adsorption at the solid/liquid interface · 1. ion exchanger adsorption at the solid/liquid...
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Adsorption at the solid/liquid interface
Adsorption at the solid/liquid interface1. Ion exchanger1. Ion exchanger
Ion exchange processIon exchange process means an exchange of ions between an electrolyte solution and a
solid (ionite).
In most cases the term is used to denote the processes of purification, separation, and
decontamination of aqueous and other ion-containing solutions with solid polymeric or
mineral 'ion exchangers'.
This process is also called ion exchange adsorption, because it takes place at the
solid/liquid interface.
Ion exchangerIon exchanger – an inorganic or organic solid substance containing ions (ionogenic groups
bounded with the exchanger which can dissociate) which can be replaced by the ions from
solution whose electric charge is of the same kind. Ion exchangers are either cation
exchangers that exchange positively charged ions (cationscations) or anion exchangers that
exchange negatively charged ions (anionsanions).
There are also amphoteric exchangers that are able to exchange both cations and anions
simultaneously.
Adsorption at the solid/liquid interface
Ion exchangers practically do not dissolve in the solution. The amount of exchanged ions
must be electrically equivalent to prevent electroneutrality.
Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer),
zeolites, montmorillonite, clay, and soil humus.
Fig. 1.1. Ion exchanger
Fig. 1.2. Ion exchange resin
[http://en.wikipedia.org/wiki/Ion_exchange]
Adsorption at the solid/liquid interface
However, the simultaneous exchange of cations and anions can be more efficiently
performed in mixed beds that contain a mixture of anion and cation exchange resins, or
passing the treated solution through several different ion exchange materials.
IonIon--exchange capacityexchange capacity→ measure of the ability of ionite to undergo displacement of ions
previously attached and loosely incorporated into its structure by ions present in the
surrounding solution per unit mass (g, kg), or unit volume (cm3, m3) of the exchanger, and
also val/kg (val = miliequivalent), mmol/g, mol/n (n – the ion valency).
The total capacityThe total capacity of an ion exchangerof an ion exchanger is defined as the total number of chemical
equivalents available for exchange per some unit weight or unit volume of resin.
The capacity may be expressed in terms of milliequivalents per dry gram of the exchanger.
Adsorption at the solid/liquid interface
Operating capacity, also called useful capacityOperating capacity, also called useful capacity,, is the number of ion exchange sites
where exchange has really taken place during the loading run.
The ion exchange capacity is expressed as eq/L (equivalents per litre of resin).
This value is characteristic of a given process and depends on the solution concentration,
kind of ions, temperature, rate of the exchange process.
The operating capacity is always smaller than the total capacityThe operating capacity is always smaller than the total capacity.
Adsorption at the solid/liquid interface
In respect of In respect of the the chemical structure of chemical structure of the the exchangerexchanger:
⇒ inorganic
⇒ organic
In respect of In respect of the the exchanger originexchanger origin:
⇒ natural
⇒ semisynthetic
⇒ synthetic.
2. Kinds of ion exchanger2. Kinds of ion exchangerss
Adsorption at the solid/liquid interface
I. Cationic exchangersI. Cationic exchangers
Detailed classification:
Inorganic:Inorganic:
⇒⇒⇒⇒⇒⇒⇒⇒ natural (clays, aluminosilicates)
⇒⇒⇒⇒⇒⇒⇒⇒ semi-synthetic (treated glauconite)
⇒⇒⇒⇒⇒⇒⇒⇒ synthetic (synthetic zeolites)
Organic:Organic:
⇒ natural (peat, brown coal)
⇒ semi-synthetic (sulfonated coal)
⇒ synthetic (phenyl-formaldehyde resins)
II. Anionic exchangersII. Anionic exchangers
Inorganic:Inorganic:
⇒⇒⇒⇒⇒⇒⇒⇒ natural (diatomite)
⇒⇒⇒⇒⇒⇒⇒⇒ semi-synthetic (treated glauconite)
⇒⇒⇒⇒⇒⇒⇒⇒ synthetic (synthetic zeolites)
Organic:Organic:
⇒ natural (peat, brown coal)
⇒ semi-synthetic (sulfonated coal)
⇒ synthetic (phenyl-formaldehyde resins)
Adsorption at the solid/liquid interfaceNatural ion exchangersNatural ion exchangersTheir application is smaller than that of the synthetic ones because of their worse
physicochemical properties in comparison to those of the synthetic ones.
They were used to soften water (zeolites - hydrated aluminosilicates of calcium and
sodium).
The general formula of zeolites:
(Me(Me2+2+,Me,Me22++ )O; Al)O; Al22OO33⋅⋅⋅⋅⋅⋅⋅⋅nSiOnSiO22⋅⋅⋅⋅⋅⋅⋅⋅mHmH22OO
This group includes such minerals as: analcime (analcite), chabazite, natrolite,
skolecite and others.
Fig. 2.1.The microporous molecular structure of the zeolite, ZSM-5
[http://en.wikipedia.org/wiki/Zeolite]
Adsorption at the solid/liquid interface
The basic structural elements of zeolites are tetrahedrons of SiO4 and AlO4 which form 4-
or 6-element rings.
The aluminosilicate skeleton possesses an excess of negative charge which is
compensated by Me+ or Me2+ ions.
The ions are not built-in the crystal structure.
Therefore they can migrate and be exchanged by other ions from solution.
This group of natural ion exchangers includes montmorillonite and glauconite as well as
some soils. The soils are amphoteric ion exchangers.
Semi-synthetic ionic exchanger
These are natural exchangers which have been chemically treated, e.g. sulfonated coals
obtained by treatment with concentrated sulphuric acid or oleum.
They are known commercially as: Zoe-Karb-H, Permutyt, Wofatyt-Z, Eskarbo-H.
Adsorption at the solid/liquid interface
Synthetic ion exchangersSynthetic ion exchangers
These are:
synthetic aluminosilicates having the general formula: Al2O3⋅(SiO2)x⋅(Na2O)x⋅(H2O)z,
synthetic resins.
Synthetic resins are the most commonly used exchangers. They are mechanically
resistant substances, insoluble in water and some organic solvents, like alcohols, ethers,
hydrocarbons.
They can exchange ions because of the presence of active groups in their matrix.
The resins are obtained by polimerization, copolimerization or polycondensation of
appropriate monomers whose functional groups can dissociate.
The gropus can be acidic exchanging cations or basic exchanging anions.
Adsorption at the solid/liquid interface
An ion-exchange resin is in the form of small (1–2 mm diameter) beads, usually white or
yellowish.
The material has a highly developed structure of pores on the surface of which there are
sites with easily trapped and released ions.
Ion-exchange resins are widely used in different separation, purification, and
decontamination processes.
The most common examples are water softening and water purification.
Adsorption at the solid/liquid interface
The resin ionite general formula can be written:The resin ionite general formula can be written:
Cationic resin: RR––AA––MM++
Anionic resin: RR––BB++XX––
Where: R – the polimer matrix,
AA–– – the covalently bonded with the matrix anionic group, for example acidic, –COO– ;
MM++ – the ionically bonded cation with A which can dissociate, e.g. H+ or metal cation;
BB++ – the covalently bonded with the matrix cationic group, e.g. =N2+,
XX–– – ionically bonded anion with B which can dissociate, e.g. OH–.
One polymer molecule can have many functional groups. Hence ionite is a polyelectrolyte
whose ions can dissociate.
Adsorption at the solid/liquid interface
Characteristic functional groups of the ion exchange resins:
Cationic resinsCationic resins Anionic resinsAnionic resins
(–SO3)–H+ – sulphonic
(–COO)–H+ – carboxylic
(–O)–H+ – phenolic
(–S)–H+ – thiophenolic
(–NH3)+OH– – primary amine
(====NH2)+OH– – secondary amine
(≡≡≡≡NH)+OH– – tertiary amine
( N)+OH– – quaternary ammonium––––
Adsorption at the solid/liquid interface
Examples:
SO3-H+ -CH-CH
2-
-CH2-CH-CH
2-CH-CH
2-CH-
SO3-H+
-CH2-CH-CH
2-CH-
Cationic ion exchangerCationic ion exchanger –
copolymer of styrene and divinylobenzene
possessing active sulphonate groups,
whose proton H+ is capable of exchanging
with other cations.
Adsorption at the solid/liquid interface
Examples:
AnAnionic ion exchangerionic ion exchanger –
polymer obtained by polycondensation
of phenol with fromaldehyde. The amine
group whose OH- ion can be replaced
by other anions is active.
OH
-H2C
NH3+OH-
CH2
OH
CH2-
CH2
NH3+OH-
Adsorption at the solid/liquid interface
The process on a cation ion exchanger:
RMRM22 + M+ M11X X ⇔⇔⇔⇔⇔⇔⇔⇔ RMRM11 + M+ M22XX
The process on an anion ion exchanger:
RHXRHX22 + MX+ MX11 ⇔⇔⇔⇔⇔⇔⇔⇔ RHXRHX11 + MX+ MX22
Where: MX1 – the electrolyte solution subjected to the process of ion exchange.
The exchange reaction is reversible, therefore under static conditions the mass action law
can be used:
RR––MM22 + M+ M11 ⇔⇔⇔⇔⇔⇔⇔⇔ RR––MM11 + M+ M22
However, in practice the ion exchange process is conducted under dynamic conditions.
The solution flows through a bed in the column filled with both cationic and anionic ion
exchangers, or by two columns with cationic and anionic exchanger.
3. The ion exchange process3. The ion exchange process
Adsorption at the solid/liquid interface
For example, if NaCl solution passes through the column filled with a cation exchanger
whose H+ protons can be substituted by Na+ cations, three zones can be distinguished:
Fig. 3.1. Schematic representation of ion exchange process
in the column: A – the post-exchange zone; B –the
exchange zone; C – the pre-exchange zone.
postpost--exchange zone (A)exchange zone (A) – upper layer
substituted with Na+,
exchexchaanage zone (B)nage zone (B) – middle layer, where
the process takes place, both Na+ and H+ are
present in the exchanger and solution,
but their concentration depends upon the site
of the layer and the solution composition
depends on the distance from the column top.
prepre--exchange (C)exchange (C) – lower layer not yet
reached by NaCl solution.
Adsorption at the solid/liquid interface
The eluate from the column will be free from Na+ cations and contains an equivalent
number of hydrogen ions until quantity of the solution passed through the bed does not
produce any shifting of the exchange zone to the end of the column bed. If it occurs the
break-through point is reached and Na+ ions start appearing in the eluate.
Their amount in the solution usually increases rapidly and then the exchanger is fully
saturated (no exchange of H+ for Na+) and the solution passes through the bed unchanged.
To the break-through point there corresponds the break-through volume (operating
capacity), which is smaller than the total exchange capacity that occurs when the
concentration of Na+ ions is the same in the eluate as that of the input solution.
Graphical representation of the ion exchange process is shown in Fig. 3.2, where c/co is a
function of the eluate volume V, and c is the ion concentration in elate while co that in the
input solution. This curve is called isoplane. The shaded area represents the total
exchange capacity of the bed. This capacity equals the abscissa 'b' at c/c = 0.5, while the
break-through volume shows abscissa 'a'.
Adsorption at the solid/liquid interface
Fig. 3.2. Isoplane of break-through (break-
through curve);
section a – the break-through volume under
given conditions,
section b – the total exchange of the bed.
Adsorption at the solid/liquid interface44. Factors affecting the ion exchange process . Factors affecting the ion exchange process
The ion exchange process is complicated and therefore it is difficult to be described
theoretically.
It involves adsorption, absorption, chemisorption and even catalytic reactions.
Interpretation of the process takes into account:
⇒⇒⇒⇒ interaction forces in the crystal lattice (inorganic ion exchangers),
⇒⇒⇒⇒ adsorption equation of Freundlich and/or Langmuir,
⇒⇒⇒⇒ Donnan's equilibrium,
⇒⇒⇒⇒ theory of swelling – osmotic pressure.
Ion exchange process depends on properties of the exchanger and the ion undergoing
exchange as well.
Adsorption at the solid/liquid interface
Affinity of the ion for a given exchanger first of all depends on:
⇒ Electric charge of the ion – the larger charge the greater is the attracting force by
the functional groups and hence larger is its exchangeable capacity and rate
of the process.
⇒ Ion radius – the exchange capability is inversely proportional to its radius.
The hydrodynamic radii of ions decrease with the increasing atomic weight and hence
their exchange energy increases.
⇒ Degree of the ion hydration – the exchange capacity of cations is inversely
proportional to the hydrated radius.
Adsorption at the solid/liquid interfaceDegree of ion hydration depends on:Degree of ion hydration depends on:
⇒ Solution concentration
⇒ Temperature
⇒ Contaminations
⇒ Other factors.
The exchange energy of cations and anios can be arranged in series.
In the case of the sulphonated phenolic resin exchangers the series are as follows:
Cations: Na+ < NH4+ < K+ < Sr2+ < Cs+ < Mg2+ < Ca2+ < Cd2+ < Co2+ < Al3+ < Fe3+
In the case of the weakly basic exchangers the series is:
Anions: F– < Cl– < Br– < I– < CH3COO– < PO4
3– < NO3– < citrate < CrO4
2– < SO42– < OH–
Adsorption at the solid/liquid interface
The ions exchangeability depends to a great extent on pH of the solution, degree of
dissociation of the exchanger functional groups, relation between H+ and/or OH– and
other ions concentration. H+ and OH– compete with other ions in the exchange
process.
The characteristic parameter of the ion exchanger is its total exchange capacity
because it does not depend on the particular conditions of the occurring process.
For analytical purposes the break-through capacity is the most important.
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