simple semi-empirical predictions of free metal activities in contaminated soil solutions sébastien...
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Simple Semi-Empirical Simple Semi-Empirical Predictions of Free Metal Predictions of Free Metal
Activities in Contaminated Activities in Contaminated Soil SolutionsSoil Solutions Sébastien Sauvé
Université de Montréal (Montréal, QC, Canada)
email:[email protected]://mapageweb.umontreal.ca/sauves/
© Sauvé 2001
ObjectivesObjectives
Determine the free metal speciation of divalent metals in soil solutions
Identify the physico-chemical characteristics of the soil which control metal solubility and speciation
• Quantify the contributions of pH, total metal and organic matter
Propose simple semi-mechanistic regression models to estimate metal solubility and free Me2+ speciation in contaminated soils
© Sauvé 2001
SoilsSoils Multiple dataset of field-collected soils
Metals originating from smelting/battery recycling operations, long-term phosphate fertilizers, aerial deposition, sewage sludge application, diffuse and point source industrial contamination
• Montréal (QC), Ithaca (NY), Québec, France, Denmark & Colorado
Field « equilibrium », in most cases contamination has occured at least ten years before sampling
© Sauvé 2001
Soil PropertiesSoil Properties
Soil pH in 0.01 M CaCl2 or KNO3 extract (from 3.5 to 8.9)
Soil organic matter of 8.0 to 108 g C kg-1
Dissolved organic carbon 1.1 to 140 mg C L-1
Metal levels from background to high industrial range
Soil totals of 0.1 to 56 mg Cd kg-1
Dissolved Cd of 0.03 to 3500 µg Cd L-1
Free Cd2+ of 10-10 to 10-5 M
© Sauvé 2001
Analytical MethodologyAnalytical Methodology «Totals» by HNO3 reflux digestion
Soil solutions obtained using 1:2 soil:0.01 M KNO3 or CaCl2 extractions filtered to <0.22µm (or <0.45µm)
Total dissolved metal by GFAAS (Zeeman)
Electrochemically labile Cd, Pb and Zn by differential pulse anodic stripping voltammetry (DPASV)
Free Cd2+ Pb2+, Zn2+ speciation by partitioning ASV-labile metal into inorganic ion-pairs
Free Cu2+ by ion-selective electrode potentiometry
MineralMineral Solubility Solubility EquilibriaEquilibria
3 4 5 6 7 8 9pH
0
2
4
6
8
10
12
p (
acti
v ity
)
CdOH2
CdCO 3
CdSO4·2Cd(OH)2
Cd3(PO4)2
3 5 7 9
0
4
8
123 5 7 9
0
4
8
123 4 5 6 7 8 9
pH
0
2
4
6
8
10
12
Cu(OH) 2
CuO
Cu4(OH)6SO4
CuCO 3
Cu3(PO4)2·H2O
3 5 7 9
0
4
8
123 5 7 9
0
4
8
123 4 5 6 7 8 9
pH
0
2
4
6
8
10
12
Pb2(CO)2(OH)2
PbOPb(OH)2
PbSO4PbHPO4
Pb5(PO4)3OH
Pb5(PO4)3Cl
3 5 7 9
0
4
8
123 5 7 9
0
4
8
12
Sauvé S. 2002. «The Role of Chemical Speciation in Bioavailability » In: Naidu R., Gupta V.V.S.R., Kookana R.S., Rogers S., Adriano D. (Eds.), Bioavailability, Toxicity and Risk
Relationships in Ecosystems. (in press?).
© Sauvé 2001
Solid/liquid PartitioningSolid/liquid Partitioning
• Assumes a unique and constant ratio between solution and solid phases:
• Total metal is in mg/kg dry soil and dissolved metal is in mg/L, hence Kd´s are usually reported as L/kg
• Sensitive to determination method, solid:liquid ratio, extracting solution, time of extraction and filtration
MetalDissolvedMetalTotal
Kd
© Sauvé 2001
Dependence of KDependence of Kdd on pH on pH
For a compilation of literature Kd’s, 29 to 58 % of the variability depends on soil solution pH.
Soil Solution pH
2 4 6 8 10
Soil Solution pH
2 4 6 8 10
Soil Solution pH
2 4 6 8 10
Soil Solution pH
2 4 6 8 10
Soil Solution pH
2 4 6 8 10
Kd(L
kg-1
)
10-1100101102103104105106107
Cd Cu Ni
Pb
Zn
Sauvé S. Hendershot W., Allen H.E. 2000. «Solid-Solution Partitioning of Metals in Contaminated Soils: Dependence on pH, Total Metal and Organic Matter ». Environ. Sci. Technol. 34:1125-1131 .
© Sauvé 2001
Dissolved Cd - KDissolved Cd - Kdd Partitioning Partitioning(Field-collected soils only)(Field-collected soils only)
Soil Total Cd (mg/kg)
10
100
1000
10000
100000
Kd (
kg
/ L)
A
3 4 5 6 7 8 9Soil Solution pH
3 4 5 6 7 8 93 4 5 6 7 8 93 4 5 6 7 8 9
B
1 10Dissolved OM (mg C/L)
C
Janssen et al. 1996 Data Lee et al. 1996 Anderson and Christensen 1988
© Sauvé 2001
Dissolved CdDissolved Cd
Total Cd
pH
Field & spiked datasets are similar at pH<8
KOH effect on DOM at pH>8
FieldSpiked
TYPE
© Sauvé 2001
Predictive RegressionsPredictive Regressions
Field-collected dataset
Field & spiked soils (pH<7).).(,64,001.0,759.0
)(log)07.0(77.0
)04.0(54.0)28.0(23.3)41.0)((log
2
10
10
ESnpR
CdSoilTotal
pHCdDissolved
.).(),7(,70,001.0,861.0
)(log)06.0(07.1
)05.0(57.0)28.0(54.3)40.0)((log
2
10
10
ESpHnpR
CdSoilTotal
pHCdDissolved
© Sauvé 2001
Soil Solution Metal DistributionSoil Solution Metal Distribution
20-60% bound to dissolved organic matter
20-30% inorganic species
10-40% free
3 4 5 6 7 8Soil Solution pH
0
20
40
60
80
100
Pe
rce
nta
ge
Organic ComplexesFree Cd2+Inorganic Ion-pairs
© Sauvé 2001
The electrochemically active metal is reduced into the Hg drop electrode
Each metal has a specific reduction potential, peak position identifies metal, peak height is proportional to concentration
Differential Pulse Anodic Stripping Differential Pulse Anodic Stripping VoltammetryVoltammetry
© Sauvé 2001
Calibration by comparison of known standards with samples
y = 0.0119x - 0.0002
R2 = 0.9985
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 10 20 30 40 50 60 70
DPASV Peak Height (nA)
ASV-
Labi
le C
d (µ
M)
Differential Pulse Anodic Stripping Differential Pulse Anodic Stripping VoltammetryVoltammetry
© Sauvé 2001
Differential Pulse Anodic Stripping Differential Pulse Anodic Stripping VoltammetryVoltammetry
© Sauvé 2001
Free CdFree Cd2+2+ Speciation Speciation
Assuming that ASV is not sensitive to metals strongly complexed with dissolved organic matter
ASV-labile Cd is composed mainly from inorganic species
)CdCdClCdNO)Cd(COCdCO
CdHCOCd(OH)Cd(OH)(CdOH ASV)by Cd Labile(2++
3-2
2303
-3
+3
02
+
Cd) LabileASV()CdBoundOM(CdDissolved
© Sauvé 2001
CuCu2+2+ by potentiometry by potentiometry
Ion selective electrode very sensitive for Cu2+
Not prone to interferences (except very high levels of chloride or mercury)
y = -0.0299x + 10.107R2 = 0.9948
2.0
4.0
6.0
8.0
10.0
12.0
14.0
-1000100200300
Electrode potential (mV)
Free
Cu
(pC
u2+)
© Sauvé 2001
Adsorption ModelAdsorption Model
Assuming competitive binding of H+ and Me2+ to a deprotonated surface (S):
HyMeSurSurHMe y
© Sauvé 2001
Adsorption ModelAdsorption Model
Transforming into a competition coefficent:
][)()(][
y
Y
SHMeHMeS
K
© Sauvé 2001
Adsorption ModelAdsorption Model
Transformed to the logarithnic form:
pHySHMeS
KpMey
][][
loglog2
where p stands for the negative log10 of Me2+
molar activity (i.e. like pH, pCu2+ of 8 means 10-8 M Cu2+ activity)
© Sauvé 2001
Adsorption ModelAdsorption Model
Assuming that MeSur<<SurHy, then:
Assuming that adsorption capacity is dependent on organic matter content:
)log(
) log(2
Surfaced
MetalTotalcpHbapMe
) log(
) log(2
MatterOrganicd
MetalTotalcpHbapMe
© Sauvé 2001
Adsorption ModelAdsorption Model
simplified without the soil organic matter parameter to:
) log(2 MetalTotalcpHbapMe
Applied with succes to the soil solution speciationof Cd2+, Cu2+, Pb2+ and Zn2+.
© Sauvé 2001
Free CdFree Cd2+2+
Total Cd
pH
Field & spiked datasets are similar
No apparent effects of KOH-induced DOM Field
Spiked
TYPE
© Sauvé 2001
Predictive Regressions for Free Predictive Regressions for Free CdCd2+2+
Spiked dataset
Field & spiked soils
.).(,102,001.0,736.0
)(log)08.0(97.0)05.0(69.0)32.0(39.4)66.0(2
102
ESnpR
CdSoilTotalpHpCd
.).(,35,001.0,822.0
)(log)11.0(76.0)07.0(66.0)27.0(96.3)51.0(2
102
ESnpR
CdSoilTotalpHpCd
© Sauvé 2001
Free CuFree Cu2+2+
Tight relationship to soil solution pH and total metal content
N=94
© Sauvé 2001
Free PbFree Pb2+2+
For Pb…
N=84
© Sauvé 2001
Free ZnFree Zn2+2+
Preliminary speciation data for a free zinc regression
N=30 (Tambasco et al., Sauvé unpublished and and Knight et al. 1999)
© Sauvé 2001
Predictive Regressions for Free Predictive Regressions for Free MetalMetal
Pb2+
Cu2+
Zn2+
Should be possible to derive similar regressions for other divalent cationic metals or anionic elements.
.).(,94,001.0,921.0
)(log)08.0(84.1)06.0(47.1)39.0(20.3)58.0(2
102
ESnpR
CuSoilTotalpHpCu
.).(,84,001.0,643.0
)(log)10.0(84.0)05.0(84.0)28.0(78.6)47.0(2
102
ESnpR
CdSoilTotalpHpPb
.).(,30,001.0,760.0
)(log)22.0(71.1)10.0(95.0)59.0(70.4)50.0(2
102
ESnpR
ZnSoilTotalpHpZn
© Sauvé 2001
Free Ion Activity ModelFree Ion Activity Model
7.58.08.59.09.5
Free Metal (pCu2+ )
0
20
40
60
80
100
% S
urv
iva l
B0 1 2 3 4 5
Total Dissolved Cu (µM)
0
20
40
60
80
100
% S
urv
iva l
A
Ma H, Kim S, Cha D, Allen H (1999) Effect of kinetics of complexation by humic acid on toxicity of copper to Ceriodaphnia dubia. Environ Toxicol Chem 18: 828-837.
InhibitionInhibition
1 10 100 1000
Total Pb (mg·kg -1)
0
20
40
60
80
100%
Inhib
itio
n
R2=0.127
From: Sauvé et al. 1998. Derivation of soil quality criteria using predicted chemical speciation of Pb2+ and Cu2+. Environ. Toxicol. Chem. 17:1481-1489.
InhibitionInhibition
1 10 100 1000
Total Pb (mg·kg -1)
0
20
40
60
80
100%
Inhib
itio
n
R2=0.127
From: Sauvé et al. 1998. Derivation of soil quality criteria using predicted chemical speciation of Pb2+ and Cu2+. Environ. Toxicol. Chem. 17:1481-1489.
© Sauvé 2001
InhibitionInhibition
6789101112
Predicted pPb2+
0
20
40
60
80
100
% I
nh
ibi t
i on B
R2=0.409
1 10 100 1000 10000Total Pb (mg/kg)
0
20
40
60
80
100
% I
nh
ibi t
i on A
R2=0.127
From: Sauvé et al. 1998. Derivation of soil quality criteria using predicted chemical speciation of Pb2+ and Cu2+. Environ. Toxicol. Chem. 17:1481-1489.
© Sauvé 2001
Soil Quality CriteriaSoil Quality Criteria
Soil Total Content (mg kg-1)
pH 5.5 6 6.5 7
pPb2+50%=8.3 177 415 972 2276
pPb2+25%=9.5 7 16 36 84
pCu2+50%=7.7 103 265 684 1766
pCu2+25%=9.6 8 20 52 135
From: Sauvé et al. 1998. Derivation of soil quality criteria using predicted chemical speciation of Pb2+ and Cu2+. Environ. Toxicol. Chem. 17:1481-1489.
© Sauvé 2001
ConclusionsConclusions Dissolved and divalent free metal in soil
solutions can be predicted from simple regressions with total metal burden and soil solution pH
Risk assessment should minimally consider the relative impact of soil properties like pH
email: [email protected]://mapageweb.umontreal.ca/sauves/