H. Magdi Selim

Heavy metals release in soils.

Sorption and Release of Heavy Metals in Soils: Nonlinear Kinetics, H.M. Selim and M.C. Amacher Anion and Cation Transport in Zeolitized Tuffs from the Nevada Test Site: Effects of Ion Type, pH, and Ionic Strength, C. Papelis and W. Um Modeling Competitive Sorption and Release of Heavy Metals in Soils, R. Kretzschmar and A. Voegelin Heavy Metal Solubility and Transport in Soil Contaminated by Mining and Smelting, S.L. McGowen and NT. Basta Phase Plane Analysis and Dynamical System Approaches to the Study of Metal Sorption in Soils, S.F. Oppenheimer, W.L. Kingery, and F.X. Han Kinetic Study of Trace Metal EDTA-Desorption from Contaminated Soils, A. Bermond and J.P. Ghestem Soil Properties Controlling Metal Partitioning, C.A. Impellitteri, H.E. Allen, Y. Yin, S-J You, and J.K. Saxe Understanding Sulfate Adsorption Mechanisms on Iron (III) Oxides and Hydroxides: Results from ATR-FTIR Spectroscopy, D. Peak, E.J. Elzinga, and D.L. Sparks Selenium Contamination in Soil: Sorption and Desorption Processes, B. Pezzarossa and G. Petruzzelli Arsenic Behavior in Contaminated Soils: Mobility and Speciation, V. Matera and I. Le Hecho Chemical Structures of Soil Organic Matter and Their Interactions with Heavy Metals, W.L. Kingery, A.J. Simpson, and M.H. B. Hayes Index

Geochemical and Hydrological Reactivity of Heavy Metals in Soils

The hydrological and geochemical interactions between clay minerals and organic matter in soils directly influence the reaction, behavior, and mobility of heavy metals in soils. Geochemical and Hydrological Reactivity of Heavy Metals in Soils is one of only a few books that comprehensively illustrates this cause-and-effect relationship. It highlights analytical techniques such as nuclear magnetic resonance spectroscopy and environmental electron microscopy and reveals how molecular-level modeling is used to remove metal contaminants from the environment. It is thus a practical guide for soil and groundwater scientists, ecologists, and government regulators.

COMPETITIVE SORPTION-DESORPTION KINETICS OF ARSENATE AND PHOSPHATE IN SOILS

The competition between arsenate (AsO4-3) and phosphate (PO4-3) on mineral surfaces has the potential of increasing arsenic mobility and bioavailability in the soil and water environment. In this study, kinetics of competitive sorption of AsO4-3 and PO4-3 in three soils was investigated in batch systems by simultaneously introducing the ligands at different molar ratios. Adsorption was carried out at different retention times, and release was investigated using successive dilutions after adsorption. Nonlinear sorption isotherms of AsO4-3 and PO4-3 were observed for all soils. Rates and amounts of AsO4-3 adsorption were significantly reduced when PO4-3 concentrations in the soil solution increased. In addition, the relative sorption preference of AsO4-3 and PO4-3 did not exhibit changes with reaction time. Desorption and sequential extractions results indicated that a significant amount of AsO4-3 was irreversibly retained by all soils. Kinetic retention data of AsO4-3 and PO4-3 were successfully described using a mechanistic multireaction model that accounted for competitive retention. This study indicates that competition of AsO4-3 and PO4-3 for adsorption sites should be considered in models predicting arsenic release from soils receiving high phosphorus inputs.

Second-order modeling of arsenite transport in soils

Rate limited processes including kinetic adsorption-desorption can greatly impact the fate and behavior of toxic arsenic compounds in heterogeneous soils. In this study, miscible displacement column experiments were carried out to investigate the extent of reactivity during transport of arsenite in soils. Arsenite breakthrough curves (BTCs) of Olivier and Windsor soils exhibited strong retardation with diffusive effluent fronts followed by slow release or tailing during leaching. Such behavior is indicative of the dominance of kinetic retention reactions for arsenite transport in the soil columns. Sharp decrease or increase in arsenite concentration in response to flow interruptions (stop-flow) further verified that non-equilibrium conditions are dominant. After some 40-60 pore volumes of continued leaching, 30-70% of the applied arsenite was retained by the soil in the columns. Furthermore, continued arsenite slow release for months was evident by the high levels of residual arsenite concentrations observed during leaching. In contrast, arsenite transport in a reference sand material exhibited no retention where complete mass recovery in the effluent solution was attained. A second-order model (SOM) which accounts for equilibrium, reversible, and irreversible retention mechanisms was utilized to describe arsenite transport results from the soil columns. Based on inverse and predictive modeling results, the SOM model successfully depicted arsenite BTCs from several soil columns. Based on inverse and predictive modeling results, a second-order model which accounts for kinetic reversible and irreversible reactions is recommended for describing arsenite transport in soils.

A web-based simulation system for transport and retention of dissolved contaminants in soil

Abstract The movement of contaminants through the soil matrix is primarily a liquid phase process in which the chemical partitions between sorbed and dissolved phases. These phenomena have been modeled extensively and several computer models were developed. The use of those computer programs requires installation of the software in the users machine. Usually, post-processing of the numerical output provided by the software is required. In this paper, a web-based simulation environment for retention and transport of dissolved organic and inorganic compounds in soils is presented. The system was developed using Java and is based on the Multi-reaction Transport Model of heavy metals in soil. The mathematical and numerical formulation of the model is briefly sketched. The core computing components of the simulation environment were written in C or fortran for their computational efficiency. The emerging Java Native Interface (JNI) technique and the Swing interface were used to design a user-friendly simulator. Java security problems (due to the use of applets calling native libraries) are discussed and a solution to avoid security restrictions is provided. The simulation system provides interactive user control and real time visualization through standard web browsers. The Java-based simulation code was used to analyze a hypothetical soil contamination problem. The almost instantaneous visualization of results provided by the Java-based interface resulted in efficient and easy analyses. Although a performance comparison was not in mind, the evaluation of the same scenario using the original fortran code took three times as much total time (program run, evaluation of results and post-processing) as that using the Java simulation system.

On the Nonlinearity of Sorption Isotherms of Solutes in Soils

Abstract Quantifying adsorption isotherms, which measure solute affinity, is essential for understanding solute retention and transport in soils and geological media. In this study, a new general solute isotherm equation was derived for soils. The basic assumption here is that it was assumed that a soil is made up of a discrete number of constituents or site fractions each having a different strength or affinity for solute sorption on matrix surfaces. Other assumptions include the validity of linear adsorption for each site fraction or soil constituent and that the sorption capacity for each site fraction is finite. The linear assumption was based on the overwhelming evidence of observed linear isotherms for a wide range of solutes in different soils. At low concentrations where surface coverage is not limiting, this approach was capable of deriving linear two- and three-phase and nonlinear isotherms. The linear assumption was also capable of deriving the Langmuir equation. A major advantage of the new general isotherm equation is that it is valid for an arbitrary site affinity distribution (e.g., histograms) and need not be a continuous function.

Lead Mobility in Calcareous Soils: Influence of Cadmium and Copper

Abstract The mobility and reactivity of heavy metals such as Pb, Cd, and Cu in the soil system is of great significance because of their potential influences on their concentration levels in the soil solution as well as water in streams and lakes. In this study, surface and subsurface soil samples were collected from an area near Bustan in the northwestern desert of Egypt. Soils were used to investigate Pb mobility in a calcareous soil and the subsequent influence of Cd and Cu on Pb release. Miscible displacement column experiments indicated that Pb was strongly sorbed (99.5%), with less than 0.5% recovery of Pb mobility in the effluent solution. The influence of subsequent Cd and Cu pulse applications did not result in Pb release from both surface and subsurface Bustan soils. Distribution of the amount of heavy metals retained versus depth in the soil column indicated that more than 85 to 93% of Pb applied was retained in the surface 2 cm. In addition, Pb was mainly associated with oxidizable as well as the carbonate and oxide fractions. Based on effluent results, the surface and subsurface soil exhibited affinity in the order of Pb > Cu > Cd. Lead exhibited high mobility when applied to the reference sand, and the amounts retained by each reference sand column were similar regardless of Pb concentration of the input pulse. Therefore, we conclude that sorption capacity for Pb in this reference sand material was attained.

Adsorption-Desorption of Lead and Tin in Soils: Experimental and Second-Order Modeling

Abstract The fate of heavy metals, such as Pb, in the soil-water environment is significant in the assessment of their potential mobility and toxicity in the ecosystem. In this study, adsorption batch and sequential extraction experiments were carried out to assess the retention and reactivity of Sn and Pb in soils. Isotherm results exhibited highly nonlinear sorption for both heavy metals. Lead isotherms indicated a Freundlich exponent parameter b of 0.204 and 0.249 for Windsor and Olivier soils, respectively. The respective values for Sn were greater than 1 (2.46 and 3.72), which implies irreversible sorption. Moreover, Sn was completely sorbed, indicative of strong irreversible retention where more than 99% of the added Sn was retained in both soils. No Sn release after 30 days of desorption was detected. The presence of Sn resulted in reducing sorption of Pb by both soils, as evidenced by the decrease in their maximum sorption capacity; whereas, Pb exhibited kinetic behavior where 3 to 15% and 13 to 28% of Pb were released for Windsor and Olivier soils, respectively, during desorption. The use of a second-order two-site model that accounts for nonlinear equilibrium and kinetic reactions was capable of describing the kinetic behavior of Pb during adsorption and desorption in both Windsor and Olivier soils. Sequential extraction results revealed that the most susceptible Pb fraction for release was highest corresponding to highest Pb input concentrations. This concentration-dependent release is consistent with the observed strong nonlinearity of Pb retention and implies that mobility of Pb tends to increase as the input Pb concentration is elevated.

Retention of Nickel in Soils: Sorption-Desorption and Extended X-ray Absorption Fine Structure Experiments

Abstract Adsorption and desorption of heavy metals in soils are primary factors that influence their bioavailability and mobility in the soil profile. To examine the characteristics of nickel (Ni) adsorption-desorption in soils, kinetic batch experiments were carried out followed by Ni release using successive dilutions. Two soils of distinctly different properties were used: one acidic (Olivier) and one neutral (Webster) soil, where a wide-range Ni concentration was implemented. Adsorption of Ni by both soils was kinetic and increased with increasing initial (input) Ni concentration. The rate of sorption was initially rapid and was followed by gradual retention over time. A sequential extraction procedure and extended X-ray absorption fine structure (EXAFS) spectroscopy were implemented to characterize Ni kinetic sorption behavior. Five sequential extractions were quantified: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter, and residual fraction. The exchangeable fraction showed a slight increase as initial Ni concentration increased, indicating weakly bound Ni adsorption complexes. Extended X-ray absorption fine structure analyses indicated that Ni hydroxide precipitate formed over time for the neutral Webster soil. This precipitate was likely bound to the Fe/Al oxide fraction. We conclude, based on EXAFS analyses and sequential extractions, formation of Ni hydroxide precipitate depends on soil pH and the amount of Ni sorbed. No Ni hydroxide precipitate was formed on the acidic Olivier soil.

Bioturbation-Driven Particle Transport in Surface Soil: The Biodiffusion Coefficient Mobility Parameter

Abstract Macrofauna-induced bioturbation of soil is dominated by earthworms, among other invertebrates, in most grassland and forest soil. First observed by Darwin, bioturbation drives particle mixing in the upper surface layers, leading to beneficial results to agricultural soils, including enhanced porosity, water permeability, and aeration and improved organic matter and nutrient distributions. Applied pesticides and other chemicals residing on surface soils are transported downward into the soil column by a random mixing of particles. This physical particle diffusion, which conceptually mimics the random mixing of molecular species in fluids, is treated as a Fickian chemical flux mechanism. Using this mechanism, for the mobility rate, while extending the soil-water advection-dispersion model to particles, yields a theoretical approach for obtaining the biodiffusion coefficient (Db). The Db is a numerical soil parameter reflecting biology-induced particle movement and differs significantly from the conventional physical and chemical diffusion coefficients. It is a kinetic parameter with units of square centimeters per year and when used with the bulk density gradient quantifies the soil particle mobility rate within the bioturbated surface layer. Field measurements on soil turnover rates and mixing depth from the literature, including Darwin’s work, were used to produce Db data sets for earthworms, ants, termites, and so on. The highly variable coefficients necessitate lognormal statistics to summarize the findings. However, the average Db values for the three invertebrates were 2.12, 0.39, and 0.75 cm2 year−1, respectively, and surprisingly similar. The need for more field and laboratory data, process-based and species-specific theoretical models, and chemical-based soil Db are discussed.