David A. Dzombak

Metal-humate interactions. 1. Discrete ligand and continuous distribution models.

In this paper the authors examine critically the theory underlying discrete and continuous multiligand models for metal-humate binding. The concepts and equations that unify the various models are presented, and a general solution to the fundamental integral equation for ion binding in a multiligand system is given. Particular attention is paid to the continuous distribution models (normal distribution, affinity spectrum, and continuous stability function) which are relatively new tools in the field of metal-humate complexation. It is shown that the lower half and extreme right of the Gaussian ligand distribution assumed in the normal distribution model never affect metal speciation measurably and hence are not knowable. It is also shown that an affinity spectrum does not correspond to an actual distribution of ligands; rather, each peak in the spectrum indicates the most probable stability constant controlling metal binding in a particular region of the experimental formation function. Application of the affinity spectrum model leads to a set of discrete ligands. A close examination of the continuous stability function model shows that it contains implicitly the same assumption as the affinity spectrum approach and thus leads also to discrete ligands.

Effects of simple organic acids on sorption of Cu2+ and Ca2+ on goethite

Effects of phthalic acid and chelidamic acid on sorption of Cu2+ and Ca2+ were examined in goethite (α-FeOOH)/water suspensions. Single-sorbate sorption of Cu2+ and Ca2+ on goethite over a wide range of conditions (pH, I, sorbate/sorbent ratio) was described reasonably well by the Generalized Two Layer Model (GTLM). In the presence of the organic acids, particularly chelidamic acid, sorption of Cu2+ was significantly enhanced at low pH values. Sorption of Cu2+ in Cu2+-phthalic acid and Cu2+-chelidamic acid binary systems was quantitatively reproduced over a wide range of conditions (pH, Cu/organic acid concentration ratio) with the GTLM by using a unique set of Cu2+ -organic acid ternary surface complexes for each binary system. Sorption of Ca2+ remained unaffected in the presence of phthalic acid, but decreased in the presence of chelidamic acid due to the formation of nonsorbing Ca2+-chelidamic acid solution complexes. These effects were predicted reasonably well by the GTLM. Although the exact sorption mechanisms are unconfirmed, available data suggest that enhanced sorption of Cu2+ resulted from sorption of Cu2+-organic acid complexes in a ligand-like manner, i.e., involving oxide surface sites and carboxyl groups of the organic acids not participating in Cu2+ complexation. Results from this work suggest that the reported mineral-humate-cation interactions are consistent with the formation of cation-humate ternary surface complexes.

Colloid release and transport processes in natural and model porous media

Abstract Colloid release was observed from packed columns for two natural porous media (sands) and one model system (glass beads with deposited latex colloids). Colloid release was found to occur in all cases when the ionic strength was reduced in columns that were in equilibrium with Na + ions. Most of the released colloids from the natural porous media were smaller than 1 μm in size, and comprised pure and impure forms of silica (60–70% by mass) and clay minerals (20–30% by mass). For greater reductions in ionic strength, the total mass of released colloids increased, although the shape of the effluent mass concentration profile did not change. Release rate coefficients were obtained by fitting a colloid transport model (an advection-dispersion transport model with source/sink terms for colloid release and deposition) to the column effluent data. To fit the data for different ionic strengths, the total available mass of releasable colloids had to be adjusted, and fitted release rate coefficients were not sensitive to the ionic strength. In contrast, calculations based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory indicate a strong dependence of release rate constants on ionic strength for homogeneous colloids. This discrepancy can be attributed to charge and size heterogeneity in the colloids, and to our inability to determine accurately interparticle forces at small separations. The trend of greater mass release for greater reductions in ionic strength could be explained qualitatively by computing interparticle interactions with a constant-charge boundary condition (albeit with a charge density much lower than that experimentally determined) which showed a decreasing energy barrier for particle detachment with decreasing ionic strength.

Interactions of copper, organic acids, and sulfate in goethite suspensions

Abstract Sorption of copper and sulfate onto goethite (α-FeOOH) in aqueous solution is examined in Cu SO 4 binary-sorbate systems and in Cu-SO 4 -organic acid (either phthalic acid or chelidamic acid) ternary-sorbate systems. Compared to single-sorbate systems, sorption of Cu onto goethite was enhanced at low pH values in the presence of sulfate. Sorption data for Cu and SO 4 in Cu SO 4 binary-sorbate systems were described with the Generalized Two Layer Model by proposing formation of a Cu SO 4 ternary surface complex. Addition of sulfate to a Cu-phthalic acid binary sorbate system had little effect on Cu sorption. However, addition of sulfate to Cu-chelidamic acid binary-sorbate systems resulted in significant reduction of Cu sorption at low pH values, primarily due to competition for surface sites between sulfate and Cu-chelidamic acid ternary surface complexes. While organic acids such as humic substances can potentially influence sorption of metal ions, results from this study suggest that the extent of such influence may be strongly dependent on the presence of other sorbing anions, such as sulfate. Sorption of Cu and SO 4 in Cu SO 4 -organic acid ternary-sorbate systems was predicted reasonably well, based on surface reactions and equilibrium constants derived from fitting of sorption data from single- and binary-sorbate systems. These modeling results provide a validation of the extrapolation of sorption from simple systems to multicomponent systems through surface complexation modeling.

Metal-humate interactions. 2. Application and comparison of models.

Discrete and continuous multiligand models for metal-humate interactions are compared and analyzed by using both synthetic and experimental data. Discrete ligands (typically two or three) are shown to be a simple and accurate means of predicting metal-humate binding within the range of calibrating titrations. Selection of discrete ligand parameters is best achieved via nonlinear regression. The continuous affinity spectrum model is highly sensitive to experimental error, and thus its usefulness as an aid for selection of discrete ligands is limited. As anticipated, only the weakest, most abundant ligands in a ligand mixture can be identified with the continuous stability function model. The continuous normal ligand distribution model is capable of fitting metal-humate binding data with only three parameters, but only the stronger ligands in the assumed distribution are important for fitting observed data. The discrete ligand approach is probably the most useful way to model metal-humate binding because of the ease with which discrete ligands can be incorporated into chemical equilibrium computer programs.

Estimating adsorption of polycyclic aromatic hydrocarbons on soils

This paper presents a synthesis of information available on the adsorption of polycyclic aromatic hydrocarbons (PAH) in water/soil systems. Included is an analysis of how limited adsorption data for PAH on soils may be used in conjunction with PAH molecular characteristics to predict adsorption properties for a wide range of PAH and soils. Also presented is an evaluation of procedures that enable prediction of adsorption characteristics of PAH on soils based on soil organic carbon content and physical-chemical or structural characteristics of the particular compound.

Surface complexation modeling : gibbsite

FOREWORD. PREFACE. CHAPTER 1 - Aluminum Oxides and Hydroxides under Environmental Conditions. 1. INTRODUCTION. 1.1 Occurrence of Aluminum Oxides and Hydroxides in the Subsurface. 1.2 Occurrence of Aluminum Oxides and Hydroxides in Surface Water. 1.3 Use of Aluminum Hydroxide in Water Treatment. 1.4 Summary. CHAPTER - Formation and Properties of Gibbsite and closely-related minerals. 2. Formation and properties of Gibbsite and closely-related minerals. 2.1 Al Polymerization Models. 2.2 Formation of Gibbsite and Other Al-Hydroxides and Oxyhydroxides. 2.3 Aluminum Hydroxide Polymorphs: Structure and Nomenclature. 2.4 Gibbsite. 2.5 Bayerite. 2.6 Nordstrandite. 2.7 Doyleite. 2.8 Other forms of aluminum oxides and oxyhydroxides. 2.9 Other forms that manufactured under high temperature and pressure. CHAPTER 3 - Types of Available Data. 3.1 Gibbsite Structure Verification. 3.2 Physical-Chemical Properties. 3.3 Acid-Base Titration Data. 3.4 Cation and Anion Sorption Data. 3.5 Spectroscopic Data for Sorption on Gibbsite. 3.6 Proton Release/Uptake Data. 3.7 Electrokinetic Data. 3.8 Summary. CHAPTER 4 - Data Compilation and Treatment Methods. 4.1 Collection of data. 4.2 Assessment of Data Quality. 4.3 Compilation of Surface Properties. 4.4 Extraction of Equilibrium Sorption Constants. 4.5 Optimal Fit Simulations. 4.6 Presentation of Results. CHAPTER 5 - Surface Properties of Gibbsite. 5.1 Surface Area. 5.2 Site Density. 5.3 Point of Zero Charge. 5.4 Surface Acid-Base Chemistry. 5.5 Effects of Dissolution on Gibbsite Surface Acid-Base Chemistry. 5.6 Summary. CHAPTER 6 - Cation Sorption on Gibbsite. 6.1 Modeling Methodology and Reactions. 6.2 Available Spectroscopic Data and Use in Modeling. 6.3 Copper. 6.4 Lead. 6.5 Cobalt. 6.6 Cadmium. 6.7 Manganese. 6.8 Iron. 6.9 Calcium. 6.10 Zinc. 6.11 Mercury. 6.12 Uranium. 6.13 Thorium. CHAPTER 7 - Anion Sorption on Gibbsite. 7.1 Modeling Methodology and Reactions. 7.2 Available Spectroscopic Data and Use in Modeling. 7.3 Phosphate. 7.4 Arsenate. 7.5 Arsenite. 7.6 Molybdate. 7.7 Selenate. 7.8 Chromate. 7.9 Borate. 7.10 Sulfate. 7.11 Fluoride. 7.12 Silicate. CHAPTER 8 - Coherence and Extrapolation of Results. 8.1 Cation Sorption on Gibbsite. 8.2 Anion Sorption on Gibbsite. 8.3 Comparison of Gibbsite surface complexation constants with those of Goethite, Hydrous Ferric Oxide and Hydrous Manganese Oxide. 8.4 Summary. REFERENCES. APPENDIX A. Summary of Experimental Details.

Sorption nonequilibrium effects on colloid-enhanced transport of hydrophobic organic compounds in porous media

This paper presents the results of modelling and experimental investigations of factors affecting enhanced transport of hydrophobic organic compounds (HOCs) on colloids in porous media, especially effects of nonequilibrium sorption/desorption. Simulations were performed with a model that considers equilibrium and rate-limited exchange of contaminant between the true dissolved phase, the suspended and attached colloids, and the fixed solid phase as well as the advection, dispersion, deposition, and release of colloidal particles from the porous medium. The model was also applied to laboratory column data for colloid-facilitated transport of a common HOC, phenanthrene. Simulations of colloid-enhanced transport and fitting of experimental data indicated that colloid-enhancement of hydrophobic organic contaminant transport can be significant for sufficiently high colloid concentrations, a high partition coefficient of contaminant on colloids with respect to the fixed porous medium, a low deposition efficiency of the colloids on the fixed porous medium surfaces, and a slow desorption rate of contaminant from the colloids. The last of these conditions was identified as probably the most important in enhancing transport distances of contaminants in natural systems with mobile colloids which usually occur at concentrations not exceeding a few mg/l.

POTENTIAL WATER-QUALITY EFFECTS FROM IRON CYANIDE ANTICAKING AGENTS IN ROAD SALT

Water-soluble iron cyanide compounds are widely used as anticaking agents in road salt, creating potential contamination of surface and groundwater with these compounds when the salt dissolves and is washed off roads in runoff. This paper presents a summary of available information on iron cyanide use in road salt and its potential effects on water quality. Estimates of total cyanide concentrations in snow-melt runoff from roadways are also presented as simple mass-balance calculations.

Rare Earth Element Distributions and Trends in Natural Waters with a Focus on Groundwater

Systematically varying properties and reactivities have led to focused research of the environmental forensic capabilities of rare earth elements (REE). Increasing anthropogenic inputs to natural systems may permanently alter the natural signatures of REE, motivating characterization of natural REE variability. We compiled and analyzed reported dissolved REE concentration data over a wide range of natural water types (ground-, ocean, river, and lake water) and groundwater chemistries (e.g., fresh, brine, and acidic) with the goal of quantifying the extent of natural REE variability, especially for groundwater systems. Quantitative challenges presented by censored data were addressed with nonparametric distributions and regressions. Reported measurements of REE in natural waters range over nearly 10 orders of magnitude, though the majority of measurements are within 2-4 orders of magnitude, and are highly correlated with one another. Few global correlations exist among dissolved abundance and bulk solution properties in groundwater, indicating the complex nature of source-sink terms and the need for care when comparing results between studies. This collection, homogenization, and analysis of a disparate literature facilitates interstudy comparison and provides insight into the wide range of variables that influence REE geochemistry.