CHROMIUM (VI) REMOVAL
1. OBJECTIVE AND IMPORTANCE OF EXPERIMENT
The properties of Cr are highly dependent on the molecular structure of the Cr compound,
particularly on the oxidation state (or oxidation number) of the Cr. Cr is an element that exists
primarily in two different oxidation states, hexavalent and trivalent. These oxidation states are
symbolized as Cr (VI) and Cr (III), respectively. Except for the rarely, naturally-found,
elemental Cr with an oxidation number of zero, Cr (0), other oxidation states of Cr are
unstable and therefore, are not found in the natural environment. The oxidation state of the Cr
has a significant effect on the transport and fate of Cr and on the type and cost of treatment
required to reduce Cr concentrations less than regulatory health-based standards [1].
Cr (VI) is far more mobile than Cr (III) and more difficult to remove from water. It is also the
toxic form of Cr, approximately 10 to 100 times more toxic than Cr (III) by the acute oral
route, presumably owing to the stronger oxidizing potential and membrane transport of Cr
(VI). The EPA classifies Cr (VI) as a known human carcinogen via inhalation, but classify Cr
(III) as not known to cause cancer. The most common Cr (VI) forms are chromate (CrO42–),
and hydrogen chromate (HCrO4–) also calledbichromate. The relative amount of these two
species depends on pH. Dichromate (Cr2O72–) can also occur. Cr (VI) compounds are anions
[1].
In the water environment, soluble chromium exists primarily in the form of chromate (Cr+6).
Trivalent forms (Cr+3) are hydrolyzed completely in natural waters, and chromium
precipitates as the hydroxide, leaving minor amounts in solution. Furthermore, there is no
evidence to indicate that the trivalent form is detrimental to human health. Chromium is used
extensively in industry to make alloys, refractories, catalysts, chromic oxide, and chromate
salts. Chromic oxide is used extensively to produce chromic acid in the plating industry.
Chromate salts are used in paints and to produce “cleaning solution” in laboratories. Most of
the latter eventually reaches the sewer system. Chromate poisoning causes skin disorders and
liver damage. There is some reason to believe that chromates are oncogenic (carcinogenic).
For this reason, the permissible level in drinking waters has been restricted to 0.1 mg/L [2].
In recent years, contamination of the environment by chromium, especially hexavalent
chromium (Cr+6), has become a major area of concern. The hexavalent form is 500 times
more toxic than the trivalent form (Cr+3) and is toxic to microorganisms, plants, animals, and
humans. Chromium is used on a large scale in many different industries, including
metallurgical, electroplating, production of paints and pigments, tanning, wood preservation,
chromium chemicals production, pulp and paper production. Tanning industry is an especially
large contributor of chromium pollution to water resources [3].
In view of the pollution hazard caused by hexavalent chromium, several methods of removal
have been reported, including filtration, chemical precipitation, adsorption,
electrodeposition, and membrane systems, or even ion exchange process [3].
Res. Assist. Elif Sekman
Yildiz Technical University, Environmental Engineering Deparment
The purpose of the experiment is to reduce Cr (VI) to Cr (III) in low acidic conditions and
then to remove Cr (III) by precipitation in high alkali conditions.
1.1 Reduction and Precipitation of Chromium:
Equilibration between solid and dissolved forms of Cr is a third physical–chemical interaction
that is used in treatment processes. Precipitation of Cr (III) occurs as Cr(OH)3(s), FeCr2O4(s),
or FexCry(OH)3(s). The solubility of Cr (III) governs its migration. Precipitation/dissolution is
a function of pH, complexation by organic matter, and the presence of other ions. As pH
increases, OH– concentration increases and more Cr precipitates. Organics can complex with
dissolved Cr, making removal by precipitation or adsorption difficult [1].
An example to chromium removal [4]:

Chromium reduction reaction:
4CrO3 + 3H2SO4 + 6NaHSO3  2Cr2(SO4)3 + 3Na2SO4 + 6H2O
2CrO3 + 6FeSO4 + 6H2SO4  3Fe2(SO4)3 + Cr2(SO4)3 + 6H2O

Chromium precipitation reaction:
Cr+3 + 3OH−  Cr(OH)3 ↓
2. EXPERIMENTAL PROCEDURE
2.1 Required Chemicals and Tools:
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K2Cr2O7 Stock Solution (Add 1 mL K2Cr2O7 solution to 1 liter distilled water)
FeSO4
H2SO4
NaOH
AAS (Atomic Absorption Spectrometry)
Beaker
Pipette
Flasks
Magnetic Stirrer (Jar Test)
pH meter
2.3 Steps of the Experiment:
1)
2)
3)
4)
5)
Measure the pH value of K2Cr2O7 solution.
Calculate the Cr+6 amounts in the solution.
Get 100 mL of K2Cr2O7 solution to a beaker.
Calculate the required reducing agent (FeSO4) amount.
Adjust the pH values:
Res. Assist. Elif Sekman
Yildiz Technical University, Environmental Engineering Deparment
Group No
1
2
3
4
5
pH values
10
8
6
4
2
6) Add a little bit more than what you calculate for FeSO4.
7) Place the beakers in Jar Test Apparatus and let them mixed for 5 min.
8) Adjust the pH values to 9 – 10 by adding NaOH.
9) Place the beakers again in Jar Test Apparatus and mixed them for 15 more min.
10) Leave the beakers stable for precipitation.
11) After 30 min, take samples from upper (clear) phase.
12) Measure final Cr+6 values at AAS and calculate the removal efficiencies.
2.4 Calculations of Removal Efficiency:
C0: Initial chromium concentration (mg/L)
C1: Final chromium concentration (mg/L)
2.5 Calculation of FeSO4:

10.216 gr K2Cr2O7 presence in 1 L K2Cr2O7 solution for COD experiment.

1000 mL K2Cr2O7 Solution
10.216 gr K2Cr2O7
1 mL K2Cr2O7 Solution
X
X = 0.010216 gr K2Cr2O7 = 10.216 mg K2Cr2O7

Mw (K2Cr2O7) = 294.185 gr
Mw (Cr) = 52 gr

294.185 gr K2Cr2O7
10.216 mg K2Cr2O7
104 gr Cr
X
X = 3.6116 mg Cr
Res. Assist. Elif Sekman
Yildiz Technical University, Environmental Engineering Deparment

Mw (CrO3) = 100 gr
Mw (FeSO4) = 152 gr

100 gr CrO3
X
52 gr Cr
3.6116 mg Cr
X = 6.9454 mg CrO3

2 x100 gr CrO3
6 x 152 gr FeSO4
6.9454 mg CrO3
X
X = 31.671 mg FeSO4
3. SOURCES
[1]
Hawley, E.L., Deeb, R.A., Kavanaugh, M.C. and Jacobs, J., 2004. “Chromium (VI)
Handbook, Chapter 8, Treatment Technologies for Cr (VI)”, pp. 273 – 308.
[2]
Sawyer, C.N., McCarty, P.L. and Parkin, G.F., 2003. “Chemistry for Environmental
Engineering and Science”, Fifth Edition.
[3]
Bellú, S., García, S., González, J.C., Atria, A.M., Sala, L.F. and Signorella, S., 2008.
“Removal of Chromium (VI) and Chromium (III) from Aqueous Solution by Grainless Stalk of Corn”, Seperation Science and Technology, 43, 11 – 12.
[4]
YTÜ Çevre Mühendisliği Bölümü, Kimyasal Temel İşlemler Laboratuvarı, 2005.
Res. Assist. Elif Sekman
Yildiz Technical University, Environmental Engineering Deparment