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DIVISION S-2-SOIL CHEMISTRY

Modeling Arsenic(III) Adsorption and Heterogeneous Oxidation Kinetics in Soils

^a Dep. of Chem. and Biochem., San Francisco State Univ., 1600 Holloway Ave., San Francisco, CA 94132 USA
^b USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507-4617 USA

bmanning{at}sfsu.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Theory Materials and methods Results and discussion Conclusions REFERENCES

 
Arsenite [As(III)] is a soluble and toxic species of arsenic that can be introduced into soil by geothermal waters, mining activities, irrigation practices, and disposal of industrial wastes. We determined the rates of As(III) adsorption, and subsequent oxidation to arsenate [As(V)] in aerobic soil–water suspensions using four California soils. The rate of As(III) adsorption on the soils was closely dependent on soil properties that reflect the reactivity of mineral surfaces including citrate–dithionite (CD) extractable metals, soil texture, specific surface area, and pH. Heterogeneous oxidation of As(III) to As(V) was observed in all soils studied. The recovery of As(V) from As(III)-treated soils was dependent on levels of oxalate-extractable Mn and soil texture. After derivation of rate equations to describe the changes in soluble and recoverable As(III) and As(V) in soil suspensions, soil property measurements were used to normalize the empirically derived rate constants for three soils. The fourth soil, which had substantially different soil properties from the other three soils, was used to independently test the derived soil property–normalized model. The soil property–normalized consecutive reaction model gave a satisfactory description of the trends seen in the experimental data for all four soils. Understanding the effects of soil properties on the kinetics of chemical reactions of As(III) and As(V) in soils will be essential to development of quantitative models for predicting the mobility of As in the field.

Abbreviations: CD, citrate–dithionite • DI, deionized • HPLC–HGAAS, high performance liquid chromatography–hydride generation atomic absorption spectrometry • Me_T, total citrate–dithionite extractable metals (Al + Fe + Mn) • Mn_OX, oxalate-extractable manganese • SA, specific surface area • XAS, x-ray absorption spectroscopy • XRD, x-ray diffraction

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Theory Materials and methods Results and discussion Conclusions REFERENCES

 
THE OXIDATION of trace metals and metalloid oxyanions by soils has an important role in determining their mobility and toxicity. Reduced species such as As(III), Se(IV), and Cr(III) can be oxidized in soils to produce As(V), Se(VI), and Cr(VI), respectively. In the case of Se and Cr, the oxidized Se(VI) and Cr(VI) species are less strongly adsorbed to soils than Se(IV) and Cr(III) and thus are more mobile and bioavailable. The oxidation of As(III) produces As(V), which is strongly adsorbed in soils and is less toxic than As(III) (Knowles and Benson, 1983). Despite the fact that the reactions of As(III) with soil have been studied, the rates and mechanisms of As(III) adsorption, as well as oxidation to As(V), are still not well understood. The As(III) species can also be present in groundwater (Korte and Fernando, 1991), and therefore the stability of As(III) after coming in contact with aerobic soils is of interest in environmental management.

Previous work on the reactions of As(III) with soil has focused primarily on As(III) adsorption rather than As(III) oxidation (Manning and Goldberg, 1997a; McGeehan and Naylor, 1994; Elkhatib et al., 1984). Iron oxides have been shown to be the most important mineral component in determining a soil's overall capacity to adsorb As(III) and As(V) (Jacobs et al., 1970; Fordham and Norrish, 1974, 1979; Elkhatib et al., 1984; Livesey and Huang, 1981; Manning and Goldberg, 1997a). It has also been concluded, using x-ray absorption spectroscopy (XAS), that As(V) is specifically adsorbed on synthetic Fe(III) oxide mineral surfaces (Waychunas et al., 1993; Fendorf et al., 1997). Electrophoretic mobility work (Pierce and Moore, 1982) has shown that As(III) is also specifically adsorbed on the Fe(III) oxide surface, and this has now been confirmed using Fourier transform infrared spectroscopy (Sun and Doner, 1996) and XAS (Manning et al., 1998).

The oxidation of As(III) to As(V) in soils has not been extensively studied despite the fact that this is an important reaction in the cycling of As in the environment. The oxidation of As(III) by lake sediments has been investigated (Oscarson et al., 1980, 1981) and it was concluded that an abiotic process involving Mn oxide minerals was directly responsible for As(III) oxidation. Heterogeneous oxidation of As(III) by synthetic Mn oxides (Oscarson et al., 1983; Scott and Morgan, 1995; Sun and Doner, 1998), and clay minerals (Manning and Goldberg, 1997b) has been shown, though very little is known about the rates or mechanisms of As(III) oxidation in whole soils. An improved understanding of the rate of As(III) oxidation in whole soil is necessary for the application of predictive models to describe As transport in the field. In addition, linking measurements of important soil properties with a quantitative description of As(III) adsorption and oxidation would improve the predictive capability of reactive transport models.

Given the need for a better understanding of the oxidation and adsorption rates of As(III) in soils, the objectives of this study were: (i) to determine the As(III) adsorption and oxidation rates in soil by measuring the As(III)–As(V) speciation and fractionation in As(III)-treated soil as a function of time; (ii) to develop a simple, verifiable kinetic rate law for describing the time-dependent reactions of As(III) in soil for eventual use in solute transport models; and (iii) to incorporate soil property measurements into kinetic expressions to improve their general applicability to different soils.

	Theory

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Two techniques can be used to describe time-dependent reactions in well-mixed soil suspensions: (i) mechanistic rate law determination and (ii) apparent rate law determination (Sparks, 1989, p. 5–11; Skopp, 1986). Mechanistic rate law determination describes elementary chemical reactions at the molecular level, whereas an apparent rate law recognizes that unknown elementary reactions may be contributing to an overall, apparent rate. Even in well-mixed, dilute soil suspensions, modeling the rate of As(III) adsorption and oxidation is complicated by intraparticle diffusion, unknown reaction intermediates, and irreversible adsorption of both reactants and products. Therefore, modeling As(III) adsorption and oxidation in soil suspensions is best treated by defining apparent rate laws for the measurable processes after treatment of the soil with As(III).

We used the following mass balance expression for the measurable fractionation of As(III) in soil suspensions:

(1)

where [As(III)]₀ is initial As(III) added to the suspension; [As(III)]_sol and [As(V)]_sol are soluble As(III) and As(V); [As(III)]_exch and [As(V)]_exch are exchangeable As(III) and As(V); and [As]_immobile is immobile, or unrecoverable, As. The [As(III)]_exch and [As(V)]_exch fractions were operationally defined by recovery using a phosphate extraction procedure (see below). The [As]_immobile fraction was operationally defined as all "unrecoverable" As, or the difference between [As(III)]₀ and total recoverable As, which can be expressed by rearrangement of Eq. [1]:

(2)

Using the mass balance expressions in Eq. [1] and [2], two apparent rate laws were developed to describe the fractionation of As(III) after addition to the soil: Case 1, rates of change in soluble As(III) and As(V), and Case 2, rates of change in recoverable As(III) and As(V).

Case 1: Soluble As(III) and As(V)
The rates of change in soluble As(III) and As(V) were described with the following expression:

(3)

where k₁ and k₂ are the forward rate constants (h^-1) for the reactions. Equation [3] is an apparent rate expression because it does not describe elementary chemical reactions and only accounts for the measurable changes in the soluble As(III) and As(V) species. Modeling the transport of As(III) and As(V) in the field will require a kinetic expression to describe the time-dependent changes in [As(III)]_sol and [As(V)]_sol. In addition, the inclusion of [As]_immobile in Eq. [3] is important because this is a long-term sink for As in soil.

Case 2: Total Recoverable As(III) and As(V)
A second rate expression was used to describe the rate of change in the amounts of total recoverable As(III) and As(V) in soil suspensions treated with As(III):

(4)

where [As(III)]_Recov and [As(V)]_Recov are total recoverable (soluble + exchangeable) As(III) and As(V), respectively. Case 2 is an attempt to describe the total As(III) and As(V) in the suspension (soil + solution) and conforms to the mass balance of the system in Eq. [1] where:

(5a)

where:

(5b)

and

(5c)

Equation [4] closely describes the rate of change in total As(III) in the soil suspension system ([As(III)]_sol + [As(III)]_exch).

Consecutive Reaction Mechanisms
The mathematical treatment of Eq. [3] and [4] uses the theory of consecutive reaction mechanisms (Laidler and Meiser, 1982, p. 406–409). Using Eq. [4] as an illustration, the first-order kinetic expression for the rate of decrease in [As(III)]_Recov is:

(6)

Equation [6] was then integrated using the boundary condition when , which gives:

(7)

A natural log transformation of experimental data (ln[As(III)/[As(III)₀]) was made and plotted vs. t (h) and the following linearized version of Eq. [7] was fit to the [As(III)]_Recov data:

(8)

The value of k₁ was derived from linear regression analysis. The rate of change in [As(V)]_Recov is described by:

(9)

Substituting Eq. [7] into [9] gives:

(10)

Integrating and rearranging Eq. [10] gives the following expression:

(11)

Equation [11] was fitted to experimental [As(V)]_Recov data using k₁ from Eq. [8] and optimizing k₂ through minimization of the sums of squares of differences between the experimental and model values ({[As(V)]_Recov - [As(V)]_Model}²) (Steel and Torrie, 1980). To predict the formation of [As]_immobile in soil suspensions as the final reaction product of As(III) with soil, Eq. [5a] was rearranged as:

(12)

Equations [7] and [11] were then combined with Eq. [12] to give:

(13)

This expression was used to predict [As]_immobile using [As(III)]₀, k₁, and k₂.

Incorporating Soil Property Data into Rate Equations
When applied to different soil types, the predictive capability of rate models will improve if important soil properties are used in the development of model equations. In our study, soil properties that influence the reactions of As(III) with soil such as CD-extractable Al + Fe + Mn (Me_T), oxalate-extractable Mn (Mn_OX), pH, and soil texture were used to normalize soil-specific rate constants. This allowed the derivation of normalized rate constants to be used in combination with measured soil-specific properties in rate expressions. Rate constants (k_1i and k_2i) for three soils (Fallbrook, Panoche, and Indio) were first fit directly to experimental data in the consecutive reaction model as described above. This was followed by correcting the fitted, empirical k_1i and k_2i values for three soils ( ) with soil property values:

(14)

where k^N₁ is the soil property–normalized rate constant for the first step of the consecutive reaction model, k_1i is the empirically fitted k₁ for soil i, and X_i and Y_i are soil properties values for soil i. For the final modeling step, a new rate constant was recalculated for each soil:

(15)

where k^soili₁ is the final calculated k₁ for soil i. Incorporating Eq. [15] into Eq. [7], for example, gives:

(16)

At most, two soil properties were used for rate constant normalization and incorporation into the consecutive reaction rate model. The influence of soil properties on the adsorption and oxidation of As(III) will be addressed in more detail below.

	Materials and methods

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Soil Characterization
Four soils from California were used in this study to represent a range of soil properties. Surface (<20 cm) samples of a Fallbrook sandy loam (fine-loamy, mixed, thermic, Typic Haploxeralf), Indio very fine sandy loam (coarse-silty, mixed, calcareous, hyperthermic Typic Torrifluvents), Panoche clay loam (fine-loamy, mixed, superactive, thermic Typic Haplocambid), and Aiken clay soil (fine, parasesquic, mesic Xeric Haplohumult) were sieved (<2 mm) and handled so as to maintain field soil moisture conditions by refrigeration at 4°C in sealed high-density polyethylene tubs. Though natural As(III)–As(V) speciation may change during storage, the As(III) oxidizing capacity of the soils did not change when the soils were stored this way for several months. The Aiken soil was used as an independent test soil for the kinetics model after model optimization using the Fallbrook, Indio, and Panoche soils. The pH of the soil–As(III) solution suspensions was determined with a Corning semi-micro glass combination pH electrode and a Corning Ion Analyzer 150 (Corning Glass Works, Corning, NY).¹ Soil texture was determined by the hydrometer method (Gee and Bauder, 1986). Specific surface area (SA) of the <2-mm soil fraction was determined by single-point Brunauer-Emmett-Teller N₂ adsorption with a Quantisorb Jr. surface area analyzer (Quantachrome Corp., Syosset, NY). The dominant mineralogy of the soils was analyzed by separating the sand and clay fractions by sedimentation in deionized (DI) water and recovery of the suspended clay particles by centrifugation. The sand fraction was then repeatedly rinsed with DI water and both oriented slide and random powder mounts were analyzed by x-ray diffraction (XRD) for major mineral components.

Selective dissolution analyses were performed to determine the Al, Fe, and Mn content and crystallinity of the soils. A CD extraction was employed to extract total crystalline or free Fe oxides by shaking 4 g of soil in 120 mL of 0.57 M sodium citrate (Na₃C₆H₅O₇ · 5H₂O) and 0.1 M sodium dithionite (Na₂S₂O₄) for 16 h (Holmgren, 1967). Dissolution of Fe oxides by CD extraction also releases coprecipitated Al and Mn. Amorphous or poorly crystalline Al, Fe, and Mn oxides were determined by extracting 4 g of soil in 120 mL of acidified ammonium oxalate buffer [0.175 M (NH₄)₂C₂O₄ + 0.1 M H₂C₂O₄, pH 3.0] for 2 h (Loeppert and Inskeep, 1996). For both extraction procedures, the suspensions were shaken, centrifuged (4100 g, 10 min) and 15-mL aliquots were removed, filtered (0.1 µm), and acidified with high purity HNO₃. Samples were then analyzed for Al, As, Fe, and Mn by inductively coupled plasma atomic emission spectrometry. A low background level of As was detected in the CD extracts of the soils (<10 mg kg^-1 As), which was substantially below the amount of As(III) added during adsorption experiments.

As(III) Reaction Kinetics
The reaction kinetics of As(III) with the soils were investigated in batch experiments using 40-mL polycarbonate centrifuge tubes. Two grams of soil were reacted with 20 mL of 66.7 µM As(III) in 5 mM CaCl₂ on a reciprocating shaker for 0.25, 0.5, 1, 1.5, 2, 3, 4, 8, 18, 24, and 48 h. All sample treatments were processed in duplicate and the averages of duplicates are reported in all figures. The As(III) adsorption and oxidation reactions were terminated by centrifuging (14500 g, 5 min), decanting the supernatant, and filtering (0.1 µm). This process took 10 min, and it was confirmed that oxidation of soluble As(III) ceased after separation of the solution from the soil solids. The solids remaining in the reaction tubes were then resuspended and shaken for 1 h in 20 mL of 10 mM KH₂PO₄:K₂HPO₄ buffer at pH 7 (10 mM PO₄) to recover exchangeable As(III) and As(V) from the soil. This extraction procedure has been used in previous work with As(III)-treated clay minerals (Manning and Goldberg, 1997b), and the As(III)–As(V) speciation in the filtered, refrigerated extract was preserved for several days. Addition of 10 mM PO₄ to the As(III)-treated soil did not cause additional As(III) oxidation, but actually may have slowed the oxidation by adsorbing on available oxidative surfaces.

The Fallbrook soil, which gave the greatest recoverable As(V), was chosen for more detailed work to investigate the effects of pH on As(III) adsorption and oxidation. These experiments used the same reaction conditions as given above except that the soils were first equilibrated for 24 h in 5 mM CaCl₂ containing an appropriate quantity of HCl or NaOH to make the soil suspension pH of 5.0, 6.0, 7.0, and 8.0. The pH-adjusted Fallbrook suspensions were treated with As(III) by adding 0.1 mL of 13.3 mM As(III) to commence the timed experiment.

Speciation of As(III) and As(V) in all soil solutions and extracts was determined by high performance liquid chromatography (HPLC) using a Dionex AS-11 IonPac anion exchange column (Dionex Corp., Sunnyvale, CA) coupled with hydride generation atomic absorption spectrometry (HGAAS) detection (Manning and Martens, 1997). This technique allowed direct, trace-level detection of As(III) and As(V) in complex soil solution matrices. The majority of samples were analyzed by HPLC-HGAAS immediately after filtering, without sample storage.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Theory Materials and methods Results and discussion Conclusions REFERENCES

 
Soil Characteristics
Selected soil properties for the four soils are shown in Table 1 . The soil pH values (1:10 soil/DI water) show a range from 9.3 for the Indio soil to 5.9 for the Aiken soil, indicative of the wide range in properties of these soils. The Indio and Fallbrook soils had similar particle-size distribution, dominated by sand, whereas the Panoche and Aiken soils were higher in clay-size particles. Specific surface area, clay content, and amount of extractable metals were closely related soil properties in these samples. Both the Fallbrook and Indio soil clay fractions were composed of predominantly mica and illite with moderate amounts of kaolinite. However, the Indio soil, with both alkaline and sodic properties, also contained quartz and some calcite in the clay fraction. The clay fraction of Panoche clay loam was composed predominantly of illite. The XRD patterns of sand fractions were dominated by quartz and other primary minerals, including plagioclase.

View larger version (23K): [in this window] [in a new window]   Fig. 1 Fractionation of As(III) and As(V) in As(III)-treated Indio, Panoche, and Fallbrook soils as a function of time. The pH values in the As(III) reaction solutions for the three soils were 6.5, 7.9, and 9.3 for Fallbrook, Panoche, and Indio soil, respectively

View larger version (44K): [in this window] [in a new window]   Fig. 2 Hypothetical reaction pathways for As(III) and As(V) in soil. In this figure, Mn denotes a manganese oxide surface complex, whereas Me denotes all other oxide mineral surfaces that bind As in soil

View larger version (23K): [in this window] [in a new window]   Fig. 3 (a) Effect of pH and time on soluble As(III), (b) PO4-exchangeable As(III), and (c) total recoverable As(III) in As(III)-treated Fallbrook soil

View larger version (21K): [in this window] [in a new window]   Fig. 4 (a) Effect of pH and time on soluble As(V), (b) exchangeable As(V), and (c) total recoverable As(V) in As(III)-treated Fallbrook soil

Determination of the rapid adsorption reaction rate of As(III) on the Fallbrook, Panoche, and Indio soils for t < 0.5 h showed a roughly linear decrease in soluble As(III) in normalized data [ln(C/C₀)] for this short time (Fig. 5a) . Three soil-specific rate constants (k^soili_1fast) were calculated using the three slopes derived from the experimental data in Fig. 5a. The (k^soili_1fast) values were then normalized to determine a constant for the three soils:

(17)

View larger version (21K): [in this window] [in a new window]   Fig. 5 Fitting a rate expression to soluble As(III) data evaluated for : (a) normalized soluble As(III) data in As(III)-treated Indio, Panoche, and Fallbrook soils with linear regression fits and (b) comparison of the experimental data (points) with the prediction using Eq. [18] (lines)

(18)

The results of inserting the values for Me_T and pH from Table 1 into Eq. [18] are shown as lines in Fig. 5b and good agreement between experimental data and the model was found for the rapid decrease in [As(III)]_sol observed in the soils. Because SA and Me_T were correlated in the four soils (Table 1) it was not necessary to simultaneously include both of these properties in Eq. [18]. Though (k^N_1fast) may not be appropriate for all soils, it probably represents soils containing similar mineral phases as the soils from which the values were obtained.

Prediction of [As(III)]_sol across longer time periods ( ) was achieved by deriving the rate constants directly from [As(III)]_sol data (Fig. 6) followed by normalization of the rate constants with soil properties. The plots of ln(C/C₀) vs. t showed two time regions at t < 2 h and t > 2 h, which necessitated the designation of a fast rate (k_1fast, t < 2 h) and a slow rate (k_1slow, t > 2 h) for accurately describing the rate of removal of As(III) from solution. This two-rate behavior probably results from rapid initial adsorption of As(III) on high-energy surface sites, followed by slower processes that affect [As(III)]_sol, such as intraparticle diffusion into soil particles and oxidation on mineral surfaces.

View larger version (27K): [in this window] [in a new window]   Fig. 6 Normalized soluble As(III) data in As(III)-treated Indio, Panoche, and Fallbrook soils with linear regression fits to the fast(t < 2 h) and slow (t > 2 h) time ranges

View larger version (29K): [in this window] [in a new window]   Fig. 7 Fit of the consecutive reaction rate model to soluble As(III) and As(V) in As(III)-treated Indio, Panoche, and Fallbrook soils

View larger version (26K): [in this window] [in a new window]   Fig. 8 Fit of the consecutive reaction rate model to recoverable As(III) and As(V) in As(III)-treated Indio, Panoche, and Fallbrook soils

View larger version (30K): [in this window] [in a new window]   Fig. 9 Fit of the consecutive reaction rate model to (a) soluble As(III) and immobile As and (b) soluble As(V) in As(III)-treated Aiken soil

View larger version (28K): [in this window] [in a new window]   Fig. 10 Fit of the consecutive reaction rate model to recoverable As(III), recoverable As(V), immobile As in As(III)-treated Aiken soil

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Theory Materials and methods Results and discussion Conclusions REFERENCES

Heterogeneous oxidation of As(III) will increase with increasing amounts of Mn oxide surfaces in the soil. However, due to the tendency of As(V) to form strong, inner-sphere complexes on metal oxide mineral surfaces, the recovery of As(V) will be more difficult in finer-textured soils due to their greater overall adsorption capacity. Though there was measurable heterogeneous As(III) oxidation in all four soils, the Fallbrook soil sample yielded the most recoverable As(V). We suspect that this was due to a combination of Fallbrook soil properties (a critical amount of Mn oxide content and a coarse soil texture). The Aiken soil, which had elevated Mn compared with the other soils and was fine textured, had very low As(V) recovery. The long-term fate of As(III) in mineral soils under aerobic conditions involved oxidation to As(V) followed by the formation of unrecoverable or strongly adsorbed As(V). It is also possible that some unrecoverable (immobile) As(III) remained in the soil.

Developing a model to describe the kinetics of As(III) reactions in soils will be useful for predicting the fate and transport of As(III) and As(V) in column studies as well as the long-term disposition of As in the field. In addition, other elements such as Cr and Se, whose solubility and toxicity depend on redox state, will require a similar type of model development. The kinetics model used in this study (the consecutive reaction rate model) was developed for mineral soils and was used in combination with soil property measurements to describe both the adsorption and oxidation of As(III). Though model development will clearly be improved with the incorporation of more soil measurements, the model gave satisfactory descriptions of the behavior of both soluble and recoverable As(III) and As(V) in soil suspensions. Application of kinetic expressions to the reactions of As(III) with soil underscores the need for understanding the effects of soil physical and chemical properties on the adsorption and oxidation capacities of soils.

	ACKNOWLEDGMENTS

 
This research was supported by the USDA National Research Initiative Competitive Grants Program (Project 9604171).

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Theory Materials and methods Results and discussion Conclusions REFERENCES

 
Contribution from the USDA-ARS U.S. Salinity Lab.

¹ Trade names are used only for the benefit of the reader and do not imply endorsement by the USDA.

Received for publication August 19, 1998.

	REFERENCES

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