General Approach to Phosphate Stabilization

The overall phosphate stabilization of hazardous waste streams may be put in two categories simple phosphate washing to stabilize most of the contaminants and actual formation of a CBPC waste to treat more difficult contaminants. These two approaches are described below. 

Most hazardous waste streams contain contaminants (Pb, Cd, Ni, and Zn) that can be treated by contacting with a phosphate solution. This treatment entails simply washing the waste stream with a phosphate solution. The amount of phosphate stabilizer is small (5 wt%). At the end of the treatment, the waste stream retains its free-flowing characteristics and hence is easy to load on a truck, or to pump at a disposal site. Because the waste volume does not increase due to this treatment, disposal cost is low. Furthermore, the binder cost is also kept to a minimum. 

Phosphate washing has been in use for many years. Over a decade ago, Wheelabrator Technologies [8,9] patented a process to treat waste incineration residue ashes from 

Over the last decade, extensive research has been conducted on conversion of hazardous metals from different waste streams using phosphate stabilizers. Eighmy and Eusden [10] searched the literature on phosphate amendment of various industrial waste streams. They found 39 patents in this area since 1994. An updated summary of the various waste streams, either treated or tested by phosphate amendment or solidification, is given in Table 16.3. 

In addition to phosphoric acid, other acid phosphates have been used as a source of phosphate [8,9,21,22,30,31]. These include triple super phosphate [22,34], diammonium phosphate [22], and sodium dihydrogen phosphate [27]. These phosphates dissolve in water, release H2PO4 ions, and drive the pH of the solution to the acidic side. The chemical compounds of Pb, Cd, and Zn dissolve in the acidic solution and react with H2PO4 ions to form stable phosphates. 

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