Rafal Sitko

Adsorption of divalent metal ions from aqueous solutions using graphene oxide

The adsorptive properties of graphene oxide (GO) towards divalent metal ions (copper, zinc, cadmium and lead) were investigated. GO prepared through the oxidation of graphite using potassium dichromate was characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FT-IR). The results of batch experiments and measurements by flame atomic absorption spectrometry (F-AAS) indicate that maximum adsorption can be achieved in broad pH ranges: 3-7 for Cu(II), 5-8 for Zn(II), 4-8 for Cd(II), 3-7 for Pb(II). The maximum adsorption capacities of Cu(II), Zn(II), Cd(II) and Pb(II) on GO at pH = 5 are 294, 345, 530, 1119 mg g(-1), respectively. The competitive adsorption experiments showed the affinity in the order of Pb(II) > Cu(II) ≫ Cd(II) > Zn(II). Adsorption isotherms and kinetic studies suggest that sorption of metal ions on GO nanosheets is monolayer coverage and adsorption is controlled by chemical adsorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of GO. Chemisorption was confirmed by XPS (binding energy and shape of O1s and C1s peaks) of GO with adsorbed metal ions. The adsorption experiments show that the dispersibility of GO in water changes remarkably after complexation of metal ions. After adsorption, the tendency to agglomerate and precipitate is observed. Excellent dispersibility of GO and strong tendency of GO-Me(II) to precipitate open the path to removal of heavy metals from water solution. Potential application of GO in analytical chemistry as a solid sorbent for preconcentration of trace elements and in heavy metal ion pollution cleanup results from its maximum adsorption capacities that are much higher than those of any of the currently reported sorbents.

Green Approach for Ultratrace Determination of Divalent Metal Ions and Arsenic Species Using Total-Reflection X-ray Fluorescence Spectrometry and Mercapto-Modified Graphene Oxide Nanosheets as a Novel Adsorbent

A new method based on dispersive microsolid phase extraction (DMSPE) and total-reflection X-ray fluorescence spectrometry (TXRF) is proposed for multielemental ultratrace determination of heavy metal ions and arsenic species. In the developed methodology, the crucial issue is a novel adsorbent synthesized by grafting 3-mercaptopropyl trimethoxysilane on a graphene oxide (GO) surface. Mercapto-modified graphene oxide (GO-SH) can be applied in quantitative adsorption of cobalt, nickel, copper, cadmium, and lead ions. Moreover, GO-SH demonstrates selectivity toward arsenite in the presence of arsenate. Due to such features of GO-SH nanosheets as wrinkled structure and excellent dispersibility in water, GO-SH seems to be ideal for fast and simple preconcentration and determination of heavy metal ions using methodology based on DMSPE and TXRF measurement. The suspension of GO-SH was injected into an analyzed water sample; after filtration, the GO-SH nanosheets with adsorbed metal ions were redispersed in a small volume of internal standard solution and deposited onto a quartz reflector. The high enrichment factor of 150 allows obtaining detection limits of 0.11, 0.078, 0.079, 0.064, 0.054, and 0.083 ng mL(-1) for Co(II), Ni(II), Cu(II), As(III), Cd(II), and Pb(II), respectively. Such low detection limits can be obtained using a benchtop TXRF system without cooling media and gas consumption. The method is suitable for the analysis of water, including high salinity samples difficult to analyze using other spectroscopy techniques. Moreover, GO-SH can be applied to the arsenic speciation due to its selectivity toward arsenite.

Determination of rare earth elements by spectroscopic techniques: a review

An overview of publications focussed on the period since 2000 and outlining modern methods of sample preparation as well as advanced techniques for determination of rare earth elements (REE) in various matrices is presented in this paper. The review discusses the problems of REE determination in diverse samples i.e. from biological through environmental and geological to advanced materials. The preferable procedure of sample digestion and the most frequently applied methods of sample preparation for determination of trace elements are discussed in this paper. The case of direct analysis of samples for REE determination is also discussed. The review outlines determination of REE employing many techniques such as, inter alia, flame or graphite furnace atomic absorption spectrometry, atomic absorption with chemical vapor generation, X-ray fluorescence spectrometry, inductively coupled plasma optical emission spectrometry, inductively coupled plasma mass spectrometry and neutron activation analysis. This article summarizes and classifies materials in which rare earth elements are present, main places of their occurrence and the methods of their analysis.

Dispersive micro solid-phase extraction using multiwalled carbon nanotubes combined with portable total-reflection X-ray fluorescence spectrometry for the determination of trace amounts of Pb and Cd in water samples

In this paper the combination of dispersive micro solid-phase extraction (DMSPE), using multiwalled carbon nanotubes (MWCNTs) as solid sorbents, with total-reflection X-ray fluorescence spectrometry (TXRF) is proposed for preconcentration and determination of lead and cadmium ions in water samples. The proposed sample preparation is quite simple and economic. After the sorption processes of the metals on the MWCNTs, the aqueous sample is separated by centrifugation and the metal loaded MWCNTs are suspended using a small volume of an internal standard solution and analyzed directly by TXRF. Parameters affecting the extraction process (complexing agent, pH of the aqueous sample, amount of MWCNTs) and TXRF analysis (volume of the deposited suspension on the reflector, drying mode, and instrumental parameters) have been carefully evaluated to test the real capability of the developed methodology for the determination of Cd and Pb at trace levels. For both elements the linear range is observed up to 50 ng mL−1. Under optimized conditions detection limits are 1.0 ng mL−1 and 2.1 ng mL−1 for Cd(II) and Pb(II) ions, respectively. Both of the examined elements can be determined with quantitative recoveries (ca. 100%) and with an adequate precision (RSD = 6.0% and 10.5% for Cd(II) and Pb(II), respectively). Our results give insight into the possibilities of the combination of DMSPE and TXRF for trace metal determination in different types of environmental waters (sea, river and waste water).

Graphene oxide as a solid sorbent for the preconcentration of cobalt, nickel, copper, zinc and lead prior to determination by energy-dispersive X-ray fluorescence spectrometry

A new method for sample preparation using graphene oxide (GO) as a novel sorbent was developed for the preconcentration of trace amounts of Co(II), Ni(II), Cu(II), Zn(II) and Pb(II). The proposed preconcentration procedure is based on dispersive micro-solid phase extraction (DMSPE). It means that GO was dispersed in aqueous samples containing trace elements to be determined. During the stirring of the analyte solution containing the GO suspension, metal ions were sorbed by GO. After the sorption, the solution was filtered under vacuum and GO with the metal ions was collected onto a membrane filter. The obtained samples were analyzed directly by energy-dispersive X-ray fluorescence spectrometry (EDXRF). The parameters affecting the extraction and preconcentration process were optimized. The pH of the analyte solution, the amount of GO, the sample volume, the contact time between analytes and sorbent (stirring time), and the effects of foreign metals are discussed in detail in this paper. The proposed procedure allows us to obtain the detection limits of 0.5, 0.7, 1.5, 1.8 and 1.4 ng mL−1 for Co(II), Ni(II), Cu(II), Zn(II) and Pb(II), respectively. The linearity of the method is in the range of 5–100 ng mL−1. The proposed method was successfully applied in the analysis of water. The accuracy of the method was verified using spiked samples and inductively coupled plasma optical emission spectrometry (ICP-OES) as a comparative technique. The recoveries over the range of 94–106% were obtained. This paper shows the great potential of GO as an excellent sorbent in the preconcentration field of analytical chemistry. The proposed method meets green chemistry rules.

Suspended aminosilanized graphene oxide nanosheets for selective preconcentration of lead ions and ultrasensitive determination by electrothermal atomic absorption spectrometry.

The aminosilanized graphene oxide (GO-NH2) was prepared for selective adsorption of Pb(II) ions. Graphene oxide (GO) and GO-NH2 prepared through the amino-silanization of GO with 3-aminopropyltriethoxysilane were characterized by scanning electron microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The batch experiments show that GO-NH2 is characterized by high selectivity toward Pb(II) ions. Adsorption isotherms suggest that sorption of Pb(II) on GO-NH2 nanosheets is monolayer coverage, and adsorption is controlled by a chemical process involving the surface complexation of Pb(II) ions with the nitrogen-containing groups on the surface of GO-NH2. Pb(II) ions can be quantitatively adsorbed at pH 6 with maximum adsorption capacity of 96 mg g(-1). The GO-NH2 was used for selective and sensitive determination of Pb(II) ions by electrothermal atomic absorption spectrometry (ET-AAS). The preconcentration of Pb(II) ions is based on dispersive micro solid-phase extraction in which the suspended GO-NH2 is rapidly injected into analyzed water sample. Such features of GO-NH2 nanosheets as wrinkled structure, softness, flexibility, and excellent dispersibility in water allow achieving very good contact with analyzed solution, and adsorption of Pb(II) ions is very fast. The experiment shows that after separation of the solid phase, the suspension of GO-NH2 with adsorbed Pb(II) ions can be directly injected into the graphite tube and analyzed by ET-AAS. The GO-NH2 is characterized by high selectivity toward Pb(II) ions and can be successfully used for analysis of various water samples with excellent enrichment factors of 100 and detection limits of 9.4 ng L(-1).

Carbon nanotubes as a solid sorbent for the preconcentration of Cr, Mn, Fe, Co, Ni, Cu, Zn and Pb prior to wavelength-dispersive X-ray fluorescence spectrometry.

The preconcentration of trace elements on multiwalled carbon nanotubes (MWCNTs) followed by a wavelength-dispersive X-ray fluorescence analysis (WDXRF) has been investigated. The proposed preconcentration procedure is based on the sorption of trace elements on MWCNTs dispersed in analyzed solution. After sorption, the MWCNTs with the metal ions were collected onto the filter, and then the preconcentrated elements were determined directly by WDXRF. The preconcentration method was optimized, and in consequence, in order to obtain satisfactory recoveries using 100 mL of samples, the sorption process was performed with 1mg of MWCNTs within 5 min. Some conditions of the preconcentration process such as the pH of analyte solution, amounts of MWCNTs, the volume of the sample, the contact time between analytes and MWCNTs (stirring time), and the effects of foreign metals are discussed in detail in the paper. Adsorption onto raw and oxidized MWCNTs was also studied. The proposed procedure allows obtaining the detection limits of 0.6, 0.6, 1.0, 0.7, 0.6, 0.5, 0.9 and 1.9 ng mL(-1) for Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Pb(II), respectively. The recoveries of determined elements were about 100%. Because the analytes are not eluted from the sorbent before WDXRF analysis, the risk of contamination and loss of analytes is reduced to minimum. Moreover, because the samples are analyzed as a thin layer, the matrix effects can be neglected. The proposed preconcentration method using MWCNTs coupled with WDXRF spectrometry was successfully applied to determine trace elements in natural water samples.

Spherical silica particles decorated with graphene oxide nanosheets as a new sorbent in inorganic trace analysis.

Graphene oxide (GO) is a novel material with excellent adsorptive properties. However, the very small particles of GO can cause serious problems is solid-phase extraction (SPE) such as the high pressure in SPE system and the adsorbent loss through pores of frit. These problems can be overcome by covalently binding GO nanosheets to a support. In this paper, GO was covalently bonded to spherical silica by coupling the amino groups of spherical aminosilica and the carboxyl groups of GO (GO@SiO2). The successful immobilization of GO nanosheets on the aminosilica was confirmed by scanning electron microscopy and X-ray photoelectron spectroscopy. The spherical particle covered by GO with crumpled silk wave-like carbon sheets are an ideal sorbent for SPE of metal ions. The wrinkled structure of the coating results in large surface area and a high extractive capacity. The adsorption bath experiment shows that Cu(II) and Pb(II) can be quantitatively adsorbed at pH 5.5 with maximum adsorption capacity of 6.0 and 13.6 mg g(-1), respectively. Such features of GO nanosheets as softness and flexibility allow achieving excellent contact with analyzed solution in flow-rate conditions. In consequence, the metal ions can be quantitatively preconcentrated from high volume of aqueous samples with excellent flow-rate. SPE column is very stable and several adsorption-elution cycles can be performed without any loss of adsorptive properties. The GO@SiO2 was used for analysis of various water samples by flame atomic absorption spectrometry with excellent enrichment factors (200-250) and detection limits (0.084 and 0.27 ng mL(-1) for Cu(II) and Pb(II), respectively).

Determination of selenium by X-ray fluorescence spectrometry using dispersive solid-phase microextraction with multiwalled carbon nanotubes as solid sorbent

A dispersive solid-phase microextraction (DSPME) with multiwalled carbon nanotubes (MWCNTs) as solid sorbent and ammonium pyrrolidinedithiocarbamate (APDC) as chelating agent was developed for determination of selenium. In the proposed procedure, the Se(IV)–APDC complex is adsorbed on MWCNTs dispersed in aqueous samples. After the adsorption process, the aqueous samples are filtered and MWCNTs with selenium chelate are collected onto a filter. The loaded filters are directly measured using X-ray fluorescence (XRF) spectrometry. In order to obtain high recovery of the Se ions on MWCNTs, the proposed procedure was optimized for various analytical parameters such as pH, amounts of MWCNTs and APDC, sample volume and time of the sorption process. Under optimized conditions Se ions can be determined with very good recovery (97 ± 3%), precision (RSD = 3.2%) and detection limits (from 0.06 to 0.2 ng mL−1, depending on counting time and XRF equipment). The effect of common coexisting ions was also investigated. Se(IV) can be determined in the presence of heavy metal ions and alkali metals. The chemical interferences observed for high concentrations of Cu(II), Fe(III), and Zn(II) can be completely eliminated using precipitation with NaOH. The proposed method was applied for the determination of Se in mineral water and biological samples (Lobster Hepatopancreas). The proposed method can also be applied for selenium speciation. The concentration of selenate can be obtained as the difference between the concentration of selenite after and before prereduction of selenate to selenite.

Determination and speciation of trace and ultratrace selenium ions by energy-dispersive X-ray fluorescence spectrometry using graphene as solid adsorbent in dispersive micro-solid phase extraction.

A dispersive micro-solid phase extraction (DMSPE) with graphene as a solid adsorbent and ammonium pyrrolidinedithiocarbamate (APDC) as a chelating agent was proposed for speciation and detemination of inorganic selenium by the energy-dispersive X-ray fluorescence spectrometry (EDXRF). In developed DMSPE, graphene particles are dispersed throughout the analyzed solution, therefore reaction between Se(IV)-APDC complexes and graphene nanoparticles occurs immediately. The concentration of Se(VI) is calculated as the difference between the concentration of selenite after and before prereduction of selenate. A central composite face-centered design with 3 center points was performed in order to optimize conditions and to study the effect of four variables (pH of the sample, concentration of APDC, concentration of Triton-X-100, and sample volume). The best results were obtained when suspension consisting of 200 µg of graphene nanosheets, 1.2 mg of APDC and 0.06 mg of Triton-X-100 was rapidly injected to the 50 mL of the analyzed solution. Under optimized conditions Se ions can be determined with a very good recovery (97.7±5.0% and 99.2±6.6% for Se(IV) and Se(VI), respectively) and precision (RSD=5.1-6.6%). Proposed DMSPE/EDXRF procedure allowed to obtain low detection limits (0.032 ng mL(-1)) and high enrichment factor (1013±15). The proposed methodology was successfully applied for the determination of Se in mineral, tap, lake and sea water samples as well as in biological materials (Lobster Hepatopancreas and Pig Kidney).