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Cyanide as a menace to earth's surface environment in relation to gold mining and future alternatives.


Cyanide is a carbon-nitrogen radical that can be found in a wide range as organic and inorganic compounds. This radical, the cyano group (C[equivalent to]N) can form solid, liquid or gaseous compounds. Among these, those that can easily release the cyanide ion ([CN.sup.-]) are highly toxic.

Once called Prussic acid, hydrogen cyanide (HCN) was first isolated from the Prussian blue dye in 1783 by the Swedish chemist Carl Wilhelm Scheele. Nowadays, hydrogen cyanide is a high volume production chemical with an annual production of 1.4 million tons worldwide. Despite its toxicity, cyanides are produced in small concentrations by several life forms such as animals, some plants and food. Some examples are microorganisms (certain bacteria, fungi and algae), centipedes, beetles, some butterflies, and are also found in apple seeds, almonds and beans. At this point the bulk of cyanide is produced by human industry [1].

Its toxicity is related to its physicochemical specification. It has been used for a long time in order to extract metals such as gold, silver, copper and zinc from ores all around the world [1]. At pH<9.2 free cyanide in aqueous solution exists as hydrogen cyanide (volatile form). Volatilization and sorption contribute to the loss of cyanide from surface water, and this volatilization increases with aeration and as pH decreases. Alkali metal cyanides such as sodium and potassium cyanide are lost from surface waters mainly by volatilization, while other soluble metal cyanides are removed primarily by precipitation and biodegradation [2]. Alternatives to cyanide can be found in gold mining industry (see Table 5), but some authors still believe that more studies should be done to improve those alternatives economical and efficiently.

Use of cyanide and background of disasters

Cyanide can pollute the environment by entering the surface waters after being released by humans. Currently the bulk of residual cyanide is produced by manufactured gas plants (MGP), mining industry and metallurgic industry (mainly metal finishing and electroplating) [2,4]. These three along with illegal fishing and war generate devastating effects on the earth's flora and fauna all around the world [3].

Even though those industries generate the biggest amount of cyanide, other daily used products should also be taken into account. These products are for example pesticides, plastics, photography, road salts, adhesives, cosmetics and electronics [1,3,4].


In gold mining, cyanide is currently the preferred lixiviant worldwide used at 90% of the gold mines [5]. Cyanidation as a chemical method for leaching of gold was studied in 1880s by John Steward MacArthur and due to instant good results, it eventually replaced chlorination processing. Nowadays, 20% of total production of cyanide is used annually to manufacture sodium cyanide (NaCH) [4], which is used in dilute solutions in gold mining operations (100-500 ppm). It is highly soluble in water and is one of the most toxic cyanide compounds [5]. In the gold leaching process, cyanide ions ([CN.sup.-]) dissolve the gold contained in the ore forming a solution from which gold is later extracted [6], as it is explained in Elsner's equation:

4Au + [8CN.sup.-] + [O.sub.2] + 2[H.sub.2]O [right arrow] 4Au[(CN).sub.2.sup.2] + 4O[H.sup.-]

Although gold mining industries are plagued with wildlife deaths due to its cyanide wastes, there is still few published data relating the actual destruction in fauna and flora with cyanide toxicity. When wildlife deaths have been studied, these have also been underestimated. In Nevada (USA) between 1990 and 1991, 9512 carcasses of over 100 different species were reported (annually 80-91% of the vertebrate carcasses were birds), although this was underestimated due to researching being voluntary. In Northparkes (Australia) in 1995 a mining incident ended with an estimation of 100 carcasses. Eventually, a precise count was conducted and 2700 birds died in the first 4 months. Avian deaths are always difficult to detect, and even after it, results are usually underestimated [8].

In some recent environmental accidents, cyanide from processing operations was spilled to the environment either by leakage through tears and punctures or by spillage from overflowing solution in tailings storage facilities (TSF) [4]. Table 1 collects information about cyanide spills and resulting impacts [4,8].

A concentration of weak-acid-dissociable (WAD) cyanide below 50mg/L is considered safe to wildlife, but outside TSF with different conditions this WAD can release cyanide ions and change previous concentrations. Therefore a distinction between free, WAD and total cyanide forms (other compounds such as cyanates or thiocyanates are also present) in tailings water is useful for legal regulatory purposes [8]. Some authors studied toxicity in several species [4]. In these cases, sensitivity to NaCN was not due to body size, but is considered to be associated with diet [7]. Carnivorous species give lower LD50 values compared to hervivorous ones. This smaller resistance in carnivores is due to their lower digestive pH used to breakdown flesh, fur or bones. A lower pH liberates cyanide ions faster and consequently releases faster a lethal dose [8].

Later, some chemical alternatives for leaching ore in mining industries will be presented.

Manufactured gas plants

Wastes containing cyanide can be found in soils at former manufactured gas plant (MGP) sites, also known as town gas sites [9]. Cyanide is generated in gas from coal pyrolysis [4]. The pressence of free cyanide in soils (and thus in air due to volatilization) suggested potential danger coming from a possible inhalation. A simple modeling analysis was carried out following very conservative suppositions, and the final result proved that the concentration in air due to volatilization is so low that does not represent a problem for humans [9]. Blue-stained soils and rocks are usually found when diverse cyanide compounds have been released into the environment. Usually concentrations of cyanide in wastewaters at MGP can reach values up to 2000 ppm, but concentrations of over 20,000 ppm have also been detected. These compounds are mainly iron-complexed forms, less toxic than free cyanide but still dangerous. An understanding of the chemistry, transport and toxicology of these cyanide-containing compounds is critical to precisely evaluate human health risks. Some authors found that most of the cyanide compounds at former MGP sites are the less toxic iron-complexed forms. These authors believe that magnified health hazards are expected when there is a lack of measurement techniques, and cyanide compounds in wastes are considered free cyanide when they are actually less toxic iron compounds [9].


Cyanide fishing has been used since the early 1960s to collect tropical marine fish for aquarium and pet shops. It began in 1962 in the Philippines to export mainly to Europe and North America. In this practice, fishermen insert sodium cyanide pellets into bottles filled with seawater. Then they dive down to coral reefs and distribute the cyanide solution where the animals hide, stunning them. This makes it easier and faster to capture the fish. This practice damages irreversibly the coral reefs and is illegal in most countries, but usually exporting and importing countries do not have accurate tests and certificate systems. This harming method was widely used in the early 1970s to export from Philippines to Hong Kong, and as the trade expanded, it spread to many other countries such as Indonesia, Cambodia, the Maldives, Thailand and Vietnam. Every year thousands of hectares of reefs are destroyed by cyanide fishing. By 2000, only 4.3% of the coral reefs in the Philippines and 6.7% in Indonesia were in perfect conditions. Healthy reefs produce 35 t of fish per square kilometer every year, while affected reefs produce much less. Sometimes, after cyanide solution is distributed some fish escape into reef crevices. Fishermen must then hammer and tear the reefs apart in order to capture the hidden fish. This irreversibly destroys the reefs and is also fatal to various marine life forms near the area. Although this proccess is illegal in most of the countries, corrupt civilians and military officials take advantage of poor rural workers and few countries take legal actions against responsibles. In some countries more than 90% of the fish vessels boarded by enforcement authorities were found to be using cyanide. Some methods have been developed in order to detect cyanide in live-caught fish, but this is difficult due to the fast detoxification mechanism in fish. A very sensitive and time consuming test is needed, and where fish have been kept for a while in clean sea water, cyanide is impossible to detect. Eventually, the best and safest way to prevent cyanide fishing might be to improve education and laws [10].

Metal finishing

Metal finishing industry is responsible for being one of the largest users of many toxic chemicals, and among those, cyanide is the most toxic. This toxicity, along with the high cost of wastewater treatment, is the biggest disadvantage of using cyanide. Several alternatives based on non-cyanide compounds have been developed, but have proven to be more expensive and difficult to use [11]. Table 2 shows concentration of cyanide compounds emitted from some industries [1]. Comparing those values, the highest are related to plating bath sites:

War and human poisoning

The toxicity of cyanide varies depending on the types of complexes that are present [8], being free cyanide the most toxic [4]. Cyanide can be lethal to humans and animals because it impedes the extraction of oxygen from the blood, binding itself to the iron-carrying enzymes that cells require to use oxygen. The body begins feeling oxygen starvation, irregular heartbeat, chest pains and vomiting. Within 10 minutes, muscle tremors, lacrimation, urination, defecation, salivation and hard breathing occur. Then, death follows quickly. Lots of animals (mainly migratory birds) die every year after stopping to drink at gold mining facilities. Poisoning may occur due to inhalation, ingestion or absorption either through mucous membranes or intact skin [4].

Every year hundreds of cyanide poisoned patients appear all around the world. Acute cyanide poisoning is not common but results in high fatality rates when caused by ingestion or inhalation. Elevated whole blood cyanide and thiocyanate have been found in smokers [12]. This has been used many times throughout history in order to attempt murder or commit suicide. Between 1985 and 1992, the Taipei Veteran General Hospital recorded a total of 138 cases of cyanide exposure, from which 85 were intentional [13]. Notable suicides by cyanide include Adolf Hitler, Eva Brown, Wallace Carothers and the mass suicide of Jonestown, where over 900 people died. Some examples of murder with cyanide include the use of hydrogen cyanide (Zykklon B) in the German Nazi during the Holocaust and judicial execution taken part in some states of the United States.

Sadly, cyanide has also been used at war. As an example, it was used in the 1980s in Halabja (northeast of Iraq) during the Iraq-Iran war [14]. Its devastating effects are not only suffered by fauna, but also by flora: The United States of America used cyanide along with agent orange during the Vietnam War in order to defoliate whole Vietnamese forests. In this process, those chemicals, were combined and sprayed to fatally accelerate plant growth, causing destruction within days of the spraying.

Manufacturing of a wide variety of products

Cyanide is used in the production of a wide range of products, such as adhesives, dyes, computer electronics, cosmetics, pharmaceuticals, insecticides, jewelry, road salts and in the production of organic chemicals such as nylon and acrylic plastics [3,5].

Cyanide removing techniques

Removing and recovering of cyanide is widely studied and discussed due to its potential toxicity and environmental impact. Two common methods to treat cyanide-contaminated water are alkaline chlorination and biological oxidation, but these techniques are only effective to degrade free cyanide and cyanide that is weakly bonded to metals. Cyanide strongly bonded or complexed with metals cannot be treated by those methods [1]. To protect the environment, many countries have imposed limiting standards for discharge of cyanide-containing wastewaters. Table 3 shows some examples of those limits, where total cyanide includes both free and complexed cyanide [1,15,16]. After devastating incidents, other countries such as Czech Republic and Hungary have definitely banned the industrial use of cyanide for mining.

Cyanide treatment systems are necessary in order to reduce the cyanide level in disposal effluents below 0.2 mg/L.

Chemical techniques

The most common methods currently used for treating cyanide containing wastewater are chemical oxidation techniques. One example is the alkaline chlorination oxidation process, where wastes are first treated with chlorine or hypochlorite to produce cyanogen chloride, that then reacts to form sodium cyanate. Compared to free cyanide, this sodium cyanate is less but still toxic. Finally, cyanate is oxidized by chlorination to carbon dioxide and nitrogen. This method can seem very convenient, but it suffers from important disadvantages. When treating cyanide complexes where cyanide is bonded to metals such as nickel or silver, chlorination is very slow. Also, wastes such as sludge or wastewater are produced. This water is toxic to aquatic life due to the high content of chlorine. The disposal of these wastes and the high quantity of chlorine required make this process relatively expensive. Chlorinated organics can also be produced if the wastewater contains organic substances [1].

Another possible chemical method for the removal of cyanide is oxidation with hydrogen peroxide ([H.sub.2][O.sub.2]) [5]. An interesting research to remove free cyanide was carried out recently using [H.sub.2][O.sub.2], in the presence of activated carbon prepared from olive stones [15]. This activated carbon plays the role of a catalyst increasing the reaction rate, and has also been used to develop other similar techniques (such as the removal of cyanide by plain and metal-impregnated activated carbons). Without it, the removal of free cyanide is very slow. This process has the advantage of not using soluble metal catalyst and hydrogen peroxide is the only chemical product consumed. On the other hand, additional tests should still be carried out to confirm the technical and economic advantage of this method compared to other alternatives [15,17].

Recent studies investigated the removal of cyanide by air and oxygen oxidation and absorption. Unfortunately, it was found that the removal was very limited, even in the presence of pure oxygen and activated carbon and copper sulphate as catalysts [18].

Physical techniques

Transforming free cyanide and its compounds into less toxic wastes by chemical or biological reactions are not always the only alternatives. The recovery of cyanide for a new use can be accomplished by physical techniques such as volatilization and absorption. In this process, sulfuric acid is used for lowering pH. At optimum conditions of 25[degrees]C and pH 2.7, 100% of free cyanide and 48% of complex cyanide can be recovered in 23 hours. The problem is that 2.4 kg of [H.sub.2]S[O.sub.4] and 1.65 kg of NaOH are consumed for each kg of CN-recovered, and even though the process can be done at higher pH values, CN-recovery will decrease. At pH<3, [H.sub.2]S[O.sub.4] consumption clearly increases [6].

Biological techniques

From an economic point of view, the biological treatment method is cost-effective compared to chemical and physical methods for cyanide removal [1,5]. When necessary conditions are maintained, some microbial species are known to degrade cyanide into less toxic products, using it as a nitrogen and carbon source and generating ammonia and carbonate. In this process, bacteria or alga convert free and metal-complexed cyanides to bicarbonate and ammonia, while the free metals are either adsorbed within the biofilm or precipitated from solution. Recent studies of biological degradation using the alga Scenedesmus obliquus, have proven to reduce WAD cyanide concentration in gold mill effluents from 77.9 mg/L to 6 mg/L in 77 hours [2].

The microorganisms that carry out the biological treatment of cyanide and thiocyanate are usually algal cultures or a mixture of indigenous soil bacteria that after extended exposure, have been adapted to the treatment of these compounds. Cyanide is easily degraded by anaerobic bacteria, but the removal of thiocyanate with anaerobic biological treatment is difficult and slower. Also, anaerobic biological treatment is more susceptible to toxic upsets due to exposure to other components present in the solution. The better way of removing thiocyanate is to use an attached or suspended growth aerobic process. The most important environmental factors include pH, temperature, oxygen levels, and nutrient availability [1].

The removal of cyanide by plants can also be done for treatment of wastewater from gold mining. Free cyanide and its compounds have proven to be lethal in certain amounts to plants and trees, but useful studies also demonstrated that cells from several different woody plants (willow, poplar, elder, rose, birch) can remove cyanide and use it as a source for nitrogen. After the exposure to different amounts of [CN.sup.-], basket willow trees showed various results. Low levels of cyanide were detected in plants after an exposure of 0.4 mg/L [CN.sup.-]. An amount of 2 mg/L of CN- in aqueous solution decreased transpiration in about 50% after 72 hours. Doses of 8 and 20 mg/L were quickly lethal. It is assumed that those levels are related to toxicity, capability of assimilation and resistance of those plants to cyanide. In order to build a "bioreactor" for cyanide removal in mining wastewater, the plants must be selected according to conditions at the mine, such as climate and soil. With a maximum cyanide removal rate of 14.5 mg [kg.sup.-1] [h.sup.-1], a total of 1100 kg of free cyanide could be removed by one hectare with willows during a growth period of 200 days [3].

Compared to other methods, biodegradation has several advantages and disadvantages. Wastewater tailings can be used as reactors reducing the total washed volume. Cyanides can be treated without generating additional waste and toxic byproducts. No chemical handling equipment or expensive control are needed, and compared to other alternatives it is relatively inexpensive. Also, it has been received well by regulators and public opinion. On the other hand, a combination of metallurgy, biology and process engineering is required, and the technology involved is not well established. A very specific study is required for each case. Even though free cyanide as well as thiocyanates and several metal complexes can be removed, high concentrations cannot be treated [1].

Future expectations and alternatives

Although cyanide removal by physical, chemical or biological techniques is possible, the process is expensive and important amounts of toxic by-products from cyanide oxidation are generated (e.g. ammonia). In Japan, a thermo hydrolysis process was recently applied to waste water treatment for effluents containing ferro-cyanide. This process is effective for neutralizing cyanide, but ammonia, which is regulated for limiting nitrogen released to water, is produced [19]. For biological treatment methods, initial costs are higher but operating costs are clearly lower; consequently, eventual costs can be significantly lower. Biotreatment has been successfully adopted in United States and Canada since the mid-1980s [5]. Despite all this, some authors remain skeptical and still think that more studies remain to be done in order to develop and effectively transfer biological treatment from the lab bench to full industrial scale [20]. Maybe, the best solution to end with cyanide pollution due to gold mining processes could be to end with the use of cyanide itself. Developing an equally effective and non-toxic leaching reagent is truly a difficult challenge. Alternatives to cyanide should be [4]:

1. Inexpensive and recyclable

2. Selective

3. Non-toxic

4. Compatible with the type of ore

5. Compatible with downstream recovery processes

Due to catastrophic cyanide spills in accidents, some countries such as Czech Republic, Hungary and several states in United States have banned the use of cyanide in mining industry. These countries among others could be especially interested in developing substitutes. Among those, the most promising alternatives are currently thiourea, thiocyanate, thiosulphate, halides and ammonia [4]:

Thiourea (N[H.sub.2]CSN[H.sub.2]) was recognised as a solvent for gold in 1906 by Moir, but has only received serious consideration in the past 20 years. It is an interesting option for treating refractory ores and floatation concentrates that can quickly extract up to 99% of gold. It has some advantages compared to cyanide including low sensitivity to residual sulphur in calcines and base metals such as Pb, Cu, Zn or As, high gold recovery from pyrite and chalcopyrite concentrates and acceptable gold recovery from carbonaceous (refractory) ores. On the other hand, its disadvantages have impeded its commercial adoption. Thiourea is more expensive than cyanide and its consumption in gold processing is high. The gold recovery step still requires more development. Even though it is less toxic than cyanide, it is a suspected carcinogen and must be treated with caution. Another drawback is that the rate of gold dissolution is strongly determined by pH, and a pH range of 1-2 is usually used.

Thiocyanate ([SCN.sup.-]) [4] can effectively dissolve gold when pH is between 1 and 2 in the presence of a suitable oxidising agent such as Fe, forming both [Au.sup.+1] and[ Au.sup.+3]. It is considered an effective lixiviant for gold with comparable dissolution rates to those of thiourea with much greater stability against oxidative decomposition.

After its use, thiocyanate complexes can be recovered on carbon adsorbents and ion exchangers.

Thiosulphate ([S.sub.2][O.sub.3.sup.2-]) can also dissolve gold in alkaline solutions. These alkaline conditions are necessary to prevent thiosulphate decomposition. Dissolution rates depend on the concentrations of thiosulphate and dissolved oxygen, the process temperature, and can be improved with copper ions. Acceptable gold dissolution rates can be achieved in the presence of ammonia and copper and causes fewer environmental impacts than cyanide. Gold extraction of 72% can be achieved after 50 days, or 90% in just a few minutes if the flotation ore concentrate is mechanicaly treated. In spite of this, the high consumption of reagent during extraction clearly rises overall costs and makes the process complex and unaffordable. Besides, this method will be slow if copper is not used as an oxidant.

Halide systems were used for gold dissolution before the inception of cyanidation [4]. Chlorine was used for recovering gold from calcines, ores and concentrates. It can disolve gold faster than cyanide, but low pH is needed and low concentrations of sulphides can greatly increase the consumption of reagent and reduce the amount of gold dissolved. The process is more complex than cyanidation, and caution is advised to avoid any health risk due to the highly poisonous chlorine gas. Bromine can also achieve high gold dissolution rates in the presence of a protonic cation. Fast extraction, non-toxicity and the possibility of being used in a wide range of pH values are some advantages, but there is usually high reagent consumption. Iodide solutions can also achieve faster gold leaching rates than cyanidation, and gold iodide complexes are the most stable in aqueous solutions among halogens. However, the excessive cost of the process hinders its use in the mining industry.

Ammonia has been found to be a very effective leaching agent for sulfidic and carbonaceous refractory gold ores under certain circumstances [22]. Solutions containing ammonia, ammonium and proper oxidants at 160-200[degrees]C have managed to recover over 95% of gold within 2 hours.

Some years ago, Nakata, Mariko, Shikazono and Honma [21] studied the dissolution of gold in hydrochloric acid. HCl was the preferred solvent for gold until 1900, when cyanide was discovered as being faster. The main problem of using HCl is the low dissolution rate of ore compared with cyanide. An interesting article about properties of Teflon was published some years ago by Clemens and McKibben [23]. There, it was shown that hydrogen can pass through a Teflon vessel [21,24,25]. This concept was the key factor of Nakata's experiment. In gold dissolution, [Au.sup.+1] and [Au.sup.+3] combines with Cl and the following reactions should be taken into account:

Au + [H.sup.+] + [2Cl.sup.-] [right arrow] Au[Cl.sub.2.sup.-] + 1/2[H.sub.2]

Au + [3H.sup.+] + [4Cl.sup.-] [right arrow] Au[Cl.sub.4.sup.-] + 3/2[H.sub.2]

Au + [4H.sup.+] + [4Cl.sup.-] [right arrow] HAu[Cl.sub.4] + 3/2[H.sub.2]

[Au.sup.3+] was identified with UV spectra as the predominant state of gold. It is a fact that the removal of [H.sub.2] from the system will change reaction kinetics and force the reaction to advance from left to right, increasing the amount of gold dissolved. This capability of Teflon to allow the leak of hydrogen at high temperatures [23] could make HCl a possible alternative to cyanide. Currently, a research about gold dissolution is being made. Nakata (1996) analyzed gold dissolution with different HCl concentrations in different time intervals [21]. 100% of gold was dissolved when using a Teflon vessel at 150[degrees]C for 100 days [25]. Since several types of Teflon exist, the combination of best factors will be searched in order to make hydrochloric acid compete with cyanide. By now, HCl is still far from being as fast solvent as cyanide. Results show that cyanide can be 10 times faster than HCl, but speed is not the only important thing in the process: hydrochloric acid is very easy to neutralize, since a simple treatment with alkaline compounds would create innocuous salts and water. This could also make it interesting from an economic point of view [21,23].

Finally, the coal-oil-gold agglomeration (CGA) method will be reviewed. The CGA method is a physical process that can be an alternative to cyanide in some cases [4]. CGA is useful for recovering hydrophobic/oleophilic free gold particles ranging from one to 100 microns from ore slurries. It is also considered an alternative to mercury amalgamation in the small-scale (artisanal) gold mining industry. Laboratory results showed gold recoveries up to 95%, while pilot test recovered 62-75%. So far these results are promising, but the main problem of CGA is that only free gold particles can be recovered.


A global use of environmental friendly ore leaching solvents different from cyanide is still far from reality. Several countries are currently using other alternatives, but sadly some of those countries (e.g. Czech Republic and Hungary) had to suffer big environmental disasters before changing laws and prohibiting the use of cyanide. Although methods with faster leaching rates and less toxic could be discovered, actual alternatives have existed for centuries. More efforts should be made for a definite change in the use of cyanide in mining industry. Cyanide containing wastes are generated in the production of some daily-used products, but that amount is very small compared to the one generated in metal finishing and ore leaching industry. At this point, corresponding engineers responsible for the design at a certain mining complex, have to choose between cyanide and other slower (and thus maybe more expensive) but less polluting options.


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[4] Gavin Hilson, A.J. Monhemius, Alternatives to cyanide in the gold mining industry: what prospects for the future?, Journal of Cleaner Production 14 (2006), 1158-1167

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[24] Masataka Nakata, Naotatsu Shikazono, Hisahide Honma, Dissolution of Ir and Os in Hydrothermal 1N and 3N HCl Solutions at 150oC, Shigen to Sozai 110 (1994), 253-255

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Diego Garijo Marcos and Naotatsu Shikazono

Laboratory of Geochemistry, Graduate School of Science and Technology, Keio University, Hiyoshi 3-14-1, Yokohama, 223-8522, Japan

Table 1: Examples of mine accidents and resulting environmental

Location and date Impacts

Nevada (USA), 1989 and 1990 Almost 900 lbs of cyanide were released
 into the environment by eight leaks

Colorado (USA), 1992 Severe environmental problems along a
 17-mile of the Alamosa River

Guyana, 1995 2.9 [m.sup.3] of cyanide tailings and
 wastewater were released into the Omai

Montana (USA), 1997 Severe contamination of groundwater with
 substantial wildlife deaths

Kyrgyzstan, 1998 Almost 2 tons of sodium cyanide was
 released into surface waters

Romania, 2000 Over 100 t of cyanide were released and
 eventually reached the Danube River

Table 2: Cyanide in industrial wastewater.

 Amount of cyanide
Wastewater source compounds (mg/L)

Gold ore extraction 18.2-22.3
Electroplating plants 0.1-14.24
Plating rinse 1.4-256
Plating bath 45,000-100,000
Alkaline cleaning bath 4,000-8,000
Colour film bleaching process 71
Paint and ink formulation 0-2
Chemical industry 10.4-50.9
Explosives manufacture 0-2.6
Oil refinery 2.25
Petroleum refining 0-1.5_

Table 3: Limiting standard for discharge of cyanide.

Country Limit (mg/L)

United States 0.2 (drinking), 0.5 (aquatic-biota)
Germany 0.01 (surface waters), 0.5 (sewers)
Switzerland 0.01 (surface waters), 0.5 (sewers)
Mexico 0.2
India 0.2
Algeria 0.1
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Author:Marcos, Diego Garijo; Shikazono, Naotatsu
Publication:International Journal of Applied Environmental Sciences
Date:Jul 1, 2011
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