This article presents first a brief overview of cyanide and mercury use in gold extraction processes, impacts of these chemicals on health and the environments. The paper provides a concise comparison of toxicity, cellular targets and mechanisms of actions for selected metallic and cyanide pollutants released from mining plants. The last part of the paper discusses the “expert panel’s safety claims that was briefly mentioned by Mr. Motumma Maqasa, the Minister of the Ethiopian National Defense Force (ENDF). The paper suggests the way forward and also warns the public (stakeholders) never to let MIDROC, and its accomplice- the Ethiopian government, off the hook in the fight to bring lasting solutions to the problems.
Cyanide and mercury based gold extraction processes.
Gold is found in various concentration in ores, special type of naturally occurring rocks, containing a particular metal or valuable mineral in a large amount. Abundance is an important factor for the extraction to be profitable. To extract gold from the ore, first the ore needs to be excavated, and then crushed/ pulverized. The milled ore is then treated with either mercury or sodium cyanide, oxygen and lime solution to dissolve or leach gold and silver from the ore. The process is done in a reactor in vat leach technique. However, large mines now use heap leach techniques in which cyanide solution is run through a heap, to leach or dissolve gold but also silver. In fact, sodium cyanide efficiently and effectively pulls gold from the ore, dissolve it into solution (Fields, 2001). The gold bearing solution (gold-cyanide complex) is retrieved while the insoluble parts are sent to holding dams, waste reserve or tailings. Cyanide is then released into the environment through a waste reservoir or tailing dam while gold is precipitated out of the solution (Acheampong, Meulepas & Lens, 2009) and further refined for marketing.
Like cyanide, elemental mercury is used in gold extraction. Mercury is mixed with milled gold-containing materials, forming a mercury-gold amalgam. The mercury-gold amalgam is then heated to evaporate the mercury and to separate out the gold. This way, a large amount of mercury (Hg) is released to the environment contaminating air, soil and water bodies (Hidayati, Juhaeti & Syarif, 2009).
While there are other methods of extracting gold from ore, such as gravitational method, mercury is still in use particularly in artisanal and small scale mining in some countries because mercury is inexpensive, easy to access and easy to use despite its toxicity to the people and the environment. But what is cyanide? What is mercury? What are the dangers these chemicals pose to health and the environment?
Cyanide is a compound consisting of one atom of carbon connected to one atom of nitrogen by three molecular bonds (C≡N), or any compound containing the cyanide bond (ASTDR, 2006). Cyanide can exit as a gas, liquid, or crystals. Exposure occurs by inhalation, ingestion or transdermal absorption (Bishop, Fody & Schoeff, 2018).
Cyanide is a rapidly acting toxic compound. It binds with ferric iron and inhibits cytochrome oxidase. This inhibition causes impaired oxygen utilization and cellular energy production leading to progressive histotoxic tissue hypoxia and metabolic acidosis (Baskin, S. I et al, 2008). Cyanide ions also inhibit other enzymes such as glutamate decarboxylase, xanthine oxidase, superoxide dismutase, NO synthase and nitrite reductase (Cassel, 1993).
Cyanide toxicity varies by dose, duration, and route of exposure (ATSDR, 2006). Most sensitive tissues are those with the fastest metabolism of oxygen (Jaszczak, E et al., 2017). Thus, exposure to high levels of cyanide harms the brain and heart first, and may cause coma and death (ATSDR, 2006). Exposure to lower levels may result in breathing difficulties, heart pains, vomiting, blood changes, headaches, and enlargement of the thyroid gland (ATSDR, 2006), confusion, hallucination, abdominal pain, and slurred speech. Baskin, S. I et al, (2008) stated that chronic cyanide exposure may lead to a kinetic rigid syndrome, tremors, pathological reflexes, disorders of sensitivity, intellectual deficits, and significant neurological morbidity arising from the apoptotic demise of neurons of the basal ganglia and sensory-motor cortex.
However, cyanide is biodegradable and does not bio-accumulate (Akcil, 2010). It is removed from water bodies primarily by volatilization as well as by aerobic or anaerobic biodegradation, according to Akcil (2010). Cyanide is cleared from the body mainly by enzymatic conversion to thiocyanate, a non-toxic product that is exerted in urine (Bishop, Fody & Schoeff, 2018). Furthermore, children exposed to cyanide are likely to exhibit the same effects as adults (ATSDR, 2006) and no known cases of birth defects or congenital deformation has been linked to cyanide. However, cyanide is fairly mobile in soil and is able to passes through soil into underground water (ATSDR, 2006).
On the other hand, cyanide is highly reactive with many heavy metals. When milled ore is treated with cyanide solution, cyanide extracts or dissolves gold, silver, and other heavy metals. While gold and silver are retrieved, the rest of the heavy metals are released into the environment.
Cyanide also combines with sulfur and forms acid mine drainage (AMD)—an acidified runoff—that can contaminate streams, and underground water. Acid mine drainage (AMD), according to Fields (2001), is a process in which acidic water is produced from the combination of sulfide minerals, water, air, and highly specialized bacteria. Once those sulfide materials are exposed to a steady supply of water and air, they begin producing sulfuric acid, and that in turn provides a medium in which the microbes thrive and further oxidize the minerals, producing a self- perpetuating chain reaction. AMD seeps out of fields of tailings, waste damps, piles of displaced surface matter and rock being slowly processed for gold removal. If left unchecked, AMD can contaminate groundwater and entire watersheds, contributing not just acidity but heavy metals—such as arsenic, lead, cadmium, mercury, zinc, iron, copper, aluminum, manganese, and chromium—which it releases from the ore it passes through (Fields 2001). These heavy metals are ]=[-=-f special concern due ttheir toxicity, bioaccumulation tendency and persistency in nature and for causing birth defects (Agrawal, 2012).
Mercury exists in three forms: elemental (or metallic), inorganic, and organic mercury compounds. Metallic mercury is liquid at room temperature and is used, among others, in mining. Inorganic mercury is formed when mercury combines with other elements, such as sulfur or oxygen, to form salts. Through methylation by microorganisms, metallic and inorganic mercury can be converted to organic mercury compounds, such as methylmercury. Organic mercury tends to bio-accumulates in the food chain.
Exposure to mercury occurs from breathing contaminated air, ingesting contaminated water and food. Exposure to high levels of mercury can permanently damage the brain, kidneys, and developing fetus (ATSDR, 1999). Chronic exposure over a long time causes neurological disturbances, memory problems, skin rash, and kidney abnormalities and damage to the nervous system. Infants born to women who were poisoned with methylmercury had developmental abnormalities and cerebral palsy, according to (ATSDR, 1999). Very young children are more sensitive to mercury than adults. Mercury in the mother’s body passes to the fetus and may bio-accumulate, possibly causing damage to the developing nervous system. Mercury’s harmful effects that may affect the fetus include developmental abnormalities, brain damage, mental retardation, incoordination, blindness, seizures, and inability to speak (ATSDR, 1999). Furthermore, organic mercury tend to bio-accumulate in the food chain and at high levels has been linked to tremors, paralysis, anemia, bone deformities, and death (Fields, 2001).