Environ. Sci. Techno/. 1995, 29, 979-984
Emulsisn Liquid Membranes for Wastewater Treatment: Equilibrium Models for Lead- and Cadmium-di-2-ethylheKyl Phosphoric Acid Systems BHAVANI J . R A G H U R A M A N , NEENA P. TIRMIZI, BYOUNG-SIK KIM,' AND JOHN M. WIENCEK* Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway, New Jersey 08855
Extraction and recovery of heavy metals from wastewater is more attractive than methods such as precipitation which result in sludges that have to be disposed in landfills. Emulsion liquid membranes are capable of extracting metals from dilute waste streams to levels much below those possible by equilibrium-limited solvent extraction. Binary equilibrium data are reported for lead and cadmium with extractants that can be used in emulsion liquid membrane formulations. Predictive models that incorporate aqueous phase nonidealities and all aqueous phase ionic reactions have been developed. Equilibrium constant values have been reported for Pb- and Cd-di-2-ethylhexyl phosphoric acid systems. Emulsion liquid membrane extractions of Pb and Cd from aqueous phases are successfully demonstrated.
Introduction In recent publications from our laboratory, we have demonstrated the advantages of using emulsion liquid membranes (ELMS)to extract heavy metals like mercury, copper, nickel, and zinc from aqueous waste streams ( 1 3). ELMS, first invented by Li (4), are made by forming an emulsion between two immiscible phases. Usually stabilized by surfactants, the water-in-oil emulsion contains the extracting agent in the oil phase and the stripping reagent in the aqueous receiving phase. This emulsion is then dispersed by mechanical agitation into a feed phase containingthe metal to be extracted. Figure 1is a schematic representation of an emulsion liquid membrane extraction of lead(I1). Combining the extraction and stripping processes removes equilibrium limitations and reduces metal concentrations in the feed to very low levels. Demulsification by application of high voltage electric fields has proven to be most efficient (5). Heavy metals concentrated in the receiving phase can be recovered by electroplating or crystallization (as a single pure salt). The oil phase can be recycled. In this paper, we report binary partitioning data for lead and cadmium extraction from dilute waste streams using an extractant, di-2-ethylhexyl phosphoric acid (DBEHPA), that can be incorporated into an ELM formulation. Models have been developed that describe the partitioning behavior for these systems, and successful ELM extractions have been demonstrated. High molecular weight amines have been used to extract lead from aqueous chloride media where lead exists as negatively charged halide complexes. McDonald et al. (€9 report equilibrium extraction and stripping data for lead with Aliquat 336 (tri-(C&lo) methylammonium chloride, Henkel) and Alamine 336. Equilibrium constants for this system were not estimated. Extraction of lead with D2EHPA has been reported by Li et al. (7 and Lee and Nam (8). Draxler et al. (5) report simultaneous extraction of Zn (6000 ppm), Cd (14 ppml, and Pb (4 ppm) using 5% bis(2ethylhexyl) dithiophosphoric acid (DTPA, Hoechst) as the extracting agent in the membrane and sulfuric acid (250 g/L) as the internal phase stripping reagent. The Pb concentration in the waste stream was reduced to 0.2 ppm; however, analysis of the internal phase indicated that the extracted lead was staying in the membrane phase and not being stripped. Emulsion liquid membrane extraction of lead using various macrocycle ligand carriers has also been reported (9, 10). Grimm and Kolarik (11) report equilibrium data for cadmium extraction with D2EHPA dissolved in n-dodecane from an aqueous phase containing 1 M (Na, H)N03 as a buffer. McDonald and Moore (12)report equilibrium data for cadmium extraction from iodide solutions using high molecular weight amines. Aliquat 336 displayed extraction over a wide pH range. Danesi et al. (13)report extraction equilibrium data for cadmium from chloride solutions using trilaurylammonium chloride (TLAHC1). Kitagawa et al. ( I 4) report successful extraction of cadmium from wastewaters with coarse emulsions using Aliquat 336 as an extracting * To whom correspondence should be addressed; e-mail address:
[email protected]. t Present address: 3-GaPil-Dong,Chung-gu, Seoul 100-715,Korea.
0013-936W95/0929-0979$09.00/0
@ 1995 American Chemical Societv
VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
979
ficients. The organic phase nonidealities are initially ignored. If the postulated equilibrium model is correct and assumption of ideal organic phase is true, then the experimental equilibrium data should yield a single average value ofK. Even ifthe organic phase is nonideal, a constant Kvalue could be obtained if the ratio of the organic phase activities as they appear in eq 2 is a constant. In this case, the constant activity ratio is included in the Kvalue. Minor deviations in the K value would reflect experimental measurement errors. However, any major deviations in K would indicate the possibility of nonidealities in the organic phase, aggregation of DZEHPA and metal complexes in the organic phase beyond the normal dimer state (as has been observed in Ni, U extractions (3, l a ) ,or a different reaction mechanism from what has been postulated.
R W W N G PHASE (Low pH to Strlp)
\
/ M'cRoDRops
Experimental Section +
I \
I
MACRODROPS
FEED PHASE (High pH to Extract) MEM8RANE PHASE (OIL)
FIGURE 1. Schematic representation of lead ion extractionwith an emulsion liquid membrane. Lead(l1) is transported to the emulsion/ feed phase interface and reacts with the complexing agent (RHh to form a soluble lead complex (PbRr2RH). This complex diffuses to the interior of the emulsion droplet until it encounters a microdroplet of the internal phase where the metal ion is exchanged for a hydrogen ion. The net effect is a unidirectionalmass transport of the cation from the original feed to the receiving phase with countertransport of hydrogen ions. The dispersion is then allowed to settle, and the lower aqueous stream is withdrawn for discharge. The upper emulsion phase is then demulsified to split the membrane and the enriched stripping phases.
agent and ethylenediaminetetraacetic acid (EDTA) as the stripping reagent. Izzat et al. (15) and Cho et al. (10)have studied emulsion liquid membrane extraction of cadmium using various macrocycle ligand carriers. Equilibrium Model. Predictive equilibria models accounting for all ionic equilibria involving the metal salts and buffers in the aqueous phase as well as for nonidealities are presented below for Pb-D2EHPA and Cd-D2EHPA systems. D2EHPA can be successfullyincorporated in ELMS and used in metal removal from wastewaters. Some equilibrium data for these systems have been reported in literature, but scant attention has been given to modeling the equilibrium behavior. D2EHPA is reported to exist as a dimer in the organic phase and complexes as a dimer with the metal (16,17). Typical metal (M2+)complexation with YZ dimers of D2EHPA ((RH),) is described by:
M2+
+ n(RH),
MR,*(n - l)(RH),
+ 2H'
(1)
The thermodynamic equilibrium constant, K,is defined as
K=
(MR,*(n - l)(RH),).(H+}'
W+I-(RH) ( ,in
(2)
where terms in braces (( }) represent activities that are products of concentrations and activity coefficients. The reader is referred to an earlier publication (3) for equations used to calculate aqueous phase activity coef980 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4 , 1 9 9 5
Materials. The sources for lead and cadmium were lead nitrate and cadmium sulfate, respectively (FisherScientific). D2EHPAwas obtained from Sigma Chemicals. Tetradecane (technicalgrade) was obtained fromHumphrey Chemicals. Surfactantused for the emulsions was ECA 5025 (polyamine supplied by Exxon). Hydrochloric acid and sulfuric acid (stripping agent in emulsions) were purchased from Fisher Scientific. Procedures. Aqueous solutions of metals were prepared by dissolving the salt in deionized water. For the PbD2EHPA system, the pH was adjusted with HCl. For the Cd-D2EHPAsystem, the pH was adjustedwith sulfuric acid or sodium acetate-acetic acid buffer. The organic phases containing the extracting agent in tetradecane solvent were prepared on a weight/weight basis. Equilibrium distribution experiments required contacting known volumes of aqueous phase and organic phase in test tubes on a tube rotator for about 2 h at 20 "C. A laboratory centrifuge was used to ensure complete disengagement of the two phases at the end of the extraction. The metal concentrations were measured using flame atomic absorption spectroscopy on a Perkin-Elmer Model 3030 spectrophotometer. The wavelengths used for lead and cadmium were 217.0 and 228.8 nm, respectively. The precision of multiple readings of the same sample was generally within 5% while that of duplicated experiments ranged from 3 to 10%. Organic phase metal concentration was calculated by material balance. Coarse emulsions were formulated by blending the organic membrane solvent (tetradecane) containing the extractant (D2EHPA) and surfactant (ECA 5025) and the internal aqueous stripping phase in a high speed blender for 2 min. The exact formulation of the emulsions for the different systems are reported with the results. Extraction was carried out in a stirred contactor using a turbine impeller at a constant stirring speed in the range of 300400 rpm. Samples were collected periodically, and the aqueous phase metal concentration was measured. Leakage of the internal phase into feed phase was monitored by analyzing for the SO4*- or C1- ion (when sulfuric acid or hydrochloric acid was used as the stripping agent) by ion chromatography on a Dionex 4000i instrument.
Results and Discussion Lead. Figure 2 shows extraction of lead with 10 wt % D2EHPA in tetradecane as a function of pH. The pH was adjusted with 6 N HCl solution. Relevant ionic equilibria in the aqueous phase for the Pb-D2EHPA system are
10oJJ
e .Z,'Z.
?
80 -
I
0
;
'
2
-
I'
e('
1
.
,"f---Cd-10 W h DPEHPA
P
f
'0 /
I
I
I
3
4
5
6
Equilibrium Pb in aqueous phase (ppm)
FIGURE 2. Equilibrium extraction of lead and cadmiumwith DZEHPA as a function of pH. lead: Aqueous phase: loo0 ppm P b hydrochloric acid used for adjusting pH. Organic phase: 10 wt YO D2EHPA in tetradecane. YorganiJVaqusour = 1. Cadmium: Aqueous phase: loo0 ppm Cd; sulfuric acid and sodium acetate/acetic acid usedto adjust pH. Organic phase: 5 or 10 wt % DZEHPA in tetradecane; YoqaniJ vaqurour = 1.
described below. Equation 8 for lead extraction with D2EHPA indicates that two hydrogen ions are released for every mole of lead extracted. Thus, the extraction is higher at higher pH while stripping dominates at the lower pH values.
-
+ NO3- PbNO,' Pb2++ 2N03- Pb(NO,), Pb2++ H,O Pb(OH)+ + H+
-
Pb2+
H,O Pb2+
- + -
+ 2H,O
+ 2(RH),
Pb(OH),
H+
Kl = 14.79 K2 = 25.11
(3) (4)
K3 = 2.00 x (5)
+ 2Hi K4 = 1.78 x lo-'* (6)
OH-
1
0
Equilibrium pH
Pb2+
0.2
K5 = 1.00 x
(7)
+
PbR2*2(RH) 2H' K = 0.0691 (calculated) (8)
The equilibrium constants for the ionic equilibria were taken from Lindsay (18). The above equilibria together with mass balance equations for lead and nitrate and a charge balance equation were solved numerically using the IMSL subroutine NEQNF. Experimental results were used as input to predict the equilibrium constant (fl for the extraction-stripping equilibria. D2EHPA has been reported to exist as dimers in organic solvents (3). Various stoichiometries of D2EHPA dimers-Pb were tried for the equilibrium data, and a stoichiometry of 2 D2EHPA dimers-1 Pb was found to give the least deviation. This is in agreement with the work of Li et al. ( 7 ) and Lee and N a m (8). Li et al. (7) studied IR spectra of D2EHPA and its lead salt in the organic phase (xylene) and report that two dimers of DZEHPA complex with one ion of lead. Lee and N a m (8)studied extraction of lead from acetate solutions with D2EHPA in CCh. They report that for loading (fraction of D2EHPA dimers complexed with lead) less than 0.05, two dimers of D2EHPA complex with one ion of lead, and the equilibrium constant was determined to be 3 x
FIGURE 3. Comparing predicted and experimental equilibrium data for Pb-DPEHPA. Organic phase: 5 wt YO or 10 wt YO DZEHPA in tetradecane. Aqueous phase: Initial Pb concentration = 900 ppm for 10 wt YOand 1048 ppm for 5 wt YODZEHPA YoqaniJVqusow varied from 0.1 to 10. Solid lines are model predictions. Unit for loading is moles of Pb per mole of initial dimerized D2EHPA.
At higher loadings, the extracted species is reported to be The (PbR2),(RH)2with an equilibrium constant of 6 x value of it is not reported in the abstract. Based on our experimental equilibrium data and a stoichiometryof two D2EHPA dimers per Pb ion, an average Kvalue of 0.0691 (38%standard deviation) was obtained. Some data points at the extremes of the extraction curves (close to 0% and 99% extractions) were excluded as even small errors in aqueous phase concentration measurement could lead to large errors in K estimates. This value of K is significantly larger than that reported by Lee and Nam (8). It is not known whether any aqueous phase ionic equilibria or nonidealities were considered in their calculation ofthe equilibrium constants, and such omissions could be a possible explanation for the difference in values. Another plausible reason for the difference could be the use of a different organic solvent in their experiments. Solid lines in Figure 3 are model predictions. There is good agreement between the predicted and experimental loadings for 10wt % D2EHPA. It is not clear why the agreement is not as good for 5 wt % D2EHPA. Organic phase nonidealities andlor aggregation of organic species should be more pronounced at the higher organic species concentrations obtained using 10 wt % D2EHPA as compared to that obtained using 5 wt % D2EHPA and hence cannot be the reason for the poorer prediction. Further, any errors in postulated reaction model or aqueous phase activity estimation should be reflected in both the predictions and hence also cannot be the cause for the deviations. Figure 4 demonstrates the advantage of combining extraction and stripping into a single step in an ELM extraction as compared to equilibrium-limited solvent extraction. The residual lead concentration in the aqueous feed is 700 times and 150times lower after an ELM extraction with 6 N H2S04 and 6 N HC1 as the stripping reagents, respectively, as compared to the equilibrium values for solvent extraction. The increase in lead concentration in the feed phase at longer extraction times is due to leakage of internal phase contents into the feed phase. Measurement of Sod2and C1- concentrations in the feed samples indicated that the leakage was only 1.4% when sulfuric acid was used as the stripping reagent as compared to 26% when HCl was used to strip the organic lead complex. VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 981
0.06 -Emulsion
r.
0.02 o.oO01
I
0
10
I
I
20
I
I
I
30
I
0
40
50
60
70
Time (min)
FIGURE 4. Coarse emulsion extraction of lead with DSEHPA. Coarse emulsion formulation: 82 wt YO organic phase (5 wt % DSEHPA 3 wt % ECA surfactant 5wtY0light mineral oil in tetradecane) and 18 wt K 6 N hydrochloric acid or sulfuric acid. Aqueous phase: 1020 ppm Pb. Initial pH = 4.W, speed = 400 rpm; treat ratio (volume of feed phase/volume of emulsion) = 10. Emulsion formulated with sulfuric acid as stripping reagent is more effective because of lower leakage of internal phase into feed.
+
+
Cadmium. Figure 2 shows extraction of Cd with D2EHPA as a function of equilibrium pH. The pH was adjusted with sulfuric acid and sodium acetate-acetic acid. The ionic equilibria for this system can be described by the following equations:
+
Cd2+ SO:-
+
- + -
-
+ H+ + H+
= 281.80
(9) (10)
K8 = 0.0105
(11)
OH-
& = 1.00 io-',
(12)
CdRy3RH
+ 2H'
HS0,-
HS0,-
SO:-
H'
I&
CdSO,
K7 = 95.49
H,SO,
Cd2+ 2.5(RH),
K = 0.0258 (calculated) (13)
The equilibrium constants for the ionic equilibria were taken from literature (18).The above equilibria together with mass balance equations for cadmium and sulfate and a charge balance equation were solved numerically using the IMSL subroutine NEQNF. Experimental results were used as input to predict the equilibrium constant ( K ) for the extraction-stripping equilibria. For an average Kvalue over the entire loading range, the standard deviation was calculated to be minimum for a stoichiometry of 2.5 D2EHPA dimers per cadmium ion. Grimm and Kolarik (11)have also reported a stoichiometry of 2.5 for this system and represented the organic species as CdR2-3RH. They did not report an equilibrium constant. The K value for our data was calculated as 0.0258with a standard deviation of 45%. Most of the data scatter was observed in the low loading range. Figure 5 compares predicted and experimental loadings using 10 wt % D2EHPA for two sets of data. Set 1 was generated by keeping the initial total cadmium concentration constant and varying the volume ratio of the two phases to cover a wide range of loadings. Set 2 was generated by keeping the volume ratio of the two phases constant and varying the initial total cadmium concentration to cover a wide range of aqueous cadmium concentrations. Figure 5 shows that there is considerable 982
---L
Set 1
h
,
H,O
-rp
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 4, 1995
-:, , , 0
I , , ~, r,,, , , , , , , 1 , , , , I ,I , , , ,100200300400500600700800 Equilibrium Cd in aqueous phase (ppm) 1
I
I
I
,
,
FIGURE 5. Comparing predicted and experimental equilibrium data for Cd-DZEHPA. Set 1: Organic phase: 10 wt YO MEHPA in tetradecane. Aqueous phase: initial Cd concentration = 1010.1 ppm; ,&,I,V, varied from 0.3 to 20. Set 2 Organic phase: 10 wt YOD2EHPA in tetradecane. Aqueous phase: initial Cd concentration in feed was varied from 0 to le00 ppm; Voq,niJV,qulour = 1. Solid lines are model predictions. Unit for loading is moles of Cd per mole of initial dimerized DZEHPA.
deviation in K at very low cadmium concentrations. Possible reasons for the deviations are analyzed below: (a)Ignoringorganicphase nonidealities. Organic phase activity coefficients are normally exponential or power functions of component mole fractions. The model was modified to include expressions for organic phase activity coefficients of the type log y = Axz;however, this did not improve or change the predictions. The reason why organic phase nonidealities may not be significant is due to the fact that the mole fractions of D2EHPA dimers and the metal complex in the organic tetradecane phase are very low (
