Around the 1950s/1960s, even when it was scientifically proven that the Minamata Disease (previously termed by laymen as the strange disease) was caused by water-borne mercury, Chisso was aware that their wastewater contained traces of mercury. This was due in part to an experiment whereby Chisso authorized local researchers to feed cats wastewater contaminated food, resulting in cat #400 developing symptoms typical of the Minamata Disease (movement dis-coordination). Despite this, Chisso refused to acknowledge and amend their faults. Unknowing to them, the buried truth of the experiment would eventually come back to affront them in the future. Today, a tombstone commemorating the death of the cats used in Chisso’s wastewater toxicity experiment stands outside Soushisha’s Minamata Disease Museum as a permanent reminder of Chisso’s fault as perpetrator in the Minamata Disease Incident.
Besides the aforementioned experiment, other evidence exist to trace Chisso’s responsibility in causing the Minamata Disease outbreak in 1953. According to research by Professor Nishimura (The University of Tokyo), Chisso initially (1931) utilized their own process for oxidizing the Mercury Oxide catalyst reduced during the process of acetaldehyde synthesis in order to avoid paying the high patent cost attached with the prevalent commercial method. Chisso’s process involved using Permanganate (MnO4–) as an oxidizing agent while the prevalent commercial method has Ferric iron (Fe3+) . However, the reason behind the effectiveness of Chisso’s method was ironically due to the presence of Ferric iron impurities in the Permaganate oxidizing agent. Chisso’s method also coincidentally lowered the production of Methyl Mercury to about a tenth of what would have been produced if the commercial method had been used. During the post-war years, when rebuilding and hence, material production (e.g. PVC) was important, Chisso switched their method of acetaldehyde synthesis to the commercial method, which utilized Ferric iron, in order to increase their production output. This method required the use of another oxidizing agent (Nitric Acid), to oxidixe the reduced Ferric Iron (Fe2+) back to Fe3+ (Figure 3), producing even more Methyl Mercury in the process. However, why did this commercial and widely used method result in a serious case of mercury poisoning only in the vicinity of Minamata city?
Figure 3: Chisso’s acetaldehyde synthesis process and two alternate routes to oxidize Mercury Oxide catalyst, and the corresponding amounts of Methyl Mercury (the organic compound which causes Mercury toxicity in the Minamata Disease).
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