TABLE 7.4 Precipitation of Sulfides

Precipitated

at pH 7

Not Precipitated

at Low pH

Soluble Complex

at High pH

Ag+

 

 

As3+a

 

x

Au+a

 

x

Bi3+

 

 

Cd2+

 

 

Co2+

x

 

Cr3+a

 

 

Cu2+

 

 

Fe2+a

x

 

Ge2+

 

x

Hg2+

 

x

In3+

x

 

Ir4+

 

x

Mn2+a

x

 

Mo3+

 

x

Ni2+

x

 

Os4+

 

 

Pb2+

 

 

Pd2+a

 

 

pt2+a

 

x

Re4+

 

 

Rh2+a

 

 

Ru4+

 

 

Sb3+a

 

x

Se2+

 

x

Sn2+

 

x

Te4+

 

x

Tl+a

x

 

V4+a

 

 

Zn2+

x

 

NOTE: Precipitation of ions listed without an x is usually not pH-dependent.

a Higher oxidation states of this ion are reduced by sulfide ion and precipitated as this sulfide.

SOURCE: Swift and Schaefer (1961)

waste going to a landfill. Waste that contains mercury, thallium, gallium, osmium, selenium, or arsenic should not be incinerated because volatile, toxic combustion products may be emitted.

7.D.3.8.4
Chemicals in Which an Anion Presents a Relatively High Hazard

The more common dangerous anions are listed in Table 7.2. Many of the comments made above about the disposal of dangerous cations apply to these anions. The hazard associated with some of these anions is their reactivity or potential to explode, which makes them unsuitable for landfill disposal. Most chemicals containing these anions can be incinerated, but strong oxidizing agents and hydrides should be introduced into the incinerator only in containers of not more than a few hundred grams. Incinerator ash from anions of chromium or manganese should be transferred to a secure landfill.

Some of these anions can be precipitated as insoluble salts for landfill disposal, as indicated in Table 7.2. Small amounts of strong oxidizing agents, hydrides, cyanides, azides, metal amides, and soluble sulfides or fluorides can be converted into less hazardous substances in the laboratory before being disposed of. Suggested procedures are presented in the following paragraphs.

7.D.3.8.5
Procedure for Reduction of Oxidizing Salts

Hypochlorites, chlorates, bromates, iodates, periodates, inorganic peroxides and hydroperoxides, persulfates, chromates, molybdates, and permanganates can be reduced by sodium hydrogen sulfite. A dilute solution or suspension of a salt containing one of these anions has its pH reduced to less than 3 with sulfuric acid, and a 50% excess of aqueous sodium hydrogen sulfite is added gradually with stirring at room temperature. An increase in temperature indicates that the reaction is taking place. If the reaction does not start on addition of about 10% of the sodium hydrogen sulfite, a further reduction in pH may initiate it. Colored anions (e.g., permanganate and chromate) serve as their own indicators of completion of the reduction. The reduced mixtures can often be washed down the drain. However, if large amounts of permanganate have been reduced, it may be necessary to transfer the manganese dioxide to a secure landfill, possibly after a reduction in volume by concentration or precipitation. Do not dispose of chromium salts in the sanitary sewer.

Hydrogen peroxide can be reduced by the sodium hydrogen sulfite procedure or by ferrous sulfate as described earlier for organic hydroperoxides. However, it is usually acceptable to dilute it to a concentration of less than 3% and dispose of it in the sanitary sewer. Solutions with a hydrogen peroxide concentration greater than 30% should be handled with great care to avoid contact with reducing agents, including all organic materials, or with transition metal compounds, which can catalyze a violent reaction.

Concentrated perchloric acid (particularly when stronger than 60%) must be kept away from reducing agents, including weak ones such as ammonia, wood, paper, plastics, and all other organic substances, because it can react violently with them. Dilute perchloric acid is not reduced by common laboratory reducing agents such as sodium hydrogen sulfite, hydrogen sulfide, hydriodic acid, iron, or zinc. Perchloric acid is most easily disposed of by stirring it gradu-



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