Acid Mine Drainage
Pollution issues have been the main focus by the government and people around the world due to the global warming issue which is getting serious by the day. The major forms of pollution are water, air and soil contamination. Acid Mine Drainage or AMD has been categorized as the most serious pollution towards environment. Historically, AMD has caused major implications on the country and currently to the mining industries (Johnson and Hallberg, 2005). The acid drainage from mines has caused major distressed on the chain life cycle of the aquatic ecosystem and also may affect on the human health, for example water supply for drinking purposes. In this report, incidence of acid mine drainages which happened at national or international level were discussed. Moreover, the environment impacts and causes of acid drainage have also been discussed. In addition, any potential solutions which can mitigate the occurrence of acid mine drainage also been discussed.
Incident of acid mine drainage (national/international)
Environment impact of acid mine drainage (particular of role bacteria)
The mining activities can give many major impacts towards environment around the world because the mining activities naturally produced pyretic minerals during and after the operations (Zalack et al., 2009). These types of minerals will generated sulfuric acid and flows into the water streams that gives negative impact on ecological degradation (Cherry et al., 2001; Gerhard et al.,2004; Lin et al., 2007) and also on the human health problems that live near to the coal industry (Lin et al., 2005; Chen et a.,l 2007). According to WHO (2004), the acceptable drinking water is between range of pH 5.8-8.0, however there was a research found that the groundwater which surrounding by AMD site has pH of 1 or less (Gerke et al., 1998). This show the groundwater is dangerous and toxicity which containing high concentration of arsenic, iron and nickel.
Consequently, when the groundwaters have been affected with AMD this have affected on the aquatic life cycle and also the marine ecosystem, for example, Howe Sand and Squamish River which located in Britannia which is productive places that used by juvenile chum (Oncorhynchusketa), chinook (Oncorhynchus tshawytscha) and salmonids. This entire species migrate from Squamish River to open ocean and due to the AMD contamination, the migration and the growth of this species are decreasing. AMD also have been identified as high toxicity to the fish, invertebrate and macroalgae (Marsden et al., 2003). There was a studied found that endemic plankton near shore at Britannia have been affected by the AMD and this will gradually make the species extinction slowly and vanished (Levings et al., 2005).
Cause of acid mine drainage (particular of role bacteria)
Acid mine drainage or AMD is a serious environmental pollution which particularly occurred in many developed countries that is involved in a continuous development in the industrial sectors. This type of pollution was believed to develop from the mining industry (Bernardin, 2006); Pinetown et al., 2007; Zhao et al., 2007) and has caused a long-term impairment to waterways and ecosystem (Akcil and Koldas, 2004). The cause of AMD comes about when the sulfide compound was exposed to the water and oxygen (Hughes, 1994). The most common sulfide minerals which produce AMD are iron sulfide (Akcil et al., 2006). As a result of this, sulfuric acid will be produced and released heavy metal to the drainage system. The release of the heavy metal tend to be associated with pyrite (FeS2) which the most abundant sulfide on this earth (Johnson et al., 2005). The chemical reactions of AMD are various and complex, as shown below. Chemical reactions have been summarized as follows (Johnson and Hallberg, 2005):
4FeS2 + 15O2+14H20 4Fe (OH)3+ 8SO2-4+16H+
Based on above chemical reaction, the regeneration of ferric iron will promote the ongoing oxidation of the mineral. There is one research paper that found the consequence of AMD is mostly dependent of pH and acidity (Akcil et al., 2006). The environmental that has been affected with AMD often considered as it has wide range of concentration which is acidic and solute which often includes iron and arsenic (Cheng et al., 2009).
In addition to the above, it was believed that there is diverse range of microorganisms that lives within AMD environment that cause the formation of AMD. These groups of organisms come from a chemoautotrophically-based biosphere (Baker et al., 2003). The microorganism will often increase the rate of AMD and produce much more generation of AMD inside the contaminated water (Silverman and Ehrlich, 1964). According to Mahmoud et al., 2005, one type of bacterium which usually occurred and mostly found in AMD site is Acidithiobacillus ferrooxidans. The bacterium will accelerate the rate and the formation of AMD when the environment is favorable, for example A.ferrooxidans is active in water with a pH less than 3.2 (Akcil et al., 2006). Whereas, if the environment is not favorable, bacterium will produce minimum acid generation. These types of bacterium use the sulfur, ferrous iron and carbon as source energy (Chen et al., 2007). This type of bacterium is gram- negative, non spore forming rod shaped acidophilic bacterium (Leduc and Ferronic, 1994). Based on previous research, there many various types of strains of Acidithiobacillus ferrooxidan that have been isolated from mine water sources which is generally presence in sulfide mineral with lower pH and highly toxic compound in the liquid phase (Mahmud et al., 2005). This microorganism will remains active for many decades if the AMD site not been untreated (Sheoran et al., 2006). Thus, the water which is contaminated with AMD usually have high level of iron, aluminum and sulfuric acid which showed that the formation of the water is in the form of orange or yellowish-orange color and sometimes it can smell like rotten eggs (Cheng et al., 2008).
Potential solution which can mitigate the occurrence of acid mine drainage
AMD is continuous environmental pollution problems which occur in all countries which have active and abandoned sulphide or coal mine sites. The untreated AMD site will gives dramatic effect on aquatic life, ecosystem and human health. Fortunately, there are some potential treatments which can mitigate the occurrence of AMD. According to Hughes (1994), there are two basic forms of AMD treatment which are active system and passive system. The active treatment is a treatment which requires a continuous maintenance of the system while for passive system is treatment that required less or without maintenance; and also using self contained with regards to treatment and waste.
Passive system has been used rapidly around the country (Gazea et al., 1995) because the cost of maintenance is low and minimal operation is needed than active system which is expensive and sometimes can be impractical (Johnson et al., 2005). The most frequent and popular technique that has been used in the mining industry (Kuyucak, 2002) for passive system is the use of anoxic limestone drains (Kleinmann et al., 1998). The purpose for this technique is to add the alkaline into AMD and maintaining the level of iron in its reduced form which to avoid the oxidation of ferrous iron and precipitate of ferric hydroxide on the limestone (Johnson et al., 2005). The concentration of ferrous iron which is less than 50mg/L are treated to a pH range of 6.5- 8.0 and compound which have higher concentration that have pH 8 to 10 will be passed through an aeration tank which the ferrous hydroxide precipitate is converted to ferry hydroxide ( Akcil et al., 2006). However, there is drawback in using this technique because will result in the formation of ferrous carbonate and manganese carbonate (Evangelou, 1998). This will require a special waste disposal facilities which is expensive and costly (Hughes, 1994).
Another passive system treatment is aerobic wetlands which effectively used to treat the mine waters which are net alkaline (Gazea et al., 1995). According to Gazea et al., 1995, oxidation reaction will happen and there will be metal precipitate which primarily is hydroxide, oxyhydroxides and oxides. Aerobic wetlands are relatively shallow system because to maintain the oxidizing condition and due to that macrophytes were planted to regulate the water flow and to filter and stabilize the ferric precipitate. (Johnson, 1995). Aerobic wetland also containing a bacteria which can oxidize arsenic (III) to arsenic (V) and reduced sulfur compounds (Battaglia-Brunet et al., 2002; Coupland et al., 2003).
Lastly, for the passive system is a permeable reactive barrier which is used for treating the polluted ground-waters and the technique has the same basic function as compost bioreactors (Benner et al., 1997). The permeable reactive barriers involved with the digging of pit which allow the flow of polluted groundwater and filling the void with reactive materials which is permeable (Johnson et al., 2005). This treatment will remove metals such as sulfides, hydroxides and carbonates. According to Young et al., 2003, the largest permeable reactive barrier been used is in Shilbottle, northeast England.
There are two types of treatment for active system which is chemical neutralizing agent and off-line sulfidogenic bioreactors. Chemical-neutralizing agent (Coulton et al., 2003) is a process whereby using agent components such as lime (calcium oxide) slaked slime, calcium carbonate, sodium carbonate, sodium hydroxide and magnesium oxide. However, there are some disadvantages for this system which is the expensive cost involved for this treatment and the bulky iron sludge was produced depending on the chemistry of the mine been treated (Johnson et al., 2005).
While, off-line sulfidogenic treatment is the biogenic production of hydrogen sulfide which used to generate alkalinity and removed all metal which is insoluble sulfide (Johnson et al.,2005). These system showed three potential advantages over passive treatment which the performance is easily to control and manage, allow some of the heavy metal that present inside the AMD to be selectively recovered and can be used again ( Johnson, 2000; Boonstra et al., 1999). However, the drawback for this system is that the construction and operational cost are considerable high (Johnson et al., 2005).
Beside treatment for AMD sites there also some of the prevention techniques that can be applied on the mining sites. According to Akcil and Koldas (2004), the main factor that we should be aware about AMD problem is the movement of water flow into the contaminated sites and this site should be controlled or monitored because water is the basic medium transportation for contaminants. Both of the researchers believed that the water entry into the site of AMD formation can be controlled by changing the water flows towards the site contaminated with AMD.
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