Fe3O4 Nanoparticles Dispersed on Douglas Fir Biochar for Phosphate

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Cite This: ACS Appl. Nano Mater. 2019, 2, 3467−3479

Fe3O4 Nanoparticles Dispersed on Douglas Fir Biochar for Phosphate Sorption Akila G. Karunanayake,†,‡,§ Chanaka M. Navarathna,†,§ Sameera R. Gunatilake,⊥ Morgan Crowley,† Renel Anderson,‡ Dinesh Mohan,∥ Felio Perez,¶ Charles U. Pittman, Jr.,*,† and Todd Mlsna*,† †

Department of Chemistry, Mississippi State University (MSU), Starkville, Mississippi 39762, United States Biochar Supreme Inc., Everson, Washington 98247, United States ⊥ College of Chemical Sciences, Institute of Chemistry Ceylon, Rajagiriya 10107, Sri Lanka ∥ School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India ¶ Material Science Laboratory, Integrated Microscopy Center, University of Memphis, Memphis, Tennessee 38152, United States

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‡

S Supporting Information *

ABSTRACT: Surface area, particle aggregation, pressure drop in columns, nanotoxicity, and commercialization difficulties limit the use of nanoparticle adsorbents. Magnetic primary nano-Fe3O4 particles (∼16.7 nm diameter) were dispersed on high-surface-area (695 m2/g) Douglas fir biochar (MBC). A cheap, commercial fast pyrolysis biochar, a syngas byproduct, was modified by chemical coprecipitation of Fe3O4 from Fe3+/Fe2+ aqueous NaOH, served as a matrix, aiding magnetite nanoparticle dispersion and reducing the extent of particle aggregation. This MBC removed ∼90.0 mg/g of phosphate from water, approximately 20 times the capacity reported for neat (∼39 nm) magnetite particles (∼5.1 mg/g). MBC was robust in fixed-bed column sorption with 82.5 mg/g (at pH 3) capacity, showing no significant equilibrium or kinetic limitations in flow versus batch sorption. The biochar support serves as an added adsorption phase for heavy metals and organic contaminants, adsorbing poorly on magnetite. MBC enables magnetic separation of exhausted adsorbent from a batch process, an alternative to filtration. The neat and phosphate-laden hybrid sorbents were was characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, point of zero charge, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), elemental analysis, vibrating sample magnetometry, and Brunauer−Emmett−Teller surface-area and pore-volume measurements. The chemisorption mechanism versus pH, evaluated by XPS and existing literature, characterized the dominant phosphate complexes adsorbed on magnetite. The pH effect on phosphate sorption and the P 2p XPS binding energy shifts at pH from 1 to 13 are reported. A solution pH of 1−3 facilitates the formation of bidentate monoprotonated phosphate complexes [(Fe−O)2-PO2H]− at Fe−OH surface functions. H2PO42− predominates in solution at pH ∼4−6.5, which favors the formation of [Fe−O−PO3H]− at these pH values. At strongly basic pH (10−13) values, PO43− predominates and forms deprotonated chemisorbed monodentate [Fe−O− PO3]2− and bidentate [(Fe−O)2PO2]2−. Multilayer phosphate sorption and precipitation of iron phosphates were considered. KEYWORDS: magnetite, nano, Douglas fir biochar, phosphate, adsorption, breakthrough, XPS

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INTRODUCTION Phosphate is a major cause of eutrophication.1 It is often the limiting nutrient for explosive algal growth.1 Concentrations as low as 100 μg/L can cause eutrophication,1 the dense growth of blue green algae and hyacinth-like plants resulting in short and long-term ecological effects.2,3 Cyanobacterial blooms can release soluble neurotoxins and hepatotoxins, killing fish or livestock when ingested and causing severe hazardous health effects in humans.4−6 A typical raw domestic wastewater has a total phosphorus (P) concentration of approximately 10 mg/ L.7 Maximum P discharge limits in municipal treatment plant effluent are l.0 mg/L P discharging into the Upper Great Lakes and 0.5 mg/L P into the Lower Great Lakes in the United States.8 The Baltic and Mediterranean Seas are impacted by treated and untreated P runoffs.9 Dead zones due to hypoxia © 2019 American Chemical Society

exist along the U.S. East Coast and within the Gulf of Mexico because of fertilizer runoff into rivers and streams.10 Iron (Fe)-based nanomaterials adsorb phosphate well.11,12 They are abundant and include magnetite (Fe3O4),12 hematite (Fe2O3),13,14 goethite (α-FeOOH),11 and akaganeite (βFeOOH).11 All except akaganeite adsorb similar amounts of phosphates (∼5.1 P mg/g). Akaganeite has the highest capacity (59.6 P mg/g) because of its tunnel-shaped morphology, which provides a high surface area, but is structurally far more fragile,15 and goethite exhibits poor recyclability (13−14%).11 Received: March 6, 2019 Accepted: May 10, 2019 Published: May 10, 2019 3467

DOI: 10.1021/acsanm.9b00430 ACS Appl. Nano Mater. 2019, 2, 3467−3479

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ACS Applied Nano Materials Scheme 1. Illustration of MBC Preparation in This Work

surface adsorbs negligible phosphate, enabling future comparisons of the various methods of preparing, depositing, and dispersing magnetite (or other oxides) on the kinetics and capacities of phosphate uptake. Phosphate adsorptions at different concentrations, temperatures, and pH values are reported and supplemented by a mechanistic study of pHdependent phosphate sorption by X-ray photoelectron spectroscopy (XPS).

In addition to iron oxides, sorptive phosphate removal has been reported for fly ash,16 biochar (BC),17 activated carbon,18 and layered double hydroxides (LDHs).19 Sorbents including calcite,20 alunite,21 red mud,22 and dolomites have also been employed. Ion exchange23 and electrocoagulation1 have also been used for phosphate removal. The current methods are costly and not efficient enough for use in eutrophicationaffected areas. Direct application of LDH, nanooxides, or hydroxides in sorption is limited by particle agglomeration, nanotoxicity, weak mechanical strength, and high pressure drops in fixed-bed columns.24 Zeolites, bentonite, diatomite, sand, resin, activated carbon, and tea waste have been used as supports to reduce nanoparticle agglomeration. Recently, we employed BC dispersants to generate composite adsorbents with small particle phases dispersed on larger carrier particles with improved sorption features.25 BCs are excellent widely available sorbents for metal cations, pharmaceuticals, dyes, and organic sorbates.26,27 Nano-Fe3O4-modified biochar (MBC) used here for phosphate adsorption was previously used to remove both organic pollutants and heavy metals from water,28−31 where the BC surfaces were the primary adsorption locations. The deposited Fe3O4 enabled the magnetic separation of spent adsorbent from remediated water. With MBC, however, the magnetite particles serve as the adsorption site and permit magnetic separation. To the best of our knowledge, the importance of dispersing nano-Fe3O4 particles on a highsurface-area BC to permit flow through columns and higher sorption capacity, along with providing additional carbonaceous surfaces for the removal of additional sorbates, has not been highlighted before. Herein, we report MBC’s surface-dispersed Fe3O4 nanoparticles as the adsorption site for aqueous phosphate removal and remediation of real eutrophic water samples. Magnetite is robust, and MBC was easily recycled. A high magnetite surface area was achieved by dispersing primary nanoparticles onto the high-surface-area BC while reducing the extent of magnetite particle aggregation. Rapid phosphate uptake and robust performance in fixed-bed column sorption were achieved along with higher phosphate capacities than those previously reported for neat iron oxide particles in the literature. The BC’s

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METHODS

Preparation of Magnetic Douglas Fir Biochar (MBC). MBC was prepared by iron oxide wet precipitation onto commercial Douglas fir BC (Biochar Supreme Inc. Environment Ultra), hereafter designated as nonmagnetic BC (NBC), by a method described in our previous studies (Scheme 1).29,30 Briefly, NBC (particle size 1−2 mm, surface area 695 m2/g, and porosity 0.264 cm3/g) was mixed in an iron(III) chloride and iron(II) sulfate solution at ∼70 °C. Then precipitation of magnetite was triggered by the dropwise addition of 10 M NaOH, maintaining the pH at 10, followed by aging for ∼24 h. The precursor NBC was produced by a proprietary process in an updraft wood gasifier at ∼900 °C for short (1−30 s) residence times from wet Douglas fir chips. The weight of iron salts versus the BC weight might appear high, but Fe3O4 has a much higher density (∼5.15 g/cm3) than the highly porous lower particle density (∼0.4 g/ cm3) of BC. A substantial (∼30 wt %) coverage of tiny dispersed iron oxide particles on the BC surface was sought to provide a high Fe3O4 surface area. Phosphate Adsorption Studies. A 1500 mg/L phosphate stock solution was made by dissolving potassium phosphate dibasic anhydrous, KH2PO4 (Sigma-Aldrich), in deionized water (MilliporeQ water system). The pH was adjusted using either 0.1 M HCl or 0.1 M NaOH, and pH measurements employed a pH meter (Hanna Instruments HI 2211 pH/ORP meter). Batch sorption studies on NBC and MBC were performed by varying the phosphate concentrations from 5 to 1500 mg/L at 25, 35, and 45 °C, using deionized water from a Millipore-Q water system at pH 3. A weighed amount of NBC or MBC was added to 40 mL amber glass vials each containing 25 mL of adsorbate solutions of different phosphate concentrations. Adsorption studies were carried out inside a static water bath (BUCHI 461), and a vortex mixer (Scientific Industries T3-1280) was used to stir the samples for 1−5 min. Equilibrium was achieved within only 2 min for MBC and NBC. No further uptake occurred between 2 and 5 min. MBC was removed from the solution magnetically, while NBC was removed using filtration (Whatman filter paper No. 1). The phosphate concentrations remaining in the filtrate were determined by the ascorbic acid method32 using a 3468


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ACS Applied Nano Materials Table 1. Elemental and Proximate Analyses and Surface-Area Data for NBC and MBC sample

%C

%H

%N

%S

%O

% ash

% Fe

surface area (m2/g)

pore volume(cm3/g)

NBC MBC

74.60 56.70

1.90 1.40

0.12 0.11

0.03 0.05

20.95 9.34a

2.40 32.40

0.10 26.20

695.1 312.6

0.264 0.135

a

Combustion analysis does not account for the O present in the iron oxide phases of MBC. The percentage of Fe is reported for the digested adsorbent. Assuming that all of the Fe in MBC is Fe3O4, then the weight fraction of deposited Fe3O4 would be ∼30%.

Figure 1. SEM micrographs of (a) NBC, (b−e) MBC, and (f) MBC after phosphate sorption. TEM micrographs of (g) MBC and (h) MBC after phosphate sorption. double-beam UV−visible spectrophotometer at 830 nm. The phosphate adsorption per unit of adsorbent (qe) was calculated using eq 1. qe =

V (C0 − Ce) M

MBC and to observe any changes in the iron oxide structure after sorption of phosphates using a Rigaku Ultima III [using Cu Kα (λ = 1.54 Å)]. XPS measurements were conducted with a Thermo Scientific K-Alpha XPS system equipped with a monochromatic X-ray source at 1486.6 eV, corresponding to the Al Kα line, with a spot size of 400 μm and a maximum penetration depth of 100 Å, with more electrons ejected closer to the surface, providing high surface sensitivity. MBC samples equilibrated with phosphate solutions at pH 1, 3, 7, 10, and 13 were used for XPS analysis. The carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S) contents of NBC and MBC were measured by combustion analysis using a CHNS elemental analyzer (EAI CE-440). The ash content was determined by heating at 650 °C for 15 h in an open-top porcelain crucible in a muffle furnace. The content of organic O was calculated using the equation O % = 100 − (C + H + N + S + ash). The weight percentage of Fe in both NBC and MBC was determined using atomic absorption spectroscopy (AAS; Shimadzu AA-7000). Acid digestion was performed on 0.1 g of each adsorbent using 50.0 mL of 1:1 95% H2SO4/70% HNO3. Fe was dissolved from the oxidizing BC into the acid for 24 h (70 °C) with stirring and then diluted 5-fold with deionized water prior to AAS analysis. Magnetic hysteresis measurements were carried out on a Lake Shore 7304 vibrating sample magnetometer. The magnetic properties of NBC and MBC are represented by the plots of magnetization (M) against the field strengths (H), giving a hysteresis loop. The saturation magnetization was measured from the hysteresis curve.

(1)

Here, C0 and Ce (mg/g) are the initial and equilibrium phosphate concentrations in the solution, V (L) is the solution volume, and M (g) is the total mass of the adsorbent added. All experiments were carried out three times, and the standard deviation error bars are from these three replicates. Characterization. NBC, MBC, and MBC after phosphate sorption were characterized. MBC samples with sorbed phosphate were obtained from the batch equilibrium adsorption in which 0.1 g of BC was vortexed in 25 mL of solution containing 1000 mg/L phosphate at pH 3. The N2 Brunauer−Emmett−Teller (BET) specific surface area was determined using a N2 adsorption isotherm at ∼77 K (Micromeritics Tristar II Plus) and the Dubinin−Astakhov equation P0 P

( ), where a denotes the amount of gas

[log a = log a0 − D log n

adsorbed per unit mass of adsorbent (mol/g), a0 is the micropore capacity (mol/g), D is a constant, P is the equilibrium pressure, and P0 is the saturation vapor pressure of the adsorbate at temperature T (K)]. Density functional theory (DFT) was used to calculate the 44000a micropore volume [W0 = ρ 0 , where W0 is the limiting micropore

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volume (cm3/g), a0 is the micropore capacity (mol/g), and ρ is the density of adsorbed gas (g/cm3)].33 BC morphology and surface textures were observed by scanning electron microscopy (SEM) using a JEOL JSM-6500F FE instrument at 5 kV coupled with a Zeiss EVO 40 scanning electron microscope containing a Bruker energydispersive X-ray (EDX) system. Transmission electron microscopy (TEM) studies of MBC and NBC were obtained using a JEOL model 2100 transmission electron microscope operated at 200 kV. TEM− EDX analysis was carried out using an Oxford X-max-80 detector. Xray diffraction (XRD) analysis was performed to identify the iron oxide phase crystallographic structure upon precipitation to make

RESULTS AND DISCUSSION Elemental and Proximate Analyses and Surface-Area Data of NBC and MBC. Combustion elemental analyses of MBC (Table 1) found lower C and H contents versus NBC because of the significant weight fraction of MBC that deposited Fe3O4 comprises. The amount of O in MBC drops for the same reason and because the iron oxide O is not determined by combustion. The NBC used originated as a byproduct from the timber industry gasification of wet waste 3469


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ACS Applied Nano Materials

Figure 2. (a) XRD spectra for NBC, MBC, and MBC after phosphate sorption at pH 3. TEM−EDX elemental mapping images of C, O, Fe, and P elements of (b) NBC, (c) MBC, and (d) MBC after phospate sorption. Magnetic moments of (e) BC and NBC and (f) MBC at 10 and 300 K by VSM.

wood (Douglas fir), produced at residence times of 1−30 s at 900 °C within an updraft gasifier. This produces a very highsurface-area BC (695 m2/g) with a high pore volume (0.264 cm3/g). The high C/H (39.3) ratio and observed O/C (0.28) ratio of NBC are attributed to the pyrolytic loss of oxygenated and hydrogenated functionalities at high temperature. Precipitating iron oxide on NBC to form MBC leads to partial pore blockage where deposited nanoparticles and their aggregates block access of N2 into a portion of the BC micropores and ultramicropores, which intersect with the char surface. This causes a loss of ∼55% of its original surface area (from 695 to 313 m2/g) and almost half of its pore volume (from 0.264 to 0.135 cm3/g). This occurs despite the addition of the surface area generated by the small magnetite particles.29 It well-known that magnetite nanoparticles are not porous,12 and measured surface areas in the literature correspond to the surface areas of hard particles with their observed diameters. The ash content in NBC is primarily composed of stable oxides and carbonates formed from sodium, potassium, magnesium, calcium, and iron salts in the wood feed.34 MBC had a high ash content because of added iron oxide deposits.

Surface Morphology. Quasi-spherical magnetite particles and their agglomerates were formed on the NBC surface to generate MBC during the precipitation of iron oxide (Figure 1). Both SEM and TEM micrographs (Figure 1) show quasispherical-shaped particles with an average fundamental particle size of 16.7 ± 3.5 nm for MBC. The particle sizes were determined using the ImageJ particle sizing software. Some primary particles are aggregated. A previous similar preparation of magnetite nanoparticles using (CH3)4N+OH− as the base, versus NaOH used here, reported an average particle size of 12 ± 2 nm with a specific surface area of 117 m2/g. Because MBC is a hybrid adsorbent, only its total surface area can be measured, belonging to both magnetite particles and the porous BC. The Fe3O4 primary particles grew to ∼16.7 nm diameter with a 69.5 m2/g calculated surface area on MBC. Using FeCl3 as the Fe3+ precursor for precipitation primarily produces quasi-spherical-shaped iron oxide particles.35 The effect of NBC surfaces on the magnetite nucleation and growth during precipitation is not known. The SEM images illustrated that iron oxides deposit into and on NBC lumen, while creating blockages of some micropores. 3470


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Figure 3. (a) Effect of the solution pH on the phosphate adsorption on NBC and MBC (the phosphate concentration was 25 mg/L, the total volume was 25 mL, the mass of NBC and MBC used were 0.1 g, and the fractional speciation composition was plotted against the pH data for phosphoric acid).57 (b) Langmuir and (c) Freundlich adsorption isotherms on MBC for phosphate at 25, 35, and 45 °C (the adsorbent concentration was 10 g/L at pH 3). (d) Continuous-flow fixed-bed column breakthrough curve for phosphate adsorption onto MBC. Here, C is the effluent concentration at time t, and C0 is the initial concentration (mg/L) of the effluent. (e) Comparison of the phosphate adsorption from distilled water versus lake water, both at the lake water’s pH 6.4 and also at pH 3.0 at 25 °C (MBC concentration of 0.4 g/L; phosphate concentration of 25 mg/L). Error bars are the standard deviation of three replicates.

taking the average sizes of peaks D220, D311, D400, D422, D511, and D440. These sizes are 17.5 ± 0.8 nm for MBC and 17.1 ± 0.8 nm for phosphate-laden MBC and quite close in size to the particles (16.7 ± 3.5 nm) determined by SEM. The lattice parameter (a) and interplanar spacing (dhkl) values were estimated for the most intense 311 phase using the Bragg λ a equation39 [dhkl = 2 sin θ = 2 2 2 , where λ is the wave-

NBC, MBC, and phosphate-laden MBC were studied by XRD (Figure 2a). The broad peak at 2θ = 22.7° for NBC comes from the deformed cellulose crystal structure. The cellulose crystallinity is reduced during biomass pyrolysis.36 The XRD peak pattern for MBC corresponds to that of the precipitated iron oxide particles, which correspond to magnetite (Figure 2a). The locations (2θ) and intensities of the diffraction peaks are consistent with the standard pattern for magnetite (JCPDS 79-0417). The major peak at 35.45° is for the crystalline plane of Fe3O4 with Miller indices of (311). Other peaks were observed at 30.10° (220), 43.08° (400), 53.45° (422), 56.98° (511), 62.57° (440), and 74.02° (622).37 The crystallite size was analyzed using the Debye−Scherrer equation,38 [Dhkl = 0.9λ/(β cos θ), where λ is the wavelength used in the X-ray experiment (nm), β is the full-width at halfmaximum value (rad) for XRD lines, and θ is half of the diffraction angle of 2θ]. The crystallite size was determined by

h +k +l

length of X-ray (nm), θ is half of the diffraction angle of 2θ, and h, k, and l are the Miller indices]. The lattice parameter (8.391 nm) and interplanar distance (0.2530 nm) values determined for the iron oxide deposited on MBC are consistent with those for bulk magnetite (JCPDS 79-0417; a = 8.394 nm and d311 = 2.531). This unequivocally confirms that the precipitated iron oxide is magnetite. Both the SEM and TEM images obtained after phosphate adsorption (Figure 1f,h) show a change in the Fe3O4 particle 3471


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exhausted adsorbent to be readily collected with a magnet at a wide range of temperatures. Effect of the Solution pH on the Phosphate Removal and Adsorption Mechanism. The effect of the solution pH on phosphate adsorption at 25 °C by both MBC and NBC was studied from pH 2 to 10 using a dose of 0.4 g/L (Figure 3a). Figure 3a also shows phosphate speciation from pH 0.5 to 14. The phosphate concentration was 25 mg/L. The percentage of phosphate adsorbed onto MBC decreased from ∼100 to 18% as the pH rose from 2 to 10. In contrast, NBC removed hardly any phosphate over this pH range. Clearly, it is the iron oxide surface that adsorbs the phosphate, in agreement with the TEM−EDX results. The phosphate interaction with MBC (hence, on magnetite) is pH-dependent. The magnetite surface iron hydroxide functional groups (FeOH) are increasingly protonated as the pH becomes more highly acidic. The pKa1 value of magnetite is 5.6.42 The protonation increases as the pH progressively drops below 5.6 (eq 2). This makes the surface more attracting to H2PO4− groups. Also, the protonated hydroxyl bound to Fe is a better leaving group during H2PO4− chemisorption. The H3PO4 dissociation constants are pKa1 = 2.12, pKa2 = 7.21, and pKa3 = 12.67. In the pH range 0 to 4.7, dominant P species are H3PO4 and H2PO4−, pH range 4.7−9.7 are H2PO4− and HPO42−, and pH 9.7−14 are HPO42− and PO43− (Figure 3a).

morphology or texture. The surfaces of the magnetite particles and their aggregates after phosphate adsorption appear rougher and darker. SEM micrographs show two different types of quasi-spherical particles and their aggregates. The brighter particles and aggregates might have sorbed less or no phosphate, while the darker particles and aggregates could be more phosphate-laden, where surface conductivity differences could influence the differences in charging. Their calculated (ImageJ) average size is 18.7 ± 3.1 nm. The surface textural changes observed for phosphate-laden MBC versus MBC are not likely due to magnetite morphology changes because the SEM appearance is highly dependent on the surface conductivity. This leads to a charging effect related contrast variation in the SEM images.40 No changes occurred in the XRD spectrum (Figure 2a) after phosphate adsorption. Thus, no iron or hydrogen phosphate mineral phases formed, which could generate XRD peaks. This is consistent with monolayer chemisorption or monolayer chemisorption with additional hydrogen phosphate salt deposition held by weaker hydrogen bonding on top of the chemisorbed layer. One possibility is that small amounts of Fe dissolution could occur at pH 3, and the resulting Fe3+ ions could react with H2PO42− to form poorly crystalline or amorphous insoluble iron phosphate or hydrogen phosphates, which precipitate at the Fe3O4 particle surfaces.12,41 It is widely accepted that phosphate adsorption on magnetite occurs only at Fe3+ octahedral (Oh) sites on the (111) plane.12 The XRD peak of this (111) plane where phosphate chemisorbs42 appears only as a very weak peak, both before and after phosphate sorption. Thus, it could not be further analyzed. Phosphate is adsorbed only at the magnetite surface, so this will not cause changes in the phase structure, XRD peak positions, or their intensities.12 Very small, poorly crystallized iron phosphate precipitates are not adsorbed complexes but might enhance phosphate removal from solution (see section 3.6 and Figures S8 and S9). TEM−EDX elemental mapping images (Figure 2b−e) of the MBC after phosphate sorption exhibited the presence of 1.6 wt % P as purple dots on the MBC surface region. P was only present on the iron oxide particles within the ability of EDX to discriminate locations. NBC and MBC surfaces away from iron oxide particles contained either negligible amounts or no P ( 0.99) than the three-parameter Sips, Redlich−Peterson, and Toth models (Table S1). The Langmuir plots were used to calculate the maximum Langmuir monolayer adsorption capacities (Q0) at pH 3 on MBC of 91.3, 91.0, and 90.0 mg/g at 45, 35, and 25 °C, respectively. Thermodynamics of Adsorption. Phosphate adsorption onto MBC varied only slightly from 25 to 45 °C. A possible physical reason for this tiny rise in the capacity with increasing temperature might be increased diffusion into char pores below the surface where some Fe3O4 may have formed.43 Neglecting this possibility, the Gibbs free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) were calculated using van’t Hoff’s equation for both the Langmuir and Freundlich constants. First, the isotherm constants for each temperature were converted to dimensionless constants (Kads) via multiplication by the density of the liquid phase (∼10 × 106 mg/L) and also considering the adsorption intensity (n). The ΔG0, ΔH0, and ΔS0 values (Table S2) from the Langmuir model isotherm constants were −18.8 to −20.5, 5.5, and 0.08 kJ/mol, respectively. The negative ΔG0 values indicate a spontaneous adsorption, and its magnitude increased slightly with the temperature. The positive ΔH0 confirms an endothermic adsorption. A positive value of ΔSο and low positive ΔH0 suggest that the adsorption is an entropy-driven process. Equilibrium was achieved in all cases within 2 min. This shows that MBC has very fast adsorption kinetics, consistent with our previous findings with organic sorbates.29

thus decreasing the phosphate adsorption as the pH rises. The crystal chemistry and DFT calculations presented by Sherman and Randall46 nicely defined the possible surface structure of arsenates and their conjugate acids chemisorbed onto various iron oxides (goethite, ferrihydrite, hematite, and lepidocrocite) as bidentate corner-sharing (2C), edge-sharing (2E), and monodentate corner-sharing (1V) complexes. These have chemical and structural analogies with phosphates. Similar surface complexes are plausible for the chemisorption of phosphate species with magnetite surfaces. The possible chemisorbed phosphate surface species versus solution pH are shown in Scheme 2. The 3D octahedral structure of magnetite, where at the surface Fe has a hydroxyl group. This is represented in Scheme 2 as structure 1. These chemisorbed phosphate species have been widely proposed from several experimental and theoretical studies.12,47−51 Monodentate neutral 2, monodentate monoanion 3, and dianion 4 can exist along with bidentate neutral 5 and bimetallic bidentate anion 6. Also, bidentate monometallic complexes at edges or corners are possible, but their stabilities have not been established. Sorption Equilibrium Studies and Modeling. Phosphate sorption equilibrium studies at 25, 35, and 45 °C were conducted on MBC at pH 3 (the MBC doses were 10 g/L). The common two-parameter Langmuir52 and Freundlich53 isotherms (Figure 3b,c) and three-parameter Sips,54 Redlich− Peterson,55 and Toth56 isotherms (Table S1 and Figure S2) were used. The parameters from the experimental data fitting to these models were evaluated using nonlinear regression (Origin 2016 software). Table S1 summarizes all five of the 3473


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ACS Applied Nano Materials Table 2. Comparison of Phosphate Adsorption Capacities adsorbent

temp (°C)

brown marine macroalgae BC porosity-enhanced MgO/BC MgO/BC nanocomposite from sugar beet tailingsa DSTC (digested sugar beet tailing BC) STC (raw sugar beet tailing BC) BC/Mg/Al-assembled nanocompositesa

20

2:1 Mg/Al-LDHs sugar cane leaf BC composite 3:1 Mg/Al-LDHs sugar cane leaf BC composite 4:1 Mg/Al-LDHs sugar cane leaf BC composite nanosized magnetite/orange peel BC composite Fe-impregnated granular sludge BC waste-derived fungal biomass magnetite BC MBC

22 55 10 20 30 25

25 25 35 45

pH

equil time

BET surface area (m2/g)

adsorption capacity (mg/g)

2.4 56.4 70.0 336.0 2.6 14.1

3.3 20.0 835.0

48 h

9.95 9.45 6

24 h 24 h 24 h

3

1h

3

20 h 24 h 24 h 2 min

10.17 11.41 12.25 19.4 10.4 53.0 312.6

335.0 480.0 727.0 53.4 72.1 81.8 1.2 23.9 91.3 91.0 90.0

ref 61 62 63 64

65

66 67 68 this study

These high phosphate removals resulted from stoichiometric reactions between Mg2+, Al3+ and HPO42−,H2PO4− and PO43− to form water insoluble compounds within the BC or at its surface. a

Column Sorption Breakthrough Studies. Batch adsorption isotherm data do not provide all of the needed information for flow systems because of the kinetic and equilibrium factors.58 Fixed-bed continuous-flow tests are required for obtaining design models to scale-up for continuous water treatment.58 Designing adsorption columns requires predicting how much effluent the bed can treat or how long the bed will last before regeneration is necessary. The packed-bed performance is described by its breakthrough curve.58,59 Figure 3d shows the breakthrough curve observed for a 100 mg/L phosphate solution (pH 3 and 25 °C) passed through a laboratory-scale, glass-wool-plugged fixed-bed column (length = 2 cm and diameter = 1.1 cm) packed with 1.0 g of MBC. The column was packed using a warm mixture of MBC/water and removing any low-density MBC that floats due to trapped air. The mixture was slowly added to the column through a glass funnel. The MBC packed column was vibrated to ensure an even packing and also to remove air bubbles. Approximately 1.5 L of a phosphate solution was passed through the column during the experiment, while the flow rate was maintained at 1.5 mL/min. The breakthrough time was reached at C/C0 ∼ 0.01, and then the concentration ratio (C/ C0) rose gradually to 1.0. This is the end of the breakthrough curve where the bed is judged to be ineffective.58 The masstransfer zone width and shape of the breakthrough curve depend on the adsorption isotherm, flow rate, adsorbate masstransfer rate, and diffusion in the pores.58 The ratio between the usable and total time (tu/tt) is the fraction of the total bed capacity or length utilized up to the breakthrough point. Hence, for a bed length of Ht, Hb is the length of the bed used up to the break point. The ratio of Hb/Ht is equal to the ratio of tu/tt (eq 3).58 Hb = (tu/t t)Ht

tt =

∫0

C yzz jj jj1 − z dt j C0 zz{ k

∞i

C yzz jj jj1 − z dt j C0 zz{ k

(5)

The usable and unusable bed lengths were 1.64 and 0.36 cm, respectively. The fraction of the total capacity used prior to the breakthrough point was 0.82, and the capacity at saturation was 82.5 mg/g, which is ∼90.0% of the Langmuir isotherm capacity for batch sorption at 25 °C. This ratio between the usable and total column lengths (0.82) is approximately equal to the fixed-bed column adsorption capacity to the batch sorption capacity ratio (0.92). Therefore, the phosphate sorption kinetics and equilibrium conditions during column and batch sorption processes have similar characteristics. Thus, these process differences will not greatly affect the sorption performance with respect to the equilibration method and technique (vortex mixing, mechanical agitation, or elution rate). This enables MBC, prepared in the laboratory scale for batch adsorption, to be used for scale-up applications without losing much of its original performance. This model’s predicted capacity was then verified by mass-balance calculations. A total of 150 mg of phosphate was passed through the column, and 55.2 mg of phosphate was eluted from the column. Hence, 94.9 mg of phosphate was adsorbed by 1.0 g of MBC in the column. This corresponds to a 94.9 mg/g capacity. This capacity is comparable to the breakthrough model’s predicted capacity of 82.5 mg/g. Application of MBC for Lake-Water Treatment. The efficiency of MBC in an authentic environmental water system was investigated by collecting water from Oktibbeha County Lake, Starkville, MS. Lake-water samples were filtered through MF-Millipore (0.22 μm, GSWP04700) filter paper. Its pH was ∼6.4. Lake- and distilled-water samples were spiked with 25 mg/L phosphate at 25 °C. MBC doses of 0.01 g were then added to each of these 25 mL samples, followed by vortexing for 2 min. The phosphate levels were determined by the ascorbic acid method32 using UV−visible spectrophotometry. Lake and distilled water were subsequently adjusted to pH 3, and the above procedure was repeated again. Figure 3e compares the phosphate adsorption capacities using MBC in lake and distilled water. The phosphate

(3)

The times tu and tt were calculated by integrating the area above the breakthrough curve using eqs 4 and 5. tu/b =

∫0

tb i

(4) 3474


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Figure 4. HR-XPS spectra of P 2p, O 1s, C 1s, and Fe 2p for NBC, MBC, and phosphate-laden MBC (MBC+P) at different initial solution pH values. Because of the complex nature of Fe 2p, the curve-resolved peaks are not labeled here (see Fe 2p spectra in Figure S13 for MBC and S14 for MBC after phosphate was sorbed at pH 3).

S13 and S14 (Fe 2p for MBC before and after phosphate uptake, respectively). To interpret the XPS and identify the possible P compounds present on MBC (chemisorbed species, hydrogen-bonded physisorbed multilayers, and iron phosphate stoichiometric compounds), a detailed description of additional experiments and calculations that were performed is given in the Supporting Information. Space limitations required this placement. The general approach followed is summarized below. (1) The specific surface area of magnetite nanoparticles on MBC was calculated using the SEM particle diameters (section 3.3 and Table S3). (2) The phosphate monolayer capacity was estimated in three ways; (a) from magnetite’s specific surface area (m2/g) and the area of the phosphate ion, (b) from the specific surface area (m2/g) of magnetite and the maximum phosphate monolayer capacity on magnetite reported in the literature and, (c) from the total surface area (m2) of magnetite and the area of the phosphate ion (section 3.4 and Table S4 and Figure S5). (3) Possible hydrogen-bonded phosphate multilayers were stripped from MBC by rinsing experiments. The phosphate removed and that remaining were determined and compared with the capacities estimated in part 2 (section 3.5 and Figures S6 and S7). (4) Iron dissolution and phosphate stripping experiments from MBC investigated the possible formation of water insoluble of iron phosphate stoichiometric compounds by Fe2+/Fe3+ dissolution from magnetite, followed by reaction with H2PO4−, HPO42−, and/or PO43− and precipitation back onto MBC (section 3.6 and Figures S8 and S9). The XPS P 2p signal for phosphate-laden MBC was strongest for samples prepared at pH 1 and 3 and decreased

adsorption capacities in lake water decreased by 25.4% at pH 3 and by 48.1% at pH 6.4, compared to the capacities in distilled water. This decreased adsorption capacity in lake water is due to other interfering adsorbates.60 Comparison of Phosphate Adsorption Capacities. Table 2 compares the MBC adsorption capacities and remarkable phosphate uptake rates on MBC versus other adsorbents in the literature. Rows 3 and 5 of Table 2 (MgO and Mg/Al/BC) are examples of stoichiometric uptake by the formation of insoluble phosphates and not true chemisorption. The high phosphate capacities of MBC compare well with those of other adsorbents. Considering the low potential MBC cost and its exceptionally rapid uptake rates, this adsorbent is very promising. Also, magnetic removal allows batch applications with fast magnetic removal that avoid the need for filtration steps. Adsorption Mechanism and XPS Data. The pHdependent adsorption mechanism was investigated utilizing the XPS data obtained for NBC, MBC, and phosphate-laden MBC at different initial pH values. XPS probes the elemental composition and chemical oxidation states of surface and nearsurface species. Low-resolution wide-scan (LS) spectra of NBC, fresh MBC, and phosphate-laden MBC are illustrated in Figure S4. Significant changes appeared after phosphate adsorption; a peak appeared at a 134.2 eV binding energy (BE) in this LS spectra, which corresponds to the ejected 2p electrons from P.12 The high-resolution XPS (HR-XPS) data were obtained and resolved by curve resolution for P 2p, O 1s, C 1s, and Fe 2p on NBC, MBC, and phosphate-laden MBC as a function of the pH from 1 to 13 to provide chemical insight (Figure 4). Example, expanded, deconvoluted, and assigned HR-XPS spectra are shown in Figures S10 (P 2p for MBC after phosphate adsorption), S11 (O 1s for MBC after phosphate adsorption), S12 (C 1s for MBC after phosphate uptake), and 3475


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rose from 1 to 13, following the observed adsorption pH dependence (Figure 3a). The amount of phosphate adsorption drops due to electrostatic repulsion as the magnetite surface becomes more negatively charged, and only HPO42− and PO43− are present in the solution at pH 11−13. The BE trends are similar to those reported for arsenite species adsorbed onto magnetite-doped activated C fiber as the pH increases.70 Magnetite’s surface hydroxyl groups increasingly deprotonate as the pH rises, so adsorption on MBC drops because of electrostatic repulsion of the H2PO4−, HPO42−, and PO43− species in solution from pH 4 to 13. Hence, the dominant surface species of adsorbed P detected in XPS should be Fe− O−PO3H−, Fe−O−PO32−, and Fe−O−PO33− and their bidentate analogues. As the pH rises above the PZC of MBC, the surface-chemisorbed Fe−O−P atomic percentages decrease, and the complete disappearance of the Fe−O−P peak at pH 13 can be attributed to the strong negative surface repulsion of negative HPO42− and PO43−. The high-resolution O 1s envelope for MBC and MBC after phosphate adsorption was resolved into four signals (peaks I− IV), which are listed and assigned in Table S7 and shown for pH 3 in Figure S11. These occur in the range 530.0−533.7 eV. The highest energy contribution (533.0 −533.7 eV) fitted is assigned for surface carbonates and carboxylate groups at the MBC.71 The BEs in the range of 531.6−532.5 eV can be attributed to P hydroxy,12 Fe hydroxy (Fe2+−OH or Fe3+− OH), and ether71 groups, while the BEs in the 530.5−531.3 eV range arise from phosphonyl,12 carbonyl,71 and surface iron− oxygen−phosphorus (Fe2+−O−P or Fe3+−O−P)12 functionalities. The lowest BE contribution (530.0−530.5 eV) occurs from the iron−oxygen (Fe3+−O−Fe3+, Fe2+−O−Fe3+, or Fe2+−O−Fe2+) surface region functionalities.12 The surface atomic percentages for the P−OH, PO, and Fe−O−P peaks increase as the pH drops from 13 to 3 and 1. This agrees with the higher phosphate species uptake versus pH shown in Figure 3a and is similar to the results reported by others.12,72 Further, the atomic percentages from the Fe−O−Fe peak (530.0−530.5 eV) decreased upon going from pH 7 to 3 and 1 because of the marked rise in adsorption due to magnetite surface coverage by phosphate at the lower pH values. Conversely, as the pH rises, the Fe−O−Fe atomic percentage of O goes up because less phosphate is adsorbed onto magnetite. It should be noted that XPS is a very surfacesensitive method, with the predominant contribution of the signals arising from the outer 10 Å region of the surface. The atomic percentage of C on MBCs decreased versus NBC in their respective C 1s HR-XPS spectra because of magnetite deposition (Table S8).72 Magnetite covers portions of the BC surface, reducing the net ejection of electrons from the carbonaceous surface. Because the amount of P that adsorbs on the carbonaceous surface (of NBC and MBC) is negligible (Figures 2a,c and 3a) and the BC surface area is high, C 1s peaks modified by such P adsorption will not be observed. The high-resolution C 1s spectra were curveresolved into four peaks, with the higher BE resolutions being very broad (Figure S12). These broad resolutions obviously involve more than one type of C oxidation state. For example, peak I converts both CO32− present from calcium(2+) and magnesium(2+) carbonates formed during the pyrolysis and the −CO2H, −CO2R, and possibly −COO− functions. Similarly, peak II is assigned to carbonyl C atoms. Overlapping of the peak I and III regions with that of peak II suggests the presence of many carbonyl variations (quinones,

for samples made at pH 7 and 10. This was expected from the plot of the percent removal vsersus pH (Figure 3a). A lower P 2p signal-to-noise ratio was found when sorption occurred at pH 13. At pH 13, a substantial negative surface charge on magnetite will lead to interaction of the predominant PO43− solution species and HPO42−, which is the only other P species present at this pH. At pH 13, another lower BE peak is resolved at ∼131.5 eV. Peak II is centered at 134.9 eV (pH 10) and disappears at pH 13. These changes must be analyzed versus the known XPS BEs of the possible phosphate species, which could be chemisorbed on magnetite and other important P model species (Table S5). Each P 2p peak in Figure 4 was curve-resolved into two peaks (see Figure S10 for the P 2p peak in the pH 3 sample). The peak I (H3PO4 analogue) BE maximum shifts from ∼134 to 134.5 eV (Table S6) upon going from pH 1 and 3 to pH 7. This peak broadens toward the lower BE direction as the pH increases from 1 to 13. Peak II corresponds to the BE of the P 2p states, resulting from surface-bound Fe−O−P monodentate and (Fe−O)2P bidentate species (Figures 4 and S10 and Table S6).12 To the best of our knowledge, the influence of the degree of protonation on the XPS peak BEs of the chemisorbed (Fe−O−PO3Hx)(2−x)− functions was never reported in the literature. Therefore, our interpretation of the surface-chemisorbed species protonation states employed the model BEs of potassium, sodium, magnesium, and calcium phosphates listed in Table S5. Varying the solution pH will alter aqueous phosphate chemical speciation (shown in Figure 3a). Hence, it will also alter the surface speciation and BEs of the surface-chemisorbed P species (e.g., Fe−O−PO3H2, Fe−O−PO3H−, and Fe−O− PO32− or their related bidentate-bound analogues). The BEs vary as the energy of the P 2p orbitals is influenced by the P electron density, which is influenced by all atoms bound to P. The crystal field applied to P and its asymmetry influences the BE required to be overcome during ejection of a 2p electron. The negative formal charge on phosphate O atoms increases the P 2p orbital energy levels, lowering their BEs. Upon protonation of these negative O atoms, the electron density around P will drop slightly and the P 2p BE rises. At pH 13 in solution, PO43− is the major solution ion, while at pH 10, HPO42− dominates. At pH 4−6, H2PO4− dominates (see Figure 3a). This same trend is expected for chemisorbed surface species Fe−O−PO3H2, Fe−O−PO3H−, and Fe−O− PO32− (and their bidentate surface analogues). Peak shifts to lower BE upon deprotonation were reported for sodium hydrogen phosphates [NaH2PO4 (134.7 eV) and Na2HPO4 (133.0 eV) versus Na3PO4 (132.5 eV)].69 The experimental P 2p peak BEs assigned for surfaceadsorbed Fe−O−P phosphate derivatives are shown in Figure 4 and Table S6 as the pH rises from 1 to 13. These values were obtained from the broadened peaks of monodentate and bidentate species, where the Fe−O−PO3H2, Fe−O−PO3H−, Fe−O−PO32−, (Fe−O)2−PO2H, and (Fe−O)2PO2− individual P 2p components of these peaks are not resolved. As the pH goes up, the P 2p BEs decrease from 133.9 eV (pH 1) to 133.5 eV (pH 13). This is consistent with Fe−O−PO32− dominating the surface speciation at high solution pH values. As the pH drops, more Fe−O−PO3H− must form, and it dominates until at the most acidic very low pH values Fe−O− PO3H2 dominates. Thus, as protonation occurs, the BE values increase. Also, the intensity of the P 2p peaks significantly dropped from 1.36% to 0.26% as the phosphate solution pH 3476


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reclamation. Magnetic removal avoids slow filtration in batch applications, and the BC phase enables the adsorption of secondary contaminants. pH-dependent surface chemisorption, its speciation, and the electrostatic factors governing the adsorption were systematically described, aided by the use of XPS. Possible multilayer, particularly at low pH and high phosphate concentrations, deposition can occur, where some Fe3+ dissolution from the magnetite can occur to form insoluble iron phosphates. This study effectively demonstrated a strategy to combat the limitations of using nanoiron oxides in commercial sorption applications.

aryl ketones, etc.), and peak II corresponds to several types of ether and hydroxyl functions within aromatic (sp2) and nonaromatic (sp3) structures. This is typical for BCs. The high-resolution Fe 2p peak curve resolution, BEs, and peak assignments are complex and are provided in Figures S13 and S14 for MBC and phosphate-laden MBC, respectively. The satellite peak positions for the Fe 2p1/2 and Fe 2p3/2 peaks are very sensitive to oxidation states. These peaks have been used for the qualitative determination of the ionic states of Fe.73 Satellite peaks are found on the high BE side of the main XPS peaks. These are associated with discrete energy losses from certain photoelectrons by “shake-up” or “shake-off” processes, caused by sudden changes in the effective charge that accompanies electron ejection.74 XPS BEs are consistent with the theoretical multiplet peaks for magnetite (Table S8 and Figure S13). Also, several additional peaks corresponding to the presence of maghemite were also observed (Table S8 and Figures S13 and S14).75 Clearly, the surface region of the Fe3O4 particles contain some maghemite in addition to magnetite. When phosphate is adsorbed, XPS can detect changes in the Fe 2p region due to the formation of surface Fe−O−PO3H3, Fe−O−PO3−H2−, Fe−O−PO3H2−, and Fe−O−PO33− or their bidentate analogues. Indeed, at pH 3, where the adsorption capacity is high, the Fe 2p 2p3/2 peaks for Fe3+ magnetite (712.6 eV) and Fe2+ magnetite (710.3 eV) shift to higher BEs (712.8 and 710.6 eV, respectively) after adsorption (Table S9 and Figures S13and S14). Here, the P species in solution are H3PO4 and H2PO42−. The Fe 2p peak BE analysis, together with the P 2p XPS results, strongly suggests that surface hydroxyl groups of iron oxide provide the adsorption binding site, resulting in Fe−O− PO33− and Fe−O−PO3H− surface species and their binuclear analogues in the pH 6−13 range. These functions donate a net electron density to surface Fe atoms versus Fe−OH and Fe− OH2+ sites, in turn lowering the Fe 2p BEs.12,72 Phosphate uptake on magnetite primarily occurs at surface octahedral sites,12 with the formation of monoprotonated binuclear complexes at pH 3 and the diprotonated neutral complexes prevailing at pH 1 (also see Scheme 2).12 In addition, monodentate coordination can occur for diprotonated and triprotonated phosphate speciation at pH 3 and 1. Moreover, the tetrahedral vacancies can result in the formation of trinculear complexes for completely deprotonated phosphates.46

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00430. TEM−EDX, additional isotherm and thermodynamic studies, magnetite nanoparticle surface-area calculations and SEM particle diameters, methods for estimating the phosphate monolayer capacities, rinsing experiments to remove hydrogen-bonded phosphate, investigations of Fe stripping during adsorption and redeposition of insoluble phosphates, HR-XPS P 2p, C 1s, O 1s, and Fe 2p data, and model compounds for XPS peak interpretation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(C.P.) E-mail: [email protected]. Tel: 662323-9297 and Fax: 662-325-1618. *(T.M.) E-mail: [email protected]. Tel: 662325-6744. Fax: 662-325-1618. ORCID

Akila G. Karunanayake: 0000-0002-2533-3106 Dinesh Mohan: 0000-0002-3251-2946 Todd Mlsna: 0000-0002-4858-1372 Author Contributions §

These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work is funded by NSF REU Grant 1659830 awarded to T.M.

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Notes

CONCLUSIONS Nano-Fe3O4 particles (16.7 nm average diameter) were formed using a cheap commercial biproduct Douglas fir BC of high surface area and a pore-volume support material to adsorb phosphate. It successfully removed phosphate at high rates from aqueous solution. Adsorption was favored at low pH values. The maximum Langmuir adsorption capacities at pH 3 and 45, 35, and 25 °C respectively were 91.3, 91.0, and 90.0 mg/g for this MBC, which are of approximately 20-fold greater capacity than that reported for bare magnetite. The Langmuir and Freundlich two-parameter mode models gave best fits (R2 > 0.99) versus other models. The breakthrough capacity for fixed-bed column sorption was 82.5 mg/g, which was robust compared to the isotherm-predicted capacity. The high uptake rate (batch equilibrium reached in 2 mins) and promising adsorption capacities occur in both batch and column sorptions. MBC would be advantageous for phosphate

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Griffin Burk, Department of Chemistry, MSU, and Dr. Rooban Venkatesh Kulandaivelu Govindarajulu Thirumalai, Institute for Imaging & Analytical Technologies, MSU, for their assistance with SEM imaging and Timothy Dowel, MSU, for his support in the preparation of the graphical abstract.

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REFERENCES

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Fe3O4 Nanoparticles Dispersed on Douglas Fir Biochar for Phosphate

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