Adsorption of hydrogen sulfide (H2S) by BaFe2O4-activated clay nanocomposite: preparation and evaluation

INTRODUCTION

Recently, BaFe2O4 nanostructure has received increased attention due to the unique physical and chemical properties. BaFe2O4 nanostructure can be utilized in many fields such as sensors and catalysts [1,2]. To date, BaFe2O4 have been broadly used to modify of support surfaces such as silicon rubber [3]. Among the supporting materials, activated clay is the best candidate because of its appropriate acidity and super adsorption capability.

In recent years, the widespread demand of hydrogen sulfide is responsible for environmental pollutions, H2S is a colorless gas with a rotten egg smell. This gas is extremely toxic, corrosive and flammable. Great amount of H2S is found in biological and industrial gases that causes corrosion in the pipes and other equipment [4]. Releasing of hydrogen sulfide is a physical/chemical process which is occurred in wastewater treatment system. H2S can seriously threat the human health [5]. Hence, control of gas emission is necessary not only for the public health and safety but also for protecting the environment [6]. To date, different methods and techniques have been developed to control the odor such as chemical scrubbers, burning the odorant compounds, adsorption, biological methods including biofilters, trickling biofilters, bio scrubbers and activated sludge reactors [7-10].

Among the different methods for H2S removal, absorption is very effective because of its simplicity, efficiency, flexibility and sensitive to toxic pollutants [11]. Recently, mesoporous materials such as active carbon and silica can be used as adsorbent due to the amazing adsorption ability. Furthermore, the results of several studies showed that using small particles, specially in the nanometer scale, makes a significant improvement in the process of removing pollutants. Dagaonkar indicated that adding nanometer particles of titanium oxide (TiO2) to different supports increased absorption of CO2 gas [12]. In other study Mostafaii et al, showed that zinc oxide nanoparticles can remove coliforms in the concentration of 1.1 gr/L at 90 min [13]. Labrada et al. removed some amount of hydrogen sulfide using nano TiO2 and ZnO [14]. Khaleghi et al., indicated benzenesulfonic acid- graphene (BS-rGO) nano absorbent could remove hydrogen sulfide [15]. Sub Song et al. showed that nanoparticles of zinc oxide/reduced graphite oxide composite could remove a significant amount of hydrogen sulfide gas [16]. Nour et al. showed silver / polydimethylsiloxane (PDMS) nanocomposite has excellent activity for H2S gas removal [17]. Mandizadeh et al. reported desulfurization efficiency can be improved by using barium ferrite nanoparticles (BaFe18O18) [18].

In the present work, BaFe2O4, combined with activated clay nanocomposites were prepared by procedure method for removing hydrogen sulfide. This research aims to achieve high desulfurization from wastewater by ferrite-activated clay nanocomposite. Nanocomposites were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR), energy dispersive X-ray (EDS) analysis, and X-ray powder diffraction (XRD).

MATERIALS AND METHODS

The starting cationic sources, Ba (NO3)2 (Mw: 261.337 g.mol-1), Fe(NO3)3.9H2O (Mw: 404 g.mol-1, Glucose (Mw: 180.156 g.mol-1) and activated clay were purchased from Sigma-Aldrich. All chemicals were supplied in analytical grades and utilized without further purification.

Preparation of barium ferrite with auto-combustion sol-gel

For synthesis barium ferrite with auto-combustion sol-gel, specific amount of Ba (NO3)2 (0.2 gr), Fe(NO3)3.9H2O (3gr) and glucose (4.2gr) were dissolved in the minimum amount of distilled water. The mixture was heated to the temperature of 100°C. After the evaporation all water, gel was dried and calcined at 700°C. The preparation conditions are summarized in Table 1.

Preparation of activated clay

We placed 10 g of clay in a three-necked flask with a solution of HNO3 and H2SO4 in 1 to 3 stoichiometric ratio. Flask was at a controlled temperature of 85˚C for 3 h. Then, the extra acid was decanted and washed 5 times with distilled water. Finally, it was dried at 100°C for 3 hours.

Preparation of BaFe2O4-activated clay nanocomposite

Nanocomposites of monoferrite- activated clay were prepared by mechanosynthesis technique. This experiment occurred on the FRITSCH planetary mill brand. A ratio of activated silica and nanoferrite (2,4 and 6%) were added to the flask and mixed at 400 rpm for 3 h. Finally, the product was washed with deionized water and dried at room temperature. All of the preparation conditions were illustrated in Table 1. Regeneration of nanocomposites was performed at 2000C in air.

Catalytic evaluation

We prepared a model sample from Kashan University of Medical sciences wastewater with a specific amount of sulfur compound. In order to measure the sulfur amount in the wastewater, Petrotest Calorimetric Bomb C5000 according to ASTM D-1266 was used. The flask was joined to a reflux- system with nanocomposite in the absorbent column at the temperature 60-100°C. In this study, different concentration of nanocomposites were investigated. The removal efficiency of hydrogen sulfide was calculated by the following formula:

(1)

A: Sulfur amount in the real sample

B: Sulfur amount of the product

Characterization

EDX analysis was performed using Philips Scanning Electron Microscope model EM208 equipped with x-ray spectroscopy. Microscopic morphology of the samples was characterized by Philips Electron Microscope model XL-30ESEM. FT-IR spectra were recorded on Magna-IR, spectrometer 550 Nicolet in KBr pellets in the range of 400–4000 cm–1. The XRD pattern was recorded from diffractometer of the Philips Company with X’PertPro monochromatized Cu Kα radiation (λ = 1.54 Å)

RESULTS AND DISCUSSION

Morphology

Morphology of samples were characterized by SEM images. Fig. 1a is related to the prepared BaFe2O4 without glucose (Sample 1). We observed in Fig 1a that the as-prepared monoferrites have spherical morphology. The conjunction of particles is due to the magnetic properties of ferrite. Fig 1b is related to the synthesized BaFe2O4 in the presence of glucose (Sample 2). Using sugar, semi-spherical particles formed, in a homogeneous texture because of the presence of hydroxyl and carboxylic acid group as capping agents. From its respective particle histograms, the average sizes of nanostructures are in the range of 36-72 nm. The Fig.2 shows the SEM of barium ferrite-activated clay nanocomposites. Fig. 2a,b and c are related to Sample 3, Sample 4 and Sample 5, respectively. As results, we observed regular arrangement of nanostructures for all samples. .The results from morphological observations are tabulated in Table 1.

Structural analysis

Phase type, crystal structure, product purity and the size of crystalline grains were measured by XRD pattern. Fig.3a,b and c are related to Sample 3, Sample 4 and Sample 5 respectively. In Fig 2a the diffraction peaks in 2θ=22.890, 26.600, 29.210, and 43.950 are related to activated clay with the standard diffraction pattern (JCPDS Card Nos. 00-031-0783). Other diffraction peaks of Sample 3 related to BaFe2O4 (JCPDS Card Nos. 00-046-0113) growth at 23.3°, 33.20, 40.10 and 49.360. As shown in Fig 3, for all samples, no peaks were detected as impurities and high purity of the products were observed. Sharp peaks in the diffraction pattern are due to the high crystallization of the achieved products. Using the Scherrer equation [18], the crystal size of BaFe2O4 were 30, 35, 48 nm for Sample 3, Sample 4 and Sample 5 respectively.

EDX and FT-IR

Fig.4a shows EDX spectrum of barium ferrite- activated clay nanocomposites (Sample 5). This spectrum shows the presence of Ba, Fe, O, Si and Al in the product. According to the results, the high purity of the product was observed. Fig 4b shows FT-IR spectra of barium ferrite- activated clay nanocomposites (Sample 5). The important peaks were observed at 3421.75, 1653.67 and 1031.80 cm-1, are related to mode stretching O-H, Si=O and C-O respectively. Two intense bands of 592.57 and 423.95 cm-1 are related to the stretching vibration Fe-O and Ba-O, respectively.

Desulfurization efficiency

Effect of varying concentration and loading of barium ferrite for desulfurization efficiency and its statistics study

According to Fig.5, by increasing loading of barium ferrite in nanocomposite, removal efficiency of hydrogen sulfide was increased. The average efficiency for hydrogen sulfide removal in 3 different loading of barium ferrite with F=413.07 and sig<0.001 showed a statistically significant difference. The results indicated a significant difference (P<0.001) between the nanocomposites in loading 2 and 4%, 2 and 6% and 4 and 6%. Hence, the BaFe2O4- activated clay nanocomposite can be utilized as a new adsorbent for sulfur removal. On the other hand, the existence of metal ion (barium) in nanocomposite can be enhanced the adsorption properties. It seems that porosity of activated clay and the reaction between iron with sulfur are two main reasons for desulfurization efficiency. Deep desulfurization removal from wastewater is reported by Pourreza et al. They showed by increasing of dopting molybdenum oxide from 10 to 50%, sulfur removal was increased due to chemical and physical adsorption [19-21]. In other study, Liua et al., indicated by increasing loading activated carbon in compacted kaolin (6%), absorption capacity was enhanced [22]. Compared to other studies, BaFe2O4- activated clay nanocomposite (6%) has strong potential for adsorptive desulfurization.

As shown in Fig. 6 increasing of BaFe2O4- activated clay nanocomposite concentration from 100 ge.L-1 to 300 gr.L-1 improved the adsorption rate of hydrogen sulfur from wastewater. The adsorption rate of sulfur compound in 300 gr.L-1 nanocomposite was estimated about 73.36% for 30 min. The average removal efficiency in 3 different concentration with F=76.45 and sig<0.001 showed a statistical significant difference. The results of the comparison of two concentrations showed a significant difference between the different loading of 2 and 4%, 2 and 6% and 4 and 6% (P<0.001). The results of one-way ANOVA with independent variables of concentration and the dependent variable of removal efficiency showed (F=215.96, sig<0.001), (F=11.89, sig=0.008) (F=30.08, sig<0.001) for different loading 2%, 4% and 6% respectively.

The highest removal efficiency can be obtained in the loading of 6% and the concentration of 300 gr.L-1 (92.79%) (Table 2). Two-way ANOVA with different concentration, loading and sulfur removal efficiency had no statistical significance with F=2.264 and sig=0.102. In one study, Guanghua Xia et al., showed hydrogen sulfide can be removed from viscose fiber wastewater by using biological trickling filter (BTF) that was in associated with our results. In industrial processes, production shut down, power failure or repair of electrical equipment are important problems. These can reduce the enzyme activities for desulfurization process [23] while in our study, these problems can not stop nanocomposite activity.

Also, according to Fig.7, the average removal hydrogen sulfide for the concentration of 100, 200 and 300 gr.L-1 and the different loading of 2, 4 and 6% were investigated. Loading of 2% had the lowest removal efficiency while the loading of 6% had the highest efficiency.

To date several studies have been reported for hydrogen sulfide removal by nanocomposites such as GO-ZnO and zinc oxide- MWCNT nanocomposites [24-25]. Among the different studies, our study has some advantages such as having a simple method for synthesis nanocomposites (auto-combustion sol gel), and simple method for desulfurization.

CONCLUSION

In summary, barium monoferrite nanostructures have been synthesized successfully by auto-combustion sol-gel method. It is understood that by choosing the glucose as capping agent and 700°C and 2 h for calcination temperature and time, barium monoferrite can be obtained. Then BaFe2O4- activated clay nanocomposites were prepared by mechanosynthesis technique

Adsorptive desulfurization results showed barium monoferrite- activated clay nanocomposite is one of the best candidates for sulfur removal. Increasing loading barium ferrite from 2 to 6% can promote adsorption rate of hydrogen sulfide from wastewater in concentration 300 gr.L-1 which was confirmed by statistical results.

ACKNOWLEDGMENTS

This study has been prepared based on the research plan number 97145 approved by the Research Affairs of Kashan University of Medical Sciences whose funds has been used. Here by, the authors would like to extend their deepest gratitude to the chancellor of the Research Department, and honorable Head of the faculty of Health and honorable vice chancellor of the faculty of Health.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.