introduction: Elements with an atomic weight between 63. 5 and 200. 6 and a density more than 5 grams per cubic meter are considered heavy metals 1. Copper (Cu) even though is important for biochemical processes in the human body, Excessive amount of this metal ion can be dangerous. The presence of some other metal ions such as cadmium (Cd) and lead (Pb), even at parts per billion (ppb) levels, due to their indestructibility and accumulation in the main systems of the human body, they are dangerous 2. The common techniques such as coagulation–flocculation, precipitation, ionic exchange, membrane separation and adsorption have been used to removal of heavy metals 3. Among the mentioned methods, adsorption is one of the effective methods for removal of heavy metal ions from water and wastewater due to its low consumption of reagents, fast and economical. In adsorption process due to large specific surface area, active binding site, metal ions are adsorbed through several forces such as electrostatic attraction, chelation, pore cave size, etc4. One of the important issues in adsorption is the separation of adsorbent from the solution. Compared to traditional methods such as filtration, sedimentation and centrifugation, magnetic separation is a fast and cost-effective method that adsorbent can be reused many times 5, 6. In magnetic solid phase extraction (MSPE) method for removal of heavy metal ions, magnetic nanosorbents are easily separated from sample solutions by a magnetet 7. For high adsorption capabilities, electrostatic interactions, easy operation, the short equilibrium time and functional groups such a hydroxyl and amine of carbon nanotubes (CNTs), they have been widely studied for removal of heavy metal ions in water and wastewater 8-10. Multi-walled CNTs (MWCNTs), carbon nanotubes are composed of multilayer graphene and the number of tubes of rolled sheets with diameters from 2 to 50 nm and they are highly pure and structurally complex 11. Nowadays, magnetic iron oxide nanoparticles are widely used in MSPE adsorbents due to their small size and high surface-to-volume ratio7. The efficiency of CNTs as adsorbent has been improved through modification with polymeric materials such as chitosan 12. In fact, chitosan is a derivative of N-deacetylation of chitin. It is a type of naturally polysaccharides that is found in the exoskeleton of insects, crabs, shrimps, crabs and in the internal structure of other invertebrates is present. Due to biodegradability, biocompatibility, non-toxicity and antibacterial activity of chitosan, it has received special attention 13, 14. Due to the limited adsorption sites of chitosan, the adsorption capacity is not very favorable. This factor limits its effectiveness in removing heavy metal ions 15, 16. Chitosan modification increases _COOH, _NH2, _CN and _S__ groups as functional groups as functional groups to adsorb soft cations 17. l-Arginine (l-Arg) is a biocompatibility and low cost amino acid with several amino groups and a carboxyl group for improving binding sites on the sorbent for removal of metal ions18. In the present study, a new modified multiwalled carbon nanotubes (l-Arg-Chi/MWCNTs-COOH/Fe3O4) was synthesized successfully which for the first time is used to preconcentrate Cu (II), Pb (II), and Cd (II). With this low-cost modification, the active sites in multi-walled carbon nanotubes were improved and after adsorption of metal ions, adsorbent separation was easily achieved. The characteristics of nano magnetic adsorbent were performed by FT-IR, XRD and SEM. Then, the effective parameters on Cu (II), Pb (II), and Cd (II) adsorption using the proposed method were optimized. The experimental data were fitted on isotherm and adsorption kinetic models to understand the adsorption mechanism. Also, thermodynamic parameters for Cu (II), Pb (II), and Cd (II) adsorption on l-Arg-Chi/MWCNTs-COOH/Fe3O4 was obtained. Finally, the synthesized l-Arg-Chi/MWCNTs-COOH/Fe3O4 showed that this method was simple, efficient, low cost and successfully applied in the removal of Cu (II), Pb (II), and Cd (II) in water and wastewater samples. materials and methods: Chitosan (deacetylation rate > 90%) and multiwalled carbon nanotubes (MWCCNTs) were obtained from Sigma-Aldrich (Darmstadt, Germany). Solutions of Cu (II), Pb (II) and Cd (II) were prepared individually by dissolving appropriate amount of the Lead (II) nitrate (Pb (NO₃) ₂), copper (II) nitrate trihydrate (Cu (NO₃) ₂. 3H₂O) and cadmium nitrate (Cd (NO3) 2) by purity over 99. 5% that were prepared from Merck (Darmstadt, Germany) ) in 1% HNO3 and then mix working solution was prepared by Step by step dilution of the stock solutions. Glutaraldehyde 25% (Purity over 99. 5%) and ethylene diamine tetra acetic acid (EDTA) (Purity over 99. 5%), l-Arginine (Purity over 99. 5%) were prepared from Merck (Darmstadt, Germany). 2 ml of 1. 0 mol L-1 acetate buffer was used to adjust the pH of the solutions. Apparatus Fourier transform infrared (FTIR) of samples were recorded in the range 4000–400 cm-1 using the KBr pellet technique (Thermo, AVATAR, Massachusetts, USA). A supermagnet with 1. 2 Tesla magnetic field (N35 model from Tehran Magnet, Tehran, Iran) was used for separation of magnetic nanosorbent. A graphite furnace atomic absorption spectrometer (GF-AAS) was used to determine metal ions. (Varian, SPECTRA AA200, Australia). A scanning electron microscope (SEM) images were carried out on PHILIPS, CM120 (Amsterdam, Netherlands) for study on the crystallinity of the magnetic nanosorbent. Synthesis of the l-Arg-Chi/MWCNTs-COOH/Fe3O4 0. 04 g MWCNTs-COOH was added to a homogeneous mixture of 0. 08 g Iron (II) chloride (Fecl2) and 0. 216 g Iron (III) chloride (Fecl3) dissolved in 200 mL of deionized (DI) water and heated under N2 atmosphere to 50 °C for 20 min. The cooled solution was dispersed in ultrasound for 20 min. 1. 0 mL of NH3 was added and heated to 50°C for 40 min under N2 atmosphere. After three washes with deionized water, the solution was separated from the Fe3O4 /MWCNTs-COOH composite precipitate by a magnet and dried at 80°C by a vacuum oven. In 100 ml of aqueous solution of acetic acid, 1. 0 g of Chi powder was dissolved. 100 mL of NaOH 0. 25 mol L-1 was added to the previous solution. After forming the precipitate, it was washed with acetone. 5 ml of epoxychloropropane was added to the previous suspension and stirred for 24 hours at 25°C. Then, 2. 00 g of l-Arg dissolved in 40 ml of DI water, was added to the solution and refluxed for 7 hours at 50°C. To 0. 25 g of l -Arg dissolved in 10 ml of DI water, 30 ml of NaOH 1. 00 mol L-1 and 0. 05 g of KI were added and the mixture was stirred for 5 hours. After cooling, the product was washed with DI water and acetone. Finally, the synthesized l-Arg-Chi was dried at 50°C by vacuum oven. 1. 0 g of each of the previous products and 0. 4 ml of glutaraldehyde were dissolved in 400 ml of acetic acid and stirred in N2 atmosphere at 40°C for 40 min. Then 1. 0 L of DI water and 120 ml of 0. 10 M NaOH were added to the mixture and the mixture was stirred again for another 30 min. After cooling the solution, the water was removed from the sponge product and the l-Arg-Chi/MWCNTs-COOH/Fe3O4 was dried at 80°C for 12 hours in a vacuum oven and finally powdered. Scheme 1 shows the shape of the adsorbent. Scheme 1. Modified MWCNTs-COOH (l-Arg-Chi/MWCNTs-COOH/Fe3O4) Adsorption experiments 10 mg of l-Arg-Chi/MWCNTs-COOH/Fe3O4 were added into of heavy metals solution (100 mL, 20 mg L-1). After adjusting pH to 6, the mixture stirred for 6 min on a shaker. Then the magnetic adsorbent was separated from the solution by a magnet. Residual concentration of Cu (II), Pb (II), and Cd (II) was measured on GF-AAS. The amount of Cu (II), Pb (II), and Cd (II) adsorbed by per unit mass of l-Arg-Chi/MWCNTs-COOH/Fe3O4 was obtained by the following equation: qₑ= (V (C₀-Cₑ) ) ⁄W (1) qe (mg g-1) is the equilibrium adsorption capacity of adsorbent for the metal ions, C0 (mg L−1) and Ce (mgL-1) are the initial concentration and the equilibrium concentrations of the metal ions, respectively. V (L) and W (g L-1) are the volume of the metal ions solution and the mass weight of the adsorbent, respectively 17, 18. results: Results Characterization of the adsorbent IR spectra analysis Fig. 1a, b, c and d, demonstrated FTIR spectra of Fe3O4 /MWCNTs-COOH, l-Arg-Chi, l-Arg-CS/MWCNTs-COOH/Fe3O4 and l-Arg-Chi/MWCNTs-COOH/Fe3O4-Cu, respectively. In Fig. 1a, the stretching vibrations corresponding to hydroxyl, aliphatic O_H, C_H, _C_O, C ̳ ̳ ̳ O and Fe_O, appeared of peaks at 3435 cm−1, 2919 cm−1, 1575 cm−1, 1113 cm-1 and 574 cm-1, respectively 19-24. In Fig. 1b, asymmetry and symmetry stretching vibration of _COO__, probably corresponding to peaks in areas at 1520 cm−1 and 1302cm−1. Also, the stretching vibration of _C_N, which is due to the reaction between _Cl and _NH2, can be attributed to the peak in the area at 1073 cm-1 18. In Fig. 1c, N_H bending vibration of amino, probably corresponding to peak in area at 1441 cm−1 and C ̳ ̳ ̳ O and the bridge between OH of the Chi and C_O_C groups, probably corresponding to peaks in areas at 1385 cm−1 and 1030 cm−1, respectively 19. In Figure 1d, a decrease in the intensity of two peaks at 1441 cm-1 and 1073 cm-1 was observed, which may be related to the engagement of active sites for Cu (II) adsorption7. In the end, all the evidence indicates that the l-Arg-C
DAHAJI et al. (Fri,) studied this question.