Overlooked effects of chlorides and bicarbonates on the intensity of peroxydisulfate activation in Fe(II)/citric acid-S2O82− process

The influence of chlorides (Cl−) and bicarbonates (HCO3−) on the Fe(II)/citric acid peroxydisulfate based (Fe(II)/CA-S2O82−) process has been commonly attributed to the formation of secondary reactive species, such as Cl• and CO3•, and their different reactivity with the target pollutant compared to primary reactive species (e.g., SO4•−). However, this conclusion has been entirely based only on analyzing target pollutants removal without the context of S2O82− consumption and dissolved Fe concentration throughout the process. To address this knowledge gap, we conducted a series of batch experiments to investigate the influence of Cl− (2 mmol∙L-1) and HCO3− (1 mmol∙L-1) on the intensity of S2O82− activation within the Fe(II)/CA-S2O82− process with tetrachloroethene (PCE) as the target pollutant. Throughout the experiments, PCE removal efficiency, S2O82− consumption, pH, redox potential, and dissolved Fe were analyzed. Equilibrium hydrochemical modelling (Visual MINTEQ 4) and high-resolution transmission electron microscopy (TEM) were employed to analyze and interpret the data obtained. We found that both Cl− and HCO3− significantly affected the intensity of S2O82− activation. While Cl− influenced the steady-state dissolved Fe(II) concentration, HCO3− primarily increased solution pH, which led to the formation of CA-stabilized ferrihydrite nanoparticles. This resulted in the inaccessibility of dissolved Fe(III) to the Fe(III)/Fe(II) regeneration cycle, consequently suppressing S2O82− activation intensity and PCE removal. Overall, the findings in this study have deepened the fundamental knowledge of how Cl− and HCO3− influence the Fe(II)/CA-S2O82− process, which may help improve the design and operation of S2O82−-based advanced oxidation processes using the Fe(II)/CA activation method.