The metal chromium, exists ionically in two forms, trivalent (+3) and hexavalent (+6). The hexavalent form is very soluble at all pH values and is quite toxic to microbial growth. The hexavalent is an anion and commonly exits as chromate (CrO4)–. The trivalent (+3) is not nearly as toxic and as a hydroxide reaches its least solubility at a pH of 8.5. The approximate solubility of trivalent chromium is the following:
To chemically precipitate chromium from solution first the hexavalent chromium (+6) must be reduced to trivalent (+3) then the trivalent will precipitate as a hydroxide compound.
Reduction of hexavalent chromium to trivalent is conducted in a baffled reaction chamber utilizing quadravalent sulfur as the reducing agent. The reaction zone maintains a pH of less than 3 with at least 5 minutes reaction time and a mixing rate of 2.5-3.0 tank turnovers per minute. The quadravalent sulfur is supplied by a solution of sodium metabisulfite or sulfur dioxide gas. A platinum element senses the oxidation and reduction potential.
Cr+6 + S+4r Cr+3 + S+6
(CrO4)– + (SO3)– + H+r Cr+++ + (SO4)– + H2O
When the pH is increased to 8.0-8.5, with either sodium hydroxide or lime (calcium hydroxide), chromium hydroxide precipitates from solution.
Cr+++ + 3(OH)-r Cr(OH)3_
Raising the pH to 7.5-8.0 reduces the theoretical solubility of chromium to less than 0.1 mg/l.
The addition of certain inorganic coagulants will not only assist in the flocculation and precipitation of trivalent chromium, but also tends to suppress the ions solubility. Data is available demonstrating that the solubility of chromium may be reduced to less than 0.010 mg/l.
Hoffland Environmental has manufactured over 200 chromate reduction systems, both continuous flow and batch treatment units. The continuous flow systems have processed from 5 to over 2,000 gpm. HEI manufactures and installs all of the equipment required to reduce hexavalent chromium and precipitate chromium hydroxide. Kindly contact HEI for any of the following equipment.
Chemical treatment modules:
- High rate inclined plate clarifiers
- Gravity flow sand filters with automatic back flush
- High pressure filter press
- Turn-key systems