Seasonal variations of cadmium (Cd) speciation and mobility in sediments from the Xizhi River basin, South China, based on passive sampling techniques and a thermodynamic chemical equilibrium model

Highlights

• Sulfates reduction enabled Cd concentrations to remain at µg·L⁻¹ levels in pore water.

• Sulfides oxidation mainly accounted for remobilization of sediment Cd.

• Sediment acted as a sink of Cd to water in winter and shifted to a source in summer.

• Sediment Cd was released into river and migrated downstream mainly in Cd-S complexes.

Abstract

Understanding the speciation and mobilization mechanisms of potentially toxic metals in sediments is critical to aquatic ecosystem health and contamination remediation in urban rivers. In this study, chemical sequential extraction, a thermodynamic chemical equilibrium model (Visual MINTEQ ver. 3.1), diffusive gradient in thin films (DGT), and high-resolution dialysis (HR-Peeper) techniques were integrated to identify seasonal variations in cadmium (Cd) mobility in sulfidized sediments. Acid-soluble Cd was the dominant geochemical fraction in sediments, followed by residual, oxidizable, and reducible Cd. The DGT-labile Cd concentration was associated with various geochemical processes and was independent of the total concentration and geochemical fractionation of Cd in sediments. Sulfate reduction facilitated the formation of insoluble CdS and induced low Cd concentrations in sediment porewater. Sulfide oxidation was principally responsible for lowered porewater pH and elevated Cd concentrations in summer. Strongly acidic conditions promoted release of sediment Cd but might reduce the binding efficiency of Chelex resin gel for dissolved Cd, leading to underestimation of the mobility of sediment Cd. Sediments generally functioned as a sink for Cd in winter and shifted to acting as a source in summer, releasing Cd into the overlying water mainly as Cd-S complexes with high potential to migrate downstream.

Introduction

Cadmium (Cd), one of the most toxic and carcinogenic chemical elements, is a widespread contaminant in the environment worldwide (Balistrieri and Blank 2008; USEPA 1989). Trace Cd concentrations in environmental media can exert acute or chronic toxic effects on organisms and threaten human health via bioaccumulation in food webs (Croteau et al., 2005; Zhang and Reynolds 2019). Due to the deleterious impacts of Cd, it has been listed as a priority contaminant by the United States and the European Union (Claudia 1993, The European Parliament and the Council of the European Union 2013). In aquatic environments, dissolved Cd derived from anthropogenic discharge tends to adsorb onto suspended particles followed by deposition in sediments, which are generally considered a major sink for metals (Gao et al., 2016; Zheng et al., 2013). In turn, sediment can act as an important Cd source in the water column due to resuspension and changes in physico-chemical conditions (Caetano et al., 2003; Xu et al., 2016). With such changes, release of sediment Cd might lead to secondary contamination of the overlying water, posing risks to aquatic organisms and domestic water supplies. Consequently, seasonal release characteristics and remobilization mechanisms of sediment Cd have attracted considerable concern worldwide.

The chemical sequential extraction procedure developed by the European Community Bureau of Reference (BCR) has been intensively utilized to explore the mobility and bioavailability of trace metals in sediments (Gao et al., 2018; Gao et al., 2017). However, this traditional research approach has been reported to have two major deficiencies: analytical errors resulting from changes in the chemical properties of sediment samples during transportation, storage, and pretreatment processes; and overestimation of metal mobility induced by advanced leaching of acid-volatile sulfide-bound metals during the extraction process (Billon et al., 2001). Therefore, passive sampling techniques such as diffusive gradient in thin films (DGT) and high-resolution dialysis (HR-Peeper) have been conducted in situ for measurement of labile species and the concentrations of dissolved target chemicals, respectively (Chen et al., 2021; Li et al., 2021; Zhang et al., 1998). Recently, the Chelex DGT probe has been widely employed to identify key factors and characterize the effects of geochemical cycling of crucial elements on the mobility of Cd in sediments or soils based on field monitoring campaigns. Some reports have indicated that differences in physico-chemical properties, including cation exchange capacity (CEC), pH, and grain size composition of soils, affect the distribution characteristics of Cd between the solid and liquid phases, as well as its mobilization potential (Muhammad et al., 2012; Perez and Anderson 2009). For sediment, rapid degradation of organic matter (OM) and reductive dissolution of Fe/Mn oxides were found to be mainly responsible for the remobilization and release of Cd (Chen et al., 2021; Parker et al., 2017). In the presence of trace levels of sulfides in sediment porewater, Cd tends to be immobilized due to the formation of insoluble CdS (Wang and Wang 2017). Application of Chelex and AgI DGT probes revealed concurrent release of Cd and sulfides, confirming the vital role of sulfur cycling in controlling Cd mobility in sediments (Zhang et al., 2020b). Furthermore, in DGT, assembly of diffusive gel layers with various pore sizes allows for the direct measurement of labile inorganically and organically bound Cd species in natural waters (Zhang and Davison 2000). However, DGT measurements are unsuitable for the quantitative determination of various labile Cd species in complex aqueous environments due to the large uncertainties surrounding ligand composition.

In general, dissolved trace metals are present in a variety of chemical forms or species, with marked differences in bioavailability and mobility depending on the physico-chemical properties and ligands. Therefore, thermodynamic chemical equilibrium models such as Visual MINTEQ and WHAM have been used to predict metal speciation in various water bodies (Charriau et al., 2011; Unsworth et al., 2005). The results showed that the predicted concentrations of Cd²⁺ and CdCl⁺ species decreased with increasing salinity, leading to the formation of CdCl₂ in the overlaying water column, whereas dissolved Cd tended to complex with sulfides in sediment porewater (Charriau et al., 2011; Ramteke et al., 2021). When dissolved organic matter was taken into account, the contribution of free Cd²⁺ to dissolved Cd decreased, as calculated using Visual MINTEQ (Cornu et al., 2009). In neutralized acid mine drainage (NAMD) water, the dissolved Cd, consisting principally of labile species (sum of free ion and inorganic and weak organic complexes), was roughly equivalent to the DGT-measured Cd, indicating that the dissolved Cd was fully available for DGT uptake (Omanovic et al., 2015). In contrast, complexation of humic substances with Cd may result in low uptake of Cd by DGT in freshwater (Sierra et al., 2017). Various physico-chemical properties of the aquatic environment can affect the chemical speciation characteristics of dissolved Cd, altering its migration potential as assessed by DGT. Integrating DGT measurement into chemical speciation modeling can facilitate establishment of linkages between labile metal species (e.g., free ion and inorganic and weak organic complexes) and the fraction measured by DGT to elucidate the remobilization mechanisms of sediment metals. Fick's first law of diffusion has been broadly applied to estimation of the diffusive flux of individual metals across the sediment-water interface (SWI) for identification of sources and sinks (Blasco et al., 2000). However, estimation based on the total concentrations of dissolved Cd might be problematic due to apparent differences in diffusion coefficients between Cd-inorganic and Cd-organic complexes in aqueous environments (Zhang and Davison 1999). Estimation of individual Cd species can eliminate such errors, enabling precise quantification of Cd flux at the SWI.

With an area of 56,000 km² and a population of more than 70 million, the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), located in the Pearl River estuary in South China, is one of the four largest bay areas worldwide in terms of gross domestic production. Due to the rapid urbanization and social-economic development that has occurred since the 1990s, the GBA has suffered from serious trace metal contamination associated with anthropogenic discharge, particularly riverine sediment contamination with the toxic element Cd (Gao et al., 2021; Gao et al., 2017; Wang et al., 2015). Based on hypotheses of seasonal cycling of Cd source and sink phases between the sediment and water column resulting from changes in natural conditions and the impacts of anthropogenic activities, this study selected the highly urbanized Xizhi River basin, located near an important water source area in the GBA, as a study area to reveal the seasonal characteristics of Cd migration and immobilization behaviors in sediment, identify key driving factors or geochemical reactions that control remobilization of sediment Cd and quantify the migration processes of various chemical forms of Cd across the SWI.