Mercury in historical silver refining wastes in Guanajuato, Mexico
Silver (Ag) production in the Americas between the 16th and 19th century is thought to be one of the largest sources of anthropogenic mercury (Hg) emissions in history. A closer look at the chemistry of the patio process involved in silver refining during that time suggests that the production of calomel (Hg2Cl2), a solid by-product in which Hg is present as Hg(I), has been overlooked, resulting in an overestimation of Hg emissions into the atmosphere. However, calomel has yet to be reported in the environmental wastes from such processes. This thesis examines whether there is evidence of calomel in wastes generated from the patio process in Guanajuato, Mexico, using a combination of in-situ gaseous elemental mercury (Hg(0) or GEM) analysis, total Hg, qualitative Hg(I) analysis, scanning and transmission electron microscopy (SEM and TEM), and a calomel photodegradation experiment. The results showed widespread, elevated GEM concentrations in the ground-level air (range: 8 to 454 ng m–3) and in the interstitial air of reprocessed mineral wastes, sediments, and soils (maximum: 44,700 ng m–3) in Guanajuato. The qualitative Hg(I) analysis indicated the presence of Hg(I) in all analyzed samples, which is diagnostic of the presence of calomel. This is further supported by the results of the calomel photodegradation experiment, in which the GEM pattern agreed with our field observations when mineral wastes were exposed to the sunlight. The microscopy analyses of a waste-containing soil sample suggested that Hg was present in all valence states, with HgS being the dominant species. Overall, our results are consistent with calomel degradation via disproportionation, forming GEM and Hg(II). GEM is held in the interstitial air of wastes until released to the ground-level air upon disturbance, while Hg(II) is likely fixed in the environment as HgS. The findings of this work stress the need for further investigation of the fate of Hg used in the historical patio process, especially the environmental stability of the calomel by-product. The work should also provide grounds for the implementation of long-term mercury monitoring in Guanajuato and in other historical silver mining regions to assess the health and safety risk.
Mercury, Calomel, Patio process, Amalgamation, Silver, Guanajuato