Metal sulfides, precipitation

An alternative procedure for removing an ion from solution is to change its identity by changing its oxidation state. The metal ions in very insoluble heavy metal sulfide precipitates can be dissolved by oxidizing the sulfide ion to elemental sulfur. For example, copper(II) sulfide, CuS, takes part in the equilibrium... 

It is reasonable to assume that the surface of metal sulfides show amphoteric behavior and it has been shown that uptake of bivalent cations is pH-dependent. Metal sulfide precipitates are efficient scavengers for heavy metal ions. 

Reductive Precipitation. Reductive precipitation involves the production of reduced species with limited solubility. An example of reductive precipitation in the environment involves the reduction of S04 to H2S and the precipitation of metals as metal-sulfides. In nature, the process of reductive precipitation is mostly microbiologically controlled. Production of H2S is the rate-controlling reaction of metal-sulfide precipitate formation. 

Benner S. G., Blowes D. W., Ptacek C. J., and Mayer K. U. (2002) Rates of sulfate removal and metal sulfide precipitation in a permeable reactive barrier. Appl. Geochem. 17, 301-320. 

Tip The hydrogen sulfide solution can be used to precipitate certain metal solutions and to show that insoluble deposits are produced metal sulfide precipitation for cation analysis ... 

Mining operations can generate acidic wastewater that contain harmful levels of dissolved transition metals, including manganese, iron, copper, zinc, silver, and cadmium. One treatment method uses naturally occurring anaerobic bacteria to remove all of the oxygen. Then sulfate-reducing bacteria convert sulfuric acid in the mine waste to sulfide. Sulfide reacts with metals in the wastewater to form metal sulfide precipitates, which can be recovered and processed for commercial use. 

The limitations in removing selenium are as follows. As opposed to many metals and metalloids in acidic wastewaters, selenium exists as an anion, that is SeOs (selenite) or SeOa (selenate). As such, selenium cannot be precipitated by hydroxide (OH ) or sulfide (S ) to form metal hydroxide or metal sulfide precipitates. At the same time, neither selenite nor selenate form stables precipitates with the common cations (Ca, Mg, Fe, Na, etc.) which are used in wastewater treatment. 

Besides, since H2S reacts rapidly with many metal salts (e.g., CUCI2) to form a metal sulfide precipitate and generate a strong acid (i.e., HCl), based on these experimental facts, Virji et al. [73] reported that when the PANI composite film mixed with CUCI2 is exposed to H2S vapor, it becomes doped by the HCl generated as a byproduct of CuS formation within a few seconds that leads to the sensitivity of PANI nanofibers to H2S vapor. 

A solution contains the metal ions listed below, all at the same concentration. The addition of thioacetamide, CH3CSNH2, produces sulfide ion, S , causing the ions to precipitate as the sulfides. Each sulfide begins to precipitate at a different concentration of sulfide ion. As the concentration of sulfide ion increases (starting from 0), in what order will the metal sulfides precipitate (Which one will start to precipitate first, second, third, and fourth ) iron(II) ion, Fe lead(II) ion, Pb ... 

The other metal ions remain in the filtrate, the solution that passes through the filter. Many of these ions can be separated by precipitating them as metal sulfides with H2S. This separation takes advantage of the fact that only the least soluble metal sulfides precipitate in acidic solution. After these are removed and the solution is made basic, other metal sulfides precipitate. ... 

Metal sulfides precipitated from dilute HCl may be dissolved in aqua regia, evaporated just to diyness, treated with N02 and CH3CO2H (aiding to Ii ), heated 5 min at 60-70 °C to form [(Rh,Ir)(N02)6] , yellow, and then treated with OH , no excess, to separate all those other metals as solid hydroxides fairly completely from the Rh and Ir ions ... 

We must notice that the metallic sulfide precipitation is often accompanied by the formation of various complexes with some anions present in the medium, for example, with sulfide anion itself or with the anion of the added acid or with some species resulting from the hydrolysis of the metallic ions. The preceding calculations do not take into account these complications and, for this reason, may be considered as being weakly realistic (see Chap. 35). 

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