Speciation and Bioavailability of Trace Elements in Contaminated Soils Sébastien Sauvé Université de Montréal (Montréal, QC, Canada)

© Sauvé 2003 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 Me 2+ speciation in contaminated soils Link within the context of a large data acquisition projet

© Sauvé 2003 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 »

© Sauvé 2003 Soil Properties Soil pH in 0.01 M CaCl 2 or KNO 3 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 Cd 2+ of to M

© Sauvé 2003 Analytical Methodology  «Totals» by HNO 3 reflux digestion  Soil solutions obtained using 1:2 soil:0.01 M KNO 3 or CaCl 2 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 Cd 2+ Pb 2+, Zn 2+ speciation by partitioning ASV- labile metal into inorganic ion-pairs Free Cu 2+ by ion-selective electrode potentiometry

© Sauvé 2003  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 Differential Pulse Anodic Stripping Voltammetry

© Sauvé 2003 Calibration by comparison of known standards with samples Differential Pulse Anodic Stripping Voltammetry

© Sauvé 2003 Free Cd 2+ Speciation Assuming that ASV is not sensitive to metals strongly complexed with dissolved organic matter ASV-labile Cd is composed mainly from inorganic species

© Sauvé 2003 Cu 2+ by potentiometry Ion selective electrode very sensitive for Cu 2+ Not prone to interferences (except very high levels of chloride or mercury)

© Sauvé 2003Fractionation/Speciation Soil total Bound to DOM Free metal Cl complexes SO 4 complexes 1 mg Cd kg -1 pH ~ 5

Mineral Solubility Equilibria Sauvé S «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

© Sauvé 2003 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 K d ´s are usually reported as L/kg Sensitive to determination method, solid:liquid ratio, extracting solution, time of extraction and filtration

© Sauvé 2003 Dependence of K d on pH For a compilation of literature K d ’s, 29 to 58 % of the variability depends on soil solution pH. Sauvé S. Hendershot W., Allen H.E «Solid-Solution Partitioning of Metals in Contaminated Soils: Dependence on pH, Total Metal and Organic Matter ». Environ. Sci. Technol. 34:

© Sauvé 2003 Dissolved Cd - K d Partitioning (Field-collected soils only) Janssen et al Data Lee et al Anderson and Christensen 1988

© Sauvé 2003 Dissolved Cd Total Cd pH Field & spiked datasets are similar at pH<8 KOH effect on DOM at pH>8

© Sauvé 2003 Predictive Regressions Field-collected dataset Field & spiked soils (pH<7)

© Sauvé 2003 Adsorption Model Assuming competitive binding of H + and Me 2+ to a deprotonated surface (S):

© Sauvé 2003 Adsorption Model With some assumptions, then: Assuming that adsorption capacity is dependent on organic matter content: But could be oxyde content, clays, sulfides, etc.

© Sauvé 2003 Adsorption Model simplified further to: Applied with succes to the soil solution speciation of Cd 2+, Cu 2+, Pb 2+ and Zn 2+.

© Sauvé 2003 Free Cd 2+ Total Cd pH Field & spiked datasets are similar No apparent effects of KOH- induced DOM

© Sauvé 2003 Predictive Regressions for Free Cd 2+ Spiked dataset Field & spiked soils

© Sauvé 2003 Free Cu 2+ Tight relationship to soil solution pH and total metal content N=94

© Sauvé 2003 Free Pb 2+ For Pb… N=84

© Sauvé 2003 Free Zn 2+ Preliminary speciation data for a free zinc regression  N=30 (Tambasco et al., Sauvé unpublished and and Knight et al. 1999)

© Sauvé 2003 Predictive Regressions for Free Metal Pb 2+ Cu 2+ Zn 2+  Should be possible to derive similar regressions for other divalent cationic metals or anionic elements.

© Sauvé 2003 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:

Inhibition From: Sauvé et al Derivation of soil quality criteria using predicted chemical speciation of Pb 2+ and Cu 2+. Environ. Toxicol. Chem. 17:

Inhibition

© Sauvé 2003 Inhibition From: Sauvé et al Derivation of soil quality criteria using predicted chemical speciation of Pb 2+ and Cu 2+. Environ. Toxicol. Chem. 17:

© Sauvé 2003 Inhibition From: Sauvé et al Derivation of soil quality criteria using predicted chemical speciation of Pb 2+ and Cu 2+. Environ. Toxicol. Chem. 17:

© Sauvé 2003 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.