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.