Determination of Mercury Species in Dental Wastewater

Objective: Our previous study identified the presence of monomethyl mercury in dental-unit wastewater (Dental Materials 2003, 19(7):677-681). The current project was undertaken to gather more complete speciation data by measuring the concentrations of Total (Hg^T), Inorganic (Hg⁺²), Monomethyl (MeHg) mercury and elemental mercury (Hg⁰) in chairside dental-unit wastewater samples. Methods: Dental-unit wastewater samples were collected at the chair over six consecutive days when amalgam restorations were placed and/or removed. Settled supernatants were kept unpreserved at 4°C until processed. Hg⁰samples were undigested and analyzed by Cold Vapor Atomic Fluorescence (CVAFS). Hg⁺² samples were analyzed by aqueous phase ethylation, Tenax® trap collection, GC separation and CVAFS detection (modified EPA method 1630). MeHg samples were prepared by solvent extraction, aqueous phase ethylation, Tenax® trap collection, GC separation and CVAFS detection. Samples for Hg^T were prepared by oxidation with BrCl for 24 hours at room temperature and then heated to 80°C for 3 hours. Digested samples were analyzed by SnCl reduction, gold trap preconcentration and CVAFS detection (modified EPA method 1631). Amalgam bound Elemental Hg was calculated as the difference between Hg^Tand the sum of Hg⁺² and free Hg⁰. Results: Results are presented in the table below:

Sample Number	Hg^T (µg/L)	MeHg (µg/L)	Hg⁰ (µg/L)	Hg⁺² (µg/L)	Particulate Bound Hg⁰ (µg/L)
1	43,081	0.31118	177.128	144.96	42,759
2	828	0.06733	34.080	13.97	780
3	79,751	0.40850	244.127	84.22	79,422
4	3,010	0.15809	80.552	54.09	2875
5	1,005	0.11718	59.092	12.14	934
6	953	0.10424	52.596	14.65	886
Mean	21,438	0.19442	107.93	54.01	21,276

Conclusions: The data show that 99.2% of the Hg in the settled samples is in the form of Hg⁰ bound to particulate. This study was supported by the US Navy Bureau of Medicine and Surgery and a grant from the USEPA Great Lakes National Program Office (IAG# DW17947929-01-0).