Removal of a mixture of metal nanoparticles from natural surface waters using traditional coagulation process

Highlights

• Traditional coagulation process is effective in the removal of metal-based ENPs mixture.

• Removal above 93% of TiO₂, Ag and CuO NPs were achieved by C/F/S.

• Treated water ENPs concentrations depend on water characteristics and type of ENPs.

• ENPs did not hampered the natural treated quality for turbidity and NOM.

Abstract

Engineered nanoparticles (ENPs) present in natural water systems and wastewater treatment plants, due to their wide use in industrial products and consumer goods, represent a hazard to human health, especially if occur in surface waters used for human consumption. Different nanoparticles may co-exist in the natural waters and until now no works have been developed on the impact of the co-existing nanoparticles on water treatment processes. Thus, this work evaluates the removal of different co-existing metal-based ENPs (TiO₂, Ag and CuO) from natural surface waters applying the most used water treatment process, coagulation/flocculation/sedimentation (C/F/S). Results showed that C/F/S is effective in the removal of the co-existing ENPs from the studied hydrophilic natural waters (low/medium turbidity and moderate/moderate-high natural organic matter content) with efficiencies above 93% for TiO₂, Ag and CuO nanoparticles. The formation of destabilised nanoparticle-organic matter aggregates in C/F promoted their precipitation. For the lowest turbidity and organic content water, Ti presented the lowest concentration, followed by Ag and Cu, because of titanium properties. For the highest turbidity and organic content water, the residual Cu concentration was the lowest and Ag the highest, due to the natural water characteristics. Water characteristics played an important role on the coagulant demand and removal of ENPs mixture by C/F/S. Results also demonstrated that ENPs mixture did not hampered the treated water quality for turbidity and natural organic matter. For the lowest turbidity and natural organic matter water, ENPs appear to contribute to a higher removal of these parameters.

Introduction

Over the past decades, widespread nanotechnologically-enabled materials have raised concerns about environmental protection, industrial and municipal wastewater treatment, and drinking water production [1]. These compounds or products are widely applied in distinct areas such as medicine, pharmaceutics, cosmetics, textiles, agriculture, energy, electronics or environment [2,3], due to their tailoring potential for specific applications [4]. However, waste streams, discharges of inadequately treated wastewaters, runoff, and releases from contaminated sludge placed in landfills, applied as fertiliser or dumped in aquatic environments, are important causes of surface water contamination by engineered nanoparticles (ENPs) [5,6]. ENPs have already been detected in drinking water sources and even in tap water at concentrations in ng/L and μg/L range making it a potential risk for humans [[7], [8], [9]]. ENPs appellative properties, such as the large surface area to volume ratio and small size, provide unique materials with new applications compared to the corresponding bulk materials. However, these same properties may also result in the enhancement of the bioavailability, increasing their toxicity and becoming a threat not only for the well-being of the aquatic organisms but also to the security of sources used for drinking water production [[10], [11], [12], [13]]. The ingestion of ENPs through drinking water may pose a direct hazard to human health or an indirect risk due to the release of trace metal ions [14]. Exposure to ingestible metal ENPs or metal ions released can result in adverse effects, such as kidney damage, increased blood pressure, gastrointestinal inflammation, neurological damage and cancerous implications [[15], [16], [17]].

Therefore, the presence of ENPs in raw waters raises the question of whether current drinking water treatment plants (DWTPs) are prepared to handle this problem being imperative to evaluate their removal effectiveness. Conventional coagulation/flocculation/sedimentation (C/F/S) is an important water treatment process dedicated to remove small particles, suspended solids, natural organic matter (NOM), microorganisms/pathogens and some inorganic compounds. Thus, it may also be a valuable process to remove nanoparticles. Earlier studies already investigated the removal of ENPs (namely ZnO, Ag, and TiO₂) by C/F/S treatment applying aluminium and iron-based coagulants to different types of contaminated waters [14,[18], [19], [20], [21], [22]]. Although some ENPs removal had been observed, efficiency rates showed high variability. Removals for TiO₂ ENPs ranged approximately between 32 and 99% [14,19,19,20,21,22], Ag ENPs from approximately 79 to 99% [14,18] and ZnO ENPs between 0.46 and 99% [14,22]. The results also demonstrated that ENPs removal by C/F/S depends largely on the type of ENP, the dosage and type of coagulant/flocculant, and the water matrix, but little is known about the removal mechanisms. Moreover, the mentioned studies used mostly synthetic waters, for each ENPs individually, optimizing coagulant dose for turbidity and NOM removal. Therefore, the present work aims to go further by evaluating the ability of C/F/S to remove a mixture of ENPs from natural surface waters. The focus was on the optimization of coagulant dose for maximal nanoparticles removal, while maintaining a sufficient turbidity and NOM removal efficiencies. Thus, the removal of co-existing ENPs mixture and dispersed in natural surface waters was investigated, since in natural aquatic systems different ENPs co-exist. The aims of this study are: (1) evaluate the removal efficiency of different co-existing ENPs in natural waters by C/F/S, (2) understand the ENPs removal behaviours and mechanisms when different nanoparticles are present in water, (3) assess the water quality parameters that may affect or be affected by the ENPs mixture removal during C/F/S, mainly turbidity and NOM, and (4) explore the C/F/S overall performance in the simultaneous presence of different ENPs. Three commercial metal-based ENPs were tested (Ag, TiO₂ and CuO NPs). TiO₂ and Ag ENPs were chosen because the former are among the most produced nanomaterials and the latter are globally applied to a larger number of consumer products [23]. CuO ENPs were chosen because they are currently emerging as solution for water treatment [[24], [25], [26], [27]]. Two natural waters from dams currently used to public supply were chosen based on turbidity and NOM.