1. Introduction
Plastic pollution has become one of our most urgent environmental issues. Due to its escalating manufacturing and widespread application in numerous industries, plastic waste has permeated practically every region of our world, from the deepest oceans to the most isolated wilderness areas. The ramifications of this development are extensive, profoundly affecting ecosystems, human health, and the overall welfare of our planet [
1]. Recognizing the necessity of solving this global issue, academics and environmentalists have been aggressively pursuing creative solutions to mitigate plastic pollution.
Plastics also referred to as organic polymers, consist of elongated carbon chains that form the foundation of their molecular architecture. These synthetic materials are mostly sourced from fossil fuels and comprise carbon, hydrogen, nitrogen, and sulfur, along with different inorganic and organic chemicals [
2].
Plastics are categorized into various types: natural, semi-synthetic, synthetic, thermoplastics, and thermosetting plastics. Plastic mass production commenced in the 1950s, with the majority of polymers first engineered for single-use purposes [
3].
Although beneficial in numerous industrial and consumer applications, the durability and versatility of plastics have prompted environmental concerns. Non-biodegradable plastics can endure in the environment for millennia, substantially exacerbating global garbage accumulation. In 2010, China generated 8.8 million tonnes of plastic garbage annually, representing 27% of global production [
4]. Indonesia generated 3.2 million tons of plastic garbage annually, accounting for 10% of the global total. Plastic has emerged as a substantial element of Indonesia's everyday refuse, comprising roughly 15% of its municipal waste [
5]. In 2018, the European plastics industry indicated that global plastic production reached 335 million tons, with Europe accounting for 60 million tons of this significant figure. Furthermore, these output figures are anticipated to rise significantly in the forthcoming decades [
6].
Although plastic is now essential to our daily existence, its durability and prevalence provides a significant environmental hazard. Due to their remarkable endurance, plastics remain insoluble for years, causing significant environmental damage as they gradually decompose into tiny particles. This persistent problem has led to an increase in plastic trash that jeopardizes numerous animals, including humans. In addition to contaminating our landscapes and waterways, plastic garbage imposes considerable environmental expenses and public health risks through incineration. Incinerating plastic trash emits toxic chemicals, such as carbon dioxide and dioxins, which are associated with respiratory disorders and cancer [
7].
Plastic pollution, a widespread problem that crosses borders and cultures, requires efficient waste management and mitigation techniques. Although minimizing, recycling, and reusing plastics have become prevalent strategies for addressing the issue, there is still a necessity for more effective techniques, especially for mixed plastic trash. Plastic garbage disposal in landfills or incinerators occupies considerable space and poses the risk of releasing toxic gases into the environment. Consequently, it is essential to devise recycling technologies that are both efficient and environmentally sustainable. Biodegradation has surfaced as a viable and economical solution to this global issue [
8].
As almost all waste materials flow through the sewage, sewage wastewater contains bacterial strains that have the potential to degrade plastic—the current study aimed to isolate polyethylene-degrading bacteria from a sewage wastewater treatment plant. This enzymatic method of plastic biodegradation is economical, environmentally friendly, and sustainable.
4. Discussion
When plastic pollution entering an area surpasses the rate of natural elimination processes or cleanup operations, plastic accumulates in the ecosystem. Natural degradation processes for plastics can take decades or even millennia [
25]. There is an increasing problem with marine contamination due to solid waste on a global scale. This problem affects generations to come [
26].
Marine litter is defined as any human-made solid waste in the ocean, whether on land or at sea. This includes materials transported to the ocean via rivers, drains, sewage, wind, or water systems but does not include organic materials like food and vegetable scraps [
27].
The current study aimed to isolate and characterize polyethylene-degrading bacteria in sewage wastewater. This is because sewage is one of the biggest sources of natural water pollution and potential microbes for removing toxicants. We collected the sewage samples from the wastewater treatment plant to isolate plastic-degrading bacteria. After initial screening and 120 days of incubation with polyethylene, we found that sample SH4 reduced plastic by 21.6%. After 16S rRNA gene sequencing, this strain was detected as Bacillus tropicus.
The study by Mukhaifi et al. (2021) sought to isolate and characterize bacteria capable of degrading polyethylene terephthalate (PET) from Shatt al-Arab water and sewage in Basra, identifying the bacteria as
Klebsiella pneumoniae. The results indicated a statistically significant difference in PET degradation, with a 24% reduction over 7 days, which grew to 46% after 4 weeks in comparison to the control group. These results are correlated with the current study [
28].
Meng et al. (2024) identified a novel marine strain of
Pseudalkalibacillus sp. MQ-1 is capable of degrading polyethylene (PE) up to 6.37% in 60 days. Scanning electron microscopy and water contact angle analyses demonstrated that MQ-1 may cling to polyethylene films, rendering them hydrophilic [
29].
GC-MS analysis was used to determine which polyethylene products had deteriorated. They were found when the compounds that had the strongest correlation with the Wiley Library database and the MS fragmentation pattern were compared [
Figure 3]. We performed the GC-MS analysis to detect compounds in MS media produced due to the biodegradation of polyethylene. Many new compounds were produced by
B. tropicus activity [
Table 2]. We neglected the compounds that were common with the control.
Plastic polymers' biodegradation byproducts following 140 days of incubation were investigated using gas chromatography-mass spectrometry. Three compounds were identified in this study: cis-2-chlorovinyl acetate (7.11 min), tri-decanoic acid (21.43 min), and octa-decanoic acid (22.46 min). The presence of tri-decanoic and octa-decanoic acids in the material under investigation implies the biodegradation process involves the creation of carbonyl groups. These groups are further oxidized to produce ketones and aldehydes, as confirmed by nuclear magnetic resonance (NMR) analysis. Thus, the current research shows that fatty acids and other metabolic intermediates are important for microbial consortiums to biodegrade plastic. It was determined that the end-products in this investigation did not pose any health risks [
30].
Shahnawaz et al. (2016) identified 1-trimethylsilylmethanol, 1,2,3-trimethylbenzene, ethyl-3,5-dimethylbenzene, hexadecanoic acid, 1,4-dimethyl-2-ethylbenzene, and 1,2,3,4-tetramethylbenzene [
31]. Roy et al. (2008) cultivated a consortium of
Bacillus pumilus, Bacillus halodenitrificans, and
Bacillus cereus on polyethylene particles, revealing the presence of both oxygenated chemicals and unoxidized low molecular weight hydrocarbons [
32]. Roy et al. (2008) identified alkanes, fatty acids, and ester-containing compounds as products of biodegradation by
Pseudomonas putida, Pseudomonas syringae, and
Pseudomonas aeruginosa [
32].
Our study also incorporated FTIR analysis to observe the changes in plastic chemical structure after degradation. FTIR spectra showed bond bending and bond stretching of the C-C bond of alkane, C=C of alkene, and N-O of the nitro compounds. In the study of Khandare et al. (2021), the chemical changes in the LDPE structure were investigated using FTIR. If you want to know what functional groups are in LDPE or any other biodegradable molecules, you can use the FTIR analysis to see changes in the carbon backbone [
33].
Previous research has made heavy use of this method, as seen by the work of Nadeem et al. (2021), who demonstrated a decrease in transmittance between 1100 and 1150 cm
−1, suggesting the formation of new (-C-O-C) bonds due to the weakening of C-C bonds. Except for the untreated control, all LDPE film FTIR spectra exhibited the production of a characteristic carbonyl peak at 1,712 cm
-1, which was significantly diminished following 90 days of bacterial incubation. The process of plastic biodegradation can be better understood by observing the presence and disappearance of carbonyl peaks [
34]. In another investigation, Selke et al. (2015) also noticed a reduction in the creation of a carbonyl peak at 1,712 cm
-1 after 90 days of treatment with bacteria [
35]. In order to understand the basic process of biodegradation, previous studies have shown that new functional groups can appear and disappear in both treated and control LDPE films, which supports the current study's conclusions [
24,
36,
37].
The SEM examination allowed us to observe the surface morphological changes on the LDPE film following 120 days of treatment with sewage bacteria. The scanning electron micrographs (SEMs) revealed surface degradation, fragility, damaged layers, cracks, and scratching in the LDPE film treated with any of the four marine bacteria compared to the control film, which remained smooth, undamaged, and clear. Sanin et al. (2003) found that FE-SEM images of synthetic polymer (LDPE) fragmented into monomeric forms when incubated with certain bacterial strains. These strains included
Rhodococcus corallinus strain 11,
Pseudomonas sp. strain A, and
Pseudomonas sp. strain D. Using AFM images, it was also noted that all LDPE films treated with bacterial isolates developed cracks, grooves, and roughness after the same incubation condition, whereas the control film, which was not supplemented with any bacteria, remained smooth and intact. This confirms that the degradation is caused by enzyme activity. Results from scanning electron microscopy (SEM) and atomic force microscopy (AFM) demonstrate that, under low-nutrient conditions, bacterial isolates cling to surfaces and use C-source from LDPE substrates [
33,
38].
Author Contributions
For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “Conceptualization, S.A. and I.; methodology, S.A.; software, S.A.; validation, Y.-C.C., S.A. and I.; formal analysis, Y.-C.C.; investigation, S.I.; resources, S.A.; data curation, I.; writing—original draft preparation, I.; writing—review and editing, I.; visualization, I.; supervision, Y.-C.C.; project administration, S.A.; funding acquisition, Y.Y. All authors have read and agreed to the published version of the manuscript.” Please turn to the CRediT taxonomy for the term explanation. Authorship must be limited to those who have contributed substantially to the work reported.