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Determination of cadmium in sewage sludge by differential pulse anodic stripping voltammetry
Talanta 49 (1999) 725 – 733
Determination of cadmium in sewage sludge by differential pulse anodic stripping voltammetry Richard A. Pacer *, Cynthia K. Scott Ellis, Ruzhan Peng Department of Chemistry, Indiana Uni6ersity Purdue Uni6ersity Fort Wayne, Fort Wayne, IN 46805 -1499, USA Received 24 August 1998; received in revised form 20 January 1999; accepted 28 January 1999
Abstract A procedure was developed for the determination of cadmium in sewage sludge by differential pulse anodic stripping voltammetry. A sodium peroxide fusion carried out in zirconium crucibles was found to give satisfactory results, based on analysis of standard reference materials. Samples collected from the municipal sludge lagoon in Fort Wayne, Indiana were found to have cadmium abundances ranging from 120 to 250 ppm, with most samples falling in the 120 to 170 ppm range. Interference from zinc is easily eliminated by carrying out the deposition step at − 0.95 V vs. Ag/AgCl. Lead-to-cadmium ratios as high as 50:1 (ppm basis) have no effect on the height of the cadmium peak. © 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction The impetus for this work, begun as an undergraduate research project, stemmed from two articles appearing in the Fort Wayne, Indiana newspapers. The January 15, 1995 edition of the Journal Gazette [1] described an operation carried out by the U.S. Army in which germ-sized particles of zinc cadmium sulfide were dropped from the air over Fort Wayne as a part of a top-secret program studying biological warfare. The second article appeared in the January 16, 1997 edition of The News Sentinel [2] in which it was stated that the cadmium levels of bio-solids in the sludge at the Lake Avenue sludge lagoons exceeded the * Corresponding author. fax: + 1-219-481-6070. E-mail address: [email protected] (R.A. Pacer)
limits set by the Indiana Department of Environmental Management. Sewage sludges contain some valuable resources, such as nitrogen, phosphorus, and organic matter. Hence their use in both home gardening and in agricultural applications is widespread [3]. From a practical standpoint, the ability to safely use municipal sewage sludge for home gardening and other applications depends, at least in part, on its lack of toxic chemicals, such as cadmium. There is evidence that soil treated with sewage sludge has resulted in cadmium being taken up from the soil by food crops. Elevated Cd concentrations (3–7- or 8-fold enhancement when compared to normal crops) have been found in lettuce, cabbage, spinach, carrots, potatoes, wheat, corn, and soybeans grown in soils containing greater than 1 mg/kg of cadmium [4–10].
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(Peas, beans, and tomatoes, on the other hand, tend not to accumulate cadmium [6]). Is there a ‘typical’ or narrow range of cadmium abundance in sewage sludge? Evidently not, given the results of Jing and Logan [5], who examined the trace element content of 17 different sludges, and found a 30-fold variation in the total cadmium content of the sludges. The values fell into three distinct groups: the Chicago sludge (227 ppm Cd); the Ashtabula, Elyria, Bellefontaine, and Fremont sludges (34 – 140 ppm Cd); and the remaining sludges (8 – 25 ppm Cd). In what chemical form does cadmium exist in sewage sludge? Theoretical considerations and indirect methods suggest that sludge Cd probably exists as a combination of organically complexed metal, adsorbed forms, and coprecipitates with Fe, Al, and Ca solid phases [5]. As a result, plant uptake of Cd is strongly dependent on pH (inverse correlation between plant uptake and soil pH), temperature, chloride salinity, organic matter, and calcium concentration [8]. Turning next to the analysis of the sewage sludge samples for cadmium, differential pulse anodic stripping voltammetry (DPASV) is ideal for determining heavy metals, such as zinc, cadmium, lead, and copper, routinely capable of achieving detection limits down to the parts per billion level. However, interelement interferences are possible in DPASV. Thus it was necessary to ensure that the specific DPASV procedure adopted was free of interelement interferences as well as free of matrix effects in general. This latter concern is normally taken care of by a standard addition procedure. A major concern in any analytical procedure is finding an optimum sample dissolution technique. Wet ashing, dry ashing, and extraction techniques [3,11–14] have been used for sewage sludge samples. Regulations in some countries often specify an aqua regia treatment [13]. A total sample digestion using oxidative acids followed by hydrofluoric acid treatment is too time-consuming and regarded as unnecessary in providing a reliable estimate of important trace metal concentrations [13]. Ackers et al. [14] report that metals which are most readily released from the sample matrix (Cu, Cd and Zn) give analytical results
with the greatest precision; and therefore for these metals, the choice of digestion technique from among a wide range used by participants in a study appeared not to matter. While Jenniss et al. [4] report that losses of cadmium and lead greater than 10% can occur when muffle furnace ignition is used on sludge samples, Ritter [15] found no such problem with using a dry-ashing method of preparing sewage sludge for AAS analysis of cadmium, lead, and other elements. Among the dissolution techniques for sewage sludge samples reported in the literature were a CaCl2 extraction procedure, described by Delschen and Werner [16]; a wet ashing procedure given by Murphy [17]; and a sodium peroxide fusion procedure as given by Kaye, Strebin, and Nevissi [18]. Of the three, the sodium peroxide fusion procedure was found to be most promising. A comparison by us of the wet ashing and fusion procedures found the latter to give better precision and to be much less time-consuming. Consequently, it was decided to adopt the fusion procedure, to modify it so as to ensure complete oxidation of our actual samples, and to test it by analyzing several NBS (now NIST) standard reference materials for cadmium. The fusion/ DPASV procedure was used to analyze several actual municipal sewage sludge samples. The results for these samples are given.
2. Experimental
2.1. Apparatus An EG & G Princeton Applied Research Model 174 A Polarographic Analyzer with a Model RE0074 X–Y recorder and an EG & G PARC Model 303 SMDE (static mercury drop electrode) were used for all of the DPASV scans. All pH measurements were made with an Orion Research Model 701A/digital Ionalyzer, using a combination electrode. A Thermolyne Type 1500 furnace was used for the initial fusion work.
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2.2. Reagents Sodium peroxide, \93% ACS reagent, was obtained from Aldrich Chemical Co., Inc. Hydrochloric acid used was Baker ‘Instra-analyzed’ grade for trace metal analysis. The zirconium crucibles used were new, purchased from B-J Scientific Products, Inc. The 0.50 M acetic acid/0.50 M sodium acetate buffer was made from Baker ‘Instra-analyzed’ CH3CO2H and ACS reagent grade CH3CO2Na. All other materials were of ACS reagent grade quality.
2.3. Procedure Samples of sewage sludge were collected from the sludge lagoon located at 5510 Lake Avenue in Fort Wayne, IN. Six samples were taken altogether. The area where the sludge was located had piles of sludge arranged in an almost horseshoelike formation. The sludge samples were collected approximately three feet from each other around the edge of the sludge piles. The samples were labeled A, B, C, D, E, and F, respectively. The order in which the samples were collected was from left to right (while facing into the mouth of the U-shape of the horseshoe-like formation). Sludge samples C and D were used to evaluate the wet ashing and Na2O2 fusion methods. This preliminary work indicated that the fusion method gave greater precision and required considerably less time than the wet ashing procedure. For the fusion procedure, 0.25 g samples of sludge were mixed with 2.5 g of Na2O2 in a nickel crucible and allowed to heat for 30 min in a furnace which had been heated to 500°C. After the crucibles cooled to room temperature, 6 M HCl was used to extract the melt. However, in some cases, a rather dark product resulted, indicating that oxidation of the sample may have been incomplete. A revised fusion procedure was developed, as follows:
2.3.1. Fusion procedure 1. Weigh three zirconium crucibles (crucibles were newly purchased for this work).
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2. Subdivide Sludge Sample A by a mixing/coning and quartering technique (sample is ground in a mortar and pestle, mixed, piled into a cone, after which the cone is quartered and alternate quarters are discarded; the process is repeated with the quarters retained until a suitable-sized lab sample remains). 3. Weigh three : 0.2 g samples (to 9 0.0001 g) into Zr crucibles. 4. Add : 2.0 g Na2O2 to each and mix thoroughly. 5. Heat over a Bunsen burner for 10 min after the mixture melts, with swirling, using the full heat of the burner during this 10-min period. 6. After allowing the crucible to cool, a 2nd fusion is carried out, this time using 1.0 g Na2O2 and 5 min of full burner heating after melting occurs. 7. To each crucible, : 18 ml of 6 M Baker ‘Instra-Analyzed’ HCl was added, dropwise, followed by quantitative transfer to a 250 ml beaker. 8. Next, the pH of each solution was adjusted to 4.0090.02 by the addition of solid sodium acetate. 9. Each solution was then treated with 20.00 ml of 0.50 M CH3CO2H/0.50 M CH3CO2Na buffer, followed by volumetric dilution to 100.0 ml. 10. Blank solutions were prepared following this same procedure. Sewage Sludge Samples B, E, and F were treated in exactly the same way. The triplicate samples of each were designated by letter and number, such as B-1, B-2 and B-3. Each solution was then run by DPASV, using a standard addition procedure. The following parameters and conditions were used:
2.3.2. DPASV parameters Initial EAPPLIED = − 0.95 V vs. Ag/AgCl; rate, 10 mV/s; direction, + ; range, 1.5 V; operating mode, differential pulse; modulation amplitude, 25 mV(PP); clock, 0.5 s; low pass filter, OFF; mode, HDME; drop size, small; purge (N2) time, 4 min; sensitivity, 2 mA full-scale (adjusted as needed to give peaks of reasonable height); sample size, 10.00 ml; spikes: three additions of 1.00
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ml each of 5.00 ppm Cd2 + (1.00 ppm Cd2 + when spiking the blank). In order to evaluate the method for possible bias, two NBS (now NIST) standard reference materials were analyzed by the above method: NBS Standard Reference Material 1648, Urban Particulate Matter; and NBS Standard Reference Material 1645, River Sediment. Because SRM 1648 has a much higher abundance of lead than the sewage sludge samples we analyzed, a separate study was conducted to see if high abundances of lead would have any effect on the DPASV peak for cadmium. We looked at solutions in which the Pb2 + /Cd2 + ppm ratio varied from 0 to 100 and observed the effect (if any) on the current due to the Cd2 + peak.
3. Results and discussion In developing the fusion procedure, it was noted that the melt had a rather dark brown color, even after heating the Zr crucible with the Na2O2/sewage sludge mixture for 10 min at the full heat of a
Bunsen burner. Consequently, the second fusion step was added (1.0 g Na2O2 additional, plus 5 min of full heat). After cooling, the solidified material had a pale yellow color. After the dropwise addition of 6 M HCl, dilution, and transfer to a volumetric flask, the solution was observed to be yellow in color, with a white, flocculent solid settled out on the bottom (presumably silica). The initial pH of the HCl solution of the melt was : 0.5. As NaC2H3O2 was added, the solution took on an orange color when the pH rose above :3 (probably a hydroxy species of Fe(III)). No further change in appearance was noted as the pH was brought to 4.009 0.02. (Blank solutions were all colorless when their pH was similarly adjusted to 4.00). If an initial EAPPLIED (vs. Ag/AgCl) of − 1.20 V is used for the deposition (pre-concentration) step in the DPASV procedure, four distinct peaks may be observed, as noted in Fig. 1, which shows the results for Sample B-2. The peaks, from left to right, are assigned (vs. Ag/AgCl) to:
Fig. 1. Analysis of sewage sludge sample B-2 by DPASV initial EAPPLIED = −1.20 V vs. Ag/AgCl.
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Fig. 2. Analysis of sewage sludge sample B-2 by DPASV initial EAPPLIED = −0.95 V vs. Ag/AgCl.
2+
Zn Cd2+ Pb2+ Cu2+
−1.13 V −0.765 V −0.575 V −0.310 V
While the Zn2 + and Cd2 + peaks are well separated, the much greater current due to the Zn2 + peak limits the current sensitivity setting which may be used; a more sensitive setting would be
desirable to enhance the Cd2 + peak and minimize uncertainties in measuring peak height. By using an initial EAPPLIED of −0.95 V (vs. Ag/AgCl) for the deposition step, the Zn2 + peak is eliminated. This is shown in Fig. 2, which is a DPASV run at −0.95 V for the same solution (B-2), but at a 5-fold greater sensitivity setting (2 mA full-scale vs. 10 mA full-scale for Fig. 1). Note in Fig. 2 that the Cd2 + and Pb2 + peaks are well-resolved, and somewhat similar in height. This was typical of all the sludge samples analyzed. Fig. 3 shows the
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R.A. Pacer et al. / Talanta 49 (1999) 725–733
results of the standard addition procedure, in which Solution B-2 is spiked with three successive 500 ml portions of 5.00 ppm Cd2 + solution. Before plotting current vs. mg Cd2 + added, for the standard addition procedure, all current values were corrected for dilution and for the Cd2 + abundance in the blank. A typical calibration curve is shown in Fig. 4. Table 1 illustrates the nature of the calculations carried out, using Sewage Sludge Sample A-1 as typical. A linear least squares plot of current in
mA (corrected for dilution) vs. mg Cd2 + added to the sample yielded an intercept of 0.57254 mA and a slope of 0.177oo mA/mg Cd, with an RVAL of 0.99964. Thus, y=mx+ b 0=(0.17700)x + 0.57254 X= − 3.235 mg Cd and
Fig. 3. Standard addition spiking of sewage sludge sample B-2 with 500 ml portions of 5.00 ppm Cd2 + .
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Table 2 Results for sewage sludge samples analyzed Sample
Cd2+ (ppm)
Sample
Cd2+ (ppm)
A-1 A-2 A-3 A (ave) sA
163 199 151 171 25
E-1 E-2 E-3 E (ave) sE
301 230 210 247 48
B-1 B-2 B-3 B (ave) sB
118 133 122 124 8
F-1 F-2 F-3 F (ave) sF
139 115 138 131 14
Fig. 4. Calibration curve for sewage sludge sample B-2.
3.235 mg Cd 100.0 ml 0.155 mg Cd X − (blank value) 9.969 ml sample sample 0.1981 g (mass of sample A1-1)
= 163 mg Cd/g sample A-1 =163 ppm Cd in Sewage Sludge Sample A-1 The results obtained for the four sewage sludge samples, each analyzed in triplicate, are given in Table 2, which includes mean and standard deviation data as well. The results show cadmium abundance values ranging from 120 to 250 ppm, with most values falling in the 120 – 170 ppm range. In order to evaluate any possible bias in the method, two NBS (NIST) standard reference materials were analyzed in triplicate. The results and standard deviations are given in Table 3. The data show no conclusive evidence of bias, giving results which are 23% low (SRM 1648) in one case and 35% high (SRM 1645) in the other. Rather, they suggest that the results are associated with a fairly large relative uncertainty. Thus,
the results given for the sewage sludge samples may well have uncertainties of the order of 30%. If a standard reference material sewage sludge had been available for analysis, it would be possible to make a more definite statement about the results. Another point of concern was the possibility that the Pb2 + peak might interfere with the peak for Cd2 + , in the case of SRM 1648. The certified values for SRM 1648 are: 7597 mg/g 6099 27 mg/g 0.6559 0.008% (or 65509 80 mg/g)
Cadmium Copper Lead
Thus there is 87 times as much lead as cadmium (ppm basis) in SRM 1648. Does this high ratio of Pb to Cd have any effect on the size of the Cd peak? (Again, no such concern existed with regard
Table 1 Summary of DPASV results for sewage sludge sample A-1 5.00 ppm Cd2+ added to sample (ml)
Cd added to sample (mg)
Peak height (div.)
Full-scale (fs = 100 div.) (mA)
Gross current (mA)
Current corrected for dilution (mA)
0.00 1.00 2.00 3.00
0.00 5.00 10.00 15.00
29.6 65.8 38.4 50.1
2 2 5 5
0.592 1.316 1.920 2.505
0.592 1.4476 2.304 3.2565
R.A. Pacer et al. / Talanta 49 (1999) 725–733
732 Table 3 Results for standard reference materials SRM
Reported value (ppm Cd2+)
1648, urban particu759 7 late matter 1645, river sediment 10.29 1.5
4. Conclusion This work (ppm Cd2+) 58.19 6.9 13.89 4.7
to the sewage sludge samples themselves, where abundances of Cd and Pb are comparable, based on peak height; and there is no evidence whatsoever of mutual peak interference). To test this possibility solutions were prepared in which the Pb2 + /Cd2 + ppm ratio ranged from 0 to 100 and the current due to Cd2 + was measured. The results are given in Table 4. It may be seen that for Pb2 + /Cd2 + ratios ranging from zero to 50 (ppm/ppm basis), increasing ratios have no effect on the height of the Cd2 + peak. There may be a peak suppression effect at higher ratios, but this would need to be investigated further. If one looks at Fig. 5, which gives the Pb2 + and Cd2 + voltammograms for Pb2 + /Cd2 + ratios (ppm/ppm) of 0, 1 and 50, it is obvious that there is an increasing overlap of the peaks as the Pb2 + /Cd2 + ratio increases. However, if one uses the baseline to the left of the Cd2 + peak as the reference baseline, this overlap has no effect on the height of the Cd2 + peak and its calculated current. For example, Table 4 shows that the current for a 1.00 ppm Cd2 + solution, in which Pb2 + /Cd2 + ratios vary from 0 to 50, remains essentially constant at 1.709 0.06 mA.
Sewage sludge samples obtained from the municipal sludge lagoon in Fort Wayne, IN were found to contain 171, 124, 247, and 131 ppm Cd, for an average of 168 ppm Cd. A sodium peroxide fusion in zirconium crucibles, followed by leaching with HCl and pH adjustment to 4.00, gave solutions which were then analyzed by differential pulse anodic stripping voltammetry. By carrying out the deposition step at −0.95 V vs. Ag/AgCl, interference from zinc was easily eliminated. Lead was not a problem, inasmuch as lead-to-cadmium ratios as high as 50:1 (ppm) have no effect on the current due to Cd; and the actual sludge samples had much lower Pb:Cd ratios. Analysis of standard reference materials indicated the procedure used was satisfactory. The average value of 168 ppm Cd falls within the range reported for sewage sludges by Jing and Logan [5]; that is, it is less than the 227 ppm Cd reported for the Chicago sludge, but greater than the upper end of the range of 34–140 ppm Cd reported for the Ashtabula–Elyria–Bellefontaine–Fremont sludges. It is, in fact, very similar to the 170 ppm value reported back in 1975 as typical of the sewage sludge used by Braude et al. [19] in their work on the uptake of Cd by soybeans grown on sludge-amended soil. There is no evidence, based on this work, that the abundance of Cd in Fort Wayne municipal sewage sludge was enhanced by the zinc cadmium sulfide dropped on Fort Wayne. In view of the elevated Cd concentrations in selected crops grown on sewage sludge treated soil (lettuce, cabbage, spinach, carrots, potatoes, wheat, corn, and soybeans), which could be as great as an 8-fold
Table 4 Investigation of possible lead interference on cadmium measurement Cd2+ concentration (ppm)
Pb2+/Cd2+ ratio (ppm/ppm)
Cd2+ current (mA)
Cd2+ current (mA) per ppm Cd2+
1.00 1.00 1.00 1.00 1.00 0.50
0.00 1.00 5.00 10.00 50.00 100.00
1.675 1.78 1.64 1.67 1.755 0.685
1.675 1.78 1.64 1.67 1.755 1.37
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733
Fig. 5. Effect of the Pb2 + /Cd2 + ratio on the current for the Cd2 + peak.
enhancement when compared to normal crops, it is recommended that Fort Wayne municipal sewage sludge not be used when growing such crops.
Acknowledgements We wish to thank Stacey J. Petrovas, agronomist for the City of Fort Wayne Utilities, for his part in the sewage sludge acquisition.
References [1] D. Noell, L. Stedman, The Journal Gazette, Fort Wayne, Indiana, January 15, 1995, 1A. [2] A Jarosh, The News Sentinel, Fort Wayne, Indiana, January 16, 1997, 1A, 9A. [3] S.B. Sterrett, R.L. Chaney, C.H. Gifford, H.W. Mielke, Environ. Geochem. Health 18 (1996) 135–142. [4] S.W. Jenniss, S.A. Katz, T. Mount, Am. Lab. 12 (1980) 18, 20, 22–23. [5] J. Jing, T.J. Logan, J. Environ. Qual. 21 (1992) 73 – 81. [6] J.C. Sherlock, Proceedings of the 4th Int. Cadmium Conference, Munich, 1983, pp. 107–109.
.
[7] M.J. Dudas, S. Pawluk, Can. J. Soil Sci. 55 (1975) 239 – 243. [8] A.P. Jackson, B.J. Alloway, Plant Soil 132 (1991) 179– 186. [9] L. Linnman, A. Andersson, K.O. Nilsson, B. Lind, T. Kjellstro¨m, L. Friberg, Arch. Environ. Health 27 (1973) 45 – 47. [10] C.S. Reddy, C.R. Dorn, Environ. Res. 38 (1985) 377– 388. [11] A. Meyer, U. De la Chevallerie-Haaf, G. Henze, Fresenius Z. Anal. Chem. 328 (1987) 565 – 568. [12] J.F.N. Maaskant, A.H. Boekholt, P.J. Jenks, Fresenius J. Anal. Chem. 360 (1998) 406 – 409. [13] B. Griepink, H. Muntau, E. Colinet, Fresenius Z. Anal. Chem. 318 (1984) 490 – 494. [14] C.J. Ackers, M.J. Gardner, J.E. Ravenscroft, Fresenius J. Anal. Chem. 354 (1996) 596 – 601. [15] C.J. Ritter, Am. Lab. 14 (1982) 72 – 73. [16] T. Delschen, W. Werner, Landwirtschaft. Forsch. 42 (1989) 29 – 39. [17] T.J. Murphy, in: R. Mavrodineau (Ed.), Procedures used at the National Bureau of Standards to Determine Selected Trace Elements in Biological and Botanical Materials, National Bureau of Standards, 1977, pp. 6 – 7. [18] J.H. Kaye, R.S. Strebin, A.E. Nevissi, J. Radioanal. Chem. 180 (1994) 197 – 200. [19] G.L. Braude, A.M. Nash, W.J. Wolf, R.L. Carr, R.L. Chaney, J. Food Sci. 45 (1980) 1187 – 1189, 1199.
Determination of cadmium in sewage sludge by differential pulse anodic stripping voltammetry Richard A. Pacer *, Cynthia K. Scott Ellis, Ruzhan Peng Department of Chemistry, Indiana Uni6ersity Purdue Uni6ersity Fort Wayne, Fort Wayne, IN 46805 -1499, USA Received 24 August 1998; received in revised form 20 January 1999; accepted 28 January 1999
Abstract A procedure was developed for the determination of cadmium in sewage sludge by differential pulse anodic stripping voltammetry. A sodium peroxide fusion carried out in zirconium crucibles was found to give satisfactory results, based on analysis of standard reference materials. Samples collected from the municipal sludge lagoon in Fort Wayne, Indiana were found to have cadmium abundances ranging from 120 to 250 ppm, with most samples falling in the 120 to 170 ppm range. Interference from zinc is easily eliminated by carrying out the deposition step at − 0.95 V vs. Ag/AgCl. Lead-to-cadmium ratios as high as 50:1 (ppm basis) have no effect on the height of the cadmium peak. © 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction The impetus for this work, begun as an undergraduate research project, stemmed from two articles appearing in the Fort Wayne, Indiana newspapers. The January 15, 1995 edition of the Journal Gazette [1] described an operation carried out by the U.S. Army in which germ-sized particles of zinc cadmium sulfide were dropped from the air over Fort Wayne as a part of a top-secret program studying biological warfare. The second article appeared in the January 16, 1997 edition of The News Sentinel [2] in which it was stated that the cadmium levels of bio-solids in the sludge at the Lake Avenue sludge lagoons exceeded the * Corresponding author. fax: + 1-219-481-6070. E-mail address: [email protected] (R.A. Pacer)
limits set by the Indiana Department of Environmental Management. Sewage sludges contain some valuable resources, such as nitrogen, phosphorus, and organic matter. Hence their use in both home gardening and in agricultural applications is widespread [3]. From a practical standpoint, the ability to safely use municipal sewage sludge for home gardening and other applications depends, at least in part, on its lack of toxic chemicals, such as cadmium. There is evidence that soil treated with sewage sludge has resulted in cadmium being taken up from the soil by food crops. Elevated Cd concentrations (3–7- or 8-fold enhancement when compared to normal crops) have been found in lettuce, cabbage, spinach, carrots, potatoes, wheat, corn, and soybeans grown in soils containing greater than 1 mg/kg of cadmium [4–10].
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(Peas, beans, and tomatoes, on the other hand, tend not to accumulate cadmium [6]). Is there a ‘typical’ or narrow range of cadmium abundance in sewage sludge? Evidently not, given the results of Jing and Logan [5], who examined the trace element content of 17 different sludges, and found a 30-fold variation in the total cadmium content of the sludges. The values fell into three distinct groups: the Chicago sludge (227 ppm Cd); the Ashtabula, Elyria, Bellefontaine, and Fremont sludges (34 – 140 ppm Cd); and the remaining sludges (8 – 25 ppm Cd). In what chemical form does cadmium exist in sewage sludge? Theoretical considerations and indirect methods suggest that sludge Cd probably exists as a combination of organically complexed metal, adsorbed forms, and coprecipitates with Fe, Al, and Ca solid phases [5]. As a result, plant uptake of Cd is strongly dependent on pH (inverse correlation between plant uptake and soil pH), temperature, chloride salinity, organic matter, and calcium concentration [8]. Turning next to the analysis of the sewage sludge samples for cadmium, differential pulse anodic stripping voltammetry (DPASV) is ideal for determining heavy metals, such as zinc, cadmium, lead, and copper, routinely capable of achieving detection limits down to the parts per billion level. However, interelement interferences are possible in DPASV. Thus it was necessary to ensure that the specific DPASV procedure adopted was free of interelement interferences as well as free of matrix effects in general. This latter concern is normally taken care of by a standard addition procedure. A major concern in any analytical procedure is finding an optimum sample dissolution technique. Wet ashing, dry ashing, and extraction techniques [3,11–14] have been used for sewage sludge samples. Regulations in some countries often specify an aqua regia treatment [13]. A total sample digestion using oxidative acids followed by hydrofluoric acid treatment is too time-consuming and regarded as unnecessary in providing a reliable estimate of important trace metal concentrations [13]. Ackers et al. [14] report that metals which are most readily released from the sample matrix (Cu, Cd and Zn) give analytical results
with the greatest precision; and therefore for these metals, the choice of digestion technique from among a wide range used by participants in a study appeared not to matter. While Jenniss et al. [4] report that losses of cadmium and lead greater than 10% can occur when muffle furnace ignition is used on sludge samples, Ritter [15] found no such problem with using a dry-ashing method of preparing sewage sludge for AAS analysis of cadmium, lead, and other elements. Among the dissolution techniques for sewage sludge samples reported in the literature were a CaCl2 extraction procedure, described by Delschen and Werner [16]; a wet ashing procedure given by Murphy [17]; and a sodium peroxide fusion procedure as given by Kaye, Strebin, and Nevissi [18]. Of the three, the sodium peroxide fusion procedure was found to be most promising. A comparison by us of the wet ashing and fusion procedures found the latter to give better precision and to be much less time-consuming. Consequently, it was decided to adopt the fusion procedure, to modify it so as to ensure complete oxidation of our actual samples, and to test it by analyzing several NBS (now NIST) standard reference materials for cadmium. The fusion/ DPASV procedure was used to analyze several actual municipal sewage sludge samples. The results for these samples are given.
2. Experimental
2.1. Apparatus An EG & G Princeton Applied Research Model 174 A Polarographic Analyzer with a Model RE0074 X–Y recorder and an EG & G PARC Model 303 SMDE (static mercury drop electrode) were used for all of the DPASV scans. All pH measurements were made with an Orion Research Model 701A/digital Ionalyzer, using a combination electrode. A Thermolyne Type 1500 furnace was used for the initial fusion work.
R.A. Pacer et al. / Talanta 49 (1999) 725–733
2.2. Reagents Sodium peroxide, \93% ACS reagent, was obtained from Aldrich Chemical Co., Inc. Hydrochloric acid used was Baker ‘Instra-analyzed’ grade for trace metal analysis. The zirconium crucibles used were new, purchased from B-J Scientific Products, Inc. The 0.50 M acetic acid/0.50 M sodium acetate buffer was made from Baker ‘Instra-analyzed’ CH3CO2H and ACS reagent grade CH3CO2Na. All other materials were of ACS reagent grade quality.
2.3. Procedure Samples of sewage sludge were collected from the sludge lagoon located at 5510 Lake Avenue in Fort Wayne, IN. Six samples were taken altogether. The area where the sludge was located had piles of sludge arranged in an almost horseshoelike formation. The sludge samples were collected approximately three feet from each other around the edge of the sludge piles. The samples were labeled A, B, C, D, E, and F, respectively. The order in which the samples were collected was from left to right (while facing into the mouth of the U-shape of the horseshoe-like formation). Sludge samples C and D were used to evaluate the wet ashing and Na2O2 fusion methods. This preliminary work indicated that the fusion method gave greater precision and required considerably less time than the wet ashing procedure. For the fusion procedure, 0.25 g samples of sludge were mixed with 2.5 g of Na2O2 in a nickel crucible and allowed to heat for 30 min in a furnace which had been heated to 500°C. After the crucibles cooled to room temperature, 6 M HCl was used to extract the melt. However, in some cases, a rather dark product resulted, indicating that oxidation of the sample may have been incomplete. A revised fusion procedure was developed, as follows:
2.3.1. Fusion procedure 1. Weigh three zirconium crucibles (crucibles were newly purchased for this work).
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2. Subdivide Sludge Sample A by a mixing/coning and quartering technique (sample is ground in a mortar and pestle, mixed, piled into a cone, after which the cone is quartered and alternate quarters are discarded; the process is repeated with the quarters retained until a suitable-sized lab sample remains). 3. Weigh three : 0.2 g samples (to 9 0.0001 g) into Zr crucibles. 4. Add : 2.0 g Na2O2 to each and mix thoroughly. 5. Heat over a Bunsen burner for 10 min after the mixture melts, with swirling, using the full heat of the burner during this 10-min period. 6. After allowing the crucible to cool, a 2nd fusion is carried out, this time using 1.0 g Na2O2 and 5 min of full burner heating after melting occurs. 7. To each crucible, : 18 ml of 6 M Baker ‘Instra-Analyzed’ HCl was added, dropwise, followed by quantitative transfer to a 250 ml beaker. 8. Next, the pH of each solution was adjusted to 4.0090.02 by the addition of solid sodium acetate. 9. Each solution was then treated with 20.00 ml of 0.50 M CH3CO2H/0.50 M CH3CO2Na buffer, followed by volumetric dilution to 100.0 ml. 10. Blank solutions were prepared following this same procedure. Sewage Sludge Samples B, E, and F were treated in exactly the same way. The triplicate samples of each were designated by letter and number, such as B-1, B-2 and B-3. Each solution was then run by DPASV, using a standard addition procedure. The following parameters and conditions were used:
2.3.2. DPASV parameters Initial EAPPLIED = − 0.95 V vs. Ag/AgCl; rate, 10 mV/s; direction, + ; range, 1.5 V; operating mode, differential pulse; modulation amplitude, 25 mV(PP); clock, 0.5 s; low pass filter, OFF; mode, HDME; drop size, small; purge (N2) time, 4 min; sensitivity, 2 mA full-scale (adjusted as needed to give peaks of reasonable height); sample size, 10.00 ml; spikes: three additions of 1.00
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ml each of 5.00 ppm Cd2 + (1.00 ppm Cd2 + when spiking the blank). In order to evaluate the method for possible bias, two NBS (now NIST) standard reference materials were analyzed by the above method: NBS Standard Reference Material 1648, Urban Particulate Matter; and NBS Standard Reference Material 1645, River Sediment. Because SRM 1648 has a much higher abundance of lead than the sewage sludge samples we analyzed, a separate study was conducted to see if high abundances of lead would have any effect on the DPASV peak for cadmium. We looked at solutions in which the Pb2 + /Cd2 + ppm ratio varied from 0 to 100 and observed the effect (if any) on the current due to the Cd2 + peak.
3. Results and discussion In developing the fusion procedure, it was noted that the melt had a rather dark brown color, even after heating the Zr crucible with the Na2O2/sewage sludge mixture for 10 min at the full heat of a
Bunsen burner. Consequently, the second fusion step was added (1.0 g Na2O2 additional, plus 5 min of full heat). After cooling, the solidified material had a pale yellow color. After the dropwise addition of 6 M HCl, dilution, and transfer to a volumetric flask, the solution was observed to be yellow in color, with a white, flocculent solid settled out on the bottom (presumably silica). The initial pH of the HCl solution of the melt was : 0.5. As NaC2H3O2 was added, the solution took on an orange color when the pH rose above :3 (probably a hydroxy species of Fe(III)). No further change in appearance was noted as the pH was brought to 4.009 0.02. (Blank solutions were all colorless when their pH was similarly adjusted to 4.00). If an initial EAPPLIED (vs. Ag/AgCl) of − 1.20 V is used for the deposition (pre-concentration) step in the DPASV procedure, four distinct peaks may be observed, as noted in Fig. 1, which shows the results for Sample B-2. The peaks, from left to right, are assigned (vs. Ag/AgCl) to:
Fig. 1. Analysis of sewage sludge sample B-2 by DPASV initial EAPPLIED = −1.20 V vs. Ag/AgCl.
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Fig. 2. Analysis of sewage sludge sample B-2 by DPASV initial EAPPLIED = −0.95 V vs. Ag/AgCl.
2+
Zn Cd2+ Pb2+ Cu2+
−1.13 V −0.765 V −0.575 V −0.310 V
While the Zn2 + and Cd2 + peaks are well separated, the much greater current due to the Zn2 + peak limits the current sensitivity setting which may be used; a more sensitive setting would be
desirable to enhance the Cd2 + peak and minimize uncertainties in measuring peak height. By using an initial EAPPLIED of −0.95 V (vs. Ag/AgCl) for the deposition step, the Zn2 + peak is eliminated. This is shown in Fig. 2, which is a DPASV run at −0.95 V for the same solution (B-2), but at a 5-fold greater sensitivity setting (2 mA full-scale vs. 10 mA full-scale for Fig. 1). Note in Fig. 2 that the Cd2 + and Pb2 + peaks are well-resolved, and somewhat similar in height. This was typical of all the sludge samples analyzed. Fig. 3 shows the
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results of the standard addition procedure, in which Solution B-2 is spiked with three successive 500 ml portions of 5.00 ppm Cd2 + solution. Before plotting current vs. mg Cd2 + added, for the standard addition procedure, all current values were corrected for dilution and for the Cd2 + abundance in the blank. A typical calibration curve is shown in Fig. 4. Table 1 illustrates the nature of the calculations carried out, using Sewage Sludge Sample A-1 as typical. A linear least squares plot of current in
mA (corrected for dilution) vs. mg Cd2 + added to the sample yielded an intercept of 0.57254 mA and a slope of 0.177oo mA/mg Cd, with an RVAL of 0.99964. Thus, y=mx+ b 0=(0.17700)x + 0.57254 X= − 3.235 mg Cd and
Fig. 3. Standard addition spiking of sewage sludge sample B-2 with 500 ml portions of 5.00 ppm Cd2 + .
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Table 2 Results for sewage sludge samples analyzed Sample
Cd2+ (ppm)
Sample
Cd2+ (ppm)
A-1 A-2 A-3 A (ave) sA
163 199 151 171 25
E-1 E-2 E-3 E (ave) sE
301 230 210 247 48
B-1 B-2 B-3 B (ave) sB
118 133 122 124 8
F-1 F-2 F-3 F (ave) sF
139 115 138 131 14
Fig. 4. Calibration curve for sewage sludge sample B-2.
3.235 mg Cd 100.0 ml 0.155 mg Cd X − (blank value) 9.969 ml sample sample 0.1981 g (mass of sample A1-1)
= 163 mg Cd/g sample A-1 =163 ppm Cd in Sewage Sludge Sample A-1 The results obtained for the four sewage sludge samples, each analyzed in triplicate, are given in Table 2, which includes mean and standard deviation data as well. The results show cadmium abundance values ranging from 120 to 250 ppm, with most values falling in the 120 – 170 ppm range. In order to evaluate any possible bias in the method, two NBS (NIST) standard reference materials were analyzed in triplicate. The results and standard deviations are given in Table 3. The data show no conclusive evidence of bias, giving results which are 23% low (SRM 1648) in one case and 35% high (SRM 1645) in the other. Rather, they suggest that the results are associated with a fairly large relative uncertainty. Thus,
the results given for the sewage sludge samples may well have uncertainties of the order of 30%. If a standard reference material sewage sludge had been available for analysis, it would be possible to make a more definite statement about the results. Another point of concern was the possibility that the Pb2 + peak might interfere with the peak for Cd2 + , in the case of SRM 1648. The certified values for SRM 1648 are: 7597 mg/g 6099 27 mg/g 0.6559 0.008% (or 65509 80 mg/g)
Cadmium Copper Lead
Thus there is 87 times as much lead as cadmium (ppm basis) in SRM 1648. Does this high ratio of Pb to Cd have any effect on the size of the Cd peak? (Again, no such concern existed with regard
Table 1 Summary of DPASV results for sewage sludge sample A-1 5.00 ppm Cd2+ added to sample (ml)
Cd added to sample (mg)
Peak height (div.)
Full-scale (fs = 100 div.) (mA)
Gross current (mA)
Current corrected for dilution (mA)
0.00 1.00 2.00 3.00
0.00 5.00 10.00 15.00
29.6 65.8 38.4 50.1
2 2 5 5
0.592 1.316 1.920 2.505
0.592 1.4476 2.304 3.2565
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732 Table 3 Results for standard reference materials SRM
Reported value (ppm Cd2+)
1648, urban particu759 7 late matter 1645, river sediment 10.29 1.5
4. Conclusion This work (ppm Cd2+) 58.19 6.9 13.89 4.7
to the sewage sludge samples themselves, where abundances of Cd and Pb are comparable, based on peak height; and there is no evidence whatsoever of mutual peak interference). To test this possibility solutions were prepared in which the Pb2 + /Cd2 + ppm ratio ranged from 0 to 100 and the current due to Cd2 + was measured. The results are given in Table 4. It may be seen that for Pb2 + /Cd2 + ratios ranging from zero to 50 (ppm/ppm basis), increasing ratios have no effect on the height of the Cd2 + peak. There may be a peak suppression effect at higher ratios, but this would need to be investigated further. If one looks at Fig. 5, which gives the Pb2 + and Cd2 + voltammograms for Pb2 + /Cd2 + ratios (ppm/ppm) of 0, 1 and 50, it is obvious that there is an increasing overlap of the peaks as the Pb2 + /Cd2 + ratio increases. However, if one uses the baseline to the left of the Cd2 + peak as the reference baseline, this overlap has no effect on the height of the Cd2 + peak and its calculated current. For example, Table 4 shows that the current for a 1.00 ppm Cd2 + solution, in which Pb2 + /Cd2 + ratios vary from 0 to 50, remains essentially constant at 1.709 0.06 mA.
Sewage sludge samples obtained from the municipal sludge lagoon in Fort Wayne, IN were found to contain 171, 124, 247, and 131 ppm Cd, for an average of 168 ppm Cd. A sodium peroxide fusion in zirconium crucibles, followed by leaching with HCl and pH adjustment to 4.00, gave solutions which were then analyzed by differential pulse anodic stripping voltammetry. By carrying out the deposition step at −0.95 V vs. Ag/AgCl, interference from zinc was easily eliminated. Lead was not a problem, inasmuch as lead-to-cadmium ratios as high as 50:1 (ppm) have no effect on the current due to Cd; and the actual sludge samples had much lower Pb:Cd ratios. Analysis of standard reference materials indicated the procedure used was satisfactory. The average value of 168 ppm Cd falls within the range reported for sewage sludges by Jing and Logan [5]; that is, it is less than the 227 ppm Cd reported for the Chicago sludge, but greater than the upper end of the range of 34–140 ppm Cd reported for the Ashtabula–Elyria–Bellefontaine–Fremont sludges. It is, in fact, very similar to the 170 ppm value reported back in 1975 as typical of the sewage sludge used by Braude et al. [19] in their work on the uptake of Cd by soybeans grown on sludge-amended soil. There is no evidence, based on this work, that the abundance of Cd in Fort Wayne municipal sewage sludge was enhanced by the zinc cadmium sulfide dropped on Fort Wayne. In view of the elevated Cd concentrations in selected crops grown on sewage sludge treated soil (lettuce, cabbage, spinach, carrots, potatoes, wheat, corn, and soybeans), which could be as great as an 8-fold
Table 4 Investigation of possible lead interference on cadmium measurement Cd2+ concentration (ppm)
Pb2+/Cd2+ ratio (ppm/ppm)
Cd2+ current (mA)
Cd2+ current (mA) per ppm Cd2+
1.00 1.00 1.00 1.00 1.00 0.50
0.00 1.00 5.00 10.00 50.00 100.00
1.675 1.78 1.64 1.67 1.755 0.685
1.675 1.78 1.64 1.67 1.755 1.37
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Fig. 5. Effect of the Pb2 + /Cd2 + ratio on the current for the Cd2 + peak.
enhancement when compared to normal crops, it is recommended that Fort Wayne municipal sewage sludge not be used when growing such crops.
Acknowledgements We wish to thank Stacey J. Petrovas, agronomist for the City of Fort Wayne Utilities, for his part in the sewage sludge acquisition.
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