Preliminary assessment of soil contamination by trace metals in peri-urban municipal landfills in Ibadan, Nigeria

Spectrometry for metal analysis

In carrying out a laboratory analysis of environmental samples for the presence of trace metals, there are two commonly used methods, namely: atomic absorption spectrometry (AA), and the inductively coupled plasma optical emission spectrometry (ICP-OES), both of which are based on emission spectroscopy^27,28. ICP-OES is a special type of atomic emission technique which employs inductively coupled plasma to generate excited atoms that emit electromagnetic radiation when heated to a very high temperature, with wavelengths that correspond to a particular element The sample atoms are subjected to a temperature of around 6727 °C²⁹. ICP-OES is based on the principle of atomic light emission properties of samples; when they are excited at high enough temperature, the concentration of the sample is directly proportional to the intensity of the emitted light.

To carry out spectroscopic analysis for the detection of trace metals in environmental samples, acid digestion is commonly performed on the collected sample. For example, in soil and sediment analysis, and in the determination of total phosphorus, and total nitrogen under heated conditions²⁸. The digestion procedure breaks down an organically bound substance and converts it to the analysable form by using oxidising agents such as sulphuric acid (H₂SO₄), nitric acid (HNO₃), perchloric acid (HClO₄), or hydrochloric acid (HCl), or using oxidising mixtures such aqua regia. Sometimes, addition of bromide (Br₂) or hydrogen peroxide (H₂O₂) to mineral acids will increase their solvent action and speed up the release of bound organic materials in the sample.

Sample collection

A detailed description of the study area as well as the geographical location of the sampling sites is available elsewhere^25,30–32. Soil sampling, pre-treatment and preservation was carried out according to³³. Several grab samples of the top soil were collected using hand-held augers and tube samplers from a depth of about 15 cm at two sub-urban landfill sites, namely: Aba-Eku and Lapite (Table 1, Figure 1), located on the outskirts of the Ibadan metropolis. The samples were immediately transferred into 50 ml airtight plastic containers (Burkle GmbH, Rheinauen, Germany) with screwed cap and labelled appropriately as samples A and B depending on the landfill sites. The soil was in dry condition at the time of sampling, with an ambient temperature of about 29 °C. The sample containers were stored in the laboratory at room temperature without any further pre-treatment, before carrying out the ICP-OES analysis.

Figure 1. Map of the study area, showing the locations of the landfill sites.

Preparation of test solutions

Test solutions were prepared according to the standard method³⁴. A weighed portion of the soil sample (1.0 g) was placed in three separate glass beakers that had been thoroughly rinsed with dilute nitric acid and thereafter deionized water. 20 ml of nitric acid (7 mol/l) was added to each beaker and thereafter transferred into an autoclave for acid digestion. The beakers were kept in the autoclave at 121°C for about 30 minutes. The content of the beakers was carefully filtered into three 100 ml volumetric flasks after cooling to room temperature.

Preparation of standard solution

The mixed standard solution for the calibration step was prepared following Rezic & Stefan’s method³⁵. Six 100 ml volumetric flasks were carefully rinsed with dilute nitric acid followed by deionized water. A calculated volume of the prepared test solution with 0.1 % nitric acid solution (Table 2) was added to each flask using a pipette, and filled to the 100 ml mark with distilled water. The test solution and the calibration solutions were subsequently transferred for ICP-OES analysis.

Heavy metal analysis

The samples of the test solutions were analysed for trace metals according to²⁹. A test sample (100 ml) was placed in the measurement chamber of the iCAP^™ 7000 spectrometer (ThermoFisher Scientific GmbH, Germany) after allowing sufficient time for initial start-up of the machine. The relevant test parameter values were given via a PC connected to the machine. The entire process is fully automated and the measured quantities of trace metals were generated as printed output. A total of two different measurements were performed on the samples from each landfill site and the result was given as the average of all measurements (Figure 2).

Figure 2. ICP-OES analysis in the laboratory showing prepared test and standard (calibration) solutions.

Bio-availability of trace metals in plant species

Trace metal uptake in plants has been widely reported in several studies, and many bio-remediation techniques are based on this principle. The danger to public health arises from the fact that trace metals can end up in the food chain as a result of bio-accumulation in plant species, especially those grown for human consumption^3,31. The key to breaking this chain is to ensure that fruits and vegetables are not grown in areas prone to trace metal contamination such as landfill sites. A more detailed discussion on the bioavailability of trace metals in the environment can be found elsewhere^36,37.

Risk assessment and remediation need

The results of the quantified levels of trace metals and their comparison with the regulatory limits are shown graphically in Figure 3–Figure 6. The implication of these results can better be understood by making a direct comparison to specified values in the appropriate regulatory standard, or by comparing with quantified values from an uncontaminated sample. The test results show that the quantified levels of copper and zinc exceeded both the threshold and the lower guideline values specified by the Finnish government decree on the assessment of soil contamination and remediation needs (Table 3), while the values for lead exceeded only the threshold values at both landfill sites. The lower and higher guideline values is an indication of soil contamination under different land use conditions³⁸. The reference document, ‘Government Decree on the Assessment of Soil Contamination and Remediation Need’ was adopted as a source for threshold and lower guideline values for levels of copper and zinc in the tested soil samples as there was no applicable regulatory standard in Nigeria with respect to soil contamination and remediation.

Figure 3. Quantified values of trace metals in soils from Aba-Eku landfill.

Figure 4. Quantified values of trace metals in soils from Lapite landfill.

Figure 5. Comparison of quantified values with the regulatory standards for Aba-Eku landfill.

Figure 6. Comparison of quantified values with the regulatory standards for Lapite landfill.

These results also confirms findings from previous communications that reported poor leachate management at the Aba-Eku landfill site^1,26. Based on these findings and owing to increasing risk of human exposure, there is an urgent need for comprehensive risk assessment at the landfill sites, and if possible a remediation measure needs to be put in place in order to protect the rapidly expanding residential areas around these sites as well as the groundwater source. However, it may be worthwhile to point out that this study does not take into account the background values of the trace metals at the sampling sites. Therefore, a direct comparison with a regulatory limit only serves as a guide, and a more comprehensive study of the landfill sites is needed in order to establish the full ecological and health risks, including the risk of groundwater contamination.