Hydroxyl radical generation linked with iron dissolution and dissolved oxygen consumption in zero-valent iron wastewater treatment process

• The OH radical generated by zero-valent iron (ZVI) was systematically examined.
• The iron dissolution dominantly controlled OH radical generation by ZVI.
• The OH radical generation was also largely affected by the passive layer formation.
• The amount of generated OH radical increased with ZVI dosage and decreased with pH.
• The OH radical generation by ZVI could be simulated by the proposed kinetic model.

In zero-valent iron (ZVI) wastewater treatments, organic pollutants are degraded by hydroxyl radical (OH radical) generated via the Fenton reaction besides by the reductive reaction. The generation of OH radical is initiated by the dissolution of ZVI. The strong linkage of OH radical generation by ZVI with eluted ferrous ion and dissolved oxygen (DO) through the iron dissolution and the formation of passive iron oxide/hydroxide layer on the ZVI surface was found. While OH radicals were gradually generated by microscale ZVI (mZVI) until the termination of iron dissolution, the OH radical generation by nanoscale ZVI (nZVI) was very quick at the initial phase, subsequently slowed down and terminated. Although the rate of OH radical generation with nZVI was much faster than that with mZVI, the amount of generated OH radical with nZVI was less than mZVI under the same iron dosage conditions. With increasing ZVI dosage and controlled solution pH value, the amount of generated OH radical for both mZVI and nZVI increased and decreased, respectively. For nZVI under the oxic condition, the quick depletion of DO at the initial phase accompanied with the sequential recovery to the saturation concentration was found. A reaction kinetic model was developed to quantify a linkage of OH radical generation with the iron dissolution and the formation of passive iron oxide/hydroxide layer on the ZVI surface. The linkage could be reasonably simulated by the proposed kinetic model with the correlation coefficient of >0.828.

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