CHEMICAL OXIDATION The use of oxidizing agents without the need of microorganisms for the reactions to proceed oxidizing agents : O 3, H 2 O 2, Cl 2 or HOCl or O 2 etc catalysts : ph, transition metals, light,..etc
Types of oxidation Processes 1. Conventional oxidation processes - Use common oxidizing agents such as ozone, chlorine without production of highly reactive species 2. Oxidation processes carried out at high temperature/ high pressure - Produce highly reactive species, hydroxyl radicals (HO )
3. Advanced oxidation processes (AOPs) - Produce highly reactive species, hydroxyl radicals (HO ) using oxidizing agents and catalysts Oxidation rate observed HO > O 3 > H 2 O 2 > HOCl > ClO 2 > KMnO 4 > Cl 2 > O 2
Application o o For nonbiodegradable, toxic organic compounds For compounds that inhibit microbial growth o Effective for destruction of many inorganic compounds o Elimination of odorous compounds e.g. H 2 S
Extent of degradation 1. Primary degradation - a destruction change in the parent compounds 2. Acceptable degradation - a structural change in the parent compounds to the extent that toxicity is reduced 3. Ultimate degradation (mineralization) - conversation of organic carbon to inorganic carbon CO 2 4. Unacceptable degradation (fusing) - a structural change in the parent compounds resulting in an increase in toxicity
Stoichiometry Stoichiometric relationship between an oxidant and the compounds to be treated To estimate the required oxidant dosages for the treatment General approach Express the half reaction for each oxidant in terms of free reactive oxygen O Exp. Oxygen, O 2 2O
For convenient : 1. Use the electrochemical half-reaction (reduction reaction) of the oxidant 2. Balance e - with half reaction of water (that gives O in this approach) H 2 O O + 2 H + + 2 e - ( O = ½ O 2 ) Exp. Ozone O 3 + 2H + + 2e - O 2 + H 2 O H 2 O O + 2 H + + 2 e - (O = ½ O 2 ) O 3 O + O 2
For ultimate conversion C a H b O c + d O a CO 2 + (b/2) H 2 O Balance equation using free reactive oxygen produced from an oxidant moles oxidant required for conversion of a mole of pollutant Ex. The ultimate conversion of phenol using O 3 C 6 H 5 OH + 14 O 3 6 CO 2 + 14 O 2 + 3 H 2 O
Note : Wastewater contains a wild variety of compounds Adapt free reactive oxygen approach to the surrogate COD Oxidant demand = (2/n)(MW/32) COD COD = Chemical oxygen demand (mg O/L) MW = Molecular weight of oxidant n = mole O/mole oxidant Exp. For O 3 O 3 stoichiometric demand = (2/1)(48/32) COD = 3 COD
Exp. Estimate stoichiometric dosage of HClO needed to treat an effluent having COD = 288 mg/l HOCl + H + + 2 e - Cl - + H 2 O H 2 O O + 2 H + + 2 e- (O = ½ O 2 ) 1. Mole of O per mole of HOCl HOCl Cl - + O + H + 2. Oxidant demand Oxidant demand = (2/n)(MW/32) COD = (2/1) (52/32) (288) = 936 mg/l
How to choose an oxidant for treatment the stoichiometric requirement -- yield of free reactive oxygen ( e.g. Moles of O per kg oxidant) Establish a Ranking of oxidants based on - cost - effectiveness of oxidant Choose a suitable oxidant
Applicability Apart from O 2, most oxidants are expensive and not competitive with biological treatment for high strength, large-volume wastewater. Chemical oxidation processes are designed for toxic, inhibitory and refractory compounds - to reduce toxicity - to increase biodegradability of the parents compounds at dosages far less than required for ultimate degradation Coupling chemical oxidation and biological treatment
Oxidants O 2 (in wet air oxidation) Ozone (O 3 ) Hydrogen peroxide (H 2 O 2 ) Fenton s reagent (Fe(II)/ H 2 O 2 ) H 2 O 2 /UV O 3 /H 2 O 2 Advanced Oxidation Processes (AOPs)
Ozone : * powerful oxidant * unstable gas which decomposes to O 2 at normal temperature * decomposition is accelerated by contact with solid surfaces, chemical substances and by heat * generated by electric discharge of air, O 2
The corona discharge device can be fabricated and configured in many different ways. The primary feature is to generate a corona between two electrode surfaces air or oxygen pass between these electrodes high-energy electrons bombard gas molecules gas molecules are ionized forming a light emitting gaseous plasma referred to as a corona
Reaction of ozone O 3 (g) O 3 (g) Added to water O 3 (aq) M Direct reaction oxidized M Φ S OH M R Indirect reaction OH - M : organic / inorganic molecule S : scavenger Φ : by-products
Direct reactions of ozone Ozone can be electrophilic and nucleophilic I. Reaction with organic compounds Examples : - oxidation of alcohols to aldehydes, organic acids - substitution of an oxygen atom onto an aromatic ring - clevage of carbon double bonds II. Reactions with inorganic compounds a. Oxidation of ammonia 4 O 3 + NH 3 NO - 3 + 4 O 2 + H 3 O +
b. Oxidation of iron and manganese Fe 2+ (aq) 2 Fe 3+ (aq) 2Fe(OH) 3(s) O 3 O 2 Mn 2+ (aq) Mn 4+ (aq) MnO 2 (s) O 3 O 2 c. Oxidation of nitrite NO 2 - + O 3 NO 3 - + O 2 Indirect reactions of ozone Free radicals reactions Radicals produced will react with compounds in water
Applications Phenol and aromatic hydrocarbon destruction Color removal Drinking water purification Water bottling plants Swimming pools Laundry recycling
Hydrogen peroxide - Colorless liquid at room temperature - decomposes easily to give O 2 - the decomposition is slow in dilute solution or in pure solution well conserved in dark Reactivity of H 2 O 2 : 2 types Direct or molecular reactivity indirect or radical reactivity
As an oxidant, H 2 O 2 can react with number of organic and inorganic pollutants. Examples are shown below. I. Direct reaction a) Treatment of sulfide, H 2 S rapid oxidation : H 2 S + H 2 O 2 S + 2H 2 O b) Treatment of cyanides H 2 O 2 + CN - + H + CNO - + H 2 O c) Purification of iron and manganes containing groundwater H 2 O 2 + 2Fe 2+ + 2H + 2Fe 3+ + 2H 2 O Fe(OH) 3 (S)
II. Direct reaction : Reaction of hydroxyl radicals formed in H 2 O 2 decomposition Limit : - oxidation by H 2 O 2 alone is not effective for high concentration of certain contaminants e.g. Highly chlorinated compounds : - Low rate of reaction at reasonable [H 2 O 2 ]
Advanced Oxidation Processes (AOPs) The advanced oxidation process can be used to decompose many hazardous chemical compounds to acceptable levels. The term advanced oxidation processes refers specifically to processes in which oxidation of organic contaminants occurs primarily through reactions with hydroxyl radicals. Source: http://www.spartanwatertreatment.com/advanced-oxidation-processes.html
The most widely applied AOPs are: Peroxide/ultraviolet light (H 2 O 2 /UV), Ozone/ultraviolet light (O 3 /UV), Hydrogen peroxide/ozone (H 2 O 2 /O 3 ) Hydrogen peroxide/ozone/ultraviolet (H 2 O 2 /O 3 /UV)
H 2 O 2 /UV process - Irradiation of UV light ( λ < 365 nm) can break H-O bond chain reactions Initiation : H 2 O 2 + hν 2 OH ε = 18.6 L mol -1 cm -1 Propagation : H 2 O 2 + 2 OH H 2 O + HOO Termination : HOO + HOO H 2 O 2 + O 2
O 3 /UV process O 3 decomposes to produce radicals at high ph values (photolysis). Initiation : O 3 + OH - OH 2 + O 2 - OH 2 O 2 - + H + Propagation : O 3 + O - 2 2O 2 + OH O 3 + OH OH 2 + O 2 Termination : OH 2 + OH O 2 + H 2 O hν Radicals produced will react with compounds in water Source: http://www.spartanwatertreatment.com/advanced-oxidation-uv-ozone.html
Chlorine Use in water and wastewater treatment e.g. Color removal At room temperature, Cl 2 can dissolved in water giving HClO. Cl 2 + H 2 O HClO + Cl - + H + HOCl + H 2 O ClO - + H 3 O + k a = 1.6-3.2 x 10-8 Hypochlorous acid hypochlorite ion
Nature of species in water depends on ph (HClO, ClO -, H 2 ClO +, Cl 2 O) Cl 2 and its derivatives can react with organic Matters in water by : - oxidation reaction - addition / substitution reaction ==> results in formation of organochlorinated compounds ==> reduce the use of chlorine
Applications of chlorine Treatment of CN - - change CN - to CNO - and finally to N 2 Reactions with NH 3, NH 4 + - result in formation of chloroamines (NH 2 Cl, NHCl 2, NCl 3 ) - possible reactions leading to disappearance of chloroamines : 4 NH 2 Cl + 3Cl 2 + H 2 O N 2 + N 2 O + 10HCl 2 NH 2 Cl + HClO N 2 + H 2 O + 3HCl NH 2 Cl + NHCl N 2 + 3HCl
Overall reactions for total degradation of NH 3 2 NH 3 + 3 HOCl N 2 + 3H 2 O + 3HCl 2 NH 3 + 3 Cl 2 N 2 + 6H + 6Cl - CHLORINE DEMAND Molar ratio Cl 2 : NH 3 = 3 : 2 Mass ratio Cl 2 : NH 3 = 7.6 : 1 amount of Cl 2 used in reactions = Cl 2 added to water - total available residual chlorine remaining at the end of the specific time (Cl 2, HClO, ClO - )
CHLORINE DEMAND Chlorine residuals A B Zero demand residual Chlorine demand D Cl 2 added (mg/l)
Point A : low chlorine residual = > consumption of chlorine by reducing compounds e.g. Fe 2+, Mn 2+, H 2 S and organic matters Above point A : Formation of chlorinated org compounds e.g. chloroamines Point B : All compounds has been reacted B D : some chlorinated org compounds are oxidized (then chlorine residual is reduced to Cl - )
Point D : Breakpoint (most of chlorinated organic Compounds are oxidized) Beyond D : - presence of some resistant chlorinated compounds - all added chlorine residual is free available chlorine (Cl 2, HClO, ClO - )
Chemicals Hypochlorites (salt of HClO) - NaOCl, Ca(OCl) 2 give OCl - - use in small installation e.g. Swimming pools Liquid chlorine - used in water treatment plant e.g. In US ClO 2 - prepared by following reactions 2 NaClO 2 + Cl 2 2ClO 2 + 2NaCl 2 NaClO 2 + 4HCl 4ClO 2 + 5NaCl + 2H 2 O
CHOICE OF USAGE - Sterilization of water ClO 2 is superior to chlorine in destruction of spores, bacteria s, virus and other pathogen organisms - Industrial water treatment - chlorine are more reactive than ClO 2 and will react with most organic compounds (ClO 2 does not react with NH 3 or NH 4+ )
References http://www.iwawaterwiki.org/xwiki/bin/view/articles/chemicaloxidatio NAPPLICATIONSFORINDUSTRIALWASTEWATERS http://www.lenntech.com/products/chemicals/water-treatment-chemicals.htm http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399252§ion=1.4.7 ( 6 Feb 2011)