DETERMINATION OF SULFATE, NITRATE, AND CHLORIDE IN THROUGHFALL USING ION-EXCHANGE RESINS SAMUEL M. SIMKIN1 , DAVID N. LEWIS1,2 , KATHLEEN C. WEATHERS1∗ , GARY M. LOVETT1 and KIRSTEN SCHWARZ1 1 Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545 U.S.A.; 2 Present address: University of Massachusetts, Amherst, MA 01003 U.S.A. (∗ author for correspondence, e-mail:
[email protected]; fax: (845) 677–5976)
(Received 11 June 2003; accepted 29 October 2003)
Abstract. Throughfall, the solution that falls from the forest canopy, is an important and commonly measured flux in forest ecosystem studies. Throughfall water and chemistry are highly variable spatially, requiring large numbers of collectors to quantify it. This and the fact that the solution can be chemically unstable make throughfall sampling very labor intensive, thus we have developed a method to reduce the field labor portion of this effort. Our throughfall collection method uses compact ion exchange resin columns that need only be collected every 1–2 months. The resin columns are subsequently extracted with 1.0 M potassium iodide (KI), releasing anions back into solution, with extraction efficiencies > 94% for sulfate, nitrate, and chloride. The extracts are analyzed by ion-chromatography (IC) to determine the total microequivalents of anions per unit area of collector surface collected over the period of resin column exposure. This ion exchange resin method was originally developed for a project in which we needed to deploy over 300 throughfall collectors to quantify throughfall variability across mountainous terrain with heterogeneous vegetation. Keywords: atmospheric deposition, heterogeneous terrain, ion exchange, landscape, resin column, sulfate, throughfall
1. Introduction Throughfall is an important and commonly measured flux in forest ecosystem studies. It is useful for canopy process studies (Draaijers et al., 1997; Eaton et al., 1973; Lindberg and Lovett, 1992; Lovett and Lindberg, 1993), solution chemistry profiles (Piirainen et al., 1998; Rustad and Cronan, 1995), ecosystem budgets (Likens et al., 2002), and atmospheric deposition studies (Erisman et al., 1994). Throughfall is spatially heterogeneous, varying at local scales (Kostelnik et al., 1989; Manderscheid and Matzner, 1995; Robson et al., 1994), as well as responding to predictable landscape features such as elevation, vegetation type, and edges (Lindberg and Owens, 1993; Lovett et al., 1997, 1999; Shubzda et al., 1995; Weathers et al., 1992, 1995, 2001). Other atmospheric deposition indices have also been shown to be a function of landscape features (Lovett and Kinsman, 1990; Weathers et al., 2000). Therefore, quantifying fine resolution spatial patterns in throughfall requires high-density sampling. Water, Air, and Soil Pollution 153: 343–354, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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Conventional aqueous throughfall samples can be voluminous and are usually collected on an event or weekly basis in order to maintain chemical integrity of the sample. The high frequency of collections, combined with the large number of samples required to capture spatial throughfall variability, make conventional throughfall sampling a very labor-intensive procedure in the field. A collection technique that requires less field labor per collector permits large arrays of throughfall collectors to be serviced by a small number of people. Preserving the chemical integrity of ions of interest for a longer period of time and therefore reducing the frequency of sampling provides additional savings in field labor expenditures as well. Ion-exchange resins are one tool that could help save labor in the field. Ionexchange resins have been used in hundreds of studies involving soil and other environmental samples (Skogley and Dobermann, 1996), but there have been only a few studies that measured throughfall chemistry using ion-exchange resins. In a study of throughfall in pine forests, nitrate and ammonium were extracted with 2 M KCl and measured using colorimetric methods (Fenn et al., 2002). In a study focused on 15 N composition of throughfall, ammonium and nitrate resin extractions were made using K2 SO4 and 2 M KCl, respectively, followed by steam distillation and measurement by colorimetric methods (Garten, 1992). In a study of throughfall under grassland canopies, sodium and potassium was extracted from resin with 1 M HCl and measured with flame emission methods, and ammonium, nitrate, sulfate, magnesium, and calcium were extracted with 1 M KCl and measured with colorimetric methods (Van Dam et al., 1991). However, the methods listed above prohibit the analysis of chloride (Cl), nor were they designed to extract sulfate, which is an anion that has been used successfully to quantify total deposition (e.g., Weathers et al., 1992, 2001; Lovett, 1994). A method that measured chloride in throughfall would be advantageous for studies in coastal areas with potentially high chloride inputs in throughfall as a result of fog drip and sea salts. Here we describe an anion-resin throughfall collector, field and laboratory tests of the technique, and laboratory methods for chemical analysis of sulfate, nitrate, and chloride. We compare our field data with anion fluxes from a wet deposition monitoring site of the U.S. National Atmospheric Deposition program (NADP) (site ME98) where we co-located our anion-resin collectors. 2. Materials and Methods 2.1. OVERVIEW The resin collector consists of a funnel attached to an ion-exchange resin column (Figure 1) to capture anions from the solution that passes through the column. The ionic bonds between anions in the sample and the positively charged exchange sites on the resin produce a more chemically stable sample than anions in solution, which allows for monthly rather than event-based sampling. The compact ion
ANION RESIN THROUGHFALL METHOD
Figure 1. Resin throughfall collector design.
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TABLE I Experimental design Treatment (µeq)
# of reps
Anion conc. (µeq L−1 )
Treatment vol. (L)
0 (DDI H2 0) 50 125 250 500 2500 5000
3 3 3 3 3 3 3
0 2500 2500 2500 2500 2500 2500
0.05 0.02 0.05 0.1 0.2 1 2
exchange resin samples can be conveniently retrieved from inaccessible locations, so bulky water samples need not be collected and subsampled or transported from the field. Subsequent extraction of resin columns in the lab releases anions back into solution. These extracts can then be analyzed by ion-chromatography (IC) to determine the flux of anions over the period of resin column exposure. 2.2. P REPARATION OF RESIN COLUMNS We used Dowex Monosphere 550-A (OH− form) anion-exchange resins (Ultrapure water grade, 32 mesh) with styrene-divinyl benzene matrices and quaternary ammonium functional groups. The resins were prepared by rinsing 1 L batches of resin with three bed volumes (approximately 3 L) of double de-ionized (DDI) water (minimum 18.1 mega-Ohm resistance) to remove both excess NaOH and ‘fines’ (broken beads of resin). The resins were poured into disposable polypropylene BioRad Econo-Pac columns (20 mL capacity). Prior to adding resin, we replaced the 1 mm thick filter at the bottom of the column with a 30–35 micron pore size, 3 mm thick filter since this appeared to decrease physical clogging of the column. Columns were filled to 20 mL with a slurry of anion resin in DDI water, taking care to eliminate air pockets from the resin bed. Since this resin has a fixed exchange capacity of 1100 µeq L−1 , each column had ∼ 22,000 µeq of exchange capacity. Resin columns were protected from heat and light exposure by storing them in a cool, dark location before deployment to the field. 2.3. T EST OF ANION CAPTURE EFFICIENCY FROM SIMULATED THROUGHFALL We prepared a mixed anion solution containing 2500 µeq L−1 each of sulfate (SO4 2− ), nitrate (NO3 − ), and chloride (Cl− ) using DDI water and (NH4 )2 SO4 , KNO3 , and NaCl salts. We then poured different volumes of this solution through
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resin columns to achieve the following six treatments, each with three replicates: 50, 125, 250, 500, 2500, and 5000 µeq SO4 2− , NO3 − and Cl− (Table I). DDI water (50 mL) was poured through three replicate resin columns as a control (0 µeq anion). This range of anion loading levels encompasses the range of throughfall flux that we would expect for a one-to-two month period at most locations in the northeastern United States. The anion loading concentration of 2500 µeq L−1 is higher than we would expect for an event sample and therefore provides a conservative test of the capacity of the resin to capture ions. For the first replicate of each treatment and the control, the solution poured through the resin column was collected to determine its remaining anion content, if any. Both the anion loading solution and the ‘post resin’ solutions were diluted 10-fold and analyzed along with resin extracts using the methods below. 2.4. KI- EXTRACTION OF ANION - LOADED RESINS Potassium chloride (KCl) is commonly used elsewhere as a resin-extractant for nitrate (Kjonaas, 1999; Fenn et al., 2002; Templer et al., 2003), but since we wished to quantify chloride fluxes we used potassium iodide (KI) instead. Anions captured on the resin columns were extracted into solution with ACS purity 1.0 M KI. Prior to the first extraction, the resin was transferred from its column to a polypropylene extraction cup. Each resin sample received three successive extractions; each extraction was shaken on an orbital shaker table for 30 min at 120 rpm in 50 mL of the KI. At the end of each extraction period, the supernatant from the KI solution was decanted through the plastic BioRad Econo-Pac column into an amber high-density polyethylene bottle, leaving the resin at the bottom of the extraction cup. The second and third extractions were performed similarly with 50 mL of KI; all three extractions were decanted into the amber bottle, producing a composite sample of KI extract solution with a total volume of 150 mL. Amber bottles were used to protect the photosensitive KI solution from transforming to elemental iodine. As anion-exchange resins and KI solution can contain trace levels of anions, three resin column ‘blanks’ were extracted and analyzed along with the anion loaded resin samples. The average ‘blank’ resin column values for SO4 2− , NO3 − and Cl− were calculated and subtracted from the total anions recovered from each loaded resin column. 2.5. I ON - CHROMATOGRAPH SET- UP AND ANALYSIS Because the ionic strength of a 1.0 M KI sample matrix was high enough to interfere with IC peak resolution, each composite sample received an approximately 30-fold dilution with DDI water using an automatic pipette with an accuracy of 0.1% and a CV of 0.22%. At this dilution, analyte anions eluted at normal times and had normal peak shapes and heights compared to anion standards in a water matrix. To stay above the IC’s low detection limit of approximately 0.2 mg L−1 , blanks received a smaller, approximately 20-fold dilution.
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Diluted resin extracts were analyzed on a Dionex DX-500 Ion-Chromatograph, fitted with Dionex Ion-Pac AG9-HC (guard) and AS9-HC (analytical) columns. We used a 12.5 mM sodium carbonate (Na2 CO3) eluent made from ACS purity salts and DDI water. The eluent flow rate was 1.6 mL min−1 , the sample injection volume was 25 µL, and peaks were collected over a 21-min elution window. Background conductivity of the eluent was auto-suppressed with a current of 100 mA, and a temperature compensation factor of 1.7%/◦ C was applied to conductivity detection to minimize baseline drift caused by changes in ambient temperature. Samples were bracketed by standard curves, with no more than 25 samples between curves. Standards for SO4 2− , NO3 − , and Cl− were made from a 1000 mg L−1 stock solution (purchased from Alfa-Aesar), using Class A pipettes and volumetric flasks. For every 25 samples, we included a standard-as-sample, an independent NIST-traceable ‘known’ solution, and two sample re-runs to check the accuracy and precision of IC analysis and dilution methods. 2.6. R ESIN THROUGHFALL COLLECTORS Each anion-resin collector consisted of a plastic, unpigmented HDPE funnel (20.32 cm outer diameter), short sections of R-3603 Tygon tubing, and HDPE connectors attached to a resin column (see description of column above). These plastic pieces were cleaned with DDI water, assembled and placed in reclosable plastic bags for transport to the field. In the field, a 1 m section of ABS plastic pipe (11/2 inch outer diameter) was driven into the ground and a hole was drilled just above ground level to ensure adequate drainage from the pipe. Resin columns, prepared in advance in the same fashion as the lab tests described above, were attached to the bottom of the funnel assembly and lowered into the ABS pipe until the funnel could be leveled and seated on top of the pipe (Figure 1). A foam collar was wrapped around the neck of the funnel to insure a good friction fit between the funnel and the pipe. 2.7. C OLOCATION OF RESIN BULK COLLECTORS AT NADP PRECIPITATION MONITORING SITE
Although the resin collector we describe here was designed primarily for throughfall, it can also be used as a bulk-deposition collector, which samples wet deposition plus a small proportion of dry deposition. For comparison purposes we co-located two anion resin collectors in an open field
