Elsevier

Carbon

Volume 39, Issue 11, September 2001, Pages 1635-1642
Carbon

Effect of modification with HNO3 and NaOH on metal adsorption by pitch-based activated carbon fibers

https://doi.org/10.1016/S0008-6223(00)00290-6Get rights and content

Abstract

Pitch-based activated carbon fibers (ACFs) were modified with nitric acid and sodium hydroxide. Their physicochemical properties were investigated in terms of N2 adsorption, mass titration, FTIR and metal adsorption. The specific surface area of the ACFs decreased after oxidation with 1 M nitric acid, but the total acidity increased three times compared to the untreated ACFs, resulting in an improved ion-exchange capacity of the ACFs. The points of zero charge of the ACFs that affect the selectivity for the ionic species changed from pH 6 to pH 4 by 1 M nitric acid and to pH 10 by 1 M sodium hydroxide treatment. Upon oxidation of the ACFs with nitric acid, surface oxide groups were observed in the FTIR spectra by absorption peaks at 1750–1400 cm−1. The carboxyl groups of ACFs decreased, while the lactone groups increased after the sodium hydroxide treatment. The adsorption capacity of copper and nickel ion is mainly influenced by the lactone groups on the carbon surface pH<pHzpc, and by the total acidic groups at pH>pHzpc.

Introduction

The main aim of oxidation of a carbon surface is obtaining a more hydrophilic surface structure with a relatively large number of oxygen-containing surface groups. In general, the oxygen-containing groups behave as acids or bases, which possess ion-exchange properties. This can be used in the preparation of carbon-supported noble metal catalysts by exchange with cationic metal complexes, resulting in highly selective catalysts [1]. For other carbon-supported catalysts, modification of the carbon surface results in a significant change of the loading capacity and of the catalytic properties [2], [3], [4].

The chemical oxidation of carbon materials is a frequently used method in the preparation of carbon-based ion-exchangers. Various reagents have been used as oxidizers: concentrated nitric or sulfuric acid, sodium hypochlorite, permanganate, bichromate, hydrogen peroxide, transition metals and ozone-based gas mixtures [5], [6], [7], [8], [9], [10]. Among oxidation treatments, nitric acid oxidation is the most widely used method to increase the total acidity in a wet oxidation treatment. There are reports that the oxidation treatment of PAN-based carbon fibers in boiling nitric acid results in a significant increase in the acidic surface groups such as carboxyl groups [8]. However, most chemical oxidation of carbon fibers has been carried out to increase the adhesion interfacial force in polymeric matrix composite systems [11], [12], [13], [14]. There are few reports on the modification of activated carbon fibers (ACFs) to enhance the loading capacity or the catalytic properties.

Most metal ions exist not only as a single component but also in the form of several species in the waste solution from many industrial sources. In our previous study for removing metal ions by ACFs from three components co-existing in solution, the removal efficiency of copper and nickel was much lower than that for chromium [15]. This difference was due to the forms of the metal complexes and the oxygen-containing surface groups on ACFs.

The objective of the present work is to enhance the adsorption capacity of co-existing copper and nickel ions from solution on ACFs modified with nitric acid and sodium hydroxide. Pitch-based ACFs were prepared by following an earlier method [16]. The changes of physicochemical properties of modified ACFs are investigated in terms of BET specific surface area, total acidity and oxide functional groups with changing the treatment conditions. The adsorption capacity of metals on ACFs is analyzed by the change of surface oxide groups according to the chemical treatment.

Section snippets

Experimental

Pitch-based carbon fibers were activated by steam diluted with nitrogen in a cylindrical quartz glass tube (I.D.=5 cm, L=50 cm) at 900°C for 30 min (ACF15). These ACFs were washed with deionized water and dried overnight at ambient temperature. This dry ACF was inserted into a three-necked flask, equipped with stirrer, condenser, dropping funnel and heating mantle. As reported from other research [14], 1 M diluted nitric acid at boiling temperature was used in the oxidation treatment to

Physicochemical properties

The nitrogen adsorption–desorption isotherms of various ACFs are shown in Fig. 1, and the porous structure of the ACFs is summarized in Table 1. All the ACFs gave type I isotherms characterized by a plateau that is nearly horizontal to the p/p° axis. This means that all the modified ACFs are microporous. However, the specific surface areas of ACFs are decreased by the chemical modification. Kutics [9] reported that the BET surface area is considerably decreased due to the blocking of the narrow

Conclusions

Activated carbon fibers are sensitive toward modification by 1 M nitric acid, which results in a decrease of pore volume and surface area. The decrease of surface area is mainly due to decreased micropore volume resulting from pore blocking by surface oxide groups existing in some of the micropores. However, this oxidation treatment gives rise to a large increase in the amount of total acidity resulting from the increase of surface oxide groups such as carboxyl, lactone and phenol. The increase

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