The removal preference of Fe(III), Fe(II), and in situ Fe(III) in coagulation when treating NOM and TC

Abstract

Tetracyclines (TCs) are a representative class of emerging organic pollutants. Their presence may affect the coagulation process commonly used in water treatment units. Fe salts are widely employed as coagulants in the coagulation process. However, there are few systematic studies of Fe(III), Fe(II), and in situ Fe(III) (oxide-assisted Fe(II) coagulation) for the treatment of natural organics (NOMs) in the presence or absence of tetracycline (TC). In this study, the coagulation process and performance of Fe(III), Fe(II) and in-situ Fe(III) generated by the oxide of potassium persulfate (PS) for the removal of humic acid (HA), NOM, NOM with TC were systematically studied. The flocculation index showed that the presence of TC lagged the floc growth. It was due to the reaction of TC with Fe, hindering the hydrolysis, aggregation and precipitation of Fe to form flocs. The removal performance evaluated by the EEM, SEC and LC-MS suggested the removal preference of Fe(III) and Fe(II) was NOM and TC, respectively. And the in-situ Fe(III) was superior than Fe(III) and Fe(II) in removing NOM and TC. It was attributed to the free radical oxidation by PS and the coagulation through in-situ Fe(III). The concentration of PS determined which of them was the main reason responsible for the enhanced organics removal. The PS at the concentration of 0.02 mM was the key point. The oxidation of organics by the free radicals dominated the enhance removal of organics when PS concentration was lower than 0.02 mM. Otherwise, it was the coagulation of the in-situ Fe(III) responsible for the enhanced removal of organics. The results of floc characterization by TEM, SEM and FTIR further indicated that the in-situ Fe(III) retained higher proportion organics with the increased concentration of PS. This study provides information on which Fe salt coagulant to choose for the treatment of TC-contaminated water and the insights of the interactions fate of Fe flocs with NOM and TC from the perspective of coagulation in natural environments.