My appreciation is extended to the water Resources. Center at Texas Tech ...... compartment and check the zero transmissivity adjustment. Adjust if necessary.
KINETICS AND MECHANISMS OF ADSORPTION OF HEAVY METAL IONS ON ACTIVATED CARBON by CHIEH-CHIEN LIN, B.S. A THESIS IN CHEMICAL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OP SCIENCE IN CHEMICAL ENGINEERING Approved
Accepted
August, 1979
r^-j /' /
^
ACKNOV/LEDGMENTS To Dr. L. D. Clements, chairman of my committee, I express appreciation for his patience and guidance throughout this project.
Dr. W. H. Smith and Dr. S. R.
Beck, members of the committee, are acknowledged for their valuable suggestions and criticism of this work. My appreciation is extended to the water Resources Center at Texas Tech University for financial support during this study.
11
TABLE OF COI^TEI^TS ACOOWLEDGMEx^'TS
ii
LIST OF TABLES
vi
LIST OF FIGURES
ix
CHAPTER I. II.
INTRODUCTION
1
RSVIE'v^ OF I4ETALS ADSORPTION LITERATURE
3
Metal Ion Removal Studies Metal Adsorption Mechanisms III.
THEORY OF ADSORPTION Carbon Surface Chemistry
3 11 15 15
Systemic Factors Affecting Rates of Adsorption
16
(a) Hydrogen Ion Concentration
16
(b)
17
Temperature
Kinetic Effect of the Nature and Concentration of Adsorbate (a)
Concentration of Adsorbate
(b) Molecular Size of Adsorbate Effects of Particle Size and Concentration of Carbon on Rates of Adsorption (a) Particle Size (b) Concentration of Carbon Kinetic Studies (a) Diffusion Control (Physical) Adsorption 1) Langmuir and BrunauerEmmet-Teller Models 2)
Freundlich Model iii
18 18 19 19 19 20 21 21 21 23
(b) IV.
Reaction Control (Chemical) Adsorption
EXPERIME'.;TAL S Y S T E M S
26
Adsorbents
26
Adsorbates
26
Procedure
26
Analytical Methods
27
(a)
Sample Preparation
28
(b)
Analysis Procedure
29
Calibration Curve Determination V.
23
EXPERII^IENTAL RESULTS Adsorption Isotherms for Cadmium (a)
Experimental Procedure
32 32
At 35*^C
32
2)
At 60^C
32 37
Rates of Adsorption 1) 2)
(b)
32
1)
Kinetic Data (a)
29
37
The First-Order Reversible Model
41
The Second-Order Reversible Model
43
Activation Enersj
52
Determine the Adsorption Capacities for Candidate Complexing Agents
53
(a)
Experimental Procedure
5;
(b)
Adsorption Da^a
54
Adsorption Mechanism Investigation (a) Mechanism I Studies iv
55 55
(b) Mechanism II Studies Optimum Tartrate Concentration
56 58
(a) Preparation of Solutions
38
(b)
Adsorption Data
58
(c)
Intermediate Complex Theory
59
Adsorption Isotherms of Cadmium with the Aid of Tartrate
VI.
62
(a)
At 35°C
62
(b)
At 60^C
62
Freundlich Isotherms
69
The Cadmium-Tartrate-Carbon System
72
CONCLUSIONS AND RECOI-LMENLATIONS
74
Conclusions
74
Recommendations
74
LIST OF REFERENCES
76
V
LIST OF TABLES Table 1 2 3 4 5 6
7 8
9
10
11
12
Page Metals of High Carbon Adsorption Potential
5
Metals of Good Carbon Adsorption Potential
6
Elements of Fair-to-Good Carbon Adsorption Potential
7
Elements of Low or Unknown Carbon Adsorption Potential
8
Heavy Metals Removed by the Orange County V/ater District Pilot Plant
yj
Removals of Trace Elements in Sand Filtration, Activated Carbon, Cation Exchange, and Anion Exchange Cadmium Calibration Table Concentration vs % iransmittance The Adsorption Data of Cadmium Solution with Different Amounts of Carbon Loading at 35°C
12 31
33
The Adsorption Data of Cadmium Solution with Different Amounts of Carbon Loading at 60°C
35
The Values of X/M and C for Cadmium Adsorption at 35°C
37
The Values of X/M and C for Cadmium Adsorption at 60°C
38
The Calculated Data for the First-Order Reversible Model at 35^C
42
vi
Table 13
14 15
16
17 18 19
20
21
22
23
Page The Calculated Data for the First-Order Reversible Model at 60°C
43
The Forward Rate Constant for the First-Order Reversible Model
46
The Calculated Data for the Second-Order Reversible Model at 35°C
47
The Calculated Data for the Second-Order Reversible Model at 60^0
Ad
The Forward Rate Constant for the Second-Order Reversible Model
51
The Activation Energy for tne Second-Order Reversible Model The Adsorption Data for the Candidate Complexing Agents at 35^C
54
Comparison of Adsorption Data for Mechanism I and Mechanism II at 60°C
57
Adsorption Data for Different Amounts of Sodium Tartrate Contained in 1000 ml, 0.1 ppm Cadmium. Solution with 0.5 g of Activated Carbon at 60^0
59
The Adsorption Data of Cadmium Tartrate Solution with Different Amounts of Carbon Loading at 35^C
63
The Adsorption Data of Cadmium Tartrate Solution with Different Amounts of Carbon Loading at 60°C
65
Vll
53
Table 24
25
26
27
28
Pa^e o The Adsorption Data of Cadmium Solution with and without Tartrate at 35'^C
67
The Adsorption Data of Cadmium Solution with and without Tartrate at 60°C
68
The Values of X/M and G for the Cadmium-Tartrate-Carbon System at 35^C
69
The Values of X/M and C for z'ne Cadmium-Tartrate-Carbon System at 60^C
70
Percent of Original Cadmium Being Directly Adsorbed by Carbon
73
Vlll
LIST CF FIGURES Figure 1
Page Effect of Crushing Granular Activated Carbon On the N^-BET Surface Area
20
2
Cadmium Calibration Curve
30
3
Adsorption Isotherms for Cadmium at 35'^C
34
Adsorption Isotherms for Cadmium at 60°C
36
A Plot of X/M vs C for Cadmium Adsorption at 35°C
39
4 5
6
A Plot of X/M vs C for Cadmium Adsorption at 60°C
39
7
The Plot of -ln(l-(X/X^)) vs t at 35°C
44
8
The Plot of -ln(l-(X/X^)) vs t at 60°C
45
9
The Plot of ln((X^-(2Xg-1)X)/(X^-X)) vs t at 35^C
49
The Plot of ln((Xg-(2X^-l)X)/(ig^X)) at 60°C
50
Adsorption Data for Different Amounts of Sodium Tartrate at 60°C
61
Adsorption Isotherms for Cadmium Tartrate at 35°C
64
Adsorption Isotherms for Cadmium Tartrate at 60°C
6r
10 11
12
13
IX
Figure 14
15
Pa^e o
A Plot of X/M vs G for Cadmium Tartrate Adsorption at 35^C
71
A Plot of X/M vs C for Cadmium Tartrate Adsorption at 60^C
71
CHAPTER I INTRODUCTION Activated carbon's enormous capacity to adsorb and retain significant quantities of many organic compounds has led to its use as a purifying medium for everything from air in a space capsule to certain distilled spirits. As a result of this property, there has been a long and active interest in describing the kinetics and mechanisms of organic compound adsorption on activated carbons. The early studies of Phelps and Peters (l) showed how the adsorption of the lower fatty acids and simple aliphatic amines depends on the pH of the aqueous solution and the ionic dissociation constant of the acids and bases.
Later, Chedlin and Williams (2) found 1) the
adsorption of a number of amino acids, vitamins and related compounds by activated carbon fits Freundlich adsorption isotherms; and 2) the presence of aromatic nuclei are important factors in aqueous adsorption of organics by activated carbons. The merit of activated carbon for removal of organic compounds from water has been well documented in the literature, but the potential for adsorption of inorganic compounds has received little publicity in the waste treatment literature.
Studies in the field of metallurgy
have indicated reasonably good adsorption capacities for 1
many metallic compounds. Some of the recent publications (3,4) indicated that activated carbon adsorption was very effective in reducing the concentration of cadmium in wastewater.
However, the
extremely different results were also obtained (5). These findings indicate that certain trace elements can be removed along with organics, suspended solids, and major inorganic ions by the carbon adsorption process. The purpose of this study was to search for a complexing agent which could aid the adsorption of cadmium by activated carbon, to determine the optimum ratio of the complexing agent to cadmium, to find out the rate of adsorption of cadmium by activated carbon at two different temperatures, and to investigate the possible mechanism.
CHAPTER II REVIEW OF I^-IETALS ADSORPTION LITERATURE One of the first definitive studies of the equilibria and kinetics associated with organic adsorption on activated carbon was done by Weber and Morris (6,7). Significant results from their work include 1) for large carbon particles, the rate of adsorption is limited oy intraparticle diffusion; 2) the rate of adsorption is proportional to the square root of concentration; 3) there is a significant correlation between increasing molecular size and decreasing adsorption rate; and 4) equilibrium adsorption from dilute solution follows the Langmuir adsorption theory.
Metal Ion Removal Studies Numerous methods have been proposed at various times for extraction of microquantities of elements from solutions.
Methods involving isomorphous carriers have
been widely used in radiochemistry (8). Starik et al. (9) isolated microquantities of uranium and other elements from aqueous solutions by coprecipitation of barium sulfate with ferric hydroxide.
Other methods proposed for
the purpose included the method of "sinking particles" (10), coprecipitation with the sulfide of cadmium or bismuth (10,11), contacted deposition on magnesium, zinc,
4 or iron powders (12,13), extraction with solutions of complex-formers in immiscible solvents (10,12,14), Sigworth and Smith (5) had reported tne result of many year's accumulation of data from published literature, studies in the research and technical service laboratories of Westvaco Carbon Department^, and a 2-yr research study conducted by the Colorado School of Mines Research Foundation. Some of the metals having high adsorption potential on carbon are shown in Table 1,
Table 2 covers several
metals of good adsorption potential.
Table 3 lists
several metals of fair to good adsorption potential. Table 4 lists several elements of low or unknown adsorption potential.
The previous data indicate a very broad
applicability of activated carbon in removing trace quantities of widely different chemical species.
This
indicates that several mechanisms are probably involved. It should be remembered that carbon will physically adsorb on their tremendous internal surfaces molecular compounds such as acids, complex ions, high molecular weight polymers, or other nonpolar species. By virtue of a relatively small number of oxygen complexes and other functional groups fixed in the carbon surfaces, a limited ion exchange action can take place.
Thus, the behavier
^Formerly Industrial Chemical Sales, West Virginia Pulp & Paper Co.
5
0
d
s o
o
CQ
o U Xi O ^ « H Cd
o
rH Cd >> •H ^ -P
oj Cd -p >
o o S
cd
Cd r-H = H
O Q) O > •H
< +^
o
eq &H
Td
0
o
•H
0 H X^
M
Cd ^ ^
u
^ •a (i> ?-i
o xi
>> rH
•H
•rH
bO
o
to
T i CQ
cd cl
o >>
H rH P
XH C! • H CQ Q) • o -P
o O
CQ Cd 0?
T3
cd -P
'J^ CQ
o o iiD
«h
xi
qO •H ^
t>>
u 0) >
c; a> > o
>i
TJ
^ 0 >
o o CJ3
^1
P
> 0)
d
o s;
P< O
in
0)
d
o s;
a o
o r—
^
Jd
•H >^ O -p
•H
O
BH
Q:;
o M
O CO
>> -p •H O •H
>> (H X 5 ^
X O
O ^
EH
o
Cd hO JQ - H •H rH 'J2
^
PM
O
B
O •H
xi
•H
s
s
•H CQ
H
O
CO •H
d •H &H
cq
VD 0) O d
\D
O
a CO
jo CO
CQ
> • H rH +> JD • H -H - P CQ >> d CQ
0 O Cd
VH
d
o ^ ^ O ;H ^ Cd
o
rH Cd > , •H ^ •P
d
Cd +* 0 ;i
P3 O ^ !^
cd
o
drH 0 Cd
d
O O
T3 0 O ^
+^ > ^ ^
CQ
>> rH •H CQ
^
(^ B 0
O
O
CQ rQ T J ^ 0
O U
CQ 0 T^ - P Cd rH •H rH«H O i^OrH
M Cd
0
top 35 0
ffl
CL> !13
^ 04
rO
-d EH O
JH
cd O
Q) :i
Pk
> a*
O
\X>
*
(D 0xiCQ
O
o cd ^
d
O
X
?H ?H O EH O - H 0 CQ
&.
> Ti X •H • cd 0
o rH
Cd O H
+J Cd
-P O
CQ rH .13 >> P4
O 0 H • H tlO'T^
xi d Cd
s
O O
cd cd 0 CQ ^r; T^ H cd
^
nO O CJ
•P
O M SH
Cd CNJ
pq
o CO
p O =q
0 ^ tiO O 'H Cd r H cH Cd
O 0 C3 > •H -=XJ -P : 5 cd
cu o LA
o* o* LA
0
d
O •^
0
d
O
s;
> +j
C5
•H
+J
O •H
xi hO
o CO
JK!
O EH
bO
•rH
H CO
•H 3^
+^
X! > j
u 0 >
bO
rH rH CO
IS O
s -p
;H
>» ^H
0
0
pi
0
> r-i •H CO
o ?H
d
s
r-i
0
s
•H -p rH Cd X^ O
o
d O
o •H vD
0 O
d 0
rH O
s >> CO
t5D
x:
o o
0 0 Cd
7
II o
O Oi
d
o
CM 0 + !S> 0 -H ^ 'XJ •H
U Xi
o u i H cd
M EH '^
o
rH Cd >» •H ^ 4-> drH 0 Cd
+» > O
O
PM
S
0 H
O O
?H •H Cd
0
PH
Ti O O C5
Td O O O
Cd •H
s rA
Cd
>j
^cd
+
0
PH
-P ;i rQ
O
M EH
O
-p 0 O
?H Cd
CO Cd
O 0
o pq
pq O
C23 > •H
o
+^ •H
-p
O •rH
xj
I
EH I M O O
PM S 0
Pi 0
CO I
vi> •H CQ T 3 CQ 0
o
x; -H
o X 0
Pi
O H Q^ 0 rH •- P
i
u
cd rH Q*
-P -d oO
0 >
•H rH CO
c
;Q Pi
-p a xi o £lO O •H r-i CQ CO cd
rH 0 •H ^
QO
-P O O d
-H r-\
"^ ^
ro
O M EH PH
o CO
o pq
r-M
o
oq
rO
P4 P4
Xi Qi
o o CM
en o r—
c!> > •H 5 : Cd :s4->
a 04
0
P4
d
r-
0 ^
p*
PH
P4
0 T -
T—
0
0
d
d
0 :5
o
-p
-p ^ JaO •H rH 00
> +i •H O •H O SH
r^^ •rH
^
xi to •H x:
rH
•H 25
o CO EH '^
pq
pq
-d
5aO •H X!
X5 •H
tsO
xi to •H
x;
K
jd
a +3
d
?4
0
0
s
Pi Pi
0 H
0 0
a ps •H a t:i cd
0
pj •H r-i rH
0
d •H SJ
>> ^ 0 pq
g
a
PJ •H
to
•H rH
^ 0
0
S
'^ yx\
O U Cd rHEH Cd O 0
um
cd
4^ 0 ?H baO
d 0
T3
•H
d
0
^ >>
JH
rH 0 CO
r^
pi cd
pq
0
s
CQ
d
0
0
d cd
-p OQ
tiO
d cd
s;
to d d EH
VD 0 O ,-^ 0
O XJ
a CO
P3
o
d (S3
0 pq
cd
0
pq
CO
o
d
H
0 «H 0 Cd
and higher-valence ions can displace H"*", Na"*", Ca"*"^, and Other such ions to fix certain metals.
Third, carbon can
induce precipitation of a supersaturated solution by nucleation and can reduce the solubility of a metallic salt.
Colloidal suspensions also can be broken by
upsetting the surface-charge structure protecting the colloidal particles.
A layer of powdered or granular
carbon also will exert an excellent filtering action under proper conditions.
Last, commercial activated carbons
contain traces of reduced forms of iron and other metals, which can enter into metathetical reactions with metallic ions lower in the electromotive series, causing the heavy metal to be deposited on the surface.
Thus, one sees that
it is very difficult to predict in a given instance what will happen or to be sure what mechanism is responsible for metal removal.
An example may point out the value of
an possibility. While the results in Table 4 indicate a slight potential for cadmium adsorption, an extremely different set adsorption data for cadmium was reported by Argo and Culp (4). Their results, shown in Table 5, are for the Orange County Water District's wastewater reclamation pilot plant's efficiency for removing cadmium concentration found in the plant's influent ranged from a low of 0.011 ppm to a high of 0,022 ppm. Following treatment, effluent concentrations were reduced to a range of 0.000 ppm to
1r 0.005 ppm.
The operating efficiency of the plant for
removing cadmium varied between 74 and 100 percent. The average removal was 89 percent.
Argo and Culp suggested
that higher efficiency appears to be a function of influent concentration.
As influent concentration
increases, so does removal.
The results of their investi-
gation indicate that certain heavy metals can be removed
TABLE 5 HE^VI METALS liE/iOVED 3Y THE ORAi>JGE COUNTY WATER DISTRICT PILOT PLA.iT
Influent Concentration (ppm)
Effluent Concentration (ppm)
Percent Removed
June 1971
0.011
0.004
74
May
0.015
0.003
80
April
0.130
0.002
98
March
0.022
0.002
91
February
0.020
0.000
100
Month
January
0.005
Dec. 1970 November
0.003 0.000
October
0.000
11 along with organics, suspended solids and other inorganics by lime coagulation due to the precipitation of CdCC^, mixed media filtration and activated carbon adsorption. They made a conclusion: "Futher research is needed to more fully define the mechanism of these removals". Linstedt (15), and co-workers have reported the adsorption data of cadmium by activated carbon as shown in Table 6.
High removal efficiency was observed for cadmium.
Also the authors have postulated that the mechanism might be a direct chemical interaction between organic substances in water and trace inorganics, followed by adsorption of the organics onto the carbon. Treatment of sorbents with reagents capable of forming compounds with sorbable ions has been used for separation of elements and for extracting them from liquid phases (16,17).
It was reported in the literature that germanium
could form complexes with tartaric acid (18-21), and iron germanates with ferric hydroxide (22-24).
Arsenic (III)
also formed complexes with tartaric acid (25,26) and was coprecipitated with ferric hydroxide (27).
Metal Adsorption Mechanisms There are two modes of carbon adsorption which appear to be unique to metal ions.
One is the phenomenon where
an anion is adsorbed onto the activated carbon, and then the metal ion interacts with the anion, effectively being
12 absorbed by an absorbate.
Tausnkanov, et al. (28) found
that the proper concentration of sodium bromide can increase the amount of lead and zinc adsorbed by a factor of 4 to 5.
Similarly, Amelin and Karyalin (29) noted that
a ten percent NaCl solution can increases the adsorption of vanadium by as much as a factor of four.
They also
used a carbon which has been pretreated with phosphoric acid, with significant increases in total vanadium adsorption.
TABLE 6 REMOVALS OF TRACE ELEMENTS IN SAND FILTRATION, ACTIVATED CARBON, CATION EXCHANGE, AI^D ANION EXCHANGE
Cumulative Removal after Given Process Trace Metal
Ag(Ag'^) Cd(Cd-^^) _2 Cr(Cr20^ ) Se(SeO,"^)
Sand Filtration
Activated Carbon
Cation Exchange
Anion Exchange
11.6
97.1
98.8
99.4
6.2
98.8
98.5
99.1
2.5
96.6
98.5
98.6
9.5
43.2
44.7
99.9
13 The second mode of metal ion adsorption is the way that organic complexing agents and preadsorbed anions can effect the amount of adsorption.
Carbons which had oeen
impregnated with ferric hydroxide and with tartaric acid gave mixed results as far as the effectiveness of such treatments (30). Tartaric acid significantly improved the adsorption of germanium, but actually decreased the loadings achieved with arsenic.
The ferric hydroxide
treatment did not particularly affect the germanium adsorption, but it increased the arsenic sorption. The effects of certain anions or complexing agents are not limited to specially pretreated carbons.
They are
also seen in adsorption studies of solutions of metals and active agent.
Kuzin and co-workers (31,32) found that
except for uranium (VI), the sorption capacity for heavy metals increases with increasing acetate concentration. A variety of anions (tartrate, oxalate, citrate, thiocyanate, flouride and phosphate) used either in conjunction with a commercial complexing agent, Trilon B with pH adjustment, or with 1-nitroso-2-naphol gave sharp separation between iron and uranyl cations (33). Also, the combined use of a salt solution (0.1 A Rochelle salt) and a carbon pretreated with a complexing agent (ditnizone, diethyldithiocarbamate, and 8-hydroxyquinoline) yield a carbon which adsorbed all the zinc, lead, copper, iron, manganese, cadmium and cobalt from an aqueous solution (34).
u Taken together, the several results summarized above seem to indicate that a primary sorption mechanism for heavy metals on activated carbon is to build up a linkage between the active carbon and tne metal ion by some adsorbable bridging group.
Direct adsorption of the metal
ion to the activated carbon appears to be a secondary factor, at least in some cases.
The other mode of metal
ion adsorption, of particular interest for the studies investigated here, is the way that organic complexing agents react with metal ion to form an intermediate which can be well removed by virgin activated carbon.
CHAPTER III THEORY Oi ADSORPTION Carbon Surface Chemistry Extensive studies have been completed to characterize the functional groups on the surface of activated carbon. Methods used include direct titration, neutralization, reaction with organic functional group reagents, infrared internal spectrometry, and polarographic methods.
Seven
types of functional group most commonly to be on the carbon surface are carboxyl groups (35), (36), quinone type carbonyl groups (36,37,38,39,40), normal lactones (36), fluoresceintype lactones (41), carboxylic acid anhydride (35), and cyclic peroxide (35). It has been suggested (42) that carbonyl oxygens on the carbon surface act as electron donors, and the aromatic rings of the carbon are the acceptors.
Ishizaki et al. (43) and Pan
(44) studied the surface chemistry of activated carbons. They found that the Filtrasorb 200 (Calgon Corporation) carried negative potential on the carbon surface.
It is
apparent to conclude that the negative charge could result from the ionization of carboxyl and phenolic acidic functional groups on the carbon surface or the specific sorption of hydroxyl and hydrogen ions. It is generally assumed that the large surface area of activated carbon and the chemical functionality of the available pores and surface, are the major factors
16 responsible for the carbon's adsorptive action.
It is of
interest to interpret qualitive variations of activated carbons in tne concept of active sites, as developed in the field of contact catalysis.
However, the adsorptive
power does not exist on all portions of tne surface. Different active sites can be similar in some respects and yet be dissimilar in others.
A previous investigation
(45) showed that two types of surface which adsorb similar amounts of a given dye under the same experimental conditions may have different adsorptive powers when conditions are altered.
Even on any one carbon, there may have
different species of active sites capable of adsorbing a given adsorbate (46). This seems to be the reason to cause the complexity of carbon's adsorptive power.
The
basic forces causing adsorption could be reasonably separated to two types : 1) intermolecular or van der Waals force, and 2) electron transfer between adsorbate and carbon surface.
Depending on which of these two force
types acts the major factor in the adsorption process, we distinguish between physical and chemical adsorption.
For
neutral organic adsorbate, the van der Waals force would play the role.
On the other hand, the chemisorption for
cations would be expected in this study.
Systemic Factors Affecting Rates of Adsorption (a) Hydrogen Ion Concentration
17 Zogorski et al. (47) suggested that the rate of movement of the mass transfer zone would be pH dependent. Huang and Wu (48) studied the effect of pH on Cr^"*" and Cr ^ adsorption by Filtrasorb 400 activated carbon. results indicated that Cr 3+ Cr
6+
The
is more readily removed than
, by at least two times.
The optimum pH for adsorptive
removal is 5.5 to 6 for Cr^"^ and 5 for Cr^"^.
An explana-
tion that appears more reasonable is that the change in adsorption rate for changing pH may be due to alterations in the carbon surface.
In this thesis, all the reaction
solution v/ill be run at about pH 7. (b)
Temperature The increase in rates of reaction with increasing
temperature is described by the Arrhenius equation,
k = A exp ~^/^^
(1)
in which k is the specific rate constant, A refers to a temperature-independent factor sometimes called the frequency factor, and E describes the activation energy, representing the minimum energy required for initiating the reaction.
The values of the quantities A and E may be
considered to remain constant within moderate temperature range. Equation 1 may be written in logarithmic form as
13 E In k = In A -
-
(2)
KT indicating that there should be a linear relation between In k and 1/T.
Equation 2 may be written for any two rates,
k^ and k^, and combined as k.
E (T.-T^)
in which R is equal to 1.987 cal/mole.
Then considering
any two temperatures, lying within the experimental range, the activation energy can be expressed as
E =
(In k./kp) (1.987) T.Tp
^—^
Li-
(4)
T^-T^ Several previous experiments have been done to evaluate the activation energy of adsorption on activated carbon. Glasstone et al. (49) reported that the activation energy for alkylbenzene sulfonate is 3500 cal/mole.
The activa-
tion energy, 4800 cal/mole, of diquat was also determined (50).
The rate of adsorption is strongly temperature-
dependent and higher temperature usually promotes better adsorption.
Kinetic Effect of the Nature and Concentration of Adsorbate (a) Concentration of Adsorbate
19 In processes that include a concentration stage, the adsorptive efficiency in a dilute solution often differs from that obtained in a more concentrated solution. The removal of trace quantities of adsorbate from dilute aqueous solution should be no obstacle because the adsorption relative to initial concentration is more rapid.
The
dilute solution has a greater fraction of the adsorbate originally exposed to the activated carbon.
(b) Molecular Size of Adsorbate If diffusion of adsorbate within the niicropore structure of activated carbon is the rate-deternmin^ step in the adsorption process, then the size of the adsorbate should affect the over-all rate of adsorption.
The larger
the molecule, the lower should be the rate of adsorption. In this work, cadmium is the only adsorbate under study.
Effects of Particle Size and Concentration of Carbon on Rates of Adsorption (a) Particle Size Since pore diffusion is a major contributor to the over-all diffusion rate, selection of the particle size could be critical.
The smaller the particle, tne
faster tne diffusion, and the shorter the mass transfer zone.
Diffusion through a surface film is very fast
compared to diffusion into the interior.
The powder
2C activated carbon could reduce the interior diffusion as well as the over-all diffusion effect.
So, the chemical
reaction rate would be the major factor for the rate of adsorption of powder activated carbon.
(b)
Concentration of Carbon Mark (51) confirmed that crushing the carbon
particles will result in an increase in the surface area. The increase in surface area is linear with log(1/particle size).
The BET method was applied for measuring the
surface area and the result is shown in Figure 1,
cd
0
1000
d
o •H Pi PH
X tH O
O CQ
a o
-P PJ o o •H O rH OH
a ^
-P Pi > : PH >
o 0
Ti
a o o
+^ d
O -H rH a
-p cd I
0 U d •H
(S/^ui) w/X
rti
cd
40 2 A surface area of 500 ra /g can be estimated for the activated carbon used here from the intercept in Figure 1. Because activated carbon used in this study has a large surface area, the chemical-control model can be tested even though the diffusion-control model, Freundlich isotherm, fits the data reasonably well.
The general
forward rate expression for two reactants is described as,
a Cd"^^ H- b C
* p Cd^^—C
(10)
^2 n_
forward rate = k^ (cd^"*] ^
- ^^p
[s]
where k^ = the forward rate constant kp = the reverse rate constant [Cd J = the concentration of cadmium in the system [s] = the concentration of carbon active sites C
if
= the activated carbon
n = the reaction order of cadmium r m = the reaction order of carbon r Kuzin et al. (31) studied the sorption of metals by SKT carbon from 0.45M acetic acid solution.
They found
the adsorptive capacity for cadmium was 1.9 mg/g. Because of the relatively large amount of activated carbo was used in this study (initial cadmium concentration is
41 0.1 mg/j^ and the minimum carbon loading is 0.1 g ) , the assumption that the concentration of carbon active sites may be considered constant was made.
Based on this
assumption Equation 10 can be simplified to Equation 11.
forward rate = k.J Cod^"^] "^
(ll)
where
Also, since the reaction solution was stirred at 1160 rpm and powdered activated carbon was used, a homogeneous reaction system is assumed.
The reaction is
apparently reversible because of the non-zero equilibrium concentration of cadmium for higher carbon loading system (see Figure 7).
However, the reversibility of the adsorp-
tion process was not tested experimentally. Since n
in Equation 11, and hence the reaction order,
is not known, first-order and second-order reversible rate equations were tested. 1)
The First-Order Reversible Model The first-order rate equation was already
expressed in Equation 8 and the values of -ln(l-(X/Xg)) were calculated and tabulated in Tables 12 and 13.
u y
TABLE 12 THE CALCULATED DATA FOR THE FIRST-ORDER REVERSIBLE MODEI AT 35 C
1/3
Time ( h r ) 0 . 1 g C*
1/2
1.0
2.0
X
0.225 0.298 0.403 0.450
(X^=0.46)
-ln(l-(X/X^))
0 . 6 7 2 1.044 2 . 0 8 8 3.829
0 . 2 g C*
X
0.267 0.352 0.450 0.435
-ln(l-(X/Xg})
0 . 7 7 1 1.232 2 . 3 5 8 3 . 7 2 4
X
0.379 0.457 0.594 0.631
(X^=0.64)
-ln(l-(X/Xg))
0 . 8 9 7 1.252 2 . 6 3 3 4 . 2 6 4
1.0 g c""
X
0.546 0.588 0.640 0.675
(Xg=0.68)
-ln(l-(X/X^J)
1.624 2 . 0 0 0 2 . 8 3 3 4 . 9 1 3
2 . 0 g C*
X
0.650 0.692 0.720 0.728
(X^=0.497) 0 . 5 g C*
(X^=0.728)
-ln(l-(X/Xg))
2.234 3.007 4.511
43 TABLE 13 THE CALCULATED DATA FOR THE FIRST-ORDER REVERSIBLE MODEL AT 60°C
Time (hr)
1/4
1/2
1.0
2.0
0.600 0.650 0.680 0,700
0.5 g C (X^=0.708)
-ln(l-(X/X^)) X
1.0 g c* (X = 0 . 7 8 6 ) e
-ln(l-(X/X^))
Figures
0.686 0.720 0.742
0.777
2.062 2 . 4 7 7 2 . 8 8 3 4 . 4 7 0
X
0.830 0.868 0.890
0.906
-ln(l-(X/X^))
2 . 3 5 5 2 . 9 2 9 3.525
4.423
2 . 0 g C* (Xg=0.9l7)
1.880 2,502 3.230 4.483
7 and
-ln(l-(X/X )) vs t.
8 were obtained by plotting The linear-regression method was
selected to determine the straight lines and the rate constant was shown in Table 14. Equation 1 showed that the reaction rate should be increased by increasing temperature.
Therefore, the lower forward rate constants
for 60°C as shown in Table 14 indicate that the firstorder reversible model is not satisfied. 2) The Second-Order Reversible Model The rate equation for the second-order
LL
5.0 -
4.5
4.0 -
3.5 -
3.0 -
X
0
^^
2.5
X
I "d rH
2.0 -
I
1.5 -
1.0 -
0.5
0
2 3
Figure
2
Time (hr)
7 - The Plot of -ln(1-(X/X^)) vs t at 35 0.
45
5.0
-
4.5
4.0
/i c
3.5
3.0
X
0
2.5
X
I
r-i I
2.0
1.5
0,5 g/i C 1.0 s/jl 0* 2 . 0 g/i C"
1.0
0.5
0 4
Figure
8
2
Time ( n r )
0^ - The P l o t of - l n ( l - ( X / X ) ) v s t a t 60
46 TABLE 14 THE FORWARD RATE CONSTAIIT FOR THE FIRST-ORDER REVERSIBLE MODEL
Carbon L o a d i n g ( g / ^ )
k:| (35°C) k^ (60^0)
0.1
0.2
0.5
1.0
2.0
0.881
0.919
1.361
1.539
3.200
1.372
1.464
1.650
reversible model can be integrated and expressed as Equation 9.
The values of ln((X^-(2X^-1)X)/(X^-X)) were
calculated and tabulated in Tables 15 and 16.
47
TABLE 15 THE OSCULATED LATA FOR THE SECOND-ORDER REVERSIBLE MODEL AT 35°C
1/3
Time ( h r )
In
Xe-X
0 . 3 7 9 0 , 4 5 7 0 . 5 9 4 0.631 X -(2X - 1 j x In - ^ — , ^
w
2.0 g C
(X = 0 . 7 2 8 ) e
0.716 1.029 2.333 3.941
0.546 0.588 0.640 0.675
1.0 g C
(X = 0 . 6 8 )
0.710 1.094 2.156 3.904
0.774 1.236 2.364 3.730
In
0.5 g C
(Xg=0.64)
2.0
0.267 0.352 0,450 0,480
0,2 g C
(Xg=0.497)
1.0
0.225 0.298 0.403 0.450
0.1 g C
(X = 0 . 4 6 )
1/2
X -(2X^-i;X In -^—rr-^
X X^-(2X^-1jX I n -^—,r^.
1.283
1.627 2.419 4 . 4 7 0
0.650 0.692 0.720 0.728 1.711 2.439 3.911
T*
TABLE 16 THE CALCULATED DATA ECi^ THE SECOND-ORDER REVERSIBLE MODEL AT 60^C
Time (hr)
1/4
Figures
0.686 0.720 0.742
0.777
1.370
1.735 2,106
3.636
0.830 0.868 0.890
0.906
In
2.0 g C (X^=0.917)
2.0
1.446 2.021 2.720 3.953
In
1.0 g C (Xg=0.786)
1.0
0.600 0.650 0.680 0.700
0.5 g C (X =0.708)
1/2
In
0.949 1.371 1.867 2.686
3 and 9 were obtained by plotting
ln((X -(2X -1)X)/(X^-X)) vs t. e e e
The linear-regression
method was applied to determine the straight lines and the forward rate constants were calculated and listed in Table 17.
AQ
T
r
5.0
4.5
1.0 g/£ C
4.0
3.5 X
I
X
0
3.0
I XI OJ
0
2.5
I
0 >. rH "-^
d
O KN
o
^ -p ^
cd EH
c?
•H +J
•H
d
-d
rH
o
d
o
CO
CO
T—
c= Pi Pi
o. o
' • — '
un o o o. o
o
.
CM
OJ
O
o
o
o
o
.
.
o
o
CO
d
o
o
a •H p o -p d P i o O -H
rH a OH
r^
u VD o o CQ
cd Ti
1 0 0 a -p -p d (d cd •H U U a -p -p Ti u u cd cd cd 0 EH EH 1
£H
0
a a d d
•H r-i a d Xi > cd 0 c^
a 0
-p pci 0 ^
d
a d •H
>^ 0 rH Pi
d 0
a x^
a 0
-p -H CQ a >> Ti U^ cd 0 d 0 a ^ 0 ^ u cd
^ - ^ ' + H c:>
T d ?H Cd Cd
a 0
0
0
0 1 «H a 0 d •H rH a Cd Ti > Cd 0 0
a
a 0
0 a 4^ =^ 0 CQ u >> X ' H CO
0 p» cd •H Ti 0 a ?H 0 -P d M
>3 ^ 0 0 J:^ EH
CHAPTER VI CONCLUSIONS AND RECOI^^IENDATIONS Conclusions (a) Of the several complexing agents investigated, tartrate is best able to increase the percent removal of cadmium by Filtrasorb 200 activated carbon.
(b) is 1.5:10.
The optimum mole ratio of tartrate to cadmium Approximately 15 % of the total cadmium
appears to be adsorbed as a neutral complex, cadmium tartrate, with the balance adsorbed as cadmium ion.
(c)
The cadmium adsorption and cadmium tartrate
adsorption on powdered activated carbon appears to be diffusion (physical) controlled and is adequately represented using a Freundlich isotherm.
Recommendations (a) Other types of carbon may yield different results for cadmium removal because of changing proportions of neutral and charged active sites.
To help
characterize these effects, carbon from a number of sources should be used.
(b)
The pH value for the solutions used in this 74
7: study were about 7.0.
Since the solation pH has a
significant effect on adsorption, solutions of different pH values should be investigated.
(c)
To approach a chemical reaction controlled
system, much finer carbon particles should be used in order to expose more active sites on the carbon surface.
(d)
The strong chelating agents, such as nitrilo-
triacetate (NTA) and ethylenediamine tetraacetate (EDTA), may have use in cadmium removal and should be investigated.
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•p
