The HSAB Concept as a Means to Interpret the Adsorption

The HSAB concept as a means to interpret the adsorptionof metal ions onto activated carbons

Ahmad Alfarraa, Elzbieta Frackowiakb, Francois Beguina,*

aCRMD, CNRS-University, 1B rue de la Ferollerie, 45071 Orleans Cedex 02, FrancebPoznan University of Technology, ul. Piotrowo 3 60-965 Poznan, Poland

Received 15 April 2003; received in revised form 23 December 2003; accepted 24 December 2003

Abstract

While activated carbons (ACs) are extensively used for metal ions trapping in aqueous medium, the physico-chemical factors

responsible of this phenomenon are not yet clearly understood, that is an important drawback for improving the adsorption

properties of these materials. The main interpretations are related either to a cationic exchange with the acid surface groups or to

an interaction with the p-orbitals of the surface polyaromatic units. However, it remains very unclear why some ions interactmainly with one type of site and rather not the other, and why the interaction might depend whether the ions are coordinated or

not. In this paper, we show that the hard and soft acids and bases (HSAB) concept allows quite a large number of published data

dealing with ions adsorption on activated carbons to be interpreted. In this concept, the surface of the basal structural units of

carbon is soft and can trap soft ions, whereas the oxygen surface groups are the hard sites that fix hard metal ions. On the other

hand, using either special treatments for the activated carbon or the coordination of metal ions by various ligands allows the

hardness of the two interacting species to be matched for a better control of adsorption. According to the HSAB concept, one is

able to predict the potential sites of adsorption on the carbon surface as a function of the hardness of each ion. Changing the

experimental conditions, metal ions with a borderline hardness can be adsorbed by the hard and/or the soft sites of carbon.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Activated carbons; Adsorption; HSAB concept; Metal ions; Cationic exchange

1. Introduction

Activated carbons (ACs) are widely used for the

adsorption of metal cations in aqueous medium. This

phenomenon is controlled by different factors, such as

the contact time, the solution concentration and pH,

the structural and microtextural characteristics of

carbon. Most studies are applied to the removal of

one kind of ion where ACs are very often considered to

have the behavior of a cationic exchanger [16] with

weak acidic surface groups. In this case, the adsorption

is almost independent of the carbon microtexture and

it increases with the pH value, the amount of surface

groups and the solution concentration. The desorption

of the trapped ions is only possible by cationic

exchange with other ions. This process works for

the adsorption of alkali, alkaliearth and several tran-

sition ions.

Some papers simplify the mechanism of cationic

exchange where it is presented as a physical interaction

[7,8]. In this concept, when the solution pH is higher

than the pH of zero charge, pHpzc, the surface groups are

Applied Surface Science 228 (2004) 8492

* Corresponding author. Tel.: 33-2-38-25-53-75;fax: 33-2-38-63-37-96.E-mail address: [email protected] (F. Beguin).

0169-4332/$ see front matter # 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2003.12.033

ionized, that charges the AC negatively, allowing the

cations to be trapped by an electrostatic effect.

For other ions such as heavy metals, the adsorption

seems to be independent both of the pH value and the

surface groups concentration; since it only depends on

the specific surface area and porosity of carbon, it

means that the ions are rather trapped on the surface of

the basal planes [9,10].

An overall study which predicts whether a given ion

could be adsorbed on AC or not, and the kind of

mechanism among those mentioned above is absent.

The main question to which an answer should be given

is: why some ions are only trapped by surface groups,

others by the basal planes of carbon, or even by both

sites? In this paper we will show that the HSAB

principle can be applied as an universal concept to

interpret the adsorption properties of ACs for all kinds

of ions. Pearsons hard and soft acid and bases

(HSAB) principle, introduced in 1963 [11,12], has

become one of the fundamental constructs of modern

chemistry. It has been widely used because its bold

Lewis acid/base statement that soft likes soft, hard

likes hard is easily understood, straightforwardly

applied, and this idea rationalizes a range of otherwise

very disparate facts. The HSAB principle was first

applied to selective organic synthesis in 1967, and

then for homogenous catalysis in transition metal

coordination chemistry. Taking into account a docu-

mented review of ions adsorption studies on ACs, we

found that ions which are trapped by the surface

groups are always hard, whereas those which are

adsorbed on the basal planes surface are always

considered to be soft.

In this paper, we will remind the main statements of

the HSAB concept taken from a rich literature review

and it will be applied to interpret the peculiar adsorp-

tion properties of a wide range of ions on activated

carbons. Finally, we will see the possibilities to modify

the hardness of an ionic species in order to enhance or

to reduce its adsorption.

2. The HSAB concept

At the same time that Brnsted proposed his acid

base theory, a broader theory was introduced by

Lewis. A Lewis base is a compound with an available

pair of electrons, either unshared or in a p-orbital,

whereas a Lewis acid is any species with a vacant

orbital. In a Lewis acidbase reaction, the base

unshared pair forms a covalent bond with the acid

vacant orbital. A specific example is given by the

following reaction:

BF3 NH3 ! F3BNH3

Far fewer quantitative measurements have been made

of Lewis acid strength compared to that of Brnsted

acids. A simple estimation of Lewis acidity based on

any quantitative measurement is not feasible because

it is related with the nature of the parent base. The

facility for an acidbase reaction to take place depends

of course on the strength of both acid and base, and

also on quite another quality, called their hardness or

softness [1114]. Hard and soft acids and bases have

the following characteristics.

Soft bases: the donor atoms have a low electro-negativity and a high polarizability and they are

easy to oxidize. They hold their valence electrons

loosely: example I. Hard bases: the donor atoms have a high electro-

negativity and a low polarizability and they are

difficult to oxidize. They hold their valence elec-

trons tightly: example F. Soft acids: the acceptor atoms are large, have a low

positive charge density, and contain unshared pairs

of electrons (p or d) in their valence shells. They

have a high polarizability and a low electronega-

tivity: example Cu, Ag. Hard acids: the acceptor atoms are small, have a

high positive charge density, and do not contain

unshared pairs in their valence shells. They have a

low polarizability and a low electronegativity:

example Li, H, Mg2.

Relative hardness or softness of some acids and

bases are listed in Table 1.

To make a quantitative treatment of hardness, Parr

and Pearson used the density functional theory [15

17]. According to this, let E(N) to be the electronic

energy of the ground-state as a function of the number

of electrons N. It is known that the derivative of E(N)

with respect to N is the chemical potential m, or thenegative of the absolute electronegativity w:

m @E@N

vr;T

w (1)

A. Alfarra et al. / Applied Surface Science 228 (2004) 8492 85

where vr is the potential acting on an electron at adistance r due to the nuclear attraction plus other

external forces that may be present. The second

derivative of E(N) gives the hardness Z:

2Z @m@N

vr;T

@2E

@N2

vr;T

(2)

The corresponding operational definition is the differ-

ence finite formula:

Z 12I A (3)

where Z, the absolute hardness (always positive), ishalf the difference between I, the ionization energy,

and A, the electron affinity. The softness, s, is simplythe inverse of the hardness, s 1=Z. The form ofhardness in Eq. (3) can be further approximated to be

the difference of orbital energy between the lowest

unoccupied molecular orbital (LUMO) and the highest

occupied molecular orbital (HOMO), which is a mea-

sure of the stability. Values of Z for some moleculesand ions are given in Table 2. It is important to notice

that cations, due to their higher ionization energy, have

a higher hardness than atoms or anions. Eq. (3) cannot

be applied to anions, because their electron affinity A

is difficult to be measured. Instead, it is assumed that Zfor anions X is the same as that for the radical X.

Once acids and bases have been classified as hard or

soft, a simple rule of the HSAB principle can be given:

hard acids prefer to bond to hard bases, and soft acids

prefer to bond to soft bases [18]. This rule has nothing

to do with acid or base strength but merely says that

the product AB will be particularly stable if both A

and B are hard or if they both are soft. Another rule is

that a soft Lewis acid and a soft Lewis base tend to

form a covalent bond, while a hard acid and a hard

base tend to bond ionically. One application of the first

rule is found in complexes between alkenes or aro-

matics (soft bases) and metal ions. Thus, complexes of

soft cations such as Ag, Pt2 and Hg2 are common,whereas complexes of Na, Mg2, or Al3 are rare.Chromium complexes are also common, however in

such complexes chromium is in low or zero oxidation

state, or softer being attached to other soft ligands, like

Cr(CO)3C6H6. In many cases, organic or aqueous

solution chemistry is based on an exchange reaction:

A : B A0 : B0 A : B0 A0 : B (4)Taking into account the above rules, it can be con-

cluded that such reactions occur as follows [19]:

HS H0S0 HH0 SS0 DE < 0 (5)

Table 1

Hard and soft acids and bases [12,13]

Hard bases Soft bases Borderline bases

H2O, OH, F, CH3COO

, SO42,

Cl, CO32, NO3

, RO,RNH2, ROH, R2O

R2S, RSH, RS, I, (RO)3P,

CO, C2H4, C6H6, H, R

ArNH2, pyridine, NO2

Hard acids Soft acids Borderline acids

H, Li, Na, K, Mg2, Ca2, Al3,Cr3, Fe3, BF3, AlCl3, CO2, HX

Cu, Ag, Pd2, Pt2, Hg2, I2 Fe2, Co2, Cu2, Zn2, Cd2,

Sn2, Sb3, Bi3

Table 2

Experimental hardness values for ions and molecules (eV) [13,15]

Cations Molecules Anions

Ion Z Compound Z Ion Z

H 1 HF 11.0 F 7.0Al3 45.8 CH4 10.3 H

6.4Li 35.1 BF3 9.7 OH

5.7Mg2 32.6 H2O 9.5 CN

5.1Na 21.1 N2 8.9 Cl

4.7Ca2 19.5 NH3 8.2 Br

4.2Fe3 13.1 (CH3)2O 8.0 SH

4.1Mn2 9.3 CO 7.9 I 3.7Cr3 9.1 C2H2 7.0Cu2 8.3 CO2 6.9Pb2 8.5 (CH3)3N 6.3Pt2 8.0 H2S 6.2Hg2 7.7 C2H4 6.2Fe2 7.2 SO2 5.6Ag 6.9 HI 5.3Pd2 6.8 Butadiene 4.9Cu 6.3 Cl2 4.6Au 5.7

86 A. Alfarra et al. / Applied Surface Science 228 (2004) 8492

where H, H0 and S, S0 are read as the harder and thesofter of the two acids (bases), respectively.

For a polyatomic system, the local softness s(r) (s isthe global softness) is proportional to the Fukui func-

tion f(r) [18]. This function represents the sensitivity

of the chemical potential of a system to an external

perturbation applied locally, such as a magnetic or an

electrostatic field. In this case, the HSAB principle is

locally applied: hard regions of a system prefer to

interact with hard reagents whereas soft regions prefer

soft species. This principle was applied by Lee et al.

[20] who calculated the local softness s(r) for three

speciesformaldehyde, the thiocyanate ion and car-

bon monoxide. They confirmed that: (1) a nucleophilic

reagent approaches the carbon atom in formaldehyde

from the direction perpendicular to the molecular

plane, while an electrophilic reagent approaches the

oxygen atom in the molecular plane; (2) the sulfur end

is softer than the nitrogen end in the thiocyanate ion;

and (3) carbon monoxide behaves like a Lewis acid in

bonding with transition metals. In another study,

Galvan et al. [21] could establish a formal relationship

between local softness and scanning tunneling micro-

scopy (STM) images. They showed that, under appro-

priate conditions, the STM images can be used to

measure the local softness for surfaces and they pre-

sented a potential application of those images as

reactivity criteria within the context of the HSAB

principle. More recently, Krishnamurty et al. [22]

have demonstrated that in the case of gases interacting

with zeolite surfaces, the reactions follow the local

HSAB principle. In the next paragraph, we will

describe the surface of activated carbons in terms of

local hardness and softness to understand the adsorp-

tion of different species.

3. Local softness and hardness of a carbonsurface

In active carbons, atoms are grouped into stacks of

flat aromatic sheets (basic structural units, BSU)

which are randomly crosslinked [23]. The mutual

arrangement of these aromatic sheets, called the

microtexture, is irregular and therefore leaves free

interstices (pores) between the sheets making active

carbons excellent adsorbents. Their adsorption capa-

city is determined by the porous microtexture but is

also strongly influenced by the chemical structure of

the surface. Active carbons are almost invariably

containing appreciable amounts of heteroatoms such

as oxygen and hydrogen, and in addition, chlorine,

nitrogen and sulfur. These heteroatoms are derived

from the starting material and become a part of the

chemical structure as a result of imperfect carboniza-

tion, or they become chemically bonded to surface

during activation or subsequent treatments. They

either form surface groups at the edge of the aromatic

sheets, or they can be incorporated in the microtexture

forming heterocyclic ring systems within the carbon

layers or bridges connecting the BSUs.

Carbonoxygen surface functional groups are by far

the most important structures that influence the sur-

face characteristics and behavior of activated carbons.

They are easily introduced by oxygen chemisorption

even on mere exposure to air or oxygen. Oxygen is

fixed firmly and comes off only as carbon oxides at

relatively high temperature. Similarly all carbons have

chemically bonded hydrogen. Activated carbons can

also bond nitrogen under ammonia treatment, sulfur

under hydrogen sulfide, carbon sulfide or sulfur treat-

ment, and halogens under gas or solution phase treat-

ment. Hence, carbonnitrogen, carbonsulfur and

carbonhalogen surface groups give rise to some

specific adsorption properties as well.

According to their electronegativity, heteroatoms

form more or less polar bonds with carbon. The p

orbitals of all the given examples of heteroatoms

except hydrogenoverlap with those of carbon and

form bonding and antibonding molecular orbitals

(MOs) extended on the whole basic structural unit.

Nevertheless, being more electronegative than carbon,

heteroatoms have lower energies for their valence

orbitals. Therefore, bonding MOs have their highest

contribution at heteroatoms, giving them a partial

negative charge d. The opposite charge d will bedelocalized over all the carbon atoms of the graphene

layer.

There are two reasons for considering that an

activated carbon has soft sites on the BSUs surface

and hard sites on the edge carbonoxygen bonds

(Fig. 1).

1. The difference between carboncarbon and car-

bonoxygen bonds. Table 2 shows that carbon

monoxide or dimethylether are harder than


ethylene or butadiene. Generally, the CO or CObonds are more polar and less polarizable, hence

harder, than the CC or CC bonds. One candeduce that the edges which contain electronega-

tive heteroatoms are harder than the graphene

layers surface.

2. The strong delocalization of MOs on the BSUs

and donor or acceptor groups. In Table 3 we can

see the effect of MOs delocalization on com-

pounds hardness. Polyaromatic hydrocarbons are

softer than benzene, and hardness decreases with

the number of cycles. Hence, it is reasonable to

attribute a very strong softness to the surface of

graphene layers, especially far from the edge

heteroatoms.

By application of the Huckel method of calculation

for aromatic compounds, the energies of the HOMO

and LUMO orbitals a xb and a xb are practicallysymmetric, i.e. the layers present approximately an

amphoteric acidbase character. Furthermore, by

comparison of different substituted aromatic com-

pounds in Table 3 we observe that the electronega-

tivity is increased by acceptor groups like COOH, NO2compared to benzene whereas it is decreased by donor

groups like OH, NH2 or OCH3. Experimentally, at low

pH (pH < 3) most carbons are positively charged,primarily due to the acceptor/donor interactions

between the graphene layers and the hydronium ions

in spite of their hardness [24]. We observe here how

the HSAB principle gives a tendency for a reaction

course. It does not allow to conclude that a reaction

between the soft graphene layers and the hard hydro-

nium ions is excluded, but simply says that the product

of this reaction can decompose easily by the presence

of another hard base (for example, OH) or a soft acid(like polyaromatic compounds which will be trapped

by graphene layers). On the other hand, the conduc-

tivity of activated carbons decreases after their oxida-

tion because this treatment destroys the large BSUs to

produce smaller ones with a higher amount of ether

bridges. Hence, the polarizability of BSUs is reduced,

and the electron transfer is more difficult along the

ether bridges due to their hardness.

Table 3

Experimental parameters for aromatic and polyaromatic molecules

(eV) [17]

Molecule w Z

C6H6 4.1 5.3

Naphthalene 4.0 4.2

Anthracene 3.8 3.3

Fullerene C60 4.8 2.0

Graphite 4.6 0.0

C6H5OH 3.8 4.8

C6H5NH2 3.3 4.4

C6H5OCH3 3.55 4.65

C6H5CO2CH3 4.7 4.6

C6H5CO2H 4.9 4.8

C6H5COCH3 4.8 4.5

C6H5NO2 5.5 4.4

OH C

OHO O CO

O O CO

COO

O

O

HO

OOO

Soft sites Hard sites

Fig. 1. Hard and soft zones of a graphene layer.


Hence, according to the previous remarks, we may

generally predict that hard acids or bases would be

adsorbed on surface groups of carbon, and that the

trapped amount would depend on the nature and

concentration of surface groups. Soft acids or bases

would be adsorbed on basal planes and their adsorp-

tion is supposed to be a function of the specific surface

area. In the following part of this paper, taking experi-

mental results from literature, we will demonstrate the

possibility of an overall interpretation of ions adsorp-

tion by activated carbons, applying the HSAB con-

cept. Especially this concept may explain why some

ions are exclusively trapped by surface groups, others

by the surface of basal planes, or even on both sites.

4. Application of the HSAB concept to cationsand anions adsorption on activated carbons

All the data of Tables 1 and 2 correspond to isolated

ions or molecules in the gas phase. To apply the HSAB

principle in water medium, it is essential to take the

ions hydration into account. In 1987, Pearson sug-

gested the maximum hardness principle which

states that there seems to be a rule of nature that

molecules arrange themselves so as to be as hard as

possible [25]. Chattaraj et al. proved the validity of

this observation for over 40 different reactions even-

tually involving cations hydration or complexation

[26]. Water is a hard base, and it is easy to predict

that its reaction with hard alkali or alkaliearth cations

(Tables 1 and 2) would produce hard hydrated cations

as well. Therefore these cations may be preferentially

adsorbed by surface groups and cannot be trapped by

the graphene layers in water medium.

Generally, the formation of complexes between

hydrated metal ions and ligands in aqueous solution

in terms of HSAB concept is complicated. Taking into

account the reaction

A B ! ABthe most accurate treatment for a quantitative work can

be given by the equation:

log K EAEB CACB DADB (7)where K is the equilibrium constant (in H2O, at 25 8C)for reaction (6), the energetic non-dimensional para-

meters EA and EB measure the ionic bonding, CA and

CB the covalent bonding; the D parameters are neces-

sary to account for steric effects. The last term in

Eq. (7) can be neglected for large cations, but it is

important for small cations. It emphasizes the impor-

tant difference between reactions in solution and in

gas phase. In an aqueous solution, a Lewis acid is

multicoordinated by a number of water molecules.

Hence, the steric effects arise when the ligands replace

these water molecules [19]. The E, C and D para-

meters are related to hardness, but not in any simple

way, since they represent electrostatic and covalent

bonding, primarily. However, when ions have the

same coordination with water molecules, the terms

EB, CB and DB (B is H2O) are the same and the values

of ions hardness between the gas phase and water

medium are simply shifted. This is the case of

hydrated transition metals that always have an octa-

hedral coordination with six water molecules. They

conserve their relative order of hardness in aqueous

solutions.

The main conclusion from Pearsons notes about

hydrated ions is that complexes of soft cations with

soft ligands are also soft while complexes of these ions

with hard ligands like water (which is the reference

ligand of complexation reactions) would be borderline

between soft and hard.

4.1. Trapping of ions by a redox process

The most simple mechanism for ions trapping by

activated carbons is the chemical adsorptionreduc-

tion. It occurs when the redox potential of metal ions is

high compared to that of carbon; platinum [27], gold

[2830] and silver [29,30] ions (E0(AuCl4/Au)

1.00 V), (E0(Ag/Ag) 0.80 V) and (E0([PtCl4]2/Pt) 0.755 V) are trapped by this mechanism, accord-ing to the following reaction taken as an example:

PtCl42 2e ! Pt 4ClHowever, these metal ions behave more or less dif-

ferently. The soft AuIII ions are strongly trapped by the

basal planes of the activated carbon where they are

extensively reduced because of their high redox poten-

tial, independently of the activation degree and con-

centration of surface groups. On the other hand,

Ag(NH3)2 ions have a borderline behavior. While

they are weakly trapped and reduced by activated

carbon fibers, this phenomenon is strongly enhanced


by the oxidation of carbon with nitric acid, giving

surface groups on which these ions are first trapped

before being reduced to the metal state. Finally

[PtCl4]2 has the behavior of a soft base. The adsorp-

tion step on the carbon surface determines the amount

of reduced ions and it is independent of the concen-

tration of surface groups. In this case, the amount of

trapped platinum increases considerably with the spe-

cific surface area [27]. Actually, on the economical

point of view, the deposited metals are more difficult

to recover after this reaction than using cyanide com-

plexes, Au(CN)2 and Ag(CN)2

, that is the base of theindustrial method for silver and gold extraction. The

redox potential of these complexes (E0(Ag(CN)2/

Ag) 0.31 V, E0(Au(CN)2/Au) 0.58 V) islow enough for their reversible trapping without reduc-

tion by the activated carbon. Jia et al. indicated two

possible sites of chemical adsorption for Au(CN)2

and Ag(CN)2: the surface of basic structural units,

and surface groups, but they did not mention the HSAB

concept [9]. They noticed that the adsorption of these

ions is independent of oxygen and nitrogen contents

and it increases with the BET specific surface area of

carbon, demonstrating that it occurs on the BSU sur-

face. Accordingly to the HSAB concept, these com-

plexes are soft bases due to their delocalized MOs, and

they are adsorbed on the surface of graphene layers

through pp interactions.

4.2. Trapping on the BSUs surface

The mercuric ion, Hg2, is a typical example of softacid. It seems to be less adsorbed on an activated

carbon of acidic character when the pH increases [10].

However, if the adsorption would occur on surface

groups, which are more dissociated by a pH increase,

one would expect a better trapping at high pH. On the

other hand, after a post treatment of carbon by CS2 the

adsorption of Hg2 ions is considerably enhanced. Nointerpretation for this result was given by the reference

[10]. Hence, the sulfur organic compounds are soft

bases (Table 2) such as RS, and their reaction withthe soft acid, Hg2, produces a steady covalent bond.The HSAB concept also explains that in aqueous

medium HgS is insoluble (pKs 53) because thedissociation of the Hg S covalent double bond isdifficult. For the same reason, when activated carbons

contain phosphonic groups, they have a better affinity

for lead than for nickel or cadmium because the lead

phosphate is insoluble [4]. A kind of similar effect is

observed when some metal ions, such as copper, zinc

or lead, are complexed by EDTA. They are better

adsorbed on activated carbons, and it has even been

proved that the formed complexes are trapped by the

BSUs surface [31]. Hence, using the organic EDTA

ligand, containing delocalized MOs, for the complexa-

tion of metal ions, the hardness of the ions is reduced,

and consequently their adsorption on the BSUs is

enhanced.

4.3. Trapping by cationic exchange

Most of the metal ions are adsorbed by cationic

exchange with the weak acid surface groups of acti-

vated carbons [3234]. The higher the pH value, the

stronger the dissociation of surface groups and the

higher the amount of trapped ions from a solution.

Many studies in this field showed the profitable effect

of carbon oxidation on the ionic exchange adsorption

of transition metal ions like zinc Zn2 [1], copperCu2 [2], chromium III and VI [3] or nickel Ni2 [4],or rare earth metal ions like dysprosium Dy3 [5]. Inthis mode of adsorption, the effect of specific surface

area is almost negligible, and even using a charcoal

with a BET surface as low as 46 m2/g, the cations

trapping is effective and it increases with the surface

oxygen content [6].

The cationic exchange phenomenon is compatible

with the HSAB concept. Indeed, all the hard acids

like Cr3 [3], Na (the acidbase titration ofsurface groups is a cationic exchange reaction) or

even borderline acids like Zn2 (they are relativelyhard in the hydrated form) are trapped by the dis-

sociated surface groups that can be considered as hard

bases [1].

In some papers, a more physical vision of adsorp-

tion is presented, that hinders the true mechanism

because of the global vision of the activated carbon.

This interpretation is based on the fact that a carbon

rich in surface groups has a negative charge above a

certain pH, known as the pH of zero charge (pHpzc)

due to the dissociation of acid surface groups, and a

positive charge below the pHpzc. Hence, cations like

Co2 [7] and Pb2 [8] are considered to be electro-statically trapped by the negatively charged surface of

activated carbons at high values of pH.


4.4. Trapping of cations with a borderline behavior

An interesting example is the adsorption of cad-

mium Cd2 which is trapped even at a pH of 1.1 on anactivated carbon. Furthermore, the amount of trapped

ions is almost independent of pH when it is lower than

3. At higher values of pH, cadmium ions are progres-

sively trapped by the surface groups which are more

and more dissociated and the amount of fixed ions

increases with pH. The result at low pH was attributed

to the physisorption of cadmium, but this cannot

explain why an isotopic exchange of trapped cadmium

is slower at a pH of 1.1 than 4.05 [35]. This behavior is

easily interpreted taking into account that cadmium

Cd2 is a borderline acid. In this case the reactionbetween Cd2 and the surface of BSUs (soft base)produces bonds with a marked covalent character,

which are dissociated with difficulty. On the other

hand, at higher pH, the cationic exchange between the

oxygenated surface groups (hard base) and Cd2 givesionic bonds which are more easily dissociated.

5. How to modify the hardness and theacid/base character of activated carbons fora better control of adsorption

As shown before, the adsorption process of cations

on the activated carbons surface is controlled by the

relative hardness of the two species. The hardness of

ACs is generally increased by oxidation with classic

reagents like nitric acid, ammonium peroxodisulfate,

hydrogen peroxide, etc. The ammonia treatment at

high temperature allows an important part of oxygen

to be substituted by nitrogen. Since pyrones are trans-

formed into pyridine groups, the activated carbon has a

stronger basic character, but the hardness of the sur-

face groups is unchanged. The reduction of the oxy-

genated surface groups by di-hydrogen treatment or by

sodium borohydride reduces the amount of hard sites

on carbon. Some other chemical treatments may also

produce a softer carbon, like carbon disulfide, nitrogen

dioxide or iodine.

Electrochemical grafting of organic species allows

a specific group to be fixed at the carbon surface,

which should enable to obtain a better control of the

adsorption process [36,37]. By choosing a precise

ligand to be fixed, it is possible to reduce or to

increase considerably the ACs hardness. Unfortu-

nately, applications of this method are still missing

in literature.

A direct complexation of metal ions by hard

ligands like NH3 and amines or by soft ligands like

EDTA, thio-urea, cyanide, iodide or organic ligands

with strongly delocalized orbitals is certainly easier

than the modification of the activated carbon hard-

ness.

Generally, the modification of the ACs hardness is

often difficult to realize, but it is required for some

applications such as water treatment. Complexation of

metal ions is more easy to apply, as it is well known in

industrial applications for the metals separation or

recovery.

6. Conclusion

The hard and soft acids and bases concept was

successfully used to interpret literature data on the

adsorption behavior of different metal ions on acti-

vated carbons. The surface groups of ACs are con-

sidered to be their hard sites whereas the surface of

basal planes represents their soft sites. Using this

concept permits to explain whether a given ion may

be adsorbed on the surface of basal planes or by

oxygen surface groups, and also to explain qua-

litatively the efficiency of adsorption. On the other

hand, one should be able to control the adsorption

of metal ions modifying either their hardness or that

of ACs.

The use of the HSAB concept would be also inter-

esting for the optimization of molecules adsorption by

activated carbons, and for a better control of their

removal as well. Several additional parameters inter-

vene in this phenomenon, compared with ions adsorp-

tion. Among them, the most important is the molecular

size that determines a different number of adsorption

sites on the molecule, and which is also responsible of

steric effects with the activated carbon surface. In

other words, the trapping is in this case controlled

by the competition between the different sites of the

molecule for its adsorption on particular sites of the

activated carbon. In the future, more experimental

results will be needed to verify the applicability of

the HSAB concept to a wide range of adsorption

phenomena on ACs.


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The HSAB concept as a means to interpret the adsorption of metal ions onto activated carbonsIntroductionThe HSAB conceptLocal softness and hardness of a carbon surfaceApplication of the HSAB concept to cations and anions adsorption on activated carbonsTrapping of ions by a redox processTrapping on the BSUs surfaceTrapping by cationic exchangeTrapping of cations with a borderline behavior

How to modify the hardness and the acid/base character of activated carbons for a better control of adsorptionConclusionReferences

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The HSAB Concept as a Means to Interpret the Adsorption