INTRODUCTION
Potable water is the most precious resource. However, it is one that is increasingly
under threat from several polluters. Amongst the countless man-made contaminants
that infiltrate our water sources are heavy metals. Heavy metals are toxic inorganic
contaminants. These metals cannot be degraded or readily detoxified biologically
and have tendency to accumulate in living material (Khan
et al., 2004). Heavy metals discharge in the wastewater can be toxic
to aquatic life and make natural water unsuitable for human consumption. Unlike
organic contaminants that can be degraded by microorganism, heavy metals must
be removed from wastewater before being discharged to the environment (Ilhan
et al., 2004). The presence of heavy metal ions such as lead, mercury,
chromium and other heavy metals in the water is a major concern due to their
toxicity to many life forms (Khan et al., 2004;
Ngah et al., 2006). The levels of these contaminants
must be strictly controlled to protect the environment from being destroyed
(Khan et al., 2004). This is because excessive
release of heavy metals into the environment due to industrialization and urbanization
has posed a great problem worldwide. Pollution by chromium is one of worrisome
environmental concerns as the metal has found widespread use in many industrial
activities such as electroplating, metal finishing, leather tanning, mining,
dyeing and fertilizer, photography industries, nuclear power plant and textile
industries (Aroua et al., 2007; Ngah
et al., 2006; De Castro Dantas et al.,
2001). Chromium occurs in aqueous systems as both trivalent, Cr (III) and
hexavalent, Cr (VI) forms (Venkateswarlu et al.,
2007; Selvi et al., 2001; Ngah
et al., 2006). The Cr (VI) is of particular concern because of its
toxicity. Hexavalent chromium compounds are carcinogenic and corrosive on tissue
and are found to be toxic to bacteria, plant, animals and people even at low
concentrations (Demirbas et al., 2004). Human
toxicity includes lung cancer, as well as kidney, liver and gastric damage (Aroua
et al., 2007). In 2005, Department of Environment (DOE) Malaysia
had monitored 88 wells at 48 sites in Peninsular Malaysia, 19 wells in Sarawak
and 15 wells in Sabah (DOE, 2006). Based on their investigation,
they found that the chromium levels exceeding the benchmark were recorded in
municipal groundwater supply (13 mg L-1), solid waste landfills (3
mg L-1), ex-mining areas (gold mine) (17 mg L-1) and golf-courses
(4 mg L-1) (DOE, 2006). However, based on DOE
effluent discharge requirements, the tolerance limit for Cr (VI) to discharge
into inland surface water and potable water is 0.05 mg L-1. While,
for Cr (III), the standard limit is 0.20 mg L-1 for standard A and
0.1 mg L-1 for standard B.
This study attempts to evaluate the removal of chromium from aqueous solution using chitosan as an adsorbent.
MATERIALS AND METHODS
Materials
Chitosan powder (88% deacytelation) which had been provided by Department
of Biotechnology Engineering, Faculty of Engineering at the International Islamic
University Malaysia was used for this experiment as adsorbent. Standard solution
of chromium (1000 mg L-1) was used to prepare the stock solution.
Deionized water was used to dilute the stock solution in order to get the desired
concentration. NaOH (1 M) and HCl (1 M) solution was used to adjust the pH.
This study was conducted at International Islamic University Malaysia, Kulliyyah Engineering, Department of Biotechnology Engineering, Bioenvironmental Engineering Research Unit (BERU), from 2007 to 2009.
Equipment
A 6 place jar test of the model R000100174 from Flocculator SW6 was used
in this experiment to run coagulation process. Other equipment used include
fine balance (model College B204-S), pH meter (model SevenEasy) and Atomic Absorption
Spectrometer (AAS) model A Analyst 400.
Experimental Design
Design of the experiment and statistical analysis in this study was carried
out by using statistical software Design Expert (Version 6.0.8). The optimization
was done using Central Composite Design (CCD) with a quadratic model. Four independent
variables namely dosage of chitosan (x1), pH (x2), contact
time (x3) and agitation speed (x4) were chosen. The low,
middle and high levels of each independent variable were designated as –1,
0 and +1, respectively. The actual design of experiments is shown in Table
1. A total 25 different combinations including 2 replications completely
randomized order according to a CCD configuration for 4 factors.
Experimental Procedure
Chromium stock solution was prepared from a 1000 mg L-1 standard
chromium solution. For this study, 20 mg L-1 stock solution was prepared
according to the chromium concentration of 20 mg L-1 in ex-mining
water (DOE, 2006).
| Table 1: |
Experimental design for process |
 |
| Experimental runs: 25. Center point: 1. Replications: 2. Option:
Small. Completely randomized design |
About 800 mL deionized water was added into 1000 mL volumetric flask. Next
20 mL of standard chromium solution was pipetted into the volumetric flask.
Two percent nitric acid (HNO3) was then added to the solution. Lastly,
deionized water was added to make up to the total volume (1000 mL). Adjustment
of pH was then carried out using NaOH or HCl. To maximize chromium removal by
the chitosan, batch experiments was conducted at ambient temperature by varying
all pertinent factors such as dosage, pH, agitation speed and contact time according
to the experimental design which are shown in Table 1. For
each experiment, chitosan powder was added to the chromium solution and the
metal bearing suspension was kept under magnetic stirring at varying speeds
in the jar test until the equilibrium conditions were reached. Then the solution
was allowed to settle down. The sample of water after flocculation and sedimentation
was collected and the concentration of chromium was analyzed. The chromium concentration
after the adsorption was measured. The removal efficiency (E) of adsorbent on
Cr is defined as:
| Where: |
| C0 |
= |
Initial concentration of chromium solution |
| C |
= |
Final concentration of chromium solution after the experimental run |
RESULTS AND DISCUSSION
The results of concentration of residual chromium due to the effects of chitosan
dosage, pH, contact time and agitation speed. The highest removal of chromium
by chitosan occurred at run 24 with 99.7% removal, while the lowest removal
at run 16 with 36% removal. Analysis of Variance (ANOVA) test result for 25
runs under CCD experimental design is shown in Table 2.
| Table 2: |
Analysis of variance (ANOVA) |
 |
Analysis of Variance is used to investigate and model the relationship between
a response variable and one or more independent variables. Values of Prob>F
less than 0.0500 indicate model terms are significant. Values greater than 0.1000
indicate the model terms are not significant. The principal variables are A
(chitosan dosage), B (pH), C (contact time) and D (agitation speed). B, C and
D are significant, while A is not significant. B indicates a very significant
model term. B2 and D2 are significant but A2
and C2 are not significant. B2 indicates a very significant
quadratic model terms. AB, AC, AD, BC and BD show significant model terms but
CD show insignificant model terms.
The regression equation obtained after analysis of variance gave the level of adsorption of chromium onto chitosan as a function of chitosan dosage, pH, contact time and agitation speed. All terms, regardless of their significance, are included in the following equation:
where, Y is the predicted response. Square regression (R2) was significant at the level of 99.63%. The coefficients were calculated using design expert. Equation 2 shows final equation in terms of actual factors. This equation is used to calculate the predicted value of residual chromium concentration, the coefficient of pH gives the highest value. This indicates that effect of pH contributes more to the successful of lowering the residual chromium concentration.
Effect of Chitosan Dosage
The rate of chromium sorption on chitosan dosage was studied by varying
the amount of adsorbents from 10 to 30 mg, while keeping other parameters (pH,
agitation speed and contact time) constant. As in Fig. 1,
the concentration of residual chromium decreases with the increasing of chitosan
dosage. This is expected due to the fact that the higher dose of adsorbents
in the solution, the greater availability of exchangeable sites for the ions.
Thus, chromium removal can be increased by using high dosage of chitosan.
|
| Fig. 1: |
Effect of chitosan dosage on chromium removal |
|
| Fig. 2: |
Effect of pH on chromium removal |
|
| Fig. 3: |
Effect of contact time on chromium removal |
Effect of pH
The sorption onto chitosan is basically electrostatic attraction type. As
Fig. 2 shows, increasing pH while fixing other factors enhance
chromium removal. However, the removal is not effective at pH greater than 7.
The low sorption efficiency at pH less than 6 is probably due to a weak interaction
between the ammonium groups of chitosan and chromium ions. Furthermore, chitosan
is soluble in pH less than 6. The optimum pH for removal of chromium is at pH
6.5.
Effect of Contact Time
As shown in Fig. 3, the concentration of chromium residual
decreases with the increase of contact time. When time less than 50 min, concentration
of residual chromium decreases gradually and decreases instantaneously when
time is greater than 50 min. Therefore, the removal of chromium is effective
when chromium interact with chitosan for longer time.
|
| Fig. 4: |
Effect of agitation speed on chromium removal |
| Table 3: |
Numerical solution for optimization |
 |
Effect of Agitation Speed
The effect of agitation speed on removal efficiency of chromium was studied
by varying the speed of agitation to 50, 150 and 250 rpm, while keeping the
optimum dose of adsorbents and optimum pH as constant. From Fig.
4, it is observed that the concentration of residual chromium increases
as the agitation speed increases. These results can be associated to the fact
that the increase of the agitation speed improves the diffusion of chromium
towards the surface of the chitosan.
Under optimization in the design expert, a set of numerical solutions will be recommended by the software according to the range and desirability of each factor and response. As shown in Table 3, a set of 10 solutions was suggested by the software according to the concentration of residual chromium which was set to achieve the target value of 0.05 mg L-1. To check the validity of the model, one of these solutions can be selected and then an experiment is carried out by modifying the values of the factors according to the numerical solution that we have selected. The selection is based on the desirability. The selection of the solution is preferable when desirability = 1.
The preferable solution is solution number 1 and number 2 since desirability is equal to 1. However, the solution number 2 is most preferable. This is because the contact time in solution number 2 is shorter than contact time in solution number 1. Furthermore, solution number 2 has higher agitation speed compare to solution number 1. This is consistent with the theory of mixing where good mixing is achieved at higher speed and shorter time. Therefore, solution number 2 with 29 mg chitosan dosage, pH 6.39, 20.16 min contact time and 156 rpm agitation speed has been selected to validate the model. The optimization result that had been generated using design expert is shown in Table 4. For the validation of the model, the value of residual chromium concentration that has been selected was higher (0.210 mg L-1) than the predicted. This is because the analysis of residual chromium concentration depends on the calibration made in the AAS machine.
| Table 4: |
Optimization solution |
 |
| Table 5: |
Comparison of various adsorbents and its percentage uptake
of Cr (III) |
 |
| Not mentioned |
The value of optimized residual chromium concentration achieves the DOE standard limit for chromium which is 0.05 mg L-1.
The literature showed that chromium can be removed by different techniques.
Several researchers investigated the removal of chromium using chitosan coated
with oil palm shell charcoal. From their research, they found that, the practical
problems of chitosan solubility at low pH aqueous systems, gel forming behavior
and mass transfer limitations can be solved by coating chitosan on oil palm
shell charcoal. Tirgar et al. (2006) carried
out a study on the removal of airborne hexavalent chromium mist using chitosan
gel beads, the highest Cr (VI) removal obtained at pH 6, low Cr (VI) concentration
(<50 μg m-3) and high sorbent concentration (20 mg mL-1).
The higher ions removal efficiencies were achieved at lower levels of air velocities,
pollution concentrations and higher levels of solution pH values, temperatures
and sorbent concentrations. Several reports evaluated the use of chitosan flakes
for the removal of heavy metal ions. The results showed that a 0.4 g portion
of activated alumina can retain 0.6 mg Cr (III) from 20 mL sample adjusted at
pH 4 and stirred for 30 min.
Table 5 shows that there are many studies on the removal of Cr (III and IV) using various types of adsorbent. However, the percentage removal for each adsorbent is different due to the variation in the operating parameters (pH, agitation speed, dosage, temperature and many more). Thus, this comparative study was conducted to further understand the mechanism of adsorption and compare the types of adsorbents that were previously used to remove Cr (VI). In comparison with the adsorption kinetics of the various adsorbents, it was concluded that most of the removal process occurred very fast within the first 20 min. Thus, from Table 5 comparisons, it can be concluded that the adsorption capability of the adsorbent is highly dependent on many factors such as surface functional groups, the specific surface area and the solution components. In order to achieve optimal removal of Cr (VI) ions, the pH of the solution must be maintained in normal conditions for the complete removal of Cr (VI) as it has been found in this research by having the optimum condition obtained to achieve 0.05 mg L-1 residual Cr concentration or 99.75% removal that meets Department of Environment (DOE) Malaysia effluent discharge requirement.
CONCLUSION
The performance of chitosan for the removal of chromium from water is 99.75% good since percentage of removal is high. It can be concluded that chitosan is a good candidate for biosorption of chromium. Thus, the use of chitosan as an adsorber for heavy metal removal (Cr) from aqueous solution is highly an efficient alternative. Besides that, the operating condition for the removal of chromium using chitosan was successfully optimized. The optimum condition is 28.79 mg of chitosan dosage, 6.39 pH, 20.16 min contact time and 156.23 rpm agitation speed. This optimum operating condition was achieved at initial chromium concentration of 20 mg L-1 which then reduced to 0.05 mg L-1. The minimum residual chromium concentration was found to be 0.059 mg L-1 and the percentage of removal was 99.7%. Therefore, this residual chromium concentration meets the permissible limit of chromium effluent discharge that has been regulated by Department of Environment (DOE) which is 0.05 mg L-1.
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