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Rhodamine B Adsorptive Removal and Photocatalytic Degradation on MIL-53-Fe MOF/Magnetic Magnetite/Biochar Composites

  • Chanaka M. Navarathna
  • Narada B. Dewage
  • Akila G. Karunanayake
  • Erin L. Farmer
  • Felio Perez
  • El Barbary Hassan
  • Todd E. MlsnaEmail author
  • Charles U. PittmanJr.Email author
Article
  • 118 Downloads

Abstract

MIL-53-Fe metal–organic framework (MOF) was grown using the terephthalic acid linker and FeCl3 into an already prepared, high surface area, magnetic, Douglas fir biochar/Fe3O4 (MBC) adsorbent hybrid. This resulting triphase hybrid, multifunctional, magnetically recoverable, sorptive, photocatalytic and degradative, adsorbent (MOF–MBC) was used both to remove and catalyze the photodegradation of Rhodamine B (Rh B) with or without Cr6+ present. Rh B is a widely used colorant in textile, printing and tanning industries that is also associated with deleterious health effects. Batch aqueous sorption studies were performed at various pHs, Rh B concentrations and temperatures in-order to determine the optimum adsorption pH, kinetics, thermodynamics and sorption capacity. This adsorption followed pseudo-2nd-order kinetics and exhibited a Rh B Langmuir adsorption capacity of ~ 55 mg/g at pH 6, 200 rpm agitation and 25 °C. This MOF–MBC hybrid was characterized by SEM, TEM, EDS, XRD, FT-IR, TGA, BET, Elemental Analysis and XPS. Deethylated and carboxylic compounds were identified as photodegradation intermediates. Electrostatic and π–π stacking interactions are thought to play a significant role in Rh B sorption. Hexavalent chromium (Cr6+) and Rh B often co-exist in tannery and printing waste water. Cr6+ can trigger the photo-degradation of Rh B into CO2 and H2O in the presence of both MIL-53-Fe MOF and MOF–MBC. Hence, adsorbent stripping regeneration can be minimized in real world applications. The biochar phase, aids to disperse the MOF, to minimize particle aggregation, to provide extra stability to the MOF, and serves as secondary adsorption site for heavy metal, oxy anion and organic contaminants. Large biochar particles allow reasonable flow through column beds while supporting other nanophases, which would cause large pressure drops when used alone.

Keywords

Rhodamine B Photodegradation Adsorption Chromium(VI) MIL-53-Fe MOF/Fe3O4/biochar adsorbents Magnetite nanoparticles 

1 Introduction

Water remediation has become a huge problem worldwide, due to rising contamination levels [1]. Efficient contaminant removal from wastewater is a crucial environmental need. Colorant dyes contaminate waters from various textile industry effluents [2] and should be addressed.

Metal–organic frameworks (MOFs) are crystalline structures consisting of metal center units connected by organic framework linker units which spontaneously assemble by forming strong bonds into three-dimensional regular crystalline structures [3, 4]. They have highly porous characteristics that are beneficial for molecular separations based on their range of pore sizes, pore volumes, large surface areas, ease of assembly and wide variability of interactions that are possible for guests to undergo within the host cavities of their inorganic and organic structural components [4]. Many MOFs also possess semiconductor properties that allow them to reduce contaminants and degrade dyes in the presence of light [5, 6, 7]. These properties suggest that some MOFs might be used for simultaneous or in-series adsorption/degradation of contaminants. Ideally, such MOFs might adsorb and catalyze destruction of a pollutant in either batch or continuous flow processes and never require regeneration [8]. MOFs have already been used in acid catalysis, esterification, hydrolysis of esters, coupling reactions, reduction reactions etc. [9]. A major drawback of many MOFs for use in aqueous adsorption is poor stability in water [10]. The MOF’s framework structure could be destroyed by displacement of bound ligands [10]. Competitive blockage of internal binding sites by water might prevent adsorption of targeted adsorbents [10]. Many MOFs have been compared to such semiconductors as n-doped TiO2 due to their remarkable photocatalytic degradation properties [11]. Some MOFs [11, 12] possess high chemical stabilities similar to TiO2 [13, 14] and have proved efficient for use in degradation. These example MOFs were easy to use [11, 12].

Biochar is a readily available byproduct from both fast biomass pyrolysis during bio-oil production or slow pyrolysis processes at a variety of temperatures [15, 16]. The H/C and O/C ratios drop as the pyrolysis time and temperatures are increased, leading to a plethora of biochar compositions, morphologies, surface areas, pore volumes etc. Biochars are being widely studied as low cost adsorbents [17, 18, 19], soil modifiers [20], carbon sequestrants [21], and nanoparticle dispersants [22, 23, 24, 25, 26, 27]. We envisioned biochar serving as a surface dispersant for MOFs to reduce particle agglomeration, to increase mechanical strength, and to form larger particle size biochar-MOF hybrids which allow rapid water flow through fixed beds, while dispersing tiny attached MOF particle sizes. Beds of tiny MOF particles alone would result in large pressure drops and slow flows in columns. For many years we have also made magnetically manipulatable adsorbents by decorating Douglas fir biochar (BC) with small magnetic Fe3O4 nanoparticles [22, 23, 24] to produce MBC. These Fe3O4-containing biochar surfaces exhibit Fe–OH functions which enhance the adsorption of oxyanion contaminants (AsO43−, AsO32−, PO43− and others) [28]. At lower pH, these surface Fe–OH hydroxyl groups are protonated, attracting and binding oxyanions while also allowing the adsorbent to be magnetically manipulated [29]. In this study, the initial Douglas fir, high surface area biochar (BC) was produced as a biproduct of wood gasification, (Biochar Supreme, Inc). Then BC was first decorated with magnetite (Fe3O4) magnetic nanoparticles and then was further decorated with MIL-53-Fe MOF (Fig. 1) to form a dual hybrid (Fig. 2). This sorbent was employed to adsorb and catalytically photodegrade the dye Rhodamine B (Rh B) (Fig. 2).
Fig. 1

Structure of MIL-53-Fe MOF

Fig. 2

Schematic representation of a MOF–MBC and b the chemical structure of Rhodamine B

Rh B is a dye and often used as a tracer to determine flow direction and transport rates [30]. Rhodamine dyes have many fluorescence applications [30]. Studies of contaminant dye separations and aqueous adsorption motivate this current study [31], including Rhodamine B effluents from textile, printing and tannery industries.

2 Materials and Methods

All the chemicals and reagents used were analytical grade (Sigma-Aldrich), unless otherwise noted.

2.1 Synthesis of MIL-53 Fe MOF and MOF–MBC Composite

A mixture of terephthalic acid (p-C6H4(CO2H)2) (~ 1.6 g), iron(III) chloride hexahydrate (FeCl3·6H2O) (~ 2 g) and N,N-dimethylformamide (DMF) (~ 20 mL) was sonicated for 1 h at 25 °C and transferred to a Teflon-lined stainless-steel bomb (~ 23 mL). This mixture was heated at 150 °C for 15 h [32]. Once cooled, the resulting MIL-53-Fe MOF suspension was filtered, thoroughly washed with methanol (3 × 50 mL), and dried under vacuum at 100 °C for 12 h. MIL-53-Fe MOF occurs as a poorly crystalline light orange powder. Washing with excess methanol exchanges the DMF solvent trapped in the MOF. The crystallinity can be enhanced by using HF acid for the synthesis mixture [33], but HF can dissolve the Fe3O4 in MBC, so it was not used in either the MIL-53-Fe MOF or MOF–MBC preparation. MBC was prepared first as a substrate for MIL-53-Fe MOF according to our previously described procedure [24]. Then, ~ 50 g of Douglas fir biochar (Black Owl, Biochar Supreme Inc.) (particle size was sieved to 1–2 mm, surface area 687 m2/g, and porosity 0.251 cm3/g) was added to water and homogenized (~ 1950 mL water in total) with a mixed iron(III) chloride (~ 18 g) and iron(II) sulfate (~ 36.6 g) solution. Then magnetite precipitation onto the “Black Owl” biochar was triggered by the drop-wise addition of 10 M NaOH, maintaining the pH at 10 for ~ 24 h. The resulting Fe3O4-magnetized biochar (MBC) was filtered, then repeatedly washed with distilled water, followed by three ethanol washes. The MBC was then vacuum-filtered and dried overnight at 50 °C in a hot air oven. The MBC was used as a substrate onto which MIL-53-Fe MOF was both deposited and nucleated during synthesis of this MOF. The same amounts of MOF precursor reagents and identical procedures as stated above were employed for the preparation of MOF–MBC, except that 8 g of dried MBC was added into the MOF synthesis mixture in a DMF slurry that was prepared as described above. Similar to the independent MOF preparation, the resulting MOF–MBC composite was washed with 3 × 50 mL of methanol and dried under vacuum at 100 °C for 12 h. The weight gain of MIL-53-Fe onto MBC after the MOF formation and deposition was ~ 3.8 g. This corresponds to ~ 89% yield of MIL-53-Fe MOF out of the total amount formed that became deposited or grew from nucleation onto MBC carrier. Since complete DMF exchange with methanol is difficult, MOF–MBC may contain some DMF that contributes to this measured weight gain despite thorough MeOH extractive washings. Traces of MeOH from washings may also remain. It is possible that some traces of terephthalic acid and iron chloride may also have deposited on MOF–MBC.

2.2 Adsorption Kinetics and Isotherm Experiments

Aqueous solutions of 50 mg/L Rh B (20.0 mL) at 25 °C were made to study the effects of pH on Rh B adsorption on MOF–MBC (50.0 mg). The solution pH values were adjusted to values ranging from pH 1 to pH 13 in pH intervals of 2 by using different molarities of HCl and NaOH. The optimal pH for adsorption was found to be 8, but a pH of 6 was chosen for kinetic and isotherm experiments because it is closer to the natural pH of water. Moreover, the percentage Rh B removal values between pH 6 and 8 were similar, so this adsorption difference was negligible. The adsorption of Rh B dependence on pH is discussed in Sect. 3.2.1.

Rh B adsorption kinetics on MOF–MBC were determined using 50.0 mg MOF–MBC doses with 20.0 mL of Rh B at separate concentrations of 25, 50 and 75 mg/L, placed into 50 mL polypropylene containers at pH 6 and 25 °C. The filled containers were shaken in an orbital shaker at 200 rpm for specific times and then removed, filtered through a 0.22 μm filter, and the remaining Rh B concentrations, were determined using UV–Vis at 554 nm versus a predetermined, calibration curve. The limit of the detection (LOD) for the method was 0.01 mg/L. The matrix effect was assessed by tests performed using matrix-matched calibration curves. Briefly, standard Rh B solutions were spiked into water pre-equilibrated with biochar and the calibration curve slopes were compared with the calibration curves prepared in ultra-pure water. Slope differences were ~ ± 6% and hence, an external calibration curve was used in the analysis. Kinetics study results are discussed in Sect. 3.2.2.

Isotherm experiments employed 5 to 1000 mg/L Rh B solutions (25.0 mL) which were equilibrated with MOF–MBC (50.0 mg) and then adjusted to pH 6 using aqueous HCl. Rh B adsorption values per unit of adsorbent (qe) were each calculated using Eq. 1:
$${\text{q}}_{\text{e}} = \frac{{{\text{V}}\left( {{\text{C}}_{0} - {\text{C}}_{\text{e}} } \right)}}{\text{M}}$$
(1)
Here, Co and Ce are the initial and equilibrium concentrations of Rh B in units of mg/g, V is the volume of the solution in L, and M is the total mass in g of adsorbent added. Each experiment was performed three times in Figs. 9, 10, 11, 12 and 13. Isotherm results appear in Sect. 3.2.3.

2.3 Characterization of BC, MBC, MIL-53-Fe MOF and MOF–MBC

BC, MBC, MIL-53-Fe MOF, MOF–MBC and Rh B-MOF–MBC were each individually characterized. A batch equilibrium adsorption experiment consisting of 50.0 mg of MOF–MBC that was equilibrated in 20 mL of solution composed of 1000 mg/L Rh B at pH 6 was the source of Rh B samples sorbed on MOF–MBC. The N2 Brunauer-Emmet-Teller (BET) specific surface area, pore volume, and pore sizes were determined using a N2 adsorption isotherm at ~ 77 K (Micromeritics Tristar II Plus). MIL-53-Fe MOF surface area was also determined by CO2 adsorption at ~ 273 K. C, H, O, N and S contents of the adsorbents and Rh B-laden-MOF–MBC were measured by combustion analysis using a CHN elemental analyzer (CE-440). Ash content was determined through heating in air at 650 °C for 15 h in an open-top porcelain crucible in the muffle furnace. The content of organic oxygen was calculated using the equation \(\left( {{\text{O }}\% = 100 - \left( {{\text{C}} + {\text{H}} + {\text{N}} + {\text{S}} + {\text{ash}}} \right)} \right)\). The weight percentages of iron in BC, MBC, MIL-53-Fe, MOF, and MOF–MBC were determined using atomic absorption spectroscopy (AAS) (Shimadzu AA-7000). Complete acid digestions were performed on 0.1 g of biochar using 50.0 mL of 1:1 95% H2SO4/70% HNO3. Iron dissolved from both the oxidizing biochar and MOF samples into the acid for 24 h (70 °C) with stirring. These solutions were then diluted tenfold with deionized water prior to AAS analysis. A recovery test was performed with Fe2O3 and produced ~ 94% recovery.

A JEOL JSM-6500F FE instrument run at 5 kV generated the scanning electron microscopy (SEM) images of the MBC and MOF–MBC surface morphologies. A Zeiss, EVO 40 SEM containing a BRUKER EDX system was used to obtain SEM/EDS surface region elemental compositions and their distributions. A JEOL model 2100 TEM electron microscope operated at 200 kV produced transmission electron microscopy (TEM) micrographs of MBC and MOF–MBC. An Oxford X-max-80 detector was used to perform TEM/Energy-dispersive X-ray spectroscopy (EDX) analysis (elemental mapping) of MBC, MIL-53-Fe MOF, MOF–MBC, and Rh B-laden MOF–MBC. A Rigaku ultima III (using Cu–K(λ = 1.54 Å X-rays) was used to conduct X-ray diffraction (XRD) analysis and determine the crystallographic structures.

X-ray photoelectron spectroscopy (XPS) analyses were run on 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 µm2. Photoelectrons were collected from a takeoff angle of 90° relative to the overall sample’s fractal particle surface. Measurements were done in the constant analyzer energy mode. The survey spectra were taken at a pass energy of 200 eV, while the high resolution (HR) core level spectra were taken at a 40 eV pass energy. Fourier Transformed Infrared spectroscopic (FT-IR) analysis (ATR mode) was carried out using a Thermoscientific iD-5 (Nicolet) instrument. Adsorbent thermogravimetric analysis (TA instruments, TGA Q 50) was performed under a 100 mL/min nitrogen flow over 30–800 °C. An Agilent 1200 series LC–MS equipped with a UV–Vis diode array and mass spectrometric (Agilent 6120 Quadrupole) detector separated (C18 reverse phase 150 mm * 4.6 i.d, column) and then analyzed intermediate products. Methanol and 5 mM ammonium acetate were used in the mobile phase. The programmed gradient was 55% methanol flow was held for 10 min, linearly increased to 65% within 0.5 min, and then maintained for 12.5 min before returning to 55% in the last 1 min. The flow rate was 1 mL/min and mass scan was conducted in 10–500 m/Z range at positive ion mode.

3 Results and Discussion

3.1 Characterization of MIL-53-Fe MOF, MBC and MOF–MBC

3.1.1 Surface Morphology

MIL-53-Fe MOF grew in octahedral rod-like shapes in ~ 0.5–1.1 µm diameters and ~ 07–1.5 µm lengths (Fig. 3a) which are aggregated as in Fig. 3b. The MBC surface exhibited magnetite primary nanoparticles (~ 16–19 nm) and their aggregates (Fig. 3c) [29]. The MOF–MBC surface exhibited octahedral rod-like MOF structures after the MIL-53-Fe MOF synthesis/deposition step onto the MBC present (Fig. 3d). Extensive coverage of the original MBC surface by MIL-53-Fe MOF rods was observed on MOF–MBC. Many of these were observed to be fractured in this dual hybrid. MIL 53-Fe MOF crystals or deposits that begin to grow within some biochar pore sizes may be not be able to complete growth due to wall boundaries or lack of component diffusion. The MBC surfaces (both Fe3O4 and biochar regions) also may initiate MOF nucleation differently than nucleation in the solutions, giving the MOF–MBC hybrid surface a different mixture of MIL-53-Fe MOF textures. Other possible causes of MOF defects generated during MOF–MBC formation are cation substitution, mixed valence and cation vacancies, ligand defects and anion or ligand substitution as implicated in other systems [34].
Fig. 3

SEM images of a, b MIL-53-Fe MOF c MBC and d MOF–MBC

Successful MIL-53-Fe MOF formation onto MBC surfaces was further confirmed using TEM–EDX and elemental mapping (Figs. S1 and S2) and EDX (~ 1–2 µm penetration depth) patterns can sample MOF–MBC below the surface. TEM elemental mapping images were produced (Fig. S1) to show surface-near surface region elemental composition in the MBC, MIL-53-Fe MOF, MOF–MBC, and the Rh B-laden MOF–MBC samples. MBC contains Fe and O on the biochar surface phase due to the Fe3O4 deposition. The MIL-53-Fe MOF sample exhibits its regular elemental composition, with some residual Cl present from FeCl3 that was not reacted and had not been washed away in the preparation. The nitrogen likely originated from DMF that had not been exchanged with methanol during purification. In MOF–MBC, the C content was larger than in MBC. This large C contribution is from the terephthalic acid linker within the MOF which covers much of the surface. Rh B has ~ 70% carbon and upon Rh B adsorption, the C content of MOF–MBC has further been increased from 44 to 68.5% (Fig. S2).

3.1.2 X-Ray Diffraction Analysis

The XRD peak pattern for MBC corresponds to that of the precipitated magnetite particles on BC, (Fig. 4). Locations (2θ) and intensities of the diffraction peaks in MBC are consistent with the standard pattern 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). The characteristic XRD peaks (Fig. 4) for MIL-53-Fe MOF alone in the 8 to 27° 2θ range (9.36°, 12.50°, 17.34°, 18.74°, 21.94°, 25.21° and 27.98°) were observed. The XRD pattern for MOF–MBC exhibited all the key peaks for both magnetite and MIL-Fe-53 MOF [35]. Since the MOF–MBC sample’s XRD pattern contains peaks characteristic of both MIL-53-Fe MOF and the magnetite peaks for MBC, the production of the dual hybrid MOF–MBC is confirmed. Some weaker peaks were difficult to resolve from substantial broad amorphous peaks from biochar [36] and the peaks from magnetite. Several peaks (12.50°, 18.74° and 27.98°) originally observed on MIL-53-Fe MOF were not seen in the XRD spectrum of MOF–MBC composite, and this is possibly due to the formation of amorphous phases of MIL-53-Fe MOF during the MOF–MBC synthesis. A review of the literature shows the XRD of MIL-53-Fe MOF is very sensitive to its growth conditions and differ in composites [32, 33, 37].
Fig. 4

XRD data for MBC, MIL-53-Fe MOF and MOF–MBC

3.1.3 FTIR Analysis

The FTIR spectra of MBC, MIL-53-Fe MOF, Rh B, MOF–MBC, and Rh B-laden MOF–MBC are displayed in Fig. 5. The FTIR spectrum of MBC (Fig. 5a) like that of other biochars does not give sharp bands. This is due to the unavailability of significant amounts of functionality to produce strong, sharp vibrational bands in IR absorption. During the pyrolysis at 400 °C and above decarboxylation, deamination etc. occur [38]. The Douglas fir biochar, precursor of MBC, was originally pyrolyzed at 900 °C, although for a short residence time of a few seconds. The FTIR spectrum of the Douglas fir biochar (BC) is almost identical to that of MBC. Figure 5b shows the IR spectrum of MIL-53-Fe MOF with a broad peak centered at ~ 3300 cm−1 which can be attributed mostly to the stretching vibrations of the O–H from adsorbed and surface-bonded water or residual methanol. Bands at ~ 1600 cm−1 and ~ 1300 cm−1 are due to the asymmetric stretching vibrations of Fe(III)-coordinated carboxylate groups and stretching vibrations of –O–C–O– framework, respectively [37]. The peak at ~ 400 cm−1 corresponds to C–H bending vibrations on the aromatic rings of the terephthalic acid linker [37]. Figure 5c shows the FTIR spectrum of Rh B with broad, H-bonded carboxyl OH vibrations centered at ~ 3300 cm−1, symmetric carboxylic carbonyl stretching at ~ 1600 cm−1 regions and sp2-hybridized C–H in plane bending on aromatic rings at ~ 400 cm−1. The MOF–MBC surface (Fig. 5d) contains functional groups corresponding to those from MIL-53-Fe MOF, which further confirms the MOF has been successfully attached to MBC. The RhB-laden MOF–MBC spectrum (Fig. 5e) shows the peak intensity enhancements in many locations associated with Rh B (especially in –C–H stretching, bending and carboxylic –C=O). However, distinguishing these from MIL-53-Fe MOF is difficult. However, in the “finger print” region from 1200 to 900 cm−1 peaks of Rh B are observed in the spectra (Fig. 5e) of Rh B–MOF–MBC, confirming the successful adsorption of Rh B on MOF–MBC.
Fig. 5

FT-IR characterization of MBC, MIL-53-Fe MOF, Rh B, MOF–MBC and Rh-laden MOF–MBC

3.1.4 Elemental, Proximate and Surface Area Analysis

The combustion elemental analyses of MBC (Table 1) found less C and H than in BC due to the added Fe3O4 weight fraction (~ 29%) that MBC contains. The high C/H (45.6) ratio and observed O/C (0.14) ratio of BC is attributed loss of oxygenated and hydrogenated functionalities during the high temperature pyrolysis of the precursor Douglas fir wood. Precipitating iron oxide onto BC to form MBC leads to partial surface coverage and partial pore blockage of the BC portion of MBC. This causes a loss of ~ 48% of its original surface area (687 to 327 m2/g) and almost half of its pore volume (0.251 to 0.129 cm3/g), despite adding the new surface generated by the small magnetite particles [23, 29]. The ash content in BC (2.65%) is primarily composed of stable oxides and carbonates formed from sodium, potassium, magnesium, calcium and iron salts in the wood feed [29, 39]. Magnetic biochar had a high ash content (~ 31.6%) mostly due to added magnetite deposits are illustrated by the amount of iron (27.8%) in MBC.
Table 1

Elemental, proximate analysis and surface area data for BC, MBC, MOF, MOF–MBC and Rh-laden MOF–MBC

Sample

% C

% H

% N

% O

% Ash

% Fe

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (Å)

BC

82.98

1.84

1.28

11.26

2.65

0.08

687.1

0.251

14.47

MBC

54.66

1.42

1.03

11.34

31.56

28.71

326.9

0.129

13.56

MOF

44.87

3.95

3.52

29.66

18.01

20.90

2.1b

42.2c

0.000053

0.007797

1.01

7.39

MOF–MBC

46.18

5.23

6.78

25.83

16.00

27.49

7.8

0.00166

8.57

Rh B-laden MOF–MBCa

62.35

2.34

2.28

33.05

aAsh and Fe analysis were not carried out for Rh B-laden MOF–MBC

bFrom N2 BET adsorption isotherm at 77 K

cFrom CO2 BET adsorption isotherm at 273 K

Elemental composition for the MIL-53-Fe MOF corresponds well with the empirical formula, Fe(III)(OH)(O2C–C6H4–CO2)·H2O (Fe = 21.9%, C = 37.7%, H = 2.77% and O = 37.6%) [33]. Trace DMF in this MOF may have contributed to the slightly higher C, H and N content versus its empirical formula. The IR spectra further confirmed the presence of a hydrated MOF structure, with a broad -OH peak centered at 3300 cm−1 regions. The N2 BET surface area of MIL-53-Fe MOF is very low (2.1 m2/g) compared to the values reported in previous studies (7.3–38.2 m2/g) [37]. This could be due to N2 not being able to access the MOF pores because of its low pore diameter. However, the CO2 BET surface area (42.2 m2/g) was very close to the higher reported values in literature. CO2 isotherms were conducted at 273 K providing sufficient energy for CO2 to move from initial pore adsorption locations near the surface to occupy surfaces deep within ultramicro-pores. This is an alternative option for materials containing very small pores in which nitrogen and argon molecules cannot access at cryogenic temperatures, since they condense in the pores near the surface, thereby blocking access to surface area deeper within the pores [40]. Poorly crystalline MOF structures may also lead to the low surface area [37].

MOF–MBC’s elemental composition reflect that the MOF has been successfully incorporated onto MBC. The iron percentage decreased when MIL-53-Fe MOF was added to MBC as expected because the %Fe in the MBC is greater than that the MIL-53-Fe MOF being added. MOF crystalline growth on MBC during MOF–MBC synthesis may further block biochar pore access on MOF–MBC. The dual hybrid’s biochar surface region is now covered with both magnetite and MIL-53-Fe MOF particles, which result in further N2-BET surface area loss from 326.9 to 7.8 m2/g. Rh B-laden MOF–MBC has a greater C content (61%) than MOF–MBC (46%) as expected because Rh B has ~ 70% C in its structure. This further confirms the successful adsorption of Rh B, but it does not define how much Rh B is adsorbed on each of the three surface phases of MOF–MBC.

3.1.5 Thermogravimetric Analysis (TGA)

TGA weight loss versus temperature plots (10 °C/min under 100 mL/min N2) for BC, MBC, MOF and MOF–MBC are reproduced in Fig. 6. BC and MBC show little mass loss up to 800 °C, because BC was originally pyrolyzed at ~ 900 °C for a short time. Also, the Fe3O4 deposited, when making MBC from BC, is stable to 800 °C in TGA [41]. MIL-53-Fe MOF undergoes a small mass loss in 100–290 °C temperature range, attributed mostly to adsorbed water and residual DMF removal. Drastic losses occur between 350–450 °C and 520–620 °C from a two stage decomposition of terephthalic linkers and their carbonization, collapse of the MOF and partial iron oxide formation [42]. MOF–MBC shows the expected lower mass drop versus MIL-53-Fe MOF since ~ 32% wt of this specific MIL-53-Fe MOF was deposited onto the more thermally robust MBC to form MOF–MBC. Similar results were observed on carbon nanotubes functionalized with MIL-53 Fe MOF [35].
Fig. 6

Thermogravimetric analysis profiles of BC, MBC, MIL-53-Fe MOF and MOF–MBC (10 °C/min heating rate under 100 mL/min N2)

3.1.6 X-Ray Photoelectron Spectroscopy (XPS)

Low resolution wide scan XPS spectra for BC, MBC, MOF, MOF–MBC and Rh-laden MOF–MBC are given in Fig. 7. All show Fe2p peaks except BC, where the original iron content in the biochar is low. These survey spectra exhibit changes in the elemental composition upon deposition of magnetite onto BC to give MBC and MIL-53-Fe MOF formation on MBC during conversion to MOF–MBC. The surface region’s Fe atomic percentage (4.2%) in MIL-53-Fe MOF is lower than MBC (9.3%). Upon deposition/growth of the MIL-53 Fe MOF on MBC, the Fe atomic percentage has reduced to 7.6%. This is because MOF–MBC only contains approximately ~ 68% by wt. of MBC. Residual N and Cl peaks appear in MIL-53-Fe MOF and MOF–MBC from small amounts of DMF and chloride that was not exchanged or not washed away, respectively, during the MOF preparation. The formation of fluorinated MIL-53 MOF, Fe(III)(OH)0.8F0.2(O2C–C6H4–CO2)·H2O has previously been reported [33], so some chlorinated MIL-53-Fe might have formed during MOF–MBC synthesis. TEM–EDX elemental mapping also supports this (Fig. S1).
Fig. 7

Low resolution (LS) survey scan XPS spectra for BC, MBC, MIL-53-Fe MOF, MOF–MBC and Rh B-laden MOF–MBC (CPS counts per second)

The C1s, O1s and Fe2p High Resolution (HR) spectra of BC, MBC, MIL-53-Fe MOF, MOF–MBC and Rh B-laden MOF–MBC were each resolved and displayed in Fig. 8. The expanded labelled version of C1s, O1s and Fe2p of these HR-XPS are shown in the supporting material (Figs. S3–S7). The C1s and O1s spectra were each resolved into four peaks and these are assigned in Table 2. The Roman numerals in Fig. 8 and Table 2 designate the resolved peaks and coordinate the resolved peaks in Fig. 8 with their binding energies (BE) and atomic percents in Table 2. Binding energies for C1s at 289.7–288.4 eV, 287.7–286.4 eV, 285.8–285.3 eV and 284.7–284.2 eV range correspond to –CO2R(H) and CO32−, C=O, C–O and (C–H, C–C). However, C–C and –CH3, –CH2, –C4– (quaternary) and aromatic C in rings (the sp2 vs. sp3 are all found in the 284.7–284.2 eV peak, but this peak for C–H and C–C vs those for C–O, C=O, –CO2R, CO32− are different carbon oxidation states and are resolved. MIL 53-Fe MOF has a higher C=O and C–O atomic percentage compared to BC and MBC. The C=O and C–O atomic percentages (from peak intensities) increased going from MBC to MOF–MBC and this further confirms successful deposition of MOF onto MBC.
Fig. 8

High resolution (HR) C1s, O1s and Fe2p XPS spectra for a BC, b MBC, c MIL-53-Fe MOF, d MOF–MBC and e Rh B-laden MOF–MBC (CPS counts per second). The Roman numerals given on the C1s and O1s HR XPS spectra correspond to those assigned in Table 2. *Due to the complex nature of the Fe2p HR-XPS, the curve-resolved peaks are not labeled here. These HR-XPS figs are given in expanded form in the supporting materials (Figs. S3, S4, S5, S6 and S7) with the assignments shown

Table 2

High resolution (HR) C1s, O1s and Fe2p XPS data for BC, MBC, MIL-53-Fe MOF, MOF–MBC and Rh B-laden MOF–MBC

Peaka

BC

MBC

MIL-53-Fe

MOF

MOF–MBC

Rh-laden MOF–MBC

C1s

 I

CO32−, O–C=O

BE (eV)

289.7

289.0

289.0

289.1

288.4

Atomic %

2.4

3.9

7.7

4.1

4.5

FWHM (eV)

1.8

2.0

1.3

1.5

1.6

 II

C=O

BE (eV)

287.7

286.7

286.4

286.6

286.4

Atomic %

3.8

4.7

12.9

10.7

7.4

FWHM (eV)

1.8

2.0

1.5

1.9

1.6

 III

C–O

BE (eV)

285.8

285.3

285.1

285.5

285.0

Atomic %

13.9

14.9

45.9

12.2

19.2

FWHM (eV)

1.8

1.7

1.2

1.3

1.5

 IV

C–C

C=C, C–H

BE (eV)

284.5

284.6

284.7

284.2

Atomic %

72.2

42.7

 

32.1

29.7

FWHM (eV)

1.0

1.1

 

1.3

1.2

O1s

 I

CO32−

BE (eV)

533.8

533.3

533.7

534.4

533.5

Atomic %

1.1

1.3

2.5

1.2

1.5

FWHM (eV)

1.5

1.4

1.4

1.5

1.5

 II

O–C=O

BE (eV)

532.8

531.9

532.8

533.1

532.0

Atomic %

2.2

4.4

3.2

4.1

5.6

FWHM (eV)

1.5

1.5

1.1

1.5

1.5

 III

O–C, C=O

BE (eV)

531.7

530.7

532.2

532.0

531.2

Atomic %

2.9

5.4

7.5

11.5

8.8

FWHM (eV)

1.5

1.5

1.0

1.5

1.5

 IV

Fe–O

BE (eV)

530.5

530.3

531.7

530.4

529.9

Atomic %

1.2

12.2

5.4

6.5

6.8

FWHM (eV)

1.5

1.9

1.5

1.4

1.4

aThe Roman numerals represent the peaks shown in Fig. 8 designated by these same numerals

Peaks were resolved in the O1s high resolution XPS binding energy (BE) ranges of (534.4–533.3) eV, (533.1–532.0) eV, (532.2–530.7) eV and (531.7–529.9) eV. These regions are assigned to (CO32−, O–C=O), O–C, C=O and Fe–O, respectively [29]. The atomic percentage of Fe–O drops as MIL-53-Fe MOF is generated and deposited on MBC. It covers a large fraction of the surface including some of the surface magnetite. XPS does not detect elements that are 100 Å below the surface and is quite surface specific to the top 15–20 Å. Likewise, the high resolution C1s spectra show that the atomic percent of C=O drops after MIL-53-Fe MOF deposited on MBC (Table 2).

The Fe2p curve-resolved envelop was not interpreted due to its complexity. The Fe2p1/2 (724.7 eV) and Fe2p3/2 (711.3 eV) doublet BEs found for MBC match the standard BEs reported for magnetite [43]. Also, the Fe2p1/2 725.5 eV and Fe2p3/2 711.7 eV BEs for MIL-53-Fe MOF match its literature values [35]. After this MOF is generated on MBC, the Fe2p1/2 and Fe2p3/2 BEs exhibit peaks 725.5 eV and 711.8 eV, respectively. The maximum intensities of these two iron envelops in MOF–MBC are close to those of MIL-53-Fe MOF because such a large amount of the MOF was deposited and also it covers over much of the magnetite nanoparticles.

3.2 Adsorption Kinetics and Isotherms on MOF–MBC

3.2.1 pH Dependence of Rh B Adsorption

The maximum Rh B uptake on MOF–MBC occurred at pH 8 with a removal percent of ~ 75% (Fig. 9). However, uptake was 70% or more over the pH range 4–10, illustrating the excellent pH window offered by this adsorbent. The pKa value of Rh B is 3.7. So below pH 3.7 it is predominantly a positive ion (Rh B+). A pH value above 3.7 the carboxylic acid function increasingly deprotonates to produce its zwitterionic form (Rh B±) [44]. The points of zero charge (PZC) determined in this work for MIL-53-Fe MOF and MOF–MBC are 7.1 and 6.7, respectively. PZC of this MIL-53-Fe MOF is consistent with reported values in literature [45]. MOF–MBC is slightly more acidic because Fe3O4 is available on the surface (which is not covered by the MOF). The PZCs of BC and MBC, reported in our previous work [23], are ~ 9.3 and ~ 6.3, respectively. Analysis of Rh B adsorption versus pH is a complex task, because three different phases on MOF–MBC can adsorb Rh B (MIL-53-Fe MOF, the biochar surface after adding magnetite, and the magnetite nanoparticles) and each phase has a different PZC value and different chemical functionality. Lower Rh B removal percentages at pH 2 most likely occur due to electrostatic repulsions between the positive MOF–MBC surface, where all three surface phases carry net positive charge, and cationic Rh B+ which is present at pH 2. Since pH 6 gives a removal percentage almost as large as pH 8 and is closer to the natural pH of water, pH 6 was selected for kinetics and isotherm experiments instead of pH 8. At pH 14 the removal is ~ 100%, but the Rh B color in solution disappears prior to the adsorption at pH 14, perhaps due to its degradation or to tautomerization to UV inactive spirolactam form [46]. Thus, no Rh B remained in the solution at pH 14 when it was contacted with MOF–MBC (Fig. 9).
Fig. 9

pH dependence of aqueous Rh B adsorption onto MOF–MBC (50.0 mg MOF–MBC dose, 20 mL of 50 ppm Rh B, 1 h equilibration, 200 rpm agitation at 25 °C)

MOF–MBC’s three phases each might contribute to the Rh B+ cation or zwitterion adsorption. These include biochar (BC) regions, Fe3O4 nanoparticles and MIL-53-Fe MOF particles, all of which are positively charged below pH 6.7. The distribution of adsorbed Rh B among these phases are not known. Above pH 7.1 both Fe3O4 and MIL-53-Fe particle surfaces become negative. The biochar phase’s PZC after loading with magnetite at pH 10 followed by extensive washing is unknown and will not be the same as the original PZC of BC (9.3). The zwitterionic form of Rh B, increasingly predominant above pH 4, will adsorb at neutral surface locations and some changed sites. At pH ~ 7 and above, the overall MOF–MBC will be net negatively charged. The ability of Rh B’s –NEt2, ether and –CO2 groups to be H-bond acceptors, its –CO2H and –N+HEt2 to be H-bond donors, and its aromatic regions to be π-donors and π-acceptors provides this sorbate with a wide pH window (4–10) to be highly adsorbed on MOF–MBC.

3.2.2 Kinetics

Figure 10 plots Rh B removal versus time at initial concentrations of 25, 50 and 75 mg/L at 25 °C, illustrating a similar adsorption behavior in each case. Equilibrium was attained within 120–180 min. The experimental data fitted well to the pseudo-second-order kinetics model (Fig. 10 and Eq. 2) [47] with high (> 0.99) correlation coefficient values.
$$\frac{\text{t}}{{{\text{q}}_{\text{t}} }} = \frac{1}{{{\text{k}}_{2} {\text{q}}_{\text{e}}^{2} }} + \frac{\text{t}}{{{\text{q}}_{\text{e}} }}$$
(2)
Here, t is contact time, qe is the equilibrium capacity of Rh B (mg/g), qt is the capacity of Rh B (mg g−1) at time t (min), and k1 (g/mg min) is 2nd order rate constant.
Fig. 10

Effect of contact time on Rh B adsorption at 25 °C onto MOF–MBC (50.0 mg dose MOF–MBC, 50 ppm Rh B (20 mL), pH 6, and 200 rpm agitation)

3.2.3 Isotherms and Thermodynamics of Adsorption

Isotherm studies were performed at 10°, 25°, and 40 °C to determine the temperature dependence of Rh B adsorption per unit weight of adsorbent (qe). The data fit well to the Langmuir isotherm model with correlation coefficients > 0.99 for all three temperatures [48]. The Langmuir plot shows that capacity (qe) increased significantly with temperature 40 °C (73 mg/g) > 25 °C (48 mg/g) > 10 °C (40 mg/g) (Fig. 11).
Fig. 11

Adsorption isotherms at 10, 25 and 40 °C of Rh B uptake on MOF–MBC and thermodynamics (50.0 mg MOF–MBC, 3 h equilibration, pH 6, 20 mL solution and 200 rpm agitation)

The Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS), calculated using the van’t Hoff equation (3), were − 25.05 kJ/mol, 5.62 kJ/mol and 0.10 kJ/mol.K, respectively.
$${\text{lnK}} = - \frac{{\Delta {\text{H}}}}{\text{R}}\left( {\frac{1}{\text{T}}} \right) + \frac{{\Delta {\text{S}}}}{\text{R}}$$
(3)
Adsorption is spontaneous and the positive ΔH confirms the adsorption is endothermic, but its magnitude suggests physisorption is occurring [49].

3.2.4 Comparison of adsorbents for simultaneous removal of Rh B and Cr(VI)

Rh B only (~ 104 µM) and simulataneous Rh B (~ 104 µM) and Cr(VI) (~ 231 µM) (1:2 concentration ratio) adsorptive removal were studied at 25 °C and pH 6 (Fig. 12) on BC, MBC, MIL-53-Fe MOF and MOF–MBC. Equal weights of BC, MBC and MOF–MBC showed similar removal percentages (~ 66–71%) of Rh B. MIL-53-Fe MOF adsorbed less Rh B, ~ 36%, possibily related to its lower surface area or difficulty of the large Rh B molecule to penetrate the MIL-53-Fe MOF porous crystalline structure.
Fig. 12

Comparison of adsorbents for removal of Rh B (~ 104 µM) and Cr(VI) (~ 231 µM) (1:2 mol ratio) (50.0 mg adsorbent, 3 h equilibration, pH 6, 20 mL solution, 25 °C and 200 rpm agitation)

The amount of Rh B adsorption from a solution containing both RhB and Cr(VI) mixture dropped from ~ 66 to ~ 41% on BC, suggesting competition from Cr(VI) adsorption. In contrast, MBC and MOF–MBC did not exhibit a significant change in Rh B removal when Cr(VI) was present. Cr(VI) exists as both Cr2O72− and HCrO4 oxyanions in experiments at pH 6. The magnetite particles adsorb Cr(VI) at neutral or protonated surface sites (–Fe–OH and Fe–OH2+). Magnetite is reported to be a good adsorbent for Cr(VI) under slightly acidic conditions [50]. The MIL-53-Fe MOF is a poorer Cr(VI) adsorbent. However, Cr(VI) adsorption increases on both MIL-53-Fe MOF and MOF–MBC when Rh B is also present and being adosrbed.

All four adsorbents generally adsorb more Rh B than Cr(VI) from a neat solution or a mixture of Cr(VI) and Rh B. The biochar (BC) used to make MBC, MOF–MBC lacks functionality that can drive large amounts of Cr(VI) oxyanion adsorption. MIL-53-MOF is not a very good Cr(VI) sorbent. However, MOF–MBC removes ~ 26% of the Cr(VI) and ~ 62% of Rh B simultaneously showing it can function to remove both.

3.2.5 Rh B Degradation

Photo-induced Rh B degradation was conducted on pure aqueous Rh B (50 mg/L) and simulated tannery waste solutions (50 mg/L Rh B and 50 mg/L Cr(VI)) [51] by placing samples in front (3 cm distance) of a long range UV wavelength (365 nm) TLC UV-light (placed inside a box) for their respective times. The vials used (borosilicate) can transmit light down to 180 nm [52]. BC, MBC, MIL-53-Fe MOF, and MOF–MBC were added as heterogenous adsorbents/photocatalysts to each of these two types of solutions. Vials were covered with Al foil and pre-equilibrated in the dark with each adsorbent/catalyst in a shaker (at 200 rpm) for 3 h to establish the adsorptive removal equilibrium before UV exposure. The foil was removed to permit exposure to irradiation from the UV light, while magnetic stirring (50 rpm) was carried out during the catalytic photodegradation experiments. Figure 13a, b shows the pseudo first order degradation kinetics for Rh B in these experiments. Degradation of Rh B is relatively insignificant (both with (a) and without (b) Cr(VI) present) in the absence of any adsorbent/catalyst.
Fig. 13

Pseudo first order plots for photodegradation of a Rh B (50.0 mg catalytic adsorbent, pH 6, 25 °C and 20 mL of the solution) and b Rh B in the presence of Cr(VI) during adsorption both in a and b on BC, MBC, MIL-53-Fe MOF and MOF–MBC (Initial Rh B and Cr(VI) concentrations were each 50 mg/L before sorptive equilibrium)

BC modestly accelerated Rh B photo-degradation, both with or without Cr(VI) present. Photocatalytic activity has been reported previously for activated carbons via the formation of hydroxyl radicals from hydroxide ions at carbon surfaces or by capturing water molecules [53]. Thus, similar behavior might be expected with BC and MBC. However, MBC shows much higher photocatalytic activity than BC due to some role played by the dispersed magnetite nanoparticles. In situ formation of reactive oxygen species (ROS); superoxide (O2), hydroxyl (OH) radicals and hydrogen peroxide (H2O2) might be possible. These radicals are known to be produced by the photocatalytic reduction of oxygen and oxidation of water on the surface of magnetite by electron \({{\left( {{\text{e}}_{\text{CB}}^{ - } } \right)} \mathord{\left/ {\vphantom {{\left( {{\text{e}}_{\text{CB}}^{ - } } \right)} {{\text{hole}}\,\left( {{\text{h}}_{\text{VB}}^{ + } } \right)}}} \right. \kern-0pt} {{\text{hole}}\,\left( {{\text{h}}_{\text{VB}}^{ + } } \right)}}\) pair generation [53]. Some further degradation effect could be expected from the biochar phase in MBC. Adsorptive removal by the biochar phase also occurs during the photodegradation experiments. The sorbed Rh B may continuously degrade and less strongly sorbed products go into the solution reversibly, making available more surface to adsorb Rh B.

Rh B degradation on MBC was slightly slower in the model tannery waste (Fig. 13). Perhaps surface adsorption of Cr(VI) onto magnetite particles decreased the availability of free hydroxyl functional groups to generate ROS. Rh B alone exhibited high photodegradation rates on MIL-53-Fe MOF and MOF–MBC. However, both their performances are further accelerated by the presence of Cr(VI) (simulated tannery waste). This enhancement can be attributed to the more complete oxidation of intermediate degradation products to CO2 and H2O by Cr(VI) [42, 54, 55]. MIL-53-Fe MOF acts as a heterogenous photocatalyst to degrade Rh B (Fig. 13) and it can also adsorb some Rh B (Fig. 12). Thus, both magnetite nanoparticles and MIL-53-Fe MOF are the active catalysts, but the relative importance of the roles they play using MOF–MBC is unknown. The MOF was deposited after magnetite. Thus, much of the magnetite surface has been covered and this portion may not play a role in adsorption or photodegradation. Differences in surface areas of the active phases, their availability to light, their porosity, and other factors currently complicated straight forward explanations and understanding.

3.2.6 Degradation Product Identification

The photodegradation product solutions from the simulated tannery waste of all four adsorbents (BC, MBC, MIL-53-Fe, MOF and MOF–MBC) were analyzed by LC–MS (Figs. S4 and S5) using Electro Spray Ionization (ESI) in the positive ion mode, where most compounds provide their (M + H)+ fragments. N-Deethylated intermediates such as N,N-diethyl-N′-ethyl-rhodamine (C26H26N2O3, m/z 415) were identified [56]. In addition, 2-(2,5-dihydroxypheny)acetic acid (C8H8O4, m/z 168), malonic acid (C3H4O4, m/z 104), oxalic acid (C2H2O4 m/z, 90), acetic acid (C2H4O2, m/z 60) and formic acid (CH2O2, m/z 46) were identified [57]. These products originate from chromophore cleavage from deethylated Rh B intermediates and ring-opening [56, 58]. Some intermediates reported in previous Rh B degradation studies over MIL-53-Fe MOF were not observed [57, 59]. This could be due to the rapid further oxidation of these unstable intermediates by Cr(VI). Importantly, when the remaining solution was analyzed after adsorptive photodegeneration of Rh B in tannery waste [Cr(VI) present] over MOF–MBC no, organic degradation products could be detected. This suggests conversion to water and CO2 might be close to quantitatively achieved.

3.2.7 Iron Leaching Experiments

Some iron leaching was expected at acidic pH based on our previous studies with MBC during Arsenite and phosphate adsorption on MBC [29]. Thus, iron leaching was studied at pH 1–13 at a pH interval of 2 from all four adsorbents (BC, MBC, MIL-53-Fe MOF and MOF–MBC). BC did not leach iron in significant amount, which was expected since BC contained only 0.08 wt% Fe. In the leaching experiments (Fig. S10a and b), 50 mg of each adsorbents was immersed in 20.0 mL of water at each pH and stirred at 200 rpm in a shaker for 3 h. In Fig S10a) only Rh-B 50 mg/L was present, which in Fig S10b) 50 mg/L of both Rh-B and Cr(VI) 50 mg/L were present.

In all cases the amount of iron leached into solution increased as the pH becomes more acidic. In both the presence and absence of Cr(VI), the amount of iron leached increased in the order MBC < MOF–MBC < MIL-53-Fe MOF. More iron leaches when Cr(VI) is present at all equivalent pH values. The pH dependence of iron leaching was higher for MIL-53-Fe MOF and MOF–MBC. Clearly MBC (with Fe3O4 nanoparticles) resists acidic leaching better than the MIL-53-Fe MOF.

These studies teach us that MBC will be a more desirable adsorbent and can be used over a wide pH range (Fig. S10) and that MIL-53-Fe MOF and MOF–MBC may erode faster in following acidic solutions.

3.2.8 Comparison of Rh B Sorption and Photocatalytic Degradation Adsorbents

Table 3 summarizes previous reports Rh B uptake and photocatalytic degradation on different adsorbents/catalysts.
Table 3

Comparative evaluation of various adsorbents/catalysts for Rh B sorption and degradation

Adsorbents

Temp./°C

Conc.

Kinetics

pH

Surface area (m2/g)

Adsorption capacity, Q0 (mg/g)

References

MIL-100(Fe)

30

400 mg/L

2 h

7

35.77

[60]

Fe3O4

1.52

Fe3O4/MIL-100 (Fe)

206.14

28.36

Activated carbon

40

 

2 h

2.1

5.33

[61]

Rice husk

5.87

MOF–MBC

10

5–1000 mg/L

3 h

6

42.2

40.24

This study

25

47.69

40

73.05

Catalyst

WL (nm)

Temp/°C

Conc.

Kinetics

pH

Surface area (m2/g)

Degradation percentage

References

TiO2

254

30

 

14.4

[61]

MIL-53-Fe MOF (1:2:190)

420

25

10 mg/L

3 h

7

38.17

80

[37]

Fe3O4/H2O2

420

25

10 mg/L

70 min

20

[62]

MIL–53(Fe) hybrid magnetic composites

55

H2O2/UV

254

25

10 µM

30 min

7

7

[63]

MOF–MBC

365

25

50 mg/L

20 h

6

42.2

70

This study

50 mg/L with + 50 mg/L Cr

95

To best of our knowledge reports on Rh B sorption onto MIL-53-Fe MOF were not reported in the literature

4 Conclusion

For the first time an MOF has been successfully immobilized on a high surface area biochar carrier surface to serve as both an adsorbent and UV photodegradation catalyst to degrade the pollutant (Rh B) being adsorbed. A second dispersed Fe3O4 nanoparticle component on the biochar surface serves to allow magnetic adsorbent manipulation, allowing facile adsorbent removal from batch solutions for regeneration [29, 64]. The presence of three different adsorbing surface phases permits a wider application of this single sorbent. High surface area biochar allows dispersion of other small co-adsorbent particle phases (Fe3O4 and MIL-53-Fe MOF) on biochar adsorbent particles. These biochar carriers are large enough to allow column flow. In contrast, use of the tiny Fe3O4 and MOF particles alone would create larger pressure drops and slow flow. Many MOF properties can be improved by functionalization followed by immobilizing derivatized MOF on biochar or other carriers. In this work, toxic Rh B was photodegraded and its toxicity neutralized in the same adsorption remediation step, Chromium(VI), is reduced to Cr(III). This is a preliminary study of a very complex catalytically active adsorbent. It is very far from being optimized and well understood. Iron is leached into the Rh B and Rh B/Cr(VI) solutions. Thus, it and Cr(VI) may absorb impinged UV–Vis light along with Rh B in the solvent phase. Adsorbed Rh-B can also absorb light or a variety of radical intermediate species may be generated at the surface. We see no advantage in currently speculating about mechanisms. Future studies which decrease the amount MIL-53-Fe MIF deposition should permit greater Cr(VI) adsorption capacity by uncovering and exposing Fe3O4 surface. Improved MIL-53-Fe MOF crystallization generating smaller well formed crystals should increase its surface area. The independent photocatalytic behavior of the Fe3O4 nanoparticle adsorbent was initially unforeseen, as it was prepared for magnetic manipulation. However, the simultaneous high surface area exposure of both Fe3O4 and MIL-53-Fe MOF needs to be explored. Also layering MOF adsorption columns to promote Rh B photodegradations followed by MBC or MBC-MOF for adsorption will be explored.

Notes

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant No. 1659830.

Supplementary material

10904_2019_1322_MOESM1_ESM.docx (8.8 mb)
Supplementary material 1 (DOCX 8996 kb)

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  • Chanaka M. Navarathna
    • 1
  • Narada B. Dewage
    • 1
  • Akila G. Karunanayake
    • 1
    • 2
  • Erin L. Farmer
    • 1
  • Felio Perez
    • 3
  • El Barbary Hassan
    • 4
  • Todd E. Mlsna
    • 1
    Email author
  • Charles U. PittmanJr.
    • 1
    Email author
  1. 1.Department of ChemistryMississippi State UniversityMississippi StateUSA
  2. 2.Biochar Supreme Inc.EversonUSA
  3. 3.Material Science Lab, Integrated Microscopy CenterUniversity of MemphisMemphisUSA
  4. 4.Department of Sustainable BioproductsMississippi State UniversityMississippi StateUSA

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