Hexavalent chromium is first reduced to the trivalent state by adding a reducing agent, with proper adjustment for acidity. This is followed by precipitation of the reduced chromium as the hydroxide, which is then physically removed from the system by settling. The reactions are:
SO2 or Na2S2O5
cycled into the cooling tower water treatment system, and the resulting chromate-free water may be disposed of or further demineralized and reused.
A successful process contacts the chromate-laden wastewater after proper pH adjustment, with a weak-base anion exchange resin in the sulfate form. The chromate (CrOf") ion exchanges with the sulfate (SO|_) ion and is incorporated in the resin. The chromate is recovered as a mixture of sodium chromate (Na2CrO4) and sodium dichromate (Na2Cr2O7) upon regeneration of the resin. The regenerant is a 5% (by weight) solution of caustic soda (NaOH) added in an overall quantity equivalent to 10% in excess of the stoichiometric amount.
Sodium hydroxide restores the chromate as sodium salts and temporarily places the resin in the hydroxyl form. The sodium hydroxide on the resin is neutralized by adding the stoichiometric quantity of 0.1N sulfuric acid. This neutralization step also restores the resin to the sulfate form. The reactions are as follows:
S04 + Na2Cr2Q7
Cr207 + NaOH (Na2Cr204
Reaction 8.3(8) proceeds almost instantaneously at a pH of 2.0 or less. Each reducing agent shown in the reaction is effective; Fe2+, however, requires an excess of about 21/2 times the stoichiometric quantity, resulting in an excess of Fe(OH) 3 sludge from neutralization. In small treatment systems, sodium metabisulfite (Na2S2O5) is usually the preferred reagent. In water it hydrolyzes to sodium bisulfite, and sulfuric acid must be added to lower the pH for the reducing reaction. Excess reagent must be added if dissolved oxygen is present in the wastewater. Larger systems, on a batch or continuous basis, use sulfur dioxide, which hydrolyzes to sulfurous acid. Additional acid for pH adjustment is not always required.
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