Heavy Metals Pollution in Agricultural Soil of Adogo in Ajaokuta Local Government Area of Kogi State, Nigeria

Headnote

Abstract: Data referring to abundance, contamination level and content of heavy metals in labile and stable fractions of agricultural soils in Adogo in Ajaokuta local Government Area of Kogi State, Nigeria are presented. Six heavy metals were analyzed (Cd, Cu, Mg, Ni, Pb, and Zn) using Flame Atomic Absorption Spectroscopy. Assessing the concentration of potentially harmful heavy metals in agricultural soil is imperative in order to evaluate the potential risks to residents. Total metal concentrations fit in the typical concentration ranges for unpolluted soils. Speciation studies were carried out by applying Tessiers procedure, which allows subdividing the total metal content into five fractions, representing portions bound to different components of the soil. The average contents of Cd, Cu, Mg, Ni, Pb, and Zn in surface soils (0-15 cm depth) were Cd <0.01, Cu 0.89±0.25, Mg 0.07±0.001, Ni 0.18±0.03, Pb 0.44±0.16 and 0.04±0.003 µg/g. The soils could not be said to be contaminated for now because metal content levels conformed to the world-wide background content of metals range in the soil.

Keywords: Heavy Metals, Speciation, Agricultural soils, Contamination

1. Introduction:

Soil represent a major sink for metals released into the environment from a variety of anthropogenic activities such as agricultural practices, transport, industrial activities and waste disposal. (Makino et al., 2010, Alloway, 2004, Abollino et al., 2002) Heavy metals are natural constituents of the Earth's crust but human activities have altered the balance and biochemical and geochemical cycles of some of them. (Alloway 2004, Abollino et al., 2002) Once in soil, some of these metals will persist in soils for a very long time because of their fairly immobile nature leading to environmental pollution in aquatic systems. (SOIL QUALITY, 2000) Environmental risk due to soil pollution is of particular importance for agricultural areas because heavy metals, which are potentially harmful to human health, may enter the food chain in elevated amounts which may affect food quality and safety. (Makino et al., 2010, Alloway, 2004, Mwegoha and Kihampa, 2010) Thus pollution of heavy metals poses a threat to a country's food production.

Total metal content of soil is useful for geochemical purposes but their speciation (bioavailability) is of more interest agriculturally and this entails the identification and quantification of the different, defined phases in which the metals occur which can help assess how strongly they are retained in the soil. (Zimmerman and Weindorf, 2010, Mwegoha and Kihampa, 2010, Abollino et al., 2002) The different chemical forms in which they are present in soils influence their reactivity and hence their mobility and bioavailability. (Abollino et al., 2002, Makino et al., 2010, Mwegoha and Kihampa, 2010)

There is a growing public concern over the potential accumulation of heavy metals in agricultural soils globally owing to rapid urban and industrial development and increasing reliance on agrochemicals in the last decades. The present study is aimed at studying heavy metal concentrations of Cd, Cu, Mg, Ni, Pb and Zn in agricultural soils to evaluate the environmental quality of the soils and the potential risks to residents. To date, no research on soil pollution in Adogo has been conducted.

2. Materials and Methods:

Adogo is located in Ajaokuta local Government Area of Kogi State, Nigeria. The study area is a farm land 50 meters from the Okene Ajaokuta road. It lies within longitude 7029'50.3'N and latitude 60 29'43.3'E. The control sample was collected from a farmland -one kilometre from the Okene Lokoja road lying within longitude 7041.821'N and latitude 60 20.925'E. The soil samples (0-15 cm dept) were collected from the vicinity of the farms at 50 meters interval using a stainless steel knife. Sampling locations are shown in Figure 1.

The collected soil samples were air-dried for seventy-two hours, ground in a mortar and passed through < 2 mm and 0.005 mm sieve and stored in clean acid treated polythene bags.

A. Tessier et al., (1979) Sequential Extraction

1. The fraction Exchangeable-1 g (< 2 mm) soil sample, 8 ml of 1 M MgCl2 (pH 7), 1 h, continuous agitation, room temperature,

2. The fraction bound to Carbonates- 8 ml of 1 M NaOAc (pH 5 with HOAc), 5 h continuous agitation, room temperature.

3. The fraction bound to Fe and Mn-oxides- 20 ml of 0.04 M NH2OH.HCl in 25% (v/v) HOAc (pH 2), 6 h at 96±30C with occasional agitation.

4. The fraction bound to Organic Matter-3 ml of 0.02 M HNO3 and 5 ml of 30% H2O2 (pH 2 with HNO3) at 85±20C, 2 h, occasional agitation. A second 3 ml 30% H2O2 (pH 2 with HNO3), 85±20C, 3 h, occasional agitation, cooling, 5 ml of 3.2 M NH4OAc in 20% (v/v) HNO3, diluted to 20 ml with 5 ml H2O, agitated continually for 30 min, 3.2 M NH4OAc.

5. Residual-The residue from (4) was digested with a HF-HClO4 mixture according to the procedure for Total metal analysis.

The extraction solution was centrifuged after each extraction step and the supernatant removed by filtration into a plastic bottle. Two drops of HNO3 was added to sample. Sample was kept for AAS analysis. The residual soil sample was shaken in 8 ml of deionized water, after each extraction, centrifuged and the supernatant discarded.

B. Tessier et al., (1979) Total metal analysis procedure was carried out by digesting 1 g (<0.005 mm) of soil sample with a mixture of 5 ml HF and 1 ml HClO4 to near dryness; a second addition of 10 ml HF and 1 ml HClO4 was made and again the mixture was evaporated to near dryness. Finally 1 ml HClO4 alone was added and the soil sample evaporated until the appearance of white fumes. The residue was dissolved in concentrated HCl and diluted to 25 ml. The resulting solution was filtered into a plastic bottle ready for AAS.

Quality control was implemented through three replicate samples, (performed for each water, plant and use of international soil reference sample (Soil Reference Material 989 Netherland). Precision for the determination of heavy metals in soil ranged 8-12 %. Analysis of soil Reference Material 989 from Netherland gave Cu 144.5±11.6 and Pb 253.8±13.8 µg/g against the standard values of 153±3.9 and 282±3.6 µg/g for Cu and Pb respectively.

3. Result and Discussion:

The summary of the concentration of metals in the soil sample is listed in Table 1. The mean concentration of the studied metals were Cd <0.01, Cu 0.89±0.25, Mg 0.07±0.001, Ni 0.18±0.03, Pb 0.44±0.16 and 0.04±0.003 µg/g. Generally the metal levels were low. Apart from Cd and Zn the other studied metals were higher in the Adogo farm than the control farm. The soils can be considered unpolluted, since the studied metals levels fit in the typical ranges of the average concentration in the Earth's crust (Yobouet, 2010, Makino et al., 2010,)

The concentration of heavy metals in soil fractions is shown on Table 2. The mean percentage value for Cd ranged 0.00-42.52 %, Cu 4.56-32.65±9.24 %, Mg 9.07-37.89 %, Ni 8.96-27.95 %, Pb 5.38- 30.58 %, and Zn 0.61-36.37±0.59 %. Cd was highest in the carbonate fraction (42.52±4.91) and lowest in the organic fraction (0.00 %), Cu was highest in the residual fraction (32.65±9.24 %) and lowest in the exchangeable fraction (4.56±0.15 %), Mg was highest in the Fe-Mn oxide fraction (37.89±6.96 %) and lowest in the exchangeable fraction (9.07±2.17 %), Ni was highest in the Fe-Mn oxide fraction (27.95±0.94%) and lowest in the exchangeable fraction (8.96±0.61 %), Pb was highest in the exchangeable fraction (30.58±2.86 %) and lowest in the carbonate fraction (5.38±0.43 %), Zn was highest in the organic fraction (36.37±0.59 %) and lowest in the exchangeable fraction (0.61±0.01 %).

All soils have been reported to contain trace metals derived from the parent material and, in most cases, also from environmental pollution, though concentrations may vary with some soil parent materials geochemically and enriched in certain elements. (Alloway, 2004) Variations in concentration and toxicity of a metal is related to variations in soil properties such as pH, redox potential, temperature, mineralogy, organic matter, microbial activity, leaching, ion exchange and seasonal changes which modifies the speciation of a metal and, thereby, its bioavailability. (Jian-Min et al., 2007, Makino et al, 2010, Alloway (2004) Iwegbue et al., 2007)

We investigated concentrations and distribution in soil phases of Pb, Zn, Cd, Cu, Mg, Ni, Pb and Zn in samples of soil from Adogo, Nigeria. The measured metallic levels observed could be due to the nature of the parent material, history of mining in the vicinity, soil with past applications of waste water or municipal sludge, use of agrochemicals, effect of rain water, and atmospheric deposition of material sourced from urban anthropogenic activities. Such findings have been reported in previous research. (Abollino, 2002, Alloway, 2004, SOIL QUALITY, 2000, Mwegoha and Kihampa, 2010, Makino et al., 2010) Alloway (2004) reported Pb, Zn and Cu as components of pesticides. There is report by Alloway (2004) that fires used to burn wood and other combustible materials in agricultural soil can lead to relatively high concentrations of metals in soil. According to Alloway (2004) some rural villages could have a history of metalliferous mining which have resulted in soils enriched in several elements due to a combination of the presence of weathering ore under the soil, contamination from solid and liquid wastes from mining and possible deposition of metal aerosols and dusts from smelting nearby. Makino et al (2010) reports that the annual amount of Cd input to agricultural lands from rainwater in Japan is approximately 650 mg ha-1

Cd level was observed to be relatively low. Similar report was given by Mwegoha and Kihampa, (2010) in the research on Heavy metal contamination in agricultural soils in Dar es Salaam city, Tanzania where Cd concentrations were reported to be consistently low at all sampling locations as compared to the rest heavy metals. According to Wong et al (2002) flooding of agricultural soil could lead to dissolution of Mn oxides and through leaching and percolation, results in the loss of Cd while Makino et al (2010) attributes the decrease to Cd solubility due to the formation of Cd sulphides which precipitates out of the soil solution.

There was no specific pattern in the distribution of metals in the different soil phases. This disagrees with the report of previous researchers that the Fe-Mn oxide fraction stores the highest level of metals. (Richtera et al. 2004, Abollino, 2002), Tessier et al., 1979, Hesterberg, 1998) The percentage of the studied metals Cd, Cu, Mg, Ni, Pb and Zn present in the exchangeable and carbonate fractions were observed to be low except Cd 26.93±9.10 % and Pb 30.58±2.86 % in the exchangeable and Cd 42.52±4.91 and Cu18.79±6.50 in the carbonate phases. Such report has been given by previous researchers. (Ameh and Akpah, 2011, Abollino, 2002, Iwegbue et al. 2007, Rao et al., 2008) Ameh and Akpah, (2011) reported carbonate fraction of low levels (11-18.6 %). This disagrees with high concentration level (30% of Cd) in the exchangeable phase reported by Jung and Thornton (1997). Jian-Min et al (2007) reported high levels of metals in the carbonate fraction. The presences of high level of Cd in the exchangeable and carbonate phases of the Adogo soil indicate that the risk of transfer to plant crops is high.

The high levels observed in the Fe-Mn Oxide phase Zn 33.71±7.62 % and Ni 27.95±0.94 could be as a result of reduction of the oxides which could cause metals to be released. The presence of Zn in high levels in the Fe-Mn Oxide phase agrees with Vanìk, et al (2008) and Jian-Min et al. (2007). According to Jian-Min et al (2007) under reducing conditions metals present in the Fe-Mn Oxide phase are unstable and easily released through dissolution releasing soluble metals. This implies that the potential risks of metal pollution will increase with time. (Jian-Min et al., 2007) The relatively high levels observed in the organic phase (Cu 29.74±7.26, Ni27.67±5.56, and Zn 36.37±0.59 %) could be due to the oxidation of Sulphide ores and the decomposition of organic matter under oxidizing condition to release metals. Such findings have previously been reported. (Jian-Min et al., 2007, Zhou et al., 2007, Rao et al. 2008)

The high levels of metals observed at the residual phase (Cu 32.65±9.24 and Pb 27.43±3.95 %) indicate that these metals are bound to the primary and secondary minerals. Cu was mainly bound to the organic and residual fraction (29.74±7.26 and32.65±9.24 %) This agrees with Makino et al (2010) and Wong et al (2002) findings that there is strong association between soil Cu and the organic fraction but disagrees with Adie and Osibanjo (2009). Zn levels were highest in the organic phase. This disagrees with previous report that majority of Zn is found in the residual fraction. (Udovic et al., 2009, Rao et al., 2008, Wong et al., 2002)

Pb levels were observed to be concentrated in the residual fraction and the exchangeable fraction. This agrees with Miller et al (2008) report but disagrees with Adie and Osibanjo (2009), Wong et al (2002) and Jian-Min et al. (2007) report that Pb is largely associated with the Fe-Mn oxide and residual fractions. According to Wong et al (2002) there is strong association between Pb and the Fe- Mn oxide and organic fractions. Its presence in high level in the exchangeable phase indicates that the risk of its transfer into food crops is high.

4. Conclusion:

The results showed that the soils could not be said to be contaminated for now because metal content levels conformed to the world-wide background content of metals range in the soil. Although the heavy metal concentrations measured in this study do not pose a serious health risk, they do affect the quality of agricultural products. The presence of Pb in high level in the exchangeable phase indicates that the risk of its transfer into food crops is high. Pollution of heavy metals poses a threat to a country's food production. The monitoring of heavy metals in soils is necessary if better management practices are to be established for polluted soils.

References

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