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EDTA as a kinetic inhibitor of copper(II) sulfide precipitation
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EDTA AS A K I N E T I C I N H I B I T O R OF COPPER(II) SULFIDE PRECIPITATION GEORGE R. HELZ and LEWIS M. HORZEMPA Department of Chemistry, University of Maryland, College Park. MD 20742. U.S.A. (Received July 198 l I Abstract--Even in billion-fold supersaturated solutions, EDTA has been found to inhibit CuS precipitation for periods of at least a few hours in some cases. The rate of precipitation increases with increasing total copper and sulfide, but decreases with increasing EDTA/Cu ratio. Colloidal sulfur accelerates precipitation rates in acidic solutions. EDTA may affect the behavior of sulfophile trace metals during wastewater treatment. It is speculated that some natural chelaters may also inhibit sulfide precipitation, thus accounting for a number of reports that sulfidic natural waters are grossly supersaturated with respect to sulfide minerals.
INTRODUCTION EDTA (ethylenediaminetetraacetic acid) and some related synthetic chelating agents are widely used as detergent additives, food stabilizers, cleansing agents in radiochemical laboratories, etc. C o n c e n t r a t i o n s of E D T A in the range 0.1-10.0,uM have been f o u n d in wastewater treatment plants, rivers, and groundwaters near radioactive waste disposal sites (Gardiner, 1976; Means et al., 1978). Furthermore, EDTA appears to be only slowly degradable in the environment (Tiedje, 1977; Means et al., 1980). The previous workers who have discussed the probable effects of EDTA in the e n v i r o n m e n t have all focused on shifts that it can cause in solubility and adsorption equilibria. In this paper, we will present evidence that E D T A is furthermore an extremely powerful kinetic inhibitor of sulfide precipitation reactions. This property may permit EDTA to affect the behavior of certain trace metals during and after wastewater treatment.
liminary experiments indicated that turbidity measurements made at this wavelength were not significantly affected by changes in light absorption by either sulfide ion or the blue Cu-EDTA complex. Samples for transmission electron micrography were prepared by a modification of the method of Chiu & Meehan 0974). A drop of the reaction solution was placed on a formvar coated grid. Soluble salts were removed by washing in distilled water. Sample grids were then gold shadowed and examined with a Hitachi transmission microscope. In several experiments, the test solutions were ultrafiltered (Amicon PM-10) after precipitation was complete to separate dissolved and precipitated copper. X-ray diffractometry indicated the precipitate to be covellite (CuS).
INTERPRETATION OF TURBIDITY MEASUREMENTS W h e n light passes through a cell of pathiength, 1, the turbidity, z, is defined as Io
1
= 7 log,o 7
EXPERIMENTAL Cu-EDTA complexes were prepared by mixing CuC12 solutions with solutions containing the disodium salt of EDTA. Copper concentrations in the test solutions ranged from 10 -~ M to 10 -3 M. To the Cu-EDTA solutions were added varying quantities of fresh l0 -3 M Na2S solutions prepared from deoxygenated water. Solution pH levels were controlled by additons of HCI (0.5-5.0 x l0 -s M), or TRIS buffer (2-amino-2-hydroxymethyl-l-3-propanediol; 3 x 10 -3 M). The test solutions were diluted to constant volume and transfered to 10cm spectrophotometer cells which were capped to minimize air contact. All experiments were performed at room temperature (23 + 3°C). Solutions were monitored for relative turbidity changes using a Bausch and Lomb Spectronic 700 spectrophotometer coupled to a Dana digital voltmeter (Model 4430). Measurements of relative turbidity changes were made by subtracting turbidity changes in the reaction solutions from turbidity measurements in control solutions containing no copper. Turbidity measurements were made at 2 = 500nm and begun 5 min after solution mixing. Pre167
(i)
where I o and I are the incident and transmitted light intensities. The turbidity is a function of the number, size, shape, refractive index a n d absorption coefficient of particles in the light beam. For small absorbing spheres of radius, r, the turbidity per cm of cell path may be expressed, -
nxr2 Ke
(2)
2.303 where n is the n u m b e r of particles per cm 3 of suspension and Ke is the extinction coefficient, possessing b o t h absorption and scattering c o m p o n e n t s (Meehan, 1968). According to Van De Hulst (1957; p. 270), Ke for a b s o r b i n g particles can be calculated with the following expression derived from Mie scattering theory.
168
GEORGE R. HELZ and LEWIS .M. I-{ORZEMP~.
I
( m / : - l)
~
3i'm~-
This is then used to evaluate equation {31 with the following results.
15:
K< = 1.31B + 0.93 B 3 + O.ll B a.
.z7L - 27m~: + 38 t Im × 2m~ - 3
+B
I,.i
:l +
(3)
Re.
In this equation, 2rer~Lo
B -
Electron micrographs indicated the spherical CuS particles in this study were <20 nm in radius, For 20 nm particles, the second term in equation (8) is only 77,~,, and the third term only 0.2'~o, of the first term. Thus, for particles below 20 nm. K~ is well-approximated by only the first term in equation (8). Therefore, from (2):
(4) r -~
and ik
t~-
mi -
'
(5)
,rio Here, it and I~o are the refractive indices of the particles and medium, respectively, ). is the wavelength of light, k is the absorption coefficient of the particles, and i = ,/~-[(. When evaluating equation (3), only the imaginary part of the first term and the real part of the second term are used. The coefficient of absorption, k, for covellite was calculated from the theoretical relationship between the reflectivity, R i , the refractive index and the coefficient of absorption (Cameron. 1961, p. 165): (~l R r -
(8)
~Lo) 2 + k'-
(,u + /~o)2 + k2.
(6)
Values for the reflectivity of polished sections of covellite have been taken from data gathered by Winchell & Winchell (1964) for the appropriate wavelength. A refractive index of 2.00 has been taken from data in Palache e t a L (1957) based on thin sections (0.0005 mm or less) of covellite. At 500 nm. a value of k = 0.90 has been calculated. Substituting this into equation (5) yields a value for m~. mi = 1.50 - 0.675i.
(7)
1.3 l
Bn~r z
2.303
= 3,00 X l05 #zr3 (r in cm).
(9)
An important implication of this approximation is that turbidity is directly proportional to the total volume (and total mass) of the precipitate. Consequently, turbidity is a valid measure of reaction progress. For larger particles, this would not be true because the higher order terms in (8) cannot be neglected. Equation (9) was tested in one experiment where the final measured turbidity was 0 . 1 8 c m - t . The particle radius was found by electron micrography to be about 11 nm. The total mass of precipitate was estimated by ultrafiltering the solution and then analyzing the copper in the concentrate. The number of particles was calculated to be 2 x 10 't cm -3 by computing the mass of an individual particle, assuming a density of 4.7, and dividing this into the total mass of precipitate. The turbidity calculated from equation (9) using these values is 0.08 c m - t about a factor of two below the observed value. This is probably reasonable agreement considering the imprecision of the optical data needed for the calculation. RESULTS Table I indicates the range of typical copper and
Table l. Initial composition and degree of saturation in selected experiments
Experiment
pH
Total Cu-" * (mMI
A
8.1 8.1 8.1 8.1 8. l 8.1 8.1 8.1 33 3,3 3.3
0.22 0. ll 0.22 0.11 0.22 0.22 0.10 0.10 0.10 0,10 0.10
13
C D E F G H 1 J K
Total EDTA (mM)
Total S z(mM}
log ac~2-*
log as-,-*
log (ac~:- as-,-/K,v)*
0.24 0.12 0.44 0.22 0.24 0.24 0.20 0.20 0.20 0,20 0.20
0.36 0.36 0.36 0.36 0.27 0.72 0.18 0.36 0.36 0.18 0.09
- 16,4 -16.4 - 17.4 - 17.4 - 16.4 - 16.4 - 17.4 - 17.4 -5.5 -5.5 -5.5
- 8.3 -8.3 - 8.3 -8.3 -8.4 -8.0 -8.6 -8.3 -16.9 - 17.2 - 17.5
10.4 10.4 9.4 9.4 10.3 10.7 9.1 9.4 12.7 12.4 12.1
*Ionization constants of H.,S and the solubility product of CuS are consistent with the thermodynamic data of Robie et al. (1978). Acidity and complexing constants for EDTA are from Schwarzenbach & Akermann (1947), and Anderegg (1964), corrected to 10-3M ionic strength using the Davies equation. In calculating acu.,-, copper hydroxide and chloride complexes were considered, but found to be negligible. Bisulfide and polysulfide complexes were neglected; increasing precipitation rate with increasing total sulfide (Fig. 2) suggests that such complexes have, at most, a minor role in the copper speciation.
EDTA as a kinetic inhibitor of CuS precipitation
I69
0.20
pH-8. t
. f
o.~ -
o,o , _
tB~ -//"
(c)'
cu,.o/ 2
EOTAr.O.12/ /
..,. ./" .....;-~2~° :
I
.
,
/
,.5,/ ° " I 2
Fig. I. The effect on precipitation rate of varying EDTA and copper at constant pH {S.ll and total sulfide (0.36mM). Letters beside the curves refer to Table I. Cu r and EDTA r refer to initial total concentrations of these components. sulfide concentrations investigated. The activities of free copper (acL,-,-) and free sulfide (as:-) initially present in the reaction solutions are also presented. While the physical existence of the S a- ion is doubtful (Ellis & Giggenbach, 1971: Goldhaber & Kaplan, 1975), as--- is reported here because it remains a convenient and valid thermodynamic variable for assessing the state of the solution with respect to saturation. The initial degree of saturation in the experiments is indicated in the right-hand column of Table 1. All solutions discussed here were initially supersaturated by more than 109 . Figure 1 indicates the results of several experiments conducted to observe the effects of copper and EDTA variations on the precipitation process. For these experiments, the pH was maintained at 8.1 _+ 0.1 using Tris buffer (1 × 10 -3 M). Based on the data of Bai & Martell (1969), Tris complexing will have a negligible effect on aco:- under the conditions of the present study. It can be seen that solutions possessing higher initial degrees of supersaturation and lower total EDTA to total copper ratios (curves A and B), display more rapid rates of precipitation. In the absence of EDTA, precipitation was instantaneous on the time scale of our measurements. Greater amounts of CuS were precipitated from solutions containing greater total copper concentrations (curves A and B). In addition the curves suggest a two stage precipitation process. Finally, note that curve (D) shows only slight evidence of CuS precipitation (~ = 0.003 c m - ~), despite a high degree of supersaturation. The effect of varying total sulfide is shown in Fig. 2. Increasing the total sulfide ion concentration while maintaining total Cu and total EDTA constant, resulted in increases in both the rate and extent of precipitation. In experiments (A) and (F), unreacted copper was separated from CuS by ultrafiltration after the conclusion of the experiment. Atomic absorption analysis for Cu indicated that at ST = 3.6 x 10 -¢ M, 50-60% w.R. 172 D
(F)
St = 0.72 mM
/
E
,~'--'~'-
;
.
/
/
(A)
S-'0.36 m M
OJ5
~,-__
o,o
• 0 44 E. r . CO r - 0.22
h
.......
I
0.20
7
• 0.05
CUr'O.II (O) EDTAT'0"22 I • I =+" 3 4 5
Time,
/
0.25
cu..o,,/ / EDTA
. , o /" /
o
(A)
/
J:
.~ I--
. . . .
• Sr" o.27mM
e /
,-(E}
L~I/~IS..- I / I I
o
1 2
1 3
I 4
Time,
} 5
6
h
Fig. 2. The effect on precipitation rate of varying total sulfide at pH 8.1. Letters beside the curves refer to Table 1. Sr is the initial total sulfide concentration. Total copper (0.22 mM) and total EDTA {0.24 mM) were the same for all three experiments. of the available copper had reacted while at ST = 7.2 x 10-'* M more than 902~, had reacted. Calcttlations indicate, however, that even if 90'!,o of the available material were precipitated as CuS. the solutions would remain strongly supersaturated. Hence, the turbidity plateaus observed in Fig. 2 apparently are not attributable to a transition from supersaturation to equilibrium. Subsequent experiments have shown that leakage of air into the optical cells caused the colloidal sulfide particles eventually to be oxidized as fast as they precipitated. To test for the effects of pH on the precipitation process, experiments were conducted at pH 7.4 + 0.I using a phosphate buffer ( 1 0 - 3 M ) and at pH3.3 using HCI to control pH. At pH 7.4 the effects of variations in CUT, EDTAT, and S T w e r e found to be similar to those observed at pH 8.1 suggesting that the nature of the buffer may not be important. At pH 3.3. however, significant differences were observed.
(I) /
j~T-
~ "
/ ( J )
o
-2
/-----A-- "I . . . . . . . . . .
l" ~-2 a
.--_
/
; ~
0.18 mM pH - 3 . 3
/
7
E
S T - 0 . 3 6 mM pH- 3.3
(H) ST" 0.36ram
/
pH " 8. I
f
/ -~
"J
f
•
(G) ST" O'lSmM (
~ i //m ~
Ii~ m~"
....
.-----I ---i---l"
8.1
pH
~e
•
•
(K)
ST" ;13 -09 mM
•
Detection limit -4
l I
I 2 Time,
I 3
h
Fig. 3. Comparison of precipitation experiments at pH 3.3 {solid curves) and pH8.1 (dashed curves). Total copper = 0.1 mM and total EDTA = 0.2 raM.
[70
GEORGE R. HELZ and LEWIS M. HORZEMP~, 0.12
0,10
//
• /
0.08
/
50 % O x i d i z e d / >,
/
0.06
004
O.OZ
/ /m . /
/ 25 % Oxidized sulfur
/' /
IO0 % Oxidized sulfur (N)
~ ,
I
-
2 Time,
,
,
3
4
h
Fig. 4. The effect of using oxidized sulfide stock solutions to prepare test mixtures. All experiments initially contained 0.18 mM total sulfur. 0.1 mM total copper and 0.2 mM EDTA at pH 3.3. However. the sulfur was obtained by mixing a fresh sulfide stock solution with an oxidized sulfide stock solution which was cloudy due to colloidal sulfur. For comparable EDTA, copper and sulfide concentrations, greater amounts of precipitation occurred at pH 3.3 (Fig. 3). In addition, as total sulfide values were increased from 0.09 to 0.36 m M, there appeared to be an abrupt onset of precipitation following induction periods of up to an hour. Solutions containing less than 0.1mM total sulfide displayed no observable precipitation over a period of several hours. It was observed that in the acidic solutions the sulfide concentrations required to produce precipitation appeared to correlate with those required to produce colloidal sulfur in air-oxidized control solutions containing no copper. Colloidal sulfur is formed from the precipitation of dissolved sulfur, the primary sulfide oxidation product in acidic solutions (Kenyon & La Met, 1949). To check the hypothesis that colloidal sulfur serves as a heterogeneous nucleus for CuS precipitation, test solutions were prepared from mixtures of fresh sulfide stock solutions and previously air-oxidized sulfide stock solutions. The latter were milky due to colloidal sulfur. The results are given in Fig. 4. Solutions containing larger fractions of oxidized sulfide (e.g. curve L) yielded more rapid initial rates of CuS precipitation than solutions containing smaller fractions of oxidized sulfide (curve M). However, virtually no growth occurred when oxidized stock solution was used as the sole source of sulfur (curve N); presumably this solution contained virtually no remaining sulfide. The addition of small quantities of gelatin (0.1-10.0 mg1-1) to acidic solutions was observed strongly to retard precipitation. Previous studies (Horzempa & Helz, 1979) have demonstrated that gelatin can prevent flocculation of colloidal CuS particles apparently through a surface coating process (steric stabilization).
Electron micrographs showed CuS particles to be essentially spherical and quite homogeneous in size. In a typical low-pH experiment (J. Table 1), the particles had radii on the order of 11 + 2 nm. Particles produced in a typical neutral-pH experiment (A, Table 1) possessed somewhat somewhat smaller and more varied radii: 7 + 3nm. Thus the product obtained consists of extremely fine particles, which are characteristic of precipitation from grossly supersaturated solutions and distinct from coarse-grained products often obtained by slow crystallization near saturation. DISCUSSION
Our results show that CuS precipitation, which is practically instantaneous in the absence of EDTA, can be completely blocked in some cases for a period of at least hours by moderate concentrations of EDTA (e.g. Runs D and KI. Yet, it is clear from the saturation calculations in the right-hand column of Table I that these solutions are tremendously supersaturated. Therefore the role of EDTA is clearly that of kinetic inhibitor. Preliminary work in our laboratory suggests that EDTA is similarly effective at inhibiting precipitation of CdS and PbS. The mechanism of inhibition is not yet known but the rate apparently is not limited by a direct reaction between free Cu-'" ions and dissolved sulfide species. Comparison of runs A vs B or C vs D indicates that different rates are obtained even when the concentrations of free Cu-' + and the dissolved sulfide species do not change. Furthermore, any reaction that involved free Cu 2 + at 10- ' 6 I0- l 7 M would necessarily be much slower than the rate observed here even if it proceeded at the diffusion limit. Thus the kinetically active metal species apparently is the C u - E D T A chelate, and inhibition must occur because of a slow
E DTA as a kinetic inhibitor of CuS precipitation step somewhere in the process that transfers Cu from the chelate to the growing CuS surface. The inhibitory role of E D T A is of potential importance with regard to the behavior of trace metals during wastewater treatment. Morel et al. (19751 have examined trace data from the Joint Water Pollution Control Project of the Los Angeles County Sanitation District. and they conclude from chemical equilibrium modelling that sulfide precipitation may control the partitioning of trace metals between the dissolved and particulate phases. In that treatment plant, primary sludge was anerobically digested and then re-mixed with the effluent. If sulfide supersaturation is produced generally in this type of treatment system, then the presence of precipitation inhibitors, such as EDTA, could be an important factor in the behavior of sulfophile trace metals in the wastewater effluent. It would lie beyond the capability of equilibrium modelling to account for such behavior. In order for EDTA to inhibit precipitation of a trace metal sulfide, two requirements must certainly be met: the EDTA concentration must exceed the trace metal concentration and the concentration of competing cations, such as Ca-'- and Mg z÷, must be low enough in comparison to the trace metal so that the latter is not displaced from EDTA. How generally these conditions are met in wastewater effluents cannot be definitely determined. Too few EDTA analyses are currently available (a situation which should be changed) and cation abundances in wastewater are notoriously variable even when measured as a function of time at a single treatment plant. However, Gardiner (1976) evaluated the equilibrium distribution of Cu z + between several ligands for one plausible set of assumed concentrations (EDTA = l/.tM, Cu = 0.05,uM) and found that below p H 9 , Cu -'+ would be almost entirely bound to EDTA even in the presence of 2.5 mM Ca-' +. Thus it may be said that the conditions required for EDTA chelation are not unreasonable ones, although there will probably be wastewater effluents and receiving streams in which they are not met. It is interesting to speculate that some of the polymeric organic chelating agents known to be common in wastewaters, and in natural waters, generally (Sunda & Hanson, 19791, may also possess EDTA's capacity to inhibit sulfide precipitation. There is considerable evidence that sulfide-bearing natural waters appear to achieve very high degrees of supersaturation with respect to many sulfide minerals (Barnes & Clarke, 1969: Spencer & Brewer, 1971; Helz & Sinex, 1979; Shanks & Bischoff, 1977; Boulegue, 1977, 1978, Boulegue et al., 19791. Our observation that gelatin (a protein sol) inhibited CuS precipitation in acid solutions may be relevant in this regard. Perhaps for kinetic reasons, precipitation of sulfides, a mechanism often invoked to explain fixation of metals in anaerobic soils, sediments and sludges, is not so important as has previously been believed.
I7l REFERENCES
Anderegg G. 11964) Komplexone XXXVI. Reaktionsenthalpie und-entropie bei der Bildung der Metallkomplexe der hoheren EDTA-Homotogen. Helc. chim. Acta 47, 1801-1814. Bai K. A. & Martell A. E. (1969~ The interaction of 2-amino-2-(hydroxymethyll-t-3-propanediol with copper IIll and nickel (1I) ions. J. inor9, nucl. Chem. 31, 1697-1707. Barnes I. & Clarke F. E. 119691 Chemical properties of groundwater and their corrosion and encrustation effects in wells. U.S. Geol. Surv. Prof. Paper 498-D: 58 pp. Boulegue J. (19771 Equilibrium in a sulfide rich water from Enghien-les-Bains. France. Geochim. cosmochim. Acta 41, 1751-1758. Boulegue J. (19781 Metastable sulfur species and trace metals (Mn, Fe, Cu, Zn, Cd. Pb) in hot brines from the French Dogger. Am. a. Sci. 27g, 1394-I411. Boulegue J., Ciabrini J. P., Fouillac C., Michard G. & Onzounier E. (1979~ Field titrations of dissolved sulfur species in anoxic environments--Geochemistry of Puzzichello waters. Chem. Geol. 25, 19-29. Cameron E. (19611 Ore Micro,~copy. Wiley, New York. Chiu G. & Meehan E. J. (1974) The preparation of monodisperse lead sulfide sols. J. Colloid lmerjiwe Sci. 49, 160-162. Ellis A. J. & Giggenbach W. (1971) Hydrogen sulfide ionization and sulphur hydrolysis in high temperature solution. Geochim. cosmochim. Acta 35, 247-260. Gardiner J. (19761 Complexation of trace metals by ethylenediaminetraacetic acid (EDTA) in natural waters. Water Res. 10, 507-514. Goldhaber M. B. & Kaplan I. R. (19751 Apparent dissociation constants of hydrogen sulfide in chloride solutions. Mar. Chem. 3, 83-104. Helz G. R. & Sinex S. A. {19741 Chemical equilibria in the thermal spring waters of Virginia. Geochim. cosmochim. Acta 38, 1807-1820. Horzempa L. M. & Helz G. R. (19791 Controls on the stability of sulfide sols: colloidal covellite as an example. Geochim. cosmochim. Acta 43. Kenyon A. A. & LaMer V. K. (1949) Light scattering studies of monodisperse sulfur sols. J. Colloid lnterfi~ce Sci. 24, 163-184. Meehan E. J. (19681 Light absorption and scattering in colloidal suspensions of strongly absorbing substances. J. Colloid. Interface Sci. 27, 388-394. Means J. L.. Crerar D. A. & Duguid J. D. (19781 Migration of radioactive wastes: radionuctide mobilization by complexing agents. Science 200, 1477-1481. Means J. L., Kucak T. & Crerar D. A. (19801 Relative degradation rates of NTA, EDTA and DTPA and environmental implications. Encir. Pollut. I, 45-60. Morel F. M. M., Westall J. C., O'Melia C. R. & Morgan J. J. (19751 Fate of trace metals in Los Angeles County Wastewater Discharge. Encir. Sci. Technol. 9, 756-761. Palache C., Berman H. & Frondel C. (1957) Da~,a's System of Mineraloyy. Wiley, New York. Robie R. A., Hemingway B. S. & Fisher J. R. (19781 Thermodynamic properties of minerals and related substances at 298. 15'K and 1 Bar (105 pascals) pressure and at higher temperatures. U.S. Geol. Sure. Bull. 1452. 456 pp. Schwarzenbach G. & Ackermann H. (1947) Komplexone V. Die Athylendiamintetraessigsaure. Heir. chim, Acta 30, 1798-1804. Shanks W. C. & Bischoff J. L. (1977) Ore transport and deposition in the Red Sea geothermal system: A geochemical model. Geochim. cosmochim. Acta 41, 1507-1519. Spencer D. W. & Brewer P. G. (197l) Vertical advection diffusion and redox potential as controls on the distribu-
172
GEORGE R. HELZ and LEWIS M. HORZEMP.-~
tion of manganese and other trace metals dissolved in ,*aters of the Black Sea. J. yeophys. Res. 76, 58775892. Sunda W. G. & Hanson P. J. 11979) Chemical speciation of copper in river water: Effect of total copper, pH. carbonate and dissolved organic matter. In Chemical Modeling in Aqueous Systems tEdited by Jenne E. A.I. American Chemical Society Symposium Series 93, 147-180.
Tiedje J. M. 11977) lntluence of environmental parameters on EDTA in soils and sediments. J. et;vir. Q,a~l. 6, 21-26. Van De Hulst H. C. {19571 Light Scattering B)' Small Partitles. Wiley. New York. Winchell A. & Winchetl H. {1964J The Micro Scopical Characters of Artificial Inorganic Solid Substances. Academic Press, New York.
0043-I3548302016--06$03.000 Pergamon Press Ltd
EDTA AS A K I N E T I C I N H I B I T O R OF COPPER(II) SULFIDE PRECIPITATION GEORGE R. HELZ and LEWIS M. HORZEMPA Department of Chemistry, University of Maryland, College Park. MD 20742. U.S.A. (Received July 198 l I Abstract--Even in billion-fold supersaturated solutions, EDTA has been found to inhibit CuS precipitation for periods of at least a few hours in some cases. The rate of precipitation increases with increasing total copper and sulfide, but decreases with increasing EDTA/Cu ratio. Colloidal sulfur accelerates precipitation rates in acidic solutions. EDTA may affect the behavior of sulfophile trace metals during wastewater treatment. It is speculated that some natural chelaters may also inhibit sulfide precipitation, thus accounting for a number of reports that sulfidic natural waters are grossly supersaturated with respect to sulfide minerals.
INTRODUCTION EDTA (ethylenediaminetetraacetic acid) and some related synthetic chelating agents are widely used as detergent additives, food stabilizers, cleansing agents in radiochemical laboratories, etc. C o n c e n t r a t i o n s of E D T A in the range 0.1-10.0,uM have been f o u n d in wastewater treatment plants, rivers, and groundwaters near radioactive waste disposal sites (Gardiner, 1976; Means et al., 1978). Furthermore, EDTA appears to be only slowly degradable in the environment (Tiedje, 1977; Means et al., 1980). The previous workers who have discussed the probable effects of EDTA in the e n v i r o n m e n t have all focused on shifts that it can cause in solubility and adsorption equilibria. In this paper, we will present evidence that E D T A is furthermore an extremely powerful kinetic inhibitor of sulfide precipitation reactions. This property may permit EDTA to affect the behavior of certain trace metals during and after wastewater treatment.
liminary experiments indicated that turbidity measurements made at this wavelength were not significantly affected by changes in light absorption by either sulfide ion or the blue Cu-EDTA complex. Samples for transmission electron micrography were prepared by a modification of the method of Chiu & Meehan 0974). A drop of the reaction solution was placed on a formvar coated grid. Soluble salts were removed by washing in distilled water. Sample grids were then gold shadowed and examined with a Hitachi transmission microscope. In several experiments, the test solutions were ultrafiltered (Amicon PM-10) after precipitation was complete to separate dissolved and precipitated copper. X-ray diffractometry indicated the precipitate to be covellite (CuS).
INTERPRETATION OF TURBIDITY MEASUREMENTS W h e n light passes through a cell of pathiength, 1, the turbidity, z, is defined as Io
1
= 7 log,o 7
EXPERIMENTAL Cu-EDTA complexes were prepared by mixing CuC12 solutions with solutions containing the disodium salt of EDTA. Copper concentrations in the test solutions ranged from 10 -~ M to 10 -3 M. To the Cu-EDTA solutions were added varying quantities of fresh l0 -3 M Na2S solutions prepared from deoxygenated water. Solution pH levels were controlled by additons of HCI (0.5-5.0 x l0 -s M), or TRIS buffer (2-amino-2-hydroxymethyl-l-3-propanediol; 3 x 10 -3 M). The test solutions were diluted to constant volume and transfered to 10cm spectrophotometer cells which were capped to minimize air contact. All experiments were performed at room temperature (23 + 3°C). Solutions were monitored for relative turbidity changes using a Bausch and Lomb Spectronic 700 spectrophotometer coupled to a Dana digital voltmeter (Model 4430). Measurements of relative turbidity changes were made by subtracting turbidity changes in the reaction solutions from turbidity measurements in control solutions containing no copper. Turbidity measurements were made at 2 = 500nm and begun 5 min after solution mixing. Pre167
(i)
where I o and I are the incident and transmitted light intensities. The turbidity is a function of the number, size, shape, refractive index a n d absorption coefficient of particles in the light beam. For small absorbing spheres of radius, r, the turbidity per cm of cell path may be expressed, -
nxr2 Ke
(2)
2.303 where n is the n u m b e r of particles per cm 3 of suspension and Ke is the extinction coefficient, possessing b o t h absorption and scattering c o m p o n e n t s (Meehan, 1968). According to Van De Hulst (1957; p. 270), Ke for a b s o r b i n g particles can be calculated with the following expression derived from Mie scattering theory.
168
GEORGE R. HELZ and LEWIS .M. I-{ORZEMP~.
I
( m / : - l)
~
3i'm~-
This is then used to evaluate equation {31 with the following results.
15:
K< = 1.31B + 0.93 B 3 + O.ll B a.
.z7L - 27m~: + 38 t Im × 2m~ - 3
+B
I,.i
:l +
(3)
Re.
In this equation, 2rer~Lo
B -
Electron micrographs indicated the spherical CuS particles in this study were <20 nm in radius, For 20 nm particles, the second term in equation (8) is only 77,~,, and the third term only 0.2'~o, of the first term. Thus, for particles below 20 nm. K~ is well-approximated by only the first term in equation (8). Therefore, from (2):
(4) r -~
and ik
t~-
mi -
'
(5)
,rio Here, it and I~o are the refractive indices of the particles and medium, respectively, ). is the wavelength of light, k is the absorption coefficient of the particles, and i = ,/~-[(. When evaluating equation (3), only the imaginary part of the first term and the real part of the second term are used. The coefficient of absorption, k, for covellite was calculated from the theoretical relationship between the reflectivity, R i , the refractive index and the coefficient of absorption (Cameron. 1961, p. 165): (~l R r -
(8)
~Lo) 2 + k'-
(,u + /~o)2 + k2.
(6)
Values for the reflectivity of polished sections of covellite have been taken from data gathered by Winchell & Winchell (1964) for the appropriate wavelength. A refractive index of 2.00 has been taken from data in Palache e t a L (1957) based on thin sections (0.0005 mm or less) of covellite. At 500 nm. a value of k = 0.90 has been calculated. Substituting this into equation (5) yields a value for m~. mi = 1.50 - 0.675i.
(7)
1.3 l
Bn~r z
2.303
= 3,00 X l05 #zr3 (r in cm).
(9)
An important implication of this approximation is that turbidity is directly proportional to the total volume (and total mass) of the precipitate. Consequently, turbidity is a valid measure of reaction progress. For larger particles, this would not be true because the higher order terms in (8) cannot be neglected. Equation (9) was tested in one experiment where the final measured turbidity was 0 . 1 8 c m - t . The particle radius was found by electron micrography to be about 11 nm. The total mass of precipitate was estimated by ultrafiltering the solution and then analyzing the copper in the concentrate. The number of particles was calculated to be 2 x 10 't cm -3 by computing the mass of an individual particle, assuming a density of 4.7, and dividing this into the total mass of precipitate. The turbidity calculated from equation (9) using these values is 0.08 c m - t about a factor of two below the observed value. This is probably reasonable agreement considering the imprecision of the optical data needed for the calculation. RESULTS Table I indicates the range of typical copper and
Table l. Initial composition and degree of saturation in selected experiments
Experiment
pH
Total Cu-" * (mMI
A
8.1 8.1 8.1 8.1 8. l 8.1 8.1 8.1 33 3,3 3.3
0.22 0. ll 0.22 0.11 0.22 0.22 0.10 0.10 0.10 0,10 0.10
13
C D E F G H 1 J K
Total EDTA (mM)
Total S z(mM}
log ac~2-*
log as-,-*
log (ac~:- as-,-/K,v)*
0.24 0.12 0.44 0.22 0.24 0.24 0.20 0.20 0.20 0,20 0.20
0.36 0.36 0.36 0.36 0.27 0.72 0.18 0.36 0.36 0.18 0.09
- 16,4 -16.4 - 17.4 - 17.4 - 16.4 - 16.4 - 17.4 - 17.4 -5.5 -5.5 -5.5
- 8.3 -8.3 - 8.3 -8.3 -8.4 -8.0 -8.6 -8.3 -16.9 - 17.2 - 17.5
10.4 10.4 9.4 9.4 10.3 10.7 9.1 9.4 12.7 12.4 12.1
*Ionization constants of H.,S and the solubility product of CuS are consistent with the thermodynamic data of Robie et al. (1978). Acidity and complexing constants for EDTA are from Schwarzenbach & Akermann (1947), and Anderegg (1964), corrected to 10-3M ionic strength using the Davies equation. In calculating acu.,-, copper hydroxide and chloride complexes were considered, but found to be negligible. Bisulfide and polysulfide complexes were neglected; increasing precipitation rate with increasing total sulfide (Fig. 2) suggests that such complexes have, at most, a minor role in the copper speciation.
EDTA as a kinetic inhibitor of CuS precipitation
I69
0.20
pH-8. t
. f
o.~ -
o,o , _
tB~ -//"
(c)'
cu,.o/ 2
EOTAr.O.12/ /
..,. ./" .....;-~2~° :
I
.
,
/
,.5,/ ° " I 2
Fig. I. The effect on precipitation rate of varying EDTA and copper at constant pH {S.ll and total sulfide (0.36mM). Letters beside the curves refer to Table I. Cu r and EDTA r refer to initial total concentrations of these components. sulfide concentrations investigated. The activities of free copper (acL,-,-) and free sulfide (as:-) initially present in the reaction solutions are also presented. While the physical existence of the S a- ion is doubtful (Ellis & Giggenbach, 1971: Goldhaber & Kaplan, 1975), as--- is reported here because it remains a convenient and valid thermodynamic variable for assessing the state of the solution with respect to saturation. The initial degree of saturation in the experiments is indicated in the right-hand column of Table 1. All solutions discussed here were initially supersaturated by more than 109 . Figure 1 indicates the results of several experiments conducted to observe the effects of copper and EDTA variations on the precipitation process. For these experiments, the pH was maintained at 8.1 _+ 0.1 using Tris buffer (1 × 10 -3 M). Based on the data of Bai & Martell (1969), Tris complexing will have a negligible effect on aco:- under the conditions of the present study. It can be seen that solutions possessing higher initial degrees of supersaturation and lower total EDTA to total copper ratios (curves A and B), display more rapid rates of precipitation. In the absence of EDTA, precipitation was instantaneous on the time scale of our measurements. Greater amounts of CuS were precipitated from solutions containing greater total copper concentrations (curves A and B). In addition the curves suggest a two stage precipitation process. Finally, note that curve (D) shows only slight evidence of CuS precipitation (~ = 0.003 c m - ~), despite a high degree of supersaturation. The effect of varying total sulfide is shown in Fig. 2. Increasing the total sulfide ion concentration while maintaining total Cu and total EDTA constant, resulted in increases in both the rate and extent of precipitation. In experiments (A) and (F), unreacted copper was separated from CuS by ultrafiltration after the conclusion of the experiment. Atomic absorption analysis for Cu indicated that at ST = 3.6 x 10 -¢ M, 50-60% w.R. 172 D
(F)
St = 0.72 mM
/
E
,~'--'~'-
;
.
/
/
(A)
S-'0.36 m M
OJ5
~,-__
o,o
• 0 44 E. r . CO r - 0.22
h
.......
I
0.20
7
• 0.05
CUr'O.II (O) EDTAT'0"22 I • I =+" 3 4 5
Time,
/
0.25
cu..o,,/ / EDTA
. , o /" /
o
(A)
/
J:
.~ I--
. . . .
• Sr" o.27mM
e /
,-(E}
L~I/~IS..- I / I I
o
1 2
1 3
I 4
Time,
} 5
6
h
Fig. 2. The effect on precipitation rate of varying total sulfide at pH 8.1. Letters beside the curves refer to Table 1. Sr is the initial total sulfide concentration. Total copper (0.22 mM) and total EDTA {0.24 mM) were the same for all three experiments. of the available copper had reacted while at ST = 7.2 x 10-'* M more than 902~, had reacted. Calcttlations indicate, however, that even if 90'!,o of the available material were precipitated as CuS. the solutions would remain strongly supersaturated. Hence, the turbidity plateaus observed in Fig. 2 apparently are not attributable to a transition from supersaturation to equilibrium. Subsequent experiments have shown that leakage of air into the optical cells caused the colloidal sulfide particles eventually to be oxidized as fast as they precipitated. To test for the effects of pH on the precipitation process, experiments were conducted at pH 7.4 + 0.I using a phosphate buffer ( 1 0 - 3 M ) and at pH3.3 using HCI to control pH. At pH 7.4 the effects of variations in CUT, EDTAT, and S T w e r e found to be similar to those observed at pH 8.1 suggesting that the nature of the buffer may not be important. At pH 3.3. however, significant differences were observed.
(I) /
j~T-
~ "
/ ( J )
o
-2
/-----A-- "I . . . . . . . . . .
l" ~-2 a
.--_
/
; ~
0.18 mM pH - 3 . 3
/
7
E
S T - 0 . 3 6 mM pH- 3.3
(H) ST" 0.36ram
/
pH " 8. I
f
/ -~
"J
f
•
(G) ST" O'lSmM (
~ i //m ~
Ii~ m~"
....
.-----I ---i---l"
8.1
pH
~e
•
•
(K)
ST" ;13 -09 mM
•
Detection limit -4
l I
I 2 Time,
I 3
h
Fig. 3. Comparison of precipitation experiments at pH 3.3 {solid curves) and pH8.1 (dashed curves). Total copper = 0.1 mM and total EDTA = 0.2 raM.
[70
GEORGE R. HELZ and LEWIS M. HORZEMP~, 0.12
0,10
//
• /
0.08
/
50 % O x i d i z e d / >,
/
0.06
004
O.OZ
/ /m . /
/ 25 % Oxidized sulfur
/' /
IO0 % Oxidized sulfur (N)
~ ,
I
-
2 Time,
,
,
3
4
h
Fig. 4. The effect of using oxidized sulfide stock solutions to prepare test mixtures. All experiments initially contained 0.18 mM total sulfur. 0.1 mM total copper and 0.2 mM EDTA at pH 3.3. However. the sulfur was obtained by mixing a fresh sulfide stock solution with an oxidized sulfide stock solution which was cloudy due to colloidal sulfur. For comparable EDTA, copper and sulfide concentrations, greater amounts of precipitation occurred at pH 3.3 (Fig. 3). In addition, as total sulfide values were increased from 0.09 to 0.36 m M, there appeared to be an abrupt onset of precipitation following induction periods of up to an hour. Solutions containing less than 0.1mM total sulfide displayed no observable precipitation over a period of several hours. It was observed that in the acidic solutions the sulfide concentrations required to produce precipitation appeared to correlate with those required to produce colloidal sulfur in air-oxidized control solutions containing no copper. Colloidal sulfur is formed from the precipitation of dissolved sulfur, the primary sulfide oxidation product in acidic solutions (Kenyon & La Met, 1949). To check the hypothesis that colloidal sulfur serves as a heterogeneous nucleus for CuS precipitation, test solutions were prepared from mixtures of fresh sulfide stock solutions and previously air-oxidized sulfide stock solutions. The latter were milky due to colloidal sulfur. The results are given in Fig. 4. Solutions containing larger fractions of oxidized sulfide (e.g. curve L) yielded more rapid initial rates of CuS precipitation than solutions containing smaller fractions of oxidized sulfide (curve M). However, virtually no growth occurred when oxidized stock solution was used as the sole source of sulfur (curve N); presumably this solution contained virtually no remaining sulfide. The addition of small quantities of gelatin (0.1-10.0 mg1-1) to acidic solutions was observed strongly to retard precipitation. Previous studies (Horzempa & Helz, 1979) have demonstrated that gelatin can prevent flocculation of colloidal CuS particles apparently through a surface coating process (steric stabilization).
Electron micrographs showed CuS particles to be essentially spherical and quite homogeneous in size. In a typical low-pH experiment (J. Table 1), the particles had radii on the order of 11 + 2 nm. Particles produced in a typical neutral-pH experiment (A, Table 1) possessed somewhat somewhat smaller and more varied radii: 7 + 3nm. Thus the product obtained consists of extremely fine particles, which are characteristic of precipitation from grossly supersaturated solutions and distinct from coarse-grained products often obtained by slow crystallization near saturation. DISCUSSION
Our results show that CuS precipitation, which is practically instantaneous in the absence of EDTA, can be completely blocked in some cases for a period of at least hours by moderate concentrations of EDTA (e.g. Runs D and KI. Yet, it is clear from the saturation calculations in the right-hand column of Table I that these solutions are tremendously supersaturated. Therefore the role of EDTA is clearly that of kinetic inhibitor. Preliminary work in our laboratory suggests that EDTA is similarly effective at inhibiting precipitation of CdS and PbS. The mechanism of inhibition is not yet known but the rate apparently is not limited by a direct reaction between free Cu-'" ions and dissolved sulfide species. Comparison of runs A vs B or C vs D indicates that different rates are obtained even when the concentrations of free Cu-' + and the dissolved sulfide species do not change. Furthermore, any reaction that involved free Cu 2 + at 10- ' 6 I0- l 7 M would necessarily be much slower than the rate observed here even if it proceeded at the diffusion limit. Thus the kinetically active metal species apparently is the C u - E D T A chelate, and inhibition must occur because of a slow
E DTA as a kinetic inhibitor of CuS precipitation step somewhere in the process that transfers Cu from the chelate to the growing CuS surface. The inhibitory role of E D T A is of potential importance with regard to the behavior of trace metals during wastewater treatment. Morel et al. (19751 have examined trace data from the Joint Water Pollution Control Project of the Los Angeles County Sanitation District. and they conclude from chemical equilibrium modelling that sulfide precipitation may control the partitioning of trace metals between the dissolved and particulate phases. In that treatment plant, primary sludge was anerobically digested and then re-mixed with the effluent. If sulfide supersaturation is produced generally in this type of treatment system, then the presence of precipitation inhibitors, such as EDTA, could be an important factor in the behavior of sulfophile trace metals in the wastewater effluent. It would lie beyond the capability of equilibrium modelling to account for such behavior. In order for EDTA to inhibit precipitation of a trace metal sulfide, two requirements must certainly be met: the EDTA concentration must exceed the trace metal concentration and the concentration of competing cations, such as Ca-'- and Mg z÷, must be low enough in comparison to the trace metal so that the latter is not displaced from EDTA. How generally these conditions are met in wastewater effluents cannot be definitely determined. Too few EDTA analyses are currently available (a situation which should be changed) and cation abundances in wastewater are notoriously variable even when measured as a function of time at a single treatment plant. However, Gardiner (1976) evaluated the equilibrium distribution of Cu z + between several ligands for one plausible set of assumed concentrations (EDTA = l/.tM, Cu = 0.05,uM) and found that below p H 9 , Cu -'+ would be almost entirely bound to EDTA even in the presence of 2.5 mM Ca-' +. Thus it may be said that the conditions required for EDTA chelation are not unreasonable ones, although there will probably be wastewater effluents and receiving streams in which they are not met. It is interesting to speculate that some of the polymeric organic chelating agents known to be common in wastewaters, and in natural waters, generally (Sunda & Hanson, 19791, may also possess EDTA's capacity to inhibit sulfide precipitation. There is considerable evidence that sulfide-bearing natural waters appear to achieve very high degrees of supersaturation with respect to many sulfide minerals (Barnes & Clarke, 1969: Spencer & Brewer, 1971; Helz & Sinex, 1979; Shanks & Bischoff, 1977; Boulegue, 1977, 1978, Boulegue et al., 19791. Our observation that gelatin (a protein sol) inhibited CuS precipitation in acid solutions may be relevant in this regard. Perhaps for kinetic reasons, precipitation of sulfides, a mechanism often invoked to explain fixation of metals in anaerobic soils, sediments and sludges, is not so important as has previously been believed.
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172
GEORGE R. HELZ and LEWIS M. HORZEMP.-~
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Tiedje J. M. 11977) lntluence of environmental parameters on EDTA in soils and sediments. J. et;vir. Q,a~l. 6, 21-26. Van De Hulst H. C. {19571 Light Scattering B)' Small Partitles. Wiley. New York. Winchell A. & Winchetl H. {1964J The Micro Scopical Characters of Artificial Inorganic Solid Substances. Academic Press, New York.