Species distribution and potential bioavailability of exogenous Hg (II) in vegetable-growing soil investigated with a modified Tessier scheme coupled with isotopic labeling technique
Highlights
► Bioavailability of soil Hg can be reflected with an isotopic labeling technique. ► Exogenous Hg (II) can be readily bound with humus substances in soil. ► Mobility and bioactivity of Hg species in F1 and F2 fractions are remarkable. ► More than 70% of soil Hg is found to be present in the residual fractions.
Introduction
With the development of industrial and agricultural activities, large amounts of mercury (Hg) from various anthropogenic sources have been continually poured into soil especially in vegetable-growing soil, which results in a series of poisoning symptoms for human beings through food chain because of its strong toxicity even at a very low concentration (Cheng et al., 2006, Rasmussen, 1994). It is a well-known conception that a total concentration of Hg in soil is insufficient to be used to predict the metal's actual toxic and ecological impact on plant growth (Krishnamurti and Naidu, 2003, Meers et al., 2007, Park et al., 2011). The different chemical species of Hg occurring within the soil matrix determine their mobility, bioavailability and toxicity. Therefore, it is essential to investigate the Hg species in vegetable-growing soil for predicting the metal's mobility, bioavailability and toxicity.
Sequential extraction procedures (SEPs) have been commonly used to partition the different chemical species of heavy metals and to evaluate their mobility and bioavailability in soil. Although there is no general agreement on the chemical reagents performed for the extraction of Hg species in soil, the SEPs protocols based on different reagents and extractants often subdivide the Hg species in soil into several operationally defined chemical fractions (Hlavay et al., 2004, Rauret et al., 1999, Sánchez et al., 2005, Tessier et al., 1979, Ure et al., 1993). Among the SEPs protocols, the Tessier scheme, originally developed for the fluvial bottom sediments, has been widely used to partition off Hg species in soil (Tessier et al., 1979). Many protocols of modified Tessier schemes have been proposed and successfully applied to achieve the purpose (Emmerson et al., 2000). However, some determining controversy occurs on the Tessier scheme, and there is no suitable reference material available to guarantee the accuracy of the SEPs protocol to date (Boszke et al., 2006). The quality assurance on the determination of Hg species in soil by Tessier scheme is commonly achieved by comparing the total Hg concentration directly measured with the summed value of Hg contents in all the subsequential fractions in the SEPs protocol including residual species.
Similar to radio‐isotopic labeling technique, stable isotopic labeling method is usually carried out by administering an amount of enriched stable isotopic tracer in the studied system to induce a detectable shift of the tracing isotopic abundance in the sample of interest (Sterckeman et al., 2009, Stürup et al., 2008). Some advantages have been found by the stable isotopic labeling technique over the traditional radioisotope labeling method, such as a non-radioactive environment for the operator, less disposal and waste restrictions, and without toxicological concerns for the living organisms. In addition, the determination of isotopic tracers including isotope ratios based on inductively coupled plasma mass spectrometry (ICP-MS) is becoming increasingly popular with several advantages compared with the traditional thermal ionization mass spectrometric technique (TIMS), including excellent sensitivity, high precision, and simple sample preparation steps (Dobney et al., 2000, Becker, 2002, Munksgaard and Parry, 1998, Pomiès et al., 1998, Rosman et al., 1998, Stürup, 2004). At present, the stable enriched isotopic labeling technique is mostly performed using “light” isotope tracers such as C, O, N and H, and has been widely applied to living organisms (Chen et al., 2011, Gao et al., 2011). However, the method by using “heavy” isotopic tracers has been gradually applied to various scientific areas. For example, 201Hg2 + and CH3200Hg+ were used to investigate the influence of algal biomass on Hg accumulation through food web (Pickhardt et al., 2002); The enriched 111Cd2 + tracer was used to label eels in water to investigate Cd accumulation and the distribution of Cd species (Rodríguez-Cea et al., 2006); The enriched isotopic 207Pb2 + tracer was sprayed on the forest floor to simulate rainfall in order to investigate the transmit of atmospherically depositing Pb in soil profile (Kaste et al., 2003). The previous results described above demonstrate that the stable enriched isotopic labeling method is suitable for investigating the distribution, bioactivity, mobility and potential bioavailability of heavy metals in soil–vegetable system with the merits of accuracy, precision and convenience. In addition, the SEPs protocol combined with the stable enriched isotopic tracers has been proved to be a powerful approach to characterize the metals' distribution and mobility in soil, which can provide the useful information of metal's bioactivity and toxicity in soil (Huang et al., 2011).
However, there are few reports about the distribution, mobility and potential bioavailability of exogenous Hg (II) in vegetable growing contaminated soil. The aim of this study is to evaluate the different chemical species of Hg occurring within the vegetable growing soil by a modified Tessier scheme. In addition, an exogenous Hg (II) is simulated to contaminate the soil. The distribution and mobility of the different Hg species in soil are investigated by the SPEs protocol coupled with the stable isotopic labeling technique. The bioactivity and the potential bioavailability of different chemical species of Hg in soil are adequately discussed and evaluated in the present study.
Section snippets
Soil samples
Three topsoil samples were collected from large pieces of vegetable-growing land in Fujian Province, China. The sampling site of GKLL soil, located at the rural area of Xiamen City, is designated as a pollution-free vegetable-growing land. The sampling site of JMHX soil located at the outskirts of Xiamen City has been rapidly urbanized in the recent years. The last sampling site of SMNS soil is located in the suburb of Shima City, where an industrial estate is located nearby. At least six
Soil characterization
As shown in Table 2, the pH values indicated that all the soil samples were acidic, which may attribute to the high frequency of acid rain in Fujian Province, China recently. The contents of soil organic matters (SOM) indicated that all the samples were of medium soil fertility. The results in Table 2 also showed that all the soil samples were contaminated with Hg because the total Hg concentrations in the three samples were greatly higher than the soil background values of 0.081 mg kg− 1 in
Discussion
Comparing the total Hg directly measured with the summed value of Hg concentrations in all the subsequential fractions by Tessier scheme is usually carried out to validate the accuracy of the SEPs protocol due to the unavailable convincing reference material. High consistency of the total Hg directly measured with the summed value of Hg concentrations in a SEPs protocol of Tessier scheme has been reported (Hou and Yin, 2007). However, the summed values of Hg concentrations in the similar SEPs
Conclusions
Based on the present study, the summed value of Hg concentrations in all fractions partitioned with a modified Tessier scheme was lower than the total soil Hg directly measured. More than 70% of intrinsic Hg in soil was found to be present in the residual fractions, and the exogenous Hg (II) was found to be difficult to distribute in the residual fractions. The results indicated that Hg species in residual fractions might be biologically inert for plants in soil. The exogenous Hg (II) in soil
Acknowledgments
The authors would like to thank the National Natural Science Foundation of China (40771185), the Natural Science Foundation of Fujian Province of China (2012J01046), the Science and Technology Planning Project of Fujian Province, China (2012Y0052), the Science and Technology Planning Project of Xiamen, China (3502Z20113024), the Foundation of the Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences (KLUEH201006), and the Foundation for
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