Mobility and sulfidization of heavy metals in sediments of a shallow eutrophic lake, Lake Taihu, China

Abstract

The technique of DGT (diffusive gradients in thin films) using three diffusive gel thicknesses was applied to estimate the mobility and bioavailability of heavy metals in sediments and porewater of Lake Taihu, China. The DGT results showed significantly positive correlations between Co, Pb, Cd and Mn, and Ni and Fe concentrations in porewater. Cu and Zn showed a significantly negative correlation with Mn, due to Cu combination with carbonates and Zn derived from agricultural pollution, respectively. The rank order of average concentrations of Co, Ni and Cd at each station was DGT_1.92 > DGT_0.78 > DGT_0.39, suggesting stronger resupply from sediments to porewater when using thicker diffusive gels. Comparing centrifugation and DGT measurements, Co, Ni and Cd are highly labile; Mn and Fe are moderately labile; and Cu, Zn and Pb are slightly labile. The variations of AVS concentrations in sediment cores indicate that metal sulfides in deeper layers are easily diffused into surface sediments.

Introduction

Heavy metals accumulate in sediments mainly from natural diagenesis, and as a function of anthropogenic activities, including mineral resource extraction, industrial manufacturing, and other non-point sources (Kishe and Machiwa, 2003, Torres et al., 2013, Zan et al., 2011). Metals from different sources often accumulate with fine-grained sediments, and can then diffuse into porewater, mostly as free ions, and can form metal complexes, and metal precipitates (e.g., metal sulfides) (Lehto et al., 2006, Prica et al., 2008). The distributions of these species, as well as the processes controlling them are critical to understanding the impacts and fates of heavy metals in the environment. Generally, in net depositional areas, heavy metal concentrations in surface sediments reflect contemporary sediment quality. Free metals (e.g., Cu, Zn, Cd, Hg, Pb) in sediments may harm aquatic ecosystems, because of their toxicity, environmental persistence, mobility and availability (Sakan et al., 2009). Depending on the reduction conditions in deeper sediment layers, metal sulfides can be common, are relatively stable and insoluble, and are not toxic (until/unless the sulfide is depleted). Additionally, metals co-precipitated with clay minerals and Fe/Mn-oxides are also stable under anaerobic conditions (Yu et al., 2001). When oxygen concentrations and redox potential (Eh) values increase in surface sediments, metal sulfides and metals co-precipitated with Fe/Mn-oxides may be released into porewater and overlying water (Gao et al., 2009), increasing metal bioavailability for benthos in sediments and aquatic life in overlying waters. Thus, it is important to study the biogeochemical behavior of heavy metals during early diagenesis.

DGT (diffusive gradients in thin films) is an in situ technique for the measurement of heavy metals in natural waters, avoiding speciation change during sampling and storage periods (Davison and Zhang, 1994). The DGT device consists of a filter membrane, diffusive gel and binding gel. During DGT deployment, metals move through the filter membrane and diffusive gel, and are bound to the binding gel, and their concentrations are calculated based on Fick's first law of diffusion. Thus, DGT measured concentrations are correlated with the diffusive gel thickness, which determines the path length between the binding resin and the bulk solution (Wu et al., 2011). For a 24 hr deployment, a very low detection limit, ranging from 0.001 to 1 μg/L, is achieved using DGT (Garmo et al., 2003). Unlike DET (diffusive equilibrium in thin films) and in situ dialysis measurements, DGT accumulates metals that are mobile and bioavailable, and excludes metal complexes with humic substances (Gimpel et al., 2003). Free metals and ligands that pass through the membrane filter may form metal complexes in the diffusive gel with lowering moving rate. This costs amounts of time to establish steady state using DGT. However, this effect is negligible for 24 hr deployments, when diffusive layer thicknesses are less than 1 mm (Lehto et al., 2006). This technology has been widely used for metal measurements in natural waters, soils and sediments (Gimpel et al., 2003, Roulier et al., 2008, Zhang et al., 1998). High-resolution profiles of heavy metals in porewater can be obtained by cutting the binding gel into about 1 mm vertical intervals, followed by inductively coupled plasma mass spectrometry (ICP-MS) analysis (Gao et al., 2009). Higher resolution profiles of heavy metals are achieved by drying gels and analyzing them using Laser Ablation (LA)-ICP-MS at 100 μm vertical resolution (Motelica-Heino et al., 2003, Warnken et al., 2004). DGT is a useful tool to characterize the mobility and bioavailability of heavy metals in sediments and porewater during early diagenesis because of its negligible effect on redox conditions in deeper sediment layers. Given the co-precipitation relationship between metals and sulfides, DGT probes with a combined chelex-AgI gel have been developed to simultaneously describe the profiles of metals and sulfide at the same spatial location (Naylor et al., 2004). Heavy metal concentrations were determined using DGT and AVS (acid volatile sulfide) in both laboratory and field studies to characterize metal mobility and bioavailability during early diagenesis (de Jonge et al., 2012, Lesven et al., 2008). However, few studies have considered the importance of diffusive gel thickness in metal concentration determination using DGT.

This study is focused on evaluating the mobility and sulfidization of heavy metals in sediments of Lake Taihu, a large freshwater lake in the Yangtze Delta plain, China. The bioavailability of free metals in sediments and porewater has been described here using DGT and centrifugation measurements. Metal sulfidization in sediments is also characterized via AVS and SEM (simultaneously extracted metals). Furthermore, three kinds of DGT probes (with diffusive gel thicknesses of 0.39 mm, 0.78 mm and 1.92 mm) are deployed to estimate the metal relaxation extent from sediments to porewater.