Post on 28-Feb-2019
ALUMINIUM MOBILITY
AT A RECOVERING ACID SENSITIVE SITEWITH HIGH S-DEPOSITION
R.D. VOGT1*, H.M. SEIP1, H. OREFELLEN1, G. SKOTTE1, C. IRGENS1, and J. TYSZKA2
1 University of Oslo, Dept. of Chemistry, P.O.Box 1033 Blindern, N-0315 Oslo,
Norway
2 Forest Research Institute,ul. Bitwy Warszawskiej 1920r Nr. 3, PL-00-
973 Warszawa, Poland
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Outline
IssueShould a Jurbanite-like model be used to predict Al-activity in soil solution with high {SO4
2-}in soils with high AlS?
Al models FindingsProblems Al controls
The study site Conclusions
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Al models
Reaction pAl =Ion Exch. With BC 3Ca2+ + 2AlX = 2Al3+ + 3CaX 1.5pCa + ½pKex
With H+ 3H+ + AlX = Al3+ + 3HX 3pH + pKex
Solubility Gibbsite Al(OH)3 (S) + 3H+ = Al3+ + H2O 3pH + pKsp
Kaolinite ½Al2Si2O5(OH)4 +3H+ = Al3+ + H4SiO4 + ½H2O 3pH + pKsp
Jurbanite Al(OH)SO4 + H+ = Al3+ + H2O + SO42- pH - pSO4 + pKsp
Co-desorp. Al & SO4 Al(OH)SO4 + H+ = Al3+ + H2O + SO42- pH - pSO4 + pKsp
SOM SOM-Alm+ + nH+ = SOM-Hn(m+n-3)+ + Al3+ pK + npH +
p(CTot/Alorg)
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The Gibbsite model does not work
The Al mobility in the soils (pH < 4.5) does not follow the pAl = pK + 3pH model
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Reduction in S-deposition does not always lead to a recovery
In some regions theANC= Ca2+,Mg2+,Na+,K+- SO4
2-,NO3-,Cl-,F- in µeq
L-1
decreases andRCL={Al3+}/{Ca2++Mg2+}) increases ⌧Due to a stronger decline in base cation concentrations
than in Al
This is contradictory to a simple cation exchange model:
32
23
T }{Ca}{AlK +
+
=
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The study site
Czerniawka in theKarkonoski National park in Poland
Coniferous
Deciduous
Mixed
Deteriorated stands
Urban area
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Decreasing large S-deposition
Avg. 5.4g S m-2yr-1
between 1985 - 1996Decreased >50% since 1987No large local sources
Precipitation pH =4.3Throughfall KL =1.7
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Soil chemistry
The %AlS in the soil is very high
0
20
40
60
80
100
%
O B1 B2
Soil horizon
BSHSAlS
Horizon pHBaCl2 CECE
O 2.8 233B1 4.1 25B2 4.1 24
meq kg-1
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Soil water
Low pH values in O-horizons (average pH = 3.7)Mainly Al release buffer the pH in the mineral soilAl equivalents account for more than half of the total charge in the mineral soil solution.
0
100
200
300
400
500
600
O A B1 B2 BCHorizons
ueq
L-1
0
100
200
300
400
500
600
ueq
L-1
H+ Al ekv Ca2+Mg2+ Na+ K+SO4 2- NO3 - Cl-
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Empirically fitted models -Agreement with observations for pAl
Optimized constant (pKsp):
Optimized constant and slope:⌧Gibbsite based model
• pAl=1.6pH-3.6 r2=0.66⌧Jurbanite based model
• pAl=1.3pH-2.6pSO4-12.5 r2=0.78⌧SOM
• r2= 0.77{ } 4.6-1.8pHAlppY 3 =
= +
org
tot
AlC
3 5 7 90
2
4
6
8
10
Observed pAl
Mod
eled
pAl
SOMGibbsiteJurbanite
1:1
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A multivariate approachSoils with AlS> 90%
PC1Recovery & dilutionRCL is positively related to dilution
PC2 Spatial variation within each soil horizon
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Increase in RCL with decreasing {SO42-}
may occur in soils with AlS>90%
The {Al3+} decreases less than {Me2+} causing an increase in RCL= {Al3+}/{Me2+} with decreasing {SO4
2-}Possible explanation:
Jurbanite dissolution⌧Al(OH)SO4 + H+ =
Al3+ + H2O + SO42-
Concurrent desorption of Al and sulphate
00.5
11.5
22.5RCL
60-89 90-100
<500500-700
>700
AlSSO4 2-
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Conclusions
Soils with high AlS that are receiving decreasing S-deposition, may experience deteriorating ecological conditions In spite of lack of knowledge of the Al-control mechanisms, the use of a jurbanite model instead of a gibbsite model is recommended for soils with AlS≥90% and high sulphate deposition. This conclusion is supported by three major findings:
i Less deviation from equilibrium with jurbanite than for gibbsite ii Increase in the RCL with decreasing [SO4
2-] iii Net mobilisation of Al and SO4
2- from soil profiles