Metal sulfides, solubility

The water dissociation equilibrium constant, Kw, and the first acid dissociation constant of H28, Ki, are both well documented. 8everal studies have confirmed that K2, the second acid dissociation constant of H28, has a value of 10 [38-43], and not as previously assumed 10 [38]. This results in a substantial revision of known metal sulfide solubilities [44]. Noticeably, the concentrations of... 

Sulfide produces an undesirable rotten-egg odor and is toxic when in the HjS gas form. Since the first pKa of H2S is 7.24, it is necessary to maintain pH 9 or above to completely prevent evolution of H2S gas (Fig. 12.7). Although excess H2S is necessary for the precipitation reaction, the excess must be kept to a minimum. Furthermore, although metal-sulfide solubility with respect to pH exhibits U-shaped behavior (Fig. 12.8), its solubility within the desirable pH range is extremely small (MCLs are met) (Fig. 12.9). Precipitation of metal-sulfides is normally carried out using Na2S or NaHS. However, not all metals precipitate effectively by sulfide. For example, chromium (Cr3+) precipitates effectively as a hydroxide rather than sulfide. 

A quantitative calculation of metal sulfide solubility requires treating several simultaneous equilibria, as the following example illustrates. 

Metal Sulfide Solubility Product Constant (Ksffi... 

Exposure to nickel metal and soluble compounds (as Ni) should not exceed 0.05 mg/cms (8-hour time-weighted average - 40-hour work week). Nickel sulfide fume and dust is recognized as being potentially carcinogenic. 

The heavy metal salts, ia contrast to the alkah metal salts, have lower melting points and are more soluble ia organic solvents, eg, methylene chloride, chloroform, tetrahydrofiiran, and benzene. They are slightly soluble ia water, alcohol, ahphatic hydrocarbons, and ethyl ether (18). Their thermal decompositions have been extensively studied by dta and tga (thermal gravimetric analysis) methods. They decompose to the metal sulfides and gaseous products, which are primarily carbonyl sulfide and carbon disulfide ia varying ratios. In some cases, the dialkyl xanthate forms. Solvent extraction studies of a large number of elements as their xanthate salts have been reported (19). 

Many of the most important naturally occurring minerals and ores of the metallic elements are sulfides (p. 648), and the recovery of metals from these ores is of major importance. Other metal sulfides, though they do not occur in nature, can be synthesized by a variety of preparative methods, and many have important physical or chemical properties which have led to their industrial production. Again, the solubility relations of metal sulfides in aqueous solution form the basis of the most widely used scheme of elementary qualitative analysis. These various more general considerations will be briefly discussed before the systematic structural chemistry of metal sulfides is summarized. 

Metal sulfides vary enormously in their solubility in water. As expected, the (predominantly ionic) alkali metal sulfides and alkaline earth metal sulfides are quite soluble though there is appreciable hydrolysis which results in... 

By contrast with the water-soluble sulfides of Groups 1 and 2, the corresponding heavy metal sulfides of Groups 11 and 12 are amongst the least-soluble compounds known. Literature values are often wildly discordant, and care should be taken in interpreting the data. Thus, for black HgS the most acceptable value of the solubility product [Hg +][S ] is 10- mol2l-2, i.e. 

In type A reaction soluble sulfide ions combine with metal ions to form a precipitate of insoluble metal sulfide. Sulfur s oxidation state of minus two does not change in this reaction. The reaction is... 

The solubility of a solid can be increased by removing one of its ions from solution acid can be used to dissolve a hydroxide, sulfide, sulfite, or carbonate precipitate and nitric acid can be used to oxidize metal sulfides to sulfur and a soluble salt. 

Sulfides with widely different solubilities and solubility products can be selectively precipitated by adding S2 ions to the solution removed from the chlorides in the first step (see Fig. 11.20). Some metal sulfides (such as CuS, HgS, and Sb2S3) have extremely small solubility products and precipitate if there is the merest trace of S2" ions in the solution. Such a very low concentration of S2 ions is achieved by adding hydrogen sulfide, H2S, to an acidified solution. A higher hydronium ion concentration shifts the equilibrium... 

The solubility product (JCsp) for the precipitation of metal sulfide (MmSn) from a solution containing m Mz+ and n S2 ions is... 

This new method can eliminate the potential hazard of excess sulfide in the effluent and the formation of gaseous hydrogen sulfide. In operation, the FeS is added to wastewater to supply sufficient sulfide ions to precipitate metal sulfides that have lower solubilities than FeS. Typical reactions include the following51-62 ... 

Chemical precipitation/coagulation methods transfer the target substances (mainly metals) in solution into a solid phase. Many heavy metal hydroxides and sulfides have very low solubility (within a certain pH range) and are therefore insoluble. The metal sulfides have significantly lower solubility than their hydroxide counterparts over a broad range of pH.26 Precipitation/coagulation is also applicable for removing certain anionic species such as phosphate, sulfate, and fluoride. 

For the heavier alkaline earth elements the characterized compounds are octahedral monomers, dimers, or separated-ion species. The complexes M(SR)2 (M=Ca, Sr, Ba) have been obtained by aminolysis and proved to be soluble in solvents such as py these compounds decompose rather cleanly to their metal sulfides.86 The monomeric species include [M(EMes )2(thf)4] (M=Ca, Sr, Ba E=S, Se),87 89 [Ca(SC6F5)2(py)4],90 [Ca(SMes )2(18-crown-6)] thf,90 or [Ba(Se-Trip)2(18-crown-6)].87 Only a dimeric barium derivative, [Ba(SeTrip) (py)3(thf)]2, has been described.87 For these elements separated ion-triple species, such as [M(18-crown-6)(hmpa)2][EMes ]2 (M=Ca, Sr, Ba E=S, Se)87,88,91 or contact/separated ion-triple species as [Ba(SMes )(18-crown-6)(hmpa)][SMes ] have been reported.91... 

Hydrogen sulfide is a weak acid that reacts with most heavy metals and produces a corresponding metal sulfide with a low solubility in water. With a divalent metal, the total reaction is as follows ... 

The metals sulfides are the most dangerous since after aerobic weathering they are transformed into water-soluble sulfates of different metals. Accordingly, in the areas of non-ferrous and rare metal ore exploration and treatments, the acid sulfate landscapes are formed with high content of toxic metals. The biogeochemical technogenic provinces are known, for instance, copper-nickel provinces in the Kola Peninsula, Fennoscandia molybdenum provinces in the Caucasian region, copper and chromium-nickel ones in the South Ural, poly-metal ones, in the Pacific coast of eastern Eurasia (Russia, China, and Korea), etc. 

A catalytic example of C-S bond breakage in benzothiophene has been reported by Bianchini [47], A catalytic desulfurisation was not yet achieved at the time as this is thermodynamically not feasible at such mild temperatures because of the relative stability of metal sulfides formed. Bianchini used a water-soluble catalyst in a two-phase system of heptane-methanol/water mixtures in which the product 2-ethylthiophenol is extracted into the basic aqueous layer containing NaOH. Figure 2.43 gives the reaction scheme and the catalyst. The 16-electron species Na(sulfos)RhH is suggested to be the catalyst. Note that a hydrodesulfurisation has not yet been achieved in this reaction because a thiol is the product. Under more forcing conditions the formation of H2S has been observed for various systems. 

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