Hazardous Waste Treatment
Hazardous wastes can be categorized based on their hazardous characteristics such as ignitable, reactive, toxic, corrosive, explosive, poisonous, and radioactive (Mudgal et al., 2007). Basically, there are three types of methods to treat hazardous wastes biological, chemical and physical. Chemical treatment methods use different properties of a chemical to alter its hazardous compounds. Chemical reactions alter the chemicals, destroying the hazardous elements or producing new compounds that are easier to treat or dispose of (WRC, 2001). Chemical treatment can be further divided into several techniques.
a) Neutralization
Neutralization is a process of neutralizing waste acid with an alkali. Hazardous wastes are classified as corrosive if their
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Feedback control system regulates the rate of addition of lime by monitoring pH during addition. Lime is widely used for treating acidic wastes and is least expensive and the solution of excess lime do not reach extremely high pH values as solubility of lime in water is limited. Alkaline wastes could be neutralised by adding inexpensive acid, such as sulphuric acid. Acetic acid is preferable for some applications since it is nontoxic and biodegradable. Alkaline wastes can also be neutralised by bubbling gaseous carbon dioxide to form carbonic acid. The advantage of using CO2 is due to the availability at treatment sites (Mudgal et al., …show more content…
(8)
Ca+ 2H2O → Ca(OH)2+H2……………………. (9) (Mudgal et al., 2007; Rahuman et al., 2006 ; WRC, 2001).
d) Chemical precipitation
Chemical precipitation is one of the useful chemical treatment to convert hazardous heavy metals to insoluble form prior to disposal to a landfill and less mobile. This technique is applicable to any liquid waste stream that consist of precipitable hazardous constituent. The solubility of toxic metals can be decreased by adjusting pH and this leads to the synthesisation of a precipitate which can be removed by filtration and settling. For instance, precipitation of cadmium and its hydroxide by the use of sodium hydroxide.
Cd2+ (aq) + 2OH- → Cd(OH)2(s) ……………………. (10)
Generally, lime [Ca(OH)2] or caustic soda is applied for precipitation of the metal ions as metal hydroxides. For example,
M2+ +Ca(OH)2←⎯⎯⎯⎯→M(OH)2 +Ca2+ ……………………. (11)
This reaction suggests the use of lime to precipitate the metal as hydroxide.
Chromium is precipitated as hydroxide.
Cr3+ +3(OH−) → Cr(OH)3 ……………………. (12)
Sodium carbonate also has been used to precipitate metals as hydroxides
(Fe(OH)3•XH2O), carbonates (CdCO3), basic carbonate salts
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