Adsorption at the solid/liquid interface · 1. Ion exchanger Adsorption at the solid/liquid interface Ion exchange process means an exchange of ions between an electrolyte solution

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.


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]


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.


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.


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


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]


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.


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.


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.


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.


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––––


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.


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-


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


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.


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'.


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.


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–


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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Adsorption at the solid/liquid interface · 1. Ion exchanger Adsorption at the solid/liquid interface Ion exchange process means an exchange of ions between an electrolyte solution