After the outbreak of Minamata disease, Kumamoto University research team had suspected the contribution of Chisso’s wastewater to the causation of mercury poisoning. However, Chisso had firmly denied the accusation by posing the questions of – if it were them, then 1) why the disease only happened until year 1953 but their operations were dated back to year 1932; 2) why it only happened in Minamata but not elsewhere with the same industrial process. This blog post is intended to explain the science concisely based on the 20-year investigation of Prof. Nishimura, recorded in his book the Science of Minamata Disease.
Why the disease only happened until 1953 but not earlier?
Figure 1. Catalytic processes that produce methyl mercury in Chisso’s factory. Based on the book of Science of Minamata（水俣病の科学、日本評論社).
An event that is overlooked by investigators is a major change of acetaldehyde production process of Chisso in year 1951. The Kucheroff’s method involves the use of mercury oxide to catalyst the acetaldehyde production, but somehow the acetaldehyde product degrades the mercury oxide to metal mercury. To reactivate the catalyst, or oxidize the mercury, Chisso has invented their unique process that uses manganese oxide (MnO2). The more common way to reactivate the catalyst at that time is using ferric ion (Fe3+), which involves an additional step of reactivating ferrous to ferric ion; such co-catalyst method has been patented. Chisso has changed to the latter method only until 1951. Because of changing into a more popular method, investigators have overlooked the impact of this change for a long time. In fact, the experiments done in recent days show that, the Chisso’s original method involved the permanganate (MnO4-) that may suppress the formulation of methyl mercury; comparing to a condition without permanganate, the observed methyl mercury is 10 times higher. Therefore, the lower methyl mercury concentration in discharged water before 1951 operation might explain why the disease only happened until later. The explanation is illustrated in Figure 1.
Figure 2. Formation of organic mercury in Minamata and Niigata’s cases in response to Cl- concentration. Based on the book of Science of Minamata（水俣病の科学、日本評論社).
Why the disease only happened in Minamata but not in a factory elsewhere?
Mercury poisoning does not happen in other factory only hold true for sometimes. In 1965, it has happened in Niigata (Showa Denko’s plant) but the impact scale is much smaller. This evidence sheds light to the study of why methyl mercury is magnified in Minamata. Recent chemistry shows that, the chemical species of methyl mercury is different in response to chlorine (Cl-) concentration of reactor water and discharging environment, i.e. in molecular form or ionic form. For the case of Minamata, the factory is located by the bay; seawater has been reported to use as cooling water, also the wastewater is discharged to seawater environment; the abundances of chlorine in both condition will promote the formation of methyl mercury chloride molecule, which is stable or hard to degrade over time. For the case of Niigata, the wastewater is discharged to river instead; without chlorine, methyl mercury ion has high affinity to protein that make it easily attached on phytoplankton, zooplankton etc. With such particulate matter, mercury is likely to settle down to the bottom of river. The pollution becomes less obvious in this case, until 1) the benthic animal consumes the polluted sediments and passes on through the food chain, and 2) the sediment is transported from river to sea, where the chlorine becomes abundant. The explanation is illustrated in Figure 2.
Author: Heng Yi Teah; Figure by Heng Yi Teah; Photo taken by Minamata Unit 2015; Contributors: Mahdi Ikhlayel, Angeli Guadalupe, Joanne Khew