Chemical speciation and behaviour of cyanide in contaminated soils

Cyanide is present as a contaminant of the soil on several hundred (former) industrial sites in the Netherlands. The risk for the occurrence of adverse effects on human health and the environment strongly depends on the chemical form in which cyanide is present and on the behaviour of this cyanide in soils.The research reported in this thesis aimed to elucidate the predominant forms of cyanide in contaminated soils and the main processes which govern the behaviour of this cyanide. First an automated method of chemical analysis was developed which differentiates the two main forms of cyanide: free cyanide and hexacyanoferrate, and corrects for the presence of thiocyanate. Using this method it was established that the groundwater on former gasworks sites only contains hexacyanoferrate, although this form of cyanide is not thermodynamically stable. The decomposition kinetics of hexacyanoferrate to free cyanide were studied. In daylight complete decomposition appears to proceed within hours. In the dark it proceeds very slowly, with a half-life of hexacyanoferrate ranging from years to hundreds of years, depending on pH and redox potential. These findings were used to improve the separation of free cyanide, hexacyanoferrate and thiocyanate in chemical analysis.The behaviour of hexacyanoferrate in soils was found to be dominated by the precipitation and dissolution of Prussian blue, a mineral occurring in several chemical forms. The exact composition and the solubility product of this mineral were determined, which enabled calculation of hexacyanoferrate concentrations in equilibrium with Prussian blue as afunction of pH and redox potential. Calculations revealed that the solubility of Prussian blue ranges from completely soluble in soils with a pH>ca. 6.5 to slightly soluble in more acid soils. This causes a large difference between the mobility of cyanide in alkaline and in acid soils which was also observed in the field situation

Modeling the binding of fulvic acid by goethite: The speciation of adsorbed FA molecules

Under natural conditions, the adsorption of ions at the solid-water interface may be strongly influenced by the adsorption of organic matter. In this paper, we describe the adsorption of fulvic acid (FA) by metal (hydr)oxide surfaces with a heterogeneous surface complexation model, the ligand and charge distribution (LCD) model. The model is a self-consistent combination of the nonideal competitive adsorption (NICA) equation and the CD-MUSIC model. The LCD model can describe simultaneously the concentration, pH, and salt dependency of the adsorption with a minimum of only three adjustable parameters. Furthermore, the model predicts the coadsorption of protons accurately for an extended range of conditions. Surface speciation calculations show that almost all hydroxyl groups of the adsorbed FA molecules are involved in outer sphere complexation reactions. The carboxylic groups of the adsorbed FA molecule form inner and outer sphere complexes. Furthermore, part of the carboxylate groups remain noncoordinated and deprotonated. Copyright (C) 2003 Elsevier Science Ltd

Evaluation of a Generic Multisurface Sorption Model for Inorganic Soil Contaminants

The performance of a multisurface sorption model approach, composed of well-accepted surface complexation models in combination with published “generic” parameter sets, is evaluated for its possible use in risk assessment. For that purpose, the leaching of a broad range of potential soil contaminants (Ni, Cu, Zn, Cd, Pb, Ba, Cr, Co, Mo, V, Sn, Sb, S, As, Se) from eight diffusely and industrially contaminated soils is predicted simultaneously without any parameter optimization. The model approach includes aqueous speciation in combination with sorption to organic matter (NICA-Donnan model), Fe/Al-(hydr)oxides (Generalized Two-Layer Model), and clay (Donnan model). Independent data generated by pH-static leaching experiments, performed with individual subsamples over a wide pH range (pH 0.4-12), provide a sensitive evaluation of the model performance. Root-mean-squared error values between predicted and measured log concentrations over the entire pH range, RMSElog, ar

Leaching of heavy metals from contaminated soils: An experimental and modeling study

In this paper, we characterize the leaching of heavy metals (Ni, Cu, Zn, Cd, and Pb) from eight contaminated soils over a wide range of pH (pH 0.4-12) using an original approach based on batch pH-static leaching experiments in combination with selective chemical extractions and geochemical modeling. The leached concentrations of the heavy metals are generally much lower than the total concentrations and show a strong pH dependency, resulting in "V-shaped" leaching curves with orders of magnitude changes in solution concentrations. The "multisurface" model used incorporates adsorption to dissolved and solid organic matter (NICA-Donnan), iron/aluminum (hydr)oxide (generalized two-layer model) and clay (Donnan model). These models were applied without modifications, and only the standard set of binding constants and parameters was used (i.e., without any fitting). The model predictions of heavy metal leaching are generally adequate and sometimes excellent. Results from speciation calculations are consistent with the well-recognized importance of organic matter as the dominant reactive solid phase in soils. The observed differences between soils with respect to element speciation in the solid phase correspond to the relative amounts of the reactive surfaces present in the soils. In the solution phase, complexes with dissolved organic matter (DOM) are predominant over most of the pH range. Free metal ions (Me2+) are generally the dominant species below pH 4. The combination of the experimental and modeling approach as used in this study is shown to be promising because it leads to a more fundamental understanding of the pH-dependent leaching processes in soils. The "multisurface" modeling approach, with the selected sorption models, is shown to be able to adequately predict the leaching of heavy metals from contaminated soils over a wide range of conditions, without any fitting of parameter