Table of Contents
Background
Mercury (Hg), a heavy metal that mainly occurs naturally in the environment including the natural occurrence of Mercury in water bodies due to various geological processes, or Hg can be present in rocks, minerals, and soil from where it can leach into groundwater or surface water through erosion and weathering. However, there is a strong evidences that also suggest its significant increase in levels due to various anthropogenic activities. These activities include mining, industrial processing, coal burning, runoffs, agricultural practices, wastewater treatment plants, and incineration of solid wastes. Because of its volatile nature, Hg can evaporate and can be transported to long distances from its point source and can contaminate the environment through atmospheric deposition phenomenon.
Chemically, Mercury (Hg) can exist in both inorganic and organic forms. The inorganic Mercury includes either elemental Hg or any other inorganic compounds bonded to Hg such as HgCl2, Hg2Cl2 etc. Whereas, the organic Mercury refers to different carbon compounds attached to the elemental Mercury. Among various organic forms, methylmercury (CH3Hg+) is widely studied compound in various environmental matrices and studies show a strong evidence of adverse health impacts associated with Hg exposure among humans. It is important to note that the toxicity of Hg is mainly related to its transformations through various processes that determine the extent of toxicity in different environmental compartments. In the aquatic environment, this transformation is triggered through certain bacteria that carry out the methylation of Mercury by adding the CH3 group to the elemental mercury through the process of biomethylation. This gives rise to two possibilities including either contamination of ground and surface water or it can bioaccumulate in the aquatic organisms from where it can enter into the human food chain.
The volatile nature of Hg and its ability to chelate with various compounds makes it among the most toxic element and humans can be exposed to Hg through ingestion of contaminated drinking water or food. It is also worth mentioning that Hg toxicity varies among individuals, for example, pregnant women, infants, and children are more susceptible to Mercury toxicity compared to others. The drinking water contamination of Mercury (Hg) takes through similar sources as described above. In order to ensure a Hg-free drinking water supply to the consumers, various monitoring programs and regulations have been made to control and maintain the Hg levels under safe limits. For example, in the US, the Environmental Protection Agency (EPA) is the main regulatory body for setting and enforcement of regulations to protect public health. For Mercury, the MCL in drinking water set by the EPA is 2 ppb (0.002 mg/L). Importantly, the public water systems in the US meet the EPA standards for Hg, and all the domestic water is either Hg-free or its limits are under the prescribed MCL. More specifically, NYC and NJ follow similar guidelines and regulations set by the EPA to regulate Hg levels.
The state department of Health (NYSDOH) and the New Jersey Department of environmental protection (NJDEP) are the main local regulating bodies in NYC and NJ respectively to ensure federal safety limits for Hg in drinking water supply. If found, any elevated levels in drinking water are subjected to suitable treatment to lower the contamination levels below MCL before its supplied to public systems. Adverse health impacts associated with Mercury exposure are well documented in humans. Some potential health impacts associated with elevated and prolonged Hg exposure includes neurological and developmental effects, kidney damage, and gastrointestinal and cardiovascular impacts. Hg can adversely affect fetuses and young children resulting in learning disabilities, reduced IQ levels, and delayed motor skills. However, there is limited epidemiological evidence in context of population consuming Hg-contaminated drinking water and its associated adverse health outcomes. A systematic investigation focusing the regions with elevated Hg levels in drinking water and targeting the individuals consuming high Hg can provide a better understanding of extent of Hg toxicity across populations.
Several mechanisms have been proposed to demonstrate Mercury (Hg) toxicity resulting in harmful impacts on the human body system. It should be noted that the toxicity mechanisms associated with Mercury purely depend on the available form of Mercury i.e. inorganic or organic. Moreover, the exposure duration also plays a significant role in determining the extent of toxicity among individuals. However, the general mechanisms of Hg toxicity include its binding to sulfhydryl groups (-SH) present in proteins because of its high affinity for -SH which may results in impaired enzyme activity, altered proteins functions as well as disrupted cellular processes (Ajsuvakova et al. 2020).
Mercury can also result in the generation of reactive oxygen species (ROS) that can cause oxidative stress leading to cellular, lipids, proteins, and DNA damage (Kim and Sharma 2004). Cellular signaling is an important process that regulates various metabolic functions in the cells. Mercury can interfere with various signaling pathways particularly neurotransmission and hormone regulation resulting in neurological impairments and disturbing hormonal balance (Branco et al. 2022). Among others, Mercury possesses a high potential for mitochondrial dysfunctioning which is an essential organelle for energy production and cellular metabolism. This results in decreased adenosine triphosphate (ATP) production causing health issues including fatigue, muscle weakness, and organ dysfunction (Houston 2011).
It is well documented that Mercury (Hg) can directly alter the DNA leading to genetic mutations and altered gene expression hence increasing the risk of cancer development and other genetic disorders (Yang et al. 2020). Having the ability to cross the blood-brain barrier, Mercury can directly affect the central nervous system causing neurological impairments, and behavioral abnormalities (Peplow and Augustine 2014).. Furthermore, recent findings suggest that Mercury can infer cellular and molecular toxicity through epigenetic mechanisms involving DNA methylation and miRNAs (Cardenas et al. 2017). However, further research on this is required to develop a better understanding related to Hg toxicity in humans.
Once entered into the human body, various metabolic processes have been associated with Hg exposure including absorption, distribution into different body parts, transformation into different forms to reduce the toxicity, and elimination from the body through excretion. The chemical form of Hg mainly determines the metabolism for instance, inorganic Mercury (Hg) has a poor absorption rate in the gastrointestinal tract compared to organic form. Similarly, the inorganic form binds to RBCs and plasma proteins while organic Mercury has an affinity for binding to proteins and can cross the brain barrier resulting in nervous disorders. Many factors determine the Hg metabolism in an individual. This includes age, genetics, gender, nutritional status, and overall health. Because of the complexity associated with metabolism and mechanisms associated with Hg toxicity, an understanding of Hg metabolism is crucial for assessing human health risks associated with exposure through drinking water.
Detection Methods and Removal Strategies
Monitoring of Hg in drinking water involves a suitable quantification technique to measure its levels. Various analytical methods are available to detect Mercury in drinking water and the choice of selection mainly depends on the availability of the instrument, affordability, and desired accuracy. Among the widely used methods involve Cold Vapor Atomic Absorption Spectrometry (CVAAS) which involves the conversion of Hg into its elemental form using chemical reduction. This is followed by measuring light absorption at a specific wavelength by Hg vapors (Thongsaw et al. 2019). Furthermore, HGAAS is also used widely for Mercury detection in water samples. For high desired sensitivity and lower detection limits, Inductively coupled plasma mass spectrometry (ICPMS) is a widely adopted method for Mercury detection that can detect Hg at ppb levels with high accuracy (Surucu 2022). However, the method is expensive and applicable where the desired results are at very low levels. More recently direct mercury analyzer (DMA) is also used widely as it can accurately detect Hg levels in a given sample without the need for sample digestion or pre-treatment (Maggi et al. 2009). Other methods for Hg detection include gold amalgamation and electrochemical methods. However, these methods are less applicable for Mercury detection in drinking water because they provide estimated results in a given water sample.
Removal of Hg from drinking water can be done by adopting various techniques to minimize its potential health risks. Some commonly used removal techniques involve activated carbon filtration which involves the absorption of Hg on the carbon surface and is effective mainly for removing dissolved Hg in water (Zhang et al. 2005). Biological processes involving specific microbes and algae are also applied for Hg removal and are considered cost-effective and environmentally friendly methods for Mercury removal (Rani et al. 2021). The use of a semi-permeable membrane in reverse osmosis (RO) is a widely used method for Hg removal and can efficiently remove Mercury from drinking water (Rani et al. 2022). Other adopted methods for Hg removal include coagulation/flocculation, the use of ion exchange resins, and biochar that have been proven to remove Mercury from drinking water effectively. It is important to note that different treatment processes are often combined to treat water in order to achieve optimal removal efficiency. However, it is highly recommended to consult professionals involved in water treatment to determine the most suitable treatment method for Hg removal. Regular monitoring of drinking water is also essential to ensure effective treatment and maintain Hg levels below MCL.
Public Perspective
Following frequently asked questions (FAQs) try to address some general public concerns in the US, especially the NYC and NJ region.
Under the Safe Drinking Water Act, EPA in 1991 set an enforceable regulation for inorganic Hg, called a maximum contaminant level (MCL), at 0.002 mg/L or 2 ppb. Public water systems must ensure that your drinking water does not exceed the MCL for Hg.
Hg is one of the most serious contaminants threatening human health. It is a global pollutant that ultimately makes its way into every aquatic ecosystem through natural and anthropogenic sources.
When Hg enters the aquatic ecosystem, certain bacteria can change it into a form called methylmercury, which is absorbed by tiny aquatic organisms and can become part of the food chain. Hg can also become part of surface and groundwater and contaminate the drinking water.
Many types of water filters and water purifiers can reduce levels of Mercury in drinking water. However, some are more effective than others. Some of the best water filters for removing Hg are reverse osmosis systems, activated carbon filters, and water distillers.
If you have Hg poisoning with a very high level of Hg in your blood, your doctor will probably recommend chelation therapy. This method involves using medications, called chelators, that bind to Hg in your body and help it to exit your system. Chelators can be taken as a pill or injected.
Most of the metallic Hg will accumulate in your kidneys, but some metallic Hg can also accumulate in the brain. Most of the metallic Hg absorbed into the body eventually leaves in the urine and feces, while smaller amounts leave the body in the exhaled breath.
Lime softening involves the use of Ca(OH) (limewater or calcium hydroxide) to increase the pH of the water, causing the heavy metal contaminants to precipitate out as Hg(OH). This method of mercury removal is also used as a remedy for water hardness, which removes calcium and magnesium as precipitates.
Avoid buying products that contain Hg except for fluorescent light bulbs. Fluorescent bulbs use less electricity than incandescent bulbs. Keep Hg-containing items out of the trash (including fluorescent light bulbs). Use appropriate disposal methods or recycle Hg-containing products.
Inorganic Hg is the most common form that is present in drinking water but is not considered to be very harmful to human health, in terms of the levels found in drinking water. However, prolonged exposure to Hg should be avoided even at very low levels.
A diet rich in various nutrients and vitamins has been shown to promote Hg elimination from the body. Some of these nutrients include selenium, glutathione, and vitamins, including vitamins C and E. Besides, green algae called chlorella is also helpful in removing Hg from the body.
Conclusion
Mercury (Hg) is a widely recognized toxic element that can contaminate drinking water through various sources. It can exist in various chemical forms and its volatile nature can be responsible for contamination at long distances through atmospheric deposition. In the US, levels of Hg in drinking water are regulated by EPA, and public water supply systems are found safe from Mercury contamination. However, regular monitoring of supply systems is warranted to ensure safe drinking water supply to US residents. The prolonged exposure even at low concentrations should be avoided for public health safety since Mercury has been shown to trigger adverse health effects at low concentrations.
References
Ajsuvakova OP, Tinkov AA, Aschner M, Rocha JB, Michalke B, Skalnaya MG, Skalny AV, Butnariu M, Dadar M, Sarac I. 2020. Sulfhydryl groups as targets of mercury toxicity. Coordination chemistry reviews. 417:213343.
Branco V, Coppo L, Aschner M, Carvalho C. 2022. N-acetylcysteine or sodium selenite prevent the p38-mediated production of proinflammatory cytokines by microglia during exposure to mercury (ii). Toxics. 10(8):433.
Cardenas A, Rifas-Shiman SL, Agha G, Hivert M-F, Litonjua AA, DeMeo DL, Lin X, Amarasiriwardena CJ, Oken E, Gillman MW. 2017. Persistent DNA methylation changes associated with prenatal mercury exposure and cognitive performance during childhood. Scientific Reports. 7(1):1-13.
Houston MC. 2011. Role of mercury toxicity in hypertension, cardiovascular disease, and stroke. The Journal of Clinical Hypertension. 13(8):621-627.
Kim SH, Sharma RP. 2004. Mercury-induced apoptosis and necrosis in murine macrophages: Role of calcium-induced reactive oxygen species and p38 mitogen-activated protein kinase signaling. Toxicology and Applied Pharmacology. 196(1):47-57.
Maggi C, Berducci MT, Bianchi J, Giani M, Campanella L. 2009. Methylmercury determination in marine sediment and organisms by direct mercury analyser. Analytica Chimica Acta. 641(1-2):32-36.
Peplow D, Augustine S. 2014. Neurological abnormalities in a mercury exposed population among indigenous wayana in southeast suriname. Environmental Science: Processes and Impacts. 16(10):2415-2422.
Rani L, Srivastav AL, Kaushal J. 2021. Bioremediation: An effective approach of mercury removal from the aqueous solutions. Chemosphere. 280:130654.
Rani L, Srivastav AL, Kaushal J, Nguyen XC. 2022. Recent advances in nanomaterial developments for efficient removal of hg (ii) from water. Environmental Science and Pollution Research. 29(42):62851-62869.
Surucu O. 2022. Electrochemical removal and simultaneous sensing of mercury with inductively coupled plasma-mass spectrometry from drinking water. Materials Today Chemistry. 23:100639.
Thongsaw A, Sananmuang R, Udnan Y, Ampiah-Bonney RJ, Chaiyasith WC. 2019. Immobilized activated carbon as sorbent in solid phase extraction with cold vapor atomic absorption spectrometry for the preconcentration and determination of mercury species in water and freshwater fish samples. Analytical Sciences. 35(11):1195-1202.
Yang L, Zhang Y, Wang F, Luo Z, Guo S, Strähle U. 2020. Toxicity of mercury: Molecular evidence. Chemosphere. 245:125586.
Zhang F-S, Nriagu JO, Itoh H. 2005. Mercury removal from water using activated carbons derived from organic sewage sludge. Water research. 39(2-3):389-395.
- Yasir A. Rehman, Ph.D.
Dr. Rehman was born in Rawalpindi, Pakistan. He completed his MSc from PMAS – Arid Agriculture University Rawalpindi in 2011 where his thesis comprised a health risk assessment of subjects living in the vicinity of wastewater channels in urban settings and its relationship with the incidence of Malaria.
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OTHER RESEARCH ON WATER CONTAMINANTS BY DR. YASIR
