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