Sampling
The study was carried out in Ala River. Samples were carefully collected at different points along the river flood plain at nine locations in December 2021 when the flood water had receded completely into the river channel. The Soil samples were collected from nine locations namely (Aule, Ayedun, Alagbaka, Fiwasaye, Sijuwade, St. Thomas, Okuta Elerinla, Barracks and Adebowale) along the floodplains of Ala River with the aid of soil auger to a depth 0-20 cm. At each site, random samples of soil were collected at 100m from each other; the random samples were wet at the point of collection. and stored in foil paper, it was air dried, combined and quartered to form a composite. Rice samples were collected from four locations in dry form, these are Aule A, Barracks C, Adebowale D, and Okuta Elerinla G. Vegetable samples were gotten from six locations namely Aule, Ayedun, Adebowale, Alagbaka, Okuta Elerinla and St. Thomas. The vegetables were uprooted in green and wet form, sorting was done to separate root from stem, the leave and stem was air dried and stored for analysis. Laboratory samples were air-dried for two weeks. They were then ground into fine particles in a mortar, sieved through a 2 mm mesh and quartered to get a composite sample. The soil and food samples were analyzed in duplicate.
Soil pH Determination
The pH of the soil sample was determined using a pH meter that had been calibrated with buffer solutions at pH 4 and 9. To create a soil-water solution in a 1:2 ratio, 5 grams of soil sample were measured and mixed thoroughly with 10 cm3 of distilled water in a beaker. This mixture was allowed to equilibrate for 1 hour, with stirring at 15-minute intervals. The pH value of this solution was then measured using a Jenway 2015 pH glass electrode system.
Determination of Soil Organic Matter
The total organic carbon content of the samples was evaluated according to the Walkley and Black (Walkley and Black, 1934). A 1-gram sample of air-dried soil was carefully weighed and placed into a 250 cm3 conical flask. Next, 10 cm3 of a 0.167 M K2Cr2O7 solution (prepared by dissolving 49.14 g of K2Cr2O7 in 1 liter of distilled water) was added to the flask and gently swirled to mix. Subsequently, 20 cm3 of concentrated sulfuric acid was added to the mixture, swirled to ensure thorough mixing, and then left to stand on a pad within a fume cupboard for a period of 30 minutes. Subsequently, 100 cm3 of distilled water was introduced into the mixture, followed by the addition of 3 drops of ferroin indicator. The content was then subjected to titration using a 0.5 M iron (II) ammonium sulfate solution until the color changed was observed from green to dark green shade. Once this transition occurred, ferrous sulfate was cautiously added drop by drop until the color shifted sharply from green to a distinct brownish-red colour. A blank titration was also performed following the same procedure in the absence of the sample.
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)
%TOM = %TOC X 1.724
Where, B represents the blank titre value..
T is the sample titre value.
(B-T) is the difference between the blank titre volume and sample titre volume used.
M is the molarity of ferrous sulphate used.
W is the weight in gram of the sample.
% TOC is the percent amount of total organic carbon of the sample.
%TOM is the percent amount of total organic matter in the sample
Source; Schulte 1995
Determination of Particle Size
A 51-gram air-dried soil sample was carefully placed in a 250 cm3 beaker and 100 cm3 of a 5% sodium hexametaphosphate solution (calgon) was added to the beaker and left to soak overnight. Afterward, the suspension was stirred, and the entire mixture was quantitatively transferred to a sedimentation cylinder. The cylinder was then filled up to the 1-liter mark using distilled water. To ensure thorough mixing and to dislodge the sediment, a stirrer with a circular end was inserted into the cylinder and moved both upwards and downwards within the cylinder. This process was carried out to achieve a homogeneous mixture. Stirring was concluded with two deliberate and gradual strokes. To eliminate the foam present on the surface of the suspension, a small amount of isopropyl alcohol was added. A blank determination tank was prepared and treated in the same manner as the sample sedimentation tank. Subsequently, a hydrometer was carefully lowered into the sedimentation tank, and its reading was noted after forty seconds (Ra40 secs). This procedure was repeated two hours later, and the temperature of the suspension was also recorded. The percentage sand, silt and clay were thus calculated
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)
Ra = 40 seconds blank hydrometer reading.
Rb = 2 hours blank hydrometer reading.
Rc = 40 seconds correction factor (temp >20ºC x 0.360)
Rd = 2 hours correction factor (temp>20ºC x 0.360)
W = weight of soil.
%Sand + %Silt + %Clay = 100
Sheldrick and Wang 1993
Extraction of PCBs in soil samples
10 g aliquot of well mixed soil sample was weighed into a solvent rinsed 200 ml beaker, the sample was spiked with 50 ml of 50:50 solvent mix of acetone and dichloromethane in 500 ml volumetric flask transferred to a sonicator for 20 minutes at 40 0C. The extract was filtered into a well labelled clean 100 ml round bottom flask using Whatman No. 41 filter paper. The process was repeated with an additional 50 ml solvent mix and filtered. The extract was concentrated to 2 ml with the aid of a rotary evaporator and the solvent was interchanged with n-Hexane and re-concentrated to 2 ml using gentle stream of Nitrogen gas. The sample extract was transferred to a 2 ml sample vials using a graduated pipette, ready for clean-up. Standard operating procedure was in accordance to Aziza et al., (2021) with little modification.
Cleanup of soil sample extracts
The concentrated extract was subjected to purification in a 100-200mesh multi-layer silica gel (which had earlier been activated at 130 oC in the oven for 16 hrs, cooled in a desiccator before packing the polypropylene cartridge for analysis) and alumina column consisting of 2 g of anhydrous Na2SO4, 4 g of alumina and 3 g of silica gel packed from top to bottom. The 50 ml polypropylene cartridge was mounted on the retort stand and the base was plug with sterile cotton wool. The cartridge was conditioned with 10 ml n-hexane, the eluate was collected as waste in a beaker. 1-2 ml of the sample extract was added to the head of the column using a narrow tip 2 ml graduated pipette. The column was eluted with 5 ml of n-hexane, the eluate was collected in a clean 100 ml flask. Before the column dried up, 40 ml of Hexane/Diethyl ether (50:50) was added to the head of the cartridge. This fraction was also collected and added to the 100 ml flask. The combined fraction was evaporated using rotary evaporator to 2 ml at 40oC. The extract was solvent exchanged into n-hexane by addition of 5 ml n-hexane. The extract volume was reduced to 2 ml using gentle stream of N2 gas. This was transferred into a 2 ml GC Vials with screw cap, ready for GCMS analysis (Aziza et al., 2021, Hakanson 1980).
Clean up of food sample extracts (Rice and vegetables)
The concentrated extract was subjected to purification in a 100-200 mesh multi-layer silica gel (which had earlier been activated at 130 oC in the oven for 16hrs, cooled in a desiccator before packing the polypropylene cartridge for analysis) and alumina column consisting of 2 g of anhydrous Na2SO4, 4 g of alumina and 3 g of silica gel packed from top to bottom. The 50 ml polypropylene cartridge was mounted on the retort stand and the base was plug with sterile cotton wool. The cartridge was conditioned with 10 ml n-hexane, the eluate was collected as waste in a beaker. 1-2 ml of the sample extract was added to the head of the column using a narrow tip 2 ml graduated pipette. The column was eluted with 5 ml of n-hexane, the eluate was collected in a clean 100 ml flask. Before the column dried up, 40 ml of Hexane/Diethyl ether (50:50) was added to the head of the cartridge. This fraction was also collected and added to the 100 ml flask. The combined fraction was evaporated using rotary evaporator to 2 ml at 40oC. The extract was solvent exchanged into n-hexane by addition of 5 ml n-hexane. The extract volume was reduced to 2 ml using gentle stream of N2 gas. This was transferred into a 2 ml GC Vials with screw cap, ready for GCMS analysis (Olatunji 2019).
GC-MS instrumentation and analysis
The analysis was carried out using a Hewlett Packard Model 5890 Series II plus gas chromatograph, which was equipped with a Hewlett Packard 5972 Mass Selective Detector (GC/MS). For the analysis, a HP-5 fused silica capillary column was utilized, with specifications including a length of 30 meters, an internal diameter of 0.25 mm, and a film thickness of 0.25 µm. This column was coated with a mixture of 5% phenyl and 95% methylpolysiloxane.
The temperature program for the column oven was initiated at 120 °C with a holding time of 1 minute. It was then raised to 190 °C at a rate of 20 °C per minute, further increased to 230 °C at a rate of 5°C per minute, and finally elevated to 300 °C at a rate of 25°C per minute, with a holding time of 10 minutes. The injector and detector temperatures were maintained at 280 °C and 300 °C, respectively, throughout the analysis. Helium was employed as the carrier gas, maintaining a consistent flow rate of 0.8 ml per minute. The injection process was carried out in split-less mode. For mass spectrometry, the instrument operated in electron impact ionization (EI) positive-mode, utilizing automatic gain control. The storage window was configured to encompass the mass-to-charge ratio (m/z) range from 200 to 500, employing selected ion monitoring (SIM). During data acquisition, the scan time was set at 1.0 second, with four micro-scans performed each second. The quantification of the target compounds was achieved by monitoring the characteristic ions at m/z 258, 292, 326, 362, 396, and 430. These ions were selected considering the parent ions of the group of PCBs present in PCB congener mixture Calibration was performed by injections of standard solution of MIX 19 (PCB-1, 5, 18, 31, 52, 44, 66, 101, 87, 110, 151, 153, 141, 138, 187, 183, 170, 180, 206) at 6 calibration levels (0.05, 0.1, 0.25, 0.5, 1 and 2 µg ml-1). In the investigated PCBs, no peak areas were detected in the blank samples, and the limit of detection was established as the concentration at which a signal-to-noise ratio of 3 was achieved. (LOD = 0.1 µg kg-1). Linearity was investigated within the range between: 0.01–1 µg ml-1 and the R2 values were found to be between 0.992 and 0.999.