Сведения об авторах

A Comparison Study on Synthesized Sulfur Nanoparticles and Sulfur Microparticles as a New Solid Phase Extractor for Extraction of Nickel from Environmental and Waste Water Samples

 

Majid Soleimani, Imam Khomeini International University, Qazvin, Iran

 

Abstract

A new method based on adsorption of nickel ions on synthesized sulfur nanoparticles and microparticles is described for extraction and preconcentration of Ni(II) from environmental and waste water samples followed by flame atomic absorption spectrometric determinations. Sulfur nanoparticles were successfully synthesized via novel water in oil (W/O) microemulsion system. The effects of the analytical parameters including pH of the sample solution, eluting solution conditions, flow rate of sample solution, sample volume and interfering ions on the solid phase extraction of Ni(II) were investigated.

 

Introduction
Heavy metals are toxic and harmful, to human beings and natural environment, even at low concentrations. One of them is nickel which is toxic and carcinogenic. It´s toxicity can cause asthma, disorders of central nervous system, cancer of lungs and nasal cavity. Moreover, nickel can cause a skin disorder known as nickel- eczema which is a considerable health problem especially among women. However, direct determination of metal ions at trace levels by FAAS is limited due to their low concentrations in real samples and matrix interferences. Preconcentration and separation methods have been routinely used to replace the original sample matrix with a new non-interfering one and cope with low metal levels. Currently, solid phase extraction (SPE) is the most common technique used for preconcentration of analytes in environmental waters because of its advantages of low consumption of organic solvents, low cost, high enrichment factor, rapid phase separation, high recovery and the ability of combination with different detection techniques in the form of on-line or off-line mode. In SPE procedure the choice of appropriate adsorbent is an important parameter to reach high enrichment factor and full recovery and it can control the analytical parameters such as capacity, selectivity and affinity.

 

Sulfur and sulfur containing compounds can be used to remove heavy metals. It is because they have excellent sorption selectivity for divalent transition metal ions due to the strong affinity between sulfur atoms and metal ions. The size of the adsorbent can affect its efficiency. Recently, nanosized sulfur have been used as solid phase extractor. Since they have high surface area, they have high adsorption capacity and also they can adsorb metal ions with great adsorption speed.

 

Synthesis of sulfur nanoparticles

Sulfur powder (particle size < 40µm) was ground fully in a mortar. 12.8 g of grounded sulfur powder was added into a flask that has been filled with 100 ml of sodium sulfide solution (2 mol/l). The reaction was occurred at room temperature for 60 min under stirring (Eq. 1)

 

(X−1) S + Na₂S → Na₂S_x .      

 

The color of the solution changed slowly to orange with the dissolving of sulfur and sodium polysulfide (Na₂S_x) solution was prepared.

                                                                                                                                                  

Stable reverse microemulsions were obtained by mixing cyclohexane as oil phase, butanol as co-surfactant, the nonionic surfactant Triton X-100 and sodium polysulfide solution 2 mol/l (microemulsion I) or hydrochloric acid solution 4 mol/l (microemulsion II) as aqueous phase. The mole ratio of the surfactant to co-surfactant was 1 : 4. The volume ratio of cyclohexane, Triton X-100, butanol and aqueous phase was 6 ml/2 ml/1.3 ml/1 ml. Cyclohexane, Triton X-100 and butanol were mixed under constant stirring with the mentioned proportion until the mixture became transparent. Then appropriate amount of sodium polysulfide solution 2 mol/l (microemulsion I) or hydrochloric acid solution 4 mol/l (microemulsion II) was added drop wise under vigorous stirring until the mixture became transparent. Then the microemulsion II was added drop wise to the microemulsion I under stirring, at room temperature. After the reaction, acetone was added to break the microemulsion to cause precipitation of the sulfur nanoparticles synthesized in the microemulsion system. Then the precipitate was separated by centrifugation at 4 000 rpm for 40 min and repeatedly washed with acetone, methanol and water to remove the remaining of organic materials and the produced salt (NaCl) from the product to make it pure; then the product was dried in an oven at 60°C for 6 h.

Eq. 2 represents the reaction occurred by mixing these two microemulsions

Na₂S_x  + 2HCl → 2NaCl + H₂S + (x−1) S.

 

Characterization of synthesized sulfur nanoparticles

The prepared sulfur nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and fourier transform infrared spectroscopy (FTIR). The results showed that monoclinic sulfur nanoparticles prepared via this method have spherical shape, high purity with an average size of about 22 nm.

 

Column preparation

1.5 g of synthesized sulfur nanoparticles or sulfur microparticles (Merck, particle size < 40 µm) was made slurry in methanol and then packed in a typical polypropylene extraction tube (6cm length and 1cm id). A polyethylene frit was placed at both ends to prevent loss of the adsorbent during the samples loading.

 

Extraction and preconcentration procedure

The proposed method was tested with model solutions prior to application to real samples. A suitable aliquot of a solution (50 ml) containing 10 µg of the nickel (II) was placed in a glass beaker. The pH of the solution was adjusted to 7.5 using diluted NaOH solution. The column was washed and preconditioned with double distilled water and then the model solution was passed through the column at a flow rate of 5 ml/min. The adsorbed nickel (II) ions on the column was eluted with 3 ml of 0.1 mol/L EDTA with pH = 7.5. The eluent was analyzed for the determination of nickel concentration by FAAS. Using the described procedure, the recovery of the nickel was calculated from the ratio of the concentration found by FAAS and the concentration calculated theoretically.

 

Results and discussion

Elemental sulfur can exist in a wide variety of allotropic forms. Orthorhombic and monoclinic sulfur are the most stable forms which contain crown-shaped S₆, S₈ and S₁₂ molecules. The interaction between these crown-shaped molecules and Mⁿ⁺ is similar to host-guest interaction such as those occurring between crown ethers and metal ions. Then, sulfur nano and microparticles can be a good extractor for metal ions. In this work, the effects of the analytical parameters including pH of the sample solution, eluting solution conditions, flow rate of sample solution, sample volume and interfering ions on the solid phase extraction of Ni(II) were investigated.

 

Conclusion

Sulfur micro and nanoparticles were introduced as a new metal extractor. Using this new adsorbent has several advantages such as simplicity, accuracy, rapidity, precision, high preconcentration factor and high reusability of adsorbent. There is no need to load any chelating and/or complexing agent onto the adsorbent before the extraction and preconcentration method to obtain full recovery of nickel ion, which minimizes the probably contamination and interferences due to the reagents. A comparison study was performed between synthesized sulfur nanoparticles and sulfur micro particles (particle size < 40 µm and purity > 99% bought from Merck, Darmstadt, Germany) for extraction and preconcentration of nickel ions from different water samples prior to determinations by FAAS. The effects of the analytical parameters including sample pH, elution, sample flow rate, sample volume and interfering ions were investigated for each adsorbent. Under the optimum experimental conditions, preconcentration factor, analytical detection limit, adsorption capacity and reusability of the sorbent were obtained for micro sulfur as 100, 0.0018 µg/mL, 15.75 µg/g, 10 cycles, respectively. Whether, using sulfur nanoparticles these amounts were obtained as 166.67, 0.00108 µg/ml, 30.08 µg/g, and > 70 cycles, respectively. These results show that, sulfur nanoparticles adsorbent will cause to higher preconcentration factor, lower detection limit, higher adsorption capacity and more reusability of the sorbent. The analytical performance of this nanosorbent is comparable or better than some of the previously reported SPE methods (Table 1). The proposed method was successfully applied for preconcentration and determination of traces of Ni(II) in water samples.

 

Table 1

Comparative data on various sorbents for nickel preconcentration

 

 

Сведения об авторах

Majid Soleimani, Associate Professor, Department of Chemistry, Imam Khomeini International University, Qazvin, P. O. Box 288, Iran. Tel./fax +98 (281) 378-00-40. E-mail