Hydroxylamine promoted Fe(III)/Fe(II) cycle on ilmenite surface to enhance persulfate catalytic activation and aqueous pharmaceutical ibuprofen degradation

Article abstract of DOI:10.1016/j.cattod.2019.04.081

This study demonstrates a new system for the degradation of emerging pharmaceutical contaminants (e.g., ibuprofen) in water by coupling the naturally occurring ilmenite with hydroxylamine (HA) and persulfate (PS). Ilmenite was able to activate persulfate to generate sulfate radicals (SO₄^?·) and hydroxyl radicals (HO·). The radical generation was greatly improved by adding small amount of hydroxylamine into the solution, due to the efficient Fe(III)/Fe(II) cycle on the ilmenite surface promoted by HA, which was confirmed by X-ray photoelectron spectroscopy and electron paramagnetic resonance (EPR) spectroscopy analysis. SO₄^?· and HO· contributed comparably to ibuprofen degradation, which was verified by the radical scavenging tests. The degradation was enhanced with increasing ilmenite, PS and HA dosages, but the HA exhibited strong scavenging effect at its high concentrations. The ilmenite/PS/HA process worked well in the real treated wastewater, because the surface-controlled radical generation was less affected by the water matrix. However, the formation of bromate in the bromide-containing water by this process should be concerned. Ibuprofen was partially mineralized, and the degradation products were identified by ESI-tqMS. A radical-induced degradation pathway was proposed based on the product identification. This work provides the mechanistic insights on persulfate activation based on the surface-controlled catalytic processes. It also offers a new strategy to degrade emerging contaminants in water and sheds light on the environmental functions of natural minerals.

Full text of DOI:10.1016/j.cattod.2019.04.081

Accepted Manuscript
Title: Hydroxylamine Promoted Fe(III)/Fe(II) Cycle on
Ilmenite Surface to Enhance Persulfate Catalytic Activation
and Aqueous Pharmaceutical Ibuprofen Degradation
Authors: Ran Yin, Lingling Hu, Dehua Xia, Jingling Yan,
Chun He, Yuhong Liao, Qing Zhang, Jia He
PII:
S0920-5861(19)30215-9
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.04.081
CATTOD 12176
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
25 March 2019
28 April 2019
30 April 2019
Please cite this article as: Yin R, Hu L, Xia D, Yan J, He C, Liao Y, Zhang Q,
He J, Hydroxylamine Promoted Fe(III)/Fe(II) Cycle on Ilmenite Surface to Enhance
Persulfate Catalytic Activation and Aqueous Pharmaceutical Ibuprofen Degradation,
Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.04.081
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Hydroxylamine Promoted Fe(III)/Fe(II) Cycle on Ilmenite
Surface to Enhance Persulfate Catalytic Activation and Aqueous
Pharmaceutical Ibuprofen Degradation
Ran Yin b#, Lingling Hu a#, Dehua Xia a*, Jingling Yang a, Chun He a *, Yuhong Liao a,
Qing Zhang a, Jia He c *
a
School of Environmental Science and Engineering, Sun Yat-sen University,
Guangzhou, China
b
Department of Civil and Environmental Engineering, The Hong Kong University of
Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
c Zhujiang Hospital of Southern Medical University, Guangzhou, 510280, China
__________________________________________________________________
# Ran Yin and Lingling Hu contributed equally to this work.
* Corresponding author: School of Environmental Science and Engineering, Sun Yat-
sen University, Guangzhou, 510275, China. Tel.: +86 20 39332690. Email address:
xiadehua3@mail.sysu.edu.cn (D. Xia); hechun@mail.sysu.edu.cn (C. He);
hejia201011@sina.com (J. He).
Graphical Abstract
1

Highlights
 Ilmenite coupled with hydroxylamine showed high reactivity in persulfate
activation.
 Hydroxylamine enhanced radical generation by promoting surface
Fe(III)/Fe(II) cycle.
-
 SO4 · and HO· were generated and contributed comparably to ibuprofen
degradation.
 The performance under various conditions and in real wastewater was
evaluated.
-
 An ibuprofen degradation pathway induced by SO4 · and HO· was proposed.
2

Abstract:
This study demonstrates a new system for the degradation of emerging
pharmaceutical contaminants (e.g., ibuprofen) in water by coupling the naturally
occurring ilmenite with hydroxylamine (HA) and persulfate (PS). Ilmenite was able to
-
activate persulfate to generate sulfate radicals (SO4 ·) and hydroxyl radicals (HO·).
The radical generation was greatly improved by adding small amount of
hydroxylamine into the solution, due to the efficient Fe(III)/Fe(II) cycle on the
ilmenite surface promoted by HA, which was confirmed by X-ray photoelectron
-
spectroscopy and electron paramagnetic resonance (EPR) spectroscopy analysis. SO4
· and HO· contributed comparably to ibuprofen degradation, which was verified by
the radical scavenging tests. The degradation was enhanced with increasing ilmenite,
PS and HA dosages, but the HA exhibited strong scavenging effect at its high
concentrations. The ilmenite/PS/HA process worked well in the real treated
wastewater, because the surface-controlled radical generation was less affected by the
water matrix. However, the formation of bromate in the bromide-containing water by
this process should be concerned. Ibuprofen was partially mineralized, and the
degradation products were identified by ESI-tqMS. A radical-induced degradation
pathway was proposed based on the product identification. This work provides the
mechanistic insights on persulfate activation based on the surface-controlled catalytic
processes. It also offers a new strategy to degrade emerging contaminants in water
and sheds light on the environmental functions of natural minerals.
Keywords: Emerging contaminant; Hydroxylamine; Natural ilmenite; Persulfate
3

1. Introduction
Contaminants of emerging concern (CECs), including pharmaceuticals and
personal care products (PPCPs), are increasingly being detected at low levels in
surface water, and there is concern that these compounds may have negative impacts
on aquatic organisms and human beings [1–3]. They pose potential chronic toxicities,
cause endocrine disruptions and induce antibiotic resistance to aquatic life and human
beings [4,5]. These chemicals require additional considerations when applying
existing ambient water quality criteria for the protection of aquatic organisms and
human beings [6,7].
Advanced remediation technologies are required to deal with these chemicals, due
to the relatively poor removal efficiency of the conventional water and wastewater
treatment methods [3]. Advanced oxidation processes (AOPs), relying on the
activation of oxidant precursors (e.g., hydrogen peroxide, persulfate,
peroxymonosulfate, chlorine) and generation of reactive radical species, have
emerged as good alternatives to treating the CECs [8–11]. Among them, oxysulfur
.-
.-
.-
radical-based AOPs (e.g., SO3 , SO4 , SO5 ) are promising because the oxidant
precursors (e.g., sulfite and persulfate) are easier to handle and has relatively lower
cost [12–15]. These oxysulfur radicals can oxidize hydroxide/water to hydroxyl
radicals, which complement with each other for the destruction of organic pollutants
[13,16,17]. A number of methods have been reported to activate persulfate, including
by UV light, electrolysis, ultrasonic radiation, and numerous metallic/non-metallic
catalysts/materials [12,13,18–22]. Many of the current activation methods require
4

high energy input (e.g., by UV, sonication and electrolysis) and/or noble materials,
which limit their engineering applications in real-world practice. Instead, some
naturally occurring materials have become increasingly popular to be used as the
persulfate activators and/or catalysts [12,13]. Natural minerals, which contain
transition metals, and are ubiquitous, easy-to-be obtained and relatively cheap, pose
great potential to serve as persulfate activators and/or catalysts. The literature on the
use of natural minerals for persulfate activation purposes are very limited in the
current literature [23–29]. More experimental and pilot investigations are required to
evaluate the feasibility of different minerals for persulfate activation, the process
optimization under different environmental and operational conditions, the
mechanisms for radical generation and the potential niche applications on
environmental remediation.
Fundamentally, the radical generation mechanisms in the heterogeneous
persulfate-activation systems are less understood [13,24]. Whether the radicals are
generated in bulk solution or catalyst surfaces needs to be elucidated, and in fact some
of the surface-bound catalysts are reported to be more reactive than those in the bulk
solution [25–27]. Both sulfate radicals and hydroxyl radicals are generated
theoretically, but whether both of them and how much do they respectively contribute
to the degradation of certain organic compounds remain unknown and depends on the
target compounds. This is critical to be figured out because it affects the degradation
pathways as well as the intermediates/products, considering the rather complicated
degradation mechanisms in such persulfate-based systems [13]. The organic
5

pollutants are usually transformed to other compounds instead of fully mineralized
[24,28]. Therefore, the identification of the degradation products and mechanistic
pathways are also crucial to guarantee the used AOP does help detoxify the
contaminated water, instead of producing more hazardous products. Moreover,
whether the co-existing water matrices (e.g., dissolved organic matter and common
anions) affect the process performance and will they be transformed to some
undesired products in real water/wastewater scenarios need to be explored as well.
Persulfate has been reported to be effectively activated by the Fe2+/surface-bound
Fe(II), because these species are easy to donate one electron to persulfate for radical
generation [29,30]. The reduction of Fe(III) by persulfate and generate radicals via
subsequent chain reactions have also been reported, but the initial step (e.g., Fe(III)
reduction) has a much lower reaction rate constant [17]. Most of the naturally
occurring materials are under the aerobic conditions and the transition metals (e.g., Fe,
Cu and Mn) inside/on the surface are at relatively higher valence state (e.g., Fe(III)
and Mn(IV)) [17,31]. To accelerate the catalytic reactions, some reducing agents are
used as the electron donor, to promote the reduction of metals from higher valence
state to lower ones. Hydroxylamine (HA) is a common reducing chemical and has
been used to enhance the Fe(III)/Fe(II) cycle of Fenton and Fenton-like reactions for
the degradation of some model compounds (e.g., benzoic acid) [26,32,33]. Moreover,
HA was reported to work better than other reductive agents (e.g., ascorbic acid,
oxalate, and humic acid) in some heterogeneous Fenton systems, because it is a
relatively weaker radical scavenger [25,26]. Despite these advantages, the chemical
6

interactions between HA and natural minerals is less understood, which limits the
application of mineral-dominant processes. Moreover, in the persulfate-based
advanced oxidation processes, the role of HA on the persulfate activation and radical
generation is anticipated to be interesting and deserve further investigation.
Therefore, this study was designed to fill the knowledge gaps as aforementioned.
Natural ilmenite and ibuprofen were selected as the catalyst and target emerging
contaminant respectively in this study. The role of hydroxylamine on the persulfate
activation and radical generation in the ilmenite/persulfate system was specifically
investigated and discussed. The radical generation mechanisms in this heterogenous
system was elucidated. The performance of the process under different operational
and environmental conditions was evaluated and the mechanistic pathway on
ibuprofen degradation was explored.
2. Materials and methods
2.1 Chemicals and materials
Ibuprofen, sodium persulfate, sodium hydroxide, ethanol, hypochlorous acid,
hydroxylamine, tert-butanol alcohol (TBA), and 5,5-dimethyl-1-pyrrolidine-n-oxide
(DMPO) of reagent-grade were obtained from Sigma-Aldrich, USA. Pristine ilmenite
was collected from a mining site in China. The raw mineral was mechanically crushed,
and then selected by an electromagnetic and a gravity separator. Before the
experiments, the ilmenite was sieved at 300-mesh and the final size of the ilmenite
powder was smaller than 30 μm. The real wastewater samples were taken from a local
7

wastewater treatment plant (Shatin WWTP) after the biologically treatment process.
The water quality of the wastewater samples after 4.5-µm membrane filtration was
provided in Table S1.
2.2 Experimental procedures
In batch tests, the ilmenite particles (0.05–1.0 g L-1) were added to 100-mL flask
containing ibuprofen at an initial concentration of 10 μmol L-1. Sodium persulfate
solution (0.5–4.0 mmol L-1) and/or hydroxylamine solution (0.1–5.0 mmol L-1) were
then spiked to the solution to initiate the reaction. The pH of the reacting solution was
unbuffered and measured as 6.50.5 throughout the reaction. Samples (1 mL) were
collected by syringes at pre-determined time intervals, filtered with 0.45-μm
membrane filters (Darmstadt, Germany), and then subjected to the measurement of
residual oxidant and ibuprofen concentrations. Scavenging tests were conducted in the
similar manner except that additional ethanol or tert-butanol at 50 mmol L-1 were
added into the solution [34,35]. Tests in real wastewater were conducted in the similar
manner except that the deionized water was replaced by the real treated wastewater
after filter by 0.45-μm membrane filters.
To identify the degradation intermediates/products of ibuprofen, 50-mL liquid
samples were collected, filtered with 0.45-μm membrane filters, enriched by liquid-
liquid extraction and rotary evaporation, and analyzed by the electrospray ionization-
triple quadruple mass spectrometry (ESI-tqMS) [36]. In addition, tests were
performed to investigate the repeatedly use of ilmenite for ibuprofen catalytic
8

degradation. In each reuse cycle, after 60-min reaction, the reacted ilmenite was
magnetically separated and then gently rinsed by deionized water for three times. And
it was reused for ibuprofen degradation in the same manner as described above.
Parallel tests were conducted in the same manner except that the pristine and used
ilmenites were magnetically separated, vacuum-freeze dried and subjected to
characterization of several surface properties.
2.3 Analytical methods
The ibuprofen concentration was determined using an ultra-performance liquid
chromatograph (HPLC, Shimadzu 6A) equipped with a C18 column (4.6 mm  150
mm, 5 µm particle size) and a UV detector at 270 nm. The eluents for ibuprofen
determination were methanol (70%) and pH-3 water (30%, prepared by diluting 687-
L phosphoric acid in 1-L deionized water) and was set at a flow rate of 1.0 mL/min.
The intermediates and products of ibuprofen degradation were identified with a
Waters HPLC/ESI-tqMS system equipped with an HSS T3 column (50 mm  2.1 mm,
1.8 μm particle size). The eluents were methanol and water at a flow rate of
0.5 mL/min and the column temperature was set at 40 °C [29]. The concentrations of
dissolved Fe(II) and Fe(III) were measured by the phenanthroline method [37]. An
Electron Paramagnetic Resonance (EPR) Spectrometer (Bruker A300) was used to
identify the radicals. Dissolved organic carbon was determined with a total organic
carbon (TOC) analyzer (TOC-VCPH, Shimadzu). Inorganic anions including bromate
were measured using an ion chromatograph (Dionex, ICS 3000).
9

The elemental composition of ilmenite was determined by an X-ray photoelectron
spectroscopy (XPS) analysis performed using an X-ray photo- electron spectroscope
(PHI 5600, Physical Electronics Inc., USA) with Al Ka radiation (1486.6 eV). The
crystal structure of ilmenite was characterized by a powder X-ray diffraction (XRD)
spectrometer (PW-1830, Philips) with Cu Ka radiation (l1⁄41.5406 A) over a 2q range
of 5–70°. The specific surface area and pore volume of ilmenite were measured using
a BET surface area analyzer (SA 3100, Beckman Coulter Inc., USA). The magnetism
of ilmenite was determined using a superconducting quantum interference
magnetometer (MPMS-5S, Quantum Design Inc., USA).
3. Results and discussions
3.1 Characterization of ilmenite
XPS spectroscopy was firstly applied to identify the elemental composition and
the valence states of the surface-bound metals of ilmenite. As shown in Fig. 1a, the
XPS full spectrum suggests that O, Ti and Fe are the major components of virgin
ilmenite, and the Fe, Ti and O account for 36, 32.2 and 29.6 wt.% respectively. Apart
from Fe, Ti and O, other elements (e.g., Si, Al, K, Ca, Mg, Zn) are identified at trace
amounts. In the Fe2p spectrum (Fig. 1b), the two peaks at the binding energy of 711.5
and 725.4 eV are identified, representing Fe(II) (40%) and Fe (III) (60%) respectively
on virgin ilmenite surface [38]. Accounting for Ti2p and O2p orbitals, the major
component in virgin ilmenite is anticipated to be FeTiOx. The crystal form of virgin
ilmenite was further confirmed by the XRD spectrum, as shown in Fig. 1c. The major
10

peaks are identified at 2-theta of 32.65°, 35.3°, 43°, 56° and 63° in the XRD spectrum,
and by comparing with the JCPDS card no. 21-1276, the main form of the crystal is
FeTiO3 [39]. More interestingly, the ilmenite has a saturation magnetization of
approximately 0.6 emu/g (Fig. 1d). The magnetic feature suggests that the ilmenite
can be easily magnetically separated from the treated water by an external magnet.
The BET specific surface area and total pore volume are determined to be 1.2466 m2
g-1 and 29.5 nm respectively, indicating the ilmenite has a relatively less porous
structure, and is likely not to be a good adsorbent when applied in water treatment
(Fig. S1) [24]. Table 1 summarizes the basic properties of virgin ilmenite as
characterized and discussed here.
3.2 Degradation of ibuprofen in different systems
The degradation of ibuprofen (IBP) by different processes, including persulfate
(PS) alone, ilmenite alone, PS + ilmenite and PS + ilmenite + HA system, was shown
in Fig. 2a. Persulfate itself was not able to oxidize IBP under the tested conditions,
which was consistent with reported literature [24]. Ilmenite alone hardly oxidized nor
adsorb IBP, which was consistent with the fact that it has small specific surface area
and poor porous structure, as discussed in Section 3.1. The coupling of HA and PS
was not able to degrade IBP, indicating the HA itself could not activate PS. The
coupling of PS with ilmenite could degrade IBP by 30% under the experimental
conditions., which was hypothesized to be attributed to the radicals generated from
the PS and ilmenite interaction [13]. The IBP degradation was largely improved by
11

adding 0.5-mM HA in the PS + ilmenite system, with more than 95% of removal
under the same tested conditions. The degradation of IBP followed the pseudo first-
order kinetics and the degradation rate constants were determined to be 0.0048 and
0.0499 min-1 in the absence and presence of HA respectively. The 10-times higher
reaction rate constant was anticipated to be attributed to the enhanced radical
generation from PS and ilmenite interactions in the presence of HA. To verify this,
tests were firstly conducted to identify the radical species and their contributions to
IBP degradation in the PS + ilmenite + HA system. Ethanol (EtOH) and tert-butanol
(TBA) were used as the radical scavengers. EtOH is highly reactive towards HO· (1.9
-
 109 M-1s-1) and SO4 · (5.6  107 M-1s-1), and it was used to scavenge both two
radicals [40]. TBA has a much higher reaction rate constant towards HO· (6.0  108
-
M-1s-1) than that towards SO4 · (4.0  105 M-1s-1), and it was used to scavenge
HO· only [39]. As shown in Fig. 2b, the IBP degradation was completely hindered in
the presence of 50-mM EtOH, verifying that the removal of IBP in the PS + ilmenite
+ HA system was attributed to the oxidation by radicals. On the other hand, the
pseudo first-order rate constant decreased by 42.89% (from 0.0499 to 0.0285 min-1) in
-
the presence of 50-mM TBA, which demonstrated that both SO4 · and HO· were
generated in the PS + ilmenite + HA system, and their relative contributions to the
IBP degradation were 57% and 43%, respectively. Considering the second-order rate
-
constants of IBP towards SO4 · and HO· have been reported to be 1.66 × 109 and 3.43
× 109 M-1 s-1, respectively [41], the steady-state concentration (equal to the overall
-
degradation rate divided by the second order rate constant) of SO4 · is calculated to be
12

around 2.75 times higher than that of HO· in the PS + ilmenite + HA system.
3.3 Mechanisms of radical generation in the PS/ilmenite/HA system
The radical generation mechanisms in the heterogenous Fenton and Fenton-like
processes remain less understood, and both the dissolved metal ions (e.g., Fe2+) and
the surface-bound metals have been reported to activate persulfate and generate
radicals [13]. In this study, the concentrations of dissolved Fe2+ during ilmenite itself
dissolution and/or ilmenite + PS interactions were monitored, and the maximum
concentration of Fe2+ was 0.29 mg L-1 for the dosage of 0.2 g L-1 ilmenite and 1.0
mmol L-1 PS. Tests were firstly conducted to compare the capability of 0.2-g L-1
ilmenite and 0.29-mg L-1 Fe2+ in persulfate activation and radical generation under the
same tested conditions. As shown in Fig. 3a, in the PS + Fe2+ + HA system, the IBP
degradation rate constant was only 0.0118 min-1, which was 4.22 times lower than the
PS + ilmenite + HA system (0.0499 min-1). The results indicated that dissolved Fe2+
was not the main activator of persulfate. With increasing ilmenite dosage from 0.05 to
1.0 g L-1, the dissolved Fe2+ concentration increased from 0.12 to 2.96 mg L-1. Fig. 3b
correlates the IBP degradation rate constants with the dissolved Fe2+ concentrations,
which was not in a linear, but a convex-type relationship. This result supported the
anticipation that the dissolved Fe2+ was not the major catalyst in the PS + ilmenite +
HA system. ESR spectra displays the signal strength of radical generation in the PS +
Fe2+ + HA (purple line), PS + ilmenite (blue line) and PS + ilmenite + HA systems
(red line). The signals had comparable strength in the PS + Fe2+ + HA (purple line)
13

and PS + ilmenite (blue line) systems, but was much stronger in the PS + ilmenite +
HA system, which further proved that instead of dissolved Fe2+, other species was the
dominant contributor to persulfate activation and radical generation.
In the heterogenous Fenton processes, the surface-bound Fe(II) has been reported
to play an critical role in hydrogen peroxide activation [25,26]. And the
HO· generation rate by the surface-bound Fe(II) was 102−104 times higher than the
homogeneous Fenton processes [25]. The catalytical decomposition of hydrogen
peroxide on the surface of minerals was also reported to be faster than that in the
homogeneous systems [42]. Therefore, the surface-bound Fe(II) on the ilmenite was
also anticipated to be the major catalytic activator to persulfate in the PS + ilmenite +
HA system. The XPS Fe2p spectra (Fig. 3d) provides a strong proof that the surface-
bound Fe(II) content was improved from 30% to 60% in the presence HA, which was
consistent with the enhanced radical generation and IBP degradation as observed in
previous sections. Detailed investigations are recommended to more accurately
qualify and quantify the surface-bound Fe(II) species by using some advanced
characterization methods (e.g., extended X-ray absorption spectroscopy), quantum
chemical calculation (e.g., density functional theory) and mathematical modelling
approaches.
3.4 Process performance under various conditions and in real wastewater matrix
Fig. 4 shows the effects of PS, ilmenite and HA dosages on the degradation of IBP
in the PS + ilmenite + HA system. The degradation rate constants increased from
14

0.0135 to 0.1902 min-1 with increasing FeTiO3 dosage from 005 to 1.0 g L-1 (Fig. 4a).
The rate constants increased linearly with FeTiO3 dosages (Fig. 4b). With increasing
PS dosage from 0.5 to 4.0 mmol L-1, the degradation rate constants increased from
0.0101 to 0.3682 min-1 (Fig. 4b). However, the rate constants did not show a linear
relationship with the PS dosages (Fig. 4d), but in a concave-type relationship, and it
was mainly due to the decreased solution pH (i.e., from 7.0 to 6.5, 5.3 and 4.0) in the
presence higher concentration of PS, which favored the radical generation and IBP
degradation [12]. The degradation rate constants increased with increasing HA
dosages (Fig. 4e), because higher concentration of HA promoted the reduction of
Fe(III) to Fe(II) on the ilmenite surface and enhanced the radical generation. However,
the radical scavenging by HA was enhanced at higher HA concentrations as well,
which explained the convex-type relationship between degradation rate constants and
HA dosages (Fig. 4f). Results also indicated that the optimal dosages of PS, ilmenite
and HA should be found, with due consideration of process efficiency and cost-
effectiveness [13].
The process performance on IBP degradation was also tested in the real treated
wastewater and compared with that in the synthetic water, as shown in Fig. 5a. The
IBP degradation rate constant was 0.0499 and 0.0342 min-1, respectively, in the
synthetic water and real treated wastewater samples. The only 30% decrease in rate
constant was unexpected, with due consideration of such high contents of TOC (7.65
mg C L-1 of TOC) and salts (2.5 g L-1 of salinity) in the treated wastewater (Table S1),
which are considered as typical radical scavengers [13]. On the other hand, the results
15

were reasonable, considering the surface-bound Fe(II) dominant catalytic activation of
persulfate may less be affected by water matrix components [25], and the fact that the
salts (e.g., chloride and bicarbonate) are also potentially transformed to other radicals
-
-
-
(e.g., Cl·, Cl2 ·, and CO3 ·) by HO·/SO4 · and contribute to IBP degradation [15][43].
Considering the treated wastewater also contains relatively high level of bromide at
4.09 mg L-1 (because sweater is used for toilet flushing in Hong Kong), the potential
generation of bromate from bromide oxidation in the PS + ilmenite + HA system was
also examined. Around 200 µg/L of bromate was detected under the tested conditions,
which suggested that the bromate formation should be concerned, when adopting the
persulfate-based advanced oxidation process to treat bromide-containing water. The
catalytic performance of ilmenite in the presence of HA was evaluated and shown in
Fig. 5b. Under the experimental conditions, the ilmenite exhibited stable catalytic
performance within the four cycles of tests, with less than 5% decrease in radical
generation and IBP degradation in each cycle. Considering the magnetic feature, the
ilmenite exhibits promising potentials as repeated-use catalyst in real-world
applications.
3.5 Products and proposed pathways of ibuprofen degradation
Although was completely degraded, only 50% of the IBP was mineralized by the
PS + ilmenite + HA process under the experimental conditions. The results indicated
that the IBP was transformed to some intermediate/products. ESI-tqMS technique was
thus applied to identify degradation products. Six major products were identified
16

according to the full scan spectra, at mass to charge ratios (m/z) of 221, 237, 205, 175,
137 and 133, respectively (Fig. S2a). Product ion scan spectra was further adopted to
analyze the possible structures of these products (Fig. S2b–d). The initial step of the
reaction was anticipated to be the hydroxylation of IBP by OH· [44], which produced
mono- and di-hydroxylated IBP, with a m/z of 221 and 237 respectively. The
compound with a m/z of 133 was identified as 4-ethylbenzaldehyde, which was
derived from mono-hydroxylated IBP via decarboxylation, hydroxylation and loss of
2-propanol [24]. 4-Hydroxybenzoic acid was proposed to represent the compound
with a m/z of 137. For the compound with a m/z of 175, it was probably 4-
isobutylacetophenone that was derived from the decarboxylation and sequential
-
oxidation of IBP by both HO· and SO4 · [45]. It should be noted that this compound
was reported to be even more toxic than the original ibuprofen, which raised another
concern about the persulfate-based advanced oxidation process [46]. Fig. 6 illustrates
a scheme for radical-induced IBP degradation in the PS + ilmenite + HA process,
based on the products identified above. More studies are suggested on the control of
degradation products/by-products of the persulfate-based advanced oxidation
processes, to further pave the ways for real-world applications.
4 Conclusions
Persulfate-based catalytic processes have emerged as promising advanced
oxidation processes for environmental remediation purposes. The development of
high-efficiency, high-stability and low-cost persulfate catalyst/activator, the better
17

understanding of the working mechanisms, and the subsequent process optimization
are thus necessary to promote the real-world applications of persulfate-based AOPs.
This study highlights a newly-identified, highly-efficient and stable, and low-cost
catalyst for persulfate activation and degradation of aqueous emerging contaminant,
by coupling the naturally occurring mineral (e.g., ilmenite) with hydroxylamine. Both
-
SO4 · and HO· were generated from the catalytic persulfate activation, and they
contributed comparably to ibuprofen degradation in water. The major catalytic species
was the surface-bound Fe(II), instead of the dissolved Fe2+, which was proved by
several comparison tests and EPR and XPS analysis. The degradation was enhanced
with increasing ilmenite, PS and HA dosages, but the optimized dose should be
considered to avoid the excessive scavenging and wasting. The process worked
relatively well in the real treated wastewater, but the formation of by-products should
be concerned. A radical-induced degradation pathway was proposed to provide more
mechanistic insights on the system.
Acknowledgement
The authors wish to thank the National Natural Science Foundation of China (No.
51578556, 21876212, 41603097, 41573086), Natural Science Foundation of
Guangdong Province (No. 2015A030308005，S2013010012927, S2011010003416),
Science and Technology Research Programs of Guangdong Province (No.
2014A020216009), and the Fundamental Research Funds for the Central Universities
(No. 13lgjc10) for financially supporting this work. Dr. Xia was also supported by the
18

Start-up Funds for High-Level Talents of Sun Yat-sen University (38000-18821111).
Yin Ran also thanks the support from the Shanghai Tongji Gao Tingyao
Environmental Science and Technology Development Foundation (STGEF).
19

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250000
200000
150000
100000
50000
0
16000
14000
12000
10000
8000
(a)
(b)
O1s
Fe(III)
Fe(II)
Fe(III)
Fe(II)
Fe 2p1/2
Fe2p
C1s
Fe 2p3/2
Ti2p
Si2p
S2p
6000
700
710
720
B.E.(eV)
730
0
200
400
600
800
1000
Binding Energy (eV)
60
(c)
38
38-75
75-106
106-150
150-300
150-300
106-150
(d)
40
20
75-106
38-75
0
-20
-40
-60
£¼38
-8000 -6000 -4000 -2000
0
2000
4000
6000
8000
20
30
40
50
60
70
80
Magnetic field (Oe)
2theta (degree)
Fig. 1. (a). XPS full spectrum of virgin ilmenite; (b). Fe2p spectrum of virgin
ilmenite; (c). XRD spectra of virgin ilmenite at different sieving sizes; (d). Magnetic
hysteresis loop at 300 K of virgin ilmenite at different sieving sizes.
29