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Speciation and Bioavailability of Trace Elements in Contaminated Soils

Speciation and Bioavailability of Trace Elements in Contaminated Soils. Sébastien Sauvé Université de Montréal (Montréal, QC, Canada) email:sebastien.sauve@umontreal.ca http://mapageweb.umontreal.ca/sauves/. Objectives. Determine the free metal speciation of divalent metals in soil solutions

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Speciation and Bioavailability of Trace Elements in Contaminated Soils

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  1. Speciation and Bioavailability of Trace Elements in Contaminated Soils Sébastien Sauvé Université de Montréal (Montréal, QC, Canada) email:sebastien.sauve@umontreal.ca http://mapageweb.umontreal.ca/sauves/

  2. Objectives • Determine the free metal speciation of divalent metals in soil solutions • Identify the physico-chemical characteristics of the soil which control metal solubility and speciation • Quantify the contributions of pH, total metal and organic matter • Propose simple semi-mechanistic regression models to estimate metal solubility and free Me2+ speciation in contaminated soils • Link within the context of a large data acquisition projet

  3. Soils • Multiple dataset of field-collected soils • Metals originating from smelting/battery recycling operations, long-term phosphate fertilizers, aerial deposition, sewage sludge application, diffuse and point source industrial contamination • Montréal (QC), Ithaca (NY), Québec, France, Denmark & Colorado • Field « equilibrium », in most cases, contamination has occured at least ten years before sampling • Uncontaminated « controls »

  4. Soil Properties • Soil pH in 0.01 M CaCl2 or KNO3 extract (from 3.5 to 8.9) • Soil organic matter of 8.0 to 108 g C kg-1 • Dissolved organic carbon 1.1 to 140 mg C L-1 • Metal levels from background to high industrial range • Soil totals of 0.1 to 56 mg Cd kg-1 • Dissolved Cd of 0.03 to 3500 µg Cd L-1 • Free Cd2+ of 10-10 to 10-5M

  5. Analytical Methodology • «Totals» by HNO3 reflux digestion • Soil solutions obtained using 1:2 soil:0.01 M KNO3 or CaCl2 extractions filtered to <0.22µm (or <0.45µm) • Total dissolved metal by GFAAS (Zeeman) or ICP-AES • Electrochemically labile Cd, Pb and Zn by differential pulse anodic stripping voltammetry (DPASV) • Free Cd2+ Pb2+, Zn2+ speciation by partitioning ASV-labile metal into inorganic ion-pairs • Free Cu2+ by ion-selective electrode potentiometry

  6. Differential Pulse Anodic Stripping Voltammetry • Electrochemically active metals are reduced into the Hg drop electrode • Each metal has a specific reduction potential, peak position identifies metal, peak height is proportional to its concentration

  7. Differential Pulse Anodic Stripping Voltammetry • Calibration by comparison of known standards with samples

  8. Free Cd2+ Speciation • Assuming that ASV is not sensitive to metals strongly complexed with dissolved organic matter • ASV-labile Cd is composed mainly from inorganic species

  9. Ion selective electrode very sensitive for Cu2+ Not prone to interferences (except very high levels of chloride or mercury) Cu2+ by potentiometry

  10. Fractionation/Speciation 1 mg Cd kg-1pH ~ 5 Soil total Bound to DOM Free metal Cl complexes SO4 complexes

  11. MineralSolubility Equilibria Sauvé S. 2002. «The Role of Chemical Speciation in Bioavailability » In: Naidu R., Gupta V.V.S.R., Kookana R.S., Rogers S., Adriano D. (Eds.), Bioavailability, Toxicity and Risk Relationships in Ecosystems. Science Publishers Inc., Enfield, NH, pp 21-44.

  12. Solid/liquid Partitioning • Assumes a unique and constant ratio between solution and solid phases: • Total metal is in mg/kg dry soil and dissolved metal is in mg/L, hence Kd´s are usually reported as L/kg • Sensitive to determination method, solid:liquid ratio, extracting solution, time of extraction and filtration

  13. Dependence of Kd on pH • For a compilation of literature Kd’s, 29 to 58 % of the variability depends on soil solution pH. Sauvé S. Hendershot W., Allen H.E. 2000. «Solid-Solution Partitioning of Metals in Contaminated Soils: Dependence on pH, Total Metal and Organic Matter  ». Environ. Sci. Technol. 34:1125-1131 .

  14. Dissolved Cd - Kd Partitioning(Field-collected soils only) • Janssen et al. 1996 Data • Lee et al. 1996 • Anderson and Christensen 1988

  15. Dissolved Cd • Total Cd • pH • Field & spiked datasets are similar at pH<8 • KOH effect on DOM at pH>8

  16. Predictive Regressions • Field-collected dataset • Field & spiked soils (pH<7)

  17. Adsorption Model • Assuming competitive binding of H+ and Me2+ to a deprotonated surface (S):

  18. Adsorption Model • With some assumptions, then: • Assuming that adsorption capacity is dependent on organic matter content: • But could be oxyde content, clays, sulfides, etc.

  19. Adsorption Model • simplified further to: Applied with succes to the soil solution speciation of Cd2+, Cu2+, Pb2+ and Zn2+.

  20. Free Cd2+ • Total Cd • pH • Field & spiked datasets are similar • No apparent effects of KOH-induced DOM

  21. Predictive Regressions for Free Cd2+ • Spiked dataset • Field & spiked soils

  22. Free Cu2+ • Tight relationship to soil solution pH and total metal content • N=94

  23. Free Pb2+ • For Pb… • N=84

  24. Free Zn2+ • Preliminary speciation data for a free zinc regression • N=30 (Tambasco et al., Sauvé unpublished and and Knight et al. 1999)

  25. Predictive Regressions for Free Metal • Pb2+ • Cu2+ • Zn2+ • Should be possible to derive similar regressions for other divalent cationic metals or anionic elements.

  26. Free Ion Activity Model Ma H, Kim S, Cha D, Allen H (1999) Effect of kinetics of complexation by humic acid on toxicity of copper to Ceriodaphnia dubia. Environ Toxicol Chem 18: 828-837.

  27. Inhibition From: Sauvé et al. 1998. Derivation of soil quality criteria using predicted chemical speciation of Pb2+ and Cu2+. Environ. Toxicol. Chem. 17:1481-1489.

  28. Inhibition From: Sauvé et al. 1998. Derivation of soil quality criteria using predicted chemical speciation of Pb2+ and Cu2+. Environ. Toxicol. Chem. 17:1481-1489.

  29. Inhibition From: Sauvé et al. 1998. Derivation of soil quality criteria using predicted chemical speciation of Pb2+ and Cu2+. Environ. Toxicol. Chem. 17:1481-1489.

  30. Inhibition From: Sauvé et al. 1998. Derivation of soil quality criteria using predicted chemical speciation of Pb2+ and Cu2+. Environ. Toxicol. Chem. 17:1481-1489.

  31. Conclusions • Reasonable predictions of the speciation of metals in soils can be realized from simple regressions with total metal burden, soil solution pH and other soil characteristics • Within a large sampling program, as much care should be devoted to « other » physicochemical parameters as to soil metal analyses per se. email: sebastien.sauve@umontreal.ca http://mapageweb.umontreal.ca/sauves/

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