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

Abstract

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 μ m fraction was characterized using SEM and the < 100 μ m 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) = b × ln (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.

Keywords:

• Bioaccessibility
• respiratory uptake
• lead

Acknowledgment

Professor Barry Smith, BGS, originally suggested this work. Tony Milodowski (BGS) carried out the SEM work and Doris Wagner (BGS) was responsible for the quantitative XRD determinations. This paper is published by permission of the Executive Director of the British Geological Survey, NERC.

Notes

*Number in brackets refer to Munsell Soil Colour Chart codes.

^a Diethylenetriaminepentacetic acid, not present in blood serum.

^b Alkylbenzyldimethylammonium chloride, added as an antibacterial agent.