Mercury speciation in preserved historical sludge: Potential risk from sludge contained within reclaimed land of Minamata Bay, Japan

Abstract In the latter half of the 1950s, a large amount of methylmercury (MeHg) was discharged into Minamata Bay, Japan by a chemical plant, resulting in the contamination of the fish and shellfish. Ultimately, an outbreak of MeHg intoxication, called Minamata disease, occurred. A multi-year (1977–90) remediation project dredged and transferred sediments exceeding 25 μg/g (dry basis) into a strictly segregated area of the bay near the wastewater outlet, and this portion was landfilled. We conducted analyses of the mercury speciation in preserved sludge samples (n = 4) collected from the reclamation land prior to the termination of the remediation project and recent sediments (n = 5) collected outside the dredging area of the Minamata Bay to evaluate the potential risk of the sludge/sediment leaking from the reclaimed land. Median total mercury (THg, dry basis) concentrations were 241 μg/g for the preserved sludge, 6.1 μg/g for the recent Minamata Bay sediments, and 0.18 μg/g for a single control sample; median MeHg concentrations (percentage of MeHg in THg) were 108 ng/g (0.031%), 3.7 ng/g (0.12%), and 0.71 ng/g (0.41%), respectively. In all samples, the MeHg% decreased exponentially with increasing THg concentration. The extractability of THg from each sample into seawater was shown to be much lower than that of MeHg. The extracted MeHg was 7.89% for the control, 4.57% for the recent Minamata Bay sediments, and 0.86% for the preserved sludge. The main chemical form of mercury in the preserved sludge containing the highest THg concentration was found to be stable β-mercury sulfide (HgS) based on transmission electron microscopy linked with energy-dispersive X-ray spectroscopy (TEM-EDX) and X-ray absorption fine structure (XAFS).