Kinetics and equilibrium adsorption study of lead(II) onto activated carbon prepared from coconut shell

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Abstract

Removal of lead from aqueous solutions by adsorption onto coconut-shell carbon was investigated. Batch adsorption experiments were performed to find out the effective lead removal at different metal ion concentrations. Adsorption of Pb2+ ion was strongly affected by pH. The coconut-shell carbon (CSC) exhibited the highest lead adsorption capacity at pH 4.5. Isotherms for the adsorption of lead on CSC were developed and the equilibrium data fitted well to the Langmuir, Freundlich, and Tempkin isotherm models. At pH 4.5, the maximum lead adsorption capacity of CSC estimated with the Langmuir model was 26.50 mg g−1 adsorbent. Energy of activation (Ea) and thermodynamic parameters such as ΔG, ΔH, and ΔS were evaluated by applying the Arrhenius and van't Hoff equations. The thermodynamics of Pb(II) on CSC indicates the spontaneous and endothermic nature of adsorption. Quantitative desorption of Pb(II) from CSC was found to be 75% which facilitates the sorption of metal by ion exchange.

Introduction

The presence of heavy metals in the aquatic environment has been of great concern to scientists and engineers because of their increased discharge, toxic nature, and other adverse effects on receiving waters. Unlike most organic pollutants, heavy metals are generally refractory and cannot be degraded or readily detoxified biologically. Hence the safe and effective disposal of wastewater containing heavy metals is always a challenge to industrialists and environmentalists, since cost-effective treatment alternative are not available. In recent years, Pb has been introduced into natural water from a variety of sources such as storage batteries, lead smelting, tetraethyllead manufacturing, mining, plating, ammunition, and the ceramic glass industries [1].

The permissible limit of lead in drinking water is 0.05 mg l−1 [2]. The presence of excess lead in drinking water causes diseases such as anemia, encephalopathy, and hepatitis. Lead ions have an affinity for ligands containing thiol and phosphatic groups and they inhibit the biosynthesis of heme, causing damage both to the kidney and liver; this behavior of lead is similar to that of calcium. However, Pb can remain immobilized for years, and hence it is difficult to detect the metabolic disorders it causes [3]. The problems connected with heavy metal pollution are curtailed by processes such as precipitation, electrode deposition, ultrafiltration, cementation, selected solvent extraction, activated carbon adsorption, ion exchange, and biological processes [4]. Adsorption is an attractive process, in view of its efficiency and the ease with which it can apply to the treatment of waste water containing heavy metals.

The use of adsorbents such as modified groundnut husk [5], olive stones [6], lignite material [3], bagasse and fly ash [7], peanut hull carbon [8], Fe(III)/Cr(III) sludge [9], betonite [10], water biogas residual slurry [11], crude coniferous bark [12], modified sawdust [13], and sugar beet pulp [14] for the removal of Pb(II) from aqueous solution has been reported.

In this paper, systematic laboratory investigations of the removal of Pb(II) from aqueous solutions using the low-cost material coconut-shell carbon as the adsorbent by batch adsorption techniques have been reported. The main objective of this work is to study adsorption on CSC in the case of Pb(II). The work reported here examines in detail the influence of temperature on the rate constant. The activation parameters calculated from the adsorption measurements in the present study are very useful in elucidating the nature of adsorption.

Section snippets

Adsorbate

A stock solution of Pb(II) (1000 ppm) was prepared by dissolving Pb(NO3)2 in deionized water and acidified with 5 ml of concentrated HNO3 to prevent hydrolysis.

Adsorbent

Coconut shell was collected from oil industries located in Pollachi, Tamil Nadu, India. It was crushed, washed with deionized water, and dried. A sample of 250 g of the dried material was mixed with 450 g of concentrated sulfuric acid and kept at atmospheric temperature for about 24 h with occasional stirring. The product,

Effect of agitation time and initial Pb(II) concentration on CSC

Fig. 1 shows the effect of agitation time on Pb(II) adsorption by CSC. The figure reveals that increased agitation time increased the uptake of lead ions and attained equilibrium in 105 min for 10 and 20 mg l−1 Pb(II) and 120 min for 30, 40, and 50 mg l−1 Pb(II) solutions. The removal curves are single, smooth, and continuous, indicating the possibility of the formation of monolayer coverage of Pb(II) at the outer surface of CSC [9]. The equilibrium time required for the metal ion is very

Acknowledgments

The authors wishes to express thanks to the Secretary, the Principal, and the management of N.G.M. College, Pollachi, India for providing all the facilities to carry out this research work.

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