Ind. Eng. Chem. Res. 1990,29, 1907-1914
U , = characteristic slip velocity, slip velocity at low dispersed-phase flow rate, cm/s Greek Symbols Ap = density difference, g/cm3 e = void fraction of packing { = dimensionless tortuosity factor, a,d,,/2 p = liquid viscosity, g/(cm s) 7 = drop size correction factor p = liquid density, g/cm3 u = interfacial tension, dyn/cm & = fraction of dispersed-phase holdup in the contacting
section 3 = criterion for determining the applicability of either the
Handlos and Baron or the Laddha and Degaleesan dispersed-phase-film mass-transfer coefficient model Subscripts c = continuous phase d = dispersed phase w = water
Literature Cited Crawford, J. W.; Wilke, C. R. Limiting Flows in Packed Extraction Columns. Chem. Eng. Prog. 1951,47(8), 423. Eckert, J. S.Extraction Variables Defined. Hydrocarbon Process. 1976,55 (31,117.
1907
Gayler, R.; Pratt, H. R. C. Liquid-Liquid Extraction V. Further Studies of Droplet Behaviour in Packed Columns. Trans. Inst. Chem. Eng. 1953,31,69. Gayler, R.; Pratt, H. R. C. Liquid-Liquid Extraction X. Overall Mass Transfer Coefficients on an Area Basis for Extraction of Acetone in Packed Columns. Trans. Inst. Chem. Eng. 1957,35, 273. Grace, J. R.; Wairegi, T.; Nguyen, T. H. Shapes and Velocities of Single Drops and Bubbles Moving Freely Through Immiscible Liquids. Trans. Inst. Chem. Eng. 1976,54,167. Handlos, A. E.; Baron, T.Mass and Heat Transfer from Drops in Liquid-Liquid Extraction. AZCh J. 1957,3,127. Laddha, G. S.;Degaleesan, T. E. Transport Phenomena in Liquid Extraction; McGraw-Hill: New York, 1978. Leibson, I.; Beckmann, R. B. The Effect of Packing Size and Column Diameter on Mass Transfer in Liquid-Liquid Extraction. Chem. Eng. Prog. 1953,49 (S), 405. Misek, T.; Berger, R.; Schroter, J. Standard Test Systems for Liquid-Liquid Extractions; Institution of Chemical Engineers: Rugby, England, 1985. Nemunaitis, R. R.; Eckert, J. S.; Foote, E. H.; Rollison, L. H. Packed Liquid-Liquid Extractors. Chem. Eng. Prog. 1971,67 (ll),60. Seibert, A. F.; Fair, J. R. Hydrodynamics and Mass Transfer in Spray and Packed Liquid-Liquid Extraction Columns. Znd. Eng. Chem. Res. 1988,27,470.
Received for review January 8, 1990 Revised manuscript received May 29, 1990 Accepted June 9, 1990
Extraction of Mercury(I1) with Sulfurized Jojoba Oil Jaime Wisniak,*it Gal Schorr,t Dov Zacovsky,+and Sofia Belferl Department of Chemical Engineering and The Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva, Israel
Sulfurized jojoba oil containing 12% by weight S has been tested as an extractant for Hg(1I) from aqueous solutions. Experiments have been performed with the extractant dissolved in a solvent (liquid-liquid extraction) or adsorbed in an appropriate resin matrix (solid-liquid extraction). The extraction characteristics of both systems have been measured and show that sulfurized jojoba oil exhibits very good possibilities as an extractant. T h e performance of several resins treated with sulfurized jojoba oil for adsorbing mercury(I1) was studied. T h e morphology of the different resins was examined by using scanning electron microscopy. The sulfurized oil is attached to the resin sites through the sulfur atoms; it is estimated that there are about 2 mol of S active sites per kilogram of resin.
Introduction Mercury metal and most of its compounds are poisons that can be fatal to all living organisms. In the context of present-day governmental regulations, the control, recovery, and disposal of mercury-bearing wastes are as important as the manufacturing process. EPA regulations normally require that liquid effluents contain no more than 5 ppb. The most significant source of pollution is the mercury-containing brines from chloralkali industries. Cleaning procedures include the use of activated carbon impregnated with silver, contact of the brine solution with a strong anion-exchange organic resin of the quaternary ammonium cross-linked type, precipitation of mercury salts from alkaline solutions with soluble sulfides or water-soluble reducing agents, etc. Solvent extraction and separation techniques using liquid membranes are considered to be the most effective and energy-saving separation techniques for the purpose mentioned above (Kirk-Othmer, 1981). Swanson et al. (1973) have suggested that treating solutions containing Hg(I1) ion with starch xanthate-polycation complex can reduce the residual mercury content to extremely low levels (3.8 ppm)
without introducing large amounts of other contaminants. In the past, solvent extraction of mercury(I1) has been investigated using extractants like trioctylphosphine oxide, tributyl phosphate, trioctylamine, and tricaprylmethylammonium chloride (Aliquat 336). Some of these extractants contain nitrogen or oxygen as a donor atom and have only poor selectivity to mercury(I1)over other metals; much effort is being spent today on the possible use of extractants containing sulfur as a donor atom, since some such as dialkyl sulfides and trialkyl thiophosphate are known to be very powerful and selective extractants for mercury( 11). The medium-to-low selectivity of neutral oxygen-containing extractants of metal complexes with inorganic ligands is due to the fact that these extractants may extract the metal both in the form of coordinately solvated compounds and in the form of complex metal acids. The selectivity of sulfur-containing neutral extractants must be higher. They are usually protonized with difficulty; therefore, their solutions in inert diluents do not extract complex metal acids. On the other hand, these extractants give coordinately solvated neutral complexes only with
0888-5885/9012 629- l907$02.50/0 0 1990 American Chemical Society
1908 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
those elements that are strongly bound with sulfur-Ag, -Au, -Hg, and -Cu(I) (Zolotov et al., 1974). Mercury, in its two oxidation states, I and 11, is considered to be a soft acid and gives way to stable complexes with ligands that contain sulfur, oxygen, or nitrogen. Mercury produces complexes with all the halides, except fluoride. These are very stable complexes, and their stability increases with an increase in the molecular weight of the ion. The halide complexes can be extracted with several solvents. The extraction ability increases in the order C1- < Br- C I-, but an increase in the concentration of the ligand produces a decrease in the degree of extraction because of the formation of anionic complexes of the types HgX,'- and HgXy, which have a lower capability than the neutral complex HgX2 (Sekine and Hasegawa, 1977). Soft metals have a high sensitivity to nucleophilic ligand character, or polarizability, and a low sensitivity to basicity of ligands toward protons. According to Handley and Dean (1960), triisooctyl thiophosphate and tributyl thiophosphate, neutral esters of monothiophosphoric acid that contain an isolated P=S group, are highly selective extractants for silver and Hg(I1) ions in nitric acid medium. The semipolar S atom is the sole structural difference between the phosphates and the monothiophosphates. In the ion association complex, bonding occurs through the P=S group. The semipolar S atom of the thiophosphoryl group has little electrondonating capability and will bond apparently only to ions of high field strength. Silver and Hg(I1) are ions of high field strength, and on the scale of electronegativity, silver and Hg are among the several metals that prefer sulfurcontaining ligands. The insolubility of their sulfides is well-known. The special affinity of silver and mercury for S contrasts with their reluctance to combine with oxygen, and this holds true for the trialkyl phosphates as compared with the trialkyl thiophosphates. The extraction of Hg(I1) from various acid solutions and from HNOBin the presence of other anions, added as their sodium salts, indicates that the partition coefficient of Hg is largest in acetic acid probably due to the formation of mercury(I1) acetate, which is slightly ionized. Neutral esters of thiophosphoric acid, thiophosphonic acid, and thiophosphinic acids tend to function through the oxygen atom toward hard metal ions and through the S atom toward soft metal ions. Similar arguments can be given on the basis of the HSAB principle: metals like Ag(1) and Hg(I1) are considered to be soft acids; they have large atoms, have low positive charge, and contain unshared pairs of electrons in their valence shell. They also have high polarizability and low electronegativity. They prefer to bond to soft bases, which hold their valence electrons loosely (March, 1985). Baba and Inoue (1985) compared the extraction of Hg(11) with dihexyl sulfide (DHS), 1,2-bis(hexylthio)ethane (BHTE),and w(buty1thio)lauric acid (BTLA) from HNO, and HCl and found that, in the case of extraction from nitric acid, Hg(I1) is almost quantitatively extracted by these extractants, except DHS and that, in the case of HCl acids, the extractability was in the order BHTE > DHS > BTLA. The distribution coefficient in BTLA was found to be independent of pH, suggesting that BTLA does not behave as a cation-exchange extractant but as a solvating extractant because of the strong affinity of the sulfur atom for the soft metal mercury. Sevdic and Meider-Gorican (1971) compared the extraction of Zn, Cd, and Hg with organic compounds containing the groups P=O, P=S, PSH, and O=PC=S. Their results showed that the affinity of metals for sulfur increases in the order Zn < Cd < Hg. Zn and Cd could
be extracted only by thiophosphorus compounds with a PSH group, while it was possible to extract Hg also with ligands that contained a P=S or O=PC=S group. Jojoba, Simmondsia chinensis, is a desert shrub that grows mild in Arizona, northwestern Mexico, and other dry areas. Interest in jojoba stems from the unusual properties of the oil that can be extracted from its nuts. Jojoba is unique among plants in that its seeds contain about 50% by weight a practically colorless oil, composed mainly of the straight-chain monoesters of the Cz0and CZ2alcohols and acids, with two double bonds, one on each side of the ester group. The almost complete absence of glycerin indicates that jojoba is not a fat but a wax (Wisniak, 1987). Sulfurization of jojoba oil with elementary sulfur and sulfur monohalides, as well as the physical properties of the derivatives, has been reported in the literature (Gisser et al., 1975; Wisniak and Benajahu, 1975; Miwa et al., 1979) in studies related to the lubrication properties of the product. High sulfur content products that remain in the liquid state can be obtained. The attack of the double bond by elementary sulfur or sulfur monohalides produces an S bridge between two different oil molecules. Each molecule of the ester contains two double bonds, so that, theoretically, it is possible to achieve a large degree of intermolecular polymerization. The complexity of the sulfurization reaction makes elucidation of the amounts and structures of the resulting products unattainable. Sulfurization of simple and pure fatty acids or alkenes, containing one terminal bond, has indicated that the structure of the main products is -C-S,-C- where n = 1 and 2 (Gisser et al., 1975). It was then assumed that sulfurization of jojoba would produce as major components a mixture of diester thioethers and diester dithioethers for each of the two original double bonds. These thioethers should act as softer Lewis bases and have an affinity for softer Lewis acids such as Hg(I1). The ability of jojoba derivatives to pick up metal ions has already been demonstrated by Wisniak (1987) in liquid-liquid systems. In the last 10 years, the advantages of liquid-liquid extraction have been widely applied in column technology by means of impregnated ion-exchange resin or their matrices. Extraction with several-impregnated resins has been thoroughly described by Warshawsky (1981).
Experimental Section Materials and Methods. The jojoba oil used in this work was obtained from Negev Jojoba and had the following characteristics: refractive index (20 "C), 1.4652; iodine value (Wijs), 83.2; composition by gas chromatography, CM0.1%, CS61.6%, (2%770,Ca 3270, C42 49%, CU 9%, C460.9% C4*0.1%. The oil was sulfurized without further purification. Kerosene, methylene chloride, and dichloroethane (DCE), CP grade, and were used as solvents. All the inorganic reagents were of analytical grade. The following resins were employed in the inmobilization experiments: Amberlites XAD-4 and XAD-8, manufactured by Rohm and Haas, and Poly Sep, from Polyscience, Inc. Before use, they were washed successively in a Soxhlet with acetone and chloroform. The polymeric adsorbents of the Amberlite family are macroporous styrene (XAD-4)or acrylic (XAD-8)-divinylbenzene copolymers, exhibiting very high surface areas, 140 and 750 m g, respectively, a wide pore diameter range, 50 and 250 respectively, a volume porosity of 0.5, and excellent adsorption characteristics. Poly Sep is a polystyrene resin cross-linked with 2 % divinylbenzene. Sulfurization. Refined jojoba was sulfurized by mixing the appropriate weight of jojoba oil with 15.0% w/w ele-
1,
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1909 mental surfur, followed by heating and stirring the mixture slowly under reduced pressure for 30 min to about 120 "C and then slowly increasing the temperature to about 180 "C, where the mixture was allowed to cook for 4 h. Heating was then stopped and the mixture allowed to cool to about 93 "C,at which point nitrogen was blown through in order to eliminate hydrogen sulfide and other sulfur-containing volatiles. Nitrogen flow was continued until the entrained gas gave a negative reaction with lead acetate paper. The product was medium dark amber in color, and any unreacted sulfur was removed by filtration. A stock of sulfurized jojoba oil (JS) containing 12% by weight S was prepared this way. The S content was determined according to ASTM method E-443 (1973) with the minor modifications reported previously (Wisniak and Benajahu, 1975). Immobilization. A known weight of resin was placed in a flask, and the volume was completed by addition of a solution of 25% by weight sulfurized jojoba oil in the appropriate solvent (kerosene or dichloroethane). After 24 h, the solid phase was separated and dried by slow evaporation of the solvent under reduced pressure. Excess extractant was removed by drying with filter paper. Extraction. 1. Liquid-Liquid Extraction. Equal volumes (50 mL) of solutions of known concentration, of mercury(I1) nitrate in water and sulfurized jojoba dissolved in the appropriate diluent, were placed in a 250-mL Erlenmeyer secured in a thermostated (20 OC) mechanical shaker. After equilibration time, a measured aliquot of the aqueous phase was removed for analysis by atomic absorption spectrometry, using a Model 5000 Perkin-Elmer apparatus with tracer. The concentration of Hg(I1) in the organic phase was obtained by calculating the difference. A clean two-phase system, with no intermediate layer, was achieved only at pH near zero (high concentration of HN03). 2. Liquid-Solid Extraction. A given weight of resin was contacted with the appropriate volume of aqueous solution at room temperature for 24 h. The concentration of metal in the aqueous phase was analyzed before and after equilibration. The amount of Hg(I1) extracted was calculated from the difference between the two analyses.
Results and Discussion 1. Liquid-Liquid Extraction. Equilibration Time. The time required for equilibration was first determined. For this purpose, a solution containing 1130 ppm Hg(I1) was equilibrated with a series of organic phases containing 35-40 g/L of sulfurized jojoba oil dissolved in kerosene. Samples of the aqueous phase were taken at different times and analyzed for the metal. The analytical results showed that equilibrium was attained after 20 min; a contact time of 30 min was then used for all the runs. Effect of Temperature. Extraction runs were performed in the temperature range 10-60 "C with the same solutions described in the previous paragraph. The results indicated that the temperature had no measurable effect on the distribution coefficient, and thereafter, all runs were made at room temperature. Effect of Extractant, Metal Concentration, and Diluent. Pure non-sulfurized jojoba oil does not extract Hg(I1). The distribution coefficient was studied as a function of composition of the organic and aqueous phases. A series of 35 runs were made to test the effect of the concentration of Hg(I1) and sulfurized jojoba oil on the distribution coefficient D in the following ranges: Hg(I1) from 800 to 4000 ppm, sulfurized jojoba oil from 20 to 40 g/L, in kerosene and methylene chloride. The results are described in Figures 1-4, where each point represents the
0
I
0
301
JS / Kerosene Q/L 20
l
a
201
b
b
+
+
-
e o
25
+d0
0
30 35 40
0
b
L 2.8
3.2
3.0
3.0
Log [Hg
3.6
(n)]
Figure 1. Variation of the distribution coefficient with the concentration of Hg(I1) and sulfurized jojoba dissolved in kerosene.
I
I S IN METHYLENE CHLORIDE
OIL e o
25
b
35 40
30
+ b
b
D
+
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b
0
+
0
4
-
O
I
e
t 0
e
1.oL
28
I
I
I
30
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3.0
5
Log [Hg(n)] Figure 2. Variation of the distribution coefficient with the concentration of Hg(I1) and sulfurized jojoba dissolved in methylene chloride. V
3.01 Ha 0
0
+
0
v
(U) ppm 3981 3660 2818 2090 845
2.0.
V 0
V
-
0 0
o
p
20
25
30
10
35
SULFURIZED J O I O B A O I L I N KEROSENE
,OIL
Figure 3. Variation of the distribution coefficient with the concentration of sulfurized jojoba oil dissolved in kerosene.
average of three runs. It can be seen that an increase in the original concentration of Hg(I1) in the aqueous phase produces a decrease in the distribution coefficient, while an increase in the concentration of JS in the organic phase leads to an increase in the coefficient. Comparison of the curves in Figures 1 and 3, or Figures 2 and 4, shows that
1910 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
3ot
0
o
+ A
045 1656 2078
3000
!0
0
0
+
0
-
+
0
t
-
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B. Pure Jojoba Oil
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Table 11. Characteristics of the Impregnated Beads % S peak in surfacea wt% resin solvent increase increase external internal A. Sulfurized Jojoba Oil XAD-4 kerosene 106 48 34 1 DCE 78 36 71 387 XAD-8 kerosene 100 58 171 DCE 100 57 375 387 0 0 Poly Sep kerosene 4
