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Published online 12 March 2007
Published in Soil Sci Soc Am J 71:322-328 (2007)
DOI: 10.2136/sssaj2006.0124
© 2007 Soil Science Society of America
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SOIL CHEMISTRY

Dissolution of Zinc-Cadmium Sulfide Solid Solutions in Aerated Aqueous Suspension

K. A. Barrett and Murray B. McBride*

Dep. of Crop and Soil Sciences, Cornell Univ., Ithaca, NY 14853

* Corresponding author (mbm7{at}cornell.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Anoxic conditions in Zn- and Cd-contaminated soils and sediments result in the formation of highly insoluble metal sulfides. Little is known, however, about the ability of mixed Zn–Cd sulfides, the most likely solid phases to form in these environments, to persist and limit Zn and Cd solubility under intermittently oxidizing conditions that are common with fluctuating water tables in wet soils. We therefore conducted laboratory experiments to measure the zero-order rate constants for oxidative dissolution of synthetic solid solutions of Zn–Cd sulfides under aerated conditions, and the release of SO42–, Zn2+, and Cd2+ into solution. It was found that ultrafine synthetic metal sulfide particles with moderately high Zn/Cd mole ratios (≥20) were relatively stable under fully aerated conditions, and oxidized at much slower rates than sulfides with low Zn/Cd ratios. At the same time, high-Zn sulfides were very efficient at retaining Cd in insoluble form even at very high solid-phase Cd concentrations (>10000 mg kg–1) and low pH (<5), preferentially releasing Zn into solution. The preferential retention of Cd in sulfide particles with high Zn/Cd ratios was indicated by the persistence of much higher Zn/Cd mole ratios in solution than in the solid as the sulfide suspensions were equilibrated with atmospheric O2. In contrast, Cd-rich Zn–Cd sulfides were readily oxidized under aerated conditions with the release of high concentrations of both Zn and Cd into solution. We concluded that colloidal Zn–Cd sulfide solid solutions high in Cd would be unlikely to limit the solubility of Cd or Zn to levels that would be nontoxic to soil biota or plants, whereas metal sulfides with high Zn/Cd ratios may be sufficiently persistent to retain Cd in an insoluble form in intermittently aerobic as well as anaerobic heavy-metal-contaminated wetlands.

Abbreviations: EC, electrical conductivity • FAA, flame atomic absorption • ICP, inductively coupled plasma


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In anoxic environments, including many sediments and waterlogged soils, the solubility and toxicity of Zn, Cd, and other chalcophilic heavy metals can be limited by metal sulfide formation (Klerks and Bartholomew, 1991; DiToro et al., 1996; Hesterberg et al.,1997; Van den Berg et al., 1998). For example, spherules of ZnS have been identified in biofilms found in high-Zn groundwater, formed by sulfate-reducing bacteria (Labrenz and Banfield, 2004). In soils, however, temporal changes in the redox conditions, such as occur with fluctuating water tables, modify the bioavailability and potential toxicity of these metals. In areas of the world with paddy rice (Oryza sativa L.) production, uptake of Cd into the grain is a health concern where soil contamination by mining has occurred. Oxic conditions that are maintained in the rhizosphere of paddy rice roots, but also arise from soil drainage that occurs as harvest is approached and commonly at midseason as well, can result in increased soil Cd solubility and uptake into rice (Jung and Thornton, 1997; Liu et al., 2006). These observations are consistent with an important role of metal sulfide oxidation during soil aeration causing greater Cd solubility and bioavailability. In general, seasonal changes in Cd and Zn concentrations measured in the pore water of soils and Cd and Zn uptake by plants have been attributed to the dissolution of metal sulfides with changing groundwater levels (Van den Berg et al., 1998; Mertens et al., 2006). Nevertheless, direct evidence for this potentially important mechanism of Cd and Zn dissolution in soils is lacking, and the tendency for soil pH to increase under reducing conditions can also contribute to lower Cd and Zn solubility (Jung and Thornton, 1997) under flooded conditions.

Cadmium contamination in soils is typically associated with high Zn concentrations, as these two metals are strongly associated geochemically, and therefore in high-Zn industrial waste materials as well. The average Zn/Cd ratio in earth surface rocks (by weight) is about 500:1, but much higher and lower ratios can be found in particular rocks and minerals (Alloway, 1990). Specifically, sphalerite (ZnS) and other sulfide minerals tend to be enriched in Cd, which readily substitutes into the metal sulfide structure. Thus, any immobilization of Cd as metal sulfide in contaminated soils and sediments is unlikely to be the result of pure CdS formation, but rather the substititution of Cd2+ into ZnS (or more common metal sulfides such as FeS). This could have important implications for the solubility of Cd, as solid solution theory predicts a lower solubility for Cd substituted in ZnS than for Cd in pure CdS (McBride, 1994). Both ZnS and CdS are kinetically much more stable than FeS in aerated suspensions, but synthetic CdS is more susceptible to oxidative dissolution over several hours than ZnS despite its lower solubility product (Simpson et al., 1998). Therefore, Cd dissolution on aeration of anaerobic soils or sediments could be kinetically limited when Cd is entrained within ZnS. Toxic trace metals introduced into soils containing metal sulfides could also be effectively immobilized by metal displacement reactions in the metal sulfide phase, as has been shown for Ag+ and Cu2+ added to ZnS (Rozan et al., 2000) as well as for Cd2+ added to FeS (DiToro et al., 1990, 1992).

Studies of a peat soil naturally high in amounts of Zn and S have shown microbial processes to release soluble Zn2+ and Cd2+ by the oxidation of organic S to SO42– (Qureshi et al., 2003). Much higher Zn, Cd, and reduced S are present in the intermittently submerged subsoil layers than in the surface of these soils (Martinez et al., 2002), and laboratory leaching experiments have shown that the potential for oxidative mobilization of Cd and Zn is higher in the subsoil than in the topsoil (McBride et al., 2006). Zinc is mobilized to a relatively greater degree than Cd, however, consistent with the fact that Cd has a higher affinity for these peat soils than Zn as measured by Kd, the metal distribution coefficient (Martinez et al., 2002). These experimental results for this metalliferrous organic soil point to an important role of reduced S species, and particularly metal sulfides, in controlling Zn and Cd mobility and toxicity. Consequently, it is important to understand the kinetics of Zn sulfide oxidation and the extent to which Cd can be retained or mobilized during the oxidative dissolution of Zn and SO42–. We therefore conducted experiments to measure the rates of dissolution and oxidation of laboratory-synthesized mixed Zn–Cd sulfide systems.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Synthesis of Zinc–Cadmium Sulfide Solid Solutions
Solutions of Cd and Zn sulfate salts with Zn/Cd mole ratios ranging from 100:1 to 0:1 were prepared by combining measured volumes of aqueous 0.10 M CdSO4 and 0.10 M ZnSO4 solutions to a total volume of 100 mL in 250-mL Erlenmeyer flasks. To each metal solution, 100 mL of 0.1 M Na2S was added from a burette with continuous stirring during a time period of about 2 min. The metal sulfide suspensions were transferred to glass centrifuge bottles and centrifuged, followed by decanting the supernatants. The solids were then freeze-dried to remove water, and stored under N2 until needed for batch dissolution studies.

The synthesized metal sulfides were analyzed for their Zn and Cd contents by dissolving 50 mg of the freeze-dried powders in concentrated HNO3 and measuring Zn and Cd concentrations in the acid digests by inductively coupled plasma (ICP) spectrometry. X-ray powder diffraction of the prepared metal sulfides was done using a Scintag diffractometer and Cu K{alpha} x-rays ({lambda} = 1.54 Å). Scanning electron microscopy of selected metal sulfide preparations (pure Zn and 5:1 Zn/Cd mole ratio) was done to estimate particle size and morphology, with energy dispersive x-ray spectroscopy analysis confirming the expected elemental composition (S, Cd, and Zn) of the preparations.

Batch Dissolution Studies of Zinc–Cadmium Sulfides
Duplicate 0.100-g samples of each of the dry Zn–Cd sulfide powders were placed in centrifuge tubes, and 30 mL of distilled water was added. After shaking, the suspensions were centrifuged and the supernatants discarded to remove excess Na2SO4 salt. A 30-mL volume of 0.01 M KNO3 was then added, followed by shaking, centrifuging, and discarding of the supernatant. Finally, the metal sulfides were transferred quantitatively into 125-mL Erlenmeyer flasks and 100 mL of a 0.01 M KNO3 solution was added. To determine the rate of metal sulfide dissolution, the flasks were continuously agitated at 20°C on a rotary shaker. Periodically, small volumes of the suspensions were removed, put through 0.2-µm membrane filters, and the filtrates were analyzed for total dissolved Zn, Cd, and S using ICP spectrometry. At the sampling times, the pH values of the metal sulfide suspensions were measured with a pH electrode. In addition, the colorimetric methylene blue method for dissolved sulfide (Nusbaum, 1965) was used at several time intervals to test for the presence of free sulfide in the solution phase. Trace amounts of sulfide could be detected (≤40 µg L–1) in the suspensions of the metal sulfides with high Zn/Cd ratios, but free sulfide concentrations were below the detection limit (≤10 µg L–1) in the metal sulfide suspensions with high Cd contents, presumably a consequence of the lower solubility of CdS relative to ZnS.

Dissolution of the metal sulfides was monitored at time intervals up to about 12 d by measuring dissolved S, Zn, and Cd in the solution phase. The oxidative dissolution rates were quantified as the slope of the increase in dissolved S with time (measured in units of µmol S L–1 h–1). For selected Zn–Cd sulfide batch systems, dissolved SO42– in solution was also measured using the standard BaSO4 turbidimetric method (USEPA Method 375.4) for comparison with dissolved S by ICP, and dissolved Zn was measured by flame atomic absorption (FAA) spectrometry for comparison with Zn measured by ICP. Although studies of the oxidative dissolution of pyrite have shown that SO42– is the only detectable S species to appear in solution (Paschka and Dzombak, 2004), this observation needed to be verified for the Zn–Cd sulfide systems studied here before dissolved S could be used as a proxy for SO42– in determining the rate of sulfide oxidative dissolution.

Sulfide solids remaining in the flasks at the end of the batch dissolution experiments were collected by centrifugation, washed in distilled water, freeze-dried, and saved for x-ray diffraction analysis to determine if aging in suspension had changed their crystallinity.

Consumption of molecular O2 by freshly prepared suspensions of Zn–Cd sulfide was measured by monitoring dissolved O2 with a polarographic electrode mounted in a sealed stirred cell containing 1 mmol (~100–200 mg) of freshly precipitated, distilled-water-washed metal sulfide suspended in 25 mL of 0.01 M KNO3. These metal sulfides were prepared with Zn/Cd mole ratios somewhat different from, but covering a range similar to, those used in the initial batch oxidative dissolution experiments, and were not freeze-dried to minimize exposure to air.

Metal sulfide dissolution was also monitored over 250 h by measuring the increase in electrical conductivity (EC) of metal sulfide suspensions as dissolved concentrations of SO42– and metal ions increased. Freeze-dried Zn–Cd sulfide samples (0.100 g) with different Zn/Cd mole ratios were weighed out in duplicate, washed twice in distilled water, and transferred into 125-mL Erlenmeyer flasks with 100 mL of distilled water. The flasks were continuously agitated at 20°C on a rotary shaker, and periodically the EC was measured with a conductivity meter.

Initial investigations of Zn and Cd sulfide dissolution in aerated aqueous suspensions showed the pH to rise during reaction. Consequently, a subsequent series of batch dissolutions was conducted by the same procedure described above, except that autotitrators were used to maintain a constant pH of 4.0 by 0.1 M HNO3 additions. The metal sulfide suspensions in flasks were continuously agitated on magnetic stirrers, and dissolved Zn in filtered suspension samples was measured at time intervals up to 120 h by FAA spectrometry.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Properties of Synthetic Zinc–Cadmium Sulfides
The initially prepared series of metal sulfides had Zn/Cd mole ratios of 97.9:1, 55.3:1, 21.8:1, 4.75:1, 0.33:1, 0.19:1, and 0.15:1 measured by ICP analysis of acid digests. The high Zn/Cd mole ratios were close to the intended ratios based on the Zn/Cd ratios in the solutions before precipitation on the addition of Na2S. At the lowest ratios, however, there was evidently a loss of Zn during the removal of the solution phase after metal sulfide precipitation. This enrichment in Cd relative to Zn in the solid sulfide phase has been observed by Sato et al. (1995), and is attributed to the higher solubility product of ZnS compared with CdS, which leads to higher residual Zn2+ left in solution after metal sulfide precipitation.

X-ray diffraction revealed a poorly crystalline metal sulfide phase, evidenced by very broad diffraction peaks, regardless of the Zn/Cd ratio. The most intense peak was identified as the 001 diffraction (at a lattice spacing of 3.12 Å for metal sulfides with high Zn/Cd ratios), matching the 100 diffraction of sphalerite and wurtzite. This peak, however, shifted to a higher spacing as a function of Cd mole fraction in the metal sulfide, according to the equation:

Formula 1[1]
where x is the mole fraction of Zn in the metal sulfide, and Cd1–xZnxS represents the chemical formula of the solid solution series. (The difference in spacing between the pure Zn and Cd end members is 0.20 Å, close to the difference in ionic radii of Cd2+ and Zn2+). Because the prepared Zn–Cd sulfides showed only a single broad 100 reflection that shifted to higher spacing with increasing Cd2+ content, it is concluded that the metal sulfides formed a true solid solution rather than a mixture of separate Cd-rich and Zn-rich phases. Aging of the metal sulfides in aqueous solution up to 12 d during the batch dissolution studies had no discernable effect on the poor crystallinity, as indicated by x-ray diffraction. In fact, the intrinsic particle size of the metal sulfides was estimated to be only 2.2 nm from the diffraction peak breadths using the Scherrer equation, similar to the particle size estimated by a different method for the Cd1–xZnxS system (Sato et al., 1995). Scanning electron microscopy confirmed extremely small particle size, on the order of nanometers, as discrete particles were not evident at 25000x magnification.

An attempt was made to prepare more crystalline ZnS by the slow diffusion of ZnSO4 and Na2S solutions through separate dialysis bags into a common aqueous bath over a period of several days. This produced only a slightly more crystalline metal sulfide (particle size of 3.3 nm based on x-ray diffraction peak width), which was not used in the subsequent batch dissolution studies. Synthesis of pure Zn and Cd sulfides by other methods have similarly shown the particles to be polycrystalline, composed of subcrystals with diameters on the order of 5 to 10 nm (Sugimoto et al., 1995, 1996). It appears that metal sulfide crystals of larger dimensions are unlikely to form under the low temperature and pressure conditions of soils and other earth-surface environments. For example, the microbially formed ZnS spherules observed by Moreau et al. (2004) are nanocrystalline, with crystallite size of about 1 to 5 nm. In contrast, large sphalerite crystals form in high-temperature hydrothermal fluids, and can be found in rocks such as dolomite.

Zinc–Cadmium Sulfide Dissolution Kinetics
The rate of dissolution of Zn–Cd sulfides in aerated aqueous solution was monitored based on the rate of increase in dissolved S and Zn. More detailed analysis of selected filtrates from several metal sulfides with a range of Zn/Cd ratios from the batch oxidative dissolution studies revealed that 77% of the dissolved S was in the form of SO42–, based on turbidimetric detection of SO42–. Furthermore, Zn concentrations measured in the filtrates by FAA spectrometry were, on average, only 70% of the dissolved Zn measured by ICP spectrometry. Finally, Zn measured by FAA was very closely correlated to SO42–, as shown in Fig. 1 , evidence for the stoichiometric release of SO42– with a Zn/SO42– mole ratio in the solution phase constant at a value very close to unity. The data for this relationship were combined from three different reaction times (43, 114, and 232 h) and three metal sulfides with very different Zn mole fractions (pure ZnS, 5:1, and 1:1 Zn/Cd ratios). Preliminary batch dissolution studies had shown little difference in oxidative dissolution kinetics for metal sulfides having Zn/Cd mole ratios >5:1 or <0.5:1, so that these three metal sulfide compositions represent the range of oxidative dissolution behavior observed for all the sulfide Zn/Cd ratios tested.


Figure 1
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  		Fig. 1. Dissolved concentrations of Zn2+ and SO42– measured simultaneously in several Zn–Cd sulfide suspensions undergoing oxidative dissolution.

 
Although these results clearly indicate the predominance of the metal sulfide oxidative dissolution reaction to release Zn2+ and SO42– into solution, they also reveal another form of S to be present in the analyzed filtrates. Because 23% of the filtrate S was not SO42–, and 30% of the ICP-measured Zn in filtrates was not detectable by FAA, the most plausible explanation is that ultrafine colloidal or clustered forms of Zn–Cd sulfide remained in the analyzed filtrates despite having been passed through 0.2-µm membrane filters. Zinc in the form of metal sulfide particles is unlikely to be detected by FAA because the relatively low temperature of the flame would be insufficient to atomize the particles.

Consistent with studies of the dissolution of pyrite (Paschka and Dzombak, 2004), intermediate oxidation states of S, such as SO32– or S2O32–, did not appear to be significant in solution during Zn–Cd sulfide oxidation. In addition, sulfide (S2–) in solution was found to be at very low concentrations both initially and after 42 h reaction time (based on the methylene blue method) at all Zn/Cd mole ratios. The highest S2– concentration measured was at the beginning of the reaction (time zero) with the highest Zn/Cd mole ratio tested (95:1), but was very low (~40 µg L–1). Therefore, S–2 was a negligible fraction of the dissolved S measured by ICP in the solution phase during Zn–Cd sulfide dissolution.

As shown in Fig. 2 , the metal sulfide dissolution rate, as measured by an increase in dissolved S, was strongly a function of the initial Zn/Cd ratio in the solid phase. The dissolution rates of the metal sulfides with high Zn/Cd ratios (>4.8) were low but measurable, whereas metal sulfides with low Zn/Cd ratios oxidized more rapidly. Pure CdS dissolved somewhat less rapidly than those metal sulfides with limited Zn content (Zn/Cd ratios of 0.15 and 0.33). This suggests that, while CdS is much more prone to dissolution than ZnS, substitution of a small fraction of Zn into CdS may destabilize this mineral phase even further.


Figure 2
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  		Fig. 2. Concentration of dissolved S in aerated Zn–Cd sulfide suspensions as a function of time. The figure legend notes the Zn/Cd mole ratio of each of the metal sulfides.

 
The experimental evidence indicates the predominant reaction of the Zn–Cd sulfides in aerated suspensions to be

Formula 2[2]
where M2+ is Zn2+ or Cd2+. This reaction is expected to follow the rate law

Formula 3[3]
where (MS) and (O2) are the concentrations of metal sulfide solids and dissolved O2 in suspension, respectively, k is the reaction rate constant, and d(SO42–)/dt is the rate of increase of dissolved SO42– concentration. Because only a small fraction of the metal sulfides dissolved during the reaction time (at least in the case of the more stable high-Zn sulfides), however, and the suspensions were continuously agitated to maintain constant dissolved O2, the last term in Eq. [3] is expected to be a constant. As a result, the reaction should follow pseudo-zero-order kinetics, with d(SO42–)/dt equal to a constant. Thus, the essentially linear increases in dissolved S shown in Fig. 2 are consistent with zero-order reaction kinetics, and the slopes of these lines represent the zero-order rate constants. Those metal sulfide dissolution rate constants are shown in Fig. 3 as a function of Zn/Cd ratio in the metal sulfide. There was a one to two orders of magnitude difference in rate constants across the range of composition from high Zn to high Cd when pH was not adjusted during the reaction. Higher but less variable dissolution rates of metal sulfides were measured at a controlled pH of 4.


Figure 3
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  		Fig. 3. Zero-order rate constants for oxidative dissolution of metal sulfides as a function of initial metal sulfide Zn/Cd molar ratio, measured under conditions of constant pH (4.0) and variable (unadjusted) pH.

 
The increase of dissolved S concentrations during metal sulfide dissolution was strongly correlated positively to an increase in dissolved Zn2+ (for high-Zn sulfides) and Cd2+ (for high-Cd sulfides), as shown in Table 1. Dissolution of Zn, shown in Fig. 4 for Zn–Cd sulfides with a range of Zn/Cd mole ratios, was nearly stoichiometric (slightly less than 1 mol of metal dissolved per mole of S dissolved) from metal sulfides high in Zn, as would be expected for complete oxidation of metal sulfide to sulfate. Similar stiochiometric behavior was observed for Cd dissolution from metal sulfides high in Cd. This tendency is shown by the slopes of the linear dissolution functions in Table 1, which range from 0.85 to 0.92 for Zn dissolution from high-Zn sulfides, and from 0.84 to 0.96 for Cd dissolution from high-Cd sulfides. The relationship of dissolved Zn to dissolved S, however, was nonlinear for metal sulfides with a Zn mole fraction below 0.25. This may have been due at least in part to reprecipitation or sorption of Zn (see discussion below), with the large pH increase of the low-Zn, high-Cd sulfides potentially promoting precipitation of Zn initially dissolved by oxidation.


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  		Table 1. Zn and Cd dissolution functions, represented by either the linear equation (M2+) = a + b(S) or the power function (M2+) = a(S)b, where (M2+) is the dissolved Cd or Zn concentration (mmol L–1) and (S) is the total dissolved S concentration (mmol L–1).

 

Figure 4
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  		Fig. 4. Concentration of dissolved Zn in aerated Zn–Cd sulfide suspensions as a function of time. The figure legend notes the Zn/Cd mole ratio of each of the metal sulfides.

 
There was little dissolution of Cd from metal sulfides with Zn/Cd mole ratios of 5 or higher, and there was no correlation between dissolved Cd and dissolved S at the lowest Cd mole fractions (0.010–0.044), which correspond to Zn/Cd ratios >20:1 (Table 1). Thus, in these metal sulfides with relatively low Cd content, any Cd released must have been readsorbed by the residual ZnS phase, as the pH was too low (<5) to allow the precipitation of Cd in hydroxide or carbonate form. It has been shown experimentally that the process of metal displacement occurs for Cd added to FeS (DiToro et al., 1990,1992) and Ag+ or Cu2+ added to ZnS (Rozan et al., 2000). This process could be of general importance in soils containing metal sulfides. The immobilization and detoxification of those heavy metals forming the most thermodynamically stable sulfide phases (e.g., Hg, Cd, Ag) could be expected as a result.

Metal sulfides with high Zn/Cd ratios released less of both Zn and Cd into solution, consistent with limited dissolution of these metal sulfides as shown in Fig. 2. Metal sulfides with lower Zn/Cd ratios (4.8 and 0.33), however, released much more Zn with time, a result of greater dissolution rates. The apparent exception to this pattern is the metal sulfide with the lowest Zn/Cd ratio (0.15), which released little Zn (Fig. 4) despite having one of the highest dissolution rates based on the amount of dissolved S. It is suspected that Zn precipitation resulted from a pH shift during the metal sulfide dissolution. As shown in Fig. 5 , the pH tended to increase with reaction time, particularly for those metal sulfides with Zn/Cd <4.8. For metal sulfides with Zn/Cd = 0.33, 0.15, and 0.0, the pH increased to 6.5 or higher, and may have led to the precipitation of released Zn2+ as Zn5(OH)6(CO3)2, a form of Zn more stable than Zn(OH)2 in fully aerated aqueous systems at this pH (Schindler et al., 1969; Baes and Mesmer,1976). In fact, solubility calculations show that Zn hydroxycarbonate would limit Zn solubility at pH 6.6 to 7.0 between 39 and 250 mg L–1, spanning the maximum solubilities actually observed. Thus, precipitation of Zn may explain nonlinear release of dissolved Zn, particularly evident in the Zn/Cd = 0.33 system (Fig. 4), although sorption of Zn on the residual Cd-rich metal sulfide solid may also be possible.


Figure 5
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  		Fig. 5. Measured pH of aerated Zn–Cd sulfide suspensions as a function of time. The figure legend notes the Zn/Cd mole ratio of each of the metal sulfides.

 
Increases in soluble Zn and dissolved S of the solution phase of these Zn–Cd sulfide suspensions are consistent with the process of oxidative dissolution of Zn and Cd sulfides by dissolved O2 to form sulfate salts of Zn and Cd; however, direct evidence of oxidation by O2 was obtained by measurement of the consumption of O2 by the metal sulfide suspensions. Figure 6 shows the initial rates of depletion of dissolved O2 from metal sulfide suspensions having a wide range of Zn/Cd mole ratios. There is a clear tendency for metal sulfides with higher Cd concentrations (low Zn/Cd ratios) to consume O2 more rapidly, confirming the observations based on S and Zn dissolution data that the high-Cd sulfides are less stable in aerated environments and more susceptible to oxidative dissolution.


Figure 6
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  		Fig. 6. Depletion rates of molecular O2 from metal sulfide suspensions with different Zn/Cd mole ratios ranging from pure ZnS to pure CdS.

 
The measurement of EC in suspension provided an independent measure of oxidative dissolution rate, as reaction Eq. [2] leads to an increase in EC that should be linearly proportional to SO42– and Zn2+ release into solution. The zero-order oxidative dissolution rates calculated from increases in EC measured over 200 h reaction time were 0.50 (pure ZnS), 0.32 (500:1), 2.5(5:1), 2.0 (2:1), 1.7(1:1) and 10.1 (0.19:1) µmol ZnSO4 L–1 h–1, where the Zn/Cd ratio of the metal sulfide tested is given in parentheses. The magnitude of these rates are consistent with those based on ICP analysis of solutions for S and Zn (see Fig. 3) and confirm that oxidative dissolution rates are strongly sensitive to the initial solid-phase Zn/Cd ratio, with high-Cd sulfides oxidizing about 20 times more rapidly than pure ZnS.

The increase in pH associated with metal sulfide dissolution, most apparent for the high-Cd sulfides, could have been due to the initial partial oxidation of sulfide, forming elemental S0 on the surface:

Formula 4[4]
This reaction would consume H+ and raise the pH, although subsequent oxidation of S0 to SO42–, if complete, would result in no net production of alkalinity:

Formula 5[5]
as the sum of the two oxidation reactions consumes no H+ ions. Surface oxidation of ZnS is known to form some S0 as an intermediate product (Durán et al., 1995). Because the more rapid oxidation associated with high-Cd sulfides produced the greatest pH rise, it is hypothesized that reaction Eq. [4] is the rate-limiting step, leading to a buildup of elemental S during the O2-mediated oxidation of sulfides having low Zn/Cd ratios. Conversely, high-Zn sulfides are sufficiently stable that their initial oxidation rates are too slow for reaction Eq. [4] to become rate limiting, and therefore there is no large pH change.

Dissolution rates at a constant pH of 4.0 were greater than those measured when pH was not controlled and allowed to rise (Fig. 3). This result indicates that oxidative dissolution by O2 is favored by lower pH, possibly because reaction Eq. [4] should, in principle, be favored by greater H+ ion activity.

Cadmium dissolution was strongly a function of Zn/Cd ratio in the metal sulfide, as shown by the much higher dissolved Cd concentrations associated with the metal sulfides with high Cd content (Fig. 7 ). Those metal sulfides with Zn/Cd mole ratios in the 22 to 98 range had very limited Cd solubility (generally well below 0.1 µmol L–1). It should be noted that there was high variability in replicate Cd analyses of the solutions from experiments in which metal sulfides with high Zn/Cd ratio were tested. This variability may be a result of the difficulty in filtering out ultrafine metal sulfide particles, even using 0.2-µm membrane filters. Because of high concentrations of Cd in the metal sulfide particles, the passage of even a small fraction of particles through membrane filters into solution could have led to erratic and sometimes high values for Cd in solution. Thus, it is likely that the average solubilities of Cd in the low range (<0.1 µmol L–1) of Fig. 7 overestimate the true Cd solubilities for these experiments. Nevertheless, if we consider the metal sulfide with the lowest Cd content (Zn/Cd = 98), the average Cd solubility during the batch dissolution experiment did not exceed about 0.06 µmol L–1. This metal sulfide had a Cd content of about 11600 mg kg–1. Consequently, the effective Cd distribution coefficient, Kd, defined in this mixed Zn–Cd sulfide–solution system as the concentration ratio of Cd in the solid to that in solution, is about 1.7 x 106 L–1 kg–1, an extremely efficient degree of immobilization of Cd compared with sorption on organic matter or on common soil minerals such as silicates and oxides (Sauvé et al., 2000, 2003).


Figure 7
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  		Fig. 7. Concentration of dissolved Cd in aerated Zn–Cd sulfide suspensions as a function of time. The figure legend notes the Zn/Cd mole ratio of each of the metal sulfides.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Batch studies of the oxidative dissolution of laboratory-synthesized solid solutions of poorly crystalline Zn–Cd sulfides in the pH range of 4 to 7 have shown high-Zn sulfides to be much more stable under fully aerated conditions than high-Cd sulfides. The predominant oxidation reaction for the high-Zn sulfides involves complete oxidation of the metal sulfide to release SO42– and Zn2+ into solution with little pH change, but high-Cd sulfides produce an increase in pH that is attributed to partial oxidation of surface sulfide to elemental S. The dissolution of Cd2+ is very limited for solid solutions with Zn/Cd mole ratios >20, implying a potentially important role of Zn sulfides in sequestering Cd in contaminated soils that are permanently or intermittently flooded. Since most Cd-contaminated soils have Zn/Cd mole ratios >100:1, these results indicate that pure CdS is much less likely to limit Cd solubility and toxicity in hydric soils than Zn–Cd sulfide solid solutions. It may be possible to take advantage of the Cd-immobilizing property of ZnS in certain agricultural systems such as paddy rice fields to reduce Cd uptake by crops. Addition of Zn salts to permanently or intermittently anaerobic soils could reduce Cd bioavailability by forming relatively stable Zn sulfides with Cd substituted into their structure.

Received for publication March 21, 2006.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome