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DIVISION S-7—FOREST & RANGE SOILS

Adsorption and Recovery of Dissolved Organic Phosphorus and Nitrogen by Mixed-Bed Ion-Exchange Resin

^a McGill University, Dep. of Natural Resource Sciences, Macdonald Campus, 21111 Lakeshore Road, Sainte Anne de Bellevue, Quebec, Canada, H9X 3V9
^b Environmental and Resource Sciences, College of Agriculture, Biotechnology, and Natural Resources, University of Nevada–Reno, Reno, Nevada, 89557

^* Corresponding author (jlangl2{at}po-box.mcgill.ca)

	ABSTRACT

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

 
A laboratory study was conducted to investigate the efficacy of using various mixed-bed anion-cation exchange resins in absorbing dissolved organic and inorganic N (DON and DIN) and P (DOP and DIP). Leachate from foliage of aspen (Populus grandidentata) was passed through columns containing three brands of mixed-bed resin contained in nylon bags, with and without pretreatment of the resins by rinsing with KCl. After leaching, the resins were extracted with either 2 M KCl or 2 M HCl, and recoveries of DIN, DON, DIP, and DOP were calculated. The results showed that all brands of resin adsorbed more DIP (91–98.5%) than DOP (55–70%) and more NO₃ (87–100%) than NH₄ (0–14%) and DON (18–49%). In general, pretreating the resin significantly decreased absorption. The recovery of DIP and DOP was influenced by the pretreatment and the extracting solution whereas the recovery of DIN and DON was problematic because of the release of amine groups. Overall, the use of mixed-bed resin seems adequate for P studies but not for N studies on a short-time scale. Longer exposure periods are needed for studies of N so that the signal/noise (blank) ratio is higher than in this study.

Abbreviations: DIN, dissolved inorganic N • DOC, dissolved organic C • DON, dissolved organic N • DIP, dissolved inorganic P • DOP, dissolved organic P • TP, total P

	INTRODUCTION

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

 
MIXED-BED cation- and anion-exchange resins are used in soil science in many contexts such as studies on nutrient-uptake simulations by plant roots (e.g., Yang et al., 1991; Tran et al., 1992), ion leaching (e.g., Dodd et al., 2000), and N transformation (e.g., Binkley et al., 1986; Kjønaas, 1999a). Until now, most studies have concentrated on DIP and DIN. The adsorption of DOP and DON on ion-exchange resin has not been adequately investigated. The adsorption of DOP and DON by ion-exchange resin would be useful in the context of monitoring P and N transport from soils to streams and lakes because organic constituents are often major components of dissolved N and P in natural waters. These can be a major source of P and N for algae if they are hydrolyzed into their inorganic counterpart (Kuhl, 1974). The question also arises as to whether DON and DOP are absorbed to resins and later hydrolyzed, potentially giving an overestimation of available P and N.

While testing various cation and anion resin beads as a sink for nutrient ions in a Spodosol, Krause and Ramsal (1987) suggested that low-molecular-weight humic substances and organometal complexes might reduce the efficiency of ion-exchange resins to adsorb inorganic ions. Rubaek and Sibbesen (1993) showed that anion-exchange resin beads (Lewatit MP500A) retained all the P from five organic compounds (glycerophosphate, ribonucleic acid, adenosine monophosphate, adenosine triphosphate, and inositol hexaphosphate), and 90 to 100% of the DOP was recovered. However, partial hydrolization of up to 20% of the DOP was also encountered during the recovery process. Using anion-exchange resin membranes, Cooperband et al. (1999) observed that P source, that is, phytase and glucose-6-P, and P concentration influenced the recovery of DOP. Moreover, they found negligible competition for membrane exchange sites between DIP and low-molecular-weight organic acids and humic substances at concentrations expected in the field.

These three studies used mono-bed resin systems (targeting only anions or cations), which perform differently than mixed-bed resin systems (targeting both anions and cations) (Skogley and Dobermann, 1996). To date, little is known about the removal and recovery of DIP, DIN, DOP, and DON in the presence of dissolved organic C (DOC) on mixed-bed ion exchange resin. Therefore, the objectives of this study were to evaluate (i) the removal of DIP, DIN, DOP, and DON by mixed-bed exchange resins in the presence of DOC, (ii) the effects of pretreatment, and (iii) the effects of the stripping solution used to recover P and N from resins.

	MATERIALS AND METHODS

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

 
Foliage Leachate
Leaves of big-toothed aspen were sampled in April 2001 at the Morgan Arboretum of McGill University (Sainte Anne de Bellevue, Quebec, Canada). One kilogram of freshly collected leaves was put in a bottle and agitated with 10 L of deionized water for 2 h. The resulting leachate was passed through a series of filters down to 0.45 µm. Drops of toluene were added to the leachate to counter microbial activity. The leachate was stored at 4°C until utilization.

Ion-Exchange Resin Bags
The resin bags were created by placing 10 g of mixed-bed exchange resin beads (H-OH, 16-50 mesh) in a ball shape between two layers of nylon sheets (20 deniers). A cable was tied at each end to close the bags. Three brands of mixed-bed resin were used: Rexyn I-300 (lot #991860, Fisher Scientific Co., Fair Lawn, NJ), J.T. Baker (lot #N44611, IONAC Chemical Co., Birmingham, NJ) and Amberlite (lot #651, 1:1.5 mixture of types IR-120 and IRA-400 resins, Caledon Laboratories LTD, Georgetown, ON, Canada).¹ For the three brands, the cation-exchange resin is sulfonated, while the anion-exchange resin is aminated. For each brand, half (10) the resin bags were rinsed and shaken for 1 h three times with 100 mL of 2 M KCl in Erlenmeyer flasks, and pH was measured after each rinsing with a pH-meter (Fisher Scientific Co., Accumet Research AR10) equipped with a combination glass electrode. Finally, pretreated bags were rinsed with deionized water.

Leaching Experiment
The leaching experiments were performed using a Centurion (model #24-01, Centurion Inc, Lincoln, NE) automatic vacuum extractor placed in an incubator set at 4°C to simulate field temperature conditions. Fifty milliliters of the foliage leachate was added in an upper syringe and a resin bag was placed in a middle syringe. The foliage leachate was vacuumed at a rate of 4.17 mL h^-1 (12 h extraction) and the extracted solution was collected in lower syringes and then filtered at 0.45 µm. Each pretreatment-leachate combination included ten replicates.

Recovery of Adsorbed Phosphorus and Nitrogen on Ion-Exchange Resin Bags
Half (i.e., five) of the resin bags of each brand and pretreatment combination were stripped with 2 M KCl, while the other half were stripped with 2 M HCl using a dropwise extraction as described by Yang et al. (1991) with the same automatic vacuum extractor mentioned above but at room temperature. This procedure was recommended by Kjønaas (1999b) for complete recovery of adsorbed ions. To evaluate the background levels of P and N on the resin itself, five resin bags for each pretreated and non-pretreated brand were stripped with 2 M KCl. All stripping solutions were filtered at 0.45 µm and their pH was measured.

Analyses
Foliage leachate was analyzed for DIP, as dissolved reactive P, and total P (TP) using a LACHAT Quickchem automatic flow injection ion analyzer (LACHAT Instruments, Milwaukee, WI). Because we found no analytical method describing how to measure P either in a KCl or a HCl matrix on this instrument, we analyzed DIP and TP in extracted solutions and stripping solutions using a Technicon AutoAnalyzer type II (Technicon Instrument Corp., Tarrytown, NY). Nitrate, ammonium, and total N (TN) for all solutions were also measured with the LACHAT Quickchem automatic flow injection ion analyzer. Total P and TN were measured after performing digestions by autoclave on the leachate, extractant, and stripping solutions (Cabrera and Beare, 1993; Centre Saint-Laurent, 1994). For the leachate, the analytical detection limits were 0.02 mg P L^-1 for DIP and TP, 0.02 mg L^-1 for NO₃–N and TN, and 0.01 mg L^-1 for NH₄–N. In the KCl and HCl extracts, the analytical detection limits were 0.05 mg P L^-1 for DIP and TP and 0.1 mg L^-1 for NO₃–N, NH₄–N, and TN. Dissolved organic P was determined as the difference between TP and DIP and DON as the difference between TN and DIN (DIN = NO₃–N + NH₄–N).

Statistics
Percentage of adsorbed amounts of DIP, DOP, NO^-₃, NH⁺₄, and DON were calculated as follows:

[1]

Percentages of recovery of DIP, DOP, NO^-₃, NH⁺₄, and DON were calculated as:

[2]

Comparisons of background content and percentage of adsorption were done using a two-way factorial design with replicates in which resin brand and pretreatment were considered as fixed treatments. Comparison of percentage of recovery was done using a three-way factorial design in which brands, pretreatment and stripping solutions were all considered fixed treatments. These tests were done using the general linear model (GLM) procedure of SYSTAT (SPSS Inc., 2000). Brand multiple comparisons were also included using Fisher's LSD test in SYSTAT but to lighten reading of tables, results from this test will be only discussed in the text. For all statistical analyses, a probability value of 0.05 was used to determine significance, and a probability value 0.01 was considered highly significant.

	RESULTS AND DISCUSSION

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

 
Foliage Leachate Composition
The analysis of five aliquots of the foliage leachate showed that its average concentration of DIP was 0.507 mg P L^-1 with a standard deviation of 0.007 mg P L^-1. The mean concentration of DOP was 0.195 mg P L^-1 with a standard deviation of 0.016 mg P L^-1. The average concentrations of NO^-₃, NH⁺₄, and DON in the foliage leachate were 0.053, 0.068, and 5.627 mg N L^-1 with standard deviations of 0.007, 0.009, and 0.139 mg N L^-1, respectively.

Background Levels of Phosphorus and Nitrogen on Bags
Almost all background levels of DOP and DIP on the resin bags were below the detection limits (Table 1). The different brands of resin had different background levels of N regardless of pretreatment. All resin brands without pretreatment contained appreciable amounts of NO^-₃, NH⁺₄, and DON. Dissolved organic N values were sometimes quite high (up to 14.2 mg N g^-1 dry resin). Without pretreatment, the Amberlite brand had the highest level of NO^-₃ (P 0.01) compared with the J.T. Baker and Rexyn brands, which were not significantly different from each other. No significant statistical differences among brands were observed for NH⁺₄ (Table 2). The Amberlite and J.T. Baker brands had respectively about 15 and 9.5 times greater background DON than the Rexyn brand.

The pH of the extracted solutions was also different among brands whether pretreated or not (Table 1). When not pretreated, the stripped solutions were either very alkaline (Rexyn) or very acidic (Amberlite and J.T. Baker). Pretreating the bags neutralized the pH of the Rexyn solution but had little effect on the other two brands. In the stripping process, an acidic or alkaline pH of the stripping solutions could result in part from the release of OH^- and H⁺ still found on active groups and also from the release of amine that either buffers OH^- from the solution or is partially transformed into NO^-₃ or NH⁺₄, thus modifying the pH. Therefore, one way to solve the problem of N background contamination might be to pretreat the resin until the pH of the rinsing solution is close to the pH of the extracting solution, which would indicate that no further large amounts of amine would then be released. Because the present paper was not designed to test this hypothesis (we pretreated the bags with three 2M KCl rinses only), a complete test of this hypothesis will have to await future experimentation.

Adsorption of Phosphorus and Nitrogen on Resin Bags
The three brands of mixed-bed resin adsorbed between 91.2 and 98.5% of DIP (Table 3). Statistical analysis showed that Rexyn resin was significantly more efficient in adsorbing DIP than the other two brands. When the bags were pretreated, significantly less DIP was absorbed but this decrease was not similar among brands because of highly significant interactions between pretreatment and brands (Table 4). The reduction of sorption varied from 5.7% for Amberlite resin and 7.2% for J.T. Baker resin to only 1.5% for Rexyn resin. A similar situation was also found for DOP absorption in which Amberlite resin significantly adsorbed less DOP than the other two brands. When the bags were not pretreated, the three brands had a similar DOP sorption capacity (68–70%). However, when the bags were pretreated, Rexyn resin adsorbed significantly less DOP than Amberlite and J.T. Baker. Again, significant interactions existed because the decrease of DOP adsorption between pretreated and non-pretreated resin varied among brands: 4.4% for Rexyn, 8% for J.T. Baker, and 13.5% for Amberlite.

Two hypotheses can explain the lower adsorption of DOP and DON compared with their inorganic counterparts. The first hypothesis is that inorganic anions (DIP and NO^-₃) are more competitive for resin exchange sites than organic anions. However, even though we did not test a maximum load, we do not believe that hypothesis is applicable in this case because there were enough exchange sites (in other words, enough resin beads) to adsorb all the P and N present in the foliage leachate. The second hypothesis is based on the assumption that the foliage leachate is probably not only composed of low-molecular-weight organic acids but also partially of larger organic molecules such as humic and fulvic substances. Because Skogley and Dobermann (1996) reported that the ability of organic ions to exchange with inorganic counterions is inversely related to their sizes, it is likely that DON and DOP found within humic and fulvic molecules are less competitive than the anions already occupying the exchange sites and this competitiveness is even less when Cl^- occupies these sites (pretreated bags).

Also, the differences in adsorption between anions (DIP and NO^-₃) and cations (NH⁺₄) could be explained by low affinity of NH⁺₄ for exchange with the sulfonated groups. Because in some cases more NH⁺₄ was released than adsorbed, however, we believe that this is not the primary explanation, and that the lower adsorption of cations compared with anions could be attributed in fact to the high adsorption of the latter. Indeed, active groups on the anion resin are made of amino compounds, which will be released into solution by DIP and NO^-₃. Once in solution, a part of these amino compounds could interfere with NH⁺₄ analysis as will be discussed at greater length below.

Recovery of Phosphorus and Nitrogen from Bags
Recovery of DIP was significantly influenced by the resin brand and the pretreatment (Tables 5 and 6). No difference of DIP recovery between Amberlite and J.T. Baker resins was observed but less DIP was recovered from Rexyn resin compared with Amberlite. Moreover, even though pretreating the bags decreased their ability to adsorb DIP, pretreating the bags helped to recover up to 35% more DIP (P 0.01). The stripping solution had no significant effect on DIP recovery and this could be attributed to highly significant interactions between pretreatment and stripping solution.

For N, almost all results from the recovery process are difficult to interpret. Indeed, recoveries were either >100% (more release than adsorbed in the first part of the experiment) or <0% (more in background level than recovered) (Table 5). The recovery of NO^-₃ and DON is influenced by the brand (P 0.05), the pretreatment (P 0.01) and the stripping solution (P 0.01) whereas NH⁺₄ recovery is influenced by none of these factors (Table 6). In detail, Amberlite resin released less NO^-₃ than J.T. Baker and Rexyn resins and the latter released more DON than the other two brands (P 0.01). Also, pretreating the resin significantly decreased the recovery of NO^-₃ whereas it significantly increased the recovery of DON. The stripping solution had an influence only on the recovery of NO^-₃ (less was recovered when HCl was used). Finally, no interactions among factors were observed for NO^-₃ and NH⁺₄ whereas interactions among brands, pretreatment, and stripping solutions were observed at P 0.01 for DON.

The problems of over and under-release could be explained by the fact that the active groups on the anion resin are made of amino compounds. Using untreated mixed resin bags, Binkley and Matson (1983) showed that these amino compounds are also released during the stripping process and may interfere with the NH⁺₄ analysis. To counter this problem, Hart and Binkley (1984) suggested rinsing the bags once with a 1M KCl solution to eliminate this interference. However, Kjønaas (1999b) observed that rinsing resin once in a solution of 2 M NaCl was unsuccessful in removing all N impurities on Amberlite resin. Just as Kjønaas (1999b) stated, we do not believe that rinsing the bags once with 1M KCl would solve the problem. We believe that these problems of over- and under-release of N are related more to the fact that, even after three rinsings with 2M KCl, release of amino groups on resin, whatever the brand, is still large (Table 1). Therefore, during the stripping process, part of these amino groups could be transformed to NH⁺₄ and NO^-₃ and the use of HCl might even further enhance these transformations compared with KCl.

For field studies, the high N background levels mean that the use of mixed-bed resin is not suitable to obtain a "snapshot" of N ion supply such as Qian and Schoenau (1996) did by using resin membranes. Therefore, mixed-bed resin should be deployed in the field for long-term studies so that the signal/noise ratio is higher (blanks are less significant relative to treated resins). The resins can then provide information on medium to large time scale dynamics of N supply such as its slow release from mineralization (Qian and Schoenau, 2002). Mixed-bed resins contained within zero tension lysimeters (resin lysimeters) have been used to measure leaching rates of N in soils when they are left in the field for long periods of time (6–12 mo), allowing greater rates of N accumulation relative to background (blank) levels (Johnson et al., 2001; Susfalk and Johnson, 2002). The resin lysimeters clearly show differences in mineral N leaching, including NH⁺₄, when treatment effects are pronounced and leaching rates are high (Johnson et al., 2001).

The high sorption of DOP by mixed-bed resin suggests that the latter could be a useful tool to monitor DOP movement in soils. However, two problems could arise from the sorption of DOP and DON. First, if present in high concentrations in the soil solution, DON and DOP could contribute to saturation of the mixed-bed resin and change its function from a sink to a dynamic exchanger, and thereby trigger the removal of previously sorbed nutrients. Second, it is possible that sorption of DOP and DON on mixed-bed resin could be problematic for the study of inorganic nutrients because this kind of resin could sustain microbial growth as suggested by Schnabel (1983). This means that microorganisms could mineralize organic molecules sorbed on the resin, and thus creates an environment around the resin richer in inorganic dissolved nutrients than in the bulk solution. Finally, results of the present study prove that, to determine background (blank) levels of N, one should always extract mixed-bed resin that has not been previously placed in the field, as has been suggested by Kjønaas (1999b).

	CONCLUSION

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

 
This study showed that monitoring subtle patterns of either inorganic or organic dissolved N in soils should not be performed using mixed-bed resin because of large background contamination. This problem could be solved by monitoring pH during the rinsing step, and could also be overcome by allowing resins to accumulate N over longer periods of time (better signal/noise ratio). On the other hand, background levels of DIP and DOP are low and the use of mixed-bed resin seems to be adequate for the study of short-term movement and availability of DIP and DOP in soils.

	ACKNOWLEDGMENTS

 
We thank Hélène Lalande (McGill University) and Mary Miller (Desert Research Institute) for helping us solve many analytical difficulties. We thank Dr. François Courchesne and Dr. William Hendershot for equipment loan. We also thank anonymous reviewers who provided relevant comments, which helped to significantly increase the quality of this paper. This work was supported by FCAR, Nevada Agricultural Experiment Station, NSERC and Water Ecosystem Sustainability.

	NOTES

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

 
¹ Trade names are mentioned to provide specific information, and are not an endorsement of the product by the authors.

Received for publication February 4, 2002.

	REFERENCES

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

 

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This Article Abstract 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 ISI Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Langlois, J. L. Articles by Mehuys, G. R. Search for Related Content PubMed Articles by Langlois, J. L. Articles by Mehuys, G. R. Agricola Articles by Langlois, J. L. Articles by Mehuys, G. R. Related Collections Nitrogen Phosphorus Soil Chemistry