2066
Anal. Chem. 1984, 56,2066-2069
(5) Hambrick, G. A.; Froelich. P. N.; Andreae, M. 0.; Lewis, B. L. Anal. Chem. 1984, 56, 421. (6) Cutter, G. A. Anal. Chim. Acta 1978, 9 8 , 59. (7) Andreae, M. 0.; AsmodB, J.-F.; Foster, P.; Van? dack, L. Anal. Chem. 1981, 53, 1776. (8) Lee, D. S. Anal. Chem. 1982, 5 4 , 1682. (9) Thompson, M.; Pahlavanpour, B. Anal. Chim. Acta 1979, 109, 251. (IO) Hashimoto, Y.; Kobayashi, R.; Chiou, K. Y.; Winchester, J. W.
"Proceedings of VIth World Congress on Air Quality"; Sepic: France, 1983; Vol. 1, p 329.
RECEIVED for review March 26,1984. Accepted May 21,1984. This work was supported in part by the Science Foundation, Grant No. OCE-8200931.
Ion Chromatographic Determination of Nitrate and Sulfate in Natural Waters Containing Humic Substances Gyorgy Marko-Varga,* Istvan Csiky, and Jan Ake Jonsson University of Lund, Department of Analytical Chemistry, P.O. Box 740, S-220 07 Lund, Sweden
Determinations of anions In natural waters by various methods, lncludlng Ion chromatography, are disturbed by the lnevltable presence of humlc substances. A cleanup column, packed with a chemically bonded amine material (Nucleosll 5 NH,) was found to effectively remove lnterferlng humic substances. This materlal Is superior to other types of chemlcally bonded materials for the Intended purpose. No Influence was found from humlc substances In concentrations up to 45 mg/L on Ion chromatographic analysls of nitrate and sulfate (10-100 mg/L) after passage through the cleanup column. The method was applled to the determination of nitrate and sulfate in sol1 lysometer waters.
In the study of the pollution of natural waters, the concentrations of nitrate and sulfate are important factors. The levels of these ions are related to such problems as acidification and eutrophication of the environment. A review of the commonly used methods for nitrate and sulfate determination in soil and natural waters can be found in a recent book (1). Nitrate is determined with a nitrate-selective electrode, spectrophotometrically, or from the total nitrogen content of the sample. Sulfate can be determined by turbidimetry, nephelometry, colorimetry, or titrimetry or from the total sulfur content. All these analytical methods are to varying extents influenced by the presence of humic substances in natural waters. In recent years, the technique of ion chromatography (IC) has developed into a convenient, sensitive and efficient means for anion analysis. The determination of small amounts of nitrate and sulfate in various matrices is already routine (2). However, the application of IC to the analysis of anions in natural waters is seriously impeded by the presence of humic substances, as those substances are strongly adsorbed on the IC column, eventually destroying it irreversibly. Humic substances (humic acid) are the major organic constituents of soil and they occur in almost all terrestrial and aquatic environments. The chemical structure is not known in detail, but it is known to involve aromatic polymers of high molecular weight, to which an unusally high number of functionalities are attached, such as carbonyl, carboxyl, hydroxyl, amine, phenolic, and quinone groups (3). Most of these groups contribute to the ion-exchange properties of the humic substances. The acidic character is generally accepted, and it is mainly due to carboxyic and phenolic groups (4). The humic substances form colloidal complexes with silica par-
Table I. Materials for Adsorption Columns material
type0
a
5 SA 5 Cia 100-5 5 SR 5 NH2
b C
d e
functional group
-so3-CldH3-
-SOH -N(CH3)3+
-"*
adsorption capacity'
0.005 0.01 0.01 1.51
13'
a All materials are of the Nucleosil series (Macherey-Nagel and Co., Duren, FRG). 'Milligrams of humic aid per gram adsorbent. 'This refers to "fraction 2" (see the text).
ticles. The ions in the water thus take part in a complicated equilibrium system, the details of which are largely unknown. Several studies of the removal of organic matter from water, as well as the separation of the organic matter into different fractions, have been presented. Techniques such as ultrafiltration (5, 6), steric exclusion chromatography (7), adsorption on Amberlite XAD-2 (8),adsorption on a bipolar ion exchanger (9),and adsorption or DEAE-cellulose (10) have been employed. Some of these methods can probably (10) also be used to remove the interference from humic substances prior to determination of ions. To selectively adsorb the interfering humic substances we suggest the use of a short cleanup column, packed with a bonded-phase material for liquid chromatography. After passage through this column, the anions in the water sample can be subjected to conventional ion chromatographic analysis.
EXPERIMENTAL SECTION Chemicals. Stock solutions of nitrate and sulfate, as well as the phthalate solution used as mobile phase, were prepared from their corresponding potassium salts. The pH of the phthalate solution was adjusted with NaOH. The water used was purified with a Milli-Q/RO-4 unit (Millipore, Bedford, MA). Humic acid (Fluka AG, Buchs, Switzerland) with a molecular weight of 600-1000 and an ash content of 10-1570 was used to prepare reference solutions of humic substances. The solution was stirred for 20 h at 60 OC and filtered through a 0.65-pm membrane filter (Millipore). Elemental analysis of an aliquot of the reference humic solution, evaporated to dryness, gave the following results: C, 43.5%; H, 3.0%; N, 0.95%. The content of dry matter was 45 mg/L. Thus, the content of dissolved organic carbon (DOC) was 20 mg carbon per L. Chromatographic Apparatus. The chromatographic system consisted of a pump (Constametric 111, LDC, Riviera Beach, FL), a loop (100or 500 pL), an injector (Model 7000, Rheodyne, Cotati, CA), a column (see below), two detectors in series (a conducto-
0 1984 American Chemical Society 0003-2700/84/0356-2066$01.50/0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
0” O8
o’2 0,o
I
1
t I I
I
0.0
0,5
by the materials d and e, while the major part (fraction 2) was considerably retarded. As fraction 1 is not appreciably adsorbed on the -N(CH3)3+ material which is analogous to the separation column, it is unlikely to interfere with the analysis. Under the conditions actually used for separation of nitrate and sulfate, fraction 1moves with the solvent front through the separation column as expected and causes no interference. Consequently, only fraction 2 disturbs the ion chromatographic analysis and must be removed. This is best accomplished with the amine material (e). It is possible to load ca. 13 mg of humic acid (5.6 mg carbon) per g of the adsorbent before saturation. This corresponds to roughly 500 injections of 500-pL samples containing 50 mg/L of humic substances. The adsorption behavior observed in Figure 1 is in good agreement with the generally accepted structure of the humic acid (3,4): As the humic substances are preferably adsorbed on the amine material, it can be supposed that acidic groups are mainly responsible for the adsorption. Hydrophobic interactions, which would have caused adsorption on the CIS material seem to have no appreciable influence. Basic or cationic groups expected to adsorb on the -SiOH material and on the -SO3- material, respectively, can also be ignored. The relatively strong adsorption onto the quaternary ammonium material indicates the presence of anionic groups in the humic acid. Several other mechanisms could contribute to the strong adsorption of the humic acid on the amine material: I t is well-known (3) that humic substances bind considerable amounts of metal ions. This suggests the possibility for formation of chelates with the amine groups of the absorbent (11,12),further increasing the adsorption. Also, the formation of covalent bonds (e.g., between carbonyl groups in the humic acid and amine groups on the adsorbent, leading to Schiff’s bases) cannot be excluded. The adsorption of nitrate and sulfate on material e was studied by injection of a solution of 10 ppm of each ion before the breakthrough experiment. The mobile phase was water. After saturation of the resin with humic acid as described above, the column was again equilibrated with water and a new injection was made. The adsorption of humics on the column only slightly increased the affinity for nitrate and sulfate; both ions virtually traveled in the solvent front. The low affinity of material e for the anions is expected as only a small fraction of the amine groups of the ion exchanger are protonated in pure water (13). However, with increased ionic strength, protonation is considerably increased due to electrostatic shielding, causing appreciable retention of anions. Therefore, it is necessary to employ water as an eluent to separate anions from humic acid. The use of the same eluent as was used to separate the anions (5 X lo4 M phthalate, pH 5.5) caused unacceptable retention in the cleanup column. Material d (-N(CH,)S+), analogous to the separation column, strongly adsorbs nitrate and sulfate even with water as an eluent due to its permanent charge and can consequently not be used for a cleanup column. Regeneration of the -NH2 adsorbent, totally loaded with humic acid, was attempted by washing with ethanol. Results were not encouraging, only about 10% of the adsorbed material could be removed. Also, regeneration with phthalic acid solutions was unsuccessful. Regeneration with stronly acidic or alkaline solutions might destroy the bonded-phase material and was accordingly not tried. Analysis of Solutions of Nitrate and Sulfate Containing Humic Substances. A series of model solutions with 10, 30, 50, 70, or 100 mg/L of both NO3- and S042-,additionally containing 0.00,0.09, and 0.90,4.5,9.0,27, or 45 mg/L of reference humic acid, were prepared. By use of the procedure described in the Experimental Section, nitrate and
/ u i
,Y, 1,0
1,5
I
2.0
2.5
3,O
3,5
4.0
4,5
5.0
log m / p g
Figure 1. Breakthrough curves of reference humic acid SOlUtiOnS on five different adsorbent materials a-e (see Table I). Observed concentration C relatlve to steady-state plateau concentration Co vs. amount m of humic acid.
metric detector, “Conductomonitor”, LDC, and a variablewavelength UV detector, “Spectromonitor 111”, LDC), and a dual-channel recorder (Model 2066, LKB, Bromma, Sweden). A reporting integrator (Model 3390 A, Hewlett Packard, Avondale, PA) and a fraction collector (Model 2112, LKB) were used in some of the experiments. Columns. For the adsorption of humic substances we used five stainless steel columns (50 mm X 4.9 mm id.) filled with 0.8 g of 5-pm HPLC materials according to Table I. The materials were structurally similar, differing only in the type of the chemically bonded organic chains and functionalities. Anions were separated by an anion exchange resin (Dionex, Sunnyvale, CA) packed into a glass column (5 mm i.d., bed length 50 mm) (Pharmacia Fine Chemicals, Uppsala, Sweden) which permits the length of the resin bed to be easily adjusted. Separation of Anions. Potassium hydrogen phthalate, 5 X M, pH 5.5, was used as mobile phase. The flow rate was 2 mL/min and the conductometric detector was used in the “absolute” mode (10 pmho full scale deflection). The conductance of the eluent was electronically offset. Under the chosen conditions, nitrate and sulfate were well resolved from other anions expected to be found in natural waters. Typical k’-values were as follows: HC03-, HzPO4-, C1-, NO; < 0.5; NO3- = 1.4; S042- = 7.2. Breakthrough Curves of Humic Acid Solutions. The adsorption capacities for humic substances were determined by pumping the humic reference solution through the column until a breakthrough was sensed by the UV detector set at 225 nm. Determination of Nitrate and Sulfate in the Presence of Humic Substances. A 500-pL water sample was injected onto the adsorption column and the eluate was collected in a fraction collector. The fraction (ca. 2.5 mL) which contained the anions, as sensed by the conductometric detector, was evaporated to dryness (80 O C overnight) and redissolved in 250 pL of water. Aliquots of 100 pL were analyzed on the IC system using the ion-exchange separation column and conductance detection as described above.
RESULTS AND DISCUSSION Adsorption of Humic Substances. Three demands are placed on a cleanup column for the present purpose: the adsorption of disturbing humic material must be essentially complete, the capacity of adsorption must be sufficient for a practical number of analyses, and the ions under study should not be significantly adsorbed. The adsorption capacity, i.e., the amount of humic acid which can be loaded onto 1 g of adsorbent material until the breakthrough occurs, can be found from the breakthrough curves in Figure 1. Values are listed in Table I for the various materials. The adsorption capacities depend strongly on the type of adsorbent, being largest for the weak anion exchange material e (-NH2) followed by the strong anion exchange material d (-N-(CH,),+). The other materials had insufficient adsorption capacities. As can be seen from the nearly horizontal line in Figure 1, a part of the humic acid (fraction 1)was only weakly adsorbed
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 12,OCTOBER 1984
Table 11. Percent Recovery of Nitrate and Sulfate from Solutions Containing Humic Acid ion concn, mg/L
0.09
% recovery for concn of humic acid added (mg/L) 0.9 4.5 9 27
45
102 101 107 101 106
98 109 104 106 101
105 99 103 106 108
94 108 105 108 104
103 103 99 106 102
94 105 106 102 108
96 95 109 101 101
94 112 102 98 97
94 101 100 99 97
90 93 94 91 94
106 93 90 90 90
110 103 93 93 93
NO310 30 50 70 100 SO>10 30 50 70 100
Table 111. Determination of Nitrate and Sulfate in Surface Water Samples samples
rP
10-32 2-32
4 3
NO; RSDc meanb 6.8 17.0
Number of replicates. standard deviation, %.
10 4.1
2
sotmeanb
RSD‘
4.2 5.9
3.1 8.5
*Concentration, mg/L.
Relative
sulfate were determined in the solutions. The calibration curve was constructed from the solutions without added humic acid. A separate analysis of 45 mg/L reference humic acid (without anions added) gave the result: NO3-, 3.7 mg/L and S042-,5.2 mg/L. This is considered as the background level of these ions in the reference humic acid. Thus, significant amounts of NO3-and SO>- are added to the model solutions together with the humic acid. Considering these background levels, the percent recovery (concentration found/expected concentration) was calculated for all model solutions analyzed and is presented in Table 11. Each value in the table is the mean of six determinations (triplicate analyses of two independent sample preparations). The relative standard deviation within each set of six values was typically 2-3% both for NO3- and for S042.-. Most of the recoveries in Table I1 are not significantly different from 100%. For sulfate, recoveries tend to be about 10% low at high concentrations of humic acid while an opposite tendency can be noted for nitrate. One reason for this behavior might be the competition between sulfate and nitrate (in our model solutions always present in equal concentrations) and other ions present in the humic acid for the binding sites of the humic substances. It is well-known that divalent anions are more strongly adsorbed than monovalent ones (3). Thus a large amount of humic acid might significantly adsorb added sulfate while releasing nitrate. An approach, similar to ours, could probably be used to investigate these little known ion-exchange equilibria. Analysis of Surface Water Samples. As an application of the method, nitrate and sulfate were analyzed in two samples of soil lysometer water taken at different places in a forest in southern Sweden. These samples were strongly colored, indicating a high level of humic substances. Particles were removed by centrifugation before analysis. After passage through the cleanup column the samples were colorless. The results of the determinations are shown in Table 111. The concentrations found are difficult to verify as virtually no reliable reference methods for such samples exist. Figure 2 shows representative examples of chromatograms from the analysis of a water sample and a corresponding blank. Similar chromatograms were obtained from the model solu-
Y 0
70 time
1
20
I min I
Figure 2. Determination of nitrate and sulfate in water containing humic substances: (A) natural water sample (2-32);(B) water blank. Peak O : (5.9 identities: 1, NO,- (17 mg/L); 2, unknown (see text); 3, S
mg/L).
tions. In all chromatograms, an unknown peak (peak 2 in the figure) appears. This is probably due to a disturbance in the phthalate protonation equilibrium created by the injection of aqueous samples. The retention time of the peak is independent of pH and the concentration of the eluent, but the intensity is affected by these factors. It is possible to adjust the eluent composition so the peaks of interest are separated from the “ghost peak”. Similar effects were pointed out for the carbonate system by Stevens (14).
CONCLUSIONS A cleanup column, packed with chemically bonded amine material for HPLC, has been shown to be useful for the selective removal of disturbing humic substances prior to anion analysis by ion chromatography. For all the development work described here, the same separation column was used. We could not notice any change in color of the resin nor any deterioration of the chromatographic properties after more than 500 injections and thus conclude that the separation column was effectively protected from the disturbing fraction of humic acid. It can be supposed that a similar technique to the one described here also can be used to discriminate metal ions associated with humic substances from free metal ions. Registry No. NO3-, 14797-55-8; Sod2-,14808-79-8; Nucleosil 5 NH2, 91423-91-5; water, 7732-18-5. LITERATURE CITED (1) Smith, K. A,, Ed. “Soil Analysis”; Marcel Dekker: New York, Basel, 1983. (2) Fritz, J. S.; Gjerde, D. T.; Pohlandt, C. “Ion Chromatography”; Dr. Alfred Hutig Verlag: Heldelberg, Basel, New York. 1982.
Anal. Chem. 1984, 56,2069-2073 (3) Bohn, H. L.; McNeal, E. L., O'Connor, 0. A. "Soil Chemistry"; John Wiley and Sons: New York, Chichester, Brisbane, Toronto, 1979. (4) Dubach, P.; Metha, N. C.; Jakab, T.; Martin, F.; Roulet, N. Geochim. Cosmochim. Acta 1984, 28, 1567-1578. (5) Buffle, J.; Deladoey, P.; Haerdi, W. Anal. Chim. Acta 1978, 701, 339-357. (6) Chian, E. S. K.; DeWalle, F. E. Environ. Sci. Technoi. 1977, 7 1 , 158- 163. (7) Miles, C. J.; Brezonik, P. L. J. Chromatogr. 1983, 259, 499-503. (8) Mantoura, R. F. C.; Riley, J. P. Anal. Chim. Acta 1975, 7 6 , 97-106. (9) Wolf, F.; Laqua, E. \/om Wasser 1977, 4 8 , 273-281.
2089
(IO) Miles, C. J.; Tuschall, J. R.; Brezonik, P. L. Anal. Chem. 1983, 55, 410-41 1. (11) Helfferich, F. "Ion Exchange"; McGraw-HIII: New York, 1962; Chapter 5. (12) Berge, D. G.; Going, J. E. Anal. Chim. Acta 1981, 723, 19-24. (13) Shaw, D. J. "Introduction to Colloid and Surface Chemistry", 3rd ed.; Butterwofths: London, 1980; Chapter 7. (14) Stevens, T. S. I n d . Res. Dev. 1983, 25(9), 96.
RECEIVED for review January 26,1984. Accepted May 14,1984.
Separation of Tellurium from Gold(1I I), Indium, Cadmium, and Other Elements by Cation Exchange Chromatography in Hydrochloric Acid-Acetone F. W.E. Strelow National Chemical Research Laboratory, CSIR, P.O. Box 395, Pretoria 0001, Republic of South Africa
Trace amounts and up to 120 mg of tellurium can be separated from 1.3 g amounts of gold and 0.6 g amounts of Indium and cadmlum by absorptlon from 0.2 M HCI containing 60 % acetone and elutlng these elements with the same reagent from a column of AG 50W-X8 cation exchange resin of 200-400 mesh partlcie slze. The retalned teilurlum Is effectively eluted wlth 1 M aqueous HCI. Separatlons are sharp and quantltatlve. Only between 1.3 and 3.0 pg of gold was found in the tellurlum fraction when 1.3 g was present origlnally. Bl( 111), Sn( IV), Se( IV), As( I I I ) , Pt( IV), Pd( I I), and Rh( I I I ) are also separated together wlth gold, though some of these elements show extended low level talllng of concentratlons of about 1 ppm or less. Relevant elutlon curves and results for the analysls of synthetic mixtures are presented.
The separation of tellurium from other elements by ion exchange chromatography has received relatively little attention so far. It has been shown by Aoki that water will elute selenium from a strongly acid cation exchanger after adsorption from less than 0.05 M acid solution. Most of the tellurium then can be eluted with aqueous 0.3 M HC1 ( I ) . The tellurium is retained not very strongly and only microgram amounts can be separated because of the very limited solubility of tellurium(1V) oxide in very dilute hydrochloric acid or water. Other methods include anion exchange in oxalate media for separation from antimony and tin (2), anion exchange in acetate for separation from selenium (3), cation exchange in 0.1 M HC1 for separation from platinum metals ( 4 ) ,elution with NHIOH from a cation exchange column for separation from copper, nickel, iron, and lead ( 4 ) , cation exchange in 12 M HBr for separation from selenium ( 5 ) ,and anion exchange in LiCl for separation from lead and gold (6). Probably the most generally useful of the methods described is anion exchange in aqueous HC1 (7,8). Selenium can be eluted with 3 M HC1 followed by tellurium with between 0.2 and 0.5 M HC1. Many other elements should be eluted together with selenium, some should accompany tellurium partially, while others should still be retained according to available information on distribution coefficients.
No method seems to have been described for the separation of traces as well as larger amounts of tellurium from large amounts of gold(II1) and many other elements forming stable chloride complexes such as indium, bismuth(III), cadmium, and the platinum metals. In the anion exchange method in HC1 (7,8) mentioned above gold(II1) would still be retained when tellurium(1V) is eluted with 0.2 M aqueous HC1. Yet gold(II1) absorbed as chloride complex on a strongly basic resin is notorious for the difficulties encountered when its quantitative recovery is attempted using an elution procedure. Furthermore, it would be much more attractive to retain tellurium and elute gold when traces of tellurium have to be separated from large amounts of gold. One possibility would be to elute gold(II1) from a cation exchange resin with very dilute aqueous hydrochloric acid, about 0.1 M or less. Unfortunately the solubility of tellurium(1V) is rather limited in this medium and the distribution coefficient in 0.1 M HC1 also is not very large with a value of about 39 (9). In addition a small but significant part of the gold(II1) is retained by the resin (9-11). Beamish and co-workers found losses of a few tenths of a percent even under the most favorable conditions (11). Though it is believed that a reduction of gold(II1) is involved, the actual mechanism of the adsorption is unknown. It has been shown that the amount of gold retained by the resin becomes negligible when 90 or 60% acetone is present in the eluting agent (9,12). A procedure has been described for the separation of gold(II1) from cadmium, indium, and other elements which are retained by the column (12). In order to elute cadmium and indium together with gold(III), the acetone concentration or the concentration of HC1 or both will have to be increased. In addition tellurium still must be retained relatively strongly. Available data on distribution coefficients (9) show that coefficients for teUurium(1V) increase significantly with acetone concentration at low concentration of HC1, reach a maximum, and then decrease again. The most favorable conditions for separating tellurium(1V) from gold(1111,indium, cadmium, bismuth, tin(IV), the platinum metals, selenium(IV), and some other elements seem to be at about 0.2 M HC1 containing 60% acetone. The distribution coefficient of tellurium(1V) is about 51 under these conditions as compared with a coefficient of 17 in aqueous 0.2 M HCl. The possibility for using this eluting agent to separate tellurium
0003-2700/84/0356-2069$0 1.50/0 -. . . 0 1984 American Chemical Society
