If activated carbon is added to soil then the concentration of highly-soluble compounds, specifically nitrate compounds, will be decreased as measured from captured runoff because activated carbon has a large surface area as well as a hydrophobic surface which allows it to absorb highly soluble compounds in water.

Dead zones

Dead zones, also called hypoxia, are areas in large bodies of water where the amount of dissolved oxygen is under 2-3 mg  of dissolved oxygen per litre of water. While these have existed for millions of years, hypoxic areas have become more common and more severe in recent years. Although there are areas that will naturally become dead zones due to high saline levels or thermal gradients, the most common cause is mostly a side effect of human activity. The majority of hypoxic areas are caused by the decomposition of algae in algal blooms, which are in turn caused by sea level rise or fertilizer pollution. Sea level rise can cause algal blooms by causing deposits of iron and other minerals found on continental shelves to be dissolved into the ocean, thus giving algae enough nutrients to quickly bloom. In areas where fertilizer is commonly used (mainly agricultural areas or golf courses), excess nutrients from fertilizers that are not absorbed by plants can be introduced to nearby aquatic ecosystems, also causing algal blooms due to excess nutrients.

Health effects of nitrates

Excess nitrates from fertilizer runoff in drinking water can have serious effects on human health and is described as primary toxicity (Fewtrell, 2004). For example, nitrates can lead to the formation of nitrosamines, mutagenic compounds, which may cause many types of cancer as well as other dangerous diseases (Science Direct, 2018). Furthermore, excess nitrates in groundwater can lead to eutrophication; the process of nutrient accumulation in water which leads to algal blooms. 

Activated Carbon

One of the things that determines a filtration’s success is its pose size distribution (PSD). The PSD refers to the number of particles a pore in a membrane can absorb (Science Direct, 2023). The PSD of activated carbon is one of the main factors that make activated carbon a good absorbent. This, with its high surface area, allows it to absorb highly-soluble compounds in water. 

Slow-release Fertilizer

The fertilizer we used for this experiment was slow-release, meaning it has a coating that prevents it from giving the plant all the nutrients it needs at once. Slow-release fertilizers rely on either microbial activity to eat the coating and release nutrients, osmosis in which water dissolves the nutrients and forces them out, or diffusion through a concentration gradient. Slow-release fertilizer can also be affected by temperature. Every 10℃ change will cause a 2x fold effect in nutrient release. 

Control Variables:

• Soil type
• Fertilizer type
• Fertilizer amount
• Water source
• Amount of water
• Lighting
• Temperature of growing area

Manipulated Variable:

• Amount of carbon in soil

Responding Variable:

• Concentration of NO₃ in captured water run-off

Set up procedure:

1. Set up two containers (per sample). One container should have holes in the bottom, and the other should be able to collect the runoff from the first container.
2. In each container, we added two cups of soil, mixed with the appropriate amount of activated carbon to achieve the carbon to soil ratio for each sample.
3. Add three teaspoons of grass seed evenly over the top of each sample
4. For all samples except the control, add ~ 9.0g of fertilizer to each sample
5. Water samples (carefully to avoid disrupting the grass seed) with ~118 mL of water

Testing procedure:

1. Empty all remaining runoff water from collection container
2. Clean out container to remove any residue
3. Water sample, then wait 2-3 minutes for runoff to collect
4. Using a cloth as a filter, add water to test tube
5. Add 10 drops of test solution 1 to the test tube, then invert 10 times to mix
6. Add 10 drops of test solution 2 to the same test tube, then shake for approx. one minute
7. Wait five minutes for color to develop
8. Record data from test
9. Clean all supplies
10. Repeat steps 1-7 for all samples

Our first experiment was designed to test if our project was feasible. Throughout this experiment, we were able to observe many trends that were then used to design a more effective second phase. The grass sprouted on the eighth day of the experiment, and reached a height of over three inches within three weeks, whereupon the grass was trimmed to maintain a consistent height.

The nitrate levels in the runoff water in the two experimental samples both followed a common trend of starting at moderate levels of NO₃ in the runoff water, then decreased to low levels as the fertilizer began to be fully used, then increased after more fertilizer was added. Additionally, nitrate levels rose significantly towards the end of the experiment, which led us to believe that the carbon in the soil was also being used up. Throughout the experiment, while the nitrate levels varied quite a bit, the concentration of NO₃ in the runoff of the sample without carbon was higher than the sample with carbon consistently, usually 5-20 ppm higher. 

For the second experiment, we experimented with four different carbon ratios in order to determine which one was the most efficient. There were some issues with the weather, and so it took longer for the grass to sprout. Furthermore, the added activated carbon resulted in more issues with growth, but more information on this can be found in the Analysis portion of this paper. 

Nitrate levels were very low for the first two months due to weather conditions. The fertilizer did not receive enough Sunlight to dissolve quickly, and as it is a slow-release fertilizer, it took longer to give the grass the nutrients it needed to grow. However, the data from February shows a more accurate result that is closer to the data from our first experiment, when the samples received the needed amount of Sunlight. 

Results 

Table 1: PPM of nitrates (NO₃) in runoff water of experiment 1 (Nov 17 - Dec 15)

The first experiment was used to determine if the concept of adding activated carbon to soil would be effective at reducing the concentration of nitrates dissolved in the runoff water. We were able to successfully determine that adding the activated carbon to the soil did effectively decrease the amount of nitrates in the runoff water. Thus, through the results of this experiment, it was proven that this project does have merit, and a second phase to determine what the most effective concentration of carbon in soil would be run. 

The second experiment involved experimentation with different ratios of activated carbon in soil. Due to unfavorable weather conditions, data from the last month of our experiment is the most useful. This data shows that out of the four activated carbon ratios, the 8:1 and 16:1 samples have the least nitrate concentration. Thus, these two are the ones with optimal activated carbon to soil ratios, but further experimentation under better growing conditions would have to be conducted to determine which of the two is better. 

Table 2: PPM of nitrates (NO₃) in runoff water of experiment 2 (Dec 02 - Mar 01)

Analysis 

There were many challenges experienced during this experiment. Firstly, the activated carbon, much like fertilizer, wears off after 2-3 weeks, and so we decided to re-add activated carbon when we fertilize (every three weeks). However, the assumption that the amount of activated carbon added should be the same, much like fertilizer, resulted in too much activated carbon. According to our research, any addends to soil should not amount to more than 40% of the soil (LibreTexts Biology, 2023). Thus, only one of our samples was affected; the 4:1 carbon sample, as it amounted to 50% of added activated carbon after the second addition. After further research it was found that the amount of activated carbon added should amount to 10% of the amount of soil added. As such, one of our samples (8:1 ratio) was at its maximum, while the 16:1 and 32:1 ratios were lower than the maximum amount. 

Secondly, the weather conditions were not ideal when the second experiment was run, especially with the cold spike, so the samples did not receive enough Sunlight to grow. Furthermore, the activated carbon needs to be left out in the Sun for at least one hour per month for it to remain activated, which is what makes it a good absorbent for highly soluble compounds. As such, the samples were moved to a different place in the house with a bigger window whenever there was Sunlight, which helped the process as seen in our data. 

Thirdly, the cold spike also affected the release of nutrients to the plant, which is why the first two months of data show a low NO₃ concentration in all samples. This was fixed towards the end of the cold spike when the samples received more Sunlight and were in an overall warmer environment. 

Overall, the experiment did have lots of challenges; however, the project does provide consistent results when run under optimal conditions for plant growth. Based on our experiment, a 16:1 and 32:1 activated carbon to soil (cups) ratios seem to be the best in both growth and nitrate absorption, but the 16:1 ratio may be better as it has consistent results of 3.0 ppm average when compared to the 32:1 ratio’s 4.0 ppm average. The difference would be larger, around 5.0-6.0 ppm ratio average, if the samples had access to more Sunlight. 

The issue that we are aiming to solve through this project is dead zones, which are aquatic areas with less than 2 mg of dissolved oxygen per liter of water. One of the biggest contributors to dead zones is nitrate pollution caused by fertilizer runoff. By reducing the amount of nitrates dissolved in aquatic ecosystems, the number and size of dead zones could be significantly decreased, as nitrates cause drastic algal growth which absorb large amounts of oxygen to live as well as decompose.

Our solution to this problem was to filter the dissolved nitrates out of the runoff water before they reach aquatic ecosystems. We did this by adding activated carbon directly into the soil. The carbon then bonded to the nitrates, effectively ensuring that they were no longer harmful to the environment by causing algal blooms, which lead to dead zones.

We ran two experiments to determine whether this concept would be effective at reducing nitrate runoff. The first experiment was solely to determine if adding activated carbon to soil would make a difference in the nitrate concentration of runoff. As the amount of nitrates in the sample with activated carbon in the soil was consistently lower than the sample without carbon, we were able to determine that the experiment does have merit. The second experiment demonstrated the effects different activated carbon ratios had on nitrate concentrations in captured runoff. According to data (specifically during the week in which samples had access to Sunlight), samples 8:1, 16:1 and 32:1 had better results when compared to the 4:1 sample, and within those 8:1 and 16:1 had better results when compared to the 32:1 sample. As such, our experiment proved the potential of activated carbon in reducing nitrate contamination due to fertilizer runoff. 

Thus, our experiment suggests that using carbon filtration in soil to reduce nitrate contamination in drinking water is feasible and delivers desired results; however, the second round of experiments unfortunately did not show these results in the expected manner when compared to the first round of experiments due to the lack of Sunlight during the months in which the experiment was conducted. 

While our experiment was run on an extremely small scale, it would be possible to use in real world settings. Two main areas where carbon filtration in soil could make a big difference are in agriculture and golf courses. These are two of the largest producers of nitrate pollution caused by fertilizer runoff. By adding activated carbon into the soil of major producers of nitrate pollution, this could significantly decrease the harmful effects of fertilizer runoff, and thus decrease the amount of dead zones. However, adding activated carbon into agricultural practices would likely increase the cost of goods produced using it, as activated carbon is quite expensive. Additionally, the process to produce activated carbon can be harmful to the environment. While using activated carbon filtration in soil to reduce the amount of nitrate pollution in aquatic systems would certainly have positive impacts in reducing the impacts of dead zones, it may not be a viable solution in the near future. However, as methods of producing activated carbon become more effective, it could be a solution to a serious problem.

Our experiment involved working with plants, which are a living organism, and thus can have issues that are out of our control, yet still affect the results of the experiment. If the number of grass seeds that were functional in one sample was much higher than another sample, this could lead to biased results. Additionally, some samples may have grass that reacts differently with the fertilizer, which could also lead to different results than expected. Another possible source of error in this experiment would be contamination. Despite taking measures to reduce the risk of contamination, keeping the samples close together (especially in experiment one) will always increase the risk of contamination.

A Review of Bio-Based Activated Carbon Properties Produced from Different Activating Chemicals during Chemicals Activation Process on Biomass and Its Potential for Malaysia. (2023, November 27). NCBI. 

Retrieved March 1, 2024, from 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10707299/

Bies, S. (2022, February 28). Activated carbon, Why, What & How to use it. Sybotanica. Retrieved March 1, 2024, from https://www.sybotanica.com/blogs/soil/activated-carbon-why-what-how

CARBON ABSORPTION definition | Cambridge English Dictionary. (2023, October 25). Cambridge Dictionary. Retrieved October 30, 2023, from https://dictionary.cambridge.org/us/dictionary/english/carbon-absorption

Cheung, J. (2020, June 12). Factors affecting the release of nutrients from coated fertilizers. LinkedIn. Retrieved February 15, 2024, from https://www.linkedin.com/pulse/factors-affecting-release-nutrients-from-coated-jane-cheung

Diaz, R. J., & Rosenberg, R. (n.d.). Spreading Dead Zones and Consequences for Marine Ecosystems. ads. Retrieved February 15, 2024, from https://ui.adsabs.harvard.edu/abs/2008Sci...321..926D/abstract

Forbes, R. (2017, October 25). Algae blooms increasing. KenoraOnline. Retrieved October 30, 2023, from https://www.kenoraonline.com/articles/algae-blooms-increasing

Functional Groups Names, Properties, and Reactions – Introductory Chemistry. (n.d.). UEN Digital Press with Pressbooks. Retrieved October 30, 2023, from https://uen.pressbooks.pub/introductorychemistry/chapter/functional-groups-names-properties-and-reactions/

Jern, M. (2018, January 15). What does activated carbon filters remove from tap water? - EN. TAPP Water. Retrieved October 30, 2023, from https://tappwater.co/en/what-activated-carbon-filters-remove/

Joyce, S. (2000, March 1). The dead zones: oxygen-starved coastal waters. EHP Publishing. Retrieved February 15, 2024, from https://ehp.niehs.nih.gov/doi/epdf/10.1289/ehp.108-a120

Kershner, K. (2023, May 12). How does reverse osmosis work? | HowStuffWorks. Science | HowStuffWorks. Retrieved October 30, 2023, from https://science.howstuffworks.com/reverse-osmosis.htm

LibreTexts Biology. (2023, October 31). 31.2A: Soil Composition. Biology LibreTexts. Retrieved February 15, 2024, from https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/31%3A_Soil_and_Plant_Nutrition/31.02%3A_The_Soil/31.2A%3A_Soil_Composition

National Centers For Coastal Ocean Science. (2023, August 2). Below Average Summer 2023 'Dead Zone' Measured in Gulf of Mexico. NCCOS. Retrieved February 15, 2024, from https://coastalscience.noaa.gov/news/below-average-summer-2023-dead-zone-measured-in-gulf-of-mexico/

Patterns in sources and forms of nitrogen in a large eutrophic lake during a cyanobacterial harmful algal bloom. (2022, July 2). Patterns in sources and forms of nitrogen in a large eutrophic lake during a cyanobacterial harmful algal bloom. Retrieved October 30, 2023, from https://aslopubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1002/lno.12311

Reverse Osmosis Water: Pros & Cons (2023 Guide) – Forbes Home. (2023, April 20). Forbes. Retrieved October 30, 2023, from https://www.forbes.com/home-improvement/home/reverse-osmosis-water-pros-cons/#benefits_of_reverse_osmosis_water_filtration_section

Robinson, B. (2020, January 6). How to Remove Nitrates from Water – Fresh Water Systems. Fresh Water Systems. Retrieved October 30, 2023, from https://www.freshwatersystems.com/blogs/blog/how-to-remove-nitrates-from-water

salisbury. (2021). The Science Behind Slow-Release Fertilizers: How They Work and Why They Matter. Salisbury Greenhouse. Retrieved February 15, 2024, from https://salisburygreenhouse.com/the-science-behind-slow-release-fertilizers/

The Science Behind Activated Carbon Water Filters | CB Tech. (2018, October 19). Carbon Block Technology. Retrieved October 30, 2023, from https://www.carbonblocktech.com/the-science-behind-activated-carbon-water-filters/

Science Direct. (2018, July 15). Investigation of rice husk derived activated carbon for removal of nitrate contamination from water. YouTube: Home. Retrieved February 15, 2024, from https://www.sciencedirect.com/science/article/abs/pii/S0048969718307290

Science Direct. (2023, May 19). Nitrate removal from aqueous solution using polyaniline modified activated carbon: Optimization and characterization Author links open overlay panel. Science Direct. Retrieved February 15, 2024, from https://www.sciencedirect.com/science/article/abs/pii/S0167732220301586

Science Direct. (2023, May 19). Nitrosamines. Science Direct. Retrieved February 15, 2024, from https://www.sciencedirect.com/topics/chemical-engineering/nitrosamines

Science Direct. (2023, May 19). Pore Distribution Size. Science Direct. Retrieved February 15, 2024, from https://www.sciencedirect.com/topics/materials-science/pore-size-distribution

Science Direct. (2023, May 19). Pore Size Distribution. Science Direct. Retrieved February 15, 2024, from https://www.sciencedirect.com/topics/chemistry/pore-size-distribution

scientific reports. (2023, May 19). Development of activated carbon for removal of pesticides from water: case study. nature briefing. Retrieved February 15, 2024, from https://www.nature.com/articles/s41598-022-25247-6

Sears, C. (2023, August 19). Do Grow Lights Work? Everything You Need to Know. The Spruce. Retrieved October 30, 2023, from https://www.thespruce.com/how-to-use-grow-lights-for-indoor-plants-5221241

Stricklin, T. (n.d.). Activated Carbon Filters: What Do They Remove from Water? SpringWell Water Filtration Systems. Retrieved October 30, 2023, from https://www.springwellwater.com/activated-carbon-filters-remove/

What Is Nitrocarburizing? How It Compares to Nitriding (and the Benefits). (n.d.). Miheu. Retrieved October 30, 2023, from https://www.miheuprecision.com/blog/nitrocarburizing

What is sorption give an example? (n.d.). BYJU'S. Retrieved October 30, 2023, from https://byjus.com/question-answer/what-is-sorption-give-an-example/

World Economic Forum & Green Matters. (2021, June 10). Ocean: What are 'dead zones' and why are they getting worse? The World Economic Forum. Retrieved February 15, 2024, from https://www.weforum.org/agenda/2021/06/dead-zones-have-been-around-for-millions-of-years-but-study-shows-theyre-getting-worse/

There are many people that we would like to thank for their help while running this experiment. The first is our mentors, Dr. Shahin Jabbari and Mrs. Kim O’Keffe for their assistance and advice. We would also like to thank Dr. Sara Bourke for her help while designing this experiment. Finally, we would like to acknowledge our friends and family for supporting us while running this experiment.