Mechanistic Insights into the Markedly Decreased Oxidation Capacity of the Fe(II)/S₂O₈^2– Process with Increasing pH

Abstract

The poor oxidation capacity of the Fe­(II)/S2O8 2– [Fe­(II)/PDS] system at pH > 3.0 has limited its wide application in water treatment. To unravel the underlying mechanism, this study systematically evaluated the possible influencing factors over the pH range of 1.0–8.0 and developed a mathematical model to quantify these effects. Results showed that ∼82% of the generated Fe­(IV) could be used for pollutant degradation at pH 1.0, whereas negligible Fe­(IV) contribution was observed at pH 7.5. This dramatic decline of Fe­(IV) contribution with increasing pH dominantly accounted for the pH-dependent performance of the Fe­(II)/PDS process. Unexpectedly, Fe­(II) could consume ∼80% of the generated SO4 •– non-productively under both acidic and near-neutral conditions, while the larger formation of Fe­(III) precipitates at high pH inhibited the SO4 •– contribution mildly. Moreover, the strong Fe­(II) scavenging effect was difficult to be compensated for by slowing down the Fe­(II) dosing rate. The competition of dissolved oxygen with PDS for Fe­(II) was insignificant at pH ≤ 7.5, where the second-order rate constants for reactions of Fe­(II) with oxygen were much lower than or comparable to that between Fe­(II) and PDS. These findings could advance our understanding of the chemistry and application of the Fe­(II)/PDS process.

Gallic Acid Accelerates the Oxidation Ability of the Peracetic Acid/Fe(III) System for Bisphenol A Removal: Fate of Various Radicals

“…Interestingly, as depicted in Figure b, there was no obvious difference in BPA removal and 7-hydroxycoumarin generation in the air-saturated solution and N 2 -purged solution within 1 min. This phenomenon may be explained by the fact that Fe(II) reacted preferentially with PAA but not with O 2 within 1 min, as k PAA‑Fe(II) (1.10 × 10–1.56 × 10 4 M –1 s –1 ) is higher than k O2‑Fe(II) (0.15–121.4 M –1 s –1 ); , thus, O 2 barely participated in the first minute, and PAA was consumed quickly within 1 min (Figure S13). normalO 2 • − + CH 3 COOOH → • OH + CH 3 COO − + normalO 2 Fe ( II ) + normalO 2 • − + 2 normalH + → Fe ( III ) + normalH 2 normalO 2 …”

Were Persulfate-Based Advanced Oxidation Processes Really Understood? Basic Concepts, Cognitive Biases, and Experimental Details

“…423 With increasing pH, the quenching effect of Fe 2+ on Fe(IV) might become much more obvious. 452 If η(PMSO 2 ) still remained unchanged at nearly 100% in the presence of alcohols, it implied that RHVMOC might be the predominant reactive species. 10,104 Alcohols (e.g., eq 130, k Fe(IV),EtOH = 2.51 × 10 3 M −1 s −1 ; 453 k Co(IV),MeOH = 6.1 × 10 3 M −1 s −1 10 ) and the generated HO 2 • (eq 131, k Fe(IV),HOd 2 • = 2.0 × 10 6 M −1 s −1 ) 104 might also consume RHVMOC.…”

“…Common filter membranes include (i) polytetrafluoroethylene (PTFE), 77,375,408,436 (ii) polyethersulfone, 264,410,414 (iii) nylon, 412,440,508,509 (iv) glass fiber, 336,409,490,492 and (v) acetate cellulose. 132,218,222,452 The effect of some filter membranes on the adsorption of some substances might not be neglected.…”

Challenges Relating to the Quantification of Ferryl(IV) Ion and Hydroxyl Radical Generation Rates Using Methyl Phenyl Sulfoxide (PMSO), Phthalhydrazide, and Benzoic Acid as Probe Compounds in the Homogeneous Fenton Reaction

“…Dimethyl sulfoxide (DMSO) is also commonly used as a probe for HO • by quantification of methylsulfinic acid, one of the major products formed on reaction of DMSO with HO • (methanesulfonic acid and formaldehyde are additional oxidation products), with a particular advantage of DMSO being its inability to chelate iron ions. − DMSO reacts by oxygen atom transfer with [Fe IV O] 2+ to form dimethyl sulfone (DMSO 2 ), which is not formed when HO • is the oxidant. , This method is considered to be of particular value because HO • and [Fe IV O] 2+ react with most substrates to form identical oxidation products but the oxidation of sulfoxides to sulfones is recognized to be an exception. , Among sulfoxides, methyl phenyl sulfoxide (PMSO) is the most popular probe to characterize [Fe IV O] 2+ due to the possibility of convenient simultaneous quantification of PMSO and its [Fe IV O] 2+ oxidation product, methyl phenyl sulfone (PMSO 2 ), using high-performance liquid chromatography (HPLC). Relying on the use of PMSO, investigations of various Fe-based advanced oxidation processes (AOPs), including Fe­(II)/peroxydisulfate, − Fe­(II)/peroxymonosulfate, Fe­(II)/hypochlorous acid, Fe­(II)/peracetic acid, Fe­(II)/periodate, and iron (oxyhydr)­oxide-catalyzed heterogeneous Fenton reaction, , have used the formation of PMSO 2 as evidence for the generation of [Fe IV O] 2+ . However, compared with DMSO, the oxidation products of PMSO that are formed, especially in systems where [Fe IV O] 2+ and other strong oxidants such as HO • and/or sulfate radicals are concomitantly produced, ,,,, are more likely to interfere with the probe system by influencing the redox reactions of iron despite the chemical inertness of PMSO itself toward iron.…”