Current World Environment

Introduction

The Twentieth century started with an extensive damage to the natural resources.¹ Unplanned industrialization, urbanization, pollution explosion, change in life-style, over exploitation of natural resources, commercial establishment and modern agriculture practices have degraded the quality of environment.

The main effects being faced are:

• Continental invasion of air and water.
• Marine pollution through waste discharges.
• Release of variety of chemical and biological contaminants into the water bodies, on land and in air.
• Ground water pollution.
• Acid rains and nuclear fallout.

These effects are not only covering the pollution of environment but also are responsible in creating genetic erosion in plants, animals including human beings and microorganisms. Water is a prime natural resource and is a basic human need. The availability of adequate water supply in terms of its quality and quality is essential for the existence of life.

Water is available in nature as surface water and ground water through the self-purification mechanisms like physical, chemical and microbiological processes an 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 concern². There is an immediate need to introduce cleaner technologies to minimize 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.
 

Table - 1: Isothermal Constants Click here to view table

Among the toxic heavy metal ion, which present in potential health hazard to aquatic animals and human like Pb, Cd, Cr, V, Bi and Mn are important.

The maximum to tolerance limit for Lead II for public water supply are 0.1mg/L. Toxicity of metal depends on the type of metal, does and the ionic form. Lead is extensively used in printing, manufacture of paints, water pipes, storage battery manufacture, pottery and soldering operation etc. Besides it is used as a antiknock agent in gasoline. Toxicity of Lead³ includes anemia poisoning like acute abdominal colic and syndrome of acute encephalopathy. It cause mental deterioration convulsive seizures, serve central nervous system depression and death.

Results and Discussion

Literature survey reveals that, there are many methods namely coagulation, precipitation, ion exchange and adsorption, for removal of Lead II â€‹â€‹metal ions from aqueous medium. However, adsorption is an easy and economical process for removal and retrieval of caption from aqueous medium. Efficiency of adsorption process mainly depends on nature or adsorbent, adsorbate, pH, concentration, temperature, time of agitation etc.

These cheap and efficient adsorbents can carry to cater to 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, Lead (II) is removed by using coconut shell^4-9 as a adsorbent.
 

Figure - 1: Equilibrium Adsorption isotherm for coconut shell - Pb++ Click here to view figure

Adsorbent

The coconut shell was first dried at a temperature of 160°C for 6 hours. After grinding it was sieved to obtain average particle size of 200 mesh. It was then washed several times with distilled water to remove dust and other impurities. Finally it was dried again in an over at 50°C for 6 hours. The adsorbent was then stored in desiccators for final studies.

Batch Study

The dried amount of 0.5gms of coconut shell was taken in 250ml reagent bottle any synthetic solution 200ml containing various concentration of Lead (II) ion was added and system is equilibrated by shaking the contents of the flasks at room temperature so that adequate time of contact between adsorbent and final concentration of metal ion. Lead (II) was determined by spectro-photometry¹⁰ using Dithizone method at 515nm against a reagent blank. The spectrophotometer, systronic (model 104) was used to measure the concentration of Lead (II) ions.
 

Figure - 2: Langumir adsorption isotherm Click here to view figure

Equilibrium adsorption isotherm for C_e verses q_e, plotted for coconut shell are shown in Figure1. The adsorption capacity in mg/L was calculated then the equation.

q_e = (C_o-C_e) V/M

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

Figure - 3: Freundilich adsorption isotherms Click here to view figure

Adsorption Isotherms

Equilibrium isotherms was studied for both Langmuir and Freundlich isotherms. The results are shown in figure 2 and 3, which illustrate the plot of Langmuir and Freundlich isotherms of coconut shell for Lead (II). The saturated monolayer can be represented by:



The lineraised form of the Langmuir isotherms is



where Q⁰ and b are Langmuir constants. The plot of 1/C_e Vs 1/q_e was found to be linear, indicating the applicability of Langmuir model. The parameters Q_o and b have been calculated and presented in Table 1. The Langmuir constant Q^o is a measure of adsorption capacity and b is the measure of energy of adsorption. In order to observe whether the adsorption is favorable or not, a dimensionless parameter ‘R’ obtained form Langmuir isotherm is.

R = (1+b× cm)^-1 

where b is Langmuir constant and Cn is maximum concentration used in the Langmuir isotherm. The adsorption of Lead (II) on coconut shell 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_e^B

linearised form of this equation is

log q_e = B.logC_e + log k

where B & k are Freundlich constants. These constants represent the adsorption capacity and the adsorption intensity respectively.

Plot of log q_e vs log C_e was 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 - 4: Effect of pH on % removal of Pb++ Click here to view figure

Results and Discussion

Effect of concentration of metal ion and contact time

The response of Adsorbate does and contact time on the removal of Lead (II) is presented in figure 1. The observation reveal that an increase in the adsorbate dose, rate of adsorption increase upto certain level and then it become constant. Also as the time of contact increase, adsorption increase and then it become constants.

Effect of pH on the removal of Lead (II)

The effect of pH on the removal of Lead II is shown in figure 4. Experiment were conducted at the constant initial Lead (II) concentration, adsorbent dose (coconut shell) of 0.5gm/100 and the contact time of 4hours. The pH of the aquoues solution is an important controlling parameter in the adsorption process. It was observed that the percentage removal of Lead (II) is higher at pH = 5 and then decrease with increase of pH.
 

Figure - 5: Lagergren plot system : coconut shell - Pb++ Click here to view figure

Effect of Particle Size

The adsorbent particle size has significant on the kinetics of adsorption. The influence of particle size furnishes important for achieving optimum utilization of adsorbent. Four-particle size 50, 100, 150, 200 micron size (Indian Standard Sieves) under optimum condition. It is found that, as the particle size increase the rate of adsorption decrease.

Kinetic of Adsorption

0.5 gm of coconut shell and 200ml Pb⁺⁺ solution was taken in 1000ml R.B. and shake vigorously for about hours. After every 15 minutes, 5ml sample of the solution was withdrawn for the first hour and subsequently the interval between the sample withdrawn was increased to 30minutes. The concentration of the metal ions in the sample, withdrawn were determined by the spectrophotometry and were designated as C_t and the value of the concentration of the metal ion on the coconut shell as the same time interval estimated using the relation.

q = (C_o – C_t) V/W

The rate of adsorption of Lead (II) on coconut shell was studied by using the first order rate equation proposed by Lagergren.¹¹



where K_ad is the rate constant for adsorption. The plote of log C_t vs t is shown in Fig. 5.

Conclusion

The following conclusion have been drawn from the present study:

1. The percentage retrieval of Lead (II) is formed to be increase with decrease the initial concentration of Lead (II). The removal is found rapid in initial stages followed by slow adsorption up to saturation limit.
2. The developed technique of retrieval of Lead (II) ions using coconut shell appears to be a cheap and practically viable for the use of semiskilled worker in the villages.
3. The present work on adsorption process in good agreement with Langmuir isotherm indicating monolayer adsorption process.
4. The result on adsorption process reveals that at pH = 5.0, Lead (II) uptake capacity is better.
5. The straight lines plots of log C_t vs t for the adsorption show the validity of Lagergren equation and suggest the first order kinetics.
6. Regeneration studies are not necessary with the view that the cost of the adsorbent is very low and it can be disposed of safely.

References