A contribution of nanoscale particles of road-deposited sediments to the pollution of urban runoff by heavy metals

Highlights

• Nanoscale particles (NSPs) of road-deposited sediments (RDS) were separated and analyzed.

• Concentration factors for Ni, Cu, Zn, Cd, Sn, Pb in NSPs are from 2 to 10.

• Contents of water-soluble heavy metals in RDS were also determined.

• Contribution NSPs of RDS to the pollution of runoff by heavy metals was evaluated.

Abstract

Road-deposited sediments (RDS) present a sink for traffic-related pollutants including heavy metals (HMs). HMs associated with RDS particles enter the urban aquatic environment during rainfall events and have adverse effects for biota. RDS nanoscale particles (NSPs) require special consideration due to their specific properties, extremely high mobility in the environment, and ability to penetrate into living organisms. In the present work, the contribution of NSPs of RDS to the pollution of urban runoff by HMs has been evaluated for the first time. It has been shown that bulk RDS samples are polluted by HMs as compared to background urban soils (geo-accumulation indexes of Cu and Zn may attain 2-3). Meanwhile, NSPs of RDS are enriched by HMs as compared to bulk samples; concentration factor for Ni, Cu, Zn, Cd, Sn, and Pb in NSPs being varied from 2 to 10. The water-soluble fractions of RDS samples were also analyzed. Results have shown that the content of water-soluble HMs in RDS is insignificant and rarely exceeds 0.5% of the total contents of HMs in the bulk samples; the highest contents are identified for Cu and Pb. It should be noted that the water-soluble fraction is nearly free from Zn and this element is almost entirely present as particulate matter (NSPs). In general, the overall contribution of NSPs and water-soluble fraction of HMs to the pollution of urban runoff is comparable.

Introduction

The rapid urbanization inevitably leads to the environmental pollution. In urban areas, the growth of number of vehicles causes an increase of exhaust and non-exhaust particulate emissions, which can be deposited and accumulated on roads and other impervious surfaces (e.g. sidewalks, parking lots, etc.). Road-deposited sediments (RDS) derived from soils and other natural sources (e.g. atmospheric deposition) present a significant sink for toxic and potentially toxic metals and metalloids (MMs), which are derived from wear of tires, brakes, asphalt, road marking, etc. (Aryal et al., 2010; Murakami et al., 2008). MMs enter into the urban terrestrial, atmospheric, and water environment with particles of RDS as a carrier and have adverse effects. Atmospheric re-suspension of RDS has significant implications for human health, and stormwater transport can directly affect aquatic biota. Therefore, in urban systems, RDS is an important environmental medium for assessing levels of contaminants.

RDS are also considered to be an important carrier for nonpoint urban pollutants (Sansalone et al., 1998; Sutherland, 2003). Nonpoint source pollution is recognized as a pertinacious illness of urban rivers because of its permanent contribution to their pollution (Wang et al., 2017). MMs (especially heavy metals (HMs)) in urban runoff are of special concern due to their direct adverse effects on the receiving waters, aquatic ecosystem, and human health.

The biological toxicity of runoff pollutants is directly related to their concentrations in various physicochemical forms (Tanizaki et al., 1992; Florence, 1982). During the rainfall, dissolved fraction of MMs being leached from RDS is bioaccessible immediately and is very mobile in the environment. It is reported that in urban runoff metals are transported primarily as species adsorbed to solids (Herngren et al., 2006), and concentrations of certain metals are much higher in solids than in solution (Lau and Stenstrom, 2005). It is considered that particulate-bound MMs does not possess immediate risk and remain in the system as a potential source of bioaccessible MMs until going into dissolved forms under adverse environmental conditions, e.g. acidification by acid rains, industrial wastewater discharge, etc. The latter is reasonable for relatively coarse fraction (>1 μm) of runoff solids, while the ultrafine particulate matter (nanoparticles) may pose a direct risk due to high penetration ability into living organisms.

Nanoparticles require a special attention due to their specific properties, mobility in the environment, and related risk assessment (Bakshi et al., 2015; Nowack and Bucheli, 2007; Biswas and Wu, 2005). The chemical properties and hence composition of nanoparticles may differ significantly for micron-sized particulate and/or bulk matter. It is known that nanoparticles are taken up by a wide variety of mammalian cell types, are able to penetrate through the cell membrane and become internalized (Nowack and Bucheli, 2007). For example, ZnO nanoparticles can be internalized by bacteria, and CeO₂ nanoparticles can be adsorbed onto the cell wall of E. coli. Ecotoxicological studies show that NPs are also toxic to aquatic organisms, both unicellular and animals (Nowack and Bucheli, 2007). According to their origin, nanoparticles in the environment can be divided into three categories: natural, incidental, and engineered. Natural nanoparticles exist on the Earth from time immemorial and derive from volcanic eruptions, dust storms, forest fires, sea salt aerosols, etc. Incidental nanoparticles are byproducts of human activities and sourced from vehicle emissions, mining, fires, etc. Engineered nanoparticles, which are synthesized by human for certain purposes, also can be released into the environment. At present, numerous studies are focused on the investigation of fate of engineered nanoparticles in the environment (Bhatt and Tripathi, 2011; Montaño et al., 2014; Dwivedi et al., 2015; Zhang et al., 2016), while natural and incidental nanoparticles undeservingly attract less attention.

RDS is a complex matter, which includes both natural (mainly sourced from soil erosion) and incidental particles (from exhaust and non-exhaust vehicle emissions) including nanoscale ones. Besides, high specific surface and reactivity of natural nanoparticles may lead to changes in their trace element composition, for example, as a result of sorption of metals from the environment.

Trace element composition of RDS is studied thoroughly. Recent studies have shown that the mobility of pollutants during rainfall depends on particle size (Gunawardana et al., 2012; Zhao and Li, 2013; Li et al., 2015). Besides, pollutants tend to be accumulated mainly onto fine particles that can be easily transported by rainfall (Gunawardana et al., 2015; Zhao et al., 2016). However, the studies on the contribution of RDS to urban runoff pollution are only confined to the investigation of micron-size particle fractions (Sutherland, 2003; Wang et al., 2017; Li et al., 2015; Gunawardana et al., 2015; Zhao et al., 2016; Yuen et al., 2012; Padoan et al., 2017), while nanoparticles are disregarded. Despite the rapid development of analytical instrumentation and corresponding methodologies, there is a gap in studies on the chemical composition and properties of environmental nanoparticles as well as their behavior in the environment. The main reason is the difficulty to separate nanoparticles from environmental samples for further characterization and quantitative analysis (Ermolin and Fedotov, 2016).

A rotating coiled column (RCC) is a conventional instrument in countercurrent chromatography (Berthod et al., 2009). In addition, in the last fifteen years RCC has been also applied to the sedimentation field-flow fractionation of nano- and microparticles (Fedotov et al., 2015) as well as to the dynamic extraction of trace MMs from soils and sediments (Fedotov et al., 2006; Fedotov, 2012). Besides, RCC has been used for studies on the mobility of MMs in atmospherically deposited dust affected by copper-smelter, and their association with dust particles of different size (Ermolin et al., 2016; Fedotov et al., 2016). Among different separation methods, the fractionation of nano and microparticles as well as soluble compounds in RCC has certain advantages for the study on particulate environmental samples. Firstly, the fractionation in RCC occurs under the environmentally relevant (dynamic) conditions in the continuous flow of carrier fluid. Secondly, the method can be applied to the separation of both nanoparticles and soluble fractions for their subsequent investigation and analysis.

The present work is aimed at the study on the elemental composition of nanoparticles of RDS and their contribution to the pollution of urban runoff by heavy metals. To the best of our knowledge it is the first research in the area. The original combined analytical approach, which is based on the separation of nanoparticle and water-soluble fractions of RDS in RCC under environmentally relevant conditions followed by the characterization and quantitative elemental analysis of the separated fractions, will be used.