JEQ Grow Your Career With ASA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 1 March 2008
Published in J Environ Qual 37:501-508 (2008)
DOI: 10.2134/jeq2007.0174
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Kwon, J.-S.
Right arrow Articles by Nightingale, M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Kwon, J.-S.
Right arrow Articles by Nightingale, M.
Agricola
Right arrow Articles by Kwon, J.-S.
Right arrow Articles by Nightingale, M.
Related Collections
Right arrow Isotopes
Right arrow Sulfur
Right arrow Soil Chemistry

SHORT COMMUNICATIONS

The Use of Ion Exchange Membranes for Isotope Analyses on Soil Water Sulfate: Laboratory Experiments

Jang-Soon Kwona, Bernhard Mayerb,*, Seong-Taek Yuna and Michael Nightingaleb

a Dep. of Earth Environmental Sciences and the Environmental Geosphere Research Lab (EGRL), Korea Univ., Seoul 136-701, Korea
b Dep. of Geoscience, Univ. of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4

* Corresponding author (bmayer{at}ucalgary.ca).

Received for publication April 7, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
To investigate the potential use of anion exchange membranes (plant root simulator [PRS] probes) for isotope investigations of the soil sulfur cycle, laboratory experiments were performed to examine the sulfate exchange characteristics and to determine the extent of sulfur and oxygen isotope fractionation during sulfate sorption and desorption on the probes in aqueous solutions and simulated soil solutions. The sulfate-exchange tests in aqueous solutions under varying experimental conditions indicated that the amount of sulfate exchanged onto PRS probes increased with increasing reaction time, initial sulfate concentration, and the number of probes used (= surface area), whereas the percentage of removal of available sulfate was constant irrespective of the initial sulfate concentration. The competition of nitrate and chloride in the solution lowered the amount of exchanged sulfate. The exchange experiments in a simulated soil under water-saturated and water-unsaturated conditions showed that a considerable proportion of the soil sulfate was exchanged by the PRS probes after about 10 d. There was no evidence for significant sulfur and oxygen isotope fractionation between soil sulfate and sulfate recovered from the PRS probes. Therefore, we recommend the use of PRS probes as an efficient and easy way to collect soil water sulfate for determination of its isotope composition.

Abbreviations: PRS, plant root simulator


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
SULFUR compounds are important air pollutants, essential nutrients for plants and microorganisms, and major constituents in natural waters. In the pedosphere, lithosphere, and hydrosphere, sulfur occurs in numerous forms and valence states including sulfate, elemental sulfur, sulfide, and organic sulfur. The major sulfur transformation processes in near-surface environments include sulfur mineralization, immobilization, assimilatory and dissimilatory sulfate reduction, sulfide oxidation, adsorption/desorption, and precipitation/dissolution of sulfates. Stable isotope ratios of sulfur and oxygen have been successfully used to identify the sources of sulfur and the predominant sulfur transformation processes in terrestrial and aquatic ecosystems (e.g., Hitchon and Krouse, 1972; Krouse and Grinenko, 1991; Cravotta, 1995; Mitchell et al., 1998; Krouse and Mayer, 2000; Rock and Mayer, 2002).

Throughout the last several decades, sulfur cycling in ecosystems has received considerable attention from researchers and policymakers due to concerns over the response of sensitive forest and aquatic ecosystems to atmospheric sulfur emissions and deposition (e.g., Alewell, 2001; Likens et al., 2002). Numerous studies have used sulfur isotope ratios to investigate the fate of atmospheric sulfate in the biosphere, pedosphere, and hydrosphere (Krouse and Case, 1981; Jäger et al., 1989; Krouse and Grinenko, 1991; Mayer et al., 1995a; Novák et al., 1996; Novák et al., 2000; Mayer et al., 2001; Knöller et al., 2005) because atmospheric sulfur deposition results in increased export of base cations (K+, Ca2+, Mg2+, and Al3+) from the soil (Driscoll et al., 2003) and may cause soil acidification with increasing SO42– concentrations in the soil water. In addition, oxygen isotope ratios have proven to be a reliable indicator for distinguishing between sulfate from atmospheric deposition with elevated oxygen isotope ratios and sulfate formed by mineralization of organic sulfur in the pedosphere, which is often characterized by significantly lower oxygen isotope ratios (Gelineau et al., 1989; Van Stempvoort et al., 1992; Mayer et al., 1995b). Hence, the ability to accurately determine the isotopic composition of seepage water sulfate and soil sulfate is important to obtain information about sulfur sources and transformation processes in soil systems.

Water in the soil environment plays a crucial role as a carrier of dissolved and solid species and is an essential medium facilitating transformation processes in the carbon, nitrogen, and sulfur cycles that are strongly tied to hydrological conditions. Diverse approaches exist for the in situ sampling of soil water (Wilson, 1995), including suction cups or lysimeters and passive capillary samplers (Grossmann and Udluft, 1991; Brandi-Dohrn et al., 1996; Louie et al., 2000). Previous studies using sulfur and oxygen isotope ratios to investigate sulfate transport in soils relied on soil water collected using suction cups or lysimeters (Van Stempvoort et al., 1990; Mayer et al., 1995a; Novák et al., 1995). The installation of such devices is labor intensive and causes significant disruptions of the soil environment. In addition, sulfate concentrations in many soil solutions are low, and collecting sufficient quantities of water to yield enough sulfate for sulfur and oxygen isotope analyses may be challenging. Therefore, a nondestructive rapid and simple technique for extracting dissolved sulfate from soil water samples for sulfur and oxygen isotope analyses is desirable.

Since the work of Saunders (1964), ion exchange resins in sheet or membrane form have gained increasing popularity for collecting dissolved ions in soils to measure the amounts of plant-available nutrient ions (Qian et al., 1992; McLaughlin et al., 1993) because of their simplicity, cost-effectiveness, and low impact on the soil environment. The commercial plant root simulator (PRS) probes, which consist of a membrane plate of ion exchange resin within a plastic holding device, are designed to adsorb nutrients directly from soils in a similar fashion to the way plant roots take up nutrients. The PRS probes have been used in many scientific soil investigations addressing issues such as salinity and salt contamination, heavy metal contamination, and amount and distribution of cations and anions in forest soils (Schoenau et al., 1993; Huang and Schoenau, 1996a,b; Huang and Schoenau, 1997; Li et al., 2001). The probes have several advantages, including easy insertion with minimal soil disturbance due to their flat structure, which ensures a constant adsorptive surface area. They are easy to remove and to clean, greatly facilitating subsequent analytical procedures in the laboratory. Moreover, the PRS probes can be reused, making them an easy and cost-effective tool for collecting dissolved ions from soil environments.

The objective of this study was to test whether commercial ion exchange membranes (PRS probes) can be used for collection of soil sulfate for subsequent accurate and precise sulfur and oxygen isotope ratio determinations. Several laboratory-scale experiments with simulated aqueous solutions and soil solutions were performed to examine the sulfate exchange characteristics and the extent of sulfur and oxygen isotope fractionation during sulfate exchange reactions onto the anion exchange resins.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Materials
Ion Exchange Membranes and Pretreatment
The PRS probes (15 x 3 x 0.5 cm; Western Ag Innovation, Saskatoon, Saskatchewan, Canada) used in this study consist of a thin anion exchange resin membrane (~5.5 x 1.6 cm, 17.5 cm2 including both sides of the membrane) with fixed cationic groups (i.e., permanent charges) within a polystyrene matrix encased in a plastic holding device. Before the ion exchange experiments, the anion exchange PRS probes were cleaned by soaking in 0.5 mol L–1 HCl for 1 h and rinsing with deionized water. The cleaned PRS probes were regenerated with bicarbonate (HCO3–) as a counterion by shaking the probes repeatedly in a well stirred beaker with 0.5 mol L–1 NaHCO3 solution for 4 h. The regenerated PRS probes were rinsed with deionized water.

Aqueous Solutions
Test solutions with varying concentrations of anions (sulfate, nitrate, and chloride) for anion exchange experiments were prepared by diluting stock solutions that were made using crystalline (NH4)2SO4, NaNO3, and NaCl (Fisher Scientific Ltd., Pittsburgh, PA), respectively.

Simulated Mineral Soil
For the sulfate-exchange experiments in a simulated mineral soil, we washed and graded fine mortar sand (Burnco; Rock Products LTD, Calgary, Alberta, Canada). Ten kilograms of this simulated mineral soil with particle diameters ranging from 149 µm to 2 mm were treated with 5 L alkaline solution (pH 12) in a large plastic container for 12 h to desorb sulfate attached to sesquioxides. The mineral soil was rinsed repeatedly with deionized water to remove water-soluble impurities. The cleaned mineral soil was air-dried at room temperature for 3 d. To adsorb sulfate onto anion exchange sites, 10 kg of dried mineral soil were soaked in a 5 L (NH4)2SO4 solution with a sulfate concentration of 200 mg L–1 in a glass container for 12 h. The sulfate-treated mineral soil samples were placed in a nylon sieve to remove the (NH4)2SO4 solution.

To determine the sulfate contents of the prepared mineral soil samples, sulfate desorption tests were performed using deionized water and 1% K3PO4 solution (1 L kg–1 soil) for 1 h. The supernatant solution obtained from the desorption experiments was centrifuged and filtered using a 0.45-µm cellulose nitrate membrane, and the sulfate concentration and the {delta}34S and {delta}18O values of sulfate in the solution were determined.

Experimental Procedures
Anion-Exchange in Aqueous Solutions
All anion exchange experiments with the PRS probes in aqueous solutions were performed in acid-washed, 1-L beakers covered with watch glasses under laboratory conditions. Plant root simulator probes were inserted into sulfate-containing test solutions without stirring under varying experimental conditions (Table 1 ). The contact (reaction) time between probes and solutions varied from 18 h to 7 d, and the number of probes used ranged from 1 to 5 (Table 1). At the end of the reaction time, the PRS probes were removed from the test solution and rinsed with deionized water to remove residual test solution. The remaining test solutions after anion-exchange reactions and the initial solutions before insertion of the PRS probes were analyzed for the concentrations of sodium and anions (sulfate, nitrate, and chloride). The bicarbonate alkalinity in the solutions was measured by acid titration with 0.05 N HCl.


View this table:
[in this window]
[in a new window]

  		Table 1. Experimental conditions for the anion exchange experiments using commercial plant root simulator (PRS) probes in simulated aqueous solutions (1 L for all experiments).

 
Sulfate-Exchange in a Simulated Soil
Sulfate-exchange experiments with PRS probes inserted in simulated soil solutions were performed in acid-washed, 1-L, Nalgene wide-mouth bottles (9 cm diameter, 19 cm height) at room temperature. Seven bottles were prepared by placing 1.0 kg of sulfate-containing moist soil in each bottle (height of soil, 10.2 cm). In four bottles we simulated water-unsaturated conditions with the moist mineral soils, and in three bottles we created water-saturated conditions by adding 300 mL of deionized water to each bottle. In each bottle, five PRS probes were completely inserted into the mineral soils for 1, 3, and 7 d in the water-saturated soils and 1, 3, 7, and 10 d in the water-unsaturated mineral soils. Thereafter, the PRS probes were removed from the mineral soils, thoroughly scrubbed with a coarse brush, and rinsed with deionized water to ensure complete removal of residual soil. Water-soluble soil sulfate was extracted by adding 700 mL and 1000 mL deionized water to the saturated and unsaturated soils, respectively, and shaking the soil/solution mixtures for 1 h in an overhead shaker. The supernatant solution was filtered through a 0.45-µm membrane filter, and the sulfate concentration and the {delta}34S and {delta}18O values of sulfate in the extracted solution were determined.

Extraction of Sulfate from PRS Probes and Soil Solutions for Isotope Analyses
The cleaned PRS probes after the tests in aqueous solutions and after removal from simulated mineral soils were transferred to Ziploc bags, and 50 mL of 0.5 mol L–1 HCl was added per bag and permitted for 6 h to elute the adsorbed anions quantitatively. Under these conditions, PRS probes require a 1-h elution to remove ≥95% of the adsorbed ions from the ion exchange membrane (Duarte, 2002). After 6 h, the PRS probes were removed from the bag, and the eluted solution was poured into a clean beaker. The eluted sulfate was precipitated as BaSO4 by adding 5 mL of 10% BaCl2 solution.

Dissolved sulfate in the initial and remaining test solutions used for anion exchange experiments without and with mineral soils was precipitated as BaSO4 by adding 5 mL of 10% BaCl2 solution after acidification of the solutions to a pH value between 3 and 4 with dilute HCl. For solutions with sulfate concentrations below 20 mg L–1, the solution volume was reduced via slow evaporation in a beaker on a hot plate to preconcentrate sulfate before precipitation of BaSO4 because of its solubility of 2.3 mg L–1. Precipitated BaSO4 was recovered via filtration with 0.45-µm membranes, washed thoroughly with deionized water, and dried before sulfur and oxygen isotope analyses.

Concentration and Isotope Analyses
The concentrations of sodium and anions (sulfate, nitrate, and chloride) in the solutions were determined using atomic absorption spectrometry (PerkinElmer Analyst 100; PerkinElmer, Waltham, MA) and liquid chromatography, respectively. The analytical uncertainties were better than ±5%.

For sulfur isotope ratio determinations, the BaSO4 was converted to SO2 in an elemental analyzer and swept with a He stream into an isotope ratio mass spectrometer (VG Prism II, Manchester, UK) (Giesemann et al., 1994). All isotope ratios are expressed as delta ({delta}) values in per mil ({per thousand}):

Formula
where [34S/32S] is the ratio of the numbers of 34S to 32S atoms in a sample and a standard (Vienna Cañon Diablo Troilite), respectively. Calibration and normalization for all measurements was performed by repeatedly analyzing three IAEA reference materials (IAEA-S1, {delta}34S = –0.3{per thousand}; IAEA-S5, {delta}34S = 0.49 ± 0.3{per thousand}; IAEA-S6, {delta}34S = –34.05 ± 0.3{per thousand}) (Coplen and Krouse, 1998). Analytical precision and accuracy for sulfur isotope measurements, including sulfate extraction, BaSO4 precipitation, gas preparation, and isotope ratio measurement, was ±0.3{per thousand} (SD of 10 laboratory standard measurements).

Oxygen isotope ratios (18O/16O) of sulfate were determined on CO generated from BaSO4 using a Finnigan MAT TC/EA pyrolysis reactor at 1450°C coupled to a delta plus XL isotope ratio mass spectrometer in continuous flow mode. Oxygen isotope ratios are also expressed using the {delta}-notation. Calibration and normalization for the {delta}18O values was performed using three international reference materials (NBS 127, {delta}18O = 9.3 ± 0.4{per thousand}; IAEA-S5, {delta}18O = 12.0 ± 0.2{per thousand}; IAEA-S6, {delta}18O = –11.0 ± 0.2{per thousand}) (Coplen et al., 2002). The analytical precision and accuracy for {delta}18O values of sulfate was ±0.5{per thousand} (SD of 10 laboratory standard measurements).


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Anion Exchange in Aqueous Solutions
We empirically determined various parameters that control the sulfate exchange characteristics and the extent of sulfur and oxygen isotope fractionation by PRS probes during sulfate exchange reactions in different aqueous solutions.

Sulfate Exchange by PRS probes
Table 1 summarizes the experimental conditions used to examine the ion exchange characteristics of sulfate on the PRS probes, including contact (reaction) time between probes and solutions (18 h to 7 d), the number of probes used (1–5), and the initial sulfate concentration in the test solutions (1–100 x 10–2 mmol L–1). Figure 1 shows the removal percentage and the amount of sulfate exchanged by the PRS probes versus initial sulfate concentration. The removal percentage and the amount of exchanged sulfate increased with increasing reaction time and number of probes used (Fig. 1). Although the amount of exchanged sulfate increased with increasing initial sulfate concentration, the percentage of sulfate removal was constant (8.2 ± 2.3, 50.6 ± 2.1, and 95.7 ± 3.0% at run no. 1, 2, and 3, respectively) (Fig. 1). These results indicate that the anion exchange behavior of sulfate onto the PRS probes in aqueous solutions is mainly governed by a diffusion-controlled ion exchange mechanism. The extent of sulfate removal by the PRS probes depends on reaction time and number of PRS probes used (= surface area).


Figure 1
View larger version (19K):
[in this window]
[in a new window]

  		Fig. 1. Removal efficiency (in %) and uptake of exchanged sulfate (in mg) onto plant root simulator (PRS) probes in aqueous sulfate solution.

 
Sulfur isotope ratios of sulfate exchanged onto the PRS probes throughout the experiments were analyzed and compared with the {delta}34S values of the dissolved sulfate in the test solutions (Fig. 2 ). In the experiments with >90% sulfate removal, the average {delta}34S value of sulfate was 15.7 ± 0.3{per thousand}, which is identical to that of the dissolved sulfate ({delta}34S = 15.6{per thousand}) within the analytical measurement uncertainty. In the experiments that achieved ~50% sulfate removal, the average {delta}34S value of sulfate on the PRS probes was 15.2 ± 0.4{per thousand}, which was within 0.4{per thousand} of that of the dissolved sulfate. The {delta}34S values of the remaining sulfate in solution was not significantly different from that of sulfate on the PRS probes (average difference, –0.5 ± 0.8{per thousand}), which indicates that there is no evidence for significant isotope fractionation associated with sulfate exchange if >50% of the sulfate has exchanged. In the experiment with <20% sulfate exchange, the {delta}34S values for sulfate on the PRS probes ranged from 14.7 to 15.5{per thousand} (Fig. 2), and the average {delta}34S value was within 0.3{per thousand} of that of the dissolved sulfate (one unexplained exception: initial sulfate concentration, 0.01 mmol L–1; 1 PRS probe used; 18 h contact time; {delta}34S value of sulfate on the PRS probe, 13.0{per thousand}). This suggests that even at the initial stage of the sulfate exchange reaction, the sulfur isotope ratio of the sorbed sulfate reflects that of the sulfate solution despite the low efficiency of sulfate removal by the ion exchange membrane.


Figure 2
View larger version (14K):
[in this window]
[in a new window]

  		Fig. 2. {delta}34S values for sulfate exchanged by plant root simulator (PRS) probes from the aqueous sulfate solutions versus removal efficiency (in %).

 
Competitive Anion Exchange
Because natural solutions contain several different anions, including SO42–, NO3–, Cl–, and HCO3–, we tested sulfate exchange reactions in a multicomponent system. These additional exchange experiments with aqueous solutions comprising sulfate, nitrate, and chloride were performed using initial anion concentrations of 0.1, 1.0, and 5.0 mmol L–1 (equal concentrations for all anions) using three PRS probes for 24 h (run no. 4 in Table 1) to examine the selectivity sequence and its impact on the obtained sulfur isotope ratios. The exchanged anionic contents and {delta}34S values of exchanged sulfate versus initial anion concentrations in the test solutions are shown in Fig. 3 . Although nitrate occupied a significant amount of the exchange sites, the sulfur isotope ratios of the adsorbed sulfate were 15.9 ± 0.4{per thousand}, which is similar to that of the dissolved sulfate ({delta}34S = 15.6{per thousand}) independent of anion concentration. This suggests that the competition of other anions, including chloride and nitrate, for sorption sites does not affect the sulfur isotope ratio of the sorbed sulfate.


Figure 3
View larger version (23K):
[in this window]
[in a new window]

  		Fig. 3. Competitive anion exchange and {delta}34S values for sulfate exchanged onto plant root simulator (PRS) probes (run no. 4 in Table 1).

 
Kinetic Tests
Ion exchange reactions generally are reversible and proceed toward an equilibrium state. To understand the kinetics for anion exchange by the PRS probes, a set of six 1-L anionic solutions with sulfate concentrations of 1.0 mmol L–1, nitrate concentrations of 0.3 mmol L–1, and chloride concentrations of 2.8 mmol L–1 were prepared, and three PRS probes were inserted for time periods ranging from 10 min to 20 h (run no. 5 in Table 1). Anion concentrations in the test solutions, including SO42–, Cl–, NO3–, and HCO3–, were determined and are plotted versus reaction time in Fig. 4 . The sum concentrations (µmolc L–1) of sorbed anions (SO42–, Cl–, and NO3–) in the test solutions gradually increased with increasing contact time, indicating their removal through ion exchange onto the probes. The concentrations of released HCO3– in solution also increased progressively due to the release from the probe as a counterion. The amount of released HCO3– into the solutions exceeded that of the sum of sorbed anions onto the probes (Fig. 4). There are two reasons for this discrepancy: (i) diffusive release of HCO3– from the probes and (ii) dissolution of NaHCO3 attached onto the probes. The latter was confirmed through the analysis of Na+ concentrations in the solutions (data not shown). Independent of the contact time, the {delta}34S values of sulfate exchanged onto the probes varied little and had an average value of 15.3 ± 0.5{per thousand}. The difference between the average {delta}34S value of exchanged sulfate and that of dissolved sulfate in the test solutions was –0.3 ± 0.5{per thousand}, which is close to the analytical measurement uncertainty. This indicates that there is no significant sulfur isotope fractionation during sulfate exchange between PRS probes and aqueous solutions.


Figure 4
View larger version (27K):
[in this window]
[in a new window]

  		Fig. 4. Comparison of the absolute concentrations between sorbed anions and released counterion and {delta}34S values for sulfate exchanged by plant root simulator (PRS) probes with respect to reaction time (run no. 5 in Table 1).

 
Sulfate-Exchange in a Simulated Mineral Soil
The extent of sulfur and oxygen isotope fractionation during sulfate sorption on PRS probes in a simulated mineral soil was determined under water-unsaturated and water-saturated conditions to examine their potential use as an efficient sulfate collector for sulfur isotope studies in soils. A set of five PRS probes were inserted vertically into 1 kg soil and were left for contact times of 1, 3, 7, and 10 d at room temperature.

Sulfate Sorption onto Mineral Soil
For the sulfate exchange tests using PRS probes in mineral soils, the anion concentrations and sulfate-sulfur isotope ratios of the solutions obtained from each soil preparation procedure were determined. Alkaline NaOH (pH 12) solutions were used to desorb sulfate from the initial mineral soil. The sulfate content of the initial soil was 9.4 mg kg–1, and the {delta}34S value of the extracted sulfate was –1.7{per thousand} (Table 2 ). The simulated test soil was prepared to have an estimated sulfate content of 37 mg kg–1 (Table 2); this value was calculated as the difference of sulfate concentrations of the solution before and after equilibration with the mineral soil and is assumed to have an uncertainty of ±10%. The measured contents of water-soluble sulfate in the test soil, as determined through extraction using deionized water, was 33 mg kg–1, and the {delta}34S value of the extracted sulfate was 13.4{per thousand} (Table 2).


View this table:
[in this window]
[in a new window]

  		Table 2. Summary of treatment procedures during the test soil preparation and selected chemical and isotopic parameters.

 
Sulfate sorption generally occurs on the surface of hydrous Al and Fe oxides and/or related amorphous phases by a combination of electrostatic attraction and specific surface reactions including ligand exchange. During soil sorption experiments, sulfate is readily desorbed by the addition of a competing anion such as phosphate or bicarbonate, but often some irreversibly adsorbed sulfate cannot be recovered due to slow desorption (Harrison et al., 1989). This is consistent with results reported in Table 2 that indicate that only 33 mg kg–1 of sulfate (4 mg kg–1 less than expected) were recovered by extraction with deionized water. In addition, isotope analyses revealed that some of the extracted sulfate was derived from the initial soils. Although the test soil was pretreated with a (NH4)2SO4 solution with a {delta}34S value of 15.6{per thousand}, the {delta}34S value of water-soluble sulfate extracted from the simulated mineral soils was 13.4{per thousand}. Phosphate extraction recovered a slightly higher amount of sulfate, and the {delta}34S value of the extracted sulfate was 12.6{per thousand}. Van Stempvoort et al. (1990) found no detectable isotopic fractionation associated with sulfate sorption by upland forest Podzols and synthetic goethite based on field and laboratory data. Assuming no sulfur isotope fractionation during sulfate adsorption and desorption, mass and isotope balance calculations based on our data indicate that the extracted sulfate from the test soils contained 13% initial soil sulfate ({delta}34S = –1.7{per thousand}) in the water-soluble extract (~4 mg kg–1) and 17% in the phosphate extract (~6 mg kg–1), with the remainder (~29 mg kg–1) being sulfate derived from the (NH4)2SO4 solution with a {delta}34S value of 15.6{per thousand}. The contribution from soil-derived sulfate was higher than expected based on the water-extractable sulfate content of the clean soil (Table 2), and it is hypothesized that conversion of organic sulfur or oxidation of reduced inorganic sulfur compounds may have occurred.

Effect of Soil Moisture on Sulfate Exchange
Soil moisture content has a significant effect on the ion supply to adsorption sites in soils and the ion exchange resins in our experiments. Figure 5 shows the removal efficiency for water-soluble sulfate as a function of reaction time under different moisture conditions. The percentage of sulfate exchanged onto the probes was generally higher in saturated soils (Fig. 5a) than in unsaturated soils (Fig. 5b). Under saturated conditions, the sulfate removal increased linearly from about 18% (= % of total sulfate amount in the soils) after 1 d to 58% after 7 d (Fig. 5a). Under unsaturated conditions, sulfate removal increased gradually from about 9% after 1 d to 37% after 10 d (Fig. 5b). These differences in ion supply dependent on soil moisture content reflect the relationship between soil moisture content and diffusive flux of ions: As the soil becomes drier, the diffusion path becomes longer and more tortuous (Qian and Schoenau, 2002). Schaff and Skogley (1982) reported similar findings in their study of the moisture effects on diffusion of K, Ca, and Mg to ion exchange resins in bead form. If it is desired to apply the PRS probes in dry soils in the field, it may be advantageous to add deionized water to the dry soil before insertion of the probe to facilitate the exchange of dissolved ions.


Figure 5
View larger version (19K):
[in this window]
[in a new window]

  		Fig. 5. Sulfate uptake (in %) and {delta}34S values of sulfate desorbed from the probes (closed diamonds) and of water-soluble sulfate remaining in the test soil (closed squares) after sulfate exchange (a) at saturated conditions and (b) at unsaturated conditions. The {delta}34S value (13.4{per thousand}) for water-soluble sulfate in the initial test soil is indicated by the dashed line.

 
Sulfur Isotope Composition of Exchanged Sulfate in Test Soils
The {delta}34S values of sulfate recovered from the PRS probes and sulfate in the remaining soil solution after the reaction are summarized in Fig. 5. The {delta}34S values of sulfate recovered from the probes varied between 13.4 and 14.3{per thousand} and were within 1{per thousand} of those of sulfate in the soil solutions ({Delta}{delta}34S at saturated condition, 0.9 ± 0.1{per thousand} [Fig. 5a]; {Delta}{delta}34S at unsaturated condition, 0.6 ± 0.4{per thousand} [Fig. 5b]). The {delta}34S values for sulfate recovered from the probes decreased slightly with increasing reaction time, approaching the {delta}34S of water-soluble sulfate in the initial test soil (dashed line; 13.4{per thousand} in Fig. 5). This indicates that sulfate recovered from the probes has the same {delta}34S value as water-soluble soil sulfate within the analytical uncertainty of the method. Hence, the PRS probes are an excellent tool for assessing the sulfur isotope ratios of water-soluble soil sulfate.

Oxygen Isotope Composition of Exchanged Sulfate
The oxygen isotope ratio of sulfate can be an important parameter for tracing sources and the fate of sulfate in forest soils. Oxygen isotope ratios versus sulfur isotope ratios of sulfate obtained from sorption/desorption onto PRS probes from aqueous solutions (n = 17) and simulated soil solution (n = 7) experiments are shown in Fig. 6 . In the experiments with the aqueous solutions, the average {delta}18O value was 2.0 ± 0.4{per thousand}, which is within 0.9{per thousand} of that of the dissolved sulfate ({delta}18O = 2.9{per thousand}), indicating no significant oxygen isotope fractionation. In the experiments with the simulated mineral soil solutions, the average difference between the {delta}18O value of sulfate recovered from the PRS probes and the water-soluble soil sulfate was –2.1 ± 0.8{per thousand}, most likely due to the contributions of initial soil sulfate as indicated by the sulfur isotope measurements. Oxygen isotope exchange between water and sulfate cannot account for the observed differences because this process is rather slow at low temperatures (Chiba and Sakai, 1985) and would favor the heavier isotope 18O in the sulfate (Lloyd, 1968; Mizutani and Rafter, 1969), resulting in elevated {delta}18O values for sulfate. Our laboratory studies indicate that there is little evidence for significant oxygen isotope fractionation of sulfate during the sulfate exchange reactions and that the obtained {delta}18O values are therefore suitable to differentiate between atmospheric-derived sulfate and sulfate formed via mineralization of organic S in soil environments.


Figure 6
View larger version (16K):
[in this window]
[in a new window]

  		Fig. 6. {delta}34S and {delta}18O values of sulfate retrieved from the probes (open diamonds: in simulated soil solutions; open triangles: in aqueous solutions) and of water-soluble sulfate remaining in the test soil (closed triangles) after sulfate exchange. The isotopic composition of sulfate in the test solution ({delta}34S = 15.6{per thousand}; {delta}18O = 2.9{per thousand}) is shown by the star.

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
Various experiments on the sulfate exchange characteristics and the extent of sulfur and oxygen isotope fractionation during sulfate retention on commercial ion exchange membranes (PRS probe) in aqueous solutions and mineral soil solutions revealed that the probes are highly suitable for collection of soil sulfate for an accurate and precise determination of its isotopic composition ({delta}34S and {delta}18O). In particular, we made the following observations: (i) The sulfate content exchanged onto the PRS probes increases with increasing reaction time, initial sulfate concentration, and the number of probes used, which is in agreement with known ion exchange principles. (ii) The percentage of sulfate removal from solutions was constant regardless of the initial sulfate concentration because of the diffusion-controlled ion exchange mechanism in solution. (iii) The competition of nitrate and chloride in the solution lowers the amount of sulfate retained on the exchange resins but does not affect the isotopic composition of the sulfate. (iv) A considerable proportion (>35% under our experimental conditions) of sulfate sorbed onto the test soil was exchanged by the PRS probes after 10 d even under unsaturated soil conditions. (v) Although the exchanged sulfate contents vary with the experimental conditions, no significant sulfur and oxygen isotope fractionation was observed between sulfate in (soil) solutions and sulfate recovered from the exchange resin. The sulfur isotope ratio of sulfate recovered from PRS probes was within ±1.0{per thousand} of that of sulfate in aqueous solutions or soil sulfate. Average {delta}18O values of sulfate derived from PRS probes deviated by <1{per thousand} from those of sulfate in aqueous solutions and by approximately 2{per thousand} from those of sulfate in soil solutions, the latter possibly being influenced by contributions from soil sulfate.

We recommend the use of PRS probes as an efficient and easy way to collect sulfate from soils and aqueous solutions for an accurate and precise determination of its isotopic composition.


   ACKNOWLEDGMENTS
 
This work was supported by Korea Research Foundation (KRF) through the Environmental Geosphere Research Laboratory (EGRL) of Korea University. Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) to Bernhard Mayer is also acknowledged.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Kwon, J.-S.
Right arrow Articles by Nightingale, M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Kwon, J.-S.
Right arrow Articles by Nightingale, M.
Agricola
Right arrow Articles by Kwon, J.-S.
Right arrow Articles by Nightingale, M.
Related Collections
Right arrow Isotopes
Right arrow Sulfur
Right arrow Soil Chemistry

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome