Abstract: Microbial fuel cells are devices that generate electricity from soil using bacteria. The bacteria transfer electrons from the soil to the circuit of the microbial fuel cell. The microbial fuel cell is a sealed container to prevent oxygen from entering. This causes the bacteria inside to use anaerobic respiration, which causes them to produce energy in the form of ATP. Different substances are usually added to the microbial fuel cell in order to stimulate the bacteria so more energy is produced. However, different substances affect the energy output of the microbial fuel cell differently. Therefore, different substances should be tried to find the one which has the greatest affect on the electricity output. In this experiment, there were three trials. In the first trial, water was added, in the second trial, salt was added, and in the last trial, citric acid was added. Overall, the microbial fuel cell generated the most electricity when salt was added to it. The conclusion from this was that ionic compounds like salt would generate the most energy. However, it is hard to know if this is accurate since there was only one type of ionic compound used, so the success of salt might have been a coincidence.
Introduction:
The microbial fuel cell is an energy generating container that generates energy through mud and water. The experiment that was conducted was an experiment to see which medium would work best mixed into the soil to generate the energy. The experiment consisted of salt, tap water, and citric acid. They made up the independent variable of the experiment. The dependent variable was the amount of energy generated. The hypothesis was that if we used acid type chemicals, then the energy generated would be greater than if we used other types of chemicals. The MFC kit is a way to generate green electricity.
The MFC can use a bacteria known as geobacter, which has the ability to transfer electrons which enables it to extract energy from biomass. (Adrian Covert, 2009) Biomass is the total mass of organisms in a given area or volume. Basically, this bacteria can extract energy from things like mud and dirt. Topsoil is what was used during the experiment. Other bacteria can be used to generate electricity. MFC’s are devices that use bacteria as catalysts (a substance that causes or accelerates a chemical reaction without itself being affected) to generate electricity. (Bruce Logan, 2006)
Recovery for electrical energy is a key point for evaluating a MFC. (Li Xiao, 2014) MFC’s are used for green energy, green energy is green because it saves energy, and recovery of energy is also part of saving it. It is also green since it uses things like wastewater, instead of other non-reusable resources, or unused water. (Rita Martin, 2014)
Methods
The materials used in the experiment were a microbial fuel cell, purchased from Keego technologies, fine grained salt, tap water, citric acid, from Carolina Biological, a notebook, a multimeter, also from Keego Technologies, and finally topsoil, which was collected from Battery Park, in New York City. At first, the topsoil was mixed with water until it was of a doughy consistency. Next, the topsoil was poured into the bottom of the microbial fuel cell container until the layer was one centimeter high. Next, the anode was placed on top of the soil, and then more soil was added until it was up to four centimeters above the bottom of the container. After that, the cathode was placed on top of that and air bubbles were removed from the container. Finally, the wires were placed through holes in the lid and the lid was sealed onto the top of the container. Each day for five days, starting from the first day the microbial fuel cell was assembled, a multimeter was used to measure the electricity output of the microbial fuel cell by placing the two wires of the multimeter over the two wires of the microbial fuel cell so that the metal parts touched. The results were written down in a notebook. The same steps were repeated two more times, except for in the second trial, 15 milliliters of salt were mixed into the soil along with the water, and in the third trial, 15 milliliters of citric acid were mixed in. Between each trial, the microbial fuel cell was thoroughly cleaned with water and the soil that was in it was thrown out.
Results
Graph 1: This graph shows the microvolts generated by the microbial fuel cell over five days for each type of medium put in. The mediums were water, salt, and citric acid.
Discussion:
The hypothesis was that acids added to a microbial fuel cell would cause it to produce more energy than if other types of substances were added. This hypothesis was proven wrong by the data in graph 1. The graph shows that every single day of the experiment, when salt was added to the microbial fuel cell it produced the most energy, more than what the acid in the experiment, citric acid, produced. This is against the hypothesis, so the hypothesis is therefore invalid.
Also, the graph shows that for each consecutive day when water was in the microbial fuel cell, the microbial fuel cell produced more energy than the last day. This might have occurred because the bacteria were more active as time went on. In contrast, for salt, after two consecutive days of increasing energy production, on the fourth day there was a sudden drop in energy production, and on the fifth day the energy production was even lower than that of the fourth day. This might have happened because the soil had run out of enough electrons for the bacteria to transfer to the circuit. For citric acid, there was a spike in energy production on the second day, but on the third day the energy production had fallen drastically. From there, the energy production slowly increased for the remaining two days. This might have happened because the soil ran out of electrodes for all the bacteria, but then the bacteria started transferring energy a bit more effectively for the coming days.
Human error might have changed the results of the experiment. First, the amount of soil or water mixed into the soil might have varied slightly, and the amount of each substance added to the microbial fuel cell could also have varied. This could have changed the results. Furthermore, the fact that the soil was from the same park but from different parts of the park could have caused the decreases in energy made in the citric acid and salt trials. The multimeter might have measured the energy output with a margin of error.
A problem faced in the experiment was that the ground was often frozen over, which made it impossible to gather soil, even with the use of a metal shovel. This caused there to be less trials, which made the results be less accurate. Furthermore, the chemicals ordered from Carolina Biological never got shipped, so there was a smaller variety of chemicals to choose from to conduct trials with. Also, since only three substances were tested, of which one was the control, the power of any specific type of substance to cause the microbial fuel cell to produce more energy than from other substances could not be proven. It could be, for example, that out of all the ionic compounds, only salt helped, and that other types of acids would help the microbial fuel cell generate more energy than citric acid did.
If the experiment were conducted again, there would be more trials of each substance, and more types of substances would be used. The soil would be collected in the fall when it would be easy to collect but wouldn’t change too much before the experiment. Also, to prevent error, a more powerful multimeter would be used.
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