Separation and Purification Technology 247 (2020) 116975
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Separation and Purification Technology
journal homepage: www.elsevier.com/locate/seppur
Recovery of magnetite from fluidized-bed homogeneous crystallization of
iron-containing solution as photocatalyst for Fenton-like degradation of RB5
azo dye under UVA irradiation
Nicolaus N.N. Mahastia, Yu-Jen Shihb, Yao-Hui Huanga,
a
b
T
⁎
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords:
Decolorization
Mineralization
Fluidized-bed homogeneous crystallization
Supersaturation
Superparamagnetic
Iron recovery from wastewater as magnetite (Fe3O4) pellets was performed in a fluidized-bed homogeneous
crystallization (FBHC) process. The catalytic effectiveness of magnetite in the heterogeneous degradation of RB5
azo dye was evaluated. The key parameters of FBHC, including effluent pH, cross-section loading (L, kg/m2.h),
initial iron concentration ([Fe(II)], mg/L), and inlet supersaturation (S), were optimized to maximize the total
iron removal (TR, %) and crystallization ratio (CR, %) from a fluidized-bed reactor. Under the conditions of
pH = 8.5–9.5 and [Fe(II)]in = 100 – 500 mg/L, the TR and CR values reached 99% and 77%, respectively. The
magnetization and coercivity of the recovered Fe3O4 pellets were analyzed to confirm the magnetic characteristic. A Fenton-like degradation of RB5 azo dye was activated using the Fe3O4 pellets under UVA irradiation,
pH = 2.75 – 3.0, initial peroxide concentration of 10 mM, and catalyst loading of 2 g/L. The efficiencies of
decolorization and mineralization reached about 98 – 99% and 60 – 68%, respectively. FBHC method benefited
the recovery of high-purity metal oxides and the production of a low quantity of sludge. Magnetic pellets
transformed from the dissolved iron in wastewater were promising materials to reuse as a photochemical reactant for organic pollutant degradation.
1. Introduction
Iron (Fe) is abundant in surface waters due to the leaching of ironcontaining rocks by rain [1]. The dumping of domestic and industrial
waste has polluted water with a significant level of iron. The iron and
steel industries, mining, and metal corrosion are the main activities that
discharge iron-containing wastewater [2]. Iron concentrations of 0.3
and 5 mg/L are recommended for drinking water and irrigation water,
respectively, according to the World Health Organization and the Food
and Agriculture Organization [3,4]. Excess ferrous ions can reduce light
intensity underwater and disturb the photosynthesis of aquatic plants.
Chemical oxidation followed by precipitation is applied to remove iron
from wastewater. An increase in pH stimulates iron oxidation and
precipitation, which can yield 1.2 m3 of iron sludge from 10 m3 of
wastewater. The purity of iron in waste sludge can be as low as 2 wt%
[5]. A large amount of the sludge was also produced from the heavy
metal removal in the wastewater treatment facilities.
Waste sludge is considered a secondary pollutant that increases the
cost of waste disposal [6].Fluidized-bed crystallization (FBC) has been
⁎
developed to solve the problem of sludge production in the removal of
heavy metals and inorganic acids processes. The inert seeds such as
silica sand was applied to collect the precipitation of the target pollutant. The low degree of supersaturation was set to initiate the heterogeneous nucleation from the reaction between target pollutant and
precipitant, and to grow the metal salt crystal [7,8]. However, the seed
material as an impurity of the crystal significantly limited the reusability of the product. Fluidized-bed homogeneous crystallization
(FBHC) has been developed to remove various kinds of metal pollutant
from wastewater and recover high purity metal-salt or metal-oxide
granules with low water content [9–12]. The pellets collected from
FBHC in the removal of metal ions or anions pollutants could be valuable products, such as calcium carbonate and hydroxyapatite from
the water softening and phosphate removal, respectively [10]; nickel
carbonate hydroxide [12] and lead carbonate [13] from heavy metal
removal. The authors have previously studied FBHC for the recovery of
FeOOH crystals from iron-contaminated solution [14]. In addition,
Bailey (1992) reported the adsorption of Cr(IV) using iron oxide coatedsand product from batch co-precipitation method in a packed column
Corresponding author.
E-mail address: yhhuang@mail.ncku.edu.tw (Y.-H. Huang).
https://doi.org/10.1016/j.seppur.2020.116975
Received 30 December 2019; Received in revised form 23 March 2020; Accepted 23 March 2020
Available online 29 April 2020
1383-5866/ © 2020 Elsevier B.V. All rights reserved.
Separation and Purification Technology 247 (2020) 116975
N.N.N. Mahasti, et al.
and solute-poor liquid phases occurs. With increasing supersaturation
degree, the nucleation of amorphous nanoclusters within the solute-rich
liquid phase occurs, followed by the crystallization of the amorphous
clusters [27]. In one study, after a sufficient number of nuclei formed,
the recirculation stream was increased to reduce the supersaturation
condition and increase the collision chance among the small particles to
make them grow. The aggregation of molecules followed by structural
modification on the surface of the existing particles led to crystal
growth [28].
The magnetite nanoparticles have been utilized to catalytically
oxidize some organic pollutants, such as hexabromocyclododecane
[29], dichlorophenol [30], triethylenetetramine [31], and azo dye [32].
Magnetite can induce the Fenton-like process due to its spinel structure.
The Fe(II) in the tetrahedral site is an electron donor for activating
H2O2 to create %OH and various reactive oxygen species (ROS) such as
superoxide radicals and singlet oxygen. Then, the octahedral site accommodates the oxidized Fe(II) [33,34]. The degradation of RB5 azo
dye was initiated by breaking the azo bonding (ReN]NeR), followed
by the cleavage of the benzenic and napthalenic cycle, which resulted
in a reduction of absorption intesity at 597 nm and 310 nm, respectively, on the UV–vis spectrum. The final products dissociated from dye
molecules would be nontoxic CO2, H2O, NO3−, and SO42− [35].
In this work, the iron removal from the iron-containing solution was
recovered as photocatalyst for the degradation of organic pollutant. The
unseeded crystallization of magnetite was evaluated in FBHC process in
terms of optimizing experimental parameters, including effluent pH and
cross-sectional loading, to maximize the crystallization ratio (CR) and
total iron removal (TR). RB5 azo dye was utilized as a model contaminant to assess the effectiveness of magnetically separable magnetite pellets in activating the UVA photo-assisted Fenton-like reaction.
[15]. Magnetite (Fe3O4) was applied in wastewater treatment, such as,
adsorbent for arsenic and other various heavy metal such as Pb2+,
Zn2+, Cd2+, and Cu2+ [16–18] or catalyst for photochemical degradation of organic pollutants [19,20].
Accordingly, an increase in solution pH ferrous ion would be hydrolyzed as different Fe(II) species, which leads to the partial oxidation
to Fe(OH)2 with different rates to Fe(III). The formation of Fe3+
strongly depends on the dissolved oxygen, which is subject to partial
pressure of oxygen gas [21]. (Detailed derivatives are added in supporting materials)
−d [Fe 2 +]
dt
−d [Fe 2 +]
dt
= k [Fe 2 +][OH−]2 PO2 (1)
= (k 0 [Fe 2 +] + k1 [Fe (OH )+] + k2 [Fe (OH )20] + k3 [Fe (OH )3]) D
(2)
O
The hydrolysis of Fe(II) produces the Fe3O4 phase:
O2 + 4H+ + 4e− ⇄ 2H2 O ; K O = 1083.14 (E 0 = 1.23 V )
(3)
Fe3 O4 + 8H+ + 2e− ⇄ 3Fe 2 + + 4H2 O ; KM = 1033.84 (E 0 = 1.001 V )
(4)
The spontaneous oxidation of ferrous ions results in their precipitation as magnetite. Near the thermodynamic equilibrium, the solubility of total iron is affected by the atmospheric oxygen pressure
[22,23].
2Fe3 O4(s) + 12H+(aq) ⇄ 6Fe 2 +(aq) + O2(aq) + 6H2 O(l) ;
Ksp1 = KM 2/ K O = 10−15.46
(5)
The solubility of magnetite therefore strongly depends on the dissolved oxygen (Eq. (5)).
Ksp1 =
6
[FeT ]6 αFe
[Fe 2 +]6 [O2]
(II ) [O2]
=
+
12
[H ]
[H+]12
2. Materials and methods
(6)
The degree of supersaturation can be determined as:
S=
2.1. Chemicals
αFe
Sat
αFe
All chemicals were laboratory-grade and used without any further
purification. The iron-containing solution was prepared by dissolving
FeSO4·7H2O (99%, Panreac, Germany) in deionized water. Na2CO3
(99.2% Sesoda, China) was applied as a feed to condition the buffering
of pH change during precipitation. Sodium hydroxide (97%, Showa,
Japan) and sulfuric acid (95 – 97%, Merck, Germany) were applied to
adjust the pH in the inlet tank. The azo dye solution was prepared by
dissolving Reactive Black 5 (RB5, 55%, Sigma-Aldrich, Germany) in
deionized water·H2O2 (50%, Chang Chun Petrochemical, Taiwan) was
applied as oxidant. For the quenching experiment, Tert-butanol
((CH3)3COH, 99.5%, Merck, Germany) and furfuryl alcohol (C5H6O2,
98%, Acros Organics, Belgium) were prepared as hydroxyl radical and
singlet oxygen scavengers respectively. The water was deionized using
a reverse osmosis ultrapure water system (resistance > 18.3 MΩ·cm).
(7)
where
sat
sat
aFe = γFe ∗ [Fe]T and aFe
= γFe
∗ [Fe]Tsat
(8)
In the presence of carbonate, however, the precipitation of ferrous
carbonate competes with that of hydroxides, in which both phases are
transformed to magnetite by dissolved oxygen [24].
FeCO3(s) + H+ ⇄ Fe 2 + + HCO3−; Ksp2 = 10−0.18
Fe (OH )2(s) +
2H+
⇄
Fe 2 +
+ 2H2 O ; Ksp3 =
1011.69
(9)
(10)
6FeCO3(s) + O2 + 6H2 O ⇄ 2Fe3 O4(s) + 6H+ + 6HCO3− ; K = 1022.55
(11)
6Fe (OH )2(s) + O2 ⇄ 2Fe3 O4(s) + 6H2 O ; K =
1093.78
(12)
2.2. Fe3O4 synthesis via FBHC
The speciation of ferrous and ferric ions as hydroxyl ions depends on
pH [25,26]. The theoretical solubility of total iron (FeT) can be estimated from the solubility products of the two main precursor phases,
i.e., Eqs. (11) and (12):
K Sp2 =
K Sp3 =
[FeT ] αFe (II ) CT αHCO3−
[Fe 2 +][HCO3−]
=
+
[H ]
[H+]
[Fe 2 +]
[H+]2
=
[FeT ] αFe (II )
[H+]2
Iron recovery (as magnetite) from the iron-containing solution was
conducted in a cylindrical FBHC reactor (450 ml) made of Pyrex, as
shown in Fig. 1. The FBHC reactor had two cross-sectional areas, one
each for the upper (diameter: 4 cm; height: 15 cm) and lower (diameter:
2 cm; height: 80 cm) parts. The upflow velocity suddenly dropped when
the solution was uplifted to the upper region, thus increasing the collision chance among the formed primary crystals. The flow rates of the
stock solutions, namely 3.5 mM FeSO4·7H2O (pH 2.8) and 4.2 mM
Na2CO3 (pH 11.6), pumped into the reactor were 5 ml/min. At effluent
pH 8.5 – 9.5, small black particles formed. The rate of recirculation was
increased to 90 ml/min to reduce the degree of supersaturation and
grow particles.
(13)
(14)
where CT is the total carbonate, and αHCO3- and αFe(II) are fractionation
of HCO3− and Fe2+ species, respectively.
Once the concentration of the solute exceeds the solubility (i.e.,
supersaturation is reached), spinodal decomposition into solute-rich
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Separation and Purification Technology 247 (2020) 116975
N.N.N. Mahasti, et al.
Fig. 2. Configuration of three-phase fluidized-bed Fenton reactor.
of RB5 and H2O2 were 50 mg/L and 10 mM, respectively. During the
chemical reaction, 98% H2SO4 was dropwise added to maintain pH at
2.75 – 3. The ranges of pH and initial H2O2 concentration have been
tested in the preliminary experiment (supporting material) under constant temperature (25 – 30 °C) using a water bath. A UVA lamp (15 W,
UVP BL-15 365 nm, Philips, Poland) was turned on to activate the
catalytic reaction. The chemical reaction was stopped by adding 1 M
NaOH to the sample before total organic carbon (TOC) analysis.
Fig. 1. Fluidized-bed crystallization apparatus.
2.3. Iron removal experiments
The FBHC process was initiated at a bed height of about 25–30 cm.
Stock solutions of Fe(II) at various concentrations (100, 300, 500 mg/L)
and CO32–, which was adjusted to yield a molar ratio of [total carbonate]/[Fe(II)] = 1.2, were pumped into the reactor at 50 ml/min. In
the real case, iron pollution level in the water existed in a very wide
range depends on the area and the pollution source, such as: 1.5 –
80 mg/L and 15 mg/L in the groundwater in Denmark and China respectively [36,37], 25 – 55 mg/L in the fracking wastewater from
natural gas refinery [38], and 500 – 700 mg/L in the acid mine drainage
[5]. The initial iron concentration was set at 100 – 500 mg/L to represent the effect of the different inlet iron concentration and the
process limitation for the certain iron loading. The recirculation at
50 ml/min was set to maintain the upflow velocity in the lower part of
FBHC at 28.6 m/h. The effluent pH (5 – 12) was conditioned by adjusting the pH of the stock solution using NaOH (40%) and H2SO4
(98%). For each variable adjustment, the FBHC reaction lasted at least
40 times the hydraulic retention time (HRT) to obtain steady-state effluent quality. Two samples were taken at specific intervals; one was
filtered using a filter with a 0.45-μm pore size and the other was unfiltered and digested using 1 N H2SO4. The filtered sample was used to
estimate TR, which was in the form of either fine nuclei that had
drained out or coarse pellets that had fluidized in the reactor. The aciddigested sample was used to calculate the CR, which reflects the
quantity of iron crystallized on the surface of the particles.
CR% = 1 −
[Fe]T × Qt
CFe × QFe
(15)
TR% = 1 −
[Fe]s × Qt
CFe × QFe
(16)
2.5. Analytical methods
The total iron concentration in the samples was measured using
inductively coupled plasma-optical emission spectrometry (ICP-OES
ULTIMA 2000, HORIBA Ltd., Japan). The carbonate and TOC were
determined using a TOC/inorganic carbon analyzer (Sievers Innovox
Laboratory, USA). The dissolved oxygen was measured using a WTW
ProfiLine Oxi 3210 Portable DO meter. Absorbance of RB-5 at 310 and
597 nm was analyzed using a diode array UV–Vis spectrometer (Agilent
8453, USA). The magnetite crystal product was taken after draining out
the solution in FBHC reactor. The collected crystals were dried at
30–35 °C overnight in an oven. The crystalline phases were characterized using X-ray diffraction (XRD, DX III, Rigaku Co., Japan) (diffraction angles: 10° − 90°; scan step: 0.02°; scan rate: 10°/s). The morphology was observed using scanning electron microscopy (SEM, JEOL
JSM-6700F, HITACHI, Japan) at 10 kV and 7 mA. The magnetic
properties of the magnetite product were analyzed using a superconducting quantum interference device vibrating sample magnetometer (SQUID VSM, MPMS-3, American Quantum Design, USA) in the
magnetization range of ± 10000 gauss. The surface compound were
determined using Fourier-transform infra-red (FT-IR, Thermo Scientific
Nicolet 6700, USA) and X-ray photoelectron spectroscopy (XPS, British
Thermo VG-Scientific / Sigma Probe, UK)
3. Results and discussion
2.4. Fenton-like degradation of RB5 azo dye
3.1. Iron recovery as Fe3O4 pellets via FBHC
The magnetite pellets recovered from the FBHC process were applied as a heterogeneous catalyst to remove RB5 azo dye under UVA
irradiation. A glass cylindrical reactor (diameter: 8 cm; height: 44 cm)
with a membrane diffuser was applied in this experiment, as shown in
Fig. 2. The 2 g/L of magnetite added in the solution was fluidized by
pumping homogeneous air at the bottom of the reactor at a flow rate of
6 sL/min (standard liter for air per minute). The initial concentrations
The FBHC process for the removal of iron was optimized using the
bed of magnetite pellets produced in the preceding step as the fluidized
medium. The flow rates of the FeSO4 and Na2CO3 streams fed into the
reactor were both 50 ml/min. The recirculation rate was adjusted at
50 ml/min to create a bed expansion of 25%, corresponding to an upflow velocity of 28.6 m/h, which was greater than the minimum fluidization velocity Umf based on the Ergun equation [39]:
3
Separation and Purification Technology 247 (2020) 116975
N.N.N. Mahasti, et al.
2
Δp
(1 − εm)2 μu 0
1 − εm ρf μ0
= 150
+ 1.75
2
2
3
L
εm
εm
(Φs dp)
Φs dp
5Fe (OH )2 + Fe 2 + + CO32 − +
(17)
[CO3. H2 O]
For the minimum fluidization condition:
g
Δp
= (1 − εmf )(ρs − ρf )
gc
Lmf
1
O2 + 3H2 → [Fe4(II ) Fe2(III ) (OH )12 ]
2
[Fe4(II ) Fe2(III ) (OH )12 ][CO3. H2 O] + O2 → 6α − FeOOH + H2 CO3 + 4H2 O
(24)
(18)
2α − FeOOH + Fe (OH )2 → Fe3 O4 + H2 O
Combining Eqs. (17) and (18) yields:
2
dp3ρf (ρs − ρf ) g
150(1 − εmf ) ⎛ dp umf ρf ⎞
1.75 ⎛ dp umf ρf ⎞
⎜
⎟ +
⎜
⎟ =
Φs εmf 3 ⎝
μ
Φs 2εmf 3 ⎝
μ
μ2
⎠
⎠
(23)
(25)
Cross-sectional loading represents the fluidized-bed capacity in the
removal of a unit weight of iron per square meter and time as plots in
Fig. 4. The degree of supersaturation increases with increasing iron
loading (Eqs. (6) and (7)). The supersaturation is related to the Gibbs
free energy that drives the saturated solute to precipitate. When the
loading to supersaturation in the region of homogeneous nucleation is
increased, the solute is converted to fine particles that are easily
drained out from the reactor by the upflow, leading to an increase in the
total iron concentration in the effluent. However, the soluble iron level
in the effluent stream remains low under high supersaturation. As a
result, the CR decreases as the loading increases, while the TR remains
constant when the cross-sectional loading increases to beyond 1.5 kg/
m2·h.
(19)
where
dp = particle diameter
Φs = sphericity
εmf = porosity
μ = viscosity
ρf = fluid density
ρs = particle density
umf = up-flow velocity
g = gravity acceleration
3.2. Characterization of FBHC pellets
For small particles:
Re < 20 umf =
(Φs dp)2 ρs − ρf ⎛ εmf 3 ⎞
.
g⎜
⎟
150
μ
⎝ 1 − εmf ⎠
Re > 1000 umf 2 =
Re =
Φs dp ρs − ρf
.
. gεmf 3
1.75
ρf
The XRD peaks of the FBHC particles indicate a mineral composition
of magnetite (JCPDS standard #19-0629), as shown in Fig. 5c. Two
samples were taken, respectively from the top and bottom parts of the
particle bed; the sample collected from the upper region consisted of
the initial particles, whereas that taken from the bottom region consisted of coarse crystals [45,46].
Fig. 5a–b show the SEM micromorphology of an FBHC pellet. The
particles grew up to 1.5–2 mm in diameter. Under high magnification,
the surface of the pellet is relatively smooth because of the collisions
and abrasion between particles promoted by the up flow of the solution
[47]. The primary particles of Fe3O4, which are around 100 nm in
diameter and irregularly aggregated, constitute the core of the pellet
(Fig. 5b). There are two steps in the crystal growth: initiation by the
solute diffusion from the bulk liquid to the solid-liquid interface, followed by the integration of the solute into the crystal lattice on the
surface [48]. The growth of small crystals was driven by a lowering of
the surface energy in the total interfacial area, resulting in an increase
in the size of aggregated pellets [49]. FTIR analysis also introduced to
the magnetite product as depicted in Fig. 5d. A dominated peak appeared at 584 cm−1 indicated the existence of Fe-O stretching in Fe3O4
[50]. Two additional peaks also appear at 796 and 890 cm−1 show the
deformational (δ) modes of hydroxyl group [51].Three similar peaks
could be observed in the commercial Fe3O4 indicated the similar surface structure between the magnetite from FBHC product to the standard commercial magnetite. The surface analysis of the magnetite
product was determined by XPS to verify the existence of Fe(II) and Fe
(III). As shown in Fig. 6, the peaks at 711.6 and 724.9 eV represented
the binding energies of Fe2p1/2 and Fe2p3/2 on the Fe3O4 structure,
which evidenced FBHC product consitsted of the crystal phase of Fe3O4
[52,53]. Fig. 7 plots the effect of the degree of supersaturation on the
CR at various input iron concentrations. The supersaturation (log S) was
defined as the difference between the measured iron activity at the
effluent (aFe) and the thermodynamically determined solubility of a
specific precipitate (aFeSat). The magnetite was a much stable material,
which should be phase-transformed from Fe(II) species at long-term
operation, compared to carbonate and hydroxide phase. Since Fe
(OH)2(s) or FeCO3(s) precipitated at very early stage, the supersaturation
was estimated based on solubility products of Fe(II) species. The ideal
solubility was obtained from eq. (13) and (14), which gave
(20)
(21)
ρUmf D
μ
(22)
As a result, the Umf is 11.40 m/h when the density of water (ρf) is
1 g/cm3, solution viscosity (μ) is 1 cp, and measured particle density
(ρs) is 2.66 g/cm3, and void ratio (ε) is 0.44, resulting in a Re number of
1.58 (Re < 20).
Fig. 3a shows that both CR and TR increase with increasing pH for
pH values of 6 to about 8.5; at pH beyond 8.5, TR remains constant and
CR drops. The ability of FBHC to remove iron in crystal form depends
on the degree of supersaturation, which is affected by pH (Eqs. (13) and
(14)). Under low supersaturation, the particles grow via the assembly of
nuclei onto the existing surface of seeds [7,40]. In contrast, the high
Gibbs energy of the system that results from high supersaturation
(ΔG = −RTlnS) drives new nuclei to form homogeneously in bulk
solution [41]. The TR of iron was ascribed to precipitation in the form
of either small particles or crystallized pellets. The generation of fine
particles, which were drained out from the reactor by the upstream,
increased the total iron level in the effluent, and thus the CR declined
significantly.
Fig. 3b shows the dissolved oxygen consumption as a function of pH
for the precipitation of magnetite. The oxygen consumption is related to
the formation of Fe(II) precipitate, followed by the oxidative transformation into Fe3O4. As shown in Fig. 3c, with increasing pH, Fe(II)
consumption increases, reaching a maximum at pH above 9 for the
selected range of initial concentrations of 100 – 500 ppm, at which the
solubility of Fe(II) is minimized. However, the residual inorganic
carbon as carbonate species increases with increasing pH during the
FBHC process, as shown in Fig. 3d. Accordingly, Fe(OH)2 became predominant in the removal of Fe(II) as the solubility of FeCO3 increased at
pH greater than 10 (Ksp2 vs. Ksp3) (Fig. 7c). The carbonate ligand acts as
a buffer that prevents Fe(OH)3 acute precipitation in the presence of
dissolved oxygen; the green rust of ferrous carbonate as the intermediate transform to magnetite in alternative pathways [42–44].
[Fe 2 +] =
K Sp2 [H +]
CT αFe (II ) αHCO −
3
, and [Fe 2 +] =
K Sp3 [H +]2
αFe (II )
for precipitation of ferrous
carbonate and hydroxide, respectively. S is strongly related to the bulk
4
Separation and Purification Technology 247 (2020) 116975
N.N.N. Mahasti, et al.
Fig. 3. Effect of input Fe concentration on (a) effluent total removal (TR) and crystallization ratio (CR) of Fe, (b) dissolved oxygen (DO) removal, (c) ferrous removal,
and (d) inorganic carbon (C) residual as a function of pH (flow rates QFe = QCO3 = QR = 50 ml/min; U = 28.6 m/h; HRT = 3 min; molar ratio of [carbonate]/[Fe
(II)] = 1.2).
(v). The value of rC can be determined as the bulk energy is balanced
out by the surface energy (γ). The energy gap (ΔG*) that has to be
overcame to form a new nucleus should be:
ΔG∗ =
Gibbs free energy (ΔGb) for the formation of crystals with a critical
radius rC [54,55]:
and rC = −
(26)
3.3. Magnetite product application for treating azo-dye-containing
wastewater
2γv
aFe
⎞
kT ln ⎛ Sat
⎝ aFe ⎠
(28)
At a specific S, the deposition of iron onto the seed surface increases.
However, once S reaches a level where the nucleus can be created
homogeneously, the iron in the FBHC system prefers to precipitate as
sludge [56,57]. As indicated in Fig. 7, the FBHC should operate at relatively low supersaturation for Fe(OH)2 compared to FeCO3, namely
pH around 9 that FeCO3 is the predominant phase to oxidatively
transform as Fe3O4 (Eq. (11)). In addition, the FBHC was optimized at
around 80% of CR under a supersaturation degree range of 0.28 – 2.15.
Fig. 8 plots the magnetization curve of Fe3O4 pellets obtained from
the FBHC process in a size range of 25–35 mesh against the magnetic
field, revealing that the magnetization and coercivity are about
79.45 emu/g and 5.60 Oe, respectively, at room temperature. The
magnetization of FBHC product was similar to that of the commercial
magnet sample; the FBHC pellets are magnetically soft possibly due to
their relatively low coercivity. The low magnetization is ascribed to a
decrease in crystalline size and crystallinity. Further, coercivity is
proportional to the number of crystal defects [58–61]. The high magnetization and low coercivity confirm the super paramagnetism of the
FBHC pellets in this work [59–62].
Fig. 4. Effect of cross-sectional surface loading on total removal (TR) and
crystallization ratio (CR) of iron at total flow rate Q = 150 ml/min,
U = 28.6 m/h, HRT = 3 min and molar ratio of [carbonate]/[Fe(II)] = 1.2.
aFe ⎞
ΔGb = −RT ln S = −jkT ln ⎜⎛ Sat
⎟,
⎝ aFe ⎠
16πγ 3v 2
3(kT ln S )2
(27)
A photo-assisted Fenton-like reaction was carried out to demonstrate the ability of FBHC pellets to degrade RB5 azo dye solution. The
where j is the ratio of the particle volume to the molar volume of nuclei
5
Separation and Purification Technology 247 (2020) 116975
N.N.N. Mahasti, et al.
Fig. 5. XRD analysis and SEM image of magnetite crystal product: (a) SEM image of magnetite whole particle, (b) inner structure of magnetite particle, and (c) XRD
patterns for magnetite product, (d) FTIR analysis for magnetite product.
about 82%; whereas the efficiencies without magnetite catalyst could
achieve 66% and 47%, respectively (Fig. 9a). In the dark reaction, the
removal rate was 14% in 6 h, which represented the specific adsorption
of dye molecules onto the catalyst surface. As shown in Fig. 9b, the TOC
removal which represents the mineralization of RB5 into carbon dioxide and water reaches less than 40% without magnetite catalyst at
the end of the reaction. By contrast, with the aid of the magnetite
catalyst, the efficiency of TOC removal increases up to 68%. The hydroxyl radicals %OH are produced through either the Fenton-like reaction, in which the adsorbed H2O2 is activated with the surface oxidation
of Fe(II) to Fe(III) on the iron oxide particles [63], or the homogeneous
Fenton reaction, which is induced by the dissolved iron and H2O2
[64,65]. Another ROS (such as HO2%, O2%−, and O21) also produced by
the chain reaction of radicals or stimulated by the dissolved oxygen
experimental conditions were controlled at 2 g/L Fe3O4 pellets, 10 mM
H2O2, and UVA irradiation for treating 50 mg/L RB5. The particles were
fluidized by pumping the air at a flow rate of 6 sL/min through the glass
membrane at the bottom of the reactor. The Fenton-like reaction was
performed for 360 min. Afterwards, the UVA light was turned off, and
the solution pH was adjusted to 12 to desorb the organic residues on the
magnetite surface. The desorption step (for 60 min) can verify whether
TOC removal was overestimated due to adsorption.
Fig. 9 plots the degradation and mineralization of RB5 azo dye. The
degradation is represented by the decolorization (i.e., breaking of azo
compound at 597 nm) and the breaking of aromatic ring (310 nm) of
azo-dye molecular structure. Under UVA irradiation, the Fenton reaction catalyzed with magnetite pellets decolorized the RB5 solution
completely in 60 min, but the aromatic ring cleavage only reached
Fig. 6. XPS analysis for magnetite product: (a) Survey XPS pattern, (b) The details of the Fe2p1/2 and Fe2p3/2.
6
Separation and Purification Technology 247 (2020) 116975
N.N.N. Mahasti, et al.
Fig. 7. Effect of supersaturation degree (S) on crystallization ratio (CR) of FBHC for precipitates of (a) Fe(OH)2; (b) FeCO3 at various input Fe concentrations; and (c)
Solubility curve of FeCO3 and Fe(OH)2 (QFe = QCO3 = QR = 50 ml/min; U = 28.6 m/h; HRT = 3 min; molar ratio of [carbonate]/[Fe(II)] = 1.2).
degraded by photolysis to produce %OH radicals [66]. Because of the
lack of active radicals in the absence of UVA irradiation or H2O2, the
decolorization and TOC removal were substantially lowered. In other
words, photolysis was required to activate the Fenton-like reaction in
the formation of radicals from H2O2.
Because the Fe(II)/Fe(III) redox in the spinel structure of Fe3O4
mediates the photo-assisted Fenton-like reaction, at low pH, the lattice
Fe(II) may dissolve into the solution after transferring the electron for
ROS generation. Fig. 10 shows the amount of iron dissolved from
magnetite particles during RB5 degradation. Two conditions produced
soluble Fe accounting for homogeneous Fenton reaction. First, in the
dark experiment, the leached iron increased up to 0.8 mg/L due to the
Fenton reaction between H2O2 and surface Fe(II) species at low pH
(2.8). Second, with UVA irradiation, the photolysis of surface metalligand complex may release Fe(II), which reached above 1 mg/L. Accordingly, the photolysis of H2O2 reduced the concentration of free Fe
(II) to around 0.5 mg/L, and thus limited the effectiveness of the
homogeneous Fenton reaction.
Quenching experiment was conducted to evaluate the predominance of reactive oxygen species (ROS) in the photo-fenton like
reaction, as depicted in Fig. 11. Tert-butyl alcohol (TBA) and furfuryl
alcohol (FA) were chosen as the %OH radical and singlet oxygen scavengers, respectively [67,68]; Fenton reaction was performed for two
hours, followed by 30 min of desorption process. Since the signal at
310 nm as representing aromatic ring may be inferred by furfuryl alcohol, the data of decolorization at 579 nm were compared. Accordingly, both of %OH radical and singlet oxygen oxidized RB5. In the
absence of singlet oxygen (FA system), %OH removed the color of RB5
Fig. 8. Magnetization curve of Fe3O4 pellets from FBHC.
from the air bubbling. However, in this work, the ineffectiveness of
Fenton-like reaction (black full and empty symbols) suggested that Fe
(II)/Fe(III) are less active to H2O2. In the photo-assisted Fenton-like
process, the surface sites on existing magnetite pellets induced the
catalytic reaction that converted the adsorbed H2O2 to %OH radicals.
The UVA light irradiation accounted for the metal–ligand charge
transfer, which mediated the reduction of the Fe(III)-organic complex
to Fe(II) for the reproducibility of the Fenton reaction·H2O2 was also
7
Separation and Purification Technology 247 (2020) 116975
N.N.N. Mahasti, et al.
Fig. 9. (a) Decolorization (Color, reduction in 597 nm band) and breaking of aromatic ring (AR, reduction in 310 nm band), and (b) TOC removal of RB5 (initial
RB5 = 50 mg/L; [H2O2] = 10 mM; Fe3O4 dosage = 2 g/L; air flow = 6 sL·min; pH = 2.8; photocatalytic reaction for 360 min, followed by desorption for 60 min).
(UVA) [69]:
DOM1 + hv → DOM1 ∗
(29)
DOM1 ∗ → DOM 3 ∗
(30)
DOM 3 ∗ + O2 → DOM1 + O21
(31)
The yield of singlet oxygen also resulted from the side reactions of
hydroxyl and superoxide radical evolved from Fenton-like process. The
heterogeneous redox reaction can be predicted as following equations
[65,70,71].
H2 O2 + s → H2 O2 − s
(32)
≡ Fe (II ) + H2 O2 − s → ≡Fe (III ) − OH + ∙OH
(33)
≡ Fe (II ) + O2 + H2 O → ≡Fe (III ) − OH + HO2 ∙
(34)
HO2 ∙ ⇄
Fig. 10. Leached iron from magnetite particles (RB5 = 50 mg/L;
[H2O2] = 10 mM; Fe3O4 dosage = 2 g/L; air flow = 6 sL/min; pH = 2.8).
H+
+
∙O2−
(35)
∙OH + H2 O2 → HO2 ∙ + H2 O
(36)
∙OH + O2∙− → O21 + OH−
(37)
HO2 ∙ +
O2∙−
→
O21
+
HO2−
(38)
HO2 ∙ + HO2 ∙ → O21 + H2 O2
≡ Fe (III ) − OH +
HO2 ∙/ ∙O2−
(39)
→ ≡Fe (II ) + H2
O/OH−
+ O2
≡ Fe (III ) − OH + hv → ≡Fe (II ) + ∙OH
(40)
(41)
The reaction kinetics of RB5 decolorization and mineralization are
shown in Fig. 12. The experimental data were fitted with pseudo-first
and second-order rate models, given by the following equations
[72–74]:
s + R ↔ sR ; K1 (adsorption)
sH2 O2 + sFe (II ) → s∙OH +
sOH−
(42)
+ sFe (III ) ; k2 (Fenton − like )
(43)
s + O21 → sO21
By using the total ROS attached on the catalyst surface
sROS = s∙OH + sO21
sR + sROS → sR∙ or sR + sROS → sCO2 + sH2 O ; k3
Fig. 11. Quenching experiment of RB5 decolorization ([RB5] = 50 mg/L,
[H2O2] = 10 mM, pH = 2.5 – 3.0, tert-butyl alcohol [TBA] = 20 mM, furfuryl
alcohol [FA] = 20 mM, air flow = 6 sL/min).
(44)
The component sR, which denotes surface-adsorbed RB5, is supposed to be a steady-state species; the concentration of sR approaches a
constant because the adsorption rate is balanced by the consumption
rate.
about 60%. By contrast, singlet oxygen could decolorize RB5 up to 88%.
Singlet oxygen may derived from the reactions of dissolved organic
matter (which was the intermediate of RB5 degradation), dissolved
oxygen (from the air bubble as fluidization media), and light irradiation
d [sR]
= k1+ [s][R] − k1− [sR] − k3 [sR][sROS ] = 0
dt
8
(45)
Separation and Purification Technology 247 (2020) 116975
N.N.N. Mahasti, et al.
Fig. 12. Linear regression of decolorization and mineralization of RB5 using (a) pseudo first-order and (b) pseudo second-order rate models (RB5 = 50 mg/L;
[H2O2] = 10 mM; Fe3O4 dosage = 2 g/L; air flow = 6 sL/min; pH = 2.8).
conventional chemical precipitation was solved by FBHC. Meanwhile,
the reuse of iron from the wastewater as granular catalysts of metal salt
or oxide for further application in treatment of organic pollutants was
also demonstrated.
Then, the rate of R removal in the bulk solution is related to the
adsorption-desorption equilibrium.
−
k +k3 [s][sROS ][R]
d [R]
= k1+ [s][R] − k1− [sR] − k3 [sR][sROS ] = 1 −
k1 + k3 [sROS ]
dt
(46)
CRediT authorship contribution statement
By assuming an excess of surface sites (s) and radicals (s•ROS), the
concentration profile of R should be:
−
d [R]
= k1+ [s][R] = kapp [R]
dt
Nicolaus N.N. Mahasti: Methodology, Formal analysis,
Investigation, Writing - original draft, Visualization. Yu-Jen Shih:
Validation, Writing - review & editing. Yao-Hui Huang:
Conceptualization, Resources, Supervision, Project administration,
Funding acquisition.
(47)
For the pseudo first-order rate model:
ln
[R]
= −kapp1 t
[R]0
Declaration of interests
(48)
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence the work reported in this paper.
For the pseudo second-order rate model:
1
1
−
= −kapp2 t
[R]0
[R]
(49)
Acknowledgement
The constant kapp represents the apparent rate of RB5 removal.
Fig. 12a and b summarize the fitting of experimental data with the
pseudo first- and second-order rate models, respectively. The pseudo
first-order reaction better described the decolorization and mineralization of RB5 than second-order reaction, according to its relatively
high R2 value. As shown in Fig. 12a, the rate constants of decolorization
and mineralization with the aid of Fe3O4 are about two times (from
k = 0.0197 to k = 0.0248 min−1) and five times larger (from
k = 0.001 to k = 0.005 min−1), respectively, than those without
Fe3O4, which is mainly attributable to the effective production of the
ROS under UVA irradiation.
The authors would like to thank to the Ministry of Science and
Technology, Taiwan, for the financial support under contract no. MOST
107-2221-E-006-004-MY3.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.seppur.2020.116975.
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