Photocatalytic Evaluation of Ag2co3 for Ethylparaben Degradation in Diﬀerent Water Matrices

water

Article Photocatalytic Evaluation of Ag2CO3 for Ethylparaben Degradation in Diﬀerent Water Matrices

Athanasia Petala 1 , Athanasia Nasiou 1, Dionissios Mantzavinos 1 and Zacharias Frontistis 2,*

1 Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, GR-26504 Patras, Greece; [email protected] (A.P.); [email protected] (A.N.); [email protected] (D.M.) 2 Department of Chemical Engineering, University of Western Macedonia, GR-50132 Kozani, Greece * Correspondence: [email protected]

Received: 23 March 2020; Accepted: 17 April 2020; Published: 20 April 2020

Abstract: The present study examines the photocatalytic properties of silver carbonate (Ag2CO3) for ethyl paraben (EP) degradation under simulated solar irradiation. Ag2CO3 was prepared according to a solution method and its physicochemical characteristics were studied by means of X-ray diffraction (XRD), the Brunauer–Emmett–Teller (BET) method, diffuse reflectance spectroscopy (DRS), and transmission electron microscopy (TEM). Complete EP (0.5 mg/L) removal was achieved after 120 min of irradiation with the use of 750 mg/L Ag2CO3 in ultrapure water (UPW), with EP degradation following pseudo-first-order kinetics. The effect of several experimental parameters was investigated; increasing catalyst concentration from 250 mg/L to 1000 mg/L led to an increase in EP removal, while 1 increasing EP concentration from 0.25 mg/L to 1.00 mg/L slightly lowered kapp from 0.115 min− to 1 0.085 min− . Experiments carried out with the use of UV or visible cut-off filters showed sufficient EP degradation under visible irradiation. A series of experiments were performed in real water matrices such as bottled water (BW) and wastewater (WW), manifesting Ag2CO3’s equally high photocatalytic activity for EP degradation. To interpret these results different concentrations of inorganic anions (bicarbonate 100–500 mg/L, chloride 100–500 mg/L) present in aqueous media, as well as 10 mg/L organic matter in the form of humic acid (HA), were added sequentially in UPW. Results showed accelerating effects on EP degradation for the lowest concentrations tested in all cases.

Keywords: advanced oxidation; endocrine disrupting compounds; waters; visible-light active materials

1. Introduction A walk through the supermarket or pharmacy is enough to observe the phrase “paraben free” that appears today in some self-care mass market products. Parabens are p-hydroxybenzoates containing alkyl-side chains linked to an ester group that constitute a large group of preservatives capable of prolonging shelf life in many health and beauty products. Common parabens used are methylparaben, ethylparaben, butylparaben, and propylparaben. Their popularity is attributed to the fact that they possess a great number of desirable characteristics, such as broad antimicrobial spectrum, high eﬃciency, and low cost [1]. However, in recent years, many studies have pointed out the potential of these substances to act as endocrine disruptors or even carcinogens [2]. People are exposed to parabens through skin absorption or swallowing and then are excreted by urination. This, together with industrial activity, results in the entrance of parabens into the water cycle, leading to an indirectly continuous exposure of humans to parabens [3]. Despite the fact that removal eﬃciencies of parabens in wastewater treatment plants exceed 90%, they have still been detected in river water samples at low concentrations of the ng/L range [4], thus multiplying the

Water 2020, 12, 1180; doi:10.3390/w12041180 www.mdpi.com/journal/water Water 2020, 12, 1180 2 of 14 adverse effects both at humans and to the environment. The threat becomes bigger as parabens can react with bromides or chlorides present in waters, resulting in the generation of chlorinated and brominated species of high toxicity [5]. In order to address this issue, development of more effective technologies for decontamination of water and wastewater is of vital importance. Among many research efforts, advanced oxidation processes (AOPs) have come up with encouraging efficiencies [6–8]. Amongst AOPs, the photocatalytic approach seems to be the least energy-consuming and friendly to the environment. Of course, photocatalysis is not new, as it dates back to 1972, when Fujishima and Honda set up their photoelectrochemical cell [9]. However, the holy grail in recent years is the development of visible light active materials and their configuration under realistic conditions. Up-to-date publications focusing on visible light active materials have pointed out bismuth vanadate (BiVO4), graphitic carbon nitride (g-C3N4), and silver phosphate (Ag3PO4) as candidate photocatalytic materials with high efficiencies for degradation of recalcitrant micropollutants [10,11]. Apart from Ag3PO4, other silver-based materials hold a prominent position in the list of candidate photocatalytic materials, such as silver vanadate (Ag3VO4)[12], silver oxide (Ag2O) [13], and silver halides (AgX, X:Cl, Br, I) [14]. In addition, Ag2CO3 has recently been reported to possess high activity toward the photocatalytic degradation of dyes such as methylene blue and rhodamine B [15]. Its satisfying photocatalytic performance is derived from the fact that the bottom of the conduction band (CB) in Ag2CO3 consists mainly of hybridized Ag s-Ag s states responsible for its high dispersity, which allows high photogenerated electrons mobility and prevents photogenerated charge carrier’s recombination [15]. In order to increase its photocatalytic properties, a wide range of heterojunctions have also been synthesized, such as Ag2CO3/TiO2 [16], ZnO/Ag2CO3/Ag2O[17], MoS2/Ag2CO3 [18], and ZnO/Ag2CO3 [19]. However, most of these studies focus on optimizing the physicochemical and optical properties of Ag2CO3-based photocatalysts and tests are limited to dyes degradation, where there is always the possibility of photo-sensitization of the catalyst. Thus, the generalization of these results to other organic pollutants should be performed with caution [20]. Unlike this, the present study concentrates on Ag2CO3 photocatalytic activity in real and synthetic water matrices of environmental concern. The degradation of ethylparaben, a characteristic EDC agent, is studied under simulated solar light irradiation. In addition, several experimental parameters, such as catalyst concentration and ethylparaben initial concentration, are discussed in detail.

2. Materials and Methods

2.1. Materials and Water Matrices

Silver nitrate ((AgNO3, CAS: 7761-88-8) and sodium bicarbonate (NaHCO3, CAS: 144-55-8) were purchased from Sigma-Aldrich and used for catalyst preparation as received, without further puriﬁcation. Ethylparaben (EP) (HO-C6H4-CO-O-CH2CH3, CAS: 120-47-8), humic acid (HA, CAS: 1415-93-6), sodium chloride (NaCl, CAS: 7647-14-5), sodium bicarbonate (NaHCO3, CAS: 144-55-8), and acetonitrile (CH3CN, CAS: 75-05-8, for HPLC analysis) were also obtained from Sigma-Aldrich. The main characteristics of water matrices used in the present study are (i) ultrapure water (UPW: 0.059 µS/cm conductivity, pH = 6.0), (ii) commercial bottled water (BW: 300 mg/L bicarbonate, 15 mg/L chloride, 5 mg/L nitrate, 12 mg/L sulphate, 387 µS/cm conductivity, pH = 7.4), and (iii) secondary treated wastewater (WW: pH = 8, conductivity = 0.38 mg/L, total organic carbon = 9 mg/L, total suspended solids = 2 mg/L, chemical oxygen demand = 26 mg/L).

2.2. Photocatalyst Preparation

Silver carbonate was prepared according to a simple solution method [21]. Half a gram of NaHCO3 was added to 60 mL UPW (D1) under continuous stirring, while 2.04 g AgNO3 were added in 60 mL UPW forming solution D2. D2 solution was then added dropwise in D1 solution and remained under Water 2020, 12, 1180 3 of 14 stirring at room temperature for 240 min. The precipitate was collected by centrifugation, washed with UPW and dried at 60 ◦C for 12 h.

2.3. Photocatalyst Characterization

The crystallographic structure of Ag2CO3 was investigated by means of X-ray diﬀraction (XRD) using an A Brucker D8 Advance instrument (Bruker, Karlsruhe, Germany) with a Cu Kα source 1 (λ = 1.5496 Å). Data were collected in the 2θ range of 10◦ to 80◦ at a scan rate of 0.05◦ s− and a step size of 0.015◦. Phase identiﬁcation was based on JCPDS cards. Primary crystallite size was determined with the use of Debye–Scherrer’s equation:

0.9λ d = (1) B cos θ where B is the line broadening (in radians) at half of its maximum, θ is the diffraction angle and λ is the X-ray wavelength corresponding to Cu Ka radiation (0.15406 nm). Specific surface area was determined according to the Brunauer–Emmett–Teller (BET) method with the use of a Micromeritics (Gemini III 2375) instrument (Norcross, GA, USA). For the determination of optical properties, diffuse reflectance spectroscopy (DRS, Varian Cary 3, Agilent 5301 Stevens Creek Blvd, Santa Clara, CA 95051, USA) was adopted. Spectrophotometer was equipped with an integration sphere, using BaSO4 as a reference. Photocatalysts were loaded into a quartz cell, and spectra were obtained in the wavelength range of 200–800 nm. Transmission electron microscopy (TEM) images were recorded using a JEOL JEM-2100 system (JEOL, Akishima, Tokyo, Japan) operated at 200 kV (resolution: point 0.23 nm, lattice 0.14 nm). Films (Kodak SO-163) were used for recording HR-TEM images. The specimens were prepared by dispersion in water and spread onto a carbon-coated copper grid (200 mesh).

2.4. Photocatalytic Experiments A typical photocatalytic degradation experiment can be described as follows: A measured volume of a stock EP solution (10 mg/L) is added to 120 mL of the water matrix (usually UPW) in order to achieve the desired initial EP concentration (0.5 mg/L in most cases). A pre-weighted amount of photocatalyst is then added to the solution (typically 750 mg/L). The photoreactor (a glass cylindrical vessel) is stirred at 400 rpm for 15 min in the dark to achieve complete adsorption/desorption equilibrium of EP on the photocatalyst surface. After this period, the suspension is irradiated employing a solar simulator (Oriel, model LCS-100, Newport, Irvine, CA, USA) equipped with a 100 W xenon ozone-free lamp) at an incident intensity of 1.3 10 4 einstein/(m2 s)., as measured by means of chemical actinometry [22]. × − · Simulated solar radiation contains about 5% UVA radiation, and 0.1% UVB radiation. A 420 nm cut-off filter (Newport FSQ-GG420, 50.8 mm 50.8 mm, Newport, Irvine, CA, USA) was employed × for experiments performed under visible light. For experiments performed under visible light, the intensity was measured at 7 10 5 einstein/(m2 s). × − · Most experiments were performed at matrix’s inherent pH (pH = 6) except those where the initial pH was adjusted to acidic or alkaline conditions. At regular time intervals, 1.2 mL volume samples are taken, filtered through a 0.2 µm PVDF filter and analyzed using high performance liquid chromatography (HPLC). Separation was achieved on a Kinetex XB-C18 100A column (Phenomenex, Torrance, CA, USA) (2.6 mm, 2.1 mm, 50 mm) and a 0.5 mm inline filter (Phenomenex, Torrance, CA, USA) (KrudKatcher Ultra) both purchased from Phenomenex. The mobile phase consisting of 75:25 water:acetonitrile eluted isocratically at 0.35 mL/min and 45 ◦C, while the injection volume was 40 mL. Detection was achieved through a photodiode array detector (Waters, Milford, MA, USA) (Waters 2996 PDA detector, detection l 1⁄4 254 nm). Details about HPLC measurements can be found elsewhere [23]. Most of the experiments were run in duplicate, and mean values are quoted as results, whose deviation never exceeded 5% in the range of concentrations tested. Water 2020, 12, x FOR PEER REVIEW 4 of 15

photodiode array detector (Waters, Milford, MA, USA) (Waters 2996 PDA detector, detection l ¼ 254 nm). Details about HPLC measurements can be found elsewhere [23]. Most of the experiments were run in duplicate, and mean values are quoted as results, whose deviation never exceeded 5% in the range of concentrations tested.

3. Results

3.1. Characterization Water 2020, 12, 1180 4 of 14 The X-ray diffraction pattern of the as prepared Ag2CO3 is presented in Figure 1A. It is observed that all diffraction patterns can be indexed to the monoclinic phase of Ag2CO3 (JCPDS 12-766) without 3. Resultsimpurities. Characteristic peaks are indicated in Figure 1. The sharpness of the peaks indicates the high crystallinity of the sample, while the primary crystallite size is 44 nm (based on Equation (1)). 3.1.Specific Characterization surface area according to BET method is lower than 1 m2/g. The optical properties of Ag2CO3 were studied with the use of (UV-vis ) diffuse reflectance The X-ray diffraction pattern of the as prepared Ag2CO3 is presented in Figure1A. It is observed spectroscopy and the corresponding spectrum is shown in Figure 2. It is observed that Ag2CO3 has that all diffraction patterns can be indexed to the monoclinic phase of Ag CO (JCPDS 12-766) without an absorption threshold at ~467 nm that allows to absorb a significant portion2 of visible3 light. Energy impurities. Characteristic peaks are indicated in Figure1. The sharpness of the peaks indicates the band gap was determined according to Tauc method and found equal to 2.6 eV (inset of Figure 2). highFurther crystallinity information of theabout sample, the morphology while the of primary Ag2CO3 was crystallite collected size from is the 44 TEM nm (basedimage (Figure on Equation 1B). (1)). 2 SpecificIt is surfaceobserved area that according crystal size to BET varies method from is 100–40 lower nmthan and 1 m is/g. of almost spherical shape.

(A) (130) (-101) (031) Intensity (a.u.) (200) (020) (110)

15 30 45 60 75 Diffraction angle (2θ)

Figure 1. (A) X-ray diﬀraction (XRD) pattern and (B) transmission electron microscopy (TEM) image

ofFigure Ag2CO 1.3.(A) X-ray diffraction (XRD) pattern and (B) transmission electron microscopy (TEM) image of Ag2CO3. The optical properties of Ag2CO3 were studied with the use of (UV-vis) diffuse reflectance spectroscopy and the corresponding spectrum is shown in Figure2. It is observed that Ag 2CO3 has an absorption threshold at ~467 nm that allows to absorb a significant portion of visible light. Energy band gap was determined according to Tauc method and found equal to 2.6 eV (inset of Figure2). Further information about the morphology of Ag2CO3 was collected from the TEM image (Figure1B). It is observed that crystal size varies from 100–40 nm and is of almost spherical shape. Characterization Water 2020, 12, x FOR PEER REVIEW 5 of 15 results are in accordance with the literature for pure Ag2CO3 [16,21].

1/2

(F(R)*hv) 2.6 eV

2.4 2.6 2.8 3.0 3.2 3.4

F(R) Energy (eV)

250 300 350 400 450 500 550 600 Wavelength (nm)

Figure 2. UV-vis diﬀuse reﬂectance spectra of Ag2CO3. Inset: Tauc plot of Ag 2CO3. Figure 2. UV-vis diffuse reflectance spectra of Ag2CO3. Ιnset: Tauc plot of Ag2CO3. 3.2. Photocatalytic Activity of Ag2CO3

3.2. Photocatalytic Activity of Ag2CO3 Figure3 presents the photocatalytic performance at di ﬀerent concentrations of Ag2CO3 for 0.5 mg/LFigure EP degradation 3 presents the in UPWphotocatalytic under simulated performance solar at different irradiation. concentrations It is observed of Ag that,2CO3in for the 0.5 absence mg/L EP degradation in UPW under simulated solar irradiation. It is observed that, in the absence of photocatalyst, practically no EP degradation takes place due to photolytic phenomena. The addition of only 250 mg/L Ag2CO3 results in almost 50% removal after 30 min, proving its high photocatalytic activity. Considering that EP degradation follows pseudo-first-order kinetics, the reaction rate can be expressed as follows [24]: 푑[EP] rate = - = k [퐸푃] (2) dt app

Apparent rate constants (kapp) were calculated from the linearized form of Equation (2) and are shown in brackets. Doubling photocatalyst dosage from 250 mg/L to 500 mg/L accelerates EP degradation, resulting in 90% EP removal in the same time period. Further increase in Ag2CO3 concentration to 750 mg/L leads to an increase in kapp from 0.075 min−1 to 0.104 min−1. In contrast, raising Ag2CO3 concentration to 1000 mg/L only slightly favors EP degradation, increasing kapp to 0.116 min−1. The optimum photocatalyst dosage constitutes a special feature of each photocatalytic system highly connected not only to the activity of the photocatalytic material but also to the light penetration into the solution. Αs a result, the optimum value varies widely depending on the geometry and operating conditions of the photoreactor. It is common knowledge that photocatalytic degradation of organic substances proceeds not only by direct reaction with photogenerated holes (h+) but also indirectly through the formation of hydroxyl radicals (•OH). A prerequisite for •OH formation is the valence band (VB) oxidation potential of the semiconductor to be located below the potential of •OH formation (2.38 V vs NHE, pH = 7). In the case of Ag2CO3, VB is at 2.78 V vs NHE, pH = 7 [25], suggesting the participation of •OH in EP degradation. Since the reaction takes place in the presence of oxygen, photogenerated electrons can also react with adsorbed O2 resulting in the formation of additional oxidative species (mainly superoxide radical (O2•−). Since optimization was not the goal of this work, a catalyst loading of 750 mg/L was selected for the continuation of this study.

Water 2020, 12, 1180 5 of 14 of photocatalyst, practically no EP degradation takes place due to photolytic phenomena. The addition of only 250 mg/L Ag2CO3 results in almost 50% removal after 30 min, proving its high photocatalytic activity. Considering that EP degradation follows pseudo-ﬁrst-order kinetics, the reaction rate can be expressed as follows [24]: Water 2020, 12, x FOR PEER REVIEW d[EP] 6 of 15 rate = = kapp[EP] (2) − dt

1.0 250 mg/L (0.024) 500 mg/L (0.075) 0.8 750 mg/L (0.104) 1000 mg/L (0.116) 0.6 photolysis

C/Co 0.4

0.2

0.0 0 5 10 15 20 25 30 Irradiation time (min)

Figure 3.FigureEﬀect 3. of Effect photocatalyst of photocatalyst concentration concentration on on 0.5 0.5 mg mg/L/L EP EP degradationdegradation in in ultrapure ultrapure water water (UPW). (UPW). 1 NumbersNumbers in brackets in brackets show show apparent apparent rate rate constants constants in in min min− −1..

ApparentFigure rate 4 constantsshows the results (kapp) obtained were calculated varying EP from concentration the linearized in the range form 0.25–1.00 of Equation mg/L. (2)It is and are shownobserved in brackets. that a kapp Doubling equal to 0.115 photocatalyst min−1 is obtained dosage for the from lower 250 EP mg concentration/L to 500 mg tested./L accelerates Increasing EP −1 degradation,EP concentration resulting to in 1.00 90% mg/L EP resulted removal in inonly the a slight same decrease time period. in kapp to Further 0.085 min increase. in Ag2CO3 Comparable irradiation times have been reported in case of organic1 dyes degradation1 under concentration to 750 mg/L leads to an increase in kapp from 0.075 min− to 0.104 min− . In contrast, visible light irradiation with the use of Ag2CO3; H. Dong et al [15] showed that 10 mg/L methyl orange raising Ag2CO3 concentration to 1000 mg/L only slightly favors EP degradation, increasing kapp to and1 10 mg/L rhodamine B was removed from ultrapure water in 15 min and 35 min, respectively, 0.116 min− . The optimum photocatalyst dosage constitutes a special feature of each photocatalytic with 0.5 g/L Ag2CO3. Moreover, the as prepared Ag2CO3 photocatalyst has similar activity with system highlyheterostructures connected that not include only it. to Specifically, the activity Ag of2CO the3/ZnO photocatalytic led to complete material rhodamine but B also degradation to the light penetrationafter into 120 min the solution.of visible light As a irradiation result, the [19], optimum while Ag value2CO3/MoS varies2 composite widely depending photocatalysts on thedegraded geometry and operatingLanasol conditions Red 5B in 25 of min the [18]. photoreactor. However, these works study photocatalytic degradation of dyes and It isnot common micropollutants, knowledge in which that photocatalyticthe results are often degradation overrated due of organic to photo-sensitization substances proceeds of the catalyst. not only by direct reaction with photogenerated holes (h+) but also indirectly through the formation of hydroxyl radicals (•OH). A prerequisite for •OH formation is the valence band (VB) oxidation potential of the semiconductor to be located1.0 below the potential of •OH formation (2.38 V vs NHE, pH = 7). In the case of Ag2CO3, VB is at 2.78 V vs NHE, pH = 7 [25], suggesting the participation of •OH in EP degradation. Since the reaction takes place0.8 in the presence of oxygen, photogenerated electrons can also react with adsorbed O2 resulting in the formation of additional oxidative species (mainly superoxide radical 0.25 mg/L (0.115) (O2•−). Since optimization was not the goal of this work, a catalyst loading of 750 mg/L was selected 0.6 1.00 mg/L (0.085) for the continuation of this study. 0.50 mg/L (0.104) Figure4 shows the results obtained varying EP concentration in the range 0.25–1.00 mg /L. It is C/Co 0.4 1 observed that a kapp equal to 0.115 min− is obtained for the lower EP concentration tested. Increasing 1 EP concentration to 1.00 mg0.2/L resulted in only a slight decrease in kapp to 0.085 min− . Comparable irradiation times have been reported in case of organic dyes degradation under visible light irradiation with0.0 the use of Ag2CO3; H. Dong et al [15] showed that 10 mg/L methyl orange and 10 mg/L rhodamine B was0 removed 5 from 10 ultrapure 15 water 20 in 1525 min 30and 35 min, respectively, with 0.5 g/L Ag2CO3. Moreover, the as preparedIrradiation Ag2CO time3 photocatalyst (min) has similar activity with heterostructures that include it. Speciﬁcally, Ag2CO3/ZnO led to complete rhodamine B degradation

Figure 4. Effect of EP concentration on its photocatalytic degradation with 750 mg/L Ag2CO3 in UPW. Numbers in brackets show apparent rate constants in min−1.

Water 2020, 12, x FOR PEER REVIEW 6 of 15

1.0 250 mg/L (0.024) 500 mg/L (0.075) 0.8 750 mg/L (0.104) 1000 mg/L (0.116) 0.6 photolysis

C/Co 0.4

0.2

0.0 0 5 10 15 20 25 30 Irradiation time (min)

Figure 3. Effect of photocatalyst concentration on 0.5 mg/L EP degradation in ultrapure water (UPW). Numbers in brackets show apparent rate constants in min−1.

Figure 4 shows the results obtained varying EP concentration in the range 0.25–1.00 mg/L. It is observed that a kapp equal to 0.115 min−1 is obtained for the lower EP concentration tested. Increasing EP concentration to 1.00 mg/L resulted in only a slight decrease in kapp to 0.085 min−1. Comparable irradiation times have been reported in case of organic dyes degradation under visible light irradiation with the use of Ag2CO3; H. Dong et al [15] showed that 10 mg/L methyl orange and 10 mg/L rhodamine B was removed from ultrapure water in 15 min and 35 min, respectively, Water 2020with, 12 0.5, 1180 g/L Ag2CO3. Moreover, the as prepared Ag2CO3 photocatalyst has similar activity with6 of 14 heterostructures that include it. Specifically, Ag2CO3/ZnO led to complete rhodamine B degradation after 120 min of visible light irradiation [19], while Ag2CO3/MoS2 composite photocatalysts degraded after 120Lanasol min ofRed visible 5B in light25 min irradiation [18]. However, [19], whilethese works Ag2CO study3/MoS photocatalytic2 composite photocatalystsdegradation of dyes degraded and Lanasolnot Red micropollutants, 5B in 25 min in [18 which]. However, the results these are often works overrated study photocatalytic due to photo-sensitization degradation of the of dyescatalyst. and not micropollutants, in which the results are often overrated due to photo-sensitization of the catalyst.

1.0

0.8

0.25 mg/L (0.115) 0.6 1.00 mg/L (0.085)

0.50 mg/L (0.104)

C/Co 0.4

0.2

0.0 0 5 10 15 20 25 30 Irradiation time (min)

Figure 4. Effect of EP concentration on its photocatalytic degradation with 750 mg/L Ag CO in UPW. Water 2020, 12, x FOR PEER REVIEW 2 3 7 of 15 Figure 4. Effect of EP concentration on its photocatalytic degradation1 with 750 mg/L Ag2CO3 in UPW. Numbers in brackets show apparent rate constants in min− . Numbers in brackets show apparent rate constants in min−1. 3.3. Effect of Type of Irradiation 3.3. Eff ect of Type of Irradiation Diffuse reflectance spectroscopy showed that Ag2CO3 can absorb a significant portion of light in Diffuse reflectance spectroscopy showed that Ag CO can absorb a significant portion of light in the visible region (Figure 2). To confirm this, additional2 3 experiments were carried out under visible the visible region (Figure2). To confirm this, additional experiments were carried out under visible or UV light with the use of appropriate cut-off filters. As seen in Figure 5, an almost 55% removal is or UVachieved light with after the 20 use min of appropriate under visible cut-o lightff alonefilters. and As thisseen increases in Figure to5, 75%an almost under 55% UV removallight, thus is achievedimplying after20 that min the under photocatalyst visible light is responsive alone and in this the increasesvisible part to of 75% the underspectrum. UV light, thus implying that the photocatalyst is responsive in the visible part of the spectrum.

1.0

0.8

0.6

C/Co 0.4

0.2 solar (0.104) UV (0.060) visible (0.042) 0.0 0 5 10 15 20 Irradiation time (min)

Figure 5. Eﬀect of type of irradiation on 0.5 mg/L EP degradation with 750 mg/L Ag CO in UPW. Figure 5. Effect of type of irradiation on 0.5 mg/L EP degradation with 750 mg/L Ag22CO33 in UPW. 1 NumbersNumbers in brackets in brackets show show apparent apparent rate rate constants constants in minin min− .−1. 3.4. Eﬀect of the Water Matrix 3.4. Effect of the Water Matrix In order to test the present photocatalytic system under more realistic conditions, a new set of In order to test the present photocatalytic system under more realistic conditions, a new set of experimentsexperiments were were conducted conducted in real in real water water matrices, matrices, such such as as BW BW and and WW. WW. Both Both organicorganic and and inorganic inorganic components present in aqueous media could modify the route of micropollutants photocatalytic degradation pathways, thus either accelerating or slowing down the overall performance [26]. In most cases, hindering effects become more significant as the water matrix complexity increases. However, this cannot be considered as a rule, as highly promoting effects have also been reported in complex aqueous media. Figure 6 presents the degradation of 0.5 mg/L EP with 750 mg/L Ag2CO3 in UPW, BW, and WW. Noticeably, there is only a minor decrease in kapp from 0.104 in UPW to 0.080 min−1 in BW and WW. In order to interpret these results, further experiments were performed in UPW spiked with (i) humic acid (HA), an analogue of the natural organic matter found in waters/wastewaters, and (ii) bicarbonate and chloride as the most common inorganic anions. The effect of 10 mg/L HA on EP degradation is also shown in Figure 6. It is observed that EP removal was greatly enhanced, with kapp increasing to 0.320 min−1. This may be attributed to the fact that (i) organic compounds naturally occurring in effluents can act as photosensitizers of the photocatalyst (through electron transfer from HA to semiconductor) [27] and/or (ii) electron transfer from photocatalyst conduction band to HA may occur, thus suppressing electron-hole recombination and, consequently, enhancing photodegradation rates [28]. Other enhancing effects may include (iii) the photosensitization activity of organic species that can promote the production of reactive oxygen species (ROS) by UV irradiation, resulting in indirect photolysis of micropollutants [29], and (iv) the homolysis of organic matter producing additional reactive radicals able to oxidize micropollutants either directly or indirectly through reaction with oxygen and formation of additional ROS [30].

Water 2020, 12, 1180 7 of 14 components present in aqueous media could modify the route of micropollutants photocatalytic degradation pathways, thus either accelerating or slowing down the overall performance [26]. In most cases, hindering effects become more significant as the water matrix complexity increases. However, this cannot be considered as a rule, as highly promoting effects have also been reported in complex aqueous media. Figure6 presents the degradation of 0.5 mg /L EP with 750 mg/L Ag2CO3 in UPW, BW, and WW. 1 Noticeably, there is only a minor decrease in kapp from 0.104 in UPW to 0.080 min− in BW and WW. In order to interpret these results, further experiments were performed in UPW spiked with (i) humic Water 2020, 12, x FOR PEER REVIEW 8 of 15 acid (HA), an analogue of the natural organic matter found in waters/wastewaters, and (ii) bicarbonate and chloride as the most common inorganic anions.

1.0

0.8

0.6

C/Co 0.4

UPW (0.104) 0.2 BW (0.081) WW (0.082) HA (0.320) 0.0 0 2 4 6 8 10 Irradiation time (min)

Figure 6. Effect of water matrix (BW, WW) and 10 mg/L HA on 0.5 mg/L EP degradation with 750 mg/L Figure 6. Effect of water matrix (BW, WW) and 10 mg/L HA on 0.51 mg/L EP degradation with 750 Ag2CO3. Numbers in brackets show apparent rate constants in min− . mg/L Ag2CO3. Numbers in brackets show apparent rate constants in min−1. The effect of 10 mg/L HA on EP degradation is also shown in Figure6. It is observed that EP The effect of bicarbonate on EP degradation with the use of 1750 mg/L Ag2CO3 is presented in removal was greatly enhanced, with kapp increasing to 0.320 min− . This may be attributed to the fact thatFigure (i) 7A. organic Addition compounds of 100 mg/L naturally bicarbonate occurring increases in significantly effluents can EP act removal as photosensitizers with kapp increasing of the from 0.104 to 0.578 min−1. A further increase to 250 mg/L or 500 mg/L lowers EP degradation rate, photocatalyst (through electron transfer from HA to semiconductor) [27] and/or (ii) electron transfer which is, however, greater than in UPW. This contradicts the majority of previous studies that usually from photocatalyst conduction band to HA may occur, thus suppressing electron-hole recombination report retardation effects associated with the scavenging of hydroxyl radicals by bicarbonate and the and, consequently, enhancing photodegradation• rates [28]. Other enhancing effects may include (iii) formation of carbonate radicals (퐻퐶푂) with lower oxidation potential [31]. In a previous work, we the photosensitization activity of organic species that can promote the production of reactive oxygen found that the presence of bicarbonate quenched the recombination of photogenerated carriers and species (ROS) by UV irradiation, resulting• in indirect photolysis• of micropollutants [29], and (iv) the this led an increased production of OH [32]. Moreover, (퐻퐶푂) is a one-electron oxidant that acts by homolysisboth electron of organic transfer matter and producing hydrogen additional abstraction reactive mechanisms radicals producing able to oxidize ROS from micropollutants the oxidized eithercompounds, directly or which indirectly could through partly counteract reaction with the scavenging oxygen and of formation •OH [33]. of additional ROS [30]. The effect of bicarbonate on EP degradation with the use of 750 mg/L Ag2CO3 is presented in

Figure7A. Addition of 100 mg /L bicarbonate increases signiﬁcantly EP removal with kapp increasing (A) (B) 1.0 1 UPW (0.104) 1.0 from 0.104 to 0.578 min− . A further increase to 250 mg/L or 500 mg/L lowers EP UPW degradation (0.104) rate, 100 mg/L (0.578) 100 mg/L (0.403) 250 mg/L (0.381) which is, however,0.8 greater than in UPW. This contradicts the majority of previous studies 250 mg/L (0.160) that usually 500 mg/L (0.232) 0.8 500 mg/L (0.041) report retardation eﬀects associated with the scavenging of hydroxyl radicals by bicarbonate and 0.6 0.6 the formation of carbonate radicals (HCO•) with lower oxidation potential [31]. In a previous work,

3 C/Co we found that0.4 the presence of bicarbonate quenched the recombinationC/Co 0.4 of photogenerated carriers and this led an increased production of •OH [32]. Moreover, (HCO3•) is a one-electron oxidant that acts by both electron0.2 transfer and hydrogen abstraction mechanisms0.2 producing ROS from the oxidized compounds,0.0 which could partly counteract the scavenging of0.0 •OH [33]. 0.0 1.5 3.0 4.5 6.0 0 5 10 15 20 Irradiation time (min) Irradiation time (min)

Figure 7. Effect of (A) bicarbonate and (B) chloride on 0.5 mg/L EP degradation with 750 mg/L Ag2CO3. Numbers in brackets show apparent rate constants in min−1.

The effect of chloride on EP degradation is depicted in Figure 7B. Addition of 100 mg/L chloride leads to complete EP removal in less than 8 min, which is almost three times faster than in UPW. Increasing chloride concentration to 250 mg/L hampers EP removal, decreasing kapp from 0.403 to 0.160 min−1, but this still is faster than in UPW. Only at 500 mg/L chloride, does EP degradation slow

Water 2020, 12, x FOR PEER REVIEW 8 of 15

1.0

0.8

0.6

C/Co 0.4

UPW (0.104) 0.2 BW (0.081) WW (0.082) HA (0.320) 0.0 0 2 4 6 8 10 Irradiation time (min)

Figure 6. Effect of water matrix (BW, WW) and 10 mg/L HA on 0.5 mg/L EP degradation with 750 mg/L Ag2CO3. Numbers in brackets show apparent rate constants in min−1.

The effect of bicarbonate on EP degradation with the use of 750 mg/L Ag2CO3 is presented in Figure 7A. Addition of 100 mg/L bicarbonate increases significantly EP removal with kapp increasing from 0.104 to 0.578 min−1. A further increase to 250 mg/L or 500 mg/L lowers EP degradation rate, which is, however, greater than in UPW. This contradicts the majority of previous studies that usually report retardation effects associated with the scavenging of hydroxyl radicals by bicarbonate and the • formation of carbonate radicals (퐻퐶푂) with lower oxidation potential [31]. In a previous work, we found that the presence of bicarbonate quenched the recombination of photogenerated carriers and • • this led an increased production of OH [32]. Moreover, (퐻퐶푂) is a one-electron oxidant that acts by both electron transfer and hydrogen abstraction mechanisms producing ROS from the oxidized Water 2020, 12, 1180 8 of 14 compounds, which could partly counteract the scavenging of •OH [33].

(A) (B) 1.0 UPW (0.104) 1.0 UPW (0.104) 100 mg/L (0.578) 100 mg/L (0.403) 250 mg/L (0.381) 0.8 250 mg/L (0.160) 500 mg/L (0.232) 0.8 500 mg/L (0.041)

0.6 0.6

C/Co 0.4 C/Co 0.4

0.2 0.2

0.0 0.0 0.0 1.5 3.0 4.5 6.0 0 5 10 15 20 Irradiation time (min) Irradiation time (min)

FigureFigure 7. EEffectﬀect of of ( A (A) bicarbonate) bicarbonate and and (B ) ( chlorideB) chloride on 0.5 on mg 0.5/L mg/L EP degradation EP degradation with 750 with mg /L 750 Ag mg/L2CO3 . Numbers in brackets show apparent rate constants in min 1. Ag2CO3. Numbers in brackets show apparent rate constants− in min−1. The eﬀect of chloride on EP degradation is depicted in Figure7B. Addition of 100 mg /L chloride The effect of chloride on EP degradation is depicted in Figure 7B. Addition of 100 mg/L chloride leads to complete EP removal in less than 8 min, which is almost three times faster than in UPW. leads to complete EP removal in less than 8 min, which is almost three times faster than in UPW. Increasing chloride concentration to 250 mg/L hampers EP removal, decreasing kapp from 0.403 to Increasing chloride1 concentration to 250 mg/L hampers EP removal, decreasing kapp from 0.403 to 0.160 min−1− , but this still is faster than in UPW. Only at 500 mg/L chloride, does EP degradation slow 0.160 min , but this still is faster than in UPW. Only at1 500 mg/L chloride, does EP degradation slow down considerably with the kapp becoming 0.041 min− . In general, addition of chloride (Cl−) in UPW could lead to the following reactions:

HO• + Cl− Cl• + HO− (3) →

Cl− + Cl• Cl•− (4) → 2 Moreover, several consecutive reactions may take place leading to the formation of additional chlorine-containing radicals such as Cl•, Cl2•−, ClOH•− [26,34]. While these reactions may be responsible for scavenging •OH or blocking catalyst active sites, beneficial effects have also been reported in similar systems [35,36]; these may be related to the higher selectivity of chlorine radicals, thus altering degradation pathways. Overall, major organic and inorganic, non-target constituents inherently found in waters seem not to inhibit the photocatalytic performance of Ag2CO3 and this has been demonstrated in experiments with environmental samples, i.e., BW and WW. A synergistic but yet complex interplay amongst the matrix’s organic and inorganic constituents, the photocatalyst and the substate under consideration may explain this observation.

3.5. Effect of pH The effect of changing the initial pH in the range 3–9 on 0.5 mg/L EP degradation in UPW was also tested and the results are shown in Figure8. It seems that a shift from inherent, near-neutral conditions (i.e., pH = 6) to an alkaline environment does not have a considerable effect on EP degradation, with kapp 1 only slightly decreasing from 0.104 to 0.081 min− . This could be explained considering the matching or mismatching between the catalyst’s surface charge and the organic molecule’s ionization state. Indeed, the pKa value of EP is 8.34 [4], while Ag2CO3 has been reported to change surface charge under highly basic conditions [37]. These observations agree with M. Guo et al. [37] and E. Nyankson et al. [38] who studied the photocatalytic performance of AgBr/Ag2CO3 and Ag2CO3-halloysite nanotubes respectively, showing the dependence of photocatalytic activity on pH, with favoring effects taking place under neutral and basic conditions. EP removal is significantly accelerated at acidic conditions. In general Ag-based semiconductors are unstable under acidic media [37], as shown in reuse experimental runs (Figure9A,B). Dissolution of Ag2CO3 in acidic media to silver ions may trigger homogeneous photocatalytic phenomena, thus explaining the increased reactivity. Water 2020, 12, x FOR PEER REVIEW 9 of 15

down considerably with the kapp becoming 0.041 min−1. In general, addition of chloride (Cl−) in UPW could lead to the following reactions: 퐻푂• + 퐶푙 → 퐶푙• + 퐻푂 (3)

• • 퐶푙 + 퐶푙 → 퐶푙 (4) Moreover, several consecutive reactions may take place leading to the formation of additional • • • chlorine-containing radicals such as 퐶푙 , 퐶푙 , 퐶푙푂퐻 [26,34]. While these reactions may be responsible for scavenging •OH or blocking catalyst active sites, beneficial effects have also been reported in similar systems [35,36]; these may be related to the higher selectivity of chlorine radicals, thus altering degradation pathways. Overall, major organic and inorganic, non-target constituents inherently found in waters seem not to inhibit the photocatalytic performance of Ag2CO3 and this has been demonstrated in experiments with environmental samples, i.e., BW and WW. A synergistic but yet complex interplay amongst the matrix’s organic and inorganic constituents, the photocatalyst and the substate under consideration may explain this observation.

3.5. Effect of pH The effect of changing the initial pH in the range 3–9 on 0.5 mg/L EP degradation in UPW was also tested and the results are shown in Figure 8. It seems that a shift from inherent, near-neutral conditions (i.e., pH = 6) to an alkaline environment does not have a considerable effect on EP degradation, with kapp only slightly decreasing from 0.104 to 0.081 min−1. This could be explained considering the matching or mismatching between the catalyst’s surface charge and the organic molecule’s ionization state. Indeed, the pKa value of EP is 8.34 [4], while Ag2CO3 has been reported to change surface charge under highly basic conditions [37]. These observations agree with M. Guo et al. [37] and E. Nyankson et al. [38] who studied the photocatalytic performance of AgBr/Ag2CO3 and Ag2CO3-halloysite nanotubes respectively, showing the dependence of photocatalytic activity on WaterpH, with2020, 12favoring, x FOR PEER effects REVIEW taking place under neutral and basic conditions. 10 of 15 EP removal is significantly accelerated at acidic conditions. In general Ag-based semiconductors 3.6.are Catalystunstable Reuse under acidic media [37], as shown in reuse experimental runs (Figures 9A and 9B). Dissolution of Ag2CO3 in acidic media to silver ions may trigger homogeneous photocatalytic Reusability tests were carried out using 750 mg/L Ag2CO3 under neutral (pH = 6) and acidic phenomena, thus explaining the increased reactivity. conditions (pH = 3) in UPW using solar and UV light irradiation. To do so, 750 mg/L of fresh Ag2CO3 This said, most environmental matrices have pH values between 6 and 8, i.e., a pH range where was added in 120 mL of ultrapure water containing 0.5 mg/L EP and the mixture was then subject to silver leaching is unlikely to occur to a considerable degree. Waterirradiation2020, 12, 1180 for 120 min; at the end of this period, EP conversion was monitored and the mixture9 of was 14 added the necessary amount of EP to bring its initial concentration back to 0.5 mg/L. This cycle was

repeated twice, while no other conditioning took place. Figure 9A shows the performance of the recycled Ag2CO3 catalyst 1.0for EP degradation for three consecutive runs. It is observed that under neutral conditions EP removal reduces to ~80% after the first run. This is probably attribute to stability pH=6 (0.104) issues related with silver-based0.8 catalysts. This reduction is more pH=3 pronounced (0.490) in the third cycle were EP removal drops to 60%. On the other hand, under acidic conditions,pH=9 (0.081) it is observed that after the first run, EP removal drops0.6 practically to zero, implying complete dissolution of Ag2CO3 at this pH.

The same experimental runs were carried out with the use of the appropriate visible light cut off

filter and results are presentedC/Co 0.4 in Figure 9B. It is observed that results are qualitatively the same under UV and solar irradiation, showing that stability issues of Ag2CO3 are probably related with the UV part of solar irradiation0.2 and not to the visible. From these preliminary results, it is clear that future research should be directed to a systematic 0.0 check of the stability of the Ag02CO catalyst in 5 a continuous 10 flow reactor 15and to examine whether the amount of silver ions exceeds the environmentalIrradiation limits. time (min)Also, technologies such as adsorption to biochar [39] can be used to recover silver, thus providing a complete solution in the context of the circularFigure economy. 8. Eﬀect of initial solution pH on 0.5 mg/L EP degradation with 750 mg/L Ag2CO3. Numbers in 1 bracketsFigure show8. Effect apparent of initial rate solution constants pH in on min 0.5− mg/L. EP degradation with 750 mg/L Ag2CO3. Numbers in brackets show apparent rate constants in min−1. 100 100 (B) pH=6 (A) pH=6 pH=3 pH=3 80 80

60 60

40 40

20 20 120-min removal EP120-min (%) 0 (%) removal EP 120-min 0

n n n u u u r r r t d t s n s 1 2 3rd run 1 2nd run 3rd run

Figure 9. Change of photocatalytic activity of Ag2CO3 upon repeated use for the degradation of 0.5 mg/L EP in UPW in neutral and acidic pH under (A) solar irradiation and (B) UV irradiation. Figure 9. Change of photocatalytic activity of Ag2CO3 upon repeated use for the degradation of 0.5 Thismg/L said, EP in most UPW environmental in neutral and acidic matrices pH haveunder pH(A) valuessolar irradiation between and 6 and (B) 8,UV i.e., irradiation. a pH range where silver leaching is unlikely to occur to a considerable degree. 3.7. Comparison with Different Advanced Oxidation AOPs 3.6. Catalyst Reuse The results presented in this work are in accordance with previous studies of our group investigatingReusability the tests photocatalytic were carried degradation out using 750 ofmg EP/ L with Ag2 CO (i) 3 CuOunderx/BiVO neutral4 photocatalysts, (pH = 6) and yielding acidic conditionscomplete (pHremoval= 3) inafter UPW 60 usingmin [40], solar and and (ii) UV ZnO light suspensions irradiation. leading To do so, to 750 complete mg/L of removal fresh Ag after2CO 320 wasmin added [41]. These in 120 yields mL of are ultrapure of the same water order containing of magnitude 0.5 mg/ Lor EP even and higher the mixture than the was state then of subjectart TiO2 to(Degussa irradiation P25) for photocatalyst, 120 min; at the showing end of thiscomplete period, 1 EPmg/L conversion EP removal was in monitored ~30 min and[41]. theIt should mixture be wasnoted added here the that necessary ZnO and amount TiO2 are of UV-light EP to bring responsive its initial materials concentration with efficiencies back to 0.5 under mg/L. visible This cycle light wasbe repeatedclose to zero. twice, In while two recent no other studies, conditioning Fernandes took et place. al [3] Figureachieved9A showscomplete the EP performance removal after of the ~20 recycledmin using Ag lab-made2CO3 catalyst g-C3N for4, while EP degradation Guo et al [42] for showed three consecutive that AgCl/Ag runs.3PO It4 photocatalysts is observed that led under to 98% neutralEP removal conditions (20 mg/L) EP removal in 40 reducesmin of solar to ~80% irradiation. after the The first run.superiority This is probablyof the present attribute system to stability derives issuesfrom relatedthe fact withthat it silver-based retains its activity catalysts. in real This water reduction matrices, is more increasing pronounced its practical in the thirdvalue. cycle Moreover, were EPAg removal2CO3 showed drops topractically 60%. On the same other hand,activity under under acidic visible conditions, and UV itirradiation, is observed establishing that after the itself first as run,a true EP removal visible drops light practically active photocatalytic to zero, implying material. complete Information dissolution regarding of Ag2CO 3 EPat this photocatalytic pH. degradationThe same is experimental collected in Table runs were 1. carried out with the use of the appropriate visible light cut off filter and results are presented in Figure9B. It is observed that results are qualitatively the same under

UV and solar irradiation, showing that stability issues of Ag2CO3 are probably related with the UV part of solar irradiation and not to the visible. From these preliminary results, it is clear that future research should be directed to a systematic check of the stability of the Ag2CO catalyst in a continuous ﬂow reactor and to examine whether the Water 2020, 12, 1180 10 of 14 amount of silver ions exceeds the environmental limits. Also, technologies such as adsorption to biochar [39] can be used to recover silver, thus providing a complete solution in the context of the circular economy.

3.7. Comparison with Diﬀerent Advanced Oxidation AOPs The results presented in this work are in accordance with previous studies of our group investigating the photocatalytic degradation of EP with (i) CuOx/BiVO4 photocatalysts, yielding complete removal after 60 min [40], and (ii) ZnO suspensions leading to complete removal after 20 min [41]. These yields are of the same order of magnitude or even higher than the state of art TiO2 (Degussa P25) photocatalyst, showing complete 1 mg/L EP removal in ~30 min [41]. It should be noted here that ZnO and TiO2 are UV-light responsive materials with eﬃciencies under visible light be close to zero. In two recent studies, Fernandes et al [3] achieved complete EP removal after ~20 min using lab-made g-C3N4, while Guo et al [42] showed that AgCl/Ag3PO4 photocatalysts led to 98% EP removal (20 mg/L) in 40 min of solar irradiation. The superiority of the present system derives from the fact that it retains its activity in real water matrices, increasing its practical value. Moreover, Ag2CO3 showed practically the same activity under visible and UV irradiation, establishing itself as a true visible light active photocatalytic material. Information regarding EP photocatalytic degradation is collected in Table1.

Table 1. EP photocatalytic degradation with diﬀerent materials.

EP Catalyst Time Period for Type of Photocatalyst Concentration Concentration Complete References Irradiation (mg/L) (mg/L) Degradation (min)

0.75(% wt.) CuOx/BiVO4 2 1000 solar 60 [40] ZnO 1 250 solar 20 [41] TiO2(P25) 1 250 solar 30 [41] g-C3N4 13 1000 visible 20 [3] AgCl/Ag3PO4 20 500 visible 40 [42] Ag2CO3 0.5 750 solar 30 [This work]

Apart from photocatalysis, EP removal has also been tested by other AOPs and characteristic examples are presented in Table2. In the case of heat-activated persulfate (PS), complete EP removal (3 mg/L) was achieved after 240 min of reaction in UPW, but the reaction was impeded when the matrix was spiked with organic matter [43]. In addition, UVC-activated persulfate has been adopted for EP removal resulting in complete degradation in UPW in 90 min [44]. EP degradation (0.2 mg/L) has also been achieved by anodic oxidation over boron-doped diamond yielding complete removal within 2 20 min at 70 mA/cm in 0.1 M Na2SO4 electrolyte [45].

Table 2. EP removal with diﬀerent advanced oxidation processes (AOPs).

Time Period for EP Concentration System Complete Experimental Parameters References (mg/L) Degradation (min) [EP]:20 mM, Heat-activated/PS 3 240 [43] pH: 7, 60 ◦C [persulfate]:1mM. [EP]:30 mM, UV-C/PS 5 90 [44] [PS =PMS]:0.25-2.5 mM [EP]:200 mg/L in Electrochemical oxidation 0.2 20 2 [45] 0.1 M Na2SO4,i:70 mA/cm Ag2CO3/solar irradiation 0.5 30 [catalyst]:750 mg/L, pH:6 [This work] Water 2020, 12, 1180 11 of 14

4. Conclusions

In this work, a visible light responsive Ag2CO3 photocatalyst was successfully prepared by a simple solution method. A series of experiments were carried out for ethylparaben degradation under simulated solar light irradiation. The main conclusions are as follows:

Ag CO shows substantial activity for EP degradation in UPW under both solar and visible • 2 3 light irradiation. The presence of ions such as bicarbonates and chlorides in concentrations found in environmental • samples improves performance. Of particular interest is that the presence of humic acid significantly increases the degradation • of EP. As a result, Ag CO retains its activity in more complex, environmentally relevant matrices, like • 2 3 secondary treated wastewater and bottled water. Ag CO photocatalytic properties are similar with other photocatalytic systems studying organic • 2 3 dyes degradation under visible light irradiation with the use of Ag2CO3. Moreover, its activity is of the same order of magnitude as very promising photocatalytic materials such as CuOx/BiVO4 and g-C3N4. In terms of efficiency, the proposed system appears to be competitive with other energy-intensive • processes such as electrochemical oxidation and heat-activated persulfate and further research is needed in this direction. Future research should move towards exploring the stability of Ag CO in a continuous flow • 2 3, configuration, as well as examining environmental and low-cost methods of silver recovery.

Author Contributions: Conceptualization, A.P. and Z.F.; methodology, A.P. and D.M.; investigation, A.P. and A.N.; resources, Z.F. and D.M.; writing—original draft preparation, A.P. and D.M.; writing—review and editing, A.P., Z.F., and D.M.; visualization, A.P. and A.N.; supervision, Z.F. and D.M.; funding acquisition, Z.F. and A.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by H.F.R.I., the Hellenic Foundation for Research and Innovation and General Secretariat for Research and Technology (GSRT). This work is part of the project “2De4P: Development and Demonstration of a Photocatalytic Process for removing Pathogens and Pharmaceuticals from wastewaters” which is implemented under the Action “H.F.R.I.–1st Call for Research Projects to Support Post-Doctoral Researchers,” funded by H.F.R.I. Hellenic Foundation for Research and Innovation and General Secretariat for Research and Technology (GSRT). Acknowledgments: The authors wish to thank M. Kollia staff of the Laboratory of Electron Microscopy and Microanalysis (L.E.M.M.) at University of Patras for TEM images. Conflicts of Interest: The authors declare no conflict of interest.

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