Environmental Engineering Reference
In-Depth Information
The produced chlorine undergoes a dismutation reaction at pH higher than
3 to 4.
Cl
2
+ H
2
OHClO + H
+
+ Cl¯
(6)
HClOClO¯ + H
+
(7)
The aluminum hydroxide flocs act as adsorbents and/or traps the metal
ions and hence eliminate them from the solution (Cenkin and Belevstev, 1985;
Ogutveren et al., 1994). Simultaneously, the hydroxyl ions which are produced
at the cathode increase the pH in the electrolyte and may induce co-
precipitation of Cd in the form of their corresponding hydroxides (Vik et al.,
1984; Biswas and Lazarescu, 1991). This acts synergistically to remove metal
pollutants from water
.
The process of electrocoagulation encumbers electricity
and electrode costs, especially for the developing countries and also often the
parasitic reactions can supersede the actual reaction limiting their efficacy.
3.5.Precipitation by Chemicals
Double precipitation method could be a possible solution for disposal of
cadmium from waste water. Double precipitation reaction can be divided in to
two important reactions: (1) Primary precipitation/Hydroxide precipitation,
and (2) Secondary precipitation/Sulfide precipitation. In the primary
precipitation or hydroxide precipitation, cadmium could be treated with
sodium hydroxide resulting in theformation ofcadmium hydroxide [Cd(OH)
2
].
In the second step, sulfide precipitation could be carried out with sodium
sulfide underalkaline condition and cadmium sulfide is produced as a resultant
precipitate (Janusz et al., 2005). Some of the limitations pertaining to this
technique are: formation ofhydrogen sulfide, pH dependency, sulfur
toxicityand highercost requirement due to use ofchemicals (Peters and Ku,
1985). The sludge formed during theprecipitation process can be discardedin a
suitable landfill.
3.6. Solidification/Stabilization
The term ―solidification/stabilization‖ refers to a general category of
processes that are used to treat a wide variety of wastes, including solids and
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