I n d . E n g . C h e m . R e s . 1989,28, 577-584
577
Segregation of Metals at the Oil-Water Interface: Results and Implications Peter B. Lloyd, Sudalaimuthu Ganesan, and P. K. Lim* D e p a r t m e n t of Chemical Engineering, N o r t h Carolina S t a t e University, Raleigh, N o r t h Carolina 27695- 7905
Experimental evidence and theoretical consideration are presented to draw attention to the adsorptive power of the oil-water interface and the promising potential i t provides for the development of an interfacial emulsion technique t h a t can be useful t o metal recovery and materials conversion. It is shown that dissolved and suspended metals can be segregated into a compact emulsion in between water and an immiscible oil phase and that such segregation facilitates metal recovery and interfacial catalytic reactions. On account of its property range being wider than that of solid-water and gas-water interfaces, the oil-water interface could provide a flexibility and versatility t h a t would make the envisioned interfacial technique more useful than packed-bed and foam techniques. Other possible advantages include high capacity and high enrichment and the availability of surface variation as a convenient process parameter. Specific applications of the proposed interfacial technique are considered with reference to metal recovery and photocatalytic waste treatment.
I. Introduction The ability of an oil-water (henceforth, 0-W) interface to attract and collect dissolved and suspended matters is a property of significance to separation and materials conversion. Until recently, interest in the properties of the 0-W interface has been confined primarily to emulsification (Becher, 1965; Lissant, 1984) and demulsification (Lissant, 1983). Emulsion stabilization is desired in the manufacturing of emulsion products, of which there are innumerable examples (Lissant, 1984), whereas demulsification is a prerequisite to the phase separation of liquid mixtures (Lissant, 1983). In recent years, however, increasing attention has been paid to the engineering potential of the 0-W interfacial phenomena in separation and materials conversion. It now appears that the adsorptive power of the 0-W interface may be just as useful as that of solid-fluid and gas-liquid interfaces. The latter interfaces have received long and sustaining attention that has resulted in such well-established applications as activated-carbon adsorption (Cheremisinoff and Ellerbursch, 1978), chromatographic and membrane separation (Meares, 1976; Drioli and Nakagaki, 1986),foam separation (Clarke and Wilson, 1983; Sebba, 1962; Somasundaran, 1972), detergency (Cutler and Davis, 1972), and heterogeneous catalysis (Thomas, 1970; Davis and Hettinger, 1983). Though recent in development, the 0-W interface has already spawned an impressive list of applications that include liquid-membrane separation (Li and Shriver, 1972; Terry et al., 1982; Bargeman and Smolders, 1986), liquid-aphron separation (Sebba, 1985,19871, interfacial and micellar catalysis (Menger, 1972; Fendler and Fendler, 1975; MacKay, 1981), and emulsion polymerization (Millich and Carraher, 1977). In this paper, we report data that show that dissolved and suspended metals can be segregated at the 0-W interface. The segregation facilitates metal recovery and makes possible the selective occurrence of catalytic reactions at the 0-W interface. There are distinct advantages of running a catalytic reaction interfacially, and these are considered in section 111-1-2. 11. Experimental Section Interfacial segregation was studied in Nalgene 125-mL separatory funnels. The funnels were chosen to be made
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of transparent Teflon fluorinated ethylene propylene resins to minimize the wall adsorption effect. In standard runs, liquid mixtures of known overall compositions were prepared by adding to the funnels the predetermined amounts of the desired ingredients: water, kerosene (or some other water-immiscible organic liquid), metal compound, collector, pH buffer, and additive (used to probe interference effect). In actual waste-solution runs, plating-bath solutions were obtained from the Lufkin-Cooper plant (located at Apex, NC) and used as received. The plating-bath solutions contained the metals of interest-copper, nickel, and chromium-and some undisclosed brightener components. Appropriate amounts of collector, oil phase, and pH buffer were added to waste solutions. In both the standard and waste-solution runs, the typical volumes of the organic and aqueous phases were 40 and 50 mL, respectively. Each mixture was mixed thoroughly with the aid of a magnetic stirrer and then allowed to equilibrate and settle for at least 24 h. Samples were taken from the two liquid phases, centrifuged, and analyzed. Any metal which may be present in the organic phase was extracted with a known volume of a 0.0010 M EDTA/NaOH solution and then analyzed. The interfacial emulsion in between water and the oil phase was isolated, and the solid material was filtered, digested in concentrated nitric acid, and then analyzed. Mass balance was made on the three phases to ensure closure of material balance. Analyses of metals were performed with the aid of a Perkin-Elmer Model 603 atomic absorption spectrometer that is equipped with various lamps. With titanium dioxide and cadmium sulfide, the analysis was performed gravimetrically. Analyses of trialkylamines were made with a Perkin-Elmer Model 2000 gas chromatograph equipped with a 60/80 Carbopack B/4% Carbowax 20M/0.8 KOH column. Experiments with and analyses of trialkylamines were performed under a nitrogen atmosphere to prevent their oxidation to trialkylamine oxides. 111. Experimental Results and Discussion
(A) General Observations. In Table I, we present evidence of dissolved and suspended metals segregating at the 0-W interface. The segregation is manifested by the formation of a compact emulsion in between water and a water-immiscible oil phase. Under microscopic examination, the interfacial emulsion appears as metal clusters 0 1989 American Chemical Societv
578 Ind. Eng. Chem. Res., Vol. 28, No. 5 , 1989 Table I. Segregation of Metals at Oil-Water Interface
M, concna copper(II), 0.0050 M
oil phase aq PH L" carbon tetrachloride 8.0 tripropylamine hexane 8.0 tripropylamine 8.0 tripropylamine kerosene
copper(II), 0.0020 M nickel(II), 0.0025 M
nickel(II), 0.0020 M cobalt(II), 0.0050 M cobalt(II), 0.0020 M chromium(III), 0.0050 M chromium(III), 0.0020 M gallium(III), 0.0050 M palladium(II), 0.00011 M
calcium(II), 0.0050 M iron(III), 0.0050 M manganese(II), 0.0050 M mercury(II), 0.0050 M silver(I), 0.0050 M cuprous chloride in disulfide, 0.010 M titanium dioxide suspension, 0.015 g/L
kerosene
nitrobenzene kerosene kerosene kerosene
kerosene n-octane
kerosene kerosene kerosene kerosene kerosene dipropyl disulfide hexane
3.0 8.0 6.4 8.0
8.7 3.0 8.0 8.5 7.0 8.0 7.9 7.8 7.5 8.9 8.0 4.5
7.0 6.0 7.0 6.5 7.5 7.0 6.0
trimethylamine triethylamine tributylamine tripentylamine n-decylamine n-pentylamine 2-ethylhexanoic acid n-tridecanoatec trimethylacetatec trimethylamine triethylamine tripropylamine tributylamine tripentylamine n-pentylamine ethylhexanoic acid triethylamine trimethylacetatec tripropylamine trimethylacetatec trimethylamine triethylamine tripropylamine trimethylacetateC trimethylamine 4,7-dimethyl- 1 , l O phenanthroline 5,6-dimethyl-l,10phenanthroline tripropylamine tripropylamine tripropylamine tripropylamine tripropylamine
collectormetal mole ratio 1.0 1.0 1.2 2.0 1.1 1.1
1.0 1.0 1.0 2.2 2.2 1.1 1.0 5.5 5.2 5.1 5.2 6.2 10.4 11.1
5.5 1.1
1.0 1.1
1.2 0.5 1.0 1.2 1.0 1.1 1.1
98
0.8 1.0 1.0 1.0 1.0
0 39 12 7 38 90 60 95
hexadecyltrimethylammonium bromide, 2.0 X M cadmium sulfide suspension, 0.015 g/L hexane
6.0 hexadecyltrimethylammonium bromide, 2.0 X M
" M= metal; L
metal recovery at interface,* % 65 62 63 92 81 67 58 9 5 8 (90% precipitate) 0 35 (63% in kerosene) 79 100 95 47 2 0 30 (65% precipitate) 0 98 76 10 79 53 43 98 23 87 96
55 90
= collector. *Volume of interfacial emulsion
