Nitrite ion is one of the most important wide-spread contaminants of aqueous environment and serves as a significant indicator of natural water quality. The increasing level of nitrite in surface or ground waters results mainly from agricultural application of fertilizers as well as from many industrial processes [1].

Removal of nitrite ions from waters has received increasing attention in recent years because of their potential harmful impact on human health. NO₂^- is highly toxic to certain species of fish. Nitrite ions enter the bloodstream through gills by a mechanism that normally transports chloride [2]. After entering the bloodstream, NO₂^- oxidizes iron in the hemoglobin molecule from ferrous state (Fe²⁺) to the ferric state (Fe³⁺). The resultant product, called methemoglobin, is incapable of reversibly binding with oxygen, so exposure to NO₂^- causes respiratory distress because of the loss in oxygen- carrying capacity of blood.

Nitrite is one of the major components of wastewater from nuclear power production and involved in the corrosion and bacterial process known as the nitrogen cycle. Nitrite and its salts have many industrial and commercial applications. Sodium nitrite is a food additive that has been used for decades to preserve meat, poultry and fish. When used alone or in conjunction with sodium nitrate, nitrite gives cured meats their characteristic reddish-pink color, flavor and texture. The allowed concentration of nitrite in drinking water and food reported by the public health service is 0.06 mg L^-1 [3]. Elevated concentrations of nitrite can be found in water receiving nitrogenous effluents, in various hypoxic environments or in effluents from agricultural application of fertilizers and industries producing metals, dyes and celluloid [4]. Ion exchange, reverse osmosis, electrodialysis, distillation and activated carbon adsorption are processes that have been employed for uptake of nitrite from drinking water supplies [4], but these processes are expensive and a brine of difficult management is generated.

Recently, we have found that polypyrrole and polyaniline conducting electroactive polymers (CEPs) can be employed for removal of some toxic metal ions from aqueous solutions [5, 6]. Synthesis of these reactive and functional polymers have been carried out by either electropolymerization at a conductive substrate (electrode) through the application of an external potential, or chemical polymerization in solution by the use of a chemical oxidant [7-10]. The overall polymerization reaction of pyrrole can be shown as the following (Fig. 1).

Fig. (1) Overall polymerisation reaction of polypyrrole.

Where, the A^- (termed as dopant or counterion) is the anion of oxidant in monomer solution or reduced product of oxidant. The value of n has been determined to be between 2.2 and 2.4. The level of oxidation of ppy is 0.25-0.32 per pyrrole unit, depending on the type and the charge of the incorporated anion, corresponding to one anion for every 3-4 pyrrole units in order to achieve electroneutrality, and this makes up 30-40% of the final weight of the polymer [10]. However, chemical polymerization remains of interest for processing purposes, because it may be easier to scale up this batch process for bulk production.

The redox behavior of PPy electroactive polymers have been shown in Fig. (2), for cases where the counterion (A^-) is able to freely leave the polymer matrix and where it is immobilized within the CEP matrix. In this representation, A^- is necessary to balance the charge on the polymer backbone. Anion movement predominates in cases where small mobile dopant, e.g., Cl^–, is used [11, 12]. If large anion dopants such as polyelectrolytes are employed, cation movement will predominate.

Fig. (2) The electrodynamic character of polypyrrole.

Polyaniline (PAni) is another class of conducting electroactive polymers that has attracted a great deal of attention in recent years [13]. It is now accepted to have the general polymeric structure as shown in Fig. (3). It differs from most of the other inherently conducting polymers, such as polypyrroles (PPys) and polythiophenes, in that it possesses three readily accessible oxidation states. These range from the fully reduced (y=1) Leucoemeraldine state to the half oxidized (y = 0.5) Emeraldine form to the fully oxidized (y = 0) Pernigraniline state. Emeraldine is the state with highest conductivity.

Fig. (3) General structure of polyaniline (PAni).

PAni is unique among inherently conducting polymers in that it can be rapidly converted between base and salt forms by treatment with acid or base. These reversible redox and pH switching properties, together with the electrical conductivity of its emeraldine salt (ES) form (Fig. 4), its easy synthesis from aqueous solutions, processibility, and its high thermal and environmental stability, have led to its becoming the most extensively studied conducting organic polymer over the past decade, and a wide range of potential applications are currently being developed.

Fig. (4) Emeraldine salt (ES) form of polyaniline.

Almost all the previous applications of conducting polymers rely on electrical and electrochemical behaviors which are the most important properties of these polymers. This paper deals with a new application of PPy and PAni conducting polymers for removal of toxic anions such as nitrite ion. The removal of nitrite ion by these polymers seems to be mostly due to the interesting ion exchange or redox properties of the employed polymers.

Sawdust, an environmentally friendly and very low cost agricultural waste material was found to be a good substrate for coating by PPy and PAni polymers from aqueous solutions. By doping of conducting electroactive polymers such as polypyrrole and polyaniline with small and releasable anions such as Cl^-, removal of nitrite ions is possible. Removal of nitrite ions using PPy and PAni is supposed to occur mostly via ion exchange process. However, the possibility of the other mechanisms can not be ignored. Further precise study is needed for achievement of more accurate conclusions. It was found that higher sorption of NO₂^- ion was observed at relatively acidic pH values, using both PPy/SD and PAni/SD. Among the examined adsorbents employed in this research, it was found that PPy/SD is more efficient for the removal of nitrite ions from aqueous solutions. The results obtained in this paper are promising for the practical application of currently introduced adsorbents for the removal of some toxic anions in the treatment of waste effluents.