Determination of Toxic and Trace Heavy Metals in the Soil of Two Different Dumpsites: A Case Study of Ojota and Oko Filling Dumpsites, Lagos State

: Heavy metals are metallic elements with a relatively high density that are dangerous or hazardous at low concentrations and pose a serious threat to people and the environment. The purpose of this research is to ascertain the concentrations of heavy and trace metals in the Lagos State communities of Ojota and Oko Filling. The soil samples taken from the dumpsites were treated using the digestion process. After being placed into a sampling vial, the mixed solution from the digestion process was then taken for measurement using inductively coupled plasma optical emission spectroscopy (ICP-OES). Samples taken from the dumpsites at Ojota and Igando contained 12.52% and 4.44% of moisture, respectively. The analysis's findings showed that the average concentrations of Fe (42.429 mg/L), Zn (2.070 mg/L), Ca (7.140 mg/L), P (9.06 mg/L), Cd (0.022 mg/L), Cr (0.042 mg/L), Cu (0.135 mg/L), Pb (1.280 mg/L), and Ni (0.123 mg/L) at the Ojota dumpsite were all within the WHO/FAO standards' tolerable limits and did not pose any risk. While, Fe (94.784 mg/L), Zn (11.564 mg/L), Ca (45.243 mg/L), Cd (2.838 mg/L), Cr (0.046 mg/L), Cu (0.215 mg/L), and Ni (0.22 mg/L) are the mean concentrations, respectively. In conclusion, the results from the Oko – filling dumpsites shows that the heavy metals from the soil is highly toxic and are such harmful to the environment and detrimental to human life than Ojota dumpsites.


Introduction
Heavy metals are inert, naturally occurring elements of the crust of the Earth [1]. They sporadically get into our systems through food, water, and air. Certain heavy metals, such as copper, selenium, and zinc, are necessary as trace elements to keep the human body's metabolism running smoothly. High ambient air concentrations near emission sources, ingestion through the food chain, drinking water, or sewage disposal are all scenarios where exposure at higher concentrations can result in poisoning. Examples include Pb, As, Hg, Cd, Zn, Ag, Cu, Fe, Cr, Ni, Pd, and Pt. The mines emit the heavy metals into the environment through a variety of anthropogenic and natural sources, including mining, industrial discharge, cars, and exhaust [3,4]. Heavy metals are non-biodegradable and have a propensity to accumulate in living things, in contrast to organic contaminants. In fact, the majority of them have been linked to cancer in the past. Long-term and ongoing exposure to heavy metals is recognized to pose a number of negative health risks [2,3,4]. Appropriate techniques must be created for their efficient removal from the environment because they are nondegradable and have a propensity to bio accumulate. As it is made up of both inorganic and organic materials, soil is the top layer of the ground. In addition to providing structural support, it also serves as a supply of water and nutrients for plants [5]. Minerals, soil organic matter, living organisms, gas, water, and water and water body included water, fish, and plants are the five primary components of soil [6]. The parent materials of the soil and numerous anthropogenic sources, the majority of which involve several metals, are the sources of heavy metals found in soils [7]. Both agricultural and urban soils can get contaminated with heavy metals from a variety of anthropogenic sources [7,8]. The health of the local people may also be significantly impacted by localized contamination from a dominant single source, such as a metal smelter, especially in nations with insufficient emission controls and requirements for soil quality.. In general, different categories of heavy metal pollutants can be found in soils at industrial sites, depending on the relevant industries and their raw materials and outputs [9,10]. All metropolitan areas typically include lead (Pb), zinc (Zn), cadmium (Cd), and copper (Cu) contamination of the soil due to traffic, paint, and numerous other non-specific urban factors. Although the parent material tends to have a stronger influence on the heavy metal composition of agricultural soils, other inputs from sources like the application of organic materials, contaminants in fertilizers, and the deposition of long-distance, atmospherically transported aerosol particles from fossil fuel combustion and other sources can also be significant. Humans and the environment have been very concerned about heavy metal contamination in soil [11]. In addition to negatively affecting several aspects of plant quality and yield, heavy metal pollution also alters the microbial community's size, composition, and activity [12]. As a result, heavy metals are one of the main causes of soil pollution. Several metals, including Cu, Ni, Cd, Zn, Cr, and Pb, are the main culprits for heavy metal contamination of the soil [13]. Heavy metals have been shown to negatively affect soil biological and biochemical characteristics [14,15,16,17]. The purpose of this research is to ascertain the levels of heavy metals in the Lagos State, Nigeria, areas of Ojota Dumpsite and Oko Filling Dumpsite.

Materials and Methods
Lagos, Nigeria's Oko-filling, Igando, and Ojo are the study locations for this research. Along Lagos's Lagos Isheri Road is where the dump is located. This landfill is known as Solous Dumpsite, and according to a consulting company with experience in waste-to-energy and biomass energy, Olusosun Dump Site in Lagos' Ojota neighborhood is the largest landfill in Lagos and Nigeria, spanning over 43 hectares of land. Igando Dump is ranked as the secondlargest landfill in Nigeria, occupying about 8 hectares of land.

Sample Collection and Preparation
Samples of soil were taken from landfills in the cities of Ojota and Igando. Six (6) different locations on the dumpsite were used to collect the samples. With the aid of a soil auger, these samples were obtained at a depth of 14 cm. The samples were then packaged in polyethene bags and transported to the lab for analysis. The samples were pulverized using a mortar and pestle to pass through a 2 mm sieve after being air dried, and the leftover particles were thrown away. The powdered form was then kept in storage for later digestion and analysis.

Sample Digestion
A hot plate in a fume cupboard was used to digest 5.0 g of dried, sieved soil samples in 12 ml of aqua regia (HCl/HNO 3 v/v, ratio 3:1) until white vapors were visible. The sample was diluted with 20 ml of 2% nitric acid (v/v) after being allowed to cool to room temperature. After filtering the liquid through Whatman No. 42 filter paper and making it mark with distilled water, the mixture was then put into a sample vial. For both samples, the experiment was run a second time. Using an inductively coupled plasma optical emission spectrometer, heavy metal concentrations in the digested soil waste samples were determined (ICP -OES).

Moisture Content
When soil samples from Ojota and Igando dumpsites were examined for moisture, it was discovered that the dry season values were 8.56 and 6.97, respectively, while the wet season values were 13.75 and 10.7, respectively. The rainy season had a larger moisture content than the dry season, and both seasons' Ojota moisture contents were greater than those of the dry season. Figure 2 displays the amount of moisture in the soil samples taken from the dumpsite.

Dumpsites
The samples were taken from two dumpsites in the Lagos State communities of Igando and Ojota throughout both the rainy and dry seasons. Toxic heavy metals and hazardous trace metals were analyzed in both samples for the two seasons, and high and low quantities of each were discovered, as shown in Figures 3 and 4 for the respective Igando and Ojota locations.

Ojota Dumpsite
Toxic and Trace Heavy Metals Figure 3a revealed the concentrations of some of the toxic heavy metals found in the sample which includes Barium (Ba) and Manganese (Mn) and lead (Pb) which has higher concentration than chromium (Cr) and cadmium (Cd) and the Figure 3 also shows that concentration of toxic heavy metals are higher during the rainy days than the sunny days, e.g Barium has the concentration of 0.41mg/L during the rainy days and has concentration of 0.293 mg/L during the sunny days. Zinc (Zn) and magnesium (Mg), which were found in the soil of the dump site in the highest concentrations both during the rainy and dry seasons, are shown in Figure 3b. Zn has a higher concentration of trace metals both during the rainy and dry seasons than calcium. Concentrations of Zn during the dry and raining seasons are 21.24 mg/L and 14.85 mg/L respectively while the concentrations of Cupper during the dry and raining seasons are 2.01 mg/L and 7.01 mg/L respectively.

Igando Dumpsite Toxic and Trace Heavy Metals
The results from the samples taken from the Ojota dumpsite showed that the mean concentrations (mg/L) for hazardous heavy metals were as follows: Th (0.005 mg/L), U (0.149 mg/L), As (0.012 mg/L), Hg (0.003 mg/L), and Cd (0.022 mg/L) are the toxic heavy metals that are within the WHO/acceptable FAO's limits and do not represent a major concern. The WHO/FAO have set acceptable limits for the mean concentrations of trace heavy metals for Cu (0.13 mg/L), Fe (40.43 mg/L), Ni (0.12 mg/L), Zn (6.48 mg/L), and Mo (0.01 mg/L) as indicated in Figure 3. Iron (Fe) and calcium (Ca), which were found in the soil at the dump site in the highest concentrations during both the rainy and the dry seasons, are shown in Figure 4b. Iron has a higher concentration of trace metals than calcium both during the rainy and the dry seasons. Whereas calcium concentrations are 2.01 mg/L and 7.01 mg/L during the dry and rainy seasons, respectively, respectively, the concentrations of iron are 21.24 mg/L and 14.85 mg/L.

Conclusion
Conclusively, the results of the hazardous and trace heavy metals found in both dumpsites showed that they did not include the same types of metals. The heavy metals in their soil are extremely poisonous, destructive to the environment, and deleterious to human life, as demonstrated by the Ojota and Igando dumpsites. The Ojota dumpsites location is surrounded by residents who have provided evidence that the level of heavy metals from the dumpsites is below EPA guidelines and that the pH from the six sites is essentially neutral, meaning that neither man nor agricultural products are being endangered.