Elsevier

Science of The Total Environment

Volume 416, 1 February 2012, Pages 127-136
Science of The Total Environment

Diffusive gradients in thin films (DGT) for the prediction of bioavailability of heavy metals in contaminated soils to earthworm (Eisenia foetida) and oral bioavailable concentrations

https://doi.org/10.1016/j.scitotenv.2011.11.007Get rights and content

Abstract

The applicability of diffusive gradients in thin-films (DGT) as a biomimic surrogate was investigated to determine the bioavailable heavy metal concentrations to earthworm (Eisenia foetida). The relationships between the amount of DGT and earthworm uptake; DGT uptake and the bioavailable concentrations of heavy metals in soils were evaluated. The one-compartment model for the dynamic uptake of heavy metals in the soil fitted well to both the earthworm (R2 = 0.641–0.990) and DGT (R2 = 0.473–0.998) uptake data. DGT uptake was linearly correlated with the total heavy metal concentrations in the soil (aqua regia), the bioavailable heavy metal concentrations estimated by fractions I + II of the standard measurements and testing (SM&T) and physiologically based extraction test (PBET, stomach + intestine). The coefficients of determination (R2) of DGT uptake vs. aqua regia were 0.433, 0.929 and 0.723; vs. SM&T fractions (I + II) were 0.901, 0.882 and 0.713 and vs. PBET (stomach + intestine) were 0.913, 0.850 and 0.649 for Pb, Zn and Cu, respectively. These results imply that DGT can be used as a biomimic surrogate for the earthworm uptake of heavy metals in contaminated soils as well as predict bioavailable concentrations of heavy metals estimated by SM&T (I + II) and PBET as a human oral bioavailable concentrations of heavy metals.

Highlights

► Diffusive gradients in thin-films (DGT) can estimate bioavailable heavy metal concentrations in contaminated soils. ► Earthworm uptake can reflect the bioavailability of heavy metals in the soils. ► SM&T (I+II) and PBET (stomach+intestine) also reflect the bioavailability of heavy metals in the soils. ► Good relationship between DGT and earthworm uptakes is estimated by one-compartment model analysis. ► DGT can be used as a biomimic surrogate for the earthworm uptake of heavy metals in the soils.

Introduction

In the risk assessment of heavy metal-contaminated soil, the total heavy metal concentration in the soil does not reflect the bioavailable fraction considering the toxicity to plants and organisms (Sanchez-Martin et al., 2007). Toxicity measurement of heavy metals on earthworms gives a more realistic estimation of the bioavailability of heavy metals in the soil than that given by chemical extractions (Harmsen, 2007, Lu et al., 2005).

Earthworms have been used as test organisms to determine bioaccumulation of heavy metals from contaminated soils. Among various earthworm species, Eisenia foetida was selected as the reference earthworm in the international toxicity tests (OECD, 2004) and by other researchers (Conder and Lanno, 2000, Spurgeon and Hopkin, 1999) because E. foetida can be cultured in large quantities and can mature within 6–8 weeks with a high reproductive rate. In addition, these species respond to a wide range of toxicants such as Pb, Ni, Cd, Cu, Zn and Cr (Conder and Lanno, 2000, Nahmani et al., 2009). In the earthworm uptake, the dynamic approach towards the bioavailability in soils was considered (Peijnenburg et al., 1999). In this approach, as a physico-chemically driven desorption process heavy metal uptake occurs through the soil and porewater to the earthworm, which is called bioavailability. Metal bioavailability to earthworms can be evaluated in terms of the relative toxicity, the biota-to-soil accumulation factor (BSAF) and the tissue concentration (Vijver et al., 2005). Vijver et al. (2005) explained that the replenishment of the labile phase heavy metals from the soil into the porewater can be calculated by comparing the absolute uptake by the earthworm with the absolute amount of metals in the porewater.

Total heavy metal concentration in the soil is a poor predictor of the bioavailability of metals to plants and organisms because soil properties such as pH, soil texture and organic matter content affect the bioavailability (McLaughlin et al., 2000). Diffusive gradients in thin films (DGT), an in situ device, can measure the flux of labile species in water, soil and sediment with undisturbed conditions and without pre-treatment. In the uptake process, heavy metals from the soil and porewater diffuse through the diffusion layer to the binding phase (Chelex gel) (Davison et al., 1994, Zhang et al., 1995). As in organism uptake, DGT uptake also lowers the heavy metal concentrations in the porewater in the vicinity of the DGT unit, which leads to have the heavy metal desorbed from the soil into the porewater at the same time. Thus, DGT can measure the metal available from the complete soil system (Zhang et al., 2001). Toxicity or bioaccumulation of heavy metal is usually predicted by considering the soil solution concentration, free metal ions in the soil solution and some operationally defined extractable fraction. These predictions do not account for the buffer capacity of the soil, i.e., its ability to replenish the supply to the porewater (Nowack et al., 2004). This method was applied for correlating the metal uptake by plants (Black et al., 2011, Perez and Anderson, 2009, Roulier et al., 2008). It was also used in in vitro unified barge method (UBM) solution containing Pb, Zn and Cu to predict the heavy metals accumulated through the intestine in human body (Pelfrene et al., 2011). However, the DGT was not studied to compare uptake by earthworms and other animals or estimation of the heavy metals in terms of bioavailability such as standard measurements and testing program (SM&T) and physiologically based extraction test (PBET), the oral bioavailable concentration to human beings.

The one-compartment model was used to estimate the heavy metal concentrations in the biota body over time (Peijnenburg et al., 1999). In this model, the bioavailability of heavy metals to earthworms is evaluated in terms of the BSAF, determined based on the concentrations of heavy metals accumulated in the earthworm tissue and concentrations of heavy metals in the soil. Thus, the BSAF of the earthworm gives an indication of the bioavailable concentrations of heavy metals present in the soil. The one-compartment model was also used to fit the DGT uptake data over time. Just like the BSAF, we attempted to evaluate the bioavailability of heavy metals to DGT uptake by defining a new concept of DGT-to-soil-accumulation factor (DSAF) as an indicator for the bioavailable concentrations of heavy metals in soils.

The bioavailable concentrations of heavy metals in the soils were estimated by SM&T (formerly BCR) and PBET. The SM&T was used for evaluating the heavy metal mobility in soil and sediment using four sequential extraction steps (Gismera et al., 2004, Janos et al., 2004). From the first two-steps of SM&T (SM&T I + II), the readily bioavailable fractions and concentrations of heavy metals could be estimated (Alvarez et al., 2002, Nemati et al., 2009). These sequential extraction procedures help to distinguish fresh and aged soils. PBET was used as an in vitro extraction method to determine the oral bioavailability of heavy metals to humans. It estimates the heavy metals available to the human body in the stomach and intestine (Ruby et al., 1996).

The major objectives of this study are: i) to determine bioavailability of heavy metals in contaminated soils (former metal refinery site and abandoned mine tailings) to earthworm (E. foetida), ii) to elucidate that DGT can be used as a biomimic surrogate of earthworm via one-compartment model, iii) to verify that DGT can estimate the total concentration (aqua regia), the bioavailable (SM&T) and in vitro human bioavailable concentrations (PBET) of heavy metals in the contaminated soils.

Section snippets

Chemicals

Glacial acetic acid (98%) was purchased from Yakuri Pure Chemical Co., Japan. 1 N HNO3, hydroxylamine hydrochloride (NH2OH HCl, 98%) and ammonium acetate (CH3COONH4, > 97%) were purchased from Daejung, Korea. HCl (35–37%) was purchased from Junsei Chemical Co., Japan and HNO3 (64–66%) from Duksan Chemical Co., Korea. Pepsin (1:1000 from porcine stomach mucosa), citrate (citric acid, > 99.5%), bile salt (bile extract porcine) and pancreatin (pancreatin from porcine pancreas) were purchased from

Physico-chemical characteristics of the soils

The physico-chemical characteristics of the seven different soils are summarized in Table 2. The organic carbon content (foc) and clay content in the soils ranged from 0.381 to 2.81% and from 5.30 to 53.0%, respectively. The CEC of the soil samples was between 0.220 and 23.3 mmol/100 g. The pH value was the highest in H soil (8.71) and the lowest in M soil (5.20). The WHC of the soil samples ranged from 40.0 to 65.0% of the dried soil. DOC was the highest in H soil (104 mg/L) and was the lowest in

Conclusions

In the risk assessment of heavy metal contaminated soil, the bioavailable concentrations of heavy metals are more important than the total concentrations of heavy metals. In this study, the DGT technique was used as a biomimic surrogate of heavy metal uptakes in earthworm (E. foetida) and to predict the bioavailable concentrations of heavy metals in soils. The one-compartment model results showed that the DGT uptake was as good as earthworm uptake and can predict the bioavailable concentration

Acknowledgment

The authors would like to acknowledge the Korean Institute of Environmental Science and Technology (KIEST) for the financial support of this work (project #080030046).

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