Mercuric Iodide Complexes in Dental-Unit Wastewater

Objective: We have informally observed increased mercury levels in wastewater from dental clinics where commercial iodine systems were installed to control biofilm formation in dental-unit waterlines. This study was undertaken to quantify the relationship between iodine and mercury concentrations in dental-unit wastewater. Methods: For laboratory studies, 5 gram samples of ground and sieved amalgam (particle size distribution of >105 microns and < 710 microns) were exposed to solutions of iodine at concentrations varying from 0 (control) to 20 mg/liter.  Exposure to the solutions was for either twenty-four hours or seven days. Mercury levels in settled supernatants were analyzed with USEPA method 245.1 (CVAA spectrometry). For clinical studies, 18 samples were taken from wastewater at two dental chairs both before (baseline) and after installation of commercial iodine releasing systems which claim to release iodine at levels between 2-to-6 mg/liter.  Mercury levels were analyzed with USEPA method 245.1.  Results:  Data from twenty-four hour and seven-day exposure samples showed linear correlations between iodine and mercury concentrations (r = 0.957 and 0.973, respectfully, both P<0.001).  At twenty-four hours, there was a 9.9 fold increase in mercury levels in the 20 mg/liter iodine solution compared to control, and a 31.6 fold increase at day seven. Data from the clinical study showed a 2.5 fold increase in dissolved mercury levels with the iodine releasing system online when compared to baseline (85.3 μg/liter, n=18, SD=44.1 and 34.5 μg/liter, n=18, SD=14.5, respectively;  P<0.001).  Conclusions:  Data are consistent with the hypothesis that iodine can increase levels of mercury in dental-unit wastewater.  Equilibrium studies done with a computerized modeling application suggest the formation of soluble anionic mercuric iodide complexes, HgI₄^-2.  This study was supported by the U.S. Navy's Bureau of Medicine and Surgery and by a grant from the U.S. Environmental Protection Agency's Great Lakes National Program Office (DW17947929-01-0).