Removal of Safranin O from Water by UV/TiO₂/IO₃⁻ Advanced Oxidation Process: Parametric Study and Impact of Inorganic Ions and Humic Acid

Abstract

In the present paper, the influence of iodate (IO₃⁻) ions on the photocatalytic process using titanium dioxide (UV/TiO₂) was systematically examined. The innovative studied system (UV/TiO₂/IO₃⁻) demonstrated high performance in the elimination of safranin O (SO), a model dye, from water due to the implication of iodine radicals (IO₃^•, IO₂^•, IO^•, etc.). The degradation was assessed by monitoring the change in initial substrate concentration (5–30 mg∙L⁻¹), TiO₂ loading (0.01–3 g∙L⁻¹), IO₃⁻ concentration, liquid temperature, and initial pH. Further enhancement of oxyanion amount was beneficial for the initial rate of degradation (r₀) over the range 0.1–50 mM, such that r₀ rose from 0.724 to 1.12 mg∙L⁻¹∙min⁻¹. However, a concentration of 100 mM IO₃⁻ slowed the removal kinetics. Low pH values were found to be favorable for pollutant removal. Furthermore, a variety of inorganic and organic substances was employed to clarify the effect of the UV/TiO₂/IO₃⁻ process in natural waters. The findings revealed a negative impact of chloride excess in seawater and a decline in SO degradation in mineral water as a result of HCO₃⁻ presence at more than 1 mM. At high concentrations of natural organic matter, the heterogeneity and the strong adsorption of humic acid on the TiO₂ surface were found to be harmful for SO decay.

1. Introduction

The world is currently suffering from natural water contamination due to the detrimental effects of industrial activities. Wastewater from textile manufacture is highly contaminated with significant amounts of chemicals. The release of these compounds into water streams can cause complex ecological and environmental problems owing to their toxicity, non-biodegradability, and potential carcinogenicity [1]. Safranin O is regarded as a model compound representative of dyes that are released in the effluent of the textile and food industries [2]. It is extensively used for dyeing tannin, mordanted cotton, bast fibers, wool, silk, leather and paper, and for coloring candies and cookies [2,3,4]. This new paradigm of recalcitrant pollutants and their associated problems requires researchers to improve their in-depth understanding of the remediation technologies. Heterogeneous photocatalysis is one of the more promising advanced oxidation processes (AOP) developed in the past 20 years in alternative to conventional processes for water treatment [5]. Based on the in situ production and use of highly reactive species such as ^•OH and O₂^•− with rate constants in the range of 118–1011 M⁻¹∙s⁻¹ [6], irradiated photocatalysts could efficiently degrade a wide range of hazardous contaminants. The high activity, non-toxicity, low cost, and excellent durability of titanium dioxide (TiO₂) make it the most widely employed photocatalyst in environmental applications. The intensification of the photocatalytic effectiveness by the involvement of inorganic oxidants such as H₂O₂, ClO₃⁻, BrO₃⁻, S₂O₈²⁻, IO₄⁻, and IO₃⁻, has attracted increasing attention. These compounds could enhance the quantum efficiencies either by charge separation through scavenging conduction-band electrons at the surface of TiO₂ or offering an additional oxygen atom as an electron acceptor to form the superoxide radical ion (O₂^•−) or reactive radical intermediates [7]. Chen et al. [1] investigated the system involving visible light TiO₂ photocatalysis (Vis/TiO₂) in the presence of peroxymonosulfate (PMS, HSO₅^-). The proposed system (Vis/TiO₂/PMS) demonstrated high efficiency in the removal of Acid Orange 7 compared to the conventional process [1]. In their study, Wu et al. [8] evaluated the effects of different oxidants, including H₂O₂, Na₂S₂O₈, NaBrO₃ and NaIO₄, on the degradation ability of UV/TiO₂ toward Reactive Red 198 and reported that the implication of periodate was more effective. Similar studies confirmed the beneficial effect of oxyanions on photocatalytic treatment in water [9,10,11,12]. However, limited research can be found about the use of iodate anion in TiO₂-mediated photocatalysis for eliminating emergent pollutants. To improve the decay rates of o-chloroaniline (o-ClA) by the illuminated Degussa P25, Choy et al. [13] used ClO₃⁻, BrO₃⁻, and IO₃⁻. Substantial enhancement was observed in the order IO₃⁻ > BrO₃⁻ > ClO₃⁻. In complementary work, Choy et al. [7] discussed the effect of iodate in successive dosing, and the degradation pathway of o-ClA was proposed according to the detection of organic intermediates. In a recent work, Zhang et al. [14] investigated the degradation of carbamazepine (CBZ) using UV/IO₃⁻ and UV/TiO₂/IO₃⁻ systems. They indicated that iodine radicals (IO^•, IO₂^• and IO₃^•) were mainly involved in the degradation of CBZ in the UV/TiO₂/IO₃⁻ process, which was found to be the most efficient process when compared to some other photocatalytic systems (UV/ClO₃⁻/TiO₂, UV/BrO₃⁻/TiO₂).

The UV/TiO₂/IO₃⁻ process has received very limited attention. Moreover, while the improving impact of using a UV/TiO₂/IO₃⁻ system has been shown to be significant, very little information is known about the influence of operating parameters on process effectiveness. Additionally, the viability of the process in the presence of inorganic and organic substances, as well as in real water matrices, such as mineral water and seawater, has not been examined earlier. All of these tasks have not been investigated previously, undoubtedly due to the novelty of the technique.

The objective of this study was to evaluate the aqueous degradation of safranin O (SO) dye using the UV/TiO₂/IO₃⁻ process. For the first time, the process performance was examined under diverse experimental parameters, namely, substrate charge, photocatalyst mass, NaIO₃ loading, liquid temperature, and initial pH. The degradation efficiency was also tested in the presence of some inorganic anions (Cl⁻, SO₄²⁻, HCO₃⁻, and NO₂⁻) and humic acid as natural organic matter. In the last part, the removal of SO from seawater and mineral water was discussed, utilizing our findings on the effect of additives.

2. Results

2.1. Photocatalytic Degradation of Safranin O through Iodate Process

In order to improve the performance of the TiO₂-mediated photocatalytic process to treat an aqueous solution of 10 mg∙L⁻¹ of safranin O dye, the iodate anions (IO₃⁻) were added. First, the effect of iodate was tested in the presence of the oxyhalogen (IO₃⁻) alone, under UV irradiation, and with the TiO₂ suspension. Figure 1 exhibits the obtained results. According to these results, no change in initial concentration of dye was noticed over time under IO₃⁻ alone, under illuminated iodate, or with the TiO₂/IO₃⁻ system. These findings can be attributed to the fact that the photon energy provided at 365 nm was not sufficient to photo-activate the oxyanion for producing efficient free radicals for dye destruction. Choy et al. [13] found a negligible effect of illuminated iodate (300 nm) in the degradation of o-chloroaniline.

The introduction of 0.15 mM of iodate into the UV/TiO₂ system substantially accelerated the SO degradation. After one hour of reaction, almost the entirety of the organic compound disappeared through the UV/TiO₂/IO₃⁻ process, versus 72% in the absence of iodate. Kinetically, the initial rate of SO degradation was substantially improved in the presence of 0.15 mM of the oxyanion, which is probably due to the involvement of new free radicals in the degradation process. Amichai and Treinin [15] detected the formation of IO₃^•, IO₂^•, and IO^• in an iodate-containing solution that undergoes a flash photolysis at neutral pH. These radicals can be yielded by the photodecomposition of iodate, as well as by secondary reactions involving the parent ions [15]. In the second way, IO₃⁻ reacts with ^•OH and IO₂^• to produce IO₃^• and IO^•, respectively (Equations (1)–(3) [15,16]), and other reactive species.

IO₃⁻, H₂O + hν → IO₂^• + ^•OH + OH⁻

(1)

IO3⁻ + ^•OH → IO3^• + OH⁻  (2.9 × 108 M⁻¹ s⁻¹_, k = 5 × 10⁷ M⁻¹ s⁻¹)

(2)

IO₃⁻ + IO₂^• → IO^• + IO₄⁻   (k = 3.5 × 10⁵ M⁻¹ s⁻¹)

(3)

In support of these findings, Haddad et al. [17] reported that the photodegradation of light green SF yellowish (LGSFY) dye was significantly improved in the presence of iodate, attributing these results to the efficient contribution of IO₂^• and IO^•. Choy et al. [7] investigated the removal of o-chloroaniline by iodate, chlorate, and borate-assisted photocatalysis. They found a considerable enhancement in degradation rate in the order of UV/TiO₂/IO₃⁻ > UV/TiO₂/BrO₃⁻ > UV/TiO₂/ClO₃⁻. This observation was justified by the ability of iodate to trap more conduction band electrons compared to the other oxyhalogens. In another study, Choy et al. [7] deduced that the target molecule was mainly degraded by the attack of ^•OH radicals. Recently, Zhang et al. [14] indicated that IO₃⁻ can delay the recombination of electrons and holes through trapping e⁻ ejected from the surface of TiO₂. Furthermore, the longer half-life of h_VB+ and the additional reactive radical can create additional pathways to boost the degradation of substrate.

Figure 2 shows the evolution of the UV–vis. spectra of SO solution (10 mg L⁻¹) prior to and during the oxidative treatment with UV/TiO₂/IO₃⁻ using 0.15 mM of IO₃⁻ and 0.4 g∙L⁻¹ of TiO₂ at 25 °C and pH~6. As can be observed from this figure, the initial SO solution has two distinct bands: the first is that of the chromophoric group and is located in the visible region, with λ_max = 519 nm, and the second is that of the aromatic rings and is located in the UV region, with λ_max = 275 nm. The characteristic band in the visible region declined with increasing reaction time until no peak was further observed. This implies the degradation of the dye upon attack by oxidative moieties. The increase in absorption in the UV region 228–238 nm is explained by the formation of by-products absorbing in this UV region.

2.2. Effect of Operational Conditions

2.2.1. Effect of Initial Concentration of SO

This section elucidates the effect of initial concentration of dye on the performance of the UV/TiO₂/IO₃⁻ process to remove SO from water. For that, batch experiments were carried out at four initial substrate concentrations: 5, 10, 20, and 30 mg∙L⁻¹. The other operational parameters were fixed at 0.4 g∙L⁻¹ of TiO₂, 0.15 mM of iodate, 25 °C, and natural liquid pH. The experimental findings are presented in Figure 3. This figure shows that the degradation efficiency was reduced in high dye concentration. For example, for an initial concentration of 5 mg∙L⁻¹, the abatement rate after 10 min of irradiation was almost total (92%), while it was only 13% for C₀ = 30 mg∙L⁻¹. The same behavior was reported in the presence of periodate instead of iodate and under classical photocatalysis [12].

Table 1. Dependence of initial degradation rate on SO initial concentration in the range (5–30 mg∙L⁻¹) through UV/TiO₂/IO₃⁻ process.

Initial SO Concentration (mg∙L−1)	5	10	20	30
r0 (mg∙L−1∙min−1)	1.5	0.765	0.342	0.1

presents the variation of initial degradation rate (r₀) as a function of initial concentration of SO. The initial degradation rate values decreased gradually from 1.131 to 0.308 mg∙L⁻¹∙min⁻¹ with the increase in dye quantity in water from 5 to 30 mg∙L⁻¹. These results can be explained by the strong competition of the substrate molecules with the reactive species, whose number remained unchangeable as long as the same operational conditions were kept in all trials.

2.2.2. Effect of Photocatalyst Concentration

Optimizing the mass of the catalyst is an essential step in the study of photocatalytic processes. Therefore, it is necessary to determine the value for which the quantity of the photocatalyst is minimal and the reaction rate is the highest. The efficiency of the UV/TiO₂/IO₃⁻ (0.15 mM) system to eliminate 10 mg∙L⁻¹ of SO was examined at different concentrations of TiO₂, namely, 0.01, 0.05, 0.1, 0.4, 1, and 3 g∙L⁻¹. The mixture temperature was kept constant at 25 °C with a pH of approximately 6. The abatement of SO concentration over the reaction time in every suspension of TiO₂ is displayed in Figure 4. Contrary to our previous study related to periodate-assisted photocatalysis [12], the greater the amount of the photocatalyst, the higher the degradation yield in the adopted range (0.01–3 g∙L⁻¹). After 15 min of irradiation, approximately 78% of the initial concentration was removed from the solutions containing 0.4 and 1 g∙L⁻¹ of titanium dioxide, versus 90.5% when [TiO₂]₀ = 3 g∙L⁻¹. The dependence of initial degradation rate on TiO₂ loading was also noticed (

Table 2. Dependence of initial degradation rate on TiO₂ loading in the range 0.01–3 g∙L⁻¹ through UV/TiO₂/IO₃⁻ process.

[TiO2]0 (g∙L−1)	0.01	0.05	0.1	0.4	1	3
r0 (mg∙L−1∙min−1)	0.15	0.397	0.526	0.765	1.017	1.32

). The increase in photocatalyst mass led to the acceleration of the destruction reaction, so that r₀ increased from 0.15 to 1.32 mg∙L⁻¹∙min⁻¹ when TiO₂ concentration increased from 0.01 to 3 g∙L⁻¹. These findings are the consequence of an increase in the active surface of TiO₂ by increasing the number of illuminated particles, thus producing a greater number of “electron-hole” pairs and, therefore, a greater quantity of oxidizing entities effective in the destruction of the pollutant. Additionally, the increase in photocatalytic activity with catalyst loading is indicative of a heterogeneous catalytic regime since the fraction of incident light absorbed by the catalyst increases as the amount of TiO₂ increases in a suspension [14,18].

In the present work, because only a slight increase in the degradation rate of SO occurred when the TiO₂ concentration was increased from 0.4 to 3 g/L, Degussa P25 concentration was set to 0.4 g∙L⁻¹ for the next experiments.

2.2.3. Effect of Iodate Loading

The investigation of IO₃⁻ loading effect on the efficiency of photocatalytic treatment has not yet been reported. In order to examine the photocatalytic response as a function of oxyhalogen dose for SO degradation, experiments were conducted in an aerated aqueous solution of dye (10 mg∙L⁻¹) containing 0.4 g∙L⁻¹ of TiO₂ at a controlled temperature (25 °C) and near-neutral pH. The concentration of IO₃⁻ was varied over the range 0.1–100 mM. Results were reported in Figure 5.

As shown in Figure 5, the removal efficiency increased proportionally with the amount of iodate involved over 0.1–50 mM. After 15 min of illumination, the system revealed its highest performance in the presence of iodate at 15 and 50 mM with a degradation rate of 92%. However, a slight drop in degradation extent was seen using 100 mM of oxyanion, estimated at 2.5% after a half hour of treatment. Kinetically, the initial degradation rates were also calculated a few minutes after starting the reaction. The dependence of the initial degradation rate on IO₃⁻ concentration was depicted in Figure 6. It was observed that the dye destruction kinetics were significantly improved by increasing the amount of iodate added. For IO₃⁻ contents of 0.1, 0.15, 0.5, and 3 mM, the r₀ were, respectively, equal to 0.724, 0.764, 0.87, and 0.93 mg∙L⁻¹∙min⁻¹. The initial rate of degradation continued to evolve very rapidly up to a value of 1.12 mg∙L⁻¹∙min⁻¹ which corresponds to [IO₃⁻]₀ = 50 mM. This trend is the result of the increasing production of active oxygen species (^•OH, O₂^•−, IO₃^•, IO₂^•, IO^•…) in the presence of more oxyhalogen in the solution. Otherwise, a detrimental effect of iodate concentration up to 10 mM was observed by Haddad et al. [17] on the photo-decomposition of LGSFY dye. In excess of IO₃⁻, the iodate radicals could react with each other to produce low efficient species in SO destruction, which explains the decline of r₀ at 100 mM of KIO₃. For the rest of the manipulations, a concentration of 0.15 mM iodate was adequate.

2.2.4. Effect of Liquid Temperature

For the first time, the impact of medium temperature on the iodate-assisted photocatalysis potential to eliminate organic pollutants is discussed. To examine the influence of this parameter on the removal of SO, five aqueous solutions of dye (10 mg∙L⁻¹) were set at different temperatures: 15, 25, 35, and 45 °C. The photocatalytic treatment was performed in the presence of 0.15 mM of IO₃⁻, 0.4 g∙L⁻¹ of photocatalyst, and at the natural pH of the solutions. Figure 7 depicts the change in SO concentration during illumination time.

The process seemed to be less sensitive to the temperature variation in the studied interval, according to Figure 7. However, the degradation was slightly inhibited at 15 °C, such that 66.5% was eliminated after 15 min, compared to 76% when the solution was heated to 25, 35, and 45 °C. Like periodate, iodate is thermally stable and does not decompose under the conditions adopted in this study. In heterogeneous photocatalysis, low temperatures affect the adsorption of molecules on the surface of the photocatalyst [19,20], which may justify the small reduction in degradation efficiency at 15 °C.

2.2.5. Effect of Initial Solution pH

Since initial solution pH is a controlling factor in photocatalytic reactions for dye removal, the effect of media acidity on safranin O (10 mg∙L⁻¹) decay in the presence of iodate (0.15 mM) was examined. The change in media protonation can modify the surface charge of both the pollutant and the photocatalyst, as well as the speciation of an aqueous inorganic oxidant. Figure 8 shows the plot of degradation efficiency during the experiment at every pH value, ranging in the interval of 2–12.

The results revealed that the process was more efficient in acidic medium. The dye was completely degraded after only 30 min at pH 2, 40 min at pH 4, and 50 min at pH 6, while 92%, 90%, and 59.5% of the initial concentration of pollutant were destroyed, respectively, at pH 8, 10, and 12 after one hour of illumination. These findings are opposed to what has been reported in conventional and periodate-assisted photocatalysis, where the alkaline pH was the most favorable [9,21,22].

Buxton et al. [23] reported that the iodic acid anion (HIO₃⁻) could be indirectly formed when IO₃⁻ captured an electron in a protonated medium, following Equation (4). At lower pH, the presence of HIO₃⁻ was probably effective in the degradation of the SO dye. The amount of iodic acid produced decreases in alkaline media, which may justify the decline in treatment efficiency at pH 8, 10, and 12. Additionally, according to reactions 1–3, the increase in OH⁻ concentration resulted into a low yield of IO₂^•, or IO₃^• and IO^•. In a recent study, synthesized defective TiO₂-HIO₃⁻ revealed a higher reactivity towards organic pollutants compared to the nicked titania [24]. Furthermore, Fkhami et al. [25] observed that the reaction of IO₃⁻ with iodide was pH-dependent and only took place in acidic medium. The accumulation of iodide ions (I⁻) with a pH higher than 5.5 could probably trap a large number of reactive radicals, resulting in low system efficiency. At pH 12, the possibility of agglomeration of the TiO₂ particles was one more reason to have a low conversion percent of the pollutant.

$${{\mathrm{IO}}_3}^{-}+{\mathrm{e}}_{\mathrm{CB}}^{-}\to {{\mathrm{IO}}_3}^{2-}\stackrel{{\mathrm{H}}^{+}}{\to }{{\mathrm{HIO}}_3}^{-}$$

(4)

2.3. Mineral and Organic Additives Impact on Degradation Efficiency

This part of the study examined the influence of some inorganic ions (Cl⁻, SO₄²⁻, HCO₃⁻ and NO₂⁻) and natural organic matter (humic acid HA) on the degradation of SO by the UV/TiO₂/IO₃⁻ process. An appropriate amount of salt was added to 10 mg∙L⁻¹ of aqueous dye in the range of 0.1–100 mM. The organic substance was used in three concentrations: 5, 10, and 20 mg∙L⁻¹. Corresponding experiments were run in a suspension of 0.4 g∙L⁻¹ of photocatalyst, 0.15 mM of iodate at 25 °C, and natural pH. Experimental results are shown in Figure 9.

According to Figure 9, the presence of salts negatively affected the degradation efficiency in the order Cl⁻ < SO₄²⁻ << HCO₃⁻ < NO₂⁻. The effect of both Cl⁻ and SO₄²⁻ was not very significant, even at 100 mM, i.e., a reduction of only 2% and 5% of degradation was observed for the ions, in order, after 60 min of treatment. These results suggest that chloride and sulfate ions are not reactive with effective iodine radicals. The weak effect of their addition can be attributed to the trapping of ^•OH, whose contribution to SO degradation is relatively weak according to our previous studies (not yet published). In their study, Haddad et al. [17] reported that the degradation of light green SF yellowish (LGSFY) by the UV/IO₃⁻ process was retarded by about half in the presence of SO₄²⁻ at more than 0.1 mM.

However, the implication of nitrite and bicarbonate ions drastically inhibited the SO destruction. As shown in Figure 9c,d, the negative impact increased with the increase of salt concentration while the entire inhibition was achieved at 100 mM of NO₂⁻ against 58.6% in the presence of the same dose of HCO₃⁻. In classical photocatalysis (not documented), the negative effect of nitrite was also found to be important, which was probably due to the strong adsorption of these ions on the surface of the catalyst, thus, less oxidant species could be formed. As for bicarbonate (Figure 9d), the change of solution alkalinity (pH > 8) was probably responsible for the degradation rate reduction, since the addition of this ion at different a concentration had a negligible effect on the ability of the UV/TiO₂/IO₄⁻ system to eliminate the SO dye. It should be noted that no available data related to the effect of NO₂⁻ ions on photocatalysis effectiveness.

The presence of HA also altered the pollutant removal, as shown Figure 10. At 20 mg∙L⁻¹, half of the initial mass of the SO was degraded after 20 min; however, the degradation of the same quantity took 10 min at 5 mg∙L⁻¹ and 12 min at 10 mg∙L⁻¹ of natural organic matter, compared to 7 min in the HA-free solution. Humic substances are a mixture of compounds that absorb light more intensely in the UV field. At high concentrations, the presence of organic matter hampered the photocatalytic degradation as the number of photoactivated TiO₂ particles decreased. Palmer et al. [26] attributed the dramatic effect of these compounds to their strong adsorption on the TiO₂ surface.

2.4. Degradation in Real Water Matrices

Natural waters can also be contaminated by colored effluents. Thus, the efficiency of the studied process (UV/TiO₂/IO₃⁻) to remove SO from different water matrices, including mineral water and seawater, was tested. The seawater was collected in the region of Annaba, Algeria. Chemical composition and acidity of each matrix are given in

Table 3. Principal characteristics of water matrices used in this study.

	Mineral Water	Seawater
pH	7.2	7.5
Ca2+	99 mg∙L−1	0.4 g∙L−1
Mg2+	24 mg∙L−1	1.3 g∙L−1
K+	2.1 mg∙L−1	-
Na+	15.8 mg∙L−1	11.0 g∙L−1
HCO3−	265 mg∙L−1	-
SO42−	68 mg∙L−1	3.0 g∙L−1
Cl−	72 mg∙L−1	20.0 g∙L−1
NO3−	15 mg∙L−1	-
NO2−	<0.02 mg∙L−1	-

. The experiments were carried out at natural pH under the same operational conditions adopted in distilled water. The corresponding results are compared and illustrated in Figure 11. As observed, the process performance diminished in natural waters where the negative effect was more pronounced in seawater. After one hour of reaction, the degradation percentage was reduced by 11% in mineral water and by about 40.5% in seawater. The influence of the different inorganic ions revealed that the bicarbonate and sulfate anions at more than 1 mM retarded the kinetics of the pollutant removal. Consequently, the presence of HCO₃⁻ at 4 mM and SO₄²⁻ at 0.7 mM (Table 3) are probably responsible for the reduction in degradation rate of SO in mineral water. In seawater, the excess of chloride (>500 mM) hampered the penetration of light, thus reducing the efficiency of the photon. Additionally, the strong adsorption of organic matter on the TiO₂ surface may be another reason for the diminution of SO degradation in seawater.

3. Materials and Methods

3.1. Reagents

All chemicals used in these experiments were of analytical reagent grade. The stock solutions of safranin O were prepared using distilled water. Safranin O (SO) was provided by Fluka (CAS 477-73-6, CI 50240, India). TiO₂ Degussa P-25 (80% anatase and 20% rutile, average particle size: 30 nm, specific surface area: 50 m²∙g⁻¹) was employed as a photocatalyst and purchased from Evonik. KIO₃ (Sigma-Aldrich, USA, CAS 7758-05-6), potassium nitrite (Sigma-Aldrich), sodium chloride (Prolabo), sodium sulfate (Sigma-Aldrich), and humic acids (Sigma-Aldrich) were used as received. For pH adjustment, NaOH (Sigma-Aldrich) and H₂SO₄ (Sigma-Aldrich) were used.

3.2. Photocatalytic Experiments and Apparatus

The photocatalytic trials were performed using the experimental setup described by Bendjama et al. [12]. The reactor consisted of a 500 mL Pyrex-jacketed cylinder. The irradiation was performed using a low-pressure mercury lamp (15 mW cm⁻², Oriel 6035) emitting mainly at 365 nm, placed in the middle of the reactor. The liquid temperature was controlled using a thermostatic immersion heater at 25 ± 2 °C in almost all experiments. Prior to irradiation, 200 mL of a solution/suspension of 10 mg∙L⁻¹ safranin O (SO) and 0.4 g∙L⁻¹ TiO₂ were magnetically stirred for 30 min in the dark to ensure adsorption–desorption equilibrium on the semiconductor surface. The concentration after the adsorption process is denoted as initial concentration (C₀). Determined quantities of iodate and other additives were added to the reactor and the UV lamp was turned on for initiating the oxidation. During irradiation, samples were withdrawn and TiO₂ particles were separated by centrifugation (10,000 rpm, 30 min), then analyzed using a UV-Vis spectrophotometer (Biochrom Lightwave II) at 519 nm. Moderate stirring was maintained throughout all experiments. The pH of the solution was adjusted using 0.1 N HCl and 0.1 N NaOH. In blank experiments, the influence of iodate was examined in the presence of IO₃⁻ alone, under UV irradiation, and with the TiO₂ suspension.

4. Conclusions

The primary aim of this study was to evaluate the performance of iodate-assisted photocatalysis to purify an aqueous solution of safranin O (SO). Based on what was presented in the experimental results, the studied system demonstrated a high efficiency of SO degradation compared to the other systems, namely IO₃⁻ sole, UV/IO₃⁻, and UV/TiO₂. The degradation rates significantly accelerated with increasing IO₃⁻ and photocatalyst concentrations in the studied range. However, beyond 50 mM of IO₃⁻, the degradation slowed. Though not expected, the acidic media was more favorable for SO destruction, whereas the temperature variation had a negligible influence. The investigation of the impact of inorganic anions on the process efficiency showed an adverse effect, which was less important in the presence of chloride (Cl⁻)- and sulfate (SO₄²⁻)-, and more pronounced in bicarbonate (HCO₃⁻)- and nitrite (NO₂⁻)-containing solutions. The addition of humic substances negatively affected the dye decay, especially at 20 mg∙L⁻¹, as a result of their strong adsorption on the TiO₂ surface. These findings allowed us to clearly demonstrate the behavior of the UV/TiO₂/IO₃⁻ system toward SO decay in various water matrices. The degradation inhibition in mineral water could be due to the presence of HCO₃⁻ and SO₄²⁻ at more than 1 mM. In seawater, the excess of chloride anions hampered the UV light penetration, thus limited oxidation occurred.