Sequential ClO2-UV/chlorine process for micropollutant removal and disinfection byproduct control

Performance and Mechanism of Chlorine Dioxide on BTEX Removal in Liquid and Indoor Air

With the development of the chemical industry, benzene, toluene, ethylbenzene, and xylene (BTEX) have gradually become the major indoor air pollutants. Various gas treatment techniques are widely used to prevent the physical and mental health hazards of BTEX in semi-enclosed spaces. Chlorine dioxide (ClO₂) is an alternative to chlorine as a secondary disinfectant with a strong oxidation ability, a wide range of action, and no carcinogenic effects. In addition, ClO₂ has a unique permeability which allows it to eliminate volatile contaminants from the source. However, little attention has been paid to the removal of BTEX by ClO₂, due to the difficulty of removing BTEX in semi-enclosed areas and the lack of testing methods for the reaction intermediates. Therefore, this study explored the performance of ClO₂ advanced oxidation technology on both liquid and gaseous benzene, toluene, o-xylene, and m-xylene. The results showed that ClO₂ was efficient in the removal of BTEX. The byproducts were detected by gas chromatography-mass spectrometry (GC-MS) and the reaction mechanism was speculated using the ab initio molecular orbital calculations method. The results demonstrated that ClO₂ could remove the BTEX from the water and the air without causing secondary pollution.

A novel solar-activated chlorine dioxide process for atrazine degradation in drinking water

New technologies using advanced oxidation processes (AOPs) with low energy-input to address the presence of micro-contaminants and the formation of disinfection byproducts (DBPs) are required for drinking water safety. In this work, the activation of chlorine dioxide with solar (solar/ClO₂ process), a type of renewable and inexhaustible energy, was developed to degrade atrazine (ATZ) and control the formation of DBPs. Results revealed that solar/ClO₂ process was effective in degrading ATZ. Hydroxyl radicals (•OH) and chlorine radicals (Cl•) produced in solar/ClO₂ process were found to be the predominant agents for ATZ degradation with contribution rates of 55.9% and 44.1%, respectively, based on radical quenching tests and competition kinetics. Reaction pH did not affect the total amount of Cl• and •OH (i.e., [•OH]_exp) and [Cl•]_exp), while the conversion of Cl• to •OH was responsible for the depressed ATZ degradation efficiency with the increasing pH in solar/ClO₂ process. The presence of bicarbonate (HCO₃^-), chloride (Cl^-) and humic acid (HA) retarded the ATZ degradation mainly due to they decreased [•OH]_exp) and [Cl•]_exp. Using the UPLC-MS/MS analysis, six degradation intermediates of ATZ were tentatively identified, and the three-stage degradation pathway as well as the stepwise detoxification of ATZ were confirmed by the condensed Fukui function (CFF) calculation and ECOSAR prediction. Applying solar/ClO₂ as a pretreatment of HA-containing water, the formation of DBPs during post-chlorination was significantly reduced. However, the presence of ATZ during solar/ClO₂ pretreatment of HA significantly lowered the control efficiency of DBPs. The major degradation intermediate, i.e., deethyldeisopropylhydroxyatrazine (DEIHA), of ATZ could incorporate into HA and therefore providing more precursors for DBPs. The acute toxicity recorded by the behavior of zebrafish larvae revealed that using chloramine instead of chlorine downstream the solar/ClO₂ pretreatment of ATZ and HA could significantly reduce the acute toxicity by decreasing the formation of total DBPs. This study demonstrated the great potential of applying solar/ClO₂ process followed by chloramination to simultaneously degrade micro-contaminants and reduce DBPs formation as well as toxic risk in practical applications.

Impact of water matrices on oxidation effects and mechanisms of pharmaceuticals by ultraviolet-based advanced oxidation technologies: A review

The binding between water components (dissolved organic matters, anions and cations) and pharmaceuticals influences the migration and transformation of pollutants. Herein, the impact of water matrices on drug degradation, as well as the electrical energy demands during UV, UV/catalysts, UV/O₃, UV/H₂O₂-based, UV/persulfate and UV/chlorine processes were systemically evaluated. The enhancement effects of water constituents are due to the powerful reactive species formation, the recombination reduction of electrons and holes of catalyst and the catalyst regeneration; the inhibition results from the light attenuation, quenching effects of the excited states of target pollutants and reactive species, the stable complexations generation and the catalyst deactivation. The transformation pathways of the same pollutant in various AOPs have high similarities. At the same time, each oxidant also can act as a special nucleophile or electrophile, depending on the functional groups of the target compound. The electrical energy per order (EEO) of drugs degradation may follow the order of EEO_UV > EEO_UV/catalyst > EEO_UV/H2O2 > EEO_UV/PS > EEO_UV/chlorine or EEO_UV/O3. Meanwhile, it is crucial to balance the cost-benefit assessment and toxic by-products formation, and the comparison of the contaminant degradation pathways and productions in the presence of different water matrices is still lacking.

Chlorine dioxide-based oxidation processes for water purification：A review

Chlorine dioxide (ClO₂) has emerged as a broad-spectrum, safe, and effective disinfectant due to its high oxidation efficiency and reduced formation of organochlorinated by-products during application. This article provides an updated overview of ClO₂-based oxidation processes used in water treatment. A systematic review of scientific information and experimental data on ClO₂-based water purification procedures is presented. Concerning ClO₂-based oxidation derivative problems, the pros and cons of ClO₂-based combined processes are assessed and disinfection by-product (DBP) control approaches are proposed. The kinetic and mechanistic data on ClO₂ reactivity towards micropollutants are discussed. ClO₂ selectively reacts with electron-rich moieties (anilines, phenols, olefins, and amines) and eliminates certain inorganic ions and microorganisms with high efficiency. The formation of chlorite and chlorate during the oxidation process is a crucial concern when utilizing ClO₂. Future applications include the combination of ClO₂ with ferrous ions, activated carbon, ozone, UV, visible light, or persulfate processes. The combined process can reduce by-product generation while still ensuring ClO₂ sterilization and disinfection. Overall, this research could provide useful information and new insights into the application of ClO₂-based technologies.

Dosing low-level ferrous iron in coagulation enhances the removal of micropollutants, chlorite and chlorate during advanced water treatment

Drinking water utilities are interested in upgrading their treatment facilities to enhance micropollutant removal and byproduct control. Pre-oxidation by chlorine dioxide (ClO₂) followed by coagulation-flocculation-sedimentation and advanced oxidation processes (AOPs) is one of the promising solutions. However, the chlorite (ClO₂^-) formed from the ClO₂ pre-oxidation stage cannot be removed by the conventional coagulation process using aluminum sulfate. ClO₂^- negatively affects the post-UV/chlorine process due to its strong radical scavenging effect, and it also enhances the formation of chlorate (ClO₃^-). In this study, dosing micromolar-level ferrous iron (Fe(II)) into aluminum-based coagulants was proposed to eliminate the ClO₂^- generated from ClO₂ pre-oxidation and benefit the post-UV/chlorine process in radical production and ClO₃^- reduction. Results showed that the addition of 52.1-µmol/L FeSO₄ effectively eliminated the ClO₂^- generated from the pre-oxidation using 1.0 mg/L (14.8 µmol/L) of ClO₂. Reduction of ClO₂^- increased the degradation rate constant of a model micropollutant (carbamazepine) by 55.0% in the post-UV/chlorine process. The enhanced degradation was verified to be attributed to the increased steady-state concentrations of HO^· and ClO^· by Fe(II) addition. Moreover, Fe(II) addition also decreased the ClO₃^- formation by 53.8% in the UV/chlorine process and its impact on the formation of chloro-organic byproducts was rather minor. The findings demonstrated a promising strategy to improve the drinking water quality and safety by adding low-level Fe(II) in coagulation in an advanced drinking water treatment train.

ClO2 pre-oxidation changes dissolved organic matter at the molecular level and reduces chloro-organic byproducts and toxicity of water treated by the UV/chlorine process

The formation of undesirable chloro-organic byproducts is of great concern in the UV/chlorine process. In this study, chlorine dioxide (ClO₂) pre-oxidation was applied to control the formation of chloro-organic byproducts and the toxicity in UV/chlorine-treated water. The molecular-level changes in dissolved organic matter (DOM) were tracked by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and ClO₂ pre-oxidation was found to preferentially react with DOM moieties with high aromaticity level and with a carbon number of > 18, producing compounds with a higher degree of oxidation and lower aromaticity. The ClO₂-treated DOM was found to be less susceptible to attack by radicals and free chlorine in the UV/chlorine process compared to the raw DOM. ClO₂ pre-oxidation resulted in a significant decrease in the number of unknown chloro-organic byproducts (i.e., -17%) and the total intensity of organic chlorine detected by FT-ICR-MS (i.e., -31%). The molecular characteristics, such as O/C, aromaticity index, and the average number of chlorine atoms, of these unknown chloro-organic byproducts generated in the scenarios with and without ClO₂ pre-oxidation were also different. Additionally, ClO₂ pre-oxidation reduced the genotoxicity (SOS/umu test) and cytotoxicity (Hep G2 cytotoxicity assay) of UV/chlorine-treated water by 26% and 20%, respectively. The findings in this study highlight the merits of ClO₂ pre-oxidation for controlling chloro-organic byproducts and reducing the toxicity of water treated by the UV/chlorine process in actual practice.