AJKOER - 1-4-2018 at 16:08
An interesting, but advanced, 2013 paper on Fenton, "Fenton chemistry at aqueous interfaces" by Shinichi Enamia, Yosuke Sakamotod and Agustín J.
Colussie, available at http://www.pnas.org/content/pnas/111/2/623.full.pdf .
The work suggests the reaction of fenton agents on the surface of aqueous FeCl2 microjets promotes the fenton reaction by 3 orders of magnitude!
The acceleration is due to interfacial Fe(H2O)n (2+) ions reacting with H2O2 (and also with zone), a thousand times faster than Fe(H2O)6 (2+) ions in
bulk water, via a process that favors inner-sphere two-electron O-atom over outer-sphere one-electron transfers.
Interestingly, the main reaction (Equation R1) is the classic Fenton reaction which is, claimed to be correct except for coefficients, to quote:
"A comparison of Figs. 1 and 5 confirms that H2O2 and O3 react with interfacial Fe2+ along reactions R1–R2 and R3a,b–R4, respectively, leading to
(formally) the same products, albeit in different proportions."
and is seemingly (see below on the acidity of water at the interface) not an acid catalyzed reaction:
R1 Fe(ll) + H2O2 → Fe(lll) + OH- + ·OH
Another source (see Table S-1, Reaction R1 also, at https://www.atmos-chem-phys.net/13/11259/2013/acp-13-11259-2... ) notes how slow the above reaction is, in bulk solution, with a rate constant of
just 7.0×10. However, the same source notes the following:
R3 H2O2 + Fe(OH)+ → Fe(OH)2+ + ·OH + OH− (moderately fast, k=1.9x10^6)
E1 Fe2+ + H2O ⇆ Fe(OH)+ + H+ (very slow)
So, perhaps the whole paper may just be a confirmation of the faster reaction R3 having, via microjets with Fe(H2O)n (2+) ions, moved the slow
reaction E1 to the right, which produces both Fe(OH)+ (which, with H2O2 per R3 is a much improved fenton) and even H+ to acid catalyze the Fenton
reaction R1 (which is now really R3 above)!?
So there may be some practical significance for fenton applications employing micro jets based on this 2013 work.
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An interesting mentioned feature of the 2013 work is that the use of an aerosol changes the dielectric constant and effective pH, to quote from the
paper:
"Whereas bulk water is neutral at pH 7, the aerial surface is neutral on bulk water at pH ∼3.5 (63, 65, 78). This caveat prevents direct comparisons
from being made between the pH dependences observed herein and those previously reported for similar experiments in bulk solution. "
As such, the superoxide, which could be formed form the action of oxygen on ferrous (or metallic iron, so called metal auto-oxidation) by:
Fe(0)/Fe(ll) + O2 = Fe(ll)/Fe(lll) + .O2-
which can be converted into the .OOH radical, and thereby move the above reaction to the right, via:
H+ + .O2- = .HO2 (pKa = 4.88)
And the latter into more hydrogen peroxide:
R39 .HO2 + .HO2 → H2O2 + O2 (k=8.6×10^5, fair to moderately fast)
And, also by:
R10 .HO2 + Fe2+ + H+ → Fe3+ + H2O2 (k= 1.2×10^6)
The two above reactions could potentially and effectively recycle some H2O2 (at the expense of some ferrous) for a bulk solution pHs above 4.88, where
normally .HO2 is pH unfavored (if not for the use of microjets).
On pH, one further quote:
"surface of more concentrated Fe2+ solutions. In this context, it is relevant to point out that we recently found that hydronium (H3O+) emerges at the
surface of water less than pH 4 as a “superacid” that protonates impinging gases having proton affinities larger than water (62, 64, 90).
Thermodynamics dictates that this is possible only if interfacial H3O+ is weakly hydrated. If Fe2+ behaves similarly, the enhanced reactivity of IF
relative to B and its emergence at the surface of more concentrated solutions could be alternatively ascribed to an incomplete hydration shell of IF."
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Here is a 2008 work noting the important aspect of pH especially with regard to cycling of reagents.
"pH Effects on Iron-Catalyzed Oxidation using Fenton’s Reagent", by Christopher K. Duesterberg, Steven E. Mylon and T. David Waite, at School of
Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia, and Department of Chemistry, Lafayette College,
Easton, Pennsylvania 18042. Published in Environ. Sci. Technol., 2008, 42 (22), pp 8522–8527, DOI: 10.1021/es801720d .
"Synopsis
The effect of pH on the Fenton-mediated oxidation of formic acid is investigated and shown to be dominated by the extent of catalytic cycling of iron
between II and III oxidation states at a particular pH.
Abstract
This work extends investigations into the development and use of a kinetic model to simulate and improve the iron-catalyzed oxidation of organic
compounds using Fenton’s reagent. While a number of recent studies have successfully modeled the kinetics and species behavior in simple Fenton
systems, none have extended and applied the model to examine the effect of operating parameters such as pH on treatment performance. The purpose of
this work is to investigate the effect of pH in Fenton-based oxidation systems and to use kinetic modeling to gain insight into the reaction mechanism
and speciation of the iron catalyst. Laboratory experiments were conducted across a range of starting concentrations of Fe(II) and H2O2 at pHs of 2.5,
3.0, and 4.0, both in the presence and absence of a target organic, formic acid (HCOOH). With minor modifications, the model presented is capable of
accurately describing changes in Fe(II) concentrations over a wide range of reaction conditions and, provided account is taken of additional hydroxyl
radical scavenging pathways, also accounts for the oxidation of formic acid over extended reaction times at all pHs considered. The use of composite
values for rate constants of reactions involving weakly acidic species is shown to be appropriate, and analysis of the model reveals the catalytic
role iron plays in the oxidation process. Experimental and simulated data at the different pHs highlights the effect the catalytic redox cycling of
iron has on the performance and oxidation capacity of the Fenton system."
Link: https://pubs.acs.org/doi/abs/10.1021/es801720d
[Edited on 2-4-2018 by AJKOER]
[Edited on 2-4-2018 by AJKOER]
AJKOER - 2-4-2018 at 08:12
Found another 2014 work based on the above model, "Fenton Oxidation of Gaseous Isoprene on Aqueous Surfaces" by F. Rifkha Kameel, et al., in J. Phys.
Chem. C, 2014, 118 (50), pp 29151–29158, DOI: 10.1021/jp505010e , July 10, 2014. To quote:
"Abstract
We report that gaseous isoprene ISO(g) is oxidized into soluble species on the surface of aqueous acidic FeCl2 solutions simultaneously exposed to
H2O2(g). In our experiments, ISO(g) and/or H2O2(g) streams intersect aqueous pH ∼ 2 FeCl2 microjets for ∼10 μs. The products formed in these
reactive encounters are identified in situ via online electrospray ionization mass spectrometry. We found that the (ISO)nH+ oligomers generated from
ISO(g) on the surface of pH < 4 water are oxidized into myriad products whose combined yields exceed 5%. MS2 analysis reveals that the positive
ions derived from the protonation of neutral products split H2O and O neutrals, whereas the less abundant negative carboxylate ion products undergo
CO, H2O, and CO2 losses. Significantly, all products are fully quenched by ·OH scavenger tert-butyl alcohol. These results are consistent with an
oxidation process initiated by the addition of ·OH from (Fe2+(aq) + H2O2(g)) to (ISO)nH+, followed by fast reactions involving dissolved H2O2, HO2·,
and O2 that lead to polyols; carbonyls; and, to a lesser extent, carboxylic acids. Our experiments demonstrate that gas-phase olefins are oxidized
upon colliding on the surface of Fe-containing acidic aqueous media under typical tropospheric conditions."
Link: https://pubs.acs.org/doi/abs/10.1021/jp505010e
Note, the citing of the active role of dissolved H2O2, HO2·, and O2 is in line with my added comments previously. Also, the original paper notes:
"The modest concentration enhancements of many gases at the air–water interface predicted by theoretical simulation (70) and demonstrated
experimentally (71–74)"
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Below is a patent citation combining cavitation and micro-jets. To quote:
"The present invention addresses the above problems, to provide a method for the advanced oxidation dye wastewater rapid decolorization of dye
wastewater. Since the reinforcing effect of ultrasound, it produces cavitation bubbles with shock wave at the time of the crash and Qiang Lie
microjets phenomena, causing intense local turbulence, thereby enhancing the mass transfer rate of the homogeneous and heterogeneous systems the · OH
radical timely react with the dye molecules. Thus, the COD removal efficiency in the same conditions, the high efficiency, in particular consume less
hydrogen peroxide, ferrous ion and acid, sludge production can be reduced, reducing operating costs."
Patent CN1546395A "Advanced oxidation method for treatment of dye waste water", available at https://patents.google.com/patent/CN1546395A/en .
[Edited on 3-4-2018 by AJKOER]