This is the author's final version of the contribution published as:
1)
[
Valeria Polliotto, Rita Pomilla, Valter Maurino, Giuseppe Marcì, Alessandra Bianco Prevot*, Roberto Nisticò*, Giuliana Magnacca, Maria Cristina Paganini, Laura Ponce Robles, Leonidas Perez, Sixto Malato, Different approaches for the solar photocatalytic removal of micro-contaminants from aqueous environment: titania vs. hybrid magnetic iron oxides. Catalysis Today, 328 2019 164– 171https://doi.org/10.1016/j.cattod.2019.01.044]
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[https://www.sciencedirect.com/science/article/pii/S0920586118310046]
When citing, please refer to the published version.
Different approaches for the solar photocatalytic removal of micro-contaminants
from aqueous environment: titania vs. hybrid magnetic iron oxides.
V. Polliotto1, F.R. Pomilla3, V. Maurino1, G. Marcì4, A. Bianco Prevot1*, R. Nisticò2*, G.
1University of Torino, Department of Chemistry, Via P. Giuria 7, 10125 Torino, Italy
2Polytechnic of Torino, Department of Applied Science and Technology DISAT, C.so Duca degli Abruzzi 24, 10129 Torino, Italy.
3University of Calabria, Department of Environmental and Chemical Engineering DIATIC, Via Pietro Bucci, 87036
Rende, CS, Italy
4University of Palermo, Department DEIM, Viale delle scienze, 90128 Palermo, Italy
5Plataforma Solar de Almería (CIEMAT), Carretera de Senés, km. 4, Tabernas, Almería, 04200, Spain
Abstract
This work reports on the light-induced heterogeneous photodegradation of four micro-contaminants (MCs): Carbamazepine (C), Flumequine (F), Ibuprofen (I), and Sulfamethoxazole (S), using two different heterogeneous advanced oxidation processes. The first one is the semiconductor photocatalysis, run in the presence of the suspension of a home prepared TiO2 (TiO2 HP); the
second one is an heterogeneous photo-Fenton process run in the presence of a hybrid magnetic nanomaterial (MB3) with an iron oxides core and an organic shell made of bio-based substances (BBS) isolated from urban biowaste. The two materials work upon two different mechanisms and were already tested (and the action mechanism hypothesized) at the lab scale under model conditions: TiO2 acts as photocatalyst through the photo-generation of hole/electron pairs able to
give rise to oxidation and reduction reactions, whereas hybrid magnetic nanomaterial acts in the presence of H2O2 by a photo-Fenton like mechanism. The results evidenced the better performances
of TiO2 HP (also better than the well-known reference TiO2 P25). Preliminary photodegradation
experiments carried out in a pilot plant under natural solar radiation confirmed the good results obtained with TiO2 HP. Moreover, in the adopted experimental conditions, the Fe(II) leached from
MB3 can be considered as responsible of the MCs degradation through a homogeneous photo-Fenton reaction, where MB3 act as iron reservoir.
Keywords: Micro-contaminants; TiO2; Photocatalysis; Water treatments; Photo-Fenton; Magnetic
materials.
1. Introduction
According to the Food and Agricultural Organization of the United Nations (FAO), the global water consumption has been raising twice the rate of the demographic growth [1]. Even if the principal source of fresh water is the continental rainfall, this is not enough to accomplish the global water demand, especially in the arid regions [2]. In this context, the remediation of contaminated water and, consequently, its reuse becomes a fundamental issue that caught the attention of worldwide experts. Although the treatments for the removal of standard contaminants continue to be developed, micro-contaminants (MCs or contaminants of emerging concern, i.e., a wide class of chemicals from anthropogenic derivation) are still hard to remove and have been widespread detected in the environment because of their extensive use in the modern society [3-4]. Hence, the scientific community worked on several innovative solutions to solve this issue. Among the different methods to be adopted, advanced oxidation processes (AOPs) could represent a suitable approach, since they have been largely studied, giving very promising results in the removal from waters of organic compounds recalcitrant to biological treatment [5-8].
Among AOPs, the use of TiO2 in heterogeneous photocatalysis for water remediation is
well-documented by the state-of-art literature. Indeed, for the physico-chemical properties, the photoactivity, the easy and wide synthesis and no toxicity, TiO2 is the most studied photocatalyst to
many processes such as water cleaning, total or partial oxidation and, photo-synthesis of added value compounds, becoming the reference to compare the catalyst photoactivity for many literature studies. TiO2 acts as photocatalyst through the photo-generation of hole/electron pairs able to give
specie, responsible for the degradation of organic substrate until their complete mineralization, in most cases. Moreover, as heterogeneous process it features, in principle, the possibility of recovery and re-use the catalyst, even if its separation after the water treatment represents an additional cost and makes the process more complicate.
Another heterogeneous AOP that is gaining increasing interest is represented by heterogeneous (photo)-Fenton methods, that exploit the hydroxyl radicals generation from the reaction of iron based materials and H2O2 with(out) UV light irradiation [9]. Compared to the traditional
(photo)-Fenton processes they aim to overcome the drawbacks of requiring very acidic pH and producing sludge at the end of the treatment; in particular, several efforts have been done by performing heterogeneous (photo)-Fenton processes in the presence of iron complexing agents, to run the process at milder pH values [10-13].
Recently, bio-based substances (BBS) extracted from urban biowaste have attracted increasing attention due to their very peculiar properties [14]. These BBS are supramolecular aggregates with a complex humic-like structure and, as evidenced in the literature, they are able to enhance the photocatalytic performances of (photo)-Fenton-like processes at circumneutral pH [15-17]. Moreover, BBS have been tested with promising results in heterogeneous (photo)-Fenton-like process, after immobilization on magnetic iron oxides nanoparticles. BBS-functionalized magnetic nanomaterials were used for the degradation of caffeine in aqueous environment evidencing promising results [18,19]. Indeed, innovative processes involving the use of iron-containing magnetic materials are becoming very promising due to the double action exerted by the inorganic nanomaterial, namely working both as iron-source in the (photo)-Fenton process [20] and as magnet-sensitive easily-removable heterogeneous support [21-23].
The present work aims to study the effectiveness of the photocatalytic degradation of four different MCs, namely Carbamazepine (C, an anticonvulsant and analgesic drug), Flumequine (F, an antibiotic), Ibuprofen (I, a non-steroidal anti-inflammatory drug), and Sulfamethoxazole (S, an antibiotic), whose chemical structures are reported in Scheme S1. Moreover, two different heterogeneous advanced oxidation processes were compared: the first one based on the home prepared TiO2 (TiO2 HP) semiconductor (photo-generation of hole/electron pairs), and the second
one based on heterogeneous photo-Fenton process run in the presence of a hybrid magnetic nanomaterial (MB3) with an iron oxides core and an organic shell made of bio-based substances (BBS) isolated from urban biowaste.
Degradation experiments were carried out at lab-scale in a solar simulator. Additionally, preliminary experiments were carried out in a pilot plant under natural solar radiation, to check the scale-up feasibility and effectiveness of the proposed approaches with a cheap and clean irradiation source.
2. Materials and methods
2.1 Reagents and chemicals
For the synthesis of the titania sample, a solution of titanium(IV) chloride (CAS: 7550-45-0, TiCl4,
98%, Carlo Erba) was used to obtain the titanium dioxide home prepared (HP) sample, whereas the TiO2 P25 reference sample was purchased from Degussa. The four microcontaminants (MCs),
selected as model compounds for this study were: Carbamazepine (C, C15H12N2O, CAS: 298-46-4),
Flumequine (F, C14H12FNO3, CAS: 42835-25-6), Ibuprofen (I, C13H18O2, CAS: 15687-27-1) and
Sulfamethoxazole (S, C10H11N3O3S, CAS: 723-46-6) and they were high-purity grade (>99%)
purchased from Sigma-Aldrich (Germany). All solvents used for liquid chromatography were HPLC-grade. Reagent grade hydrogen peroxide (H2O2, CAS: 7722-84-1, 35% w/v) was purchased
from Sigma-Aldrich, whereas sulphuric acid (H2SO4, CAS: 7664-93-9, 0.1 N) was supplied by
J.T.Baker (USA). Other reagents used were: 1,10-phenanthroline (o-phenanthroline, C12H8N2, CAS:
66-71-7, ≥99.0%, Sigma-Aldrich), and ascorbic acid (C6H8O6, CAS: 50-81-7, Sigma-Aldrich). The
filters used were Millipore 0.2 μm syringe-driven Millex nylon membrane filters. All aqueous solutions for HPLC analysis were prepared using ultrapure water Millipore Milli-QTM. All
chemicals were used without further purification. For the synthesis of magnetic hybrid nanomaterials, anhydrous ferric chloride FeCl3 (CAS: 7705-08-0, ≥98%, Fluka Chemika) and
ferrous sulphate heptahydrate FeSO4·7H2O (CAS: 7782-63-0, ≥99.5%, FlukaChemika) were used as
magnetite precursors, whereas ammonium hydroxide solution (CAS 1336-21-6, NH3 essay 28–30%,
Merck) was used as base for the coprecipitation reaction. Bio-Based Substances (BBS) isolated from composted urban biowastes were extracted following a laboratory procedure carried out on compost Florawiva [24] obtained from the ACEA Pinerolese Industriale S.p.A. (Pinerolo, Italy).
2.2 Preparation and characterization of the photoactive nanomaterials
The titanium dioxide home prepared (HP) tested in this work was synthesized by following the same procedure reported by Bellardita et al [25]. In particular, 17 mL of TiCl4 solution was slowly
added to 170 mL of distilled water (volume ratio 1:10) at room temperature. After about 12 hours a clean solution was obtained. This solution was boiled for 2 hours under stirring and then a milky TiO2 dispersion obtained was filtered and dried under vacuum at 50°C.The obtained white powder
was named TiO2 home prepared (HP). The physicochemical characterization of the TiO2 sample has
been reported previously [25]. Bulk and surface of TiO2 HP catalyst and commercial TiO2 (P25,
Evonik) were characterized for defining the physicochemical properties of the powder.
Their crystalline phase structures were analyzed at room temperature by powder X-ray diffraction by using a PANalytical Empyrean instrument, equipped with Cu Kα radiation and a PixCel-1D (tm) detector. The Brunauer−Emmett−Teller specific surface area of the synthesized catalysts was measured by nitrogen adsorption−desorption isotherms using a Micromeritics ASAP 2020 instrument. Infrared spectra of the samples in KBr (Aldrich) pellets were obtained with an FTIR-8400 Shimadzu spectrophotometer and recorded with 1 cm−1 resolution and 256 scans. The diffuse
reflectance spectra were measured in air at room temperature in the 200−800 nm wavelength range using a Shimadzu UV-2401 PC spectrophotometer, with BaSO4 as the reference material.
The magnetic hybrid nanomaterial investigated in this study was produced following the procedure already reported in the literature [18]. Namely, Fe(II) and Fe(III) salts, with a ferric/ferrous molar ratio of 1.5, were dissolved in deionized water and heated up to 90°C under mechanical stirring. Afterwards, two solutions were added consecutively: a 25 vol.% solution of ammonium hydroxide (to generate the basic environment necessary for the coprecipitation reaction), and a BBS-rich aqueous solution (3wt.%). The final dispersion containing the dark-brown particles was maintained under stirring at 90°C for 30 minutes and then cooled down to room temperature (RT). Lastly, the hybrid nanoparticles were magnetically separated (by means of a commercial neodymium magnet), and purified by washing several times with deionized water. The nanomaterial was oven-dried at 80°C overnight in the dark and stored dried at RT before use: in these conditions, it shows long-term stability. The magnetic hybrid nanomaterial was coded as MB3 (3 wt.% BBS). The complete physicochemical characterization of the MB3 sample has been reported previously [18].
The specific surface area was determined as already described for TiO2 HP. Fourier transform
infrared (FTIR) spectra were recorded on samples dispersed in KBr (1:20 wt ratio) in transmission mode by means of a Bruker Vector 22 spectrophotometer equipped with Globar source, DTGS detector, and working with 128 scans at 4 cm-1 resolution in the 4000–400 cm-1 range. X-ray
diffraction (XRD) patterns were obtained by means of an X’Pert PRO MPD diffractometer from PANalytical, equipped with Cu anode, working at 45 kV and 40 mA, in a Bragg-Brentano geometry performing experiments on flat sample-holder configurations.
Thermo-gravimetric analyses (TGA) were carried out by means of an ultra-microbalance (sensitivity 0.1 g) connected with a time-resolved FTIR detector (Perkin-Elmer Pyris 1 TGA instrument, Waltham, MA, USA equipped with Spectrum 100, Perkin-Elmer).
2.3 Preliminary tests
In order to estimate the drugs adsorption phenomena on TiO2 HP and on MB3 surface, dark tests
of each MC in the presence of 0.2 g L-1 of either TiO
2 HP or MB3 . The adsorption phenomena were
tested for TiO2 HP catalyst at natural pH (that was equal to 3.5 due to the acidity of the catalyst) and
at natural pH (ca. 6) and pH=3 for MB3.
The release of iron in solution from MB3 was followed at pH=3, with and without addition of H2O2,
since at this pH the magnetite dissolution could be higher; the iron leaching was studied in different experimental conditions, that is in the dark and under irradiation. The determination of both the total Fe and Fe(II) in solution has been done by means of a colorimetric test, following the standard ISO 6332, o-phenanthroline essay [26]. In detail, after a previous calibration, two sampling were done and in both were added an acetate buffer solution (pH = 4) and the o-phenanthroline indicator. In order to determine the amount of total Fe, ascorbic acid (a reducing agent) was added in one sample (thus, inducing the Fe(III)-to-Fe(II) conversion). The absorbance of the orange-red ferrous-tris-o-phenanthroline complex was measured by using a spectrophotometer (PG Instruments Ltd. T-60-U).
To evaluate the photolysis of all MCs experiments were performed in a glass Pyrex bottle reactor exposed to natural solar irradiation maintaining under stirring 1.5 L of aqueous solution containing 100 g L-1 of the MCs tested, in the absence of the photoactive materials. Photolysis experiments of
all MCs were performed at natural pH (pH = 6.1) and at acid pH (pH = 3, adjusted with H2SO4).
Another preliminary (blank) experiment was performed by irradiating with natural solar light 1.5 L of aqueous solution containing 100 g L-1 of the MCs tested, in the presence of H
2O2 (1 mM) at
circumneutral pH and at pH=3 in an open reactor maintaining under stirring, in the absence of the photoactive materials. The H2O2 was added just before the irradiation. In all of the cases, the
temperature was maintained constant at 20°C.
2.4 Photodegradation tests performed in solar simulator
Several photocatalytic tests were carried out in an Atlas XLS Suntest solar simulator under constant illumination from a Xenon lamp with a constant UV irradiance of 30 W∙m−2 (i.e., the average
typical solar UV power during a sunny day). The UV radiation was monitored with a SOLARLIGHT PMA2100 radiometer placed within the simulator.
For the TiO2 HP performances study, three different amounts of powders were added to 1.5 L of
solutions containing 100 g L-1 of each MCs in order to obtain loading equal to 0.05, 0.1 and 0.2 g
L-1. In this way it was possible to gain information about the influence of the catalyst amount on the
catalytic reactivity.
As for the test performed in the presence of MB3, 0.3 g of powder were added to 1.5 L (0.2 g L-1
MB3 loading) of aqueous solution containing 100 g L-1 of the MCs tested. Two experiments were
run: the former at natural pH, and the second one by adjusting the pH at 3 with H2SO4 conc.
solution. Both suspensions were left stirring in the dark at 20°C for 20 min. Then, after taking the first sample (0 time), H2O2 was added to reach a 1 mM concentration, and the lamp of the solar
simulator was turned on. Both experiments were performed considering 2 hours of exposure time. The presence of H2O2 is essential to have a photo-Fenton reaction with the iron supplied by the
hybrid nanomaterials.
In all cases, the suspensions were left stirring in the dark at 20°C for 30 min before taking the first sample (0 time) and starting the irradiation in the solar light simulator.
2.5 Photodegradation tests performed in solar CPC pilot plant photo-reactors
Finally, photocatalytic experiments in the presence of either TiO2 HP or MB3 (with H2O2) were
performed in a CPC pilot plant under natural solar radiation at the Plataforma Solar de Almeria (Spain) located at 37.097005 N and 2.364750 W. The set-up consisted of a plug flow photoreactor (PFP) in a total recycle loop with a non-reacting stirred tank whose function is providing aeration and sample withdrawing for analyses. The photoreactor having two solar-UV-transparent glass tubes (inner diameter 0.028 m and length 1.5 m) connected in series and placed on a fixed support inclined 37º (latitude of the PSA) with respect to the horizontal plane and facing South to maximize
the daily incidence of solar radiation was equipped with CPC (compound parabolic collector). The aqueous suspension was continuously fed to the PFP upwards from the non-reacting tank by means of a centrifugal pump (mod. NH-50PX) producing a flow of 2.5 L min-1.
The total irradiated area (Ai) was 0.26 m2 and the total volume (V
T) was 8 L, 2 L of which irradiated
(Vi) [6-7].
The studies performed in the presence of TiO2 HP were carried out in presence of 100 g L-1 for
each MC, at natural pH, that in this case was 3.5. In order to study the influence of the catalyst amount on the catalytic reactivity, the TiO2 HP loading was equal to 0.05, 0.1 and 0.2 g L-1.
Additionally, for the sake of comparison, experiments were also performed using the reference commercial TiO2 P25 (Degussa) at the highest loading of 0.2 g L-1.
For both TiO2 HP and P25 catalysts, the tests were performed as follows: the suitable amount of
catalyst was added to 8 L of solution containing all MCs, to reach the desired loading amount. The suspension was left recirculating in the reactor for 20 min in the dark and then the first sample (time 0) was taken. Subsequently, the reactor was exposed to solar irradiation.
As for the experiments performed in the presence of MB3, they were accomplished only at pH = 3 (adjusted with H2SO4), with 0.2 g L-1 of MB3, and 100 g L-1 of each MCs. The suspension was left
recirculating in the reactor for 30 min in the dark. Then the first sample (time 0) was taken and H2O2 was added to reach a 1 mM concentration. Subsequently, the reactor was exposed to solar
irradiation.
All experiments were carried out considering 2 hours of exposure time to solar radiation: starting between 12:00 A.M. local time on completely sunny days. The water temperature in the reactor was monitored during all the experiments and was always below 35.8°C.
2.6 Sampling conditions during the photocatalytic experiments
During all the experiments (i.e., both in bottle reactor and CPC reactor), samples of 7 mL of suspension were taken and filtered through a Millipore 0.2 μm syringe Millex nylon membrane filters before analysis. The filters were cleaned with 3 mL of acetonitrile to recover contaminant possibly adsorbed on the catalysts. The quantitative evaluation of the selected MCs was done by means of an Ultra-Performance Liquid Chromatography (UPLC, Agilent Technologies, Series 1200) with a UV-DAD detector and a XDB C-18 (4.6×50 mm, 1.8 μm) analytical column. The initial conditions were 90% water with 25 mM formic acid (mobile phase A) and 10% acetonitrile (mobile phase B). A linear gradient from 10% to 85% B in 13 min was used. The re-equilibration time was 3 min with a flow rate of 1 mL min-1. The injection volume was 100 μL.
Additionally, in MB3 tests the concentration of iron (Fe(II) and Fe(III)) released in solution was measured by the spectrophotometric method described above.
3. Results and discussion
3.1 Material characterization
Analysis of TiO2 HP and TiO2 P25 photocatalysts were undertaken using FTIR spectroscopy, X-ray
diffraction (XRD), and UV-Vis diffuse reflectance spectroscopy (DRS).
As for the results of the XRD analysis, the principal position peaks at diffraction angles 2 = 25° and 27° indicate the presence of anatase and rutile phase on both samples. In particular, the commercial TiO2 P25 showed narrow peaks due to the high crystallinity of the material. On the
contrary the diffraction peaks width of TiO2 HP evidenced the predominance of amorphous phase
according to Bellardita et al. [25]. The chemical structure is confirmed by FTIR, featuring a strong wide band from 400 to 900 cm–1 that corresponds to Ti–O and Ti–O–Ti stretching vibration modes
in both samples, thus confirming the TiO2 presence.
The Specific Surface Area (SSA), obtained as single point measurements, showed the same value equal to 52 m2g-1 for both titanium dioxide types.
From the DRS it was calculated the energy band gap by using a Kubelka Munk modified equation, obtaining the values of about 3.08 and 3.19 eV for TiO2 HP and TiO2 P25, respectively.
Concerning MB3 characterization [18], a value of specific surface area of 33 m2g-1 was determined;
the FTIR spectrum confirms the presence of BBS in MB3 structure mainly by the carboxylate stretching mode at ca. 1600 cm-1. Significant differences were observed by comparing the FTIR
spectra of BBS and MB3 in the region relative to carboxylic/carboxylate functionalities confirming the interaction between iron oxide and BBS-carboxylate moieties. XRD patterns are consistent with the presence of magnetite/maghemite phase. TGA was performed on MB3 and on neat BBS samples, allowing to estimate a BBS/Fe3O4 weight ratio in MB3 of about 0.1 [18].
3.2 Blank experiments: MCs adsorption on both nanomaterials and iron release from MB3
Investigating the MCs adsorption is worth to be done in order to understand if the MCs disappearance observed under irradiation can be ascribed to their degradation.
Figure S1, panel A, reports the adsorption behavior of all MCs in the presence of MB3 at pH=6.1.
Based on the obtained results, both S and C do not undergo any adsorption phenomenon, whereas ca. 50% of F and 30% of I are adsorbed onto MB3 surface after about 1 hour of contact time. The same experiments carried out at pH=3 (not reported for the sake of brevity) indicates an almost negligible adsorption for S and C and about 10 and 20% for F and I, respectively.
Figure S1, panel B, shows the iron release in solution from MB3 in the dark at pH = 3, since at this
pH there are both the highest release of iron ions [27] and the highest efficiency of the photo-Fenton reactions [9-10]. The analysis indicated a very low release of iron ions (ca. 0.3 mg L-1) from MB3 in
the aqueous medium after 120 minutes (corresponding to the longest MCs photo-degradation runs), almost entirely attributable to Fe(II) ions. If the test is performed at pH=3 under irradiation, the Fe(II) leaching slightly increases (ca. 0.5 mg L-1). On the contrary, there is no evidence of iron ion
leaching when operating at pH=6.1 (either in the dark or under irradiation), reasonably because of the very low iron solubility at this pH.
The amount of iron ions released in solution by MB3 under irradiation at pH=3 is not affected by the addition of H2O2 (1mM).
Concerning the adsorption test over TiO2 HP, only F exhibited a strong adsorption, approximately
70%, while I and C show only a slight adsorption (less than 20%, see Figure S1, panel C).
3.3 Experiments under solar irradiation: effect of pH and H2O2 addition on MCs photo
degradation in the absence of both nanomaterials
The photo-stability of the selected MCs was checked by irradiating their aqueous solutions in the absence of any nanomaterials. In particular, four blank conditions were investigated: i) photolysis of MCs at circumneutral pH, ii) effect of added H2O2 on the photolysis at circumneutral pH, iii)
photolysis of MCs at pH = 3 and iv) effect of added H2O2 on the photolysis of MCs at pH=3.
As described in the literature [28-31] S, C and I are stable under solar (or simulated solar) irradiation, I can undergo indirect photolysis in the presence of radical species, whereas F can undergo direct photolysis as well as indirect photolysis in the presence of radical species. In agreement with the previous assumptions, it can be observed in Figure 1, panel A, that at pH = 6.1 the MCs investigated undergo a limited photolysis after 1 hour of irradiation (ca. 13%, 12%, 8%, and 12% degradation of F, S, C and I, respectively). On the other hand, after the first 60 min of irradiation, only F started to degrade and after two hours of irradiation the amount left in solution was 45% (inset in Figure 1 A). The addition of H2O2 significantly enhanced the degradation extent
of F and I (see Figure 1B). In fact, after 1 hour of irradiation, the degradation percentage of I raised to ca. 50%, whereas the degradation of F was almost complete. This finding can be explained by considering the phosensitizing action of flumequine; upon light absorption F can form an excited triplet state able to react with H2O2 enhancing its decomposition [32]. Analogous behavior can be
envisaged for ibuprofen. Both S and C photolysis was not affected by the addition of H2O2 giving a
percentage of abatement of ca. 15% and 8%, respectively. Based on these results, we confirm the expected MCs stability order: C ≈ S > I >> F.
Figure 1. Panel A: MCs relative concentration vs. irradiation time at pH=6.1 in the absence of H2O2/nanomaterial. Inset: irradiation time extended up to 120 min. Panel B: MCs relative concentration vs. irradiation time at pH=6.1 with H2O2 (1 mM) and in the absence of both nanomaterials.
Higher stability towards photolysis was showed by all MCs when working at pH=3, with the only exception of S, featuring a slight decrease (as also reported in [28]), as can be observed in Figure
S2. Analogously to what observed at pH=6.1, the addition of H2O2 significantly enhanced the MCs
degradation extent of F and I also operating at pH=3, yielding, after 60 min, a degradation percentage of ca. 30% for I and of ca. 90% for F; conversely, no significant effect of pH was
observed for the photolysis of S and C in the presence of added H2O2, in the limit of experimental
error.
3.4 Degradation experiments in the solar simulator: loading effect of TiO2 HP
The influence of the nanomaterial loading (0.05, 0.1 and 0.2 g L-1) on the MCs photocatalytic
degradation was studied by using the TiO2 HP powder (Figure 2). In the first 20 minutes the
experiments were carried out in the dark in order to reach the adsorption equilibrium (see Figure
S1A), after that the light was switched on in order to follow the degradation. The photocatalytic
tests lasted 60 minutes. As evidenced by the trend dark-light, and already discussed in paragraph 3.2, the adsorption effect is important in the presence of F molecule, as the MC disappears in large extent in the first 20 minutes of the experiment carried out in the dark. The adsorption, however, is not the only mechanism involved in the elimination of the MC from the solution, but rather it can be considered the preliminary step to the MC photodegradation, as the light switch on clearly induces a further decrease of F in solution. As shown in Figure 2A, in the presence of the lowest amount of TiO2 HP (0.05 g L-1) the total MCs degradation was achieved after 30 min for I and 45 min for F,
whereas small amounts (< 2 % of the initial ones) of S and C were still present after 60 min. By increasing the amount of TiO2 HP from 0.05 to 0.2 g L-1, 100% of degradation was reached after
only 30 min of irradiation time in the case of 0.2 g L-1.
By comparing all tests carried out at different loading of TiO2 (Figure 2A-C), the faster degradation
of the MCs was observed at 0.2 g L-1. In particular, F and I disappeared after 10 minute of
irradiation while C and S after ca. 20-30 minute, respectively. From the perusal of Figure 2A-C, it is evident that the reaction rates of MCs degradation increased by increasing the TiO2 amount
present in the suspension. The increase of the reactivity by increasing the catalyst loading in the suspension is usual in photocatalysis because more catalyst means more photon absorbed and consequently more electron-hole pairs generated and higher reaction rate. Due to the fact that the amount of 0.2 g L-1 of catalyst was enough to absorb all photon reaching the suspension, it was
decided do not perform runs with higher loading of catalyst. Indeed, for higher amount of catalyst a shielding effect can occurs reducing the reaction rate.
3.5 Degradation experiments in solar CPC pilot plant in the presence of TiO2 HP
The effect of TiO2 HP loading on MCs photo-degradation was studied also in the solar pilot plant.
For the sake of comparison, the degradation of MCs was reported versus the cumulative irradiation energy entering the system. All runs lasted 45-90 minutes corresponding to an amount of solar UV irradiation energy (Q) entering in the system per liter of MCs solution, ranging between 5.5-9.6 kJ L-1.
In the presence of the lowest amount (0.05 g L-1) of TiO
2 HP only I and F were totally degraded
after 1h of irradiation (5.78 kJ L-1) while at the same time S and C showed an abatement of 37% and
20%, respectively (Figure 3A). At a catalyst loading of 0.1 g L-1, the photodegradation rate
increased for all of the MCs. In particular, as reported in Figure 3B, F was completely abated after 15 min of irradiation (1.7 kJ L-1), I after 20 min (2.3 kJ L-1), whereas both S and C after 1.5 h (9.6 kJ
L-1). At the highest TiO
2 HP loading (0.2 g L-1), all MCs disappeared after ca. 10-20 minute of
irradiation, thus confirming that the rate of MCs removal increases by increasing the amount of the suspended TiO2 HP. Interestingly, the MCs degradation kinetics obtained in both CPC pilot plant
Figure 2. MCs photo-induced degradation tests at three TiO2 HP loading equal to 0.05, 0.1 and 0.2 g L-1 in A, B and C panel, respectively. Runs carried out under solar simulated irradiation.
This result is very encouraging since the system performance is comparable even if experiments were performed in different reactor geometry, thus envisaging the possibility to easily scale-up the process after its optimization at lab scale. An additional experiment was run using the reference TiO2 P25 (0.2 g L-1, Figure 3D) for the sake of comparison. It is necessary to remark that maximum
catalyst loading was 0.2 g L-1 as this is the optimum for P25 in this type of solar photoreactor
[33,34]. Therefore, comparison with TiO2 HP should be done under the same catalyst loading.
Experimental results evidenced that TiO2 HP resulted more active than P25.
3.6 Degradation experiments in the solar simulator: pH effect on MB3
The performance of MB3 in promoting the photo-Fenton like process was explored at two different pH conditions: circumneutral (pH ca. 6) and acid (pH = 3) and Figure 4 reports the obtained results. At circumneutral pH, MB3 promoted the photodegradation of all MCs, at different extents: compared to MCs photolysis (Figure 1 A) a great degradation enhancement occurred for all MCs. On the contrary, when comparing the photo-Fenton like results with the photodegradation in the presence of only added H2O2 (Figure 1 B) it can be noticed that the degradation of F and I is
delayed while the degradation of C and S is enhanced. It can be hypothesized that different mechanisms are operating in the two processes: as discussed above, when irradiating MCs solution in the presence of H2O2, the degradation is mainly driven by the H2O2 photolysis yielding hydroxyl
radicals (•OH), photosensitized by F and I. The addition of MB3, that is able to absorb the incident light, could instead result in an internal shielding effect, negatively affecting the H2O2 photolysis
Figure 3. MCs photo-induced degradation tests at three TiO2 HP loading equal to 0.05, 0.1 and 0.2 g L-1 in A, B and C panel, respectively. The panel D shows the test carried out with the reference TiO2 P25 (loading equal to 0.2 g L-1). Runs were carried out in CPC pilot plant. The concentration reduction of MCs at Q=0 kJ L-1 (strong in some cases) showed in figure is due to the adsorption of the molecules on the catalyst surface occurring during the first 20 min in the dark.
On the other hand, the process in the presence of MB3 let us envisage an active role of the organic moiety (forming the MB3 shell), that could be able to form photoactive complexes with iron, in analogy with the findings (and related interpretation) of Huang and co-workers [22]. Such iron complexes could be responsible of the formation of superoxide radical anions, able to enhance the generation of Fe(II) and therefore the production of •OH. This implies the consideration also in the present work of the possible role of superoxide anion. The formation of carboxylate-iron complexes can also justify the initial significant adsorption of flumequine on MB3 material occurred during 20 min of suspension stabilization in the dark, explaining its higher reactivity compared to the other MCs. Moreover the organic shell itself should be able to generate reactive species under light excitation, as shown for other humic-like substances extracted from compost [35].
Figure 4B reports the degradation profiles obtained working at pH close to 3. At this pH the
adsorption of MCs on MB3 is drastically reduced, even still rather relevant for F, and the almost complete degradation was attained after two hours of irradiation for all MCs, with very similar kinetic profiles. Compared to the results obtained in the presence of H2O2 under irradiation at pH=3,
a slight efficiency decrease can be observed only for F, while a beneficial effect can be observed for the other MCs. Compared to the results obtained at pH 6.1 (Figure 4A), the faster degradation rate observed at acidic pH is in agreement with what is generally reported for the homogeneous photo Fenton process. Therefore a contribution to MCs photo-degradation deriving from a homogeneous photo-Fenton process involving the iron released in solution (even if at very low concentration), has to be carefully considered.
Figure 4. MCs photo-induced degradation tests at circumneutral (A, left) and acid (B, right) pH in the presence of 0.2 g
L-1 MB3 and H2O2 (1mM).
In order to give insights into this aspect, a photo-Fenton degradation of MCs in the presence of 0.5 mg L-1 ofFe(II) andH
2O2 (1 mM) at pH=3 was performed (Figure S4). Surprisingly the degradation
profile obtained working in homogeneous system with an added amount of Fe(II) equal to the amount leached by MB3 was almost the same than the one obtained in the heterogeneous photo-Fenton process run in the presence of MB3.
Such evidence is in contrast with all previous studies performed using MB3 [15,18,19]; in those cases, higher concentration of contaminants (5-10 mg L-1) were treated, and a synergic effect of
adding MB3 compared to the homogeneous photo-Fenton was evidenced. In the present case, it seems that when decreasing the concentration of MCs, the very low amount of dissolved Fe(II) is sufficient to promote their degradation through homogeneous photo-Fenton, making negligible the effect of MB3. This is in agreement with former results obtained for conventional photo-Fenton [36]. Optmising photon capture by a high iron concentration is crucial when the need of hydroxyl radicals is large such as for macropollutants (mg/L–g/L) removal. When the treatment is aimed at micropollutant oxidation though, the pollutant concentration is at least a hundred times lower. Therefore, the process needs less hydroxyl radicals and, consequently, less active photon to achieve removal [37]. From this arises the question that at very low contaminant concentration, very low iron concentration is sufficient to promote their degradation.
3.7 Degradation experiments in solar CPC pilot plant in the presence of MB3.
Since the present work aims to compare the TiO2-based photocatalytic degradation of a mixture of
MCs with another heterogeneous approach, an experiment was performed in the CPC pilot plant also in the presence of 0.2 g L-1 of MB3, working at pH=3, and H
2O2 (1 mM). For the sake of
comparison, the degradation of MCs was reported versus the cumulative solar UV irradiation energy entering the system. The run lasted ca. 90 minutes corresponding to an amount of irradiation energy (Q) entering in the system per liter of MCs solution of 8.5 kJ L-1.
The experiment carried out in the solar CPC pilot plant (Figure S5, panel A) was compared to the one performed at the lab-scale normalized for the cumulative energy entering the reactor (Figure
S5, panel B). Working in the CPC reactor yields to overall lower degradation rate, indicating that in
this case the reactor geometry affects the efficiency of the system. To obtain a good dispersion of MB3 particles could be, at this purpose, a relevant issue.
4. Conclusions
The obtained results evidences the good performance of TiO2 HP for the degradation of the selected
MCs in solar simulator as well as under natural solar irradiation, allowing to envisage the use of sustainable light source. Furthermore, TiO2 HP (without any doping or whatever other added
material) exhibited a clear higher activity than P25, a very unusual finding in photocatalysis for conventional TiO2.
Concerning the heterogeneous photo-Fenton run in the presence of MB3, this would be an interesting heterogeneous AOP for the abatement of the selected MCs at circumneutral pH, even if the reduced efficiency compared to the process run in acidic conditions has to be carefully considered. On the other hand, if choosing the best operating condition in term of degradation efficiency (i.e. pH=3), the same results can be obtained either in the presence of MB3 or performing a traditional homogeneous photo-Fenton. Nevertheless, MB3 can be employed as Fe(II) reservoir, easily recoverable at the end of the treatment.
5. Acknowledgements
SFERA program (EC-CIEMAT-PSA contract n°312643) is acknowledged for allowing to access the DETOX Facilility of Plataforma Solar de Almeria, Spain, whereas Polytechnic of Torino is gratefully acknowledged for funding project Starting Grant RTD (project number: 54_RSG17NIR01). This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Maria Sklodowska-Curie grant agreement No 645551 (Mat4Treat) and from Compagnia di San Paolo and University of Torino through ‘‘Bando per il finanziamento di progetti di ricerca di Ateneo – anno 2014’’, Project Microbusters Torino_call2014_L2_126.
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