Environmental Engineering Reference
In-Depth Information
Eh (Volts)
2.0
Cr - Na - H2O - System at 25.00 C
Cr2O7(-2a)
1.5
CrO4(-2a)
1.0
0.5
H
2
O Limits
0.0
Cr2O3
-0.5
H
2
O Limits
-1.0
-1.5
Cr(OH)2
-2.0
0 2 4 6 8 10 12 14
pH
C:\HSC5\EpH\TcN25.iep
ELEMENTS
Cr
Na
Molality
1.000E+00
1.000E+00
Pressure
1.000E+00
1.000E+00
FIGURE 8.5
E
h
-pH diagram for Cr(VI)-Cr(III) (generated using the HSC 5® software).
As a second example, chromium exists predominantly in either the +III or +VI
oxidation state. Cr(VI) exists as the oxyanions chromate (CrO
4
2-
) and dichromate
(Cr
2
O
7
2-
), and their salts, all of which are soluble. The trivalent state, Cr(III), readily
forms a relatively insoluble hydroxide and is the desired form for environmental
management. The conventional process for management of Cr(VI)-containing waste
streams is a two-step process: 1) reduction of Cr(VI) → Cr(III), with a suitable reducing
agent, and 2) precipitation as the hydroxide, by addition of a suitable base. Reducing
agents commonly used to treat the Cr(VI) include ferrous sulfate, FeSO
4
, sodium met-
abisulfite, Na
2
S
2
O
5
, and sodium dithionite, Na
2
S
2
O
4
. Figure 8.5 gives the E
h
-pH dia-
gram for Cr(VI)-Cr(III) calculated by HSC 5 using Na
+
as the counterbalancing
cation in aqueous solution.
8.2.4
S
ORPTION
AND
I
ON
E
XCHANGE
Reagents used to sorb metal contaminants (and organic contaminants) include acti-
vated carbon, ion exchange resins, metal oxides, natural materials (such as clays
and zeolites), and synthetic materials. Flyash and rice hull ash also have sorptive
properties. Some of these sorbents such as the ion exchange resins, zeolites, clays,
and flyash both sorb and function as ion exchangers. Reagents that sorb work well
for neutral, complexed, or organometallic compounds. The reagents with active ion
exchange sites can work well to remove cationic metals from solution.
Search WWH ::
Custom Search