Do we have a problem with freshwater Kd values?

B. Howard and E. TippingCEH, UK

Analysis for discussion only

– do not quote

ERICA

• ERICA uses Kd values to predict unknown water or sediment concentrations

• Water conc is used with CR to predict wholebody conc and internal dose

• Sediment conc is used for estimation of external dose

• Some ERICA values are based on sea water– -does this introduce larger error than for the other

values used?

Nuclide

Distribution Coefficient (Kd) Source type

aquatic system original source

ERICAAg 1.2E+05 TRS rev ads fw Ciffroy et alAm 5.3E+05 TRS rev in situ fw Ciffroy et alC 5.0E+00 SRS 19 / TRS 364 fw Onishi 81Cd 3.0E+04 TRS 422 sw ocean margin Table IICe 3.8E+05 TRS rev Ciffroy et alCl 1.0E+00 educated guessCm 5.0E+03 SRS 19 / TRS 364 fw Onishi 81Co 1.1E+05 TRS rev in situ fw Ciffroy et alCs 1.4E+05 TRS rev in situ fw Ciffroy et alEu 5.0E+02 SRS 19 / TRS 364 fw Onishi 81H 1.0E+00 Copplestone et al 2001I 3.0E+02 AA BalkemaMn 9.1E+04 TRS rev in situ fw Ciffroy et alNb 8.0E+05 TRS 422 sw ocean margin Table IINi 2.0E+04 TRS 422 sw ocean margin Table IINp 1.0E+01 SRS 19 / TRS 364 fw Onishi 81

P 5.0E+01SRS 19 / TRS 364 + Onishi pers comm fw Onishi 81

Pb 1.0E+05 TRS 422 sw ocean margin Table IIPo 2.0E+07 TRS 422 sw analoguePu 1.4E+06 TRS rev in situ fw Ciffroy et alRa 1.5E+04 TRS rev Ciffroy et alRu 4.0E+04 TRS rev Ciffroy et alS 5.0E-01 TRS 422 sw ocean margin Table II

Sb 1.2E+04SRS 19 / TRS 364 + Onishi pers comm fw Onishi 81

Se 3.0E+03 TRS 422 sw ocean margin Table IISr 2.0E+03 TRS rev in situ fw Ciffroy et alTc 5.0E+00 SRS 19 / TRS 364 fw Onishi 81Te 1.0E+03 TRS 422 sw analogueTh 1.8E+07 TRS rev Ciffroy et alU 5.0E+01 SRS 19 / TRS 364 fw Onishi 81Zr 1.0E+03 SRS 19 / TRS 364 fw Onishi 81

ERICA values from Ciffroy are AM from the reported GM

Comparison with TRS 364 (Onishi 81)

Oxidising conditions

Nuclide ERICA TRS 364 value ratio ERICA/364

Am 530000 5000 106Ce 384000 10000 38.4Co 106000 5000 21Cs 137000 1000 137I 300 10 30Mn 90800 1000 90.8Pu 1390000 10000 139Ra 15200 500 30Sr 2000 1000 2Th 18400000 1000 18400

KD

mol bound (g colloid)-1

mol L-1 in solution=

But KD depends on:

pH competing solutescompeting ligandsloading of the colloidionic strength

Modelling tries to explain variability in KD

H+

Mz

+

H+

Mz

+

Mz

+

Mz

+

-

-

-

-

Mz

+

N

NModel VI

Specific & non-specific

proton & metal binding

WHAM

Key assumption – binding to organic matter dominates for metal

ions

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9

metal conc

bin

din

g s

tren

gth

Humic substances

• Partial decomposition products of plants etc

• Principally composed of C, H and O, + N & S

• Possess weak acid groups - COOH, phenolic-OH

• Fulvic acid MWt ~ 1000dominant in waters• Humic acid MWt ~ 10 000 dominant in soils

• Heterogeneous, recalcitrant, yellow-to-brown

The most The most abundant abundant

macromolecules macromolecules on the planet!on the planet!

Database for WHAM / Model VI

• ~ 20 data sets for protons ~ 100 data sets for metals

• Average proton binding for FA and HA• Average binding for 23+ metals (Mg…Cu…Eu...Cm)

• Laboratory studies with isolated HA and FAGAP FILLING: binding strength correlations for metal ions

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9

log K acetic acid

log

K h

um

ic a

cid

Esp actinides

Model VI and cation binding : summary

• Proton and metal binding as function of [H+], [Mz+]

• Proton-metal competition (pH dependence)

• Metal-metal competition (esp at high [M+])

• Ionic (eg Na, Cl, )strength dependence of H and M binding due to interference with binding

Ion-binding models and their combinations

“simple” solution

chemistry

Oxide model

AlOx SiOx MnOx FeOx

Clay cation

exchanger

Humic Ion-Binding

Models V & VIWHAM

SCAMP

Na, Cl, OH, CO3, SO4

Wham 6 set up

assumed ph8 for fw values also

•Freshwaters are for 3 different [DOC] - 1, 3 and 10 mg/L

•A range of pH's is generated by titrating an initially acid solution with Ca, to take us from pH ~ 4 to pH ~ 8.5

•Seawater is assumed to be at pH 8, and with 2 mg/l DOC

WHAM IV

• Calculations assume that – DOC can be represented by average isolated fulvic

acid, – OM in particulate matter (SPM) can be represented

by average isolated humic acid• Only organic matter in the SPM has any binding

properties (oxides, clay etc ignored)• Calculations take into account

– competition between the element of interest and major ions (H+, Mg, Ca, Al, Fe etc),

– complexation by inorganic ligands and natural organic matter (dissolved and particulate)

Kd estimates

• The Kd's are calculated for suspended particulate matter containing 10% organic matter

• results give some idea of – how Kd can vary with pH and [DOC],– comparisons between FW and SW

Health warning

• Elements which form hydrolysis reactions in solution at low pH may not be represented well as the model assumes organic complexation (eg Pu)

• The element concentrations are set to low levels and will be sensitive to the model's assumptions about small numbers of strong binding sites

• The model default database has differences in the binding strengths of fulvic and humic acid towards most metals, – these difference may not be real. (e.g. UO2 and PuO2)

• Some elements affected by redox, models assumes specifi oxidation state– Cr, Mn, Fe, Tc, Np

CrIII

1

10

100

1000

10000

100000

1000000

10000000

3 4 5 6 7 8 9

pH

Kd

l/kg [DOC] = 1

[DOC] = 3

[DOC] = 10

sw pH 8

FeIII

1

10

100

1000

10000

100000

1000000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

sw pH 8

Zn

1

10

100

1000

10000

100000

3 4 5 6 7 8 9

pH

Kd

l/kg [DOC] = 1

[DOC] = 3

[DOC] = 10

sw pH 8

No Erica value (just WHAM)

sw value similar to fw predictions at relevant pH

Onishi

Fe – 5000

Cr low

Zn - 500

Th

1

10

100

1000

10000

100000

1000000

10000000

100000000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

Am

1

10

100

1000

10000

100000

1000000

10000000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

Erica - CiffroyAm – ERICA high over most pH range

Sw – lower

Onishi 100x lower than ERICA

Th – ERICA much higher

Sw – lower, similar to fw model

TRS – much lower Onishi (c.20000)

PuIV

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

PuO2

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

3 4 5 6 7 8 9

pH

Kd

l/k

h

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

ERICA - Ciffroy

Onishi – 100x lower

Co

1

10

100

1000

10000

100000

1000000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

Sr

1

10

100

1000

10000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

Mn

1

10

100

1000

10000

100000

3 4 5 6 7 8 9

pH

Kd

l/k

g

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH8

Erica - Ciffroy

Onishi

Mn 100 x lower

Co 20 x lower

Sr - same

Eu (Onishi)

1

10

100

1000

10000

100000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

ERICA - Onishi

Cm (Onishi)

1

10

100

1000

10000

100000

1000000

10000000

100000000

1000000000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

UO2

1

10

100

1000

10000

100000

1000000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

U IV

0.01

0.1

1

10

100

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value

sw pH 8

ERICA - Onishi

Cd

1

10

100

1000

10000

100000

3 4 5 6 7 8 9

pH

Kg

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value (sw)

sw pH 8

Erica – sw value

Ni

1

10

100

1000

10000

100000

3 4 5 6 7 8 9

pH

Kd

l/k

g

[DOC] 2

[DOC] 5

[DOC] 10

ERICA value (sw)

sw pH 8

Pb

1

10

100

1000

10000

100000

1000000

3 4 5 6 7 8 9

pH

Kd

l/kg

[DOC] = 1

[DOC] = 3

[DOC] = 10

ERICA value (sw)

sw pH 8

Changes with pH increase in Wham

• rises – Cr, Zn, Eu, Cm, Pb (Fe III, Am)

• rise and fall – Mn, Co, Sr, UO2, Ni, Cd

• decrease – U IV (Th, Pu IV , PuO2)

Not possible to attribute differences systematically to only one causal factor – this would be misleading

Effect of DOC conc on Kd in FW in Wham IV

ratio 1 / 10 mg/l DOC

0.0

2.0

4.0

6.0

8.0

10.0

12.0

CrM

nFeI

IICo Ni

Zn SrCd Pb

UO2

UIVPuI

VPuO

2 ThAm Cm Eu

rati

o o

f K

d

High values are all metal ions with have the strongest binding to OM

So more DOC = more metal in solution

less DOC = less metal in soluton

SW vs FW – Erica vs model

• FW much higher than Wham SW– Am, Co, Mn, Sr, Th, PuIV, PuO2 (Ciffroy)

– Ni, Cd (sw values)– UIV (Onishi)

• Similar – Pb (ERICA is sw)

• FW much lower than Wham SW– UO2, Eu, Cm (Onishi)

FW vs SW– model

• Wham FW higher than Wham SW– Cd, Mn, Sr, PuIV, PuO2 UIV (Co, Eu, Ni,)

• Similar – Am, Cr, Cm, Fe III, Pb, Th, Zn

• FW lower than Wham SW– UO2

Erica vs FW model

• Erica always higher than Wham– Co, Mn, Th, PuIV, PuO2 (Ciffroy)

• Erica higher than Wham at low pH– Am, Sr, Ni, Cd, Pb (sw values)

• Erica lower than Wham– UO2, Eu, Cm (Onishi) – except at pH 4

• Similar at low pH, higher at high pH– UIV (Onishi)

Conclusions

• ERICA AM values often high

• Model rarely predicts SW > FW, often FW higher

• pH has large effect for many elements

• DOC important for Cr, Fe III, Pb, Am, Cm, Eu

Does it matter

• Too High Kd values– Will give low water conc – low whole body

conc – therefore NOT conservative but more sensitive to error

– Will give high sediment conc – higher external exposure - as >90% of most metals in sediment – less sensitive to error

• Can we “do” something in ERICA to assist user?