ENVIRONMENTAL LEAD CONTAMINATION: HEAVY METALS AND CERUSSITE IN NIGERIAN “GOLD” ORES
The processed ground ore sample contained high levels of lead (Pb), mercury (Hg), arsenic (As), antimony (Sb), and copper (Cu). The Pb levels were ~61,000 mg/kg in the ground ore and ~34,000 mg/kg in the washed ore. High As (>200 mg/kg) was associated with a ground ore whereas high levels of Hg, Sb, and Cu were found in both ground and washed ores. Hg concentrations were 73 mg/kg in ground ore, and 9.9 mg/kg in the washed ore.
Pb concentrations exceed the US EPA Residential Soil Screening Level of 400 ppm in children’s sleeping areas (as high as 1,400 mg/kg), but not in play areas. XRD analyses identified cerussite (PbCO3), a secondary weathering and alteration product of galena (PbS), in the processed ore. No Pb-bearing crystalline phases have yet been found in swept dust samples, suggesting likely abundances of <0.5 wt% cerussite (as expected with Pb <2000 mg/kg).
The presence of cerussite provides a probable explanation for the Pb poisoning and could be the primary source of Pb in the mining villages. As a carbonate mineral, cerussite is easily dissolved in biological fluids; hence, the Pb is readily bioavailable. Inhalation and/or ingestion of either cerussitic dust or amalgamation products are likely Pb exposure pathways.
These results provide a key step in understanding the risks associated with gold ore processing in Nigerian villages. Further work will entail the determination of the concentrations and geochemical partitioning of Pb, As, Hg, and Sb, especially in relation to the presence of cerussite as well as any other minerals discovered.