A network perspective reveals decreasing material diversity in studies on nanoparticle interactions with dissolved organic matter

Abstract

Dissolved organic matter (DOM) strongly influences the properties and fate of engineered nanoparticles (ENPs) in aquatic environments. There is an extensive body of experiments on interactions between DOM and ENPs and also larger particles. [We denote particles on the nano-and micrometer scale as particulate matter (PM).] However, the experimental results are very heterogeneous, and a general mechanistic understanding of DOM-PM interactions is still missing. In this situation, recent reviews have called to expand the range of DOM and ENPs studied. Therefore, our work focuses on the diversity of the DOM and PM types investigated. Because the experimental results reported in the literature are highly disparate and difficult to structure, a new format of organizing, visualizing, and interpreting the results is needed. To this end, we perform a network analysis of 951 experimental results on DOM-PM interactions, which enabled us to analyze and quantify the diversity of the materials investigated. The diversity of the DOM-PM combinations studied has mostly been decreasing over the last 25 y, which is driven by an increasing focus on several frequently investigated materials, such as DOM isolated from fresh water, DOM in whole-water samples, and TiO 2 and silver PM. Furthermore, there is an underrepresentation of studies into the effect of particle coating on PM-DOM interactions. Finally, it is of great importance that the properties of DOM used in experiments with PM, in particular the molecular weight and the content of aromatic and aliphatic carbon, are reported more comprehensively and systematically.nanoparticles | dissolved organic matter | environmental fate | network analysis | experimental design T he development and use of nanotechnology results in the release of engineered nanoparticles (ENPs) to the environment (1, 2). Much effort in current research is put into assessing the environmental risks of ENPs, which requires the understanding of the fate and distribution of ENPs in different environmental media, as well as their potential toxicity (3-6). Dissolved organic matter (DOM) (e.g., humic substances, polysaccharides, and proteins) is ubiquitous in aquatic systems and is known to interact with ENPs and modify their surface properties (7). DOM has been shown to either stabilize or destabilize aqueous ENP suspensions, depending on DOM properties and medium composition (8, 9). The effects of DOM on the stability of ENP suspensions are expected to strongly influence the mobility and distribution of ENPs in the environment (10).The extraordinary variability in DOM effects on ENPs originates from the fact that the interactions between DOM and ENPs depend on numerous interrelated factors, such as pH and ionic strength of the media, the particles' surface properties and size, and the chemical composition and concentration of the DOM (11). Some types of DOM, such as fulvic and humic acids, electrostatically stabilize ENPs via the adsorption of the electrically charged DOM on the ENP surface (12-16), where the str...

Nanoparticles in the environment: where do we come from, where do we go to?

“…Colloidal stability of NP is one of the key factors controlling their fate and effects [ 12 , 89 ]. When released into the environment, NP interact with the variety of dissolved or particulate, inorganic or organic compounds influencing NP aggregation dynamics and thus colloidal stability [ 90 ]. Ultimately, exposure conditions are controlled by NP aggregation.…”

The influence of past research on the design of experiments with dissolved organic matter and engineered nanoparticles

“…For this reason, recent reviews have called to diversify the types of DOM and PM studied in the experiments [ 10 , 12 ]. However, in our previous work we found that there is an overall decrease, rather than an increase, in the diversity of studied DOM-PM combinations [ 11 ]. PM types studied most often are titanium dioxide and silver nanoparticles; rarely studied are, e.g., quantum dots (note that PM and ENPs are not distinct categories, but PM includes ENPs).…”

“…To obtain insights applicable to a broad range of DOM and ENP types, both from correlations and from mechanistic studies, there is a need to study the interactions between DOM and particulate matter (PM), i.e., ENPs and particles of larger size, for a range of chemically and structurally diverse DOM and ENP types [ 11 ]. For this reason, recent reviews have called to diversify the types of DOM and PM studied in the experiments [ 10 , 12 ].…”

“…On the DOM side, river humic acids are predominantly investigated, whereas humic substances from peat and seawater are much less often studied. An overview of the DOM and PM types investigated in different years is provided in Figure 3 of Sani-Kast et al (2017) [ 11 ]. Experiments with particles of different size were included here because (i) size is a relevant factor that may affect DOM-PM interactions and (ii) there are also many other relevant factors (particle properties other than size; properties of the DOM; conditions of the aqueous medium) and a sufficiently broad empirical basis that reflects the influences of these factors is needed.…”

“…Here, in response to this call, we aim to understand the dynamics behind the choices of DOM and PM to be studied and discuss possible driving forces that account for the observed imbalance in the choice of materials. Going beyond the empirical findings presented by Sani-Kast et al (2017) [ 11 ], we present an agent-based model (ABM) that simulates the choices of DOM and PM types made by researchers in experiments between 1990 and 2015. This type of model allows us to analyze how the choices made collectively by many agents drive the spectrum of the DOM and PM types investigated (the actions of individual researchers and, in particular, how the pattern of the investigated PM and DOM types emerges from these actions were not addressed by Sani-Kast et al (2017) [ 11 ]).…”