Table of Contents  

Al Kawas: Quantitative analysis of mercury burden in the wastewater released from dental clinics in the United Arab Emirates

Introduction

Amalgam is one of the most commonly used materials in restorative dentistry. The amalgam alloy is composed of many metals, including mercury, silver, tin, copper and zinc.1,2 Amalgam waste from dental practices and clinics is a significant source of mercury released into the environment when it is washed down a drain or disposed of improperly. Mercury is of particular concern owing to its potential adverse effects on humans and the environment. Several studies have shown that exposure to mercury may lead to health complications such as impairment of the developing central nervous system, pulmonary and nephrotic damage and impairment of osmoregulatory functions.35 Additionally, mercury is very mobile in the environment and is known to accumulate in fish and other marine organisms, and therefore has the potential to contaminate the entire food chain.58 It is also known that dentists and their assistants and patients are directly exposed to mercury, with significant increases in their plasma mercury concentrations reported in comparison with those of control groups.912 The public and the environment are also indirectly exposed to this element via mercury emissions from incinerators and mercury in wastewater from dental clinics and households.4,6,13,14

Globally, hundreds of tonnes of mercury are used annually in dental amalgam15 and, according to many investigations, dental clinics appear to be responsible for most of the mercury found in the sludge generated by sewage treatment plants. As a result, dental clinics are considered an indubitable source of environmental pollution with mercury.14,1618

Increasing knowledge of the health risks associated with exposure to mercury and its accumulation in the food chain and in ecosystems has led to growing pressure to reduce emissions of this metal into the environment. Consequently, the mercury waste generated by dental clinics has received increased attention. Additionally, restrictions on the handling and discharge of contaminated waste have been established in several countries. For instance, the United States Environmental Protection Agency (USEPA) has set a maximum contaminant level of inorganic mercury in drinking water of only 2 μg/l.19 Environmental standards set by Dubai Municipality state that the maximum allowable discharge of mercury-containing waste into the sewerage system is 10 μg/l, with no more than 1 μg/l to be released into the land for irrigation.20 In Saudi Arabia, the maximum level of mercury permissible in source water discharged into the public sewer is 50 μg/l,21 and the limit for treated wastewater intended for use in agriculture is just 1 μg/l.22 As a consequence, several measures have been adopted by dental clinics worldwide to reduce quantities of mercury discharged into the environment, including the use of amalgam separators and filters, improvements in the design of the waste discharge system, and the use of high-pressure water cleaning.14,23,24 However, implementation of such measures to reduce mercury waste from dental clinics in the United Arab Emirates (UAE) is still unregulated.

The aim of this study was to quantitatively assess the mercury burden in the wastewater discharged from a number of dental clinics in the UAE.

Materials and methods

Equipment

An atomic absorption spectrometer (SpectraAA 220 FS, Varian) equipped with a cold vapour generation accessory (VGA-77, Varian) and a T-shaped quartz absorption cell was used for mercury determination. The instrument has been used according to the parameters listed in Table 1.

TABLE 1

Instrumental parameters for the cold-vapour atomic absorption spectrometry (CVAAS)

Parameter Unit
Wavelength 253.7 nm
Slit width 0.5R nm
Lamp current 4.0 mA
Measurement time 5.0 seconds
Background correction BC On
Vapour type Cold vapour
Pre-read delay 70 seconds
Sampling mode AutoMix
Calibration mode Concentration
Measurement mode Peak area
Replicates standard 3
Replicates sample 3
Smoothing 5 point
Calibration algorithm Linear
Flow rate of reducing agent 1.0 ml/min
Flow rate of samples 7.0 ml/min

Reagents and solutions

All chemicals used were of analytical reagent grade unless otherwise stated. Water used was obtained from a Milli-Q reagent system (resistivity 18.2 MW cm; Millipore, Bedford, MA, USA). All plastic and glassware used was soaked in 4 M nitric acid for a minimum of 12 hours, washed with distilled water and finally rinsed with Milli-Q water before use.

The nitric acid (68–70%) and sodium chloride (99.5%) used were purchased from Panreac Quimica (Barcelona, Spain); the sulphuric acid (GPR), hydrochloric acid (37%), hydroxyl ammonium chloride (99%) and stannous chloride (98–103%) from BDH (VWR International Ltd, UK); the potassium permanganate (99.5%) from Surechem Products Ltd (Suffolk, UK); the potassium persulphate (98%) from Fluka (USA); mercury (for calibration, 1001 ± 5mg/l solution in 2 mol/l HNO3) from BDH (England); and CRM (20.0 μg/l ± 0.5% in 2% HNO3) from High Purity Standards (Charleston, SC, USA).

The stannous chloride (25% weight per volume) reducing solution was prepared by adding 25 g of stannous chloride to 20 ml of concentrated hydrochloric acid. The mix was heated to dissolve the stannous chloride then allowed to cool, before diluting the solution to 100 ml with water and mixing.

Sample collection, treatment and analysis

Wastewater samples were collected in acid-washed plastic containers from the academic dental centre at the University of Sharjah in Sharjah and Zayed Military Hospital in Abu Dhabi. At Zayed Military Hospital, the samples were collected from four different general dental practitioners over a period of 10 days. All the clinicians used dental units fitted with built-in amalgam separators. The samples were collected directly from the final drain outlet of the dental unit prior to its joining the central drainage duct of the hospital sewage system. Clinical dental procedures were not limited to the removal or placement of dental amalgam restorations. The clinicians routinely performed comprehensive dental care which, to an appreciable extent, involved removal of existing amalgam restorations and, less frequently, placement of dental amalgam. The samples were collected continuously from the beginning of the clinical session at 0730 h until the end of the session at 1330 h.

At the University of Sharjah academic dental centre, the samples were collected from 24 clinics in two different ways. The first of these was to collect samples directly from the outlets of the dental chairs into the collection containers. These samples were collected from 12 clinics at the end of the day (Table 2). The second method was to take representative samples from 12 dental chairs connected to a central suction unit. Here, the wastewater generated during dental treatments passes into the collection tank and then on through an amalgam separator, and finally reaches the main sewer with no further treatment. These representative samples were collected over a period of 2 weeks (from 7 April to 21 April 2010), after passing through the amalgam separator and before being drained into the main sewer.

TABLE 2

Concentrations of mercury in the samples

Sample number Mercury concentration (μg/l) Collection date and time Location
1 528.1 7 April 2010 at 1300 h UoS clinic A – unit 3
2 < MDLa 7 April 2010 at 1310 h UoS clinic – central suction unit
3 674.7 8 April 2010 at 1420 h UoS clinic – central suction unit
4 284.0 8 April 2010 at 1620 h UoS clinic A – unit 1
5 98.2 10 April 2010 at 1330 h UoS clinic D – unit 4
6 153.9 10 April 2010 at 1400 h UoS clinic – central suction unit
7 177.0 10 April 2010 at 1620 h UoS clinic – central suction unit
8 245.7 13 April 2010 at 1130 h UoS clinic D – unit 5
9 435.2 13 April 2010 at 1340 h UoS clinic – central suction unit
10 498.9 13 April 2010 at 1630 h UoS clinic – central suction unit
11 131.5 14 April 2010 at 1320 h UoS clinic – central suction unit
12 86.6 14 April 2010 at 1000 h UoS clinic D – unit 2
13 58.3 14 April 2010 at 1600 h UoS clinic – central suction unit
14 84.3 15 April 2010 at 1300 h UoS clinic – central suction unit
15 82.3 15 April 2010 at 1420 h UoS clinic D – unit 5
16 119.7 15 April 2010 at 1620 h UoS clinic – central suction unit
17 1028.0 18 April 2010 at 1200 h UoS clinic – central suction unit
18 4.6 18 April 2010 at 1400 h UoS clinic D – unit 3
19 1148.0 18 April 2010 at 1615 h UoS clinic – central suction unit
20 1095.7 19 April 2010 at 1330 h UoS clinic – central suction unit
21 7.2 19 April 2010 at 1430 h UoS clinic D – unit 7
22 349.6 19 April 2010 at 1645 h UoS clinic – central suction unit
23 865.4 20 April 2010 at 1320 h UoS clinic – central suction unit
24 9.7 20 April 2010 at 1420 h UoS clinic D – unit 1
25 746.1 20 April 2010 at 1600 h UoS clinic – central suction unit
26 1535.2 21 April 2010 at 1320 h UoS clinic – central suction unit
27 59.8 21 April 2010 at 1430 h UoS clinic D – unit 3
28 406.5 21 April 2010 at 1620 h UoS clinic – central suction unit
29 74.8 0730–1330 h Zayed Military Hospital
30 332.3 0730–1330 h Zayed Military Hospital
31 93.4 0730–1330 h Zayed Military Hospital
32 80.7 0730–1330 h Zayed Military Hospital
33 34.6 0730–1330 h Zayed Military Hospital
34 136.4 0730–1330 h Zayed Military Hospital
35 206.3 0730–1330 h Zayed Military Hospital
36 75.1 0730–1330 h Zayed Military Hospital
37 68.6 0730–1330 h Zayed Military Hospital
38 55.9 0730–1330 h Zayed Military Hospital
Average 317.7
Standard deviation 379.7
Range < MDL–1535.2

UoS, University of Sharjah.

a The MDL is 0.2 μg/l, calculated as three times the standard deviation of nine blank replicates.

Collected samples were treated with nitric acid. Concentrated acid was added to each sample container to achieve a final acid concentration of 1%, before storing at 4°C in a refrigerator until the time of analysis. The wastewater samples were then digested according to USEPA Method 245.1, by shaking the samples' containers before taking subsamples for analysis. Subsamples were gravity filtered using 595 filter 45 papers (Schleicher & Schuell), and 50 ml of filtered sample was placed in a 150-ml, clean plastic bottle. This was followed by the addition of 5 ml of concentrated sulphuric acid, 2.5 ml of concentrated nitric acid, 5 ml of potassium persulphate (5%) and 5 ml of potassium permanganate (5%). Samples treated in this way were then placed inside an oven preset to 95°C for 2 hours. After cooling to room temperature, hydroxyl amine hydrochloride (12% in 12% sodium chloride) was added dropwise to each bottle until the colour of potassium permanganate disappeared. More hydroxyl amine hydrochloride was added if the potassium permanganate colour reappeared before 15 minutes had elapsed since the first addition. Approximately 1 ml of hydroxyl amine hydrochloride was found to be enough to completely reduce any excess potassium permanganate. Finally, water was added to each bottle to obtain a final volume of 100 ml. All calibration solutions, blanks and CRM were treated in the same way. Concentrations of mercury in the samples were determined using cold-vapour atomic absorption spectrometry (CVAAS).

Results and discussion

The mercury concentrations in the wastewater samples are listed in Table 2. This table also includes the average, standard deviation and range of the mercury concentrations in the samples.

The average concentration of mercury in all samples, as shown in Table 2, was 317.7 μg/l, with a standard deviation of 379.7 μg/l and a range from < MDL to 1535.2 μg/l. These results show some variation in mercury concentrations between samples. This variation is expected, and can be explained by differences in the nature of dental operations undertaken at each of the clinics from which samples were collected (e.g. placement or extraction of amalgam fillings, placement of non-amalgam fillings, teeth extraction, and scaling and polishing). Similar findings and explanations have been reported by other investigators.1,16 It should be noted here that the obtained mercury results represent only the soluble fraction of the metal; the actual values are certainly higher because much of the mercury released from the clinics remains insoluble as solid amalgam, which stays trapped inside the chair filters, vaporizes to the air25,26 or settles on the bottom of sample containers. It is also obvious from Table 2 that most of the wastewater samples contain unacceptable concentrations of mercury according to Dubai Municipality standards.20 This means that these samples are considered as hazardous waste (in terms of metals contamination) and should not be discharged into the main sewer without proper treatment to reduce the mercury content to acceptable levels.

The levels of mercury found in the study samples are lower than those reported in the literature in several parts of the world.1,14,27,28 The low average level of mercury in these results can be attributed to the use of new alternatives to mercury amalgam (e.g. non-mercury composite), which are available on the market and already employed by dentists in the UAE and other countries. Other factors include the use of amalgam separators in most of the clinics investigated, and the large number of private and public dental clinics in the UAE, which results in fewer patients attending each clinic. A more comprehensive investigation, covering more clinics, should be conducted to build a clearer idea of the total levels of mercury released from dental clinics in their wastewater.

Conclusion

Most of the wastewater released from dental clinics in this study contains concentrations of mercury which, although lower than what has been reported in the literature, are unacceptable according to local and international environmental standards. This implies that such wastewater should not be discharged without adequate pretreatment. The authorities concerned must monitor the mechanisms employed by dental clinics for the disposal of mercury wastes, and their efficiency in the sorting and handling of amalgam, to reduce the levels of mercury in medical wastes. A more comprehensive investigation, involving a larger number of dental clinics studied over a longer period of time, will certainly provide a clear idea of the levels of mercury in the wastewater from dental clinics, and the impact of mercury pollution on the environment.

Acknowledgements

This study was supported by full funding from Sheikh Hamdan Bin Rashid Al Maktoum Award for Medical Sciences (Grant No. MRG-18/2007–2008). The author would like to thank Dr Imad Abu-Yousef, Dr Amjad Shraim, Dr Abu baker Siddique and Mr Naser Abdo for their valuable assistance in this study.

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