In reality, the Minamata Disease did not just occur in Minamata City (and its vicinity). In 1965, Methyl Mercury was also released by a factory into a river in Niigata, resulting in people around the vicinity exhibiting a range of Mercury toxicity symptoms. However, what was different in the case of Methyl Mercury discharge in the case of Showa Denko (Niigata) and Chisso (Minamata City) was that the latter was situated nearer to the sea and hence wastewater was discharged directly into the ocean waters. According to Professor Nishimura’s research (The Science of the Minamata Disease -水俣病の科学), the concentration of Chlorine ion present in solution would affect the solubility of the Methyl Mercury compound. When wastewater containing Methyl Mercury is discharged directly into seawater (as in Chisso’s case), the high concentration of chlorine ions present readily combine with it to form the soluble Methyl Mercury Chloride compound. This compound is highly mobile in water and can be easily taken into body tissue of sea-creatures and humans that directly or indirectly ingest the polluted water.
In the case of mercury pollution in Niigata, mercury-containing wastewater was discharged into a river where concentration of chlorine ion is comparatively lower than seawater. The Methyl Mercury ion then conjugates with abundant organic debris present in the river water (e.g. protein colloids, algae) and sinks into the riverbed. As Methyl Mercury in this form is insoluble and therefore, not mobile, it is not as readily transferred to the human body. However, mobility of this form of Methyl Mercury can occur when it is consumed by benthic organisms as sediment; and biologically magnified up the food chain; or when it is transported to the sea along with riverbed sediment through erosion (Figure 4). Once at the sea, the Methyl Mercury ion trapped in the insoluble compound can recombined with the abundant supply of chlorine ions in the seawater, forming soluble Methyl Mercury Chloride. In Chisso’s case, seawater was also used for the cooling of factory reactors, resulting in the formation of more Methyl Mercury Chloride when seepage occurred. As such, this post, combined with the previous one [Examining the cause of the Minamata disease (1)], thus answers the two questions posed by Chisso on the post: An Introduction to the Minamata Disease (2) ｜Chisso Factory and Mercury Pollution.
Figure 4: The different forms of Methyl Mercury: i. The case of Minamata City where Methyl mercury is released directly into the sea; ii. The case of Niigata where Methyl Mercury is released into the river and conjugated with sediment
Source: Adapted from the Science of Minamata (水俣病の科学)
Author: Joanne Khew; Figure by Heng Yi Teah; Contributors: Mahdi Ikhlayel, Heng Yi Teah, Angeli Guadalupe