Mercury in the Environment

Mercury is a kind of heavy metal that occurs naturally in the crust of the Earth. In the periodic table, it is represented by the symbol “Hg” and has an atomic number of 80. Due to its bio-accumulative property and toxicity towards the environment and living beings, mercury has become a serious issue when it is released into the environment, both in the atmosphere and in water (Kidd et al., 2003).

5.2.1 Sources of Mercury

Over the years, there have been reports that the emissions and levels of mercury in the environment are increasing day by day. An estimation of the global emission of mercury into the environment was in the range of 5000 to 8000 metric tonnes each year (Romanov et al., 2017). The sources of the emission of mercury into the environment can be divided into two categories.

The first category is induced by emissions from natural geological activities, including volcanic activities and eruptions, forest fires and the weathering of rocks (Pirrone et al., 2010).

However, it has been reported that the main factor which causes the release of mercury into the environment is due to anthropogenic activities, which comprises at least 50 percent of all mercury emissions (Gworek et al., 2017). According to data reported in 2010, the amount of mercury released into the atmosphere caused by anthropogenic activities alone was estimated to be 1960 metric tonnes, while emission into oceans was at least 1000 metric tonnes (UNEP, 2013). Different activities conducted by humans that can cause the emission of mercury includes the combustion of fossil fuels and coal, the mining of metals, and the emission of hazardous chemical waste from industries and medical processes (Gade et al., 2015).

5.2.2 Impacts of Mercury

Mercury is an element that is very dangerous, and can lead to significant issues to both the environment and human health. Due to the high persistency of mercury, all sources of mercury are of concern as it can be re-introduced into the environment through natural processes such as evaporation of water. On the other hand, those that are emitted into the atmosphere will be transported and deposited on the surface of Earth again by rain (Lourie et al., 2003). Mercury is hazardous because of its ability to accumulate and bio-magnify in the ecosystems and bodies of living beings, in other words, its

concentration tends to increase with time (Schmitt et al., 2011). Mercury contamination has long been known to be toxic to both public health and environment.

5.2.2.1 Environmental Impacts

Ecological disturbance is the major environmental impact found due to the contamination of mercury. Food, primarily fish, has become the most significant source of mercury exposure for the general population. Fish and aquatic life are known as the main source of food for many animals including birds. When water sources such as oceans and rivers are contaminated by mercury, it will then be ingested by aquatic life and fish. The health of predators that mainly rely on aquatic life as a food source such as loons and eagles will be damaged. For instance, the ability to reproduce of Minnesota loons might be affected and impaired due to the intake and accumulation of mercury (Ensor et al., 1992). On the other hand, another example of the influence of mercury to the environment was seen at the Minamata Bay, after the serious mercury poisoning incident caused by a chemical plant. Serious neurological effects were observed in the animals and wildlife that lived near the Minamata Bay (Yorifuji et al., 2013). Birds were found to be experience difficulty in flying, and exhibited abnormal behaviour. Furthermore, wildlife that was exposed to mercury were found to have organ impairment such as liver and kidney damage. Also, the emission of mercury is suggested to have affected the ecosystem of the nature. For example, the contamination of mercury caused the reduction in microbiological activity, which is crucial to

the terrestrial food chain. As a result, all the other species within that food web are then affected and causing an ecological imbalance (Driscoll et al., 2007).

5.2.2.2 Health Impacts

All humans are exposed to a certain low level of mercury in their daily lives, often through chronic exposure (continuous or intermittent long-term contact). However, due to the increase of the pollution caused by mercury, the exposure of humans to mercury has greatly accelerated. Human exposure to mercury can occur through different channels. One of the major routes is through the consumption of aquatic life, especially fish (Ruiz-Guzman et al., 2014). As mentioned earlier, the mercury contaminants that are emitted into the environment and water bodies will be ingested, then bio-accumulate in the tissues of aquatic life and fish. Consequently, the contaminant is passed up the natural food chain and eventually reaches humans (Driscoll et al., 2007).

Hence, fish have become the most significant source of mercury exposure for the other population.

Exposure to mercury can lead to a variety of health impacts. First and foremost, mercury is a well-known neurotoxin, which can cause serious damage to the neurological system. High exposure to mercury may cause adverse effects to both the brain and spinal cord of human bodies. Symptoms such as tremors, loss of memory, insomnia and motor dysfunction might occur in human bodies due to the high exposure to mercury (Bernhoft et al., 2012).

A research was conducted in 2000 to observe the effect of mercury exposure

to the nerve system, where they found out that the presence of mercury inhibits the binding of GTP to the brain tubulin, thereby inhibiting the polymerization of tubulin into microtubules. Also, the research has demonstrated that the exposure to mercury ions causes the failing of neuronal somata to sprout. The research has concluded that mercury is a potential etiological factor in neurodegeneration (Leong et al,. 2001).

The greatest risk is for foetuses or young children because of the on-going developing process of their brain and nervous systems. The exposure of the foetus towards mercury usually occurs when the contaminant in the pregnant mother’s bloodstream is transported across the placenta into the foetus (Minai et al., 2016). Low doses of mercury in the bodies of pregnant women have been shown to have impacts on the foetuses which includes the poor performance on neurobehavioral tests, particularly on tests of attention, fine-motor function as well as verbal memory, language, visual-spatial abilities. In addition, the exposure to mercury can destroy and impair different organs in human bodies, including the gastrointestinal and renal organs.

Moreover, the exposure to mercury can alter the heart rate of human bodies which then lead to the increment of heart attack risk and cause disturbances to the human immune processes. Recent epidemiological studies found out the associations between exposure to low level of mercury content and adverse cardiovascular effects. One of the most serious mercury poisoning cases happened in Minamata, Japan, in 1956, due to the release of mercury in the industrial wastewater by a chemical factory. Symptoms such as muscle weakness, loss of peripheral vision and hearing were first discovered within

the population who stayed near the Minamata Bay. The poisoning then worsened and issues such as insanity, coma and fatalities were discovered within weeks of the discovered mercury poisoning symptoms (Semionov et al., 2018).

Realizing the severe consequences of this issue, an international treaty was designed and approved in 2013 by approximately 140 countries as a global efforts to protect the natural ecosystem and human health from further threats by mercury, which named as the “Minamata Convention on Mercury”.

This convention addressed the control and reductions of the content of mercury used and the waste emitted in industries and different processes (Selin et al., 2018). Apart from this, the management of mercury has become an important issue to the world due to the harmful effects of mercury exposure.

Particularly, attention has been drawn in finding measurements, not only for reducing and limiting the release of mercury into the environment, but also for the monitoring and detection of mercury in water bodies. Different techniques and methods to monitor mercury content have been proposed and demonstrated over the last few decades.

5.3 History of Mercury Detection

Over the years, many researches were conducted to detect mercury in water. Among the sensors proposed, optical-based type sensors were the most commonly used devices, and they can be classified into colourimetric, Fluorescent and Surface Enhanced Raman Scattering (SERS) sensors.