*2.4.4. Metal analysis*

collected for PAHs and organochlorine pesticides analyses were not air-dried to minimise

Approximately 20.0 g each of the air-dried and sieved soil samples (< 2 mm) were weighed, 50 ml distilled water was added and then mixed and allowed to stand for 30 minutes. Sus‐ pension was stirred every 10 minutes during this period. The suspension was allowed to settle for another 30 minutes and a pH meter electrode (Jenway 3510) was placed in the suspension and the reading was taken after some seconds. The electrode was removed from the suspension

About 0.70 g each of the air-dried and sieved soil samples were weighed and 10.0 ml of 1.0 N K2CrO7 solution and 20.0 ml concentrated H2SO4 were added immediately [69]. The solution was mixed for 1 minute by swirling until the soil and reagents were thoroughly mixed. The mixture was allowed to stand for 30 minutes after which 100 ml distilled water was added and then allowed to cool under running water. 3-4 drops of Ferroin indicator was added and then titrated with 0.5 N Ferrous Ammonium Sulphate. Near the end-point, the solution took a greenish colour and later changed to dark green. At this point, the 0.5 N Ferrous Ammonium Sulphate was added drop by drop until the colour changed from green to reddish-brown at the end point. The Ferrous Ammonium Sulphate solution was standardized using 1 N potassium dichromate before use. Blank titration was carried out in the same manner, but

and rinsed thoroughly with distilled water and excess water was carefully dried.

*% Organic Carbon* <sup>=</sup> (*meq*. *FAS for blank* <sup>−</sup> *meq FAS for sample*) (0.003 *<sup>x</sup>* <sup>100</sup> *<sup>f</sup>* )

meq- Milligram equivalent = normality of solution x ml of solution used.

*2.4.3. Determination of soil particle size by the Bouyoucos hydrometer method*

*Weight in g of air* −*dried soil*

This is a measure of the size distribution of individual particles in a soil sample. Particle size distribution analysis is often used in soil science to evaluate soil texture. The soil texture is based on different combination of sand, silt, and clay separately, that makes up the particle size distribution. The size range of sand is 2.0 mm-0.05 mm, silt is 0.05 mm-0.002 mm and clay is less than 0.002 mm (< 0.002 mm). Particle size distribution analysis result can be used to predict the water retention capacity and unsaturated hydraulic conductivity of soils [70]. About 50.0 g of air-dried soil sample sieved to < 2 mm was weighed into 250 ml beaker. 20 ml

loss of those components which can vaporise easily [68].

**2.4. Analytical procedure**

without the soil sample.

Where, f = 1.334

*2.4.1. Determination of soil pH*

236 Environmental Risk Assessment of Soil Contamination

*2.4.2. Determination of soil organic matter*

The % organic Carbon was determined as follows:

% Organic matter = 1.729 x % Total Organic Carbon

Acid digestion procedure was employed for the determination of the concentrations of the environmentally available metals in the soil samples. Prior to the determination of the metal concentration in soil, sample digestion is a necessary pre-treatment step. The most common method for determining the concentrations of metal contained in a soil sample is the total elemental analysis. The review on total metal concentration determination in soil samples revealed that heavy metals bound to non-silicates and silicate structures were determined with concentrated acids such as HCl, HClO4, and HF [71]. However, in recent times, strong acid extractant such as concentrated nitric acid, aqua regia (a mixture of concentrated HCl and concentrated HNO3 in ratio 3:1) and dilute form of the acids are used to determine total metals in contaminated soils [72]. The acids dissolve almost all elements that could become environ‐ mentally available especially metal oxides and carbonates [73].

However, heavy metals bound to silicate structures are not normally dissolved since they are not usually mobile in the environment [74] but the extractant normally give a reliable measure of metals added to soils as non-silicates from industrial sources that has potential for natural leaching and biological processes. The use of perchloric acid and hydrofluoric acid is generally fading away since complete dissolution of the soil is no longer required for total metal analysis in environmental work [75].

Most current environmental analysis work that involved total metal analysis has employed the use of 2 M HNO3 [3,76,77]. The levels of heavy metals extracted with 2 M HNO3 have been reported to represent maximum contents of potentially available metals for plants [77]. In this study, approximately 1.0 g each of the composite soil samples were digested with 20 ml of 2 M HNO3 by heating the vessel in a water bath between 90-100o C for 2 hours with shaking every 20 minutes. The vessel was allowed to cool to room temperature. The digestate was filtered and made up to mark with distilled water. Two replicate samples were digested in the same way together with a reagent blank and all the samples were analyzed using Buck 200A Atomic Absorption Spectrophotometer using air-acetylene flame.
