Bioavailability and risk assessment of arsenic in surface sediments of the Yangtze River estuary

Highlights

• Arsenic bioavailability in sediment was discussed based on fraction and speciation.

• The main fraction was residual fraction which attenuated the overall bioavailability.

• The reduction from As(V) to As(III) might aggravate the bioavailability in wet seasons.

• Total concentration was insufficient to estimate the potential ecological risk.

Abstract

The bioavailability and risk assessment of As were studied in sediments of the Yangtze River estuary (YRE). Results showed that residual fractions dominated the As partition (> 85%), which attenuated overall bioavailability. After the residual fraction, As mainly partitioned into the Fe-Mn oxides fraction (3.16–4.22%). Arsenic bound to Fe-Mn oxides was higher in wet seasons. The carbonate fraction was minimal, which may result from the negative state presence of As in sediments. According to the risk assessment code, the YRE was classified as low risk. Additionally, the reduction of As(V) to As(III) may occur due to the reducing condition in wet seasons. Considering As(III) is more toxic and mobile, As bound to the exchangeable and Fe-Mn oxides fractions may have more potential ecological risk. Thus, the speciation and fraction should be both considered on the ecological risk of As in sediments of the YRE.

Introduction

Arsenic (As) is a toxic metalloid that poses a health threat in many countries (Amstaetter et al., 2010). Arsenic has acute and chronic toxic and carcinogenic effects on aquatic organisms and humans (Oremland and Stolz, 2003). Elevated levels of arsenic present in the environment have caused public concern (Yang et al., 2009). Arsenic is ubiquitous in groundwater, soil and sediments as a result of natural processes including mineral weathering, dissolution and geothermal activities (Manning et al., 1998, Yamamura et al., 2013). Anthropogenic sources include insecticides, pigment production, leaded gasoline manufacture, fossil fuel combustion, mining and electronic industries. Of these, mining and the use of groundwater abundant in As for crop irrigation are the main sources of higher levels of As in the environment (Ehlert et al., 2014). The sources in estuarine environment include terrestrial input, wet and dry atmospheric deposition, release during the resuspension of sediments, decomposition and regeneration of phytoplankton, and anthropogenic activities (Li et al., 2014). In an aquatic environment, As is typically much more concentrated in sediments than in water (Ahmann et al., 1997). Sediments act as both a source of groundwater contamination and a sink for As, which can be re-partitioned to the water column due to changes in the chemical environment (Choi et al., 2009).

To quantify the extent of pollution, total concentration is widely used for assessing contamination in sediments. However, total concentration provides insufficient information to estimate the potential environmental effect on sediments (Nemati et al., 2011, Sundaray et al., 2011). A metal's mobility, bioavailability and toxicity depend considerably on the chemical fraction (Díaz-de Alba et al., 2011, Huang et al., 2013, Fan et al., 2014). According to sequential extraction procedure, metals in sediments can be classified into exchangeable, carbonate, Fe-Mn oxide, organic and residual fractions (Tessier et al., 1979). Arsenic, when bound to different fractions, exhibits different bioavailabilities. The metal bound to the exchangeable fraction is the most mobile and bioavailable, as well as the most harmful to aquatic organisms (Feng et al., 2014). Many studies have assessed metal pollution using different methods (Díaz-de Alba et al., 2011, Nemati et al., 2011). Use of the risk assessment code (RAC) is one of the methods that has been applied to assess the potential ecological risk of a metal with the consideration of bioavailability to a marine ecosystem by considering the exchangeable and carbonate fractions (Sundaray et al., 2011, Duan et al., 2013, Huang et al., 2013).

In sediments, As predominantly exists in two speciations: (1) arsenate (As(V)), which dominates in oxidizing conditions, and (2) arsenite (As(III)), which dominates in reducing conditions (La Force et al., 2000). In natural conditions, As is preferentially adsorbed on Fe-oxides and Mn-oxides. Many studies have investigated the micro mechanism of As, including sorption and desorption on the surface of mineral oxides, the process of partitioning between the solid and aquatic phases and the transformation between As(III) and As(V) under different redox conditions (Jay et al., 2005, Kocar and Fendorf, 2009, Ying et al., 2011, Muehe et al., 2013, Ohtsuka et al., 2013, Singer et al., 2013). However, these studies mainly focus on the speciation of As overall whereas less attention has been paid to the speciation in different fractions. Considering that arsenite is much more mobile and toxic than arsenate (Keon et al., 2001, Ying et al., 2011), each fraction of As with different ratio of arsenite and arsenate may have a different bioavailability.

In order to manage environmental pollution, it is not sufficient to study only the sorption and desorption on the surface of mineral oxides, the process of partitioning between the solid and aquatic phases and the transformation between As(III) and As(V) under different redox conditions. The spatio-temporal changes of As also should be carried out to analyse the distribution, seasonal variation, and pollution assessment. The combination of both approaches on mechanism and spatio-temporal studies could give a better understanding of the behavior of As in the environment. Geostatistics and geographic information system (GIS) are also useful tools to study the distribution of contaminants and analyse the extent of pollution in large scale regions (Mamat et al., 2014, Wang et al., 2015). Geostatistics has been proven to be an effective methodological approximation for studying metal pollution in soil, sediment, mining areas and groundwater (Lee et al., 2009, Antunes and Albuquerque, 2013, Chica-Olmo et al., 2014). Ordinary Kriging is also widely used in predicting pollution and has also been utilized in the present study (Liu et al., 2014, Wang et al., 2014, Wang et al., 2015).

An estuary is an environmentally sensitive area with varying environmental attributes, which may have an influence on the partitioning of a metal into different fractions. The large amounts of organic and inorganic effluents in an estuary and the interaction between these complexes result in an estuary presenting multiple redox conditions (Yao et al., 2006). In addition, seasonal variations might change the oxidizing environment in estuary (Wang et al., 2012). An estuary also contains distinctive particle sizes due to various environmental conditions, hydrological events and human activities (Yao et al., 2007, Balzer et al., 2013, Feng et al., 2014). Particle size is very important in the transportation and cycling of metals. The process of transformation between exchangeable and residual fractions has reportedly been observed on clay minerals (Lim et al., 2002).

In this study, the Yangtze River estuary (YRE) was chosen to investigate the behavior of As in estuarine sediment. The aims of this study were to: (1) describe the fraction distributions in three seasons; (2) investigate the impact of seasonal variations on fractions; (3) analyse bioavailability by examining the relationship between fraction and speciation; (4) assess the potential ecological risk of As in the YRE.