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The problems presented in wastewater when hexavalent chrome is introduced requires special attention and experience in reducing to less hazardous trivalent chrome. Here are 2 recommended ways to reduce chrome after precipitation.
1). While holding the pH at 2.0- 2.5 with sulfuric acid, sodium meta bisulfite needs to see the ORP value at positive 250 Mv. When the change in ORP value is noted, this indicates the reaction is complete. This process takes 15 minutes so it is crucial that the pH and ORP of the water be held tightly as not to mistake a rise in pH for the completion of the reduction reaction. Once reduction has occurred the color will also change from orange to blue and the pH can then be adjusted up using caustic soda to 9.0-9.5 for the best removal of the trivalent chrome.
2). If you are at a pH > than 2.5 and don’t want to add acid, reduction of hexavalent chrome can be done by the addition of wastewater treatment chemicals. Typical additions are 1-3 mls/gallon of rinses contaminated with hexavalent chrome but may require more. Jar testing should be done to determine the best dosing.
In all cases, water needs to meet local, state and federal guidelines for chrome removal.