7 The Adsorption of Aqueous Metal on Colloidal Hydrous Manganese Oxide
Downloaded by NORTHWESTERN UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch007
D. J.
MURRAY1
Department of M i n e r a l Technology, University of California, Berkeley, Calif. T. W .
HEALY
Department of Chemistry, University of Melbourne, Victoria, Australia D. W.
FUERSTENAU
Department of M i n e r a l Technology, College of Engineering, University of California, Berkeley, Calif.
The adsorption manganite
of Groups
in aqueous
strongly
dependent
pendent
of pH at higher
adsorption double
indicates
the disordered
layer of the manganite
studies
and especially
at the
In dilute
with the transition
as evidenced
solution, diffuse
at higher
Co
+2
exhibit
lattice.
be
but is inde-
ions in the
uptake
(II) to
that these ions are incorporated
Ni ,
sorption
concentrations.
the extensive
adsorption 2+
found
on pH at low concentrations
layer, whereas
Cu
has been
must then occur as counter
centrations
2+
I and II cations on manganese suspension
In
coninto
addition,
metal ions show that marked
by the finite adsorption
specific
ad-
of these
ions
zero-point-of-charge.
' T p h e a b i l i t y of c o l l o i d a l , h y d r o u s manganese oxides to a d s o r b l a r g e q u a n t i t i e s of aqueous m e t a l ions has b e e n a c o n t i n u i n g subject of s t u d y since v a n B e m m e l e n s w o r k of 1881 ( 5 ) .
W h i l e c e r t a i n aspects of
the subject h a v e b e e n w e l l established—e.g., h y d r o g e n ions are r e l e a s e d ( o r h y d r o x i d e ions a d s o r b e d ) i n p r o p o r t i o n to the q u a n t i t y of m e t a l i o n a d s o r b e d (11)—there
is s t i l l c o n f u s i o n as to the details of the m e c h a n i s m
of i o n a d s o r p t i o n . 1
Presently in the U . S. Army.
74
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
7.
M U R R A Y
E T
Adsorption
A L .
T h e recent
comprehensive
of Aqueous s t u d y of
75
Metal
the g e n e r a l
colloid-chemical
properties of a h y d r o u s manganese d i o x i d e b y M o r g a n a n d S t u m m ( J O ) d i d m u c h to c l a r i f y m a n y aspects of the a d s o r p t i o n process. assignment of the zero-point-of-charge phous hydrous M n 0 S o r p t i o n of M n
2 +
2
A tentative
( z . p . c . ) of the essentially a m o r
of less t h a n p H 3 w a s m a d e b y these w o r k e r s .
on M n 0
2
a b o u t the z.p.c. w a s i n t e r p r e t e d either as
surface c o m p l e x f o r m a t i o n or as i o n exchange.
U s e of the mass a c t i o n
analysis of K u r b a t o v ( 9 ) , was s h o w n b y M o r g a n a n d S t u m m (10)
to b e
of l i m i t e d q u a n t i t a t i v e significance for the extensive a d s o r p t i o n of cations on M n 0 . Downloaded by NORTHWESTERN UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch007
2
F o l l o w i n g M o r g a n a n d S t u m m (10),
the surface properties of
five
manganese oxides w e r e e x a m i n e d b y H e a l y , H e r r i n g , a n d F u e r s t e n a u ( 7 ) . Zero-points-of-charge
of each oxide w e r e d e t e r m i n e d b y
electrophoresis
a n d c o a g u l a t i o n t e c h n i q u e s a n d f o u n d to range i n a p r e d i c t a b l e m a n n e r f r o m p H 1.5 for 8 - M n 0 , p H 1.8 for M n ( I I ) m a n g a n i t e , 4.5 for 2
«-Mn0 , 2
p H 5.5 for y - M n 0 , to p H 7.3 for / ? - M n 0 . 2
2
T h e present s t u d y w a s i n i t i a t e d i n o r d e r to o b t a i n q u a n t i t a t i v e d a t a o n the r e l a t i v e a d s o r p t i o n potentials of m e t a l ions i n the r e g i o n of t h e z.p.c. of h y d r o u s manganese oxide.
T h i s i n f o r m a t i o n is of
considerable
i m p o r t a n c e i n a v a r i e t y of p r a c t i c a l p h e n o m e n a r a n g i n g f r o m the m e c h a n i s m of trace m e t a l i n c l u s i o n i n ocean-floor
manganese
nodules
and
p i s o l i t i c manganese ores to the s o r p t i o n b e h a v i o r of manganese p r e c i p i tates i n n a t u r a l w a t e r a n d waste systems. Experimental A s t u d y of the zero-point-of-charge of v a r i o u s manganese d i o x i d e p r e p a r a t i o n s has b e e n r e p o r t e d p r e v i o u s l y ( 7 ) . T h e o x i d e selected for the present s t u d y w a s manganese ( I I ) m a n g a n i t e or " 1 0 A . m a n g a n i t e " h a v i n g a B . E . T . surface area of 70 m e t e r / g r a m . T h e w a s h e d oxide w a s k e p t u n d e r t w i c e d i s t i l l e d w a t e r at a l l times. R e a g e n t g r a d e nitrates of lithium, potassium, sodium, calcium, b a r i u m , copper ( I I ) , n i c k e l ( I I ) , and cobalt (II) were used without further purification. T h e p H was adjusted w i t h reagent grade n i t r i c a c i d . 2
C o n d i t i o n i n g of the manganese oxide suspension w i t h e a c h c a t i o n w a s c o n d u c t e d i n a thermostatted c e l l ( 2 5 ° ± 0 . 0 5 ° C . ) d e s c r i b e d p r e v i o u s l y (13). A n a l y s e s of r e s i d u a l l i t h i u m , p o t a s s i u m , s o d i u m , c a l c i u m , a n d b a r i u m w e r e o b t a i n e d b y s t a n d a r d flame p h o t o m e t r y t e c h n i q u e s o n a B e c k m a n D U - 2 spectrophotometer w i t h flame attachment. A n a l y s e s of c o p p e r , n i c k e l , a n d cobalt w e r e c o n d u c t e d o n a Sargent M o d e l X R r e c o r d i n g p o l a r o g r a p h . Samples for analysis w e r e r e m o v e d u p o n e q u i l i b r a t i o n of the system, the s o l i d c e n t r i f u g e d off a n d a n a l y t i c a l c o n c e n t r a tions d e t e r m i n e d f r o m c a l i b r a t i o n curves. I n contrast to M o r g a n a n d S t u m m (10) w h o r e p o r t f a i r l y r a p i d e q u i l i b r a t i o n , final a t t a i n m e n t of e q u i l i b r i u m at constant p H , for e x a m p l e , u p o n a d d i t i o n of m e t a l ions w a s often v e r y slow, i n some cases of the o r d e r of several h o u r s .
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
76
ADSORPTION F R O M
AQUEOUS SOLUTION
Results and Discussion T h e a d s o r p t i o n o f m e t a l ions i n s u c h systems c a n b e t r e a t e d i n o n e
Downloaded by NORTHWESTERN UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch007
of three w a y s , d e p e n d i n g o n w h e t h e r the a d s o r b i n g i o n is ( a ) P o t e n t i a l d e t e r m i n i n g — i . e . , t h a t conjugate p a i r o f ions w h i c h define the t o t a l d o u b l e l a y e r p o t e n t i a l , o r ( b ) S u r f a c e i n a c t i v e — i . e . , a n i o n w h i c h does n o t enter t h e S t e r n l a y e r a n d w h i c h cannot reverse t h e p o t e n t i a l o f t h e outer H e l m h o l t z p l a n e o f the e l e c t r i c a l d o u b l e layer, o r ( c ) Surface active, o r specifically adsorbing—i.e., a n i o n w h i c h c a n enter t h e S t e r n l a y e r a n d c a n reverse t h e sign o f t h e p o t e n t i a l o f t h e outer H e l m h o l t z p l a n e . P r e v i o u s studies (7, 10) o n manganese oxides h a v e established that H a n d O H " are p o t e n t i a l - d e t e r m i n i n g f o r the M n 0
2
+
series; f o r 10A. m a n g a
n i t e , electrophoresis a n d c o a g u l a t i o n measurements ( F i g u r e 2 ) b o t h gave a v a l u e o f p H 1.8 ± 0.5 f o r the z.p.c. ( 7 ) . T h e n a t u r e o f a d s o r p t i o n o f G r o u p I ions o n the oxide is i l l u s t r a t e d i n F i g u r e 1 i n w h i c h the a d s o r p t i o n isotherms for p o t a s s i u m ions o n M n 0
2
are g i v e n as a f u n c t i o n o f p H . S i m i l a r b e h a v i o r w a s o b s e r v e d f o r t h e a d s o r p t i o n o f s o d i u m ions. T h e p H d e p e n d e n c e of a d s o r p t i o n at l o w e r concentrations
suggests
that t h e a d s o r p t i o n occurs p r i m a r i l y as counterions i n t h e diffuse p a r t 10"
r
Mn(E) MANGANITE
o c S °
i
IO"
2
25 ° C
-
5 O UJ GO
or o . v> io" o
