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Young Scientist Journal

The Addition of Various Metal-Oxide Coatings on g-C3N4 and Its Correlation with Its Photolytic Ability to Degrade Polyvinyl Chloride (PVC) Plastic

ABSTRACT

Plastic pollution of macroplastics is becoming an ever-increasing issue as plastics begin to reach the end of their shelf life and degrade. Photocatalyst graphitic carbon nitride has shown promise as a method to degrade the carbon carbon bonds within the molecular structure of polyvinyl chloride plastic study aims to optimize this reaction through the use of various metal coatings. Iron (III) Nitrate Nanohydrate, Cobalt (II) Nitrate, Copper (II) Nitrate Hemi-pentahydrate, and Nickel (II) Nitrate were the metal oxides that were added to the surface of g-C3N4.The solution was dried, baked, and then pressurized with oxygen before the resulting supernatant was examined using gas chromatography. The metal coated graphitic carbon nitride was found to have an effect on the PVC, with Nickel Nitrate having the highest secondary peak and being the most likely contender for further study.

INTRODUCTION.

Plastic pollution has been an ever-increasing issue in today’s world [1]. As of right now, research suggests that the global waste generation has climbed to approximately 1.7-1.9 billion metric tons per year [2]. This number is expected to keep rising to about 27 billion tons by 2050 [2]. With this crisis, a good fraction of the waste comes from single use plastics [1]. In fact, globally, 450 million tons of plastic are being produced annually [3]. Microplastics and macroplastics have only recently become an emerging issue due to plastic from decades ago decomposing into smaller pieces [4]. Plastic tends to degrade into microplastics due to exposure to ultraviolet radiation and weathering. Because of this exposure, these microplastic pollutants have become a more present issue due to research showing that microplastics compound in the bodies of organisms [4]. In light of this issue, there have been increased efforts to eliminate the still growing macro plastic pollution from our oceans (as well as other bodies of water) to decrease the presence of microplastics in the future. To prevent microplastics from entering the environment, the best way to combat this environmental crisis is by going to its source, which is plastic pollution. PVC plastic is difficult to recycle and for this reason it is not economically viable without an improved method, such as a catalyst. The aim of this study is to investigate the effectiveness of modifications of photocatalyst graphitic carbon nitride (g-C3N4) to degrade polyvinyl chloride (PVC) plastic. It is hypothesized that, at the conclusion of this study, all of the metal-oxide coated catalysts will showcase a positive correlation with PVC plastics’ degradation.

MATERIALS AND METHODS.

Making g-C3N4.

To make the g-C3N4 [3,5,6,7] photolytic catalyst, analytical grade urea was baked. The temperature started at room temperature ramping at 300 ⷪC/hour until reaching 450ⷪC. Then the temperature was held at 550 ⷪC for 2 hours [14].

Making the Solution.

To prepare the solution, 0.00645 moles of each metal was used equating to 0.3602 grams of Iron (III) Nitrate Nonahydrate [2], 0.3801 grams of Cobalt (II) Nitrate, 0.4099 grams of Copper (II) Nitrate Hemi-pentahydrate [6], and 0.3786 grams of Nickel (II) Nitrate. The amount was weighed on a scale reading out to the hundredth place, for accuracy purposes. Once the respective metal was placed into a 10-milliliter test tube, the same amount of 1, 10 phenanthroline was measured and added to the solution. The amount of 1, 10 phenanthroline was changed to mirror the amount of its respective metal added (for example: 0.3786 grams of 1, 10 phenanthroline was added into the solution with Nickel (II) Nitrate. After the 1, 10 phenanthroline is added, 15 mL of 95% lab grade ethanol was added to the solution.

Preparing the Catalyst.

Once the solution was made, it was placed on a hot plate and a magnetic stir bar was added to a standard 15mL test tube. The solution was stirred for 30 minutes before adding 1 gram of g-C3N4 to the test tube. The sample was transferred to a round bottom flask and refluxed for 5 hours at 60 ⷪC [8]. The catalyst was then moved into a conical centrifuge tube where it was run for 5 minutes. The solvent was decanted, leaving the solute in the conical tube. The mixture was then resuspended with ethanol and transferred into a porcelain crucible. A stir bar was added into the porcelain crucible as the crucible was placed onto a hot plate and gently stirred until the mixture was 50 ⷪC [6,9]. The precipitate was heated for 2 hours until dried. If the precipitate failed to dry within 2 hours, it was then placed into an oven at 80 ⷪC to heat overnight [10]. After this, the dry solid was ground manually in a porcelain spice grinder to increase the surface area of the catalyst. The dry solid was then baked at 450 ⷪC for 2.5 hours [5]. The temperature was ramped at 700 ⷪC/hour, stopping the ramping once it reached 450 ⷪC (ramping stopped after 30 minutes). The crucible was kept in the kiln until it reached room temperature.

Pressurization.

After the solution was cooled to room temperature, 0.025 of the dried solid catalyst was measured and placed into a pressure vessel. 1 gram of lab grade PVC plastic [3,7] was measured and added into the pressure vessel. Finally, 15 mL of methanol was measured using a 10 mL graduated cylinder and was added into the pressure vessel along with a small round magnetic stir bar. The pressure vessel was then pressurized with oxygen at 200 kPa (2 atmospheres) at 100ⷪC for 4 hours [11]. The apparatus was set under simulated sunlight in the form of a warm lighted lamp for 24 hours in order to induce photolytic degradation of the PVC [12].

Setting Up Catalyst for Analysis.

Once the pressurization step was completed, the solution was given time to separate out, and the supernatant was removed. The cleaned supernatant for the chromatography sample was prepared using a 0.22 micrometer PTFE syringe filter. The supernatant was then placed into a test tube and capped.

Gas Chromatography Prep for Analysis.

3 ul of the solvent was extracted from each sample for analysis via SRI instruments gas chromatography. The ramping parameters used for the gas chromatography analysis is as follows: an initial temperature of 35°C, with a hold on that temperature for 5 minutes. Then the temperature is ramped at 100 ⷪC/minute until reaching 200 ⷪC. The temperature is then held at 200 ⷪC for 15 minutes. The samples were run in a silica packed column with a helium flow rate at 5 PSI. Unfortunately, mass spectrometry was not able to occur during this study due to the only mass spectrometer in the lab being broken.

RESULTS.

The gas chromatograms below were all run under the same conditions listed above. The y-axis of each sample is representative of the height of the peaks noted in each sample, while the x-axis is indicative of the retention time each peak occurred at. This helped to see if there were trends between samples when it came to the time each peak occurred at.

Controls.

Water served as a negative control (Figure 1). It was used to ensure that the chromatograms being produced had as little error as possible. Water was also used to flush out any remaining compounds or materials to minimize possible contaminants in the sample. As seen in this sample, while a peak was visible, it was minimized after multiple flushing attempts. Methanol served as a positive control in this study (Figure 2). It was the baseline for all other samples as it allowed for the identification of the approximate time the solvent peak would appear on the chromatogram.

Figure 1. A gas chromatogram of source water as control. A small negligible contamination peak at 7.083 minutes was noted in the water sample. The peak seen at 7.083 was present after multiple flushing attempts of water. This image was the closest attempt to achieving no visible peaks.

 

Figure 2. A gas chromatogram of methanol as a control. The methanol peak was observed at 8.183 minutes, the time noted at 13.150 minutes indicates the end run time of this sample.

Metal Coatings.

For the iron (III) nitrate coated catalyst, no additional peaks were observed alongside the methanol solvent peak (Figure 3). A lack of additional peaks displays that no other compound had successfully passed through the column besides the initial methanol (as seen by the similar results of figure 2 and 3). This contradicts the findings of the study done by Pengfei Guo due to how his study positively saw iron (III) nitrate coated g-C3N4 degrade PET plastic [11], while in figure 3 there is no observable evidence to suggest a reaction took place. Of course, it is necessary to mention how there may have been a reaction that took place, but it was simply one that was unable to be identified by the gas chromatography machinery. Because of this possibility, more testing will be needed in the future to fully determine if iron is a feasible metal-oxide to coat g-C3N4 with.

Figure 3. Gas chromatogram of iron (III) nitrate coated g-C3N4. The chromatogram shows only a single peak at 9.166 minutes that can be attributed to methanol (see Fig 2), with no additional peaks from iron (III) nitrate.

The cobalt (II) nitrate coated catalyst, 3 additional peaks were observed alongside the initial methanol solvent peak (figure 4). These additional peaks support the findings of Pengfeu Guo. The additional peaks display the presence of additional unidentified compounds, shown in the peaks presented at 9.450 minutes, 10.266 minutes, and 13.016 minutes (figure 4). These unidentified peaks display that there is a compound within the catalyst-methanol solution that is not methanol, prompting us to draw the conclusion that figure 4 is evidence supporting the conclusion that a reaction did take place. However, due to limited resources in the form of analysis equipment, there was no further way to identify what these unidentified compounds were. Further testing with adequate equipment will be needed in the future.

Figure 4. Gas chromatogram of cobalt (II) nitrate coated g-C3N4. Two secondary peaks are noted to the right side of the initial methanol peak (see figure 2). Methanol peak observed at 9.183 minutes, next peak is observed at 10.208 minutes, last peak observed at 13.018 minutes.

With the nickel (II) nitrate coated catalyst, 4 additional peaks were identified. Similarly to figure 4, in figure 5 there were peaks present at similar times. In figure 5 there were peaks identified at 9.416 minutes, 9.783 minutes, 10.223 minutes, and 10.083 minutes. The 1st, 3rd, and 4th additional curves were very similar to the ones present in the cobalt sample, prompting the conclusion that these specific peaks are similar compounds, if not the same ones. While this cannot be confirmed due to limits in analysis methods, due to the close time proximity, it can be believed that these are similar compounds. The 2nd additional peak may be a different unidentified compound that could have resulted from the differing metal coating. The results from figure 5 also support the conclusion that additional reactions occur when metal coatings are added.

Figure 5. Gas chromatogram of nickel nitrate. A significantly higher secondary peak is noted in this sample. Initial methanol peak observed (see figure 2 for reference) between 9.300-9.416 minutes, next peak observed at 10.233 minutes, final peak observed at 13.083 minutes.

Finally, the copper (II) nitrate hemi-pentahydrate coated catalyst. In figure 6, 3 additional peaks were observed alongside the initial methanol solvent peak (figure 2). This sample was different from those in figure 4 and figure 5 due to how there is no peak present within the 10-minute interval, which offers the conclusion that whatever unidentified compound was present at the 10-minute interval in figure 4 and figure 5, was not present in the copper (II) nitrate sample. However, there was still the consistent peak present at 13.083 minutes, meaning that there are some consistent unidentified compounds being produced for figures 4,5, and 6. These additional peaks present in this sample also support the conclusion that a reaction took place in this sample.

Figure 6. Gas chromatogram copper nitrate. Secondary peaks noted towards the left side of the graph. The initial methanol peak was observed at 9.300 minutes (see figure 2 for reference), the next peak was observed at 9.883 minutes, and the final notable peak was observed at 13.150 minutes.

Limitations.

During this study, multiple limitations were present. The first mentionable limitation would be lack of access to a working mass spectrometer. This limitation played a very big part in the lack of analysis that was able to be done. Due to the lack of a mass spectrometer, it was impossible to figure out what the secondary peaks in the sample represented. This drastically stunted the analysis that was able to happen in this study. While it was a shame that mass spectrometry was not available, it was still possible to make do with only gas chromatography, though the analysis is weakened due to the absences of mass spectrometry. Another limitation that could have influenced the analysis of the catalysts was that the gas chromatography instrument used was also used by several other students within my science research department. Because of this, it is possible that the chromatography instrument had not been cleaned properly after every other student, which could have caused unwanted contaminants in the sample, therefore producing unexpected peaks or variations in the results of the samples.

CONCLUSION.

From the peaks shown through various trials, it can be concluded that the Nickel Nitrate coated g-C3N4 showed the most promise for future studies due to it being the sample with the highest secondary peak. Furthermore, the Copper Nitrate and Cobalt Nitrate samples also showed a lot of promise with notable secondary peaks. However, the Iron Nitrate showed little to no secondary peak in these samples, which should be investigated further as it seems to indicate an inconsistency with a similar study done by Guo [11]. In conclusion, Nickel Nitrate proved to be the metal-oxide coating that produced the most of an unknown material after having reacted with PVC plastic.

Implications.

The goal of this catalyst was to identify if the metal-oxide catalysts were producing unusual results. The original reason for this was in hopes to aid recycling efforts, when it came to PVC plastic, in the future. The results discussed in this study proved that some unknown compound is being made after reacting with PVC plastic. While further analysis and testing is needed, as of now, this provides a current positive correlation between potential degradation and metal-oxide coated g-C3N4. It can’t be proven as of now that the catalyst is degrading PVC plastic, but by looking at the trends visible in the samples, it can be inferred that there is potential degradation. However, there is still much more work to be done in this field of study before the proper implications can be considered when speaking on the long term. The experiment should be replicated to further enforce validity of the results obtained. However, if during future studies, metal-oxide g-C3N4 is shown to degrade PVC plastic, recycling for this previously stubborn catalyst could increase the rates of recycling in the long term. This could, in theory, reshape how the packaging industry and other plastic manufacturers view recycling as a whole. This would be an optimistic implication to consider when speaking of possible future discoveries. However, there is also the worry that the compound being produced during this reaction is toxic or harmful to the environment, in which case it would be doing more harm than good. In the end, I had predicted that there would be unknown compounds produced for all the metal coated catalysts. This hypothesis turned out to be partially correct as 3 of the 4 catalysts did display secondary peaks. So, the conclusion and analysis of this study supported the initial hypothesis.

Future Directions.

When it comes to this study, there are a lot of directions that could be taken in the future in order to modify the study. Studies such as the one done by Ariza-Tarazona suggested that the metal-oxide of titanium could also yield interesting results [6]. Due to this study, titanium could prove to be an interesting alternative metal to analyze and incorporate in the future. Another thing that could have been changed was the temperature. There are several studies that cite different temperatures that can be used. Such as Chen’s study in which g-C3N4 was baked at 135°C to have an LMPET sheath stick onto the catalyst [10]. 135°C is only one example of the potential temperatures that could be used in the future. However, it is possible that different temperatures could lead to differing and interesting results. Finally, one major flaw of this study was the lack of mass spectrometer results. Due to this, in the future, it would be a wise decision to continue this study by doing mass spectrometer analysis. This would allow for more accurate analysis and would give a better idea as to the compounds that are being produced.

ACKNOWLEDGMENTS.

A special thank you to Massapequa High School’s science department for allowing me the privilege to be in such an amazing science research program. Furthermore, I would like to thank my science research professor, Dr. Paul Hesleitner, for aiding me with this project over these past 3 years. Lastly, I would like to thank Mrs. Alice Hadad for supporting me over the last 2 years and for encouraging me to pursue publishing my work. Thank you to all the amazing people that have allowed me to be where I am today.

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Posted by on Tuesday, June 30, 2026 in May 2026.

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