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:
pH |
7.0 |
0.1 |
mg/l |
|
7.5 |
0.08 |
mg/l |
8.0 |
0.12 |
mg/l |
8.5 |
0.50 |
mg/l |
|
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
