Geochemical Processes at Mineral Surfaces - ACS Publications

16 Adsorption of Metal Ions and Complexes on Aluminosilicate Minerals B. A. Goodman

Downloaded by COLUMBIA UNIV on October 6, 2014 | http://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch016

The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, Scotland Adsorption of metal ions and complexes on the various types of aluminosilicate mineral is briefly reviewed along with contributions made by several spectroscopic methods to understanding the nature of adsorbed species. A knowledge of the chemical forms of adsorbed species is an important preliminary to any understanding of their reactivities in either natural or artificial situations and, although significant progress has been made in some systems, there is clearly still a great deal of work necessary in order to characterize fully the environments of many adsorbed species. Examples of the types of reaction that may be carried out specifically by metal-exchanged clays are given and serve as illustrations of the importance of such species in natural systems and of the tremendous potential that such systems have in performing novel chemical reactions. The

surfaces

other

cases

usually of,

compensated

equivalent

adsorption (which play

the

clays the

and

is

science

toxic

surface

of

areas)

of

importance agricultural

models of

often

Therefore,

nature

of

the

i n many

areas

of

industries,

and c o n s t r u c t i o n of adsorption

ions, the

complex adsorbent

have

of

and waters

an influence

and

for clay

systems.

is

adsorption

charge

mobility

and i n

charge

involving

opposite

in soils

a charge

surface

importance

the

clay.

the

carry

Any

i n natural

c a n have the

and

of metal

properties

ions

properties

to the engineering

adsorption

with,

regulating

species

great and

Theoretical the

i n

processes of

of

minerals exist.

are of particular

surface

properties

chemical

may

by a s s o c i a t i o n

role

adsorbed

adsorption

centres

amounts

high

major

chemical

aluminosilicate

processes

have a

beneficial of

o f many reactive

Clay ions

such

minerals minerals with

both

and the nature

on the p h y s i c a l and an understanding

of

species

on

science

through

adsorbed ranging

from

environmental

industries. been

formation

reviewed

along

with

and the i n f l u e n c e

and s o l u t i o n

of

pH o n a d s o r p t i o n

0097-6156/ 86/ 0323-0342S06.00/ 0 © 1986 American Chemical Society

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

GOODMAN

processes take

a

in

a

number

there

is

Adsorption

on Aluminosilicate

Minerals

previous

ACS

(1).

of

chemical

cheraisorption,

or may

3-dimensional

growth

discussed detail sites

at

simple species the

in

of

cannot

be

that from

a

attention

and

of

some

energy

are

value

energy

nuclear

resonance

or

spin

is

+

information and to

on

liquids spin

with

1/2)

of the

priniple

electron

unpaired

of

observed

whilst

is

related

also

spin

echo

is

whose

performing to is

amenable

spectroscopy spectroscopy

to

the

are in

study

of

surface

electrons

occur

at

these

sorption

there

are

the

solid has

that

Again

spin

and

also

spectroscopy.

X-ray

involving

sensitive and

of a is

the

ESEM

electron

produce

a

decay

neighbouring IR

OH)

and

techniques and

Raman

and

IR

In

the

and

adsorbed

involving

valence

IR

enough

regions have to

solid-state

be

of

the

absorptions of

value

samples

amenable

photoelectron

is

transition

applications

species

inner-shell

ESR

observed

vibrations

near

being

usual remove

essentially

samples.

(particularly

where

are

case to

Transitions and

to

resonance

wide-ranging

instances are

liquid

solids

is

excitation.

of

molecular

groups

liquid-

pulses

these

it

environment(s)

this

nature

UV-visible

many

of

states provides

Both

case

is

the

with

latter

transitions

of

organic).

the

frequencies

photoacoustic probes

in

studies.

investigated,

of

by

spin and

resonance

in

and

Raman

(UV)-visible

(54.7°)

resonance

and

series

All

concerned

(particularly and

a

atoms.

molecules spectrum

by

particular

characterization

nuclear

electron

determined

and

information.

ENDOR

of

review

spectroscopy.

transitions

chemical

ESR s p e c t r o s c o p y

modulated shape

a

order

(ENDOR)

nucleus.

former

the have

paramagnetic

(IR)

field

angle"

that

the

that

or

nuclear

the

"magic

probed. an

spin

resonance

solution-like

states,

NMR i n

in

except

being

next-nearest-neighbour are

NMR

spin

ESR and

the

produce

to

electron(s)

combination

curve

and

this

magnetic

environment

effects

In

in

and Mossbauer

although at

will

most

adsorption.

between

external

rapidly

and

ultra-violet

studied,

sample

review

virtually

infra-red

chemical

similar between

(ESEM), and

of

obtained

mentioned

double

spectroscopy

an

cover

electron

spin

this

to one

number

c a n be

radiation

be

anisotropic in

that

(NMR),

a

and

a of

usage.

surface

transitions

in

and

have forms

task

In

of the

in addition

approaches. information

the

can

the

of

in

external

chemical

formidable

techniques

electron

modulation

the

minerals

a

techniques

resonance

photoelectron

concerned

(usually

on

with

of

rarely

is

their

of

photoacoustic

spectroscopy,

variations

onto

exist

studies

considered

chemical nature

solutions

electromagnetic

various

EPR),

echo

spectroscopy, NMR

in

the

magnetic

(ESR

electron

techniques of

understanding

This

of

is

our

the

of

precipitation

improve of

their

deal

resultant problem

be

conventional

great

the The

i.e.

addition

not

adsorbed

examples

range

potential

increasing

by a

with

In

and w i l l

required

spectroscopic

some

Spectroscopic complete

is

can

whether

surface,

species.

surface

since

involved.

achieved

on

and

Also,

those

however,

variety

to

occurs

information

mineral

surface

and

knowledge

pH.

metals

cases,

focus

order

processes

physisorption.

molecular

volume)

need

adsorption

solution

amounts

specific

in

we

of

solid

new

Sorption

d i s t i n c t i o n being

the

case

the

a

(this

especially

composition,

with

adsorption

However,

which

broadest

the

on

of

processes

conditions,

in

occur

Sposito

here.

adsorption

the

reaction

between

by

publication

with

not,

precipitation distinguishing


forms,

343

to

can

study

spectrosopy

electrons.


It

in be by

(XPS) has

344

G E O C H E M I C A L PROCESSES AT M I N E R A L S U R F A C E S

applications

to

information in

on

sorption

investigations creates

situations.

the

to


very

a

insensitive produce the

a

with

and

volume). of

texts

The

nature

As

far

as

minerals (i)

of

carry

a

cage

surfaces

hours

the

the

of

type

of

in

the

the

various

interested

chemical

Doppler

but

by are

is

solid

approach It

even

experiment.

chapter is

only

spectroscopy

is an

days)

to

Fe-57

is

However,

of

Ambe

of

is

nucleus

examples

its

generated

purposes,

methods

reader

velocity

Information

(or

in the

from

being

source

s e n s i t i v i t y and

given

the

can

be

use

of

et

al.

(this

beyond

the

scope

referred

to

some

of

the

literature.

surfaces

divided

properties into

layer

negative

(ii)

are

practice

processes.

practical

radioactive

adsorption

increase

many

most

transitions

decaying

nucleus

sorption

the

and

expanding

greatly

to

vacuum

practical

occurring

conventional

involving greater

a

Mossbauer

this

clay

may b e

the

for

details

in

the

to

In

nucleus

suitable

Sb-119

article

many

In

requiring

by

high

with

precursor. of

solid-state a

energy

nucleus

absorber

for

the

state

radioactive

often

to

into

spectrum.

a

environment as

provides technique

concerned

by

spectrum.

also

valuable

results

modulating

applications

much

Further

this

very

a

sample

states,

excited

spectrum and,

studies

performed Co-57

method isotope

sorption

the

material its

single

only

to

by

a

limited

the

is

excited

and

it

relating

emitted

chemical

in

in

obtained

state,

unknown

of

electromagnetic

suitable

the

limited

the

is

contribute

the

and

radiation

of on

materials using

of

ground

decay

provided

elements making

however,

spectroscopy

ground

of

is,

difficulties

radiation

excited by

Insertion

region

energy

of

It

the

Mossbauer

"tf-ray

number

chemical nature,

studies.

nuclear

exciting

large

and

possible

between the

a their

their charge

structures, surface over

structures,

accessible

are

3 general

a

only

to

such

area very

such

on

as

range

can

generally

pHs,

which

molecules

which

and which

of

zeolites, or

aluminosilicate

smectites,

solvation

wide

as

ions

concerned, groups:-

below

have a

internal

certain

size,

and (iii)

structures

by

chemical natures

the

The either which at

surface

from is

pH

the

especially amounts

or

from

under

different

that,

unless

an

kaolins

impurities

on

oxide-like a l . ,

is

have

taken

kaolinite source. (J3)

In

the

solely

may

a

low

of

in

been

of

the

Bolland

et

on

the

principal

mistakenly

with

a of

number x-ray

small surface

during

kaolin

the

minerals

have

structural

basis

of

high

a l . ,

dissolution of c a n be

The

mineral for

The

problems,

presence with

reported

contrast

zeolites.

charge. the

former

minerals,

present

minerals

surfaces

concluded,

net

either

reactions The

smectite

sometimes

from

Thus of

in

arise

structure,

pH d e p e n d e n t .

important

decomposition

circumstances.

charge LIm e t

have

is

dominate

can

arise

situations

account

which be

also charge

that

minerals

protonation/deprotonation

may

partial

Both

by

groups

cases

undetected

experiment.

negative

these

determined

substitution within

to

surface

are

aluminosilicate

considered

minerals

in

of or

properties

surfaces.

cation

surface

latter of

with

of

charge

charge

generally

determination difficulties

their

independent,

Is

whereas

adsorption of

isomorphous

oxide/hydroxide

mechanism

to

whose

shown

Al,

the

attributed of

natural

diffraction


16.

GOODMAN

(XRD)

and

cation

Adsorption

chemical

exchange

external

surface

The

CEC of

of

e.g. with

underestimate

the

fully

theoretical exclusion

model

has

recently

of

The

like

concentrations distinguish

be

between such

as

but

requiring

the

calculations

rationalize

acid

type

demonstrate

Lewis

pyridine

by

sites

and could

surfaces.

However, frequently

groups

in

on

information surfaces

investigations

of

Cu(II)

ionic

broadening exchange Cu the

(12) of

sites.

Recent

levels

of

adsorption that

the sites

Mn(II) that

most of

was

and

this of

the

type cross

Bronsted

of N-15 enriched followed

hydroxyl

by

groups

mineral

of

hydroxyl

that

in

on the

occurs

the

has

been

surface

For

minerals.

of

of

sites the

preferentially

than

arise

dipolar divalent

surfaces.

the clay

The with

sheet,

with

edge

and C u ( I I )

h a s shown

that

associated

for oxide

measurements at

and

dependent

Cd(II)

dependence

the

talc The

with

orientation

the plane

for

from ESR

example,

was n o t a s s o c i a t e d

From these

sites

kaolinite,

on t h e m i n e r a l

4 - 8 . 5 (13) pH

of

the exchange

the

a n d much l o w e r

reactions. pH

of

adsorption range

however,

adsorption

to

exchange

pH

reversible

ions.

h u m i d i t y were

the

and nature

consistent

11-12 Â apart

cation

work

i n this

to of

published

c a n be o b t a i n e d

exchanged

each

perpendicular

the

metals, Is

in

spectra

about

axis

precipitation

edge

Mn

minerals

paramagnetic

and

low r e l a t i v e

that

montmorillonite

for

the

principal

suggesting

zeolites

the technique

on the o r i g i n

demonstrate

being

at

is

groups

i n the characterization of

of

adsorbed

substitutions

ions

spectra

10,11)

Quantum

been

Silylation with

IR

minerals.

results

pyrophyllite

spectroscopy

in

conditions

used

by

difficult,

hydroxyl

can distinguish

be

not

i n an attempt

have

the surface

also

Total

does

adsorption

are

displacement (£) ·

of

surfaces

in resolving

under

spinning

presumably

(e.g.

the

from

surface

papers

a probe

By the

(6)

used

molecules

n-butylamlne as

IR

Experimental adsorption

of

NMR

assumed.

solid

absorptivities.

been

adsorbent

N-15

and

c a n be d i s t i n g u i s h e d

molar

some

the

potentials

but t h i s

sites.

and

angle"

used

used

from

of

co-ion

that

generally

measurements

by t h e s y s t e m a t i c

unenriched

(8,9)

hydroxyl

with

and "magic

acid

most

sites

recently

example,

NMR h a s b e e n

silica

acid

deriving

from

suggest

on

simple

characterization.

t h e u s e o f NMR s p e c t r o s c o p y

For

polarization

Si-29

have

aluminosilicates,

problem.

and

these

is

titration,

t h e i n t e r a c t i o n mechanisms

Lewis

of

by

quantitative

determination

as

layers

a

for

(b)

work

sites

grossly

problem

constant

i n mineral

and Lewis

pyridine,

(5),

acidic

problem

Bronsted

have

present

the

interface

this

surfaces

of

sometimes

this

the

g.

i n s o l u t i o n when

of

observed

measurements

developed

results

determined

mechanical

amorphous

been

constant-charge

can

spectroscopy

which

The

fundamental

may

t o overcome

m o n t m o r i l l o n i t e and I l l i t e

a

bases,

area

adsorption

2

the

impurities,

0 - 1 meq/100

the c l a y - s o l u t i o n

characterization

represents

the

for

much o f

smectite

from

i s exposed

In an attempt

potentials

n o t behave

of

that

N

that

from

ranging

surface

measurements.

surfaces do

area

expanded.

procedures,

smectites

345

Minerals

(CEC) a r i s e s

kaolinite

assessment

double-layer


extraction

capacity

difficulties, are

on Aluminosilicate

at

with

minerals

i t was

the constant

on low or

argued

potential

range.


GEOCHEMICAL PROCESSES AT MINERAL SURFACES

346

Adsorption

of

metal

Selectivity

of

variety

factors,

of

on

minerals

adsorption

can

the

vary

most

form

of

the

adsorbate,

external

solution

and

the

investigation.

contributing

to

interpreting examples the

the

from

the of

Exchange divalent identical

at

reversible

the

exchange

found

to

with when with

this

sodium-trace

the

metal

exchanging

ions

are

of

ion

analysis

structures work

may

the

related data

involving action

ions

have

exchange range

strong

halloysite The

has

(19). Ni

When by

the

the

much

less

reported

has

clay

been

to

half

of

solutions

montmorillonite follow In

(23).

competition

preferentially to

Na

and

Κ but

a

smaller

anion,

sulphate

and

species

uranyl

roughly

with

and

being

preference

over

strong Mg,

have

at

to

very of

been

increasing

clay

alkaline a

and

by the

adsorption

curves alkali

to

(20,21),

solutions

the

in

ions

equal

solutions for

of was

nitrate of

a Ni

This

solutions

Isotherms

there

with of

(22).

CEC

of

adsorption

nitrate.

adsorbing

nitrate

with

adsorbed,

as

exert

adsorption

although

CEC f o r

the

cation,

minerals

form

acetate

from

the

strength,

pairs

the

Langmuir-type

adsorption relative

the

of

Ca-saturated with

the

reported

and

wide

individual

medium c a n

ionic

Ion

in

of

a

changing

particular

e.g.

with

of

with

where

a

the

solution

neutral

form

for

(17,18).

obtained

activity

variously

approaching

was

of

uranium

concentrations, U

that

nitrate

sulphate

on

dominant to

in

of

for

one

for

ions

the

Zn

No

exchange

that

desorbing

fluctuation

for

of

than

latter

suggest and

trends

behaviour,

Na-

from

tactoid

this

charge.

cases

increasing

with

formation

Adsorption

approximately uranyl

was

reduction

montmorillonite of

with

relative

for

strength

adsorption

occurred

smectites adsorbing

although

selectivity

In

of

was

Ion

also

(JUS),

formation

on

When

and

and

adsorption

(15).

shown

the

phase

exchange

consistent are

ionic

on

decreased

consequent

CEC

There

high

sulphate

solution.

followed

and

depressed

explained

conditions,

been

principles,

undergoing

on

energies

phase.

has

exchange

the

described

specific

influence

adsorption

was

the

a

nature

kaolinites

greater

hydration

it

exchanger

been

in was

mass-action

ions

when

metal

exchange

adsorption.

adsorption

successfully

conditions.

show

ion

of

trace

mixture

the

virtually

coefficient

the

an

has

ideal

the

approximation

coefficient

significant by

poor

experimental

experimental minerals e.g.

a

different

selectivity

from of

it

of

several

be

non-ideal,

ideal that

selectivity

deviation

trivalent

is

data,

and

selected

stoichiometric

the

thermodynamic

an

factors

for to

were

of in

as

charge,

dissimilarity

equation of

unequal

(14)

found

produces

as

exchange

of

the

mass

of

to

the

illustrations

selectivity

metal

exchange

behaves

and

reactions

of

in

mineral

understanding

as

a

and

processes.

fraction the

the

section,

produced

described

according

influence

degree to

amount

cation

that

and

was

that,

montmorillonite an

0.01 than

Adsorbability

was

on

Cd

Pickering

but

on

exchange

particularly

often,

have

on

a

types

metals

(27)

pH r a n g e ion

formation

have

Sr

the

Co.

ligands

effect

the

loading

ligands

trace

Inskeep

which

there

Cd a n d

the

and

to

different

distibution of

ion.

different

Co

chloride

in

pH,

increased

that

hydroxy-Al

simple

distribution

levels

with

suggesting

solvated

due

to

or

concentrations

result

concentrations,

loading

coefficient

a

as

clay

Cd,

in a

montmorillonite

(14,24)

widely

from

CdCl^"",

Their

salt

low

salt

increasing

concentrations. very

type

free

of

with

increase as

Co o n

the

with

lower

the

with

on

adsorption

high was

possibly the

groups

investigated

to

of

interpreted

consistent

Cd

347

Minerals

adsorption

solutions

been

moderate

solutions

clay

work,

was

of

the been

hydroxyl

from

Adsorption

of

has

other

adsorbability


5-6

structural

montmorillonite

(26) .

on Aluminosilicate

was

complex. with

pH u p

were

found

excess

respectively,

behaviour

controlling on

hydroxy

ions

complexation

of

process

species

with

the

of

The to to Mg

of

its

i l l i t e

was

apparently

particular is

to

greater

surface

controlled the

âmerican Chemical Society Library In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

by

ligands.


348


Montmorillonite, i n contrast, appeared to behave as an ion-exchanger with results being Interpreted i n terras of competition between a l l p o s i t i v e l y charged solution species for the adsorption s i t e s . The retention of Cu by allophane i s enhanced by phosphate regardless of the sequence of Cu and phosphate adsorption, although Cu has been found to have no e f f e c t on the simultaneous and subsequent adsorption of phosphate on surface bound Cu (32). ESR results suggest that the Cu binds to surface A10H groups of the allophane i r r e s p e c t i v e of the presence of phosphate and i t was proposed that the enhanced Cu retention was the result of the formation of a ternary complex by the binding of phosphate to the a x i a l position of the surface-bound Cu i o n . The formation of polymeric metal ions on mineral surfaces readily occurs i n nature and aluminosilicate clay minerals i n t h e i r natural state are often associated with surface coatings of iron and/or aluminium oxides. Mossbauer spectroscopy has been used extensively i n the i d e n t i f i c a t i o n and characterization of such iron oxide species In minerals (e.g. 33), but l i t t l e work has been carried out on the s i m i l a r aluminium oxide species. However, the production of hydroxy-aluminium i n t e r l a y e r s i n expanding layer minerals, possibly involving the Al^O^OH^ "*" ion, has received considerable attention (34). The polymeric cations p a r t i a l l y neutralize the charge of the aluminosilicate sheets, which, therefore, exhibit a reduced CEC, but at the same time the i n t e r l a y e r ions function as p i l l a r s which prevent the mineral from collapsing on heating. Thus structures resembling those of z e o l i t e s are formed and have attracted interest because of their molecular sieve and c a t a l y t i c properties. Hydroxy-magnesium interlayers also occur extensively i n p h y l l o s i l i c a t e s . Such interlayers can be prepared s y n t h e t i c a l l y (35,36) by t i t r a t i n g MgCl and NaOH into a suspension of a layer s i l i c a t e mineral, although the extent of i n t e r l a y e r formation Is strongly dependent on pH and the nature of the mineral (37). 7

Characterization of chemical forms of adsorbed metal ions Information on the nature of the chemical environment of trace metal ions adsorbed on clay minerals can be obtained by a number of spectroscopic methods, but the p r i n c i p a l applications have used either XPS or ESR spectroscopy, or one of i t s related techniques, such as ENDOR and ESEM spectroscopy. With XPS i t i s possible to obtain good a n a l y t i c a l information on the amount of metal adsorbed and, i n favourable cases, to i d e n t i f y the chemical form of that metal. Oxidation states are readily determined and I t can be shown, for example, that adsorption of Co(II) on manganese oxides results i n oxidation to Co(III) (38,39), whereas adsorption of Co(II) on z i r c o n i a and alumina leads to the formation of cobalt(II) hydroxide (40). With Y-type z e o l i t e s hexaaquacobalt(II) i s adsorbed as Co(II), and cobalt(III) hexaamraine i s adsorbed as Co(III). The XPS spectrum of Co(II) adsorbed on c h l o r i t e was consistent with the presence of the hexaaquacobalt(II) ion f o r pH 3-7 and indicated that no cobalt(II) hydroxide was present (41). With k a o l i n i t e and i l l i t e , Co i s adsorbed as Co(II) over the pH range 3-10 (39,42), i t being bound as the aqua Ion below pH 6 and as the hydroxide above pH 8. Measurements involving Pb have


16.

GOODMAN

Adsorption

shown

that

(43),

whereas

Results

adsorbed the

(44)

adsorbed

indicate

chroraium(III) structure state.

is

able

and As

those

an

differences Mg

compounds,

such

magnesium


be

has on

made

aqueous

their

natural

solution

their

methods

for

only

a

small

much

of

the

the

and

Auger

exchangeable

and

typical

in

short

equilibrium

supercages.

low

ionic

resembles

are

the

set

small of

ESR

a

ions

shown the

there

under from

Another

decrease

but

this

susceptibility

of

ions

of

other

case

is

of

that

line

broadening

i t

was

Y-zeolites

a

this of

of

to

between

has also

of

spectrum

ion.

By

ESR s p e c t r a

of

occurred

is

been

dipolar the

these

ions pairs

and,

from

Cu(II)

exchanged

and S o r i a

was

to a reduction

a

similar

of

Cu(II)

in

interpreted

in trigonal

temperatures by

(48) 100°C,

show no c h a n g e

The e f f e c t

Cu(II)

upon double

i n t e n s i t y minimum a t

at

indeed,

dependent

performing

Conesa

measurements

accompanied

and corresponded

to

are the last

an ESR spectrum

conditions.

i n intensity

slow

different

coupling

the presence

a n ESR s p e c t r a l

t h e same

ions

but

a

in

i n the

the Cu(II)

exhibit

dehydration,

susceptibility

found

ions

ESR parameters,

Dipolar

were

temperature

were

allows

with

parameters

lattice.

to produce

degrees

varies

Cu(II)

a 4.2 Â separation

produce

of

ions

the

magnitude

reveal

the zeolite

Na-Y

evacuation

The

the 1st d e r i v a t i v e

magnetic

arising

with

a t ambient

Cu(II)

leaves

exchanged

(47).

calculation

of

varying

that

paramagnetism

hydrated

distinctive

Cu/Ce

the a b i l i t y

of

Cu(II)

temperature.

environment

with

and

adsorption

zeolites

experiments

cavities

integrations Y-zeolite

with

to

though,

When

of

latter

a l l paramagnetic

chemical

cases,

investigated

literature. (46),

spectra

permits

the

of

other

of

These

oxygen-broadening

many

than

ESR s p e c t r a

parameters. i n

with

in

both

in addition

and a range

they

ions

Not

from

orientation

experiments

where

Cu(II)

the large

the

cages,

high

interaction

exists

obtained

c a n be

the Cu(II)

prolonged

at

i n

of

exhibit by

reduction of

which

use samples

In these

of

to

to the

the mineral,

dehydration

they

the

of

times

followed

identified

is

concerned

reports

that

metal

have

as

sensitively

ions

is

to

provided

more

trace

a question

on is

components.

work

of

measurements

techniques

information

concentrations

Rapid

where

into

ESR

study

investigated

paramagnetic

the nature

the

in

mineral

oxidation

considerable

to

always

surfaces

trace

hydration

and

as

4 and

ions.

reported

distinctive

of

there

with

supercages

the

Information

clay

of

published

exchanged

while

in a

the l a t t e r

because

is

environments

number

of

dehydration

being

phases.

on

the study

degree

have

the

whilst

ESR and r e l a t e d

solid

zeolites

been

sample

and c h e m i c a l

metal

With

in

ions

t h e same

reveal

of

in

there

state

although

transition

and

with

similar

However,

vacuum,

chemical

the

Cu(II),

pairs

the

below

photoelectron-

fluoride,

applications

conditions.

species

undergo

and

6.

Is

being

(39).

and that

pH v a l u e s

between m e t a l

states

former

surfaces.

high

and

adsorbed

migrate

above

Mg

electronic

general

of

under

have

ion at

Pb(II) 2

kaolinite

as C r ( I I I )

montmorillonite

magnesium

mineral

relationship

the

on c h l o r i t e ,

on the s u r f a c e the

the

as

under

probing

pH v a l u e s

distinguish

as

oxide.

XPS species

at

Mg

the

(45),

aqua

remains

on/3- and δ-Μη0

Cr remains

Cr(III)

of

in

skeletal

occurs

Cr(III)

the

example,

electron-spectra

of

adsorbed

349

montmorillonite

the to

Minerals

Pb(IV)

that

hydroxide

is

on

to

adsorption

species

XPS

Pb(II)

oxidation

from

i l l i t e

on Aluminosilicate

symmetry.

above

300°C,

decrease to


Cu(I).

in

350

GEOCHEMICAL PROCESSES AT MINERAL SURFACES Small

ESR

amounts

spectra

hexaaquacopper(II) oriented

with

the

mineral

the

amount

as

a

1

formation

of

copper

formation

of

this

unit

lower

lowering

IR

indicated

and

could

be

surface

where

at

A1-0H and A1(0H)

when

a

Cu(II)

layer

has

silicates from

axial and i s

spectra

in

from

films

of

terms

aligned

at

approximately

results

i t

is

not

relative

smectites,

where

water

molecules,

again

adopted

perpendicular With

In can

dipolar

Mn

ions

which

It

was

hexaaquaraanganese(II) than

are

found

approximately molecules air,

for 30%

(52).

thus

decreased

when

positions

in

crystalline

hectorite

(53). was

The

of

that

was

with

Cu(II)

i n t o Mg of

that

three

Cu once

principal

line

present

with

axis

amounts

widths

of

somewhat were

between

broader

consistent

significantly of

exchanged conditions

collisions

to

(52). effects

t h e Mg

hydrated

mobility

structure

of

preferred

Fe, dipolar

fully

ions

such

type

F e a n d Mn a n d b e t w e e n

anisotropic

limited

was being

(51).

structural

Mn

with

no

significant

between

the

i n the

Cu

this

is

its

o f Mn i n t o

is

in

result

However,

between Doping

i.e.

with

two w a t e r

and a spectrum

an

water

on d r y i n g

the i o n i s

t o move

lines

with

in

greatly

molecules.

into

hexagonal

typical

o f Mn i n

observed.

varying

quantities

i n hydrolysis product having

partially

and t h i s

amounts

the

water

there

to

layer

tetrahedral

orientation

under

the

of

the adsorbed

sheets

that,

adsorbed

charge,

demonstrated

and which

(50)

an i n t e r l a y e r

having

increased

that

caused

hydrolyzed

and

of

retain

time

interlayer

resulted as

in

widths

the s i l i c a t e

interpreted

aqueous

increase

the faces

perpendicular

of

with

structure.

alignment

were

that

space,

or

layers

the s i l i c a t e

solutions

shown

the layer

observed,

smectites,

ions

200°C

Exchange Mg

clay

small

the

hydroxide

by a range

the s i l i c a t e .

l i t t l e

Line

matrices

of

axis

samples

that

Because

on the (001)

axis

distinguish

i n large

fluid

the

at

were

result

demonstrating

Dehydration

two

contains

found

from

copper

between

copper

hectorite

interactions

be m i n i m i z e d by d o p i n g

form.

the

was p r o p o s e d

exhibited

ESR s p e c t r a of

on the gibbsite

the metal.

steps

changes

to

preferential

iron,

hectorite,

is in

to

the

produced

a

i t

and t h e s i t u a t i o n where to

promote

of

t h e symmetry

With

the a i r - d r i e d

on

precipitation

i t has been

the o r i g i n

45°

to the plane

neighbouring

of

polymers

phase

the interlayer

the p r i n c i p a l

Mn(II)-exchanged

structural

with

behaviour

possible

alignment

in

intensity

present.

the clays

of

of

interaction

crystal

appreciable

in

orientation

are

saponite. no

interpreted

preferential

of

substitutions

space,

no

microscopy,

occupies

independent

octahedral

interlayer

water Such

was

structures,

symmetry

layers.

substitutions

groups

of

plane

a n d no s e p a r a t e - p h a s e

the edges 2

of

surfaces

solubility

there

silicate

monolayer

silicate

apparent

that

by e l e c t r o n

occurs

With

the

the

t h e commencement

free Cu(II)

i n an increase

hydroxide

hydroxide

the g i b b s i t e

hydroxyls

detected

adsorption

than

that

of

monomeric

i n t h e ESR s i g n a l

the

and i n d i c a t e s

l o w pH g a v e

to the (001)

5 resulted

The

hydrolysis, copper


but a decrease

at

presence

from

perpendicular

Cu adsorbed of

the

component

t h e pH a b o v e

of

results

axis

on g i b b s i t e

with

rigid-limit

its principal

pH

solution

adsorbed

consistent

and a

surface.

occurs

Cu(II)

were

(49)« Raising

result

mineral

of

that

a

that

the oxovanadium(IV)

V at

low l e v e l s

was a d s o r b e d

ligand

hydroxide

of

of

environment

i n

nature.

of

on the c l a y that With

i o n on

adsorption

was

surface

partially

increasing


V

GOODMAN


16.

Adsorption

on Aluminosilicate

Minerals

351

adsorption on w e t t e d h e c t o r i t e the p a r t i a l l y h y d r o l y z e d p r o d u c t was obscured by a spectrum from the s o l v a t e d oxovanadium(IV) i o n which had a linewidth greater than i n aqueous s o l u t i o n as a r e s u l t o f restricted m o b i l i t y . Under strongly dehydrating conditions the adsorbed V was observed to align with the principal V=0 a x i s p e r p e n d i c u l a r to the plane of the c l a y p l a t e l e t s . ESR has a l s o been used i n the c h a r a c t e r i z a t i o n o f s p e c i e s adsorbed on pillared clays, i . e . smectites with hydroxy-aluminium interlayers. Adsorption of Cu(II) on hydroxy-aluminium h e c t o r i t e produced mobile hexaaquacopper(II) and Cu(II) cheraisorbed to discrete s i t e s of the 0H-A1 interlayerÇ54)· The r a t i o o f c h e m i s o r b e d to m o b i l e Cu i n c r e a s e d w i t h i n c r e a s i n g pH, but even a t p H > 7 , when the solubility product of copper(II) hydroxide was exceeded, cheraisorbed Cu(II) remained the dominant species. This is in complete contrast to the results with g i b b s i t e (49), where precipitation of copper hydroxide was observed a t p H > 5 . S p e c t r a from air-dried f i l m s showed t h a t the C u ( I I ) had a x i a l symmetry w i t h principal axis perpendicular to the O H - A l - h e c t o r i t e ab p l a n e . At higher pH, a spectrum similar t o t h a t o f Cu(OH)^~ on a l u m i n a was observed, suggesting a l i g a n d exchange mechanism for Cu(II) adsorption on the complex. W i t h h y d r o x y - a l u m i n i u m m o n t m o r i l l o n i t e there was an i n c r e a s i n g c a p a c i t y f o r Na a d s o r p t i o n w i t h i n c r e a s i n g pH (55) and Na was not d i s p l a c e d by a d s o r p t i o n o f low l e v e l s o f Cu, indicating the existence of C u - s p e c i f i c s i t e s . I n c o n t r a s t t o the hectorite system, there was evidence of a hydroxy or hydroxy carbonate precipitate analogous to the situation with g i b b s i t e , although there was no g i b b s i t e d e t e c t e d i n t h i s system. ESR s p e c t r a showed the existence of a cheraisorbed species as well as electrostatically bound h e x a a q u a c o p p e r ( I I ) , but r a t h e r u n e x p e c t e d l y the Cu c o u l d be s o l u b i l i z e d w i t h ammonia o n l y a f t e r e x t r a c t i o n w i t h barium c h l o r i d e . The specific a d s o r p t i o n o f C u ( I I ) and C o ( I I ) by i m o g o l i t e , an a l u m i n o s i l i c a t e m i n e r a l w i t h t u b u l a r morphology o f c o m p o s i t i o n Al Si0 (OH) , was found t o be lower than f o r a l l o p h a n e s , which a r e related amorphous a l u m i n o s i l i c a t e m a t e r i a l s covering a range o f Al:Si ratios. Cu a d s o r p t i o n on s y n t h e t i c a l l o p h a n e s was dependent on the Si:Al ratio (increasing with i n c r e a s i n g A l ) (56) but no consistent effect was found with n a t u r a l a l l o p h a n i c c l a y s or f o r Co(II) adsorption. ESR s p e c t r a were i n t e r p r e t e d as i n d i c a t i n g t h a t adsorption of monomeric C u ( I I ) o c c u r r e d on an a l u m i n a - l i k e s u r f a c e , where OH was c o o r d i n a t e d to a s i n g l e A l i o n and a t a second type of site, which was thought t o be a s i n g l e SIOH o r A10H group, w i t h the distribuion o f C u ( I I ) between the s i t e s b e i n g dependent on the S i : A l ratio, pH and a d s o r b a t e c o n c e n t r a t i o n s , e x p o s u r e o f the a d s o r b e d Cu to ammonia resulted in l i g a n d exchange and the formation of Cu(II)-NH -surface complexes. W i t h imogolite some C u ( I I ) was desorbed from the surfaces as tetraamminecopper(II) ions and desorption of both Cu(II) and C o ( I I ) i o n s was r e a d i l y e f f e c t e d by c o m p l e x a t i o n w i t h EDTA o r by c o m p e t i t i o n w i t h P b ( I I ) o r p r o t o n s . 2

3

4

3

In the ESR spectra of adsorbed oxovanadium(IV) i o n s on minerals, information on the nature of the a d s o r b e d s p e c i e s Is obtained from the g - v a l u e s and V h y p e r f i n e c o u p l i n g c o n s t a n t s , but ligand hyperfine s t r u c t u r e i s seldom, i f e v e r , o b s e r v e d . W i t h ENDOR much s m a l l e r h y p e r f i n e s p l i t t i n g s can be observed than w i t h ESR and it i s p o s s i b l e t o measure h y p e r f i n e c o u p l i n g from n u c l e a r s p i n s i n



352

the

neighbourhood

splittings

for

oxovanadium(IV) those

from

matrix

ion

ENDOR

and

on

in

peak,

with

paramagnetic Y-zeolite

solution,

indicating

protons

outside

was

no

close

not

be

i n the centre

one

more

significant

lost

its

spectrum axial

(57)·

water

structure, occur

change

proton

sample.

This

and

and

there

although

showed binds

species

spectra

bulk

exhibited

illustrating

adsorbed

structure

result

molecule

ENDOR

and

are is

the more

great

(58),

a

did not

in

by

cause

spectrum N a ENDOR

V loses

of

chemical result

the

surrounded

complex

oxygens

sensitive value

molecules,

on dehydration of

as

from

sphere(57),

t h e ENDOR

more

four

type

simply

water

to

was no

shielded

evacuation a

that

to

was

there

the supercage

the ESR spectrum

signal,

thus

in

in

of

of

with

t o be s i m i l a r

1st coordination

approach Also,

V

proton

associated

found

the

its

there

layers.

were

that of

could

solvent

F o r example,

water

but i n the z e o l i t e

Because or

cation.

equatorial

complex a


the axial

adsorbed

the

interaction

of

both

the

change

its

zeolite that

can

of dehydrating

the

than ESR t o changes performing

both

in

types

of

measurement. The small

ESEM t e c h n i q u e

hyperfine

information

on

interaction

in

provides

interactions the

number

addition

an a l t e r n a t i v e

and of

to

has

spins

distinguishing

to the hyperfine

coupling

of

domain

can

the

time papers

adsorption shown

of

that

with

have

results been

cations

Cu(II)

ammonia

or

water,

coordination

spheres.

into

is

silica

gel,

where ESR

of

and

mixed

The

triaqua

or

present

with

influence Cu(II)

the

the

interaction migrations ESR

of

have

same

as bulk

because

from

is

is

(61).

freezable

unfreezable, of

the surface

silica

of

alkali

metal

the

in

Influence

Cu(II)

(60).

t o form

the formation

that

a of

both

species

are

both

cations

can

geometry

of

C u ( I I ) . The

Tl-rich

NaTl-X and

ethanol

a n d DMSO

adsorbates

with

was

CsTl-X

interpreted

his with

with

with

of

observing

Measurements shown but at

that

zeolites. in

This

terms

of

paramagnetic

surfaces.

of

a resonance Cu(II)

temperatures

two t y p e s

of

from

Mn(II) at

behaves i t

room

i n the

experiences

water

are

found;

c r y s t a l l i z a t i o n and the

the ice structure

and

although

on s i l i c a

water

ions

Cu(II)

investigation,

adsorbed

lower

On f r e e z i n g which

adsorbed

mineral

type

and undergoes in

onto

of

observed

be used

i n

water,

their

the/S-cages.

can

have

in

exchanged

the«-cage

zeolite,

dehydration

associated

above

mobility

which

was

encountered

manner which

adsorbed

result

with

have

present.]

indicating

adsorbates

used

decreased

one

that

into

NaTl-X

(59)

molecules

the Cu(II)

coordination

after

environments. and

Κ

to interact

polar

liquid

are

distorted

temperature a

was a b l e

been

and In

and

the copper

the

et a l

to demonstrate

Cu(II)

intensity,

interaction

difficulties in

Na

of

species.

same

spectroscopy

study

Mn(II)

whereas

observed

with

difficulties,

g e l and i n t e r a c t i n g

i n a series

able

dipolar analysis

o f ESEM t o t h e

that

is

Cu(II)

were

forces

location

such

of

Ichikawa ligand

to

species

and

on the c o o r d i n a t i o n and l o c a t i o n o f

Tl(l)

raonoaqua

zeolites no

to

zeolites

species

KT1-X but

studies

Tl-X

species,

two

providing particular

Although

present

silica

be noted

different

ESEM

of

into

only

should

completely

co-cations

presence

hexaaqua

have

[It

the hexaaquacopper(II)

ion-exchanged

solids.

exchanged

measuring

of

for a

on the a p p l i c a t i o n

on i n o r g a n i c

ions

for

isotropic

constants.

sometimes

published

method

advantage

responsible

contributions several

the

cannot

be

i n t e r a c t i o n . NMR, I R a n d d i f f e r e n t i a l


other formed

thermal

16.

GOODMAN

Adsorption

analysis

(DTA)

strucure

of

mineral field

also

adsorbed

propertes, gradient

influenced

by

Adsorption

and

The

have

on Aluminosilicate

by

around

the

Al,

degree

of

of


i l l i t e

and

occurred,

leading

similar

manner,

and hence

in

ZSM-5

of

complex

metal Thus

the

XPS

showed

formation

dehydration

of

of

conveniently

followed

Demetallation

of

and

(69)·

luminescence

Sn(IV) after

cation

destruction

possibility

as

well

of

as

pH

mono-

or

as

4-9.

a

kaolinite

and

polymeric

hydroxy

In

contrast,

an

ion

competing of 6

for

showed

resembling two

the on

association

of

imogolite ESR

adsorbing

the

ESEM a

range

were

able

for in

N-

and

number.

all

the

of

the

in

the

on

that

one

the

two

metal

to

Na the

another

ion

to

and

have

shown

surface

the

O-coordinated

a

gives photon

investigate that

complexes

the

in

determine

copper

involve on

the

whether

the

from

to

ESR

being

proceed

measurements

different

involve

the

organic

to

that

matter

for

a to

was

pH one the

complex, physical

surface,

which

at

species,

Cu humate

mineral

via

species

imogolite

adsorbed

of

(30,31).

solution

(32).

on

formation

clays

appeared

to

the

complexes

the

charged

large

appeared

whereas

produce

an

obtained

by

(71).

to size and

ligands

of

that the

or

two

a

form

these

seem

difference

Cu(II)

adsorbate

depending

Differences

but

not

silica-exchanged

one

Cu

(72).

ligands,

symmetry

other

with

Interaction

and

to

novel

the

IETS

mineral

with

showed

coordinate

in

a and

IR-allowed

used

sites

different

identical

the

is

spectra

Raman- and/or

however,

positively

on

the

N-coordinated

the the

UV-visible

indicating to

a

and to

as

using

on m o n t m o r i l l o n i t e and

humate

polarity

coordination

in

particular

Cu was

on

In

(68),

occur,

close

different

complex,

adsorbates to

interaction,

at

of

uncomplexed

results of

led

absorbance

forbidden

have

shown

montmorillonite

that

(67).

porphyrin adsorbed

on a l u m i n a

Furthermore,

the

IR

observed

are

are

of

sites

solution

extracted

spectrum

that (70)

been

adsorbed

(71).

adsorption

the

by

reactions

ions

vibrational

possible,

with

presence

minerals

complexes

montmorillonite adsorption

availabile

the

of

formed.

has

process

bisglyclneCu(II)

adsorption

aqua

reversible

ligand

number

species

with

exchange

Mazor

not

a

i l l i t e

often

chromium(III)

spectroscopy(IETS)

bands

adsorbed

were

of

is

environment

hydrolysis

remained

that

and

was

diglycinates

Adsorption

minerals

can a l s o

was

produces

bidentate

It

was

tunneling that

observing

Hipps

be

complex.

glycinates

acts

range

the

by

that

by

which anion

transitions

spectroscopies. Co

of

technique

the

and

porphyrin

to

minerals

the

tetra(4-pyridyl)

reaction,

electron

spectroscopic

glycine

tin

on

ammonium

complexes

spectroscopy,

and

Inelastic

Ni

of

The

on

electric

shown

on m o n t m o r i l l o n i t e and

being

dehydration

the

hexaamminecobalt(III)

adsorbed

hydroxide

ions.

of

been

of

effect

the

coordination

in

reaction case

species

the

ammonium hectorite

has

complexes

change

kaolinite,

cobalt(II)

zeolites

an

(66)·

tris(ethylenediamine)

chloropentaaraminecobalt(III) formation

width,

ion.

to

Investigation

NMR l i n e

hydration

and

the

can have

which

metal

in

which

transition

reactions

hexaamminechroraium(III) chlorite,

the

employed

(62-65),

c h a r a c t e r i z a t i o n of

adsorption

followed

e.g.

around

the

been

water

353

Minerals

on

the

chemical

in

A^were

to

reflect

in

adsorbate

approximately

with

molecules

square


observed a

change ligand planar


354 complexes

with

two

O-coordinated octahedral four

oxygens,

coordinated

Complexes affinity stability

Cu(II) for

indicated

of


smectite

large

clay fit

a

of

amounts

bis

complex

pH

these

the

on

spectroscopies

is

hectorite

interlamellar indicating

using

shown lost

the

surface

field

be

of with

explained

towards ions

the

are too

50% h u m i d i t y , a n d ,

the c l a y ,

the

square

d i e t h y l e n e t r i a m i n e and i s more

complicated,

i n s o l u t i o n as a and N i ( I I )

hectorite

function

complexes

surface

complexes

prefers water

a r e formed

Ni(II)

complexes

are

functions

as only

a very

of

absorption

Axially-coordinated

and planar

The planar

the c l a y

With

orbitals

increasing

E S R , IR a n d U V - v i s i b l e

that

on d - d

number

solvent Cu(II)

at

to

a

with

could

anionic

Cu(II)

complexes.

surface.

that

of

aqueous

and to decrease

results

the s i t u a t i o n

studies

in

based

stabilization

on

linearly

existing

high

crystal

field

adsorbed

(76).

complexes

readily

The

crystal

a r e added

a

overall

bis(ethylenediamine)

(74).

These

adsorbed complexes

have

are

for

the minerals

the complex

tetragonally-distorted molecules

the complexes

tris(ethylenediamine)

Adsorption

ligands

of

(75).

have

with

calculations

montmorillonite sheets

of

different

(77).

with

t h e G a n d i n - p l a n e IT

heterogeneous

that

between

large

several

involving

formed

minerals

to increase

density

as

shows

with

for

for

and

the c l a y

tetraethylenepentamine with

than

complexes

lODq,

of

acts

XRD

planar

ammines

orbital

values

adsorbed

charge

loading

to

when

ions clay

parameters

was o b s e r v e d

negative

the

distorted

complexes, were

bis(ethylenediamine)Cu(II)

increased complex.

ESR

similar and

minerals

average

higher molecular

and

parameter,

energy

if

much

energies

splitting

whereas

in

molecule,

3d t r a n s i t i o n m e t a l

although

solution

oxygens

oxygens

5-coordinated

montraorillonite-type

(73),

transition

Distorted

lattice

lattice

and one a d s o r b a t e

constants

solution

two

four

ligands.

of

for

and

involve

complexes.

lattice

TT-bond

adsorbates

ligands

on the

diamagnetic, weak

axial

ligand. Cu(II) show

and

a high

the

case

with

complex. the

that

Fe(phen) as

3

2

twice the

of

a

(80). the

7VM(phen)

3

whereas

as F e ( p h e n )

2 +

,

to

a

to

resolve

having

3

+

of

in

(79;. This

property

of

3

2 +

of

exploited

a labile

metal

with A-Ru(phen)

3

2

"**

but not

t h e Co o r F e

presence

achiral

complexes

of A.-Ru(phen)

molecule,

spectrum of

- m o n t m o r i l l o n i t e complex enantiomers

3

2 +

*

such as the

* -montmorillonite resulted

i n the e l e c t r o n i c

o r one a r o m a t i c

2 +

in

(M=Ru,Fe,Ni),

2 +

either

the

an

3

3

adsorbed

2

mixture

chromatographically

two a r o m a t i c

Fe(phen)

is

3

+

were

enantiomeric

to produce

Fe(phen)

in

couple,

complexes of

racemic

of

2

the

as a b i s 3 +

3

the CEC has been

introduced

The ^ - N i ( p h e n )

Fe(phen)

when

A s was

an increase

to twice

mixtures

Adsorption

been

-

montmorillonite

antiraceraize

being

*

shown

racemic

c a t i o n by A ~ N i ( p h e n )

orange

2

Adsorption

A-M(phen)

of

adsorbate. molecules

seen

3

(phen)

(78).

exists

complex,

X"" i o n p a i r s

2 +

of

and racemic

activity

used

3

of

adduct

adsorption

-montmorillonite. acridine

was

surface.

racemic mixture An

observed

optical

solvent,

surface

Cu(II)

in a tris

the F e ( p h e n )

mineral

species,

1,10-phenanthrollne

(hectorite)

adsorbed

bound

of

antiracemization

accepted

with

on m o n t m o r i l l o n i t e has been

t h e amount

complex

were

the

the adsorbed

adsorption in

pure

with

"*"

Fe remains

potential

in

associated

complexes

for a smectite

ethylenediamine,

Although

oxidation

above

Fe(II)

affinity

has of

recently organic

and one a l i p h a t i c

(81).


in the

ring

16.

GOODMAN

Reactions The

Adsorption

involving

the

chemical

quality.

of

as

natural of

papers

molecules

have

with

condensation

of

warming

wetting

drying, observed

Levi

and

(82)

the was

incorporated

into

detected.

Less

dimer

greater

2

and

adsorption

over Two

the types

a

the

environmental

presented

in

the

2 of

(85,86).

ESR s p e c t r a

to

the

terms

of

electron

observed

(87),

anisole

of

the

Type

g-value from

Cu(I)

occur

and

with

and

although arene some

Na

monomer largest and

found

ATP

yields

of

in

the

peptide.

Mg-montraorillonite although was

the

not

divalent than

adsorbed

(84).

2'-,

for

the

peptide

metal

by

ions

Also

pyrimidine

between

be

decomposed

accelerated

small

only to

montmorillonite

Pure

was

or

alanine

ADP was

for

preference

related

reaction

be

is

an

2 complex

the

purine

nucleotides

3'-

and

5*-AMP

adsorption

this

the

organic

an

of

the

to

aromatic

to

the

aromaticity

of

can

radical

a

be

Type

complex

narrow

to

in

Cu(II),

cation.

Similar

However

with

1 complexes

were

were

to

peak

the

seen

montmorillonites on

Cu(II),

explained

molecules.

ethers

Cu its

of

only of

with

retains

bonded

molecule

organic

Cu(II)

benzene

absence

result

aromatic

types

of

benzene

consist

and

benzenes on

the

edge

there

other

both

molecules

on

species to

complex

donation of

1

alkyl-substituted

adsorption

the

Mg

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