Modification of activated carbon using sodium acetate and its regeneration using sodium hydroxide for the adsorption

Modification of activated carbon using sodium acetate and its regeneration using sodium hydroxide for the adsorption

of copper from aqueous solution

Dan Mugisidi

^a,*

, Aria Ranaldo

^a

, Johny W. Soedarsono

^b

, Muhammad Hikam

^c

aPostgraduate Material Science Program, Faculty of Mathematics and Natural Science, University of Indonesia, Jakarta, Indonesia

bDepartment of Metallurgy and Materials, Faculty of Engineering, University of Indonesia, Jakarta, Indonesia

cDepartment of Physics, Faculty of Mathematics and Natural Science, University of Indonesia, Jakarta, Indonesia

Received 17 September 2006; accepted 8 December 2006 Available online 20 December 2006

Abstract

Activated carbon from coconut shell was modified with sodium acetate at concentrations of 10% and 15%, and used in a fixed-bed column to study the adsorption of copper ions. Synthetic wastewater containing 258 mg/l of Cu was passed through plain activated carbon and modified activated carbon. Plain activated carbon was able to adsorb 20 mg of Cu, and activated carbon modified by treat- ment with 10% sodium acetate adsorbed 33 mg of Cu. The highest adsorption capacity was found for the activated carbon modified by treatment with 15% sodium acetate, which adsorbed 45 mg of Cu; i.e. 2.2 times as much as the plain activated carbon. After regeneration with 0.71 M NaOH, the activated carbon modified by treatment with 15% sodium acetate was able to adsorb 60 mg of Cu; i.e. three times as much as the plain activated carbon.

Ó2006 Elsevier Ltd. All rights reserved.

1. Introduction

Heavy-metal pollution from the electroplating industry is a serious problem. Many industries use Cu, which has negative effects on the water environment.

Various studies have been done to investigate the adsorption of heavy metals using activated carbon. Recent studies have shown that activated carbon is able to adsorb heavy metals because of its large surface area, micro-pore character and acidic functional groups.

Ouki and Newfeld [1] used a column technique to adsorb Cr(VI) and found that the capacity increased signif- icantly at acidic pH values, where a redox reaction occurred on the carbon surface when Cr(VI) was reduced to Cr(III). The activated carbon reactive sites were oxi- dized, resulting in an increased removal capacity.

Aggarwal et al.[2]reported that only a small fraction of the surface area of activated carbon is occupied by Cr(III) ions. After oxidation of the carbon surface, the area covered increased but was still considerably smaller than the BET surface area. Furthermore, they mentioned that interaction might occur during the removal of metal ions with plain activated carbon (PCA), and that metal ions are exchanged with the acidic functional groups (phenolic, carboxylic, lac- tonic, hydroxyl and carbonyl) on the carbon surface.

Adsorption by activated carbon is much more effective for removing organic compounds than it is for removing metals and other inorganic pollutants. Modification with suitable chemicals to increase the adsorption capacities of activated carbon has been investigated. Leyva Ramos et al. [3] used aluminum-impregnated carbon for the removal of fluoride from aqueous solutions, and observed that the adsorption capacity for fluoride was about three to five times than that of PCA.

Chen and Wang [4] obtained results for the effective removal of copper, zinc and lead from synthetic wastewater

0008-6223/$ - see front matter Ó2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2006.12.009

* Corresponding author. Tel./fax: +62 21 8502057.

E-mail address:olefin@indosat.net.id(D. Mugisidi).

www.elsevier.com/locate/carbon Carbon 45 (2007) 1081–1084

using a fixed bed of granulated activated carbon. They pre- treated the carbon with deionized water at a certain pH, and found that the removal of metal ions decreased when other metal ions were added.

Sodium diethyl dithiocarbamate (SDDC) has been used to modify activated carbon for multi-species metal ions (Cu, Zn, Cr) removal and has an effective removal capacity for Cu (four times), Zn (four times) and Cr (two times) greater than plain carbon[5].

Citric acid was used to modify a commercially available activated carbon to improve the adsorption of copper ions from aqueous solutions. The results showed that this mod- ification reduced the specific surface area from 650 m²/g to 430 m²/g, while increasing the adsorption capacity from 6 mg of Cu/g to 15 mg of Cu/g[6].

The objective of this study was to remove the Cu(II) ions from synthetic industrial wastewater using modified acti- vated carbon, knowing its removal mechanism and regen- eration of modified activated carbon.

2. Approach and methods 2.1. Materials

Granular activated carbon from coconut shell was purchased from Indo Karbon, Indonesia, with 60lm and a density of 0.4–0.5 g/l. Sodium acetate and copper sulphate were purchased from Merck (Germany).

2.2. Preparation

Plain activated carbon was sieved and washed with deionized water to remove any fine powder, then dried in an oven at 60°C for 24 h.

One sample of the PCA was reacted with 10% sodium acetate for 72 h and another sample was reacted with 15% sodium acetate for 72 h, then dried in the oven at 60°C for 24 h to produce modified activated carbon types MCA10 and MCA15, respectively.

2.3. Method

Activated carbon was packed into a glass column (inner diameter 1 cm and height 30 cm). Feed solution was pumped in a down-flow mode at a flow-rate of 2 ml/min. The samples were taken of the solution, to act as a reference, and of the effluent.

The concentrations of metal ions were determined with an atomic absorption spectrophotometer (Perkin Elmer 3110). The pH measured with an Orion model 525 A pH meter. Specific surface areas were mea- sured with AUTOSORB QUANTACHROME AS - 69/USA.

3. Results and discussion 3.1. Metal ion removal

This study was designed to increase the adsorption capacities and rates of Cu(II) removal by the modification of the activated carbon. When the modifying chemicals are immobilized at the surface of the activated carbon, the removal mechanism is changed[2], because the functional group at the activated carbon surface is replaced with a functional group from the modifier chemicals.

The results inTable 1show that the amounts of Cu(II) ions removed are in the order PCA < MCA10 < MCA15.

The modifier chemicals increased the activated carbon capacity with increased concentration of the modifier.

This result is similar to that reported by Chen et al.[6]

that the modification of activated carbon increased its capacity. The adsorption capacity for MCA10 was 2.2 mg/g and for MCA15 it was 3 mg/g.

The ratio between the influent and the effluent concen- tration (Ceff/Cinf)versus time (in min) is plotted inFig. 1.

The higher the value of the ratio, the fewer metal ions are removed. Fig. 1 shows the values of the ratio versus time for PCA and for MCA10 and MCA15. Cu(II) ion removal by the PCA was less than that by the MCAs, and the amount of Cu(II) ions removed from the solution increased with increased concentration of sodium acetate.

The higher concentration of sodium acetate contained a greater number of carboxylic functional groups. As a result, metal ion removal increased because of the addition of the carboxylic functional groups at the surface of the activated carbon, and because the affinity of the carboxylic functional group for metal ions is very high[6].

Fig. 2 shows the pH of the effluent after passage over PCA, MCA10, and MCA15. The patterns are similar for all three, but the values are lowest for PCA. As can be seen, pH increased significantly at first contact and decreased gradually thereafter. Increased effluent pH was reported also by Chen and Wang [4]; in their study, the pH inc- reased sharply to 7.3 at the beginning of the experiments, and decreased gradually to the pH value of the influent.

Table 1

Removal capacity of plain and modified carbon

No. Adsorbent Cu(II) ion

mg mg/g

1 PCA 20 1.4

2 MCA10 33 2.2

3 MCA15 45 3.0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 15 30 45 60 75 90 105 120 135 150 165

Minutes Cinf/Ceffof Cu

MCA10 MCA15 plain Activated Carbon

Fig. 1. Cu(II) ion removal using the plain activated carbon, MCA10, and MCA15.

1082 D. Mugisidi et al. / Carbon 45 (2007) 1081–1084

Adsorption of hydrogen ions from the solution and disso- lution of some impurities from the granulated activated carbon contribute to an increase in the pH of the effluent.

The reaction of the functional group on the activated car- bon surface with metal ions also reduces the concentration of hydrogen in the solution.

Our experiment yielded similar changes of pH. After increasing at the first contact, the pH decreased gradually while the concentration of Cu(II) ions in the effluent solu- tion increased. In this study, the change of pH was influ- enced by the reaction between the functional groups at the surface of the activated carbon and metal ions, which is related to the removal capacity.

Faur-Brasquet et al.[7]reported that the activated carbon from coconut shell contained phenolic functional groups at the surface. The reaction between the phenolic functional groups on PCA and metal ion adsorbed H ions from the solution, and the pH of the solution increased initially and then decreased with decreased removal of metal ions.

The increase in pH with the MCAs has a different expla- nation. The reaction between sodium acetate and metal ions released Na ions, which came from a strong base, and increased the pH of the solution. The result indicates that the pH of the effluent is related to the removal of metal ions.

3.2. Mechanism for the removal of Cu(II) ions

The specific surface area of PCA in our experiment, which measures with gas sorption BET analyzer, was 480 m²/g, and this was reduced to 300 m²/g after modifica- tion by treatment with sodium acetate but the adsorption increased from 1.4 mg of Cu(II)/g to 3 mg of Cu(II)/g.

Chen reported that the surface area was reduced from 650 m²/g to 430 m²/g after modification, but increased the adsorption capacity from 6 mg of Cu(II)/g to 15 mg of Cu(II)/g[6]. It was an indication that the pore network adsorption was not the only mechanism involved.

In order to understand the mechanism underlying the process, an experiment was conducted in which a sample

of the column effluent was taken to measure the concentra- tion of Cu(II) and Na ions. As can be seen in Fig. 3, Na and Cu(II) ions have the same trends; Na ions in the efflu- ent decreased while Cu(II) adsorbed in the activated car- bon decreased. There was 250 mg/l of Cu(II) ions in the influent, which was reduced to 5 mg/l in the effluent after 7.5 min. The result was different for Na ions; 1 mg/l of Na ions was found in the influent, which was increased to 174 mg/l in the effluent.

The number of Cu(II) ions removed from the solution was almost equivalent to the number of Na ions added to the solution. From the following reaction, it can be seen that the result agreed with calculations, where Cu(II) was 3.85 mol and Na was 7.54 mol.

2CH₃COONa + CuSO₄!Cu(CH₃COO)₂+ NaSO₄ ð1Þ The hydrophobic groups were adsorbed onto the surface of activated carbon, while the ionic groups remained in the bulk solution and acted as a cationic exchanger [5]. Simi- larly, in our experiment, the organic group was adsorbed onto the surface of the activated carbon and the Na ion acted as the cationic exchanger.

3.3. Regeneration of MCA15

Regeneration should be a part of the process of using MCA in the process line. When a breakthrough occurred, certain chemicals flowed into the column to take the adsorbed Cu(II) ions from the MCA, which was later returned to service for another use. We used 0.7 M NaOH to take the Cu(II) ions from the activated carbon and to recover the Na ions at the surface functional groups.

Fig. 4plots the concentration of Cu(II) ions in the efflu- ent of the regeneration process. In the first run, the effluent contained 2550 mg/l of Cu(II) ion. In the second sample, the Cu(II) ions in the effluent decreased significantly. The experiment was stopped after 30 min because the amount of Cu(II) ions contained in the effluent became small.

The end result of regeneration was 39 mg of Cu(II) ions, which is 87% of the Cu(II) ions adsorbed by MCA15.

1 3 5 7 9 11

0 7. 5 15 22.5 30 37.5 45 52.5 60 67.5 75 82.5 90 97.5 105 113

Minutes

pH

pH MCA15 pH MCA10 pH Plain Activated Carbon

Fig. 2. pH at the effluent of the plain activated carbon, MCA10, and MCA15.

0 1 2 3 4 5 6 7 8 9

7.5 15 22.5 30 37. 5 45 52.5 60 67.5 75 82.5 90 97. 5 105 113 minutes

mol

Cu(II) in Activated carbon Na in effluent solution

Fig. 3. Ion exchange mechanism of Na and Cu(II) in MCA15.

D. Mugisidi et al. / Carbon 45 (2007) 1081–1084 1083

In Fig. 5, the Cu(II) ions in the effluent from the acti- vated carbon after the regeneration of MCA15 (MCA15Reg) is plotted and compared with MCA15. The adsorption by MCA15Reg was 60.1 mg of Cu(II), which is an increase by about 1.5 times of MCA15 and 3 times of PCA. The activated carbon contained surface functional groups that make a large contribution to metal ion adsorp- tion onto the activated carbon [4,5]. The addition of

sodium acetate increased the number of functional groups on the activated carbon surface. In MCA15Reg, the Na ions took the place of the Cu(II) ions that were adsorbed on the activated carbon surface. In this process, the Na ions also react with other surface functional groups and thereby increase the number of functional sites on the acti- vated carbon surface. This is the reason why the adsorption by MCA15Reg was greater than that by MCA15.

4. Conclusions

An experiment to remove Cu(II) ions from synthetic wastewater with MCA was conducted. The results showed that modification of activated carbon using 15% sodium acetate (MCA15) had a capacity greater than 3.0 mg of Cu(II)/g, while the capacity of PCA was only 1.4 mg Cu(II)/g. The mechanism of removal was cationic exchange, which occurred because the Na ions act as cat- ionic exchangers. After regeneration, the adsorption by MCA increased, from 45.1 mg of Cu(II) to 60.1 mg of Cu(II). This was because there were more Na ions on the activated carbon reactive sites after regeneration.

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0 500 1000 1500 2000 2500 3000

7.5 15 22.5 30

Minutes

mg/l

Cu(II) in regeneration solution of MCA15 Fig. 4. Cu(II) in effluent of regeneration process of MCA15.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 15 30 45 60 75 90 105

120 135

150 165 180 195 210

Minutes Cinf/Ceff

MCA15 MCA 15 Reg

Fig. 5. Cu(II) ion removal using MCA15 and MCA15 Reg.

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