Heterogeneous Fenton-like oxidative degradation of sulfanilamide catalyzed by RuO2-rectorite composite

Article abstract of DOI:10.1007/s11164-021-04547-y

RuO₂-rectorite (RuO₂-Rec) was prepared by intercalation, deposition and calcination. Its structure was characterized by XRD, XPS, SEM and EDS. It was used as a catalyst for the sulfanilamide (SA) degradation in the presence of H₂O₂. Unlike sodium-rectorite and RuO₂ which couldn’t catalyze the degradation of SA, RuO₂-Rec could effectively catalyze the decomposition of H₂O₂ into hydroxyl radicals to degrade SA. The degradation rate could reach ~ 100% under the optimal conditions of 58?μmol/L of SA, 1.16?mmol/L of H₂O₂, 0.133?g/L of RuO₂-Rec, pH 3.5 and 25?°C in 5?h. The degradation process conformed to pseudo-first-order kinetic correlation. This degradation was affected by pH, the amount of RuO₂-Rec and the concentrations of H₂O₂ and SA. However, under the optimal pH value of 3.5, a high degradation rate could be achieved with the increase in SA concentration from 58?μmol/L to 290?μmol/L as long as the optimal ratio of RuO₂-Rec, H₂O₂ and SA kept unchanged. In addition, RuO₂-Rec was stable and possessed low ruthenium leaching rate and excellent reusability. Therefore, RuO₂-Rec is expected to be an active catalyst for the pollutant removal in the heterogeneous Fenton-like system.

Full text of DOI:10.1007/s11164-021-04547-y

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04547-y
Heterogeneous Fenton‑like oxidative degradation
of sulfanilamide catalyzed by RuO ‑rectorite composite
2
Feifei Pan · Jianhong Yang1 · Jun Cai · Lianye Liu
1
2
1
Received: 17 February 2021 / Accepted: 15 July 2021
©
The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
RuO -rectorite (RuO -Rec) was prepared by intercalation, deposition and cal-
2
2
cination. Its structure was characterized by XRD, XPS, SEM and EDS. It was
used as a catalyst for the sulfanilamide (SA) degradation in the presence of H O .
2
2
Unlike sodium-rectorite and RuO which couldn’t catalyze the degradation of SA,
2
RuO -Rec could eꢀectively catalyze the decomposition of H O into hydroxyl
2
2
2
radicals to degrade SA. The degradation rate could reach ~100% under the opti-
mal conditions of 58 μmol/L of SA, 1.16 mmol/L of H O , 0.133 g/L of RuO -Rec,
2
2
2
pH 3.5 and 25 °C in 5 h. The degradation process conformed to pseudo-ꢁrst-order
kinetic correlation. This degradation was aꢀected by pH, the amount of RuO -Rec
2
and the concentrations of H O and SA. However, under the optimal pH value of
2
2
3
.5, a high degradation rate could be achieved with the increase in SA concentration
from 58 μmol/L to 290 μmol/L as long as the optimal ratio of RuO -Rec, H O and
2
2
2
SA kept unchanged. In addition, RuO -Rec was stable and possessed low ruthenium
2
leaching rate and excellent reusability. Therefore, RuO -Rec is expected to be an
2
active catalyst for the pollutant removal in the heterogeneous Fenton-like system.
Keywords Rectorite · Ruthenium (IV) oxide · Catalytic oxidation · Sulfanilamide ·
Degradation
Introduction
Antibiotics are widely used in humans, veterinary medicine and aquaculture to pre-
vent or treat microbial infections [1, 2]. So far, hundreds of diꢀerent antibiotics have
been used [3]. Sulfanilamide is one of these antibiotics, which is commonly used in
*
Jianhong Yang
yangshenyang3@163.com
1
2
School of Environmental and Safety Engineering, Changzhou University, Changzhou 213164,
China
Key Laboratory of Fermentation Engineering (Ministry of Education), Hubei University
of Technology, Wuhan 430068, China
Vol.:(012
1
3456789)
3

F. Pan et al.
animal husbandry to control bacterial diseases [4–8]. However, its metabolic rate in
the body is very low, and most of it is discharged into the environment in its original
form. The concentrations of sulfanilamide in pig slurry, surface water, groundwater
and puriꢁed waste water were found to be up to 500 mg/L, 0.04 μg/L, 0.02 μg/L and
1
.0 μg/L, respectively [3, 9]. Because of its good stability, it is diꢂcult to degrade
and remove naturally and it accumulates excessively in the aquatic environment,
which not only results in the drug resistance of bacterial pathogens and seriously
threatens human and animal health, but also aꢀects chlorophyll synthesis, enzyme
secretion and root growth of plants [10–13]. Therefore, the removal of sulfanilamide
from wastewater has attracted much attention.
As an advanced oxidation process to remove sulfonamides and their derivatives,
Fenton reaction has been extensively studied in wastewater treatment [14, 15]. In
homogeneous Fenton system, iron ion is used as catalyst to decompose the hydro-
gen peroxide (H O ) into hydroxyl radicals (·OH) at pH 2–4 and ·OH eꢀectively
2
2
destroys sulfonamides to CO , H O and inorganic ions [14, 16, 17]. Although the
2
2
homogeneous Fenton system is highly eꢂcient, the stoichiometric requirements of
2
+
Fe have a very negative impact on Fenton chemistry as a solution for pollutant
2
+
remediation. One alternative to stoichiometric Fe is the use of UV-photo-Fenton
3
+
2+
reaction [18–20]. Due to the existence of UV light, the conversion of Fe to Fe
with low concentration is also realized. However, the wastewater is usually opaque
and most organic compounds show strong absorption band in the ultraviolet region,
3
+
which hinders the absorption of photons by low concentration Fe [21]. These limit
the application of UV-photo-Fenton reaction in the wastewater treatment. In order to
solve the problems mentioned above, the development of heterogeneous Fenton-like
system that can take place in the dark has become the focus of attention.
Considering that Fenton reaction is mainly promoted by iron and copper ions,
iron and copper oxides are usually used as solid catalysts for heterogeneous Fen-
ton-like systems. Iron oxide showed the best catalytic performance for the degrada-
tion of organic pollutants at pH 3–4 [22, 23]. Obviously, the leaching of iron ion
was inevitable under such conditions due to its alkaline oxide characteristics. Feng
et al. [23] found that for the degradation of sulfanilamide, CuO/H O oxidation was
2
2
best at pH 6.5 and CuFe O nanoparticle also showed good catalytic eꢀect under
2
4
neutral conditions due to the existence of copper ions. However, although CuO and
CuFe O nanoparticles revealed relative stability under neutral conditions, metal
2
4
leaching still existed. For the CuFe O nanoparticles, the leaching amount of cop-
2
4
per ion was much higher than that of ferric ion. In order to improve the stability of
catalysts, iron and copper oxides were usually deposited on the supporter as solid
catalysts for heterogeneous Fenton-like system [21, 24]. A convenient and important
way was to immobilize iron and copper oxides on clays. Iron and copper oxides pil-
lared layered aluminosilicates (clays) (Fe/Al- and Cu/Al- PILCs) showed high cata-
lytic activity in Fenton oxidation of organic pollutants. Making use of Fe/Al-PILCs
as catalyst in Fenton-like system, the removal rate of sulfanilamide with H O was
2
2
9
5–99% at pH 3.1 and 4.1 for 6 h [25]. Achma et al. [26] reported that Cu/Al-PILCs
showed high catalytic activity for the H O oxidation of tyrosol. When 500 mg/L of
2
2
catalyst was used, total conversion of 500 mg/L tyrosol was achieved in 1 h at 40 °C
with stoichiometric amount of H O . Three metal oxides Fe/Cu/Al-PILCs had also
2
2
1
3

Heterogeneous Fenton‑like oxidative degradation of…
been prepared and used as heterogeneous Fenton-like catalyst, which showed good
catalytic degradation eꢀect on phenol and sulfanilamide [8, 18]. However, even
if oxides of iron and copper were anchored to the aluminum pillars, metal leach-
ing was inevitable at pH 2–4 because they are alkaline oxides. This often caused a
prominent reduction in the catalytic activity of these catalysts after being reused for
2
–5 times [8, 18, 25, 26]. So far, people have been looking for new catalysts for the
heterogeneous Fenton-like reaction system.
Ruthenium, a transition metal, is located in Group VIIIB of the Periodic Table
like iron. However, unlike iron oxides, RuO is an acidic oxide and insoluble in acid
2
solution. RuO was often loaded on various carriers and used as electrode materi-
2
als and catalysts for sensors [27], supercapacitors [28, 29], water splitting [30–32]
and oxidation of organic compounds [33]. In the ꢁeld of environmental applica-
tions, RuO had been deposited on the surfaces of TiO for photocatalytic degrada-
2
2
tion of pollutants in water [34, 35] and for the catalytic total oxidation of volatile
organic compounds [36]. Co-loaded RuO and Pt on the t-BaTiO showed a syner-
2
3
gistic enhancement eꢀect on the piezo-degradation of tricyclazole [37]. However,
RuO as a solid catalyst for heterogeneous Fenton-like reactions hasn’t been hitherto
2
reported. Recently, it has been conꢁrmed that protein-supported RuO nanoparticles
2
can catalyze the decomposition of H O into ·OH [38]. In addition, rectorite is a
2
2
layered clay mineral, which consists of alternating pairs of dioctahedral mica-like
layer and dioctahedral smectite-like layer at ratio of 1:1 [39]. Because of its cation
exchange capacity, adsorptive capacity, good hydrophilicity and excellent dispers-
ibility in water, it was often used to support catalysts in aqueous solution [40–45].
Liu et al. [43] immobilized CuO on rectorite to achieve eꢂcient aerobic oxidation
of alcohol in water. It was reported that loading (N, Cu) co-doped TiO [41], Ag O
2
2
[
46] and nano-Fe O [47] on rectorite, respectively, could also eꢀectively improve
3 4
their catalytic degradation performances for organic pollutants and their stability in
aqueous solution. In this study, RuO was loaded on rectorite and used as a het-
2
erogeneous catalyst for Fenton-like reaction. Its catalytic performance was evaluated
by the degradation of sulfanilamide (SA) in aqueous solution and compared with
sodium-rectorite and RuO . Furthermore, the active species produced in the degra-
2
dation reaction, the degradation kinetics, the factors aꢀecting the catalytic degrada-
tion and the reusability of RuO -Rec were also investigated.
2
Experimental
Materials
Calcium-rectorite (Ca-Rec) was from Hubei Zhongxiangmingliu Development Co.,
Ltd (China). Its element composition is listed in Table 1. Ruthenium trichloride
hydrate and ruthenium (IV) oxide were purchased from Shanghai Macklin Biochem-
ical Co., Ltd (China). Sulfanilamide and hydrogen peroxide (30%) were obtained
from Sinopharm (China). Ruthenium standard solution (1000 μg/mL) was purchased
from Guobiao (Beijing) Testing & Certiꢁcation Co. Ltd. (GBTC, China). All other
chemicals were used as received.
1
3

F. Pan et al.
Table 1 Contents of various elements in rectorite samples based on the EDS analysis
Sample
Content of element (wt%)
Ru
C
O
Na
Mg
Al
Si
S
K
Ca
Ti
Fe
Ca-Rec
H-Rec
0
0
12.73 55.15 0.66 0.21 12.84 13.66 0.51 0.55 1.66 1.27 0.95
0
0
63.10 1.07 0.23 15.00 17.04 0.28 0.82 1.16 1.01 0.28
61.05 0.87 0.19 12.33 13.57 0.53 0.90 0.97
RuO -Rec 9.59
0
0
2
Preparation of RuO ‑rectorite composite
2
Ca-Rec was ꢁrst modiꢁed by acid treatment. 10 g of Ca-Rec was dispersed into
50 ml of HCl (1.2 mol/L) and stirred at 90 °C for 12 h. The stirring speed was
maintained at 280 rpm. After standing overnight, the mixture was centrifuged at
000 rpm (×2,900 g). The precipitate was washed with H O until no chloride ions
1
4
2
were detectable. The washing was repeated approximately 30 times. The acidiꢁed
rectorite (H-Rec) was obtained after drying at 90 °C. Then 5 g of H-Rec was dis-
persed in 150 mL water, and 250 mL of 0.01 mol/L ruthenium chloride aqueous
solution was put in. The mixture was stirred at 25 °C for two hours. Then 0.2 mol/L
of NaOH was added dropwise to maintain pH at 5.5–6.5. After 24 h, the suspension
was ꢁltrated. The precipitate was rinsed with H O, dried at 90 °C and then calcined
2
at 500 °C for ꢁve hours. The product RuO -rectorite composite was abbreviated to
2
RuO -Rec. The preparation process is illustrated in Scheme 1. In addition, sodium-
2
rectorite (Na-Rec) was also prepared by using H-Rec. 1 g of H-Rec was dispersed in
1
00 mL of NaCl solution (1.2 mol/L) and stirred at 80 °C for 10 h. The mixture was
centrifuged and the precipitate was washed with H O until no chloride ions were
2
detectable. Na-Rec was obtained after drying at 105 °C for 5 h.
Characterization
The XRD patterns were recorded on a Rigaku D/max 2500 PC X-ray diꢀractometer
(
Japan) with a Cu-target tube (λ=1.5406 Å). The test was carried out at 40 kV of
voltage and 100 mA of current. The scan range was 2θ=2–70°. The surface mor-
phology and element distribution of samples were observed and recorded on a Zeiss
SUPRA 55 ꢁeld emission scanning electron microscope equipped with QUANTAX
Scheme 1 The Preparation process of RuO -Rec. In the surface of RuO -Rec, part of RuO came from
2
2
2
Ru(OH) deposited on the surface of Rec from the solution when NaOH was added
3
1
3

Heterogeneous Fenton‑like oxidative degradation of…
energy-dispersive X-ray spectrometer (Germany). XPS was analyzed using an
ESCALAB 250XI spectrometer (Thermo, USA).
Catalytic oxidative degradation of sulfanilamide
The catalytic oxidative degradation of SA was carried out in a glass ꢃask equipped
with a magnetic stirrer at 25 °C. RuO -Rec was added into 30 mL of SA aqueous
2
solution with a certain pH. After stirring for 10 min, 35 μL of H O aqueous solu-
2
2
tion with appropriate concentration was added to start the reaction. The amount of
RuO -Rec, the concentration of SA aqueous solution and the molar ratio of H O to
2
2
2
SA were controlled in the range of 0.033–0.233 g/L, 58–290 μmol/L (10–50 mg/L)
and 2.5:1–100:1, respectively. The pH of the solution was controlled at 2.5–6.5 by
adding 0.2 mol/L of NaOH and 3.0 mol/L of H SO . During the reaction process,
2
4
the solution was detected using a Shimadzu LC-20AT HPLC (Japan) equipped with
an InertSustain C18 (250 mm×4.6 mm) column (GL Sciences, Japan) at 258 nm.
The column temperature, the eluent and the ꢃow rate were 35 °C, a mixture of water
and acetonitrile (v/v, 4:1) and 1.0 mL/min, respectively. Meanwhile, the reaction
solution was also monitored by a Shimadzu 1601 UV–Vis spectrometer (Japan),
and TOC in the solution was measured using a Multi N/C 2100s analyzer (Analy-
tik Jena, Germany). The experiments were triplicate. For comparison, the catalytic
properties of Na-Rec and pure RuO were also investigated. In addition, under pH
2
3
.5 and 25 °C, 3.0 mg of RuO -Rec (or Na-Rec) and 30 mL of SA aqueous solution
2
(
58 μmol/L) were mixed and stirred. The adsorption of RuO -Rec (or Na-Rec) for
2
SA was evaluated.
Leaching test of ruthenium ion
Under pH 3.5 and 25 °C, 4.0 mg of RuO -Rec and 30 mL of SA aqueous solution
2
(
58 μmol/L) were mixed. After 5-h reaction, the solution was ꢁltered. Nitric acid
was added to the ꢁltrate until its concentration was 2 wt%. Then the concentration
of ruthenium ion was determined by NexION 350X ICP-Mass spectrometer (Perki-
nElmer, USA). Calibration curve was made using ruthenium standard solution.
Results and discussion
Characterization of RuO ‑rectorite composite
2
Generally, in order to realize the substitution of calcium ions in Ca-rectorite by other
metal ions, a preferential acid activation is necessary. Here, Ca-Rec was ꢁrst acti-
+
3+
vated in 1.2 mol/L of HCl at 90 °C for 12 h, and then Na and Ru ions were
introduced into rectorite, respectively (see Scheme 1). Figure 1a shows the XRD
patterns of original rectorite and its modiꢁed products. After Ca-Rec was acidi-
ꢁ
ed, its basal reꢃection at 2θ=3.541° shifted to 2θ=3.560°, and its basal spacing
decreased from 2.493 to 2.480 nm. The decrease in the basal spacing indicated
1
3

F. Pan et al.
Fig. 1 a XRD patterns of Ca-Rec, H-Rec, Na-Rec and RuO -Rec; b Ru 3p XPS spectra of RuO -Rec;
2
2
c SEM images of Ca-Rec, H-Rec and RuO -Rec; d quantitative analysis and element mapping of Ru by
2
SEM–EDS of RuO -Rec
2
+
that H had been introduced into the interlayer of rectorite [48–50]. Moreover, the
results of EDS analysis showed that the content of Ca element decreased from 1.66
2
+
to 1.16 wt%, further suggesting that some Ca in Ca-rectorite had been replaced
+
by H after acid treatment. Further, after H-Rec was treated with NaCl solution,
the basal reꢃection was shifted to 2θ=4.0°, and its basal spacing decreased to
+
2
.207 nm. This decrease indicated that Na had been introduced into the interlayer
of rectorite [42, 51]. After H-Rec was treated with ruthenium chloride solution, its
basal reꢃection further shifted to 2θ=3.989° and its basal spacing also decreased to
2
.213 nm. The intensity of the reꢃection also became very weak. These indicated
3
+
that Ru ions had been introduced into the interlayer of rectorite. In addition, when
3
+
3+
the pH value was adjusted to 5.5–6.5, Ru was converted to Ru(OH) , and the Ru
3
1
3

Heterogeneous Fenton‑like oxidative degradation of…
in the solution was partly deposited on the surface of Rec in the form Ru(OH) .
3
Then Ru-rectorite was calcined at 500 °C for 5 h. Under such calcination, stable
RuO would be formed because of oxidation of oxygen [52] (see Scheme 1). In the
2
XRD pattern of RuO (Fig. 1a), there are three characteristic reꢃections 2θ=28°,
2
3
5.1° and 54.1° assigned to RuO (110), RuO (101) and RuO (211), respectively
2
2
2
[
36, 53–55]. In comparison with the XRD pattern of H-Rec, a new strong reꢃection
at 2θ=28° appeared in the XRD pattern of RuO -Rec, and the intensities of two
2
reꢃections at 2θ=35.1° and 54.1° also increased from 2008 to 2390 and from 619 to
1
017, respectively. These indicated that RuO was loaded on the rectorite. Further-
2
more, RuO -Rec was also characterized by XPS (Fig. 1b). For RuO -Rec, the bind-
2
2
ing energy value of Ru (3p ) was 463.1 eV, which was exactly the same as that of
3
/2
4
+
Ru in the literature [56, 57]. This further indicated that Ru element existed on the
rectorite in the RuO species.
2
The results of SEM and EDS analysis of rectorite samples are shown in Fig. 1c
and d. All of them showed a typical layered structure (Fig. 1c), indicating that acid
treatment and the loading of RuO did not aꢀect the layered structure of rectorite.
2
The EDS analysis showed that the content of Ru element in RuO -Rec was 9.59 wt%
2
(
Table 1), indicating RuO deposited on the surface of rectorite. The elemental
2
mapping analysis by EDS showed RuO was evenly distributed over the rectorite’s
2
surface (Fig. 1d). In addition, there was 12.73 wt% of carbon element in Ca-Rec
(
Table 1), but it disappeared after acid treatment, suggesting carbon element in the
rectorite existed in the form of carbonate. Ca-Rec also contained a small amount of
Ti element, which was also conꢁrmed by the Ti (2p ) peak at 458.6 eV of binding
1
/2
energy value in XPS spectrum (Fig. 1b) [58, 59]. No copper was detected in the
rectorite sample. There was a small amount of Fe element in Ca-Rec (0.95 wt%)
and H-Rec (0.28%). However, the Fe element was completely removed during the
loading of RuO (Table 1 and Fig. 1d). Therefore, when RuO -Rec was used as the
2
2
catalyst for the heterogeneous Fenton-like reaction, there was no interference from
copper and iron ions.
Catalytic degradation performance of RuO ‑rectorite composite
2
The catalytic degradation of sulfanilamide by RuO -rectorite composite was evalu-
2
ated under 25 °C, pH 3.5, 58 μmol/L of SA, 1.16 mmol/L of H O and 0.1 g/L of
2
2
RuO -Rec. The results are shown in Fig. 2a. It is observed that the concentration of
2
sulfanilamide decreased swiftly with increasing the reaction time. 88.7% of sulfa-
nilamide was removed in 5 h. The reaction process was tracked by HPLC (Fig. 2b).
In the HPLC proꢁles, the peak at 4.21 min was related to the sulfanilamide. The
intensity of the peak declined rapidly as the reaction time increased, further indicat-
ing the sulfanilamide was eꢀectively removed under such conditions. At the same
time, the degradation of sulfanilamide without RuO -rectorite was investigated as
2
well (Fig. 2a). It showed that in the absence of RuO -rectorite, only 5.1% of sulfa-
2
nilamide was removed in 5 h. This indicated that H O itself could also react with
2
2
SA to cause its degradation, but the degradation rate was very slow. In addition,
+
considering that the amino group of sulfanilamide existed in the form of NH under
3
1
3

F. Pan et al.
Fig. 2 a The removal of SA in the presence of H O and RuO -Rec (●), H O without RuO -Rec
2
2
2
2
2
2
(
▼), H O , RuO -Rec and tert-butanol (▲), and H O and Na-Rec (♦), and the absorption of SA by
2 2 2 2 2
RuO -Rec in the absence of H O (▪); b HPLC proꢁles of reaction solution in the degradation process
2
2
2
of SA; c UV–Vis spectra of the reaction solution in the degradation process of SA; d the kinetic curve
plotted as ln(C/C ) versus time for the catalytic degradation of SA. Reaction conditions: 25 °C, pH 3.5,
0
1
0 mg/L SA (58 μmol/L), 1.16 mmol/L H O and 0.1 g/L RuO -Rec or Na-Rec. In addition, C and C
2
2
2
0
t
are the concentration of SA at 0 h and t h, respectively
acidic condition and it could be adsorbed by rectorite, the adsorption properties of
the catalyst RuO -Rec were evaluated at 3.5 of pH. But only a small amount of sul-
2
fanilamide was adsorbed by the RuO -Rec and its removal rate was 5.7% in 5 h by
2
adsorption (Fig. 2a). These results indicated that RuO -Rec had a signiꢁcant cata-
2
lytic eꢀect on the oxidative degradation of sulfanilamide with H O .
2
2
Active species in catalytic degradation and possible degradation mechanism
In the HPLC proꢁles, the peak at 2.56 min was related to H O (Fig. 2b). The
2
2
intensity of the peak decreased with the degradation of the sulfanilamide and
6
5.5% of H O was consumed in 5 h, indicating that H O was involved in the
2 2 2 2
degradation process. Further, the active species in the degradation reaction was
investigated. Tert-butanol was added into the reaction solution to trap the active
species and it initial concentration was 11.6 mmol/L. The results showed that
addition of tert-butanol greatly reduced the degradation rate of sulfanilamide and
only 11.2% of sulfanilamide was removed in 5 h (Fig. 2a). Because tert-butanol
was mainly used to trap the generated hydroxyl radicals (·OH) [60], it could be
inferred that ·OH was the active species in the reaction system, which was con-
sistent with the results of protein-supported RuO nanoparticles catalyzing the
2
1
3

Heterogeneous Fenton‑like oxidative degradation of…
decomposition of H O to produce ·OH [38]. Thus, the degradation of sulfanila-
2
2
mide could be considered as the result of Fenton-like reaction.
For the Fenton-like degradation of sulfanilamide, it was generally believed
that it was subject to the cleavage of C-N, N-S and C-S bonds [23, 25]. Some
intermediates such as phenol, catechol, resorcinol, hydroquinone, maleic acid,
fumaric acid, acetic acid and formic acid were formed in the reaction process,
and ꢁnally, SA was completely mineralized [23, 25]. We tracked the degradation
process of SA with UV–Vis spectrometer, and the results are shown in Fig. 2c. In
the UV–Vis spectra, the adsorption bands at 203 nm and 259 nm were assigned
to K-band and B-band of sulfanilamide [61]. In the ꢁrst hour of the reaction, the
absorption band at 259 nm shifted to 256 nm and its intensity decreased swiftly.
Considering that the molar extinction coeꢂcients (ε) of B bands of protonated
+
aniline (C H NH , λ = 254 nm) and benzenesulfonamide (C H SO NH ,
6
5
3
max
6
5
2
2
−
1
−1
−1
−1
λ
= 264.5 nm) were 160 M ·cm and 740 M ·cm [61], respectively, it
max
could be reasonably inferred that the degradation reaction began with the sul-
fonamido group and the C-S bond was cleaved preferentially, which was consist-
ent with the results in the literature [23]. With further increase in reaction time,
the intensities of B-band continued to decrease and a shoulder peak at~ 290 nm
appeared. It was considered to be due to the cleavage of C-N bond and the for-
mation of catechol, resorcinol, hydroquinone and p-benzoquinone [23, 25]. In
addition, with the decrease in B-band intensity, K-band intensity also decreases
gradually. After 5 h, the K-band also became very weak, indicating the ring open-
ing reaction of phenyl ring occurred. Although the degradation mechanism of SA
had not been investigated in detail here, combined with the reaction of HO· with
SA reported in the studies [23, 25], its possible degradation course is shown in
Scheme 2.
Kinetics of catalytic degradation
The kinetic behavior of catalytic degradation of SA by RuO -Rec was also inves-
2
tigated under 25 °C, pH 3.5, 58 μmol/L of SA, 1.16 mmol/L of H O and 0.1 g/L
2
2
of RuO -Rec. According to the ꢁtting experimental data, the degradation process
2
conformed to pseudo-ꢁrst-order kinetic correlation (Fig. 2d), which accorded
with the pseudo-ꢁrst-order kinetics of Fenton-like reactions reported in the litera-
ture [62]. Under the above experimental conditions, the ꢁtting equation was ln(C/
−
1
C ) = − 0.00841t + 0.08579 (apparent rate constant k = − 0.00841 min ) and the
0
2
correlation coeꢂcient(R ) was 0.99056.
Scheme 2 Possible degradation course of sulfanilamide
1
3

F. Pan et al.
The role of rectorite and RuO2
The role of rectorite and RuO in the degradation of SA was also investigated.
2
If RuO -Rec was replaced by 0.1 g/L of Na-Rec under the reaction conditions
2
in the “Catalytic degradation performance of RuO -rectorite composite” section,
2
the removal rate of sulfanilamide was only 9.8% in 5 h (Fig. 2a). In comparison
with the degradation of SA by H O itself (Fig. 2a) and the adsorption of SA by
2
2
Na-Rec (6.2% of removal rate in 5 h), it could be found that rectorite had almost
no catalytic activity for SA degradation in the presence of H O . When RuO -Rec
2
2
2
was replaced by pure RuO , the removal rate of sulfanilamide was also very low
2
(
Fig. 3a). When the amount of RuO was 0.003 g/L, the removal rate of SA was
2
4
.7% in 5 h. As the amount of RuO increased to 0.1 g/L, the removal rate of SA
2
only increased to 20% in 5 h. However, the consumption rate of H O increased
2
2
rapidly with the increase in RuO dosage (Fig. 3b). When the dosage of RuO
2
2
was 0.033 g/L and 0.1 g/L, H O had been completely consumed in 0.5 h, while
2
2
SA was only removed by 2.5% and 10.3%, respectively. Even though there was
no H O in the solution, the removal of SA increased gradually with the further
2
2
increase in reaction time, and the removal rate of SA reached 9.4% and 20.0% at
5
h, respectively (Fig. 3b). In addition, the adsorption of SA by RuO was also
2
investigated (Fig. 3c). When the dosage of RuO was 0.033 g/L and 0.1 g/L, it
2
was found that 9.6% and 26.2% of SA were adsorbed by RuO in 5 h, respec-
2
tively. These results indicated that the removal of SA mainly resulted from the
adsorption of RuO and the rapid consumption of H O did not contribute to the
2
2
2
removal of SA.
Further, the dosage of RuO was reduced to 0.013 g/L and 0.003 g/L. It was
2
found that the consumption rate of H O greatly declined, especially when
2
2
0
.003 g/L of RuO was used (Fig. 3b). However, diꢀerent from RuO -Rec, the
2
2
consumption of H O was not in company with the catalytic degradation of SA by
2
2
RuO (Fig. 3a). In the presence of 0.013 g/L and 0.003 g/L of RuO , the removal
2
2
rates of SA were only 3.4% and 4.7%, respectively. This indicated that H O was
2
2
not involved in the degradation of SA. It was speculated that for the pure RuO
2
particles, due to the concentration of catalytic active sites, H O was rapidly
2
2
Fig. 3 a Eꢀects of pure RuO on the removal of SA; b eꢀects of diꢀerent amount of RuO on H O
2
2
2
2
consumption; c removal of SA by RuO in the absence of H O (reaction conditions: 25 °C, pH 3.5,
2
2
2
5
8 μmol/L of SA, 1.16 mmol/L of H O and 0.003~0.1 g/L of RuO ). In addition, C and C are the
2 2 2 0 t
concentration of SA at 0 h and t h, respectively. C’ and C’ are the concentration of H O at 0 h and t h,
0
t
2
2
respectively
1
3

Heterogeneous Fenton‑like oxidative degradation of…
decomposed into HO·, high concentration of HO· was accumulated on the surface
of RuO , and then they were consumed swiftly via the following reactions [23,
2
2
5, 63, 64]:
HO ⋅ + HO⋅ → H O
2
2
(1)
(2)
(3)
H O + HO⋅ → HO ⋅ + H O
2
2
2
2
HO ⋅ + HO⋅ → H O + O
2
2
2
This resulted in the decomposition of H O , but the degradation of SA was not
2
2
initiated (Scheme 3a). However, for the RuO -Rec, RuO was uniformly distributed
2
2
on the surface of rectorite as mentioned earlier (Fig. 1d). Due to the big distance
between RuO active sites, the concentration of produced HO· locally was not high
2
enough, thus greatly reducing the consumption of HO· via the reactions (1)–(3). In
this way, the hydroxyl radicals generated could react with SA and caused its degra-
dation (Scheme 3b). Therefore, RuO supported on rectorite could solve the problem
2
that pure RuO only decomposed H O without degrading organic pollutants. Fur-
2
2
2
thermore, it was possible that rectorite endowed RuO with good hydrophilicity and
2
excellent dispersibility in water [42, 43], which should be conducive to the catalytic
degradation of SA by RuO -Rec as well.
2
Inꢀuencing factors of catalytic degradation of sulfanilamide
Eꢀect of pH
For Fenton-like systems, the pH of the reaction system strongly inꢃuenced the
activity of the catalyst [8, 18, 22–26]. The eꢀects of pH on the SA removal were
investigated under 25 °C, 58 μmol/L of SA, 0.1 g/L of RuO -Rec and 1.16 mmol/L
2
of H O . The results are shown in Fig. 4a. The highest degradation eꢂciency of
2
2
SA appeared at 3.5 of pH. The pH value was basically consistent with the reported
heterogeneous Fenton systems with Fe/Cu/Al and (Al–Fe) pillared clays and nano-
Fe O /rectorite composite as catalysts [8, 25, 47]. The characteristic of these clays
3
4
was assumed to be due to their special environment, which made them the best
activity at this pH value [8]. In addition, it was observed in our experiment that the
Scheme 3 Possible interaction of pure RuO a and RuO -Rec b with H O
2
2
2
2
1
3

F. Pan et al.
Fig. 4 Eꢀects of pH value on sulfanilamide degradation a and H O consumption b. Reaction conditions:
2
2
2
5 °C, pH 2.5–6.5, 58 μmol/L of SA, 1.16 mmol/L of H O and 0.1 g/L of RuO -Rec. C and C are the
2 2 2 0 t
concentration of SA at 0 h and t h, respectively
decomposition rate of H O was accelerated with the increase in pH value from 2.5
2
2
to 6.5 (Fig. 4b). Obviously, the accelerated decomposition of hydrogen peroxide did
not necessarily lead to the increase in SA degradation. This was because the HO·
free radicals formed could be consumed via the reactions (1)–(3) as mentioned pre-
viously. Because these HO· radicals were not involved in the decomposition of SA,
they did not contribute to the degradation of SA. Therefore, controlling the decom-
position rate of H O was a key to the degradation of SA. In this study, the decom-
2
2
position rate of H O was most suitable at pH 3.5.
2
2
Eꢀects of H O and SA concentrations
2
2
The eꢀects of H O concentration on the degradation of SA were investigated under
2
2
2
5 °C, pH 3.5, 58 μmol/L of SA and 0.1 g/L of RuO -Rec. The results are shown in
2
Fig. 5a. As the concentration of H O and the molar ratio of H O to SA increased
2
2
2
2
from 0.145 to 2.32 mmol/L and from 2.5 to 40, respectively, the degradation rate of
SA increased from 48.4% to 100% in 5 h. However, as the molar ratio of H O to
2
2
Fig. 5 a Eꢀects of H O concentration on the degradation of SA (58 μmol/L); b H O consumption
2
2
2
2
under diꢀerent molar ratio of H O to SA (58 μmol/L); c eꢀects of H O and SA concentrations on the
2
2
2
2
degradation of SA under 20:1 molar ratio of H O to SA. Reaction conditions: pH 3.5, 25 °C and 0.1 g/L
2
2
of RuO -Rec. C , C are the SA concentration at 0 h and t h, respectively
2
0
t
1
3

Heterogeneous Fenton‑like oxidative degradation of…
SA further increased, the degradation rate of SA decreased slightly. When the ratio
was 100, the degradation rate of SA was 94.4% in 5 h. In theory, the molar ratio of
H O to SA is 18 for the full mineralization of SA [25]. From the point of view of
2
2
H O consumption within 5 h, the consumption of H O gradually increased with
2
2
2
2
the increase in molar ratio of H O to SA (Fig. 5b). When the ratio of H O to SA
2
2
2
2
reached 40, the ratio of consumed H O to SA exceeded 18. But further increase in
2
2
H O consumption led to a slight reduction in SA degradation. Actually, in addition
2
2
to reacting with SA, the HO· free radicals formed by RuO -Rec could be consumed
2
by Eqs. (1)–(3). When the molar ratio of H O to SA was less than 20:1, the HO·
2
2
free radicals formed by RuO -Rec could mainly interact with SA to leading to its
2
degradation. Once the molar ratio exceeded 30:1, more HO· free radicals would be
reacted with H O molecules and be consumed, thus reducing the number of HO·
2
2
free radicals in the reaction with SA. When the molar ratio of 100:1 was used, some
gas production could be observed during the experiment as well. This could be
due to the oxygen produced by the consumption of the HO· free radicals according
to the reactions (2) and (3). In addition, when the molar ratio of H O to SA was
2
2
2
0:1, complete degradation of SA could also be achieved by prolonging the reaction
time to 9 h (Fig. 5c). Therefore, for the catalytic degradation of SA by RuO -Rec, it
2
should be appropriate that the molar ratio of H O to SA was kept at 20:1.
2
2
If the molar ratio of H O to SA was kept at 20:1 with 0.1 g/L of RuO -Rec, and
2
2
2
the concentration of SA was increased from 58 to 290 μmol/L, the degradation rate
of SA decreased from 88.7 to 64.6% in 5 h (Fig. 5c). At the same time, the consump-
tion of H O decreased from 75.2 to 45.5% in 5 h. Obviously, this decrease in SA
2
2
degradation with the increase in SA concentration was due to the formation of insuf-
ꢁ
cient hydroxyl radicals. In order to improve the degradation rate of SA, the method
of prolonging reaction time could be used. However, when the concentration of SA
was 290 μmol/L, the reaction time was prolonged to 10 h and the degradation rate
only reached 89.7%. Therefore, it was not a good method to increase the degradation
rate only by prolonging the reaction time at high SA concentration.
Eꢀect of catalyst
The eꢀects of RuO -Rec content on the degradation of SA were investigated under
2
2
5 °C, 3.5 of pH, 58 μmol/L of SA and 1.16 mmol/L of H O . The results are shown
2
2
in Fig. 6a. When the concentration of catalyst increased from 0.033 to 0.133 g/L,
the degradation rate of sulfanilamide increased rapidly. This was mainly due to
the increase in hydroxyl radical formation by RuO -Rec catalyst. However, further
2
increase in catalyst content caused a slight decline in the SA degradation. As men-
tioned above, this could also be due to the formation of excessive hydroxyl radi-
cals, resulting in side eꢀects [23, 25]. Therefore, under such a reaction condition, the
appropriate amount of catalyst is 0.133 g/L.
If the molar ratio of H O to SA was kept at 20:1 and the amount of catalyst also
2
2
increased proportionally with the increase in the concentration of SA from 58 to
2
90 μmol/L, it was found that all SA the degradation rates could reach almost 100% in
h (Fig. 6b). This could be mainly that the hydroxyl radicals were increased propor-
5
tionally with the increase in catalyst. Obviously, compared with prolonging the reaction
1
3

F. Pan et al.
Fig. 6 Eꢀects of RuO -Rec content on the SA degradation under 25 °C and 3.5 of pH. a 58 μmol/L of
2
SA and 1.16 mmol/L of H O ; b 20:1 molar ratio of H O to SA remained unchanged and the amount of
2
2
2
2
RuO -Rec increased proportionally with SA. C and C are the concentration of SA at 0 h and t h, respec-
2
0
t
tively
time, it was a better way for the SA degradation with high concentration to increase the
amount of RuO -Rec.
2
Heterogeneous nature of catalyst and its recycling
In order to investigate whether leached ruthenium or RuO -Rec acted as a catalyst, the
2
degradation of SA was carried out under 25 °C, pH 3.5, 58 μmol/L of SA and 0.133 g/L
of RuO -Rec. After the solution was reacted for 5 h and the catalyst was ꢁltered out,
2
H O and SA was added to achieve their initial concentration. The mixture continued
2
2
to react for 5 h. It was observed that the degradation rate was only 7.8%. The result was
basically consistent with that mentioned above which was from direct oxidation with
hydrogen peroxide in the absence of RuO -Rec. These indicated that the SA degra-
2
dation was heterogeneously catalyzed by RuO -Rec. In addition, ruthenium leaching
2
from RuO -Rec was determined by ICP-MS spectrophotometry as well. The leaching
2
amount of ruthenium was 0.30 μg/L in 5 h. It was obvious that the leached ruthenium
was not enough to catalyze the degradation of SA. Meanwhile, in comparison with
CuO, CuFe O nanoparticle, Fe/Al-PILCs, Cu/Al- PILCs and Fe/Cu/Al-PILCs, the
2
4
leaching amount of ruthenium from RuO -Rec was far less than that of Fe or Cu from
2
these catalysts [8, 18, 23, 25, 26]. Further, for the heterogeneous Fenton-like system,
the recyclability of RuO -Rec was also investigated under 25 °C, pH 3.5, 58 μmol/L
2
of SA, 1.16 mmol/L H O and 0.133 g/L of RuO -Rec (Table 2). After reaction for
2
2
2
5
h, RuO -Rec was separated by centrifugation, washed with water and dried at 90 °C
2
for 2 h for the next use. It was observed that the activity of the recycling RuO -Rec
2
was only very slightly decreased in four consecutive cycles, which could be due to the
Table 2 The recyclability of
Number of reuse
1
2
3
4
RuO -Rec
2
Degradation rate of sulfanilamide (%) 100.0 100.0 98.2 98.1
1
3

Heterogeneous Fenton‑like oxidative degradation of…
fact that ruthenium was diꢂcult to leach in the reaction process. These indicated that
RuO -Rec had excellent stability and reusability.
2
Removal of TOC
The TOC change in the solution was investigated under 25 °C, pH 3.5, 0.133 g/L
RuO -Rec, 1.16 mmol/L H O and 58 μmol/L of SA. Although SA was completely
2
2
2
removed in 5 h (Fig. 6a, b), it was found that only 17.0% of TOC removal was
achieved. At this time, the consumption of H O was 76.7%. When the reaction time
2
2
was further prolonged to 11 h, H O was completely consumed, but the removal rate
2
2
of TOC was only 32.8%. After adding the same amount of H O and repeating the
2
2
above process for three times, the TOC value was ꢁnally reduced to 0. Obviously,
a large amount of H O was consumed ineꢀectively in the reaction process. This
2
2
should be related to the fact that the amount of RuO deposited on the rectorite and
2
its distribution on the rectorite surface were not optimized.
Conclusions
In this paper, RuO -Rec was prepared and used in a heterogeneous Fenton-like sys-
2
tem. RuO supported on rectorite solved the problem that pure RuO only catalyzed
2
2
decomposition of H O without degrading organic pollutants. RuO -Rec could
2
2
2
eꢀectively catalyze the degradation of sulfanilamide in the presence of H O . This
2
2
degradation was aꢀected by many factors, such as pH, the amount of RuO -Rec
2
and the concentration of H O and SA. In order to achieve excellent degradation
2
2
eꢂciency, in addition to keeping the optimal pH at 3.5, the control of the ratios
of H O , SA and RuO -Rec was particularly important. In addition, Ru was hardly
2
2
2
leached in the reaction process and RuO -Rec possessed excellent reusability as
2
well. However, although RuO -Rec could be a promising catalyst for the pollutant
2
removal, the preparation of RuO -Rec still needs to be optimized to realize the eꢀec-
2
tive utilization of H O .
2
2
Acknowledgements This work was supported by the Key Laboratory of Fermentation Engineering (Min-
istry of Education) and Hubei Key Laboratory of Industrial Microbiology Open Foundation in Hubei
University of Technology.
Declarations
Conꢁict of interest The authors declare no conꢃict of interests.
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