Indian Journal of Chemical Technology Vol. 18, September 2011, pp. 414-420

Removal of Ni (II) from aqueous solutions by adsorption onto Cajanus cajan L Milsp seed shell activated carbons P Thamilarasu1*, P Sivakumar2 & K Karunakaran3 1 Department of Chemistry, AMS Engineering College, Namakkal, 637 013, India Department of Chemistry, A A Govt. Arts College, Namakkal, 637 002, India 3 Department of Chemistry, Sona College of Technology, Salem, 636 005, India Received 23 September 2010; accepted 7 July 2011 The adsorptive removal of Ni(II) from aqueous solution using Cajanus cajan L Milsp seed shells activated carbon (CCC) and polypyrrole coated Cajanus cajan L Milsp seed shells activated carbon (PPy/CCC) has been carried out under various experimental conditions. Quantity of Ni(II) uptake at 50 mg of activated carbon is 25.75 mg/g for CCC and 29.60 mg/g for PPy/CCC. Adsorption data are modeled with Freundlich, Langmuir and Temkin adsorption isotherms. Thermodynamics parameters, such as ∆Ho, ∆So, and ∆Go have been calculated and the findings indicate that the adsorption is spontaneous and endothermic. Enthalpy change values range from 8.90 kJ/mol to 23.04 kJ/mol, and based on these values the adsorption of Ni(II) by CCC could be a physisorption. A mechanism involving intra particle diffusion and surface adsorption has been proposed for the adsorption of Ni(II) onto the adsorbent. Adsorbent used in this study is also characterized by FT-IR and SEM before and after the adsorption of metal ions. 2

Keywords: Activated carbon, Adsorption, Cajanus cajan L, Isotherm, Kinetics, Nickel(II)

Water pollution mainly occurs due to organic and inorganic wastages, sediments, radioactive materials, effluents, sewage and heavy metals. Among these pollutants, contribution of heavy metals is a major concern because of its toxicity, bioaccumulation, persistence and non-biodegradable nature. Large scale extraction of metals releases more amount of hazardous wastes. High concentrations of heavy metals in rivers discharged from metal extraction have adversely affected fisheries in more than 21,000 km of rivers1. The metals which are often cited for their impact on the environment are nickel, cadmium and selenium2. In India, acceptable limit of Ni in drinking water is 0.01 mg/L and maximum permissible limit is 0.1 mg/L (ref. 3). According to Environmental Protection Agency, USA, permissible limits of Ni(II) in wastewater is 1.0 mg/L and by Bureau of Indian Standards it is 3.0 mg/L (ref. 4). Nickel poisoning causes headache, dizziness, nausea and vomiting, chest pain, tightness of the chest, dry cough and shortness of breath, rapid respiration and extreme weakness to human beings. Toxic nature of nickel to fish, lentil plants, crops and algae were also reported5. ___________ *Corresponding author. E-mail: [email protected]

Hence, it is essential to remove Ni(II) before discharging them into water bodies. Conventional treatment technologies like precipitation and coagulation have become less effective and more expensive. It is essential to explore the feasibility of the application of several low cost, non-conventional adsorbents obtained from agricultural wastes. Removal of metals by low cost and easily available materials like flyash6, orange peel7, kyanite8, baker’s yeast9, bagasse fly ash10, geothite11 and sewage sludge12 were studied recently. In this study, Cajanus cajan L Milsp seed shell, an economical and low cost material, has been used to adsorb nickel from aqueous solutions. Experimental Procedure Preparation of adsorbents

Raw material (Cajanus cajan L Milsp seed shell) was procured from local vendor. The material was washed in hot distilled water to remove earthy matter, cut into small pieces and dried in sunlight. The dried material was soaked in concentrated sulphuric acid (1:1 ratio of acid volume and weight of the material). Then, charred material was washed several times with distilled water until pH of the washing becomes neutral. The material was dried and activated at 500°C using muffle furnace. Finally, activated carbon was

THAMILARASU et al.: REMOVAL OF Ni (II) FROM AQUEOUS SOLUTIONS BY ADSORPTION

ground and sieved into a particle size of 180-300 micron size sieves. All reagents used for this study are commercially available Analar grade (Merck, SRL, India and SD-fine, India). Cajanus cajan L Milsp seed shells activated carbon (CCC) and polypyrrole coated Cajanus cajan L Milsp seed shells activated carbon (PPy/CCC) were used for the removal of Ni(II). Pyrrole was used as a monomer for preparing PPy/CCC. For the preparation of PPy/CCC, 5.0 g CCC (180-300 micron) was immersed in 50 mL of 0.2 M freshly prepared pyrrole solution for 12 h before polymerization. Excess monomer solution was removed by simple decantation. Then 50 mL of 0.5 M ferric chloride (oxidant solution) was gradually added into the mixture and then the reaction was allowed to continue for another 2 h at room temperature. The polymer coated CCC was filtered, washed in distilled water, then dried at 60oC in a hot air oven and sieved again before use13. Preparation of adsorbate solution

A stock solution of the adsorbate containing 1000 mg/L of Ni(II) ion was prepared by dissolving required quantity of Analar grade salt of nickel sulphate hexahydrate in doubly distilled water. Double distilled water was used throughout experiments. Stock solution was further diluted with distilled water as and when required. Batch mode adsorption studies

Adsorption experiments were carried out by batch equilibrium method. Exactly 50 mg of adsorbent mixed with 50 mL of adsorbate solution in iodine flask was taken and the mixture was agitated with different time intervals in a temperature controller water bath shaker (Technico). The solutions were centrifuged at regular intervals, and the residual concentration of Ni(II) ion present in the filtrate at each stage was determined using spectrophotometer at 470 nm (Systronics 169 model). Experiments were repeated at three different temperatures, viz. 30, 40 and 50°C for the removal of Ni(II). Results and Discussion Adsorbent characterization

All the reported physico-chemical characteristics of the adsorbent was analyzed using standard testing methods14. Activated carbon is widely used as adsorbent due to its high adsorption capacity, high surface area, micro porous structure and high degree

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of surface heterogeneity. The chemical nature and pore structure usually determine the sorption activity. Some important physico-chemical characteristics of CCC and PPy/CCC are presented in Table 1. The lower ash content values attributed to lower inorganic content and higher fixed carbon. The high surface area is considered to be most suitable for adsorption of adsorbates in aqueous solution. The lower bulk density value indicates highly branched and porous carbon with more void space. Acid soluble matter content was found to be high in carbon because of incorporated carbonate groups in the pores. Sodium and potassium contents may be due to presence of mineral sodium and potassium in the CCC seed shell. Effect of adsorbent dosage

Results of adsorption experiments carried out by using various dosages of CCC, PPy/CCC and 30 mg/L of Ni(II) solution is shown in Fig. 1. Result indicates that the adsorption capacity increases with increase of CCC and PPy/CCC dosages. This is due to the increase in number of available adsorption sites of adsorbent, which results in enhanced removal of Ni(II). Rate of adsorption of Ni(II) by CCC is moderate at an initial adsorbent dose of 50 mg and also shows an increasing trend at higher adsorbent doses like 200 and 250 mg. In view of this observation, it was decided to fix the dose of activated carbon as 50 mg. Quantity of Ni(II) uptake at 50 mg of activated carbon was 25.75 mg/g for CCC and 29.60 mg/g for PPy/CCC. Table 1Characteristics of CCC and PPy/CCC activated carbon Parameters

CCC

PPy/CCC

pH pHzpc Moisture content (%) Bulk density (g/mL) Solubility in water (%) Solubility in 0.25M HCl (%) Porosity (%) Specific gravity Volatile matter (%) Ash content (%) Fixed carbon (%) Sodium (mg/L) Potassium (mg/L) Phenol adsorption capacity (%) Conductivity (mS) Surface area (m2/g) Iodine number (m2/g)

7.48 5.24 2.80 0.48 0.81 4.91 59.10 1.07 8.15 3.86 85.19 67.00 3.30 36.20 1.85 532 453

8.06 5.78 2.60 0.36 0.69 2.86 71.43 1.26 5.26 3.65 88.49 62.00 3.10 39.34 1.89 560 440

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INDIAN J. CHEM. TECHNOL., SEPTEMBER 2011

Effect of pH

The pH is one of the most important parameters controlling the uptake of metal ions from their aqueous solution by adsorbents. Effect of pH for Nickel(II) removal was studied with carbon dosage of 50 mg, equilibrium time of 30 min and Ni(II) initial concentration of 30 mg/L for CCC and PPy/CCC. Figure 2 shows the effect of pH on the removal of Ni(II). The adsorption increases with the increases of pH from 1 to 5 and there is no appreciable change above the pH of 5.0. Influence on the Nickel(II)

removal at lower pH is due to the higher concentration of hydrogen ions present in the mixture, this competes with the positively charged metal ion for adsorption sites, which results in reduced uptake of metal ions. In view of this, pH 5 was fixed as an optimum pH for further adsorption studies15. Effect on contact time

Figure 3 represents the effect of initial concentration on the adsorption of Ni(II) by CCC. There is an increase in adsorption in initial stages of reaction as seen in curve. Maximum adsorption occurs at 25 min after that the adsorption remains uniform. Based on this result, it was decided to fix the equilibrium contact time of activated carbon as 25 min for the remaining experiments16. Effect of initial concentration

Fig 2Effect of pH

Fig. 3Effect of contact time

% of (Ni((II) Removal

Fig. 1Effect of adsorbent dose

For the analysis of effect of initial concentration, 50 mg of adsorbent (CCC and PPy/CCC) was treated with 50 mL of Ni(II) solutions of different concentrations. Sorption experiments were carried out at most suitable pH 5 for each sorbent. On increasing the initial concentration and temperature, removal of Ni(II) moderately increases in case of CCC and PPy/CCC. Based on the results, PPy/CCC is very efficient sorbent for removal of Ni(II) from aqueous solutions. The results are summarized in Table 2. All the kinetic and isotherm study of CCC and PPy/CCC are found to be almost similar, and hence the results of kinetic and isotherm studies of CCC alone are discussed in this paper.

THAMILARASU et al.: REMOVAL OF Ni (II) FROM AQUEOUS SOLUTIONS BY ADSORPTION

Adsorption isotherms

Linear form of Freundlich equation is

Adsorption isotherms generally used for design of adsorption system. The Langmuir17and Freundlich18 equations are commonly used for describing the adsorption isotherm. Linear equation of Langmuir and Freundlich are represented as follows. The Langmuir isotherm can be expressed as:

qe =

Q0 .bL .Ce (1 + bL .Ce )

… (1)

Linear form of the rearranged Langmuir model is

1 C Ce = + e qe Q0 .bL Q0

… (2)

The constants Q0 and bL can be calculated from the slope and intercept of the plot of Ce/qe versus Ce and they are presented in Table 3 (Figure not shown). Freundlich equation is given below:

qe = k f C e1 / n

… (3)

Table 2Effect of the initial concentration on removal of Ni(II) by CCC and PPy/CCC Co mg/L 10 20 30 40 50

% of Ni(II) removal using CCC 30oC 89.02 88.63 86.91 87.33 86.19

40oC 91.26 90.09 88.99 88.03 87.38

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50oC 93.46 91.57 90.92 88.86 88.60

% of Ni(II) removal using PPy/ CCC 30oC 94.63 94.45 92.51 90.65 89.71

40oC 95.76 95.20 95.20 93.71 91.65

50oC 96.45 96.03 95.72 95.29 93.97

log q e = log k f +

1 log C e n

… (4)

where qe is the equilibrium adsorption capacity, Ce is the final residual concentration of Ni(II), Qo and bL are Langmuir constants related to adsorption capacity and energy of adsorption, k and n are empirical constants of the Freundlich isotherm measuring the adsorption capacity and intensity of adsorption respectively. The Freundlich constants kf and n were calculated from the plot log qe versus log Ce as shown in Fig. 4 and the results are given in Table 3. The intensity of adsorption (n) is greater than unity signifies that the forces between the Ni(II) and adsorbent surface are attractive which leads to the favourable adsorption. Essential characteristics of Langmuir equation can be described by the following dimensionless equilibrium parameter RL : Equilibrium parameter (RL) = 1/ (1+bL.C0)

… (5)

where bL is the Langmuir constant, and Co is the initial concentration of Ni(II) in mg/L. Value of RL indicates the shape of isotherms to be either unfavorable (RL>1), linear (RL = 1), favorable (o< RL