A Google Scholar (GS) literature review was conducted using Boolean operators. Quotation marks were employed to refine the search and results. The specific query was "microplastics in rivers." The Boolean operator "AND" was used, followed by the target country in quotation marks ("microplastics in rivers" AND "country"). Since the study focuses on Latin America and the Caribbean, we reviewed research from 33 countries: 12 in South America, eight in Central America (including Mexico), and 13 in the Caribbean (Table 1), searching in English, Spanish, and Portuguese to assess the regional research landscape, with a focus on South America.

The initial results were refined by excluding citations, patents, reviews, theses, reports, documents lacking microplastic content or distribution results, marine studies, duplicates, articles from unlisted countries or other continents, and those in different languages. Despite using specific keywords, some irrelevant results appeared. Articles meeting the criteria were reviewed in depth, focusing on methodology, especially sampling and laboratory tests.

Table 1

Region	Countries
Central America	Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, Panama
Caribbean and Bahamas	Antigua and Barbuda, Bahamas, Barbados, Cuba, Dominica, Grenada, Haiti, Jamaica, Dominican Republic, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Trinidad and Tobago
South America	Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Guyana, Paraguay, Peru, Suriname, Uruguay, Venezuela

Source: The authors (2024).

The search yielded 314 articles: 19 from Central America, 29 from the Caribbean and The Bahamas, and 266 from South America. Occasionally, there were no results for the three languages. Several articles did not contain keywords, and some were non-research articles, or merely mentioned in the grey literature, necessitating a more meticulous review. A more thorough examination allowed for a selection of the information obtained, reducing the number of articles developed in South America to nine (Figure 1). Further verification ensured that they involved collecting and processing water or river sediment samples.

Figure 1 Source: The authors (2024).

This selection was thoroughly reviewed, considering: the year of publication, country/locality, sample unit, sample collection, laboratory analysis, reported abundance, and polymer determination.

To research global trends in river water sampling and microplastics analysis, a GS search was conducted using the phrases "microplastics abundance" AND "rivers" excluding citations and patents and selecting only works published in 2023 (from September to December 2023), yielding 224 pre-selected articles. A subsequent review eliminated those lacking the keywords in the title or abstract provided by the search engine, as well as books, reports, repositories, and reviews. Articles exclusively analyzing microplastics in marine environments or determining their presence in organisms were also discarded.

The review focused on water sample studies, resulting in a detailed analysis of seven articles. Manual collection using glass bottles of varying capacities was the most common method, though plankton and Nansen nets were also reported. Only two studies reported preserving samples via refrigeration during transport and before laboratory testing (Table 2).

Table 2

Sampling with
Bottles	4
Bucket	1
Plankton/Nansen net	2
Preservation
Refrigeration	2

Source: The authors (2024).

These seven articles show notable differences in laboratory procedures for microplastic extraction, identification, and classification, ranging from four to eight steps (Table 3). A key disparity is the absence of chemical analyses in three studies. The primary technique for polymer identification was Fourier transform infrared spectroscopy (FTIR), often supplemented by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Some studies used multiple techniques to gather chemical information on plastic particles. Discrepancies in chemical analysis choices likely stem from financial constraints or the need to outsource analyses. Though particle classification methods varied, all papers used stereo microscopy of varying magnification levels to characterize properties like color, size, and shape.

Further disparities are evident in the sequencing and execution of certain steps, including the method of filtration, the treatment of organic matter that involved reagents, digestion temperature, and the application of density separation, not universally implemented across all investigations. Additionally, the material obtained from filtration underwent drying or sieving at varying temperatures, although room temperature was the prevailing choice. It is important to underscore that the laboratory phase is intricately linked to the resources available to researchers, equipment, materials, reagents, and their familiarity with assays adaptable to microplastic study.

Table 3

Steps	a	b	c	d	e	f	g
	(Forero-López et al., 2021)	(Truchet et al., 2022)	(de Faria et al., 2021)	(Panizzon et al., 2022)	(Capparelli et al., 2021)	(Chavés-Velasco et al., 2023)	(Donoso; Rios Touma et al., 2020)
Filtration or sieve	Low vacuum (Nitrocellulose 0.45mm)	-	-	-	63mm (Sieve)	Filters 0.45mm (In situ)	Sieve (5, 1.1 and 0.3 mm)
Removal of organic matter	Fenton reagent 75°C	Fenton reagent 70°C	-	KOH 10% 40°C, 24h	H2O2 30% 60°C 100 rpm 2h	H2O2 50% (in situ)	Fenton reagent 75°C
Density separation	NaCl 5M (24h)	-		-	NaCl (24h)		NaCl (Overnight)
Filtration	Nitrocellulose filters (0.45mm)	Nitrocellulose filters (0.45mm)	Vacuum filtration Nitrocellulose filters (0.45mm)	Millipore cellulose acetate membranes (0.45mm)	Membranes (0.45mm) vacuum system		Glass fiber Filter (0.7mm)
Drying filters	60°C	Room temperature	Room temperature	Room temperature	Not specified	Desiccator	Not specified
Visual sorting	Stereomicroscope; optical microscope	Stereomicroscope	Stereomicroscope (45x)	Optical Stereomicroscope	Stereomicroscope (20x)	Microscope	Microscope (40x, 100x)
Categories	Color Size (<0.5, 0.5-1.5, 1.5-2.5, 2.5-3.5, 3.5-5 and > 5 mm) Shape (fragments, pellets, fibers, and films)	Size (<0.5, 0.5-1 and 1-5mm) Shape (Fragments, fibers, films, paint sheets, foam, microbead)	Shape (Fragments, fibers, pellets, films, closed-cell extruded polystyrene foam)	Shape Color	Shape Color	Shape (Fragments, spheres, film, and fiber)	Shape (fragments, foam, pellets, film and fiber)
Chemical analysis	FT-IR SEM-EDX	mATR-FTIR SEM-EDX XRD	-	SEM EDS	-		-

Source: The authors (2024)

Quality assurance and contamination management are crucial in microplastic studies due to their ubiquity in environments like tap water. However, three of the articles reviewed lack quality control measures. Before sample collection, sampling materials and instruments were washed and prepared, including rinsing with local water and washing glassware and metalware with filtered deionized water or 70% methanol. Reagents were prepared using distilled/deionized water and filtered, with materials and samples covered in aluminum foil to prevent contamination. Sampling bottles were rinsed three times before use, and cotton clothing, lab coats, and face masks were used to avoid textile contamination.

Table 4 summarizes some of the findings reported in this review. The contents vary depending on the characteristics of each body of water evaluated and its features (dimensions, morphology, others), the proximity and characteristics of population centers (urban or rural areas), potential sources of contamination, specifically the influx of untreated wastewater (Forero-López et al., 2021), and even the sampling and analysis conditions in terms of the size limits of the particles to be evaluated.

These methods did not follow the same protocols due to the lack of a unified convention as in the case of marine environments. Although such protocols do not exist, and this complicates comparisons between studies conducted around the world, making them can be helpful to have a reference of the pollution status in different locations.

The highest abundance of microplastics was reported in Argentina with an average of 6162 items/m³ (Forero-López et al., 2021), followed by Colombia, 3704.64 items/m³ (Chavés-Velasco et al., 2023). Fibers and fragments are the most frequently reported shapes in South American rivers, with transparent plastic particles prevailing. The polymers found included cellulose (CE), polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), polyacrylonitrile (PAN), polyvinyl chloride (PVC), and alkyd resins (AR).

Table 4

N°	Authors	Finds
1	Forero-López et al. (2021) (Argentina)	• MPs average abundance: 6162 items/m3
		• Main shape: Fibers
		• Main colors: Black, transparent, blue
		• MPs size: 0.5-1.5mm (44.21%) and ˂0.5mm (40.21%)
		• Polymers: CE, PAN, PET, PP, polyester (PES), AR, PVC
		• MPs content was higher near untreated effluent discharge
		• Detected elements: C, O, Si, Al, K, Ca, Cl, Ti, Fe, S, and P (potential pollutant carriers in the water)
2	Truchet et al. (2022) (Argentina)	• MPs abundance: 8-68 items/L
		• Main shape: Fibers
		• Main color: Transparent
		• MPs size: <0.5mm accounted for 53%
		• Polymers: Cotton-polyamide (PA), PE, and PET
3	de Faria et al. (2021) (Paraguay)	• MPs average size: 192±142μm.
		• MPs average abundance: 9.6±8.3 items/m3
		• Main shapes: Fibers (50%), fragments (19%), granules (22%), and extruded polystyrene foam (9%,)
4	Panizzon et al. (2022) (Brazil)	• MPs were found in all samples and sites
		• Abundance: 65 items/L
		• MPs size: 67µm-4.2mm
5	Capparelli et al. (2021) (Ecuador)	• Main shapes: Fibers (43%) and fragments (58%)
		• Main color: Transparent (42%)
6	Chavés-Velasco et al. (2023) (Colombia)	• MPs abundance: 79-3728 items/L
		• Main shapes: Fragments, fibers, spheres
7	Donoso and Rios-Touma (2020) (Ecuador)	• MPs abundance: 1.42-3704.64 items/m3
		• Main shapes: Fragments, fibers (most frequent), and films

Source: The authors (2024).

As observed, there is a growing interest within the Latin American scientific community in tackling this research topic, with notable findings reported in Ecuador, which has the highest number of publications.

However, while the reviewed articles demonstrate a rigorous approach in sample collection, pretreatments, laboratory assays including chemical characterization or identification, and quality control measures, there are differences in the methods employed to report microplastic abundance in river water samples. These discrepancies pose challenges that must be addressed to advance research on plastic pollution and contribute to potential solutions.

This review on GS was carried out between September and December 2023, allowing the final selection of 49 articles after the exclusion criteria. Figure 3 illustrates the sifting process for the articles subsequently reread to know the methodological details applied. Figure 4 illustrates the geographical distribution of the articles returned by the search, observing that most of these were developed in Asia (80%), followed in equal percentage by the American, European, and African continents (6%), and the lowest, Oceania (2%).

Eighty percent of the studies analyzed river waters, either alone (20 articles) or with sediment samples (19 articles), while 20% focused solely on sediments (10 articles). Furthermore, all primarily emphasized the spatial factor, with only 60% examining the temporal or seasonal influence on riverine microplastic content. Subsequently, a detailed review highlighted discrepancies in sample collection and laboratory analyses.

Figure 3 Source: The authors (2024).

Figure 4 Source: The authors (2024).

Regarding sampling methods, an examination of the procedures for collecting river water samples revealed various techniques, both manual and automatic methods (pumping and ultrafiltration). Manual water collection is the most common, including the use of buckets (20%), glass bottles (15%), or stainless-steel water samplers (12%). Interestingly, some articles reference nets traditionally used for sampling marine organisms (27%; Figure 5). However, the amount of water collected is inconsistent across studies, leading to high variability in volume.

Some authors report collecting 500 mL (Alam et al., 2023), while others mention volumes of 1L (Qaiser et al., 2023), 2L (Ragoobur et al., 2023), or even 5L (Wu et al., 2023; Karing et al., 2023), with one study even referring to the collection of 5L to complete a total of 25L (Shen et al., 2023). Additionally, it is important to note that all samples were collected at a surface level between 0.0-0.3m, as indicated in studies reporting sampling depth.

Twenty-six of 39 articles reported sieving or filtering (67%) either during water sample collection or laboratory analysis, using stainless steel sieves with different mesh sizes and filter paper (e.g., glass microfiber, pretreatment), regardless of the collection method. Both techniques are referenced in the European JPI Oceans protocol (Gago et al., 2018) and the NOAA manual (Masura et al., 2015), subsequently, the samples underwent drying. Of the 26 studies that applied initial water sample filtration, only 14 mentioned the drying process lasting between 30 minutes and 72 hours at an ambient temperature of 105°C.

Figure 5 Source: The authors (2024).

Organic matter (OM) removal was documented in 28 of the 39 articles (72%). Among these, 30% hydrogen peroxide (H₂O₂) emerged as the predominant reagent for OM degradation, utilized in 18 articles (64.29%), with reaction times from 12 hours to 7 days and digestion temperature between 25 °C and 70 °C. Some studies used Fenton reagent (six articles, 21.43%) or KOH (14.29%).

Following OM degradation, filtration was employed in 11 articles with pore sizes from 1μm to 200μm, the most common being 0.45mm. Filter materials included glass microfiber, steel, Whatman paper, and nitrocellulose. Density separation was employed in 19 articles to isolate plastics, with sodium chloride or zinc chloride as the most frequently used reagents (Table 5).

Filtration post-density separation was performed in 13 articles (33.4%) and drying was reported after OM removal or post-filtration in eight articles (20.5%). After obtaining the solid residue, plastic particles were visually classified regardless of the preceding steps, with 12 articles using stereomicroscopes or microscopes. Notably, OM was removed in one of the studies after density separation (Xu et al., 2023), optimizing reagent use and waste generation.

Table 5

Reagent	Chemical formula	Density (g/ cm3)	Articles reporting reagents	Toxicity1	Reference price (€ x 250 g)
Sodium chloride	NaCl	1.0-1.2	8	1 (low)	3
Zinc chloride	ZnCl2	1.6-1.8	7	3 (high)	45
Sodium iodide	NaI	1.80	2	2 (moderate)	130
Potassium formate	HCOOK	1.91	1	Not available	30

¹

Health hazard retrieved from NFPA/HMIS forms and toxicity values retrieved from MSDS forms. ² Reference prices consulted online. These may vary according to brand, country, etc.

To observe, separate, and characterize the plastic particles found in the samples, 31 articles used microscopes/stereoscopes with magnifications from 5x to 200x, and only one used direct visualization. The choice of devices and magnification levels depended on equipment availability and particle size under study. In some cases, a microscope with a digital camera (Priyanka et al., 2023) or an electron microscope (Shi et al. 2022) was used. Particle classification primarily considered shape (26 articles), color (24 articles), and size (17 articles).