Removal of Cd(II) and Pb(II) ions, from aqueous solutions, by adsorption onto sawdust of Pinus sylvestris

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Abstract

Fixation of heavy metal ions (Cd(II) and Pb(II)) onto sawdust of Pinus sylvestris is presented in this paper. Batch experiments were conducted to study the main parameters such as adsorbent concentration, initial adsorbate concentration, contact time, kinetic, pH solution, and stirring velocity on the sorption of Cd(II) and Pb(II) by sawdust of P. sylvestris. Kinetic aspects are studied in order to develop a model which can describe the process of adsorption on sawdust. The equilibrium of a solution between liquid and solid phases is described by Langmuir model. Scanning electronic microscopy (SEM) coupled with energy dispersive X-ray analysis (EDAX) and X-ray photoelectron spectroscopy (XPS) shows that the process is controlled by a porous diffusion with ion-exchange. The capacity of the metal ions to bind onto the biomass was 96% for Cd(II), and 98% for Pb(II). The sorption followed a pseudo-second-order kinetics. The adsorption of these heavy metals ions increased with the pH and reached a maximum at a 5.5 value. From these results, it can be concluded that the sawdust of P. sylvestris could be a good adsorbent for the metal ions coming from aqueous solutions. Moreover, this material could also be used for purification of water before rejection into the natural environment.

Introduction

The removal of toxic heavy metals ions from sewage, industrial and mining waste effluents has been widely studied in recent years. Their presence in streams and lakes has been responsible for several health problems with animals, plants, and human beings. Numerous metals such as Sb, Cr, Cd, Pb, Mn, Hg, etc. have toxic effects on man and his environment. The consumption of food containing traces of these elements may lead to various diseases, for example, the consumption of (i) rice containing high concentrations of cadmium (0.37 at 3.6 mg l−1) led to a surge in the Itaı̈-Itaı̈ disease in Japan in 1955 [1], [2], and (ii) fish contaminated with methylmercury led to Minamata disease [3]. For these reasons, the legislation is progressively becoming more stringent; for example, the maximum concentrations of metals in drinking water according to guideline values are now: 0.01 mg l−1 for Pb(II) and 0.02 mg l−1 for Cd(II) [4]. However, these values are higher due to accumulation phenomena observed when the spreading operations are used instead of direct overflowing into rivers.

Among the many methods available to reduce heavy metals concentrations from wastewater, the most common are chemical precipitation, ion-exchange, and reverse osmosis. Precipitation methods are particularly reliable but require large settling tanks for the precipitation of voluminous alkalines sludges and a subsequent treatment is needed [5]. Ion-exchange has the advantage of allowing the recovery of metallic ions, but it is expensive and sophisticated. This has encouraged research into discovering materials that are both efficient and cheap. The by-products and wastes from the agricultural and forest industries were studied as these materials could be assumed as low-cost since they (i) require little processing and (ii) are abundant in nature. Commonly, it concerns vegetal materials, then the term of biosorption is used to designate the fixation of contaminants onto biomaterials. The low-cost adsorbent used in our study is the sawdust of Pinus sylvestris, a by-product of the wood industry. It contains various organic compounds (lignin, cellulose, and hemicellulose) with polyphenolic groups that might be useful for binding heavy metals ions [6].

The objective of this preliminary study was to investigate the feasibility of metallic ions removal from aqueous solution by sawdust of P. sylvestris. The kinetics of the process was determined, especially in relation to the effects of various factors on the removal. The factors investigated included aqueous metallic concentration, stirring, contact time, pH of solution, and adsorbent concentration. The mechanism of metallic ions adsorption at sawdust–solution interface was also studied.

Section snippets

Adsorbate

All the compounds used to prepare reagent solutions were of analytic reagent grade. The mother solution of Cd(II) and Pb(II) (1000 mg l−1 for each ion) is prepared by dissolving a weighed quantity of the respective nitrate salts in twice distilled water. The concentrations of metals solutions ranged from 1 to 10 mg l−1. Before mixing with the sawdust, the pH of each solution was adjusted to the required value for the sorption of Cd(II) and Pb(II) ions, by adding 0.1 M NaOH or 0.1 M HNO3. Constant

Effect of biomass pre-treatment on Cd2+ and Pb2+ sorption

The effect of the treatment on the sorption was examined by using the experimental procedure previously described. The solution with an initial concentration of 100 mg l−1 was stirred with 1 g l−1 of treated sawdust. Another experiment was carried out with untreated sawdust. The pH of each solution was regulated by adding dilute NaOH or HNO3. Table 1 shows the retention capacities of treated sawdust and untreated sawdust for lead and cadmium ions sorption after a contact time of 1 h.

The results show

Conclusion

The present study clearly establishes that sawdust of P. sylvestris is an effective adsorbent for Pb(II) and Cd(II) removal from aqueous solutions. The results obtained show that the sorption of lead is higher than that of cadmium. Moreover, both sorption capacity and sorption rate are strongly dependent on the initial concentration in heavy metals, initial pH of solution, and sawdust concentration.

Adsorption equilibrium can be simulated by Langmuir isotherms. These isotherms are influenced by

Acknowledgements

The authors would like to thank the Agence Nationale pour la Valorisation de la Recherche (ANVAR) for their financial support. We are also sincerely grateful to Dr. Monin (Methodes Physico-Chimiques d’Analyses/Cnam Laboratory) for conducting the absorption atomic analysis. Our thanks also go to Dr. R. Caplain (Laboratoire Matériaux Minéraux/Cnam Laboratory) for his recording and interpretation of the X-ray photoelectron spectroscopy (XPS) as well as to P. Le Francois (Laboratoire de

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