Geochemical Processes at Mineral Surfaces - American Chemical

contiguous solid phase may be termed a sorption process. Among .... 1,. B m = K m ; n = l o r 2 ) , the van Bemmelen-Freundlich equation. -. 0 ,. 0 < ...
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Distinguishing A d s o r p t i o n f r o m Surface Precipitation Garrison Sposito

Downloaded by MONASH UNIV on November 9, 2012 | http://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch011

Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521 Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-à-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecularlevel probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. The loss of a chemical species from an aqueous solution phase to a contiguous solid phase may be termed a sorption process. Among the mechanisms by which sorption processes occur, the three principal ones are: precipitation, the growth of a solid phase exhibiting a primitive molecular unit (a complex) that repeats itself in three dimensions; adsorption, an accumulation of matter at the interface between an aqueous solution phase and a solid adsorbent without the development of a three-dimensional molecular arrangement; and absorpt ion, the diffusion of an aqueous chemical species into a solid phase (1,2). A precipitation mechanism may be initiated by either homogeneous or heterogeneous nucleation, may involve the formation of a solid mixture either by inclusion or by coprecipitation, or may take place on the surface of a pre-existent solid phase (surface precipitation). Regardless of these variations, the essential characteristic of precipitation is the development of a solid phase whose molecular ordering is intrinsically three-dimensional (2). An adsorption [strictly speaking, positive adsorption (1_)] mechanism, on the other hand, involves only two-dimensional molecular arrangements 0097-6156/86/0323-0217$06.00/0 © 1986 American Chemical Society

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

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

218

on

a surface.

adsorbates eliminate sional,

solid

even

growth

for

on

layer

2:1

layers,

of

Downloaded by MONASH UNIV on November 9, 2012 | http://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch011

form

it

i t s periphery

vein,

that

from

of

or

review,

the adsorption/surface on the c o n c e p t u a l made

distinguishing

above:

Adsorption

isotherms.

is

of

calculated

from

n^ is

with

solid,

of

solution

phase.

metric species

i

water

assigned Q ) ;

is

w

In b a t c h

hence,

to (2) be

to

-

a chemical

an

set o f is

six

in

not

a

com­

approaches

The emphasis to

the

here

defining

capable

species

an aqueous

of

i

adsorbed

solution

(1):

M m. w ι

(1) species

of

water

M

i n column

w

is

i i

the

inverse

where

M

w

of

is

is

the

the

side

gravi­

excess

no n e t

w on the l e f t

of

aqueous

the surface

there

suspension

mass

in the

experiments

1 represents

i n the

per unit

the adsorptive

the superscript

is

written as

result

an approach

of

interface

has read

follow

t h e mass of

region.

distinguish

popular

o f moles

Equation

t o an

likely to

depth

systems

for h i s

approach

is

penetration

precipitation?

of

experiments

whereas

content.

Corey

contacting

= n. ι

the m o l a l i t y

density,

water

M

which

should

each

the equation

number

solid,

a n d m^ i s

suspension

of

the t o t a l

kilogram

water

by

adsorbed

interfacial

which

extent

material

η

per

the

on three

surface

The q u a n t i t y

q. ι where

of

view,

only

to the

(1-3).

The d i s c u s s i o n

To what

a solid

of

adsorbate

t h e nanometer

particularly

focuses

point

previously

natural

conditions

their hydroxides

of

refer

with

in

metal

p r e c i p i t a t i o n dichotomy.

adsorption

Methods

phase

essay,

relationship

Solubility

mass

methods

does

influenced

beyond

defines

problem

instead

is

unit

this

precipitation. but

must

phase

precipitation of

to

per

solid

set out general

adsorption

c a n be

in the chemistry of

to the present

prehensive

statements

a

experimental

review

that

ordering

it

three-dimen­

From t h i s succession

and not b y any o t h e r

operationally

surface

comprehensive

interlayer

to a

mixed

but

inherently

(1)].

refer

(2)],

and a r e h i n d e r e d

[e.g.,

"absorption"

into

problem

establishment

conclusions

must

forms

species

is

on surfaces reasons

not p r e c l u d e

solutions"

structure

whose m o l e c u l a r

I n t h e same

introduction

solid

aluminosilicates

on w h i c h

adsorption a

they

type

A central the

r e s t r i c t i o n does

whose

adsorption"

a chemical

from

phases

if

each

layer

layers. of

latter

stereochemical

"multilayer the

This

["two-dimensional

of

accumulation of

the

equa­

tion. Adsorption the

change

phase. can

be

phenomena

frequently

in concentration of

Simple

mass-balance

rewritten

in a

form

q^ ι Δπΐ£

kilograms

= m° -

m^ a n d m °

o f water

are

species

studied i

considerations compatible

w )

η

where

a

is

with

by measuring

i n t h e aqueous (1)

show t h a t

this

solely

solution Equation

= Am.M ι Tw the m o l a l i t y

i n t h e aqueous

solution

1

methodology:

(2) of

species

phase

prior

_ i _ i n M.p to

its

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

w

being

SPOSITO

11.

brought vides

Distinguishing

into

solution.

1 is

analysis ous

critical

the

total

is

solid of

Cd

not 2 +

variable,

Data

special

cases

instead

q^ )

of

a

of

this

of

m^,

temperature kind

the

is

(m =

l,...,n)

adsorption pression B

m

= K

0 < 3 y

m

.

(w>

=

m

or

l / ^ fit

m

are

η =

; η =

Σ m=l

adjustable (_3,j4) t h e

1), 1).

In

it

is

to

be

possible

classical

of

of

to

Langmuir

L

derive

the

basis

of

Popular

yet

Langmuir

been

more

shown

van

to

the

task

becomes

of

corre­

fitted

iso­

numerically

to

^

c . ι

(3) r

m

b ,

m

i

1^,

m

in B ^

special

the

larger

is

=

m

is

of

=