Chromate Reduction

Hexavalent chromium is toxic, and most chromates are water-soluble. Only Ba2 +, Pb2 +, Hg+, and Ag+ form insoluble chromates. Pollution with chromates is especially troublesome, since chromates are not adsorbed by soil and thus reach the groundwater table rapidly.

Most chemical control strategies are based on the reduction of hexavalent chromium (Crvl) to trivalent chromium (Cr111). Trivalent chromium is much less toxic than chromates and forms a hydroxide with a solubility minimum in the pH range 8.0-9.0. Common reducing agents used are sulfur dioxide (S02) and its salts, ferrous salts, and iron sulfide.

1. Chromate Reduction with Metabisulfite

The most widely used reducing agent in S02-based processes is metabisulfite (Na2S205). It crystallizes from solutions containing NaOH and S02 in a mole ratio of 1:1. In aqueous solutions it forms an equilibrium with the disulfite ion:

metabisulfite disulfite

Aqueous solutions of sodium metabisulfite react acidicly. The solubility in water is 54 g/100 mL at 20°C [12].

Metabisulfite reduces chromates in acidic solutions:

4 H2Ci04 + 3 Na2S205 + 6 H2S04 Cr2(S04)3 + 6 NaHS04 + 7 H20 (9)

The sulfite (S4 + ) is oxidized to sulfate (S6 + ). The reaction consumes acid and is strongly temperature- and pH-dependent. The reaction rate increases with decreasing pH. The sulfur dioxide equilibrium pressure also increases with decreasing pH, causing S02 gases to escape that are toxic and cause corrosion. Therefore a pH range of 2-3 is generally maintained in the reduction reactor as a compromise between S02 backpressure and reaction rate.

2. Chromate Reduction with Fe" in Acidic Medium

Chromate reduction is feasible with ferrous salts in acidic solutions. The redox potential of the Fe"l/Fe" chain is pH-independent in the acidic region, while the oxidation potential of chromate is strongly dependent on the hydrogen ion concentration. Chromate reduction using ferrous ions in the acid region is performed at pH values <3 [13]. The overall reaction,

2 H2Cr04 + 6 FeS04 + 6 H2S04 -» Cr2(S04)3 + Fe2(S04)3 + 8 H20 (10)

consumes 3 mol of Fe" per mol of Crvl reduced to Cr"1. Fe" is oxidized to Fe1" in the reaction. This stoichiometry is less favorable than using S02-based reducing agents where 1.5 mol

Disadvantage Cyanide
Figure 1 Electrolytic decomposition of cyanide. Effect of cyanide concentration (g/L NaCN) on process efficiency (kWh/g CN" destroyed). (Data from Ref. 9.)

of SOz is oxidized for every mol of Crvl being reduced. An additional disadvantage is the introduction of iron into the solution, which increases the sludge production in the chromium precipitation step.

These disadvantages must be weighed against the advantages offered by the iron process. Chromate reduction by iron is odorless. No toxic and corrosive gases escape from the reactor as in S02-based processes. In addition, heavy metals are precipitated to concentrations one to two orders of magnitude lower than is achievable by hydroxide precipitation of these metals in the absence of iron. These advantages are compounded if the chromate reduction is performed in the alkaline region. Chromate reduction and metal precipitation can be achieved in one reactor at one pH under alkaline conditions.

3. Chromate Reduction with Fe"in Alkaline Medium

Iron (II) and iron (III) precipitate in the neutral and alkaline region as hydroxides. The Fe"1 hydroxide is by 24 orders of magnitude less soluble than the Fe" hydroxide [A:sp(Fe(OH)3) = 10"37 4; Ksp(Fe(OH)2) = 10""13 8]. The Nernst equation describes the redox potential of the Fe"I/Fe" chain:

which reduces to

The Fe111 and Fe11 activities are expressed as functions of pH using the solubility products of the hydroxides, the ion product of water (£„,), and the definition of pH.

aw = A"sp(Fe(OH)3)/ 1 N flft" ~ tfsp(Fe(OH)2)UoH-

The potential described by Equation (12) is more negative in the entire alkaline range than the oxidation potential of the Crvl/Cr"' chain. This makes the reduction of chromate feasible in the pH range of 7-11. Fe11 can be added in the form of ferrous sulfate, which is a by-product from acid pickling in steel mills. The Fe" can also be produced electrochemically by anodic dissolution of steel anodes [7].

4. Chromate Reduction with Iron Sulfide

Chromate can be reduced by the addition of FeS at pH 7. Fe" is oxidized to Fe"1, and the sulfide ion is oxidized to elemental sulfur [4],

H2Cr04 + FeS + 2 H20 -» Cr(OH)3 + Fe (OH)3 + S (18)

The reaction time needed for the completion of reaction (18) is several hours [14].

Table 2 Theoretical pH for Metal Precipitation to 0.1 ppm

Metal hydroxide

Ksp"

Equilibrium pH for 0.1 ppm metal in solution

AgOH

1(T77

12.3

Mg(OH)2

10"10 9

11.2

Fe(OH)2

10-'13 5

10.1

Ni(OH)2

,0-13.8

10.0

Cd(OH)2

io-13 9

9.6

Mn(OH)2

10"14 2

9.8

Pb(OH)2

10-I5.6

8.9

CO(OH)2

,0-15.7

9.0

Zn(OH)2

,0-16.8

8.5

Be(OH)2

,0-18.6

7.2

Cu(OH)2

,0-19.8

7.0

Sn(OH)2

,0-25.3

3.4

Cr(OH)3

,0-3O.2

5.8

Al(OH)3

,0-32.7

4.9

Fe(OH)3

,0-37.4

3.4

Sb(OH)3

,0-41.4

1.6

"AfSD values from Ref. 15.

"AfSD values from Ref. 15.

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