Accepted Manuscript Synthesis and Characterization of Hybrid Materials Containing Iron Oxide for Removal of Sulfides from Water Irena Jacukowicz-Sobala, Łukasz J. Wilk, Krzysztof Drabent, Elżbieta Kociołek-Balawejder PII: DOI: Reference:

S0021-9797(15)30130-2 http://dx.doi.org/10.1016/j.jcis.2015.08.035 YJCIS 20669

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

14 May 2015 17 August 2015 19 August 2015

Please cite this article as: I. Jacukowicz-Sobala, Ł. Wilk, K. Drabent, E. Kociołek-Balawejder, Synthesis and Characterization of Hybrid Materials Containing Iron Oxide for Removal of Sulfides from Water, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.08.035

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Synthesis and Characterization of Hybrid Materials Containing Iron Oxide for Removal of Sulfides from Water Irena Jacukowicz-Sobalaa, Łukasz J. Wilka, Krzysztof Drabentb, Elżbieta KociołekBalawejdera a

Department of Industrial Chemistry, Wroclaw University of Economics, 118/120

Komandorska St., 53-345 Wroclaw, Poland b

Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland

Corresponding author: Irena Jacukowicz-Sobala, Department of Industrial Chemistry, Wroclaw University of Economics, 118/120 Komandorska St., 53-345 Wroclaw, Poland Tel. 48713680460, e-mail adress: [email protected] E mail addresses: Irena Jacukowicz-Sobala: [email protected] Łukasz Wilk: [email protected] Krzysztof Drabent: [email protected] Elżbieta Kociołek-Balawejder: [email protected] Abstract Hybrid materials containing iron oxides based on macroporous and gel-type sulfonic and carboxylic cation exchangers as supporting materials were obtained. Multiple factors, including the kind of functional groups, ion exchange capacity, and polymer matrix type (chemical constitution and porous structure), affected the amount of iron oxides introduced into their matrix (7.8-35.2% Fe). Products containing the highest iron content were obtained using carboxylic cation exchangers, with their inorganic deposit being mostly a mixture of iron(III) oxides, including maghemite. Obtained hybrid polymers were used for removal of sulfides from anoxic aqueous solutions (50-200 mg S2-/dm3). The research showed that the form (Na+ or H+) of ionic groups of hybrid materials had a crucial impact on the sulfide removal process. Due to high iron oxide content (35% Fe), advantageous chemical constitution and porous structure, the highest removal efficiency (60 mg S2-/g) was exhibited by a hybrid polymer obtained using a macroporous carboxylic cation exchanger as the host material. The process of sulfide removal was very complex and proceeded with heterogeneous oxidation, iron(III) oxide reductive dissolution and formation of sulfide oxidation and precipitation products such as iron(II) sulfides, thiosulfates and polysulfides. Key words: hybrid polymers, maghemite, cation exchangers, sulfide removal, reductive dissolution

Synthesis and Characterization of Hybrid Materials Containing Iron Oxide for Removal of Sulfides from Water Irena Jacukowicz-Sobalaa, Łukasz J. Wilka, Krzysztof Drabentb, Elżbieta KociołekBalawejdera a

Department of Industrial Chemistry, Wroclaw University of Economics, 118/120

Komandorska St., 53-345 Wroclaw, Poland b

Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland

Corresponding author: Irena Jacukowicz-Sobala, Department of Industrial Chemistry, Wroclaw University of Economics, 118/120 Komandorska St., 53-345 Wroclaw, Poland Tel. 48713680460, e-mail adress: [email protected] Abstract Hybrid materials containing iron oxides based on macroporous and gel-type sulfonic and carboxylic cation exchangers as supporting materials were obtained. Multiple factors, including the kind of functional groups, ion exchange capacity, and polymer matrix type (chemical constitution and porous structure), affected the amount of iron oxides introduced into their matrix (7.8-35.2% Fe). Products containing the highest iron content were obtained using carboxylic cation exchangers, with their inorganic deposit being mostly a mixture of iron(III) oxides, including maghemite. Obtained hybrid polymers were used for removal of sulfides from anoxic aqueous solutions (50-200 mg S2-/dm3). The research showed that the form (Na+ or H+) of ionic groups of hybrid materials had a crucial impact on the sulfide removal process. Due to high iron oxide content (35% Fe), advantageous chemical constitution and porous structure, the highest removal efficiency (60 mg S2-/g) was exhibited by a hybrid polymer obtained using a macroporous carboxylic cation exchanger as the host material. The process of sulfide removal was very complex and proceeded with heterogeneous oxidation, iron(III) oxide reductive dissolution and formation of sulfide oxidation and precipitation products such as iron(II) sulfides, thiosulfates and polysulfides. Key words: hybrid polymers, maghemite, cation exchangers, sulfide removal, reductive dissolution Introduction Sulfides (H2S(aq), HS- and S2-) are considered to be among the most undesirable and troublesome contaminants of various aqueous environments. When present in water even at negligible levels, sulfides are distinctly perceptible and give water an obnoxious taste and odor, making it usless for municipal use in organoleptic respects. Water containing sulfides

should also not be used for industrial purposes. Sulfides in the form of H2S(aq) have a toxic influence on microbes performing fermentation processes. They are also reactive toward metals and metal oxides, which are active ingredients of numerous catalysts, and as a result shorten the time of their utilization or completely deactivate them. Moreover, if present in industrial water, sulfides have corrosive effects on pipelines, steam boilers, and heat exchangers. Since sulfides can be emitted from the solution in the form of toxic hydrogen sulfide, their presence in aqueous solutions constitutes a serious risk to human life and health [1]. Therefore, the concentration of sulfides in wastes drained away to natural water reservoirs has to be kept at a very low level (e.g. below 0.2 mg/dm3). Sulfides are, inter alia, constituents of wastewaters from coke and steel production processes, refining of petroleum products, pulp and paper production, and tanning of raw hides. Sulfides are also formed in the natural environment as byproducts of degradation of organic matter and metabolic activity performed by sulfate reducing bacteria. These processes are ubiquitous and take place in anaerobic conditions in environments such as municipal, industrial (e.g. from meat processing) and agricultural wastes (e.g. in liquid manures). Sulfides are also constituents of one of the renewable fuels, biogas (up to 0.4% v/v H2S), which therefore has to be desulfurized before use [1,2]. There are some well-known methods used for removal of sulfides from industrial wastewaters. The simplest of them is to lower the pH and drive away sulfides from the solution in the form of hydrogen sulfide with air (at pH ~5.0 sulfides are present in aqueous solutions only in the molecular form of H2S(aq)) followed by its absorption in alkaline media or adsorption on bog ore. Sulfides can also be oxidized in the solution by means of lowmolecular weight oxidizers, such as chlorine and its compounds, oxygen, ozone, hydrogen peroxide or potassium manganate(VII). It is also possible to precipitate sulfides using iron(II) or iron(III) salts. Nevertheless, basic approaches for sulfide removal are usually not suitable to completely remove sulfides from aqueous solutions. Therefore after the treatment there are some microquantities of sulfides left that can cause technical problems and are a source of odor [1,3,4]. Adsorption processes are a well-established technology which is highly valuable for removal of contaminants present in wastewater streams in low concentrations. However, the most widely used adsorbent – activated carbon – exhibits low selectivity towards sulfides and requires modification before use [5,6]. Recent research in this area has also focused on the use of metal oxides as adsorbents for removal of residual sulfides. Metal oxides such as iron(III), manganese(IV), copper(II), and zinc(II) oxides are considered to show adequate reactivity toward sulfides (being able to oxidize/adsorb them) in aqueous environments. Currently Fe(III) oxides have significant practical relevance, mostly because they are nontoxic, cheap, and easy to obtain, they are also often constituents of hard to reuse industrial

wastes (adsorbents with iron(III) oxides are widely used for desulfurization of biogas). However, the process of chemisorption of sulfides by means of reactive metal oxides has some serious drawbacks, in particular when powdered metal oxides are used. These oxides tend to agglomerate in aqueous solutions, and in consequence their active surface area is considerably decreased. Moreover, separation of metal oxide suspended matter after the process is also considered a serious technical problem [4]. In order to prevent technical problems associated with agglomeration and separation of iron oxides during and after the water purification process, the particles of iron oxides are deposited in the structure of porous, water-insoluble, chemically resistant carriers (such as silica, montmorillonite) and organic polymers (e.g. cellulose or ion exchangers) [7-10]. Ion exchangers are particularly good raw materials for producing these types of hybrid adsorbents, since they are characterized by large surface areas, a well-developed pore structure and the presence of ionic functional groups (through which an inorganic deposit can be introduced into the structure of an organic polymer). Among them, various reactive functional polymers including anion exchangers [11,12] cation exchangers [13-16] and macromolecular oxidizers [17] were used as polymeric supporting materials for iron oxide deposition. Iron oxide precipitation within the structure of a polymeric matrix containing different functional groups and porous characteristics makes the process more complex than precipitation conducted in the solution. Therefore using different methods of synthesis and supporting polymers, products containing different polymorphic forms of iron oxides (magnetite, maghemite, ferrihydrite, amorphous hydrous iron oxide) with different iron content were obtained. Through conversion of the macroporous sulfonic cation exchanger into the Fe2+ form and then through alkalization with NaOH solution and oxidation with H2O2, Ziolo et al. obtained a hybrid polymer with adequate optical properties containing iron oxides in the form of maghemite [13]. In the present study the same method of iron oxide deposition within the matrix of different carboxylic and sulfonic cation exchangers was used. Since the properties of the products may depend not only on the reactions used to produce them, but also on the properties of polymer matrix, the partial aim of the present work was to determine the influence of ion exchange capacity, chemical constitution and the morphologic structure of cation exchangers on the form and quantity on iron oxide deposit. Simultaneously, the structure of iron oxide and distribution of its deposition within polymer matrix, and properties of the polymeric support, may have a significant bearing on sorption capacity of the obtained products. Since, to the best of the authors’ knowledge, these types of materials were not used for removal of sulfides from aqueous solutions, the aim of this study was to evaluate adsorption/oxidation properties of the hybrid polymers containing iron oxide obtained using different cation exchangers as the host polymers.

Experimental section Materials The polymeric support for dispersed iron oxide were cation exchange resins being commercial products sourced by Dow Chemicals. The characteristics of these materials are described in Table 1. SM, SG, CM and CG stand for respectively a macroporous cation exchanger with sulfonic acid functional groups, a gel-type cation exchanger with sulfonic acid functional groups, a macroporous cation exchanger with carboxylic acid functional groups and a gel-type cation exchanger with carboxylic acid functional groups. Synthesis The ion exchangers used in all the syntheses were in the Na+ form. Encapsulation of iron oxide within the polymer matrix was effected by batch method. Samples of cation exchangers (10.0 g) were converted into the Fe2+ form, using 0.25 M FeSO4 in 0.001 M H2SO4 solution. After rinsing with water the samples of ion exchangers were placed in 200 cm3 of 10 M NaOH solution subjected to stirring for 30 min. Then the polymer beads were thoroughly washed with water and treated with 200 cm3 of 1.5% (w/w) H2O2 solution under stirring for 30 min. After rinsing with water and methanol the products were dried at 60°C for 24 h. The whole cycle was repeated twice. The capitals in the sample names stand for the polymeric support and the number stands for the number of cycles. In order to determine the iron(II) and (III) content in the polymer matrix the hybrid polymer samples were treated with 2 M H2SO4 or 18% (w/w) HCl solution. The obtained solutions were analyzed for Fe(II) and Fe(III) ion content. Characterization Microscopic examinations were performed by means of a HITACHI S-3400N scanning microscope equipped with an EDS microanalyzer (4 nm, BSE detector). X-ray powder diffraction (XRD) patterns were recorded by an ULTIMA IV/Rigaku/2008 instrument operated at 40 kV and 40 mA with Cu Kα radiation at a wavelength of 0.15406 nm. The Mössbauer spectra of the powdered samples were recorded on a Mössbauer 2330 spectrophotometer (Polon, Poland) at room temperature. The isomer shift values were referenced to α-Fe standard. 57Co in the Rh matrix of 25 mCi activity was used as the source.23 The porous characteristics of the polymeric supports and hybrid polymers were determined from the adsorption isotherms for liquid nitrogen at 77K using Accelerated Surface Area and Porosimetry Analyzer ASAP 2020, 2007, Micrometrics. Adsorption experiments Adsorption experiments were carried out in the batch regime, in nitrogen atmosphere, by mixing 0.3 or 0.5 g of the hybrid polymer with 100 cm3 Na2S·9H2O (Sigma-Aldrich) in deoxygenated distilled water with ~50-200 mg S2-/dm3 concentration. The concentration of dissolved oxygen was measured using WTW Multi 3410 multimeter with FDO 925 optical

probe. Nitrogen was inserted to glass stoppered conical flasks with hybrid polymers and sulfide solutions in periodic regime by flushing. In order to quantify possible loss of sulfides caused by insertion of nitrogen and sampling their concentration, apart from samples with sorbent, was also measured in blank samples (constantly sealed, without sorbent). All experiments were carried out in three repetitions. Regeneration experiments Regeneration experiments were carried out by bubbling the suspension of exhausted sorbent and distilled water with air through ceramic frit for 24 h (Method I), drying the exhausted sorbent in a laboratory dryer at 55°C for 24 h (Method II) or mixing it with wellaerated 0.01 M HCl solution in a non-stoppered conical flask on a laboratory shaker for 24 h (Method III). Sorbents were regenerated three times using each mentioned method, followed by sorption experiments after every regeneration cycle. The sorption for regeneration studies was carried out by mixing 0.3 g of regenerated hybrid polymer with 100 cm3 of Na2S·9H2O in deoxygenated distilled water with ~100 mg S2-/dm3 concentration in a nitrogen atmosphere. Analysis The concentrations of Fe(II) and Fe(III) were measured by spectrophotometric methods using a Spekol 1200 spectrophotometer (Analytik Jena, Germany). Fe(II) and Fe(III) were determined with respectively 1,10-phenantroline and potassium thiocyanate. Absorbance measurements were taken at 510 nm, and 480 nm. The concentrations of sulfides were determined using a Uv-Vis Specord 210 PC spectrophotometer by methylene blue method with N,N-dimethyl-p-phenylene sulfate, 660 nm. Concentrations of sulfides, sulfites and thiosulfates were measured by an ion chromatography method using a MerckHitachi liquid chromatograph equipped with a conductometric detector (non-suppressed conductivity), Knauer SmartLine 1000 gradient pump and Shodex IC I-524A column. The analyses were performed at a temperature of 50°C using 0,25 mM p-hydroxybenzoic acid and 1.2 mM N,N-diethylethanolamine aqueous solution with 10% methanol as the mobile phase. The flow rate of the eluent was 1.5 cm3/min. Total sulfur content adsorbed on the hybrid polymer surface was determined by Eschka mixture (Fluka) method according to ISO 334:1992 (E) international standard. Results and discussion The synthesis of the hybrid polymers containing iron oxides, described in [12] proceeded in three stages as shown in scheme 1. [P]−(G-Na+)2

FeSO4 Ion exchange

H2O2 Oxidation

[P]−(G-)2Fe2+

NaOH Alkalization

[P]−(G-Na+)2#Fe2O3

[P]−(G-Na+)2#Fe(OH)2

[P] – the polymer matrix, # − within the polymer matrix, G- − a carboxyl or sulfonate functional group. The introduction of iron oxides into the structure of the polymers was accompanied by changes in the appearance of their beads (Table 1 and 2). After the synthesis and drying, hybrid polymers SM3, SG3 and CG3 were black with a claret tint and the typically opaque beads of the macroporous sulfonic polymer would gain luster. The appearance of CM3 changed differently than that of the other products: after the synthesis its beads were redbrown and remained opaque. The introduction of iron oxides into the cation exchanger’s structure was conducted in three cycles in order to incorporate the highest inorganic load. The results presented in Table 2 show that in each successive cycle new portions of Fe2+ would be introduced through ion exchange reaction into cationic functional groups, which subsequently through alkalization and oxidation were successfully deposited in the inner structure of both S/DVB and PA/DVB copolymers. When the data for the first synthesis cycle are examined, it becomes apparent that in the case of sulfonic cation exchangers (SM, SG) practically the total amount of iron introduced through the ion exchange reaction was deposited within the polymer matrix. Since the iron deposit increased with each cycle, the contact between the reagents and the cation exchanger’s functional groups would become more difficult, whereby the increase in the iron content would become increasingly smaller. A comparison of the Fe content in SM3 and SG3 (20.0% and 20.3% respectively) shows that the matrix structure did not have a noticeable effect on the effectiveness of the synthesis, i.e. the more compact gel structure did not constitute a barrier for iron deposit introduction. Due to higher content of functional groups in the structure of carboxylic cation exchangers in comparison to sulfonic ones, in each successive cycle a considerably higher iron oxide load was deposited, which after the third cycle increased to 35.2 and 30.8% Fe (determined for CM3 and CG3 respectively). However, the amount of the iron bound in the functional groups of the carboxylic cation exchangers already in the first cycle was not equivalent to the iron deposit in the polymer structure. This could have been due to the washing out of some of the deposit during rinsing, to which the loosened hydrophilic structure of PA/DVB matrix was conducive. Carboxylic cation exchangers in the sodium form are characterized by a swelling index of 100% (resulting from the conversion of their functional groups from the H+ form into Na+) while the swelling index of sulfonic cation exchangers with the hybrid S/DVB matrix is low (10%). The results (Table 2) also show that the obtained products contained some iron(II) (0.09-4.15%) whose content increased with the number of synthesis cycles. This could be residues of unreacted Fe(II) or the products of the side reactions occurring as part of the Fenton process [18]: Fe3+ + H2O2

Fe2+ + H+ + HO2•

(1)

Fe3+ + HO2•

Fe2+ + O2 + H+

(2)

The hybrid polymers CM3 and CG3 contained much more iron(II) in comparison with products SM3 and SG3. Since carboxylic cation exchangers exhibit significantly higher swelling capacity, which results in facilitated contact of the reagents during the oxidation step, the Fenton process is a more likely cause of higher Fe(II) content within the polymer matrix. Characterization The SEM images (Fig. 1) of the macroporous sulfonic cation exchanger and the hybrid polymers obtained after one and three synthesis cycles revealed that the polymer beads have undergone visible transformation. This could have been caused by a zonal change in the swelling capacity of the ion exchanger beads after the introduction of each successive iron portion. Consequently, as the reaction was conducted in different reaction environments the structure would shrink and swell, which could lead to the splitting of the beads. After the introduction of iron oxide the appearance of the surface of the beads did not change. There are no iron oxide deposit agglomerations. An EDS analysis of the Fe content at different points showed similar values (3.84-4.43%). This means that the iron oxides were well dispersed in the polymer structure. According to Table 3, after the third synthesis cycle the end product contained large amounts (65%) of iron on the surface of beads, while inside it the Fe content was 10-14%. The images in Figure 2 show the hybrid polymer obtained (after three synthesis cycles) using the gel carboxylic cation exchanger. It is apparent that the appearance of the outer surface of the beads has changed – there are layers of iron oxides (in these places the mass fraction of Fe would reach 77.5%). No significant changes were observed on the inner surface of beads. The iron inside the structure is well dispersed, and its content at different points ranges from 10.2 to 16.2%. Figure 3 shows isotherms of the nitrogen adsorption/desorption on the porous sulfonic cation exchanger and on the hybrid polymers obtained with its participation. The raw material was characterized by poorly developed specific surface SBET, and the hysteresis loop starting at p/po values closer to unity indicates that it was a mesoporous sorbent with inkpot-shaped pores (Table 4). The introduction of iron deposit into the polymer matrix in one synthesis cycle caused an increase in specific surface SBET and the formation of mainly mesopores. Also the shape of the hysteresis changed, indicating a smaller difference in pore diameter between the inside of the pores and their mouths, which may mean that iron oxides would be deposited on the inner walls of the polymer pores. The introduction of the next large portions of iron oxides in the second and third synthesis cycle had an opposite effect on the development of the sorbent’s surface (which underwent a considerable reduction). At the same time the shape of the hysteresis changed, indicating the narrowing of the pore mouth

radius. This may mean that in the successive cycles iron oxides would be deposited mainly on the surface of the beads, which in this case was confirmed by SEM examinations (the high Fe content on the surface of the beads, Table 3). Figure 4 shows adsorption isotherms measured for the porous carboxylic cation exchanger and its iron oxide deposit containing the conversion product obtained after three cycles of the synthesis of CM3. The raw material was characterized by poorly developed and small surface SBET. The introduction of iron oxides into its matrix caused a substantial increase in specific surface area, and the shape of the hysteresis loop indicates that the deposition took place at the mouths of the polymer pores. The plots of nitrogen adsorption/desorption isotherms determined for gel-type raw materials (SG, CG) and products (SG3, CG3) were straight lines without a hysteresis loop, which indicate the adsorption of nitrogen on the outer surface of the beads or in the open accessible and uniform sized pores. Since the measurements are carried out with the samples in the dry state, the porous structure of the gel-type polymers is probably collapsed. Therefore under these conditions it is impossible to determine their porous characteristics. The diffraction patterns (Fig. 5) indicate that the obtained products contained a certain amount of iron oxides characterized by a crystal structure. The diffraction patterns of the hybrid polymers SM3, CM3 and CG3 show reflections characteristic of magnetite (ICSD 20596) or maghemite (ICSD 172906), which appear at the same values of angle 2θ: 30, 35, 43, 54, 57 (Fig. 5). In the case of hybrid polymer CM3 the diffraction lines are characterized by lower intensity and greater broadening, which may indicate lower content of crystalline iron oxide forms. Simultaneously, the broadened diffraction lines are indicative of the deposition of iron oxides in the form of nanoparticles. Figure 6 shows the Mössbauer spectra of the hybrid polymers. In the case of the product obtained using the macroporous sulfonic cation exchanger (a) the spectrum is a superposition of two overlapping sextets, which may correspond to Fe3+ in the tetrahedral and octahedral sites of maghemite. The presence of magnetite could be excluded, since this form of iron oxide gives two Zeeman components with different isomer shifts resulting in a well-separated first line, which is not observed in this spectrum [19]. The dominant sextets are characterized by broad lines, due to which it is difficult to determine their parameters. The spectral broadening may be a consequence of the spread in iron oxide particle size. Reducing the nanoparticle size usually results in a collapse of the magnetic six-line structure to a paramagnetic quadrupole doublet, which is not the consequence of thermodynamic transition but occurs when the superparamagnetic relaxation time of the nanoparticles becomes comparable with the time scale of Mössbauer spectroscopy [21,21]. The spread in particle sizes results in spread of relaxation times and substantially broadened spectra. In the spectra of hybrid polymers CM3 and CG3, besides the same two sextets, additional

components appear. The spectrum of the product based on the gel-type polymer includes a doublet with a quadruple splitting of 0.84 mm/s and isomeric shift IS = 0.32 mm/s due to the presence of atoms of high-spin Fe(III) (Fig. 6c). This may indicate the presence of the paramagnetic form of iron oxide deposited in the polymer structure. In the case of CM3, this component is characterized by a much stronger effect than in the case of CG3. Moreover, in the spectrum of hybrid polymer CM3 there appears one more quadruple doublet with parameters QS = 3.01 mm/s and IS = 1.35 mm/s, originating from the high-spin atoms of Fe(II), which may indicate the presence of Fe(OH)2 [22]. The significant difference in the effects in the spectra of the products based on the carboxylic cation exchangers CM3 and CG3 with a similar Fe(III) content (about 30%) may indicate a looser structure of the crystal lattice of the iron oxides deposited in the matrix of the gel cation exchanger than the one in the matrix of the macroporous cation exchanger. Adsorption studies Hybrid polymers after three cycles of synthesis were preliminarily tested to evaluate their capacity for removal of sulfides from aqueous solutions. Since the form of functional groups has an influence on pH conditions of Na2S aqueous solutions, before preliminary adsorption experiments the sulfonic and carboxylic functional groups of a portion of SM3, SG3, CM3, and CG3 were transformed from the Na+ to the H+ form using 0.002 M HCl. In consequence, eight different products were used as adsorbents for sulfides. As can be seen in Figure 7, hybrid polymers with functional groups in the H+ form have shown a sulfide removal capacity up to 97.7%, which is considerably higher in comparison with their counterparts with functional groups in the Na+ form, while in the case of SG3 the sulfides in treated solution were even more persistent than in the blank sample. There is also an evident dependency between the form of functional groups and final pH of purified solutions. When the hybrid polymers in acidic forms were used, the pH of sulfide solution during the adsorption process dropped from 12.0 to 8.0, while negligible pH changes were observed during the adsorption process using hybrid polymers in the Na+ form. There are several possible reasons why the form of ionic functional groups of the sorbents has a significant impact on the sulfide removal process. Since sulfide solutions were prepared using Na2S·9H2O during the process of sulfide removal, the functional groups of SM3H, SG3H, CM3H, and CG3H underwent an ion exchange reaction from the H+ to the Na+ form, resulting in a pH decrease of the solution. The lower the pH is, the more sulfides are present in the reactive molecular form of H2S(aq), due to the properties of hydrogen sulfide, which is a weak acid with a pKa1 value of ~7.0. It is also likely that the mentioned ion exchange reaction facilitated the transport of sulfide anions originally coupled with sodium cations and allowed them to diffuse against the Donnan membrane equilibrium caused by the presence of covalently bonded sulfonic or carboxylic functional groups within the polymer

matrix, enhancing the reaction with iron oxide deposited within the inner structure of the adsorbent beads [23]. Hybrid polymers obtained using as host polymers carboxylic cation exchangers showed the best chemisorption capacity towards sulfides. This can be explained by the fact that these products – CG3 and CM3 – had the highest overall iron load, respectively 30.0 and 35.0%, after three cycles of synthesis, and their polymeric supports exhibited a significantly higher swelling index in comparison with the sulfonic cation exchangers. Furthermore, due to high affinity of carboxylic functional groups toward H+, these materials may exhibit a smaller Donnan exclusion effect toward HS- contained in solutions with lower pH (under pH=4.0 weak acidic carboxylic groups are present in undissociated -COOH form). Simultaneously, due to well-developed mesoporous structure of the hybrid polymer CM3H, it also showed higher sorption capacity than hybrid polymer CG3H obtained using gel-type carboxylic cation exchanger. Further studies were performed using the adsorbent that showed the best properties during the preliminary experiments: CM3H. In order to examine the effect of concentration of sulfides on the efficiency of their removal, kinetic studies were carried out (Fig. 8a). When a solution of 50 mg S2-/dm3 was treated, sulfides were undetectable in the liquid phase after 6 h. In the case of more concentrated solutions being used (100-200 mg S2-/dm3), equilibria of the reactions were reached after 7 h. These results also showed that sulfide removal efficiency reached high values from 95 to 100% regardless of their initial concentrations in the relatively wide range from 50 to 200 mg/dm3. As a result, the overall removal capacity of CM3H rose to 59 mg S2-/g with the increase of sulfide concentration in the solution (Fig. 8b). The increase of overall sulfide removal capacity can be explained by the mechanism of this process using iron(III) oxides proposed earlier in the literature [24-28], which is based on iron oxide (as a heterogeneous oxidant) reductive dissolution. The first fast stage of the process was formation of iron-sulfide surface complexes: =FeIIIOH + HS- → = FeIIIS- + H2O (3) Then within that complex the iron(III) atom was reduced to iron(II), with one electron being transferred to a sulfur atom which was partially oxidized to a S·- free radical: =FeIIIS- → = FeII S· (4) Afterwards the S·- free radical was released from the surface complex. Since pH of the solution was above 7, the presence of the S·- free radical was more likely rather than the HS· radical [27]:

=FeII S· → = FeIIOH2+ + S·- (5) The release of the free radical was followed by Fe2+ dissolution, which was accelerated by protonation of the iron(III) oxide surface caused by the presence of carboxylic functional groups in the H+ form in the structure of the hybrid polymer: =FeII OH2+ → [new surface active site] + Fe2+ + H2O (6) Dissolution of Fe2+ ions exposed new active sites on iron(III) oxide, which were able to react with further sulfide species. Simultaneously the S·- free radicals could react with active sites on iron(III) oxide and released Fe2+ could precipitate in the form of FeS as a result of reaction with sulfides present in the solution: Fe2+ + HS- → FeS↓ + H+ (7) As can be seen in Figure 9, the process of sulfide removal proceeded with pH changes, which occurred during the whole process before the equilibrium; pH of the solutions decreased considerably within the first 1-2 h of the process. Afterwards pH showed a slight increase, with a peak between 4 and 5 h, and dropped again, reaching a plateau after 7 h of the treatment. The significant drop of pH to the level of 8.4-9.0 in the first stage of the process was mostly caused by the ion exchange reaction between carboxylic functional groups in the H+ form and the solution containing Na+ ions and partially also was a result of reaction (7), since the dominant sulfide form at pH~8-12 was HS-. In the second step (4-5 h of kinetic experiments) elemental sulfur became visible (ring precipitation, white turbidity), which was coupled with a slight increase of pH. This phenomenon was probably caused by gradual formation of polysulfides and homocyclic sulfur molecules according to a (simplified) chain of reactions between constituents of the solution [29]: 2S·- → S22(8) S22- + S·- + H+ ↔ S2·- + HS(9) 2S2·- → S42(10) S2·- + S·- → S32(11) S32- + S·- + H+ → S3·- + HS- (12) S42- + S·- + H+ → S4·- + HS- (13) 2 S3·- ↔ S62(14) S3·- + S4·- ↔ S72(15) 2 S4·- ↔ S82(16) 2 S62- ↔ S52- + S72(17) S52- + S42- ↔ S92(18) S72- + H+ ↔ HS7- ↔ HS- + S60 (19) S82- + H+ ↔ HS8- ↔ HS- + S70 (20) S92- + H+ ↔ HS8- ↔ HS- + S80 (21)

Hydrosulfide ions were simultaneously precipitated by Fe2+ ions according to equation 7, and therefore the pH values of the solutions were lowered and reached plateaus after 7 hours of the process. However, due to the presence of ionic functional groups bound to polymer matrix of the hybrid material significantly affecting the pH of the solution, it is impossible to definitively identify the influence of particular reactions on the conditions in the solution during the sulfide removal process. After 8 hours of the sulfide removal process using the hybrid polymer CM3H, spent adsorbent/heterogeneous oxidant and treated solution were examined in order to determine sulfide products’ chemisorption and oxidation (Table 5). As can be seen from mass balance of the sulfide removal process, only 17-30% of removed sulfur was detected on the hybrid polymer surface. The majority of sulfur compounds were present in the liquid phase as sulfide oxidation and precipitation products. Therefore the process of sulfide removal is most likely heterogeneous oxidation. During the process the beads of hybrid polymer became black, which might lead to an assumption that reaction products chemisorbed in their structure were iron(II) sulfides. Due to leaching out of inorganic deposit from the polymeric matrix during the process (caused inter alia by swelling of polymer beads), black iron sulfide precipitate was also a constituent of the liquid phase. Products of sulfide oxidation present in the solution were thiosulfates (probably also some polysulfides) and elemental sulfur. According to literature data [30,31], thiosulfates can be products of sulfide oxidation in strongly reducing environments with a low sulfide to oxygen ratio. Simultaneously, some studies have also shown that pH is a crucial factor influencing the final products of the redox reaction between hydrosulfides and iron oxides [32]. In mildly acidic solutions the oxidation of sulfides leads to formation of sulfates, while under mildly alkaline conditions the main final products are thiosulfates. Since the process in this study was conducted under anoxic and mildly alkaline conditions, thiosulfates were most likely formed from intermediate polysulfides obtained in the previous steps: 7[=FeIII OH] +S22- +H+ +3H2O → Fe2+ + S2O32- + OH- + 6[=FeII (OH2)+] + [new surface active site] (22) The complex mechanism of sulfide removal from aqueous solutions has been confirmed in many studies exhibiting the presence of FeS, thiosulfate, sulfate and polysulfides among S2- oxidation products [24-28,30,31]. Poulton et al., using a bed of zeolite pellets coated with ferrihydrite (500 g of zeolite containing 2.0 g Fe(III)), obtained 160 dm3 of purified water with high efficiency of sulfide removal from aqueous solutions (87%, at initial concentration of 1.44 mg S2-/dm3), simultaneously with insignificant loss in efficiency even after prolonged process duration [26]. Although hybrid polymer obtained in the present study contained only 30% Fe, its sulfide removal capacity reached a value of 60 mg/g, which is

comparable with the adsorption capacity of pure CuO and ZnO particles (respectively 64.0 and 28.3 mg S2-/g) [33]. Regeneration studies Regenerations studies were conducted using three methods: aeration in distilled water, drying at 55°C, and treatment with acid solution under aerated conditions. The first two methods chosen for regeneration of exhausted sorbents were in general similar to those used for regeneration of exhausted hydrogen sulfide adsorbents containing iron(III) oxides in their structure (e.g. bog ore) [34]. The principle of the mentioned regeneration process is to heat and/or purge spent moist sorbent with air or oxygen, which can be described by the following simplified reaction: 2Fe2S3 + 3O2 → 2Fe2O3 + 6S (23) Since during sorption experiments the functional groups underwent an ion exchange reaction from the H+ to the Na+ form, regeneration of spent sorbent in aerated 0.01 M HCl was chosen in order to reconvert carboxyl functional groups into the H+ form which, as stated before, is crucial for efficiency of the sulfide removal process. As can be seen in figure 10, in the case of the first and second regeneration method a significant decrease of overall average sorption capacity was observed after the second cycle of the regeneration process. When using 0.01 M HCl as the regeneration agent a slight decrease of overall average sorption capacity was noted between fresh and regenerated sorbent even after the third regeneration cycle (33.25 and 28.68 mg S2-/g respectively for a 100 mg/dm3 sulfide solution). Relatively high sulfide removal efficiency was correlated with a drop of pH of the solution from ~12.00 to ~9.00, which confirmed that protonation of functional groups of the hybrid polymer ensured optimal pH of the solution, which has a significant bearing on the sulfide removal process with CM3H. There was also a slight increase of Fe(II) content between fresh and regenerated sorbents, whereas Fe(III) content decreased on average by 7% during all three cycles of regeneration studies regardless of the regeneration method used. Presumably due to high content of Fe(III) in the structure of CM3H, repeated effective cycles of adsorption/regeneration are possible until a sufficient amount of inorganic deposit is present in its polymeric matrix. Conclusions Gel-type and macroporous sulfonic and carboxylic cation exchangers were used to produce hybrid polymers containing a mixture of iron oxides, including maghemite with Fe content of 7.8-35.2%. Products with the highest iron deposit content were obtained using carboxylic cation exchangers. The deposition of large amounts of iron oxides in the structure of the macroporous polymers would cause changes in their pore structure and BET surface

area (mainly pore mouth narrowing). As regards removal of sulfides from aqueous solutions, the highest efficiency (60 mg S2-/g) was exhibited by a hybrid polymer obtained using the macroporous carboxylic cation exchanger, due to the high iron oxide load (Fe content 35%), high swelling index and well-developed mesoporous structure of polymeric matrix. It was also found that the form of functional groups in the polymer matrix also played a crucial role in the sulfide removal process – sulfonic or carboxylic groups in the H+ form enabled advantageous modification of the pH level due to the concurrent ion exchange reaction. The process of sulfide removal with obtained hybrid polymers was very complex and proceeded with heterogeneous oxidation, reductive dissolution of iron(III) oxides and formation of many sulfide oxidation and precipitation products and byproducts. Release of Fe2+ ion from the iron oxide surface exposed new active sites, and therefore overall sorption and heterogeneous oxidation capacity of the sorbent increased with the increase of initial sulfide concentration. Simultaneously the obtained hybrid polymer underwent effective regeneration with HCl solution under aerated conditions. Due to combination of properties of iron oxide and polymer matrix the sulfide removal process occurred with high efficiency. The physical form of obtained material allowed its easy separation from the treated solutions, and repeated use of the sorbent after regeneration. In the future we intend to study the performance of the obtained hybrid polymer for purification of wastewater from pulp and paper production. References [1] Zhang, L., De Schryver, P., De Gusseme, B., De Muynck, W., Boon, N., Verstraete, W., Chemical and biological technologies for hydrogen sulfide emission control in sewer systems: A review. Water Res. 2008, 42, 1-12. [2] Kociołek-Balawejder, E., Wilk, Ł.J.,. Sulfides in industrial systems. Technical and environmental problems [in polish], Przem. Chem. 2011, 90, 825-830. [3] Kociołek-Balawejder, E., A copolymer with N-chlorosulfonamide pendant groups as oxidant for residual sulfides, React. Funct. Polym. 2002, 52, 89-97. [4] Kociołek-Balawejder, E., Wilk, Ł.J., Sulfides in aqueous environment. Part 2. Preventing the emission and removal [in polish], Przem. Chem. 2012, 91, 2345-2350. [5] Bagreev, A., Bandosz, T.J., On the Mechanism of Hydrogen Sulfide Removal from Moist Air on Catalytic Carbonaceous Adsorbents, Ind. Eng. Chem. Res. 2005, 44, 530-538. [6] Abatzoglou N., Boivin S., A review of biogas purification processes, Biofuels, Bioprod. Bioref. 2009, 3, 42-71. [7] Suber, L., Foglia, S., Ingo, G.M., Boukos, N., Synthesis, and structural and morphological charcterization of iron oxide- ion exchange resin and –cellulose nanocomposites, Appl. Organometal. Chem. 2001, 15, 414-420.

[8] Chen, X., Lam, K.F., Zhang, Q., Pan, B., Arruebo, M., Yeung, K.L., Synthesis of highly selective magnetic mesoporous adsorbent, J. Phys. Chem. C, 2009, 113, 9804-9813. [9] Mishra, A.K., Ramaprabhu, S., Magnetite decorated multiwall carbon nanotube based supercapacitor for arsenic removal and desalination of seawater, J. Phys. Chem. C, 2010, 114, 2583-2590. [10] Hasanzadeha, M., Farajbakhshb, F., Shadjoucd, N., Jouybana, A., Mesoporous (organo) silica decorated with magnetic nanoparticles as a reusable nanoadsorbent for arsenic removal from water samples, Environ. Technol. 2015, 36, 36-44. [11] Sarkar, S., Blaney, L.B., Gupta, A., Ghosh, D., SenGupta, A.K., Use of ArsenXnp, a hybrid anion exchanger, for arsenic removal in remote villages in the Indian subcontinent, React. Funct. Polym., 2007, 67, 1599-1611. [12] Iesan, C.M., Capat, C., Ruta, F., Udrea, I., Characterization of hybrid inorganic/organic polymer-type materials used for arsenic removal from drinking water, React. Funct. Polym., 2008, 68, 1578-1586. [13] Ziolo, R.F., Giannelis, E.P., Einstein, B.A., O’Horo, M.P., Ganguly, B.N., Mehrotra, V., Russel, M.W., Rayn, D.R., Hoffman, R.H., Matrix-mediated synthesis of nanocrystalline γFe2O3: a new optically transparent magnetic material, Science, 1992, 257, 219-223. [14] Leun, D., SenGupta, A.K., Preparation and characterization of magnetically active polymeric particles (MAPPs) for complex environmental separations, Environ. Sci. Technol. 2000, 34, 3276-3282. [15] Rodriguez, A.F.R., Coaquira, J.A.H., Santos, J.G., Silveira, L.B., Marmolejo, E.M., Trennenpohl, W., Rabelo, D., Oliveira, A.C., Garg, V.K., Morais, P.C., Characterization of magnetite nanoparticles supported in sulfonated styrene-divinylbenzene mesoporous copolymer, Hyperfine Interact., 2009, 191, 87-93. [16] Sarkar, S., Guibal, E., Quignard, F., Polymer-supported metals and metal oxide nanoparticles: synthesis, characterization, and application, J. Nanopart. Res. 2012, 14:715, DOI 10.1007/s11051-011-0715-2. [17] Jacukowicz-Sobala, I., Ciechanowska, A., Kociołek-Balawejder, E., Hybrid polymer containing ferric oxides obtained using a redox polymer. Part I. Synthesis and characterization, Polimery 2014, 59, 131-135. [18] Rodriguez, E., Fernandez, G., Ledesma, B., Alvarez, P., Beltran, F.J., Photocatalytic degradation of organics in water in the presence of iron oxides: Influence of carboxylic acids, Appl. Catal. B, 2009, 92, 240-249. [19] Layek, S., Pandey, Anjana, Pandey, Ashutosh, Verma, H.C., Synthesis of γ-Fe2O3 nanoparticles with crystallographic and magnetic texture, Int. J. Eng., Sci. Technol., 2010, 2, 33-39.

[20] Gabbasov, R.R., Cherepanov, V.M., Chuev, M.A., Polikarpov, M.A., Panchenko, V.Y., Size effect of Mössbauer parameters ion iron oxide nanoparticles, Hyperfine Interact. 2014, 226, 383-387. [21] Ramos Guivar, J. A., Bustamante, A., Flores, J., Mejía Santillan, M., Osorio, A. M., Martínez, A. I., De Los Santos Valladares, L., Barnes, C. H. W., Mössbauer study of intermediate superparamagnetic relaxation of maghemite (γ-Fe2O3) nanoparticles, Hyperfine Interact. 2014, 224, 89-97. [22] Polyakov, A.Yu., Goldt, A.E., Sorkina, T.A., Perminova, I.V., Pankratov, D.A., Goodilin, E.A., Tretyakovab, Y.D., Constrained growth of anisotropic magnetic δ-FeOOH nanoparticles in the presence of humic substances, CrystEngComm, 2012,14, 8097-8102. [23] Cumbal, L., SenGupta, A.K., Arsenic removal using polymer supported hydrated iron(III) oxide nanoparticles: role of Donnan membrane effect, Environ. Sci. Technol. 2005, 39, 65086515. [24] Dos Santos Afonso, M., Stumm, W., Reductive Dissolution of Iron(III) (Hydr)oxides by Hydrogen Sulfide, Langmuir 1992, 8, 1671-1675. [25] Davydov, A., Chuang, K.T., Sanger, A.R., Mechanism of H2S Oxidation by Ferric Oxide and Hydroxide Surfaces, J. Phys. Chem. 1998, B 102, 4745-4752. [26] Poulton, S.W., Krom, M.D., Van Rijn, J., Raiswell, R., The use of hydrous iron (III) oxides for the removal of hydrogen sulphide in aqueous systems, Water Res. 2002, 36 825-834. [27] Poulton, S.W., Sulfide oxidation and iron dissolution kinetics during the reaction of dissolved sulfide with ferrihydrite, Chem. Geol. 2003, 202, 79-94. [28] Poulton, S.W., Krom, M.D., Raiswell, R., A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulfide. Geochim. Cosmochim. Ac. 2004, 68, 3703-3715. [29] Steudel, R., Mechanism for the Formation of Elemental Sulfur from Aqueous Sulfide in Chemical and Microbiological Desulfurization Processes. Ind. Eng. Chem. Res. 1996, 35, 1417-1423. [30] Yao, W., Millero, F.J., Oxidation of hydrogen sulfide by hydrous Fe(III) oxides in seawater. Mar. Chem. 1996, 52, 1-16. [31] Herszage, J., Dos Santos Afonso, M.,The autooxidation of hydrogen sulfide in the presence of hematite, Colloid. Surface. A. 2000, 168, 61-69. [32] Petre, C.T., Alkaline oxidation of hydrosulfide and methyl mercaptide by iron/cerium oxide-hydroxide in presence of dissolved oxygen. Possible application for removal of Total Reduced Sulfur (TRS) emissions in Pulp & Paper industry. Département De Génie Chimique Faculté Des Aciences Et De Génie Université Laval Québec, Québec 2007. [33] Haimour, N., El-Bishtawi, R., Ail-Wahbi, A., Equilibrium adsorption of hydrogen sulfide onto CuO and ZnO, Desalination 2005, 181, 145-152.

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Table 1. Characterization of polymeric support Amberlite Amberlite IR120 Amberlite IRC50 252 Matrix* S/DVB S/DVB PA/DVB Matrix structure macroporous gel macroporous −SO 3Na −SO3Na −COOH Functional group Exchange capacity [meq/g] 4.3 4.4 10.8 Appearance beige, yellow, creamy, opaque translucent opaque Code SM SG CM *S/DVB – Styrene-divinylbenzene Copolymer, PA/DVB – Polyacrylic-divinylbenzene -Copolymer Property

Amberlite IRC86

PA/DVB gel −COOH 10.7 brownish-red translucent CG

Table 2. Fe(II) and Fe (III) content in hybrid polymers Polymeric support

Cycle number

SM

SG

CM

CG

Ion exchange capacity 2+ [mmol Fe /g]

Fe(III) content [mmol/g]

Appearance of hybrid polymer beads

Fe(II) content [%]

[mmol/g]

[%]

red-black lustrous

1

1.40

1.38

7.73

0.016

0.09

2

1.27

2.60

14.6

0.035

0.20

3

0.98

3.53

19.8

0.039

0.22

1

1.44

1.43

8.01

0.013

0.07

red-black

2

1.43

2.78

15.6

0.085

0.48

lustrous

3

1.27

3.40

19.0

0.224

1.26

1

4.49

2.66

14.9

0.004

0.02

red-brown

2

2.32

4.94

27.7

0.223

1.25

opaque

3

1.83

5.55

31.1

0.741

4.15

1

4.03

2.25

11.3

0.339

1.90

red-black

2

2.23

4.10

23.0

0.580

3.25

lustrous

3

1.90

4.99

27.0

0.678

3.79

Table 3. EDS analysis of Fe content in hybrid polymer SM3 Point

1 2 3 4 5 6 7

Fe content [%] 64.98 14.18 11.12 11.80 10.41 14.37 14.12

Table 4. Porous structural data of polymeric raw materials and hybrid products 2

3

Sample

SBET [m /g]

VTotal [cm /g]

SM

8.37

0.0157

SM1

21.8

0.0254

SM3

3.54

0.0084

CM

1.63

0.0047

CM1

18.7

0.0566

Initial concentration 23 [mg S /dm ] 50 100 150 200

Sulfides removed [%]

Table 5. Mass balance of sulfide removal process with hybrid polymer CM3H Product of sulfides removal Sorbent Solution 20* FeS S2O3 FeS+S 17.62 25.36 57.02 29.90 17.41 47.93 26.76 19.57 59.82 29.73 18.41 48.47

∑ [%]

100 95.24 96.15 96.61

Overall average removal capacity 2[mg S /g] 19.83 33.25 48.69 59.80

Figure captions Figure 1. SEM images of a) polymeric support SM and its products containing iron oxide, b) after 1 cycle of synthesis (SM1), c), d) after 3 cycles of synthesis (SM3) Figure 2. SEM images of a), b) polymeric support CG, and its product containing iron oxide c), d) CG3 after 3 cycles of synthesis Figure 3. Nitrogen adsorption-desorption isotherms for polymeric support SM and hybrid polymers SM1 and SM3 Figure 4. Nitrogen adsorption-desorption isotherms for polymeric support CG and hybrid polymer CG3 Figure 5. XRD patterns of hybrid polymers a) SM3, b) SG3, c) CM3, d) CG3 Figure 6. Mössbauer spectra of hybrid polymers, recorded at 298 K: a) SM3, b) CM3, c) CG3 Figure 7. Sulfide removal efficiency from solutions with concentration 100 mg S2-/dm3 using obtained hybrid polymers at dose 0.5 g/0.1 dm3 Figure 8. Sulfide removal capacity of hybrid polymer CM3H versus time at different initial sulfide concentrations expressed in a) [%], b) mg S2-/g Figure 9. pH changes in treated solutions during the sulfide removal process using hybrid polymer CM3H Figure 10. Removal capacity of hybrid polymer CM3H after regeneration with: method 1 – aeration in distilled water, method 2 – drying at temp. of 55°C, method 3 – treatment with 0.01 M HCl solution under aerated conditions

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Synthesis and characterization of hybrid materials containing iron oxide for removal of sulfides from water.

Hybrid materials containing iron oxides based on macroporous and gel-type sulfonic and carboxylic cation exchangers as supporting materials were obtai...
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