Comparison of five in vitro digestion models to in vivo experimental results: lead bioaccessibility in the human gastrointestinal tract.

This paper presents a multi-laboratory comparison study of in vitro models assessing bioaccessibility of soil-bound lead in the human gastrointestinal tract under simulated fasted and fed conditions. Oral bioavailability data from a previous human in vivo study on the same soil served as a reference point. In general, the bioaccessible lead fraction was significantly (P<0.05) different between the in vitro methods and ranged for the fasted models from 2% to 33% and for the fed models from 7% to 29%. The in vivo bioavailability data from literature were 26.2+/-8.1% for fasted conditions, compared to 2.5+/-1.7% for fed conditions. Under fed conditions, all models returned higher bioaccessibility values than the in vivo bioavailability; whereas three models returned a lower bioaccessibility than bioavailability under fasted conditions. These differences are often due to the method's digestion parameters that need further optimization. An important outcome of this study was the determination that the method for separating the bioaccessible lead from the non-bioaccessible fraction (centrifugation, filtration, ultrafiltration) is crucial for the interpretation of the results. Bioaccessibility values from models that use more stringent separation methods better approximate in vivo bioavailability results, yet at the expense of the level of conservancy. We conclude from this study that more optimization of in vitro digestion models is needed for use in risk assessment. Moreover, attention should be paid to the laboratory separation method since it largely influences what fraction of the contaminant is considered bioaccessible.

The bioaccessibility of lead from Welsh mine waste using a respiratory uptake test.

The objective of this study was to develop an in vitro respiratory uptake test to determine the bioaccessibility of lead derived from mining waste tailings and dusts. Samples were collected from an abandoned mining area in mid-Wales, UK, the <10 microm fraction was characterized using SEM and the <100 microm fraction using XRD techniques. Gamble's Solution was employed as the synthetic lung fluid and tests were run for 630 hours in a specially designed water bath. The long test duration was specified because of the long duration of particulates in the lung after inhalation. Bioaccessible lead was determined throughout the test and the final values ranged from 15 to 41% of total lead. The extraction profile of the lead could be modeled by: Pb-extracted (M)=bxln (time, t)+c, where b and c are sample specific constants, M is the mass extracted in mg and t is the time in hours. However, despite acceptable values of R2, the standardised residuals of simple regression suggest that lead extracted is under predicted at early time and over predicted at later time. Clearly from the regression model presented the dissolution rate is declining with time and the dissolution rate decreases by an order of magnitude for the tailings tested over the duration of the test. The explanation for this is the deposition of an insoluble lead phosphate mineral during the extraction onto lead mineral surfaces that effectively limits dissolution. Based on this finding it is suggested that the in vitro extraction method described can provide a conservative estimate of bioaccessible lead for a shorter duration test of 100 hours.

Bioaccessibility of arsenic in mine waste-contaminated soils: a case study from an abandoned arsenic mine in SW England (UK).

This study characterises the total As concentrations and As bioaccessibility in 109 soils from Devon Great Consols Mine, an abandoned Cu-As mine in Devon, SW England, UK and discusses the soil and mineralogical factors that influence the bioaccessibility of this element. These data provide the basis for developing more accurate exposure estimates for use in human health risk assessments. The median value of the percent bioaccesible As of 15% for these As rich soils contaminated by mining activities indicated that relatively little of the total As is present in a bioaccessible form. Spatial variability of As bioaccesibility in the soils was also recognised throughout the mine site as a function of mineralogy. Multivariate statistical analysis identified a sulphide component responsible for the reduced As bioaccessibility of one cluster of soils. In the larger cluster of acidic mine soils covered by woodland As is mainly hosted in Fe oxyhydroxides whose partial dissolution is responsible for the bioaccessible As fraction. It was highlighted that the degree of Fe oxyhydroxide crystallinity might represent an important factor influencing arsenic bioaccessibility. Mine soils from Devon Great Consols Mine showed overall higher As bioaccessibility (15%) than other mineralised soils not affected by mining activities and background soils within the Tamar Catchment whose percent bioaccessible As median values were 9%.