Simultaneous adsorption of nickel ions and humic acid onto an
activated carbon
Yu Ji
Department of Environmental Science and Engineering, ational University of
Singapore, 10 Kent Ridge Crescent, 119260 Singapore
Abstract
Simultaneous adsorption of nickel ions and humic acid (HA) from Aldrich onto
an activated carbon is studied in this research. It is found that the nickel adsorption in
the absence of HA increases as the pH is increased, but decreases after pH reaches
about 6. Nickel-HA adsorption also varies when HA concentration differs. The ionic
strength also has a positive effect on the nickel adsorption. The zinc and copper ions
do not affect the nickel adsorption significantly.
1. ITRODUCTIO
The objective of this study was to investigate the mechanisms of the adsorption
of nickel ions and HA onto an activated carbon simultaneously. In this study, nickel
was selected to be studied, because it always exists in the industrial wastewater due to
its wide application in a variety of industries. The important factors in the adsorption,
such as pH, HA concentration, ionic strength, and competitive metal ions, were all
considerate.
Nickel is widely used in varies industries (Bansal, 1996) and the concentrations
in industrial wastewaters can range from 3.4 to 900 mg/L Compared to the maximum
contaminant limit in potable water is only 50 μg/L in accordance with the standards
of European Economic Community (Eswed, 2006), Such high concentration causes
harmful effect on creatures.
Humic acid is one of the major components of humic substances. Due to its
strong affinity to metal ions and complex ability, the HA in aqueous solutions can
change the metal adsorptive behavior. So the simultaneous adsorption of nickel ions
and HA onto activated carbons is worth to be studied.
2. EXPERIMETAL SECTIOS
2.1 Materials
Charcoal activated granular carbon which was produced by SINO Chemical CO
(PTE) LTD was used. After being washed for several times by HCL and DI water to
remove the impurities, the carbon was dried in a drying oven at 65◦C for 24 h and
stored in a desiccator for use. sodium chloride, zinc chloride, copper chloride,
hydrochloric acid, nitric acid, and sodium hydroxide are from Merck (Germany). HA
(sodium salt) and was obtained from Aldrich (Milwaukee, WI). All chemicals used
were of reagent grade.
2.2 Adsorption Experiments
A series of batch adsorption equilibrium experiments was conducted to obtain
the factors to influence the nickel adsorption with or without HA The general
experimental procedures are described below: Nickel and/or HA solutions with or
without pH control by dropping HCl and NaOH solutions were prepared. The ionic
strength was adjusted by the NaCl. The carbon was then added into the solutions. The
flasks were placed in a shaker and shaken for 96 h at 25 ◦C to reach equilibrium. The
concentration of nickel ions was measured by inductively coupled plasma emission
spectroscopy (ICP-ES) (Perkin– Elmer Optima 3000, USA). The pH was measured by
an Accumet Basic pH meter (Fisher Scientific, USA).
The Langmuir equations was used to fit the isothermal adsorption data,
q =
(1)
Where qe is the concentration of adsorbate in the solid phase at equilibrium (mg/g),
Ce is the concentration of adsorbate in liquid phase at equilibrium (mg/L), qmax and b
are the Langmuir constants.
3. RESULTS AD DISCUSSIO
3.1 pH dependence on Nickel adsorption
pH dependence on Nickel adsorption
Ni adsorption
percentage (%)
20
15
10
5
0
3
4
5
pH
6
7
8
Figure 1 pH dependence on Nickel adsorption
After shaking for 96 hours, all the samples are tested. The results can be
observed from Figure 1. At the same time period, the adsorption efficiency increases
to 15% as the pH increases from 3.5 to 6, and then decrease a little bit after pH
reaches 6. According to the old studies done before, the maximum adsorption was at
both acidic and basic pH ranges. However the rate of adsorption in basic pH was
much faster (Bansal, 1996).
3.2 Effect of humic acid (HA) concentration on nickel adsorption
Effect of HA concentration on ickel adsorption
i adsorption percentage (%)
40
35
30
25
20
15
10
5
0
No pH buffer (pH 3.9-4.6)
pH=5
pH=6
0
2
4
6
8
10
12
HA concentration (mg/L)
Figure2a Effect of HA concentration on nickel adsorption
Effect of HA on nickel adsorption
0.8
0.7
qe (mg/g)
0.6
0.5
[HA]=0,pH=5
0.4
[HA]=5,pH=5
0.3
[HA]=0,Ph=6
0.2
[HA]=5,Ph=6
0.1
0
0
2
4
6
8
10
Ce (mg/L)
Figure 2b effect of HA on nickel adsorption
The effect of HA concentration on nickel adsorption is demonstrated in Figure
2a and 2b. Different concentrations of HA play different roles in nickel adsorption.
An increase in pH values can also help to enhance the nickel uptake. As shown in the
figure 2a, in the metal adsorption, there are two critical HA concentration amounts
(C1 and C2) of 2 mg/L and 6mg/L respectively. When the HA concentration is lower
than the C1, the nickel adsorption decreases as the HA concentration is increased;
however,when the HA concentration is between C1 and C2, the nickel uptake is
enhanced as the HA concentration is increased. However,after HA concentration
reached C2, an increase of HA concentration causes a decrease in nickel uptake. The
similar results can be found in the previous study on the copper-HA adsorption (Chen,
2004).
3.3 Effect of ionic strength on Nickel adsorption in the presence of HA
Effect of ionic strength on ickel adsorption in the presence of HA
1.2
1
qe (mg/g)
0.8
0.6
pH=5,[NaCl]=0.01M
Ph=6,[NaCl]=0.01M
pH=5,[NaCl]=0.1M
pH=6,[NaCl]=0.1M
0.4
0.2
0
0
2
4
Ce (mg/L)
6
8
10
Figure 3 Effect of ionic strength on nickel adsorption in the presence of HA
It is found in Figure 3 that an increase in solution ionic strength (NaCl) can
enhance the nickel adsorption onto the HA–carbon. It has been reported that higher
ionic strength can slightly increase cation adsorption onto activated carbons due to
compression of the electrostatic double layer (Bjelopavlic, 1999; Zhou, 1994). The
increase in ionic strength can increase the HA adsorption (Stumm, 1996), which is
also due to the compression of the EDL (Bjelopavlic, 1999; Zhou, 1994). As more
HA macromolecules are adsorbed onto the surfaces of carbon at higher ionic strength,
more adsorptive sites are thus formed for the metal adsorption. As a result, higher
metal uptake onto the HA–carbon is observed.
3.4 Effect of competitive metal species on nickel–HA adsorption
0.8
Effect of competitive metals on nickel-HA adsorption
qe(mg/g)
0.6
[Zn]=0,[Cu]=0
0.4
[Zn]=4.6mg/L,[Cu]=
0
[Zn]=4.6mg/L,[Cu]=
6.4mg/L
0.2
0
0
2
4 C (mg/L) 6
e
8
10
Figure 4 Effect of competitive metals on nickel-HA adsorption
Figure 4 demonstrates that existence of zinc or copper ions has very slightly
effects on the nickel adsorption in the presence of HA. This finding is similar to that
in the presence of zinc or cobalt (in the absence of HA) found in other study (Chen,
2001). The effect of the presence of competing metal ions on the nickel adsorption
can be attributed to the different affinities of these metal ions to the adsorptive sites
on the HA–carbon.
i adsorption percentage
(%)
3.5 Nickel adsorption kinetics with/without HA
30
25
20
15
Kinetics without HA
10
Kinetics With 5mg/L HA
5
0
0
20
40
60
80
100
Time(hours)
Figure 5 nickel adsorption kinetics with/without HA
Nickel adsorption kinetic experiments were carried out at last. The results of
nickel adsorption kinetics in the absence and the presence of HA are shown in Figure
5. It is observed that the presence of HA extends the equilibrium time required. In the
absence of HA, the nickel adsorption achieves equilibrium in approximately 65 hours,
while the adsorption cannot reach the equilibrium until 80 hours in the presence of
HA. It can be observed that high percentage of the nickel adsorption occurs in the first
25 to 28 hours in the absence and the presence of HA, followed by a long period of
slow adsorption process.
4. COCLUTIO
It is found that the nickel adsorption in the absence of HA increases as the pH is
increased, but decreases after pH reaches about 6. There exist two critical HA
concentration amounts for nickel adsorption. The absorption effect is decreased at
first when HA concentration increases until it reaches 3mg/L; then the adsorption
increases as the HA concentration increases until it reaches the 6mg/L. The ionic
strength also has a positive effect on the nickel adsorption. However, the presences of
zinc or copper ions do not affect the nickel adsorption significantly.
5. ACKOWLEDGEMET
The great help from Assoc Prof J.P. Chen and Dr. Zheng Yu-Ming are
appreciated.
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