Insights into the effects of bromide at fresh water levels on the radical chemistry in the UV/peroxydisulfate process

Underestimated role of hydroxyl radicals for bromate formation in persulfate-based advanced oxidation processes

In persulfate-based advanced oxidation processes (PS-AOPs), sulfate radicals (SO4•-) have been recognized to play more important roles in inducing bromate (BrO3-) formation rather than hydroxyl radicals (HO•) because of the stronger oxidation capacity of the former. However, this study reported an opposite result that HO• indeed dominated the formation of bromate instead of SO4•-. Quenching experiments were coupled with electron paramagnetic resonance (EPR) detection and chemical probe identification to elucidate the contributions of each radical species. The comparison of different thermal activated persulfates (PDS and PMS) demonstrated that the significant higher bromate formation in HEAT/PMS ([BrO3-]/[Br-]0 = 0.8), as compared to HEAT/PDS ([BrO3-]/[Br-]0 = 0.2), was attributable to the higher concentration of HO• radicals in HEAT/PMS. Similarly, the bromate formation in UV/PDS ([BrO3-]/[Br-]0 = 1.0), with a high concentration of HO•, further underscored the dominant role of HO•. As a result, we quantified that HO• and SO4•- radicals accounted 66.7% and 33.3% for bromate formation. This controversial result can be reconciled by considering the critical intermediate, hypobromic acid/hypobromate (HOBr/BrO-), involved in the transformation of Br- to BrO3-. HO• radicals have the chemical preference to induce the formation of HOBr/BrO- intermediates (contributing ∼ 60%) relative to SO4•- radicals (contributing ∼ 40%). This study highlighted the dominant role of HO• in the formation of bromate rather than SO4•- in PS-AOPs and potentially offered novel insights for reducing disinfection byproduct formation by controlling the radical species in AOPs.

High-valent metal-oxo species transformation and regulation by co-existing chloride: Reaction pathways and impacts on the generation of chlorinated by-products

High-valent metal-oxo species (HMOS) have been extensively recognized in advanced oxidation processes (AOPs) owing to their high selectivity and high chemical utilization efficiency. However, the interactions between HMOS and halide ions in sewage wastewater are complicated, leading to ongoing debates on the intrinsic reactive species and impacts on remediation. Herein, we prepared three typical HMOS, including Fe(IV), Mn(V)-nitrilotriacetic acid complex (Mn(V)NTA) and Co(IV) through peroxymonosulfate (PMS) activation and comparatively studied their interactions with Cl- to reveal different reactive chlorine species (RCS) and the effects of HMOS types on RCS generation pathways. Our results show that the presence of Cl- alters the cleavage behavior of the peroxide OO bond in PMS and prohibits the generation of Fe(IV), spontaneously promoting SO4•- production and its subsequent transformation to secondary radicals like Cl• and Cl2•-. The generation and oxidation capacity of Mn(V)NTA was scarcely influenced by Cl-, while Cl- would substantially consume Co(IV) and promote HOCl generation through an oxygen-transfer reaction, evidenced by density functional theory (DFT) and deuterium oxide solvent exchange experiment. The two-electron-transfer standard redox potentials of Fe(IV), Mn(V)NTA and Co(IV) were calculated as 2.43, 2.55 and 2.85 V, respectively. Due to the different reactive species and pathways in the presence of Cl-, the amounts of chlorinated by-products followed the order of Co(II)/PMS > Fe(II)/PMS > Mn(II)NTA/PMS. Thus, this work renovates the knowledge of halide chemistry in HMOS-based systems and sheds light on the impact on the treatment of salinity-containing wastewater.

Elucidating the enhanced role of carbonate radical in propranolol degradation by UV/peroxymonosulfate system

Carbonate radical (CO3•-) has been proved to be an important secondary radical in advanced oxidation processes due to various radical reactions involved HCO3-/CO32-. However, the roles and contributions of CO3•- in organic micropollutant degradation have not been explored systematically. Here, we quantified the impact of CO3•- on the degradation kinetics of propranolol, a representative pollutant in the UV/peroxymonosulfate (PMS) system, by constructing a steady-state radical model. Substantially, the measured values were coincident with the predictive values, and the contributions of CO3•- on propranolol degradation were the water matrix-dependent. Propranolol degradation increased by 130% in UV/PMS system containing 10 mM HCO3-, and the contribution of CO3•- was as high as 58%. Relatively high pH values are beneficial for propranolol degradation in pure water containing HCO3-, and the contributions of CO3•- also enhanced, while an inverse phenomenon was shown for the effects of propranolol concentrations. Dissolved organic matter exhibited significant scavenging effects on HO•, SO4•-, and CO3•-, substantially retarding the elimination process. The developed model successfully predicted oxidation degradation kinetics of propranolol in actual sewage, and CO3•- contribution was up to 93%, which in indicative of the important role of CO3•- in organic micropollutant removal via AOPs treatment.

Far-UVC Photolysis of Peroxydisulfate for Micropollutant Degradation in Water

Increasing radical yields to reduce UV fluence requirement for achieving targeted removal of micropollutants in water would make UV-based advanced oxidation processes (AOPs) less energy demanding in the context of United Nations' Sustainable Development Goals and carbon neutrality. We herein demonstrate that, by switching the UV radiation source from conventional low-pressure UV at 254 nm (UV254) to emerging Far-UVC at 222 nm (UV222), the fluence-based concentration of HO• in the UV/peroxydisulfate (UV/PDS) AOP increases by 6.40, 2.89, and 6.00 times in deionized water, tap water, and surface water, respectively, with increases in the fluence-based concentration of SO4•- also by 5.06, 5.81, and 55.47 times, respectively. The enhancement to radical generation is confirmed using a kinetic model. The pseudo-first-order degradation rate constants of 16 micropollutants by the UV222/PDS AOP in surface water are predicted to be 1.94-13.71 times higher than those by the UV254/PDS AOP. Among the tested water matrix components, chloride and nitrate decrease SO4•- but increase HO• concentration in the UV222/PDS AOP. Compared to the UV254/PDS AOP, the UV222/PDS AOP decreases the formation potentials of carbonaceous disinfection byproducts (DBPs) but increases the formation potentials of nitrogenous DBPs.

Development of a Five-Chemical-Probe Method to Determine Multiple Radicals Simultaneously in Hydroxyl and Sulfate Radical-Mediated Advanced Oxidation Processes

Advanced oxidation processes (AOPs), such as hydroxyl radical (HO•)- and sulfate radical (SO4•-)-mediated oxidation, are attractive technologies used in water and wastewater treatments. To evaluate the treatment efficiencies of AOPs, monitoring the primary radicals (HO• and SO4•-) as well as the secondary radicals generated from the reaction of HO•/SO4•- with water matrices is necessary. Therefore, we developed a novel chemical probe method to examine five key radicals simultaneously, including HO•, SO4•-, Cl•, Cl2•-, and CO3•-. Five probes, including nitrobenzene, para-chlorobenzoic acid, benzoic acid, 2,4,6-trimethylbenzoic acid, and 2,4,6-trimethylphenol, were selected in this study. Their bimolecular reaction rate constants with diverse radicals were first calibrated under the same conditions to minimize systematic errors. Three typical AOPs (UV/H2O2, UV/S2O82-, and UV/HSO5-) were tested to obtain the radical steady-state concentrations. The effects of dissolved organic matter, Br-, and the probe concentration were inspected. Our results suggest that the five-probe method can accurately measure radicals in the HO•- and SO4•--mediated AOPs when the concentration of Br- and DOM are less than 4.0 μM and 15 mgC L-1, respectively. Overall, the five-probe method is a practical and easily accessible method to determine multiple radicals simultaneously.

The diverse roles of halide ions in the degradation of bisphenol A via UV/peracetic acid process at different pH values: Radical chemistry, and transformation pathways

UV/Peracetic Acid (UV/PAA), as an innovative advanced oxidation process (AOP), is employed to treat bisphenol A (BPA) in water through the generation of hydroxyl radicals (•OH) and carbon-centered radicals (R-C•). The impact of halide ions (Cl-; Br-; I-) on the efficiency of UV/PAA was investigated for the first time under varying pH levels. The presence of halide ions exerted an influence on the reactivity of •OH and R-C•, exhibiting varying degrees of impact across different pH conditions. It was discovered that pH exerts a significant influence on its efficiency, with optimal removal performance observed at a pH 9. The degradation of BPA was inhibited by Cl- through the generation of reactive chlorine species (RCS), which triggers the interconversion between •OH and R-C•. Reactive bromine species (RBS) were produced in the presence of Br-, facilitating BPA degradation and generating HOBr as a supplementary source of •OH radicals. I- primarily generate reactive iodine species (RIS) through photolysis, which facilitates the degradation of BPA. The transformation of BPA involves hydroxylation, demethylation, halogenation, and cleavage reactions to form various products and pathways. The toxicity test demonstrates that the UV/PAA treatment of BPA exhibits lower toxicity, thereby indicating its environmentally friendly.

Ball-milled layer double hydroxide as persulfate activator for efficient degradation of organic: Alkaline sites-triggered non-radical mechanism

Considerable efforts have been put into enhancing the activation performance of peroxydisulfate (PDS) by catalysts toward oxidative degradation of organic pollutants, while the oxidative selectivity is somehow overlooked. Here, we reported an enhanced non-radical oxidation pathway of PDS, activated by ball-milled Mg/Al-layered double hydroxide (BM-LDH), to reconcile the selectivity and reactivity. EPR and quenching experiments suggested that 1O2 dominated the oxidative pathway for phenol degradation without generating carcinogenic halide by-products. Multiple interfacial characterizations and density functional theory (DFT) calculations revealed that BM-LDH played dual roles in PDS activation: (1) the interlaminar BM-LDH allowed PDS intercalation to form complexed PDS, resulting in decreases in the activation barrier of PDS; (2) abundant terminal hydroxyls in the layers of BM-LDH acted as alkaline-activation sites that can efficiently activate PDS to generate 1O2 toward phenol degradation. Ball-milling treatment of LDH refined the structural hierarchy of LDH to create pore volumes, which greatly enhanced the diffusion of phenol to the intercalated PDS, resulting in more than twice the reaction rate for phenol degradation. This study provided a promising approach to simultaneously control over the reactivity and selectivity toward PDS activation that are critical for the degradation of organic pollutants particularly in drinking water treatment.

Factors hindering the degradation of pharmaceuticals from human urine in an iron-activated persulfate system

This study investigated the degradation of clofibric acid (CFA), bezafibrate (BZF), and sulfamethoxazole (SMX) in synthetic human urine using a novel mesoporous iron powder-activated persulfate system (mFe-PS system), and identified the factors limiting their degradation in synthetic human urine. A kinetic model was established to expose the radical production in various reaction conditions, and experiments were conducted to verify the modeling results. In the phosphate-containing mFe-PS system, the 120 min removal efficiency of CFA decreased from 95.1% to 76.6% as the phosphate concentration increased from 0.32 to 6.45 mmol/L, but recovered to 90.5% when phosphate concentration increased to 16.10 mmol/L. Meanwhile, the increased concentration of phosphate from 0.32 to 16.10 mmol/L reduced the BZF degradation efficacy from 91.5% to 79.0%, whereas SMX removal improved from 37.3% to 62.9%. The mFe-PS system containing (bi)carbonate, from 4.20 to 166.70 mmol/L, reduced CFA and BZF removal efficiencies from 100% to 76.8% and 80.4%, respectively, and SMX from 83.5% to 56.7% within a 120-min reaction time. In addition, alkaline conditions (pH ≥ 8.0) inhibited CFA and BZF degradations, while nonacidic pH (pH ≥ 7.0) remarkably inhibited SMX degradation. Results of the kinetic model indicated the formation of phosphate (H2PO4·/HPO4·-) and/or carbonate radicals (CO3·-) could limit pharmaceutical removal. The transformation products (TPs) of the pharmaceuticals revealed more incompletely oxidized TPs occurred in the phosphate- and (bi)carbonate-containing mFe-PS systems, and indicated that H2PO4·/HPO4·- mainly degraded pharmaceuticals via a benzene ring-opening reaction while CO3·- preferentially oxidized pharmaceuticals via a hydroxylation reaction.

Comprehensive Assessment of Reactive Bromine Species in Advanced Oxidation Processes: Differential Roles in Micropollutant Abatement in Bromide-Containing Water

Reactive bromine species (RBS) are gaining increasing attention in natural and engineered aqueous systems containing bromide ions (Br-). However, their roles in the degradation of structurally diverse micropollutants by advanced oxidation processes (AOPs) were not differentiated. In this study, the second-order rate constants (k) of Br•, Br2•-, BrO•, and ClBr•- were collected and evaluated. Br• is the most reactive RBS toward 21 examined micropollutants with k values of 108-1010 M-1 s-1. Br2•-, ClBr•-, and BrO• are selective for electron-rich micropollutants with k values of 106-108 M-1 s-1. The specific roles of RBS in aqueous micropollutant degradation in AOPs were revealed by using simplified models via sensitivity analysis. Generally, RBS play minimal roles in the UV/H2O2 process but are significant in the UV/peroxydisulfate (PDS) and UV/chlorine processes in the presence of trace Br-. In UV/PDS with ≥1 μM Br-, Br• emerges as the major RBS for removing electron-rich micropollutants. In UV/chlorine, BrO• contributes to the degradation of specific electron-rich micropollutants with removal percentages of ≥20% at 1 μM Br-, while the contributions of BrO• and Br• are comparable to those of reactive chlorine species as Br- concentration increases to several μM. In all AOPs, Br2•- and ClBr•- play minor roles at 1-10 μM Br-. Water matrix components such as HCO3-, Cl-, and natural organic matter (NOM) significantly inhibit Br•, while BrO• is less affected, only slightly scavenged by NOM with a k value of 2.1 (mgC/L)-1 s-1. This study sheds light on the differential roles of multiple RBS in micropollutant abatement by AOPs in Br--containing water.

Enhancing Ferrate Oxidation of Micropollutants via Inducing Fe(V)/Fe(IV) Formation Needs Caution: Increased Conversion of Bromide to Bromate

This study explores the formation of bromate (BrO3-) in the copresence of Fe(VI) and bromide (Br-). It challenges previous beliefs about the role of Fe(VI) as a green oxidant and highlights the crucial role of intermediates Fe(V) and Fe(IV) in the conversion of Br- to BrO3-. The results show that the maximum concentration of BrO3- of 48.3 μg/L was obtained at 16 mg/L Br- and that the contribution of Fe(V)/Fe(IV) to the conversion was positively related to pH. The study suggests that a single-electron transfer from Br- to Fe(V)/Fe(IV) along with the generation of reactive bromine radicals is the first step of Br- conversion, followed by the formation of OBr- which was then oxidized to BrO3- by Fe(VI) and Fe(V)/Fe(IV). Some common background water constituents (e.g., DOM, HCO3-, and Cl-) significantly inhibited BrO3- formation by consuming Fe(V)/Fe(IV) and/or scavenging the reactive bromine species. While investigations proposing to promote Fe(V)/Fe(IV) formation in Fe(VI)-based oxidation to enhance its oxidation capacity have been rapidly accumulated recently, this work called attention to the considerable formation of BrO3- in this process.

Degradation of antiviral drug acyclovir by thermal activated persulfate process: Kinetics study and modeling

Pharmaceutical and personal care products (PPCPs) pose a great threat to water environment security. In this study, acyclovir (ACV) was efficiently degraded by thermally activated persulfate (TAP) system. The ACV degradation increased with rising reaction temperature and persulfate dosage. With the existence of inorganic anions and humic acid, ACV removal was retarded to varying degrees. Under strong alkaline condition, it was observed that the degradation of ACV was significantly inhibited. In addition, Kintecus software was employed to simulate ACV removal and achieved a good fit with the experimental results. The contribution rates of main reactive radicals under acidic, neutral, and alkaline conditions were investigated, and the contribution of hydroxyl radical (⋅OH) increased significantly under alkaline condition. The main active species were identified as sulfate radical (SO₄⋅^-) and ⋅OH through quenching experiment, and the second-order reaction rate constants of SO₄⋅^- and ∙OH reacted with ACV were calculated to be 9.17 × 10⁹ M^-1 s^-1 and 2.74 × 10⁹ M^-1 s^-1, respectively. The main degradation pathways included addition of free radicals, oxidation of branch chain and ring opening. The acute and chronic toxicity of intermediates to organisms predicted by ECOSAR were significantly reduced compared with that of ACV.

Impact of aqueous environments on hydrogen peroxide activation by manganese oxides: Kinetics and the critical role of bicarbonate

MnO₂ activating H₂O₂ is a promising way in the field of advanced oxidation processes (AOPs) and in situ chemical oxidation (ISCO) to remove contaminants. However, few studies have focused on the influence of various environmental conditions on the performance of MnO₂-H₂O₂ process, which restricts the application in real world. In this study, the effect of essential environmental factors (ionic strength, pH, specific anions and cations, dissolved organic matter (DOM), SiO₂) on the decomposition of H₂O₂ by MnO₂ (ε-MnO₂ and β-MnO₂) were investigated. The results suggested that H₂O₂ degradation was negatively correlated with ionic strength and strongly inhibited under low pH conditions and with phosphate existence. DOM had a slight inhibitory effect while Br^-, Ca²⁺, Mn²⁺ and SiO₂ placed negligible impact on this process. Interestingly, HCO₃^- inhibited the reaction at low concentrations but promoted H₂O₂ decomposition at high concentrations, possibly due to the formation of peroxymonocarbonate. This study may provide a more comprehensive reference for potential application of H₂O₂ activation by MnO₂ in different water systems.

Comparison of UV and UV-LED activated sodium percarbonate for the degradation of O-desmethylvenlafaxine

As an active metabolite of venlafaxine and emerging antidepressant, O-desmethylvenlafaxine (ODVEN) was widely detected in different water bodies, which caused potential harm to human health and environmental safety. In this study, the comparative work on the ODVEN degradation by UV (254 nm) and UV-LED (275 nm) activated sodium percarbonate (SPC) systems was systematically performed. The higher removal rate of ODVEN can be achieved under UV-LED direct photolysis (14.99%) than UV direct photolysis (4.57%) due to the higher values of photolysis coefficient at the wavelength 275 nm. Significant synergistic effects were observed in the UV/SPC (80.38%) and UV-LED/SPC (53.57%) systems and the former exhibited better performance for the elimination of ODVEN. The degradation of ODVEN all followed the pseudo-first-order kinetics well in these processes, and the pseudo-first-order rate constant (k_obs) increased with increasing SPC concentration. Radicals quenching experiments demonstrated that both ·OH and CO₃·^- were involved in the degradation of ODVEN and the second-order rate constant of ODVEN with CO₃·^- (1.58 × 10⁸ (mol/L)^-1 sec^-1) was reported for the first time based on competitive kinetic method. The introduction of HA, Cl^-, NO₃^- and HCO₃^- inhibited the ODVEN degradation to varying degrees in the both processes. According to quantum chemical calculation, radical addition at the ortho-position of the phenolic hydroxyl group was confirmed to be the main reaction pathways for the oxidation of ODVEN by ·OH. In addition, the oxidation of ODVEN may involve the demethylation, H-abstraction, OH-addition and C-N bond cleavage. Eventually, the UV-LED/SPC process was considered to be more cost-effective compared to the UV/SPC process, although the UV/SPC process possessed a higher removal rate of ODVEN.

Formation of halogenated macromolecular organics induced by Br- and I- during plasma oxidation/chlorination of DOM: Highlighting competitive mechanisms

Understanding the effects of halogens on the production of macromolecular disinfection byproducts (DBPs) is critical for drinking water safety. The effects of Br^- and I^- on the chemical diversity of dissolved organic matter (DOM) during plasma preoxidation and the subsequent formation of macromolecular halogenated DBPs after chlorination were deciphered. Plasma preoxidation changed DOM diversity from aromatic component-oriented to lignin and tannin component-oriented, resulting in 62.0% and 21.2% decreases in N-DBPs (C_kH_nO_mN_zCl_x formulas) and C-DBPs (C_kH_nO_mCl_x formulas) after chlorination, respectively. Br^- could induce the formation of organobromine compounds (OBrCs) during plasma oxidation; however, the intensities of OBrCs decreased by 56.3% (CHO formulas) and 75.2% (CHON formulas) after further chlorination. OBrCs still accounted for 79.8% of the total organohalogen compounds (OXCs, X=Cl or Br) due to the higher substitutability of bromine. I^-promoted OIC production in the DOM preoxidation process, and OICs acted as intermediates to form OClCs during chlorination. When Br^-and I^-coexisted, Br^- promoted OIC production in the DOM preoxidation process; therefore, more OBrCs and OClCs were generated due to intermediates of OICs in subsequent chlorination. Connections between OXCs and their precursors were established using network computation. The precursors of OClCs were located in the aromatic structure region (0.2 < H/C ≤ 0.7; O/C ≤ 0.67); those of OBrCs and OICs were located in the lignin (0.7 < H/C ≤ 1.5; 0.1 < O/C < 0.67) and tannin (0.6 ≤ H/C ≤ 1.5, 0.67 < O/C < 1.0) regions with relatively greater H/C and O/C ratios, respectively.

Degradation of bisphenol A by UV/persulfate process in the presence of bromide: Role of reactive bromine

Bromide (Br^-), a ubiquitous species in natural water, is capable of reacting with sulfate radical (SO₄∙^-) and hydroxyl radical (∙OH) to form secondary reactive bromine species (RBS). The reaction routes can influence the degradation mechanisms and performance of these radicals for removal of target pollutants and may also form harmful bromine-containing disinfection by-products (Br-DBPs) during subsequent chlorination. In the present research, the UV-activated persulfate (PS) degradation of bisphenol A (BPA) was systematically examined in the presence of Br^-. Results indicated that the presence of Br^-enhanced the BPA degradation and both UV/PS and UV/PS/Br^- processes followed the pseudo-first-order kinetics. At 0-0.8 mM Br^-, 0.2 mM Br^- exerted the best enhanced effect on BPA degradation, while RBS functioned as the major contributor in the presence of 0.05-0.5 mM Br^-. Solution pH (6.0-8.0) barely affected the BPA degradation in the UV/PS system, but the introduction of Br^- augmented the pH dependence. In the UV/PS/Br^-system, the reaction rate constant of BPA increased/decreased with increasing PS/HA dosage, and was affected slightly in the presence of bicarbonate and chloride. According to the quantum chemical calculation, the second-order rate constants of BPA with ∙OH, SO₄∙^-, Br∙ and Br₂∙^- were calculated as 7.65 × 10¹⁰, 1.67 × 10⁹, 1.77 × 10⁸ and 2.83 × 10² M^-1 s^-1, respectively. Additionally, three degradation pathways of BPA were proposed based on DFT calculation and HPLC/MS analysis, and the formed bromine-containing products exhibited higher toxicity than BPA. Br-DBPs, particularly tribromomethane and tribromoacetic acid, generated from UV/PS/Br^-pre-oxidation during BPA chlorination significantly increased the toxicity of total DBPs.

Misconceptions about the Chemistry of Aqueous Chlorine Atoms and HClOH•(aq), and a Revised Mechanism for the Photochemical Peroxydisulfate/Chloride Reaction

It is widely considered that aqueous chlorine atoms (Cl•) convert to the species HClOH• with a half life of about 3 µs and that this species plays an important role...

Rate Constants and Mechanisms for Reactions of Bromine Radicals with Trace Organic Contaminants

Bromine radicals can pose great impacts on the photochemical transformation of trace organic contaminants in natural and engineered waters. However, the reaction kinetics and mechanisms involved are barely known. In this work, second-order reaction rate constants with Br^• and Br₂^•- were determined for 70 common trace organic contaminants and for 17 model compounds using laser flash photolysis and steady-state competition kinetics. The k_Br^• values ranged from <10⁸ to (2.86 ± 0.31) × 10¹⁰ M^-1 s^-1 and the k_Br2^•- values from <10⁵ to (1.18 ± 0.09) × 10⁹ M^-1 s^-1 at pH 7.0. Six quantitative structure-activity relationships were developed, which allow predicting additional unknown k_Br^• and k_Br2^•- values. Single-electron transfer was shown to be a favored pathway for the reactions of Br^• and Br₂^•- with trace organic contaminants, and this was supported by transient spectroscopy and quantum chemical calculations. This study is essential in advancing the scientific understanding of halogen radical-involved chemistry in contaminant transformation.