Fenton Chemistry

8/2/2019 Fenton Chemistry

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Reference Library/Applications.Industrial Wastewater - Fenton's Reagent 1 Iron-Catalyze... Page 1 of 6Ieference LibPeroxide AppI industrial,. ._ _ . , . . . . (I(c':.!

. / 1 _ "_ ( ( C - ) (.~.f, '-FENTON'S REAGENT

iron-catalyzed hydrogen peroxide

IntroductionMany metals have special oxygen transfer properties which improve the utifity of hydrogen peroxide. By far, themost common of these is iron which, when used in the prescribed manner, results in the generation of highlyreactive hydroxyl radicals (. OH). The reactivity of this system was first observed in 1894 by its inventor H.J.H.Fenton, but its utility was not recognized until the 1930's once the mechanisms were identified. Today, Fenton'sReagent is used to treat a variety of industrial wastes containing a range of toxic organic compounds (phenols,formaldehyde, BTEX, and complex wastes derived from dyestuffs, pesticides, wood preservatives, plasticsadditives, and rubber chemicals). The process may be applied to wastewaters, sludges, or contaminated soils,with the effects being:

Organic pollutant destruction Toxicity reduction Biodegradability improvement BOD / COD removal Odor and color removal

This entry presents the general chemistry of Fenton's Reagent, and discusses the parameters which affect itsperformance.

General Overview

Fe z+ + HzOz -->Fe 3+ + OH - + . OHFe 3+ + HzOz ---> Fe z+ + . OOH + H+

The procedure requires: adjusting the wastewater to pH 3-5; adding the iron catalyst (as a solution of FeS04); and adding slowly the H202. If the pH is too high, the iron precpltates as Fe(OH)3 and catalyticallydecomposes the H202 to oxygen -- potentially creating a hazardous situation.

Reaction rates with Fenton's Reagent are generally limited by the rate of . OH generation,O& .. concentration ofimn r.atalvsQ..a.ndless so by the specific wastewater being treated. Typical Fe:H202 ratios are 1:5-1 0 wVwL,tbollgh iron. leyels < 25-50 mglL can require ~ ~Y fL re .a &ti.on Jim e.s_ ..lQ:21..IlQurs1 Lhis is particularly true wherethe oxidation products (organic acids) sequester the iron and remove it from the catalytic cycle. Fenton's Reagentis most effective as a pretreatment tool. where COD's are> 500 mg/L. This is due to the loss in selectivity aspollutant levels decrease:

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Reference Library/Applications.Industrial Wastewater - Fenton's Reagent 1 Iron-Catalyze... Page 2 of6

. OH + H202 - - - > . 02H + H20

. OH + Fe 3+ - - - > Fe 2+ + OH -

In addition to free radical scavengers, the process is inhibited by (iron) chelants such as phosphates, EDTA,formaldehyde, and citric/oxalic acids. Because of the sensitivity of Fenton's Reagent to different wastewaters, it isrecommended that the reaction always be characterized through laboratory treatability tests before proceeding toplant scale.

Discussion

The hydroxyl radical is one of the most reactive chemical species known, second only to elemental fluorinein its reactivity (see below).Reactive SpeciesFlourineHydroxyl radicalAtomic oxygen (singlet)Hydrogen peroxidePerhydroxyl radicalPermanganateHypobromous acidChlorine dioxideHypochlorous acidHypiodous acidChlorineBromineIodine

Relative Oxidation Power (CI2=1.0)2.232.061.781.311.251.241.171.151.101.071.000.800.54

The chemical reactions of the hydroxyl radical in water are of four types:

where the hydroxyl radical adds to an unsaturated compound, aliphatic or aromatic, to form afree radical product (cyclohexadienyl radical shown above).

where an organic free radical and water are formed.Electron Transfer: .OH + [Fe(CN)sl4- --> [Fe(CN)613- + OH -

where ions of a higher valence state are formed, or an atom or free radical if a mononegativeion is oxidized.

Radical Interaction: .OH + . OH ---> H202where the hydroxyl radical reacts with another hydroxyl radical, or with an unlike radical, tocombine or to disproportionate to form a stable product.

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eference Library/Applications:lndustrial Wastewater - Fenton's Reagent / Iron-Catalyze... Page 3 of 6In applying Fenton's Reagent for industrial waste treatment, the conditions of the reaction are adjusted sothat first two mechanisms (hydrogen abstraction and oxygen addition) predominate. Typical rates ofreaction between the hydroxyl radical and organic materials are 109 - 1010 k (M-1 5-1).

1. A minimal threshold concentration of 3-15 mg/L Fe which allows the reaction to proceed within areasonable period of time regardless of the concentration of organic material;

2. A constant ratio of Fe:substrate above the minimal threshold, typically 1 part Fe per 10-50 partssubstrate, which produces the desired end products. Note that the ratio of Fe:substrate may affectthe distribution of reaction products; and3. A supplemental aliquot of Fe which saturates the chelating properties in the wastewater, therebyavailing unsequestered iron to catalyze the formation of hydroxyl radicals.

Iron dose may also be expressed as a ratio to H202 dose. Typical ranges are 1 part Fe per 5-25 partsH202 (wtIwt).

For most applications, it does not matter whether Fe2+ or Fe3+salts are used to catalyze the reaction -- thecatalytic cycle begins quickly i f H202 and organic material are in abundance. However, if tow doses ofFenton's Reagent are being used (e.g., < 10-25 mglL H202). some research suggests ferrous iron may bepreferred. Neither does it matter whether a chloride or sulfate salt of the iron is used, although with theformer, chlorine may be generated at high rates of application.It is also possible to recycle the iron following the reaction. This can be done by raising the pH, separatingthe iron floc, and re-acidifying the iron sludge. There have been some recent developments in supportedcatalysts that facilitate iron recovery and reuse.

Because of the indiscriminate nature by which hydroxyl radicals oxidize organic materials, it is important toprofile the reaction in the laboratory for each waste to be treated. For example, in a typical application thefollowing series of reactions will occur:

Oxidized Oxidized Oxidized OxidizedSubstrate ->Intermediate ->Intermediate ->Intermediate --> Intermediate -->"A ll 11B" IICJ~ 0"

OxidizedIntermediate ---> CO2IIEII

Each transformation in this series has its own reaction rate and, as the case of phenolics illustrates, theremay occur build-up of an undesirable intermediate (quinones). which requires sufficient H202 to be addedto push the reaction beyond that point. This js fr~!J!l:!l!ly seen when pretreating a complex organicwastewater for toxicity reduction. As the H202 dose is increased, a steady reduction in COD may occurwith little or no change in toxicity until a threshold is attained, whereupon further addition of H202 results in

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Reference Library/Applications:Industrial Wastewater - Fenton's Reagent I Iron-Catalyze... Page 5 of 6

Typical pH Profile of Fenton Reactions7 ~~~====~==~~~~~~~--~----~6 . .. .. .. .. .~::i:5:c

a. 432~~~~~~~~~~~=r~~~~~~

o o 3 10 30

The first inf~ion is caused.b.}Ltbe.additjon..olEeSQ4 ..catalyst.whlch typically,00Iltair.1s.residuaLH2S04. A- second, ..more prooounce.d.dropJn.pHo!:curs ..as.theJ;"I202.is added.andcontlnues grClc:lu~lIyat a ratewhich is large.1v;lt;!Pfl"n.o.eotoncatalyst concentration ..This drop in pH is attributed to the fragmenting oforganic material into organic acids. This pH change is often monitored to ensure that the reaction isprogressing as planned -- the absence of such a pH decrease may mean that the reaction is inhibited andthat a potentially hazardous build-up of H202 is occurring within the reaction mixture ..

Reaction time-min

In highly concentrated waste streams (>10 gIL COD), it may be necessary to perform the oxidation insteps, readjusting the pH upwards to pH 4-5 after each step so as to prevent low pH from inhibiting thereaction.

The time needed to complete a Fenton reaction will depend on the many variables discussed above, mostnotably catalyst dose and wastewater strength. For simple phenol oxidation (less than ca. 250 mg/L ) ,typical reaction times are 30 - 60 minutes. For more complex or more concentrated wastes, the reactionmay take several hours. In such cases, performing the reaction in steps (adding both iron and H202) maybe more effective (and safer) than increasing the initial charges.Determining the completion of the reaction may prove troublesome. The presence of residual H202 willinterlere with many wastewater analyses {see Interferences with Analytical Methods).~92 may'"be removed . . H to e.... H 7-10 or b neutralizing with bisulfite solution. Often, observingco or changes can used to assess the reaction progresslon'~_"d'"~'" _.~y'PI~~y(j?rKElI1,up,onH202addition and clear up as the reaction reaches completion. . "' ,.

f f f fJ_ct QtPC)$trre~tmentAs a result of degrading complex organic materials into organic acid fragments, the pre-oxidized effluent isgenerally more amenable to conventional treatment, e.g., flocculation and biotreatment. The presence ofiron in the reaction mixture makes it particularly suited to subsequent lime flocculation. In many cases, itmay be possible to remove up to 80% of the wastewater COD through a combination of Fenton's Reagentand lime flocculation. Significantly, this may be achieved with an H202 dose of 50-75% of thestoichiometry.

Process EquipmentHistorically, Fenton's Reagent has been applied in batch mode. Today, however, it is being used in bothcontinuous and sequential batch processes. A typical batch operation would consist of chemical storage and

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" Reference Library/Applications:Industrial Wastewater - Fenton's Reagent I Iron-Catalyze... Page 6 of 6

dosing modules (for H202, FeS04, acid, and lime/NaOH); a primary reactor and (optional) holding tank; a solidsdewatering device (optional); and miscellaneous temperature and pH controls. The materials of construction torthe reactor and holding tank are typically Types 304 or 316 stainless steel, while those for the chemical storagesystems may also be HDPE. Packaged Fenton's systems are available from some equipment providers, or USPeroxide can provide you with engineering guidance on custom designs and retrofits.

References

Bishop, D.F. et.al, "Hydrogen Peroxide Catalytic Oxidation of Refractory Organics in Municipal Waste Waters", inIndLe:!lg,_C_t!~m" Pr9C~Sl;)Oe~!gn &P~ye.lQPment, vol.7, pp. 1110-117 (1968).Walling, Cheves "Fenton's Reagent Revisited", in A G . c l s QLGhem, He~earch, vol. 8, pp. 125-131 (1975).

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