Removal of Copper (II) From Aqueous Solution by Khangar

Removal of Copper (II) From Aqueous Solution by Khangar

Garima Nagpal1, A. Bhattacharya2 and N. B. Singp*

1Research and Technology Development Centre, Sharda University, Greater Noida, India

2Department of Chemistry, University of Petroleum & Energy Studies, Dehradun-248007 India

Abstract

Process of removal of Cu(II) from aqueous solution of copper sulphate by adsorption on the surface of Khangar has been studied by orthogonal array L16. The adsorption process parameters viz pH, adsorbent dose, metal ion concentration and contact time have been optimized by using Taguchis method. The results have been analyzed using signal to noise ratio and ANOVA at 95% confidence level for all considered parameters. By the study it is observed that pH is the most important parameter for removal of Cu(II) from aqueous solution. The optimum results are obtained on pH 7; adsorbent dose 0.7 g/50ml; metal ion concentration 20 mg/L and contact time 120 minutes. To support the analysis, SEM and FTIR examination of Khangar has been done before and after adsorption of Cu(II) from aqueous solution.

Keywords: Cu(II), FTIR, Khangar, Optimization, Taguchis method.

1. Introduction

Industrialization has resulted in release of heavy metals in the environment in larger amounts. These contaminants can be accumulated by living organisms throughout the food chain as a non biodegradable pollutant [1]. Among heavy metals, copper poses a significant threat to the environment and public health due to its toxicity. Copper is introduced into natural waters by variety of industrial wastewaters such as electroplating, paper manufacturing, pesticides, herbicides and tannery industries. The continuous intake of Cu by human being leads to necrotic changes in the liver, kidney, mucosal irritation, wide spread capillary damage, depression, weakness, lethargy and lung cancer etc. [2, 3]. To remove heavy metals from aqueous solution many physico-chemical methods such as membrane filtration, coagulation, chemical precipitation, ion exchange and electrodialysis have been used [4-7]. The application of such processes is often limited because of technical/economical

constraints. However the adsorption technique is one of the preferred methods for removal of heavy metals because of its efficiency and low cost. The most common adsorbent materials are: alumina silica, metal hydroxides and activated carbon [8]. Many reports have appeared on the development of low-cost activated carbon adsorbents developed from cheaper and readily available materials. Activated carbons with their large surface area, microporous character and chemical nature of their surface have made them potential adsorbents for the removal of heavy metals from industrial waste water. In this paper carbon waste obtained from coal refining industry (known as Khangar) has been used as an adsorbent for removal of Cu(II) from aqueous solution under different conditions. Adsorption studies have been made and optimum conditions for Cu(II) removal have been established. The effect of various operational parameters such as pH, adsorbent dose, metal ion concentration and time were investigated by Taguchi method using MINITAB software [9].

2. Material and Methods

1. Preparation of adsorbent and adsorbate

   In this study the adsorbent used was carbon waste obtained from coal refining industry (Ghaziabad) commonly known as Khangar. The collected material was washed three times with deionized water and then air dried for several days. It was oven dried at 110°C for 2 hrs. The dried carbon waste was crushed in a mechanical grinder and sieved through 350µm mesh sieve to obtain fine powder. Cu(II) solution was prepared by dissolving copper sulphate in deionized water.

2. Characterization of adsorbent (Khangar)

   The FTIR spectra of Khangar before and after adsorption of Cu(II) were recorded in KBr phase in the wave length range 400 4000 cm-1.

   SEM pictures of gold coated Khangar before and after Cu(II) adsorption were also recorded. Gold coating was done by sputtering technique.

3. Design of Experiment by Taguchi Method

   Taguchis optimization technique is a unique and powerful technique that allows optimization with minimum number of experiments [9]. Taguchi developed a method for designing experiments to investigate how different parameters affect the mean and variance of a process performance characteristic that defines how well the process is functioning [10]. To design the experiment Taguchi used the orthogonal array approach. In our experiment we have used L16 Taguchis orthogonal array. The decision of orthogonal array depends upon the number of process parameters and their levels [11-12]. In this study we used the effect of four parameters pH of the solution, adsorbent dose, initial metal ion concentration and contact time and their four levels as shown in Table-1 and Table-2. Taguchi method has been used to assess the interaction of parameters to maximum adsorption of metal ions through the signal to noise (S/N) Ratio by software MINITAB 15. Usually, three types of S/N ratio analysis are applicable: (1) lower is better (LB), (2) nominal is better (NB) and (3) higher is better (HB) [13]. The target of this study is to maximize the metal removal efficiency, so the S/N ratio with HB characteristics are required which is given by following equation:

   Where Y is value of characteristic in an observation j, R is number of repetition in a trial.

   Table 1. Factors and Levels in experimental design

   Analysis of Variance (ANOVA) is also a part of Taguchis analysis to identify the most influential factor in the process. It is a powerful statistical analytical tool that separates the total variability within the data. The Fischer ratio (or F test) of the ANOVA is used to determine significant process factors. The F value for each process factor is simply a ratio of the mean of the squared deviations to the mean of the squared error. In ANOVA F- test is used to calculate expected values of process parameters to identify the variation of results from limits [13-14].

4. Experimental procedure

Experiments were performed in batch manner by synthetic solution. A stock solution containing 1000 mg/L of Cu(II) was prepared using copper sulphate in double distilled water. The pH of the solution were adjusted using 0.1N NaOH and 0.1N HCl. All reagents used were of analytical grade. The experiment was conducted in batch of three experiment and the process parameters were adjusted as per Table-2.

Table 2. Experimentation data of OA L16

The sorption capacity and the percent removal of Cu(II) from aqueous solutions by Khangar were calculated by following equations:

Where q percent

eq is the adsorption capacity (mg/g), n is removal (%), C0 is the initial Cu(II)

two new bands (Figure.4) appearing at around 3180 and 1400 cm-1. First band suggests that Cu is adsorbed on Khangar where hydroxyl groups are present. Appearance of a new band at 1400 cm-1 also suggests association of copper with carboxylate group.

concentration (mg/L), Ce is the equilibrium Cu(II) concentration (mg/L), m is the sorbent mass (g) and V is the volume of metal solution put in contact with the adsorbent [15-16].

To analyze the obtained data we used regression technique, statistical tool ANOVA and the confidence intervals as suggested by Taguchi, i.e., CIPOP (confidence interval for the population), and CICE (confidence interval for a sample group). They were used at 95% of confidence level. It is the estimated mean of the optimal treatment conditions.

In order to verify the conclusions drawn from the experiments, certain confirmation experiments were carried out at the optimum conditions for the significant parameters. The average of the results of the confirmation experiments are compared with the anticipated average based on the parameters and levels tested.

3. Results and Discussion

Figure 2. Khangar after adsorption

52

50

567.01cm-1

3421.86cm-1 618.87cm-1

2923.68cm-1

45

40

Scanning electron microscopic pictures of Khangar

%T

%T

before and after adsorption of Cu(II) are shown in 35

Figures. 1 and 2 respectively. 30

794.00cm-1

471.36cm-1

25

20

15

14

4000

1091.21cm-1

3500 3000 2500 2000 1500 1000 500450

cm-1

Name Description

KHA-1_1 Sample 003 By pharmacy Date Tuesday, August 30 2011

Figure 3. FTIR Image of Khangar Before Adsorption

34

32

30 618.11cm-1

474.83cm-1

28

26 7 9 5 . 0 8 c m – 1

%T

%T

24

Figure 1. Khangar before adsorption

It appears that the surface morphology is slightly changed after Cu(II) adsorption. The FTIR spectrum of

22 3130.03cm-1

20

18

16

3500 3000 2500 2000 1500 1000 500450

cm-1

3500 3000 2500 2000 1500 1000 500450

cm-1

14

4000

4000

1400.28cm-1

1094.61cm-1

Name Description

Khangar (Figure.3) shows a broad vibrational band at 3420.18 cm-1 indicative of the existence of bound OH group due to adsorption of water molecules. Band at about 2900 cm-1 is due to CH stretching modes. A strong band at 1091.09 cm-1 may be due to =C=O stretching vibrations. In the Cu (II) treated Khangar,

WE(cu)_1 Sample 007 By pharmacy Date Tuesday, February 14 2012

Figure 4. FTIR image of Khangar after Cu(II) Adsorption

Analysis of the mean and S/N ratio for each experiment give the optimum level of process parameters for the maximum removal of the metal ions from the aqueous solution. For raw data, mean response and S/N ratio is calculated and shown in Table-3. Response of various parameters on percentage removal of Cu(II) ions clearly shows that the most influential pH is 7 at level

Table 3. Experimentation results of OA L16 of S/N Ratio and Mean

contact time for metal removal is most influential at third level i.e. 120 min as shown in Table-4. The contribution of individual factors on the adsorption of Cu(II) ions onto Khangar are given in Table-5&6. It shows ANOVA results for S/N ratio data and Mean. The percentage contribution of each parameter is also given in these tables. It is observed that pH parameter is the most significant factor with 87.97 and 86.8% contribution to mean and S/N ratio data, respectively. The value of coefficient of correlation R2 is near to 1.0 and it shows good agreement with R2 (adj) 0.996.

Main Effects Plot for SN ratios

Data Means

pH

39.0

38.5

Mean of SN ratios

Mean of SN ratios

38.0

37.5

37.0

adsorbent dose

1

39.0

38.5

38.0

37.5

37.0

1

3 5

10 20

7 0.1

30 60

0.3

90

0.5

120

0.7

150

Signal-to-noise: Larger is better

85

80

75

70

85

80

75

70

Figure 5. Variation of S/N ratio with pH, Adsorbent dose, Initial Metal ion concentration and Time

Main Effects Plot for Means

Data Means

pH adsorbent dose

Main Effects Plot for Means

Data Means

pH adsorbent dose

1 10 20 30 60 90 120 150

1 10 20 30 60 90 120 150

four, adsorbent dose is most influential i.e., 0.7 gm

/50ml at level four, the initial metal ion concentration is most influential at the third level i.e. 20 mg/L and

1

1

3

3

5 7 0.1 0.3 0.5 0.7

5 7 0.1 0.3 0.5 0.7

initial metal ion conc. time

initial metal ion conc. time

85

80

75

70

85

80

75

70

Mean of Means

Mean of Means

Figure 6. Variation of Means of means with pH, Adsorbent dose, Initial Metal ion concentration and Time

"Table 4. Effects of various factors on S/n Ratio and Average Response

Table 5. ANOVA for S/N Ratio (% Removal)

S/N: Signal to Noise; ANOVA: Analysis of Variance; DF: Degree of freedom S = 0.04783 R-Sq = 99.9% R-Sq(adj) = 99.6%

Order of significance: 1. pH of the solution, 2. Adsorbent Dose, 3. Initial Metal Ion Conc., and 4. Contact time

*Significance at 95% level.

Mean response and S/N ratio (Fig 5, 6) clearly indicate that the removal of Cu(II) from aqueous solution is maximum at near to neutral pH. It can be inferred that at higher H+ conc., the adsorbent surface becomes more positively charged, thus reducing the attraction between adsorbent and metal ion. As pH increases, negatively charged surface become available, thus promoting greater metal uptake [17, 18]. At higher pH, the Cu(II) ions precipitated as their hydroxides which decrease the rate of adsorption and subsequently the percent removal of metal ions.

Fig 5 & 6 shows an increase in percentage removal of Cu with the increase in dose of adsorbent up to a certain limit and then it remains almost constant.

Increase in adsorption with adsorbent dosage can be attributed to the availability of more adsorption sites [19, 20].

From plots of mean and S/N ratio it is found that the percent removal increased with time. However the removal was found to be the maximum at 120 minutes. The time required to attain this value is termed as the equilibrium time [21]. From the figure, it is evident that the percentage removal shows an increasing trend up to 20 ppm and then decreases continuously. It appears that at lower concentrations, all Cu(II) ions present in the sorption could interact with the binding sites of Khangar, hence higher removal. However at higher concentrations, because of the saturation of the sorption

Table 6. ANOVA for Mean (% Removal)

S/N: signal-to-noise; ANOVA: analysis of variance; DF: Degree of freedom

S = 0.2287 R-Sq = 100.0% R-Sq(adj) = 99.9%

Order of significance: 1. pH of the solution, 2. Adsorbnet Dose, 3. Initial Metal Ion Conc., and 4. Contact time

*Significance at 95% level.

sites, the percentage uptake of the copper by Khangar shows a decreasing trend. A similar trend was reported by [22].

F- Test was used to predict the results with optimization [10]. The mean at the optimal condition (optimal value at response characteristics) is calculated from following equation:

Where, is the overall mean of the response = 78.43 (Table 3), and is the average removal of metal ion concentration at level four of parameter pH of the solution = 86.70 (Table 4), is the average removal of metal ion concentration at level four of parameter Adsorbent dose = 80.75 (Table 4), is the average removal of metal ion concentration at level three of parameter Initial metal ion concentration = 79.28 (Table 4), and is the average removal of metal ion concentration at level three of parameter contact time =

78.97 (Table 4). Hence,

90.40 MPa

The confidence interval for the predicted mean for the confirmation experiment can be calculated by following equations

Where,

F (1, fe) = F ratio at a confidence level of (1 ) against Degree of freedom (DF) 1& error DF, fe; Ve = Error variance (from ANOVA); and eff is calculated from following equation:

Where, N = Total number of results

R = Sample size for the confirmation experiment

By substituting values N: total no of results = 16 X 3= 48; fe =3 and Ve =0.052 (from Table 6); F0.05(1,3) =

10,1 (tabulated F-value) in equations 5 to 7. The 95%

confidence intervals (Clpop and ClCE) of the predicted ranges for adsorption of Cu(II) ions onto Khangar are given in Table 7.

In Taguchi method confirmation experiment is the last step to verify the results, which we get from Taguchis experiment approach. The experiment was conducted by setting all the process parameters at optimum level in a batch experiment. The average

Table 7 Responses at optimum levels of process parameters

values obtained are compared with the predicted values (Table 7). The results are in range of predicted values. This shows the successful optimization of experiment.

4. Conclusion

From the results it is concluded that 91% Cu(II) can be removed by using Khangar as an adsorbent. This is much better adsorbent as compared to conventionally used carbon powders. The optimized conditions for Cu(II) as found by Taguchi method are pH – 7; adsorbent dose – 0.7g/50ml; initial metal ion conc. – 20 mg/L and contact time – 120 minutes. It is also found that the experimental values are in the predicted range. To access the commercial feasibility of Khangar as an adsorbent for removal of heavy metal ions particularly Cu(II) ions further experiments are required.

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