Advanced Oxidation Processes (AOPs)Presented by

Shangali Pius Anagu Chidi Charles Abang Roger Hongwei Li Tudor Zankov

definition Advanced oxidation processes (AOPs) are used for the destruction of synthetic organic chemicals (SOCs) in water, apart from disinfection and deactivation of pathogenic microorganisms that are difficult to degrade biologically. These processes include the application of ozone, hydrogen peroxide, and ultraviolet light, either individually or in combination O3/UV; H2O2/UV; and O3/H2O2/UV.

These processes are mediated by free-radical reactions, and are becoming increasingly complex from the standpoint of scientific analysis. Oxidation processes based on generation of hydroxyl radical (HO) intermediates. Combinations based on H2O2, O3 and UV have been investigate most heavily

All these AOPs have an item in common: the generation of hydroxyl radical, which is extremely reactive, and possesses a very high oxidation potential. As a matter of fact, the hydroxyl radical shows an oxidation potential over 3 Volts, surpassed only by the negative oxidation potential of fluorine (see Table 1).

Table 1: Comparison of various oxidising Potentials Electrochemical Oxidation Potential, (EOP)V 3.06 2.80 2.42 2.08 1.78 1.49 1.36 1.27 1.23

Oxidizing agent Fluorine Hydroxyl radical Oxygen (atomic) Ozone Hdrogen Peroxide Hypochlorite Chlorine Chlorine dioxide Oxygen (Molecular)

EOP relative to Chlorine 2.25 2.05 1.78 1.52 1.30 1.10 1.00 0.93 0.90

Sources: www-rcf.usc.edu/~pirbazar/group_homepage/resear/adv.html www.iupac.org/publications/pac/1998/pdf/7012x2271.pdf

Examples of AOP Methods

Theory of AOPs Involves the generation and use of the HO as a strong oxidant to destroy compounds that cannot be oxidized by conventional oxidants such as O2, O3 and Cl HO reacts with dissolved constituents in a series of oxidation reactions until the constituents are completely mineralized They are non selective in their mode of attack and are able to operate at normal temperature and pressure Able to oxidize almost all reduced materials present in wastewater without restriction to specific classes or groups of compounds AOP differs from other treatment processes because wastewater compounds are degraded rather than concentrated or transferred into a different phase Because no secondary materials are generated, there is no need to dispose of or regenerate material

Ultra Violet (UV) Ultraviolet light (UV) is a general term for electromagnetic radiation emitted from the region of the spectrum lying beyond the visible light (400 nm) and before x-rays (100 nm). UV is widely accepted as a reliable, environmentally friendly solution for water disinfection.

UV disinfection system UV exhibits strong bactericidal and virucidal properties, and unlike some chemical disinfection processes, UV does not produce any harmful by-products. Innovative reactor design allows for optimal UV radiation efficiency. A broad range of equipment capabilities provides for best suited solution depending on water quality and flow rate.

Chlorine Chlorination is used for many reasons in wastewater treatment such as in disinfection, taste and odor control, color removal, oxidation of ammonia, iron, manganese and sulfide and BOD removal. Most common chlorine compounds used in waste water treatments are chlorine gas ( Cl2), calcium hypochlorite [Ca(OCl2)], sodium hypochlorite (NaOCl) and chlorine dioxide (ClO2). The rate and efficiencies of reactions when the chlorine is added to the wastewater depends on temperature, pH, buffering capacity and the form in which the chlorine is supplied. Chlorine in aqueous solution produces hypochlorus acid and hypochlorite ion. Cl2 + H2O HOCL + H+ + Cl- (hypochlorous acid) HOCL H+ + OCL- (hypoclorite ion)

Chlorine contd Amount of HOCL and OCL- present in water called free chlorine residual. The disinfection power of HOCL is 40 80 times greater than that of OCL-. That is why wastewater at lower PH is easy to disinfect by chlorination. While the chlorine gas lower the pH hypochlorite in solution raises the pH and favors the formation of OCL-, which is less effective than HOCL. The disinfection efficiency of chlorine depends on contact time, chlorine dosage, temperature, pH, nature of liquid and suspended matter and type of microorganism. Chlorine is cheap, effective, available in large quantities, non toxic in low concentration to higher forms of life the basic disadvantage include acid generation, build up of total dissolved salts and formation of potentially carcinogenic halogenated organic compounds

Hydrogen peroxide H2O2 A powerful, safe and effective oxidant that looks like water in its appearance, chemical formula and reaction products2 H2O2 ----> 2 H2O + O2 (hydrogen peroxide ----> water + oxygen)

Application in odor control, BOD/COD removal, metal oxidation and precipitation in wastewater treatment, flocculation and disinfection/bio-control. Used mostly in combination with ozone and UV in AOPs

Technologies Hydrogen Peroxide/UV HO formed when water containing H2O2 is exposed to UV light ( = 200 280nm) The photolysis of H2O2 given as H2O2 + UV (or h = 200 280nm) HO + HO

Applied in the oxidation of trace constituents found in treated water such as Sex and steroidal hormones Human prescription and non-prescription drugs Veterinary and human antibiotics Industrial and household wastewater products

Technologies Ozone/UV Production of free radical HO with UV light by the photolysis of ozone O3 + UV (or h,