H. M. Selim

CORRELATION OF FREUNDLICH Kd AND n RETENTION PARAMETERS WITH SOILS AND ELEMENTS

We studied the retention of 15 elements by 11 soils from 10 soil orders to determine the effects of element and soil properties on the magnitude of the Freundlich parameters Kd and n. The magnitude of Kd and n was related to both soil and element properties. Strongly retained elements, such as Cu, Hg, Pb, V, and P had the highest Kd values. The transition metal cations Co and Ni had similar Kd and n values, as did the group IIB elements Zn and Cd. Oxyanion species tended to have lower n values than did cation species. Soil pH and CEC were significantly correlated with log Kd values for cation species. High pH and high CEC soils retained greater quantities of the cation species than did low pH and low CEC soils. A significant negative correlation between soil pH and the Freundlich parameter n was observed for cation species, whereas a significant positive correlation between soil pH and n for Cr(VI) was found. Greater quantities of anion species were retained by soils with high amounts of amorphous iron oxides, aluminum oxides, and amorphous material than were retained by soils with low amounts of these minerals. Several anion species were not retained by high pH soils. Despite the facts that element retention by soils is the result of many interacting processes and that many factors influence retention, significant relationships among retention parameters and soil and element properties exist even among soils with greatly different characteristics.

Retention and release of metals by soils — Evaluation of several models

Several kinetic models, including irreversible and reversible 1st, 2nd, and nth order models, and several equilibrium models, including the linear, Langmuir, two-surface Langmuir, and Freundlich models, were evaluated for their ability to describe the retention/release of Cr, Cd, and Hg by various soils. The retention/release data were obtained using a batch reaction method. In general, no single-reaction kinetic model fit the data over the entire time and concentration ranges studied for any of the metals or soils. The relationship between the amount of metal retained by the soil and the concentration of metal in solution was described by either the two-surface Langmuir or Freundlich models. A significiant fraction of the metals retained by the soil was not released to solution and was not exchangeable, indicating that some irreversible retention of the metals occurred. The results suggest that a multi-reaction model consisting of irreversible and reversible kinetic models is needed to fit all the data.

Modeling the Transport and Retention of Inorganics in Soils

Publisher Summary The interactions of dissolved chemicals and their transport in the soil profile play a significant role in their leaching losses beyond the root zone, availability for uptake by plants, and the potential contamination of groundwater supplies. To predict the transport of reactive solutes in the soil, models that include retention and release reactions of solutes with the soil matrix are needed. This chapter discusses the major features of retention models that govern retention reactions of solutes in the soil. Single-reaction models of the equilibrium type and kinetic type are discussed. Retention models of the multiple-reaction type, including the two-site equilibrium-kinetic models, the concurrent- and consecutive-multireaction models, and the second-order approach are presented. This is followed by multicomponent or competitive-type models wherein ion exchange is considered the dominant retention mechanism. Selected experimental data sets are described for the purpose of model evaluation and validation and necessary (input) parameters are also discussed. The chapter presents the formulations of the transport equations that govern the transport of solutes in a water-saturated and water-unsaturated porous medium. Retention reactions of the reversible and irreversible types are incorporated into the transport formulation. Boundary and initial conditions commonly encountered under field conditions are also presented.

Reaction and Transport of Arsenic in Soils: Equilibrium and Kinetic Modeling

Arsenic contamination of the soil and groundwater poses great risk to human and animal health. There is a growing public interest in developing risk assessment framework, environment regulations, and remedial strategies for protecting ecosystems and human from arsenic poisoning. Although extensive research efforts have been made over the past four decades, the prediction of the fate and transport of arsenic in soils are often inaccurate due to the complex biogeochemical reactions of various arsenic species in soil and water environments. In-depth knowledge of factors that influence the behavior of arsenic in aqueous and solid phases are critical in making accurate determinations of the mobility, bioavailability, and toxicity of arsenic in the soil root zone. In this contribution, we present a review of the state of knowledge on reactions and transport of arsenic in soils with emphasis on modeling of the physical, chemical, and biological interactions of arsenic in soil environment. Specifically, we present an overview of (i) biogeochemical mechanisms of arsenic adsorption desorption, oxidation reduction, and precipitation dissolution; (ii) reactive transport mechanisms of arsenic in the natural environment as affected by factors including arsenic species, redox potential, solution chemistry, flow regime, and colloid-facilitated transport; and (iii) equilibrium and kinetic modeling approaches to simulating the geochemical reactions and transport mechanisms of arsenic in porous media. A range of remedial technologies have been reviewed and their effectiveness and feasibility in the removal or in situ stabilization of arsenic in contaminated soils are discussed. Future research needs are also outlined.

Adsorption-desorption of 2,4,6-trinitrotoluene and hexahydro-1,3,5-trinitro-1,3,5-triazine in soils

We studied the adsorption-desorption behavior of TNT (2, 4, 6-trinitrotoluene) and RDX (hexahydro-1, 3, 5-trinitro-1, 3, 5-triazine) in a bentonite/sand reference material (Swy-1 montmorillonite clay mixed with acidwashed sand) and two selected soils (Norwood and Kolin). Release of TNT, RDX, and other compounds from a contaminated soil obtained from the Louisiana Army Ammunition Plant (AAP) site was also investigated. The kinetics of TNT and RDX retention were measured using batch methods for a range of input concentrations. For RDX, the adsorption isotherms were distinctly linear. The TNT adsorption isotherm for bentonite/sand mixture appeared linear and was described equally well using linear, Freundlich, Langmuir, and a modified Langmuir model. For the Norwood and Kolin soils, TNT adsorption isotherms exhibited distinct nonlinearity and the Freundlich model provided the best fit. As indicated by the Kd values, TNT exhibited stronger retention or affinity to all soils and the bentonite/sand mixture than for RDX. The RDX retention data indicated little time-dependent behavior. The TNT retention data indicated a continued decrease in TNT concentration with time in the Norwood and Kolin soils. This was possibly caused by the formation and subsequent adsorption of transformation products because transformation products, such as amino nitro toluene compounds, were identified during batch experiments. For the bentonite/sand mixture, TNT retention was rapid initially and reached apparent equilibrium within 1 day. Unlike Kolin and Norwood soils, there was no hysteretic behavior of TNT adsorption-desorption by the bentonite/sand mixture and a mass balance suggested fully reversible retention mechanisms.

Transport of 2,4,6-trinitrotoluene and hexahydro-1,3,5-trinitro-1,3,5-triazine in soils

We investigated the fate and transport of explosives in soils. Transport experiments were conducted to describe the mobility of 2, 4, 6-trinitrotoluene (TNT) and hexahydro-1, 3, 5-trinitro-1, 3, 5-triazine (RDX) in a SWy-1 reference clay (bentonite mixed with sand) and two selected soils (Norwood and Kolin). Miscible displacement experiments in packed soil columns under steady flow were used. For the bentonite/sand column, TNT was highly mobile and fully reversible when methanol was used as the background solution. In contrast, with 0.005 M Ca (NO3)2 as the background solution, the TNT pulse was strongly retarded with as much as 50% of that applied remaining within the bentonite/sand, Norwood, or Kolin columsn. Products of the transformation of TNT to 4-Am-DNT and other compound were identified in the effluent solution. A 7-day flow interruption during the TNT pulse application resulted in decreased TNT levels in the effluent solution. This decrease corresponded to a sudden increase in the 4-Am-DNT concentration in the effluent, with peak concentrations of 0.60 mg ml-1. For RDX, only limited retention as observed in all columns. These findings are consistent with results from adsorption-desorption batch experiments. The TNT and RDX transport results were successfully described by a nonlinear multireaction and transport model (MRTM), which accounted for equilibrium and kinetic (reversible and irreversible) retention mechanisms. However, efforts to describe RDX transport were more successful than efforts to describe TNT when idependently determined (batch) parameters were used. The mobility of TNT, RDX, and other compounds from a contaminated soil obtained from a Louisiana Army Ammunition Plant (AAP) site was also investigated. A gradual release and subsequent movement of various contaminants, including HMX, TNT, RDX, TNB, 2-Am-DNT, and 4-Am-DNT, was observed. The leaching patterns were consistent with results from uncontaminated Kolin soil columns and reflected the affinity of contaminants during leaching in the AAP soil.

Modeling the impact of ferrihydrite on adsorption-desorption of soil phosphorus

Ferrihydrite is an Fe-oxide mineral with a high phosphorus (P) sorption capacity. Modeling the P adsorption and desorption mechanisms of soil amended with ferrihydrite is necessary to predict the movement of dissolved and sediment-bound P. The objective of this study was to model the multi-reaction P sorption properties of soil amended with ferrihydrite. Soil samples were treated with 0, 6.72, and 11.20 Mg ha−1 of ferrihydrite. The <2-mm-size fraction of each treatment was tested for P sorption properties by the batch isotherm method. The Langmuir and Freundlich equations were applied as a single-site, instantaneous equilibrium approach for describing adsorption, and the multi-reaction (MRTM) model was applied with various combinations of equilibrium, reversible-kinetic, and irreversible sorption sites to describe the adsorption-desorption. Application of ferrihydrite increased the rapid P adsorption, but equilibrium was not reached after 1440 h because of the highly kinetic nature of P sorption. The one-site, instantaneous equilibrium approach was deemed inappropriate because of strong time-dependence in the Langmuir and Freundlich parameters. The reversible-kinetic sorption approach was superior to the instantaneous-equilibrium sorption approach for describing the rapid reactivity. The two-site model involving a reversible-kinetic site with either a concurrent irreversible or independently irreversible site was superior to the one-site and multi-site approaches. Application of ferrihydrite increased the rapid adsorption rate coefficients and the irreversible rate coefficients. These results suggest that ferrihydrite can be an effective soil amendment for enhancing P sorption and reducing P release.

Modeling Approaches of Competitive Sorption and Transport of Trace Metals and Metalloids in Soils: A Review

Competition among various heavy metal species for available adsorption sites on soil matrix surfaces can enhance the mobility of contaminants in the soil environment. Accurate predictions of the fate and behavior of heavy metals in soils and geologic media requires the understanding of the underlying competitive-sorption and transport processes. In this review, we present equilibrium and kinetic models for competitive heavy metal sorption and transport in soils. Several examples are summarized to illustrate the impact of competing ions on the reactivities and mobility of heavy metals in the soil-water environment. We demonstrate that equilibrium Freundlich approaches can be extended to account for competitive sorption of cations and anions with the incorporation of competition coefficients associated with each reaction. Furthermore, retention models of the multiple-reaction type including the two-site nonlinear equilibrium-kinetic models and the concurrent- and consecutive-multireaction models were modified to describe commonly observed time-dependent behaviors of heavy metals in soils. We also show that equilibrium Langmuir and kinetic second-order models can be extended to simulate the competitive sorption and transport in soils, although the use of such models is limited due to their simplifying assumptions. A major drawback of the empirically based Freundlich and Langmuir approaches is that their associated parameters are specific for each soil. Alternatively, geochemical models that are based on ion-exchange and surface-complexation concepts are capable of quantifying the competitive behavior of several chemical species under a wide range of environmental conditions. Such geochemical models, however, are incapable of describing the time-dependent sorption behavior of heavy metal ions in competitive systems. Further research is needed to develop a general-purpose model based on physical and chemical mechanisms governing competitive sorption in soils.

Adsorption/Desorption Kinetics of Zn in Soils: Influence of Phosphate

The reactivities of heavy metals such as zinc in the soil environment are affected by several rate-limiting processes, including kinetic sorption and release reactions. In this study, kinetic batch experiments were carried out to evaluate sorption and desorption of Zn for soils having different properties. Sorption isotherms exhibited strong nonlinearity as well as kinetic behavior. Distinct differences in the amounts of Zn sorbed among the different soils were observed where lowest sorption was associated with acidic soils (Windsor and Olivier) soil. In contrast, Webster (neutral) soil exhibited highest sorption capacity. Discrepancies between adsorption and successive desorption isotherms indicate considerable hysteresis for Zn release, the extent of which varied among the three soils. For the two acidic soils, Zn release as a percentage of that total sorbed was 47% to 51% and 42% to 49% for the Windsor and Olivier soils, respectively; whereas for the neutral soil, only 9% to 11% of sorbed Zn was released over time. A second objective was to test the hypothesis that phosphate addition to soils increases Zn adsorption. The influence of the presence of P on increased Zn sorption was clearly manifested by the isotherms where similar trends were observed for all three soils. Moreover, P additions reduced Zn desorption, which indicates that the presence of P increased the affinity or specific sorption sites for Zn. A multireaction model that accounts for nonlinear equilibrium and kinetic retention reactions was successfully used to simulate the time-dependent behavior of Zn sorption-desorption for all three soils and at different P levels. Moreover, the addition of different levels of P did not affect the kinetic sorption-desorption behavior of Zn in all soils.

Competitive Sorption of Nickel and Cadmium in Different Soils

Competing ions strongly affect heavy-metal retention and release in soils. In this study, we evaluated the sorption of Ni and Cd in single and binary Ni-Cd systems for one neutral and 2 acidic soils. Ni and Cd sorption isotherms were obtained using batch techniques over a wide range of concentrations. Both Ni and Cd adsorption isotherms exhibited strong nonlinear behavior with similar overall patterns for all soils. Sorption of Ni and Cd correlated well the cation exchange capacity for all soils. Moreover, Cd sorption by the two acidic soils was greater than Ni, whereas for the neutral soil, Ni sorption was greater than Cd. The Freundlich model was utilized to describe the family of Ni and Cd isotherms in the presence of competing ions. For both Ni and Cd isotherms, the Freundlich distribution coefficient K decreased with increasing concentration of the competing ion. The dimensionless parameter n was within a narrow range of 0.50 to 0.64 for both Ni and Cd and was not affected by the competing ion. Competitive sorption was described using the Sheindorf-Rebhun-Sheintuch (SRS) equation, which is based on multicomponent Freundlich-type models. Estimates of SRS competitive sorption coefficients indicated that for the neutral soil, Ni sorption was least affected by Cd. The SRS equation was also capable of describing the overall competitive Ni and Cd sorption patterns for all soils. However, SRS model modifications are needed to improve sorption prediction for the highest competing concentrations.