Tetravalent Manganese Feroxyhyte: A Novel Nanoadsorbent

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Tetravalent Manganese Feroxyhyte: A Novel Nanoadsorbent Equally Selective for As(III) and As(V) Removal from Drinking Water Sofia Tresintsi,† Konstantinos Simeonidis,‡ Sonia Estradé,§,∥ Carlos Martinez-Boubeta,∥ George Vourlias,⊥ Fani Pinakidou,⊥ Maria Katsikini,⊥ Eleni C. Paloura,⊥ George Stavropoulos,# and Manassis Mitrakas*,† †

Analytical Chemistry Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece ‡ Department of Mechanical Engineering, School of Engineering, University of Thessaly, 38334 Volos, Greece § TEM-MAT, CCiT-Universitat de Barcelona, Solé i Sabarís, 1, 08028 Barcelona, Spain ∥ Departament d’Electrònica and MIND/IN2UB, Universitat de Barcelona, 08028 Barcelona, Spain ⊥ Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece # Laboratory of Chemical Technology, Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece S Supporting Information *

ABSTRACT: The development of a single-phase Fe/Mn oxy-hydroxide (δ-Fe0.76Mn0.24OOH), highly efficient at adsorbing both As(III) and As(V), is reported. Its synthesis involves the coprecipitation of FeSO4 and KMnO4 in a kilogram-scale continuous process, in acidic and strongly oxidizing environments. The produced material was identified as a manganese feroxyhyte in which tetravalent manganese is homogeneously distributed into the crystal unit, whereas a second-order hollow spherical morphology is favored. According to this structuration, the oxyhydroxide maintains the high adsorption capacity for As(V) of a single Fe oxy-hydroxide combined with enhanced As(III) removal based on the oxidizing mediation of Mn(IV). Ion-exchange between arsenic species and sulfates as well as the strongly positive surface charge further facilitate arsenic adsorption. Batch adsorption tests performed in natural-like water indicate that Mn(IV)-feroxyhyte can remove 11.7 μg As(V)/mg and 6.7 μg As(III)/mg at equilibrium pH 7, before residual concentration overcomes the regulation limit of 10 μg As/L for drinking water. The improved efficiency of this material, its low cost, and the possibility for scaling-up its production to industry indicate the high practical impact and environmental importance of this novel adsorbent.

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INTRODUCTION

effective As(III) removal. The preoxidation significantly increases the capital cost of the arsenic removal process. In this direction, a number of studies have incorporated an oxidant in the form of composite arsenic adsorbents. Specifically, it has been shown that MnO2 effectively oxidizes As(III),11,12 despite its rather low adsorption capacity for As(V).13 Zhang et al.14,15 prepared such an Fe−Mn adsorbent consisting of a mixture of Fe oxy-hydroxide and MnO2, by adding a FeSO4 solution in KMnO4 at the pH range 7−8. Alternatively, Huang et al.,16 produced a composite adsorbent by coating iron oxide granules obtained from a real Fenton fluidized bed with MnO2. The main drawback of these materials is that although MnO2 effectively oxidizes As(III) and increases removal capacity for this species, it decreases the total arsenic capacity proportionally to the MnO2 percentage since its

To address adverse health effects arising from human exposure to arsenic by drinking water consumption, numerous arsenic removal techniques have been developed. Chemical coagulation/filtration using ferric salts1 and adsorption onto ferric oxy-hydroxides2 or titanium dioxide3 are the most prevalent and comply with the recently established arsenic maximum contaminant level of 10 μg/L in drinking water.4 The decision to apply one of these two methods depends strictly on the total operation cost as defined by the flow rate demands of the treatment plant and the level of arsenic concentration.5,6 However, adsorption is often preferred due to its simplicity, stability, compact facilities, and easy handling of waste. Nevertheless, the above-mentioned processes remove effectively only the As(V) oxy-anions (H2AsO4−/HAsO42−) and not the uncharged H3AsO3 of As(III), which appear as the dominant forms at natural water pH.7 Therefore, a preoxidation step, activated by a chemical reagent,8 a granular solid (e.g., MnO29) or biological oxidation10 is usually required for © XXXX American Chemical Society

Received: March 5, 2013 Revised: July 26, 2013 Accepted: July 26, 2013

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dx.doi.org/10.1021/es4009932 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

left to thicken in an Imhoff tank for 24 h. The sludge was washed/centrifuged with water, dried at 110 °C and sieved to various sizes. Fine powder ( 11). In this way, Mn-goethite was synthesized by Sun et al., 1 7 a Mn0.13Fe0.87OOH oxy-hydroxide by Lakshmipathiraj et al.,18 and a series of Mn-ferrihydrites by Mohanty et al.19 However, the proposed materials have several drawbacks including both cost-efficiency of the preparation procedure and the arsenic removal capacity. More specifically, the production cost may rise significantly by using batch operations, long reaction times (>1 day), and, in some cases, high temperatures to ensure Mn(II) oxidation to Mn(III) using a mild oxidant such as oxygen. On the other hand, synthesis at high pH-values (high OH− concentration) results in the limited presence of positive charges on the adsorbent’s surface implying reduced adsorption capacity concerning As(V) oxy-ions.20 Additionally, the formed Mn(III) has low oxidizing ability, which, along with the slow oxidation rate of As(III), is responsible for poor As(III) adsorption. Oxidation of As(III) by Mn(III) may also cause the leaching of Mn2+ into the treated water. Even a modified route for synthesizing a hydrated oxide MnxFe2‑xO3·yH2O (x = 0.18) at acidic pH 6 using NaOCl as oxidizing agent21 did not produce an efficient material since its adsorption capacity at equilibrium concentration equal to the maximum contaminant level of 10 μg/L was significantly lower than that of commercial iron oxy-hydroxides.20 The purpose of this study was to prepare an adsorbent capable of high simultaneous As(III) and As(V) removal, comprising a binary Fe(III)−Mn(IV) oxy-hydroxide, efficient at both As(III) oxidation due to Mn(IV) presence, and As(V) capture due to the high density of positive surface charge. Based on our previous optimization study on the preparation of single Fe oxy-hydroxides with improved arsenic removal capacity,20 a novel nanostructured Fe−Mn oxy-hydroxide, identified as tetravalent manganese feroxyhyte (δ-Mn(IV) FeOOH), was developed. In the proposed production route, the material is synthesized after the reaction of FeSO4 with KMnO4 in acidic and high oxidative environments fixed by KMnO4 itself. The mechanisms determining the fast, efficient adsorption of both arsenic species observed on such solids, were examined in water with realistic arsenic concentrations and with compliance to the regulation limit.

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MATERIALS AND METHODS Synthesis. The studied Mn(IV)-feroxyhyte (δFe0.76Mn0.24OOH) was synthesized at kilogram-scale in a laboratory two-stage continuous flow reactor with a 1 h retention time (Supporting Information). The typical chemical process included the coprecipitation into water of the iron source (FeSO4·H2O) and the manganese source (KMnO4), which also played the role of the oxidant, at pH 4. The FeSO4· H2O inflow (40 g/L solution) was set to 10 L/h, which corresponds to the production of around 0.3 kg/h dry product, while reaction’s redox was adjusted to 850 mV by KMnO4 addition (20 g/L solution), which controlled the measured manganese valence close to +4. Drops of NaOH (30 wt %) were added to regulate the pH value in both reactors. The product was received in the form of a suspension, which was B



Article

Arsenic adsorption efficiency was not evaluated according to the maximum adsorption capacity but to the capacity that corresponds to a residual As concentration of 10 μg/L (Q10index) signifying the ability to reduce As below the regulation limit. Adsorption isotherms were recorded after batch experiments where 5−25 mg of fine powder (6544 eV), is related to metal−metal interactions.31 The existence of Mn(III) or Mn(IV) does not seriously affect the position of the M1 and M2 constituents. However, variations in their relative intensity that cause a blue shift of the pre-edge peak centroid by approximately 0.5 eV is indicative of the variation of the Mn oxidation state.29 Thus, although the oxidation state of Mn cannot be accurately determined by the centroid position of the pre-edge peak, the predominance of Mn(IV) in the studied sample is verified. The oxidation state of Mn, as determined by XANES spectroscopy, coincides to that measured by titration (Table 1). Table 1. Comparison of Physicochemical Characteristics and Arsenic Removal Efficiency for Single Fe Oxy-hydroxide and Mn(IV)-feroxyhyte FeOOH synthesis pH redox (mV) structure Fe (wt %) Mn (wt %) SO42− chemisorbed/total (wt %) Mn valence (titration/XAFS) IEP PZC surface area (m2/g) Q10 for As(III) (μg/mg)a distilled (pH 6/7/8) NSF (pH 6/7/8) Q10 for As(V) (μg/mg)a distilled (pH 6/7/8) NSF (pH 6/7/8)

Figure 3. Fe- (a) and Mn−K-edge (b) XANES spectra of the studied Fe/Mn oxy-hydroxide (TMF). The pre-edge peak (raw data shown with circles), and the fitting (solid line) obtained using Lorentzian or Voigt functions, are shown in the insets. Corresponding Fourier Transform amplitudes of the k3 χ(k) EXAFS spectra recorded at the Fe- (c) and Mn−K- edge (d). The spectra of the reference feroxyhyte (Fx) and Mn-feroxyhyte (Mn-Fx) samples are also included. The raw data and the fitting curves in c and d are shown in solid and dashed lines, respectively.

a

4.0 ± 0.05 +410 ± 10 schwertmannite 50.4 ± 1.2

Fe/MnOOH

5.2 ± 0.1/14.0 ± 0.5

4.0 ± 0.05 +850 ± 30 feroxyhyte 38.1 ± 0.9 11.5 ± 0.3 4.8 ± 0.2/11.1 ± 0.5

7.1 ± 0.1 2.9 ± 0.1 120 ± 5

4.0 ± 0.1/4.0 ± 0.1 7.2 ± 0.1 3.1 ± 0.1 187 ± 7

5.7/8.0/10.1 1.7/1.9/2.1

13.7/12.3/10.9 8.8/6.7/4.6

31.1/24.0/16.9 21.4/13.5/5.5

25.4/17.8/10.3 18.0/11.7/5.4

Mean value of 3 replicates.

The low intensity of the pre-edge peak in XANES spectra is characteristic for the octahedral coordination of both Fe and Mn.30 The way these octahedra are linked was investigated using EXAFS spectroscopy. Fourier transforms (FT) of the experimental k3-weighted χ(k) EXAFS spectra are shown in Figure 3. Analysis of the Fe−K-EXAFS spectra was performed using the feroxyhyte model in the three nearest neighboring shells: oxygen atoms in octahedral coordination, two Fecentered polyhedra that share faces, and six that share edges. For Mn−K-EXAFS spectra, a modified model of feroxyhyte was used where Mn atoms substitute for Fe. It should be pointed out that due to the slight difference in the atomic number of Mn and Fe that yields similar backscattering characteristics, the splitting of the scattering paths in Mn and Fe subshells would not yield more accurate results. The fitting results verify that both Fe and Mn are octahedrally coordinated (coordination number = 6). The Fe−O distance found equal to 1.99 Å, is characteristic of Fe(III) forming octahedra being similar to the respective value in the reference feroxyhyte sample. The Mn− O distance was found equal to 1.90 Å as in the Mn-feroxyhyte reference sample. The Fe−Fe scattering paths that correspond to face- and edge sharing polyhedra at distances 2.27 and 3.02 Å, respectively, fit well to the Fe−K-edge spectra. The

pre-edge peaks were fitted by three components. Three Lorentzian functions were used for the fitting of Fe−KXANES spectra (inset Figure 3a). F1 and F2 are located at 7113.3 ± 0.2 eV and 7115.1 ± 0.2 eV, respectively, and are characteristic of octahedrally coordinated Fe(III).26 F3 appearing at higher energies (>7116.5 eV) is attributed to Fe−Fe interactions between adjacent polyhedra and was not considered in the pre-edge peak area calculations.27,28 The total area under the F1 and F2 peaks in the Fe/Mn oxy-hydroxide is 0.20 ± 0.02 arb. units and is consistent with the respective value in the feroxyhyte reference sample (0.17 ± 0.02 arb. units). In the Mn−K-XANES spectra, the energy position of both the absorption edge and the pre-edge peak vary linearly with the oxidation state of the Mn atoms.29 However, certain characteristics of the pre-edge peak (e.g., the relative intensity of the various peak contributions) are also strongly affected by the bonding geometry of Mn.30 Thus, the oxidation state of Mn was determined from the position of the Mn−K- absorption edge, using the calibration curve proposed by Belli et al.29 It was found that the valence reaches +4.0 and the complete formation of Mn(IV) occurs for the sample prepared at pH 4 D



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by the surface charge density. To verify the assumed SO42−As(V) ion-exchange mechanism, adsorption experiments were carried at solution pH 5, where As(V) is met exclusively in the H2AsO4− form, using high As(V) concentrations (2.5−100 mg/ L) and comparing As(V) adsorption and SO42− release into the solution (Figure 5). Results indicate that the molar ratio of

corresponding shells at the Mn−K-edge spectra were detected at 2.38 Å (face sharing) and 2.93 Å (edge sharing). However, for the reference Mn feroxyhyte, the face-sharing linkage contribution was diminished. This result is in agreement with previously published results on manganese-feroxyhyte, where Mn forms chains by edge sharing octahedra which are incorporated in the feroxyhyte structure.32 Arsenic Removal Efficiency. To gain a reliable assessment of arsenic adsorption efficiency in the presence of the usual interfering-competing ions occurring in drinking water, the synthesized Mn(IV)-feroxyhyte was evaluated in NSF water, although their adsorption capacity was significantly amplified when distilled water was the equilibration medium. The data points of adsorption isotherms were fitted using the Freundlich equation, and the Q10-index was derived by setting the equilibrium concentration of Ce = 10 μg/L in the Freundlich equation Q = KFCe1/n, where Q is the amount of arsenic adsorbed per mass of adsorbent, and KF and n are constants related to adsorption capacity and intensity, respectively. At equilibrium pH 7, for example, the Q10-values of 17.8 μg As(V)/mg and 12.3 μg As(III)/mg measured in distilled water were reduced to 11.7 and 6.7 μg/mg in NSF, respectively (Figure 4). The reduction of the Q10-index when equilibrium

Figure 5. Correlation of released SO42− to the adsorption of As(V) (adsorbed-residual) in the sample. The test distilled water was adjusted to pH 5. The dashed line indicates the sulfate ions located in the Stern layer (excluding those in the diffuse layer).

SO42− released in the solution to the H2AsO4− captured is around 1:2, implying that each SO42− ion leaves the surface and offers two positive sites for the adsorption of two H2AsO4−. In other words, chemisorption of SO42− facilitates adsorption by “preserving” positively charged sites during oxy-hydroxide drying. Thermodynamic Studies. The effect of temperature on the efficiency and strength of arsenic adsorption for Mn(IV)feroxyhyte may be explained considering thermodynamic parameters, such as Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°). The change of Gibbs free energy in the adsorption reaction was calculated by ΔG° = −RT ln K ads Figure 4. As(III) and As(V) adsorption isotherms of the sample at low equilibrium concentrations using distilled and NSF water adjusted to pH 7. Lines represent the Freundlich function fitting.

(3)

where R is the ideal gas constant 8.314 J/(mol K), T the absolute temperature in K, and Kads is the equilibrium adsorption constant, which can be approximated by the Langmuir’s equilibrium constant KL when expressed in L/ mol.33 ΔH° and ΔS° were calculated (Table 2) from van’t Hoff’s equation:

pH increases (Table 1) is appointed to the gradual predomination of HAsO42− against H2AsO4−, which demands two active sites to be adsorbed, and to the increased concentrations of anions such as OH−, PO43−, and H3SiO4−, which strongly compete for adsorption sites. A combination of parameters regarding surface charge and ion-exchange mechanisms is considered to understand the improved efficiency of both As(III) and As(V) adsorption in the studied sample synthesized at pH 4. The SO42−, K+, and Na+ rich environment during synthesis plays an important role determining morphology and surface charge. The significant divergence of IEP (7.2) and PZC (3.1) indicates that the highly positive surface of the material prepared in acidic environments attracts sulfates that are strongly adsorbed covering most of the available sites. Indeed, the estimated total sulfate in the sample is equal to 11.1 wt % with almost half of them appearing as chemically adsorbed in the Stern layer. The presence of chemisorbed SO42− is related to the ion-exchange potential of the samples to the charged arsenic species, which is quantified

ΔH ° ΔS° + (4) RT R by plotting ln(Kads) versus 1/T and estimating ΔH° and ΔS° from the slope and intercept, respectively. The positive enthalpy change with temperature indicates the endothermic character of the As(III) and As(V) adsorption. More specifically, the ΔH° value for As(V) adsorption on Mn(IV) feroxyhyte is in agreement with the corresponding Fe oxyhydroxide (Table 2) and those estimated for iron oxyhydroxides (36.37 kJ/mol).34 The same enthalpy value was found for low and high equilibrium concentrations, while the ΔH° for As(III) showed strong dependence on equilibrium concentration. At low As(III) concentrations commonly found in drinking water, the calculated ΔH° value (35.9 kJ/mol) is close to that calculated for As(V), which supports complete As(III) oxidation and adsorption in As(V) form. On the ln K ads = −

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Table 2. Summary of the Thermodynamic Parameters for the Mn(IV)-feroxyhytea −ΔG° (kJ/mol) species As(III) As(V)

As(III) As(V) a

equilibrium concn. (μg/L) 5−100 100−10000 5−100 100−10000 5−100 100−10000 5−100 100−10000

Qmax (20 °C) (μg As/mg)

114.9 49.1

108.7 48.8

ΔH° (kJ/mol)

Mn(IV)-feroxyhyte 35.9 21.9 34.1 34.4 Fe oxy-hydroxide 14.7 14.3 34.6 33.9

ΔS° (J/mol·K)

283 K

293 K

303 K

257 172 251 227

36.8 26.8 36.9 29.8

39.4 28.5 39.5 32.1

42.0 30.2 41.9 34.4

179 139 254 227

35.9 25.0 37.3 30.3

37.7 26.4 39.8 32.6

39.5 27.8 42.4 34.9

Data are compared to those of a single Fe oxy-hydroxide prepared at pH 4 (adsorbent dose 100 mg/L).

contrary, at high As(III) equilibrium concentrations, a ΔH° value of 21.9 kJ/mol was estimated, which stands between those for As(V) and As(III) adsorption (14.3 kJ/mol) onto the corresponding Fe oxy-hydroxide (Table 2) and GFH.34 Similar values (23.23 kJ/mol) were estimated by Gupta et al.21 when practicing manganese associated agglomerates of iron oxide and equally high equilibrium concentrations. This value suggests partial As(III) oxidation which possibly explains the diverse behavior in the As(III) adsorption isotherm (Supporting Information). Concurrently, an increase in randomness at the adsorbent/solution interface during adsorption occurrence is verified by the positive ΔSo, while the negative values of ΔGo in the temperature range studied show the spontaneous nature of the process for both arsenic species. Arsenic Bonding Configuration. The improved As(III) uptake by the Mn(IV)-feroxyhyte and the sorption geometry was studied by means of As−K-edge XAFS spectroscopy. XANES spectra for the Fe/Mn adsorbent compared to the single Fe adsorbent are shown in Figure 6a. The energy position of the characteristic “white line” in the XANES spectra is a signature of the As oxidation state as it appears at distinctly lower energy for As(III) than As(V).35 In the case of the single Fe oxy-hydroxide, the oxidation state of As remains unaffected after adsorption of As(III) and As(V). In contrast, when the Fe/Mn adsorbent was used for the As(III) adsorption, the

captured form was identified as As(V). This is evidence of the mediating role of Mn(IV) that oxidizes As(III) and facilitates the adsorption of As(V) by Fe atoms. The oxidation step of As(III) in Mn(IV)-feroxyhyte is also verified by the EXAFS analysis. Experimental As−K-edge EXAFS spectra fitted in R-space are shown in Figure 6b. The As−O distance is found equal to 1.68−1.69 Å for adsorption of As(V) by the single Fe or for adsorption of As(III) and As(V) by Mn(IV)-feroxyhyte, whereas it increases to 1.80 Å for adsorption of As(III) by single-Fe oxy-hydroxide. The larger As−O distance is consistent with the oxidation state of As(III).35 Concerning the adsorption geometry of the As oxyanions, the small contribution of scattering shells beyond the first nearest neighbor in Figure 6b indicates that the As oxyanions are adsorbed by forming inner sphere complexes.36 In samples where As is adsorbed in the form of As(V), the As−Fe distance ranges from 2.83 to 2.85 Å. This corresponds to the bidentate mononuclear configuration (1E) of the edge sharing tetrahedra of the As oxy-anions with the Fe octahedra (inset Figure 6b). The As−Fe distance is found equal to 2.96 Å when As(III) is adsorbed by the pure Fe oxy-hydroxide due to the larger dimensions of the arsenite oxy-anion.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details on NSF water preparation, determination of IEP, PZC, sulfate percentage, Mn valence. Adsorption isotherms extended at higher concentrations. Details on XAFS analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax +30 2310 996248. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the European Commission FP7/ Research for SMEs “AquAsZero”, project No. 232241. K. Simeonidis thanks the Action “Supporting Postdoctoral Researchers” of Operational Program “Education and Lifelong Learning”, cofinanced by the European Social Fund (ESF) and the Greek State (GSRT). C.M. Boubeta was supported by the Spanish Government under the “Ramón y Cajal” Fellowship program. The measurements at BESSY were funded by the

Figure 6. As−K-edge XANES spectra of a single-Fe prepared at pH 4 and the Mn(IV)-feroxyhyte after the sorption of As (a). FT amplitude of the As−K-edge EXAFS spectra (b). Solid and dotted lines correspond to the experimental and fitting curves, respectively. F



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European Community’s seventh Framework Programme (FP7/ 2007-2013) under grant agreement No. 226716.

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Tetravalent Manganese Feroxyhyte: A Novel Nanoadsorbent

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