Generation of Electricity from Acid Mine Drainage

9.5.1 Introduction

Over the years, microbial fuel cells (MFCs) have emerged as a promising yet challenging technology for converting organic waste including low-strength wastewaters and lignocellulosic biomass into electricity through the metabolic activities of the microorganisms that act as catalysts (Pant et al., 2010; Simate et al., 2011). In other words, MFC-based wastewater systems employ bioelectrochemical catalytic activities of microbes to produce electricity from the oxidation of organic and in some cases inorganic substrates present in urban sewage, agricultural, dairy, food, and industrial wastewaters (Gude, 2016). In principle, the MFC can enable the simultaneous treatment of wastewater such as AMD while generating electricity from organic matter in the wastewater. In other words, the MFCs treat wastewater and generate electricity at the same time (Bennetto, 1984; Habermann and Pommer, 1991). The MFC is a combined system with anaerobic and aerobic characteristics (Simate et al., 2011). The MFCs are designed for anaerobic treatment by bacteria in the solution near the anode, with the cathode exposed to oxygen (or an alternative chemical electron acceptor). Electrons released by bacterial oxidation of the organic matter are transferred through the external circuit to the cathode where they combine with oxygen to form water (Feng et al., 2008). It is noted that a combination of anaerobic-aerobic process can be constructed using a double-chamber MFC, in which effluent of anode chamber could be used directly as the influent of the cathode chamber so as to be treated further under aerobic condition to improve wastewater treatment efficiency (Wen et al., 2010).

The MFC is considered to be a promising and sustainable technology that would meet increasing energy needs, especially when using wastewaters as substrates (Du et al., 2007; Lu et al., 2009). Since it results in the production of electricity and clean water as final products, it may offset the operational costs of wastewater treatment plants (Lu et al., 2009).

9.5.2 Selected Typical Studies of the Generation of Electricity from Acid Mine Drainage

A study by Cheng et al. (2007) used fuel cell technologies to generate electricity while removing iron from the AMD. The AMD fuel cell (AMD-FC) was constructed from two plastic (Plexiglas) cylindrical chambers each 2 cm long by 3 cm in diameter separated by an anion-exchange membrane. The anode was a carbon cloth (non-wet-proofed) with a projected surface area of 7 cm2 (one side). The cathode electrode was made by applying platinum (0.5 mg/cm2) to a commercially available carbon cloth (30 wt% wet-proofed). The membrane was held between the two cylindrical chambers with a rubber О-ring to prevent leakage. The anode electrode was located at the end of one chamber and covered with a plastic end plate (5 x 5 x 0.6 cm). The cathode electrode was placed at the end of another chamber and covered with another end plate with a centre hole (3 cm in diameter), with the platinum- catalyst side facing the solution and another side facing the air (Figure 9.16). Platinum wires (1 mm in diameter) were used to connect both electrodes and used as terminals of the cell. The reactor was operated in open circuit mode for 0.5 h before connecting an external resistor (1000 ft) to measure electricity generation.

The AMD-FC operated in fed-batch mode generated a maximum power density of 290 mW/m2 at a Coulombic efficiency greater than 97%; and electricity generation was reported to be abiotic in nature. In the fuel cell system, ferrous iron (Fe2+) was oxidised to ferric iron (Fe3+) at the anode, and oxygen from the air was reduced to water at the cathode. In the best possible way, ferrous iron was completely removed through oxidation to insoluble Fe3+, forming a precipitate at the bottom of the anode chamber and on the anode electrode. Several factors were examined to determine their effect on operation, including pH, ferrous iron concentration, and solution chemistry. Optimum conditions were reported at a pH of 6.3 and a ferrous iron concentration above 0.0036 M. These results suggested that fuel cell technologies can be used not only for treating AMD through removal of metals from solution, but also for producing useful products such as electricity and recoverable metals.

In a related study, Cheng et al. (2011) showed that fuel cell technologies are not only used for simultaneous treatment of AMD and power generation, but also they can generate useful products such as iron oxide particles having sizes appropriate for use in pigments and other applications. In the study, Cheng et al. (2011) used AMD-FC technique to generate spherical

FIGURE 9.16

Laboratory scale prototype (A) and schematic (B) fuel cell system used to generate electricity. (From Cheng et al., 2007.)

nanoparticles of iron oxide that, upon drying, were transformed to goethite (a-FeOOH). This approach, therefore, provided a relatively straightforward way to generate a product that has commercial value. In other words, the results provided a method that could easily produce iron oxide particles that are essentially used in pigments and other products.

Hai et al. (2016) coupled membrane-free MFC with permeable reactive barrier (PRB) to treat AMD and generate electricity. The MFC-PRB system was carried out by employing parallel acrylic material columns, which were separated by a plate with a centre hole (3 cm inside diameter). The exterior chamber was used as PRB packed with corn cob media and inoculated with sulphate-reducing bacteria, and the cathode electrode was placed at the end of an exterior chamber and covered with another end plate. The inner chamber was directly used as an anode area that was filled with excess sewage sludge. In general, AMD lacks organic matter, therefore, additional organic substrate such as sewage sludge would need to be added so that it serves as microbial carbon sources for AMD treatment by MFCs (Jiang et al., 2009; Zhang et al., 2012; Peng et al., 2017). Sewage sludge is a by-product of biological wastewater treatment that requires treatment and disposal, but it contains high concentrations of organic matter, mainly protein and carbohydrate (Jiang et al., 2009; Zhang et al., 2012; Peng et al., 2017). The anode and cathode electrodes were made from a piece of 43.4-cm carbon rod and carbon felt without any pre-treatment and which were connected through a 1000 П resistor. The MFC-PRB system was continuously fed with synthetic AMD in a down flow mode using multiport peristaltic pumps, and it was operated for five periods at room temperature of 25 ± 3°C. The results showed that the MFC-PRB could continuously generate electricity from AMD, and the average sulphate removal rates of 51.2%, 39.8%, and 33.1% were obtained in effluents of 1000, 2000, and 3000 mg/L, respectively. Fligh Cu2+, Pb2", and Zn2+ removal efficiencies (99.5%) were also obtained during the operation, with most of the results being in the range of 0.01-0.05 mg/L which are far below the discharge level required by the Chinese government legislation of 0.5 mg/L, for example.

Peng et al. (2017) made use of the MFC to remove metals and sulphate from AMD using sewage sludge organics and simultaneously generated electricity. A total of six identical MFC reactors consisting of a vertical cylinder built using plexiglass were operated simultaneously and the reactors were closed during operation. To start up the MFC, 300-mL sludge was used to inoculate the MFC containing 700-mL AMD. To accelerate microbial growth, sodium acetate was added once into the reactors with an initial concentration of 2.0 g/L at the beginning of the start-up phase. After start-up, experiments were conducted in fed-batch mode at room temperature (25 ± 2°C), and the reactors were replenished with fresh sludge and AMD every 10 days to initiate a new cycle. The results showed that under anaerobic conditions, 71.2% sulphate (from 2100 to 605 mg/L), 99.7% heavy metals, and 51.6% total chemical oxygen demand were removed at an electrode spacing of 4 cm and a sludge concentration of 30% (v/v) after 10-day treatment. A maximum power density of 51.3 mW/m2 was obtained. Approximately 79.5% of the dissipated sulphate was converted to elemental sulphur or polysulphides. The sulphide concentration was kept below 20 mg/L. The concentrations of heavy metals were in the range of 0.02-0.06 mg/L in the effluent, which were far below the levels required by the Chinese government legislation. This study was one of many studies that showed the potential of synchronous degradation of residual sludge and treatment of AMD with electricity harvesting.

Lefebvre et al. (2012) investigated the bioelectrochemical treatment of AMD dominated by iron using acetate solution (as substrate component of the reactive mixture) in the anode compartment and simulated AMD (FeCl3 solution) in the cathode compartment. The study was performed with two

FIGURE 9.17

Schematic diagram of the (a) salt bridge and (b) membrane-microbial fuel cells. (From Lefebvre et al., 2012.)

different MFC designs: a salt bridge MFC and a membrane MFC. Overall, the salt bridge design - despite its rudimentary architecture - was intended to demonstrate the principle and feasibility of AMD bioelectrochemical treatment, while the membrane design was meant to improve iron recovery, thus demonstrating the potential of the technology. Figure 9.17 shows the two types of dual-chamber MFCs used in the study which only differed in the nature of the separator. The reactive mixture - used as the anolyte - consisted of a solution of nutrients, minerals, and vitamins, to which sodium acetate was added as the carbon source and electron donor (substrate). An artificial AMD, with 500 mg/L of Fe3+, prepared using ferric chloride hexa- hydrate (FeCl3-6H20), was used as the catholyte. The selected Fe3+ concentrations corresponded to values found in natural AMD. The pH of the artificial AMD was left unadjusted at 2.4 + 0.1. The experiment was carried out in batch mode. At the start of a batch test, the anode and the cathode chambers of the MFCs were filled with fresh solutions of anolyte and catholyte and the batch test was considered completed when the voltage recorded over an external resistance of 5 П dropped below 0.2 mV. The anode chamber was kept anaerobic throughout the batch testing, while the cathode chamber was constantly aerated using an aquarium air pump connected to an air diffuser.

Based on the findings from the study by Lefebvre et al. (2012), the AMD showed a potential to generate substantial amount of power (up to

8.6 ± 2.3 Wirr3) in an MFC, which could help reduce the costs of full-scale bioelectrochemical treatment of AMD dominated with iron. In this study, Fe3+ was reduced to Fe2+ at the cathode of the MFC, followed by Fe2+ reoxidation and precipitation as oxy(hydroxi)des. In a broader perspective, the treatment process developed in this study could be attractive as a sustainable alternative for the treatment of AMD with high iron concentration.

This could involve the precipitation of iron prior to other challenging metals (e.g., Mn) or the co-precipitation of Fe-oxy(hydroxi)des with other dissolved metals in AMD. The optimum conditions were found at a charge of 662 Coulombs, which was achieved within 7 days at an acetate concentration of

1.6 g/L in a membrane MFC. This caused the pH to rise to 7.9 and resulted in iron removal of 99%. Treated effluent met the pH discharge limits of 6.5-9.0.

 
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