UV-Based Advanced Oxidation Processes of Remazol Brilliant Blue R Dye Catalyzed by Carbon Dots

UV-based advanced oxidation processes (AOPs) (UV/H2O2 and UV/S2O82−) with a titanium(IV)-doped carbon dot, TiP-CD, as a catalyst were developed for the decomposition of Remazol Brilliant Blue R (Reactive Blue 19), an anthraquinone textile dye (at T = 25 °C and pH = 7). The Ti-CD, with marked catalytic UV properties, was successfully synthesized by the one-pot hydrothermal procedure, using L-cysteine as carbon precursor, ethylenediamine as nitrogen source, PEG (polyethylene glycol) as a capping agent, and titanium(IV) isopropoxide (precursor of TiO2 doping). Contrary to azo dyes (methyl orange, orange II sodium salt, and reactive black 5), which achieved complete degradation in a time interval less than 30 min in the developed AOP systems (UV/H2O2, UV/S2O82−, and UV/TiO2), the RBB-R showed relatively low degradation rates and low discoloration rate constants. In the presence of the catalyzer, the reaction rate significantly increased, and the pseudo-first-order rate constants for the RBB-R discoloration were UV/3.0 mM H2O2/TIP-CD-0.0330 min−1 and UV/1.02 mM S2O82−/TIP-CD-0.0345 min−1.


Introduction
The textile industry is one of the most polluting industries in the world, consuming 93 billion cubic meters of water. In fact, 20% of wastewater worldwide comes from fabric dyeing and treatment [1,2]. The increase of fast-fashion trends complicates the environmental impact of textile business, and the lack of garment eco-design inhibits efficient circular economy strategies [3]. About 80% of the global emissions of this industry are discharged into environmental water, and, although textile effluents are constituted by a huge number of chemical substances, natural fibers, and microplastics, of greater concern are the large amount of non-biodegradable organic compounds, especially textile dyes [4,5]. The treatment of these substances in wastewaters requires advanced oxidation processes (AOPs) [6]. One of the types of AOPs that can be used for the degradation of selected pollutants present in wastewater is based on ultraviolet light (UV); that is a standard technology for water disinfection, but it can also be coupled to several other chemical systems, such as, for example, hydrogen peroxide and persulfate anion, enhancing the performance in AOP [7,8].
The direct exposition of a dye (Dy) to high-energy UV light creates an excited state molecule ( # Dy) or the photolysis of a chemical bond, resulting in its fragmentation and radical formation (Dy1 * + Dy2 * ). In the presence of molecular oxygen, reactive oxygen species can form, such as the superoxide anion (O 2 *− ) [9]: # Dy → Dy1 * + Dy2 * (2) (3) By coupling UV with hydrogen peroxide (H 2 O 2 ), the following chain mechanism is obtained [10,11]:  (9) By coupling UV with persulfate anion (S 2 O 8 2− ), the following chain mechanism is obtained [11][12][13] Coupling UV with H 2 O 2 and S 2 O 8 2− will generate the hydroxyl radical and, in the case of S 2 O 8 2− , the sulfate radical, and these radical species are active oxidants in the dyes' degradation processes [11]. Besides these two homogeneous catalysts, the heterogeneous photocatalyst and semiconductor TiO 2 is also commonly used as an AOP, and hydroxyl radical is produced according to the following mechanism [14][15][16][17][18]: After TiO 2 UV absorption, an electron (e CB− ) in the valence band is transferred to the conduction band, leaving a hole in the valence band (h VB+ ). The TiO 2 (h VB+ ) will oxidize water, and TiO 2 (e CB− ) will reduce oxygen-generating reactive oxygen species such as hydroxide and superoxide radicals.
Nanomaterials are being coupled with UV-based AOP to make treatment processes more sustainable, i.e., to eliminate or reduce the amount of chemical substances involved in those processes, while increasing the pollutants degradation yields [6]. Taking into consideration natural resources, environmental sustainability, and toxicity issues, carbonbased nanomaterials are being preferred to metallic/semi-metallic materials in real-world applications [19]. Among the carbon-based nanomaterials, the synthetic versatility of carbon dots (CDs) [20][21][22][23][24][25][26][27][28][29] makes them the preferable choice to be coupling to an AOP for the treatment of textile wastewater.
Carbon dots are synthesized to achieve a desirable reactivity. In this paper, we describe the synthesis of a CD to be a catalyzer of the dye decomposition upon UV coupled with H 2 O 2 and S 2 O 8 2− AOPs. Because we were developing catalytic-based AOP, the objective was to increase the concentration of the hydroxyl radical and improve the dye decomposi-tion or wastewater discoloration. In this work, the rational for the synthesis design had the following criteria:

1.
To promote the decomposition of an oxidant molecule such as H 2 O 2 and S 2 O 8 2− , an antioxidant is necessary. We have been using L-cysteine as a carbon precursor in CD synthesis [5]. Moreover, due to the existence of SH functional groups, it has antioxidant properties [30]; 2.
To increase the surface reactivity of the CD, the doping with a semiconductor such as TiO 2 , which is excited when exposed to UV light, was used. Indeed, this strategy has already been used for the production of photocatalytic carbon-based nanoparticles [31][32][33].
In this paper, we describe the synthesis of two titanium(IV)-doped carbon-based nanomaterials (carbon dot), one with PEG (TiP-CD) and another without PEG (Ti-CD), that were used as catalysts in the UV based AOP discoloration of the anthraquinone dye, the Remazol Brilliant Blue R (also known as Reactive Blue 19) (RBB-R). Moreover, the performance of UV-based AOP (UV, UV/H 2 O 2 , UV/S 2 O 8 2− , and UV/TiO 2 ) was assessed in the discoloration of aqueous solutions of azo dyes, methyl orange (MO), orange II sodium salt (O-II), and reactive black 5 (RB-5), besides RBB-R. The dyes used in this work were similar to those described in reference [5]. The chemical structure of the anthraquinone dye RBB-R is shown in Figure 1.
Carbon dots are synthesized to achieve a desirable reactivity. In this paper, we describe the synthesis of a CD to be a catalyzer of the dye decomposition upon UV coupled with H2O2 and S2O8 2− AOPs. Because we were developing catalytic-based AOP, the objective was to increase the concentration of the hydroxyl radical and improve the dye decomposition or wastewater discoloration. In this work, the rational for the synthesis design had the following criteria: 1. To promote the decomposition of an oxidant molecule such as H2O2 and S2O8 2− , an antioxidant is necessary. We have been using L-cysteine as a carbon precursor in CD synthesis [5]. Moreover, due to the existence of SH functional groups, it has antioxidant properties [30]; 2. To increase the surface reactivity of the CD, the doping with a semiconductor such as TiO2, which is excited when exposed to UV light, was used. Indeed, this strategy has already been used for the production of photocatalytic carbon-based nanoparticles [31][32][33]. 3. Polyethylene glycol (PEG) has been used in CD synthesis to increase the water solubility and to coat the CD with a soft surface [5]. Indeed, PEG undergoes caramelization, leading to the production of PEGylated nanoparticles [7,[34][35][36][37].
In this paper, we describe the synthesis of two titanium(IV)-doped carbon-based nanomaterials (carbon dot), one with PEG (TiP-CD) and another without PEG (Ti-CD), that were used as catalysts in the UV based AOP discoloration of the anthraquinone dye, the Remazol Brilliant Blue R (also known as Reactive Blue 19) (RBB-R). Moreover, the performance of UV-based AOP (UV, UV/H2O2, UV/S2O8 2− , and UV/TiO2) was assessed in the discoloration of aqueous solutions of azo dyes, methyl orange (MO), orange II sodium salt (O-II), and reactive black 5 (RB-5), besides RBB-R. were used. The dyes used in this work were similar to those described in reference [5]. The chemical structure of the anthraquinone dye RBB-R is shown in Figure 1.

Synthesis of Titanium(IV)-Doped Carbon Dots (Ti-CD)
A one-pot button-up hydrothermal process was used for the Tip-CD and Ti-CD synthesis. The following solution was prepared for hydrothermal treatment: 0.400 g of L-cysteine was dissolved in 30 mL of water; after, only for the Tip-CD sample, 6.00 mL of PEG was used, and 0.60 mL of ethylenediamine was added and homogenized; lastly, 1.00 mL

Synthesis of Titanium(IV)-Doped Carbon Dots (Ti-CD)
A one-pot button-up hydrothermal process was used for the Tip-CD and Ti-CD synthesis. The following solution was prepared for hydrothermal treatment: 0.400 g of L-cysteine was dissolved in 30 mL of water; after, only for the Tip-CD sample, 6.00 mL of PEG was used, and 0.60 mL of ethylenediamine was added and homogenized; lastly, 1.00 mL of titanium(IV) isopropoxide was added to the previous solution. The resulting mixture was placed in a Parr Series 4700 Pressure Vessel and treated hydrothermally for 5 h at 200 • C. The purification procedure was similar to that described in Reference [5].

UV Advanced Oxidation Process (AOP)
A 254 nm UV lamp system for water disinfection (25 W low-pressure mercury lamp S463RL, UV model S5Q-PA/2, VIQUA, Guelph, ON, Canada) was used. Experiments were carried out at pH = 3.00, 7.00, and 10.00, using the same methodology as described in Reference [5]. At the end of the experiments, the pH of the treated solutions was measured, and at pH = 3 experiments, it remained constant, but, at the pH = 7 and 10 experiments, it decreased to about 1 pH unit.
The initial concentration of the dye aqueous solutions was similar to that described in Reference [5]. For the analysis of the effect of the Tip-CD and Ti-CD, 0.500 mL of the solution was used.

Equipment
The synthesized carbon dots were characterized by using the following the equipment. Fluorescence analysis was measured in a 10 mm fluorescence quartz cell by using a Horiba Jobin Yvon FluoroMax spectrofluorimeter (New Jersey, NJ, USA), using 5 nm slit widths. The Zeta Potential was measured by using a particle analyzer Anton Paar Litesizer TM 500 (Graz, Austria) and a polycarbonate Omega Cuvette (Ref. 155765). Fourier-transform infrared (FTIR) spectra were measured with a Perkin-Elmer (Waltham, MA, USA) Spectrum Two with an ATR sampling accessory. UV-Vis spectra were acquired with a VWR UV3100-PC spectrophotometer (Avantor, Radnor, PA, USA). AFM analysis was carried out by using a Veeco (Plainview, NY, USA) Metrology Multimode/Nanoscope IVA by tapping mode, using a Bruker (Billerica, MD, USA) silicon probe (model TESP-SS, resonant frequency of 320 kHz, nominal force constant of 42 N/m, and estimated tip radius of 2 nm). Scanning electron microscopy, SEM, was performed with a Quanta 200 from FEI Company (Hillsboro, OR, USA) coupled with energy dispersive spectroscopy (SEM-EDS).

Data Analysis
The registry of the absorbance of the dyes' aqueous solutions as a function of the time of exposition to the UV light was carried out at the maximum absorbance of the dyes [5]. The percentage of dye removal (%DR), the apparent pseudo-first-order rate constant (k ap without catalyst, k ap c with H 2 O 2 , Na 2 S 2 O 8 , or TiO 2 catalyst and k ap N with nanomaterial), the percentage of increase of the rate constant when the catalyst is present (%Inc) and the percentage of increase of the rate constant when the nanomaterial is present (%Inc N ) were calculated by using Equations (21)-(24), respectively (a linear relation between the absorbance and the aqueous concentration of the dyes was observed): %DR = 100 × (Abs 0 −Abs t )/Abs 0 (21) ln(Abs t /Abs 0 ) = − k ap t where Abs 0 and Abs t are the absorbances of the dye aqueous solution at initial and t time of UV exposition.

UV AOP of Dyes
The UV exposition of the aqueous solutions of the dyes resulted differently for the four dyes and pH values under analysis ( Figure 2 shows an example of the pH = 10 experiments). The analysis of the spectra as function of the time of UV exposition shows that the visible part of the spectra decreases the absorbance (discoloration), but, in the UV section of the spectra, absorption bands show different trends, suggesting the formation followed by degradation of other intermediate molecular fragments. Table 1 presents the discoloration kinetics parameters resulting from the UV AOP of the aqueous solutions of the dyes at three pH values: 3, 7, and 10. azo dye show a lower decomposition time and has the biggest rate constant at pH = 3, at about 0.03 min −1 , with a %DR of about 60% in 30 min. The anthraquinone dye RBB-R has a relatively lower rate constant, as can clearly be observed in Figure 2, achieving a maximum %DR of about 20% (30 min) at pH = 3.
In order to try to improve the decomposition rates of the dyes, UV was coupled with H2O2, S2O8 2− , and TiO2. Because the pH of wastewaters of textile is close to neutral [5], the following work focused on aqueous solutions at pH = 7.   The analysis of Table 1 shows that the discoloration apparent rate constants of the azo dyes are about ten times bigger than those of RBB-R. Moreover, for the two azo dyes, O-II and RB-5, the discoloration rate is bigger at pH = 10, with a rate constant value of about 0.04 min −1 and with a percentage of dye removal of about 70% in 30 min. The third azo dye show a lower decomposition time and has the biggest rate constant at pH = 3, at about 0.03 min −1 , with a %DR of about 60% in 30 min. The anthraquinone dye RBB-R has a relatively lower rate constant, as can clearly be observed in Figure 2, achieving a maximum %DR of about 20% (30 min) at pH = 3.
In order to try to improve the decomposition rates of the dyes, UV was coupled with H 2 O 2 , S 2 O 8 2− , and TiO 2 . Because the pH of wastewaters of textile is close to neutral [5], the following work focused on aqueous solutions at pH = 7.   Figure 3 show the results obtained for the four dyes under the UV + H 2 O 2 AOP. Coupling UV with a relatively low concentration of hydrogen peroxide of 0.3 mM allowed the AOP to reach almost complete discoloration of the azo dyes, but within different exposition times: MO, 30 min; O-II, 10 min; and RB-5, 4 min. However, at this H 2 O 2 concentration, the decomposition of the RBB-R achieved only about 14% in 30 min. For the decomposition of RBB-R to reach about 96%, it was necessary to use a 100-times-higher H 2 O 2 concentration.
The discoloration-rate values of the dyes studied in this work are higher than most results reported in the literature for the same dyes when UV/H 2 O 2 AOP was used. One study evaluated the effectiveness of RB-5 bleaching through the UV/H 2 O 2 system, in which a k ap value of 0.105 min −1 was obtained (pH = 7) [38]. The degradation of MO was studied by using a UV/H 2 O 2 process, and the almost complete degradation of MO was achieved in just 3 min when 0.1 mL of H 2 O 2 was added to 20 mL of MO solution [39]. Four Fe-containing materials were used as heterogeneous photochemical catalysts for the discoloration of O-II in the presence of H 2 O 2 and UV-C (pH values of 3 and 6), and the k ap increased from 0.050 to 0.193 min −1 [40]. The discoloration of RB-5 was investigated through the UV/H 2 O 2 process, which resulted in k ap values ranging from 0.01 to 0.042 min −1 [41]. The discoloration capacity of RB-5 was evaluated through the advanced photochemical process ferrioxalate/H 2 O 2 /UV-C, in which a k ap value of 0.0922 min −1 was obtained [42].   The discoloration-rate values of the dyes studied in this work are higher than most results reported in the literature for the same dyes when UV/H2O2 AOP was used. One study evaluated the effectiveness of RB-5 bleaching through the UV/H2O2 system, in which a kap value of 0.105 min −1 was obtained (pH = 7) [38]. The degradation of MO was studied by using a UV/H2O2 process, and the almost complete degradation of MO was achieved in just 3 min when 0.1 mL of H2O2 was added to 20 mL of MO solution [39]. Four Fe-containing materials were used as heterogeneous photochemical catalysts for the discoloration of O-II in the presence of H2O2 and UV-C (pH values of 3 and 6), and the kap increased from 0.050 to 0.193 min −1 [40]. The discoloration of RB-5 was investigated through the UV/H2O2 process, which resulted in kap values ranging from 0.01 to 0.042 min −1 [41]. The discoloration capacity of RB-5 was evaluated through the advanced photochemical process ferrioxalate/H2O2/UV-C, in which a kap value of 0.0922 min −1 was obtained [42]. Table 3 shows the results obtained for the four dyes under the UV+Persulfate AOP. Coupling UV with a concentration of persulfate higher than 0.1 mM allowed the AOP of the azo dyes to reach a discoloration percentage higher than 90% (at 30 min). However, for the decomposition of RBB-R to reach about 90%, an 80-times-higher persulfate concentration is necessary. This result shows that a similar relatively low efficiency, when compared with the azo dyes, for the RBB-R degradation is achieved with the UV/persulfate AOP when compared with the UV/H2O2 AOP.  Table 3 shows the results obtained for the four dyes under the UV + Persulfate AOP. Coupling UV with a concentration of persulfate higher than 0.1 mM allowed the AOP of the azo dyes to reach a discoloration percentage higher than 90% (at 30 min). However, for the decomposition of RBB-R to reach about 90%, an 80-times-higher persulfate concentration is necessary. This result shows that a similar relatively low efficiency, when compared with the azo dyes, for the RBB-R degradation is achieved with the UV/persulfate AOP when compared with the UV/H 2 O 2 AOP. The discoloration-rate values of the dyes, as shown in Table 3, are higher than most results reported in the literature for the same dyes when using AOPs based on the sulfate radical: namely, a UV/persulfate process was used for the degradation of MO, with a k ap of 0.7 min −1 (25 • C) after 15 min [43]; the degradation of O-II by the AOP based on sulfate radical mediated by Fe o @Fe x O y particles was investigated, and a k ap of 0.057 min −1 was obtained [44]; Fe 3 O 4 magnetic nanoparticles were used to activate peroxymonosulfate (PMS) in the degradation of RB-5, and when the PMS dosage increased from 0.25 to 2 mM, the kinetic rate constant showed the same trend, with an increase of 0.0085 min −1 to 0.0387 min −1 [45]; for the degradation of O-II, copper phosphide (Cu 3 P) was used as a heterogeneous catalyst for the activation of peroxydisulfate (PDS), and the k ap was 0.1306 min −1 (pH = 7) [46]; and a persulfate system, activated by a nanocompound (α-FeOOH@GCA + K 2 S 2 O 8 ), was applied for the discoloration of O-II, and a k ap of 0.0502 min −1 was obtained [47]. Table 4 shows the results obtained for the four dyes under the UV + TiO 2 AOP, using a concentration of 0.050 g of TiO 2 /L. Comparing these results with the simple UV AOP, we see that large increases (between 151 and 331) in the discoloration are observed for the azo dyes. On the contrary, the presence of TiO 2 almost does not affect the discoloration of RBB-R. Table 4. Apparent rate constant, percentage of the rate constant when the catalyst is present, and percentage of dye removal of the UV + TiO 2 AOP of dyes (the standard deviations of at least three independent repetitions are shown). In parentheses, the reaction time necessary to achieve the percentage of dye removal is shown. The fitting of the integrated first-order kinetics resulted in R 2 > 0.98. As shown in Table 4, the presence of TiO 2 increased the rate of discoloration of the dyes. The catalytic effect of the UV/TiO 2 AOP is well-known, and, for the dyes under investigation, the following results from the literature can be found: the effectiveness of RB-5 bleaching through the UV/TiO 2 system had a k ap value of 0.033 min −1 (pH = 7) [38]; TiO 2 nanoparticles (NPs) were used in the degradation of MO under UV radiation, and the k ap ranged from 0.005 to 0.0063 min −1 [48]; one study demonstrated efficient degradation of the O-II and RB-5 dyes through the UV/TiO 2 photocatalytic system, with a k ap of 0.025 and 0.005 min −1 , respectively [49]; the photocatalytic discoloration of RB-5 in the presence of TiO 2 nanoparticles under UV radiation was studied, and the k ap values ranged from 0.01 to 0.125 min −1 [50]; a photocatalytic reactor with an ozone-generating mercury vapor lamp and TiO 2 was used for the degradation of MO, and the k ap was 0.0473 min −1 [51]; and the photocatalytic degradation of RBB-R was studied with thin films of nitrogen-doped TiO 2 catalysts under UV-A radiation, and the values were k ap = 5.1 ×10 −5 (undoped), k ap = 1.8 × 10 −4 (doped with 0.72% of N), and k ap = 2.0 × 10 −4 (doped with 0.84% of N) [52].

Dye
The above UV-based AOPs are not very effective for the anthraquinone dye RBB-R decomposition unless relatively high concentrations of hydrogen peroxide and persulfate are used; however, this raises sustainability problems. As an alternative, a carbon-based nanocatalyst was developed to be coupled to UV and hydrogen peroxide or persulfate to increase the efficiency of the RBB-R discoloration.

Preliminary Analysis of the Catalytic Performance of the Synthesized CD
The basic precursors of the CD developed in this project were L-cysteine and ethylenediamine, which were used as carbon and nitrogen sources, respectively. The other two components, PEG and the titanium compound, were analyzed for their essential role in the final photocatalyst. Taking into consideration the critical role of PEG previously observed in the ozonation catalyst [5], two CDs were synthesized with (TiP-CD) and without PEG (Ti-CD). The preliminary comparative analysis of the two photocatalysts confirmed that PEG is essential to the observed photocatalysis, and this observation is further supported below. In the case of titanium doping (namely TiO 2 ), a CD was synthesized by using the composition described for TiP-CD (Section 2.2), but without the titanium compound. The catalytic discoloration properties of this non-doped CD was markedly worse than the TiP-CD. For this reason, the work focused mainly on TiP-CD investigation.

Characterization of the Nanomaterials
In aqueous solution, the nanomaterials become agglomerated and hydrated, as seen the DLS results shown in Figure 4a, with an average hydrodynamic size of about 310 and 297 nm for Ti-CD and TiP-CD, respectively. Figure 4b shows that the mean Zeta potential is −0.3 and −3.5 mV for Ti-CD and TiP-CD, respectively, with a charge distribution ranging from slightly positively charged to slightly negatively charged nanoparticles. Figure 5 shows the AFM images of the TiP-CD. The analysis of the AFM results shows that they have a spherical shape, with an average size of 22 ± 3 nm. aterials 2022, 12, x FOR PEER REVIEW 10 of 16 similar to other CDs' spectra, with a strong absorption in the ultraviolet that tails into the visible region due to the n-* transition of multi-conjugate C=O [53,54]. The FTIR spectra of Ti-CD and TiP-CD (Figure 7b) are rather different due to the PEG capping of sample TiP-CD. The spectrum of Ti-CD is characterized by the following main bands [53][54][55]   The fluorescence emission spectra characterized by a broad band with a maximum at 384 and 394 nm for Ti-CD and TiP-CD, respectively, that are responsible for a typical blue emission of CD [58][59][60]. The lifetime decay of the TiP-CD fits (with a  2 = 1.25) a threefluorophores model with the following lifetimes (pre-exponential factors under parenthesis): 0.69 ± 0.02 ns (0.0508 ± 0.0003), 2.67 ± 0.08 ns (0.0241 ± 0.0001), and 2.67 ± 0.08 ns  Figure 6 shows the SEM and EDS of Ti-CD (Figure 6a,d) and TiP-CD (Figure 6b,c,e) samples. The morphology of the two samples, as revealed by SEM, is rather different. While Ti-CDs (Figure 6a) have a nanotube shape, with a diameter of about 35 nm, and are highly agglomerated into bigger clusters, the TiP-CDs (Figure 6b,c) are dispersed with a PEG capping (particularly clear in Figure 6b) that confirm the size determined by AFM; Figure 6c shows as an example a nanoparticle with a size of 21 nm. This result shows the role of PEG on the capping, dispersion, and shape of the nanoparticles. The EDS of the two samples confirm the titanium doping, particularly in the Ti-CD sample (Figure 6d). In the case of TiP-CDs, because they are highly dispersed and capped with PEG, titanium is hardly detected, but, as expected, the presence of carbon, oxygen, and sulfur is clearly observed. The fluorescence emission spectra characterized by a broad band with a maximum at 384 and 394 nm for Ti-CD and TiP-CD, respectively, that are responsible for a typical blue emission of CD [58][59][60]. The lifetime decay of the TiP-CD fits (with a  2 = 1.25) a threefluorophores model with the following lifetimes (pre-exponential factors under parenthesis): 0.69 ± 0.02 ns (0.0508 ± 0.0003), 2.67 ± 0.08 ns (0.0241 ± 0.0001), and 2.67 ± 0.08 ns (0.00583 ± 0.00003).    (Figure 7d). The UV-Vis spectra of both samples (Figure 5a) are characterized by a main band at 317 nm and a shoulder at 286 nm. These spectra are similar to other CDs' spectra, with a strong absorption in the ultraviolet that tails into the visible region due to the n-χ* transition of multi-conjugate C=O [53,54].

UV-Based AOP of RBB-R in the Presence of TiP-CD
The effect of the addition of the TiP-CD in the UV/H2O2 and UV/persulfate degradation kinetics of the RBB-R dye is shown in Figure 8 and Table 5. The presence of Ti-CD or TiP-CD on the single UV AOP of RBB-R inhibits the discoloration rate. However, in the UV/H2O2 and UV/persulfate AOP, a marked increase in the discoloration rate is observed when TiP-CD is present, reaching a 263% increase. However, as expected from the initial catalytic evaluation of both nanomaterials, the effect of Ti-CD is null. This result supports the important role of the PEG capping in the nanocatalysts of AOP [5]. The fluorescence emission spectra characterized by a broad band with a maximum at 384 and 394 nm for Ti-CD and TiP-CD, respectively, that are responsible for a typical blue emission of CD [58][59][60]. The lifetime decay of the TiP-CD fits (with a χ 2 = 1.25) a threefluorophores model with the following lifetimes (pre-exponential factors under parenthesis): 0.69 ± 0.02 ns (0.0508 ± 0.0003), 2.67 ± 0.08 ns (0.0241 ± 0.0001), and 2.67 ± 0.08 ns (0.00583 ± 0.00003).

UV-Based AOP of RBB-R in the Presence of TiP-CD
The effect of the addition of the TiP-CD in the UV/H 2 O 2 and UV/persulfate degradation kinetics of the RBB-R dye is shown in Figure 8 and Table 5. The presence of Ti-CD or TiP-CD on the single UV AOP of RBB-R inhibits the discoloration rate. However, in the UV/H 2 O 2 and UV/persulfate AOP, a marked increase in the discoloration rate is observed when TiP-CD is present, reaching a 263% increase. However, as expected from the initial catalytic evaluation of both nanomaterials, the effect of Ti-CD is null. This result supports the important role of the PEG capping in the nanocatalysts of AOP [5].  Taking into consideration that the decomposition of the dyes occurs due to the reaction with the hydroxyl (mainly Equations (4), (6), (11), and (18)) and sulfate (Equation (10)) radicals, the presence of TiP-CD will somehow promote the formation of these highly oxidant species. The enhancement of photodegradation of RBB-R in the presence of TiP-CD and H2O2 or S2O8 2− may be attributed to the combined effects of photodissociation of the oxidant species in the presence of UV light and TiP-CD surface, the presence of the hydroxyl radical species on the reactant mixture, and the PEG coating together with the dye [61]. A hypothesis of the direct role of TiP-CD may be in the surface catalyzed hydroxyl radical formation, as described in Equation (25) for the case of H2O2 [62]: TiP-CD + H2O2 → TiP-CD + + OH − + *OH (25)

Conclusions
A titanium(IV)-doped and PEG-capped carbon dot synthesized from L-cysteine and ethylenediamine (TiP-CD) showed itself to be an efficient catalyst for the UV/H2O2 and UV/persulfate AOP. Indeed, marked increases of up to 263% of the discoloration rate of the anthraquinone dye RBB-R were observed when the TiP-CD was present; this can open up new sustainable AOP strategies for the degradation of families of dyes that are difficult to degrade. Discoloration pseudo-first-order rate constants in the order of 0.034 min −1 were obtained with both UV/H2O2 and UV/persulfate AOP systems.
AOP, UV/H2O2, and UV/persulfate generate the hydroxyl radical upon UV exposition, and, in the case of persulfate, the radical sulfate, the observed catalytic role of the carbon-based nanomaterial is related to an increase of the hydroxyl radical production. Moreover, because PEG is crucial for the observed catalytic effect, and PEG is capping the  Taking into consideration that the decomposition of the dyes occurs due to the reaction with the hydroxyl (mainly Equations (4), (6), (11), and (18)) and sulfate (Equation (10)) radicals, the presence of TiP-CD will somehow promote the formation of these highly oxidant species. The enhancement of photodegradation of RBB-R in the presence of TiP-CD and H 2 O 2 or S 2 O 8 2− may be attributed to the combined effects of photodissociation of the oxidant species in the presence of UV light and TiP-CD surface, the presence of the hydroxyl radical species on the reactant mixture, and the PEG coating together with the dye [61]. A hypothesis of the direct role of TiP-CD may be in the surface catalyzed hydroxyl radical formation, as described in Equation (25)

Conclusions
A titanium(IV)-doped and PEG-capped carbon dot synthesized from L-cysteine and ethylenediamine (TiP-CD) showed itself to be an efficient catalyst for the UV/H 2 O 2 and UV/persulfate AOP. Indeed, marked increases of up to 263% of the discoloration rate of the anthraquinone dye RBB-R were observed when the TiP-CD was present; this can open up new sustainable AOP strategies for the degradation of families of dyes that are difficult to degrade. Discoloration pseudo-first-order rate constants in the order of 0.034 min −1 were obtained with both UV/H 2 O 2 and UV/persulfate AOP systems. AOP, UV/H 2 O 2 , and UV/persulfate generate the hydroxyl radical upon UV exposition, and, in the case of persulfate, the radical sulfate, the observed catalytic role of the carbon-based nanomaterial is related to an increase of the hydroxyl radical production.
Moreover, because PEG is crucial for the observed catalytic effect, and PEG is capping the nanoparticle, the active substrate, H 2 O 2 , and persulfate anion will become trapped in the gelified external layer of the nanoparticle, and the UV-activated core will decompose the substrate into hydroxyl and sulfate radicals.