Optimised Measurement of the Solid-Phase Distribution of Iron
and Arsenic in Soils by Acid Extraction and Chemometric Data
Processing
Vijay Mistry
Department of Chemistry, Loughborough University, Loughborough, LE11 3TU, UK
Modified HMET
Centrifugation
The Devon Great Consols mine set lies on the east bank of the River Tamar in the Tavistock district of Devon. Between 1848
and 1909 the mine was used for arsenic production with over 70000 tons being collected during this period. In addition to this the
natural geochemical dispersion from the copper-tin-arsenic mineralisation has resulted in a vast amount of arsenic contaminated
soil. This contamination is deemed to be natural as it is a part of the geochemistry of the area (M. R. Cave et al., 2002 Technical
Report).
•Separate aliquots of nitric acid of increasing concentration.
In natural systems arsenic can exist in four oxidation states, however arsenate, As (V), and arsenite, As (III), are the most
common forms. Arsenite is of greater concern to human health as arsenate is able to sorb strongly to other minerals hence
reducing its mobility. Naturally occurring arsenic may be found in a number of phases within the soil, however it is most
commonly associated with iron oxides (M. R. Cave et al., 2002 Technical Report).
The southern part of
the catchment was
heavily mined in the
19th century.
•Identification of physico-chemical hosts and the metal distributions
within the sample under test.
•Passed through the sample under centrifugal force.
•Determination by ICP-AES.
•Chemometric data processing .
Tube
no
1
2
1
DI
DI
3
DI
4
DI
5
DI
Nitric acid
Reagent number
3
4
0.05 M HCl
0.1 M HCl
0.05 M
0.1 M
HNO 3
HNO 3
0.05 M aq.
0.1 M aq.
reg.
reg.
0.05 M
0.1 M
HNO 3
HNO 3
0.05 M
0.1 M
HNO 3
HNO 3
2
0.01 M HCl
0.01 M
HNO 3
0.01 M aq.
reg.
0.01 M
HNO 3
0.01 M
HNO 3
5
0.5 M HCl
0.5 M
HNO 3
0.5 M aq.
reg.
0.5 M HCl
6
1 M HCl
1 M HNO 3
7
5 M HCl
5 M HNO 3
1 M aq.
reg.
1 M HCl
5 M aq.
reg.
5 M HCl
0.5 M HCl
1 M HCl
5 M HCl
MPS
32
C o m p o n e n t 5 (F e -A s )
3000
HNO 3
2000
A qua regia
1500
M ix
1000
M ix duplic ate
500
0
1
•To find the most efficient reagent (HCl, HNO3 or oxalic acid) for the extraction of iron from a Devon Great Consols soil sample,
using experimental design and analysis of extracts using ICP-AES analysis.
•To look at the distribution of arsenic within the different forms of iron identified.
Experimental Design Results
Pareto Chart of Standardized Effects; Variable: ARSENIC
•HCl has the largest
effect on the
extraction of iron.
DV: ARSENIC
(3)HCL(L)
19.04607
(2)NITRIC(L)
13.95655
HCL(Q)
•HNO3 also has a
significant effect.
7.327599
1Lby3L
-5.91862
1Lby2L
•Effect of oxalic
acid is questionable
in comparison and
may cause negative
effects.
-5.59409
(1)OXALIC(L)
4.540919
2Lby3L
-3.42849
OXALIC(Q)
2.937961
NITRIC(Q)
2.057116
2
4
6
8
10
12
14
16
Effect Estimate (Absolute Value)
Pareto chart for the extraction of arsenic from the samples that were agitated for 30 minutes
3
4
5
6
7
8
9 10 11 12 13 14
18
20
22
Component
1
2
3
4
5
6
7
8
16000
14000
12000
10000
8000
6000
4000
2000
0
Composition Tentative assignment
Humic material
Mg chlorite
Calcite
Exchangeable
Amorphous iron oxide
Epidote
Crystalline iron oxide
Crystalline iron oxide
Fe-Na-S
Fe-Mg
Ca
Ca-Al-S-K
Fe-As
Fe-Ca-As
Fe
Fe-As
Tentative assignment of the soil components
T h e d istrib u tio n o f a rse n ic a m o n g st th e va rio u s co m p o n e n ts
o f so il sa m p le 06851-032
C omponent 8 (Fe-As)
Fe- A s
Fe
HCl
HNO3
Aqua regia
M ix
M ix duplic ate
Fe- Ca- A s
Fe- A s
Ca- A l- S -K
Ca
Fe- Mg
Fe - Na -S
1 2 3 4 5 6 7 8 9 10 11 12 13 14
3 3-level factors, 1 Blocks, 15 Runs; MS Residual=2694.669
p=.05
leachate
Extraction profile for amorphous iron oxide
Total extracted solids
(mg/kg)
•To use principle component analysis (PCA) to examine the data collected to determine the physico-chemical forms of iron
present in the test soil.
2
Ex tra ctio n n u m b e r
•To modify the existing HMET methodology to incorporate the optimum conditions for iron extraction and to test the new method
on a soil sample.
0
HCl
2500
(mg/kg)
Total extracted solids
3500
Project Aims
0.45 µ filter
mem brane
Modified HMET reagents table
MINE
There are several small
towns and villages in the
most contaminated area.
Soil
Component
Introduction
0 .00
1 0.0 0
2 0.0 0
Ex tra ction num be r
Extraction profile for crystalline iron oxide
3 0.0 0
4 0.0 0
5 0.0 0
Distribution of arsenic amongst the components of the soil
Conclusions
•Experimental design showed HCl to extract the most iron. Oxalic acid was the least effective.
•Modified HMET results indicated that the mixed reagent HMET extracted the greatest amount of iron and arsenic.
•The standard BGS HMET method proved to be the least efficient.
•Arsenic was only found amongst the iron bearing phases.
•The majority of the extracted arsenic was associated with the crystalline iron oxide component.
References
•Cave, M R, Wragg, J, Palumbo, B and Klinck, B A, 2003. Measurement of the bioaccessibility of arsenic in UK soils. R&D Technical Report P5-062/TR02.
•Klinck, B A, Palumbo, B, Cave, M R and Wragg, J, 2003. Arsenic occurrence, mobility and bioaccessibility from mine contaminated soils: a case study from an abandoned mine in Devon, UK. Draft Report.
6 0.0 0
Pe r ce n t a g e a r s e n ic
7 0.0 0
8 0.0 0