Titanium and zinc-containing nanoparticles in estuarine sediments: Occurrence and their environmental implications

Highlights

• Vast amounts of Ti and Zn-NPs in estuarine sediments were detected by SP-ICP-MS.

• Ti-NPs were derived from non-bioavailable (traditionally safe) fraction in sediments.

• Zn-NPs were derived from the bioavailable fraction of Zn in sediments.

• Salinity was the environmental factor regulating the concentrations of Ti-NPs.

Abstract

Understanding the behavior and risk of nanoparticles (NPs) in the aquatic environment is currently limited by the lack of quantitative characterization of NPs in the environmental matrices, such as sediments. In this study, based on the single particle (SP)-ICP-MS technique, metal-containing NPs, including Ti- and Zn-containing NPs, were analyzed in sediments taken along the Yangtze Estuary. Combined with the traditional sequential extraction method that has been widely used for metal risk assessment, different single extraction methods were used to understand the association of NPs with different chemical fractions in sediments and their potential environmental implications. Ti-containing NPs, with an average size of 81 nm, ranged from 3.02 × 10⁷ parts/mg to 9.61 × 10⁷ parts/mg, and Zn-containing NPs, with an average size of 41 nm, ranged from 2.47 × 10⁶ parts/mg to 1.21 × 10⁷ parts/mg. Both correlation and redundancy analyses showed that particle concentrations of Ti-containing NPs in sediment were significantly correlated to the Ti-containing NPs in the residual fraction and salinity, indicating that Ti-containing NPs in sediments may be dominated by Ti-containing NPs in the residue fractions of sediments. Large amounts of these NPs may be released from the residual fraction that has been considered to be not bioavailable and “environmentally safe” in the traditional environmental risk assessment of metals in sediments. Zn-containing NPs, mostly associated with carbonates, were positively correlated to all the bioavailable fractions of Zn in sediments, suggesting that these NPs may be largely presented in the bioavailable fraction. This study showed that, vast numbers of NPs with minute sizes were present in estuarine sediments, and that they were associated with different chemical fractions with different potential environmental risks. The study findings call for further research to update the traditional risk assessment method.

Introduction

With the development of nanotechnology, an increasing number of nanoparticles (NPs) have inevitably entered the aquatic environment through various paths, such as atmospheric deposition, storms, runoff and wastewater treatment plant effluent, and are finally deposited into sediments (Benn et al., 2010; Kaegi et al., 2010; Kim et al., 2017; Kuenniger et al., 2014; Mackevica et al., 2017b). For example, ZnO and TiO₂ are two commonly used nanomaterials and are widely used in architectural coatings, catalysts, fiber fabrics, cosmetics and consumer products. These NPs can enter wastewater treatment plants from our daily life activities, or be corroded after exposure to complex and changeable climate conditions (such as rain, snow and wind), further entering the aquatic environment for deposition into sediments (Gondikas et al., 2014; Kaegi et al., 2008; Mackevica et al., 2017a). Sediments can act as a significant sink for NPs, further controlling their stability, fate and toxicity. Quantitatively characterizing the occurrence and potential bioavailability of NPs in sediments is critical to better understand the fate and eco-environmental risk assessment of NPs in aquatic systems. However, such studies are extremely lacking, and are limited by the standard method for the separation and extraction of potential dispersible nanoscale particles in complex environmental matrices, such as sediments (Li et al., 2012; Mahdi et al., 2017; Schwertfeger et al., 2017). Single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) is a relatively new technique that is used to quantify the size distribution and particle concentration of NPs at low concentrations in environmental samples. It can determine metal concentrations in both the dissolved and particulate forms simultaneously, and has been applied to determine the particle sizes and concentrations of spiked NPs in drinking water, biological tissues, sunscreen, soil and sewage sludge (Dan et al., 2015a; Dan et al., 2015b; Donovan et al., 2016; Gray et al., 2013; Pace et al., 2012; Tou et al., 2017). Recently, based on the SP-ICP-MS technique, a simple method for the extraction and determination of metal-containing NPs (e.g. Ti- and Zn-containing NPs) in sediments has been developed as summarized in an associated paper. In the present study, we further used this method to detect the particle sizes and their concentrations in sediment samples.

Metal chemical extraction methods, consisting of sequential extraction methods and single extraction methods, were developed to determine the metals involved in various geochemical equilibrium processes such as sorption, dissolution, precipitation and redox, and could be used to predict the bioavailability of metals and their implications on ecosystems (Feng et al., 2005; Houba et al., 2000; Kashem et al., 2007; Menzies et al., 2007; Peijnenburg and Jager, 2003; Peijnenburg et al., 2007; Quevauviller et al., 1994; Rao et al., 2008; Sahuquillo et al., 2003; Ure et al., 1993). For example, the sequential extraction method proposed by the European Community Bureau of Reference (BCR sequential extraction) and its variations have been widely used for characterizing the bioavailability of metals in soils, sediments and related environmental samples. A modified BCR sequential extraction method was used in the present study, and metals were divided into four metal chemical fractions: an acid-exchangeable fraction, a reducible fraction, an oxidizable fraction and a residual fraction. The acid-exchangeable fraction, mainly consisted of water-soluble, exchangeable and carbonate-bound metals that were readily released into the environment and taken up by organisms, representing the readily bioavailable fraction of metals (Quevauviller et al., 1994; Ure et al., 1993). Single extraction methods can imitate the leaching of metals and extract the readily bioavailable fraction of metals in soils/sediments. CaCl₂ extraction (0.01 M, pH 5.0) and DTPA (diethylenetriaminepentaacetic acid, 0.005 M, pH 7.3–7.6) extraction are commonly used to evaluate the bioavailability of metals such as Cu, Zn, and Cd in soils/sediments. The CaCl₂ extraction method evaluates the bioavailability of metals by imitating the chemistry of pore water, containing the water soluble and exchangeable metals in soils/sediments (Houba et al., 2000; Peijnenburg and Jager, 2003; Peijnenburg et al., 2007). DTPA is a strong chelating agent that can imitate the chelation of plant root exudates, and contain not only free ions but also the carbonate-bound and organic-bound fractions of metals in soils/sediments. However, metal-containing NPs have not been considered in all of the abovementioned metal chemical fractions (Menzies et al., 2007; Peijnenburg et al., 2007; Sahuquillo et al., 2003).

Estuaries are the joint area between the rivers and sea and act as a buffer between the land and sea, serving as the main route for terrestrial materials entering the sea and the acceptor of large amounts of pollutants from rivers, including anthropogenic metals and metal-containing NPs. The Yangtze Estuary, which covers an area of 1.8 million kilometers, is one of the largest estuaries in the world. Over the last several decades, it has continued to be subject to heavy metal contamination due to the rapid economic and industrial development of the Yangtze River basin, especially the megacity, Shanghai, which is located on the Yangtze Estuary. With many shipyards, petrochemical plants and steel plants around/along it, a total amount of more than 5 × 10⁶ tons of industrial and domestic wastewater is discharged into this estuary every day (Du et al., 2013; Zhang et al., 2009). Currently, metal contamination in the sediments of the Yangtze Estuary has been discussed in many studies, but none of these studies is related to the occurrence and environmental implications of NPs in the sediments (Chen et al., 2017; Feng et al., 2014; Han et al., 2017; Wang et al., 2015; Wang et al., 2014a; Wang et al., 2014b; Zhang et al., 2009).

This study was designed to quantitatively characterize the distribution of metal-containing NPs, such as Ti- and Zn-containing NPs in sediments along the Yangtze Estuary and elucidate the controlling factors, considering different metal chemical fractions and other environmental factors, such as salinity and TOC. To this end, surface sediment samples were taken along the Yangtze Estuary in different seasons (summer and winter) and the specific objectives of this study are (1) to quantify the particle sizes and particle concentrations of Ti- and Zn-containing NPs in sediments by SP-ICP-MS, (2) to elucidate the key factors affecting the occurrence and distribution of Ti- and Zn-containing NPs in sediments by RDA and correlation analysis, and (3) to elucidate the environmental implications of these metal-containing NPs based on metal chemical speciation analysis. The information provided by this study will have important environmental implications and is significant for environmental risk assessments of NPs in environmental samples.