Ind. Eng. Chem. Res. 1992,31,1111-1115
1111
Removal of Chelated Copper from Wastewaters by Iron Cementation Young Ku* and Chi-Hwa Chen Department of Chemical Engineering, National Taiwan Institute of Technology, 43, Keelung Road, Sec. 4, Taipei, Taiwan, R.O.C.
The treatment of wastewaters containing the chelated copper species by cementation on powdered iron was studied. The removal of heavy metals was found to be a function of solution pH, amount of EDTA and iron, concentration of copper ion, and contact time. In the presence of EDTA, reaction rate of cementation was found to be strongly dependent on solution pH values, and the global reaction rate was found to be half-order with respect to the concentration of copper ion and first-order with respect to the amount of iron. These experimental results can be explained successfully by the distribution of complexed copper ion species in solutions.
Introduction The presence of heavy metals in the aquatic environment has been a subject of increasing importance because of their toxicity and possible accumulation in the environment. Chelating agents in heavy-metal-laden wastewaters are of great concern primarily because of their interaction with heavy metal ions holding them in solution. The presence of chelating agents is believed to play an important role in transporting the heavy metals in the environment and preventing their removal by conventional treatment processes (Nilsson, 1971; Reuter and Perdue, 1977; Ku and Peters, 1986). This fact is of vital importance to certain metal finishing industries such as printed circuit board manufacturers because of the increasing application of strong chelating agents such as ethylenediaminetetraacetate (EDTA),Quadrol, or nitrilotriacetate ("A) in the plating solution (Karra et al., 1985). Growing concern on the removal of chelated heavy metals from wastewaters has stimulated vigorous research activities in the development of a number of specialized treatment technologies. A cementation process utilizing an electropositive sacrificing solid metal for the recovery of the electronegative dissolved metal species regarding wastewater treatment has been demonstrated to be an effective alternative to conventional treatment processes removing heavy metals from wastewaters due to its relatively simplicity, ease of control, and the possible recovery of valuable metals. However, the consumption of sacrificing metal as reducing agents and the redox potential of metals are the main limitations of the process (Agelidis et al., 1988; Agrawal, 1988). Although several experimental studies have been conducted successfully for the removal of various heavy metals from wastewaters by a cementation process (Nadkarni and Wadsworth, 1967; Mackinnon and Ingraham, 1971; Gould, 1982; Agelidis et al., 1985; Addy and Fletcher, 1987; Gould et al., 1987), no previous research work has been reported about the possible adverse effect of the interaction between chelating agents and heavy metals on the dynamics of the process. In this particular research, a cementation process of using iron powder for recovering EDTA-chelated copper species from aqueous solution was studied. The cementation rate and temporal behavior are determined, and the kinetic parameters obtained from experimental rate data are expressed in terms of deactivation kinetics. Background The metal speciation in aqueous solutions is very cumbersome, especially in the presence of chelating agents. Various soluble chelated or complexed metal species may
Table I. Stability Constants for Cu-EDTA Chelating System (Inczedy, 1976) cations cu2+ ligands H+ HL 1 X lo4 C U L 1 x 106 OHCuL2: 6.31 X 10" c~L,:3.16 x 1014 CuL,: 3.98 X 10l6 K ~ +1.6 x 10-19 EDTA HL 2.19 X 1O'O CuL: 6.31 X lo1* H2L: 3.8 X 10l6 C u H L 6.3 X loz1 H,L: 2.14 X 1019 C u O H L 1.58 X loz1 H4L: 2.51 X loz1
be present as expressed by the following equations from which charges of ions have been omitted for convenience. M+L=ML ML + L = ML2 ML,-l
+ L = ML,
+ H = MHL, ML, + OH = MOHL, M + OH = MOH MOH + OH = M(OH)2 ML,
M(OH),-1+ OH = M(OH), where m and t are the maximum number of ligand and hydroxyl ion on metal cation, respectively. Most chelating agents are the conjugated base of Bronsted acids, and therefore the protonation of chelating agent should be considered and given by the following equations: H,L = H + H,-1L H,-1L = H
+ H,-2L
HL=H+L Thus, the equilibrium concentration of various metal species present in aqueous solution can be calculated by solving a number of simultaneous equations. The calculated results of copper species distribution in the presence of EDTA are plotted as a function of pH in Figure 1 using the appropriate equilibrium constants aa shown in Table I. For highly acidic conditions (pH 12),the predominant
0 1992 American Chemical Society 0888-5885/92/2631-1111$03.00/0
1112 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 0 ,
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EDTA.
copper species are the free copper ion, Cu2+, and the protonated copper chelate, CuHEDTA. Very stable copper chelates, CUEDTA, form over a broad pH range from 3 to 11. At highly alkaline solutions, formation of hydroxyl copper chelates, CuOHEDTA, predominates. The removal of copper ions by iron cementation is achieved by the following redox equation: Fe Cu2+= Fez+ + Cu Several redox side reactions relevant to the cementation of copper by iron may occur in the aqueous solution and are expressed as (Nadkarni et al., 1967; Mackinnon and Ingraham, 1971; Agrawal, 1988) Fe + 2H+ = Fe2++ H2
+
2Fe + 4H+ + O2 = 2Fe2++ 2Hz0 2Fe3+ + Fe = 3Fe2+
+ Y202= 2Fe3++ HzO Cu + 2H+ + YZO2= Cu2++ H,O
2Fe2++ 2H+
The experimental works of this research were carried out under nitrogen-purged conditions and the dissolved oxygen levels in solutions were kept under 0.5 mg/L to avoid the possible side effects of dissolved oxygen as described by the above expressions. The only significant redox side reaction is assumed to be the reaction between iron and hydrogen ion at low solution pH levels. As discussed above, the overall reactions involved for copper removal by iron cementation in the presence of a chelating agent is very cumbersome: various metal species may be present in the aqueous solution through successive complexing reactions. The cementation of copper ion by solid iron is thus interfered with by the multiple reactions involved.
Experimental Procedures Iron powder of >99.99% purity with a medium size of 150 pm was used as sacrificing metal. All experimental solutions were prepared with certified reagent grade chemicals and double-distilled water. The reaction kinetic studies were performed in a 2-L reaction vessel apparatus as shown in Figure 2. The vessel was filled with copper and EDTA solution of known concentrations and then purged with nitrogen gas to warrant the dissolved oxygen levels below 0.5 mg/L throughout the course of reaction. By use of an automatic pH controller,
the solution pH was adjusted and maintained at the desired level by periodic addition of acid or base to the solution. A predetermined amount of iron powder was added to the copper feed solution. Because several researchers (Karra et al., 1985; Agelidis et al., 1988) indicate that cementation is a diffusion-controlled reaction at low mixing conditions, a mixing device with Teflon mixer was used and the mixing speed was kept a t 400 rpm after a series of testa were performed to suspend the iron powder in solution and eliminate the effect of mass-transfer control on the reaction rate. Appropriate temperature control was maintained for the reaction by containing the whole apparatus setup in a constant-temperature water bath. Typical reaction runs lasted 40 min to 1 h. At intermittent periods of reaction time, a portion of the reactor solution was transferred and filtered using a 0.45-pm fdter. The filtrate was analyzed for copper and iron concentrations by atomic absorption spectroscopy with a Varian Spectra AA-20 spectrophotometer. Total sample volumes were kept below 3% of the total reactor solution volume. After the experiment was finished, the reaction solution was filtered for metal analysis, and the residual iron powder was rinsed with acetone, dried, and kept in a nitrogen environment prior for study of the surface morphology by scanning electron microscopy (SEM) using a Cambridge S360 and the composition of the cemented deposits by X-ray diffraction (XRD) using a Philips MP710.
Results and Discussion A series of jar tests were carried out to study the residual copper concentrations after cementation with iron for 7 days under various pH conditions at 25 f 2 OC. The experimental results indicated that the residual copper concentrations in solutions can be removed substantially using a sufficient amount of iron powder as a sacrificing metal even in the presence of EDTA. Throughout the solution pH range studied, the copper concentrations were reduced from about 100 mg/L to less than 1.0 mg/L. The effect of solution temperature on the removal of copper ion by iron cementation in the pressure of EDTA was studied at 15, 25,35, and 45 "C. The experimental results are shown in Figure 3. Elevated solution temperature slightly increased the cementation reaction rate. The Arrhenius plot based on the first-order kinetics indicates that the apparent activated energy for the Cu2+Fe-EDTA cementation system is about 12.0 kcal/g-mol (Table 11) for the temperature range of 15-45 "C. Comparing with the apparent activated energy of about 2.0 kcal/g-mol (Ku and Chen, 1992) for copper cementation on iron without any chelating agents, the values of activation energy indicate that the reaction is possibly mixed
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1113 120 Initial conc. of Cu" ion: 100f5 rng/l Particle size of Fe powder: 150 urn DO: under 0.5 mg/l pH: 210.15 ,
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Time(min) Figure 5. Effect of solution pH on the cementation of copper ion on iron powder in the presence of 914 mg/L EDTA. 120 conc. of Cu ion: 1 0 0 t 5 mg/1 le size of Fe powder: 150 urn
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Table 11. Apparent Activated Energies for Copper Ion Cementation on Iron Obtained by Arrhenius Plots in the Presence of EDTA app activated energy, temp range, DH EDTAme/L kcal/e-mol O C 2 467 11.580 15-45 12.870 15-45 2 914 ~
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Time(min) Figure 4. Effect of solution pH on the cementation of copper ion on iron powder in the presence of 457 mg/L EDTA.
-
oeeeo Fe(.)=2.51/1
0 0
20
40
60
80
100
120
Time( min) Figure 6. Effect of amount of iron powder on the cementation of copper ion in the presence of 457 mg/L EDTA.
~~~
( 1
controlled in the presence of EDTA. Figures 4 and 5 indicate the effect of solution pH on the removal of copper by cementation on iron surface in the presence of EDTA concentration of 457 and 914 mg/L, respectively. The cementation rate of copper was found to be significantly lower with increasing solution pH, which may be explained as the formation of copper chelates lead to the inhibition of copper inhibition on the iron powder surface. A series of 48 batch reaction experiments were performed to study the global kinetics of the cementation of
copper ion on iron. The effect of the amount of iron powder on the cementation of copper ion is shown in Figures 6 and 7 for EDTA concentration of 457 and 914 mg/L, respectively. The cementation can be considered as a solid-liquid reaction in essence. The deposit of reduced metal, which nucleated at cathodic sites on the iron particle surface, gradually coalesced as the cementation proceeded. The reaction would cease after the surface site of sacrificing metal was totally occupied by the deposit of reduced metal. The active-site-losing behavior of sacrificing metal is similar to the deactivation of catalyst by coking. The polarographic studies indicate that the dissociation potentials of the CuHEDTA and CuEDTA are -0.46 and -0.76 V, respectively. Both values exceed the metal iron oxidation potential of 0.44V; that is, only free
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992
1114
Table 111. Kinetic Parameters Obtained by Regression for Copper Ion Cementation on Iron in the Presence of EDTA EDTA concn 457 mg/L 914 mg/L init soln pH 104k I a
b correln coeff
2.0 270 1 1.14 1.00
3.0 9.7 0.5 0.86 1.00
4.0 6.2 0.5 0.98 0.99
5.0 5.8 0.5 0.88 0.99
7.0 2.8 0.5 1.1 0.98
9.0 1.6 0.5 1.39 0.95
2.0 230
3.0 5.4 0.5 0.98 0.99
1
1.36 1.00
120
4.0 2.6 0.5 1.05 1.00
5.0 1.3 0.5 1.19 0.99
7.0 5.4 0.5 1.62 0.97
--- 4
9.0 3.2 0.5 0.98 1.00 I
Initial conc. o
100
EDTA: 914 mg I
h
1niti.1 corn. of CU" ion: loor5 m p / ~ Particle size of Fc powder: 150 um Temp.: 25tO.Z'C DO: under 0.5 mg/l
h
5E v
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+
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Figure 8. Consumption of iron powder in the presence of 457 EDTA at pH 2. 0
50
100 150 200 250 300 350 400 450
Time(min)
I2O
Figure 7. Effect of amount of iron powder on the cementation of copper ion in the presence of 914 mg/L EDTA.
copper ion can be reduced by metallic iron. Assuming that cementation only occurs on the surface of sacrificing metal powders which are spherical with uniform size and ignoring the codeposition of both sacrificing and reducing metals, the global deactivation kinetic for Cu2+-Fe system can be described as
where [CUI = concentration of free copper ion to be deposited; k = reaction rate constant; A = surface area of sacrificing metal expressed as the weight of metal for uniform size of metal powder; V = reactor volume; m = amount of iron; and a, b = empirical constants. Assuming only free copper species can deposit on the surface of sacrificing metal, the concentration of free copper ion, [CUI, is [CUI = [CU2+] =
1+
[CUI, CKi(OH)' + CK,[Lp
i = 1,2 (2)
where [Cu], = total copper concentration, and Ki, K, = equilibrium constants of complexing reactions between M2+and OH- and L, respectively. Because the reaction takes days to reach equilibrium, the kinetic measurement obtained should be initial rate data. Using the method of initial rate, the apparent reaction rate constants, k', for copper cementation on iron power obtained under various solution pH conditions and EDTA concentrations are shown in Table 111. The calculated apparent global reaction rate based on the experimental results poses very satisfactory correlations and seems to support the assumption that the distribution of
100
T
1
2E 1 80
v
~ n i t ~ rconc. l of CU" ion: 1 0 ~ mr/l 5 Particle rlze of Fe powder: 150 urn Temp.: 2St0.2'C DO: under 0 , 5 mg/l l2.15
p3::
EDTA: 457 mg/l
Figure 9. Consumption of iron powder in the presence of 457 mg/L EDTA at pH 5.
various copper species may present different adsorption and reaction properties. At very acidic solutions (pH 12), free copper ion (Cu2+)is the primary copper species in solution despite the presence of EDTA and the k'values are almost independent of EDTA concentration. However, for solution pH greater than 3, the distribution of copper species may change significantly due to the formation of various copper chelates; thus the reaction rate would appear to be a strong function of solution pH and EDTA concentration. The residual iron concentrations under various experimental conditions are presented in Figures 8 and 9. Unlike the results found by several researchers for different systems without chelating agents, the discrepancies between the experimental residual iron concentrations in solutions and that which could be accounted for by the copper cementation were not very significant. Comparing
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1115 with the experimental renulta for copper cementation on iron without the presence of chelating agents (Ku and Chen, 1992), both the iron consumption rates caused by copper and hydrogen ion reduction were slower with increased amounts of EDTA. The discrepancies of residual iron concentrations and the observed gas evolution on the iron surfam indicate that the residual iron concentrations were cawed mainly by the reduction of copper and hydrogen ions. That is Fq = Fe,, + FeH, (3) where Fe, is the total iron consumption, Few is the iron consumption caused by copper reduction, and F%, is the iron consumption caused by hydrogen reduction. Iron consumed by copper cementation at specfic initial pH conditions was discussed with apparent half-order kinetics with appropriate stoichiometry:
where [CUI, and [CUI are copper ion concentrations at the beginning of reaction'and time t, renpectively. Excess iron consumption due to the reduction of hydrogen ion is assumed to be dependent on solution pH, the amount of EDTA, and the amount of iron powder. Aa shown in Figures 8 and 9, the iron consumption cawed by hydrogen reduction is approximately h e a r with reaction time at constant solution pH and EDTA concentration indicating that the hydrogen reduction rate poses a zeroorder relationship to the amount of iron powder. The apparent reaction order to hydrogen ion (H+)was obtained and expressed by the following empirical equation: FeH, = k;[H+lO."t
(5)
The resulta of XRD analysis indicate that copper dep i t a on the iron surface almost all as formsof the copper molecule; no indications of any Cu-Fe alloy or metal oxidea/hydmxidea were found on the surface. Examiition of the cemented surfaces using scanning-electron microscopy (SEM) shows the formation of fine, nodular copper deposits of relatively uniform size with irregular channeln penetrating the iron surface possibly caused by the dissolution (etching)of iron as shown in Figure 10 for solution pH 5 in the presence of 457 mg/LEDTA. Agrawal(1988) indicated that a better deposit structure on the cemented surface is mainly determined by a slower reaction rate. The formation of the dense, uniform surface observed by SEM is thus possibly caused by the reduced reaction rate in the presence of EDTA.
SummarY Cementation was shown to be a feasible alternative to achieve a high degree of removal of copper from chelated solution over a broad operational condition. Apparent reaction kinetics based on deactivation model fita well with the experimental results. The copper concentration and the surface area of iron powder were found to be half- and fmt-order with resped to the cementation rate of copper,
Figure 10. SEM picture of cemented iron surface in the presence of EDTA a t pH 5.
respectively. The deposition rate of copper was found to be dependent on the concentration of chelating agent and solution pH. Excess iron consumption is caused mainly by the reduction of hydrogen ion, which is vastly speeded up by the presence of a chelating agent. Registry No. Fe, 7439-89-6;copper EDTA, 51395-10-9. Literature Cited Addy, S.; Fletcher, A. J. The Deposition of cobalt on Iron Powder by Means of Cementation Reaction. Hydrometallurgy 1987,17. 269. Agelidis, T.;Fytianoa, K.; Vasilikiotis, G. Kinetic atudy of Lead Cementation by Iron Powder in Wastewater. Chemosphere 19%. 18, 1001. Agelidis, T.; Fytianos, K.; Vasilikiotis, G.; JannaLoudahis. D. Lead Removal from Wastewater by Cementation U w i S i i a Fixed Bed of Iron Sphere. Environ. Pollut. 1988.50.243. Agrawal, R. D. Cementation-A Critical Renew. J. Mines. Met. Fuels 1988,36(3),138. Gould, J. P. The Kinetics of Hexavalent Chromium Reduction hy M e d i c Iron. Water Rea. 1982,16,871. Gould, J. P.; Escovar, I. B.; Khudenko. B. M.Examination of the Zinc Cementation of Cadmium Aqueous Solutions. Water Sci. Teehnol. 1987,19,333. Innedv. J. Analvtical ADDlication of ComDIex Eauilibricl: wilev: .. New York, 1956. Karra, S. B.; Haas, C. N.; Tam. V.; Men, H. E. Kinetic Limitations on the Selective Precipitation Treatment of Elsetmnica Waste. Water, Air Soil Pollut. 1985,24,253. Ku,Y.; Peters. R. W. The Effect of Weak Chelating Agents on the Removal of Heavy Metals by Precipitation Proeeasea. Enuiron. Prog. 1986,5 (3), 147. Ku, Y.; Chen, C. H. Kinetic Study of Copper Deposition on Iron by Cementation Reaction. Sep. Sci. Technol. 1992. in press. Mackinnon, D. J.; Ingraham, T. R. Copper Cementation on Aluminum Canning Sheet, Can. Metall. Q. 1971, 10 (3), 197. Nadkarin. R. M.; Wadsworth, M. E. A Kinetic Study of Copper Cementation Precipitation of Iron-Part 11. Trans. Metall. SOC. AIME 1967,W9,1066. N h n , R. Removal of Metals by ChemicaJ Treatment of Municipal Wastewaters. Water Res. 1911,5,51. Reuter, J. H.; Perdue, E. M. Importanm of Heavy Metal-Organic Matter Interactions in National Waters. Geochim. Cosmochim. Acto 1977, 41. 325. Received for review September 17,1991 Revised manuscript received December 6, 1991 Accepted December 18,1991
