Sulfides, precipitation of heavy

Bhattacharyya D., and Ku Y., Sulfide Precipitation of Heavy Metals Effect of Complexing Agents. EPA-600/S2-84-023. Cincinnati, Ohio, 1984. 

Sulfide Precipitants. Sulfide precipitation of heavy metals from 10-34-0 prepared from Western U.S. superphosphoric acid was also investigated. Portions of the 10-34-0 were then treated with varying amounts of Na2S and Na2S2.32 solutions. The results of these tests are shown in Table VI. 

Table VI. Sulfide Precipitation of Heavy Metals From 10-34-0...

Peters. R.W., Young, K., Dlbakar, B., Llh-Fen Chen Crystal size distribution of sulfide precipitation of heavy metals. In Industrial Crystallization 84 (eds. JanCld, S.J., de Jong, E.J.), p. Ill, Elsevier, Amsterdam 1984... 

The most important complex-type reaction for controlling toxicity in anaerobic waste treatment is the precipitation of heavy metals by sulfides. This was noted by Barth et al. (7) and Masselh and Masselli (8). The reason for the preeminence of sulfides is the extreme insolubility of heavy metal sulfides. The solubility product of heavy metal sulfides ranges from 3.7 X 10" for FeS to 8.5 X 10" for CuS (9). Thus, in the presence of sulfides, the heavy metal concentration is expected to be virtually zero in an anaerobic waste treatment unit. 

Applications Sodium sulfide is mainly used in tanning as a hair-removing agent (so-called asher). It is also utilized in ore flotation, dyeing with sulfur dyes in the textile industry and in the chemical industry e.g. for the precipitation of heavy metal ions. Anhydrous sodium sulfide is also utilized in the manufacture of polyphenylenesulfide (PPS), a high temperature-resistant plastic. The current production of sodium sulfide in Western Europe is estimated to be ca. 50 10- t/a. 

Table IV. Sulfide Precipitation of Selected Heavy Metals From WPA ...

If the solubility of the metal is reduced, the leaching potential is then also reduced. Robinson [19] has studied sulfide precipitation and hydroxide precipitation of heavy metals, including lead, chromium, and cadmium he saw less leaching among the sulfides, which also had lower solubility. Robinson also reported that certain sulfide processes could stabilize hexavalent chromium without reducing it to trivalent chromium (but does not call it sulfide precipitation and does not describe the mechanism). Others in the field have not reported this. 

Robinson, A.K., Sulfide vs. hydroxide precipitation of heavy metals from industrial wastewater, in Proc. First Annual Conference on Advanced Pollution Control for the Metal Finishing Industry, Report EPA-600/8-78-010, Environmental Protection Agency, Washington DC, 1978, 59. 

The iodate is a poison potassium iodide, however, is used in foodstuffs. Thus the iodate must be completely removed frequently by a final reduction with carbon. After re-solution in water, further purification is carried out before recrystallization. Iron, barium, carbonate, and hydrogen sulfide are used to effect precipitation of sulfates and heavy metals. 

Hydrogen sulfide causes the precipitation of sulfides from many heavy-metal salts. The classical quaUtative analysis scheme depends on precipitation of the sulfides of Hg, Pb, Bi, Cu, Cd, As, Sb, and Sn under acid conditions and the sulfides of Co, Ni, Mn, Zn, and Fe under ammoniacal conditions. 

Plating Bath Purification. Purification, often needed once a plating bath is made, is used periodically to maintain the plating solutions. Alkaline ziac plating solutions are sensitive to a few mg/L of heavy-metal contamination, which can be precipitated usiag sodium sulfide and filtered out. 

Heavy Rare Earth Element). Therefore, it is considered that negative Ce and positive Eu anomalies in hydrothermally altered volcanic rocks, Kuroko ores, and ferruginous chert and LREE enrichment in the Kuroko ores have been caused by hydrothermal alteration and precipitations of minerals from hydrothermal solution responsible for sulfides-sulfate (barite) mineralization. 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

Precipitation of nickel and other heavy metals, besides chromium, as metallic sulfide, followed by separation by settling and filtration of the wastewater containing the metals, is an improved treatment process of wastewater in the nickel sulfide industry. 

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. 

The most important applications of hydrogen sulfide involve the production of sodium sulfide and other inorganic sulfides. Hydrogen sulfide obtained as a by-product often is converted into sulfuric acid. It also is used in organic synthesis to make thiols or mercaptans. Other applications are in metallurgy for extracting nickel, copper, and cobalt as sulfides from their minerals and in classical qualitative analytical methods for precipitation of many metals (see Reactions). It also is used in producing heavy water for nuclear reactors. 

Baryta was first distinguished from lime in 1779 by Scheele, who prepared it from heavy spar, a naturally occurring barium sulfate. He reduced the sulfate to the sulfide by heating a sticky, pasty mixture of heavy spar, powdered charcoal, and honey. After decomposing the barium sulfide with hydrochloric acid, he added excess potassium carbonate to precipitate the barium as the carbonate (15). 

There are two treatability groups of dissolved metals for chemical precipitation, complexed and non-complexed metals. Non-complexed metals can be removed by a direct precipitation with such a chemical as lime (Ca(OH) ), caustic (NaOH), sodium sulfide (Na2S), ferrous sulfide (FeS), or sodium carbonate (NajCOj). Complexed metals require coprecipitation with ferrous sulfate (FeS04), ferrous chloride (FeClj), or sodium dimethyl dithiocarbamate (DTC) in addition to a regular precipitant such as caustic or lime. Electrochemically generated ferrous ion is also effective in removing a wide variety of heavy metals, including hexavalent chromium. 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

If undistilled sample is used, the addition of zinc sulfate in presence of NaOH would precipitate out iron, magnesium, calcium, and sulfide as a heavy flocculent,... 

Sodium dithionite has been used in combination with sodium citrate and sodium bicarbonate in a range of concentrations (Beckett, 1989, pp. 163-164), and usually at pHs between 5.8 and 7.3, for the reduction of both crystalline and amorphous (McKeague and Day, 1966 Gupta and Chen, 1975) iron oxide phases and release of sorbed trace metals. It is little used for heavy metal studies because of contamination of the dithionite with zinc and the possibility of precipitation of metal sulfides. 

Metal-Sulfides. Sulfide precipitation has been one of the most widely used methods to precipitate many of the heavy metals. The low MCLs required for some of the highly toxic metals (e.g., mercury), are often achievable only by precipitation as sulfides, since they generally have solubilities several orders of magnitude lower than the hydroxides, carbonates, or phosphates throughout the pH range of 6-9. However, metal sulfides can resolubilize in an oxidizing environment. 

Land disturbance and exposure of buried geologic strata to the open environment leads to sulfide oxidation (if present) and, as a consequence, water-quality degradation of runoff. For water-quality-control purposes, sedimentation ponds required by law are used as water treatment basins. Often, the pH of such basin waters is below 6, and the concentration of heavy metals is above acceptable levels. Water treatments include neutralization and removal of heavy metals as precipitates. Similar water-quality problems arise from other industrial sources, including heavy steel industries, electronics, food processing, mineral processing, and waste-disposal leachates. This portion of the chapter deals with some of the chemical agents used for neutralization purposes and some of their limitations. 

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