General Chemistry of Fenton's Reagent Fe
2+
+ H2O2 → Fe
3+
+ OH - + . OH
Fe 3+ + H2O2 → Fe 2+ + . OOH + H+
The procedure requires:
•
adjusting the wastewater to pH 3-5;
•
adding the iron catalyst (as a solution of FeSO4); and
•
adding slowly the H2O2. If the pH is too high, the iron precipitates as Fe(OH)3 and catalytically decomposes the H2O2 to oxygen -- potentially creating a hazardous situation.
Reacti Reaction on rates rates with with Fenton Fenton’s ’s Reagen Reagentt are genera generally lly limite limited d by the rate rate of . OH generation generation (i.e., concentration of iron catalyst) and less so by the specific wastewater being treated. Typical Fe:H2O2 ratios are 1:5-10 wt/wt, though iron levels < 25-50 mg/L can require excessive reaction times (10-24 hours). This is particularly true where the oxidation products (organic acids) sequester the iron and remove it from the catalytic cycle. Fenton’s Reagent is most effective as a pretreatment tool, where COD’s are > 500 mg/L. This is due to the loss in selectivity as pollutant levels decrease: .
OH + H2O2 → . O2H + H2O
.
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 is recommended that the reaction always be characterized through laboratory treatability tests before proceeding to plant scale.
Hydroxyl radical reactivity The hydroxyl radical is one of the most reactive chemical species known, second only to elemental fluorine in its reactivity (see below).
Reactive Species
Relative Oxidation Power (Cl2=1.0)
Flourine
2.23
Hydroxyl radical
2.06
Atomic oxygen (singlet)
1.78
Hydrogen peroxide
1.31
Perhydroxyl radical
1.25
Permanganate
1.24
Hypobromous acid
1.17
Chlorine dioxide
1.15
Hypochlorous acid
1.1
Hypiodous acid
1.07
Chlorine
1
Bromine
0.8
Iodine
0.54
The chemical reactions of the hydroxyl radical in water are of four types: Addition: . OH + C6H6 → (OH)C6H6 . where the hydroxyl radical adds to an unsaturated compound, aliphatic or aromatic, to form a free radical product (cyclohexadienyl radical shown above). Hydrogen Abstraction: . OH + CH3OH → . CH2OH + H2O where an organic free radical and water are formed. Electron Transfer: . OH + [Fe(CN)6] 4- → [Fe(CN)6] 3- + OH where ions of a higher valence state are formed, or an atom or free radical if a mononegative ion is oxidized. Radical Interaction: . OH + . OH → H2O2 where the hydroxyl radical reacts with another hydroxyl radical, or with an unlike radical, to combine or to disproportionate to form a stable product. In applying Fenton’s Reagent for industrial waste treatment, the conditions of the reaction are adjusted so that first two mechanisms (hydrogen abstraction and oxygen addition) predominate. Typical rates of reaction between the hydroxyl radical and organic materials are 109 - 1010 k (M-1 s-1).
Effect of Iron Concentration
In the absence of iron, there is no evidence of hydroxyl radical formation when, for example, H2O2 is added to a phenolic wastewater (i.e., no reduction in the level of phenol occurs). As the concentration of iron is increased, phenol removal accelerates until a point is reached where further addition of iron becomes inefficient. This feature (an optimal dose range for iron catalyst) is characteristic of Fenton’s Reagent, although the definition of the range varies between wastewaters. Three factors typically influence its definition:
1.
A minimal threshold concentration of 3-15 mg/L Fe which allows the reaction to proceed within a reasonable 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 parts substrate, which produces the desired end products. Note that the ratio of Fe:substrate may affect the distribution of reaction products; and
3.
A supplemental aliquot of Fe which saturates the chelating properties in the wastewater, thereby availing unsequestered iron to catalyze the formation of hydroxyl radicals.
Iron dose may also be expressed as a ratio to H2O2 dose. Typical ranges are 1 part Fe per 5-25 parts H2O2 (wt/wt).
Effect of Iron Type (Ferrous or Ferric) For most applications, it does not matter whether Fe2+ or Fe3+ salts are used to catalyze the reaction -the catalytic cycle begins quickly if H2O2 and organic material are in abundance. However, if low doses of Fenton’s Reagent are being used (e.g., < 10-25 mg/L H2O2), some research suggests ferrous iron may be preferred. Neither does it matter whether a chloride or sulfate salt of the iron is used, although with the former, 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, separating the iron floc, and re-acidifying the iron sludge. There have been some recent developments in supported catalysts that facilitate iron recovery and reuse.
Effect of H2O2 Concentration Because of the indiscriminate nature by which hydroxyl radicals oxidize organic materials, it is important to profile the reaction in the laboratory for each waste to be treated. For example, in a typical application the following series of reactions will occur: Substrate →
Oxidized
→
Oxidized
→ Oxidized
→ Oxidized
→ Oxidized
Intermediate
Intermediate
Intermediate
Intermediate
Intermediate
"A"
"B"
"C"
"D"
"E"
→ CO2
Each transformation in this series has its own reaction rate and, as the case of phenolics illustrates, there may occur build-up of an undesirable intermediate (quinones), which requires sufficient H2O2 to be added to push the reaction beyond that point. This is frequently seen when pretreating a complex organic wastewater for toxicity reduction. As the H2O2 dose is increased, a steady reduction in COD may occur with little or no change in toxicity until a threshold is attained, whereupon further addition of H2O2 results in a rapid decrease in wastewater toxicity.
Effect of Temperature The rate of reaction with Fenton’s Reagent increases with increasing temperature, with the effect more pronounced at temperatures < 20 deg-C. However, as temperatures increase above 40-50 deg-C, the efficiency of H2O2 utilization declines. This is due to the accelerated decomposition of H2O2 into oxygen and water. As a practical matter, most commercial applications of Fenton’s Reagent occur at temperatures between 20-40 deg-C. Applications of Fenton’s Reagent for pretreating high strength wastes may require controlled or sequential addition of H2O2 to moderate the rise in temperature which occurs as the reaction proceeds. This should be expected when H2O2 doses exceed 10-20 g/L. Moderating the temperature is important not only for economic reasons, but for safety reasons as well.
Effect of pH The effect of pH on reaction efficiency is illustrated below:
The optimal pH occurs between pH 3 and pH 6. The drop in efficiency on the basic side is attributed to the transition of iron from a hydrated ferrous ion to a colloidal ferric species. In the latter form, iron catalytically decomposes the H2O2 into oxygen and water, without forming hydroxyl radicals. There have been some recent developments using nonradical scavenging sequestering agents (e.g., NTA and gallic acid) to extend the useful pH range to pH 8-9, but no commercial applications are known. The drop in efficiency on the acid side is less dramatic given the logarithmic function of pH, and is generally a concern only with high application rates.
A second aspect of pH deals with its shift as the reaction progresses. Provided an initial wastewater pH of 6.0, the following profile is typical of Fenton reactions.
The first inflection is caused by the addition of FeSO4 catalyst which typically contains residual H2SO4. A second, more pronounced drop in pH occurs as the H2O2 is added, and continues gradually at a rate which is largely dependent on catalyst concentration. This drop in pH is attributed to the fragmenting of organic material into organic acids. This pH change is often monitored to ensure that the reaction is progressing as planned -- the absence of such a pH decrease may mean that the reaction is inhibited and that a potentially hazardous build-up of H2O2 is occurring within the reaction mixture. In highly concentrated waste streams (>10 g/L COD), it may be necessary to perform the oxidation in steps, readjusting the pH upwards to pH 4-5 after each step so as to prevent low pH from inhibiting the reaction.
Effect of Reaction Time The time needed to complete a Fenton reaction will depend on the many variables discussed above, most notably 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 reaction may take several hours. In such cases, performing the reaction in steps (adding both iron and H2O2) may be more effective (and safer) than increasing the initial charges. Determining the completion of the reaction may prove troublesome. The presence of residual H2O2 will interfere with many wastewater analyses. Residual H2O2 may be removed by raising the pH to e.g., pH 7-10, or by neutralizing with bisulfite solution. Often, observing color changes can used to assess the reaction progression. Wastewaters will typically darken upon H2O2 addition and clear up as the reaction reaches completion.
Effect of Post Treatment As a result of degrading complex organic materials into organic acid fragments, the pre-oxidized effluent is generally more amenable to conventional treatment, e.g., flocculation and biotreatment. The presence of iron in the reaction mixture makes it particularly suited to subsequent lime flocculation. In many
cases, it may be possible to remove up to 80% of the wastewater COD through a combination of Fenton’s Reagent and lime flocculation. Significantly, this may be achieved with an H2O2 dose of 5075% of the stoichiometry.
Process Equipment Historically, Fenton’s Reagent has been applied in batch mode. Today, however, it is being used in both continuous and sequential batch processes. A typical batch operation would consist of chemical storage and dosing modules (for H2O2, FeSO4, acid, and lime/NaOH); a primary reactor and (optional) holding tank; a solids dewatering device (optional); and miscellaneous temperature and pH controls. The materials of construction for the reactor and holding tank are typically Types 304 or 316 stainless steel, while those for the chemical storage systems may also be HDPE. Packaged Fenton’s systems are available from some equipment providers, or US Peroxide 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", in Ind. Eng. Chem., Process Design & Development, vol.7, pp. 1110-117 (1968). "Fenton’s Reagent Revisited", in Accts of Chem. Research, vol. 8, pp. 125-131 (1975).
Fenton’s Reagent
Many metals have special oxygen transfer properties which improve the utility of hydrogen peroxide. By far, the most common of these is iron which, when used in the prescribed manner, results in the generation of highly reactive 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 1930s once the mechanisms were identified. Today, Fentons Reagent 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, plastics additives, 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
Fenton's Reagent Case Studies •
A Louisiana chemical manufacturer used Fenton's process to pre-treat a 40 GPM wastewater stream, removing more than 90% of phenols.
•
A refinery in the southeast US effectively pre-treated a 15 GPM scrubber blowdown stream prior to discharge to on-site biological treatment.
•
A western US wood treating facility used Fenton's Reagent to remove phenols, napthols and cresols and eliminate inhibition of biomass and discharge violations.
•
An aircraft paint stripping facility in the Midwest US pre-treated a wastewater stream to reduce targeted toxic organics by more than 95% prior to discharge.
•
An Alabama chemical plant performed emergency treatment of phenol-contaminated wastewater, reducing levels from 200-300 mg/L to 0.2 mg/L overnight.
Hydrogen Peroxide (H2O2) As simple as it may seem, the treatment of contaminated waters is as diverse and complicated as the operations from which it comes. In today's environment, where merely transferring contaminants from one medium to another is no longer acceptable, it is no surprise that a powerful oxidizer that looks like water -- in its appearance, chemical formula and reaction products -- should be so widely used. This is hydrogen peroxide (H2O2) -- a powerful yet versatile oxidant that is both safe and effective.
H2O2 Advantages
Powerful - H2O2 is one of the most powerful oxidizers known -- stronger than chlorine, chlorine dioxide, and potassium permanganate. And through catalysis, H2O2 can be converted into hydroxyl radicals (.OH) with reactivity second only to fluorine. Oxidant
Oxidation Potential, V
Fluorine
3.0
Hydroxyl radical
2.8
Ozone
2.1
Hydrogen peroxide
1.8
Potassium permanganate1.7
Chlorine dioxide
1.5
Chlorine
1.4
Safe - Despite its power, H2O2 is a natural metabolite of many organisms, which decompose the H2O2 they produce into oxygen and water. H2O2 is also formed by the action of sunlight on water -- a natural purification system for our environment. Consequently, H2O2 has none of the problems of gaseous release or chemical residues that are associated with other chemical oxidants. And since H2O2 is totally miscible with water, the issue of safety is one of concentration. Industrial strength H2O2 is a strong oxidizer and as such requires special handling precautions. Versatile - The fact that H2O2 is used for seemingly converse applications proves its versatility. For example, it can inhibit microbial growth (as in the biofouling of water circuits) and encourage microbial growth (as in the bioremediation of contaminated groundwaters and soils). Similarly, it can treat both easy-to-oxidize pollutants (iron and sulfides) and difficult to oxidize pollutants (solvents, gasolines and pesticides). Selective - The reason why H2O2 can be used for such diverse applications is the different ways in which its power can be directed -- termed selectivity. By simply adjusting the conditions of the reaction (e.g., pH, temperature, dose, reaction time, and/or catalyst addition), H2O2 can often be made to oxidize one pollutant over another, or even to favor different oxidation products from the same pollutant. Widely Used - Since it was first commercialized in the 1800's, H2O2 production has now grown to over a billion pounds per year (as 100%). In addition to pollution control, H2O2 is used to bleach textiles and paper products, and to manufacture or process foods, minerals, petrochemicals, and consumer
products (detergents). Its use for pollution control parallels those of the movement itself -- municipal wastewater applications in the 1970's; industrial waste/wastewater applications in the 1980's; and more recently, air applications in the 1990's. Today, H2O2 is readily available throughout the U.S. in drum, tote, mini-bulk, and bulk quantities in concentrations of 35% or 50% by weight.
End Use Industries
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Landfills
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Food processing
•
Oil refining
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Electronics
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Mining / metallurgy
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Pulp and paper
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Machining
•
Timber products
•
Textiles
•
Hazardous wastes
•
Power production
•
Site remediation
•
Composting
•
Potable water
•
Municipal wastewater
•
Chemicals and resins
Environmental Applications of H 2O2 H2O2 applications span the range of possible media: air, water, wastewater, soils and sludges. Depending on the objective, H2O2 may be used either alone or in combination with other processes to enhance their performance.
Stand-Alone Applications Odor control - Oxidizes hydrogen sulfide, mercaptans, amines and aldehydes. H2O2 may be applied directly to aqueous wastes containing these odorants, or to wet scrubbers used to remove them from airstreams. If the odors are the result of biological activity, H2O2 may instead be added as a preventative to eliminate the anoxic conditions which favor the generation of odors. Corrosion control - destroys residual chlorine and reduced sulfur compounds thiosulfates, sulfites, and sulfides) which form corrosive acids when condensed onto processing equipment and oxidized by air. BOD/COD removal - Oxidizes both organic and inorganic pollutants which contribute to BOD and COD -- catalytic, H2O2 may be needed to oxidize the more resistant substances. H2O2 may also affect BOD/COD removal by enhancing the performance of other processes (see below). Inorganic oxidation - Oxidizes cyanides, NOx/SOx, nitrites, hydrazine, carbonyl sulfide, and other reduced sulfur compounds mentioned above (odor/corrosion control).
Organic oxidation - Hydrolyzes formaldehyde, carbon disulfide, carbohydrates, organophosphorus and nitrogen compounds, and various water-soluble polymers; and (with catalysis) destroys phenols, BTEX pesticides, solvents, plasticizers, chelants, and virtually any other organic requiring treatment. Metals oxidation - Oxidizes ferrous iron, manganese, arsenic, and selenium to improve their adsorption, filtration, or precipitation from process waters and wastewaters. Toxicity reduction/Biodegradability improvement - With catalysis, chemically digests complex organics into smaller, less toxic and more bi odegradable fragments. Disinfection/Bio-control - Checks excess biogrowth in water supplies and cooling circuits, and (with catalysis) disinfects process waters and biological effluents.
Enhancement (Combination) Applications Flocculationlprecipitation - Oxidizes metal complexes and improves the performance of inorganic flocculants. Air Flotation - Releases evenly dispersed microbubbles which entrain emulsified fats, oils and greases to enhance their removal in air flotation units and grease traps. Biotreatnent - As a pretreatment - degrades toxic, refractory or bio-inhibitory organics, rendering them more amenable to biodegradation. In con junction with - provides a supplemental source of dissolved oxygen in-situ (penetrating both soil columns and bioflocs, eliminating the sludge bulking phenomenon). As a polishing step - destroys trace levels of organics that pass through biotreatment, providing the ancillary benefit of disinfection. Filtration - Controls biofouling of UF and RO membranes while eliminating foul odors from media filters. Carbon adsorption - Enhances the adsorption of many pollutants while providing dissolved oxygen to support biologically-active carbon beds (improving removal efficiencies still further). Air scrubbers - Replaces chlorine for deodorizing offgases and controlling VOC's. Depending on the target pollutant(s), catalytic or Advanced Oxidation Processes may be required. Incineration - Provides supplemental oxygen to improve combustion efficiencies and lower operating temperatures.
H2O2 Processes Simple H2O2 - Most H2O2 applications involve its simple injection into the water stream with no requirement for additional chemicals or equipment. These include the control of biogrowth (slime), the supply of supplemental oxygen, the removal of FOG and chlorine residuals, and the oxidation of sulfides/sulfites, metals, and other easy-to-oxidize components of BOD/COD. Activation of H2O2 in these applications may be affected by the adjustment/control of pH, temperature, and/or reaction time.
Catalytic H2O2 - The more difficult-to-oxidize pollutants may require the H2O2 to be activated with catalysts such as iron, copper, manganese, or other transition metal compounds. These catalysts may also be used to speed up H2O2 reactions that may otherwise take hours or days to complete. H2O2 catalysis may occur either in solution (using soluble catalysts) or in packed columns (using solid catalysts). Solution catalysis - The most commonly used solution catalyst is iron, which when used with H2O2 is referred to as Fenton's Reagent. The reaction requires a slightly acidic pH and results in the formation of highly reactive hydroxyl radicals (.OH) which are capable of degrading most organic pollutants. Another solution catalyst is copper, which is often used to destroy cyanides. Other metals also show catalytic activity with H2O2 and may be used to selectively destroy specific pollutants. Packed column catalysis - Solid catalysts eliminate the need to add soluble metals to the wastestrearn, and may offer greater flexibility in terms of reaction rates, selectivity, and the need for pH adjustment. This is an active area of research and many new developments are underway for a variety of applications. Advanced Oxidation Processes (AOP's) - AOP's represent the newest development in H2O2 technology, and are loosely defined as processes that generate highly reactive oxygen radicals without the addition of metal catalysts. Typically, this means combining H2O2 with ozone or ultraviolet light. The result is the on-site total destruction of even refractory organics without the generation of sludges or residues. This technology is being widely applied to treat contaminated groundwaters, to purify and disinfect drinking waters and process waters, and to destroy trace organics in industrial effluents.
Hydrogen Peroxide Applications •
Municipal Wastewater & Drinking Water Treatment
•
Industrial
•
Remediation
