The Importance of Constructed Wetlands in the Process of Mine Pollution Amelioration - Literature review Example

Describe and assess the importance of constructed wetlands in the process of mine pollution amelioration Name: Lecturer: Course: Date: Table of Contents Table of Contents 2 Introduction 3 Importance of constructed wetlands 4 Discussion 10 Conclusion 14 References 15 Introduction Mineral extraction and processing are critical sources of contaminants responsible for polluting the soil, water, and air at the mine sites. They also lead to vast lands of unproductive and infertile land (Smith 1997). As a result, land reclamation strategies are required to minimize the destruction of the pollutants and bring back environmental productivity to the mine sites. However, a large number of remediation strategies are costly and timely. As Smith (1997) points out, acid mine drainage (AMD) is among the mining industry’s most difficult challenges to solve. It is formed due to exposure of sulfide minerals to oxidising states in metal or coal mining. AMD can persist for many years leading to waste of mine lands. This requires that AMD treatment to be sustained long after mining has ceased at the site. In such a case, several researchers have suggested the use of constructed wetlands (Lizama et al 2011; Lorion 2001). Skousen et al. (2000) describe constructed wetlands as artificial ecosystems that containing soils and sediments saturated with water. They are usually integrated with vegetation, and contain shallow excavations. These excavations overflow with soil, gravel and organic, as well as supporting aquatic plants, such as Typha. Smith (1997) points out that constructed wetlands provide an effective means in addressing long-term amelioration of acid mine drainage in mine sites. It consists of an artificial wetland built for purposes of purifying and discharging wastewater (Kleiman 2006). This report describes and assesses the significance of constructed wetlands in the process of mine pollution amelioration. Importance of constructed wetlands The wetlands are appropriate for reclamation of wasteland after mining. They pre-treat water through filtration, settling, as well as bacterial decomposition. Constructed wetlands are a potential solution to remediation of AMD for the long-term (Norton 2006). Studies have established positive outcomes in respect to using constructed wetlands to ameliorate human-induced pollutants (Vymazal 2008). In the past study, Smith (1997) acknowledged that this environmental technology treats acid mine drainage by providing low-cost, continuous and efficient solution to mining pollution. Basically, AMD drainage happens after reaction of sulfide oxidation with air and water in rocks to create sufate, hydroxide, and hydrogen ions (Ziemkiewicz et al 1997). The mineral necessitating the reaction is called pyrite (Younger et al 2002). The mining processes and activities trigger pyrite to weathering water, air, as well as microbial processes. This intensifies the contaminated water with acid leading to high concentrations of sulphate, heavy metals, as well as other dissolved solids. The contaminants that are of specific concern include iron, acidity, aluminium, and manganese (Sadak 2008). Chemosynthetic bacteria also cause acid mine. These include Ferrobacillus ferroxidans, Thiobacillus ferooxidans, and T. thiooxidans. Such bacteria also accelerate the oxidation of pyrite. The figure below shows the composition of a model constructed wetland for remediation of mine wastewater polluted with AMD. Figure 1: Constructed wetland (Sadak 2008) Constructed wetlands are cost-effective biogeochemical means of treating wastewater generated from mining (Khan et al 2010). Several wetlands have been constructed as restoration projects at gravel, coal, sand and phosphate mines across the United States. Examples of the large-scale constructed wetland projects are restoration, creation, or riparian wetlands, such as the Florida phosphate-mining district, where more than 5,000 of hectares have been reclaimed from coalmine ponds (NCSU Water Quality Group 2014). Constructed wetlands provide an effective means of lowering the acidity and reducing the heavy metals in AMD polluted mine water (Smith 1997). These technologies eliminated the heavy metals and eliminate the mine water’s pH through hydrology, vegetation, and soil in the underlying environment. The function may be through aerobic or anaerobic process. Hence, constructed wetlands may be aerobic or anaerobic. The type depends on condition or type of mine water to be treated. Sadak (2008) shows that aerobic wetlands are suitable for mine water containing high manganese and iron concentration while anaerobic wetlands are effective in situations where the mine water’s acidity is more than 300 mg/L. To this end, aerobic wetlands rely on available oxygen for iron oxidation. The oxidation leads to settling of iron precipitates that form sediments at the bed of the constructed wetland (Skousen et al 2000). Constructed wetlands also ameliorate the mine wastewater. Unlike aerobic wetland, anaerobic wetland relies on limestone and organic materials to ameliorate the mine wastewater (Skousen et al 2000). Presence of limestone and organic materials in the constructed wetland promote the growth of sulphate-reducing bacteria (Desulfovibrio sp.), which eats up the iron. In return, it is reduced to sulphide. As a result, production of bicarbonates takes place that raise the pH level of the mine water. Ultimately, the sulphides react with the heavy metals leading to formation of precipitates that form sediments at the bed of the wetland. Examples of organic materials that can be used include sawdust, manure and mushroom compost (Sadak 2008). Constructed wetlands have the capacity to reduce metals within mine drainage and to neutralize acid mine drainage (Lorion 2001). An underlying principle for their efficacy is since wetlands are mainly self-sustaining ecosystems. Constructed wetlands are capable of remediating contaminated mine drainage on condition that they are generated. Hence, they may signify long-term solution to acid mine drainage (Lizama et al 2011). The wetland activities that lower metal and acid concentration within the wetlands have been suggested by several studies as capable of remediating the contaminants within AMD. The wetlands contain organic-rich substrates that exchange dissolved metals. Such exchange happens between the large fulvic and humic acids and dissolved metals found in the substrate. In general, wetland sediments are anaerobic when under the thin oxidised surface layer containing organic carbon that promotes bacterial growth. On the other hand, the anoxic layer of the sediments offers an environment that favors chemical and microbial reducing activities. They also transform sulphates and iron, as well as sulphides and hydrogen. Soluble metals become transformed into insoluble forms due to the anoxic environments necessitated by the wetland sediments. Conversely, the suspended insoluble materials settle due to water velocity control of the wetland vegetation. Studies have confirmed that Typha wetlands are effective removers of manganese and iron. Indeed, a study by Smith (1997) showed that the wetlands could reduce iron and manganese concentrations substantially. The plant roots grown at the constructed wetlands also assist effectively in oxidation and reduction processes by creating microenvironments. Constructed wetlands help in remediation of mine drainage. In this regard, vegetation is often used in construction of wetlands because of their vital role in remediation of AMD. This may include establishing perennial vegetation, such as direct placement of rhizomes in saturated subsurface zone (Lone et al 2008). The main species used for this purpose include aquatic moss (Sphagnum spp), and cattails (Typha spp.) are usually used (Smith 1991). Recently, the use of water hyacinth (Eichhornia crassipes) has been widely embraced. Aquatic moss and cattails are acid tolerant, which makes them capable of thriving in adverse conditions (Vymazal 2008). These species help in absorption of manganese and iron, which makes them appropriate for remediating mine drainage. Constructed wetlands are important as they usually influence the quality and flow of water. They improve the quality of water through interception of surface runoff and removal. They also process organic waste, retain organic matter, and reduce sediments that are suspended. These processes are illustrated below. Figure 2: Constructed wetland pollutant removal mechanism The constructed wetlands contain aquantic plant life, such as water hyacinth (Eichhornia crassipes) with living cells that absorb substances from the environment. These aquatic plants later use the substances for purposes of synthesising own energy source and cellular components. Hence, they reduce the nutrients in their immediate environment to a considerable degree. This mechanism of subtracting nutrients within the reed (Phragmites australis) bed system range from chemical, physical and biological processes that take place within the plant ‘rhizospehere,’ as well as solid-water matrix (Potgieter 2002). Constructed wetlands remove heavy metals and minerals from their dissolved states in the drainage and fix them to the underneath soils or incorporate them into the cell structures. Past studies have indicated that constructed wetlands effectively remove heavy metal from water (Khan et al 2010). As Khan et al (2010) established, the constructed wetland has natural activities of aquatic macrophytes, which directly collect pollutants and absorb them into their tissues. These macrophytes also catalyse purification processes that happen within a plant’s rhizosphere (Potgieter 2002). Inside the rhizosphere, biological and physiological activities are facilitated through interaction of microorganisms and sediments, as a result eliminating heavy metals from the mine wastewater. Mining contaminate the underground water with heavy metals. As a result, constructed wetlands provide inexpensive yet effective means for amelioration of mines from heavy metal pollution (Heal & Sal 2000). Constructed wetlands also enable growth of periphyton. Smith (1997) explained that the vegetation grown in the constructed wetland offers a substrate in the form of stems, leaves, and roots through which growth of microorganisms, known as periphyton, take place. Together with natural chemical composition, the periphyton removes some 90% of the pollutants, as well as breakdown of 90% of the waste matter. The plants eliminated between 7% and 10% of the pollutants while simultaneously functioning as source of carbon needed by microbes in the process of decay. Therefore, degradation of the constituents of wastewater happens in the artificial reed bed system triggered by microbial processes, within the soil layer subject to influences from rhizomes or reed roots. The reeds offer oxygen to the rhizosphere through the branches, leaves and stems. This leads to creation of a space around the roots where oxidisation of ammonia to nitrate can be facilitate by aerobic bacteria (Cooper & Findlater 2013). Constructed wetlands also create grounds for process of bioremediation, which correct the imbalances in the soil property, as well as prevent the need to treat the mine site with chemicals. Bioremediation is rooted in the physical principles and natural biological principles that facilitate sedimentation of the ore components (Potgieter 2002). In this process, microorganisms, which are biologically active, extract the metals and minerals from their dissolve conditions in water, and fix them underneath the soil in insoluble condition by integrating them in the structures of the cells. This process promotes accumulation and sedimentation of metals and minerals (Cooper & Findlater 2013). Hence, bioremediation gets to correct an imbalance, as well as do away with the need to use chemicals for treatment of the mine drainage, such as active chemical treatment, which are relatively costly in the long-term. Skousen, et al (2000) comment that one reason why constructed wetlands are important for treating AMD is since they do not call for continual chemical application. Rather, they make use of the naturally occurring biological and chemical processes to purify polluted mine water. Constructed technology is therefore a passive technology. Discussion Constructed wetlands perform several functions that help in amelioration of AMD. Such features are needed for specific processes to happen. These include ion absorption and exchange, abiotic and bacterial oxidation, bioaccumulation, carbonate mineral dissolution, and mineral neutralisation (Potgieter 2002). They provide benefits, such as improvement of water quality with advantages over design, management and location to maximise the functions of water quality. While the constructed wetlands are not usually designed to restore the entire functions of a natural wetland, they bring about particular benefits that ensure pollutant elimination efficiency. They also reduce point source pollution before eventually releasing water to the streams or other components that receive waters. However, their effectiveness is not always guaranteed (Skousen et al 2000). According to Younger et al (2002), the effectiveness of treatment and reliability of the constructed wetlands vary depending on the type of the wetland. Hence, they may not be used solely for on-site treatment of AMD. In Heal and Sal’s (2000) view, however, well-built wetlands can eliminate concentration of dissolves heavy metals to the desired level that meets the effluent limits. In most cases however, constructed wetlands have been mostly used when looking to reduce the long-term costs of using the traditional chemical treatments. They can effectively remove aluminium (Al), iron (Fe) and Manganese (Mn). In the order of importance, the method retention of metals in the constructed wetlands are development oxyhyrdroxides and metal oxides, development of metal sulphides, reactions of organic ‘complexation,’ switch over with cations on sites that are negatively charged and absorption by aquatic plants (Skousen et al 2000). Additionally, the method the constructed wetland is built also affects the degree of ADM treatment. To this end, two techniques prevail. These include aerobic wetland and anaerobic wetland. Aerobic Wetlands encourage hydrolysis and oxidation, which lead to physical retention and precipitation of Al, Mn and Fe oxyhydroxides (Sadak 2008). Effective elimination of metals is dependent on the concentration of dissolved metal, the level of ph of the mine water, the level of dissolved oxygen, the period of time water is detained in the wetland and the occurrence of microbial biomass (Potgieter 2002). For the constructed wetland to eliminate heavy metals effectively, the water’s net alkalinity/acidity and pH are critical, as they both determine the extent to which metal hydroxide has dissolved precipitates, as well as the kinetics of metal hydrolysis and oxidation. In which case, the hydrolysis serves to lower pH to allow the water’s alkalinity level to maintain the pH. These allow oxidation to persist (Skousen et al 2000). On the other hand, anaerobic wetlands provide suitable conditions for hydrolysis and oxidation of metal within the aerobic surface layers. However, they basically depend on microbial and chemical reduction processes, which neutralise acidity. Because anaerobic wetlands generate alkalinity, their application can be broadened to the low pH, net acidic as well as high iron (Smith 1997). Still, Sadak (2008) demonstrates that the efficiency and mechanism of treating AMD ranges across seasons and wetland age. At any rate, microbial mechanisms of production of alkalinity are critical for continual treatment of ADM. Additionally, when the constructed wetlands are provided with elevated acid loads, the capacity of the microbes to detect pH falls. These show that like anaerobic wetlands, aerobic wetlands are most effective when applied in treatment of AMD flows that have moderate quality of water (Skousen et al 2000). Presently, the value for elimination of Fe in a typical constructed wetland is 10GDM. Cattails (Typha spp.) and aquatic moss (Sphagnum spp.) can eliminate between 50% and 80% of the heavy metals in AMD. A study by Smith (1997) established that substrates (such as acid wetland soil, topsoil, non-acid wetland soil and clay) with vegetation are similar to elimination of Fe without vegetation after a single growing season. These show that for any substrate material, provided that it has a capacity to retain metal, it may not be as important as the conditions that the vegetation in the constructed wetland creates (Skousen et al 2000). During the aerobic process, the reaction between the oxygen (O2) and Fe lowers the wetland’s pH, which may harm the aquatic plants with low tolerance of extreme shifts in pH. The iron precipitates that form in the sediments can be removed manually (Sadak 2008). When the wetland pressures iron from the sediment, it is possible that the wetland will attain its optimal holding capacity, which would reduce the effectiveness of the constructed wetland in removing heavy metal. Hence, for the constructed wetland to function effectively, its aquatic plants that are tolerance to extreme changes in pH should be used. Examples of these plants include common reed (Phragmites australis) and cattail (Typha latifolia) (Skousen et al 2000). In anaerobic process, it is significant to prevent the aquatic plants from invading the entire wetland if the technology has to be effective. In situations of vast presence of plants, the oxygen introduced for the anaerobic conditions and substrate will lack. Additionally, eliminating precipitates is difficult within anaerobic wetlands since they combine with the organic matter, which makes it difficult for the wetland to drive the precipitates out (Sadak 2008; Skousen 2000). For the constructed wetlands to remediate environments in the long-term, it needs persistent maintenance. Without which they are less likely to be effective in reduction of wastewater acidity, and removal of heavy metals (Kleiman 2006). Hence, the precipitates that build up have to be eliminated before the constructed wetland can attain its maximum holding capacity. At any rate, the microorganisms and aquatic vegetation play a significant role in absorbing the heavy metals occurring in the form of AMD in the sediments (Norton 2006). It should also be noted that the aquatic vegetation could only absorb heavy metals at varied rates dependent on the plant’s growth cycle and heavy metal concentration (Skousen et al 2000). As Sadak (2008) noted, the aquatic vegetation can sustain 200,000 times the level of metal concentration in the mine wastewater. The plants have the capacity to hold such high heavy metal concentration, as they store them in their cell walls. Sadak (2008) suggests that most appropriate plants with heavy concentration of lead, zinc, copper, manganese, and cadmium include umbrella plants (Cyperus alternifolius). Conclusion Constructed wetlands are potential solution to remediation of AMD for the long-term. Their basic functions include ion absorption and exchange, abiotic and bacterial oxidation, bioaccumulation, carbonate mineral dissolution, and mineral neutralisation. However, their effectiveness is not always guaranteed. They help in amelioration of AMD to reclaim wasteland after mining. They are cost-effective biogeochemical means of treating wastewater generated from mining. They pre-treat water through filtration, settling, as well as bacterial decomposition. They also provide an effective means of lowering the acidity and reducing the heavy metals in AMD polluted mine water. Constructed wetlands have the capacity to reduce metals within mine drainage and to neutralize acid mine drainage. They also help in remediation of mine drainage. In this regard, vegetation is often used in construction of wetlands because of their vital role in remediation of AMD. Additionally, they influence the quality and flow of water. In this regards, they improve the quality of water through interception of surface runoff and removal. Constructed wetlands also create grounds for process of bioremediation, which correct the imbalances in the soil property, as well as prevent the need to treat the mine site with chemicals. 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