Do we have a problem with freshwater Kd values?

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Do we have a problem with freshwater Kd values?
Analysis for discussion only do not quote

• B. Howard and E. Tipping
• CEH, UK

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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?

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ERICA values from Ciffroy are AM from the
reported GM
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Comparison with TRS 364 (Onishi 81)
Oxidising conditions
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KD
But KD depends on
Modelling tries to explain variability in KD
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WHAM
Key assumption binding to organic matter
dominates for metal ions
Model VI Specific non-specific proton metal
binding
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The most abundant macromolecules on the planet!
Humic substances

• Partial decomposition products of plants etc

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

• Heterogeneous, recalcitrant, yellow-to-brown

• Possess weak acid groups - COOH, phenolic-OH

• Fulvic acid MWt 1000 dominant in waters

• Humic acid MWt 10 000 dominant in soils

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Database for WHAM / Model VI

• Laboratory studies with isolated HA and FA

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

Esp actinides

• Average proton binding for FA and HA
• Average binding for 23 metals (MgCuEu...Cm)

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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

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Ion-binding models and their combinations
simple solution chemistry
Humic Ion-Binding Models V VI
Na, Cl, OH, CO3, SO4
Oxide model AlOx SiOx MnOx FeOx
Clay cation exchanger
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Wham 6 set up



assumed ph8 for fw values also assumed ph8 for fw values also assumed ph8 for fw values also 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

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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)

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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

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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

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No Erica value (just WHAM)
Onishi Fe 5000 Cr low Zn - 500
sw value similar to fw predictions at relevant pH
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Erica - Ciffroy
Am 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)
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ERICA - Ciffroy
Onishi 100x lower
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Erica - Ciffroy
Onishi Mn 100 x lower Co 20 x lower Sr - same
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ERICA - Onishi
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ERICA - Onishi
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Erica sw value
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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
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Effect of DOC conc on Kd in FW in Wham IV
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
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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)

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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

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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)

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Conclusions

• ERICA AM values often high
• Model rarely predicts SW gt FW, often FW higher
• pH has large effect for many elements
• DOC important for Cr, Fe III, Pb, Am, Cm, Eu

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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 gt90 of most metals in sediment
  less sensitive to error
• Can we do something in ERICA to assist user?