Iron Oxide Catalyst for

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The materials were tested as filters for sulfur dioxide, on sulfur-rich coals fume. ... Except sulfur dioxide (SO2), also sulfur-trioxide (SO3), sulfuric acid (H2SO4) ...

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ScienceDirect Materials Today: Proceedings 7 (2019) 920–929

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NanoFIS 2017

Novel Nanoporous Carbon/Iron Oxide Catalyst for SO2 Degradation Maja Stanisavljevića, Savka Jankovića, Dragana Milisavića, Marko Čađob, Zoran Kukrićb, Dragana Stevićc, Radovan Kukobatc, Predrag Ilićd, Denis Međedd, Magdalena Parlinska Wojtane and Suzana Gotovac Atlagića* a

Faculty of Natural Sciences and Mathematics, University of Banja Luka, Mladena Stojanovića 2, Banja Luka 78000, Bosnia and Herzegovina b Faculty of Technology, University of Banja Luka, Bulevar vojvode Stepe Stepanovića 73, Banja Luka 78000, Bosnia and Herzegovina c Center for Energy and Environmental Science,Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan d Institute for Protection and Ecology of the Republic of Srpska, Vidovdanska 43, Banja Luka 78000, Bosnia and Herzegovina e Institute of Nuclear Physics, Polish Academy of Science, Krakow PL-31-342, Poland

Abstract A green chemistry approach for fabrication of active carbon, enriched with remediated iron is shown. Detailed characterization (HRSTEM, STM, and N2 adsorption) was performed, followed by active carbon soaking in iron and manganese ionic solutions obtained from remediated mining sludge. Novel forms of active carbons with round or needle-like oxide particles on the surface were obtained by combining these materials. The materials were tested as filters for sulfur dioxide, on sulfur-rich coals fume. After adsorption, the iron structures seem to play a stronger catalytic role in the filter regeneration, transforming the adsorbed sulfur dioxide into sulfuric acid. © 2018 Elsevier Ltd. All rights reserved. Selection and/or peer-review under responsibility of NanoFIS 2017 - Functional Integrated nano Systems. Keywords: active carbon; mining sludge; metal remediation; organometallic materials; SO2 filtration

1. Introduction In today’s society high demand and consumption of energy has caused serious pollution of air, soil and water. The sources of pollution are fossil fuels, industries, traffic, agriculture and many other human activities, but regardless of its origin the consequences of human reluctant behavior are getting more and more apparent as well as

* Corresponding author. Tel.: +387-51-319-142; fax: +387-51-319-142. E-mail address:[email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or peer-review under responsibility of NanoFIS 2017 - Functional Integrated nano Systems.

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serious, leaving the pollution as one of the biggest issues among environmentalists, ecologists and scientists. In pollution prevention nanoporous active carbon (AC), produced from cellulose waste, is historically one of the best adsorption materials in water and air treatment and various other fields [1]. It is a well-known, but still widely explored nanoporous adsorbent material which can vary in pore sizes and is a subject of different improvement methods in order to gain enhanced adsorbing characteristics. Nowadays, the unique properties of AC are explored for even further improvements [2,3]. As shown in the present study, fabrication of new hybrid organometallic materials often opens new possibilities for existing functional nanomaterials, such as active carbon. The present study deals with improvement of the AC functionality in sulfur dioxide (SO2) filtration. Besides SO2, the most common air pollutants are carbon monoxide (CO), particles, toxic metals-containing particles, nitrogen oxides (NOX), organic compounds such as hydrocarbons, chlorofluorocarbons and many more. Except sulfur dioxide (SO2), also sulfur-trioxide (SO3), sulfuric acid (H2SO4) and sulfuric acid salts are pollutants usually emitted by anthropogenic activities such as burning fossil fuels and many industrial processes (oil refining, thermal plants, iron-smelting plants). Sulfur dioxide concentration in the atmosphere varies for different places. Urban and industrial regions contain bigger concentration of this pollution material with rated concentration of 0.010.02 ppm [4]. However, momentary concentration can be much bigger, for example one-hour concentration can be 4-7 times higher than rated annual concentration. High concentrations of these pollutants found in urban areas cause serious health problems among the inhabitants, increasing necessity for pollutants removal from the air. Emitted pollutants can be present in atmosphere for several days reaching the distances of up to few thousand kilometers until they get transformed into acid rains and reach the ground and surface waters. The detrimental effects of the acid rains are seen in different forms such as acidification of the ground and water causing serious damage to the ecosystems in that area as well as to human health. Moreover, they cause enormous damage in the corrosion of infrastructure and other buildings. Worldwide damage from corrosion is estimated to 1.4 trillion of US dollars in the last decade according to the World Corrosion Organization [5]. The present study was performed in Bosnia and Herzegovina (B&H), a country with an evident problem of sulfur dioxide in the air and air pollution overall. B&H, Bulgaria, Albania and Ukraine have the highest European mortality rates attributed to the air pollution. B&H is listed as the worst European performer and had European highest average of 55.1 μg particular matter per m3 and in 2012 B&H registered nearly 231 deaths per 100,000 people and had European highest death rate related to the air pollution [6]. The average annual value with the aim of protection of human health for sulfur dioxide prescribed by Directive 2008/50/EC, for sampling period of 1 hour is 350 μg/m3 and period of 24 hours is 125 μg/m3 [7]. The 24-hour average for sulfur dioxide recommended from World Health Organization (WHO) is 125 μg/m3 [8]. Following recommendation of EU Directive and WHO, B&H law recognizes 125 μg/m3, the 24-houraverage, as limit value [9]. Despite the legal obligation, monitoring of the air pollution is at very low level in B&H although there are significant researches done regarding the air pollution [4,10–14]. In B&H most of the pollution with SO2 is coming from the coal thermal power plants, the city thermal plants for municipal water heating systems and due to the fact that the used coal naturally has high content of the sulfur. The problem will further increase with newly opened thermal plant Stanari and Gacko II plant for which construction agreement was recently signed. However, B&H does not have only problems with the large industrial scale pollution but likewise within small scale pollution, such as households, which are using coal in their individual heating systems. In order to prevent SOx pollution in general industry has developed methods of sulfur removal and reduction from mineral sources, before, during and after combustion. Flue gas desulphurization (FGD), either regenerable or nonregenerable, is considered as the most effective for SO2 removal on large scale [15,16]. However, all the aforementioned methods have advantages as well as disadvantages related to their costs, regarding their overall efficiency and waste produced during the desulfurization of mineral sources. Thus there is plenty of room for improvements and developments. Some of the methods, when talking about small scale desulfurization, rely on adsorbent materials such as zeolites and active carbons. The present paper will show results in development of new nanoporous carbon impregnated with iron oxide forming needle-like nanoparticles at the outer surface, as enhanced material for SO2 removal on the small scale (household use). There have been few attempts in the literature to combine the metallic species with active carbon and carbon fibers for different purposes. Sumathi et al. [17] examined the impregnated carbon-based sorbents for simultaneous removal of SO2 and NOx from simulated flue gas. The carbon-based sorbents were prepared using

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palm shell active carbon (PSAC) impregnated with several metal oxides (Ni, V, Fe and Ce). The removal of SO2 and NOx from simulated flue gas was investigated in a fixed-bed reactor. The results showed that PSAC impregnated with CeO2 (PSAC-Ce) reported the highest sorption capacity among other impregnated metal oxides for the simultaneous removal of SO2 and NOx [17]. Also, Marbán et al. [18] impregnated different types of the carbon fibers and pitch with manganese acetate, converting the adsorbed Mn into the manganese dioxide and conducted study of their effectiveness in NOx gases selective catalytic reduction. They also used the Fe, Cr, Ni and V for comparison gaining results that the iron species-containing fiber supports were the most active [18]. Davini et al. [19] studied active carbons derived from a petroleum pitch impregnated with certain iron derivatives by pyrolysis and subsequent activation with CO2 show good SO2 and NOx sorbent characteristics. These types of carbons have shown better sorbent characteristics, better than those of similarly active carbons impregnated with iron after the activation process. However, despite promising results and iron’s non-toxicity as additive, in international scientific literature there are no recent reports regarding AC/Fe materials for removal of SO2. Moreover, another specificity of the presented work is the raw materials origin-only cellulose waste and mining waste metals were used, which is in accordance of the green chemistry principles. Active carbon was prepared from olive pits, while iron is remediated from waste ponds of active iron mine Omarska, Bosnia and Herzegovina. Recent research work of the authors has shown that it is possible to effectively remediate iron ions from mining waste and even to produce nanoparticles of high quality [20,21]. Overall, the present study is an example showing how it is possible to make advanced nanomaterials in accordance to the sustainable development, successfully converting waste into valuable materials. 2. Materials and methods 2.1. Preparation of the ionic solution from the sludge 60 g of dense mining sludge was placed into digestion vessels together with 10 ml of concentrated nitric acid and 5 ml of concentrated sulfuric acid. Digestion process was performed under gentile boiling of the mixture for 5 hours at approximately 140ºC. In the third hour of the digestion additional 10 ml of concentrated nitric acid was added. During digestion the mass changed color from brick-red, through brown to turquoise color with the brown traces at the end. After digestion the obtained product was dried at 180ºC for 2 h, and then 20 g of the obtained product was transferred to the Erlenmeyer flask and 2000 ml of distilled water was added. The pH of this solution was 2-3, indicating presence of acids traces. The solution was heated to 60ºC, changing its color from brown to intense brickred indicating oxidation of the Fe2+ to Fe3+ ions. The solution was filtered first through standard filter paper to remove micro sized impurities and through the glass fiber microfilter with mesh < 45 µm. After filtration a goldcolored solution was obtained. For separation of the iron from manganese and other ions in solution, standard analytical chemistry cation separation procedure was used. The solution was boiled and cooled before adding NH4Cl and NH4OH, in excess. Iron ions have precipitated due to iron hydroxide formation, while manganese and traces of others metals remained in the solution. Iron hydroxide was separated by standard filter paper followed by glass fiber microfilter with mesh < 45 µm obtaining clear solution of manganese and other metals. Precipitated iron hydroxide was separately dissolved in 1000 ml of distilled water with addition of concentrated sulfuric acid to neutralize ammonium and promote dissolution. The measurements of the metal concentrations were performed at atomic absorption spectrometer (AAS) Analyst 400, Perkin Elmer, according to the standard method 3311 C [22]. The preparation for the measurements was according to the standard procedure already described in the literature [22]. The concentration of the obtained iron solution was 2.9 g/l, while the concentration of the manganese solution was 205 mg/l. Therefore the iron solution was diluted 10times in order to balance the concentrations. It is important to note that the traces of manganese and other metals could be found in the separated iron-dominant phase, and viceversa, the traces of iron probably could be found in the predominantly manganese-dominant phase of the ionic solution. However, other metals were around detection limits for AAS so they are not discussed here.

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2.2. Impregnation of active carbon Active carbon from collaborating laboratory [23] was washed with distilled water and dried at 105°C for 24 h in a vacuum oven. For impregnation 10 g of active carbon was placed in three Erlenmeyer flasks respectively. The first sample was a blank, filled only with 100 ml of distilled water. The second sample was soaked with 100 ml of pure remediated iron solution (this sample was abbreviated as AC/Fe). Third sample was soaked with 100 ml of manganese+trace metals solution (this sample was abbreviated as AC/Mn). All flasks were placed in a shaker at the 100 rpm for 24 hours at 25 ºC. After shaking, all flasks content has been filtered through standard filter paper and heat treated at 180ºC for 24 h. 2.3. Characterization of active carbon The morphological observation has been done using a FEI Osiris instrument (STEM) equipped with a FEG cathode and operating at 200 kV. The TEM samples were prepared by placing a drop of the of the impregnated carbon containing solution on a carbon foil coated copper TEM grid and subsequently drying them at room temperature for several minutes. Nitrogen adsorption isotherms and the pore size distributions were measured by Autosorb iQ Station at temperature of 77 K. 2.4. SO2 adsorption experimental setup Adsorbing characteristics of the pristine and impregnated active carbon was tested on SO2 filtration after combustion of the sulfur rich coal extensively used in thermal power plant Ugljevik, Bosnia and Herzegovina. Previously prepared active carbons were placed in hand-made filter carriers (Fig. 1.), specifically made for the purposes of the experiments, forming three different filters Fig. 1. Filters setup

with pristine active carbon, manganese impregnated active carbon (AC/Mn) and iron impregnated carbon (AC/Fe). The coal burning unit scheme with positioning of the SO2 analyser is shown in 2.The fire was first set with wooden clips and after obtaining strong glow 1 kg of the sulfur rich coal was put into the unit. In B&H, there is no recommendation or regulations how long should measurement take for household stoves; however in the nearest countries existing recommendation for industrial measurements is 30 min. Based on the experience in the field work and concentration measurements by accredited institute (co-author), measurement was adapted to 20 min for household chimney which was used in present study. To prevent nanoporous carbon combustion, temperature had been controlled around the point where filters were placed by infrared thermometer type PCE-890U, PCE Instruments. Filters have been closed and placed at the upper part of the chimney. Time dependent concentration of SO2 was measured by Gas Analyser PSSS Horiba, Japan for 20 min to be sure that peak SOx concentration has been

Fig. 2. Scheme of the coal burning unit with positions of filters, SO2 analyser and laser thermometer

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released. After measurements, filters have been carefully removed from chimney and active carbons were soaked into 100ml of distilled water in three Erlenmeyer flasks. 2.5. SO2 recovery and filter recycling The samples have been placed to the shaker on 150 rpm, 24h for more efficient desorption of SO2. Also pH values were measured immediately after water adding, after 1h and after 24h of shaking. After 24 h desorption process samples were filtrated through black ribbon filter paper into the 100ml flasks and prepared for classical gravimetric determination of sulfates [22]. Namely, in this procedure of the SO2 recovery, the SO2 is expected to be transformed into H2SO4 as shown in equation 1 and 2. The samples were manipulated in the open system along with shaking, therefore O2 from the air is easily available to oxidize SO2 which is fairly water soluble, to SO3, which has strong affinity to water and provides complete transformation of the SO2 to H2SO4. SO + 1⁄2 O SO 1 SO H O H SO 2 The precipitates were dried at the 80ºC for 2h and the content of the adsorbed SO2 was calculated as defined in the classical gravimetric determination of sulfates referenced above. 3. Results and discussion The samples characterization started from the microscopy analytics. Fig. 3. is showing STEM HAADF images of the three samples. The magnifications are not parallel; they were appropriately chosen to point out the characteristics of each sample respectively. First, at the images of the pristine carbon, it can be seen that the sample is rather homogenous, irregularly shaped as the granule (observed at the micro-level), with developed nanoporosity (observed at the nano-level). Next, the specific of the AC/Mn, a Mn impregnated sample is the presence of the round or triangular new structures ranging in size approximately between 100 nm to more than 1 m. These are assumed to be the new, manganese-dominated structures. Finally, for the AC/Fe sample, rich needle-like new structures were observed emerging from the armature of the activated carbon granules. Also, at high magnifications very finely Fig. 3. STEM HAADF images with adequate magnifications for pristine carbon, AC/Mn and dispersed, apparently metallic AC/Fe samples. nanoparticles were observed ranging in size from 5-20 nm. Qualitatively these are expected to be the iron oxide or similar species. This is in good agreement with findings of Marbán et al. [18] who found that Fe had quite higher dispersion degree on active carbon base comparing to Ni, Cr, V and Mn,

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which is quite comparable to present investigation due to similarity in structures, surface chemistry and morphology of the active carbon fibers and active carbon itself.

Fig 4. Nitrogen adsorption isotherms and the pore size distributions for pristine, Fe-ion and Mn-ion impregnated active carbon

Summary of the N2 adsorption measurements and recalculation of the specific surface according to the BET and s plots [24, 25] are given in Table 1, while the isotherm and the pore size distribution are shown at Fig 4. (left and right respectively). From the adsorption isotherms it is immediately notable that the pore volume had decreased with manganese and iron treatment, with more pronounced decrease in the case of iron. Overall BET specific surface decreased 9.4% in the case of AC/Mn, and a whole 76 % for AC/Fe. Results are slightly different but partially in good agreement when analyzed by s method, with 22% decrease for AC/Mn and 77% for AC/Fe. Table 1. Surface properties of active carbons obtained from N2 adsorption isotherms at 77 K.

SBET

SαS

SαS external

[m /g]

[m /g]

[m /g]

Pristine carbon

1170

1200

92

AC/Mn

1060

926

85

AC/Fe

290

280

140

Sample

2

2

2

The results are indicating that most of the inner surface (micro and nanopores) is covered with Fe3+ ions during Fe impregnation treatment. This is even clearer from the s method-determined external surface, showing that most of the total surface in AC/Fe sample is the external surface (around 50%) comparing to 8% for pristine carbon and7% for AC/Mn sample. The smaller pores (so called inner surface) were for sure more occupied after the Feimpregnation and this is also comparable from the pore volume change represented in Fig 4. (right). Since the carbon used contains 1.8 mol/g oxygenated sites at the surface as previously confirmed [19], it is likely that the metal ions were adsorbed at these sites and during the heat treatment some forms of the oxides or even alloys at these sites were obtained. The toxic gas concentrations variations are visible from the Figure 5. It was not possible to stoichiometrically determine the exact concentrations which have passed through the filters since there was about 1/5 of the free surface between the three filters on the top of the chimney so some of the toxic gases have certainly escaped. From Fig. 5. it is possible to conclude that used coal was indeed sulfur-rich and that it also developed plenty of the CO at the same time. The development of the CO is especially pronounced in first 8 min. Interestingly, NOx gases development was much less pronounced being most of the time around 20 times less than that of SO2 and CO, and even lower. These data pronounces the need for resolution of the SO2 problem in the regions where this type of the coal is used (the municipality of Bijeljina and Ugljevik).

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Fig. 5.Toxic gases concentration change with time throughout the chimney

Although, the thermal plant Ugljevik is in the process of building the modern desulfurization unit at the moment of this paper publishing (sponsored by the JICA grant in aid), there are still thousands of households in the region buying and using this sulfur-rich coal. The behavior of the active carbon filters enriched with Fe and Mn ions after the chimney fume filtration and possibility for the effective recovery of the filter was first followed by the pH change. The results of the pH change are given in the Table 2. It is notable that the pH of the AC/Fe sample was strongly acidic immediately after soaking and remained to certain extent the same during the

first 24 hours. The increase in acidity is probably due to the expected mechanism of the transformation of the adsorbed (and here desorbed) SO2 into the sulfuric acid as explained in Equation 1 and 2. Although the concentrations of CO were also very high, even if it was co-adsorbed with SO2 on the AC samples, the CO has very low solubility in water and has only very slight potential for formation the weak complexes with water, which would not change the pH of the medium [26, 27]. The AC/Mn sample have shown more pronounced acidity (more desorption of the SO2) at the beginning, however, it values were balanced after 24 hours with the pristine AC filter. Table 2. pH changes in sample washout water after sulfate recovery Sample pH after soaking pH after 1 hour pH after 24 hours AC; SO42 AC/Mn; SO4 AC/Fe; SO42

4 2¬

2,95

3,03

3

2,57

2,64

1

0,65

0,75

All three samples were held symmetrically on the stand at the top of the chimney and it is assumed that they were exposed to the same flow of fume and therefore approximately the same concentration of the adsorbed SO2 is expected. This is supported by recommendations from the EN standard for this type of measurement [28]. Namely, this European norm recommends that for the homogeneous mixing of the gasses, the measurement devices are set at least on 5 diameters of flat stream (in present case 5x125mm=625 mm of the height, upstream). However, after the 24 h recovery, the results clearly show that largest quantity of the SO2 in the indirect for of the SO42- ions was recovered from the AC/Fe filter according to the gravimetric determination as described above. The percentage of the sulfate recovery (results shown in Table 3.) was 21.18% for AC/Fe, while for pristine AC and AC/Mn were 13,47% and 13,8% respectively. From the data it can be concluded that recovery from the AC/Fe filter has increased for around 60%. The approximate value of the sulfate recovery from the AC and AC/Mn is in accordance with observed pH changes shown in Table 2. However, it is notable that the pH changes and probably the sulfate recovery responsible for it, was faster in the case of the AC/Mn indicating that presence of the manganese on the surface was slightly catalyzing the recovery process. However, the catalytic effect was much stronger for the AC/Fe which probably had turned most of the SO2 into the sulfate ions during the first minutes or even more so during the first hour. Obviously, the catalytic role of the iron-species formed at the surface is very strong. This might be due to the specific “needle-like” structures which were formed as shown by electronic microscopy observation.

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Table 3. Percentage of the sulfate recovered from samples Sample Percent of the sulfate recovery (%) AC; SO42 AC/Mn; SO4

13,47 2¬

AC/Fe; SO42

13,80 21,18

They have probably in some way helped the stronger adsorption which could have tendencies of the chemisorption during the chimney fume filtration, but on the other hand “needle-like structures” also played a role in catalyzing the desorption process once the samples were soaked in water for the filter recycle (SO2 recovery). This conclusion is derived due to the significantly decreased specific surface and nanopores volumes of the AC/Fe sample as discussed related to the Table 1 and Fig. 4. These have shown that the simple nanopores confinement effect in adsorption of the SO2 is not happening, but more complex mechanism is present, which led to the conclusion that adsorption is tending towards chemisorption in nature.

4. Conclusion The present paper demonstrated that impregnation of the active carbon with Fe ions leads to the decrease of the nanopores volumes due to the formation of the “needle-like” iron based particles which are probably creating highly active sites at the outer surface of active carbon. This way enhanced materials exhibit good filtering properties for SO2 removal from the coal fume, as well as good properties for desorption of SO2 (recycling of the filter for multiple uses). Development of the small filters for household application based on these materials, could reduce extreme values of the SO2, measured in B&H cities [4]. Additionally using the mining waste as Fe ion source shows that these results could lead to the partial resolution of the pollution cause by mining, to the improving of the metal extraction processes as well as giving to the nanotechnology new and greener sources of the raw materials. The exact structure and the nature of the Fe species will be studied in the future work together with study of the influence of the concentrations of Fe ions during impregnations on effectiveness of the filter and general economic effects of the mining waste recycling.

Acknowledgements The study is realized in the frame of the postdoctoral grant for M. S. from PhosAgro/UNESCO/IUPAC Partnership in Green Chemistry for Life contract number 4500355075, and was also supported by the EIT raw materials project number 16320.

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