chemical oxidation and reduction

PIERO M. ARMENANTENJIT

Chemical Oxidationand Reduction


Oxidation State and OxidationNumber

• The oxidation state of an atom in a reactant isassociated with the electronic structure of theatom resulting from the chemical bond inwhich it is involved

• In ionic bonds each atom is clearly associatedwith a net charge due to the acquisition or lossof electrons during the formation of the bond(e.g., in NaCl the sodium ion has an electronwhich it received from the chlorine atom)



• In covalent bonds the charge is shared by theatoms involved in the bond. However, sinceeach atom will have a different level of affinityfor electrons (electronegativity) it isconventionally assumed that the electronsinvolved in the bond reside with the mostelectronegative atom in the bond

• The oxidation number or (electro)valence isthe number of electrons that the atom in thebond has gained or lost during the formationof the bond



• In each non-ionized molecule the total sum ofthe formal charges must be zero

• The same element can have different oxidationnumber in different molecules


Examples of Oxidation Numbers ofAtoms in Different Molecules

Chemical Species Valence

HNO3 H=+1; N=+5; O=-2

HNO2 H=+1; N=+3; O=-2

NH3 H=+1; N=-3

HCN H=+1; C=+2; N=-3

CH4 H=+1; C=-4

CO2 C=+4; O=-2

C2H5OH H=+1; C=-4; O=+2


Oxidation and Reduction Reactions• Oxidation and reduction reactions, or redox

reactions, are those chemical reactions in whichthe oxidation state of the reactants changes duringthe reaction

• Two types of reactions are always simultaneouslyinvolved in every redox reactions:- oxidations, i.e., the loss of electrons by a

chemical species (which increases its oxidationnumber) which becomes oxidated

- reductions, i.e., the acquisition of electrons bya chemical species (which decreases itsoxidation number) which becomes reduced


Oxidation and Reduction ReactionsSince the total number of electrons during areaction must remain constant the number ofelectrons gained by the molecules containing theoxidising species (which becomes reduced) mustequal the number of electrons lost by themolecules containing the reducing species (whichbecomes oxidated)


Oxidation and Reduction ReactionsExample: in the reaction

a H2S + b HNO3 → c H2SO4 + d NO2 + e H2O

• S has changed oxidation state from -2 to +6losing 8 electrons

• N has changed oxidation state from +5 to +4gaining 1 electron

Hence it must be that:

a/b = 1/8

The balanced reaction is:

H2S + 8 HNO3 → H2SO4 + 8 NO2 + 4 H2O


Oxidation and Reduction Reactionsin Wastewater Treatment

• A number of wastewater pollutants canundergo redox reactions with the appropriateaddition of a oxidizing or reducing agent to thewastewater

• The result of such reaction it typically theprecipitation of contaminants (especially in thecase of inorganic pollutants such as heavymetals) or their conversion to a much lesstoxic form (e.g., an organic waste mineralizedto CO2 and H2O)


Common Oxidation Agents Used inWastewater Treatment

The following strong oxidants find application inwaste treatment:• Sodium hypochlorite ...................... NaClO

• Calcium hypochlorite ..................... Ca(ClO)2

• Chlorine ........................................... Cl2• Potassium permanganate .............. KMnO4

• Hydrogen peroxide ......................... H2O2

• Ozone............................................... O3

• Oxygen............................................. O2


Common Reduction Agents Used inWastewater Treatment

The following strong reducing agents findapplication in waste treatment:

• Sulfur dioxide.................................. SO2

• Sodium bisulfite .............................. NaHSO3

• Ferrous sulfate................................ FeSO4


Chlorine as an Oxidation Agent• Chlorine gas dissolves in water where it

hydrolyzes according to the reaction:

Cl2 + H2O ↔ HOCl +H+ + Cl-

• Hypochlorous acid is a weak acid whichdissociates forming:

HOCl ↔ H+ + OCl-

• The equilibrium equations are:

KHOCl H Cl

ClMh = = ⋅

+ −−

b gdidibg2

4 24 5 10. KH OCl

HOClMi = = ⋅

+ −−

did ib g 2 7 10 8.


Sodium Hypochlorite and CalciumHypochlorite as Oxidation Agents• Sodium hypochlorite and calcium hypochlorite

also hydrolyze when placed in solution:NaOCl ↔ Na++ OCl-

Ca(OCl)2 ↔ Ca+++ 2 OCl-

H+ + OCl- ↔ HOClpartially reforming the undissociated acid.

• The sum of the (OCl-) and (HOCl) concentrationis called the free available chlorine.

• The distribution of the ionic species inequilibrium is a strong function of pH


Ozone as an Oxidizing Agent• Ozone is a gas at normal pressure and

temperature. Its solubility in water is afunction of its partial pressure andtemperature

• Ozone is generated by high voltage dischargein air or oxygen

• Ozone is unstable and tends to react to form:O3 + H+ + e- → O2 + H2O

• Ozone is a very strong oxidizing agent

• Ozone is very effective as a decoloration agentand as a oxidant of organic material


Hydrogen Peroxide as an OxidizingAgent

• Hydrogen peroxide (H2O2) is a colorless water(30 - 70 % solution) which in the presence of acatalyst (such as iron) reacts to form:

H2O2 +2 H+ + 2 e-↔ 2 H2O

• Hydrogen peroxide is a strong oxidanttypically used in the treatment of cyanides andwastewaters containing organic materials


Oxygen as Oxidizing Agent in WetOxidation

• Wet oxidation is a process in which oxygendissolved in the wastewater under pressure isused as an oxidizing agent at high temperatureand pressure

• Typical temperatures and pressures are 150 - 325oC and 2000 to 20,000 kPa (gauge pressure),respectively

• The process is extremely effective in oxidizingorganic materials, organic sulfur, cyanides,pesticides, and other toxic compounds withremoval efficiencies of the order or 99+%


Wet Oxidation Flow Diagram

After Freeman, Standard Handbook of Hazardous Waste Treatment andDisposal, 1989, p. 8.78


Typical Applications of RedoxReactions to Wastewater Pollutants

• Inorganic Pollutants- heavy metals- cyanides- sulfides

• Organic Pollutants- phenol and chlorophenols- pesticides- ammonia nitrogen and amines- sulfur-containing organic compounds (e.g.,

mercaptans)


Cyanide Removal from Wastewaters• Cyanide (CN-) is a common pollutant found in

many industrial applications, especially metalplating

• Because of its extreme toxicity even at lowconcentrations cyanides must be removedprior to wastewater discharge

• Chemical treatment of cyanide typicallyinvolves its oxidation with a strong oxidizingagent to cyanate (CNO-), followed by theoxidation of the cyanate to carbon dioxide,nitrogen, water and NaCl


Oxidation Reactions for theConversion of Cyanide to Cyanate

Oxidizing Agent Reaction

Sodiumhypochlorite

2 NaCN + 5 NaOCl + H20 → N2 +2 NaHCO3 + 5 NaCl

Calciumhypochlorite

4 NaCN + 5Ca(OCl)2 + 2 H20 → 2N2+ 2 Ca(HCO3)2 + 3 CaCl2 +4 NaCI

Chlorine 2 NaCN + 5 CI2 + 12 NaOH → N2 +2 Na2CO3 + 10 NaCI + 6 H20


Oxidation Reactions for theConversion of Cyanide to Cyanate

Oxidizing Agent Reaction

Potassiumpermanganate

NaCN + 2 KMnO4 + 2 KOH →2 K2MnO4 + NaCNO + H20

Hydrogenperoxide

NaCN + H2O2 → NaCNO + H2O

Ozone NaCN + O3 + H20 → NaCNO + O2


Mineralization of Cyanide withChlorine or Hypochlorite

Mineralization (i.e., conversion to CO2, H2O and N2) is carriedout sequentially in two steps. Reaction time = 10 - 30 min.

• Step 1: Cyanide conversion to cyanate (pH = 9 -11)

NaCN + 2 NaOH + Cl2 → NaCNO + 2 NaCl + H2O

using chlorine, or

NaCN + NaOCl → NaCNO + NaCl

using hypochlorite

• Step 2: Cyanate mineralization with Cl2 (pH = 8.5)

2 NaCNO + 4 NaOH + 3 Cl2 →N2 + CO2 +6 NaCl + H2O


Removal of Iron and Manganese fromWastewater

• Soluble ferrous (Fe2+) and manganous (Mn2+)ions are removed by precipitation as Fe(OH)3and MnO2, respectively via oxidation

• The reaction rate is a function of pH, alkalinity,and impurities that can act as catalyst

• As oxidation agents oxygen (O2), chlorine (Cl2),or permanganate (MnO4

-)are typically used


Oxidation Reactions Involved in theRemoval of Iron and Manganese

• With oxygen (pH = 7 for Fe; pH = 10 for Mn; 10 - 20 min):

2 Fe2+ + 1/2 O2+ 5 H20 = 2 Fe(OH)3↓ + 4 H+

Mn2+ + 1/2 O2+ H20 = MnO2↓ + 2 H+

• With Cl2 (fast reaction):

Fe2+ + 1/2 CI2 + 3H20 = Fe(OH)3↓ + Cl- + 3 H+

Mn2+ + Cl2 + 2 H20 = MnO2 + 2 Cl- + 4 H+

• With KMnO4 (pH = 6 - 9; very fast reaction):

3 Fe2+ + MnO4- + 7 H20 = 3 Fe(OH)3↓ + MnO2↓ + 5 H+

3 Mn2+ + MnO4- + 2 H20 = 5 MnO2↓ + 4 H+


Removal of Residual Nitrogen(as Ammonia) via Chlorination

Ammonia nitrogen can be chemically removed viareaction with chlorine according to the reaction:

2 NH3 + 3 HOCl → N2 3 H2O + 3 HCl

The reaction pH is typically in the range 6 - 7


Oxidation of Organic Material in WastewaterThe BOD or COD of wastewater can be chemicallyreduced using oxidation agents such as Cl2 orozone. The typical dosage to be used forchemical oxidation of organic material is:

Dosage (lb/lb removed)

Chemical Use Range Typical

Chlorine BOD5Reduction

0.5 - 2.51.0 - 3.0

1.752.0

Ozone CODReduction

2 - 43.0 - 8.0

3.06.0

After Metcalf and Eddy, Wastewater Engineering, 1991, p. 755


Additional Information and Examples on ChemicalOxidation and Reduction in Wastewater

Additional information and examples can be found in thefollowing references:

• Sundstrom, D. W. and Klei, H. E., 1979, WastewaterTreatment, Prentice Hall, Englewood Cliffs, NJ, pp.368 - 384.

• Wentz, C. W., 1989, Hazardous Waste Management,McGraw-Hill, New York, pp. 152 - 153.

• Corbitt, R. A. 1990, The Standard Handbook ofEnvironmental Engineering, McGraw-Hill, New York,pp. 9.27 - 9.28.


Additional Information and Examples on ChemicalOxidation and Reduction in Wastewater

• Metcalf & Eddy, 1991, Wastewater Engineering:Treatment, Disposal, and Reuse, McGraw-Hill, NewYork, pp. 739 - 740; 755.

• Weber, W. J., Jr., 1972, Physicochemical Process forWater Quality Control, Wiley-Interscience, John Wiley& Sons, New York, pp. 363 - 411.

• Freeman, H. M. (ed.), 1989, Standard Handbook ofHazardous Waste Treatment and Disposal, McGraw-Hill, New York, pp. 7.41 - 7.76.

• Eckenfelder, W. W., Jr., 1989, Industrial WaterPollution Control, McGraw-Hill, New York, pp. 300 -311.

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