HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (11) Citing Articles via Google Scholar Google Scholar Articles by Almås, A.R. Articles by Singh, B.R. Search for Related Content PubMed Articles by Almås, A.R. Articles by Singh, B.R. GeoRef GeoRef Citation Agricola Articles by Almås, A.R. Articles by Singh, B.R.

DIVISION S-2-SOIL CHEMISTRY

Changes in Partitioning of Cadmium-109 and Zinc-65 in Soil as Affected by Organic Matter Addition and Temperature

^a Agricultural University of Norway, Department of Soil and Water Sciences, Post Box 5028, 1432 Aas, Norway
^b Agricultural University of Norway, Department of Chemistry and Biotechnology, Post Box 5026, 1432 Aas, Norway

asgeir.almas{at}ijvf.nlh.no

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soil collected from the A horizon of a soil developed from alum shale was added organic matter at the rate of 0 and 4% (w/w) and placed in temperature-controlled climate chambers at 9 and 21°C. After spiking the soil with ¹⁰⁹Cd and ⁶⁵Zn, sequential extraction of the tracers was performed on soil subsamples collected at time intervals ranging from 0.5 to 8760 h (1 yr). The activity concentrations of ¹⁰⁹Cd and ⁶⁵Zn determined in the seven extracts were used in a three-component model to calculate the kinetics of ¹⁰⁹Cd and ⁶⁵Zn transfer between the water-soluble, reversibly sorbed and irreversibly sorbed model components. The rates and the time-dependent distribution coefficient constants for ¹⁰⁹Cd and ⁶⁵Zn distribution among the three components were determined, and the impacts of organic matter addition and increasing temperature on these constants were assessed. The reversible sorption of the tracers occurred rapidly, within 0.5 h, while the transfer of reversibly sorbed ¹⁰⁹Cd and ⁶⁵Zn into the irreversibly sorbed fractions was a significantly slower process. The addition of organic matter reduced the rate of ¹⁰⁹Cd and ⁶⁵Zn transfer into the irreversibly sorbed fractions, whereas the transfer rate increased with increasing temperature. The interaction between organic matter and temperature affected both the rates and the pseudoequilibrium constants, and the temperature regimes and rate of organic matter addition may thus influence the potential mobility of the investigated metals.

Abbreviations: TOC, total organic C

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
WHEN ANTHROPOGENIC trace metals enter the soil, information on their relocation and transformation within the affected soil system is required for assessing the short- and long-term environmental consequences. Among the soil environmental factors, soil temperature and organic matter are known to affect the trace metal transfer between soil phases (Almås et al., 1999; Barrow, 1992; Harter and Naidu, 1995). Organic substances are essentially a mixture of compounds with different molecular weights, and the total number of base-titratable functional groups is in the range of 1000 to 2000 cmol kg^-1 C (Stumm and Morgan, 1996). Complexes with monodentate ligands are usually less stable than those with multidentate ligands; therefore, large soil organic molecules are more important for metal retention than low molecular weight organics. However, at higher soil pH levels, dissolved organics can increase the solubility of metal ions by formation of soluble organometallic complexes, which compete with the solid phases for metal ions (Almås et al., 1999; Harter and Naidu, 1995; Sauve et al., 1998). Since the metal reaction with a ligand is endothermic (Hodgson et al., 1964; Taylor et al., 1995), formation of metal complexes with organic and inorganic ligands are generally favored at higher temperatures, and the relationship between temperature and reaction rate, described by Arrhenius, indicates that the reaction rate increases with temperature.

Sorption of trace metals may or may not require a significant activation energy (McBride, 1989), and the activation energy (E_a) calculated from the Arrhenius equation is a measure of the magnitude of forces to be overcome during the reversible ion exchange process. It has also been suggested that low values of E_a indicate that the reacting material consists of major defects, which are facilitating metal diffusion (Barrow et al., 1989). This is in agreement with activation energies reported to be substantially lower for the trace metal transfer in soil (Barrow, 1986) than in goethite (Barrow et al., 1989) and magnesium oxide (Barr and Lidiard, 1970). On oxides like goethite, the potential for a strong redox reagent to reverse sorption of Ni²⁺, Zn²⁺, and Cd²⁺ diminishes with time, a fact that also has been attributed to slow diffusion into the solid phases (Gerth et al., 1993). The hypothesis is that the metals slowly may diffuse into extremely small pores of particle aggregates, a process that probably displays a high degree of apparent nonreversibility (Gerth et al., 1993; McBride, 1989). The rate and extent of metal sorption in soil increases with increasing temperature; however, the positive sorption effect of temperature is most likely to occur when soil pH is low (Barrow, 1992). Low values for activation energy, observed in many cases, indicate that diffusion-controlled exchange may contribute greatly to trace metal penetration in soil (Gerth et al., 1993; Sparks, 1986).

Although the composition of the reacting material may greatly influence the activation energy, chemisorption processes, such as polymerization, nucleation, precipitation, and crystallization of the metals applied, also may significantly contribute to the overall fixation, and the participating mechanisms may be difficult to describe individually. Trace metal interaction with soil particles has previously been reported to increase with temperature (Barrow, 1986; Barrow, 1992; Johnson, 1990; Pehlivan et al., 1995), and according to Barrow (1992), temperature also influences the metal affinity for surfaces. In support of this, Bergseth (1982) reported that increasing temperature not only increased the sorption of Cd²⁺ and Zn²⁺ by vermiculite, but it also increased the selectivity for Zn²⁺ over Cd²⁺.

It has previously been reported that diffusion is involved in the reaction of metals with soil components (Barrow et al., 1989; Barrow, 1992; Gerth and Tiller, 1988). It is suggested that the rapid surface sorption reactions are followed by a subsequent slow transfer of the metals into soil fractions with tendency towards irreversibility, or at least a desorption rate that is significant slower than the adsorption rate. The rate of metal reversible sorption and further transfer into the slowly reversibly sorbed, or irreversibly sorbed, soil fraction can be separated and estimated by an adequate model. Such a model provides valuable estimations on the mobility and retention of trace metals in the soil system. However, these calculations do not necessarily provide a unique identification of reaction mechanism.

The purpose of this study was to determine the kinetics of reactions related to the geochemical partitioning of Cd and Zn in soil. Tracers of Cd and Zn were used to identify the metals added from those naturally existing (¹⁰⁹Cd and ⁶⁵Zn do not exist in natural soils). The method is very precise and the gamma emission from the radioactive metals enables detection of extremely low metal concentrations (Salbu, 1987). In previous studies, subsamples were withdrawn from the alum shale soil at increasing contact time between ¹⁰⁹Cd and ⁶⁵Zn and the soil (Alms et al., 1999). These results showed that increasing contact time lead to an accumulation of ¹⁰⁹Cd and ⁶⁵Zn in the immobile soil fractions. Our study was therefore aimed at (i) using results from sequential extraction analysis to study the exchange kinetic of ¹⁰⁹Cd and ⁶⁵Zn partitioning in the soil, by using a simple three-component kinetic model, and (ii) investigating the effect of temperature and the addition of organic matter on the rate of ¹⁰⁹Cd and ⁶⁵Zn sorption and partitioning into irreversibly sorbed soil fractions.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soil and Soil Treatment
The A horizon of an alum shale soil was used for these investigations. This soil is classified as a Typic Cryoboroll (Soil Survey Staff, 1992), and was collected from an agricultural school farm at Jønsberg in southeastern Norway. The most important properties of the soil are presented in Table 1 , and the detailed description of this soil is reported elsewhere (Jeng and Bergseth, 1992; Singh et al., 1995). The collected alum shale soil was mixed with 0 and 4% of organic material (w/w), and the source of organic material was manure from pigs (Sus scrofa, Norwegian Landrace), mixed with wood shavings. The chemical composition of the organic matter source is also shown in Table 1. A tracer solution of ¹⁰⁹Cd²⁺ (specific activity: 1.24 x 10⁸ Bq mg^-1) and ⁶⁵Zn²⁺ (specific activity: 1.45 x 10⁸ Bq mg^-1) (DuPont, Mechelen, Belgium) was diluted with MilliQ-water (Millipore, Bedford, MA) to 25 mL, acidified to pH 2 to prevent adsorption of the tracers to the equipment surfaces, and mixed with the total soil volume of about 2.2 L (2500–2800 g, air-dry weight). This did not change the soil pH noticeably. The small spike solution volume was completely adsorbed by the dry soil, and immediately after spiking the soil was moistened to 60% of field capacity. The specific activities of ¹⁰⁹Cd and ⁶⁵Zn in soil were estimated to be 500 and 600 Bq g^-1 soil, respectively. This soil was utilized for a pot experiment conducted in temperature-controlled climate chambers (phytotron) at 9 and 21°C to simulate the soil temperature range of the growth season in southern Norway. The moisture content of the soil in pots was maintained at 60% of field capacity throughout the experimental period.

• F1. H₂O, pH 5.5, for 1 h at room temperature;
  
• F2. 1 M NH₄OAc, soil pH, for 2 h at room temperature;
  
• F3. 1 M NH₄OAc, pH 5, for 2 h at room temperature;
  
• F4. 0.04 M NH₂OH·HCl in 25% HAc, for 6 h at 80°C;
  
• F5. 30% H₂O₂ at pH 2 (HNO₃), for 5.5 h at 80°C, then added 3.2 M NH₄OAc in 20% HNO₃ for 0.5 h at room temperature;
  
• F6. 7 M HNO₃ for 6 h at 80°C;
  
• F7. Residue.
  

After each extraction step, all the solid phases were shaken and washed with 10 mL of MilliQ-water. The supernatant was separated from the solid by high-speed centrifugation for 30 min at 11000 g, and evacuated for analysis. The concentrations of ¹⁰⁹Cd and ⁶⁵Zn recovered in the washing step were added to the extracted fraction prior to washing. A Packard Minaxi 3'' through hole NaI Gamma Counter, 5000 Series, (Packard Instrument Co., Downers Grove, IL) was used to determine the activity of ¹⁰⁹Cd and ⁶⁵Zn in the extracts and in the residue left after the 7 M HNO₃ extraction step. The stabile Cd and Zn concentrations were also determined in the extracts by using a Perkin-Elmer graphite furnace AAS (Perkin-Elmer, Norwalk, CT) and a Thermo Jarrell Ash ICP (Thermo Jarrell Ash, Franklin, MA), respectively. All funnels and containers used were of polycarbonate and all the equipment was soaked in 10% HCl and thoroughly rinsed in MilliQ-water prior to use. A Shimadzu TOC-5000 analyzer (Shimadzu Scientific, Columbia, MD) provided the analysis of total organic C (TOC) in the water extracts.

The amount of acid-oxalate (Saunders, 1965) and dithionate-citrate-extractable Fe (Holmgen, 1967) in the soil was also determined, and the soil pH was measured in deionized water with solid/solution ratio at 1:2.5 (Table 1).

Other Soil Characterization Procedures
The particle-size distribution was determined by the method of Elonen (1971), whereas C (total C and TOC) and total N were determined by the use of a LECO CHN 1000 analyzer (Leco, St. Joseph, MI) as described by Nelson and Sommer (1996), and Bremner (1996), respectively.

Modeling the Rate of Metal Exchange between Fractions
The results from the different extractions were combined according to the three-component model described in Fig. 1 . The calculations, provided by ModelMaker 2.0 (SB Technology, 1994) are, in general, based on the relative distribution among the three components. Since plant uptake of metal ions occurs primarily through soil solution, the water soluble fraction (F1) is considered separately. Reversibly sorbed metals recovered in the extracts from the F2 and F3 steps in the sequence made up the second component of the model (reversibly sorbed). The apparent irreversibly sorbed fractions (F4, F5, F6, and F7) from where sorbed metals are released using redox agents, made up the third component of the model (irreversibly sorbed). Metals retained by slowly reversible electrostatic sorption are also mainly dissolved by the redox reagents used, because of reasons discussed above (limited time of extraction). However, for the reader's convenience, these metal fractions (which were dissolved by redox reagents) are referred to as irreversibly sorbed throughout the text and in the figures. It is expected that mechanisms of fixation include chemisorption, (co)precipitation, and solid-state diffusion of metals, or a combination thereof. Metals associated to these fractions show a high degree of irreversibility, since redox reagents are required to dissolve them. The model incorporates the calculated rate of ¹⁰⁹Cd and ⁶⁵Zn transfer among the three components as influenced by the organic matter content and temperature change. Ideally, the initial values used in the model should be 0% for the reversibly sorbed and the irreversibly sorbed components and 100% for the water-soluble component at the moment of spiking. Due to rapid reversible sorption of ¹⁰⁹Cd and ⁶⁵Zn, the initial values in the model refer to those obtained after the first withdrawal (0.5 h) in the extraction sequence. The results from the calculations are therefore valid only for the time span of 0.5 to 8760 h. As outlined below, the processed apparent rate constants k₁, k₂, k₃, and k₄ define the time-dependent distribution coefficients K_r and K_i (Bq kg^-1 solid/Bq L^-1 solution) between the phases. Rates of metal transfer between the phases followed a first-order reaction process kinetics, and according to Børretzen (1995), Eq. [2], [3], and [4] can be used to describe the metal flux among the three components shown in Eq. [1] and Fig. 1.

(1)

(2)

(3)

(4)

where N_w, N_r, and N_i denote the relative metal concentrations in the water, reversibly sorbed fractions, and irreversibly sorbed fractions, respectively. The k₁, k₂, k₃, and k₄ factors are treated as apparent rate constants (h^-1), and they describe fluxes between components in the model. Ideally, at equilibrium, and thus the pseudoequilibrium constants K_r and K_i (L kg^-1) can be expressed in Eq. [5] and [6], respectively, by adjusting the k₁/k₂ and k₃/k₄ ratios for solution volume (V) and weight of soil (m). The V/m ratio in this investigation was 8.

(5)

(6)

View larger version (17K): [in this window] [in a new window]   Fig. 1 Distribution of 109Cd and 65Zn in the three-component soil-soil water system model

The Arrhenius-postulated relationship between temperature and reaction rate shown in Eq. [7] can be used to estimate the activation energy (E_a).

(7)

The k value is the rate constant, A is the frequency factor, R is the gas constant, T is the absolute temperature, and E_a is the activation energy. By taking the natural logarithm of both sides, we obtain

(8)

This expression was used to compare the apparent rate constant value (k₃) for 9 and 21°C in combination as shown in Eq. [9].

(9)

We used Eq. [9] to calculate E_a for the transfer of ¹⁰⁹Cd and ⁶⁵Zn into the irreversibly sorbed fractions as a function of the apparent rate constant value (k₃) at 9 and at 21°C.

Statistical Analysis and Graphical Presentations
The experiment is a two-factorial design with two levels of temperature (9 and 21°C) and two levels of organic matter addition (0 and 4%). The observations were hence treated as a 2² factorial design with two levels for each of the factors (high and low) and three replicates of each factor combination. Statistical analysis was done using JMP 3.1 for Windows (SAS Institute, 1995), and the level of significance is 95%, if not otherwise specified. Origin 5.0 (Microcal Software, 1997) was used for graphical presentations and curve fitting, whereas the apparent rates constants were calculated by ModelMaker 2.0 (SB Technology, 1994).

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Sorption of Cadmium-109 and Zinc-65
The spiked ¹⁰⁹Cd and ⁶⁵Zn were totally sorbed by the soil components within a short time after their application. Although adsorption of ¹⁰⁹Cd and ⁶⁵Zn was rapid, the transfer of ¹⁰⁹Cd (Fig. 2) and ⁶⁵Zn (Fig. 3) from the reversibly sorbed fractions to the irreversibly sorbed fractions increased with contact time, irrespective of organic matter and temperature levels. The amount of ¹⁰⁹Cd and ⁶⁵Zn recovered in the water fraction was <0.3% after 0.5 h of contact time, and <25 to 35% in the reversibly sorbed fractions (Fig. 2 and 3). Although not shown here, the accumulation of ¹⁰⁹Cd and ⁶⁵Zn with time occurred mainly in the F5 fraction (Alms et al., 1999). Equations [2], [3], and [4] satisfactorily described the rate of ¹⁰⁹Cd and ⁶⁵Zn transfer among the three components (Eq. [1]), (P < 0.001).

View larger version (25K): [in this window] [in a new window]   Fig. 2 The observed (symbols) and calculated (lines) distribution of 109Cd with time among the water-soluble, reversibly sorbed, and irreversibly sorbed fractions as affected by temperature (9 and 21°C) and the addition of organic matter (OM) (4%). Each symbol represents the mean of

View larger version (25K): [in this window] [in a new window]   Fig. 3 The observed (symbols) and calculated (lines) distribution of 65Zn with time among the water-soluble, reversibly sorbed, and irreversibly sorbed fractions as affected by temperature (9 and 21°C) and the addition of organic matter (OM) (4%). Each symbol represents the mean of

View larger version (27K): [in this window] [in a new window]   Fig. 4 Effects of temperature and organic matter addition (OM) on the time-dependent distribution coefficient Ki (L kg-1) for 109Cd and 65Zn

Transfer of Cadmium-109 and Zinc-65 into the Irreversibly Sorbed Fractions
Based on the Arrhenius equation (Eq. [7]), the activation energy (E_a) was calculated by using Eq. [9] for the transfer of ¹⁰⁹Cd and ⁶⁵Zn into the irreversibly sorbed fractions (k₃). Since the temperature-induced increase in the k₃ values is moderate (6–8% increase) in the soil without organic matter addition, the calculated values for E_a are low (3.5 kJ mol^-1 for ¹⁰⁹Cd and 4.6 kJ mol^-1 for ⁶⁵Zn). These E_a values are much lower than the values previously reported by Bruemmer et al. (1988) (90 and 55 kJ mol^-1 for Zn and Cd, respectively) and by Barrow et al. (1989) (91 and 103 kJ mol^-1 for Zn and Cd, respectively) for the metal interaction with goethite, and the E_a values (66 kJ mol^-1) reported for the interaction of Zn with a loamy sand soil (Barrow, 1986). When organic matter was added at a level of 4% to the soil in our study, the E_a values increased from 3.5 to 61 kJ mol^-1 for ¹⁰⁹Cd and from 4.6 to 42 kJ mol^-1 for ⁶⁵Zn. These differences suggest that the effect of temperature on the interaction between metals and soil solids is dependent on soil condition and composition. Sparks (1986) concluded that since pure chemical reactions (chemisorption in soils) have higher energies of activation, low values of E_a indicate that diffusion may be important for the metal penetration in soil. The metals may slowly diffuse into extremely small pores of particle aggregates (Gerth et al., 1993; McBride, 1989), and the composition of the reacting material may thus greatly influence the activation energy. Barrow et al. (1989) suggested that low values of E_a indicate that the reacting material consists of major defects, facilitating metal diffusion. Thus, the greater the amount of such major defects in the reacting soil material the lower the values of E_a.

The results from the sequential extraction scheme show that retention of ¹⁰⁹Cd and ⁶⁵Zn are mainly contributed to the F4 (40–55%) and F5 (20–30%) soil fractions (Alms et al., 1999). The amount of dithionate-citrate extractable Fe is high (3.3%, Table 1), and dithionate-citrate is a reducing agent that should dissolve both amorphous and redox sensitive phases. This may exhibit favorable properties for metal diffusion due to major pores and defects in reacting amorphous Fe-bearing minerals and in residual organic matters (oxidized by H₂O₂). Previous published results have shown that diffusion may be involved in trace metal interaction both with a sandy loam soil (Barrow, 1986) and crystalline materials (goethite) (Barrow et al., 1989). However, it is important to note that reactions such as polymerization, nucleation, precipitation, and crystallization also may significantly contribute to the overall fixation. Diffusion cannot be concluded unequivocally to be the principal mechanism at work in the slowly reversible and irreversible sorption process seen here.

Effect of Organic Matter on the Reaction Kinetics
The rate of transfer among the three components in the model is significantly affected by the addition of organic matter. The presented values for E_a emphasized how important the organic matter addition was for diminishing the ¹⁰⁹Cd and ⁶⁵Zn transfer into the irreversibly sorbed fractions. The E_a values increased from 3.5 to 61 kJ mol^-1 for ¹⁰⁹Cd and from 4.6 to 42 kJ mol^-1 for ⁶⁵Zn, when organic matter was added. The organic matter induced reduction of the reaction rate k₃ was more pronounced for ¹⁰⁹Cd than for ⁶⁵Zn; hence, the organic matter induced increase of E_a was higher for ¹⁰⁹Cd than for ⁶⁵Zn. Since the forward reaction rate (k₃) between Component II and III in the model (Fig. 1) was counteracted by the reverse reaction rate (k₄, Table 2), the organic matter induced shift in K_i is hardly detectable for ¹⁰⁹Cd (Fig. 4), and the K_i decrease of only 2 to 7% is not significant. It may be recalled that the equilibrium constants are based on the ratio of rate coefficients, as shown in Eq. [5] and [6]. The decrease in K_i is statistically significant for ⁶⁵Zn, both at 9 and 21°C. This work indicates that the addition of organic matter also increased the potential mobility of ¹⁰⁹Cd, although the effect on K_i was not found to be significant. Despite the element of uncertainty connected to the equilibrium constant K_r, a significant reduction of K_r in the organic matter treated soil indicates that dissolved organics may have increased the solubility of ¹⁰⁹Cd, and also for ⁶⁵Zn at 9°C, by formation of dissolved metal–organic complexes. The concentrations of ¹⁰⁹Cd and ⁶⁵Zn in the water extracts are significantly higher in the organic matter treated soil. Analysis of TOC in these water extracts showed that the TOC concentrations were significantly higher in the extracts of the organic matter treated soil than in the untreated soil during the whole experimental period (paired t test). Hence, the correlation between the concentrations of TOC and ¹⁰⁹Cd and ⁶⁵Zn in the water extracts is positive and significant . This is treated as a corroborative evidence for suggesting that the potential mobility of the ¹⁰⁹Cd and ⁶⁵Zn is increased by formation of dissolved metal–organic complexes. Due to the decrease in K_i in the organic matter treated soil, mobile organic ligands may have diminished the fixation of metal ions by formation of soluble metal–organic complexes. The addition of organic matter thus increased the potential mobility of ¹⁰⁹Cd and ⁶⁵Zn (Fig. 2 and 3).

Effect of Temperature on the Reaction Kinetics
The effect of temperature on the rate of ¹⁰⁹Cd and ⁶⁵Zn transfer into the irreversibly sorbed fractions (k₃) is statistically significant, as discussed in the section on activation energy. In addition to increasing the k₃ values, the increased temperature also significantly shifted the K_i values to higher levels for both metals, irrespective of the organic matter addition (Table 2). Although the k₃ values either remained the same or decreased with increasing temperature, the K_i value increased because the rate coefficient that describes the reverse reaction (k₄) decreased even more. This resulted in increased concentrations of ¹⁰⁹Cd and ⁶⁵Zn in the irreversibly sorbed fractions at 21°C, and the corresponding decrease in the amount of reversibly sorbed metals and metals in the water fractions (Fig. 2 and 3). These observations agree well with previously published results showing increasing rates of metal reaction with soils (Barrow, 1986) and oxides (Gerth and Tiller, 1988) with time and temperature. It also appears from the K_i values that the effect of temperature is more pronounced for ⁶⁵Zn than for ¹⁰⁹Cd. The temperature-induced increase in the K_i values are 9 and 3% for ¹⁰⁹Cd, and 23 and 19% for ⁶⁵Zn, in the untreated and in the organic matter treated soils, respectively. Bergseth (1982) reported temperature-induced selectivity for Zn²⁺ to Cd²⁺ during the process of reversible sorption by vermiculite. He found that increasing temperature not only increased the sorption of Cd²⁺ and Zn²⁺ by vermiculite, but it also increased the selectivity for Zn²⁺ over Cd²⁺. As discussed above, the addition of organic matter may have mobilized the metals, but the effect is more pronounced at 9 than at 21°C. It can be seen in Fig. 4 that the values for K_i are lowest in the organic matter treated soil at 9°C. This implies that the organic matter induced mobilization of ¹⁰⁹Cd and ⁶⁵Zn is more efficient at the lower temperature. It is possible that the aggregation of suspended metal–organic complexes has increased with temperature, which would subsequently increase the solid phase fraction of ¹⁰⁹Cd and ⁶⁵Zn. Since complexation is more important for metal retention in soil than exchange sites on organic matter (Basta et al., 1993), it is likely that solid organic and inorganic matter increasingly chemisorbs labile metal ions associated with dissolved organics as temperature increases.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
In this study, a soil was spiked with ¹⁰⁹Cd and ⁶⁵Zn to determine the kinetics of reactions affecting the geochemical partitioning of trace metals with time as influenced with by temperature and organic matter addition. The results show that the initial reversible sorption of ¹⁰⁹Cd and ⁶⁵Zn occurred rapidly (within 0.5 h), and therefore the estimated rate of this process is uncertain. This study showed, however, that ¹⁰⁹Cd and ⁶⁵Zn were subsequently transferred from the reversibly sorbed fractions into the irreversibly sorbed fractions with time. Temperature and the addition of organic matter affected the rate of transfer and the distribution at pseudoequilibrium condition. Because increasing temperature increases the rate of ¹⁰⁹Cd and ⁶⁵Zn interaction with the soil, the forward reaction rate (k₃) is facilitated by temperature. This results in higher ¹⁰⁹Cd and ⁶⁵Zn concentrations in the irreversibly sorbed fractions, which is reflected by a temperature-induced increase of the pseudoequilibrium constant K_i. On the other hand, the k₃ and the K_i values are reduced by the addition of organic matter. These results indicate that the potential mobility of ¹⁰⁹Cd and ⁶⁵Zn is reduced with increasing temperature, whereas it is increased with the addition of organic matter. The effect of these two treatment factors on the exchange rates and pseudoequilibrium constants for ¹⁰⁹Cd and ⁶⁵Zn distribution is hence antagonistic in the model used.Technology Ltd 1994

	ACKNOWLEDGMENTS

 
The Research Council of Norway supported this research through a fellowship to the senior author as well as through research funding, and this assistance is gratefully acknowledged. The first author wishes to thank Peer Børretzen at Department of Chemistry and Biotechnology, for his valuable assistance during the model setup and its execution.

Received for publication July 1, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

• Alms .R., Singh B.R., Salbu B. Mobility of cadmium-109 and zinc-65 in soil influenced by equilibration time, temperature and organic matter. J. Environ. Qual. 1999;28:1742-1750.[Abstract/Free Full Text]
• Barr, L.W., and A.B. Lidiard. 1970. Defects in ionic crystals. In W. Jost. (ed.) Physical chemistry, an advanced treatise. Academic Press. New York.
• Barrow N.J. Testing a mechanistic model. II. The effect of time and temperature on the reaction of zinc with a soil. J. Soil Sci. 1986;37:277-286.
• Barrow N.J. A brief discussion on the effect of temperature on the reaction of inorganic ions with soil. J. Soil Sci. 1992;43:37-45.
• Barrow N.J., Gerth J., Brümmer G.W. Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. II. Modelling the extent and rate of reaction. J. Soil Sci. 1989;40:437-450.
• Basta N.T., Pantone D.J., Tabatabai M.A. Path analysis of heavy metal adsorption by soil. Agron. J. 1993;85:1054-1057.[Abstract/Free Full Text]
• Bergseth H. Effect of increasing temperature on the selectivity of a suspended vermiculite for Cu²⁺, Zn²⁺ and Cd²⁺. (In German.) Acta Agric. Scand., Sect. B, Soil Plant Sci. 1982;32:373-377.
• Børretzen, P.E. 1995. Mobility of trace elements in sediments. An overview of the physico-chemical forms of mainly the trace elements Cd and Zn, in sediments from the Karah sea area. (In Norwegian.) M.S. thesis. Agricultural Univ. of Norway.
• Bremner J.M. Nitrogen—Total. In: Sparks D.L., ed. Methods of soil analysis. Part 3. Madison, WI: SSSA, 1996:1085-1122 SSSA Book Ser. 5..
• Bruemmer B.W., Gerth J., Tiller K.G. Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. I. Adsorption and diffusion of metals. J. Soil Sci. 1988;39:37-52.
• Christensen T.H. Cadmium soil sorption at low concentrations: I. Effect of time, cadmium load, pH, and calcium. Water Air Soil Pollut. 1984;21:105-114.
• Domenico P.A., Schwartz F.W. Physical and chemical hydrogeology. New York: John Wiley & Sons, 1990.
• Elonen P. Particle-size analysis of soil. Helsinki, Finland: Acta Agralia Fennica, 1971.
• Gerth J., Brümmer G.W., Tiller K.G. Retention of Ni, Zn and Cd by Si-associated goethite. Z. Pflanzenernaehr. Bodenkd. 1993;156:123-129.
• Gerth J., Tiller K.G. Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. I. Adsorption and diffusion of metals. J. Soil Sci. 1988;39:37-52.
• Harter R.D., Naidu R. Role of metal–organic complexation in metal sorption by soils. Adv. Agron. 1995;55:219-263.
• Hodgson J.F., Geering H.R., Fellows M. The influence of fluoride, temperature, calcium, and alcohol on the reaction of cobalt with montmorillonite. Soil Sci. Soc. Am. J. 1964;28:39-42.[Abstract/Free Full Text]
• Holmgen G.S. A rapid citrat-dithionite exractable iron procedure. Soil Sci. Soc. Am. J. 1967;31:210-211.[Abstract/Free Full Text]
• Jeng A.S., Bergseth H. Chemical and mineralogical properties of Norwegian alum shale soils, with special emphasis on heavy metal content and availability. Acta Agric. Scand., Sect. B, Soil Plant Sci. 1992;42:88-93.
• Johnson B.B. Effect of pH, temperature, and concentration on the adsorption of cadmium on goethite. Environ. Sci. Technol. 1990;24:112-118.
• Kennedy V.H., Sanchez A.L., Oughton D.H., Rowland A.P. Use of single and sequential chemical extractants to assess radionuclide and heavy metal availability from soils for root uptake. Analyst 1997;122:R89-R100.
• McBride M.B. Reactions controlling heavy metal solubility in soils. Adv. Soil Sci. 1989;10:1-56.
• Microcal Software. 1997. Origin, data analysis and technical graphics. 5. Northampton, MA.
• Narwal R.P., Singh B.R. Effect of organic materials on partitioning, extractability and plant uptake of metals in an alum shale soil. Water Air Soil Pollut. 1998;103:405-421.
• Nelson D.W., Sommer L.E. Total carbon, organic carbon, and organic matter. In: Sparks D.L., ed. Methods of soil analysis. Part 3. Madison, WI: SSSA, 1996:961-1010 SSSA Book Ser. 5..
• Pehlivan E., Ersoz M., Pehlivan M., Yildiz S., Duncan H.J. The effect of pH and temperature on the sorption of zinc (II), cadmium (II), and aluminum (III) onto new metal–ligand complexes of sporopollenin. J. Colloid Interface Sci. 1995;170:320-325.
• Salbu B. Radioactive tracer techniques in speciation studies. Environ. Tech. Lett. 1987;8:381-392.
• Salbu, B. 1998. Speciation of radionuclides. In Encyclopedia of analytical chemistry. John Wiley & Sons, Chichester, UK.
• Salbu B., Krekling T., Oughton D.H. Characterisation of radioactive particles in the environment. Analyst 1998;123:843-849.
• SAS Institute. JMP, Statistical Discovery Software. 3.1. Cary, NC: SAS Inst, 1995.
• Saunders W.M.H. Phosphate retention by New Zealand soil and its relationship to free sesquioxides, organic matter and other soil properties. N.Z. J. Agric. Res. 1965;8:30-57.
• Sauve S., McBride M.B., Hendershot W. Soil solution speciation of lead (II): Effects of organic matter and pH. Soil Sci. Soc. Am. J. 1998;62:618-621.[Abstract/Free Full Text]
• Technology Ltd S.B. SB ModelMaker. 2.0b. Basingstoke, UK: SB Technology Ltd, 1994.
• Singh B.R., Narwal R.P., Alms .R. Crop uptake and extractability of cadmium in soils naturally high in metals at different pH levels. Commun. Soil Sci. Plant Anal. 1995;26:2123-2142.
• Soil Survey Staff. Keys to soil taxonomy, 5th ed Washington, DC: USDA-SCS, 1992.
• Sparks D.L. Soil physical chemistry. Boca Raton, FL: CRC Press, 1986.
• Stumm W., Morgan J.J. Aquatic chemistry: chemical equilibria and rates in natural waters, 3rd ed New York: Wiley Interscience, 1996.
• Taylor R.W., Hassan K., Mehadi A.A., Shuford J.W. Zinc sorption by some Alabama soils. Commun. Soil Sci. Plant Anal. 1995;26:993-1008.
• Tessier A., Campbell P.G.C., Bisson M. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 1979;51:844-851.

This article has been cited by other articles:

				C. W. Gray, R. G. McLaren, D. Gunther, and S. Sinaj An Assessment of Cadmium Availability in Cadmium-Contaminated Soils using Isotope Exchange Kinetics Soil Sci. Soc. Am. J., July 1, 2004; 68(4): 1210 - 1217. [Abstract] [Full Text] [PDF]

				S. Brown, R. Chaney, J. Hallfrisch, J. A. Ryan, and W. R. Berti In Situ Soil Treatments to Reduce the Phyto- and Bioavailability of Lead, Zinc, and Cadmium J. Environ. Qual., March 1, 2004; 33(2): 522 - 531. [Abstract] [Full Text] [PDF]

				A.R. Almas and B.R. Singh Plant Uptake of Cadmium-109 and Zinc-65 at Different Temperature and Organic Matter Levels J. Environ. Qual., May 1, 2001; 30(3): 869 - 877. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (11) Citing Articles via Google Scholar Google Scholar Articles by Almås, A.R. Articles by Singh, B.R. Search for Related Content PubMed Articles by Almås, A.R. Articles by Singh, B.R. GeoRef GeoRef Citation Agricola Articles by Almås, A.R. Articles by Singh, B.R.