Carbon nanotubes-iron oxides magnetic composites as adsorbent for removal of Pb(II) and Cu(II) from water

Carbon nanotubes (CNTs), a novel kind of carbon, were first reported by Iijima in 1991 [1]. CNTs can be thought of as cylindrical hollow micro-crystals of graphite [2]. Because they have relatively large specific area, CNTs have attracted researchers interest as a new type of adsorbent and offer an attractive option for the removal of organic and inorganic contaminants from water. Li et al. found that acid-refluxed CNTs can be good adsorbents for Pb and Cd [3,4]. Peng et al. reported that CNTs can also be used to remove organic pollutants from water [5]. Because of their relatively large specific area, CNTs can also be used as supports for adsorption materials. Li et al. prepared amorphous Al2O3 supported on carbon nanotubes (Al2O3/CNTs) and found that it has a much higher adsorption capacity of fluoride than c-Al2O3 [6]. Our previous research has found that ceria nanoparticles supported on carbon nanotubes can be effective adsorbent for the removal of arsenate from water [7]. In this work, a novel kind of magnetic composites based on CNTs was prepared and the obtained composites show good adsorption capacity for Pb(II) and Cu(II) from aqueous solution. CNTs-iron oxide magnetic composites can be used to adsorb contaminants from aqueous effluents and after the adsorption is carried out, the adsorbent can be separated from the medium by a magnetic process. The preparation of carbon-iron oxides magnetic adsorbent based on activated carbon has been reported in the literature [8]. But magnetic adsorbent based on CNTs has not been reported and is the topic of the present study. CNTs were prepared by catalytic pyrolysis of the propylene-hydrogen (C3H6:H2 = 2:1) mixture at 750 C in a ceramic tube furnace using Ni particles as catalysts. The CNTs were then washed with deionized water and ground. The specific area of as-grown CNTs is 134.0 m g . For the preparation of CNTs-iron oxide magnetic composites, CNTs were first immersed in concentrated nitric acid and then hydrofluoric acid for 24 h each to dissolve the catalyst particles. Then they were oxidized by refluxing the as-prepared CNTs with 50% nitric acid at 140 C for 1 h. The magnetic composites were prepared from a suspension of 1.0 g oxidized CNTs in a 300 ml solution of 0.585 g FeCl3 AE 6H2O and 0.390 g FeSO4 AE 6H2O at 70 C under N2 atmosphere. NaOH solution (18.7 ml, 0.5 mol l ) was added dropwise to precipitate the iron oxides. After the addition of NaOH solution, the mixture was adjusted to pH 11.0 and stirred for 30 min. Then the mixture was aged at 70 C for 2 h and washed 3 times with deionized water. The obtained material were dried in an oven at 45 C and ground (particle size <0.075 mm). The magnetic composites were characterized by powder XRD using a D/max-RB X-ray diffractometer (CuKa, k = 1.540598 A) and SEM using a Hitachi model S-3000N scanning electron microscope. Magnetization measurements were carried out on a LDJ model 9600 vibrating sample magnetometer at room temperature. Batch adsorption experiments were performed in glass bottles. To each bottle, 50 ml Pb(II) or Cu(II) solution and 0.03 g adsorbent were added. For the effect of pH on adsorption experiments, 20 mg l 1 Pb(II) or Cu(II) solution was used and the experiments were performed at 20 C. Adsorption isotherms were obtained by varying the initial Pb(II) and Cu(II) concentration with the initial pH of 5.0 at 20 C. The pH was adjusted using 1 M HCl or 1 M NaOH. The bottles were capped with glass screws and then were shaken on a HZQ-C shaker for 3 h. After equilibrium the adsorbent was * Corresponding author. Tel.: +86 10 62849198; fax: +86 10 62849198. E-mail address: luanzk@mail.rcees.ac.cn (Z. Luan).