Page 543 - Materials Chemistry, Second Edition

CAT3525_C16.qxd 1/27/2005 12:42 PM Page 514
514 Waste Management Practices: Municipal, Hazardous, and Industrial
SOLUTION

From Equation 16.9, the molar ratio of CNO /CN 1. From Equation 16.10, the molar ratio of

NaOH/CNO 4/2 2. Therefore, the molar ratio NaOH/CN 2. The mass ratio NaOH/CN

2 40/26 3.08 kg/kg. The mass of NaOH required daily is 111.7 3.08 kg/kg 344.0 kg/day.
Hexavalent chromium-containing wastewater is produced in chromium electroplating and in
metal-finishing operations carried out on chromium as the base material. Chromium wastes are typ-
ically treated in a two-stage batch process. In the initial stage, the highly toxic hexavalent chromium
6
3
(Cr ) is reduced to the less toxic trivalent form (Cr ). There are several ways to reduce the hexa-
valent chromium to trivalent chromium including the use of sulfur dioxide, bisulfate, or ferrous sul-
fate. The Cr 3 can then be precipitated as chromic hydroxide and removed. Most processes use
caustic soda (NaOH) to precipitate chromium hydroxide. Hydrated lime (Ca(OH) ) may also be
2
used. The key reactions are as follows:
SO H O → H SO
2 2 2 3 (16.12)
3H SO 2H CrO → Cr (SO ) 5H O (16.13)
2 3 2 4 2 4 3 2
Addition of NaOH will result in Cr precipitation:

6NaOH Cr (SO ) → 2Cr(OH) 3Na SO
2 4 3 3(s) 2 4 (16.14)
Some generic oxidation and reduction reactions are depicted in Table 16.4.

16.3.4 SORPTION
Sorption involves the use of a sorbent to remove a soluble hazardous contaminant (the sorbate) from an
aqueous waste solution. Sorption is not a chemical process; rather, it involves the physical adhesion of
molecules or particles to the surface of a solid sorbent. Sorption is solely a surface phenomenon.
One of the most popular sorbents for the removal of both organic and some inorganic substances
from aqueous waste is activated carbon. Carbon possesses a high surface area and hydrophobic sur-
face characteristics, thus making it an excellent sorbent for removing contaminants from water.

TABLE 16.4
Examples of Oxidation and Reduction Reactions Used to Treat Wastes
Waste Type Reaction with Oxidant or Reductant
Oxidation of Organics
Organic matter, (CH O) {CH O} 2{O} → CO H O
2
2
2
2
Aldehyde CH CHO {O} → CH COOH
3
3
Oxidation of Inorganics

Cyanide 2CN 5OCl H O → N 2HCO 3 5 Cl
2
2
Iron(II) 4Fe 2 O 10H O → 4Fe(OH) 8H
2
2
3
Sulfur dioxide 2SO O 2H O → 2H SO 4
2
2
2
2
Reduction of Inorganics

Chromate 2CrO 4 2 3SO 4H → Cr (SO ) 2H O
2
4 3
2
2
Permanganate MnO 4 + 3Fe 2 7H O → MnO (s) 3Fe(OH) (s) 5H
3
2
2
Reproduced with kind permission of Manahan, S.E., Environmental Chemistry, 6th ed., Lewis Publishers, Boca Raton, FL,
1994. Copyright Lewis Publishers, an imprint of CRC Press. Boca Raton, FL