Page 264 - Adsorption, Ion Exchange & Catalysis- 2007, Elsevier - Copy

Else_AIEC-INGLE_cH004.qxd 7/1/2006 6:53 PM Page 260
260 4. Adsorption and Ion Exchange

Table 4.12

Selectivity of ion-exchange resins in decreasing order of preference
xchanger Strong acid cation e Strong base anion e xchanger
Barium Iodide
Lead Nitrate
Calcium Bisulfite
Nickel Cyanide
Chloride Bicarbonate
Cadmium Hydroxide
Copper Fluoride
Zinc Sulfate
Magnesium
Potassium
Ammonia sodium
Hydrogen

their regeneration compared to strong acid resins. The dissociation of a weak acid resin
and thus its capacity is greatly affected by the pH of the solution.
Strong base anion resins : These resins are used in the hydroxide ( OH) form for w a ter
deionization and their capacity is not influenced by pH. So, they can be used oer the v
entire pH range. They can conert an acid solution into pure w v ater:

R H C 3 R NH Cl H O 3

H
NH O Hl
It is easily understood that their regeneration is achieed by treating them with concen-
v
trated sodium hydroxide (NaOH).
Weak base anion resins : Just lik the operation of weak base anion e for weak acid resins,
resins is greatly affected by pH. They exhibit their maximum exchange capacity in the pH
range up to 7.0. They hardly adsorb any strong acids: they cannot split salts.
Heavy-metal-selective chelating resins : The behavior of chelating resins resembles that
of weak acid cation resins. They differ in that they are much more selective in heavy metal
es with the hea x cations. Chelating resins tend to form stable complevy metals and contain
EDTA as the functional group. In the sodium form, they are referred to as EDTA-Na. Their
v groups hae donor atoms such as sulfur and nitrogen and may deelop bonds with metals v
that exhibit both covalent and ionic characteristics. Chelating resins have been extensively
studied and their sorption properties are well known. Besides their selectivity in removing
transition metals, another feature in f y can be easily re a or of their use is that thegenerated v
with mineral acids. There are also resins that do not contain nee ionic groups but are gati v
x
also capable of forming complees with metals. For e picolylamine resins create xample,
stable complexes at low pH values. These kinds of complex-forming resins are suitable for
removing trace amounts of metal cations from solutions containing high amounts of com-
peting alkaline and alkaline-earth metal ions at acidic pH.
v
In Table 4.13, the relatie selectivity of a commercial chelating resin for heavy metal
wn.
cations is shoThe relatie selectivity is based on the selectivity for calcium ions. F v or