Sulfur Nanoparticles Synthesis and Characterization from H2S Gas, Using Novel Biodegradable Iron Chelates in W/O Microemulsion
© to the authors 2008
Received: 4 February 2008
Accepted: 4 June 2008
Published: 3 July 2008
Sulfur nanoparticles were synthesized from hazardous H2S gas using novel biodegradable iron chelates in w/o microemulsion system. Fe3+–malic acid chelate (0.05 M aqueous solution) was studied in w/o microemulsion containing cyclohexane, Triton X-100 andn-hexanol as oil phase, surfactant, co-surfactant, respectively, for catalytic oxidation of H2S gas at ambient conditions of temperature, pressure, and neutral pH. The structural features of sulfur nanoparticles have been characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), diffused reflectance infra-red Fourier transform technique, and BET surface area measurements. XRD analysis indicates the presence of α-sulfur. TEM analysis shows that the morphology of sulfur nanoparticles synthesized in w/o microemulsion system is nearly uniform in size (average particle size 10 nm) and narrow particle size distribution (in range of 5–15 nm) as compared to that in aqueous surfactant systems. The EDS analysis indicated high purity of sulfur (>99%). Moreover, sulfur nanoparticles synthesized in w/o microemulsion system exhibit higher antimicrobial activity (against bacteria, yeast, and fungi) than that of colloidal sulfur.
KeywordsSulfur nanoparticles H2S gas Iron chelates W/O microemulsion Antimicrobial activity
Sulfur finds extensive technological applications such as in production of sulfuric acid, plastics, enamels, antimicrobial agent, insecticide, fumigant, metal glass cements, in manufacture of dyes, phosphate fertilizers, gun-powder and in the vulcanization of rubber, etc. [1–4]. Sulfur nanostructures are also used in synthesis of sulfur nanocomposites for lithium batteries [5, 6], modification of carbon nanostructures [7, 8], in synthesis of sulfur nanowires with carbon to form hybrid materials with useful properties for gas sensor and catalytic applications , Metal-sulfur compounds like ZnS and CdS play important role in nonlinear optical and electroluminescent devices, etc. [10–15].
The synthesis of nanoparticles can be carried out by various methods. Among them, use of microemulsion system is an attractive and simple method as it allows greater control over nanoparticle morphology (size and shape) [16, 17]. Recently, Guo et al.  reported the synthesis of monoclinic sulfur nanoparticles using the mixture of two w/o microemulsion systems. However, during synthesis the H2S gas was released as hazardous by-product. In this study we report for the first time synthesis of sulfur nanoparticles in the range of 5–15 nm from H2S gas by catalytic conversion using biodegradable iron chelates in w/o microemulsion system.
At present, use of iron chelates has been extensively commercialized in Lo-CAT, Sulferox process . However, these chelating agents (e.g., EDTA, DTPA, NTA, Cyclohexane-1,2-diaminetetraacetic (CDTA), etc.) have very low rate of biodegradation and therefore cause environmental pollution. Alternative chelating agents for gas sweetening should meet three main criteria, viz. (1) should possess equal to or better complex forming properties compared to commercial chelating agents, (2) should possess better biodegradability, and (3) should contain low nitrogen to minimize the nitrogen content in effluents. It has been reported that carboxylic acids (e.g., citric acid, malic acid, gluconic acid, etc.) have good chelating properties and have faster rate of biodegradation [28, 29].
In this study use of novel biodegradable iron chelates, in particular FeCl3–malic acid chelate system, has been extensively studied in w/o microemulsion (cyclohexane/n-hexanol/Triton X-100/water) for the catalytic conversion of H2S gas to sulfur nanoparticles. The sulfur nanoparticles have been systematically characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), diffused Reflectance Infra-red Fourier transform technique (DRIFT-IR), and BET surface area measurements. Furthermore, the potency of antimicrobial activity of sulfur nanoparticles has been determined by plate assay and compared with that of colloidal sulfur.
Materials and Methods
All chemicals were of analytical grade and used without further purification. Ferric chloride, ferric sulfate, ferric nitrate, gluconic acid, malic acid, citric acid, sodium hydroxide, sodium sulfide, cyclohexane,n-hexanol, Triton X-100 were purchased from Merck India. All solutions were prepared by deionized milli-Q water (Q-H2O with 18.2 MQ cm resistivity, Millipore corporation).
All microbial cultures (Pseudomonas areuginosa NCIM 2036 andStyphylococcus areus NCIM 2079;Candida albicans NCIM 3102,C. albicans NCIM 3466,Aspergillus flavus NCIM 535, andAspergillus niger 545) were procured from National Collection of Industrial Micro-organisms (NCIM), NCL-Pune (India).
Preparation of Iron Chelate Solution
Screening of iron chelate systems
Reaction time (min)
Sulfur recovered mg/gm of iron chelate
% purity Sulfura
General Method of Sulfur Recovery from H2S Gas
Synthesis of Sulfur Nanoparticles
Preparation of FeCl3–malic acid (Fe3+–malic acid) Chelate Solution
Iron chelate solution (0.05 M) was prepared by dissolving ferric chloride and malic acid (1:3 mole ratio) in deionized water. The pH of the solution was adjusted to 7.0–7.5 with sodium hydroxide solution. This aqueous chelate solution was used in the w/o microemulsion system for synthesis of sulfur nanoparticles.
Synthesis of Sulfur Nanoparticles in W/O Microemulsion System
The sulfur nanoparticles were characterized by XRD using Rigaku Dmax 2500 diffractometer equipped with graphite monochromatized CuKa radiation (λ = 1.5406 Å) employing a scanning rate of 5°/min in the 2θ range from 10 to 80°. The EDS was carried out by using EDAX of Phoenix Company. The morphology of the sulfur nanoparticles was observed by TEM. The samples for TEM analysis were prepared by deposition of an ultrasonically dispersed suspension of the sulfur product in methanol on collidon and carbon-coated copper grids. The TEM measurements were performed on a JEOL model 1200EX instrument. Infrared (IR) spectra were recorded by Parkin-Elmer system 2000 Infrared spectroscope employing a potassium bromide (KBr) using DRIFT-IR.
Evaluation of Antimicrobial Activity of Sulfur Nanoparticles
The antimicrobial activity of sulfur nanoparticles was determined by plate assay and compared with that of colloidal sulfur. Here, nutrient agar (HiMedia, India), MGYP [malt extract (0.3 g), glucose (1.0 g), yeast extract (0.3 g), peptone (0.5 g), and agar (2.0 g/100 mL medium)], and potato dextrose agar (HiMedia, India) were used as growth media for bacterial, yeast, and fungal strains, respectively. The antimicrobial activity of sulfur nanoparticles and colloidal sulfur was determined by measuring the inhibition zones on agar plates inoculated with bacterial (P. areuginosa,S. areus), yeast (C. albicans), and fungal strains (A. flavus,A. niger). An aliquot part of 0.5 mL freshly prepared microbial cell suspension was added on sterile Petri plates containing solidified agar medium. An aliquot part of 30 μL of sulfur suspension was added on agar plates followed by incubation at specified temperature (28 °C for yeast and fungi, 30 °C for bacteria). Being antimicrobial agent, sulfur inhibited the growth of microorganism and inhibition zones were formed. The inhibition zones were measured after 24 h for bacteria and after 48 h for yeast and fungi. The efficiency of antimicrobial activity is expressed in terms of average diameter of inhibition zones obtained by triplicate results.
Results and Discussion
Selection of Optimum Iron Chelate System
Different iron chelates were evaluated for catalytic conversion of H2S gas to elemental sulfur. Table 1reports the values of reaction time indicating the breakthrough of H2S uptake since the beginning of experiments. In all experiments, the volume of the reaction mixture and the rate of H2S gas addition were kept constant. The values of ‘reaction time’ therefore give a measure of the rate of reaction for various iron chelates. Besides the reaction time, purity and recovery of sulfur also need to be considered to establish the best iron chelate system. Among various iron chelates, FeCl3–gluconic acid chelate system required much less reaction time than the others (5 min); however, the purity and recovery of product was very poor. Over all, FeCl3–malic acid chelate system has been observed to give maximum recovery of sulfur (499 mg/g of iron chelate) along with high purity (>99%) and reasonable extent of reaction time (15 min). Hence it was selected for further evaluation in w/o microemulsion system.
XRD Analysis of the Sulfur Particles
where D is the crystallite size, k the Scherrer constant usually taken as 0.89, λ the wavelength of the X-ray radiation (0.154056 nm for Cu Kα), and β is the full width at half maximum of the diffraction peak measured at 2θ. The estimated crystallite size of the sample prepared in microemulsion from line broadening of the most intense diffraction peak is approximately 10 nm.
Antimicrobial Activity of Sulfur Nanoparticles
Antimicrobial activity of sulfur nanoparticles
Type of micro-organism
Microbial strain (μg/mL)
Zone of inhibition (mm)
Sulfur synthesized in aqueous system (80–100 nm)a
Sulfur nanoparticles (5–15 nm)a
P. areuginosa NCIM 2036
S. areus NCIM 2079
C. albicans NCIM 3102
C. albicans NCIM 3466
A. spergillus flavus NCIM 535
A. niger NCIM 545
In case of Fungi, 30 and 150 μg/mL suspensions of sulfur gave no inhibition zones. Therefore the antifungal activity was determined at higher concentrations. The distinct inhibition zones were observed when sulfur nanoparticles concentration was increased up to 1,500 and 3,000 μg/mL (Table 2). In the case ofA. niger, sulfur nanoparticles (at 1,500 and 3,000 μg/mL concentrations) were found to retard normal fungal growth. Here the formation of spores (late stage of fungus growth cycle) was observed to be delayed by about 48 h. Thus the reduction in particle size enhances the effectiveness of sulfur particles as antimicrobial agent. These results are significant and indicate the greater efficacy of sulfur nanoparticles than normal sulfur particularly as an antifungal agent.
To the best of our knowledge, the detail study of iron chelating property of malic acid has not yet been reported. Due to low biodegradability (e.g., BOD5 of EDTA: 0.01%), the disposal of iron chelates used in Lo-CAT process raised serious environmental concern . In order to overcome this environmental pollution, there is need to look for alternative iron chelating agents. The carboxylic acids are known to possess high rate of biodegradation (e.g., BOD5 of malic acid: 65%) along with good iron chelating property. The results of the present investigation indicated that Fe+3–malic acid iron chelates in w/o microemulsion can effectively oxidize hazardous H2S gas to produce sulfur nanoparticles. Besides reduction in environmental pollution (use of biodegradable chelating agents) and waste utilization (H2S abatement), the described process also has high commercial significance.
Synthesis of α-sulfur nanoparticles by catalytic oxidation of H2S gas using biodegradable Fe+3–malic acid chelate system in w/o microemulsion has been reported first time. The described process gives highly crystalline pure α-sulfur with uniform shape and size of 10 nm of sulfur nanoparticles. The synthesized nanoparticles are showing very high antimicrobial and antifungal activity compare to sulfur synthesized in aqueous phase only. The sulfur nanoparticles synthesis was carried out at ambient temperature and atmospheric pressure and between pH 7 and 7.5. The described process serves mainly two objectives: waste utilization for preparation of commercially important product and reduction in environmental pollution.
A.S.D would like to thank CSIR, India, for Senior Research Fellowship.
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