pH-Dependent Degradation of Diclofenac by a Tunnel-Structured Manganese Oxide

: The mechanism of diclofenac (DIC) degradation by tunnel-structured γ -MnO 2 , with superior oxidative and catalytic abilities, was determined in terms of solution pH. High-performance liquid chromatography with mass spectroscopy (HPLC–MS) was used to identify intermediates and ﬁnal products of DIC degradation. DIC can be e ﬃ ciently oxidized by γ -MnO 2 in an acidic medium, and the removal rate decreased signiﬁcantly under neutral and alkaline conditions. The developed model can successfully ﬁt DIC degradation kinetics and demonstrates electron transfer control under acidic conditions and precursor complex formation control mechanism under neutral to alkaline conditions, in which the pH extent for two mechanisms exactly corresponds to the distribution percentage of ionized species of DIC. We also found surface reactive sites (S rxn ), a key parameter in the kinetic model for mechanism determination, to be exactly a function of solution pH and MnO 2 dosage. The main products of oxidation with a highly active hydroxylation pathway on the tunnel-structured Mn-oxide are 5-iminoquinone DIC, hydroxyl-DIC, and 2,6-dichloro-N-o-tolylbenzenamine.


Introduction
Diclofenac (DIC), one of the most commonly used nonsteroidal anti-inflammatory drugs (NSAIDs) worldwide, is discharged in large amounts from wastewater treatment plants because of its high hydrophilic nature [1] and low biodegradability [2]. Thus, DIC is widely found in the aquatic environment in a range from ng/L to µg/L and is one of the most frequently detected pharmaceutical and personal care products in water [3,4]. No evidence suggests that DIC is harmful to humans; however, it might be toxic to aquatic organisms and harmful to embryos, infants, children, and adults with low immunity and being sensitive to pharmaceuticals [5][6][7][8]. Most of the evidence was focused on its adverse effects on the aquatic and terrestrial organisms, which might cause ecological damage [9][10][11]. Besides, the transformation products of diclofenac might be more toxic than diclofenac [12,13], which needs to be investigated further.

Batch Experiments
Experiments were conducted as a function of pH (4)(5)(6)(7)(8)(9). For each batch system, various amounts of a γ-MnO 2 suspension solution were added to 15-mL glass centrifuge tubes. In the solution, 0.005 M NaH 2 PO 4 and NaH 2 BO 3 were added as a buffer. Various proportions of 0.1 M HCl and NaOH were used to adjust pH to the designed value within a controlled range (±0.07). After each reaction course, the solution pH was remeasured to confirm that it remained within the controlled range. For simplification, the pH value is indicated as the designed value in the following paragraphs. The initial concentration of sodium diclofenac (CAS 15307-86-5) prepared was 100 µM, which was confirmed to be completely soluble, with a water solubility of 10 −5.1 -10 −1.78 M [39,40]. The centrifuge tubes were covered with an aluminum foil to prevent light exposure. The suspensions were rotated at 25 • C through end-over-end rotations at 10 rpm for a specific time in kinetic trials and 24 h in thermodynamic tests. All experiments were conducted in duplicate. Moreover, controls (no MnO 2 powder) were established using a similar preparatory process to account for sorption on glass tubes and other reactions in the solution.

Sample Preparation and Analysis
After reactions, the suspensions were centrifuged (Pico 17, Thermo Scientific, Waltham, MA, USA) at 8000 rpm for 40 min, and then the supernatant was quantified using high-performance liquid chromatography (HPLC, L-7200, Hitachi, Japan) with a diode array (DAD) detector (L-7455, Hitachi) at 270 nm. Chromatographic separation was conducted using an RP-18 column (150 µm × 4.6 µm and an internal diameter of 5 µm) purchased from Mightysi with an eluent comprising 60% acetonitrile and 40% acidified water (25 mM phosphoric acid). The flow rate and injection volume were 1 mL min −1 and 20 µL, respectively.

Identification of Oxidation Products
Major oxidation products were identified using HPLC with mass spectrometry (HPLC-MS). The HPLC-MS system comprised an Agilent 1100 Series liquid chromatography system (LC, Agilent, Palo Alto, CA, USA) with a CTC PAL auto-sampler (CTC Analytica, Carrboro, NC, USA) separation module interfaced with an API 4000 triple quadrupole mass spectrometer (Applied Biosystems AB/MDS Sciex, Foster City, CA, USA). The LC column was a Luna Polar RP (150 mm × 2.1 mm internal diameter) column purchased from Phenomenex (Torrance, CA, USA). The flow rate and injection volume were 0.5 mL min −1 and 10 µL, respectively. An HPLC gradient was established by mixing two mobile phases: acetonitrile and deionized water, with 10 mM formic acid. Chromatographic separation was achieved with the following gradient: 0 to 1 min: 0% acetonitrile; 1-5 min: linear-gradient to 100% acetonitrile; 5-10 min: 100% acetonitrile; 10 to 10.1 min: 0% acetonitrile; and 10.1-15 min: 0% acetonitrile. Mass spectrometer parameters operated in a positive ion mode were as follows: curtain gas, 20 psi; ion source gas 1, 30 psi; ion source gas 2, 40 psi; source temperature, 500 • C; entrance potential, 10 V; and nebulizer current, 5 µA, and the interface heater was turned on. Positive ions in the range of 100-500 m/z were scanned at a cycle time of 1 s. The data obtained were processed with Analyst 1.4.2 software. Figure 1 presents the kinetic data of DIC degradation on the tunnel-structured Mn-oxide denoted as dot symbols. Interfacial reactions between DIC and γ-MnO 2 were highly pH-dependent and initially involved a rapid removal of DIC, followed by gradual slowdown and eventual approach to a plateau. In the acidic medium (pH 4-6), the gradual slowdown period was longer than that in neutral and alkaline conditions (pH 7-9) within the tested pH range. The complicated and multistep reactions between the organic micropollutant interface and Mn-oxides result in limitations of kinetics studies; therefore, only initial reaction rates have been explored in most studies and have been generally characterized with a pseudo-first-order degradation model [27,41]. However, the pseudo-first-order kinetics may not satisfy conditions for the later stage of the interfacial reaction. In general, the interfacial reaction can be initiated by the formation of a precursor complex between the Mn(IV) of oxide surface and target organic pollutants, subsequently followed by electron transfer within the precursor complex, redox product formation (Equations (1)-(5)). Either formation of the precursor complex (Equation (1)) or electron transfer within the precursor complex (Equation (2)) is likely to be the rate-limiting step [42]. The formation of redox products, including the surface Mn(III) and Mn(II) (Equation (4)), and further oxidization or combination of organic radicals to form products (Equation (5)), was rapid because of the unstable nature of intermediates.

DIC Degradation Kinetics on the Tunnel-Structured Mn-Oxide
An integrated kinetic model [42] was applied to examine the DIC reaction over γ-MnO 2 . The kinetic equation can be expressed as follows: Total reactive surface sites (S) for DIC degradation can be represented as follows: Both ≡Mn III -DIC and ≡Mn III are negligible in Equation (7) because of their high instability. The ≡Mn II concentration can be calculated with the concentration difference between parent DIC at initial (C 0 ) and specific (C) times on account of two electrons transferred from parent DIC to Mn-oxide [43]. To verify the rate-limited step and degradation mechanism, k 1 C >> k -1 + k 2 and k −1 + k 2 >> k 1 C, respectively, were assumed for electron transfer control and precursor complex formation control, and the analytical solution of Equation (6) for electron transfer control kinetic model is as follows: where k et equals to k 2 and represents the rate constant of the electron transfer control mechanism. The analytical solution for the precursor complex formation control model is as follows: where k pf equals to k 1 k 2 k −1 +k 2 and denotes the rate constant of the precursor formation control mechanism.

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The analytical solution for the precursor complex formation control model is as follows: where kpf equals to and denotes the rate constant of the precursor formation control mechanism.

Correlation between pH and Oxidative Kinetic Constants
The pH of the solution shifted the degradation kinetics from electron-transfer control to precursor complex-formation control ( Figure 1). The electron-transfer control mechanism model successfully described degradation evolution with time under acidic conditions (pH 4-6), whereas under neutral-to-alkaline conditions (pH 7-9), the precursor complex-formation control mechanism was highly fitting to the experimental data (high r-value in Table 1). As Table 1 indicates, the pHs (7-9) with precursor complex-formation control mechanism exactly correspond to the DIC existing as 100% ionized species. Figure S1 (supplementary materials) displayed the DIC species distribution versus solution pH based on the calculation of DIC's pKa 4.15 [43].
When pH was higher than 7, the ionized species account for 100% of DIC in solution. Since the formation of precursor complex of DIC with the γ-MnO 2 surface is coupled with a release of OH − ions [34], the anionic species of DIC confront with the competition of OH − ions for the surface sites under pH 7-9. Thus, adsorption was not favored by higher pH value (7)(8)(9) and the precursor complex formation becomes the control mechanism.
Moreover, the rate constant (k et or k pf ) and surface reaction site (S) for DIC degradation decreased when pH increased ( Table 1). The inverse relationship between k and pH for DIC could be partially attributed to a decrease in the reduction potential of MnO 2 with an increase in pH (Equation (10)). In addition to reducing the potential, solution pH alters the amounts of surface reactive sites (S, Table 1). Under acidic conditions, a large amount of S was expected because of the relatively strong affinity of anionic DIC (pKa = 4.15 [44]) species on the surface of net positively charged MnO 2 . Consequently, electron transfer was limited against sufficient active reaction sites for DIC within the tested acidic pH (4-6). When pH increased, electrostatic attraction between the net negatively charged surface and anionic DIC species decreased. Furthermore, OH − strongly competed against DIC for surface-bound Mn(IV). A lower amount of S at higher pH represented insufficient active reaction sites for DIC attachment, leading to the removal mechanism to shift to precursor complex-formation control mechanism. In addition, each component was actually derived from the initial dosage of MnO 2 (Equation (7)); therefore, surface reaction sites were presumed to be functions of initial dosage of MnO 2 and solution pH: To present the H + concentration as pH, the log form can be written as follows: To investigate the influence of pH on S and the kinetic mechanism shift, the log S values extracted from Table 1   To investigate the influence of pH on S and the kinetic mechanism shift, the log S values extracted from Table 1   Compared with the Mn-oxide dosage yield (total mole of DIC removal per mole of MnO2 dosage), that for DIC removal by employing -MnO2 in this study was 0.07, which falls in relatively higher than 4.49 × 10 -4 -0.14 reported in studies on DIC degradation using other structured Mn-oxides [27,45,46], at 24 h under similar pH conditions. Despite varying the DIC concentration and Mn-oxide dosage from μM to mM in these studies, degradation efficiencies could be compared in a unified manner when the oxide dosage yield was introduced. The remarkable differences in dosage yields indicated that the structure of Mn-oxides substantially influenced their degradative capacity toward DIC, and striking differences were observed for their sorption, oxidative, catalytic, and electrochemical properties [47][48][49]. The higher oxide dosage yield of -MnO2 could be ascribed to the higher amounts of more flexible corner-shared MnO6 sites dominated in Mn-oxide bulk, which may facilitate oxidation for target pollutant degradation [49][50][51] Compared with the Mn-oxide dosage yield (total mole of DIC removal per mole of MnO 2 dosage), that for DIC removal by employing γ-MnO 2 in this study was 0.07, which falls in relatively higher than 4.49 × 10 -4 -0.14 reported in studies on DIC degradation using other structured Mn-oxides [27,45,46], at 24 h under similar pH conditions. Despite varying the DIC concentration and Mn-oxide dosage from µM to mM in these studies, degradation efficiencies could be compared in a unified manner when the oxide dosage yield was introduced. The remarkable differences in dosage yields indicated that the structure of Mn-oxides substantially influenced their degradative capacity toward DIC, and striking differences were observed for their sorption, oxidative, catalytic, and electrochemical properties [47][48][49]. The higher oxide dosage yield of γ-MnO 2 could be ascribed to the higher amounts of more flexible corner-shared MnO 6 sites dominated in Mn-oxide bulk, which may facilitate oxidation for target pollutant degradation [49][50][51].

Identification of Oxidation Products Using HPLC-MS
HPLC-MS was used to determine the M/Z ratio of parent DIC, oxidation intermediates (reaction time of 2 h), and products (reaction time of 24 h), and Figures 3-5, respectively, present their MS chromatograms. Figure 3a has a peak with a very pronounced tailing and this phenomenon should be due to the pH mismatch effect mentioned in a previous study [35]. This fact should not affect the identification of the oxidation intermediates and products because the pronounced tail did not appear after reaction (because the concentration of DIC decreased significantly) and most of the compounds did not appear in this region. System peaks were observed in Figure 4 (Figure 4a,b) and Figure S2. This phenomenon reflected some compounds which are strongly absorbed to the stationary phase were generated after reaction [36]. These compounds cannot be identified using this effluent procedure.
Under neutral-to-alkaline conditions, no intermediates were detected at a reaction interval of 2 h, and compared with acidic conditions, fewer oxidative products were obtained at 24 h according to MS analysis results ( Figure S2). Because of the relatively low degradation of DIC under neutral and alkaline conditions, the MS analyses of intermediates and final products were mainly conducted under the acidic condition.
Because of the ionic nature of DIC, two electrospray ionization (ESI) methods, ESI+ and ESI−, were employed to study degradation products, and Table 2 presents the results. Under acidic conditions, three intermediates (I 1 , I 2 , and I 3 ) were formed after 2 h of the reaction, and four final products (F 1 , F 2 , F 3 , and F 4 ) were obtained after a day of the reaction. Because of the ionic nature of DIC, two electrospray ionization (ESI) methods, ESI+ and ESI−, were employed to study degradation products, and Table 2 presents the results. Under acidic conditions, three intermediates (I1, I2, and I3) were formed after 2 h of the reaction, and four final products (F1, F2, F3, and F4) were obtained after a day of the reaction.                According to Monteagudo et al. [52,53], I 1 (RT = 2.09, m/z = 346) correspond totri-hydroxyl-DIC (m/z = 346) or di-hydroxyl-DIC (m/z = 328). I2 (RT = 2.48, m/z = 298) should be a hydrolyzeddecarboxylated DIC (296 − 14 + 16 = 298). The molecular weight of I 3 (RT = 2.92, m/z = 597) is considerably higher than that of DIC. Moreover, I 3 exhibited numerous isotopic peaks, and its intensity ratio of (M + 1)/Z to (M + 3)/Z was approximately 3:4, which revealed that these compounds contained four chlorine atoms. Thus, I 3 should be a dimmer of 5-iminoquinone DIC (m/z = 308) and another intermediate. This finding indicated that polymerization or dimerization, which was found during the reaction of other aromatic compounds with Mn-oxides, may occur during DIC degradation by γ-MnO 2 . Similar results were reported by Huguet et al. [26]. F1 (RT = 3.02, m/z = 503) is a new product, and its molecular weight is substantially higher than that of DIC. Therefore, it should be a transformation product of I 3 . F 2 (RT = 3.58, m/z = 308) and F 3 (RT = 3.93, m/z = 312) correspond to 5-iminoquinone DIC and hydroxyl-DIC, respectively, which have been reported in literature [26]. The peak of hydroxyl-DIC (F 3 ) split into two and the m/z ratio (255) in negative mode was considerably lower than the m/z ratio (312) in the positive mode. The split of the peak could be attributed to the different sites of the hydroxyl group of the compound (structure isomers) leading to different hydrophilicity, and the observed difference of m/z ratio for F 3 between in positive mode and negative mode should result from the carbon chain (-CH 2 COOH, M = 57) broke during ionization. F 4 (RT = 2.59, m/z = 250) should be a decarboxylation product of DIC (2,6-dichloro-N-o-tolylbenzenamine), which was reported by Martínez et al. [54].

I1 I2 I3
The intensity of F 3 was much higher than that of F 2 , and multiple hydroxyl intermediates (I 1 ) were found. According to studies, decarboxylation, hydroxylation, and dimerization are the three main pathways of DIC transformation by Mn-oxides [26]. The pathways of DIC transformation by γ-MnO 2 are the same as those of birnessite or other natural manganese oxides [26], and compared with the layer-structured birnessite that is widely used in studies, hydroxylation of DIC by γ-MnO 2 was more active than that through other pathways. This phenomenon could corroborate that the large amounts of highly flexible corner-shared MnO 6 may provide abundant reactive hydroxyl groups and facilitate oxidation for target pollutant degradation [50,51]. Therefore, the dimerization products of DIC obtained through γ-MnO 2 are highly hydrophilic and can be detected without extraction. Hydroxylation intermediates were not detected after 1 day because they were oxidized to smaller or hydrophilic compounds due to further hydroxylation. According to Monteagudo et al. [52,53], I1 (RT = 2.09, m/z = 346) correspond totri-hydroxyl-DIC (m/z = 346) or di-hydroxyl-DIC (m/z = 328). I2 (RT = 2.48, m/z = 298) should be a hydrolyzeddecarboxylated DIC (296 − 14 + 16 = 298). The molecular weight of I3 (RT = 2.92, m/z = 597) is considerably higher than that of DIC. Moreover, I3 exhibited numerous isotopic peaks, and its intensity ratio of (M + 1)/Z to (M + 3)/Z was approximately 3:4, which revealed that these compounds contained four chlorine atoms. Thus, I3 should be a dimmer of 5-iminoquinone DIC (m/z = 308) and another intermediate. This finding indicated that polymerization or dimerization, which was found during the reaction of other aromatic compounds with Mn-oxides, may occur during DIC degradation by -MnO2. Similar results were reported by Huguet et al. [26]. F1 (RT = 3.02, m/z = 503) is a new product, and its molecular weight is substantially higher than that of DIC. Therefore, it should be a transformation product of I3. F2 (RT = 3.58, m/z = 308) and F3 (RT = 3.93, m/z = 312) correspond to 5-iminoquinone DIC and hydroxyl-DIC, respectively, which have been reported in literature [26]. The peak of hydroxyl-DIC (F3) split into two and the m/z ratio (255) in negative mode was considerably lower than the m/z ratio (312) in the positive mode. The split of the peak could be attributed to the different sites of the hydroxyl group of the compound (structure isomers) leading to different hydrophilicity, and the observed difference of m/z ratio for F3 between in positive mode and negative mode should result from the carbon chain (-CH2COOH, M = 57) broke during ionization. F4 (RT = 2.59, m/z = 250) should be a decarboxylation product of DIC (2,6-dichloro-N-o-tolylbenzenamine), which was reported by Martínez et al. [54].
The intensity of F3 was much higher than that of F2, and multiple hydroxyl intermediates (I1) were found. According to studies, decarboxylation, hydroxylation, and dimerization are the three main pathways of DIC transformation by Mn-oxides [26]. The pathways of DIC transformation by -MnO2 are the same as those of birnessite or other natural manganese oxides [26], and compared with the layer-structured birnessite that is widely used in studies, hydroxylation of DIC by -MnO2 was more active than that through other pathways. This phenomenon could corroborate that the large amounts of highly flexible corner-shared MnO6 may provide abundant reactive hydroxyl groups and facilitate oxidation for target pollutant degradation [50,51]. Therefore, the dimerization products of DIC obtained through -MnO2 are highly hydrophilic and can be detected without extraction. Hydroxylation intermediates were not detected after 1 day because they were oxidized to smaller or hydrophilic compounds due to further hydroxylation. According to Monteagudo et al. [52,53], I1 (RT = 2.09, m/z = 346) correspond totri-hydroxyl-DIC (m/z = 346) or di-hydroxyl-DIC (m/z = 328). I2 (RT = 2.48, m/z = 298) should be a hydrolyzeddecarboxylated DIC (296 − 14 + 16 = 298). The molecular weight of I3 (RT = 2.92, m/z = 597) is considerably higher than that of DIC. Moreover, I3 exhibited numerous isotopic peaks, and its intensity ratio of (M + 1)/Z to (M + 3)/Z was approximately 3:4, which revealed that these compounds contained four chlorine atoms. Thus, I3 should be a dimmer of 5-iminoquinone DIC (m/z = 308) and another intermediate. This finding indicated that polymerization or dimerization, which was found during the reaction of other aromatic compounds with Mn-oxides, may occur during DIC degradation by -MnO2. Similar results were reported by Huguet et al. [26]. F1 (RT = 3.02, m/z = 503) is a new product, and its molecular weight is substantially higher than that of DIC. Therefore, it should be a transformation product of I3. F2 (RT = 3.58, m/z = 308) and F3 (RT = 3.93, m/z = 312) correspond to 5-iminoquinone DIC and hydroxyl-DIC, respectively, which have been reported in literature [26]. The peak of hydroxyl-DIC (F3) split into two and the m/z ratio (255) in negative mode was considerably lower than the m/z ratio (312) in the positive mode. The split of the peak could be attributed to the different sites of the hydroxyl group of the compound (structure isomers) leading to different hydrophilicity, and the observed difference of m/z ratio for F3 between in positive mode and negative mode should result from the carbon chain (-CH2COOH, M = 57) broke during ionization. F4 (RT = 2.59, m/z = 250) should be a decarboxylation product of DIC (2,6-dichloro-N-o-tolylbenzenamine), which was reported by Martínez et al. [54].
The intensity of F3 was much higher than that of F2, and multiple hydroxyl intermediates (I1) were found. According to studies, decarboxylation, hydroxylation, and dimerization are the three main pathways of DIC transformation by Mn-oxides [26]. The pathways of DIC transformation by -MnO2 are the same as those of birnessite or other natural manganese oxides [26], and compared with the layer-structured birnessite that is widely used in studies, hydroxylation of DIC by -MnO2 was more active than that through other pathways. This phenomenon could corroborate that the large amounts of highly flexible corner-shared MnO6 may provide abundant reactive hydroxyl groups and facilitate oxidation for target pollutant degradation [50,51]. Therefore, the dimerization products of DIC obtained through -MnO2 are highly hydrophilic and can be detected without extraction. Hydroxylation intermediates were not detected after 1 day because they were oxidized to smaller or hydrophilic compounds due to further hydroxylation.

Conclusions
This study demonstrated that the pH of media highly influences DIC oxidative degradation on the tunnel-structured Mn-oxide (-MnO2). The reduction potential of Mn-oxide, the number of surface reactive sites (S), and electrostatic affinity between DIC and -MnO2 increase with a decrease in pH value. Consequently, the electron-transfer control mechanism model successfully described degradation evolution with time under acidic conditions (pH = 4-6). While under neutral-to-alkaline conditions (pH = 7-9), the precursor complex-formation control mechanism was highly fitting to the experimental data. At pH 7-9 the anionic species account for 100% DIC in solution and hence confront with the competition of OH − ions for the complex formation on the -MnO2 surface. In contrast, the acid form of DIC with a substantial ratio under pH 4-6 is favored for the surface complex formation with less competition. The results of the analysis of oxidative intermediates and products by using HPLC-MS revealed decarboxylation, hydroxylation, and dimerization as the three main pathways of DIC transformation by -MnO2. Although the oxidation products obtained by -MnO2 are similar to those obtained by other Mn-oxides, hydroxylation of DIC by -MnO2 is more active than other pathways because of an abundance of flexible corner-shared MnO6 for target pollutant degradation.

Supplementary Materials:
The following are available online: www.mdpi.com/xxx/s1, Figure S1: DIC species distribution versus solution. HA and A − represent the acid and ionized form of DIC, respectively. The black and gray line were calculated based on pKa = 4.15 of DIC [44]. When pH higher than 7, the ionized species (A − ) accounts for 100% of DIC in solution pH. Figure

Conflicts of Interest:
The authors declare no conflict of interest. [26,55]

Conclusions
This study demonstrated that the pH of media highly influences DIC oxidative degradation on the tunnel-structured Mn-oxide (-MnO2). The reduction potential of Mn-oxide, the number of surface reactive sites (S), and electrostatic affinity between DIC and -MnO2 increase with a decrease in pH value. Consequently, the electron-transfer control mechanism model successfully described degradation evolution with time under acidic conditions (pH = 4-6). While under neutral-to-alkaline conditions (pH = 7-9), the precursor complex-formation control mechanism was highly fitting to the experimental data. At pH 7-9 the anionic species account for 100% DIC in solution and hence confront with the competition of OH − ions for the complex formation on the -MnO2 surface. In contrast, the acid form of DIC with a substantial ratio under pH 4-6 is favored for the surface complex formation with less competition. The results of the analysis of oxidative intermediates and products by using HPLC-MS revealed decarboxylation, hydroxylation, and dimerization as the three main pathways of DIC transformation by -MnO2. Although the oxidation products obtained by -MnO2 are similar to those obtained by other Mn-oxides, hydroxylation of DIC by -MnO2 is more active than other pathways because of an abundance of flexible corner-shared MnO6 for target pollutant degradation.

Supplementary Materials:
The following are available online: www.mdpi.com/xxx/s1, Figure S1: DIC species distribution versus solution. HA and A − represent the acid and ionized form of DIC, respectively. The black and gray line were calculated based on pKa = 4.15 of DIC [44]. When pH higher than 7, the ionized species (A − ) accounts for 100% of DIC in solution pH. Figure

Conflicts of Interest:
The authors declare no conflict of interest. [26,55]

Conclusions
This study demonstrated that the pH of media highly influences DIC oxidative degradation on the tunnel-structured Mn-oxide (-MnO2). The reduction potential of Mn-oxide, the number of surface reactive sites (S), and electrostatic affinity between DIC and -MnO2 increase with a decrease in pH value. Consequently, the electron-transfer control mechanism model successfully described degradation evolution with time under acidic conditions (pH = 4-6). While under neutral-to-alkaline conditions (pH = 7-9), the precursor complex-formation control mechanism was highly fitting to the experimental data. At pH 7-9 the anionic species account for 100% DIC in solution and hence confront with the competition of OH − ions for the complex formation on the -MnO2 surface. In contrast, the acid form of DIC with a substantial ratio under pH 4-6 is favored for the surface complex formation with less competition. The results of the analysis of oxidative intermediates and products by using HPLC-MS revealed decarboxylation, hydroxylation, and dimerization as the three main pathways of DIC transformation by -MnO2. Although the oxidation products obtained by -MnO2 are similar to those obtained by other Mn-oxides, hydroxylation of DIC by -MnO2 is more active than other pathways because of an abundance of flexible corner-shared MnO6 for target pollutant degradation.

Conclusions
This study demonstrated that the pH of media highly influences DIC oxidative degradation on the tunnel-structured Mn-oxide (γ-MnO 2 ). The reduction potential of Mn-oxide, the number of surface reactive sites (S), and electrostatic affinity between DIC and γ-MnO 2 increase with a decrease in pH value. Consequently, the electron-transfer control mechanism model successfully described degradation evolution with time under acidic conditions (pH = 4-6). While under neutral-to-alkaline conditions (pH = 7-9), the precursor complex-formation control mechanism was highly fitting to the experimental data. At pH 7-9 the anionic species account for 100% DIC in solution and hence confront with the competition of OH − ions for the complex formation on the γ-MnO 2 surface. In contrast, the acid form of DIC with a substantial ratio under pH 4-6 is favored for the surface complex formation with less competition. The results of the analysis of oxidative intermediates and products by using HPLC-MS revealed decarboxylation, hydroxylation, and dimerization as the three main pathways of DIC transformation by γ-MnO 2 . Although the oxidation products obtained by γ-MnO 2 are similar to those obtained by other Mn-oxides, hydroxylation of DIC by γ-MnO 2 is more active than other pathways because of an abundance of flexible corner-shared MnO 6 for target pollutant degradation.
Supplementary Materials: The following are available online: http://www.mdpi.com/2073-4441/12/8/2203/s1, Figure S1: DIC species distribution versus solution. HA and A − represent the acid and ionized form of DIC, respectively. The black and gray line were calculated based on pKa = 4.15 of DIC [44]. When pH higher than 7, the ionized species (A − ) accounts for 100% of DIC in solution pH. Figure