The promoting effect of bisulfite on pollutant abatement by Fe(II)/peroxydisulfate: Dual roles of bisulfite as the accelerator of FeII-(FeIII/FeIV) recycling and radical precursor

[Display omitted] •HSO3– significantly promoted the pollutants abatement in Fe(II)/PDS process.•HSO3– shifted the distribution of reactive species from Fe(IV) to SO4−.•HSO3– increased the exposure dosage of SO4− mainly via the reaction with Fe(IV).•The in situ formed PMS contributed to SO4− formation considerably.•CaS2O5 was used as a slow-releasing source of HSO3– to further optimize the process. The advanced oxidation processes (AOPs) with Fe(II) as homogenous activator suffer from the accumulation of insoluble Fe(III) oxides which results in the decrease of available catalyst and increase of iron sludge. Herein, with Fe(II)/peroxydisulfate (Fe(II)/PDS) process as an example, HSO3– was proposed to enhance the regeneration of Fe(II). The results showed that 250 µM of HSO3– increased the removal of benzoic acid (BA) from 18.8% to 41.3% while further increasing HSO3– dosage promoted its scavenging effects and decreased BA removal. Accordingly, multiple-dosing mode of HSO3– was used which enhanced BA removal to 63.8%. Addition of HSO3– recovered Fe(II) from Fe(III)/Fe(IV) and shifted the distribution of reactive species from Fe(IV) to SO4−. Besides as the reductant, the reaction of HSO3– with Fe(III) and Fe(IV) severed as the AOPs and produced SO4−. A mathematic model was constructed for the Fe(II)/PDS/HSO3– process, and it revealed that ∼ 78% of dosed HSO3– was transformed to SO4−, but ∼ 81% of the generated SO4− was consumed by HSO3–. The reaction of Fe(IV) with HSO3– and the activation of in situ formed peroxymonosulfate by Fe(II) are chiefly responsible for SO4− formation with the contributions of ∼ 47% and ∼ 27%, respectively, in the Fe(II)/PDS/HSO3– process. Considering the complexity of the multiple-dosing mode of HSO3– in the real practice, the sparingly soluble CaS2O5 which released HSO3– gradually was synthesized and used. The single dosing of CaS2O5 stood out in terms of promoting pollutant abatement by the Fe(II)/PDS process compared to that adding equivalent of HSO3–. These results will benefit the optimization of the Fe(II)-based AOPs.