Thermodynamic Behavior of Electrolytes in Mixed ... - ACS Publications

25 The Removal of Organic Lead from Aqueous Effluents by a Combined Chemical Complexing-Solvent Extraction Technique Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch025

A. J. B A R K E R and S. R. M . E L L I S Department of Chemical Engineering, The University of Birmingham, P.O. Box 363, Birmingham B15 2TT England A. B. C L A R K E Colworth Welwyn Laboratory, Unilever Ltd., Colworth House, Sharnbrook, Bedford MK44 1LQ England

The removal of organic lead from aqueous solution can be achieved by a method of chemical complexing followed by sol vent extraction. The mechanism of removal depends first on the formation of a coordinate complex; this is determined by the distribution of organic lead species in the aqueous phase in the presence of NaCl. The second step in the process is the transfer of the coordinate or neutral species from the aqueous to the solvent phase. The extent of this transfer is determined by the distribution of these species between the two phases. The continued formation of the complex depends on the law of mass action, and the rate of extraction is determined by factors controlling the physical distribution in the process. The major operating parameters determining the level of organic lead re moval are the organic-to-aqueous phase ratio and the com plexing reagent-to-organic lead ratio.

W

aste waters containing l o w concentrations of soluble organic lead i n the presence of high concentrations of other diverse ions such as C l ~ pose a p a r t i c u l a r l y d i f f i c u l t treatment p r o b l e m . Generally, organic lead exists i n solution as the tri- or dialkyl lead chloride species. These salts are not amenable to the conventional methods used to remove inorganic lead, v i z . , those of p H adjustment f o l l o w e d b y settling. T h e technique of c h e m i c a l conversion of the 381

Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

382

THERMODYNAMIC

BEHAVIOR O F E L E C T R O L Y T E S

organic lead salts to the inorganic state, w h i l e possible, is precluded on economic grounds because of the very high capital and energy costs of effective processes. A n alternative process w h i c h d i d not possess these major disadvantages was therefore sought. T h e method of chemical complexing followed by solvent extraction has been used for some time as an analytical technique for metal determination b y chelate extraction (I). T h e chelating agent d i e t h y l d i t h i o c a r b a m i c a c i d s o d i u m salt is an established reagent for the determination of inorganic lead.

T h e chelating

power of the dithiocarbamates a n d other potentially interesting reagents such as the thiazoles a n d xanthates is based u p o n the a f f i n i t y of the sulfur-bearing groups of the chelating agent for heavy metals. Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch025

Initially the application of this analytical technique to an effluent treatment problem would not appear to be very favorable.

In the analytical technique high

reagent concentrations relative to the metal salt are employed, and the extraction is dependent u p o n p H level of the aqueous phase (1,2). F u r t h e r m o r e , organic lead salts i n the presence of h i g h concentrations of ions such as C l " w i l l f o r m a series of complexes.

As a consequence, any method of extraction must take into

account the e q u i l i b r i u m of these species.

H o w e v e r , these apparent objections

to the use of the analytical technique can be resolved a n d so the process of c o m b i n e d c h e m i c a l complexing-solvent extraction does have the potential of successful d e p l o y m e n t i n the f i e l d of the large scale treatment of organic-leadcontaminated waste waters.

Experimental F o r the investigation of the potential of a c h e m i c a l c o m p l e x i n g - s o l v e n t extraction technique for the treatment of waste waters c o n t a i n i n g organic lead, a sequence of experiments was performed using synthetic effluents. As the major organic lead contaminant i n these waste waters is generally trialkyl lead chloride i n the presence of a h i g h concentration of chloride ions, the synthetic effluents m a d e usually contained u p to 100 p p m of t r i a l k y l lead c h l o r i d e a n d 0.83 m sod i u m chloride. T h e c h e m i c a l complexing-solvent extraction technique e m p l o y e d i n this work involved the formation of a neutral complex i n the aqueous phase between t r i a l k y l lead chloride a n d a dithiocarbamate reagent such as s o d i u m d i e t h y l d i thiocarbamate. T h e complex was subsequently removed either as a precipitate or b y extraction into an organic solvent. T h e extent of lead removal was traced b y analysis of the aqueous phase for residual t r i a l k y l lead using a P y e - U n i c a m 8000 spectrophotometer. A n y d i a l k y l lead present was d e t e r m i n e d spectrophotometrically as the d i a l k y l l e a d 4(2-pyridylazo)resorcinol (P. A . R . ) complex at p H 9. T r i a l k y l lead does not f o r m a complex w i t h P . A . R . ; therefore its concentration was obtained by conversion to the d i a l k y l lead f o r m i n iodine monochloride solution followed b y determination as the P . A . R . complex. A n y inorganic lead i n solution was


25.

BARKER E T A L .

383

Removal of Organic Lead

masked b y the a d d i t i o n of excess ethylenediaminetetraacetic salt.

Characterization

acid, disodium

of the Complex

O f the c o m p l e x i n g reagents investigated, s o d i u m d i e t h y l dithiocarbamate proved on a n u m b e r of counts to be the most effective.

T h e bulk of experiments

therefore used this compound. T h e nature of this complex between the reagent and trialkyl lead chloride was first characterized i n the absence of sodium chloride i n the aqueous phase. A sample of complex sufficient for elemental analysis was obtained b y adding 0.5 g ( C H ) N C S S N a . 3 H 0 i n 50 m l distilled water to 0.5 g of solid ( C H ) P b C l

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch025

2

5

2

2

2

dissolved i n 50 m l distilled water.

5

3

A substantial precipitate was f o r m e d w h i c h

flocculated sufficiently for a large percentage of it to be separated i n a laboratory filter.

T h e precipitate was washed w i t h distilled water a n d d r i e d i n a v a c u u m

desiccator for 30 hr. Examination of the precipitate showed that it consisted of floes of irregularly shaped particles 1—5 ^ i n size.

A c c o r d i n g l y , at l o w organic lead concentrations

of, e.g., 10 p p m , the use of C e n t r i f l o membrane filters was required for efficient separation of the precipitate f r o m solution. T h e filters, h o l d i n g 7 - m l aliquots of the aqueous phase were c e n t r i f u g e d for 3 m i n at 1500 r p m . E l e m e n t a l analysis of the product f o r m e d between ( C H s ) P b C l a n d 2

3

( C H ) N C S S N a « 3 H 0 showed the product to contain stoichiometric quantities 2

5

2

2

of reagent a n d organic lead corresponding to c o m p l e x i n g o n a 1:1 m o l a r basis; that is the formation of ( C H ) P b S C S N ( C H ) - 3 H 0 . 2

5

3

2

5

2

2

T h e results of the analysis

were compared with those calculated for a 1:1 and 2:1 complex as shown i n Table I. T h e f o r m a t i o n of a 1:1 complex was c o n f i r m e d b y examination of the rem o v a l of organic lead f r o m the aqueous phase as a f u n c t i o n of the reagent-toorganic lead ratio C R / C L at a temperature of 30° C .

Figure 1 shows that removal

of organic lead corresponds to what is calculated.

F o r a m o l a r reagent-to-lead

ratio ( C R / C L = 1), complete r e m o v a l of organic lead is achieved.

This dem-

onstrates that, i n the absence of N a C l , the complex is essentially insoluble i n the aqueous phase, at least to w i t h i n the accuracy of analysis, ± 0 . 1 p p m . F r o m the results of a f actorially designed experiment, variance analysis of the complexing reaction showed that over the range 1 5 ° - 6 0 ° C , temperature had

T a b l e I.

Analysis of Complex of Triethyl Lead Chloride and Sodium Diethyl Dithiocarbamate

Element C H S

Calculated (2:1) 27.5 6.7 18.6

Calculated (1:1)

Found

26.6 6.3 12.7

26.9 5.8 12.9


384

T H E R M O D Y N A M I C BEHAVIOR O F

ELECTROLYTES


100

REAGENT

Figure

1.

TO

ORGANIC LEAD RATIO,(m)

Complex of {C H^PhCl (C H ) NCSSNa 2

2

with

5 2

no significant effect on either the level or rate of organic lead removal, reaction b e i n g v i r t u a l l y instantaneous i n a l l cases. Characterization of the c o m p l e x i n g reaction was then conducted i n the presence of 0.83 m s o d i u m chloride i n the aqueous phase over a temperature range 1 5 ° - 6 0 ° C . In addition, r e m o v a l of ^ H s ^ P b C l as a f u n c t i o n of the reagent-to-organic lead ratio C R / C L i n the presence of 5 wt % N a C l was examined at 30° C using i n turn three other dithiocarbamate reagents besides that of sodium d i e t h y l dithiocarbamate. In a l l cases, the results demonstrate that i n the presence of s o d i u m chloride i n the aqueous phase, the level of organic lead removed f r o m solution is significantly reduced. F i g u r e 2 shows that for complete precipitation of organic lead a ratio, C R / C L , of at least 350 is necessary. This is over two orders of magnitude greater than i n the absence of sodium chloride. Clearly such an excess of reagent is undesirable i n both economic a n d environmental terms. The high reagent-to-organic lead ratio required is attributed to the formation of the series of chloro complexes, ( C 2 H ) P b C l , n = 0 , 1 , 2, 3, i n the presence of the chloride ion. A h i g h excess of reagent is required to cause a s w i n g i n the equilibrium, viz. 5

(C H ,) Pb 2

r

3

3

n

1 _ n

+

complex with

+ n CF

^

(QJUPba -" 1

(C H ) N C S S N a - 3 H 0 2

5

3

2


25.

BARKER

385


ET AL.

T h e existence of the anionic species ( C H 5 ) P b C l ~ a n d ( C H ) P b C l ~ has previously been established b y paper chromatography (3, 4), i o n exchange (5, 6, 7), a n d a m i n e extraction techniques (8). 2

3

2

2

5

3

3

2

T h e work of Barker and C l a r k e (9) has demonstrated the influence of the chloride i o n on the relative concentration of i n d i v i d u a l chloro species. F i g u r e 3 shows the concentration of the species ( C H ) P b a n d ( C H ) P b C l ~ rem a i n i n g i n the aqueous phase after a m i n e extraction, assuming ( C H 5 ) P b C l ~ to be the dominant species (as suggested b y values of the stability constants) a n d assuming the neutral species ( C H ) P b C l ° to be absent f r o m the aqueous phase or i n a concentration too low to measure. T h e latter assumption is consistent w i t h a h i g h distribution of the neutral species between organic a n d aqueous phases. 2

5

3

+

2

5

3

2


2

5

2

3

2

3

O w i n g to the inadequacy of the mathematical model available for analysis of the amine extraction system (7), accurate values of the stability constants could not be evaluated f o r the ( C H ) P b system i n the presence of N a C l . H o w e v e r , using the values of stability constants obtained b y B e r t a z z i f o r the system ( C H ) P b C l - i n L i C l at 8.0 m (10), v i z . ft = 3.5, 0 = 1.0, j8 = 0.1, the neutral species ( C H 5 ) P b C l ° (n = 1) is seen to be dominant. Therefore a simple solvent extraction w o u l d be expected to remove a certain amount of triethyl lead f r o m solution. As shown i n Table II, this is seen to be so. H o w e v e r , 2

2

5

3

n

1

2

10

3

1 - n

n

r i

2

3

3

10

1

REAGENT

Figure 2.

5

10

2

3

TO ORGANIC LEAD RATIO (m)

Complex of ( C H s ) P f c C / with various dithiocarbamate reagents 2

3



386

THERMODYNAMIC BEHAVIOR O F ELECTROLYTES

Table II.

Solvent E x t r a c t i o n o f ( C H ) C 1 f r o m Synthetic 2

S

3

Effluent

Initial Concentration of (C HJ PbCl 10 ppm Concentration of NaCl = 0.833m V, /V =5 No reagent 2

q

:

3

on

Solvent Benzene Chloroform CC1 Xylene Trichloroethylene Diethylether rc-Octane Isooctane 4

C

14 30 H

C. , H „

Final Concentration (C H ) PbCl (ppm) 2

5 3

6.5 2.5 6.6 7.5 6.5 10 10 10 10

Solvent Solubility (per 100 parts water) 0.07 0.82 0.08 0.05 0.10 7.50 0.002 insignificant insignificant insignificant


25.

387


BARKER E T A L .

this is true for only certain solvents w h i c h i n general are those h a v i n g a h i g h solubility i n the aqueous phase (relative to desirable limits for toxic solvents such as benzene, c h l o r o f o r m , carbon tetrachloride). F o r m a t i o n a n d subsequent

extraction

of a neutral species such as

(C2H )3PbCl° suggest that a c o m b i n e d chemical complexing-solvent extraction 5

technique m i g h t be more effective i n terms of a lower C R / C L ratio than direct precipitation.

T h i s is c o n f i r m e d b y the results of the c h e m i c a l c o m p l e x i n g sol-

vent-extraction studies.

Chemical Complexing-Solvent

Extraction Studies


C h e m i c a l complexing-solvent extraction studies were conducted using a l iquots of synthetic effluents w h i c h contained v a r y i n g quantities of ^ H s ^ P b C l and ( C 2 H ) 3 N C S S N a - 3 H 0 w i t h the sodium chloride concentration maintained 5

2

at 0.83 m . T h e ratio of reagent-to-organic lead i n the effluent was varied between 0.1 and 10.0.

F o r an initial [ ( C 2 H ) P b C l ] = 10 p p m , 50-ml aliquots of effluent were 5

3

shaken for 15 sec w i t h 10 m l of an organic solvent i n a 250-ml separating funnel. T h e choice of the phase ratio, V

a q

/V

o r g

, was a n arbitrary one.

A f t e r phase

separation, the aqueous phase was r u n off a n d analyzed for organic lead.

T a b l e III.

This

Chemical Complexing-Solvent Extraction of ( C H ) P b C l from Synthetic Effluent 2

s

3

Initial Concentrations of (C H ) PbCl Concentration of NaCl = 0.833m 2

Reagent

= 67ppm

5 3

(C H ) NCSSNa3H 0 2

5 2

2

C /C R

2

Solvent Kerosene Petroleum Ether D i e t h y l Ether Xylene Benzene /2-Pentane Isooctane CC1 Chloroform n-Nonane, 99% 4

C H Toluene Cyclohexane 1 S

3 2

L

Final concentration (C H ) PbCl (ppm)

= 0.75

R

2

5 3

21.0 17.2 4.8 6.4 4.6 27.0 20.0 6.6 0.0 12.0 20.3 18.1 9.6 12.6

C /C

% Removal 70.0 74.6 93.0 90.3 93.2 59.7 69.0 90.0 100.0 82.0 70.0 72.0 86.0 82.3

L

Final concentration (C H ) PbCl (ppm)

= 1.00

5 3

% Remoi

9.6 10.0 — — 0.0 7.0 12.0 — — 10.2 — 12.0 — 8.4


81.0 80.0 — — 100.0 86.0 76.0 — — 79.0 — 76.0 — 83.0

388

T H E R M O D Y N A M I C BEHAVIOR O F E L E C T R O L Y T E S

Theoretical "1-1 Complex o ^ o

(CH ) PbCl. . =10ppm 2 53 init. K

(NaCl] V

aa'

V

aq

org

K

=0-833 m =

5

o Xylene


* Iso-Octane

10"'

10 REAGENT

Figure 4.

Solvent extraction

u

TO ORGANIC

10 LEAD

RATIO (m)

of the complex by xylene and

isooctane

procedure was repeated for the solvents diethyl ether, isooctane, xylene, kerosene, C14H30 a n d C15H32, a l l at ambient temperature 1 6 ° C ± 2 ° C . F u r t h e r experiments were conducted w i t h a w i d e range of organic solvent for the i n i t i a l conditions: [ ( C H 5 ) P b C l ] = 67 p p m , V / V = 1.0. C / C = 0.75 m , [ N a C l ] = 0.83 m . 2

3

a q

o r g

R

L

Reference to Table III shows that i n the presence of 0.83 m sodium chloride and for a n aqueous-to-organic phase ratio V / V = 1.0, a ratio C R / C L of 1.0 is sufficient to remove at least 75% organic lead. F o r solvents such as benzene and chloroform this ratio is sufficient to achieve complete organic lead extraction. a q

REAGENT

Figure 5.

o r g

TO ORGANIC LEAD RATIO

Solvent extraction

(m)

of the complex by diethyl

ether


25.

BARKER

E T AL.

389


Table I V .

C o m p a r i s o n o f Solvent Strength


Solvent Most Effective

Chloroform Diethyl ether Benzene Xylene Toluene Carbon Tetrachloride Cyclohexane Kerosene Petroleum ether (30-40%) rc-Nonane 99%

Least Effective

Isooctane Pentane

Table V .

R e m o v a l o f ( C H ) P b C l as a F u n c t i o n o f t h e R a t i o s 2

5

3

âq/ôrg,

CR/CL

Initial concentration of (C H ) PbCl = 27ppm Concentration of NaCl = 0.833m Reagent (C H ) NCSSNa • 3H 0 Solvent C H pH7.2 Aqueous phase shaken with two aliquots of organic phase to give residual lead concentrations ofPbf^ and Pbf 2

2

5 2

XS

s 3

7

32

2

C /C R

(m)

L

0.75 0.75 0.75 0.75 1.5 1.5 1.5 1.5 2.2 2.2 2.2 2.2 4.0 4.0 4.0 4.0 4.0 4.0 4.0

âq/ôrg

25 50 75 100 25 50 75 100 25 50 75 100 25 50 75 100 150 200 300

Pb

fi

(ppm) 9.0 11.5 13.5 19.0 3.0 2.7 4.4 3.5 0.1 1.3 1.3 2.7 0.3 0.3 1.6 2.4 4.5 7.4 9.1

Pbf (ppm) 2

9.0 11.4 11.2 11.2 1.7 2.7 2.8 3.3 0.0 2.7 1.3 1.3 0.0 0.0 0.5 0.5 2.7 5.3 6.1


390

T H E R M O D Y N A M I C BEHAVIOR O F E L E C T R O L Y T E S

Figures 4 and 5 for V

a q

/V

o r g

= 5.0 show that removal of organic lead corresponds

closely to that w h i c h w o u l d be obtained for a theoretical 1:1 c o m p l e x i n the absence of s o d i u m chloride.

Variations i n extraction e f f i c i e n c y are observed f o r

d i f f e r e n t solvents, b u t for a l l of the solvents e m p l o y e d a ratio C R / C L = 1.0 is sufficient to reduce an i n i t i a l t r i e t h y l lead c h l o r i d e level of 10 p p m to

Thermodynamic Behavior of Electrolytes in Mixed ... - ACS Publications

Recommend Documents