Comparison of chlorine and chloramine in the release of mercury from dental amalgam

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Abstract

The purpose of this project was to compare the ability of chlorine (HOCl/OCl) and monochloramine (NH2Cl) to mobilize mercury from dental amalgam. Two types of amalgam were used in this investigation: laboratory-prepared amalgam and samples obtained from dental-unit wastewater. For disinfectant exposure simulations, 0.5 g of either the laboratory-generated or clinically obtained amalgam waste was added to 250 mL amber bottles. The amalgam samples were agitated by end-over-end rotation at 30 rpm in the presence of 1 mg/L chlorine, 10 mg/L chlorine, 1 mg/L monochloramine, 10 mg/L monochloramine, or deionized water for intervals of 0 h, 2 h, 4 h, 8 h, and 24 h for the clinically obtained amalgam waste samples and 4 h and 24 h for the laboratory-prepared samples. Chlorine and monochloramine concentrations were measured with a spectrophotometer. Samples were filtered through a 0.45 µm membrane filter and analyzed for mercury with USEPA standard method 245.7. When the two sample types were combined, the mean mercury level in the 1 mg/L chlorine group was 0.020 mg/L (n = 25, SD = 0.008). The 10 mg/L chlorine group had a mean mercury concentration of 0.59 mg/L (n = 25, SD = 1.06). The 1 mg/L chloramine group had a mean mercury level of 0.023 mg/L (n = 25, SD = 0.010). The 10 mg/L chloramine group had a mean mercury level of 0.024 mg/L (n = 25, SD = 0.011). Independent samples t-tests showed that there was a significant difference between the natural log mercury measurements of 10 mg/L chlorine compared to those of 1 mg/L and 10 mg/L chloramine. Changing from chlorine to chloramine disinfection at water treatment plants would not be expected to produce substantial increases in dissolved mercury levels in dental-unit wastewater.

Introduction

Mercury is a potent neurotoxin and is of interest due to its continued presence at levels of concern in the environment. Exposure to high levels of mercury can irreversibly damage the brain, kidneys, and the developing fetus. Lower dose exposure to mercury can result in irritability, shyness, tremors, changes in vision or hearing, and memory problems (ATSDR, 1999). Amalgam, a mercury-containing restorative material, is widely used by dentists to restore decayed tooth structure. As a consequence of the placement and removal of amalgam restorations, mercury can contaminate wastewater leaving dental facilities. Mercury concentrations in this waste stream can reach substantial concentrations and in one well documented instance, a large dental treatment facility was disconnected from a municipal wastewater treatment system as a result of exceeding mercury discharge limits (Stone et al., 1999). Dental-unit wastewater is increasingly being recognized as a source of anthropogenic mercury emission and this has resulted in an escalating trend to regulate the discharge of mercury from dental offices across the United States (Stone et al., 2008).

The ability of chlorine (and other oxidizing disinfectants) to mobilize mercury from dental amalgam is well recognized (Batchu et al., 2006). Chlorine has a long history in the disinfection of drinking water, with the first recorded use occurring in 1896 (AWWA, 2006). Its utilization was linked with a dramatic decrease in water-borne illnesses, including typhoid, dysentery, and cholera (AWWA, 2006). Recently, chlorine has come under scrutiny because its by-products such as trihalomethanes (THM) and haloacetic acids (HAA) – produced when surface waters are disinfected with chlorine – are thought to be carcinogenic (AWWA, 1998, AWWA, 2006, USEPA, 2001, Yang and Shang, 2004). The U. S. Environmental Protection Agency (USEPA) has promulgated a regulation under the Disinfectants and Disinfection By-Products Rule that sets maximum levels for THM and HAA (USEPA, 2001). In an effort to meet the new drinking water regulations, many water treatment utilities have begun switching to chloramination because of the diminished tendency for chloramine to produce halogenated by-products (AWWA, 1998, AWWA, 2006).

The switch to disinfection by chloramination has been accompanied by reports of lead spikes exceeding the USEPA's action level for lead in drinking water (15 µg/L) (Renner, 2004, Switzer et al., 2006, Miranda et al., 2007). The chloramine-mediated mobilization of lead from plumbing lines raises the possibility that chloramine may mobilize mercury from amalgam trapped in plumbing lines. To investigate this issue, a study was designed to compare the capacity of chlorine and chloramine to mobilize mercury from dental amalgam.

Section snippets

Materials and methods

This study was designed to compare chlorine with chloramine in the mobilization of mercury from dental amalgam. Laboratory and clinically derived amalgam samples were exposed to 1 mg/L and 10 mg/L solutions of chlorine (HOCl/OCl) and monochloramine (NH2Cl). Exposure took place over 24 h with sampling intervals of 0 h, 2 h, 4 h, 8 h, and 24 h for the clinically obtained amalgam samples and 4 h and 24 h for the laboratory-prepared amalgam samples. Parameters measured at each sampling interval

Descriptive statistics

Data describing the mercury levels leached from the four different experimental groups and the deionized water control are presented here and summarized in Table 1. Both laboratory-derived and clinically derived amalgam samples were grouped together for each concentration. The mean mercury level in the 1 mg/L chlorine group was 0.020 mg/L (n = 25, SD = 0.008). The mean mercury mobilized from clinically obtained amalgam was found to be 0.020 mg/L (n = 16, SD = 0.009). The mean mercury mobilized from

Discussion

Chlorine has been used to disinfect drinking water since the 1860s, but only recently has the significance of chlorine-mediated production of disinfectant by-products been appreciated (AWWA, 2006). The presence of THM and HAA in chlorine-treated water led the USEPA to promulgate a regulation aimed at reducing THM and HAA levels in drinking water (USEPA, 2001). The new regulation has prompted some water treatment plants to switch to chloramine disinfection to decrease the amount of halogenated

Conclusion

Data presented here do not support the suggestion (Hammarback et al., 2007) that the change from chlorine to chloramine disinfection triggered the dramatic increase in dissolved mercury levels seen in the one large dental treatment facility. Even at a concentration 2.5 times higher than the USEPA's maximum disinfection level, chloramine is less vigorous in mobilizing mercury than chlorine. While chloramine does appear to be able to increase lead levels in drinking water in some locations, and

Disclaimer

The views expressed herein are those of the authors and do not necessarily reflect the official policy or position of the Departments of the Navy or Defense, nor the U.S. Government. The use of commercially available products does not imply endorsement.

Acknowledgements

This study was supported by the U.S. Navy Bureau of Medicine and Surgery (BUMED) and by a grant from the United States Environmental Protection Agency's Great Lakes National Program Office. The authors would like to thank Gerry Bargren and Robin Holmlund for assistance with sample preparation and analysis.

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