*2.3. Experimental Conditions*

Dissolution tests were performed using spherical 80 nm citrate-coated Ag NPs (NanoXactTM) obtained from Nanocomposix, San Diego, CA, USA (JRD0035). In order to assess the feasibility of the flow-through dissolution method, a set of experiments were performed at pH 5 with 10 mM NaNO3 (Merck, Darmstadt, Germany) as exposure medium. These experiments were performed in order (1) to define optimal experimental parameters for the test and (2) to investigate the influence of extrinsic parameters (i.e., injection velocity, flow rate and ENMs loading). Table 1 summarizes the experimental conditions tested for the dissolution experiments. Ag\_0.2mL/min\_2.2μg\_NaNO3 and Ag\_0.2mL/min\_8.2μg\_NaNO3 were performed with 2.2 and 8.2 μg of Ag NPs loaded onto the filter, corresponding to 2.99 % and 0.91 % of the filter area covered by Ag NPs, respectively. For these experiments, the injection flow rate was set to 1 mL.min−<sup>1</sup> during 1 h. After injection, the flow rate was reduced to 0.2 mL.min−<sup>1</sup> until the end of the experiment. Ag\_0.5mL/min\_8.2μg\_NaNO3 was performed with 8.2 μg of Ag NPs loaded onto the membrane at a flow rate of 0.5 mL.min−1, constant during all the time of the experiment. Ag\_0.2mL/min\_2.2μg\_NaNO3, Ag\_0.2mL/min\_8.2μg\_NaNO3 and Ag\_0.5mL/min\_8.2μg\_NaNO3 were performed in duplicates. All other experiments were performed with an Ag NPs loading of 8.2 μg and a constant flow rate of 0.5 mL.min−1. For the latter, 2 mM Ca(NO3)2 and 0.5 mM MgSO4 solutions (Merck, Darmstadt, Germany) were used in order to have a Ca2+/Mg2+ ratio of 4:1, as recommended by the OECD guideline 318 [33]. To simulate acidic surface waters (ASW), Ag\_0.5mL/min\_8.2μg\_ASW was performed at pH 5. The pH of the eluent was adjusted using 0.2 M HNO3. To simulate nearneutral surface waters (NSW), Ag\_0.5mL/min\_8.2μg\_NSW was performed at pH 7.5 by adding 5 mM NaHCO3 − to the Ca(NO3)2-MgSO4 solution, acting as pH buffer. As part of this work, no DOC and orthophosphate were included, the aim being to show a proof of concept of the continuous flow dissolution setup instead of discussing the impact of ligands on the dissolution kinetic of Ag NPs.

**Table 1.** Experimental parameters and media composition used to perform the flow-through experiments.


1 the Ag NPs concentration injected was determined by acid digestion of 500 μL of the 20 mg.L−<sup>1</sup> Ag NPs stock solution. For Ag\_0.2mL/min\_2.2μg\_NaNO3, the volume of the 20 mg.L−<sup>1</sup> Ag NPs stock solution used to prepare the 5 mg.L−<sup>1</sup> solution was acid digested and Ag NPs concentration was thus assume from the dilution factor.

### *2.4. Samples Measurement and Data Treatment*

For all experiments, samples were taken at 5, 10, 15, or 20 min intervals directly into 15 mL PP vials, acidified with 20 μL of 65%HNO3 (AnalaR Normapur®, VWR, Austria), and analyzed by inductively coupled plasma-mass spectrometry (Agilent 7900 ICP-MS, Agilent Technologies, Tokyo, Japan) for dissolved Ag concentrations. The ICP-MS was equipped with a Ni cone, a MicroMist nebulizer and a scott-type double pass spray chamber. It was

operated on standard mode, under continuous Ar gas flow (UHP, Nebulizer gas flow rate at 0.8 L.min−1; Dilution gas flow rate of 0.4 L.min−1). The two stable isotopes 107Ag and 109Ag were measured for higher accuracy, and Rhodium (Rh, solution of 10 μg.L−1) was used as internal standard. The ICP-MS was calibrated with dissolved Ag standards ranging from 5 ng.L−<sup>1</sup> to 50 μg.L−<sup>1</sup> prepared from a single-element Ag standard (1000 μg.mL−1; CGAG1 −125 ml, Inorganic Ventures, Christiansburg, VA, USA) diluted with 2% HNO3. The acid blanks for all measurement times show an averaged limit of detection (LOD; 3× standard deviation + mean) of 0.05 μg.mL−1and an averaged limit of quantification (LOQ; 10× standard deviation + mean) of 0.12 μg.mL−1. Measurements were carried out in triplicates.

ExperimentAg\_0.2mL/min\_2.2μg\_NaNO3 wasperformed for 5h. Ag\_0.2mL/min\_8.2μg\_NaNO3, Ag\_0.5mL/min\_8.2μg\_NaNO3, Ag\_0.5mL/min\_8.2μg\_ASW and Ag\_0.5mL/min\_8.2μg\_NSW were performed for 8 h. For each experiment, apparent dissolution rates *k* were calculated from the outflow concentrations using Equation (1):

$$k = \text{[Ag]}\_{\text{outlet}} \times \text{F/SA} \tag{1}$$

where *k* in μg.s<sup>−</sup>1.m−2, [Ag]outlet the Ag concentration measured at the outlet in μg.mL−1, F the flow rate of the eluent in mL.s−1, and SA the combined surface area of the NPs deposited on the membrane in m2. SA was determined considering the surface area of an 80 nm diameter spherical NP, NPSA and the total number of NPs deposited onto the membrane, NPsmembrane (SA = NPSA × NPsmembrane).
