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Background
Water is crucial to survive all living organisms on the earth, and access to safe drinking-water is important as health and development issue at a national, regional and local levels (1). One of today's environmental challenges is the excessive use of dyes for industrial and domestic activities which it contaminates the ground and surface water; therefore, it is important to remove these dangerous organic compounds from water and wastewater prior to discharge into the environment (2-5). Water pollution by synthetic organic dyes used in textile, cosmetics, pulp and paper industries etc., is mainly regarded as environmental and ecological critical issues worldwide (4-7). These industries produce considerable unfixed dyes that in turn impart strong color, expelling of these dyes into water bodies, produce a large quantity of dyeing wastewater which needs to treatment before being discharged into waterways.
 Due to the aromatic structures, synthetic dyes have chemical stability; thus, they are difficult to biodegrade, oxidizing and photo degradation, giving water undesirable color, diminish light penetration and oxygen gas solubility, so, photosynthetic reactions of aquatic plants, will be decreased, having toxic effects on human health (8,9).
Sulfur dyes are synthetic organic ones, manufactured by the thionation or sulphurisation of organic compounds containing nitro or amino group, and is mainly used for dyeing cellulose fibers in terms of volume. They are extremely colored, water insoluble and have to be converted to water soluble (leuco form) before application to textile material (Eq.1).
  (Eq.1)
Due to their good wet- light fastness, low-cost and rapid processing, consumption of these kinds of dyes remains high. An approximate production of these dyes was more than105 tons in the world annually. Averagely 25% of sulfur dyes are remained in waste streams, so it is important environmentally and ecologically, to remove these pollutants from waste water before discharging into hydrosphere (10,11).
There are two major remediation methods for color removal, including physico-chemical and biological treatments. As mentioned above, due to stability of dye molecule, biological treatment is not effective for removal of all dyes. Among physico-chemical methods, adsorption is one of the most effective, promising and superior to the others. Some characteristics such as easy operation, flexibility, economically feasible etc, make that this method has been used for good removal of wide variety of dyes in wastewater streams (10-12). Moreover dye chromospheres will be changed or collapsed by adsorption; this process can remove the whole dying molecules and their fragments and also control the bio- chemical oxygen demand in the effluent (10-13). Silica gel as an inert and non-toxic material with high specific surface area (~800 m²/g), porosity and high efficiency for functionalization with various ligands allocated to a wide range of biomedical and treatment applications.
 The hydroxyl (OH) groups on the surface of silica can be functionalized to improve specialty silica gels with unique stationary phase characteristics. These properties make silica as a convenient material for iron oxide shielding and functionalization (14,15). Magnetic modification of low-cost adsorbent could result in materials which is useful for biotechnology and environmental applications (16). Adsorption process by magnetic separation technology combined with inexpensive adsorbents has been widely used in environmental purification.
Aims of the study:
In the current investigation, iron oxide/silica gel nanocomposite (Fe₃O₄/SG)) was synthesized, then used as a low-cost, cost-effective, and environmentally friendly to the treatment of sulfur dye. With their large surface areas, these particles can readily adsorb sulfur dye from aqueous solutions.
 

Materials & Methods

All chemicals which used in the study were from analytical grade and obtained from Merck Co. The reagents were used as received without further purification.
 The stock solution was prepared by adding of 0.10 M Na₂S solution and 0.50 g of the sulfur blue 15 (C.I. 53540, CAS 1327-69-1, dye content 95%) in 1.0 L of distilled water under constant stirring, and the temperature was slowly increased to 90 ^oC, which was maintained at this state for 30 min. Finally, we were obtained practical solutions of varying concentrations for further experiments.
Iron oxide magnetic coated silica was prepared by co-precipitation method as described before after some modifications (17): 20 g of silica powder (in spherical sizes of 0.50 to 1.0 mm diameters), 1.48 g FeCl₂.4H₂O and 4.0 g FeCl₃. 6H₂O, were loaded into a 250 mL flask containing 60 mL of de-ionized water, at determined temperature, under refluxing condition in N₂ atmosphere and adding 25% ammonia solution with continuous stirring. Final particles were then rinsed with de-ionized water and separated by low speed centrifugation from un-reacted magnetite. The resulting particles then dried at temperature about 60 ^oC for 4 h in an oven and were stored in a glass vial for further use.
The sulfur dye in aqueous media was determined spectrophotometrically by a single beam Perkin-Elmer UV-VIS spectrophotometer in λ_max=520 nm at pH 10.0 and quantified using a calibration graph which was obtained from the standard solutions prepared from the standard sulfur dye solution in distilled water in the concentration range of 1.0–100.0 mg L⁻¹.
 The calibration curve shows that the Beer’s law is obeyed in this concentration range. A Metrohm pH meter (model 827) with a combined double junction glass electrode was used for pH measurements. Dilute NaOH and HCl solutions were used for pH adjustment.
 All experiments were done in the batch system to study the effects of solution pH, initial concentration, adsorbent dosage and temperature on the adsorption of sulfur dye on Fe₃O₄/SG composite.
 

Results

Morphological structure
The Scanning Electron Microscopy technique (SEM) has been known as a primary tool for characterizing the surface morphology and fundamental physical properties of the adsorbent. Fig.1 (a) and (b) shows the SEM images of the Fe₃O₄/SiO₂ composite adsorbent before and after the dye treatment.
 

 

(a)	(b)
Figure 1) a: SEM micrographs of the Fe3O4/SiO2 (before dye treatment), b: SEM Micrographs of the Fe3O4/ SiO2 (after dye treatment).

Effect of solution pH
The dye solution pH, as one of the most important factors, effects on the surface charge of the adsorbent and the dissociation of functional groups on the active sites of the adsorbent (18). The percentage removal of sulfur dye at different pH values (2 to 8) is plotted in Fig. 2.
 

Figure 2) Effect of solution pH on dye removal
(initial dye concentration=100 mg/L, adsorbent dosage=0.25 g, agitation speed=150 rpm, and temperature=298 K).
Effect of sorbent dosage
Different weights of Fe₃O₄/SG (varied from 0.10 to 0.30 g) were exposed to the dye solution for 1 hour. The obtained results are illustrated in Fig. 3.

Figure 3) Effect of sorbent dosage on dye removal (initial sulfur dye concentration= 100 mg/L, pH=5, and temperature=298 K).
Effect of contact time
In this investigation, the effect of agitation time on dye removal at the various initial dye concentrations was studied. The obtained results are shown in Fig. 4.

Figure 4) Effect of dye concentration on sorption of sulfur dye onto Fe3O4/SG (adsorbent dosage=0.20 g, pH=5, agitation speed=150 rpm, temperature=298 K).
Adsorption isotherms
The plot versus  related to the adsorption of sulfur dye on the Fe₃O₄/SG composite adsorbent at room temperature is shown in Fig. 5.

Figure 5) Langmuir isotherm obtained for sorption of sulfur dye at 298K (initial dye concentration=25-100 mg/L, sorbent dosage=0.20g and agitation speed=150 rpm).
Adsorption kinetics
Adsorption processes and suitability of an adsorbent for removal of a special adsorbate is usually described through graphs known as isotherm. For isotherm investigation, the initial concentration of adsorbate is varied then the amount of adsorbate on the adsorbent as a function if its pressure or concentration at constant temperature is plotted. From linear plot of log (q_e-q_t) versus t for the pseudo-first-order model (not shown), and t/q_t versus t for the pseudo-second-order (shown in Fig.6), the kinetic parameters in both models were calculated. Table 1 gives the values of the parameters for the pseudo-first-order and second-order kinetic models.
 
 
 
 

Figure 6) Pseudo-first-order kinetics plots for sulfur dye adsorption at different initial dye concentration (T=298 K).

 
 
Table 1) Adsorption kinetic parameters of sulfur dye onto Fe₃O₄/SG at 298 K.

Co	Pseudo- first-order kinetic model								Pseudo- second-order kinetic model
	qe,exp		qe1,cal	k1		R2			qe2,cal	k2	R2
25	3.10	3.59			0.159	0.969		3.30		0.119	0.998
50	6.16	2.88			0.089	0.985		6.53		0.052	0.999
75	9.03	3.81			0.066	0.990		9.80		0.026	0.999
100	11.20	14.52			0.075	0.988		16.66		0.002	0.990

 
Table 2) Thermodynamic parameters for adsorption of sulfur dye onto Fe3O4/SG

T (K)		∆Go (kJ/mol)	∆Ho (kJ/mol)		∆So [kJ/(mol k)]			R2
298		- 5.44
308			-77.92		-0.24			0.991
318