Chemical Oxidation - How Oxidants Work

Transcription

1 Chemical Oxidation - How Oxidants Work Charles Blanchard, PE Regional Engineer Groundwater & Environmental Services, Inc.

2 Chemical Oxidation Overview Direct oxidation is a reaction between a compound and the oxidant Advanced oxidation describes a reaction between a compound and a radical Biodegradation is a common desirable byproduct of chemical oxidation Total mass oxidized = advanced oxidation + direct oxidation + biodegradation

3 Oxidation Basics Oxidation is always coupled with reduction Oxidation = losing electrons Reduction = gaining electrons Oxidizer gains electrons during the reaction becoming reduced 2H 2 + O 2 2H 2 O Each hydrogen loses 1 electron and is oxidized leaving a charge of +1 Each oxygen gains two electrons becoming reduced Total net charge is zero before and after reaction

4 Remediation Oxidation Chemistry 1. Reactants + oxidant products 2. Reactants include COCs, humic matter, other reduced ions 3. Typical oxidants include: oxygen, ozone, hydrogen peroxide, sodium persulfate 4. Ideal products are carbon dioxide, water and halogen ions (e.g Cl - ). 5. Non-ideal products include anything not listed in Products are typically less toxic than COCs

5 Reaction Completion 1. Reactions continue until the reactant, oxidant, or both are expended 2. Reactions terminate for a number of reasons: Limited contact between oxidant and reactants Oxidant and reactant don t directly react Byproducts don t react further 3. Partially oxidized products can commonly be biodegraded.

6 Oxidizer Comparison Oxidizing Species Oxidation Potential (Volts) Relative Oxidation Power (Chlorine as reference) Hydroxyl Radical Activated Persulfate Ozone Persulfate Hydrogen Peroxide Perhydroxyl Radical Permanganate Chlorine Oxygen Ref: Solvey Interox., Hydrogen Peroxide, Fenton s Reagent, 6/93

7 Is an oxidizer a good choice? Do the COC and oxidizer react? Reaction speed Oxidizer cost Electrons transferred per unit mass Byproducts Impurities Reaction mechanism (not always available)

8 Oxidant Mass Efficiency Hydrogen Peroxide Oxidant e - MW e - per Pound {moles per lb} Ozone Regenox Sodium Persulfate Oxygen

9 Potassium Permanganate Sometimes listed as non-specific in chemistry texts Does not react with aromatic compounds i.e. BTEX Strong reaction with double-bonded carbons (VC, TCE, PCE) Final products of ethene oxidation: ethylene glycol or CO 2 depending on ph

10 Permanganate Alkene Reaction

11 Ozone Reactions Less specific than permanganate due to higher oxidation potential Reacts directly with many compounds Reacts with water to form highly oxidizing hydroxyl radicals Breaks down to form oxygen enhancing biodegradation

12 Ozone Reactions (cont.) Can react with water to form hydrogen peroxide Peroxide can react with ozone to form hydroxyl radicals

13 Radical Reactions Sulfate (2.6 ev) and hydroxyl (2.8 ev) radicals are highly oxidizing Consist of hydroxide and sulfate ions that are short one electron Truly non-specific oxidants Exact reaction mechanisms frequently not known

14 Hydroxyl Radical Formation 1. Hydrogen peroxide will react with ozone to form hydroxyl radicals: 2 O 3 + H 2 O 2 2 ( OH) + 3 O 2 2. Hydrogen peroxide will react with iron to form hydroxyl radicals: H 2 O 2 + C OH+ OH - + C + C = Iron or Metal Catalyst; OH = Hydroxyl Radicals 3. Hydrogen peroxide will react with persulfate to form sulfate radicals and hydroxyl radicals: S 2 O H 2 O 2 2SO 4 + 2( OH)

15 Sodium Persulfate Provides persulfate anion (2.1 ev) Can be activated to form sulfate radicals Activators include heat, high ph, metals and hydrogen peroxide Oxidation potential nearly as high as hydroxyl radicals Does not liberate oxygen although sulfate residuals can aid in biodegradation

16 Persulfate (cont.) Persulfate anion reaction S 2 O e- 2SO 4 2- Sulfate radical formation S 2 O [C] 2SO 4 - No oxygen generated by either reaction

17 ph Changes Water broken down to provide oxygen Hydrogen atoms are therefore converted to H + instead of water 3S 2 O C 2 HCl 3 + 4H 2 O 2CO 2 + 9H + + 6SO 4 2-2MnO 4- + C 2 HCl 3 2CO 2 + 2MnO 2 + H + + 3Cl -

18 NOD and Reaction Rate Natural oxidant demand consumes a portion of the added oxidant If NOD had to be fully reacted, oxidation would never be cost effective Reactions have a rate and occur simultaneously COC + [O] NOD + [O] products products Can be determined through bench or pilot scale testing

19 Typical Reaction and Oxidant Mass Calc. C 8 H O 2 9H 2 O + 8CO 2 Electrons transferred 50 Mass O 2 / mass octane 3.5 Mass peroxide per mass octane: 3.5 X / = 7.4 lb peroxide / lb octane

20 Oxidant Demand Calculation Determine mass of COC in soil and groundwater Ensure that LNAPL is accounted for Select oxidant Determine moles of electrons required Determine mass of oxidant required Estimate efficiency

21 Analytical Recommendations Soil Samples Vadose zone Upper saturated zone Lower saturated zone Fraction Organic Carbon Water Samples COCs Chemical oxygen demand Transition metals Carbonates

22 Oxidant Efficiency Oxidant efficiency is a function of: Oxidant type Subsurface distribution Presence of catalysts Natural humic matter Reaction rate with the COCs Oxidant efficiencies can by 30% or lower depending on these variables

23 Summary Understand the reaction between the particular COC and oxidant Determine the final disposition of the products Evaluate if other processes such as bioremediation will come into play Calculate mass of COC Determine the mass of oxidant required Use a reasonable efficiency