Removal chromate

X-ray diffraction measurements indicated that the zeolite rock consisted primarily of clinoptilolite (60-70%), volcanic glass (10%), feldspar (10%) and minor quantities of cristobalite, quartz and plagioclase (20%). Fig. 1 represents the XRD pattern of ODA-Clinoptilolite-rich tuff used for arsenate or chromate removal from aqueous solutions. 

Some differences in arsenate and chromate adsorption on ODA-clinoptilolite and Pb-(Ag-linoptilolites) as well were recorded (Figs. 5 and 6). ODA-clinoptilolite exhibited more efficient arsenate and chromate removal from aqueous solutions than the inorganically exchanged modifications. However, silver exchanged clinoptilolite revealed higher capacity values for both oxyanions uptake than lead exchanged clinoptilolite did. This phenomenon supports preferred silver treated clinoptilolite utilization for specific water purification process even on the base of environmental acceptability. 

Arsenate removal by ODA-clinoptilolite proceeded almost analogous as chromate removal did, however the front part of the breakthrough curve is fairly shallow and indicates earlier leakage of pollutant into adsorbate (Fig. 

Continuous Belt Ion Exchanger Chromate Removal from Cooling Tower Blowdown Waters... 

Zero-valent iron can also be used for heavy-metal removal. Applications for cadmium and chromate removal have already been shown to be successful. When combined with chloride ions, iron has been shown to be a simple and inexpensive method to remove mercury from wastewater (Grau and Bisang, 1995). Nitrates also degrade in the presence of zero-valent iron, but the application of treating nitrate-contaminated water has not been extensively studied (Siantar et al., 1996). 

In the intensification of chromate removal from water, a double-chain cationic surfactant, dioctadecyl-dimethylammonium chloride (DODDMAC), was used as a carrier and a cross-flow electrofiltration was used, in which both the transient and the steady-state fluxes and the rejection of metal ions and surfactant were measured.Dioctadecyldimethy-lammonium chloride in water forms multilamellar droplets, even at very low concentrations. This structure is shown in Fig. 10. Metal ions are entrapped within the water layers and organic toxins can be immobilized within the surfactant bilayers. Under an electric field. 

Eary, L. E., and D. Rai. 1988. Chromate removal from aqueous wastes by reduction with ferrous ion. Envir. Sci Technol 22(8) 972-77. 

In the range of environmental applications of natural zeolites based on ion exchange, also of interest is the possible use of surfactant-modified zeolites (see Sub-sec. 5.2.3.) for the removal of a series of water contaminants. A recent multipurpose investigation demonstrated good performances of these materials for chromate removal from water in a pilot-plant. In addition, laboratory tests demonstrated excellent selectivitics for organics present in oilfield wastewaters and even for bacteria and viruses present in sewage effluents [ 101J. 

Eary LE, Rai D. (1988). Chromate removal from aqueous waste by reduction with ferrous iron. Environmental Science and Technology 22 972-977. 

The dichromate(VI) salts may be obtained by the addition of acid to the chromate(VI) salts. However, they are better prepared by adding one-half the acid equivalent of a metal hydrate, oxide, or carbonate to an aqueous solution of CrO, then removing the water and/or CO2. Most dichromates(VI) are water-soluble, and the salts contain water(s) of hydration. However, the normal salts of K, Cs, and Rb are anhydrous. Dichromate(VI) compounds of the colorless cations are generally orange-red. The geometry of Ci2 is described as two tetrahedral CrO linked by the shared odd oxygen (72). 

Modem manufacturing processes quench the roast by continuous discharge into the leach water held in tanks equipped with agitators. At this point the pH of the leach solution is adjusted to between 8 and 9 to precipitate aluminum and siHcon. The modem leaching operations are very rapid because no or htde lime is used. After separation of the ore residue and precipitated impurities using rotary vacuum filters, the cmde Hquid sodium chromate may need to be treated to remove vanadium, if present, in a separate operation. The ore residue and precipitants are either recycled or treated to reduce hexavalent chromium to Cr(III) before disposal. 

Sodium chromate can be converted to the dichromate by a continuous process treating with sulfuric acid, carbon dioxide, or a combination of these two (Fig. 2). Evaporation of the sodium dichromate Hquor causes the precipitation of sodium sulfate and/or sodium bicarbonate, and these compounds are removed before the final sodium dichromate crystallization. The recovered sodium sulfate may be used for other purposes, and the sodium bicarbonate can replace some of the soda ash used for the roasting operation (76). The dichromate mother Hquor may be returned to the evaporators, used to adjust the pH of the leach, or marketed, usually as 69% sodium dichromate solution. 

Potassium chromate is prepared by the reaction of potassium dichromate and potassium hydroxide. Sulfates are the most difficult impurity to remove, because potassium sulfate and potassium chromate are isomorphic. 

Excess NaOH is used to start the reaction and not over 35% of the chromium is added as dichromate. At the end of the reaction, the thiosulfate is removed by filtration and recovered. The hydrous oxide slurry is then acidified to pH 3—4 and washed free of sodium salts. On calcination at 1200—1300°C, a fluffy pigment oxide is obtained, which may be densifted and strengthened by grinding. The shade can be varied by changes in the chromate dichromate ratio, and by additives. 

A dichromate or chromate solution is reduced under pressure to produce a hydrous oxide, which is filtered, washed, and calcined at 1000°C. The calcined oxide is washed to remove sodium chromate, dried, and ground. Sulfur, glucose, sulfite, and reducing gases may be used as reducing agent, and temperatures may reach 210°C and pressures 4—5 MPa (600—700 psi). 

Preparation. The mother liquors from strychnine manufacture are concentrated and the alkaloids precipitated as neutral oxalates. The precipitate is dried and extracted with dry alcohol in which the strychnine salt is the more soluble. The less soluble salt dissolved in water is decolorised with charcoal, the alkaloid regenerated with ammonia and purified by crystallisation as the sulphate. According to Saunders, pure brucine may be obtained by slow crystallisation from a solution of the pure hydrochloride in alcoholic ammonia. A method of separation depending on the greater solubility in water of strychnine hydriodide was employed by Shenstone, whilst others have made use of the sparing solubility of strychnine chromate for the removal of small quantities of this alkaloid from brucine. For a large scale process see Schwyzer. ... 

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