Case 2: Bio-solubility of Lead in Mine Waste Contaminated Soils

Background

Elevated levels of lead in children's blood that results from environmental exposure may have serious implications for the health of the child. Pb in soil and dust is considered to be the most important source of such exposure. Pb in these media is inadvertently consumed as a result of normal hand-to-mouth activity by the child. A necessary prerequisite for the absorption of this ingested Pb is gastrointestinal solubility. This is because the amount of Pb absorbed across the child's intestinal wall into the bloodstream will be determined by the quantity of dissolved Pb which leaves the stomach and enters the intestine. The amount of Pb solubilized in the stomach is in turn controlled by the ease with which various forms of ingested Pb are dissolved under stomach acid conditions.

Aims of the Study

A simple in vitro dissolution model was used to assess the behavior of individual soil Pb-bearing particles present in the low pH environment of the stomach. The primary objective of this study was to identify specific particle phases which have the potential to be solubilized under these weak acid conditions, and hence, possibly, be bioavailable if ingested by the young child.

Materials and Methods

Fine particulate material was sampled from the surface soil of a mine waste tailings heap (Hams Pile) in Leadville, Colorado. Computer controlled SEM analyses were performed on particles screened through a <40 µm mesh. The fine particulate material in dust and soil is preferentially retained on the hands of young children. To determine the weak stomach acid solubility of the different constituent particulate Pb phases, computer controlled SEM analysis was performed on both pristine soil and on soil after acid treatment. This "differential" particle analysis makes it possible to determine the proportion of Pb present in different forms and to assess any variations in solubility between phases.

The simulated stomach fluid which was used in the in vitro tests consisted of a dilute solution of HCl (pH 1.2) with additional small quantities of: a) the protein-splitting stomach enzyme pepsin, b) citric, acetic, malic and lactic acid. The protocol required the addition of 100mg of sieved soil to a test tube containing 15 ml of the simulated gastric fluid. Samples were placed in a rotary action shaker incubator for 2 hr at 37°C at 250 rpm. The pH was checked and, if necessary, adjusted every 15 minutes for the first 60 minutes and then again at 90 minutes. After 2 hours, a 200 µl aliquot of suspension was removed for computer controlled SEM analysis and the remaining sample processed for total Pb analysis. The solid and liquid phases were separated by vacuum filtering the suspension through a 0.8 µm pore size polycarbonate membrane filter. The fluid which passed through this filter was then filtered a second time through a 0.1 µm pore size membra ne. Both filters were consolidated for subsequent analysis. The Pb content of the supernatant and the solid residue were determined, following a strong nitric acid digest, by atomic emission spectroscopy.

Discussion

Computer controlled SEM analysis was confined to the high atomic number particles in the samples. Initial data obtained from over 500 particles in the pristine sample permitted the classification (by element composition) of the particles into 9 major types using a divisive hierarchical clustering procedure. Of the 9 particle groupings, 5 were identified as Pb-bearing, while the other 4 groups contained little or no Pb (see table). Total sample analysis of duplicate soils gave values for the solid residue of 780 and 830 µg of Pb, and for the filtered fluid of 142 and 151 µg/ml of Pb. For an initial volume of fluid in each case of 15 ml the amount of Pb solubilized was 2130 and 2265 µg, or roughly 3 times the amount left in the residue. This dissolution process was refl ected in the computer controlled SEM analysis of the residue particles. Following the acid attack, 3 of the Pb-bearing particle types (classes Pb 1,2,3) were essentially no longer present while the other particle types were unchanged (see figure). The computer controlled SEM analysis clearly demonstrated which Pb particle phases in the tailings soil were susceptible to simulated stomach acid dissolution (Pb, Pb+P/Ca/Cl, Pb+Mn), and which were not (Pb+ Mn/P, Pb+Mn/Fe/S). Clearly, certain particle types (approximately 50% of the Pb particles, but which likely account for most of the Pb) pose a greater risk if ingested than other particle types in this soil.