Adsorption of Dl Alanine by Allophane

7/30/2019 Adsorption of Dl Alanine by Allophane

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C l a y M i n e r a l s (1999) 34,233~38

A d s o r p t i o ne f f e c t o f p H

o f D L - a l a n i n e b y

and unit partic le

a l lophane:a g g r e g a t i o n

H. HAS HI ZU ME AND B. K. G. TH EN G *'1

N a t i o n a l I n s t i tu t e f o r R e s e a r c h i n I n o r g a n i c M a te r ia ls ', T s u k u b a 3 0 5 , J a p a n , a n d * L a n d c a r e R e s e a r c h , P r i v a t e B a g

1 1 0 52 , P a l m e r s t o n N o r th , N e w Z e a l a n d

( R e c e i v e d 2 4 F e b r u a r y 1 9 98 ," r e v i s e d 1 5 J u n e 1 9 9 8 )

ABSTRACT: The adsorption of DL-alanine at pH 4, 6 and 8 by a soil allophane has been

determined. Two sets of experiments were carried out: (1) in which the allophane had been kept in a

moist state throughout; and (2) in which the mineral had previously been dried at 50~ In both

instances, the adsorption isotherms showed three distinct regions as the concentration of alanine in

solution was increased: (1) an initial, nearly linear, rise at low equilibrium concentrations; (2) a

levelling off to a plateau at intermediate concentrations; and (3) a steep linear increase at highconcentrations. For comparable concentrations of alanine in solution, adsorption decreased in the

order pH 6 > pH 8 > pH 4. Irrespective of pH, however, more alanine was adsorbed by the 'wet'

allophane than by its 'dry' counterpart. These observations are interpreted in terms of the

morphology and aggregation of allophane unit particles together with the pH-dependent chargecharacteristics of allophane and alanine. The results are compared with published data on the

adsorption of alanine by montmorillonite.

Amino acids as such, or as peptides, or associated

with humic substances (organic matter), occur

widely in soils and sediments (Stevenson, 1982).

The persistence and survival of amino acids in these

environments have been ascribed to adsorption and

physical protection by clays and other fine-grained

mineral constituents (Theng, 1974a; Bada, 1991;

Curry e t a l . , 1994). Clays have also been reported

as being capable of catalysing peptide bond

formation, and of differentiating between the L-

and D-optical isomers of some amino acids (Degens

e t a l . , 1970; Jackson, 1971; Lahav e t a l . , 1978;

Siffert & Naidja, 1992; Bujdak e t a l . , 1996). For all

these reasons, the clay-amino acid interaction has

received a great deal of attention. The focus of

research, however, has been on the adsorption and

intercalation of amino acids by crystalline layer

silicates with pH-independent ('permanent' ) charge,

among which montmorillonite has received much

1 Corresponding author

attention (Theng, 1974a,b; Siffert & Kessaissia,

1978; Dashman & Stotzky, 1982; Hedges & Hare,

1987; Naidja & Huang, 1994).

On the other hand, little is known about the

interactions between amino acids and short-range

order (poorly crystalline) alumino-silicates with

variable charge (e.g. allophane) which seems

surprising as these minerals are widespread in

volcanic ash soils (Wada, 1989; Parfitt, 1990),

and have a large propensity for accumulating and

stabilizing organic matter in soil (Oades e t a l . ,

1989; Theng e t a l . , 1989; Andreux & Theng, 1990).

Although the reactivity of allophane towards

organic compounds, in general, may be ascribed

to the size, shape, and peculiar structure of its unit

particles, the underlying mechanisms are not well

understood (Tate & Theng, 1980; Theng e t a l . ,

1982; Parfitt, 1990). Here we investigate the

adsorption of DL-alanine by a soil allophane from

New Zealand as part of a larger programme of

research on the stabil ization of organic matter in

allophanic soils (Parfitt e t a l . , 1997), and the

possible role of allophane in discriminating

~) 1999 The Mineralogical Society



2 3 4 H . H a s h i z u m e a n d B. K . G. T h e n g

b e t w e e n e n a n t i o m e r i c f o r m s o f a m i n o a c id s

( H as h i zu me & T h en g , 1 9 9 6 ) .

M A T E R I A L S A N D M E T H O D S

T h e a l l o p h a n e w a s o b t a i n e d f r o m a s o i l n e a r T e

K u i t i , N e w Z e a l a n d , a n d i s d e r i v e d f r o m t h e

r h y o l i t i c Ro t o e h u t ep h r a , d a t ed a t 4 2 0 0 0 y e a r s

B . P . ( T h e n g e t a l . , 1 9 8 2 ) . T h e f i e l d - mo i s t s o i l w as

s amp l ed f r o m a f r e s h l y ex p o s ed p r o f i l e a t a d ep t h

o f 2 m , a n d t a k e n t o t h e l a b o r a t o ry i n a d o u b l e

p l a s t i c b ag . T h e c l ay ( < 2 g m eq u i v a l en t s p h e r i ca l

d i ame t e r ) f r ac t i o n w as s ep a r a t ed b y d i s p e r s i n g t h e

b u l k s a m p l e i n w a t e r a t p H 3 . 5 w i t h a n u l t r a s o n i c

p r o b e , s e d i m e n t i n g u n d e r g r a v it y , a n d c o a g u l a t i n g

w i t h 1 M N aC1 a t p H 6 . A f t e r d e can t i n g t h e b u l k

s o l u t i o n , t h e c o a g u l a t e d m a t e r i a l w a s d i a l y s e d

a g a i n s t d e i o n i z e d w a t e r u n t i l f r e e o f c h l o r i d e , a n d

s t o r ed a s an aq u eo u s s u s p en s i o n i n a s t o p p e r ed

g l a s s co n t a i n e r . A p o r t i o n o f th i s 'w e t ' a l l o p h an e

w a s d r i e d i n a n o v e n a t 5 0 ~ t o o b t a i n t h e ' d r y '

s amp l e . T h e A 1 /S i r a ti o o f t h e s amp l e , d e r i v ed f r o m

e l e m e n t a l a n a l y s i s o f a n a c i d a m m o n i u m o x a l a t e

ex t r ac t, i s 1 . 57 , w h i l e i ts p o i n t o f z e r o ch a r g e

( P Z C ) , d e t e r m i n e d b y a d s o r p t io n o f N a + a n d C I - , i s

5 . 7 ( T h e n g e t a l . , 1982).

A d s o r p t i o n i s o t h e r ms a t 20 _ + I ~ w e r e d e t e r -

m i n e d b y e q u i l ib r a t i n g t h e ' w e t ' o r ' d r y ' a l l o p h a n e

w i t h 0 . 0 0 6 - 0 . 2 M s o l u t io n s o f a l a n i n e i n st o p p e r ed

p o l y t h e n e t u b e s. R e a g e n t g r a d e a l a n i n e w a s

s u p p l i e d b y W a k o P u r e C h e m i c a l s , J a p an , a n d

u s e d a s r e c e i v ed . T h e s o l u ti o n s w e r e m a d e u p i n

0 . 0 0 4 M N aC1, co n t a i n i n g 1 0 3 M N aN 3 t o i n h i b i t

mi c r o b i a l g r o w t h . I n t h e f i r s t s e t o f ex p e r i men t s w e

u s e d 1 0 0 m g o f ' w e t ' a l l o p h a n e to 1 0 m l o f a l a n i n e

s o l u t io n , w h i l e i n t h e s e c o n d s e t w e a d d e d 2 0 m l o f

s o l u t io n t o 2 0 0 m g o f ' d r y ' a l l o p h a n e . T h e

s u s p e n s i o n s w e r e a d j u s te d t o t h e d e s i r e d p H b y

d r o p w i s e a d d i t io n o f 0 . 1 M H C 1 o r N a O H u s i n g a n

O x f o r d m i c r o p i p e t t e . A f t e r s h a k i n g e n d - o v e r - e n d

f o r 6 6 h , a n d c e n t r i f u g i n g , - t h e c o n c e n t r a t i o n o f

a l a n i n e i n t h e s u p e r n a t a n t w a s m e a s u r e d i n a

S h i m a d z u T O C - 5 0 0 0 a n a l y s er . T h e a m o u n t

a d s o r b e d w a s e s t i m a t e d f r o m t h e d i f f e r e n c e

b e t w e e n t h e a m o u n t i n i t i a l l y a d d e d a n d t h a t

m e a s u r e d a t e q u i l i b r i u m w i t h th e a l l o p h a n e .

R E S U L T S

Fi g u r e 1 s h o w s t h e ad s o r p t i o n i s o t h e r ms f o r t h e

'w e t ' a l l o p h an e ( A w ) , an d F i g . 2 f o r t h e s amp l e t h a t

h a s p r ev i o u s l y b ee n d r ied a t 5 0 ~ ( A d) . I n b o t h

cas e s ad s o r p t i o n d ec r ea s ed i n t h e o r d e r p H 6 > p H

8 > p H 4 . T h e cu r v es a l s o s h o w t h a t ap p r ec i ab l y

m o r e a l a n i n e w a s t a k e n u p b y A w t h a n b y A d a t

c o m p a r a b le v a l u e s o f p H a n d e q u i l i b r iu m c o n c e n -

t r a t i o n . H o w ev e r , a l l t h e i s o t h e r ms a r e s i mi l a r i n

s h ap e h av i n g t h r ee d i s t i n c t r eg i o n s o f ad s o r p t i o n . I n

r eg i o n I ad s o r p t i o n i n c r ea s e s mo r e o r l e s s l i n ea r l y

u p t o a c e r t a i n co n c en t r a t i o n o f a l an i n e i n s o l u t i o n ,

r each es a p l a t eau i n r eg i o n I I , an d s h o w s a s t e ep ,

l i n e a r i n c r e a s e i n r e g i o n I I I . H o w e v e r , t h e l i m i t o f

c o n c e n t r a t i o n to w h i c h e a c h r e g i o n e x t e n d s t e n d s t o

b e w i d e r f o r A w t h an f o r A d . Reg i o n I , f o r ex amp l e ,

r an g e s f r o m 0 t o 0 . 0 3 M f o r A w b u t o n l y f r o m 0 t o

0.01 M for Ad.

3

pH

62

8

"o 1<

4

0

0.1 0.2Equilibrium oncentrationmol/I)

FIG. 1 . I so therms fo r the adsorp t ion o f DL-alan ine by

'w e t ' a l l o p h an e .

1 . 0 I I p H

0.8 ~ 68

0.6 4g

S- 0 . 4

0 . 2

0 , I

0 0.10 0.20

Equilibrium oncentrationmol/I)

FIG. 2 . I so therms fo r the adsorp t ion o f DL -alan ine by

'd ry ' a l lophane .



Adsorption of alanine by allophane 235

D I S C U S S I O N

Morphology , charge charac ter i s t i c s , and

a g g r e g a t i o n o f a l l o p h a n e u n i t p a r t i c l e s

Allophane is a collective term for a series of

hydrated alumino-silicates with short-range order,

and an A1/Si ratio typically ranging from 1:1 to 2:1

(Wada, 1989). The primary or un it particle of

allophane is a hollow spherule with an outer diameter

of 3 .5-5.0 nm, and a wall thickness of -0.7 nm.

Aluminium-rich (soil) allophanes apparently have the

imogolite structure over a short range with the

spherule wall being composed of an A1-O,OH

octahedral (gibbsitic) sheet to which orthosilicate

(03 SiOH) groups are attached on the inside (Partqtt,

1990). In Si-rich allophanes, some of the silicate is

polymerized, and a large proportion of the silicate

may be bound to the outside surface of the alumina

octahedral sheet (Wada, 1989; Parfitt, 1990).

Structural defects within the spherule wall give

rise to ~0.3 nm-wide perforations (Wada & Wada,

1977). Theng et al. (1982) have suggested that

(OH)AI(H20) groups, exposed at such defect sites,

are responsible for the variable charge character-

istics of the allophane used here. That is, these

groups can gain protons on the acid side, and lose

protons on the alkaline side, of the PZC (pH 5.7).

In other words, the net surface charge of the

mineral would be positive at pH < PZC, and

negative at pH > PZC.

Rheological measurements suggest that in aqueous

suspensions the unit particles of allophane tend to

form small aggregates through electrostatic and van

der Waals interactions (Wells & Theng, 1985). The

extent of aggregation ( 'flocculation') is close to

maximal at pH 6 when the net surface charge

approaches zero. On the other hand, at pH 8 and pH

4, the particles tend to repel each other, leading to a

reduction in aggregate size. In line with these

suggestions, transmission electron micrographs of

an aqueous suspension at pH -6 of the allophane

used, show hollow spherules with an average

diameter of 4.3 nm, forming 0.030-0.0 35 ~tm

spheroidal aggregates which, in turn, coalesce into

globular clusters of varying size (Hall et al. , 1985).

For spherical particles, the specific surface area,

S (in mZ/g), may be derived from the relationship

S = (6/pD) x 103 (1)

where p is the density (in g/cm3), and D the

diameter (nm), of the particles. Taking a densi ty of

2.6 g/cm3 for allophane, a value of 537 m2/g is

obta ined for the total (external) spherule area, and

66-77 m2/g for the surface area of aggregates.

I s o t h e r m s h a p e a n d a d s o r p t i o n r e g i o n s

For the sake of convenience, we will first

consider the 'wet ' system at pH 6. With an

isoelectric point of 6.11, alanine would exist in

the zwitteri0nic form at this pH. At the same time,

the net surface charge on allophane is essentially

zero. Adsorption would therefore be primarily

controlled by electrostatic interactions involving

the COO and NI-I~ groups of alanine, on the one

hand, and the (OHz)+AI(H20) and (OH)AI(OH)-

groups of allophane, on the other.

We propose that region I of the isotherms

describes adsorption on the external surface of

allophane aggregates, and that the plateau (region

II) indicates full coverage of this surface by alanine

(Fig. 1). Assuming a molecular area of 0.28 nm2 for

alanine, the amount adsorbed at the plateau

(0.42 mmol/g) corresponds to a surface coverage

of 71 m2/g. The good agreement between this value

and the external surface area of allophane

aggregates (66- 77 mZ/g), estimated from electron

micrographs, lends further support to the proposal.

On this basis, it seems reasonable to suppose that

region III of the isotherms represents intra-

aggregate penetration by alanine, followed by

adsorption on surfaces of unit particles making up

an aggregate. Despite the limited data available, the

shape of the isotherms in this region accords with

this interpretation, since a linear increase in uptake

with concentration indicates that fresh sites are

continuously created as adsorption progresses (Giles

et al., 1974a,b). In this case, the process involves

intra-aggregate expansion, possibly accompanied by

some dissociation of uni t particles within aggre-

gates. Linear (C-type) isotherms further suggest

'constant ' partition of solute (here alanine zwitter-

ions) between the bulk and intra-aggregate solution,

terminating in a (second) flat plateau when all the

available (external) spherule surface is occupied by

the solute. The extent of this surface, derived from

equation (1), is 537 m2/g. To achieve complete

surface coverage would require the adsorption of

~3.2 mmol alanine per gram allophane. Since the

concentration of alanine in solution was clearly

insufficient to achieve this level of adsorption

(Fig. 1), no second plateau was observed. Figure 3

gives a schematic representation of the adsorption

process for the three regions of the isotherms.



236 H. Hashizume and B. K. G. Theng

The isotherms at pH 8 and pH 4 are similar in

shape to that obtained at pH 6 but s ignificantly less

alanine is adsorbed at comparable concentrations in

so lu t ion . These observa t ions may l a rge ly be

explained in terms of the pH-dependent charge

characteristics of the reactants . At pH 8 both

allophane and alanine have a net negative charge,

while at pH 4 the net charge on both reactants is

posit ive. Because of electrostatic repulsion between

surface and solute, uptake at either pH is less than at

pH 6. The reason for the larger uptake at pH 8 than

at pH 4 is not obvious. However, at pH 8 the

allophane sample has a net negative charge of

20 cmol( ) /kg whereas at pH 4 the net posit ive

charge is 18 cmol(+)/kg (Theng et al. , 1982). It

seems likely, therefore, that charge~zharge repulsion

between allophane unit particles is greater at pH 8

than at pH 4. As a result , the size of allophane

aggregates at pH 8 would be smaller , and the surface

area available for adsorption larger, than at pH 4.

The isotherms for 'dry' allophane ( A d) , a s shown

in Fig. 2, are similar in shape to those observed for

the 'wet ' sample (Aw) as well as showing the same

dependence on pH. However, at comparable equili-

br ium concentrat ion and pH, the capaci ty of A d to

adsorb DL-alanine is appreciably diminished. This

observation is consistent with the structural changes

that drying and dehydration tend to induce. I t is well

known that allophane fails to rehydrate to i ts f ield-

moist s tate after air-drying (Warkentin & Maeda,

1980; Wells & Theng, 1985), while drying at l l0~

causes the aggregates of allophanc to coalesce into a

sheet-like structure (Kitagawa, 1971). Drying at 50~

would therefore be expected to enhance particle-

par t ic le interact ions and aggregat ion, caus ing a

reduction in surface area (Wells & Theng, 1988).

As already proposed, the ammmt adsorbed at the

plateau (region II) represents saturation of the

externa l surface o f allopha ne aggregates (Fig. 3).

For Ad at pH 6, this amount corresponds to a surface

coverage (S) o f -1 7 m2/g. Inserting this value of S in

eqn. (1) gives D - 0.136 ~tm wh ich is a four-fold

increase from the value of 0.030 0035 lam calcu-

lated for the aggregate diameter in Aw (cf. Fig. 1). It

would appear that the size of allophane aggregates

can be subs tant ia lly increased even by mild o ven-

drying (50~ as a result of which the reactivity of

allophane towards organic species is greatly reduced.

Similarly, Ishida (1991) found that prior oven-drying

of allophane-rich soils led to a marked decrease in

their capacity to adsorb polyethylene glycols. On the

basis of s tatis t ical thermodynamics, he was able to

o

Alanine

Equilibrium concentration

FIG. 3. Schem atic illustration of the allop hane-a lanine

interaction in regions 1, lI, and 111 of the adsorption

isotherms.

explain this observation in terms of a reduction in

both the reactivity and extent of the surface that is

accessible to the polymer.

C o m p a r i s o n w i t h m o n t m o r i l l o n i t e

As shown in Figs. 1 and 2, uptake by both Aw

and Ad is maximal at pH 6, close to the isoelectr ic

point (pI ) of a lanine, wi th appreciably less being

adsorbed at pH 4. In marked contrast to allophane,

uptake by m ontmo r i l loni te increases as the med ium

pH fal ls below the pI of the amino acid (Theng,

1974a) . Th i s d i f f e r ence in r eac t iv i ty be tween

a l lophane and montmor i l lon i te may be exp la ined

in terms of the inf luence of pH on the charge

characteristics of the reactants . Unlike allophane,

the negat ive sur face charge o f montmor i l loni te i s

essent ia l ly independent of pH. On the other hand,

amino acids become pos i t ively charged at pH < pI ,

and negat ively charged at pH > pI which in the case

of DL-alanine may be depicted as fol lows:

.H + _HT

C H 3- ~ H - C O O H ~ C H 3- ~ H - C O O ~ - C H 3- ~ H -C O O (2)

NH~ NH~ NH2

+H + +H +

pK t 2,35 pKz 9,87

(cation, acid pH) (zwitterion,near-neutralpH) (anion, alkalinepH)

pH<pI pH=pI=6.11 pH>pl

where K1 and K: are the corresponding equi l ibr ium

constants . Since neutral amino acids, in general,

exist as the corresponding cationic species at low

pH (<pI) , they are s t rongly at t racted to the



Adsorp t ion o f a lan ine by a l lophane 237

negatively charged surface of crystalline layer

silicates. The majority of published data on

montmoril lonite (Theng, 1974a,b), therefore, refer

either to adsorption at pH <3 or to uptake by a

hydrogen-saturated clay when cation exchange or

proton transfer is the dominant process.

The adsorption of ~- and 13-alanine by H-

montmoril lonite yielded L-type isotherms, reaching

a plateau at an equilibrium concentration of

~0.02 M (Greenland et a l . , 1965a; Cloos et a l . ,

1966). Although both compounds were apparently

intercalated as the cationic species, the amount

adsorbed at the plateau was appreciably less than

the cation exchange capacity of the clay. For ~- (or

DL-) alanine the plateau adsorption of 0.44 mmol/g

(Greenland et a l . , 1965a) is closely similar to that

shown by 'wet ' allophane at pH 6 (Fig. 1).

However, since the operative mechanism in mont-

moril lonite (dominantly proton transfer) is fund-

amentally different from that in allophane (electro-

static interactions), this similarity is more apparent

than real.

On the other hand, it seems valid to compare

uptake by allophane at pH 6 with that by

montmorillonite at the same pH. In both instances

alanine is adsorbed in the zwitterionic form. As

with allophane, adsorption by Ca-montmorillonite

(at pH 5.6-6.6) up to an equilibrium concentration

of 0.05 M yielded a linear isotherm (Greenland e t

al . , 1965b). This observation was explained in

terms of physical adsorption of alanine by constant

partition between the solution phase and surface-

adsorbed (Stern-layer) water. At the highest

equilibrium concentration of 0.05 M, -0.07 mmol

of alanine was adsorbed per gram of montmorillon-

ite. Even in the absence of intercalation, this

amount is much less than would be required to

saturate the external surface area (106 mZ/g) of the

clay. This would explain why the isotherm did not

level off to a plateau as observed with allophane in

region II (Figs. 1 and 3).

Furthermore, alanine is known to enter the

interlayer space of montmorillonite, forming

single-layer intercalation complexes (Greenland e t

al . , 1965a; Cloos et a l . , 1966). As a result, an extra

(760-106)/2 = 327 m2/g of (interlayer) surface area

would be available for adsorption, assuming a total

area of 760 m2/g for montmorillonite. In other

words, the highest adsorption (0.07 mmol/g)

measured by Greenland et a l . (1965b) was at least

an order of magnitude smaller than what can be

accommodated. In the case of allophane, alanine

began to penetrate the interspherule space when its

concentration in solution (at pH 6) exceeded

-0.075 M (Figs. 1 and 2). Interspherule (or intra-

aggregate) solute penetration in allophane may be

likened to intercalation in montmorillonite since, in

both instances, the process leads to the creation

(exposure) of fresh sites as adsorption progresses,

giving rise to linear isotherms.

CO N CL U SIO N S

The adsorption by allophane of a neutral amino

acid, like DL-alanine, is sensitive to variations in

the pH of the medium as well as to the aggregation

state of the mineral. Adsorption is highest at or near

the isoelectric point (pI) of the amino acid, and

decreases on either side of the pI. Irrespective of

pH, adsorption occurs initia lly on the external

surface of allophane aggregates. When this surface

is completely covered, the organic solute penetrates

the interspherule space within individual aggre-

gates, creating fresh adsorption sites. At comparable

solute concentration and pH, more alanine is

adsorbed by allophane that has been kept moist

than by a pre-dried sample. Even mild drying (at

50~ of allophane appears to enhance partic le-

particle interaction and aggregation, causing a

reduction in surface area and adsorptive capacity.

Neutral amino acids, like alanine, can thus serve as

a probe to assess the extent and mode of interaction

between unit particles of allophane under different

experimental conditions.

ACKNOWLEDGMENTS

Financial support from the Foundation of Research,

Science and Technology to BKGT is gratefully acknowl-

edged. We thank C. Feltham of Landcare Research forassistance with the TOC analysis, and E. Hagenaars of the

same institute for finishing the figures.

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B.M. (1996) Investigation on the mechanism of

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2 3 8 H. Hashizume and B. K. G. Theng

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