Ferrate Oxidation of Phenolic Compounds in Iodine-Containing Water: Control of Iodinated Aromatic Products

Differential Impacts of Pyrophosphate on Ferrates(VI, V, and IV): Through Its Unique Inhibition to Identify Fe(V) Species

High-valent iron species [Fe(V) and Fe(IV)] exhibit remarkable oxidative activity in environmental chemistry. However, the distinctions between the properties of Fe(V) and Fe(IV) remain poorly understood due to the challenges of distinguishing them. Herein, using pyrophosphate as a model ligand, we comprehensively investigated the influence of oxo-ligands on the reactivity of high-valent iron(VI, V, IV) species. An innovative strategy to selectively generate Fe(IV) using the Fe(VI)-initiated system was proposed, enabling an in-depth investigation of the interaction between Fe(IV) and pyrophosphate. The results reveal that pyrophosphate strongly inhibits Fe(V) oxidation, while it has minimal impact on the reactivity of Fe(VI) and Fe(IV). Based on ligand field theory, pyrophosphate complexation can induce iron 3d orbital resplitting, leading to spin electron rearrangement. Specifically, the hexa-coordinated Fe(V)-oxo complex ligated by pyrophosphate exhibits higher orbital energy, reducing its stability and effective collisions with contaminants, whereas, the potential Jahn-Teller distortion of the Fe(IV)-oxo complex could enhance its stability and preserve its significant reactivity. Given its selective inhibition of Fe(V) oxidation, pyrophosphate can emerge as a promising targeted quenching agent for Fe(V) species. This study provides valuable theoretical insights to guide the identification and characterization of intermediate iron species in iron-based oxidation processes.

Phenols coupled during oxidation upstream of water treatment would generate higher toxic coupling phenolic disinfection by-products during chlorination disinfection

Phenolic compounds usually produce coupling phenols during the upstream oxidation treatment of water, but the disinfection by-products (DBPs) of coupling phenols in the subsequent disinfection process have been overlooked. Herein, we demonstrated the generation and higher toxicity of these DBPs. In Songhua River water, 0.355 ng/L 4-(4-bromophenoxy)phenol and 122.67 ng/L phenol were detected. Pre-oxidation with K2FeO4 and KMnO4 resulted in the formation of 0.68 ng/L and 0.506 ng/L 4-(4-iodophenoxy)phenol in subsequent disinfection, respectively, which were 2-3 times that without pre-oxidation. Coupling of phenolic compounds during pre-oxidation and then halogenated during chlorination was shown to be the main pathway for the generation of coupling phenolic DBPs. The maximum rate constants for the reactions of hypochlorous acid with phenol and its coupling products (4-phenoxyphenol, 2,2'-biphenol, and [1,1'-biphenyl]-2,4'-diol) were 115.61 M-1s-1, 65.85 M-1s-1, 143.13 M-1s-1, and 212.52 M-1s-1, respectively, with 2,2'-biphenol and [1,1'-biphenyl]-2,4'-diol breaking through lower energy barriers and releasing more energy than phenol. This indicated coupling phenols have a higher potential to form DBPs. Additionally, coupling phenolic DBPs (4-(4-chlorophenoxy)phenol, 4-(4-bromophenoxy)phenol, and 4-(4-iodophenoxy)phenol) was 1-3 orders of magnitude more toxic than their precursors (4-phenoxyphenol, 2,2'-biphenol, [1,1'-biphenyl]-2,4'-diol, and phenol) and uncoupled DBPs (4-iodophenol). Therefore, the formation and hazards of coupling phenolic DBPs require more attention.

Freeze-induced acceleration of iodide oxidation and consequent iodination of dissolved organic matter to form organoiodine compounds

Freeze-induced acceleration of I- oxidation and the consequent iodination of dissolved organic matter (DOM) contribute to the formation of organoiodine compounds (OICs) in cold regions. The formed OICs may be a potentially important source of risk and are very closely with the environment and human health. Herein, we investigated the acceleration effects of the freeze process on I- oxidation and the formation of OICs. In comparison to reactive iodine species (RIS) formed in aqueous solutions, I- oxidation and RIS formation were greatly enhanced in frozen solution and were affected by pH, and the content of I- and O2. Freeze-thaw process further promoted I- oxidation and the concentration of RIS reached 45.7 µmol/L after 6 freeze-thaw cycles. The consequent products of DOM iodination were greatly promoted in terms of both concentration and number. The total content of OICs ranged from 0.02 to 2.83 µmol/L under various conditions. About 183-1197 OICs were detected by Fourier transform ion cyclotron resonance mass spectrometry, and more than 96.2% contained one or two iodine atoms. Most OICs had aromatic structures and were formed via substitution and addition reactions. Our findings reveal an important formation pathway for OICs and shed light on the biogeochemical cycling of iodine in the natural aquatic environment.

Rapid and Highly Selective Fe(IV) Generation by Fe(II)-Peroxyacid Advanced Oxidation Processes: Mechanistic Investigation via Kinetics and Density Functional Theory

High-valent iron (Fe(IV/V/VI)) has been widely applied in water decontamination. However, common Fe(II)-activating oxidants including hydrogen peroxide (H2O2) and persulfate react slowly with Fe(II) and exhibit low selectivity for Fe(IV) production due to the cogeneration of radicals. Herein, we report peroxyacids (POAs; R-C(O)OOH) that can react with Fe(II) more than 3 orders of magnitude faster than H2O2, with high selectivity for Fe(IV) generation. Rapid degradation of bisphenol A (BPA, an endocrine disruptor) was achieved by the combination of Fe(II) with performic acid (PFA), peracetic acid (PAA), or perpropionic acid (PPA) within one second. Experiments with phenyl methyl sulfoxide (PMSO) and tert-butyl alcohol (TBA) revealed Fe(IV) as the major reactive species in all three Fe(II)-POA systems, with a minor contribution of radicals (i.e., •OH and R-C(O)O•). To understand the exceptionally high reactivity of POAs, a detailed computational comparison among the Fenton-like reactions with step-by-step thermodynamic evaluation was conducted. The high reactivity is attributed to the lower energy barriers for O-O bond cleavage, which is determined as the rate-limiting step for the Fenton-like reactions, and the thermodynamically favorable bidentate binding pathway of POA with iron. Overall, this study advances knowledge on POAs as novel Fenton-like reagents and sheds light on computational chemistry for these systems.

A novel visible light-driven oxygen doped C3N4/Bi12O17Cl2/ferrate(VI) system for Bisphenol A degradation: Radical and nonradical pathways

In this study, visible light-activated photocatalyst oxygen-doped C3N4@Bi12O17Cl2 (OCN@BOC) and Fe(VI) coupling system was proposed for the efficient degradation of bisphenol A (BPA). The comprehensive characterization of the OCN@BOC photocatalyst revealed its excellent photogenerated carrier separation rate in heterogeneous structures. The OCN@BOC/Fe(VI)/Vis system exhibited a remarkable BPA removal efficiency of over 84% within 5 min. Comparatively, only 37% and 59% of BPA were degraded by single OCN@BOC and Fe(VI) in 5 min, respectively. Reactive species scavenging experiments, phenyl sulfoxide transformation experiments, and electron paramagnetic resonance experiments confirmed the involvement of superoxide radicals (⋅O2-), singlet oxygen (1O2), as well as iron(V)/iron(IV) (Fe(V)/Fe(IV)) species in the degradation process of BPA. Furthermore, density functional theoretical calculations and identification of intermediates provided insights into the potential degradation mechanism of BPA during these reactions. Additionally, simulation evaluations using an ecological structure activity relationship model demonstrated that the toxicity of BPA to the ecological environment was mitigated during its degradation process. This study presented a novel strategy for removing BPA utilizing visible light photocatalysts, highlighting promising applications for practical water environment remediation with the OCN@BOC/Fe(VI)/Vis system.

Revisiting the Electron Transfer Mechanisms in Ru(III)-Mediated Advanced Oxidation Processes with Peroxyacids and Ferrate(VI)

The potential of Ru(III)-mediated advanced oxidation processes has attracted attention due to the recyclable catalysis, high efficiency at circumneutral pHs, and robust resistance against background anions (e.g., phosphate). However, the reactive species in Ru(III)-peracetic acid (PAA) and Ru(III)-ferrate(VI) (FeO42-) systems have not been rigorously examined and were tentatively attributed to organic radicals (CH3C(O)O•/CH3C(O)OO•) and Fe(IV)/Ru(V), representing single electron transfer (SET) and double electron transfer (DET) mechanisms, respectively. Herein, the reaction mechanisms of both systems were investigated by chemical probes, stoichiometry, and electrochemical analysis, revealing different reaction pathways. The negligible contribution of hydroxyl (HO•) and organic (CH3C(O)O•/CH3C(O)OO•) radicals in the Ru(III)-PAA system clearly indicated a DET reaction via oxygen atom transfer (OAT) that produces Ru(V) as the only reactive species. Further, the Ru(III)-performic acid (PFA) system exhibited a similar OAT oxidation mechanism and efficiency. In contrast, the 1:2 stoichiometry and negligible Fe(IV) formation suggested the SET reaction between Ru(III) and ferrate(VI), generating Ru(IV), Ru(V), and Fe(V) as reactive species for micropollutant abatement. Despite the slower oxidation rate constant (kinetically modeled), Ru(V) could contribute comparably as Fe(V) to oxidation due to its higher steady-state concentration. These reaction mechanisms are distinctly different from the previous studies and provide new mechanistic insights into Ru chemistry and Ru(III)-based AOPs.

Ferrate(VI)/percarbonate for the oxidation of micropollutants: Interactive activation and release of low-concentration hydrogen peroxide for efficient electron utilization

A novel "ferrate/percarbonate (Fe(VI)/SPC) co-oxidation process" was used to treat ciprofloxacin (CIP) and various micropollutants (MPs), which owned better performance than mixture of Fe(VI), Na2CO3 and H2O2. The mechanism investigation found that the low-concentration H2O2 (1-2 µM) released by SPC can promote the high-valent iron intermediates (Fe(IV)/Fe(V)) of Fe(VI) to the MP oxidation, and Fe(VI) products can also activate SPC to produce hydroxyl radical (·OH). The interactive activation of Fe(VI) and SPC was realized, which retained the high selectivity of Fe(VI) to electron-rich pollutants, and also made up the oxidation of electron-deficient pollutants through •OH, improving the degradation effect of various MPs by 20-30%, and the rate constant was increased by 1 to 3 times. Moreover, non-purgeable organic carbon (NPOC) determination confirmed that •OH participation reduced the NPOC value of CIP from 5.43 mg/L to 4.37 mg/L. The transformation pathway of CIP showed that Fe(VI)/SPC resulted in more hydroxylation intermediates of CIP than Fe(VI) alone. Acute toxicity assays found that the photoinhibition rate of CIP treated with Fe(VI) alone was 14.5%, while the sample treated with Fe(VI)/SPC showed no significant photoinhibition effect, which proved that the new process had good detoxification properties for CIP.

Ferrate(VI) assists in reducing cytotoxicity and genotoxicity to mammalian cells and organic bromine formation in ozonated wastewater

Ozonation of wastewater containing bromide (Br-) forms highly toxic organic bromine. The effectiveness of ozonation in mitigating wastewater toxicity is minimal. Simultaneous application of ozone (O3) (5 mg/L) and ferrate(VI) (Fe(VI)) (10 mg-Fe/L) reduced cytotoxicity and genotoxicity towards mammalian cells by 39.8% and 71.1% (pH 7.0), respectively, when the wastewater has low levels of Br-. This enhanced reduction in toxicity can be attributed to increased production of reactive iron species Fe(IV)/Fe(V) and reactive oxygen species (•OH) that possess higher oxidizing ability. When wastewater contains 2 mg/L Br-, ozonation increased cytotoxicity and genotoxicity by 168%-180% and 150%-155%, respectively, primarily due to the formation of organic bromine. However, O3/Fe(VI) significantly (p < 0.05) suppressed both total organic bromine (TOBr), BrO3-, as well as their associated toxicity. Electron donating capacity (EDC) measurement and precursor inference using Orbitrap ultra-high resolution mass spectrometry found that Fe(IV)/Fe(V) and •OH enhanced EDC removal from precursors present in wastewater, inhibiting electrophilic substitution and electrophilic addition reactions that lead to organic bromine formation. Additionally, HOBr quenched by self-decomposition-produced H2O2 from Fe(VI) also inhibits TOBr formation along with its associated toxicity. The adsorption of Fe(III) flocs resulting from Fe(VI) decomposition contributes only minimally to reducing toxicity. Compared to ozonation alone, integration of Fe(VI) with O3 offers improved safety for treating wastewater with varying concentrations of Br-.

Wastewater Treatment Using Membrane Bioreactor Technologies: Removal of Phenolic Contaminants from Oil and Coal Refineries and Pharmaceutical Industries

Huge amounts of noxious chemicals from coal and petrochemical refineries and pharmaceutical industries are released into water bodies. These chemicals are highly toxic and cause adverse effects on both aquatic and terrestrial life. The removal of hazardous contaminants from industrial effluents is expensive and environmentally driven. The majority of the technologies applied nowadays for the removal of phenols and other contaminants are based on physio-chemical processes such as solvent extraction, chemical precipitation, and adsorption. The removal efficiency of toxic chemicals, especially phenols, is low with these technologies when the concentrations are very low. Furthermore, the major drawbacks of these technologies are the high operation costs and inadequate selectivity. To overcome these limitations, researchers are applying biological and membrane technologies together, which are gaining more attention because of their ease of use, high selectivity, and effectiveness. In the present review, the microbial degradation of phenolics in combination with intensified membrane bioreactors (MBRs) has been discussed. Important factors, including the origin and mode of phenols' biodegradation as well as the characteristics of the membrane bioreactors for the optimal removal of phenolic contaminants from industrial effluents are considered. The modifications of MBRs for the removal of phenols from various wastewater sources have also been addressed in this review article. The economic analysis on the cost and benefits of MBR technology compared with conventional wastewater treatments is discussed extensively.

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.

pH-dependent bisphenol A transformation and iodine disinfection byproduct generation by peracetic acid: Kinetic and mechanistic explorations

Peracetic acid (PAA) is regarded as an environmentally friendly oxidant because of its low formation of toxic byproducts. However, this study revealed the potential risk of generating disinfection byproducts (DBPs) when treating iodine-containing wastewater with PAA. The transformation efficiency of bisphenol A (BPA), a commonly detected phenolic contaminant and a surrogate for phenolic moieties in dissolved organic matter, by PAA increased rapidly in the presence of I-, which was primarily attributed to the formation of active iodine (HOI/I2) in the system. Kinetic model simulations demonstrated that the second-order rate constant between PAA and HOI was 54.0 M-1 s-1 at pH 7.0, which was lower than the generation rate of HOI via the reaction between PAA and I-. Therefore, HOI can combine with BPA to produce iodine disinfection byproducts (I-DBPs). The transformation of BPA and the generation of I-DBPs in the I-/PAA system were highly pH-dependent. Specifically, acidic conditions were more favorable for BPA degradation because of the higher reaction rates of BPA and HOI. More iodinated aromatic products were detected after 5 min of the reaction under acidic and neutral conditions, resulting in higher toxicity towards E. coli. After 12 h of the reaction, more adsorbable organic iodine (AOI) was generated at alkaline conditions because HOI was not able to efficiency transform to IO3-. The presence of H2O2 in the PAA solution played a role in the reaction with HOI, particularly under alkaline conditions. This study significantly advances the understanding of the role of I- in BPA oxidation by PAA and provides a warning to further evaluate the potential environmental risk during the treatment of iodine-bearing wastewater with PAA.

High efficiency removal of organic and inorganic iodine with ferrate[Fe(VI)] through oxidation and adsorption

I- is a halogen species existing in natural waters, and the transformation of organic and inorganic iodine in natural and artificial processes would impact the quality of drinking water. Herein, it was found that Fe(VI) could oxidize organic and inorganic iodine to IO3-and simultaneously remove the resulted IO3- through Fe(III) particles. For the river water, wastewater treatment plant (WWTP) effluent, and shale gas wastewater treated by 5 mg/L of Fe(VI) (as Fe), around 63 %, 55 % and 71 % of total iodine (total-I) had been removed within 10 min, respectively. Fe(VI) was superior to coagulants in removing organic and inorganic iodine from the source water. Adsorption kinetic analysis suggested that the equilibrium adsorption amount of I- and IO3- were 11 and 10.1 μg/mg, respectively, and the maximum adsorption capacity of IO3- by Fe(VI) resulted Fe(III) particles was as high as 514.7 μg/mg. The heterogeneous transformation of Fe(VI) into Fe(III) effectively improved the interaction probability of IO3- with iron species. Density functional theory (DFT) calculation suggested that the IO3- was mainly adsorbed in the cavity (between the γ-FeOOH shell and γ-Fe2O3 core) of Fe(III) particles through electrostatic adsorption, van der Waals force and hydrogen bond. Fe(VI) treatment is effective for inhibiting the formation of iodinated disinfection by-products in chlor(am)inated source water.

Key role of Fe(VI)-activated Bi2WO6 in the photocatalytic oxidation of sulfonamides: Mediated electron transfer mechanism

The widespread use of sulfonamides (SAs) in animals and human infections has raised significant concerns regarding their presence in ambient waterways and potential for inducing antimicrobial resistance. Herein, we report on the capacity of ferrate (VI) (Fe^VIO₄^2-, Fe(VI)) to facilitate the photocatalytic degradation of sulfamethazine (SMT) via bismuth tungstate (Bi₂WO₆, BWO) under blue LED light (Vis/BWO/Fe(VI)) exposure, at rates that were 45-fold faster than BWO photocatalysis. Both the stepwise and time-series addition of Fe(VI) contributed to the degradation. Multiple lines of evidence confirmed that the common reactive species (RSs) in BWO-based photocatalytic systems and Fe(VI)-involved systems (e.g., •OH/h⁺, O₂^•-, ¹O₂ and Fe(V)/Fe(IV)) played subtle roles in our study system. Herein, for the first time, it was discovered that the precursor complex (BWO-Fe(V)/Fe(IV)* )) was the main contributor to induce electron transfer of SAs through the "conductive bridge" effect of BWO. The studied system was able to effectively degrade SMT in synthetic hydrolyzed urine (SHU) with low interference from background substances in water. This work not only offers a novel facilitation strategy for BWO, but also holds a great application prospect for contamination remediation in urine.

Occurrence of Iodophenols in Aquatic Environments and the Deiodination of Organic Iodine with Ferrate(VI)

Toxic and odorous iodophenols are commonly identified as disinfection by-products (DBPs) in drinking water. Herein, ng/L levels of iodophenols were identified in river water, wastewater treatment plant effluent, and medical wastewater, with the simultaneous identification of μg/L to mg/L levels of iodide (I^-) and total organic iodine (TOI). Oxidation experiment suggested that the I^-, TOI, and iodophenols could be oxidized by ferrate [Fe(VI)], and more than 97% of TOI had been transformed into stable and nontoxic IO₃^-. Fe(VI) initially cleaved the C-I bond of iodophenols and led to the deiodination of iodophenols. The resulted I^- was swiftly oxidized into HOI and IO₃^-, with the intermediate phenolic products be further oxidized into lower molecular weight products. The Gibbs free energy change (ΔG) of the overall reaction was negative, indicating that the deiodination of iodophenols by Fe(VI) was spontaneous. In the disinfection of iodine-containing river water, ng/L levels of iodophenols and chloro-iodophenols formed in the reaction with NaClO/NH₂Cl, while Fe(VI) preoxidation was effective for inhibiting the formation of iodinated DBPs. Fe(VI) exhibited multiple functions for oxidizing organic iodine, abating their acute toxicity/cytotoxicity and controlling the formation of iodinated DBPs for the treatment of iodide/organic iodine-containing waters.

Enhanced removal of ammonia in Fe(VI)/Br- oxidation system: Kinetics, transformation mechanism and theoretical calculations

This work systematically examined the capability of ferrate (Fe(VI)) for ammonia oxidation, revealing for the first time that bromide ions (Br^-) played an important role in promoting the removal of ammonia in Fe(VI) system. In the presence of 10.0 mM Br^-, the removal efficiency of ammonia was nearly 3.4 times that of the control, and 1.0 mM ammonia was almost completely removed after two rounds addition of 1.0 mM Fe(VI) in 60 min. PMSO probe test, electron paramagnetic resonance spectra and radical quenching experiments were employed to interpret the underlying promotion mechanism of Br^-, and it was proposed that the formation of active bromine (HOBr/OBr^-) played a dominant role in the enhanced oxidative removal of ammonia by Fe(VI). Further kinetic model simulations revealed that HOBr/OBr^- and Fe(VI) were the two major reactive species in Fe(VI)/Br^- system, accounting for 66.7% and 33.0% of ammonia removal, respectively. As the target contaminant, ammonia could quickly consume the generated HOBr/OBr^-, thereby suppressing the formation of brominated disinfection byproducts. Finally, NO₃^- was identified as the dominant transformation product of ammonia, and density functional theory (DFT) calculations revealed that six reaction stages were involved in ammonia oxidation with the first step as the rate-limiting step. This work would enable the full use of coexisting bromides for effective removal of ammonia from natural waters or wastewaters by in situ Fe(VI) oxidation method.

Degradation of Organic Contaminants by Reactive Iron/Manganese Species: Progress and Challenges

Many iron(II, III, VI)- and manganese(II, IV, VII)-based oxidation processes can generate reactive iron/manganese species (RFeS/RMnS, i.e., Fe(IV)/Fe(V) and Mn(III)/Mn(V)/Mn(VI)), which have mild and selective reactivity toward a wide range of organic contaminants, and thus have drawn significant attention. The reaction mechanisms of these processes are rather complicated due to the simultaneous involvement of multiple radical and/or nonradical species. As a result, the ambiguity in the occurrence of RFeS/RMnS and divergence in the degradation mechanisms of trace organic contaminants in the presence of RFeS/RMnS exist in literature. In order to improve the critical understanding of the RFeS/RMnS-mediated oxidation processes, the detection methods of RFeS/RMnS and their roles in the destruction of trace organic contaminants are reviewed with special attention to some specific problems related to the scavenger and probe selection and experimental results analysis potentially resulting in some questionable conclusions. Moreover, the influence of background constituents, such as organic matter and halides, on oxidation efficiency of RFeS/RMnS-mediated oxidation processes and formation of byproducts are discussed through their comparison with those in free radicals-dominated oxidation processes. Finally, the prospects of the RFeS/RMnS-mediated oxidation processes and the challenges for future applications are presented.

Formation of iodinated aromatic DBPs at different molar ratios of chlorine and nitrogen in iodide-containing water

Variations in iodinated aromatic disinfection byproducts (DBPs) in the presence of I^- and organic compounds as a function of reaction time in different molar ratios (MRs) of HOCl:NH₃-N were investigated. Up to 17 kinds of iodinated aromatic DBPs were identified in the breakpoint chlorination of iodide (I^-)/organic (phenol, bisphenol S (BPS) and p-nitrophenol (p-NP)) systems, and the possible pathways for the formation of iodinated aromatic DBPs were proposed. The reaction pathways include HOCl/HOI electrophilic substitution and oxidation, while the dominant iodinated DBPs were quantified. In the I^-/phenol system (pH = 7.0), the sum of the concentrations of four iodinated aliphatic DBPs ranged from 0.32 to 1.04 μM (triiodomethane (TIM), dichloroiodomethane (DCIM), diiodochloromethane (DICM) and monoiodoacetic acid (MIAA)), while the concentration of 4-iodophenol ranged from 2.99 to 12.87 μM. The concentration of iodinated aromatic DBPs remained stable with an MR = 1:1. When the MR was 6:1, iodinated aromatic DBPs decreased with increasing reaction time, in which the main disinfectant in the system was active chlorine. This study proposed the formation mechanism of iodinated aromatic DBPs during the breakpoint chlorination of iodide-containing water. These results can be used to control the formation of hazardous iodinated aromatic DBPs in the disinfection of iodine containing water.

Mechanism insight into the role of clay particles on enhancing phosphate removal by ferrate compared with ferric salt

The application of ferrate (Fe(VI)) and ferric chloride as coagulants for treating phosphate wastewater in the presence of kaolin clay particles was comparatively studied. The phosphate removal processes by ferrate and ferric chloride assisted with kaolin clay particles were investigated under different Fe/P molar ratios. At neutral pH, complete removal of phosphates by ferrate and ferric chloride was observed at 2:1 and 6:1 of Fe/P molar ratio, respectively. The effect of kaolin clay particles on the phosphate removal process was discussed by zeta potential, size particle distribution, FTIR and XPS. We showed that with the increase of Fe/P molar ratio, the interaction intensity of kaolin clay particles with Fe flocs was decreased by ferric chloride coagulation while firstly increased and then decreased by ferrate. This depends on the Fe species with positive charge from ferric chloride hydrolysis and ferrate decomposition. Phosphate can inhibit the formation of FeOH²⁺ and Fe(OH)₂⁺ in the ferric chloride hydrolysis but promote the formation of FeOOH and Fe(OH)₂⁺ in the ferrate decomposition. Kaolin clay particles can more remarkably promote phosphate removal by ferrate than by ferric chloride.

Straw biochar enhanced removal of heavy metal by ferrate

This study demonstrated that As(III) was appreciably removed by ferrate in the presence of straw biochar. Removal efficiency of As in ferrate/biochar system was over 91%, increased by 34% compared with ferrate alone ([biochar]₀ = 10 mg/L, [ferrate]₀ = 6 mg/L, [As(III)]₀ = 200 μg/L). In the reaction process, As(III) was oxidized to As(V) mainly by ferrate, while ferrate was reduced into ferric (hydr)oxides and coated on the biochar. Biochar was oxidized in the reaction and its surface area, pore volume and the amount of Lewis acid functional groups were substantially improved, which provided interaction sites for As adsorption. Analysis of hydrodynamic diameter and zeta potential revealed that biochar interacted with the ferrate resulted ferric oxides and enlarged the Fe-C-As particle/floc, which promoted their settlement and thus the liquid-solid separation of As. As(V) was adsorbed on the surface of biochar and ferric (hydr)oxides through hydrogen bond, electrostatic attraction and As-(OFe) bond. Ferrate/biochar was not only effective for As removal, but removed 73.31% of As, 50.38% of Cd, and 75.27% of Tl when these hazardous species synchronously existed in polluted water (initial content: As, 100 μg/L; Cd, 50 μg/L; Tl, 1 μg/L). The combination of ferrate with biochar has potential for the remediation of hazardous species polluted water.

Iodinated trihalomethanes formation in iopamidol-contained water during ferrate/chlor(am)ination treatment

Iopamidol is a commonly used iodinated X-ray contrast media in medical field, and its residue in water can react with disinfectants to form highly toxic iodinated disinfection by-products (I-DBPs). This study investigated the degradation of iopamidol and formation of DBPs, especially iodinated trihalomethanes (I-THMs), during ferrate (Fe(VI)) pre-oxidation and subsequent chlor(am)ination under raw water background. It was found that iopamidol degradation efficiency in raw water by Fe(VI) at pH 9 could reach about 80%, which was much higher than that at pH 5 and pH 7 (both about 25%). With Fe(VI) dose increasing, iopamidol removal efficiency increased obviously. During the iopamidol degradation by Fe(VI), IO₃^- was the dominant product among all the iodine species. After pre-treated by Fe(VI), yields of THM4 and I-THMs can be reduced in subsequent chlor(am)ination. Besides, pH was a crucial factor for Fe(VI) pre-oxidition controlling DBPs. With the pH increasing from 5 to 9, the yield of THM4 kept increasing in subsequent chlorination but showed the highest amount at pH 6 in subsequent chloramination. The yield of I-THMs increased first and then decreased with the increase of pH in both subsequent chlorination and chloramination. I-THM concentrations in chlorinated samples were lower than chloraminated ones under acidic conditions but became higher under neutral and alkaline conditions. The total CTI of THMs during Fe(VI)-chloramination was higher than that during Fe(VI)-chlorination under neutral condition, but sharply decreased under alkaline conditions. In summary, Fe(VI)-chloramination subsequent treatment under alkaline conditions should be an effective method for iopamidol removal and DBP control.

How Much of the Total Organic Halogen and Developmental Toxicity of Chlorinated Drinking Water Might Be Attributed to Aromatic Halogenated DBPs?

Although >700 disinfection byproducts (DBPs) have been identified, >50% of the total organic halogen (TOX) in drinking water chlorination is unknown, and the DBPs responsible for the chlorination-associated health risks remain largely unclear. Recent studies have revealed numerous aromatic halo-DBPs, which generally present substantially higher developmental toxicity than aliphatic halo-DBPs. This raises a fascinating and important question: how much of the TOX and developmental toxicity of chlorinated drinking water can be attributed to aromatic halo-DBPs? In this study, an effective approach with ultraperformance liquid chromatography was developed to separate the DBP mixture (from chlorination of bromide-rich raw water) into aliphatic and aromatic fractions, which were then characterized for their TOX and developmental toxicity. For chlorine contact times of 0.25-72 h, aromatic fractions accounted for 49-67% of the TOX in the obtained aliphatic and aromatic fractions, which were equivalent to 26-36% of the TOX in the original chlorinated water samples. Aromatic halo-DBP fractions were more developmentally toxic than the corresponding aliphatic fractions, and the overall developmental toxicity of chlorinated water samples was dominated by aromatic halo-DBP fractions. This might be explained by the considerably higher potentials of aromatic halo-DBPs to bioconcentrate and then generate reactive oxygen species in the organism.

Chlorination decreases acute toxicity of iodophenols through the formation of iodate and chlorinated aliphatic disinfection byproducts

Highly toxic iodinated phenolic by-products were frequently detected in the oxidative treatment and disinfection of iodine-containing water. Herein, it was found that three model iodinated phenolic disinfection byproducts (DBPs), 2-iodophenol, 4-iodophenol and 2,4,6-triiodophenol, were reactive with HOCl, and the reaction rate constants (at pH 7.0 and 25℃) were 1.86 ×10², 1.62 ×10² and 7.5 ×10¹ M^-1s^-1, respectively. When HOCl was in excess (HOCl/iodophenol = 40/1, [iodophenol]₀ = 20 μM), acute toxicity of water sample containing iodophenols could be largely eliminated (> 85%), with the conversion of iodophenols into stable and non-toxic iodate (IO₃^-) and iodinated and chlorinated aliphatic DBPs. Besides IO₃^-, seven kinds of aromatic intermediate products including iodophenols, chloroiodophenols, iodoquinones, chloroiodoquinones, chloroquinones, chlorophenols, and coupling products were detected. C-I bond of iodophenols was cleaved in the reaction and the resulted aromatic products were further transformed into chlorinated aliphatic DBPs [trichloromethane (TCM), trichloroacetic acid (TCAA), dichloroacetic acid (DCAA), and chloral hydrate (CH)] (mg/L level) and iodinated trihalomethanes (μg/L level). HOCl was effective for converting iodophenols into IO₃^- and less toxic chlorinated aliphatic DBPs. Considering that chlorine was widely used as disinfectant, transformation and toxicity alteration of emerging DBPs during chlorination/booster chlorination warrant further investigations.

Ferrate(VI) pretreatment of water containing natural organic matter, bromide, and iodide: A potential strategy to control soluble lead release from PbO2(s)

Lead dioxide (PbO₂(s)) is a corrosion product of lead-containing plumbing materials in water distribution pipelines. The presence of reductants in water could cause the release of soluble lead (mainly Pb(II)) from PbO₂(s). Lead in drinking water is detrimental to public health. This paper presents the first application of ferrate (Fe^VIO₄^2-, Fe(VI)) to decreasing the generation of soluble lead in water containing PbO₂(s) and common reducing constituents (e.g., natural organic matter (NOM), iodide (I^-), and bromide (Br^-)) at different pH conditions (i.e., 6.0, 7.0, and 8.0). The released soluble lead from PbO₂(s) was found to be dominantly controlled by NOM in water, via the redox dissolution of PbO₂(s) and the reduction of PbO₂(s) by reducing moieties of NOM. The feasibility of both processes increased when pH decreased. The I^- and Br^- in water played minor roles in generating soluble lead. Fe(VI) reacted with reducing functional groups of NOM, as determined by ¹³C nuclear magnetic resonance spectroscopy. Water pretreatment with Fe(VI) inhibited the reaction of NOM with PbO₂(s) and therefore, caused lower soluble lead concentrations compared to water samples without Fe(VI) treatment. This study indicates that Fe(VI) pretreatment is a potential approach to controlling soluble lead in drinking water.