Fe3O4 Nanoparticles Dispersed on Douglas Fir Biochar for Phosphate

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Fe3O4 Nanoparticles Dispersed on Douglas Fir Biochar for Phosphate Sorption Akila G Karunanayake, Chanaka Navarathna, Sameera Gunatilake, Morgan Crowley, Renel Anderson, Dinesh Mohan, Felio Perez, Charles U. Pittman, and Todd E. Mlsna ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00430 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 11, 2019

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ACS Applied Nano Materials

Fe3O4 Nanoparticles Dispersed on Douglas Fir Biochar for Phosphate Sorption Akila G Karunanayakea,b, ‡ , Chanaka M Navarathnaa,‡, Sameera R. Gunatilakec, Morgan Crowleya, Renel Andersonb, Dinesh Mohand, Felio Pereze, Charles U. Pittman, Jr.a and Todd Mlsnaa,* a

Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA

b

Biochar Supreme Inc., Everson, WA 98247, USA

c

College of Chemical Sciences, Institute of Chemistry Ceylon, Rajagiriya, CO, 10107, Sri Lanka

d

School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India

e

Material Science Lab, Integrated Microscopy Center, University of Memphis, Memphis, TN

38152, USA ‡

These authors contributed equally.

* Corresponding Author (Tel: 662- 325-6744; fax: 662-325-1618; email: [email protected]) Full postal address Department of Chemistry Box 9573 Mississippi State University Mississippi State, MS 39762-9573 Keywords Magnetite Nano Douglas fir biochar Phosphate Adsorption Breakthrough XPS

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GRAPHICAL ABSTRACT

ABSTRACT (300 word limit) Surface area, particle aggregation, pressure drop in columns, nanotoxicity and commercialization difficulties, limit using of nanoparticle adsorbents. Magnetic primary nanoFe3O4 particles (~16.7 nm dia) were dispersed on high surface (695 m2/g) Douglas fir biochar (MBC). This cheap, commercial fast pyrolysis biochar, a syngas biproduct on, chemical coprecipitation of Fe3O4 from Fe3+/Fe2+aqueous NaOH served as a matrix, aiding magnetite nanoparticle dispersion, 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 separations of exhausted adsorbent from batch process, an alternative to filtration. This neat and phosphate-laden hybrid sorbent were characterized by scanning electron microscopy (SEM), transmission electron 2


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microscopy (TEM), energy dispersive X-ray (EDX), point of zero charge (PZC), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), elemental analysis, vibrating sample magnetometry (VSM) and Brunauer–Emmett–Teller (BET) 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 P2p XPS binding energy shifts at pH from 1 to 13 are reported. Solution pH of 1-3 facilitates the formation of bidentate mono-protonated phosphate complexes [(Fe-O)2-PO2H]- at Fe-OH surface functions. H2PO42- predominates in solution at pH ~4-6.5 favors formation of [Fe-O-PO3H]- at these pHs. At strongly basic pH (10-13) values, PO43predominates and forms deprotonated chemisorbed monodentate [Fe-O-PO3]2- and bidentate [(Fe-O)2PO2]2-. Multilayer phosphate sorption and precipitation of iron phosphates was considered.

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 waste water has a total phosphorus concentration of approximately 10 mg/L.7 Maximum phosphorus discharge limits in municipal treatment plant effluent is l.0 mg/L of P discharging into the upper Great lakes and 0.5 mg/L P into the lower Great lakes in the USA.8 The Baltic and

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Mediterranean seas are impacted by treated and untreated phosphorus runoffs.9 Dead zones due to hypoxia exist are along the US East Coast and within the Gulf of Mexico due to fertilizer runoff into rivers and streams.10

Iron-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 (β–FeOOH).11 All except akaganeite adsorb similar amounts of phosphates (~5.1 P mg/g). Akaganeite had the highest capacity (59.6 P mg/g), due to its tunnel-shaped morphology, which provides a high surface area, but akaganeite is structurally far more fragile15 and goethite exhibits poor recyclability (13 – 14%).11

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 mud22 and dolomites have also been employed. Ion exchange23 and electrocoagulation1 have also been used for phosphate removal. Current methods are costly and not efficient enough for use in eutrophication-affected areas. Direct application of LDH, nanooxides or hydroxides in sorption are 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 biochar dispersants to generate composite adsorbents with small particle phases dispersed on larger carrier particles

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with improved sorption features.25 Biochars are excellent widely available sorbents for metal cations, pharmaceutical, dye and organics 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 biochar surfaces were the primary adsorption locations. The deposited Fe3O4 enabled magnetic separation of spent adsorbent from remediated water. With MBC, however, the magnetite particles serve as the adsorption site and permit magnetic separation. To best of our knowledge the importance of dispersing nano-Fe3O4 particles on a high surface area biochar to permit flow through columns, higher sorption capacity along with providing additional carbonaceous surfaces for removing 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 biochar, 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 previously reported for neat iron oxide particles in literature. The biochar’s surface adsorbs negligible phosphate, enabling future comparison of various methods of preparing, depositing and dispersing magnetite (or other oxides) on the kinetics and capacities of phosphate uptake. Phosphate adsorption at different concentrations, temperatures and pH values are reported and supplemented by a mechanistic study of pH-dependent phosphate sorption by XPS.

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Methods Preparation of magnetic Douglas fir biochar (MBC). MBC was prepared by iron oxide wet precipitation onto commercial Douglass fir biochar (Biochar Supreme Inc. Environment Ultra), hereafter designated as non-magnetic biochar, (NBC), by a method described in our previous studies (Scheme1).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 the precipitation of magnetite was triggered by the drop-wise addition of 10 M NaOH, maintaining the pH at 10, followed by ageing 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 biochar surface was sought to provide a high Fe3O4 surface area. Scheme1: Illustration of MBC preparation on this work

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Phosphate adsorption studies A 1500 mg/L phosphate stock solution was made by dissolving potassium phosphate dibasic anhydrous, KH2PO4 (Sigma-Aldrich) in de-ionized water (Millipore-Q 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 instrument HI 2211 pH/ORP meter). Batch sorption studies on NBC and MBC were performed by varying phosphate concentrations from 5 – 1500 mg/L at 25, 35 and 45°C, using de-ionized 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 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 between 1 and 5 min. Equilibrium was achieved within only 2 min for MBC and NBC. No further uptake occurred between 2 to 5 min. MBC was removed from 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 double beam UV-Visible spectrophotometer at 830 nm. The phosphate adsorption per unit of adsorbent (qe) was calculated using equation, (1).

q

(1)

Here, C0 and Ce (mg/g) are initial and equilibrium phosphate concentrations in the solution, V (L) is the solution volume, and M (g) is the total mass of adsorbent added. All experiments were carried out three times and the standard deviation error bars are from these 3 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 biochar 7



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was vortexed in 25 mL of solution containing 1000 mg/L phosphate at pH = 3. The N2 BrunauerEmmet-Teller (BET) specific surface area was determined using a N2 adsorption isotherm at 273.15 K (Micromeritics Tristar II Plus) and the Dubinin-Astakhov equation log a Dlog

log a

, where ‘a’ denotes amount of gas adsorbed per unit mass of adsorbent (mol/g), a0 is

the micropore capacity (mol/g), D is constant, P is the equilibrium pressure and P0 is the saturation vapor pressure of adsorbate at temperature T (K)). Density functional theory (DFT) was used to calculate micropore volume ( W

, where W0 is limiting micropore volume

(cm3/g), a0 is the micropore capacity (mol/g), and ρ is the density of adsorbed gas (g/cm3)).33 Biochar 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 SEM containing a BRUKER EDX system. Transmission electron microscopy (TEM) studies of MBC and NBC were obtained using a JEOL model 2100 TEM electron microscope operated at 200 kV. Transmission electron microscopy/energy-dispersive X-ray spectroscopy (EDX) analysis were carried out using an Oxford X-max-80 detector. X-ray diffraction (XRD) analysis was performed to identify the iron oxide phase crystallographic structure upon precipitation to make 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). X-ray photoelectron spectroscopy (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 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.

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C, H, O, N and S contents of NBC and MBC were measured by combustion analysis using a CHNS elemental analyzer (EAI CE-440). Ash content was determined through heating at 650 °C for 15 hrs in an open-top porcelain crucible in the muffle furnace. The content of organic oxygen was calculated using the equation (O %

100

C

H

N

S

ash ). The weight

% of iron in both NBC and MBC biochars was determined using atomic absorption spectroscopy (AAS) (Shimadzu AA-7000). An acid digestion was performed on 0.1 g of biochar using 50.0 mL of 1:1 95% H2SO4 /70% HNO3. Iron dissolved from the oxidizing biochar into the acid for 24 h (70 °C) with stirring and was 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 (VSM). The magnetic properties of NBC and MBC are represented by plots of magnetization (M) against the field strengths (H) giving the hysteresis loop. The saturation magnetization was measured from the hysteresis curve. Results and discussion Elemental, proximate and surface area analysis of NBC and MBC The combustion elemental analyses of MBC (Table 1) found lower C and H contents versus NBC due to the significant weight fraction of the MBC that deposited Fe3O4 comprises. The amount of oxygen in MBC drops for the same reason and because the iron oxide oxygen is not determined by combustion. The NBC used originated as a by-product from timber industry gasification of wet waste wood (Douglas fir), produced at residence time of 1-30 s at 900 ºC within an updraft gasifier. This produces a very high surface area biochar (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 is 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 9



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deposited nanoparticles and their aggregates block access of nitrogen into a portion of biochar micropores and ultra micropores which intersect with the char surface. This causes a loss of ~55 % of its original surface area (695 to 313 m2/g) and almost half of its pore volume (0.264 to 0.135 cm3/g). This occurs despite adding the surface area generated by the small magnetite particles.29 It well known that magnetite nanoparticles are not porous12 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 Magnetic biochar had a high ash content due to added iron oxide deposits. Table 1: Elemental, proximate analysis and surface area data for NBC and MBC Pore Sample % C % H % N % S % O % Ash % Fe volume (cm3/g) 74.60 1.90 0.12 0.03 20.95 2.40 0.10 695.1 0.264 NBC a 32.40 26.20 312.6 0.135 56.70 1.40 0.11 0.05 9.34 MBC a Combustion analysis does not account for the oxygen present in the iron oxide phases of MBC. Percentage of iron is reported for the digested adsorbent. Assuming all the Fe in MBC is Fe3O4, then the weight fraction of deposited Fe3O4 would be ~30%. Surface area (m2/g)

Surface morphology Quasi-spherical magnetite particles and their agglomerates were formed on the NBC surface to generate MBC during precipitation of iron oxide (Fig. 1). Both SEM and TEM micrographs (Fig. 1) show the quasi-spherical shaped particles with an average fundamental particle size of 16.7±3.5 nm for MBC. Particle sizes were determined using the Image J 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. Since MBC is a hybrid adsorbent, only its total surface area can be measured, belonging to both magnetite particles and 10


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the porous biochar. 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 qausi-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.

Fig. 1. Scanning electron micrographs of (a) NBC; (b, c, d, e) MBC; and (f) MBC after phosphate sorption and, Transmission Electron Micrographs of (g) MBC and (h) MBC after phosphate sorption. NBC, MBC and phosphate-laden MBC were studied by XRD (Fig. 2a). The broad peak at 2θ = 22.7° for NBC, comes from the deformed cellulose crystal structure. Cellulose crystallinity is reduced during the biomass pyrolysis.36 The XRD peak pattern for MBC corresponds to that of the precipitated iron oxide particles, which correspond to magnetite (Fig. 2a). Locations (2θ) and intensities of the diffraction peaks are consistent with the standard pattern

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for magnetite, JCPDS Card No. (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 D

0.9λ

βcosθ where λ is the wavelength used in

the X-ray experiment (nm), β is the full-width at half-maximum (FWHM) value in radians for XRD diffraction lines and θ is half of the diffraction angle of 2θ. The crystallite size was determined by taking the average sizes of peaks D220, D311, D400, D422, D511 and D440. These sizes were 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 inter-planar spacing (dhkl) values were estimated for the most intense 311 phase using the Bragg equation,39 d

λ

2sinθ

a

√h

k

l

, where λ is the wavelength of X-ray (nm), θ is half of the

diffraction angle of 2θ, h, k and, l are the Miller indices. The lattice parameter (8.391 nm) and inter-planar distance (0.2530 nm) values determined for the iron oxide deposited on MBC are consistent with those for bulk magnetite JCPDS Card No. (79-0417) (a = 8.394 nm and d311 = 2.531). This unequivocally confirms that the precipitated iron oxide is magnetite.

Both SEM and TEM images obtained after the phosphate adsorption (Figs. 1f and h) show a change in the Fe3O4 particle morphology or texture. The surface of the magnetite particles and their aggregates after phosphate adsorption appear rougher and darker. SEM micrograph shows two different types of quasi-spherical particles and their aggregates. The brighter particles and aggregates might be have sorbed less or no phosphate, while the darker particles and aggregates could be more phosphate-laden, where surface conductivity differences could influence differences in charging. Their calculated (Image J) average size is 18.7 ± 3.1 nm. 12


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The surface textural changes observed for phosphate-laden MBC versus MBC are not likely due to magnetite morphology changes as the SEM appearance is highly dependent on surface conductivity. This leads to charging-effect related contrast variation in the SEM images.40 No changes occurred in the XRD spectrum (Fig. 2a) after phosphate adsorption. Thus, no iron 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 H-bonding on top of the chemisorbed layer. One possibility is that small amounts of iron 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 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 S 3.6, Figs. S8 and S9)

TEM-EDX elemental mapping images (Fig. 2b, c, d and e) of the biochar after phosphate sorption exhibited the presence of 1.6 wt% phosphorus as purple dots on the MBC surface region. Phosphorus 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 phosphorus (

Fe3O4 Nanoparticles Dispersed on Douglas Fir Biochar for Phosphate

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