Sulfur Nanoparticles Synthesis and Characterization from H2S Gas, Using Novel Biodegradable Iron Chelates in W/O Microemulsion

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.

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. [18] reported the synthesis of monoclinic sulfur nanoparticles using the mixture of two w/o microemulsion systems. However, during synthesis the H 2 S gas was released as hazardous byproduct. In this study we report for the first time synthesis of sulfur nanoparticles in the range of 5-15 nm from H 2 S gas by catalytic conversion using biodegradable iron chelates in w/o microemulsion system.
The catalytic conversion of H 2 S gas to elemental sulfur can be achieved by various chemical [19][20][21][22] and biological [23,24] means for gas sweetening. Nagal [25] has reported the gas desulfurization based on liquid redox chemistry, as follows where 'L' denotes an organic ligand, which are usually a polyaminocarboxyllic acid, such as ethylenediamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), hydroxy, diethylenetriamine pentaacetic acid (DTPA), etc. [26] and 'n' denotes the charge on the organic ligand. Since the active ferric chelate is converted to inactive ferrous chelate, the later component has to be regenerated by oxidation according to the reactions, At present, use of iron chelates has been extensively commercialized in Lo-CAT, Sulferox process [27]. 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 FeCl 3 -malic acid chelate system, has been extensively studied in w/o microemulsion (cyclohexane/n-hexanol/ Triton X-100/water) for the catalytic conversion of H 2 S 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
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-H 2 O with 18.2 MQ cm resistivity, Millipore corporation).

Preparation of Iron Chelate Solution
Iron chelates were synthesized using ferric salts (namely ferric chloride, ferric sulfate, and ferric nitrate) in combination with different carboxylic acids (namely monodenatate gluconic acid, bidenatate malic acid, and tridenatate citric acid) as a chelating agent in various molar ratios as given in Table 1. These iron chelates were used in the concentration range of 500-20,000 ppm iron for catalytic conversion of H 2 S to elemental sulfur at ambient conditions of temperature, pressure and pH 7-7.5. FeCl 3 -malic acid chelate was found to give elemental sulfur of high purity and better recovery and has therefore been selected for further studies. General Method of Sulfur Recovery from H 2 S Gas Figure 1 shows schematic flow diagram for catalytic conversion of H 2 S to elemental sulfur. Pure H 2 S gas (99.9%) was sparged in the absorber unit for about 35-45 min. In the absorber unit, Fe 3+ -malic acid chelate solution oxidized H 2 S gas to elemental sulfur and simultaneously active ferric chelate was reduced to inactive ferrous chelate (reaction 2). After complete reduction of ferric chelate, H 2 S conversion to elemental sulfur ceases. The outlet gas from absorber unit was checked intermittently by formation of black precipitate of Ag 2 S on bubbling through 0.1 N AgNO 3 solution. The regeneration of inactive ferrous chelate to active ferric chelate was carried out by sparging pure oxygen (reaction 4). The regenerated ferric chelate was recycled for further H 2 S conversion.

Preparation of FeCl 3 -malic acid (Fe 3+ -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 oil phase was prepared by mixing 52 wt.% of cyclohexane, 22 wt.% Triton X-100 (as surfactant), and 11 wt.% n-hexanol (as co-surfactant) under constant stirring. Then 15% (0.05 M) aqueous Fe 3+ -malic acid chelate solution was added drop wise to the oil phase under vigorous stirring in order to prepare optically clear and stable system. The sulfur nanoparticles synthesis was carried out by the procedure described earlier in section ''General method of sulfur recovery from H 2 S gas.'' The oxidation of H 2 S occurred inside aqueous micelle containing iron chelate as shown in Fig. 2. The precipitated sulfur was separated by centrifugation at 8,000 rpm for 20 min and washed with water, methanol, and dried under vacuum at 60°C for 4 h.

Instrumentation
The sulfur nanoparticles were characterized by XRD using Rigaku Dmax 2500 diffractometer equipped with graphite monochromatized CuKa radiation (k = 1.5406 Å ) employing a scanning rate of 5°/min in the 2h 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 lL 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.

Selection of Optimum Iron Chelate System
Different iron chelates were evaluated for catalytic conversion of H 2 S gas to elemental sulfur. Table 1 reports the values of reaction time indicating the breakthrough of H 2 S uptake since the beginning of experiments. In all experiments, the volume of the reaction mixture and the rate of H 2 S 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, FeCl 3 -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,

Reaction -2
H 2 S (g) + 2Fe 3+ L n- FeCl 3 -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
The XRD analysis of sulfur nanoparticles synthesized in w/o microemulsion and aqueous solution is shown in Fig. 3. The position and intensities of the diffraction peaks of all samples were compared with standard a-sulfur particle diffraction pattern [30]. The presence of sharp peaks in Fig. 3b (and in the insight) indicates highly crystalline nature of the sulfur nanoparticles synthesized using w/o microemulsion system compared to particle synthesized by normal aqueous phase (Fig. 3a). The determination of the mean particle diameter (D) was done by the XRD analysis using Debye-Scherrer formula, where D is the crystallite size, k the Scherrer constant usually taken as 0.89, k the wavelength of the X-ray radiation (0.154056 nm for Cu Ka), and b is the full width at half maximum of the diffraction peak measured at 2h.
The estimated crystallite size of the sample prepared in microemulsion from line broadening of the most intense diffraction peak is approximately 10 nm.

EDS Analysis
The synthesized nanoparticles were characterized by EDAX for the evaluation of their composition and purity. Figure 4 shows the spectrum of the EDAX analysis. It is evident from the peaks that the product is completely pure and corresponds to sulfur element only, which are synthesized in w/o microemulsion system and aqueous system.

TEM Analysis
The morphology of the sulfur nanoparticles synthesized in w/o microemulsion system and aqueous system was analyzed using TEM (Fig. 5). Sulfur particles prepared in aqueous phase showed larger particle size and a wider particle size distribution (*80-100 nm) (Fig. 5a), However, sulfur nanoparticles synthesized in w/o microemulsion system show very fine particle size (*10 nm) and a relatively narrow particle size distribution (*5-15 nm) (Fig. 5b). Figure 6 shows the histogram for the sulfur nanoparticles size distribution synthesized in W/O microemulsion.  (Fig. 7b) [31].

Antimicrobial Activity of Sulfur Nanoparticles
The sulfur element is known to possess potent antimicrobial activity [4]. Due to their smaller particle size (*10 nm), sulfur nanoparticles are expected to exhibit antimicrobial action at lower concentrations than sulfur particles synthesized in aqueous phase (80-100 nm). Furthermore, BET analysis shows that there was significant increase in surface area of sulfur nanoparticles (177 m 2 /g) compared to sulfur (48 m 2 /g) synthesized in aqueous phase only as shown in BET isotherms as shown in Figs. 8 and 9, respectively. The results of antimicrobial activities of nanoparticle and sulfur are summarized in Table 2. The inhibition zones for bacteria and yeast were determined at 30 and 150 lg/mL of sulfur suspension. In bacterial strains, sulfur nanoparticles gave larger inhibition zones compared to that of sulfur. Similarly, sulfur nanoparticles were found to be more effective than sulfur in inhibiting growth of yeasts strains (C. albicans NCIM 3102 and 3466). This indicates that at the same concentration, the sulfur nanoparticles have better  In case of Fungi, 30 and 150 lg/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 lg/mL (Table 2). In the case of A. niger, sulfur nanoparticles (at 1,500 and 3,000 lg/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., BOD 5 of EDTA: 0.01%), the disposal of iron chelates used in Lo-CAT process raised serious environmental concern [28]. 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., BOD 5 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 H 2 S gas to produce sulfur nanoparticles. Besides reduction in environmental pollution (use of biodegradable chelating agents) and waste utilization (H 2 S abatement), the described process also has high commercial significance.

Conclusions
Synthesis of a-sulfur nanoparticles by catalytic oxidation of H 2 S 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 a-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.