Activated PDS of Single-Atom-Fe(III): Nonradical Oxidation of Pollutants

Prof. Jiang et al., from (1. Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing, China; 2. University of Chinese Academy of Sciences, Beijing, China, 3. Institute of Environmental and Ecological Engineering, Guangdong University of Technology, Guangzhou, China) and their research collaboration team have reported on the single-atom Fe(III)- and nitrogen doped carbon (Fe−N−C) which can eﬃciently activate peroxydisulfate (PDS) to selectively remove some organic pollutants following an unreported nonradical pathway.

Generally, PDS is used as an oxidant for the degradation of polluted water/soil, it generates sulfate radical intermediate for the degradation pollutants, and this radical unselectively reacts with most of the substances in water/soil. PDS can be activated by base, heat, UV, low valent transition metal ions (e.g., Co^2+, Fe^2+, Cu^2+, and Ag⁺), and zero valent iron to produce a sulfate radical (SO₄^•−). All these PDS activation processes consume extensive energy or chemicals. Also, it is found that PDS is much cheaper and does not significantly reduce water pH upon addition. Sulfate radical is not preferred for purification of water. Therefore, recent research focus on the PDS activation without generating radicals is preferred to maximize its oxidation capacity for targeted pollutants.

This research work was published in Environmental Science & Technology on October 23, 2020 [1]. They find a new pathway for the degradation of pollutants without sulfate radicals, particularly, the coordinated Fe(III) is readily converted to Fe(V) through two-electron abstraction by PDS, and Fe(V) is responsible for the selective degradation of organic pollutants. The selectivity of the PDS/Fe−N−C oxidation process was examined with various pollutants, the PDS activation clearly follows a nonradical pathway. The highly dispersed Fe(III) sites on the Fe−N−C were proposed and converted to Fe(V) for pollutant degradation. This new approach is more effective than sulfate radical formation and also other reported nonradical oxidation like PDS/CuO under the same experimental conditions. This approach is also used to selectively degrade some organic pollutants through PDS activation.

Figure 1. Mechanism of nonradical oxidation of pollutants by single-atom-Fe(III) activated PDS [1].

If we use chlorinated water for purification process, chloride consumes a sulfate radical to produce a chlorine radical (Cl₂^•−), which not only reduces the oxidation potential toward pollutants but also produces hazardous chlorinated by-products [2]. Therefore, to avoid sulfate radical formation approach, a new approach is needed for water purification system with PDS. Lee et al., proposed similar approach that PDS can be attached to carbon nanotubes (CNTs) and react with phenolic compounds [3]. CNTs posses a high specific surface area (200−500 m²g⁻¹), however, their effectiveness for PDS activation still needs to be improved further.

Single-atom catalysts (SACs), with active metal sites dispersed in the single-atom state, have attracted a greater attention and wide interest in recent years for catalytic reactions. Some of the SACs were reported for the catalytic application with good efficiency. Particularly, a single cobalt-atom catalyst (Co−N−C), metal and nitrogen co-doped carbon materials [M−N−Cs (M = Fe, Co, Mn, etc.,)] and Fe(III)-doped g-C₃N has been successfully prepared, which showed good activity in peroxymonosulfate (PMS) activation. Also, the SACs have utilized its metal sites, that shows an excellent catalytic activity, structural stability and becomes a frontier in catalysis, especially for hydrogen evolution, oxygen reduction, and CO₂ conversion. These results inspired us to search for an appropriate M−N−C to activate for the cheaper PDS.

As an ﬁrst attempt, Jiang et al. and his team have apply a single atom Fe(III) for eﬀective PDS activation without generating radicals. Further, they have prepared single atom site of Fe−N−C characterized by Aberration-corrected high-angle annular dark-ﬁeld scanning transmission electron microscopy, X-ray adsorption spectra (XAS) including X-ray adsorption near-edge structure (XANES) and extended X-ray adsorption ﬁne structure (EXAFS).

Oxidation of pollutants was tested with batch reactions. They used phenol, bisphenol A, chlorophenols, sulfamethoxazole, atrazine, benzoic acid, EDTA, p-chloro benzoic acid, and benzothiazole to test the selectivity of the PDS/Fe−N−C-coupled oxidation. Further, they observed that all the compounds were degraded in this process except for benzothiazole, p-chloro benzoic acid, and benzoic acid. Benzoic acid and p-chloro benzoic acid have low reactivity possibly because of no substituent group existing at their α-C, which is not favorable for Fe(V) attack. Because of the high stability of the thiazole ring, benzothiazole was barely degraded here. The selectivity also shows that PDS/Fe−N−C has lower oxidation power as compared with sulfate and hydroxyl radicals.

Important points regarding environmental significance of this research work:

The single-atom Fe(III) material, Fe−N−C, can eﬃciently activate PDS to degrade some organic pollutants.
The PDS activation follows a nonradical pathway with Fe(V) as the possible intermediate oxidant for pollutant removal.
This reported SACs were used for the remediation of polluted groundwater and soil, and the treatment of drinking water and some industrial wastewater without any toxicity.
A low dosage of the Fe−N−C can achieve relatively high eﬃciency in pollutant degradation.

Our SNB team recommended this research article to enrich our viewer’s knowledge to know about the single-atom Fe(III)- and nitrogen doped carbon (Fe−N−C) can eﬃciently activate PDS to selectively remove some organic pollutants followed with an unreported nonradical pathway. The research studies showed that PDS can be activated while not generating radicals, that is, the pollutants can be degraded just through electron transfer to PDS. Hence, the future work is needed to develop an low cost approaches to obtain a large amount of single-atom Fe−N−C materials (e.g., through pyrolysis of biosolids) and to investigate the selectivity of the PDS/Fe−N−C oxidation and the transformation pathway of pollutants.

References

N. Jiang, et al., Environ. Sci. Technol. (2020) https://dx.doi.org/10.1021/acs.est.0c04867.
T. Zhang, et al., Environ. Sci. Technol. 48, 5868 (2014).
H. Lee, et al., Chem. Eng. J. 266, 28 (2015).

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National College, Tiruchirappalli

Tamil Nadu, India

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