Characterization, origin and aggregation behavior of colloids in eutrophic shallow lake
Graphical abstract
Introduction
Natural colloids, defined as particles with sizes ranging between 1 nm and 1 μm, are ubiquitous in aquatic environments (Buffle and Leppard, 1995; Gibson et al., 2007). Due to the small sizes, these colloidal particles usually have a long residence time and substantially impact transparency and recreational value of waters. The properties of high specific surface area and reactivity endow them strong adsorption potential to many trace elements, including nutrients and pollutants, and as a result, influence their chemical speciation, behavior and environmental fate (Gibson et al., 2007). Moreover, the persistence of aquatic colloids can also cause potential toxicity to microorganism (Shang et al., 2017) and impacts on aquatic ecosystems (Colman et al., 2014. Owing to the significant influence on the fate and transport of pollutants and on ecosystem balance, characterization of aquatic colloids have received increasing attention over the past years.
Environmental colloidal particles in lake waters originate from either allochthonous or autochthonous sources, depending on human activity and local meteorological/hydrological conditions. Compared with deep lakes, shallow lakes are usually characterized by higher turbidity and colloidal abundance due to high primary production and sediment re-suspension (Xing and Kong, 2007; Zheng et al., 2015). The size, composition, and spatio-temporal distribution of lake water colloids had previously been reported, but mainly focused on inorganic colloidal particles (ICPs) (Chanudet and Filella, 2007, 2009; Filella et al., 2009). However, ICPs in natural waters are usually coated with dissolved organic ligands to form organic-inorganic colloidal particles (Schäfer et al., 2007). In addition, dissolved organic matter in natural waters had been shown to occur mostly in the form of colloids (Guéguen et al., 2006; Guo et al., 2009; Shirokova et al., 2013; Stolpe et al., 2013). Therefore, the organic, inorganic and organic-inorganic colloids collectively constitute the abundance of natural colloidal particles (NCPs) in lake waters. Nevertheless, knowledge on the properties and sources of colloidal particles (including NCPs and ICPs) in lake waters, especially the high-turbidity shallow lakes, is lacking, but vital for understanding the roles of colloidal particles in regulating water quality, ecosystem function and pollutant transport.
The environmental behaviors of colloids in natural waters are controlled by their stability/aggregation propensities, which are highly dependent on environmental conditions. Many parameters, including electrolytes, ionic strength, pH, and natural organic matter (NOM), have been reported to influence the stability of inorganic colloids like kaolinite (Kretzschmar et al., 1998; Aurell and Wistrom, 2000; Sequaris, 2010), montmorillonite (Garcia-Garcia et al., 2007; Sequaris, 2010; Borgnino, 2013), illite and quartz (Jiang et al., 2012). Among them, electrolytes and NOM, including quantity and composition, play critical roles in affecting the stability of colloids (Philippe and Schaumann, 2014). It was generally found that electrolytes (including mono- and divalent) enhanced colloidal aggregation, but presence of NOM can significantly inhibit the aggregation potential of colloids/nanoparticles (Chen et al., 2006; Dong and Lo, 2013). However, the NOM-induced aggregation inhibition can be highly related to its inherent chemical structure and molecular size. For instance, bovine serum albumins were found to be more effective than humic and fulvic acids in stabilizing the manganese dioxide colloids due to the difference in molecular sizes of the NOM (Huangfu et al., 2013). In fact, NOM in aquatic environments is a highly heterogeneous mixture with various organic components and a continuous size spectra (Philippe and Schaumann, 2014; Xu and Guo, 2017), and different functional groups and molecular weight (MW) fractions may interact differently with colloidal particles. Within the bulk NOM pool, the high MW (HMW) NOM fraction has been shown to play a central role in regulating the concentration and speciation, and hence the fate, transport and bioavailability of many trace elements in aquatic ecosystems (Guo et al., 2002; Alasonati et al., 2010). In addition, recent studies also highlighted the influence of NOM on colloidal stability and found the MWs and functionalities of NOM an important parameter in controlling aggregation kinetics and stability for nanoparticles, including silver (Yin et al., 2015), gold (Louie et al., 2015), fullerene (Shen et al., 2015), and ZnO (Kteeba et al., 2017). However, these studies usually selected one or two synthesized or engineered nanoparticles as the model materials, while the NCPs are polyfunctional and contain compounds with various compositions and structures. In addition, it is not clear in the aggregation heterogeneities between the NCPs and ICPs in natural waters, leaving a knowledge gap regarding the aggregation behaviors of colloids (including NCPs and ICPs) in environmentally relevant conditions.
The objectives of this study were to: (1) characterize the physicochemical properties of aquatic colloids, including NCPs and ICPs, in shallow lakes; (2) define the origins of these colloidal particles; (3) explore the NOM- and electrolyte-related aggregation profiles in ambient conditions and reveal the specific MW fractions in NOM matrix that regulate colloidal stability. Both spectral and microscopic techniques were used to characterize the chemical properties of colloidal particles, and a simulation experiment was carried out to quantify the contribution of sediment re-suspension processes to the colloid abundance in lake waters. In addition, ultrafiltration was used to fractionate the bulk NOM into LMW- and HMW-NOM fractions, whose specific effects on colloidal aggregation under mono- and divalent electrolyte conditions were determined via the dynamic light scattering (DLS) technique.
Section snippets
Collection of aquatic colloids and sediment core samples
Lake Taihu, located between 30°55′40″–31°32′58″ N and 119°52′32’’ −120°36′10″ E, is one of the largest shallow lakes in China (mean water depth: 1.9 m) with high turbidity and low transparency in the water column (Qin et al., 2007). Another feature of Lake Taihu is that it has two distinctive ecological regions: macrophyte- and algae-dominant regions. Surface water samples were collected from the two regions (Fig. S1 in the Supporting Information, SI) with an average wind speed (based on 30 min
Characterization of aquatic colloids
Fig. 1 shows typical TEM micrographs and EDX spectra for the colloids collected from different ecological regions of the lake. No significant difference in shapes was observed for NCPs between the two regions. They both contained a mixture of spherical, polygonal and elliptical-shaped colloids with an uneven size distribution, ranging from 20 nm to 200 nm. In addition, some organic-coated boundary can be found on the surface of NCPs, which would be attributed to the chemical and/or physical
Conclusion
Aquatic colloids, including NCPs and TICPs, in eutrophic shallow lake waters, were characterized with high abundance, broad size distribution, and Fe/Al/Si-containing mineralogical properties. Sediment re-suspension can have a significant contribution to colloidal particles at 54–55% of the NCPs and 78–80% of the TICPs in lake waters. The colloidal particles exhibited obvious DLVO-like aggregation profiles in mono- and divalent electrolytes, but presence of FA significantly inhibited the
Acknowledgements
We gratefully thank three anonymous reviewers for their constructive comments and suggestions which improved the manuscript. This study was supported, in part, by the National Natural Science Foundation of China (51479187) and Youth Innovation Promotion Association CAS (2016286). We also thank Dr. Cheng Liu and Qiushi Shen for their help in sediment re-suspension simulation.
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