Heavy metals are non-biodegradable. They accumulate in the environment and subsequently contaminate the food chain. It is therefore essential to monitor heavy metals content in the soil, so as to prevent too much accumulation in human beings and animals through food chain. Test soil samples were collected from Nigeria Ports Authority (NPA) Expressway open dump, and background (control) soil samples were also collected at about 2 km from the open dump and spatial test samples were also collected. The physicochemical properties of the soil were determined. Tessier’s sequential extraction protocols were used to assess the geochemical forms of Cr, Pb, Zn, and Mn in the soil of the open dump. The concentrations of the heavy metals in the extracts were determined in a pre-calibrated atomic absorption spectrophotometer and they are above the background sample values. The metal assessment index (Igeo) evaluated, indicated that the soil in the vicinity of the open dump was highly polluted. The results obtained showed the mobile metal pools which are available to plants roots.
Pollution by heavy metals occurs largely from industrial, domestic and agricultural wastes as well as from combustion of fossil fuel by automobiles and industries [
Warri is a beehive of commercial and industrial activities. There are major industries in Warri metropolis and also numerous shops, markets and schools, which generate both degradable and non-degradable solid wastes. These wastes are comprised of paper, metals, glass, plastics, wood, rag, rubber, leather, food, scraps, damaged bicycles, coins, stainless steel, zinc roofing sheets, computer monitors, printer wiring board (PWB), relays, switch, broken mercury thermometer, dental amalgams, fluorescence lamps, etc. They decay and break into small pieces, are mineralized overtime and release their metallic contents into the soil. Soil pollution is caused by misuse of the soil, such as poor agricultural practices, disposal of industrial and urban wastes [
Therefore, the study aimed at assessing the heavy metal hazard potentials of the soil from NPA Expressway open dump by determining the varying concentrations of the toxic metals constituents and evaluating the mobile forms of the metals.
Different types of indices are used to assess the pollution status of soil/sediment and select the best index to describe the soil quality. Some of the indices used are geoaccumulation index (Igeo) and metal mobility factor (MF).
The Geoaccumulation index (Igeo) introduced by Muller in 1979 was used to assess metal pollution in soil/sediments [
Igeo = Log2 (Cn/1.5 Bn)
The geoaccumulation index includes seven grades.
It includes various degrees of enrichment above the background value, ranging from unpolluted to very highly polluted soil/sediment quality.
The highest grade (class six) reflects 100-fold enrichment above the background values [
Source: Singh et al. (2003) [
Intervention Values indicate the quality for which the functionality of soil for human, animal and plant life are threatened with being seriously impaired or are being impaired. Concentration levels in excess of the intervention values correspond to serious contamination. Intervention values are soil-type specific in that it relates to the content of organic matter and clay in the soil of concern.
Trace metals may be distributed or fractionated among many components of the soil and may be associated with them in different ways. Chemical speciation is defined as distribution of an individual chemical element among different species or groups.
Tessier’s extraction scheme allows the division of the total metal content into five fractions: Exchangeable bound, carbonate bound, iron/manganese oxide bound, organic matter bound and residual fraction [
The relative index of metal mobility is calculated as mobility factor [
MF = F 1 + F 2 F 1 + F 2 + F 3 + F 4 + F 5 × 100
The mobility of metals in soil is assessed on the basis of absolute and relative content of fractions weakly bound to soil components.
High MF values are symptoms of relatively high lability and biological availability of heavy metals in soils to plants and other ecological receptors.
The purpose of this study is to evaluate the contribution of solid wastes to heavy metal pollution of the urban soils of Warri. This is because the plant crops growing close to such open dumps will be harvested for food or for sale to the public by the owner of the land, and heavy metals will be transferred by food chain to man. The NPA expressway open dump was chosen as it is one of the popular dump sites in the city of Warri.
Objectives of the StudyThis work is set to:
1) Measure the total concentration levels of heavy metals in soils in the vicinity of NPA Expressway open dump in Warri.
2) Evaluate anthropogenic pollution status of the soil in the vicinity of the open dump using the geoaccumulation index.
3) Determine the chemical forms of the heavy metals in the soil samples in the NPA Expressway open dump using sequential extraction technique.
4) Determine the spatial distribution of the heavy metals in the soil samples in the vicinity of the open dump.
5) Evaluate the mobile metal forms of the heavy metals of environmental concerns.
Warri metropolis is the headquarters of Warri South Local Government Area. Warri is situated in the Southern part of Delta State precisely about the intersection of longitude 5.45˚East and latitude 5.30˚North of the Equator. Warri is about 256 square kilometres in Area, with a total population of 303,417 [
1) Aggregate Test Soil Samples of NPA Expressway Open Dump
Aggregate soil samples were collected from NPA Expressway open dump using a stainless steel auger.
Background (control) soil samples were also collected at about 2km from the open dump. The background samples are the control samples which are collected far away from the open dump to avoid possible contamination of the soil.
2) Spatial Test Soil Samples
Spatial test samples were collected at various increasing distances 10 m ? 109 m from the open dump. Sampling was done in the wet season.
The soil samples were air dried, ground and sieved through a 2 mm sieve. Dried soil samples that fell below the sieve were stored in polythene bags and properly labeled for subsequent analyses. They were used for all the analyses. The total heavy metal concentrations of the test soil and background soil samples were determined using the procedure described by Tessier et al., 1979 [
The physico-chemical properties, pH, TOC, CEC, the soil textural class and the total metal content of the soil samples were determined using standard procedures/methods.
20 g of soil sample was weighed into a 50 ml beaker and 20 ml distilled water was added. The soil/water mixture (ratio 1:1) was allowed to stand for 30 minutes and stirred occasionally with a glass rod. The Suntex pH meter was calibrated using buffer 4 and 7. The electrodes were rinsed and subsequently immersed into the soil/water mixture and the pH was recorded as pH (H20) [
5 ml of K2Cr207 solution was added to one gram soil sample in a 250 ml Erlenmeyer flask and swirled gently to wet the sample thoroughly. 20 ml of concentrated H2SO4 was then added and the mixture was allowed to cool. 100 ml of distilled water was added, followed by few drops of ferrion indicator and the mixture was titrated against 0.5 M iron (II) sulphate solution to a green endpoint. A blank determination was also performed. The blank contained 5 ml of the K2Cr207 solution, 20 ml concentrated H2SO4, 100 ml distilled water and 5 drops of Ferroin indicator [
% Organic carbon = ( B − T ) × M × 0.003 × 1.33 × 100 W
B = Blank titer (ml)
T = Sample titer (ml)
M = Molarity of Iron (II) sulphate
0.003 = Factor for equivalent weight of carbon
1.33 = Factor for proportion of active organic carbon
W = Weight of soil (1 g)
% organic matter in soil = % organic carbon × 1.729.
5 g of soil sample was weighed into a 250 ml polypropylene bottle and 100 ml 5 M NH4OAc solution added. The mixture was shaken 30 mins in a mechanical shaker (Heldoph) at 200 rpm for 30 mins. The supernatant was filtered through Whatman No. 1 filter paper. The concentration of K+ and Na+ in the extract were determined by flame photometer while Mg2+ and Ca2+ were determined using Atomic Absorption Spectrophotometer (Buck Scientific VGP 210 model). The equipment was calibrated using various concentrations of the metals ranging from 2 - 20 ppm prepared from analytical grade reagents (Sigma, BDH and Buck Scientific). The summation of the various cations was reported as cation exchange capacity [
50 g of soil sample was weighed into a 250 ml beaker, 100 ml distilled water and 10 ml of concentrated H202 were added to the soil. The content of the beaker was heated until frothing stopped. The mixture was cooled and transferred into shaking bottles, 20 ml of sodium hexametaphosphate solution was added and mixture shaken for 1 hr. The suspension was transferred to 1 L sedimentation cylinder and brought to mark with water. A plunger was used to agitate the suspension. The hydrometer was then lowered into the suspension and its reading was taken after 40 seconds. The temperature was noted. The first reading, R1, gave the percentage clay and silt. The suspension was allowed to stand for 2 hours before the hydrometer reading was taken again. The second reading, R2, gave the percentage clay [
Calculation
% ( Clay + Silt ) = R 1 + 0.3 x × 100 DryWeightofsoil
% Clay = R 2 + 0.3 x × 100 W
R1 = Hydrometer reading after 40 seconds.
R2 = Hydrometer reading after 2 hours.
X = Degree rise above calibration point of hydrometer
0.3 = Correction factor for every degree rise above the calibration temperature of the hydrometer.
W = Weight of soil sample (50 g).
% sand = 100 ? (Clay + Silt)%
% Silt = 100 ? (Sand + Clay)%
In order to assess the geochemical forms and bioavailability (Cr, Pb, Zn and Mn) in the soil of the open dumps, Tessier et al., (1979) [
1 g of each of the soil samples was used for the sequential extraction. After each successive extraction process, centrifuging the mixture at 1500 rpm for 15 mins affected the liquid-solid phase separation. The supernatant was decanted into a polypropylene bottle for metal analysis while the residue was carried through the whole extraction process.
The concentrations of heavy metals (Cr, Pb, Zn and Mn) in the various extracts were determined in a pre-calibrated atomic absorption spectrophotometer.
Exchangeable (F1): The residue from above is extracted with 8 ml 1 M MgCl2 (pH7) for 1 hour.
Carbonate Bound (F2): The residue from (F1) extracted with 8 ml 5 M NaOAC (pH5) for 5 hours.
Fe ? Mn Oxide bound (F3): The residue from (F2) extracted with 0.04 M NH2OH. HC1 for 6 hours at 96˚C ± 2˚C.
Organic matter bound (F4): The residue from (F3) extracted with 30% H2O2 (pH2), 3.6 M NH4OAC for 5 hours + 30 mins at 85˚C ± 2˚C.
Residual (F5): The residue from (F4) extracted with Aqua-regia, HC1O4 until white fumes appeared.
In order to ensure the accuracy and reliability of the results obtained, all reagents used for the preparation of standard solutions and analysis were analytical grades (BDH, Sigma and Buck Scientific).
All glass wares and plastics were acid-washed and rinsed thoroughly with deionized water.
Buck scientific standard solutions were used to calibrate the atomic absorption spectrophotometer as required. Buck scientific (VGP210) atomic absorption spectrophotometer was used for the analyses.
Procedural blank samples were subjected to similar extraction method using the same amounts of reagents. Blank determinations of the elements were below the detection limits of the atomic absorption spectrophotometer.
The analyses were carried out in triplicates.
The analytical results were compiled to form a multi-element data base using excel.
Tables 1-4 and
The physicochemical properties of the soil samples from NPA Expressway open dump showed that the soil is predominantly sandy loam - 57.00% ± 2.00% being sand, 27.00% ± 2.00% being silt. Clay is made up of 16.00% ± 2.00% as shown in
Metals | Mean values of heavy metal content (mg/kg) | |||
---|---|---|---|---|
NPA Expressway sample values | Background sample values | Intervention values | Uncontaminated soil values | |
Cr | 292.28 ± 7.82 | 1.80 ± 0.20 | 190.4 | 100 |
Pb | 10.00 ± 2.00 | 1.00 ± 0.04 | 312 | 14 |
Zn | 456.00 ± 5.00 | 3.80 ± 0.80 | 257.5 | 75 |
Mn | 111.00 ± 7.00 | 5.00 ± 0.30 | NL | 960 |
Metals (mg/kg) | 10 m | 11 m | 12 m | 13 m | 14 m | 29 m | 59 m | 109 m | Background values |
---|---|---|---|---|---|---|---|---|---|
Cr | 234.00 ± 1.00 | 208.20 ± 10.00 | 184.00 ± 5.00 | 158.00 ± 0.00 | 111.00 ± 2.00 | 20.00 ± 4.00 | 18.00 ± 3.70 | 14.80 ± 2.80 | 1.80 ± 0.20 |
Pb | 7.20 ± 1.00 | 5.90 ± 1.00 | 3.20 ± 0.80 | 1.90 ± 0.80 | 1.20 ± 0.10 | 1.10 ± 0.00 | 1.10 ± 0.20 | 1.10 ± 0.40 | 1.00 ± 0.04 |
Zn | 396.80 ± 9.00 | 301.00 ± 6.00 | 233.20 ± 5.00 | 134.10 ± 12.00 | 76.00 ± 10.00 | 23.80 ± 5.20 | 19.60 ± 3.60 | 15.00 ± 3.00 | 3.80 ± 0.80 |
Mn | 104.00 ± 1.00 | 88.40 ± 1.00 | 81.70 ± 3.00 | 65.00 ± 0.00 | 54.00 ± 2.00 | 23.90 ± 4.00 | 24.40 ± 3.00 | 20.00 ± 0.00 | 5.00 ± 0.30 |
Uncontaminated soil values from around the world [
Distances from epicentre of the open dump (m) | Metals | Geoaccumulation Index (Igeo) | Pollution status |
---|---|---|---|
10 | Cr Pb Zn Mn | 6.4 2.3 6.1 3.8 | Very highly polluted Moderately polluted Very highly polluted Moderately highly polluted |
11 | Cr Pb Zn Mn | 6.3 2.0 5.7 3.6 | Very highly polluted Moderately unpolluted Very highly polluted Moderately highly polluted |
12 | Cr Pb Zn Mn | 6.1 1.1 5.4 3.8 | Very highly polluted Moderately to unpolluted Very highly polluted Moderately highly polluted |
13 | Cr Pb Zn Mn | 5.9 0.3 4.6 3.5 | Very highly polluted Unpolluted Highly polluted Moderately highly polluted |
14 | Cr Pb Zn Mn | 5.4 −0.3 3.7 2.9 | Very highly polluted Background concentration Moderately highly polluted Moderately polluted |
29 | Cr Pb Zn Mn | 2.9 −0.6 2.1 1.7 | Moderately polluted Background concentration Moderately polluted Moderately unpolluted |
59 | Cr Pb Zn Mn | 2.7 −1.3 1.8 1.7 | Moderately polluted Background concentration Moderately unpolluted Moderately unpolluted |
109 | Cr Pb Zn Mn | 2.5 −0.5 1.4 1.4 | Moderately polluted Background concentration Moderately unpolluted Moderately unpolluted |
The mean values of Cr and Zn exceeded the 190.4 and 257.5 intervention values
respectively, and the 100 mg/kg and 75 mg/kg values respectively stated for uncontaminated soil from around the world [
The high values of Cr and Zn may be associated with anthropogenic activities ranging from the waste disposal into the open dump, vehicular emissions and particulate matter while the low value of Pb may be due to the fact that NPA Expressway is far from human settlement as a result Pb containing materials are few in the open dump. Atmospheric deposition may have contributed to the amount determined.
The total metal content determined as shown in
The levels of Lead and Manganese in all the distances (10 m - 109 m) away from the epicentre of the open dump are below the 14 mg/kg and 960 mg/kg values for uncontaminated soils from around the world as in
The spatial variation of Cr, Pb, Zn and Mn as in
At distance 59 m away from the epicentre of NPA Expressway open dump, Mtotal was 61.60 ± 10.50 mg/kg (and contained Cr: 18.00 ± 3.70 mg/kg, Pb: 0.6 ± 0.20 mg/kg, Zn: 19.60 ± 3.60 mg/kg and Mn: 24.40 ± 3.00 mg/kg), suggesting a spatial reduction in M total of about 97.7%.
The metal assessment index (Igeo) used to assess the contamination status of the soil of the NPA Expressway open dump as shown in
However, the heavy metal assessment indices are not to be used as the only indicator for soil/sediment quality. Risk assessment relies on the mobile fraction of the metal.
Mobility Factor (MF in %) of Cr, Pb, Zn and Mn in the representative aggregate soil samples in the vicinity of NPA Expressway open dump evaluated showed that the MF values ranged from 6% - 14%. Zn had the highest MF values of 14%. The low MF values of the metals may be attributed to the high organic matter content of the soils and the textural class of the soil. This is consistent with other reporters [
Chemical Speciation of the metals as shown in
The percentage chemical forms of Cr, Zn and Mn showed in
Evaluation of the mobile metal forms showed that the most mobile metal in the open dump is Zn (63.42 ± 2.00 mg/kg) while the least mobile metal is Pb (0.54 ± 0.10 mg/kg) and the levels of the mobile metals are above the background sample values.
The mobile metals pools in the soil sample 59 m away from the open dump indicated that the mobile metals pools have reduced drastically and the most mobile metal in the open dump is Mn (7.70 ± 4.00 mg/kg) while the least mobile metal is Pb (0.10 ± 0.00 mg/kg).
Mean values of Cr, Pb, Zn and Mn reported in this study exceeded the background samples values. The heavy metal index (Igeo) indicated that the soils in the vicinity of the NPA Expressway open dump were very highly polluted. A large amount of the metals were bound to the Fe-Mn oxide fraction. Pb was bound mostly to the organic carbon. The most mobile metal in the vicinity of the open dump was Zn, but at 59 m away from the open dump, the most mobile metal was Mn, while the least mobile metal was Pb. At about 59 m away from the open dump, there was a reduction in the mobile metal pools from 63.42 ± 2.00 mg/kg to 6.70 ± 2.60 mg/kg Zn, 0.54 ± 0.10 mg/kg to 0.10 ± 0.00 mg/kg Pb and 9.62 ± 0.20 mg/kg to 7.70 ± 4.00 mg/kg Mn.
The open dump induces contamination on the nearer environment by metal mobility. Spatial contamination profile of the heavy metals showed that the levels of the metals decreased with increasing distances from the open dump. Nevertheless, while the impact of the open dump was clear up to the 109 m covered, the pollution impact of the metals was observed mostly in the soils at very close proximity to the open dump.
This study has provided data and information on:
1) The chemical forms of the heavy metals of environmental concerns in the soils in the vicinity of NPA Expressway open dump.
2) The mobile metal pools which could be absorbable by plants.
3) The spatial contamination profile of the heavy metals in the soils in the vicinity of NPA Expressway open dump, Warri.
I wish to acknowledge the head, central analytical laboratory, Nigerian Institute for Oil Palm Research (NIFOR), Benin city, Edo state, Nigeria for the analytical work. The same acknowledgment also goes to the Tertiary Education Trust Fund (TETFUND) for the grant given towards this research.
The authors declare no conflicts of interest regarding the publication of this paper.
Ataikiru, H. and Okieimen, F.E. (2019) Risk Assessment of Trace Metals in Soils in the Vicinity of NPA Expressway Open Dump in Warri Metropolis, Delta State, Nigeria. Journal of Environmental Protection, 10, 562-576. https://doi.org/10.4236/jep.2019.104032