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Article
Evaluating How the Concentration of Chitosan Affects the Properties of Bioplastic in Terms of its Biodegradation Rate
Czarina Royce H. Rebanal*
IGEE Proc 2025;2(1):43-52.
DOI: https://doi.org/10.69841/igee.2025.010
Published online: February 27, 2025

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*Corresponding Author: Czarina Royce H. Rebanal, E-mail: czarinaroyce@yonsei.ac.kr
• Received: January 16, 2025   • Revised: February 13, 2025   • Accepted: February 14, 2025

© 2025 by the authors.

Submitted for possible open-access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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  • We encounter plastic materials daily. Many companies opt to use plastic in manufacturing their products to keep production costs low, hence making it affordable for consumers. Though this makes it easier for consumers to buy daily necessities, plastic industries release 1.34 billion tons of greenhouse gas annually. Additionally, plastic persists in the environment because of its extremely low rate of biodegradation. The plastic build up has led to dangerous levels of plastic pollution which has affected wildlife, climate, and human health. A prominent solution to this problem is the ongoing research on bioplastics, plastics made from renewable sources (like microbes and plants) instead of petroleum. The current issue with these bioplastics though, is that they are not as durable as normal plastic since these renewable materials easily degrade. The rigorous search for a material that combats synthetic plastic’s durability has led to the research on chitosan, a biodegradable sugar found in the exoskeleton of shellfish. Due to its antimicrobial properties, it has a lower degradation rate compared to other bioplastics. This makes the durability of chitosan-based bioplastics comparable to synthetic plastics. Through this research, the durability of chitosan-based bioplastics will be tested through a soil burial test and tensile strength test. This research discusses how varying the concentration of chitosan in chitosan-based bioplastics mimics natural biodegradation mechanisms whilst combating the durability of synthetic plastics.
The plastic industry is vital to the Philippine economy. The Philippines Transparent Plastics Market is valued at $540 million in 2022, and it is expected to grow at a significant CAGR of over 5.2% over the forecast period of 2023-2030 (RationalStat LLC, 2023). Plastic is a cheap material to manufacture since it is produced from leftover oil production. This is why many companies opt to use plastic for packaging to keep costs low and to make it affordable to consumers. This idea of plastic as a cheap and durable material that can be used in manufacturing is true for companies worldwide as well, which is why the intensity of natural disasters has increased rapidly. Though beneficial to consumers, plastic industries release 1.34 billion tons of greenhouse gas annually. Greenhouse gases are gases in the earth’s atmosphere that trap heat in our environment which then leads to the increase of global temperatures. This then results in more violent natural disasters. Furthermore, plastic does not biodegrade which is why when it is discarded, it persists in our environment. Many researchers are looking for materials to replace synthetic plastics in order to combat its negative impact. Most bioplastics easily degrade since they contain essential materials for bacteria, such as starch. This fast degradation rate is the main reason why big companies are hesitant or are not even considering a more eco-friendly approach. Synthetic plastic has always been reliable due to its durability. To make bioplastics more comparable to synthetic plastics in terms of degradation rates, researchers have studied chitosan, a biodegradable sugar found in the exoskeleton of shellfish. Chitosan is found to lower degradation rate due to its antibacterial properties and this makes the durability of chitosan-based plastic comparable to synthetic plastic. This research paper discusses how varying the concentration of chitosan (0 g, 5 g, 10 g, 15 g, and 20 g) mimic natural biodegradation mechanisms whilst decreasing biodegradation rate in soil for durability, as measured by weight loss over 10 days.
This research is worth investigating because it can help reduce plastic pollution by opening people and companies to use more eco-friendly materials. This could help in determining if chitosan’s antibacterial properties increase its durability greatly which can make it a viable eco-friendly alternative to synthetic plastic.
Plastic Pollution in the Philippines
The Philippines had the largest share of global plastic waste discarded in 2019. It was recorded that the country was responsible for 36.38% of the global oceanic plastic waste. Plastic cannot be broken down by biological processes because it is composed of long chains of polypropylene which have extremely strong carbon-carbon bonds. Due to the chemical composition of plastic, it persists in the environment and this leads to harmful effects to marine life and soil quality.
There are multiple reasons why the problem of plastic pollution is common in the country. The main factors leading to plastic pollution are the prevalent use of plastic products and the lack of access to disposal facilities. The Philippines is a third world country, and it is reported by the Philippine Statistics Authority that there are approximately 20 million Filipinos who are living under the poverty line in 2021. Though plastic is seen to cause adverse effects to the environment, the affordability and accessibility of plastic is often prioritized by the majority of people. The lack of access to disposal facilities on the other hand can be attributed to government mismanagement. There are many laws surrounding the waste disposal but it is observed that there is failure in enforcing these laws.
Plastic is vital to the economy of the country but it is not biodegradable and its disposal is poorly managed. This is why the research of plastic alternatives has been emerging.
Biomimicry
As the world changes, many challenges emerge. Problems such as overpopulation lead to higher consumption of goods, and in order to keep up with demand, unsustainable practices are being implemented. These unsustainable practices have led to environmental issues which have then caused extreme weather events and destruction of habitats. These negative changes have altered the world’s equilibrium and it may get to the point where the earth’s conditions, which is why many researchers are finding ways that encourage people to switch to a sustainable lifestyle. One method that is used for sustainability is biomimicry.
Ghasemlou, a researcher from RMIT university, used lotus leaves as a material for bioplastics. Normally bioplastics are created using starch since it is a versatile natural polymer, but it is fragile and highly susceptible to moisture. This is why it is often seen as impractical even though it is better for the environment due to its biodegradability. As he researched natural resources that could be used for bioplastics, he observed the ability of lotus leaves to repel liquid and dirt and its ability to be degraded by natural processes. According to Ghasemlou (2021), they have replicated the phenomenally water-repellant structure of lotus leaves to deliver a unique type of bioplastic that precisely combines both strength and degradability. This is an example of what biomimicry is and how it reduces the resource consumption in various industries.
From its literal meaning, biomimicry is the practice of imitating life and nature, but it is so much more than that. It is the application of biological processes in order to create sustainable solutions to the world’s most pressing challenges. It can be seen in history that organisms adapt to new environments in order to survive. As time goes on, the world changes and more problems emerge, which is why biomimicry is a key factor in sustainability. In the case of bioplastics, biomimicry is used through the observation of properties of different natural materials, such as the lotus leaves, in order to compare to commercially produced plastics. The creation of bioplastics that can compare to the durability of synthetic plastics may become a solution to the pressing challenge of waste management of pollution not only seen in the Philippines, but in the whole world
There are currently many types of bioplastic that are commonly being researched on and since the Philippines is surrounded by numerous bodies of water, the type of bioplastic that will be investigated in this research is the chitosan-based bioplastic since chitosan can be found in seafood, specifically crustaceans. According to Popović et. al. (2023), 8 million tons of waste crab, shrimp and lobster shells are produced globally every year. These shells are mostly disposed of at sea or are sent to landfills, but when disposal gets uncontrolled, it can cause environmental damage. This is why shell waste reutilization is currently being explored and one of its current uses is through bioplastics.
Chitosan
One of the key themes in biology is that structure determines function and the application of this key theme is seen in proteins. Each protein has a unique sequence of amino acids; the interactions between these amino acids determine the protein’s structure. This is the same for the many materials that researchers have tried to create bioplastics with. The main difference of chitosan from other polysaccharides is its nitrogen content. According to Priyadarshi & Rhim (2020), the presence of amino groups in chitosan renders it with functional properties; the electronegative amino group takes up protons and develops a positive charge providing chitosan with various physical, chemical, and biological properties. This paper will focus on biological properties, which is its antimicrobial activity.
The main cause for the degradation of bioplastics are microbial enzymes. Microorganisms such as firmicutes, proteobacteria, ascomycetes, etc. can degrade bacteria. These bacteria are distributed throughout ecosystems which is why bioplastics can easily degrade in any environment where it is used. Bioplastics have lower durability since they can be easily degraded, which is why many people opt to use synthetic plastic instead. Due to the antibacterial properties of chitosan though, this may allow bioplastics to last longer. Using chitosan can create a bioplastic that has the durability comparable to synthetic plastic but still be able to be naturally degraded.
In a research done by Anggraini et. al. (2017), she increased the concentration of chitosan (0 g, 5 g, 10 g, 15 g, 20 g) to see how the increase of chitosan affected the bioplastic’s thickness, density, and tensile strength. The results show that as the researcher’s increased the concentration of the chitosan, its thickness, density, and tensile strength also increased. As an extension of this research, the rate of biodegradation will be observed to support the practicality of creating chitosan-based bioplastic.
A trial for the creation of chitosan-based bioplastic was executed in order to ensure that the increments for the controlled variables and the initial methodology would yield the desired results. It can be seen in Figure 1, that mold and cracks were formed in the bioplastic during the preliminary research. I first investigated the formation of mold. There were many factors that led to this error such as the amount of vinegar added, disinfection of materials used, and the thickness of the bioplastic created.
In the process of creating bioplastic, vinegar breaks down the molecular structure of starch so that the bioplastic can be homogenous. Aside from this, it also prevents bacterial growth. Initially the vinegar to water used ratio was 1:100, but after the trial the ratio changed to 10:100 to increase the amount of vinegar which would then increase antibacterial activity. The petri dish used in the trial was also only rinsed with water and not thoroughly disinfected. For the final experiment, the mold where the bioplastic was casted in was thoroughly washed with dishwashing soap.
The cracking on the other hand was caused due to the bioplastic being too thick. Bioplastics are composed of mostly water and during the drying process, the majority of the water evaporates. The evaporation of water leads to the shrinkage of the bioplastic. Since the bioplastics created were too thick, there was uneven shrinkage in different parts of the bioplastic as it dried. This led to the cracks that are seen in Figure 1. For the final experiment, the bioplastics were casted into circle molds that were 3.4 cm in width and 0.5 cm in thickness.
The preliminary trial led to the identification of errors in the increments and methodology. After adjusting the methodology based on the results of the preliminary trial, the final experiment could then start.
Independent Variable
The independent variable is the concentration of chitosan powder and the increments used is 0 g, 5 g, 10 g, 15 g, 20 g. The chitosan powder will be weighed using an electronic balance to ensure that the correct amount is added each trial.
Dependent Variable
The dependent variable for this experiment is the weight loss (%) of bioplastic after soil burial test.
The soil burial test is a standard test used to determine the rate of biodegradation of bioplastics. The presence of microorganisms in soil breaks down bioplastic and will then convert this material into different organic compounds such as water, carbon dioxide, and compost. This will lead to a loss in weight of the bioplastic. The more weight the bioplastic loses, the easier the material degrades. Since chitosan is seen to have antimicrobial properties, it should help prolong the biodegradation process. This test will show how effective the antimicrobial properties of chitosan are and if chitosan increases bioplastic durability.
The plastic will be measured before and after the soil burial test and then weight loss (%) will be calculated using this formula:
Weightloss(%)=W1-W2W1×100W1=massofbioplasticbeforedegradationW2=massofbioplasticafterdegradation
Controlled Variables
The volume of vinegar to be used for the creation of bioplastics is 10 mL. Vinegar helps in breaking down amylopectin and this leads to bioplastic to become more flexible. This should remain constant for all trials since it should only be the chitosan that affects the physical properties of the bioplastic.
The mass of potato starch used for this experiment is 5 grams. In a research done by Wankhade et. al (2025), the increase of starch increased the elongation of bioplastics. This means that it increases the durability of bioplastics which is why it should stay constant for all trials.
The mass of glycerin used for this experiment is also 5 grams. Glycerin is a plasticizer that can reduce internal hydrogen in intermolecular bonds in bioplastics. It can affect the biodegradability and mechanical properties of bioplastics which is why it is important to keep it constant for all the trials.
The volume of water will also be kept constant. The water is the base for the creation of the bioplastics. The ratios for each ingredient used, relies on how much water will be used for the creation of the bioplastics which is why it must stay constant.
Materials needed:
- Chitosan powder - 50g
- Vinegar - 50 mL
- Water - 500 mL
- Glycerin - 25g
- Beakers - 5
- Electronic balance - 1
- Potato starch - 25g
- Hot plate - 1
- Flowerpot - 25
- 10 mL Graduated cylinder - 1
- Spring balance - 1
- Metal rod - 1
- Tables - 2
- Tape - 1
- Laboratory spatula - 3
The uncertainty of the measuring devices, namely the electronic balance and the graduated cylinder, was calculated to ensure the accuracy of the results of the experiment. The uncertainty of the electronic balance is 0.001 g while the uncertainty of the graduated cylinder is 0.05 mL. The low uncertainty of the measuring devices shows that they are appropriate to be used as it minimizes random error.
Bioplastic Preparation
Five grams of the chitosan powder and a 10% vinegar solution (10 mL of vinegar in 100 mL of water) is first prepared. The chitosan is then mixed with the 10% vinegar solution in a beaker over a hot plate so chitosan can dissolve. Five grams of potato starch and five grams of glycerin is then added to this solution. This mixture is stirred until it becomes homogeneous and thick. In order to form uniform plastic samples, the mixture is poured into a circular mold that is 3.4 cm in width and 0.5 cm in thickness five times. This process is repeated for the other increments of chitosan (10 g, 15 g, and 20 g). The increment of 0 g however, will have no addition of chitosan.
Soil Burial Test
Before conducting the soil burial test, the initial weight of each bioplastic sample is weighed. After this, soil is put into the flower pots and then the samples with 0 g of chitosan are inserted into the soil. Each increment has five samples which also means that each increment has five trials for the soil burial test. The samples are then checked after 10 days. This process is repeated for the other increments (5 g, 10 g, 15 g, and 20 g)
The samples are removed from the soil and are weighed again. For each trial, the weight loss (%) is calculated. The average mass lost is then calculated for each increment and analyzed to see if there is a correlation between the increase of chitosan content and the rate of biodegradation.
There are safety concerns when creating the bioplastic since a metal plate is used. If handled incorrectly, it can result in injury. I ensured that I kept a distance from the hot plate while working and used laboratory tongs to remove the beaker from the hot plate when it was done. After heating, the bioplastic was left to cool before pouring into the mold.
Creating bioplastics can also cause health hazards since there is a possibility of forming mold. Starch is an ingredient for the creation of bioplastics and it is a great source of nutrients for mold to grow and thrive. The exposure to a large number of mold spores may cause allergic symptoms such as headache and fatigue. When mold starts to form in bioplastic, ensure to wear protective equipment such as masks and gloves and to dispose of it right away.
Qualitative Observations
1. When creating the bioplastic the mixture became thicker when more chitosan was added.
2. After the soil burial test, the samples with higher concentration of chitosan were able to retain their color while the samples with lower concentration had a duller color.
3. The samples with a lower concentration of chitosan were thinner when dried. This is because chitosan increases the intermolecular strength of the bioplastics which prevents evaporation. It is noticeable that the samples with 15 g and 20g of chitosan were the thickest.
4. Bioplastics with higher concentration of chitosan were harder but were more brittle.
Purpose of Getting Mean and Standard Deviation
Getting the mean shows the central tendency of a variable since it incorporates all the values from a data set. The mean becomes the representation of how one variable affects another. It is also important in determining outliers in data sets. It allows the researcher to find important information on the study and can indicate incorrect methods of experimentation. This allows a deeper analysis on the phenomenon being researched and suggestions on how to improve the methodology for other researchers.
Standard deviation on the other hand is to see how dispersed data is to the mean. This can be used as an indicator on how strong the data collected is. A low value for standard deviation indicates less dispersion in data which means that the results are all relatively similar. A high value for standard deviation indicates the opposite. The higher the standard deviation, the more dispersed the data set is and this could mean that there is variation in values. From the results of the soil burial test and the tensile strength test, the values of standard deviation for each increment are relatively low. This means that there is consistency in the data.
Pearson Correlation
The hypothesis for how the concentration of chitosan affects the rate of biodegradation of the bioplastic is that, if the concentration of chitosan increases, then weight loss after soil burial test decreases. In Figure 2, there is a downward trend and this is inline with the hypothesis. In order to analyze this data quantitatively, the Pearson correlation for the weight loss (%) was calculated using socscistatistics.com. The Pearson correlation (R) is used in order to measure the strength of the linear relationship between two variables. This is to see if one variable strongly affects the other. The R-value ranges from -1 to 1. When the value is closer to -1, this indicates a negative correlation between the two variables, when it is closer to 1 on the other hand, this means that there is a positive correlation. The R-value for average weight loss is -0.8651 with the P-value of 0.058323 meaning that the result is significant at p < 0.10. This indicates that there is a strong negative correlation between the X and Y values. This means that as the X value increases, the Y value decreases.
Antimicrobial Properties of Chitosan
The data from the experimentation proved the initial hypothesis. The data gathered showed that the increase in chitosan, decreased the rate of biodegradation using weight loss of plastic as an indicator. The reason behind this is most likely due to chitosan’s antibacterial properties. Other starch-based bioplastics easily get degraded since bacteria use the sugars in starch in order to get energy. This phenomenon is seen in the preliminary trial (Figure 1), wherein mold was more evident in the sample with 0g of chitosan when compared to the samples with 10g and 20g. The chitosan slowed down the rate of biodegradation during the soil burial test since microorganisms are responsible for starting the biodegradation process. Though the antibacterial activity of chitosan is not yet fully understood, the most prevalent mechanism of antibacterial activity of chitosan is proposed by Nagy et al. (2011). She proposed that chitosan binds to negatively charged bacterial cell walls causing disruption of the cell, thus altering the membrane permeability, followed by attachment to DNA causing inhibition of DNA replication and subsequently cell death.
Other Possible Reasons for Decrease in Biodegradation Rate
Another reason may be because of chitosan’s low solubility in water. Soil is composed of many components and one of them is water. According to Wu et. al. (2014), chitosan is insoluble at neutral and high pH due to its molecular structure. This makes it comparable to real plastic and it shows that it can be used in different conditions such as for grocery shopping wherein some products may have moisture (e.g. meat and fish).
Evaluation of Procedure
A limitation of this research is that the chitosan powder was bought instead of being manually extracted since the process of chitosan extraction is too time consuming and produces a small yield. Though the supplier of the chitosan powder indicated that it was 100% chitosan, there may be other chemicals used in the creation of the powder. These chemicals may change the bioplastic’s properties. A modification to better increase the accuracy of results is to ensure that there is sufficient time to do both the experiment and the chitosan powder extraction.
Another limitation is that the temperature was not maintained constant throughout the soil burial test. According to Zekri & Chaalal (2005), increasing the temperature leads to the acceleration of bacterial growth which means that temperature affects biodegradation rate. Due to the increase of bacteria, there is a heightened rate of biodegradation. A modification to this limitation is to conduct the soil burial test in an area where temperature can easily be controlled.
The last limitation is that some of the bioplastics broke before doing the tensile strength test which led to an unequal amount of trials among the increments. A modification to this limitation is to create sheets of bioplastic instead of making pieces of bioplastic. Making plastic sheets would make the bioplastic less brittle. This allows it to be more manageable when doing the tensile strength test.
For additional analysis on the effect of chitosan on the properties of chitosan, the tensile strength was measured using a spring balance. Though not directly related to the research question, it is a good indicator to see if it can compare synthetic plastic’s durability.
The results of the tensile strength test (Figure 3) shows that there is a peak in the tensile strength at the chitosan concentration of 15 g. It can also be noticed in Table 4, that there is a lack in a data point in the 15 g increment. This is due to the bioplastic breaking when the metal rod was inserted into it during the tensile strength test. It was noticeable that the bioplastic had a decrease in flexibility and became more brittle when chitosan was added. In research by Ginting et. al. (2018), it was also found that the increase in chitosan had a decrease in bioplastic flexibility. It explained in this research paper that the insoluble chitosan particles reduce tensile strength since these particles reduce the hydroxyl that interacts with amylose and amylopectin. It can be seen in Figure 3 that the chitosan with the concentration of 10 g has high tensile strength and all of the trials were successful. This indicates that this is the ideal concentration of chitosan for durability since it can withstand high forces and is not brittle.
Another extension to this research can be about looking into how to prevent the bioplastic from becoming brittle when a large amount of chitosan is used in order to keep its practical use.
It can be seen in this experiment that chitosan acts as a biodegradation inhibitor as a way to increase bioplastic durability. The decrease of weight loss of bioplastic after the soil burial test (as seen in Figure 2) indicates that there is less biodegradation. Without checking the bioplastic’s weight, it was already evident that the bioplastic had less biodegradation due to its appearance. The bioplastics with higher concentration of chitosan retained its shape and color while those with lower concentration appeared to have decreased in size and had a duller color. This is supported by research done by Dewi et. al. (2023) wherein they tested the effect of additional bioplastic and cellulose on the performance of bioplastic. Their results on the effect of chitosan on biodegradation of bioplastic is that it made it stronger and it took longer to decompose. This is due to chitosan having antibacterial properties and this makes the material resistant to decomposing microorganisms found in soil.
The creation of chitosan-based bioplastic can be a solution to the impending problem of waste management and sustainability in the country and even in the world. Chitosan-based plastic can be used in industries when creating their products since it is durable like synthetic plastic. Unlike synthetic plastic though, it is biodegradable. It can be consumed by soil and other natural components which means that there will be less waste. Finding alternatives to non-biodegradable products is important, since waste pollution can lead to even more issues, such as contamination of drinking water which would then lead to infections and diseases.
A possible extension to this research is to test chitosan-based bioplastic’s water resistance and moisture content in order to have an even deeper understanding on the durability of chitosan-based bioplastics.

Funding

This research was conducted without extra financial support. The costs associated with the study were funded by the author.

Conflict of interests

The author confirms that there are no conflicts of interest related to this research and there has been no significant financial support that could have affected this outcome.

Data Availability

All data generated or analyzed during this study are included in this paper and the supplementary materials can be found in the appendix section.

Figure 1.
Preliminary Trial for Creation of Chitosan-based Plastic
igee-2025-010f1.jpg
Figure 2.
Concentration of Chitosan (g) Against Average Weight Loss (%)
igee-2025-010f2.jpg
Figure 3.
Concentration of Chitosan (g) Against Average Tensile Strength (N)
igee-2025-010f3.jpg
Table 1.
Concentration of Chitosan, and Mass Before and After Soil Burial Test for Each Trial
Concentration of Chitosan (g) ∆W = 0.01g Mass before and after soil burial test (g) ∆W = 0.01g
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5
Initial Final Initial Final Initial Final Initial Final Initial Final
0 0.37 0.24 0.55 0.38 0.50 0.39 0.52 0.40 0.49 0.34
5 0.87 0.76 0.77 0.64 0.72 0.70 0.85 0.72 0.82 0.79
10 0.82 0.80 0.72 0.68 0.77 0.75 0.82 0.79 0.75 0.71
15 1.16 1.14 1.12 1.10 1.20 1.17 1.03 0.98 1.10 1.04
20 1.06 1.05 1.25 1.23 1.07 1.05 1.06 1.03 1.02 1.01
Table 2.
Concentration of Chitosan, Mass Loss (%) of Bioplastics, and the Average Weight Loss (%)
Concentration of Chitosan (g) ∆W = 0.01g Mass loss (%) Average Weight Loss (%)
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5
0 35.14 30.91 22 23.08 30.61 28.35
5 12.64 16.88 2.78 15.29 3.66 10.25
10 2.44 5.56 2.67 3.66 5.33 3.93
15 1.72 1.79 2.5 4.85 5.45 3.26
20 0.94 1.6 1.87 2.83 0.98 1.64
Table 3.
Concentration of Chitosan, Mass Loss in Percent for Each Trial, and Average Mass Loss of Each Increment in Newton
Concentration of Chitosan (g) Average Weight Loss (%) Standard Deviation for Average Weight Loss M%ove - σ M%ove + σ
0 28.35 5.02 23.33 33.37
5 10.25 5.90 4.35 16.15
10 3.93 1.30 2.63 5.23
15 3.26 1.58 1.68 4.84
20 1.64 0.69 0.95 2.33
Table 4.
Concentration of Chitosan, Tensile Strength of Each Trial in Newtons, and Average Tensile Strength of Each Increment in Newton
Concentration of Chitosan (g) ∆W = 0.01g Tensile Strength ∆F = 0.05N Average Tensile Strength (N)
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5
0 10 6.8 8.6 8 8.35
5 5.4 3 5.4 7.8 3.4 5
10 7.8 8 9.4 10 10 9.04
15 10 10 8.4 10 9.6
20 9.2 9.2
Appendix 1.
Samples before soil burial test
igee-2025-010f4.jpg
Appendix 2.
Samples after soil burial test
igee-2025-010f5.jpg

Figure & Data

References

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