Department of GeologyDepartment of Geology
.An Analytical Analysis of the
Experimental Protocol for my
thesis (Preliminary findings):
Effects of Temperature and
Incubation Time on sorption Of
Lead in 2 tropical soils.
BY
FATAI OLABANJI BALOGUN
OUTLINE•INTRODUCTION
•METHODOLOGY
•RESULTS
•POSSIBLE ERRORS
•CONCLUSIONS
MAP OF SOIL ORDERS IN AFRICA
INTRODUCTION
•Lead (Pb) is a major heavy metal in Gold and Galena mining districts, which leaches out of mine tailings and can cause significant health problem in humans.
•Lead occurs in a variety of Mineral phases such as Galena (PbS), Cerussite (PbCO3), Anglesite (PbSO4), Pyromorphite (PbCl3Pb3(PO4)3 and Mimetesite (PbCl3Pb3(AsO4)3
•The Mineral phases of Lead in the environment are dictated by factors such as weathering, Eh and pH, Mineralogy, etc.
•For the purpose of this work, the incooperation of PbCO3 into tropical environments where pedogenic processes have formed Ultisol and Mollisol will be considered. The soils were obtained from a depth of (0-13cm)
•The work is aimed at assessing the impact of Temperature and Incubation time on the sorption of lead in these two distinct soil types.
INTRODUCTION
Fig A: Eh-pH diagrams of field of Stability of Water and Cerussite in Different Environments
METHODOLOGY
•180 grams each of Ultisol and Mollisol was weighed in a tarred glass beaker and oven dried at a Temperature of 60 C for 24hours.⁰
•After 24hours of drying, the soils were sieved using ASTM 10 sieve (<2mm) in order to sieve out coarse grained soil particles and roots.
•The fractions of each soil type passing through the 2mm sieve was subdivided into portions. 85 grams of each soil type was weighed into 4 different tarred glass beaker, labelled U1, U2, M1, M2.
•Each batch of soils was ground in a ceramic casket, using a ball pestle impact grinder for 5minutes.
•20 grams from each soil batch was weighed into 16 different petri dishes labelled (C1UD0, C1UW1, C1UW4, C1UW8) and (C2UD0, C2UW1, C2UW4, C2UW8). (C1MD0, C1MW1, C1MW4, C1MW8) and (C2MD0, C2MW1, C2MW4, C2MW8)
METHODOLOGY
• Petri dishes labelled C1 and C2 were spiked with 0.052grams (2000ppm) and 0.10 grams (4000ppm) of pure PbCO3 (dispensed on 5ml dilute H2SO4) and the soil was properly homogenized. 5ml of 0.05M NaNO3 was also added
•The soils were then incubated at 0days, 1 week (33°C), 4 weeks (45°C) and 8weeks (70 C).⁰
•After each period of incubation, 3 replicate measurements (5grams each) was taken from 20 grams of spiked soil and were placed in 3 different 50ml centrifuge tubes.
•30ml of “rain water” (dilute H2SO4) at pH of 4.41(prepared in 1L volumetric flask) was added to extract/digest each soil. The samples were equilibrated in a shaker for 24 hours, then centrifuged at 8500rpm for 15minutes.
•The supernatants were collected in a clean 50ml beaker and centrifuged using a 0.45μm and 0.22 μm PES syringe filter and sent off for ICP-MS analysis.
FIGURE B: FLOW DIAGRAM OF THE EXPERIMENTAL SET UP
85 grams of Ultisol
20 grams unaged (day 0) + 2000ppm of PbCO3
20 grams aged at 33 C for ⁰1week +2000ppm of PbCO3
20 grams aged at 70 C for 8 ⁰weeks + 2000ppm of PbCO3
20 grams aged at 45 C for ⁰4weeks + 2000ppm of PbCO3
3 replicate samples (5grams each) collected at each stage of incubation
30ml of H2SO4 (pH 4.41) was added to each/ Samples were equilibrated for 24hrs.
Samples were centrifuged at 8500rpm for 15 minutes and the supernatants were centrifuged using a Polyethersulfone 0.45μm and 0.22μm filter
20μl of trace metal grade HNO3 was added, and samples were sent for ICP-MS analysis.
RESULTSTable1: Table showing the concentration of (Pb) in solution for unaged samples.
Sample ID Concentration of Pb in Solution (x10) (ppm)
Mean and S.D (x10) (ppm) Pb
SU1doC1 1.12 1.07 ± 0.05
SU2doC1 1.06
SU3doC1 1.02
SU1doC2 3.90 3.7 ± 0.4
SU2doC2 3.95
SU3doC2 3.24
SM1doC1 0.22 0.19 ± 0.03
SM2doC1 0.17
SM3doC1 0.19
SM1doC2 0.27 0.27 ± 0.02
SM2doC2 0.26
SM3doC2 0.29
QC 20 ppb 19.60
RESULTSTable2: Concentration of (Pb) in solution for samples aged for 8weeks at
. Sample ID Concentration of Pb in Solution (x10) (ppm)
Mean and S.D (ppm) (x10) Pb
SU1W8C1 5.8 3.79 ± 1.76
SU2W8C1 3.02
SU3W8C1 2.55
SU1W8C2 3.87 4.62 ± 1.24
SU2W8C2 6.05
SU3W8C2 3.93
SM1W8C1 1.43 1.37 ± 0.19
SM2W8C1 1.53
SM3W8C1 1.16
SM1W8C2 1.78 1.70 ± 0.16
SM2W8C2 1.52
SM3W8C2 1.80
QC 20 ppb 19.90
RESULTSTable2: Concentration of (Pb) in solution for samples aged for 8weeks at
. Sample ID Concentration of Pb in Solution (x10) (ppm)
Mean and S.D (ppm) (x10) Pb
SQ1doC1 0.66 0.79 ± 0.20
SQ2doC1 0.69
SQ3doC1 1.02
SQ1doC2 1.30 1.23 ± 0.10
SQ2doC2 1.12
SQ3doC2 1.28
SQ1W8C1 1.83 2.04 ± 0.43
SQ2W8C1 2.53
SQ3W8C1 1.76
SQ1W8C2 1.48 2.34 ± 1.51
SQ2W8C2 4.08
SQ3W8C2 1.45
QC 20 ppb 19.90
RESULTS
POSSIBLE SOURCES OF ERROR
•The impact grinder milled the samples for only 5 minutes. There is a possibility that the resulting powder has unequal grain sizes.
– Samples could have been grinded for a longer time to ensure homogeneity of the grains.
•The desired pH of the extracting solution was 4.0. However, the resulting pH after 5.55μl of 18M H2SO4 was titrated against 1L of D.I water was less than 4.0
– This could be as a result of instrument drift. The Eppendorf pipette used might be out of calibration.
•Working with a spiking agent such as Cerussite in its powdered form is challenging; small quantities of PbCO3 can become easily air-borne when transferring it into the delivering solution. Therefore, starting concentration in the soil could be less than had been earlier anticipated.
– I would suggest that for the purpose of a similar type of experiment a soluble of Lead could be use probably Pb(NO3)2.
POSSIBLE SOURCES OF ERROR
• H2SO4 of pH 4.41 corresponds to 3.89 x 10-5 M (i.e. H+ and SO4 2- have equal
concentration in solution. If SO4 2- reacts with the Pb in the soil, there is a
probability of PbSO4 being formed, and Pb precipitating in the solution.
KspPbSO4= [Pb2+] [SO4 2-] ---- 1.8 x 10-8 = [Pb2+] [3.89 x 10-5 ]
[Pb2+] =4.63 x 10-4 M However, Ksp of PbCO3 = (4 x 10-14)
• Samples were filtered about 10 minutes after centrifuging. This time interval could have a significant impact on the concentration of Pb sorbed.
• Lastly, for a meaningful statistical correlation to be established between the treatments (incubation time, temperature and concentration of Lead) and the concentration of Pb desorbed (in solution), the treatments should have been replicated. Replications were not done on the treatments.– The whole experimental set up could be repeated to replicate the treatments. Otherwise,
intermediate treatments such as those at 1 and 4 weeks could be replicated.
Muscovite in Association with Quartz grains
CONCLUSIONS
•Working with solid spikes is fraught with a lot danger due to the nature of the particles making up PbCO3, (i.e. particles become easily airborne).
•Before embarking on a experiment it is imperative to confirm that, the instruments to be used are well calibrated.
•In conclusion, the Fundamental error committed by failing to replicate the treatments are also irreversible unless the experiments are repeated. As the results of the experiment stands, only confidence limits can be determined to ascertain, the precision of the measurements.
•However, it can be seen that soils aged at 8 weeks definitely released more Pb into solution than those that were unaged.
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