TABLE 7.1 High-and Low-Toxicity Cations and Preferred Precipitants

High Toxic Hazard

Low Toxic Hazard

Cation

Precipitanta

Cation

Precipitanta

Antimony

OH-, S2-

Aluminium

OH-

Arsenic

S2-

Bismuth

OH-, S2-

Barium

SO42-, CO32-

Calcium

SO42-, CO32-

Beryllium

OH-

Cerium

OH-

Cadmium

OH-, S2-

Cesium

 

Chromium(III)b

OH

Copperc

OH-, S2-

Cobalt(II)b

OH-, S2-

Gold

OH-, S2-

Gallium

OH-

Ironc

OH-, S2-

Germanium

OH-, S2-

Lanthanides

OH-

Hafnium

OH-

Lithium

 

Indium

OH-, S2-

Magnesium

OH-

Iridiumd

OH-, S2-

Molybdenum(VI)b,e

 

Lead

OH-, S2-

Niobium(V)

OH-

Manganese(II)b

OH-, S2-

Palladium

OH-, S2-

Mercury

OH-, S2-

Potassium

 

Nickel

OH-, S2-

Rubidium

 

Osmium(IV)b,f

OH-, S2-

Scandium

OH-

Platinum(II)b

OH-, S2-

Sodium

 

Rhenium(VII)b

S2-

Strontium

SO42- CO32-

Rhodium(III)b

OH-, S2-

Tantalum

OH-

Ruthenium(II)b

OH-, S2-

Tin

OH-, S2-

Selenium

S2-

Titanium

OH-

Silverd

Cl, OH-, S2-

Yttrium

OH-

Tellurium

S2

Zincc

OH-, S2-

Thallium

OH-, S2-

Zirconium

OH-

Tungsten(VI)b,e

 

 

 

Vanadium

OH-, S2-

 

 

a Precipitants are listed in order of preference: OH-, CO32- =base (sodium hydroxide or sodium carbonate), S2- = sulfide, SO42- = sulfate, and Cl- = chloride.

b The precipitant is for the indicated valence state.

c Very low maximum tolerance levels have been set for these low-toxicity ions in some countries, and large amounts should not be put into public sewer systems. The small amounts typically used in laboratories will not normally affect water supplies, although they may be prohibited by the local publicly owned treatment works (POTW).

d Recovery of these rare and expensive metals may be economically favorable.

e These ions are best precipitated as calcium molybdate(VI) or calcium tungstate(VI).

f CAUTION: Osmium tetroxide, OSO4, a volatile, extremely poisonous substance, is formed from almost any osmium compound under acid conditions in the presence of air. Reaction with corn oil or powdered milk will destroy it.

tion of 1 M sulfuric acid, or 1 M sodium hydroxide or carbonate. The pH can be determined over the range 1 through 10 by use of pH test paper.

The precipitate is separated by filtration, or as a heavy sludge by decantation, and packed for disposal. Some gelatinous hydroxides are difficult to filter. In such cases, heating the mixture close to 100 °C or stirring with diatomaceous earth, approximately 1 to 2 times the weight of the precipitate, often facilitates filtration.

As shown in Table 7.1, precipitants other than a base may be superior for some metal ions, such as sulfuric acid for calcium ion. For some ions, the hydroxide precipitate will redissolve at a high pH (Table 7.3). For a number of metal ions the use of sodium carbonate will result in precipitation of the metal carbonate or a mixture of hydroxide and carbonate.

7.D.3.8.3 Chemicals in Which the Cation Presents a Relatively High Hazard from Toxicity

In general, waste chemicals containing any of the cations listed as highly hazardous in Table 7.1 can be precipitated as their hydroxides or oxides. Alternatively, many can be precipitated as insoluble sulfides by treatment with sodium sulfide in neutral solution (Table 7.4). Several sulfides will redissolve in excess sulfide ion, and so it is important that the sulfide ion concentration be controlled by adjustment of the pH.

Precipitation as the hydroxide is achieved as described above. Precipitation as the sulfide is accomplished by adding a 1 M solution of sodium sulfide to the metal ion solution, and then adjusting the pH to



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