A Review on the Degradation of Pollutants by Fenton-Like Systems Based on Zero-Valent Iron and Persulfate: Effects of Reduction Potentials, pH, and Anions Occurring in Waste Waters

Abstract

Among the advanced oxidation processes (AOPs), the Fenton reaction has attracted much attention in recent years for the treatment of water and wastewater. This review provides insight into a particular variant of the process, where soluble Fe(II) salts are replaced by zero-valent iron (ZVI), and hydrogen peroxide (H2O2) is replaced by persulfate (S2O82−). Heterogeneous Fenton with ZVI has the advantage of minimizing a major problem found with homogeneous Fenton. Indeed, the precipitation of Fe(III) at pH > 4 interferes with the recycling of Fe species and inhibits oxidation in homogeneous Fenton; in contrast, suspended ZVI as iron source is less sensitive to the increase of pH. Moreover, persulfate favors the production of sulfate radicals (SO4•−) that are more selective towards pollutant degradation, compared to the hydroxyl radicals (•OH) produced in classic, H2O2-based Fenton. Higher selectivity means that degradation of SO4•−-reactive contaminants is less affected by interfering agents typically found in wastewater; however, the ability of SO4•− to oxidize H2O/OH− to •OH makes it difficult to obtain conditions where SO4•− is the only reactive species. Research results have shown that ZVI-Fenton with persulfate works best at acidic pH, but it is often possible to get reasonable degradation at pH values that are not too far from neutrality. Moreover, inorganic ions that are very common in water and wastewater (Cl−, HCO3−, CO32−, NO3−, NO2−) can sometimes inhibit degradation by scavenging SO4•− and/or •OH, but in other cases they even enhance the process. Therefore, ZVI-Fenton with persulfate might perform unexpectedly well in some saline waters, although the possible formation of harmful by-products upon oxidation of the anions cannot be ruled out.

Austenitic Stainless Steel as a Catalyst Material for Photo-Fenton Degradation of Organic Dyes

“…Photodecoloration in the presence of persulfate ions also occurs with a high efficiency in the case of MB (88.4% at a 999 mg/L persulfate concentration, Figure 2a), although in this case, after the aforementioned concentration, there is a sharp efficiency decrease. This could be due to the fact that the higher amount of iron ions generated from stainless steel through persulfate-aided oxidation hinders the efficient harvesting of visible light [29,30]. In the experiments where the photodecoloration occurs in the presence of the sulfate and hydroxyl radicals scavenger tert-butanol (TBu), a decrease of up to 55% (for MB, Figure 2a) and up to 62% (for MO, Figure 2b) in the photo-Fenton efficiencies was registered for both oxidants, indicating the critical role that these species play in the degradation of organic matter.…”

“…Photodecoloration in the presence of persulfate ions also occurs with a high efficiency in the case of MB (88.4% at a 999 mg/L persulfate concentration, Figure 2a), although in this case, after the aforementioned concentration, there is a sharp efficiency decrease. This could be due to the fact that the higher amount of iron ions generated from stainless steel through persulfate-aided oxidation hinders the efficient harvesting of visible light [29,30].…”

“…Avoiding surface passivation in this step is crucial for achieving a high photodegradation yield [32]. The sulfate radicals can react with water to generate hydroxyl radicals, so for the persulfate oxidant, the dyes can be oxidized by both type of radicals [29,34,35], according to the following Mechanism (5):…”

Degradation Efficiency and Kinetics Analysis of an Advanced Oxidation Process Utilizing Ozone, Hydrogen Peroxide and Persulfate to Degrade the Dye Rhodamine B

“…Solid particles can increase the consumption of reagents needed for the oxidation of pollutants and absorb UV radiation. In turn, the ions can inhibit RhB degradation as a result of scavenging SO •− 4 and/or • OH radicals [49].…”

Systematic Performance Comparison of Fe3+/Fe0/Peroxymonosulfate and Fe3+/Fe0/Peroxydisulfate Systems for Organics Removal

“…Zero-valent iron (Fe 0 , ZVI) is regarded as one of the most versatile catalysts in environmental catalysis [9]. It is relatively efficient, cost-effective, and environmentally friendly [10]. ZVI has been widely used for many applications, including as a catalyst for Fenton oxidation [11] and sequestration [12].…”