Assessment of Phthalate Esters in the Sediment of OBA River and the Associated Ecological Risks

Phthalate esters (PEs) are esters of benzene-1,2-dicarboxylic acid and are widely recognized as a class of synthetic chemicals commonly used as plasticizers in various industrial and household products [1]. These compounds are typically stable, colorless, odorless, and tasteless, with solubility in liquids across a broad temperature range [2]. PEs are incorporated into polymers such as polyethylene, polyethylene terephthalate, polyvinyl acetate, and polyvinyl chloride (PVC) in proportions that can reach up to 60% by weight, to enhance the flexibility, durability, and extensibility of plastic materials [3]. Their widespread applications span multiple sectors, including food packaging, building materials, children’s toys, medical devices, personal care products, adhesives, detergents, agricultural inputs, paints, printing inks, coatings, textiles, and more [1,2,4]. Unlike chemically bound additives, most phthalates are physically blended into plastics, which facilitates their gradual leaching into the environment during manufacturing, usage, or disposal [5]. As a result, PEs are now frequently detected in diverse environmental matrices such as surface water, drinking water, sediments, soils, air (indoor and outdoor), dust, food, and even human urine [6,7]. Among them, compounds like diethylhexyl phthalate (DEHP) and diethyl phthalate (DEP) have been identified as endocrine-disrupting chemicals, with exposure linked to reproductive toxicity, hypertension, developmental issues, and childhood obesity [8]. Sources of phthalate pollution in aquatic environments include discharges from industrial processes, urban runoff, and the improper disposal of plastic-containing materials such as worn-out toys, plastic bottles, vehicle tires, rubber flooring, electrical cables, and paints [9,10]. Once introduced into water bodies, PEs tend to accumulate in sediments, which act as long-term sinks and potential secondary sources of contamination [11]. Sediments are composed of particulate materials that settle at the bottom of aquatic systems over time [12]. These sediments can adsorb and retain organic pollutants like phthalates, resulting in prolonged exposure risks to benthic organisms, which play key roles in nutrient cycling and aquatic food webs [13]. PEs in sediments may persist for extended durations, potentially re-entering the water column and threatening aquatic ecosystems and human populations dependent on these resources [14]. Therefore, assessing PE concentrations in sediment is crucial for understanding contamination patterns and guiding the development of effective pollution control and remediation strategies. Phthalate-induced toxicity in aquatic environments can lead to reproductive and developmental impairments in fish and invertebrates, thereby altering community dynamics and reducing biodiversity. Furthermore, phthalates may disrupt microbial populations responsible for organic matter decomposition, impacting sediment quality and ecosystem functioning [1]. Due to their bioaccumulative nature, PEs pose risks not only to aquatic organisms but also to higher trophic levels, including humans [15]. The Oba River, located in southwestern Nigeria, is an ecologically and socioeconomically important water body that supports a range of aquatic biodiversity and serves as a vital resource for domestic, agricultural, and industrial activities. Although previous studies have highlighted the river’s susceptibility to various forms of pollution [16,17], data on specific contaminants such as phthalate esters remain limited. Thus, evaluating the levels and associated risks of PEs in the sediments of the Oba River is essential for informing sustainable environmental management and public health protection efforts.

MATERIALS AND METHOD

Sample Collection

Sediment samples were collected from four designated sites along the Oba River (Figure 1). At each site, two samples (one upstream and one downstream) were obtained using a stainless-steel Peterson grab sampler. Samples were air-dried at room temperature in the laboratory and subsequently sieved through a 2 mm mesh to remove coarse particles and debris.

Physicochemical Analysis

The physicochemical parameters analyzed in the sediment samples included pH, electrical conductivity (EC), total organic carbon (TOC), and alkalinity. pH and EC were measured using the electrometric method with a Hanna instrument (Model HI 9813-5). Alkalinity and TOC were determined via standard titrimetric methods.

Extraction of Phthalate Esters

The extraction of phthalate esters (PEs) was carried out following the United States Environmental Protection Agency (USEPA) Method 3550C. Ten grams of each sediment sample were placed in a centrifuge tube with 30 mL of dichloromethane and subjected to ultrasonic extraction at 30°C for 20 minutes. This procedure was repeated three additional times using fresh solvent portions. After each sonication cycle, the solvent was decanted, and the residue was filtered using Whatman No. 41 filter paper. Care was taken to prevent the extracts from drying out to avoid loss of analytes.

Clean-up and Fractionation

The combined extracts were concentrated and purified using column chromatography. The column (10 mm internal diameter) was packed with 12 cm of activated silica gel, 6 cm of neutral alumina, and 1 cm of anhydrous sodium sulfate. Prior to sample loading, the column was preconditioned with 30 mL of a 1:1 (v/v) mixture of n-hexane and dichloromethane. The concentrated extract was then loaded onto the column and eluted sequentially with 20 mL of n-hexane and 30 mL of a 1:1 (v/v) mixture of n-hexane and dichloromethane to isolate saturated hydrocarbons and polar, respectively. The polar fractions were then concentrated using a rotary evaporator for subsequent analysis.

Instrumental Analysis of Phthalate Esters

Quantification of phthalate esters was performed using gas chromatography–mass spectrometry (GC-MS). The instrument was calibrated using a certified phthalate ester standard (2000 ppm; Catalog No. M-606, AccuStandard), containing six phthalate congeners. A four-point serial dilution (0.3, 0.6, 1.5, and 3.0 ppm) was prepared for calibration. Before analysis, the mass spectrometer was auto-tuned to perfluorotributylamine (PFTBA) to verify instrument sensitivity and the abundance of m/z 69, 219, and 502. The GC-MS was operated in both selective ion monitoring (SIM) and scan modes for high sensitivity detection. Helium served as the carrier gas at a constant flow rate of 1.2 mL/min with a nominal pressure of 0.26 psi and an average linear velocity of 40.00 cm/sec. Samples (1 µL) were injected in splitless mode at an injector temperature of 250°C. Purge flow to the split vent was set at 30.0 mL/min after 0.35 minutes, with a total flow of 31.24 mL/min. The gas saver mode was disabled. The oven temperature was programmed to start at 100°C (held for 1 min), then ramped at 20°C/min to 280°C (held for 7 min). The total run time was 12 minutes with a 3-minute solvent delay.

Statistical and Risk Analysis

Concentrations of phthalate esters in sediment samples were reported as means, standard deviations, and ranges using Microsoft Excel for Windows 10. The total concentration of PEs was obtained by summing the mean concentrations of individual congeners. A one-way analysis of variance (ANOVA) was used to assess statistically significant differences in PE concentrations across the four sampling sites. Ecological risk was evaluated using the risk quotient (RQ) approach developed by the USEPA, defined as:

RQ =  MECPNEC

Where:

MEC (Measured Environmental Concentration) is the average concentration of a specific PE in sediment,

PNEC (Predicted No Effect Concentration) for sediment is estimated using:

PNECsediment = (0.783 + (0.0217 × Koc) × PNECwater

LogKoc = 0.00028 + (0.983 × logKow)

Here, Koc is the organic carbon–water partition coefficient, and Kow is the octanol–water partition coefficient, which reflects the hydrophobicity of the compound [18].

Risk categories were defined as follows:

High risk: RQ > 1

Medium risk: 0.01 ≤ RQ ≤ 1

Low risk: RQ < 0.01

This classification helps in evaluating the ecological threat posed by phthalate ester contamination in sediment environments.

Figure 1: Location Oba River showing Sampling Points [17]

RESULT AND DISCUSSION

 Physicochemical Parameters of Sediment

The physicochemical properties of sediment samples from Iluju, Onitinrin, Obada, and Ajaawa were assessed to characterize the sediment environment and its suitability for ecological studies. The measured parameters (Table 1) were compared against United States Environmental Protection Agency (USEPA) guidelines to evaluate their environmental compliance. The pH values across all sites ranged within the USEPA-recommended range of 6.2 to 8.5, indicating a neutral to mildly alkaline environment conducive to aquatic life and sediment health [17]. Total organic carbon (TOC) content in all locations also fell within the acceptable range of 1–5%, reflecting moderate organic matter presence that can support microbial activity and nutrient cycling [19]. Electrical conductivity (EC) values ranged between 10–20 µS/cm, aligning with USEPA standards and indicating low salinity and mineral ion content typical of freshwater, non-saline environments [20]. Alkalinity values were within the permissible range (20–200 mg/L) at three sites, with Iluju (15 mg/L) and Onitinrin (12.5 mg/L) slightly below the threshold. Although marginally lower, these values may still provide some buffering capacity against acidification. Ajaawa exhibited the highest alkalinity (22.5 mg/L), suggesting a greater ability to neutralize acidic inputs and maintain pH stability [21].

Table 1: Result of Physicochemical Parameters in the Sediment

Concentration of Phthalate Esters in Sediment

Phthalate ester (PE) concentrations were analyzed at all four sites, with DBP, DEP, BBP, DEHP, DnOP, and DMP detected in all sediment samples (Table 2). The total concentration of the six phthalates (∑6PAEs) was highest at Obada (452.61 mg/kg), followed by Iluju (405.56 mg/kg), Onitinrin (281.48 mg/kg), and Ajaawa (279.36 mg/kg). Variations in concentration may reflect differences in anthropogenic activities such as plastic waste disposal, population density, and the nature of agricultural and commercial practices. Environmental factors like microbial degradation, hydrolysis, and photodecomposition may also influence the persistence and spatial distribution of PEs in sediments [22]. The average concentrations of individual phthalates in mg/kg were as follows: DMP (0.002–0.191), DEP (0.049–0.920), DBP (0.454–8.069), BBP (0.075–2.092), DEHP (90.614–734.732), DnOP (1.403–69.593) The order of abundance was: DEHP > DnOP > DBP > BBP > DEP > DMP. DEHP dominated all sites, accounting for 86.5% to 94.9% of total PEs, followed by DnOP (3.5%–12.6%). This predominance is consistent with their extensive use in the production of PVC, coatings, and flexible plastics due to their low volatility and high plasticizing efficiency. DBP and BBP occurred at lower concentrations, with Iluju recording the highest levels of both. DMP and DEP exhibited the lowest concentrations across all sites. Comparing these values with USEPA's sediment Ecological Screening Levels (ESLs), DMP (0.81 mg/kg), DEP (1.3 mg/kg), and BBP (0.81 mg/kg) were below guideline values, indicating minimal ecological concern. However, DBP (0.81 mg/kg), DEHP (2.4 mg/kg), and DnOP (1.1 mg/kg) all exceeded their respective limits—DEHP, in particular, exceeded the standard by over 230 times, highlighting a severe contamination risk.  Comparative analysis with global studies (Table 3) shows that PE concentrations in Oba River sediments are considerably higher than those reported in other regions, with the exception of Harike Wetland in India [18]. Consistent with prior findings by [7,23,24,25], DEHP emerged as the dominant plasticizer in this study. Its persistence in sediment is likely due to its high log Kow value and the increased affinity of its branched alkyl chains for sediment particles, which enhances sorption and reduces biodegradation [18].

Table 2: Summary of the individual concentration of PEs (mg/kg) in sediment of Oba river

Table 3: Comparison of PEs concentration in sediments Oba River with other areas in the world (mg/kg)

Ecological Risk Assessment

The ecological risks posed by PEs in sediments were evaluated using the risk quotient (RQ) approach (Table 4). RQ values were calculated for DMP, DEP, DBP, and DEHP to assess potential harm to aquatic life. The descending order of risk was DBP > DEP > DEHP > DMP, with the following RQ values: DBP (0.456), DEP (0.227), DEHP (0.040), DMP (0.023) All RQ values fell within the medium risk category (0.01 ≤ RQ < 1), implying that these phthalate esters may pose moderate ecological risks to benthic organisms in the Oba River ecosystem. Among the four, DBP presented the highest risk, likely due to its relatively high concentration and known toxicity to aquatic invertebrates and fish, including endocrine disruption and reproductive impairment [1]. The cumulative risk, represented by the Total RQ value of 0.746, approaches the threshold for high ecological risk (RQ ≥ 1). This indicates that combined exposure to multiple phthalate esters even when individual compounds fall below high-risk levels can synergistically increase environmental stress, especially with long-term exposure. Although DEHP showed a relatively low RQ in this study (0.040), it was the most abundant compound detected in sediments, accounting for over 85% of total PEs. Its long-term presence may pose chronic risks not fully captured by short-term RQ calculations. Furthermore, the low RQ values of DMP and DEP do not necessarily imply safety, as chronic low-level exposure can still disrupt hormonal systems, especially in early life stages of fish and invertebrates. DMP and DEP are known to interfere with enzyme activity and hormone regulation even at sublethal concentrations, potentially affecting growth, development, and reproductive success in exposed populations [1]. The ecological implications of these findings are concerning. Prolonged exposure to moderate levels of phthalates can lead to reduced species diversity, altered community structures, bioaccumulation in aquatic food chains, potentially impacting higher trophic levels including fish and humans, Sediment toxicity, affecting benthic fauna that play critical roles in nutrient cycling and sediment aeration [1,7,10].

Table 4: Ecological Risk Assessment of phthalates esters (mg/kg)