Environ. Sci. Technol. 2003, 37, 1365-1370
In Vitro Gastrointestinal Method To Estimate Relative Bioavailable Cadmium in Contaminated Soil JACKIE L. SCHRODER, NICHOLAS T. BASTA,* AND JITAO SI Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, Oklahoma 74078 STAN W. CASTEEL AND TIM EVANS Veterinary Medical Diagnostic Laboratory, University of Missouri-Columbia, Columbia, Missouri 65211 MARK PAYTON Department of Statistics, Oklahoma State University, Stillwater, Oklahoma 74078
The capacity of an in vitro gastrointestinal (IVG) method to predict relative bioavailable Cd from soil ingestion was evaluated. Bioaccessible Cd determined by the IVG method was compared with relative bioavailable Cd measured from dosing trials using juvenile swine for 10 soils contaminated with Cd from 23.8 to 465 mg kg-1. The effect of the food-dosing vehicle (i.e., dough) in the IVG method was evaluated. Bioaccessible Cd was measured in the gastric extraction and intestinal extraction steps of the IVG method. Means for bioaccessible Cd, in parentheses, fell in the following order: gastric extraction step without dough (63.0%) > intestinal extraction step without dough (39.1%) > gastric extraction step with dough (38.2%) > intestinal extraction step with dough (12.9%). It is possible that phytic acid associated with the addition of dough decreased bioaccessible Cd. In vivo relative bioavailable Cd ranged from 10.4 to 116% with a mean of 63.4%. Linear relationships between the IVG gastric extraction step without dough (r ) 0.86), the IVG intestinal extraction step with dough (r ) 0.80), and the in vivo relative bioavailable Cd were found. Inexpensive in vitro methods might be useful in estimating the relative biovailability of Cd in soils from contaminated sites.
smelting of (Zn) ores, atmospheric deposition from metallurgical industries, incineration of plastics and batteries, sewage sludge application to land, and burning of fossil fuels (9). Total releases of Cd to the environment due to anthropogenic activities are estimated to range from 2000 to 6500 MT year-1, with major contributions from mining activities and the burning of fossil fuels (3). Cadmium is considered a human carcinogen by the International Agency for Research on Cancer (2), as well as a probable human carcinogen by the United States Environmental Protection Agency (10). Human exposure to Cd can occur through the consumption of contaminated foods or drinking water, the incidental ingestion of soil or dust, the inhalation of Cd-containing particles from ambient air, or the inhalation of vaporized Cd in cigarette smoke or from working in occupations involving exposure to Cd fumes and dust (1). Chronic exposure to Cd can result in obstructive pulmonary disease, emphysema, and kidney disease (3, 11). Cadmium is absorbed and retained by both terrestrial and aquatic plants and, as a consequence, concentrated in the liver and kidneys of animals that consume these plants. Extremely high dietary intake of Cd has been associated with osteomalacia, osteoporosis, and spontaneous fractures, which are conditions collectively termed “itai-itai” (ouchouch) that were originally documented in postmenopausal women living in the Cd-contaminated Fuchu area of Japan prior to and during World War II (12). Cadmium pollution of soil has been reported in 433 of the 766 National Priorities List hazardous waste sites, and concentrations as high as 750 mg kg-1 have been reported in soils in the vicinity of Zn smelters (3, 13). Incidental ingestion of soil by children is an important pathway in the assessment of public health risks due to exposure to metalcontaminated soils. Most risk from Cd in ingested soil or waste materials is associated with the fraction of the ingested soil or waste material that is available for absorption from the gastrointestinal tract into the circulatory system. The amount of Cd absorbed through the gastrointestinal tract (bioavailable Cd) can be described in absolute or relative terms. Absolute bioavailability (ABA), also referred to as the oral absorption fraction, is given by
ABA )
absorbed dose ingested dose
(1)
Relative bioavailability (RBA) is the ratio of the ABA of Cd present in some test material (study soil) to the ABA of Cd in an appropriate reference material, that is
Introduction Cadmium (Cd) is a naturally occurring metal found as a mineral combined with other elements such as oxygen (e.g., CdCO3) or sulfur (CdS, CdSO4). Cd typically ranges from 0.1 to 1.0 mg kg-1 in the earth’s crust (1-3). The typical mean Cd content for surface soils worldwide averages 0.53 mg kg-1 and ranges from 0.06 to 1.1 mg kg-1 (4). Cd levels in uncontaminated surface soils of the United States range from 0.005 to 2.4 mg kg-1, with a geometric mean of 0.27 mg kg-1 (5). Cadmium is used for a variety of industrial and consumer materials, including nickel-cadmium batteries; stabilizers for poly(vinyl chloride); pigments used in plastics, ceramics, and glasses; engineering coatings on steel and some nonferrous metals; and components of specialized alloys (2, 6-8). Cadmium contamination of soil can result from mining and * Corresponding author phone: (405) 744-9568; fax: (405)7445269; e-mail:
[email protected]. 10.1021/es026105e CCC: $25.00 Published on Web 02/21/2003
2003 American Chemical Society
RBA )
ABA (study soil) ABA (reference material)
(2)
Cadmium chloride (CdCl2), a readily soluble form of Cd and thus easily absorbed from the gastrointestinal tract, is used as the reference material in the critical toxicity study reported in the Integrated Risk Information System (10). Relative bioavailability can be determined experimentally without specifically measuring absolute bioavailability. For example, the tissue concentration of Cd in animals dosed with study soil can be compared with the tissue concentration of Cd in animals dosed with reference material. In this case, the relative bioavailability is defined by
RBA )
tissue Cd (study soil) tissue Cd (reference material)
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Often, baseline risk assessments used for contaminated sites assume that the relative bioavailability of Cd is 100% (i.e., the bioavailability of Cd in contaminated soil/media is the same as the bioavailability of CdCl2 used in the IRIS critical toxicity study). However, because of the different geochemical and physical forms of Cd present in contaminated soils and waste, the relative bioavailability of Cd is likely to be less than 100% and might pose less risk to humans than highly soluble forms of ingested Cd. The soil matrix lowered the relative bioavailability of Cd to rats in a dosing study using Cd-spiked artificial soil (14). The relative bioavailability of metal contaminants (e.g., Pb and As) in waste materials from hazardous waste sites has been assessed using in vivo animal dosing trials and used for risk assessment. In vivo dosing trials using animal models (juvenile swine, monkeys) are both lengthy and extremely expensive. Most dosing studies using contaminated soil have focused on Pb and As. Fewer studies have focused on measuring Cd relative bioavailability in contaminated soil using animal models (15). Less expensive in vitro chemical extraction methods that simulate gastrointestinal biochemistry have been developed to estimate relative bioavailable Pb (16, 17) and As (18). The amount of contaminant dissolved in the gastrointestinal environment that is soluble and available for absorption is termed “bioaccessible”. Several in vitro methods are sequential extractions with two distinct extraction steps: a gastric-phase extraction that simulates the acidic biochemical stomach environment and a subsequent intestinal-phase extraction that simulates the biochemical environment of the small intestine. The fraction of the contaminant dissolved by the in vitro procedure, the bioaccessible contaminant, has been used to estimate the relative bioavailability of the contaminant in soil (19). Hamel et al. (20) reported an in vitro method for estimating bioaccessible Cd in soils, but they did not relate their method to relative bioavailable Cd measured by an animal model. The in vitro physiologically based extraction test (PBET) of Ruby et al. (17), which does not use food in the extraction to mimic fasting conditions, has been shown to predict accurately relative bioavailable Pb in contaminated soil and media (19). The in vitro gastrointestinal (IVG) method developed by Rodriguez et al. (18) is an accurate predictor of relative bioavailable As in contaminated soils and waste materials that uses food in the extraction procedure. To our knowledge, an in vitro method for estimating relative bioavailable Cd associated with soil ingestion has not been reported. Also, the effect of the presence or absence of food in in vitro extraction procedures on the capacity to estimate the relative bioavailability of Cd in contaminated soil has not been reported. The objective of this study was to determine the capacity of the IVG method of Rodriguez et al. (18), with and without food, to predict relative bioavailable Cd in contaminated soil as measured in vivo in juvenile swine.
Methods and Materials Contaminated Soils. Ten contaminated soils from seven different hazardous waste sites were evaluated using the IVG method of Rodriguez et al. (18). Air-dried soil was sieved through nylon mesh (
