Holistic Strategy to Study Nanoparticles and Metallic Trace Elements in Surface Waters

Keywords: Nanoparticle, Metallic Trace Elements, Surface Water, In-situ sensors, Analytical, Colloids, Dissolved, Labile, Concept, Holistic, Aggregation, Disaggregation 1 Introduction Nanoparticles (NPs) and Emerging Contaminants (ECs) are a real preoccupation both for scientific, industry, environmental agencies, political authorities as well as the public society. A debate about the advantages and disadvantages of NPs started some years ago. Our purpose it not to discuss of this but evaluate NPs and ECs trends, when they are disseminated in the biosphere. Just some facts concerning the NPs: the global nanotechnology market has doubled between 2011 ($20.1 billion) to 2016 ($39.2 billion) [1]. Global consumption of nanocomposites was estimated at 118768 tons for a value of over $800 million in 2010. By 2016, the market reached around 308,000 tons for a value of $1.6 billion [2]. Recent data about fine particles (size < 1 µm) in the atmosphere reveal high impact on the human health. With 7 million premature deaths, including almost 600 000 for Europe in 2012, the global cost for WHO Europe is estimated for the year 2010 at $1575 trillion [3]. According to the UN, 23 m3/s of wastewater is discharged directly into the environment. In developing countries, 90% of wastewater is released into the natural environment without any treatment. According to the WHO, 3.4 million people die each year from aquatic pollution. With 3.1% of deaths in the world is explained by poor water quality in terms of sanitation and hygiene, it should be related to the 2.6 billion people without sanitary facilities. If the main source of water contamination is due to biological form (fecal peril), the exposure to "emerging" contaminants, become a societal problem. Indeed, in the case of Metallic Trace Elements (MTE), it is a diffuse long-term pollution with bio-accumulative effects throughout the trophic chain. The presence of NPs in waste stream is a fact which increases each year [4]. Despite abundant literature on NPs, their effects on cells and DNA remains largely undervalued. Producers communicate little about the impact of their products on the living and our feedback is still scarce. Some authors suggest significant effects on health and the environment [5]. The main difficulties related to the study of MTE and NPs in surface waters are threefold: their sampling and detection, their qualitative and quantitative characterization and the interpretation of the data according to which concept(s)? Our objective is to propose a holistic approach covering sampling, analysis and concept(s). Sampling: A crucial point due to the representativeness of the samples with respect to water mass (notion of flux) whose matrix integrates organic matter, chemical elements (from the major to the trace) ligands, biological compounds and contaminants such as MTEs and NPs? Analysis: Once a sample is obtained, how to analyze with minimum perturbation (fractionation, metastable complexes over time, endogenous reaction between the different components, etc.)? What set of parameters are necessary for understanding the fate of TMEs and NPs in the water column? Concept(s): or approaches should be holistic, which means being able to describe the main scenario allowing consideration of the main or principal parameters controlling the fate and behavior of MTE and NPs. Holistic approach is not related only to a discipline but it is a global way integrating and updating methods, concepts and tools to investigate novel materials, compounds in all the human and natural compartments. In this paper we will describe sampling strategies, methods and tools dedicated to MTEs and NPs analysis. It consists of sampling devices adapted to the specific target elements, user-friendly and easy to analyze using a dedicated analytical platform that is fit to the physico-chemical speciation approach. By using such conceptual approach, we are able to measure important characteristics controlling of the MTE and NPs concentrations, properties, behavior, transformation, and potential impact in the environment [7]. By considering continental waters as a complex and dynamic matrix including a mixture of particles, colloids and dissolved species, aggregation/ disaggregation and NPs surface coating must be apprehended to explain the evolution of MTE and NPs into this system [8]. Their comportments identified it should be possible to determine a soft, green and efficient treatment of contaminated waters 2 Materials and Methods 2.1 Sampling strategy Transport and fate of metallic trace elements (MTE) and nanoparticles (NPs) require sophisticated tools and approaches. Meanwhile the concepts have to be defined especially for surface water quality assessment. Classic approaches are based on total fraction and filtrated ones (< 0.45 µm). A bias is often observed by considering the filtrated fraction equivalent to dissolved one. In fact it recovers a part of particle, colloidal, nanoparticulate and dissolved fractions. To precise which fraction carries traces metals or nanoparticles, it is crucial to use another protocol. Few studies are based on ultra-filtration (by-passed at 0.01 µm) allowing true dissolved fraction separation; and so what about the other fractions able to trap contaminants? Another problem encountered by hydro-geochemist is the transfer from a fraction to another one. This phenomenon is called colloidal pumping [8] occurring in all surface waters when physical and/or chemical parameters change such as NOM concentration, pH, Ionic strength [9] or by the influence of biological activities induced by bacteria and micro-organisms [10, 11]. In such case, it is fundamental to work onto samples well identified in term of fractions. First strategy is based on separative techniques: passive (filtration, centrifugation) or dynamic (field flow field fractionation or 4F) to perform analysis on “perfectly” identified fractions. One limitation by operating with separative techniques resides into the possibility (or not) to identify the relationships between traces elements, nanoparticles and colloids and particles. On the over hand, whole-sample analysis is difficult due to the extrema heterogeneity and complexity of environmental samples, in term of size, chemical composition, etc. The role and impact of NOM in determining the fate and transport of emergent pollutants [12], trace metals [13, 14] and nanoparticles is highly important [15]. If metals or nanoparticles concentrations in total fraction can be done more or less in a user-friendly manner, to getting the information of on the labile or dissolved fraction is more complex. Sample preparation protocols differ and can induce some bias. Generally, to access to the labile trace metal fraction, it is recommended to add some drops of acid (HCl, HNO₃, mix HCl-HNO₃). Some authors [16] recommend digestion activation under microwave, sonic agitation etc. All of them are empirical and nor necessary fully validated to do what they are claimed to do. For example, to get MTEs, NPs into the total fractions of water exhibiting NOM high concentration, it is recommended to destroy organic part of the NOM by addition of hydroxide peroxide (H ₂O₂) followed by acid addition and microwave digestion. In such case what really do we measure? Are we sure to determine to extract the real true dissolved fraction, called dissolved or labile? What is the signification of MTEs and NPs obtained on the total fraction: how to discriminate neogenic from anthropic origin? In fact, it becomes hard to know exactly, on many studies, what has been done. Thus, studies without errors estimation should be rejected. On other hand we have to inform in detail the protocol used and to present data with internal errors based on standard deviation (sd), percentage (±x %), confidence interval, etc. As soon as the same protocol has been used, it is not so much important (depending of the purpose of course) to get “exact” values but to inform about the values plus error to be able to determine or precise the trends of the trace metals, nanoparticles concentration evolution into water matrix. The rules and protocols can become complex, time overload, expensive, and in developing countries quite impossible to apply high quality level analysis due to lack in some basic infrastructures, or qualified technicians/staffs. Recently, some progresses have been made on dedicated in-situ sample based on nanogel encapsulated with a geometry minimizing artefact during their deployment [6]. Nevertheless, some strategic alternatives can be considered. Based on our experiences, we introduce point of view from analytical platform up to the concept. First to avoid or to minimize sampling transport and conservation, we recommend to use in-situ passive sensors (Table 1) to get data about labile fraction associated with grab samples (minimum of 4 times 250 ml) . Of course we recommend using the suggested protocol which should allow getting reliable high value data (Fig.1). Actually there are many different kinds of “In-situ passive” samplers or probes (electrochemical, biological and diffusive ones) available. To avoid extensive discussion about the different solutions, our study is based on our own experience on diffusive gradient in thin-films. Recent technological development is to move from film or 2D geometry to bulk ones (3D). They allow better performance by minimizing some inherent default of 2D ones. Main advantages and limitations of different sampling solutions integrating the Diffusive Milli-Gel (DMG) are listed above (table1). Figure 1 provides a conceptual schematic of the options and strategies allowing reliable data collections. Scientists involved in water analysis have to keep in mind that water media is a complex matrix Type (target) Appearance Advantages Disadvantages Grad sample (any contaminants) Recipients adapted to the contaminants Traditional Rapid Total/Colloid/Labil Detection $$$ (technician, ) Transport/Conservation Punctual If NOM : complex DGTs (main trace metals, Hg, P₂O ₅, NH₄) 2D Feedback Fluxes 3F (friendly, flexible & few $ Time of deployment Detection limit Breakable 2D Selective Extraction procedure POCIS (polar compounds) 2D Feedback Fluxes 2F ± $$ LDP (hydrophobic compounds) 2D Feedback Fluxes 2F ± $$ DMG (any contaminants) 3D Fluxes 3F No extraction Adapted/dedicated $ New Not commercial Lab scale production Few feedback Table 1: Recommended protocols for estimating concentrations of Labile and Total fraction in waters containing organic, inorganic, biological particles. These compounds have size from the nm up to several µm. While in chemistry, classification is based on concentrations named major, minor and trace elements. Their physical states are classified as follow: (truly) dissolved (when size < 1 nm), colloids (1nm