Spatial and environmental characteristics of colloidal trace Cu in the surface water of the Yellow River Estuary, China

https://doi.org/10.1016/j.marpolbul.2021.112401Get rights and content

Highlights

  • Colloidal Cu heterogeneity in size and variation was studied in the YRE.

  • LMW-Cu migrated more violently than HMW-Cu and stabilized with increasing salinity.

  • <1 kDa Cu was double supplemented from LMW decomposition and particulate ligands addition.

  • Strong ligand, mixing process and desorption co-regulated colloidal Cu of the YRE.

Abstract

Dynamic variations in chemical composition and size distribution of dissolved copper (Cu) along the river-sea interface in the Yellow River Estuary (China) were investigated. On average, ~64% and ~8% of bulk dissolved Cu (<0.45 μm) were partitioned in the <1 kDa fraction and 1–100 kDa, respectively. The other 28% were in the 100 kDa–0.45 μm colloids, which indicates that this fraction may dominate the overall morphology of colloidal Cu. The <3 kDa Cu fraction was susceptible to environmental parameters and the >3 kDa fraction was related to the behavior of dissolved organic carbon. 1–100 kDa Cu migrated more violently than >100 kDa Cu and tended to be a stable polymer, with stability increasing towards the sea. The source of <1 kDa Cu was complex and may be supplemented by the decomposition of small molecular colloids and the addition of the sediments or particles ligands.

Introduction

Trace metals play a vital role in the biogeochemical cycles of estuarine, offshore, and open waters (Bruland and Lohan, 2003; Waeles et al., 2008; Lu et al., 2020). As an important connection between freshwater and seawater, estuaries have a strong hydrodynamic force and numerous physical and chemical parameter gradients that greatly affect the distribution and flux of metals (Wang et al., 2018; Lu et al., 2020). Many trace metals are nutrients that contribute to the primary productivity of aquatic systems, and when the concentration is too low, they become an important limiting factor for primary productivity (Maldonado et al., 2006; Han et al., 2021). On the other hand, if the concentration of these metals exceeds a certain level, they will become toxic metals that accumulate along the food chain and affect human health (Li et al., 2015; Lu et al., 2019). For example, copper (Cu) is characteristic of both a nutrient and toxic metal and its potential toxicity in coastal waters requires further research (Martin et al., 1994; Morel et al., 2003; Rivera-Duarte et al., 2005). Furthermore, Cu plays an important role in marine ecosystems (Peers and Price, 2006): most phytoplankton requires Cu to perform key redox reactions (Illuminati et al., 2017; González-Álvarez et al., 2020).

Most research on dissolved metals in aquatic environments filters water samples through a 0.2 μm or 0.45 μm pore filter membrane, and divides it into the dissolved state and particulate state for further analysis (Li et al., 2015; Wang et al., 2018). However, this technique may underestimate the behavior of dissolved metals, that is, colloidal metals (Guo and Santschi, 2007; Lu et al., 2019). Due to their large specific surface area and diverse species content, aquatic colloids play an important role in the biogeochemical cycle of elements and are receiving increasing attention (Waeles et al., 2008; Liu et al., 2013). In recent years, with the rapid development of various colloid separation technologies, research is no longer limited to the detection of concentration and distribution, but is rapidly extending in the direction of toxicology and environmental behavior (Xu et al., 2018a, Xu et al., 2018b, Xu et al., 2018c; Lu et al., 2020). The study of colloidal behavior in estuarine areas is an important research direction (Waeles et al., 2008; Savenkoa and Pokrovsky, 2019) because the dynamic behavior of estuary colloids is different from that of open water. Estuary colloids are also sensitive to environment variation, which makes this research important for understanding the biogeochemical behaviors of trace elements in estuaries (Gueguen et al., 2002; Lasareva et al., 2017; Xie et al., 2018).

The Yellow River is the largest river in Northern China. It has a large drainage area and contains a large amount of sediment and nutrients from the upper and lower reaches of the Bohai Sea. Therefore, it has the highest sediment content in the world and is the main creator of the world's youngest wetland (Zhang and Huang, 1993; Tang et al., 2010; Wang et al., 2011; Wang et al., 2016; Chen et al., 2020). In addition, the Yellow River Estuary (YRE) is a fast-growing aquaculture area and an important part of the Shandong Peninsula Blue Economic Zone in the national development strategy of China, making research on this area particularly important (Huang et al., 2013; Yuan et al., 2016).

Previous studies on the YRE focus on the concentration, distribution, and potential pollution of traditional dissolved metals (i.e. Zn, Cd, and Pb) (Qiao et al., 2007; Lin et al., 2016; Wang et al., 2018). However, biogeochemical studies on colloidal trace metals in this region, especially colloidal Cu, are limited. As an important biologically active metal, Cu can be easily adsorbed on inorganic/organic ligands in the aquatic environment, thereby increasing its bioavailability. This maybe an important environmental factor in red tides, green tides, and algal blooms in the nearshore area (Hirose, 2006). Therefore, dissolved Cu in the surface water of the YRE was investigated. The main objectives of this study were: (1) to investigate the spatial concentration variations of Cu in five molecular weight fractions of the dissolved pool in surface water of the YRE; (2) to assess the status of Cu as a toxic metal and the resulting ecological risks; (3) to explore the relationship between dissolved Cu behavior and environmental factors in the five molecular weight fractions in the dissolved phase; and (4) to investigate the partition coefficient of Cu between colloidal and total dissolved (TD) phases. These five dissolved state classifications cover the truly dissolved phase (<1 kDa), the low-molecular-weight (LMW) colloidal phase and the high -molecular-weight (HMW) colloidal phase, to facilitate the detailed evaluation of Cu migration between each fraction. The new insights brought about by the application of clean sampling and pretreatment technologies are of great significance for in-depth evaluations of colloid heterogeneity and dynamic behaviors in the river-sea mixing area. In addition, the results obtained from this study would provide a better insight into the physicochemical properties of aquatic colloids and their contributions in metal transportation.

Section snippets

Sampling and pre-filtration

A survey of the YRE was carried out in July 2020: 10 stations in the river-sea mixing zone were selected (Fig. 1; bright yellow area). All vessels were pre-cleaned before sampling, modified from the cleaning process in Li et al. (2015) and Lu et al. (2020). Briefly, all utensils were rinsed three times with 10% Decon 90™ detergent (v/v) and ultra-pure water (R = 18.2 MΩ-cm), soaked in 10% nitric acid (HNO3) and 10% hydrochloric acid (HCl) for 48 h, then placed in a clean bench to dry, and

Dissolved Cu concentrations and CF values

The mean concentrations and CF value distribution of the TD Cu in the YRE are presented in Fig. 2. The CF values of the sampling sites and seawater standard are summarized in Table 2. The TD Cu range from 41.96–73.29 nmol L−1, with an average of 59.03 nmol L−1. The highest concentration of TD Cu was found at site 2 (73.29 nmol L−1), followed by sites 5 (72.24 nmol L−1) and 9 (63.81 nmol L−1). The lowest TD concentration was found at site 7 (41.96 nmol L−1), followed by sites 3 (48.77 nmol L−1)

Conclusions

In this study, the spatial distribution, sources and character of <1 kDa truly dissolved phase, colloidal phase (1–3 kDa, 3–10 kDa, 10–100 kDa and 100 kDa −0.45 μm), and TD phase (<0.45 μm) Cu in the YRE, China, was investigated using the modified CFU method and clean sampling technique. The water quality of TD Cu was examined, and the dynamic changes in size distribution and the relationship between mixing behavior and environmental parameters were evaluated. Grade-one seawater quality

CRediT authorship contribution statement

Yuxi Lu: Investigation, Formal analysis, Conceptualization, Methodology, Validation, Writing – original draft. Dawei Pan: Funding acquisition, Resources, Conceptualization, Methodology, Validation, Supervision, Writing – review & editing. Tingting Yang: Formal analysis. Chenchen Wang: Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We gratefully thank anonymous reviewers for their constructive comments and suggestions. This work was financially supported by the Original Innovation Project (ZDBS-LY-DQC009) and the Strategic Priority Research Program (XDB42000000) of Chinese Academy of Sciences, and the National Key R&D Program of China (2019YFD0901103).

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