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Introduction

Water is a Prime natural resource and is a basic human need. It is available in nature as surface water and ground water through the self purification mechanisms like physical, chemical and microbiological processes at natural bodies are carried out in nature. However, natural water is rarely suitable for direct consumption to human beings. Rapid industrialization and population growth resulted to generation of large quantities of wastewater and causing problem of their disposal. Industrial waste constitutes the major source of various kinds of metal pollution in natural water. The presence of heavy metals in the environment has been of great concern because of their increased discharge, toxic nature and other adverse effects on the receiving streams. When the concentration of toxic metal ions exceed tolerance limit. They may become real health concern1. There is an immediate need to introduce cleaner technologies to minimise the pollution and to protect the degrading environment. It is not possible to achieve zero waste discharge, but it is an essential to treat the waste.

Among the toxic heavy metal ion which present in potential health hazard to aquatic animals and human life, Pb, Cd, Cr, V, Bi and Mn are important. Toxicity of manganese (II) and its salt include Psychological disorders, Neurological disorder, Manganese Pneumonia, Bronchitis, nose and throat infection increased respiration, hypo- sexuality, tremor of the fingers, muscular rigidity, chronic bronchitis and decrease liver activity. Manganese ethylene-bis-dithiocarbomide is found to be carcinogenic and cause cancer. Divalent manganese has been found to be 2.5 to 3 times more toxic than trivalent form.

Literature survey reveals that, there are many methods namely coagulation, precipitation, ion exchange and adsor ption for removal of manganese metal ions from aqueous medium. However, adsorption is an easy and economical process for removal and retrieval of cation from aqueous medium. Efficiency of adsorption process mainly depends on nature of adsorbent, adsorbate, pH, concentration, temperature, time of agitation etc. There is a need to enhance the adsorptive power of an adsorbant by using the same with various type of chelating agents. These cheap and efficient adsorpbents can carry to cater the need of population in the rural areas and the population in the industrial area where safe drinking water is not available. In the present study, and attempt has been made to increase the adsorption efficiency of G.A.C. by using chelating agents namely EDTA and NTA.

Materials and Methods

Preparation of Adsorbents (Granuler Activated Carbon)

The commercially available activated Carbon was selected as an adsorbant for the present study. The carbon was sieved through a mesh size (- 12 X 18) (M/S Jayant Test sieves, Mumbai) and washed with distilled water, several times until the leachate was free from any suspended impurities. The washed sample was then dried in an oven at 100-110°C and stored in a calcium chloride desiccator until use.

Table 1: Isotherm constants Click here to view table

Batch Study

The dried amount of 0.5 gms of GAC was taken in 1000 ml in round bottom flask and synthetic solution (200ml) containing various concentrations of various amount of manganese (II) ion was added and was stirred using remistirrer at 1000 r.p.m. in a thermostat maintained at (25+1)° C for six hours.

Figure 1 Click here to view figure

Similarly 0.5 gms. of GAC was taken in shaking bottle and 100 ml of 0.002 M of ligand (EDTA or NTA) was adsorbed on it. This process of fixing a ligand (EDTA or NTA on GAC is denoted as "Loading of GAC". Then the bottles were shaken at room temperature (25+1)°C. Once the loading of GAC with ligand (EDTA or NTA) was done, synthetic solution (200 ml) containing various concentrations of Manganese ions was added to the loaded activated carbon in 1000 ml, round bottom flask, and stirred using remistirrer at 1000 r.p.m. in thermostat maintained at temp (25+1)oC, for six hours.

Figure 2 Click here to view figure

The initial and final concentrations of Manganese (II) metal ion was determined by spectrophotometry³ using periodate method and measured absorbance at 460 nanometer. The spectrophotometer, Systronic (model 104) was used to measure the concentration of Manganese (II) ions.

Figure 3 Click here to view figure

Results and Discussions

Equilibrium adsor ption isother ms for C_eVs q_eplotted for activated carbon are shown in Fig. 1. The adsor ption capacity in mg/L was calculated from the equation.

q_e= (C_o-C_e)V/M

where,C_ois the initial concentration of Manganese (II) C_eis the concentration of Manganese (II) at equilibrium in mg/L V is the volume of solution in litre and M is the mass of adsorbent in grams

Similarly equilibrium adsorption isotherm for Ce Vs qe plotted for EDTA and NTA adsorbed on GAC is shown in Fig. 2 The adsorption capacity was calculated from the equation:

qe = (Co-Ce)V/M

where,

q_erepresents the maximum amount of Manganese that millimole of EDTA and NTA can hold.

C_o= initial concentration of Manganese ion in solution in mg/L

C_e= concentration of Manganese ion at equilibrium in solution in mg/L

V = Volume of solution in litre and

W = Millimole of ligand (EDTA or NTA)

Figure 4 Click here to view figure

Adsorption Isotherms

Equilibrium isotherms was studied for both Langmuir and Freundlich isotherms. The results are shown in Fig. 3, 4 and 5 which, illustrate the plot of Langmuir and Freundlich isotherms of activated carbon loaded with EDTA and NTA for Manganese. The saturated monolayer can be represented by:
q_e= Q⁰.b.C_e1+b.C_e
The linearised from of the Langmuir isotherm isWhere, Q⁰and b are Langmuir constants.

The plot of 1/C_eVs 1/q_ewas found to be linear, indicating the applicability of Langmuir model. The parameters Q⁰and b have been calculated and presented in Table 1. The Langmuir constant Q⁰is a Measure of adsor ption capacity and b is a measure of energy of adsorption. In order to observe whether the adsorption is a favourable or not, a dimensionless parameter 'R' obtained from Langmuir isotherm is R = (1 + b x Cm)^-1​​​​​​​ Where, b is Langmuir constant and Cm is maximum concentration used in the Langmuir isotherm.

Figure 5 Click here to view figure

The adsorption of Manganese on GAC loaded with ligand is a favourable process as 'R' values lie between zero to one. Coefficients of co- relation (r) are also shown in Table 1. The applicability of Freundlich isotherm was also tried using the following general equation: q_e= k.C_eB linearised form of this equation is log q_e=B.logC_e+logk. Where, B and k are Freundlich constants.

Figure 6 Click here to view figure

These constants represent the adsorption capacity and the adsorption intensity respectively. Plot of log q_eVs log C_ewas also found to be linear.The values of B and k are presented in Table 1. Since the values of B are less than 1, it indicates favourable adsorption.

Figure 7 Click here to view figure

Effect of pH on the Removal of Manganese (II)

The effect of pH on the removal of Manganese (II) is shown in Fig. 6 Experiment were conducted at the constant initial Manganese (II) concentration, adsorbent dose (GAC) of 0.5 gm/100 ml and the contact time of 3 hours. Results indicate that GAC loaded with EDTA has the maximum adsor ption capacity for removal of Manganese (II) as compared to GAC loaded with NTA and GAC used independently.

This establishes that the pH of the aqueous solution is an important controlling parameter in the adsorption process. It was obser ved that the percentage removal of Manganese is higher at pH= 8.6 and then decreases with increase of pH.

Kinetics of Adsorption

The kinetics modelling for the removal of Mn²⁺by GAC, GAC loading with EDTA and NTA has been carried out and rate constant for adsorption was determined by using lagergren equation. log C_t= log C_o-x t

The straight line plots of log C versus time for the adsorption show the validity of lagergren equation suggest the first order kinetics (Fig. 7)