S1: Soil analysis
Soil texture
Soil samples large enough to make into individual balls were collected and ultra-pure deionized water 18.2MΩ from a Milli-Q analytical reagent grade water purification system (Millipore) was added drop by drop till the soil got to a sticky point. The step by step instruction on the ‘key for finger assessment of soil texture worksheet was followed to determine the soil texture (Thien, 1979).
pH
Sieved soil sample (1g) was placed in a beaker and mixed with ultra-pure deionized water 18.2MΩ from a Milli-Q analytical reagent grade water purification system (Millipore) in the ratio 1:10. The mixture was placed on a mechanical shaker for twenty minutes adopting the USEPA method 9045D (2004). Jenway 3505 pH meter was calibrated with buffer solution at pH 7 and pH 10. The calibrated pH electrode was placed in the suspension and the pH readings were taken immediately.
Organic matter content (OMC)
This was determined by Loss on Ignition (LOI) (ASTM, 1993). Firstly, the moisture content was determined; soil (5g) was heated to 105°C for 24 h in a muffle furnace then cooled in the desiccators and weighed. The samples were then heated to 440°C for 6 h or (till completely ashed), allowed to cool in desiccators and weighed. The percentage organic matter content was then calculated as:
% Moisture content= A-BA*100 ------Equation 3.1
% Ash content= C*100B ------Equation 3.2
% Organic matter content = 100- % Ash content------Equation 3.3
Where; A= pre ignition weight (g), B= post ignition weight (g), C= weight of ash (g).
Cation exchange capacity (CEC)
The method used was adopted from Chapman (1965) and the USEPA method 9081. 5g of air dried soil was measured into 50 ml centrifuge tubes and 1M sodium acetate (30 ml) was added. The samples were agitated in an ultrasonic bath for 10 mins then centrifuged at 1500 x g[1] for 5 minutes. The resultant supernatant was decanted and discarded, and then 30 ml of ethanol was added to the samples, agitated, centrifuged and then decanted again. This procedure was carried out twice to ensure the removal of excess sodium acetate. The washed soil samples was extracted thrice using 20 ml portions of ammonium acetate using the ultrasonic bath and centrifuged each time. The supernatant from each extraction was filtered and collected in a 100 ml volumetric flask then made up to the mark.
Data analysis and calculation
Sample concentration = C*D*VS/R ------Equation 3.4
Where: C= concentration in extract (mg l-1), V = volume of extract (ml), D = dilution factor, S = dry weight of the sample (mg), R = relative atomic mass of element (Na=22.99, Mg=24.3, K=39.1, Ca= 40)
Cation exchange capacity (cmolcKg-1) = Na+Mg+K+Ca------Equation 3.5
Total heavy metals content
The total metal concentration was determined according to the EPA 3051a protocol (US EPA, 2007). 0.5g of dried soil sample was weighed in Teflon microwave tubes, 20 ml of 70% nitric acid (HNO3) added, tubes were placed microwave apparatus (CEM, Model MARS Xpress) at 175°C for 10 min. Cooled samples were centrifuged for 5 min at 1500 x g, filtered using Whatman’s No 42 into 50 ml volumetric flasks and the extract was made up to the total volume of 50 ml with deionised water, placed in the fridge at 4 °C till ready for analysis.
ICP-OES analysis
A Thermos ICP-OES (iCAP 1600) was used for triplicate readings of blank sample, nitric acid solution, deionised water and soil sample extract. The instrument was calibrated and profiled using a mixed metal standard solution with concentrations ranging from 0.2 mg kg-1 to 1.0 mg kg-1. The system was rinsed for one minute with 3% nitric acid solution in between the analysis of each sample. Details of the QA/QC are given in section 3.10 below.
Sequential extraction procedure
The extraction procedure used in this study classified heavy metals into three chemical fractions:
Fraction 1 (exchangeable fraction): 5.00g ±0.01g soil sample is extracted with 1M magnesium chloride of pH 7 in the ratio 1:10 (w/v). The soil was extracted at room temperature for 1 hour by sonication. The extract was centrifuged and the residual soil was washed with deionised water and dried.
Fraction 2 (bound to organic compounds): The air dried residual soil is extracted with 0.05M 0f EDTA in the ratio 1:10 (w/v) at room temperature for 2 hours using sonication. The residual soil is again washed with deionized water and allowed to dry.
Fraction 3 (residual fraction): the residual soil is extracted nitric acid and hydrochloric acid following USEPA method 3051a (2007)
S2: Operating parameters of ICP- OES (iCAP 1600)
Table 1: Operating parameters of ICP- OES (iCAP 1600)
Operating parameters of the thermos ICP-OES (iCAP 1600)Power (W) / 1150
Auxiliary gas flow (L/min) / 0.5
Nebuliser gas flow (L/min) / 0.75
Coolant gas flow(L/min) / 12
View / Axial
Purge gas flow / Normal
Flush pump rate (rpm) / 100
Analysis pump rate (rpm) / 50
Camera temperature / -47
Optics temperature / 38
Table 2: Wavelengths used on the ICP-OES of the elements investigated
Elements / Wavelength (nm)Cd / 228.802
Cr / 283.563
Cu / 324.754
Mn / 257.610
Ni / 221.647
Pb / 220.353
Sb / 206.833
Zn / 213.856
S3: Pollution indices
Table 3: Potential ecological risk assessment
Eri / Single pollutant degree of environmental risk / PERI / Comprehensive environmental risk levelEri ≤40 / low ecological risk / RI ≤150 / low ecological risk
40 <Eri ≤80 / moderate ecological risk / 150 < RI ≤300 / moderate ecological risk
80 <Eri ≤160 / considerable ecological risk / 300< RI ≤600 / considerable ecological risk
160 < Eri ≤320 / high ecological risk / RI >600 / very high ecological risk
Eri > 320 / very high ecological risk
Table 4: Risk assessment code (RAC)
% exchangeable fraction / Risk<1% / No risk
1-10% / Low risk
11-30% / Medium risk
31-50% / High risk
>50% / Very high risk
S4: Soil guideline values
Table 5: Soil guideline values of various regulatory bodies
Soil guideline values of various regulatory bodiesMetals / USEPA
(mgkg-1) / ESDAT
(mgkg-1) / DEFRA
(mgkg-1) / Dutch
(mgkg-1)
CSSL / TV / IV / Residential
IV / Industrial
IV / TV / IV
Cd / 70 / 0.8 / 12 / 1-8 / 1400 / 0.8 / 12
Cu / - / 36 / 190 / - / - / 36 / 190
Pb / 400 / 85 / 530 / 450 / 750 / - / -
Cr / 230 / 100 / 380 / - / - / 100 / 380
Ni / 1600 / 35 / 210 / 50 / 5000 / 35 / 210
Zn / - / 140 / 720 / - / - / 140 / 720
Sb / - / 3 / 15 / - / - / 3 / 15
TV= Target value, IV= Intervention level, CSSL= Contaminated soil screening level. DEFRA= Department for Environment Food & Rural Affairs, ESDAT= Environmental Data Management System, USEPA= United States Environment Protection Agency. ‘-‘ = not available
S5: Recovery of metals in certified reference material
Table 6: Percentage recovery of metals in certified reference material.
Measured values (mg/kg) and % recovery of metals in certified reference material SQC001-050G (lot 011233) Mean ± S.E. (n=5)Cd / Cr / Cu / Mn / Ni / Pb / Sb / Zn
Certified total / 134±2.57 / 176±4.08 / 56.1±1.20 / 183±3.74 / 65.1±1.54 / 134±3.02 / 73±10.5 / 473±9.21
Measured total / 133.4±1.21 / 169.15±1.4 / 56.14±0.44 / 182.89±2.11 / 63.19±1.15 / 132.58±3.15 / 71.67±0.63 / 483±1.03
% Recovery / 99.6 / 96.11 / 100.3 / 99.9 / 97.1 / 98.9 / 97.2 / 102.1
3
[1] Where g is the relative centrifugal force. It depends on the revolutions per minute (RPM) and radius of the rotor.