Acid Mine Drainage - Essay Example

Table of contents ................................................................................................................................. Introduction........................................................................................................................... Main causes of AMD phenomena.......................................................................................... Results and discussion............................................................................................................. Problems posed by acid mine drainage.................................................................................. Acid mine drainage remediation............................................................................................. Recommendations for the site remediation........................................................................... References............................................................................................................................. Abstract: This paper sets out to examine the causes of acid mine drainage of a site under investigation-a long-abandoned coal mine. The analyses of the samples collected were done to ascertain the alleged source of the river contamination. Authorities reported about the river passing by the spoil becoming orange. A comparison between the analysed levels of the pH of the site to determine the source of contamination based on the Acid generating Potential and the Acid Neutralizing Capacity (ANC) which is calculated directly by adding acid to a slurry of the spoil until the pH falls below some specified value (often pH 4.5) is expressed directly as meq (of acid added) per kg (of sample used). This analysis will help to pin point the source of acid mine, whether it is coming from the spoil heap or mine adit. This information can be used to devise remediation measures in order to reduce the damage on the environment-river pollution. Knowledge of acid mine drainage is not only significant to authorities tasked with environmental protection but also to consultancy agencies in acid mine drainage. Lastly, mining companies need to reduce the damage to the environment by employing technologies for proper treatment of their discharges. Wetlands engineering can reduce the damages of acid mine aquatic plants and animals. It is theorized that building wetlands can mitigate AMD pollution and have lower or no future operations and maintenance costs. Introduction: 1.0 Acid mine drainage can be defined as both ground and surface waters of pH is ≤ 4.5. The Acid mine drainage is mainly caused by the oxidation reaction of surface waters with pyrite containing rocks or ores. Sulphate is found almost everywhere on this earth even in natural fresh waters (Drever, 1997). It is usually the second or third most abundant anion in rainwater, where it is derived both from natural aquatic sources and from atmospheric pollution - the sulphur component of “acid rain” as a result of fossil fuel combustion. 1.01 Apart from human activities, pyrite oxidation also produces acidity, enhancing other weathering reactions. Such as, production of waters with very low pH and high sulphate concentration as a result of buffering the PH. Sulphate available comes from many anthropogenic sources: they can be coming from farms and agricultural sources. Sulphate fertilizes could be chief source of such contaminants. Sometimes they can come from other cleaning agents agents; and industrial point emissions of sulphuric acid utilised in manufacturing. Main causes of AMD phenomena: 2.0 Understanding the geochemical reactions that generates AMD provides an insight into the three most important reagents that must present – Fe 3+, oxygen (O2) and water. Moreover, “runaway” AMD production needs Fe3+ dissolved in solution and the activity of different species of bacteria “ Thiobacillus ferrooxidans” to catalyse the reactions ten thousands times. 2.01 In more details, pyrite is oxidised by the oxygen molecules dissolved in water as shown in equation (1) below: FeS2 + 3.5O2 + H2O => Fe2+ + 2SO42- + 2H+ (1) This reaction produces acidity, hence the pH drops if buffer reactions are lacking. At first, when the pH is greater than 3, in the presence of water, the Fe2+ generated is oxidised to form an insoluble precipitate of Fe(OH)3: Fe2+ + 0.25O2 + 2.5H2O => Fe(OH)3 + 2H+ (2) Figure1. Feedback loop This reaction result to acidity and lowers the pH (Berner & Berner, 1987) Reaction (1) is sparingly slow and when neutralization occurs at this phase,(often by the presence of carbonate compounds) or the rate of flow is high enough to remove water and acidity from the system then pH may not reach levels of 3 or less and the system remains stabilizes as shown in figure 2 on the next page. Figure2. Feedback loop However, in oxygen deficient environments both reactions If the system evolves to a pH of Fe3+ + OH- (3) Fe3+ is thus able to take place in oxidation of pyrite via the reaction: FeS2 + 14Fe3+ + 8H2O => 15Fe2+ + 2SO42- + 16H+ (4) - a reaction which yields a large amount of acidity. In oxygen deficient both reactions 3 and 4, required to maintain pyrite oxidation, are relatively slow. However, both are catalyzed by iron oxidizing and sulphide oxidizing bacteria and the rate of reaction is increased by orders of magnitude. Although, both are catalyzed by iron oxidizing and sulphide oxidizing bacteria and the rate of reaction is increased by orders of magnitude. These bacteria are acidophilic and the reaction can essentially “run away”, producing severe acid mine drainage; pH commonly falls to 2 or less, sometimes negative pH values are recorded. 2.2 The interaction of pyrite ore and surface water happens in various fashions. First, when mining activities changes groundwater channels facilitating oxygenated water flow into rock outcrops that were formerly stationery. Second, when pyrite mining soil is dumped on the surface hence exposes it to infiltrating oxygenated water. Third, when pyrite rock mineral recycling trash is dumped in water bodies. Reactions generating acid mine drainage entail the oxidation of pyrite (Domenico, & Schwartz, 1997). Results and discussion: 3.0 Acid Generating Potential - AGP If water > pH 3 and for BH1/2.0m (TABLE 2): each unit of pyrite generates 4 units of acidity (H+) so at 4 wt% S 0.625 mol FeS2 × 4 mol acidity = 2.5 mol acidity / kg spoil kg spoil mol FeS2 ACID generating Potential is given by (wt% S in the spoil) x (mol acidity / kg spoil) so at 4 wt% S, the acid generating potential is 4 wt% × 2.5 mol acidity = 10 mol acidity / kg spoil kg spoil Acid Neutralising Capacity - ANC is measured directly by adding acid to a slurry of the spoil until the pH falls below some specified value (often pH 4.5) is expressed directly as meq (of acid added) per kg (of sample used). As already mentioned , Acid Neutralising Capacity - ANC is measured directly by adding acid to a slurry of the spoil until the pH falls below some specified value (often pH 4.5) is expressed directly as meq (of acid added) per kg (of sample used) Thus, at BH1/2.0m, 4 units of soil sample is added At this level ANC =4/2560 mEq kg-1 =0.0015625 mEq kg-1 As a rule, in case the value of AGP is greater than the ANC, then the spoil has the potential to generate acidic drainage. On the other hand, if the AGP is lower than the ANC then the spoil will naturally neutralise the acid that it produces. Therefore, for the acid mine case; AGP is greater than ANC i.e. 10 mol acidity / kg spoil ˃0.0015625 mEq kg-1 Conclusion: The acid mine is the source of acid mine drainage in the river because AGP is greater than ANC hence the spoil cannot naturally neutralize the acid. Table 1. Results of average of Ten pairs of samples Site Flow L s-1) pH Fe mg L-1 Al mg L-1 Cu mg L-1 Zn mg L-1 S1 220 6.1 0.20 3, Fe(OH)3, precipitation reaction occurs (Equation (2)) sometimes generating huge quantities of orange-brown sediment. This can submerge organisms inhabiting the bottom of the aquatic environments thereby acting as a barrier to direct suns ray reaching the ocean bottom, consequently, hinder the process of photosynthesis- ecological damage. Acid mine drainage remediation: 5.0 In summary, the geochemical reactions that generate AMD illustrates remediation generation of AMD can be realized by inhibiting or eliminating any one or all of driving forces at a particular area (Younger et al 2002). Minimize the production of O2 engineering capping of the spoil using either clay or geotextile. Re-vegetation and deep burial of spoil can reduce the amount of oxygen produced. Secondly, eliminate Fe 3+ from solution by adding phosphates and biocides. Also, inhibiting AMD-generating bacteria is critical. Finally, treatment of waste water either using engineering chemical treatment (low technology or high technology) or wetlands. In wetlands microbial activities produce alkaline condition (eliminating acidity). Recommendations for the site remediation The company can use wetlands whereby microbial processes in reducing wetlands generate alkalinity hence remove acidity. Also the built wetlands could mitigate AMD pollution and is cheaper because ones it constructed, there are no future maintenance or operation costs. It can also adopt a technology of waste water treatment especially low technology engineered chemical treatment because it relatively cheap References: Appelo, C.A.J. and Postma, D. (1996) Geochemistry, groundwater and pollution .Balkema.  Berner, R.A. and Berner, E.K. (1987). The global water cycle : geochemistry and environment, Prentice-Hall, Domenico, P. and Schwartz, F. (1997). Physical and chemical hydrogeology, Second Edition, John Wiley and Sons.  Drever, J.I.. (1997). The geochemistry of natural waters : surface and groundwater environments, Third Edition, Prentice-Hall,.  Younger, P.L. Banwart, S.A. and Hedin, R.S. 2002. Mine water: Hydrology, pollution, renmediation. Kluwer Academic Publishers. Read More