EnVkOn. SCL Techno/. 199 1, 25, 1262- 1266
Simultaneous Determination of Chromium( I I I ) , Aluminum( I I I), and Iron( 11) in Tannery Sludge Acid Extracts by Reversed-Phase High-Performance Liquid Chromatography Antonio Lopez,+ Taddeo Rotunno,t Francesco Palmisano,t Roberto Passlno, * * +Giovanni Tiravanti,t and Pier Giorglo Zambonid
Consiglio Nazionale delle Ricerche, Istituto di Ricerca Sulle Acque, Via Reno, 1, 00185 Roma, Italy, and Dipartimento di Chimica, Laboratorio di Chimica Analitlca, Universitti degli Studi di Bari, Trav. 200 Re David, 4, 70126 Bari, Italy
A new chromatographic method for the simultaneous determination of Cu(II), Zn(II), Cr(III), Al(III), and Fe(II1) or Fe(I1) has been developed. The method is based on precolumn formation of stable metal-8-hydroxyquinoline chelates, their separation on a C-18 revened-phase column by HPLC, and their UV-vis detection a t 400 nm. The experimental conditions giving the highest chelate yields resulted: pH 4.2; T = 90 "C; reaction time 30 min; reaction mixture composition methanol (66.7% )/acetonitrile (13.3%)/water (20%) (v/v/v) plus 10 mM &hydroxyquinoline. The mobile-phase composition giving the best resolution of Cr(II1)- and A1(III)-8-hydroxyquinoline chromatographic peaks has been optimized by the simplex algorithm: acetonitrile (13.5%)/methanol (29%)/O.l acetate buffer pH 6.8 (13.5%) (v/v/v) plus 100 mM 8hydroxyquinoline. The method has been applied to synthetic solutions as well as, after sample pretreatment on XAD-7 resin, to real sulfuric acid extracts of tannery sludges. As for this latter matrix, additional information on Cr and Fe oxidation states has been obtained, combining the proposed method with atomic absorption spectroscopy and ion chromatography.
Introduction The disposal of tannery sludges releasing organic and inorganic toxic contaminants is a serious threat for the environment. In particular, in Italy, every year 120000 tons of such sludges, as dry solid, are wasted. Because of their high chromium content [Cr(III) ranges from 1% to 5% while Cr(V1) is practically absent ( I ) ] ,according to the current Italian regulations, the disposal of these sludges calls for controlled landfilling. On the other hand, due to the progressive exhaustion of such landfills, land disposal of such sludges is increasingly attractive for economic as well as logistic reasons. However, spreading tannery sludges on land (raw or after composting with urban solid wastes) is not allowed as chromium could affect metabolism and/or accumulate in living organisms. The unavoidable oxidation of Cr(II1) to more soluble and toxic Cr(V1) species, furthermore, could cause groundwater pollution (2). Accordingly, several efforts aimed at the removal and recovery of chromium from tannery sludges have been made (2-4). Particularly interesting is a recently developed process based on metals extraction from the sludges by acid treatment followed by selective ion exchange for recovering chromium from the obtained extracts. The recovered chromium is than recycled back to the tanning process ( 5 ) . For checking the different steps of such process, simultaneous determination of the predominant metallic
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'Istituto di Ricerca Sulle Acque. Universith degli Studi di Bari.
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species (Cr, Al, and Fe) in the different process streams is needed. Previously, several methods had been proposed for the analytical characterization of tannery wastes (6-8). For metallic species, the most common technique is atomic absorption spectroscopy (AAS) employing flame or graphite furnace atomization. Several drawbacks, however, are related to AAS, including the matrix effect, with consequent need of background correction and difficulty in the choice of compatible standards, and the impossibility of simultaneous analysis of different elements. On the other hand, even though inductively coupled plasma emission spectroscopyovercomes some of these drawbacks, it requires expensive instrumentation. Consequently, alternative methods for determining metallic species in tannery sludges and liquors have been investigated: Zn and A1 have been quantitated by a combined two-stage titration with EDTA (9); Cr has been quantified by colorimetry (IO),volumetric analysis (II), ion selective potentiometry (12),ion chromatography (13), and gel filtration chromatography (14). Furthermore, in simpler matrices, the gel chromatographic behavior of trace amounts of Cr(V1) and hydrolyzed Cr(II1) has been investigated by Osaki et al. (15). As for multielement simultaneous analysis, both ion chromatography and high-performance liquid chromatography (HPLC) are considered the most suitable techniques (16, 17). Increasing attention has been recently focused on analytical applications of 8-hydroxyquinoline (HQ) as chelating agent of many transition metals. These chelates are thermodynamically stable, chromatographically separable, and absorb in the UV-vis region (18). Several chromatographic procedures, based on different stationary as well as mobile phases, have been tested (19-24) for separating various mixtures of metal-HQ chelates by HPLC. So far, however, no HPLC method for the simultaneous determination of Cr(III), Al(III), and Fe(II1) as HQ chelates has been reported. This is not surprising if we consider (i) the high degree of overlapping of Cr(II1)-HQ and Al(II1)-HQ chelate peaks, (ii) the slow kinetics of Cr(111)-HQ chelate formation compared to that of the other metal-HQ chelates, and (iii) the different yields of the chelate formation reaction for the various metals. This paper describes a procedure for the separation and the simultaneous detection of Zn(II), Cu(II), Al(III), Cr(111),and Fe(II1) or Fe(I1) by reversed-phase HPLC, after precolumn chelation of the metals with HQ. Particular attention has been paid to the optimization of the chelate formation yields. The chromatographic conditions giving the best peak resolutions have been also optimized by mean of the simplex algorithm (25). The method has been applied to the simultaneous quantitative determination of Cr, Al, and Fe in synthetic solutions as well as in sulfuric acid extracts of tannery sludges.
0013-936X/91/0925-1262$02.50/0
0 1991 Ametlcan Chemical Society
Experimental Section
Instrumentation. A Perkin-Elmer (Norwalk, CT) Series 10 pump module equipped with a Rheodyne 7125 injection valve and a (150 X 4.6 mm id.) Supelcosil LC-18, 5-pm packing reversed-phase column (Supelco Inc., Bellefonte, PA) was used as the chromatographic system. A Supelcosil (20 X 4.6 mm i.d.) guard column was used to protect the analytical column. A Varian Model 2550 UVvis detector (Varian Instruments Group), interfaced to an Epson microcomputer (Epson, Milan, Italy) was used to collect the chromatographic data. A Dionex Model 4000i ion chromatograph (Dionex Co., Sunnyvale, CA) equipped with a CG-2 precolumn, a CS-5 analytical column, and a RDM-1 reagent delivery module with membrane reactor (all items from Dionex) was used for the determination of Fe(I1) and Fe(II1) in the acid extracts, according to the method reported in ref 26. The program used for the simplex optimization was written in GWBASIC and run on a COMPAQ 386 microcomputer (COMPAQ Computer Corp.). An atomic absorption spectrometer, Model IL 951 (Instrumentation Laboratory Inc., Wilmington, MA), was used for the analysis of the total metal content in the acid extracts of tannery sludge. Chemicals. The solvents used were HPLC grade (Carlo Erba, Milan, Italy). The 8-hydroxyquinoline (HQ), acetic acid, and ammonium acetate (analytical grade reagent, Carlo Erba) were used without further purification. The mobile phase was filtered through a 0.45-km membrane (HATF 04 7000 Millipore) and vacuum degassed. The standard solutions of AUIII), Fe(III), Cu(II), and Zn(I1) were prepared from Carlo Erba atomic absorption grade stock solutions (lo00 mg/L). Standard solutions of Cr(1II) were prepared from chromium basic sulfate (Carlo Erba) immediately before use. Fe(I1) standards were prepared from Fe(NH4)2(S04)2. 6H20 (Aldrich) with deareated ultrapure water and protected from exposure to air and light. Precolumn Chelation. As for synthetic solutions, the precolumn formation of metal-HQ chelates was carried out by adding the sample solution to a reaction mixture (1:5 v/v) containing acetonitrile/methanol/ 10 mM acetate buffer pH 4.2 (13.366.7:20 20 v/v/v) plus 10 mM HQ. The resulting solution, heated at 90 "C for 30 min, was cooled and later diluted with the reaction mixture to a final 1:lO (v/v) dilution ratio. The sulfuric extracts of tannery sludges (pH 5 1)were filtered on Whatman No. 41 filters to remove the precipitated CaS04. The filtrate was diluted (1:lO v/v) with water. A 25-mL aliquot of the resulting solution was loaded (2.5 mL/min) onto 5 mL of Amberlite XAD-7 synthetic adsorbent (Rohm and Haas Co., Philadelphia, PA) to remove the organic matter, which could foul the chromatographic column. After the elution step, the sample was submitted to the chelation process as previously described for synthetic solutions. Chromatographic Conditions. The mobile phase was a mixture of acetonitrile/methanol/O.l M acetate buffer pH 6.8 (13.5:29.0:57.5 v/v/v) plus 100 mM HQ; its flow rate was 2 mL/min. The sample injection volume was 20 pL, room temperature. Chelate detection wavelength was 400 nm because of the minimal HQ contribution to absorption spectra at this X value (23). According to Moses et al. (26),the mobile phase in the ion chromatographic runs was a 6 mM pyridine-2,6-dicarboxylic acid (PDCA)/5O mM acetic acid solution at pH 4.5; its flow rate was 1.0 mL/min. The sample injection volume was 50 pL, room temperature. The postcolumn
Table 1. Effect of the Reaction Mixture Composition on the Chelate Formation Normalized Yielda organic
organic-phase
0.7 1.5 1.5 4.0
29.0 + 13.5 30.0 + 30.0 50.0 + 10.0 66.7 + 13.3 83.3 + 16.7
(C)
80 64 64 65 100
73 85 100 98 55
48 85 85 100
88
The normalized yield values were obtained according to the following procedure: in each chromatogram, metall-HQ chelate formation yields were measured as peak areas; then, for each metal, the largest peak area among those obtained using the five reported different compositions was set equal to 100; the other peak area values were normalized to this one. Each reported value is the mean of three replicates. bMeOH, methanol; ACN, acetonitrile. The remaining component was acetate buffer at pH 4.2. Organic phase only.
reagent was a 0.2 mM 4-(2-pyridylazo)resorcinol(PAR)/3 M ammonia/l M acetic acid solution, prepared just before use. Detection wavelength was 520 nm.
Results and Discussion The influence of the reaction mixture composition, pH, and temperature on metal-HQ chelate formation was evaluated with synthetic solutions containing known amounts of the investigated metallic ions. Previous studies had demonstrated that chelate formation yields were highest in the pH range between 4.2 and 4.5 (18) and that temperature affected only Cr(II1)HQ formation as the kinetics of this reaction is rather slow (24). In contrast, the fast kinetics of Zn(II), Cu(II), Al(III), and Fe(II1) ions with HQ allows "on-column" chelation at room temperature. To overcome the drawback due to the slow kinetics of Cr(II1)-HQ formation, in the present investigation the samples containing such ions were heated at 90 OC for -30 min. The composition of the reaction mixture plays the main role in determining the chelation yield. Table I shows some of the most significant results concerning the influence of the reaction mixture composition on the chelation yield of Cr(III), Al(III), and Fe(II1). As can be observed, the organic modifier/water-phase ratio has a remarkable and peculiar influence on each species. Furthermore, for a given value of such ratio, the chelation yield is influenced by the organic-phasecomposition (% ACN/ 9'0 MeOH). If the first and the last organic-phase compositions reported in Table I, which gave, respectively, for Fe and Al, very poor chelate formation yields were disregarded, then among the remaining three compositions, all giving the same yield for Cr (-65%), that corresponding to methanol/acetonitrile (66.7% / 13.3% v/v) was considered the best as the corresponding normalized yields for A1 and Fe were both very high, respectively, 98% and 100%. The metal chelate separation was carried out on a C-18 colugn using, in the first attempt, an eluent mixture having the same composition, reported above, of the reaction mixture. However, very poor chromatographicresults were obtained. The composition of the mobile phase was then optimized in order to improve, in particular, the resolution of Cr(II1)- and Al(II1)-HQ chelate peaks. To this purpose the simplex algorithm (25) was applied using % acetonitrile, % methanol, and pH of the mobile phase as refining parameters. Envlron. Sci. fechnol., Vol. 25,
No. 7, 1901 1263
In
I
1
I
0
15
30
TIME ( r n i n )
Flgure 1. Typical chromatogram of a synthetic mixture of metal-HQ chelates. Metals concenvatbns: Zn(II), 8 ppm; Cu(II), 2 ppm; Cr(III), 2 ppm; AI(III), 2 ppm; Fe(III), 2 ppm. Reaction mixture: ACN/ MeOH/10 mM acetate buffer pH 4.2 (13.3:66.7:20.0 v/v/v) plus 10 mM HQ. Moblle phase: ACN/MeOH/O.l M acetate buffer pH 6.8 (13.5:29.0:57.5 vlvlv) plus 100 mM HQ. Flow rate 2.0 mL/min; sample injection volume 20 pl.; detector wavelength 400 nm. Other conditions as in the Experimental Section.
5
0
10
metal concentration ( p p m )
The success of the simplex optimization depends on the choice of an appropriate chromatographic response function (CRF). In order to obtain the minimum acceptable degree of separation of the peaks in the maximum allowable time of analysis, the following response function was used (27):
+
CRF = In (Pj/Po) a(t, - tl)
(1)
where Pj is the peak separation for the j t h pair of peaks; Po is the desired peak separation; t , is the maximum acceptable analysis time (fixed to 30 min in this study); tl is the retention time of the last eluted peak; a is an arbitrary weighting factor. Standard solutions of Zn(II), Cu(II), Cr(III), Al(III), and Fe(II1) were used for the simplex optimization. In addition to the four experiments required for the starting conditions, all the significant improvements were attained in the first 10 of the 15 experiments carried out for the simplex optimization. The results showed that values of pH of the mobile phase in the range 4.5-7 did not cause significant changes in the peak resolution of the chelates. The CRF, and especially the resolution of Cr(II1) and Al(II1) chelate peaks, were found to be strongly dependent on the aqueous-phase/organic-phaseratio of the eluent. Best results were obtained when the mobile phase was water enriched (see the optimized mobile-phase composition reported in the Experimental Section). This behavior is typical of the elution of chelates having very electronegative atoms in the chelating groups (24). Figure 1 shows a typical chromatogram relative to a synthetic solution of Zn(II), Cu(II), Cr(III), Al(III), and Fe(II1). Under the optimized experimental conditions, the degree of peak overlap between Cr(II1)- and Al(II1)-HQ chelates allowed the use of a common peak integration algorithm (perpendicular dropping) instead of more sophisticated deconvolution algorithms (27). Calibration curves for Al(III), Cr(III), and Fe(II1) (see Figure 2) were obtained in the 0.1-10 ppm concentration range, by three replicate measurements for each concentration. The mean relative standard deviations were 4.7%, 1264
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Figure 2. Calibration graphs for AI(II1) (A),Cr(II1) (O), and Fe(II1) (0).Chromatographic conditlons same as reported in the Figure 1 caption.
Table 11. Average Composition of the Investigated Tannery Sludge Sulfuric Acid Extractsa
element Cr Fe AI Ca Mg Zn Ni S2TOC
PH
concn, mg/L 160 221 375
535 500
38 ndc
