Charge neutralization of the ionic species present in
wastewater takes place due to the counter ion produced
by the electrochemical dissolution of the sacrificial anode.
These counter ions reduce the electrostatic inter particle
repulsion to the extent that the van der Waals attraction
predominates, thus causing coagulation.
The floc formed as a result of coagulation creates a sludge
blanket that entraps and bridges colloidal particles still
remaining in the aqueous medium [30].
For hexavalent chromium concentrations < 520 mg/L, by
taking into account the pH range (0.9 < pH < 6.5) it may
be concluded that chemical reduction of hexavalent chromium
by iron (II) ions electrogenerated at the anode occurs according
to the following reaction:
HCrO
4 ðaqÞ þ3Fe2þ
ðaqÞ þ7Hþ
ðaqÞ !Cr3þ
ðaqÞ þ3Fe3þ
ðaqÞ þ4H2Oð1Þ ð11Þ
when the medium pH values are in the range of 6.5–7.5, the
reaction can be written as:
CrO2
4 ðaqÞ þ3Fe2þ
ðaqÞ þ4H2Oð1Þ !3Fe3þ
ðaqÞ þCr3þ
ðaqÞ þ8OHðaqÞ ð12Þ
For pH values above 7.5, the corresponding reaction
scheme is [31]:
CrO2
4 ðaqÞ þ3FeðOHÞ2 þ4H2Oð1Þ !CrðOHÞ3 þ3FeðOHÞ3 þ2OHðaqÞ
ð13Þ
For hexavalent chromium concentrations > 520 mg/l, by
taking into account the pH range (0.9 < pH < 6.5) it may
be concluded that iron (II) ions can also reduce hexavalent
chromium ions, the reaction can be written as:
Cr2O72
ðaqÞ
þ6Fe2þ
ðaqÞ þ14Hþ
ðaqÞ ! 2Cr3þ
ðaqÞ þ6Fe3þ
ðaqÞ þ7H2Oð1Þ ð14Þ
However, the iron (III) ions may undergo hydrolysis
depending on the pH of the solution Fe(OH)2+, Fe(OH)2
+
and Fe(OH)3 species may be present under acidic conditions.
The reactions involved are:
Fe3þ
ðaqÞ þ H2Oð1Þ ! FeðOHÞ
þ
ðaqÞ þ 2Hþ
ðaqÞ ð15Þ
Fe3þ
ðaqÞ þ 2H2Oð1Þ ! FeðOHÞ
2þ
ðaqÞ þ 2Hþ
ðaqÞ ð16Þ
Fe3þ
ðaqÞ þ 3H2Oð1Þ ! FeðOHÞ3 þ 3Hþ
ðaqÞ ð17Þ
Under alkaline conditions, Fe(OH)6 and Fe(OH)4 ions
may also be present [30].
2. Me