Advanced oxidation processes

A presentation
on
Advanced Oxidation Processes

Advanced Oxidation Processes
• Advanced oxidation processes (AOPs) are used to oxidized organic compound
that are not degrade in biological or secondary stage of water treatment.
• In oxidation process, It may not be necessary to oxidize a given compound or
group of compound completely.
• Partial oxidation is sufficient to render specific compound more amenable to
subsequent biological treatment or to reduce their toxicity.

The oxidation of specific compound may be characterized extent of degradation of
final products as follow:
• Primary degradation : Structural change in the patent compound
• Acceptable degradation : Structural change in the patent compound to the extent
that toxicity is reduced.
• Ultimate degradation : Conversion of organic carbon to inorganic CO2
• Unacceptable degradation: Structural change in the patent compound resulting
in increased toxicity.
Advanced Oxidation Processes cont.…..

Theory of Advanced Oxidation
• Advanced oxidation processes typically involve the generation and use of
hydroxyl free radical (OH*) as a strong oxidant to destroy the compounds that
cannot be oxidized by conventional oxidants such as ozone, oxygen and chlorine
expect fluorine.
• The hydroxyl free radical reacts with the dissolved constituent and initiating series
of oxidation reaction until the constituent mineralized.
• Hydroxyl free radical are capable of oxidizing all reduced materials present
without restriction of class or group of compound as compared to other oxidants.

• Ozone base AOPs
Ozone/UV AOPS
Ozone/Hydrogen peroxides AOPs
• UV base AOPs
Hydrogen peroxide/UV AOPs
Ozone/UV AOPS
Ozone/Hydrogen peroxides AOPs
• Fenton related AOPs
• Photocatalysis based AOP
• Ultrasound assisted AOPs
Technologies to produce Hydrogen free
radicals

• Ozone is classified as an AOP because the ozonation process generates hydroxyl
radicals and target compounds are oxidized both by the direct reaction with ozone
and by reaction with hydroxyl radicals.
• The reaction with hydroxyl radicals is much more important because the rate
constant for the reaction of the target compound with hydroxyl radicals is typically
several orders of magnitude higher than the apparent rate constant for the reaction
of the target compound with ozone.
Ozone based AOPs

Ozone/UV AOPs
Production of Hydroxyl free radicals with UV light can be illustrated by the
following reaction for photolysis of ozone.
Ozone based AOPs cont….

• Ozone/Hydrogen peroxide AOPs
• For the compound that do not adsorb UV, AOPs involving ozone/H2O2.
• Compounds in water such as trichloroethylene (TCE) and perchloroethylene
(PCE) have been reduced significantly with AOPs using hydrogen peroxides and
ozone to generate hydroxyl free radicals.
Ozone based AOPs cont….

• UV/photolysis is a process in which compounds absorb photons and the energy
released drives oxidation processes induced by light.
• The photolysis rate of a compound can be estimated based on the compound’s
light absorption rate and quantum yield.
• Some organic compounds, such as NDMA, can be reduced by photolysis alone.
UV based AOPs

Hydrogen peroxide/UV AOPs
• Hydroxyl radicals are also formed when water containing H2O2 is exposed to UV
light (200 to 280 nm).
UV based AOPs cont…..
• Hydrogen peroxide/UV process has been applied to oxidation of trace constituents
found in treated water.
• The process has been applied for removal of N-Nitrosodimethylamine (NDMA)
and other compounds of concern in treated water including
Sex and steroidal hormones
Drugs
Veterinary and human antibiotics
Industrial and household wastewater product

• Fenton’s oxidation was one of the best known metal catalysed oxidation reactions of
water-miscible organic compounds.
• The mixture of FeSO4 or any other ferrous complex and H2O2 (Fenton’s reagent) at
low enough pH, results in Fe2+ catalytic decomposition of H2O2
• Proceeds via a free radical chain process that produces hydroxyl radicals
• The hydroxyl radicals have extremely high oxidizing ability and could oxidize hard
to decompose organic compounds in a short time.
Fenton based AOPs

• The classical Fenton radical mechanisms primarily involve the following
reaction:
Fenton based AOPs cont..

Photo-Fenton process
• This process is combination of ultraviolet or visible light with conventional Fenton
process that can enhance the catalytic effect of iron and increase the degradation
efficiency of organic pollutants and reduce the iron sludge production.
• In this process reduction of Fe3+ to Fe2+ is accelerated by energy provided from
light source.
• Under light irradiation, Fe3+ which will be present as an intermediate of
[Fe(OH)]2+ undergoes metal charge transfer excitation.

Electro-Fenton Process
• This process is developed to overcome the limitations of classical Fenton process which is
related to the high costs and risks associated with handling, transportation and storage of
reagents and also the accumulation of iron sludge.
• Combine concept of Electrochemistry and classical Fenton process is called as Electro-
Fenton process.
• Reactants of Fenton process H2O2 & Fe2+ are in-situ generated via the electrochemical
reaction on cathode and anode.
• H2O2 is generated via reduction of O2 on the cathode, similarly Fe2+ is generated via
oxidation of Fe ion.

Merits
• Works on ambient Temperature and Pressure.
• Less energy inputs required.
• Rapid degradation and complete mineralization of
organic matter.
• Reduces organic loading in terms of chemical
oxygen demand and removal of recalcitrant and
toxic pollutants
• is a relatively economical method since it requires
no additional energy and both iron and hydrogen
peroxide are relatively cheap and safe.
Demerits
• large amount of chemicals are required to maintain
the pH of wastewater as wastewater from different
industries have different pH values
• As it includes hydrogen peroxide and ferrous-ion
chemical as a reactant, storage and handling of
them is needed to be addressed. Both these
reactants are reactive and unstable in chemical
properties thus may detonate in storage, resulting
into waste of reagents. Also, H2O2 is explosive
and toxic thus harmful to humans
• It is difficult to separate and recover iron sludge,
which not only causes serious loss of iron species
and reduction of catalytic activity, but also causes
secondary pollution to the environment

Photocatalysis related AOPs
• The use of photocatalytic processes in the water treatment is a relatively new area
with great features.
• The photocatalytic oxidation process leads to degrade organic contaminants from
the wastewater.

Photocatalysis related AOPs cont.…
 In the stages of photocatalytic AOPs, Photocatalyst
adsorbed photon that has a higher energy than its band gap.
 Next, an electron-hole pair (h+) and electrons (e-) are
formed at the valence band and the conduction band
respectively.
 The photoexcited electrons will promote reduction
reactions in conduction band while h+ participates in
oxidation process in valance band.
 This can lead to the activation of dissolved O2 and H2O2,
releasing active radicals (e.g., hydroxyl radicals).
 In aqueous media, the *OH and h+ can effectively oxidize
organic/inorganic pollutants.

Classification of photocatalysis process
 Homogeneous photocatalysis: When both the photocatalyst and reactant are in the
same phase, i.e. gas, solid, or liquid, such photocatalytic reactions are termed as
homogeneous photocatalysis. Metal ion such as Mn(II), Ni(II), Zn(II), Cu(II), Co(II),
Cd(II), Ag(I) and Cr(III) are the most common homogenous photocatalyst.
 Heterogeneous photocatalysis: When both the photocatalyst and reactant are in
different phases, such photocatalytic reactions are classified as heterogeneous
photocatalysis. TiO2, WO3, Fe2O3, ZnO, CdS, ZnS, SnO2, SrTiO3, Cu-TiO2,
Fe2O3/Al2O3, Ru-CeO2, etc are mostly used heterogenous photocatalyst used in
oxidation process.
Photocatalysis related AOPs cont.…

Ultrasound assisted AOP
• Ultrasound assisted AOP is a highly efficient process for removal of this kind of
contaminants and their degradation.
• In aqueous medium, sonolysis involves the production of waves through sound at a
specific frequency, leading to the formation of cavitation bubbles.
• These bubbles allow the decomposition of the water molecule to generate HO•, which
is capable of oxidizing pollutants and degradation of the toxic compounds.

Ultrasound assisted AOPcont….
• Reaction mechanism

Ultrasound assisted AOPcont….
Advantage of Ultrasound assisted oxidation process
 Enhance the rate of mass transfer and chemical reactions
 Improve the surface and reducing the size of solid particles
 Lower the consumption of chemical reagents and generation of waste sludge
 Utilize AOPs or other advanced treatment technologies for the management
of recalcitrant micropollutants

Advantages of AOPs
 Unlike conventional technologies, AOPs can provides the complete
mineralization of pollutants.
 It can be used in combination with other processes for pre-or post-treatment.
 Possess strong oxidizing power with high reaction rates.
 The formation of by-products can be minimized if optimized amounts of
reactants are employed.
 In many cases, AOPs consume less energy compared to thermal destruction
processes (incineration) of liquid wastewaters

Factors affecting the performances of AOP
• Presence of carbonate species - Bicarbonate and carbonate ions present in the
background water matrix will scavenge hydroxyl radicals and reduce the reaction rate
with organics.
• Presence of natural organic matter - Natural organic matter (NOM) present in the
background water matrix will scavenge hydroxyl radicals and reduce the reaction rate
with organics.

Factors affecting the performances of AOP cont.….
• pH - The pH dictates the level at which certain ions important to AOPs will be present,
including carbonate ion, bicarbonate ion, and the anion of hydrogen peroxide.
• Presence of reduced metal ions - Reduced metal ions, such as Fe(II) and Mn(II), present
in the background water matrix will scavenge hydroxyl radicals and reduce the reaction
rate with organics.
• Reactivity of the target compound with hydroxyl radicals - The second order
hydroxyl radical rate constant is an indication of how the AOP reactions will proceed.
AOP reactions tend to be quite rapid with second order hydroxyl radical rate constants on
the order of 108 to 1010 L/mole.s.

Advanced oxidation processes

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