Impact of a nanofiltration system on microplastic contamination in Geneva groundwater (Switzerland)

Microplastics (MPs) have been observed in the oceans, fresh waters, karstic water and remote water bodies. However, little is known on groundwater contamination, which is a natural resource of utmost importance for millions of people and is often perceived as a reliable source of water. Moreover, nanofiltration is perceived as a reliable technology to remove contaminants from water. In this study, large sample volumes of a silty-sandy gravel aquifer and the corresponding nanofiltered water were analysed for the presence of MPs (> 20 µm) using Fourier transform infrared (FTIR) microscopy. Concentration in ground water was 8 ± 7 MPs/m3 and increased to 36 ± 11 MPs/m3 in nanofiltered water. All MPs had a maximum Ferret diameter lower than 500 µm. Size distribution of MPs was towards the small size class (20-50 µm). In groundwater, 33% of MPs were detected in the smallest size class (20-50 µm) and 67% in the 50-100-µm-size class. In comparison, around 52% of MPs in nanofiltered water were observed in the 20-50 µm size class. Moreover, 33% of the MPs observed in nanofiltered water were in the 50-100 µm size class and 15% in the 100-500-µm-size class. From a chemical point of view, different plastic polymers were identified in groundwater and in nanofiltered water, such as polypropylene (PP), polyvinyl chloride (PVC), ethylene (vinyl acetate) copolymer (EVA), polyethylene (PE), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA) and other polymer materials (such as polystyrene-based copolymers, vinyl-based copolymers). Fibres were observed in all samples, but only a small number of fibres (near 1%) were identified as PP synthetic fibres in nanofiltered water. Furthermore, no clear difference of fibre concentrations was observed between groundwater (232 ± 127 fibres/m3) and nanofiltered water (247 ± 118 fibres/m3). Groundwater had extremely low levels of microplastics, and although the nanofiltration effectively removes suspended particulate matter, it slightly contaminates the filtered water with MPs.

Contamination and removal efficiency of microplastics and synthetic fibres in a conventional drinking water treatment plant in Geneva, Switzerland

Although it is known that freshwater resources are contaminated with microplastics (MPs), still limited information is known about the efficiency of large drinking water treatment plants (DWTP) to remove microplastics. Moreover, reported concentrations of MPs in drinking water variates from some units to thousands of units per litre and the sampling volumes used for MPs analysis are generally heterogeneous and limited. The present study evaluates the removal of MPs and synthetic fibres in the main DWTP of Geneva, Switzerland, by considering large sampling volumes at different time intervals. Furthermore, contrary to other studies, this DWTP does not count with a clarification process before sand filtration and coagulated water is sent directly to sand filtration. In this study a distinction is made between microplastics as fragments, films, pellets, and synthetic fibres. Raw water and effluents of each filtering mass (sand and activated carbon filtration) are analysed for the presence of MPs and synthetic fibres with sizes ≥63 μm using infrared spectroscopy. Concentrations of MPs in raw water range from 25.7 to 55.6 MPs/m³ and in treated water from 0 to 4 MPs/m³, respectively. Results show that 70 % of MPs are retained during sand filtration and total removal is equal to 97 % in treated water after activated carbon filtration. Concentration of identified synthetic fibres is low (average value of 2 synthetic fibres/m³) and constant in all steps of water treatment. Chemical composition of microplastics and synthetic fibres is found more heterogeneous in raw water than after sand filtration and activated carbon filtration, indicating the persistence of some types of plastics (like polyethylene and polyethylene terephthalate) in water treatment processes. Heterogeneities in MP concentrations are observed from one sampling campaign to another, indicating significant variations of MP concentrations in raw water.

Removal efficiency and adsorption mechanisms of CeO2 nanoparticles onto granular activated carbon used in drinking water treatment plants

The presence of NPs in drinking water resources raises a global concern on their potential risk for human health, and whether or not drinking water treatment plants are able to effectively remove NPs to prevent their ingestion by humans. In this study, we investigate the efficiency of granular activated carbon (GAC), commonly used in conventional municipal water treatment processes, for the removal of CeO₂ NPs. In ultrapure water, NPs are found to have a good affinity for GAC and results indicate an increase in the adsorption capacity from 0.62 ± 0.10 to 5.05 ± 0.51 mg/g, and removal efficiency from 35 % ± 4 to 54 % ± 5 with increasing NPs concentration. Kinetic studies reveal that intraparticle diffusion is not the only rate controlling step indicating that mass transfer effect is also playing a role. Adsorption mechanisms are mainly controlled by the electrostatic attractions between the positively charged NPs and negatively charged GAC. Although electrostatic conditions in Lake Geneva water are less favorable for NPs adsorption, the adsorption capacity and removal efficiency are higher than in ultrapure water with values raising from 0.41 ± 0.17 to 7.13 ± 1.13 mg/g and 26 % ± 8 to 75 % ± 11, respectively. Furthermore, the external mass transfer process onto GAC surface is more important than for ultrapure water. NPs adsorption mechanism is explained by the presence of divalent cations and natural organic matter (NOM) which promote the formation of CeO₂ NPs-NOM-divalent cation heteroaggregates increasing both adsorption and removal efficiency by cation bridging.

Fate and removal efficiency of polystyrene nanoplastics in a pilot drinking water treatment plant

Occurrence of microplastics and nanoplastics in aquatic systems, as well as in water compartments used to produce drinking water have become a major concern due to their impact on the environment and public health. Nanoplastics in particular, in regard to their fate and removal efficiency in drinking water treatment plants (DWTP), which ensure water quality and supply drinking water for human consumption have been, by far, rarely investigated. This study investigates the removal efficiency of polystyrene (PS) nanoplastics in a conventional water treatment plant providing drinking water for 500'000 consumers. For that purpose, a pilot-scale DWTP, located within the main treatment plant station, reproducing at a reduced scale the different processes and conditions of the main treatment plant is used. The results show that filtration process through sand and granular activated carbon (GAC) filters in the absence of coagulation achieves an overall nanoplastic removal of 88.1%. The removal efficiency of filtration processes is mainly attributed to physical retention and adsorption mechanisms. On the other hand, it is found that coagulation process greatly improves the removal efficiency of nanoplastics with a global removal efficiency equal to 99.4%. The effective removal efficiency of sand filtration increases considerably from 54.3% to 99.2% in the presence of coagulant, indicating that most of PS nanoplastics are removed during sand filtration process. The higher removal efficiency with the addition of coagulant is related to nanoplastics surface charge reduction and aggregation thus significantly increasing their retention in the filter media.

Nanoplastics adsorption and removal efficiency by granular activated carbon used in drinking water treatment process

In this study Granular Activated Carbon (GAC) used in drinking water treatment processes is evaluated for its capacity to adsorb and remove polystyrene (PS) nanoplastics. Batch experiments are conducted in ultrapure and surface water from Lake Geneva, currently used as drinking water resources. Equilibrium and kinetic studies are conducted to understand adsorption mechanisms and limiting factors. Our results show that in ultrapure water the adsorption and removal of PS nanoplastics are mainly due to electrostatic interactions between the positively charged nanoplastics and negatively charged GAC. It is found that the adsorption capacity increases with nanoplastic concentration with a maximum adsorption capacity of 2.20 mg/g. The adsorption kinetics follows a pseudo-second-order model and indicates that the intra-particle diffusion is not the only rate-controlling step. The Langmuir isotherm indicates that nanoplastics are adsorbed as a homogeneous monolayer onto the GAC surface with a maximum monolayer adsorption capacity of 2.15 mg/g in agreement with the experimental value. In Lake Geneva water the adsorption capacity and removal efficiency of PS nanoplastics are found three times higher than in ultrapure water and increase significantly with increasing PS nanoplastics concentration with a maximum adsorption capacity of 6.33 mg/g. This improvement in adsorption capacity is due to the presence of Dissolved Organic Matter (DOM), resulting in PS surface charge modification, presence of divalent ions making possible the adsorption of PS-DOM complexes, and, aggregation of PS nanoplastics. The kinetic pseudo-second-order and intra-particle diffusion provide a good correlation with the experimental data. In contrast, neither Langmuir nor Freundlich isotherms describe in a satisfactory way the adsorption of nanoplastics by GAC. This study reveals that GAC produced from renewable sources can be considered as a moderate adsorbent for the removal of PS nanoplastics in water treatment plants and that the presence of DOM and cationic species play a major role.

Coagulation of TiO2 , CeO2 nanoparticles, and polystyrene nanoplastics in bottled mineral and surface waters. Effect of water properties, coagulant type, and dosage

Intensive use of engineered nanoparticles (NPs) results in their release into aquatic systems and consequently into drinking water resources. Therefore, it is important to evaluate how NPs can be effectively removed through water treatment processes, such as coagulation, to control environmental and health risks associated with NP exposure. This work investigates the effect of two conventional coagulants, polyaluminum chloride (PACl) and iron chloride (FeCl₃ ), on NPs. Three bottled mineral and Lake Geneva waters, currently used as drinking water resources, were considered to get an insight into coagulation efficiency. TiO₂ , CeO₂ NPs, and polystyrene (PS) nanoplastics were selected, owing to their large number of applications and contrasting surface charge and aggregation behavior at environmental pH. Our findings indicate that PACl is more efficient compared with FeCl₃ since lower dosages are required to coagulate all nanoparticles. On the other hand, nanoplastic coagulation is found less efficient compared with TiO₂ and CeO₂ NPs. This is an important outcome indicating that nanoplastic stability and dispersion state will be more pronounced and therefore more challenging to eliminate. Results highlight the key role of NP and PS nanoplastic surface charge, as well as water properties, coagulant type, and dosage on nanoparticle elimination from aquatic systems. PRACTITIONER POINTS: pH, water hardness, and NOM are playing roles in final coagulant dosage concentration. PACl is more efficient than FeCl₃ in most conditions. Positively charged nanoplastics are more difficult to eliminate by coagulation. NP surface properties in bottled mineral and surface waters are controlled by pH, divalent cations, and NOM. NP surface charge and coagulation efficiency depend on water properties.