Investigation of heavy metals adsorbed on microplastics in drinking water and water resources of Zabol

Abstract

This study investigates the presence and adsorption of heavy metals (HMs) on microplastics (MPs) in the drinking water and water resources of Zabol, Iran. Sampling was conducted at five stations in Zabol and two Chah-Nimeh reservoirs (CHWRs), the primary drinking water sources, using glass samplers and polyvinylidene fluoride membrane filters. Fourier-transform infrared spectroscopy, scanning electron microscopy, and optical microscopy identified MPs, while inductively coupled plasma optical emission spectrometry quantified heavy metals. Results revealed that polyethylene, polystyrene, polyamide, and polypropylene were the predominant polymers, with Fe, Mn, Zn, Cu, and Ni adsorbed on their surfaces. Iron (Fe) exhibited the highest concentration, reaching 84 µg.L⁻¹ in CHWRs and 85 µg.L⁻¹ in the distribution network. In CHWRs the highest percentage (61%) of size of MPs were between 500 and 1000 μm, while in water distribution network maximum range of obtained MPs were between 50 and 100 μm. The average concentration of the other detected heavy metals were as: zinc (Zn) 43.9 µg.L^− 1 and 7.75 µg.L^− 1, manganese (Mn) 11.48 µg.L^− 1 and 6.5 µg.L^− 1, arsenic (As) 20.22 µg.L^− 1 and 0.26 µg.L^− 1, cadmium (Cd) 3.16 µg.L^− 1 and 2.8 µg.L^− 1, copper (Cu) 15.18 µg.L^− 1 and 0.03 µg.L^− 1, and nickel (Ni) 0.10 µg.L^− 1 and 2.25 µg.L^− 1 in distribution network and CHWRs, respectively. World Health Organization recommended the permissible limit of these cations in drinking water as 10 µg.L^− 1 for As, 5 µg.L^− 1 for Cd, 50 µg.L^− 1 for Cr, 20 µg.L^− 1 for Ni, 10 µg.L^− 1 for Mn, 500 µg.L^− 1 for Zn, 300 µg.L^− 1, 2000 µg.L^− 1 for Cu and 300 µg.L^− 1 for Fe. The study highlights MPs as carriers of toxic heavy metals, presenting significant health and environmental risks. This novel research emphasizes the impact of secondary pollution and water treatment processes on MP fragmentation and HM contamination. Recommendations include adopting enhanced water treatment protocols to mitigate MP and HM risks, implementing stricter quality monitoring at all stages of water distribution, and promoting public awareness of plastic pollution. Future studies should explore the health effects of MPs and HMs, optimize sampling methods, and focus on long-term monitoring under diverse environmental conditions to address this emerging issue comprehensively.

Introduction

This study revealed critical insights into the presence of microplastics (MPs) and their role as carriers of heavy metals (HMs) in the drinking water sources and distribution network of Zabol, Iran. Key findings include the dominance of polyethylene and polystyrene among MPs, significant differences in MP size and shape between different sampling locations, and the high concentration of iron as the most prevalent heavy metal. Additionally, the adsorption of HMs onto MPs underscores the risk of these pollutants acting as vectors for toxic elements in drinking water systems.

Microplastics (MPs) are small plastic debris, often ranging in size from several millimeters to less than 5 mm. They are emerging pollutants that enter drinking water, water sources, and sewage, posing a threat to humans and aquatic organisms¹. Recent research has shown that the consumption of MPs by aquatic organisms can cause serious harm such as reproductive damage and organ blockage. MPs ingested by fish can be transferred to humans through the food chain, thereby exposing people to toxic additives used in plastic production^2,3. Additionally, recent studies have showed that plants can absorb pollutants from the soil, which are then consumed by insects and accumulate in their bodies as nanoplastics⁴. Scientists have reported finding MPs in human feces, confirming that humans are part of the food chain and directly affected by MPs,⁵. Zhu et al.⁶ stated that MPs between 20 and 100 micrometers have accumulated in all body tissues. The respiratory system has shown the highest presence of MPs⁷. Hu et al.⁸ revealed the presence of MPs in human testes and their potential association with sperm count and the weights of testes and epididymis, raising significant health concerns.

The main concern with MPs is the toxic compounds and elements added during the plastic production. MPs and their additives, such as stabilizers or pigments, can enter to the environment and cause many dangers in the lives of animals For example; additives such as calcium carbonate and sodium stearate are included to reduce costs⁹. These additives are released from MPs after human consumption and can cause serious health problem for the human such as cancer or corrosion of internal organs. Bisphenol A is added to plastic as a stabilizer and used as an additive in many consumer products such as water bottles, soft drink bottles, disposable tableware and electronic products. If bisphenol A ingested, it can act as a hormone blocker and directly harm human health¹⁰.

One of the important characteristics of MPs is their surface hydrophobicity, which causes the absorption of many contaminations such as heavy metals (HMs), aromatic and chlorinated compounds. HMs are compounds that are introduced into the environment from different sources, including from MPs¹¹. Song et al. have reported that there was a relatively good significant linear relationship between the risk index of HMs that confirmed MPs can adsorbed and carried HMs¹².

Industries, automobiles, polluted soils and sewage are some sources of HM emissions. Exposure to HMs can result in a variety of health issues, including cancer, neurological disorders, skin conditions, blood problems, cardiovascular problems, kidney damage, and tissue accumulation. Lead, even in small amounts, causes learning disorders, hyperactivity and reduced intelligence^13,14. Mercury is an important factor in the occurrence of depression, fatigue, headache, and forgetfulness due to its special affinity for brain tissue¹⁵. In addition, cadmium causes adverse effects on the kidney and bone lesions such as osteoporosis and osteomalacia¹⁶. Arsenic is highly toxic, and absorption of large amounts causes gastrointestinal symptoms. Cardiovascular disorders and death are complications of arsenic ingestion. Lung, bladder and kidney cancer have been seen in people who have consumed water containing arsenic¹⁷.

MPs are found in oceans, seas and other water sources. Various amounts of these pollutants have been found in drinking water. Bottles containing drinking water, urban and rural water distribution networks and water storage tanks also have many evidences of the presence of MPs^18,19.

The city of Zabol is located in the northeastern part of Sistan and Baluchestan province, Iran. This region experiences a hot and dry climate with an average annual temperature of 22.9 °C. The only source of water supply in the area is the Hirmand River, which originates in Afghanistan. To conserve water during droughts, four natural reservoirs, called Chah-Nimeh, were engineered to store water. The water for the Chah-Nimeh reservoirs (CHWRs) comes from the Hirmand River. A 220-kilometer pipeline connects one of the CHWRs to Zahedan, the provincial capital, supplying drinking water to approximately 600,000 people. Additionally, the CHWRs are the primary water source for agriculture in the Sistan region. In recent years, the interruption of the Hirmand River’s water flow has significantly reduced the water volume in the CHWRs, resulting in the drying up of two out of four CHWRs at the time of this research. This reduction in water, along with the increasing intensity of dust storms and human activities, has led to an increase in pollution levels in the CHWRs, raising concerns among researchers about the rising levels of HMs and other pollutant such as MPs. By examining the geochemical properties of the CHWRs, Hosseini et al., showed that human activities, evaporation, and dust storms have increased the pollution of the water resources. They indicated that Chah-Nimeh number 4 has the highest level of pollution²⁰.

Due to the above-mentioned reasons, this research was conducted to investigate the presence of MPs in the CHWRs, as well as the urban water distribution network of the city of Zabol. Additionally, the concentration of HMs was measured in both CHWRs and the urban water distribution network. Finally, the number of HMs adsorbed on MPs was determined in the distribution network and drinking water supply resources.

Materials and methods

Sampling areas

This study was conducted in spring and winter of 2023. Thirty samples were collected from the water sources of CHWRs. The reduced water level in the reservoirs limited the sampling areas. Additionally, for the evaluation of the number of MPs and HMs, 40 samples were collected from the water distribution network in different parts of Zabol city. Sampling was performed from consumption taps without any protective equipment’s or net. Figure 1 shows the geographical coordinates station in CHWRs and water distribution network.

Fig. 1.

Geographic location of the studied and sampling points (Zabol city: red).

Sampling containers

500 mL glass containers were used for sampling, which were thoroughly washed with dilute hydrochloric acid and then distilled water 24 h before sampling. During sampling, the container was filled with water and the pH of the sample was stabilized with 65% nitric acid near 2.

Separation of MPs from water samples

Water samples were filtered using a polyvinylidene fluoride filter. A vacuum pump was used for better smoothing. When all the water samples were filtered, the membrane was dried for 24 h. Then, MPs were removed from the membrane using a tweezers for analysis. Different standard protocols such as pressure and hot needle tests were used to detect MPs²¹.

Quality assurance and control

To minimize the risk of sample contamination, strict protocols were applied throughout the experimental procedure. All apparatus were accurately cleaned with distilled water, dried, and sealed with aluminum foil prior to use. Laboratory personnel adhered to wearing cotton lab coats, using glass containers, and performing experiments with gloves. The workspace was continuously cleaned with ethanol, and air currents were minimized by closing windows during the analytical phase. Control samples were prepared concurrently to assess background contamination levels, which were found to be negligible.

Validation of the method

To validate the microplastic (MP) extraction method, the recovery rate was estimated to ensure efficiency. A total of 60 plastic particles, including polystyrene (PS), polyamide (PA), polypropylene (PP), and polyethylene (PE), were spiked into water samples free of MPs. The plastic particles were shaved and crushed with a razor and scissors to achieve different sizes and shapes, and then added to the samples. This mixture was then extracted from the water according to the described separation method. The recovery rate of the microplastic particles ranged from 98 to 100%.

Heavy metals analysis

The amount of HMs was determined using an inductively coupled plasma - optical emission spectrometry instrument (model GENESIS, Spectrum Arcos, Germany). The measurement conditions of instrument were as, power output of 1400 watts for generating radio frequency, a plasma gas flow rate of 14.5 L. min^− 1, an auxiliary gas flow rate of 0.8 L. min^− 1, and nebulizer gas flow rate of 0.8 L. min^− 1. Other measurement specification of ICP-OES were set as, totally 240 s for sample uptake time, rinse time of 45 s and measurement replicates of 3. To determine the levels of HMs zinc, lead, cadmium, cobalt, and nickel, analytical wavelengths of 324.7 nm, 240.7 nm, 226.5 nm, 220 nm, and 213.9 nm were used, respectively.

Statistical analysis

SPSS version 24 software was used for statistical analysis. The Shapiro-Wilk test (a more common normality test than the Smirnov-Kolmogorov test) was used to check for the normal distribution of the data. Descriptive statistics, including means, and paired t-tests were used to compare the average frequency of MPs between two different seasons. Pearson correlation coefficients were used to determine the relationship between the abundance of MPs at different sampling points.

Results and discussion

Presence of MPs

The results of this research showed that MPs are present in all the samples collected from the CHWRs. Figure 2 shows the number of MPs in these water sources. As can be seen, the number of detected MPs in spring for CHWRs No. 1 and CHWRs No. 3 were 49, 47 MPs/m^3, respectively. In winter number of MPs were increased and were 55, 61 MPs/ m³. Also, MPs were observed in all samples in the water distribution network. Also, results showed that average abundant of MPs in water distribution network were 56 and 66.2 MPs/ m³ in spring and winter, respectively.

Fig. 2.

Presence of MPs in water resources (A) and water distribution network (B) (blue: spring, brown: winter).

Shape and size of MPs

The most common shapes of MPs in CHWRs No. 1 and No. 3 during spring were fragmented (53% and 47% frequency, respectively), while in winter, the fragmented shapes reached 60% and 50% for CHWRs No. 1 and No. 3, respectively. Other MP shapes in CHWRs during spring included fibers (average frequency of 37% in both reservoirs), foam (average of 6% in both reservoirs), and granular (average of 6% in both reservoirs).

In the water distribution network, fragmented, fiber, and granular MPs dominated in spring, with average frequencies of 49.2%, 38.4%, and 8.6%, respectively. Winter showed similar shapes, with fragmented MPs having the highest percentage detected in both CHWRs and the distribution network. However, at sampling station 3 (water treatment plant), fiber MPs outnumbered fragmented MPs in spring. Figure 3 illustrates the shapes of MPs in the water reservoirs and distribution network. The maximum average size of MPs in the distribution network ranged from 50 to 100 μm, with average frequencies of 47.85% and 48.71% in spring and winter, respectively. In water reservoirs, the maximum sizes of MPs ranged from 500 to 1000 μm, with average frequencies of 45.5% and 49.5% in spring and winter, respectively. Figure 4 shows the range sizes of MPs in the water distribution network.

Fig. 3.

Shapes of MPs in water reservoirs (A) and water distribution network (B).

Fig. 4.

Sizes of MPs founded in water distribution network and CHWRs.

Types of MPs

A Fourier-transform infrared spectroscopy (Shimadzu, Japan) was used to identify MPs. Figure 5 shows the types of obtained MPs. As can be seen, the most common types of MPs in CHWRs are PE with average frequency of 32% in both spring and winter. Figure 6-a depicts the shapes of PE MPs fiber under a scanning electron microscope image (CIQTEK, China), and Fig. 5-b shows the FT- spectrum of a PE MP that was founded in water resources.

Fig. 5.

Types of MPs in water distribution network (S1-S5) and water resources (CH1, CH3).

Fig. 6.

Scanning electron microscope image of a MP fiber (a), FT-IR spectrum of a poly ethylene MP (b).

After that, PS (26%), PP (25.5%), PA (12.5%) and PET (3.5%) are the other types of MPs in CHWRs. In the water distribution network, the most detected polymers were PS with an abundant of 32.6%. PE (29.2%), PP (21.6%) and polyamide (11.8%) are other dominant types of polymers.

Presence of heavy metals

Heavy metals in water resources and water distribution network

Figures 7 and 8 show the amount of HMs in CHWRs and in the water distribution network. As shown in Fig. 8, the highest amount of HMs in the water distribution network, corresponds to Fe and Zn with concentration of 84 µg.L^− 1 and 65 µg.L^− 1. Similarly, in water reservoirs, the highest concentrations are is related to Fe and Cd with concentrations of 85 µg.L^− 1 and 35 µg.L^− 1 respectively.

Fig. 7.

Concentration of HMs in water distribution network.

Fig. 8.

Concentration of HMs in water resources.

Heavy metals on MPs

After detecting and analysis of MPs, measurement of HMs was performed. For this purpose, all recorded MPs were used for sample preparation and measurement process. Results are shown in Fig. 9. Among the HMs, Fe with a concentration of 0.04 µg. L^− 1, Ni with of 0.008 µg. L^− 1 and Pb with 0.07 µg. L^− 1 have the highest concentration on MPs in CHWRs. Other extracted HMs from MPs were Cr, Cd, Zn and cu with concentration of 0.002 µg. L^− 1, 0.001 µg.L^− 1, 0.0068 µg. L^− 1 and 0.005 µg. L^− 1, respectively. Figure 9 also shows that amount of HMs that extracted from MPs in water distribution network. As can be seen, Fe with concentration of 0.035 µg.L^− 1, Mn, 0.03 µg.L^− 1, Pb, 0.008 µg.L^− 1 and Zn, 0.0054 µg.L^− 1 were metal ions that measured on MPs, respectively.

Fig. 9.

Average concentration of the adsorbed HMs on MPs in water distribution network and CHWRs.

Discussion

Water pollution is one of the major problems and challenges in the world. One of the main causes of diseases and mortality globally is water pollution. Groundwater sources are typically exposed to various pollutants. Depending on the nature and origin of the pollutants, whether natural or anthropogenic, different classifications for the pollutants found in CHWRs can be considered. These pollutants can be divided into three groups: organic substances, inorganic substances, and physical factors. The main organic pollutants in CHWRs include detergents, agricultural runoff, pesticides and herbicides, petroleum products from motor vehicles such as boats, plant and tree debris, and volatile organic compounds. The main inorganic pollutants in CHWRs include acidity from chemical fertilizers, HMs, and salts. Inorganic pollutants cause water turbidity and can sometimes be observed as suspended particles in the water. Sudden changes in the acidity and temperature of water sources due to human activities are also considered a type of physical water pollution. Additionally, some pollutants such as viruses, bacteria, and parasites have a biological origin. Pollution from agricultural lands, leachate from wastewater, the use of chemical fertilizers and pesticides, and the discharge of petroleum pollutants onto permeable land and water sources can be sources of pollutants entering CHWRs. Furthermore, the proximity of local livestock grazing areas to water sources is another way that waste can enter the natural reservoirs of CHWRs.

Microplastics abundance

The abundance of MPs in water sources depends on various factors such as the presence of cosmetics and health industries, erosion of tires, synthetic fibers and materials used in building²². Another important source of production and release of MPs in environment is the decomposition of plastic waste left in the environment due to physical, chemical and biological destruction processes²³. The drying of Hirmand has limited the water resources of Sistan. At the time of conducting this research, out of four water resources, only two reservoirs could be used. These conditions have caused an increase in the number of MPs as one of the main places of deposition of atmospheric MPs in Sistan. Compared to the results of Taghipour et al.,(43 MPs/m3), the number of MPs in both CHWRs has increased²⁴. Statistical analysis showed that the difference between the number of MPs in the two reservoirs was not significant (P > 0.05). But in spring and winter, the difference of MPs in both reservoirs were significant (p < 0.05). The increase in the intensity of dust storms and, as a result, the increase in the amount of atmospheric MPs and settling in the reservoirs, was one of the reasons for the increase in the number of MPs in the water reservoirs. In water distribution network, the number of detected MPs were more than from CHWRs. The use of disinfectants such as chlorine in the long term causes oxidation of water pipes. In addition, scratches, breaks and other defects in the transmission pipes cause the presence of MPs in the water distribution network. Furthermore, the use of household water storage tanks, which are used in people’s lives, is another reason for the presence of MPs in the water distribution network. Because these tanks are placed in roof or yard and directly exposed to the UV radiation of sunlight and these conditions caused to the photo- oxidative of plastic tanks that is one way to release of MPs in drinking water. No statistically significant difference was observed between the number of MPs in different sampling station (p > 0.05).

Recently, Studies have consistently found MPs in rivers, lakes, and oceans. Lu et al., demonstrated that that urban rivers had higher concentrations (1.8–2.4 items. L^− 1) of MPs compared to rural rivers (0.9 items.L^− 1), with polyethylene and polypropylene being the most common types²⁵. In our study, the number of founded MPs in distribution network, were more than from rural CHWRs water.

Esfandiari et al., revealed that MPs are also present in groundwater, though at lower concentrations (0.1 to 1.3 MP.L^− 1) than in surface water. The study suggested that MPs can infiltrate groundwater through soil and aquifer systems²⁶.

Size, shape and type of polymers

Figure 3 shows that the maximum obtained size of MPs with an average frequency of 61.5% in spring and winter were in the range of 50 to 100 μm. There is no significant difference observed between the size of MPs in the spring and winter seasons in the water distribution network(p > 0.05). The results of previous research indicated that in raw water sources such as lakes and natural reservoirs, the size of MPs is larger than the MPs in treatment water.

The shape of MPs depended on the environmental conditions such as heat, amount of displacement and type of crushing and abrasion. As resulted in this research, the fragmented form of MPs was the dominant shape of MPs in all stations of the distribution network and water sources. Fiber, granular and other forms such as foam were also among the detected MPs.

The small size of MPs makes them more likely to be ingested by aquatic organisms, potentially leading to bioaccumulation and biomagnification in the food web. The diversity in composition also suggests that different sources contribute to MPs pollution, necessitating targeted mitigation strategies. MPs in water sources vary in size, with most studies reporting particles in the range of 1 μm to 5 mm. A study by Bakir et al., revealed that smaller MPs (< 100 μm) were more abundant in marine environments, posing a greater risk to marine life²⁷.

The composition of MPs varies depending on the source. For instance, a study by Ben-David demonstrated that fibers from synthetic textiles were the dominant form of MPs in urban wastewater, while fragments from degraded plastic items were more common in rural areas²⁸. simillary, in our study fragmented form of MPs in rural CHWRs were more than other forms.

Determining of polymers is chemically important, because it influenced the ecological risk of MPs. Results of FT-IR analysis showed that polyethylene, polypropylene, polyamide and polystyrene are the most abundant polymers in water sources of water reservoirs.

Polymers of this type are frequently utilized in textile applications. O’Brien et al. demonstrated that the MP polymer composition found in the environment primarily originates from wastewater sources, such as laundry effluent²⁹. Polypropylene and polyethylene are commonly employed in the manufacture of ropes and fishing nets³⁰. Additionally, polypropylene fibers are extensively used in the production of floor coverings, carpets, rugs, and sports apparel. Polypropylene also finds applications in agricultural mulch, packaging bags, and ropes. Due to its high strength and durability, polyamide is used in the production of equipment like ropes, safety belts, parachutes, and fishing nets, as well as in the manufacture of synthetic fibers for clothing, socks, and carpets³¹. Therefore, the use of fishing nets and ropes by fishermen, deposition of atmospheric MPs and sewage can be one of the factors that enter these MPs into of CHWRs. Shahraki et al. evaluated atmospheric MPs of Sistan and results showed that PE are maximum type of airborne MPs in Sistan³².

In the water distribution network, the most polymer was polystyrene. Investigating the types of MPs in drinking water showed that the presence of different types of MPs is related to their production and consumption.

Ecological Impacts of MPs surveyed by Du et al. Their research indicated that MPs can cause physical harm to aquatic organisms, such as blockages in the digestive tract and reduced feeding. Additionally, MPs can adsorb harmful chemicals, which can be transferred to organisms upon ingestion³³.

The ecological impacts of MPs could disrupt aquatic ecosystems and reduce biodiversity. For humans, the long-term health effects of microplastic ingestion are still not fully understood, but there is growing concern about their potential to cause inflammation, oxidative stress, and other health issues..

Concentration of HMs in water reservoirs

The concentration of HMs in water reservoirs and water distribution network are shown in Figs. 7 and 8. As depicted in these Figures, Fe ions has highest concentration of HMs in CHWRs (85 µg.L^− 1) and water distribution network (84 µg. L^− 1). Arsenic has lowest concentration among the measured HMs in CHWRs with concentration of 0.1 µg.L^− 1, but in water distribution network concentration of arsenic is increased and was about 34 µg. L^− 1. Cu, Pb, Cr and Zn are also other HMs that have higher concentration in CHWRs than the water distribution network. Multiple fractures of transmission and distribution pipes, leaks from worn pipes, contamination of domestic water tanks, existence of aged metal piping network and defects in the purification process are factors that influenced presence and variety of HMs in the drinking water distribution network^34,35. The concentration of none of the HMs were higher than the permissible values according to the Environmental protection agency (EPA).

Adsorption of HMs by MPs are spontaneously and surface of MPs are controlled adsorption process. MPs do not have porosity for adsorption of HMs, but after photo- oxidation, abrasion and biodegradation in natural water, they get negatively surface charge and can adsorbed chemically HMs³⁶ Amount of extracted HMs from MPs in CHWRs and water distribution network indicated that some HMs are adsorbed and transmitted with MPs. highest concentration was related to the Fe ions in CHWRs and water distribution.

HMs in drinking water pose significant health risks due to their toxicity, persistence, and ability to accumulate in the human body. Health effects of some HMs in water resources can be concluded as bellow:

Neurological damage, especially in children (reduced IQ, learning disabilities, and behavioral problems). Cardiovascular effects in adults (increased blood pressure, hypertension). Kidney damage and reproductive issues³⁷ from Pb, skin lesions, cancer (skin, lung, bladder), and cardiovascular diseases from As³⁷, Kidney damage and bone demineralization (osteoporosis) and Gastrointestinal and respiratory issues from exposure to Cd³⁷ and other effect such as Gastrointestinal distress (nausea, vomiting, diarrhea), Allergic reactions (dermatitis) were reported.

Comparison with other reported research

Table 1 shows a comparison of this research with other similar reported work in the world. As shown in Table 1 several types of HMs were adsorbed on MPs. Different in type and physical characteristic of MPs in other places indicated that geographic condition influenced the MPs properties. Also, sampling methods, sampling season and type of water reservoirs (river, lake and well) are affected abundant of MPs. HMs are being discharged into the environment from several sources such as effluents of various industries, releasing by vehicle exhausts and roadside soils and from transmitted dust by wind. In this study, some HMs Fe, MN, Zn and Cu detected and measured on MPs. Concentration of these metals in comparison of other location showed that measured HMs in CHWRs were less than other places. PE, PP and PS were highest obtained polymers in this study that are similar to other reported work in Table 1.

Table 1.

Comparison of characteristics of mps, HMs and adsorbed HMs on MPs in this study and other location of world.

Polymer	Shape	Abundant	Heavy metals concentration (water sources)	Heavy metals on MPs	Location	Refs.
PP, PE, PS	Fragmented, pellets, films, fibers	155 MPS/m3	Cr (1.1), Cu (0.05), Ni (0.05), Pb (0.01)	Cr(2.95), Cu(13.02),Ni(0.78),Pb(17.61)	Chao Phraya River, Thailand	38
PP, PE, PS	Fragmented, pellets, films, fibers	63MPs/m3	Cr (0.01), Cu (0.005), Ni (not detected), Pb (0.005), Cd (0.004), Zn (0.003)	Cr (17.3), Cu (14.5), Ni (3.52), Pb (38.67), Cd (2.81), Zn (391.2)	Chao Phraya River Estuary, Thailand	39
Not determined	Pellets, fragments, granules, filaments, films, foams	33,561 MPs/m3	Al (0.85), Cu (0.845), Fe (0.917), Ni (0.87), Mn (0.929), Ti (0.85), Zn (0.917)	Not determined	Coastal areas of Bandar Abbas, Iran	40
PP, PE, PES, PVC, nylon	-	-	Pb (0.068), Cu (0.018)	Pb (2.21), Cu (0.36)	Musi River, Indonesia	41
PE, PP, PA, PS, PET	Fragmented, fibers, granular, foam	61 in water resources	As(0.42), Cd(35), Cr(3), Ni(4), Pb(0.95), Mn(7.5), Zn(8.5), Cu(0.04), Fe(85)	As(0.002), Cd(0.0010), Cr(0.005), Ni(0.005), Pb(0.008), Mn(0.002), Zn(0.0068), Cu(0.005), Fe(0.04)	Chah-Nimeh reservoirs	This study

Limitation of the study

The sampling process for water from the CHWRs faced several challenges due to extreme environmental conditions. Harsh weather, including high winds and lightning, posed safety hazards during fieldwork, while stormy conditions in summer often introduced contaminants such as debris, sediment, and runoff into the samples. Accessibility to sampling sites was further complicated by flooded or muddy terrain, and rapidly changing water levels and flow rates made it difficult to obtain representative samples for time-sensitive analyses. Equipment also faced risks of damage under these conditions, while high turbidity during storms required additional filtration or pretreatment steps to ensure reliable results. Furthermore, the limited availability of water in CHWRs during droughts made it challenging to collect adequate sample volumes, with high temperatures and evaporation altering the chemical composition of some samples before analysis. Seasonal variability in water quality added another layer of complexity, highlighting the need for long-term monitoring to address these limitations effectively.

Conclusion

This study is the first to comprehensively investigate MPs and HMs in drinking water from source to distribution in the Zabol region, Iran. The results revealed a significant presence of MPs and HMs in both the CHWRs and the water distribution network, with fragmentation of MPs and secondary contamination contributing to the observed patterns. Notably, polyethylene was the dominant polymer in CHWRs, while polystyrene was prevalent in the distribution network, indicating differing pollution sources and degradation processes. Among the HMs, iron exhibited the highest concentration, both in water samples and adsorbed onto MPs, underscoring the role of MPs as carriers of toxic substances.

The novelty of this study lies in its holistic approach to linking MP fragmentation, polymer composition, and HM adsorption in a unique geographic setting. This is among the first reports to highlight the interplay of atmospheric deposition, water distribution infrastructure, and climatic factors on MP and HM contamination. These findings contribute valuable data to the growing global concern about MPs and their role in transporting hazardous materials.

The study is not without limitations. Extreme weather, high turbidity, and limited sampling windows impacted the data collection process. Seasonal variations and environmental challenges also underscore the need for more extensive, long-term studies to better understand MP behavior and HM dynamics under varying climatic conditions.

Future research should focus on advancing sampling methodologies and exploring the implications of MP-HM interactions on human health and aquatic ecosystems. Improved water treatment and monitoring strategies are essential to mitigate the risks posed by MPs and their associated contaminants. This study provides a foundation for further exploration of MPs as vectors for environmental pollutants in regions facing similar climatic and ecological challenges.

Author contributions

Mohammad Reza Rezaei Kahkha: Conceptualization, Methodology, Writing – Original Draft Preparation, Data Curation; Jamshid Piri: Writing – Review & Editing, Visualization; Ali Faghihi Zarandi: Investigation, Writing – Review & Editing; Massoud Kaykhaii: Conceptualization, Writing – Review & Editing. All authors reviewed the manuscript.

Data availability

All data is provided within the manuscript.

Declarations

Competing interests

The authors declare no competing interests.

Ethical

All authors have read, understood, and have complied as applicable with the statement on “Ethical responsibilities of Authors” as found in the Instructions for Authors.