Precipitation of heavy metal

Phosphonic acid and hydrogen phosphonates are used as strong but slow-acting reducing agents. They cause precipitation of heavy metals from solutions of their salts and reduce sulfur dioxide to sulfur, and iodine to iodide in neutral or alkaline solution. 

Precipitation of heavy metals with hydroxides, carbonates or sulphides. Selective precipitation of the different types of heavy metals is also possible... 

Many industrial waste acid generating processes have conventionally employed lime for the precipitation of heavy metals and the concurrent neutralization of the acid(s). This procedure is not environmentally acceptable in the long run because even the low level of contamination of heavy metals may cause the solid waste to be classified as a toxic waste, requiring transportation to special dumps at considerable expense. Additionally, the discharge of the filtrate to the water ways will undoubtedly be controlled by stricter regulations. 

The data above were presented to demonstrate that in treating acid heavy-metal-rich colloidal suspensions with NH3, the latter introduces some undesirable complexities owing to the potential of the heavy metals to form metal-ammine complexes in solution and/or the exchange complex. Disposal of such waters and/or such sludges requires prior knowledge. For example, the formation of metal-ammine complexes in solution would not permit precipitation of heavy metals in the treated water. On the other hand, formation of precipitate-NH4 complexes and/or metal-ammine complexes on the surface of colloidal particles causes disposal problems for such sludges. Nitrification of this NH4 would release NOa and heavy metals owing to acidification. 

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. 

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

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. 

Oxidation of Cyanide Effluent and Precipitation of Heavy Metals... 

C. C. Nesbitt, J. L. Hendrix, and J. H. Nelson, Use of Thiourea for Precipitation of Heavy Metals in Metallurgical Operation Effluents, in Extraction Metallurgy 85, The Institute of Mining and Metallurgy, London England, pp. 355-375, 1985. 

A. Mdller, A. Grahn, and U. Welander, Precipitation of Heavy Metals from Landfill Leachates by Microbially Produced Sulphide, Environmental Technology, 25(1), 69-77 (2004). 

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...

When a sample is dissolved, the phosphorus usually passes into solution as P(V). Rather than isolate the phosphate, it is often better to isolate the interfering elements, leaving the phosphate to be determined in the mother liquor. Examples of such separations include distillation of Si, As, and Ge as volatile halides [1] or of boron as trimethyl borate [2], precipitation of heavy metals as sulphides from an acid medium, retention of cations on a strongly acidic cation exchanger, and electrolytic separation of metals. 

Precipitation is an important factor in leachate attenuation processes within landfills and is especially important in attenuating heavy metals. Most heavy metals are characteristically more soluble at low pH values and therefore may be expected to be more mobile during the early stages of waste stabilisation. However, as the waste decomposition proceeds, the pH rises causing the solubility of most heavy metals to decrease. Also under conditions of lower redox potential, the precipitation of heavy metals as insoluble sulphides and carbonates may occur (Rees, 1982), thus reducing the concentration of heavy metals in the leachate. According to Pohland (1991) inorganic heavy metals codisposed with MSW are attenuated by the microbially-mediated processes of reduction, precipitation. 

Solvent extraction, chromatographic separation, distillation and precipitation of heavy metals as sulphides are among the techniques employed for this purpose. Isobutyl acetate, for example, extracts molybdophosphate but not molybdosilicate at pH = 0.3-1.0. Butanol and chloroform can be used to separate molybdophosphate from molybdoarsenate. Under the appropriate circumstances. Si, As and Ge can be distilled off from the sample as volatile halides, and B removed as McjB. 

Dual systems were also used for the flocculation of mineral suspensions [27,28] or for the precipitation of heavy metals such as Cu ", Co ", Zn, Ni ", and Pb " [29, 30]. Glover [27] investigated the effect of a dual system on the compressive yield stress and... 

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... 

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 primary raw materials used in the surface finishing process include surface cleaning agents, metals for hot-dip deposition, fluxes, chemicals for plating and anodizing baths, and treatment reagents for process effluents. The use of effluent chemicals is required to reduce hexavalent chromium to trivalent chromium to allow for the precipitation of heavy metal content and to ionize the cyanide content. 

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