close

Вход

Забыли?

вход по аккаунту

?

The Influence of the Properties of Activated Bagacillo on Cr(VI) Adsorption.

код для вставкиСкачать
Dev. Chem.Eng. Mineral Process., 7(5/6),pp.537-549, 1999.
The Influence of the Properties of Activated
Bagacillo on Cr(VI) Adsorption
M. Valix *, J. Siliezar and K. Zhang
Department of Chemistry and Chemical Engineering
James Cook University, Townsville, Queensland 481 1, Australia
The present study deals with the adsorption of Cr(V1) in a series of carbons prepared
from bagacillo, which is a lignocellulosic by-product from a sugar milling operation.
Activated carbons were prepared by chemical activation using ZnCl,, CaCl,, H,PO,
and Ca(OH), and physical activation with steam and carbon dioxide. The effects of
the chemical nature and the pore structures on the adsorption behaviour of these
activated carbons were investigated. The functional groups present on the surface of
the carbons were differentiated and related to the observed adsorption behaviour of
the carbons. It appears that adsorption is strongly influenced by the pH of the carbon,
where acidic functional groups appear to enhance the adsorption behaviour. Fine
micropore structures appear to exhibit a molecular sieve effect. Restrictive pores,
present in carbons with high surface areas, appear to inhibit the access of Cr(V1) into
the micropore structures resulting in low adsorption.
Introduction
The effects of hexavalent chromium to humans and animals are well established [ 11.
Exposure to Cr(V1) can have damaging effects on the human physiology, neurology
and biological system when tolerance levels are exceeded. Naturally occurring
chromium is in the form of chromium sesquioxides (Cr,O,). Chromite or the spinel of
chrome-iron ore (FeO-Cr,O,) is the chief ore of chromium and poses little threat
*Authorfor correspondence.
537
M. Valix,.I.
Siliezar and K. Zhang
because of its low solubility in potable waters. The increasing applications of
tanning, plating, photographic sensitization pigment, wood preservation and
corrosion inhibitors have introduced Cr(V1) into natural water bodies. Treatment of
chromium waste can be achieved by various techniques including chemical reduction
and precipitation, ion exchange, evaporation and concentration, electrolysis and
electroplating, ion flotation, and carbon adsorption [2,3]. It is the latter which is of
interest in this study. These techniques meet with various degree of success in the
removal of Cr(VI), however adsorption in carbon, appear to be the most effective in
the removal of low concentrations of metals ions. The presence of Cr(VI), even in
trace levels, in waste water is considered hazardous because it has the tendency to
concentrate on the surface films of water bodies. Removal of the trace Cr(V1) or
other metals in waste waters are often seen as the challenge in the treatment of
industrial waste water.
In this investigation, activated carbons were prepared from bagacillo which is the
fme particle component of bagasse. Bagasse is the lignocellulosic by-product from
the sugar cane milling operation. It is underutilised, present in large quantities and
has a stable supply, which makes it an ideal commercial precursor for activated
carbons. Previous studies have demonstrated the potential of activated bagacillo as an
adsorbent for Cr(V1) [4]. These studies, however, have simply shown the effects of
the manufacturing conditions on the adsorption behaviour of the carbon. To optimise
the adsorption capacity of the carbon, it has been emphasized by previous workers
[5,6], that an understanding of the pertinent carbon properties which influence
chromium removal is required. In the present study, the influences of both the pore
characteristics and the chemical nature on the Cr(V1) adsorption capacities of the
series of activated carbon prepared from bagacillo, were investigated.
Experimenta1
Preparation of Active Carbons
The samples of bagacillo were collected from the sugar mill using a cyclone.
Samples were dried and activated by physical and chemical methods.
538
Influence of activated bagacillo properties on Cr(VI)adsorption
Chemical Activation of Bagacillo
The procedure used for the chemical activation of bagacillo is s d a r to those
reported in literature [7,8]. Bagacillo was impregnated with a solution of activating
agent using an activating ratio of 0.75. (7.5 grams of activating agent in 10 grams of
dned bagacillo). These activating agents include ZnCl,, CaCl,, H,PO, and Ca(OH),
The solvent was boiled off and the residual mixtures were dried in an oven at 105°C
overnight. The samples were then pyrolysed at 500°C under nitrogen. The activated
samples were washed with distilled water to remove and recover any residual
activating chemicals. Finally the carbons were dried and ground.
Physical Activation of Bagacillo
Preparation of physically activated bagacillo includes two steps:
I) CurbonZzution
Raw dried bagacillo was treated with concentrated sulfuric acid in a 4:3 by weight
ratio respectively. The mixture was then carbonized in a pyrex reactor at 160°C for a
period of two hours. The carbon was cooled, washed with distilled water and dried in
a muffle oven at 105°C.
11) Gusification
Activation was acheved by gasification with steam or CO, at 900°C in a 2.6 cm I.D.
vycor tube. Details of the gasification equipment and the procedure are described in
detail elsewhere [9]. The gasification was carried out for different periods of time (115 hours) to achieve various degree of burn off.
Characterisation of the Active Carbons
The carbons were characterised to elucidate their physical properties which include
surface area and pore size and the chemical properties whch include the
carbonaceous content, pH and surface functional groups.
The micropore structure of the carbons were analysed using N, adsorption at 77K
using a Micromeritics Accelerated Surface Area Analyser and Porosimetry System
(ASAP 2000). The apparent total surfaces area of the carbons were determined by
applying the BET equation to the N, adsorption isotherm. The micropore size
539
M,Valix, J. Siliezar and K.Zhang
dmibutions were also determined from the N, adsorption isotherm using the Kelvin
equation [lo] which were corrected for the thickness of the adsorbed gas layer
estimated using the Halsey equation [ 113.
The ash content and the pH of the raw bagacillo and the activated carbons were
determined using the ASTM standard test D2866-94 and D3838-80 [12] respectively.
The quantity of the acidic functional groups were determined by neutralization
against standardised bases, NaOH, and Na,CO, and basic functional groups using
HC1 and CH,COOH.
Preparation of the Synthetic Hexavalent Chromium Solution
The synthetic chromium solution was prepared by dissolving potassium dichromate
(K,Cr,O,) in distilled water, which was adjusted to a pH of 2. The pH previously
found to provide an optimum Cr(VI) adsorption in activated carbons [13]. About 0.5
grams of activated carbon is mixed with 500 ml of the synthetic Cr(VI) effluent. The
slurry was continuously stirred during the test. Samples of the slurry were obtained at
predetermined time interval, filtered and analysed using atomic absorption
spectroscopy.
Results and Discussion
Physical Characteristics and C r o Adsorption of the Carbons
In this study, chemically, physically activated bagacillo and a series of commercially
available carbons were tested as adsorbents for Cr(VI). The relationship between the
Cr(VI) adsorption capacities of the carbons and their corresponding total surface
area and mean micropore sizes are shown in Figures 1 and 2 respectively. Note that
the adsorption capacities reported in this paper are all based on an ash free basis. It
appears, that the adsorption capacities of the carbons decreased with increasing
surface area. These results suggest that the surface area may not be the single
criterion for the adsorption capacity of the carbon. Figure 2 indicates that carbons
with mean micropore greater than 40-50
A, appears to promote high adsorption.
Rivera-Utrilla and Ferro-Garcia [5 ] suggests that the microporous structure of the
540
Injluence of activated bagacillo properties on Cr(VI) adrorption
j
1
I
chemically activated
CO, activated
steam activated
commercial
A
v
0
T
I
0
I
1
0
200
400
600
800
1000
1400
1200
1600
Total surface area (m'lg)
Figure 1. Cr(V4 aakorption capacity as a@nction of the total su$ace area of the
carbons.
T
c
.-
I
Pl
T
T
A
A'A
V A
I
chemicallyactivated !
C0,activated
steamactivated
cornmeraal
iI
A
T
T
0
20
30
40
50
60
70
80
90
I
1
100
110
mean micropore size (Angstrom)
Figure 2. Cr(V4 adsorption capacity as a function of mean micropore size.
541
M. Valix, J. Siliezar and K. Zhang
160
140
i
7
,
0
A
v
0
chemically activated
C0,activated
steam activated
commercial carbons
I
1
0
200
400
600
800
1000
1200
1400 1600
Total Surface Area (m2/g)
Figure
The micropore size of the carbons as ajiinction of tofua,;u$ace area.
activated carbons may contain restricted pores which may exhibit a molecular sieve
effect. Figure 2, appear to suggest that the micropores, associated with the carbon in
this study, contain pores which restricts the flow of Cr(VI) species into the carbon.
The inverse relationship of the micropore sizes to the total surface area in Figure 3
suggest that the low adsorption observed in high surface area carbons may be
attributed to the presence of restricted pores.
Chemical Characteristics and C r O Adsorption of the Carbon
The surface functional groups on the surface of the carbon are reflected by the pH of
the carbons. Acidic or the type L carbons are shown to contain acidic functional
groups which include phenolic, carboxylic, lactonic, cyclic peroxides and carbonyl
groups [7]. The basic functional groups in type H carbons have been suggested to
have chromene structures which could be oxidised to give carbonium structures and
in turn adsorb anions [14]. The type of functional groups present on the carbon are a
542
InfZuence of activated bagacillo properties on Cr(Vl) aakorption
strong function of the history of activation process. Chemical activating agents which
are acidic, typically H,S04 and H3P04, appear to create acidic functional groups.
Whereas basic activating agents, for example, Ca(OH), and NaOH, appear to form
basic groups. Since complete removal of the activating agents is often difficult,
residual chemical agents may also contribute to the pH of the carbons. Bansal [7]
suggest that carbons gasified below 200 "C and above 700 "C develop basic
functional groups and that between 200 and 700 "C develop acidc functional groups
[7]. The activated bagacillo gasified with steam and carbon dioxide at 900 "C, in this
investigation, developed basic surfaces which is in agreement with the above
observations.
In this study, measurement of the pH and differentiation of the functional groups
were attempted by neutralization. Acidic functional groups were titrated with bases
of different basicities. Differentiations were based according to the method proposed
by Boehm [15]. Boehm suggested that NaHCO, neutralizes carboxylic groups,
Na,CO, neutralizes carboxylic and lactonic groups and NaOH neutralizes carboxylic,
lactonic, and phenolic groups. It is envisaged that NaOH provided a measure of the
total surface acidity. The basic functional groups were neutralized using hydrochloric
acid. These basic functional groups were found to have an insigdicant mfluence on
the adsorption and were not reported in this paper.
The relationship between the pH of the carbons and their adsorption capacities is
shown in Figure 4. It appears that adsorption is favoured in carbons which shows
acidic surfaces. Smith [16] suggests that acidc surface oxygen complexes will
o x i h e water molecules accordmg to the following reactions:
RCO,
+2H20+
RC(02H):- + 2 H f
(1)
Studies by Huang and Wu [ 131, reveal that under neutral to acidic conditions and at
concentration of chromium below 10 M, the bichromate (HCrO,) are the dominant
543
M. Valix, J. Siliezar and K.Zhang
ions in solution. These species are removed by carbon with basic surfaces. It appears
from this study that the removal of Cr(VI) by this mechanism does not appear
sigmfkant. In the presence of protons these bichromate species will react to form
chromic acid as follows [ 171:
10
-
91
I
chemicallyactivated
C0,achvated
1 8 1
3
Q
0
7:
6 -
0
8
8
C
c
0
v
v
A
A AA:
v
A
A 8
AA
2-
6
.
v
v
1
v
1-
0
I
I
I
Figure 4. C r ( V . adsorption as afunction of carbon pH
2H’
+ 2HCr04- + 2H,CrO,
(3)
At high concentration of chromium, chromic acid can easily dissociate into
dichromate (Cr,O,”):
2 H 2 C r 0 4w Cr,Of-
+ 2H’ + H,O
(4)
Dichromate ions, as reported by Huang and Hu [13], are not removed by carbon,
which suggest that high concentration of Cr(VI) is prohibitive to the efficient
544
Influence of activated bagacillo properties on Cr(Vl) adsorption
chromium adsorption in carbons and that the dichromate ions must undergo a
transformation to enable its removal fiom solution. In an acidic environment,
dichromate ions will also react with protons to form chromium trioxide [ 171:
Cr,O;-
+ 2H' +
2Cr0,
+ H,O
(6)
Chromium trioxide is very soluble in water, which suggest that the adsorption of this
species in the carbon may be low. In the case where precipitation is not the
mechanism for the removal of ions in solutions, Weber [ 181 elaborates that the active
sites on the carbon compete with the solvent water molecules for the solute. The
active sites are envisaged to break the solute-solvent interaction and establish a
solute-solid association. If the solute is strongly water soluble, the potential for the
formation of the carbon-solute bond will be greatly reduced.
Chromium trioxide is a vigorous oxidising agent and would easily reduce to
chromic oxide (Cr,OJ, a less soluble species:
4Cr0,
+ 2 C r 2 0 , + 30,
(7)
The stability hagram for chromium indicates that chromic oxide is stabihzed by a
reducing condition, or a low oxygen environment [19]. Carbon surfaces are known to
easily adsorb oxygen, it is envisaged that the porous microstructure of the activated
carbon may support a reducing environment, which would enhance the removal of
Cr(V1) by precipitation as postulated above.
The total surface areas of the various activated carbons are plotted as a function of
the carbon pH in Figure 5. It appears that high surface area coincides with high pH,
which may suggest that the chemical nature of the carbon is also causing the low
adsorption observed in carbons with high surface area and in turn small micropore
size. Adsorption results of carbons with relatively similar pH are related to the pore
size in Figure 6 . The trend observed is slmilar to that shown in Figure 2. These
results support the existence of restricted pores which appears to contribute to the
mhibition of chromium adsorption.
545
M. Valix, J. Siliezar and K. Zhang
1200
1
T
T
0
0
commercial
T
1
0 ,
I
0
2
4
I
8
6
1
0
1
2
1
4
PH
Figure 5. The total suflace area of the carbon as afirnction of the carbon pH.
6 -
!
A
-
pH = 7.4 7.7
pH = 5 . 4 - 5.6
pH = 6.3 6.6
I
A
0
A
A
I
1
25
30
,
I
35
40
I
1
45
50
55
60
micropore size (Angstrom)
Figure 6. Cr(Vr) adsorption capacity as afirnction ofmicropore size of carbons with
relatively narrow p H distributions.
546
Influence of activated bagacillo properties on Cr( VI) adrorption
10
-
,
+ phenol groups (meq/gC)
-A Cr(VI) adsorption capacity (mg Cr/g C )
k
0
.
2
- , - ,
4
6
I
,
8
10
I
12
14
16
Time of carbon gasification with CO, (hr)
The Cr(VI) adsorption capacity and the active site densities of the carbons
as a finction of the time of carbon dioxide gasification.
The active site densities and the adsorption capacities of the carbon prepared by
CO, gasification are plotted as a function of gasification period in Figure 7. The
adsorption capacity appears to correlate closely to the combined carboxylic and
lactonic groups. Whereas, the phenol groups do not appear to have a si@cant
influence on the adsorption capacity. These results appear to indicate that the Cr (VI)
adsorption capacity is strongly dependent on the strong acidic functional groups and
weak acidic groups have very little influence on the adsorption behaviour.
Conclusions
The data obtained from this study led to the following conclusions:
1. Cr(VI) adsorption appears to be dependent on both the surface chemistry and
physical structure of the pores of the carbons.
2. Cr(VI) adsorption is favoured by low carbon pH (-1).
547
M. Valix,J. Siliezar and K. Zhang
3. The dominant acidic functional groups, responsible for Cr(VI) adsorption, appear
to be lactonic and carboxylic groups. Weak acids associated with phenolic groups
do not appear to have a si&icant
influence on the adsorption capacity.
4. The low adsorption capacities observed in high surface area carbons appear to be
associated with the existence of molecular sieve structure and to the high pH of
the carbon surface. Restricted micropores appear to inhibit access of Cr(VI)
groups into the pore structure of the carbons. These pores appear to be associated
with carbon with mean micropore sizes below 40-50 A.
5. The mechanism of Cr(V1) removal at the low pH is postulated to be the reduction
of dichromate ions into chromic oxide which is accommodated by the anaerobic
conditions within the carbon.
Acknowledgement
This study was conducted through the support of CSR Sugar Mills Group and the
assistance of the particle characterisation laboratory in the School of Chemical
Engineering and Industrial Chemistry, University of New South Wales.
References
1.
2.
3.
4.
5.
6.
7.
Agency for Toxic Substances and Disease Registry (ATSDR). “ Toxicological Profile Of
Chromium.” Department Of Health, Human Services And Public Health Services. Atlanta, US 1993.
Marshall, S. 1990, “Metal and Inorganic Waste Recycling Encyclopedia”, Wayers Bata Corporation,
Dan-ridge, New Jersey.
Patterson, J.W., 1975, “Treatment Technology for Hexavalent Chromium Waste: Water Treatment
Technology”, Ann.Anbor.Science Pub. Inc.
Siliezar, J, Amal, R, Valix, M, 1997, “ Adsorption of Cr(V1) Using Activated Carbon Prepared from
Sugar Cane Bagacillo”, 25* Australian and New Zealand Chemical Engineering Conference, 29
September- 1 October 1997, Rotorua.
Rivera-Utrilla, J., and FerroGarcia M.A., 1987, “Study of Cobalt Adsorption from Aqueous Solution
on Activated Carbons from Almond Shells”, Carbon, 25,645-652.
Khalfaoui, B., Meniai, A.H., and Boja, R, 1995, “ Removal of Copper from Industrial Wastewater
by Raw Charcoal Obtained from Reeds”, J.Chem. Tech. Biotechnol. 64, 153-156.
B a n d , RC. and Donnet, J.B. and Stoeckli F., 1988,“Active Carbon”, Marcel Dekker Inc. New
York.
8. Dean B. E. 1993 “Preparation and Characterisation of Carbonaceous Adsorbents from Rice Hull
Char. PhD thesis submitted to UNSW.
9. Siliezar J 1996, “Adsorption of Cr(VI) Pollutants from Synthetic Solutions Using Activated Carbon
Prepared from Sugar Cane Bagacillo”, Honour’s thesis submitted to J a m s Cook University
10 Thompson, W.T.,
1871, Phil. Mag., 42,448.
1 1 Halsey, G.D 1948, J. Chem. Phys.. 16,931.
12 ASTM 1996 Annual Book of ASTM Standards. ASTM Vol. 15.01, Easton USA.
13 Huang, C., Wu, M. H. 1975, “Chromium Removal by Carbon Adsorption”, J. Wat. Pollut. Control.
Fed., 47,2437.
548
Injluence of activated bagacillo properties on Cr(VI) adsorption
14. Ganen, V.A., and Weiss, D.E.1957, Rev. Pure Appl. Chem., 7,69.
15. Boehm, H.P. 1966, “Advances in Catalysis”, Vol 16, (Edited by D.E..Eley, H. Pines, and P. Weisz),
Academic Press, New York.
16. Smith, R.N. 1959,Quart. Rev. 13,287.
17. Mellor’s Modem Inorganic Chemistry 1958, Edited by G.D. Parkes. Longmans, Green and Co.
18. Weber, W.J., and Moms, J.C., 1963, “Kinetics of Adsorption on Carbon from Solution”, Jour. San.
End. Div., Proc. Amer. SOC.Civil Engr., 89, SA2,31.
19. O’Neriaga, J and Niebeoer E. (1988), “Chromium in the Natural and Human Environment”. Adv.
Environ. Sci. Technol. 20.
549
Документ
Категория
Без категории
Просмотров
2
Размер файла
453 Кб
Теги
properties, adsorption, bagacillo, influence, activated
1/--страниц
Пожаловаться на содержимое документа