NICKEL AND COPPER EXTRACTION FROM LIQUID SAMPLES USING CARBON NANOTUBES

Novel carbon-based materials find nowadays increasing use in analytical science, mainly due to their unique properties. Carbon nanotubes (CNTs), one of the carbon-based materials, could possibly be used for the preconcentration and separation of trace amounts of heavy metal from liquid samples. Method called solid phase extraction (SPE) is used as extraction method for metal preconcentration, and carbon nanotubes (CNTs) are considered to be suitable solid sorbent material for this technique. Multiwalled carbon nanotubes were used as a solid phase extractor for Ni 2+ and Cu 2+ ions from liquid samples in this study. Experimental parameters including, pH of sample solution, oxidation technique of MWCNTs, amount of solid sorbent and analytes, concentration of eluent were investigated, focusing on the influence of these parameters on quantitative recoveries of the analytes. Analyte content in each sample or extract was determined by flame atomic absorption spectrometry (FAAS).


Introduction
Solid-phase extraction plays an important role in sample preparation process by replacing the classic liquid-liquid extraction in environmental and food analyses.The main objectives of sample preparation are the removal of potential interferences, analyte preconcentration, or converting the analyte into a more suitable form for detection or separation [1].Carbon nanotubes are relatively new material, which, in the field of analytical chemistry, has obtained wide attention in recent years.Strong interactions of hexagonally grouped carbons in graphene sheets of CNTs, with other molecules and large specific surface area make this material a promising solid sorbent for preconcentration procedures [2].In contrast to the activated carbon, CNTs have a well-defined structure at atomic scale, so reproducible results of metal preconcentration can be achieved [3].Carbon nanotubes can be divided into two categories: singlewalled CNTs (SWCNTs) which look like hexagonal sheets of graphite rolled into cylindrical tubes and multiwalled CNTs (MWCNTs) consisting of several concentric SWCNTs [4,5].Usually metal ions in an aqueous medium are not capable of spontaneous adsorption on the surface of non-modified carbon nanotubes, and therefore it is necessary to modify them.The simplest form of modification of the carbon nanotube surface is by oxidation process.This process forms free oxygen functional groups on the surface of CNTs (for example -OH, -C=O, -COOH), and consequently causes their increased solubility in aqueous media [3] [6].The literary overview of carbon nanotubes oxidation methods, before utilizing them as a solid sorbent, is given in Table 1.The metal analytes adsorbed onto oxidized CNTs can be quantitatively desorbed with the aid of acidic eluting agents.Low pH of the eluent provides quantitative elution of analytes due to the competition between H + and metal ions [3].Most commonly used elution agents and their concentrations are shown in Table 2.
As was mentioned above, nitric acid oxidation of carbon nanotubes leads to functionalization of the surface, and hence to the creation of functional groups containing oxygen radical.The surface charge of nanotube depends on the pH of surrounding electrolyte [2].The sorption of metal ions on surface of the oxidized nanotubes increases in general with increasing pH.The surface of carbon nanotubes at higher pH is more deprotonated (-COOH →-COO -), which causes electrostatic interactions between metal ion Me 2+ and oxygen functional group [9].Sorption of the analyte onto surface of CNT varies with the metal ion to be sorbet.It was shown [2,9], that within the range of pH 7 -9, affinity of metal ion to CNTs surface is decreasing in this order: Cu  laboratory shaker, the sample after elution was filtered through filter paper and analyzed by FAAS.The determination of copper and nickel in samples after extraction and in samples after elution was carried out on Perkin Elmer 3030 atomic absorption spectrometer with flame atomization with these conditions: slit width: 0.7 nm, gas flow: 45 dm 3 hour -1 (air), 17 dm 3 hour -1 (acetylene), wavelength: 324.8 nm (Cu), 231.9 nm (Ni).The microwave-assisted and ultrasoundassisted oxidation process of CNTs was performed by high performance microwave digestion system (Ethos ONE, MILESTONE) and ultrasound probe (UZD 500, Person-Ultragen).

Results and discussion
The aim of this work was to optimize selected experimental conditions of novel and simple method of preconcentration for selected metal ions Cu 2+ and Ni 2+ using carbon nanotubes as a solid sorbent, before their determination by atomic absorption spectrometry with flame atomization.

Oxidation of CNTs
Second most important parameter of SPE-MWCNTs extraction is the oxidation process of CNTs material.Four types of CNTs oxidation were chosen: microwave-assisted oxidation (MW), ultrasound-assisted oxidation (UE), oxidation using reflux (RF), and laboratory temperature oxidation (LT).Experimental properties of this four oxidation processes are given in   We have chosen oxidation with reflux for pretreatment of CNTs in our further studies.This was mainly due to the fact, that it was possible to oxidize higher amount of CNTs at once, than it was possible with the microwave technique.
Fig. 2 The influence of CNTs oxidation process on the extraction recoveries of Cu and Ni

Amount of analyte
Next step was the determination of analytes amount that could be quantitatively adsorbed onto 200 mg of oxidized CNTs in the presence of other macro and micro components (components content are given in Table 3).Experimental conditions of these experiments were as follows: 40 cm 3 First condition that needs to be optimized in SPE-MWCNTs extractions is the pH value of sample solution.Model distilled water solutions containing 2 µg cm -3 of Cu and Ni were prepared and their pH values were modified with appropriate buffer solution to achieve pH values from 4.0 to 9.0.Twenty milliliters of model solutions were then added to 200 mg of oxidized CNTs sorbents and shaken for 4 hours.The dependences of Cu and Ni recoveries on sample solution pH in pH range of 4.0 -9.0 are shown in Fig 1.The highest recoveries were achieved by extracting metal ions at pH value of 9.0 in both cases.This pH was used as constant in our further experiments.

Fig. 1
Fig. 1 The influence of sample solution pH on the extraction recoveries of Cu and Ni

2 .
The dependences of analytes recoveries according to the type of CNTs oxidation process used in SPE-CNT extractions are shown in Fig.It is clear that microwave assisted oxidation of CNTs and their oxidation with reflux were two methods best suitable for CNTs treatment before their use in solid phase extraction of Cu and Ni from liquid solutions.The extraction efficiency of Ni was only around 80 %, by the use of other two methods.

3 .Fig. 3 4 .Fig. 4 Fig. 5
Fig. 3 The influence of analyte amount on the extraction recoveries of Cu and Ni oxygen radicals are capable of binding metal ions at a suitable pH.The oxidation of nanotubes can be carried out with acid reflux with HNO 3 , H 2 SO 4 , HCl, or with oxidizing agents such as KMnO 4 and H 2 O 2 . These DOI 10.12776/ams.v21i2.486p-ISSN 1335-1532 e-ISSN 1338-1156 functional groups containing

Table 1
Overview of CNTs oxidation processes

2+ , Pb 2+ , Zn 2+ , Co 2+ , Ni 2+ , Cd 2+ . 2 Experimental conditions 2.1 Reagents and materials Synthetic
aqueous solutions of copper, nickel and other accompanying metals were prepared from water stock solutions with certified content 1.000 ± 0.002 g dm-3of given element in 2 % HNO 3 (ANALYTIKA ® ).Analytical grade salts (Mg(NO 3 ) 2 .6H 2 O, CaCO 3 , Na 2 SO 4 , Na 2 HPO 4 .12H 2 O, NaCl) were added to deionized water to create model solution containing main macro components of natural water at their average contents.The model solution content is listed in

Table 2
Overview of SPE-CNT experimental conditions

Table 4
Experimental properties of CNTs oxidation