Optimal design and characterization of sulfide-modified nanoscale zerovalent iron for diclofenac removal

•Sulfidation suppresses aggregation and sedimentation of nZVI.•S-nZVI/O2 system has much higher DCF removal ratio than nZVI/O2.•Sulfidation can restrict the direct reaction between oxygen and Fe0 core of nZVI.•Surface-bound Fe(II) can produce hyroxyl radicals through one electron transfer.

Noble metal catalyzed nanoscale zerovalent iron (nZVI) has shown some promise in degrading pharmaceuticals, but it still suffers from inactivation caused by common anions. In this study, sulfidation was adopted to enhance diclofenac (DCF, an emerging groundwater pollutant) removal under aerobic conditions in the presence of common anions. X-ray adsorption near edge structure (XANES) analysis shows that dithionite is not only able to sulfurize nZVI, but also stimulates the crystal growth of Fe(0) and restrains FeOOH formation to some degree. Except in CaCl2 solution, certain extent of sulfidation can inhibit the aggregation and sedimentation of nZVI in aqueous media with common ions and anions. While pristine nZVI achieves only 21.2% DCF removal, the optimal sulfide-modified nZVI (S-nZVI) shows 73.5% DCF removal under near neutral condition (pH ∼6.5), and the maximum removal could reach 85.9% at pH 4.5. Mechanism study shows that a heterogeneous layer composed of iron sulfide and iron oxide restricts the direct reaction between oxygen and Fe(0), but facilitates electron transfer from Fe(0) core to Fe(III), producing considerable amount of surface bound Fe(II). Electron Paramagnetic Resonance spectroscope (EPR) analysis and quenching experiments further demonstrate that sulfidation catalyzes dissolved molecular oxygen activation through one-electron transfer. Moreover, sulfidation can lower the negative impact of common anions and humic acid on DCF removal, and S-nZVI is capable of removing DCF in simulated groundwater efficiently. This study provides fundamental understanding on the sulfur catalyzed oxidation of DCF under aerobic conditions.

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