Speciation of aqueous palladium(II) chloride solutions using optical spectroscopies

Geochimica et Cosmochrmrcn Copy&t 0 1991 Pergamon

Acra Vol. 55, pp. 1253-1264 Press pk. Printed in U.S.A.

Speciation

0016.7037/91/$3.00

+ 00

of aqueous palladium(I1) chloride solutions using optical spectroscopies C. DREW TAIT, DAVID R. JANECKY,and PAMELAS. Z. ROGERS

Isotope Geochemistry Group (INC-7), Mailstop J5 14, Los Alamos National Laboratory, Los Alamos, NM 87545, USA (Received June 5, 1990; accepted in revised form February 26, 199 1)

Abstract-Spectroscopic measurements of palladium(I1) chloride solutions have been performed under ambient to elevated temperature conditions with systematic changes in pH and [Cl-]. Spectral signatures from electronic absorption spectra (Uv/Vis) and Raman vibrational spectra were determined, and these were subsequently used to systematically map out species along several paths of the predominance diagram. The species PdClAH20):1$ (x = 2,3, or 4) and Pd(OH)2 were observed, along with a precipitation product formed from > 10 PM [Pd”] solutions under near-neutral conditions. The elemental composition of the precipitation product was examined with a scanning electron micrograph (SEM) and was found to contain chloride as well as palladium. While sub-stoichiometric amounts of available OH- ligand produced UV/ Vis solution spectra likely to be from mixed Pd-Cl-OH species, the pH field of these species would be vanishingly small at low, geologically relevant palladium ion concentrations where [OH-] is no longer the limiting reagent in the transformation. In mildly acidic media, where chloropalladium(I1) species predominate, elevated temperatures (up to 90°C) cause lower charged palladium-chloride species to be favored, consistent with the lowered dielectric constant of water at higher temperatures. provide a starting point for planning and interpreting spectroscopic observations. Furthermore, the proclivity toward square planar palladium(II) complexes (COTTON and WILKINSON, 1988) makes the study of these complexes a natural extension of previous work on the square planar gold(II1) chloride system (PECK et al., 1990a,b). In fact, the neglect of possible mixed chloro-hydroxo palladium complexes in the near-neutral region (see, for instance, the thermodynamic speciation diagrams as calculated by MOUNTAINand WOOD, 1988, and SASSANIand SHOCK, 1990) was in marked contrast to the mixed complexes found for gold(II1) chloride solutions (PECK et al., 1990a,b). Raman vibrational spectroscopy offers a structure-specific method to investigate aqueous speciation. Peak positions, numbers, and symmetry depend on the charge of the central metal atom, the number of counterions/ligands, and their spatial arrangements (e.g., cis vs. tram ligand arrangement). Therefore, vibrations are expected to occur at different frequencies for different species, allowing direct species fingerprinting, especially since subtle changes can be detected owing to the narrowness of the bands (typically several wavenumbers full width at half maximum height). This spectral characterization of the palladium species is an important tool, especially in determining the species actually present, as distinct Pd-Cl and Pd-OH stretching peaks give direct evidence of the complex identity. Although little solvent interference is expected in Raman spectra, relatively high concentrations (tens of millimolar) are required for good signal levels. Observations of dilute (micromolar concentration) aqueous palladium(I1) chloride solutions can be made with conventional electronic absorption spectroscopy. The ultraviolet/ visible (Uv/Vis) wavelength region, 200 to 700 nm, is dominated by halide ligand-to-metal charge transfer (LMCT) transitions in which electron density is transferred from the ligand orbitals to the metal upon absorption of a photon. Peaks are typically several hundred wavenumbers broad and poor spectral resolution can result. Nonetheless, Uv/Vis ab-

INTRODUCTION PROCESSESINVOLVEDIN THE concentration of platinum group elements (PGEs) into ore deposits by aqueous processes continue to receive attention (See, for inStanCe, COUSINS, 1973; FUCHS and ROSE, 1974; MIHALIK et al., 1974; MCCALLUM et al., 1976; ROWELL and EDGAR, 1986; MOUNTAIN and WOOD,

1988; HYLANDand BANCROFT,1990; SASSHOCK, 1990, and refs. therein). Transport by hydrothermal brine solutions and deposition by either reprecipitation or adsorption/reduction onto sulfide minerals is of particular interest, following description of extensive hydrothermal systems cross-cutting large ultramafic PGE-bearing bodies. The solubilities of the PGEs under hydrothermal conditions have therefore been a matter of interest, and the solubilities depend on the exact identity of the dissolved (chloride, hydroxide, bisulfide, etc.) species. Speciation and solubility of PGE species is also of interest in marine, estuary, and fresh water environments, where they may exist as inorganic chlorides, organic chelates in sediment waters, or both (LEE, 1983; KUMP and BYRNE, 1989; GOLDBERG, 1990). In either case, the chloride speciation under ambient conditions is a prerequisite for this understanding. While thermodynamic speciation calculations are a tractable method for attacking this problem, they require self-consistent input data and also require that significant assumptions be made about the system of interest when extensions to new conditions are made. We have used optical spectroscopic techniques under easily obtained laboratory conditions (room temperature to 9O”C, ambient fo,) to gather data needed to help constrain the calculations and their assumptions. Our experimental work on PGEs has concentrated on the palladium(II) chloride system for several reasons. Firstly, the recent thermodynamic calculations of MOUNTAINand WOOD (1988) and SASSANIand SHOCK (1990), summarizing conflicting data on palladium(I1) chloride solutions by mapping out the resulting thermodynamic predominance diagrams, SANI and

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C. D.

Tait. D. R. Janecky, and P. S. Z. Rodgers

sorption spectral shifts, peak intensity changes, and the presence or absence of isobestic points (i.e., wavelength points where different species have equal extinction coefficients) furnish information about changes in the speciation of solvated palladium. Indeed, careful work by ELDING ( 1972) on the absorption spectra of pa.lladium(II) chloride species under extremely acidic (1 M HC104) conditions was used as a preliminary guide to spectral interpretation under the more geochemically relevant conditions studied here. A very recent report has also used Elding’s conclusions to analyze palladium in seawater, where the chloride concentration was maintained at 0.558 M (KUMP and BYRNE, 1989). Therefore, detailed species identification was determined by Raman vibrational spectroscopy in concert with electrochemical (Cl--electrode) and endpoint data. Once a predominance region was identified, Uv/Vis spectral changes under more dilute conditions could be used to determine transition regions of the predominance diagram. Further changes in the predominance diagram with temperature (19 to 90°C) were then examined. EXPERIMENTAL Solutions were made from palladium(I1) dichloride, 99.9%, Pd 60%, obtained from Aesar-Johnson Matthey. Dilutions were made with deionized water, chloride concentration set with A.C.S. grade NaCl, and pH set with A.C.S. grade HC104, HNOs, and NaOH. pH was monitored with an ORION SA 250 pH meter equipped with a Ross pH electrode. Free chloride concentrations were measured with an ORION combination Cl- electrode, in conjunction with an ORION Sure Flow Ross reference electrode. The voltage readings from the Cl- electrode were first calibrated using standard samples of known Cl- concentrations spanning the range of interest. Conductivity measurements were performed with a VWR mode1 604 conductivity meter. While most of the solutions were prepared under atmospheric f02, preparation under Nz in a glovebag did not produce changes in speciation regions. Uv/Vis absorption spectra were taken using I.0 cm pathlength quartz cuvettes and a Perkin-Elmer Lambda 2 Uv/Vis spectrometer. Spectra from micromolar Pd2+ concentrations could be probed at 19 f 1“C (thermostated clean-room temperature) with conventional absorption spectroscopy. For elevated temperatures, the metal cell holders in the spectrometer were thermostated with flowing water from a water bath/temperature controller (VWR model 1180) to within +O. 1“C. The basic Raman instrumentation has been described previously (MARLEYet al., 1988).The excitation source was an argon ion laser (Spectra-Physics model 2025-05), focussed onto the sample with a cylindrical lens to match the monochromator slit. The Raman scattering was viewed in a 135” backscattering configuration, with the scattered light collimated with a fast plane-convex lens (f/1.3, planar side toward focussed spot on sample) and then subsequently focussed onto the slit of the monochromator (Spex Triplemate, f/6.3 input optics) with a second piano-convex f-matching lens (f/6.5, planar

side toward monochromator). The first two stages of the monochromator are locked in an additive-subtractive mode and function as a tunable bandpass filter, especially against the Rayleigh scattered (i.e., elastically scattered) photons. The final stage dispersed the inelastically scattered photons across a Princeton Applied Research model 1420 intensified silicon photodiode array detector, which was read with an optical multichannel analyzer (Princeton Applied Research model 1460 with a 1461 detector interface). Spectra were calibrated with Raman standard peaks oftoluene and dichloromethane samples and with neon emission lines (Oriel Corporation model 6032 calibration lamps). Raman spectra were taken using the 5 14.5 nm Ar+ excitation line, which is outside the absorption envelop of the d -W d transitions of palladium. Excitation into these transitions (e.g., with the 457.9 and 488.0 nm lines) did not result in resonance enhancement of the Ra-

man signal. No sample degradation (e.g., metal photoredox process) was observed even with several hundred mW of incident laser power. RESULTS AND DISCUSSION Initial experimental conditions were based on calculated predominance diagrams (MOUNTAINand WOOD, 1988; SASSANI and SHOCK, 1990). Because the calculations used different data sets and also require extrapolations from these data sets, there are some discrepancies between the two works. Specifically, MOUNTAIN and WOOD (1988) based their calculations on work done by ELDING (1972) and IZATT et al. (1967), while SASSANIand SHOCK (1990) based theirs on work done by DROLL et al. (1957), BIRYUKOVand SCHLENSKAYA (1964), and IZATT et al. (1967). [Besides ELDING (1972), DROLL et al. (1957), and BIRYUKOVand SCHLENSKAYA ( 1964), other data sets (not mutually consistent) also exist for dissociation constants of Pd-chlorides, all of which

were derived from extremely acidic conditions (pH - 0). These other sets include WEED (I 964), LEVANDA( 1968), and BURGER (1964).] In the resulting predominance diagrams calculated by SASSANIand SHOCK (1990) and MOUNTAIN and WOOD (1988), one striking difference arising from the different choice of data sets is the narrower chloride activity area covered by the species PdC13(H20)- in the diagram of SASSANIand SHOCK(1990, hereafter referred to as the SSDI model for Sassani, Shock, Droll, and Izatt). Another significant difference between the two models is the general increased persistence of chloropalladium(I1) species to higher pH values in the SSDI model. For example, the PdCl,(H20)2/ Pd-OH boundary at log acl- = -3 should occur at pH N 3 (SASSANIand SHOCK, 1990) versus ~2.5 (MOUNTAIN and WOOD, 1988). Subsequently, the Mountain and Wood calculations have been revised (WOOD et al., 1989) with the inclusion of stability constants for the species Pd(OH)2 and Pd(OH); as determined by NABIVANETSand KABINA(1970). In this revised study (hereafter referred to as WMFEINK for Wood, Mountain, Fenlon, Elding, Izatt, Nabivanets, and Kabina), the chlorides were calculated to exist to much higher pH values; in the example above, the PdC12 + Pd(OH)2 transformation was calculated to occur at pH = 5. To constrain thermodynamic models and to provide a consistent starting point for future geochemical studies (e.g., temperature effects in speciation), we have used optical spectroscopies under relevant geochemical conditions to directly probe speciation issues. The interpretation of Raman and Uv/Vis optical spectroscopic techniques in speciation studies is assisted tremendously from endpoint spectra which can be definitely correlated with known species. Literature spectra of PdCl:- (GRAY, 1965; HENDRA, 1967; GOGGIN and MINK, 1974) matched the low pH, high [Cl-] (note that the [X] nomenclature denotes molal concentration of X) endpoint spectra, while the Cl- electrode was used to establish limits on possible species under low [Cl-] conditions (vi& infiu). Furthermore, electronic absorption spectra for the series PdCI,(H,O)::c have been reported under concentrated acid (1 M HC104) solutions (e.g., WEED, 1964; ELDING, 1972). However, as this study attempts to directly probe species in the near-neutral region in the search for mixed Pd-Cl-OH species, further characterizations of the solutions with other,

1255

Speciation of Pd chloride complexes

Palladium( II)

chloride

added [Cl-]

342 296

pH=2

h

PdC12(H20)2 O.OOM

0.03M

0.07M

0.18M

0.37M

350 Wavenumbers

460 (cm-11

FIG. I. Room temperature Raman scattering vibrational spectra of palladium(U) chloride species at pH = 2 under increasing amounts of added chloride. Species transformation can be observed from the change in vibrational spectra under the systemmatically varied conditions. The total palladium ion concentration is 30 mM, and laser excitation is from the 5 14.5 nm Arf line.

more structure-specific techniques were performed the spectra to the palladium complexes.

Structure-Specific

to confirm

Characterizations

Figure 1 shows the Raman vibrational spectra of 30 mM chloropalladium(I1) solutions at pH = 2 under varying [Cl-] concentrations. The spectrum obtained after the addition of 0.37 M sodium chloride (Fig. le) matches that reported previously for PdCl:- (HENDRA, 1967; GOGGIN and MINK, 1974). The two observed peaks are attributed to Pd-Cl stretches (symmetric stretch at 300 cm-’ and the out-of-phase b’g mode at 279 cm-‘) for the planar tetra-chloro species. [Note that wavenumbers (cm-‘) are used in vibrational spectroscopy as frequency units (cm-’ = hz (cycles/sec)/3 X 10” (cm/set)) and b’, is the symmetry notation for the vibration in which the adjacent chlorides are moving exactly out-ofphase (i.e., in vs. out) as one moves around the Pd center.] The two peaks for the tetra-chloro species are replaced by peaks at 335 and 296 cm-’ at lower chloride concentration (Fig. Id and c). No other peaks out to 800 cm-’ are observed. Because Pd-OH stretches should be observed if they are present (however, see below), their absence suggests that the new species formed is not a mixed chloro-hydroxo palladium complex. Other possible species that might form would include PdC13(HZO)- or, in light of the very limited predominance range predicted for the tri-chloro species by SASSANI and SHOCK (1990) PdClz(HzOh. Note that Pd-(H20)

stretches are expected to have much weaker Raman signals, and the failure to detect them does not imply a non-aquated palladium cation. A third candidate is the dimeric form of the tri-chloro species, Pd#Zla-, with two bridging chlorides and two terminal chlorides on each Pd. Some of these possible species structures can be eliminated on the basis of the Raman spectra alone. For instance, the two Pd-Cl stretching peaks rule out PdCl(H20); and Pd(H*O):’ as the dominant species, as these would contribute only one and zero peaks, respectively. Consideration of the dimeric palladium complex can be eliminated based on the Raman peak positions, since the observed peaks are not consistent with those previously observed for the dimer (strong peaks at 346 and 302 cm-‘, G~GGIN and MINK, 1974). In terms of the vibrational selection rules operating in Raman spectroscopy, two peaks would be consistent with the spectrum for planar PdCls(HaO)- or cis (but not truns) PdC12(H20)2. The latter possibility is less likely, since the out-of-phase Raman peak for cis PdClz(Hz0)2 would not be expected to yield a peak nearly as strong as that for the symmetric stretch. Hence, the peaks are probably due to PdCls(H@-, but cis PdC12(H20)2 cannot be ruled out from these Raman spectra alone. Note that although the symmetry rules predict two polarized peaks (from two totally symmetric modes) for PdC13(H20)- and only one for PdC12(H20)2, the peaks were too weak to yield depolarization ratios. A third spectrum, characterized by Pd-Cl stretching peaks at 296 and 342 cm-‘, is observed at very dilute chloride concentrations (Fig. la). Again, no peak associated with a PdOH stretch was observed. To help determine the species contributing to this spectrum, a chloride-sensitive electrode was employed. As noted above, the Pd2+ source was PdCl*. Upon dissolution under conditions identical to those from which Fig. la was determined (i.e., pH = 2, no added chloride, 243 rmol PdC12in solution), only 5 rmol of free Cl- was detected. Therefore, the chloride ligands are held tightly to the palladium cation. The straightfonvard implication of this result is that the spectrum in Fig. 1a is due to PdClz(Hz0)2 formed upon dissolution of PdCIZ. However, the persistence of a peak at 296 cm-’ was suspicious, as it is also a peak seen in the species yielding Fig. lc. While this could be coincidental, another explanation might be the presence of a slow reaction upon dissolution, such as PdClz(s) + PdCla(H,O)-(aq)

+ PdC1(H20):(aq).

The 296 cm-’ peak would, in this scenario, be from the trichloro species, and the 342 cm-’ peak from unresolved peaks of the mono-chloro species and PdC13(H20)- (335 cm-‘). To determine the likelihood of such a reaction upon dissolution, the conductivity of the solution was also measured. If the PdC12(H20)2 species were predominant, the conductivity should remain similar to a pH = 2 solution without chloropalladium, while if the charged tri- and mono-chloro species were formed, the conductivity should be significantly increased. While the conductivity of a pH = 2 “blank” water solution measured 2000 w-mhos, the conductivity of a 12 mM solution of the Pd-Cl at pH = 2 measured 2400 p-mhos. In comparison, the conductivity of a pH = 2 solution with 12 mM NaCl (to mimic ion formation) measured 2900 pmhos. The intermediate value of the chloropalladium solution

C. D. Tait, D. R. Janecky, and P. S. 2. Rodgers

1256

is a rather ambiguous result. However, the electronic absorption spectrum for this solution, especially the d,d absorption bands, is not the sum of the spectra for the tri- and mono-chloro-species (vide infa), and the best explanation for the data is that the predominant species present upon dissolution of PdCl2 is indeed simply PdCl2(H,O), . Hence, the peaks observed in the Raman spectra are best ascribed to PdCl:- (Fig. le), PdC13(H20)- (Fig. lc), and PdClz(HpO)r (Fig. la). The small predominance field of the tri-chloro species predicted by the SSDI model does not fit the observed data, which instead favor the WMFEINK model. Furthermore, PdC12(H20), cannot exist only in the tram form, as that would yield only one Raman peak where two are observed. The two peaks actually observed would indicate a mixture of cis and tram (each contributing one peak to the overall spectrum), since the out-of-phase Cl-W-Cl stretch of the cis form, although Raman active, should be weak. A mixture of cis and tram dichloro species was also postulated by ELDING (1973). Such Raman vibrational characterizations (Fig. 1, Table I) of these palladium(B) chloride species may also prove useful in future studies of palladium sorption and reduction on mineral surfaces in support of work such as reported by HYLAND and BANCROFT(1990). For instance, the existence of the less sterically hindered cis (for adsorption purposes) as well as tram PdC12(H20)2 may be important to the adsorption of palladium onto surfaces in the first step in forming hydrothermally placed palladium deposits. Finally, several attempts were made to obtain a Raman spectrum of the complex under very basic (pH > 10) conditions, No vibrational peaks were observed, despite (super)saturated palladium concentrations and high laser powers used. We are forced to conclude that the Pd-OH species is a poor scatterer. ExpIo~tion

of the ~edomi~nce

Diagram

With the Raman and electrochemical species characterization of PdCl:-, PdC13(H@-, and PdC12(H20)2, the association of these species with Uv/Vis electronic absorption spectra can be made, with strong absorbances at specific waveiengths associated with specific coordinations (Table I). The isobestic points (i.e., wavelength points of equal absorbances as chloride concentration is changed) associated with the loss of absorbance at 280 nm and gain at 252 nm (see Fig. 2a for typical spectra) demonstrate that only two species (and no intermediates) are involved in the transformation

from PdCl:- to PdC13(H20)-. The transformation of PdClx(H20)- to PdC12(Hz0)2 (Fig. 2b) is characterized by a general loss of absorbance throughout the wavelength range. Furthermore, the spectroscopic signature of a hydroxopalladium(I1) complex was observed, with a peak at 366 nm. This peak is tentatively ascribed to a (d,d) transition because of its wavelength position and low extinction coefficient (222 M-’ cm-‘). Note that a hydroxide-paliadium(I1) ligand-tometal charge transfer transition, the principle alternative to a metal-centered transition, is likely to occur at lower wavelengths than the chloride-palladium(B) CT bands previously described due to the higher electronegativity of oxygen over chlorine (RARINOWITCH,1942). The identity of the hydroxypalladium(B) species is probably Pd(OH)z, consistent with recent hydroxy~lladium speciation reports for approximately this pH range (WM~INK model; WOOD, 1990). To obtain predominance ranges from the absorption experiments, absorbances at complex-specific wavelengths were plotted as a function of added chloride concentration or pH change (see Fig. 3). Concentrations were found from absorbances through the application of Beer’s Law (A = ~bc, where A = absorbance, E = extinction coefficient (M-’ cm-‘), b = path length of light through sample (cm), and c = concentration (M); STROBEL, 1973; WILLARD et al., 1974). When overlapping peaks from different spectra interfere with direct concentration determination, concentrations can still be calculated since the absorbances from the individual species i are simply additive, so Ah = b C Cicr,h.Hence, by judicial choice of species-characteristic wavelengths to minimize overlap interference (see Table I, Fig. 3a), relative concentrations and areas of species predominance were determined from the absorption data. Figure 3a plots out absorbances at 280 and 252 nm (the 252 nm data shows PdC13(H20)- replacing PdCI$) as a function of [Cl-] at several pH values. Because the 280 nm absorbance is due almost entirely to PdCIi-, initial estimates for curve fittings could be obtained by treating this data graphically as a photometric titration (WILLARDet al., 1974). A curve simulation (“+” signs) with log k4 = 1.23, log k3 = 2.60, log kZ = 3.54, log k, = 4.34, ~,(280) = 10500, ~(280) = 1500, ~(280) = 465, ~~(280) = 194, and CO= 1 (initial extinction coefficients from ELDING, 1972; important only as relative values due to curve normalization) is shown in the last, composite plot of Fig. 3a. Because the 236 nm absorbance of PdC12(H20)2 (plotted in Fig. 3b) is intermediate

TABLE I. Spectroscopic Characterizations of Pd-Cl-OH species Uv/Vis absorptiona Raman scattering species ==========-====E=======I==F===========rO====~====== s=sm===== PdC14*- : 221, 280 (LMCT),474 (d,d) nm; 279, 300 cm-l PdC13(H20)-:

207, 236 (LMCT),426 fd,d) nm;

296, 335 cm-1

PdC12(H20)2:

350 (d,d), 422 id,d) nm;

296, 342 cm-l

Pd(OHJ2:

366 (d,d) nm

LMCT = ligand-to-metal charge transfer band (d,d)= ligand field absorption band achloropalladium peaks assigned previously (see ELDING, 1972)

Speciation of Pd chloride complexes

Palladium(ll)

Chloride

T = iaoc

pH = 1.7

5.000 M I [N&I] s 0.007 M

280 300 Wovelength (nm)

Palladium(ll) pH = 1.7

Chloride

T = laoc

0.0100 M e [NaCI] e 0.00005 M

Wovelength (em)

FIG. 2. Typical room temperature electronic absorption spectra of palladium(I1) chloride species (total [Pd*‘] = 65 JIM) at pH = 2.7 with systemmatic change in added chloride ligand. The strong peaks in the blue/ultraviolet region (~350 nm) are from ligand-to-metal charge transfer bands. (a) shows the transformation between

PdCti- and PdC13(HZO)-, while (b) showsthe transformation between PdCI,(H@- and PdC12(H20)Z.The arrows show the spectral trends from solutions with successively lower chloride concentration. to that of PdC13(H20)- and PdCI(H20)$, no plateau in the absorption vs. [Cl-] is reached (i.e., there is no endpoint at PdC1z(H,Ofz) and the photometric titration method cannot

1257

be used, Nonetheless, k3 can be estimated by numerical methods based on Beer’s law as outlined above: the normalized curve simulation with log k4 = 1.23, log k3 = 2.60, log k2 = 3.54, log k, = 4.47, t4(236) = 10800, ~(236) = 13300, ~~(236) = 8970, ~~(236) = 1670, and ~(236) = 59 fits the data adequately. Finally, Fig. 3c shows the absorbances at several wavelengths as pH is changed for various fixed [Cl-] (see discussion below). Although lower signal-to-noise resulted, experiments in which pH was changed were performed with as low a palladium concentmtion as possible (3 PM) rather than the typical SO-90 PM concentrations used in the more acidic regions when [Cl-] was varied. This low concentration was necessitated by precipitate formation at higher concentration and also to minimize the zone in which [OH-] i [Pd*‘], where lack of transformation may be from limiting-reagent rather than from stability considerations (vi& infra). When these precipitates formed, they would appear as a slight cloudiness in the solution. The absorption spectrum of this product consisted of a very broad “absorbance” from the near Uv tailing throughout the visible to beyond 600 nm. This featureless spectrum is consistent with scattering from a suspended solid (precipitate) rather than true absorption, as more scattering is expected at lower wavelengths (STROBEL, 1973). A harvestable precipitate was collected after neutralizing (to pH = 8) a solution with high [Cl-] concentrations (thereby allowing high palladium concentrations). SEM examination of the filtered product showed a prominant chloride peak as well as palladium peak. One possibility for this precipitate, then, is the formation of mixed chlorohydroxypalladium(H) insoluble complex(es). Such an interpretation was assumed in the speciation analysis of Pd2+ in seawater ([Cl-] = 0.558 M) as a function of pH (KUMP and BYRNE, 1989) and in a potentiomet~c hydrolysis study of PdClz(KAZAKOVAand PTITSYN, 1967). Other hydrolysis studies, however, used a non-coordinating perchlorate medium (IzATT et al., 1967; NABIVANETSand KALABINA, 1970) or ignored the effects of chloride on the hydrolysis (and pre-hydrolysis) products (MILIC and BUGARICIC,1984). To investigate this region further, we repeated the experimental conditions (i.e., [Cl-] = 0.558 M) used by Ku~~and BYRNE( 1989) for high (124 PM) and low (3 PM) palladium concentrations (Fig. 4). From starting pH value of 7.8 to 9.7 (the pH would slowly decrease as the palladium cations acted as a buffer), the absorption spectrum for the high concentration series changed during a 24-hour period. A recent paper reports similarly slow stepwise hydrolysis kinetics of tetrachloroplatinate(I1) in base (Wu et al., 1990). One day after the solution was mixed, a yellow precipitate had settled to the bottom of the cuvettes, allowing good solution spectra to be taken. The original strong LMCT peaks of PdCl:- at 280 and 221 nm were replaced (with non-isobestic behavior) by other peaks at 276, 269, and 237 nm (Fig. 4a). The nonisobestic behavior implies a transformation into more than one complex, and these other peaks may be LMCT bands of mixed Pd-Cl-OH species corresponding to the precipitated Pd-Cl-OH species (precipitate identity assumed by analogy to the SEM results described above). Similar behavior was not as pronounced for the low concentration series (Fig. 4b): the LMCT peaks of PdCl:- were bleached (i.e., lost intensity)

1258

C. D. Tait, D. R. Janecky, -

and P. S. Z. Rodgers

A(252) A(280)

06

040

-060 -050 -040 -030 -0x) -0

10

TOOO

5

.4

-3

.2

-1

0

1

-5

.4

-3

log[NaCI]

-2

-1

0

1

log[NaCI]

06

o.asr)-

pti = 4.0

- 0 055

0.020 -:

.a&:

-0035

0.4 0.010

-0015

02 0.0

-5

-4

-3

-2

-1

0

-0.005

0.000

1

-5

-4

-3

log[NaCI]

-2

-1

0

1

log[NaCI]

055.H=s.s . x Cl0

1

l.O-

g 'y

0.6 -

:

0 03504525-

0151

-5

j

0.6-

; f a

0.4 -

1

.

numcak

l

mpH-1.7

l

norm pH-J.0

l

mPHm27

.

ncimpH-4.0

#%a+

.*.:

r;‘

C#

a .* _**

- 0.6 -06 0.4

I

I

-4

-3

I

-2

I

-1

I

0

.

-I

t-o.2

1

log[NaCI]

:

-3

-4

-8

-2

-1

0

1

log[NaCI]

FIG. 3. Plot of absorptivity of species-characteristic wavelengths as a function of chloride concentration at several pH values (total [Pd”] between 50 and 90 FM) or pH (total [Pd”] = 3 PM). (a) Absorptivity at 280 nm, due primarily to PdCl:-, and at 252 nm, due primarily to PdC13(H20)-. The final plot shows the 280 nm peak heights from several pH values and the results of a fit with log /Q = 1.23, log k, = 2.60, log kg = 3.54, log k, = 4.47, ~~(280) = 10500, ~(280) = 1500, ~(280) = 465, and ~(480) = 194. (b) Absorptivity at 236 nm, due primarily to PdCI,(H20)) as a function of added chloride concentration at two different pH values. The accompanying fits used the following parameters: log k4 = 1.23, log k, = 2.60, log k2 = 3.54, log k, = 4.47, ~(236) = 10800, t,(236) = 13300, ~(236) = 8970, t,(236) = 1670, co(236) = 59. (c) Absorptivity at 280 nm, due primarily to PdCh-, 236 nm, due primarily to PdC13(H20)-, and at 252 nm, due primarily to mixed chlorohydroxypalladium species formed when [Pd”] > [OH-].

with some increased absorbance between 240 and 270 nm as pH was increased to an initial reading of between pH = 8 and 9. Furthermore, a slow buffering reaction took place even for the 3 PM [Pd2’] solution: the initially raised pH would decrease, presumably because of a decrease in free [OH-] as OH- displaced chloride. (Note that the final pH values of the solution in the UV cuvettes could not be measured because

of the small volume available). Therefore, mixed chlorohydroxypalladium(I1) species probably exist as a solute as well as a precipitate, but only when [OH-] serves as a limiting reagent (i.e., only when not enough OH- ligand is present to completely replace the chlorides). However, since [Pd’+] in hydrothermal systems is < 100 nM (- 10 ppb) and in marine/ lake systems is
1259

Speciation of Pd chloride complexes

O.OlS-

Qf 0.4 -

.* : **

0.2 -

. 0.0,

l ++

/’

-5

B

A(236) pH=1.7

v

A(2521

*

A(236) pH=1.7 talc

v

A(2601

.* .

fools

0.018,

I

-

.4

I.1

-3

b -

-2

.l

I

-,

0

:

1

-1.1 -0016

log[NaCI]

0.017- 0.014 -0.012

0.016 -

-0.010 - 0.008 i

0.014 f 2.5

.

, 4.5

*

) 6.5

.

, 6.5

I

co.006 10.5

FH A(252) 0.8 0.6 0.4 -

A(2801

k” . B :

El A(236) pH=3

l

.*’

l

A(236) pH9.0

talc

0.2 -

*.** l+*+ 0.0 . 1 -5 -4

.

I -3

.

, .2

I -1

.

I 0

. 1

iog[NaCl]

-0.oQs-i -

,

2.5

4.5

.

I

6.5

.

I

6.5

.

1 .5

PH _

A(236)

FIG. 3. (Continue)

small. Hence, in natural systems, the step-wise replacement of chlorides by hydroxides would not be observed because OH- readily replaces Cl- over a ne&gibIe pH change. An important example of the near-neutral transition zone between chloropalladium and hydroxopalladium species predominance is seawater, where [Cl-] 2 0.6 M and pH z 8 (i.e., [OH-] E 1 PM). Both the experimental data presented here and the WMFEINK model indicate that considerable fractions of both PdC@ and W-OH species (vi& infa) could exist under these conditions. In contrast to the work O~KUMP and BYRNE (1989), we do not believe that mixed chloroishingly

hydroxo palladium compounds predominate at this or higher pH values in seawater. However, as noted by KUMP and BYRNE (1989), actual transport in seawater may involve organic compiexation rather than strictly inorganic complexation, so this conclusion is onIy a prerequisite for understanding palladium transport in seawater. The ultimate species likely to form upon base hydrolysis of the chlorides is Pd(OH), (also formulated as PdO * nH20), as suggested by the work Of&ATT et al. ( I967), NABIWIVANE~~ and KALABINA (1970), WESTLAND (19X l), and WOOD (1990). Further characterization of this species is difficuit because of

C. D. Tait, D. R. Janecky, and P. S. 2. Rodgers

1260

mixed chlorohydroxypalladium complexes diminish enough to observe a peak at 366 nm. Transition conditions determined from these considerations are plotted (as X’s) on the experimentally determined predominance diagram shown in Fig. 5. The lines drawn between the X’s are interpolations between these experimental points. The ordinate for Fig. 5 is shown both as the log of [NaCl] (i.e., experimental concentration) and the log of the activity of NaCl, where the activity was calculated from concentration from Pitzer’s equations (PITZER, 198 1; HARVIE et al., 1984). Note that the descrepancies between the two are minimal, especially when [NaCl] < 0.1 M. Several conclusions can be drawn from the constructed predominance diagram for ambient conditions. In general, the diagram is essentially that predicted by the WMFEINK model: the [Cl-] and pH predominance ranges of acidic solutions of the PdCldH,O)::$ species conform to those shown in WOOD et al. (1989). Specifically, the boundary between the tetra- and tri-chloropalladium(I1) species has been ex-

.6

277

‘pHz9.1

\

\\

I

240

260

280

Wavelength

300

--7-v 320

340

(nm)

perimentally determined here to be log [NaCl] = -1.23 + 0.15 (log ac,- = - 1.32 * 0.14) while that between the triand bis-chloro species = -2.60 + 0.15 (log ac,- = -2.62 + 0.15) at 19°C. Table II compares our results to other studies. Most importantly, our work disagrees with the narrow range of PdCls(H20)- predominance suggested by the SSDI model. While our k4 value is not significantly different from

those of WEED ( 1964) BURGER ( 1964) and ELDING(1972) our k3 is different and more closely resembles that reported by SHCHUKAREV et al. ( 196 l), resulting in a slightly larger predominance

area for PdC13(H20)) than reported previously.

Temperature Dependence To extend the 19°C data just described, samples were heated by circulating thermostated water through the LJv/ Vis metal cuvette holders. The major item to be investigated was the temperature dependence of the chloropalladium pre-

:: s e 8 P .l 4

log[NaCI]

:80

’ cl0

-2 [mixed Pd-Cl-OH]

,

0 260

280 Wavelength (nm)

3bo

-4

sio

FIG. 4. Uv/Vis spectra of chloropalladium(II) species (a) [Pd*‘] = 124 pM, [Cl-] = 0.558 M and (b) [Pd*+] = 3 PM, [Cl-] = 0.558 M) under near neutral (pH = 8) conditions. The reported pH values are starting pH values and do not account for the slow buffering of the solution as OH- ligands coordinate to the palladium ions. New peaks at 237,26 1,269, and 290 nm observed in the more concentrated solution are attributed to mixed chlorohydroxypalladium(I1) species trapped due to a limiting supply of hydroxide ligands (i.e., 2[Pd*‘] > [OH-]).

its low solubility and lack of characteristic electronic absorption peaks (see Fig. 4b). A qualitative change occurs near pH = 10, where solubility rises enough and interference from

2

4

6

0

10

12

PH FIG. 5. Experimentally determined predominance diagram for Pd(II) chloride species. X mark transformation points where two species have equal concentrations, while the lines are interpolated between these points. Uncertainties in the marked points are approximately *O. 1 log [NaCl] units and f0.2 pH units. The neutral pH region between the chloropalladium complexes and the dashed line contains mixed chlorohydroxypalladium complexes at the palladium concentrations studied here, but should become negligibly small under natural conditions (i.e., the dashed line should move to lower pH values when [OH-] is no longer a limiting reagent). Note that coordinating water molecules have been omitted from the species labels for greater clarity (i.e., PdClj = PdC13(H20)) and PdCIZ = PdC12(H20)2).

1261

Speciation ofPd chloride complexes TABLE

II. Comparisons of palladium chloride stepwise association Constants log k, = log~[PdCl,z-n]/([PdCl~-13-nJ [Cl-l) I; n=l-4.

Droll et al. (1957)

Shchukarev et al. (1961)

Weed (1964)

Burger (1964)

Buryukov and Shlenskaya (1964)

logkl logkz logk3 logk4

6.2 4.7 2.5 2.6

4.34 3.54 2.68 1.68

4.4 3.34 2.34 1.38

3.88 3.06 2.14 1.34

3.98 3.24 2.30 2.00

PH [Fd2+l

0.7 786 uM

0.2 1.35 mt4

TV%)

21

20

0.0 0.218 to 4.35 mM 25

0.0 0.66 to 1.99mM 25

0.2 to 0.8 0.3 to 1.22 mM 25 IIeE==P113=

--IDPEPIIIPr-lPIPl=fI=IP

IlfPIILIII=II=lllllf==~==*=====---===-===-Shlenskaya and Grinberg Buryukov (1966) et al. (1967)

Levanda (1968)

Elding (1972)

1.43

4.47 3.28 2.42 1.37

logkl logkz logk3 logk4

4.00 3.45

PH

0.1

1.0

0.7

0.0

1.7 to 5.5

fPd2+1 T(OC)

42 v 25

30 mu 20

100 &lM 25

95 )iM 25

5 90 w 19

avalues

5.1

this work

-

in parantheses

correspond

(2.62ja t1.321a

to logacl- rather than log[NaCll

regions in acidic media, and the determination of the equimolar point between two adjacent species was determined by the ratio of absorbances at different wavelengths. In this way, any changes in absolute absorbances from factors such as density changes would cancel. Specifically, the criterion for the PdCl~-/PdCl~(H~O)- equimolar point occurs, as determined from the 19°C data above, at ~(280)/~(252) = 0.89, while that for the PdCl~(H*O)-/PdCl*(H~O)~ equimolar point occurs at A(236)/A(220) = 1.05. Figure 6 shows typical spectral effects of increasing the temperature on Pd(II)-Cl systems. The first series (Fig. 6a) shows PdCl:- - PdClI(H20)- as the temperature is increased at pH = 5.4, while the lower series (Fig. 6b) shows PdC&(HZO)- --, PdC12(Hz0)2 at pH = 4.2. The isobestic points in Fig. 6 demonstrate that each spectral series repredominance

TABLEIII.

2.60 1.25

Temperature

dependence

sents the transformation of one species into the other, with no intermediate or side reaction. While none of the chloropalladium peaks shift with increasing temperature, the absorbance increases to the extreme Uv side of the spectra are caused by a red shift of the charge transfer to the solvent band of free chloride ions (SEWARD, 1984). The observed reactions for the chlorides were fast, occurring within minutes. In each case, a net gain for the less-charged species (PdCl&H20)- in Fig. 6a and PdC12(H20)2 in Fig. 6b) is observed, consistent with a decrease in the dielectric constant of water and subsequent “hardening” of solutes (BRIMHALL and CRERAR, 1987; SEWARD, 1984). The dielectric constant effect can be summarized in the Born equation, AG = -([zie]*/2rJ(l - l/D) (zie = charge of ion i, ri = radius of ion i, and D = solvent dielectric constant), which represents

of the boundaries between

PdClq2- and PdC13(H201- 1413) and PdC13(H20)- and PdC12(H20)2 (3/2).

4/3

(pa = 5.4)

T('C) -log[cl-1

3/2

19

41

66

79

90

1.23

1.10

1.00

0.90

0.85

19

45

52

72

76

2.60

2.50

2.40

2.10

2.15

(pH = 4.2)

T('Ct -loglcl-I

1262

C. D. Tait, D. R. Janecky, and P. S. Z. Rodgers

the free energy change associated with moving an ion from a vacuum to a solvent environment. Consequently, the solvent provides less stabilization for charged species at higher temperatures (D = 58 at 9O“C as opposed to 80 at 19”(Z),

Palladium(ll) Chloride pHS.4 29 11~82 log[NaCI] = -1 .lO

‘X

Temperature

Dependence

of logkll

and logkd

{PdCl4}2-

-3.5

I

0

20

40

Temperature

60

(C)

80

100

FIG. 7. Plot of log [NaClJ of the equivalence points (points ofequal concentrations ofadjacent species) as a function of temperature. Note that the boundary points between PdC$ and PdCl,(H20)- (4/3) were determined at pH = 5.4 while the 312 points were determined at pH = 4.2.

220

240

260

200

300

320

340

Wavelenqlh (nm)

07

Palladiuni(ll) t

pHz4.2 log[NaCI]

Chloride 19rTls.71 = -2.40

OC

and the boundaries in the predominance diagram shift from the ambient temperature boundaries. These shifts can be followed by noting the chloride concentration necessary to reestablish the equimolar point at higher temperature (i.e., higher chloride concentrations are needed to force the equilibrium back to the equivalence point at higher temperatures). Table III and Fig. 7 show this temperature effect on the PdCl$-/PdC13(H20)- (4/3) and the PdC13(HZO)-/PdC&(H-Izo)z (3/2) equivalence points (i.e., [Cl-] points where the two species have equal concentrations). Because of the limited temperature range studied, an adequate linear fit can be drawn through the (T, log [NaCl]) data points. For the 4/3 boundary, the best line has slope = (5.3 + 2.6) X 10m3T-‘. y-intercept = - 1.33 + 0.02, and ? = 0.99; and for the 312 boundary, the corresponding values are slope = (9.0 + 1.6) X 10v3 T-l, y-intercept = -2.83 f. 0.09, and ? = 0.91. The faster rate of change for log k3 than for log k4 is somewhat unexpected, as the higher charged PdCI$- should be destabilized more effectively with decreased dielectric constant (from the Born equation). A larger temperature range is required to determine if this effect is indeed significant. CONCLUSIONS

220

240

260

280

300

320

340

Wovelenqih (nm)

FIG. 6. Uv/Vis absorption spectra as a function of temperature. The arrows show the spectral trends with higher temperature. (a) [Pd2’] = 79 PM, log [NaCI] = -1.10, and pH = 5.4. The 280 nm peak of PdClt- loses intensity while the 252 and 236 nm peaks of PdCIs(H20)- gain intensity as the temperature is increased. (b) [Pd”] = 62 pM, log [N&l] = -2.40, and pH = 4.2. The 236 peak of PdCIs(H20)- loses intensity with increased temperature.

Spectroscopic signatures, including Raman vibrational and Uv/Vis electronic absorption spectra, have been collected and verified for the complexes PdClJH,O)~:~ (x = 2-4) and Pd(OH)fex (x = 2, 3, or 4) under acidic and basic endpoint conditions (Table I). These signatures in turn have been used to map out the predominance diagram of p~ladium(I1) chloride species as functions of pH and [Cl-‘] (Fig. 4). Under acidic conditions to neutral conditions, the W~FEINK model has been generally verified experimentally. Furthermore, evidence for one or more mixed W-OH-Cl species was found, both as a precipitate (SEM evidence) and as a (dilute) solute (shifts in Uv absorption spectra from known PdCl(H,O)::$ spectra). However, these species were formed under near-neutral conditions where [OH-] < [Pd2’], a condition which would exist only in a vanishingly small stability region in geologically relevant solutions. Temperature dependence for the acidic equilibria of the Pd-Cl series also has been presented. As expected (BRIMHALL

Speciation of Pd chloride complexes and CRERAR, 1987), higher temperatures

lead to increasing predominance of lower-charged species consistent with the decrease in solvent dielectric constant. In this respect, note that the dielectric constant of water is changing very rapidly from 15 to 9O”C, so the constraints offered by this study are important for modeling calculations. The chloropalladium(I1) complexes persist to high pH values and thereby occupy a large percentage of the predominance diagram. This persistence gives some (limited) laboratory support to claims of chloropalladium transport under oxidizing, acidic to neutral, saline conditions. Such conditions exist in marine (KUMP and BYRNE, 1989), estuary, and some “fresh water” lakes (e.g. Lake Algi in the USSR Lake Baikal region, POGREBNYAKand TATYANIUNA,1979). Furthermore, these conditions (at elevated temperatures) may have existed in sediment-hosted stratiform deposits like the Kupferschiefer (MOUNTAIN and WOOD, 1988; KUCHA, 1975, 1982), the Zambia-Zaire copper belt with accompanying palladium deposits (MOUNTAINand WOOD, 1988; MERTIE(1969), in some unconformity-related uranium deposits with accompanying high but sub-economic deposits of palladium in the Northern Territory of Australia such as the Jabiluka deposit (WILDE and BLOOM, 1988; WILDE et al., 1989), and possibly contempary hydrothermal systems like the Salton Sea system (MCKIBBEN, 1990). Finally, chloride complexes (esp. PdCl,(H,0)2) could have been important in high temperature remobilization of originally magmatic Pd (and Pt) observed in the Bushveld (South Africa) and Stillwater (Montana, USA) Complexes (MOUNTAIN and WOOD, 1988; COUSINS, 1973; KINLOCH, 1982; SCHIFFRIES, 1982; VOLBORTHand HousLEY, 1984; BOUDREAUand MCCALLUM, 1985; BOUDREAU et al., 1986a,b; WATKINSON et al., 1986; SCHIFFRIESand SKINNER,1987). Solubility studies also indicate the possibility of chloro complexes in PGE redistribution under acidic and oxidizing conditions (GAMMONSand BLOOM, 1990). The other large area of the predominance diagram was attributed to Pd(OH)2, with possible importance to surface marine environments (along with chloropalladium(I1) and possibly organic complexes). Furthermore, high palladium concentrations have been found along a redox front boundary at Pocos de Caldas uranium mines in southeastern Brazil (CURTIS, pers. comm.). These pH = 5 to 6 oxidizing waters are low in chloride, bisulfide, and ammonia concentrations (the latter two are low because of oxidation to SOi- and NO:) (CHAPMAN, 1988), leaving Pd(OH)2 as a possible palladium mobilizer. Acknowledgments-This study was supported by the DOE Basic Energy Science’s Division of Engineering and Geosciences contract W7405-ENG-36. We also acknowledge the SEM work oerformed bv Greg Bayhurst of INC-7. Finally, we would like to thank Prof. E. Shock and D. Sassani for a preprint of their paper and the reviewers of the original manuscript, including Profs. S. A. Wood, M. S. Bloom, and E. Shock, for their critical reviews and helpful comments. Editorial handling: S. A. Wood

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1263

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Halogen geochemistry of the Stillwater and Bushveld Complexes: Evidence for transport of the platinum-group elements by Cl-rich fluids. J. Petrol. 27, 967-986. BRIMHALL G. H. and CRERARD. A. (1987) Ore fluids: Magmatic to supergene. In Thermodynamic Modeling of Geological Materials: Minerals, Fluids, and Melts: (eds. I. S. E. CARMICHAEL and H. P. EUGSTER);Reviews in Mineralogy 17 pp. 255-32 1. BURGERK. (1964) Determination of the stability constants of palladium(U) complexes. Mugy. Kern. Folyoirat 70, 179-184. BIRYUKOVA. A. and SHLENSKAYAV. I. (1964) Composition and stability constants of chloro-complexes of palladium(II). Russian J. Inorg. Chem. 9,450-452. CHAPMANN. A. (ed.) (1988) Pocos de Caldas Project: Second annual report and Proceeding of the project workshop held at Pocos de Caldas, Minas Gerais, Brazil Feb. 22-26, 1988. Swedish Nuclear Fuel and Waste Management Co., Box 5864, S-102 48, Stockholm, Sweden. COTTON F. A. and WILKINSONG. (1988) Advanced Inorganic Chemistry, 5th edn. Wiley Interscience. COUSINSC. A. (1973) Notes on the geochemistry of the platinum group elements. Geol. Sot. S.A. Trans. 76, 77-81. DROLL H. A., BLOCKB. P., and FERNELIUSW. C. (1957) Studies on coordination compounds. XV. Formation constants for chloride and acetylacetonate complexes of palladium(I1). J. Phys. Chem. 61, 1000-1004. ELDINGL. I. (1972) Palladium(U) halide complexes. I. Stabilities and spectra of palladium(I1) chloro and bromo aqua complexes. Inorg. Chim. Acta 6, 647-65 1. ELDING L. I. (1973) Palladium(U) halide complexes. III. Acid hydrolyses and halide anations of cis- and trans-dichlorodiaquapalladium(I1) and dibromodiaquapalladium(I1). Inorg. Chim. Acta 7, 581-588. FUCHS W. A. and ROSE A. W. (1974) The geochemical behaviour of platinum and palladium in the weathering cycle in the Stillwater complex, Montana. Econ. Geol. 69, 332-346. GAMMONSC. H. and BLOOMM. S. (1990) Experimental investigations on the stability and stoichiometry of Pd(II), Pt(I1) and Pt(IV) chloride complexes to 300°C (abstr.). 1990 Ann. Mtng. Geol. Sot. Amer. Abstr. Prog., A 158. GCMXIN P. L. and MINK J. (1974) Vibrational spectra of squareplanar tetrahalogeno- gold(lII), -palladium(II), and -platinum(B) anions in solution. J. Chem. Sot., Dalton Trans. 1479-1483. GOLDBERGE. D. (1990) Chemical descriptions of seawater: past, present, and future. (abstr.). 200th ACS Natl. Mtng. Abstr. Pap. Amer. Chem. Sot., GEOC 1. GRAY H. B. (1965) Electronic structure of square planar metal complexes. In Transition Metal Chemistry: A Series of Advances, 1 (ed. R. L. CARLIN),pp. 239-287. GRINBERGA. A., GEL’FMANM. I., and KISELEVAN. V. (1967) Instability constants of halogeno-complexes of palladium. Russian J. Inorg. Chem. It, 620-621. HARVIEC. E., MOLLERN., and WEAREJ. H. (1984) The prediction of mineral solubilities in natural waters: The Na-K-Mg-Ca-H-ClS04-OH-HCO#Z02-HzO system to high ionic strengths at 25°C. Geochim. Cosmochim. Acta 48, 723-75 1. HENDRAP. J. (1967)The Raman spectra of complex anions of formula MX!- where M is Au”‘, Pt”, or Pd”, and X is a halogen atom. J. Chem. Sot. (A), 1298-1301. HyLAND M. M. and BANCROF~G. M. (1990) Palladium sorption and reduction on sulphide mineral surfaces: An XPS and AES study. Geochim. Cosmochim. Acta 54, 117- 130. IZATTR. M., EATOUGHD., and CHRISTENSEN J. J. (1967) A study ofPd2+ (aq) hydrolysis. Hydrolysis constants and the standard potential for the Pd, Pd*+ couple. J. Chem. Sot. London (A), 130 l1304. KAZAKOVAV. I. and PTITSYN B. V. (1967) Hydrolysis of halogenocomplexes of palladium. Russian J. Inorg. Chem. 12, 323-326. INLOCH E. D. (1982) Regional trends in the platinum-group min-

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