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DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Stability of Ion Exchange Resin Under Freeze–Thaw or Dry–Wet Environment

^a Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 68583
^b USDA-ARS, Lincoln, NE 68583

^* Corresponding Author (mmamo3{at}unl.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Ion exchange resins have widely been used in mineralization studies of organic materials. However, the stability of resin (anionic and cationic) under changing physical environmental conditions is not well known. Our objective was to evaluate N and P adsorption or desorption characteristics of resins exposed to freeze–thaw or dry–wet cycles. Mixed bed resins (1:1 oven-dry mass strong base anion A464-D and strong acid cation C-249) were subjected to 0, 1, and 30 freeze–thaw or dry–wet cycles. To accomplish the dry–wet cycles, fresh resin was kept in a forced-air oven at 25 (±2)°C for 28 h and rewetted to initial moisture condition for 20 h. To accomplish the freeze–thaw cycle, fresh resin was frozen for 16 h and thawed to room temperature for 8 h daily. At the end of the freeze–thaw or dry–wet cycles, resin was equilibrated with 3.2 mM L^–1 NH₄–N, 3.2 mM L^–1 NO₃–N, or 0.97 mM L^–1 PO₄–P for a period of 1 h. Dry–wet cycles induced desorption of N and P associated with shrinkage of resins and expulsion of interstitial liquid. At the highest dry–wet cycle, 3.3, 0.35, and 0.15% of the total adsorbed PO₄–P, NH₄–N, and NO₃–N was desorbed, respectively. Scanning electron microscopy (SEM) revealed that the dry–wet or freeze–thaw cycles did not alter the physical integrity of these resins. Freeze–thaw cycles had no effects on N and P adsorption or desorption characteristics of resins of the specific resins used in this study. Ion exchange resins used for in situ nutrient monitoring should be screened using similar techniques to assess its adsorption and desorption stability and physical integrity to fluctuating environmental conditions. Resin types and stability should be mentioned when comparisons are made to other studies.

Abbreviations: SEM, scanning electron microscope

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SOLID PHASE ION exchange resins are used to extract plant-available nutrients in situ and/or in the laboratory as a possible substitute for liquid extraction (Skogley and Dobermann, 1996; Giblin et al., 1994; Yang et al., 1991). Ion exchange resins are also widely used to measure nutrient movement or fluxes in agricultural fields (Zou et al., 1992; Sakadevan et al., 1994; Lehmann et al., 2001). In most soil studies, resins are used as a nutrient sink rather than a source to monitor seasonal fluctuations in nutrient status over the growing season.

Resin offers the advantage of representing the actual soil environmental conditions (i.e., temperature and moisture) and minimizes the need and cost of periodic soil sampling throughout the growing season. Under field conditions, resins have been used in the soil for periods ranging from 4 (Eghball, 2000) to 48 wk (Giblin et al., 1994). In the field, resins are exposed to a variety of changing physical conditions including freeze–thaw or dry–wet cycles. Kjonaas (1999) found that drying reduced adsorption of NO₃–N and NH₄–N by anionic and cationic resins, respectively. Kjonaas (1999) also observed no detectable effect of freezing on resin stability. Lehmann et al. (2001) have observed that the amount of NO₃–N and NH₄–N removal from solution was specific to resin type.

Studies have also shown minimal effect of storage conditions on recovery of ions adsorbed on resin (Skogley et al., 1997; Kjonaas, 1999). However, knowledge of the stability of resins under changing physical environments for in situ measurements of nutrient mineralization year-round in regions with soil freezing and fluctuating soil moisture is not well documented. The hypothesis of this experiment was that cycles of alternating freeze–thaw or dry–wet may not shift resin ion-adsorption/desorption characteristics or alter its effectiveness as a nutrient trap. For example, as the resin experiences dry–wet cycles in the summer, its efficacy as a nutrient sink should not be reduced or nutrients that had been previously trapped should not be released during the dry–wet cycles. The objective of this experiment was to measure changes in N and P adsorption and desorption of mixed bed resins that have undergone several freeze–thaw or dry–wet cycles.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Resin Characteristics and Preparation
Anionic and cationic resins, strong base anion A464-D and strong acid cation C-249 (US Filter, Warrendale, PA) were used in the experiment. These resins were selected because they are currently being used to detect seasonal N mineralization in plots treated with manure (Eghball, 2000). Both anion and cation resins had a styrene-divinyl benzene matrix with an exchange capacity of 1.2 and 1.9 mol_c kg^–1, respectively (Table 1).

Adsorption Characteristic of Mixed Resins
Treatment of Mixed Resins with Freeze–Thaw or Dry–Wet Cycles
The mixed resin was exposed to 0, 1, and 30 freeze–thaw cycles. For each freeze-thaw cycle, a sealed container of mixed resin in triplicate was placed in a freezer at –20 (±3)°C for 16 h then thawed for 8 h at 21(±3)°C. Upon completion of a given freeze–thaw treatment cycle, the resin was stored at 7 (±3)°C until the adsorption isotherm experiments.

The dry–wet treatment also consisted of 0, 1, and 30 dry–wet cycles. The term wet in this experiment represents resin at fresh water content (509 g kg^–1 oven-dry resin). The dry resin was achieved by air-drying resin in open containers placed in a forced air oven at 25 (±3)°C for 28 h (oven-dry moisture ranges from 290 to 380 g kg^–1). Drying was done at 25 (±3)°C to minimize resin shrinkage and irreversible chemical change before initiating the adsorption isotherm experiment on the resin. Dry condition was assumed when the oven-dry water content reached 55 to 70% of the water content at field capacity. These levels of drying were considered to be realistic especially for the widespread irrigated field conditions found in Nebraska. Under these field conditions, irrigation will be triggered when the water content reaches 60 to 70% of the water content at field capacity; however, we expect lower water content under dryland field conditions. We do not expect the resin to desiccate because even at 1.5 MPa (wilting point), the soil is near 99% relative humidity (Hanks and Ashcroft, 1980).

For the wetting cycle, the amount of water lost to reach the dry cycle was calculated and de-ionized water was added to the containers to bring the water content to the original fresh resin water content. The container was sealed and the resin and water left to equilibrate for 20 h before the next cycle. Upon completion of the final dry-wet cycle, de-ionized water was added to the starting moisture content and the resin was stored at 7 (±3)°C until the adsorption isotherm experiments.

Adsorption Isotherm of Treated Mixed Resin
The adsorption solution ion concentrations were selected based on soil measurements in field experiments that used the same mixed bed resins (Eghball, 2000). Two solutions were prepared, one containing NH₄–N and NO₃–N at 3.2 mM L^–1 and the other containing PO₄–P at 0.97 mM L^–1. The solution to resin ratio needed for the adsorption experiment was established by preliminary experiments where resins were mixed with 3.2 mM L^–1 NH₄–N, 3.2 mM L^–1 NO₃–N, or 0.97 mM L^–1 PO₄–P solutions for 24 h by varying resin mass with constant solution volume. Equilibrium solutions from each ratio (volume/oven-dry mass), 5:1, 10:1, 25:1, 70:1, 240:1, 600:1, 800:1, and 1200:1 were analyzed for NH₄–N and NO₃–N or PO₄–P. The 800:1 solution/resin ratio (volume/oven-dry mass) was selected for the NH₄–N and NO₃–N adsorption and a 600:1 ratio (volume/oven-dry mass) was selected for the PO₄–P adsorption. The ratio selection is based on ion adsorption >10 and <90% to ensure accurate and precise measurement of ion in solution and quantification of adsorption.

The mixed resins previously treated with cycles of freeze–thaw or dry–wet were equilibrated with solutions containing the N or P ions for periods of 1 h. The 1-h adsorption period was selected because testing of equilibrium periods (1, 24, 48, and 72 h) showed that equilibration was reached after 1 h (data not presented). The N adsorption was achieved by mixing 0.239 g of mixed resin (oven-dry basis at 105°C) with 200-mL of 3.2 mM L^–1 N solution in 250-mL polyethylene bottles. The P adsorption was achieved by mixing 0.335 g (oven-dry) of mixed resin with 200-mL of 0.97 mM L^–1 PO₄–P solution in 250-mL polyethylene bottles. The control mixed resin (not exposed to freeze–thaw or dry–wet cycles) was included simultaneously in the adsorption isotherm experiment. At the end of the adsorption period, the solution was decanted and analyzed for NH₄–N and NO₃–N using a Lachat (Zellweger Analytics, Milwaukee, WI) system and PO₄–P was analyzed using the molybdate blue method (Murphy and Riley, 1962).

Desorption Characteristics of Mixed Resins
Presaturation of Mixed Bed Resin with PO₄–P, NH₄–N, and NO₃–N
This experiment was conducted to determine if dry–wet or freeze–thaw cycles affected desorption of previously trapped resin N and P. In this case, the resins were subjected to different time lengths of freeze–thaw or dry–wet cycles after preloading resin with NO₃–N, NH₄–N, or PO₄–P. Sixty-five grams of fresh resin (32 g oven-dry equivalent) were saturated in 19 L of 0.97 P mM L^–1 solution (as KH₂PO₄) or 25 L of 3.2 mM N L^–1 (as NH₄NO₃) for 48 h at solution/resin ratios of 600:1 and 800:1, respectively. The 48-h equilibrium was used for ion saturation of resin with N and P since there was agitation to allow for fast equilibrium. After saturation, excess solutions were decanted and any entrained salt solution removed by placing the ion saturated resin on a filter paper covered funnel and rinsing it with 40 L of de-ionized water. The resin was air-dried to its original fresh water content by pouring it on an absorbent paper towel and monitoring its mass. To estimate the amount of P or N preloaded on the resin before the environmental cycles and subsequent desorption experiment, the resin was extracted by three consecutive 20 min extractions using 2 M HCl (Dobermann et al., 1997). The final total extractant volume and the mass of saturated resin used were 600:1 for PO₄–P and 800:1 for NO₃–N and NH₄–N. The extract was neutralized to near neutral pH with NaOH and analyzed for PO₄–P or NH₄–N and NO₃–N. The extractable amount of PO₄–P, NH₄–N, and NO₃–N preloaded on the resin was 245, 655, and 1070 mM kg^–1, respectively. Freeze–thaw or dry–wet treatments were applied on the resins preloaded with N or P in cycles of 0, 1, and 30 as described above. At the end of the treatment cycles, 0.835 g oven-dry equivalent weight resin was extracted with 20 mL of de-ionized water for 1 h. The solution was separated from the resin to measure desorbed PO₄–P or NH₄–N and NO₃–N.

Resin Physical Integrity
Preliminary observation during determination of resin water-holding properties indicated that fresh resin expanded during saturation and shrunk during drying. To evaluate swelling of fresh resin, 13 to 15 g of fresh resin were placed into a 50-mL graduated-flask; 25 mL distilled water was added, and the total volume was immediately recorded. The initial volume of fresh resin was determined by the difference between the total volume minus 25 mL. The resin was left in the flask for 24 to 48 h to allow for saturation. After saturation, the water was decanted carefully from the flask (not to loose any resin grains), and the excess was removed from the resin by drying with a paper towel. Immediately, the volume of saturated resin was determined with the same procedure of determining the volume of fresh resin. The swelling was expressed (in percentage) as the ratio of the difference of volume between saturated minus fresh resin and the volume of fresh resin.

Scanning electron microscopy was performed on fresh resin and resin treated with 10 dry–wet or freeze–thaw cycles. Six to eight resin samples were attached with carbon adhesive tabs onto stubs and coated with 20- to 30-nm thick gold/palladium in a Hummer II (Anatech Ltd., Union City, CA) or an Emitech K575X (Emitech Products Inc., Houston, TX) sputtering unit. Specimens were examined and digitized with a Hitachi 3000N (Hitachi, Japan) SEM operated at 15 kV. It was observed that the Hummer II sputtering unit cracked the surface of the resin and therefore images prepared with the Emitech K575X unit are presented.

Data Analysis
The experimental design was a randomized complete block (RCB) with three replications. Freeze–thaw or dry–wet cycle effects on resin N and P adsorption or desorption were compared with the untreated (check) resin using a GLM model on SAS version 8 (Statistical Analysis Software, Statistical Analysis System, 1999). Mean separation was done using the Duncan–Waller test within each nutrient. Significance level was set at a probability of 5%.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Moisture Condition of Resin
Fresh resin water content was 509 (g kg^–1), which was slightly drier than the water content at 0.033 MPa (Table 1). The majority of resin water was interstitial, that is, within the resin granule rather than between the granules. The significant proportion of interstitial water explained why it took a long drying time (28 h) to deplete 25 to 43% of the fresh resin water content. As will be discussed in a later section, the interstitial water property was very important in controlling both shrinking and swelling and desorption of ions as the resins underwent drying and wetting.

Adsorption of Nitrogen and Phosphorus on Ion Exchange Resins
Freeze–Thaw and Dry–Wet Cycles
The concentration of NO₃–N (3.2 mM) mixed with the resin was three times higher than the PO₄–P (0.97 mM L^–1), thus, NO₃–N adsorption by resin was four to five times more than PO₄–P under both dry–wet and freeze–thaw cycles (Fig. 1 and 2) . Under field conditions, more NO₃–N is found in soil solution compared with PO₄–P; therefore, NO₃–N recovery by resin is expected to be higher than PO₄–P. Although a 1:1 ratio was used to make the mixed bed resins and the concentration of NO₃–N and NH₄–N added were similar; there was less NH₄–N than NO₃–N adsorbed. The lower NH₄–N adsorption in comparison with NO₃–N is not well understood. Perhaps higher resin impurities of NH₄–N (92 mM kg^–1) compared with NO₃–N (3 mM kg^–1) may have contributed to the differences.

View larger version (25K): [in this window] [in a new window]   Fig. 1. Effect of dry–wet cycles on ion exchange resin adsorption of PO4–P, NO3–N, and NH4–N. Bars with the same letter for each ion are not significantly different at P < 0.05.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Effect of freeze–thaw cycles on ion exchange resin adsorption of PO4–P, NO3–N, and NH4–N. Bars with the same letter for each ion are not significantly different at P < 0.05.

Desorption of Phosphorus and Nitrogen from Ion Exchange Resins
Dry–Wet Cycles
The amount of PO₄–P desorbed increased as the dry–wet cycle increased (Fig. 3) . The amount of NO₃–N and NH₄–N desorbed increased only at the highest (30) dry–wet cycle. Under each dry–wet cycle (Fig. 3), PO₄–P desorption was higher than that of NO₃–N. This may be due to higher affinity of resin toward NO₃–N than PO₄–P. Compared with NO₃–N, the increase in PO₄–P desorption was much higher with increasing dry–wet cycles. The PO₄–P desorbed were 2.7, 4.5, and 5.1 times of NO₃–N desorption at 0, 1, and 30 dry–wet cycles, respectively. There was also higher desorption of NH₄–N under the dry–wet cycles compared with NO₃–N.

View larger version (15K): [in this window] [in a new window]   Fig. 3. Effect of dry–wet cycles on ion exchange resin desorption of PO4–P, NO3–N, and NH4–N. Bars with the same letter for each ion are not significantly different at P < 0.05.

Freeze–Thaw Cycles
Unlike the dry–wet cycles, the freeze–thaw cycles did not significantly affect the desorption of PO₄–P, NO₃–N, or NH₄–N (Fig. 4) . The swelling and shrinking mechanism of resins that resulted in expelling nutrient solution out of the interstitial sites during shrinking (from wet to dry or from frozen to thawed condition) may have been greater during the dry–wet cycles compared with the freeze–thaw cycles. This was confirmed by the expansion of fresh resin after saturation with distilled water. The average expansion of resin volume was 18.8% (ranged from 13.3–23.2%). The influence of resin shrink–swell upon dry–wet cycles and the subsequent ion desorption implied that the integrity of the resin may be affected. The physical integrity of resin surfaces was then observed with SEM.

View larger version (22K): [in this window] [in a new window]   Fig. 4. Effect of freeze–thaw cycles on ion exchange resin desorption of PO4–P, NO3–N, and NH4–N. Bars with the same letter for each ion are not significantly different at P < 0.05.

View larger version (55K): [in this window] [in a new window]   Fig. 5. Scanning electron microscopy of (a) fresh resin, (b) resin treated with dry-wet cycles, and (c) resin treated with freeze–thaw cycles.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
For the resin used in this study, adsorption of N and P was not changed after resins had undergone several dry–wet or freeze–thaw cycles. Freeze–thaw cycles did not induce desorption of ions from resin, however, dry–wet cycles induced desorption of N and P (<3.3% of total) associated with shrinking of resins. The extent of desorption after a prolonged dry–wet cycle was not due to disintegration of resin structure. While ion exchange resins are good tools to employ in the field for in situ short- or long-term monitoring of nutrients, the specific resin type should be screened using similar techniques to assess its adsorption and desorption stability and physical integrity to fluctuating environmental conditions before deployment.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A contribution of the University of Nebraska Agricultural Research Division, Lincoln, NE 68583. Journal Series No. 14151.

Received for publication February 14, 2003.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

• Dobermann, A., M.F. Pampolino, and M.A.A. Adviento. 1997. Resin capsules for on-site assessment of soil nutrient supply in lowland rice fields. Soil Sci. Soc. Am. J. 61:1202–1213.[Abstract/Free Full Text]
• Eghball, B. 2000. Nitrogen mineralization from field-applied beef cattle feedlot manure or compost. Soil Sci. Soc. Am. J. 64:2024–2030.[Abstract/Free Full Text]
• Giblin, A.E., J.A. Laundre, K.J. Nadelhoffer, and G.R. Shaver. 1994. Measuring nutrient availability in arctic soils using ion exchange resins: A field test. Soil Sci. Soc. Am. J. 58:1154–1162.[Abstract/Free Full Text]
• Hanks, R.J., and G.L. Ashcroft. 1980. Applied soil physics. Springer-Verlag, New York.
• Kjonaas, O.J. 1999. Factors affecting stability and efficiency of ion exchange resins in studies of soil nitrogen transformation. Commun. Soil Sci. Plant Anal. 30:2377–2397.
• Lehmann, J., K. Kaiser, and I. Peter. 2001. Exchange resin cores for the estimation of nutrient fluxes in highly permeable tropical soil. J. Plant Nutr. Soil Sci. 164:57–64.
• Murphy, J., and J.P. Riley. 1962. A modified single solution method for determination of phosphate in natural waters. Anal. Chim. Acta 27:31–36.
• Sakadevan, K., M.J. Hedley, and A.D. Mackay. 1994. An in situ mini lysimeter with a removable ion exchange resin trap for measuring nutrient losses by leaching from grazed pastures. Aust. J. Soil Res. 32:1389–1400.
• Skogley, E.O., and A. Dobermann. 1996. Synthetic ion-exchange resins: Soil and environmental studies. J. Environ. Qual. 25:13–24.
• Skogley, E.O., A. Dobermann, J.E. Yang, B.E. Schaff, M.A.A. Adviento, and M.F. Pampolino. 1997. Methodologies for resin capsules: Capsule storage and ion recovery. Sciences of Soils, Rel. 2. http://www.hintze-online.com/sos/1997/Articles/Art3 (verified 25 Nov. 2003).
• Statistical Analysis System. 1999. SAS/STAT user's guide. Version Windows 8. SAS Institute Inc., Cary, NC.
• Yang, J.E., E.O. Skogley, and B.E. Schaff. 1991. Nutrient flux to mixed-bed ion exchange resin: Temperature effects. Soil Sci. Soc. Am. J. 55:762–767.[Abstract/Free Full Text]
• Zou, X., D.W. Valentine, R.L. Sanford, and D. Binkley. 1992. Resin-core and buried-bag estimates of nitrogen transformations in Costa Rican lowland rainforests. Plant Soil 139:275–283.

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