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DIVISION S-2-SOIL CHEMISTRY

Assessment of Isotopically Exchangeable Zinc in Polluted and Nonpolluted Soils

^a Institute of Plant Sciences, Swiss Federal Institute of Technology (ETH), Versuchsstation Eschikon, Eschikon 33, CH-8315 Lindau, Switzerland

sokrat.sinaj{at}ipw.agrl.ethz.ch

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
In this study, an isotope exchange kinetics (IEK) approach was used to study soil Zn exchangeability in five polluted and six nonpolluted soils. Results were used in a compartmental analysis to quantify the amount of Zn isotopically exchangeable within 1 min, the amount of Zn exchangeable between 1 min and 15 d, and the amount of Zn that could not be exchanged within 15 d. Results derived from short-term IEK experiments (100 min) allowed for a precise prediction of the increase of exchangeable Zn throughout 15 d in most of the soils studied. The compartmental analysis conducted on the 11 samples allowed clear separation of the polluted from the nonpolluted soils and showed that most of the soil Zn is either very slowly or not at all exchangeable. The proposed approach yielded simultaneously the intensity factor, the quantity factor, and a parameter related to soil properties governing Zn sorption onto soil [r(1)/R]. The amount of Zn exchangeable within 15 d was found to be very close to Zn extractable by 0.005 M diethylene triamine pentaacetic acid (DTPA) + 0.01 M CaCl₂ + 0.1 M triethanolamine (TEA) (Zn_DTPA) in all soils, suggesting that both methods were characterizing the same pool of soil Zn.

Abbreviations: C_Zn, concentration of free Zn in a soil water extract • C_Zn3d, concentration of free Zn in a soil water extract after 3 d of preequilibration in ultrapure water • DTPA, diethylene triamine pentaacetic acid • E_{(1 min)}, Zn isotopically exchangeable within one min • E_{(1 min-15 d)}, Zn isotopically exchangeable between 1 min and 15 d • E_{(>15 d)}, Zn which cannot be isotopically exchanged within 15 d • E_(t)exp, amount of Zn isotopically exchangeable after t min derived from experimental results • E_(t)pred, amount of Zn isotopically exchangeable after t min predicted using parameters derived from a 100-min-long isotope exchange kinetic experiment together with C_Zn3d and Zn_HNO3 • EDTA, ethylene diamine tetraacetic acid • IEK, isotope exchange kinetic • TEA, triethanolamine • Zn_HNO3, Zn extractable by 2 M HNO₃ • Zn_DTPA, Zn extractable by 0.005 M DTPA + 0.01 M CaCl₂ + 0.1 M TEA

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
TRACE METALS such as Cu, Mn, or Zn are essential for plant growth (Marschner, 1995). However, when taken up in excessive quantities, those elements are transferred to the food chain, where they may have adverse effects on the health of animals and humans (Adriano, 1986; Chaney, 1993; Ebbs and Kochian, 1997). A precise evaluation of the availability of trace metals for plants is therefore needed for optimizing crop production in deficient soils through the appropriate addition of fertilizers and for assessing the danger of excessive metals uptake by crops in polluted soils.

Since plants take up most of their trace metals as free ions from the soil solution, which is then replenished by ions located on the solid phase of the soil, their availability can be described using the concepts of intensity, quantity, and buffering capacity (Brümmer, 1986; Marschner, 1993; Barber, 1995; Nair, 1996). The intensity is the activity of free ions in the soil solution (Brümmer, 1986). It is estimated by measuring the concentration of free ions in the soil solution (Winistoerfer, 1995; Holm et al., 1995). The quantity is the amount of ions that can be released from the solid phase of the soil into the soil solution during the interval of time considered for plant growth, and it is usually assessed for trace metals by extracting the soil with DTPA or ethylene diamine tetraacetic acid (EDTA) (Lakanen and Erviö, 1971; Tiller et al., 1972b; Sinah et al., 1977; Lindsay and Norvell, 1978; Fujii and Corey, 1986; Sheppard and Evenden, 1989). Finally, the buffering capacity defines the changes in quantity per unit change in concentration of metals in solution (Holford and Mattingly, 1976).

Until now, the isotope dilution technique has been used in trace metals studies to determine simultaneously two of those three factors, either the intensity and the quantity factors (Weir and Miller, 1962; McLaren and Crawford, 1974; Checkai, 1979; Do Vale, 1982) or the buffering capacity and the quantity factor (Sinah et al., 1977; Sheppard and Evenden, 1989). Tiller et al. (1972a) and Checkai (1979) showed that the amount of Zn isotopically exchangeable in dilute CaCl₂ or SrNO₃ was strongly related to the amount of isotopically exchangeable Zn measured in pot experiments (L value) in acidic soils, suggesting that the isotopically exchangeable Zn was the main source of Zn for the studied plants. However, most of those studies, with the exception of the work done by Weir and Miller (1962), did not pay a lot of attention to the isotopic exchange kinetics of these trace metals: all the measurements were carried out after a few days of isotopic exchange. It has been recently shown that the three factors describing P availability (intensity, quantity, and buffering capacity) could be obtained by studying the kinetic transfer of ³²PO₄ from the solution to the soil's solid phase in systems at a steady state (Morel et al., 1994; Fardeau, 1996). Furthermore, Fardeau et al. (1985)(1994) showed that the parameters obtained from an isotope exchange kinetic experiment conducted during 100 min could be used in a model to predict the increase of soil exchangeable PO₄ during at least 12 wk.

The objectives of this study were to assess whether the method of isotope exchange kinetics (Fardeau et al., 1985) could provide relevant information on Zn exchangeability in nonpolluted and polluted soils and to establish relations between the parameters obtained from the isotope exchange kinetic experiment and some soil properties.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soil Samples
The surface horizons (0–20 cm) of eleven agricultural soils were collected for this study. All samples were air dried and sieved to 2 mm. Some of their physical and chemical properties are presented in the Table 1 . According to the Swiss ordinance on pollutants in soils (Verordnung des schweizerischen Bundesrates, 1986), Samples 7 to 11, which were sampled in the vicinity of smelters, are considered polluted.

(1)

where R is the total introduced radioactivity (MBq) and r(1) and r() are the radioactivity (MBq) remaining in the solution after 1 min and at infinite time, respectively. The value n is a parameter describing the rate of disappearance of the tracer from the solution after 1 min.

Since the system is at steady state, the decrease of radioactivity in the solution is ascribed to a homoionic exchange between radioactive ions added to the solution and exchangeable stable ions located on the solid phase of the soil. The quantity E_(t) (mg ion considered kg^-1 soil) of isotopically exchangeable ions at a time t can be calculated for a given compound using the following equation, assuming that (i) the stable and radioactive ions have the same fate in the system and (ii) whatever the time t, the specific activity of the considered ions in the soil solution is identical to that of the isotopically exchanged ions in the entire soil–solution system (Lopez and Graham, 1970; Tiller et al., 1972a; McLaren and Crawford, 1974; Sinah et al., 1977; Fardeau et al., 1985).

(2)

where C is the concentration of the considered ion in the solution (mg L^-1). The factor 10 arises from the soil/solution ratio of 1g of soil in 10 mL of water, so that 10C is equivalent to the concentration of water-soluble ions of the soil (mg kg^-1).

The IEK method gives information on the three factors characterizing PO₄ availability in soils (Morel et al., 1994; Fardeau, 1996). The PO₄ concentration in the soil solution represents the intensity factor. The quantity of isotopically exchangeable P E_(t) gives information on the quantity factor. The r(1)/R ratio and n parameter, which represent, respectively, the ratio between the radioactivity remaining in the solution after 1 min of exchange and the total introduced radioactivity and the rate of disappearance of the radioactivity from the solution for exchange times longer than 1 min, give information on the soil P-buffering capacity (capacity factor) (Tran et al., 1988; Salcedo et al., 1991; Frossard et al., 1992).

Data Analysis
The results obtained from the isotopic exchange kinetic technique can be interpreted by two complementary analyses: (i) stochastic and (ii) compartmental. Only the compartmental analysis will be considered here, information on the stochastic analysis can be found in Fardeau et al. (1991).

A compartment is defined as a homogeneous unit in which all the ions have the same kinetic properties and exchange at the same rate with the same ions present in other compartments (Sheppard, 1962; Atkins, 1973). A pool can be defined as a volume that contains at least one compartment (Atkins, 1973). We have chosen to analyze soil isotopically exchangeable Zn with a three-pool model.

Pool of Zinc Exchangeable within One Minute
The Zn present in this pool is composed of Zn²⁺ in the soil solution and adsorbed or chelated Zn that is very rapidly exchangeable. This pool is considered analogous to the pool of PO₄ exchangeable within 1 min [E_{(1 min)}], which has been shown to contain PO₄ ions located in the soil solution and on the soil's solid phase that present the same kinetic properties (Tran et al., 1988; Salcedo et al., 1991). The ions present in this compartment may be directly exchanged with ions located in the other pools.

Pool of Zinc Exchangeable between One Minute and Fifteen Days
Fifteen days corresponds with the maximum period during which isotopically exchangeable Zn has been experimentally measured. This pool contains Zn that is exchangeable between 1 min and 15 d [E_{(1 min-15 d)}].

Pool of Zinc that Cannot be Exchanged within Fifteen Days
This pool [E_{(>15 d)}] contains Zn that is either slowly or not exchangeable, for instance Zn occluded in minerals or strongly adsorbed onto soil particles.

Experiment
The isotopic exchange kinetic technique has been recently described by Fardeau (1996). In our work, the experiments were conducted by adding carrier-free ⁶⁵Zn (1.0 MBq L^-1) as ZnCl₂, originally stored in a solution 0.1 M HCl, to soil solution suspensions that had been preequilibrated by continuous shaking for 3 d. This solution of ⁶⁵Zn contained a very low level of nonradioactive Zn (its specific activity ranged between 3.7 and 37 GBq mg^-1 Zn), and was free from other radionucleides. The use of this radioactive solution resulted in a slight contamination of the soil solution on the order of magnitude of 0.001 mg Zn L^-1, which did not show any significant effect on the isotope exchange kinetics. The experiment was conducted on all samples using a soil/solution ratio of 1:10 during 15 d, with the exception of Sample 4 for which the isotope exchange experiment was only conducted for 100 min. For Samples 1, 7, and 10 two additional soil/solution ratios were considered (1:5 and 1:20). The amount of radioactivity, r(t), and the concentration of Zn²⁺ in the solution, C_Zn, were measured after 1, 10, 40, and 100 min, and 1, 2, 3, 7, 11, and 15 d of isotopic exchange by scintillation counting and ion chromatography, respectively. The concentration of ⁶⁵Zn was determined by mixing 1 mL of radioactive solution in 10 mL of scintillation liquid (Ultima Gold, Packard Instrument Co., Downers Grove, IL.) in order to detect its ß⁺ radiation (energy of emission: 325 KeV; half-life: 243.8 d). The initial amount of added ⁶⁵Zn (R) was always counted together with the concentration of radioactivity remaining in the soil solution after each time of exchange [r(t)] for each sample; therefore, there was no need to correct for radioactive decay. The maximum counting time for solutions containing a low activity was 10 min. The actual counts per minute (cpm) were always corrected for quenching in order to obtain an absolute measure of the activity (Bq) (Kessler, 1989; L'Annunziata, 1979). Finally, samples were held at maximum 3 h before all the radioactivity determinations were completely carried out.

These experimental data [r(t)/R and C_Zn] were used in Eq. [2] to calculate directly the experimental isotopically exchangeable Zn E_(t)exp. Then, E_(t)exp values were compared with the predicted values of isotopically exchangeable Zn E_(t)pred. Values of E_(t)pred were calculated using Eq. [1] and [2] for 1 min and 1, 2, 3, 7, 11, and 15 d of isotopic exchange in all samples, except Samples 2 and 3, using r(1)/R, n, Zn_HNO3, and C_Zn3d. The parameters r(1)/R, and n were derived from a linear regression between log[r(t)/R] and log[t] during the first 100 min of exchange (Fardeau et al., 1985). The ratio r()/R, the maximum possible dilution of the added isotope, was approximated as follows (Fardeau et al., 1985):

(3)

where V is a factor representative of water/soil ratio and VC_Zn3d is the quantity of water-soluble Zn of the soil (mg kg^-1).

However, this approach was not possible for Samples 2 and 3 because of the very rapid decrease in radioactivity from the solution and because of the very low concentrations of free Zn in the solution, close to our detection limit. Using a nonlinear regression, the decrease of radioactivity in solution was very well described in both samples by Eq. [1] . Furthermore, the isotopic steady state was very rapidly attained in both samples, allowing for an experimental measurement of the term r()/R on at least three subsequent points. Equation [3] was then used to calculate C_Zn. This method of estimating C_Zn for very low Zn concentrations gives both lower results and lower standard deviations than direct measurements made using ion chromatography (Table 5) . This suggests either that direct measurements were not precise enough because they were reaching the detection limit or that a slight net Zn sorption occurred during the 15-d-long isotope exchange experiment. The nonlinear regression based on Eq. [1] also allowed us to assess r(1)/R and n in Samples 2 and 3.

Statistical Analysis
All analyses were replicated at least three times. Coefficients of variation ranged from 0.5 to 9, 2 to 7, 1 to 5, 1 to 8, and 0.01 to 8% for C_Zn, r(1)/R, n, E_{(1 min)}, and Zn_DTPA, respectively. Analysis of variance and Duncan tests were used to evaluate the effect of the soil/water ratio on E_(t)exp, while regression analysis was used to compare E_(t)exp and E_(t)pred, and to derive relationships between soil properties and parameters obtained from the isotope exchange kinetic experiment (Snedecor and Cochran, 1982).

	Results and discussion

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Adaptation of the Isotopic Exchange Kinetic Technique to Assess Soil Zinc Exchangeability
Effect of Shaking Time on the Concentration of Free Zinc in Soil Water Extracts
After 3 d of shaking, a steady state was reached for C_Zn in Samples 1 and 7, whereas C_Zn was still decreasing in Sample 10 after 14 d (Fig. 1) . The steady-state concentration found in Sample 1 is within the range of C_Zn given by Checkai (1979), Do Vale (1982), and Barber (1995) for nonpolluted soils, while the solution concentrations of Zn observed after 14 d in the two polluted soils (Samples 7 and 10) are within the range of water-soluble Zn concentration given by Ma and Rao (1997) for polluted soils. In both polluted soils, the Zn²⁺ concentration decreased with time. This decrease can possibly be explained by the presence of Zn-rich colloids in these samples (Table 2) deposited during smelter emissions. The suspension of both samples in ultrapure water probably induced a release of colloid-associated Zn to the solution, which then was increasingly readsorbed onto soil particles with time. Similar results were obtained by Jopony and Young (1994), who observed a decrease in 0.05 M CaCl₂–extractable Pb concentration with time in a contaminated soil and who found that 10 d of shaking were necessary to reach a steady state.

View larger version (19K): [in this window] [in a new window]   Fig. 1 Changes of free Zn concentration in the solution with time (CZn) during the extraction of three soils (Samples 1, 7, and 10) with ultrapure water (only the results for the soil/solution ratio of 1:10 are presented; the other soil/solution ratios gave similar results)

View larger version (20K): [in this window] [in a new window]   Fig. 2 Comparison between the observed [E(t)exp] and predicted [E(t)pred] increase of isotopically exchangeable Zn with time for three samples (Samples 1, 7, and 10) that were preequilibrated 3 d in ultrapure water prior to adding the radioactive Zn (only the results for the soil/solution ratio of 1:10 are presented; the other soil/solution ratios gave similar results)

View larger version (14K): [in this window] [in a new window]   Fig. 3 Relation between E(t)exp and E(t)pred for all exchange times, all soil samples (except Samples 2 and 3 for which this prediction was not possible and Sample 4 for which only a short term isotope exchange kinetic experiment was made), and all soil/solution ratios

The compartmental analysis conducted on the 11 soil samples allows for a clear distinction between nonpolluted and polluted soils (Table 7). For the six nonpolluted soils, the amount of Zn located in the E_{(1 min)}, E_{(1 min-15 d)}, and E_{(>15 d)} pools had an average of 3.5, 13.9, and 63.1 mg Zn kg^-1 soil, respectively. While for the five samples polluted by smelter emissions, the amount of Zn located in the E_{(1 min)}, E_{(1 min-15 d)}, and E_{(>15 d)} pools reached an average of 49.9, 112.1, and 392.2 mg Zn kg^-1 soil, respectively. However, when expressed on a percentage basis, the results obtained in nonpolluted soils were similar to those obtained on polluted soils. An average of 4.0 to 8.2% of the Zn_HNO3 was isotopically exchangeable within 1 min for nonpolluted and polluted soils, respectively; 15.6 to 17.9% of the Zn_HNO3 was to be found in the pool E_{(1 min-15 d)} of nonpolluted and polluted soils, respectively; and 80.4 to 73.9% of the Zn_HNO3 was to be found in the pool E_{(>15 d)} of nonpolluted and polluted soils, respectively. These results indicate that (i) most of the Zn present in those samples is either slowly exchangeable or not exchangeable at all, which is in agreement with the well-known fact that Zn is very strongly sorbed onto soil particles (McBride, 1989; Ross, 1994) and that (ii) although pollution due to smelter emissions substantially increased all the pools of exchangeable Zn, it did not drastically change its relative distribution within the different pools.

Relationships Between r(1)/R, E_{(1 min)}, E_{(15 d)}, Zn_DTPA, Zn_HNO3, and Some Soil Properties
Regression analysis showed that r(1)/R was significantly related to the soil clay and oxalate-extractable Mn contents, pH, and percentage of cationic exchange capacity saturation (Table 8) . Because these soil characteristics have been shown to be related to the sorption of Zn in soils (Brümmer, 1986; Barrow, 1993; Ross, 1994), r(1)/R may provide valuable information on the short-term sorption of Zn onto soil particles. We hypothesize that r(1)/R could also provide information on the soil buffering capacity for Zn.

Limits of the Isotope Exchange Kinetics Approach
An important assumption underlying the use of the isotope exchange kinetics approach is that the tracer and the stable element have to be measured in the same compartment (i.e., where the tracer has been added) (Sheppard, 1962; Fardeau, 1996). Here, this assumption has not been strictly adhered to since the radioactivity was measured in solution and may have included ⁶⁵Zn in a free form as well as some ⁶⁵Zn chelated in organic complexes, while ion chromatography would not have detected strongly complexed Zn. On the other hand, ion chromatography measurements include weakly complexed Zn (complexes of Zn²⁺ with Cl^- or SO^2-₄) and may overestimate the intensity factor (C_Zn) in soils with high Cl^- or SO^2-₄ concentrations.

Another important assumption when studying isotope exchange kinetics is that the soil solution system should be kept at a steady state throughout the course of the experiment (Fardeau, 1996). This condition was met for the three soils studied, since C_Zn did not vary significantly during the 100 min of exchange following the 3 d of preequilibration. Furthermore, it might be argued that steady state cannot be met in natural systems because plants, microorganisms, leaching, and erosion continuously remove metals from the soils. However, the rate of metal removal is several orders of magnitude lower than the rate of exchange between the soil and its solution and could be a problem only in soils containing a very low reserve of available Zn.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Our results show that the isotope exchange kinetics approach allows for assessing the exchangeability of Zn both in nonpolluted and polluted soils:

1. The increase in isotopically exchangeable Zn [E_(t)] can be precisely described by Eq. [1] and [2] for soils having a range of different properties and for three soil/solution ratios. Furthermore, results derived from short-term IEK experiments (100 min) allowed for a precise prediction of the increase of exchangeable Zn throughout 15 d in most of the soils studied. The only samples for which this prediction was not possible were the samples with a high pH and a low Zn_HNO3 content. However, in those soils the isotopic steady state was reached quickly.
   
2. The compartmental analysis conducted on the 11 soil samples clearly separated the polluted from the nonpolluted samples and showed that most of the soil Zn is either slowly or not at all exchangeable with the Zn present in the solution.
   
3. The proposed approach yielded simultaneously the intensity factor (C_Zn), the quantity factor [E_(t)], and a parameter related to soil properties governing Zn sorption onto soil [r(1)/R]. Furthermore the amount of Zn exchangeable during 15 d [E_{(15 d)}] was found to be very close to Zn_DTPA in our 11 soils, suggesting that both methods were characterizing the same pool of soil Zn.
   

Although, no single approach will completely reflect the availability of trace metals because of the complexity of the soil environment and the diversity of plant and microorganism responses (McBride, 1989), this approach provides a means to further understand and quantify the effect of various soil conditions on metal availability.Frossard Fardeau Brossard Morel 1994; Verordnung des schweizerischen Bundesrates. 1986

	ACKNOWLEDGMENTS

 
The authors thank Dr. J.C. Fardeau (INRA, Versailles, France) and Mr. P. Zdruli (European Soil Bureau, ISPRA, Italy) for valuable discussions; Dr. R. Fujii (Water Resources Division, Sacramento, CA) for his helpful comments on the manuscript; Dr. J. Martinez (CEMAGREF, Rennes, France) for providing Sample 4; Dr. S.K. Gupta (FAL-IUL, Bern, Switzerland) for providing Samples 7 and 10; and Mr. G. Gini (Sezione agricoltura/Ufficio pianificazione agricola, Bellinzona, Switzerland) for providing Samples 8 and 9.

Received for publication July 21, 1997.

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