Environmental context. The toxicity of metals in the environment is controlled by several parameters including total metal concentration, pH and organic and inorganic ligands (type and concentration). The characterisation of different metal pools in natural matrices (e.g. seawater, soil) is important for the evaluation of their toxic impact. The copper ion-selective electrode (Cu-ISE) is a method of choice for the analytical determination of the speciation (i.e. chemical forms) of divalent copper in natural matrices. This paper clarifies several operational parameters in the hope of decreasing variability of results and increasing the application domain of the Cu-ISE.Abstract. The determination of free copper concentrations in natural matrices is critical for the evaluation of copper toxicity. The ISE is one of the few analytical means for determining the direct speciation of free metal species. We have refined the method for low salinity and low ionic strength solutions for application with soil water extracts or fresh waters. Moreover, we have detailed and standardised a method for using a Cu-ISE with an autotitrator. The standardisation shows a good response and allows significant time saving (under 2 h for the calibration). The results obtained using the ISE are compared with those predicted in the presence of different organic ligands or even the lower free Cu 2+ activities resulting from the formation of Cu hydroxyl species. The method was validated for the determination of Cu speciation at environmentally relevant free Cu 2+ activity, i.e. ranging between 10 −14 and 10 −4 M. The chemical equilibrium calculations were made using the MINEQL+ software and the results agree well for pH values between 3 and 10. In terms of precision, the standard deviations of the measured values never exceed 0.1 units, and in terms of accuracy, the measured values were very close to the nominal values, within a range of 0.1. Outside the optimal pH range, the electrode yields higher activity than expected.

Environmental context. The speciation of metals in soils is controlled by the equilibrium between the solid and aqueous phases and by several parameters such as pH and total metal concentrations. The integration of affinity constants between several cations and active soil surfaces of different soils in the chemical equilibrium modelling software MINEQL+ allows a good evaluation of the chemical speciation of the metals. Abstract.A new approach, derived from the concept of the biotic ligand model, was used for the determination of the affinity constants of Ca, Cu, Cd and H to the active surfaces of different kinds of soils. This approach allowed us to obtain consistent data and to integrate these values in the chemical equilibrium modelling software MINEQL+ and eventually into a solid phase-controlled soil ligand model. This could then very easily be transformed into a terrestrial biotic ligand model by adding constants for biological components. We obtained the chemical speciation of the metals of interest by integrating the initial characteristics of the soil (pH; cation exchange capacity, CEC; total metal concentrations in soil extracts; ionic strength; and CO 2 pressure). Comparison of the predicted and measured values of free Cu 2+ is excellent using soil-specific affinity constants as well as average values. The average affinity constants between the active soil surfaces (S) and the target cations are log K Ca-S = −0.84 (±0.01), log K Cu-S = 5.3 (±0.1), log K Cd-S = 4.4 (±0.2) and log K H-S = 4.1 (±0.2). External soils have been used to validate the conceptual model and the results show a very good correlation between the predicted and the measured free Cu (pCu) except for an acidic soil (pH < 5.2), highlighting the importance of integrating Al into the model.

Classical analytical methods limit understanding of the dynamics of geochemical processes in soils. The technique of diffusive gradients in thin films (DGT) allows the quantification of the mobilization fluxes of traces metals in soils and more specifically the metal supply from the soil's solid phase. Diffusive gradients in thin films, measuring fluxes from soil solids to solutions, were reported in three different cadmium (Cd)-contaminated soils with different levels of soil organic matter (SOM). The soil solution concentration ratio between the labile Cd, determined using differential pulse anodic stripping voltammetry, and the total Cd obtained by inductively coupled plasma-atomic emission spectrometry was compared. The data suggest that SOM affected the complexation of Cd in the soil solution, and the values obtained by DGT also demonstrated that the sorption of the Cd to the solid phase was also affected. The fluxes of Cd into the DGT were 1619 decreased when organic matter was added to the soils but were also decreased when SOM was reduced using hydrogen peroxide (H 2 O 2 ).

The emerging technique of DGT (diffusive gradients in thin films) can be used to measure porewater concentrations and remobilization fluxes of trace metals in soils. This study reports the fluxes from soil solids to solutions in a soil contaminated with different levels of cadmium (Cd) at the naturally acidic pH and after CaCO 3 treatment. The comparison of labile dissolved Cd, determined by using differential pulse anodic stripping voltammetry, and the concentration obtained by DGT shows that pH has little effect on the complexation of Cd in solution and that the effect of liming is more pronounced on the sorption of Cd to the soil particles. The flux from ORDER REPRINTS soil to solution seems to be reduced by 10-50 times following liming from pH 5.6 to 7.0. The final concentrations of labile Cd represent 70-95% of the initial labile concentrations and the Cd fluxes varied from 2.8 to 54 mg cm À2 h À1 at pH ¼ 5.6 and from 0.6 to 5.5 mg cm À2 h À1 at pH ¼ 7.0.

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