1. INTRODUCTION:

Chromium (VI) has been considered to be more hazardous due to its toxic and carcinogenic properties¹. Chromium can be released to the environment through a large number of industrial operations, including metal finishing industry, iron and steel industries and inorganic chemicals production². Chromium discharged to the environment, which causes objectionable effects, impairing the welfare of the environment, reducing the quality of life and may eventually cause death. Such a substance has to be present in the environment beyond the limit or unacceptable limit, which may be poisonous or toxic and will cause harm to living things. The permissible limit of chromium for drinking water is 0.1 mg/L (as total chromium) in EPA standard³. There are various methods to remove Cr (VI) including chemical precipitation, membrane process, ion exchange, liquid extraction and electro dialysis⁴. These methods are non-economical and have many disadvantages such as incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal or treatment. In contrast, the adsorption technique is one of the preferred methods for removal of heavy metals because of its efficiency and low cost⁵.

For this purpose in recent years, investigations have been carried out for the effective removal of various heavy metals from solution using natural adsorbents which are economically viable such as agricultural wastes and by product. In this study, an activated carbon prepared from Prosopis juliflora bark carbon had been used to remove the Cr (VI) from aqueous solution. The aims of this study are to investigate the effect of different parameters such as contact time, pH, adsorbent dose and initial chromium concentration and to find the optimum adsorption isotherm as well as the adsorption kinetics.

2. MATERIALS AND METHODS:

2.1. Preparation of Prosopis Juliflora Bark Carbon (PJBC) adsorbent:

Prosopis Juliflora Bark Carbon (PJBC) was collected from local market. The collected PJBC materials were finely powdered with grinding machine. Then the powder was treated con. sulphuric acid for 2 hours then filtered and then washed with water. The black product was kept in a furnace for about 24 hours and the temperature was maintained at 800^oC. The resulting powder was used for adsorption experiment. All other chemicals used for the experiments were highly pure.

2.2. Adsorbate:

The Chromium (VI) ions used in this study was purchased from Sigma-Aldrich company. Chromium (VI) ion from K₂Cr₂O₇. The stock solution was prepared by dissolving 5.658g of amount of K₂Cr₂O₇ in 1000 ml distilled water. The solutions of different concentrations used in batch experiments were obtained by dilution of the stock solutions.

2.3. Batch Method:

Batch method⁶ was used for adsorption and kinetic studies. The test solutions such as adsorbent, adsorbate were taken in 250 ml conical flasks. The solution pH was adjusted using a 0.1N HCl or 0.1N NaOH solution. The solution temperature was monitored using thermostatic water bath with constant stirring. The residual concentration of metal ions were measured using spectrophotometer at 375 nm. The equilibrium parameters were used for the adsorption and kinetic studies

The amount of adsorption q_t(mg/g) at time “t”, amount of adsorption at equilibrium q_e(mg/g), and % removal of metal ions were calculated by

(1)

(2)

(3)

Where, C_o, C_t(mg/L) and C_e (mg/L) are the liquid phase concentrations of Nickel ions at initial, at time and equilibrium respectively .V (L) is the volume of the solution. W (g) is the mass of dry adsorbent used.

3. RESULTS AND DISCUSSION:

3.1. Effect of contact time:

The effect of contact time between adsorbent (PJBC) and adsorbate Chromium (VI) ions was determined by keeping initial Chromium (VI) ions concentration, adsorbent dosage, pH and temperature were constant. The percentage removal of metal ions were determined with different time interval such as 10,20,30,40,50,60,70,80 and 90 minutes. The effect of contact time was shown in figure.1. The experimental results indicate that adsorption equilibrium established within 60 minutes. Similar experiments were carried out by changing the parameters such as metal ion concentration, adsorbent dose, pH and temperature to determine the optimum condition. So the further batch experiments were carried out up to 60 minutes.

3.2. Effect of PJBC Adsorbent dose:

This study was carried out by varying the adsorbent dose from 5 to 250 mg/50ml were added into a 250 ml stopper glass containing a definite volume (50ml in each flask) of fixed initial concentration (25mg/L) of chromium (VI) ions solution and keeping all other parameters constant. The Chromium (VI) ions concentrations were measured at equilibrium. The experimental results indicate that adsorbent dose of 25 mg/50ml was sufficient for this metal ion concentration range as shown in Figure.2. So the further batch experiments were carried out using 25 mg/50ml of adsorbent. The adsorption increases with increase in adsorbent concentration; this is due to the increase in surface area and availability of more adsorption site. The maximum adsorption takes place at the range of 25-150 mg/L.

Fig. 1. Effect of contact time for the removal of Cr (VI) ions

[Cr (VI)]=25mg/L: Temp=30^oC; Adsorbent dose=25mg/50ml

Fig. 2. Effect of adsorbent dose for the removal of Cr (VI) ions

[Cr (VI)]=25 mg/L; pH=6.5; Temp=30^oC; Contact time=60 min

3.3. Effect of solution pH:

The solution pH is one of the most important factors that control the adsorption of metal ions on the adsorbent material. The solution pH greatly alters the adsorbent surface (acidic or basic) as well as metal ions (free metal ion or metal hydroxide form). Therefore any change in solution pH might be an increase or decrease in the adsorption capacity of the adsorbent

To examine the effect of pHfor the adsorption of chromium(VI) ions using PJBC was studied by mixing 25 mg/L initial Chromium (VI) ions concentration, 25mg/50ml of adsorbent with different pH values (2.0-10.0) at 30^oC.The pH was adjusted with 0.1N NaOH and 0.1N HCl solutions and measured by using a pH meter. The % removal of Cr (VI) ion with pH as shown in figure. 3. Similar experiments were carried out by changing the others parameters.

The % removal increases may be due to the presence of negative charge on the surface of the adsorbent that may be responsible for metal binding. However the pH is very lower, the hydrogen ions may compete with the metal ions for the adsorption sites. On the other hand, the % removal decreases as pH > 7 may be due to the anionic species retards the approach of metal ions toward the adsorbent surface or the metal hydroxide formation. From the experimental results, the optimum pH range for adsorption of the chromium (VI) ions is 5.0 to 6.5

Fig.3. Effect of initial pH for the removal of Cr (VI) ions using PJBC

[Cr (VI)]=25 mg/L; Temp=30°C; Adsorbent dose=25mg/50ml; Contact time=60 minute

3.4. Adsorption Isotherms:

The Freundlich and Langmuir isotherm equations were used to calculate the adsorption capacity and energy of adsorption or intensity of adsorption. The linear form of Freundlich⁷ (1906) isotherm is represented by the equation

(4)

Where , q_e (mg/g) is the amount of metal ions adsorbed per unit weight of the adsorbent and C_e (mg/L) is the equilibrium concentration of the adsorbate, K_f (mg/g) is measure of adsorption capacity and 1/n is the adsorption intensity. The value of K_f and n are calculated from the intercept and slope of the plot of log q_evs log C_e respectively.

The linear form of the Langmuir⁸ (1918) isotherm is given by

(5)

Where, C_e (mg/L) is the equilibrium concentration of the adsorbate, q_e (mg/g) is the amount of adsorbate per unit mass of adsorbent at equilibrium. The Q_m(mg/g) and b (L/mg) are Langmuir constants related to adsorption capacity and energy of adsorption. The Q_mand b are calculated from the intercept and slope of the plot C_e/q_e vs. C_e. The Langmuir and Freundlich isotherm parameters were shown in Table.1.

Table 1.Comparison of Langmuir and Freundlich parameters for Cr (VI) ions using PJBC.

Temp	Langmuir		Fruendlich
Temp	Q_m	b	K_f	n
30^ᵒC	125.40	0.2229	4.80	3.2911
40^ᵒC	133.52	0.2222	4.67	3.1851
50^ᵒC	227.61	0.0939	4.48	1.9858
60^ᵒC	148.89	0.2406	5.07	3.0236

From the adsorption data, it was clear that Langmuir adsorption capacity (Q_m) value linearly increases up to 50^ᵒC after that slightly decrease with increase in temperature. The increase in temperature creates more active sites on adsorbent surface. Hence monolayer adsorption and pores diffusion of Cr (VI) ions were possible. Above 50^ᵒC the monolayer was disturbed, but adsorption intensity linearly decreases up to 50^ᵒC and then increase with raise in temperature. The bond energies decreases when there was increase in temperature. The Freundlich isotherm indicates that the adsorption capacity and n value decreases up to 50^ᵒC and then increases with increase in temperature. The n values indicates that the adsorption was highly favorable process.

3.5. Equilibrium parameter:

The favorability of the Langmuir adsorption process expressed in terms of equilibrium parameter (R_L) which is defined as

(3)

Where, b is the Langmuir constant and C₀(mg/L) is the initial concentration of the metal ions. The R_L values indicate the nature of the adsorption isotherm is either linear or favorable or unfavorable or irreversible. The R_Lvalues at different temperature were calculated and given in Table. 2. The R_L values between 0 and 1 indicates favorable adsorption for all the initial Cr (VI) ion concentrations were used.

Table 2. Equilibrium parameter (RL)

C₀	Temperature
C₀	30^ᵒC	40^ᵒC	50^ᵒC	60^ᵒC
25	0.1521	0.1525	0.2988	0.1426
50	0.0823	0.0826	0.1756	0.0768
75	0.0564	0.0566	0.1244	0.0525
100	0.0429	0.0431	0.0963	0.0399
125	0.0346	0.0348	0.0785	0.0322

3.6. Thermodynamic Parameters:

The standard free energy change (ΔG⁰), standard enthalpy change (ΔH⁰) and standard entropy change (ΔS⁰) were calculated using the value of adsorption equilibrium constants (K₀) and temperature. The value of ΔH⁰ and ΔS⁰ can be calculated from the slope and intercept of the plot of lnK₀against 1/T. The free energy of adsorption process related to the equilibrium constant K₀ ( L/mol ) is given by the equation

(3)

Where, ΔG⁰ is the free energy change of adsorption (kJ/mol), T is the temperature in Kelvin and R is the universal gas constant (8.314 J K^-1mol^-1). The equilibrium constant K₀for the adsorption were determined from the slope of the plot of ln(q_e/C_e) against C_e at different temperature according to the method suggested by Khan and Singh⁹. The equilibrium constant may also be expressed in terms of enthalpy change (ΔH⁰) and entropy change (ΔS⁰) as a function of temperature.

(4)

Where, ΔH⁰is the standard heat change of adsorption (kJ/mol) and ΔS⁰ is standard entropy change (J K^-1mol^-1). These parameters values were given in Table 3. The negative ΔG⁰ value confirmed the feasibility of the adsorption process and the spontaneous nature of adsorption. The negative value of ΔH⁰indicates the exothermic nature of adsorption process. The positive ΔS⁰value indicates the increase in the randomness of the Cr (VI) on the PJBC surface. In this condition the adsorbed water molecules gain more translational entropy due to the displacement of the Cr (VI) ions, thus allowing the increase in the randomness in the system. The magnitude of ΔS⁰ suggest that the process was physical adsorption

Table 3.Thermodynamic parameters

C₀	ΔG⁰				ΔH⁰	ΔS⁰
C₀	30^ᵒC	40^ᵒC	50^ᵒC	60^ᵒC	ΔH⁰	ΔS⁰
25	-5417.00	-5879.04	-6338.76	-6721.41	-7.89	43.83
50	-3557.81	-4030.59	-4711.80	-5323.13	-14.54	59.59
75	-2308.94	-2789.60	-3311.90	-3844.51	-13.23	51.22
100	-836.60	-1179.30	-4273.24	-1990.51	-19.78	68.71
125	-361.16	-65.46	-2451.66	-558.48	-16.97	55.40

3.7. Adsorption Kinetics:

The adsorption kinetics describes the solute uptake rate. The kinetic models such as pseudo second order and Intra-particle diffusion were used. The pseudo second-order equation explained by Ho and McKay ^{10, 11} is

(5)

Where k₂ is the pseudo second order rate constant (g mg^-1min^-1). The plot of (t/q_t) against time should give a linear relationship from which q_e and k₂ can be determined from the slope and intercept of the plot, respectively.

The intra-particle diffusion model proposed by Weber and Morris ¹² is

(6)

Where k_id is the intra-particle diffusion rate constant (mg g^-1min^-1/2) and C is constant. The slope of the plot of q_t against t^1/2 will give the value of the intra-particle diffusion constant. The kinetic data were shown in table.4. From the kinetic data, the q_evalue calculated from the pseudo second order is almost equal to the experimentally calculated q_evalue. The rate constant value is almost same at particular initial chromium (VI) ions concentration with high R²value. In the intraparticle diffusion model, the pore diffusion decreases with temperature raises from 30 to 60^oC and R²value is always greater than 0.9000. Since these two model explain the mechanism of the chromium (VI) ions adsorption.

4. CONCLUSIONS:

The adsorption of Cr (VI) ions from aqueous solution using low cost adsorbent PJBC was investigated under different experimental conditions in batch process. The Freundlich and Langmuir adsorption capacity were found to be increases. The thermodynamic parameters were found to be thermodynamically favourable physical adsorption process. The kinetic data fit with the pseudo second order and intra-particle diffusion models. The results of the present studies indicate that the PJBC could be employed for the removal of Cr (VI) ions from aqueous solution.

Table 4. Kinetic parameters

C₀	Temp (⁰C)	Pseudo second order			Intraparticle diffusion
C₀	Temp (⁰C)	q_e	K₂x 10^-3	R²	K_id	R²
25	30	44.79	2.86	0.9888	3.300	0.9914
	40	45.27	2.67	0.9867	3.160	0.9849
	50	45.69	2.49	0.9874	3.022	0.9821
	60	45.95	2.44	0.9881	3.080	0.9853
50	30	80.41	3.47	0.9852	6.206	0.9903
	40	82.48	3.34	0.9834	6.157	0.9835
	50	85.25	3.09	0.9875	6.018	0.9890
	60	87.25	2.98	0.9898	6.162	0.9941
75	30	107.15	3.87	0.9789	9.267	0.9864
	40	111.75	3.70	0.9831	9.108	0.9899
	50	116.16	3.59	0.9877	9.080	0.9920
	60	120.06	3.42	0.9053	8.864	0.9260
100	30	116.45	4.23	0.9667	12.434	0.9885
	40	122.28	4.14	0.9679	12.290	0.9850
	50	166.16	1.92	0.9726	12.277	0.9863
	60	134.48	3.91	0.9833	11.995	0.9942
125	30	116.06	4.35	0.9404	15.184	0.9870
	40	123.43	4.37	0.9438	15.448	0.9854
	50	178.40	2.70	0.9368	16.154	0.9805
	60	137.57	4.33	0.9616	15.725	0.9891

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Received on 19.01.2014 Modified on 05.05.2014

Asian J. Research Chem. 7(6): June 2014; Page 565-569