Formation Of Chlorine Research Articles

Understanding Breakpoint Chlorination during in-Situ Electrochlorination: Influence of Mass Transport

Based on the United Nations of 2023, approximately 26% of people lack access to reliably managed drinking water services, and around 44% of water worldwide is not safely treated before releasing into the environment. Water scarcity is induced not only by impact of water stresses, but also by rapid spreading of freshwater pollution. Decentralized electrochemical water treatment technologies are becoming a sustainable and easily scalable solution for water treatment in areas experiencing water stresses, especially when powered by alternative energy sources. Electrochemical water treatment has received profound interest among researchers due to its high efficiency in oxidizing wide range of pharmaceuticals, disinfecting water from pathogens, reducing nitrate content, and removing ammonia. The electrochemical oxidation of ammonia is particularly interesting because ammonia is a widely common species in water. Dealing with high level of ammonia, either from initially contaminated water or water with elevated levels of ammonia generated during other water treatment processes presents severe challenges. Indirect electrochemical oxidation of ammonia can become a feasible solution that exploits the unavoidable oxidation of chloride ions (ubiquitous in all types of waters) to chlorine.Despite indirect ammonia oxidation being present in many research works that utilize real water matrices, the mechanism of electrochemical breakpoint chlorination still requires further research efforts. This study aimed to examine the mechanism of breakpoint chlorination during electrochemical generation of chlorine active species and determine the main differences when comparing with homogeneous ammonia oxidation. A dimensional stable anode of Ti/IrO2, identified as one of the most selective anodes for chlorine evolution, was used. The study found that operating parameters influenced generation of intermediates and products. The difference in current density did not impact significantly on the species formation and mechanism. One of the most interesting findings revealed the significant role of operating hydraulic conditions on the electrochemical breakpoint chlorination process. Contradictory to many electrochemical water treatment reactions, electrochemical breakpoint chlorination under low mass transport conditions in batch reactor resulted in more efficient reaction performance. Two reasons were proposed that could justify the observation including (i) higher chlorine species availability on the anode surface that resulted to the elevated chlorine to ammonia ratio leading to faster ammonia oxidation, and (ii) decreased mass transport that led to formation of thicker diffusion layer on the anode surface with a low pH profile resulting in more efficient chlorine evolution reaction. The Pourbaix diagram of chlorine species formation indicates more effective chlorine evolution under lower pH conditions. The experiments with only chlorine evolution reaction without ammonia present confirmed that lower mass transport in the reactor led to more efficient chlorine evolution performance. Cyclic voltammograms confirmed more effective chlorine evolution at lower water pH. In addition, there was a significant difference between the electrochemical indirect ammonia oxidation and homogeneous breakpoint chlorination. Results showed that electrochemical treatment resulted in negligible chlorine and monochloramine formation, and simultaneous pH decrease due to immediate ammonia oxidation, which differs from homogeneous breakpoint chlorination. Heterogeneous chlorine availability on the anode surface during electrochemical reaction and corresponding local ammonia oxidation due to high chlorine to ammonia ratio suggest a more environmentally friendly approach when comparing with homogeneous oxidation. This might result to lower number of halogenated compounds in water due to electrochemical breakpoint chlorination resulted in diminutive chlorine species presence in the bulk solution until breakpoint was achieved. These findings opened a new perspective on electrochemical breakpoint chlorination. In addition, faster and more economically feasible ammonia oxidation might be achieved by lowering mass transport during chlorine evolution to maintain more acidic pH on the anode surface. The saddle point of the most efficient electrochemical breakpoint chlorination might be found through the mass transport optimization. Figure 1

Consequences of weakening of dynamic barrier of the Arctic polar vortex

The dynamic barrier is one of the physical characteristics of the polar vortices; it prevents subpolar air masses from penetrating into the vortex and contributes to a temperature decrease inside the vortex in the lower stratosphere. In the presence of a dynamic barrier in winter, chlorine compounds involved in the ozone destruction cycle accumulate on particles of polar stratospheric clouds (PSCs) and heterogeneous reactions occur with the formation of molecular chlorine, and with the appearance of solar radiation over the polar region, photochemical reactions begin, leading to large-scale ozone depletion. When the dynamic barrier is weakened in winter, the temperature inside the vortex rises, PSC melts and, thus, the accumulation of chlorine cycle reagents on PSC is interrupted. We proposed dividing the Arctic polar vortex dynamics into 3 types according to the consequences: (1) the strong vortex, whose activity results in ozone depletion, (2) the weak vortex with breakdown in winter, marked by a sudden stratospheric warming, and (3) the stable vortex with an episode (episodes) weakening of the dynamic barrier in winter without ozone depletion in the period from late winter to spring. We have for the first time proposed a characteristic of the dynamic barrier of the polar vortex at all pressure levels from 100 to 1 hPa and described the consequences of its weakening. Using the vortex delineation method based on the data of the ERA5 and MERRA-2 reanalyses, we showed that in all cases when the polar ozone depletion was not recorded from late winter to spring under the conditions of the stable polar vortex, the dynamic barrier weakening and PSС melting was observed in midwinter.

Immobilization of heavy metals with chlorine and liquid phase formation during thermal treatment of MSWI fly ash

Due to effective detoxification, thermal treatment is a preferred treatment technology of municipal solid waste incineration (MSWI) fly ash. However, immobilization behavior of heavy metals during thermal treatment is unclear. In this work, a circulated fluidized bed fly ash with SiO2 addition and a grate furnace fly ash with high silica-aluminum coal ash addition were used to investigate combined effect of active chlorine content, liquid polymerization degree, temperature of initial liquid phase formation and liquid phase formation rate on the immobilization ratios of Zn, Pb and Cd by principle component analysis (PCA). The results show that the decrease in active chlorine content favors the immobilization of Zn, Pb and Cd because chlorination volatilization is reduced. Both the increase in liquid polymerization degree and the formation of initial liquid phase at low temperature promote the immobilization of Zn, Pb and Cd for physical encapsulation. However, a rapid formation of liquid phase is not favorable for the immobilization of Zn, Pb and Cd, because transfer of heavy metals between crystals and liquid phase is impeded by high viscosity of liquid phase during fusion. The results can provide a reference for how to achieve the immobilization of heavy metals by adjusting the chemical composition of MSWI fly ash during thermal treatment.

Solar-driven chloride activation via 3D oxygen-vacancy-rich TiO2 network photoanode for ultrafast and durable purification of saline wastewater

Photoelectrochemical chloride activation (PEC-Cl) is a promising and reagent-free strategy to treat saline organic wastewater by activation of chloride (Cl–) using sunlight. However, the low efficiency and Cl– corrosion of photoelectrode, and the limited mass transfer of pollutant hinder its application. Herein we successfully solve these challenges by developing porous oxygen-vacancy-rich TiO2 nanoneedle arrays (TiO2 NNs-r400) photoanode. Under illumination, the photoinduced holes in TiO2 NNs-r400 oxidize Cl– to reactive species (e.g., HOCl, Cl•, Cl2•− and ClO•), resulting in the degradation rate of tetracycline hydrochloride (HTC, 0.861 min−1) by TiO2 NNs-r400/PEC-Cl system is 2–3 orders of magnitude higher than the state-of-the-art. Experiments and computational analysis indicate that oxygen vacancies (Ovs) on TiO2 NNs-r400 promote the photocarriers separation efficiency and more importantly, facilitate Cl– adsorption with intermediary strength and reduce the energy barriers of Cl– oxidation. Furthermore, we design a continuous flow-through PEC reactor for TiO2 NNs-r400/PEC-Cl technology. The process exhibits 100% HTC degradation (TOC = 60.23%) for over 2000 h flow-through operation, which maintains high activity in the treatment marine aquaculture wastewater with nearly zero energy consumption by directly utilize natural sunlight. In the final effluent, the toxicity of almost all degradation intermediates is decreased, and the formation of total organic chlorine is low (17.2 µg L–1). This work provides insights into Cl– activation on Ov-rich photoanode and strategy for practical application of solar energy in efficient, sustainable and eco-friendly wastewater decontamination.

Unraveling the competition between the oxygen and chlorine evolution reactions in seawater electrolysis: Enhancing selectivity for green hydrogen production

Selective oxidation of water without production of chlorine during the electrolysis of seawater is a critical impediment towards obtaining green hydrogen. Indeed, understanding the complex competitive mechanisms of oxygen and chlorine formation at the anode is an analytic challenge. An argument for direct seawater electrolysis is presented with a dissection of the complications that arise at the anode in the presence of seawater ionic constituents such as the chloride ion. Electrolyser system durability and the impact on the current and voltage efficiencies are discussed. Critical challenges at the anode under the acidic conditions of proton exchange membrane water electrolysis interrelate the thermodynamic and kinetic constraints of the oxygen evolution reaction (OER) and the chlorine evolution reaction (CER) to elucidate the heterogenous mechanisms of the OER and the CER and the crucial stability predicament under selective OER electrocatalysis. The selectivity circumstances resolved by Density Functional Theory computations shed insight onto the reaction conditions that select for the preferred OER adsorbate chemisorption on the surface and substantiates the observed overpotentials required for the OER and CER; experimental rotating ring disk electrode analysis further indicate competitive adsorption of OER and CER reactants under an assumed Langmuir isotherm model. Identifying the rate determining step and breaking the scaling relationship of the AEM OER pathway may both improve the stability of the catalyst and achieve lower OER overpotentials. Critical insight is given into designing the heterogeneous electrocatalyst structure with selective facets, additional point defects, and augmented active site density through single atom catalysts. An argument for the utilization of ruthenium for its high natural ubiquity and modulable valence states that can facilitate atomic configurations with optimal active site d-band center energies to promote selective adsorbate binding is presented. Studies of in-situ filtration of the chloride ion under acidic conditions highlight the utility of manganese oxide and silica; the augmentation of the conductivity through manipulation of polaronic interactions; and the design of heterogeneous electrocatalysts with self-healing characteristics demonstrated in select molecular catalysts that may decrease the overpotential of the OER and achieve selectivity. It is with the hope that these design strategies provide insights into future research efforts to uncover an electrocatalytic surface selective for OER evolution under the perilous acidic conditions and reveal an effective solution as serendipitous as the abundancy of a natural resource such as seawater.

A time-resolved study of PbCl2-induced corrosion of low-alloyed steel in the presence of water vapour at 400 °C

A laboratory study of a low-alloyed steel (T22) exposed to an 5% O2 + 20% H2O + N2 bal. gas in the presence of PbCl2(s) and PbO(s) at 400 °C is presented. The presence of PbCl2(s) strongly accelerates corrosion by promoting oxide delamination and crack formation. The corrosion attack is explained according to an electrochemical mechanism, involving the inward diffusion of chlorine ions and formation of metal chlorides at the metal/oxide interface. The role of Cl in PbCl2-indcued corrosion of low-alloyed steels is argued to be the major driving force while the role of Pb in the corrosion attack is minor.

Formation of Chlorine in the Atmosphere by Reaction of Hypochlorous Acid with Seawater.

The highly reactive dihalogens play a significant role in the oxidative chemistry of the troposphere. One of the main reservoirs of these halogens is hypohalous acids, HOX, which produce dihalogens in the presence of halides (Y-), where X, Y = Cl, Br, I. These reactions occur in and on aerosol particles and seawater surfaces and have been studied experimentally and by field observations. However, the mechanisms of these atmospheric reactions are still unknown. Here, we establish the atomistic mechanism of HOCl + Cl- → Cl2 + OH- at the surface of the water slab by performing ab initio molecular dynamics (AIMD) simulations. Main findings are (1) This reaction proceeds by halogen-bonded complexes of (HOCl)···(Cl-)aq surrounded with the neighboring water molecules. (2) The halogen bonded (HOCl)···(Cl-)aq complexes undergo charge transfer from Cl- to OH- to form transient Cl2 at neutral pH. (3) The addition of a proton to one proximal water greatly facilitates the Cl2 formation, which explains the enhanced rate at low pH.

Pyrolysis of municipal plastic waste: Chlorine distribution and formation of organic chlorinated compounds

The release of chlorine during the pyrolysis of actual municipal plastic waste (MPW) was studied. Firstly, thermogravimetry-Fourier transform infrared (TG-FTIR) was analyzed to investigate the chlorine release behavior. Then, the effect of temperature on chlorine migrations was investigated by fast pyrolysis experiments in a fixed bed reactor. Results showed that chlorine released mainly between 241 and 353 °C in the form of HCl or chloroesters during MPW pyrolysis. After pyrolysis, chlorine was mainly distributed in the pyrolytic gas (74.34–82.89 %) and char (10.17–21.29 %). However, the release of chlorine was inhibited due to the melting behavior of MPW at <350 °C. Besides, the relative contents and types of organic chlorinated compounds in liquid products were both decreased with temperature. It was observed that polyethylene terephthalate (PET) was the greatest contributor to the formation of organic chlorinated compounds during MPW pyrolysis. Meanwhile, the pyrolysis of PET was significantly promoted by the HCl released from polyvinyl chloride (PVC). Subsequently, the pathways for the formation of organic chlorinated compounds through the co-pyrolysis of PVC and PET were proposed, including the initial degradation and subsequent chlorination of PET. These findings provided new insights into the release and regulation of chlorine-containing pollutants during actual MPW pyrolysis.

Investigation of the Impact of Chloride Contamination on Degradation in PEM Water Electrolyzer Cells

Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology for hydrogen production from renewable energy sources. However, current systems require ultra-pure water (>18 MΩ*cm) and fail in the presence of impurities like dissolved ions or chemical contaminants [1,2]. Due to the cost of purification and local scarcity of groundwater, there is a rising interest in creating electrolyzers that can function using water of inferior quality [3]. Additionally, feedwater used in offshore applications that is generated by the desalination of seawater still contains low amounts of ionic contaminants [4].Among the most detrimental impurities responsible for performance loss in PEMWE, Cl− is present in high concentrations in both, sea- and potable water [3]. Cl− can reduce the efficiency of the electrolysis process by accelerating component degradation and lower the purity of the produced hydrogen. A competing reaction between the oxygen evolution reaction (OER) and the chlorine evolution reaction (CER) results in the formation of gaseous chlorine that lowers the quality of the produced hydrogen gas. Additionally, Cl− blocking layers can form on the catalyst surfaces which result in a reduced electrochemical surface area. For PEM fuel cells, a technology facing related problems, Cl− adsorption has also been reported to accelerate the formation of hydrogen peroxide [5], promoting chemical degradation of the PFSA-based membrane.The scope of this work is to quantify the reduction of efficiency based on different Cl− concentrations in the feedwater of a PEMWE cell. Therefore, a single cell PEMWE with commercial catalyst coated membranes (CCMs) is exposed to different concentrations of Cl−. The PEMWE single cell is operated up to 100 h and investigated by various characterization techniques, including in-situ and ex-situ cell tests. Electrochemical performance characterization by means of polarization behaviour and impedance spectroscopy is carried out throughout a range of currents (0.01 A cm− 2 to 3.0 A cm− 2) at 60°C. Furthermore, the fluoride emission rate (FER) is monitored during the experiments and optical investigation of the CCMs is carried out after the tests. Based on this complementary investigation, a deeper insight into degradation phenomena associated with Cl− contamination in PEMWE cells is presented. Feng, Q.; Yuan, X.; Liu, G.; Wei, B.; Zhang, Z.; Li, H.; Wang, H. A Review of Proton Exchange Membrane Water Electrolysis on Degradation Mechanisms and Mitigation Strategies. Journal of Power Sources 2017, 366, 33–55, doi:10.1016/j.jpowsour.2017.09.006.Kuhnert, E.; Hacker, V.; Bodner, M. A Review of Accelerated Stress Tests for Enhancing MEA Durability in PEM Water Electrolysis Cells. International Journal of Energy Research 2023, 2023, 1–23, doi:10.1155/2023/3183108.Becker, H.; Murawski, J.; Shinde, D.V.; Stephens, I.E.L.; Hinds, G.; Smith, G. Impact of Impurities on Water Electrolysis: A Review. Sustainable Energy Fuels 2023, 7, 1565–1603, doi:10.1039/D2SE01517J.Bacquart, T.; Moore, N.; Wilmot, R.; Bartlett, S.; Morris, A.S.O.; Olden, J.; Becker, H.; Aarhaug, T.A.; Germe, S.; Riot, P.; et al. Hydrogen for Maritime Application—Quality of Hydrogen Generated Onboard Ship by Electrolysis of Purified Seawater. Processes 2021, 9, 1252, doi:10.3390/pr9071252.Katsounaros, I.; Meier, J.C.; Mayrhofer, K.J.J. The Impact of Chloride Ions and the Catalyst Loading on the Reduction of H2O2 on High-Surface-Area Platinum Catalysts. Electrochimica Acta 2013, 110, 790–795, doi:10.1016/j.electacta.2013.03.156.

Comparative formation of chlorinated and brominated disinfection byproducts from chlorination and bromination of amino acids

Amino acids are the main components of dissolved organic nitrogen in algal- and wastewater-impacted waters, which can react with chlorine to form toxic halogenated disinfection by-products (DBPs) in the disinfection process. In the presence of bromide, the reaction between amino acids and secondarily formed hypobromous acid can lead to the formation of brominated DBPs that are more toxic than chlorinated analogues. This study compares the formation of regulated and unregulated DBPs during chlorination and bromination of representative amino acids (AAs) (e.g., aspartic acid, asparagine, tryptophan, tyrosine, and histidine). In general, concentrations of brominated DBPs (trihalomethanes, haloacetonitriles, and haloacetamides, 24.9–5835.0 nM) during bromination were higher than their chlorinated analogues (9.3–3235.3 nM) during chlorination. This indicates the greater efficacy of bromine as a halogenating agent. However, the formation of chlorinated haloacetic acids during chlorination was higher than the corresponding brominated DBPs from bromination. It is likely that an oxidation pathway is required for the formation of haloacetic acids and chlorine is a stronger oxidant than bromine. Moreover, chlorine forms higher levels of haloacetaldehydes (74.4–1077.8 nM) from amino acids than bromine (1.0–480.2 nM) owing to the instability of brominated species. The DBP formation yields depend on the types of functional groups in the side chain of AAs. Eight intermediates resulting from chlorination/bromination of tyrosine were identified by triple quadrupole mass spectrometer, including N-chlorinated/brominated tyrosine, 3-chloro/bromo-tyrosine, and 3,5-dichloro/dibromo-tyrosine. These findings provided new insights into the DBP formation during the chlorination of algal- and wastewater-impacted waters with elevated bromide.

Active chlorine electrosynthesis from dilute chloride solutions in a flow cell equipped with a Ti|Ti-Ru-Ir-oxides anode

This investigation reports the electrosynthesis of active chlorine from diluted chloride solutions using an undivided filter-press lab scale electrolyzer equipped with Ti|Ti-Ru-Ir-oxides anode and stainless-steel cathode. The interest in studying dilute chloride solutions to yield active chlorine (mixture of Cl2, HClO and ClO−) is because such chloride amounts are typically found in wastewater samples. The influence of the initial chloride concentration (20 ≤ CCl− ≤ 50 mM), anodic applied potential (1.5 ≤ E ≤ 1.8 V vs. SHE), and mean linear flow velocity (2.5 ≤ u ≤ 12.5 cm s−1) on the active chlorine accumulation was systematically examined. The initial electrolyte also contains 50 mM Na2SO4 at pH 3; such pH favors the formation of HClO (with the highest oxidizing power). The best electrolysis regarding the active chlorine accumulation (202 mg L−1) gave 34.4 % current efficiency, and 0.009 kWh (g active chlorine)−1 in 50 mM Cl− solution using E = 1.6 V vs. SHE, and u = 10 cm s−1. The modest current efficiency is due to the concurrent active chlorine formation with the oxygen evolution reaction (OER) at the Ti|Ti-Ru-Ir-oxides anode. The pH increases from 3 to 7.6 over the electrolysis (t = 240 min) due to the hydrogen evolution reaction (HER) occurring at the cathode; at these pHs, a mix of HClO and ClO− species are produced. The well-engineered electrolyzer used herein to form active chlorine species envisages its future use in electrochemical advanced oxidation processes to incinerate persistent organic pollutants.

Gas-Phase and Surface-Initiated Reactions of Household Bleach and Terpene-Containing Cleaning Products Yield Chlorination and Oxidation Products Adsorbed onto Indoor Relevant Surfaces.

The use of household bleach cleaning products results in emissions of highly oxidative gaseous species, such as hypochlorous acid (HOCl) and chlorine (Cl2). These species readily react with volatile organic compounds (VOCs), such as limonene, one of the most abundant compounds found in indoor enviroments. In this study, reactions of HOCl/Cl2 with limonene in the gas phase and on indoor relevant surfaces were investigated. Using an environmental Teflon chamber, we show that silica (SiO2), a proxy for window glass, and rutile (TiO2), a component of paint and self-cleaning surfaces, act as a reservoir for adsorption of gas-phase products formed between HOCl/Cl2 and limonene. Furthermore, high-resolution mass spectrometry (HRMS) shows that the gas-phase reaction products of HOCl/Cl2 and limonene readily adsorb on both SiO2 and TiO2. Surface-mediated reactions can also occur, leading to the formation of new chlorine- and oxygen-containing products. Transmission Fourier-transform infrared (FTIR) spectroscopy of adsorption and desorption of bleach and terpene oxidation products indicates that these chlorine- and oxygen-containing products strongly adsorb on both SiO2 and TiO2 surfaces for days, providing potential sources of human exposure and sinks for additional heterogeneous reactions.

Byproduct Formation of Chlorination and Chlorine Dioxide Oxidation in Drinking Water Treatment

Increasing water scarcity caused by population growth, climate change, pollution from natural and anthropogenic sources, etc. is likely to impact the occurrence of water-associated infectious diseases. Nowadays, access to clean and safe water is a growing concern worldwide. Therefore, disinfection of drinking water is a vital step in public treatment systems as it ensures the removal of various contaminants, including pathogenic microorganisms (protozoa, viruses, bacteria, and intestinal parasites) that give rise to waterborne diseases. Nevertheless, undesirable disinfection byproducts (DBPs) are formed during disinfection as a result of reactions between chemical disinfectants and natural organic matter (NOM), and/or anthropogenic contaminants, and/or bromide/iodide that are present in the raw water. The chemical complexity and heterogeneity of matters in the raw water makes the characterization and the mechanism of DBPs formation quite difficult and ambiguous regardless of the previous hundreds of studies on DBPs generation. As chlorination is still the most economic and most often used disinfection method, and beside chlorination, the application of chlorine dioxide is becoming more widespread, this paper investigates the possible DBPs generated using chlorine and chlorine dioxide with highlighting their adverse health effects. It overviews the reactions of those disinfectants with inorganic and organic compounds. It is important to note that in order to better understand the performance of disinfectants in water treatment, further investigations on the mechanisms of them with inorganic and organic compounds found in water are critically needed.

Nighttime NO emissions strongly suppress chlorine and nitrate radical formation during the winter in Delhi

Abstract. Atmospheric pollution in urban regions is highly influenced by oxidants due to their important role in the formation of secondary organic aerosol (SOA) and smog. These include the nitrate radical (NO3), which is typically considered a nighttime oxidant, and the chlorine radical (Cl), an extremely potent oxidant that can be released in the morning in chloride-rich environments as a result of nocturnal build-up of nitryl chloride (ClNO2). Chloride makes up a higher percentage of particulate matter in Delhi than has been observed anywhere else in the world, which results in Cl having an unusually strong influence in this city. Here, we present observations and model results revealing that atmospheric chemistry in Delhi exhibits an unusual diel cycle that is controlled by high concentrations of NO during the night. As a result of this, the formation of both NO3 and dinitrogen pentoxide (N2O5), a precursor of ClNO2 and thus Cl, are suppressed at night and increase to unusually high levels during the day. Our results indicate that a substantial reduction in nighttime NO has the potential to increase both nocturnal oxidation via NO3 and the production of Cl during the day.

Production of reactive species during UV photolysis of chlorite for the transformation of micropollutants in simulated drinking water

Chlorite (ClO2−) is a dominant product during chlorine dioxide (ClO2) disinfection. This study investigated the mechanism for UV photolysis of ClO2− and the production of reactive species. The apparent quantum yield of ClO2− is 1.21 ∼ 1.29 mol einstein−1 by UV photolysis at 254 nm at pH 7. UV photolysis primarily transforms ClO2− to ClO2, and then to HOCl/OCl−, ClO3− and Cl−. Both ClO2 and HOCl/OCl− are major reactive chlorine species, which maximum concentrations are 8.4 and 11.0 μM, respectively, at 50 μM ClO2− and pH 7. Meanwhile, hydroxyl radical (HO) and chlorine atom (Cl) are also formed with the concentrations of 3.1 × 10−14 and 7.8 × 10−15 M, respectively. The abatement of structurally diverse pharmaceuticals and personal care products (PPCPs) by UV/ClO2− is compound specific, depending on their reactivity toward ClO2, HOCl/OCl− and HO. The first-order rate constants of PPCPs primarily relying on ClO2 and HOCl/OCl− range from 0.043 to 1.533 min−1 and from 0.240 to 2.071 min−1, respectively, while those relying on HO range from 0.012 to 0.020 min−1. As for phenolics, phenol and bisphenol A (BPA) are completely removed within 15 min and ClO2 plays a dominant role. Natural organic matter (NOM) inhibits the degradation of phenol and BPA in UV/ClO2−, while chloride and bicarbonate shows slight impacts. The transformation pathways of BPA in the UV/ClO2− system initiate with electron transfer, and then hydroxylation, carbonylation, chlorination, and coupling reactions. The formation of disinfection byproducts (DBPs) and total organic chlorine (TOCl) is not significant during UV/ClO2− treatment, and over 90% TOCl are unknown DBPs. This study improves the understanding of the water chemistry of UV photolysis of ClO2−.

Roles of Activated Carbon in UV/Chlorine/Activated Carbon-TiO2 Process for Micropollutant Abatement and DBP Control.

The ultraviolet (UV)/chlorine process has attracted increasing attention for micropollutant abatement. However, the limited hydroxyl radical (HO•) generation and the formation of undesired disinfection byproducts (DBPs) are the two major issues in this process. This study investigated the roles of activated carbon (AC) in the UV/chlorine/AC-TiO2 process for micropollutant abatement and DBP control. The degradation rate constant of metronidazole by UV/chlorine/AC-TiO2 was 3.44, 2.45, and 1.58 times higher than those by UV/AC-TiO2, UV/chlorine, and UV/chlorine/TiO2, respectively. AC acted as an electron conductor and dissolved oxygen (DO) adsorbent, resulting in the steady-state concentration of HO• that was ∼2.5 times that of UV/chlorine. Compared with UV/chlorine, the formation of total organic chlorine (TOCl) and known DBPs in UV/chlorine/AC-TiO2 was reduced by 62.3 and 75.7%, respectively. DBP could be controlled via adsorption on AC, and the increased HO• and decreased chlorine radical (Cl•) and chlorine exposure reduced DBP formation. UV/chlorine/AC-TiO2 efficiently abated 16 structurally different micropollutants under environmentally relevant conditions owing to the enhanced generation of HO•. This study provides a new strategy for designing catalysts with photocatalytic and adsorption properties for UV/chlorine to promote micropollutant abatement and DBP control.

Reaction Mechanisms of Chlorine Dioxide with Phenolic Compounds─Influence of Different Substituents on Stoichiometric Ratios and Intrinsic Formation of Free Available Chlorine.

Chlorine dioxide (ClO2) is an oxidant applied in water treatment processes that is very effective for disinfection and abatement of inorganic and organic pollutants. Thereby phenol is the most important reaction partner of ClO2 in reactions of natural organic matter (NOM) and in pollutant degradation. It was previously reported that with specific reaction partners (e.g., phenol), free available chlorine (FAC) could form as another byproduct next to chlorite (ClO2-). This study investigates the impact of different functional groups attached to the aromatic ring of phenol on the formation of inorganic byproducts (i.e., FAC, ClO2-, chloride, and chlorate) and the overall reaction mechanism. The majority of the investigated compounds reacted with a 2:1 stoichiometry and formed 50% ClO2- and 50% FAC, regardless of the position and kind of the groups attached to the aromatic ring. The only functional groups strongly influencing the FAC formation in the ClO2 reaction with phenols were hydroxyl- and amino-substituents in ortho- and para-positions, causing 100% ClO2- and 0% FAC formation. Additionally, this class of compounds showed a pH-dependent stoichiometric ratio due to pH-dependent autoxidation. Overall, FAC is an important secondary oxidant in ClO2 based treatment processes. Synergetic effects in pollutant control and disinfection might be observable; however, the formation of halogenated byproducts needs to be considered as well.

Interactions Between Calcium Hypochlorite and Irrigants Commonly Used in Endodontic Practice: A Chemical Analysis

IntroductionThis study aimed to identify possible products resulting from chemical interactions between calcium hypochlorite (Ca[OCl]2) and other irrigants for endodontic use using electrospray ionization quadrupole time-of-flight mass spectrometry. MethodsThe 5.25% Ca(OCl)2 was associated with either 70% ethanol solution, distilled water, saline solution (0.9% sodium chloride), 5% sodium thiosulfate, 10% citric acid, 17% ethylenediaminetetraacetic acid (EDTA), or 2% chlorhexidine (CHX). The reaction ratio was 1:1 and the products obtained were analyzed by electrospray ionization quadrupole time-of-flight mass spectrometry. ResultsThe interactions between Ca(OCl)2 and CHX generated an orange-brown precipitate, without identification of para-chloroaniline and between Ca(OCl)2 and sodium thiosulfate, a milky-white precipitate. Furthermore, when the oxidizing agent was associated with EDTA and citric acid, chlorine gas was released. As for the other associations, 70% ethanol, distilled water, and saline solution, no precipitation or gas release occurred. ConclusionsThe orange-brown precipitate occurs due to the chlorination of guanidine nitrogens, and the milky-white precipitate is due to the partial neutralization of the oxidizing agent. The release of chlorine gas occurs due to the low pH of the mixture, which results in the rapid formation and decomposition of chlorine. In this context, an intermediate rinsed with distilled water, saline solution, and ethanol between Ca(OCl)2 and CHX, citric acid, and EDTA seems to be appropriate to prevent the formation of by-products when these irrigants need to be used in the canal. Furthermore, if it is necessary to use sodium thiosulfate, a larger volume of the solution must be used compared to that used for the oxidizing solution.

Changes of dissolved organic matter fractions and formation of oxidation byproducts during electrochemical treatment of landfill leachates: Development of spectroscopic indicators for process optimization

Electrochemical oxidation (EO) is an attractive option for treatment of dissolved organic matter (DOM) in landfill leachate but concerns remain over the energy efficiency and formation of oxidation byproducts ClO3− and ClO4−. In this study, EO treatment of landfill leachates was carried out using representative active and nonactive anode materials, cell configurations and current densities. Size exclusion chromatograms coupled with 2D synchronous and asynchronous correlation analysis showed that the sensitivity of DOM fractions to EO degradation was dependent on the anode material. The nonactive boron-doped diamond (BDD) anode demonstrated the best performance for DOM oxidation. The humic acid-like fraction (HA, 2.5–20 kDa) predominated the visible absorbance of landfill leachates at λ ≥400 nm, and it generally had the highest reaction rates except the occurrence of the pH-induced denaturation and precipitation of the proteinaceous biopolymer fraction (BP, >20 kDa). During the EO treatment of landfill leachate with BDD anode, the UV absorbance spectra of landfill leachates at wavelengths <400 nm were affected by the formation of free chlorine. Instead, the decrease of Abs420 was found to be a good indicator of the shift of the oxidation from predominantly HA fraction to the proteinaceous BP fraction. The behavior of the Abs420 parameter was also indicative of the transition from the energy-efficient oxidation of DOM to the dominance of side reactions of chlorine evolution and the subsequent formation of ClO3− and ClO4−. These findings suggest that the EO treatment of landfill leachate can be optimized by adjusting the current density with feedback signals from the online monitoring of Abs420, to achieve a trade-off between degradation of DOM and control of ClO3− and ClO4−.

Synergistic removal of ammonia nitrogen by UV photo-electrocatalytic process: Heterogeneous reaction pathways and mechanism

UV photo-electrocatalytic process has emerged as a promising approach for the abatement of ammonia nitrogen in wastewater. However, the radical chemistry for ammonia nitrogen removal in this process requires further clarification. The heterogeneous reaction pathways and mechanism of ammonia nitrogen oxidation in the UV photo-electrocatalytic process were investigated in this study. Photocurrents of the Ti/RuO2-IrO2-TiO2 electrodes with good electrocatalytic behavior were enhanced by UV radiation. Compared with the electrocatalytic process, the removal rates of ammonia nitrogen were improved by 12.4%∼20% at the Cl− concentration of 50 mM in the photo-electrocatalytic process. The ammonia nitrogen was mainly transferred into N2, meanwhile, the formation of nitrogenous by-products, active chlorine, and chloramines was inhibited in the photo-electrocatalytic process. The contribution of UV radiation was to photo-decompose the active chlorine and chloramines, generating free radicals. The removal efficiency of ammonia nitrogen reached more than 64% after the elimination of free radicals. The electro-generated HOCl was the principal species for ammonia nitrogen removal, and the synergistic effect for the acceleration of ammonia nitrogen oxidation resulted from the photo-generated chlorine radicals. This investigation provides insight into the electrochemistry, photochemistry, and radical chemistry for the heterogeneous catalytic degradation of ammonia nitrogen by UV photo-electrocatalytic process.