 ± 1 mV), and nanoplastics 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 stabilization effects. NOM coating is illustrated from a morphological point of view in the SEM image that is presented in Figure 10. PS nanoplastic shapes are no more spherical, butslightly ellipsoidal and fuzzy membranes surrounding the particles are producing less contrasted images. . 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. In Lake Geneva water, high concentrations of NOM lead to charge reversal (−13 ± 1 mV), and nanoplastics 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 stabilization effects. NOM coating is illustrated from a morphological point of view in the SEM image that is presented in Figure 10. PS nanoplastic shapes are no more spherical, butslightly ellipsoidal and fuzzy membranes surrounding the particles are producing less contrasted images. 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 potential is observed. As can be seen in Figure 9a, the surface charge of nanoplastics in drinking water significantly decreases but remains positive at about +10 mV, resulting in aggregation, as indicated in Figure 9b.
In Lake Geneva water, high concentrations of NOM lead to charge reversal (-13 ± 1 mV), and nanoplastics 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 stabilization effects. NOM coating is illustrated from a morphological point of view in the SEM image that is presented in Figure 10. PS nanoplastic shapes are no more spherical, butslightly ellipsoidal and fuzzy membranes surrounding the particles are producing less contrasted images. . 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 3b) where, pH > pH pzc . PS nanoplastic aggregates are found coated with a complex organic matter matrix resulting in a fuzzy particle contrast.

Impact of Water Hardness and Organic Matter
To better highlight the influence of both water hardness and NOM on the aggregation behavior of NPs and nanoplastics, stability diagrams were calculated from the z-average hydrodynamic diameter values that were obtained in the different conditions, and they are presented in Figure 11 for TiO 2 and CeO 2 NPs, and nanoplastics. All of the results dealing with aggregate formation obtained in MWs, drinking and Lake Geneva waters have merged together. Red color indicates that NPs are dispersed, whereas blue color indicates strong aggregation i.e., the formation of aggregates with sizes that are greater than 2000 nm. For TiO 2 NPs, aggregation is found to be promoted by increasing water hardness so long as the DOC concentration is less than 0.6 mg/L. This indicates that divalent ions, such as Ca 2+ and Mg 2+ , are effective in neutralizing TiO 2 NPs negative surface charge. As TiO 2 NPs are strongly negatively charged at environmental pH, Ca 2+ and Mg 2+ are strongly adsorbed onto TiO 2 due to the attractive electrostatic forces between NPs and divalent cations, resulting in TiO 2 NPs charge neutralization. Increasing DOC concentration is found to reduce the aggregation of TiO 2 NPs, which is probably due to the formation of Ca 2+ and Mg 2+ complexes reducing the impact of these ions on the TiO 2 surface charge. On the other hand, water hardness is found to have a less significant role on CeO 2 NPs aggregation. At environmental pH, the surface charge of CeO 2 NPs is close to the PZC (pH pzc = 6.9 ± 0.1) and, consequently, in most conditions, NPs are already aggregated. The stability of CeO 2 NPs with regards to water hardness is caused by the repulsive forces between NPs and divalent ions (Ca 2+ and Mg 2+ ). DOC increase (above 0.8 mg/L) is also found to reduce aggregation. On the other hand, polystyrene nanoplastics, which are strongly positively charged at environmental pH, are found to be more stable with regards to water hardness and natural organic matter. PS nanoplastics are stabilized against water hardness due to electrostatic repulsive interaction between the amidine functional group and ions Ca 2+ and Mg 2+ . Negatively charged NOM can adsorb onto positively charged NPs promoting aggregation by charge neutralization and via bridging mechanisms that are induced by the presence of divalent cations [33]. However, PS nanoplastics aggregation is observed in a limited domain with specific water hardness and DOC. This is an important issue, indicating that, in general, the polystyrene nanoplastics stability and dispersion state are more pronounced in aquatic systems.

Impact of Water Hardness and Organic mMatter
To better highlight the influence of both water hardness and NOM on the aggregation behavior of NPs and nanoplastics, stability diagrams were calculated from the z-average hydrodynamic diameter values that were obtained in the different conditions, and they are presented in Figure 11 for TiO2 and CeO2 NPs, and nanoplastics. All of the results dealing with aggregate formation obtained in MWs, drinking and Lake Geneva waters have merged together. Red color indicates that NPs are dispersed, whereas blue color indicates strong aggregation i.e. the formation of aggregates with sizes that are greater than 2000 nm. For TiO2 NPs, aggregation is found to be promoted by increasing water hardness so long as the DOC concentration is less than 0.6 mg/L. This indicates that divalent ions, such as Ca 2+ and Mg 2+ , are effective in neutralizing TiO2 NPs negative surface charge. As TiO2 NPs are strongly negatively charged at environmental pH, Ca 2+ and Mg 2+ are strongly adsorbed onto TiO2 due to the attractive electrostatic forces between NPs and divalent cations, resulting in TiO2 NPs charge neutralization. Increasing DOC concentration is found to reduce the aggregation of TiO2 NPs, which is probably due to the formation of Ca 2+ and Mg 2+ complexes reducing the impact of these ions on the TiO2 surface charge. On the other hand, water hardness is found to have a less significant role on CeO2 NPs aggregation. At environmental pH, the surface charge of CeO2 NPs is close to the PZC (pHpzc = 6.9 ± 0.1) and, consequently, in most conditions, NPs are already aggregated. The stability of CeO2 NPs with regards to water hardness is caused by the repulsive forces between NPs and divalent ions (Ca 2+ and Mg 2+ ). DOC increase (above 0.8 mg/L) is also found to reduce aggregation. On the other hand, polystyrene nanoplastics, which are strongly positively charged at environmental pH, are found to be more stable with regards to water hardness and natural organic matter. PS nanoplastics are stabilized against water hardness due to electrostatic repulsive interaction between the amidine functional group and ions Ca 2+ and Mg 2+ . Negatively charged NOM can adsorb onto positively charged NPs promoting aggregation by charge neutralization and via bridging mechanisms that are induced by the presence of divalent cations [33]. However, PS nanoplastics aggregation is observed in a limited domain with specific water hardness and DOC. This is an important issue, indicating that, in general, the polystyrene nanoplastics stability and dispersion state are more pronounced in aquatic systems. Figure 11. Aggregation intensity diagrams of TiO2, CeO2, and amidine PS nanoplastic particles as a function of water hardness and Dissolved Organic Carbon (DOC) in MW and SW at a 10 mg/L TiO2, CeO2 and PS nanoplastic concentration. The color and respective values in the legend represent the zaverage diameter of NPs and PS nanoplastic particles in nanometers that were obtained after 1h. Blue color indicates significant aggregation, whereas red color indicates limited aggregation.

Conclusions
In this study, the importance of several relevant environmental parameters on the stability of NPs and PS nanoplastics in various waters was evaluated. In ultrapure water, TiO2 and PS 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 Ca 2+

Conclusions
In this study, the importance of several relevant environmental parameters on the stability of NPs and PS nanoplastics in various waters was evaluated. In ultrapure water, TiO 2 and PS nanoplastic were found stable, while CeO 2 NPs was found to aggregate with time. In bottled mineral waters, TiO 2 and CeO 2 NPs were found to be aggregated. The presence of divalent ions, such as Ca 2+ and Mg 2+ , 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.