INTRODUCTION:

The commonly used procedures for removing metal ions from aqueous streams include chemical precipitation, lime coagulation, ion exchange, reverse osmosis and solvent extraction^1.Hence the disadvantages like incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require careful disposal has made it imperative for a cost-effective treatment method that is capable of removing heavy metals from aqueous effluents. Conventional techniques to remove toxic metals and radionuclides, such as ion exchange and precipitation, lack specificity and are ineffective at low metal ion concentrations. The need for effective and economically viable technologies is driven by environmental factors such as Stricter regulations with regard to the metal discharges are being enforced, particularly in industrialized countries. Toxicology studies confirm the dangerous impacts of heavy metals and Current technologies for the removal of heavy metals from industrial effluents often create secondary problems with metal-bearing sludge.

Biosorption is highly competitive with the presently available technologies like ion exchange, electrodialysis, reverse osmosis, etc. Biosorption is considered to be a fast physical or chemical process. The biosorption rate depends on the type of the process. According to literature, biosorption can be divided into two main proceses: adsorption of the ions on cell surface and bioaccumulation within the cell².

Recent biosorption experiments have focused attention on waste materials, which are by-products or the waste materials from large-scale industrial operations. For e.g. the waste mycelia available from fermentation processes, olive mill solid residues³, activated sludge from sewage treatment plants⁴, biosolids⁵, aquatic macrophytes⁶, etc.

Biosorption is being demonstrated as a useful alternative to conventional systems for the removal of toxic metals from industrial effluents. The development of the biosorption processes requires further investigation in the direction of modeling, of regeneration of biosorbent material and of testing immobilized raw biomasses with industrial effluents. Due to the extensive research and significant economic benefits of biosorption, we have selected this research work, our research work was investigated the removal of chromium (VI) from the aqueous solutions by two adsorbents, like Acid treated Pongamia Leaf Powder (APLP) and Acid treated Neem Leaf Powder (ANLP), in a batch system.

MATERIALS AND METHODS:

Preparation of Chromium Solution:

Standard hexavalent chromium solution was prepared from dried potassium dichromate salts by dissolving 5, 10, 15, 20 and 25 g into 100 mL of double distilled water.

Preparation of Biosorbent:

Activated Pongamia Leaf Powder:

Fresh Pongamia leaves were obtained from nearby agricultural land. The fresh leaves were sun-dried for 4 hours to save the energy expended in hot air oven drying and powdered it by using domestic grinder. The powder was sieved to get uniform particle sizes (50 to 60 mesh size). Later, 1 g of sieved Pongamia leaf powder was digested by chemical methods. Leaf powder sample and 100 mL of 1N HCl(Hydrochloric acid) were taken in a 100 ml conical flask. The mixture was gently heated on burner for 20 min after boiling starts. The acid treated leaf powder was washed with distilled water. Washing was done until maximum colour was removed and clear water obtained. The activated Pongamia leaf powder (APLP) was used in further experiments.

Activated Neem Leaf Powder:

Fresh Neem leaves were obtained from nearby land. The fresh leaves were sun-dried for 4 hours to save the energy expended in hot air oven drying and powdered it by using domestic grinder. The powder was sieved to get uniform particle sizes (50 to 60 mesh size). Later, 1 g of sieved Neem leaf powder was digested by chemical methods. Leaf powder sample and 100 mL of 1N HCl(Hydrochloric acid) were taken in a 100 ml conical flask. The mixture was gently heated on burner for 20 min after boiling starts. The acid treated leaf powder was washed with distilled water. Washing was done until maximum colour was removed and clear water obtained. The activated Neem leaf powder (ANLP) was used in further experiments.

Batch Biosorption Studies:

The batch adsorption experiments were carried out by varying initial dye concentrations, pH and adsorbent dosage. Experiments were performed in 100 mL Erlenmeyer flasks, using 20 mL of standard chromium solution and 0.2 gram of dry adsorbent. The flasks were maintained at 35±1ºC under constant agitation (120 rpm) in a rotary shaker. Samples (5 mL) were collected at different time intervals, filtered by using Whatmann filter paper No. 42 and analysed for chromium by titrimetric method. The percentage removal of Cr (VI) ions %R from standard solution was calculated using Eq. (1):

%R_t = ((C_o-C_t)/C_o) x 100 ----- (1)

The amount of Cr (VI) adsorbed q in mg/g was computed by using Eq. (2):

q = ((C_o-C_t)/m) x V ------------- (2)

Where, %R_t is percentage dye removed at time t (min), q_tis amount of dye adsorbed per unit mass of adsorbent at time t (mg g−1), C_o and C_t are the Cr (VI) concentrations in g/L initially and at a given time t, respectively, V is the volume of the standard Cr (VI) solutions in mL, and m is the mass of adsorbent in g.

Adsorption isotherms:

Equilibrium data commonly known as adsorption isotherms are basic requirements for the design of adsorption systems. These data provide information on the capacity of the adsorbent or the amount required to remove a unit mass of pollutant under the system conditions. Langmuir and Freundlich isotherms were used to describe the equilibrium characteristics of adsorption. An accurate isotherm is important for design purposes. Linear regression is commonly used to determine the best fit model, and the method of least squares has been widely used for obtaining the isotherm constants. It is however, well known that obtaining the parameters of a non-linear equation using its linear form may introduce large errors⁷. The non-linear regression of untransformed data is therefore preferred. One of the most popular adsorption isotherms used for liquids to describe adsorption on a surface having heterogeneous energy distribution is Freundlich isotherm. It is given as:

q_e = K_f.C_e^1/n ------------ (3)

Freundlich isotherm is derived assuming heterogeneity surface. K_F and n are indicators of adsorption capacity and adsorption intensity, respectively⁸.Rearranging Eq. (3) we get,

lnq_e = lnK_F + 1/n lnC_{e -----------------}(4)

A plot of log q_e versus log C_e yields a straight line, with a slope of 1/n and intercept of ln KF. The value of Freundlich constant (n) should lie in the range of 1–10 for favorable adsorption⁸.

Langmuir isotherm, applicable for homogenous surface adsorption, is given as:

Langmuir:

q_e= (q_oK_LC_e) / (1+ K_LC_e) ---------(5)

Eq. (5) can be rearranged into linear form:

1/qe = (1/q_mK_LC_e) +1/q_m --------- (6)

By plotting 1/qe versus 1/Ce, the Langmuir constants can be obtained. The essential characteristics of Langmuir isotherm can be expressed by a separation or equilibrium parameter, which is a dimensionless constant defined as:

R_L =1/ (1 + K_LC₀) ---------------- (7)

R_L indicates the nature of adsorption⁸ as indicated below:

Unfavorable RL >1; linear RL =1; favorable 0 < RL <1; irreversible RL =0.

The effect of initial concentration of chromium, contact time and pH on chromium removal was investigated by varying any one of the process parameters and keeping the other parameters constant at a particular value.

RESULTS AND DISCUSSIONS:

Effect of contact time:

The percentages of colour removal obtained for different contact time for both adsorbents were shown in Fig 1 and 2. The amount of chromium removed from the aqueous solutions was 53.23, 61.12, 64.24, 70.25 and 76.58% for APLP dose in 5, 10, 15, 20, 25g/100ml, respectively. The amount of chromium removed from the aqueous solutions was 52.86, 56.47, 61.85, 68.21 and 73.21% for APLP dose in 5, 10, 15, 20, 25g/100ml, respectively. This is basically due to adsorbent sites remaining unsaturated during the adsorption process. Since 1g/100ml both adsorbent dosages were not sufficient to achieve optimum removal of dye from a solution of 5g/100ml further increase in dosage was required. At 1g/100ml of dosage of both plant was attained the equilibrium within a short time interval (plateau is obtained in 15 min). It is obvious that an increase in adsorbent dosage increases available surface area and active sites. The maximum colour removal occurred at 15 min for both adsorbents. Beyond this time, increase in colour removal was not significant.

Fig 1. Effect of contact time on colour removal for APLP at different concentrations

Fig 2. Effect of contact time on colour removal for ANLP at different concentrations

Effect of pH:

Effect pHwas performed by taking a specific concentration of 5 g of Cr (VI) per 100 mL and contact time of 15 min. The pH values are varied from 1 to 12 using dilute NaOH/ HCl solutions. The samples were agitated for specific time, filtered and then analyzed for the residual Cr (VI) concentration. The Effect of pH on adsorption is shown in Fig. 3. It was found that the percentage color removal was less at high pH and maximum at the acidic pH (2.5). At high pH (11) the maximum dye removal was 60.1 and 58.2% for APLP and ANLP respectively. Dye removal was 74.25, 69.25, and 65.84% at pH 3, 6 and 8 respectively for APLP. Dye removal was 71.25, 67.28, and 63.25% at pH 3, 6, and 8 respectively for ANLP. Optimum condition of pH was found to be 2.5 and chromium removal rate was found to be 74.30% in optimum conditions. The effect of contact time on percentage colour removal was more pronounced than that of pH.

Fig 3. Effect of pH on colour removal for APLP and ANLP at a specific concentration of 5 g of Cr (VI) per 100 mL and contact time of 15 min

Adsorption Isotherms:

Batch experiments were carried out at different dye concentrations varying from 5 to 25gm/100ml. Other process parameters were kept constant. Samples were taken and analyzed at regular time intervals till equilibrium was attained. The fitted equilibrium data in Freundlich and Langmuir isotherm expressions for APLP and ANLP were shown in fig 4, fig 5, fig 6 and fig 7 respectively. The data fits well with both Freundlich & Langmuir isotherms yielding high R2 values, close to 1.0. However, the values of other error functions indicate that the non-linear regression of Langmuir model provides the best fit for the experimental data. The calculated Freundlich and Langmuir constants K_f, n, q_o and K_L for APLP are given by 4.9 (mg/g)(mL/g)ⁿ, 1.7, 25 mg/g and 4.44 mL/mg respectively and that of ANLP are given by 3.2 (mg/g)(mL/g)ⁿ, 1.4, 28.57 mg/g and 1.52 mL/mg respectively. The higher adsorption capacity, q_o confirms that the pongamia and neem leaf powders can be effectively used as adsorbents for the removal of chromium from its aqueous solutions. Also the higher value of q_o (q_o>>1) confirms that the sorbates are strongly adsorbed onto sorbent by electrostatic attraction. The coefficient of determination (R²) of both Langmiur and Freundlich models were mostly greater than 0.9. This observation is in agreement with results reported by Ho et al., who had suggested⁹ Chisquare analysis for choosing the best fitting isotherm model, rather than the use of coefficient of determination (R²). Conformation of the experimental data into Langmuir isotherm model indicates the homogeneous nature of adsorbent surface. The value of dimensionless separation parameter, RL, was found to be in the range of 0–1, suggesting favorable adsorption process. Comparison of adsorption capacity of APLP and ANLP for chromium clearly indicates that the capacity of APLP for adsorption of chromium is quite high than ANLP. It can be expected that APLP and ANLP would have similar capacities for dyes with similar molecular weight, structure and/or ionic load. Thus, the naturally defoliated the pongamia and neem leaf powders a low-cost natural resource, can be effectively used to remove pollutants from effluents.

Fig 4. Freundlich isotherm for APLP

Fig 5. Freundlich isotherm for ANLP

Fig 6. Langmuir isotherm for APLP

Fig 7. Langmuir isotherm for ANLP

CONCLUSIONS:

Removal of chromium (VI) from its aqueous solutions by two adsorbents, Acid treated Pongamia Leaf Powder (APLP) and Acid treated Neem Leaf Powder (ANLP), was investigated in a batch biosorption system. The percentage dye removal was found to increase with increase in contact time, pH. Equilibrium data were well explained by Langmuir isotherm. The study confirms that APLP and ANLP, an inexpensive and easily available material, can be used as an alternative for more costly adsorbents used for dye removal in wastewater treatment processes. Comparison of specific uptake of APLP and ANLP for adsorption of chromium indicates that APLP is quite efficient for the removal of chromium from aqueous solutions than ANLP. It can be expected that APLP and ANLP would have similar capacities for dyes with similar molecular weight, structure, and/or ionic load. Thus, the naturally defoliated the pongamia and neem leaf powders a low-cost natural resource, can be effectively used to remove pollutants from effluents, instead of being disposed arbitrarily.

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Received on 06.12.2009 Modified on 09.02.2010

Asian J. Research Chem. 3(2): April- June 2010; Page 346-350