Reversible precipitation

Treatment of dye wastewater involves physical, physico-chemical, chemical, and biological methods. Physical processes are dilution, filtration, and gamma radiation. Physico-chemical includes adsorption, coagulation, flocculation, precipitation, reverse osmosis, ion exchange, etc. 

A substance which passes into the colloidal state, simply by bringing it in contact with water is known as hydrophilic colloid (hydro = water, philic = loving). But if any solvent like organic liquid is used instead of water, then the more general term lyophilic colloid (lyo = solvent philic = loving) is used. Gum, starch, soap are lyophilic colloids. These colloids when once precipitated can again be brought back directly into the colloidal state. Hence, they are also known as reversible colloids. (Colloidal state precipitate). Reversible colloids are also termed as resoluble or non-electrocratic colloids. 

Consider the mineral AB (Reaction 7.1), where A denotes any cation (A+) and B denotes any anion (B ). Upon introducing H20, the mineral undergoes solubilization (forward reaction) until precipitation (reverse reaction) becomes significant enough so that the two rates (forward and reverse) are equal ... 

The figure below (from Rimstidt and Barnes, 1980) shows the energy profiles in the dissolution (forward direction) and precipitation (reverse direction) reactions of amorphous Si02 and quartz, Q -Si02. Use the concept of detailed balancing (microscopic reversibility principle) to explain why the dissolution rate constant is directly proportional to the solubility of the Si02 solid phase. 

The overall trend calculated in carbon isotopes is toward heavier isotope ratios for carbonate in solution after sulfide oxidation and neutralization. Dolomite dissolution introduces heavier carbon ((5 C = -17oo i )- Calcite precipitation reverses this trend by preferentially removing = 11.8 to 27oo Ht urc, where e, is the isotopic fractionation between calcite precipitated (p) and (s) the solution), but the mass transfer for this step tended to be small. The effect of CO outgassing above pH = 5.5 is to make the in solution heavier by preferentially removing light carbon = 0.5 to -9.07oo where is the isotopic... 

The need for high purity in a separations process is common in many industries semiconductor manufacture, pharmaceuticals processing, and the foods industry, as weil as in many cases of more-conventional chemical processing. It is also very important in separation processes that are oriented to cleaning gas, liquid, and solid streams for environmental purposes. The low concentrations required of many environmentally significant compounds prior to discharge from a chemical plant have created a need for a new class of separation methods and have focused attention on many techniques that often have been ignored. Adsorption, ultraflitration, electrostatic precipitation, reverse osmosis, and electrodialysis are just a few examples of separation processes in which there has been an increased level of interest partly because of their potential in environmental applications. 

When evaluating the commercial potential and feasibility of a biosorption application the following aspects should be considered assessment of the competing technologies (precipitation, reverse osmosis, ion exchange, and bioreduction), assessment of the market size and the cost of new biosorbents (including purchase/cultivation, pre-treatment and immobilisation). 

A white gelatinous precipitate of aluminium hydroxide is obtained when an alkali is added to an aqueous solution of an aluminium salt. Addition of an excess of caustic alkali causes the precipitate to redissolve, the whole process being reversed by the addition of a strong acid the actual substance present at any time depending on... 

Here again the simple formulation [Sb ] is used to represent all the cationic species present.) The hydrolysis is reversible and the precipitate dissolves in hydrochloric acid and the trichloride is reformed. This reaction is in sharp contrast to the reactions of phosphorus(III) chloride. 

Solutions of many antimony and bismuth salts hydrolyse when diluted the cationic species then present will usually form a precipitate with any anion present. Addition of the appropriate acid suppresses the hydrolysis, reverses the reaction and the precipitate dissolves. This reaction indicates the presence of a bismuth or an antimony salt. 

Alkali hydroxide gives a white precipitate solubie in excess. The white precipitate, Zn(OH)2, gives the oxide when dehydrated the white yellow reversible colour change observed on heating the oxide is a useful confirmatory test. 

The resulting oligonucleotide is often of surprising purity as judged by analytic HPLC or electrophoresis, and up to 30 mg of a deoxyeicosanucleotide (20-base DNA) can be routinely obtained. Nevertheless small amounts of short sequences, resulting from capping and from base-catalysed hydrolysis, must always be removed by quick gel filtration, repeated ethanol precipitation from water (desalting), reverse-phase HPLC, gel electrophoresis, and other standard methods. 

In the equilibrium treatment of precipitation, however, the reverse reaction describing the dissolution of the precipitate is more frequently encountered. 

The reverse of coagulation in which a coagulated precipitate reverts to smaller particles. 

Inorganic Analysis The most important precipitants for inorganic cations are chromate, the halides, hydroxide, oxalate, sulfate, sulfide, and phosphate. A summary of selected methods, grouped by precipitant, is shown in Table 8.1. Many inorganic anions can be determined using the same reactions by reversing the analyte... 

Cuprous iodomercurate [13876-85-2] Cu2Hgl4, is a bright red water-insoluble compound prepared by precipitation from a solution of K Hgl with cuprous chloride. It is used in temperature-indicating paints because it reversibly changes color to brown at 70°C (see Chromogenic materials). 

Silver iodomercurate [36011-71-9] Ag2HgI, is a bright yellow compound and is prepared similarly to the cuprous salt where silver nitrate is the precipitant. The silver salt, which darkens reversibly at 50°C, is used for the same appHcation as cuprous iodomercurate. 

Precipitation. The precipitation of aluminum tribydroxide ia the recovery step of the Bayer process is achieved either by loweting the temperature or by diluting the pregnant Hquor and reduciag its pH. Both methods reverse the direction of equation 35, but seeding with previously precipitated crystals is required ia order to initiate nucleation. 

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

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