Do we have a problem with freshwater Kd values?

1. Analysis for discussion only – do not quote Do we have a problem with freshwater Kd values? B. Howard and E. Tipping CEH, UK

2. 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? ERICA

3. ERICA values from Ciffroy are AM from the reported GM

4. Comparison with TRS 364 (Onishi 81) Oxidising conditions

5. mol bound (g colloid)-1 = mol L-1 in solution pH competing solutes competing ligands loading of the colloid ionic strength KD But KD depends on: Modelling tries to explain variability in KD

6. N N Mz+ Mz+ Mz+ Mz+ Mz+ H+ H+ - - - - WHAM Key assumption – binding to organic matter dominates for metal ions Model VI Specific & non-specific proton & metal binding

7. 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

8. 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 (Mg…Cu…Eu...Cm)

9. 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

10. WHAM SCAMP 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

11. Wham 6 set up • 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

12. 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) WHAM IV

13. 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 Kd estimates

14. 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 Health warning

15. No Erica value (just WHAM) Onishi Fe – 5000 Cr low Zn - 500 sw value similar to fw predictions at relevant pH

16. 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)

17. ERICA - Ciffroy Onishi – 100x lower

18. Erica - Ciffroy Onishi Mn 100 x lower Co 20 x lower Sr - same

19. ERICA - Onishi

20. ERICA - Onishi

21. Erica – sw value

22. 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) Changes with pH increase in Wham Not possible to attribute differences systematically to only one causal factor – this would be misleading

23. 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

24. 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) SW vs FW – Erica vs model

25. 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 FW vs SW– model

26. 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) Erica vs FW model

27. 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 Conclusions

28. 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? Does it matter