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Environmental fate & pathways

Adsorption / desorption

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Reference
Endpoint:
adsorption / desorption: screening
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
Study conducted to a current guideline. Ther Kd median result is considered to be sufficiently representative at present to use in risk assessment.
Reason / purpose for cross-reference:
other: Target record
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)
Deviations:
no
GLP compliance:
not specified
Remarks:
GLP compliance not specified.
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
yes
Test temperature:
Room temperature
Analytical monitoring:
yes
Details on sampling:
- Concentrations: Carrier-free 54Mn2+ was added at about 143 Bq/g for acid soils (pH < 6) and 667 Bq/g for pH neutral soils (pH > 6) and thoroughly mixed.
- Sampling interval: Two consecutive extractions.
- Sample storage before analysis: After 3 days, 100 days and 6 months.
Matrix type:
other: Soil
% Org. carbon:
>= 0.2 - <= 23.3
pH:
>= 3 - <= 8.5
CEC:
>= 1.2 - <= 35 other: Cmolc/kg
Details on matrix:
COLLECTION AND STORAGE
- Geographic location: See table below.
- Collection procedures: Stones and vegetation were cleared from the soil samples.
- Sampling depth (cm): Soils were collected with a metal spade from the plough layer.
- Storage conditions: The soil was put in 60-L plastic drums. Soils were stored at 4 °C until drying and sieving.
- Storage length: The time between sampling and cold storage was never more than one week.
- Soil preparation: The soils were dried in a thin layer at 25 °C in a plant growth cabinet with continuous illumination. After partial drying, soils were sieved through a 4-mm sieve. The sieved soil was thoroughly mixed to ensure homogeneity and further dried. The air-dry sieved soils were stored in 60-L plastic drums.

PROPERTIES
- Soil texture: Selected soil properties were measured for the 35 uncontaminated topsoils. Results are expressed on an oven dry basis. All analyses were performed in duplicate. In Table 1, the average values of the analyses are shown.
- Organic carbon (%): Total carbon (C) was measured by ignition using a Variomax CN analyser. The CaCO3 content of the soils was determined from pressure increases after addition of HCl to the soil in closed containers (including FeSO4 as a reducing agent). The organic carbon content was determined by difference between total and inorganic carbon content. The pH of the soils was measured in 0.01 M CaCl2 (1:5 soil/solution ratio) after shaking for 1 h and allowing to settle for 30 minutes before pH measurement.
- CEC (meq/100 g): The silver-thiourea method (Chhabra et al., 1975) was used to measure cation exchange capacity (CEC) at soil pH and exchangeable cations, with concentrations in extracts determined by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES, Perkin Elmer Optima 3300 DV). Total metal concentrations in soil, including Mn, were determined for all soils by boiling aqua regia extraction and analysis with ICP-OES.
- Carbonate as CaCO3: Our analytical laboratory forms part of an international soil-analytical exchange program (ISE, University Wageningen) where %C, %N, exchangeable cations, %CaCO3 and acid extractable metals are measured. Our methods generally yield good correspondence with median values obtained by other laboratories.
Details on test conditions:
Soils were collected with a metal spade from the plough layer. Stones and vegetation were cleared from the soil samples before being put in 60 litre plastic drums. Soils were stored at 4°C until drying and sieving. The time between sampling and cold storage was never more than one week. The soils were dried in a thin layer at 25°C in a plant growth cabinet with continuous illumination. After partial drying, soils were sieved through a 4 mm sieve. The sieved soil was thoroughly mixed to ensure homogeneity and further dried. Air dry, sieved soils were stored in 60 litre plastic drums.

Of each soil, about 25 g dried soil sample was wetted to field capacity and incubated during 2 weeks in aerobic conditions. After that, carrier-free 54Mn2+ was added at about 143 Bq/g for acid soils (pH < 6) and 667 Bq/g for pH neutral soils (pH > 6) and thoroughly mixed. After 3 days and 100 days two consecutive extractions were performed with soil subsamples of 5 g:

-CaCl2 10 mM (5 g wet soil:10 mL; 2 replicates, 24 h end-over-end shaking), centrifuged at 6750 RCF and 5 mL of the liquid phase were assayed for metals, including Mn (ICP-OES, Perkin Elmer 3300 DV) and 54Mn activity (Minaxi 5,530 auto-gamma counter, decay corrected).

-NH4Ac 1.25 M (pH 4.8): 20 mL was added to each soil subsample in the centrifuge tube (combined with the CaCl2 10 mM remaining), shaken for 2 hours and centrifuged at 6750 RCF. The liquid phase was assayed for metals and 54Mn activity and Mn as described. This extraction is assumed to extract the adsorbed Mn(II) completely (Guest et al., 2002).
Five soils (3, 8, 14, 21, 29), selected to cover a range of pH values, were additionally assayed by adding 200 mg MnO2/kg. All soils were incubated moist (closed vessels, opened regularly to aerate) at room temperature. At 3 and 100 days after spiking, soils were extracted in duplicate. Extractions will also be performed after 6 months.

Selected soil properties (Table 1) were measured for the 35 uncontaminated topsoils. Results are expressed on an oven dry basis. All analyses were performed in duplicate. In the table the average values of the analysis are shown.
Total carbon (C) was measured by ignition using a Variomax CN analyser. The CaCO3 content of the soils was determined from pressure increases after addition of HCl to the soil in closed containers (including FeSO4 as a reducing agent). The organic carbon content was determined by difference between total and inorganic carbon content. The pH of the soils was measured in 0.01 M CaCl2 (1:5 soil/solution ratio) after shaking for 1 h and allowing to settle for 30 minutes before pH measurement.
The silver-thiourea method (Chhabra et al., 1975) was used to measure cation exchange capacity (CEC) at soil pH and exchangeable cations, with concentrations in extracts determined by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES, Perkin Elmer Optima 3300 DV). Total metal concentrations in soil, including Mn, were determined for all soils by boiling aqua regia extraction and analysis with ICP-OES.

Computational methods:
DATA ANALYSIS
The base set is the solid–liquid distribution of the added 54Mn (quantified by the distribution coefficient Kd) in the 35 soils. The Kd expresses the distribution of an element between the solid phase and solution phase:

Kd = Msolid / [M]

where
Msolid is the solid phase concentration, expressed on a soil-weight basis, and
[M] the concentration in solution, expressed on a solution-volume basis.

Thus, the Kd value has a unit of volume per mass, liter kg-1.
The Kd of the isotope, Kd*, is calculated from the ratio of added 54Mn minus 54Mn in solution divided by solution activity concentration of 54Mn. Secondly, also the partitioning of the stable Mn (‘desorption Kd’) is calculated and compared with that of the added 54Mn. This comparison will be used to interpret the fate of Mn on the long time scale. Finally, the Kd of the divalent Mn (KdII) was calculated. The concentration of divalent Mn on the solid phase was estimated by the NH4Ac extraction. Comparison of the Kd and KdII allows interpreting the total soil reaction, i.e. how much Mn is sorbed as MnII and how much is precipitated as MnIII/IV oxides.
For the data interpretation, regression analysis of the Kd with soil properties was carried out: if a correlation with soil pH and CEC or OC is significant, then this ‘model’ might be used to better estimate the EU average Kd value for Mn since the EU soil properties are mapped.
Type:
Kd
Value:
>= 1 - <= 20 550 L/kg
Temp.:
25 °C
Matrix:
Range from 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: 3 days incubation
Remarks:
pH ranged from 3.0 to 8.5
Type:
Kd
Value:
>= 3 - <= 6 516 L/kg
Temp.:
25 °C
Matrix:
Range from 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: 100 days incubation
Remarks:
pH ranged from 3.0 to 8.5
Type:
Kd
Value:
>= 1 - <= 10 818 L/kg
Temp.:
25 °C
Matrix:
Range from 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: 180 days incubation
Remarks:
pH ranged from 3.0 to 8.5
Type:
Kd
Value:
4 131 L/kg
Temp.:
25 °C
Matrix:
Average for 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: Average for 3 days incubation
Remarks:
pH ranged from 3.0 to 8.5
Type:
Kd
Value:
1 184 L/kg
Temp.:
25 °C
Matrix:
Average for 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: Average for 100 days incubation
Remarks:
pH ranged from 3.0 to 8.5
Type:
Kd
Value:
2 180 L/kg
Temp.:
25 °C
Matrix:
Average for 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: Average for 180 days incubation
Remarks:
pH ranged from 3.0 to 8.5
Type:
Kd
Value:
1 598 L/kg
Temp.:
25 °C
Matrix:
Median for 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: Median for 3 days incubation
Remarks:
pH ranged from 3.0 to 8.5
Type:
Kd
Value:
650 L/kg
Temp.:
25 °C
Matrix:
Median for 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: Median for 100 days incubation
Remarks:
pH ranged from 3.0 to 8.5
Type:
Kd
Value:
601 L/kg
Temp.:
25 °C
Matrix:
Median for 35 soil samples
% Org. carbon:
>= 0.2 - <= 23.3
Remarks on result:
other: Median for 180 days incubation
Remarks:
pH ranged from 3.0 to 8.4
Adsorption and desorption constants:
DETERMINATION OF MANGANESE ADSOPRTION Kd
Previous studies (Sheppard & Evenden, 1989) have recommended the use of geometric means to summarize Kd data. The variation in pH with time was 0.28 units in average.
The adsorption Kd* values ranged 1.0–20550 L/kg (median 1598 L/kg) at 3 days and 1.0–6516 L/kg (median 601 L/kg) at 180 days. The Kd geometric mean increased circa 1.2-fold between day 3 and 180.
In general, the Kd* increased or remained constant over time for acid soils (pH< 6), whereas the Kd* decreased or remained constant for neutral-basic soils. The changes in Kd* with time were strongly related to the change in the fraction of isotope that was extractable with NH4Ac. A decrease in NH4Ac-extractable 54Mn corresponds to an increase in the Kd* and is most likely due to transformation of the added 54Mn2+ to 54MnIII/IV, i.e. the isotope equilibrated with the natural Mn that is partly present as MnIII/IV oxides. An increase in NH4Ac-extractable 54Mn and decrease in Kd (observed for some high pH soils) suggests that transformation of 54MnIII/IV to 54MnII occurred. Presumably, there was a fast oxidation of the added 54Mn2+, resulting in a low NH4Ac extractable fraction at 3 days of incubation. However, upon incubation of these samples, reduction of 54MnIII/IV presumably occurred in some of these soils.
There was a strong positive correlation (P< 0.05) between logKd* and pH (measured at time of extraction): the correlation coefficient r was 0.85 at 3 days, 0.83 at 100 days and 0.65at 180 days. Thus, pH explains most of the variation in (log)Kd*, in agreement with previous studies that pointed pH as main factor explaining Kd values of metals in soils (Gerritse & van Driel, 1984; Goldberg & Smith, 1984; Sauvé et al., 2000).
Transformation products:
not measured
Details on results (Batch equilibrium method):
PREDICTION OF THE SOLID-LIQUID PARTITIONING OF Mn IN SOILS
Partitioning of Mn in soil
To predict the partitioning of Mn in soils, some understanding of the speciation of Mn in the whole soil is necessary. Manganese in soil may be in different redox states, most commonly +2, +3 or +4. In soil, MnIII and MnIV occur as insoluble oxides. At low pH or under reduced conditions, MnII is the most stable form which has much higher solubility than MnIII/IV. It is expected that MnII will adsorb on sorption sites (of organic matter, clay minerals and oxides). The Mn redox chemistry therefore plays an important role in Mn solubility.

Solid–liquid partitioning of MnII
To estimate the partitioning of MnII between sorbed and solution pool, extraction with1 M NH4Ac was carried out after the CaCl2 extraction. The Mn concentration in the 10 mM CaCl2 extract is a measure of the soil solution concentration. The NH4Ac extract also extracts sorbed MnII. The solid–liquid distribution coefficient of MnII (KdII) was therefore calculated as the ratio between adsorbed 54MnII (=extractable with NH4Ac) and the 54Mn activity in the CaCl2 extract. The KdII was strongly related to pH and showed no consistent variations over time. Since there were no consistent variations over time, regression analysis was carried out taking the results for all three sampling occasions together. The Kd of MnII was related to pH and organic C content.

Partitioning of Mn between MnII and MnIII/IV
Only part of Mn is present as MnII. A large fraction of Mn in soil can be precipitated as MnIII/IV oxides. The fraction of Mn in soil extracted by NH4Ac (assumed to correspond to MnII) varies between 1 and 100% of total soil Mn and showed a negative correlation with total Mn in soil. The NH4Ac-extractable fraction was comparable at 3 and 180 days, though in some soils, more Mn was extracted at 180 days, presumably because Mn reduction occurred in some of the soils. The fraction of isotope added that was NH4Ac-extractable was generally a bit larger than for the in situ Mn at 3 days after the addition. However, after 180 days of incubation, the NH4Ac-extractable fraction of the isotope and of the in situ Mn was of the same order, indicating nearly full equilibration of the isotope with the soil Mn.

Solid-liquid partitioning of total Mn and suggested models for risk assessment
The Kd* gives the partitioning between total solid phase 54Mn (including sorbed and precipitated pools) and 54Mn in solution. The Kd* is larger than the KdII and is related as follows:

Kd*=KdII/f*NH4Ac

where f*NH4Ac is the fraction of solid phase associated isotope that is NH4Ac-extractable (=MnII). The Kd* is also positively correlated with pH, but there is more variation than for the KdII, because of the variation in the NH4Ac extractable fraction.

The ‘desorption’ Kd gives the partitioning of total soil Mn between solid and solution (=10 mM CaCl2 extract) phase. As the isotope is nearly fully equilibrated with the soil Mn at 180d, the Kd is similar to the Kd* at 180 days. However, at 3 days after addition of the isotope, the Kd is generally larger than the Kd*, as the isotope has not fully equilibrated with the soil Mn yet.
Even though the distribution of soil (in situ) Mn and the added isotope (54Mn) was similar at 180 days of incubation, the regression analysis was not carried out with the isotope data at 180 days, since in some soils, reduction processes apparently occurred, as evident from the increase in NH4Ac extractable and CaCl2-extractable Mn with time. Therefore, we chose to use the Kd values of total soil Mn (‘desorption’ Kd) at shorter incubation times. Since 3 days of incubation probably did not allow for full equilibration of added Mn in the MnO2 amended soils, the 100-days results were selected for the regression analysis. However, also at 100 days, there were four soils where reduction processes apparently occurred, and for these soils (15, 21 with and without MnO2, and 23), the Kd after 3 days incubation was used. Following equation was tested:

logKd= a +b pH + c logOC +d logMnsoil

Only pH was a significant (P<0.05) regressor, and following equation was obtained:

logKd (L/kg)= -2.21 +0.92 Ph (R2=0.76, n=40)

Thermodynamically, it is expected that the Mn solubility will be controlled by sorption processes at low pH and by precipitation at higher pH. If that is the case, it is expected that the dissolved Mn concentration is proportional to the total Mn concentration in soil at low pH (equilibrium between sorbed Mn and Mn in solution) and that the Kd is therefore independent of total Mn concentration at low pH. At high pH, it is expected that the dissolved concentration is independent of total Mn (since the dissolved Mn concentration will be at saturation with the precipitate) and that the Kd will therefore increase with increasing total Mn concentration.
The experimental data were in agreement with these theoretical considerations. For the soils with pH below 5.6, solution concentrations increased with total Mn concentration in soil, and there was no indication that the Kd depends on total soil Mn. However, for soils with pH above 5.6, the Kd indeed appeared to depend on total Mn concentration in soil. This was for instance clearly observed for the MnO2 amended soils: addition of MnO2 increased the dissolved Mn in the low pH soils, but had no effect in the high pH soil.

Therefore, the regression analysis was repeated for the soils with pH<5.6 and soils with pH  5.6 separately. Two soils with pH 3.1 and 3.5 were excluded from the regression analysis, because there Kd was >100 L/kg despite the very low pH. The isotopic K was much smaller for these soils, even after 180 days of incubation, in contrast with the other soils. Most likely, a large fraction of Mn in these soils may be present in the crystal lattice of clay minerals (and is not isotopically exchangeable). For the low pH soils, pH was again the only significant (P<0.05) regressor. For the soils with pH  5.6, also total Mn was a significant regressor.
Following equations were obtained:

pH <5.6: logKd (L/kg)= -2.95 + 0.99 pH (R2=0.65, n=16)
pH ≥5.6: logKd (L/kg)= -2.01 + 0.67 pH + 0.71 logMnsoil (R2=0.64, n=22)

Using the predicted Kd, solution concentrations can be estimated as:

cpred = Mnsoil / (Kd,pred + L/S) ≡ Mnsoil/Kd,pred

where L/S is the liquid:solid ratio (in L/kg).

In general, a better prediction is obtained with the equations above, despite the lower R2 values. This lower R2 can be explained by the lower variation in logKd when grouping the soils by pH. Also the results for the MnO2 amended soils illustrate that the equations above give a conceptually more sound prediction of the Mn solubility, since little effect of MnO2 addition on dissolved Mn concentrations was observed experimentally for the high pH soils, as is predicted.
These models are proposed here for implementing in risk assessment on the fate of either MnII or MnO2 that is added to soil.
It should be noted that all predictions are for aerobic soils. Under saturated conditions, reduction of MnIII/IV oxides may occur, resulting in an increase in MnII concentration and dissolved Mn concentrations.

Kdvalues of added54Mn (mean and standard deviation of 2 replicates) obtained for a collection of 35 European soils. The Kdwas determined at 3, 100 or 180 days after addition of the isotope.

Soil nr

Soil name

Total54Mn Kd*(L/kg)

Factor change

3 – 180 days

3 days

100 days

180 days

mean

SD

mean

SD

mean

SD

1

Gudow

1.4

0.8

8.6

2.1

10

0.8

7.2

2

Nottingham

3.8

0.9

3.0

0.0

10

0.5

2.7

3

Houthalen

1.0

0.1

15

3.4

32

16

31

4

Lommel

1.1

0.3

1.1

0.2

1.0

0.3

0.9

5

Kasterlee

14

1.6

45

3.7

55

2.2

4.1

6

Zegveld

30

9.3

28

7.3

14

1.7

0.5

7

Kövlinge I

5

0.5

40

0.4

147

6.3

32

8

Rhydtalog

16

4.2

32

4.9

601

8.8

39

9

Wageningen

34

9.0

933

67

420

138

12

10

Montpellier

698

40

113

12

689

577

1

11

De Meern

346

28

3094

486

5720

0.6

17

12

Zwijnaarde

7.8

0.8

77

23

251

21

32

13

Hygum

1800

1501

435

10

2320

394

1.3

14

Aluminusa

1931

304

1462

10

8017

1512

4

15

Lovenjoel

1788

362

14

0.2

18

2.1

0.01

16

Zeveren

18

4.9

23

7

743

173

42

17

Plombières

14

4.0

224

13

486

189

34

18

Wilsele

2570

526

883

618

2152

801

0.8

19

London

3052

736

2349

302

3425

249

1.1

20

Woburn

88

10

959

15

2841

1692

32

21

Rijswijk Uzimet

15

1.0

8.5

0.9

9.4

1.0

0.6

22

Lille Exide

1598

781

1203

380

73

18

0.05

23

Ter Munck

10519

3992

775

869

86

27

0.01

24

Bordeaux

20550

11373

688

124

2930

450

0.1

25

Florival Exide

3591

363

502

258

232

84

0.1

26

Paris

4331

2451

650

58

151

19

0.03

27

Rots

8755

5308

1943

106

192

107

0.02

28

Vault de Lugny

13584

2562

3764

90

9053

1773

0.7

29

Souli

1497

711

605

254

2146

1152

1.4

30

Barcelona

19180

8256

1900

100

4268

943

0.2

31

Brécy

14176

8256

5965

196

10818

1836

0.8

32

Marknesse

1810

297

1790

824

3081

735

1.7

33

Guadalajara

11595

11907

3292

344

6081

2895

0.5

34

Nagyhörcsök

17512

10615

6516

6809

4066

1166

0.2

35

Granada

3454

663

1101

795

5155

562

1.5

 

 

 

 

 

 

 

 

 

Average

4131

 

1184

 

2180

 

 

Median

1598

 

650

 

601

 

 

Geomean

344

 

265

 

424

 

 

Concentrations of NH4Ac-extractable Mn and dissolved Mn in five soils with background Mn concentrations (MnBG) or amended with MnO2at a rate of 200 mg Mn/kg. The soils were moistened and incubated for three days before extraction.

Soil

pH

MnBG

Kd*(L/kg)

MnNH4Ac(mg/kg)

Mndiss(mg/L)

-MnO2

+ MnO2

-MnO2

-MnO2

+ MnO2

+ MnO2

3

3.7

17

1.0

1.1

0.3

47

0.03

14.3

8

4.7

86

16

16

53

83

4.4

8.8

14

5.7

49

1931

778

1.1

1.5

0.02

0.01

21

5.8

352

15

18

15

16

0.87

0.74

29

7.3

102

1497

2705

26

27

0.08

0.07

Concentrations of NH4Ac-extractable Mn and dissolved Mn in 5 soils with background Mn concentrations (MnBG) or amended with MnO2at a rate of 200 mg Mn/kg. The soils were moistened and incubated for 180 days before extraction.

Soil

pHa

MnBG

Kd*(L/kg)

MnNH4Ac(mg/kg)

Mndiss(mg/L)

-MnO2

+ MnO2

-MnO2

+ MnO2

+ MnO2

+ MnO2

3

4.0

17

32

3.2

0.03

54

0.009

11.1

8

4.9

86

601

128

9.3

126

0.17

1.3

14

5.9

49

8017

12442

2.7

3.3

0.003

0.17 (0.01)b

21

5.8

352

9.4

9.1

80

54

8.05

5.3

29

7.7

102

2146

2659

36

36

0.02

0.20 (0.05)b

aat 180 days of incubation;

bmost likely, there were inert Mn colloids in solution.

The labile dissolved Mn (in brackets) was estimated based on the specific activity in the NH4Ac extract.

Coefficients for regression equation relating the logarithm of the distribution coefficientof MnII(logKdII, in L/kg) to pH and OC content. The regression analysis was carried out for all soils at the three sampling occasions (R2=0.81,n=114).

 

Coefficients

Standard Error

P-value

Intercept

-3.01

0.24

2.0´10-23

pH

0.80

0.04

3.2´10-40

logOC (%)

0.33

0.09

0.0007

Coefficients for regression equation relating the logarithm of the distribution coefficient of total Mn in soil(logKd, in L/kg) to pH. The regression analysis was carried out for all soils at 100 days of incubationa(R2=0.76,n=40).

 

Coefficients

Standard Error

P-value

Intercept

-2.21

0.50

8.8´10-5

pH

0.92

0.08

3.3´10-13

Coefficients for regression equation relating the logarithm of the distribution coefficient of total Mn in soil(logKd, in L/kg) to pH and total Mn in soil for soils with pH<5.6 and soils with pH³5.6. The regression analysis was carried out for results obtained after 100 days of incubation.a(R2and number of samples given in the table).These models are proposed here for implementing in risk assessment on the fate of either MnIIor MnO2that is added to soil.

 

Coefficients

Standard Error

P-value

Soils with pH <5.6 (R2=0.65,n=16)

Intercept

-2.95

0.94

0.0007

pH

0.99

0.19

0.0001

Soils with pH³5.6 (R2=0.64,n=22)

Intercept

-2.01

1.06

0.07

pH

0.67

0.14

9.3´10-5

logMnsoil(mg/kg)

0.71

0.21

0.003

aData at 3 days of incubation used for soils 15, 21 and 23. Soils 1 and 3 excluded.

Predicted effect of pH and total Mn concentration (Mnsoil, mg/kg) on the Kd(L/kg) and on the dissolved Mn concentrationc(mg/L). Cells for conditions outside the calibration range have been left blank. These values are proposed here for implementing in risk assessment on the fate of either MnIIor MnO2that is added to soil.

Mnsoil(mg/kg)

pH

3.5

4

5

6

7

7.5

 

Kd(L/kg)

10

3.3

10

101

509

 

 

32

3.3

10

101

1158

5385

11613

100

3.3

10

101

2636

12259

26436

316

3.3

10

101

6001

27908

60181

1000

 

 

101

13662

63531

137001

3162

 

 

 

 

144626

311878

 

c(mg/L)

10

3.0

1.0

0.1

0.02

 

 

32

10

3.1

0.3

0.03

0.006

0.003

100

30

9.7

1.0

0.04

0.008

0.004

316

96

31

3.1

0.05

0.011

0.005

1000

 

 

9.9

0.07

0.02

0.007

3162

 

 

 

 

0.02

0.010

Validity criteria fulfilled:
not specified
Conclusions:
Sorption strength increases with pH. A median Kd of 1355 was obtained across all 35 soils (pH 3.0-8.5). Since Manganese dioxide comprises of Mn4+, this form of the substance can be expected to adsorb to soil to a slightly greater extent than Mn 2+, and the Kd median result is considered to be sufficiently representative at present to use in risk assessment.
At 3 days incubation the median Kd was 1598 L/kg. At 100 days incubation the median Kd was 650 L/kg and at 180 days incubation the median Kd was 601 L/kg.
Executive summary:

The adsorption of the test material manganese sulphate to soil was assessed using a method similar to OECD test guideline 106 using 35 European soil samples to encompass the range of properties that may affect the Kd values. 

One of the REACH testing requirements is the screening of the adsorption-desorption reactions of compounds. This screening is required to describe the fate of a given compound in the environment and to predict the environmental concentration resulting from a particular emission. This screening refers to the measurements of the partitioning coefficients, i.e. Kd or Kp values, that are defined as the ratio of the concentrations in the solid phase (of soil, sediment or suspended particles) to that in the liquid phase.

This proposal addressed the solid–liquid partitioning of manganese (Mn) in soils. To be most relevant for the risk assessment, the ‘adsorption Kd’ was measured in soils. No such Mn adsorption studies have been made for European soils before and no comparison has been found between adsorption (of added Mn) and desorption (of native Mn) Kd values.

Manganese in soil is present in different redox states, most commonly +2, +3 or +4. In soil, MnIII and MnIV occur as insoluble oxides. At low pH or under reduced conditions, MnII is the most stable form and this species has much higher solubility than MnIII/IV. The Mn redox chemistry therefore plays an important role in mobility and availability of Mn. The Kd concept that is used in traditional exposure estimates for risk assessment, refers to adsorption reactions, for which solution concentrations increase as the solid-phase concentrations increase. For Mn, this Kd concept does not hold when MnIII and MnIV are present as precipitates (oxides), because the solubility of a precipitate does not depend on the amount of precipitate. Mathematically, the Kd can still be derived in case of precipitation reactions, but this value increases with increasing total concentration once the precipitate is formed. For pragmatic reasons for risk assessment, we have derived the Kd values in this study, even when precipitates formed. Practically, this means that Kd values of Mn in soil not only depend on pH or redox but also on the total Mn in soil.

Thirty-five European soils were selected from our database to make a comprehensive assessment of the Kd values in European soils and to encompass the range of properties that may affect the Kd values, i.e. the cation exchange capacity, pH and organic matter content. Soils were amended with (carrier free)54MnII (a Mn2 +salt) and subsamples were extracted with 10 mM CaCl2at 3, 100 and 180 days after spiking. The adsorption Kd values ranged from 1.0 to 20550 L/kg at 3 days and from 1.0 to 6516 L/kg at 180 days. The geometric mean Kd increased 1.2-fold between day 3 and day 180, from 334 to 424 L/kg. On a restricted set of soil, MnO2was added. The added Mn reached the same speciation as native Mn in most soils by day 100.

The fraction of total Mn that is present as MnIIwas estimated by extraction with 1 M NH4Ac. At 180 days, the partitioning of the isotope and the soil Mn between extractable (MnII) and non-extractable (MnIII/IV) pools was similar for the isotope and the native Mn, indicating that the isotope nearly fully equilibrated within this period. Effectively, this means that the solid-liquid distribution of native Mn in soil may be used to predict the Kd of added Mn.

 

For risk assessment, regression models with R2of 0.64-0.65 were derived from which appropriate Kd values can be selected that are valid for Mn added to aerobic soil as MnII salts or MnO2. The Kd in low pH soils (pH<5.6) is strongly related to pH and did not depend on the total Mn concentration in soil. In high pH soils (pH>5.6), both pH and total Mn concentration affects the partitioning of Mn in the soil. Practically, the Kd values can be implemented in the EU risk assessment schemes if generic values of soil pH and total Mn are selected. A look-up table allows selecting appropriate Kd values depending on pH and total Mn. These predictions are only valid for aerobic soils. Under saturated conditions, reduction of MnIII/IV oxides may occur, resulting in an increase in MnII concentration and dissolved Mn concentrations.

 

At 3 days incubation the median Kd was 1598 L/kg. At 100 days incubation the median Kd was 650 L/kg and at 180 days incubation the median Kd was 601 L/kg.

Description of key information

Data is provided on the surrogate substance manganese sulphate. Sorption strength increases with pH. However, in several soils, residual 54Mn in solution was below values expected for sorption, suggesting oxidation. The Kd median result is considered to be sufficiently representative at present to use in risk assessment.

At 3 days incubation the median Kd was 1598 L/kg. At 100 days incubation the median Kd was 650 L/kg and at 180 days incubation the median Kd was 601 L/kg.

Key value for chemical safety assessment

Koc at 20 °C:
32 500

Additional information

This proposal addressed the solid–liquid partitioning of manganese (Mn) in soils. To be most relevant for the risk assessment, the ‘adsorption Kd’ was measured in soils. No such Mn adsorption studies have been made for European soils before and no comparison has been found between adsorption (of added Mn) and desorption (of native Mn) Kdvalues.

Manganese in soil is present in different redox states, most commonly +2, +3 or +4. In soil, MnIIIand MnIVoccur as insoluble oxides. At low pH or under reduced conditions, MnIIis the most stable form and this species has much higher solubility than MnIII/IV. The Mn redox chemistry therefore plays an important role in mobility and availability of Mn. The Kdconcept that is used in traditional exposure estimates for risk assessment, refers to adsorption reactions, for which solution concentrations increase as the solid-phase concentrations increase. For Mn, this Kdconcept does not hold when MnIIIand MnIVare present as precipitates (oxides), because the solubility of a precipitate does not depend on the amount of precipitate. Mathematically, the Kdcan still be derived in case of precipitation reactions, but this value increases with increasing total concentration once the precipitate is formed. For pragmatic reasons for risk assessment, we have derived the Kdvalues in this study, even when precipitates formed. Practically, this means that Kdvalues of Mn in soil not only depend on pH or redox but also on the total Mn in soil.

Thirty-five European soils were selected from our database to make a comprehensive assessment of the Kdvalues in European soils and to encompass the range of properties that may affect the Kdvalues, i.e. the cation exchange capacity, pH and organic matter content. Soils were amended with (carrier free)54MnII(a Mn2 +salt) and subsamples were extracted with 10 mM CaCl2at 3, 100 and 180 days after spiking. The adsorption Kdvalues ranged from 1.0 to 20550 L/kg at 3 days and from 1.0 to 6516 L/kg at 180 days. The geometric mean Kdincreased 1.2-fold between day 3 and day 180, from 334 to 424 L/kg. On a restricted set of soil, MnO2was added. The added Mn reached the same speciation as native Mn in most soils by day 100.

The fraction of total Mn that is present as MnII was estimated by extraction with 1 M NH4Ac. At 180 days, the partitioning of the isotope and the soil Mn between extractable (MnII) and non-extractable (MnIII/IV) pools was similar for the isotope and the native Mn, indicating that the isotope nearly fully equilibrated within this period. Effectively, this means that the solid-liquid distribution of native Mn in soil may be used to predict the Kdof added Mn.

 

For risk assessment, regression models with R2of 0.64-0.65 were derived from which appropriate Kdvalues can be selected that are valid for Mn added to aerobic soil as MnIIsalts or MnO2. The Kdin low pH soils (pH<5.6) is strongly related to pH and did not depend on the total Mn concentration in soil. In high pH soils (pH>5.6), both pH and total Mn concentration affects the partitioning of Mn in the soil. Practically, the Kdvalues can be implemented in the EU risk assessment schemes if generic values of soil pH and total Mn are selected. A look-up table allows selecting appropriate Kdvalues depending on pH and total Mn. These predictions are only valid for aerobic soils. Under saturated conditions, reduction of MnIII/IVoxides may occur, resulting in an increase in MnIIconcentration and dissolved Mn concentrations.