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  • Title: Dosing low-level ferrous iron in coagulation enhances the removal of micropollutants, chlorite and chlorate during advanced water treatment.
    Author: Cassol GS, Shang C, Li J, Ling L, Yang X, Yin R.
    Journal: J Environ Sci (China); 2022 Jul; 117():119-128. PubMed ID: 35725064.
    Abstract:
    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.
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