Application of Novel Fulvic Acid-Coated Magnetite Nanoparticles for CO2−-Mediated Photoreduction of Cr(VI)

Valeria B Arce,Daniel O Mártire,Rosa M Torres Sánchez,Carla R Mucci,Alejandra M Fernández Solarte

doi:10.1007/s11270-018-3693-5

Abstract

We here isolate fulvic acids from vermicompost to prepare and characterize novel fulvic acid-coated magnetite nanoparticles. UV-A irradiation of suspensions of the nanoparticles under different experimental conditions led to photo-reduction of Cr(VI). In anoxic conditions in the presence of formic acid, after 60 min of irradiation ca. 100% of Cr(VI) was reduced. Under these conditions, the carbon dioxide radical anions, CO2 .- , mediated the photo-reduction of Cr(VI). However, the high reduction potential of Cr(VI) and the variety of reactive species generated upon UV-A irradiation make this nanomaterial also suitable for Cr(VI) photo-reduction also under aerobic conditions in the presence of formic acid or under anoxic conditions without the addition of formic acid. The possible photodegradation routes involved are discussed in detail.

Highlights

We have previously shown that anaerobic UV-A irradiation of 1,4-naphthoquinone (NQ) solutions containing formic acid results in the oxidation of formic acid by the triplet state of NQ to yield the carbon dioxide radical anion, CO2., which reduced HgCl2(aq) to Hg2Cl2(s) (Berkovic et al 2012)
We here clearly demonstrate that fulvic acids (FA) can be transformed into a magnetic additive for the photochemical treatment of water contaminated with Cr(VI)
The high reduction potential of Cr(VI) and the variety of reactive species generated upon UV-A irradiation make this nanomaterial suitable for Cr(VI) photoreduction under several conditions

Summary

Introduction

We have previously shown that anaerobic UV-A irradiation of 1,4-naphthoquinone (NQ) solutions containing formic acid results in the oxidation of formic acid by the triplet state of NQ to yield the carbon dioxide radical anion, CO2.-, which reduced HgCl2(aq) to Hg2Cl2(s) (Berkovic et al 2012).Humic acids (HA) and fulvic acids (FA) are operationally defined fractions of humic acids based on their solubility difference in acid and base. We have previously shown that anaerobic UV-A irradiation of 1,4-naphthoquinone (NQ) solutions containing formic acid results in the oxidation of formic acid by the triplet state of NQ to yield the carbon dioxide radical anion, CO2.-, which reduced HgCl2(aq) to Hg2Cl2(s) (Berkovic et al 2012). Mixed-valence nanoscale iron oxides, such as magnetite, which present high surface areas and sorption capacities, are potential materials suitable for environmental remediation (Auffan et al 2007). These insoluble magnetic materials can be employed as nanoadsorbents of metal ions (Carlos et al 2013) and organic dyes (Inbaraj and Chen, 2011) among other contaminants. Since bare magnetite nanoparticles are susceptible to air oxidation (Maity and Agrawal 2007) and are aggregated in aqueous systems, their stabilization is desirable. HA-coatings enhance the stability of nanodispersions of the Fe3O4 particles by preventing their aggregation (Shahid et al 2017)

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