1. Speciation and Complexation of Cd in Soil Solutions S.Sauvé, W. Norvell, M. McBride and W. Hendershot Cornell University (Ithaca, NY, USA) McGill University (Montréal, QC, Canada) QSAR Risk Assessment (Montréal, QC, Canada) (email:sebastien.sauve@umontreal.ca)

2. © Sauvé 1999 Objectives Determine the free metal speciation of divalent Cd 2+ in soil solutions  Identify the physico-chemical characteristics of the soil which control Cd solubility and speciation Quantify the contributions of pH, total metal and organic matter Propose simple semi-mechanistic regression models to estimate Cd solubility and free Cd 2+ speciation in contaminated soils

3. © Sauvé 1999 Soils 2 datasets  64 Field-collected soils Cd originating from long-term phosphate fertilizers, aerial deposition, sewage sludge application, diffuse and point source industrial contamination Ithaca (NY), Montréal (QC), Québec & Colorado  46 Metal salt spiked soils One soil spiked up from 1 to 50 mg Cd kg -1 soil pH adjusted with HNO 3 and KOH

4. © Sauvé 1999 Soil Properties Soil pH CaCl 2 of 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 Cd levels  Soil totals of 0.1 to 56 mg Cd kg -1  Dissolved Cd of 0.03 to 3500 µg Cd L -1  Free Cd 2+ of 10 -10 to 10 -5 M

5. © Sauvé 1999 Methodology «Totals» by HNO 3 reflux digestion Soil solutions obtained using 1:2 soil:0.01 M KNO 3 extractions filtered to <0.22µm  Dissolved Cd by GFAAS (Zeeman)  Electrochemically labile Cd by differential pulse anodic stripping voltammetry (ASV)  Free Cd 2+ speciation by partitioning ASV-labile Cd into inorganic ion-pairs

6. © Sauvé 1999 Free Cd 2+ Speciation ASV is not sensitive to metals strongly complexed with dissolved organic matter ASV-labile Cd is mainly inorganic species

7. © Sauvé 1999 Soil Solution Metal Distribution 20-60% bound to dissolved organic matter 20-30% inorganic species 10-40% free

8. © Sauvé 1999 Dissolved Cd Total Cd pH Field & spiked datasets are similar at pH<8 KOH effect on DOM at pH>8

9. © Sauvé 1999 Predictive Regressions Field-collected dataset Field & spiked soils (pH<7)

10. © Sauvé 1999 Dissolved Cd - K d Partitioning (Field-collected soils only) Janssen et al. 1996 Data Lee et al. 1996 Anderson and Christensen 1988

11. © Sauvé 1999 Free Cd 2+ Total Cd pH Field & spiked datasets are similar No obvious effects of KOH induced DOM

12. © Sauvé 1999 Predictive Regressions for Free Cd 2+ Spiked dataset Field & spiked soils

13. © Sauvé 1999 Conclusions Dissolved and free Cd 2+ in soil solutions can be predicted from simple regressions with total metal burden and soil solution pH Risk Assessment should consider the relative impact of soil properties like pH