Simple Semi-Empirical Predictions of Free Metal Activities in Contaminated Soil Solutions Sébastien Sauvé Université de Montréal (Montréal, QC, Canada)
© Sauvé 2001 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
© Sauvé 2001 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
© Sauvé 2001 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 2001 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) 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
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. (in press?).
© Sauvé 2001 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 2001 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 2001 Dissolved Cd - K d Partitioning (Field-collected soils only) Janssen et al Data Lee et al Anderson and Christensen 1988
© Sauvé 2001 Dissolved Cd Total Cd pH Field & spiked datasets are similar at pH<8 KOH effect on DOM at pH>8
© Sauvé 2001 Predictive Regressions Field-collected dataset Field & spiked soils (pH<7)
© Sauvé 2001 Soil Solution Metal Distribution 20-60% bound to dissolved organic matter 20-30% inorganic species 10-40% free
© Sauvé 2001 The electrochemically active metal is reduced into the Hg drop electrode Each metal has a specific reduction potential, peak position identifies metal, peak height is proportional to concentration Differential Pulse Anodic Stripping Voltammetry
© Sauvé 2001 Calibration by comparison of known standards with samples Differential Pulse Anodic Stripping Voltammetry
© Sauvé 2001 Differential Pulse Anodic Stripping Voltammetry
© Sauvé 2001 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 2001 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 2001 Adsorption Model Assuming competitive binding of H + and Me 2+ to a deprotonated surface (S):
© Sauvé 2001 Adsorption Model Transforming into a competition coefficent:
© Sauvé 2001 Adsorption Model Transformed to the logarithnic form: where p stands for the negative log 10 of Me 2+ molar activity (i.e. like pH, pCu 2+ of 8 means M Cu 2+ activity)
© Sauvé 2001 Adsorption Model Assuming that MeSur<<SurH y, then: Assuming that adsorption capacity is dependent on organic matter content:
© Sauvé 2001 Adsorption Model simplified without the soil organic matter parameter to: Applied with succes to the soil solution speciation of Cd 2+, Cu 2+, Pb 2+ and Zn 2+.
© Sauvé 2001 Free Cd 2+ Total Cd pH Field & spiked datasets are similar No apparent effects of KOH- induced DOM
© Sauvé 2001 Predictive Regressions for Free Cd 2+ Spiked dataset Field & spiked soils
© Sauvé 2001 Free Cu 2+ Tight relationship to soil solution pH and total metal content N=94
© Sauvé 2001 Free Pb 2+ For Pb… N=84
© Sauvé 2001 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 2001 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 2001 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 2001 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 2001 Soil Quality Criteria From: Sauvé et al Derivation of soil quality criteria using predicted chemical speciation of Pb 2+ and Cu 2+. Environ. Toxicol. Chem. 17:
© Sauvé 2001 Conclusions Dissolved and divalent free metal in soil solutions can be predicted from simple regressions with total metal burden and soil solution pH Risk assessment should minimally consider the relative impact of soil properties like pH