Formation Of ClO3 Research Articles

Simultaneous removal of chlorite and coexisting emerging organic contaminants by sulfite: Kinetics and mechanisms

The inevitable formation of chlorite (ClO2–) and the ineffective degradation of some emerging organic contaminants (EOCs) during chlorine dioxide disinfection have aroused increasing concern about their adverse effects on human health. In this study, the kinetics and mechanisms of simultaneous removal of ClO2– and coexisting EOCs by sulfite (S(IV)) were investigated. Under aerobic conditions, ClO2– and coexisting EOCs could be effectively removed by S(IV) at pH 6.5–7.5, and their removal rates decreased as pH increased. ClO3– was generated in this process but its amount decreased with increasing pH. Many lines of evidence verified that the reaction of ClO2– with HSO3– and H+ generated SO3•− and ClO•, and the generated ClO• was rapidly quenched by S(IV) to form OCl– and SO3•−. Subsequently, SO3•− quickly reacted with O2 to form SO5•−, which was further transformed into SO4•−. SO4•− was the dominant reactive oxidant contributing to the degradation of coexisting EOCs and the formation of ClO3–. Using S(IV) to simultaneously control ClO2– and coexisting EOCs in water treatment is a promising approach because its performance was slightly affected by the water matrix.

Formation of chlorate and perchlorate during electrochemical oxidation by Magnéli phase Ti4O7 anode: inhibitory effects of coexisting constituents

Formation of chlorate (ClO3−) and perchlorate (ClO4−) as by-products in electrooxidation process has raised concern. In the present study, the formation of ClO3− and ClO4− in the presence of 1.0 mM Cl− on boron doped diamond (BDD) and Magneli phase titanium suboxide (Ti4O7) anodes were evaluated. The Cl− was transformed to ClO3− (temporal maximum 276.2 μM) in the first 0.5 h on BDD anodes with a constant current density of 10 mA cm2, while approximately 1000 μM ClO4− was formed after 4.0 h. The formation of ClO3− on the Ti4O7 anode was slower, reaching a temporary maximum of approximately 350.6 μM in 4.0 h, and the formation of ClO4− was also slower on the Ti4O7 anode, taking 8.0 h to reach 780.0 μM. Compared with the BDD anode, the rate of ClO3− and ClO4− formation on the Ti4O7 anode were always slower, regardless of the supporting electrolytes used in the experiments, including Na2SO4, NaNO3, Na2B4O7, and Na2HPO4. It is interesting that the formation of ClO4− during electrooxidation was largely mitigated or even eliminated, when methanol, KI, and H2O2 were included in the reaction solutions. The mechanism of the inhibition on Cl− transformation by electrooxidation was explored.

Kinetic and mechanistic understanding of chlorite oxidation during chlorination: Optimization of ClO2 pre-oxidation for disinfection byproduct control

Chlorine dioxide (ClO2) applications to drinking water are limited by the formation of chlorite (ClO2–) which is regulated in many countries. However, when ClO2 is used as a pre-oxidant, ClO2– can be oxidized by chlorine during subsequent disinfection. In this study, a kinetic model for the reaction of chlorine with ClO2– was developed to predict the fate of ClO2– during chlorine disinfection. The reaction of ClO2– with chlorine was found to be highly pH-dependent with formation of ClO3– and ClO2 in ultrapure water. In presence of dissolved organic matter (DOM), 60-70% of the ClO2– was transformed to ClO3– during chlorination, while the in situ regenerated ClO2 was quickly consumed by reaction with DOM. The remaining 30-40% of the ClO2– first reacted to ClO2 which then formed chlorine from the DOM-ClO2 reaction. Since only part of the ClO2– was transformed to ClO3–, the sum of the molar concentrations of oxychlorine species (ClO2– + ClO3–) decreased during chlorination. By kinetic modelling, the ClO2– concentration after 24 h of chlorination was accurately predicted in synthetic waters but was largely overestimated in natural waters, possibly due to a ClO2– decay enhanced by high concentrations of chloride and in situ formed bromine from bromide. Understanding the chlorine-ClO2– reaction mechanism and the corresponding kinetics allows to potentially apply higher ClO2 doses during the pre-oxidation step, thus improving disinfection byproduct mitigation while keeping ClO2–, and if required, ClO3– below the regulatory limits. In addition, ClO2 was demonstrated to efficiently degrade haloacetonitrile precursors, either when used as pre-oxidant or when regenerated in situ during chlorination.

Potential catalysts for the production of NaClO3 in the decomposition of HOCl

By pursuing the aim of identifying new types of catalysts in the electrolytic production of NaClO3 from NaCl (chlorate process), we report a detailed study on the decomposition of hypochlorous acid accelerated by yttrium(III) chloride (YCl3), yttrium(III) oxide (Y2O3) and telluric acid (Te(OH)6) at 80°C. The results were compared with those obtained in the uncatalyzed and chromium(VI) catalyzed reactions. In general, the decomposition of HOCl occurs via two competing paths toward the formation of ClO3− or O2. In the case of YCl3, the decomposition proceeds via the oxygen path over the entire studied pH range. Y2O3 slightly catalyzes the chlorate path under acidic conditions, however the noted catalytic effect is probably due to the “self-buffering” of the reaction mixture (Y2O3 suspension). Although, a real catalytic process takes place in the presence of Te(OH)6, a significant pH-effect is also observed which is most likely associated with the acid–base equilibria of telluric acid. pH dependent studies demonstrate that the optimum pH of decomposition is at around 6.7–6.9 in this case. The comparison of the results obtained in the presence of chromium(VI) and Te(OH)6 reveals that the former is a more active catalyst. On the basis of kinetic and stoichiometric results, it is reasonable to assume that Te(OH)6 may be utilized as an alternative catalyst in the chlorate process.

Simultaneous removal of chlorite and contaminants of emerging concern under UV photolysis: Hydroxyl radicals vs. chlorate formation

It is well known that using chlorine dioxide (ClO2) as a disinfectant inevitably produces a common disinfection byproducts chlorite (ClO2‒). In this study, we found that UV photolysis after ClO2 disinfection can effectively eliminate both ClO2‒ and contaminants of emerging concern (CECs). However, the kinetic mechanisms of UV/ClO2‒ process destructing CECs, as well as transformation of ClO2‒ in UV/ClO2‒ system are not clear yet. Therefore, we systematically investigated the UV/ClO2‒ system to assist us appropriately design this process under optimal operational conditions. In this work, we first investigated the impact of water matrix conditions (i.e., pH, bicarbonate and natural organic matter (NOM)) and ClO2‒ dosage on the UV/ClO2‒ process. We found that bicarbonate and NOM have inhibition effects, while lower pH and higher ClO2‒ dosage have enhancement effects. Besides, hydroxyl radical (HO•) and reactive chlorine species (RCS) are generated from UV/ClO2‒ system, and RCS are main contributors to CBZ degradation. Then we proposed a possible degradation pathway of CBZ based on the determined products from experiments. Additionally, we found that photolysis of ClO2‒ resulted in the generation of chloride (Cl‒) and chlorate (ClO3‒). As the ClO2‒ dosage increases, the yield of ClO3‒ increased while that of Cl‒ decreased. Finally, we elucidated the second order rate constant of the target organic compound with HO• has a strong correlation with the formation of ClO3‒.

Mechanistic and Kinetic Understanding of the UV254 Photolysis of Chlorine and Bromine Species in Water and Formation of Oxyhalides.

This study investigated the UV254 photolysis of free available chlorine and bromine species in water. The intrinsic quantum yields for •OH and X• (X = Cl or Br) generation were determined by model fitting of formaldehyde formation using a tert-butanol assay to be 0.61/0.45 for HOCl/OCl- and 0.32/0.43 for HOBr/OBr-. The steady-state •OH concentration in UV/HOX was higher than that in UV/OX- by a factor of 23.3 and 7.8 for Cl and Br, respectively. This was attributed to the different •OH consumption rate by HOCl versus OCl-, while for HOBr/OBr-, both the •OH formation and consumption rates were implied. This was supported by a k of 1.4 × 108 M-1 s-1 for the •OH reaction with HOCl, which was >14 times less than the k for •OH reactions with OCl-, HOBr, and OBr-. Formation of ClO3- and BrO3- was found to be significant with apparent quantum yields of 0.12-0.23. A detailed mechanistic study on the formation of XO3- including a new pathway involving XO• is presented, which has important implications as the level of XO3- can exceed the regulation (BrO3-) or guideline (ClO3-) values during UV/halogen oxidant water treatment. Our new kinetic models well simulate the experimental results for the halogen oxidant decomposition, probe compound degradation, and formation of ClO3- and BrO3-.

DFT study of the reduction reaction of calcium perchlorate on olivine surface: Implications to formation of Martian’s regolith

Perchlorates have been found widespread on the surface of Mars, their origin and degradation pathways are not understood to date yet. We investigate here, from a theoretical point of view, the potential redox processes that take place in the interaction of Martian minerals such as olivine, with anhydrous and hydrated perchlorates. For this theoretical study, we take as mineral substrate the (1 0 0) surface of forsterite and calcium perchlorate salt as adsorbate. Our DFT calculations suggests a reduction pathway to chlorate and chlorite. When the perchlorate has more than 4 water molecules, this mechanism, which does not require high-temperature or high energy sources, results in parallel with the oxidation of the mineral surface, forming magnesium peroxide, MgO2, and in the formation of ClO3, which through photolysis is known to form ClO-O2. Because of the high UV irradiance that reaches the surface of Mars, this may be a source of O2 on Mars. Our results suggest that this process may be a natural removal pathway for perchlorates from the Martian regolith, which in the presence of atmospheric water for salt hydration, can furthermore lead to the production of oxygen. This mechanism may thus have implications on the present and future habitability of the Martian surface.

Formation of reactive chlorine species in saline solution treated by non-equilibrium atmospheric pressure He/O2 plasma jet

The aqueous-phase chemistry of reactive chlorine species initiated in saline solution by the gas phase discharge plasma above the liquid surface was investigated. A micro-atmospheric pressure 13.56 MHz plasma jet operating with a 0.6% O2 in He was used to promote reactions of plasma-generated oxygen atoms with chloride anions at the gas-liquid interface. Physiological NaCl solution and phosphate buffered saline (PBS) were used. Three chlorine oxoacids or their conjugate bases were detected in the plasma treated saline: hypochlorites HOCl/OCl−, chlorites ClO2−, and chlorates ClO3−, being formed in the concentration ratio 4.5:1:1, respectively. In addition, chlorine dioxide ClO2 was formed in minor amounts. The final pH value of the plasma treated saline was typically 10.2. The total hypochlorite production, [HOCl] + [OCl−], was directly proportional to the initial NaCl concentration, supporting the idea of a direct reaction of Cl− with O atoms. Subsequent post-discharge chemical processes in the bulk plasma treated saline solution were found to lead to the disproportionation of hypochlorite into different oxidation states of chlorine with formation of ClO3− as the final product. The reactivity of O atoms in NaCl solution was studied using taurine and phenol as chemical probes. Experiments with 50 mM taurine revealed a very high initial formation rate of OCl− in PBS (up to 4 μM s−1). Chlorinated products of phenol (2- and 4-chlorophenol) in addition to the hydroxylated products (hydroquinone and catechol) demonstrated chlorination of phenol by hypochlorite. The rate of Cl− oxidation by O atoms was estimated to be 2–3 orders slower than the reaction of O atoms with phenol.

Comparison between OCl−-Injection and In Situ Electrochlorination in the Formation of Chlorate and Perchlorate in Seawater

To prevent biofouling from occurring in the cooling systems of coastal power plants, chlorine is often added to the cooling water. In this study, we have evaluated the fate of the total residual oxidants and the formation of inorganic chlorination byproducts including ClO3− and ClO4− during in situ electrochlorination with seawater. Then, the results were compared with those during direct OCl−-injection to seawater. The in situ electrochlorination method based on Ti/RuO2 electrodes produced much less ClO3−, while a similar level of total residual oxidants could be achieved with a reaction time of 5 min. Moreover, no ClO4− was observed, while the direct OCl−-injection system could still result in the production of ClO4−. The less or no production of ClO3− or ClO4− by the electrochlorination of seawater was mainly attributed to two reasons. First, during the electrolysis, the less amount of OCl− is available for ClO3− formation. Secondly, the formation of ClO3− or ClO4− is affected by the electrode material. In other words, if the electrode material is carefully chosen, the production of harmful reaction byproducts can be prevented or minimized. In short, based on the results from our study, electrochlorination technology proves to be a marine environmentally friendly method for controlling biofouling in the pipes of the cooling system in a coastal power plant.

Evidence of Fenton-like reaction with active chlorine during the electrocatalytic oxidation of Acid Yellow 36 azo dye with Ir-Sn-Sb oxide anode in the presence of iron ion

The degradation of 2.5L of Acid Yellow 36 solutions at pH 3.0 by electro-oxidation (EO) has been studied in a flow plant with a reactor containing an Ir-Sn-Sb oxide anode and a stainless steel cathode. The anode was prepared onto Ti by the Pechini method and characterized by SEM-EDX and XRD. It showed a certain ability to electrocatalyze both, the generation of adsorbed OH from water oxidation in sulfate medium and, more largely, the production of active chlorine in a mixed electrolyte containing Cl− ion. The EO treatment of the dye solution in the latter medium led to a rapid decolorization because active chorine destroyed the colored by-products formed, but color removal was much slower in pure NaClO4 or Na2SO4 due to the limited formation of OH. In contrast, greater mineralization was obtained in both pure electrolytes since the by-products formed in the presence of Cl− became largely persistent. The effect of liquid flow rate, current density and dye content on the EO performance in the mixed electrolyte was examined. The drop of absorbance and dye concentration obeyed a pseudo-first-order kinetics. Interestingly, the decolorization rate, dye concentration decay and TOC removal were enhanced upon catalysis with 1.0mM Fe2+. Such better performance can be accounted for by the formation of OH in the bulk from the electro-Fenton-like process between electrogenerated HClO and added Fe2+. Even larger mineralization was achieved by the photoelectro-Fenton-like process upon irradiation of the solution with UVA light due to photolysis of some refractory intermediates. Maleic and acetic acids were detected as final short-chain linear carboxylic acids. The loss of Cl− and the formation of ClO3−, ClO4−, SO42−, NO3− and NH4+ were evaluated as well.

Phototransformation of Perchlorate to Chloride in the Presence of Polysilanes

Perchlorate is a uniquely stable chemical described as an emerging thyroid disrupting agent that is presently detected in several terrestrial and aquatic matrices. The present study was undertaken to deoxygenate perchlorate to the chloride anion photolytically in the presence of dodecamethylcyclohexasilane (Me2Si)6 1. It is found that photolysis of 1 in the presence of dry NaClO4 in tetrahydrofuran (THF) at 254 nm leads to the disappearance of the salt. The removal of ClO4– occurred with the concurrent formation of ClO3– and ClO2–, which disappear to eventually produce the chloride anion quantitatively. The two cyclic silanes (Me2Si)5 3 and (Me2Si)4 4 in addition to several other siloxanes that include (Me2SiO)3, (Me2SiO)4, and (Me2Si)xO2 (x = 4 and 5) were also detected. When the reaction was repeated using uniformly labelled 18O-[ClO4–] it was found that oxygen incorporated in the siloxane products was derived from perchlorate. Mixing 1 with perchlorate in THF in the dark or adding the salt to 1 after the latter being photolyzed in THF did not deoxygenate ClO4–. Based on experimental evidence gathered thus far it is concluded that dimethylsilylene, Me2Si: 2, a reactive intermediate produced by the photolysis of 1, is in part responsible for the deoxygenation of perchlorate. Direct oxygen transfer from ClO4– to the silanes during photolysis is also suggested as a potential route of deoxygenating ClO4–.