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.