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Article

Microplastic Pollution in Tropical River: Fourier Transform Infrared Spectroscopy-Based Characterization of Abundance and Polymer Composition in Water and Sediments from Filobobos River, Mexico

by
Gleybis Hernández-Morales
1,
María Cristina López-Mendez
2,
Alan Antonio Rico-Barragán
3,
Jesús Pérez-Moreno
4,
Carolina Peña-Montes
5,
Luis Alberto Peralta-Pelaez
5,* and
Humberto Raymundo González-Moreno
2,*
1
Laboratorio de Microplásticos, Tecnológico Nacional de México, Instituto Tecnológico Superior de Misantla, Km 1.8 Road to Loma del Cojolite, Misantla 93821, Veracruz, Mexico
2
División de Estudios de Posgrado e Investigación, Tecnológico Nacional de México, Instituto Tecnológico Superior de Misantla, Km 1.8 Road to Loma del Cojolite, Misantla 93821, Veracruz, Mexico
3
Departamento de Ingeniería Ambiental, Tecnológico Nacional de México, Instituto Tecnológico Superior de Misantla, Misantla 93821, Veracruz, Mexico
4
Laboratorio de Microbiología del Suelo, Postgrado de Edafología, Colegio de Postgraduados, Campus Montecillo, México-Texcoco Road Km 36.5, Montecillo 56264, Texcoco Estado de México, Mexico
5
Tecnológico Nacional de México, Instituto Tecnológico de Veracruz, Unidad de Investigación y Desarrollo en Alimentos, MA de Quevedo 2779, Veracruz 91897, Mexico
*
Authors to whom correspondence should be addressed.
Hydrology 2025, 12(5), 124; https://doi.org/10.3390/hydrology12050124
Submission received: 23 March 2025 / Revised: 11 May 2025 / Accepted: 13 May 2025 / Published: 21 May 2025
(This article belongs to the Special Issue Recent Research Advances in Microplastics in Water and the Environment)
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Abstract

Veracruz is a megadiverse state facing great water resource management challenges. The contamination of water bodies with external materials of anthropogenic origin stands out, including those derived from plastic products, which are deemed ubiquitous, emerging contaminants that have gained notoriety in recent decades due to the extent and effects of their presence, persistence and distribution in aquatic ecosystems. Being a significant environmental threat, their presence, persistence and distribution in aquatic ecosystems are deserving of a more detailed study. This research focused on analyzing microplastic (MP) retention and characterization in environmental matrixes (water and sediment) in the Bobos River’s lower basin, also taking into account other water physicochemical parameters, including a pH range from slightly acidic (5.17) to slightly alkaline (8.94) as the maximum value and an average temperature of 28.87 °C (83.96 °F). MPs are most frequently found in the form of blue-colored fibers. A polymer analysis by Fourier Transform Infrared Spectroscopy (FTIR) revealed that the most common polymer was polyethylene (PE), which is the main component of most agricultural mulch and agrochemical containers. This research aims to enhance the understanding of the plastic matter contamination of water bodies, pointing out the need for further and deeper research on this subject.

Graphical Abstract

1. Introduction

Rivers are dynamic systems in which hydrology and human activity determine the mobility and retention of contaminants by runoff behavior. Rivers work as transport pathways from terrestrial ecosystems to the final repository—the ocean. Between both ends, microplastics (MPs) have emerged as a growing environmental concern due to their persistence and dispersibility in aquatic ecosystems [1,2,3,4]. Almost every form of life in all and any ecosystems are affected by plastics, as these have become a major source of noxious effects [5]. Their transport throughout the hydrological basin is determined by flow regimes, which vary in time and space depending on volume, terrain slope, and channel structure. In high-energy rivers—those in the upper basin—turbulent flow promote suspended plastic particle removal and transport, while in the lower basin, where slower flow regimes and meander formation predominate, microplastics can accumulate in sediments, riparian vegetation and floodplain soils [6,7,8]. In the Bobos River’s case, anthropogenic activities in the region, particularly the citrus industry, which represents a key economic activity in the area, directly impact its hydrology. The intensive use of agrochemicals has severe repercussions on water and soil quality, and as containers are not disposed of correctly, they end up being incorporated into the environment and turning into high-toxicity plastic contamination. Agricultural discharges and surface runoff contribute to the transport of plastic particles from greenhouse coverings, agrochemical packaging and residues from citrus processing plants, which generate large volumes of plastics in their production chain. In addition, heavy rainfall and flooding events can redistribute these contaminants, increasing their deposition in areas of low hydraulic energy, where sedimentation processes integrate them into the fluvial ecosystem [9,10].
This study aimed to analyze the retention and characterization of microplastics in water and sediments of the Bobos River, assessing their interaction with hydrological regimes and citrus farming activities in the region. Although no continuous measurements of historical flow rates or contaminant loads have been made, available evidence indicates that tropical river systems show seasonal patterns of microplastic transport, with higher concentrations during the rainy season due to agricultural and urban runoff [11]. In addition, studies show that river sediments act as sinks for microplastics, especially near cultivated areas [11,12]. This historical–hydrological dynamic is fundamental for understanding the mechanisms of microplastic dispersion and accumulation and for assessing the role of rivers as vectors of pollution to aquatic ecosystems and their impact on water quality [13] (see Figure 1).

2. Materials and Methods

2.1. Study Area

The Bobos River basin is exorheic and is located in the northeast of the Gulf of Mexico, as shown in Figure 2, with a predominantly mountainous terrain in an area of 3544 km2 (1368.346 mi2) with an altitude ranging from 50 to 3200 m above sea level (164.04 to 10,500 ft). It comprises 10 municipalities between the Veracruz and Puebla states and stands as a relevant waterway running through urban and industrial areas, in which diverse economic activities are developed in different productive sectors [14], among which citrus production in the Martínez de la Torre municipality stands out. Other activities in which water quality plays a very important role are carried out as well, impacting the current state of the water body [15,16].

2.2. Sampling Areas

Nine sampling areas in the lower basin of the Bobos River (also Filobobos) were defined (Table 1) for the months of November and December of 2023 after the rainy season in the region, at which a total of 25 sediment samples and 9 water samples were collected. Site selection was determined according to the tributary order of the runoffs that make up the lower basin of the Bobos River, considering only order 3 or higher runoffs, meaning that two order 2 runoffs join together in an order 3 runoff, increasing the gathering area, in addition to considering the access to the channel according to the orography and the economic activities developed on the margin, as shown in Figure 3 [13,16].

2.2.1. Water Sampling

Water quality in water bodies largely depends on the interaction between physical, chemical and biological parameters as a working system that determines the actual condition of the water body [17]. A water sample was collected from the center of the channel by introducing a plankton net (60 µm pore diameter) against the current from a boat at constant speed. The sample contained in the net codend was collected and stored in a properly labeled glass jar for transport to the laboratory. In addition, three water samples were collected in different containers, from which information from each sampling site was recorded in situ, such as pH, electrical conductivity, temperature, dissolved oxygen, total solids and salinity, using a HANNA Instruments portable meter (model HI98130, Smithfield, RI, USA) and a dissolved oxygen meter (Milwaukee MW600 Mulund West, Bombay, Maharashtra, India), respectively. Chemical parameters (Chemical oxygen demand, total nitrogen, total phosphorus, nitrate, phosphate) were calculated at the Tecnológico Nacional de México-Campus Misantla lab, according to the established Mexican Standards (NMX-AA-030/1-SCFI-2012 [18], NOM-001-SEMARNAT-2021 [19]), and the suspended sediment concentration (SSC) analysis method based on the standard methods for water analysis [17,20,21], using the Statistical Package for the Social Sciences (SPSS, ver. 25.0; SPSS Inc, Chicago, IL, USA). The parameters obtained from the collected water samples were related. In the laboratory, water samples were passed through MF-Millipore glass fiber filters of 0.45 µm pore size and 47 mm diameter.

2.2.2. Sediment Sampling

A transect was mapped parallel to the riverbed and a site without evidence of human activity was randomly selected, delineating a 50 by 50 cm (19.685 by 19.685 in) plot where the pH was measured by means of the Hanna Groline Soil Test Ec Tester, and whose top two centimeters (0.78 in) of sediment were collected and passed through metallic sieve number 10 (2000 µm) to eliminate materials exceeding these dimensions. The samples were placed in aluminum envelopes duly identified for transfer to the lab. Once at the lab, the sediment samples were placed on aluminum trays so that exposure to direct sunlight would remove moisture so that the samples could be passed through a tower of metal mesh sieves of different pore sizes, 20 (841 µm), 40 (420 µm), 60 (250 µm), 100 (149 µm), and 200 (74 µm), and then each fraction was measured in grams [21,22].

2.3. Sample Chemical Digestion

A digestion process of the organic matter in the sediment samples was carried out using a mixture of potassium hydroxide (KOH 10% w/v), hydrogen peroxide (H2O2, 30% w/v) and iron (Fe2+) as a catalyst to accelerate the digestion reaction and properly removing any organic material from environmental matrixes, stirring at 100 revolutions per minute. By adding a saturated solution of sodium chloride (NaCl, 1.2 g/cm3) [23], the sedimentation and separation of materials by different densities was achieved, bearing in mind that the average density of the plastic materials was in the range of 0.8 to 1.6 g/cm3 (49.94 to 99.88 lb/ft3), while the density of the sediment was around 2.7 g/cm3 (168.55 lb/ft3), which was left to settle for a 24 h period to be filtered later through MF-Millipore glass fiber filters of 0.45 µm pore size and 47 mm in diameter [24].

2.4. Extraction and Classification of MPs

With the help of a model VE S-1, VELAB stereoscopic microscope, 4× magnification, visual assessments were made of the plastic particles in the samples processed in the laboratory based on their physical features (shape, color and number). According to their shape, they were classified into fiber, sphere, granule or film, and according to color, they were classified into transparent, black, white, red, yellow and blue, as per the standard Crawford identification guide [25]. Once particles were located, they were transferred to glass Petri dishes and slides with the support of tweezers and metallic dissection needles for observation via an Optika Microscopes Italy model B-150D-BRPL optical microscope, OPTIKA S.r.l., Lombardy, Italy. Using Fourier Transform Infrared Spectroscopy with an attenuated total reflectance module (ATR-FTIR), we identified the types of polymers present by way of a spectrophotometer equipped with a Perkin Elmer ATR module, with a diamond crystal, a sampling area of 1.5 mm (0.059 in), and an optical window of 400 to 3500 cm−1. Spectra were acquired with a resolution of 4 cm−1 and compared against available spectral libraries [26,27].

3. Results

Nine sampling areas were defined in the Bobos River’s lower basin, where nine water samples were collected in the river channel together with twenty-five sediment samples at sites on the river margin, with no apparent presence of footprints or human activity.

3.1. Water Physicochemical Parameters

From the water samples collected in the Bobos River’s lower basin, the average dissolved oxygen was 6.36 mg L−1 ± 0.165 (0.371 ± 0.0096 gpg) and the temperature was 28.76 °C ± 0.105 (83.17 ± 0.19 °F). According to [28,29,30], the optimum concentration of dissolved oxygen in a healthy water body is between 8 and 12 mg/L−1 (0.46736 and 0.70104 gpg), values above those obtained in this study. Electrical conductivity ranged between 0.25 and 3.98 mS cm−1, and the peak value was tracked in the area of the mouth of the basin, in areas SA1 and SA2. The average pH value obtained in this study is 7.62, above the optimal range for a freshwater river (pH 7.4), as shown in Table 2.

3.1.1. Statistical Analysis

The Pearson correlation matrix (Table 3) is presented as a descriptive environmental context, revealing significant relationships between the physicochemical parameters of the aquatic system. A very strong positive correlation is observed between total dissolved solids (TDS) and electrical conductivity (EC) (r = 0.937, p < 0.01), suggesting a direct relationship between the increase in ionic solutes and the conductive capacity of the water. Likewise, temperature (T) shows a significant correlation with EC (r = 0.811, p < 0.01) and TDS (r = 0.618, p < 0.05), indicating that thermal variations could influence the dynamics of these parameters.
A key finding is the strong association between nitrate (NO3) and total nitrogen (TN) (r = 0.911, p < 0.01), as well as between phosphate (PO4) and TN (r = 0.975, p < 0.01), reflecting coupled biogeochemical processes in nutrient cycling. On the other hand, dissolved oxygen (DO) exhibits moderate negative correlations with SDT (r = −0.580) and COD (r = −0.556), suggesting possible effects of organic loading and salinity on oxygen availability. These results highlight the interdependence between physical (T,EC), chemical (SDT) and biogenic (NO3, PO4) factors, providing an interpretative framework for assessing water quality in tropical river systems under anthropogenic stresses.

3.1.2. Microplastics (MPs) in Water Samples

A total of nine water samples were collected in the Bobos River, in which 91 microplastic particles were found. The sampling area with the highest frequency of microplastics was identified as SA6, followed by SA4 and SA7. MPs were classified and physically characterized according to the Crawford classification [25], visually distinguishing MPs according to light reflection, the absence of cellular or organic structures and crystallinity through a stereoscopic microscope. Figure 4 describes observations in line with previous studies [31,32,33] in which plastic materials of similar colors were recovered, the most frequently found form of which were fibers with 81%, followed by flake with 14% (Table 4).

3.2. MPs in Sediment Samples

A total of 25 sediment samples were collected from the Bobos River, in which 156 microplastic particles were found. They were later classified and physically characterized according to the Crawford classification [25], whereby the MPs were visually distinguished according to light reflection, the absence of cellular or organic structures and crystallinity through a stereoscopic microscope. Figure 5 observations are in line with previous studies [34,35,36] that recovered plastic materials of similar colors, the most frequently found form of which were fibers (36%), followed by films (35%) as shown in Figure 6. The area with the highest number of microplastic particles was identified as SA1-1, which corresponds to the mouth of the river, followed by SA8-2 which corresponds to the sample collected in the municipality of Martínez de la Torre (Table 4).

3.3. Chemical Identification of MP Types Present in Samples

The MPs found were analyzed using the FTIR technique, through which the functional groups in the polymeric structures were identified. The most frequent polymer was polyethylene (PE) with 38%, in second place was polyethylene terephthalate (PET) with 28%, followed by polyvinyl chloride (PVC) with 23%, and finally polybutylene terephthalate (PBT) was found in 11% of the samples analyzed. The infrared spectra in Figure 7 show the different chemical compositions.

4. Discussion

4.1. Hydrodynamics Influence on Microplastic Retention

The transport and deposition of microplastics in rivers are highly influenced by flow dynamics and channel geomorphology [37]. In systems with high hydraulic energy, microplastics tend to remain in suspension on account of their density, and due to the turbulence of the flow, their transport downstream is most likely consistent with the result of this study, which reports the highest number of MPs at the river mouth. However, in rivers with a lower gradient and the presence of meanders, low-velocity zones are generated where MPs can sediment and accumulate in river beds and riparian vegetation [37,38]. According to information generated by the National Water Commission (CONAGUA), the government agency in charge of weather forecasting in Mexico, the lower basin and mouth of the Filobobos River is characterized by a permanently turbulent and sub-critical flow due to its low gradient (5%) and high depths (20 m), with strong sediment transport dynamics (mean annual flow 115 m3/s) and susceptibility to extreme events where the bottom load is high during the rainy season (mean annual rainfall 2800 mm), forming sand bars and shoreline displacement, which is very evident in similar tropical river mouths [37,38]. Tropical rivers represent one of the most diverse freshwater ecosystems in the world and are the most affected by anthropogenic activities to the extent of interfering with the optimal development of aquatic species by altering water quality parameters such as temperature, dissolved oxygen and electrical conductivity [39,40], which is an indicator of the amount of salts and carbonates dissolved in the water—the more ions in the water body, the higher the conductivity level. In this study, conductivity values ranged from 0.25 to 3.98 mS cm−1, which is above the USEPA-provided range. For a water body to support fishing, it must have a range of 0.15 mS cm−1 to 0.50 mS cm−1, so the river in this study demonstrated that it is not suitable for this activity [41,42] The lower Nautla River basin is a region of high hydrological dynamics due to its proximity to the coast of the Gulf of Mexico, where its low slope with intense sedimentation, fluvial–marine interaction and increasing risks of seasonal flooding converge [43], conditions that together led to the detection of MPs in all water and sediment samples in this study. Fibers were the most frequent form of MPs in the Bobos River’s lower basin, consistent with previous studies [44,45], followed by flake and film, showing consistency in color frequency concordance [46]. Our results obtained in this research are in line with previous studies, finding that the most frequent form of MPs in sediment and water are mostly fibers, fragments and films. We also point out polyethylene (PE) and polyethylene terephthalate (PET) as the most frequent polymer types [47,48,49,50]. Table 5 below summarizes how hydrodynamics affect the retention of MPs in rivers.

4.2. Relationship Between Citrus Industry and Microplastic Contamination

Intensive agriculture is a recognized source of microplastic pollution, MPs, due to the widespread use of plastics in agricultural activities [12]. In citrus cultivation, MPs are likely to come from multiple sources, such as plastic greenhouse covers, fruit packaging and agrochemical containers [55]. Although this study did not directly quantify MP fluxes from citrus orchards to the Bobos River, empirical evidence from similar tropical agroecosystems suggests that these inputs may degrade water quality [56,57].
The water quality of the Bobos River, according to our results, appears suboptimal for sensitive activities such as fishing, coinciding with previous studies linking agricultural runoff with elevated turbidity and nutrients in this watershed [58]. It is worth noting that tropical rivers such as the Bobos are among the most biodiverse freshwater ecosystems, although their physicochemical parameters are frequently altered by anthropogenic pressures [46,48]. Although some papers propose an inverse relationship between DO and temperature [59,60], our data at sites SA1 and SA4 showed identical DO concentrations at different temperatures, suggesting that local factors (e.g., photosynthetic activity or subsurface inputs) may mask this trend. To better visualize the main sources of MPs in the citrus industry, the following Table 6 is presented.
The implementation of specific regulations is crucial to address this problem. In Mexico, NOM-001-SEMARNAT-2021 [19] and NOM-003-ECOL-1997 [65] dispositions regulate and protect national waters and assets by establishing maximum permissible limits for contaminants and conventional physicochemical and microbiological parameters for wastewater discharges into bodies of water, in addition to the General Law for the Prevention and Integral Management of Waste, which regulates the trade of single-use plastics. It is important to highlight that some states in the country have made progress by focusing on specific products such as straws and bags. However, regulations are focused on the final part of the plastic life-cycle, undermining the need for implementing comprehensive legislation that stresses prevention. In the European Union, Regulation (EU) 2023/2055 was developed to limit the release of microplastics in several sectors, restricting their use in different products of daily use [66,67,68].

5. Conclusions

The integration of the water and sediment data obtained in this study reveals a complex scenario that requires multi-sectoral approaches, from public policies to community participation, to reduce the entry and persistence of MPs in the Bobos River. According to the results obtained, it is concluded that there is widespread contamination in the water and sediment, with the presence of microplastics in all samples obtained, evidencing a widespread problem in the river [8,35,47]. Providing an interpretative framework for assessing water quality in tropical river systems under anthropogenic stress, the interdependence between physical (T,EC), chemical (SDT) and biogenic (NO3, PO4) factors in the water samples was observed in this study, reflecting typical patterns of freshwater chemistry; however, for the case of the Bobos River, further hydrodynamic or sedimentation analysis is required to explore possible connections between the above factors, with the transport or retention of microplastics.
For the case of water, sampling sites SA6, SA4 and SA7 were identified as critical areas that exhibited the highest concentration of MPs, suggesting point sources or local hydrological dynamics. For the presence of MPs in sediment, the highest concentration was found in the site identified as SA1-1, which corresponds to the mouth of the river, and in site SA8-2, findings that coincide with previous studies in similar river systems [31,32,36]. The distribution of microplastics in the river differs between upstream/downstream and urban areas due to their multiple sources and the influence of the surrounding environment. Recognizing the key elements influencing the abundance of MPs is complex as they are numerous and interrelated; therefore, exploring the impact of physicochemical factors on aspects such as the color and shape of MPs allows for a better understanding [55,62]. However, regarding their association with anthropogenic activities such as urban discharges, industry, hydrological dynamics and river transport, it is recommended that new lines of research and the development of additional studies where sources of MP emissions are considered complement current research.
The detection and characterization of microplastics in environmental matrices represents a challenge for science itself, becoming an area of opportunity for new lines of research for their study through the standardization of analytical methods [48,49,69]. The persistence of microplastics in the environment and their interaction with organisms represent a major ecological threat as the main impact is their ability to be ingested by a wide variety of aquatic and terrestrial organisms, causing toxic effects [60,61,70]. Despite growing scientific evidence on the negative effects of microplastics on the environment and human health, in Mexico, there is still no specific regulation that regulates their presence in water bodies and sediments, so it is necessary to develop research that allows us to discern the overall picture of them, establishing standardized methodologies and defining permissible limits along with clear strategies for their mitigation based on scientific evidence and international cooperation to reduce their long-term ecological consequences [71,72,73].

Author Contributions

Conceptualization, project administration, resources, writing—review: H.R.G.-M. and L.A.P.-P.; data curation, methodology: C.P.-M., M.C.L.-M., G.H.-M. and J.P.-M.; supervision: A.A.R.-B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported through the postdoctoral grant CVU 429551 and the doctoral grant CVU 493558 from the (Secretaría de Ciencia, Humanidades, Tecnología-SECIHTI) at the Tecnológico Nacional de México Campus Misantla, Misantla 93821, Veracruz, Mexico.

Data Availability Statement

The original contributions presented in this study are included in the article. For more information, please contact the corresponding author.

Acknowledgments

The Tecnológico Nacional de México Campus Misantla, Misantla 93821, Veracruz, Mexico.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Process of MP retention in vegetation, due to storm effects and OMWL, Ordinary Maximum Water Levels (NAMO, niveles de aguas máximas ordinarias in Spanish), and EMWL, Extraordinary Maximum Water Levels (NAME, nivel de aguas máximas extraordinarias in Spanish).
Figure 1. Process of MP retention in vegetation, due to storm effects and OMWL, Ordinary Maximum Water Levels (NAMO, niveles de aguas máximas ordinarias in Spanish), and EMWL, Extraordinary Maximum Water Levels (NAME, nivel de aguas máximas extraordinarias in Spanish).
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Figure 2. Study area location.
Figure 2. Study area location.
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Figure 3. A close-up view of the water and sediment sampling sites at the Bobos River’s lower basin. (a) Martínez de la Torre. (b) Carrillo Puerto. (c) Jicaltepec. (d) Bobos River Estuary.
Figure 3. A close-up view of the water and sediment sampling sites at the Bobos River’s lower basin. (a) Martínez de la Torre. (b) Carrillo Puerto. (c) Jicaltepec. (d) Bobos River Estuary.
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Figure 4. MP frequency in water samples according to color and shape (n = 91).
Figure 4. MP frequency in water samples according to color and shape (n = 91).
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Figure 5. MPs in sediment samples. (a) Blue flake in sediment sample from site SA1-3 (Maracaibo Beach), (b) transparent fiber in sediment sample from site SA3-2 (Nautla), (c) blue film in sediment sample SA5-2 (Chapa-Chapa), (d) transparent fiber in water sample from site SA2 (Maracaibo River), (e) blue fiber in water sample from site SA9 (La Capilla), (f) blue fiber in water sample from site SA5 (Chapa-Chapa).
Figure 5. MPs in sediment samples. (a) Blue flake in sediment sample from site SA1-3 (Maracaibo Beach), (b) transparent fiber in sediment sample from site SA3-2 (Nautla), (c) blue film in sediment sample SA5-2 (Chapa-Chapa), (d) transparent fiber in water sample from site SA2 (Maracaibo River), (e) blue fiber in water sample from site SA9 (La Capilla), (f) blue fiber in water sample from site SA5 (Chapa-Chapa).
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Figure 6. MP frequency in sediment samples according to color and shape (n = 156).
Figure 6. MP frequency in sediment samples according to color and shape (n = 156).
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Figure 7. Transmittance spectra by the Fourier Transform Infrared FTIR technique. (a) CH2 bands are seen in the region 2915 cm−1 (asymmetric), 2850 cm−1 (symmetric), the angular deformation of CH2 at 1460 cm−1, and 720 cm−1, which distinguish polyethylene (PE) from other polymers, denoting a film-shaped microplastic particle found in the sample identified as SA5-2. (b) The infrared spectrum shows the presence of an intense C=O band in the region of 1720 cm−1, in addition to multiple aromatic bands between 1600 and 1450 cm−1 and 730 and 870 cm−1 and C-O bands of the ester in the region of 1240 to 1260 cm−1 and 1100 cm−1, which distinguish polyethylene terephthalate (PET) from other polymers, corresponding to a transparent fiber-shaped microplastic particle from a sediment sample identified as SA3-2. (c) The spectrum shows C-Cl bond vibrations with an intense band in the region 600 to 700 cm−1 forming two peaks, the absence of complex functional groups, and a simplified CH2 pattern between 2900 and 3000 cm−1 with asymmetric stretches at 2915 cm−1 and symmetric stretches at 2848 cm−1, characteristic of polyvinyl chloride (PVC). (d) The infrared spectrum of the C=O band at 1720 cm−1, indicating an ester conjugated with an aromatic, the presence of CH3 at 1380 cm−1 caused by the butyl chain and a distinctive aromatic pattern in the region 1600 to 1450 cm−1, which differentiate polybutylene terephthalate (PBT) from other materials.
Figure 7. Transmittance spectra by the Fourier Transform Infrared FTIR technique. (a) CH2 bands are seen in the region 2915 cm−1 (asymmetric), 2850 cm−1 (symmetric), the angular deformation of CH2 at 1460 cm−1, and 720 cm−1, which distinguish polyethylene (PE) from other polymers, denoting a film-shaped microplastic particle found in the sample identified as SA5-2. (b) The infrared spectrum shows the presence of an intense C=O band in the region of 1720 cm−1, in addition to multiple aromatic bands between 1600 and 1450 cm−1 and 730 and 870 cm−1 and C-O bands of the ester in the region of 1240 to 1260 cm−1 and 1100 cm−1, which distinguish polyethylene terephthalate (PET) from other polymers, corresponding to a transparent fiber-shaped microplastic particle from a sediment sample identified as SA3-2. (c) The spectrum shows C-Cl bond vibrations with an intense band in the region 600 to 700 cm−1 forming two peaks, the absence of complex functional groups, and a simplified CH2 pattern between 2900 and 3000 cm−1 with asymmetric stretches at 2915 cm−1 and symmetric stretches at 2848 cm−1, characteristic of polyvinyl chloride (PVC). (d) The infrared spectrum of the C=O band at 1720 cm−1, indicating an ester conjugated with an aromatic, the presence of CH3 at 1380 cm−1 caused by the butyl chain and a distinctive aromatic pattern in the region 1600 to 1450 cm−1, which differentiate polybutylene terephthalate (PBT) from other materials.
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Table 1. Sampling areas.
Table 1. Sampling areas.
IDSampling AreaGeographic Coordinates
SA-1Maracaibo BeachE 732099.806252
N 2236577.718221
SA-2Maracaibo RiverE 732099.806252
N 2236577.718604
SA-3NautlaE 724696.863808
N 2231664.942291
SA-4JicaltepecE 724659.198550
N 2231566.645207
SA-5Chapa-ChapaE 732259.440263
N 2236838.257247
SA-6Carrillo PuertoE 724659.198550
N 2231566.645207
SA-7Máximo GarciaE 732099.806252
N 2236577.718604
SA-8MartínezE 704710.236904
N 2219794.298297
SA-9La CapillaE 6804710.246904
N 2219794.298297
Table 2. Water physicochemical parameters, Bobos River’s lower basin.
Table 2. Water physicochemical parameters, Bobos River’s lower basin.
SampleDissolved Oxygen
mg/L−1 (gpg)		Temperature °C (°F)Electrical Conductivity
(mS cm−1)		pH
SA15.43 ± 0.15 (0.318 ± 0.009)29.93 ± 0.06 (85.87 ± 0.11)3.98 ± 0.015.17 ± 0.72
SA26.17 ± 0.12 (0.360 ± 0.007)33.83 ± 0.06 (92.89 ± 0.11)3.98 ± 0.017.51 ± 0.50
SA37.23 ± 0.31 (0.422 ± 0.018)29.37 ± 0.12 (84.87 ± 0.22)1.14 ± 0.738.47 ± 0.55
SA46.17 ± 0.23 (0.360 ± 0.013)27.67 ± 0.15 (81.81 ± 0.27)0.29 ± 0.068.94 ± 1.79
SA57.00 ± 0.20 (0.409 ± 0.012)27.43 ± 0.12 (81.37 ± 0.22)0.65 ± 0.078.02 ± 0.85
SA66.37 ± 0.23 (0.372 ± 0.013)29.37 ± 0.12 (84.87 ± 0.22)0.25 ± 0.037.21 ± 0.69
SA76.57 ± 0.25 (0.384 ± 0.015)27.00 ± 0.20 (80.60 ± 0.36)0.37 ± 0.126.82 ± 1.02
SA85.53 ± 0.31 (0.323 ± 0.018)26.53 ± 0.25 (79.75 ± 0.45)0.37 ± 0.128.23 ± 0.68
SA96.77 ± 0.23 (0.396 ± 0.013)27.73 ± 0.15 (81.91 ± 0.27)0.52 ± 0.058.21 ± 0.71
Table 3. Bobos River’s physicochemical parameter correlation matrix.
Table 3. Bobos River’s physicochemical parameter correlation matrix.
ParameterpHECTTDSDOCODTNNO3TPPO4
pH1
EC−0.6031
T−0.2830.811 **1
TDS−0.772 *0.937 **0.6181
DO0.471−0.409−0.102−0.5801
COD−0.0450.4520.3100.334−0.5561
TN−0.3380.3550.4250.3380.154−0.1551
NO3−0.4260.4090.5840.3890.081−0.2310.911 **1
TP−0.5900.5750.4200.666−0.4110.1960.678 *0.5971
PO4−0.2990.4740.5330.4350.120−0.1080.975 **0.898 **0.697 *1
* Correlation is significant at 0.05 level (bilateral). ** Correlation is significant at 0.01 level (bilateral).
Table 4. Frequency of microplastics found in water samples.
Table 4. Frequency of microplastics found in water samples.
IDSampling AreaMP Shape—Sediment
SphereFlakeFilmFiber
SA1-1Maracaibo Beach 1-2214
SA1-2Maracaibo Beach 2-1-8
SA1-3Maracaibo Beach 3-122
SA2-1Maracaibo River 1--24
SA2-2Maracaibo River 2-321
SA2-3Maracaibo River 3-2-2
SA3-1Nautla 1-6-2
SA3-2Nautla 2-4-5
SA3-3Nautla 3--22
SA4Jicaltepec1224
SA5-1Chapa-Chapa 11231
SA5-2Chapa-Chapa 2-221
SA5-3Chapa-Chapa 3--21
SA6-1Carrillo Puerto 1--1-
SA6-2Carrillo Puerto 2--6-
SA6-3Carrillo Puerto 3--1-
SA7-1Maximo García 1--1-
SA7-2Maximo García 2-24-
SA7-3Maximo García 3--1-
SA8-1Martínez 1-262
SA8-2Martínez 2-374
SA8-3Martínez 3--33
SA9-1La Capilla 1-62-
SA9-2La Capilla 2-31-
SA9-3La Capilla 3-23-
MP Shape—Water
SA1Maracaibo Beach-1-9
SA2Maracaibo River-1-10
SA3Nautla-1-7
SA4Jicaltepec-2-10
SA5Chapa-Chapa---7
SA6Carrillo Puerto22-9
SA7Maximo García-228
SA8Martínez-3-7
SA9La Capilla-1-7
Table 5. Microplastics retention in rivers.
Table 5. Microplastics retention in rivers.
Hydraulic FactorEffect on MP MobilityReference
High-Speed FlowDownstream Dispersal and Transportation[51]
Lower Speed in MeandersRetention and Accumulation of Sediments[52]
TurbulenceMP Suspension in Water Column[53]
Sediment Bank FormationMP Adsorption in Finer Particulates[54]
Table 6. Microplastics source in citrus industry.
Table 6. Microplastics source in citrus industry.
Microplastics Source in Citrus IndustryExampleEnvironmental ImpactReference
Fruit plastic packagingPolyethylene bags, PET boxesContamination by MP fragments[61]
Greenhouse coveringsPolyethylene plasticsWear and release of microfibers[62]
Pesticides and herbicidesFormulations with MPs as carrier agentsEntry of MPs into soil and water[63]
Irrigation systemsWater contaminated with MPs transported from other sourcesRedistribution of MPs in crops and water bodies[64]
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Hernández-Morales, G.; López-Mendez, M.C.; Rico-Barragán, A.A.; Pérez-Moreno, J.; Peña-Montes, C.; Peralta-Pelaez, L.A.; González-Moreno, H.R. Microplastic Pollution in Tropical River: Fourier Transform Infrared Spectroscopy-Based Characterization of Abundance and Polymer Composition in Water and Sediments from Filobobos River, Mexico. Hydrology 2025, 12, 124. https://doi.org/10.3390/hydrology12050124

AMA Style

Hernández-Morales G, López-Mendez MC, Rico-Barragán AA, Pérez-Moreno J, Peña-Montes C, Peralta-Pelaez LA, González-Moreno HR. Microplastic Pollution in Tropical River: Fourier Transform Infrared Spectroscopy-Based Characterization of Abundance and Polymer Composition in Water and Sediments from Filobobos River, Mexico. Hydrology. 2025; 12(5):124. https://doi.org/10.3390/hydrology12050124

Chicago/Turabian Style

Hernández-Morales, Gleybis, María Cristina López-Mendez, Alan Antonio Rico-Barragán, Jesús Pérez-Moreno, Carolina Peña-Montes, Luis Alberto Peralta-Pelaez, and Humberto Raymundo González-Moreno. 2025. "Microplastic Pollution in Tropical River: Fourier Transform Infrared Spectroscopy-Based Characterization of Abundance and Polymer Composition in Water and Sediments from Filobobos River, Mexico" Hydrology 12, no. 5: 124. https://doi.org/10.3390/hydrology12050124

APA Style

Hernández-Morales, G., López-Mendez, M. C., Rico-Barragán, A. A., Pérez-Moreno, J., Peña-Montes, C., Peralta-Pelaez, L. A., & González-Moreno, H. R. (2025). Microplastic Pollution in Tropical River: Fourier Transform Infrared Spectroscopy-Based Characterization of Abundance and Polymer Composition in Water and Sediments from Filobobos River, Mexico. Hydrology, 12(5), 124. https://doi.org/10.3390/hydrology12050124

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