VIETNAM JOURNAL OF CHEMISTRY VOL. 51(5) 551-555 OCTOBER 2013
REMOVAL OF Cr(VI) IONS IN AQUEOUS SOLUTION USING BAMBOO CHARCOAL/CHITOSAN BIOCOMPOSITE
Nguyen Van Cuong'', Nguyen Thi Hong Anh^
Department of Chemical Engineering, Industrial University of Ho Chi Minh City
^Department of Chemical Engineering, Ho Chi Minh City University of Food Industry Received 25 February 2013
Abstract
In this study, the adsorption of chromium (VI) ions from aqueous soluUon by chitosan/bamboo charcoal biocomposite was studied m a batch adsorption system. Biosorbents including charcoal/chitosan beads and acid treated charcoal/chitosan beads were utilized for the removal of chromium (VI) ions from aqueous solution at different initial concentrations, agitation time and mitial pH. The equilibrium adsorption data were described by the Langmuir and Freundlich models. The results indicate that the adsorpUon efficiency of acid treated charcoaFchitosan beads is higher thao that of charcoaFchitosan beads. In addition, the adsorption capacity of the biocomposite decreased with the increase of pH.
Keywords: Biocomposite, chitosan, heavy metal, charcoal, chromium (VI) ions.
1. INTRODUCTION
Many heavy metals, such as chrome, have been found in harmful concenfrations in surface waters due to contamination infroduced from industrial pollution. Once present chromium compoimds are very persistent in water and often, are present in particulate form as sediment. Some of the particulate chromium will remain as suspended matter and ultimately be deposited in sediments.
Industrial use of chromium includes in metal alloys such as stainless steel, protective coatings on metal, magnetic tapes, and pigments for paints, cement, paper, rubber, composition floor covering and other matenals. The hexavalent compounds of chromium have been shown to be carcinogenic to public health and are conosive to tissue. The United Nations Food and Agriculture Organization recommended maximum level for irrigation waters is 100 pg/l. The U.S. EPA pnmary drinking water standard MCL is 0.1 mg/1 for total chromium. Therefore, removal of chrome from wastewater is also needed to achieve the water quality level needed for reuse and recycling. Many industries are turning to reuse and recycling practices to reduce the cost and volume of makeup water. Various methods of chromium removal include filtration, chemical precipitation, adsorption, elecfrodeposition and
membrane systems or even ion exchange process.
Among these methods, adsorption is one of the most economically favorable and technically easy method [1].
Recently biopolymeric adsorbents have been emerging as highly effective alternatives to activated carbons for pollutant removal from industrial effluents [2]. Among these biopolymeric adsorbents, chitosan is becoming increasingly important natural polymers because of its unique combination of properties like biodegradability, biocompatibility and bioactivity, in addition to atfractive physical and mechanical properties [3-6].
Chitosan, a nifrogenous polysaccharide is widely applied in industrial wastewater freatment and in recovery of feed grade material from food processing plants. Chitosan and its derivatives have been used to remove toxic heavy metals commonly found in the global environment, e.g. Hg(n) [7], Cu(II), Zn(II) [8] and Pb(II) [9].
In the present work, biocomposite of bamboo charcoal/chitosan and acid freated bamboo charcoal/chitosan were prepared, and to evaluate the removal of Cr(VI) ions from aqueous solution. The adsorptive capacity of the biocomposite by varying pH of solution, agitation time, adsorbent concentration, and initial ion concentration were also investigated.
VJC, Vol. 51(5), 2013 2. EXPERIMENTAL SECTION 2.1. Materials
Synthetic industrial wastewater was employed for the adsorption studies. Stock solutions (1000 g/l) of chromium (VI) was prepared by dissolving 2.829 g kali cromat (K2Cr207) in 1000 ml distilled water.
The solutions of different concenfrations used in various experiments were obtained by dilution of the stock solutions, Chitosan powder was prepared from our lab with degree of deacetylation of 90% and was purified before using. Bamboo charcoal based active carbon (200-300 mesh with specific surface area of 950 m^/g) was prepared by physical activation from bamboo using steam as an activating agent [10].
2.2. Preparation of Charcoal/Chltosan (CCB) and Acid Treated Bamboo Charcoal/Chitosan Biocomposite (ACB)
Preparations of CCB abd ACB were described elsewhere [10]. In brief, a solution of chitosan (2%) was prepared by dissolving determined chitosan in 50 ml of 1.0 M acetic acid. Next, 12 ml of chitosan was mixed with activated carbon or acid freated charcoal (3 g) and agitated for 1 h. Then, the viscous solution was sprayed dropwise through a syringe, at a constant rate, into neufralization solution containing 10% NaOH and methanol in a volume ratio of 3:1. The resulting solution was left for 2 h.
The formed biocomposite was washed with boiled and chilled deionized water until solution become neufral, and then dned at 80°C.
2.3. Adsorption Experiments
Adsorption of biocomposite was performed in a set of Erlenmeyer flasks (250 ml), where solutions of Cr(VI) ion (20 ml) with initial concentrations of 20 mg/1 were place in these flasks. The original pH 1 of the solution was used. 0.4 g of CCB or ACB (adsorbents) with particle size (2-3 mm) were added to ion solutions, and the mixtures were then kept at room temperature. The aqueous samples were taken at present time intervals, and the concenfrations of Cr ions was analyzed specfrophotometncally at 540 nm using 1,5-diphenyl carbazide as the complexing agent (using standard methods recommended for examination of water and wastewater) [11]. The amount of adsorption at time t, q, (mg/g), is calculated by q, = (Co - CO*V/W where Co and Ct (mg-l) are the liquid-phase concenfrations of Cr ions at initial and any time t, respectively; V is the
Nguyen Van Cuong, et al.
volume of the solution (I); W is the mass of dry adsorbent used (g).
Isotherm data were obtamed by placmg 0.4 g bicomposite in chromium solutions of different initial concentration (5-25 mg/1). The efTect of pH on the adsorption of Cr(IV) ions was examined by mixing 0.4 g biocomposite, 20 ml of Cr ion solution (20 ppm), equilibrium time 6 h and the pH ranging from 1 to 6. The amount of adsorption was determined in the same way as described above. All the experiments were performed in triplicates.
3. RESULTS AND DISCUSSION 3.1. Effect of Agitation Time
The effect of agitation time on removal efficiency of Cr was studied by varying the agitation time from 0 to 7 h. Removal efficiency of Cr(IV) ions by biocomposite is shown in figure 1. The results indicate that removal of Cr(VI) by sorption on both biocomposites from aqueous solution increase with time till equilibrium is attained in 5 h. Moreover, the acid freated-charcoal/chitosan biocomposite presented a higher removal efficiency than that of nonacid freated-charcoal/chitosan biocomposite at first four hours of contact. This may be due to the freatment of charcoal with phosphoric acid would introduce more acidic C=0 groups on the surface of charcoal. This would enhance the elecfrostatic interaction between chitosan and the more negatively charged acid freated charcoal/chitosan.
Similar result was reported by other group [11]. The removal efficiency is similar for both biocomposites after 5 h of contact. Further increase in contact time did not increase the uptake due
IflO-
- ^ BU-
E 6 0 -
- « •
s
D
l | ^ - " '
j
- • - C C B - • - A C B ;
Fig. 1: Effect of time on the adsorption of Cr(VI) ion on two adsorbents
VJCVol. 51(5), 2013
to deposition of metal ions on the available adsorption sites on the adsorbent materials. At 0.5 h, the removal efficiencies are 45.99% and 50.33% for BCC and ACB. respectively. However, the removal efficiencies increase to 95.25% and 96.1% for BCC and ACB, respectively; then get equilibrium at 99% for both biocomposites at 7 h.
It noted that the amount of Cr(VI) adsorbed onto CCB increases from 0.27 mg/g to L26 mg/g by increasing Cr(VI) concenfration from 5 mg/1 to 25 mgA. More interesting, equilibrium time increased with increasing of initial Cr(IV) ions concentration, e.g. the equilibrium time was 3 h for 5 and 10 ppm, and 4 h for 15, 20 and 25 ppm (figure. 2).
Removal ofCr(VI) ions in aqueous...
shown, the removal efficiency of Cr(VI) ions decreased with mcrease of pH from I to 6. The maximum removal was at pH 1 for both ACB and CCB. Additionally, ACB presented a higher removal efficiency than that of CCB. It can be explained that because the pH of the aqueous solution affects the speciation of chromium and the surface charge of the adsorbent. Cr(VI) exists in different forms in aqueous solution and the stability of these forms is dependent on the pH of the system. At pH less than 1.0, the chromium ions exist in the form of H2Cr04, while in the pH range of 1.0-6.0, different forms of chromium ions such as Cr207~, HCr04~, CraOjo' and so on. These forms change to Cr207^~ and Cr04^~
when pH increases. At pH lower than 4.0, the amino groups (-NH2) of chitosan would be in protonated cationic form (-NHj*) and Cr(VI) exist predominantly as HCrO*' in aqueous solution which result in a sfronger elecfrostatic interaction occurs between the sorbent and HCr04~ ions resulting in high chromium removal. The higher pH decreases adsorption capacity, it may be explained by existence of the Cr04^~ anions.
Fig. 2: Effect of initial concentration on the adsorption of Cr(VI) ion on BCC adsorbent 3.2. Effect of Initial Ion Concentration
The dependence of initial Cr(VI) ions on the sorption efficiency was studied by varying the amount of Cr(VI) ion from 5 ppm to 25 ppm, while keeping other parameters (pH, contact time and amount of biocomposite) constant. Figure 3 that the removal efficiency of Cr(VI) increases from 95% to 99% by increasing Cr(VI) concenfration from 50 mg/1 to 25 mg/1 for both ACB and CCB. The ACB presents the maximum removal of Cr (VI) at 25 mg/ml (99%), while CCB shows maximum one at IS mg/ml (98%) and decrease when the initial concenfration increase from 15 mg/ml to 25 mg/ml (97.5%). This may be explained by the ACB has more carboxylic groups on the surface led to more Cr(VI) ions to be adsorbed (figure 3).
3.3. Effect of pH
The effect of pH on the removal efficiency of Cr(VI) ions by biocomposite was determmed at pH 1-6. The results were demonsfrated in figure 4. As
Fig. 3: Effect of initial concenfration on the removal of Cr(VI) ion 3.4. Adsorption Isotherms
Adsorption isotherms can be generated based on numerous theoretical models such as Langmuir, Temkin and Freundlich models. In this study, Lang- muir, Freundlich models were used to determine the adsorption equihbrium between the adsorbent and metal ions. The results were shown in table 1. The Langmuir model assumes that a monomolecular layer is formed when adsorption takes place without any interaction between the adsorbed molecules.
The Langmuir model can be represented as:
VJC, Vol, 51(5), 2013 K,.C.
Nguyen Van Cuong, et al.
?,=
I l i a , ( l ) o r — = , — + —^
q, K, C, K, (2) l + a , , Q
Where Q is the equilibrium concentration (mg/L), q^
the amount of metal ion sorbed (mg/g), KL and a^
are a constant related to the affinity of the bmding sites.
m . Fig. 4: Effect of pH on the adsorption of Cr(VI) ion
on adsorbent
The Freundlich isotherm is an empirical equation assuming that the adsorption process takes place on heterogeneous surfaces and adsorption capacity is related to the concenfration of Cr(VI) at equilibrium. This isotherm model is defined below.
\_
n Where Ce is the equilibrium concenfration (mg/L), q^
the amount of metal ion sorbed (mg/g), Kf and n are the Freundlich constants which are related to adsorption capacity and intensity, respectively.
• CCB BACB
3.5 3 - 2.5 - 5" 2 1.5
1 0.5
y=0,494x-0.011 , • / • R'=0,98 y ^
. - • • > '
yy^ V = 0,427)i + 0.047 4>» R= = 0,968
Fig 5 Langmuir plot for adsorption of Cr(VI) upon CCB and ACB
y=l,OS3x-0,334 M R' = 0.948 _^ •
Fig. 6: Freundlich plot for adsorption of Cr(VI) upon CCB and ACB Table I: Physico-sorption constants for the
adsorption of Cr(VI) ion
Adsorbent
ACB CCB
Langmuir constant
K , 2.34 2.02
" L
0.11 0.02
R^
0.968 0.984
Freundlich constant Kp 2.15 2.0
1 n 1.05 1.0
R ' 0.948 0.970 As results, the adsorption of Cr(VI) ions by both biocomposite obey Freundlich and Langmuir equations indicating beneficial adsorption occurring through a monolayer mechanism and heterogeneous surfaces involving physisorption and/or chemisorption.
4. CONCLUSION
In this present work, the biocomposite of charcoal/chitosan and acid freated charcoal/chitosan were as effectively adsorbents for removal of Cr(VI) ion from aqueous solution. Batch experiments showed that the initial Cr(VI) ions concenfration, contact time and the pH affect the adsorption capacity of Cr(VI) ions onto biocomposite. These bioadsorbents can be a good candidate for removal of heavy metal ions in wastewater.
Acknowledgments: 77ie authors would like to thank Ho Chi Minh City University of Industry and Ho Chi Minh City University of Food Industry for financial support under project 2012. The authors thankMiss Ngoc Han for help on sample preparation
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Corresponding author. N g u y e n V a n C u o n g
D e p a r t m e n t of Chemical Engineering, Industrial University of H o Chi M i n h City 12 N g u y e n V a n B a o , G o Vap, H o Chi M m h City
E m a i l : n v c @ h u i , e d u . v n T e l : 0 9 8 5 7 7 8 6 9 2 .
