STEM 1:

Chemical Acidification Process for the Removal of CO2 from Seawater​

Overview:

The purpose of this project is to examine the effects of varying additions of CO2 on the growth of the marine microalgaeTetraselmis chuii in a controlled environment. In addition, it serves to examine whether this additional CO2 could be generated from oceanwater, which would be acidified to convert bicarbonate to CO2 due to the ocean’s buffer system. This water would need a base additive to prevent an exacerbation of ocean acidification. My data collection serves to analyze this process, its potential, and its demands.

Research Proposal

Literature Review

Materials and Procedure:

During the experiments, cultures of marine microalgae Tetraselmis chuii sp. were grown in a controlled environment. The lighting used to grow the microalgae consisted of 660nm red light and 460 nm blue light. The temperatures were maintained inside a bioreactor at and around 25 degrees Celsius. The algae were grown in a medium that consisted of Atlantic Ocean water with an f/2 media containing phosphates and nitrates to support microalgae growth. The water was sterilized and boiled to remove any predation, and the salinity of ocean water the microalgae was grown was 35ppt (+- 0.2%). The pH of the water, in which the microalgae was grown in, was measured with a Aridea liquid pH sensor, and displayed using an Arduino with an LCD display. In the early phases, the pH was determined by using standard litmus paper. The microalgae were grown in 3.15 x 3.15 x 10.04-inch transparent cylindrical containers. CO2 induced acidification was achieved inside the containers using a CO2 generator system with pressure gauge and cotton filter. The CO2 was supplied inside the generator system with water, citric acid, and sodium bicarbonate. For the sample analysis of the microalgae, transmittance values were taken with a portable Vernier spectrometer at a wavelength of 750nm, above chlorophyll absorbance. The transmittance values were compiled with the Logger Pro software program. Dry weight analysis was determined using filter paper, which was dried in oven at 165 degrees Fahrenheit for half less than an hour. the Seawater, 2M HCl, and a Vernier CO2 Sensor were also used for further measurements. In addition, 0.5 M NaOH was used to adjust the pH values within the algae containers when necessary.

Tetraselmis chuii was grown in a bioreactor with photoperiods ranging from twenty-four hours to eighteen hours. 250mL of the microalgae cultures were mixed with 750mL of sterilized seawater with the f/2 growth media. The microalgae were grown over the period of fourteen days. Every twenty-four hours the containers were mixed to prevent oxygen saturation above 300% and to break the nutrient boundary layers formed by the algae settling at the bottom. After this, the Aridea pH sensor was used to measure the pH levels. Since microalgae converts bicarbonate into oxygen and glucose, the pH of the water decreases overtime. So, CO2 was bubbled through the seawater and the pH was measured to maintain relevantly consistent pH levels in the four different samples. These samples had an unaltered pH of 9, and pH’s of around 5.5,6.5,7.5. And in addition to maintaining the pH of the water by adding CO2, small amounts of 0.5 NaOH were added to the water to maintain pH values consistent of those numbers. Following these procedures, the transmittances of the microalgae samples were all determined. The dry mass of the microalgae samples was also estimated separately by finding the dry mass of an algae cultures at a 100% and finding the transmittance for the concentrations with dilutions of 10, 20 ,30, 40 ….90% dilutions. Using this information, the transmittance and the concentrations were plotted, and a residual regression line was determined. The transmittances are an accurate way of indirectly measuring microalgae growth since more light is scattered, as cell populations/concentrations increase inside of the container.

In a separate phase, 100 mL of seawater was added to a 300 mL container, and the Vernier CO2 sensor was placed in the container, which was then sealed airtight. An initial measurement for the ppm of CO2 inside the container was taken and following that a pipette was used to add 50, 100, 190, 200, 400, 600 uL to the different seawater samples. Over the span of fifteen-sixty minutes, CO2 measurements in ppm were taken as a function of time inside the containers. Then, using the ideal gas law, the amount of CO2 in moles/L could be determined in relation to the volume of the container, or in other words, the amount of sodium bicarbonate converted to CO2 in the water could be determined. In addition, the final pH was measured in a few of the samples, to get an estimate for how much bicarbonate would be needed to neutralize the water.

Results and Figures:

As described, different dilutions of microalgae were measured transmittance-wise, and their dry weight was determined to create a linear regression line. Figure 1 shows the relationship between the two measurements, representing the equation g-DW/L=-1.1114(%T)+11.029. More specifically, the dry weight for a sample with 52% transmittance at 100 percent concentration was determined to have a mass of 5g per Liter after its dry weight was determined to be 0.25 grams for 50mL of water. The same sample was diluted to percent of 10, 20, 30… 90 and the transmittance odf all those samples was measured at 750nm. And, because these are dilutions, the g-DW/L in each of the samples in equal to the percent dilution times the g-DW/L at with the 100% concentration. Furthermore, these values for dry weight could be used to determine the growth rate of the algae.

Figure Two displays the ppm of CO2 released from 100 mL of Seawater in relation to time. Specifically, this graph displays the changing rate at which CO2 is released within a contained environment within an environment that had an empty volume of 200 mL. Using the Ideal Gas Law, the change in ppm can be used to determine the moles of CO2 in the container/ the amount of bicarbonate converted to CO2. Figure 2 refers to a case in which 190mL of 2M HCl was added to the seawater sample that had an initial pH of 7.48. 190 mL of bicarbonate was added to the water because that is the amount of HCl with an equal amount of moles HCl compared to the bicarbonate in the water going from the assumption that the water has 140 mg/L of bicarbonate. Overall, in the course of an hour, 1.81E-05 moles of CO2 were released in the container. In addition, the release rate represented the equation logarithmic equation 1185*log(60.56x) for this period. The rapid rate in release in the first quarter of an hour demonstrate the possible effectivity of just collecting the CO2 released from seawater within a time frame such as that for the efficiency of a system that collects CO2 by acidifying seawater. In addition, the pH dropped within the sample to a pH value of 1.82. This though indicates that a more than necessary amount of HCl was added to the water, since at pH of 4.5, bicarbonate would be of no concentration in the water because of the ocean’s buffer system. In addition to this data, the CO2 release for half an hour with HCL quantities being 100 mL and 50mL was determined to be 1.27E-05 and 1.19E-05 respectively. And HCL additive quantities of 200 mL, 400 mL, and 600mL had respective CO2 release values of 7.85E-06, 7.85E-06, and 7.49E-06 moles. Comparatively, going off the assumption again that their typical seawater has bicarbonate quantities of roughly 140 mg per liter, in the 100 mL seawater sample, there would be 0.000318 moles of CO2 in the sample. So, in the case of an hour, when the 190 mL of bicarbonate was added to the water, only 0.057 percent of the bicarbonate in the water was converted to CO2, which suggests the occurrence of a steady release rate in the time following.

The microalgae g-DW/L were determined using the transmittances of the algae samples. Figure three on the left shows the release the growth of the algae in the separate samples in the final week, where the most growth is expected to occur following a lag phase in the algae growth. For the samples with average pH’s of 5.62, 7.28, 6.48, and 9.09 the respective growth rates from day through day fourteen were 26.7%, 68%, 143.7%, and 68.41%, So, at the very end of the fourteen days, the group with the average pH of 7.28 had the greatest final dry weight biomass estimated to be at 4.68 g-DW/L. In each of the containers, the pH was never exactly consistent because the growth of the microalgae meant that bicarbonate was continuously being converted to oxygen and glucose, so as the algae grew the pH in the environment increased. CO2 was added routinely every 24 hours, the initial pH was measured at that point, and so to be the pH of the algae at the end of the acidification period. The mean pH of the environment was taken from these values, which represented the maximum and minimum pH for each twenty-four hours. The variation amongst the pHs can be represented by standard deviation of the pH values, which in accordance with the previous lists, had standard deviations 0.38, 0.71,0.69, 0.47. Furthermore, this indicates that there was variability among the pHs, but it was aimed to be kept at a minimum.

Analysis:

The tests indicate that CO2 induced acidification had a positive effect on the growth rate of the marine microalgae Tetraselmis chuii. However, it is important to identify some of the possible confounders that could have affected the data. Most of these confounders are evident of the growth of the microalgae. In first week, the algae the growing algae was not measured using the Vernier spectrometer, and transmittance values were not consistent. This is evident in the transmittances measured on the first and second day of testing in which the transmittances dropped in a single day by values as low 12%. However, this confounder was not influential in the second week. In addition, the 6.48 and 5.62 pH groups were over acidified, which required the addition of 0.5 M NaOH >30mL, which could furthermore have had an influence on the CO2 buffer this therefor would have meant that the pH of the groups in the second week were not consistent with that of the first week. And one final possible confounder relates to the growth of the microalgae in the control group. The culture medium containing the f/2 media consisted of had saturated levels of bicarbonate, this would have influenced the growth of the microalgae in control group compared to the other groups, which when acidified would have driven out some of the bicarbonate out of the water. However, the water was acidified on the eighth day, and by the ninth day the pH levels were back to levels that remained constant for the final four days. In addition, the recorded values for the ppm of CO2 released with 400microL was greater than that released with 600microL, this in turn shows that this difference did not have a significant effect on the CO2 release. And, since pH for the 190 microL induced a pH that was less than 4.5, it indicates than less HCl could be added to the water in order to maximize the pH because at 4.5 all the bicarbonate is driven out of the water. However, may have an effect on the CO2 that is released form the water in relation to time.

Discussion/Conclusion:

Despite some of the possible confounders, the growth of the microalgae, and the CO2 generation from this system show the potential for the physical design of a model that works under this designs principal. At the same time there needs to be further, less hypothetical analysis for CO2 generation rate of the both systems, both with accordance to time. Furthermore, the system, would need to designed in order to allow for the CO2 generated from the system to be transferred to the controlled seawater with the algae in the photobioreactor. In addition, there are many future extensions that could be made to this project. Microalgae is high in lipid content, meaning that it has the potential to be used in the creation of biofuels and other marketable products. In addition, a system such as this, which is not combined with electrodialysis would require a lot of resources. So, an aim if this were the path, would be to make the processe more cyclic. Would it be possible to generate acids from the algae? Would it be possible to have the system increase the pH of different seawater batches after acidification? Would it be possible to recylce nutrients for algae cultures grown with the seawater? Overall, there are a lot of possible contributions that could be made to this project.

References:

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