Singlet-Oxygen Generation in Alkaline Periodate Solution

Nov 23, 2015 - A nonphotochemical generation of singlet oxygen (1O2) using potassium periodate (KIO4) in alkaline condition (pH > 8) was investigated ...
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Singlet-Oxygen Generation in Alkaline Periodate Solution Alok D. Bokare and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea S Supporting Information *

ABSTRACT: A nonphotochemical generation of singlet oxygen (1O2) using potassium periodate (KIO4) in alkaline condition (pH > 8) was investigated for selective oxidation of aqueous organic pollutants. The generation of 1O2 was initiated by the spontaneous reaction between IO4− and hydroxyl ions, along with a stoichiometric conversion of IO4− to iodate (IO3−). The reactivity of in-situ-generated 1O2 was monitored by using furfuryl alcohol (FFA) as a model substrate. The formation of 1O2 in the KIO4/KOH system was experimentally confirmed using electron spin resonance (ESR) measurements in corroboration with quenching studies using azide as a selective 1O2 scavenger. The reaction in the KIO4/KOH solution in both oxic and anoxic conditions initiated the generation of superoxide ion as a precursor of the singlet oxygen (confirmed by using superoxide scavengers), and the presence of molecular oxygen was not required as a precursor of 1O2. Although hydrogen peroxide had no direct influence on the FFA oxidation process, the presence of natural organic matter, such as humic and fulvic acids, enhanced the oxidation efficiency. Using the oxidation of simple organic diols as model compounds, the enhanced 1O2 formation is attributed to periodate-mediated oxidation of vicinal hydroxyl groups present in humic and fulvic constituent moieties. The efficient and simple generation of 1O2 using the KIO4/KOH system without any light irradiation can be employed for the selective oxidation of aqueous organic compounds under neutral and near-alkaline conditions.

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INTRODUCTION The widespread applicability of various advanced oxidation processes (AOPs) for the removal of organic pollutants is mainly ascribed to the efficient generation of strongly oxidizing free radicals at near-ambient temperature and pressure.1,2 AOPdriven generation of reactive oxygen species (ROS) such as the hydroxyl radical (HO•) and the sulfate radical (SO4•−) has been extensively utilized due to their high nonselective reactivity toward most organic pollutants.3,4 These nonselective oxidants are also easily generated through various activation methods using UV photolysis, ozonation, ultrasound irradiation, and transition metal catalysts, either separately or in combination.5 However, singlet oxygen (1O2) (a nonradical ROS) has been largely used for the selective oxidation of organic substrates such as pharmaceuticals and endocrinedisrupting chemicals (EDCs)6−8 and the inactivation of toxic pathogens, such as E. coli and the MS-2 bacteriophage.9−11 Because the electrophilic nature of 1O2 enables the rapid oxidation of electron-rich functional groups,12 high-priority pollutants such as EDCs, antibiotics, and pharmaceuticals present in common wastewaters in low concentrations can be selectively oxidized and removed in the presence of background organic matters. The efficient production of 1O2 using the photoinduced energy transfer to molecular oxygen (photosensitization) remains the most popular and efficient method for watertreatment applications. This pathway essentially involves the excitation of a photosensitizer molecule and the subsequent energy transfer from its excited state to molecular oxygen. © XXXX American Chemical Society

Various synthetic photosensitizers, such as organic dyes (Rose Bengal, Eosin Y, and Methylene Blue),13,14 metal porphyrins,15,16 and C60-fullerene derivatives6−12,17 have been employed for 1O2-mediated pollutant degradation under both UV and visible light irradiation. Singlet-oxygen generation using natural organic matter (NOM) as a primary photosensitizer has also been demonstrated in both natural and synthetic aquatic systems under sunlight illumination.18,19 The photocatalytic activation of dioxygen on semiconductor metal oxides such as TiO 2 is also an alternative pathway for efficient 1 O 2 generation.20,21 Despite the continuous research interests in photosensitizing methods for generating 1O2 under visible light or sunlight irradiation, studies on nonphotochemical methods of 1O2 formation are limited. Although inorganic chemicals such as potassium perchromate and triphenyl phosphite ozonide decompose spontaneously to form singlet oxygen,22 their extreme toxicity and low stability make them unsuitable for water-treatment applications. However, 1O2 generation through the release of lattice oxygen from bismuth-based oxides such as BiAgxOy23 and Bi(V)−Bi(III) composite24 has been used to oxidize pollutants such as Rhodamine B and bisphenol A. However, the formation of 1O2 only through lattice oxygen Received: August 25, 2015 Revised: October 13, 2015 Accepted: November 11, 2015

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DOI: 10.1021/acs.est.5b04119 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

nitrogen gas was first purged through the aqueous solution under continuous ultrasonication for 30 min before periodate stock solution (separately purged without ultrasonication) was added to start the reaction. Purging was then continued without ultrasonication for the entire reaction time. All experiments were carried out in triplicate for a given condition. The quantitative analysis of substrates was done using a highperformance liquid chromatograph (HPLC Agilent 1100) using a C-18 column (Agilent Zorbax 300SB) and a diode-array detector. The eluent compositions and detection wavelengths were: (a) 0.1% phosphoric acid aqueous solution and methanol (85:15 v/v) with λ = 218 nm for FFA and with λ = 230 nm for CMT and RANI, (b) 0.1% phosphoric acid aqueous solution and acetonitrile (80:20 v/v with λ = 218 nm for periodate, iodate, and 4-CP), (60:40 v/v with λ = 214 nm for PPL) and (c) water and acetonitrile (50:50 v/v) with λ = 280 nm for BPA. Electron spin resonance (ESR) trapping of singlet oxygen was carried out using the standard spin trap 2,2,6,6-tetramethyl4-piperidinol (TMP−OH, Aldrich). TMP−OH reacts with singlet oxygen and yields a paramagnetic product, 4-hydroxy2,2,6,6- tetramethyl-4-piperidine 1-oxyl (TEMPOL). For ESR measurements, reaction aliquots (5−10 μL) were sampled from the solution containing KIO4, KOH, and 25 mM TMP−OH after 3 h, transferred into quartz capillary tubes, and sealed on both ends. ESR analyses were carried out using an A200 spectrometer (Bruker) equipped with a rectangular mode TE102 cavity. For each experimental point, seven-scan field-swept ESR spectra were recorded. The instrumental settings were: microwave frequency of 9.44 GHz, sweep width of 100 G, modulation frequency of 100 kHz, modulation amplitude of 0.5 G, receiver gain of 1 × 104, time constant of 20.48 ms, conversion time of 40.96 ms, and time per single scan of 41.9 s.

release is a major disadvantage because a large excess of catalyst is required for sustained 1O2 reactivity. In the present study, we report an efficient generation of 1O2 using alkali-mediated activation of inorganic periodate (IO4−) for the selective oxidation of aqueous pollutants. AOPs based on periodate activation have been previously developed to generate oxidizing radical intermediates, such as IO3• and HO•.25−28 However, these activation processes require shortwavelength UV irradiation or bimetallic nanoparticle catalysts for inititating the oxidation process. Using the KIO4/KOH reaction system, this work proposes a nonphotochemical and metal-free method of periodate activation for 1O2-based AOP applications. The spontaneous reaction between IO4− and OH− in both oxic and anoxic conditions generates superoxide radical (O2•−) as a precursor for singlet oxygen, along with the stoichiometric formation of iodate (IO3−). This method of 1O2 generation in alkaline periodate solutions has been previously utilized as a chemiluminescence-based sensing platform, wherein the intrinsic luminescence (energy) emission associated with 1O2 decay allows sensitive and selective analyte estimation.29−31 However, despite the development of such periodate-based optical sensors, the oxidative power of 1O2 generated in the KIO4/KOH system has not been exploited for AOP applications. To understand the detailed mechanism of 1 O2 formation and the important factors affecting its oxidizing capacity, we employed various probe substances and varying experimental conditions to provide critical insights. The use of base-activated periodate transformation offers a new and simple method by which to generate 1O2 for the selective removal of labile organic contaminants.

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EXPERIMENTAL SECTION Chemicals and Materials. Chemicals that were used as received in this study include: potassium periodate (KIO4, Aldrich), sodium iodate (Aldrich), potassium hydroxide (Samchun), furfuryl alcohol (FFA, Aldrich), 4-chlorophenol (Sigma), bisphenol A (BPA, Aldrich), propanolol (PPL, Aldrich), cimetidine (CMT, Aldrich), ranitidine (RANI, Aldrich), catechol (Aldrich), hydroquinone (Aldrich), resorcinol (Aldrich), sodium carbonate (Aldrich), sodium azide (Aldrich), nitroblue tetrazolium (Aldrich), hydrogen peroxide (Kanto), methanol (Daejung), and acetonitrile (Merck). Suwannee River humic acid (SRHA, 2S101H) and Suwannee River fulvic acid (SRFA, 2S101F) were purchased from the International Humic Substance Society, whereas the sodium salt of humic acid (AHA) was purchased from Aldrich. Stock solutions of all humic substances (200 mg/L) were prepared by stirring in the dark for 24 h, stored at 4 °C, and discarded after 1 month. Periodate stock solutions (10 mM) were freshly prepared every week. All solutions were prepared in ultrapure water (18 MΩ cm) obtained from a Barnstead purification system. Procedure and Analytical Methods. The reactions were carried out in 50 mL glass beakers stirred on a magnetic stirrer. An aliquot of stock solution of FFA (or other substrate, 1 mM) was added to make a desired concentration (typically 0.1 mM), and the reaction was then initiated by the sequential addition of KIO4 and KOH. Unless otherwise mentioned, all experiments were carried out in air-equilibrated solution open to the air. Sample aliquots (1 mL) were withdrawn at regular time intervals from the reactor and injected into 4 mL glass vials containing 100 μL of sodium sulfite (Na2SO3, 2 M) to quench residual periodate. For experiments under anoxic conditions,

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RESULTS AND DISCUSSION Singlet-Oxygen Generation in KIO4/KOH System. To understand the catalytic reactivity of the KIO4/KOH system, we first investigated the oxidation of different organic pollutants under air-equilibrated conditions. As shown in Figure 1, pharmaceutical compounds such as cimetidine (CMT),

Figure 1. Degradation kinetics of aqueous organic compounds in alkaline periodate solutions. [KIO4]0 = 1 mM, [KOH]0 = 1 mM, pHi = 8.5, and [substrate]0 = 50 μM. (CMT: cimetidine, RANI: ranitidine, PPL: propranolol, BPA: bisphenol A, and 4-CP: 4-chlorophenol.) B

DOI: 10.1021/acs.est.5b04119 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology ranitidine (RANI), and propranolol (PPL) were degraded, whereas phenolic species, such as bisphenol A (BPA) and 4chlorophenol (4-CP), exhibited very low or negligible reactivity. The selective degradation of pharmaceuticals compared to phenolic compounds in near alkaline conditions (pHi = 8.6) strongly suggest the primary involvement of 1O2 because furan and imidazole moieties in pharmaceuticals, such as CMT and RANI, are vulnerable to electrophilic oxidation by 1 O2 [k(furan + 1O2) = 1.4 × 107 M−1 s−1 and k(imidazole + 1 O2) = 3.4 × 107 M−1 s−1].8,32 In contrast, 1O2 reacts slowly with neutral phenol compounds (two orders of magnitude slower) compared to their phenolate anion counterparts. In the KIO4/KOH system (pHi = 8.5), both 4-CP and BPA exist mainly in their neutral form (pKa values of 9.4 and 9.7, respectively) and show higher resistance to oxidation by 1O2 (see Figure 1). Because no pharmaceutical degradation was observed in the absence of either KIO4 (only KOH) or KOH (only KIO4), the formation of 1O2 must be initiated by a chemical reaction between IO4− and OH−. To investigate the reaction mechanism for 1O2 generation in the KIO4/KOH system, we used the oxidation of furfuryl alcohol [FFA, k(FFA + 1O2) = 1.2 × 108 M−1 s−1] as a selective probe reaction.21 Similar to the pharmaceutical oxidation process, no FFA oxidation was observed in the absence of KIO4, whereas a small fraction of FFA could be oxidized by periodate in the absence of KOH. However, in the presence of both KIO4 and KOH, enhanced oxidation was achieved, with the simultaneous and stoichiometric conversion of periodate (IO4−) to iodate (IO3−) (Figure 2a). This indicates that the formation of 1O2 in the KIO4/KOH system was initiated by the base-mediated transformation of IO4− to IO3−. The generation of 1O2 in alkaline KIO4 solution was also confirmed by ESR trapping using TMP−OH as a singlet-oxygen scavenger. As shown in Figure 2b, the characteristic 1:1:1 triplet signal of the TMP−OH adduct with 1O2 is clear evidence of singlet-oxygen formation in the KIO4/KOH system. Moreover, as expected, the ESR signal was not observed in the presence of excess sodium azide (NaN3) as an efficient scavenger of 1O2. Previous studies on singlet-oxygen chemiluminescence measurements in alkaline periodate solutions proposed that the formation of 1O2 involves the recombination of superoxide radicals (O2•−), which are produced as an intermediate product in the presence of IO4− and dissolved oxygen (eqs 1 and 2):29−31 IO4 − + O2 + 2OH− → IO3− + 2O2•− + H 2O

(1)

2O2•− + 2H 2O → 1O2 + H 2O2 + 2OH−

(2)

Figure 2. (a) Comparison of furfuryl alcohol oxidation kinetics (as a probe test for 1O2 generation) and the concurrent transformation of periodate (IO4−) to iodate (IO3−) in alkaline periodate solutions. (b) ESR spectra of TEMPOL generated in alkaline periodate solution containing TMP−OH as a singlet oxygen probe. In (a), [KIO4]0 = 1 mM, [KOH]0 = 1 mM, and [FFA]0 = 100 μM. In (b), [KIO4]0 = 1 mM, [KOH]0 = 1 mM, [TMP−OH] = 25 mM, and [NaN3]0 = 75 mM.

3IO4 − + 2OH− → 3IO3− + 2O2•− + H 2O

(3)

The in situ generated superoxide radicals can then recombine to form 1O2 (eq 2) and the overall reaction (eq 3 + eq 2) stoichiometry predicts that 3 mol of periodate consumed generates 1 mol of 1O2. This is consistent with Figure 2a, which shows that 3 mol of periodate oxidize roughly 1 mol of FFA, which implies that most 1O2 reacts with FFA with a 1:1 molar ratio. Incidentally, the direct oxidation of superoxide to 1O2 by residual periodate (eq 4) is also a thermodynamically possible alternate pathway [E0(1O2/O2•−) = −0.34 VNHE and E0(IO4−/ IO3−) = +0.7 VNHE].

However, in the absence of dissolved oxygen (continuous N2 purging), the FFA oxidation efficiency was not significantly reduced in the present study (Figure 3). This observation is partially in agreement with the chemiluminescence studies,26,27 wherein nitrogen purging reduced the singlet-oxygen luminescence emission by 50% but did not completely quench the signal. Although inert gas purging at ambient temperature cannot remove all of the dissoved oxygen, the low-residual O2 concentration ( 1 mM with and without pH adjustment did not exhibit any significant difference. A possible explanation for the observed decrease in FFA oxidation at [KIO4] > 1 mM might be a scavenging of singlet oxygen by excess residual periodate. Considering that periodate is known to scavenge HO• (k ∼ 108 M−1 s−1),26 on the one hand, it might scavenge singlet oxygen at higher concentration, although there is no quantitative estimation of singlet-oxygen scavenging by periodate. On the other hand, the critical role of initial pH was evident from the effect of KOH concentration (see Figure 4b). As observed in the case of [KIO4], the oxidation was enhanced when [KOH] increased up to 1 mM (pHi = 8.5). However, at higher [KOH] (pHi > 8.5), the oxidation efficiency decreased. This pHdependent reactivity is attributed to the periodate speciation change in neutral and alkaline conditions, wherein various I(VII) species coexist in equilibrium depending on pH (eqs 8−11):25,26,36 H5IO6 ↔ H4IO6− + H+

H4IO6− ↔ H3IO6 2 − + H+

pK a = 1.6

pK a = 8.3

2H3IO6 2 − ↔ H 2I 2O10 4 − + 2H 2O

Kdehydration = 40

(11)

The calculated periodate speciation diagram (concentration− pH profile) (see Figure S5) indicates that IO4− is the dominant species at pH < 8, but the dimerized species (H2I2O104−) is predominant at higher pH. Although Lee and Yoon reported a similar photochemical reactivity for both IO4− and H2I2O104− species,26 the observed decrease in FFA oxidation at higher pH (see Figure 4b) suggests that the IO4− species is more reactive for 1O2 generation than its dimerized form. Effect of Dissolved Organic Matter and Vicinal Hydroxyl Groups. The presence of dissolved natural organic matter has a significant influence on the generation of ROS available for organic-pollutant degradation in both natural aquatic systems and synthetic wastewaters. The photochemical generation of various ROS-like hydroxyl radicals (HO•), peroxy radicals (RO2•) and singlet oxygen (1O2) using NOM as UVor sunlight-absorbing chromophoric species has been well documented.37,38 However, in the absence of incident light (dark reaction), NOM also acts as an efficient ROS scavenger and significantly inhibits the organic oxidation efficiency. Brame et al. demonstrated that the presence of NOM inhibited the efficiency of HO•-mediated organic degradation by 60% as well as that of 1O2-mediated oxidation efficiency by 20%.14 This dissimilarity in NOM reactivity toward an individual ROS-like HO• and 1O2 can be explained in terms of significant structural diversity within the constituent organic fractions.14 Nevertheless, the characteristic nature of NOM as an efficient ROS

(8) (9)

Kdimerization = 141M−1 (10) E

DOI: 10.1021/acs.est.5b04119 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology scavenger always has a negative influence on pollutant oxidation. On the contrary, the addition of NOM into the KIO4/KOH solution increased the FFA oxidation efficiency (Figure 5a,c). In the presence of both Suwannee River humic acid (SRHA) and Suwannee River fulvic acid (SRFA), used as model NOM compounds, the oxidation of FFA increased with increasing NOM concentration. The SRHA and SRFA concentrations (10−50 mg/L) used in this experiment represent the typical NOM concentration in aquatic systems.37−39 The oxidation of FFA was negligible in the absence of either KIO4 (only NOM and KOH) or KOH (only NOM and KIO4). Thus, an alkaline condition is required for the reaction between KIO4 and NOM for enhanced FFA oxidation. Accordingly, the observed enhancement in FFA oxidation (see Figure 5a,c) was also accompanied by enhanced transformation of periodate (IO4−) to iodate (IO3−) (Figure 5b,d). This indicates that the periodate-mediated oxidation of NOM functional groups led to an additional pathway of 1O2 generation for FFA oxidation. The periodate-induced oxidation of polyhydroxy aromatic compounds (simple structural analogues of NOM) such as pyrogallol, gallic acid, and tannic acid has been extensively utilized in chemiluminescence (CL) detection techniques based on 1O2 generation and photoluminescence.40 The direct oxidation of terminal hydroxyl groups in weakly alkaline periodate solution (similar to this study) leads to the formation of an excited-state oxidized species, which further decomposes to form 1O2 with an enhanced photoluminesence signal (eqs 12 and 13),40 IO4 − + R − (OH)x → IO3− + [R − (O)x ]†

(12)

[R − (O)x ]† + O2 → 1O2 + [R − (O)]y

(13)

Figure 6. Kinetics of FFA oxidation in the presence of catechol (CC), resorcinol (RSC), hydroquinone (HQ), and glucose (G). CC, RSC, and HQ were used as representative vicinal and nonvicinal benezenediols, whereas glucose was used as a representative vicinal aliphatic diol ([KIO4]0 = 1 mM, [KOH]0 = 1 mM, [FFA]0 = 100 μM, [benzenediol] = 1 mM, and [glucose] = 1 mM).

k(CC+1O2) = 8.8 × 105 M−1 s−1].43 This implies that CC can induce the additional generation of the singlet oxygen as eqs 12 and 13 suggest. Incidentally, the contribution of vicinal OH groups in carbohydrate moieties toward 1O2 generation appears negligible because no significant enhancement in FFA oxidation was observed when glucose was added in the KIO4/KOH solution (see Figure 6). Similar to the case of SRHA and SRFA, no FFA oxidation was observed in the absence of either periodate or KOH. This result is in complete agreement with chemiluminescence emission studies of alkaline periodate solution, wherein an enhancement was observed only in the presence of catechol.30 Replacing catechol by resorcinol or hydroquinone resulted in the complete quenching of the chemiluminescence signal,30 similar to the data in Figure 6. It should be noted that the addition of resorcinol or hydroquinone into the KIO4/KOH solution led to the formation of colored oxidation products (within 30 min) in the present study, whereas the aqueous solution containing catechol remained colorless until the end of the experiment (see Figure S6). Thus, on the basis of eqs 12 and 13, the periodate-induced oxidation of catechol resulted in the formation of an unstable intermediate species, which further led to the generation of both 1O2 (enhanced FFA oxidation) and unidentified colorless transformation products (as illustrated in Scheme 1). In the case of resorcinol or hydroquinone oxidation, immediate formation of the corresponding quinone species (see Scheme 1) explains the color development in the aqueous solution. The formation of highly stable quinone products prevented further decomposition and does not induce the generation of 1O2. The fact that FFA oxidation was completely inhibited by RSC or HQ (1 mM) cannot be explained only in terms of the singlet oxygen scavenging by RSC/HQ because a 1 mM concentration of RSC/HQ is not high enough to scavenge all of the singlet oxygen for the complete inhibition of the FFA oxidation [k(FFA+1O2) = 1.2 × 108 M−1 s−1]. For example, only 6% of 1 O2 should be scavenged by RSC under the condition of Figure 6, estimating from the above-mentioned rate contants (k(RSC +1O2) versus k(FFA+1O2)). It seems that all periodates are

where R−(OH)x, [R−(O)x]†, and [R−(O)]y represent the original polyhydroxyl compound, its oxidized intermediate, and the final decomposition product, respectively. The oxidation of organic diol involves the formation of a cyclic diol-periodate monoester, which further undergoes base-catalyzed rearrangement to form an unstable intermediate complex.41 Because the base-catalyzed cyclic rearrangement is the rate-limiting step, alkaline condition is needed. However, only polyhydroxy compounds containing at least one pair of vicinal hydroxyl groups (OH groups on neighboring carbon atoms) enhanced 1 O2 generation.30,40 Because humic and fulvic acids contain a variety of structurally different hydroxyl groups, such as aliphatic (alcoholic or carboxylic) hydroxyls, carbohydrate hydroxyls (sugar moieties), and aromatic hydroxyls (phenols),42 the IO4−-mediated oxidation of vicinal OH groups present in NOM can enhance 1O2-mediated FFA oxidation in alkaline condition (see parts a and c of Figure 5). To understand this mechanistic pathway in more detail, we investigated FFA oxidation in the presence of organic compounds with one pair of vicinal or nonvicinal OH groups (benzene diols). As shown in Figure 6, the addition of catechol (CC, 1,2-benzene diol or vicinal diol) into the KIO4/KOH solution enhanced the FFA oxidation whereas nonvicinal diols such as resorcinol (RSC, 1,3-benzenediol) and hydroquinone (HQ, 1,4-benzenediol) at the same concentration (1 mM) completely inhibited the FFA oxidation. The effect of CC is the opposite to that of RSC/HQ, although all of the benzenediols can scavenge the singlet oxygen [k(RSC+1O2) = 7.9 × 105 M−1 s−1 in C2H5OH; k(HQ+1O2) = 6.9 × 107 M−1 s−1 in C5H5N; F

DOI: 10.1021/acs.est.5b04119 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Scheme 1. Schematic Illustration of Proposed 1O2 Generation Pathway through Benzenediol Oxidation in Alkaline Periodate Solutiona

a

CC, RSC, and HQ represent catechol, resorcinol, and hydroquinone, respectively. P denotes an unidentified colorless product of catechol oxidation.

conversion, which does not undergo further transformation to more reduced species like iodide (I−). The oxidative conversion of iodide to hypoiodous acid (HOI) is primarily responsible for the formation of iodinated compounds, such as iodo acids and iodotrihalomethanes.28 When iodide ions were added externally (50−100 μM) in the KIO4/KOH/FFA solution, no significant iodide transformation or inhibition of FFA oxidation was observed. On the one hand, this should lower the probability of secondary contamination by iodinated products, although a complete byproduct identification and toxicity assessment evalution may still be necessary to establish process viability. On the other hand, the presence of NOMs enhanced the 1O2 formation through the selective oxidation of the vicinal OH functional groups. Considering that such compounds as NOMs and natural polyphenols (gallic and tannic acid) are commonly detected in contaminated waters, their presence might be helpful for the application of the proposed KIO4/KOH system. However, other NOM functional moieties that are also reactive toward 1O2 (double bonds, amino groups, thiols, etc.) may influence the overall competitive scavenging within a complicated wastewater matrix. Finally, despite the obvious advantages associated with this UV-free (low process cost) and metal-free (inexpensive catalyst) system for 1O2 generation, the use of periodate reagent and the production of iodate inherently limit its practical applications for water treatment. There are no established environmental discharge requirements yet for periodate and iodate, but their intrinsic toxicity (especially for iodate45) may eventually restrict permissible levels for both species. Although the concentrations used here (