Insights into the corrosion of sulfidated microscale zero valent iron in the presence of benzoic acid: Inhibited oxidation capacity and promoted zinc immobilization

Under oxic environment, the corrosion of zero valent iron (ZVI) connects with its oxidation and adsorption capacity, but little is known about the effects of sulfidation on those processes of ZVI. To assess the oxidant production from ball-milling unsulfidated and sulfidated microscale ZVI (mZVI(bm) and S-mZVI(bm)), the transformation of benzoic acid (BA) to p-hydroxybenzoic acid (p-HBA) was used as the oxidation probe reaction. During the oxidation reaction, the concentration of p-HBA in S-mZVI(bm)/O-2 systems rapidly reached equilibrium (< 12 mu M) within 40 min regardless of sulfur loading content, whereas it could gradually reach 26 mu M in mZVIbm/O-2 system within 240 min despite the slower generation of p-HBA at initial stage. Combining with the characteristic analysis, the lower oxidation capacity of S-mZVI(bm)/O-2 system might be ascribed to the enhanced Fe(II) dissolution that caused a larger solution pH rise along with the loss of Fe(0) and FeSx sites and the formation of lepidocrocite, which was not beneficial for oxygen activation. Under the identical conditions such as material dosage and co-existing anions, the oxidation capacity of mZVI(bm)/O-2 system also exceeded S-mZVI(bm)/O-2 system. In addition, the comparative experiment of Zn2+ immobilization by S-mZVI(bm) and S-mZVI(bm) with or without BA was investigated. mZVI(bm) showed insignificant Zn2+ immobilization, whereas BA could enhance Zn2+ immobilization by S-mZVIbm due to the enhanced corrosion that could provide more adsorption sites. Overall, this work is valuable for assessing the oxidation capacity of S-mZVI(bm)/O-2 system and BA effect on heavy metal immobilization of S-mZVI in wastewater treatment.