Kinetic studies of adsorption of Pb(II), Cr(III) and Cu(II) from aqueous solution by sawdust and modified peanut husk

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Abstract

Sawdust and modified peanut husk were used as adsorbents to remove Pb(II), Cr(III) and Cu(II) from aqueous solution. Results of kinetic experiments demonstrated that the adsorption was effective and rapid. Three different kinds of kinetic models (i.e., intraparticular diffusion model, Lagergren-first-order and second-order equations) were used to investigate the adsorption mechanisms. The adsorption of heavy metals on sawdust and modified peanut husk is not an intraparticular diffusion control. The kinetic adsorption data can be described by the second-order equation and the adsorption might be a rate-limiting control. The suitability of the adsorbent was tested by fitting the adsorption data with Langmuir and Freundlich isotherms, which gave good fits with both isotherms.

Introduction

With the rapid increase in global industrial activities, heavy metal pollution has become serious [1]. Heavy metals may come from various industrial sources such as electroplating, metal finishing, textile, storage batteries, lead smelting, mining, plating, ceramic and glass industries. Lead, chromium and copper are common contaminants of industrial wastewaters. Because they pose serious environmental problems and are dangerous to human health, considerable attention has been paid to methods for their removal from industrial wastewaters [2], [3].

There are various methods for removing heavy metals including chemical precipitation, membrane filtration, ion exchange, liquid extraction or electrodialysis [4], [5]. However, these methods are not widely used due to their high cost and low feasibility for small-scale industries [6]. In contrast, the adsorption technique is one of the preferred methods for removal of heavy metals because of its efficiency and low cost. Conventional adsorbents such as granular or powdered activated carbon are not always popular as they are not economically viable and technically efficient [7]. Non-conventional materials have been tested in a large scale for this purpose, such as fly ash [8], lignite [9], tree fern [10], etc.

In this research, sawdust and modified peanut husk have been used as adsorbents to remove Pb(II), Cr(III) and Cu(II) from wastewater. Sawdust and peanut husk are often considered as solid wastes of agriculture and widely available [11], [12]. These materials cause a significant disposal problem. Efforts have been made to use the cheapest and unconventional adsorbents to adsorb heavy metals from aqueous solution. A series of experiments have been performed to characterize heavy metals adsorption equilibrium on sawdust and modified peanut husk. Three different kinds of kinetic models (Intraparticular diffusion models, Lagergren-first-order and second-order equations) were selected to determine which of these mechanisms predominates in heavy metals adsorption by sawdust and modified peanut husk. The experimental data was compared with Langmuir and Freundlich isotherms. This information will be useful for further application of the treatment system.

Section snippets

Materials and equipment

The sawdust and peanut husk were applied from the suburb of Nanjing, China. The sawdust is of poplar trees. The sawdust and peanut husk were dried at an oven temperature of 40 °C, and sieved into 40–60 meshes (0.45–0.60 mm). The sawdust was washed with deionized water to remove dust and other materials. The peanut husk was modified with formalin to reduce organic pigment. 10 g peanut husk was mixed with 250 ml 0.2 mol/l formalin and 1.0 mol/l H2SO4, reacted 3 h at a temperature of 50 °C. The modified

Effects of adsorption time

Fig. 1 shows the time profiles of sawdust and modified peanut husk adsorption of different heavy metals. Adsorption of Pb(II), Cr(III) and Cu(II) showed an increasing trend up to a reaction time of 1 h beyond which adsorption appeared to have approached equilibrium. After 1 h, the adsorption capacity of Cr(III) by sawdust was still increasing. The others became constant. After 6 h, all got equilibrium. The rate of uptake of heavy metals by modified peanut husk was faster than sawdust.

Intraparticular diffusion model

The relation

Conclusion

The capacity of sawdust and modified peanut husk in adsorption of Pb(II), Cr(III) and Cu(II) has been investigated, and the experimental data demonstrate that sawdust and modified peanut husk are two effective adsorbents of heavy metals in solutions. Intraparticular diffusion models, Lagergren-first-order and second-order equations were selected to determine which of these mechanisms predominates in heavy metals adsorption by sawdust and modified peanut husk. The following conclusion can be

Acknowledgments

This work was supported by the Analysis and Measure Foundation of Nanjing University (06HK10). We also thank two anonymous reviewers and the associate editor whose thoughtful comments helped to improve the clarity of the manuscript.

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