α-Sulfur as a metal-free catalyst to activate peroxymonosulfate under visible light irradiation for decolorization

Article abstract of DOI:10.1039/c5ra22947b

While transition metals have been frequently used to activate peroxymonosulfate (PMS) for chemical oxidation reactions, recently metal-free activation of PMS has also drawn great attention considering that no metal is required and the environmental impact can be minimized. In this study, orthorhombic α-sulfur (α-S), for the first time, is employed as a metal-free photocatalyst to activate PMS under visible (vis) light irradiation. To study the activation of PMS by the α-S/vis process, decolorization of rhodamine B (RhB) dye is selected as a model reaction. Parameters affecting the decolorization were investigated, including α-S loading, PMS dosage, temperature, pH, salt and inhibitors. The decolorization using PMS activated by α-S/vis was much faster than the self-activation of PMS. A higher α-S loading also facilitated the activation of PMS; however, over-loading of α-S led to the shielding effect, thereby decreasing the decolorization extent. Higher PMS dosages and temperatures were both preferable for the decolorization. While the decolorization was improved under acidic conditions, the activation of PMS was hindered under alkaline conditions. When high concentrations of NaCl were added to RhB solutions, the decolorization extent still remained almost the same. Electron paramagnetic resonance (EPR) spectroscopic analysis was performed to probe into the mechanism of PMS activated by the α-S/vis process. The α-S/vis process was found to be recyclable and stable over multiple cycles, even though α-S did not undergo any regeneration treatments. Considering these features, the α-S/vis process appears to be a promising and environmentally friendly process to activate PMS for chemical oxidation reactions.

Full text of DOI:10.1039/c5ra22947b

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Page 1 of 29
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αꢀSulfur as a Metalꢀfree Catalyst to Activate
Peroxymonosulfate under Visible Light Irradiation
for Decolorization
KunꢀYi Andrew Lin* and ZhiꢀYu Zhang
Department of Environmental Engineering, National Chung Hsing University,
250 KuoꢀKuang Road, Taichung, Taiwan, R.O.C.
*Corresponding Author. Tel: +886ꢀ4ꢀ22854709, Eꢀmail address: linky@nchu.edu.tw
(KunꢀYi Andrew Lin)
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Abstract
While transition metals have been frequently used to activate peroxymonosulfate
(PMS) for chemical oxidation reactions, recently metalꢀfree activation of PMS has
also drawn great attention considering that no metal is required and the environmental
impact can be minimized. In this study, orthorhombic αꢀsulfur (αꢀS), as the first time,
is employed as a metalꢀfree photocatalyst to activate PMS under visible (Vis) light
irradiation. To study the activation of PMS by αꢀS/Vis process, decolorization of
Rhodamine B (RhB) dye is selected as a model reaction. Parameters affecting the
decolorization were investigated, including αꢀS loading, PMS dosage, temperature,
pH, salt and inhibitors. The decolorization using PMS activated by αꢀS/Vis was much
faster than the selfꢀactivation of PMS. A higher αꢀS loading also facilitated the
activation of PMS; however, overꢀloading of αꢀS led to the shielding effect, thereby
decreasing the decolorization extent. Higher PMS dosages and temperatures were
both preferable for the decolorization. While the decolorization was improved under
the acidic condition, the activation of PMS was hindered under alkaline conditions.
When high concentrations of NaCl were added to RhB solutions, the decolorization
extent still remained almost the same. Electron paramagnetic resonance (EPR)
spectroscopic analysis was performed to probe into the mechanism of PMS activated
by αꢀS/Vis process. αꢀS/Vis process was found to be recyclable and stable over
multiple cycles, even though αꢀS did not undergo any regeneration treatments.
Considering these features, αꢀS/Vis process appears to be a promising and
environmentally friendly process to activate PMS for chemical oxidation reactions.
Keywords: αꢀsulfur, peroxymonosulfate, decolorization, Rhodamine B, visible light
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1. Introduction
Chemical oxidation reaction is one of the most important reactions, especially in
chemical synthesis [1] and contaminant degradation [2ꢀ4]. Wet chemical oxidations,
like Advanced Oxidation Processes (AOPs) [2, 5ꢀ7], typically involve highly reactive
•−
oxygenic species such as hydroxyl (OH•) and sulfate (SO4 ) radicals [8, 9]. As
hydroxyl radicalꢀgenerated technologies (e.g., Fenton reaction, photoꢀFenton,
sonoꢀFenton, electroꢀFenton, etc.) have been extensively investigated and developed,
sulfate radicalꢀgenerated processes recently have been also focused considering
several advantageous features of sulfate radicals [10] as follows. First, the oxidation
•−
potential of SO4 (2.5−3.1 V vs NHE) is comparable or even higher than that of OH•
•−
(2.8 V vs NHE) [11]. Second, SO4 exhibits a highly selective reactivity toward
•−
unsaturated and aromatic electrons [11, 12]. Third, the halfꢀlife period of SO4 (i.e.,
30−40 ꢁs) is much longer than that of OH• (i.e., < 1 ꢁs) [13, 14].
To obtain sulfate radicals, peroxymonosulfate (PMS) has become an essential
source, since it is commercially available and environmentalꢀfriendly [10]. Therefore,
PMS has been frequently used for bleaching, cleaning, disinfection as well as
chemical synthesis [10, 15]. However, PMS must be activated in order to facilitate the
generation of sulfate radicals [10, 16ꢀ22].
To date, a number of techniques have been used to activate PMS, including
metallic catalysts, thermal treatments, and sonication [16ꢀ22]. Among these
techniques, addition of transition metals (e.g., cobalt oxide [20, 22], iron oxide [20, 23,
24], and manganese oxide [25, 26]) is recognized as a highly efficient approach [20],
especially considering that the thermal and ultrasound activation of PMS require
continuous and intensive energy input. Nevertheless, in view of the usage of metals,
especially noble metals, and the subsequent environmental concerns, nonꢀmetal or
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metalꢀfree activation of PMS has been focused lately [27]. Sun et al. demonstrated
using reduced graphene oxide (RGO) to accelerate the activation of PMS [28], since
oxygenic species of RGO could initiate the activation. Duan et al. further modified
RGO with sulfur and nitrogen to improve the activation efficiency of PMS [29, 30].
Tao et al. employed a nonꢀmetal semiconductor, carbon nitride (gꢀC3N4), to activate
PMS under visible light irradiation [27]. Considering these studies, nonꢀmetal and
metalꢀfree activation of PMS appears to be a promising and sustainable alternative to
activate PMS and it should be further developed.
As ionic sulfur is commonly employed in sulfide photocatalysts and doped in
oxide photocatalysts, recently element sulfur is also evaluated as a photocatalyst [31].
Although elemental sulfur has more than 30 allotropes, S8 is recognized as the most
stable configuration under standard pressure and temperature. S8 can form crystals
and comprise three allotropes, including orthorhombic αꢀsulfur (αꢀS), monoclinic
βꢀsulfur (βꢀs) and γꢀsulfur [31]. Orthorhombic αꢀsulfur (αꢀS) is considered as the most
stable form of elemental sulfur under standard pressure and temperature and it is also
confirmed to exhibit visibleꢀlightꢀdriven photocatalytic activity [31]. Therefore, it is
possible to employ αꢀS together with visible light to generate electrons and in turn to
activate PMS. Nevertheless, such a metalꢀfree approach, to our best knowledge, has
not been explored as a technique to activate PMS. Considering that sulfur is one of the
most abundant elements on earth and visible light can be provided from the Sun, αꢀS
with visible light irradiation (αꢀS/Vis) process can be a promising, sustainable and
environmentalꢀfriendly approach to activate PMS, and therefore must be investigated.
In this study, αꢀS was prepared via a wet chemistry method under a mild
condition and characterized by scanning electron microscopy (SEM), Xꢀray
diffractometer (XRD) and diffused reflectance spectroscopy (DRS). To examine the
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PMS activation by αꢀS/Vis process, decolorization of Rhodamine B (RhB) dye was
selected as a model reaction. Parameters influencing the decolorization using PMS
activated by αꢀS/Vis process were examined, including αꢀS loading, PMS dosage,
temperature, pH, salt as well as inhibitive scavengers. To probe in the mechanism of
PMS activated by αꢀS/Vis, Electron paramagnetic resonance (EPR) spectroscopic
analysis was performed. The recyclability of αꢀS without regeneration treatment was
also evaluated by testing αꢀS/Vis for the decolorization over 10 cycles.
2. Experimental
2.1 Synthesis and characterization of α-S
The orthorhombic αꢀsulfur (αꢀS) crystal was synthesized according to a reported
procedure [32]. In brief, 3 g of Na2S2O3 (SigmaꢀAldrich, USA) was dissolved in 0.5 L
of DI water, followed by an addition of 6 ml of Triton TXꢀ100 (SigmaꢀAldrich, USA)
aqueous solution (1 wt%) at 50 °C. The mixture was then heated to 70 °C and 24 ml
of concentrated HCl aqueous solution was added to the mixture, which was stirred for
5hr. The precipitate was collected via centrifugation, washed thoroughly with DI
water and dried at 65 °C for 12 hr to obtain the yellowish powder, αꢀS.
Physical and chemical characteristics of the asꢀprepared αꢀS powder were
determined first by a field emission SEM (JEOL JSMꢀ6700, Japan) to visualize its
morphology. The XRD pattern of αꢀS was obtained using an Xꢀray diffractometer
with copper as an anode material (40 mA, 45 kV) (PANalytical, the Netherlands).
DRS measurement of αꢀS was performed using a UVꢀVis spectrophotometer equipped
with integrating spheres (Jasco V650, Japan).
2.2 Dye decolorization using PMS activated by α-S/Vis process
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Decolorization of RhB dye in water was conducted as a model test to evaluate the
activation of PMS and it was performed in a batch reactor as follows. First, 0.1 g of
αꢀS was dispersed in a 0.2 L of RhB aqueous solution with an initial concentration (C0)
of 10 mg L−1. Subsequently, 0.06 g of PMS powder (SigmaꢀAldrich, USA) was
immediately added to the RhB solution which was irradiated by visible light lamp
(150W Philips, Netherland) with UV light cutꢀoff. The light intensity irradiating the
reactor was 110 × 103 Lux (equivalent to 0.016 watt cm−2) measured by a light meter
(TES 1335, China). The batch reactor was equipped with a waterꢀcirculation system
to maintain solution temperatures. At preꢀset times, sample aliquots were withdrawn
from the batch reactor and the remaining RhB concentration (Ct, mg L−1) was
analyzed by a UVꢀVis spectrophotometer (eꢀChomTech CTꢀ2000, Taiwan).
To evaluate effects of catalyst loading and oxidant dosage, the addition of αꢀS
was varied from 250 to 1500 mg L−1. PMS dosage was also changed from 100 to 300
mg L−1. The decolorization test was conducted at different temperatures to study the
effect of temperature and to determine the activation energy (Ea) of the activation of
PMS by αꢀS/Vis process. Effects of pH and salt on the RhB decolorization were also
evaluated. pH values were changed to 3, 7, 9 and 11 to investigate the decolorization
under acidic, neutral, weakly basic and basic conditions, respectively. The effect of
salt was evaluated by adding different amounts of NaCl (i.e., 1000, 1500 and 2000 mg
L−1) to RhB solutions. Several inhibitors were also added to RhB solutions during the
decolorization to evaluate their inhibitive effects and to provide insights into the
activation mechanism. The inhibitors included ascorbic acid (0.01 M), methanol (0.2
M) and tertꢀbutyl alcohol (TBA) (0.2 M). Since the inhibitive effect of ascorbic acid
was significantly stronger than methanol and TBA, the dosage of ascorbic acid was
much less than that of methanol as well as TBA. To further probe into the mechanism,
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EPR spectroscopy was employed using 5,5ꢀDimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) as
a radicalꢀtrapping agent. EPR spectra of PMS activated by αꢀS with and without the
visible light irradiation were measured by Bruker EPR spectrometer (EMXꢀP,
Germany). To examine the recyclability of αꢀS/Vis process to activate PMS, αꢀS was
reꢀused for 10 cycles without any regeneration treatments.
3. Results and Discussion
3.1 Characterization of α-S
A macroscopic view of αꢀS is shown on the right side of Fig. 1(a). One can see that
αꢀS crystals are fine yellowish powder. The chemical structure of αꢀS is typically
represented as a ringꢀlike structure from the top view (Fig. 1(b)) and a crownꢀlike
structure from the side view (Fig. 1(c)). The morphology of αꢀS crystals is further
revealed in detail in Fig. 2(a). αꢀS particles were irregular and the size of αꢀS particles
was around 5 micrometers. A closer view of αꢀS (Fig. S1, see ESI†) shows that αꢀS
exhibited surface roughness and no obvious porosity was observed. The crystalline
structure of αꢀS is revealed in Fig. 2(b) and its XRD pattern can be readily indexed for
the orthorhombic sulfur (JCPDS Standard 08ꢀ0247) [33], confirming the formation of
αꢀS crystals. The optical absorption property of αꢀS is shown in Fig. 2(c), which
displays an onset of adsorption of αꢀS at 500 nm as reported in a previous literature
[31]. The experimental band gap of αꢀS has been determined to be 2.79 eV [31].
3.2 Decolorization using PMS activated by α-S under visible light irradiation
Since the RhB dye might be decolorized by αꢀS via adsorption, we first evaluated the
adsorption of RhB to αꢀS in Fig. 3. It can be seen that no RhB was decolorized
through the adsorption to αꢀS, indicating that there was no strong affinity between αꢀS
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and RhB. Furthermore, the decolorization by αꢀS under visible light irradiation was
also evaluated (Fig. 3). Unlike the adsorption to αꢀS, RhB was slightly decolorized by
the photocatalysis of αꢀS. This was possibly because αꢀS under the visible light
irradiation resulted in electrons which subsequently became certain reactive oxygenic
species (e.g., OH•) to degrade RhB. Furthermore, the RhB decolorization using PMS
alone (under visible light irradiation) was examined as a reference for comparison
with PMS activated by αꢀS/Vis process. Even though PMS itself was able to
decolorize RhB (Fig. 3), Ct/C0 only approached ~0.35 in 100 min, indicating that the
activation of PMS without catalysts was quite slow. Nevertheless, when PMS was
added to the RhB solution in the presences of αꢀS and visible light irradiation, the
kinetics of the RhB decolorization became significantly faster and Ct/C0 reached zero
in 50 min. UVꢀvisible spectroscopic analysis (Fig. S2, see ESI†) also shows that RhB
dye diminished alone the reaction time. This suggests that the activation of PMS in
the presences of αꢀS and visible light irradiation was remarkably accelerated. In order
to determine the enhancement in the decolorization using αꢀS/Vis process, the
decolorization kinetics was analyzed using the pseudo first order rate law (Eq. (1)):
Ct = C0 exp(−k1t)
(1)
where k1 is the first order rate constant of the decolorization. The k1 values of PMS
alone and PMS activated by αꢀS/Vis were calculated and listed in Table 1. When PMS
alone was used, k1 was 0.008 min−1. After αꢀS was added to the solution, k1 became
0.064 min−1. This comparison validates that αꢀS together with visible light irradiation
was able to activate PMS for the generation of sulfate radicals.
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3.3 Effects of α-S loading and PMS dosage on the decolorization
To further investigate the roles of αꢀS and PMS in the PMS activation for the
decolorization, αꢀS loadings and PMS dosages were varied to study their respective
effects. Fig. 4(a) shows the decolorization using different αꢀS loadings. When αꢀS
loading was 250 mg L−1, the decolorization proceeded quite slowly and Ct/C0 did not
reach 0 even after 100 min. Once αꢀS loading became 500 mg L−1, the decolorization
kinetics became faster and Ct/C0 approached 0 in 50 min. To quantitatively compare
the kinetics using different αꢀS loadings, the pseudo first order rate law was employed.
k1 for αꢀS loading = 250 mg L−1 was 0.022 min−1, whereas k1 was 0.064 min−1 in the
case of αꢀS loading = 500 mg L−1, indicating that αꢀS loading can affect the activation
of PMS. When αꢀS loading was increased to 1000 mg L−1, the decolorization kinetics
(k1 = 0.0534 min−1) and efficiency, however, did not improve significantly.
Interestingly, as αꢀS loading was further raised to 1500 mg L−1, the decolorization
kinetics (k1 = 0.0344 min−1) and efficiency became slower and lower, respectively.
This result suggests that even though a higher αꢀS loading facilitated the PMS
activation, overꢀloading of αꢀS might lead to adverse effects. This could be due to that
αꢀS particles suspending in the solution blocked the visible light irradiation and the
shielding effect decreased the catalytic activity of αꢀS for the activation of PMS.
The effect of PMS dosage was evaluated in Fig. 4(b), in which the PMS dosage
was changed from 100 to 400 mg L−1. An obvious trend can be observed that a lower
PMS dosage led to the slower decoloriztion kinetics, whereas a higher PMS dosage
improved the decolorization. The decolorization was also decreased with a lower
PMS dosage as Ct/C0 did not reach 0 even after 100 min with PMS dosage of 100 and
200 mg L−1. The corresponding k1 values are summarized in Table 1. When PMS
dosage changed from 100 to 200, 300 and 400 mg L−1, k1 increased from 0.010, 0.016,
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0.064 and then 0.094 min−1. Although the decolorization kinetics was indeed faster
using PMS of 400 mg L−1, the improvement in kinetics from PMS of 300 to 400 mg
L−1 was not as significant as that from 200 to 300 mg L−1. Nevertheless, the higher
PMS dosage leads to a higher amount of PMS consumption, increasing operation cost.
Therefore, an optimal PMS dosage has to be traded off between decolorization
performance and operation cost.
3.4 Effects of temperature and pH on the RhB decolorization
Additionally, the effect of temperature on the decolorization was also studied in Fig.
5(a). When temperature was changed from 40 to 20 °C, RhB dye was still fully
decolorized after 100 min. Nevertheless, the kinetics became much slower as k1
decreased from 0.064 to 0.025 min−1. In contrast, as the temperature varied from 40 to
60 °C, the decolorization kinetics was improved significantly and Ct/C0 approached 0
within 20 min. The rate constant (k1) at 60 °C was 0.143 min−1, much higher than k1
obtained at 40 °C. This suggests that the activation of PMS by αꢀS/Vis process can be
greatly facilitated at a higher temperature as reported in literatures [19].
The rate constants (k1) at different temperatures can be further analyzed using the
Arrhenius equation to determine the activation energy (Ea, kJ mol–1) of PMS activated
by αꢀS/Vis process using the following equation (Eq.(2)):
ln k1 = ln k – Ea/RT (2)
where k represents the temperatureꢀindependent factor (g mg–1 min–1); R is the
universal gas constant; T is the solution temperature in Kelvin (K). A plot of 1/T
versus ln k1 is shown in Fig. S3 (see ESI†), in which the data points are perfectly
fitted by the linear regression with R2 = 0.999. This indicates that the kinetics of the
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decolorization using PMS activated by αꢀS/Vis process can be well correlated to
temperature using the Arrhenius equation. According to the slope and intercept of the
linear plot in Fig. S3, the Ea and k were determined to be 35.2 kJ mol–1 and 47.5 g
mg–1 min–1, respectively.
Furthermore, pH of RhB solution was also varied to investigate the effect of pH.
Fig. 5(b) shows the RhB decolorization at pH = 3, 7, 9 and 11, and there was a
notable trend that the decolorization and the kinetics became lower and slower,
respectively, as pH increased. When pH was adjusted to 3, the RhB was fully
decolorized within a very short time (i.e., 10 min). k1 was found to be 0.522 min−1
(Table 1), which was much higher than k1 obtained at the unadjusted pH (i.e., 3.7).
Nevertheless, at pH = 7, even though RhB was still fully decolorized, the
decolorization kinetics became much slower with k1 = 0.048 min−1. Once pH was
further raised to 9 and 11, the adverse effect of alkaline condition became even more
obvious. k1 values were found to be 0.024 and 0.005 min−1 for the decolorization tests
at pH = 9 and 11, respectively. These results suggest that PMS activated by αꢀS/Vis
process is more favorable under acidic conditions, possibly because PMS is relatively
stable at low pH [34]. On the other hand, the alkaline conditions might lead to the
selfꢀdecomposition of PMS without formation of sulfate radicals [34ꢀ36] and
consequently decreased the decolorization extent.
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3.5 Effects of salt and inhibitors and the proposed mechanism
Considering that dyeꢀcontaining wastewater may contain other compounds, especially
salts, we also investigated the effect of NaCl as a typical salt. By varying NaCl
concentration, the effect of ionic strength on the PMS activation was also examined.
Fig. 6(a) shows the RhB decolorization with different concentrations of NaCl.
Compared to the test without NaCl, the presence of NaCl did not cause substantial
changes on the decolorization. The decolorization tests with NaCl = 1000 and 1500
mg L−1 remained almost the same to the test without NaCl. Although the
decolorization kinetics appeared to be slower when NaCl was 2000 mg L−1, the
change was not significant, indicating that the activation of PMS by αꢀS/Vis process
was quite stable even with a high concentration of salt.
Besides, we also evaluated the effects of several inhibitors which might scavenge
radical species and hinder the decolorization. Through the examination of particular
inhibitors, the mechanism of PMS activated by αꢀS/Vis process might also be
revealed. As shown in Fig. 6(b), when ascorbic acid was added to RhB solutions, the
decolorization extent and kinetics both decreased. This was because ascorbic acid is
considered as a universal radical scavenger which considerably hindered the
generation of sulfate radicals. This also reveals the crucial role of radicals in this
radicalꢀdriven decolorization reaction. Subsequently, another inhibitor, tertꢀbutyl
alcohol (TBA), was added to RhB solutions and TBA has been recognized as a probe
reagent specifically for hydroxyl radicals. As revealed in Fig. 6(b), the decolorization
extent and kinetics were indeed decreased, suggesting that the RhB decolorization
using PMS activated by αꢀS/Vis process partially involved hydroxyl radicals. On the
other hand, methanol was also added to RhB solutions as it is a probe reagent for both
hydroxyl and sulfate radicals. In the presence of methanol, the decolorization extent
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and kinetics both were significantly decreased. This reveals that the activation of PMS
by αꢀS/Vis process certainly involved with hydroxyl and sulfate radicals.
Considering the semiconducting property of αꢀS, the visible light irradiation on
αꢀS can lead to generation of electrons (e−) and holes (h+) as follows (Eq.(3)):
αꢀS + hv → e− + h+ (3)
The resultant electrons and holes may react with PMS to form sulfate radicals as
follows (Eqs.(4)ꢀ(5)) [37]:
HSO5 + e− → SO4−• + HO−
(4)
(5)
−
−
HSO5 + h+ → SO5−• + H+
The result sulfate radical (i.e., SO4−• ) might react with H2O to generate the hydroxyl
radical (i.e., OH•) as follows (Eq. (6))[38]:
SO4−• + H2O → SO42− + OH• + H+ (6)
Both sulfate radicals (i.e., 2.5−3.1 V) and hydroxyl radicals (i.e., 2.8 V) possess
high oxidizing powers to attack chromophoric groups of dye and cause the
decolorization. Based on the aboveꢀmentioned possible reaction routes, the EPR
analysis was also performed to observe occurrence of radical species during the
activation of PMS by αꢀS/Vis process. We first measured EPR spectra of a mixture of
αꢀS and DMPO with and without the visible light irradiation (Fig. 7). When the light
was off, no significant peaks were observed. However, once the visible light was
introduced, several peaks can be observed, corresponding to occurrence of hydroxyl
radicals. This validates that αꢀS exhibited photocatalytic activity and utilized photons
to generate electrons which reacted with water to form hydroxyl radicals. On the other
hand, we also measured EPR spectra of mixtures of αꢀS, PMS and DMPO with and
without visible light irradiation. In the absence of visible light irradiation, almost no
peak was observed from the spectrum, indicating that no noticeable amount of
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radicals generated when αꢀS was mixed with PMS. This suggests that αꢀS alone was
ineffective to accelerate the PMS activation. In contrast, when the visible light
irradiation was given, a number of peaks appeared and these peaks were identified as
occurrences of sulfate radicals as well as hydroxyl radicals [30, 39]. This result
indicates that αꢀS together with visible light irradiation considerably accelerated the
PMS activation, leading to generation of sulfate radicals and subsequently hydroxyl
radicals. Based on the result of the effect of inhibitors and the aboveꢀmentioned EPR
analysis, a proposed mechanism is also schematically presented in Fig. 8.
3.6 Multiple-cycle of α-S/Vis process for the RhB decolorization
Since αꢀS was proposed as a catalyst to activate PMS, it is critical to examine its
recyclability and stability. To evaluate the recyclability of αꢀS, the used αꢀS powder
was recovered from a previous experiment and added to a subsequent test without any
regeneration treatments. Fig. 9 shows the decolorization efficiencies (defined as
(1ꢀCt/C0) %) over 10 cycles using αꢀS/Vis process and the decolorization efficiency
remained almost the same during the 10 cycles. The pseudo first order rate constants
(k1) of the multiꢀcycle test are also determined and shown in Fig. 9. While slight
variation in k1 can be noticed, k1 still remains quite comparable over 10 cycles. These
results reveal that αꢀS can be a stable and recyclable catalyst to facilitate the
activation of PMS under the visible light irradiation. Thus, αꢀS/Vis process can be
considered as a sustainable and environmentally friendly process for the PMS
activation, which is employed in wastewater treatment and chemical synthesis
considering that no metal is required and solar light can be a renewable energy input.
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4. Conclusion
In this study, αꢀS, as the first time, was used as a metalꢀfree photocatalyst to activate
PMS under visible light irradiation. Through RhB decolorization, the activation of
PMS by αꢀS/Vis process was successfully demonstrated. The decolorization using
PMS activated by αꢀS/Vis was much faster than the selfꢀactivation of PMS. A higher
αꢀS loading was found to facilitate the PMS decomposition while overꢀloading of αꢀS
led to the shielding effect which blocked the visible light, thereby decreasing the RhB
decolorization. A higher PMS dosage and a higher temperature were both preferable
for the activation of PMS for the decolorization. While the acidic condition improved
the RhB decolorization, the activation of PMS was greatly hindered under alkaline
conditions. Nevertheless, the RhB decolorization remained almost the same even
though high concentrations of NaCl were added to RhB solutions. The mechanism of
PMS activated by αꢀS/Vis process might be attributed to the generation of sulfate
radicals derived from the reaction between the resultant electrons of αꢀS/Vis process
and PMS. Such a process was also found to be recyclable and stable over multiple
cycles even though αꢀS did not undergo any regeneration treatments. Considering
these features, αꢀS/Vis process can be employed as a sustainable and environmentally
friendly process for the PMS activation as no metal is required and visible light can be
obtained from a renewable energy input, the Sun.
15

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Table 1 The kinetic parameters derived from the pseudo first order rate law for the
decolorization using αꢀS with visible light irradiation (αꢀS/Vis) under various
conditions.
Conditions
Temp. (°C)
The pseudo first order rate law
αꢀS
0
PMS
300
300
300
300
300
100
200
400
300
300
300
300
300
300
pH
3.7
3.7
3.7
3.7
3.7
3.7
3.7
3.7
3.7
3.7
3
k1 (min−1)
0.008
0.064
0.022
0.053
0.034
0.010
0.018
0.094
0.025
0.143
0.522
0.048
0.024
0.005
R2
40
40
40
40
40
40
40
40
20
60
40
40
40
40
0.984
0.950
0.972
0.970
0.990
0.959
0.994
0.998
0.990
0.972
0.950
0.949
0.960
0.954
500
250
1000
1500
500
500
500
500
500
500
500
500
500
7
9
11

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(a)
(b)
(c)
Fig. 1. The ringꢀstructure αꢀS: (a) preparation scheme, (b) top view and (c) side view
of αꢀS.

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(a)
5 µm
(b)
10
15
20
25
30
35
40
2-theta (degree)
(c)
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Characteristics of αꢀS: (a) SEM image, (b) XRD pattern and (c) DRS
spectrum.

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1.0
0.8
0.6
0.4
0.2
0.0
adsorption to
-S + light
PMS+ light
-S + PMS + light
α-S
α
α
0
20 40 60 80 100
t (min)
Fig. 3. Comparison of decolorization by adsorption to αꢀS, αꢀS under visible light
irradiation (αꢀS + light), PMS under visible light irradiation (PMS + light), and PMS
activated using αꢀS under visible light irradiation (αꢀS + PMS + light) (PMS = 300 mg
L−1, αꢀS = 500 mg L−1, T = 40 οC; C0 of RhB = 10 mg L−1)

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(b)
(a)
α
α
α
α
-S = 250 mg L−1
-S = 500 mg L−1
-S = 1000 mg L−1
-S = 1500 mg L−1
PMS = 100 mg L−1
1.0
1.0
0.8
0.6
0.4
0.2
0.0
PMS = 200 mg L−1
PMS = 300 mg L−1
PMS = 400 mg L−1
0.8
0.6
0.4
0.2
0.0
0
20 40 60 80 100
0
20 40 60 80 100
t (min)
t (min)
Fig. 4. Effects of (a) αꢀS loading (PMS = 300 mg L−1, T = 40 οC), (b) PMS dosage
(αꢀS = 500 mg L−1, T = 40 οC) on the decolorization. C0 of RhB = 10 mg L−1.

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(a)
(b)
ο
20 C
pH = 3
pH = 7
pH = 9
pH = 11
1.0
0.8
0.6
0.4
0.2
0.0
1.0
ο
40 C
ο
60 C
0.8
0.6
0.4
0.2
0.0
0
20 40 60 80 100
0
20 40 60 80 100
t (min)
t (min)
Fig. 5. Effects of (a) temperature and (b) pH on the decolorization. PMS = 300 mg
L−1; αꢀS = 500 mg L−1; C0 of RhB = 10 mg L−1; T = 40 οC; C0 of RhB = 10 mg L−1.

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(a)
(b)
NaCl = 1000 mg L−1
NaCl = 1500 mg L−1
NaCl = 2000 mg L−1
1.0
1.0
0.8
0.6
0.4
0.2
0.0
Ascorbic acid
MeOH
0.8
0.6
0.4
0.2
0.0
No NaCl
TBA
No inhibitors
0
20 40 60 80 100
0
20 40 60 80 100
t (min)
t (min)
Fig. 6. Effects of (a) NaCl and (b) inhibitors (ascorbic acid = 0.01 M, methanol =
0.2 M and TBA = 0.2 M) on the decolorization. PMS = 300 mg L−1; αꢀS = 500 mg L−1;
C0 of RhB = 10 mg L−1; T = 40 οC.

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α
ꢀS (no light irradiation)
α
ꢀS + visible light
α
ꢀS + PMS (no light irradiation)
α
ꢀS + PMS+ visible light
3440 3460 3480 3500 3520
Magnetic field (G)
Fig. 7. Electron paramagnetic resonance (EPR) spectra of
αꢀS with and without visible light irradiation (
ꢀ: DMPOꢀSO4 and ꢁ: DMPOꢀOH)
(PMS = 300 mg L−1; αꢀS = 500 mg L−1; DMPO = 500 mg L−1; T = 25 οC).

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Fig. 8. A proposed mechanism of PMS activated by αꢀS/Vis process for the
decolorization.
0.10
0.08
0.06
0.04
100
80
60
40
0.02
0.00
20
0
0 1 2 3 4 5 6 7 8 9
Cycle #
Fig. 9. Recyclability of αꢀS/Vis to activate PMS for the RhB decolorization (PMS =
300 mg L−1; αꢀS = 500 mg L−1; C0 of RhB = 10 mg L−1; T = 40 οC).

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