Adsorption From Aqueous Solution - ACS Publications

10 Reaction of the Hydrated Proton with Active Carbon

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch010

VERNON

L . S N O E Y I N K and

WALTER

J.

WEBER,

JR.

Department of C i v i l Engineering, College of Engineering, The University of M i c h i g a n , A n n Arbor, M i c h .

Reactions

of the hydronium

have been investigated activity,

specific-anion

have been among action

ion with porous

in aqueous

concentration,

and

the major variables

verified

by activation

The results can be interpreted

of a reaction

of the

a surface

benzpyran

hydrogen

peroxide

ion with

a sorbed

sorption

group

to

and a surface benzopyrylium anion,

and partly

of the acid on the carbon

reas kcal.

partly in terms

ion and dissolved

(chromene)

dosage

Rates of

of —(2 to 3)

per mole-deg. with

carbon

by pore diffusion,

energies

hydronium

carbon

Hydronium-ion

studied.

have been found to be limited

partially

active

systems.

oxygen produce (carbonium)

in terms of

physical

surface.

T T v d r o n i u m i o n c o n c e n t r a t i o n has b e e n f o u n d to b e a significant factor i n the a d s o r p t i o n of

various organic compounds

s o l u t i o n b y a c t i v e c a r b o n (28).

from

aqueous

A p a r t i a l e x p l a n a t i o n of this effect is

afforded b y the fact that the i o n i z a t i o n — a n d therefore the m o b i l i t y a n d a d s o r p t i v e p r o p e r t i e s — o f m a n y o r g a n i c m o l e c u l e s is affected b y p H . H o w e v e r , p H changes h a v e b e e n f o u n d to affect t h e u p t a k e of c e r t a i n o r g a n i c m o l e c u l e s u n d e r c i r c u m s t a n c e s i n w h i c h these changes w o u l d not h a v e a significant effect o n the i o n i c character of t h e a d s o r b i n g species. W e b e r a n d M o r r i s (28)

h a v e f o u n d i n c r e a s e d rates of a d s o r p t i o n

w i t h d e c r e a s i n g p H for s u l f o n a t e d a l k y l b e n z e n e s i n p H regions r e m o v e d f r o m the p K r a n g e for these c o m p o u n d s .

far

T h e enhanced adsorp

t i o n rates h a v e b e e n a t t r i b u t e d to p a r t i a l n e u t r a l i z a t i o n of t h e a c t i v e carbon's n e g a t i v e surface charge, thus r e d u c i n g resistance to p o r e trans port. Studies o n rates of u p t a k e of v a r i o u s s u b s t i t u t e d phenols also h a v e i n d i c a t e d that the p H effect is m o r e t h a n c a n b e e x p l a i n e d i n terms of s i m p l e v a r i a t i o n s i n sorbate species

(II).

112

In Adsorption From Aqueous Solution; Weber, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

10.

SNOEYINK A N D W E B E R

JR.

Hydrated

113

Proton

S e v e r a l studies h a v e b e e n d i r e c t e d t o w a r d e x a m i n a t i o n of t h e i n t e r a c t i o n of acids a n d bases w i t h active carbons

( I , 8, 10, 17, 18, 1 9 ) .

B o e h m ( 3 ) , G a r t e n a n d W e i s s ( 9 ) , a n d S n o e y i n k a n d W e b e r (21)

have

presented r e v i e w s o n t h e subject.

have

Garten and Weiss

(8, 9, 10)

s h o w n that a c i d a n d a l k a l i s o r p t i o n c a n b e r e l a t e d to surface f u n c t i o n a l groups w h i c h f o r m d u r i n g t h e p r e p a r a t i o n of t h e c a r b o n . A l k a l i s o r p t i o n occurs p r i n c i p a l l y o n carbons a c t i v a t e d at temperatures near 4 0 0 ° C , a n d is a t t r i b u t e d to t h e presence of p h e n o l i c a n d lactone f u n c t i o n a l groups


o n t h e c a r b o n surface. C a r b o n s w h i c h sorb a c i d u s u a l l y are a c t i v a t e d at temperatures near 1 0 0 0 ° C ; t h e a c i d r e a c t i o n i n this case is a s s u m e d to take p l a c e w i t h c h r o m e n e ( b e n z p y r a n ) structures o n t h e surface. T h e studies r e p o r t e d i n this p a p e r h a v e f o c u s e d o n m o r e c o m p l e t e e l u c i d a t i o n of t h e n a t u r e of t h e i n t e r a c t i o n b e t w e e n t h e h y d r o n i u m i o n a n d a c t i v e c a r b o n . B o t h rate a n d extent of r e a c t i o n h a v e b e e n s t u d i e d as a f u n c t i o n of several v a r i a b l e s to o b t a i n d a t a w h i c h u l t i m a t e l y s h o u l d c o n t r i b u t e to a m e a n i n g f u l i n t e r p r e t a t i o n of p H effects o n a d s o r p t i o n of o r g a n i c solutes b y active c a r b o n . Experimental C a r b o n s . T h e a c t i v e c a r b o n u s e d for t h e m a j o r i t y of the experiments i n this s t u d y w a s a g r a n u l a r , c o m m e r c i a l c o c o n u t - s h e l l c a r b o n , c a r e f u l l y s i e v e d to a size range w h i c h i n c l u d e d discrete p a r t i c l e s p a s s i n g a U . S . S t a n d a r d Sieve N o . 50 a n d b e i n g r e t a i n e d o n a N o . 60 sieve; t h e m e a n p a r t i c l e d i a m e t e r f o r this size r a n g e is 273 m i c r o n s . A f t e r s i e v i n g , t h e c a r b o n w a s w a s h e d t h o r o u g h l y w i t h d i s t i l l e d w a t e r to r e m o v e dust arid fines, a n d t h e n d r i e d to a constant w e i g h t at 105 ° C . T h e i n o r g a n i c c o n tent w a s 0 . 7 % b y w e i g h t . O n e of t h e p r i m a r y reasons f o r c h o o s i n g this p a r t i c u l a r c a r b o n w a s its resistance to a t t r i t i o n i n t h e r a p i d l y - s t i r r e d e x p e r i m e n t a l reactors. A coal-base c a r b o n , p r e p a r e d i n t h e same f a s h i o n a n d of t h e same m e a n p a r t i c l e size, w a s e m p l o y e d i n several experiments. T h e m a x i m u m ash content f o r this m a t e r i a l , as r e p o r t e d b y t h e m a n u f a c t u r e r , w a s 8 % (20). A p o r e size d i s t r i b u t i o n w a s n o t a v a i l a b l e f o r t h e c o c o n u t - s h e l l c a r b o n , b u t , a g a i n a c c o r d i n g to t h e m a n u f a c t u r e r , a p p r o x i m a t e l y 5 5 % of the i n t r a p a r t i c l e v o l u m e of t h e coal-base c a r b o n w a s c o m p r i s e d of d i a m e ters b e t w e e n 15 a n d 20 A . (20). T h e coal-base c a r b o n w a s d e s i g n e d primarily for adsorption from solution, while the coconut-shell carbon w a s i n t e n d e d p r i m a r i l y f o r a p p l i c a t i o n i n gaseous systems ( 6 ) . E x p e r i m e n t a l Systems. R a t e - o f - r e a c t i o n studies w e r e c a r r i e d out u t i l i z i n g b o t h finite a n d infinite b a t h t e c h n i q u e s . T e s t solutions w e r e p r e p a r e d at t h e d e s i r e d i o n i c strength, t e m p e r a t u r e , a n d i n i t i a l p H . T h e s e solutions w e r e s t i r r e d r a p i d l y w i t h a m o t o r - d r i v e n p o l y e t h y l e n e - c o a t e d s t i r r i n g b l a d e . F o r e a c h test, a c a r e f u l l y m e a s u r e d q u a n t i t y of c a r b o n w a s a d d e d i n t h e d r y f o r m . T h e finite b a t h t e c h n i q u e consisted o f r e c o r d i n g p H values as a f u n c t i o n of t i m e after c a r b o n a d d i t i o n . A l l



114

ADSORPTION F R O M

AQUEOUS SOLUTION

finite b a t h p H measurements w e r e m a d e w i t h a C o r n i n g M o d e l 12, expanded-scale p H meter. T h e infinite b a t h t e c h n i q u e consisted of m a i n t a i n i n g a constant p H t h r o u g h o u t the r e a c t i o n w i t h a Sargent r e c o r d i n g p H Stat. A s the r e a c t i o n p r o c e e d e d , the p H Stat c o n t i n u o u s l y a d d e d a n d r e c o r d e d the q u a n t i t y of s t a n d a r d a c i d ( 0 . 1 N i n this case) n e e d e d to m a i n t a i n a constant p H . S t i r r i n g speeds i n a l l cases w e r e i n excess of the m i n i m u m r e q u i r e d to k e e p the c a r b o n i n suspension; separate experiments i n d i c a t e d i n d e p e n d e n c e of s o r p t i o n rate o n s t i r r i n g speeds greater t h a n this m i n i m u m . E q u i l i b r i u m studies w e r e p e r f o r m e d to d e t e r m i n e the extent of the h y d r o n i u m i o n - a c t i v e c a r b o n r e a c t i o n . T h e s e studies w e r e c a r r i e d out i n r e a c t i o n vessels c o n t a i n i n g 1.5 liters of d i s t i l l e d w a t e r a d j u s t e d to the d e s i r e d p H a n d i o n i c strength. T e m p e r a t u r e c o n t r o l was p r o v i d e d b y i m m e r s i n g the reactor i n a w a t e r b a t h , except for tests at r o o m t e m p e r a ture ( 2 5 ° ± 3 ° C ) . A t the start of e a c h e x p e r i m e n t a k n o w n q u a n t i t y of c a r b o n w a s a d d e d to the test s o l u t i o n . S t i r r i n g w a s a g a i n a c c o m p l i s h e d w i t h a m o t o r - d r i v e n p o l y e t h y l e n e s t i r r i n g b l a d e at s p e e d sufficient to k e e p the c a r b o n i n suspension at a l l times. A f t e r t w o to three days ( e x p e r i m e n t a l l y d e t e r m i n e d as the t i m e r e q u i r e d for t h e system to c o m e to e q u i l i b r i u m ) the p H w a s m e a s u r e d a n d r e c o r d e d , a n d a n a d d i t i o n a l q u a n t i t y of s t a n d a r d a c i d w a s a d d e d ; t w o to three days later the e q u i l i b r i u m p H w a s a g a i n m e a s u r e d , a n d another q u a n t i t y of a c i d a d d e d . T h i s p r o c e d u r e was r e p e a t e d several times, u s u a l l y over a p e r i o d of f r o m t w o to f o u r w e e k s , to p r o v i d e a series of a d s o r p t i o n capacities for decreas i n g e q u i l i b r i u m p H values. D i s t i l l e d w a t e r w a s a d d e d to the solutions p e r i o d i c a l l y to correct the v o l u m e f o r e v a p o r a t i o n . W h e n e v a p o r a t i o n w a s s m a l l , i t w a s necessary to correct the d a t a for i n c r e a s e d v o l u m e o w i n g to a d d i t i o n of a c i d . T h e d a t a o b t a i n e d c o u l d t h e n b e r e d u c e d to moles of a c i d r e a c t e d p e r g r a m of c a r b o n at a g i v e n h y d r o n i u m - i o n a c t i v i t y , u s i n g the e x t e n d e d D e b y e - H i i c k e l l a w for c a l c u l a t i n g a c t i v i t y coefficients. T h e a c t i v i t y coefficient for t h e 1 M N a C l s o l u t i o n , y = 0.75, w a s o b t a i n e d f r o m H a r n e d a n d O w e n (12). F r e u n d l i c h parameters w e r e o b t a i n e d f r o m l o g - l o g plots of the e x p e r i m e n t a l d a t a . E x c e p t w h e n o t h e r w i s e specified, t h e d a t a r e p o r t e d i n this p a p e r h a v e b e e n o b t a i n e d f r o m experiments at 2 5 ° C . Ash Content Analysis. A s h content c a n be a n i m p o r t a n t factor i n d e t e r m i n i n g the a d s o r p t i v e b e h a v i o r of a n a c t i v e c a r b o n . B l a c k b u r n a n d K i p l i n g ( 2 ) h a v e d e m o n s t r a t e d some of the effects of ash content o n the a d s o r p t i o n process. T o assess the effects of ash o n the i n t e r a c t i o n of strong a c i d w i t h a c t i v e c a r b o n , separate q u a n t i t i e s of the e x p e r i m e n t a l coconut carbon were washed w i t h 1 + 1 hydrochloric acid and w i t h 1 + 1 g l a c i a l acetic a c i d to r e d u c e the ash content of the c a r b o n . T h i r t y - to f o r t y - g r a m samples of c a r b o n w e r e s h a k e n w i t h the acids for a b o u t five days. T h e carbons w e r e t h e n w a s h e d c o n t i n u o u s l y w i t h d i s t i l l e d w a t e r for a p e r i o d of three m o n t h s u n t i l the c a r b o n c o u l d b e c o n t a c t e d w i t h d i s t i l l e d w a t e r for a f e w days w i t h o u t s i g n i f i c a n t l y r e d u c i n g the p H of the w a t e r . T h e carbons w e r e t h e n d r i e d at 1 0 5 ° C . to a constant w e i g h t . T h e ash content w a s m e a s u r e d b y b u r n i n g a k n o w n w e i g h t of the c a r b o n at 7 0 0 ° C . a n d w e i g h i n g the residue. T h e ash content of t h e c a r b o n w a s h e d w i t h acetic a c i d w a s r e d u c e d f r o m 0 . 7 % to 0 . 6 % , w h i l e t h a t w a s h e d w i t h h y d r o c h l o r i c a c i d w a s r e d u c e d f r o m 0 . 7 % to 0 . 3 % . A d s o r p -


10.


JR.

Hydrated

115

Proton

tion studies were then performed on the treated and untreated carbon for purposes of comparison. Stoichiometry Studies. T h e H C l - N a C l system was studied to determine if CI" ion was removed stoichiometrically with H O as the acid sorption reaction occurred. T w o 1-liter solutions were prepared at P C H — 3.00 and 1 0 " M N a C l ; two 1-liter solutions at p C = 3.00 and 2 X 10" M N a C l ; and two 1-liter solutions at p C — 2.70 and 1 0 " M N a C l . A five-gram quantity of coconut-shell carbon was added to each of the pC — 3.00 solutions, and a ten-gram quantity to each of the p C = 2.70 solutions. After one day of stirring, the residual H 0 concentration was measured with a p H meter and corrected for activity, and the residual CI" ion concentration was determined by means of the Mercuric Nitrate Method (24). T h e percent stoichiometry was then calculated from the data obtained. s

+

+

3

3

H

H

H

+

+

3

+

H


3

Results and The

+

+

Discussion

rate-limiting step for the hydronium ion-active carbon reaction

appears to be intraparticle transport.

F r o m a phenomenological point

of view, both the hydronium ion and the conjugate anion of the acid added are removed stoichiometrically. T h e studies made on the H C l N a C l systems described above show that the ratio of CI" ion removed to H O s

+

ion removed is in the range of 0.93:1 to 1:1.

This corresponds to

similar findings by Carr et al. (4) and Miller (16).

T h e data indicates

that the electroneutrality requirement for sorption is satisfied in this reaction. Since the hydronium ion and anion are removed from solution stoichiometrically, they would also be expected to diffuse through the pore in pairs with the anion limiting the rate of diffusion. T h e diffusion coefficients for different acids, calculated on the basis that intraparticle transport is rate-limiting, are on the order of those expected for the anion, thus giving support to this assumption.

Helfferich (13)

states

that film diffusion, the other likely possibility for being the rate-limiting step, will control only under extreme conditions.

A study of sorption

rate vs. stirring speed showed no increase in rate for stirring speeds above that required to keep the carbon in suspension.

Other evidence

in support of intraparticle transport as the rate controlling mechanism is the fact that the experimental data are described well by a diffusion model, as will be illustrated shortly. A n activation energy of —1.8 to —2.5 kcal./mole-°K., also discussed in more detail in a later section of this paper, falls in the range expected for a diffusion-controlled process (27). Diffusion Model. Assuming that pore diffusion is rate limiting, a diffusion model based on Fick's second law can be utilized for calculation of diffusion coefficients from the experimental data.

T h e model must


116

ADSORPTION F R O M

AQUEOUS

SOLUTION

take a c c o u n t of t h e s i m u l t a n e o u s d i f f u s i o n - r e a c t i o n process. I f t h e s o r p t i o n r e a c t i o n is n o t t a k e n i n t o a c c o u n t , the c a l c u l a t e d d i f f u s i o n coefficients w i l l d e v i a t e c o n s i d e r a b l y f r o m the a c t u a l values. S u c h a m o d e l has b e e n d e v e l o p e d b y C r a n k (7)—and

utilized later b y W e b e r a n d R u m e r

(29)

— b y m o d i f y i n g F i c k ' s s e c o n d l a w to i n c l u d e a t e r m w h i c h accounts for


n o n - l i n e a r s o r p t i o n . T h e g e n e r a l f o r m is

E q u a t i o n 1 is g i v e n i n s p h e r i c a l c o o r d i n a t e s , thus a s s u m i n g a s p h e r i c a l shape for t h e c a r b o n p a r t i c l e , a n a s s u m p t i o n w h i c h a c c o r d s r e a s o n a b l y w e l l w i t h m i c r o s c o p i c observations of the g e o m e t r y of p a r t i c l e s of t h e experimental carbon. solution; t

9

I n E q u a t i o n 1, C represents the H 0 3

activity i n

t i m e ; r , the r a d i a l d i s t a n c e f r o m the p a r t i c l e center; D , t h e

d i f f u s i o n coefficient; a n d S, t h e H 0 3

carbon.

+

+

c o n c e n t r a t i o n at t h e surface of the

F o r the present e x p e r i m e n t s , the e q u i l i b r i u m r e l a t i o n s h i p b e

t w e e n S a n d C is d e s c r i b e d i n terms of t h e F r e u n d l i c h expression 1.0

S = RC ,N< N

(2)

E q u a t i o n 1 is subject to the b o u n d a r y c o n d i t i o n s C = 0

t= 0

for

0 V

Sp)

r

St

w i t h the boundary conditions,


c= 0

^

< 1

- ' c

T

= 0

for

0

cm. /sec, 2

w h i c h is n e a r l y e q u a l t o that for H C l i n b u l k s o l u t i o n . A l t h o u g h m a n y other factors m i g h t c o n t r i b u t e t o the large difference b e t w e e n the t w o carbons, r e l a t i v e p o r e size is p r o b a b l y a m a j o r one. O t h e r factors w h i c h m a y also b e responsible t o some extent for the s m a l l coefficient o b t a i n e d for the c o c o n u t c a r b o n i n c l u d e the a p p r o x i m a t i o n o f s p h e r i c a l p a r t i c l e geometry,

the assumption of a n isotropic

m e d i u m , a n d the a s s u m p t i o n o f a r a d i a l diffusion p a t h

(13).

Specific c h e m i c a l i n t e r a c t i o n b e t w e e n the c h l o r i d e i o n a n d m e t a l ions present a t the p o r e surfaces has also b e e n c o n s i d e r e d as a possible factor c o n t r i b u t i n g t o r e t a r d a t i o n of H C l diffusion.

T o evaluate this

p o s s i b i l i t y , one p o r t i o n o f the c o c o n u t c a r b o n w a s w a s h e d w i t h acetic a c i d t o r e d u c e its ash content f r o m 0 . 7 %

t o 0 . 6 % , a n d another p o r t i o n

w i t h h y d r o c h l o r i c a c i d to r e d u c e its ash content f r o m 0 . 7 % to 0 . 3 % . T h e finite b a t h t e c h n i q u e was u s e d t o s t u d y these t w o carbons i n otherwise i d e n t i c a l systems c o n s i s t i n g o f 1 0 ~ M N a C l , 1.5 grams c a r b o n p e r liter, 2

an i n i t i a l p H of 3.50 ( H C l ) , at a t e m p e r a t u r e of 2 5 ° C . T h e c o r r e s p o n d i n g isotherms s h o w n o significant difference b e t w e e n these carbons a n d the



122

ADSORPTION F R O M

Figure 5.

AQUEOUS SOLUTION

The effect of ash removal on finite-bath adsorption rate

Data are presented for adsorption of HCl on acid-treated coconut carbon. Data for two separate runs are shown for each of the acid-treated carbons. The calculated curve has been derived from the data for adsorption on untreated coconut carbon in the same type of system

untreated coconut carbon.

T h e e x p e r i m e n t a l rate d a t a a n d c a l c u l a t e d

curves for the t w o systems are p l o t t e d i n F i g u r e 5. f o u n d to be no essential difference b e t w e e n

B e c a u s e there w a s

the c a l c u l a t e d curves

for

the a c i d t r e a t e d a n d the u n t r e a t e d c a r b o n , o n l y one c u r v e is s h o w n .

The

e x p e r i m e n t a l rate d a t a for the a c e t i c - a c i d - t r e a t e d c a r b o n are not

de

s c r i b e d as w e l l as are those for the h y d r o c h l o r i c - a c i d - t r e a t e d c a r b o n , n o r as w e l l as those for the u n t r e a t e d c a r b o n fitted b y the c a l c u l a t e d c u r v e s h o w n i n F i g u r e 1. T h e diffusion coefficient d e r i v e d f r o m the c u r v e of best fit for the a c e t i c - a c i d - w a s h e d c a r b o n is 9 X 10~ c m . - / s e c , a n d that 7

for the h y d r o c h l o r i c - a c i d - w a s h e d c a r b o n is 9.5 compares

w i t h 6.75

X

10"

7

X

10"

7

c m . / s e c ; this 2

c m . / s e c . for the u n t r e a t e d c a r b o n . 2

The

increase i n the d i f f u s i o n coefficient for the t r e a t e d c a r b o n over that for the u n t r e a t e d c a r b o n i n d i c a t e s that i n t e r a c t i o n of the H C l w i t h the i n o r g a n i c content of the c a r b o n c o u l d b e responsible to some extent for r e t a r d i n g diffusion.

A n a l y s i s has s h o w n that the ash is a p p r o x i m a t e l y

5 0 % i r o n , w h i c h i n i o n i c f o r m tends to f o r m complexes w i t h the C I " i o n . D i f f u s i o n coefficients for different i n i t i a l p H c a l c u l a t e d f r o m o b t a i n e d b y the finite b a t h t e c h n i q u e c o m p a r e r e a s o n a b l y w e l l w i t h d e t e r m i n e d b y the infinite b a t h t e c h n i q u e , as c a n b e seen f r o m the i n T a b l e I. T h e c a l c u l a t e d rate curves fit the e x p e r i m e n t a l d a t a w e l l for i n i t i a l p H values of 3.50 a n d 3.70, b u t for a n i n i t i a l p H of


data those data very 4.00,

10.

SNOEYINK A N D W E B E R J R .

finite-bath

Hydrated

123

Proton

case, t h e fit is n o t as g o o d . F o r t h e latter case, t h e r a n g e o f

the d i f f u s i o n coefficient is ( 6 — 14) X 10~ c m . V s e c . f o r a l l of the e x p e r i 7

m e n t a l d a t a to f a l l o n t h e c a l c u l a t e d c u r v e . A n o t h e r effect s h o w n b y t h e d a t a is that t h e d i f f u s i o n coefficient increases w i t h i n c r e a s i n g i n i t i a l p H , i n d i c a t i n g that t h e coefficient is a f u n c t i o n of a c t i v i t y . T h i s is c o n t r a r y to t h e a s s u m p t i o n o f a constant diffusion coefficient b u i l t into t h e rate m o d e l . H o w e v e r , except for t h e case of a n i n i t i a l p H of 4.0, finite b a t h case, t h e effect is a p p a r e n t l y n o t v e r y great f o r t h e c o n c e n t r a t i o n change


o c c u r r i n g i n a n y o f t h e rate studies d e p i c t e d i n F i g u r e 2; otherwise t h e e x p e r i m e n t a l d a t a w o u l d n o t b e d e s c r i b e d as w e l l b y t h e c a l c u l a t e d curves.

T h e decrease

i n diffusion

coefficient

a c t i v i t y is p r o b a b l y c a u s e d b y h y d r a t i o n effects. of h y d r a t e d H O A

+

with

increasing

H O M

+

T h e increased number

a n d C I " ions i n t h e pore decreases t h e a m o u n t of free

solvent a v a i l a b l e f o r diffusion of t h e ions, thus d e c r e a s i n g effective

pore

size a n d i n c r e a s i n g r e t a r d a t i o n effects caused b y i n t e r a c t i o n o f t h e diffus ing

species w i t h t h e c a r b o n f r a m e w o r k .

A decrease

i n t h e diffusion

coefficient is t h e r e b y b r o u g h t about ( 1 3 ) . S a l t Effects.

T h e H C l - N a C l system has b e e n s t u d i e d f u r t h e r to

d e t e r m i n e t h e effect of N a C l c o n c e n t r a t i o n o n t h e a c i d - c a r b o n r e a c t i o n . T h e c a r b o n dosage w a s v a r i e d f r o m 0.33 t o 2.0 grams o f 2 7 3 - m i c r o n

.6

n u

-

I 0

I 4

I 8

I 12

I 16

I 20

I 24

I 28

I 32

I

H 0 ACTIVITY x 10 3

Figure

6.

+

s

The effect of salt concentration on adsorption hydrochloric acid

capacity for

Isotherms are presented for adsorption of hydrochloric acid on coconut carbon in the presence of different concentrations of NaCl. The solid lines represent the corresponding calculated Freundlich isotherms, respective values for the parameters of which are given in Table II



124

ADSORPTION F R O M

Figure 7.

Finite-bath

rate of adsorption for hydrochloric salt concentration

AQUEOUS SOLUTION

acid at higher

The concentration of NaCl in the experiment represented by these data was two times that in the experiment for which rate data are given in Figure 2 at pH = 3.5. All other factors were the same c o c o n u t c a r b o n p e r liter for the e q u i l i b r i u m studies, a n d the t e m p e r a t u r e w a s 25 ° C .

N a C l c o n c e n t r a t i o n was s t u d i e d over a range f r o m 5 X

10~

3

to 1 . 0 M . T h e effect of salt c o n c e n t r a t i o n o n the e q u i l i b r i u m p o s i t i o n of the r e a c t i o n is s h o w n i n F i g u r e s 7 a n d 9. T y p i c a l rate d a t a , a l o n g w i t h the c o r r e s p o n d i n g c a l c u l a t e d c u r v e are s h o w n i n F i g u r e 7 for 2 X 1 0 " M 2

c o n c e n t r a t i o n of N a C l .

T h e finite b a t h m e t h o d w a s u s e d for these rate

studies. A s u m m a r y of the results is g i v e n i n T a b l e I I . It s h o u l d b e n o t e d that the range of the F r e u n d l i c h parameters u s e d for the rate c u r v e c a l c u l a t i o n s p r o d u c e d v e r y slight changes i n the shapes of the r e s u l t i n g curves. Table II.

The Effect of N a C l Concentration on Diffusion of H C l Initial pH = 3.50, Temperature = 25°C.

Freundlich Parameters R, X 10* 5.6 6.5 6.9 7.1 7.6

. „ . Concentration (moles/liter) X 10 N

N 0.125 0.127 0.133 0.120 0.105

a

C

L

2

0.5 1.0 2.0 10.0 100.0

_ D, (cm*/sec.) X 10

7

4.0 6.8 7.5 15.1 68.0


10.

SNOEYINK A N D W E B E R J R .

Hydrated

125

Proton

S i m i l a r studies h a v e b e e n c a r r i e d o u t o n t h e H C 1 0 - N a C l 0 4

4

system.

A l l c o n d i t i o n s w e r e t h e same as those for t h e H C l - N a C l tests, except that t h e c a r b o n dosage w a s 1.33 g r a m s / l i t e r f o r t h e e q u i l i b r i u m s t u d y a n d the c o n c e n t r a t i o n of N a C 1 0 r a n g e d f r o m 5 X 10" to 1 0 M . T h e effect 3

4

_ 1

o n t h e extent of r e a c t i o n is s h o w n i n F i g u r e s 8 a n d 9, a n d t y p i c a l rate data along w i t h the corresponding calculated curve for a concentration of 1 0 " M N a C 1 0 are g i v e n i n F i g u r e 10. T h e results are s u m m a r i z e d i n 2

4


Table III. Table III.

The Effect of N a C l 0

4

Concentration on Diffusion of H C l 0

4

Initial pH = 3.50, Temperature = 25°C. Freundlich Parameters R, X 10* 7.6 8.2 8.4 10.0

N 0.1050 0.1030 0.1000 0.1065

„ NaClO^ Concentration (moles/liter) X 10

2

D, (cm. /sec.) X 10" 2

0.5 1.0 2.0 10.0

4.7 5.3 6.0 10.0

T h e e q u i l i b r i u m studies w e r e p e r f o r m e d

w i t h o n l y s o d i u m salts.

R a t e studies, h o w e v e r , w e r e p e r f o r m e d w i t h L i C l a n d K C 1 as w e l l as w i t h N a C l , a n d n o significant difference w a s f o u n d . T h e r e is a m a r k e d increase i n the q u a n t i t y of a c i d s o r b e d

with

i n c r e a s i n g salt c o n c e n t r a t i o n f o r b o t h the H C l - N a C l a n d t h e H C 1 0 4

NaC10

4

systems.

A two-hundred-fold

increase i n N a C l

concentration

p r o d u c e s a 6 0 % increase i n H C l s o r p t i o n at p H 3.40, w h i l e a t w e n t y - f o l d increase i n N a C 1 0 c o n c e n t r a t i o n causes a 3 0 % increase i n H C 1 0 4

4

sorp

t i o n at t h e same p H . T h i s o b s e r v a t i o n is consistent w i t h a p h y s i c a l s o r p t i o n m o d e l . E l e c t r o p h o r e t i c m o b i l i t y measurements h a v e s h o w n t h a t the a c t i v e c a r b o n has a negative surface p o t e n t i a l . I t is possible, t h e r e fore, that t h e p r o t o n is p r i m a r i l y a d s o r b e d w h i l e t h e a n i o n is s e c o n d a r i l y a d s o r b e d i n t h e d o u b l e layer. I f t h e p r o t o n is a d s o r b e d r e a d i l y b u t h e l d b a c k because t h e a n i o n is n o t easily t a k e n i n t o t h e d o u b l e l a y e r , t h e n an increase i n salt c o n c e n t r a t i o n w o u l d h a v e the effect of i n c r e a s i n g t h e a n i o n pressure, a n d m o r e a c i d w o u l d t e n d to b e a d s o r b e d .

T h e effect is

consistent w i t h that n o t e d b y S t e e n b e r g ( 9 ) , a n d P a r k s a n d B a r t l e t t ( 1 9 ) . I n d e e d , B o e h m ( 3 ) f o u n d e v i d e n c e that p h y s i c a l s o r p t i o n of a c i d took p l a c e o n t h e b a s a l planes of t h e m i c r o c r y s t a l l i t e . O t h e r s o r p t i o n m e c h a nisms c a n n o t b e r u l e d out, h o w e v e r , because r e v e r s i b l e c h e m i s o r p t i o n c o u l d s h o w a s i m i l a r effect w i t h i n a g i v e n r a n g e of salt concentrations. A l s o , pores w h i c h m i g h t b e inaccessible at l o w salt concentrations c o u l d b e c o m e of i m p o r t a n c e at the h i g h e r a n i o n pressures.



126

ADSORPTION F R O M AQUEOUS SOLUTION

12

32

24

16

H 0 ACTIVITY x 10 3

Figure 8.

+

s

The effect of salt concentration on adsorption perchloric acid

capacity for

Isotherms are presented for adsorption of perchloric acid on coconut carbon in the presence of different concentrations of NaClO . The solid lines represent the corresponding calculated Freundlich isotherms, respective values for the parameters of which are given in Table III h

T h e increase i n t h e diffusion coefficient w i t h increased chloride concentration

for the H C l - N a C l

system

is especially n o t e w o r t h y .

This

effect c a n b e best e x p l a i n e d b y t h e fact that t h e d r i v i n g force f o r t h e r a t e - l i m i t i n g C I " i o n is increased, w i t h a r e s u l t i n g increase i n t h e H C l flux.

Since the effect of a n i o n c o n c e n t r a t i o n is n o t b u i l t into the m a t h e

m a t i c a l diffusion m o d e l , h i g h e r diffusion coefficients result. T h e reason for t h e effect b e i n g greater f o r t h e H C l - N a C l HC10 -Na00 4

4

system t h a n f o r t h e

system m a y b e r e l a t e d to relative i o n i c size a n d to the

fact that t h e diffusion coefficient f o r t h e H C l is m u c h f u r t h e r f r o m its b u l k s o l u t i o n v a l u e t h a n is that f o r H C 1 0 . 4

Because of h y d r a t i o n effects,

the C I " i o n is larger i n s o l u t i o n t h a n is t h e C 1 0 " i o n ( 5 ) . A s a result, 4

there are p r o b a b l y m a n y m o r e r e t a r d a t i o n effects o w i n g t o i o n i c size w h i c h c a n b e o v e r c o m e b y t h e increase i n C I " i o n a c t i v i t y . T h e fact that the diffusion coefficient f o r H C 1 0

4

is ten times as large as that f o r H C l

c o u l d also b e a t t r i b u t e d t o the r e l a t i v e sizes. T h e m u c h h i g h e r c a p a c i t y of the active c a r b o n f o r H C 1 0 H C l has b e e n o b s e r v e d

b y others f o r i o n exchange

4

than for

resins ( 5 ) . T h e

o b s e r v a t i o n is i n k e e p i n g w i t h the s m a l l e r h y d r a t e d r a d i u s of t h e C 1 0 " 4

i o n as n o t e d above. C h u et al. ( 5 ) have c l a i m e d that a n a d d i t i o n a l effect derives f r o m t h e i n a b i l i t y of t h e C 1 0 " i o n to orient s u r r o u n d i n g w a t e r 4

molecules i n b u l k s o l u t i o n as effectively as does t h e C I " i o n . T h e w a t e r



10.


JR.

Hydrated

127

Proton

-LOG SALT CONCENTRATION

Figure 9.

Variation of adsorption salt concentration

capacity

with

Adsorption capacities for different salt concentrations are shown for the HCIO

Adsorption From Aqueous Solution - ACS Publications

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