In Artisanal and Small-Scale Gold Mining (ASGM), the amalgamation process, tailings processing, and gold recovery from the amalgam result in substantial release of mercury into the environment. By some estimates, release of mercury from ASGM exceeds 1 million kg each year. This level of mercury pollution may exceed the combined emissions of coal combustion, cement production, chlor-alkali plant operation, and large-scale industrial mining and metallurgy. It is therefore important to look at each stage of common ASGM practices to identify how mercury is released into the environment so that measures can be taken to prevent such emissions and mitigate harm. The primary sources of these emissions are from tailings discharge to land and water and mercury gas emissions during amalgam roasting.
In the amalgamation process substantial amounts of mercury can be lost in the tailings. In particular, milling ore and mercury in trommels can result in the formation of tiny mercury droplets that become finely dispersed in the tailings. This “mercury flour” is especially problematic because it can be easily washed away with water and transported far from the mining site. In some cases, mercury-rich tailings can travel in rivers hundreds of kilometers from the mine. The floured mercury is also difficult to recover because it does not coalesce efficiently. In cases in which the tailings are released directly into the environment, the mercury contaminates water and soil, where it is difficult to recover or remediate. These tailings may contain between 50 and 5000 mg mercury per kg ore, resulting in a substantial loss of mercury. This lost mercury may also contain up to 14 % gold by mass, so significant amounts of gold are lost in tailings too. These tailings are sometimes sold for further processing in large tanks using cyanide leaching. The cyanide complexes and dissolves the residual gold. The gold is then recovered by the addition of activated carbon, which is, in turn, isolated and burned (often in open drums).
As much as 20 g of gold can be isolated for every tonne of tailings. Because mercury is also solubilized through cyanide leaching, release of this water into the environment—either through direct release or by way of drainage from unsecured tailing ponds—is a significant source of mercury pollution.
The release of mercury from tailings into soil and water is a serious hazard to human health because it can compromise food safety and contaminate drinking water. Mercury-contaminated water used for irrigation also leads to contaminated food crops, such as rice.
When mercury is pulverized in mills it can form tiny beads that bind to soil. The soil containing the floured mercury appears very similar in appearance to mercury-free soil. The floured mercury is observable in the SEM image as microbeads bound to soil. A polymer made from sulfur and recycled cooking oil is effective at removing floured mercury from the soil sample. The procedure requires mixing the polymer and soil directly.
Heating mercury–gold amalgams to vaporize mercury is another major source of mercury emissions in ASGM. Mercury gas is harmful to the lungs, kidneys, liver and nervous system so these emissions are especially dangerous.
Where miners evaporate mercury from mercury–gold amalgams directly, this is frequently done without proper ventilation or a retort. The amalgam may even be roasted over stoves in residential kitchens. With kilograms of amalgam processed by miners daily, and each amalgam containing 40–80 % mercury by mass, the exposure to mercury is extremely high. Gold shops and other amalgam smelting centers are also sources of substantial mercury pollution. Poor ventilation and inadequate extractor or fume-hood equipment puts the shop owners and operators at risk for mercury exposure. These facilities are also commonly deficient in appropriate scrubber technology, emitting mercury vapor into the public. Because gold shops are typically located in villages, cities and areas of high population density, public exposure to mercury gas is a serious concern. Even with shops equipped with retorts and filters, atmospheric levels of mercury within 10 m of the entrance can be as high as 10–20 ppb and levels exceeding 100 ppb are common in the processing rooms, with both levels far beyond the 1 ppb level considered hazardous to human health by the World Health Organization (WHO).
When no retorts or air filtration are used, extreme mercury concentrations are inevitable in amalgam roasting because virtually all of the mercury in a kilogram of amalgam is evaporated in less than one hour. Alarmingly, there are reports of mercury gas emissions from gold shops and amalgam processing stations operating in immediate proximity to residential areas, markets, health centers, schools, public gathering areas, playing children and breast- feeding mothers.
Health and Ecological Impacts
Mercury poisoning is a tremendous burden to human health, especially in ASGM communities. Mercury gas, such as that encountered in ASGM amalgam processing, is readily absorbed in the lungs and then further transported to other organs. Elemental mercury is able to cross membranes including the blood–brain barrier and the blood-placenta barrier, posing a threat to neurological function and fetal development, respectively. Acute mercury exposure (for instance to mercury vapor produced from heating mercury–gold amalgam) can lead to tremors, memory loss, respiratory distress and even death. Chronic exposure to mercury gas may lead to renal failure, tremors, movement disorders, and various psychosis and memory impairment.
Inorganic mercury, formed through oxidation of mercury metal lost during ASGM may contaminate water and also lead to kidney damage if consumed. Conversion of mercury pollution from ASGM into methyl mercury also poses a tremendous risk as this highly toxic form of mercury accumulates in food supplies, such as fish, crustaceans and mollusks. Consumption of methyl mercury is particularly harmful to the central nervous system, causing nerve and brain damage. Kidneys are also affected and methylmercury presents an extreme risk to fetal development.
There are many documented cases of miners in ASGM suffering the effects of prolonged exposure to mercury. In addition to the symptoms of mercury intoxication noted above, these individuals suffered from frequent and severe headaches, dizziness, vision and motor disorders, among other health issues. Video recorded and published clinical interviews with miners poisoned by mercury show these devastating effects, especially with respect to tremors and movement disorders.
Another troubling consequence of mercury pollution from ASGM is the effects on embryos, fetuses, and children. Mercury levels in women of child-bearing age near ASGM activities are often high due to consumption of mercury-contaminated water, seafood or rice; direct handling of mercury in mining or other gold-related processing; or through exposure to mercury gas during amalgam processing.
Because maternal transfer of mercury to the fetus is efficient for elemental and methylmercury, it is perhaps not surprising that children in ASGM communities have high incidence of physical and mental disabilities. News reports on the debilitating effects of mercury poisoning in children of artisanal gold mines (including limb deformities, brain damage, and hydrocephalus) bring this problem into confronting focus.
With recent estimates indicating there are as many as 19 million artisanal gold miners, mercury use and emissions in ASGM is clearly a global health problem.
Wildlife and plant health
While the human cost of mercury poisoning in ASGM is the most important and immediate concern, mercury pollution also damages the wider ecosystem—compromising food chains and biodiversity. For example, mercury emissions can adversely affect algal growth; crustacean health; fish growth, brain function, and reproduction; and amphibian larval health and survival. It is also known that mercury bio-accumulates in fish, which then poses a threat to any bird or mammal that consumes it. Harm to these avian and mammalian predators is relevant to ASGM as many of the mining sites are located in highly diverse regions such as the Amazon rainforest. It is also common for the people living near these ASGM areas to eat fish as a major source of dietary protein, which leads to high mercury levels even in non-miners. In this way, mercury pollution threatens food security.
Aquatic plants are bio-accumulators of mercury and uptake of the heavy metal may, in some cases, compromise plant health. Inorganic mercury in water, for instance, can lead to decreased chlorophyll content and protease activity for floating water cabbage, Pistia stratiotes. Likewise, the pond weed, Elodea densa, presented with abnormal mitotic activity upon exposure to methylmercury. For terrestrial plants, the uptake and effects of mercury seems to be plant specific and highly dependent on the mercury concentration. In fact, mercury-derived fungicides have long been used to protect wheat seed and sugarcane setts, so the effects of mercury on plant health are not universally detrimental. This may be due to low bioavailability of mercury in soil and a tendency for the mercury to accumulate in roots. However, mercury vapor uptake through the leaves of both C3 and C4 plants is possible and therefore relevant to ASGM regions.
For regions of ASGM, an understanding of mercury uptake in plants is important for protecting food crops from contamination, and also for using plants intentionally to remove mercury from soil. Regarding crop contamination, mercury uptake in rice in ASGM communities has been documented. In these cases rice paddies were irrigated with mercury contaminated water, resulting in mercury levels as high as 1.2 ppm in the edible grain—more than 10 times the limits recommended by the WHO.
While mercury uptake into crops is clearly undesirable, mercury uptake into non‐edible plants may be a useful way to remediate mercury pollution in water and soil due to ASGM. In one example, it was shown that Siam weed (Chromolaena odorata) could grow in nutrient‐supplemented ASGM tailings and accumulate mercury in its leaves without phytotoxicity. In another recent study at a Colombian gold mine, native plant species growing at mercury contaminated sites were also promising leads for removing and accumulating mercury from soil. In these and other strategies in phytoremediation of ASGM areas careful planning for the fate of the biomass is required so that the heavy metal is ultimately removed from the environment and not simply re‐emitted by means of combustion, for example.