Atomic Absorption Analysis of Strong Heavy Metal Chelating Agents in Water and Waste Water Robert Kunkel and Stanley E. Manahan' Department of Chemistry, University of Missouri, Columbia, Mo. 65207
A method for the analysis of total strong heavy metal chelating agents in water and waste water is described. The method is based upon the solubilization of copper by the chelating agents at pH 10 followed by filtration and atomic absorption analysis of soluble copper in the filtrate. The analysis gives total levels of strong chelating agents, a significant parameter insofar as the properties of natural waters and waste waters are concerned. The method is simple, sensitive, and relatively free of interferences. It can be applied to the analysis of individual chelating agents aftqr separation.
During recent years concern has grown over the presence of heavy metals in water. Natural waters and waste waters have been surveyed extensively for heavy metals. In many cases, the levels of metals are much higher than would be expected on the basis of inorganic solution chemistry and solubility considerations. Surprisingly, comparatively little attention has been given to the chelating agents, of both natural and pollutant origin, which almost certainly play an important role in the solubilization, transport, and reactions of heavy metals in water. The possible effects of chelating agents in waters and waste waters have been discussed ( I , 2). In addition to heavy metal transport in natural waters and through waste-water treatment processes, chelating agents may hinder the removal of metals from water, increase the corrosion of metal surfaces, and affect the oxidation state of metals in water. Chelating agents may have a strong effect upon aquatic biota. For example, it is generally necessary to add chelating agents to artificial algal nutrient media used for growing algae in the laboratory. These chelating agents serve to keep essential micronutrient metal ions (e.g., iron) in solution and available to algae. It is not unlikely that chelating agents serve the same function in nature, thus playing a role in algal growkh and resulting eutrophication. Chelating agents may detoxify toxic metals. Thus, although copper a t levels of around 0.5 mg/l. and above is toxic to algae and is used as an algicide, algae grow well in water containing much higher levels of copper when an excess of a strong chelating agent is present. Indeed, algae thrive in a medium which is literally blue from the presence of copper (12 mg/l.) if an excess of EDTA is present ( 3 ) . The use of strong chelating agents such as NTA for detergent phosphate substitutes would give cause for increased concern oyer the effects of chelating agents in waste waters and natural waters. All of the proposed che-
A u t h o r t o w h o m i n q u i r i e s s h o u l d b e addressed (1) S . E. Manahan, "Environmental Chemistry," Wiliard Grant Press, Boston. Mass., 1972. (2) M. Schnitzer, "Organic: Compounds in Aquatic Environments," Samuel D. Faust and Joseph V. Hunter, Ed., Marcel Dekker, New York, N . Y . , 1971. (3) M . J. Smith, Ph.D., T'hesis. The University of Missouri-Columbia, Columbia, Mo., 1972.
lating detergent phosphate substitutes are biodegradable. However, in many cases, an appreciable fraction would not be degraded in waste treatment plants and would persist for some time in receiving waters before degrading. Furthermore, given the present status of secondary sewage treatment in many areas, appreciable amounts of chelating detergent builders would not be subjected to treatment a t all before entering receiving waters. Chelating agents occur in water from natural sources. The degradation of plant material produces fulvic acids, which are chelating agents. Some water "color" is thought to be due to the presence of metal chelates, particularly iron chelates. Amino acids are a potential source of chelating agents, particularly in some waste waters. One reason for the dearth of information on chelating agents in water is the lack of analytical methodology for them. Recently several methods have been published for the analysis of NTA. One of these methods, the zinc-Zincon method ( 4 ) , is now a standard method for the analysis of NTA in water and wastes. This method is based upon the fact that zinc forms a blue-colored complex with 2carboxy-2'-hydroxy-5'-sulfoformazylbenzene(Zincon) in a solution buffered a t p H 9.2. The NTA replaces Zincon in the complex, thus reducing the absorbance in proportion to the amount of NTA present. The method works in surface waters down to 0.5 mg/l. NTA (as the trisodium salt). The most common interfering ions are calcium, magnesium, zinc, copper, iron, and manganese. These ions are chelated by NTA producing a negative interference. Batch treatment of the sample with a cation exchanger in the sodium form exchanges sodium ion for the interfering ions and eliminates the interference. The polarographic reduction of the indium(II1)-NTA chelate may be used as the basis for a polarographic analysis of NTA ( 5 ) . Anion-exchange concentration and isotope dilution enables use of the method a t very low concentrations of NTA ranging from 0.0257 to 2.57 mg/l. of trisodium S T A . Without these measures the method was reported to be directly applicable through a range of 2.57 to 257 mg/l. NTA. Application to large-scale analysis of water samples would be rather time consuming, however. There is a need for an analytical method for total strong heavy metal chelating agents in water applicable to the large-scale analysis of water samples. The method should be rapid, simple, and sensitive to levels of chelating agents below 1 mg/l. Since a mixture of chelating agents is found in most waters and waste waters, the method should not be specific for any one chelating agent. Furthermore, insofar as water quality is concerned, it is an effect, Le., the ability to form stable soluble chelates with metals, which is of concern. In the interest of simplicity and speed, the need for separation steps prior to analysis should be avoided. This paper describes a method of analysis meeting the above criteria. (4) "Methods for Chemical Analysis of Water and Wastes-1971 , " Environmental Protection Agency, Water Quality Office, Analytical Quality Control Laboratory, Cincinnati, Ohio, 1971, p 205. (5) J. P. Haberman, A n a / . Chem., 43, 63 (1971).
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
1465
SOLUBILIZATION OF COPPER(I1) BY CHELATING AGENTS Total strong chelating agents in water may be analyzed bv a method based on the solubilization of copper(I1) as a _chelate in a basic medium. Adding base to a cupric-ion solution produces a precipitate which for simplicity we will designate as Cu(OH)2, Cu2+
+
2 0 H - = Cu(OH),
At pH 10. the concentration of uncomplexed cupric ion in equilibrium with cupric hydroxide may be calculated as follows.
K,
= [ C U ~ + J [ O H -= ] ~3 X
(2)
K.
From the above calculation, it is seen that the equilibrium concentration of Cu2+ in solution a t pH 10 is extremely low. Experimental confirmation of this may be obtained by adjusting a solution containing cupric ion to p H 10 and carefully removing the resulting precipitate by membrane filtration. Copper can be detected only barely in the filtrate by conventional atomic-absorption analysis. The presence of a strong chelating agent a t pH 10 keeps some copper in solution as the copper chelate. This solubilization of copper in a basic medium enables analysis of total strong heavy metal chelating agent. The analysis is accomplished by adding cupric ion to a sample containing strong chelating agent, L*-, and adjusting the pH to 10. An amount of copper equivalent to the strong heavy metal chelating agent present is chelated cu2+
+ L"-
= cuL2-"
(4)
and the excess unchelated copper is precipitated Cu2+ + 2 0 H - = Cu(OH), (5) Careful removal of the copper-containing precipitate by membrane filtration enables analysis of the soluble chelated copper remaining in solution by atomic absorption analysis or other means.
EXPERIMENTAL Stock solutions of 5.00 X 10-2M CuSO4 and 5.00 x 10-ZM XaZCOs were prepared from reagent-grade chemicals. Commercial copper standards (Perkin-Elmer Corp.) were used to prepare the calibration plots for atomic-absorption analysis. Immediately after collection, samples were filtered through a 0.45-p membrane filter held in a pressurized filtration apparatus. If a t all possible, the samples were analyzed immediately. I t is possible t o preserve Eamples for several days in 5 x l O - 3 M CuS01. The preservation of sewage samples in nitric acid has given anomalously high results, and this method of preservation is not recommended. The atomic-absorption analyses were done with a Perkin-Elmer Model 403 atomic absorption spectrophotometer using an airacetylene flame and a single-slot burner. The basic analytical procedure is the following: To 50.0 ml of filtered sample add 5 ml of 5.00 X 10-2M CuSO4. A minimum amount of copper should he added so long as it is in excess of that required to react with the chelating agents present. Add slowly with stirring 5.00 X 10-2M Na2C03 until the solution has a stable pH between 9.8 and 10.2. Check for the presence of a coppercontaining precipitate. If it is absent, additional copper should be added. Heat the sample to boiling. Continue heating and stir until the precipitate changes color from light blue to gray or brown. A particularly long heating time generally is required for sewage samples. Dilute the cooled sample quantitatively to 100 ml. Filter a portion of the sample through a 0.45-p membrane filter held in a pressure holder. Collect sufficient filtrate for atomicabsorption analysis. The region of the filter below the filter disk 1466
*
must be free of copper contamination. Acidify the collected filtrate with nitric acid and analyze for copper by conventional atomic-absorptiontechniques. The concentration of total strong heavy metal chelating agents in the original sample may be expressed in units of m d l . copper equivalent chelating capacity k e . , the number of milligrams- of copper which can be chelated by the chelating agent present in 1 1. of solution) or other desired units.
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, N O . 8, JULY 1973
RESULTS AND DISCUSSION Recoveries of EDTA and NTA from solutions ranging from 0 to 6 X 10-4M in chelating agent are shown in Figure 1. The solid line indicates the theoretical or stoichiometric recovery expected from quantitative solubilization of copper by the chelating agents. Close agreement with stoichiometric recoveries is observed. The analysis of very low levels of EDTA is shown in Figure 2. The copper determinations for this plot were done colorimetrically with dithizone. The solid line is the theoretical curve. The blank used for the dithizone analysis of chelate filtrate was prepared from the filtrate of solutions which had been through the chelate analytical procedure in the absence of added chelate. Considering the very low levels of chelating agent involved, good agreement with stoichiometric recovery is observed. Using atomic absorption for the measurement of copper in the filtrate gives a lower limit of analysis for EDTA of approximately 1 X W 6 M (ca. 0.06 mg/l. copper equivalent chelating capacity). One of the most important potential applications of the method is the measurement of chelating agents in sewage effluents. Recovery of EDTA from secondary sewage effluent containing initially 0.65 mg/l. copper equivalent chelating capacity from materials originally present in the effluent is shown in Figure 3. Quantitative recovery of EDTA is observed and the intercept of the plot a t zero EDTA added falls very close to the level of chelating agent in the original sewage sample. Analyses of some typical samples are given in Table 1. The creek waters containing 1 to 2 mg/l. copper equivalent chelating capacity are typical of relatively unpolluted surface waters. The pool sample was taken from a fountain fed by recirculated tap water. The pool was supporting a visible, though not objectionable, growth of algae. Since the tap water used to fill the pool did not contain any chelating agent initially, it is possible that the chelating agent found was produced by the algae. The Eastern Kansas oilfield brine was analyzed for chelating agents in an effort to explain levels of lead, cobalt, and nickel present a t the 1-2 mg/l. level along with soluble sulfide. The sewage samples are indicative of relatively efficient removal of chelating agents by activated sludge treatment. The tap water analysis shows the absence of chelating agents in water properly treated for domestic use. In most waters, potentially interfering metal ions are not present at interfering levels. Metals suspected of interfering with the analysis may be removed ( 5 ) .N o interference has been observed from lead, calcium, or mercury placed in samples containing EDTA, NTA, or naturally occurring chelating agents. In general, of the metal ions commonly found a t appreciable levels in a water sample, only iron(II1) forms more stable chelates with most chelating agents than does copper. However, in the basic medium in which the copper is solubilized, the chelating agent must solubilize the metal ion from the solid metal hydroxide, or from a similar compound, such as a basic salt. As an illustrative example, NTA may be examined. If base is added to a solution containing NTA, excess CupA, and excess Fe3+, the system a t equilibrium will contain precipitated iron(III), precipitated copper(II), and the sol-
T
\ ri
P d
: 3 r&
8' 3c 1
E 8u
1.0
a
z:
I . . . . . . . . . ,
0.0
o 0
1
2
3
4
5
AGENT, %lo4 Figure 1. Recovery of EDTA and NTA as compared with stoichiometric copper recovery CONCENTRATION
( 0 )EDTA: ( 0 )NTA; (--)
2.0
EDTA ADDED, mg/l COPPER EQUIVALENT CHELATING CAPACITY
OF CHELATING
stoichiometric copper recovery
1.0
0.0
6
Figure 3. Recovery of EDTA from secondary sewage effluent containing initially 0.65 mg/l. copper equivalent chelating capacity
(a)chelating agent recovered; (-)
stoichiometric copper recovery
6
"a 4
21 d 5 c
Y 8
4
a 4
u 3 0
5
2
d E 1 2 U
"
0
0
1
2
3
4
5
6
g
C O N C E N T R A T I O N OF EDTA, g 1 0 6
( 0 )EDTA, (-) stoichiornetric copper recovery Copper analysis by the dithizone method
uble NTA chelates of these metals, FeT and CuT-. If the equilibrium of the react.ion Cu(OH),
+
FeT
+
O H - = Fe(OH),
0
1
2
CONCENTRATION
Figure 2. Recovery of low concentrations of EDTA
+
C U T - (6)
lies strongly to the right, iron should not constitute an interference. From published values of the solubility products of Cu(0H)z and Fe(OH)3 and the formation constants of FeT and CUT- ( 6 ) , the equilibrium constant of reaction 6 is calculated as 6.0 X 1015. At p H 10 the ratio Therefore, all of [CUT-]:[FeT] theoretically is 6.0 X the NTA should be present as the copper chelate and iron should not interfere. Experimentally iron(II1) interference is observed in solutions which are not aged or heated as called for in step 4 of' the procedure. In such cases, the analysis of copper and iron in the filtrate shows concentrations of both metals of a comparable magnitude, with the sum of their molar concentrations being equivalent to the concentration of chelating agent. Heating the sample to boiling after adjusting to pH 10 as called for in the procedure or aging for 48 hr will eliminate the iron interference completely. Presumably freshly precipitated iron(II1) is much more soluble than the heated or aged precipitate. Figure 4 shows the effects of icterfering iron in the analysis of EDTA. At each concentration of EDTA, two sepa(6) L . G.Sillen and A . E. Martell, "Stability Constants of Metal-Ion Complexes," The Chemical Society, London, 1964.
Figure iO-3M
3
4
5
6
OF EDTA, fiX1o4
4. Recovery of EDTA from solutions containing 1 iron(1ll)
X
(0)copper from heated samples; ( A ) iron from heated samples; ( 0 ) copper from unheated samples; ( A ) iron from unheated samples; (-) stoichiometric copper recovery
Table I. Levels of Total Strong Heavy Metal Chelating Agents in Typical Water Samples
Sample source
Bee Fork Creek, Mo. Courtois Creek, Mo. Water from a fountain pool fed with tap water and supporting algal growth Eastern Kansas oilfield brine with high transition and heavy metals levels Tap watera Raw sewagea Primary sewage effluenta Activated sludge sewage effluenta
Total chelating agent concentration, mg/l. copper equivalent chelating capacity 1.48 0.98 1.10 31.5
0.00 3.39 3.01
0.90
a From Columbia, Mo.
rate solutions, both containing iron(III), were prepared. One of the solutions was heated after formation of the precipitates as called for in the procedure; the other solution was not heated. Both iron and copper were analyzed in the filtrates. In the heated samples, iron could not be ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, J U L Y 1 9 7 3
1467
Table II. Ammonium Ion Interference with Chelate Analysis (no chelate present) Copper in filtrate, mQ/l. ["4CI]
Unheated sample
Blank 1x 10-4~
1.04 1.10
x 10-3~ 1 x 10-2M
44.80
1
2.08
Heated sample
0.00 0.00 0.00 0.00
detected in the filtrate and the solubilization of copper by EDTA was stoichiometric. In the unheated samples, measurable amounts of iron were found in the filtrate and the concentration of copper in the filtrate was less than that which would result from quantitative solubilization by EDTA. For each point, however, the sum of the molar concentrations of iron and copper solubilized was equivalent to the amount of EDTA in the sample. In the unlikely case of a chelating agent which did solubilize iron in preference to copper, both iron and copper could be analyzed in the filtrate, with the sum of their molar concentrations equalling the total chelating capacity. Although calcium has not been shown to interfere with the analysis of any chelating agents, this potential interference is eliminated by using sodium carbonate for pH adjustment. Calcium chelates are generally much weaker
than copper chelates. However, because of the relatively high solubility of calcium hydroxide, some calcium chelates might predominate over the corresponding copper chelates a t pH 10. The presence of carbonate ion lowers the concentration of soluble calcium below any possible interfering level. Theoretically the ammonia complexes of copper(I1) are not strong enough to cause interferences except a t very high concentrations of ammonium ion. In practice, however, low levels of ammonium ion do interfere if the solutions containing the precipitates are not heated. This step eliminates ammonium-ion interference as shown in Table 11. Even in the absence of any interferences, high blanks are obtained if the heating step in the procedure is omitted. Incomplete removal of colloidal copper hydroxide by filtration probably is responsible. Heating not only eliminates the high blanks, it also facilitates filtration and enables use of the same filter for a number of different samples. Received for review October 24, 1972. Accepted January 22, 1973. This research was supported by the United States Department of the Interior Office of Water Resources Research Allotment Grant A-049-MO and the University of Missouri Environmental Trace Substances Center.
Comparison of Lock-In Amplification and Photon Counting with Low Background Flames and Graphite Atomizers in Atomic Fluorescence Spectrometry M . K. Murphy,' S. A. Clyburn, and Claude Veillon2 Department of Chemistry, University of Houston, Houston, Texas 77004
A photoelectron pulse counting system is compared with a conventional phase-sensitive ("lock-in") amplifier system for use in atomic fluorescence spectrometry. Minimum detectable concentrations and relative sensitivities for Zn, Cu, and Bi are determined, using conventional, low-intensity hollow cathode lamps, a 150-W Xe continuum lamp, and high-intensity electrodeless discharge lamps as primary excitation sources. Four atomization systems having little or no background emission are employed. A sheathed Ar/02/H2 flame, a slotted graphite rod, and a graphite tube system are employed with continuous sample introduction, and a graphite rod system is employed with small individual samples. The photon counting system is superior to the lock-in amplifier system when low-intensity sources are employed, giving detection limits comparable to those reported for atomic absorption flame spectrometry. With high-intensity sources, results with the two measurement systems are about equal.
In instruments used for atomic emission, absorption, and fluorescence spectrometry, signals are derived from photomultipliers and the anode current is measured, by 1468
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
dc, ac (tuned or wide-band), or phase-sensitive ("lock-in") amplifiers. The last has become the most popular for improved signal-to-noise ( S / N ) ratios and is used in almost every commercially available atomic absorption instrument ( I ) , as well as by most researchers in analytical atomic spectrometry. In applications where small signals must be recovered from large noise signals, lock-in amplifiers allow a trade-off of time of measurement for improved S/N. In atomic emission spectrometry, fluctuations in the background emission of the atomization source ( e . g . , flame) constitute the major noise components, while in atomic absorption spectrometry, fluctuations in the primary light source, shot noise, and atomization system fluctuations are frequently the major noise components. In atomic fluorescence spectrometry, one is frequently trying to measure very small signal levels, but not necessarily in the presence of other large noise signals, so noise in the detector-amplifier-readout system often predominates. When used for atomic fluorescence spectrometry, atomization systems having very low back1 D e p a r t m e n t of C h e m i s t r y , C a l i f o r n i a I n s t i t u t e o f Technology, Pasadena, C a l i f . 91109. 2 A u t h o r to w h o m a l l correspondence s h o u l d be directed.
(1) "Handbook of Commercial Scientific Instruments. ' C.Veillon and W. W. Wendlandt, Ed., Vol. 1, Marcel Dekker, New York. N.Y.. 1972.
