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.
The catalytic conversion of H2
S gas to elemental sulfur can be achieved by various chemical [19
] and biological [23
] 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 . 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.