The Occurrence and Fate of Microplastics in Wastewater Treatment Plants in South Africa and the Degradation of Microplastics in Aquatic Environments—A Critical Review

Abstract

The occurrence of microplastics (MPs) and their omnipresence have attracted enormous attention across the globe; however, research on MPs in South Africa is still in its infancy and thus, the data are still very much lacking. Approximately 120 million tons of general waste is generated each year in South Africa, which exacerbates the pressure on the ability of municipalities to manage waste. Wastewater treatment plants (WWTPs) are at the center of this general waste that includes plastic debris and primary MPs that are discharged from households and industries. In general, the removal efficiency of MPs across the globe was found to be between 70% and 99%, with biological treatment technology common in both primary and secondary treatment steps in WWTPs. Furthermore, the current review paper has revealed that there is a wide research gap on the occurrence and fate of MPs in WWTPs across South Africa. This is a worrying factor considering the increasing rate of plastic waste generation due to rapid industrialization, urbanization, and overpopulation in the country. More so, the scarcity of data regarding the occurrence of MPs in freshwater is widely noticeable across the country. Therefore, given the amounts of MPs discharged from households, industries, and plastic debris littered into the surface waters, the data on the occurrence and fate of MPs in South Africa should be given the urgent attention they deserve. To achieve the effective and sustainable management of water resources and services set out in the National Development Plan (NDP) 2030 adopted by the government, the status and capabilities of WWTPs to remove MPs must be monitored and addressed. In addition to achieving the zero waste to landfill goal by 2030, a circular economy is regarded as the most effective model for solving the plastic waste crisis in the environment; therefore, its full implementation is required for a substantial impact.

1. Introduction

Microplastics (MPs) continue to draw attention across the globe due to their omnipresence in our natural environments; however, the research on MPs in South Africa is still in its early stages and thus, the data are still very much lacking. Meanwhile, the constant exponential increase in plastic production, especially in personal care products, car tire manufacturing, and packaging industries, has necessitated a call for strict plastic regulations as well as deep research on health effects [1]. With the current waste management in place failing, plastic waste will inevitably end up in the environment, thus resulting in a plastic pollution crisis. Due to abiotic and biotic factors, plastic debris is subjected to biodegradation, photo-degradation, mechanical abrasion, and photo-oxidative degradation, which lead to breaking down into small particles [2]. Microplastics are small plastic particles that are less than 5 mm in size and are produced from various industrial processes and the fragmentation or breakdown of plastic debris [3,4,5]. On the other hand, nanoplastics (NPs) (ranging from 1 to 100 µm) have also attracted a huge interest due to their extremely small size [6]. While a significant number of studies across the globe have reported on the occurrence of MPs in the soil [7,8] and aquatic environments [9,10], recent studies have highlighted atmospheric medium as an equally concerning MP pathway in the environment [11,12,13].

This review seeks to uncover and address concerning research gaps on MPs in South Africa, particularly on the degradation of plastics in aquatic systems and the status and capability of municipal wastewater treatment plants (WWTPs) to remove MPs. Meanwhile, it is also imperative to unpack and link human factors that aggravate the occurrence of MPs in the aquatic systems, considering the rapid increase in immigration, urbanization, and overpopulation in big metros across the country.

In recent years, the world has seen an overwhelming increase in industrialization, which undoubtedly translates to an increase in economic growth. However, the downside of this rapid increase in industrialization is the large production of plastics products (as shown in Figure 1) [14]. Millions of tons of plastic particles are reportedly produced by industries and households each year, and they mainly end up in WWTPs [15,16]. Furthermore, the increasing rate of urbanization and overpopulation in cities have been reported to further exacerbate the plastic pollution crisis, thus increasing MPs in the environment [17,18]. About two-thirds of the world’s population is estimated to live in cities in the next 15–20 years [18,19,20]. Equally concerning is the disposal of malpractices of used plastic products from health sectors, such as clinics and hospitals, and the discharge of sewage into rivers [21].

Although significant attention has been given to the marine environment in the coastal regions (e.g., Kwa-Zulu Natal and Western Cape provinces), very limited information is available on the occurrence of MPs in the inland rivers in provinces such as Gauteng province. Similar observations were made by Singh, et al. [23], where the MP studies in the coastal regions were found to be more than twice the studies in the inland provinces. Figure 2 summarizes MP research reported in the literature in South Africa. Based on this information, there is a clear wide knowledge gap on the occurrence, behavior, and toxic effects of MPs in the country. More so, the information on the occurrence of MPs in WWTPs remains untapped and poorly understood. In addition, research on MPs in the inland provinces is still lacking and very scarce, while more focus has been on coastal provinces, such as Kwa-Zulu Natal and Western Cape.

In this review paper, a systemic literature review was conducted to investigate the nature and occurrence of MPs, including their toxic effects and bioaccumulation, in the food chain. In addition, this review paper focused on the background and information deficit of MPs in South Africa. The databases used to search for publications such as peer-reviewed or refereed articles, book chapters, and abstracts for this review paper included Google Scholar (118), ScienceDirect (22), and Elsevier (7). In addition, the researchers selected publications published in English from 2010 to date to include recently published papers; meanwhile, keywords such as “Microplastics”, “Wastewater treatment plant”, “Aquatic environment”, and “Polymer” were used to collect the information. Furthermore, the common Boolean operators, which include “AND” and “OR”, were used during the search. In addition, citations within published papers were also used to find more papers.

1.1. Nature of MPs Found in the Aquatic Environment

Microplastics (referred to as plastic particles less than 5 mm) come from a wide variety of products and have different chemical compositions, specific densities, colors, and sizes. Consequently, MPs are classified into two classes: primary and secondary MPs.

1.1.1. Primary MPs

Primary MPs, which are specifically produced as small particles, originate from the pre-production of pellets, personal care products (PCPs), household cleaning products, and glitters [24,25]. Furthermore, primary MPs are added to kinds of toothpaste, face cleaners, soaps, bath/shower gels, face scrub, eye shadow, baby products, eyeliner, lipgloss, deodorant, sunscreen, and many other products [26,27,28]. The daily use of these products is estimated to be equivalent to 2.4 mg of MPs per person each day, which corresponds to over 1.5 million tons per year [29]. The last few decades have seen a drastic increase in lifestyle change where beauty and personal care are at center stage for both men and women. Based on the above information, it is possible to link this increasing habit to the fast-growing cosmetics industry. In 2013, the South African cosmetics industry accounted for 6% (ZAR 17 billion) of the manufacturing industry, according to Dalziel et al. [30]. In 2018, Gebashe et al. [31], reported that the South African beauty and personal care market industry was estimated at USD 3.5 billion, and it is expected to reach USD 6.16 billion in 2024. The ever-growing online fashion influencers are considered to be drawing strong interest in cosmetics products among young women. In addition to MPs, Wang et al. [32] described primary MPs as not only intentionally produced MPs but also by-products resulting from the use of plastic products (e.g., abrasion of tires, microfibers detached from synthetic materials, etc.). In a study by Liu and co-authors [33], concentrations of MPs in the soil along industrial roads were reported to be between 0.03% and 6.7%.

1.1.2. Secondary MPs

Secondary MPs are produced from breaking down plastic debris, and they originate from fishing activities, food packaging, clothing, medical products, textiles, carry bags, agricultural materials, beverage bottles, and construction materials [34,35,36]. Due to a wide variety of plastic debris, secondary MPs vary in terms of color, shape, and polymer type [37]. Considering the extent of plastic pollution in the environment, secondary MPs are believed to be responsible for most of the MPs in aquatic systems [38]. The most common pathways of MPs into the environment include storm water, wind, currents, sewers, and the degradation of large plastic debris from landfills and illegal dumping sites [39]. When the plastic debris is exposed to abiotic factors, such as UV radiation, temperature, microbial degradation, and atmospheric pressure, it results in stress on its structure, thus leading to degradation [40,41].

Both primary and secondary MPs have been reported to be carriers of other toxic pollutants, such as persistent organic pollutants (POPs), heavy metals, dyes, endocrine disrupting compounds (EDCs), polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), and pharmaceutical pollutants, owing to their high surface area and hydrophobic nature [42,43,44,45]. Meanwhile, the adsorption of these chemicals onto MPs, which has been extensively reported by many researchers [46,47,48], may cause adverse effects such as cancer, disturbed reproductive system, immune system dysfunction, respiratory problems, and a disturbed nervous system [49,50]. The adsorption of chemicals on MPs was studied by Du and co-authors [51]. A soluble dye (Rhodamine B) was allowed to interact with MPs (polyvinyl chloride, polyesterene, and polyterethlate) for two days. The study found that the adsorption of Rhodamine B on MPs was related to the surface charge, hydrophobicity, and functional groups, with the adsorption capacity of MPs inversely proportional to MP sizes. Furthermore, MPs, especially secondary MPs, may contain toxic additives that are added during synthesis [52]. These additives have been widely reported to be harmful due to the hazardous chemicals incorporated in them (e.g., plasticizers, flame retardants, dyes, etc.) and thus pose serious health risks to human and aquatic life [53,54,55].

1.2. Degradation of Polymers in the Environment

1.2.1. Physicochemical Degradation

Physicochemical degradation (abiotic) involves the breaking down of plastic debris as a result of light, temperature, water, and mechanical forces. Mechanical degradation, which is one of the main degradation processes, involves the collision and abrasion or traction of plastic debris with rocks and sand as a result of waves [56,57]. The majority of MP fibers in the aquatic environment are a result of abrasion and shear during domestic washing [23,56]. In addition, photo-degradation causes plastic particles to be brittle, leading to mechanical breakdown into micro and nanoplastics [58,59]. Photo-degradation is considered the most important process that initiates the degradation of plastic debris [60]. Ultraviolet (UV) rays and heat from the sunlight play a major role during the photo-oxidation reaction of plastics. Polyethylene (PE) is reportedly resistant to photo-degradation due to a lack of chromophores; however, the effects of weathering might act as chromophores [56,61].

The mechanical degradation of plastics is the breakdown of plastic debris as a result of external forces and involves the collision and abrasion of plastic debris against stones and sediment due to waves (in the water) and wind (on land) [57]. Thermal degradation, on the other hand, occurs when the plastic receives high energy as a result of exposure to elevated temperatures. The type of reaction during thermal degradation is known as a thermo-oxidative reaction [56].

1.2.2. Biodegradation

Biodegradation generally refers to the breakdown of plastic particles by microorganisms [62,63]. During the degradation of plastics, the following characteristics of plastics are considered: molecular weight, functional groups, additives, crystallinity, and mobility [64,65]. The microorganisms are thus capable of biodegrading MPs without causing harm to the surroundings where the process takes place [66]. Moreover, they can degrade plastics through the direct ingestion of plastic particles as a source of food and also by indirect actions of different microbial enzymes [67]. The biodegradation by these microorganisms leads to a reduced molecular weight of the target plastic, and thus the plastic polymer is converted into its monomers, as shown in Figure 3. Thereafter, the monomer breaks into carbon dioxide, water, and methane, a process that is known as mineralization [64,68]. The biodegradation of polymers tends to decrease as the molecular weight increases [69]. This is because of the decrease in solubility, which then makes it difficult for microbial attack. However, many studies have reported that this degradation by microorganisms can take thousands of years to complete [70]. Lastly, the growth of fungi around the polymer can lead to the swelling and breaking of the polymer [71]. Although there are significant studies that have investigated microbial degradation of MPs, a clear understanding of the interactions between microorganisms and MPs is still under scientific review.

Effects of Biofilms on MP Degradation

Microplastics can interact with various pollutants such as inorganic particles, organic matter, and microorganisms in the aquatic environment, which can result in the formation of biofilms on the surface of MPs, which is also referred to as biofouling [66]. These biofilms include microbial cells, bacteria, fungi, algae, and protozoa, which are covered by extracellular matrix [73]. Although biofilm formation may shield MPs from UV light, they have the capability to biodegrade the polymer [74], as illustrated in Figure 4. The formation of mono- or multispecies communities of microbes on the surface of the MPs leads to biocorrosion of the MPs [75]. Meanwhile, biocorrosion, which eventually leads to fragmentation of the MPs, is controlled by extracellular enzymes produced by these microbes. Plakunov and co-authors [75] noted that a diverse group of microbial communities provides enhanced biodegradation compared to an individual microbial. This finding was also reported in a study by Miao et al. [76]. In addition, the rapid formation of microbial biofilms was reported to have been observed within one or two weeks [76]. Moreover, the study revealed that the general presence of bacterial communities on MPs was significantly lower than on the natural substrate. Therefore, this may lead to the MP-associated biofilm community being less likely to maintain sufficient microbial activities required for MP biodegradation. In a study by Morohoshi and co-authors [77], biofilms were formed by soaking plastic films in freshwater samples mixed with NH₄Cl and KH₂PO₄, and the mixture was subsequently incubated for two weeks until biofilms were formed. The results revealed a loss of transparency on the poly(3-hydrobutyrate-co-3-hydroxyhexanoate) (PHBH) surface, while a rugged surface and holes were observed on the PHBH films. Furthermore, Burkholderiales, such as Acodovorax and Undibacterium, were found to be degrading bacteria. More developments of MP degradation by microbial communities have been substantially reported in the literature [69,78,79,80,81]. Even then, there is a clear research gap, indicating a need for more research on the occurrence of MPs in freshwaters in order to improve understanding of their fate and impact on the aquatic ecosystem. Additionally, the colonization of microbial communities on MPs in freshwaters has yet to receive full attention [76].

1.3. Recent Trends of Plastic Pollution in South Africa

South Africa (SA) is one of the most industrialized countries in Africa and has a population estimated at 60.6 million in 2022 [82], an increase of 2.87 million from 57.73 million in 2018 [83]. According to the SA Plastic Pact, only 43.2% of the total plastic waste that reached the SA market was recycled in 2020, while 81.2% of the total packaging plastics that reached the market were recycled in 2021 [84]. Packaging plastics, which contribute more than 50% of plastic production (Figure 5), barely reach the market, while the ones that reach the market are also difficult to recycle, according to SA Plastic Pact [84]. In addition, although the report has reported a positive trend in recycling, the lack of waste collection, particularly in townships and informal settlements, remains a great concern and thus, a setback to plastic recycling efforts. Meanwhile, the concerns from businesses regarding the effects of recycled plastic products on consumers’ choice, as well as the public’s lack of appetite for plastic recycling, hinder plastic recycling efforts and thus, contribute to plastic pollution in the environment. In efforts to curb plastic waste generation, the government has implemented a shared National Economic Development and Labour Council (NEDLAC) agreement, where the government, businesses, and organized labor signed a memorandum of understanding [85]. However, despite these efforts, plastic pollution remains a great challenge in the country, with more than 50% of plastic waste illegally dumped in dumping sites and littered on land and aquatic streams according to SA Plastic Pack [84].

Amongst the nine provinces of South Africa, Gauteng province, the economic hub of South Africa, and also the smallest province, is responsible for 26.6% of the country’s population. Considering the population and the industrial activities in the province, it is clear that the province will likely experience a huge influx of sewage containing various pollutants, including MPs, thus overburdening the already underperforming WWTPs [86]. In accordance with the government’s gazette No. 36784 of 23 August 2013, the responsibilities and well-being of the landfills and WWTPs have been placed on the local governments [87]. In addition, the local governments must ensure that waste management services, such as waste removal and disposal services, are provided. The role of local government regarding waste management has also been outlined in the government gazette of 30 March 2012 [88]. Furthermore, it is the responsibility of each municipality to integrate the waste management plan for their domestic waste in their respective by-laws [89]. It is important to note that this overpopulated province has been reported to have the second least number of WWTPs, as shown in Figure 6.

1.4. The Status of WWTPs in South Africa and Their Capabilities for the Removal of MPs

According to the South African Water Quality Guidelines volume 1 of 1991, the government is duty-bound to ensure that the quality of water in the country remains within the “No Effect Range”, such that the levels of contaminants in the water resources are negligibly low [91]. While industrialization is a vital key to the economic development of the country, industrial wastewater pollution is considered a major contribution of chemicals, such as MPs, in the environment. In addition, WWTPs are at the center of MP discharge from industries and households; thus, they are the epicenter of primary MPs presence in the environment, particularly the river bodies [92]. In a monitoring study conducted on the Leeuwkuil WWTP located in Vaal, Gauteng province, Iloms and co-authors [93] reported that the constant breakdown of the trickling filter plant had a direct impact on the ability of the WWTP to remove various pollutants entering the plant. In addition, another major problem facing the Leeuwkuil WWTP and other WWTPs across the province and the country as a whole is that these WWTPs often receive a huge flow of influents daily that is above their capacity. For instance, the Leeuwkuil WWTP reportedly receives about 42,000 m³/d flow of waste, which is way beyond its designed capacity of 36,000 m³/d [93]. Furthermore, WWTPs operate throughout the whole year in some conditions that may be corrosive, leading to the aging of the infrastructure. In light of this, a lack of maintenance, which is one of the major challenges facing most WWTPs in the country, will seriously affect the normal functioning of WWTPs. Although there is an emerging interest from researchers in understanding the occurrence of MPs and their fate in WWTPs across the country, the information is still very limited.

Despite many efforts carried out to curb the surging MP pollution, studies show that MPs from households and industries discharged into WWTPs are not completely removed and thus are discharged into surface waters [94,95]. Most WWTPs in many countries, both developed and underdeveloped countries, have been found to be incapacitated to deal with various pollutants, including heavy metals [96], organic pollutants [97], and MPs [98]. In a study conducted in the town of Thohoyandou, Limpopo province, South Africa, Dalu and co-authors [98] confirmed the effects of urbanization and thus human activities on MP concentrations along the river system. Furthermore, the authors found that the concentrations of MPs in the river after the WWTP were low in some cases, suggesting a possible impact of the WWTP on the reduction in MPs. Interestingly, overall, MP distribution was found to vary along the river system, and in different seasons, however, the impact of the WWTP was not clear [98]. Although there is considerable literature available on the capacity of WWTPs in South Africa to remove a wide variety of pollutants, such as heavy metals [96] and organic pollutants [99], the literature on the occurrence of MPs in WWTPs is very scarce, while quantities that are discharged into surface waters from sewage sludge and wastewater effluents are largely unknown yet very important in mitigating the long-term effects of MP pollution in aquatic systems.

According to a study by Cristaldi and co-authors, a considerable number of MPs can be removed during primary (78–98%) and secondary (7–20%) treatment [100]. The same findings were reported by Xu and co-authors, where it was noted that the concentration of MPs drops drastically after preliminary and primary treatment [101]. Meanwhile, tertiary treatment was reported to be less significant for the removal of MPs. In a study by Magni et al. [102], an 84% removal efficiency of MPs was reported for primary treatment. However, apart from MPs discharged with the effluents, a considerable amount of MPs was detected in the recycled activated sludge. More data on the capabilities of WWTPs across the globe for the removal of MPs are shown in

Table 1. Capabilities of WWTPs for the removal of MPs.

Country	Technologies (Primary and Secondary Treatment)	Removal Efficiency (%)	References
Italy	Biological treatment, sedimentation, sand filter treatment, disinfection	84	[102]
China	Anaerobic anoxic oxic (A2O), membrane bio-rector (MBR),	97.67	[104]
	cyclic activated sludge technology (CAST), fiber rotary filter (FRF)	98.46
Turkey	Biological and physical treatment	93.0	[105]
Germany	Gravity filters, membrane reactor	97	[106]
USA	Sedimentation, filtration, disinfection	77.7–95.9	[107]
Australia	Biological and UV treatment	76.61	[108]
South Korea	Physical and biological treatment, filtration	74.76–91.04	[109]
South Africa	Bio-filtration, disinfection	79.35	[103]

. More so, this information shows that there is still a serious research gap on the investigation and removal of MPs by WWTPs in Africa as a whole. In 2021, a study published by Vilakati and co-workers found that there were only three (03) published research articles on MPs in coastal water, two (02) on marine sand, and six (06) on marine animals, while there were only six (06) on freshwater [103]. To our knowledge, this is the first review study that comprehensively sheds light on the status and the capabilities of WWTPs for the removal of MPs in South Africa. Furthermore, the literature shows that more focus has been on the marine environment and coastal regions, as shown in

Table 2. Occurrence and analysis of MPs in South Africa.

Province	Source of MPs	Extraction Method	Type of MPs	Type of Polymer	MPs Concentration	Reference
Kwa-Zulu Natal	Beach sediment	H2O2 followed by density separation	Fibers, films	PP, HDPE, LDPE, PES, PS, PET	84 MPs/g	[110]
Western Cape	Mussels	10% KOH; oven heated at 60 °C for 48 h	Filaments and fragments	PET, PVC, HDPE	0.04 MPs/g	[111]
Kwa-Zulu Natal	Fish	Proteinase K; incubated at 39 °C overnight	Fibers and fragments	PES, PVC	0.79 ± 1.00/fish	[112]
Western Cape, Eastern Cape	Beach sediment	Saline treatment	Fibers and fragments	Not provided	688.9 ± 9 and 348.2 ± 1449 particles/m2	[113]
Western Cape	Fish	30% KOH; incubated at 40 °C for 24 h	Fibers and fragments	PP, PS, PE	1.36 MPs/fish	[114]
Western Cape	River water	10% KOH; incubated at 50 °C for 24 h	Fibers and Films	PE, PP, PET	5.13 ± 6.62 MP/L	[115]
Western Cape	Fish	10% KOH; incubated at 60 °C for 24 h	Fibers	PE	2.8–4.6 MPs/fish	[116]
Kwa-Zulu Natal	Estuarine sediment and water	Not provided	Fibers, films, foams	PE, PP, PS, PUR	18.5 ± 34.4/500 g and 11.9 ± 11.2/10,000 L	[117]

. Therefore, the data on the occurrence of MPs in freshwater from inland regions are still limited.

Effects of WWTPs on MPs

Various robust processes, which include physical, chemical, and biological, are involved during the treatment of wastewater, as indicated in Figure 7. Consequently, WWTPs have been reported to have an effect on the aging and physicochemical changes of MPs [118]. In relation to changes in shape, MP fibers can entangle with flocs and thus end up entering the sludge, after which the MPs will be deposited in the environment. In addition, MPs were reported to change in size due to friction and collision with silica, thus forming nanoplastics (NPs) [119]. According to a study by Bayo et al. [120], changes in MP size were attributed to the low pH in the primary and secondary treatment processes. Meanwhile, the use of treatment processes involving harder materials than MPs also has an impact on the fragmentation of MPs. Yang et al. [121] observed during primary and secondary treatment that the MPs continued to decrease into NPs; meanwhile, the concentration of NPs in sludge increased to more than 85%. The study further noted that while WWTPs are capable of removing large amounts of MPs, NPs are often found in the sludge in higher concentrations. This is a result of the adsorption of NPs during the sedimentation process. There are still limited data on a global scale regarding the effect of WWTPs on the physicochemical changes of MPs, which is more so due to the limited data on the occurrence of MPs in WWTPs in most countries, including South Africa, and the effects of WWTPs on MPs are thus largely unknown.

1.5. Different Polymers Used in Plastic Production in South Africa

Considering the current rapid increase in plastic production, its role in the economy, and the continued increase in plastic demand, the research on the extent and effects of plastic pollution will remain very crucial and relevant. Different polymers that are commonly used in South Africa are summarized in Figure 8.

These polymers can be classified into two classes: carbon–carbon (C–C) backbone polymers and heteroatomic (C–O) polymers [124]. Carbon–carbon polymers include polypropylene (PP), polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC) and have been reported to represent 77% of the plastic demand globally; meanwhile, heteroatomic polymers are responsible for 18% (see Figure 9), according to Samir et al. [124]. Of all the common polymers reported, PE is responsible for more than 90% of the MPs found in the environment [125,126]. This is because PE is reported to be frequently used in the packing industry as the main packaging material [127]. Moreover, the microbial biodegradation of C–C polymers has been reported to be extremely slow due to the high molecular weight and the strong C–C bonds [124].

1.6. Contamination of MPs in Groundwater and Drinking Water

Recent pieces of evidence have shown that groundwater and tap water are susceptible to MP contamination. In agreement with this, Mintenig et al. [128] detected a total of five different polymer types, polyester (PS) (62%), polyvinyl chloride (14%), polyamide (PA), epoxy resins (9%) and polyethylene (PE), in both raw groundwater and tap water. A total of 24 samples were sampled, of which 14 samples were found to contain no MPs. Meanwhile, MP concentration in the remaining samples varied between 0 and 7 MP particles per cubic meter. An investigation into the presence of MPs in bottled drinking water detected between 0 and 36 MPs particles per liter [129]. According to the study, PET particles were detected in single-use plastic bottles, whereby fragments accounted for 93 % and fibers accounted for the rest of the particles. These results bear evidence that drinking water is susceptible to MP contamination. However, the current availability of MPs in South Africa shows that there is still a very scarce information on occurrence of MPs in drinking water. Similar observations were also made by Iroegbu and co-authors, [130]. This existing knowledge need to be substantially explored to make sure that the quality of municipal drinking water, in particular, is within the “No Effect Range”, as stipulated in the South African Water Quality Guidelines volume 1 of 1991 [91].

Remediation of MPs in Freshwater

Remediation techniques include chemical, biological, and physical methods [131]. While chemical and physical methods have been widely used in previous years, setbacks such as high cost, complexity, and environmental unfriendliness have necessitated a need to explore biological methods [131,132]. Phytoremediation, a bio-technology that uses plants and their rhizospheric microorganisms to stabilize and degrade various pollutants from the environment, is considered as a preferred remediation technique [133,134]. Decades ago, plants were recognized for their ability to absorb unwanted and toxic chemicals, leading to a growing interest in the phytoremediation of heavy metals [135,136,137], organic pollutants [138,139], and recently, MPs [140,141]. Advantages of phytoremediation include economic feasibility, environmental friendliness, adaptability, and large scale application [142,143].

Due to the unprecedented and persistence of plastic pollution, more focus has shifted to developing robust, eco-friendly, and cost-effective technologies for the removal of MPs from marine and aquatic systems. However, the feasibility of phytoremediation in the degradation or remediation of MPs has yet to be fully understood [144]. Moreover, the phytoremediation of MPs can take place via one or more of the following mechanisms: phytoaccumulation, phytofiltration, phytodegradation, phytostabilization, and phytovolatilization, as shown in Figure 10. More so, the efficiency and mechanism(s) involved depend on the type of the targeted pollutant(s) and bioavailability [145].

2. Conclusions

This study concludes by summarizing the findings and highlighting the research gaps that need to be addressed further. In addition, challenges or tendencies that lead to MPs in the environment were also mentioned. Thus, although recycling is perceived as the most preferred method of solving the plastic waste crisis, the process is time consuming and it also alters the plastic material, resulting in low-quality plastic products. This might explain the very small percentage of plastic waste that successfully gets recycled across the globe. Meanwhile, due to the toxic additives used by the manufacturing of polymers, the incineration of plastic waste often generates toxic by-products, including PCBs, furans, etc.) [147]. In attempts to curb plastic pollution, there has been significant interest across the research community in improving biodegradability and reducing the toxicity of polymers by exploring biologically produced plastics that are easily degradable and environmentally friendly. These plastics, which include polyhydroxyalkanoate (PHAs), polylactic acid (PLA), polybutylene succinate (PBS), polyethylene adipate-co-terephthalate (PBAT), etc., are reported to possess similar physicochemical, mechanical, and thermal properties as the conventional plastics that are used in the market across the globe [148]. Although the capabilities of microorganisms to degrade polymer plastics have interestingly drawn attention, the formation of biofilms on synthetic polymers has been found to be insufficient, resulting in a biodegradation process that takes hundreds of years to complete under normal environmental conditions [17].

In the South African context, there is seemingly a wide knowledge gap on the occurrence, transport pathways, and toxic effects of MPs in the country. Moreover, there is also a concerning gap regarding the occurrence of MPs in WWTPs, as well as factors that affect the breakdown of plastic particles both in WWTPs and aquatic ecosystems. This review also notes that the current state of many WWTPs in the country will exacerbate the presence of both MPs and NPs in the environment. In this study, the authors strongly agree with previous authors regarding the urgent need for education, awareness, and strict legislation (which includes heavy penalties for defaulters) in addressing MP pollution in the country. Moreover, poor waste collection and lack of waste management and law enforcement are some of the main contributors of MPs in most townships across the country.

3. Limitations and Perspectives

Although there are new emerging studies focused on MPs across the country, there are still limited data on the occurrence of MPs in the inland regions. In addition, while MPs are well known to adsorb various chemicals on their surfaces, synthetic polymers are also known to contain various harmful additives that are added during the manufacturing process. However, the knowledge on the leaching of these toxic pollutants and their potential transfer to humans is still largely poorly understood, and very limited studies is available. Furthermore, many studies across the globe have concurred that WWTPs are at the center of MPs discharged from households and industries and are thus the epicenter of MPs in the environment. However, the literature on the occurrence of MPs in WWTPs is still very scarce, while quantities that are discharged into surface waters from sewage sludge and wastewater effluents are largely unknown yet very important in mitigating the long-term effects of MP pollution in aquatic systems. Meanwhile, it is important to note that the knowledge of the occurrence of MPs in WWTPs and their escape into the surface waters will help upgrade the current existing WWTPs or plan new WWTPs in the country.

4. Recommendations and Future Work

The development of suitable policies in addressing MPs will rely heavily on a clear full-scale understanding of the behaviour of MPs, their distribution in the environment, and toxicity toward human health. As reported by some organizations, the adoption of a legally binding United Nations treaty (March 2022) on plastic pollution [149] will significantly contribute to general plastic pollution in South Africa. A high number of studies carried out on MPs across the globe have emphasized an urgent need for effective policies and legislation to reduce plastic debris and MPs in the terrestrial and aquatic environment. Incentivising waste plastic collection for recycling, as well as re-using plastic waste and converting it into different products, such as bricks, will increase participation and, in turn, reduce plastic waste in the environment. Furthermore, a circular economy model is a perfect solution for achieving the “zero waste to landfill” goal by 2030, as envisaged by the government in the National Development Plan (NDP) 2030. The circular economy (CE) implies eliminating waste materials and reducing energy consumption and raw materials, closing the resources loop and allowing regeneration. However, according to a study by the Council of Scientific and Industrial Research (CSIR), this model has, so far, been partially implemented; therefore, a full scale-up is required to achieve desirable results [150].