Langmuir 1992,8, 1851-1856
1851
Electrokinetic Characteristics of Some Metal Sulfide-Water Interfaces J. C. Liut and C . P.Huang' Department of Civil Engineering, University of Delaware, Newark, Delaware 19716 Received October 10,1990. In Final Form: January 28, 1992 Electrokinetic characteristics of some environmentally important metal sulfides, namely, ZnS(s), CdS(a), PbS(s),PbS(s),HgS(s), CuS(s), CuzS(s), FeS(s),FeSds), CoS(s), MnS(s), SbzSa(s), As~Sa(s1,MoS(s), and S(s), were studied. Most metal sulfides have less than 3.0 isoelectric point (IEP)values. The electrical double layer (EDL) structures of ZnS(s), CdS(s), and HgS(s) were investigated further. Various factors that may affect the electrokinetic behaviors of metal sulfides were studied. Crystal structure, crystal field stabilization energy, and the ratio of cationic charge to ionic radii, ZIR, were found to affect IEPs, in a way analogous to that of metal oxides.
1. Introduction
Electrokinetic studies of minerals are important to the development of a flotation technique. Compared with the well-studied surface properties of oxides, however, many findings on the surface chemistry of mineral sulfides are often inconsistent. This has been indicated in the review b y Healy and Moignard.' Factors, such as surface oxidation, lattice defects, photoirradiation, and sample pretreatment, that may effect the surface properties were usually not taken into account. Considering the great benefit non-sulfide mineral flotation received from electrokinetic studies, it is surprising to notice that little research and development has been carried out on the metal sulfide system. Furthermore, recently m a n y studies have focused on the photodecomposition of organic compounds using the semiconducting property of metal sulfide. Better understanding of the chemistry of metal sulfidewater interfaces would greatly contribute to the further development of metal sulfide as an industrial mineral as well as to benefit other related fields, such as environmental pollution control. Concern for dredging and the fate and transport of pollutants in the aquatic system, for instance, are but a few of the possible implications related to the chemistry of metal sulfide surfaces. In the present work, a detailed s t u d y is reported on the electrokinetic properties of some environmentally important metal sulfides. Various factors, such as crystal structure and cationic charge affecting the electrokinetic properties of metal sulfide, are discussed. The electrical double-layer structure at some metal sulfide-electrolyte interfaces is investigated. 2. Methods and Materials The synthetic sulfide and oxide powders used were "Gold Labeled" (purity >99%), obtained from Aldrich and Fisher. Natural sulfide minerals were from Wards Natural Museum (New York). They are sphalerite, ZnS(s) (Tennessee), galena, PbS(s) (Missouri), cinnabar, HgS(s) (Idria, Yugoslavia),covellite, CuS( 8 ) (Montana), chalcocite, Cu&) (Messina, Transvaal), pyrite, FeSz(s) (Huanzala, Peru), and marcasite, FeSz(s) (Indiana). An X-ray diffractometer was used to determine the crystal structure of metal sulfides. A scanning electron microscope (SEM) was used to examine the morphology of particles. For
* To whom correspondence should be addressed.
+ Present address: Department of Chemical Engineering, National Taiwan Institute of Technology, Taipei, Taiwan, ROC.
(1)Healy, T. W.; Moignard, M. S. In Flotation; Fuerstenau, M. C., Ed.; AIME: New York, 1976; Vol. 1, p 275.
the surface content analysis, energy dispersive X-ray analysis (EDAX) was utilized. The surface area was determined by a Model 69-7 Quantasorb surface analyzer (Quantachrome Corp., New York), using N2 as adsorbate. A surface area of 9.0 mzlgwas found for synthetic ZnS(s), 14 m2/gfor synthetic CdS(s), and 4.6 mzlg for synthetic CUS(S). The metal sulfides were ground in a glovebox under nitrogen atmosphere, and then sieved to obtain particles with a diameter of less than 250 mesh (100pm). In order to understand the effect of impurities on the sulfide surfaces, the pretreatment suggested by Moses et aL2 was conducted on both CdS(s) and sphalerite ZnS(s), respectively. A 3-g portion of metal sulfide was boiled in 50 mL of 6 M HC1 for 15min and rinsed twice with 50 mL of boiling 6 M HCl and at least three times with 50 mL of warm acetone. Each sample was dried under Nz and used for the experiment within 30 min. The results are shown in Figure 1. It is evident that the oxidized layer coated on the metal sulfide surface had little effect on the electrokinetic behavior of both sulfides. Therefore, right before each experiment, samples were M HClO,) weighed and then only washed with dilute acid to remove the amorphous or disturbed layer acquired from grinding. To two 500-mL flasks of deoxygenated water, continuously bubbled with saturated Nz gas, was added 0.5 g of solid metal sulfide, and the mixture was stirred. Unless specified, the hydration time was set at 30 min. While the suspension was being stirred, the initial pH was measured and recorded. The pH was then adjusted with 0.1 M HClO, or 0.1 M NaOH to cover a range from 3 to 10 in approximately 0.5 pH unit incrementa. In this experiment, NaClO4 was used to control ionic strength. For the experiment of the metal ion effect on the surface potential, a cupric ion specific electrode was used to measure the concentration of Cu2+. An Orion Model 98-48 copper electrode and an Orion Model 90-01 single-junction reference electrode were employed. The concentrations of Cuz+ were measured simultaneously with the 5 potential measurement. The pH was kept at 5.5 throughout the experiment for the CuS(s)-Cuz+ system. In the other case, pH was kept at 3.6 for the ZnS(s)-Zn2+ system. For the concentration of Zn2+,we have used the total concentration of Zn(ClO4)z added. Since the experiment was run at a pH of 3.6, uncomplexed Zn2+was the dominant species. No further calculation was necessary.
3. Results and Discussion 3.1. IEPs of Metal S u l f i d e s . T h e typical results of the electrokinetic behavior of metal sulfides are shown in Figure 2 and summarized in T a b l e I. Hawleyite CdS(s) (2)Moes, C.0.; Nordstrom, D. K.; Herman, J. S.;Mills, A. L. Geochim. Cosmochim. Acta 1987,51, 1561. ( 3 ) Williams, R.; Labib, M. E.J.ColloidInterfacial Sci. 1986,106 (No. 1). 251. (4) Moignard, M. J.; Dixon, D. R.; Healy, T. W. Roc.-Australas. Inst. Min. Metall. 1977,263, 31.
0743-746319212408-1851$03.00/0 0 1992 American Chemical Society
Liu and Huang 40
.
Table I. Summary of Electrokinetic Studies of Metal Sulfides
u s
30
-
20
-
10
-
0
u s
*1.1,..1.,
S.W.,I,.
0
P,.lr..l.,s,".l.rlt.
samDle method IEP ZnS(s), synthetic zeta meter 3.0" ZnS(s), synthetic microelectrophoresis 6.7b ZnS(s), synthetic zetasizer 8.w ZnS(s),sphalerite zetasizer 3.w ZnS(s),synthetic microelectrophoresis 6Bd ZnS(s), synthetic (wurtzite) zeta meter 8.5O ZnS(s), sphalerite zeta meter
