Sulfate coprecipitation with carbonate

Cadmium is found naturally deep in the subsurface in zinc, lead, and copper ores, in coal, shales, and other fossil fuels it also is released during volcanic activity. These deposits can serve as sources to ground and surface waters, especially when in contact with soft, acidic waters. Chloride, nitrate, and sulfate salts of cadmium are soluble, and sorption to soils is pH-dependent (increasing with alkalinity). Cadmium found in association with carbonate minerals, precipitated as stable solid compounds, or coprecipitated with hydrous iron oxides is less likely to be mobilized by resuspension of sediments or biological activity. Cadmium absorbed to mineral surfaces (e.g., clay) or organic materials is more easily bioaccumulated or released in a dissolved state when sediments are disturbed, such as during flooding. 

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. 

Sulfate The coprecipitation of relatively few anions with carbonate minerals has been studied and, with the exception of sulfate, these studies have generally not been as detailed as many of those with cations. However, coprecipitation reactions can be important for the removal of ions such as fluoride, borate, and phosphate from seawater (e.g., Morse and Cook, 1978 Okumura et al., 1983). It is also probable that anions will eventually gain a greater stature in the study of diagenesis... 

Lithopone is a mixed zinc sulfide-barium sulfate pigment available in two types one containing 30 percent zinc sulfide and one containing 60 percent zinc sulfide. Coprecipitation is achieved by reacting an aqueous solution of zinc sulfate with barium sulfide. The barium sulfide solution is prepared by reducing barite ore (BaS04) with carbon. The equations are as follows ... 

Additionally, two cadmium chromates are known (classed with chromate pigments) along with a cadmium tungstate (classed with tungsten pigments). Lithopone forms - that is, pigments coprecipitated with barium sulfate (q.v.) - of the cadmium sulfides are also extant. The composition of cadmium white mentioned in later nineteenth cenmry literature is not well known Salter (1869) describes it as a hydrated oxide or carbonate of cadmium. 

By far the most common lead salt used for PVC stabilization is tribasic lead sulfate. It can be found either alone or combined with another lead salt in almost every lead-stabilized PVC formulation. Many of the combinations are actually coprecipitated hybrid products, ie, basic lead sulfophthalates. Dibasic lead stearate and lead stearate are generally used as costabilizers combined with other primary lead salts, particularly in rigid PVC formulations where they contribute lubrication properties dibasic lead stearate provides internal lubrication and lead stearate is a good external lubricant. Basic lead carbonate is slowly being replaced by tribasic lead sulfate in most appHcations due the relatively low heat stabiHty of the carbonate salt which releases CO2 at about 180°C during PVC processing. 

In many of its chemical properties, radium is like the elements magnesium, caldum, strontium and barium, and it is placed in group 2, as is consistent with its 6s26pcls2 electron configuraUon. Its sulfate (Ksp — 4.2 a 10-1 ) is even more insoluble in water than barium sulfate, with which it is conveniently coprecipitated, Like barium and other alkaline earth metals, it forms a soluble chloride (X p = 0,4) and bromide, which can also be obtained as dihydrates, Radium also resembles the other group 2 elements in forming an insoluble carbonate and a very slightly soluble lodate (Xsp = 8.8 x 1(T10). 

Coprecipitation of actinides with divalent-cation carbonates and sulfates. 

The pH of the solution is raised by adding, for example, nickel carbonate. Iron hydroxide coprecipitates other impurities from the anolyte, particularly lead and arsenic [43], Oxidation of iron is accomplished by using air in pure sulfate electrolyte systems and by using chlorine in chloride systems. Iron can be removed from pure chloride electrolytes by solvent extraction with tributylphosphate. 

An important part of the precipitation process is normally aggregation of the colloidal particles first formed to produce a cohesive mass. Precipitates are often relatively complicated species such as the basic salt of lead carbonate, 2PbC03-Pb(0H)2. Heavy metals, a common ingredient of hazardous waste species precipitated in the hydrosphere, tend to form hydroxides, carbonates, and sulfates with the 0H , HCOJ, and S04 ions that commonly are present in water, and sulfides are likely to be formed in bottom regions of bodies of water where sulfide is generated by anoxic bacteria. Heavy metals are often coprecipitated as a minor constituent of some other compound or are sorbed by the surface of another solid. 

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