The impact of sequestration on the bioaccessibility of arsenic in long-term contaminated soils
Introduction
Arsenic (As) is a ubiquitous element found throughout the environment. Its presence in the environment is due to mobilisation of the element through both natural processes, such as weathering of the regolith, biological activity, as well as anthropogenic activities such as mining, agriculture and non-agricultural activities. Arsenic is a commonly encountered contaminant at polluted sites and occurs in more than 30% of US superfund sites (deLemos et al., 2006) and has been extensively identified as a contaminant throughout Europe (Keegan et al., 2006, Navarro et al., 2006, Schulin et al., 2007). In Australia, As contamination has resulted from the use of As-based herbicides, pesticides, tanning solutions, timber preservatives as well as through mining and smelting processes. Surface soil As contamination in Australia has been reported extensively (McLaren et al., 1998, Smith et al., 1999, Smith et al., 2006) with As concentrations of up to 15 000 mg As kg−1 soil being detected (Ellice et al., 2001).
Exposure to As has been shown to result in the potential development of human carcinogens and the development of other numerous health disorders (Chen et al., 1999, Mandal and Suzuki, 2002). As a result, human exposure to As in the environment is of concern to regulatory agencies and the community at large. Davis et al. (2001) reported that between 1985 and 1998, 52 out of 69 (75%) remedial goals for As clean-up were based on human health risk estimates in the USA. With this premise, in vivo animal studies and in vitro chemical methods have been developed to assess the potential contaminant exposure for humans from contaminated soil. Casteel et al. (1997) utilised an in vivo swine model to determine the bioavailability of As in a number of soil types. Arsenic bioavailability ranged between 0% and 52% of the total soil-bound As concentration and was found to be dependent on the form of As in the sample and the sample matrix. Juhasz et al. (2007a) compared the bioavailability of As in contaminated soils (from geogenic and anthropogenic sources) using the simplified bioaccessibility extraction test (SBET) and an in vivo swine model. Their results demonstrated that there was a close agreement between As bioaccessibility using the in vitro method and As bioavailability using the in vivo swine model with As bioavailability generally less than 50% of the total soil-bound As content of the soil.
While it has been recognised that soil-bound As is generally less than 100% bioavailable (Davis et al., 1996, Casteel et al., 1997, Rodriguez et al., 1999, Juhasz et al., 2007a, Juhasz et al., 2007b), there are a number of regulatory issues associated with including bioavailability in human health risk assessments. A major issue is the difficulty in estimating contaminant bioavailability in a range of different soils. Yang et al. (2002) reported that As bioaccessibility was closely related to the iron (Fe) oxide content and soil pH in arsenate spiked soils and that these parameters could be used to predict As bioaccessibility in soils. Similarly, Tang et al. (2007) investigated the ageing effect of As in five Chinese soils and the resultant reduction in As bioaccessibility as determined using the physiologically based extraction test (PBET). Using correlation analysis, Tang et al. (2007) reported that non-specifically and specifically sorbed As was likely to constitute the main soil fractions that contributed to bioaccessible As in the soils studied. These studies are timely as there is considerable interest in predicting soil bioaccessibility from soil properties. However, the studies of Yang et al., 2002, Yang et al., 2005 and Tang et al. (2007) were only limited to As spiked soils aged for up to 12 months which may not be representative of As bioaccessibility behavior of As in historically (>40 years) contaminated soils. This was illustrated by Juhasz et al. (2007b) who applied the model of Yang et al. (2002) for predicting As bioaccessibility in 50 long-term As contaminated soils from mine sites, former cattle dip sites, naturally elevated regions (gossans) and former railway corridors. A poor correlation was found between measured and predicated bioaccessible As using soil pH and dithionite–citrate–bicarbonate extractable Fe oxide content (Juhasz et al., 2007a). However, better predictability of As bioaccessibility was achieved using total As concentration and Fe content of the soil. While some similarity exists between the models of Yang et al. (2002) and Juhasz et al. (2007b) further investigation is warranted for the assessment of soil properties that may influence As bioaccessibility in long-term contaminated soils. The objective of this study was to investigate which soil fractions contribute to As bioaccessibility in selected long-term As contaminated soils utilised by Juhasz et al. (2007b) using both SBET and sequential extraction procedures.
Section snippets
Soil collection and preparation
Soils used in this study were collected from four regional areas where the soil type, mode of As input and As residence time varied. Twelve soils were used in this study and their properties are described in Table 1. Of the 12 soils studied, five soils were collected from former railway corridors in South Australia contaminated with As through extensive As herbicide applications to control grass growth during the early 1940s to the late 1960s. Four soils were collected from former cattle tick
Physiochemical properties of soils
The soils investigated in this study originated from a range of sites where As contamination was present historically through mining practices (n = 2), herbicide application (n = 5), pesticide application (n = 4) or was geogenic in origin (n = 1). Arsenic was historically used for the extensive control of grass growth along railway corridors and as a pesticide to control cattle ticks but these practices ceased over 40 years ago (Smith et al., 2006). Arsenic concentrations in the contaminated soils
Acknowledgements
The authors would like to acknowledge the support of the Australian Centre for Industry and Agricultural Research funding (LWR/1998/003/) for partial support of Dr. Euan Smith, the Australian Research Council Linkage Scheme with generous support of IPOH Pacific Ltd (Grant number LP0347301). The authors would like to acknowledge the support of the University of South Australia for providing laboratory facilities to undertake this research.
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Present address: Cooperative Research Centre for Contamination Assessment and Remediation of the Environment, PO Box 486, SA 5106, Australia.