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Acid Mine Drainage: Origins and Assessment of Methods

Overview

An Acid Mine Drainage (AMD) is water that drains out from coal deposit mining areas and has a lower pH which has been occasioned by the presence of sulphuric acid. In other words, an Acid Mine Drainage is usually acidic although not all mine drainages are acidic. This acidity is due to the high concentration of sulphuric acid. Rainwater is the origin of the sulfur-rich water which usually finds its way through the rock fractures which have been left behind after coal mining has taken place (Wolkersdorfer, 2000). Throughout the history of coal mining, this type of drainage has been a challenge on Wheeling Creek water point which can be traced back about 1800 years ago.

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The growth and development of industries that dealt with Wheeling greatly depended on the use of energy derived from coal. In fact, most developments could only e carried out with the help of coal energy which had been found to be very useful then. Up to date, West Virginia’s industrial development still relies heavily on coal-bearing in mind that the mining of coal in this region is still one of the most dominant industries. Besides, most of the energy demands for industries located at the Ohio River Valley still depend on the use of energy from coal (Benner, Blowes & Ptacek, 1997).

Before the advent of 1950s, there was very little consideration put in place as far as the mining of coal and environmental conservation was concerned. Mining of this mineral resource was rampant and was done on large scale. Recently, the impact of coal mining on rivers and streams has been brought under necessary checks and balances. For instance, new and better methods of mining coal which are also environmentally friendly have been devised (Pašava & Kříbek, 1995). Besides, there is a myriad of environmental regulation policies that have been enacted to control the impact of coal mining on environmental pollution. In addition, several methodologies on effluent treatment from coal mining sites are being used to control pollution associated with Acid Mine Drainage.

Nonetheless, West Virginia still faces major challenges today related to coal mining and Acid Mine Drainage. There are quite a number of deep mines which have been abandoned for some time which are equally a threat to the environment due to the continuous discharge of acidic effluents. The level of water quality in West Virginia is of poor quality due to these acidic effluents. Moreover, the ecosystem within the rivers has been disrupted significantly. This paper attempts to explore and give an incisive report on the origin of Acid Mine Drainage as well as an assessment of methods that can be employed to clean up the drainage.

Origins and development of AMD’s

Natural impacts which result from general mining activities are responsible for Acid Mine Drainage. This process occurs due to the exposure of compounds that contain sulfur in the process of excavating through hard rocks. Excavation of the ground is usually deep and reaches the bedrock and groundwater level where most of the sulfur deposits are located (Pašava & Kříbek, 1995). Once these sulfur-containing compounds are left open, they become susceptible to the attack by moisture and oxygen. These two natural agents of rock weathering initiate the chemical reaction whereby sulfur gradually dissolves in the available water and mixes with oxygen to form sulphuric acid which has relatively high concentration.

The mining of metal ores well below the level of underground water has been a possibility since the use of steam engines was introduced. These engines have facilitated the procedure of avoiding any contact between water, oxygen and sulfur-containing compounds (Benner, Blowes & Ptacek, 1997). Once the contract is avoided, the formation of hazardous sulphuric acid and its relative impact on the environment is equally kept at bay.

Nevertheless, even with the use of steam engines, surface and sub-surface water run-offs which find their way through the open seams of exposed metal ores have led to the origin and formation of Acid Mine Drainage because a similar chemical reaction is initiated when water, oxygen and exposed sulfur ores are combined.

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Mining has undergone a myriad of stages. The advent of mining saw the possibility of extracting metal ores that were only over and above the level of groundwater. This was relatively safe and environmentally friendly because the formation of Acid Mine Drainage could be avoided at all costs. The developments made in the mining engineering however, transformed the way mineral ores were being extracted from the ground. For instance, horizontal shafts were constructed so that there could be a way out of the mining site especially in cases where the mineral deposits were far much below the surface of the ground (Pašava & Kříbek, 1995). Another important function of these horizontal shafts was to provide a channel through which groundwater could be drained to nearby streams or valleys so that the lower levels of the mine could be easily accessed. As the mining depths increased with time, it became necessary to use steam engines to pump water from the mining sites. As a result of this development, the groundwater level was artificially lowered especially within the surroundings of the mining site. The value of the minerals which were mined managed to offset the costs incurred in pumping huge volumes of water from the vicinity of the mines.

In spite of this mining procedure, there is still an environmental challenge associated with it. It is definite that the mines will begin to flood if pumping of the ground is terminated. If pumping is not resumed in due time, the groundwater level will resume its initial depth. In cases where the mines have been abandoned and there is no need to pump water from the site anymore, the rising groundwater level will eventually resume its initial natural level and it will also catch up with the level of the constructed horizontal shafts that originally served the function of draining water out of the mine to the river valleys and streams. The very groundwater will begin to drain again using the adits into river valleys. Sincerely speaking, this type of water which is being drained into river valleys is not safe or recommended quality bearing in mind that it has contaminated itself with residues at the mining site.

Due to this contamination, a biologically mediated oxidative reaction takes place (Pašava & Kříbek, 1995). This type of reaction specifically affects compounds that contain sulfur. The fact that the mining sites often remain moist-free does not mean that no oxidation reaction can take place. Usually, sulfate salts are produced by sulfur-containing compounds. Although the salts are in solidified form, they are often generated in plenty and in a form that they can be available for further chemical reactions. These sulfate salts also contain many of the metals which are mined alongside the ores. The flow of water into the mining sites initiates dissolution process of the sulfate-containing salts. It should be appreciated that this dissolved mixture also contains metal elements from the ores and it is acidic in nature. It is also composed of the discharge from Acid Mine Drainage. The available sulfur salts are oxidized by the action of chemolithotrophic bacteria. As a result, the continuous discharge of Acid Mine Drainage from the mines is maintained for a considerably long period of time after the original flush.

Environmental Threats

Acid Mine Drainage is a real environmental threat especially in consideration of the fact most living organisms flourish well at a pH of around 7. Acidic or basic conditions favor only a minuscule fraction of life. This drainage is highly acidic and leads to the acidification of local watersheds. This may further hamper, disrupt or eliminate the river ecology. Fish and other vertebrates are even prominently affected by Acid Mine Drainage more than other animal and plant species (Benner, Blowes Ptacek, 1997).

Another accompanying problem is the metal parts which are part and parcel of the Acid Mine Drainage. Most mining sites that extract coal may not escape the possibility of discharging iron components in the Acid Mine Drainage. The poisonous nature of iron compounds makes its discharge into watercourses a big threat to both plants and animals. Both ferrous and ferric forms of iron are naturally toxic and high intake levels of the same cannot be tolerated by most living organisms (McElfish & Beier, 1990).

As Acid Mine Drainage is produced, it is usually in ferrous form. This form of iron is highly soluble in water and is not found in form of precipitates but as a soluble product. However, the presence of oxygen and of course moisture oxidizes the ferrous (Fe2+ ions) to ferric (Fe3+ ions) iron. The ferrous iron is red-brown in color and is in form precipitates because it is not soluble. The iron (III) ion is a solid with a high density. If this form of iron is concentrated in water even in small volumes, large precipitate amounts will still be generated. These precipitates have the ability to create a cover on land and stream surfaces especially at the point where the content is being drained. Consequently, the environment is smothered by this iron coating prohibiting the natural flow of water, air and other nutrients. As a result, it hinders life from the process of growth and development. One specific effect on fish is that their gills are coated. It is this coating effect that causes fatalities of fish although the metal itself is not intrinsically toxic.

After the closure of the mines, water pumps are not switched off instantly. The groundwater level has to be maintained at a lower level because other mines in the vicinity which are still operating should still be protected from the groundwater interference. However, pumping of water from the mines is more likely to cease in totality as mining in other nearby sites comes to an end. Later on, the rising groundwater will fill up the empty mines and the same will be discharged in river beds and the immediate environment. It is only through thorough treatment of water from the mines that adverse effects to the environment will be avoided (Demchak, Morrow & Skousen, 2001).

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The risk of Acid Mine Drainage will extend to salmonid rivers. Since the natural watercourses and the ecosystem will have been disturbed, habitation will no longer be suitable in these rivers and this will lead to either deaths or massive migration of different animal species from the affected habitat. Moreover, the angling community will be duly affected and some monetary engagements will be necessary to offer remedy to the situation.

Even after treating the discharge from mines which are rich in acids, the challenge will still persist. For example, the Acid Mine Drainage affecting the pH of the water will still have to be treated in some way so as to reduce the level of acidity in the flowing water. Alternatively, it will also require the discarding of sludges that are rich in metal ions. These sludges are usually left as residues after water has been treated.

The Acidity of Mine Drainage

Although most mine drainages are acidic, there are some which are not. The effluent from the mining sites may be almost neutral especially if the geology of the area being mined is well endowed with calcium compounds or lime. Waters which test neutral even after sweeping through the mines are also saline or brackish in nature (Pašava & Kříbek, 1995). Nevertheless, this does not insinuate that drainages which are not acidic are environmentally friendly and do not require to be treated. Although most environmental degradation problems are associated with acidity, it should be noted that alkaline conditions also have their own share of problems and they equally need to be treated.

On the same note, it is also imperative to note that the presence of iron (III) ions is still a major threat to the environment (McElfish & Beier, 1990). An environment that is neutral will favor the rapid precipitation of iron and consequently cause the coating effect on land, watercourses and gills of fish as explained earlier. The only merit of a neutral pH is that it allows easier and quicker treatment of the discharge owing to the fact that the precipitation of iron is not very necessary. Adding lime to the mixture to precipitate iron is not required because the discharge is not acidic at all.

Nonetheless, this does not eliminate the challenge and associated financial costs of discarding waste materials that are left after treatment of mine water. All the water samples containing iron compounds are supposed to be treated before they are allowed into the environment. Since treatment of the Acid Mine Drainage remains to be the hallmark of dealing with this waste from the mines, it is crucial to identify who is specifically responsible for treating Acid Mine Drainage in addition to the best methods which can be employed in cleaning up the mine drainage.

The problem caused by the pumping of water from mining sites was not foreseen immediately even as the use of steam engines was embraced in the mining process. Better still, less concern on environmental conservation and protection prevailed especially in 19th century. Mining operations still go on and may not stop in the near foreseeable future unless the mining sites are depleted of mineral resources. The responsibility of cleaning up the mines squarely remains on the shoulder of the major key players in the mining industry such as mining companies and government agencies in whose dockets mining falls (Demchak, Morrow & Skousen, 2001).

Cleaning up of Acid Mine Drainage

Events which lead to the formation of Acid Mine Drainages are more delicate to the environment compared to other forms of pollutants like oil spillages and nitrate accumulation or flow in the environment. The main reason for this is that pollution is caused by Acid Mine Drainages are mostly non-biodegradable and as such they cannot be broken down with the passage of time. Microorganisms whose habitats are water can utilize nitrates while oil pollutants in the environment will eventually break down into other compounds namely carbon dioxide and water. However, iron and other metal pollutants will persist in the environment. The only transition they can undergo is changing from one form to the other. For example, iron (II) ions will be oxidized in the presence of moisture and active part of air into iron (III) ions which as discussed earlier, is even more hazardous in the environment due its coating ability (Benner, Blowes & Ptacek, 1997).

The problem associated with Acid Mine Drainage is not strange; it has been there and experienced for a long period of time. All the same, the AMD incidences which have existed in the past have been successfully treated using varying technologies. Industry players, academicians and governments have broadly collaborated in seeking solutions to the environmental challenge caused by AMDs.

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Although an assortment of treatment methods for Acid Mine Drainages exist, lack of consensus on the best technology to use in cleaning up AMD has led to the use of different treatment methods each year. A case study of the Wheal Jane Acid Mine Drainage in United Kingdom is a typical illustration that each AMD incidence might require a specific method for treatment. At one time, this mine happened to produce an assortment of metals besides coal which was considered to be relatively harmful to the environment. The drainage discharged high levels of cadmium and zinc metals (Pašava & Kříbek, 1995). The common iron was also spotted in the discharge. In fact, it was due to the characteristic orange color which could be seen from afar that alerted onlookers. The government had to swing swiftly into action to contain the situation. After the incident, the Wheel Jane site was adopted as a test site for Acid Mine Drainage in United Kingdom.

Treatment Alternatives

There are two main treatment methods that can be employed in an Acid Mine Drainage. These are active and passive treatment systems. Inactive treatment system, there is need to continually maintain the operations of the system. For example, the supply of calcium hydroxide needed to neutralize the discharge from AMD may be required on a regular basis to facilitate uninterrupted running of the treatment plant. Another maintenance exercise needed in an active treatment system is the elimination and transportation of waste products from the point of treatment.

On the other hand, passive treatment systems do not require any significant maintenance exercises either on-site or off-site (Demchak, Morrow & Skousen, 2001). The system is expected to contain itself. In the event that maintenance is required, then it is often as minimal as possible.

Precipitation of other metal materials by use of lime active waste treatment systems is the most commonly used technology of cleaning up Acid Mine Drainages. The method has been in application for several years. Treatment using calcium hydroxide or commonly known as lime is perceived to be simple and vigorous. Moreover, the long-term usage of this method has also made it possible for users to predict the likely drawbacks and successes of this system. However, there is a myriad of environmental challenges presented by this method. Firstly, there is a significant amount of water contained in the material generated after the AMD has been treated with lime. Besides, it is very rich in metal elements. The presence of metals in the waste products implies that some specialized waste disposal facilities will be required (Wolkersdorfer, 2000). Unfortunately, this basically multiplies the cost of treating Acid Mine Drainage and unless funds are set aside for the additional costs, the waste treatment process will not be complete.

Secondly, the high volume of water in the wastes generated increases the amount of space occupied by the waste as well as the mass or weight. Hence, additional financial costs will be incurred for transporting the excess water in the wastes as well as more charges to cater for fees incurred in landfills. These associated costs can otherwise be avoided.

Thirdly, the acquisition of lime used in the precipitation process has negative environmental impacts on the areas where crude limestone is mined. The environment is grossly degraded by both the quarrying and transportation processes of limestone (Demchak, Morrow & Skousen, 2001). Due to these constraints, it is highly questionable if this AMD clean-up method is sustainable for a long period of time bearing in mind that the challenge of mine drainage has continued to grow with time. Hence, it is common knowledge that there is need to develop alternatives to seal the loopholes in the use of this system especially in regard to the material usage, waste disposal or the generation of materials that can be used instead of being wholly dumped.

There are several advantages of using lime in the active treatment of Acid Mine Drainage. For instance, this technology has been tried, tested and approved for use and so its setbacks are well known (Bell & Donnelly, 2006). This makes it possible for stakeholders to decisively choose which method to employ. Besides, this method is most effective in treating mine discharges that are highly acidic due to the use of lime that tends to neutralize much of the acidic discharge. Moreover, fluctuations in temperature which might affect other types of technologies have no effect on this method. The treatment process can proceed normally regardless of the rise and fall in temperature. Besides, the dam which is used to settle water is relatively efficient and water that originates from this settling point can be redirected into other watercourses without any cause for alarm. The parameters used for operating the treatment plant can be adjusted accordingly so that both the quality and quantity of water are adjusted accordingly whenever there is need.

In spite of these advantages, there are quite a number of drawbacks which are associated with the use of this type of AMD treatment technology. For example, the cost of maintaining the equipment is rather high this being an active system that has to be maintained on a regular basis. Scaling of the system automatically increases the maintenance costs (McElfish & Beier, 1990).

Elimination of certain metals like manganese requires the use of a very high pH. This on the other hand may remobilize hydroxides of other metals such as aluminum. Moreover, the sludge which is obtained as waste product has chemically complicated and not stable and hence the disposal of the product in the long term may prove to be a challenge. Besides, the very sludges are rather costly both in terms of handling and disposal due to their low density.

Lack of economic value on the sludges is another problem encountered in the system in spite of the large amount of calcium hydroxide used in the process to facilitate total precipitation.

Ion exchange

This is yet another type of active treatment method. Indeed, the costs incurred in the pH modification method described above can be reversed if the value of the recovered metals can be put into consideration. Base metals such as zinc and copper can be of great use if they are fully recovered from the system. The technology behind ion exchange can be employed in extracting useful metals from the mine drains. This takes place before the Acid Mine Drainage is precipitated with lime. Through the ion exchange technology, the base metal is concentrated good enough that it can be sold off to other users in need such as smelting operators. The main challenge in the use of the ion exchange method is that materials for ion exchange are very expensive to manage economically (Wolkersdorfer, 2000). It has been found out that the costs involved in producing the metals are greater than the value of the metals themselves.

Biological Treatments

The treatment of Acid Mine Drainage using wetlands was earlier proposed as a passive methodology for cleaning up the acidic discharge into the environment. However, this method works best for acidic drainages from mines whose contamination levels are quite low. Although this method has been used to treat acidic discharges, it has not been approved universally as the best method for treating mine water. The sulfate which reduces bacterial activity on wetlands has been used by some companies. This wetland component has been used to biologically treat Acid Mine Drainages using an active system. For the bacteria to function well, it needs a substrate that is carbon-based (Pašava & Kříbek, 1995). This substrate facilitates the process of metabolism although it does not make use of oxygen as it is always the case; it however utilizes the available sulfate from wetlands. As it is well known, most biologically mediated systems require a lot of air sparging. This method does not need this requirement.

The use of biological systems in treating Acid Mine Drainage may sometimes not be the best especially when used on its own without supplementing with other methods. In order to improve the efficiency of the biological system, lime is added to the acidic mixture to remove all the acidic components of the mine water. The biogenic systems generate products that are unstable especially in environments that are toxic in nature. As a result, the disposal of products is an issue of concern and it is important to make some considerations before the end products from waste treatment are eventually disposed of. Adsorbents can also be used in treating Acid Mine Drainage using biological systems. These adsorbents are produced biologically and their purpose is to concentrate metals that have been discharged from the mine water.

Alternative Adsorption Treatment methods

Adsorbents that are non-biological in nature can also be used to treat Acidic Mine Drainage as an alternative treatment method (Pašava & Kříbek, 1995). This type of technology has been approved and proposed for use in treating acidic discharges from mining sites. The principle behind this technology is the use of particles whose density and size are known. These particles basically adsorb metals from the discharged mine water. Later in the treatment process, there are physical processes that are applied to separate the individual solid particles from the solution.

Electrochemical Treatment

Electrochemical properties of the metals in the water discharged from the mining site are very instrumental in applying electrical technology in cleaning up Acid Mine Drainages (McElfish & Beier,1990). This method of treating acidic discharges is gradually growing in terms of popularity and use. However, the use of electrochemistry of metals in treating mine water discharges that are acidic requires a continuous flow of electric energy in addition to a technical support for the purposes of maintaining the system.

Physical Process

The use of physical processes in removing individual metals from the Acid Mine Drainage has often been thought of as a better option since metals are eliminated from the acidic discharge in form of crystals compared to the use of lime which leaves behind precipitates or sludges (Wolkersdorfer, 2000).

Conclusion

In summing up this report, it is imperative to reiterate that Acid Mine Drainage refers to the acidic water discharge which originates from mining sites. Although this discharge is an environmental pollutant, not so much emphasis was laid on its management in the past due to unawareness of how much it could impact the environment. However, a recent development has witnessed the enactment of necessary regulations to countercheck the release of the discharge into the environment.

The invention of steam engines which were used to pump water from the mines accelerated the discharge of Acid Mine Drainages to watercourses. Initially, mining was basically carried on the near-surface hence there was no need to excavate deep underground. However, as time passed by, there was need to reach the inner levels of the ground where mineral deposits could be located. This led to the construction of horizontal trenches which could not only permit the extraction of minerals but also channel water out from the mining sites. In the event that pumping of water from the mines was terminated, the initial groundwater level could assume its original water level and eventually mix with the mine residues consisting of metals and sulfate deposits. This mixture could then be transported to the immediate environment and watercourses causing harm to live organisms.

In an attempt to clean up the Acid Mine Drainage, several methods have been employed. The most commonly used technology is the addition of lime (calcium hydroxide) in the acidic discharge from the mines so that precipitation can take place and the mixture neutralized, safe and ready to be released to the environment. This is an active system of treating AMDs and is relatively costly because the system has to be maintained at all times for operations to continue. There are financial costs incurred as a result of transporting sludges

On the other hand, passive methods such as the use of biologically mediated systems require very minimal or no maintenance at all. Biological treatment requires the use of certain bacteria. Other methods of treating AMDs include electrochemistry and physical technologies.

References

Bell, G.F. and Donnelly, J.L. (2006). Mining and its impact on the environment, New York: Taylor & Francis.

Benner, S.G., Blowes, D.W. and Ptacek, C.J. (1997). A full-scale porous reactive wall for prevention of acid mine drainage. Ground Water Monitoring and Remediation, 17 (4): 99-107.

Demchak, J., Morrow, T. and Skousen. J.(2001). Treatment of acid mine drainage by four vertical flow wetlands in Pennsylvania. Geochemistry: Exploration, Environment, Analysis 1(1): 71-80.

McElfish, M.J. and Beier, E.A. (1990). Environmental regulation of coal mining: SMCRA’s second decade, Washington D.C: Environmental Law Institute.

Pašava, J. and Kříbek, B. (1995). Mineral deposits: from their origin to their environmental impacts, Rotterdam: A.A. Balkema.

Wolkersdorfer, C. (2000). Water Management at Abandoned Flooded Underground Mines, Muchen: Springer.

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