Speciation of mercury in water at the bottom of Minamata Bay, Japan

Introduction
From the 1930s until the late 1960s, an acetaldehyde-producing factory in Minamata, Japan, has been releasing wastewater contaminated with mercury into the Minamata Bay. The Minamata Bay Pollution Prevention Project (1977-1990) removed sedimentary sludge that had contained high levels of mercury. After the project was over, there were reports and investigations regarding the mercury concentrations in Yatsushira Sea sediments and it was reported that sediments (containing mercury) were being transported from Minamata Bay to Yatsushira Sea. During the re-suspension and transportation of contaminated sediments, some of the mercury present in the sediments may have dissolved into the water.

Investigations on the concentration of  methylmercury in water of the upper and middle layers of Minamata Bay have been conducted. However, as for the layer of water just above the sediment, no previous investigations were done.

In this study, methylmercury levels in the water were determined by the use of HPLC-ICP-MS.



Materials & Apparatus
1) Water samples were taken in at different points as shown in the picture above

Upper layer of the water (clear) was filtered through a 0.45μm Millipore filter
Lower layer of the water (turbid) was centrifuged and the supernatant was filtered through a 0.45μm Millipore filter

2) HPLC-ICP-MS instrument

HPLC conditions
* Column: ZORBAX Eclipse XDB-C18, 2.1mm x 50mm, 5μm
* Mobile Phase: 0.06M ammonium acetate, 5% (v/v) methanol, 0.1% 2-mercaptoethanol (pH 6.8)
* Flow Rate: 0.4mL/min
* Injection volume: 100μL

Pre-conditioning of HPLC column
* HPLC grade methanol was pumped through the column at 0.4mL/min for minimum 2 hours and condition with eluent (at same flow rate) for minumum half an hour
* Without this pre-conditioning, the inorganic mercury will be subjected to contamination in the system leading to poor recovery or peak splitting

3) Reagents & apparatus

All reagents used were of the highest purity available and were of analytical reagent grade
All apparatus were cleaned with detergent, 10% potassium permanganate, 2% hydroxylammonium chloride and water. Glassware were washed and heated before use.

4) Standards

A series of calibration standards (10ng/L to 100μg/L) were prepared by dissolving the appropriate amount of HgCl2 in high purity water  (External Standard)
Bismuth (1.0μg/L) was added to the methanol eluent as an Internal Standard

Methods
1) Both the samples and external standard are injected into the HPLC
2) Obtain peak areas of sample and standards
3) Construct a calibration graph from the peak areas of external standards
4) Determine concentration of samples from the calibration graph