The solubility products of metal sulfides are several orders of magnitude lower than those of the corresponding hydroxides. This should make it feasible to precipitate the heavy metals from solutions containing complexing agents that prevent metal removal by hydroxide precipitation. Also, the pH range for the precipitation of metals as sulfides is much broader.
Table 3 lists the solubility products for metal sulfides of the type MeS, Me2S, and Me2S3 . Metal sulfide precipitation occurs if the solubility product is smaller than the product of metal concentration times sulfide ion concentration:
Table 3 Solubility Products (KSJ>) for Metal Sulfides of Type MeS, Me2S, and Me2S3 MeS K Me2S K,p Me2S3 Ksp
,0-26.7 ,0-20.5 ,0-26.0 ,0-27.O ,0-27.9 ,0-28.6 ,0-40.2
T12S Hg2S Cu2S Ag2S
As2S3 Sb2S3 Bi2S3 Fe2S3 Co2S3
Source: Data from Ref. 15.
Figure 2 Metal removal efficiency by hydroxide precipitation and Fem coprecipitation. (---) Solubilities of metals precipitated with NaOH from single-metal solutions as a function of pH. (o) Solubilities of metals precipitated with ferrous sulfate-NaOH from a solution containing 20 ppm each of CrVI, Pb, Cu, Cd, Zn, and Ni as a function of pH.
Sulfide (S2_) is the dissociation product of H2S. The solubility of H2S in water is 0.1 mol/L. H2S dissociates in two stages:
HS~ S2~ + H+ K2 = 10"12 9 molII (26) We find for the overall reaction:
From Equation (27), we calculate that at pH 7 a hydrogen sulfide solution of 0.1 mol/L contains only 10-7 mol/L of free S2_ ions. The most soluble sulfide (MnS) listed in Table 3 would shown an equilibrium concentration of [Mn] = Kspl 10-7 = 10~7 mol/L = 5.5 mg/L under these precipitation conditions.
In practice, processes are designed to operate with lower sulfide concentrations in order to minimize the H2S odor in the working environment. The odor detection level of hydrogen sulfide is 0.1-1 ppm. Because H2S is toxic, the workplace H2S limit is set by the Occupational Safety and Health Administration (OSHA) at 10 ppm.
Sulfide can be added in soluble form as Na2S or NaHS. The problem with this approach is the correct sulfide dosage. Encouraging results using a sulfide-specific electrode were reported . The Sulfex process  uses freshly precipitated FeS as the source for sulfide and operates in the neutral pH range. FeS dissolves to its equilibrium concentration. The sulfide ions liberated in this fashion precipitate all sulfides with a solubility product smaller than that of FeS. From Table 3 it is seen that only manganese would not be precipitated as MnS. The dissolved iron precipitates as Fe(OH)2.
The FeS dosage necessary in this approach is several times the theoretical amount. The amount of sludge produced is therefore large in comparison to the amount of heavy metals removed. Chromate, if present, is reduced and precipitates as Cr(OH)3.
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