Introduction
Pollution issues have been one of the main focuses by the government and nation worldwide due to the global warming concern 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 our environment. Historically, AMD has caused major problematic implications to the country, especially 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 have affected the human life, for example daily drinking water supply. In this report, incidence of acid mine drainages which happened at national or international level have been discussed. In addition, the environmental impacts and causes of acid drainage have also been addressed. Potential solutions which can mitigate the occurrence of acid mine drainage also been suggested.
Incident of acid mine drainage (national/international)
Environment impact of acid mine drainage (particular of role bacteria)
The mining activities can provide many major impacts towards the environment around the world because mining activities normally produced pyretic minerals during and after the its operations processes (Zalack et al., 2009). These types of minerals tends to generate sulfuric acid and flows into the water streams that produces negative impact on ecological degradation (Cherry et al., 2001; Gerhard et al.,2004; Lin et al., 2007) and also on human safety that lives near to the coal industry (Lin et al., 2005; Chen et al., 2007). According to the WHO (2004) report, the acceptable drinking water pH for human should be between ranges of pH 5.8-8.0, however, a research on the groundwater discovered a site surrounded by AMD has a pH of 1 or less (Gerke et al., 1998). This has indicated that groundwater is considered dangerous and toxicity, which may contained high concentration of arsenic, iron and nickel.
Therefore groundwater which have been affected with AMD, may affect the aquatic life cycle and also the marine ecosystem, for example, the Howe Sand and Squamish River which located in Britannia which is an active and productive places that is used by juvenile chum (Oncorhynchusketa), chinook (Oncorhynchus tshawytscha) and salmonids. This entire species migrated from Squamish River to the open ocean and due to AMD contamination, the migration and the growth of this species have been affected by AMD and also been identified as highly toxicity to the fish, invertebrate and macroalgae (Marsden et al., 2003). There was a study that found that endemic plankton near the shore at Britannia has been badly affected by the AMD and this may gradually make the species extinct and finally 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 have 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 developed 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 releases of the heavy metal tend to be associated with pyrite (FeS2) which is 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 is often considered to have a wide range of concentration in 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 may 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). However, if the environment is unfavorable, bacterium will produce a minimum level acid generation. These types of bacterium use 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 a previous research, there are 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 smell like rotten eggs (Cheng et al., 2008).
Possible solutions 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 treatment it requires less or no maintenance at all using self contained with regards to treatment and waste.
Passive system treatment has been rapidly used around the country (Gazea et al., 1995) because of low maintenance cost and minimal operation is required compared to the active treatment system which is considered 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 formation 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, the drawback in using this technique is 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 wetland which is effectively used to treat the mine waters which are net alkaline (Gazea et al., 1995). According to Gazea et al., 1995, oxidation reaction will occurs and leads to metal precipitate which primarily is hydroxide, oxyhydroxides and oxides. Aerobic wetlands are relatively shallow system because to maintain the oxidizing condition and due to the 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).
Finally, 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 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 in which the performance is easily to control, manage, and allow some of the heavy metal that are 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 the 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.
